Development of Radiation Detectors Based on Hydrogenated Amorphous Silicon and its Alloys.

Hydrogenated amorphous silicon and related materials have been applied to radiation detectors, utilizing their good radiation resistance and the feasibility of making deposits over a large area at low cost. Effects of deposition parameters on various material properties of a-Si:H have been studied to produce a material satisfying the requirements for specific detection application. Thick(~50 mu m), device quality a-Si:H p-i-n diodes for direct detection of minimum ionizing particles have been prepared with low internal stress by a combination of low temperature growth, He-dilution of silane, and post annealing. The structure of the new film contained voids and tiny crystalline inclusions and was different from the one observed in conventional a-Si:H. Deposition on patterned substrates was attempted as an alternative to controlling deposition parameters to minimize substrate bending and delamination of thick a-Si:H films. Growth on an inversed-pyramid pattern reduced the substrate bending by a factor of 3 ~ 4 for the same thickness film. Thin (0.1-0.2 μm) films of a-Si:H and a-Si:C:H have been applied to microstrip gas chambers to control gain instabilities due to charges on the substrate. Light sensitivity of the a-Si:H sheet resistance was minimized and the surface resistivity was successfully controlled in the range of 10^ {12}~ 10^{17} Omega/ square by carbon alloying and boron doping. Performance of the detectors with boron-doped a-Si:C:H layers was comparable to that of electronic-conducting glass. Hydrogen dilution of silane has been explored to improve electrical transport properties of a-Si:H material for high speed photo-detectors and TFT applications. Various electrical properties of the hydrogen-diluted a-Si:H showed factors of 3 ~ 10 improvements at the expense of increased residual stress. The optimal material characteristics for TFT application appeared at deposition conditions corresponding to the onset of crystalline formation, indicating that the enhancement of the electrical properties is due to the superior amorphous matrix not to the presence of crystalline phase. Material properties of the a-Si:H based materials can be easily adapted to the characteristics required for individual applications by modifying fabrication conditions and procedures. This asset, in addition to the advantages of large area capability and radiation hardness, makes a-Si:H an attractive candidate for radiation detection applications.