Optimization of Schottky Diode Submillimeter-Wave Mixer Elements via Relief of Interfacial Stress.

The detection of millimeter and submillimeter radiation, critical to many areas of research physics and communications, relies on low-noise mixer elements, such as the GaAs Schottky diode, to convert received signals to lower frequencies at which amplification is possible. Although research at the UVa Semiconductor Device Laboratory has resulted in devices which exhibit record-low noise temperatures, the work up to this time had not addressed the significant mechanical stresses inherent in the present devices. Preliminary observations had established a definite link between the stress and degraded reverse I(V) characteristics in the devices, and indicated that the stress might also affect the device noise characteristics. The goals of this research were to (a) determine experimentally the relationship between the mechanical stress and the noise performance of the diodes, and (b) use such knowledge to design a fabrication technology which yields devices with significantly improved noise characteristics. The mechanical stresses are largely due to the thin SiO_2 film which defines the individual diodes. The stress in uniform SiO_2 films is directly measured, and the stress distribution in the GaAs near a circular window in the stressed overlayer is derived. The calculated stress distributions are used as the basis for several models in an attempt to explain theoretically the relationship between the oxide-induced mechanical stresses and the diode noise characteristics which has been suggested by experimental observations. The effects of piezoelectrically induced charge, piezoelectric scattering, and deformation potential scattering are all considered. Modifications to the device structure which act only to reduce the mechanical stress are shown to significantly reduce device noise, and provide the first proof of the existance of stress-induced diode noise. Results presented indicate that similar stress-sensitive mechanisms may be responsible for nonidealities in both reverse I(V) and noise characteristics. Initial research on a new fabrication technology which obviates the need for the stressed SiO_2 film is introduced, and its feasibility is demonstrated. Additionally, a cause of unstable I(V) characteristics in standard devices is determined from this portion of the work.