N. Bottka

Photoreflectance characterization of interband transitions in GaAs/AlGaAs multiple quantum wells and modulation‐doped heterojunctions

The optical modulation technique of photoreflectance (PR) has been applied to characterize the interband transitions in GaAs/AlGaAs multiple quantum wells (MQW) and modulation‐doped heterojunctions at room temperature. The spectra of the MQW show ‘‘derivativelike’’ reflectance features due to allowed interband transitions from heavy and light hole subbands to conduction subbands, and the E0(Γ8,v→Γ6,c) transitions of the AlGaAs layers. Our data are consistent with a square well calculation using a conduction‐band offset of 60% of the band‐gap discontinuity. For modulation‐doped heterojunctions, a correlation is observed between a PR feature approximately 18 meV above the GaAs fundamental gap and the presence of a two‐dimensional electron gas.

Band‐gap determination by photoreflectance of InGaAs and InAlAs lattice matched to InP

Photoreflectance‐derived band‐gap parameters as a function of temperature for InGaAs and InAlAs lattice matched to InP are reported. The experiment was performed on a set of samples of various compositions (and strains) yielding greater reliability and ensuring self‐consistency. For InGaAs, fits to the Varshni equation gave E0(T=0 K)=803 meV, α=4.0×10−4 eV K−1, and β=226 K. For InAlAs, E0(T=0 K)=1.541 eV, α=4.7×10−4 eV K−1, β=149 K, and Δ0=338 meV.

OMVPE of GaN and AIN films by metal alkyls and hydrazine

Thin films of GaN have been grown on A12O3, Si and GaAs using trimethylgallium and hydrazine (N2H4) in N2 at atmospheric pressure. Growth proceeded by the formation of a room temperature adduct which decomposed to form GaN in the temperature range 425–960°C. Growth coefficients were about 3 μm/mmol (of TMG) for growth temperatures above 650°C. The films grown below 600°C were yellow and polycrystalline on all substrates. Hall mobilities as large as 50 cm2/V·s (n = 1×1020 cm−3) were obtained for films grown at 900°C with VIII = 20. The mobilities were about 1 cm2/V·s (n = 6×1019 cm−3) below 650°C. The impurities in the films were O (≈ 2%) and C (<1%) for all growth temperatures. Estimates were made for the room temperature LO and TO phonons of 92 and 67 meV respectively. UV transmission data and the photoresponse of the films show an impurity band about 2.5 ev below the conduction band, probably due to O related defects. A1N was also deposited via the decomposition of an adduct formed by the room temperature reaction between trimethylaluminium and N2H4. Films deposited at 575 and 785°C had a rough surface morphology.

Growth of GaN films using trimethylgallium and hydrazine

Hydrazine has been used as a nitrogen source for the organometallic vapor phase deposition of GaN using trimethylgallium and a nitrogen carrier gas in the temperature range 425–960 °C. Hydrazine and trimethylgallium form an adduct at room temperature which decomposes over the substrate with an activation energy of 1 eV for temperatures below 650 °C. No carbon has been detected by Auger spectroscopy in the films. The electrical properties are dominated by oxygen impurities, probably originating from the hydrazine. Since hydrazine readily decomposes above 400 °C, it is a better source of nitrogen for low‐temperature depositions than other, more stable nitrogen sources.

IR diode laser probing of OMVPE kinetics

Abstract Previous experiments employing in-situ, real-time infrared laser diode absorption spectroscopy as a probe of the OMVPE reactor environment [J. Crystal Growth 77 (1986) 73 and 163] have been used to characterize gaseous chemical intermediates and to tune an ultra-fast gas delivery system for producing abrupt gas switching. This work has led to the growth of modulated doped heterostructures with mobilities in excess of 90 000 cm 2 V −1 s −1 on a routine basis. In addition, these experiments observed gas phase methyl radicals from the decomposition of the methyl precursors above the heated GaAs surface. Because the concentration of the methyl radicals was found to be inversely related to the arsine concentration in the reactor, we have undertaken the study of the homogeneous gas phase reaction of: CH 3 + AsH 3 → products. Tunable infrared diode laser transient absorption spectroscopy was used to probe the methyl radical concentration versus time following excimer laser photolysis of methyl iodide in a flowing gaseous mixture of methyl iodide, arsine, and hydrogen (argon). The bimolecular rate constant at room temperature was determined to be 6.5(2.0)×10 −14 cm 3 molecule −1 s −1 . Limited measurements of the temperature dependence of this rate constant indicate an activation energy barrier of 1.65(0.50) kcal mol −1 .

Photoreflectance characterization of OMVPE GaAs on Si

Abstract Photoreflectance, a contactless, non-destructive optical characterization tool, can be of great utility to the material scientist in identifying crystal growth problems and ascertaining material quality in a very short time. It provides a precise measurement of the spectral energy of the fundamental absorption edge and higher lying critical point transitions. Shifts in these energies are a measure of the built-in strain in strain-layered heterostructures. Moreover, the spectral energy of the so-called Franz-Keldysh oscillation extrema is directly related to the net carrier concentration of the semiconductor material under study. We have used the Photoreflectance technique to determine the crystal quality, the carrier concentration, and the built-in strain of epitaxial GaAs grown by OMPVE on Si substrates. The study includes determination of these parameters as a function of various growth conditions, post-growth anneal cycle, and spot position on the wafer. Results are correlated with X-ray diffraction, and C - V measurements.

Epitaxial growth of Fe on GaAs by metalorganic chemical vapor deposition in ultrahigh vacuum

Fe epitaxial films have been grown on GaAs(100) by thermal dissociation of Fe (CO)5 in a high vacuum environment. In situ low‐energy electron diffraction (LEED) and Auger spectroscopy have been used to study the MOCVD process and to characterize the growing films. Excellent film quality is evidenced by the observed small ferromagnetic resonance linewidth.

Qualification of OMVPE AlGaAs/GaAs HBT structures using nondestructive photoreflectance spectroscopy

Abstract High performance heterojunction bipolar transistor (HBT) integrated circuits are extremely dependent upon the uniformity and quality of the III–V compound heteroepitaxial materials used in their fabrication. HBT requirements include the need for excellent control over layer thickness, uniformity, alloy composition, and intentional impurity concentration over a wide range of carrier concentrations. We have used the contactless technique of photoreflectance (PR) to evaluate HBT structures of various epitaxial layer design and correlated the measured PR results with transistor performance. PR was performed in the range 1.3–2.0 eV which is ideally suited to probing AlGaAs/GaAs HBT structures. The resulting signals gave information about the gap energy of the material probed (and thus its composition) and the built-in DC electric fields within the various epitaxial layers. Measurement of the spectral energy of the observed Franz-Keldysh oscillation extrema near the GaAs and AlGaAs edges, were used to determine the built-in electric field in the collector and the emitter regions. These measured fields were in good agreement with electric field distribution calculations obtained by solving the Poisson and the continuity equations for an HBT structure. Run-to-run uniformity of HBT structures from a multi-wafer OMVPE reactor were correlated to measured transistor characteristics.

Epitaxial growth of β‐SiC on silicon‐on‐sapphire substrates by chemical vapor deposition

Cubic (β) silicon carbide films have been grown epitaxially on silicon‐on‐sapphire (SOS) substrates by chemical vapor deposition. The β‐SiC films were grown between 1340–1370 °C on SOS substrates which have a layer of silicon deposited in situ or were grown directly on the as‐is SOS substrate. In both cases, the silicon surface was carbonized prior to growth of the β‐SiC epilayer. With a growth rate of ∼3.5 μm/h, 7 μm β‐SiC films have been obtained. The films have been characterized by infrared reflectance spectroscopy, optical and scanning electron microscopy. Specular films are obtained which have growth columns and pits. Electrical transport properties of the films were measured by the Van der Pauw–Hall method. The films are p type with carrier concentrations between 1×1018 and 2×1018 cm−3 and Hall mobilities of approximately 30 cm2/V s.

Chemical vapor deposition of β-SiC on silicon-on-sapphire and silicon-on-insulator substrates

Abstract Cubic silicon carbide (β-SiC) films have been grown epitaxially on silicon-on-sapphire (SOS) substrates by chemical vapor deposition. A fresh layer of silicon is first deposited in situ on the SOS substrate at approximately 1050°C. The silicon layer is then carbonized while being heated to 1360°C. The β-SiC layer is grown at 1360°C using silane and propane as sources. β-SiC films can also be grown directly on the SOS substrate without utilizing a fresh silicon layer. Deposition of β-SiC films on silicon-on-insulator (SOI) substrates has also been accomplished with slight modification of the growth parameters described above. The β-SiC films have been characterized by IR reflectance spectroscopy, optical microscopy and electron microscopy. Typical films are 7 μm thick and have a specular surface with some physical features. Electrical transport properties as determined by the Van der Pauw Hall method show the β-SiC films to be p-type while those grown on SOI were n-type. X-ray rocking curve measurements were obtained to determine the crystalline quality of the films. In addition, preliminary optical characterization of the films has been performed.