John M. Baker

Characterization of photochemically unpinned GaAs

The unpinning of the etched GaAs(100) surface has recently been reported by both photowashing and by Na2S⋅9H2O treatments. Both techniques hold promise of elucidating the mechanism by which GaAs passivation is achieved. We have identified as part of the photowashing process a separate photoactivation step. The activation step did not become apparent until we modified the photowash process in order to minimize light exposure when flowing water was not applied to the substrate. The degree of unpinning produced by this process has been found to be related to the thickness of this oxide, the presence of oxygen and water vapor and the integrated light flux incident on the sample. Photoluminescence experiments clearly show that a photoactivation step involving water vapor is required to achieve the flat‐band condition. This process is relatively insensitive to the surface treatment prior to the photowash. We have observed similar photoactivation and insensitivity to surface treatment on GaAs coated with Na2S⋅9H...

Oxide growth in an rf plasma

A mechanism is considered for oxide growth on an electrically isolated metal sample in the presence of an rf‐excited oxygen plasma. The assumption is made that the rate of rf oxidation is limited by transport of ionic species through the already‐formed oxide layer. Thermally activated hopping of ionic defects in the presence of electric fields due to the surface potential established by the rf discharge and modified by the space charge of the mobile ionic defects is hypothesized. The origin of the voltage across the oxide is discussed in terms of a balance between the transport of negatively charged O ions and transport of electron holes created by ion neutralization of positive ions from the plasma. This model is developed analytically and evaluated numerically by employing the continuum limit of hopping transport equations valid for the very‐high‐field limit. A three parameter fit gives excellent agreement of the theory with the published data of Greiner for the rf oxidation of lead. The fitting paramet...

Tunnel barriers on Pb–In–Au alloy films

Tunnel barrier oxides consisting primarily of In2O3 formed on Pb–In–Au alloys by various thermal and rf‐plasma oxidation techniques were characterized in situ using ellipsometry and Auger Electron Spectroscopy (AES). The results were compared to measurements of the Josephson critical current density j1 and the specific capacitance of tunnel junctions fabricated on the same wafers. It was found for junctions with oxides consisting entirely of In2O3 that j1 exceeded 3 KA/cm2 and did not depend strongly on oxide thickness from about 3 nm (for thermally‐grown oxides) to 4.5–7 nm (for rf‐grown oxides). The addition of a thin layer of PbOx at the top of the barrier, either by backscattering during rf oxidation or by deposition onto a thermal oxide, caused by a substantial decrease in j1, while the presence of PbO in the bulk of the oxide had little effect. The specific capacitance of the lower current density junctions was essentially the same for either 3‐ or 4.5–6‐nm thick oxides and was consistent with a bar...

Structure and chemical composition of water‐grown oxides of GaAs

Previously it has been shown that the electronic surface properties of GaAs can be improved by photochemical treatment in water. If this photowashing technique is carried out with intense white light, oxides several hundred angstroms thick can be grown. This paper reports the structure and composition of this photowashed oxide and one grown by soaking in stagnant water in low light. The oxide was determined by TEM cross sections to be highly porous, but with thin continuous oxide layers both at the surface and at the oxide/GaAs interface. The oxide is composed of Ga2O3 with a low concentration of As2O3. The layer is primarily a fine grain Ga oxide crystal with a structure which appears different from the common forms of Ga2O3.