P. D. Kirchner

Chromium: a stream-processing framework for interactive rendering on clusters

We describe Chromium, a system for manipulating streams of graphics API commands on clusters of workstations. Chromiums stream filters can be arranged to create sort-first and sort-last parallel graphics architectures that, in many cases, support the same applications while using only commodity graphics accelerators. In addition, these stream filters can be extended programmatically, allowing the user to customize the stream transformations performed by nodes in a cluster. Because our stream processing mechanism is completely general, any cluster-parallel rendering algorithm can be either implemented on top of or embedded in Chromium. In this paper, we give examples of real-world applications that use Chromium to achieve good scalability on clusters of workstations, and describe other potential uses of this stream processing technology. By completely abstracting the underlying graphics architecture, network topology, and API command processing semantics, we allow a variety of applications to run in different environments.

Nucleation mechanisms and the elimination of misfit dislocations at mismatched interfaces by reduction in growth area

To investigate the effect of growth area on interface dislocation density in strained‐layer epitaxy, we have fabricated 2‐μm‐high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5×105, 104, and 102 cm−2. 3500‐, 7000‐, and 8250‐A‐thick In0.05Ga0.95As layers, corresponding to 5, 10, and 11 times the experimental critical layer thickness as measured for large‐area samples, were then deposited by molecular‐beam epitaxy. For the 3500‐A layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 cm−1 for mesas as large as 100 μm. A pronounced difference in the linear interface dislocation densities along the two interface 〈110〉 directions indicates that α dislocations nucleate about twice as much as β dislocations. For samples grown on the highest dislocation density substrates, the linear interface‐dislocation density was found to vary linearly with mesa...

Mechanism and conditions for anomalous strain relaxation in graded thin films and superlattices

Compositionally graded films of SiGe/Si(100) and GaInAs/GaAs were grown under different conditions in order to investigate the different modes of strain relaxation associated with the compositional grading. We show that, when the growth conditions are very clean and the gradient is shallow enough (about 1% misfit per half micron), very good, relaxed films are obtained. This coincides with the introduction of large numbers of dislocations in the substrate itself, which is counter‐intuitive at first since the substrate is under negligible strain. We show that this introduction of dislocations is the result of the activation of novel Frank–Read‐like sources located in the graded region, and is directly correlated to the lack of other low energy nucleation sites for dislocations. We detail the conditions of growth necessary for this phenomenon to occur, and show that it operates both for the SiGe/Si system and the GaInAs/GaAs system. Pure, relaxed Ge films have been grown in this manner on Si(100), with a def...

Ohmic contacts to n‐GaAs using graded band gap layers of Ga1−xInxAs grown by molecular beam epitaxy

Ohmic contacts were studied on structures which utilize the fact that for InAs surfaces Fermi level pinning occurs at or in the conduction band. It was found that an epitaxial layer of n‐Ga1−xInxAs grown by molecular beam epitaxy on n‐GaAs which is graded in composition from x = 0 at the GaAs interface to 0.8?x?1.0 at the surface will produce a structure with a nearly zero Schottky barrier height for the metal–Ga1−xInxAs interface and hence a low resistance ohmic contact. A transmission line measurement of non‐alloyed contact resistance of 5×10−7<Rc<5×10−6 ohm cm2 was obtained for a Ag/n‐Ga1−xInxAs/n‐GaAs MESFET structure.

An In 0.15 Ga 0.85 As/GaAs pseudomorphic single quantum well HEMT

This letter describes high electron mobility transistors (HEMTs) utilizing a conducting channel which is a single In<inf>0.15</inf>Ga<inf>0.85</inf>AS quantum well grown pseudomorphically on a GaAs substrate. A Hall mobility of 40 000 cm²/V.s has been observed at 77 K. Shubnikov-de Haas oscillations have been observed at 4.2 K which verify the existence of a two-dimensional electron gas at the In<inf>0.15</inf>Ga<inf>0.85</inf>As/GaAs interface. HEMTs fabricated with 2-µm gate lengths show an extrinsic transconductance of 90 and 140 mS/mm at 300 and 77 K, respectively-significantly larger than that previously reported for strained-layer superlattice In<inf>x</inf>Ga<inf>1-x</inf>As structures which are nonpseudomorphic to GaAs substrates. HEMTs with 1-µm gate lengths have been fabricated, which show an extrinsic transconductance of 175 mS/mm at 300 K which is higher than previously reported values for both strained and unstrained In<inf>x</inf>Ga<inf>1-x</inf>As FETs. The absence of Al<inf>x</inf>Ga<inf>1-x</inf>As in these structures has eliminated both the persistent photoconductivity effect and drain current collapse at 77 K.

Unpinned (100) GaAs surfaces in air using photochemistry

We have unpinned the Fermi level at the surface of both n‐ and p‐type (100) GaAs in air. Light‐induced photochemistry between GaAs and water unpins the surface Fermi level by reducing the surface state density. Excitation photoluminescence spectroscopy shows a substantial decrease in both surface band bending and surface recombination velocity in treated samples, consistent with a greatly reduced surface state density (≂1011 cm−2). Capacitance‐voltage measurements on metal‐insulator‐semiconductor structures corroborate this reduction in surface state density and show that the band bending may be controlled externally, indicating an unpinned Fermi level at the insulator/GaAs interface. We discuss a possible unpinning mechanism.

Structure and recombination in InGaAs/GaAs heterostructures

The defect structure of lattice‐mismatched 1‐μm InxGa1−xAs (x≊0.12, misfit Δa/a≊8.5×10−3) epilayers on GaAs was studied with scanning cathodoluminescence (CL), transmission electron microscopy (TEM), high‐voltage electron microscopy, and scanning electron microscopy. CL shows that nonradiative recombination lines exist in the GaAs buffer layer as far as 4000 A from the interface. The density of these defects is independent of substrate dislocation density. Plan‐view TEM analysis indicates that the majority of these dislocations in the buffer layer are sessile edge half‐loops. Cross‐sectional TEM shows that loops also extend into the InGaAs epilayer, but the majority of the loops are located on the buffer layer (substrate) side of the interface. A model is proposed to explain sessile edge dislocation formation in the buffer layer. A comparison of CL and high‐voltage electron microscopy images from the same interface area reveals that the dark nonradiative recombination lines seen in scanning luminescence i...

Lattice contraction due to carbon doping of GaAs grown by metalorganic molecular beam epitaxy

Epitaxial layers of GaAs have been grown by metalorganic molecular beam epitaxy (MOMBE) with atomic carbon concentrations ranging from 4×1017 to 3.5×1020 cm−3. The dependences of GaAs lattice parameter and hole concentration on atomic carbon concentration have been determined from x‐ray diffraction, Hall effect, and secondary‐ion mass spectrometry measurements. For atomic carbon concentrations in excess of 1×1019 cm−3, the hole concentrations are less than the corresponding atomic carbon concentrations. Lattice parameter shifts as large as 0.2% are observed for carbon concentrations in excess of 1×1020 cm−3, which results in misfit dislocation generation in some cases due to the lattice mismatch between the C‐doped epilayer and undoped substrate. Over the entire range of carbon concentrations investigated, Vegard’s law accurately predicts the observed lattice contraction.

Photoreflectance study of the surface Fermi level at (001)n-and p-type GaAs surfaces

We report a photoreflectance study of Fermi level pinning (VF) on (001) n‐ and p‐type GaAs with large, uniform electric fields. Surface photovoltage (Vs) effects were evaluated as a function of temperature (77 K<T<450 K), pump beam wavelength (633 and 407 nm) and W–metal coverage (in situ). The dependence of the measured barrier height, Vb (=VF−Vs), on T can be explained by a modification of the theory of M. Hecht [Phys. Rev. B 41, 7918 (1990)] yielding values of VF =0.77±0.02 V and VF =0.75±0.02 V for n‐ and p‐type GaAs, respectively, at 300 K. In addition, by introducing the ratio (r) of the area of the surface states to the illuminated area into the theory of Hecht we have been able to estimate the density of surface states on the GaAs surface. The effect of metal coverage is to increase r and reduce the influence of Vs.

Photoreflectance study of surface photovoltage effects at (100)GaAs surfaces/interfaces

We report a photoreflectance study of surface photovoltage (VS) effects on the determination of Fermi level pinning (VF) on (100) n‐GaAs in air and with W‐metal coverage (in situ) as a function of temperature (77 K<T<450 K) and light intensity (I). The dependence of VS on T and I can be explained by a modification the theory of M. Hecht [Phys. Rev. B 41, 7918 (1990)] yielding a value of VF=0.73±0.02 V. The effect of metal coverage is to reduce the influence of VS.