n-ZnSelp-GaAs HETEROJUNCTION SOLAR CELLS

U. Blieske, T. Kampschulte, A. Bauknecht, M. Saad, J. Sollner a), A. Krostb), K. Schatkeb), M. Ch. Lux-Steiner

Hahn-Meitner-lnstitut Berlin, Dep. FH, Glienicker Str. 100, D-14109 Berlin, Germany; a) AIXTRON AG, Semiconductor Technologies, Kackertstr. 15-1 7, 52072 Aachen, Germany;

b) lnstitut fur Festkorperphysik, Technische Universitat Berlin, Hardenberg Str. 36, 10623 Berlin, Germany

ABSTRACT

For PV applications epitaxial layers of n-ZnSe were grown by metal-organic vapour phase epitaxy (MOVPE) at 340°C on GaAs(001) substrates. n-t pe net carrier concentration in the range between I O ’ -iO1’cm” was demonstrated. By increasing the doping concentration the double crystal x-ray diffraction full width at half maximum of 1.2 pm thick ZnSe-layers increased from 400“ to 600”. A first n-ZnSelp-GaAs solar cell achieved an open circuit voltage of 706 mV, a fill factor of 65% and a short circuit current density of 10mAcm-2 (total area, ELH-lamp, 1 OOmWcm-’, no AR-coating). J,, was increased by 4 m A ~ m ‘ ~ by depositing sputtered n’-ZnO on the n-ZnSe layer, In order to further improve the PV performance n- ZnSe will be grown on GaAs buffer layers, the doping profile will be optimised systematically and MgF2 will be deposited on the ZnO layer.

1:

INTRODUCTION

ZnSe is a potential window-layer material for GaAs solar cells. It is nearly lattice-matched to GaAs and has a bandgap of 2.76 eV at room temperature, which is significantly higher than that of standard window layers used for GaAs solar cells, as Al0.5Ga0.5As and Ga0.5ln0.5P. Additionally, ZnSe can be grown at temperatures around 340°C [ I ] , which is much lower than the growth temperature of the conventional window layers [2]. Lately, it has been shown that MOVPE grown ZnSe reduces the surface recombination in GaAs [3,4]. In this work n-ZnSelp-GaAs heterodiodes are investigated [5].

EXPERIMENTAL

ZnSe epilayers were grown using a low-pressure MOVPE system (AIXTRON AIX 200 SC) with a horizontal quartz reactor. The reactor pressure was 300

mbar. Palladium-diffused HZ was used as carrier gas. The total H2 flow rate into the reactor was varied between 1.6 and 3.5 I/min. Dimethylzinc-triethylamine (DMZn(TEN)) was used as zinc source and ditertiarybutylselenide (DTBSe) were used as selenium sources. The growth of ZnSe was studied dependent on temperature: The optimised growth temperature was 340°C. Different GaAs(001) -substrates were used: undoped (for Hall- and transmission-line-measurements) and Zn-doped (p-type for solar cell devices). The GaAs substrates were prepared by wet chemical etching with HF. Furthermore, n-type ZnSe was grown using 1- chlorobutane as dopant-source. The molar flow rate of the Cl-source during growth was varied between approximately 1 and 100 pmol/min. This corresponds to partial pressure ratios pl-CI.butane/PDTBSe between 0.01 and 1. The effect of CI molar flow rate during growth of ZnSe on the crystallographic quality, as well as optical and electrical properties were investigated. The ability to dope ZnSe with chlorine was used for the formation of an n-ZnSelp-GaAs heterodiode. Mn/Au contacts were evaporated on the p-GaAs substrate for ohmic back contacts. The front contact grid was made by evaporating a bilayer of In/Ag through a shadow mask. In order to improve the ohmic contacts and the optical properties of the device n’-ZnO was deposited on the n- ZnSe layer by rf-magnetron sputtering. The front contact grid on ZnO was made by e-beam evaporation of Ni/AI through the same shadow mask as mentioned above. Double crystal x-ray diffraction (DCXRD) in rocking mode was used to determine the crystallografic quality, photoluminescence (PL) spectra at 11 K were used to investigate the optical properties and Hall-, CV- and transmission-line-measurements were applied to determine the electrical properties of the ZnSe-layers grown on GaAs(001). In/Ag contacts were used for all electrical measurements. The device properties were studied by measuring the current-voltage characteristics and the external quantum efficiencies.

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0-7803-3767-0/97/$10.00 0 1997 IEEE 26th PVSC; Sept. 3 0 4 c t . 3,1997; Anaheim, CA

RESULTS AND DISCUSSION

CI-doping of ZnSe

The carrier type and concentration in Cl-doped ZnSe grown on undoped GaAs substrates were determined by Hall-measurements. All samples showed n-type behaviour. Hall-mobilities around 50 cm2V1s-’ and net carrier concentrations up to IO1’cm” have been measured at room temperature which is reported to be the limit due to saturation [6] . It was shown by transmission-line-measurements that the resistivity decreases steadily with increasing molar flow of I-CI- butan, i.e. partial pressure ratios PI-CI-butane/PDTBSe during growth (Fig. 1). Resistivities down to O.OlS2cm have been achieved at a partial pressure ratio pl-CI.butane/PDTBSe of approx. 0.2 (molar flow of 20 pmol/min). At higher molar flow rates of I-Cl-butan the resistivity increased drastically. It was found by XRD measurements that the crystal quality of the epitaxial layers was strongly affected by the incorporation of chlorine. Fig. 2 shows DCXRD measurements of ZnSe layers grown at different I-CI- butane partial pressures. The thicknesses of all layers was approx. 1.2pm. The ZnSe-thin film layer grown without doping shows the lowest ZnSe-peak DCXRD FWHM of 400”. A series of MOVPE experiments using various partial pressure ratio pl-CI-butane/PDTBSe started with a value of approximately 0.01 (curve b in Fig. 2). Each successive layer presented in Fig. 2 has been grown with a four times higher partial pressure of I-CI- butan than the previous one. Correspondingly the DCXRD FWHM increased from 400” to 600”. The layer grown with a partial pressure ratio pl-CI-butane/PDTESe of approximately 0.8 shows no ZnSe-peak. Nevertheless, thickness measurements indicate the growth of a 1.2 pm thick layer.

Furthermore, the optical properties of the n-ZnSe layers of the doping series were investigated by PL-

Fig. 1 : Transmission-line measurements of Cl-doped ZnSe

measurements at 11 K. Fig. 3 shows that the deep level emission (which can be attributed to self activated centers (SA)) increases with increasing chlorine incorporation. The ZnSe layer with the highest partial pressure of I-Cl-butane exhibits no near-band-edge emission, again indicating detrimental effects on the ZnSe-epilayer structure.

GaAs(004)

ZnSe(004)

Increasing

partial

pressure

1 -CI-butane

-1500 -1000 -500 0 500 1000 1500 arcseconds

Fig. 2: DCXRD measurements of ZnSe on GaAs with the following partial pressure ratio pl-CI-butana/PDTBSe: a) 0, b) 0.01, c) 0.04, d) 0.2 and e) 0.8

increasing partial pressu

of I-Cl-butane n x 5

~~

1.6 1,8 2,O 2,2 2,4 2,6 2,8

energy [eV]

Fig. 3: PL-measurements of ZnSe on GaAs with various CI-concentrations (the factors refer to the intensity scaling of each curve).

940

ZnSelGaAs solar cells

ZnSe:CI n=1 019

n=10l9

n=1017 t ZnSe:CI

ZnSe (undoped)

GaAs:Zn (substrate) ~ > 6 . 1 0 ’ ~

ZnOIZnSelGaAs solar cells

160nm

0.3ym

40nm

500pm

The ability to dope ZnSe with chlorine was used for the formation of a n-ZnSe/p-GaAs heterodiode. Simple device structures with heavily doped ZnSe (nF;.lOigcm”) were made, but also solar cells with a doping gradient in the ZnSe-layer ( n ~ 4 0 ’ ~ c m ” at the interface and n~1019c~n” at the surface of the device). Fig. 4 shows the schematic cross-section of the latter device. The first 40nm of ZnSe were grown without incorpoation of chlorine, the following 300nm of ZnSe were grown with a doping gradient and the final layer was heavily doped (n=lO” cm”) with chlorine. Mn/Au contacts were evaporated on p-GaAs and In/Ag in ZnSe, respectively.

Mn / A u 0.5ym

Fig. 4: Schematic cross section of the ZnSe/GaAs solar cell investigated in this work, the layer thicknesses are indicated at the right.

In order to achieve contact resistances below 0.1 ncm2 between n-ZnSe and In/Ag high carrier concentrations (n>1018cm-3) were needed in the ZnSe layer. However, n’-ZnSe/p’-GaAs (substrate) solar cells with heavily doped ZnSe achieved open circuit voltages of only 497mV, short circuit current densities (Jsc) around IOmAcm-’ and fill factors around 50% (ELH-lamp, total area, no AR-coating). The devices with a doping gradient in the n-ZnSe layer showed open circuit voltages around 700 mV, short circuit current densities around IOmAcm-’ and fill factors of up to 65%. The reason for low J,, values are due to recombination losses in the heavily doped absorber layer and at the interface (about 30%, -9mAcm-*), reflection losses from ZnSe (20%, -6mAcm-’), absorption in ZnSe not contributing to JSc (IO%, - 3 m A ~ m - ~ ) and high shadowing losses due to the metal grid (IO%, -3mA~m’~ ) . The numbers refer to the theoretical limit of J,, for GaAs ( 3 1 m A ~ m ‘ ~ under AM1.5 illumination). The recombination losses will be discussed briefly for the following device.

In order to improve the ohmic contacts and the optical properties of the device n’-ZnO was deposited on the n- ZnSe layer by rf-magnetron sputtering. The electrical properties of sputtered Zn0:Al-layers on glass used for ZnO/ZnSe/GaAs solar cells were measured by Hall- measurements using van-der-Pauw-geometry. The layers were found to be n-type with a carrier concentration of 5.1 02’cm”. Mobilities of 20 cm’V-’s‘’ were recorded. Fig. 5 shows the n’-ZnO/n-ZnSe/p’-GaAs device structure used in this work. It also contains a doping gradient in the ZnSe layer The thickness of this layer was varied between 50 and 800nm. The short circuit current density of the device with n’-ZnO window layer is 4 m A ~ m . ~ higher than in the case of the n-ZnSe/p’-GaAs solar cell. This is mainly due to reduced reflection losses which can be explained by the lower refractive index of ZnO (n=2.05 at 500nm and T=300K [7]) as compared to ZnSe ( ~ 2 . 7 5 at 500nm and T=300K [8]) resulting in a reduced reflection (from 22% to 10%). All other PV- parameters are comparable to the values of the device mentioned above.

0.7pm

370nm

ZnSe:CI

2nSe:CI n=lO

ZnSe (undoped) 20nm

GaAs:Zn (substrate) ~ > 6 . 1 0 ’ ~ 500ym

Mn / A u 0.5pm

Fig. 5: Schematic cross section of the ZnO/ZnSe/GaAs solar cell, the layer thicknesses are indicated at the right.

Fig. 6 shows the external quantum efficiency of two nt- ZnOln-ZnSe/p’-GaAs solar cells with ZnSe-layers of various thicknesses (the values refer to all three layers in Fig. 5). In both curves the band-edges of ZnSe and GaAs are clearly visible. It can be seen from Fig. 6 that all the light absorbed in the ZnSe layer does not contribute to the current of the device. The 100nm thick ZnSe layer transmits some of the light with h<436nm, the 800nm thick layer absorbs all the light below the ZnSe-band edge. The quantum efficiencies at wavelengths between 500 and 700nm is dominated by the interference pattern resulting from the difference in refractive indeces of ZnO, ZnSe (3 layers) and GaAs. The calculated values determined from the reflection

94 1

I , I , . ~ . I , . , I . , I

Wavelength [nm] 400 500 600 700 800 900

Fig. 6: External quantum efficiencies of ZnO/ZnSe/GaAs solar cells without bias illumination. The device structure is shown in Fig. 5. The thicknesses refer to all 3 ZnSe layers.

peaks correspond with the thicknesses measured by a profiler. At wavelengths b 7 0 0 n m the quantum efficiency decreases steadily with increasing wavelength which can be attributed to a low effective lifetime in the GaAs-absorber layer. This effect can not be explained by free carrier absorption in the heavily doped ZnSe layer as demonstrated by the independence of the layer thickness (Fig. 6). An effective diffusion length of approx. 400nm has been calculated from the quantum efficiency data between 680 and 840nm in Fig. 6. This can partly be explained by the high doping concentration of the GaAs-absorber layer (p>6,10’*~m-~). Hence, it is expected that higher current densities can be achieved if the n-ZnSe layer is deposited on an epitaxial p-GaAs buffer layer [9] with moderate doping (p=IO” ~ m - ~ ) instead of on a heavily doped p’-GaAs substrate (p>6.1Q18cm”). First n’-ZnO/n-ZnSe/p-GaAs/p’-GaAs solar cells exhibit short circuit current densities of up to 2QmA~m.~.

Quantum efficiency measurements with white bias illumination revealed a high defect concentration at the interface which could be saturated under illumination. In order to achieve a better control of the interface an in- situ-preparation of a GaAs-buffer between layer ZnSe- layer and p+-GaAs substrate is required. Oxide formation before ZnSe deposition has to be avoided,

CONGLUSlONS

It has been shown that epitaxial ZnSe-layers grown by MOVPE on GaAs (001) can be3doped up to carrier concentrations of approx 1 Oigcm by the incorporation of chlorine for partial pressure ratios PI-CI butane/ POTBSe

below 0 25 However, layers grown with a partial pressure ratio butane/PDTBSe above 0 5 show no

DCXRD-peak and no near band-edge emission related to ZnSe and high resistivities ( > I O Rcm). First MOCVD-grown n-ZnSe/p-GaAs exhibit open circuit voltages up to 706mV, fill factors around 65% and short circuit current densities of 10mAcm-2 (ELH-lamp, 100mWcm-*, no AR-coating). The J,, of the devices were increased by 4 m A ~ m - ~ by depositing sputtered n+- ZnO on the n-ZnSe layer.

In order to further improve the PV-performance n-ZnSe will be deposited on in-situ grown GaAs buffer layers, the doping profile will be optimised systematically. The next step will be to optimise the absorption in a MgF>/ZnO/ZnSe/GaAs solar cell structure.

ACKNOWLEDGE M E NTS

This work was supported by the European Commission within the frame of the Joule Ill project “Wide Gap CPV under contract no. JOR3-CT97-0135 and by the Bundesministerium fur Bildung und Forschung (BMBF). The authors would like to thank EPICHEM for their support in MOVPE- precursors. The authors are grateful to J. Albert, J. Beckmann, J. Beier, A. Gerhard and M. Kirsch for electrical and continues assistance.

LITERATURE

J. Sollner, M. Scholl, A. Schneider, M. Heuken, J. Woitok, J. Crystal Growth 138 (1994) 35 C.Y. Tsai, W.H. Bloss, K. Zieger, F. Scholz, V. Frese, U. Blieske, Proc. of 13‘h EC PVSEC (1 995) 91 8

A. Krost, W. Richter, D.R.T. Zahn, Appl. Surf Science 56-58 (1 992) 691 J.A. Yater, G.A. Landis, S.G. Bailey, L.C. Olsen, F.W. Addis, Proc. of 25‘h IEEE PVSC (1 996) 65

B. V. Zhuk, I.A. Zhuk, A.A. Zlenko, Solid State Electronics, 29(2) (1 986) 247

J. Walukiewicz, J . Vac. Sci. Techno/. 86 (1988) 1257

Y.S. Park, J.R. Schneider, J. Appl. Pbys., 39 (7) (1968) 3049 S. Adachi, Phys. Rev. €3, 43 (12) (1991) 9569 H.F. MacMillan, H.C. Hamaker, N.R. Kaminar, M.S. Kuryala, M. Ladle Ristow, D.D. Liu, G.F. Virshup, J.M: Gee, Proc. of 20“ lEEE PVSC (1 988) 462

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