Examination of In-Grown Stacking Faults in 8°- and 4°-Offcut 4H-SiC Epitaxy by Photoluminescence Imaging

Thirty-six large, up to 3-inch-diameter, epitaxial 4H-SiC samples were mapped by photoluminescence imaging. In-grown stacking faults (IGSFs) for both 8°- and 4°-offcut were examined structurally and spectrally. Imaging at various spectral bands revealed different features for IGSFs. For the 8°-offcut, IGSFs possessed two well-defined shapes, while for the 4°-offcut IGSFs appear with a variety of shapes. The difference in IGSF formations between 8°- and 4°-offcut is currently unknown. Screw dislocations displaced the IGSF basal plane, producing line defects that possessed irregular intensity. Rough estimates of the IGSF density were performed over representative regions of the whole wafers with some wafers having <1 cm⁻² while others had >100 cm⁻². Most IGSFs (>95%) originated at the epilayer/substrate interface, revealed by a small triangle in the buffer layer. Particles were responsible for the few IGSFs formed after the initial growth. The results suggested that pregrowth treatment and initial growth conditions were responsible for forming a majority of the IGSFs.     

Structural study of degraded ZnMgSSe blue light emitters

Using electroluminescence (EL) topography and transmission electron microscopy (TEM), we investigated the nonluminescent regions which form while current is being injected into ZnMgSSe/ZnSSe/ZnCdSe-based blue light emitters. Small dark spots were observed just after turn-on and spread out forming rough nonluminescent triangles in the <100> directions in the EL image of the active region. TEM studies showed that the small dark spots are pre-existing stacking faults originating at the substrate/epitaxial layer interface. The nonluminescent triangles were found to be a dense region of dislocation dipoles and dislocation loops. Each dipole was aligned along two <110> directions in the {111} planes. The Burgers vectors were of the type a/2<011> inclined at 45° to the (001) junction plane.     

Suppression of twin formation in CdTe(111)B epilayers grown by molecular beam epitaxy on misoriented Si(001)

CdTe(lll)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(lll)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[11-2] along Si[1-10] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].     

Transmission electron microscopy and x-ray topography study of mercury-cadmium-telluride

The fault structure of MCT crystals was studied by means of X-ray topography and transmission electron microscopy. Subgrain boundaries were revealed and identified by both techniques, and their tilt angle was calculated to be of the order of 10^-3 rad. The dislocations which form the subgrain boundaries were identified by TEM to have Burger‘s vectors parallel to the <110> directions. Twins in large numbers were observed by TEM. The twins were identified to have a {111} habit plane.     

The effect of substrate tilt on MOCVD growth of {100}CdTe on {100}GaAs

Epitaxial layers of CdTe were grown by metalorganic chemical vapor deposition on surfaces of single crystal, {100} GaAs which had been ground, polished, and etched to a spherically shaped done. This dome-shaped surface allowed the morphological and structural properties of the epitaxial CdTe layers to be determined for all 360° of azimuth and up to 15° of polar angle from the [100] axis within a single growth experiment. At two growth temperatures, approximately 275 and 375°C, the results show distinct twofold rotational symmetry in both morphology and crystal perfection as determined by x-ray rocking curve measurement. Surface morphology is superior at azimuths near tilts toward the <111>A pole. Four-sided pyramidal hillocks appear at other azimuths and at 0° tilt; the symmetry of the hillocks diminishes as the tilt increases. The orientations for growth which simultaneously minimize the surface defects and rocking curve full-width half-maximum appear to be at locations on the surface where the surface normal is tilted 3–4° toward the <111>A or <111>B, depending on the temperature regime chosen. Epitaxial layers grown on planar wafers of {100}GaAs tilted toward <111>Ga and <111>As show surface morphology essentially identical to the dome at these orientations. The surface morphology of CdTe growth on GaAs/Si wafers suggests that these layers are tilted toward the <111>B.     

Growth of polycrystalline silicon films on titanium and aluminum layers

Adherent, polycrystalline silicon films were vacuum deposited onto titanium passivated steel alloy substrates at substrate temperatures between 535 and 650°C and onto aluminum films at substrate temperatures between 480 and 520°C. Silicon films deposited onto titanium layers are characterized by a sub-micron grain size and a preferential orientation of the <110> direction perpendicular to the growth surface. Resistivities of ∿10⁴ ohm-cm are measured for the undoped films. Silicon films deposited onto aluminum layers have a larger grain size, ∿5μm, a columnar morphology and a preferential orientation of the <111> direction perpendicular to the growth surface. As-deposited resistivities of ∿10² ohm-cm are measured for these films. Boron and phosphorus doped silicon films on titanium layers were annealed. The behavior with annealing of the electrical properties of the films depended on which doping impurity was used. Silicon films on aluminum were annealed to reduce lattice damage within the silicon grains and to dope the films with aluminum from the aluminum layer. Resistivities of several ohm-cm were measured for the annealed films on aluminum.     

Influence of Substrate Misorientation on the Composition and the Structural and Photoluminescence Properties of Epitaxial Layers Grown on GaAs(100)

The influence of the degree of misorientation and treatment of a GaAs substrate on the structural and optical characteristics of homoepitaxial GaAs/GaAs(100) structures grown by metal–organic chemicalvapor deposition is studied. From the data obtained by a series of structural and spectroscopic techniques, it is shown that the degree of deviation of the substrate from the exact orientation towards the [110] direction by an angle of up to 4° brings about stepwise growth of the GaAs film in the initial stage and a further increase in the degree of misorienration towards the [110] direction to 10° results in an increase in the number of structural defects in the epitaxial film. At the same time, the samples of homoepitaxial structures grown by metal–organic chemical-vapor deposition on GaAs(100) substrates misoriented by 4° towards the [110] direction possess the highest photoluminescence efficiency; it is ~15% higher than the corresponding quantity for structures grown on precisely oriented GaAs(100) substrates. Preliminary polishing of the GaAs substrate (removal of an oxide layer) also yields the intensification of photoluminescence emission compared to emission in the case of an unpolished substrate of the same type. For samples grown on substrates misoriented by 4°, such an increase in the photoluminescence efficiency is ~30%.     

Molecular beam epitaxy—Grown ZnSe nanowires

Molecular beam epitaxy (MBE) via the vapor-liquid-solid (VLS) reaction was used to grow ZnSe nanowires (NWs) on (111), (100), and (110) oriented GaAs substrates. Through detailed transmission electron microscopy (TEM) studies, it was found that 〈111〉 orientation is the growth direction for NWs with size ≥30 nm, while NWs with size around 10 nm prefer to grow along the 〈110〉 direction, with a small portion along the 〈112〉 direction. These observations have led to the realization of vertical ZnSe NWs with size around 10 nm grown on a GaAs (110) substrate. An ordered ZnSe NW array fabricated on a GaAs (111) substrate with a novel prepatterning method associated with plasma etching shows a high degree of ordering and a good size uniformity of the as-grown NWs. The diameter of the NWs in the array is around 80 nm and most of them are found to orient vertically, but some tilt to one of the six possible directions of the 〈111〉 family.     

Scanning force microscopy studies of GaAs films grown on offcut Ge substrates

The surface morphology of GaAs films grown on offcut Ge substrates is studied using a scanning force microscope (SFM). We investigated the effects of the Ge buffer layer, growth temperature, film thickness, and prelayer on the GaAs surface morphology. The starting Ge substrates are offcut 6° toward the [110] direction to minimize single steps on the substrates before molecular beam epitaxial film growth. We find that comparing with GaAs samples grown without Ge buffer layers or with unannealed Ge buffer layers, samples with annealed Ge buffer layers are much smoother and contain no antiphase boundaries (APBs) on the surface. For thick (≥1 μm) GaAs films with an annealed Ge buffer layer, the surfaces display crosshatch lines and elongated mounds (along , which are associated with the substrate offcut direction. As the film thickness increases, the crosshatch lines become shorter, denser and rougher, and the mounds grow bigger (an indication of GaAs homoepitaxial growth). We conclude that annealed Ge buffer layers are crucial for growing high quality GaAs films with few APBs generated during the growth. In addition, under optimal conditions, different prelayers make little difference for thick GaAs films with annealed Ge buffer layers.     

LP-MOCVD grown (lnAs)m(GaAs)m short period superlattices on InP

(InAs)_m(GaAs)_m (1 ≤ m ≤ 12) short period superlattices (SPSs) have been grown on semi-insulated InP substrates with a 200 nm InP cap layer using low pressure metalorganic chemical vapor deposition (MOCVD). According to double crystal x-ray diffraction and transmission electron microscopy results, the critical layer thickness of (InAs)_m(GaAs)_m SPS was observed to be ～30Å (m = 5). For the SPS below the critical layer thickness, mirror-like surface morphology was found without defects, and strong intensity Fourier transformed photoluminescence (FT-PL) spectra were also obtained at room temperature. The SPS with m = 4 showed a drastic improvement in photoluminescence intensity of order of two compared to an InGaAs ternary layer. However, the SPS with a large value of m (m ≥ 6), rough surface was observed with defects, with broad and weak FT-PL spectra. The surface morphology of SPS was greatly affected by the substrate orientation. The SPS with m = 5 was grown on two degree tilted substrate from (100) direction and showed poor surface morphology as compared to the one grown on (100) exact substrate Moreover, the SPS grown on a (111)B substrate showed a rough triangular pattern with Nomarski optical microscopy. In-situ thermal annealed SPS with m = 4 showed a 18 meV increase in PL peak energy compared to the as-grown sample due to phase separation resulting from thermal interdiffusion.     

Domain epitaxial growth of TiN/Si(001), TiN/GaAs(001), and Si/TiN/Si(001) heterostructures by laser physical vapor deposition: Theory and experiment

We have successfully deposited epitaxial titanium nitride films on (001) silicon and (001) gallium arsenide substrates and multilayer Si/TiN/Si(001) epitaxial heterostructures using pulsed laser (KrF: λ = 248 nm, τ = 25 ns) physical vapor deposition. The deposition of TiN was carried out at a substrate temperature of 600°C on Si(001) and 400°C on GaAs(00l). The interfaces were sharp without any indication of interfacial reaction. The epitaxial relationships were found to be <001> TiN ‖<001> Si on the silicon substrate, <001> Si ‖<001> TiN |<001> Si on the heterostructure, and [1-10] TiN‖[110] GaAs and [001] TiN ‖[110] GaAs on the GaAs substrate. The growth in these large-mismatch systems is modeled and the various energy terms contributing to the growth of these films are determined. The domain matching epitaxy provides a mechanism of epitaxial growth in systems with large lattice mismatch.The epitaxial growth is characterized by domain epitaxial orientation relationships with m lattice constants of epilayer matching with n of the substrate and with a small residual domain mismatch present in the epilayer. This residual mismatch is responsible for a coherent strain energy. The magnitude of compression of Ti-N bond in the first atomic layer, contributing to the chemical free energy of the interface during the initial stages of growth, is found to be a very important factor in determining the orientation relationship. This result was used to explain the differences in the orientaion relationships between TiN/Si and TiN/GaAs systems. The various energy terms associated with the domain epitaxial growth are evaluated to illustrate that the domain epitaxial growth is energetically favorable compared to the lattice mismatched epitaxial growth. The results of this analysis illustrate that the observed variations in the epitaxial growth are consistent with the minimum energy configurations associated with the domain epitaxial growth.     

Morphology and defect structures of GaSb islands on GaAs grown by metalorganic vapor phase epitaxy

The initial nucleation of GaSb on (001) GaAs substrates by metalorganic vapor phase epitaxy has been investigated using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). TEM results showed that the GaSb islands experience a morphological transition as the growth temperature increases. For growth at 520°C, the islands are longer along the [110] direction; at 540°C, they are nearly square, and at 560°C, they are longer along the direction. Possible mechanisms are proposed to describe such a transition. TEM and HREM examination showed that lattice misfit relaxation mechanisms depend on the growth temperature. For the sample grown at 520°C, the lattice mismatch strain was accommodated mainly by 90° dislocations; for the sample grown at 540°C, the misfit strain was relieved mostly by 90° dislocations with some of 60° dislocations, and for the sample grown at 560°C, the strain was accommodated mainly by 60° dislocations which caused a local tilt of the GaSb islands with respect to the GaAs substrate. The density of threading dislocations was also found to be dependent on the growth temperature. Mechanisms are proposed to explain these phenomena.     

Degradation of ZnSe/ZnTe multiquantum well contacts to p-ZnSe

The work presented in this paper studied the degradation of ZnTe/ZnSe multiquantum well contacts to p-ZnSe under high current loading (1000 to 1500 A/cm²). During degradation, localized heating (up to 200°C > the bulk substrate and heat sink) was observed to occur at the point were electrical power was supplied. Auger data from degraded samples indicated that due to the localized heating, Zn and Te from the ZnTe layers and Zn from the ZnSe layer diffused through the Au metallization to the samples surface. In addition, thermal stress from the localized heating generated micro-cracks in the ZnSe which acted as high diffusivity paths for impurities. Rectangular defects were also found to form in the degraded region. These defects were oriented to the micro-cracks and had similar geometries as dislocation patches (dark line defects) which have been reported to form in the quantum well region of degraded ZnSe based laser devices. The similarities between the rectangular defects and dark line defects suggest the formation of similar dislocation patches in the quantum well region of the multiquantum well contacts.     

In situ observation of surface morphology of InP grown on singular and vicinal (001) substrates

Surface morphology of InP layers is monitored during organometallic vapor phase epitaxy using an in situ diffuse laser light scattering technique. Changes in the diffuse scatter signal are noted for several substrate orientations near the (001) plane and at various growth temperatures. The diffuse scatter signal is shown to be a semi-quantitative indicator of surface roughness through post-growth examination of the samples with phase contrast optical microscopy and atomic force microscopy. Singular substrates consistently have almost featureless surfaces and very little diffuse scattering during growth. Vicinal substrates display a more complicated morphological evolution which cannot be deduced from the diffuse scattering alone but which does produce characteristic changes in diffuse scattering.     

Crystal defects and interdiffusion behavior of Hg1−xCdxTe/(100)CdTe epitaxial layers grown by chemical vapor transport

Crystal defects of chemical vapor transport grown Hg_1−xCd_xTe on (100) CdTe structures have been investigated using chemical etching, wavelength-dispersive spectroscopy, x-ray rocking curve, and scanning electron microscopy methods The results indicate that the origin and spatial distribution of the misfit dislocations can be attributed to both the lattice parameter misfit and the inevitable interdiffusion occurring between the substrate and the epitaxial layer. It is proposed that the interdiffusion of Hg along the [100] direction is enhanced by dislocation channels and other defect cores along or near this direction owing to defects on the initial surface of the CdTe substrate. The results indicate that the subgrain boundaries in Hg_1−xCd_xTe are caused by slight misorientation of the lattices and polygonization of the defects during epitaxial layer growth, and by the propagation of the subgrain boundaries existing in the CdTe substrate.     

Energy-filtered electron diffraction and high-resolution electron microscopy on short-range ordered structure in GaAs(0.5)Sb(0.5)

Characteristic intensity distribution of diffuse scattering in III-V alloy semiconductor GaAs(0.5)Sb(0.5) epitaxially grown was observed by the energy-filtered electron diffraction method with [110] incidence. The diffuse scattering situates at the one-third positions between the fundamental reflections extending parallel to the q002 direction in the reciprocal space. A high-resolution electron microscope image shows weak contrast modulation corresponding to the diffuse scattering. The image processed with the Fourier transform by selecting the diffuse scattering and a fundamental reflection shows small regions consisting of bright dots being elongated along the (111) planes and aligning on the (002) planes, which are considered to result from the ordering of As and Sb during the growth process. The effect of including the fundamental reflection for imaging the ordered regions in the image processing method is also discussed. Finally, based on the results obtained by energy-filtered electron diffraction and high-resolution electron microscopy, a simple structure model for the short-range ordered structure in GaAs(0.5)Sb0.5 is proposed.     

Selective etch-back and growth of InGaAs ON (100) Fe:lnP by electroepitaxy

Selective etch-back prior to growth of InGaAs islands on SiO₂-masked (100)Fe-doped InP substrates was performed by electroepitaxy. The etch-back of the substrate and the growth of the layer was done at a constant furnace temperature of 640° C by passing a direct electric current from the melt to the substrate for etch-back and from the substrate to the melt for growth. The current density used was 1 to 20 A/cm² for a period from 15 to 60 min. The isolated InP regions were of various sizes (40 × 1000μm to 3000 × 3000μm), and different geometries (narrow and wide strips, square, circular). A uniform etch-back and uniform growth with excellent surface morphology was obtained on strips as wide as 200μm and on circles withd < 500μm. For islands with wider geometry, growth as well as etch-back were uniform up to 100–200μm from the periphery with excellent surface morphology. The etch-back and growth profiles are trapezoid-shaped and are not influenced by the difference in chemical activity between crystalline planes. The orientation dependence of the etch rate was {110} > {100} > {011} > {111} B > {111} A.     

Assembly of Photopolymerizable Discotic Molecules on an Aligned Polyimide Layer Surface to Form a Negative Retardation Film with an Oblique Optical Axis

Ultraviolet (UV) polymerizable discotic liquid‐crystalline (DLC) molecules (2,3,6,7,10,11‐hexakis(4′‐acryloy‐m‐alkyloxybenzoyoxy)triphenylene [HAHBT‐m, where m was the number of methylene units, and here m = 6 (HAHBT‐6)]) were assembled to form a negative retardation film with an oblique optical axis on a specifically designed rubbing‐aligned polyimide layer surface [6FDA‐11CBBP (where 11 is the number of methylene units in the side chains)]. The side chains of this polyimide were terminated by cyanobiphenyl groups. Surface‐enhanced Raman scattering (SERS) and optical second harmonic generation results showed that rubbing caused a surface structural re‐arrangement in the alignment layer resulting in a negative pre‐tilt angle (θ_s) of –8.5° (which was in the direction opposite to the rubbing direction). The molecular topology at the rubbed surface was governed by a stable fold‐like bent structure of the cyanobiphenyl side chains, in which the CN groups preferentially pointed down towards the surface. When the DLC molecules were deposited onto the alignment surface and polymerized via UV irradiation to generate a new optical film, an oblique optical axis with an average tilt angle of –18.6° with respect to the film normal was detected using ellipsometric measurements. This tilted optical axis was developed by the DLC molecules being wedged on top of the cyanobiphenyl groups when in the bent conformation. Furthermore, the tilt angle difference between the θ_s at the alignment surface and at the air interface of the DLC molecules was attributed to a splay deformation of the DLC molecules along the film surface normal. Optical modeling has also confirmed our experimental observations.     

Investigations of 3C-SiC inclusions in 4H-SiC epilayers on 4H-SiC single crystal substrates

Synchrotron white beam x-ray topography (SWBXT) and Nomarski optical microscopy (NOM) have been used to characterize 4H-SiC epilayers and to study the character of triangular inclusions therein. 4H-SiC substrates misoriented by a range of angles from (0001), as well as (1 100) and (1120) oriented substrates were used. For epilayers grown on substrates misoriented by 3.5° from (0001) toward <1120>, the triangular inclusions were identified as consisting of two 3C-SiC structural configurations which are related to each other by a 180° rotation about the [111] axis. The epitaxial relationships between the 3C inclusions and the 4H-SiC epilayers (or substrates) were also determined. No evidence was found for the nucleation of 3C-SiC inclusions at superscrew dislocations (along the [0001] axis) in the 4H-SiC substrates. Increasing the off-axis angle of the substrates from 3.5 to 6.5° was found to greatly suppress the formation of the triangular inclusions. In the case of substrates misoriented by 8.0° from (0001) toward <1120>, the triangular inclusions were virtually eliminated. The crystalline quality of 4H-SiC epilayers grown on the substrates misoriented by 8.0° from (0001) was very good. For the (1100) and (1120) samples, there is no indication of 3C-SiC inclusions in the epilayers. Possible formation mechanisms and the morphology of 3C-SiC inclusions are discussed.     

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.