TEM investigations of GaN layers grown on silicon and sintered GaN nano‐ceramic substrates

GaN nano‐ceramics were analyzed using transmission electron microscopy (TEM), showing that these ceramics are characterized by highly disoriented grains of the linear size of 100–150 nm. These GaN ceramics were used as substrates for GaN epitaxy in standard MOVPE conditions. For the comparison, MOVPE GaN layers on silicon substrates were grown using similar conditions. It is shown that MOVPE growth of GaN layers is highly anisotropic for both cases. However, the disorientation of the highly mismatched GaN layer on silicon is different from that characterizing GaN layer deposited on the ceramic substrate. In the latter case the disorientation is much higher, and three dimensional in nature, causing creation of polycrystalline structure having large number of the dislocations. In the case of the GaN layer grown on the silicon substrate the principal disorientation is due to rotation around c‐axis, causing creation of mosaic structure of edge dislocations. Additionally, it is shown that the typical grain size in AlN nucleation layer on Si is smaller, of order of 20 nm. These two factors contribute to pronounced differences in later stage of the growth of GaN layer on the ceramic. Due to high growth anisotropy an appropriately thick GaN layer can, eventually, develop flat surfaces suitable for construction of optoelectronic and electronic structures. As shown by the TEM data, this can be achieved only at the cost of creation of the relatively large density of dislocations and stacking faults. The latter defects were not observed for the GaN growth on Si substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Molecular beam epitaxy of GaN/AlxGa1−xN superlattices for 1.52 – 4.2 μm intersubband transitions

High-quality superlattice structures of GaN/AlGaN were grown on (0 0 0 1) sapphire substrates by molecular beam epitaxy. The threading dislocation density was reduced by growing low-temperature AlN layers in between the high-temperature GaN. In addition, in situ monitoring of the growth rate was achieved using pyrometric interferometry. Cross-sectional transmission electron microscopy of the superlattice structures revealed abrupt interfaces between GaN/AlGaN and excellent layer uniformity. We observed intersubband absorption at wavelengths as short as 1.52 μm in the GaN/AlGaN material system. A range of intersubband absorption peaks was observed between 1.52 and 4.2 μm by varying the well thickness and barrier Al content. In addition, the distribution of the built-in electric field between the well and barrier layers was also found to affect the intersubband transition wavelength.     

AlGaN epitaxial lateral overgrowth on Ti-evaporated GaN/sapphire substrate

AlGaN growth using epitaxial lateral overgrowth (ELO) by metalorganic chemical vapor deposition on striped Ti, evaporated GaN on sapphire, has been investigated. AlGaN/AlN films growth on GaN/AlGaN superlattices (SLs) structure on the Ti masks, with various SLs growth temperature (1030, 1060 and 1090 °C) were grown. With increasing the growth temperature, AlGaN surface became flat. The AlGaN film had a cathodoluminescence peak around 345 nm. However, in secondary ion mass spectrometry (SIMS) measurement, Ti signal was detected on the top of AlGaN surface when GaN/AlGaN SLs was grown on Ti striped masks. By inserting the AlN blocking layer on SLs, Ti diffusion was stopped at the AlN layer, and the AlGaN crystalline quality was improved.     

MOVPE of AlN-free hexagonal GaN/cubic SiC/Si heterostructures for vertical devices

The heterostructures of GaN/SiC/Si were prepared without using AlN or AlGaN buffer layers (AlN buffers) in the metalorganic vapor phase epitaxy of GaN on SiC. GaN (0 0 0 1) with specular surface was obtained. The AlN buffers are usually used in the conventional growth of GaN on SiC due to the poor nucleation of GaN on SiC. Instead, the nucleation of GaN was controlled by varying the partial pressure of H₂ in the carrier gas, the mixture of H₂ and N₂, during the low-temperature (600 °C) growth of GaN (LT-GaN). After the LT-GaN, the high-temperature (1000 °C) growth of GaN was performed using pure H₂ as the carrier gas. The epitaxial film of cubic SiC (1 1 1) on a Si (1 1 1) substrate was used as the SiC template. Increasing the partial pressure of H₂ in the carrier gas decreased the coverage of SiC surface by LT-GaN. It is suggested that the hydrogen atoms adsorbed on the surface of SiC is preventing the nucleation of GaN.     

Theoretical stress calculations in polar,semipolar and nonpolar AlGaN/GaN heterostructures of different compositions

The new comprehensive model of the process for matching epitaxial layers to substrates, in dependence of theirs crystallographic orientation, was developed to allow a theoretical prediction of the strain and stress in thin AlGaN epitaxial layers with different composition grown on GaN template. The elements of the continuous anisotropic materials strength theory was applied to develop the model. It was observed that in AlGaN/GaN heterostructures the stress was greater than the upper limit of acceptable tensile stress even for a small Al content and also that the stress could greatly vary, in a value and a direction, depending on substrate crystallographic orientation and an Al content in AlGaN layer. The obtained results theoretically explained the commonly observed technological problems occurring during the growth of AlGaN layers even with a small Al content.     

Well parameters of two‐dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

Resistivity and Hall effect measurements were carried out as a function of magnetic field (0‐1.5 T) and temperature (30‐300 K) for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two‐dimensional electron gas (2DEG) channel located at the Al_0.88In_0.12N/GaN interface with an AlN interlayer and a two‐dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al_0.88In_0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spatially resolved X‐ray diffraction as a tool for strain analysis in laterally modulated epitaxial structures

Spatially resolved X‐ray diffraction (SRXRD) is applied for micro‐imaging of strain in laterally modulated epitaxial structures. In GaAs layers grown by liquid phase epitaxial lateral overgrowth (ELO) on SiO₂‐masked GaAs substrates a downward tilt of ELO wings caused by their interaction with the mask is observed. The distribution of the tilt magnitude across the wings width is determined with μm‐scale spatial resolution. This allows measuring of the shape of the lattice planes in individual ELO stripes. If a large area of the sample is studied the X‐ray imaging provides precise information on the tilt of an individual wing and its distribution. In heteroepitaxial GaSb/GaAs ELO layers local mosaicity in the wing area is found. By the SRXRD the size of microblocks and their relative misorientation were analyzed. Finally, the SRXRD technique was applied to study distribution of localized strain in AlGaN epilayers grown by MOVPE on bulk GaN substrates with AlN mask. X‐ray mapping proves that by mask patterning strain in AlGaN layer can be easily engineered, which opens a way to produce thicker, crack‐free AlGaN layers with a higher Al content needed in GaN‐based laser diodes. All these examples show that high spatial and angular resolutions offered by SRXRD makes the technique a powerful tool to study local lattice distortions in semiconductor microstructures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of GaN crystal quality on silicon substrates using GaN/AlN superlattice structures

The crystal quality of GaN thin film on silicon using GaN/AlN superlattice structures was investigated. The growth was carried out on Si(111) for GaN(0001) in a metal‐organic vapor phase epitaxy system. Various GaN/AlN superlattice intermediate layers have been designed to decrease the dislocation density. The results showed that the etch pit density could be greatly reduced by one order of magnitude. Cross‐sectional transmission electron microscopy (XTEM) study confirmed the efficiency of GaN/AlN superlattice in blocking threading dislocation propagation in GaN crystal. The design of nine period GaN/AlN (20nm/2nm) superlattice has been evidenced to be effective in reducing the dislocation density and improving the crystal quality. In addition, the dislocation bending in GaN/AlN interface and dislocation merging is investigated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MOVPE growth of nonpolar a-plane GaN with low oxygen contamination and specular surface on a freestanding GaN substrate

We investigated unintentionally doped nonpolar a- and m-plane GaN layers grown by metalorganic vapor phase epitaxy under several sets of conditions on freestanding a- and m-plane GaN substrates. Oxygen contamination in a-plane GaN is greatly reduced by increasing the V/III ratio during growth. As a result, a high-resistivity GaN buffer layer for an AlGaN/GaN heterostructure field-effect transistor was realized.     

In situ optical monitoring of AlGaN thickness and composition during MOVPE growth of AlGaN/GaN microwave HFETs

Real-time spectral reflectometry has been implemented to monitor the MOVPE growth of AlGaN/GaN microwave HFET structures. The aim is to monitor and control the thickness and composition of the thin AlGaN layer during growth. In order to extract useful information from the in situ spectra the optical constants of AlGaN as a function of alloy composition are required at the growth temperature (1050°C). As the first step to obtaining the high temperature optical constants, a room temperature spectroscopic ellipsometry study (energy range 1.65–4.95 eV) has been carried out on thin AlGaN films of various thickness (30 and 100 nm) and aluminium content (0.15 and 0.25). The multilayer model of each sample from the ellipsometry study is used to generate a reflectance spectrum which is compared with the in situ spectral reflectometry spectrum of the same sample acquired at room temperature to verify the technique. Further work is in progress to model the bandgap and optical constants of GaN and AlGaN at growth temperature.     

Si(111)外延GaN的表面形貌及形成机理研究

用高温AlN作缓冲层在Si(111)上外延生长出GaN薄膜.通过对薄膜表面扫描电子显微镜(SEM)和高分辨率双晶X射线衍射(DCXRD)的分析,确定缓冲层对外延层形貌的影响,分析解释了表面形貌中凹坑的形成及缓冲层生长温度对凹坑的影响.结果表明:温度的高低通过影响缓冲层初始成核密度和成核尺寸来影响外延层表面形貌.     

Metalorganic vapor phase epitaxy of III–V-on-silicon: Experiment and theory

The integration of III–V semiconductors with Si has been pursued for more than 25 years since it is strongly desired in various high-efficiency applications ranging from microelectronics to energy conversion. In the last decade, there have been tremendous advances in Si preparation in hydrogen-based metalorganic vapor phase epitaxy (MOVPE) environment, III–V nucleation and subsequent heteroepitaxial layer growth. Simultaneously, MOVPE itself took off in its triumphal course in solid state lighting production demonstrating its power as industrially relevant growth technique. Here, we review the recent progress in MOVPE growth of III–V-on-silicon heterostructures, preparation of the involved interfaces and fabrication of devices structures. We focus on a broad range of in situ, in system and ex situ characterization techniques. We highlight important contributions of density functional theory and kinetic growth simulations to a deeper understanding of growth phenomena and support of the experimental analysis. Besides new device concepts for planar heterostructures and the specific challenges of (001) interfaces, we also cover nano-dimensioned III–V structures, which are preferentially prepared on (111) surfaces and which emerged as veritable candidates for future optoelectronic devices.     

Growth of GaN based structures on Si(1 1 0) by molecular beam epitaxy

The growth of GaN based structures on Si(1 1 0) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical and electrical properties of such structures are assessed and are quite similar to the ones obtained on Si(1 1 1) in-spite of the very different substrate surface symmetry. A threading dislocation density of 3.7×10⁹ cm⁻² is evaluated by transmission electron microscopy, which is in the low range of typical densities obtained on up to 2 μm thick GaN structures grown on Si(1 1 1). To assess the potential of such structure for device realization, AlGaN/GaN high electron mobility transistor and InGaN/GaN light emitting diode heterostructures were grown and their properties are compared with the ones obtained on Si(1 1 1).     

Transmission electron microscopy study of an AlN nucleation layer for the growth of GaN on a 7-degree off-oriented (0 0 1) Si substrate by metalorganic vapor phase epitaxy

We have investigated the morphology of the high-temperature-grown AlN nucleation layer and its role in the early stage of GaN growth, by means of transmission electron microscopy. The nitride was selectively grown on a 7-degree off-oriented (0 0 1) patterned Si substrate by metalorganic vapor phase epitaxy. AlN was deposited on the inclined unmasked (1 1 1) facet in the form of islands. The size of the islands varied along the slope, which is attributable to the diffusion of the growth species in the vapor phase. The GaN nucleation occurred at the region where rounded AlN islands formed densely. The threading dislocations were observed to generate in the GaN nucleated region.     

The effect of the AlxGa1−xN/AIN buffer layer on the properties of GaN/Si(1 1 1) film grown by NH3-MBE

We describe the growth of GaN on Si(1 1 1) substrates with Al_xGa_1−xN/AlN buffer layer by ammonia gas source molecular beam epitaxy (NH₃-GSMBE). The influence of the AlN and Al_xGa_1−xN buffer layer thickness and the Al composition on the crack density of GaN has been investigated. It is found that the optimum thickness is 120 and 250 nm for AlN and Al_xGa_1−xN layers, respectively. The optimum Al composition is between 0.3<x<0.6.     

Effects of temperature and carrier gas flow amount on the formation of GaN nanorods by the HVPE method

We fabricated one-dimensional GaN nanorods on AlN/Si (1 1 1) substrates at various temperatures, and carrier gas flow amount, using the hydride vapor phase epitaxy (HVPE) method. An AlN buffer layer of 50 nm thickness was deposited by RF sputtering for 25 min. Stalagmite-like GaN nanorods formed at a growth temperature of 650 °C. The diameters and lengths of GaN nanorods increase with growth time, whereas the density of nanorods decreases. And we performed the experiments by changing the carrier gas flow amount at a growth temperature of 650 °C and HCl:NH₃ flow ratio of 1:40. GaN nanorods, with an average diameter of 50 nm, were obtained at a carrier gas flow amount of 1340 sccm. The shape, structures, and optical characteristics of the nanorods were investigated by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence.     

Controlled Formation of β-Si3N4 on Native Oxidized Silicon Wafers in Ammonia Flow

Crystallography Reports - The formation of β-Si3N4 for subsequent growth of AlGaN and GaN heterostructures of silicon wafers has been studied. It is established that the native oxide layer...     

Study on optimal growth conditions of a-plane GaN grown on r-plane sapphire by metal-organic chemical vapor deposition

Non-polar a-plane GaN thin films were grown on r-plane sapphire substrates by metal-organic chemical vapor deposition. In order to obtain a-plane GaN films with better crystal quality and surface morphology, detailed comparisons between different growth conditions were investigated. The results showed that high-temperature and low-pressure conditions facilitating two-dimensional growth could lead to a fully coalesced a-plane GaN layer with a very smooth surface. The best mean roughness of the surface morphology was 10.5 Å. Various thickness values of AlN nucleation layers and the V/III ratios for growth of the a-plane GaN bulk were also studied to determine the best condition for obtaining a smooth surface morphology of the a-plane GaN layer.     

Influence of AlGaN/GaN/InGaN superlattice on the characteristics of LEDs grown by metalorganic chemical vapor deposition

This study examined the influence of strain-compensated triple AlGaN/GaN/InGaN superlattice structures (SLs) in n-GaN on the structural, electrical and optical characteristics of LEDs by analyzing the etch pits density (EPD), stress measurement, high-resolution X-ray diffraction (HRXRD), sheet resistance, photoluminescence (PL) and light–current–voltage (L–I–V). EPD, stress measurement and HRXRD studies showed that the insertion of AlGaN/GaN/InGaN SLs during the growth of n-GaN effectively distributed and compensated for the strong compressive stress, and decreased the dislocation density in n-GaN. The operating voltage at 20 mA for the LEDs grown with SLs decreased to 3.18 V from 3.4 V for the LEDs grown without SLs. In addition, a decrease in the spectral blue shift compared to the LEDs grown without SLs was observed in the LEDs grown with the SLs.     

Growth of GaN/AlN and AlGaN by MOCVD using triethylgallium and tritertiarybutylaluminium

Films of aluminium nitride (AIN) with thicknesses in the range from 200 to 3600 Å have been deposited at 1050°C by low-pressure MOCVD. Using an alternative precursor, tritertiarybutylaluminium (^tBu₃Al), and ammonia (NH₃), we have grown AlN on sapphire (c-Al₂O₃). At a growth rate of 0.35 μm/h, the FWHM of the rocking curve measured by X-ray diffraction was 150 arcsec. Therefore, we used the thin AlN films as buffer layers for the deposition of gallium nitride (GaN) at 950°C using triethylgallium (Et₃Ga). Aluminium gallium nitride (Al_xGa_1−xN) with aluminium contents x from 0 to 0.5 were grown using a mixture of Et₃Ga and ^tBu₃Al. The strctural and optical properties of GaN, AlGaN and AlN were verified by X-ray diffraction (XRD), spectrally resolved photoconductivity (SPC), photothermal deflection (PDS) and photoluminescence spectroscopies.