A method for depositing large grained polycrystalline GaAs on lattice mismatched substrates through controlled nucleation and selective growth is presented. The process was developed on Si wafers. Nucleation site formation began with nanolithography to create submicron holes in photoresist on Si. Ga metal was electrochemically deposited into the holes. Subsequent arsine anneals converted the gallium deposits into GaAs. Photoluminescence and electron diffraction verified conversion to GaAs. Metal-Organic Chemical Vapor Deposition (MOCVD) enlarged the seed crystals to coalescence without creating additional nucleation sites within the patterned field. Having successfully demonstrated the approach, subsequent work has been directed at lower cost, alternative ways to define initial nucleation sites, such as, microcontact lithography and direct decomposition of triethyl gallium to Ga metal in the MOCVD chamber.

Due to the high chemical reactivity of GaSb surfaces, many commonly used aqueous sulfide passivation techniques lead to the growth of surface oxides that degrade device performance. We have developed a non-aqueous passivation regime consisting of Na2S/benzene/15-crown-5/oxidant. The use of a non-polar, aprotic organic medium required the addition of a specific chelating agent, i.e. a 15-crown-5 ether, to solubilize sodium sulfide, and organic oxidizing agents, such as anthraquinone and benzophenone, to act as electron acceptors. The surface optical and chemical properties of GaSb surfaces after aqueous and non-aqueous sulfide treatments were compared. Non-aqueous passivation resulted in higher PL intensity, lower oxide content, and a less amount of elemental Sb than aqueous passivation.

We have made an extensive study of Si0.82Ge0.18 film relaxation on silicon on insulator (SOI) substrates having a top Si layer 40, 70, 330nm, and 10[.proportional]m thick. SiGe films were deposited with a thickness up to 1.2[.proportional]m in an ultrahigh vacuum chemical vapor deposition system at 630°C. Following growth, films were characterized by X-ray diffraction and a dislocation revealing etch. The same level of relaxation is reached for each thickness of SiGe film independent of the substrate structure. Accompanying the film relaxation is the development of a tetragonal tensile strain in the thin Si layer of the SOI substrates. This strain reached 0.22% for the 1.2[.proportional]m film on the 40nm SOI and decreases with SOI thickness. The Si thickness of the SOI substrate also effected the threading dislocation density. For 85% relaxed films the density fell from 7×106 pits/cm2 on bulk Si to 103pits/cm2 for the 40, 70, and 330nm SOI substrates. The buried amorphous layer of the SOI substrate alters the dislocation dynamics by allowing dislocation core spreading or dislocation dissociation. The reduced strain field of these dislocations reduces dislocation interactions and the pinning that results. Without the dislocation pinning, the misfit dislocations can extend longer distances yielding a greatly reduced threading dislocation density.

We demonstrate the use of low-energy electron microscopy (LEEM) as a tool for studying dis-location formation in low-Ge-content SiGe films on Si(001) and silicon-on-insulator. Compared to TEM, sample preparation for LEEM consists only of conventional surface cleaning. Yet, because of its sensitivity to local variations in surface strain on Si(001), LEEM can detect dislocations at the earliest stages of strain relaxation. In identically prepared SiGe films, the typical dislocation extends over the entire viewable region of several hundred microns in SiGe/Si, but is less than 100 microns in SiGe/SOI. In addition, dislocation cross-slip and threading segments are common in SiGe/SOI, but virtually non-existent in SiGe/Si. We have also observed dislocation formation in real-time during high temperature annealing. Preliminary results appear to demonstrate dislocation multiplication and blocking at a perpendicular glide plane. The applicability of LEEM to strain relaxation in other Si-based systems will be discussed.

The development of new chemically based growth techniques has opened the range of possible GaN applications. This paper reviews some of the challenges in the chemically based growth of GaN and related materials. Ammonothermal-based growth, hydride vapor phase epitaxy and metal organic vapor phase epitaxy (MOVPE) are chemically complex systems wherein the underlying mechanisms of growth are not well understood at present. All these systems require substantial experimental and theoretical efforts to determine the nature and kinetics of GaN growth. In the case of metal organic vapor phase epitaxy, the application of computational techniques based on density functional theory have augmented the more conventional experimental approaches to determining the growth chemistry. These chemical reaction schemes, when combined with computational thermal-fluid models of the reactor environment, provide the opportunity to predict growth rates, uniformity and eve ntually materials properties.

Structural properties of epitaxially laterally overgrown (ELO) GaN on patterned GaN ‘substrates’ by hydride vapor phase epitaxy (HVPE) have been investigated. The epitaxially lateral overgrowth of GaN on SiO2 areas is realized and a planar ELO GaN film is obtained. Scanning electron microscope, transmission electron microscope (TEM) and atomic force microscope (AFM) are used to study the structure and surface morphology of the ELO GaN materials. AFM images indicate that no observable step termination is detected over a 4 μm2 area in the ELO region. TEM observations indicate that the dislocation density is very low in the ELO region. No void at the coalescence interface is observed. Lattice bending as high as 3.3° is observed and attributed to pileup of threading dislocations coming from the underlying GaN “seeding layer” and tilting horizontally and quenching at the coalescence interface.

Metal organic vapor phase epitaxy (MOVPE) of GaN has been carried out using diethyl gallium chloride (DEGaCl) and ammonia. The growth rate and efficiency of the DEGaCl-based growth decreases with increasing temperature when compared to trimethyl gallium (TMG)-based growth under similar conditions. Both low temperature buffer and the high temperature GaN layers were grown using the DEGaCl-NH3 precursor combination on the basal plane of sapphire and compared to similar structures grown using TMG and NH3. DEGaCl-based growth reveals an improved growth behavior under identical growth conditions to the conventional TMGa and ammonia growth. X-ray, Hall, and atomic force microscopy (AFM) measurements have been carried out on these samples providing a direct comparison of materials properties associated with these growth precursors. For the DEGaCl-based growth, the x-ray rocking curve line width, using the (0002) reflection, is as low as 300 arcsec on a 2.5-micron thick film. A RMS surface roughness of ∼0.5nm measured over a 10×10 micron area.

Epitaxial lateral overgrowth (ELO) of GaN on SiO2-masked (0001) GaN substrates has been investigated by using chloride-based growth chemistries via hydride vapor phase epitaxy (HVPE) and metal organic vapor phase epitaxy (MOVPE). Diethyl gallium chloride, (C2H5)2GaCl, was used in as the MOVPE Ga precursor. The lateral and vertical growth rates as well as the overgrowth morphology of ELO GaN structures are dependent on growth temperature, V/III ratio and the in-plane orientation of the mask opening. A high growth temperature and low V/III ratio increase the lateral growth rate and produce ELO structures with a planar surface to the GaN prisms. High-quality coalesced and planar ELO GaN has been fabricated by both growth chemistries. The use of the diethyl gallium chloride source allows for the benefits of HVPE growth to be realized within the MOVPE growth environment.

Metal organic vapor phase epitaxy (MOVPE) of GaN has been carried out using diethyl gallium chloride (DEGaCI) and ammonia. The growth rate and efficiency of the DEGaCl-based growth decreases with increasing temperature when compared to trimethyl gallium (TMG)-based growth under similar conditions. Both low temperature buffer and the high temperature GaN layers were grown using the DEGaCI-NH3 precursor combination on the basal plane of sapphire and compared to similar structures grown using TMG and NH3. DEGaCl-based growth reveals an improved growth behavior under identical growth conditions to the conventional TMGa and ammonia growth. X-ray, Hall, and atomic force microscopy (AFM) measurements have been carried out on these samples providing a direct comparison of materials properties associated with these growth precursors. For the DEGaCl-based growth, the x-ray rocking curve line width, using the (0002) reflection, is as low as 300 arcsec on a 2.5-micron thick film. A RMS surface roughness of ∼0.5nm measured over a 10x10 micron area.

Epitaxial lateral overgrowth (ELO) of GaN on SiO2-masked (0001) GaN substrates has been investigated by using chloride-based growth chemistries via hydride vapor phase epitaxy (HVPE) and metal organic vapor phase epitaxy (MOVPE). Diethyl gallium chloride, (C2H5)2GaCl, was used in as the MOVPE Ga precursor. The lateral and vertical growth rates as well as the overgrowth morphology of ELO GaN structures are dependent on growth temperature, V/III ratio and the in-plane orientation of the mask opening. A high growth temperature and low V/III ratio increase the lateral growth rate and produce ELO structures with a planar surface to the GaN prisms. High-quality coalesced and planar ELO GaN has been fabricated by both growth chemistries. The use of the diethyl gallium chloride source allows for the benefits of HVPE growth to be realized within the MOVPE growth environment.

We modeled the breakdown voltage, critical current density and maximum operating frequency of several GaN and GaN/AlN based high power and high temperature electronic devices. Important model parameters which influence device design and performance are minority carrier recombination lifetime and critical field for electric breakdown.

GaN Schottky devices have been fabricated in the planar geometry. Current-voltage measurements indicated the importance of the vertical geometry for achieving large breakdown voltages. The minority carrier (hole) recombination lifetimes have been measured by electron beam induced currents (EBIC). The measured hole lifetime of 7 ns and estimate for the critical field indicate the possibility of GaN/AlGaN thyristor switch devices operating at 5KV with current densities approximately equal to 200 A/cm2 and at frequencies above 2MHz.

The GaN structural and optical materials quality and processing requirement for etching is discussed.

The reliance of the semiconductor industry on pyrophoric silane gas as the primary source of silicon entails high risk and attendant cost. In order to enhance safety and improve control we are engaged in a systematic study of the decomposition characteristics of alkyl silanes with a focus on chemical species and conditions that exhibit silane type behavior or that actually produce silane. We have prepared a series of air stable innocuous liquid alkyl silanes, RSiH3, that generate silane and stable organic (R-H) byproducts upon flow tube pyrolysis. T-buty1, t-hexy1, and t-decyl silane as well as the corresponding R-H products exhibit a range of physical properties : room temperature vapor pressures, for example, range from several Torr to over an atmosphere. Relevant physical properties as well as kinetics and mechanisms of the hydride transfer process will be described and correlated with structural characteristics of the RSiH3 precursors.

The near-infrared photoluminescence of high purity, nominally undoped MOVPE AlGaAs was investigated as a function of growth temperature, aluminum content and hydrostatic pressure. Two PL bands, observed at ˜1.1 eV and ˜0.8 eV independent of aluminum content, were attributed to oxygen-related defects based on the correlation of emission intensity and oxygen concentration. Hydrostatic pressure experiments, along with the measurement temperature dependence, suggest that the ˜0.8 eV band is due to emission from an oxygen-related mid-gap level to a shallow acceptor or the valence band, depending on temperature. A tentative defect model based on the off-center OA, defect in bulk GaAs and variations in the number of nearest neighbor aluminum atoms is proposed to explain the two PL bands and the dependence of their relative intensity on aluminum content.

A variety of metal films deposited at room temperature have been found to grow epitaxially under conventional vacuum conditions on GaN grown by metalorganic vapor phase epitaxy on sapphire substrates. The metal films have been characterized by X-ray diffraction using a thin-film Read camera and by MeV ion channeling measurements. Lattice mismatch between the epitaxial metals and the GaN basal planes ranges from ∼ 0.2% to ∼ 22%, and does not appear to be the determining factor in the epitaxy reported here. Surface structure of the epitaxial metal films has been studied by atomic force microscopy and found to differ considerably from that of nonepitaxial metal films grown on similar GaN substrates.

A model for the growth of gallium nitride in a vertical metalorganic vapor phase epitaxy reactor is presented. For a mixture of non-dilute gases, the flow temperature and concentration profiles are predicted. The results show that the growth of GaN epilayers is through an intermediate adduct of TMG and ammonia. Growth rates are predicted based on simple reaction mechanisms and compared with those obtained experimentally. Loss of adduct species due to polymerization leads to lowering in growth rate. An attempt to quantify loss of reacting species is made based on experimentally observed growth rates.

A major limitation of the current technology for GaN epitaxy is the availability of suitable substrates matched in both lattice constant and thermal expansion coefficient. One alternative for the development of GaN substrates rests in the application of halide vapor phase epitaxy (HVPE) to produce GaN films at high growth rates. In this paper, we describe the growth of thick GaN films via the HVPE technique on (0001) sapphire and (111) Si substrates. At a temperature of 1030°C, films are grown at rates between 70 and 90 μm/hr, yielding total thicknesses exceeding 200 μm on sapphire. DCXRD measurements of GaN/sapphire indicate FWHM values less than 220 arcsec on 180 μm thick films. Room temperature PL measurements of GaN/sapphire indicate strong emission at 3.41 eV, with a FWHM value of 65 meV. Moreover, no detectable deep level emission was found in room temperature PL measurement. Under optimized conditions, films are morphologically smooth and optically clear. The GaN morphology appears to be a strong function of the initial nucleation conditions, which in turn are strongly affected by the partial pressure of GaCl. HVPE growth on (111) Si substrates is accomplished using an AlN MOVPE buffer layer.

Gas phase reactions between trimethylgallium (TMG) and ammonia were studied at high temperatures, characteristic to MOCVD of GaN reactors, by means of insitu mass spectroscopy in a flow tube reactor. It is shown, that a very fast adduct formation followed by elimination of methane occurs. The decomposition of TMG and the adduct - derived compounds are both first order and have similar apparent activation energy. The pre-exponential factor of the adduct decomposition is smaller, and hence is responsible for the higher full decomposition temperature of the adduct relative to that of TMG.

ZnTe has gained attention as a promising material for short wavelength optoelectronic applications. Unlike ZnSe, ZnTe can be doped p-type but not n-type. There are few reports detailing the growth and doping of ZnTe films through the MOVPE technique. We present growth and doping studies for ZnTe formed through low pressure MOVPE. Dimethylzinctriethylamine and diisopropyltelluride were chosen as the metalorganic precursors. Growth studies were carried out on both GaAs (100) and GaSb (100) substrates. Growth morphology was very sensitive to both the lattice mismatch and the initial surface preparation. This is particularly evident on the GaSb substrates. ZnTe growth temperatures ranged from 340 to 420°C, and growth rates of up to 1 μm/hour were achieved. ZnTe was found to be readily doped using tertiarybutylphosphine, with p-type concentrations ranging from 4×1016 cm−3 to 2x1019 cm−3, and corresponding room temperature hole mobilities between 55 and 12 cm2/V-sec. Tertiarybutylphosphine was also observed to significantly improve the surface morphology for growths on GaSb substrates.

Impurities at heterointerfaces can alter the interfacial structure resulting in changes in physical, electrical and optical properties. We present a study of the interfacial roughness of GaAs/A1xGa1-xAs superlattices which were grown using controlled addition of oxygen at the interface. The interfacial properties were characterized by x-ray diffraction. The morphology of the surface was determined by Atomic Force Microscopy (AFM). X-ray diffraction measurements, both θ-2θ and rocking curves, were used to analyze the correlated and uncorrelated component of the interfacial roughness. A strong difference in the interfacial roughness was observed depending on whether the intentional oxygen incorporation occurred at the GaAs-to-A1GaAs interface or at both interfaces. When oxygen is incorporated at both interfaces, the x- ray reflectivity of the superlattice is decreased considerably resulting from a much higher interfacial roughness. The substrate miscut has a significant effect on RMS roughness, correlated roughness and its correlation length when oxygen is incorporated at the GaAs-to-A1xGa1-xAs interface.

A systematic study of the enhancement of Schottky barriers to n-GaAs diodes has been carried out using the Ni-Al binary system. The diodes, Ni2Al3/n-GaAs, Ni2Al3/Ni/n-GaAs, Ni/Al/Ni/n-GaAs and NiAl/Al/Ni/n-GaAs, have been realized by sputter deposition at a base pressure ∼2xl0-7 Torr. A high Schottky barrier height ranging from 0.95 to 0.98 eV (deduced from current-voltage measurements) was observed for all the annealed contacts except for Ni2Al3/n-GaAs contacts. The enhancement of the Schottky barrier height in all the contacts was attributed to the formation of a high Al content (Al,Ga)As layer at the metal/semiconductor interface. The formation of this (Al,Ga)As layer was explained in terms of a regrowth mechanism. In this mechanism, Ni reacts with GaAs initially at low temperatures, forming NixGaAs. The NixGaAs layer is believed to react with the Ni-Al layer to form the (Al,Ga)As layer when subjected to a high temperature annealing. A (200) dark field XTEM image of the annealed contact was used to demonstrate the existence of this (Al,Ga)As phase.