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Contract No. DE-AC36-99-GO10337 

S. Kurtz, S.W. Johnston, J.F. Geisz, 

D.J. Friedman, and A.J. Ptak 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3-7, 2005 

February 2005 • NREL/CP-520-37361 

Effect of Nitrogen Concentration 

on the Performance of 

Ga1-xInxNyAs1-y Solar Cells 
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Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 
 EFFECT OF NITROGEN CONCENTRATION ON THE PERFORMANCE OF 

Ga1-xInxNyAs1-y SOLAR CELLS 

Sarah Kurtz, S.W. Johnston, J.F. Geisz, D.J. Friedman, and A.J. Ptak 

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

ABSTRACT 

A study of Ga1-xInxNyAs1-y solar cells shows that nitrogen degrades the solar cells even for very small nitrogen concentrations. By comparing the properties of pon-n and n-on-p Ga1-xInxNyAs1-y cells as a function of y, we

find that the n-on-p cells show a more abrupt decrease in 

the open-circuit voltage and greater decrease of the 

photocurrent. The asymmetry in the performance of the 

cells reflects the differences observed for electrons and 

holes in Ga1-xInxNyAs1-y. The electron mobility is degraded 

much more than the hole mobility when nitrogen is added 

to GaAs, implying that the electron diffusion length should 

be degraded more than the hole diffusion length. An 

electron trap (observed by deep-level transient 

spectroscopy) affects p-type GaNyAs1-y more than n-type 

GaNyAs1-y, consistent with the observation that the opencircuit voltage of n-on-p cells decreases more than that of 

p-on-n cells. The effect of nitrogen on GaNAs cells is 

shown to be much greater than expected for an 

isoelectronic impurity.

 

BACKGROUND 

A four-junction solar cell using Ga0.5In0.5P/Ga(In)As/ 

Ga0.94In0.06As0.98N0.02/Ge has a theoretical efficiency of 

40%-50% [1]. However, achievement of such high efficiencies has been elusive because of poor performance of 

the Ga1-xInxNyAs1-y (hereafter, GaInNAs) subcell [2,3]. The 

cause of the poor performance is not yet understood. One 

solution to this problem is to reduce the photocurrent that 

is needed from the GaInNAs cell by dividing the 

photocurrent of the upper junctions, as in a 5- or 6-junction 

solar cell [4]. A better understanding of the effects of N on 

GaInNAs cells may lead to a method for improving the 

cells, or at least a determination of the best performance 

that can be expected. 

The addition of nitrogen to GaAs has been shown to 

reduce the optoelectronic quality of GaAs, including 

reduction of the electron mobility [5], reduction of the 

photoluminescence intensity and lifetime, and an increase 

in the background carrier concentration [6]. The reduced 

electron mobility is at least partially caused by alloy

scattering associated with the large bandgap bowing of the 

GaNAs alloy system [7]. 

Recently, we have reported the correlation between a 

degraded open-circuit voltage (Voc) and the observation of 

an electron trap using deep-level transient spectroscopy

(DLTS) [8]. The structure of the electron trap has not yet 

been identified, but its concentration correlates with the 

nitrogen concentration. The DLTS data is discussed in 

more detail elsewhere [9,10]. 

Improved device performance has been obtained by

using a GaInNAs p-i-n structure with a wider depletion 

width. This approach is relatively difficult to implement 

because of high background carrier concentrations in 

GaInNAs, but excellent photocurrent can be obtained by

growing GaInNAs by molecular-beam epitaxy to reduce 

the carbon and hydrogen contamination [11]. 

In this paper we explore the effect of variable amounts 

of N on the performance of GaInNAs solar cells. We

compare results for n-on-p and p-on-n solar cells and 

report measurements of the dark current, Voc, and 

photocurrent (quantum efficiency). The quantumefficiency data are modeled to derive the minority-carrier 

diffusion lengths. The reasons for the greater degradation 

of the n-on-p cells are discussed. 

EXPERIMENTAL METHOD 

The GaInNAs solar cells were grown by metalorganic 

chemical vapor deposition using triethylgallium, trimethylindium, arsine, and dimethylhydrazine or nitrogen trifluoride in a hydrogen carrier gas at atmospheric pressure. 

Diethylzinc and hydrogen selenide were used as the 

dopant sources. An arsine pressure of ~ 0.2 torr and a 

growth rate of 6.5 μm/hr were used. The dimethylhydrazine or nitrogen trifluoride flows were varied to adjust 

the nitrogen concentration. The growth temperature was 

also varied between 600°C and 650°C to obtain a wider 

range of nitrogen concentrations. GaAs cells were grown 

for each of the growth temperatures. The relatively high 

growth temperatures resulted in some annealing during 

growth of the devices, but no postgrowth annealing was 

applied to these samples. The effect of additional 

annealing is documented elsewhere [12]. 

The structures for the n-on-p and p-on-n solar cells 

are shown schematically in Fig. 1. Most of the cells were 

grown without any indium. A few of the n-on-p cells 

included some indium. Heterojunction structures were 

intentionally chosen to help separate the effects of 

nitrogen on minority-carrier electrons (n-on-p devices) and 

holes (p-on-n devices). The device areas were 0.25 cm2

. 

The dark-current-voltage (IV), light-IV, capacitancevoltage (CV), and quantum-efficiency (QE) measurements 

used standard techniques. The internal QE was calculated from the external QE and the reflectivity. The 

width of the depleted layer and the net acceptor or donor 

1
concentration in the base were calculated from the 3-kHz 

CV data, assuming one-sided junctions. The N 

concentrations were estimated from x-ray diffraction data 

[6]. 

 n-on-p device structure p-on-n device structure 

Fig. 1. Device structures used for this study. Both were 

grown on GaAs substrates and used GaAs contacting 

layers, with electroplated gold contacts front and back (not 

shown). The emitter (front) layers were doped ~1018 cm-3. 

The nitrogen (and indium) concentrations were varied for 

the base layers. 

GaInNAs CELL PERFORMANCE 

The effect of a very small amount of N on the dark 

current of n-on-p and p-on-n GaNAs cells is shown in Fig. 

2. Whereas, the addition of 2 X 1018 - 2 X 1019 cm-3 of N 

increases the dark current of an n-on-p cell by a factor of 

~10, N has a relatively smaller effect on p-on-n cells. 

0.01

0.1

1

10

Dark current density (mA/cm2

)

0.6 0.8 1.0

Voltage (V)

0.6 0.8 1.0

Voltage (V)

n-on-p p-on-n

 GaAs

 GaNAs

Fig. 2. The effect of < 0.1% nitrogen on the dark current 

of n-on-p (left) and p-on-n (right) GaNAs cells. The 

bandgap change caused by the addition of the nitrogen 

was less than 20 meV. 

The increase in dark current causes a decrease in the 

Voc of the cells. To differentiate the unexpected (because 

of material degradation) from the expected (because of 

bandgap reduction) decrease in Voc with addition of 

nitrogen, the Voc is plotted as a function of bandgap in 

Fig. 3. Consistent with the data shown in Fig. 2, the Voc 

decreases much more abruptly for the n-on-p cells than for 

the p-on-n cells. The addition of indium has very little 

effect on the Voc. 

The change in the photocurrent, as manifested by the 

internal QE, is shown in Fig. 4. Again, both types of cells 

show significant degradation, but the effect on the n-on-p 

cells is more dramatic. 

The net ionized acceptor and donor concentrations 

were measured for the n-on-p and p-on-n devices, 

respectively, and are shown in Fig. 5. The systematic 

increase (decrease) in acceptor (donor) concentrations 

with increasing nitrogen implies that the nitrogen is 

causing point defects that act as acceptors. These 

defects may be gallium vacancy complexes [13]. 

Nitrogen-related point defects can also be donor-like [14]. 

25 nm GaInP:Se 500 nm GaInP:Zn

100 nm GaAs:Se 100 nm GaAs:Zn 

3 μm Ga(In)NAs:Zn 3 μm GaNAs:Se 

1.0

0.9

0.8

0.7

0.6

0.5

Open-circuit voltage (V)

1.15 1.20 1.25 1.30 1.35 1.40

Bandgap (eV)

GaAs

Ga(In)NAs

 n-on-p p-on-n

GaAs 

GaNAs 

GaInNAs

150 nm GaInP:Zn 50 nm GaInP:Se 

Fig. 3. The open-circuit voltage as a function of bandgap 

for GaInNAs cells. The line has unity slope, indicating a 

linear change in Voc with bandgap. 

1.0

0.8

0.6

0.4

0.2

0.0

1.3 1.4 1.5

Photon Energy (eV)

1.0

0.8

0.6

0.4

0.2

0.0

Internal quantum efficiency

 GaAs

 GaNAs

n-on-p

p-on-n

Fig. 4. Internal quantum efficiency for n-on-p (top) and pon-n (bottom) GaAs (solid lines) and GaNAs (broken lines) 

cells with variable amounts of nitrogen. The shift to lower 

bandgap gives an indication of the nitrogen concentration. 

MODELING OF QUANTUM EFFICIENCY 

The decrease in the quantum efficiency is dependent 

both on the device structure and on the material quality.

Modeling is necessary to separate these two effects. We

chose an emitter design that is independent of nitrogen 

concentration, simplifying the modeling. The use of doped 

active layers reduces the depletion layer width; we have 

assumed that a photocarrier is collected for every photon 

2
absorbed in the depleted layer. The intentional doping 

gives some control over the base doping. In contrast, if no 

doping is added to GaNAs, the base doping is highly

variable and may be nonuniform, especially if the sample 

is annealed after growth [14], complicating the modeling. 

The photocarrier collection in the base and emitter were 

calculated using Hovel’s equations [15]. 

2

3

4

5

6

7

8

9 1017

2

Net carrier concentration (cm-3)

1.15 1.20 1.25 1.30 1.35 1.40

Bandgap (eV)

n-on-p p-on-n

GaAs

GaNAs

GaInNAs

Fig. 5. Base doping of GaInNAs cells as measured by CV 

at the short-circuit condition. 

The absorption coefficient was approximated by 4.5 X 

(E-Eg)0.5 μm-1 where E is the photon energy and Eg is the 

bandgap. The absorption coefficient of GaAs varies 

slightly with doping, but this effect was not included. 

The photocarrier collection in the base was calculated 

assuming a back-surface recombination velocity of 100 

cm/s. Changing the assumptions about the back-surface 

recombination or absorption coefficient changed the 

modeled results slightly, but the trends remained the 

same. The uncertainty increases for the cases when the 

modeled diffusion length, L, is longer than the thickness of 

the base (3 μm) or shorter than the depletion width (0.1 

μm). Depending on the assumptions made, the n-on-p 

GaAs cell data could be fit only with L > 30 μm. The 30-

μm value was selected based on literature data [16]. 

The data in Fig. 4 and QE data from other cells were 

fit with the model, and the resulting L values are shown in 

Fig. 6. Both the hole and electron L values decrease 

dramatically with the addition of a small amount of 

nitrogen. The degradation of L for the n-on-p cells is much 

greater (from >> 3 μm to < 0.1 μm, or, according to our 

best estimate, by a factor of ~1000) than for the p-on-n 

cells (from ~5 μm to ~0.5 μm, or a factor of ~10). 

DISCUSSION 

The Voc of a solar cell is a complicated function of a 

number of material and device parameters. It is degraded 

primarily when the minority-carrier lifetime is reduced, but 

is relatively independent of the mobility. The exact cause 

of the decreased lifetime in GaNAs alloys is unknown, but 

our observation that the Voc of n-on-p GaNAs cells is 

correlated with an electron trap observed by DLTS may

imply that this trap is the problem. This correlation is also 

observed for the p-on-n cells for which the trap 

concentration and change in Voc are both smaller. 

Because of the higher trap concentration and larger 

expected degradation of electron lifetime in p-type 

GaInNAs, the n-on-p cell that relies on collection of 

electrons from the p-type base is more strongly affected. 

We plan to make a more quantitative treatment in a future 

publication, including quantifying the electron trap 

concentration and the photocarrier lifetime as a function of 

bandgap for n- and p-type GaInNAs. 

0.1

1

10

Modeled diffusion length (μm)

1.20 1.25 1.30 1.35 1.40

Bandgap (eV)

n-on-p p-on-n

GaAs

GaNAs

GaInNAs

Fig. 6. Modeled electron and hole diffusion lengths. 

In contrast to the Voc, which is primarily dependent on 

lifetime, the minority-carrier diffusion length is proportional 

to the square root of both the minority-carrier lifetime and 

the mobility. Based on the Voc data, the electron diffusion 

length should be shortened more than the hole diffusion 

length because of the relative changes in the lifetime, but 

changes in mobility should also be important. In highpurity GaAs, the electron diffusion length is longer than the 

hole diffusion length because of the higher mobility of the 

electrons. The addition of nitrogen is known to decrease 

the electron mobility so that it is similar to the hole mobility

in GaNAs. The observation that nitrogen degrades the 

electron diffusion length more than the hole diffusion 

length is qualitatively consistent with the relative 

degradation of the majority-carrier transport properties. 

In the discussion above, we have contrasted the 

differences in the effects of nitrogen on n-on-p and p-on-n 

cells. Understanding these differences are useful toward 

diagnosing the problem, but the more important 

observation may be that nitrogen dramatically reduces the 

performance of all of the GaNAs cells. 

Some impurities degrade the performance of solar 

cells more than others. Impurities forming mid-gap recombination centers are the most problematic. Dopants 

are associated with shallow levels and can be tolerated in 

small quantities, but dopant concentrations >1018 cm-3 also 

reduce the photovoltaic performance. In contrast, addition 

of indium (an isoelectronic impurity) has very little effect on 

GaAs, and can actually improve solar cell performance 

[17]. 

Nitrogen in GaAs is an isoelectronic impurity. One 

might expect the addition of nitrogen to affect GaAs in a 

way similar to the addition of indium. In Fig. 7 we plot the 

electron diffusion length data from Fig. 6 as a function of 

nitrogen concentration and as a function of the electron 

trap concentration measured by DLTS and corrected by a 

factor of 20 to compensate for the small penetration of the 

minority carriers into the depleted layer [10]. We also 

compare this data to diffusion lengths expected for an 

isoelectronic impurity such as indium (negligible effect in 

this concentration range) and shallow dopant such as 

3
selenium. The effect of a deep level is expected to be 

much stronger. The data presented here show that N 

degrades GaAs much faster than In. The DLTS data 

suggest that the electron trap concentration is < 10% of 

the [N]. This raises the question of whether N can be 

added without forming the problematic defect. The 

literature shows that annealing can reduce the electron 

trap concentration [12,18,19] as well as improve the 

performance of the solar cells [3,12]. Thus, it may be 

possible to remove the electron traps without removing the 

nitrogen, giving hope that GaNAs may yet be used in highperformance devices. 

0.1

1

10

Estimated electron diffusion length (μm)

10 17 10 18 10 19 10 20

Impurity concentration (cm-3)

Estimated for indium

Estimated for Se

Nitrogen in GaAs

Electron traps 

 in GaNAs

Fig. 7. Electron diffusion lengths as a function of impurity

concentration in GaAs. The nitrogen data are from Fig. 6. 

These data are plotted both as a function of the nitrogen 

concentration and as a function of the electron trap 

concentration. The data for In and Se are estimated from 

the literature [17,20]. 

SUMMARY 

In summary, we observe a dramatic change in the 

dark current of n-on-p GaNAs solar cells with the addition 

of doping levels of N. Even with N concentrations too low

to affect the band gap, the N increases the dark current by

about a factor of 10, also reducing the open-circuit voltage 

(Voc). Addition of more N causes a change that is less 

dramatic and is more consistent with the observed change 

in band gap. The reduction in Voc is less abrupt, but of a 

similar size, for p-on-on GaNAs cells. The n-on-p cells 

exhibit higher electron-trap concentrations than the p-on-n 

cells, possibly explaining why the Voc (and lifetime) are 

degraded more for the n-on-p cells. 

The minority-carrier diffusion length also decreases 

very quickly with the addition of nitrogen, with the electron 

diffusion length changing more than the hole diffusion 

length. The larger change in electron diffusion length 

likely reflects the larger change in the electron mobility as 

well as the larger change in electron lifetime. 

Substantially more work will be needed to fully understand why nitrogen is degrading GaAs so much more 

than is expected for an isoelectronic alloy and whether the 

material quality can be improved. 

ACKNOWLEDGMENTS 

We thank C. Kramer and M. Young for the sample 

preparation and R. King and J. Olson for useful 

discussions. This work was completed at the National 

Renewable Energy Laboratory under DOE Contract No. 

DE-AC36-99GO10337. 

 

REFERENCES 

[1] S.R. Kurtz, et al., "Projected Performance of Threeand Four-Junction Devices using GaAs and GaInP," 26th 

IEEE PVSC, 1997, pp. 875-878. 

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

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

[3] S.R. Kurtz, et al., "InGaAsN Solar Cells with 1.0 eV 

Band Gap, Lattice Matched to GaAs," Appl. Phys. Lett. 74, 

1999, pp. 729-731. 

[4] R.R. King, et al., "Metamorphic III-V Materials, 

Sublattice Disorder, and Multijunction Solar Cell Approaches with Over 37% Efficiency," 19th European 

PVSEC, 2004, p. 3587. 

[5] S.R. Kurtz, et al., "Minority Carrier Diffusion, Defects, 

and Localization in InGaAsN, with 2% Nitrogen," Appl. 

Phys. Lett. 77, 2000, pp. 400-402. 

[6] J.F. Geisz, et al., "Photocurrent of 1 eV GaInNAs 

Lattice-Matched to GaAs," J. Cryst. Growth 195, 1998, pp. 

401-408. 

[7] S. Fahy and E.P. O'Reilly, "Intrinsic Limits on Electron 

Mobility in Dilute Nitride Semiconductors," Appl. Phys. 

Lett. 83, 2003, pp. 3731-3733. 

[8] S. Kurtz, et al., "Capacitance-Spectroscopy Identification of a Key Defect in N-degraded GaInNAs Solar 

Cells," submitted to Appl. Phys. Lett. 2005. 

[9] S.W. Johnston, et al., to be published. 

[10] S. Johnston, et al., "Electron Traps in p-type GaAsN 

Characterized by Deep-Level Transient Spectroscopy," 

31st IEEE PVSC, 2005. 

[11] A.J. Ptak, et al., "Enhanced-Depletion-Width GaInNAs 

Solar Cells Grown by Molecular-Beam Epitaxy," 31st IEEE 

PVSC, 2005. 

[12] A. Khan, et al., "Evolution of Defect Complexes in 

InGaAsN," 31st IEEE PVSC, 2005. 

[13] A. Janotti, et al., "Interactions Between Nitrogen, 

Hydrogen, and Gallium Vacancies in GaAs1-xNx alloys," 

Phys Rev B 6716, 2003, p. 1201. 

[14] S. Kurtz, et al., "Modeling of Electron Diffusion Length 

in GaInAsN Solar Cells," 28th IEEE PVSC, 2000, p. 1210. 

[15] H.J. Hovel, Solar Cells, Academic Press, (1975). 

[16] S. Kurtz, et al., "Modeling of Two-Junction, SeriesConnected Tandem Solar Cells Using Top-Cell Thickness 

as an Adjustable Parameter," J. Appl. Phys. 68, 1990, p. 

1890. 

[17] T. Takamoto, et al., "High Efficiency InGaP/InGaAs 

Tandem Solar Cells on Ge Substrates," 28th IEEE PVSC, 

2000, p. 976. 

[18] P. Krispin, et al., "Electron Traps in Ga(As,N) Layers 

Grown by Molecular-Beam Epitaxy," Appl. Phys. Lett. 80, 

2002, pp. 2120-2122. 

[19] R.J. Kaplar, et al., "Deep Levels in p- and n-type 

InGaAsN for High-Efficiency Multi-Junction III-V Solar 

Cells," Solar Energy Mat. and Solar Cells 69, 2001, pp. 

85-91. 

[20] D. Rode and S. Knight, Phys. Rev. B3, 1971, p. 2534. 

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4. TITLE AND SUBTITLE

Effect of Nitrogen Concentration on the Performance of 

Ga1-xInxNyAs1-y Solar Cells 

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14. ABSTRACT (Maximum 200 Words)

A study of Ga1-xInxNyAs1-y solar cells shows that nitrogen degrades the solar cells even for very small nitrogen 

concentrations. By comparing the properties of p-on-n and n-on-p Ga1-xInxNyAs1-y cells as a function of y, we find that 

the n-on-p cells show a more abrupt decrease in the open-circuit voltage and greater decrease of the photocurrent. 

The asymmetry in the performance of the cells reflects the differences observed for electrons and holes in 

Ga1-xInxNyAs1-y. The electron mobility is degraded much more than the hole mobility when nitrogen is added to GaAs, 

implying that the electron diffusion length should be degraded more than the hole diffusion length. An electron trap 

(observed by deep-level transient spectroscopy) affects p-type GaNyAs1-y more than n-type GaNyAs1-y, consistent 

with the observation that the open-circuit voltage of n-on-p cells decreases more than that of p-on-n cells. The effect 

of nitrogen on GaNAs cells is shown to be much greater than expected for an isoelectronic impurity.

15. SUBJECT TERMS

PV; nitrogen concentration; solar cells; deep-level transient spectroscopy; isoelectronic impurity; open-circuit voltage;

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