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Photoreflectance (PR) spectroscopy and Hall‐effect measurements have been used for the analysis of a molecular beam epitaxially grown AlGaAs/GaAs heterostructure. The photoreflectance spectrum provided valuable information regarding the quality of the undoped GaAs, the aluminum composition of the AlGaAs layers, impurity diffusion, and the quantum well widths. Successive layer removal aided with the identification of some photoreflectance features and provided insight into the electrical transport properties of the heterostructure. Quantum well widths obtained from PR lineshape fits are compared with layer thicknesses measured from cross‐sectional transmission electron micrographs.

Optical and electrical properties are described for bulk GaAs, grown from a melt doped with iron to create FeGa deep acceptors in a sufficient amount (exceeding the EL2 defect concentration) to make high‐resistivity p‐type rather than semi‐insulating material. Both iron photoionization and EL2+ photoneutralization contribute to the near‐infrared optical absorption. This made it possible to deduce the concentrations (NAi and NAn) of ionized and lattice‐neutral iron, and the ratio (NAi/NAn). Temperature dependent measurements of dc electrical transport yielded quantities such as the free hole density, and hence the Fermi energy, for the 290–420 K range. This information combined with (NAi/NAn) led to a determination of the iron acceptor’s free energy eA(T): about 0.46 eV above the valence band at 300 K, and ∼40 meV closer at 420 K. The temperature dependence of eA for iron is shown to differ from ev, ec, midgap, or the free energy for CrGa acceptors in GaAs.

As an acceptor dopant with a solid:liquid distribution coefficient ks

The universal drift instability and other drift instabilities driven by density and temperature gradients in a toroidal system are investigated in both linear and nonlinear regimes via particle simulation. Runs in toroidal and cylindrical geometry show dramatic differences in plasma behavior, primarily due to the toroidicity‐induced coupling of rational surfaces through the poloidal mode number m. In the toroidal system studied, the eigenmodes are seen to possess (i) an elongated, nearly global radial extent, (ii) a higher growth rate than in the corresponding cylindrical system, (iii) an eigenfrequency nearly constant with radius, and (iv) a global temperature relaxation and enhancement of thermal heat conduction. Most importantly, the measured χi shows an increase with radius and an absolute value on the order of that observed in experiment. On the basis of the present observations, it is argued that the increase in χi with radius observed in experiment is caused by the global nature of heat convection ...

Molecular beam epitaxial (MBE) layer heterostructures have been grown on GaAs wafers that received several different predeposition surface treatments. The electrical properties of the epitaxial structures have been correlated with the electrically active chemical impurities present at the substrate‐epitaxial layer interface. Direct evidence has been found that relates the interface condition to the population of deep levels in the MBE structures. Photoreflectance modulation spectroscopy was used to qualitatively determine the electric field strengths of the epitaxial buffer layer regions.

Sidegating characteristics of molecular‐beam‐epitaxially grown AlGaAs/GaAs heterostructure field‐effect transistors were obtained from layer structures containing varied AlGaAs spacer layer thicknesses. It is demonstrated that the sidegating effects for these wafers can be related to the leakage currents measured at the sidegate electrodes. The leakage currents were determined primarily to be a function of the resistance created by the isolation implants between the sidegate electrode and the device. Additionally, the various AlGaAs spacer layer thicknesses permitted the influence of silicon diffusion on device properties to be observed. The experimental results are compared with theoretical calculations.

Sidegating characteristics of AlGaAs/GaAs heterostructure field‐effect transistors, fabricated on molecular‐beam epitaxially grown layers, were investigated with emphasis on the material properties. A systematic analyses of the epitaxial layers concluded with the identification of the substrate–superlattice‐buffer‐layer interface as the predominant cause of the sidegating effect. Remnant carbon contamination on the GaAs surface was found to produce a p‐type, conducting interfacial region. Controlled oxidation of the carbon on the wafers was accomplished using ultraviolet radiation. This oxide was desorbed in situ before epitaxial growth. Secondary‐ion‐mass spectroscopy was employed to estimate the carbon concentration at the substrate–epitaxial‐layer interface for standard cleaned and ultraviolet‐ozone‐treated wafers. The carbon concentration of the interfacial region decreased by two orders of magnitude for the wafers exposed to the ultraviolet radiation. Hall‐effect measurements of standard cleaned and ...

Crystal properties such as bulk resistivity, mobility, and dislocation density which influence ion‐implantation activation are reported for liquid‐encapsulated Czochralski, horizontal gradient freeze, and vertical gradient freeze semi‐insulating GaAs crystals. The liquid‐encapsulated Czochralski crystals used in this study include as‐grown, boule‐annealed, and indium‐alloyed GaAs. Wafer maps displaying the concentration and spatial distribution of neutral EL2 are presented for each crystal. Wafers from the various crystals were implanted with 29Si at a dose of 3×1012 ions/cm2 and an energy of 60 keV. Macroscopic variations in the donor distribution and peak carrier density demonstrate that point defects impact significantly the donor implant activation processes.

Wafers obtained from undoped, semi‐insulating GaAs crystals grown by liquid‐encapsulated Czochralski and vertical gradient freeze techniques were implanted with 29Si at a dose of 3×1012 ions/cm2 and an energy of 60 keV. Activation of the implanted region was carried out by furnace or rapid thermal annealing methods, with and without the use of an SiO2 encapsulant. Profiles of the electron concentration and distribution were obtained by the capacitance‐voltage method. Significant variations in the donor distributions were found to be dependent upon the annealing technique employed. The dissimilarities in implant activation properties for GaAs wafers grown by the two techniques, and subjected to identical annealing cycles, have been attributed to variations in melt stoichiometry and native point defects.

Spatial distributions of EL2 in undoped, semi‐insulating GaAs crystals grown by a novel vertical gradient freeze (VGF) method are reported. As a result of the low‐temperature gradients present during growth and post‐solidification cooling, these crystals exhibit lower EL2 concentrations and lower dislocation densities than liquid‐encapsulated Czochralski crystals. Both the EL2 distribution and dislocation density over the area of a wafer do not display the fourfold symmetric pattern prevalent for LEC‐grown GaAs. The radial distributions of EL2 in as‐grown VGF crystals have been found to be independent of the dislocation density. Axial and radial Hall‐effect measurements are included. Thermal activation energies are also presented and the compensation mechanism for this material is discussed.

Implantations of silicon and silicon fluoride ions into semi‐insulating GaAs wafers were performed to compare and evaluate the quality of the n‐type layers that were produced. The use of SiFx (x=1,2,3) molecular ion implantations creates n‐type active regions and n+ ohmic contact layers employing a higher implant energy than that needed to achieve a similar projected range for silicon ions. The higher energy allows the implanter to operate with a more stable and reproducible ion beam for shallow implant applications. Variables affecting the net electrically active ion distributions, such as the ion beam incidence angle, and the use of furnace or rapid thermal annealing cycles for implantation activation are addressed. Capacitance‐voltage profiles were obtained for the n‐type regions, and the free carrier distributions for the n+ implantations were obtained by polaron electrolytic profiling. The implanted regions were characterized by Hall‐effect measurements, cathodoluminescence, and secondary ion mass sp...

An undoped, semi‐insulating GaAs crystal grown by the horizontal Bridgman method has been characterized by axial and radial resistivity and mobility measurements. Due to lower thermal gradients, defect densities for wafers from this crystal are less than those observed for liquid‐encapsulated Czochralski crystals. Cathodoluminescent images display uniform luminescent intensity around dislocated regions indicating an absence of impurity gettering in these areas. Concentrations of neutral EL2, a native defect and principal deep donor in undoped GaAs, are reported for seed, middle, and tail‐end wafers. The distribution of EL2 over the area of each wafer is shown. Wafers were also implanted with 29Si and furnace annealed to form shallow n layers. Capacitance‐voltage profiles demonstrate reproducible peak carrier concentrations and penetration depths.

Polygonized surface structure produced by furnace annealing GaAs wafers was studied by optical and scanning electron microscopy. The surface structure was revealed by chemically etching wafers which had been implanted with silicon and subjected to a furnace annealing cycle. Cathodoluminescence micrographs demonstrate an absence of impurity segregation to the polygon boundaries. No correlation was found between growth‐induced dislocations and the polygonal networks. It is proposed that the surface structure results from a vacancy‐condensation process. Related surface effects were observed for rapidly annealed wafers. A correlation between the furnace annealed and rapidly annealed GaAs is presented.