Nonpolar Orientations Research Articles

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.

Unravelling a solution-based formation of single-crystalline kinked wurtzite nanowires: The case of MnSe

The search for a novel strategy to sculpt semiconductor nanowires (NWs) at the atomistic scale is crucial for the development of new paradigms in optics, electronics, and spintronics. Thus far, the fabrication of single-crystalline kinked semiconductor NWs has been achieved mainly through the vapor−liquid−solid growth technique. In this study, we developed a new strategy for sculpting single-crystalline kinked wurtzite (WZ) MnSe NWs by triggering the nonpolar axial-oriented growth, thereby switching—at the atomistic scale—the NW growth orientation along the nonpolar axes in a facile solution-based procedure. This presents substantial challenges owing to the dominant polar c axis growth in the solution-based synthesis of one-dimensional WZ nanocrystals. More significantly, the ability to continuously switch the nonpolar axial-growth orientation allowed us to craft the kinking landscape of types 150°, 120°, 90°, and 60°. A probabilistic analysis of kinked MnSe NWs reveals the correlations of the synergy and interplay between these two sets of nonpolar axial growth-orientation switching, which determine the actual kinked motifs. Furthermore, discriminating the side-facet structures of the kinked NWs significantly strengthened the spatially selected interaction of Au nanoparticles. We envisage that such a facile solution-based strategy can be useful for synthesizing other single-crystalline kinked WZ-type transition-metal dichalcogenide NWs to develop novel functional materials with finely tuned properties.

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.

Comparative study of (0001) and InGaN based light emitting diodes

We have systematically investigated the doping of with Si and Mg by metal–organic vapour phase epitaxy for light emitting diodes (LEDs). By Si doping of GaN we reached electron concentrations close to 1020 cm−3, but the topography degrades above mid 1019 cm−3. By Mg doping we reached hole concentrations close to 5 × 1017 cm−3, using Mg partial pressures about 3× higher than those for (0001). Exceeding the maximum Mg partial pressure led to a quick degradation of the sample. Low resistivities as well as high hole concentrations required a growth temperature of 900 °C or higher. At optimised conditions the electrical properties as well as the photoluminescence of p-GaN were similar to (0001) p-GaN. The best ohmic p-contacts were achieved by NiAg metallisation. A single quantum well LED emitting at 465 nm was realised on (0001) and . Droop (sub-linear increase of the light output power) occurred at much higher current densities on . However, the light output of the (0001) LED was higher than that of until deep in the droop regime. Our LEDs as well as those in the literature indicate a reduction in efficiency from (0001) over semi-polar to non-polar orientations. We propose that reduced fields open a loss channel for carriers.

Development of a ReaxFF potential for Ag/Zn/O and application to Ag deposition on ZnO

A new empirical potential has been derived to model an Ag–Zn–O system. Additional parameters have been included into the reactive force field (ReaxFF) parameter set established for ZnO to describe the interaction between Ag and ZnO for use in molecular dynamics (MD) simulations. The reactive force field parameters have been fitted to density functional theory (DFT) calculations performed on both bulk crystal and surface structures. ReaxFF accurately reproduces the equations of state determined for silver, silver zinc alloy and silver oxide crystals via DFT. It also compares well to DFT binding energies and works of separation for Ag on a ZnO surface. The potential was then used to model single point Ag deposition on polar (0001¯) and non-polar (101¯0) orientations of a ZnO wurtzite substrate, at different energies. Simulation results then predict that maximum Ag adsorption on a ZnO surface requires deposition energies of ≤10eV.

Pseudomorphic ZnO‐based heterostructures: From polar through all semipolar to nonpolar orientations

AbstractWe calculate the strain and stress state and the polarization in pseudomorphic wurtzite MgZnO/ZnO and CdZnO/ZnO heterostructures as a function of the polar angle between the c‐axis and the epitaxial direction. (Cd,Zn)O/ZnO behaves qualitatively similar to the systems (In,Ga)N/GaN and (Al,Ga)N/GaN. (Mg,Zn)O/ZnO represents a unique case since the signs of the change of lattice constants and are opposite. Among the consequences are: (i) the epilayer strain energy has a minimum between and for semipolar growth plane close to ; (ii) for nonpolar growth () the strain in the epitaxial direction is close to zero; and the in‐plane polarization is large (comparable to the polar case); and (iii) the piezoelectric polarization along the growth direction has only one zero (at ) between 0 and . Also we give useful analytical formula within an isotropic approximation for the compliances.Elastic strain energy (in relative units) of pseudomorphic wurtzite (Mg,Zn)O/ZnO and (Cd,Zn)O/ZnO heterostructures as a function of the polar angle between interface normal and the c‐axis

Chemical vapor deposition of m-plane and c-plane InN nanowires on Si (100) substrate

In this paper, synthesis of indium nitride (InN) nanowires (NWs) by chemical vapor deposition (CVD) is studied. InN NWs were synthesized via a vapor–liquid–solid (VLS) growth mechanism using high purity indium foil and ammonia as the source materials, and nitrogen as carrier gas. The mixture of nonpolar m-plane oriented and polar c-plane oriented tapered InN NWs is observed grown on top of Si (100) substrate. Energy dispersive spectroscopy (EDS) showed that the tips of the NWs are primarily consisted of Au and the rest of the NWs are consisted of indium (In) and nitrogen (N). High resolution scanning electron microscopy (HRSEM) revealed that the InN NWs have both triangular and hexagonal cross sections. Transmission electron microscopy (TEM) diffraction pattern showed that the NWs are high quality single crystals having wurtzite crystal structure. High resolution transmission electron microscopy (HRTEM) showed the growth directions of the InN NWs with triangular cross section are along 〈10−10〉 nonpolar m-plane orientation and the InN NWs with hexagonal cross section are along 〈0001〉 polar c-plane orientation.

Impact of Nitridation on Structural and Optical Properties of Epitaxial GaN Films Grown on M-Plane Sapphire by PAMBE

ABSTRACTGaN epilayers were grown on m-plane (10-10) sapphire substrates using plasma assisted molecular beam epitaxy. Impact of nitridation on structural and optical properties of GaN film was investigated. The film grown on a nitridated surface resulted in a nonpolar (10-10) orientation while without nitridation caused a semipolar (11-22) orientation. The high resolution X-ray diffraction studies confirmed the orientation of the GaN films. X-ray rocking curve showed better crystallinity of semipolar as compared to nonpolar GaN. Atomic force microscopy showed smoother films in case of nonpolar GaN which might be in account of the nitridation treatment. Room temperature photoluminescence study showed nonpolar GaN to have higher value of compressive strain as compared to semipolar GaN film, which was further confirmed by room temperature Raman spectroscopy. Despite the fact that it is difficult to obtain high-quality nonpolar material due to the planar anisotropic nature of the growth mode, we hereby report the development of non-polar GaN of usable quality, on an m-plane sapphire, involving controlled steps of nitridation.

M-plane (101¯) and (202¯1) GaN/AlxGa1–xN conduction band offsets measured by capacitance-voltage profiling

The reduced polarization of nonpolar and semipolar orientations of the III-Nitrides is very attractive for optoelectronics applications. Several properties of these orientations have yet to be investigated. In particular, there is no a priori reason for the band offset to be the same for different crystallographical orientations. In the present article, we report on the extraction of the GaN/AlxGa1–xN conduction band offsets through capacitance-voltage profiling for m-plane (101¯0) and (202¯1) orientations. We extracted a ΔEc:ΔEv of 50:50–60:40. We measured a non-zero hetero-interface charge for the (202¯1) orientation which made the extraction of ΔEc less reliable.

Semipolar and nonpolar GaN epi-films grown on m-sapphire by plasma assisted molecular beam epitaxy

We hereby report the development of non-polar epi-GaN films of usable quality, on an m-plane sapphire. Generally, it is difficult to obtain high-quality nonpolar material due to the planar anisotropic nature of the growth mode. However, we could achieve good quality epi-GaN films by involving controlled steps of nitridation. GaN epilayers were grown on m-plane (10-10) sapphire substrates using plasma assisted molecular beam epitaxy. The films grown on the nitridated surface resulted in a nonpolar (10-10) orientation while without nitridation caused a semipolar (11-22) orientation. Room temperature photoluminescence study showed that nonpolar GaN films have higher value of compressive strain as compared to semipolar GaN films, which was further confirmed by room temperature Raman spectroscopy. The room temperature UV photodetection of both films was investigated by measuring the I-V characteristics under UV light illumination. UV photodetectors fabricated on nonpolar GaN showed better characteristics, including higher external quantum efficiency, compared to photodetectors fabricated on semipolar GaN. X-ray rocking curves confirmed better crystallinity of semipolar as compared to nonpolar GaN which resulted in faster transit response of the device.

Strain relaxation of thick (11–22) semipolar InGaN layer for long wavelength nitride-based device

In this study, the properties of thick stress-relaxed (11–22) semipolar InGaN layers were investigated. Owing to the inclination of growth orientation, misfit dislocations (MDs) occurred at the heterointerface when the strain state of the (11–22) semipolar InGaN layers reached the critical point. We found that unlike InGaN layers based on polar and nonpolar growth orientations, the surface morphologies of the stress-relaxed (11–22) semipolar InGaN layers did not differ from each other and were similar to the morphology of the underlying GaN layer. In addition, misfit strain across the whole InGaN layer was gradually relaxed by MD formation at the heterointerface. To minimize the effect of surface roughness and defects in GaN layers on the InGaN layer, we conducted further investigation on a thick (11–22) semipolar InGaN layer grown on an epitaxial lateral overgrown GaN template. We found that the lateral indium composition across the whole stress-relaxed InGaN layer was almost uniform. Therefore, thick stress-relaxed (11–22) semipolar InGaN layers are suitable candidates for use as underlying layers in long-wavelength devices, as they can be used to control strain accumulation in the heterostructure active region without additional influence of surface roughness.

Surface phonon polariton responses of hexagonal sapphire crystals with non-polar and semi-polar crystallographic planes.

The surface phonon polariton (SPhP) characteristics of hexagonal sapphire crystals with non-polar and semi-polar crystallographic planes are investigated. A formulation that considers the effects of crystal orientation is employed to calculate the SPhP dispersion curves of the samples. The SPhP dispersion curves indicate that the SPhP responses of sapphire crystals in non-polar and semi-polar orientations are directionally sensitive. Resonance frequencies and spectral strengths of the SPhP modes can be modulated simply by tuning the angular positions of the samples. The validity of the theoretical results is confirmed by the polarized infrared attenuated total reflection measurements.

Real-time x-ray studies of crystal growth modes during metal-organic vapor phase epitaxy of GaN on c- and m-plane single crystals

Non-polar orientations of III-nitride semiconductors have attracted significant interest due to their potential application in optoelectronic devices with enhanced efficiency. Using in situ surface x-ray scattering during metal-organic vapor phase epitaxy (MOVPE) of GaN on non-polar (m-plane) and polar (c-plane) orientations of single crystal substrates, we have observed the homoepitaxial growth modes as a function of temperature and growth rate. On the m-plane surface, we observe all three growth modes (step-flow, layer-by-layer, and three-dimensional) as conditions are varied. In contrast, the +c-plane surface exhibits a direct crossover between step-flow and 3D growth, with no layer-by-layer regime. The apparent activation energy of 2.8 ± 0.2 eV observed for the growth rate at the layer-by-layer to step-flow boundary on the m-plane surface is consistent with those observed for MOVPE growth of other III-V compounds, indicating a large critical nucleus size for islands.

Effect of high temperature thermal treatment of (100) γ-LiAlO2 substrate on epitaxial growth of ZnO films by plasma-assisted molecular beam epitaxy

Non-polar ZnO thin films were fabricated on (100) γ-LiAlO2 substrates by plasma-assisted molecular beam epitaxy. The effect of high temperature thermal treatment of substrate on the crystalline orientation and quality of ZnO thin film was investigated. The film grown on (100) γ-LiAlO2 substrate without high temperature thermal treatment consists of domains of both polar and non-polar orientations as identified by the X-ray diffraction pattern. Using high temperature thermal treatment of substrate, the growth of polar ZnO has been suppressed effectively. Besides, high temperature thermal treatment of substrate improves the crystalline quality of epitaxial ZnO thin film, which exhibits a smaller full width at half maximum value of ZnO (101−0) diffraction peak and a weaker deep level emission of photoluminescence. The suppression of polar ZnO growth and the quality improvement of the epitaxial ZnO films are due to the improvement of surface morphology and roughness of the substrate upon high temperature treatment.

Optical characteristics of nonpolar a-plane ZnO thin film on (010) LiGaO2 substrate

We investigate the optical properties of non-polar a-plane ZnO film grown on (010) LiGaO2 substrate by chemical vapor deposition. X-ray diffraction indicates the a-plane orientation and Raman spectroscopy reveals the phase purity. Four distinct features in the near band-edge range are identified as neutral-donor-bound-exciton (D°X), free-exciton (FX), free-to-bound, and donor-acceptor pair transitions. The thermal activation energy of D°X is 12 meV while an acceptor energy level of 124 meV is estimated. Temperature evolution of photoluminescence (PL) shows that the room-temperature luminescence is a mixture of free-to-bound and FX transitions. The polarization dependence of the near-band-edge transitions were investigated. The largest degree of polarization of 95.5% occurs in FX transition. It can be attributed to anisotropic in-plane strain and the nonpolar a-plane orientation.

Properties of p-type ZnO thin films with different orientations

The stable properties of N-doped p-type ZnO thin films with preferential nonpolar (100) plane orientation relative to polar (002) plane orientation are investigated. The two kinds of oriented thin films are fabricated by the methods of post heat treatment and double sources in situ, respectively. The Hall investigations demonstrate that N-doped p-type ZnO thin films with preferential nonpolar (100) plane orientation are more stable,and the results are also proved by build-in electric field model and electronic structure calculations of the films based on the first principle.

Equilibrium Strain and Dislocation Density in Nitride Based Heterostructures

This work presents a computational approach for the determination of equilibrium strain and dislocation density in nitride-based heterostructures. Verification of this approach is demonstrated in the calculation of critical layer thickness for In x Ga 1-x N / GaN (0001) as compared to other work. The present modeling approach can also be applied to heterostructures of varying compositional profiles and structure. To demonstrate this, we studied the equilibrium strain and misfit dislocation density profiles for metamorphic In x Ga 1-x N / GaN (0001) heterostructures containing uniform layers on top of linearly-graded buffers. These structures were also compared to similar arsenide-based heterostructures involving In x Ga 1-x As / GaAs (001) with the zinc blende crystal structure. The generalized energy minimization modeling tool presented here can be readily extended to arbitrarily graded nitride heterostructures with polar, semi-polar, or non-polar orientations.

Electron affinities and ionization energies of Cu and Ag delafossite compounds: A hybrid functional study

Using density functional theory with a hybrid functional, we calculate the ionization energies and electron affinities of a series of delafossite compounds ($AM$O${}_{2}$: $A=\text{Cu}$, Ag; $M=\text{B}$, Al, Ga, In, Sc). The alignments of the valence band maximum and the conduction band minimum, which directly relate to the ionization energies and electron affinities, were obtained by calculations of supercell slab models constructed in a nonpolar orientation. Our calculations reveal that the ionization energy decreases with an increasing atomic number of group-III elements, and thus suggest an improved $p$-type doping propensity for heavier compounds. For keeping both a low ionization energy and a band gap of sufficient size, CuScO${}_{2}$ is superior to the Cu-based group-III delafossites. By analyzing the electronic structures, we demonstrate that the compositional trend of the ionization energies and electron affinities is the result of a combined effect of $d$-band broadening due to Cu(Ag)-Cu(Ag) coupling and a repositioning of the $d$-band center.

Determination of lattice parameters, strain state and composition in semipolar III-nitrides using high resolution X-ray diffraction

In group-III-nitride heterostructures with semipolar or nonpolar crystal orientation, anisotropic lattice and thermal mismatch with the buffer or substrate lead to a complex distortion of the unit cells, e.g., by shearing of the lattice. This makes an accurate determination of lattice parameters, composition, and strain state under assumption of the hexagonal symmetry impossible. In this work, we present a procedure to accurately determine the lattice constants, strain state, and composition of semipolar heterostructures using high resolution X-ray diffraction. An analysis of the unit cell distortion shows that four independent lattice parameters are sufficient to describe this distortion. Assuming only small deviations from an ideal hexagonal structure, a linear expression for the interplanar distances dhkl is derived. It is used to determine the lattice parameters from high resolution X-ray diffraction 2ϑ-ω-scans of multiple on- and off-axis reflections via a weighted least-square fit. The strain and composition of ternary alloys are then evaluated by transforming the elastic parameters (using Hooke's law) from the natural crystal-fixed coordinate system to a layer-based system, given by the in-plane directions and the growth direction. We illustrate our procedure taking an example of (112¯2) AlκGa1−κN epilayers with Al-contents over the entire composition range. We separately identify the in-plane and out-of-plane strains and discuss origins for the observed anisotropy.

Inductively coupled plasma–reactive ion etching of c- and a-plane AlGaN over the entire Al composition range: Effect of BCl3 pretreatment in Cl2/Ar plasma chemistry

Inductively coupled plasma (ICP)–reactive ion etching (RIE) patterning is a standard processing step for UV and optical photonic devices based on III-nitride materials. There is little research on ICP-RIE of high Al-content AlGaN alloys and for nonpolar nitride orientations. The authors present a comprehensive study of the ICP-RIE of c- and a-plane AlGaN in Cl2/Ar plasma over the entire Al composition range. The authors find that the etch rate decreases in general with increasing Al content, with different behavior for c- and a-plane AlGaN. They also study the effect of BCl3 deoxidizing plasma pretreatment. An ICP deoxidizing BCl3 plasma with the addition of argon is more efficient in removal of surface oxides from AlxGa1−xN than RIE alone. These experiments show that AlxGa1−xN etching is affected by the higher binding energy of AlN and the higher affinity of oxygen to aluminum compared to gallium, with oxides on a-plane AlGaN more difficult to etch as compared to oxides on c-plane AlGaN, specifically for high Al composition materials. The authors achieve reasonably high etch rate (∼350 nm/min) for high Al-content materials with a smooth surface morphology at a low DC bias of ∼−45 VDC.