Off-angle Substrates Research Articles

(3̄10)-Oriented β-Ga2O3 grown on (0001) sapphire by halide vapor phase epitaxy: growth and structural characterizations

Unique (3̄10) β-Ga2O3 films have been obtained on off-angled sapphire substrates with the best crystal quality reported so far.

Solar-blind photodetectors fabricated on β-Ga2O3 films deposited on 6° off-angled sapphire substrates

In this paper, the β-Ga2O3 films were deposited on 6° off-angled (towards <11–20> direction) (0001) sapphire substrates by MOCVD, and the MSM structure solar-blind photodetectors were fabricated. The effects of growth temperature on the surface morphology, elemental composition, crystalline quality and optical properties of β-Ga2O3 thin films were investigated. As the substrate temperature increased from 650 to 850 °C, the surface RMS roughness of the film gradually increased, and the bandgap was 4.71, 4.73 and 4.76 eV, respectively. In addition, the photodetector fabricated on β-Ga2O3 film at 850 °C exhibited a large photocurrent (Iphoto) of 0.59 mA, a high responsivity (R) of 1.26 A/W, a high photo-to-dark current ratio (PDCR) of 9.3 × 104, a detectivity (D*) of 2.5 × 1012 Jones, and a fast response recovery time of 0.38 s.

Growth and characterization of β-Ga2O3 thin films grown on off-angled Al2O3 substrates by metal-organic chemical vapor deposition

Beta-gallium oxide (β-Ga2O3) thin films were deposited on c-plane (0001) sapphire substrates with different mis-cut angles along <> by metal-organic chemical vapor deposition (MOCVD). The structural properties and surface morphology of as-grown β-Ga2O3 thin films were investigated in detail. It was found that by using thin buffer layer and mis-cut substrate technology, the full width at half maximum (FWHM) of the () diffraction peak of the β-Ga2O3 film is decreased from 2° on c-plane (0001) Al2O3 substrate to 0.64° on an 8° off-angled c-plane (0001) Al2O3 substrate. The surface root-mean-square (RMS) roughness can also be improved greatly and the value is 1.27 nm for 8° off-angled c-plane (0001) Al2O3 substrate. Room temperature photoluminescence (PL) was observed, which was attributed to the self-trapped excitons formed by oxygen and gallium vacancies in the film. The ultraviolet–blue PL intensity related with oxygen and gallium vacancies is decreased with the increasing mis-cut angle, which is in agreement with the improved crystal quality measured by high resolution X-ray diffraction (HR-XRD). The present results provide a route for growing high quality β-Ga2O3 film on Al2O3 substrate.

Preparation of β-Ga2O3 films on off-angled sapphire substrates and solar-blind ultraviolet photodetectors

β-Ga2O3 films have been prepared on (0001) sapphire substrates with different off-axis angles toward by sol–gel method and the solar-blind ultraviolet photodetectors (PDs) with metal-semiconductor-metal structure have been fabricated on these films. The effects of off-angled sapphire substrates on the surface morphology and crystal quality of β-Ga2O3 films have been systematically investigated. All the results indicate that to some extent the larger off-angled sapphire substrate could lead to the smoother surface morphology and higher crystal quality of β-Ga2O3 films. Reduction in the numbers of in-plane rotational domains for β-Ga2O3 films on off-angled substrates may result in the above-mentioned improvement, which have been confirmed by x-ray diffraction ϕ-scan. β-Ga2O3 film grown on ∼7° off-angled sapphire substrate has () preferred orientation with the single-domain growth mode. Besides, the performance of β-Ga2O3 PDs fabricated on different off-angled substrates show the same trend with changing the off-axis angles. The β-Ga2O3 PD fabricated on ∼7° off-angled sapphire substrate has excellent optical characteristics with large photocurrent to dark current ratio of 430.53 and high UV–visible rejection ratio of 1.42 × 103. These results would be helpful for improving the crystal-quality of hetero-epitaxy β-Ga2O3 films by using off-angled sapphire substrates and fabricating the cost-effective mass-producible solar-blind ultraviolet PDs by sol–gel method.

Massive reduction of threading screw dislocations in 4H-SiC crystals grown by a hybrid method combined with solution growth and physical vapor transport growth on higher off-angle substrates

Propagation of threading screw dislocations (TSDs) during hybrid growth, combined with TSD conversion by solution growth and bulk growth by physical vapor transport, has been investigated for 4° off and 15° off C-face seeds. The converted defects on basal planes (BP-defects) are able to be eliminated by continuing to guide them on basal planes with a 15° off seed, while BP-defects revert to TSDs with a 4° off seed. The difference in the BP-defects propagation is discussed based on the elastic energy balance between TSDs and BP-defects in the unit growth thickness for higher off-angle seeds.

Characterization of morphological defects related to micropipes in 4H-SiC thick homoepitaxial layers

A new type of morphological defects related to substrate micropipe is observed in 4H-SiC thick homoepitaxial layers. The structure and formation mechanism are investigated by optical microscopy, laser microscopy, scanning electron microscopy, micro-Raman spectroscopy, micro-photoluminescence spectroscopy and molten KOH etching. The observed new defect has truncated hexagonal prism morphology with a closed-core hexagonal pit, and it is 4H-SiC without polytype transition. It is found that the micropipe in the substrate can not only extend to the epilayer, but also cause a morphological defect to disturb the step-flow growth on the epilayer surface. A model has been proposed to interpret the formation mechanism of the morphological defect. It show that the morphology of the observed defect is mainly determined by the wider terrace in 4° off-angle substrate, the interaction between step-flow growth and spiral growth, and the thickness of the epilayer.

Study of β-Ga2O3 films hetero-epitaxially grown on off-angled sapphire substrates by halide vapor phase epitaxy

The heteroepitaxial β-Ga2O3 films were grown on the (0001) sapphire substrates with various off-angles toward 〈112-0〉 by halide vapor phase epitaxy (HVPE). The effects of off-angled substrates on the surface morphologies and crystal quality of as-grown β-Ga2O3 films were systematically investigated. The results indicated that the surface morphologies and crystal quality of β-Ga2O3 films grown on the off-angled sapphire substrates were effectively improved, compared with that of the films grown on (0001) sapphire. The quadrilateral domains growth mode was observed by scanning electron microscope and X-ray diffraction φ-scans of as-grown β-Ga2O3 films. When the off-angle of (0001) sapphire substrate was ~7°, β-Ga2O3 films exhibited the flatter surface morphology and the better crystal quality. The results would be helpful for the hetero-epitaxy of high-quality Ga2O3 films with smooth surface.

Characterisation of defects in p-type 4H-, 6H- and 3C-SiC epilayers grown on SiC substrates

The defects in p-type 4H-, 6H- and 3C-SiC epilayers grown on SiC substrates were characterised. The surface roughness after etching and the full width at half maximum of the X-ray rocking curves of the epilayers revealed more dislocations in those grown on the on-axis substrates than on the off-axis ones; the deep levels were also significant for the epilayers grown on the on-axis substrates. These results suggested that the dislocations, double-positioning boundaries and stacking faults in the SiC epilayers grown on on-axis substrates induced deep levels in the bandgap. Step-flow epitaxy on off-angle substrates is the best method for p-type SiC epilayers fabrication, and thus we need novel growth technique to obtain high quality 3C-SiC epilayers.

The influence of growth temperature on 4H-SiC epilayers grown on different off-angle (0 0 0 1) Si-face substrates

In this paper, homoepitaxial growth was performed on on-axis, 4° and 8° off-axis 4H-SiC (0001) Si-face substrate by using our home-made vertical hot wall LPCVD reactor and SiH4 + C2H4 + H2 + HCl gas system. The influence mechanism of growth temperature on the crystal quality, the growth rate and surface morphology is studied. The growth rate increased with the increase of growth temperature, and the epitaxial wafer surface morphology is more excellent by the increasing of off-angle. The results demonstrate that growth temperature is a fundamental process parameter to optimize.

Photocathode for hydrogen generation using 3C-SiC epilayer grown on vicinal off-angle 4H-SiC substrate

We employed a 3C-SiC epilayer grown on a vicinal off-angle 4H-SiC substrate as a photocathode. The presence of 3C-SiC on the surface of this epilayer was confirmed by optical microscopy and Raman spectroscopy. Spectral responses of the photocathode show high quantum efficiencies in a wide wavelength range. By illuminating the photocathode with full-spectrum solar light of 1.1 W/cm2, the photocurrent was determined to be −13.5 mA/cm2 with a Ni counterelectrode. The generated hydrogen volumes were much larger than those reported previously for SiC photocathodes.

Influences of two-step growth and off-angle Ge substrate on crystalline quality of GaAs buffer layers grown by MOVPE

GaAs buffer layers were grown on off-angle Ge substrates with a two-step growth process by metalorganic vapor phase epitaxy in order to prepare a high quality buffer layer for the InGaAsN p–i–n solar-cell structure. In our contribution, we present results from the optimized two-step growth of GaAs buffer layers, namely with a low-temperature step at 470°C and a high-temperature step at 580°C, combined with the use of Ge substrates misoriented by 4° and 6° towards [110]. HRXRD results showed that by using the off-angle substrates, the FWHM of (004) rocking curves was decreased to 6.7sec, which is about 4 times the FWHM of the GaAs commercial substrate. A decreased FWHM indicates a narrower distribution of crystal orientation. Furthermore, a smooth surface with a RMS roughness of 0.7nm was clearly observed by AFM. Cross-sectional dark-field TEM images showed the GaAs buffer layer with 20–30nm diamond-shaped anti-phase domains (APDs) at the GaAs/Ge interface, followed by APDs-free GaAs regions on the 6° off-angle Ge substrate. However, anti-phase boundaries were generated along the [001] direction for the buffer layer on the on-axis substrate. Our results demonstrated that the use of 6° off-angle Ge substrate and a two-step growth process allow the suppression of APDs in the GaAs buffer layer.

Quasi-heteroepitaxial growth of β-Ga2O3 on off-angled sapphire (0 0 0 1) substrates by halide vapor phase epitaxy

We demonstrate the high-speed growth of β-Ga2O3 quasi-heteroepilayers on off-angled sapphire (0001) substrates by halide vapor phase epitaxy (HVPE). (2¯01) oriented β-Ga2O3 layers were successfully grown using GaCl and O2 as source gases. The growth rate monotonically increased with increasing the partial pressures of the source gases, reaching over 250μm/h. This rate is over two orders of magnitude larger than those of conventional vapor phase epitaxial growth techniques such as molecular beam epitaxy or metalorganic vapor phase epitaxy. X-ray pole figure measurements indicated the presence of six in-plane rotational domains, in accordance with the substrate symmetry, plus some minor (310) domains. By the use of off-angled substrates and thick layer overgrowth, one of the in-plane orientations was strongly favored and the (310) residuals effectively suppressed, so that quasi-heteroepitaxial growth was achieved. Therefore, these results indicate the high-potential of the HVPE technique for the growth of thick and thin β-Ga2O3 layers for the cost-effective production of β-Ga2O3 based devices.

Chloride-Based CVD of 4H-SiC at High Growth Rates on Substrates with Different Off-Angles

A review of recently achieved results with the chloride-based CVD on 8° and 4° off axis and nominally on-axis 4H-SiC wafers is done to clarify the epitaxial growth mechanisms on different off-angle substrates. The process conditions selected for each off-axis angle become even more difficult when running at growth rates of 100 µm/h or more. A fine-tuning of process parameters mainly temperature, C/Si ratio and in situ surface preparation is necessary for each off-angle. Some trends related to the surface properties and the effective C/Si ratio existing on the surface prior to and during the epitaxial growth can be observed.

Epitaxial growth of SiC with chlorinated precursors on different off-angle substrates

This study focuses on the epitaxial growth of silicon carbide (SiC) epitaxial layers, adopting the chloride-based chemical-vapor-deposition (CVD) process, which allows to achieve ten times higher growth rate compared to the standard process based on the mixture of a silicon-containing gas and a hydrocarbon.In order to improve the material quality, substrates with different off-angles were used, since low off-angle substrates result in a reduction of killer defects for specific devices. Different growth mechanisms dominate for different substrate off-cut and an accurate set up of dedicated surface preparation procedures and tuning of growth parameters are needed. This study demonstrates that silicon-rich gas inputs are favorable for lower off-angle (nominally on-axis) substrates, while carbon-rich are beneficial for higher off-angles (usually 8° off-axis for 4H–SiC).Methyltrichlorosilane (MTS) is shown to be the best precursor to achieve the presented results.

A Review on Polarization Control of Vertical-Cavity Surface-Emitting Semiconductor Lasers

VCSELs have major applications in high-speed data- and tele-communications such as local/metro area networks, optical interconnects and free-space optical links. Even though VCSEL was invented more than three decades ago and rapidly entered into system applications, a method of mass producing monolithic, polarization stable VCSELs at industry scale is still on-demand. Unfortunately, due to the high symmetry of VCSEL along its optic (emission) axis, VCSELs do not have intrinsic polarization stability. As the injection current increases above threshold, polarization randomly switches between the two orthogonal directions and leads to relative intensity noise and degrades the bit error rates during signal transmission. Until now polarization mode in VCSELs has been stabilized by growing VCSELs on high-index and off-angled substrates; introducing single/multiple asymmetric shapes in active and current aperture layers; growing asymmetric quantum nanostructures in active layers; introducing stress externally on the top DBRs; fabricating 1D shallow surface, high-contrast sub-δ gratings and integrating with SP elements on the topmost layer of VCSEL; introducing photonic nanostructures into VCSEL cavities, and so on. Our current review not only focuses on successful demonstrations and patents in the past but also identifies suitable methods for mass production of polarization stable VCSELs for practical applications. Keywords: Birefringence, in-plane anisotropy, polarization control, Semiconductor laser, Surface emitting laser, VCSEL, strained quantum well, Spin-Flip Model, Polarization Switching

Optimization of a Concentrated Chloride-Based CVD Process for 4H–SiC Epilayers

Concentrated homoepitaxial growths of 4H–SiC was performed using a chloride-based chemical vapor deposition (CVD) process on different off-angle substrates (on-axis, 4 and 8° off-axis toward the direction). A suitable combination of gas flow and process pressure is needed to produce the gas speed that yields an optimum cracking of the precursors and a uniform gas distribution for deposition over large areas. The use of low pressure and the addition of chlorinated precursors bring the added benefit of achieving higher growth rates. A systematic study of the gas speed’s effect on the growth rate, uniformity, and morphology on the 4H–SiC epitaxial layers was performed. Growth rates in excess of were achieved on 50 mm diameter wafers with excellent thickness uniformity (below 2% σ/mean without rotation of the substrate) and smooth morphology using only 1/10 of the typical gas carrier flow and process pressure demonstrating the feasibility of a concentrated chloride-based CVD process for 4H–SiC. Thermodynamic calculations showed that the improved thickness uniformity could be due to a more uniform gas phase composition of the silicon intermediates. The concentration of the intermediate increases by a factor of 8 at a reduced carrier flow, while all the other hydrogenated silicon intermediates decrease.

4H-SiC Homoepitaxial Growth on Vicinal-Off Angled Si-Face Substrate

We have carried out detailed investigations of 4H-SiC homoepitaxial growth on vicinal off-angled Si-face substrates. We found that the surface morphology of the substrate just after in-situ H2 etching was also affected by the value of the vicinal-off angle. Growth conditions consisting of a low C/Si ratio and a low growth temperature were effective in suppressing macro step bunching at the grown epilayer surface. We also demonstrated epitaxial growth without step bunching on a 2-inch 4H-SiC Si-face substrate with a vicinal off angle of 0.79o. Ni Schottky barrier diodes fabricated on an as-grown epilayer had a blocking voltage above 1000V and a leakage current of less than 5x10-7A/cm2. We also investigated the propagation of basal plane dislocation from the vicinal off angled substrate into the epitaxial layer.

Homoepitaxial growth of non-polar ZnO [formula omitted] films on off-angle ZnO substrates by MOCVD

Homoepitaxial ZnO ( 1 1 2 ¯ 0 ) films were successfully prepared on single crystal ZnO ( 1 1 2 ¯ 0 ) substrates with off-angles of 0°, 1°, 2° and 3° toward the [ 1 1 ¯ 0 0 ] direction by using an atmospheric pressure MOCVD. A striped pattern along the ZnO [0 0 0 1] direction was observed in ZnO films, and surface smoothness of ZnO films was remarkably improved by increase in off-angle. Full-width at half-maximum of XRD patterns of rocking curves and 2 θ– ω scans for (0 0 0 2) and ( 1 0 1 ¯ 0 ) planes measured by grazing incident diffraction measurement was decreased with increase in off-angles. Free exciton emissions and donor-bound exciton emissions were clearly observed in the photoluminescence (PL) spectra of ZnO films deposited on ZnO substrates with off-angles measured at 5 K. Room temperature PL spectra of ZnO films have a strong band edge emission on the samples with ZnO films deposited on off-angle substrates.

Off-Angle Dependence of Characteristics of 4H-SiC-Oxide Interfaces

SiC-Oxide interfaces on 4H-SiC epitaxial substrates of various off-angles have been characterized by C-V measurements, synchrotron radiation excited photoemission spectroscopy and atomic force microscopy. We found that the interface state density (N it ) and the amount of sub-oxides (S i ) for small off-angle substrates were smaller than those for large off-angle ones. It was also found that there is a good correlation between the amount of the interface states and that of sub-oxides. Step-terrace structures were clearly observed in the surface morphology after the removal of oxide layers for small off-angle substrates, which shows that the interfaces were atomically smooth. These results suggest that the small N it and S i values for small off-angle substrates relate to the smoothness of the interface and that high performance SiC MOS devices are expected by using small off-angle substrates.

Growth and characterization of 4H-SiC epilayers on substrates with different off-cut angles

In this paper, epitaxial growth on 4H-SiC (0001) substrates with 4° and 8° off-cut angles is investigated. Both growth rate and nitrogen doping show a dependence on the C∕Si ratio and off-cut angle. Good surface morphologies are obtained on the 8° off-angle substrates over a wide range of C∕Si ratios, while macrostep bunching is observed on the 4° off-angle substrates depending on C∕Si ratios. Step bunching behaviors on both substrate orientations are investigated by Nomarski microscopy and atomic force microscopy, and possible step bunching mechanisms are proposed to explain the results. Basal plane dislocation (BPD) densities are examined by means of molten KOH etching. Low off-cut angle is found to enhance the conversion of BPDs into threading edge dislocations. A BPD density of 2.6cm−2 is achieved on the 4° off-angle substrate under optimized growth conditions. The effects of changing the C∕Si ratio on BPD densities are also investigated.