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Contract No. DE-AC36-99-GO10337 

A.J. Ptak, D.J. Friedman, S. Kurtz, and J. Kiehl

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37479 

Enhanced-Depletion-Width 

GaInNAs Solar Cells Grown 

by Molecular-Beam Epitaxy 
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ENHANCED-DEPLETION-WIDTH GaInNAs SOLAR CELLS GROWN BY MOLECULARBEAM EPITAXY 

Aaron J. Ptak, Daniel J. Friedman, Sarah Kurtz and James Kiehl 

National Renewable Energy Laboratory, Golden, CO 80401 

1.0

0.8

0.6

0.4

0.2

0.0

QE

1.0 1.1 1.2 1.3 1.4

Energy (eV)

W=1.5 μm

W=1.0 μm

W=0.5 μm

W=0.2 μm

Eg=1.05 eV

Ln=0.01 μm

W=2.5 μm

Figure 1. Calculations for GaInNAs solar cells with 

bandgaps of ~ 1.05 eV, showing an increase of the internal quantum efficiency with increasing depletion 

width, W, for a base thickness of 3 μm. 

ABSTRACT 

GaInNAs, potentially useful in a 4-junction 

GaInP2/GaAs/GaInNAs/Ge solar cell, suffers from very low

minority-carrier collection lengths. To date, the currents 

available from GaInNAs solar cells are not high enough to 

increase the efficiency of a 3-junction device by adding this 

fourth junction. Here, we grow p-i-n GaInNAs solar cells by

MBE with wide, intrinsic base layers and internal QE’s near 

1.0. If similar 1.0-eV GaInNAs junctions can be successfully integrated into the 3-junction structure, the resulting 4-

junction cell would have a higher efficiency. 

INTRODUCTION 

The 3-junction, GaInP2/GaAs/Ge solar cell is a nonoptimized structure due to excess light absorbed by the Ge 

junction. Because of this, a fourth junction inserted between the GaAs and Ge subcells could use the excess light 

and provide an increase in device efficiency [1]. Unfortunately, the leading candidate material, Ga1–3xIn3xNxAs1–x, 

hereafter GaInNAs, suffers from very low minority-carrier 

diffusion lengths compared to its parent compound, GaAs 

[2],[3]. These low diffusion lengths do not allow for the collection of adequate photocurrent to increase the efficiency

of a 4-junction device to more than that of a 3-junction 

structure. If the photocurrents generated from the GaInNAs 

subcell are increased, practical conversion efficiencies from 

this structure may exceed 40%. 

Defects in the GaInNAs layer lead to short lifetimes for 

minority carriers. This means that diffusion lengths in this 

material are not long enough to generate significant photocurrent. One way to accomplish an increase of current from 

the GaInNAs junction is to employ a p–i–n structure with a 

wide, intrinsic base layer. The built-in field across this depleted layer sweeps minority carriers toward the junction, 

increasing the current generated by the cell. Figure 1 

shows modeled internal quantum efficiency (QE) values for 

1.05 eV bandgap GaInNAs solar cells with different thicknesses of the intrinsic base layer, or depletion layer. The 

model assumes that there is no collection from the GaAs 

emitter and no contribution due to diffusion in the GaInNAs 

base and that all carriers in the depleted layer are collected. 

Clearly, the QE is expected to improve dramatically with 

wider depletion widths. We estimate that photocarrier collection (drift plus any potential contribution from diffusion) 

from a region greater than 2 μm is necessary for a GaInNAs 

cell with a bandgap near 1 eV to provide an increased efficiency over the 3-junction device [4]. 

Growth of GaInNAs by MBE 

Collection of sufficient photocarriers from outside the 

depletion region in GaInNAs is not assured because diffusion lengths are low. To make use of field-aided collection 

in the depletion region, a p-i-n structure requires a thick 

base layer with a very low background acceptor concentration. Unfortunately, GaInNAs grown by metalorganic 

chemical-vapor deposition (MOCVD) typically shows background carrier concentrations near 1017 cm–3 [5], with corresponding depletion widths of ~0.2 μm. Carbon acceptors 

incorporated from the organic precursors used in MOCVD 

account for much of this high background level. The rest is 

likely due to gallium vacancies that are stabilized in GaInNAs by the abundant atomic hydrogen available during 

growth [6]. A positron annihilation spectroscopy report [7] 

shows that gallium vacancies are more likely to form in 

MOCVD-grown GaInNAs than in material grown by molecu1
lar-beam epitaxy (MBE) due to the lack of hydrogen present 

during MBE growth. Also, MBE uses carbon-free sources, 

thus minimizing background impurities in addition to gallium 

vacancies. As a result, material grown by MBE has the 

potential for much lower background acceptor concentrations resulting in depletion widths of several microns. 

1.0

0.8

0.6

0.4

0.2

0.0

Internal QE

1.1 1.2 1.3 1.4 1.5

Energy (eV)

 GaAs

 Eg=1.28 eV

W=0.66 μm

 Eg=1.28 eV

W=2.7 μm

 Eg=1.15 eV

W=2.3 μm

Figure 2. Comparison of QEs for similar samples with 

wide and short depletion widths. The data are corrected 

for the measured grid coverage to facilitate the comparison of devices with different grid designs. 

EXPERMINENTAL DETAILS 

GaInNAs solar cells were grown by solid-source MBE 

using elemental sources of Ga, In, and As, with active nitrogen provided by an Oxford HD25 rf-plasma source. The 

growth temperature for the nitride layers was ~530°C and 

no post-growth annealing was performed. GaAs cells were 

grown for comparison at a temperature of ~580°C. For 

these experiments, the solar cells consist of an unintentionally doped GaInNAs, 3 μm thick base layer and a Si-doped 

GaAs emitter (n~1.5x1018 cm–3). A highly Si-doped GaAs 

contact layer caps the structure. This structure is somewhat non-optimized in that it does not employ a backsurface field, has no window layer, and no way to selectively remove the contact layer after processing. 

The QE, current-voltage (IV), and capacitance-voltage 

(CV) measurements were completed using standard techniques. The internal QE was calculated from the measured 

external QE and reflectivity. The QE data presented below

were corrected for grid shadowing to facilitate the comparison of devices with different grid designs. The IV curves 

yielded the short-circuit current (Jsc) and open-circuit voltage (Voc), and the CV data provided the depletion width. 

RESULTS AND DISCUSSION 

Quantum Efficiency

1.0

0.8

0.6

0.4

0.2

0.0

Internal QE @ Eg+0.2 eV

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Depletion Width (μm)

 exp't data

 model

Figure 3. Compilation of QE values (measured at 

Eg+0.2 eV) vs. depletion width. The data resemble the 

expected values from a simple model that assumes no 

contribution from diffusion or the emitter. The data are 

corrected for estimated grid coverage to facilitate the 

comparison of devices with different grid designs. 

Utilizing the lower impurity and vacancy concentrations 

possible using MBE growth, we succeeded in growing 

GaInNAs solar cells with depletion widths greater than 

2 μm. These cells, with bandgaps down to 1.15 eV, show

greatly enhanced short-circuit currents and QEs compared 

to reports in the literature. Figure 2 shows the QE measurements of several GaInNAs solar cells, displaying the 

difference between wide and narrow depletion widths at 

Eg~1.28 eV, as well as the QE from a cell with Eg~1.15 eV. 

The QE for a comparable GaAs cell is included for reference. The QEs for the samples with the wide depletion 

widths are, to our knowledge, the best reported for a GaInNAs sample. A report by Li et al on GaInNAs samples with 

similar bandgaps, but grown by MOCVD, shows maximum 

QEs below 0.7 [8]. Indeed, the best GaInNAs solar cells 

reported anywhere have had near band-edge QEs of approximately 0.8 [9]. 

Figure 3 shows the dependence of the QE (measured 

at an energy of Eg+0.2 eV) on the depletion width. The 

data compares favorably to a simple model that assumes 

light is filtered by, but not collected by, the GaAs emitter 

and that the diffusion length of carriers in the GaInNAs base 

is negligible. The collection of photocarriers in the emitter is 

neglected because the emitter is not passivated, and most 

of the photocarriers recombine at the front surface. There 

are many data points that lie significantly above the model’s 

predictions, possibly indicating some collection from diffusion in addition to the collection due to the drift of carriers in 

the field. The contribution of this non-zero diffusion length 

to the QE may be negligible when the depletion width is 

increased to more than 2 μm. 

2
Concentrator Measurements 

The possibility exists that using a wide, intrinsically

doped layer in a solar cell device will adversely effect the 

associated series resistance, especially under concentrator 

operation. To evaluate this issue, we measured the IV 

curves as a function of concentration for a 1.28-eV junction 

with a fully depleted base of width 3 μm and doping 

p=2x1014/cm3

. The IV curves were measured using a 

pulsed solar simulator, [10] with the sample nominally at 

25°C. Figure 4 shows the resulting Voc, the ideality factor n

as deduced from the slope of the Voc curve, and the fill factor (FF) vs. Jsc. In the application with GaInP2 and GaAs top 

and middle cells, the desired one-sun current would be on 

the order of 14 mA/cm2

, i.e. a 500-sun current of 7 A/cm2 as 

indicated by the vertical dotted line in the figure. At this concentration, Fig. 4c shows that FF is barely below its one-sun 

value, indicating that series resistance is not a significant 

factor at this concentration. 

To be more quantitative, we fit the FF data by calculating FF for the standard diode equation with ideality factor n, 

including series resistance RS as the fit parameter, and 

scaled to the measured FF value at one sun. A very good fit 

is obtained with RS=1.1x10–2 Ωcm2

, as shown by the 

dashed line in Fig. 4c. This value for RS represents the sum 

of all the series resistances in the device, including not only

the base resistance of interest but also resistances through 

the emitter, the grid fingers, and the grid-semiconductor 

contact. Separate measurements of these parameters indicate that together they total 2x10–3 Ωcm2

, so RS due to the 

base alone is RS=9x10–3 Ωcm2

. At the 500-sun current of 

7 A/cm2

, the voltage drop across the base due to this RS is 

0.063 V. In other words, the wide-depletion-width device of 

Fig. 4 does not suffer significantly from series resistance 

across the base for concentrations at least as high as 500 

suns. 

-20

-10

0

10

20

Current (mA/cm 2

)

-0.2 0.0 0.2 0.4 0.6 0.8

Voltage (V)

 Eg~1.15 eV

 Eg~1.28 ev

Figure 5. Light and dark IV curves for the widedepletion-width GaInNAs cells from Fig.2. 

Open-Circuit Voltage 

Figure 5 shows light and dark IV curves for the widedepletion-width GaInNAs layers in Fig. 2. The measurements were performed without a GaAs filter. The difference 

between the bandgap and the Voc (Eg-Voc) for these cells is 

~0.62 V. Typically, Eg-Voc is about 0.4–0.5 V for GaAs and 

GaInP2, while GaInNAs cells are usually much closer to 0.6 

V, presumably due to some unidentified defect [11]. One 

concern with using a p–i–n structure is possible carrier recombination in the depletion region, lowering the Voc, and 

perhaps the QE [12] of the cell. Additional decreases in the 

Voc would reduce the usefulness of GaInNAs in a 4-junction 

structure, whether or not the current is increased [4]. Promisingly, the Voc’s for our cells degrade by less than 5% for 

depletion widths of several microns. 

W would like to know what Voc we can expect from a 

1.0-eV GaInNAs solar cell. For one-sun applications, we

could expect ~0.4 V, based on Eg-Voc~0.6. Using the data 

in Fig. 4 and assuming the same value for Eg–Voc, we estimate that a 1.0-eV junction would show Voc~0.6 V at 500 

suns illumination. There is the possibility that these voltages will increase with an optimized cell structure. 

It is important to note that the bandgaps of the samples 

in this study are only as low as 1.15 eV, much higher (that 

Table 1. Photocurrent collected from GaInNAs layers 

for photons with energies below the GaAs band edge. 

Eg (eV) AM0 (mA/cm2

) Low AOD (mA/cm2) 

1.28 3.15 2.16

1.15 6.64 5.67

0.9

0.8

0.7

VOC(V)

101 102 103 104

JSC (mA/cm2)

Voc

(a)

1.7

1.6

1.5

1.4

1.3

n

101 102 103 104

JSC (mA/cm2)

n

(b)

70

60

50

FF (%)

101 102 103 104

JSC (mA/cm2)

FF

(c) device EB252n12

Figure 4. (a) Voc, (b) ideality factor, n, and (c) FF of a 

wide-depletion-width, 1.28-eV GaInNAs cell as a function 

of concentration. The dashed line in (c) is a fit to a seriesresistance model, with fit value Rs=1.1x10–2 Ωcm2. 

3
is, much less nitrogen) than necessary for use in a currentmatched 4–junction structure. Table I shows the photocurrent collected from the GaInNAs junction (below the GaAs 

band edge) for two different spectra. Clearly, the bandgaps 

of these cells need to be pushed toward 1 eV, without significant degradation of the depletion width, to provide an 

increase in device efficiency for the 4-junction structure. It 

is possible that the addition of more nitrogen to these cells 

will adversely affect the background carrier concentrations, 

and hence the depletion widths. However, we are encouraged by the excellent QEs obtained for the bandgaps studied thus far. 

CONCLUSIONS 

High-quality GaInNAs has been sought for use in 4-

junction solar cells for several years. Although the photovoltages in this material are still lower than they ideally

should be, we show here that the photocurrents are greatly

increased with the use of an MBE-grown p–i–n structure 

with a wide depletion region. Series resistance across the 

low-doped base does not significantly degrade concentrator 

performance up to at least 500 suns. With extension to 

lower bandgaps, these increased quantum efficiencies may

provide the current needed for a higher-efficiency 4-junction 

GaInP2/GaAs/GaInNAs/Ge device.

REFERENCES 

[1] S. R. Kurtz, D. Myers, and J. M. Olson, "Projected 

Performance of Three- and Four-Junction Devices using 

GaAs and GaInP," presented at 26th IEEE Photovoltaic 

Specialists Conference, Anaheim, 1997. 

[2] J. F. Geisz, D. J. Friedman, J. M. Olson, S. R. 

Kurtz, and B. M. Keyes, "Photocurrent of 1 eV GaInNAs 

lattice-matched to GaAs," J Cryst Growth, vol. 195, pp. 401-

408, 1998. 

[3] S. R. Kurtz, A. A. Allerman, C. H. Seager, R. M. 

Sieg, and E. D. Jones, "Minority carrier diffusion, defects, 

and localization in InGaAsN, with 2% nitrogen," Appl. Phys. 

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

[4] D. J. Friedman and S. Kurtz, "Breakeven Criteria 

for the GaInNAs junction in GaInP/GaAs/GaInNAs/Ge fourjunction solar cells," Prog. Photovolt., vol. 10, pp. 331-344, 

2002. 

[5] S. Kurtz, R. Reedy, B. Keyes, G. D. Barber, J. F.

Geisz, D. J. Friedman, W. E. McMahon, and J. M. Olson, 

"Evaluation of NF3 versus dimethylhydrazine as N sources 

for GaAsN," J Cryst Growth, vol. 234, pp. 323-326, 2002. 

[6] A. Janotti, S. H. Wei, S. B. Zhang, S. Kurtz, and C. 

G. VandeWalle, "Interactions between nitrogen, hydrogen, 

and gallium vacancies in GaAs1-xNx alloys - art. no. 

161201," Phys Rev B, vol. 6716, pp. 1201, 2003. 

[7] A. J. Ptak, S. Kurtz, K. G. Lynn, and M. H. Weber, 

"Positron annihilation spectroscopy study of vacancies in 

GaInNAs," J vac Sci Technol B, vol. 22, pp. 1584, 2004. 

[8] N. Y. Li, P. R. Sharps, M. Stan, F. Newman, J. S. 

Hills, H. Q. Hou, J. M. Gee, and D. J. Aiken, "Development 

of 1.25 eV InGaAsN for Triple Junction Solar Cells," presented at 28th IEEE Photovoltaic Specialists Conference, 

Anchorage, Alaska, 2000. 

[9] S. Kurtz, J. F. Geisz, D. J. Friedman, J. M. Olson, 

A. Duda, N. H. Karam, R. R. King, J. H. Ermer, and D. E. 

Joslin, "Modeling of Electron Diffusion Length in GaInAsN 

Solar Cells," presented at 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska, 2000. 

[10] J. Kiehl and K. Emery, "Characterization of Photovoltaic Concentrators," presented at DOE Solar Program 

Review Meeting, Denver, 2004. 

[11] Sarah Kurtz, J.F. Geisz, D.J. Friedman, and A.J. 

Ptak, "Effect of nitrogen concentration on the performance 

of Ga(In)As(N) solar cells", paper submitted to the 31st

IEEE Photovoltaic Specialists Conference, 2005. 

[12] D.J. Friedman, Sarah Kurtz, A.J. Ptak, and J.F.

Geisz, "Analysis of depletion-region collection in GaInNAs 

solar cells", paper submitted to the 31st IEEE Photovoltaic 

Specialists Conference, 2005. 

4
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4. TITLE AND SUBTITLE

Enhanced-Depletion-Width GaInNAs Solar Cells Grown 

by Molecular-Beam Epitaxy 

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NREL/CP-520-37479 

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14. ABSTRACT (Maximum 200 Words)

GaInNAs, potentially useful in a 4-junction GaInP2/GaAs/GaInNAs/Ge solar cell, suffers from very low minority-carrier 

collection lengths. To date, the currents available from GaInNAs solar cells are not high enough to increase the 

efficiency of a 3-junction device by adding this fourth junction. Here, we grow p-i-n GaInNAs solar cells by molecularbeam epitaxy with wide, intrinsic base layers and internal quantum efficiencies near 1.0. If similar 1.0-eV GaInNAs 

junctions can be successfully integrated into the 3-junction structure, the resulting 4-junction cell would have a higher 

efficiency. 

15. SUBJECT TERMS

PV; enhanced depletion; solar cell; quantum efficiency; concentrator measurements; open-circuit voltage; bandgap; 

molecular-beam epitaxy; 

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