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.     

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.     

Mg segregation in a (1 1¯ 0 1) GaN grown on a 7° off-axis (0 0 1) Si substrate by MOVPE

Redistribution behavior of magnesium (Mg) in the N-terminated (1 1¯ 0 1) gallium nitride (GaN) has been investigated. A nominally undoped GaN layer was grown on a heavily Mg-doped GaN template by metalorganic vapor-phase epitaxy (MOVPE). Mg dopant profiles were measured by secondary ion mass spectrometry (SIMS) analysis. A slow decay of the Mg concentration was observed in the nominally undoped GaN layer due to the surface segregation. The calculated decay lengths of the (1 1¯ 0 1) GaN are ∼75–85 nm/decade. These values are shorter than the decay length determined in the sample grown on the Ga-terminated (0 0 0 1) GaN. This result indicates that Mg exhibited weak surface segregation in the (1 1¯ 0 1) GaN as compared to the (0 0 0 1) GaN. The weak surface segregation is in agreement with the high efficiency of Mg incorporation on the (1 1¯ 0 1) face. The high density of hydrogen was obtained in the (1 1¯ 0 1) GaN, which might enhance the Mg incorporation.     

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.     

MOVPE growth and properties of GaN on (1 1 1)Si using an AlInN intermediate layer

Using an AlInN intermediate layer, GaN was grown on (1 1 1)Si substrate by selective metalorganic vapor phase epitaxy. The variation of the surface morphology was investigated as a function of the In composition and thickness of the AlInN layer. It was found that the In composition in the AlInN layer was a function of the growth temperature and thickness. Because of the small band offset at the AlInN/Si hetero-interface, we have achieved a low series resistance of the order of 9 Ω (0.0036 Ω cm²) across the GaN/AlInN/AlN/Si layer structure.     

GaN grown on (1 1 1) single crystal diamond substrate by molecular beam epitaxy

GaN epilayers are grown on (1 1 1) oriented single crystal diamond substrate by ammonia-source molecular beam epitaxy. Each step of the growth is monitored in situ by reflection high energy electron diffraction. It is found that a two-dimensional epitaxial wurtzite GaN film is obtained. The surface morphology is smooth: the rms roughness is as low as 1.3 nm for 2×2 μm² scan. Photoluminescence measurements reveal pretty good optical properties. The GaN band edge is centred at 3.469 eV with a linewidth of 5 meV. These results demonstrate that GaN heteroepitaxially grown on diamond opens new rooms for high power electronic applications.     

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.     

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.     

Growth of thick GaInN on grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC

We succeeded in growing high-crystalline-quality thick (1 0 1¯ 1¯) Ga_0.92In_0.08N films on a grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC underlying layer. We also fabricated GaInN/GaN multiple quantum wells (MQWs) with a peak wavelength of 580 nm on a high-crystalline-quality thick GaInN film. The photoluminescence intensity of the MQWs is about six times higher than that of MQWs grown on planar GaN and twice as high as that of MQWs grown on a GaN underlying layer having the same grooved structure.     

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.     

Formation of GaN nanodots on Si (1 1 1) by droplet nitridation

GaN nanodots (NDs) are obtained by Ga metallic droplet formation on Si (1 1 1) substrates followed by their nitridation. The size and density of Ga droplets and GaN NDs can be controlled by varying the growth temperature within the range 514–640 °C. Atomic force microscopy (AFM) investigation of Ga droplets shows an increase in the average diameter with temperature. The average diameter of GaN NDs increases with growth temperature while their density decreases more than one order of magnitude. In addition, the formation of a GaN crystallite rough layer on Si, in-between NDs, indicates that a spreading mechanism takes place during the nitridation process. High-resolution transmission electron microscopy (HRTEM) is used for the investigation of shape, crystalline quality and surface distribution of GaN dots. X-ray photoelectron spectroscopy (XPS) results confirm that Ga droplets that are transformed into GaN NDs spread over the sample surface during nitridation.     

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.     

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 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).     

Molecular beam epitaxial growth of hexagonal boron nitride on Ni(1 1 1) substrate

We demonstrate hexagonal boron nitride (h-BN) epitaxial growth on Ni(1 1 1) substrate by molecular beam epitaxy (MBE) at 890 °C. Elemental boron evaporated by an electron-beam gun and active nitrogen generated by a radio-frequency (RF) plasma source were used as the group-III and -V sources, respectively. Reflection high-energy electron diffraction revealed a streaky (1×1) pattern, indicative of an atomically flat surface in the ongoing growth. Correspondingly, atomic force microscopy images exhibit atomically smooth surface of the resulting h-BN film. X-ray diffraction characterization confirmed the crystallinity of the epitaxial film to be h-BN, and its X-ray rocking curve has a full-width at half-maximum of 0.61°, which is the narrowest ever reported for h-BN thin film. The epitaxial alignments between the h-BN film and the Ni substrate were determined to be [0 0 0 1]_h−BN∥[1 1 1]_Ni, [1 1 2¯ 0]_h−BN∥[1¯ 1 0]_Ni, and [1 1¯ 0 0]_h−BN∥[1¯ 1¯ 2]_Ni.     

Subtle interplay between polytypism and preferred growth direction alignment in GaN nanorods non-catalytically grown on Si(1 1 1) substrates by using hydride vapor phase epitaxy

GaN nanorods were grown on Si(1 1 1) substrates by using hydride vapor phase epitaxy, and the crystallographic characteristics associated with their preferred growth directions were investigated by utilizing synchrotron X-ray reciprocal space mapping in a grazing incidence geometry and scanning electron microscopy. Crystallographic analysis reveals that the nanorods containing both wurtzite and zinc blende phase tend to have narrower distribution of the preferred growth directions than those containing only wurtzite phase. This tendency is partly attributed to the subtle interplay between polytypism and the preferred growth directions of GaN nanorods.     

Atomic ordering in GaAsSb (0 0 1) grown by metalorganic vapor phase epitaxy

Epitaxial GaAsSb (0 0 1) semiconductor alloys grown by metalorganic vapor phase epitaxy exhibit several spontaneously ordered structures. A superlattice structure with three-fold ordering in the [1 1 0] direction has been previously observed by different groups. CuAu structures with (1 0 0) and (0 1 0) ordering planes have also been reported. The physical origin of CuAu ordering in III–V semiconductors has not yet been explained. In this work we report the effect of growth conditions on CuAu ordering in GaAsSb, including miscut from (0 0 1), growth rate, bismuth surfactant concentration, and growth temperature. These data point to a surface kinetic mechanism not based on dimer strain, but possibly due to one-dimensional ordering at step edges.     

The effects of annealing on non-polar (1 1 2¯ 0) a-plane GaN films

Non-polar a-plane (1 1 2¯ 0) GaN films were grown on r-plane sapphire by metal–organic vapor phase epitaxy and were subsequently annealed for 90 min at 1070 °C. Most dislocations were partial dislocations, which terminated basal plane stacking faults. Prior to annealing, these dislocations were randomly distributed. After annealing, these dislocations moved into arrays oriented along the [0 0 0 1] direction and aligned perpendicular to the film–substrate interface throughout their length, although the total dislocation density remained unchanged. These changes were accompanied by broadening of the symmetric X-ray diffraction 1 1 2¯ 0 ω-scan widths. The mechanism of movement was identified as dislocation glide, occurring due to highly anisotropic stresses (confirmed by X-ray diffraction lattice parameter measurements) and evidenced by macroscopic slip bands observed on the sample surface. There was also an increase in the density of unintentionally n-type doped electrically conductive inclined features present at the film–substrate interface (as observed in cross-section using scanning capacitance microscopy), suggesting out-diffusion of impurities from the substrate along with prismatic stacking faults. These data suggest that annealing processes performed close to film growth temperatures can affect both the microstructure and the electrical properties of non-polar GaN films.     

Effects of morphologies on the field emission characteristics of GaN nanorods grown on Si (0 0 1) by MBE

GaN nanorods were grown on Si (0 0 1) substrates with a native oxide layer by molecular beam epitaxy. The changes in the morphologies and their effects on the field emission characteristics of GaN nanorods were investigated by varying growth conditions, namely, growth time of low-temperature GaN buffer layer, growth time of GaN nanorods, Ga flux during growth of GaN nanorods, and growth temperature of GaN nanorods. GaN nanorods with a low aspect ratio measured by diode configuration showed better field emission characteristics than those with a high aspect ratio, which may be due to the effects of screening and the surface depletion layer. In addition, the distance between the GaN nanorods and the anode played an important role in the field emission characteristics such as turn-on field, field enhancement factor, and field distribution on the emitter surface.