thermophotovoltaic gasb cells fabrication and characterisation

Thermophotovoltaic GaSb Cells Fabrication and Characterisation

V.P. Khvostikov1, J.L Santailler2, J. Rothman2, J.P. Bell2, M. Couchaud2, C. Calvat2, G. Basset2, A. Passero2, O.A. Khvostikova1, M.Z. Shvarts1.

Rep1 Ioffe Physico-Technical Institute, 26 Polytechnicheskaya, 194021, St. Petersburg, Russia. 2 Commissariat à l’Energie Atomique, LETI/DOPT, 17 Rue des martyrs 38054 Grenoble Cedex 9,

France. Corresponding author: V.P. Khvostikov, Ioffe Physico-Technical Institute, 26 Polytechnicheskaya,

194021, St. Petersburg, Russia,. e-mail: [email protected], Tel: +7-812-2927924, Fax: +7-812-247 10 17

Abstract. For production of highly effective photoconverters semiconductor materials with strictly determined parameters are required. For thermophotovoltaic (TPV) GaSb cells homogeneous Te-doping level of (2-7)·1017 cm-3 in the bulk semiconductor is required to produce high efficient PV cells by the Zn diffusion process [1-3]. In this paper we present data on investigation of the performance of the cells obtained on different GaSb:Te wafers of (100) and (211) orientation. Based on classical I/V measurements and external quantum efficiency (EQE) curves, we analyze cell performances in order to improve all fabrication stages like wafer surface preparation, p-type GaSb emitter elaboration by the zinc diffusion process, antireflection coating deposition and contact realization. Today good performances are obtained on both 3.5 x 3.5 mm² (211) and 10x10 mm² (100) GaSb cells. We obtained EQE of 70-76 % in the 800-1600 nm range for the first one and 80-88 % in the same spectrum for the second one. Electrical characterization gives respectively, the fill factor (FF) from 67.8 % down to 65.8 % in the 1-2 A/cm² range and 63 % at 5 A/cm2. The open circuit voltage Voc increases from 0.44 V (1 A/cm²) up to 0.49 V (5 A/cm²) for the small area cell.

Keywords: crystal growth, GaSb, thermophotovoltaics. PACS: 81.10.Aj; 84.60.Jt

INTRODUCTION

The Czochralski process is widely used for the growth of gallium antimonide semiconductor compounds [4-13]. We have developed such a technique for the growth of single GaSb:Te crystals, 2 inches in diameter and more than 100 mm in length. Crystals were grown from home made seeds oriented in the <100> direction [13]. For TPV application the tellurium concentration in the bulk material must be in the range of (2-7)·10 17

cm–3. The segregation coefficient (kTe) represents the ratio between Te concentration in the solid phase (Cs) and the Te concentration in the liquid phase (Cl) at the solid/liquid interface during the growth. Assuming that kTe = Cs/Cl is ~ 0.37 [14, 15], the Te concentration in the crystal is increasing during the growth and only part of the crystal will match the specifications. So, we have to fix the initial Te

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concentration in the raw material and then to optimize the growth conditions in order to get a Te homogeneity in both radial and axial directions in the most part of the crystal. For this it is necessary to prepare a polycrystalline GaSb charge from high purity raw materials, Ga, Sb and Te, in a closed silica ampoule sealed under secondary vacuum. The initial Te concentration in the charge is intentionally increased by a factor of 10 - 15. The following parameters are used in the growing process: crystal pulling rate of (4-15) mm/h, crystal rotation of (5-10) rpm, crucible rotation of (1-5) rpm and argon atmosphere. Basing on the work done in [3], we succeeded to grow crystals with a weight regulation. Typical crystals of 2 Kg, 150 mm in length, 55 mm in diameter were achieved (figure 1). Due to the high probability of twin formation with a <100> seed orientation in the crystal shoulder, we obtained several times crystals with the first part oriented (100) and the second one (211). A precise growth angle control helped to overcome this problem.

After the growth, crystals were cut with a multi-dicing saw. All samples were then polished and etched (see [3] for more details).

FIGURE 1. As grown GaSb:Te crystal, diameter 55-60 mm, length 150 mm.

Wafers with two different orientations were processed for cells manufacturing at

IOFFE. Sample and wafer surface preparation were done by the procedure described in [3]. The p-n junctions were realized by the two-step zinc diffusion process [16, 17]. After Zn diffusion an anodic oxidation was used to remove very accurately the front surface (by 20 nm step). The effect of the shape and depth of the p-n junction on basic characteristics of the GaSb TPV cells fabricated from (100) and (211) wafers have been studied. First 3.5х3.5 mm² cells were tested with photocurrent density of up to 5 A/cm².

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EXPERIMENTAL

As the material specification for TPV applications is fixed in a narrow range, we obtained the carrier density, the electron mobility from Hall measurements in the Van der Pauw configuration of different samples extracted from three different crystals. The schematic drawing of the Hall method [18] is done in figure 2.

FIGURE 2. Schematic drawing of a typical Hall configuration.

The basic physical principle underlying the Hall effect is the Lorentz force. When

an electron moves along a direction perpendicular to an applied magnetic field, it experiences a force acting normal to both directions and moves in response to this force and the force effected by the internal electric field. For an n-type, bar-shaped semiconductor shown in figure 2, the carriers are predominately electrons of bulk density n. We assume that a direct current I flows along the x-axis from left to right in the presence of a z-directed magnetic field. Electrons subjected to the Lorentz force initially drift away from the current line toward the negative y-axis, resulting in an excess surface electrical charge on the sample side. This charge forms the Hall voltage, a potential drop across the two sides of the sample. Note that the force on holes is toward the same side because of their opposite motion and positive charge. This transverse voltage is the Hall voltage VH and its magnitude is equal to IB/qnd, where I is the current, B is the magnetic field, d is the sample thickness, and q = 1.602 10-19 C is the elementary charge. In some cases, it is convenient to use layer or sheet density (ns = nd) instead of bulk density. One then obtains the equation ns = IB/q|VH|. Thus, by measuring the Hall voltage VH and from the known values of I, B, and q, one can determine the sheet density ns of charge carriers in semiconductors. If the measurement apparatus is set up as described, the Hall voltage is negative for n-type semiconductors and positive for p-type semiconductors. The sheet resistance RS of the semiconductor can be conveniently determined by the use of the van der Pauw resistivity measurement technique. Since sheet resistance involves both sheet charge density and mobility, one can determine the Hall mobility from the equation µ = |VH|/RSIB = 1/(qnSRS).

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Table 1 presents the Hall results and the materials properties.

TABLE 1. Summary of the main Hall results obtained at 300 K. Gasb:Te

ingot GaSb:Te sample

Orientation d (µm)

Type Mobility µ (cm²/(V·s))

ns (at/cm²)

n (at/cm3)

Se_127 Se_127_A01 (211) 1750 n 2767 9,87 1016 5,64 1017 Se_129 Se_129-37 (100) 669 n 3153 1,87 1016 2,80 1017 Se_139 Se_139-A (100) 620 n 3105 1,51 1016 2,43 1017 Se_139 Se_139-B (100) 660 n 3093 1,38 1016 2,10 1017

2500

3000

3500

4000

4500

5000

5500

1,00E+17 1,00E+18

Carrier conconcentration (At/cm3)

Hal

l Mob

ility

cm

²/(V

.s)

1,1 Tesla 300K1,1 Tesla 250K1,1 Tesla 200K1,1 Tesla 150K

GaSb_127_A01GaSb_129-37GaSb_139-A and B

GaSb_127_A01

GaSb_129-37GaSb_139-A and B

GaSb_127_A01


GaSb_127_A01


FIGURE 3. Hall mobility versus carrier concentration and temperature for four GaSb:Te samples.

Figure 3 shows the mobility variation at 1.1 Tesla with carrier concentration and temperature. All samples are n-type. One can notice, that, first, for all samples, mobility increases from 2500-3000 cm²/(V·s) at 300 K up to 5000-5500 cm²/(V·s) at 150 K. This behaviour is typical for GaSb. Second, for each sample, carrier concentration increases slightly when temperature is decreasing, we observe the same trend for four samples. At all temperatures sample Se_127_A01 oriented (211) presents a less mobility compared to (100) samples.

Figure 4 presents the contribution of the two energy bands Eg and El to the total effective Hall mobility of the four samples (at 1.1 Tesla). We observe that for all samples carrier population in Eg band contributes 70-73 % and for the El band 19.8 – 23 % to the effective samples’ mobility (~ 4-7 % are located in Ex band). For the sample Se_127_A01 oriented (211) the contribution is higher than for (100) samples in the El band and lower in the Eg band. Variation of the carrier concentration in Eg band is drawn versus temperature in figure 5. The maximum mobility of 5500-6200 cm²/(V·s) is observed at about 120 K for (100) GaSb:Te samples and at about 100 K for the (211) sample ~ 5700 cm²/(V·s).

From these measurements, we can conclude that our growth process is validated for obtaining the right Te concentration in GaSb samples. At room temperature, effective Hall mobility is 3120±50 cm²/(V·s) for (100) GaSb:Te samples and 2767 cm²/(V·s) for the (211) sample.

201


500

1000

1500

2000

2500

3000

3500

4000

4500

1,00E+17 1,00E+18Carrier conconcentration (At/cm3)

Hal

l Mob

ility

cm

²/(V

.s)

1,1 Tesla 300KEg Contribution 300KEl Contribution 300K

GaSb_127_A01

GaSb_127_A01 71,5 %

GaSb_127_A01 23%

GaSb_129-37

GaSb_129-37 73,2 %

GaSb_129-37 19,8 %

GaSb_139-A and B 70 %

GaSb_139-A and B

GaSb_139-A and B 19,8 %

FIGURE 4. Hall mobility versus carrier concentration at 300 K for four GaSb:Te samples. Contribution of two different carrier bands Eg and El.

2500

3000

3500

4000

4500

5000

5500

6000

6500

0 50 100 150 200 250 300 350

Temperature (K)

Hal

l mob

ility

Eg

carr

ier

cm²/(

V•s

)

GaSb_Se_127-A0GaSb_Se_129_37_01GaSb_Se_139_A_01GaSb_Se_139_B_01

FIGURE 5. Hall mobility (Eg contribution) versus temperature for four GaSb:Te samples.

GALLIUM ANTIMONIDE BASED TPV CELLS

Gallium antimonide wafers obtained from the ingots grown in CEA/DOPT were polished in IOFFE. Then, on these wafers, investigations of Zn diffused profile and penetration depth of p-n junction [19, 20] were carried out in order to obtain high efficient GaSb TPV cells and to compare characteristics of these cells with different orientations: (100) and (211). Zn diffusion [16, 17] was performed into n-GaSb wafers doped with tellurium with charge carrier concentration of (4-7)·1017 cm-3 in a pseudo closed graphite boat under hydrogen flow. The first diffusion process, during which the p-n junction was being formed, took 4 hours at 460 0C. Depth of the initial

202


photosensitive p-n junction was about 500 nm. Then, selective etching of the oxide layer of the samples in diluted HCl was carried out. Diffusion Zn profiles in GaSb and p-n junction penetration depths were investigated. Figure 6 presents the zinc profile in (100) and (211) samples after diffusion during the same experiment. We clearly observed that the near-surface region about 60 nm is a heavily doped layer. The maximum etching depth of the GaSb p-emitter was 140 nm at the initial junction depth of 0.5 μm.

0 100 200 300 400 500 6001017

1018

1019

1020

1021

1022

Ato

mic

con

cent

ratio

n, c

m-3

Depth, nm

Zn profile: (100) (211)

Te profile:

FIGURE 6. Diffusion profile of Zn and Te distribution in GaSb of (211) and (100) orientation.

As follows from the presented distribution, the value of the near-surface region zinc

concentration on the (211) wafers is essentially larger than that on the (100) wafers. The following technological steps of GaSb cell manufacture were alike for cells fabricated early [21]. The second diffusion was carried out with the aim to deepen the p-n junction under the contact grid. After that, 3.5×3.5 mm2 cells were formed by means of photolithography. Contact grid spacing was 200 µm. The cells had the antireflection ZnS/MgF2 coating.

0 20 40 60 80 100 120 140 16028

30

32

34

36

38

40

Pho

tocu

rren

t den

sity

(AM

1.5D

), m

A/c

m2

Etching depth,nm

(100)

(211)

FIGURE 7. Dependence of the current density on the etching depth of the GaSb p-emitter for two wafer orientations.

203


It follows from measurements of the external quantum yield (spectrum AM1.5D Low AOD, 1000 W/m2) on the cells with different depths of the p-n junction for (100) and (211) orientations (figure 7) that in increasing the etching depths the generated photocurrent density rises, and for the cells fabricated on the (211) wafers the photocurrent density is noticeably greater. At further increasing the etching depth the photocurrent density drop took place.

0 20 40 60 80 100 120 140 160 18056

58

60

62

64

66

68

FF, %

Etching depth, nm

Jsc=1A/cm2

Jsc=2A/cm2

Jsc=5A/cm2

Jsc=3A/cm2

FIGURE 8. Dependence of 3.5х3.5 mm2 GaSb cell fill factor on the etching depth of the p-emitter at different generated photocurrent densities.

As is seen from the plots presented in figure 8 and 9, the fill factor and efficiency vary with changing the p-n junction penetration depth and have two maxima at etching depths of 60 nm and 120 nm. The open circuit voltage varies in etching the GaSb cell p-emitter in the same way and has also a maximum in the 60 nm region at 0.48 V for 5 A/cm². However it has no well pronounced second maximum.

0 20 40 60 80 100 120 140 1605

6

7

8

9

10

11

12

1

2

3

Effi

cien

cy, %

Etching depth, nm

4

FIGURE 9. Dependence of 3.5х3.5 mm2 GaSb cell efficiency on the etching depth of the p-emitter at different generated photocurrent densities: 1 – 5 A/cm2; 2 – 3 A/cm2; 3 – 2 A/cm2; 4 – 1 A/cm2.

204


As follows from the presented results, the optimum etching depth of the GaSb p-emitter is 60 nm at the initial p-n junction penetration depth of 0.5 μm. Figures 10 and 11 present dependencies of the fill factor, the open circuit voltage and the efficiency of 3.5х3.5 mm2 GaSb cells fabricated with an optimal profile and penetration depths of the p-n junction in illuminating with AM1.5D Low AOD light on the photocurrent density. It is clear that for the generated current densities up to 5-7 A/cm2 the cells manufactured from the (211) wafers look more preferential compared with those manufactured from (100) wafers, other factors being the same.

At the final step of this work, 1x1 cm2 GaSb TPV cells have been manufactured and investigated. The 1x1 cm2 GaSb cell had the spectral characteristic with high value (0.8) of the external quantum yield.

1 2 3 4 5 6 7 8 9 1062

64

66

68

70

72

74

76

78

0,36

0,38

0,40

0,42

0,44

0,46

0,48

0,50

FF, %

Photocurrent density, A/cm2

VO

C, V

Voc

FF

(211) (100)

FIGURE 10. Dependencies of the fill factor and open circuit voltage of 3.5x3.5 mm2 GaSb cells on the photocurrent density.

1 2 3 4 5 6 7 8 9109,5

10,0

10,5

11,0

11,5

12,0

12,5

(100)

(211)

Effi

cien

cy, %

Photocurrent density, A/cm2

FIGURE 11. Dependence of the efficiency of 3.5x3.5 mm2 GaSb cells fabricated from (100) and (211) wafers on the photocurrent density.

205


As is seen from figure 12, the fill factor of the given cells was 70 % and the open circuit voltage was 0.42-0.45 V for the generated photocurrent of 1 A. It is clear that the 1x1 cm2 GaSb TPV cells manufactured from (211) wafers look more preferable compared with those manufactured from (100) wafers. Efficiencies of 10 % have been obtained for these cells for conversion of solar spectrum AM1.5D Low AOD, 1000 W/m2 at photocurrent densities of 1-2 A/cm2.

500 1000 1500 2000 2500 3000 3500 400040

45

50

55

60

65

70

0,44

0,45

0,46

0,47

0,48

0,49

0,50

0,51

0,52FF

, %

I, mA

UO

C,V

(100)(211)FF

UOC

FIGURE 12. Dependencies of the fill factor and the open circuit voltage on current for 1x1 cm2 GaSb cells soldered on a cupper base and measured under illumination from SiC heated by electric current.

CONCLUSION

High quality GaSb:Te crystals 2” in diameter were grown. Hall measurements were used to evaluate bulk material properties. Two-step gas Zn diffusion technique was applied for fabrication of high effective GaSb TPV converters. The effect of etching by means of anodic oxidation of Zn diffused emitter on the behaviour of characteristics of GaSb TPV cells fabricated from (100) and (211) wafers was investigated. The optimum depth of the initial p-n-junction etching has been found allowing to obtain maximal efficiencies for the cell photocurrents of up to 5 A/cm2. TPV cells based on (211) GaSb wafers look slightly preferable in terms of efficiency, other factor being the same. Efficiencies of 11 % have been obtained for conversion of solar spectrum AM1.5D Low AOD, 1000 W/m2.

ACKNOWLEDGEMENTS

Authors would like to express our gratitude to Prof. V.M. Andreev for helpful discussions. This work has been partially supported by the European Commission through the funding of the project FULLSPECTRUM (Ref. N: SES6-CT-2003-502620).

206


REFERENCES

1. L. M. Frass, G. R. Girard, B. A. Avery, V. S. Arau, V. S. Sundaram, and S. M. Tompson, J. M. Gee, Journal of Applied Physics 66, 8, 3866-3870 (1989).

2. T. Schegl, O. V. Sulima, A. W. Bett, “The Influence of Surface Preparation on Zn Diffusion Processes in GaSb,” in 6th Conference on Thermophotovoltaic Generation of Electricity, Freiburg, edited by A. Gopinath et al, AIP Conference Proceedings 738, American Institute of Physics, Melville, NY 2004, pp. 396-403.

3. S. Luca, J.L Santailler, J. Rothman, J.P. Bell, C. Calvat, G. Basset, A. Passero.V.P. Khvostikov, N.S. Potapovich, R.V. Levin, Journal of Solar Energy Engineering, 2006, to be published.

4. M. Kumagawa, Journal of Crystal Growth 44, 3, 291-296 (1978). 5. S. Miyazawa, S. Kondo, M. Naganuma, Journal of Crystal Growth 49, 4, 670-674 (1980). 6. M. Hársy, T. Görög, E. Lendvay, F. Koltai, Journal of Crystal Growth 53, 2, 234-238 (1981). 7. Kondo, S., Miyazawa, S., Journal of Crystal Growth 56, 1, 39-44 (1982). 8. Y. Ohmori, Akai K. SugiiShin-ichi, K. Matsumoto, Journal of Crystal Growth 60, 1, 79-85 (1982). 9. W. A. Sunder, R. L. Barns, T. Y. Kometani, J. M. Parsey, Jr. and R. A. Laudise, Journal of Crystal

Growth 78, 1, 9-18 (1986). 10. J. Doerschel, U. Geissler, Journal of Crystal Growth, 121, 4, 781-789 (1992). 11. A. Watanabe, A. Tanaka, T. Sukegawa, Journal of Crystal Growth, 128, 1-4, 462-465 (1993). 12. P. Boiton, N. Giacometti, T. Duffar, J. L. Santailler, P. Dusserre, J. P. Nabot, Journal of Crystal

Growth, 206, 3, 159-165 (1999). 13. T. Duffar, P. Dusserre, N. Giacometti, Journal of Crystal Growth, 223, 1-2, 69-72 (2001). 14. Y. J. Van Der Meulen, Journal of Phys. Chem. Chem. Solids 28, p. 25 (1967). 15. P. S. Dutta, A. G. Ostrogorsky, Journal of Crystal Growth, 191, 4, 904-908 (1998). 16. V.P. Khvostikov, O.A. Khvostikova, P.Y. Gazaryan, S.V. Sorokina, N.S. Potapovich, A.V.

Malevskaya, M.Z. Shvarts, N.A. Kaluzhniy, V.M. Andreev, V.D. Rumyantsev, “Photoconverters for Solar TPV Systems,” in 4th World Conference on Photovoltaic Energy Conversion, 2006, to be published.

17. V.P. Khvostikov, V.D. Rumyantsev, O.A. Khvostikova, M.Z. Shvarts, P.Y. Gazaryan, S.V. Sorokina, N.A. Kaluzhniy, V.M. Andreev, “Thermophotovoltaic Cells Based on Low-Bandgap Compounds,” in 6th Conference on Thermophotovoltaic Generation of Electricity, Freiburg, edited by A. Gopinath et al, AIP Conference Proceedings 738, American Institute of Physics, Melville, NY, 2004, pp.436-444.

18. L.J. van der Pauw, A method for measuring specific resistivity and Hall effect of discs of arbitrary shape, Philips Res. Repts. 13, 1958, pp. 1-9.

19. O.V. Sulima, A.W. Bett, Solar Energy Materials & Solar Cells 66, 533-540 (2001). 20. V.D. Rumyantsev, V.P.Khvostikov, O.A.Khvostikova, E.V.Oliva, M.Z.Shvarts, “Improvement of

GaSb based TPV cells by nano-etching of diffused emitter”, in 5th ISTC SAC Seminar: Nanotechnologies in the area of physics, chemistry and biotechnology, 2002, pp. 392-395.

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thermophotovoltaic gasb cells fabrication and characterisation

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