Optical Properties Of GaN Layers Research Articles

Growth and characterization of cubic GaN grown on GaAs (110) substrate by MOVPE

GaN layers were grown by Metal Organic Vapor Phase Epitaxy (MOVPE) on GaAs (110) substrate at temperature varying in the range of 750–900 °C. 50 nm-thick GaN layer grown at low temperature (550 °C) was used as buffer layer. Scanning electron microscopy (SEM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD) and room temperature cathodoluminescence (RT-CL) studies were carried out and showed a large dependence of the crystalline and optical properties of GaN layer with the growth temperature. A mixture of cubic and hexagonal GaN phases is evidenced by the 2θ/ω spectra which are dominated by a broad peak associated to c-GaN (220) and h-GaN (11.0) reflections. The cubic GaN phase is maximal for a growth temperature of 850 °C. At this temperature, morphological observations by AFM showed the presence of c-GaN islands and RT-CL spectra showed only c-GaN (3.23 eV) emission whereas both emissions of c-GaN (3.23 eV) and h-GaN (3.39 eV) were observed at 900 °C.

Optical Properties of GaN/SiC/por-Si/Si(111) Hybrid Heterostructures

Using a complex of structural and spectroscopic methods of diagnostics, the influence of a nanoporous-silicon (por-Si) transition layer on the optical properties of GaN layers grown on SiC/por-Si/c-Si templates by molecular-beam epitaxy with the plasma activation (MBE PA) of nitrogen is studied. It is shown for the first time that the MBE PA technology of the synthesis of GaN on a virtual SiC/por-Si/c-Si substrate provides a means for producing a GaN film of much higher structural and optical quality at a much lower growth temperature compared to those in similar studies, in which growth on porous Si substrates is demonstrated. The use of a por-Si layer makes it possible to improve the structural and morphological properties of the GaN epitaxial layer and to attain unique optical and electrical characteristics of the layer. The data obtained in the study will serve as an important basis for understanding the foundations of the physics of GaN/SiC/por-Si nanoheterostructures and for promoting their potential use in optoelectronics.

Оптические свойства гибридных гетероструктур GaN/SiC/por-Si/Si(111)

Abstract Using a complex of structural and spectroscopic methods of diagnostics, the influence of a nanoporous-silicon ( por -Si) transition layer on the optical properties of GaN layers grown on SiC/ por -Si/ c -Si templates by molecular-beam epitaxy with the plasma activation (MBE PA) of nitrogen is studied. It is shown for the first time that the MBE PA technology of the synthesis of GaN on a virtual SiC/ por -Si/ c -Si substrate provides a means for producing a GaN film of much higher structural and optical quality at a much lower growth temperature compared to those in similar studies, in which growth on porous Si substrates is demonstrated. The use of a por -Si layer makes it possible to improve the structural and morphological properties of the GaN epitaxial layer and to attain unique optical and electrical characteristics of the layer. The data obtained in the study will serve as an important basis for understanding the foundations of the physics of GaN/SiC/ por -Si nanoheterostructures and for promoting their potential use in optoelectronics.

Influence of growth temperature on structural and optical properties of laser MBE grown epitaxial thin GaN films on a-plane sapphire

Epitaxial thin GaN films (∼60 nm) have been grown on a-plane sapphire substrates at different growth temperatures (500–700 °C) using laser molecular beam epitaxy (LMBE). The effect of growth temperatures on the structural and optical properties of GaN layers grown on low temperature (LT) GaN buffer on prenitridated a-sapphire have been studied systematically. The in situ reflection high energy electron diffraction pattern revealed the three-dimensional epitaxial growth of GaN films on a-sapphire under the adopted growth conditions. The full width at half maximum (FWHM) value of x-ray rocking curves (XRCs) along GaN (0002) and (10-12) planes decreases with increasing growth temperature. The FWHM values of (0002) and (10-12) XRC for the 700 °C grown GaN film are 1.09° and 1.08°, respectively. Atomic force microscopy characterization showed that the grain size of GaN increases from 30–60 to 70–125 nm with the increase in growth temperature as GaN coalescence time is shorter at high temperature. The refractive index value for the dense GaN film grown at 600 °C is obtained to be ∼2.19 at the wavelength of 632 nm as deduced by spectroscopic ellipsometry. Photoluminescence spectroscopy confirmed that the epitaxial GaN layers grown on a-sapphire at 600–700 °C possess near band edge emission at ∼3.39 eV, close to bulk GaN. The GaN growth at 700 °C without a buffer still produced films with better crystalline and optical properties, but their surface morphology and coverage were inferior to those of the films grown with LT buffer. The results show that the growth temperature strongly influences the structural and optical quality of LMBE grown epitaxial GaN thin films on a-plane sapphire, and a growth temperature of >600 °C is necessary to achieve good quality GaN films.

MBE growth and optical properties of GaN layers on SiC/Si(111) hybrid substrate

The fundamental possibility of the growth of GaN layers by molecular-beam epitaxy on a silicon substrate with nanoscale buffer layer of silicon carbide without any AlN layers has been demonstrated for the first time. Morphological properties of the resulting system have been studied.

Influence of laser repetition rate on the structural and optical properties of GaN layers grown on sapphire (0001) by laser molecular beam epitaxy

High-quality GaN layers were grown on sapphire (0001) substrates using laser molecular beam epitaxy (LMBE) by laser ablating a solid GaN target at different laser repetition rates (10–40 Hz) under a constant supply of r.f. nitrogen plasma. The effect of laser repetition rate on the structural and optical properties of GaN layers was systematically studied using high-resolution X-ray diffraction (HRXRD), field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy. High-resolution X-ray rocking curve measurements revealed highly c-axis oriented GaN layers on sapphire grown at 30 Hz with a calculated screw dislocation density of ~1.42 × 107 cm−2, whereas the GaN layers grown at 10 or 40 Hz consisted the screw dislocation density in the range of 108–109 cm−2. Surface morphological analysis revealed a change in grain size as well as surface roughness as a function of laser repetition rate and is explained on the basis of growth kinetics. Vibrational Raman spectroscopy revealed that the GaN layer grown at 10 Hz shows an in-plane compressive stress of ~1 GPa, while the film grown at 30 Hz exhibits a minimum stress of ~0.3 GPa. The PL measurements show a highly luminescent band-to-band emission of GaN at 3.44 eV for the 10 Hz grown highly strained GaN layer and at 3.41 eV for the less strained film grown at 30 Hz along with a broad defect band emission centered around 2.28 eV. It is found that the GaN layers grown at 30 Hz have excellent structural and optical properties. We expect that the less strained thin and highly oriented GaN film grown by LMBE can further be utilized for developing prodigious low-temperature-grown nitride-based multilayer structures and devices.

Study of the partial decomposition of GaN layers grown by MOVPE with different coalescence degree

We investigated the partial decomposition of GaN layers grown with different coalescence degrees by atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) on SiN treated sapphire substrate. The decomposition was performed in AP-MOVPE reactor under nitrogen (N2) flow at 1200°C. The growth and decomposition processes were in-situ monitored by laser reflectometry (LR) at normal incidence. Surface morphology, crystalline and optical properties of GaN layers were examined before and after partial decomposition by scanning electron microscope (SEM) and high resolution X-ray diffraction (HRXRD). Low decomposition rate and low surface degradation were obtained for thick and most coalesced GaN layers. The partial decomposition did not significantly affect the optical and crystalline properties of GaN. In particular, HRXRD showed almost the same full width at halfmaximum (FWHM) of (00.2) and (10.2) rocking curves (RCs) before and after partial decomposition of coalesced GaN layer.

Cross-sectional and plan-view cathodoluminescence of GaN partially coalesced above a nanocolumn array

The optical properties of GaN layers coalesced above an array of nanocolumns have important consequences for advanced optoelectronic devices. GaN nanocolumns coalesced using a nanoscale epitaxial overgrowth technique have been investigated by high resolution cathodoluminescence (CL) hyperspectral imaging. Plan-view microscopy reveals partially coalesced GaN layers with a sub-μm scale domain structure and distinct grain boundaries, which is mapped using CL spectroscopy showing high strain at the grain boundaries. Cross-sectional areas spanning the partially coalesced GaN and underlying nanocolumns are mapped using CL, revealing that the GaN bandedge peak shifts by about 25 meV across the partially coalesced layer of ∼2 μm thick. The GaN above the nanocolumns remains under tensile strain, probably due to Si out-diffusion from the mask or substrate. The cross-sectional data show how this strain is reduced towards the surface of the partially coalesced layer, possibly due to misalignment between adjacent partially coalesced regions.

Luminescent properties of GaN films grown on porous silicon substrate

GaN films have been grown on porous silicon at high temperatures (800–1050 °C) by metal organic vapor phase epitaxy. The optical properties of GaN layers were investigated by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. PL spectra recorded at 5 K exhibit excitonic emissions around 3.36–3.501 eV and a broad yellow luminescence at 2.2 eV. CL analysis at different electron excitation conditions shows spatial non-uniformity in-depth of the yellow and the band-edge emissions. These bands of luminescence are broadened and red- or blue-shifted as the electron beam penetrates in the sample. These behaviors are explained by a change of the fundamental band gap due to residual strain and the local thermal effect. It was found that the use of AlN buffer layer improves the crystalline quality and the luminescence property of GaN.

Comparative analysis of morphology and optical properties of GaN layers on sapphire

A comprehensive analysis of gallium nitride layers grown by the metal-organic vapor-phase epitaxy technique on sapphire substrates is performed. The spectrum of photoluminescence of these layers corresponds to the width of the energy gap of gallium nitride. Comparative analysis of the layer’s surface with the interferometry, scanning electron microscopy, and atomic-force microscopy has revealed characteristic inhomogeneities on it. The deposition of a thin metal film on the surface has allowed one to identify them as the inhomogeneities of the work function and not of the surface relief.

Buffer optimization for crack-free GaN epitaxial layers grown on Si(1 1 1) substrate by MOCVD

We report the growth of GaN films on the Si(1 1 1) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(1 1 1) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(1 1 1) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6 µm. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. The resulting GaN film surfaces were flat, mirror-like and crack-free. The mosaic structure in the GaN layers was investigated. With a combination of Williamson–Hall measurements and the fitting of twist angles, it was found that the buffer thickness determines the lateral coherence length, vertical coherence length, as well as the tilt and twist of the mosaic blocks in GaN films. The PL spectra at 8 K show that a strong band edge photoluminescence of GaN on Si (1 1 1) emits light at an energy of 3.449 eV with a full width at half maximum (FWHM) of approximately 16 meV. At room temperature, the peak position and FWHM of this emission become 3.390 eV and 58 meV, respectively. The origin of this peak was attributed to the neutral donor bound exciton. It was found that the optimized total thickness of the AlN and graded AlGaN layers played a very important role in the improvement of quality and in turn reduced the cracks during the growth of GaN/Si(1 1 1) epitaxial layers.

Investigation of high mobility two-dimensional electron gas in AlGaN/GaN heterostructures grown by hydride vapor phase epitaxy (HVPE)

Abstract:Two-inch-diameter sapphire (0001) substrates were used for the growth of AlGaN/GaNstructure by hydride vapor phase epitaxy (HVPE). The two-dimensional electron gas(2DEG) structure consisted of a 2 mm undoped GaN layer substrate, followed by 30 nmthick Al0.3Ga 0.7N layer. Structural and optical properties of GaN layer werecharacterized by high resolution X-ray diffraction, transmission electron microscopy(TEM) and photoluminescence (PL) measurements. TEM images showed thedislocation density in the GaN layer was ~ 9 ´ 109 /cm2 . The full width at halfmaximum (FWHM) values of the x-ray rocking curve for GaN (0002) was 0.156 deg.PL spectra of our sample exhibited a predominant band-edge emission of the wurziteGaN epilayer near 3.36 eV with FWHM = 45 meV at 50 K and 75 meV at roomtemperature, respectively. The Hall effect measurements gave a sheet electronconcentration of 7.8´1012 cm-2 and a mobility of 850 cm2/Vs at 300K. The experimentaldata indicated that the HVPE grown GaN layers exhibited superior quality compared toGaN layers grown by metal organic chemical vapor deposition in prior art literatures.

Dislocation Reduction of GaN Layer with a SiN Mask by Using Metalorganic Chemical Vapor Deposition

GaN layers with an in-situ SiN mask were grown by using metalorganic chemical vapor deposition (MOCVD) and the physical properties of the GaN layer were studied. Also, the influence of a SiN mask on the optical property of GaN layer was investigated. By inserting a SiN mask, the (102) full width at half maximum (FWHM) decreased from 480 arcsec to 409 arcsec, and the threading dislocation (TD) density decreased from 3.21 × 10 cm−2 to 9.7 × 10 cm−2. The photoluminescence (PL) peak intensity of sample with a SiN mask increased by 2 times compared to that of sample without a SiN mask. We have shown that a SiN mask significantly improved the physical and the optical properties of the GaN layer.

Morphological, structural and optical properties of GaN grown on porous silicon/Si(100) substrate

In the present paper, we report results of GaN layers grown at 800 °C by metal organic vapour phase epitaxy (MOVPE) on porous silicon (PS) formed on Si(100) substrates. The surface morphology and the crystallinity of the GaN films were characterized by scanning electron microscope and X-ray diffraction. It was shown that GaN grows on PS preferentially on hexagonal polycrystalline form. The SEM observation reveals roughly surface textured by disoriented GaN grains having different shapes and sizes. The surface coverage and the wetting of GaN to PS are improved when the thickness of GaN layer increases. The optical properties of GaN layers were examined by PL and CL at low and room temperatures. Besides, the near edge-band (BE) emission, shows yellow (YL) and deep localized excitons bands at approximately 2.2 and 3.3–3.36 eV respectively. The depth CL analysis shows a spatial variation of the dominating YL and BE emissions as the electron beam energy rises from 3 to 25 kV.

Improvement of microstructural and optical properties of GaN layer on sapphire by nanoscale lateral epitaxial overgrowth

Nanoscale lateral epitaxial overgrown (NLEO) GaN layers were investigated for the improvement of microstructural and optical properties of GaN. Nanoporous SiO2 films on the surfaces of GaN/sapphire (0001) were fabricated by inductively coupled plasma etching using anodic alumina templates as etch masks, resulting in an average pore diameter and interpore distance of 60 and 110nm, respectively. GaN was grown over the nanoporous SiO2 layer using metal organic chemical vapor deposition to realize a continuous and smooth film. NLEO GaN was found to result in a significant reduction of threading dislocation density as characterized by transmission electron microscopy and atomic force microscopy. The threading dislocation density was ∼107cm−2 at the surface of NLEO GaN. The narrower band-edge excitonic transition in photoluminescence spectrum shows a better optical quality in the NLEO GaN film.

Crack control in GaN grown on silicon (111) using In doped low-temperature AlGaN interlayer by metalorganic chemical vapor deposition

Abstract The effects of In doped low-temperature (LT) AlGaN interlayer on the properties of GaN/Si(1 1 1) by MOCVD have been investigated. Using In doping LT-interlayer can decrease the stress sufficiently for avoiding crack formation in a thick (2.0 μm) GaN layer. Significant improvement in the crystal and optical properties of GaN layer is also achieved. In doping is observed to reduce the stress in AlGaN interlayer measured by high-resolution X-ray diffraction (HRXRD). It can provide more compressive stress to counteract tensile stress and reduce crack density in subsequent GaN layer. Moreover, as a surfactant, indium is observed to cause an enhanced PL intensity and the narrowed linewidths of PL and XRD spectra for the LT-interlayer. Additionally, the crystal quality of GaN layer is found to be dependent on the growth parameters of underneath In-doped LT-AlGaN interlayer. The optimal parameters, such as TMIn flow rate, TMAl flow rates and thickness, are achieved to obtain nearly 2.0 μm thick crack free GaN film with advanced optical and crystal properties.

Microphotoluminescence mapping of laterally overgrown GaN layers on patterned Si (111) substrates

Spatial distribution of optical properties of GaN layers grown on patterned Si (111) substrates by maskless metalorganic chemical vapor deposition has been investigated. The Si substrates were prepared with a pattern of 1.5μm diameter holes at a 3.5μm distance from each other. The holes were overgrown by GaN until coalescence, creating GaN areas with no substrate underneath. Microphotoluminescence mapping measurements with 0.8μm lateral resolution show a five-fold increase in luminescence intensity coming from the overhang areas as compared to the layer directly over the substrate. This is accompanied by a slight redshift of the luminescence peak wavelength. Photoelectrochemical etching shows that the dislocation density is much lower in those areas while the photoluminescence redshift is attributed to lesser strain relaxation resulting from a lower dislocation density.

Structural and optical characterization of GaN epilayers grown on Si(1 1 1) substrates by hydride vapor-phase epitaxy

The GaN epitaxial layer was grown by hydride vapor-phase epitaxy (HVPE) on Si(1 1 1) substrate with AlN buffer layer. Structural and optical properties of GaN layer were characterized by high-resolution X-ray diffraction, transmission electron microscopy (TEM) and photoluminescence (PL) measurements. TEM images showed that the dislocation density in the GaN layer was ∼9×10 9/cm 2. The full-width at half-maximum (FWHM) values of the X-ray rocking curve for GaN (0 0 0 2) and (1 0 1¯ 2) were 13.7 and 35.3 arcmin, respectively. The PL spectra of our sample exhibited a predominant band-edge emission of the wurzite GaN epilayer near 3.36 eV with FWHM=64.2 meV at 50 K and 76.5 meV at room temperature. The experimental data indicated that the HVPE-grown GaN layers exhibited superior quality compared to GaN layers grown by metal organic chemical vapor deposition in prior literatures.

Phonon-assisted exciton luminescence in GaN layers grown by MBE and chloride-hydride VPE

Optical properties of GaN layers grown by the molecular-beam epitaxy (MBE) and chloride-hydride vapor-phase epitaxy (CHVPE) have been studied, and the quality of two types of samples has been compared. The photoluminescence spectra have an excitonic nature for both types of layers. To determine precisely the nature of exciton transitions, the reflectance spectra were studied. A key point was the investigation of phonon-assisted exciton luminescence, which provides information on the density distribution of the exciton states. Temperature dependences of the exciton transition energy and the ratio of intensities of one- and two-phpnon replicas were studied. The high quality of both types of layers has been confirmed, though the concentration of acceptors in MBE-grown samples is higher than in CHVPE samples. © 2003 MAIK "Nauka/Interperiodica".

Microscopic evidence of point defect incorporation in laterally overgrown GaN

Electron microscopy techniques are applied to investigate structural and optical properties of GaN layers selectively grown by hydride vapor phase epitaxy on crystalline GaN seed layers deposited on (0001)Al2O3 substrates. Regions with different optical properties are observed in the cross- sections of the layers. They are defined by the crystallographic planes that serve as growth facets. We give a simple geometrical explanation of point defect incorporation occurring more easily for the {112̄2} GaN than for the {0001} GaN growth facets. Microscopic evidences supporting the model are higher concentrations of point-like defects and local strain variations in laterally grown regions that are revealed by high-resolution electron microscopy.