Off-cut Angle Research Articles

Luminescence Imaging of Extended Defects in SiC via Hyperspectral Imaging

Over the past decade, improvements in silicon carbide growth and materials has led to the development of commercialized unipolar devices such as Schottky diodes and MOSFETs, however, much work remains to realizing the goal of wide-scale commercialization of both unipolar and bipolar devices such as pin diodes or IGBTs, for high applications requiring high powers, operating in elevated temperatures or radiation environments or for many fast switching applications. Despite the great strides that have been made in reducing extended and point defect densities during this period, such defects still remain and with the push to lower off-cut angle substrates are in many cases seeing increases in prevalence. Thus, spectroscopic and imaging techniques for locating and identifying these defects are in high demand. Luminescence imaging and spectroscopy have both been utilized heavily in such work, yet simultaneously obtaining corresponding spectroscopic and spatial information from such defects is problematic. Here we report on hyperspectral imaging of electroluminescence from SiC pin diodes, whereby a stack of luminescence images are collected over a wide spectral range (400-900 nm), thereby providing the ability to both image distinct features and identify their corresponding spectral properties. This process is also equally applicable to collecting either photo- or electroluminescence from other materials or devices emitting in either the UV-Vis or NIR spectral range, as well as to reflectance, transmission or other imaging techniques.

Effect of r-plane (1–102) sapphire off-cut angle on the anisotropic strain in nonpolar Si-doped a-plane (11–20) GaN

We studied the growth and the characteristics of nonpolar Si-doped a-plane GaN grown on r-plane sapphire substrates with different off-cut angles which were changed in the range of −0.2° ∼ +0.4°. Samples grown by using −0.2° and +0.2° off-cut angles showed triangular pit-free and smooth surfaces, which resulted from enhanced lateral growth owing to the epitaxial films having a Ga face. On the other hand, the sample grown by using +0.4° off-cut angles revealed a high density of pits and low crystalline quality due to a high density of dislocations. The strain determined by using calculations with the lattice parameters also showed a dependence on the off-cut angles. We expect r-plane sapphire with off-cut angles in the range of −0.2° ∼ +0.2° to be very effective for improving the crystalline quality and the surface morphology of a-plane GaN.

Influence of off-cut angle of (0001) 4H-SiC on the crystal quality of InN grown by RF-MBE

The effect of the off-cut angle of a 4H-SiC (0001) substrate on the growth of InN thick layer by RF-plasma assisted molecular beam epitaxy (RF-MBE) has been investigated. The off-cut angle used in this study was inclined from 0° (just surface) toward the [11–20] direction of 4° and 8°. Crystalline quality and surface morphology were remarkably sensitive to the value of off-angle. Higher off-cut angles result in a reduction of the full-widths at half-maximum of HRXRD (0002) ω-scans, compared to that of the layer on the (0001)-just surface. In addition, the full-widths at halfmaximum of μ-Raman scattering spectra at 490cm-1, which is attributed to E2 (high) phonon mode, was decreased with increase in off-cut angle. Furthermore, In-droplets, which are commonly observed on the (0001) InN grown surface under In rich-growth condition, were found to be suppressed owing to an improvement of a nucleation on the off-cut angle surface. In our case, the use of 8°-off substrate increased film density and growth rate, while a surface roughness was reduced. These results clearly demonstrate that the larger off-cut angles improve the crystalline quality of InN film with reducing the In-droplets due to a higher step surface density on the off-cut angle surface.

Effect of vicinal off-cut substrates on the basal stacking fault density in nonpolar a-GaN epilayers

Nonpolar a-GaN epilayers were grown on r-plane sapphire substrates with vicinal off-cut angles by metalorganic chemical vapor deposition. The effect of the off-cut angle on the formation of basal stacking faults (BSFs) observed in the a-GaN epilayers was investigated using transmission electron microscopy (TEM). The vicinal off-cut angles of the r-plane sapphire, which were used, were ±0.4° toward the [1̄ 1 0 1] Sapp direction. Based on high-resolution TEM images, the values for the tilt angle of the epilayers were −0.3 and +3.0 for the −0.4° and +0.4° off-cuts, respectively, considering the spontaneous −2.0° tilting observed in the film on the on-axis substrate. The vicinal off-cut induced a slight difference in step height on the substrate surface. Consequently, this resulted in periodical surface steps of one monolayer and formed BSFs due to the variations in the stacking sequence of the (1 1 2̄ 0) GaN planes. It was observed that the relatively high tilt angle of the epilayer observed in the +0.4° off-cut substrate was responsible for the reduction in the BSF density of the a-GaN epilayers.

TEM Observation of 8 Deg Off-Axis 4H-SiC (0001) Surfaces Planarized by Catalyst-Referred Etching

We have developed a novel abrasive-free planarization method called catalyst-referred etching (CARE). A CARE-processed 8 deg off-axis 4H-SiC (0001) surface is investigated by cross-sectional transmission electron microscopy (TEM). The surface is composed of alternating wide and narrow terraces with single-bilayer-height steps, which are similar to the structure observed on a CARE-processed on-axis 4H-SiC (0001) surface. These results indicate that the structure appears on CARE-processed surfaces regardless of the off-cut angle.

Dislocation Revelation in Highly Doped N-Type 4<i>H</i>-SiC by Molten KOH Etching with Na<sub>2</sub>O<sub>2</sub> Additive

We have proposed a new wet etching recipe using molten KOH and Na2O2 as the etchant (“KN etching”) for dislocation revelation in highly doped n-type 4H-SiC (n+-4H-SiC). Threading screw dislocations (TSDs) and threading edge dislocations (TEDs) have been clearly revealed as hexagonal etch pits differing in pit sizes, and basal plane dislocations (BPDs) as seashell-shaped pits. This new etching recipe has provided a solution to the problem that conventional KOH etching is not effective for dislocation identification in 4H-SiC if the electron concentration is high (>mid-1018 cm-3). We have investigated the effect of SiC off-cut angle on KN etching and it has been shown that the “KN etching” is applicable for the n+-SiC substrate with off-angle from 0o to 8o.

Surface Plasmon Resonance on the Sapphire Single Crystal Substrates with 3D Nano-Textured Surface

Surface plasmon resonance (SPR) bio-sensors comprise an attenuated total reflection (ATR) system known as the Kretschmann geometry. The sensing surface comprises flat Au thin films deposited on a high index flat glass substrate (LAL-10, n = 1.72). This paper describes the effects of the 3D nano-textured Au thin films on the sensing properties are discussed. The nano-textured Au thin films were fabricated by the deposition of the uniform Au thin films on the (0001) vicinal sapphire substrates. The vicinal substrates comprise periodic atomic step lines and terraces. The step height is chiefly half lattice spacing of the sapphire (0.22 nm). The terrace width is governed by the off-cut angles. The stepped surface structure were reconstructed by the annealing at 900 to 1400oC (1 to 3 h) in air. The annealed stepped surfaces comprised stepped heights of 5 to 12 nm with the terrace width of 2.7 to 140 nm for the off-cut angles at 5 degrees. The laser diodes (635 nm) were used for the optical light source. It is found that the detection limit of the SPR bio-sensors using the flat Au thin films on the (0001) sapphire substrates is 1.6 ng/ml for the tumor marker (AFP, α-fetoprotein) by antibody/antigen reaction, which is almost the same to the performance of the flat Au thin films on the conventional high index glass substrates as expected by the simulation Winspal 2. The SPR sensitivities using the nano-textured Au thin films with the step height of 12 nm and the terrace width of 140 nm were one order of magnitude smaller than those of Au thin films on the flat (0001) sapphire, 20 ng/ml, probably due to the scattering of the surface plasmon and/or evanescent wave. However, well-designed nano-textured Au thin films will enhance the SPR sensitivity. The sapphire substrates could integrate optical devices and will provide novel optical-bio MEMS.

Optimization of growth conditions for InGaAs/InAlAs/InP quantum cascade lasers by metalorganic chemical vapor deposition

We investigate the growth conditions for lattice-matched InGaAs/InAlAs/InP quantum cascade lasers (QCLs) by metalorganic chemical vapor deposition (MOCVD). Effect of substrate misorientation, growth temperature, and V/III ratios of InGaAs and InAlAs layers on the surface morphology, optical quality, and impurity incorporation were systematically studied. It was found that epitaxial layers and multi-quantum-well structures grown at 720 °C with V/III ratios of 116 for InGaAs and 21 for InAlAs on InP substrates with an off-cut angle of ∼0.06° exhibit a stable step-flow growth and low oxygen and carbon contamination. Using these conditions, a ∼11.3-μm-thick QCL with an emission wavelength at ∼9.2 μm was grown and fabricated, which demonstrated excellent structural quality and operated at room temperature in pulsed mode with a threshold current density of 2.0 kA/cm 2 and a slope efficiency of 550 mW/A.

Dislocation Conversion and Propagation during Homoepitaxial Growth of 4H-SiC

Basal Plane Dislocations (BPDs) in SiC are thought to cause degradation of bipolar diodes with blocking voltages > 2kV by triggering the formation and expansion of stacking faults during device operation. Hence, low N doped, thick epitaxial layers without BPDs are urgently needed for the realization of long-term stable SiC bipolar diodes. Such epilayers can be achieved if the conversion of the BPD into another harmless dislocation type is supported by proper epitaxial growth parameters and use of vicinal (off-cut) substrates. In this work, the influence of the substrate’s off-cut angle and of the epilayer thickness on BPD density and surface morphology were investigated. The BPD densities of epilayers grown on 2° and 4° off-cut substrates were very low compared to growth on 8° off-axis substrates. X-Ray Topography has proved that all the Threading Dislocations (TD) propagate from the substrate to the epilayer and that BPDs in the substrate convert to Threading Edge Dislocations (TED) in the epilayer, i.e. the dislocation density (DD) of the substrate determines the epilayer’s DD. The conversion of BPDs is supported by the presence of bunched steps as for growth of thick layers on 2° and 4° off-cut substrates.

Physical Properties of AlGaN/GaN Heterostructures Grown on Vicinal Substrates

We report on the growth of Al0.25Ga0.75N/GaN heterostructures grown on low dislocation density vicinal surfaces of semi-insulating c-axis GaN substrates. Atomic force microscopy (AFM), photoluminescence (PL), cathodoluminescence (CL), high-resolution x-ray diffraction (HRXRD), secondary-ion mass spectroscopy (SIMS), Hall effect, and Raman spectroscopy have been used to assess structural and electrical properties as a function of substrate offcut. Bulk GaN substrates with vicinal offcut between 0.5° and 1.4° are optimal with respect to surface roughness and dopant incorporation. AFM, PL, and CL show decreasing Mg incorporation with increasing offcut angle. Raman spectroscopy, used to analyze biaxial strain, confirms essentially strain-free heterostructure growth on vicinal substrates with offcut angles between 0.5° and 1.4° off [0001] toward $$ [1\overline{1} 00] $$ . Aluminum (Al) incorporation in the Al x Ga1−x N barrier assessed by Raman vibration is in excellent agreement with trends found by HRXRD.

Influence of off-cut angle of r-plane sapphire on the crystal quality of nonpolar a-plane AlN by LP-HVPE

The effect of the off-cut angle of an r-plane sapphire substrate has been investigated on the growth of a-plane AlN thick layer by low-pressure hydride vapor phase epitaxy (LP-HVPE). The off-cut angle ( θ) was changed from +5.0° (close to c-axis) to −5.0° (close to m-axis). Results show that the crystalline quality and surface morphology are very sensitive to the sign of θ off-angle. The plus θ off-angle is found to be dramatically reduce the full-widths at half-maximum (FWHM) of X-ray rocking curves (XRC), compared with the minus θ off-angle. In-plane FWHM anisotropic feature marked as M- or W-shape dependence on azimuth angle was observed for a-plane AlN. The shape and degree of anisotropy depend on the sign of θ off-angle, while the plus of θ off-angle will leads to the W-shape and the decreased anisotropy. The minimum crystal tilts and twists of the films are observed for the vicinal sapphires with the plus off-angles of +0.2° to +1.0°.

Turning of Basal Plane Dislocations during Epitaxial Growth on 4° Off-Axis 4H-SiC

Epitaxial layers were grown on 4° off-axis 4H-SiC substrates by hot-wall chemical vapor deposition. The reduced off-cut angle resulted in lower basal plane dislocation (BPD) densities. The dependence of BPD reduction on growth conditions was investigated using ultraviolet photoluminescence (UVPL) imaging. With this method, it was found that the dislocations were converting to threading edge dislocations throughout the thickness of the film. A high (≥ 97%) conversion efficiency was found for all films grown with this orientation. A conversion of 100% was achieved for several films without pre-growth treatments or growth interrupts.

Carrier Generation Lifetime in 4H-SiC Epitaxial Wafers

The pulsed MOS-C (Metal Oxide Semiconductor-Capacitor) technique was used to measure generation lifetimes in 4H-SiC epitaxial wafers. The ratio of generation to recombination lifetime has been investigated to understand the dominant defect for generation lifetime. The EH6/7 defect level is considered to limit generation lifetime and field enhanced emission is proposed to explain extremely large variation of generation lifetime in a small area. Generation lifetime is limited by dislocations when they are above a threshold density of about 106cm-2. Generation lifetimes measured on 4 and 8 degree off-cut angle epi-substrates are very comparable.

Theoretical Monte Carlo Study of the Formation and Evolution of Defects in the Homoepitaxial Growth of SiC

A novel Monte Carlo kinetic model has been developed and implemented to predict growth rate regimes and defect formation for the homo-epitaxial growth of various SiC polytypes on different substrates. Using this model we have studied the generation of both point like and extended defects in terms of the growth rate and off-cut angle, finding qualitative agreement with both electrical and optical characterization and analytical results.

Si (001) surface preparation for the antiphase domain free heteroepitaxial growth of GaP on Si substrate

The present paper summarizes results of thermal annealing steps on epitaxially grown Si buffer morphology in dependence of Si offcut angle as well as miscut direction. A Si buffer layer was grown on an undulated Si substrate by vapour phase epitaxy, which provides due to its roughness deliberate off-orientations. An optimized annealing procedure under a hydrogen pressure of 950 mbar was used after growth. Atomic force investigation shows that there is a clear tendency to form bi-atomic steps along both <110> directions as long as the surface miscut is low in angle. For the off-orientations in between these two directions the surface steps delimit mono-layer high terraces. An exact (001) Si substrate with a low miscut of 0.12° towards [11̄0] was treated by the same annealing condition. The Si surface is strongly dominated by one kind of sublattice but not completely double-stepped. Using this Si wafer as a substrate for GaP heteroepitaxy by metal organic vapour phase epitaxy, an antiphase domain free GaP layer is achieved after 50 nm of overgrowth, proven by transmission electron microscopy investigation.

Study of triangular defects and inverted pyramids in 4H-SiC 4° off-cut (0 0 0 1) Si face epilayers

Growth of high-quality epilayers on low off-cut angle wafers is essential for the development of high-performance devices based on hexagonal SiC. Device killing defects such as triangular and inverted pyramid-type defects formed during homoepitaxial growth on the 4H-SiC Si face, 4° off-cut towards [1 1 2¯ 0] direction, have been investigated in this work. The goal of this research was to minimize or eliminate these defects. Growth parameters responsible for triangular defect formation were identified and optimized for its reduction. It was found that although growth at high temperatures reduces the density of triangular defects and inverted pyramid-type defects, it is not the only remedy for reducing their density; cleanliness of the susceptor along with the initial growth condition plays a major role in the formation of these defects.

Reflectance anisotropy studies of 5×2‐Au structures grown on Si(111) surfaces with different step formations

AbstractReflection anisotropy spectroscopy (RAS) studies are reported of Si(111)‐5×2‐Au structures grown on substrates with different offcuts and step densities. Such substrates produce a predominant domain on the (111) terrace that would otherwise produce a negligible RAS signal, due to domain averaging. Changes in the RAS spectra with offcut angle are observed in the 1 to 3 eV region, where Au‐induced structures appear. Results from offcuts towards [$ {\bar 1}{\bar 1} $2] of 2°, 3°, 9.5°, and ‐4° are presented. The variation in the RAS spectra is consistent with different populations of single‐ and double‐chain gold structures formed in the region of the steps. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Using reflectance anisotropy spectroscopy to characterize capped silver nanostructures grown on silicon

AbstractUsing the single domain Si(111)‐3×1‐Ag surface as a template, room temperature deposition of two or more monolayers of Ag leads to the formation of metallic nanostructures. Reflectance anisotropy spectroscopy (RAS) in the infrared (IR) spectral region is used to analyse the anisotropic conductivity of the structures. The anisotropy is found to be influenced by the offcut angle of the substrate, and hence the terrace width. The Ag nanostructures were capped with Si to form a near‐IR transparent protecting layer. The samples are stable to exposure to ambient conditions for significant periods. The RAS spectra are compared to model calculations, which support the conclusion that the buried metallic Ag nanostructures survive the capping process. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Observation of Asymmetric Wafer Bending for 3C-SiC Thin Films Grown on Misoriented Silicon Substrates

ABSTRACTWe present an experimental study of asymmetric wafer deformation for 3C-SiC layers grown on deliberately misorientated silicon substrates. An asymmetric curvature has been observed both on (100) and (111) oriented layers. In this work we focus on the (100) oriented samples. The curvature of the wafers is studied as a function of wafer thickness and offcut angle. We look for the correlations between the observed asymmetric strain relaxation and the layer morphology and microstructure. We claim that different defect pattern, measured along [110] and [1-10] direction can be at the origin of almost complete relaxation of mismatch strain along the offcut direction.

Effect of substrate misorientation on the InAs∕InAlAs∕InP nanostructure morphology and lateral composition modulation in the InAlAs matrix

The authors report the self-organized growth of InAs∕InAlAs quantum wires on nominal (001) InP substrate and (001) InP substrates misoriented by 2°, 4°, and 8° towards both [−110] and [110]. The influence of substrate misorientation on the structural and optical properties of these InAs∕InAlAs quantum wires is studied by transmission electron microscopy and photoluminescence measurements. Compared with that grown on nominal (001) InP substrate, the density of InAs∕InAlAs quantum wires grown on misoriented InP(001) substrates is enhanced. A strong lateral composition modulation effect take place in the InAlAs buffer layers grown on misoriented InP substrates with large off-cut angles (4° and 8°), which induces a nucleation template for the first-period InAs quantum wires and greatly improve the size distribution of InAs quantum wires. InAs∕InAlAs quantum wires grown on InP (001) substrate 8° off cut towards [−110] show the best size homogeneity and photoluminescence intensity.