Structural study of GaN grown on (001) GaAs by organometallic vapor phase epitaxy

Detailed transmission electron microscope (TEM) and transmission electron diffraction (TED) examination has been performed on organometallic vapor phase epitaxial GaN layers grown on (001) GaAs substrate to investigate microstructures and phase stability. TED and TEM results exhibit the occurrence of a mixed phase of GaN. The wurtzite (α) phase grains are embedded in the zinc-blende (β) phase matrix. It is shown that there are two types of the wurtzite GaN phase, namely, the epitaxial wurtzite and the tilted wurtzite. The tilted wurtzite grains are rotated some degrees ranging from ∼5° to ∼35° regarding the GaAs substrate. A simple model is presented to describe the occurrence of the mixed phases and the two types of the wurtzite phase.     

Structural and optical properties of GaxIn1−xP layers grown by chemical beam epitaxy

Chemical beam epitaxial (CBE) Ga_xIn_1?xP layers (x≈0.5) grown on (001) GaAs substrates at temperatures ranging from 490 to 580°C have been investigated using transmission electron diffraction (TED), transmission electron microscopy, and photoluminescence (PL). TED examination revealed the presence of diffuse scattering 1/2{111}_B positions, indicating the occurrence of typical CuPt-type ordering in the GaInP CBE layers. As the growth temperature decreased from 580 to 490°C, maxima in the intensity of the diffuse scattering moved from ½{111}_B to ½{?1+δ,1?δ,0} positions, where δ is a positive value. As the growth temperature increased from 490 to 550°C, the maxima in the diffuse scattering intensity progressively approached positions of $\frac{1}{2}\{\bar 110\} $ , i.e., the value of δ decreased from 0.25 to 0.17. Bandgap reduction (～45 meV) was observed in the CBE GaInP layers and was attributed to the presence of ordered structures.     

Structural properties of GaAsN grown on (001) GaAs by metalorganic molecular beam epitaxy

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs_1−xN_x layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

Analysis of the stress and interfacial reactions in Pt/Ti/SiO2/Si for use with ferroelectric thin films

The microstructure of the Pt/Ti/SiO₂/Si structure has been investigated by scanning and transmission electron microscopy. Pt films of 100 nm thickness deposited by sputtering or evaporation onto unheated substrates gave complete coverage of the underlying Ti layer and showed a granular and faceted structure with grains ∼20 nm in diameter. They did not exhibit hillocks or surface TiO_x formation. X-ray diffraction was used to examine the film stress through use of the sin²ψ method with bulk values for the elastic constants (v=0.39, E=162 GPa). The as-deposited sputtered film had a compressive stress of ∼540 MPa, while the evaporated films had tensile stresses of ∼630 MPa. The films then received a 400°C rapid thermal anneal (RTA) for 90 s and a subsequent RTA of 650°C for 30s. Further investigation of the film stresses and microstructure were made after each annealing step. After the low temperature anneal, the film stress for the sputtered film became tensile. Plan-view sections examined by transmission electron microscopy (TEM) showed that the as-deposited sputtered films were dense but became porous after annealing. Initially, the evaporated films had a less dense microstructure, but were more stable with annealing. Little change in the stress for the evaporated film was observed after this initial low temperature annealing step. Additional annealing of the evaporated and sputtered samples caused complete consumption of the Ti layer including some TiO_x formation from the underlying SiO₂ layer and marked interaction with the Pt; however, little change in the stress was found. The surface of the Pt film revealed larger grains, but otherwise remained unaffected. The underlying phase changes were minimized once the Ti layer had reacted with the Pt. Due to the ratio of the layers, Pt:Ti of 2:1, the surface of the Pt was unaffected.     

Investigation of microstructure and V-defect formation in In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N/GaN MQW grown using temperature-gradient metalorganic chemical vapor deposition

Temperature-gradient metalorganic chemical vapor deposition (MOCVD) was used to deposit In_xGa_1−xN/GaN multiple quantum well (MQW) structures with a concentration gradient of indium across the wafer. These MQW structures were deposited on low defect density (2×10⁸ cm⁻²) GaN template layers for investigation of microstructural properties and V-defect (pinhole) formation. Room temperature (RT) photoluminescence (PL) and photomodulated transmission (PT) were used for optical characterization, which show a systematic decrease in emission energy for a decrease in growth temperature. Triple-axis x-ray diffraction (XRD), scanning electron microscopy, and cross-sectional transmission electron microscopy were used to obtain microstructural properties of different regions across the wafer. Results show that there is a decrease in crystal quality and an increase in V-defect formation with increasing indium concentration. A direct correlation was found between V-defect density and growth temperature due to increased strain and indium segregation for increasing indium concentration.     

Investigation of GaNxP1−x/GaP LED structure optical properties

GaN_xP_1−x alloy represents a novel compound semiconductor that has attracted considerable interest as a candidate for realization of light emitting diodes (LEDs) in the green-red range of the visible spectra. Simple GaN_xP_1−x/GaP LED structures grown by low-preassure metalorganic vapor phase epitaxy and containing different N contents (0.6–2.3%) were investigated. The hierarchy of N complexes that generate different bound states were determined from photocurrent and electroluminescence spectra for different N concentrations in the GaN_xP_1−x layer. From the experimental measurements, it was confirmed that the electroluminescence emission peaks show discrete emission maxima at ∼608 nm and ∼628 nm with increasing N content due to formation of N clusters.     

Surface and Interface Properties of Metal-Organic Chemical Vapor Deposition Grown a -Plane Mg x Zn1– x O (0?≤ x ≤?0.3) Films

The a-plane Mg _x Zn_1−x O (0 ≤ x ≤ 0.3) films were grown on r-plane () sapphire substrates using metal-organic chemical vapor deposition (MOCVD). Growth was done at temperatures from 450°C to 500°C, with a typical growth rate of ∼500 nm/h. Field emission scanning electron microscopy (FESEM) images show that the films are smooth and dense. X-ray diffraction (XRD) scans confirm good crystallinity of the films. The interface of Mg _x Zn_1−x O films with r-sapphire was found to be semicoherent as characterized by high-resolution transmission electron microscopy (HRTEM). The Mg _x Zn_1−x O surfaces were characterized using scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). Low-energy electron diffraction (LEED) shows well-ordered and single-crystalline surfaces. The films have a characteristic wavelike surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Photoluminescence (PL) measurements show a strong near-band-edge emission without observable deep level emission, indicating a low defect concentration. In-plane optical anisotropic transmission was observed by polarized transmission measurements.     

Electron microscope studies of InxGav1−xAs/GaAs/Si grown by metalorganic chemical vapor deposition

The microstructure of In_xGa_1−xAs/GaAs (5 nm/5 nm, x < 0 to 1.0), as grown by a metalorganic chemical vapor deposition two-step growth technique on Si(100) at 450‡C, and subsequently annealed at 750‡C, is investigated using plan-view and cross-sectional transmission electron microscopy. The variations in resultant island morphology and strain as a function of the In content were examined through the comparison of the misfit dislocation arrays and moirés observed. The results are discussed in relation to the ways in which the island relaxation process changes for high In content.     

Investigation of HgCdTe p-n device structures grown by liquid-phase epitaxy

The microstructure of p-n device structures grown by liquid-phase epitaxy (LPE) on CdZnTe substrates has been evaluated using transmission electron microscopy (TEM). The devices consisted of thick (∼21-μm) n-type layers and thin (∼1.6-μm) p-type layers, with final CdTe (∼0.5 μm) passivation layers. Initial observations revealed small defects, both within the n-type layer (doped with 8×10¹⁴/cm³ of In) and also within the p-type layer but at a much reduced level. These defects were not visible, however, in cross-sectional samples prepared by ion milling with the sample held at liquid nitrogen temperature. Only isolated growth defects were observed in samples having low indium doping levels (2×10¹⁴/cm³). The CdTe passivation layers were generally columnar and polycrystalline, and interfaces with the p-type HgCdTe layers were uneven. No obvious structural changes were apparent in the region of the CdTe/HgCdTe interfaces as a result of annealing at 250°C.     

Structural and chemical characterization of as-deposited microcrystalline indium oxide films prepared by dc reactive magnetron sputtering

Microcrystalline indium oxide (InO_x) films with thickness of 120–1600 nm were prepared by dc reactive magnetron sputtering in various mixtures of oxygen in argon at room temperature. The depositions were carried out onto Corning 7059 glass and silicon substrates. The conductivity of the as-deposited films can change in a controllable and fully reversible manner by about six orders of magnitude by alternately exposing the films to ultraviolet (UV) light (hv≥3.5eV) in vacuum and reoxidizing them in ozone. The microstructure of the films was investigated using transmission electron microscopy (TEM) and electron diffraction. For this purpose, films with a thickness of about 100 nm were deposited onto NaCl substrates. The surface and depth composition of the films were examined using Auger electron spectroscopy (AES) combined with depth profiling analysis. The depth profiles showed that all the films exhibit an extremely good in-depth uniformity, all the way to the interface with the glass substrate, regardless of their thickness. Quantitative Auger and energy dispersive x-ray (EDX) analyses were employed to determine the stoichiometry of the films. An oxygen deficiency of 2–5% has been observed with respect to the stoichiometric composition. The effects of film thickness and oxygen content in the sputtering gas on the stoichiometry were examined. Both AES and EDX analyses confirmed that the stoichiometry is invariant for these parameters.     

Control of ordering in GaInP and effect on bandgap energy

Ga_xIn_1-x P layers with x ≈ 0.5 have been grown by atmospheric pressure organometallic vapor phase epitaxy on GaAs substrates with 10 micron wide, [110]-oriented grooves produced photolithographically on the surface. The [110] steps and the misorientation produced at the edges of the grooves have been found to have important effects on the formation of the Cu-Pt ordered structure (ordering on {111} planes) in the GaInP layers during growth. In this work, the groove shape is demonstrated to be critically important. For the optimum groove shape, with a maximum angle to the (001) surface of between 10 and 16°, single domains of the (-111) and (1-11) variants of the Cu-Pt ordered structure are formed on the two sides of the groove. Shallow (≤0.25 μm deep) grooves, with maximum angles of <10°, are less effective. Within the large domains on each side of the groove, small domains of the other variant are observed. The boundary between the two domains is seen to wander laterally by a micron or more during growth, due to the change in shape of the groove during growth. For deep (1.5 μm) grooves, with maximum angles to the (001) plane of 35°, only a single variant is formed on each side of the groove. However, the domains are small, dispersed in a disordered matrix. For substrates with deep grooves on a GaAs substrate misoriented by 9° toward the [-110] direction, an interesting and useful pattern is produced. One half of the groove is a single domain which shrinks in size as the growth proceeds. The other half of the groove, where the misorientation is larger, is disordered. Thus, every groove contains large (>1 μm² cross-sectional area and several mm long) regions of highly ordered and completely disordered material separated by no more than a few microns. This allows a direct determination of the effect of ordering on the bandgap of the material using cathodoluminescence (CL) spectroscopy. The 10K photoluminescence (PL) consists of three distinct peaks at 1.94, 1.88, and 1.84 eV. High resolution CL images reveal that the peaks come from different regions of the sample. The high energy peak comes from the disordered material and the low energy peak comes from the large ordered domains. Electron microprobe measurements of the solid composition demonstrate that the shift in emission energy is not due to changes in solid composition. This is the firstdirect verification that ordering causes a reduction in bandgap of any III/V alloy. Decreasing the Ga_0.5In_0.5P growth rate from the normal 2.0 to 0.5 μ/h is found to enhance ordering in layers grown on planar GaAs substrates. Transmission electron diffraction results show that the domain size also increases significantly. For material grown on exactly (001)-oriented substrates, a pronounced [001] streaking of the superlattice spots is observed. This is correlated with the presence of a dense pattern of fine lines lying in the (001) plane in the transmission electron micrographs. The PL of this highly ordered material consists of a single peak that shifts to higher energy by > 110 meV as the excitation intensity is increased by several orders of magnitude.     

Use of electron cyclotron resonance plasmas to prepare CdZnTe (211)B substrates for HgCdTe molecular beam epitaxy

Without any additional preparation, Cd_1−yZn_yTe (211)B (y∼3.5%) wafers were cleaned by exposure to an electron cyclotron resonance (ECR) Ar/H₂ plasma and used as substrates for HgCdTe molecular beam epitaxy. Auger electron spectra were taken from as-received wafers, conventionally prepared wafers (bromine: methanol etching, followed by heating to 330–340°C), and wafers prepared under a variety of ECR process conditions. Surfaces of as-received wafers contained ∼1.5 monolayers of contaminants (oxygen, carbon, and chlorine). Conventionally prepared wafers had ∼1/4 monolayer of carbon contamination, as well as excess tellurium and/or excess zinc depending on the heating process used. Auger spectra from plasma-treated CdZnTe wafers showed surfaces free from contamination, with the expected stoichiometry. Stoichiometry and surface cleanliness were insensitive to the duration of plasma exposure (2–20 s) and to changes in radio frequency input power (20–100 W). Reflection high energy electron diffraction patterns were streaked indicating microscopically smooth and ordered surfaces. The smoothness of plasma-etched CdZnTe wafers was further confirmed ex situ using interferometric microscopy. Surface roughness values of ∼0.4 nm were measured. Characteristics of HgCdTe epilayers deposited on wafers prepared with plasma and conventional etching were found to be comparable. For these epilayers, etch pit densities on the order of 10⁵ cm⁻² have been achieved. ECR Ar/H₂ plasma cleaning is now utilized at Night Vision and Electronic Sensors Directorate as the baseline CdZnTe surface preparation technique.     

Microcharacterization of composition modulations in epitaxial ZnSe1-xTex

Molecular beam epitaxial growth of the ZnSe_1-xTe_x (x=0.44-0.47) alloy on vicinal (001) GaAs substrates tilted four, six, and nine degree-[111]A or B results in partial phase separation of the alloy with a vertical modulation between different compositions. Transmission electron microscopy images of samples grown on four degree-tilted substrates showed superlattice-like structures, with periods in the range 13.4-28.9Â. Lattice images reveal diffuse interfaces between light and dark bands. Period variations were detected in isolated regions of some samples. We present evidence that the modulation develops at the growth surface, and remains stable in the bulk at temperatures up to 450°C. Satellite spot pairs with approximate indices (h k 1 + δ) were present near the zinc-blende spots in electron diffraction patterns and x-ray diffraction data, as expected from material with a sinusoidal composition profile. The orientation of the spots reveals that the modulation vector is parallel to the growth direction, rather than to [001]. The [111]A- and B-tilted samples showed significant modulation, while the five degree-[110] and on-axis material showed no detectable modulation. The modulation wavelength did not strongly depend on growth temperature in the range examined (285–335°C). Samples showing composition modulation did not exhibit significantly altered low-temperature luminescence spectra from material with no modulation.     

Reciprocal space mapping of ordered domains in InxGa1−xP

We applied high resolution x-ray diffraction techniques to determine the three dimensional domain shape in nominally lattice-matched, CuPt_B ordered, In_xGa_1−xP epitaxial layers deposited on GaAs by metal organic vapor phase epitaxy. A technique of reciprocal space mapping is described which provides three-dimensional information on the shape and size of the ordered domains. The domain shape is obtained by reciprocal space mapping along orthogonal crystallographic directions. Applying this technique shows that, at large miscut angles, the domains are oriented close to the growth direction, and are elongated by differing extents along the [110] and [ ].     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

Chemical beam epitaxy and laser-modified chemical beam epitaxy of InGaAs using tris-dimethylaminoarsenic

The growth of In_xGaj_1−xAs (x = 0.13–0.25) on GaAs by chemical beam epitaxy (CBE) and laser-modified CBE using trimethylindium (TMIn), triethylgallium (TEGa), and tris-dimethylaminoarsenic (TDMAAs) has been studied. Reflection high-energy electron diffraction measurements were used to investigate the growth behavior of InGaAs at different conditions. X-ray rocking curve and lowtemperature photoluminescence (PL) measurements were used to characterize the InGaAs/GaAs pseudomorphic strained quantum well structures. Good InGaAs/GaAs interface and optical property were obtained by optimizing the growth condition. As determined by the x-ray simulation, laser irradiation during the InGaAs quantum well growth was found to enhance the InGaAs growth rate and reduce the indium composition in the substrate temperature range studied, 440–500°C, where good interfaces can be achieved. These changes, which are believed to be caused by laser-enhanced decomposition of TEGa and laser-enhanced desorption of TDMAAs, were found to depend on the laser power density as well. With laser irradiation, lateral variation of PL exciton peaks was observed, and the PL peaks became narrower.     

MBE growth and properties of GaN and AlxGa1−xN on GaN/SiC substrates

The growth of GaN and AlGaN by molecular beam epitaxy (MBE) has been studied using GaN/SiC substrates. The GaN/SiC substrates consisted of ∼3 μm thick GaN buffer layers grown on 6H-SiC wafers by metalorganic vapor phase epitaxy (MOVPE) at Crée Research, Inc. The MBE-grown GaN films exhibit excellent structural and optical properties—comparable to the best GaN grown by MOVPE. Al_xGa_1−xN films (x ∼ 0.06-0.08) and Al_xGa_1−xN/GaN multi-quantum-well structures which display good optical properties were also grown by MBE on GaN/SiC substrates.     

Characterization survey of Ga<Subscript>x</Subscript>In<Subscript>1−x</Subscript>As/InAs<Subscript>y</Subscript>P<Subscript>1−y</Subscript> double heterostructures and InAs<Subscript>y</Subscript>P<Subscript>1−y</Subscript> multilayers grown on InP

Low-bandgap, lattice-mismatched Ga_xIn_1−xAs (GaInAs) grown using InAs_yP_1−y (InAsP) compositional-step grades on InP is a primary choice for lightabsorbing, active layers in high-efficiency thermophotovoltaic (TPV) devices. The GaInAs/InAsP double heterostructures (DHs) show exceptional minority carrier lifetimes of up to several microseconds. We have performed a characterization survey of 0.4–0.6-eV GaInAs/InAsP DHs using a variety of techniques, including transmission electron microscopy (TEM). Dislocations are rarely observed to thread into the GaInAs active layers from the InAsP buffer layers that terminate the graded regions. Nearly complete strain relaxation occurs in buried regions of the InAsP grades. The buffer-layer strain prior to deposition of the active layer is virtually independent of the net misfit. Foreknowledge of this buffer-layer strain is essential to correctly lattice match the buffer to the GaInAs active layer.     

Investigations on the nanostructures of GaN,InN and InxGa1−xN

A controllable approach to the formation of III- nitride nanocrystalline structures using hydrothermal assisted method is presented. The structural and morphological properties of the prepared nanostructures are analyzed using X-ray diffraction, Fast Fourier Transformation and transmission electron microscope techniques. The temperature dependent structural formation of nitride nanostructures have been systematically investigated using X-ray diffraction. Raman spectra of the samples grown at optimized condition exhibited different phonon modes of the respective nitrides (GaN, InN and In_xGa_1−xN). Nanoparticles and nanorods formation of the indium nitride and indium gallium nitride are observed in the TEM micrographs. FFT analysis revealed that the synthesized III-nitride nanostructures are of good crystalline quality. Nanorods of these nitrides showed better crystalline quality than the nanoparticles in the FFT reflections.