Homoepitaxial Layers Research Articles

Behavior of Threading Dislocations from GaN Substrate to Epitaxial Layer

Threading dislocations (TDs) in the GaN homoepitaxial layer were nondestructively evaluated using X-ray topography, Raman spectroscopy, and multi-photon photoluminescence imaging. TDs in the GaN substrate have been identified as threading edge dislocations (TEDs), threading mixed dislocations (TMDs), and threading screw dislocations (TSDs). In article number 1900527, Hisashi Yamada and co-workers discuss the direction and angle of inclined TDs in the GaN epitaxial layer. Stress relaxation via misfit dislocation is considered as a possible explanation for TED tilting toward the extra half plane.

An approach to remove homoepitaxially grown GaN layers by cleavage from the substrate surface

We demonstrate removal of homoepitaxially grown semi-polar gallium nitride (GaN) layers from the native substrates. The weak link at the interface of the epitaxial lateral overgrowth and cleavable m-plane of the respective native semi-polar plane is used to separate homoepitaxial GaN from its native substrate. Homoepitaxial GaN layers of the semi-polar planes, (101), (201), (301), (10), (20), and (30) are successfully removed. This approach allows the reuse of expensive semi-polar GaN substrates, eliminating one barrier to market introduction of superior optoelectronic devices grown with semi-polar orientations.

Sub-bandgap optical spectroscopy of epitaxial β -Ga2O3 thin films

Room temperature sub-gap optical absorption spectra measured by photothermal deflection spectroscopy were investigated for hetero- and homo-epitaxial β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy as well as for a bulk crystal. The absorption spectra show a pronounced exponential Urbach tail with slope parameters of 120–150 meV in the spectral region between 4.5 and 5 eV, indicating an unusually large self-trapping energy of excitons. In addition, an absorption band related to deep defects is observed in the spectral region from 2.5 to 4.5 eV. The steepness of the Urbach tail as well as the strength of the defect-related absorption can be influenced and optimized by annealing at 900–1000 °C in an oxygen atmosphere. Similar features were also observed for bulk β-Ga2O3 crystals and for homoepitaxial β-Ga2O3 layers. The present results for β-Ga2O3 are compared and discussed in the context of similar measurements for other wide-bandgap semiconductors of current interest in electronics and photocatalysis: GaN, ZnO, TiO2, and BiVO4.

Epitaxial diamond on Ir/ SrTiO3/Si (001): From sequential material characterizations to fabrication of lateral Schottky diodes

Advanced characterizations with combined analytical tools were carried out at the different stages of diamond heteroepitaxy on Ir/STO/Si (001) substrates. HRTEM and STEM-EELS revealed the presence of epitaxial nanometric diamond crystals after bias enhanced nucleation. UV Raman allowed estimating the diamond film quality and its strain at the early stages of heteroepitaxial growth. The crystalline structure and the strain within thick heteroepitaxial films were determined by XRD and CL investigations. A CL study of the cross-section provided the mapping of the dislocation network along the growth direction. Measurements performed on lateral Schottky diodes fabricated on a thick diamond film showed an excellent reproducibility on the substrate with a Schottky barrier height in good agreement with those obtained on homoepitaxial layers.

Behavior of Threading Dislocations from GaN Substrate to Epitaxial Layer

Threading dislocations (TDs) in the GaN substrate and homoepitaxial layer are nondestructively evaluated using X‐ray topography (XRT), Raman spectroscopy, and multiphoton photoluminescence (MPPL) imaging. When the XRT and Raman mapping images are compared, TDs in the GaN substrate are identified as threading edge dislocations (TEDs), threading mixed dislocations (TMDs), and threading screw dislocations (TSDs). TDs are observed to propagate from the GaN substrate to the epitaxial layer. From MPPL imaging, the TEDs are found to be inclined in the direction, which is at 90° from the Burgers vector. The TMD studied in detail is found to be inclined in the direction, which is parallel to the Burgers vector. The feasibility of identifying TDs via analysis of the inclined direction of the dislocations in the GaN epitaxial layer is suggested.

High Crystallinity and Highly Relaxed Al0.60Ga0.40N Films Using Growth Mode Control Fabricated on a Sputtered AlN Template with High‐Temperature Annealing

Herein, a method for growing AlGaN using an intermediate AlN molar fraction on a sputtered AlN template with high‐temperature annealing via an AlN homoepitaxial layer is investigated. The growth mode dependence of AlGaN on the amount of Ga doping in the AlN homoepitaxial layer is examined in detail. As a result, homoepitaxial growth of AlN on sputtered AlN improve the surface flatness. In addition, it is found that the macroscopic flatness is improved by increasing the amount of Ga doping, but this decreases the microscopic flatness. Moreover, when Al0.60Ga0.40N is grown on homoepitaxial AlN layers with different amounts of Ga doping, a difference in the growth mode is observed, and this reveal a remarkable difference in the dislocation density of Al0.60Ga0.40N. The dislocation density dependence of the lasing threshold power density in a UV–B optical pumped laser is also investigated. As a result, the lasing threshold power density is reduced from ≈220 to 50 kW cm−2 by reducing the dislocation density from 1.6 × 109 to 8.0 × 108 cm−2. It is confirmed that it is, therefore, essential to reduce the dislocation density to reduce the threshold power density of the UV–B laser.

Dual-color-sub-bandgap-light-excited isothermal capacitance transient spectroscopy for quick measurement of carbon-related hole trap density in n-type GaN

A quick method is proposed for measurement of the carbon-related hole trap (H1: +0.87 eV) density in an n-type GaN homoepitaxial layer using dual-color-sub-bandgap-light-excited isothermal capacitance transient spectroscopy. Shorter wavelength (390 nm) light irradiation is employed to cause the hole traps to be in the hole-occupied state. Longer wavelength (660 nm) light irradiation is then used to emit the hole from the trap to the valence band. The photoemission of holes is much quicker than the thermal emission, which reduces the measurement time. The trap density can be calculated from the capacitance transient.

Effect of LPHT annealing on interface characteristics between HPHT Ib diamond substrates and homoepitaxial CVD diamond layers

Abstract

Surface morphology smoothing of a 2 inch-diameter GaN homoepitaxial layer observed by X-ray diffraction topography.

We investigated the surface morphology changes in a 2 inch-diameter, c-plane, free-standing GaN wafer using X-ray diffraction topography in a grazing-incidence geometry. We observed a decrease in the peak intensity and increase in the full width at half maximum of the GaN 112̄4 Bragg peak after the deposition of a homoepitaxial layer on the same GaN wafer. However, the lattice plane bending angles did not change after homoepitaxial layer deposition. Distorted-wave Born approximation calculations near the total external reflection condition revealed a decrease in the X-ray incidence angle of the 112̄4 Bragg peak after the homoepitaxial layer deposition. The decrease in both X-ray penetration and incidence angle induced broader and weaker diffraction peaks from the surface instead of the bulk GaN.

Effect of temperature on conversion of basal plane dislocations to treading edge dislocations during 4H-SiC homoepitaxiy

Abstract 4H-SiC homoepitaxial layer grown on 4° off-axis substrates is performed at various growth temperature by using the traditional chemical vapor deposition. The effect of growth temperature on propagation of basal plane dislocations from substrate to epitaxy was studied using the deep KOH etching and Synchrotron reflection X-ray topography, and the conversion mechanism was also investigated. It is found that the majority basal plane dislocations are converted to threading edge dislocations during epitaxial growth. Meanwhile, the two types of threading edge dislocations in epitaxial layer are observed after KOH etch. By the optimization of growth temperature, high quality epitaxial wafer with extremely low basal plane dislocations density (

Defect appearance on 4H-SiC homoepitaxial layers via molten KOH etching

Abstract Epitaxial defects such as threading screw dislocation (TSD), basal plane dislocation (BPD) and so on will seriously reduce the reverse voltage or increase leakage current of power devices. We studied the classification and statistics of the above several types of 4H-SiC epitaxial defects, by using molten KOH etching the epitaxial layer to explore and trace the origin of epitaxial defects. Studies have shown that the formation principles of various epitaxial defects are related to dislocations, and the growth parameters such as carbon-to-silicon ratio and growth time have paid a significant role on epitaxial defects.

High quality 4H-SiC homo-epitaxial wafer using the optimal C/Si ratio

Epitaxial growth of 4H-SiC on 4° off-axis substrates has been performed under different C/Si ratio in order to improve the quality level of epitaxial layer. 4H-SiC homoepitaxial layer is grown in horizontal warm-wall planetary reactor. The growth rate, doping concentration, morphology defect of epitaxial layer are characterized using the Fourier transform infrared, Hg schottky contact and optical surface analyzer, respectively. Meanwhile, the epitaxial layer is etched by molten KOH and microscopy observation to understand conversion of BPDs dependence of C/Si ratio. It is suggested that the defect density in the epitaxial layer is increased under C-rich and Si-rich condition, since the increasing of 2D island nucleation and formation of silicon drops, respectively. Moreover, this low conversion ratio is considered to be induced by suppression lateral growth under the low C/Si ratio condition. As a result, epitaxial layer with high yield (99.7%) and extremely low basal plane dislocations density (＜0.05 cm−2) can be obtained by the optimization of C/Si ratio.

Highly effective activation of Mg-implanted p-type GaN by ultra-high-pressure annealing

A high activation ratio of acceptors to Mg ions implanted into a homoepitaxial GaN layer was achieved through an ultra-high-pressure annealing (UHPA) process. Capless annealing under a nitrogen pressure of 1 GPa in a temperature range of 1573–1753 K activated acceptors without thermally decomposing the GaN layer. Conventional rapid thermal annealing leads to a serious decomposition at 1573 K, even with an AlN protective cap. The sample annealed at 1673 K under UHPA exhibited very intense cathodoluminescence in near-band edge and donor-acceptor-pair band emissions. The intensities were over one order of magnitude higher than those of the sample treated by conventional annealing. A Hall-effect measurement was obtained in the temperature range of 275–500 K for the UHPA sample. The obtained hole concentration and mobility at 300 K were 3.6 × 1016 cm−3 and 24.1 cm2 V−1 s−1, respectively. The mobility value was close to that of an epitaxial p-type GaN with the same doping concentration. An Arrhenius plot of hole concentrations showed that the acceptor concentration and ionization energy were separately estimated to be (2.6 ± 0.8) × 1018 cm−3 and 212 ± 5 meV, respectively. By comparing the Mg concentrations obtained from secondary ion mass spectrometry, the acceptor activation ratio (acceptor concentration/Mg concentration) of the UHPA samples exceeded 70%. These results suggest that the UHPA process as a postimplantation annealing technique is promising for the fabrication of GaN-based power devices with selective area doping.

Vertical-Type Ni/GaN UV Photodetectors Fabricated on Free-Standing GaN Substrates

The authors report on a vertical-type visible-blind ultraviolet (UV) Schottky-type photodetector fabricated on a homoepitaxial GaN layer grown on free-standing GaN substrates with a semi-transparent Ni Schottky contact. Owing to the high-quality GaN drift layer with low-density threading dislocation and high electron mobility, the UV photodetector shows a high specific detectivity of more than 1012 Jones and a UV/visible discrimination ratio of ~1530 at −5 V. The photodetector also shows the excellent self-powered photo-response and a high signal-to-noise ratio of more than 104 at zero voltage. It is found that a relatively lower growth rate for the GaN epilayer is preferred to improve the performance of the Schottky-type photodetectors due to the better microstructure and surface properties.

Basal Plane Dislocation Conversion Enhancement in 4H-SiC Homo-Epitaxial Layers by Ion Implantation into the Wafer

We studied the impact of ion implantation into the wafer substrate prior to the epitaxy process on the basal plane dislocation conversion behavior during epitaxial layer growth. Defect density measurements show an enhancing effect of the ion implantation on the basal plane dislocation to threading edge dislocation conversion. Analysis of the lateral conversion distribution, the stress field in the material as well as the wafer topography at the onset of epitaxial growth lead us to believe, that stresses in the epitaxy layer cause the enhanced basal plane dislocation conversion.

Influence of Substrate Properties on the Defectivity and Minority Carrier Lifetime in 4H-SiC Homoepitaxial Layers

Two fully loaded epitaxial growth runs with 16 wafers in total were conducted in the AIXTRON G5 WW reactor in order to keep epigrowth conditions constant. The wafers were selected with a large spread of specific resistivity and dislocation densities. The resulting epilayers showed very good intra-wafer homogeneities as well as excellent wafer-to-wafer and run-to-run reproducibility with regard to epilayer thickness and doping concentration, point defect concentrations of Z1/2 and EH6/7 and the resulting Shockley-Read-Hall carrier lifetime. We found that the dislocation densities of the underlying substrates are influencing the stacking fault densities of the epilayers, which then vary between 0.1 and 10 cm-2. A substrate effect on the effective minority carrier lifetime was found.

Ab Initio Study of the Early Stage of Si Epitaxy on the Chlorinated Si(100) Surface

The homoepitaxial growth of Si on Si(100) covered by a resist mask is a necessary technological step for the fabrication of donor-based quantum devices with scanning tunneling microscope lithography. In the present work, the chlorine monolayer is selected as the resist. Using density functional theory, we investigated the adsorption of a single silicon atom on Si(100)-2$\times$1-Cl as the starting process of Si epitaxy. The incorporation of a silicon atom under a Cl monolayer proved to be the most energetically favorable process. Our results show that chlorine segregates on the surface during Si deposition and does not incorporate into homoepitaxial layers. In addition, we found that SiCl$_2^{\ast}$, SiCl$_3^{\ast}$, and SiCl$_4^{\ast}$ clusters can be formed above a Si(100)-2$\times$1-Cl surface while Si is adsorbed. SiCl$_2^{\ast}$ clusters are bound weakly to the substrate, and their desorption leaves the silicon surface free of chlorine. To check whether the Si epitaxy is possible on the chlorine resist, we compare our results with the well-studied case of a hydrogen resist. We find the two processes to be similar; moreover, epitaxy on chlorine resist appears to have an advantage.

Deep-level transient spectroscopy studies of electron and hole traps in n-type GaN homoepitaxial layers grown by quartz-free hydride-vapor-phase epitaxy

We studied deep levels in quartz-free hydride-vapor-phase epitaxy (QF-HVPE)-grown homoepitaxial n-type GaN layers within which three electron and eight hole traps were detected. The dominant electron and hole traps observed in the QF-HVPE-grown GaN layers were E3 (EC − 0.60 eV) and H1 (EV + 0.87 eV), respectively. We found that the E3 trap density of QF-HVPE-grown GaN (∼1014 cm−3) was comparable with that of MOVPE-grown GaN layers, whereas the H1 trap density of QF-HVPE-grown GaN (∼1014 cm−3) was much smaller than that of an MOVPE-grown GaN layer with a low-residual-carbon growth condition. A detailed analysis of the QF-HVPE-grown GaN layers revealed that the H1 trap density is almost equal to the carbon impurity concentration and other impurities that compensate the Si donors besides the carbon impurity were hardly detected in the QF-HVPE-grown GaN layers.

A comparative study of high-quality C-face and Si-face 3C-SiC(1 1 1) grown on off-oriented 4H-SiC substrates

We present a comparative study of the C-face and Si-face of 3C-SiC(111) grown on off-oriented 4H-SiC substrates by the sublimation epitaxy. By the lateral enlargement method, we demonstrate that the high-quality bulk-like C-face 3C-SiC with thickness of ~1 mm can be grown over a large single domain without double positioning boundaries (DPBs), which are known to have a strongly negative impact on the electronic properties of the material. Moreover, the C-face sample exhibits a smoother surface with one unit cell height steps while the surface of the Si-face sample exhibits steps twice as high as on the C-face due to step-bunching. High-resolution XRD and low temperature photoluminescence measurements show that C-face 3C-SiC can reach the same high crystalline quality as the Si-face 3C-SiC. Furthermore, cross-section studies of the C- and Si-face 3C-SiC demonstrate that in both cases an initial homoepitaxial 4H-SiC layer followed by a polytype transition layer are formed prior to the formation and lateral expansion of 3C-SiC layer. However, the transition layer in the C-face sample is extending along the step-flow direction less than that on the Si-face sample, giving rise to a more fairly consistent crystalline quality 3C-SiC epilayer over the whole sample compared to the Si-face 3C-SiC where more defects appeared on the surface at the edge. This facilitates the lateral enlargement of 3C-SiC growth on hexagonal SiC substrates.

Ultraviolet-B band lasers fabricated on highly relaxed thick Al0.55Ga0.45N films grown on various types of AlN wafers

In this paper, we investigated the dependence of threshold power density on the Al0.55Ga0.45N underlying layer film thickness in ultraviolet-B band (UV-B) lasers on various AlN wafers (four types). We also prepared and compared AlN templates for AlN freestanding substrates, AlN films fabricated by metalorganic vapor phase epitaxy, and annealed sputtered AlN templates at high temperature. The initial growth of AlGaN became three-dimensional by inserting a homoepitaxial Ga-doped AlN layer between the AlN template and Al0.55Ga0.45N, before it shifted to two-dimensional growth. It is possible to reduce the dislocation in Al0.55Ga0.45N using this mode. The dependence of AlGaN film thickness and that of the AlN template on samples with an inserted homoepitaxial Ga-doped AlN layer were studied. Compared with Al0.55Ga0.45N having a thickness of 5 μm, there was almost no noticeable difference between the dark spot density characterized by cathodoluminescence and the threshold power density in UV-B lasers for the AlN template. Besides, the characteristics were noticeably different for the film thickness of Al0.55Ga0.45N. The threshold power density in UV-B laser and dark spot density were reduced by increasing the film thickness. Through the optimization of the crystal growth condition, the threshold power density in UV-B laser and dark spot density were reduced to 36 kW cm−2 and 7.5 × 108 cm−2, respectively.