Low-temperature GaN Buffer Layer Research Articles

Effect of buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(0 0 1) substrates

In this letter, we investigated the effect of the buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(0 0 1) substrate. The reflection high-energy electron diffraction (RHEED) pattern of the low-temperature GaN buffer layers shows that both the deposition temperature and time are important in obtaining a smooth surface. Four-circle X-ray double-crystal diffraction (XRDCD) reciprocal space mapping was used to study the hexagonal phase inclusions in the cubic GaN (c-GaN) films grown on the buffer layers. The calculation of the volume contents of the hexagonal phase shows that higher temperature and longer time deposition of the buffer layer is not preferable for growing pure c-GaN film. Under optimized condition, 47 meV FWHM of near band gap emission of the c-GaN film was achieved.

Improved Heteroepitaxial MBE GaN Growth with a Ga Metal Buffer Layer

ABSTRACTWe demonstrate that the use of pure gallium (Ga) as a buffer layer results in improved crystal quality of GaN epilayers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. The resulting epilayers show electron Hall mobilities as high as 400 cm2/Vs at a background carrier concentration of 4 × 1017 cm−3, an outstanding value for an MBE-grown GaN layer on sapphire. Structural properties are also improved; the asymmetric (101) X-ray rocking curve width is drastically reduced with respect to that of the reference GaN epilayer grown on a low-temperature GaN buffer layer. Nitrided Ga metal layers were investigated for different Ga deposition time. These layers can be regarded as templates for the subsequent Ga main layer growth. It was found that there is an optimum Ga metal layer deposition time for improving the electron mobility in the epilayer. Heating of the Ga metal layer to the epilayer growth temperature under nitrogen plasma is found to be sufficient to produce highly oriented GaN crystals. However, nonuniform surface morphology and incomplete surface coverage were observed after nitridation of comparatively thick Ga metal layers. This is shown to be the reason for the decreasing electron mobility of the epilayers as the Ga metal layer thickness exceeds the optimum value.

Surface Modification of Cubic Gan Buffer Layer Grown by Metalorganic Vapor Phase Epitaxy

ABSTRACTAnisotropic X-Ray diffraction (XRD) and transport properties of cubic GaN grown on GaAs substrates correspond to the features of low-temperature grown GaN (LT-GaN) buffer layer. When the LT-GaN layer is grown on the surface tilted from (001) to [1-10] with annealing in arsenic ambient, the macroscopic step edges along [1-10] direction are modified by either the ambient of thermal annealing, or substrate misorientation. A parallel conduction in GaN, GaAs, and GaN/GaAs hetero-interface was observed by photoconductivity measurements. Transmission electron microscope (TEM) observation shows that self-annihilations for (-111) B stacking faults are preferentially occurred near GaAs interface when GaN film grown on the surface tilted from (001) toward [1-10] (As step edge) is annealed in arsenic ambient. TEM observation also shows that stacking faults and dislocations are preferentially generated near GaN/GaAs interface. It is suggested that anisotropic transport properties correspond to the well-like potential generated by band bending at GaN/GaAs interface. The nearly isotropic mobility of 3,000 cm2/Vsec at 77K is obtained by improving interface property.

The Application of a Low Temperature GaN Buffer Layer to Thick GaN Film Growth on ZnO/Si Substrate

The effect of a low temperature GaN (LT-GaN) buffer layer on the properties of thick GaN grown on Si substrate by hydride vapor phase epitaxy (HVPE) was investigated to obtain thick GaN films of high quality. ZnO was deposited by sputtering as the first buffer layer. LT-GaN buffer layers were grown for 2 to 7 min at 600 °C. The optimization of the LT-GaN buffer layer was done using low temperature PL, AFM and XRD analysis. A Zn-related impurity peak was observed in PL spectra of LT-GaN/ZnO/Si. A strong orientation along (0002) was observed in thick GaN/LT-GaN/ZnO/Si. A red shift in PL spectra of thick GaN due to the strain was found. The LT-GaN buffer improved the crystalline and optical properties of thick GaN grown on Si substrate.

X-ray diffraction characterization of epitaxial zinc-blende GaN films on a miscut GaAs(0 0 1) substrates using the hydride vapor-phase epitaxy method

In this study, cubic GaN epitaxial films are grown on a 2° miscut GaAs(0 0 1) substrate by hydride vapor-phase epitaxy reactor with a low-temperature GaN buffer layer before the growth of the GaN epitaxy film. X-ray diffraction spectra reveal that the epitaxial film contains most of the zinc-blende GaN with a few percent of wurtzite GaN also embedded in the film. The crystalline coherence length of the zinc-blende GaN is 30 nm and the rocking curve width is 4.8°. In addition, in the φ-scan, are found, two small peaks in two-fold symmetry which did not correspond to the substrate orientation relation of GaN(0 0 1)‖GaAs(0 0 1) and GaN(0 1 0)‖GaAs(0 1 0). These two small peaks were grown along the miscut direction of the substrate. Photoluminescence spectra confirm that a cubic GaN edge emission peak appears at 388 nm as well as a strong yellow emission at 500 nm region.

LP-MOCVD growth of GaN on silicon substrates—comparison between AlAs and ZnO nucleation layers

The impact of AlAs and ZnO nucleation layers on the low-pressure MOCVD growth of GaN on silicon substrates is the subject of this study. In both cases, prior to the high-temperature deposition of the GaN main layer a thin low-temperature GaN buffer layer has been found to improve the crystal quality. Especially in the case of a ZnO nucleation layer, no single crystalline growth of GaN was obtained without a low-temperature GaN buffer layer. Furthermore, the crystallographic quality of the GaN layer is strongly dependent on the strength of the (0001) texture of the ZnO grains. The ZnO (0001)-texture is independent of the Si(111) or Si(001) orientation and allows for single crystalline growth of GaN on both types of Si surfaces. TEM investigations show, that the ZnO nucleation layer has completely vanished after the epitaxial process. In contrast, AlAs nucleation layers remain stable as proven by X-ray diffraction and TEM measurements.

Molecular beam epitaxy (MBE) growth and structural properties of GaN and AlN on 3C-SiC(0 0 1) substrates

AlN and GaN was deposited by molecular beam epitaxy (MBE) on 3C-SiC(0 0 1) substrates on low-temperature (LT) GaN and AlN buffer layers. It is shown that not only GaN but also epitaxial AlN can be stabilized in the metastable zincblende phase. The zincblende AlN is only obtained on a zincblende LT-GaN buffer layer; on the other hand, AlN crystallizes in the wurtzite phase if it is grown directly on a 3C-SiC(0 0 1) substrate or on a LT-AlN buffer layer. The structural properties of the layers and in particular the orientation relationship of the wurtzite AlN on the 3C-SiC(0 0 1) were analyzed by conventional and high-resolution transmission electron microscopy.

Low-pressure metal organic chemical vapor deposition of GaN on silicon(111) substrates using an AlAs nucleation layer

We have investigated the growth of GaN on silicon by low-pressure metal organic chemical vapor deposition. Good quality GaN layers are grown on silicon(111) using an AlAs nucleation layer. AlAs is thermally stable even at 1050 °C and, unlike GaN and AlN buffer layers, the formation of SiNx on the Si surface is prevented. Single crystalline GaN films are obtained by introducing a thin low-temperature GaN buffer layer grown on the AlAs nucleation layer. The GaN layers are characterized by x-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy, photoluminescence, and cathodoluminescence.

Time of flight mass spectroscopy of recoiled ions studies of surface kinetics and growth peculiarities during gas source molecular beam epitaxy of GaN

High growth rate GaN thin films were successfully grown by gas source molecular beam epitaxy and studied in situ by time of flight mass spectroscopy of recoiled ions (TOF-MSRI) and reflection high energy electron diffraction (RHEED). We show that TOF-MSRI allows for in situ monitoring and control of sapphire surface chemistry and its nitridation. In the latter case, TOF-MSRI is more sensitive to the surface changes during nitridation than RHEED. Using both RHEED and TOF-MSRI, growth of low-temperature GaN buffer layers was monitored, and their recrystallization and island-like nature were demonstrated. A model describing the probable growth mechanism for gas source molecular beam epitaxy of GaN is suggested. The model explains both the chemical dissociation of ammonia at low temperature and the origin of Ga to N TOF-MSRI peak ratio changes for various Ga and ammonia fluxes. High-resolution transmission electron microscopy studies confirm that GaN films grown with a buffer layer have excellent structural quality without any evidence of interfacial defects. Those without a buffer layer are highly defective.

Effects of Surface Preparation on Epitaxial GaN on 6H-SiC Deposited Via MOCVD

A comparison was made of 6H-SiC surfaces etched with H2, C2H4/H2, and HCl/H2, and the resulting crystal quality of epitaxial GaN films deposited on these substrates. To remove the many fine scratches and to smooth the rough surfaces typical of commercial SiC substrates, the Si-face 6H-SiC substrates were etched in H2, C2H4/H2, and HCl/H2 at 145 °C. GaN was subsequently deposited on these etched surfaces after first depositing a low temperature GaN buffer layer via metalorganic chemical vapor deposition (MOCVD). The surface morphologies after etching and after GaN deposition were characterized by atomic force microscopy and Normaski differential interference contrast microscopy, while the crystal quality of the GaN films was assessed by double crystal x-ray rocking curves and x-ray topography. 6H-SiC substrate surfaces were improved in terms of the removal of scratches and the reduction of surface roughness, and both surface morphology and crystal quality of the subsequently deposited GaN films were enhanced. However, the dislocation density was not decreased by the surface etching. The best GaN film was produced by etching the substrate in pure H2 for 40 minutes before growth. Recommendations for the optimum substrate treatment are made.

Monitoring and Controlling of Strain During MOCVD of AlGaN for UV Optoelectronics

The grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AlGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AlN buffer layers results in a reduced and possibly controllable strain.

Microstructure and optical properties of epitaxial GaN on ZnO (0001) grown by reactive molecular beam epitaxy

High quality GaN epilayers have been grown on oxygen and zinc surfaces of ZnO (0001) substrates by reactive molecular beam epitaxy and the effect of the intermediate buffer layer on the structural and optical properties of the GaN films has been investigated. The optical and structural characterization of the GaN epilayers and ZnO substrates were performed using photoluminescence, reflectivity, x-ray double diffraction, atomic force microscopy, and transmission electron microscopy. The optical results indicated that GaN was grown with compressive strain due to the difference in thermal expansion coefficient between GaN and ZnO. The surface roughness has been reduced by using an intermediate low temperature GaN buffer layer. The low temperature photoluminescence spectra of GaN/ZnO epilayers did not reveal any sign of the well-known midgap yellow signal. Linear polarized reflectivity and photoluminescence indicated that GaN epilayer planes were not misoriented with respect to the ZnO substrate planes: this result was confirmed by x-ray double diffraction measurements.

Influences of initial nitridation and buffer layer deposition on the morphology of a (0001) GaN layer grown on sapphire substrates

Hexagonal GaN is grown on (0001) sapphire substrates using an atmospheric pressure organometallic vapor phase epitaxy method. We investigate the influences of the initial treatment of sapphire substrate, such as initial nitridation and low-temperature GaN buffer layer deposition, upon the surface morphology and crystallinity. The thermal stability of grown GaN layers is also investigated using a thermal etching process in a H2 atmosphere in order to obtain the information concerning the surface polarity of GaN(0001) layers. When sapphire substrates are initially nitrided, highly crystalline GaN layers with large hexagonal facets are obtained and its surface appears to be (0001)N. On the other hand, the deposition of a thicker buffer layer on the nitrided sapphire substrates improves the surface morphology, and the surface polarity of the mirror surface appears to be (0001)Ga. The initial nitridation of sapphire substrates and the GaN buffer layer deposition are considered to be important processes from viewpoints of the (0001) surface polarity.

Effects of Surface Preparation on Epitaxial GaN on 6H-SIC Deposited Via Mocvd

AbstractA comparison was made of 6H-SiC surfaces etched with H2, C2H4/H2, and HCl/H2, and the resulting crystal quality of epitaxial GaN films deposited on these substrates. To remove the many fine scratches and to smooth the rough surfaces typical of commercial SiC substrates, the Si-face 6H-SiC substrates were etched in H2, C2H4/H2, and HCl/H2 at 1450°C. GaN was subsequently deposited on these etched surfaces after first depositing a low temperature GaN buffer layer via metalorganic chemical vapor deposition (MOCVD). The surface morphologies after etching and after GaN deposition were characterized by atomic force microscopy and Normaski differential interference contrast microscopy, while the crystal quality of the GaN films was assessed by double crystal x-ray rocking curves and x-ray topography. 6H-SiC substrate surfaces were improved in terms of the removal of scratches and the reduction of surface roughness, and both surface morphology and crystal quality of the subsequently deposited GaN films were enhanced. However, the dislocation density was not decreased by the surface etching. The best GaN film was produced by etching the substrate in pure H2 for 40 minutes before growth. Recommendations for the optimum substrate treatment are made.

Monitoring and Controlling of Strain During Mocvd of AlGaN for UV Optoelectronics

AbstractThe grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AIGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AIN buffer layers results in a reduced and possibly controllable strain.

Effects of thermal treatment of low-temperature GaN buffer layers on the quality of subsequent GaN layers

The surface morphology and crystalline quality of GaN layers grown by metalorganic chemical vapor deposition on sapphire (0001) substrates were investigated for various thermal treatment conditions of low-temperature (LT)-grown GaN buffer layers. The surface morphology and crystalline quality of subsequently grown high-temperature (HT) GaN layers strongly depended on thermal effects during the temperature ramping process after LT growth of the buffer layers. We have found that the defect density and structure are affected by this temperature ramping process, and that the generation of growth pits is closely related to defects in the HT-GaN layers. High-quality HT-GaN layers with specular surface morphology were obtained with optimum growth and ramping conditions for the LT-GaN buffer layers. Furthermore, the role of thermal treatment during the temperature ramping process was identified, and mechanisms of nucleus formation, HT-growth initiation on the LT-GaN buffer layers, and defect formation are proposed and discussed.

Changes in the growth mode of low temperature GaN buffer layers with nitrogen plasma nitridation of sapphire substrates

Low temperature GaN buffer layers were grown by remote plasma enhanced metalorganic chemical vapor deposition on various pretreated sapphire substrates. The effects of the initial nitridation of sapphire substrates by rf nitrogen plasma on the subsequent growth mode and the crystallinity of the GaN buffer layers were studied. GaN buffer layers on non-nitridated sapphire substrates showed a three-dimensional island growth mode with rough surfaces and misoriented islands. On the other hand, those grown on nitridated sapphire substrates showed an enhanced two-dimensional growth mode with near-equilibrium truncated hexagonal pyramids. The structural quality of the low temperature GaN buffer layer improved significantly with nitrogen plasma nitridation.

Growth and characterization of GaN thin films on SiC SOI substrates

SiC semiconductor-on-insulator (SOI) structures have been investigated as substrates for the growth of GaN films. The SiC SOI was obtained through the conversion of Si SOI wafers by reaction with propane and H2. (111) SiC SOI have been produced by this carbonization process at temperatures ranging from 1200 to 1300°C. X-ray diffraction (XRD) and infrared spectroscopy (FTIR) are used to chart the conversion of the Si layer to SiC. Under our conditions, growth time of 3 min at 1250°C is sufficient to completely convert a 1000A layer. XRD of the SiC SOI reveals a single SiC peak at 2θ = 35.7° corresponding to the (111) reflection, with a corrected full width at half-maximum (FWHM) of ~590±90 arc-sec. Infrared spectroscopy of SiC SOI structures obtained under optimum carboniza-tion conditions exhibited a sharp absorption peak produced by the Si-C bond at 795 cm−1, with FWHM of ∼ 20–25 cm−1. Metalorganic CVD growth of GaN on the (111) SiC SOI was carried out with trimethylgallium and NH3. The growth of a thin (≤200A), low temperature (500°C) GaN buffer layer was followed by the growth of a thick (∼2 µm) layer at 1050°C. Optimum surface morphology was obtained for zero buffer layer. XRD indicates highly oriented hexagonal GaN, with FWHM of the (0002) peak of ~360±90 arc-sec. Under high power excitation, the 300°K photoluminescence (PL) spectrum of GaN films exhibits a strong near band-edge peak (at λp~371 nm, with FWHM = 100–150 meV) and very weak yellow emission. Under low power excitation, the 370 nm PL emission from the GaN/SiC SOI structure increases rapidly with SiC carbonization temperature, while the yellow band (∼550–620 nm) correspondingly decreases.

The role of mixed cubic/hexagonal nucleation layers on threading dislocation reduction in epitaxial GaN films

The defect density and their configuration in epitaxial GaN films on sapphire are strongly related to the optical and electrical properties of the films. The crystalline quality of GaN on sapphire can be greatly improved by introducing a low temperature GaN buffer layer. However, recent studies in our group demonstrate that the pre-growth substrate treatment can significantly change the defects structure and electron properties of the films. In this paper, we present our results on studies of the film microstructures and their corresponding nucleation layers. We propose a novel mechanism for dislocation reduction for GaN on sapphire.The growths for this study were achieved in a horizontal flow reactor operating at atmospheric pressure. Basal plane AI2O3 (Union Carbide) substrates were first cleaned in solvents and then heated in flowing H2 at 1050°C. Sample A was exposed to an ammonia flow of 3 1/min for 60s, sample B for 400s before the temperature was then reduced to 600°C and a nominal 190 Å thick GaN layer was grown.

Growth of GaN by gas-source molecular beam epitaxy by ammonia and by plasma generated nitrogen radicals

We will present a comparison between GaN grown on c- and r-plane sapphire by gas-source molecular beam epitaxy (GSMBE) using solid source elemental Ga with uncracked ammonia (NH 3). Improved GaN film quality was found for samples grown at substrate temperatures above 700°C with optimized temperatures at 780°C. GaN deposited on a low-temperature (∼ 350°C) GaN buffer layer grown using an rf-plasma radical beam source exhibited poor photoluminescence (PL) intensity and carrier mobility while SEM analysis showed smooth GaN surface morphologies. GaN deposited on an ammonia-grown low-temperature (∼ 550°C) GaN or AlN buffer were found to have improved PL intensity and line widths, GaN surface morphology and carrier mobility. X-ray rocking curve (XRC) data showed that the GaN crystal quality improved as a function of increasing substrate growth temperature independent of buffer layer implementation.