Jun Komiyama

Suppression of crack generation in GaN epitaxy on Si using cubic SiC as intermediate layers

We demonstrate suppression of crack generation in GaN epitaxy on Si using cubic SiC as intermediate layers. Crack-free GaN with a thickness of 2μm was obtained. Epilayers of SiC (0–1μm), thin AlN (50nm), and GaN (1–3μm) were prepared on 3in. (111)Si substrates (GaN∕AlN∕SiC∕Si) by metalorganic vapor-phase epitaxy. Cracking of GaN is suppressed with thicker SiC (1μm), whereas cracks are generated in GaN without SiC and with thinner SiC (50nm). Transmission electron microscopy analysis revealed monocrystalline wurzite structure of GaN. Current-voltage measurements showed breakdown voltage exceeding 250V, indicating its potential for high voltage application.

Stress reduction in epitaxial GaN films on Si using cubic SiC as intermediate layers

Stress in the epitaxial films of GaN on Si is reduced by using SiC as intermediate layers. The crystalline films of cubic SiC (0–1μm), thin AlN (50nm), and GaN (1–3μm) were prepared on 3in. (1 1 1) Si substrates—stacked in the order of GaN∕AlN∕SiC∕Si—by metalorganic vapor-phase epitaxy. It is revealed by Raman spectroscopy that the tensile stress in GaN is reduced to half (reduction of about 300MPa) for GaN on Si with SiC intermediate layers compared with GaN on Si without SiC intermediate layers. Because of stress reduction, crack-free GaN on Si with a thickness of 2μm was obtained by using SiC intermediate layers. Cracking was minimized even on thicker GaN on Si (3μm thick) with SiC intermediate layers. The SiC intermediate layers are promising for the realization of nitride based electronic devices on Si.

Semipolar Nitrides Grown on Si(001) Offcut Substrates with 3C-SiC Buffer Layers

The growth process of semipolar GaN(10-12) on Si(001) offcut substrates with 3C-SiC buffer layers has been investigated. From XRD analysis, the difference in the crystal orientation between GaN(10-12) and 3C-SiC(001) has been found to be around 8˚ toward the [110] direction of the 3C-SiC templates. From TEM observations, a cubic-phase AlN seed layer is found to grow on 3C-SiC(001) templates, and the swift transition from the cubic phase to a hexagonal phase leads to the stable growth of hexagonal nitrides. Using 8˚-offcut Si substrates, it is possible to obtain a mirror-like surface of GaN(10-12) using an approximately 10-nm-thick AlN seed layer, which swiftly transitions from cubic AlN to hexagonal GaN.

Defect Propagation from 3C-SiC Intermediate Layers to III–Nitride Epilayers

The propagation of crystal defects from a 3C-SiC intermediate layer (3C-SiC IL) to hexagonal III–nitride epilayers formed by a metalorganic vapor phased epitaxy (MOVPE) has been investigated by observing the interface between the 3C-SiC IL and the hexagonal III–nitride epilayers. The 3C-SiC(111) IL grown on a Si(111) substrate has many stacking faults (SFs) that form along the 3C-SiC111 planes. The density of the SFs decreases with separation from the Si substrate. The initial III–nitride epilayers have V-shaped trenches due to the SFs of the 3C-SiC IL. However, there are some SFs, that do not generate V-shaped trenches. On the basis of high-resolution cross-sectional observations by transmission electron microscopy and X-ray pole-figure analysis, an atomic model for the SFs is considered in terms of twin bands of 3C-SiC. V-shaped trenches were determined to be formed on protrusions consisting of the twin bands in the 3C-SiC IL.

Determination of Aluminium Mole Fraction in AlGaN Layers of GaN-Capped AlGaN/GaN Heteroepitaxial Wafers by Ultraviolet Reflection

Ultraviolet reflection spectroscopy is applicable to the determination of the aluminium mole fraction of GaN-capped AlGaN/GaN heteroepitaxial wafers on silicon, while conventional photoluminescence is inapplicable. AlGaN peaks in the ultraviolet reflection spectra are clearly observed regardless of the cap, but the AlGaN photoluminescence peaks of the samples with a 2-nm-thick cap are difficult to observe clearly. For some capped samples, the quantum-well emission due to the cap is observed near the AlGaN peak.

HRTEM Analysis of AlN Layer Grown on 3C-SiC/Si Heteroepitaxial Substrates with Various Surface Orientations

Epitaxial growth of AlN was carried out by MOVPE method on SiC/Si buffered substrates prepared by using various Si surfaces of (110), (211) and (001). Cross-sectional HRTEM analyses of the interfaces between SiC buffer layer and AlN epitaxial layer disclosed characteristic nanostructures related growth mechanism on the each substrate. In the case of Si(110) and Si(211) substrate, hexagonal AlN grew directly on SiC(111) plane with AlN(0001) plane parallel to it. In contrast, growth on Si(001) substrate gave complicate structure at AlN/SiC interface. Hexagonal AlN didn’t grow directly but cubic AlN appeared with a pyramidal shape on SiC(001). When the cubic AlN grew 10nm in height, structure of growing AlN crystal changed to hexagonal type on the pyramidal {111} planes of cubic AlN.

Evaluation of Stacking Faults in Single-Crystalline 3C-SiC Films by Polarized Raman Spectroscopy

Thin films of 3C-SiC were grown by vapor phase epitaxy on 2-in. Si(100) substrates to a thickness of 6000 nm. The correlation between stacking faults and the full width at half maximum (FWHM) of transverse optical (TO) Raman lines was analyzed. Transmission electron microscopy (TEM) and micro-Raman spectroscopy were performed to measure stacking fault density and FWHM, respectively. Cross-sectional TEM images show that the high-defect-density region extends to 1500 nm above the film-substrate interface. FWHM decreased sharply with increasing distance from the interface to 4000 nm in 3C-SiC, and it gradually decreased beyond 4000 nm. The correlation between stacking fault density and FWHM was found to be nearly linear.

Characterization of Dislocations in GaN Thin Film and GaN/AlN Multilayer

The dislocations in GaN thin film with GaN/AlN multilayer (ML) as the buffer layer were evaluated using transmission electron microscopy. A high density of dislocations parallel to the GaN/ML interface and a sudden decrease in the dislocation density at the GaN/ML interface were found. Dislocation propagation in the direction parallel to the GaN/ML interface by turning horizontally on the GaN/ML interface is considered to be effective in decreasing the dislocation density at the top layer of GaN.

Defect Propagation from 3C-SiC to III-Nitride

Role of 3C-SiC intermediate layer (3C-SiC IL) and a propagation of crystal defects from 3C-SiC IL to III-nitride epilayers has been investigated by observing the interface between the 3C-SiC IL and III-nitride epilayers. We found that better quality epilayers were obtained by using such intermediate layers than by direct growth on Si substrates. In the case of III-nitride epilayers grown directly on Si, the layers grown at the initial stage are not flat. The 3C-SiC IL grown on Si (111) substrates has many stacking faults (SFs) that form along the 3C-SiC {111} planes. The initial III-nitride films have V-shaped trenches due to the SFs of the 3C-SiC IL. However, some SFs do not lead to the V-shaped defects.

HRTEM study of AlN/3C-SiC heterointerfaces grown on Si(001) and Si(211) substrates

Epitaxial growth of nitride semiconductors on Si wafer is an important technique to develop electro-optical devices. However, a large lattice mismatch between Si and nitrides disturbs direct growth of defect-free nitride layer on Si substrate. Cubic SiC (3C-SiC), which can be formed directly on Si substrate [1], is attractive as a buffer layer to relieve the lattice mismatch. Nanoscopic information at interface between nitride and 3C-SiC is required to improve quality of growing nitride layer. In this work, interface structure between AlN and 3C-SiC buffer layer grown on Si(001) and Si(211) substrates is studied by high-resolution transmission electron microscopy (HRTEM).