Nonpolar Directions Research Articles

Lateral and vertical diffusion of magnesium in ion-implanted Halide Vapor Phase Epitaxy gallium nitride

In the present study, the diffusion of magnesium in gallium nitride was investigated. In order to ensure high structural quality and purity of the analyzed material, bulk crystals, grown by Halide Vapor Phase Epitaxy on ammonothermal native seeds, were used. Samples of polarity (0001), (10-10) and (11–20) were prepared from them and implanted with magnesium ions with fluence of 1 × 1016 cm−2 and 230 keV energy. X-ray diffraction was used to assess the structural quality of the samples at each stage of the study. Ultra-high-pressure annealing was carried out in the temperature range of 1250–1450 °C to activate the dopant and remove post-implantation damage. Diffusion of the implanted species was a consequence of the annealing process. Secondary ion mass spectrometry was used to assess the in-depth concentration of magnesium in the annealed samples. Strong diffusion anisotropy was observed for the crystallographic directions analyzed, with [000–1] being the most susceptible to diffusion and [10-10] being the most resistant. Using the finite element method for numerical analysis of the measured profiles, the diffusion coefficients were determined. Substitutional diffusion involving gallium vacancies was found to be the dominant mechanism in the analyzed samples. The temperature-independent diffusion coefficient was calculated. Its value was 1.7 × 10−4 cm2/s for the [0001] direction and one order of magnitude less for non-polar directions. The activation energy was similar for all cases studied and ranged from 2.26 eV to 2.38 eV.

Origin and Mechanism of Anisotropic Piezoelectric Generation on the ZnO(101̅0) Crystal Plane

The experimental observation of pressure-induced structural changes is crucial for establishing the correct piezoelectric mechanism. Here, we show the in-plane anisotropic piezoelectric generation behavior on a ZnO non-polar (101̅0) crystal plane. The piezoelectric and rectification effects are found in the [0001] polar direction on the (101̅0) crystal facet. Neither effect was observed in the [12̅10] non-polar direction perpendicular to the [0001] direction. The piezoelectric generation and rectification properties can be reduced and vanished under pressure in the [0001] direction. The spontaneous polarization in the [0001] direction is evidenced to be the origin of piezoelectric effect. Pressure-induced structural change of ZnO is observed for the first time. In the piezoelectric effect, the applied stress causes the polar structure of ZnO to change into a non-polar structure, and the piezoelectric current is generated. This novel piezoelectric model will overturn the existing piezoelectric theory and piezoelectric generation mechanism and contribute to comprehending the crystal orientation-dependent piezoelectric properties and the development of new high-performance piezoelectric materials and devices.

Facile synthesis of ZnO films with anisotropic preferred orientations: An effective strategy for controllable surface and optical property

Although one-dimensional ZnO nanodevices have already been widely used in practical applications, there still exists limitation of inevitable growth along polar orientation for wurtzite ZnO. Herein, ZnO films grown along diverse preferred orientations including the non-polar, semi-polar and polar directions could be acquired by a facile and reproducible low-temperature aqueous solution deposition method. The non-epitaxial growth pattern was realized by introducing moderate amount of sodium citrate and ammonia into the non-polar solution without any surfactant or passivated agent. Preferential growth mechanism and surface properties of as-prepared anisotropic ZnO films were investigated. Transmission spectra of ZnO films with anisotropic preferred orientations exhibited an isotropic transmittance. Visible emission spectra indicated multifarious defect types and quantities related to diverse preferred orientations of anisotropic ZnO films. This work could not merely enrich the crystallographic growth of ZnO films along anisotropic orientations, but also benefit for designing ZnO film-based nanodevices with desirable performances from the perspective of regulating anisotropic growth of ZnO.

Strain in BInGaN thin layers grown in nonpolar and semipolar directions

Strain in BInGaN thin layers grown in nonpolar and semipolar directions

Investigation of beryllium diffusion in HVPE-GaN grown in [11–20] and [10-10] crystallographic directions

The diffusion of beryllium in gallium nitride crystals grown in non-polar crystallographic directions by halide vapor phase epitaxy was investigated. The layers were crystallized on (10-10) and (11–20) planes of native ammonothermal substrates of the highest structural quality. The new-grown crystals were prepared to an epi-ready state, implanted with beryllium, and treated with ultra-high-pressure annealing. The crystallographic structure of all the samples before and after ion implantation as well as annealing processes was evaluated by X-ray diffraction measurements. The diffusion of beryllium was investigated by analyzing depth profiles obtained by secondary ion mass spectrometry. The diffusion profiles were box-shaped for both examined non-polar directions. Pre-exponential factors and activation energies for beryllium diffusion in non-polar gallium nitride were determined. A model related to point defect complexes, based on gallium vacancies and oxygen, was proposed for explaining the box-shaped beryllium diffusion profiles and the visible differences in the diffusion range between the two analyzed non-polar directions.

Optimizing K0.5Na0.5NbO3 Single Crystal by Engineering Piezoelectric Anisotropy.

K0.5Na0.5NbO3 is considered as one of the most promising lead-free piezoelectric ceramics in the field of wearable electronics because of its excellent piezoelectric properties and environmental friendliness. In this work, the temperature-dependent longitudinal piezoelectric coefficient was investigated in K0.5Na0.5NbO3 single crystals via the Landau–Ginzburg–Devonshire theory. Results show that the piezoelectric anisotropy varies with the temperature and the maximum of deviates from the polar direction of the ferroelectric phase. In the tetragonal phase, parallels with cubic polarization direction near the tetragonal-cubic transition region, and then gradually switches toward the nonpolar direction with decreasing temperatures. The maximum of in the orthorhombic phase reveals a distinct varying trend in different crystal planes. As for the rhombohedral phase, slight fluctuation of the maximum of was observed and delivered a more stable temperature-dependent maximum and its corresponding angle θmax in comparison with tetragonal and orthorhombic phases. This work not only sheds some light on the temperature-dependent phase transitions, but also paves the way for the optimization of piezoelectric properties in piezoelectric materials and devices.

Effect of Thin Film Thickness on the Electronic Properties of Wurtzite Structure (ZnO and GaN): A Density Functional Theory Study

By using first-principle density-functional theory (DFT) calculations supplemented with symmetry analysis, we investigated the effect of thin-film thickness on the electronic properties of non-polar ( ZnO-wz and GaN-wz. We find that the electronic band structures of thin-film non-polar ( surface with layer variations from two until ten bilayers show the Rashba spin splitting. Importantly, we revealed that this splitting is found to be strongly anisotropic observed in the valence band maximum (VBM) around the Γ point. We clarified the origin of the anisotropic spin splitting in the electronic band structures by considering the point-group symmetry (PGS) of the present system. We found that the changes of the PGS from C6v (for polar [0001] direction) to Cs (for non-polar ( direction) are responsible for inducing the anisotropic of the spin splitting. To further confirm the anisotropic splitting, we calculated the Rashba spin splitting parameters for different directions of the k-path. We found that these parameters are different in magnitude for a different direction of the k-path indicating the anisotropic spin splitting quantitatively, which is consistent well with symmetry analysis. Finally, we conclude that the observed Rashba spin splitting in the wurtzite surface structure is promising for spintronics applications.

Wet chemical etching of GaN or InGaN nanowires on Si substrate for micro and nano-devises fabrication

In this work, it is experimentally shown that the etching of GaN or InGaN NWs in the KOH solution allows managing the morphology and optical properties of the nanowires array. Thinning rate of GaN nanowires is 5 times slower than the rate for InGaN nanowires along semi-polar and non-polar direction for nanowires with Ga-polar crystal structure. The diameter of the InGaN NWs decreases from 200-100 to 30-40 nm. After etching process, the intensity photoluminescence (PL) of nanowires and uniformity PL signal at all sample area increase.

Accurate characterization of complete set constants of single domain 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 single crystal by resonant ultrasound spectroscopy using only one sample

In order to find out the origin of the giant piezoelectric effects of relaxor-PbTiO3 single crystals when poled along nonpolar directions, it is necessary to obtain the complete physical property data of a single domain state. Conventional characterization methods using multiple samples of different geometries often produce inconsistent data set because the domain patterns produced during poling are strongly dependent on the sample geometry. Here, we used the resonant ultrasound spectroscopy (RUS) technique to determine the material constants of a single domain 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 single crystal, which has a high mechanical quality factor (QM). Errors caused by sample-to-sample variation are avoided because all constants were measured from the same sample. The data set obtained is highly self-consistent, which provides a base for theoretical analysis as well as the input for simulation designs of piezoelectric devices. Comparison with data measured by other methods shows the advantage and higher accuracy of this characterization strategy.

Nonpolar-Oriented Wurtzite InP Nanowires with Electron Mobility Approaching the Theoretical Limit.

As an important semiconductor nanomaterial, InP nanowires (NWs) grown with a typical vapor-liquid-solid mechanism are still restricted from their low electron mobility for practical applications. Here, nonpolar-oriented defect-free wurtzite InP NWs with electron mobility of as high as 2000 cm2 V-1 s-1 can be successfully synthesized via Pd-catalyzed vapor-solid-solid growth. Specifically, PdIn catalyst particles are involved and found to expose their PdIn{210} planes at the InP nucleation frontier due to their minimal lattice mismatch with nonpolar InP{2̅110} and {1̅100} planes. This appropriate lattice registration would then minimize the overall free energy and enable the highly crystalline InP NW growth epitaxially along the nonpolar directions. Because of the minimized crystal defects, the record-high electron mobility of InP NWs ( i.e., 2000 cm2 V-1 s-1 at an electron concentration of 1017 cm-3) results, being close to the theoretical limit of their bulk counterparts. Furthermore, once the top-gated device geometry is employed, the device subthreshold slopes can be impressively reduced down to 91 mV dec-1 at room temperature. In addition, these NWs exhibit a high photoresponsivity of 104 A W-1 with fast rise and decay times of 0.89 and 0.82 s, respectively, in photodetection. All these results evidently demonstrate the promise of nonpolar-oriented InP NWs for next-generation electronics and optoelectronics.

Temperature dependent piezoelectric anisotropy in tetragonal 0.63Pb(Mg1/3Nb2/3)-0.37PbTiO3 single crystal

In this work, we studied the temperature-dependent effective piezoelectric coefficient d33* along the arbitrary direction of a tetragonal 0.63Pb(Mg1/3Nb2/3)-0.37PbTiO3 single crystal. Results show that the crystal changes from a rotator ferroelectrics with the maximum d33* occurring along the nonpolar direction to a extender type with maximum piezoelectricity along [001]C. Two polymorphic phase transitions, orthorhombic-tetragonal and tetragonal-cubic, greatly influence the PS dynamics, leading to the change of d33* anisotropy with temperature. The [011]C oriented crystals possess both improved piezoelectricity and high thermal stability, hence are the best choice for practical applications.

Dislocation bending in GaN/step-graded (Al,Ga)N/AlN buffer layers on Si(111) investigated by STM and STEM

ABSTRACTThe distribution and bending of dislocations in GaN/step-graded (Al,Ga)N/AlN buffer layers grown on Si(111) are investigated by cross-sectional scanning tunnelling microscopy (STM) and scanning transmission electron microscopy (STEM). We observe dislocations with -type Burgers vector intersecting the m-plane cleavage surface and having line directions bent off the [0001] growth direction toward non-polar directions. The spatial distribution of dislocations intersecting the m-plane cleavage surface indicates consecutive bending of dislocations due to strain at interfaces between subsequent lattice mismatched buffer layers and at doping junctions, reducing the density of threading dislocations at the (0001) growth front. No interface misfit dislocations, v-shaped defects, or loss of crystalline quality are observed, demonstrating the high performance of the step-graded (Al,Ga)N/AlN buffer layers on Si for relaxing the lattice constant without creating large defect concentrations.

Intersubband Transitions in Nonpolar m‐Plane AlGaN/GaN Heterostructures

Nonpolar AlGaN/GaN heterostructures have the potential to supplant polar heterostructures in infrared optoelectronic devices due to their theoretical advantages stemming from the absence of built‐in polarization fields along nonpolar directions of the nitride wurtzite lattice. However, development of nonpolar m‐plane infrared devices in a broad spectral range has been hampered, so far, by challenges to grow homogeneous high Al‐composition AlGaN on m‐plane GaN. AlxGa1‐xN layers with 0.6< x <0.8 are found to be kinetically unstable under metal‐rich growth conditions by plasma‐assisted molecular‐beam epitaxy. After reviewing recent progress in the field, this paper focuses on the effect of the structure of m‐plane AlxGa1−xN/GaN (x <0.6) superlattices on near‐infrared intersubband absorption. Even at these intermediate Al‐compositions, the effective growth rate of AlGaN is drastically reduced, and the AlGaN‐GaN interface roughness is unexpectedly high. Consequently, accurate determination of layer thicknesses and alloy composition necessitates structural characterization by a combination of scanning transmission electron microscopy and high‐resolution X‐ray diffraction. The energy and linewidth of near‐infrared intersubband transitions are also significantly affected by this unusual growth behavior. The experimental results for intersubband absorption of m‐ and c‐plane superlattices are compared to each other and with numerical calculations, and the main reasons for discrepancies are discussed.

Polar Imperfections in Amino Acid Crystals: Design, Structure, and Emerging Functionalities.

Crystals are physical arrays delineated by polar surfaces and often contain imperfections of a polar nature. Understanding the structure of such defects on the molecular level is of topical importance since they strongly affect the macroscopic properties of materials. Moreover, polar imperfections in crystals can be created intentionally and specifically designed by doping nonpolar crystals with "tailor-made" additives as dopants, since their incorporation generally takes place in a polar mode. Insertion of dopants also induces a polar deformation of neighboring host molecules, resulting in the creation of polar domains within the crystals. The contribution of the distorted host molecules to the polarity of such domains should be substantial, particularly in crystals composed of molecules with large dipole moments, such as the zwitterionic amino acids, which possess dipole moments as high as ∼14 D. Polar materials are pyroelectric, i.e., they generate surface charge as a result of temperature change. With the application of recent very sensitive instruments for measuring electric currents, coupled with theoretical computations, it has become possible to determine the structure of polar imperfections, including surfaces, at a molecular level. The detection of pyroelectricity requires attachment of electrodes, which might induce various artifacts and modify the surface of the crystal. Therefore, a new method for contactless pyroelectric measurement using X-ray photoelectron spectroscopy was developed and compared to the traditional periodic temperature change technique. Here we describe the molecular-level determination of the structure of imperfections of different natures in molecular crystals and how they affect the macroscopic properties of the crystals, with the following specific examples: (i) Experimental support for the nonclassical crystal growth mechanism as provided by the detection of pyroelectricity from near-surface solvated polar layers present at different faces of nonpolar amino acid crystals. (ii) Enantiomeric disorder in dl-alanine crystals disclosed by detection of anomalously strong pyroelectricity along their nonpolar directions. The presence of such disorder, which is not revealed by accurate diffraction techniques, explains the riddle of their needlelike morphology. (iii) The design of mixed polar crystals of l-asparagine·H2O/l-aspartic acid with controlled degrees of polarity, as determined by pyroelectricity and X-ray diffraction, and their use in mechanistic studies of electrofreezing of supercooled water. (iv) Pyroelectricity coupled with dispersion-corrected density functional theory calculations and molecular dynamics simulations as an analytical method for the molecular-level determination of the structure of polar domains created by doping of α-glycine crystals with different l-amino acids at concentrations below 0.5%. (v) Selective insertion of minute amounts of alcohols within the bulk of α-glycine crystals, elucidating their role as inducers of the metastable β-glycine polymorph. In conclusion, the various examples demonstrate that although these imperfections are present in minute amounts, they can be detected by the sensitive pyroelectric measurement, and by combining them with theoretical computations one can elucidate their diverse emerging functionalities.

Growth of HVPE-GaN on native seeds – numerical simulation based on experimental results

Investigation of source of stress in HVPE-GaN grown in c-direction on native seeds is reported. The study was based on experimental results. Ammonothermally grown GaN seed was used for growth of a thick HVPE-GaN layer in the c-direction. Deterioration of quality of the HVPE-GaN layer appeared. Numerical simulations were used for determining and examining radial, tangential, and axial stress distributions in the grown GaN crystal and the seed. The maximum equivalent stress (von Mises stress) was calculated and analyzed. It was shown that growth in non-polar and semi-polar directions, which appears on the edges of the crystal during crystallization process, can generate a very large tensile stress in GaN growing in the c-direction. This stress is much larger than the one generated due to a lattice mismatch between the ammonothermal seed and the deposited HVPE layer.

Influence of edge-grown HVPE GaN on the structural quality of c-plane oriented HVPE-GaN grown on ammonothermal GaN substrates

Study on the sources of stress in HVPE-GaN layer crystallized on 1-in. ammonothermally grown GaN seed is presented in this paper. Characterization by means of X-ray diffraction and transmission electron microscopy is performed. HVPE-GaN samples of high quality and those with visible quality deterioration are investigated on c-plane and m-plane cross-sections. Special attention is paid to HVPE material growing in semi-polar and non-polar directions on the edges of the seed and the growing layer. It is shown that this material generates significant stress leading to a structural deterioration of HVPE-GaN growing in the c-direction.

Fabrication of high-quality GaN substrates using the Na flux method

Gallium nitride (GaN) substrates fabricated along the nonpolar and semipolar directions are the most promising materials for realizing optical and electronic devices with low power consumption. In this study, we carry out the Na flux growth on -plane GaN templates grown heteroepitaxially on sapphires. The GaN crystals are grown at low supersaturation using the Na flux method with the dipping technique. The crystallinity of the grown GaN crystals is improved compared to that of the seed substrates. Then it improves further by lowering the supersaturation. Finally, we succeed in fabricating a 2-in. -plane GaN single crystal with high transparency and crystallinity.

Piezoelectric Anisotropy of KNbO3 by Molecular Dynamics Simulations Using a Shell Model

By molecular dynamics simulations using a shell model, we predicted the temperature dependence of the piezoelectric coefficients of KNbO3. The results were similar to those by phenomenological calculations by the Landau–Ginzburg–Devonshire (LGD) theory for KNbO3 and BaTiO3, first-principles effective Hamiltonian for the rhombohedral phase of BaTiO3, and a first-principles calculation for the rhombohedral phase of KNbO3. We also predicted the temperature dependence of the longitudinal piezoelectric surface \(d_{33}^{*}\) of KNbO3. The results were also similar to the previous calculations, since the temperature dependence of the piezoelectric constants were similar to them. The large shear piezoelectric constants especially near the phase transition temperatures resulted in the maximum of \(d_{33}^{*}\) along the non-polar directions except for the tetragonal phase near the tetragonal–cubic phase transition temperatures.

Enhanced UV detection by non-polar epitaxial GaN films

Nonpolar a-GaN (11-20) epilayers were grown on r-plane (1-102) sapphire substrates using plasma assisted molecular beam epitaxy. High resolution x-ray diffractometer confirmed the orientation of the grown film. Effect of the Ga/N ratio on the morphology and strain of a-GaN epilayers was compared and the best condition was obtained for the nitrogen flow of 1 sccm. Atomic force microscopy was used to analyze the surface morphology while the strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by reciprocal space mapping technique. UV photo response of a-GaN film was measured after fabricating a metal-semiconductor-metal structure over the film with gold metal. The external quantum efficiency of the photodetectors fabricated in the (0002) polar and (11-20) nonpolar growth directions were compared in terms of responsivity and nonpolar GaN showed the best sensitivity at the cost of comparatively slow response time.

Strong piezoelectric anisotropy d15/d33 in ⟨111⟩ textured Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 ceramics

The shear mode piezoelectric properties of Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 (PMN-PZT) ceramic with 72% ⟨111⟩ texture were investigated. The piezoelectric anisotropic factor d15/d33 was as high as 8.5 in ⟨111⟩ textured ceramic as compared to 2.0 in random counterpart. The high d15/d33 indicates the “rotator” ferroelectric characteristics of PMN-PZT system and suggests that the large shear piezoelectric response contributes towards the high longitudinal piezoelectric response (d33) in non-polar direction (d33 = 1100 pC/N in ⟨001⟩ textured ceramic vs. d33 = 112 pC/N in ⟨111⟩ textured ceramic).