Nonpolar GaN Research Articles

Impact of Surface Treatments and Post-Deposition Annealing Upon Interfacial Property of ALD-Al₂O₃ on a-Plane GaN

Optimization of interface characteristics between dielectric and non-polar GaN surface is very important and urgent for vertical GaN MOS device whose channel is perpendicular to the conventional cplane. In this work, the effects of piranha cleaning and N2 post deposition annealing (PDA) to the interface between atomic-layer-deposited (ALD)-Al 2 O 3 and a-plane GaN samples were comprehensively investigated by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and photo-assisted capacitance-voltage (C-V) measurements. The piranha cleaning and N2 annealing can improve interface characteristics through the reduction in surface roughness and Ga-O bonds, respectively. Therefore, the frequency dispersion and hysteresis are nearly suppressed with a low interface trap quantity (Qit) of 4.1 x 10 11 cm -2 and a low average interface state density (Dit) of 2.04 x 10 11 cm -2 ·eV -1 from photoassisted C-V measurements, showing the great promise of utilizing piranha pretreatment, buffered oxide etch (BOE) dip, and N 2 annealing as an effective route to improve the vertical GaN MOS interface properties.

Investigation of catalyst-assisted growth of nonpolar GaN nanowiresviaa modified HVPE process

The growth of nonpolar GaN nanowires along the [101[combining macron]0] orientation has been demonstrated via a modified hydride vapor phase epitaxy (HVPE) process using GaCl3 and NH3 as precursors. The morphology and structure evolution as a dependence of the growth parameters was thoroughly studied to elucidate the nucleation and crystallization of nonpolar GaN nanowires. It has been found that the V/III ratio and temperature are critically important for the formation of high-quality nonpolar GaN nanowires. The existence of a cubic GaN (c-GaN) transition layer between the Au catalyst and hexagonal GaN (h-GaN) nonpolar nanowires was demonstrated by high-resolution transmission electron microscopy (HRTEM) characterization, which plays an important role in the initial nucleation of nonpolar GaN nanowires and the formation of stacking faults (SFs) in the GaN nanowires grown at lower temperature. Optical investigations show that the defect-related visible emission of nonpolar GaN nanowires is closely related to the growth process and can be selectively tailored. The synthetic strategy using GaCl3 as the Ga precursor to study the vapor phase epitaxy process in this work will provide a simple and efficient approach to obtain nonpolar GaN nanowires and will thus pave a solid way for fundamental research on high-quality nonpolar GaN nanowires in optoelectronic nanodevices.

Temperature-dependence of Cl2/Ar ICP-RIE of polar, semipolar, and nonpolar GaN and AlN following BCl3/Ar breakthrough plasma

The authors report a comprehensive investigation of temperature-dependence of inductively coupled plasma reactive ion etching (ICP-RIE) of polar (0001), semipolar (11−22), and nonpolar (11−20) GaN and AlN, in the temperature range of 22–205 °C. The main objective is to study the effect of ICP etching near and beyond the boiling point of the volatile etch end-products: GaCl3 (201 °C) for GaN and AlCl3 (180 °C) for AlN. High-temperature ICP-RIE is beneficial in quicker removal of surface oxides and may permit the use of a single-step Cl2/Ar ICP-RIE for etching all orientations of GaN and AlN. However, the best results are still obtained with a combination of BCl3-plasma based surface oxide removal pretreatment and Cl2/Ar ICP-RIE etching, which provides a constant etch rate with a smooth surface morphology irrespective of the etching temperature.

High Quality, Mass‐Producible Semipolar GaN and InGaN Light‐Emitting Diodes Grown on Sapphire

The heteroepitaxy efforts to grow semipolar and nonpolar GaN on foreign substrates of the past 20 years are reviewed and summarized. With the demonstration of three representative semipolar GaN grown on sapphire, the capability to produce semipolar GaN with any orientation on sapphire is exhibited. Also, a unique growth technology called facet‐engineered orientation‐controlling growth to eliminate the stacking faults (SFs) in semipolar GaN is developed and SF‐free (201) GaN grown on sapphire is presented, demonstrating the capability of producing device‐quality, large‐area semipolar GaN. InGaN green light‐emitting diodes (LEDs) grown on the SF‐free semipolar (201) GaN/sapphire templates are performed with much higher external quantum efficiency in comparison with the typical semipolar/nonpolar LEDs heteroepitaxially grown on foreign substrates reported before.

Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds

Abstract GaN crystals were grown on non-polar and semi-polar HVPE GaN seeds by basic ammonothermal method. Stress distributions were investigated in cross-section of (1 1 −2 0) plane, (1 0 −1 0) plane, (2 0 −2 1) plane and (1 0 −1 1) plane GaN crystal. The cathodoluminescence (CL) images show cross-section information clearly and each examined object consisted of hydride vapor phase epitaxy (HVPE) seed and ammonothermal GaN (Am-GaN). The impurity concentration and free carrier concentration were estimated by secondary-ion mass spectroscopy (SIMS) and Hall. Moreover, Raman spectroscopy was used for studying stress distribution in the cross section. Shifts of E2(high) phonon lines were analyzed to determine stress. Our results indicate that the stress is about 35 MPa in bulk GaN and the stress is less than 60 MPa in the interface of Am-GaN.

Comparative Study of ZnO Nanostructures Grown on Variously Orientated GaN and AlxGa1−xN: The Role of Polarization, and Surface Pits

Through comparing ZnO directly grown on the substrates of a-plane, c-plane, and (11-22) plane GaN and AlxGa1−xN (0.06 ≤ x ≤ 1), the roles of different factors that may influence growth have been studied. Seeded by surface pits, ZnO nanowire (NW) preferentially grew along the polarized direction on top of the nonpolar GaN (laterally aligned), polar GaN and AlGaN (vertically aligned), and semipolar GaN (obliquely upward aligned). Nanosheets were easily formed when the polarized surface of the AlGaN film was not intact. The kinetic effect of polarization must be considered to explain the high aspect ratio of NWs along the polarized direction. It was found that dislocation affected NW growth through the surface pits, which provided excellent nucleation sites. If the surface pits on GaN could be controlled to distribute uniformly, self-organized ZnO NW array could be controllably and directly grown on GaN. Moreover, surface pits could also seed for nanosheet growth in AlN, since Al(OH)4− would presumably bind to the Zn2+ terminated surface and suppress the kinetic effects of polarization.

Realization of thin-film m-plane InGaN laser diode fabricated by epitaxial lateral overgrowth and mechanical separation from a reusable growth substrate.

A nonpolar edge emitting thin film InGaN laser diode has been separated from its native substrate by mechanical tearing with adhesive tape, combining the benefits of Epitaxial Lateral Overgrowth (ELO) and cleavability of nonpolar GaN crystal. The essence of ELO is mainly to weakening strength between native substrate and the fabricated laser device on top of it. We report a 3 mm long laser bar removed from its native GaN substrate. We confirmed edge emitting lasing operation after cleaving facets on a separated thin bar. Threshold current density of the laser was measured to be as low as 2.15 kA/cm2.

Intersubband Transitions in Nonpolar GaN-based Resonant Phonon Depopulation Multiple-Quantum Wells for Terahertz Emissions

We investigate the polarization effect in intersubband transitions in polar and nonpolar GaN-based multiple-quantum well (MQW) structures for terahertz (THz) emissions by using systematic comparisons and design a nonpolar GaN/Al0.2Ga0.8N two-well-based MQW structure with an emitting photon of 7.27 THz (30.07 meV). Its lower energy separation (92.7 meV) matches the resonant phonon depopulation condition for better population inversion. It shows a lower threshold current density Jth at all temperatures (1.548 kA/cm2 at 90 K) and a higher output power of up to 86.1 mW at 5.8 K and 33.6 mW at 100 K. Our results for the polar GaN MQW are very close to the experimental data in the literature. We find that the Jth of the nonpolar GaN MQW increases more slowly than that of the polar GaN MQW as temperature increases, indicating the nonpolar GaN MQW may be a worth-trying direction for improving the operation temperature. These results can provide meaningful references for the design and fabrication of nonpolar GaN-based THz MQW or quantum cascade structures.

Optical and structural characterization of GaInN/GaN multiple quantum wells grown on nonpolar a-plane GaN templates by metalorganic vapor phase epitaxy

Typical multi quantum well (MQW) grown on nonpolar a-plane GaN templates shows many pits composed of several semipolar planes, and these pits emit at different wavelengths due to the variation of indium incorporations into the QW. In this study, a surface-recovery GaN layer and high-temperature grown barrier were introduced to improve the a-plane MQW quality. We succeeded in decreasing the pits prior to the MQW growth and improving the interfaces of MQW. The structural and optical properties of the MQW were significantly improved.

Improved crystal quality of nonpolar a-plane GaN based on the nano pattern formed by the annealed thin Ni layer

Abstract A method to improve the nonpolar (11–20) a-plane GaN crystalline quality is investigated. The promoted crystal quality of a-plane GaN is achieved through growing on the a-plane GaN template with nano pattern. For on-axis (11–20) plane X-ray rocking curves, the full width at half maximum values are remarkably reduced from 2834 to 1210 arcsec, 2868 to 780 arcsec along the m-axis and the c-axis directions, respectively. Also, the improved crystalline quality results in the better optical properties according to the photoluminescence and Raman measurements. Moreover, the cross-sectional transmission electron microscopy is also used to study the mechanisms of dislocation reduction. Evidently, this technique is effective in promoting crystal quality of nonpolar a-plane GaN.

Characterization and optimization of AlN nucleation layer for nonpolar a-plane GaN grown on r-plane sapphire substrate

Abstract Nonpolar (11–20) a-plane GaN films with AlN nucleation layer were grown on (10–12) r-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The crystalline and surface qualities of a-plane GaN were found to closely depend on the growth conditions of AlN nucleation layer. With decreasing AlN growth temperature, the AlN grains became larger and sparser, which significantly reduced the defects density of a-plane GaN films. The growth time of the low temperature AlN layer was further optimized, and a-plane GaN films with reduced anisotropy in the crystalline quality, surface morphology and in-plane strains were achieved. It was found that the lateral growth lengths along different directions of GaN could be modulated by the growth time of AlN nucleation layer, thus changing the anisotropy of a-plane GaN films.

Monolithic multiple colour emission from InGaN grown on patterned non-polar GaN

A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated. Such a multiple-facet structure can be achieved by means of overgrowth on non-polar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates. Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions. Detailed cathodoluminescence measurements and excitation-power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs. It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.

Growth behavior of ammonothermal GaN crystals grown on non-polar and semi-polar HVPE GaN seeds

This paper investigated the growth behaviors of ammonothermal GaN crystals grown on non-polar and semi-polar HVPE GaN seeds.

Reduction in crystalline quality anisotropy for non-polar a-plane GaN directly grown on titanium patterned sapphire substrate

Nonpolar a-plane GaN films were directly grown on titanium patterned sapphire substrates (Ti-PSS) by metalorganic chemical vapor deposition (MOCVD). It is demonstrated that the GaN film grown on Ti-PSS has superior surface quality and less surface pits than that on flat sapphire substrate. More importantly, the anisotropic behavior of the X-ray rocking curve-full width at half maximum values for the on-axis reflections were significantly improved. It is found that, the increased mosaic block size along m-direction could be the main reason for the reduction in the crystalline quality anisotropy. This work provides a simple and effective method to improve the crystal quality of non-polar GaN films.

Non-polar (11-20) GaN grown on sapphire with double overgrowth on micro-rod/stripe templates

Non-polar (11-20) GaN with low defect density can be achieved on sapphire by means of an overgrowth on a micro-rod template recently developed, or on a conventional 〈1-100〉-oriented stripe template. The overgrowth on stripes block BSFs in the nonpolar GaN more effectively, but it is difficult to obtain a flat GaN surface due to its anisotropic pattern for overgrowth. The overgrowth on micro-rods can significantly reduce dislocations, simultaneously maintaining a smooth sample surface. Very recently, we develop a double overgrowth approach to grow (11-20) GaN on sapphire, i.e. first overgrowth on stripes and second overgrowth on micro-rods. The double overgrowth technique successfully utilizes the strengths of the two kinds of overgrowths, further improving crystal quality, which will be a very promising approach to achieve high quality (11-20) GaN for large-scale industrial production.

First-principles study of polar, nonpolar, and semipolar GaN surfaces during oxide vapor phase epitaxy growth

Stable structures of polar, nonpolar, and semipolar GaN surfaces undergoing oxide vapor phase epitaxy (OVPE) growth were examined with first-principles calculations. The relationships between temperature and pressure growth conditions and stable surface structures are described in surface phase diagrams. The results revealed that an O atom was stably incorporated into the N vacancy site in the N-polar and nonpolar surfaces. The desorption energy of the O atom from the GaN surfaces was estimated to be about 7 eV or higher. This indicates that the O atom did not readily desorb from the GaN surfaces under OVPE growth conditions at 1500 K. The desorption energy from the surface was the highest and that from the surface was the smallest among the calculated values. There was no significant difference in desorption energy among other surfaces.

Deformation and fracture in (0001) and (10-10) GaN single crystals

ABSTRACTIndentation techniques were utilised to induce deformation on polar (0001) c-plane and non-polar m-plane GaN single crystal. Cracking was more sensitively dependent on the orientation of the indenter tip, compared to hardness. The indentation-induced plastic deformation and fracture sequences were studied by cathodoluminescence imaging and optical microscopy, respectively. Polar GaN was harder than non-polar, while pop-in discontinuities occurred at lower loads in polar than non-polar GaN. Dislocation arrangements were more isotropic at the polar than the non-polar orientation. Polar GaN was more susceptible to cracking compared to non-polar. Indentation at the high load regime fostered radial and lateral crack formation at both indenter orientations in polar GaN. Post-indentation lateral crack propagation was observed in situ in polar GaN.This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.

Valence band tomography of wurtzite GaN by spectroscopic ellipsometry

The effective hole masses of wurtzite GaN are determined experimentally by an all-optical method. Therefore, we measure interband transitions as a function of the degenerate free-electron density and analyze the different many-body contributions arising from the Burstein–Moss effect and band-gap renormalization. When the known anisotropic effective electron mass is taken into account, the shape of the valence band can be reconstructed. Here, nonpolar GaN thin films doped with germanium or silicon at 9 × 1018 ≤ n ≤ 1.5 × 1020 cm−3 are investigated. Effective hole masses of , for the A valence band and for the B valence band are found (for 0.3 nm−1 < |k| < 2 nm−1) .

A systematic investigation of radiative recombination in GaN nanowires: The influence of nanowire geometry and environmental conditions

Nanowires intrinsically exhibit a large surface area, which makes them sensitive to physical and chemical interactions with their environment. Here, we investigate the surface recombination at m-plane side walls of selective area-grown GaN nanowires on Si (111) subjected to different environmental conditions. In contrast to the stable photoluminescence observed from c-plane surfaces of films, nanowires exhibit a distinct time-dependent photoluminescence quenching by over 90% within the time scale of seconds in the presence of air or dissociated liquids. This quenching is most pronounced for 50 nm diameter nanowires with interwire spacings larger than 500 nm due to internal electric field and external light field distributions. Ion- and pH-sensitive measurements, in combination with an externally applied voltage, allow the assignment of this effect to anions from the surroundings to accumulate at the nonpolar GaN side walls of the UV-exposed GaN nanowires. The decay times of the luminescence signal follow the dynamics of valence band holes, which deplete GaN surface states and positively charge the nanowire surfaces. This, in turn, induces the buildup of a capacitive anion shell around the nanowires, leading to an enhanced nonradiative surface recombination of photo-generated charge carriers from the GaN nanowire. In the absence of UV light, a recovery of the photoluminescence signal within tens of minutes indicates the dissolution of the anionic shell via charge balancing. The impact of light-induced electronic and ionic charge redistribution on photocarrier recombination represents an important mechanism of function for GaN nanowire-based devices, ranging from sensors to photocatalysts.

Overgrowth and strain investigation of (11\u201320) non-polar GaN on patterned templates on sapphire

Non-polar (11–20) GaN with significantly improved crystal quality has been achieved by means of overgrowth on regularly arrayed micro-rod templates on sapphire in comparison with standard non-polar GaN grown without any patterning processes on sapphire. Our overgrown GaN shows massively reduced linewidth of X-ray rocking curves with typical values of 270 arcsec along the [0001] direction and 380 arcsec along the [1–100] direction, which are among the best reports. Detailed X-ray measurements have been performed in order to investigate strain relaxation and in-plane strain distribution. The study has been compared with the standard non-polar GaN grown without any patterning processes and an extra non-polar GaN sample overgrown on a standard stripe-patterned template. The standard non-polar GaN grown without involving any patterning processes typically exhibits highly anisotropic in-plane strain distribution, while the overgrown GaN on our regularly arrayed micro-rod templates shows a highly isotropic in-plane strain distribution. Between them is the overgrown non-polar GaN on the stripe-patterned template. The results presented demonstrate the major advantages of using our regularly arrayed micro-rod templates for the overgrowth of non-polar GaN, leading to both high crystal quality and isotropic in-plane strain distribution, which is important for the further growth of any device structures.