Aug 16, 2019

Crack-free GaN substrates grown by the Na-flux method with a sapphire dissolution technique

GaN wafers are generally fabricated by separating a foreign substrate from a GaN layer using thermal stress; however, thermal stress also leads to the cracking of the GaN layer. In this study, we first succeeded in dissolving a sapphire substrate just after Na-flux growth by successively changing the flux content for GaN growth (Ga–Na–C) to that for dissolving sapphire (Ga–Na–C–Li) at the considered growth temperature. Hence, no thermal stress was induced in the grown GaN crystals, resulting in a crack-free GaN substrate. We concluded that this process is a good candidate technique for supplying free-standing GaN substrates.



Source:IOPscience

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Aug 9, 2019

MOVPE growth of GaN on Si substrate with 3C-SiC buffer layer

We investigated the effect of the thickness of a 3C-SiC buffer layer on the growth of GaN on a Si substrate. GaN samples with thicknesses of 2.0 and 4.5 µm were grown by metal organic vapor phase epitaxy. Islands were observed at the initial growth of the GaN samples, and they became larger and then coalesced with each other with increasing growth time. The crystalline quality of the GaN samples was affected by the thickness of the 3C-SiC buffer layer and was improved by increasing their thickness. This indicates that the crystalline quality trend for thick GaN growth is different from that for initial GaN growth. Moreover, the GaN sample with a 100-nm-thick SiC buffer layer had a low initial curvature, and the crystalline qualities on GaN(0004) and  planes were 389 and 460 arcsec for 4.5-µm-thick GaN growth, respectively.


Source:IOPscience

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Aug 1, 2019

Theoretical investigations on the growth mode of GaN thin films on an AlN(0001) substrate

The growth modes of GaN thin films on an AlN(0001) substrate are systematically investigated using our macroscopic theory with the aid of empirical interatomic potential and ab initio calculations. Using empirical interatomic potential calculations, we demonstrate that a GaN/AlN(0001) system with misfit dislocation (MD) is stabilized compared with a coherently grown system when the GaN film thickness exceeds six monolayers. According to the calculated results including the surface energy of GaN, the macroscopic free energy calculations for the growth mode imply that the growth of GaN on AlN(0001) proceeds along the lower-energy path from two-dimensional coherent (2D-coherent) to 2D-MD under Ga-rich conditions but from 2D-coherent to three-dimensional coherent with truncated hexagonal pyramid islands consisting of {${\rm{10}}\bar{1}3$} facets under N-rich conditions owing to surface energy anisotropy, which depends strongly on the growth conditions. These results suggest that surface energy anisotropy is crucial for the growth of GaN on an AlN(0001) substrate.



Source:IOPscience

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Jul 23, 2019

PEALD-deposited crystalline GaN films on Si (100) substrates with sharp interfaces

Polycrystalline gallium nitride (GaN) thin films were deposited on Si (100) substrates via plasma-enhanced atomic layer deposition (PEALD) under optimal deposition parameters. In this work, we focus on the research of the GaN/Si (100) interfacial properties. The x-ray reflectivity measurements show the clearly-resolved fringes for all the as-grown GaN films, which reveals a perfectly smooth interface between the GaN film and Si (100), and this feature of sharp interface is further confirmed by high resolution transmission electron microscopy (HRTEM). However, an amorphous interfacial layer (~2 nm) can be observed from the HRTEM images, and is determined to be mixture of Ga x O y and GaN by x-ray photoelectron spectroscopy. To investigate the effect of this interlayer on the GaN growth, an AlN buffer layer was employed for GaN deposition. No interlayer is observed between GaN and AlN, and GaN shows better crystallization and lower oxygen impurity during the initial growth stage than the GaN with an interlayer.



Source:IOPscience

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Jul 17, 2019

Growth behavior of hexagonal GaN on Si(100) and Si(111) substrates prepared by pulsed laser deposition

In this study, we investigated the microstructure and optical properties of hexagonal GaN (h-GaN) films grown by high-temperature pulsed laser deposition (PLD) on Si(100) and Si(111) substrates. The growth mechanism, crystallization, and surface morphology of h-GaN deposition on both Si(100) and Si(111) substrates were monitored by transmission electron microscopy (TEM) and scanning electron microscopy at various times in the growth process. Our results indicated that the h-GaN grown on Si(111) has better crystalline structure and optical properties than that on Si(100) owing to the smaller mismatch of the orientations of the Si(111) substrate and h-GaN film. On the Si(100) substrate, the growth principles of PLD and N2 plasma nitridation are the main contributions to the conversion of the cubic GaN into h-GaN. Moreover, no significant Ga–Si meltback etching was observed on the GaN/Si surface with the PLD operation temperature of 1000 °C. The TEM images also revealed that an abrupt GaN/Si interface can be obtained because of the suppression of substrate–film interfacial reactions in PLD.


Source:IOPscience

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Jul 9, 2019

Formation of helical dislocations in ammonothermal GaN substrate by heat treatment

GaN substrate produced by the basic ammonothermal method and an epitaxial layer on the substrate was evaluated using synchrotron radiation x-ray topography and transmission electron microscopy. We revealed that the threading dislocations present in the GaN substrate are deformed into helical dislocations and the generation of the voids by heat treatment in the substrate for the first observation in the GaN crystal. These phenomena are formed by the interactions between the dislocations and vacancies. The helical dislocation was formed in the substrate region, and not in the epitaxial layer region. Furthermore, the evaluation of the influence of the dislocations on the leakage current of Schottky barrier diodes fabricated on the epitaxial layer is discussed. The dislocations did not affect the leakage current characteristics of the epitaxial layer. Our results suggest that the deformation of dislocations in the GaN substrate does not adversely affect the epitaxial layer.



Source:IOPscience

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Jul 5, 2019

Dramatic increase in the growth rate of GaN layers grown from Ga2O vapor by epitaxial growth on HVPE-GaN substrates with a well-prepared surface

A GaN growth technique using Ga2O vapor allows high-temperature growth, which results in high-crystalline GaN. In this study, we succeeded in increasing the growth rate up to 180 µm/h, and with this technique, we maintained the crystallinity of the epitaxial layers at the same level as the crystallinity of the seed substrates [the FWHM of the (0002) GaN X-ray rocking curve was 71'']. To achieve this improvement, growth occurred on atomically smooth, damage-free seed substrates prepared by chemical mechanical polishing (CMP), and these substrates were subjected to a subsequent H2 heating process. Moreover, for this growth process, both a high temperature (1200 °C) and a H2/N2 atmosphere were also found to improve crystallinity.




Source:IOPscience

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