The orientation dependence of phase separation has been examined in detail in InGaAsP layers grown by liquid phase epitaxy on (001), (110), (111)ln and (123) InP substrates. It is shown that phase separation is two-dimensional in nature and does not occur along the growth direction for the cases examined. Further, phase separation takes place along the soft directions lying in the growth plane. These results very strongly suggest that phase separation evolves at the surface while the layer is growing. CuPt-type ordering characteristics of InGaAsP layers are presented. In addition, the Influence of growth temperature and growth rate on domain sizes have been investigated in GaInP2 layers. A model has been proposed to rationalize the formation of domains and involves steps present on the surface. Results suggest that ordering like phase separation occurs at the surface while the layers is being deposited. It is inferred that the two microstructural features evolve concomitantly at the surface during layer growth.

In directional solidification, dewetting is characterized by the lack of contact between the crystal and the crucible walls, due to the existence of a liquid meniscus at the level of the solid-liquid interface. This creates a gap of a few tens of micrometers between the crystal and the crucible. One of the immediate consequences of this phenomenon is the dramatic improvement of the quality of the crystal. This improvement is partly due to the modification of the solid-liquid interface curvature and partly to the absence of sticking and spurious nucleation at the crystal-crucible interface. Dewetting has been, commonly observed during the growth of semiconductors in crucibles under microgravity conditions where it appears to be very stable: the gap between the crystal and the crucible remains constant along several centimetres of growth. The physical models of the phenomenon are well established and they predict that dewetting should not occur in microgravity, if sufficient static pressure is imposed on the melt, pushing it towards the crucible. We present the results of InSb(Zn) solidification experiments conducted at the International Space Station (ISS) where, in spite of a spring exerting a pressure on the liquid, partial dewetting did occur. This surprising result is discussed in terms of force exerted .by the spring on the liquid and of possibility that the spring did not work properly. Furthermore, it appears that the segregation of the Zn was not affected by the occurrence of the dewetting. The data suggest that there was no significant interference of convection with segregation of Zn in InSb.

Four Te-doped InSb crystals were directionally solidified under microgravity conditions at the International Space Station (ISS). Three Te-doped InSb crystals were grown at R=5mm/h. One crystal was grown at R=3.33mm/h. The distribution of Te was measured using secondary ion mass spectroscopy (SIMS). The initial transients in Te concentration were found to be consistent, yielding a diffusivity of Te in InSb melts of D=1×10−5cm2/s. One experiment revealed a diffusion controlled final transient. In all experiments, the charge was pressurized by a piston and spring device, to prevent de-wetting.

Solidification Using a Baffle in Sealed Ampoules (SUBSA) is the first investigation conducted in the Microgravity Science Glovebox (MSG) Facility at the International Space Station (ISS) Alpha. In July, August and September of 2002, 8 single crystals of InSb, doped with Te and Zn, were directionally solidified in microgravity. Ground based tests, related numerical modeling and images of the growth process obtained in microgravity are presented.

The composition of semiconductor crystals produced in space by conventional melt-growth processes (directional solidification and zone melting) is affected by minute levels of residual micro-acceleration, which causes natural convection. The residual acceleration has random magnitude, direction and frequency. Therefore, the velocity field in the melt is apriori unpredictable. As a result, the composition of the crystals grown in space can not be predicted and reproduced. The method for directional solidification with a submerged heater or a baffle was developed under NASA sponsorship. The disk-shaped baffle acts as a partition, creating a small melt zone at the solid-liquid interface. As a result, in ground based experiment the level of buoyancy-driven convection at the interface is significantly reduced. In several experiments with Te-doped GaSb, nearly diffusion controlled segregation was achieved.

Solidification Using a Baffle in Sealed Ampoules (SUBSA) is a Microgravity Science Glovebox Investigation manifested for the UF2 flight, on the U.S. Orbiter 111, to the International Space Station (ISS). SUBSA complements the parent flight investigation CG13 (Space-and Groundbased Crystal Growth Using a Baffle). During directional solidification, the disk-shaped baffle acts as a partition, creating a small melt zone at the solid-liquid interface. As a result, the level of buoyancy-driven convection at the interface is significantly reduced. In space, the baffle will reduce convection driven by residual micro acceleration. The baffle reduces the Rayleigh number (Ra) of the melt by a factor of 103. The combined effect of the baffle and microgravity will yield a reduction in Ra by a factor of 107 to 109 approaching effectively the acceleration conditions in nanogravity. The results of ground based tests and numerical modeling will be presented. The furnace for directional solidification (flight hardware and the ground unit) was developed by Tec-Masters Inc. The flight ampoules were produced jointly at Rensselaer, Tec-Masters Inc. and Crystallod Inc.

Aluminum-free buried-heterostructure quantum-well lasers have been successfully fabricated on low-composition InGaAs:n substrates. Selective-area metalorganic chemical vapor deposition (MOCVD) was utilized to investigate a variety of InGaAs quantum wells with a wide range of composition and thickness. Compressively strained quantum wells can be deposited thicker on substrates of InGaAs than GaAs before the generation of misfit dislocations. These deeper potential wells enable laser diodes with longer wavelengths (1.1504 mu m) than GaAs-based emitters and higher characteristic temperatures (145 K) than InP-based devices.

Broad-area (W = 150 mu m) single-quantum-well (SQW) lasers have been successfully fabricated on n-type In0.03Ga0.97As substrates without incorporating aluminum-containing alloys. The strained-layer InGaP-GaAs-InGaAs heterostructure was grown by atmospheric pressure metalorganic chemical vapor deposition. Due to both the increased substrate lattice constant and the partial strain compensation supplied by the tensile-strained GaAs optical guiding layers, a peak emission wavelength of 1.1403 mu m is obtained for a 1-mm cavity length.

Single and double pulse doped pseudomorphic high electron mobility transistor structures with 110-Å-thick InGaAs channel layers have been grown on InxGa1−xAs substrates (x=0.04; 0.065) and GaAs substrates. For In0.23Ga0.77As channel layers, higher electron mobilities were obtained on In0.04Ga0.96As substrates due to reduced strain. Transmission electron microscopy micrographs on a GaAs-based sample exhibited a roughened selectively doped heterojunction but no detected misfit dislocations. Pseudomorphic structures with In0.27Ga0.73As channel layers were also grown on In0.065Ga0.935As substrates with good transport and optical properties. The properties of the analogous structure grown on GaAs were severely degraded. Transmission electron microscopy micrographs on the GaAs sample showed a very rough selectively doped heterojunction with misfit dislocations.

Double pulse doped AlGaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistor (PHEMT) structures have been grown on InxGa1−xAs (x=0.025–0.07) substrates using molecular beam epitaxy. A strain compensated, AlGaInAs/GaAs superlattice was used for improved resistivity and breakdown. Excellent electrical and optical properties were obtained for 110-Å-thick InGaAs channel layers with indium concentrations up to 31%. A room temperature mobility of 6860 cm2/V s with 77 K sheet density of 4.0×1012 cm−2 was achieved. The InGaAs channel photoluminescence intensity was equivalent to an analogous structure on a GaAs substrate. To reduce strain PHEMT structures with a composite InGaP/AlGaAs Schottky layer were also grown. The structures also exhibited excellent electrical and optical properties. Transmission electron micrographs showed planar channel interfaces for highly strained In0.30Ga0.70As channel layers.

Bulk Ga 1 − x In x As single crystals with In composition ranging from x = 0.02 to 0.1 have been grown by the LEC method from melt compositions of 26.3 to 55 mol% In. Due to segregation effects, the In content in the melt increases during growth. Crystals grown from different initial melt compositions exhibited cellular structure followed by polycrystallinity when the melt composition had reached a unique critical value of In concentration for a particular set of growth conditions. The point at which cellular structure occurs has been evaluated for a variety of growth parameters. These parameters include initial melt compositions, thermal gradients in the melt, crystal rotation and pull speed. Applying the knowledge of the conditions for the onset of cellular structure has maximized the successful growth of single-crystal ternary material and has been instrumental in increasing indium content. The AB and KOH etches were used to highlight growth striations, view onset of cellular structure and examine dislocation density. The In composition in the crystal was determined by low-temperature PL, X-ray lattice parameter measurement, absorption edge position and WDX.

Growth of In0.15Ga0.85As/GaAs multilayers are reported on lattice matched In0.04Ga0.96As buffer layers using two different starting substrates: (1) a bulk, lattice-matched, Czochralski-grown In0.04Ga0.96As wafer, and (2) a bulk (unmatched) GaAs wafer. The structures, grown by molecular-beam epitaxy, consist of a 200-nm-thick undoped buffer plus a 28-1/2 period modulation-doped multilayer having 10-nm-thick In0.15Ga0.85As quantum wells and 25-nm-thick GaAs barriers doped over their 10-nm central region. The 4 K Hall mobility of the multilayer grown on the InGaAs substrate is 57% larger than the structure grown on GaAs, despite a low dislocation density observed in transmission electron micrographs of the latter. These results suggest that bulk ternary substrates can provide enhanced performance for future electronic and optoelectronic devices.

InGaAs films have been deposited on semi-insulating InxGa1−xAs (x=0.04–0.05) substrates with improved material properties compared to similar InGaAs films grown on GaAs substrates. For near lattice matched conditions the films grown on InGaAs substrates have a smooth surface morphology compared to a dislocation-induced cross hatch morphology on GaAs substrates. The resulting film double crystal x-ray linewidths are considerably narrower. The InGaAs film photoluminescence intensity is stronger with a narrower x-ray linewidth due to the elimination of lattice mismatch dislocations. Also the Hall mobilities are higher for the films grown on InGaAs substrates.

We report the liquid encapsulated Czochralski growth and characterization of large, substrate quality single crystal Ga1-xInxAs, 0 < x < 0.10, in excess of 50 millimeters in diameter and weighing 1000 grams. This unique ability to grow large single crystals of ternary III-V compound semiconductors permits realization of the concept of substrate engineering for both homo- and heteroepitaxial applications. One area of particular interest and importance is the development of short visible wavelength (blue) lasers. Wafers with x equals 0.038 have been used for lattice matched MBE growth of ZnSe and ZnCdSe epilayers for blue emitter applications. Low temperature photoluminescence, WDX and double crystal x-ray diffraction (rocking curve) measurements have been utilized to confirm compositional uniformity and crystal quality. Characterization results for both substrate and epitaxial layer are discussed.

The orientation dependence of doping in organometallic chemical vapor deposition (OMCVD) is shown to be far more complex than previously believed, with the variation of doping with increasing misorientation from (100) towards the (111)A/B being non-monotonic. However, the ratio of the n-doping on the B face to that on the corresponding A face is always greater than 1, irrespective of whether the dopant is a group IV or VI element. For p-doping with Zn, the reverse is true. The orientation dependence of doping has been used to create current blocking layers in InP/InGaAsP double heterostructure (DH) and multiple quantum well (MQW) lasers grown in a single step on a mesa or in a V-groove.