Wurtzite GaN Research Articles

Mitigate self-compensation with high crystal symmetry: A first-principles study of formation and activation of impurities in GaN

Formation and activation energies of common impurities: silicon (Si), germanium (Ge), carbon (C), beryllium (Be), and magnesium (Mg) in wurtzite (wz-) and zincblende (zb-) GaN are investigated by first–principles calculations. Si and Ge are excellent donors with low activation energies (<30 meV). The acceptor activation energies of C and Be in wz-GaN (and zb-GaN) are found to be 590 and 205 meV (and 490 and 134 meV), respectively. Neither C nor Be is a qualified acceptor in general as the former has a large activation energy (>300 meV) and the latter has a strong self-compensation effect (the formation energy of Bei compensating donor is negative). Bei is energetically abundant in zb-GaN because it has a 0.66 eV lower formation energy than wz-GaN. This is attributed to the coherence of the symmetricity between the s-orbitals and the interstitial site. The acceptor activation energy of Mg in zb-GaN is found to be 153 meV, which is smaller than its value in wz-GaN, 226 meV. Vibrational analysis suggests that Mgi compensating donor is less favorable to form in zb- than wz-GaN, since the higher symmetricity of the interstitial site in zb-GaN renders a much smaller vibrational entropy. Contrarily, the differences are insignificant for the impurities residing on Ga sites due to the same tetrahedral symmetricity. The results evidence the importance of self-compensation effects, which can be mitigated by the higher crystal symmetry of zb-GaN.

Anisotropic Strain on GaN Microdisks Grown by Plasma-Assisted Molecular Beam Epitaxy

Lattice relaxation on wurtzite GaN microdisks grown by plasma-assisted molecular beam epitaxy was systematically studied. The lattice constants of GaN microdisks were evaluated from high-resolution transmission electron microscopy, and the anisotropic strain was then analyzed by observing the microscopic atomic layers. We found that the vertical lattice strain along the c-axis followed a linear relationship, while the lateral lattice strain along the a-axis exhibited a quadratic deviation. The lattice mismatch is about 0.94% at the interface between the GaN microdisks and the γ-LiAlO2 substrate, which induces the anisotropic strain during epi-growth.

Electrical and Optical Properties of Beryllium Deep Acceptors in GaN

Beryllium (Be) may be a substitute for GaN p-type dopant magnesium due to its smaller atomic size and higher electronegativity. However, Be has been shown to be a deep acceptor in recent experiments. By using the hybrid functional based on density functional theory (DFT), we predict two acceptors with symmetric-broken configurations (SB1 and SB2) in neutral charge state of Be substitutional doping, and analyze their optical and electronic properties. The ionization energy and emission peak of the symmetric-broken configuration SB2 are almost the same as the ground-state (neutral charge state) configuration SB1. Our work provides a comprehensive understanding of the deep acceptor behavior of Be in wurtzite GaN.

Piezoelectric Electron-Phonon Interaction from AbInitio Dynamical Quadrupoles: Impact on Charge Transport in Wurtzite GaN.

First-principles calculations of e-ph interactions are becoming a pillar of electronic structure theory. However, the current approach is incomplete. The piezoelectric (PE) e-ph interaction, a long-range scattering mechanism due to acoustic phonons in noncentrosymmetric polar materials, is not accurately described at present. Current calculations include short-range e-ph interactions (obtained by interpolation) and the dipolelike Frölich long-range coupling in polar materials, but lack important quadrupole effects for acoustic modes and PE materials. Here we derive and compute the long-range e-ph interaction due to dynamical quadrupoles, and apply this framework to investigate e-ph interactions and the carrier mobility in the PE material wurtzite GaN. We show that the quadrupole contribution is essential to obtain accurate e-ph matrix elements for acoustic modes and to compute PE scattering. Our work resolves the outstanding problem of correctly computing e-ph interactions for acoustic modes from first principles, and enables studies of e-ph coupling and charge transport in PE materials.

Band alignment at β -Ga2O3/III-N (III = Al, Ga) interfaces through hybrid functional calculations

The band alignment and the chemical bonding at the β-Ga2O3/AlN and β-Ga2O3/GaN interfaces are studied through hybrid functional calculations. We construct realistic slab models with III–O (III = Al, Ga) bonds dominating the chemical bonding at both interfaces. The epitaxial relationships between β-Ga2O3 and wurtzite AlN and GaN determined from experiments are adopted in our slab models. These models satisfy electron counting rules, and all the dangling bonds are saturated at the interfaces. β-Ga2O3 is found to form type II heterojunctions with both wurtzite AlN and GaN. For the interfaces with AlN and GaN substrates, the calculated valence band offsets are 0.74 and 0.90 eV, respectively. These are in good agreement with the experimental values. The obtained band alignments are useful for designing optical and electronic devices based on β-Ga2O3 and group III nitrides.

Gadolinium-implanted GaN studied by spin-polarized positron annihilation spectroscopy

In this study, Gd ion implantation and annealing were performed at ${900}^{\ensuremath{\circ}}\phantom{\rule{0.16em}{0ex}}\mathrm{C}$ for nominally undoped wurtzite GaN grown by metal-organic chemical vapor deposition. Spin-polarized positron annihilation measurements showed that vacancy clusters including at least 12 vacancies per cluster were the major positron-trapping centers and that the electrons in the vacancy clusters were spin-polarized. These observations could be explained by first-principles calculations. The previous speculation about the defect-assisted ferromagnetism of Gd-implanted GaN may be supported if vacancy clusters are considered.

Polarization-resolved Er emission in Er doped GaN bulk crystals

Erbium-doped GaN (Er:GaN) quasi-bulk crystals are emerging as a promising novel gain medium for high energy lasers emitting at the retina-safe wavelength window of 1.5 μm. We report the polarization-resolved photoluminescence (PL) emission spectroscopy studies, which revealed that the pumping efficiency with the excitation polarization parallel to the c-axis of GaN (E⇀||c⇀) is significantly higher than that with the excitation polarization perpendicular to the c-axis of GaN (E⇀⊥c⇀). This phenomenon is a direct consequence of the inherent polar wurtzite GaN lattice, giving rise to a net local field, surrounding each Er ion, along the c-axis of GaN. The temperature dependent behaviors of the PL emission spectra were explained in terms of the Boltzmann population distributions among sublevels within the 4I15/2 ground state and the 4I13/2 first excited state of Er3+ in GaN, thereby providing an improved understanding regarding the origin of the dominant emission lines observed near 1.5 μm. The results suggested that the polarization field in GaN can be exploited to enhance the effective Er excitation cross section by manipulating the polarization of the excitation light source.

Structural and Optical Properties of GaN Film on Copper and Graphene/Copper Metal Foils Grown by Laser Molecular Beam Epitaxy.

We report the direct growth of crystalline GaN on bare copper (Cu) and monolayer-graphene/Cu metal foils using laser molecular beam epitaxy technique at growth temperature of 700 °C. The surface morphology investigated with field emission scanning electron microscopy revealed that the size of GaN grains for film grown on bare Cu falls in range of 90 to 160 nm whereas large grains with size of ˜200 to 600 nm was obtained for GaN grown on graphene/Cu foil under similar growth condition. The transverse optical mode of cubic GaN and E₂ (high) phonon mode for wurtzite GaN phases were obtained on the GaN film grown on Cu and graphene/Cu metal foils as deduced by Raman spectroscopy. The photoluminescence (PL) spectroscopy studies showed that the near band edge emission peaks for GaN on Cu and graphene/Cu consist two major peaks at 3.26 and 3.4 eV, corresponding to cubic and wurtzite GaN, respectively. The Raman and PL studies disclosed that the mixed phase growth of GaN occurs on these foils and better structural and optical quality for GaN on graphene/Cu foil. The direct growth of GaN on two dimensional graphene on polycrystalline metal foils is beneficial various transferrable and flexible opto-electronics device applications.

Nano-spectroscopic and nanoscopic imaging of single GaN nanowires in the sub-diffraction limit

The optical spatial resolution limit prohibits a precise spectroscopic characterization of objects at the nanoscale. However, plasmonic assisted microscopy at the nanoscale can overcome the diffraction limit owing to the confinement of the localized electric field. We report the spectroscopic imaging of a single GaN nanowire (NW) of diameter ∼200 nm with the help of Au nanoparticle assisted plasmonic tip-enhanced Raman spectroscopy (TERS), well below the diffraction limit of ∼750 nm. The challenge in the achievement was that the insignificant value of change of polarizability in the partially ionic III–V nitrides made the Raman scattering intensity too low for spectroscopic studies. Consequently, the enhancement factors for different observed Raman modes for the wurtzite GaN NW were also found to be significantly low. Spectroscopic imaging of such low scattering efficiency group III-nitride single GaN NW is achieved in the sub-diffraction limit by careful selection of the TERS substrate. Similarly, the nanoscopic optical imaging of a single GaN NW of diameter ∼10 nm using the confined electromagnetic wave is also achieved with the help of the plasmonic assisted near-field scanning optical microscopic technique in the sub-diffraction limit.

Piezotronic Synapse Based on a Single GaN Microwire for Artificial Sensory Systems.

Tactile information is efficiently captured and processed through a complex sensory system combined with mechanoreceptors, neurons, and synapses in human skin. Synapses are essential for tactile signal transmission between pre/post-neurons. However, developing an electronic device that integrates the functions of tactile information sensation and transmission remains a challenge. Here, we present a piezotronic synapse based on a single GaN microwire that can simultaneously achieve the capabilities of strain sensing and synaptic functions. The piezotronic effect in the wurtzite GaN is introduced to strengthen synaptic weight updates (e.g., 330% enhancement at a compressive stress of -0.36%) with pulse trains. A high gauge factor for strain sensing (ranging from 0 to -0.81%) of about 736 is also obtained. Remarkably, the piezotronic synapse enables the neuromorphic hardware achievement of the perception and processing of tactile information in a single micro/nanowire system, demonstrating an advance in biorealistic artificial intelligence systems.

Synthesis of GaN nanowires by ammoniating Ga2O3/BN

GaN nanowires were successfully prepared on Si (111) substrate through ammoniating Ga2O3/BN films deposited by radio frequency magnetron sputtering system. The synthesized nanowires were confirmed as hexagonal wurtzite GaN by X-ray diffraction, selected-area electron diffraction and Fourier transform infrared spectra. Scanning electron microscopy and transmission electron microscopy revealed that the grown GaN nanowires have a smooth and clean surface with diameters ranging from 40 to 160 nm and lengths typically up to several tens of micrometers. The representative photoluminescence spectrum at room temperature exhibited a strong UV light emission band centered at 363 nm and a relative weak purple light emission peak at 422 nm. The growth mechanism is discussed briefly.

Mean Free Path–Thermal Conductivity Accumulation Calculations for Wurtzite Gallium Nitride: Two Approaches

ABSTRACT Understanding the mean free path distribution of the dominant heat carriers is very important in determining the ballistic to diffusive heat transport transition in nanoscale devices. This is true for the high electron mobility transistors made from GaN where both the thickness of the buffer layer and localized heating causing ballistic-diffusive heat transfer may complicate the transport properties needed to describe the device thermal response. In this work, we obtain the mean free path–thermal conductivity relation of phonons in bulk wurtzite GaN crystals using two different, ab-initio-based calculations. While the Vienna Ab-initio Simulation Package (VASP) is used in both approaches at the initial stage, the first method does not calculate the third-order force constants (FCs) and approximates the anharmonicity with a single fitting parameter in determination of discrete phonon properties thermal conductivity and relaxation time, while the second method uses third-order force constants directly. Results show that the third-order FCs are important in modeling the contribution of high-frequency optical phonons with relatively short MFPs, to the thermal conductivity of the material. Yet these effects are more significant at high temperatures and at samples without crystallographic disorders, and they can be omitted while modeling the real samples at low temperatures.

Metalorganic vapor phase epitaxy of wurtzite InP nanowires on GaN

The metalorganic vapor phase epitaxy of wurtzite InP nanowires on GaN (0001) is demonstrated. The InP nanowires exhibit the same wurtzite structure as the underlying wurtzite GaN. The photoluminescence studies indicate that the InP nanowires are single-phase wurtzite with high crystalline quality which is supported by transmission and scanning electron microscopy images. The position of the second valence band or valence band splitting energy is also deduced from the photoluminescence data to be ΔAB=30 meV at room temperature. The InP/GaN heterojunction can enable exotic optoelectronic and spintronic experiments and applications. In addition, these results can enable traditional III–V growth on III-N materials for heterojunction devices.

Core–Shell Tunnel Junction Nanowire White-Light-Emitting Diode

We have demonstrated a new class of phosphor-free white LEDs with the use of tunnel junction structure in nonpolar core-shell InGaN nanowires. It is confirmed that the tunnel junction based nanowire LEDs can eliminate the use of the resistive p-GaN:Mg contact layer, leading to significantly enhanced hole injection and dramatically reduced voltage loss. The nonpolar core-shell nanowire heterostructure showed the enhanced carrier injection efficiency through the widened shell n-GaN contact area. The TEM analysis verified that the core-shell Al tunnel junction layers were uniformly grown on nonpolar surfaces of the GaN wurtzite crystal nanowire structure. We have also showed the monolithic integration of multiple-color emission on a single chip by using the multiple-stacked tunnel junction core-shell nanowire heterostructure. Compared to the conventional film based quantum well LEDs, the demonstrated nonpolar core-shell tunnel junction nanowire LEDs will be a very promising candidate for future solid-state lighting applications as well as phosphor-free white LEDs.

Decorated Dislocations against Phonon Propagation for Thermal Management

The impact of decorated dislocations on the effective thermal conductivity of GaN is investigated by means of equilibrium molecular dynamics simulations via the Green-Kubo approach. The formation of "nanowires" by a few atoms of In in the core of dislocations in wurtzite GaN is found to affect the thermal properties of the material, as it leads to a significant decrease of the thermal conductivity, along with an enhancement of its anisotropic character. The thermal conductivity of In-decorated dislocations is compared to the ones of pristine GaN, InN, and random and ordered InxGa1-xN alloy, to examine the impact of doping. Results are explained by the stress maps, the bonding properties and the phonon density of states of the aforementioned systems. The decorated dislocations engineering is a novel way to tune, among other transport properties, the effective thermal conductivity of materials at the nanoscale, which can lead to the manufacturing of interesting candidates for thermoelectric or anisotropic thermal dissipation devices.

Controlled growth of GaN nanorods directly on flexible Mo metal foil by laser molecular beam epitaxy

Direct growth of one-dimensional (1D) GaN nanostructures on metal foils is perceived as a promising approach for the realization of futuristic flexible opto-electronic devices. Optimization of growth process to achieve 1D GaN nanostructures on metal foils is a challenging task as the growth is sensitive to the nature of individual metals. Here, we report the laser molecular beam epitaxy growth of GaN nanorods by investigating the role of growth temperature, laser energy density and laser repetition rate in the range of 500–700 °C, 3–5 J/cm2and 10–30 Hz, respectively. A higher growth temperature is noticed to be very crucial in determining the formation of 1D GaN nanorod while the laser energy and repetition rate mostly control the nanorod density. GaN nanorod ensemble with the density, length and top cross-sectional width, respectively,~1.41 × 109 cm−2, 430–620 nm and 70–160 nm is achieved at 700 °C using laser energy density of ~5 J/cm2 and repetition rate of 10 Hz. The shape of GaN nanorods is nearly same for different laser repetition rates (20–30 Hz) and the density of GaN nanorods increases with the laser repetition rate (~3.42 × 109 cm−2 for 20 Hz and ~3.51 × 109 cm−2 for 30 Hz). X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy studies revealed the c-axis oriented growth of single crystalline wurtzite GaN nanorods on Mo foil. Room temperature photoluminescence spectroscopy exhibited a high intense near band edge emission at ~3.4 eV with negligible deep-bands. The controlled growth of self-assembled GaN nanostructures on polycrystalline metal foil with good structural and optical properties has the potential for developing III-nitride based bendable optoelectronic devices.

Titanium Carbide MXene Nucleation Layer for Epitaxial Growth of High-Quality GaN Nanowires on Amorphous Substrates.

Growing III-nitride nanowires on 2D materials is advantageous, as it effectively decouples the underlying growth substrate from the properties of the nanowires. As a relatively new family of 2D materials, MXenes are promising candidates as III-nitride nanowire nucleation layers capable of providing simultaneous transparency and conductivity. In this work, we demonstrate the direct epitaxial growth of GaN nanowires on Ti3C2 MXene films. The MXene films consist of nanoflakes spray coated onto an amorphous silica substrate. We observed an epitaxial relationship between the GaN nanowires and the MXene nanoflakes due to the compatibility between the triangular lattice of Ti3C2 MXene and the hexagonal structure of wurtzite GaN. The GaN nanowires on MXene show good material quality and partial transparency at visible wavelengths. Nanoscale electrical characterization using conductive atomic force microscopy reveals a Schottky barrier height of ∼330 meV between the GaN nanowire and the Ti3C2 MXene film. Our work highlights the potential of using MXene as a transparent and conductive preorienting nucleation layer for high-quality GaN growth on amorphous substrates.

Noise characteristics of Ni/GaN Schottky barrier IMPATT diode based on polar- and nonpolar-oriented wurtzite GaN for terahertz application

Abstract This paper reports a study investigating the noise performance of Ni/GaN Schottky barrier impact-ionization-avalanche-transit-time (IMPATT) diodes based on the polar- and nonpolar-oriented wurtzite GaN by a numerical simulation. Results show that the nonpolar IMPATT diode exhibits a significant wider and higher frequency-band corresponding to the low noise measure (NM). The upper limit of the low-noise frequency-band of the IMPATT diode shifts from 159.2 GHz to 184.3 GHz, from 183.4 GHz to 245.7 GHz and from 212.2 GHz to 285.6 GHz when J0 = 1 kA/cm2, 10 kA/cm2 and 100 kA/cm2, respectively, comparing the polar with the nonpolar IMPATT diodes. Our analysis proves that the mechanism comes from excellent properties of the Negative Differential Mobility (NDM) characteristic in the nonpolar orientation wurtzite GaN. In particular, the nonpolar diode still demonstrates better unity between the noise and RF power performances and can significantly improve the stability of IMPATT diodes at terahertz frequency.

Specific heat at constant pressure from first principles: contributions from fully anharmonic vibrations

Specific heat at constant pressure (Cp) for diamond Si and wurtzite GaN is calculated using a novel, first principles method based on density functional theory. This method, termed the Beyond Quasi-Harmonic method, completely takes into account all anharmonic vibrations–both from changes in volume and phonon interactions. Our calculated values for Si show excellent agreement with the generally accepted experimental values. For GaN, however, there is a large discrepancy between two sets of experiments. From an analysis of the available literature, and our own data we develop a set of recommended values.

Defect-induced exciton localization in bulk gallium nitride from many-body perturbation theory

We present a many-body perturbation theory study of the excitonic properties of wurtzite GaN containing a single charged nitrogen vacancy. We determine that the lowest-energy exciton consists of a bulk to defect transition, resulting in a slight redshift ($&lt;0.1$ eV) of the optical absorption onset and a 50 meV increase in the exciton binding energy when compared with pristine bulk. Furthermore, by analysis of the electron-hole correlation function, we quantify the defect-induced localization of the Wannier-Mott exciton in two ways. First, we show that the electron-hole separation is reduced, and that the exciton envelope wave function can be related to a simple model of a defect-bound exciton. Second, we show that the exciton center-of-mass does not display the periodicity of the lattice due to defect-induced localization. We anticipate that our approach, which quantitatively describes the influence of a point defect on the exciton wave function, will be generally applicable.