EFFECTS OF STRESS IN Ill-V SOLAR CELLS

C.L. Chu and G.I.C. Chen TECSTAR/ASD, City of Industry, California, USA

ABSTRACT

Planar, thermal stresses were applied to GalnP/GaAs/Ge cascade structures by bonding cells to Si or AI plates. After thorough evaluation, it was found that electrical properties of the solar cell were not affected by the stress. Using other characterization methods, the bandgap of the stressed cell was widened, and PL mapping showed improvement in the electronic quality of the sample. Triple-axis X-ray measurement did not resolve the relative variation in d-spacing of the stressed epi-layers, but lateral variation of the crystal was observed. These laboratory observations were in agreement with numerical simulation.

INTRODUCTION

Stress can affect the optical, electrical and mechanical properties of Ill-V solar cells. By varying epitaxial growth parameters or by applying external stress to GaAslGe and GalnP/GaAs/Ge solar cells, changes in lattice constants, bandgap energies, photoluminescence spectra, and electrical performance have been observed. From numerical simulation, the strain and stress induced by the external forces can be calculated. The focus of this paper is to identify and estimate the stress, and to determine the consequences of stress effects in these 111- V compound solar cells.

The conditions of stress in GalnP/GaAs and GaAs epi-layers grown on Ge substrates are determined by many factors. First, the intrinsic stress is defined by the lattice mismatch between the substrate and the subsequent layers, the impurity concentration in each layer, and the growth defects in the layers. For instance, the stress present in the GalnP layer can result from composition gradient. The alloy’s sensitivity to growth condition complicates precise control over the incorporation of Ga and In. The another factor is the different coefficients of thermal expansion of grown layers and the substrate. The epi-layers are certain to have some degree of residual stress mainly at the interface, as a result of cooling from growth to room temperatures. If the thin films grown are not lattice-matched, additional stress is generated. Moreover, the stress in the films must be balanced by an opposing stress in the substrate. The stress in the films determines the mechanical stability of the films and may influence the electrical properties of the device as well. Processing steps such as

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metalization, interconnection, and other celllpanel handling, where wafers are exposed to reasonably high temperatures, can induce stress caused by contrasting thermal properties of metals and semiconductors.

The generated stress field can affect the performance of solar cells, the fabrication yield, and ultimately the useful lifetime of satellites. For single junction Ill-V cells, stress does not appear to be a significant problem. For multi-junction cells however, different thermal effects will create physical obstacles at the interfaces, and the built-in stresses are likely to affect the performance of individual junctions, and the overall efficiency of the solar cell.

CHARACTERIZATION METHODS

Several characterization techniques were employed in evaluating grown wafers and solar cells. From triple axis X-ray diffraction in (004), compositional values of AlGaAs and GalnP were found. Mapping in the reciprocal space revealed additional tilting in the layers. Photoluminescence (PL) of the samples shed light on the bandgap energies, and on the fundamental recombination process in the semiconductors. The distribution of luminescence over the entire sample area revealed the uniformity in the epi-layer, providing information related to the curve-fill factor of the solar cell. From the quantum efficiency measurements, the bandgap was also derived and confirmed; the diffusion length of minority carriers was inferred. Key parameters of the solar cells, such as open-circuit voltage (Vac), short-circuit current (Isc), curve- f i l l factor (CFF), and efficiency, were measured under AM0 simulation, and compared with cell-modeling, and with data from previous runs. Analysis of the cell characteristics, combined with the other characterization tests, showed possible effects of stress on cell properties.

A triple axis X-ray diffractometer provides higher resolution and greater details than a double crystal system. A reciprocal space map obtained by a triple axis system provides a wealth of information. Scattering from various sources can be identified. And most importantly, strain or mismatch, a distortion in the o\28 direction, may be distinguished from tilt or mosaic spread, a distortion in the o direction.

Photoluminescence (PL) is the most simple and elegant technique that provides a wealth of information on lll-V semiconductors and their alloys. PL quickly characterizes the quality in terms of uniformity and explores the fundamental recombination process in the

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26th PVSC; Sept. 30-0ct. 3,1997; Anaheim, CA

semiconductor. At room temperature, only band-to-band recombination is observed. The line width of this recombination is inherently broad due to heavy doping, lattice mismatch, and random alloy composition fluctuations, especially in the GalnP layer.

Spectral response, the measurement of cell response as the wavelength of incident light is varied, can give important operation on the quality of the internal layers which make up the cell. For multi-junction cells, an external bias light is used with a monochromator, to allow measurement of each cell with narrow bandwidth light excitation. The bandgap energies of both junctions can then be derived from the individual cell response.

NASTRAN, a finite element program developed by NASA has been widely used to simulate thermal, mechanical, and dynamic problems in various industries. The stress field and distribution of solar heat within solar panels can also be calculated by this program.

In this work, NASTRAN was used to calculate the change of stress field within the grown layers bonded to aluminum or silicon plates. For simplicity in the experimental work, a fixed temperature difference AT = 200°C was used. A plane stress generated by the difference of thermal expansion coefficients of different materials was used to calculate the deformation occurring in the epi-layers, especially in the top cell, GalnP. A 22x30 mesh was generated for calculation. Although there may be the differences between the real sample and modeling, the calculated results and the in-situ stress should be in same order.

EXPERIMENT

In this work, Ill-V solar cells grown on Ge substrate were bonded to aluminum or silicon plates at -220°C. As a result of the cooling process to room temperature considerable amount of thermal stress, either compression or tension was generated in both GaAsIGe and GalnPIGaAslGe solar cells. Changes were observed in all of the following measurements: PL spectra, bandgap energies, I-V characteristics, and X-ray scans.

The thermally built-in stresses in these bonded structures were estimated using the NASTRAN program. For simplicity, a 2-D model was assigned to the thermally stressed sample. It was found that changes in the thickness of the bonding material had a profound impact on the stresses in the epi-layers. This simulation may help in selection of cell design and bonding materials to reduce stress introduction during solar cell fabrication and panel preparation.

RESULTS AND DISCUSSION

NASTRAN Simulation

significant deformation in the crystal. The GaAs layers also were subjected to stresses of same magnitude. Since the elastic constants of GalnP and GaAs are close, the strains generated in these layers should be similar. The generated strain in the sample has changed the curvature in the vertical direction, as confirmed by the reciprocal space map.

High resolution X-ray diffraction

The acquired reciprocal space maps indicated relatively no distortion in 13\20 direction, while the o direction was significantly elongated (see Figure I). Values for the spread of scattering are summarized in Table 1. The constant 13\20 values suggest that there is no change in the relative d-spacing between all layers including the substrate. Both the curvature and mosaicity would cause broadening in the 13 direction without affecting the 01\20 direction.[2,3] For curved samples, the substrate and layers are elongated in Q direction; for mosaic layers, the substrate remains relatively unaffected. Since all layers including the substrate showed distortion in the OI direction, the mounting and cooling process has induced an overall curvature in the specimen.

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The GalnP cells mounted on Aluminum plates appeared to be compressed by a 0.5 kBar, and cells on Silicon plates showed a tension of 0.2 kBar. These stresses can generate a detectable strain but not a

Fig.1 Reciprocal space maps of (a) before and (b) after bonding. The w and 0\20 are reported in reciprocal lattice units, q,,, and q,,,, respectively.

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TEM or other techniques can measure the exact dimensions of the lattice and may reveal further variation in epi-layers resulted from the thermal effects. The lattice mismatch observed by X-ray diffraction was mainly caused by the variation of stoichiometry or other lattice effects, such as formation of mosaic or superlattice structures. The possible effects of solar cell manufacturing and panel fabrication on electrical performance are of more importance.

I-V testing AND Photoluminescence

Six cascade solar cells of efficiency greater than 23.6% (AMO) were used for stress evaluation. They were bonded to Si or AI plates and tested using a pulsed solar simulator. The results are listed in Table 2. No significant difference was observed between the bonding experiments on Si and AI. From the spectral response measurements, the bandgap energies of these cells were estimated before and after bonding. For both compressed and elongated cells, a 0.005 eV to 0.007 eV increase in bandgap was observed. This shifting of the bandgaps, however, was not consistent with the PL spectra. The mapping of the PL across the wafers has indicated a small increase, as much as 2 nm, in the average wavelength in both the GalnP and GaAs layers. Further studies are in progress to find the relationship between electrical and optical measurements of the bandgap.

CONCLUSIONS

This work has begun to evaluate the degree of stress in the grown layers of cascade solar cells, and has also increased understanding of the effects of stress in solar cells. Several highlights of the study can be listed as follows:

1. The thermal stress (AT = 200°C) generated by the difference of thermal expansion coefficients between cascade cells and AI or Si mounting plates did not affect the electrical performance of cascade solar cells. Temperatures are elevated during solar panel fabrication and thermal stresses are generated by the differential thermal expansion coefficients of solar cells and panels. The actual assembly temperature ranges are lower than the stresses experimentally applied to the solar cells in this work. This shows that the existing panel assembly technology can adapt well to the cascade panel preparation.

2. Planar thermal stresses do not significantly affect the relative difference of d-spacings between GalnP, GaAs, AIGaAs, and Ge. However, the effect of lateral variation (parallel to the epi-layer) was detected in the form of curvature by reciprocal space map. For cascade cells of different structure grown on Ge substrates of different orientation, study of the stress effects may help decide on the best electrical andlor mechanical structure for high efficiency cascade cell.

3. The residual thermal stresses in epi-layers are not easily separated from the stress effects caused by lattice mismatch. Further studies on thermal effects may provide a quantitative understanding of the stress effects within epi-layers.

REFERENCE

[I] J. Blakemore, Gallium Arsenide, (American Institute of Physics, 1987). [2] P.F. Fewster, Appl. Surf. Sci. 50,1991, pp. 9-18 [3] P.F. Fewster, J. Appl. Cryst. 24, 1991, pp.178-183.

Table 2: Efficiency of Cascade Solar Cells (Pre-Bonding and Post-Bonding)

Pre-Bonding Test: AMO, 28OC, Modified Dual-Source Simulator Post-Bonding Test: LAPSS Test, Calibrated by 1997 Balloon Flight Standard The number in parenthesis is the band gap of the cell.

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