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Contract No. DE-AC36-99-GO10337 

S.W. Johnston, S. Kurtz, D.J. Friedman, A.J. Ptak, 

R.K. Ahrenkiel, and R.S. Crandall 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37474 

Electron Traps in p-Type GaAsN 

Characterized by Deep-Level 

Transient Spectroscopy 
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ELECTRON TRAPS IN P-TYPE GaAsN CHARACTERIZED BY DEEPLEVEL TRANSIENT SPECTROSCOPY 

S. W. Johnston, Sarah Kurtz, D. J. Friedman, A. J. Ptak, R. K. Ahrenkiel, and R. S. Crandall 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 

ABSTRACT 

We have used deep level transient spectroscopy to 

detect traps in p-type GaAsN grown by metal-organic 

chemical vapor deposition. Although minority-carrier electrons are not intentionally injected into the depletion region 

of the measured samples, electron traps are detected in 

both Schottky barrier and p-n junction devices. The electron-trap signal can exist using only reverse biases during 

measurement, and checks of series resistance and minority-carrier injection using an optical source also confirm the 

electron-trap signal. For dilute-nitrogen p-n junction samples, the electron trap gives the dominant signal peak. 

The peak's magnitude, which corresponds to trap density,

correlates to amounts of nitrogen incorporated during 

growth and reduced open-circuit voltage during lightcharacterization. The p-type GaAsN layers have net acceptor carrier concentrations in the mid-1016 to low-1017

cm-3, as determined by capacitance voltage profiling. The 

electron-trap concentration is dependent on the N content, 

but values, when traps are filled to saturation, range from 

1015 to 1016 cm-3. The electron signal peak shows a 

shoulder peak on some samples, giving another close 

energy level. The electron-trap activation energy is 

somewhat dependent on the trap filling time, but ranges 

from about 0.15 to 0.30 eV, and is usually near 0.2 eV for 

the largest peak when filled to saturation. 

INTRODUCTION 

The bandgap of the GaAsN alloy can be reduced to 

near 1 eV when the nitrogen content is about 2%. Indium 

can also be added to the alloy to improve lattice matching 

to GaAs and Ge [1,2]. Both of these properties are advantageous to developing a four-junction high-efficiency solar 

cell, consisting of GaInP, GaAs, InGaAsN, and Ge. Such 

a structure has an ideal AM0 efficiency over 40%, and 

could also be used in a terrestrial concentrator module [3]. 

However, poor minority-carrier properties have limited the 

usefulness of the GaAsN alloy in such a solar cell [4]. 

We have used deep level transient spectroscopy

(DLTS) [5] to characterize traps in p-type GaAsN. We

started with GaAs solar cells that showed no DLTS peaks. 

Nitrogen was then added to the active layers of these solar 

cells so that resulting DLTS data could be used to identify

a detrimental defect level. While others have measured 

the GaAsN alloy by DLTS and report several levels of both 

hole and electron traps [6-10], we detect one dominant 

peak in Zn-doped material. The same defect signal also 

appears as the largest peak in unintentionally doped (uid) 

material. The appearance of this peak with added N correlates to a detrimental reduction in the solar cell’s opencircuit voltage beyond that associated with decreased 

bandgap [11]. 

EXPERIMENT 

The epitaxial layers studied here were grown by atmospheric-pressure metal-organic chemical vapor deposition using trimethylgallium or triethylgallium, arsine, and 

dimethylhydrazine on Zn-doped GaAs substrates. The 

growth temperature ranged from 570°C to 650°C to better 

control N content, which is estimated from x-ray diffraction 

measurement. A series of pn junction samples were 

grown with increasing amounts of N. Samples MF057, 

MF058, MF152, MF153, and MF166 have about 0.05%, 

0.1%, 0.25%, 0.6%, and 1.2% N, respectively. The bandgaps range from 1.4 to 1.2 eV. The junction is formed 

using a highly doped n-type GaAs layer and 1-mm square 

contacts deposited on top. A Schottky contact sample was 

also made by sputtering gold dots of 0.75 and 1.0 mm 

diameter on an as-grown uid GaAsN layer. This sample 

has a N content similar to MF166 and a bandgap also near 

1.2 eV. All samples had ohmic contacts on the back surface. 

Most data were collected using an Accent Optical 

Technologies DLTS system. Optical DLTS data were collected using a Sula Technologies DLTS system. Samples 

measured on either system were only reverse-biased. 

Traps were filled when the sample was biased to less reverse bias or zero bias for a defined filling pulse time. 

RESULTS AND DISCUSSION 

The addition of a small amount of nitrogen led to a 

positive peak in the DLTS spectra that was not seen for 

GaAs with no N added. As shown in Fig. 1, a positive 

peak corresponding to minority-carrier electron trapping 

occurs near 125 K for a 23 s-1 rate window. Long filling 

pulses 10 s in duration were used to assure the signal had 

reached a saturation in magnitude. The peak heights, 

which are proportional to trap density, increase with increasing N content. 

The inset of Fig. 1 shows nearly linear data points on 

an inverse capacitance squared versus voltage plot, and 

thus illustrates the uniform p-type doping density over the 

1
bias range used for the DLTS measurement (-1 V to 0 V). 

Linear fits of the data give net acceptor concentrations in 

the mid-1016 cm-3 for all samples. 

2.0x1016

1.5

1.0

0.5

0.0

NT (cm-3

)

100 150 200 250 300 350 400

Temperature (T)

 MF057 

 MF058 

 MF152 

 MF153 

 MF166 

 GaAs (offset for clarity)

1.4x10-5

1.2

1.0

0.8

0.6

1/C2 (1/pF2

)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

Bias (V)

 MF057

 MF058

 MF152

 MF153

 MF166 

Fig. 1. DLTS spectra using a 23 s-1 rate window and long 

signal-saturating filling pulses (10 s) for GaAsN pn junction 

samples with increasing amounts of N. The positive peak 

corresponds to an electron trap. Capacitance-voltage data 

plotted in the inset graph show uniform net acceptor concentrations of 3.5x1016, 3.6x1016, 4.6x1016, 5.7x1016, and 

5.8x1016 cm-3 for MF057, MF058, MF152, MF153, and 

MF166, respectively. 

The DLTS data represented by Fig. 1 are plotted in an 

Arrhenius plot in Fig. 2. Fits of the data for each sample 

give an electron trap activation energy of about 0.2 eV, 

and intercept values give corresponding capture cross 

sections. The electron trap concentration values at saturation also increase with increasing N content. We estimate these densities using a standard DLTS algorithm for 

material having uniform trap distribution. 

70

60

50

40

30

ln(τ*vth*NV)

7 8 9 10 11

1000/T (1/K)

Sample Ea (eV) σ (cm2) NT (cm-3) 

---------- --------- --------- ----------

 MF057 0.18 3x10-17 3x1015

 MF058 0.23 2x10-15 7x1015

 MF152 0.23 6x10-15 1x1016

 MF153 0.24 6x10-15 2x1016

 MF166 0.23 2x10-15 2x1016

Fig. 2. Arrhenius plot created from DLTS data of Fig. 1. 

The curves are vertically offset by +3 for clarity.

The Schottky-barrier p-type GaAsN sample was similarly measured and characterized by DLTS. Filling pulse 

widths ranging from 10 μs to 10 s were used to fill the 

traps during the 0 V bias. As shown in Fig. 3, there is a 

negative peak near 290 K, which corresponds to a majority

carrier (hole trap) with an activation energy of about 0.7 

eV. However, for this work, we will focus on the positive 

electron-trap peaks at low temperature that are similar to 

those seen in the pn junction samples. Although minoritycarrier traps are not expected to be detected when applying only reverse bias, and perhaps even less expected 

with a Schottky barrier, we nonetheless observed similar 

electron trap peaks: one that has a larger amplitude at 

about 110 to 120 K and one that is a shoulder at about 

150 K. When plotting the data on an Arrhenius plot, the 

larger peak at saturation results in an electron trap with an 

activation energy of 0.19 eV, whereas the shoulder peak 

corresponds to 0.22 eV. The peak temperature values of 

the positive peaks are seen to shift to slightly lower temperatures as the filling pulse width is increased. The corresponding activation energies then decrease as shown in 

the inset of Fig. 3, suggesting a possible band of electron 

traps [12]. The filled trap concentrations, which are proportional to the signal amplitudes, are also plotted in the 

inset of Fig. 3. 

2.0

1.5

1.0

0.5

0.0

-0.5

DLTS Signal (pF)

80 120 160 200 240 280 320

Temperature (K)

10 μs

100 μs

1 ms

10 ms

100 ms

1 s

10 s

1013

1014

1015

1016

NT (cm-3)

10-5 10-2 101

Filling pulse width (s)

0.35

0.30

0.25

0.20

0.15

Ea (eV)

Fig. 3. DLTS spectra using a 23 s-1 rate window and various filling pulse widths for a Schottky p-type GaAsN sample. The inset shows calculated trap densities and activation energies for the positive peaks as a function of filling 

pulse: circles for large peak and squares for shoulder 

peak; filled symbols for trap density on left scale and open 

symbols for activation energy on right scale. 

This Schottky sample was measured again using reverse-bias conditions of -1, -2, -3, and -4 V. The pulse 

filling bias is 1 V higher than the reverse-bias value for a 

duration of 10 s. The DLTS results are plotted in Fig. 4. 

The left inset shows the majority hole-concentration of 

2x1017 cm-3 as measured by capacitance-voltage. The 

right inset shows the electron trap signal from optical 

DLTS when using a flash lamp for trap filling. Additionally, 

we verified that the positive signal was not due to high 

series resistance [13,14] by adding a known series resistance that did invert the signal. The DLTS data are plotted 

in an Arrhenius plot shown in Fig. 5. 

Although the signal is smaller at the larger reverse biases, a positive peak is observed for all four bias conditions. However, the signal magnitude does not decrease 

proportionally to the reduction of the space charge region 

width. Instead, the peak magnitude is reduced much more 

quickly. A modeling program was used to investigate this 

2
signal reduction and address the source of electrons to fill 

the traps. 

2.0

1.5

1.0

0.5

0.0

-0.5

DLTS Signal (pF)

50 100 150 200 250 300

Temperature (K)

-4 to -3 V

-3 to -2 V

-2 to -1 V

 -1 to 0 V 2.5

2.0

1.5

1.0

0.5

0.0

ODLTS Signal (pF)

100 200 300 400

Temperature (K)

1.8x10-5

1.6

1.4

1.2

1.0

0.8

0.6

1/C2 (1/pF2) 

-4 -2 0

Bias (V)

2x1017 cm-3

Fig. 4. DLTS spectra using a 23 s-1 rate window, 10 s 

signal-saturating filling pulses, and varying bias conditions 

for a Schottky barrier GaAsN sample. The left inset shows 

the majority hole concentration from capacitance-voltage. 

The right inset shows optical DLTS results for a similar 

rate window. 

70

65

60

55

ln(τ*vth*NV)

8 10 12 14

1000/T (K-1)

 DLTS Bias Ranges

 

Filling 

Reverse Pulse

Bias (V) Bias (V) 

---------- ----------

 -1 to 0

 -2 to -1

 -3 to -2

 -4 to -3

Ea (eV) σ (cm2) NT (cm-3) 

---------- --------- ----------

0.20 7x10-16 1.1x1016

 0.19 3x10-16 5.1x1015

 0.18 1x10-16 2.7x1015

 0.20 5x10-16 1.5x1015

Fig. 5. Arrhenius plot created from DLTS data of Fig. 4. 

The curves are vertically offset by +3 for clarity.

We used the SimWindows [15] modeling program to 

solve for the spatially dependent Fermi level position in a 

zero-biased and reverse-biased Schottky barrier device. 

We calculated a Schottky barrier height of about 0.9 eV 

based on experimental measurements of bandgap (1.2 

eV), built-in potential barrier (0.8 eV), and carrier concentration (2x1017 cm-3). The numerical solutions for 0 V and 

-1 V applied bias give the relative positions of the conduction and valence band edges and the Fermi level or quasiFermi levels, as shown in Fig. 6. An estimated electron 

trap level is shown 0.2 eV below the conduction band as a 

visual aid. 

The relative position of the Fermi level, EF, or electron 

quasi-Fermi level, EFn, with respect to the conduction band 

gives the probability that states in this region are occupied 

by electrons. The electron concentration is spatially integrated and reaches its maximum value within the first few

nanometers near the interface. This gives a value of the 

total number of electrons per unit area. The difference of 

these electron densities calculated at two different biases 

corresponds to the DLTS signal of electron traps that can 

become occupied and then thermally emit when using 

those same biases as the reverse bias and filling pulse 

bias of a DLTS measurement. Without knowledge of the 

total number of electron states, we normalize these differences to give a relative amount of trapped electrons for 

various pulse conditions. The line plotted in Fig. 7 shows 

these normalized changes in electron density between 

biases having steps of 0.2 V. For example, the largest 

value corresponds to a reverse bias of -0.2 V and a filling 

pulse to 0 V and is normalized to 1.0. DLTS data for the 

sample are taken using the same steps of 0.2 V. The filling pulse width is increased until the peak amplitude saturates. These data are shown in the inset of Fig. 7. The 

normalized saturation values are plotted in Fig. 7 for each 

bias condition using the filled circles and lie very near to 

the modeled values (line). 

-5.0

-4.5

-4.0

-3.5

-3.0

Energy (eV)

0 20 40 60 80 100

Distance (nm)

 EC 

 ET 

 EV 

 EF 

 EC (-1 V)

 EFn (-1 V)

 EV (-1 V) ~0.3 eV 

 Au GaAsN

 ET (-1 V)

Fig. 6. Modeled band diagram and electron concen-tration 

for zero-bias and 1 V reverse bias. 

The model’s close fit suggests that the traps near the 

interface are filled and emptied during the changing bias 

conditions. The results are similar for the n+

p junction 

samples, as well. To fill the traps, electrons must surmount the conduction-band offset at the metal interface 

(for the Schottky barrier) and the conduction-band bending 

due to the built-in field and any applied reverse bias in 

order to penetrate a few nanometers into the GaAsN layer 

and become trapped. This energy is estibated to be about 

0.3 eV, as shown in Fig. 6. An estimate of trap filling time 

can be calculated using thermionic emission [16]. Current 

over a potential barrier is given by

 J = AT 2 exp − qΦB

kBT

⎛

⎝

⎜

⎞

⎠

⎟

, (1)

3
1.0

0.8

0.6

0.4

0.2

Normalized DLTS Saturated Peak Height

0 1 2 3 4

|Reverse Bias| (V)

 model 

 data

0.8

0.6

0.4

0.2

0.0

DLTS Signal (pF)

0.0001 0.001 0.01 0.1 1

Filling Pulse Width (s)

Au/GaAsN

T = 110 K

Fig. 7. Measured DLTS signal magnitude versus the 

modeled electron concentration. The inset shows trap 

filling data for the Schottky barrier GaAsN sample. The 

smallest signal represents biasing of -4 V to -3.8 V. 

where A is the Richardson constant of 120 A/cm2

/K2, q is 

electron charge, ΦB is the potential barrier, and kB is Boltzmann’s constant. The amount of trapped charge necessary to give the corresponding DLTS signal can be estimated from the measured values before and just after the 

filling pulse when the applied voltage is the same and 

Q1/C1=Q2/C2. Using a doping value of 2x1017 cm-3, depletion width of 150 nm, and capacitance changing from 

C1=340 pF to C2=342 pF, the trapped charge (Q2-Q1) is 

2x1010 electrons/cm2

 or 3x10-9 C/cm2. Before saturation, 

the traps continue to fill for roughly 0.1 s, and the resulting 

thermionic current is 3x10-8 A/cm2

. Using T=110 K, the 

potential barrier the electrons can surmount is ~0.3 eV, as 

speculated above and sketched on Fig. 6. Also, if the electrons are trapped in just a few nanometers near the interface instead of distributed across the changing depletion 

width, the resulting electron trap densities may actually be 

over 10 to 100 times larger than those calculated from 

standard DLTS analysis. 

The agreement of the DLTS data to the model for the 

various bias conditions suggests that electron traps at or 

near the interface are filled and emptied during measurement. The n+

p junction samples have provided a different 

interface, yet the electron trap was still present. Recently, 

we have also measured p+

n junction samples and have 

detected a negative peak, which corresponds to a majority-carrier electron trap. Preliminary analysis of this electron trap gives an activation energy in the 0.3 to 0.4 eV 

range, and may be the same or similar defect. Detection 

of this level as a majority trap in n-type material also suggests it exists in the bulk GaAsN material. 

SUMMARY 

In summary, an electron trap having an activation energy near 0.2 eV was observed in p-type GaAsN using 

both n+

p junction and Schottky-barrier samples. The same 

minority-carrier electron trap peak was confirmed using 

optical carrier generation. The minority-carrier electron 

DLTS peaks were observed using only reverse-bias conditions. Modeling showed that reduced reverse bias allowed 

increased electron-trap occupation within a few nanometers of the metal or n+

p interface. An estimate of 

thermionic emission provided adequate current to fill these 

electron traps from the metal or n-type side of the junction. 

REFERENCES 

[1] M. Weyers, M. Sato, and H. Ando, “Red Shift of Photoluminescence and Absorption in Dilute GaAsN Alloy Layers”, Jpn. J. Appl. Phys., Part 2 31, 1992, pp. 853-855. 

[2] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, 

and Y. Yazawa, “GaInNAs: A Novel Material for LongWavelength-Range laser Diodes with Excellent highTemperature Performance”, Jpn. J. Appl. Phys., Part 1 35, 

1996, pp. 1273-1275. 

[3] S.R. Kurtz, et al., “Projected Performance of Threeand Four-Junction Devices Using GaAs and GaInP”, 

Twenty-Sixth IEEE PVSC, 1997, pp. 875-878. 

[4] D.J. Friedman, et al., “1-eV Solar Cells with GaInNAs 

Active Layer”, J. Cryst. Growth 195, 1998, pp. 409-415. 

[5] D.V. Lang, “Deep-level transient spectroscopy: A new

method to characterize traps in semiconductors”, J. Appl. 

Phys. 45, 1974, pp. 3023-3032. 

[6] K. M. Chen, et al., “Deep levels in nitrogen-implanted 

n-type GaAs”, J. Appl. Phys. 78, 1995, pp. 4261-4263. 

[7] D. Kwon, et al., “Deep levels in p-type InGaAsN lattice 

matched to GaAs”, Appl. Phys. Lett. 74, 1999, pp. 2830-

2832. 

[8] P. Krispin, et al., “Origin and annealing of deep-level 

defects in p-type GaAs/Ga(As,N)/GaAs hetero-structures 

grown by molecular beam epitaxy” J. Appl. Phys. 89, 

2001, pp. 6294-6301. 

[9] P. Krispin, et al., “Electron traps in Ga(As,N) layers 

grown by molecular-beam epitaxy” Appl. Phys. Lett. 80, 

2001, pp. 2120-2122. 

[10] R. J. Kaplar, et al., “Deep levels in p- and n-type InGaAsN for high-efficiency multi-junction III/V solar cells”, 

Sol. Energy Mater. Sol. Cells 69, 2001, pp. 85-91. 

[11] S.R. Kurtz, et al., “Effect of Nitrogen Concentration on 

the Performance of GaxIn1-xNyAs1-y Solar Cells”, ThirtyFirst IEEE PVSC, 2005. 

[12] O. Yastrubchak, et al., “Capture kinetics at deep-level 

defects in lattice-mismatched GaAs-based heterostructures”, Physica B 308-310, 2001, pp. 757-760. 

[13] D. K. Schroder, Semiconductor Material and Device 

Characterization (Wiley, New York, 1990). 

[14] P. N. Grillot, et al., “Minority- and majority-carrier trapping in strain-relaxed Ge0.3Si0.7/Si heterostructure diodes 

grown by rapid thermal chemical-vapor deposition”, J. 

Appl. Phys. 77, 1995, pp. 676-685. 

[15] SimWindows is a semiconductor device simulator 

originally developed at the Optoelectronics Computing 

Systems Center at the University of Colorado, Boulder. 

Also, see http://www-ocs.colorado.edu/SimWindows. 

[16] S. M. Sze, Physics of Semiconductor Devices, 2nd ed. 

(Wiley, New York, 1981). 

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4. TITLE AND SUBTITLE

Electron Traps in P-Type GaAsN Characterized by Deep-Level 

Transient Spectroscopy 

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S.W. Johnston, S. Kurtz, D.J. Friedman, A.J. Ptak, R.K. Ahrenkiel, 

and R.S. Crandall 

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14. ABSTRACT (Maximum 200 Words)

We have used deep-level transient spectroscopy to detect traps in p-type GaAsN grown by metal-organic chemical 

vapor deposition. Although minority-carrier electrons are not intentionally injected into the depletion region of the 

measured samples, electron traps are detected in both Schottky barrier and p-n junction devices. The electron-trap 

signal can exist using only reverse biases during measurement, and checks of series resistance and minority-carrier 

injection using an optical source also confirm the electron-trap signal. For dilute-nitrogen p-n junction samples, the 

electron trap gives the dominant signal peak. The peak's magnitude, which corresponds to trap density, correlates to 

amounts of nitrogen incorporated during growth and reduced open-circuit voltage during light characterization. The 

p-type GaAsN layers have net acceptor carrier concentrations in the mid-1016 to low-1017 cm-3, as determined by 

capacitance-voltage profiling. The electron-trap concentration depends on the N content, but values, when traps are 

filled to saturation, range from 1015 to 1016 cm-3. The electron signal peak shows a shoulder peak on some samples, 

giving another close energy level. The electron-trap activation energy is somewhat dependent on the trap filling time, 

but ranges from about 0.15 to 0.30 eV, and is usally near 0.2 eV for the largest peak when filled to saturation. 

15. SUBJECT TERMS

PV; deep-level transient spectroscopy; metal-organic chemical vapor deposition; minority carrier; electron traps; 

Schottky barrier; open-circuit voltage; 

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