829 34. ii–iv semiconductors for optoelectronics: ii–iv...

829

II–IV Semicond34. II–IV Semiconductors for Optoelectronics:CdS, CdSe, CdTe

Owing to their suitable band gaps and highabsorption coefficients, Cd-based compoundssuch as CdTe and CdS are the most promisingphotovoltaic materials available for low-cost high-efficiency solar cells. Additionally,because of their large atomic number, Cd-based compounds such as CdTe and CdZnTe,have been applied to radiation detectors.For these reasons, preparation techniquesfor these materials in the polycrystallinefilms and bulk single crystals demanded bythese devices have advanced significantlyin recent decades, and practical applica-tions have been realized in optoelectronicdevices. This chapter mainly describes theapplication of these materials in solar cellsand radiation detectors and introduces recentprogress.

34.1 Background ........................................ 829

34.2 Solar Cells ........................................... 82934.2.1 Basic Description of Solar Cells .... 82934.2.2 Design of Cd-Based Solar Cells .... 83034.2.3 Development

of CdS/CdTe Solar Cells ................ 83134.2.4 CdZnTe Solar Cells ...................... 83434.2.5 The Future

of Cd-Based Solar Cells .............. 834

34.3 Radiation Detectors ............................. 83434.3.1 Basic Description of Semi-

conductor Radiation Detectors .... 83534.3.2 CdTe and CdZnTe Radiation

Detectors.................................. 83534.3.3 Performance

of CdTe and CdZnTe Detectors ...... 83634.3.4 Applications

of CdTe and CdZnTe Detectors ...... 839

34.4 Conclusions ......................................... 840

References .................................................. 840

34.1 Background

Cd-based compounds are very important semiconductormaterials in the II–VI family. The attraction of Cd-basedbinary and ternary compounds arises from their promis-

ing applications as solar cells, γ - and X-ray detectorsetc. These devices made from Cd-based materials arebeing widely applied in many fields.

34.2 Solar Cells

With the development of human society, energy sourcesin the earth are being slowly exhausted and we are facedwith a serious problem. The solar cell is one substi-tute for fossil fuels and is being realized throughoutthe world. For this reason, solar-cell technologies havebeen developed since work was started by Becquerelin 1839 [34.1]. The solar cell has now been appliedto daily life, industry, agriculture, space exploration,military affairs etc.

Solar cells have many advantages. Firstly, sunlightas an energy source for power generation is not onlylimitless but can be used freely. Secondly, since light is

directly converted to electricity, the conversion processis clean, noise-free and not harmful to the environment,unlike a mechanical power generator. Thirdly, solar cellsneed little maintenance.

34.2.1 Basic Description of Solar Cells

A solar cell is a semiconductor device that directly con-verts light energy into electrical energy through thephotovoltaic process. The basic structure of a solarcell is shown in Fig. 34.1. A typical solar cell consistsof a junction formed between an n-type and a p-type

PartD

34

830 Part D Materials for Optoelectronics and Photonics

Electron

Backelectrode

Solar radiation

LoadHole

Depletionregion

Transparentelectrode

n

p

Fig. 34.1 The principle of photovoltaic devices

semiconductor, of the same (homojunction) or differentmaterials (heterojunction), an antireflection coating andOhmic collecting electrodes. When the light irradiatesthe surface of a solar cell, photons with an energy greaterthan the band gap of the semiconductor are absorbed bythe semiconductor material. This absorption activateselectron transitions from the valence to the conductionband, so that electron–hole pairs are generated. If thesecarriers can diffuse into the depletion region before theyrecombine, they can be separated by the applied electricfield. At the p–n junction, the negative electrons dif-fuse into the n-type region and the positive holes diffuseinto the p-type region. They are then collected by elec-trodes, resulting in a voltage difference between the twoelectrodes. When an external load is connected, electriccurrent flows through the load. This is the origin of thesolar cell’s photocurrent.

Three parameters can be used to describe the perfor-mance of a solar cell: the short-circuit current density(JSC), the open-circuit voltage (VOC) and the fill factor(FF).

The short-circuit current density JSC is the photocur-rent output from a solar cell when the output terminalsare short-circuited. In the ideal case, this is equal to thecurrent density generated by the light JL and is propor-tional to the incident photon flux. JSC is determined bythe spectral response of the device, the junction depth,and the series (internal) resistance, RS. The open-circuitvoltage VOC is the voltage appearing across the outputterminals of the cell when there is no load present, i. e.when J = 0. The relationship between VOC and JSC canbe described by the diode equation, i. e.:

VOC = nkT

eln

(JSC

J0+1

), (34.1)

0Voltage (V)

Current (A/cm2)

Vmp Voc V

Jsc

Jmp

J

FF

M

A

Fig. 34.2 Photovoltaic output characteristics of a solar cell

where e, n, J0, k and T are the charge on an electron,the diode ideality factor, the reverse saturation-currentdensity, the Boltzmann constant and the absolute tem-perature, respectively. The fill factor (FF) is the ratio ofthe maximum electrical power available from the cell[i. e. at the operating point Jm, Vm] in Fig. 34.2) to theproduct ofVOC and JSC. It describes the rectangularityof the photovoltaic output characteristic.

FF = Vm · Jm

Voc · Jsc= Pm

Voc · Jsc. (34.2)

The conversion efficiency (η) of a solar cell is determinedas

η = Pm

Ps= Voc · Jsc ·FF

Ps. (34.3)

Where Ps is the incident illumination power and Pm isthe output electricity power, both being per unit area.The relationship between the short-circuit current den-sity JSC and the open-circuit voltage VOC is shown inFig. 34.2.

34.2.2 Design of Cd-Based Solar Cells

Solar cells can be made from silicon (Si), III–V com-pounds such as GaAs, or II–VI compounds such asZn-based and Cd-based compound semiconductors. Ofthese materials, CdTe is the most attractive because ofa number of advantages. CdTe is a direct-band-gap ma-terial with a band gap of 1.54 eV at room temperature.This is very close to the theoretically calculated opti-mum value for solar cells. CdTe has a high absorptioncoefficient (above 105 cm−1 at a wavelength of 700 nm),so that approximately 90% of the incident light is ab-sorbed by a layer thickness of only 2 µm (compared

PartD

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II–IV Semiconductors for Optoelectronics: CdS, CdSe, CdTe 34.2 Solar Cells 831

ITO

Backconnect

Incident light

CdTe

CdS

Glasssubstrate

Fig. 34.3 A typical CdS/CdTe solar cell

with around 10 µm for Si), cutting down the quantity ofsemiconductor required. Therefore, CdTe is one of themost promising photovoltaic materials available for usein low-cost high-efficiency solar cells.

CdTe is the only material in which both n- andp-type conductivity can be easily controlled by dop-ing acceptor or donor impurities in II–VI compoundsemiconductors. Furthermore, due to its high absorp-tion coefficient and small carrier diffusion length, thejunction must be formed closed to the surface, whichreduces the carrier lifetime through surface recombi-nation. In spite of this, the conversion efficiency hasbeen gradually enhanced from 4% [34.2] to 16% [34.3]since the first CdTe-based solar cell was fabricated.CdTe solar cells can be made from polycrystalline orsingle-crystal material. However, it was found that thisp–n homojuction structure is unstable due to the agingbehavior of dopants in CdTe films and bulk single crys-tals [34.4]. Furthermore, it is also difficult to fabricateshallow p–n junctions with highly conducting surfacelayers, and high surface recombination velocities led tosubstantial losses that could not easily be avoided. Forthis reason, the CdS/CdTe heterojunction solar cell wasproposed.

It has been proved that CdS/CdTe is a high-efficiencyheterojunction solar cell. The theoretical calculationshows that this cell has a conversion efficiency of ashigh as 29% [34.5], and it is considered a promising al-ternative to the more widely used silicon devices. Thetypical structure of a CdS/CdTe solar cell is glass/indiumtin oxide (ITO)/CdS/CdTe/back electrode, as seen inFig. 34.3. The CdS/CdTe solar cell is based on the het-erojunction formed between n-type CdS (only n-CdS isavailable) and p-type CdTe and is fabricated in a su-perstrate configuration where the incident light passesthrough the glass substrate, which has a thickness of

2–4 mm. This window glass is transparent, strong andcheap. This glass substrate also protects the active lay-ers from the environment, and it provides the mechanicalstrength of the device. The outer face of the panel of-ten has an antireflection coating to enhance absorptionefficiency. A transparent conducting oxide (TCO) layer,usually tin oxide or indium tin oxide (ITO), acts as thefront contact to the device. This is needed to reduce theseries resistance of the device, which would otherwisearise from the thinness of the CdS layer. Usually, thepolycrystalline n-type CdS layer is deposited onto theTCO. Due to its wide band gap (Eg ≈ 2.4 eV at 300 K)it is transparent down to wavelengths of around 516 nm.The thickness of this layer is typically 100–300 nm.The CdTe layer is p-type-doped and its thickness istypically around 10 µm. Generally, the carrier concen-tration in CdS layer is 2–3 orders greater than that of theCdTe layer. As a result, the potential is applied mostlyto the CdTe absorber layer, i. e. the depletion region ismostly within the CdTe layer. Therefore, the photogen-erated carriers can be effectively separated in this activeregion.

The electrode preparation is very important in ob-taining Ohmic contact or minimal series resistivity.Since there are no metals with a work function higherthan the hole affinity of p-type CdTe (−5.78 eV) [34.6],it is difficult to produce a complete Ohmic contact onp-type CdTe surface. Many methods have been triedto fabricate Ohmic contact to p-type CdTe. Differentmetals or alloy and compounds such as Au, Al, andZnTe/Cu etc. have also been used to serve as electrodematerials. Although significant effort has been appliedto this issue, the junction still inevitably displays someSchottky-diode characteristics. This is a problem thatwill be continuously studied.

34.2.3 Development of CdS/CdTe Solar Cells

The layers of CdS and CdTe for solar cells can beprepared using various techniques, which are summa-rized in Table 34.1. In these techniques, evaporation,close-space sublimation (CSS), screen printing andelectrodeposition (ED) have been demonstrated to bevery effective for fabricating high-efficiency large-areaCdS/CdTe solar cells.

Study of solar cells based on CdTe single crys-tals and polycrystalline films were started as early as1963 [34.7]. The cells were prepared with a p-typecopper-telluride surface film as an integral part of thephotovoltaic junction. The junction was thought to bea heterojunction between the p-type copper telluride and

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Table 34.1 Some fabrication techniques and conversion efficiency of CdS/CdTe solar cells

Fabrication techniques of CdS and CdTe Conversion Referenceefficiency area

Evaporation: physical vapor deposition (PVD), and 11.8% 0.3 cm2 [34.8]

chemical vapor deposition (CVD)

Close-spaced sublimation (CSS) 16% 1.0 cm2 [34.3]

8.4% 7200 cm2 [34.9]

Electrodeposition (ED) 14.2% 0.02 cm2 [34.10]

Screen printing (SP) and sintering 12.8% 0.78 cm2 [34.11]

Metalorganic CVD (MOCVD) 11.9% 0.08 cm2 [34.12]

PVD vacuum evaporation 11.8% 0.3 cm2 [34.8]

MBE 10.5% 0.08 cm2 [34.13]

Sputtering deposition (SD) 14% 0.09 cm2 [34.14]

the n-type CdTe, with the transition region extendingwell into the CdTe side. A similar structure was re-ported for single-crystal cells. A conversion efficiencyof up to 6% was obtained in the film cells, and 7.5% forsingle-crystal cells.

Dutton and Muller [34.15] fabricated CdS/CdTe so-lar cells by standard vapor-deposition procedures witha very thin CdTe layer. The existence of the interfaciallayer was confirmed using X-ray diffraction studies, andsubstantiated by diode photocurrent measurements. Theresults showed a threshold photon energy that corre-sponds to the band gap of CdTe. The thin CdTe layerresulted in excellent rectification ratios and high reversebreakdown voltages. The observed photoresponse, I–V(current versus voltage), and C–V (capacitance versusvoltage) characteristics were consistence with those pre-dicted by a semiconductor heterojunction model of theCdS–CdTe interface. Mitchell et al. [34.16] prepared avariety of CdS/CdTe heterojunction solar cells with anITO coating and a glycerol antireflection coating by vac-uum evaporation of n-CdS films onto single-crystal p-CdTe substrates. Comparisons were made between cellsprepared using different substrate resistivity, substratesurface preparations, and CdS film resistivity. The mech-anisms of controlling the dark junction current, pho-tocarrier collection, and photovoltaic properties weremodeled, taking account of the interface states of thejunction. A conversion efficiency of 7.9% was obtainedunder 85 mW/cm2 of solar simulator illumination.

At the end of the 1970s, a new process, screenprinting and sintering, was developed for fabricatingCdS/CdTe polycrystal solar cells by Matsushita andcoworkers [34.11, 17–20]. This technique proved tobe a very effective method for fabricating large-areaCdS/CdTe film solar cells. There are several importantparameters in the screen-printing technique: the viscos-

ity of the paste, the mesh number of the screen, thesnap-off distance between the screen and the substrateand the pressure and speed of the squeegee. As cadmiumchloride (CdCl2) has a low melting point (Tm = 568 C),and forms a eutectic with both CdS and CdTe at lowtemperature, and the vapor pressure at 600 C is highenough to allow complete volatilization of CdCl2 afterthe sintering process, CdCl2 is used as an ideal flux forsintering both CdS and CdTe.

Nakayama et al. [34.18] prepared the first thin-film CdS/CdTe solar cells using the screen-printingtechnique (Fig. 34.4). A glass plate, which was coatedsuccessively with In2O3 film and CdS ceramic thin film,was used as a transparent Ohmic contact substrate. Then-type CdS film with a thickness of about 20 µm anda resistivity of 0.2 Ω cm was prepared on this substrateby the screen-printing method. The CdTe paste wasprinted onto the CdS layer using the same technique andthen heat-treated in an N2 atmosphere at temperaturesof 500–800 C. As a result, the p-type CdTe layer witha thickness of 10 µm showed a resistivity of 0.1–1 Ωcm.A silver (Ag) paste electrode was applied to the p-type

CdS filmCdTe film

In2O3 film

Cu2TeLead wire

Electrode

Glass

Light

Fig. 34.4 Cross section of a ceramic thin-film CdTe solarcell

PartD

34.2

II–IV Semiconductors for Optoelectronics: CdS, CdSe, CdTe 34.2 Solar Cells 833

layer and an In–Ga alloy was applied to the CdS layer.The cell, with an active area of 0.36 cm2, showed anintrinsic solar conversion efficiency of 8.1% under anillumination of 140 mW/cm2 solar simulator (AM0).Subsequently, this group continued to work on im-proving CdS/CdTe solar cells [34.11, 19, 20]. From thepractical point of view, a modular solar cell is necessary.For this purpose, thin-film CdS/CdTe solar cells with anefficiency of 6.3% were prepared on a borosilicate-glasssubstrate of 4×4 cm2 by successively screen printing andheating (sintering) of each paste of CdS, CdTe and car-bon [34.19]. Although they fabricated some structures,including dividing the CdTe film on one substrate intofive small cells, to prevent increases in the series re-sistivity, a 1-watt module only showed an efficiency of2.9% from 25 elemental cells with a 4×4 cm2 substrate.A main reason for this low efficiency was thought to bethe high series resistivity.

In order to decrease the resistivity of the carbon elec-trode further, this group changed the heating conditionsof the carbon electrodes for CdTe and examined theeffect of impurities in the carbon paste on the character-istics of solar cells [34.11]. They found that the seriesresistance (Rs) and the conversion efficiency (η) of solarcells exhibited strong dependence on the oxygen (O2)partial pressure during the heating process. The resultsshowed that the addition of Cu to the carbon paste causesRs and the diode factor (n) to decrease, resulting in a re-markable improvement in η. Using a low-resistancecontact electrode, which was made from 50 ppm addedCu carbon paste, a cell with an active area of 0.78 cm2

displayed Voc= 0.754 V, Isc= 0.022 A, FF= 0.606 andη=12.8%.

Britt and Ferekides [34.21] reported the fabrica-tion and characteristics of high-efficiency thin-filmCdS/CdTe heterojunction solar cells. CdS films with0.07–0.10 µm in thickness were deposited on a 0.5-µm

Glass

Ag AgCdTe

Ag

ITO

MgF2

CdS

Fig. 34.5 The cross-sectional structure of a CdS/CdTe solarcell fabricating by the screen-printing method

SnO2:F transparent conducting oxide layer by chemical-bath deposition and p-CdTe films with a thicknessof 5 µm were deposited on the CdS layer by usingthe CSS method. Prior to the deposition of CdTe,CdS/SnO2:F/glass was annealed in a H2 atmosphere attemperatures of 350–425 C for 5–20 min. The charac-teristics of the CdS/CdTe solar cell were Voc= 843 mV,Jsc= 25.1 mA/cm2, FF=74.5%, corresponding to a totalarea greater than 1 cm2 with an air mass 1.5 (AM1.5)conversion efficiency of 15.8%.

With modern growth techniques, high-efficiencyCdS/CdTe solar cells have been developed using ultra-thin CdS films having a thickness of 50 nm [34.22].Figure 34.5 shows the cross-sectional structure ofa CdS/CdTe solar cell. CdS films were deposited onan ITO glass substrates by the metal organic chem-ical vapor deposition (MOCVD) technique, and CdTefilms were subsequently deposited by the CSS technique(see Fig. 34.6) for the fabrication of CdS/CdTe solarcells. A thin-film CdS/CdTe solar cell with an area of1.0 cm2 under AM1.5 conditions showed Voc= 840 mV,Jsc= 26.08 mA/cm2 and FF= 73%. As a result, a highconversion efficiency of 16.0% was achieved. Fur-thermore, they made developments in improving theuniformity of thickness and film qualities of CdS inorder to obtain more efficient solar cells and to re-alize film deposition on large-area substrates of 30×60 cm or more [34.23]. They found clear differencesin the electrical and optical properties between high-and low-quality CdS films. A photovoltaic conversionefficiency of 10.5% was achieved by a solar modulewith an area of 1376 cm2 under AM1.5 measurementconditions.

Ar

CdS/ITO/glass

substrateCdTe source

R.P.

Quartz tube

Lamp heater

Carbonsuspecter

Lamp heater

Fig. 34.6 Schematic of the close-spaced sublimation appa-ratus used for depositing CdTe films

PartD

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In addition to dry processes such as screenprinting, close-spaced sublimation etc., electrodeposi-tion [34.24–27] has been also investigated for preparingpolycrystalline CdTe layers for solar cells. Awakura andcoworkers [34.25–27] reported the cathodic depositionbehavior of CdTe thin layers under irradiation by visi-ble light using ammoniacal basic aqueous solution asan electrolytic bath. Both deposition current densityand current efficiency for the CdTe deposition wereenhanced by irradiation. The deposited rate was over10 times high than non-photoassisted electrodeposition.This was believed to represent significant progress inreducing the cost of Cd-based solar cells.

To date, solar cells with an n-CdS/p-CdTe hetero-junction have been reported with efficiencies as high as16% [34.22]. Recently, it was reported that thin-layeredn-CdS/p-CdTe heterojunction solar cells have alreadybeen manufactured industrially [34.28]. n-CdS film witha thickness range of 500–1000 Å was deposited from anaqueous solution directly onto a transparent conductiveoxide (TCO) substrate. After the CdS film was annealedfor densification and grain growing, p-CdTe layer wasformed by electrochemical deposition. The CdTe filmwas then annealed in air at 450 C. Subsequently, mono-lithic TCO/CdS/CdTe was cut into discrete cells usingan infrared laser. After other preparing process, a max-imum efficiency of 10.6% for a CdTe 0.94-m2 modulewith a power of 91.5 W was fabricated. The test re-sults showed good stability. For practical applications,a 10-MW CdTe solar-cell manufacturing plant has beenconstructed [34.28].

34.2.4 CdZnTe Solar Cells

The ternary compound cadmium zinc telluride (CdZnTe,CZT) has potential for the preparation of high-efficiencytandem solar cell since its band gap can be tuned from1.45 to 2.26 eV [34.29, 30]. McCandless et al. [34.29]deposited Cd1−xZnxTe films using the physical va-por deposition (PVD) and vapor transport deposition(VTD) techniques. The film composition was between0.35 and 0.6, corresponding to a band gap from 1.7to 1.9 eV. Post-deposition treatment of CdZnTe filmsin ZnCl2 vapor at 400 C resulted in no change to

the alloy composition and caused recrystallization. So-lar cells made from Cd1−xZnxTe films with x ≈ 0.35exhibited Voc = 0.78 V and Jsc < 10 mA/cm2. Theseresults were similar to those obtained from CdSwindow layers. Analysis of the spectral response in-dicated that Cd1−xZnxTe with x ≈ 0.35 has a bandgap of about 1.7 eV. Gidiputti et al. [34.30] usedtwo deposition technologies, co-sputtering from CdTeand ZnTe targets and co-close-spaced sublimation(CCSS) from CdTe and ZnTe powders, to prepareCdZnTe films. A structure similar to the CdTe su-perstrate configuration was initially utilized for cellfabrication: glass/ITO/CdS/CZT/graphite. Typical so-lar cell parameters obtained for CdZnTe/CdS (whenEg(CdZnTe) = 1.72 eV) solar cells were Voc = 720 mV,and Jsc = 2 mA/cm2. However, the spectral responseindicated increasing loss of photocurrent at longer wave-lengths. In order to improve the collection efficiency,CdZnTe devices were annealed in a H2 atmosphere. Thispostprocessing treatment showed that Jsc increased toover 10 mA/cm2. Study of CdZnTe solar cells is beingcarried out in various directions.

34.2.5 The Future of Cd-Based Solar Cells

From the development process of Cd-based solar cells,the study on the laboratory scale is being transferedto large-scale deposition and cell fabrication. Modulesare being developed throughout the world, using thescreen printing, evaporative deposition and close-spacesublimation [34.31–33]

In order to make a commercial Cd-based pho-tovoltaic cell with its full potential, a large-scalehigh-throughput manufacturing process is required. Theprocess must possess excellent yields and produce high-efficiency devices with good long-term stability. In orderto progress towards these goals, a pilot system for con-tinuous, inline processing of CdS/CdTe devices has beendeveloped [34.34–36]. High-quality low-cost thin-filmCdTe modules with an average total area efficiencyof 8% and cascaded production-line yield of > 70%have been manufactured, and technology-developmentprograms will further increase production-line moduleefficiency to 13% within five years [34.36].

34.3 Radiation Detectors

A radiation detector is a device that converts a radi-ation ray into electrical signal. They can be dividedinto gas-filled detectors, scintillation detectors and

semiconductor detectors. Since semiconductor radia-tion detectors have a high spectrometric performance,and can be made portable, they have been applied

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II–IV Semiconductors for Optoelectronics: CdS, CdSe, CdTe 34.3 Radiation Detectors 835

Incidentradiative ray

Chargesensitivepreamplifier

Bias

Shapingamplifier

Multi-channelanalyzer

Fig. 34.7 The principle of operation of a semiconductordetector

in nuclear safeguards, medical physics and imagingdiagnosis, industrial safety, nondestructive analysis,security and monitoring, nonproliferation and astro-physics [34.37, 38]. Meanwhile, they are also beinggradually improved.

34.3.1 Basic Description of SemiconductorRadiation Detectors

The principle of operation of a semiconductor radiationdetector is shown in Fig. 34.7. When high-energy pho-tons (X- or γ -rays) radiate the detector, they lose theenergy to induce electron–hole pairs in the semicon-ductor through photoelectric or Compton interactions,thereby increasing the conductivity of the material. Inorder to detect the change in conductivity, a bias voltageis required. An externally applied electric field sepa-rates the electron–hole pairs before they recombine, andelectrons drift towards the anode, holes to the cathode.The charges are then collected by the electrodes. Thecollected charges produce a current pulse, whose inte-gral equals the total charge generated by the incidentphotons, indicating the radiation intensity. The read-outgoes through a charge-sensitive preamplifier, followed

Table 34.2 The features of some semiconductor detectors

NaI Si Ge CdTe Cd0.9Zn0.2Te

Bandgap (eV) at 300 K – 1.14 0.67 1.54 1.58

Density (g/cm3) 3.67 2.33 5.35 6.2 5.78

Energy per e–h pair (eV) – 3.61 2.96 4.43 4.64

Detection/absorption High Low Medium High High

Operation at room temp. Yes Yes No Yes Yes

Spectrometic performance Medium High Very high High High

by a shaping amplifier. The multichannel analyzer isused for spectroscopic measurements of the radiationray. This type of detector can produce both count andenergy information. Typically, detectors are manufac-tured in a square or rectangular configuration to maintaina uniform bias-current distribution throughout the activeregion. The wavelength of the peak response depends onthe material’s band-gap energy.

There are several important parameters to describea detector performance. Detection efficiency is definedas the percentage of radiation incident on a detectorsystem that is actually detected. Detector efficiency de-pends on the detector size and shape (larger areas andvolumes are more sensitive), radiation type, materialdensity (atomic number), and the depth of the detec-tion medium. Energy resolution is one of the mostimportant characteristics of semiconductor detectors. Itis usually defined as the full-width at half-maximum(FWHM) of a single-energy peak at a specific energy.High resolution enables the system to more clearly re-solve the peaks within a spectrum. So the reliabilityof the detector is also high. Detector sensitivity im-plies minimal detectable counts. It is defined as thenumber of counts that can be distinguished from thebackground. The relative detector response factor ex-presses the sensitivity of a detector relative to a standardsubstance. If the relative detector response factor isexpressed on an equal-mass (-weight) basis, the deter-mined sensitivity values can be substituted for the peakarea.

34.3.2 CdTe and CdZnTe Radiation Detectors

Semiconductor radiation detectors are fabricated froma variety of materials including: germanium (Ge), silicon(Si), mercuric iodide (HgI2), CdTe, CdZnTe etc.. Typ-ical detectors for a given application depend on severalfactors. Table 34.2 shows the features of some semicon-ductor detectors. From Table 34.2, Ge detectors havethe best resolution, but require liquid-nitrogen cooling,which makes them impractical for portable applications.

PartD

34.3


0.1

100

80

60

40

20

00.1

110

100

90

80

70

60

50

40

30

20

10

01 10 1000.2 0.5 1.0 2.0 5.0 10.0

a) Efficiency (%)

Thickness (mm)

b) Absorption efficiency (%)

Thickness (mm)

10 keV

60 keV

100 keV

140 keV660 keV

CdTe

Ge

Si

Gamma rayabsorptionin CdZnTe

Fig. 34.8a,b γ -ray absorption in CdTe (a) and CdZnTe (b) detectors as a function of thickness

Si detectors also need cooling, and are inefficient in de-tecting photons with energies greater than a few tensof keV. CdTe and CdZnTe detectors possess their ownadvantages.

Single crystals of CdTe and CdZnTe are very im-portant materials for the development of X- and γ -raydetectors. Currently Ge and Si detectors have to be usedat liquid-nitrogen temperatures, and suffer from poor de-tection efficiency. Since the compound semiconductorsCdTe and CdZnTe with high atomic number (ZCd = 48,Z Zn = 30, ZTe = 52) have a significantly higher pho-toelectric absorption efficiency in the -100–500 keVrange, they can be fabricated into detectors that pro-vide two advantages for use in portable instrumentation:(1) the large band-gap energy (Eg > 1.54 eV at roomtemperature) results in extremely small leakage currentof a few nanoamps at room temperature. Therefore,these detectors have the potential to serve as use-ful detectors for radiation spectroscopy without theneed for cryogenic cooling as required for more con-ventional silicon and hyper-pure germanium detectors;(2) the high density of the crystal provides excellentstopping power over a wide range of energies. Theability to operate at room temperature without theneed for liquid-nitrogen cooling allows the construc-tion of compact devices. CdTe and CdZnTe detectorscan be fabricated into a variety of shapes and sizes,which makes it possible to produce detectors capa-ble of meeting the requirements of a wide assortmentof applications that are unsupported by other detectortypes. Their small size and relatively simple electronicsallow them to open new areas of detector applica-tion. In many instances, CdTe and CdZnTe detectors

can be substituted for other detector types in existingapplications.

Figure 34.8 shows the γ -ray absorption efficiencyin: (a) CdTe, and (b) CdZnTe detectors as a func-tion of thickness. It was reported that, for a detectorwith a thickness of about 10 mm detecting 100-keVγ -rays, the detection efficiency for a CdTe detectoris 100%, for a Ge detector 88%, for a Si detectoronly 12% [34.39]. In the case of a CdZnTe detec-tor, CdZnTe with a thickness of only about 7 mm canabsorb 100% of the γ -rays [34.40]. This efficiency as-sumes that the γ -ray is completely absorbed and thegenerated electron–hole pairs are completely collected.In fact, a large amount of charge loss occurs in CdTeand CdZnTe detector, depending on the quality of thematerials.

34.3.3 Performanceof CdTe and CdZnTe Detectors

Although CdTe and CdZnTe have many advantages overSi and Ge, including higher attenuation coefficients andlower leakage currents due to the wider bandgap, theirdisadvantages limit their application range [34.41]. Themain disadvantage is that their crystals usually containhigher density defects. These defects are remaining im-purities and structural defects such as mechanical cracks,twins, grain and tilt boundaries, and dislocations alllower the crystalline perfection of the materials.

In CdTe and CdZnTe detectors, the charge trans-port determines the collection efficiency of generatedelectron-hole pairs, and structural properties of mater-ials control the uniformity of the charge transport. The

PartD

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problem of poor transport and collection of carriers, es-pecially hole, can be overcome by applying high voltageto device and a range of electrode config. The formeris related to high resistivity material, and later is re-lated to the design of a detector. In addition, it wasfound that detected pulse signal intensity degraded withthe time [34.42, 43]. This phenomenon was thought tobe mainly related to the defects existing in bandgap.When the detector starts to work, these defects couldact as slowly ionized acceptors. The low resistivity andpoor quality material always exhibits such a polarizationeffect.

Summarizing above, improvement of a detectorperformance is equivalent to the improvement of thecrystallinity of CdTe and CdZnTe. The key feature ofall applications except substrate materials of CdTe andCdZnTe is the resistivity. This is because high resis-tivity can be obtained only by controlling native defectsand impurity concentration. For many years, much effortwas done in preparing high quality CdTe and CnZnTesingle crystals.

Progresses in Crystal Growthof High Quality CdTe and CdZnTe

Various techniques have been applied to grow high-quality CdTe and CdZnTe single crystals. The Bridgmanmethod [34.44, 45], the traveling-heater method [34.46,47], growth from Te solvent [34.48], the gradient-freezemethod [34.49,50], and physical vapor transport [34.51,52] are the most widely used. Crystals with applicablequality and size have become available, and have alsofostered the rapid progress of research on CdTe.

As-grown CdTe single crystals commonly con-tain high concentration of both residual impuritiesand intrinsic defects. Preparation of high-purity high-resistivity CdTe and CdZnTe crystals has been widelyattempted [34.53]. Early works was done by Tribouletand Marfaing [34.54] in obtaining high-purity lightlycompensated zone-melting growth following synthesisby the Bridgman method. The room-temperature car-rier concentrations range from 1 to 5×1013 cm−3, andthe resistivity from 100 to 400 Ω cm. The carrier mo-bility at 32 K reaches as high as 1.46 × 105 cm2/Vs.The total concentration of electrically active centers wasestimated to be about 1014 cm−3.

Usually, as-grown CdTe crystal shows p-type con-ductivity owing to the existence of remaining acceptorimpurities or native defects. For this reason, many re-sults have been reported in preparing high-resistivityCdTe. Chlorine (Cl) is thought to be a suitable donor forthe compensation of these remaining acceptors because

Cl can be doped quite uniformly due to its very smallsegregation coefficient [34.55].

The growth of high-resistivity CdTe:Cl single crys-tals with device quality was successfully performed bythe traveling-heater method (THM) [34.55]. Solvent al-loys for THM growth were synthesized in ampoulesfilled with Te, CdTe and CdCl2 so that the molar ratioof Te/Cd was the same as in the solvent zone duringthe growth. The Cl concentration in the grown crys-tal was 2 weight−ppm. According to the dynamics ofthe crystal growth, it is important to control the shapeof the solid–liquid to grow high-quality single crys-tals. Therefore, the solvent volume was optimized. Theuse of a slightly tilted seed from <111>B was alsoeffective in limiting the generation of twins with differ-ent directions. Single-crystal (111) wafers, larger than30 × 30 mm2 were successfully obtained from a growncrystal with a diameter of 50 mm. Pt/CdTe/In detectorswith dimensions of 2 × 2 × 0.5 mm3 showed better en-ergy resolution, because a higher electric field can beapplied. The effective detector resistivity was estimatedto be 1011 Ωcm.

CdTe doped with chlorine (Cl) or indium (In) witha resistivity of 3 × 109 Ωcm and CdZnTe with a resis-tivity of 5 × 1010 Ωcm were grown by the high-pressureBridgman (HPB) technique [34.56]. The material waspolycrystalline with large grains and twins. Althoughthe crystalline quality of HPB CdTe and CdZnTe ispoor, the grains are large enough to obtain volume de-tectors of several cm3. Photoluminescence (PL) spectraat 4 K showed that the free exciton could be observedand the FWHM of the bound excitons is very low. Someimpurity emissions were identified. The detectors werefabricated from HPB CdTe and CdZnTe. These detec-tors showed excellent performance. Gamma-ray spectrawere presented with high-energy resolution in an en-ergy range from 60 to 600 keV. Using a 10×10×2 mm2

HPB detector, at a bias of 300 V, the peak at 122 keVfrom 57Co had a FWHM of 5.2 KeV. Detectors withhigh-energy resolution were fabricated.

Because of the difference in vapor pressure betweenCd and Te, grown crystals contain a large number ofCd vacancies (VCd). These Cd vacancies manifest ac-ceptor behaviour. Therefore, high-resistivity CdTe canalso be obtained by controlling the concentration of Cdvacancies. However, a prerequisite is that the purity ofthe CdTe crystal is high enough that the remaining im-purities do not play a substantial role in determiningthe conductivity. Recently, the preparation of ultra-high-purity CdTe single crystals was reported [34.57, 58].In order to obtain a high purity the starting mater-

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1.58 1.585 1.59 1.595 1.6

Intensity (arb. units)

Energy (eV)

(A0, X)1.5896

FWHM = 0.31 meV

As-grown CdTe4.2 K

Fig. 34.9 A typical high-resolution PL spectrum of a high-purity CdTe sample

ials of Cd and Te, Cd were first purified by vacuumdistillation (VD) and the overlap zone-melting (OZM)method [34.57], while Te was purified using the normalfreezing method [34.58]. The results of glow-dischargemass spectroscopy (GDMS) and the measurement ofthe residual resistivity ratio (RRR) showed that refinedCd and Te had the purity of 6N-up. Using the refinedCd and Te as starting materials, extremely high-puritytwin-free CdTe single crystals were prepared by the tra-ditional vertical Bridgman technique. Figure 34.9 showsa typical high-resolution PL spectrum of a high-purityCdTe sample. Only a sharp and strong emission lineof (A0, X) (an exciton bound to a neutral acceptor) at1.5896 eV was observed. The FWHM of the (A0, X)line is 0.31 meV. This is the narrowest value ever re-ported for CdTe single crystals grown by the Bridgmanmethod. All of the above indicate that the sample is ofhigh purity and quality. For a good performance detec-tor, the leakage current must not exceed a few nanoamps,which requires a material with high resistivity. Whengrowing CdTe crystals, the CdTe contain large num-ber of Cd vacancies due to the extreme volatility of Cd.For these reason, CdTe single crystals were annealedin a Cd atmosphere in order to obtain a high-resistivitycrystal. Hall-measurement results showed that the con-ductive type changed from as-grown p-type to n-typeat a Cd pressure around 1.5 × 10−2 atm, correspondingto a Cd source temperature of 500 C. Samples an-nealed under these conditions showed a resistivity ashigh as 109 Ωcm.

Lachish [34.59] grew CdTe and CdZnTe crystalswith excess cadmium in order to avoid the Te precip-

itates found in crystals grown by Bridgman method.During crystal growth one ampoule end was kept ata low temperature, which determines a constant, nearlyatmospheric, vapor pressure in the system. The constantvapor pressure maintains a constant liquid compositionand balanced amounts of cadmium and tellurium withinthe crystal. Although the highly donor-doped crystalsare highly electrically conducting, successive annealingin Te vapor transforms the crystal wafers into a highlycompensated state showing high electrical resistivity andhigh gamma sensitivity, depending on the doping level.

Improvements of Transport and Collectionof Carriers in CdTe and CdZnTe Detectors

The transport and collection of carriers in CdTe andCdZnTe, especially holes, are the main problems thataffect their performance. Poor carriers transport causesposition-dependent charge collection and limits their ca-pability as high-resolution spectrometers [34.60]. Asa result, this limits the volume of detector. Therefore, theefficient detecting resolution and range are decreased.Amman et al. [34.61] studied the affect of nonuniformelectron trapping on the performance of a γ -ray detec-tor. An analysis of the induced charge signals indicatedthat regions of enhanced electron trapping are associatedwith inclusions, and that these regions extend beyond thephysical size of the inclusions. Such regions introducenonuniform electron trapping in the material that thendegrades the spectroscopic performance of the mater-ial as a γ -ray detector. The measurements showed thatthe degree of nonuniformity that affects detector per-formance could be at the 1% level. Consequently, anyuseful characterization and analysis technique must besensitive down to this level.

Detectors equipped with Ohmic contacts, anda grounded guard ring around the positive contact, havea fast charge-collection time. Conductivity adjustment,in detectors insensitive to hole trapping, optimizes thedetector operation by a trade-off between electron life-time and electrical resistance. Better hole collection hasbeen achieved by designing detectors and electrodes ofvarious geometries. Nakazawa et al. [34.62] improvedthe CdTe diode detectors by fabricating the guard-ringstructure in the cathode face, and the leakage current ofthe detector decreased by more than an order of magni-tude. The new CdTe detector was operated with a biasas high as 800 V at 20 C and showed a good energyresolution (0.93 keV and 1 keV FWHM for 59.5 keVand 122 keV, respectively) and high stability for long-term operation at room temperature. Furthermore, theyalso developed a large-area (20×20 mm2) detector with

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very high resolution. Owing to its high stopping powerand high resolution at room temperature, pixel imagershave also been developed in the same group, and it isthought that this large diode detector could possibly bea substitute for scintillation detectors.

Since the carriers drift slow and have a short lifetimein CdTe detectors, the number of photons in the photo-peak is reduced and the spectrum is distorted by a tailtowards lower energies. For these reasons, CdTe detec-tors are usually made into Schottky diodes, because thisstructure can withstand much higher bias voltage witha leakage current orders of magnitude lower than de-tectors with Ohmic contacts. Takahashi et al. [34.63]adopted the configuration of Schottky CdTe diode,which due to the low leaking current, makes it possibleto apply a much higher bias voltage to ensure completecharge collection in relatively thin (< 1 mm) devices.Both the improved charge-collection efficiency and thelow leakage current lead to an energy resolution ofbetter than 600 eV FWHM at 60 keV for a 2×2 mm2 de-vice without any electronics for charge-loss correction.Meanwhile, they also fabricated large-area detectorswith dimensions of 21.5×21.5 mm2, with a thicknessof 0.5 mm and an energy resolution of 2.8 keV. Stackeddetectors can measure the energies as high as 300 keV.Furthermore, a large-array detector, consisting of 1024individual CdTe diodes, was also made. Every detec-tor had a dimension of 1.2×5.0 mm2. The total area,including the spaces between the detector elements, is44×44 mm2. This array detector is expected to be usedin next-generation Compton telescopes.

34.3.4 Applicationsof CdTe and CdZnTe Detectors

Owing to the convenience of the smaller collimator,better resolution and temperature stability, CdTe andCZT detectors have been used in safeguard appli-cations by the International Atomic Energy Agency(IAEA) and some countries for over ten years [34.64,65]. With the gradual improvement in performance,CdTe and CdZnTe detectors are replacing NaI detec-tors used in spent-fuel attribute tests [34.66], thoughtheir sensitivity is still low compared to NaI and Gedetectors, although already sufficient for many appli-cations. CdTe detectors have a sensitive volume ofabout 20–100 mm3 and a probe diameter of 8–9 mm.CdZnTe detectors have a larger volume than CdTe de-tectors. The largest commercial CdZnTe detectors havea geometric volume of 1687 mm3 (15 × 15 × 7.5 mm3).Detectors are mainly of hemispheric design to obtain

high carrier-collection efficiency. These large-volumedetectors have been made into portable and hand-heldisotope-identification devices and are being used to de-tect radioactive sources.They will become commerciallyavailable in the near future.

At present, conventional X-ray film or scintillatormammograms are used in medical diagnostics such asscreening for breast cancer. However, these show a non-linear response to X-ray intensity and the detectionquantum efficiency is low. Room-temperature semi-conductor detectors such as CdTe and CdZnTe havefavorable physical characteristics for medical applica-tions. From the start of these investigations in the 1980s,rapid progress has been achieved [34.67–72]. Bar-ber [34.68] presented results concerning, first, a CdTetwo-dimensional (2-D) imaging system (20 × 30 mm2

with 400×600 pixels) for dental radiology and, sec-ond, a CdZnTe fast pulse-correction method appliedto a 5×5×5 mm3 CdZnTe detector (energy resolutionof 5% for a detection efficiency of 85% at 122 keV)for medical imaging. After that, a 2-mm-thick CdZnTedetector was fabricated for application to digital mam-mography [34.70]. The preliminary images showed highspatial resolution and efficiency. Furthermore, CdTe andCdZnTe detectors with a thickness of 0.15–0.2 mmwere fabricated [34.72]. The detectors are indium-bump-bonded onto a small version of a chip. Their detectionquantum efficiency was measured as 65%. This resultshowed that CdTe and CdZnTe detectors are superiorto scintillator-based digital systems, whose quantumefficiency is typically around 30 −-40%. This showedthat CdTe and CdZnTe detectors have potential appli-cations in medical imaging, as well as industrially fornondestructive evaluation inspection.

In universe exploration, Cd-based detectors canbe used in an advanced Compton telescope (ACT)planned as the next-generation space-based instrumentdevoted to observations of low/medium-energy γ -rays(≈0.2–30 MeV) and to the nonthermal energy ex-ploration telescope (NeXT) [34.73]. In the universe,radiation rays have a wide energy range of 0.5–80 keV.In order to detect this wide range of radiation, Taka-hashi et al. [34.74–76] proposed a new focal-planedetector based on the idea of combining an X-ray charge-coupled device (CCD) and a CdTe pixel detector as thewide-band X-ray imager (WXI). The WXI consists ofa soft-X-ray imager and a hard-X-ray imager. For thedetection of soft X-rays (10–20 keV) with high posi-tional resolution, a CCD with a very thin dead layer willbe used. For hard X-rays, CdTe pixel detectors serve asabsorbers. This study is now under way.

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34.4 Conclusions

Devices fabricated from Cd-based compounds, such assolar cells and radiation detectors, are being appliedin our daily life. In the case of solar cells, althoughrecent success have improved their conversion effi-ciency and reduced the cost, many problems remain.Fundamental understanding of the CdTe-based solar-cell properties is limited, particularly as a result oftheir polycrystalline nature. Therefore, the fundamen-tal electronic properties of polycrystalline Cd-basedthin films should be studied deeply. Other challengesare to reduce the cost and to lengthen the operat-ing life span. These problems are being studied andsolved [34.77].

In the case of radiation detectors, CdTe andCdZnTe detectors have many advantages, such as room-temperature operation, high count rates, small size,and direct conversion of photons to charge, whichmake them attractive candidates for a wide varietyof applications in industrial gauging and analysis,as well as medical instrumentation and other areas.These detectors are being made available commer-cially at present. However, they have severe problemssuch as polarization effects, long-term stability andtheir high price. Further efforts should still be focusedon the preparation of high-quality materials and im-provement of the stability and reliability of detectors.We are confident that radiation detectors made from

Cd-based compounds will achieve more widespreadapplication.

Definition of Terms• Cd-based compound semiconductor: a semicon-ductor that contains the element Cd.• Heterojunction: a junction between semiconduc-tors that differ in their doping-level conductivities,and also in their atomic or alloy composition.• Band gap: energy difference between the conduc-tion band and the valence band.• n-type conductivity: a semiconductor material,with electrons as the majority charge carriers, that isformed by doping with donor atoms.• p-type conductivity: a semiconductor material inwhich the dopants create holes as the majoritycharge carrier, formed by doping with acceptoratoms.• Solar cell: a semiconductor device that converts theenergy of sunlight into electric energy. Also calleda photovoltaic cell.• Semiconductor detector: a device that converts theincident photons directly into an electrical pulse.• Conversion efficiency: the ratio of incident photonenergy and output electricity energy.• Detection efficiency: percentage of radiation inci-dent on a detector system that is actually detected.

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