Carol I. H. Ashby

Advances in selective wet oxidation of AlGaAs alloys

We review the chemistry, microstructure, and processing of buried oxides converted from AlGaAs layers using wet oxidation. Hydrogen is shown to have a central role in the oxidation reaction as the oxidizing agent and to reduce the intermediate predict As/sub 2/O/sub 3/ to As. The stable oxide is amorphous (Al/sub x/Ga/sub 1-x/)/sub 2/O/sub 3/ which has no defects along the oxide/semiconductor interfaces but can exhibit strain at the oxide terminus due to volume shrinkage. The influence of gas flow, gas composition, temperature, Al-content, and layer thickness on the oxidation rate are characterized to establish a reproducible process. Linear oxidation rates with Arrhenius activation energies which strongly depend upon AlAs mole fraction are found. The latter produces strong oxidation selectivity between AlGaAs layers with slightly differing Al-content. Oxidation selectivity to thickness is also shown for layer thickness <60 nm. Differences between the properties of buried oxides converted from AlGaAs and AlAs layers and the impact on selectively oxidized vertical cavity laser lifetime are reported.

Low-dislocation-density GaN from a single growth on a textured substrate

The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates is typically greater than 10{sup 9}/cm{sup 2}. Such high dislocation densities degrade both the electronic and photonic properties of the material. The density of dislocations can be decreased by orders of magnitude using cantilever epitaxy (CE), which employs prepatterned sapphire substrates to provide reduced-dimension mesa regions for nucleation and etched trenches between them for suspended lateral growth of GaN or AlGaN. The substrate is prepatterned with narrow lines and etched to a depth that permits coalescence of laterally growing III-N nucleated on the mesa surfaces before vertical growth fills the etched trench. Low dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions.

Photochemical dry etching of GaAs

GaAs exhibits greatly enhanced reactivity with gas‐phase reactive Cl species when the surface is irradiated with low intensity laser light having a frequency which can excite above the band gap of GaAs. This laser‐induced reactivity is shown to be photochemical rather than thermal in origin. This is the first reported observation of a purely photochemical dry etching process for a III‐V semiconductor material.

Wet oxidation of AlxGa1−xAs: Temporal evolution of composition and microstructure and the implications for metal-insulator-semiconductor applications

Three important processes dominate the wet thermal oxidation of AlxGa1−xAs on GaAs: (1) oxidation of Al and Ga in the AlxGa1−xAs alloy to form an amorphous oxide, (2) formation and elimination of crystalline and amorphous elemental As and of amorphous As2O3, and (3) crystallization of the amorphous oxide film. Residual As can lead to strong Fermi-level pinning at the oxidized AlGaAs/GaAs interface, up to a 100-fold increase in leakage current, and a 30% increase in the dielectric constant of the oxide layer. Thermodynamically favored interfacial As may impose a fundamental limitation on the use of AlGaAs wet oxidation in metal-insulatorsemiconductor devices in the GaAs material system.

Wet oxidation of AlGaAs: The role of hydrogen

Wet oxidation of AlGaAs to form Al2O3 by the reduction of H+ from water to H produces intermediate As2O3. Reduction of As2O3 by H to elemental As enables the escape of arsenic from the oxidized film. Further reduction of As to AsH3 can provide another volatile As species. Formation of intermediate As is problematic for the use of wet oxidation in metal-insulator-semiconductor applications. The kinetic balance between As2O3 formation and As escape can explain the transition between the linear and parabolic time dependence of the wet oxidation of buried AlGaAs layers. The near-total suppression of wet oxidation by O2 is attributed to the suppression of volatile product formation through consumption of atomic hydrogen by reaction with O2 to form H2O in preference to the hydrogen reduction of As2O3.

Wet thermal oxidation of AlAsSb lattice matched to InP for optoelectronic applications

We demonstrate wet thermal oxidation of an AlAsSb layer lattice matched to an InP substrate. Oxidation in an InGaAs/AlAsSb/InGaAs structure proceeds in a lateral direction, producing an oxide layer embedded between two layers of InGaAs. Auger analysis and Raman spectroscopy indicate conversion of the AlAsSb into an aluminum oxide with an elemental antimony layer at the top oxide‐InGaAs interface. Scanning electron microscope cross‐sectional views of partially and fully oxidized samples are also presented.

Minimizing threading dislocations by redirection during cantilever epitaxial growth of GaN

A 40-fold reduction in density of vertical threading dislocations (VTDs) at the surface of GaN is obtained with cantilever epitaxy by using narrow (<1 μm) mesas etched into a sapphire substrate and conditions producing angled {11-22} facets to initiate growth by metalorganic chemical vapor deposition. These two techniques redirect VTDs over the mesas to the horizontal and away from device areas above. Further reductions appear possible if the facets uniformly cover all mesas prior to cantilever growth.

Origin of the Time-Dependence of Wet Oxidation of AlGaAs

The time dependence of the wet oxidation of high-Al-content AlGaAs can be either linear, indicating reaction-rate limitation, or parabolic, indicating diffusion-limited rates. The transition from linear to parabolic time dependence can be explained by the increased rate of the formation of intermediate As2O3 versus its reduction to elemental As. A steadily increasing thickness of the As2O3-containing region at the oxidation front will shift the process from the linear to the parabolic regime. This shift from reaction-rate limited (linear) to diffusion-limited (parabolic) time dependence is favored by increasing temperature or increasing Al mole fraction.

Doping level selective photochemical dry etching of GaAs

The first observation of a highly selective photochemical dry etching process which discriminates between semiconductor materials differing in dopant concentrations by less than a factor of 5 is reported here. A photochemical reaction of GaAs with gas phase reactive Cl species occurs when the surface is irradiated with low‐intensity light of band gap or greater quantum energies. Application of an appropriate negative bias to a GaAs sample can almost totally suppress the photochemical reaction of heavily doped n‐GaAs, while less heavily doped n‐GaAs or p‐GaAs continue to etch at undiminished rates under the same conditions. This is the first reported etching process to produce greater than 20:1 selectivity for doping levels differing by less than a factor of 5. A mechanism which may explain the origin of the photochemical reaction and the voltage‐controlled doping level selectivity which it displays is presented here. The potential significance of these observations for other semiconductor materials, such ...

YBa2Cu3O7 nanobridges fabricated by direct‐write electron beam lithography

A direct method for nondamaging, nanometer‐scale patterning of high Tc superconductor thin films is presented. We have fabricated superconducting nanobridges in high‐quality, epitaxial thin‐film YBa2Cu3O7 (YBCO) by combining direct‐write electron beam lithography and an improved aqueous etchant. Weak links with both length and width dimensions less than 20 nm have exhibited critical currents at 77 K of 4–20 μA and IcRn products of 10–100 μV which compare favorably with results for other YBCO junction technologies. We have used this technique in the fabrication of a shock‐wave pulse former as an initial demonstration of its applicability to monolithic superconductive electronics.