Studies on a-Se/n-Si and a-Te/n-Si heterojunctions

Abstract

Heterojunctions are fabricated by depositing amorphous selenium (a-Se) and amorphous tellurium (a-Te) films on n-type single (n-Si) wafers by the method of vacuum evaporation. The silicon wafers have surface orientation of (111). Resistivity of each silicon wafer is 5Ω-cm and carrier concentration of 8.30 x 10 14 cm -3 . Two of the junction devices are annealed in a vacuum for half an hour. Current-voltage measurements are made at room temperature (298K). Rectification properties are observed in all the junctions. Barrier heights of a-Se/n-Si junctions are higher than a-Te/n-Si junctions. The current density in annealed junctions is lower than in as-deposited (unannealed) counterpart. @JASEM Heterojunction devices of the metal-semiconductor, semiconductor-semiconductor and amorphous semiconductor-crystalline semiconductor types have gained considerable interest from researchers both from a fundamental physics and technological viewpoint. For example, Chandra and Prasad, 1983 found that metal-semiconductor contacts (or Schottky barriers) can be used as low temperature sensors. Cross, 1992; Xiao and Whitefield, 1992 have indicated that solar cells based on silicon (Si) and Gallium Arsenide (GaAs) semiconductors can provide power for satellites and water pumping systems. Study of Mimura and Hatanaka (1987) showed that hydrogenated amorphous silicon- crystalline silicon hererojunction has application to imaging devices. Kentaro and Nakazawa, 1988; Lovejoy, 1992 have also stated that amorphous and polycrystalline thin film semiconductors have possible application in optoelectronics. Heterojuncton devices have also been studied from the viewpoint of understanding fundamental device physics. Such studies include band structure, current transport mechanism and as tools in the analysis of other physical parameters. For example, Sinha and Misra, 1983 studied the current-voltage characteristics of the junction between amorphous germanium and monocrystal p-Si at different temperatures. The authors found that the junction exhibits almost ideal Schottky diode behaviour in a narrow region of applied bias. However, the junction current at large external bias (V>300mV) is dominated by tunneling. Amorphous Selenium (aSe) and amorphous tellurium (aTe) may be obtained by thermal evaporation of materials in a vacuum. According to Kazmerski, 1980 amorphous thin films posses a large density of dangling bonds, which defy doping attempts because dopants attach themselves to these bonds. The purpose of this study is to measure the electrical properties of thermally evaporated amorphous selenium and tellurium films on n-type crystalline silicon substrates and assess the junctions for possible device application.