Hydride vapor phase epitaxial growth of thick GaN layers with in-situ optical monitoring

An in-situ optical monitoring system made in our laboratory is set up on the horizontal hydride vapor phase epitaxy (HVPE) equipment. From the growth rate information provided by this system, some basic growth parameters are optimized and high-quality GaN layers are grown. The growth stress of the HVPE GaN layer grown on different templates is also examined through the in-situ optical measuring.     

Improvements in a-plane GaN crystal quality by AlN/AlGaN superlattices layers

Non-polar (1 1 2¯ 0) a-plane GaN films have been grown by low-pressure metal-organic vapor deposition on r-plane (1 1¯ 0 2) sapphire substrate. We report on an approach of using AlN/AlGaN superlattices (SLs) for crystal quality improvement of a-plane GaN on r-plane sapphire. Using X-ray diffraction and atomic force microscopy measurements, we show that the insertion of AlN/AlGaN SLs improves crystal quality, reduces surface roughness effectively and eliminates triangular pits on the surface completely.     

Optical properties of MOVPE-grown a-plane GaN and AlGaN

a-Plane GaN and AlGaN were grown on r-plane sapphire by low-pressure metal-organic vapor epitaxy (LP-MOVPE), and the effects of reactor pressure (from 40 to 500 Torr) and growth temperature (from 1020 to 1100 °C) on the crystalline quality and surface morphology of a-plane GaN were studied. The a-plane GaN grown under 40 Torr had a smooth-surface morphology but a poor crystalline quality; however, the a-plane GaN grown under 500 Torr had higher crystalline quality and optical properties, whose full-width at half-maximum of the X-ray rocking curve (XRC-FWHM) and intensity of yellow luminescence (YL) were smaller. Furthermore, the optical properties of a-plane GaN were investigated by photoluminescence (PL) in detail. We also studied the emission properties of a-plane Al_0.35Ga_0.65N grown at room temperature.     

One-sidewall-seeded epitaxial lateral overgrowth of a-plane GaN by metalorganic vapor-phase epitaxy

The selective regrowth of GaN during sidewall-seeded epitaxial lateral overgrowth was performed. In addition to adjusting the V/III ratio, control of offset angle of the sidewall was found to be effective for realizing one-sidewall-seeded a-plane (1 1 2¯ 0) GaN on r-plane (1 1¯ 0 2) sapphire. The number of coalescence regions on the grooves was reduced, and threading-dislocation and stacking-fault densities as low as 10⁶–10⁷ cm⁻² and 10³–10⁴ cm⁻¹, respectively, were successfully realized.     

Structural and electrical properties of Si-doped a-plane GaN grown on r-plane sapphire by MOVPE

Approximately 2-μm-thick Si-doped a-plane GaN films with different doping concentrations were grown on approximately 8-μm-thick undoped a-plane GaN/r-sapphire by metal organic vapor phase epitaxy (MOVPE). The structural and electrical properties of the Si-doped a-plane GaN films were investigated by high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and temperature-dependent Hall measurement. The results showed that a small amount of Si doping can improve the surface morphology and decrease the density of pits. Upon increasing the CH₃SiH₃ flow rate, the crystalline quality of the (0 0 0 2) plane was slightly improved. The highest room-temperature mobility of 83.4 cm²/Vs was obtained at a carrier density of 6.2×10¹⁸ with a CH₃SiH₃ flow rate of 10 sccm.     

Maskless selective growth of semi-polar (1 12¯ 2) GaN on Si (3 1 1) substrate by metal organic vapor phase epitaxy

Semi-polar (1 1 2¯ 2) GaN layers were selectively grown by metal organic chemical vapor phase epitaxy on patterned Si (3 1 1) substrates without SiO₂ amorphous mask. The (1 1 2¯ 2) GaN layers could be selectively grown only on Si (1 1 1) facets when the stripe mask width was narrower than 1 μm even without SiO₂. Inhomogeneous spatial distribution of donor bound exciton (DBE) peak in low-temperature cathodoluminescence (CL) spectra was explained by the difference of growth mode before and after the coalescence of stripes. It was found that the emission intensity related crystal defects is drastically decreased in case of selective growth without SiO₂ masks as compared to that obtained with SiO₂ masks.     

Photoluminescence study of Si-doped a-plane GaN grown by MOVPE

Si-doped a-plane GaN films with different doping concentrations were grown by metal-organic vapor phase epitaxy. A mirrorlike surface without pits or anisotropic stripes was observed by optical microscopy. Detailed optical properties of the samples were characterized by temperature- and excitation-intensity-dependent PL measurements. A series of emission peaks at 3.487, 3.440, 3.375–3.350, 3.290 and 3.197 eV were observed in the low-temperature PL spectra of all samples. The origin of these emissions is discussed in detail.     

Growth of (1 0 1¯ 3¯ ) semipolar GaN on m-plane sapphire by hydride vapor phase epitaxy

The crystalline, surface, and optical properties of the (1 0 1¯ 3¯) semipolar GaN directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) were investigated. It was found that the increase of V/III ratio led to high quality (1 0 1¯ 3¯) oriented GaN epilayers with a morphology that may have been produced by step-flow growth and with minor evidence of anisotropic crystalline structure. After etching in the mixed acids, the inclined pyramids dominated the GaN surface with a density of 2×10⁵ cm⁻², revealing the N-polarity characteristic. In the low-temperature PL spectra, weak BSF-related emission at 3.44 eV could be observed as a shoulder of donor-bound exciton lines for the epilayer at high V/III ratio, which was indicative of obvious reduction of BSFs density. In comparison with other defect related emissions, a different quenching behavior was found for the 3.29 eV emission, characterized by the temperature-dependent PL measurement.     

The origin of the residual conductivity of GaN films on ferroelectric materials

In this paper, the origin of the conductivity of GaN films grown on ferroelectric materials was investigated using XPS, AES, and XRD analysis tools. Depth profiles confirmed the existence of impurities in the GaN film originating from the substrates. Bonding energy analysis from XPS and AES verified that oxygen impurities from the substrates were the dominant origin of the conductivity of the GaN film. Furthermore, Ga-rich GaN films have a greater chance of enhancing diffusion of lithium oxide from the substrates, resulting in more substrate phase separation and a wider inter-mixed region confirmed by XRD. Therefore, the direct GaN film growth on ferroelectric materials causes impurity diffusion from the substrates, resulting in highly conductive GaN films. Future work needs to develop non-conductive buffer layers for impurity suppression in order to obtain highly resistive GaN films.     

Epitaxial lateral overgrowth on (2 1¯ 1¯ 0) a-plane GaN with [0 1¯ 1 1]-oriented stripes

Epitaxial lateral overgrowth was applied to a-plane GaN on r-plane sapphire using SiO₂ stripe masks oriented parallel to [0 1¯ 1 1]. Coalescence and defect distribution was studied using scanning electron microscopy and cathodoluminescence. Defects, i.e., threading dislocations and basal plane stacking faults from the template propagate into the overgrown layer through the mask openings. Stacking faults spread into the whole overgrown layer, whereas threading dislocations are laterally confined in the region above the mask where a part of them is terminated at the inclined coalescence boundary. Lateral overgrowth and dislocation termination at the coalescence boundary leads to an improvement in luminescence intensity and crystal quality, in comparison to the template. The measured XRD rocking curve FWHM were 453″ with incidence along the [0 0 0 1] c-direction and 280″ with incidence along the [0 1 1¯ 0] m-direction.     

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.     

HVPE growth of semi-polar (1 1 2¯ 2)GaN on GaN template (1 1 3)Si substrate

The hydride-vapour-phase-epitaxial (HVPE) growth of semi-polar (1 1 2¯ 2)GaN is attempted on a GaN template layer grown on a patterned (1 1 3) Si substrate. It is found that the chemical reaction between the GaN grown layer and the Si substrate during the growth is suppressed substantially by lowering the growth temperatures no higher than 900 °C. And the surface morphology is improved by decreasing the V/III ratio. It is shown that a 230-μm-thick (1 1 2¯ 2)GaN with smooth surface is obtained at a growth temperature of 870 °C with V/III of 14.     

Reduction of dislocations in a (1 1 2¯ 2)GaN grown by selective MOVPE on (1 1 3)Si

Two-step selective epitaxy (SAG/ELO) of (1 1 2¯ 2)GaN on (1 1 3)Si substrate is studied to reduce the defect density in the epitaxial lateral overgrowth. The first SAG/ELO is to prepare a (1 1 2¯ 2)GaN template on a (1 1 3)Si and the second SAG/ELO is to get a uniform (1 1 2¯ 2)GaN. It is found that the reduction of the defect density is improved by optimizing the mask configuration in the second SAG/ELO. The minimum dark spot density obtained is 3×10⁷/cm², which is two orders of magnitude lower than that found in a (0 0 0 1)GaN grown on (1 1 1)Si.     

Strain-reduced GaN thick-film grown by hydride vapor phase epitaxy utilizing dot air-bridged structure

A 300 μm GaN thick-film, in diameter 1.5 in, was demonstrated without any crack by hydride vapor phase epitaxy (HVPE) growth. The technique used in relaxing the residual stress caused by differences of thermal expansion coefficients (TEC) and lattice constants between GaN and sapphire substrate to prevent GaN film from crack is called a dot air-bridged structure. After the laser lift-off process, 300-μm-thick freestanding GaN wafer, in diameter 1.5 in, could be fabricated. The compressive stress in the dot air-bridged structure was measured by micro-Raman spectroscopy with the E₂(high) phonon mode. The compressive stress could be reduced to as small as 0.04 GPa, which could prevent the crack during the epitaxial process for GaN growth by HVPE. It is important to obtain a large-area crack-free GaN thick-film, which can be used for fabricating freestanding GaN wafer.     

Characterization of GaInN/GaN layers for green emitting laser diodes

An enhancement of radiative recombination in GaInN/GaN heterostructures is being pursued by a reduction of defects associated with threading dislocations and a structural control of piezoelectric polarization in the active light-emitting regions. First, in conventional heteroepitaxy on sapphire substrate along the polar c-axis of GaN, green and deep green emitting light-emitting diode (LED) wafers are being developed. By means of photoluminescence at variable low temperature and excitation density, internal quantum efficiencies of 0.18 for LEDs emitting at 530 nm and 0.08 for those emitting at 555 nm are determined. Those values hold for the high current density of 50 A/cm² of high-power LED lamps. In bare epi dies, we obtain efficacies of 16 lm/W. At 780 A/cm² we obtain 22 lm when measured through the substrate only. The 555 nm LED epi material under pulsed photoexcitation shows stimulated emission up to a wavelength of 485 nm. This strong blue shift of the emission wavelength can be avoided in homoepitaxial multiple quantum well (MQW) and LED structures grown along the non-polar a- and m-axes of low-dislocation-density bulk GaN. Here, wavelength-stable emission is obtained at 500 and 488 nm, respectively, independent on excitation power density opening perspectives for visible laser diodes.     

Growth and properties of semi-polar GaN on a patterned silicon substrate

Adopting anisotropy etching method, a (1 1 1) facet of Si is obtained on a Si substrate and selective area growth (SAG) of GaN is performed with metal-organic vapor phase epitaxy on the facet. The epitaxial lateral overgrowth of (1 1¯ 0 1), (1 1 2¯ 2) GaN is investigated on (0 0 1) and (1 1 3) Si substrate, respectively, and the incorporation properties of Si, C, and Mg elements are discussed in relation to the atomic configuration on the surface. Analyzing the optical and electrical properties of C-doped (1 1¯ 0 1) GaN layer, it is shown that carbon creates a shallow acceptor level. On the thus prepared (1 1¯ 0 1) GaN layer, a light emitting diode (LED) with a C-doped p-type layer is fabricated.     

Growth and characterization of GaAs layers on polished Ge/Si by selective aspect ratio trapping

Epitaxial GaAs layers have been deposited on polished Ge film grown on exactly (0 0 1) oriented Si substrate by metal-organic chemical vapor deposition (MOCVD) via aspect ratio trapping (ART) method. Double-crystal X-ray diffraction shows that the full-width at half-maximum (FWHM) of the (4 0 0) reflection obtained from 1 μm GaAs is 140 arcsec. Scanning electron microscopy (SEM) of the GaAs layer surface shows that the amount of antiphase domain defects (APD) raised from GaAs/Ge interface using Ge ART on Si is dramatically reduced compared to GaAs layers grown on exact (0 0 1) Ge substrate. Defect reduction and Ge diffusion at vicinal GaAs/Ge interface were investigated via cross-section transmission electron microscopy (X-TEM) and secondary ion mass spectrometry (SIMS). Film morphology and optical properties were evaluated via SEM and room temperature photoluminescence (PL).     

Revealing extended defects in HVPE-grown GaN

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.     

GaN-based nitride semiconductor films deposited on nitrified HfO2/Si substrate by molecular beam epitaxy

Nitrified HfO₂/Si substrate was used to grow GaN-based film with molecular beam epitaxy. Four-period InGaN/GaN layered structure and p/n junction were deposited on the nitrified HfO₂/Si. X-ray photoelectron spectroscopy (XPS) result shows that N was effectively incorporated into the HfO₂. The crystallographic relationship of the GaN/HfO₂/Si is GaN(0 0 0 2)∥HfO₂(1 1 1)∥Si(1 1 1). Temperature-dependent photoluminescence (PL), PL peak wavelength, PL peak intensity, and PL full-width at half-maximum of the p/n junction were investigated. Light-emitting diode was fabricated from the p/n junction. Red light was emitted at low voltage and yellow light was emitted when increasing the voltage.     

Overgrowth of GaN on Be-doped coalesced GaN nanocolumn layer by rf-plasma-assisted molecular-beam epitaxy—Formation of high-quality GaN microcolumns

GaN crystals were overgrown on GaN nanocolumn platforms with a Be-doped coalesced layer by rf-plasma-assisted molecular-beam epitaxy (rf-MBE). The overgrown GaN included large micrometer-scale hexagonal columnar crystals. These microcrystals were named ‘microcolumns’ and showed high optical properties comparable to those of GaN bulk crystals grown by hydride vapor phase epitaxy (HVPE).