Effect of growth parameters on the MOVPE of GaAs/Ge for solar cell applications

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Abstract

GaAs has been grown on Ge substrate oriented 6° off towards (1 1 0) by atmospheric pressure MOVPE. Various growth rates and growth temperatures were tried to get device quality epitaxial layers suitable for solar cell applications. It was observed that the growth temperatures and the growth rates affect the surface morphology, optical and interface properties and crystalline quality of the epitaxial layers. This was studied using optical microscope, photoluminescence, photovoltage measurements, ECV and double crystal X-ray diffractometry.

Introduction

In past few years GaAs/Ge material system have attracted considerable attention due to its close matching in lattice spacing (0.12%) and thermal expansion. But, very little work is done for MOVPE grown materials specially for studying the effect of growth conditions on interface properties and epilayer quality. Ge has higher mechanical strength as compared to GaAs and therefore thinner Ge substrate can be used, resulting in lighter weight cells. In terms of power/weight ratio, GaAs/Ge solar cells can outperform GaAs/GaAs for space applications. GaAs/Ge cells also have lower reverse breakdown voltage, which can reduce the cell degradation caused by large reverse currents.

Recently, high efficiency GaAs solar cells on Ge substrate have been reported [1], [2], [3], [4]. The growth of good quality GaAs epitaxial layers on Ge substrate require highly controlled growth conditions, as there is slight lattice mismatch in GaAs and Ge and it also involves the growth of polar GaAs on non-polar Ge substrate. Some growth conditions results in active Ge in which extra photovoltage can be generated at the GaAs/Ge interface (called active interface) [4], [5], in tandem with the GaAs p–n junction, when the GaAs p–n junction is grown on a n-type Ge substrate. This arises because Ga has higher solid solubility than As in Ge, diffuses to the Ge and behave as a p-type dopant for Ge. However, this active Ge structure does not provide any extra power output and infact either reduces or does not change the total efficiency. Careful studies show that some of the projections of air-mass-zero (AMO) efficiencies were high mainly due to the inaccurate simulated AMO spectrum. When realistic AMO spectra are used, the tandem cells shows the “kink” in the I–V curve which results in a significant decrease in the fill-factor with a decrease in the efficiency. This “kink” in the I–V curve appears due to insufficient carrier generation in the Ge when illuminated with realistic AMO spectra. As a result, the GaAs/Ge interface generates less current than the GaAs p–n junction and in operation, the GaAs/Ge cell is driven into reverse bias, leading to a kinked I–V curve. In case of other growth conditions, extra photovoltage is not generated at the interface (called passive) and Ge acts only as a robust substrate. Such a passive interface is preferred for solar cell fabrications.

Though high efficiency GaAs/Ge solar cell on passive Ge substrate have been reported, however, detailed study of growth conditions which control the interface and epilayer quality are scantily available. In this paper, we present the effect of growth conditions on interface properties and epilayer quality for high efficiency solar cells.

Section snippets

Experimental

An atmospheric pressure, horizontal MOVPE reactor was used to grow GaAs epitaxial layers. The graphite susceptor, which can hold upto 1 inch diameter wafer, was heated using RF system. The reactants used were trimethylgallium (TMGa) as group-III material and pure arsine (AsH3) as group -V material. DEZn and SiH4 were used as p- and n-type dopant sources, respectively. The ultrapure H2 was used as a carrier gas as well as for purging the system. The n-type Ge substrate having orientation (1 0 0) 6°

Surface morphology

The surface morphology was studied using optical microscope. The growth temperature below 640°C and above 680°C yielded rough surfaces for single-step growth. The epilayers grown at 660°C (Fig. 1a) shows a cross-hatch pattern indicating that the stress is completely relieved by creations of misfit dislocations. This morphology is preferred for solar cell fabrication in contrast to the rough surfaces. The surface morphology of the sample grown at 680°C shows slightly patterned surface as shown

Conclusion

It is clear from the above results that only certain growth conditions will produce GaAs epitaxial layers with good surface morphology, optical and crystal quality with passive interface on Ge substrates. The growth temperatures and growth rates are the important parameters which influence the ultimate epilayer quality. However, good quality epilayers with passive interface could be obtained with initial deposition of 1000 Å GaAs at 1 μm/h growth rate at 680°C and subsequent growth at 4 μm/h at

Acknowledgements

The authors are thankful for the help extended by Dr. Srinivasan for PL measurements and Dr. Ramjaypal for X-ray diffraction studies. The authors are also thankful to Prof. Vikram Kumar for his help and encouragement during the course of this work.

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