THE OUANTUM EFFICIECY OF InGaAsSb TPV DIODES

\

G. Charache, R. U.Martinelli, et. al.

October 1997

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TH)! QUANTUM mClENCY OF XnCaAsSb THERMOPHOTOVOLTASC DIODES

Ramon U. Marthelli, Dmitri Z Garbuzov, Ha0 Lee, Nancy Morris, Tunde Odtlbanjo, Gordon C. Taylor, and John C. C o ~ o l l y

Sarnoff Corporation M1 Washington Rd.

PMceton, NJ OS43-5300 (609) 734-2000

Abstract

Uncoated inGaAsSb/GaSb thermophotovoltaic (Tpv) diodes with 0.56 eV ( 2 2 w) bandgaps exhibit external quantum efficiencies of 59% at 2 cun, which mmqxmds to an internal quantum efficiency of 95%. The structures were grown by molecular-beam epitaxy. The devices have electron diffusion lengths as long as 29 pm in Spxn-wide p-InGaAssb layers and hole diffusion lengths of 3 pm in 6-jm- wide n-InGaAsSb layers. The electron and hole diffusion lengths appear to increase with inereass p- and n-layer widths, respectively. These excellent minority carrier transport properties of IncaAsSb are well-suited to efficient TPV diode operation.

INTRODUCTION

There has been a recent interest in thermophotovoltaic (TPV) electric power generation from bkkbody sources at about 1300 K (Cmm 1994 and Benner 1995). Candidate 111-V axnpomd materials for the TPV diode active are InGaAs grown Q\ W and InGaAsSb grown 00 GaSb. Since the bandgap of the optimal TPV diode is about 055 eV, the IrtG& diodes have a compositionally graded layer interposed between the k\p substrate and the WaAs layer to accommodate the lattice mismatch. The InGhAsSb diodes are lattice-matched tu the substrate. Latticematched devices may offer more possibilities in terms of bandgap engineering axxi the variety of material choices. Nonetheless, while InGaAs diodes currently offer superior performance in terms of quarttum efficiency and saturation Furrent, owing in part to their longer development (Charache 1996}, InGaAssb is demonstrating excellent minority-carrier transport characteristics that make it a very attractive material for TPV diodes (Wang 1997 and Martinelli 1997).

Quantum efficiencies of 59% at 2 p - t waveZength from an I&a&Sb/Casb diode were measured. The electron diffusion length, as inferred fn#n a one-dimensional diffusion model of the spectral quantum efficiency (SQE), is 29 p m in this device. c>ur SQE curves rompare favorably with recently reported results for both hGaAsSb and InGaAs TPV diodes (Charache 1996).

The TPV diodes discussed are designed to generate electrical power fmm 1300 K blackbody radiation. The active region is a nominally 0.55 eV InGaAssb p/n homojunctim To optimize the power-conuersim efficiency, radiation up to the bandgap cut-off wavelength must be maximally absorbed, and the photo-generated carriersmustbe transported to the p/njunctim with minimal loss. There is, hence, a

trade-off between widening the active region to incxease optical absorption at near-bandgap

: . ~. , .: .- . . -&.-..a i.

. - ... -. wavelengths, where the absorption constant is relatively small; and decreasing the p- and n-layer thicknesses tu maximize carrier collection at tho junction. The v d u s of the electron and hole diffusion lengths, L, and are critical to the opthizition of the TPV structure. In general, long diffusivn lengths allow wide active regions, which lead to high quantum efficiencies. We unckrtwk this study of the TPV diode SQE to determine the values of L, and Lh'

The TPV structures were gown by solidsource, molecular-beam epitaxy (MBE) and were lattice- matched to the GaSb substrates in a manner very similar to that used for the growth of antimonide- based lasers (Lee 1997 and k 1995). To study the transport properties of the diodes, 500-pm-diameter mesas were etched into the wafers, metallized, d individual mesa diodes were evaluated. F i p e 1 schematically shows the vertical structure of the TPV diode.

IMnGaAssb (2 x lo'* cm-q Base & v

FIGURE 1. Vertical Layer Structure OX a p-on-n InGaAsSb 7pV Dode.

After growing a lj00-nm-thick n-GaSb buffer layer on the n-GaSb substrate, the base region is grown x, pm thick. For must of the devices used in this work, x, is 1 to 6 pm. The n-type base layers are Tedoped to about 2 x 10'' anq On top of the base the p-type emitter, Moped to 2 x I@' ana3, is grown to

thid<nesses, q, ranging from 1 to 8 pm. A Wm-thick p' GaSb window layer, Beduped to lOI9 a', is grown <n the emitter, completing MBE growth of this p-0rt-n diode. The GaSb window layer insurw very low electron recombination at the p*-GaSb/p-InGaAsSb window-layer/emitter interface.

Following growth, the wafer is thinned to 150 pxn and metallized with standard Aw/Ge/Ni/Au. The epitaxial surface is patterned and metallized with Cr/Au, and the mesas are etched several mictw\s into the substrate using a brornine/methanol solution.

Kelative SQE a w e s in the wavelength interval d 3 to 2.5 p m were obtained Using a mm-meter with a known relative output-power spectrum The diode's absolute quantum efficiency was measured using a 1.575 pn fibered diode laser with a kncnq~ output-power-versus-laser-current characteristic. The relative !3QE data were then adjusted to agree with the 1.575 p m absolute data.

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Fi- 2 shows the W E of a TPV diode having an 8- jm emitter and a l+un base, along with model curves for external and internal SQE. The measured SQS rises sharply at wavelengths shorter than the bandgap wavelength of 2.2 pm (0.56 eV), reaching a maximum QE of !%YO at 2 m. As the wavelength decreases, the SQE slowly decreases to 500/0 at 1 gm.

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0.8

0.6

3i

E uI 0.4

0.0

----absorbing externat

iosdess internal

1 -0 1.5 2.0 2.5

Wavelength, hum)

RGURE 2. The Spectral Quantum Efficiincy of a W n TPV Diode Having an 8- Emitter and a 1-Pm Base. Mode1 Curves Show Extemai and Internal SQE for Absorbing and Lossless GaSb Window Layers.

The model used to fit the SQE data is a simple, one-disncnsional minorityi.arrier diffusion model commonly employed in SQE data analysis (Sze 1981). The measured reflectivity was used to calculate the photon flux into the diode. In some c a w we used the measured absorption constant of InGaAsSb, but mostly we usxi calculated semi-empirical absorption data (Bonego 1994). Accounting for absorption by the W-m-thick GaSb window layer, two sets of SQE curves were calculated: are set assuming that thc window layer contributes all electrons photo-generated within it to the emitter (labeled "lossless internal (or external)" in Figure 21, and the other set assuming that the window layer contributes no photo-generated electrons to the emitter (labeled "absorbing internal (or external)" in Figure 2).

Electron and hole diffusion lengths, I, and Lh. characterize minority-carrier transport. The badsubstrate and the window-layer/emitter interfaces are drwacterized by recombination velocities S, and S , respectively. Note that we normalize these recombination velocities to the "bulk" recombination velocity D/L, where D is the appropriate diffusion constant. In this way S, >> 1 describes an interface that acts like a sink for holes, and Sb << 1, desctik a nearly perfect reflecting interface. The total photocurrent comprises contribu~ons frons the emitter, base, depletion-region, and possibly from the window layer.

3

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* The model curve of external SQE with an absorbing window desaibes the measured SQE curve in Figme 2 masonably well using the following parameters: L, = 29 pm, S, * 0, = 1 pm, and S, = 0. Note that in diodes where x, > x, as in this device, the influence cn the SQE of photocurrent from the base is negligibly small, and the parameters describing hole transport are, therefore, inaccurate. The values we used m this case were obtained from diodes having 1-pm ernitten and bases. The best fit to the external W E for a ~osless window layer gives &= 25 pm, still much longex than x,. In Figure 2 the external SQE curve for a lossless window layer is calculated with L, = 29 pn, which gives a curve that is slightly higher than the data.

Setting the spectral reflectivity to zero gives the i n t e d S Q E model curves shown in Figure Z EFom 1 tu 2.1 the internal QE is above 8oo/o, and it is over 90% between 1.4 and 2.1 pm. Absorption in the GaSb window layer causes the gradual decrease in the htmal SQE begulning at 1.7 c ~ n Assuming complete! photos\ectron collection from a lossless window layer, the internal SQE is constant at 95% for wavelengths shorter than 2 pm, as indicated in Figure 2.

electron difhion lengths have also been inferred from the SQE data of diodes with different emitter widths. The foliowing relation describes the majority of aur observations: L, 2 &. Figwe 3 shows our iderred values of L, for diodes with emitter widths of 1 to 8 p. The dashed line m Figure 3 denotes L, = x, Twenty-one of the twenty-fsur data shown in Figure 3, or 88%, lie m <x above the line, stzongly suggesting that the relation Le 2 %is valid.

The model also fits the SQE data reawnably well by asftMing m-zem values of S,, but consequently, values of t, larger than 29 pm must be used. Our approach has been to assume that the smallest value of I,, consistent with the data, is the most prudent estimation.

The vertical axis at the right in Figure 3 indicates the ekdmn lifetimes 7, in the emitter. z, is calculated from the relation T, = (kT/@Jb*, in which the ininority electron mobility, is assumed qual to the majority electron mobility: namely, 1600 cm2/Vs. An estimate of the radiative lifetime in 0.56-eV InGaAsSb doped to 2 x 10" ali3 is 50 IS (Varshni 1964), which comspods to L, = 14 pm. Tbe inferred electron lifetimes in 8- emitters are between 100 and 200 nti One of the possible expbatians for these long lifetimes is photon recycling (Garbuzov 1977 and Khalfin 1978): the creation of electmn-hole pairs through the re-absorption of luminescent photons arising from radiative recombination. The lifetime-enhancing effect of photon recycting has been observed in other Itt-V compounds, such as GaAs (Ahrenkiel1989).

To a lesser extent, we observed the same phenomemm with respect to hole transport. Not as many wide-base diodes were measured, since efficient p o ~ l Tpv diodes will have wider emitters than bases, owing to the fact that electrons have the longer diffusion length. Figure 4 shows the dependence of L, on base width. As in the case of electrons, the values of I.,, inferred from the model increase with increasing base width, but the magnitudes of & for a given x, are only about one-tenth that of electrons. This result may reflect the lower rnc4iJity and non-radiative lifetime of holes. The base layers of these diodes were doped an d e r of magnitude higher than were the emitter layers, which might decrease the holes' non-radiative lifetime to an estimated 5 ns, which tOTzeSpOndS to Lt, = 1.6 p. For 6- iim wide bases the inferred lifetimes exceed this estimate, sqgesting the presence of photon recycling.

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CJ W

3 c 0 8 3 -

25 0.

Ir

4%

is o $0 e # - e

'

m N 0

0 0 0 2 4 6 8 ¶O

Emitter Width, x&m)

I . . . I . . . I . . . a . L

FIGURE 3. The Inferred Electron Diffusion Length for TPV Diodes with Emitter Widths of 1 to 8 pm.

FIGURE 4. The Inferred Hole Diffusion Length for TPV Diodes with Base Widths of 1 to 6 pm.

In condushq. we have shown that the transport of photpsenemted electmns in p-InGaAsSb is very efficient. psn-n TPV diodes with bandgaps of 0.56 eV have external quantum efficiencies d 5% a t

2 pn and exhibit electron diffusion lengths of 29 pm m M *-wide emitter. The electron diffusion length appears to increase with increasing emitter width. Similarly, the hole diffusion length appears to increase with increasing base width, although the hole diffusion lengths are about me-

5

! WUCED tT oov7 EXPENSE #. 49

.a. . I . L ' . 4 '

f tenth those of the. electron. Given these results, an opttmized Inc;aAsSb/Gasb pori-n Tpv diode with a 6- to 8-pn-wide emitter should have an excellent SQE. -

We gratefully acknowledge the technical assistance of M. G. Harvey, R. Farkas, D. R. Capewell, and R. J- Matarese.

References

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Lee, H., D. 2. Garbuwv, R U. Martixrelli, R. J. Menna, and J. C- CmoUy, (1997) "Moleclllar Beam Epitaxy of AIGaA&/hGaAsSbKa5b Mid-Mrared High-Power SeparakCmfinement Quatun- Well Lasers," p'lesented at the Electronic Materials Research Conference, Ft. Collins, CO, June 25-27, 1997.

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Semiconductors (I)" Phys. Status Solidi, 19 459-514.

Wang, C. W., H. K. Choi, G. W. Tumer, and M. J. M&a, "L.%tticeMatched Epitaxial GaInAssb Themophotvoltaic Devices," (1997) presented at Themtophotovoltaic Generation of Electricity: Third NREL Coderem?, Coutts, T. J., Allman, C. S., and Benncr, J. P.. eds, AIP Cmf. PIPX., 401, Colorado Springs, CO.

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