Browsing by Author "Cole, Eric D."

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    Electrical analysis of low energy argon ion bombarded GaAs
    Cole, Eric D. (Virginia Polytechnic Institute and State University, 1988)
    An electrical analysis was done on A1 and Au Schottky diodes fabricated on n-type (100) GaAs which had been bombarded with low energy Ar ions. The purpose of this study was to quantify electrically damage caused by the Ion Beam Etching (IBE) as functions of energy and fluence. Electrical studies included Deep Level Transient Spectroscopy (DLTS), Current-Voltage (I-V), Capacitance-Voltage (C-V), ConductanceVoltage (G-V), Capacitance-Temperature (C-T), and Activation Energy Analysis. These electrical measurements were carried out on GaAs which had been exposed to a variety of treatments after IBE (such as chemical etch removal) to determine damage depth. At the lowest energy studied, 0.5keV, Schottky reverse saturation currents (Isat) increased by over 4 orders of magnitude from the virgin case. The ideality factor, n, increased slightly while the breakdown voltage decreased. The most prominent changes occurred in the DLTS spectrum where it was observed that the native arsenic defect EL2 peak disappeared completely after ion etching. Concurrently a sharp increase in the diode conductivity with temperature was seen. It was found that chemical removal of 100Å of GaAs by chemical means could restore most of the diode parameters and the EL2 peak. It is proposed that the loss of EL2 is not related to a true physical reduction (i.e. an arsenic depletion) since calculations showed that the As loss would have extended beyond 3000Å for detectable DLTS changes. Also, the EL2 peak could be made to artificially disappear on a virgin sample with an external diode shunting resistor. The loss of the EL2 peak is, rather, attributed to a thin low resistivity surface layer having a partly amorphous nonstoichiometric crystal structure which can desensitize or mask the DLTS measurement. Surface chemical etch studies over the top of the Schottky diodes recovered 25% of the EL2 peak supporting this conclusion. Lower fluences had no effect at 0.5keV. Increasing ion bombardment energy showed a steady degradation in diode ideality factors. The reverse breakdown voltage increased past the unetched value and the DLTS spectrum began to show a very slight return of EL2. At 3keV the ideality factor was large, indicating the presence of a somewhat thicker high resistance layer. In fact recovery of diode parameters and EL2 did not occur until after 100Å removal. This was much deeper than expected at this energy, according to theory. Physical and lumped R-C electrical models are reported with an accompanying computer simulation of experimental DLTS results. The simulation used both thin low resistance and thick high resistance top layers to show that EL2 could be removed artificially. The models were also somewhat successful in explaining previously reported capacitance dispersion found in IBE GaAs.
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    On the feasibility and application of optical p to n inversion
    Cole, Eric D. (Virginia Tech, 1985-06-05)
    The feasibility of achieving carrier inversion of a properly doped crystal via optical excitation is studied. This process involves a host substrate doped with deep donors for n-type light characteristic and compensated by a shallow acceptor for p-type characteristic in the dark. This substrate is analyzed using well-known semiconductor equations. In addition conditions which must exist for carrier inversion are also specified. The solutions found are applied to a realistic set of dopants for illustrative purposes as well as indication of feasibility range. This inversion technique may possibly be used to generate bipolar junctions and thus devices. Other forms of photoconductivity are also qualitatively considered to supplement and extend the range of the inversion techniques applications. The processing of circuits using the developed concept offers possible interesting and useful advantages over existing techniques. The motivation for further research thus becomes obvious and is indeed the purpose of the thesis.