Microcrystalline Silicon Prepared by VHF-GD: Structure, Transport and Optical Properties
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MICROCRYSTALLINE SILICON PREPARED BY VHF-GD: STRUCTURE, TRANSPORT AND OPTICAL PROPERTIES, F. Finger*, R. Carius*, P. Hapke*, K. Prasad** and R. Fluickiger**
* Forschungszentrum Jtilich, Institut fir Schicht- und Ionentechnik, D-5170 Jilich, Germany
** Institut de Microtechnique, Universit6 de NeuchiteI, CH-2000 Neuchitel, Switzerland ABSTRACT Doped and compensated microcrystalline silicon prepared by Very High Frequency Glow Discharge has been investigated by optical spectroscopy and by temperature dependence of the electrical conductivity. Raman spectroscopy confirms the microcrystalline nature of all samples with only small changes of the crystalline volume fraction upon doping. Strong absorption in the infrared region, which correlates with the conductivity, is attributed to free carrier absorption. As a function of temperature the conductivity of all samples shows a deviation from a purely activated behaviour. Consequences of this observation for transport mechanisms are discussed. INTRODUCTION The use of doped microcrystalline silicon (gc-Si:H) layers in amorphous silicon (a-Si:H) based solar cells requires a careful optimization and understanding of the optoelectronic properties and the band structure of this material. At present a clear picture is still missing. Even one of the most fundamental properties - the temperature dependence of dark conductivity - is not understood. Various reports on conductivity of pc-Si:H have been published, usually covering only a small temperature range, where - trivial enough - a plot of conductivity vs. 1/T always can be approximated by a straight line. The interpretations have been in terms of an activated transport across grain boundary potential barriers similar to the transport in polysilicon [1]. Other studies have found two different activation energies, associated with two different competing current paths [2]. A deviation from a purely activated transport is found when a wider temperature range is covered [3,4]. The authors of ref. [3] propose a T- 114 characteristic typical for variable range hopping transport. The understanding is not much better for the band structure of pc-Si:H compounds. The picture that emerges from structural investigations shows pc-Si:H as a composite material of (1) crystalline grains, (2) grain boundaries and (3) amorphous phase. The band structure should change correspondingly with changes of the material "fine-"structure. Additional effects are to be expected from the fact that typically the dimensions of the grains are of the order of 100 A only. So far only ad-hoc assumptions on a detailed band structure exist. An important test for any band structure model should be if it matches with the transport characteristics of the material. In the present study we will present some more results on the properties of jic-Si:H that demonstrate the need for a further thorough treatment of the theoretical aspects of the gc-Si:H system. We have chosen a set of samples where the conductivity was varied over a wide range by doping and compensation. SA
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