matching unless the GaAs PN junction current was significantly decreased. This option is self-defeating, because the efficiency of the cascade cell derives mostly from the GaAs cell. Parallel work on cascade cells consisting of an optimized Ge PN junction cell, connected by a tunnel diode to a top cell had led to selection of an AlGaAs top cell where the Al content was 8-10%, to ensure current matching for both cells. Even if current matching was possible, growth on Ge with no intentional junction would require control of surface states and/or impurity inter-diffusion at the GaAs/Ge interface during the growth of the GaAs layers. Also, retention of the active-Ge structure would require adjustment in users' simulators, and in cell testing correlation. For these reasons, ASEC decided to pursue a passive-Ge structure, where the additional photovoltage at the GaAs/Ge interface is suppressed. There are possible methods to achieve passivation after layer growth (involving buried, shorting contacts or deep ion implantation) but these would make processing more difficult and could introduce other problems. Tests were made to change the MOCVD growth conditions especially for the early GaAs layers formed near the Ge surface. In effect, the ‘optimized' procedures developed in the previous work were reversed. At the time, it was not known if it would still be possible to grow high quality GaAs layers under these modified conditions.
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