Growth Of GaN Epitaxial Layers Research Articles

Effect of gas pre-decomposition device on the growth of GaN epitaxial layer

In previous studies, the influence of gas phase and surface reactions on the growth of GaN was mainly calculated through simulations. In this study, a novel gas pre-decomposition device (GPDD) was designed to experimentally investigate the effects of gas phase and surface reactions on GaN growth by changing the length and height of the isolation plates (IPs). By varying the structure of the GPDD, the effects on the growth rate and thickness uniformity of the GaN films were studied. The growth rate of the GaN sample slowed with the extension of the IPs because the longer partition plates led to insufficient gas mixing and premature consumption of the precursor trimethylgallium (TMG). The use of GPDD simultaneously achieves high crystal quality and smooth surface morphology of the GaN film. Owing to the use of GPDD, the decomposition of TMG in the pyrolysis pathway was promoted, which suppressed Ga vacancies and C impurities, resulting in weak yellow luminescence bands in the photoluminescence. This study provides a comprehensive understanding of the chemical reaction mechanism of GaN and plays an important role in promoting the development of metal-organic chemical vapor deposition equipment.

Optimization of the growth of GaN epitaxial layers in an indigenously developed MOVPE system

Growth of GaN epitaxial layers is optimized in an indigenously developed nitride metal organic vapour phase epitaxy (MOVPE) system. The motivation of present work is to develop a simple MOVPE reactor which is though affordable but can deliver GaN epilayers of desired quality. The design and fabrication methodology of nitride MOVPE system is briefly discussed. MOVPE growth of GaN epilayers is carried out through a two-step growth process where influence of the growth parameters of low temperature buffer layer on the crystalline properties of high temperature GaN epilayer is discussed in detail. Optimum values of the growth temperature, annealing duration and thickness of GaN buffer layer are achieved. High resolution X-ray diffraction measurements (HRXRD) confirmed that the crystalline quality of GaN epitaxial layers is reasonable. A good surface morphology of the optimized sample is strongly corroborated by the results obtained from photoluminescence (PL) and HRXRD techniques. Usefulness of a systematic Williamson-Hall analysis in the optimization of growth of GaN epitaxial layers is demonstrated where a high value of lateral coherence length of GaN buffer layer is found to be a key parameter. Such an optimization process leads to the good crystalline quality of GaN epitaxial layers with low dislocation density. It is found that the high temperature GaN epilayers grown on the optimized GaN buffer layer are compressively stressed which is also revealed by the PL measurements.

Successful Fabrication of GaN Epitaxial Layer on Non‐Catalytically‐grown Graphene

Sapphire is widely used as a substrate for the growth of GaN epitaxial layer (EPI), but has several drawbacks such as high cost, large lattice mismatch, non‐flexibility, and so on. Here, we first employ graphene directly grown on Si or sapphire substrate as a platform for the growth and lift‐off of GaN‐light‐emitting diode (LED) EPI, useful for not only recycling the substrate but also transferring the GaN‐LED EPI to other flexible substrates. Sequential standard processes of nucleation/recrystallization of GaN seeds and deposition of undoped (u‐) GaN/AlN buffer layer were done on graphene/substrate before the growth of GaN‐LED EPI, accompanied by taping and lift‐off of u‐GaN/AlN or GaN‐LED EPI. This approach can overcome the limitations by the catalytic growth and transfer of graphene, and make the oxygen‐plasma treatment of graphene for the growth of GaN EPI unnecessary.

Impact of GaN transition layers in the growth of GaN epitaxial layer on silicon**Project supported by the National Natural Science Foundation of China (Nos. 61474110, 61377020, 61376089, 61223005, 61176126), and the National Science Fund for Distinguished Young Scholars (No. 60925017).

A method for growing GaN epitaxial layer on Si (111) substrate is investigated. Due to the large lattice mismatch between GaN and AlN, GaN grown directly above an AlN buffer layer on the Si substrate turns out to be of poor quality. In this study, a GaN transition layer is grown additionally on the AlN buffer before the GaN epitaxial growth. By changing the growth conditions of the GaN transition layer, we can control the growth and merging of islands and control the transfer time from 3D to 2D growth mode. With this method, the crystalline quality of the GaN epitaxial layer can be improved and the crack density is reduced. Here, we have investigated the impact of a transition layer on the crystalline quality and stress evolution of a GaN epitaxial layer with methods of X-ray diffraction, optical microscopy and in situ reflectivity trace. With the increasing thickness of transition layer, the crack decreases and the crystalline quality is improved. But when the transition layer exceeds a critical thickness, the crystalline quality of the epilayer becomes lower and the crack density increases.

Low Temperature Growth of GaN Epitaxial Layers on Sapphire (0001) by Pulsed Laser Deposition Using Liquid Gallium Target

Smooth surface GaN epilayers have been grown on sapphire (0001) by pulsed laser ablation of liquid Ga target under radio frequency assisted atomic nitrogen ambient. Laser energy density, laser frequency and substrate-to-target distance were adjusted to optimize the Ga flux for a fixed nitrogen plasma condition. From in-situ reflection high energy electron diffraction and atomic force microscopy analyses, it is found that the Ga/N flux ratio plays a crucial role in determining the growth mode and that only the Ga-rich condition promotes a two dimensional GaN growth. Also, the increase of Ga flux by increasing the laser energy density above 8 J/cm(2) is observed to produce Ga droplets on the grown GaN surface even if the growth front is excess with nitrogen flux. Instead, the increase of laser frequency (up to 50 Hz, in this study) is preferable. The effect of growth temperature on the crystalline quality of GaN layers has also been investigated in the range from 300 to 700 degrees C. Highly c-axis oriented GaN layers, with a (0002) plane X-ray rocking curve full width at half maximum of 245 arcsec, have been obtained at 500-600 degrees C, which is about 150 degrees C lower than the conventional MBE growth.

Plasma‐assisted MBE growth of GaN on Si(111) substrates

AbstractIn this work we present details of growth of GaN epitaxial layers on Si(111) substrates by molecular beam epitaxy (MBE) with the use of RF nitrogen plasma source. We focus on preparation of silicon substrate before the growth, on procedure of AlN buffer growth initiation (aluminum or nitrogen first) and on influence of III/N ratio on structural properties of the layers. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single Crystalline GaN Epitaxial Layer Prepared on Nano-Patterned Si(001) Substrate

The authors report the growth of GaN epitaxial layers on nano-patterned Si(001) substrates prepared by the standard facilities used in integrated circuit (IC) industry. It was found that we could achieve high-quality single crystalline GaN by using the 50 nm SiO2 recess patterned Si(001) substrate. It was also found that we can reduce the tensile stress in GaN epitaxial layer by about 95% using the nano-patterned Si(001) substrate, as compared to the conventional un-patterned Si(111) substrate.

GaN Epitaxial Layers Prepared on Nano-Patterned Si(001) Substrate

We report the growth of GaN epitaxial layer on Si(001) substrate with nano-patterns prepared by dry etching facility used in integrated circuit (IC) industry. It was found that the GaN epitaxial layer prepared on nano-patterned Si(001) substrate exhibits both cubic and hexagonal phases. It was also found that threading dislocation observed from GaN prepared on nano-patterned Si(001) substrate was significantly smaller than that prepared on conventional unpatterned Si(111) substrate. Furthermore, it was found that we can reduce the tensile stress in GaN epitaxial layer by about 78% using the nano-patterned Si(001) substrate.

Epitaxial growth of GaN on (0001) Al2O3 via solution-cast seed layer formation process using Ga(mDTC)3

This paper reports an alternative method for the growth of GaN epitaxial layer on (0001) Al2O3 substrate by hot-wall vapor phase epitaxy technique. Tris (N,N-dimethyldithiocarbamato)-gallium (III), Ga(mDTC)3 was introduced as a precursor material for the seed layer formation in the growth of GaN. Optimal growth conditions with seed layers formed by the Ga(mDTC)3 concentration of 0.047 mol/L were identified: Growth temperature was found to be 850 °C, and optimal distance between the reactant outlet and substrate was determined to be 12.5 cm. Characterization results showed that this growth method produce high-crystallinity GaN epitaxial layers at a relatively lower growth temperature compared to the existing growth techniques and simplify the growth process.

Low-Temperature Growth of GaN Epitaxial Layers with Buffer Layers by Reactive Close-Spaced Method

GaN epitaxial layers with buffer layers were grown on (0001) sapphire substrates by direct reaction of Ga and NH3 at the low temperature of 750°C. The buffer layers were deposited on the substrates by using GaN-coated graphite boats at temperatures below 750°C in Ar flow before the epitaxial layers were grown, where GaN films coated on the graphite boats acted as a source material in the deposition of the buffer layers. The Hall mobility of the GaN epitaxial layers grown with the buffer layers was approximately 50 cm2/(V·s), whereas that of the GaN epitaxial layers grown without the buffer layers was approximately 2 cm2/(V·s). The crystallinity of the GaN epitaxial layer was improved by being grown on the buffer layer. The buffer layer deposited by using the GaN-coated graphite boat played a role similar to that of the buffer layer deposited at low temperature by conventional metal-organic chemical vapor deposition.

Growth of GaN epitaxial layers on sapphire with preheated ammonia and their structural and optoelectronic properties

GaN epitaxial layers were grown by metalorganic chemical vapor deposition with preheated ammonia as a group V source. The growth rates of GaN epilayers were little affected when ammonia preheater temperature was varied from room temperature to 1000 °C, however, their electrical properties as well as source materials utilization efficiency was significantly improved by using preheated ammonia. GaN epilayers grown with preheated ammonia showed excellent structural and optical properties.

Effects of thermal convection of NH 3 during growth of GaN epitaxial layers by horizontal MOCVD reactor

We have studied the effects of thermal convection during the growth of GaN epitaxial layers by horizontal MOCVD reactor. The epilayers were grown with the variation of reaction pressures in order to change the total gas velocity in the horizontal MOCVD reactor. The van der Pauw technique, SEM, double crystal X-ray diffractometry and photoluminescence were used in the characterization of their crystallographic, electrical and optical properties for investigating thermal convection effect of NH 3. The GaN epilayer which was grown under thermal convection of NH 3 related to low gas velocity shows rough surface having large islands. However, the quasi-mirror-like surface without any defect such as hillocks and islands can be seen in the sample which was grown free from thermal convection of NH 3 due to suitable gas velocity. The growth rate is also changed, which results from different thermal convections of NH 3 related to various gas velocities in the reactor during their growth. Judging from various data, it is possible to conclude that thermal convection of NH 3 is taking place when the epilayer is grown in the reactor at a gas velocity below 0.32 m/s. And, the generated thermal convection of NH 3 seriously affects the quality of GaN epilayers such as crystallinity, electrical characteristics and optoelectronic properties.

Improvement of the crystallinity of GaN epitaxial layers grown on porous Si (100) layers by using a two-step method

A new approach was used for combining GaN and porous Si with the goal of producing high-quality GaN epitaxial layers for optoelectronic integrated circuit devices based on Si substrates. Reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), photoluminescence (PL), and Van der Pauw–Hall effect measurements were performed to investigate the structural, optical, and electrical properties of the GaN epitaxial films grown on porous Si(100) by plasma-assisted molecular-beam epitaxy with a two-step method. The RHEED patterns were streaky with clear Kikuchi lines, which was direct evidence for layer-by-layer two-dimensional growth of GaN epitaxial layers on porous Si layers. The XRD curves showed that the grown layers were GaN(0001) epitaxial films. The results of the XRD and the PL measurements showed that the crystallinities of the GaN epilayers grown on porous Si by using a two-step growth were remarkably improved because the porous Si layer reduced the strains in the GaN epilayers by sharing them with the Si substrates. Hall-effect measurements showed that the mobility of the GaN active layer was higher than that of the GaN initial layer. These results indicate that high-quality GaN epitaxial films grown on porous Si(100) by using two-step growth hold promise for potential applications in new kinds of optoelectronic monolithic and ultralarge integrated circuits.

In situ real time ellipsometry for GaN remote plasma MOCVD technology

The potentiality of non-intrusive, non-destructive and non-perturbing ellipsometry in the remote plasma metalorganic chemical vapor (RP-MOCVD) technology of GaN is briefly reviewed. It is shown that in situ spectroscopic ellipsometry (SE) is a powerful technique to provide (i) accurate thickness measurements, (ii) depth-profiles of interfaces and thin films structure, (iii) the composition of layers and (iv) the microroughness of the surface layer. With respect to this, the processes of GaAs substrates cleaning and of GaAs and GaP substrates nitridation by in situ remote plasmas of, respectively, H 2 and N 2 are investigated. These processes are extremely important in the growth of GaN epitaxial layer, since they prepare the substrate to be a good template for the heteroepitaxial growth. Furthermore, it will be shown that the results obtained by SE are reliable and corroborated by the results of XPS and AFM. Single wavelength ellipsometry (SWE) is used for the real time monitoring of the kinetics of the GaAs and GaP substrate cleaning and nitridation and of GaN epigrowth by trimethylgallium and plasma pre-activated nitrogen. Thus, the chemical and kinetic model of the above processes is presented. Also, the capability of the real time ellipsometry in monitoring adsorption equilibrium is discussed through the process of sapphire plasma nitridation.