Masashi Tsukihara

Surface Smoothing Mechanism of AlN Film by Initially Alternating Supply of Ammonia

A buffer technique that initially alternates supply of ammonia (IASA) is employed for AlN film growth on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Atomic force microscopy (AFM) study reveals that step-flow-like growth morphology is achieved for the AlN film with the root-mean-square (rms) roughness as small as 0.336 nm. In contrast, the surface morphology of thick AlN film grown directly on sapphire substrate shows rough grainy feature with a large rms value of 28.9 nm. The mechanism leading to superior morphology by introducing IASA process is investigated using transmission electron microscopy (TEM), hot wet chemical etching and scanning electron microscopy (SEM) techniques. Evidence is presented that their morphological differences are attributed to strain reduction and polarity inversion. The present work provides insight into the AlN epitaxial growth and indicates that IASA is an effective method to realize atomically smooth AlN film.

Dislocation reduction in GaN layer by introducing GaN-rich GaNP intermediate layers

We report a new method of reducing the dislocation density in a GaN epilayer grown on sapphire by metalorganic chemical vapor deposition (MOCVD), by introducing GaN-rich-GaNP/GaN multiple layers (MLs) during the growth of the GaN layer. It is found that some threading dislocations bend and/or terminate near the ML region, which would suppress the dislocations propagating to the top film surface. The dislocation density in the films consequently decreases to 6×108 cm-2 in the GaN epilayer with GaNP/GaN MLs, which is a decrease of factor 2 with respect to that in a conventionally grown GaN epilayer. It is considered that the ternary GaN-rich GaNP selectively deposits in the regions of those dislocation-induced pits on the as-grown GaN surface, and then influences the propagation properties of the dislocations. This method can be applied to many devices such as laser diodes whose performance is deteriorated by dislocations.

GaN growth using a low-temperature GaNP buffer on sapphire by metalorganic chemical vapor deposition

We developed a buffer layer to grow GaN epilayers by metalorganic chemical vapor deposition. The buffer layer consists of a thin GaN-rich GaNP layer deposited at low temperature (LT) (500 °C) on sapphire substrate, using phosphine (PH3) as the phosphorus source. For high-temperature GaN epilayers grown on this type of buffer, full-width at half maximum values from both (0002) and (1012) x-ray rocking curves decrease as phosphorus composition in the GaNP buffer increases; a dislocation density observed by transmission electron microscopy is as low as 5×108 cm−2, which is a factor of 2 less compared to that in a conventional GaN buffer epilayer. These results reveal that LT GaNP can be used as an appropriate buffer for further improving quality of GaN-based films.

Al0.17Ga0.83N Film Using Middle-Temperature Intermediate Layer Grown on (0001) Sapphire Substrate by Metal-Organic Chemical Vapor Deposition

The reduction of the dislocation density in a high-temperature (HT)-Al0.17Ga0.83N epitaxial layer was achieved by a middle-temperature (MT)-intermediate layer technique, in which the HT and the MT were 1050 and 950 °C, respectively. For one set of the MT-intermediate layer, the structure was 4.5-µm-thick HT-Al0.17Ga0.83N/1-µm-thick MT-intermediate layer/100-nm-thick HT-Al0.17Ga0.83N layer/low-temperature (LT)-GaNP buffer/trenched (0001) sapphire. The full-width at half maximum values of (0002) and (1012) diffraction peaks of the X-ray diffraction for the Al0.17Ga0.83N epitaxial layer using one set of the MT-intermediate layer were improved to 359 and 486 arcsec compared with 401 and 977 arcsec for the film without the MT-intermediate layer technique, respectively. Transmission electron microscopy result showed that the dislocation density in the Al0.17Ga0.83N film using one set of MT-intermediate layer was reduced from (1–4)×1010 to 1.7×109 cm-2. The Al0.17Ga0.83N epitaxial film including two sets of MT-intermediate layers improved the most, showing a dislocation density of 9.3×108 cm-2.

Investigations of V-shaped defects and photoluminescence of thin GaN-rich GaNP layers grown on a GaN epilayer by metalorganic chemical vapor deposition

Structural defects in high-temperature GaN-rich GaNP layers grown on a thick GaN epilayer by metalorganic chemical vapor deposition were investigated by means of transmission electron microscopy and atomic force microscopy. It is found that there are inverted hexagonal pyramid V-shaped defects in the GaNP layers with a higher phosphorus (P) composition. These defects are generally related to threading dislocations propagating from the GaN layer beneath. Consequently, the dislocation density in the GaNP layers is dramatically decreased to 5–8×107 cm−2. Photonluminescence (PL) measurements show that the PL wavelength of GaNP redshifts and the integrated emission intensity significantly increases, with respect to that from GaN layers grown under identical conditions. The emission enhancement is attributed to lowering of the dislocation density and enlarging the escape cone of photons by the rough surface, which result from the V-shaped defects formed in the GaNP layer.

Dislocation reduction in GaN epilayers grown on a GaNP buffer on sapphire substrate by metalorganic chemical vapor deposition

Dislocation defects in a GaN epilayer grown on a low-temperature GaN-rich GaNP (LT-GaNP) buffer on sapphire substrate by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) were investigated by means of transmission electron microscopy and atomic force microscopy. The GaNP-buffer-based high-temperature GaN (HT-GaN) layers have a dislocation density as low as 5×108 cm-2, which can be compared to the best results for GaN epilayers grown on sapphire by atmospheric pressure MOCVD using the conventional LT-GaN or -AlN buffer. The dislocation density reduction could be predominately attributed to an enhanced lateral overgrowth for the HT-GaN layer grown on a GaNP buffer, which is confirmed by the observation of a special morphology evolution beginning from the GaNP buffer.

Growth of AlN and GaN by Metalorganic Chemical Vapor Deposition on BP Synthesized by Flux Method

BP with the size of 50 µm to 3 mm was synthesized by the Cu flux method. The BP crystals have a zincblend structure, and the lattice constant and the cathodoluminescence peak wavelength were 4.557 A and 370 nm, respectively. GaN and AlN were grown by metalorganic chemical vapor deposition on BP. It was found that AlN grown at 1150 °C on (100)BP was grown smoothly but that grown on (111)BP had a rough surface. GaN, however, was irregularly grown on both (100) and (111)BP. It was demonstrated that AlN on (100)BP is another candidate as a substrate for a UV-light-emitting diode.

Effect of GaNP buffer layer on AlGaN epilayers deposited on (0001) sapphire substrates by metalorganic chemical vapor deposition

AlxGa1-xN (x ≤0.1) epilayers grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor deposition with low-temperature (LT)-GaNP buffer have been characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the full width at half maximum (FWHM) values of the GaNP-buffer-based AlxGa1-xN (x ≤0.1) epilayers decrease are lower than that of the conventional GaN-buffer-based AlxGa1-xN (x ≤0.1) epilayers, but the difference becomes smaller as the Al composition increases. These results could be mainly attributed to a longer diffusion length with the GaNP buffer layer than with the GaN buffer layer.

Self-Separation of Sublimation-Grown AlN with AlSiN Buffer Layer

AlN was grown by a sublimation method on 6H-SiC. We found the grown AlN layer is easily separated from the substrate when Si powder is added to the AlN source powder. The formation of AlSiN layer with the Si content of 15% at the AlN/6H-SiC interface was confirmed by energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). This AlSiN layer causes the separation of AlN.