Zinc-blende GaN Research Articles

Impact of stacking faults on the luminescence of a zincblende InGaN/GaN single quantum well

Abstract In this paper, we investigate the optical properties of a zincblende InGaN single quantum well structure containing stacking faults. Cathodoluminescence studies revealed the presence of sharp emission features adjacent to stacking faults, identified as quantum wires via their spatial anisotropy. Scanning transmission electron microscopy provided evidence of indium rich regions adjacent to stacking faults which intersect the quantum well along the [110] and [1-10] directions, whilst atom probe tomography revealed that the indium rich regions have an elongated structure, creating a quantum wire. This work sheds light on the intricate relationship between stacking faults and quantum wires in zincblende InGaN single quantum well structures, offering insights into the underlying mechanisms governing their optical behaviour.

Influence of Xe+ and Ga+ milling species on the cathodoluminescence of wurtzite and zincblende GaN

III-nitride materials, such as GaN and its alloys, are essential for modern microelectronics and optoelectronics due to their unique properties. Focused ion beam (FIB) techniques play a crucial role in their prototyping and characterization at the micro- and nanoscale. However, conventional FIB milling with Ga ions presents challenges, including surface amorphization and point defect introduction, prompting the exploration of alternative ion sources. Xenon-based inductively coupled plasma or plasma FIB has emerged as a promising alternative, offering reduced damage and better sample property preservation. Despite extensive research on FIB-induced damage in GaN, systematic comparisons between Ga and Xe ion milling on the luminescence characteristics of GaN remain limited. This study aims to fill this gap by evaluating and comparing the extent of FIB-induced damage caused by Ga and Xe ions in wurtzite and zincblende GaN through cathodoluminescence measurements. Our findings indicate that Xe ion milling yields higher integrated intensities compared to Ga ion milling, attributed to shallower implantation depths and reduced lattice disorder. We also observe a decrease in integrated intensity with increasing ion beam acceleration voltage for both wurtzite and zincblende GaN layers. This study provides valuable insights into optimizing FIB-based sample preparation techniques for III-nitride materials, with implications for enhancing device performance and reliability.

Influence of excess silicon on polytype selection during metal-mediated epitaxy of GaN nanowires

We have examined the origins of polytype selection during metal-mediated molecular-beam epitaxy of GaN nanowires (NWs). High-angle annular dark-field scanning transmission electron microscopy reveals [111]-oriented zinc blende (ZB) NWs and [0001]-oriented wurtzite (WZ) NWs, with SixNy at the interface between individual NWs and the Si (001) substrate. Quantitative energy dispersive x-ray spectroscopy reveals a notably higher Si concentration of 7.0% ± 2.3% in zinc blende (ZB) NWs than 2.3% ± 1.2% in wurtzite (WZ) NWs. Meanwhile, density functional theory calculations show that incorporation of 8 at. % Si on the Ga sublattice inverts the difference in formation energies between WZ and ZB GaN, such that the ZB polytype of GaN is stabilized. This identification of Si and other ZB polytype stabilizers will enable the development of polytype heterostructures in a wide variety of WZ-preferring compounds.

Cathodoluminescence studies of the optical properties of a zincblende InGaN/GaN single quantum well

Zincblende GaN has the potential to improve the efficiency of green- and amber-emitting nitride light emitting diodes due to the absence of internal polarisation fields. However, high densities of stacking faults are found in current zincblende GaN structures. This study presents a cathodoluminescence spectroscopy investigation into the low-temperature optical behaviour of a zincblende GaN/InGaN single quantum well structure. In panchromatic cathodoluminescence maps, stacking faults are observed as dark stripes, and are associated with non-radiative recombination centres. Furthermore, power dependent studies were performed to address whether the zincblende single quantum well exhibited a reduction in emission efficiency at higher carrier densities—the phenomenon known as efficiency droop. The single quantum well structure was observed to exhibit droop, and regions with high densities of stacking faults were seen to exacerbate this phenomenon. Overall, this study suggests that achieving efficient emission from zinc-blende GaN/InGaN quantum wells will require reduction in the stacking fault density.

ScN/GaN(11̅00): A New Platform for the Epitaxy of Twin-Free Metal-Semiconductor Heterostructures.

We study the molecular beam epitaxy of rock-salt ScN on the wurtzite GaN(11̅00) surface. To this end, ScN is grown on freestanding GaN(11̅00) substrates and self-assembled GaN nanowires exhibiting (11̅00) sidewalls. On both substrates, ScN crystallizes twin-free thanks to a specific epitaxial relationship, namely ScN(110)[001]∥GaN(11̅00)[0001], providing a congruent, low-symmetry interface. The 13.1% uniaxial lattice mismatch occurring in this orientation mostly relaxes within the first few monolayers of growth by forming a near-coincidence site lattice, where 7 GaN planes coincide with 8 ScN planes, leaving the ScN surface nearly free of extended defects. Overgrowth of the ScN with GaN leads to a kinetic stabilization of the zinc blende phase, that rapidly develops wurtzite inclusions nucleating on {111} nanofacets, commonly observed during zinc blende GaN growth. Our ScN/GaN(11̅00) platform opens a new route for the epitaxy of twin-free metal-semiconductor heterostructures including closely lattice-matched GaN, ScN, HfN, and ZrN compounds.

Low field mobility in bulk GaN and its ternary AlGaN/GaN compounds (quantum kinetic approach)

Analytical expressions for the low-field mobility of charge carrier gases with three-(3D), two-(2D) and one-(1D) dimensionalities are obtained. Multi-ion ionized impurities scattering, acoustic and polar optic phonons are considered as scattering mechanisms. The calculated values of mobility are compared to known experimental data for bulk (3D) n-and p-type wurtzite, n-type zinc-blende GaN crystals and low dimensional (2D and 1D) ternary GaAlN compounds. The resulting analytical expressions give the dependences of mobility on dimensionality of charge carrier gas, its density, effective mass, temperature and confining dimensions. A comparison of the experimental and calculated temperature dependences of the mobility in bulk GaN crystals (3D) and in AlGaN/GaN nanowires (1D) shows that the mobility at 100\\,{\ ext{K}}$?> T>100K is determined by the scattering of charge carriers by polar optical phonons with an energy of 91.2 meV. The temperature dependences of mobility in Al0.25Ga0.75N/GaN heterostructures (2D) at 100\\,{\ ext{K}}$?> T>100K are in consistent with experiment for electron scattering by polar optical phonons with a noticeably higher energy of 160 meV. We associate this fact with the heterointerface, which according to well-known theoretical studies can change both the strength of electron polar optical phonons scattering and the energy of the phonons.

Crystalline phase purity and twinning of Mg-doped zincblende GaN thin films

Zincblende (zb) GaN thin films doped with Mg were grown by molecular beam epitaxy on GaAs under Ga- and N-rich conditions and different Mg cell temperatures. X-ray diffraction and electron-backscatter diffraction determined the ideal orientation of the zb-GaN, its textural relationship to the substrate, wurtzite and twin content, and their interfaces with the GaN matrix. A comparison to pole figure simulations allowed a thorough description of orientation of wurtzite inclusions and twins, forming interfaces along the 〈111〉 directions of the matrix. The results revealed zincblende volumetric purities as high as 99.9 % and twinning as low as 1.3 %. Ga-rich conditions and lower doping cell temperatures led to the highest phase purities. Moreover, crystalline and surface quality are also deeply linked to the wurtzite inclusions and twinning obtained. Films with the least amounts of these displayed the highest quality. This is likely due to excess Ga atoms on the surface helping the Mg doping atoms incorporate more orderly into the growing film. The results from this investigation provide a deeper understanding of the effects of Mg doping on the crystalline structure of zb-GaN films, paving the way for practical applications of zincblende nitrides in modern optoelectronic devices.

MOVPE studies of zincblende GaN on 3C-SiC/Si(0 0 1)

Cubic zincblende GaN films were grown by metalorganic vapour-phase epitaxy on 3C-SiC/Si (001) templates and characterized using Nomarski optical microscopy, atomic force microscopy, X-ray diffraction, and transmission-electron microscopy. In particular, structural properties were investigated of films where the growth temperature of a GaN epilayer varied in the range of 830 °C to 910 °C and the gas-phase V/III-ratio varied from 15 to 1200 at a constant reactor pressure of 300 Torr. It was observed that with increasing epi temperature at a constant V/III-ratio of 76, the film surface consisted of micrometer-sized elongated features aligned along [1–10] up to a temperature of 880 °C. The zincblende phase purity of such samples was generally high with a wurtzite fraction of less than 1%. When grown above 880 °C the GaN surface morphology degraded and the zincblende phase purity reduced as a result of inclusions with the wurtzite phase. A progressive narrowing of the 002 reflection with increasing epi growth temperature suggested an improvement of the film mosaicity. With increasing V/III-ratio at a constant growth temperature of 880 °C, the film surface formed elongated features aligned along [1–10] at V/III values between 38 and 300 but the morphology became granular at both lower and higher V/III values. The zincblende phase purity is high at V/III values below 300. A slight broadening of the 002 X-ray diffraction reflection with increasing V/III-ratio indicated a small degradation of mosaicity. Scanning electron diffraction analyses of cross-sectional transmission-electron micrographs taken of a selection of samples illustrated the spatial distribution, quantity and structure of wurtzite inclusions within the zincblende GaN matrix. Within the limits of this study, the optimum epilayer growth conditions at a constant pressure of 300 Torr were identified to be at a temperature around 860 °C to 880 °C and a V/III-ratio in the range of 23 to 76, resulting in relatively smooth, zincblende GaN films without significant wurtzite contamination.

Polarity determination of crystal defects in zincblende GaN by aberration-corrected electron microscopy

Aberration-corrected scanning transmission electron microscopy techniques are used to study the bonding configuration between gallium cations and nitrogen anions at defects in metalorganic vapor-phase epitaxy-grown cubic zincblende GaN on vicinal (001) 3C-SiC/Si. By combining high-angle annular dark-field and annular bright-field imaging, the orientation and bond polarity of planar defects, such as stacking faults and wurtzite inclusions, were identified. It is found that the substrate miscut direction toward one of the 3C-SiC ⟨110⟩ in-plane directions is correlated with the crystallographic [1–10] in-plane direction and that the {111} planes with a zone axis parallel to the miscut have a Ga-polar character, whereas the {111} planes in the zone perpendicular to the miscut direction have N-polarity. The polarity of {111}-type stacking faults is maintained in the former case by rotating the coordination of Ga atoms by 180° around the ⟨111⟩ polar axes and in the latter case by a similar rotation of the coordination of the N atoms. The presence of small amounts of the hexagonal wurtzite phase on Ga-polar {111} planes and their total absence on N-polar {111} planes is tentatively explained by the preferential growth of wurtzite GaN in the [0001] Ga-polar direction under non-optimized growth conditions.

Hexagonal GaN nanostructure via electrochemical etching

Gallium nitride GaN thin films were deposited on Si (111) substrates using electrochemical deposition technique at 20 °C. SEM images and EDX results indicated that the growth of GaN films varies with the current density. XRD and Raman analyses showed the presence of hexagonal wurtzite and cubic zinc blende GaN phases with the crystallite size around 18–19 nm. Photoluminescence spectrum showed that the energy gaps of h-GaN/Si (111) and c-GaN/Si (111) were near 3.39 eV and 3.2 eV respectively at 300 K. Raman spectrum indicated the presence of mixed phonon modes of hexagonal and cubic GaN.

Band structures and pressure dependence of the band gaps of GaN

The linear muffin-tin orbital method is used with the atomic-sphere approximation to study the ground-state properties, band structures and pressure dependence of the band gaps of zincblende GaN. The equilibrium lattice constant calculated is smaller than the experimental value by about 2%. It is found that the fundamental band gap is direct and that the value (3.90 eV) is in good agreement with the experimental value. The linear pressure coefficient for this band gap is much smaller than those for Ga.As and GaP. The total density of states and local density of. states are discussed. Moreover, the band structure (wurtzite) GaN is also studied. It is found that the band gap of wurtzite GaN is smaller than that of zincblende GaN, which agrees well with the experimental results.

Inclusion of infrared dielectric screening in the GW method from polaron energies to charge mobilities

We introduce in the many-body GW scheme the modulation of the screened Coulomb interaction W arising from the macroscopic dielectric response in the infrared. We derive expressions for the polaron binding energies, the renormalization of the effective masses and for the electron and hole relaxation times. Electron and hole mobilities are then obtained from the incorporation of appropriate scattering rules. Zinc-blende GaN and orthorhombic MAPbI3 are used as test beds finding fair agreement with results from rigorous electron-phonon coupling approaches. Although limited to polar phonons, our method has a negligible computational cost.

Investigation of wurtzite formation in MOVPE-grown zincblende GaN epilayers on AlxGa1−xN nucleation layers

The influence of AlGaN nucleation layers on zincblende GaN epilayers was studied to investigate the formation of wurtzite phase inclusions in the epilayer. GaN epilayers grown on AlGaN nucleation layers with varying aluminum contents suffer from the increasing presence of wurtzite inclusions as the aluminum content of the nucleation layer increases. High-resolution transmission electron microscopy along with four-dimensional scanning transmission electron microscopy is used to investigate the origin of the wurtzite inclusions in the nucleation layer and at the GaN/AlGaN interface. It was observed that a GaN nucleation layer and an Al0.95Ga0.05N nucleation layer grew in the zincblende and wurtzite phase, respectively. These phases were then adopted by the overgrown GaN epilayers. For a GaN epilayer on an Al0.29Ga0.71N nucleation layer, wurtzite inclusions tend to form at the GaN/ Al0.29Ga0.71N interface due to strong {111}-type faceting observed in the zincblende nucleation layer. This strong faceting is correlated with an enrichment of aluminum in the upper part of the nucleation layer, as observed in energy dispersive x-ray spectroscopy, which may influence the kinetics or thermodynamics controlling the surface morphology.

Binding energy, polarizability, and diamagnetic response of shallow donor impurity in zinc blende GaN quantum dots

In this theoretical investigation, the binding energy, the binding energy Stark-shift, the dipole moment, the polarizability, and the diamagnetic susceptibility related with a confined shallow donor impurity in zinc blende GaN conical-shaped quantum dots are calculated under the effect of an electric field applied along the z-direction. Calculations have been made by using a variational approach within the infinite confining potential model and considering the parabolic conduction band and the effective mass approximations. The results suggest important dependencies of the calculated physical properties on the variation of the dot dimensions, axial impurity position, and the intensity of the applied electric field. It is observed that: i) the binding energy Stark shift increases up to a maximum value and then decreases with increasing the strength of the electric field, and it is strongly influenced by the impurity position and geometrical parameters, ii) for a fixed electric field value, the binding energy Stark shift is always an increasing function of the dot height, and iii) for fixed electric field values, the binding energy Stark shift shows a mixed behavior concerning the dot radius, i.e., for low field strength, the binding energy is always a growing function of the radius, and for large field strengths, such physical quantity grows with the radius up to a maximum and then decreases. Furthermore, the electric field (the strong quantum confinement) enhances slightly (diminishes) the diamagnetic susceptibility of impurity.

Influence of Al x Ga1−x N nucleation layers on MOVPE-grown zincblende GaN epilayers on 3C-SiC/Si(001)

The suitability of Al x Ga1−x N nucleation layers (NLs) with varying Al fraction x for the metal organic vapour phase epitaxy of zincblende GaN on (001) 3C-SiC was investigated, using x-ray photoelectron spectroscopy, atomic force microscopy, and x-ray diffraction. The as-grown NLs exhibited elongated island structures on their surface, which reduce laterally into smaller, more equiaxed islands with increasing AlN composition. During high-temperature annealing in a mixture of NH3 and H2 the nucleation islands with low Al fraction ripened and increased in size, whereas this effect was less pronounced in samples with higher Al fraction. The compressive biaxial in-plane strain of the NLs increases with increasing AlN composition up to x = 0.29. GaN epilayers grown over NLs that have low Al fraction have high cubic zincblende phase purity and are slightly compressively strained relative to 3C-SiC. However, those samples with a measured Al fraction in the NL higher than 0.29 were predominantly of the hexagonal wurtzite phase, due to formation of wurtzite inclusions on various {111} facets of zb-GaN, thus indicating the optimal Al composition for phase-pure zb-GaN epilayer growth.

Hybrid density functional theory study on zinc blende GaN and diamond surfaces and interfaces: Effects of size, hydrogen passivation, and dipole corrections

GaN-diamond heterojunctions show promise in high electron mobility transistor applications. This prompts the need for accurate models of structural and electronic properties. We detail here a hybrid density functional theory study of zinc blende (ZB) GaN and diamond heterostructures using slab supercell models. The dependence of electronic properties on supercell size, pseudo-hydrogen passivation, and dipole corrections is detailed. The larger bulk modulus of diamond is shown to provide a templating structure for GaN to grow upon, where the large lattice mismatch is accounted for through the inclusion of a cationic Ga adlayer. Looking at both type I and II surfaces and interfaces of GaN shows the instability of zb GaN without an adlayer (type II), where increased size, pseudo-hydrogen passivation and dipole corrections do not remove the spurious interaction between the top and bottom layers in type II GaN. Layer dependent density of states, local potential differences, and charge density differences show that the type I interface (with a Ga adlayer) is stable with an adhesion energy of 0.704 eV/Å2 (4.346 J/m2); interestingly, the diamond charge density intercalates into the first layer of GaN, which was seen experimentally for wurtzite GaN grown over diamond. The type II interface is shown to be unstable which implies that, to form a stable, thin-film zb interface between GaN and diamond, the partial pressure of trimethylgallium must be controlled to ensure a Ga layer exists both on the top and bottom layer of the GaN thin film atop the diamond. We believe our results lead to a better understanding of the GaN/diamond multifaceted interface present in the GaN overgrowth on diamond heterojunction used in electronic device applications.

First principles calculation of electronic properties and effective mass of zinc-blende GaN

Based on the first principle pseudopotential plane wave method, the electronic structure of zinc-blende semiconductor GaN is calculated. Using the relativistic treatment of valence states, the spin orbit splitting energy of valence band top near the center of Brillouin region is calculated. Based on the effective mass approximation theory, the effective mass of electrons near the bottom of the conduction band and the effective mass of light and heavy holes near the Γ point along the directions of [100], [110] and [111] are calculated. These parameters are valuable and important parameters of optoelectronic materials.

Blue luminescence origin and Mg acceptor saturation in highly doped zinc-blende GaN with Mg

We study the incorporation of Mg at degenerated doping concentration in zinc-blende GaN and its effect on the optical and electrical properties. The study starts with a theoretical analysis by Density Functional Theory (DFT) and then supported by an experimental methodology employing X-Ray Diffractometry (XRD), Atomic Force Microscopy (AFM), Photoluminescence (PL), Hall effect, Secondary Ion-Mass Spectroscopy (SIMS), and X-Ray Photoelectron Spectroscopy (XPS). Employing Ga-rich growth conditions allow providing the amount of Mg able to obtain an Mg-acceptor concentration limit above 2 × 1019 cm−3. Moreover, the blue band emission centered at 2.85 eV is discussed and used as a systematic tool to detect the Mg incorporation into an undesired site. To the best of our knowledge, the blue emission mechanism is consistently explained in zb-GaN:Mg for the first time by theory and experimentation.

The calculation for quantized valence subband structure of zinc-blende GaN heterojunction quantum well based on k·p method

A quantized valence subband model is proposed to study the energy dispersion of zinc-blende GaN P-channel heterojunction quantum well under arbitrary stress. Anisotropic band structures of bulk GaN and GaN in quantum well under field confinement effect are computed using a six-band stress-dependent k·p Hamiltonian and self-consistently Poisson–Schrödinger equation. A comprehensive study of microscopic relationship between stress and hole effective mass in GaN quantum well has been performed according to the stress effects on quantized valence subband. Simulation results show that uniaxial compression can be considered as the most effective stress in the hole effective mass reduction of zinc-blende GaN heterojunction.

The Quantum Calculation for Valence Band Structure of Strained Zinc-blende GaN Using Six-Band Based k·p Method

The variations of valence band energy with stress effects in zinc-blende GaN are proposed in this paper. The calculations are based on a six-band strain dependent k·p Hamiltonian, and can be self-consistently solved by Schrödinger-Poisson equation. Accurate physical pictures are given for the quantized valence subband structure under biaxial and uniaxial stress in (001) surface along the [110] direction accounting the quantum confinement effect. The warping of the energy profile results in carrier distribution change. This research will be beneficial for improving the hole mobility and the selective of optimum stress for group-III nitride semiconductor based devices.