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ABSTRACT We report on the growth and properties of a-(Si,Ge):H films and p-i-n solar cell devices prepared using a remote, low pressure ECR plasma deposition technique. The films and devices were prepared using either He or H2 as the diluent gas. The plasma conditions were controlled so as to induce significant ion bombardment during growth. We find that there is a dramatic influence of plasma chemistry on the growth and properties of a-(Si,Ge):H films and devices. In particular, with hydrogen as the diluent gas, changing the pressure in the reactor dramatically changes both the Germanium incorporation in the film, and the electronic properties. Lower pressures lead to less Ge being incorporated, and higher mobility-lifetime product for holes for a given Tauc gap, as well as better p-i-n devices. In contrast, changing the pressure when He is the diluent gas does not produce such large changes. We speculate that the changes in device and film properties are due to the influence of ion bombardment on growth chemistry, and that both efficient energy and momentum transfer to the growing surface are necessary to achieve the best devices. The differences between He and hydrogen may simply be due to the fact that He plasma is much more energetic than a comparable hydrogen plasma, and there is more efficient momentum transfer when He is used as compared to when hydrogen is used. We have also produced very good single junction a-(Si,Ge) devices using the ECR technique. INTRODUCTION A-(Si,Ge) is an important electronic material for IR sensors and for photovoltaic devices. Extensive work has been done to improve the material properties of a-(Si,Ge):H.[1-4]. However, to date, the properties of the best a-(Si,Ge):H remain inferior to the properties of best a-Si:H. Mobility-lifetime products for both electrons and holes are lower, and subgap defect densities and Urbach energies for tail states higher in a-(Si,Ge):H than in a-Si:H. The problem is particularly severe for alloys whose composition is about 50%Si-50%Ge. Various techniques, such as high hydrogen dilution[1,2], ion bombardment[3] and ECR growth[4] have been tried to improve the material properties for alloys in this mid-range, with varying degrees of success. It is also found that the best a-(Si,Ge):H is generally made at higher temperatures than for a-Si:H. It is postulated that one of the reasons for the poorer properties of a-(Si,Ge):H alloys is that during growth, the germyl radical is less mobile than the silyl radical; consequently, it diffuses less, and as a result, an excessive density of voids or clustered Ge-Ge bonds may form in the material. In this paper, we show that controlled ion bombardment from either a H or a He plasma, generated by an ECR plasma reactor, can be used to improve the material properties of a(Si,Ge):H alloys. In particular, the use of a high bombardment, low pressure plasma seems to be beneficial in improving the material and device properties.

441 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society

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