Gallium Nitride Epilayers Research Articles

Effect of repetition nanoindentation of GaN epilayers on a‐axis sapphire substrates

AbstractIn this work, gallium nitride (GaN) epilayers were deposited on a‐axis sapphire substrate by means of metal organic chemical vapor deposition (MOCVD). Berkovich nanoindentation was used to explore the repetition pressure‐induced impairment of the GaN film. The observation of load‐displacement vs stress‐strain curves concludes that basal slip is implicated in the deformation on the A plane GaN. The increase in the hardness (H) and elastic modulus (E) was determined from cyclic nanoindentation, and resulted in a crack due to the formation of incipient slip bands and/or the to‐and‐fro motion of mobile dislocation. It is indicated that the generation of individual dislocation and residual deformation of the GaN films are showed by CL mapping analysis. From the morphological studies, it is revealed that the crack was found by means of atomic force microscope (AFM) technique at nine loading/reloading cycles even after the indentation beyond the critical depth on the residual indentation impression. Copyright © 2010 John Wiley & Sons, Ltd.

Nanotexturing of GaN light-emitting diode material through mask-less dry etching

We describe a new technique for random surface texturing of a gallium nitride (GaN)light-emitting diode wafer through a mask-less dry etch process. This involves depositing asub-monolayer film of silica nanospheres (typical diameter of 200 nm) and then subjectingthe coated wafer to a dry etch process with enhanced physical bombardment. The silicaspheres acting as nanotargets get sputtered and silica fragments are randomly deposited onthe GaN epi-layer. Subsequently, the reactive component of the dry etch plasma etchesthrough the exposed GaN surface. Silica fragments act as nanoparticles, locally maskingthe underlying GaN. The etch rate is much reduced at these sites and consequently arough topography develops. Scanning electron microscopy (SEM) and atomicforce microscopy (AFM) inspections show that random topographic features atthe scale of a few tens of nanometres are formed. Optical measurements usingangle-resolved photoluminescence show that GaN light-emitting diode material thusroughened has the capability to extract more light from within the epilayers.

Structural, optical, and electrical characteristics of 70 Mev Si5+ ion irradiation-induced nanoclusters of gallium nitride

We report the formation of nanoclusters on the surface of gallium nitride (GaN) epilayers due to irradiation with 70 MeV Si ions with the fluences of 1 × 1012 ions/cm2 at the liquid nitrogen temperature (77 K). GaN epilayers were grown using a metal organic chemical vapor deposition system. Omega scan rocking curves of (002) and (101) plane reflection shows irradiation-induced broadening. Atomic force microscopy imagery revealed the formation of nanoclusters on the surface of the irradiated samples. X-ray photoelectron spectroscopy confirms that the surface features are composed of GaN. The effects of ion-beam-produced lattice defects on the surface, electrical, and optical properties of GaN were studied and possible mechanisms responsible for the formation of nanoclusters during irradiation have been discussed.

Nanoscratch Characterization of GaN Epilayers on c- and a-Axis Sapphire Substrates

In this study, we used metal organic chemical vapor deposition to form gallium nitride (GaN) epilayers on c- and a-axis sapphire substrates and then used the nanoscratch technique and atomic force microscopy (AFM) to determine the nanotribological behavior and deformation characteristics of the GaN epilayers, respectively. The AFM morphological studies revealed that pile-up phenomena occurred on both sides of the scratches formed on the GaN epilayers. It is suggested that cracking dominates in the case of GaN epilayers while ploughing during the process of scratching; the appearances of the scratched surfaces were significantly different for the GaN epilayers on the c- and a-axis sapphire substrates. In addition, compared to the c-axis substrate, we obtained higher values of the coefficient of friction (μ) and deeper penetration of the scratches on the GaN a-axis sapphire sample when we set the ramped force at 4,000 μN. This discrepancy suggests that GaN epilayers grown on c-axis sapphire have higher shear resistances than those formed on a-axis sapphire. The occurrence of pile-up events indicates that the generation and motion of individual dislocation, which we measured under the sites of critical brittle transitions of the scratch track, resulted in ductile and/or brittle properties as a result of the deformed and strain-hardened lattice structure.

Nanoindentation characterization of GaN epilayers on A-plane sapphire substrates

Gallium nitride (GaN) epilayers was deposited on a-axis sapphire substrate by means of metal-organic chemical vapor deposition (MOCVD) method. The GaN epilayers has been investigated in their repetition pressure-induced impairment events from nanoindentation technique and, the relative deformation effect was observed from atomic force microscopy (AFM). From the morphological studies, it is revealed that none of crack and particle was found even after the indentation beyond the critical depth on the residual indentation impression. The ‘pop-in’ event was explained by the interaction of the deformed region, produced by the indenter tip, with the inner threading dislocations in the GaN films. Pop-in events indicate the generation and motion of individual dislocation, which is measured under critical depth and, no residual deformation of the GaN films is observed.

Investigations on the effect of 100 MeV Ni ions irradiated chloride vapour phase epitaxy (Cl-VPE) grown GaN epilayers

Gallium nitride (GaN) epilayers have been grown by chloride vapour phase epitaxy (Cl-VPE) technique and the grown GaN layers were irradiated with 100 MeV Ni ions at the fluences of 5 × 10 12 and 2 × 10 13 ions/cm 2. The pristine and 100 MeV Ni ions irradiated GaN samples were characterized using X-ray diffraction (XRD), UV-visible transmittance spectrum, photoluminescence (PL) and atomic force microscopy (AFM) analysis. XRD results indicate the presence of gallium oxide phases after Ni ion irradiation, increase in the FWHM and decrease in the intensity of the GaN (0 0 0 2) peak with increasing ion fluences. The UV-visible transmittance spectrum and PL measurements show decrease in the band gap value after irradiation. AFM images show the nanocluster formation upon irradiation and the roughness value of GaN increases with increasing ion fluences.

Growth behavior of nonpolar GaN on the nearly lattice-matched (1 0 0) γ-LiAlO 2 substrate by chemical vapor deposition

Nonpolar gallium nitride (GaN) epitaxial film was grown on a nearly lattice-matched (1 0 0) γ-LiAlO 2 substrate by a versatile chemical vapor deposition (CVD) method. LiAlO 2 single crystal was grown by the Czochralski (Cz) method. Epi-ready LiAlO 2 single-crystal substrate with RMS roughness of 0.24–0.32 nm were used for all experiments. The dependence of growth characteristics on the growth temperatures and deposition time was investigated. The orientation of GaN film was identified as (1 0 1¯ 0) m-plane by X-ray diffraction pattern. The characterization of detailed structure of the nonpolar GaN epilayer was done by transmission electron microscopy (TEM). Optical properties examined by photoluminescence spectra exhibit a strong near-band-edge emission peak at 3.37 eV and a weak yellow band emission.

Light Extraction Enhancement of Gallium Nitride Epilayers with Stripe Pattern Transferred from Patterned Sapphire Substrate

In this study, the epilayers of gallium nitride light-emitting diode grown by metallorganic chemical vapor deposition on the stripe-patterned sapphire substrate prepared by chemical wet etching were characterized. The stripe pattern on vertical surface transferred by wafer bonding and laser lift-off techniques from the patterned sapphire substrate contributes much stronger photoluminescence intensity.

Influence of NH3 gas for GaN epilayer on β-Ga2O3 substrate by nitridation

Gallium nitride (GaN) epilayers were successfully grown on beta gallium oxide (β-Ga2O3) single crystal in an NH3 atmosphere at 950 °C for 5 h. The thickness of the epilayers was measured using environmental scanning electron microscopy (ESEM). The structure quality of the GaN epilayers was confirmed by high resolution X-ray diffraction (HR-XRD) and Raman spectroscopy. In addition, the surface morphology of the epilayers was investigated by atomic force microscopy (AFM). The results obtained for the epilayers indicated the potential use of GaN crystal as a buffer layer for optoelectronic devices.

Chloride vapor-phase epitaxy of gallium nitride on silicon: Effect of a silicon carbide interlayer

A new approach is described, according to which the use of a thin silicon carbide (SiC) interlayer ensures the suppression of cracking and the simultaneous release of elastic strain in gallium nitride (GaN) epilayers grown by hydride-chloride vapor-phase epitaxy (HVPE) on 1.5-inch Si(111) substrates. Using this method, 20-μm-thick GaN epilayers have been grown by HVPE on Si substrates with AlN (300 nm) and SiC (100 nm) interlayers. A high quality of the obtained GaN epilayers is confirmed by the photoluminescence spectra, where an exciton band with hvmax = 3.45 eV and a half-width (FWHM) of 68 meV is observed at 77 K, as well as by the X-ray rocking curves exhibiting GaN(0002) reflections with a half-width of ωϑ = 600 arc sec.

Investigations on 40 MeV Li3+ ions irradiated GaN epilayers

The Metal Organic Chemical Vapour Deposition (MOCVD) grown n-type Gallium nitride (GaN) layers on sapphire (0001) substrates have been irradiated at low and room temperatures with 40MeV Li3+ ions at the fluence of 1×1013ionscm−2. Irradiated samples were characterised by using X-ray diffraction (XRD), photoluminescence (PL), Raman spectroscopy and atomic force microscopy (AFM). XRD results show that the formation of Ga2O3 has been observed upon irradiation. This is due to interface mixing of GaN/Al2O3, at both temperatures. Also the GaN (0002) peak splits into two at low temperature irradiation. PL measurements show a yellow emission band shift towards blue band side upon irradiation at 77K. Raman studies indicate that the lattice disorder is high at room temperature irradiation compared to low temperature irradiation. AFM images indicate the increasing surface roughness after ion irradiation at room temperature when compared to pristine GaN and low temperature irradiated GaN. These observations are discussed in detail with the use of complementary techniques.

Mechanism Underlying Damage Induced in Gallium Nitride Epilayer during Laser Lift-Off Process

A vertical-type gallium nitride blue light-emitting diode on silicon was prepared by using a laser lift-off process. A grid like surface was obtained after the laser lift-off process. The central part of the grid preserves a uniform surface morphology, but damage occurred and a crack was formed on the peripheral area of a grid. Though a comparison of the crystalline and optical properties of the central and peripheral areas, the high thermal gradient of the peripheral area under pulse laser shot irradiation was found to play an important role during the laser lift-off process.

Growth and Evaluation of GaN Grown on Patterned Sapphire Substrates

We report the microstructure and optical properties of gallium nitride (GaN) epilayers grown on lens shape patterned sapphire substrate (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth times. A lens shaped pattern was used to reduce the threading dislocation density and to improve optical emission efficiency. A scanning electron microscope (SEM) image shows flat and smooth surface of GaN grown on PSS at 80 min which could be achieved by lateral growth from the trench region. From the DCXRD spectra, full width at half maximun (FWHM) value was decreased with increasing growth time. FWHM of the sample grown at 80 min was 473.5 arc sec. This indicates there is an improvement in crystalline quality of the GaN grown on PSS as the growth time increases. From photoluminescence (PL) spectra, an increase in band edge emission intensity and a decrease in defect related yellow luminescence was observed for GaN on PSS as the growth time increased. From the PL spectra, FWHM was 82.2 meV at peak position 363.9 nm for the sample grown for 80 min. It is clearly seen that the threading dislocations can be reduced by lateral growth improving the light emission efficiency by internal light reflection on the lens surface for GaN grown on PSS.

Raman study of Epitaxial lateral Overgrowth of GaN on Patterned Sapphire Substrate

Structural and optical properties of gallium nitride (GaN) epilayers grown on lens shape patterned sapphire substrate (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth times were evaluated. From Raman spectra, a blue shift and reduction in the FWHM of Raman modes of GaN grown on PSS were observed when compared to GaN grown on unpatterend sapphire substrate (UPSS). From the DCXRD spectra, full width at half maximum (FWHM) value was decreased with increasing growth time. FWHM of the sample grown at 80 min was 473.5 arc sec. This indicates that there is an improvement in crystalline quality of the GaN grown on PSS as the growth time increases. From photoluminescence (PL) spectra, an increase in band edge emission intensity and a decrease in defect related yellow luminescence were observed for GaN on PSS as the growth time increased.

Characteristic Comparison of GaN Grown on Patterned Sapphire Substrates Following Growth Time

Gallium nitride (GaN) epilayers were grown on a patterned sapphire substrate (0001) (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth time from 10 to 80 min. Structural characteristics, surface morphology and optical properties of the GaN epilayer were investigated using double crystal X-ray diffraction (DCXRD), scanning electron microscopy (SEM) and photoluminescence (PL). A lens shaped pattern was formed on the sapphire substrate to reduce threading dislocation density and also to improve the optical emission efficiency by internal reflection on the lens. From the DCXRD rocking spectrum, full width at half maximum (FWHM) was decreased with increasing growth time. The FWHM of the sample grown at 80 min was 473.48 arcsec which indicates good crystalline quality. From the SEM images, full coalescence takes place at 80 min growth time. It was observed that the threading dislocation densities were reduced as the growth time increased thereby obtaining a smooth and defect free GaN epilayer at high growth time. From the PL spectra, a decrease in the FWHM and an increase in the peak position were observed as the growth time was increased. Similarly, the yellow luminescence was decreased as the growth time increased and disappeared at growth time 80 min. From the band edge emission spectra, FWHM was 82.2 meV at peak position 363.9 nm for the sample grown at 80 min which indicates a low threading dislocation density. Our results show that by using PSS for growing GaN epilayer with high growth time, threading dislocation density could be reduced thereby improving the crystalline and optical quality of the GaN layers.

Investigations on the influence of 100 MeV O7+ ion irradiation on the structural, surface morphology and optical studies of gallium nitride epilayers

Swift heavy ion irradiations were carried out on metal organic chemical vapour deposition- grown GaN epilayers on sapphire (0 0 0 1) substrates at room temperature, with 100 MeV O7+ ions and fluences varying from 1×1012, 1×1013 and 5×1013 ions cm−2. The pristine and irradiated GaN samples were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and UV–visible optical transmittance studies. XRD results indicate the presence of gallium oxide phases after O7+ ion irradiation and the increase in the Ga2O3 peak intensity and full-width half-maximum of irradiated GaN with increasing ion fluences. Surface morphology was observed from AFM analysis, which indicates a decrease in the root-mean square of roughness from 3.9 to 0.5 nm. PL measurements show a red shift as compared to the as-grown GaN. The UV–visible optical transmittance studies show a decrease in band gap after ion irradiation.

Low dislocation density high-quality thick hydride vapour phase epitaxy (HVPE) GaN layers

Thick high quality gallium nitride (GaN) layers presenting a dislocation density reduced to 6×10 6 cm −2 were grown by hydride vapour phase epitaxy (HVPE). Scanning electron microscopy (SEM) characterizations, X-ray double diffraction (XRD) measurements, photoluminescence and reflectivity experiments, both at 4.5 K have been carried out to reveal the optical and the structural properties of the GaN epilayers. The strain relaxation was studied as a function of increasing the thickness. Special emphasis has been placed to the control of the parasitic nucleation on the reactor wall upstream the substrate, which consistently appears for long-time growth experiments. The amount of the parasitic deposit was quantitatively assessed by theoretical modelling. Attention was paid to accurate characterizations of 450 μm thick GaN layers obtained via thickening through a regrowth step after growth interruption and strain relaxation on cooling the templates.

Aluminum Nitride Micro-Channels Grown via Metal Organic Vapor Phase Epitaxy for MEMs Applications

AbstractAluminum nitride (AlN) is a promising material for a number of applications due to its temperature and chemical stability. Furthermore, AlN maintains its piezoelectric properties at higher temperatures than more commonly used materials, such as Lead Zirconate Titanate (PZT) [1, 2], making AlN attractive for high temperature micro and nano-electromechanical (MEMs and NEMs) applications including, but not limited to, high temperature sensors and actuators, micro- channels for fuel cell applications, and micromechanical resonators.This work presents a novel AlN micro-channel fabrication technique using Metal Organic Vapor Phase Epitaxy (MOVPE). AlN easily nucleates on dielectric surfaces due to the large sticking coefficient and short diffusion length of the aluminum species resulting in a high quality polycrystalline growth on typical mask materials, such as silicon dioxide and silicon nitride [3,4]. The fabrication process introduced involves partially masking a substrate with a silicon dioxide striped pattern and then growing AlN via MOVPE simultaneously on the dielectric mask and exposed substrate. A buffered oxide etch is then used to remove the underlying silicon dioxide and leave a free standing AlN micro-channel. The width of the channel has been varied from 5 ìm to 110 ìm and the height of the air gap from 130 nm to 800 nm indicating the stability of the structure. Furthermore, this versatile process has been performed on (111) silicon, c-plane sapphire, and gallium nitride epilayers on sapphire substrates. Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM), and Raman measurements have been taken on channels grown on each substrate and indicate that the substrate is influencing the growth of the AlN micro-channels on the SiO2 sacrificial layer.

Lateral migration of dislocations in oxygen-doped GaN grown by molecular beam epitaxy

Threading dislocations in gallium nitride epilayers intermittently doped with oxygen were examined using transmission electron microscopy. Dislocations of all types were observed to incline away from the [0001] growth direction at the first introduction of oxygen, remaining inclined through subsequently doped layers. The type and the sign of the Burgers vector of inclined dislocations were characterized by large angle convergent beam electron diffraction, which revealed that the direction of inclination was not primarily driven by misfit stresses. In contrast, dislocations were observed to incline towards pronounced surface pits. It is concluded that the inclination is driven by surface roughening induced by oxygen doping, in contrast to previously published results.

Research on fabrication and properties of nanoporous GaN epilayers

Gallium nitride (GaN) epilayers with nanopore arrays were fabricated by inductive coupled plasma (ICP) etching using anodic aluminum oxide (AAO) as mask. Nanoporous AAO templates were formed by anodizing the Al films deposited on GaN epilayers. The diameter of the perforations in the AAO masks could be easily controlled by tuning the technique parameters of AAO fabrication process. Cl 2/Ar and Cl 2/He were employed as etching gas. Scanning electron microscopy (SEM) analysis shows that vertical nanoporous arrays with uniform distribution can directly be transferred from AAO masks to GaN films in some proper conditions. Photoluminescence (PL) spectra, X-ray diffraction (XRD) and Raman spectroscopy were applied to assess properties of the nanoporous GaN films with different average pore diameters and interpore distances.