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Characterization of a Dominant 

Electron Trap in GaNAs Using 

Deep-Level Transient 

Spectroscopy 

S.W. Johnston, S.R. Kurtz, and R.S. Crandall 

Presented at the 2005 MRS Fall Meeting 

Boston, Massachusetts 

November 28 – December 2, 2005 

Conference Paper 

NREL/CP-520-38334 

August 2006 

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Characterization of a Dominant Electron Trap in GaNAs Using Deep-Level 

Transient Spectroscopy 

Steven W. Johnston, Sarah R. Kurtz, and Richard S. Crandall 

National Renewable Energy Laboratory, Golden, CO 80401, U.S.A. 

ABSTRACT 

Dilute-nitrogen GaNAs epitaxial layers grown by metal-organic chemical vapor deposition 

were characterized by deep-level transient spectroscopy (DLTS). For all samples, the dominant 

DLTS signal corresponds to an electron trap having an activation energy of about 0.25 to 0.35 

eV. The minority-carrier trap density in the p-type material is quantified based on computer 

simulation of the devices. The simulations show that only about 2% of the traps in the depleted 

layer are filled during the transient. The fraction of the traps that are filled depends strongly on 

the depth of the trap, but only weakly on the doping of the layers and on the conduction-band 

offset. The simulations provide a pathway to obtain semi-quantitative data for analysis of 

minority-carrier traps by DLTS.

INTRODUCTION 

Deep-level transient spectroscopy (DLTS) [1] studies of GaNAs have reported several levels 

of both hole and electron traps [2-6]. We reported diodes exhibiting only one dominant electron 

trap peak in both Zn-doped, p-type and Se-doped, n-type GaNAs. The appearance of these 

DLTS peaks with added N correlated to a detrimental reduction in the solar cell’s open-circuit 

voltage beyond that associated with decreased bandgap [7,8]. 

In this paper, we explore this DLTS data in more detail. We use modeling to quantify the 

fraction of the depletion width that is affected by the DLTS-measurement applied biases and then 

calculate the trap density accordingly. We assess the sensitivity of this modeling to a number of 

parameters, thus developing the tools to quantify DLTS studies of minority-carrier traps. 

EXPERIMENTAL DETAILS 

The epitaxial layers studied here were grown by atmospheric-pressure metal-organic 

chemical vapor deposition using triethylgallium, arsine, and dimethylhydrazine on p- and n-type 

conductive GaAs substrates. The growth temperature was 650°C. Samples were grown with 

increasing amounts of N for both p-type and n-type GaNAs [7]. The samples were intentionally 

doped: p-type using Zn with room temperature values near 8x1016 cm-3, and n-type using Se 

with room temperature values near 7x1017 cm-3. The bandgaps ranged from 1.4 to about 1.2 eV. 

A pn junction was formed by depositing a more-heavily doped (2x1018 cm-3) n-type or p-type 

GaAs layer on the p-type or n-type GaNAs, respectively. Finally, 1-mm2 ohmic contacts were 

deposited on top. 

Capacitance-voltage (CV) and DLTS data were collected using an Accent Optical 

Technologies Fourier transform DLTS system [9]. This system uses a 1-MHz modulating signal. 

Samples were measured between 0 V and 1 V reverse bias. The leads connected to the sample 

contacts were reversed to apply opposite polarity bias to n-type samples. Traps were filled when 

the sample, which was held at a reverse bias of 1 V, was biased to 0 V for 1 s. 

1
RESULTS AND DISCUSSION 

The DLTS data for the N-varying samples are shown in Figure 1. The addition of small 

amounts of N led to peaks in the DLTS spectra for both p-type and n-type GaNAs. As shown in 

the left graph of Figure 1, a positive peak corresponding to minority-carrier electron trapping 

occurs near 125 K using a 23 s-1 rate window for the p-type GaNAs samples. The middle graph 

of Figure 1 shows a negative peak, corresponding to a majority-carrier electron trap, occurring 

between 150 K and 170 K for the n-type GaNAs samples. For all samples, the long filling pulse 

of 1 s allowed the DLTS signal to reach saturation in magnitude [10]. The peak heights, which 

are roughly proportional to trap density, increase with increasing N content. The right graph of 

Figure 1 shows DLTS data for all recorded rate windows summarized in the form of an 

Arrhenius plot [11]. The linear fits of these data are used to calculate the trap activation energies 

and capture cross-sections [11]. The p-type samples vary in N content from 0.02% to 1.2%. 

Their electron trap activation energies are 0.27±0.03 eV, and their cross sections range from 

2x10-15 to 4x10-13 cm

2

. The n-type samples vary in N content from 0.011% to 0.45%. Their 

electron trap activation energies are 0.35±0.03 eV, and their cross sections range from 2x10-13 to 

1x10-12 cm

2

. 

The left graph of Figure 2 shows CV data for a representative p-type sample. The inset 

shows temperature-dependent -1V-bias capacitance values. The right graph of Figure 2 shows 

that the p-type samples’ net hole concentrations determined from CV measurement [11] 

decreased by factors of 2 to 3 as temperature decreased from about 400 K to lower temperatures. 

The linear fits illustrate the uniform doping density over the bias range used for the DLTS 

measurement. 

1.5x10-2

1.0

0.5

0.0

ΔC/C

100 200 300 400

Temperature (K)

p-type samples

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

x10-3

100 200 300 400

Temperature (K)

n-type samples

60

56

52

ln(tau*vth*NC)

6 8 10

1000/T (K-1)

n-type

p-type 

Figure 1. The left and middle graphs show DLTS data (23 s-1 rate window and 1 s filling time) 

for the p-type and n-type GaNAs samples, respectively. The right graph shows the Arrhenius 

plot created from DLTS data. Circles correspond to majority-carrier electron traps in n-type 

material, and triangles correspond to minority-carrier electron traps in p-type material.

2
1400

1200

1000

800

600

Capacitance (pF)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

Bias (V)

 390 K

 300 K

 150 K 

4x10-6

3

2

1

1/C2 (1/pF2

)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

Bias (V)

 390 K

 300 K

 150 K 

1.0 x 1017 cm-3

2.3 x 1017 cm-3

2.7 x 1017 cm-3

1200

1000

800

600

400

C (pF)

100 200 300 400

T (K)

Figure 2. Capacitance-voltage data taken at various temperatures for a p-type GaNAs sample. 

The left graph shows CV at 390, 300, and 150 K, and the inset shows capacitance at -1V as a 

function of temperature. The right graph shows the Mott-Schottky plot with linear fits giving 

carrier concentrations at 390, 300, and 150 K. 

The CV data for the n-type samples [10] (not shown) also have good linear fits, 

representative of uniform doping over the measurement region. Unlike the p-type samples, the 

n-type samples’ net electron concentrations decreased by less than 10% when cooling from high 

to low temperature. 

Detecting minority-carrier traps without the use of forward bias or light is not a typical result; 

however, we have consistently observed electron traps in p-type GaNAs samples. We have 

previously described how experimental values of trap filling time and applied-bias-dependent 

depth profiling agree with modeling that the electrons fill and emit from trap states near the 

interface between n-type GaAs (or metal for a Schottky-barrier sample) and p-type GaNAs [12]. 

In this paper, we use modeling to calculate a correction factor for determining the electron trap 

density. 

Each p-type sample is modeled at DLTS-peak temperatures (~140 K) using simulation 

software. The conduction and valence bands and quasi-Fermi levels are determined for 0 V and 

1 V reverse-bias conditions. A trap level with the Arrhenius-plot-derived activation energy is 

included, and the occupation of this trap is calculated based on the electron quasi-Fermi level. 

The graphs of Figure 3 show the modeled band diagrams for a 2x1018 cm-3 n-type GaAs layer on 

a 1x1017 cm-3 p-type GaNAs layer, where the interface is at 0.10 μm. Band bending in the 

depletion region causes the trap level to cross the electron quasi-Fermi level near the 

metallurgical pn junction (~0.11 μm). At -1V, electron traps nearer the junction from the 

crossing point always remain filled. Then, at 0 V, the band bending is slightly lowered such that 

the crossing point moves slightly farther from the junction. This leaves empty traps below the 

electron quasi-Fermi, which electrons from the n-type side can fill by surmounting the potential 

energy due to the band bending in the depletion region [10,12]. According to a thermionic 

emission estimate [13], the time to fill these traps at low temperature where the DLTS peak 

3
0.09 0.10 0.11 0.12 0.13

x (μm)

n-type

GaAs

p-type

GaNAs

 -1 V 

0.09 0.10 0.11 0.12 0.13

x (μm)

n-type

GaAs

p-type

GaNAs

 0 V -5.5

-5.0

-4.5

-4.0

-3.5

-3.0

Energy (eV)

0.09 0.10 0.11 0.12 0.13

x (μm)

n-type

GaAs

p-type

GaNAs

 -1 V 

EC

ET

EF

EV

Figure 3. The left, center, and right graphs show the band diagrams for conditions before, 

during, and immediately after trap filling, respectively. Traps that remain filled are highlighted 

in black. Traps that fill and emit with changing bias conditions are highlighted in gray. 

occurs agrees with the experimental values of approximately 1 s observed for the p-type samples 

[10,12]. After the filling pulse, the bias is returned to -1V, and now these filled traps are above 

the electron quasi-Fermi level such that they will emit with times characteristic of the trap 

activation energy and sample temperature and be observed by the DLTS capacitance transient. 

The trap density, NT, is initially calculated based on room-temperature carrier concentration 

and the simplified equation [11], 

NT ,DLTS ≈ 2Na

ΔC

C , (1) 

where Na is net acceptor concentration, ΔC is the DLTS change in capacitance, and C is the 

reverse-bias capacitance. Equation 1 assumes the trap density is small compared to doping and 

that traps are filled throughout the total depletion width. The total charge due to filling and 

emptying traps is then given by 

Q = NT ,DLTS xd ,

 (2) 

where xd is the depletion width. From the modeled results, though, we can determine the region 

where traps are actually filling and emitting as shown in Figure 3. The trapped charge is the 

difference between the zero bias and 1-V reverse bias conditions given by 

Q = NT ,actual f (ET )

0

xd

∫

0V

dx − NT ,actual f (ET )

0

xd

∫ −1V

dx

,

 (3) 

where f(ET) is the Fermi-Dirac distribution function that gives the probability that the electron 

trap energy state at ET will be occupied by an electron. Finally, the actual trap density is given as 

a function of the simplified trap density by equating total charge: 

4
NT ,actual = NT ,DLTS

xd

f (ET ) 0V dx

0

xd

∫ − f (ET ) −1V

0

xd

∫ dx

. (4) 

This resulting factor increases the p-type samples’ trap densities by up to 40 to 60 times. For ptype samples, these trap densities range from 1x1017 to 4x1017 cm-3 for low to high N content. 

The trap density adjustment modeling in p-type samples is most sensitive to the values of 

activation energy used. Variations of up to 0.1 eV, affected possibly by high-field Poole-Frenkel 

emission [10,14], may change the adjustment factors by up to 30%. This, along with other 

uncertainty analysis calculations, is shown in Figure 4. The baseline model consisted of a 2x1018

cm-3 n-type GaAs layer on a 1x1017 cm-3 p-type GaNAs layer at 140 K, which described a 

representative p-type sample. A grid of 10 points per 1 nm was used in the simulation. The 

measured trap activation energy was 0.32 eV, and the left graph of Figure 4 shows how the trap 

density correction factor is modified if only this value is changed. Other perturbations modeled 

include simulation temperature, GaAs n-type doping, GaNAs p-type doping, and GaNAs 

bandgap. These are shown in the graphs of Figure 4, but have less effect on the correction factor 

than the trap activation energy. Finally, other factors, such as uncertainties in the DLTS peak 

fitting and CV data fitting to get carrier concentration, contribute by adding about 10% error. 

Electrons fill traps in n-type material as majority carriers. Trap densities are also adjusted to 

account for the region, which is smaller than the total depletion region, where electron traps fill 

and emit according to the bias conditions [11]. This calculation results in a factor of 4 increase 

for the n-type samples’ trap density over that calculated by Equation 1 when biases of -1V and 

0V are used in the DLTS measurement. For n-type samples, the corrected trap densities range 

from 8x1014 to 4x1016 cm-3 for low to high N content. 

60

50

40

30

20

NT Correction Factor

0.25 0.30 0.35 0.40

Activation Energy (eV)

120 130 140 150 160

Temperature (K)

 Ea

 T

4.0x1018 1.0 2.0 3.0

GaAs n-type Doping (cm-3)

5 6 7 8 9

1017

2

Net Acceptor Doping (cm-3)

 Nd

 Na 

0.00 0.05 0.10 0.15 0.20

Decreased EC and Offset (eV)

Figure 4. The left graph shows the trap density correction factor as a function of trap activation 

energy and simulation temperature. The middle graph shows the trap density correction factor as 

a function of net n-type GaAs doping level and net p-type GaNAs doping level. The right graph 

shows the trap density correction factor as a function of GaNAs bandgap, with all of the offset at 

the interface in the conduction band. 

5
SUMMARY 

An electron trap dominated the DLTS spectra for both p-type and n-type GaNAs samples. In 

p-type GaNAs, the electron trap had a measured activation energy of about 0.25 to 0.3 eV. In ntype GaNAs, the activation energy was about 0.35 eV. Trap density adjustment factors were 

calculated for both p-type and n-type samples. In n-type samples, the majority-carrier trap 

concentrations were increased by about a factor of 4, whereas in p-type samples, the minoritycarrier traps (which are usually not reported for pn junctions in reverse bias) showed very large 

correction factors of 40 to 50. Thus, we have used modeling to quantify the trap densities of 

minority-carrier electron traps in p-type GaNAs. 

REFERENCES 

1. D. V. Lang, J. Appl. Phys. 45, 3023 (1974). 

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(1995). 

3. D. Kwon, R. J. Kaplar, and S. A. Ringel, Appl. Phys. Lett. 74, 2830 (1999). 

4. P. Krispin, S. G. Spruytte, J. S. Harris, and K. H. Ploog, J. Appl. Phys. 89, 6294 (2001). 

5. P. Krispin, S. G. Spruytte, J. S. Harris, and K. H. Ploog, Appl. Phys. Lett. 80, 2120 (2002). 

6. R. J. Kaplar, D. Kwon, S. A. Ringel, A. A. Allerman, S. R. Kurtz, E. D. Jones, and R. M. 

Sieg, Sol. Energy Mater. Sol. Cells 69, 85 (2001). 

7. S. Kurtz, S. W. Johnston, J. F. Geisz, D. J. Friedman, and A. J. Ptak, Thirty-First IEEE 

PVSC, 595 (2005). 

8. S. Kurtz, S. Johnston, and H. M. Branz, Appl. Phys. Lett. 86, 113506 (2005). 

9. S. Weiss and R. Kassing, Solid-State Electron. 31, 1733 (1988). 

10. S. W. Johnston and S. R. Kurtz, unpublished. 

11. P. Blood and J. W. Orton, The Electrical Characterization of Semiconductors: Majority 

Carriers and Electron States (Academic, San Diego, 1992). 

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

Appl. Phys. Lett. 86, 072109 (2005). 

13. S. M. Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley, New York, 1981). 

14. P. A. Martin, B. G. Streetman, and K. Hess, J. Appl. Phys. 52, 7409 (1981). 

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Characterization of a Dominant Electron Trap in GaNAs Using 

Deep-Level Transient Spectroscopy 

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14. ABSTRACT (Maximum 200 Words)

Dilute-nitrogen GaNAs epitaxial layers grown by metal-organic chemical vapor deposition were characterized by 

deep-level transient spectroscopy (DLTS). For all samples, the dominant DLTS signal corresponds to an electron 

trap having an activation energy of about 0.25 to 0.35 eV. The minority-carrier trap density in the p-type material is 

quantified based on computer simulation of the devices. The simulations show that only about 2% of the traps in the 

depleted layer are filled during the transient. The fraction of the traps that are filled depends strongly on the depth of 

the trap, but only weakly on the doping of the layers and on the conduction-band offset. The simulations provide a 

pathway to obtain semi-quantitative data for analysis of minority-carrier traps by DLTS. 

15. SUBJECT TERMS

PV; deep-level transient spectroscopy; metal-organic; chemical vapor deposition; minority carrier; devices. 

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