Investigation of the suitability of amorphous semiconductors as sensors for optical process tomography
In this work, the suitability of amorphous semiconductors as hard field optical sensors for application in optical process tomography (OPT) has been established. Two amorphous semiconductors were selected for the study, these being amorphous arsenic triselenide (a-As 2 Se3) and hydrogenated amorphous silicon (a-Si:H). The a-As2Se3 device was a single layered structure of 60|Um thickness fabricated upon a 2mm thick cylindrical aluminium substrate. The a-Si:H device was a multi-layered structure of 27.1fim overall thickness fabricated upon a 4mm thick cylindrical aluminium substrate. 20mm 2 samples were cut from the cylinders, their surface being left free for a xerographic investigation. For a tomographic investigation, semitransparent gold (Au) contacts were sputtered onto the surface of the devices to produce single contacts or contact arrays. The study comprised of the two fields of xerography and tomography. The xerographic study comprised of the measurement of such parameters as charge acceptance, dark decay, residual potential, and photoinduced discharge. The research project has concurred with other workers in that the dark discharge mechanism in a-As 2 Se 3 proceeds via a xerographic depletion discharge process, and a Poole-Frenkel type emission in a-Si:H. The tomographic investigation involved the study of such parameters as detectivity, responsivity, steady state photocurrent, and photoinduced fatigue. Detectivity has found to be dependant upon the magnitude of applied electric field and level of incident irradiance. Irradiance in the order of 3.45mW/cm 2 to 9.57mW/cm 2 for a- fj f\ As 2 Se3 and 5.31mW/cm to 28.32mW/cm for a-Si:H was required in order to produce a clean and repeatable photogenerated current pulse over the range of electric fields specified (0.66xl05 V/cm to 1.66xl05 V/cm). The production of steady state current has found to be dependant upon the magnitude of electric field, the level of irradiance, and the illumination period. Irradiance of 319mW/cm 2 to 1.46W/cm2 with an illumination period of 520ns was required to produce steady state photocurrent in a-As2Se3 , and 693mW/cm2 to 2.62W/cm 2 with an illumination period of 880ns for a-Si:H. A linear relationship between electric field and responsivity has been observed in both materials over a range of irradiance of 3.45mW/cm 2 to 9.57mW/cm2 . Responsivity in the order of 87.86|LiAAV to 145.19|iA/W for a-As2Se 3 and 14.19(iAAV to 103.81)J,AAV for a-Si:H has been demonstrated. An investigation as to the effects of photoinduced fatigue in both a-As 2 Se3 and a-Si:H has been carried out by the application of pulsed visible light of various flash repetition rate (FRR) under a constant high electric field over a 30 minute illumination period. It has been shown that the rate of fatigue is dependant upon the material, time, electric field, light intensity, and FRR. A maximum operating speed of 20Hz has been determined for a-As 2Se 3 and lOOHz for a-Si:H. The maximum operating speed of 20Hz for a-As 2 Se 3 was deemed unsuitable for OPT application and the a-As 2 Se 3 material was eliminated from further tomographic investigation. Tomographic prototypes were employed to establish the a-Si:H devices ability to produce qualitative and quantitative data. The results of this investigation demonstrated that 1mm changes in water level and 0.5% changes in fluid colour could be accurately determined by the a-Si:H device at speeds required for OPT. The use of an a-Si:H device containing a Au contact array facilitated the imaging of the curvature of a pipeline and a phantom object contained within the pipeline. The results of the overall investigation have confirmed that the a-Si:H device is suitable for application as a hard field optical sensor for OPT.