Pure GaN Research Articles

(Invited) Halide Vapor Phase Epitaxy of III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting devices comprising III-V semiconductors alloys hold record solar-to-hydrogen efficiencies, achievable only by complex architectures involving tandem cells i.e an inverted metamorphic GaInAsP/GaInAs solar cell with an efficiency of 32.6% under 1-sun and the GaInAs/p-GaInP PV/PEC with a solar to hydrogen conversion efficicncy close to 17% under 1-sun as well; nanostructures and buried junctions, which leads to high manufacturing complexity. These approaches have been necessary to circumvent the shortcomings towards unassisted water splitting of state-of-the art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, such as a) inadequate band edge alignment with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) charge recombination. An alternative path to overcome these limitations while maintaining device simplicity and competitive solar fuels costs is to develop novel III-V photoabsorbers with adequate band energetics.First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb into GaN or GaP forms GaSbxN1-x and GaSbyP1-y alloys with narrowed band gap in the former case and changes the electronic band structure from indirect to direct in the latter. Theoretical computations predict that with band gaps in the order of 2 eV, these materials could be able to straddle the electrochemical potentials for water oxidation and proton reduction in acidic electrolytes.Metal organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE) have been used to synthesize GaSbxN1-x and GaSbyP1-y alloys, respectively, achieving single crystalline quality in a wide range of homogeneous Sb incorporation. Experimental measurements confirm the significant band gap reduction from 3.4 eV in pure GaN to 1.5 eV in GaSbxN1-x as determined by Tauc plot analysis of diffuse reflectance data and low temperature photoluminescence spectroscopy; and suggest reduction of the direct transition of GaP at 2.68 eV in the case of GaSbyP1-y alloys.The photoelectrochemical performance towards water splitting of GaSbxN1-x and GaSbyP1-y alloys has been assessed in 2- and 3-electrode standard methods to determine their characteristic attributes, e.g. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situ fluorescence probing. Furthermore, photoelectrochemical impedance measurements have shown the absence of charge transfer resistance at the interface between electrode and electrolyte in the case of GaSbyP1-y alloys that is clearly observed in pure GaP, suggesting antimony can also modify the charge transport properties of the material.These promising results have also motivated our efforts for better understanding of the synthesis processes, particularly for the HVPE technique given its fast growth rate is both an advantage and a challenge for precise thickness and composition control. A computational model using Chemkin Prowas used to validate the previous experimental results for the growth of Gallium Antimonide Phosphide (GaSbyP1-y) films. The model was developed for a reactor having one heating zone for precursor formation and a second zone for film deposition. The first zone was assumed to be in thermodynamic equilibrium. The second (deposition) zone was modeled using a chemistry set describing various gas phase and gas-solid kinetics. The substrate temperature and system pressure were successfully correlated with the observed experimental values for the growth rate and other important features such as the crystalline quality and degree of Sb incorporation. The results also point out the major gas phase chemical species involved in the growth, such as GaCl, SbCl, P2 and P4, which opens the door for future optimization of the synthesis process oriented towards a better precursor utilization and higher antimony incorporation at elevated substrate temperatures, which are required for single crystalline material quality. Acknowledgements: Financial support from Conn Center for Renewable Energy Research and NSF EPSCoR Program (1355438)

On the Investigations of Chip-on-Board Ultra-Violet Sensor by Screen Printing of GaN Powder

In this work, the characteristics of a chip-on-board screen printed GaN UV sensor was investigated. On the sensing element, GaN powders were obtained through ammonolysis of Ga2O3 at 1000°C under NH3 flow. The UV sensor platform was prepared using soft-lithography method, resulted in patterned circuit board. For the screen printing process, GaN powder is mixed with ethylcellulose/ethanol, subsequently deposited on the electrode pairs. The pure GaN sensor exhibited oscillations and change in amplitude upon UV sensing. This could be ascribed to high intrinsic resistance and parasitic capacitance and inductance. To mitigate this effect, rGO fillers were added and showed discern responds. Both sensors (with and without rGO) showed sensitiviy at 300 and 30% respectively, while the current magnitude for the latter was 54 times higher than that of former.

Calculation of Surface Properties of Cubic and Hexagonal Crystals through Molecular Statics Simulations

Surface property is an important factor that is widely considered in crystal growth and design. It is also found to play a critical role in changing the constitutive law seen in the classical elasticity theory for nanomaterials. Through molecular static simulations, this work presents the calculation of surface properties (surface energy density, surface stress and surface stiffness) of some typical cubic and hexagonal crystals: face-centered-cubic (FCC) pure metals (Cu, Ni, Pd and Ag), body-centered-cubic (BCC) pure metals (Mo and W), diamond Si, zincblende GaAs and GaN, hexagonal-close-packed (HCP) pure metals (Mg, Zr and Ti), and wurzite GaN. Sound agreements of the bulk and surface properties between this work and the literature are found. New results are first reported for the surface stiffness of BCC pure metals, surface stress and surface stiffness of HCP pure metals, Si, GaAs and GaN. Comparative studies of the surface properties are carried out to uncover trends in their behaviors. The results in this work could be helpful to the investigation of material properties and structure performances of crystals.

First principle study of optical properties of Cu doped zincblende GaN for novel optoelectronic applications

Current computational study is focused on calculation of optical properties of Cu doped zincblende GaN system where PBE-GGA approximation is employed in Wien2K code. We consider various Cu concentrations 6.25 %, 3.12 %, 1.56 % corresponding to 1 ×1×2, 1×2×2, 2×2×2 supercell configurations respectively. We substitute one Ga atom with one Cu atom in each concentration and we present a good comparison among optical properties of pure GaN and all Cu concentrations. TDOS and PDOS plots reveal contribution of Ga p-states, N p-states and Cu d-states. Optical absorption shows redshift in comparison to pure GaN and because of interactions of Cu and N atoms, we inspect localized d-states at minima of conduction band or Fermi level. Hence, electro-optical properties of zincblende GaN are enhanced upon addition of impurity (Cu) and the material may potentially be used in photonic, power electronics, solar cells, optoelectronics, UV photodetectors and LEDs.

Mechanism of segmentation of lead halide perovskite at interfaces with GaN and ZnO

Structural degradation is a main problem for the commercialization of the organic lead halide perovskite solar cells. A role of the interfaces with the transparent electrodes, in this process, could be made clear by the combined ab initio calculations and the X-ray absorption measurements. The junctions of MAPbI3(MA = CH3NH3) with pure GaN and ZnO crystals, oriented with the polar [0 0 0 1] and [0 0 0 −1] surfaces, are theoretically investigated by means of the density functional theory (DFT) and simulations of the X-ray absorption near-edge spectroscopy (XANES). Unwanted decomposition of the perovskite, into the layers which are parallel to the interface, coexists with a strong binding of the halide atom at GaN [0 0 0 1] and ZnO [0 0 0 1]. In contrast, at the interfaces with the [0 0 0 −1]-oriented GaN and ZnO, aforementioned segmentation of the perovskite does not occur, because binding of the halide atom is much weaker. Thus, the adsorption properties of the halide atom at semiconductor surface might be a precondition for a choice of the substrate to grow the stable perovskite film. The I-core K-edge XANES spectra at the interface atoms contain a fingerprint of the iodide adsorption strength.

Fabrication of a GaN template with an air tunnel in a patterned sapphire substrate using carbonization with a photoresist mask

We successfully fabricated a template with an air tunnel between a patterned sapphire substrate (PSS) and GaN using a photoresist mask. The photoresist mask was subjected to plasma etching to induce the growth of the GaN nucleus at the top of the mask by exposing only the top of the PSS lens to the mask height. The carbon profile of the photoresist mask was checked by secondary ion mass spectrometry to detect the presence or absence of carbon in the GaN template during the process. In order to confirm the carbon’s effect on the epitaxially grown GaN template, the XRD was confirmed by the pole figure and the rocking curve. In order to confirm the change of the energy level of GaN by forming a trap in the energy band by carbon, we confirmed the effect of the carbon by comparing the degree of pure GaN.

Exploring optical properties of Gd doped zincblende GaN for novel optoelectronic applications (A DFT+U study)

In current research, we investigate optical properties of Gd doped zincblende GaN using Wien2K code, by employing DFT+U. We consider pure GaN and we dope various Gd concentrations 3.12%, 6.25% and 12.5% into the host GaN lattice while supercell size is kept fixed (1 × 2 × 2) for all cases. We elaborate and present a detailed comparison among optical and electronic properties of pure GaN with various Gd concentrations. Interaction of Gd and N atoms and localization of d and f states of dopant are remarked near Fermi level or maxima of valence band. In comparison to pure GaN, absorption spectra for 3.12% Gd concentration shows redshift but for highest Gd concentration (6.25% 12.5%), a blueshift in absorption spectrum is noted. Absorption is pronounced and enhanced in the UV region. Study of optical spectra for various optical properties suggest that Gd:GaN system is mostly suitable for UV optoelectronics. Results for optical properties have great similarity with the existing literature works. Our unprecedented and first ever reported results on optical properties of Gd:GaN system, direct future path of this material for its potential uses in UV optoelectronic, photonic, LEDs, UV sterilization application, photosensors, thermochromic solar cells and biochemical sensing industries.

As-related stability of the band gap temperature dependence in N-rich GaNAs

GaNAs layers with a low As concentration (As ≤ 0.6%) have been grown by molecular beam epitaxy and studied by structural and optical methods. It has been observed that the incorporation of a small amount of As atoms into the GaN host leads to a significant reduction of the bandgap due to the formation of an As-related band above the valence band of the GaN host. The position of this band does not change with temperature, and therefore, a reduced temperature dependence of the bandgap is observed for As-diluted GaN compared to the pure GaN host, which is ∼40 meV vs ∼70 meV in the 10–295 K temperature range. The observed effect is explained within the band anticrossing model. It is expected that the reduced temperature dependence of the bandgap in As-diluted GaN can be utilized in lasers with improved thermal stability.

Strong suppression of In desorption from InGaN QW by improved technology of upper InGaN/GaN QW interface

The aim of this work is to elucidate how different growth mode and composition of barriers can influence the QW properties and their PL and to find optimal QW capping process, to suppress the In desorption from QWs and to maintain the QW PL efficiency. It concentrates on the technology procedure for growth of upper quantum well (QW) interfaces in InGaN/GaN QW structure when different temperature for QW and barrier epitaxy is used. We have found that optimal photoluminescence (PL) results were achieved, when the growth after QW formation was not interrupted, but immediately continued during the temperature ramp by the growth of (In)GaN capping layer with small introduction of In precursor into the reactor. Optimal barrier between QW with respect to PL results was found to be pure GaN. We have shown according to SIMS and HRTEM results that by this technological procedure the InGaN desorption was considerably suppressed and three times higher In concentration and two times thicker QWs were achieved for the same QW growth parameters without deterioration of PL intensity in comparison to sample with usually used thin GaN low temperature capping protection. Additionally, for samples covered by the QW capping layer during the temperature ramp the defect band is almost completely missing, thus we can conclude that this defect band is connected with quality of the upper QW interface.

Enhanced mobility in vertically scaled N-polar high-electron-mobility transistors using GaN/InGaN composite channels

A GaN/InGaN composite channel design for vertically scaled N-polar high-electron-mobility transistor (HEMT) structures is proposed and demonstrated by metal-organic chemical vapor deposition. In a conventional N-polar HEMT structure, as the channel thickness (tch) decreases, the sheet charge density (ns) decreases, the electric field in the channel increases, and the centroid of the two-dimensional electron gas (2DEG) moves towards the back-barrier/channel interface, resulting in stronger scattering and lower electron mobility (μ). In this study, a thin InGaN layer was introduced in-between the channel and the AlGaN cap to increase the 2DEG density and reduce the electric field in the channel and therefore increase the electron mobility. The dependence of μ on the InGaN thickness (tInGaN) and the indium composition (xIn) was investigated for different channel thicknesses. With optimized tInGaN and xIn, significant improvements in electron mobility were observed. For a 6 nm channel HEMT structure, the electron mobility increased from 606 to 1141 cm2/(V·s) when the 6 nm thick pure GaN channel was replaced by the 4 nm GaN/2 nm In0.1Ga0.9N composite channel.

Effects of heterogeneity and prestress field on phonon properties of semiconductor nanofilms

The phonon dispersion relation, phonon group velocity and phonon density of state in GaN-based wurtzite nanofilms under prestress fields are investigated theoretically. Considering the acoustoelastic effects and spatial confinement effects, the acoustic phonons properties in stressed AlN/GaN/AlN nanofilms are derived by using the elasticity model, comparing with those in the pure AlN nanofilm and pure GaN nanofilm. Numerical simulations demonstrate that the heterogeneity in GaN-based nanofilms can modify the phonon dispersion relations of nanofilms, leading to the change of phonon group velocity and phonon density of state. The stress field can significantly affect the phonon properties of GaN-based heterostructural nanofilms, e.g. the positive stress decreases the phonon energy and group velocity while enhances the density of state. The influence of the stress field is also sensitive to the thickness of each component in heterostructural nanofilms. The present work will be useful in tuning the phonon properties through phonon engineering and strain/stress engineering to control the thermal and electrical performance in GaN-heterostructure-based electronic devices.

BInGaN alloys nearly lattice-matched to GaN for high-power high-efficiency visible LEDs

InGaN-based visible light-emitting diodes (LEDs) find commercial applications for solid-state lighting and displays, but lattice mismatch limits the thickness of InGaN quantum wells that can be grown on GaN with high crystalline quality. Since narrower wells operate at a higher carrier density for a given current density, they increase the fraction of carriers lost to Auger recombination and lower the efficiency. The incorporation of boron, a smaller group-III element, into InGaN alloys is a promising method to eliminate the lattice mismatch and realize high-power, high-efficiency visible LEDs with thick active regions. In this work, we apply predictive calculations based on hybrid density functional theory to investigate the thermodynamic, structural, and electronic properties of BInGaN alloys. Our results show that BInGaN alloys with a B:In ratio of 2:3 are better lattice matched to GaN compared to InGaN and, for indium fractions less than 0.2, nearly lattice matched. Deviations from Vegard's law appear as bowing of the in-plane lattice constant with respect to composition. Our thermodynamics calculations demonstrate that the solubility of boron is higher in InGaN than in pure GaN. Varying the Ga mole fraction while keeping the B:In ratio constant enables the adjustment of the (direct) gap in the 1.75–3.39 eV range, which covers the entire visible spectrum. Holes are strongly localized in non-bonded N 2p states caused by local bond planarization near boron atoms. Our results indicate that BInGaN alloys are promising for fabricating nitride heterostructures with thick active regions for high-power, high-efficiency LEDs.

Point defects and composition in hexagonal group-III nitride monolayers: A first-principles calculation

In this paper, we systematically investigate the structural, electronic and magnetic properties of hexagonal group-III nitride monolayers with point defects and alloying on the basis of first-principles calculations. Six typical point defects including three vacancies and three antisites are modeled in pure AlN, GaN and InN monolayers. The defect-induced modifications of band gaps and magnetic properties are demonstrated. The vacancy of nitrogen, with lowest formation energy, metalizes the semiconducting nitride monolayers. The defects losing single group-III atom introduce net magnetic moment to the systems, while others maintain non-magnetic. For ordered alloy monolayers, the AlGaN and InGaN systems are taken into consideration. The compositional variation is achieved by atomic substitution in supercells with different sizes. We find that the lattice constant and cohesive energy follow good linear relation with concentration while a slight bowing effect is observed for the band gap. These results provide a development in defective and alloy nitride monolayers and extend the potential applications.

Controlling Surface Morphology and Circumventing Secondary Phase Formation in Non-polar m-GaN by Tuning Nitrogen Activity

For the development of non-polar nitrides based optoelectronic devices, high-quality films with smooth surfaces, free of defects or clusters, are critical. In this work, the mechanisms governing the topography and single phase epitaxy of non-polar m-plane gallium nitride (m-GaN) thin films are studied. The samples were grown using plasma-assisted molecular beam epitaxy on m-plane sapphire substrates. Growth of pure m-GaN thin films, concomitant with smooth surfaces is possible at low radio frequency powers and high growth temperatures as judged by the high resolution x-ray diffraction, field emission scanning electron microscopy, and atomic force microscopy measurements. Defect types and densities are quantified using transmission electron microscopy, while Raman spectroscopy was used to analyze the in-plane stress in the thin films which matches the lattice mismatch analysis. Energy dispersive spectroscopy and cathodoluminescence support a congruent growth and a dominant near band edge emission. From the analysis, a narrow growth window is discovered wherein epitaxial growth of pure m-plane GaN samples free of secondary phases with narrow rocking curves and considerable smooth surfaces are successfully demonstrated.

Si Complies with GaN to Overcome Thermal Mismatches for the Heteroepitaxy of Thick GaN on Si.

Heteroepitaxial growth of lattice mismatched materials has advanced through the epitaxy of thin coherently strained layers, the strain sharing in virtual and nanoscale substrates, and the growth of thick films with intermediate strain-relaxed buffer layers. However, the thermal mismatch is not completely resolved in highly mismatched systems such as in GaN-on-Si. Here, geometrical effects and surface faceting to dilate thermal stresses at the surface of selectively grown epitaxial GaN layers on Si are exploited. The growth of thick (19 µm), crack-free, and pure GaN layers on Si with the lowest threading dislocation density of 1.1 × 107 cm-2 achieved to date in GaN-on-Si is demonstrated. With these advances, the first vertical GaN metal-insulator-semiconductor field-effect transistors on Si substrates with low leakage currents and high on/off ratios paving the way for a cost-effective high power device paradigm on an Si CMOS platform are demonstrated.

Hydride-vapor-phase epitaxial growth of highly pure GaN layers with smooth as-grown surfaces on freestanding GaN substrates

Thick (20–30 µm) layers of highly pure GaN with device-quality smooth as-grown surfaces were prepared on freestanding GaN substrates by using our advanced hydride-vapor-phase epitaxy (HVPE) system. Removal of quartz parts from the HVPE system markedly reduced concentrations of residual impurities to below the limits of detection by secondary-ion mass spectrometry. Appropriate gas-flow management in the HVPE system realized device-quality, smooth, as-grown surfaces with an excellent uniformity of thickness. The undoped GaN layer showed insulating properties. By Si doping, the electron concentration could be controlled over a wide range, down to 2 × 1014 cm−3, with a maximum mobility of 1150 cm2·V−1·s−1. The concentration of residual deep levels in lightly Si-doped layers was in the 1014 cm−3 range or less throughout the entire 2-in. wafer surface. These achievements clearly demonstrate the potential of HVPE as a tool for epitaxial growth of power-device structures.

Phonon transport properties of bulk and monolayer GaN from first-principles calculations

The thermal conductivity of isotopically pure wurtzite GaN and the corresponding two-dimensional monolayer crystals are investigated based on first-principles calculations and phonon Boltzmann transport equation. It is found that the c-axis thermal conductivity of defect-free wurtzite GaN is 42–11% higher than that along the in-plane direction when temperature increases from 100 to 800K. Our calculation also shows that the thermal conductivity of monolayer GaN with hexagonal lattice is higher than that with haeckelite lattice by a factor of four at room temperature. The phonon dispersion relation, phonon group velocity and the phonon scattering rate are extracted to uncover the underlying physical mechanism. The thermal conductivity accumulation function with respect to the phonon mean free path and the relative contribution of each phonon branch for the two kinds of monolayer GaN are further calculated as compared to that of graphene. These investigations would provide important guidance to develop GaN-based electronic and photonic device.

A comparative ab intio study on structural evolution, stability and electronic properties of undoped and Al-doped GaxNy ( x + y = 4-25 clusters

We present a comparative study on structure evolution, binding energies, relative stabilities, electronic and vibrational properties of size-selected GaN and Al-doped GaN clusters by employing B3LYP exchange-correlation function with double- $ \xi$ basis set LANL2DZ via the density functional theory (DFT) method. Cube, ring, and rhombus stacks are constructed, and the optimization results suggest that the evolution of basic structural entities of different morphologies from planer rings to nanotubes may be realized by taking into account the stacking mode of stable clusters. We also find that the geometry of each Al-substituted GaN cluster keeps a structure similar to that of the corresponding pure GaN clusters. The stability, IR and Raman activity vary with the growth of units and the stacking mode. The energetic analysis shows that the most stable structure is the 3-dimensional cubic structure. A blue shift in the vibrational frequency of the most intense Raman activity has been observed with variation in size and structure. Moreover, the vibrational properties are significantly affected by the introduction of Al dopants. We also consider parameters, such as HOMO-LUMO gap, ionization potential (IP), electron affinity (EA), dipole moment (DM), chemical potential (CP), chemical hardness (CH), and results were compared and critically discussed for interpretation of the enhanced stability and to extract useful information for their applications.

Effects of transition metal ions doping on optical and electronic properties of GaN

Transition metal ions Co, Cr, Mn and V were implanted at 200 keV into GaN thin films at three different doses 5 × 1014, 5 × 1015 and 5 × 1016 cm−2 and then thermally annealed to recover the lattice from implantation induced damages. In order to access the modification in optical properties of the samples, detailed photoluminescence investigations were carried out at room temperature and low temperatures. The emission related to band edge of the material is observed un-shifted for all samples but lower in intensity whereas several new luminescence centers appeared after implantation. The emission observed for pure GaN at 3.44 eV is enhanced after implantation whereas newly observed line at 3.35 eV appeared only after implantation at highest dose of 5 × 1016 cm−2. The decrease in intensity of existing emission lines of the material is assigned to implantation induced non-radiative recombination centers whereas emergence of new peaks is allocated to dopant related gap states as per predictions made by our first principles calculations.

GaN nanocones field emitters with the selenium doping

The pure and selenium (Se)-doped GaN nanocones were grown via a vapor–liquid–solid (VLS) mechanism with Pt catalysts. The diameter of GaN nanocones gradually decreases from ~500 to ~100 nm along the wire axis,and the tips of GaN nanocones change to thin with sharp ends. The Se-doped GaN nanocones have exhibited impressive field emission properties and high stability with a lower turn-on field of 2.75 V/µm (at room temperature), which is lower than pure GaN nanocones (5.53 V/μm). Consequently, the incorporation of Se impurities can improve the field emission performance of GaN nanocones, which is sufficient for application in field electron emission devices and cold electron sources in display devices. Moreover, the synthesized Se-doped GaN nanocones will facilitate flexible design of device architectures for nanoelectronics.