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Current transport mechanism of p-GaN Schottky contacts

Kenji Shiojimaa)

NTT Photonics Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-0198, Japan

Tomoya Sugahara and Shiro SakaiDepartment of Electrical and Electronic Engineering, Tokushima University, 2-1 Minami-josanjima,Tokushima, 770-8506, Japan

Received 30 May 2000; accepted for publication 20 October 2000

Transient measurements ofI V and depletion layer capacitance were conducted to clarify the leaky

current flow mechanism in Ni Schottky contacts formed on Mg-doped p-GaN. We found that

carrier capture and emission from acceptor-like deep level defects cause depletion layer width

(Wdep) to vary significantly. Upon ionization of the defects by white light, which results in small

Wdep , current can go through the Schottky barrier and a leaky I V curve is observed. Upon filling

by current injection, Wdep becomes larger and the large original Schottky barrier height is seen. The

time constant of carrier emission is as long as 8.3103 min. 2000 American Institute of

Physics. S0003-6951 00 04251-0

The recent progress in GaN-based optoelectronic de-

vices, such blue light-emitting diodes,1 laser diodes,2 and

ultraviolet detectors,3 points to the need for better ohmic

contacts to p-GaN and a deeper understanding of the basic

characteristics of metal/p-GaN interfaces.

For n-GaN Schottky contacts, the Schottky barrier

height (qB) has been found to basically depend on the

metal work function due to the ionic nature of GaN,4,5 and

qB of up to 1.5 eV has been reported.6 For p-GaN, a much

higher qB above 2 eV is expected since the sum of the

qBs of n and p types adds up to the band gap (Eg) of 3.4

eV.7,8 However, the contacts tend to exhibit very leaky

Schottky characteristics, i.e., a low barrier,9 but high series

resistance.10 Consequently, the mechanism of current flow

through the interface has not been established and the exactvalue of qB has not yet been estimated.

We have reported improved leaky characteristics for

Ni/p-GaN Schottky contacts by means of low Mg doping

and obtained qB as high as 2.40.2 eV from I V measure-

ments, which is in good agreement with the previous predic-

tion (qBFEgqBn ).11 This letter reports transient mea-

surements of I V and depletion layer capacitances (Cdep)

conducted to characterize the leaky current flow mechanism

of Ni/p-GaN Schottky contacts.

2-m-thick Mg-doped GaN films were grown on 0001

sapphire substrates using metalorganic chemical vapor

deposition.11 The Mg concentration was 1.31018 cm3 ac-

cording to secondary ion mass spectrometry SIMS mea-surements. The carrier concentration was estimated from

capacitance voltage (C V) measurements at a modulation

frequency of 40 Hz to be 6.21016 cm3.

Planar-type Schottky contacts were formed by the lift-

off process and electron-beam evaporation. Ni 30 nm /

Au 150 nm ohmic contacts were deposited, and then the

samples were annealed at 450 C for 5 min. After the surface

oxide was removed in buffered hydrofluoric acid solution, Ni

100-nm-thick Schottky contacts with both circular and rect-

angular patterns were deposited. The ohmic contacts sur-

round the Schottky contacts with a gap of 10 m and are

more than 100 times larger in area than the Schottky con-

tacts.

A sample with 100 m circular dots was loaded in a

measurement box and probes were lowered onto the con-

tacts. The sample was illuminated by white light from vari-

ous directions. The light went through the GaN gap region

between the contacts, and was reflected at the metallic

sample-mounting base. This resulted in illumination of the

GaN layer and metal/semiconductor interfaces. Then, the

viewing window was closed to darken the inside of the box,

and the first measurement was conducted with forward bias-

ing from 0 to 10 V at a sweep speed of about 2 V/min.

Figure 1 a shows typical forward I V curves of Ni/p-GaN.The current linearly increased as the voltage increased to 0.5

V. In the voltage region between 0.5 and 4 V, the current

saturated around 1 nA. Above 4 V, the current gradually

increased to 2107 A at 10 V not shown in Fig. 1 a . The

time was set at t0 at the end of this measurement just

after the bias voltage was swept to 10 V , and immediately a

series of I V measurements were carried out to observe any

changes in the turn-on voltage. In order to avoid further cur-

rent injection to the depletion layer, the measurements were

stopped when the current reached 1010 A.

The second I V curve is completely different from the

first one. The current is very small when the bias voltage is

2.5 V or less. The diode turns on above 2.5 V. Taking ac-

count of the time delay of biasing from 0 V to turn-on, this

second curve is denoted as that obtained 2 min after the first

measurement. Further I V measurements were carried out

up to 800 min after the first measurement. The absolute value

of the turn-on voltage decreased as the time from the first

measurement became longer. At any time, after illuminating

the sample by white light and then putting it in the dark, the

sample reproducibly showed the first I V curve. These be-

haviors could be associated with deep level defects, such as

ionization by illumination, trap filling by forward bias, and

natural emission with a large time constant.a Electronic mail: shiojima@aecl.ntt.co.jp

APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 26 25 DECEMBER 2000

43530003-6951/2000/77(26)/4353/3/$17.00 2000 American Institute of PhysicsDownloaded 04 Mar 2009 to 132.234.251.211. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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In order to estimate the level of the change in the I V

curves, the apparent qB and ideality factors n values were

simply calculated in terms of only the thermionic emission

model12 using

JA** T2 expq

B/kT

exp

qV/nkT

1

,

1

where A** is the effective Richardson constant 72

A/cm2 K2 for p-GaN with m*0.60m 0 .12 Later, however,

we will show that the original qB does not change and

another current flow mechanism arises. Figure 1 b shows

the apparent qB , and n value from I V measurements as a

function of the time from the first measurement. At the first

measurement ( t0), qB is 0.858 eV and n is 1.93. At the

second (t2 min , qB is as high as 2.45 eV and n is as low

as 1.73. The qB gradually decreases and n increases as time

increases. Even after 800 min, qB is over 1.76 eV. It is

concluded that the apparent qB widely ranges from a large

value expected by qBPE

gq

Bnto a small value close

to reported ones with higher Mg-doping p-GaN.9 Therefore,

understanding the mechanism for the variation is very impor-

tant.

In order to investigate this behavior more quantitatively,

the transient response of Cdep was measured using a preci-

sion LCR meter HP 4284A . Prior to the measurements,

some preliminary examinations were conducted in order to

establish the proper measurement condition details are in

Ref. 13 . Huang reported that there is a modulation fre-

quency dispersion of Cdep because the activation energy lev-

els of the Mg acceptors are relatively deep 160 meV and

the ionization of the acceptors can not follow the modulation

at higher frequencies.14

Our results are very similar, i.e., asthe frequency decreases, Cdep increases and then saturates

below 100 Hz. The measurement frequency chosen was 40

Hz, which is low enough to treat Mg as a shallow dopant.

Second, the contribution of series resistance and parallel con-

ductance to the measurement was evaluated. In both series

and parallel mode measurements, Cdep was the same and the

Q value was over 30, even though the sample was illumi-nated. Therefore, the dominant component is Cdep and the

contribution of these factors can be neglected. C V mea-

surements showed good linearity in a 1/C2 plot. Finally, Cdepmeasurements for diodes with different junction areas were

carried out. Cdep was completely proportional to the area, so

that the ohmic capacitance can be neglected.

Figure 2 shows the transient response of Cdep at dc

bias0 V for the sample with 726 m395 m rectangular

dots in the dark. The sample was mounted in the measure-

ment box and illuminated as in the I V measurement. After

closing the viewing window to darken the inside of the box,

we started the measurement ( t0).

Cdep exponentially decreased as measurement time in-creased and saturated at around 375 pF. Due to the illumina-

tion, deep levels in the depletion region were ionized and

thus more carriers were generated in the intervening GaN

region. It is well known that both p- and n-GaN exhibit

persistent photoconductivity PPC ,15,16 which may affect the

series resistance. In our measurements, however, as de-

scribed before, the neutralization of some of the deep levels

from the depletion edge, not PPC, is dominant in this decay.

After applying forward bias voltage of 5 V for 10 min, Cdepdropped to 100115 pF 30 s after the biasing once, and

then exponentially increased. Cdep was still as small as 205

pF 800 min after biasing. Finally, Cdep saturated at around

375 pF. This large variation of Cdep indicates a large densityof deep level defects.

The Cdep is calculated as12

Cdep qs NANdeep x ,t /2Vbi1/2, 2

where s is a dielectric constant, NA is acceptor concentra-

tion, Ndeep is deep-level-defect concentration, and Vbi is a

built-in potential. NA should be constant and Ndeep should be

a function of both position and time. Assuming all deep lev-

els are neutralized by the forward bias (Ndeep0) and only

6.21016 cm3 Mg acceptors are ionized, Cdep is calculated

to be 125 pF, where qB and a Mg level are 2.5 and 0.16 eV

(Vbi

qB

EMg). That is close to the measured Cdep 30 safter the biasing. Trap filling by the biasing is a reasonable

FIG. 1. a Typical forward I V curves of Ni/p-GaN in a semilog plot. b

Apparent qB and n values as a function of the time from the first I V

measurement.

FIG. 2. Transient response of Cdep at V0 V in the dark. When the illumi-

nation was turned off, the measurement was started.

4354 Appl. Phys. Lett., Vol. 77, No. 26, 25 December 2000 Shiojima, Sugahara, and Sakai

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explanation. After the biasing, the deep levels are ionized.

The estimated time constant of this emission was about 8.3

103 min. As expected from the I V results, these decays

are reproducible by either forward-biasing or illumination.

Based on these results, the band diagrams of a

metal/p-GaN contact for each stage were devised. The basicconcept is that the large original qB is constant and hole

capture and emission to deep levels significantly vary deple-

tion layer width (Wdep). The band diagrams are shown in

Fig. 3. There are acceptor-like deep level defects in p-GaN

and their concentration (Ndeep) is much larger than that of

shallow acceptor impurities (NA). When the sample is

loaded into the measurement box, the deep level defects are

ionized by white light and Wdep becomes smaller. Conse-

quently, Cdep becomes large, and a current can go through

the barrier a . This would explain the apparent low qB and

large n value. In our preliminary analysis of the C t mea-

surements, Ndeep is larger than 31018 cm3. Due to this

high concentration, the tunneling current could flow via atrap level or not and it, not the thermionic emission, would

be dominant. Temperature dependence measurements of

I V characteristics would reveal the current flow mecha-

nism.

When forward bias is applied, the deep-level defects are

filled due to the current crossing the interface and become

neutral. The Wdep becomes larger and Cdep smaller, and the

original large qB is seen b . The reason the forward cur-

rent slightly decreased around 1 nA when the applied voltage

was increased from 0.7 to 3 V in the first I V curve in Fig.

1 is that trap filling occurred due to the forward current and

tunneling current was impeded. The state can be flip-flopped

by illumination or current injection at any time. Since the

time constant is very long, this contact can be used as a

memory device.

As a result of the large qB formed, very-deep-level

defects can be found, but, at present, the origin is unclear.

Our SIMS measurements showed that both Mg (1.3

1018 cm3 and O(11017 cm3) concentration were

less than Ndeep . Spectroscopic defect characterization tech-niques, such as photocapacitance and the temperature depen-

dence of transient capacitance, would provide more informa-

tion.

In conclusion, transient measurements of I V, and Cdepwere conducted to clarify the leaky current flow mechanism

of Ni/p-GaN Schottky contacts. Carrier capture and emis-

sion from deep level defects cause Wdep to vary significantly.

When Wdep is small, current can go through the barrier and a

leaky I V curve low apparent qB is observed. When

Wdep is large, the large original qB is seen. This process is

repeatable by illumination by white light or current injection.

The time constant of carrier emission is as long as 8.3

103 min.

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FIG. 3. Band diagrams of metal/p-GaN with carrier capture and emission

by deep level defects.

4355Appl. Phys. Lett., Vol. 77, No. 26, 25 December 2000 Shiojima, Sugahara, and Sakai

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