Growth Of GaN Layers Research Articles

Facet growth of self-separated GaN layers through HVPE on large square-patterned template

A self-separated GaN layer was prepared by hydride vapor phase epitaxy (HVPE) on a square-patterned template with large periodicity. Self-separation was completed by breakage of the fragile layer because of the thermal stresses generated during the cooling process after HVPE growth. The GaN layer exhibited graphical surface comprising the terrace and the concave, the shapes of which were corresponding with the mask pattern. The terrace came from the growth on window openings, and had Ga-polarity by wet etching and micro-Raman measurement. The concave over the mask was composed of large inclined facets, and was demonstrated to have N-polarity. The growth on large square-patterned template was considered to be facet growth. The polarity inversion was related to dislocation accumulation. The strain distribution regularly varied, which was interpreted based on the facet growth mode.

Growth of N-polar GaN on vicinal sapphire substrate by metal organic chemical vapor deposition

The growth and properties of N-polar GaN layers by metal organic chemical vapor deposition (MOCVD) were reported. It is found that N-polar GaN grown on normal sapphire substrate shows hexagonal hillock surface morphology. With the misorientation angles increasing from 0.5° to 2.0° toward the a-plane of the sapphire substrate, the number of the hillock becomes less and less and finally the surface becomes flat one on the sapphire substrate with the misorientation angle of 2°. It is also found that the crystalline quality and the strain in the GaN are greatly influenced by the misorientation angle.

실리콘 기판위에 금속 완충층을 이용한 GaN 성장과 특성분석

실리콘 기판 위에 GaN를 성장하기 위해서 AlN 완충층을 사용해 왔다. 그러나 AlN은 아직까지 high doping이 쉽지 않기 때문에, 이로 인해 AlN를 전자소자나 광소자 제작을 위한 완충층으로 이용하는 경우 직렬 저항의 증가라는 문제가 발생할 수 있다. 본 연구에서는 이러한 문제점을 개선하기 위해 AlN 완충층 대신에 금속 완충층을 사용하여 실리콘 기판 위에 GaN 박막 성장실험을 수행하였다. Al, Ti, Cr 그리고 Au 등을 금속 완충층으로 사용하여 실리콘 기판 위에 GaN 층을 성장하였다. 성장된 GaN 박막의 표면 특성을 분석하기 위해 광학현미경과 SEM을 사용하였고, 결정성과 광학적 특성을 평가하기 위하여 PL과 XRD 분석을 실시하였으며 AlN 완충층을 사용한 경우와 금속 완충층을 사용한 경우의 저항 차이를 확인하기 위하여 전류-전압 특성을 측정하였다. AlN buffer layers have been used for the growth of GaN layers on Si substrates. However, the doping of high concentration of carriers into AlN layers is still not easy, therefore it may cause the increase of series resistance when it is used for the electrical or optical devices. In this work, to improve such a problem, the growth of GaN layers on Si substrates were performed using metal buffer layers instead of AlN buffer layer. We tried combinations of Ti, Al, Cr and Au as metal buffer layers for the growth of GaN on Si substrates. Surface morphology was measured by optical microscope and scanning electron microscope (SEM), and optical properties and crystalline quality were measured by photoluminescence (PL) and X-ray diffractometer (XRD), respectively. Electrical resistances for both cases of AlN and metal buffer layer were compared by current-voltage (I-V) measurement.

Growth of GaN layer with preserved nano-columnar low temperature GaN buffer to reduce the wafer bowing

In this study, a nano-columnar low-temperature (LT) GaN buffer was used to reduce the wafer bowing of a GaN layer grown on a sapphire substrate. A significant reduction in the extent of wafer bowing was observed for the GaN layer for the preserved nano-columnar LT GaN layer when compared with the conventional GaN layer. These results suggest that the preserved nano-columnar structure helped relax the GaN layer strain energy associated with thermal expansion mismatch. The flow of TMGa during the temperature ramp-up from LT to high-temperature was found to be an important process parameter to preserving the nano-columnar structure of LT GaN, resulting in less bowed GaN on sapphire.

Improved Output Power of GaN-based Blue LEDs by Forming Air Voids on Ar-Implanted Sapphire Substrate

This paper investigates GaN-based blue light-emitting diodes (LEDs) grown on sapphire substrates with selective-area Ar-ion implantation. The GaN-based epitaxial layers grown on the Ar-implanted sapphire substrates (Ar-ISS) exhibited selective growth and subsequent lateral growth because of different lattice constants between the implantation and implantation-free regions. As a result, air voids were formed at the GaN/sapphire interface, above the implanted regions and below the active layers of LEDs. We proposed the GaN layer growth mechanisms on the Ar-ISS, and characterized the LEDs with embedded air voids at the GaN/sapphire interface. Using a 20-mA current injection, the light output of the experimental LEDs was found to be 15% greater than that of conventional LEDs. This enhancement can be attributed to the light scattering at the textured GaN/air void interfaces, which increases the probability of photons escaping from the LEDs.

Plasma‐assisted electroepitaxy of GaN layers

AbstractWe have studied the growth of GaN layers by plasma‐assisted electroepitaxy (PAEE). We have used an RF‐plasma source to produce high concentrations of N species in the Ga melt and a high DC electric current to transfer the N species from the Ga surface to the growth interface. We have achieved continuous GaN layers by PAEE at growth temperatures as low as ∼650oC. We have also investigated PAEE growth of GaN layers on GaN nanocolumn templates. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasma-assisted molecular beam epitaxy process combined with a liquid phase electroepitaxy, a novel method for the growth of GaN layers

We have studied a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). We have designed and built a new growth chamber which allowed us to combine a plasma-assisted molecular beam epitaxy process with a liquid phase electroepitaxy. We have demonstrated that it is possible to grow continuous GaN layers by PAEE from a liquid Ga melt at growth temperatures as low as ∼650°C, with low nitrogen overpressures of ∼3×10−5Torr.

Vertical Blocking Voltage Improvement of GaN HEMT Structures on n-SiC by Pre-Epitaxial Substrate Implantation

To overcome the use of expensive semi-insulating SiC substrates, we developed a method to increase the vertical blocking strength of n-SiC by argon implantation prior to the high temperature epitaxial growth (1150°C) of the (Al)GaN layers. We superimpose 4 argon implantation energies (30 keV + 60 keV + 180 keV + 360 keV) to a total maximal concentration of 1·1018 atoms/cm3 and observed a 90 V - 100 V increase of the vertical blocking strength by this method. Implantation profile and dose dependencies are studied showing that the results could be related to an increased substrate resistance achieved by implantation. HRXRD analysis shows that the defect density in the GaN buffer is not increased by substrate implantation.

Optimization of low temperature GaN buffer layers for halide vapor phase epitaxy growth of bulk GaN

We have studied growth and self-separation of bulk GaN on c-oriented Al2O3 using low temperature (LT) GaN buffer layers. By studying the X-ray diffraction (XRD) signature for the asymmetric and symmetric reflections versus the LT-GaN thickness and V/III precursor ratio, we observe that the peak width of the reflections is minimized using a LT buffer thickness of ∼100–300nm. It was observed that the V/III precursor ratio has a strong influence on the morphology. In order to obtain a smooth morphology, the V/III precursor ratio has to be more than 17 during the growth of the buffer layer. By using an optimized LT buffer layer for growth of a 20μm thick GaN layer, we obtain a XRD peak with a full width at half maximum of ∼400 and ∼250arcs for (002) and (105) reflection planes, respectively, and with a dark pit density of ∼2.2×108cm−2. For layers thicker than 1mm, the GaN was spontaneously separated and by utilizing this process, thick free freestanding 2″ GaN substrates were manufactured.

Optimization of GaN wafer bow grown on cone shaped patterned sapphire substrates

Strain evolution in GaN layers grown on flat (FSS) and cone shaped patterned sapphire substrates (CPSS) is studied by in-situ reflectance and curvature measurements. Intrinsic growth strain and dislocation density are investigated in relation to different steps of the growth procedure. In spite of the typical tensile strain of GaN on sapphire, a clear compressive stress during growth is found when the lateral coalescence time is delayed. From the curvature measurement, the average compressive stress during GaN layer growth on CPSS before full coalescence is determined to be below 0.21GPa. Subsequently, tensile stress of 0.25GPa builds up as soon as the surface is closed. Using optimizing growth procedures with controlled stress profile enables nearly zero wafer bow in critical steps of the growth process.

Growth of cubic GaN on 3C–SiC/Si (001) nanostructures

We report on the molecular beam epitaxy growth of cubic GaN on 3C–SiC (001) nanostructures. Transmission electron microscopy (TEM) studies show phase-pure cubic GaN crystals with a low defect density on top of the post shaped 3C–SiC nanostructures and GaN grown on their sidewalls, which is dominated by {111} planar defects. The nanostructures, aligned parallel and perpendicular to the [110] directions of the substrate, are located in anti-phase domains of the 3C–SiC/Si (001) substrate. These anti-phase domains strongly influence the optimum growth of GaN layers in these regions. TEM measurements demonstrate a different stacking fault density in the cubic GaN epilayer in these areas.

Heteroepitaxial fabrication and structural characterizations of ultrathin GaN nanotube arrays

We report on the heteroepitaxial fabrication and the structural characterizations of ultrafine GaN nanotube arrays. The ultrathin GaN nanotube arrays were fabricated by using epitaxial growth of a ultrathin GaN layer on ZnO nanoneedles and etching core Zno nanoneedles. Epitaxial growth of GaN and precise etcing control of core ZnO nanoneedles were obtained by using low-pressure metalorganic vapor-phase epitaxy. ZnO nanoneedles were grown on Si and sapphire substrates by using catalyst-free metal-organic chemical vapor deposition. Furthermore, the structural properties of the nanostructures were investigated using both synchrotron-radiation X-ray diffraction and highresolution transmission electron microscopy. The strain induced during the GaN deposition on ZnO was nearly sustained even after the removal of core ZnO.

Topography of [formula omitted] AlGaN, GaN and InGaN layers grown by metal-organic vapor phase epitaxy

The growth of AlGaN, GaN and InGaN layers on (202¯1) GaN substrates was investigated by metal-organic vapor phase epitaxy. All layers exhibit undulations along [101¯4] with a period length between 20nm and 45nm. Under certain growth conditions, GaN exhibits bunching of the undulations leading to an increase of the undulation period and amplitude. This is also observed in the ternary alloys InGaN and AlGaN. Strong impact of the growth temperature and reactor pressure on the surface roughness and photoluminescence properties is found for GaN layers, with the smoothest layers obtained at low growth temperatures and low reactor pressures. Atomic scale interpretation suggests that the undulations originate from (101¯0) and (101¯1) microfacets.

Plasma-assisted electroepitaxy of GaN layers from the liquid Ga melt

In this current paper we have studied a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). In this method, we have combined the advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface, without spontaneous crystallisation on the surface or within the solution. We have designed and built a new growth chamber which allowed us to combine the plasma-assisted molecular beam epitaxy process with a liquid-phase electroepitaxy system. We have demonstrated that it is possible to grow continuous GaN layers by PAEE from liquid Ga melt at growth temperatures as low as ∼650°C, with low nitrogen overpressures of ∼3×10−5Torr.

Plasma‐assisted electroepitaxy as a novel method for the growth of GaN layers

AbstractIn the current study we have demonstrated the feasibility of a novel approach for the growth of GaN layers, namely plasma‐assisted electroepitaxy (PAEE). In this method, we have combined the advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface, without spontaneous crystallisation on the surface or within the solution. We have designed and built a new growth chamber which allows us to combine the plasma‐assisted molecular beam epitaxy process with a liquid phase electroepitaxy system. We have demonstrated that it is possible to grow GaN layers by PAEE at growth temperatures as low as ∼650 oC and with low nitrogen overpressures of ∼3×10‐5 Torr. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of GaN Layer on Patterned Al/Ti Metal Mask by Metal–Organic Chemical Vapor Deposition

Growth of GaN Layer on Patterned Al/Ti Metal Mask by Metal–Organic Chemical Vapor Deposition

Influence of nitrogen precursor and its flow rate on the quality and the residual doping in GaN grown by molecular beam epitaxy

AbstractThis work is dedicated to the growth of GaN layers by molecular beam epitaxy. Plasma assisted and ammonia growth techniques are compared. The influence of the nitrogen source (NH3 or N2 plasma) as well as its flow rate is investigated. In particular, we show how the amount of donors incorporated in the layers depends on the growth conditions. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of GaN Layer on Patterned Al/Ti Metal Mask by Metal–Organic Chemical Vapor Deposition

We report on high-quality GaN epitaxial growth by metal–organic chemical vapor deposition (MOCVD) on a stripe-patterned GaN template using a metal mask. A multiple Al/Ti metal system with 10 µm periodicity was used as a masking layer for the epitaxial lateral overgrowth (ELOG). The overgrowth of GaN on the patterned metal mask begins at the open window region between the stripes, and then ELOG leads to the formation of a continuous layer. Micro-cathodoluminescence (µ-CL) results show the improvement of optical properties and significant strain relaxation in the overgrown GaN layer. About a 2-order reduction of threading dislocation density was observed on the overgrown GaN on metal mask regions compared with that on GaN template regions.

Pulsed metal organic chemical vapor deposition of nearly latticed-matched InAlN/GaN/InAlN/GaN double-channel high electron mobility transistors

High quality, nearly lattice-matched InAlN/GaN/InAlN/GaN double-channel heterostructures were grown on sapphire by pulsed-metal-organic-chemical-vapor-deposition (PMOCVD). High electron mobility of 1414 cm2/Vs was achieved along with a two-dimensional-electron-gas density of 2.55 × 1013 cm−2. We attribute it to the high quality PMOCVD-grown InAlN barriers and, additionally, to the novel GaN layer growth between two InAlN barriers, which consists of a thin GaN spacer to prevent indium-redistribution and indium-cluster formation during the subsequent growth and a relatively thick GaN channel to enhance electron mobility. High-electron-mobility-transistors fabricated on these heterostructures with 0.8-μm-length gate exhibit a maximum drain current of 906 mA/mm and a transconductance of 186 mS/mm.

Growth of Less Bowed GaN Epitaxial Layers on Sapphire Substrates by Formation of Low-Temperature GaN Buffer Layer with Columnar Microstructure

Low-temperature (LT) GaN layers with a columnar microstructure were grown on InN layers with various thicknesses. Subsequently, thick, high-temperature GaN layers were grown on the columnar LT GaN/InN/sapphire template. The microstructure of the InN interlayer and the LT GaN columnar structure were investigated by transmission electron microscopy. From the X-ray diffraction analysis, it was shown that the InN layer was decomposed at the initial stage of LT GaN growth. The depth profile of Ga and In atoms from the GaN/InN/sapphire structure obtained by secondary ion mass spectrometry showed that the distribution of In atoms was confined only within the columnar LT GaN and InN/GaN layer interface. Wafer bowing was measured by optical methods and a significant reduction in the extent of wafer bowing was observed for the samples with the columnar LT GaN layer compared with the GaN layer grown on conventional LT GaN buffer layers. It is suggested that the epitaxial growth of a columnar LT GaN layer induced by a polycrystalline InN interlayer helped relax the GaN layer strain energy associated with thermal expansion mismatch, resulting in a less bowed GaN epitaxial layer on the sapphire substrate.