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Investigation of dopant profiles from capacitance-voltage measurements on Schottky diodes

University of British Columbia

Measurement of the differential capacitance (C) of a Schottky diode as a function of voltage (V) is widely used to probe dopant profiles in semiconductors. However, the theory of the dopant profiling method is based on an approximation, and does not work well when the dopant concentration changes rapidly with distance. The region beyond the maximum of an implanted Gaussian profile is of particular interest in connection with ingot qualification tests for GaAs, and it is just there that the problem is the most serious. In this thesis, an investigation was made by numerical simulation on problems associated with the profiling method. Programs were written to calculate the differential capacitance-voltage relation for GaAs Schottky diodes with and without deep energy levels, and with a specified dopant distribution. The programs predict what the approximation method would indicate for the dopant profiles according to a set of canonical equations used in the profiling method. The predicted and the specified dopant profiles were then compared. Mainly ion-implanted dopant profiles in semiconductors were studied although doped epitaxial layers were also considered. For ion-implanted GaAs, the predicted dopant profiles were found to be about 10% lower near the peak region than the true dopant profiles, and the predicted profiles were confirmed to be too high in the tail region. For doped epitaxial layers, the predicted profile was found, in some cases, to give good estimates for the dopant concentrations on the high and low sides of the true step profile, but in some others, the predicted profiles were found to be totally misleading. For GaAs with deep levels, a method of calculating the differential capacitance was developed to take into account the fact that the deep levels do not respond to the 1 MHz a.c. signal normally used in the C(V) measurements. It is believed to simulate the experimental C(V) measurements more realistically. The tail sections of the predicted profiles were found to increase with the concentration of background shallow donor atoms in the deep-level-free semiconductor before ion-implantation, and with the number of impurity atoms which are channelled or diffused to the region during or after ion-implantation. This implies that although the profiling method is erroneous in the tail section, it can nevertheless be used on a comparative basis to indicate the level of background shallow dopant concentration, and the degree of channelling or diffusion. The effects of the substrate parameters in liquid encapsulated Czochralski (LEC) GaAs, which include the concentrations of EL2, net shallow acceptors, and sometimes Cr, have been investigated on the predicted dopant profiles for ion-implanted samples. Increases in Cr and net shallow acceptor concentrations were found to increase the steepness of the predicted dopant profile, while an increase in EL2 concentration has little effect. A method of estimating dopant activation efficiency in GaAs has been proposed. This method uses the author's second program to avoid underestimations of the activation efficiency in GaAs caused by the peak lowering in the predicted dopant profiles. The concept of Debye length in semi-insulating LEC GaAs was also discussed. The Debye length given by the standard formula for semiconductors with shallow donors and acceptors can become inapplicable when deep levels are present.

Text

2010-11-17

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http://hdl.handle.net/2429/29997

Electrical and Computer Engineering

University of British Columbia

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