Nonpolar M-plane Research Articles

Growth and characterization of polar and nonpolar ZnO film grown on sapphire substrates by using atomic layer deposition

We investigated the electrical and the optical properties of polar and nonpolar ZnO films grown on sapphire substrates with different crystallographic planes. High resolution X-ray results revealed that polar c-plane (0001), nonpolar m-plane (10-10) and a-plane (11-20) ZnO thin films were grown on c-plane, m- and r-sapphire substrates by atomic layer deposition, respectively. Compared with the c-plane ZnO film, nonpolar m-plane and a-plane ZnO films showed smaller surface roughness and anisotropic surface structures. Regardless of ZnO crystal planes, room temperature photoluminescence spectra represented two emissions which consisted of the near bandedge (~380nm) and the deep level emission (~500nm). The a-plane ZnO films represented better optical and electrical properties than c-plane ZnO, while m-plane ZnO films exhibited poorer optical and electrical properties than c-plane ZnO.

Semipolar $({\hbox{20}}\bar{{\hbox{2}}}\bar{{\hbox{1}}})$ InGaN/GaN Light-Emitting Diodes for High-Efficiency Solid-State Lighting

This work examines the effects of polarization-related electric fields on the energy band diagrams, wavelength shift, wave function overlap, and efficiency droop for InGaN quantum wells on various crystal orientations, including polar (0001) ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -plane), semipolar (202̅1̅), semipolar (202̅1̅), and nonpolar (101̅0̅) ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> -plane). Based on simulations, we show that the semipolar (202̅1̅) orientation exhibits excellent potential for the development of high-efficiency, low-droop light-emitting diodes (LEDs). We then present recent advancements in crystal growth, optical performance, and thermal performance of semipolar (202̅1̅) LEDs. Finally, we demonstrate a low-droop, high-efficiency single-quantum-well blue semipolar (202̅1̅) LED with an external quantum efficiency of more than 50% at 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Low Contact Resistance P-Type Electrode for Nonpolar m-Plane GaN

A low contact resistance p-type electrode for nonpolar m-plane GaN was obtained using Mg as the electrode material. Ga atoms diffused toward the Mg electrode after heat treatment. Because the Ga vacancies act as acceptors and form levels in the Schottky barrier, we consider hopping conduction through the Ga vacancy levels to be the main mechanism for the formation of the p-type ohmic contact to m-plane GaN. The specific contact resistance was reduced to 8×10-4 Ω cm2 by optimizing the heat treatment temperature and the Mg thickness. A significant reduction in the operating voltage of m-plane GaN LEDs was achieved by using this electrode technology.

Three-dimensional GaN templates for molecular beam epitaxy of nonpolar InGaN/GaN coaxial light-emitting diodes

This work highlights the development of three-dimensional (3D) GaN templates grown by metal organic vapor phase epitaxy (MOVPE). These templates are ideally suited for the subsequent growth of nonpolar (11¯00) m-plane InGaN-based coaxial wire/wall light-emitting diodes (LEDs) using molecular beam epitaxy (MBE). The use of MBE is expected to result in increased indium incorporation on the (11¯00) m-plane, compared with growth using MOVPE, which provides an attractive approach for the development of nonpolar green LEDs. While planar free-standing m-plane GaN substrates are prohibitively expensive (approximately ∼$500 per cm2), the coaxial LED approach offers an attractive lower-cost alternative. These nonpolar nanoscale LED templates are also free from threading dislocations and are expected to provide a number of benefits, including higher light extraction and a larger effective active region area. Previous work in our group has demonstrated the controlled growth of GaN nanowires using a catalyst-free selective-area MOVPE growth method. The present work extends this method to grow 3D GaN templates in various aperture geometries, resulting in smooth m-plane GaN using a scalable and industrially viable high-quality GaN growth technique. In addition to m-plane sidewalls, the authors also demonstrate semipolar (11¯01) inclined sidewalls. This orientation has been shown to incorporate indium at a higher rate during MOVPE growth, in comparison to both c-plane and m-plane.

Radial growth behavior and characteristics of m-plane In0.16Ga0.84N/GaN MQW nanowires by MOCVD

We demonstrate the growth of high-quality non-polar (m-plane) and semi-polar (r-plane) multiple quantum well (MQW) nanowires (NWs) for high internal quantum efficiency optoelectronic device applications. Highly aligned InGaN/GaN MQW layers were grown radially on the {1-100} sidewalls of c-axis n-GaN NWs on Si(111) substrates by pulsed flow metalorganic chemical vapor deposition (MOCVD) technique. The photoluminescence (PL) measurements revealed that the wavelength and intensity of the MQW structure with various pairs (5 to 20) were observed to be very stable and easily controlled with indium concentration. The cathodoluminescence (CL) spectrum of individual InGaN/GaN MQW NWs is dominated by the band-edge emission at 369 and 440 nm with a relatively homogeneous profile of parallel alignment. High-resolution transmission electron microscopy (HR-TEM) studies of radial InGaN/GaN MQW NWs measured along and zone axes revealed that the grown NWs were uniform with six non-polar m-plane facets without any dislocations and stacking faults. This kind of high-quality radial NWs are viable high surface area MQW structures for the enhanced efficiency of light emitting diodes.

Effect of temperature distribution on growth habit of AlN crystal

The effect of temperature distribution on AlN sublimation growth union to growth habit has been studied in this paper.With the larger temperature difference between sublimation region and crystalline region,the growth direction of AlN was along c-axis.On the contrary,the growth direction was along a-axis.Meantime,the temperature distribution also had an effect on crystal size and quality.The nonpolar m-plane AlN single crystals with the size about 1cm had been grown by sublimation and characterized.The growth furnace was designed by ourselves,this device contained two resistance heaters,heating sublimation region and crystalline region,respectively.

Electrical properties of Ag Schottky contacts to hydrothermally-grown polar and nonpolar bulk ZnO

The electrical properties of Ag Schottky contacts to hydrothermally-grown polar (Zn- and Opolar) and nonpolar (m-plane) bulk ZnO were investigated. Ohmic-like behavior for Zn-polar ZnO and rectifying behaviors for O-polar and m-plane ZnO were observed from current-voltage (I–V ) measurements. Schottky contacts to O-polar ZnO were found to have higher barrier heights and lower ideality factors than those to m-plane ZnO. Higher series resistances were found for both the Zn-polar and the m-plane ZnO compared to the O-polar ZnO probably due to the presence of the interfacial layer and interface states. The homogeneous barrier heights obtained from linear fitting to the effective barrier height vs. ideality factor plots were similar to those obtained from capacitance-voltage (C–V ) measurements. A higher degree of oxidation at the Ag-ZnO interface might occur for O-polar ZnO than for m-plane ZnO, increasing the barrier heights. The large dispersion in the capacitance for m-plane ZnO was related to the excess capacitance due to the interface states.

High-efficiency InGaN/GaN light-emitting diodes with electron injector

The efficiency in non-polar m-plane InGaN/GaN light-emitting diodes (LEDs) with an electron injector (EI) layer was investigated using multiband effective mass theory. The electron overflow is shown to decrease with the inclusion of an electron-blocking layer. In particular, with the inclusion of the EI layer, the electron overflow is significantly reduced even for a large bias voltage. In the case of the single EI layer, the layer thicker than 13 nm would be needed to reduce electron overflows substantially. However, it will be difficult to grow epitaxially high-quality thick single EI layer. Instead, we show that the quantum-well structure with the superlattice-like EI layer is more effective in reducing the ballistic and quasiballistic electron transport across the active region, compared with a thick single EI layer.

Epitaxial growth of non-polar m-plane ZnO thin films by pulsed laser deposition

Non-polar ZnO thin films were deposited on m-plane sapphire substrates by pulsed laser deposition at various temperatures from 300 to 700°C. The effects of growth temperature on surface morphology, structural, electrical, and optical properties of the films were investigated. All the films exhibited unique m-plane orientation indicated by X-ray diffraction and transmission electron microscopy. Based on the scanning electron microscopy and atomic force microscopy, the obtained films had smooth and highly anisotropic surface, and the root mean square roughness was less than 10nm above 500°C. The maximum electron mobility was ∼18cm2/Vs, with resistivity of ∼0.26Ωcm for the film grown at 700°C. Room temperature photoluminescence of the m-plane films was also investigated.

Time-resolved photoluminescence, positron annihilation, and Al0.23Ga0.77N/GaN heterostructure growth studies on low defect density polar and nonpolar freestanding GaN substrates grown by hydride vapor phase epitaxy

Time-resolved photoluminescence (TRPL) and positron annihilation measurements, as well as Al0.23Ga0.77N/GaN heterostructure growth by metalorganic vapor phase epitaxy were carried out on very low defect density, polar c-plane and nonpolar m-plane freestanding GaN (FS-GaN) substrates grown by hydride vapor phase epitaxy. Room-temperature photoluminescence (PL) lifetime for the near-band-edge (NBE) excitonic emission of the FS-GaN substrates increases with increasing positron diffusion length (L+); i.e., decreasing gross concentration of charged and neutral point defects and complexes. The best undoped c-plane FS-GaN exhibits record-long L+ being 116 nm. The fast component of the PL lifetime for its NBE emission increases with temperature rise up to 100 K and levels off at approximately 1.1 ns. The result implies a saturation in thermal activation of nonradiative recombination centers. The surface and interface roughnesses for a Si-doped Al0.23Ga0.77N/GaN/Al0.18Ga0.82N/GaN heterostructure are improved by the use of FS-GaN substrates, in comparison with the structure fabricated on a standard GaN template. The emission signals related to the recombination of a two-dimensional electron gas and excited holes are recognized for an Al0.23Ga0.77N/GaN single heterostructure grown on the c-plane FS-GaN substrate.

Study of Epitaxial Graphene on Non-Polar 6H-SiC Faces

We present epitaxial graphene (EG) growth on non-polar a-plane and m-plane 6H-SiC faces where material characterization is compared with that known for EG grown on polar faces. AFM surface morphology exhibits nanocrystalline graphite like features for non-polar faces, while polar silicon face shows step like features. This differing behavior is attributed to the lack of a hexagonal template on the non-polar faces. Non-polar faces also exhibit greater disorder and red shift of all Raman peaks (D, G and 2D) with increasing temperature. This is attributed to decreasing stress with increasing temperature. These variations provide evidence of different EG growth mechanisms on non-polar and polar faces, likely due to differences in surface free energy. We also present differences between a-plane ( ) EG and m-plane ( ) EG in terms of morphology, thickness and Raman characteristics.

Coaxial nanowire resonant tunneling diodes from non-polar AlN/GaN on silicon

Resonant tunneling diodes are formed using AlN/GaN core-shell nanowire heterostructures grown by plasma assisted molecular beam epitaxy on n-Si(111) substrates. By using a coaxial geometry, these devices take advantage of non-polar (m-plane) nanowire sidewalls. Device modeling predicts non-polar orientation should enhance resonant tunneling compared to a polar structure, and that AlN double barriers will lead to higher peak-to-valley current ratios compared to AlGaN barriers. Electrical measurements of ensembles of nanowires show negative differential resistance appearing only at cryogenic temperature. Individual nanowire measurements show negative differential resistance at room temperature with peak current density of 5 × 105 A/cm2.

Deep traps in nonpolar m-plane GaN grown by ammonia-based molecular beam epitaxy

Deep level defects in nonpolar m-plane GaN grown by ammonia-based molecular beam epitaxy were characterized using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) and compared with polar c-plane GaN that was grown simultaneously in the same growth run. Significant differences in both the levels present and their concentrations were observed upon comparison of both growth orientations. DLTS revealed electron traps with activation energies of 0.14 eV, 0.20 eV, and 0.66 eV in the m-plane material, with concentrations that were ∼10−50 × higher than traps of similar activation energies in the c-plane material. Likewise, DLOS measurements showed ∼20 × higher concentrations of both a CN acceptor-like state at EC − 3.26 eV, which correlates with a high background carbon concentration observed by secondary ion mass spectroscopy for the m-plane material [A. Armstrong, A. R. Arehart, B. Moran, S. P. DenBaars, U. K. Mishra, J. S. Speck, and S. A. Ringel, Appl. Phys. Lett. 84, 374 (2004)], and the VGa-related state level at EC − 2.49 eV, which is consistent with an enhanced yellow luminescence observed by photoluminescence. The findings suggest a strong impact of growth dynamics on the incorporation of impurities and electrically active native point defects as a function of GaN growth plane polarity.

Vertical nonpolar growth templates for light emitting diodes formed with GaN nanosheets

We demonstrate that nonpolar m-plane surfaces can be generated on uniform GaN nanosheet arrays grown vertically from the (0001)-GaN bulk material. InGaN/GaN multiple quantum wells (MQWs) grown on the facets of these nanosheets are demonstrated by cross-sectional transmission electron microscopy. Owing to the high aspect ratio of the GaN nanosheet structure, the MQWs predominantly grow on nonpolar GaN planes. The results suggest that GaN nanosheets provide a conduction path for device fabrication and also a growth template to reduce the piezoelectric field inside the active region of InGaN-based light emitting diodes.

A novel approach for controlled oriented growth of non-polar m-plane ZnO thin films via low temperature chemical solution route

We report a successful synthesis of high quality non-polar m-plane (100) ZnO thin films via a novel and facile two step solution growth method. The reproducible growth of polarity free films is achieved by manipulating the seed/substrate characteristics, solution chemistry and growth conditions. Based on time resolved growth reaction FESEM analysis, three step growth phenomena viz. dissolution, recrystallization & primary growth and in situ nucleation/crystallization & secondary growth is proposed and explained.

The Role of Growth-Pressure on the Determination of Anisotropy Properties in Nonpolar m-Plane GaN

This study reports the role of growth-pressure on the determination of anisotropy properties in nonpolar m-plane GaN. In the high-pressure-growth samples, unrelieved lattice misfit strain lead to larger anisotropic in-plane strains, a striated surface, lower densities of basal-plane stacking faults (BSF) and prismatic stacking faults (PSF), and a smaller anisotropic polarization. In contrast, the low-pressure-growth sample shows the opposite trend. It is suggested that the growth pressure determines the degree of relaxation of lattice misfit strain, which in turn affects the anisotropic in-plane strains, striation feature, densities of BSF and PSF, and degree of polarization. The research results can be used to optimize the growth of nonpolar m-plane GaN.

Nonpolar light emitting diode made by m-plane n-ZnO/p-GaN heterostructure

Nonpolar (100) m-plane n-ZnO/p-GaN light-emitting-diodes (LEDs) were grown by chemical vapor deposition on p-GaN templates which was grown by metalorganic chemical vapor deposition on LiAlO2(100) substrate. Direct current (DC) electroluminescence (EL) measurements yielded a peak at 458nm. The EL peak position was independent of drive current and a full width of half maximum (FWHM) of 21.8 nm was realized at 20mA. The current-voltage characteristics of these diodes showed a forward voltage (Vf) of 6V with a series resistance of 2.2 × 105 Ω.

Surface striation, anisotropic in-plane strain, and degree of polarization in nonpolar m-plane GaN grown on SiC

We report the roles of island coalescence rate and strain relaxation in the development of anisotropic in-plane strains, striation feature, and subsequent degree of polarization in NH3-flow-rate-dependent m-plane GaN. In the high-NH3-flow-rate samples, the results of cathodoluminescence, polarized Raman and in situ optical reflectance measurements reveal that a slower coalescence and unrelieved lattice misfit strain lead to larger anisotropic in-plane strains, striated surface and luminescence patterns, and a lower density of basal-plane stacking fault (BSF) and prismatic stacking fault (PSF). In contrast, a lower NH3 flow rate leads to more rapid island coalescence and fully relaxed lattice misfit strain such that relaxed in-plane strains, a reduced striation surface, and a higher density of BSF and PSF are observed. It is suggested that the anisotropic in-plane strains, striation feature, and BSF and PSF density are consequences of how rapidly coalescence occurs and the degree of relaxation of lattice misfit strain. In addition, the simulation results of the k · p perturbation approach confirm a larger anisotropic strain results in a smaller degree of polarization. The research results provide important information for optimized growth of nonpolar III-nitrides.

Epitaxy of m-plane GaN on nanoscale patterned c-plane sapphire substrates

Nonpolar ( 10 1 ¯ 0 ) m-plane GaN epilayer was grown on nanoscale patterned c-plane sapphire substrates by MOCVD. The nanoscale patterned sapphire substrates were fabricated by natural lithography and dry etching methods. The nanopatterns are defined particularly with no c-plane surfaces exposed. The ( 10 1 ¯ 0 ) m-plane GaN epilayer was characterized by X-ray diffraction and atomic force microscope, which indicated the FWHM of the rocking curve is 316 arcsec and the rms surface roughness is 0.3 nm, respectively. The SEM and TEM studies reveal that the m-plane GaN grains nucleate on the exposed n-plane facets of the etched c-plane sapphire substrates.

P-type non-polar m-plane ZnO films grown by plasma-assisted molecular beam epitaxy

Non-polar ZnO thin films were grown epitaxially on m-plane sapphire substrates using plasma-assisted molecular beam epitaxy. The film is (1 0 1¯ 0) oriented ( m-plane) as identified by the X-ray diffraction pattern. The surface of ZnO film shows highly anisotropic morphology with stripes elongated along the c-axis. The nonintentionally doped non-polar ZnO film was grown under oxygen-rich condition, which exhibits weak p-type conductivity with a hole concentration of 1.3×10 16 cm −3, a Hall mobility of 0.314 cm 2 V −1 s −1, and a resistivity of 1536 Ω cm. Room-temperature photoluminescence of the p-type non-polar ZnO shows a strong UV emission at ∼3.307 eV and negligible deep level emission.