DESIGN AND CONSTRUCTION OF A COMPUTER AIDED CAPACITANCE TRANSIENT SPECTROMETER FOR THE STUDY OF DEEP DEFECT CENTRES IN SEMICONDUCTORS

Abstract

The object of the present work has been to design and construct a computer aided transient capacitance spectrometric system for detection and characterisation of deep defect centres in Semiconductor. It is hoped that the spectrometer would be able to detect, identify and characterise the deep defect centres present in semiconductor devices. It is known that the presence of impurities or lattice defects in a semiconductor crystal creates deep defect centres. The defect states are located deep inside the forbidden zone, which may act either as <■_ «electron/hole traps or recombination centres. So these centres influence the electrical and optical properties of a semiconductor. The identification and study of their characteristics and kinetics would facilitate the understanding of physics of the semiconductor and its devices. The basic idea of a DLTS spectrometer is to implement a “rate window”. There are two methods of implementing the rate window: by the use of (i) a double box car averager or (ii) a Lock-in-amplifier. In the present work we have implemented the rate window with the computer aided electronic circuitry based on the basic principle of box car averager. DLTS spectrometer system is a complicated experimental setup and needs sophisticated instrumentation. To develop our spectrometric system we have used a 710 ACER computer. The main component i.e. the heart of our system is a rapid data acquisition system peculiar to our need and purpose which would collect and store data, for subsequent manipulation, record temperature of the system, and capacitance from a capacitance meter. For the system we need various electronics circuitry including S/H circuit^unity gain amplifier, base line restorer, and pulse amplifier. All these units have been developed and constructed in the laboratory. Other experimental devices and setups such as cryostatic set up, sample assembly etc. have been constructed in the laboratory. Interfacing between the analog world and the computer has been made via interfacing unit, the Keithley made 570 system. The variable width pulse generator and the capacitance meter formp other two major component^ our system. The capacitance meter operates at 1MHz. The necessary softwares needed to run the constructed spectrometer based on BASIC High Level language together with a special software Soft500 have been developed to suit our environment and need. In order to test the performance of our spectrometer system we have used hardware based laboratoiy made artificial and software based computer simulated DLTS signals. A few real samples i.e. Si p-n junctions have been examined and DLTS signals could be detected