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.     

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.     

Synthesis and Judd-Ofelt analysis of a sol-gel material doped with a Ln2(Benzoate)6(Phen)2 (Ln = Er/Yb) complex

In this work, we have prepared a sol-gel derived hybrid material directly doped with Er_1.4Yb_0.6(Benzoate)₆(Phen)₂ (Phen = 1,10-phenanthroline) complex, which was reported with intramolecular Yb-Er energy-transfer process in our previous work. The infrared (IR) spectra of the pure complex and hybrid gel material were investigated. The NIR photoluminescence (PL) spectrum of hybrid gel material shows strong characteristic emission of Er³⁺ with broad full width at half-maximum (FWHM) of 70 nm. Judd-Ofelt theory was used in order to analyze the optical properties of Er³⁺ ions in the hybrid gel material.     

Effect of NH3 flow rate on m-plane GaN growth on m-plane SiC by metalorganic chemical vapor deposition

This paper reports a study of the effect of NH₃ flow rate on m-plane GaN growth on m-plane SiC with an AlN buffer layer. It is found that a reduced NH₃ flow rate during m-plane GaN growth can greatly improve the recovery of in situ optical reflectance and the surface morphology, and narrow down the on-axis (1 0 1¯ 0) X-ray rocking curve (XRC) measured along the in-plane a-axis. The surface striation along the in-plane a-axis, a result of GaN island coalescence along the in-plane c-axis, strongly depends on the NH₃ flow rate, an observation consistent with our recent study of kinetic Wulff plots. The pronounced broadening of the (1 0 1¯ 0) XRC measured along the c-axis is attributed to the limited lateral coherence length of GaN domains along the c-axis, due to the presence of a high density of basal-plane stacking faults, most of which are formed at the GaN/AlN interface, according to transmission electron microscopy.     

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.     

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.     

Optical investigations of Berthelot-type properties in quaternary AlInGaN multiple quantum well heterosystems

The anomalous Berthelot-type optical properties of quaternary AlInGaN heterostructure with different quantum well pairs have been investigated systematically in this study. A microscopic model based on the luminescence observations of a comparison between an Arrhenius-type radiative process and a Berthelot-type nonradiative process has been proposed to elucidate the temperature dependence of the optical and transport behavior in nanocrystalline and amorphous semiconductors. The photoluminescence of the AlInGaN heterostructures is also found to exhibit unique luminescence features such as S-shaped emission peak energy and W-shaped full-width at half-maximum (FWHM) curves over a broad range of temperature. We investigated in detail the aspects of luminescence that would provide significant information regarding the degree of disorder in AlInGaN-based semiconductor heterosystems, leading to the appearance of the Berthelot-type behavior. The increase of quantum-well pairs apparently will cause the incorporation of indium atoms in the AlInGaN nanostructures, resulting in augmentation of the degree of crystalline randomization. In other words, the higher degree of disorder in AlInGaN heterostructures, the longer the static microbarrier width and the more obvious carrier localization effects may be observed.     

Characterizations of GaN films and AlGaN/GaN heterostructures on vicinal sapphire (0 0 0 1) substrates grown by MOCVD

GaN films and AlGaN/GaN heterostructures grown on vicinal sapphire (0 0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that surface morphologies of GaN films depend on the vicinal angle, however, they are not sensitive to the inclination directions of the substrate. The optimized vicinal angle for obtaining excellent surface morphology is around 0.5°. This conclusion is also confirmed by characterizing the electrical property of two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure.     

Optical properties of Cr ion in lithium metasilicate Li2O · SiO2 transparent glass-ceramics

The optical properties of Cr³⁺ ions in lithium metasilicate (Li₂O · SiO₂) transparent glass-ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li₂O · SiO₂) crystal. The percent crystallinity and crystalline size were ranging 65-75% and 20-35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr³⁺ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr³⁺ ions substitute for three Li⁺ ions and occupy the distorted octahedral site between single [SiO₄]_n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq = 13 088 cm⁻¹, B = 453 cm⁻¹, Dq/B = 2.89 and C = 2036 cm⁻¹. The near-infrared luminescence centered at 1250 nm was not detected in the deep pink glass-ceramics unlike emerald green glass.     

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.     

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.     

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.     

Influence of growth temperature on the selective area MOVPE of InAs nanowires on GaAs (1 1 1) B using N2 carrier gas

The influence of temperature on selective area (SA) InAs nanowire growth was investigated for metal-organic vapor phase epitaxy (MOVPE) using N₂ as the carrier gas and (1 1 1) B GaAs substrates. In contrast to the growth temperature range – below 600 °C – reported for hydrogen ambient, the optimal growth temperature between 650 and 700 °C was 100 K higher than the optimal ones for H₂ carrier gas. At these temperatures, nanowires with aspect ratios of about 80 and a symmetric hexagonal shape were obtained. The results found are attributed to the physical and chemical properties of the carrier gas.     

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.     

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.     

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.     

Spectrometric and ellipsometric studies of (1 − x)TiO2 · xLn2O3 (Ln = Nd, Sm, Gd, Er, Yb) thin films

Spectrometric and ellipsometric studies of (1 − x)TiO₂ · xLn₂O₃ (Ln = Nd, Sm, Gd, Er, Yb; x = 0.33, 0.5) thin films at room temperature were performed. The obtained dispersion dependences of refractive indices are successfully described by the optical-refractometric relation. The dependence of optical pseudogap and refractive indices on composition and molar mass of the films is investigated. The influence of compositional disordering on the energy width of the exponential absorption edge is studied.     

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.     

Layer-by-layer growth and growth-mode transition of SrRuO3 thin films on atomically flat single-terminated SrTiO3 (1 1 1) surfaces

We report on growth-mode transitions in the growth of SrRuO₃ thin films on atomically flat Ti⁴⁺ single-terminated SrTiO₃ (1 1 1) substrates, investigated by reflection high-energy electron diffraction and atomic force microscopy. Over the first ~9 unit cells, the dominant growth mode changes from island to layer-by-layer for the growth rate of 0.074 unit cells/s and the growth temperature of 700 °C. Moreover, in the course of growing SrRuO₃ films, the governing growth mode of interest can be manipulated by changing the growth temperature and the growth rate, which change allows for the selection of the desired layer-by-layer mode. The present study thus paves the way for integrations of SrRuO₃ thin layers into (1 1 1)-orientated oxide heterostructures, and hence multi-functional devices, requiring control of the sharp atomic-level interfaces and the layer-by-layer growth mode.