Nonpolar A-plane GaN Research Articles

Reduction in crystalline quality anisotropy and strain for non-polar a-plane GaN epi-layers with nano-scale island-like SiNx interlayer

The reduction in the crystalline quality anisotropy and the in-plane strain for the non-polar a-plane GaN epi-layer with nano-scale island-like SiNx interlayer was studied extensively with scanning electron microscopy, high-resolution X-ray diffraction measurement, and Raman spectroscopy. It was demonstrated that the SiNx interlayer was powerful to suppress the crystalline quality anisotropy in the a-plane GaN epi-layer since the full width at half maximum values of the X-ray rocking curves for both c-direction and m-direction were reduced from 1108 to 780 arcsec and from 2077 to 806 arcsec, respectively. The Raman measurement results also reveal that the SiNx interlayer plays a crucial role in compensating the in-plane strains. In fact, the in-plane compressive strain along m-direction and the tensile strain along c-direction were found to be decreased by approximately 62% and 78%, respectively due to the insertion of the SiNx interlayer.

Terahertz Dielectric Response of Nonpolar a-plane GaN Films

We report on the dielectric properties of non-polar a-plane (11-20) GaN (a-GaN) films in the terahertz (THz) frequency region, using THz time domain spectroscopy. We measured the transmitted THz time-domain waveforms by rotating the phi angle for 6μm-thick a-GaN films, which were lifted off from sapphire substrates. THz transmittance of a-GaN films clearly showed the angular dependence on azimuth angle, possibly due to the birefringence and optical phonon modes in a-GaN crystal. It is believed that the discrepancy in the refractive index results from the difference of the transverse-optical and the longitudinal optical phonon branches.

Terahertz Dielectric Response of Nonpolar a-plane GaN Films

The generation and the detection of the THz radiation in the frequency range of 0.1 to 10 THz have attracted much research interest, which was mainly driven by the particular applications of THz time-domain spectroscopy (TDS) in physics, chemistry, nano and life sciences. THz TDS has shown many advantages due to its non-destructive analysis feature in characterizing complex dielectric properties and understanding the carrier-dynamics in various semiconductor materials. Extensive studies have been reported on the THz dielectric and optical properties such as electro-optical effects, optical rectification, and birefringence. In addition, the ultrafast relaxation and recombination dynamics of photogenerated carriers were investigated for new materials such as carbon nanotube, graphene, and perovskites using optical pump THz-probe spectroscopy. Recently, the terahertz (THz) quantum cascade lasers and intersubband transition devices has attracted considerable attention based on AlGaN/GaN quantum well structure for far-infrared applications. It is necessary to investigate the optical and dielectric properties of GaN and sapphire crystals in the far IR as well as THz frequency regions. In the previous report, nonpolar and semipolar GaN crystals in the hexagonal crystal system are found to show multi-phonon mode behavior in the far-infrared frequency region. In addition, the polarized IR reflectance response of sapphire is reported to exhibit the dependence on the crystal orientation. In this paper, we investigated the optical properties and dielectric response of nonpolar a-plane GaN (a-GaN) films in the terahertz (THz) frequency using polarized femto-second THz TDS. The phase separation and amplitude ratio were measured by changing the crystal orientation with respect to the polarization of the incident THz pulse. In sharp contrast with polar c-plane GaN, nonpolar a-GaN films exhibited the angle dependence of the THz absorptions at room temperature, presumably due to the birefringence in nonpolar a-GaN films. The transmitted THz signal intensity, the time delay, and the Fourier-transformed amplitude were investigated by rotating the azimuth angle of the crystal orientation. Strong THz field absorptions around 1 THz frequency were observed at the azimuth angle of 0°, 90°, and 180° for a-GaN films. We believe that the birefringence in the THz frequency region originates from the difference of the transverse-optical and the longitudinal optical phonon splitting between the optical phonon branches.

무분극 a-plane 질화물계 발광다이오드에서 SiO2 전류 제한 층을 통한 발광 효율 증가

In this study, we investigate the <TEX>$SiO_2$</TEX> current blocking layer (CBL) to improve light output power efficiency in nonpolar a-plane (11-20) GaN LEDs on a r-plane sapphire substrate. The <TEX>$SiO_2$</TEX> CBL was produced under the p-pad layer using plasma enhanced chemical vapor deposition (PECVD). The results show that nonpolar GaN LED light output power with the <TEX>$SiO_2$</TEX> CBL is considerably enhanced compared without the <TEX>$SiO_2$</TEX> CBL. This can be attributed to reduced light absorption at the p-pad due to current blocking to the active layer by the <TEX>$SiO_2$</TEX> CBL.

Ohmic Contact Properties of Photo-Chemically Etched Nonpolar A-Plane GaN Films

Nonpolar a-plane GaN (a-GaN) was photo-chemically etched under ultraviolet illumination using alkali KOH solution, and the Ohmic contact properties on the etched a-GaN epitaxial layer were investigated. After photo-chemical etching, the striated surface morphology along the c-axis [0001] direction was obtained having triangular prisms consisting of m-plane facets. The properties of Ti/Al/Ni/Au Ohmic contact on surface-textured a-GaN films were studied with transmission line method patterns aligned along the specific crystal orientations. The minimum specific contact resistance of 9.97 × 10−5 Ω·cm2 was achieved along the c-axis on the etched GaN surface at the annealing temperature of 750°C. The etched a-GaN showed higher electrical conductivity along the c-axis than along m-axis. This anisotropic behavior is in contrast to that of the unetched a-GaN where carrier scattering by basal plane stacking faults in the direction perpendicular to the c-axis is a major factor in determining opposite anisotropic conductivity. The higher conductivity along the c-axis of the etched a-GaN film could be explained by more dominant effect of surface roughness scattering.

Highly sensitive nonpolar a-plane GaN based hydrogen diode sensor with textured active area using photo-chemical etching

In this work, a highly sensitive a-plane (112¯0) GaN (a-GaN) based hydrogen sensor with a large active surface area on the Schottky contact region was fabricated and characterized. By using a simple photochemical etching technique, a striated surface morphology with triangular prisms consisting of m-plane facets on the a-GaN surface was obtained. The maximum relative current change of the etched a-GaN diode was as high as 3.8×107%, and the reduction of the effective Schottky barrier height was 0.49eV upon 4% hydrogen exposure. The photo-chemically etched a-GaN sensor showed a remarkably improved hydrogen response and good repeatability for cyclic exposure to hydrogen. The photo-chemically textured GaN surface with enlarged surface area increased the number of adsorption sites available for hydrogen molecules to catalytically-decompose into surface atoms, lowering the effective Schottky barrier height, thereby increasing the measured current. Furthermore, the hydrogen sensing properties of the etched a-GaN diodes at different values of humidity and temperature were investigated.

Current mapping of nonpolar a-plane and polar c-plane GaN films by conductive atomic force microscopy

Nonpolar (11-20) a-plane GaN and polar (0001) c-plane GaN films have been grown by metal organic chemical vapor deposition on r-plane (1-102) and c-plane (0001) sapphire substrates, respectively. Conductive atomic force microscopy (C-AFM) has been used to investigate the local conductivity of the films. C-AFM shows enhanced current conduction within the etch pits of c-plane GaN and triangular pits of a-plane GaN. The results indicate that the off-axis planes are more electrically active than c-plane and a-plane. Surprisingly, the C-AFM values in triangular pit of the a-plane GaN are much smaller than that in etch pits of the c-plane GaN. The dislocations type related current leakage mechanism is revealed for polar c-plane and nonpolar a-plane GaN films.

Spatially resolved and orientation dependent Raman mapping of epitaxial lateral overgrowth nonpolar a-plane GaN on r-plane sapphire.

Uncoalesced a-plane GaN epitaxial lateral overgrowth (ELO) structures have been synthesized along two mask stripe orientations on a-plane GaN template by MOCVD. The morphology of two ELO GaN structures is performed by Scanning electronic microscopy. The anisotropy of crystalline quality and stress are investigated by micro-Raman spectroscopy. According to the Raman mapping spectra, the variations on the intensity, peak shift and the full width at half maximum (FWHM) of GaN E2 (high) peak indicate that the crystalline quality improvement occurs in the window region of the GaN stripes along [0001], which is caused by the dislocations bending towards the sidewalls. Conversely, the wing regions have better quality with less stress as the dislocations propagated upwards when the GaN stripes are along []. Spatial cathodoluminescence mapping results further support the explanation for the different dislocation growth mechanisms in the ELO processes with two different mask stripe orientations.

A study of the optical and polarisation properties of InGaN/GaN multiple quantum wells grown on a-plane and m-plane GaN substrates

We report on a comparative study of the low temperature emission and polarisation properties of InGaN/GaN quantum wells grown on nonpolar () a-plane and () m-plane free-standing bulk GaN substrates where the In content varied from 0.14 to 0.28 in the m-plane series and 0.08 to 0.21 for the a-plane series. The low temperature photoluminescence spectra from both sets of samples are broad with full width at half maximum height increasing from 81 to 330 meV as the In fraction increases. Photoluminescence excitation spectroscopy indicates that the recombination mainly involves strongly localised carriers. At 10 K the degree of linear polarisation of the a-plane samples is much smaller than of the m-plane counterparts and also varies across the spectrum. From polarisation-resolved photoluminescence excitation spectroscopy we measured the energy splitting between the lowest valence sub-bands to lie in the range of 23–54 meV for the a- and m-plane samples in which we could observe distinct exciton features. Thus the thermal occupation of a higher valence sub-band cannot be responsible for the reduction of the degree of linear polarisation at 10 K. Time-resolved spectroscopy indicates that in a-plane samples there is an extra emission component which is at least partly responsible for the reduction in the degree of linear polarisation.

Structural Anisotropy and Optical Properties of Nonpolar a-Plane GaN Epitaxial Layers.

In-plane structural anisotropy is characteristic of nonpolar (1120) a-plane GaN (a-GaN) films grown on r-plane sapphire substrates. The anisotropic peak broadenings of X-ray rocking curves (XRCs) are clearly observed with M- or W-shaped dependence on the azimuth angles. We investigated the optical properties of both M- and W-shaped a-GaN samples with room and low-temperature photoluminescence (PL) measurements. The W-shaped a-GaN film showed higher PL intensity and more compressive strain compared to the M-shaped a-GaN film, whereas the XRC peak widths of the M-shaped a-GaN film on the azimuth angles are lower than those of W-shaped specimens, indicating that better crystalline quality was obtained. We speculate that the PL intensity and strain state of a-GaN layers may be more influenced by the crystallinity of a specific crystal orientation or direction, especially along the m-axis as opposed to the c-axis. This occurrence is most likely due to anisotropic defect distributions, resulting from differences in dangling bond densities of (0001) and {1-100} facets.

Morphological and Microstructural Evolution and Related Impurity Incorporation in Non-Polar a-Plane GaN Grown on r-Sapphire Substrates **Supported by the National Natural Science Foundation of China under Grant No 61204006, and the Fundamental Research Funds for the Central Universities under Grant No K50511250002.

Effects of the growth temperature on morphological and microstructural evolution of a-plane GaN films grown on r-plane sapphires by metal organic chemical vapor deposition are investigated by atomic force microscopy and secondary ion mass spectroscopy (SIMS). Surface morphology, structural quality and related impurity incorporation are very sensitive to the growth temperature. A significant difference of yellow luminescence is observed and attributed to the incorporation of carbon into GaN films, which is confirmed by SIMS analysis. Our results show that the sample with triangular-pit morphology has significantly higher concentrations of oxygen than the other sample with pentagon-like pit morphology, which is induced by the existence of an N-face in triangular pits.

Growth of a-Plane GaN Films on r-Plane Sapphire by Combining Metal Organic Vapor Phase Epitaxy with the Hydride Vapor Phase Epitaxy**Supported by the National Natural Science Foundation of China under Grant No 61204006, the Fundamental Research Funds for the Central Universities under Grant No 7214570101, and the National Key Science and Technology Special Project under Grant No 2008ZX01002-002.

Hydride vapor phase epitaxy (HVPE) is utilized to grow nonpolar a-plane GaN layers on r-plane sapphire templates prepared by metal organic vapor phase epitaxy (MOVPE). The surface morphology and microstructures of the samples are characterized by atomic force microscopy. The full width at half maximum (FWHM) of the HVPE sample shows a W-shape and that of the MOVPE sample shows an M-shape plane with the degree of ϕ in the high-resolution x-ray diffraction (HRXRD) results. The surface morphology attributes to this significant anisotropic. HRXRD reveals that there is a significant reduction in the FWHM, both on-axis and off-axis for HVPE GaN are compared with the MOVPE template. The decrease of the FWHM of E2 (high) Raman scattering spectra further indicates the improvement of crystal quality after HVPE. By comparing the results of secondary-ion-mass spectroscope and photoluminescence spectrum of the samples grown by HVPE and MOVPE, we propose that C-involved defects are originally responsible for the yellow luminescence.

Direct spontaneous growth and interfacial structural properties of inclined GaN nanopillars on r-plane sapphire

The spontaneous growth of GaN nanopillars (NPs) by direct plasma-assisted molecular beam epitaxy on nitridated r-plane sapphire substrates has been studied. The emanation of metal-polarity NPs from inside an a-plane nonpolar GaN film was found to depend on both the substrate nitridation and the growth conditions. The density of NPs increased with increasing the duration of the nitridation process and the power applied on the radio-frequency plasma source, as well as the III/V flux ratio, while variation of the first two parameters enhanced the roughness of the substrate's surface. Transmission electron microscopy (TEM) techniques were employed to reveal the structural characteristics of the NPs and their nucleation mechanism from steps on the sapphire surface and/or interfacial semipolar GaN nanocrystals. Lattice strain measurements showed a possible Al enrichment of the first 5–6 monolayers of the NPs. By combining cross-sectional and plan-view TEM observations, the three-dimensional model of the NPs was constructed. The orientation relationship and interfacial accommodation between the NPs and the nonpolar a-plane GaN film were also elucidated. The NPs exhibited strong and narrow excitonic emission, suggesting an excellent structural quality.

Hydrothermally Grown Nonpolar a-Plane ZnO and Its Applications

There has been great interest in ZnO as promising alternative light emitting material. It has wide direct bandgap of 3.37 eV and large exciton binding energy of 60 meV. A diverse of growth methods have been developed for ZnO epitaxial layer growth such as laser-assisted plasma enhanced chemical vapor deposition (PECVD), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and hydrothermal synthesis. Among them, hydrothermal growth is the cost-effective, environmentally friendly, and low thermal budget requiring process compared to conventional MOCVD. Most of hydrothermally grown ZnOs are in the shapes of nanorod or nanowire along c-axis due to fast growth rate of c-plane. Commercially available LEDs are based on GaN related material system, which is grown along the polar c-axis of wurtzite crystal structure. In this crystal, there exists spontaneous and piezoelectric polarization inducing internal electric fields which result in spatial separation of electron and hole wave functions. This quantum confined stark effect (QCSE) causes reduction in radiative recombination efficiency under high current injection. As one of solution, extensive efforts on nonpolar a-plane GaN are exerted. Nonpolar a-plane ZnO with high crystalline quality was successfully grown on a-plane GaN tempelate by simple, low-cost, and mass producible hydrothermal method. Synthesized a-plane ZnO thin film was studied with SEM, TEM, XRD, AFM, and PL analyses. In this presentation, growth of hydrothermal ZnO and its applications will be introduced.

Effect of Basal-plane Stacking Faults on X-ray Diffraction of Non-polar (1120) a-plane GaN Films Grown on (1102) r-plane Sapphire Substrates

We report the effect of basal-plane stacking faults (BSFs) on X-ray diffraction (XRD) of non-polar (1120) a-plane GaN films with different SiNx interlayers. Complete SiNx coverage and increased three-dimensional (3D) to two-dimensional (2D) transition stages substantially reduce BSF density. It was revealed that the Si-doping profile in the Sidoped GaN layer was unaffected by the introduction of a SiNx interlayer. The smallest in-plane anisotropy of the (1120) XRD ω-scan widths was found in the sample with multiple SiNx layers, and this finding can be attributed to the relatively isotropic GaN mosaic resulting from the increase in the 3D-2D growth step. Williamson-Hall (WH) analysis of the (h0h0) series of diffractions was employed to determine the c-axis lateral coherence length (LCL) and to estimate the mosaic tilt. The c-axis LCLs obtained from WH analyses of the present study’s representative a-plane GaN samples were well correlated with the BSFrelated results from both the off-axis XRD ω-scan and transmission electron microscopy (TEM). Based on WH and TEM analyses, the trends in BSF densities were very similar, even though the BSF densities extracted from LCLs indicated that the values were reduced by a factor of about twenty.

Multiphonon resonant Raman scattering in non-polar GaN epilayers

Multiphonon resonant Raman scattering in non-polar a-plane (112¯0) GaN epitaxial layers grown on sapphire substrate were investigated. We report longitudinal optical phonon overtones up to seventh order in a-plane GaN epilayer, which has rarely been observed earlier in GaN. However, for high quality polar c-plane (0001) GaN epilayer and semi-polar r-plane (112¯2) GaN epilayer, resonant Raman spectra were not very prominent. Strong multiphonon resonant Raman scattering process is explained by exciton-mediated multiphonon Raman scattering and defect-induced Fröhlich interaction.

Effect of oxygen incorporation in a-plane GaN on p-type ohmic contact property

We report on the origin of the non-ohmic behavior of Ni/Au-based p-type contacts on a nonpolar a-plane GaN layer. The contact properties of Ga-polar c-plane GaN and nonpolar a-plane GaN are compared. While the Ga-polar c-plane shows ohmic-contact properties in the Ni/Au contact after heat treatment, the nonpolar a-plane shows rectifying characteristics both before and after heat treatment. We determined the reasons why the two planes show substantial differences in contact properties using various tools. We conclude that the differences originated from the oxygen incorporation preference resulting in gallium oxide formation at the interface of nonpolar a-plane GaN.

Anisotropic microstructure of hydrothermally-grown non-polar a-plane ZnO on a-plane GaN film

Nonpolar a-plane ZnO (a-ZnO) films with good crystalline quality have been achieved by a hydrothermal growth method on nonpolar a-plane GaN (a-GaN) films without any nucleation steps. The X-ray diffraction θ–2θ scan of the a-ZnO film grown on a-GaN film showed that the film was uniformly a-plane 112¯0 oriented. The a-ZnO films exhibited anisotropic peak broadenings of the on-axis rocking curves with X-ray in-beam orientations. The full width at half maximum of the on-axis rocking curve of nonpolar a-ZnO film along the c-axis direction was measured to be 2092arcsec, which is almost two times larger than the one along the m-axis direction (1219arcsec). The Williamson–Hall plot analysis of the symmetric 112¯0 and 224¯0 Bragg reflections showed that the mosaic tilt values were 0.647° and 0.407° along the c-axis and m-axis directions, respectively. It clearly indicates that large mosaic tilt along the c-axis in a-ZnO film is mainly responsible for the anisotropic peak broadenings of the on-axis rocking curves with respect to X-ray in-beam orientations.

Effect of additional hydrochloric acid flow on the growth of non-polar a-plane GaN layers on r-plane sapphire by hydride vapor-phase epitaxy

The effect of additional HCl flow on the growth of a-plane GaN layers on r-plane sapphire by hydride vapor-phase epitaxy was investigated. Upon increasing the additional HCl flow rate, the surface roughness of the a-plane GaN layers, as measured by atomic force microscopy, reduced. The crystal quality of a-plane GaN, however, deteriorated, as confirmed by high stacking fault density observed by transmission electron microscopy, relating to the increased nuclei density and mosaicity and a high full width at half maximum in the ω-scan X-ray rocking curve. These observations were attributed to the large difference of growth rate and etch rate along c-direction, and m-direction of a-plane GaN, which were originated from the surface energetics of the crystallographic planes of GaN.

Large third-order nonlinearity of nonpolar A-plane GaN film at 800 nm determined by Z-scan technology

We report an investigation on the optical third-order nonlinear property of the nonpolar A-plane GaN film. The film sample with a thickness of ~2 μm was grown on an r-plane sapphire substrate by metal-organic chemical vapor deposition system. By performing the Z-scan method combined with a mode-locked femtosecond Ti:sapphire laser (800 nm, 50 fs), the optical nonlinearity of the nonpolar A-plane GaN film was measured with the electric vector E of the laser beam being polarized parallel (//) and perpendicular (⊥) to the c axis of the film. The results show that both the third-order nonlinear absorption coefficient β and the nonlinear refractive index n2 of the sample film possess negative and large values, i.e. β// = −135 ± 29 cm/GW, n2// = −(4.0 ± 0.3) × 10−3 cm2/GW and β⊥ = −234 ± 29 cm/GW, n2⊥ = −(4.9 ± 0.4) × 10−3 cm2/GW, which are much larger than those of conventional C-plane GaN film, GaN bulk, and even the other oxide semiconductors.