Semipolar Orientations Research Articles

Effects of substrate self-bias and nitrogen flow rate on non-polar AlN film growth by reactive sputtering

The effects of the N2 gas flow ratio and the substrate self-bias of reactive sputtering on the polarity of AlN film on various planar sapphire substrates were investigated to obtain a non-polar oriented AlN film. On a c-plane sapphire substrate, substrate self-bias enhanced c-plane AlN growth and Al termination at the surface, as confirmed by X-ray diffraction (XRD) and time-of-flight low-energy atom scattering spectroscopy. In addition, the AlN film deposited at an N2 gas:Ar sputtering gas ratio of 14% with substrate self-bias had high crystallinity with an XRD ω-rocking curve of 17 arcsec for the 0002 diffraction. In contrast, the AlN films on a- and r-plane sapphire substrates had semi-polar and non-polar plane orientations, respectively. However, on the a-plane sapphire substrate, the c-plane-orientated AlN phase was also observed, which was increased under substrate self-bias condition and an N2 flow rate below 30%. Preferable conditions for non-polar AlN growth by reactive sputtering were non-substrate self-bias and a high N2 flow rate of 30% or more.

Thermodynamic analysis of semipolar GaN and AlN under metalorganic vapor phase epitaxy growth conditions

We perform an improved method of thermodynamic analysis for semipolar and surfaces of GaN and AlN to elucidate the relationship between growth conditions and semipolar and polar surfaces during metalorganic vapor phase epitaxy (MOVPE). The calculations for H2 carrier gas suggest that for both GaN and AlN the maximum temperature for growth on surfaces is higher than that for growth on surfaces. On the other hand, the maximum temperature for growth on GaN surfaces is comparable to that for growth on GaN surfaces, while the maximum temperature for the growth on AlN surfaces is higher than that for growth on AlN surfaces. These results could be used to provide favorable conditions for growth of group-III nitrides along a semipolar orientation during MOVPE.

Новый метод получения объемных кристаллов AlN, GaN и AlGaN с использованием гибридных подложек SiC/Si

The paper presents the main provisions of the new method of growing bulk, with a thickness of 100 μm or more, single-crystal AlN, AlGaN and GaN films on silicon poles with a silicon carbide buffer layer with their subsequent separation from the Si substrate. The main essence of this method is the combination of the method of chloride-hydride epitaxy, which ensures high growth rates of III-nitride layers using as substrate for growth, Si substrate with a buffer layer of a nanoscale atomic SiC film. The Si substrate with a SiC layer grown by the substitution method has a number of structural, physical, and chemical features as compared to SiC layers grown on Si by standard methods. It is shown that it is this feature that makes it possible to grow on its surface thick, uncracked layers of AlN, AlGaN and GaN, followed by their simple separation from the substrate. In this work, single-crystal uncracked layers were grown: AlN up to 300 microns thick; AlGaN thickness up to 400 microns; GaN thickness up to 200 microns; GaN semipolar (112 ̅4) orientation up to 35 microns.

Absolute surface energies of semipolar planes of AlN during metalorganic vapor phase epitaxy growth

We propose a new approach for calculating absolute surface energies of semipolar planes of AlN under metalorganic vapor phase epitaxy (MOVPE) growth conditions. The calculations using wedge-shape geometries demonstrate that the stable surface orientation depends on Al chemical potential. The calculated absolute surface energies of semipolar AlN112¯2 and 11¯01 are lower than that of N-polar AlN0001¯ surface under Al-rich condition. On the other hand, the surface energy of N-polar AlN0001¯ is lower than those semipolar surfaces under N-rich condition. The results on the orientation dependence of surface stability could provide valuable insights for the epitaxial growth conditions of AlN along semipolar orientations during the MOVPE growth.

Highly c-axis oriented crystal growth induced giant unusual saturable absorption behavior in wet chemically grown ZnO thin films

Wet-chemical sol-gel spin coating technique was considered to fabricate ZnO thin films on bare glass substrates with different thickness values (30 nm, 90 nm, 110 nm, 150 nm). The XRD reflections showed a transition from semi-polar crystal growth orientation to polar crystal growth orientation when the thickness was reduced to 30 nm. The nonlinear characteristics of prepared nano-films were quantized by performing open aperture and closed aperture Z-scan measurements under a continuous wavelength regime. The open aperture Z-scan transmittance curves showed a remarkable transition from normalized valley to normalized peak in 30 nm thick ZnO film. The presence of normalized valley and peak were attributed to reverse saturable absorption (RSA) and saturable absorption (SA) behavior respectively. The RSA behavior was attributed to well known two-photon absorption (TPA) mechanisms and the SA behavior was due to the reduction in electron-hole wave function overlapping along c-axis. The closed aperture Z-scan measurement of 30 nm thick ZnO film revealed the violation of two parabolic band (TPB) and Kramer-Kronig relations by exhibiting self-focusing mechanism. The self-focusing mechanism was attributed to thermal effects due to the smaller thickness of ZnO. However, self-defocusing behavior was observed in 90 nm, 110 nm, and 150 nm thick ZnO films.

Impact of thermal treatment on the growth of semipolar AlN on m-plane sapphire

The interest in semipolar orientations has been increasing because the reduced piezoelectric field can improve the performance of nitride-based optoelectronic devices. However, the crystalline quality of semipolar AlN on m-plane sapphire is still not good enough to realize light emitters with sufficiently high efficiency. We performed high-temperature annealing on AlN on m-plane sapphire to improve the crystalline quality. For (10-1-3) and (11-22) AlN on m-plane sapphire, the crystalline quality improved as the annealing temperature was increased up to 1700 °C, whereas beyond 1750 °C the AlN layer started to deteriorate and desorb. The crystalline quality was further improved by additional growth of AlN. In addition, X-ray rocking curve measurements and transmission electron microscopy confirmed that the density of stacking faults was reduced after the additional growth of AlN.

Well-ordered ZnO nanowires with controllable inclination on semipolar ZnO surfaces by chemical bath deposition

Controlling the formation of ZnO nanowire (NW) arrays on a wide variety of substrates is crucial for their efficient integration into nanoscale devices. While their nucleation and growth by chemical bath deposition (CBD) have intensively been investigated on non-polar and polar c-plane ZnO surfaces, their formation on alternatively oriented ZnO surfaces has not been addressed yet. In this work, the standard CBD technique of ZnO is investigated on and semipolar ZnO single crystal surfaces. A uniform nanostructured layer consisting of tilted ZnO NWs is formed on the surface while elongated nanostructures are coalesced into a two-dimensional compact layer on the surface. By further combining the CBD with selective area growth (SAG) using electron beam-assisted lithography, highly tilted well-ordered ZnO NWs with high structural uniformity are grown on the patterned surface. The structural analysis reveals that ZnO NWs are homoepitaxially grown along the polar c-axis. The occurrence of quasi-transverse and -longitudinal optical phonon modes in Raman spectra is detected and their origin and position are explained in the framework of the Loudon’s model. These results highlight the possibility to form ZnO NWs on original semipolar ZnO surfaces. It also opens the way for comprehensively understanding the nucleation and growth of ZnO NW arrays on poorly and highly textured polycrystalline ZnO seed layers composed of nanoparticles with a wide range of non-polar, semipolar, and polar plane orientations. Eventually, the possibility to tune both the inclination and dimensions of well-ordered ZnO NW arrays by using SAG on semipolar surfaces is noteworthy for photonic and optoelectronic nanoscale devices.

Surface, structural and optical properties of AlN thin films grown on different face sapphire substrates by metalorganic chemical vapor deposition

AlN thin films grown on different face sapphire substrates by metalorganic chemical vapor deposition were studied comprehensively by a variety of techniques including high resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS), Raman scattering (RS) and theoretical calculations. Our results showed that substrate orientation has a drastic effect on the initial growth of AlN. The AlN films showed dominant semipolar (101) orientation but with slightly weaker individual features. Optical reflectance measurements confirmed that AlN films grown on different surface orientation sapphire substrates at the same time possessed different film thicknesses. It was found that the surface orientation also has a major influence on the dislocation density, crystallite size, and surface oxygen incorporation. Raman line width exhibited good correlation with XPS measurements. The combined analyses led to a better understanding of the variation in the quality of the AlN films grown on different faces of sapphires.

(Invited) High-Speed GaN-Based Micro-Scale Light-Emitting Diodes for Visible-Light Communication

The increasing demand for wireless data communication and popularity of solid-state lighting has prompted research into visible-light communication (VLC) systems using GaN-based light-emitting diodes (LEDs). VLC is a promising candidate for next-generation (5G and beyond) light-fidelity (Li-Fi) network systems. To support multi-Gb/s data rates, VLC systems will require efficient LEDs with large modulation bandwidths. Conventional lighting-class LEDs cannot achieve high-speed operation due to their large chip size, large active region volume, and phosphor-converted output. Conversely, micro-scale LEDs (micro-LEDs) offer a viable path to high-speed operation due to their lower parasitic capacitance and ability for operation at high current densities where the carrier lifetime is significantly reduced. Furthermore, conventional c-plane LEDs suffer from polarization-related electric fields, which reduce the overlap between the electron and hole wave functions and lower the carrier recombination rate. Since modulation bandwidth is proportional to the carrier recombination rate (inversely proportional to the carrier lifetime), the overlap between the wave functions should be maximized for high-speed operation. Nonpolar and semipolar orientations have significantly reduced polarization effects and wave function overlaps approaching unity. These orientations can enable high-efficiency LEDs with simultaneously large modulation bandwidths, especially at relatively low current densities. In this work, we introduce VLC and discuss progress on the growth, fabrication, and characterization of high-speed micro-LEDs. Polar (0001), nonpolar (1010), and semipolar (2021) InGaN/GaN micro-LEDs on free-standing GaN substrates are investigated for their small-signal modulation characteristics as a function of current density, temperature, device area, and active region design. We demonstrate that the modulation bandwidths are well correlated with the wave function overlap in the active region by examining the orientation-dependence of the bandwidth. Record modulation bandwidths for GaN-based LEDs above 1 GHz are achieved for the nonpolar and semipolar orientations. We also show the modulation characteristics of an electrically injected nanowire-based LED, which achieves a modulation bandwidth of 1.2 GHz. Finally, we present a small-signal modeling method for determining the RC characteristics, differential carrier lifetime, carrier escape lifetime, and injection efficiency of the LEDs under electrical injection and discuss the results in the context of efficiency droop. Figure 1: Bandwidth vs. current density for the nonpolar LEDs (open black triangles), semipolar LED (open blue circles), and polar c-plane LED (open green squares) compared to other reported polar c-plane LEDs on sapphire or with a flip-chip design and compared to semipolar (1122) LEDs. The bandwidth characteristics for a nanowire-based LED are also included. Also shown are the crystal orientations used in the orientation-dependent bandwidth study. Figure 1

Rapid Synthesis of Semipolar ZnO Nanorod Arrays on M-Sapphire by Microwave-Assisted Chemical Bath Deposition

ZnO is a wide-band gap (3.37 eV) II–VI semiconductor with large exciton binding energy of 60 meV which is larger than other materials commonly used for light emission devices, such as ZnSe (22 meV) and GaN (25 meV). Especially ZnO nanorods due to their unique physical and chemical properties have many important applications for devices, such as transparent electronics, sensors, detectors, etc. It is well known that wurtzite ZnO in c-plane orientation has strong polarization which significantly reduces light-emitting efficiency. Though the polarization is eliminated for nonpolar ZnO, it is difficult to grow nonpolar ZnO nanorods as they are normally grown along c-axis normal to the substrate surface. Polarization can be greatly reduced for semipolar ZnO which is likely to be obtained if nanorods can be grown with c-axis in an inclined angle with substrate surface; however, well-aligned semipolar oriented ZnO nanorods have been rarely reported. Here, we report the first observations of growth of epi-ZnO nanorods with semipolar orientation on m-sapphire by microwave-assisted chemical bath deposition (CBD) with a fast growth rate. ZnO nanorods have been widely grown by CBD which is a simple, low cost and low temperature process but with a low growth rate generally in the range of 5-10 nm/min. Microwave-assisted CBD is a newly developed technique because of microwave can directly transmit its energy to dielectric medium by radiation, such that quick and uniform heating without much energy loss caused by conduction is feasible for aqueous solution. In recent years, microwave heating has been demonstrated to have much higher ZnO nucleation and growth rates than with conventional heating. Furthermore, a several microns thick epitaxial c-plane ZnO film can be deposited on MgAl2O4 (111) substrate in just a few minutes by microwave assisted CBD. In this study, 1 x 1 cm2 m-sapphire substrates were seeded with semipolar ZnO at a temperature between 500-700 °C by using mist chemical vapor deposition. The seeded substrate was then placed in a 90 °C aqueous solution for microwave-assisted CBD of ZnO nanorods. The solution was pre-formulated at room temperature with 0.0375 mol/L zinc nitrate hexahydrate and 0.0375 mol/L hexamethylenetetramine. The microwave power can be automatically tuned to maintain the solution at a constant temperature during CBD. Microstructural characterization was performed with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) to understand the growth process. Cross-sectional TEM specimens were prepared with focused ion beam. Photoluminescence (PL) spectroscopy was performed with He-Cd laser for measurements of optical properties. For ZnO nanorods formed after growth for 10 minutes at 90°C, SEM in Fig. 1 shows that tilted hexagonal columns of ZnO with diameter about 100-200 nm are formed as a well-aligned and inclined array on the sapphire. Figure 2 shows a typical bright field TEM micrograph in cross-section view, illustrating that the ZnO nanorods grown on the 30 nm thick seed layer have an inclined angle about 30° with m-sapphire surface and an average length about 500 nm which gives a growth rate of 50 nm/min and an aspect ratio of 2.5-5 for the nanorods. Furthermore, high-resolution TEM and atomic-resolution annular dark field STEM micrographs with FFT patterns of sapphire and ZnO as shown in Fig. 2 reveal that the semipolar ZnO seed layer has an epitaxial relationship with m-sapphire as ZnO[1-100] || sapphire[11-20] and ZnO(0001)|| sapphire(1-10-4). Hence, the c-plane of ZnO seed has a 30° tilt away from the surface normal of the sapphire, resulting in the following growth of nanorods along the c-axis direction. As a result, the inclined nanorods has a semipolar orientation of (-1-122) || sapphire m-plane. It should be pointed out that ZnO (-1-122) reflection is hardly seen in XRD as its interplanar spacing is very close to that of sapphire (3-300). A typical PL spectrum of the ZnO nanorods measured at 10K is shown in Fig. 3 from which a strong narrow near band emission (NBE) peak centered at 3.31 eV is clearly seen with and a weak deep-level emission peak at around 2.17 eV, implying that the nanorods have a reasonably good quality. Keywords: ZnO, nanorod, chemical bath deposition microwave Figure 1

Impact of crystal orientation on the modulation bandwidth of InGaN/GaN light-emitting diodes

High-speed InGaN/GaN blue light-emitting diodes (LEDs) are needed for future gigabit-per-second visible-light communication systems. Large LED modulation bandwidths are typically achieved at high current densities, with reports close to 1 GHz bandwidth at current densities ranging from 5 to 10 kA/cm2. However, the internal quantum efficiency (IQE) of InGaN/GaN LEDs is quite low at high current densities due to the well-known efficiency droop phenomenon. Here, we show experimentally that nonpolar and semipolar orientations of GaN enable higher modulation bandwidths at low current densities where the IQE is expected to be higher and power dissipation is lower. We experimentally compare the modulation bandwidth vs. current density for LEDs on nonpolar (101¯0), semipolar (202¯1¯), and polar 0001 orientations. In agreement with wavefunction overlap considerations, the experimental results indicate a higher modulation bandwidth for the nonpolar and semipolar LEDs, especially at relatively low current densities. At 500 A/cm2, the nonpolar LED has a 3 dB bandwidth of ∼1 GHz, while the semipolar and polar LEDs exhibit bandwidths of 260 MHz and 75 MHz, respectively. A lower carrier density for a given current density is extracted from the RF measurements for the nonpolar and semipolar LEDs, consistent with the higher wavefunction overlaps in these orientations. At large current densities, the bandwidth of the polar LED approaches that of the nonpolar and semipolar LEDs due to coulomb screening of the polarization field. The results support using nonpolar and semipolar orientations to achieve high-speed LEDs at low current densities.

Internal quantum efficiency in polar and semipolar (11–22) InxGa1-xN/InyGa1-yN quantum wells emitting from blue to red

In this work, we investigate the impact of the quantum confined Stark effect and of the carrier localization on the internal quantum efficiency of polarized single or multiple InxGa1-xN/GaN quantum well(s), and semi-polar (11–22) multiple InxGa1-xN/InyGa1-yN quantum well. We find that increasing the influence of the quantum confined Stark effect at constant indium content with increasing the well-width induces a reduction of the internal quantum efficiency onsets for a decreasing value of the photoexcitation density. Similar but no so dramatic trend is reported when increasing the indium content and thus when increasing the localization of carriers to localized fluctuations of the chemical composition of the alloy. In addition, a change of the electric field internal to active layers (quantified by using time-resolved photoluminescence spectroscopy) realized by growing samples along a semi-polar orientation leads to experimental observation of a substantial enhancement of the threshold of photoexcitation density at which onsets the reduction of the internal quantum efficiency. A correlation is found through several orders of magnitude between the photoexcitation density PT for the onset of the collapse of IQE and the values of the photoluminescence radiative decay time τrad. A scaling law is found in the investigated samples: PT ∼ τrad-n with n = 3/2 ± 0.15 which evidences that quantum confined Stark effect is the main origin for the efficiency droop in nitride light-emitting diodes based on InxGa1-xN active layers.

Optimizing GaInN/GaN light-emitting diode structures under piezoelectric polarization

We model and optimize various light emitting diode structures under bias voltage to maximize emission efficiency with particular respect to piezoelectric polarization. We compare polar and non-polar structures, namely, wurtzite c-plane, a-plane, (11–22) semi-polar, and (001) cubic crystal orientations in self-consistent Schrödinger-Poisson and drift-diffusion models. We consider both structures strained to a GaN pn-junction and strain-reduced systems based on GaInN templates. In light of numerous experimental findings of the actual electric field strength, we find it necessary to reduce the piezoelectric coefficients over those commonly cited. A weaker variation with composition or wavelength is the consequence. For the non-polar and cubic systems, we find a 22% increase of the electron-hole overlap and an 18% increase for the c-plane strain-reduced system at an InN fraction of x = 0.30 when compared to standard c-plane structures. For the green and longer wavelength range, we find that strain-reduced and cubic GaN systems should hold particular promise for higher radiative efficiency.

Explanation of low efficiency droop in semipolar (202¯1¯) InGaN/GaN LEDs through evaluation of carrier recombination coefficients.

We report the carrier dynamics and recombination coefficients in single-quantum-well semipolar (202¯1¯) InGaN/GaN light-emitting diodes emitting at 440 nm with 93% peak internal quantum efficiency. The differential carrier lifetime is analyzed for various injection current densities from 5 A/cm2 to 10 kA/cm2, and the corresponding carrier densities are obtained. The coupling of internal quantum efficiency and differential carrier lifetime vs injected carrier density (n) enables the separation of the radiative and nonradiative recombination lifetimes and the extraction of the Shockley-Read-Hall (SRH) nonradiative (A), radiative (B), and Auger (C) recombination coefficients and their n-dependency considering the saturation of the SRH recombination rate and phase-space filling. The results indicate a three to four-fold higher A and a nearly two-fold higher B0 for this semipolar orientation compared to that of c-plane reported using a similar approach [A. David and M. J. Grundmann, Appl. Phys. Lett. 96, 103504 (2010)]. In addition, the carrier density in semipolar (202¯1¯) is found to be lower than the carrier density in c-plane for a given current density, which is important for suppressing efficiency droop. The semipolar LED also shows a two-fold lower C0 compared to c-plane, which is consistent with the lower relative efficiency droop for the semipolar LED (57% vs. 69%). The lower carrier density, higher B0 coefficient, and lower C0 (Auger) coefficient are directly responsible for the high efficiency and low efficiency droop reported in semipolar (202¯1¯) LEDs.

Growth conditions of semi and non-polar GaN on Si with Er2O3 buffer layer

The GaN was grown on Si(100) with Er2O3(110) buffer by metal organic chemical vapor deposition (MOCVD). The results showed the polycrystalline nature of the layers with dominant non-polar (11–20) and semi-polar (10–13) orientations. GaN was formed in a two step process regime: low temperature growth was followed by high temperature growth regime. At a low temperature GaN tends to crystalize with dominant non-polar (11–20) orientation, while high temperature leads to preferential semi-polar (10–13) orientation growth. Additionally, the effects of parameters such as growth temperature, V/III-ratio and type of carrier gases on structural, morphological and optical properties of GaN was discussed in the current work.

Characterizations of nonlinear optical properties on GaN crystals in polar, nonpolar, and semipolar orientations

We report the basic nonlinear optical properties, namely, two-photon absorption coefficient (β), three-photon absorption coefficient (γ), and Kerr nonlinear refractive index (nkerr), of GaN crystals in polar c-plane, nonpolar m-plane, and semipolar (2021¯) plane orientations. A typical Z-scan technique was used for the measurement with a femtosecond Ti:S laser from wavelengths of 724 nm to 840 nm. For the two-photon absorption coefficient (β), similar values were obtained for polar, nonpolar, and semipolar samples, which are characterized to be ∼0.90 cm/GW at 724 nm and ∼0.65 cm/GW at 730 nm for all the three samples. For the Kerr nonlinear refractive index (nkerr), self-focusing features were observed in this work, which is different from previous reports where self-defocusing features were observed on GaN in the visible and near-UV spectral regions. At 724 nm, nkerr was measured to be ∼2.5 0×10−14 cm2/W for all three samples. Three-photon absorption coefficients (γ) were also determined, which were found to be consistent with previous reports. This study provides valuable information on the basic nonlinear optical properties of III-nitride semiconductors, which are vital for a wide range of applications such as integrated photonics and quantum photonics.

Effect of nano‐porous SiNxinterlayer on propagation of extended defects in semipolar ‐orientated GaN

Semipolar orientations of GaN have attracted considerable attention as potential platforms for high brightness light‐emitting devices. The heteroepitaxy of semipolar GaN layers results in high densities of threading dislocations (TDs) and basal‐plane stacking faults (BSFs). Various defect reduction methods have been employed to improve the crystal quality of the semipolar substrates, among which thein situepitaxial lateral overgrowth (ELO) is of great importance due to its relatively simple procedure, low cost, and high effectiveness in reducing density of TDs. We have shown thatin situdeposited nano‐porous SiNxcould act as a blocking layer preventing both TDs and BSFs from penetrating to overgrown semipolarGaN layers. In this contribution, we present microscopic study of interaction of extended defects (TDs and BSFs) with nano‐porous SiNxin‐oriented GaN layers grown by MOCVD onm‐sapphire substrates. Cross‐sectional scanning transmission electron microscopy‐cathodoluminescence (STEM‐CL) reveals reduction of BSF density by about 50% for dual insertion of SiNxinterlayers. It is also shown that the integrated CL intensity is enhanced by about two orders of magnitude indicating that the majority of TDs are blocked by the interlayers. The interaction of TDs and BSFs with nano‐porous SiNxand defect distribution in GaN nuclei are briefly discussed.

Chemically assisted ion beam etching of laser diode facets on nonpolar and semipolar orientations of GaN

We demonstrate a vertical (<1° departure) and smooth (2.0 nm root mean square line-edge roughness (LER)) etch by chemically assisted Ar ion beam etching (CAIBE) in Cl2 chemistry that is suitable for forming laser diode (LD) facets on nonpolar and semipolar oriented III-nitride devices. The etch profiles were achieved with photoresist masks and optimized CAIBE chamber conditions including the platen tilt angle and Cl2 flow rate. Co-loaded studies showed similar etch rates of ∼60 nm min−1 for and m-plane orientations. The etched surfaces of LD facets on these orientations are chemically dissimilar (Ga-rich versus N-rich), but were visually indistinguishable, thus confirming the negligible orientation dependence of the etch. Continuous-wave blue LDs were fabricated on the semipolar plane to compare CAIBE and reactive ion etch (RIE) facet processes. The CAIBE process resulted in LDs with lower threshold current densities due to reduced parasitic mirror loss compared with the RIE process. The LER, degree of verticality, and model of the 1D vertical laser mode were used to calculate a maximum uncoated facet reflection of 17% (94% of the nominal) for the CAIBE facet. The results demonstrate the suitability of CAIBE for forming high quality facets for high performance nonpolar and semipolar III-N LDs.

Semipolar InGaN/GaN nanostructure light-emitting diodes on c-plane sapphire

The fabrication of electrically injected triangular-nanostripe core–shell semipolar III–nitride LEDs (TLEDs) is demonstrated using interferometric lithography and catalyst-free bottom-up selective-area metal–organic chemical vapor deposition (MOCVD). This alternative approach enables semipolar orientations on inexpensive, c-plane sapphire substrates, in comparison with planar growth on free-standing GaN substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy reveal nonuniform quantum well thickness and composition, respectively, as a function of location on the triangular stripes. The broad electroluminescence spectra, wavelength shift with increasing current density, and nonlinear light vs current characteristics are well correlated with the observed quantum-well nonuniformities.

Critical thickness for the formation of misfit dislocations originating from prismatic slip in semipolar and nonpolar III-nitride heterostructures

We calculate the critical thickness for misfit dislocation (MD) formation in lattice mismatched semipolar and nonpolar III-nitride wurtzite semiconductor layers for the case of MDs originated from prismatic slip (PSMDs). It has been shown that there is a switch of stress relaxation modes from generation of basal slip originated MDs to PSMDs after the angle between c-axis in wurtzite crystal structure and the direction of semipolar growth reaches a particular value, e.g., ∼70° for Al0.13Ga0.87N/GaN (h0h̄1) semipolar heterostructures. This means that for some semipolar growth orientations of III-nitride heterostructures biaxial relaxation of misfit stress can be realized. The results of modeling are compared to experimental data on the onset of plastic relaxation in AlxGa1−xN/GaN heterostructures.