Investigation of gallium-arsenide solar cell structures using deep level transient spectroscopy

David Philip Rancour, Purdue University

Abstract

The isothermal current-DLTS technique was successfully applied to a 0.5cm $\times$ 0.5cm GaAs solar cell, whereas previous attempts to apply conventional thermally-scanned capacitance-DLTS to the same cell were unsuccessful. The use of current observations circumvented the problem of excessively large capacitance values, permitting DLTS measurements to be performed directly on solar cells. The benefits inherent in the new procedures hold for both current and capacitance transient measurements. In either case the isothermal analysis yields more accurate estimates for thermal emission coefficients. In addition, isothermal-DLTS measurements eliminate analysis errors caused by unaccounted for temperature dependencies, avoid the problem of temperature hysteresis, facilitate averaging of many transients for noise reduction, and enable rapid data collection. The additional development of the universal isothermal-DLTS curve, introduced herein, makes the isothermal DLTS technique that much more attractive. All normalized spectra derived from both current and capacitance transients should conform to the temperature-independent universal curve. Therefore a significant departure from the universal curve is a direct and immediate indication of a non-exponential transient. The conventional DLTS approach has no such test for non-exponential transients. The results of IV/DLTS characterization indicated that edge effects all but dominated the I-V characteristics of small area GaAs mesa diodes. For high quality films perimeter recombination current appeared to be greater in magnitude than internal SRH recombination current in the SCR. Moreover, shunt leakage current was also shown to be a perimeter effect. Deep level concentrations were greatly reduced when an AlGaAs buffer layer was used. It is suspected that the AlGaAs layer trapped impurities which were diffusing from the substrate, and/or gettered impurities from the subsequent GaAs layers during film growth. The reduced impurity concentrations in turn caused a reduction in the 2kT current. The results indicate that there is at least an indirect correlation between J$\sb{02}$ and the deep level traps. When overall trap densities are reduced, J$\sb{02}$ is brought down as well.

Degree

Ph.D.

Advisors

Pierret, Purdue University.

Subject Area

Electrical engineering

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