chapter 1 an introduction to the nbn photodetector

1

Chapter 1 An Introduction to the nBn Photodetector

Infrared detector systems are integral to a variety of surveillance and remote sensing

applications for the military. They are anticipated to become prevalent in civilian markets

supplying equipment used to monitor environmental factors and the weather, make

astronomical observations, meet the demands of law enforcement, and perform medical

diagnostics. These systems are frequently built around focal plane arrays, which consist of

as many as millions of individual photodetectors.1,2

Future infrared photodetector systems must possess reduced cooling requirements, consume

less power, exhibit longer lifetimes, and have improved manufacturability. The highest

performing infrared photodetectors are currently cooled to cryogenic temperatures, which

enables optimal operation by decreasing internal detector noise.3 Warmer temperature

operation will enable the use of lower cost and less bulky coolers with decreased power

requirements. This is anticipated to permit more extensive field use and result in a decrease

of the, sometimes critical, wait between powering on the imaging system and being able to

use it. Increasing the yields and simplifying the fabrication requirements of the detector

arrays are projected to result in substantial cost savings.2,4,5

The nBn photodetector6, the subject of this thesis, is a recently introduced class of midwave

infrared photodetectors developed as an alternative to those photodetectors currently

incorporated into infrared detector systems. The nBn photodetector architecture specifies an

n-type absorption layer, a Barrier layer, and an n-type contact layer. The barrier layer is

constrained to have both a negligible valence band energy offset with the absorption layer,

which allows the free passage minority carrier holes, and a large conduction band energy

offset with the absorption layer, which blocks the flow of majority carrier electrons. Nowhere

in the device is the Fermi level near the middle of the bandgap. This suppresses the

Shockley-Read-Hall (SRH) generation current that is a primary noise source in cooled7,8,9, as

well as some uncooled10, infrared p-n junction photodiodes; the depletion region is the

primary source of SRH generation current in p-n junction photodiodes, which is a

consequence the Fermi level in the depletion region being located in the middle of the

bandgap. In addition, the unpassivated nBn photodetector exhibits negligible surface

leakage current.6 The surface leakage current path is disrupted by the barrier layer, which is

not etched during processing. Surface leakage current is of considerable concern in p-n

1.1 Focal Plane Arrays

2

junction photodiodes,7 particularly if they are fabricated from semiconductor materials with

narrow bandgap energies. It is common to passivate p-n junction photodiodes during

fabrication to reduce surface leakage current.2,7,11 The nBn photodetector, through the near-

elimination of SRH and surface leakage currents, requires less cooling to operate optimally

and has simpler processing requirements than the p-n junction photodiodes. The nBn

architecture may be extended to different material systems, but the focus of this work is on

the InAs-based nBn photodetector intended for midwave infrared operation.6

1.1 Focal Plane Arrays: A Brief Background

Infrared imaging systems trace their origins to 1800 when William Herschel,12 using a prism

and a thermometer, discovered infrared radiation. The earliest infrared radiation detection

systems were based on thermometers, thermocouples, and bolometers.13-16 Subsequent

work led to achievements including lead salt photon-based infrared detectors,17,18 but it was

not until the introduction of the transistor in the late 1940s that the development of modern

infrared detectors became possible. Research conducted through the 1950s, spurred by the

transistor, led to improved semiconductor purification and material growth techniques. During

this time, the III-V semiconductors were also identified, and infrared detectors based on

extrinsic germanium, InSb, and variable-gap HgCdTe were fabricated.19 The III-V

semiconductor family became of particular interest to researchers working with molecular

beam epitaxy (MBE), which was introduced in the early 1970s and permitted the fine control

of semiconductor growth conditions not possible with contemporary thin film deposition

techniques.20 In the mid-1970s, silicon emerged as an important material for imaging with the

advent of charge coupled devices (CCDs), which had a substantial impact on the industry

due to the integration of detection and readout functions on the same chip.19

Photolithographic processing techniques, adapted from the printing industry, revolutionized

semiconductor device processing by enabling complex device and chip designs.19 Before

photolithography, CCDs, and other charge transfer devices (CTDs) became available, the

first infrared detector systems were limited to single elements and sparsely-populated linear

arrays of elements.2 These elements and modestly sized arrays are rastered along both the

x and y axes to create an image of the scene. The scans are complex to implement, and the

detector systems possess limited sensitivity as a consequence of short integration times.

Photolithography, combined with etching techniques, advanced the state of the art by


3

facilitating the fabrication of two-dimensional matrices of infrared detectors. The production

of the first forward looking infrared (FLIR) detector arrays occurred in the 1960s.21

Beginning in the 1970s, advances in CTD technologies supported the development of two-

dimensional detector arrays with thousands of elements.2,7 The earliest were 16 x 16 and 32

x 32 arrays, which are stepped over the entire field of view to form an image. These small

two-dimensional scanned arrays led to larger staring arrays, also called Focal Plane Arrays

(FPAs). It is not necessary to raster staring arrays, as these detector arrays contain enough

elements to image the full scene.2 Focal plane arrays are considered critical to advanced

infrared imaging systems. Staring arrays, by avoiding inefficiencies associated with

mechanical scanning, reduce the complexity and cost of infrared detection systems.7 Larger

arrays also offer better sensitivity, as a result of significantly increased integration times.

However, they have nontrivial signal processing requirements and place difficult demands on

the imaging optics. There are also producibility concerns, due to low yields, for some

material systems.2

FPAs can be monolithic or a hybrid combination of detector elements and multiplexing

readout circuitry.7 A monolithic structure, in which the detector and multiplexing functions are

integrated, is not always an option. Despite research into multiplexers produced from

narrow-gap semiconductors, silicon remains the only material system producing marketable

monolithic FPAs. For this reason FPAs are frequently hybrids, meaning the detector array

and the associated multiplexers are fabricated from different materials and are electrically

and physically joined using a technology such as indium bumps. Hybridization has the

benefit of allowing the detector and multiplexer to be optimized independently of one another.

In the case of a hybrid, once the detector array and multiplexer are produced, indium bumps

are deposited on both, the two are aligned, and then they are pressed together to make

electrical contact.1,2 The detectors may be illuminated from the frontside, with the radiation

passing through the multiplexer, or from the backside, with the radiation passing through the

substrate underpinning the detector array. The latter is usually chosen to avoid transmission

losses due to the opaque metallization on the multiplexer. If the substrate is not transparent

to the radiation of interest, it must be thinned to minimize undesirable absorption. It may also

be thinned to improve the resolution and response of the FPA.7

Uniformity in the performance of the constituent detectors is key to the overall performance of

FPAs.2 The performance of a photon detector is strongly dependent on the temperature of

the device; a small variation in operating temperature across an array substantially impacts


4

the quality of the image. Achieving lower operating temperatures consumes more energy, as

multiple cooling stages are required. Four are not uncommon. It is common for dewars to

cost more than the FPA and to be more labor intensive to produce.2,22 An accepted goal of

infrared detector system research is the reduction of cooling requirements enabling long-lived

and inexpensive coolers with modest power consumption, such as thermoelectric coolers

permitting operating temperatures of 200 K and higher, to be used.3

1.2 Emission and Atmospheric Transmission of Infrared Radiation

Infrared detectors monitor thermal radiators, and these may be approximated as blackbody

sources. The greater the thermal energy of the object, the higher the energies of the emitted

photons. The spectral radiant exitance of a blackbody source, Meλ(λ,T) is obtained from the

Planck radiation law (see for example Reference 23), and may be expressed as:

Me,λ λ,T( )=2πhc2

λ5 exp hcλkT

−1

−1

(1.2.1)

where λ is the wavelength, h is Planck’s constant, c is the velocity of light, and k is

Boltzmann’s constant. As plotted in Figure 1.2.1, the peak power emitted by hotter objects

occurs at lower wavelengths, while the power emitted at all wavelengths increases. The

spectral window spanning 2 to 12 microns coincides with a majority of the radiation emitted

by objects near room temperature. Infrared systems designed to monitor earth-bound

objects target this wavelength range.1,23

Infrared photodetection systems are often used as remote sensors, which has led to infrared

detectors commonly being designed to operate in one of two atmospheric transmission

windows as shown in Figure 1.2.2: the 3-5 micron medium wavelength infrared (MWIR)

window or the 8-14 micron long wavelength infrared (LWIR) window. Operation in the MWIR

atmospheric window is typical when the application relies on contrast resolution more than

detector sensitivity, comparatively hot objects are monitored, and operation occurs during

clear weather. High humidity conditions do not affect the atmospheric transmission of

radiation in this window, however the LWIR range is better suited for foggy, hazy, dusty, or

misty conditions.1

1.2 IR Emission and Transmission

5

Figure 1.2.1 Emission spectra of blackbody radiators at selected temperatures.

Figure 1.2.2 Atmospheric transmission spectrum of radiation. Transmission through 6000 feet at sea level.1 Reproduced with kind permission from Rogalski, A. and Chrzanowski, K., (2002). “Infrared devices and techniques,” Opto-Electronics Review, 10 (2), pp. 111-136.

1.3 Noise in MWIR Photodiodes

6

1.3 Noise in Semiconductor-Based MWIR Photodetectors

Noise in infrared detection systems is attributable to statistical fluctuations in the detected

signal as well as fluctuations in the flow of spurious charge carriers created through means

other than by the absorption of a signal photon, such as through thermal processes.24

Thermal processes are particularly important for narrow-bandgap semiconductor-based

photodetectors operating near room temperature, as the energy of the charge carriers, kT, is

comparable to the transition energy.3 Some common types of internal noise in infrared

detectors are Johnson-Nyquist, 1/f, generation-recombination, diffusion, tunneling, and

surface leakage.19,23 Noise may also arise through external sources such as background

radiation, cooling irregularities, mechanical issues, and incident optical sources.23,25

Random motion of carriers in a semiconductor results in fluctuations of the open circuit

voltage, Vj, called thermal or Johnson-Nyquist noise,24

Vj = 4kTR∆f (1.3.1)

with ∆f representing the electrical bandwidth, and R the real part of the impedance.

More precisely written

1/ f α , with α having a value of approximately unity, 1/f noise is

sometimes correlated with surface effects and is important at lower frequencies. It has not

been modeled precisely.24,25

Shot noise has a white spectrum at low frequencies and arises as a consequence of the

quantum fluctuations of the optical field, as well as statistical variations in the emission,

recombination, and flow of charge carriers. The mean square shot noise current, In, is

modeled as24

In = 2qI ∆f (1.3.2)

with I the current, and q the electronic charge.

The components of dark current important to this work are due to surface leakage, generation

processes, and those produced by the detection of background radiation. These may be


7

modeled as shot noise.19 Recombination processes are not significant for photodiodes and

nBn photodetectors. Recombination processes become important in the presence of excess

carriers, as in the case of photoconductors and when carriers are injected into a device.

Generation processes, and not recombination processes, are considered in devices without

excess carriers, which is the case for devices that extract carriers, such as photodiodes.19,26

The characteristics of generation noise depend on the constituent detector material, device

structure, and device temperature.27 The three most important types of generation

mechanisms in narrow-bandgap semiconductors are: Shockley-Read-Hall, Auger, and

radiative.7 Equation (1.3.3) is a general formalism applicable to variety of semiconductor

generation processes. The generation rate, Gk, of a particular mechanism, k, is related to the

square of the intrinsic carrier concentration, ni, through a proportionality term, gk:7

Gk = gk ni2( ). (1.3.3)

The net generation rate, G, is the sum of all Gk. The square of the intrinsic carrier

concentration is the product of the electron, no, and hole, po, concentrations under conditions

of thermal equilibrium:

ni2 = no po. Non-equilibrium carrier densities are n = no + δn, and p =

po + δp. The general expressions for the electron, n, and hole, p, concentrations are:28

n = NcF1/ 2 −Ec − E f( )

kT

(1.3.4a)

p = NvF1/ 2 −E f − Ev( )

kT

(1.3.4b)

with the Fermi-Dirac integral, F1/2[η], defined using

F j η( )=1

Γ j +1( )x jdx

1+ exp x −η( )0

∞

∫ , (1.3.5)

where Γ represents the gamma function, and the effective densities of states in the

conduction and valence bands, Nc and Nv respectively, are


8

Nc = 2 me*kT

2πh2

3 / 2

(1.3.6a)

Nv = 2 mh*kT

2πh2

3 / 2

(1.3.6b)

with electron and hole effective masses, me* and mh*. In this work, when treating the case of

degenerate semiconductors, the Fermi-Dirac integral is approximated as29

F1/ 2 η( ) ≈ exp −η( )+3π1/ 2 /4

η4 + 33.6η 1− 0.68exp −0.17 η +1( )2[ ]{ }+ 50

3 / 8

−1

(1.3.7a)

which valid over

−∞ < η < ∞ . In the case of non-degenerate semiconductors the form

F1/ 2 η( ) ≈ exp η( ) (1.3.7b)

is used.

The net generation rate of the process k, expressed in Equation (1.3.3), becomes, under

small-signal conditions and expressed in terms of the lifetime of the process, τk,7

Gk =δnτ k

(1.3.8)

with the overall recombination lifetime the parallel sum

1τ

=1τ kk

∑ . (1.3.9)

1.3 Noise in MWIR Photodetectors

9

1.3.1 Shockley-Read-Hall Noise

Shockley-Read-Hall generation-recombination processes30,31 are extrinsic, as they utilize

energy levels within the bandgap resulting from impurities and material defects. Electrons

and holes may be both captured by and emitted from these trap states, which impacts excess

carrier lifetimes. Semiconductor material with better crystalline quality exhibits less SRH

generation current. The net generation rate, which takes into account all four processes, is,

after Reference 7,

GSRH =σ nσ pvthni

2NDL

σ n n + n1( )+ σ p p + p1( ) (1.3.10)

with σn and σp the electron and hole capture cross sections, vth the carrier thermal velocity,

NDL the density of the traps, and the nondegenerate electron, n1, and hole, p1, concentrations

corresponding to a Fermi energy level coincident with the energy of the traps, ET, are

n1 = Nc exp −Ec − ET( )

kT

(1.3.11a)

p1 = Nv exp −ET − Ev( )

kT

. (1.3.11b)

The most efficient generation of SRH current occurs when the energies of the trap states and

the Fermi level coincide near the middle of the bandgap. This is found by varying the Fermi

energy and trap energy to maximize GSRH. The trap energy required to maximize GSRH is that

which maximizes 1/ (n1 + p1). Figure 1.3.1 is a normalized plot of this fraction for the case of

InAs at 200 K. The peak occurs when ET = 0.22 eV above the valence band edge energy.

This coincides with the intrinsic Fermi energy, Efi, and is near the middle of the bandgap,

which is 0.19 eV greater than the valence band energy.32 The proximity of the intrinsic Fermi

energy and the energy at the middle of the bandgap leads to the common and convenient

approximation of Efi ~ Eg/2.

It is of particular interest to note the Fermi energy lies in the middle of the bandgap in the

depletion region of p-n junction photodetectors, and SRH generation current is a primary

noise source for cooled photodiodes.


10

Figure 1.3.1 Determination of the trap energy required to maximize GSRH. The trap energy is measured from the top of the valence band edge and the computed fraction is normalized.

It is useful to simplify Equation (1.3.10) by approximating terms n, p, n1, and p1 for cases in

which the Fermi level is located at mid-gap in the depletion region of a semiconductor device.

The approximation substitutes –Eg/2kT for the arguments in the exponentials of each of these

four terms. Doing so allows n, p, n1, and p1 to be easily summed and GSRH to be expressed,

as

GSRHDepl , in terms of the intrinsic carrier concentration and a lifetime, τo:

GSRHdepl ≈

ni2

τ o NcNv exp −Eg /2kT

=ni

τ o

(1.3.12)

where

τ o =σ nσ pvthNDL NcNv

2σ nNc + 2σ pNv

−1

(1.3.13)


11

The relation between the bandgap energy, temperature, and

GSRHdepl is

GSRHdepl ∝T1/ 2ni ∝T2 exp −

Eg

2kT

, (1.3.14)

which takes into account the T1/2 temperature dependence of the thermal velocity 24 in the

lifetime term.

1.3.2 Auger Noise

Multiple types of band-to-band Auger processes are possible in semiconductor materials,33

but only one is relevant to this work. In direct bandgap n-type semiconductors, such as the

unintentionally doped InAs composing the absorption layer of the InAs-based nBn

photodetector, the most significant Auger process is known as Auger 1 or CHCC.7,34 It

involves two transitions: one between the conduction and heavy-hole bands, and the other

between two energy states in the conduction band. According to the Auger 1 process,

generation involves two electrons and one heavy hole; collision with an energetic conduction

band electron excites an electron from the heavy hole valence band into the conduction

band. The generation rate for the Auger 1 process is

GA = GA1nni2 (1.3.15)

with

GA1 =8 2π( )5 / 2q4mo

h3 4πεoεs( )2

me* /mo( )F1F2

2

1+ µ( )1/ 2 1+ 2µ( )

1ni

2kTEg

3 / 2

exp −1+ 2µ1+ µ

Eg

kT

(1.3.16)

µ = me* / mh* is the electron and hole effective mass ratio, εs is the relative static dielectric

constant, εo is the vacuum permittivity, mo is the rest mass of the electron, and F1 and F2 are

the overlap integrals of the periodic portion of the wave functions of the electrons, with 0.1 <

|F1F2| < 0.3. It is common to approximate Equation (1.3.16). One approximation comes from

recognizing narrow-bandgap semiconductor materials such as InAs have µ << 1, so that the

argument in the exponential term becomes –Eg/kT. Another approximation can be made


12

while considering n-type InAs. The difference between the Fermi and conduction band

energies for unintentionally doped n-type InAs with a carrier concentration of 1 x 1016 cm-1 are

calculated to vary from 0.06 to 0.02 eV for temperatures from 300 K to 150 K. Then, for n-

type InAs, it is reasonable to disregard the temperature dependence of the exponential

portion of the n term in Equation (1.3.15). The value of Eg can also be considered to be

approximately temperature independent. GA is then found to be proportional to the square of

the intrinsic carrier concentration,

GA ∝ ni2 ∝T3 exp −

Eg

kT

. (1.3.17)

When the first two approximations are not made, but the temperature dependence of Eg is

neglected, a more accurate proportional relationship for the case of InAs is:

GA ∝T3 exp −1.08Eg + Ec − E f( )

kT

. (1.3.18)

The equation for the lifetime associated with the Auger 1 process is not reproduced here, but

it can be found in Reference 7. Figure 1.3.2 includes a curve corresponding to the

calculated Auger 1 lifetime in InAs.

The other two Auger processes recognized as important in semiconductors with one

conduction band and heavy hole and light hole conduction bands are the Auger 7, in which

one carrier transitions between the conduction and heavy hole bands and another between

the heavy hole and light hole bands (CHLH), and the Auger S, in which one carrier transitions

between the conduction and heavy hole bands and another between the split-off and heavy

hole bands (CHSH). These processes may be disregarded in n-type material but should be

considered for p-type material. The Auger 7 may be the dominant Auger process in a p-type

material if the spin split-off energy is much larger than the bandgap energy of the material.7


13

1.3.3 Radiative Recombination

Radiative recombination occurs when a photon is emitted upon the recombination of an

electron and a hole. In direct bandgap semiconductor-based light emitting diodes, the

intrinsic radiative recombination process occurring across the bandgap is important.

Radiative recombination is not considered a limiting factor in practical photodetectors. Most

photons arising through radiative decay are reabsorbed, which lengthens the measured

radiative lifetime. An expression for the radiative lifetime, τR, is7

τ R =ni

2

GR no + po( ). (1.3.19)


14

Figure 1.3.2 Carrier lifetimes of Auger and radiative recombination processes in InAs. Lifetimes denoted τA1 for Auger 1, τR for radiative recombination, τA7 for Auger 7, and τAS for Auger S processes. Lifetimes are plotted with respect to normalized doping concentrations in InAs at 200 and 300 K.7,34-36 Reproduced with kind permission from Rogalski, A., Adamiec, K., and Rutkowski, J., (2000). Narrow-Gap Semiconductor Photodiodes, Bellingham, Washington: SPIE-The International Society for Optical Engineering, p. 99.

Auger and radiative recombination are intrinsic processes for which universal curves of the

associated lifetimes, such as those shown in Figure 1.3.2, may be generated. Universal

curves may not be produced for the lifetimes of SRH processes, as these depend on the

density of traps. Figure 1.3.2 plots the calculated lifetimes for the radiative, Auger 1, Auger 7,

and Auger S processes for InAs material of various doping densities at 200 K and 300 K.

Auger 1 is the dominant process in n-type InAs.7

1.4 Figures of Merit

Figures of merit quantify the performance of an infrared detection system; they contrast the

numbers of carriers created by incident signal photons with the numbers generated from

noise processes. The most useful figures of merit allow comparison of infrared detection


15

systems with different architectures, constituent material systems, and manufacturers. While

some figures of merit are broadly applicable, none is universally appropriate. In some cases,

experimental conditions influence the reported numerical values.2

Most of the figures of merit applied to infrared detection systems use the signal to noise ratio,

SNR, as a basis, and are therefore related to one another. One expression of SNR,26

SNR =Is

In

(1.4.1)

is the ratio of the signal current, Is, and the noise current, In.

The responsivity, Ri, is the frequency dependent ratio of the output photocurrent, Is, to the

incident optical power, Popt,24,26

Ri =Is

Popt

=ηqhν

= η qλhc

(1.4.2)

in which ν is the frequency of light, and the quantum efficiency, η, is defined as the number of

electron-hole pairs generated for each incident photon. The value of the quantum efficiency

strongly depends on the absorption coefficient of the material, α(λ).24 Responsivity is a more

attractive metric when the ratio is linear, but it is not as useful as some other figures of merit

as it contains no information about the minimum strength a signal must have to be

detectable.37

Noise equivalent power, NEP, is the ratio of the signal noise to the responsivity, and the

detectivity, D, is the reciprocal of the NEP:26,37

NEP =1D

=In

Ri

. (1.4.3)

A detector with superior sensitivity has a lower noise equivalent power and, consequently, a

higher detectivity. NEP and D are functions of detector area, A, and electrical bandwidth.


16

When D is inversely proportional to

A∆f , as is frequently the case, the spectral detectivity

D*, is defined37

D* = D A∆f . (1.4.4)

The value of D* is both wavelength and temperature dependent. Photon-based detectors

have a often have a more pronounced dependence on D* than thermal detectors.7

D* also depends on the field of view (FOV) when the current arising from the detection of

background radiation is the primary contributor to noise current,7,19,23

In2 = 2q qηAφB[ ]∆f (1.4.5)

where φB is the background photon flux density,19

φB =π sin2 θ /2( )

π2πc

λ4 exp hc /λkTB( )−1[ ]0

λc

∫ dλ . (1.4.6)

and the FOV is θ. The value of D* under these conditions is,

DBLIP* =

ηqλhc

2q2ηφB[ ]−1/ 2∝

1sin FOV /2( )

. (1.4.7)

When a single value is quoted for D*, it generally refers to the peak value of detectivity. Plots

of spectral detectivity curves for infrared photoconductive and photovoltaic detectors are

widely available, and they are typically plotted against ideal curves computed assuming peak

detectivity at each wavelength.1,2 An example is shown in Figure 1.4.1.7


17

Figure 1.4.1 D* for a selection of commercially available infrared photodetectors.1 Reproduced with kind permission from Rogalski, A. and Chrzanowski, K., (2002). “Infrared devices and techniques,” Opto-Electronics Review, 10 (2), pp. 111-136.

The noise equivalent temperature difference (NETD, NE∆T), which is expressed in units of

temperature, is the minimum discernible temperature difference present in a scene. NETD is

that temperature difference producing a system output signal equal to the noise of the

camera,1,37

NETD =∆T

I∆s / In

(1.4.8)

with

I∆s = η Is

∆T

∆T . (1.4.9)

Here I∆s is the differential current associated with the ∆T change in temperature, and Is is the

current from the detector element. Two pixels can be distinguished from one another when

one pixel images a portion of the scene that differs by the NETD from the portion of the scene

imaged by another pixel. This basic equation can be cast to more explicitly show influences

of different elements, including D*, on the NETD.2,7 The equation may be stated in terms of

the f-number of the optics, f# the focal length divided by the lens diameter, as1


18

NETD =4 f#

2 ∆ftop M* A

(1.4.10)

with the top the transmission of the optics, and M* the spectrally-dependent figure of merit,

M* =∂L∂T

λ

tatDλ*dλ

0

∞

∫ , (1.4.11)

which incorporates the partial derivative of the emitted radiance as a function of temperature,

∂L/∂T , and the atmospheric transmission tat.. This figure of merit is especially useful for

thermal imagers, which convert the temperature differences in an imaged scene into a visual

image. Temperature resolution is therefore a common figure of merit for these systems.2

However, as this work focuses on detector elements and not infrared systems, D* is the

preferred figure of merit for this work.

The RoA product of a photodiode, which is computed by multiplying the area by the

resistance computed for zero-bias voltage, is a figure of merit frequently quoted for

photodiodes. It may be calculated directly from the current density as a function of voltage

data: it is the reciprocal of the slope of the curve taken at a zero voltage bias:19

RoA=∂J∂V

−1

V= 0

(1.4.12)

It is desirable for RoA to be as large as possible; this is advantageous when mating the

detector to a readout, and it identifies a detector with less noise and lower leakage current.2

Device cooling is used to improve RoA, but it cannot be increased to an arbitrary value. The

sensitivity of RoA to temperature becomes negligible under conditions including surface

leakage current dominating the dark current, tunneling through the depletion region, and the

detection of background radiation.7,39 The RoA of a photodiode is frequently evaluated for the

case of zero voltage bias and no background radiation.39

Background limited infrared photodetection, also known as background limited in

performance, (BLIP) operation can be defined for specific background radiation conditions.


19

BLIP operation occurs when the internally-generated and temperature-dependent dark

current of a photodetector has been reduced, through cooling, to equal the magnitude of the

current produced by the detection of background radiation. Equations (1.4.5) through (1.4.7)

assume BLIP operation. Reducing the temperature of the device below the temperature for

which BLIP operation is achieved does not increase performance, as the primary noise

component is now contributed by an external stimulus.23,40

1.5 Classes of Infrared Photodetectors

Infrared detectors dominating the commercial marketplace are solid state devices that

convert infrared radiation into an electrical signal, and these are broadly classified as either

photon or thermal detectors. Thermal detectors experience changes in electrical properties

when the temperature of the constituent material changes after absorbing infrared radiation.

In photon-based detectors, incident photons interact directly with electrons. This results in a

modification of the electronic distribution in the material and the alteration of an electronic

property.1,41

Microbolometers, which are members of the thermal class of detectors, and p-n junction

photodiodes, which are photon-based detectors, are currently the most widely used MWIR

detectors, however no one type of detector is well suited to all applications.1,2 Operating

conditions, minimum required response time and detectivity, the wavelength range of interest,

and budget dictate the best choice. Whether the detector will monitor space or Earth-bound

targets substantially impacts selection. The latter assumes targets are imbedded against a

background of objects at 300 K, while background radiation in the former case may be

consistent with blackbodies having temperatures lower than a couple tens of kelvin. Whether

or not the application supports detector cooling is also a factor. Cooling is most easily

implemented in fixed laboratories and large vehicles, such as tanks. Cooling is generally not

available and/or practical for applications requiring portability, such as in the case of imaging

systems carried by personnel in the field or employed in temporary installations.

1.5.1 Thermal Detectors

1.5 IR Photodetector Classes

20

The three types of thermal detectors most important to infrared radiation sensing are

bolometers, pyroelectric detectors, and sensors based on the thermoelectric effect such as

thermocouples. Electrical resistance changes with temperature in bolometers, differences in

the spontaneous electrical polarization are monitored in pyroelectric detectors, and variable

voltage generation is the basis of thermocouples. These, like all thermal detectors, return a

signal based on the radiant power incident on the detector and are largely insensitive to the

spectral content of the radiation. The majority of thermal detectors are not cooled, which

makes them inexpensive, easy to use, and favored for use in space-based and field

operations. (Thermal photodetectors based on superconductors, which are cooled to tens of

kelvin, are a significant exception.) Benefits associated with uncooled operation are offset by

slow response times, which is a consequence of the necessary wait while the material heats

and cools, sensitivities well below those of photon-based detectors, and historically large

center-to-center spacing of the elements.2,19

Micromachined bolometers, termed microbolometers, have been fabricated and fashioned

into uncooled FPAs with monolithic and hybrid readout circuits.2,5 Infrared detector systems

based on microbolometer FPAs are becoming increasingly important for LWIR detection for

applications such as missile warning, surveillance, night vision, and thermal sights.

Microbolometer arrays with 640 x 480 elements and pixels with 17 micron side lengths are

commercially available.42

1.5.2 Photon Detectors

The class of photon-based detectors, in which photodetection relies on charge carriers

interacting directly with incident photons, includes semiconductor-based infrared detectors.

Charge carriers may be bound to lattice or impurity atoms, or they may be free. Interband

transitions have been leveraged in photoconductive and photovoltaic detectors, among

others. Carriers generated in photoconductive detectors alter the conductivity of the material,

while electron-hole pairs created in photovoltaic detectors result in a photocurrent or a

voltage difference detectable across two electrodes. Energy states residing in the bandgap

of the host semiconductor may be created by doping a semiconductor with impurity atoms.

The extrinsic absorption of a photon involves exciting a carrier from such a state into an

energy band of the semiconductor, and these transitions are used in some photoconductive

detectors. Internal photoemission, which referrs to the photon-induced ejection of an


21

electron, is the basis of Schottky barrier detector operation. Quantized energy levels may be

created with a quantum well structure, and transitons involving these levels have been used

in photoconductive and photovoltaic detectors.7,43

Unlike the majority of thermal detectors, photon-based detectors exhibit pronounced

wavelength dependence. For photon-based detectors there exists an abrupt long wavelength

cutoff and a typically more gradual drop off in responsivity for shorter wavelengths.7,24

Detection occurs when the photons composing the incident radiation possess enough energy

to transition carriers into different energy states. In the case of intrinsic photoconductive and

photovoltaic detectors, carriers are excited across the bandgap of the semiconductor. The

long wavelength cutoff,

λc =hcEg

, (1.5.1)

occurs for photons with energies less than the bandgap energy; these photons are not able to

effect a transition and are so minimally absorbed that the material is considered transparent

to them. For ideal photoconductors and photodiodes operating at wavelengths below the

cutoff, η = 1. However, responsivity for high-energy photons is actually poor as a

consequence of the large value of the absorption coefficient at these lower wavelengths. At a

penetration depth of x, the transmitted radition intensity, Iv, is reduced from the incident

intensity, Ivo.24,43

Iv x( )= Ivo exp −αx( ). (1.5.2)

Short wavelength photons are absorbed close to the surface of the semiconductor. These

are prone to quickly recombine, which prevents them from contributing to the measured

electrical signal.

Although the spectral response of photon-based detectors is more narrow than their thermal-

based counterparts, photon-based detectors possess a number of advantages.

Semiconductor material systems of most consequence to the field of infrared detectors

possess direct energy gaps, doping flexibility, high electron mobilities, and low dielectric

constants.7 This endows them with fast response times, high sensitivities, and high signal to

noise ratios.1,45


22

Photon-based detectors are frequently cooled for better performance. Detectors operating in

the MWIR are generally cooled to temperatures between 200 and 77 K, and it is common to

cool those operating in the LWIR to temperatures 77 K or lower. Cooling supresses the

thermal generation of carriers, which competes with optical generation processess and

obsures the signal. More agressive cooling is required for LWIR photodetectors, as the

smaller energy transitions are more easily triggered by thermal processes. Fulfilling cooling

requirements presents an important obstacle to the more widespread adoption of photon-

based infrared systems. There is a focus on developing devices having optimal or near-

optimal performance at room temperature or, baring that, at temperatures greater than 200 K,

which are acheivable with thermoelectric coolers. Thermoelectric coolers are lighter-weight

and reliable performers with modest power requirements.2,45

1.6 Significant Semiconductor-Based Infrared Photodetectors

A variety of semiconductor materials are used in photon-based infrared photodetectors, with

Hg1-xCdxTe and InSb being the most significant to commercial detection systems. While Hg1-

xCdxTe is used in both MWIR and LWIR systems, InSb-based systems are exclusive to the

MWIR. Other materials, such as extrinsic semiconductors and the lead salts, have not been

widely adopted for use in the MWIR and LWIR.5 Lead salts are used in a limited fashion in

the MWIR, often with minimal cooling and in applications for which high resolution is not

critical. Photodetectors based on the lead salts are inexpensive and straight-forward to

produce, but they degrade in humid and acidic environments and must be shielded from

bright visible light.2 Extrinsic silicon and germanium are featured prominently in systems

detecting wavelengths beyond 20 microns, and these detectors must be cooled below 4.2 K.5

1.6 Semiconductor-based MWIR Photodetectors

23

1.6.1 Mercury Cadmium Telluride (Hg1-xCdxTe)

Hg1-xCdxTe, of the II-VI semiconductor material system, occupies the dominant position in

infrared detector systems and is the most commonly used material in forward looking infrared

(FLIR) detection systems for the military. It is found in systems intended for both high and

low background radiation applications. No material system has been developed more

extensively for infrared photodetection, which is traceable to the enormous freedom it confers

during system design. The undisputed advantage of using Hg1-xCdxTe is the ability to tune

the wavelength sensitivity over an extensive range, from 1 to 30 microns and beyond, during

manufacture by adjusting the composition of the ternary. It also experiences notably small

changes in lattice constant with changes in composition; the 0.3% change in lattice constant

between HgTe and CdTe permits layered and graded-gap structures to be grown as high-

quality crystals. Hg1-xCdxTe does not lack for competitors, however these have not been able

to claim better performance or, with the exception of thermal detectors, operation at higher

temperatures. Instead, most rivals claim to be more manufacturable.2,5,7

The manufacturability of Hg1-xCdxTe is a significant issue. For commercial viability, it is

necessary for semiconductor materials possessing low defect densities and good uniformity

to be grown reproducibly on large-diameter wafers,. Growing these crystals with uniform

compositions is a significant challenge, as a high mercury pressure during crystal growth

complicates the regulation of the stoichiometry.7 Compositional variations across the wafer

translate to a range of cutoff wavelengths, which is broader for longer wavelength cutoffs. As

responsivity is dependent on wavelength and FPAs require uniform responsivity across the

array, it is necessary to tightly control the composition of the Hg1-xCdxTe during growth. The

difficulties in doing so at longer wavelengths, where a 5% variation in composition

corresponds to variations in cutoff wavelength of several microns, have limited the cutoff

wavelengths of commercially produced large-area FPAs to around 11 microns.2,7

Post-growth processing of Hg1-xCdxTe, a typically proprietary and onerous procedure that

includes the deposition of a passivation layer, is required to yield the high-performance

photovoltaic devices incorporated into FPAs. Passivation is critical to performance, as it

reduces the otherwise high levels of surface leakage current, suppresses tunneling, and

prevents the decomposition of the material. Without passivation, mercury evaporates from

and alters the properties of the material. Passivation also reduces surface recombination


24

velocity and increases RoA.2,7 Elimination of the passivation step would reduce costs and

increase yields.6

Infrared photodetectors fabricated from Hg1-xCdxTe exhibit quantum efficiencies of 50-60

percent without anti-reflective coatings and 70-80 percent with them. FPAs composed of

photovoltaic devices operating in the MWIR typically exhibit D* of 1 x 1011 cm·Hz1/2·W-1 at 100

K. Uniformities are strongly influenced by the cutoff wavelength of the alloy; arrays operating

at shorter wavelengths are more uniform, and uniformities are better for smaller arrays.2

Nonuniformities of less than 4 percent have been reported for a 256 x 256 array operating at

4.9 microns.46 Uncorrected nonuniformities for LWIR arrays are typically 10 to 20 percent,

and the best corrected uniformities are an order of magnitude lower.2

The RoA of MWIR photodiodes, reported to be 4 x 107 ohm·cm2 at 77 K for a 4.0 micron cutoff

wavelength, is limited by generation-recombination current originating in the depletion

region.48-50 The RoA drops to approximately 30 ohm·cm2 at 193 K, where currents originating

from radiative and Auger mechanisms are prevalent.47,48,51,52 The RoA for average devices

with cutoff wavelengths between 9-11 microns at 78 K is 300 ohm·cm2,53 and it is around 650

ohm·cm2 for the best.54 Under BLIP conditions at 40 K, the RoA is greater than 6 x 105

ohm·cm2.53

1.6.2 Indium Arsenide Antimonide (InAs1-xSbx)

InAs and InSb, which are both narrow-bandgap binaries of the III-V semiconductor family with

similar physical properties, are common MWIR photodetector materials. InSb is the more

prevalent as it has a smaller bandgap, 0.22 eV32 and a cutoff wavelength of 5.6 microns,

which is better suited to cover the whole 3 to 5 micron wavelength range of the MWIR

atmospheric transmission window. InAs possesses a larger bandgap of 0.35 eV and a

consequently shorter cutoff wavelength of 3.6 microns.32 InAs is not currently used or under

consideration for use in FPAs.2 FPAs based on InAs1-xSbx alloys are under development, as

these promise to extend to the cutoff wavelengths beyond those of the binaries; InAs0.40Sb0.60

with a cutoff wavelength of 7.0 microns has been demonstrated. An infrared detector

composed of InAs0.10Sb0.90 has the potential for 77 K operation at 9.0 microns, which is a

cutoff wavelength no other III-V semiconductor can match.55,56


25

InSb is a mature material, and it is being positioned as a less-expensive alternative to

HgCdTe. Photodetectors produced from InSb and HgCdTe perform equally well in the

MWIR, InSb is easier to manufacture than HgCdTe, and detectors fabricated from both have

similar power requirements. InSb is also easier to grow uniformly; nonuniformities across the

array, which are incurred during the manufacturing process after crystal growth, are typically

one to two percent. InSb is frequently seen as the best choice for MWIR applications

demanding high sensitivity, good corrected uniformity, and ease of manufacture. Despite the

benefits of InSb, it is a fragile material and fabrication requires specialized equipment and

training. InSb use is mostly confined to the niche FPA market.2

InSb photodetectors may be photovoltaic or photoconductive, and both monolithic and hybrid

arrays have been produced. Hybridized arrays are common; producing monolithic arrays

results in detectors having shorter cutoff wavelengths than when they are manufactured

separately as well as nonlinearities in the CMOS readout.2 While free carrier absorption is

reduced when the back surfaces of the detector arrays are thinned to 100 microns, they are

frequently polished to a 10 micron thickness to achieve higher quantum efficiency.5,7 The

diffusion length of minority carriers in n-type InSb is approximately 30 microns at 80 K,57 and

detector arrays with thicker substrates suffer from a loss of resolution and reduced

responsivity.2 Once thinned, the back surfaces are passivated and coated with antireflection

film.2,7 A report of an 128 x 128 FPA claims a D* of 7.56 x 1011 cm·Hz1/2·W-1 for illumination

conditions which have a predicted theoretical BLIP D* of 9.4 x 1011 cm·Hz1/2·W-1.59 Other

figures of merit for FPAs include RoA of 2 x 106 ohm·cm2 at 77 K,58 and quantum efficiencies

approaching 90 percent.2

InAs1-xSbx has attracted interest as it can potentially produce detectors operating out to 9

microns, but the challenge of fabricating high-quality material is substantial. The lack of

substrates with suitable lattice constants is a key issue; devices grown from alloys closely

lattice matched to and grown on GaSb substrates perform best. This material must be

passivated to suppress the otherwise high levels of surface leakage current, and the higher

levels of generation current, arising from an increased dislocation density in the strained

material, is significant. To date, InAs0.91Sb0.09 photodetectors have seen acceptance in

optical fiber communication systems operating in the 2-4 micron wavelength range.2,7


26

1.6.3 Schottky Barrier Photodiodes and the Silicides

Schottky barrier photodiodes have been under sustained development as alternatives to p-n

junction photodiodes. They consist of a thin film, which is on the order of 10-20 Angstroms

thick and contains heavy metal atoms, in contact with a semiconductor crystal. Palladium,

platinum, and iridium are popular dopants, with cutoff wavelengths of 3.2, 5.7, and 10

microns, respectively. Rectification creates a potential barrier between the metal and the

doped silicate layers. Incident signal photons are absorbed in the metal-containing film, and

those possessing enough energy excite majority carriers over the potential barrier.2 Majority

carriers are typically the carriers of signal current. Carriers may also tunnel through the

barrier, and carrier recombination may occur in the space charge and neutral regions.24,43

Platinum silicide Schottky barriers have enjoyed considerable commercial interest.2

Schottky barrier photodiodes have a number of advantages as compared with p-n junction

photodiodes. The manufacturing process for the former is notably less complex, they are not

subject to diffusion processes at high temperatures, and they have faster response times that

allow higher frequency operation.7 Forward biasing a Schottky barrier photodiode based on

an n-type semiconductor crystal injects electrons into the metal-containing film.24,43 There,

collisions between carriers result in thermalization occurring on a time scale of approximately

10-14 seconds.60 Forward biasing a p-n junction photodiode injects carriers that must instead

dissipate through recombination, and minority carrier recombination lifetimes are typically on

the order of 0.5 microseconds. The difference in lifetimes results in Schottky barrier

photodiodes having the potential for higher frequency operation.7

Schottky barriers are well suited to applications in which high resolution is required, the

background radiation is high, and cooling to temperatures below 80 K is acceptable. Material

uniformity is excellent. Uncorrected uniformity is better than 99 percent for Pt:Si, which

allows for the fabrication of large arrays.2 Arrays of Pt:Si devices with 1040 x 1040 pixels are

available.61 These detectors must be cooled to temperatures below those required by p-n

photodiodes and other detectors, due to the necessity of quelling the thermal emission of

carriers over the barrier. However, once appropriately cooled, typically to 77 K, the sensitivity

of Schottky barrier photodetectors is largely independent of temperature, which is a noted

advantage over other types of photon detectors.2,7 Sensitivity is low, with scattering

mechanisms interfering with the transport of photogenerated carriers.62 Quantum efficiencies

are 4 percent and 0.5 percent for Pt:Si at 2.5 and 4 microns, respectively.2


27

This work does not consider Schottky barrier photodiodes to be direct competitors to p-n

junction photodiodes or the nBn photodetector, as they have very low sensitivities and

operate at liquid nitrogen temperatures. They are not considered further in this work.

1.6.4 Photoconductors

Photoconductors are essentially optically variable resistors, in which conductivity increases

when absorbed photons create free carriers.19,24,43 In the case of intrinsic semiconductor

material, the cutoff wavelength of the absorbed photons is determined by the bandgap

energy. For extrinsic material, it is the difference between the energy of the donor state and

the band edge. Photoconductors are structurally simple photodetectors, consisting of a width

of semiconductor material sandwiched between two ohmic contacts.

Figure 1.6.1 Sketch of a Photoconductor. The physical structure is illustrated at left. The schematic at right shows electron-hole pair generation across the bandgap. Ec is the energy at the bottom of the conduction band and Ev is the energy at the top of the valence band.

During operation, an electric field is applied across the contacts. A current always flows in

these devices, due to the lack of barriers and the sustained voltage bias, but the magnitude

of the current increases in the presence of a signal. The conductivity is expressed as

σ = q µnn + µp p( ), (1.6.1)

where q is the charge on the carrier, µn and µp are the electron and hole mobilities, and n and

p are the electron and hole densities.

L

hν

Ev Ec

hν


28

Performance may be quantified in terms of quantum efficiency, gain, response time, and

detectivity. The internal quantum efficiency approaches unity, as an absorbed photon nearly

always increases the conductivity. The external quantum efficiency can exceed 0.9 when the

illuminated surface of the detector is coated with an anti-reflection coating. The gain,

G =Ip

Iph

=τtr

=τ

L/vd

, (1.6.3)

where L is the length of the semiconductor and vd is the drift velocity, is the ratio of the carrier

lifetime, τ, and the carrier transit time, tr, as well as the ratio of the measured photocurrent, Ip,

to the primary photocurrent,

Iph = q ηPopt

hν

. (1.6.2)

The carrier lifetime is a critical parameter to both the gain, which may range from 1 to 106,

and the response time, which varies from 10-3 to 10-8 seconds and is longer than that of

photovoltaic photodetectors.24 A benefit of gain includes lessened dependence on low-noise

preamplifiers. However, a longer response time can be a liability; photoconductive

photodetectors are limited to lower frequency operation than photovoltaic photodetectors.2,7

The primary internal noise sources in photoconductive photodetectors are Johnson-Nyquist

noise noise, given in Equation (1.3.1), generation-recombination noise, and 1/f noise, for

which there is no exact analytical model.19 Both generation and recombination processes are

active in photoconductive photodetectors, while only generation processes are significant in

the nBn photodetector and the p-n junction photodiode. As a consequence, photoconductive

photodetectors are constrained to have at least

2 times more noise than photodetectors, in

which only the generation process contributes significant noise.19,26 There are a variety of

expressions for generation-recombination noise in photoconductive detectors; each reflects

different compositional and operational characteristics. One applicable to a nearly intrinsic

photoconductor is,63

Vgr =2Vb

Atµe + µh

µen + µh pnp

n + p

1/ 2τ∆f

1+ ω 2τ 2

1/ 2

, (1.6.3)


29

where Vb is the bias voltage, t is the thickness of the absorbing material, and ω is the

modulation frequency. An expression for the voltage responsivity,

Rv =Vb

hcµe + µh

noµe + poµh

ηAt

, (1.6.4)

is applicable when sweep-out, surface recombination, edge effects, and absorption of

background radiation may be disregarded. Further assuming 1/f has been made negligible

during device manufacture and Johnson-Nyquist and generation-recombination noise are

dominant, the expression for D* becomes19

D* =Rv A∆fVj

2 + Vgr2

. (1.6.5)

The advantages of photoconductive photodetectors include high gain and ease of

manufacture. This is particularly true of those based on extrinsic silicon and germanium. In

the HgCdTe material system photovoltaic photodetectors, with microwatt power dissipation,

are generally preferred over the photoconductive, with milliwatt power dissipation. Arrays of

extrinsic silicon photoconductive photodetectors are modest in size, usually not larger than

128 x 128, highly sensitive, typically useful into the LWIR, and require cooling to

temperatures in the neighborhood of 20 K to reduce dark current to acceptable levels.

HgCdTe based photoconductive photodetectors are either operated singly or as groupings of

a few elements.2 Photoconductive photodetectors are not considered significant competitors

to the p-n junction photodiode and nBn photodetector.

1.6.5 p-n Junction Photodiodes

The p-n junction photodiode incorporates a barrier to the flow of majority carriers absent from

the photoconductive detector. A p-n junction is formed at the interface of p-type and n-type

materials. Diffusion of majority charge carriers across the metallurgical junction creates

space charge regions surrounding the junction that are depleted of free charge carriers. This

diffusion continues until the electric field arising from the exposed and fixed ions is sufficiently

large to discourage the further net migration of the majority charge carriers. In thermal


30

equilibrium, the Fermi energy, Ef, is uniform throughout the device. In the absence of an

externally applied field and under steady-state conditions, the built-in potential barrier, Vbi, is

used to define the difference between the intrinsic Fermi energies in the n-type and p-type

regions.43

Figure 1.6.2 Sketch of a p-n junction. Top: The space charge region exists at the interface of p-type and n-type materials. Bottom: Energy diagram of a p-n junction showing Auger (band-to-band) and SRH generation (trap-assisted) processes. Direction of free carrier diffusion is indicated. (After Reference 7.)

The majority of photogenerated carriers in MWIR photodiodes are produced in the material

adjoining the depletion region, rather than in the depletion region itself. Photoexcitation

occurring across the bandgap is the preferred process. Arranging for the majority of the

absorption to occur in the p-type region is typical, as electrons have a higher mobility than

holes.19 Carriers generated within a diffusion length may diffuse into the space-charge

region, where the electric field selectively sweeps the minority carriers through the depletion

region. Gain is taken to be unity, except in the case of avalanche photodiodes. Photodiodes


31

may be operated under a negative voltage bias, in the photoconductive mode, or with no

voltage bias, in the photovoltaic mode.24,28

Dark current in infrared, narrow-bandgap, semiconductor-based photodiodes arises through

mechanisms including band-to-band generation (diffusion current), generation through trap

states (SRH generation current), Johnson-Nyquist noise, band-to-band tunneling across the

depletion region, trap-assisted tunneling through the depletion region, and surface leakage.19

In the following, only diffusion currents, SRH generation currents, and Johnson-Nyquist noise

are treated.

The shot noise model describes the intrinsic noise in infrared photodiodes.19 When operating

in the low-frequency regime, the mean square noise current is

In2 = 2q Isat exp qV

kT

+ Isat

∆f , (1.6.6)

where Isat is the saturation current and V is the applied voltage bias. Under the additional

constraints of thermal equilibrium, no applied voltage, and no incident radiation, the mean

square noise current consists of only Johnson-Nyquist noise,

In2 =

4kTRo

∆f . (1.6.7)

Comparison of Equations (1.6.6) and (1.6.7) gives the saturation current,

Isat =kTqRo

. (1.6.8)

Diffusion current in p-n junctions is

ID = Isat exp qVkT

−1

, (1.6.9)


32

and SRH generation current, in which Wdep is the width of the depletion region and with the

assumptions that the Fermi and trap energies coincide and the electron and hole lifetimes are

both equal to τo, is19,43

ISRH =qWdepni

2τ o

A. (1.6.10)

An expression for D* is obtained using Equations (1.4.2) through (1.4.4),

D* =ηqλhc

A∆fIn

. (1.6.11)

Using Equation (1.6.11), two equivalent expressions for the maximum value of D* for a p-n

junction photodiode under zero voltage bias and in the absence of incident radiation are,19

Dpeak* =

ηqλc

2hcRoAkT

1/ 2

(1.6.12a)

and

Dpeak* =

ηλc

2hcq

Jsat

1/ 2

, (1.6.12b)

in which Jsat is the saturation current density.

Infrared photodetection systems, particularly those employing large FPAs, frequently

incorporate p-n junction photodiodes, which have a number of benefits over photoconductive

photodetectors.2 Stringent noise requirements can be more easily met. Only generation,

rather than both generation and recombination, processes are significant in the p-n junction

photodiode. This difference alone results in the noise in p-n junction photodiodes being

better than that in photoconductive photodetectors by a factor of

2 .19,26 In addition, bias

currents in p-n junction photodiodes are low or negligible, which results in lower thermal

power dissipation, and coupling to the input state of a silicon CCD benefits from the naturally

high impedance of the devices. Photodiodes also permit higher-frequency operation, biasing

is simpler, and responsivity can be more accurately predicted.2,7


33

The preceding discussion of dark current, noise current, and D* does not consider surface

leakage current, however the primary contributors to the noise current in narrow-bandgap

and cooled p-n junction photodiodes are surface leakage and SRH generation currents.

Surface leakage current has an ohmic current-voltage relationship, and it is approximately

temperature independent. It is enabled by a uniformly n-type layer that covers the surface of

narrow-bandgap semiconductors, regardless of the doping in the bulk. This surface layer

arises due to the presence of surface charges. Surface leakage current negatively impacts

the performance of the p-n junction photodiode, as charge carriers are able to use surface

leakage channels, which run parallel to the depletion region, to bypass the potential barrier.

The surfaces of p-n junction based devices are routinely passivated, through the application

of native oxides or other insulators, to reduce surface leakage current and surface

recombination and to protect the devices against undesired environmental effects.7

Passivation is a time consuming, and hence undesirable, process, and one suitable for

commercial InAs devices has not yet been developed.

While it is possible to mitigate the effects of surface leakage current, SRH current remains a

primary source of dark current in cooled photodiodes. The bulk of the SRH current is

generated within depletion region, and, as seen in Equation (1.6.10), the magnitude is

proportional to both the volume of the depletion region and, through the presence of ni, the

generation rate, GSRH. The key to minimizing SRH current is removing the Fermi level from

the middle of the bandgap. The Fermi level may be moved from the middle of the bandgap

by designing a photodetector composed of n-type or p-type material and without a depletion

region. The nBn detector, discussed in the next section, is designed to suppress SRH

current by displacing the Fermi level from the middle of the bandgap.6

1.6.6 The nBn Photodetector

The InAs / AlAsSb nBn photodetector is shown in Figure 1.6.3 as a representative of the

class of nBn photodetectors.6 The name of the nBn photodetector class derives from the n-

type absorption layer, the Barrier layer, and the n-type contact layer. Signal photons

interacting with the absorption layer result in band-to-band excitations, and signal current is

generated when the resulting holes travel to the contact. The nBn photodetector design

stipulates that there be no energy discontinuity in the valence band, and consequently no

barrier to the free flow of minority carrier holes. However, the intentionally large conduction


34

band energy discontinuity, which is much larger than kT, blocks the flow of the majority

carrier electrons. In the InAs-based nBn photodetector, both the zero valence band and the

large conduction band energy discontinuities may be achieved by an AlAsxSb1-x barrier layer

with a specific composition, x, which is found to be 0.14 < x < 0.17. The composition of the

AlAsSb barrier layer is the subject of Chapter 2. A pixel is defined by etching through the

contact layer; the barrier layer acts as an etch stop. Gold metal contacts are deposited on

top of the contact layer. The nBn photodetector operates under a voltage bias, and the bias

applied to the device shown in Figure 1.6.3 is referred to as a reverse bias.

Figure 1.6.3 Sketch of the voltage-biased nBn photodetector. The fabricated device structure is shown at the upper left, and the energy diagram is beneath. The contacts are gold, the substrate, absorption, and contact layers are n-type InAs, and the barrier layer is AlAsSb. In the energy diagram, the contact layer is located to the right of the barrier layer.

The design of the nBn photodetector suppresses SRH generation current and reduces

surface leakage current to negligible levels.6 Nowhere does the Fermi energy coincide with

the middle of the bandgap, which significantly reduces the SRH generation rate. Surface

leakage current is essentially eliminated by the inclusion of the wide bandgap barrier layer. It


35

breaks the surface conductivity path that would otherwise exist between the absorbing and

contact layers, which are composed of narrow bandgap semiconductors in MWIR

photodetectors. The wide bandgap barrier layer offers no shunt path for mobile majority

carriers to use to bypass the large conduction band energy discontinuity. Measurements of

temperature-dependent dark current presented in Chapter 3 show diffusion current, which

results from the thermal generation of carriers across the bandgap in the absorption region,

dominates the dark current of the lattice-matched InAs-based nBn photodetector. As a

consequence, the cooled nBn photodetector has significantly lower levels of dark current than

the cooled p-n junction photodiodes tested for comparison.

Figure 1.6.4 Temperature dependent D* of SRH generation and diffusion currents. Calculations of ISRH (blue) and Idiff (maroon) assume unintentionally doped InAs with good crystalline quality. D* is proportional to (ni)-1 and (ni

2/Nd)-1 respectively, and Nd =1x1016cm-3.

Auger 1 generation processes are the primary sources of dark current in nBn photodetectors.

The diffusion current may be expressed as


36

Idiff ∝qpo1

τ diff

AL = q ni2

Nd

1τ diff

AL, (1.6.20)

where Idiff is proportional to ni2 as expected, τdiff is the carrier lifetime, Nd is the n-type doping

density, and L is the width of the neutral region.6,28 The equations for the SRH generation

current, Equation (1.6.10), and the diffusion, or Auger, noise current, Equation (1.6.20), may

be directly compared. The lifetime tdiff is reported to be approximately 1 microsecond in

unintentionally doped InAs,7 and tSRH, is reported to be approximately 200 ns in high quality

InAs0.91Sb0.09.64 This factor of five difference and the roughly order of magnitude difference

between lengths L and Wdep are not considered significant to this analysis.6,24,28,34 Given this,

the SRH generation current, ISRH, which is the primary source of dark current in p-n junction

photodiodes, may be taken as being proportional to ni, and Idiff to ni2/Nd. This results in the D*

of the nBn photoconductor exceeding that of the p-n junction photodetector over a wide range

of temperatures, as is seen in Figure 1.6.4. The reduced noise in nBn photodetectors

benefits applications that support cooling for optimal performance and those requiring

operation at warmer device temperatures.6

The potential to increase the D* of a photodetector through cooling exists only if the

internally-generated components of dark current are greater than the current produced by the

detection of the incident background radiation. When the internally-generated and

temperature-dependent dark current of a photodetector has been reduced, through cooling,

to a magnitude equal to that arising from detection of the background radiation, the detector

has achieved BLIP operation. Reducing the temperature of the device below the BLIP

temperature does not increase performance. For temperatures cooler than the BLIP

temperature, the primary component of current is a product of the detection of the

background radiation, which originates from an external stimulus and is not dependent on the

temperature of the photodetector.40

While it is routine to cool photodetectors when logistics permit, reducing cooling requirements

remains a priority. Photodiodes and nBn photodetectors operating at BLIP temperatures

deliver similar performance, but the BLIP temperature of the nBn photodetector is higher than

that of the photodiode.6 The difference in the two BLIP temperatures derives from the

performance of the photodiodes being limited by SRH dark current while the performance of

the nBn photodetectors is limited by diffusion current.23 Conventional MWIR photodiodes are

usually cooled to below 200 K,3 and the BLIP temperatures of at least some commercial InAs


37

MWIR photodiodes are below 188 K.22 It is typical to operate InSb MWIR photodiodes at 77

K. Focal plane arrays consisting of these photodiodes are known to suffer from unacceptable

levels of dark current at temperatures in excess of 80 K.2 This is in contrast with InAs-based

nBn photodetectors, which have been observed to achieve BLIP operation when cooled to

only 230 K.6

1.7 Purpose of Current Research

Initial measurements of the InAs-based nBn photodetector have been published,6 but the

device has not been fully characterized. The purpose of this research is to better

characterize this MWIR infrared photodetector. A series of InAs / AlAsSb nBn photodetectors

are fabricated from sample crystal growths performed using molecular beam epitaxy.

Measurements, which include taking current-voltage (I-V) traces of selected pixels as a

function of temperature, are made on these photodetectors. These data are analyzed,

compared with the results of mathematical modeling, and contrasted with similar data taken

for p-n junction photodiodes. Conclusions are made on the subjects of the barrier

composition of AlAsSb (Chapter 2), surface leakage current (Chapter 3), the effects on dark

current in lattice-mismatched InAs-based nBn photodetectors possessing absorption layers

with high densities of dislocations (Chapter 4), and the temperature-dependent lateral

diffusion lengths in the absorption layers of various InAs-based nBn photodetectors (Chapter

5). Chapter 6 contains a summary of this research and suggestions for future work.

Molecular beam epitaxy (MBE)20,65 is a technique for growing crystalline epitaxial layers on

prepared substrates. In the Molecular Beam Epitaxy Laboratory, crystal growths are

performed on Riber 32P and Varian Gen II MBE systems. The crystalline epilayers consist of

III-V semiconductor compounds, and the work presented here is based exclusively on the

arsenide-antimonides. The growths are conducted under ultra-high vacuum conditions,

which are better than 10-9 Torr, and at growth rates of approximately 1 micron/hour. The

ultra-high vacuum conditions limit the levels of residual background gas, and thus the rate at

which contaminants may be incorporated into the growing layer. Ultra-high vacuum

conditions limit the growth rate of contaminants a factor of 10-5 times the growth rate of the

desired semiconductor compound. Vacuum is maintained on the Riber and Gen II through a

combination of ion pumps and cryopumps, as well as shrouds filled with liquid nitrogen.

Advantages of using MBE for crystal growth include the ability to tightly constrain the growth


38

conditions, the repeatable production of high-purity epilayers with finely controlled

compositions, and the ability to monitor and alter the growth conditions while the growth is in

progress.

Figure 1.7.1 Cut-away illustration of the growth chamber of the Riber 32P.

Figure 1.7.1 shows a cut-away illustration of the growth chamber of the Riber 32P MBE

machine.66 There are differences between the Riber 32P and Varian Gen II MBE systems,

but they are fundamentally similar. With the exception of a clearly identified commercially

sourced photodiode, all devices evaluated in this work are products of either the Riber 32P or

the Varian Gen II as described in this section.

The Riber accepts up to 3” wafer substrates, while the Gen II is limited to 2” and smaller

substrates. One eighth to one quarter pieces of 2” wafers are used as substrates for the

majority of crystal growths conducted for this research. The substrates are acquired epi-

ready from outside vendors and are loaded into the machines without being subjected to


39

additional chemical treatments. Indium-free mounting of the substrates on the substrate

mounting blocks is an option in both systems, but all crystal growths conducted as part of this

work are on substrates mounted with indium; the substrates are mounted on a silicon carrier

wafer when the growths are conducted in the Riber and on a solid molybdenum block when

crystal growth occurs in the Gen II. In Figure 1.7.1 the substrate is orange, circular in shape,

and located in the center of the growth chamber. The mounting block is depicted by the thin

cylindrical ring behind it, and this assembly is held by the L-shaped manipulator arm. In the

image the substrate faces an array of effusion cells, which contain the precursor materials for

the growth.

Both systems are similarly equipped with source material. Valved cracker sources are

capable of delivering As2 and As4; this work uses As2 exclusively. Conventional effusion cells

supply the rest of the source materials, including Sb4 and the group III elements. All source

material is elemental, with the exception of the GaTe precursor, which supplies the n-type

dopant Te. Beryllium is used as the p-type dopant. The source materials are maintained at

precisely controlled temperatures by the combination of the effusion cells and temperature

controllers. Molecular beams evaporate or sublimate from the contents of the effusion cells

and overlap at the growing surface of the substrate. The molecular, or atomic, flux

distribution is highest towards the center of the beam and decreases on either side at a rate

determined by the geometry of the cell. High vacuum conditions, 10-5 to 10-9 Torr, are

sufficient for maintaining the beam nature of the evaporated precursor materials.65

Crystal growth conditions are specified by controlling source and substrate temperatures and

the position and rotation of the substrate in the growth chamber. Growth conditions depend

on the status of the MBE machine, and they evolve as conditions in the machine change.

The temperatures of the sources control the rate of flux of the precursor elements, which

affects the growth rate and quality of the growing crystal. The temperature of the substrate is

monitored in a few ways, and all are used together to ensure repeatable growths. A

pyrometer provides an estimated temperature reading of the surface of the substrate, and a

thermocouple located on the manipulator arm approximates the temperature of the mounting

block. Reflection high energy electron diffraction (RHEED)20,65 is used to monitor the surface

of the growing crystal. In Figure 1.7.1 the RHEED gun is found at the lower left, protruding

from the growth chamber at a position just above the ‘Riber’ label. The electron beam

emitted by the RHEED gun intersects the growing crystal surface at a glancing angle. The

resulting forward-scattered diffraction pattern is incident on a phosphor screen, which is


40

shown positioned in the circular flange located to the right of the substrate. The diffraction

pattern on the phosphor screen is visible from the outside of the machine, and the pattern

changes as the surface reconstruction of the growing epilayer changes. As the surface

reconstruction changes at certain temperatures and under various growth conditions, the

forward-scattered RHEED diffraction pattern contains information about both the temperature

of the substrate and the quality of the growth.

Chapter 2 investigates AlAsxSb1-x as a barrier layer in the InAs-based nBn photodetector.

Investigations of the composition, x, which possesses the required energy band offsets with

InAs are conducted and compared with the composition of AlAsxSb1-x lattice matching InAs.

Maximum and minimum practical thickness of the barrier layer are explored, using criteria

which include: not exceeding the pseudomorphic critical thickness and minimizing electron

tunneling through the barrier.

Chapter 3 compares an InAs-based nBn photodetector with two InAs-based photodiodes as a

means of investigating the temperature-dependent dark currents in the two types of devices.

It is posited and confirmed that the performance of the photodiodes are limited by surface

leakage currents, while the nBn photodetector contains undetectable levels of surface

leakage current. Conclusions are made about the effects of the composition of dark current

on the D* figure of merit.

Chapter 4 uses temperature-dependent I-V data, measured for InAs-based nBn

photodetectors grown on lattice-mismatched substrates, to investigate the impact of an

absorption layer containing high densities of dislocations on the performance of the nBn

photodetector. The primary contributor to the temperature-dependent dark current of these

devices is the focus of this chapter.

Chapter 5 uses the temperature-dependent I-V data measured for several different InAs-

based photodetectors to estimate the values of the temperature-dependent lateral diffusion

lengths of the minority carriers in the absorbing layer.

chapter 1 an introduction to the nbn photodetector

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