Wide Band Gap a-Si:H Based Ihgh Gain Vidicon Devices Prepared by Chemical Anniealing
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ABSTRACT Previously it was shown that high quality wide gap hydrogenated amorphous silicon material could be prepared by a layer-by-layer technique involving hydrogen chemical annealing. Using this wide gap material, high electric field, n-i-p diode devices were fabricated. Reverse bias dark current was suppressed by optimization of the n-layer doping level (250ppm) and the thickness (2000A). Working vidicon type device were prepared, tested, and optimized by further reduction in the high reverse bias leakage current. Vidicon devices showed very promising performance; however, at the present stage of development some point defects were observable at the highest reverse bias
voltages probed
('-6x10
5
V/cm).
INTRODUCTION Recently, we reported that a Chemical Annealing technique involving a layer-by-layer process could be used to prepare high quality wide gap a-Si:H films [1,2,3]. This layerby-layer process employed the deposition of thin a-Si:H filmns (< 200 A) followed by an atomic hydrogen treatment. By repeated this process many times homogeneous thick amorphous wide gap silicon material could be prepared. The optical band gap of these materials could be selected from 1.8 eV to 2.1 eV by control of the substrate temperature, the deposition thickness (per cycle), and the hydrogen treatment time. Raman spectra analysis indicate the optical band gap was independent of the short range silicon-silicon order such as bond angle distribution. Neither Urbach energy nor the Tauc plot slope ofthe chemical annealed films were functions of the optical band gap. The defect densities estimated from CPM (Constant Photocurrent Method) were only _101 5 /Cm3 in the annealed state. These materials shows high photoconductivity, high g'r product, improved light soaking stability (compared to standard state ofthe art 1.7 eV band gap a-Si:H), and
very low dark conductivity
(< 10 12 S/CM)
consistent with a wide band gap intrinsic
357
Mat. Ries. Soc. Syrup. Proc. Vol. 507 ©01998 Materials
Riesearch Society
semiconductor. These properties suggested that the chemical technique could be applied to the fabrication of high performance vidicon imaging devices that employ large electric fields. Previously, Takasaki et al. reported Avalanche multiplication of photo-generated carriers in a-Se materials and imaging devices[4]. In that case, quantum efficiencies over 10 were attained at electric fields over 106 V/cm and the fabrication of high sensitivity image tubes was also reported demonstrating that avalanche multiplication can occur in an amorphous material. While, amorphous silicon has many characteristics suitable for the vidicon application including higher thermal stability than a-Se, avalanche multiplication in a-Si:H has not yet been clearly demonstrated. Recently, two groups (Sawada et. al. [5] and Miyoshi et.al. [6] reported evidence of a-Si:H Avalanche multiplication; however, these reports are still unconfirmed. To achieve the Avalanche multiplication in a-Si:H high electric field devices (>106 V/cm) [7] are required. To a
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