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Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on 6H-SiC(0001) substrates with no miscut and with 3.5° miscuts in both the [1 0 0] and [1 1 0] directions. The hydrogen-etched substrates display straight or chevron shaped steps, respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.
Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on vicinal 6H-SiC(0001) substrates with [1 1 00] and [11 2 0] miscut directions. The hydrogen-etched substrates display straight, or chevron shaped steps respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.
Predominantly 6H <0001> SiC single crystals have been implanted with nitrogen and aluminum at 300°K. The effects of implantation and post-implantation annealing at 573–1173°K have been characterized by Rutherford backscattering/channeling, microhardness measurements and transmission electron microscopy. Even at 1173°K, defect annealing was inhibited in amorphized regions. Progressive damage recovery as the anneal temperature was increased was otherwise generally observed. Fluences predicted by the critical damage energy of 2 × 1021 keV/cm3 did not quite produce amorphization to x = 0. In the as-implanted specimens, decreasing hardness was observed with increasing fluence. Significant surface softening (∼ 33% hardness reduction) was achieved at the highest aluminum fluence (5 × 1015 Al/cm2) and was stable at temperatures < 893°K.
We have used molecular beam methods and temperature programmed desorption to probe the reaction of several hydrocarbons with the Si(100) surface at cryogenic temperatures. It has been found that the kinetics of the surface reaction with the C=C bond can be strongly influenced by the production of active surface sites using prebombardment with Ar ions. The chemistry of the adsorbate is also influenced by electron bombardment of the adsorbed layer. Conversely, capping of active sites with atomic hydrogen retards the kinetics of the surface reaction. This work forms a first step in using the methods of surface kinetics and spectroscopy to probe the details of the elementary steps at work in chemical vapor deposition and plasma vapor deposition, leading to the production of SiC films.
Various thicknesses of AlGaAs are grown on GaAs substrates by MOCVD. Low temperature photoluminescence of the substrate is observed even for layers of AlGaAs 24μm thick. Direct excitation by the 488.0 nm pumping radiation and excitation by reradiation from the AlGaAs are eliminated as causes. From photoluminescence and EBIC studies, evidence is given to show that the substrate luminescence is caused by a much larger than expected electron diffusion length. A small trace of GaAs luminescence may be due to alloy segregation in the AlGaAs films themselves.
The effects of chemical etching, mechanical thinning, and ion milling on the low temperature photoluminescence spectra of MBE grown (001) CdTe films are reported. Line defects observed by TEM are correlated with photoluminescence. It is shown that X-ray D.C.R.C, measurements in these films are weighted averages over the whole thickness of the films and therefore weakly reflect the structural perfection of the samples near the surface as deduced by photoluminescence.
A short historical introduction concentrates on key experimental and theoretical developments prior to the mid-seventies which form the basis of our present understanding of the semiconducting and optical properties of the polytypes of SiC. Selected research of the last decade is reviewed. Finally, we discuss our on-going surface and defect studies in both epitaxial single crystal films of 3C SiC and other hexagonal SiC polytypes.
A combined characterization of theoretical calculation and experimental measurements, including Raman scattering, photoluminescence and cross sectional transmission electron microscopy, has been made on GaAs-AlxGa1-xAs multiple quantum wells (MQW) structures with different well widths grown by metalorganic chemical vapor epitaxy (MOCVD) with a modified reactor. Various parameters of these MQWs are obtained. The results with and without the alkyl push flow are compared. Related physical phenomena are discussed.
lons of boron, phosphorous, titanium and neon were implanted into (0001) oriented 6H SiC crystals at 300 K. Implantation energies and fluences were chosen to produce equal (calculated) displacements per atom at similar peak damage depths and a randomized (metaminct or amorphous) zone extending to the front surface. RBS/channeling was used to test the amorphization criteria. Dopant effects on regrowth kinetics and microhardness have been determined by isochronal annealing.
The formation and annealing of buried damage layers in hydrogen implanted N-type float zone <111> silicon has been studied by Rutherford Backscattering/ion channeling and cross-section transmission electron microscopy. Implantation with 50 keV or 75 keV H+ ions was conducted at temperatures of 95K, 300K and 800K at fluences of 2×1017 H+/cm2, 8×1017 H+/cm2 and 1×1018 H+/cm2. Post implantation annealing was conducted at temperatures up to 800K. The results show a temperature dependent transition from a highly hydrogen doped amorphous zone bounded by strongly diffracting (TEM) 2–5 nm diameter defects for implantation at 95K to a crystalline microstructure containing small dislocation loops and ∼40% of the implanted hydrogen for implantation at 300K. Defect production and annealing and hydrogen trapping in the damage zone are shown to depend on the relative implantation and post implantation annealing temperatures.
Single crystals of Al2O3 were implanted with chromium and zirconium to fluences of 1 × 1016 to 1 × 1017 ions cm−2. Rutherford backscattering-channeling studies showed the surface layers to be damaged but crystalline with the implanted ions randomly distributed. The microhardness and indentation fracture toughness were higher for the random solutions than for conventionally formed solid solutions. Changes in structure and properties caused by annealing in air at temperatures up to 1800°C were studied.
Energy deposition and heat-flow determine the temperature distributions in pulsed-beam irradiated structures. In laser irradiated Si, transient conductance measurements indicated a liquid/solid interface velocity of 2.8 m/sec during crystallization in agreement with a heat flow model. With pulsed ionbeam annealing of metal-Si structures, melting starts at the interface; epitaxial NiSi2 layers have been formed. Ion-beammixing experiments on polycrystalline and epitaxial Au-Ag bilayers show that intermixing is more pronounced in the polycrystalline structures as is the case with thermal annealing. Superlattice structures are formed in the epitaxial structures.
Thin surface copper-nickel alloys were prepared by ion implantation at 90 keV. During the implantation of one pure element by the other the sputtered products were collected on catcher foils at different stages from the beginning of the implant through to the steady state configuration of the target surface. The collector foils and targets were analyzed to determine the behavior of the sputtering yields during implantation and for the change in surface composition at the selected fluence. The total sputtering yield for the target and the effective elemental sputtering yields for each component appear to be functions of the changing surface fractions, the self ion sputtering yield of the implanted species, and the elemental sputtering yield of the initial target species. A model relating these parameters is presented.
Transmission electron microscopy (plan-view as well as cross-section) and high-resolution Rutherford backscattering and channeling techniques have been combined to investigate residual defects and substitutional concentrations of In and Sb in <100> and <111> orientations of ion implanted silicon layers after solid-phase-epitaxial (SPE) growth at 475–600°C. The maximum concentrations of Sb and In in substitutional sites were found to be, respectively, 1.3 × 1021 and 5.0 × 1019 cm−3 , exceeding the respective retrograde maxima by factors of 18 and 60. These results provide direct evidence of solute trapping and metastable alloying under solid-phase growth conditions. Accumulation of solute at the crystalline-amorphous interface was observed only for indium above a certain interfacial concentration. At still higher interfacial concentrations, the planar interface was observed to become unstable and SPE growth was retarded.
The rapid annealing of implant damage using thermal radiation has been shown to be a production-worthy method of achieving good activation and minimal dopant redistribution of implanted species. The method involves the shuttered exposure of a standard 3" or 4" silicon wafer to a uniform thermal radiation front produced by a graphite heater for short times (10–30 sec). The propagation of residual damage observed by TEM is significantly less than that produced by furnace annealing and device structures annealed have electrical characteristics comparable to standard furnace anneals.
A series of A15 structure V-Si films were sputtered with compositions ranging from approximately V90Si10 to V75Si25 . The as-deposited critical temperature onsets were between ~ 8 and 17K. Carbon levels of up to 5 at.%(average) were implanted and the films subsequently annealed at various temperatures between 650 and 1050°C. No Tc's above 17K were obtained in the C-implanted films. However enhancements of as much as ~ 9K, from ~ 8 to 17K, in highly Si-deficient films indicated that carbon was incorporated into the A15 structure of these films to form pseudo-binary V-Si-C alloys. Experimental details are given and some reasons why Tc values greater than 17K were not obtained are discussed.
A description is given of the profiling of CVD grown 3C SiC on undulant (001) Si using low temperature photoluminescence (LTPL). Inelastic neutron scattering (INS) and X-ray Raman scattering (XRS) are compared for acoustical modes of 4H SiC. Schottky barrier heights are obtained for 4H and 6H SiC on different crystal faces using three different measuring techniques. Scanning electron microscopy (SEM) is used to display a variety of porous SiC morphologies achieved in n-type and p-type SiC.
This paper is intended to be the introduction to the “CHARACTERIZATION” section of this volume. To serve this purpose we illustrate the subject matter with new results using four distinct experimental techniques.
Photoluminescence in the neighborhood of 1.54μm due to the 4I13/2 → 4l15/2 transitions is observed from 2 K up to 520 K in erbium implanted 6H SiC. The integrated 1.54 μm photoluminescence (PL) intensity is almost constant from 2 K up to about 400 K, with slight sample to sample variation. The shallow nitrogen donors play an important role in the excitation of the Er3+ centers. 1.54 μm electroluminescence (EL) is observed in erbium implanted 6H SiC p-n junctions under forward bias conditions. The EL spectrum is identical to the PL spectrum.
Growth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low NH3/TMAl ratio results in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4, a method reported earlier. We suggest that the Al adatoms, which have a high sticking coefficient on SiC, form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N leading to metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface, are absent on N-polarity GaN on off-axis C-SiC owing to high density of terraces on off-axis C-SiC. The properties of GaN layers grown on C-SiC are studied by X-ray diffraction.
AlN epitaxial layers in a thickness range of 0.065 to 0.6 microns have been grown on double-side polished c-plane sapphire by molecular beam epitaxy (MBE) using either RF nitrogen or ammonia as the nitrogen source. The samples were characterized by XRD, room temperature (RT) and low temperature (2K) optical absorption (transmission) measurements. The XRD (0002) peak FWHM diminishes from 380 arcsec to 84 arcsec when the thickness of the AlN films is increased from 65 nm towards 0.6 μm. All the samples grown with ammonia (NH3), representing N-rich conditions, exhibited optical bandgap exceeding 6 eV at RT. For samples grown with RF nitrogen source, we find that higher nitrogen flow (partial pressure of 8.3-9.4 × 10-5 Torr) results in optical bandgap values larger than 6.0 eV, regardless of the XRD results. The bandgap is found to be smaller than 6.0 eV for the samples grown with lower nitrogen partial pressure (1.0-3.5 x 10-5 Torr), regardless of sample thickness and the XRD data.
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