Structural Properties Of GaN Research Articles

Assessing the Effect of Electronic Pseudopotentials and Relativistic Treatments on the Structural and Electrical Properties of GaN: A DFT Study

Applying the principle of Density functional theory, we can calculate various parameters like lattice constant, band gap, band plot, dielectric function plot, refractive index plot, conductivity plot, density of state plot, loss function etc. of GaN. In this work, we use different electronic pseudopotentials with different relativistic treatment studied using Local density approximation functional (LDA-CAPZ) within DFT for GaN. We used to calculate the energy values, lattice parameters change after geometry optimisation and plot the band energies. Electronic structure calculations results are compares taking different electronic pseudopotentials of different cut-off energy having different relativistic approaches. The Density of state plot and partial density of states plot help to studied more about the electronic as well as magnetic characteristics of the GaN sample. Here, we also compare the advantages and disadvantages of different pseudopotentials with different relativistic approaches of the sample. Energy level distribution and partial density of states were compared for all the pseudopotentials with different relativistic treatments, providing insight into the orbital contributions of electrons to the density of levels. Our study provides a deeper understanding into the impact of electronic pseudopotentials and relativistic treatments on the electronic and structural properties of GaN.

GaN and InGaN nanowires prepared by metal-assisted electroless etching: Experimental and theoretical studies

We investigate the optical and structural properties of GaN and InGaN nanowires (NWs) fabricated by metal-assisted electroless etching in a hydrofluoric acid (HF) solution. The emission spectra of GaN and InGaN NWs exhibit a red shift compared to the as-grown samples resulting from an increase in the surface-to-volume ratio and stress relaxation in these nanostructures. The carrier lifetimes of GaN and InGaN NWs were measured. In addition, density functional theory (DFT) investigations were carried out on GaN and InGaN NWs using the generalized gradient approximation (GGA), including the Hubbard U parameter. The presence of compressive stress in the NWs was confirmed by the DFT calculations, which indicated that it induces a change in the lattice parameter along the c-direction. Formation energy calculations showed that In is a much more stable dopant in the GaN NWs compared to the native point defects, such as Ga and N vacancies. Moreover, electronic structure analysis revealed that the complex defects formed by the presence of In along with vacancy defects shifts the valence band maximum, thus changing the conducting properties of the NWs.

Investigation of the Anisotropy of the Structural Properties of GaN(0001) Layers Grown by MOVPE on a-Plane (11 $$\bar {2}$$ 0) Sapphire

The structural properties of GaN(0001) heteroepitaxial layers grown by metalorganic vapor phase epitaxy (MOVPE) on a-plane (11 $$\bar {2}$$ 0) sapphire substrates are investigated by X-ray diffractometry. Anisotropy of the rocking-curve width for the symmetric (0004) and asymmetric {11 $$\bar {2}$$ 4} and {10 $$\bar {1}$$ 5} reflections of gallium nitride upon rotation of the sample is observed. A comparison of the anisotropy of the rocking-curve width for GaN(0001)/Al2O3(11 $$\bar {2}$$ 0) layers with two different variants of in-plane orientation relationships suggests that the anisotropy of the structural properties is independent of the thermoelastic stress arising upon cooling the heterostructure.

First-principles study on the optical properties of Fe-doped GaN

Using hybrid density functional theory, we investigate the structural, electronic and optical properties of pristine GaN and Fe-doped GaN with a Fe concentration of 12.5%. Specifically, we first analyze the crystal lattice constant, band structure, and density of states, respectively. Then we predict the dielectric function, absorption coefficient, refractive index, reflectivity, energy-loss spectrum and extinction coefficient. Finally, we analyze the influences of the doping of Fe element on the photoelectric property of Fe doped systems. The calculated lattice constants for perfect GaN are a=b=3.19 Å, c=5.18 Å, which are in good agreement with the experimental values. Furthermore, we find that the doping of Fe element has little effect on the structural properties of GaN. The Band gap of pristine GaN is predicted to be 3.41 eV, very close to the experimental value of 3.39 eV. The band gap of Fe doped GaN (12.5%) significantly decreases to 3.06 eV. By comparing the densities of states of the systems with and without Fe doping, it is found that Fe-3 d state is mainly responsible for the decrease of band gap. The calculated static dielectric constant of perfect GaN is 5.74, and it increases to 6.20 after incorporating the Fe element. The results about the imaginary part of dielectric function show that two equal-strength perfect GaN peaks are observed to be at 6.81 eV and 10.85 eV. The first peak is closely related to the direction transition from the valence band top to the conduction band bottom. Furthermore, it is also observed that a peak is located at 4.04 eV in the low energy, which can be understood as resulting from the electron transition inside the valence band. The optical absorption edge of the intrinsic GaN is 3.25 eV, corresponding to the transition energy. The reason why this energy is smaller than the bandgap is because the electronic band gap equals the sum of optical bandgap and exciton energy. However, the maximum absorption coefficients of these two systems both occur at 13.80 eV in energy. The refractive index for intrinsic system is 2.39, and it increases to 2.48 after doping the Fe element. It is found from the energy-loss spectrum that the maximum energy-loss is at 20.02 eV for a perfect system, while it is at 18.96 eV for a doped system. Additionally, we obtain the reliable reflectivity and excitation coefficient. In conclusion, our calculated results provide a well theoretical basis for the theoretical research on the co-doping of Fe element and other elements. The analyses on the Fe-doped GaN high-voltage photoconductive switch materials and devices also provide a powerful theoretical basis and experimental support in the future research.

Crystalline structures and defects in liquid GaN during rapid cooling processes

The formation of crystalline structures and defects in GaN at three different cooling rates were investigated based on molecular dynamics simulations by using the Stillinger–Weber potential. The structural properties of GaN during the solidification processes were analyzed by several structural analysis methods. The system formed crystals at 200K when cooled at rates of 1011K/s and 1012K/s, whereas the amorphous state that formed at the cooling rate of 1013K/s included some crystal clusters. The 〈4 0 0 0〉 and 〈2 3 0 0〉 Voronoi polyhedrons were dominant in GaN during the rapid quenching processes. The 〈4 0 0 0〉 polyhedrons can aggregate to form cubic and hexagonal phases in GaN at lower temperatures. After solidification, cubic and hexagonal structures were coexistent in the system with a hybrid disordered region and crystal defects. When the temperature was decreased at a rate of 1011K/s, coherent twin boundaries, having excellent structural stability and configural continuity, were easily formed between the cubic and hexagonal crystal structures.

Theoretical Investigation of GaN

Abstract Using the Full Potential Linear Muffin Tin Orbitals (FPLMTO) method, the structural properties of GaN were studied. Different crystal structures were considered: NaCl, CsCl, wurtzite, zincblende, β-tin, Cinnabar and NiAs structures. The wurtzite is the calculated ground state structure. Results are given for lattice parameters, bulk modulus and its first derivative in the different cases. The phase transitions for GaN were also investigated. The results are compared with the available theoretical and experimental data. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of the volume, the bulk modulus, the variation of the thermal expansion α, as well as the heat capacity Cv were successfully obtained in the whole range from 0 to 30 GPa and temperature range from 0 to 1000 K.

Rapid thermal synthesis of GaN nanocrystals and nanodisks

Gallium nitride materials are at the forefront of nanoelectronic research due to their importance for UV optoelectronics. In this contribution, we present a facile and well-controlled synthesis of GaN nanodisks by rapid thermal ammonolysis of complex gallium fluoride precursor. We observed the formation of GaN nanodisks in 150 s at 800 °C. The structural properties of GaN were investigated by X-ray diffraction, Raman spectroscopy, and micro-photoluminescence. The morphology of GaN was investigated by scanning electron microscopy and the magnetic properties by superconducting quantum interference device (SQUID) techniques. The morphology of nanodisks was strongly influenced by the temperature of synthesis. The structure characterization shows a high concentration of defects related mainly to the vacancies of N and Ga. The magnetic measurement by SQUID shows paramagnetic behavior induced by structure defects. These findings have a strong implication on the construction of modern optoelectronic nanodevices.

Two-domain formation during the epitaxial growth of GaN (0001) on c-plane Al2O3 (0001) by high power impulse magnetron sputtering

We study the effect of high power pulses in reactive magnetron sputter epitaxy on the structural properties of GaN (0001) thin films grown directly on Al2O3 (0001) substrates. The epilayers are grown by sputtering from a liquid Ga target, using a high power impulse magnetron sputtering power supply in a mixed N2/Ar discharge. X-ray diffraction, micro-Raman, micro-photoluminescence, and transmission electron microscopy investigations show the formation of two distinct types of domains. One almost fully relaxed domain exhibits superior structural and optical properties as evidenced by rocking curves with a full width at half maximum of 885 arc sec and a low temperature band edge luminescence at 3.47 eV with the full width at half maximum of 10 meV. The other domain exhibits a 14 times higher isotropic strain component, which is due to the higher densities of the point and extended defects, resulting from the ion bombardment during growth. Voids form at the domain boundaries. Mechanisms for the formation of differently strained domains, along with voids during the epitaxial growth of GaN are discussed.

Structural properties of GaN(0001) epitaxial layers revealed by high resolution X-ray diffraction

High-resolution X-ray diffraction has been used to analyze GaN(0001) epitaxial layers on sapphire substrates. Several structural properties of GaN, including the lattice constants, strains, and dislocation densities are revealed by the technique of X-ray dffraction (XRD). Lattice constants calculated from the omega/2theta scan are c=0.5185 nm and a=0.3157 nm. Also, the in-plane strain is −1.003%, while out of the plane, the epitaxial film is almost relaxed. Several methods are used to deduce the mosaicity and dislocation density of GaN, showing that the edge type dislocations are the overwhelming majority.

Structural Characterization of GaN Epitaxial Layers Grown on 4H-SiC Substrates with Different Off-Cut

The influence of surface preparation of 4H-SiC substrates on structural properties of GaN grown by low pressure metalorganic vapour phase epitaxy was studied. Substrate etching has an impact on the crystallographic structure of epilayers and improves its crystal quality. The GaN layers were characterized by RBS/channelling and HRXRD measurements. It was observed that on-axis 4H-SiC is most suitable for GaN epitaxy and that substrate etching improves the crystal quality of epilayer.

Role of lateral growth on the structural properties of high temperature GaN layer

The role of lateral growth on the structural properties of high temperature (HT) GaN epitaxial layer has been investigated by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD). Variations of the lateral growth rate of HT GaN in metal-organic chemical vapor deposition (MOCVD) can be obtained by changing the V/III ratio. It is found that under higher lateral growth rate, dislocation is easier to bend into subgrains away from c axis, and the position where bend occurs is closer to the buffer layer, however, dislocation density does not show to monotonically vary with increasing lateral growth rate. A model concerning the GaN growth dynamics and dislocation bending mechanics has been proposed to explain the correlation between lateral growth and the structural properties of GaN.

Structural properties of GaN and Ga1–xMnx N layers grown by sublimation sandwich method

AbstractThe structural properties of GaN and Ga1–x Mnx N layers are reported. The GaN and Ga1–x Mnx N layers were obtained by the sublimation sandwich method (SSM). Electron microprobe measurements detected high Mn concentration of up to 4.0 at% in Ga1–x Mnx N materials. Raman investigations demonstrated that the prepared layers had a well‐ordered structure. The densities of dislocations in the template MOCVD layer, SSM GaN layers and SSM Ga1–x Mnx N layers were 3.9 × 108 cm–2, 1.4 × 108 cm–2 and 3.0 × 109 cm–2, respectively. The AFM studies demonstrated that the GaN RMS roughness increased with the increase in thickness. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of growth temperature on the luminescence and structural properties of GaN:Tm films grown by gas-source MBE

During molecular beam epitaxy of GaN:Tm films, substrate temperature strongly influences the rare earth incorporation, surface morphology and luminescence spectrum. The Tm incorporation into films grown between 730 and 830 °C was estimated by wavelength-dispersive X-ray (WDX) spectroscopy. Comparative WDX, atomic force microscopy (AFM) and cathodoluminescence (CL) mappings reveal that at an optimal growth temperature between 775 and 780 °C, a high Tm content (∼2.2 at%) and a smooth surface morphology can be obtained, leading to an intense sharp Tm 3+ emission. For lower substrate temperatures, Ga droplets and large (∼8–15 μm) circular pits mar the sample surface; for higher temperatures, the sharp CL lines disappear due to low Tm content (⩽0.8 at%).

Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy

Single-crystalline layers of GaN and related alloys such as AlGaN and InGaN were grown on Al 2O 3 (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N 2 composition ratio in N 2/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N 2 composition ratios, and the GaN layer grown at 30% N 2 condition was transparent and colorless. Al x Ga 1− x N layers with x = 0.06–0.08 and In x Ga 1− x N layers with x = 0.45–0.5, were obtained at 30–40% and 30–50% N 2 composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys ( x = 0.15 in both Al x Ga 1− x and In x Ga 1− x alloys).

Enhanced performance of p-GaN by Mg δ doping

The electrical and structural properties of Mg δ-doped GaN epilayers grown by MOCVD were investigated. Compared to uniform Mg-doping GaN layers, it has been shown that the delta-doping (δ-doping) process could suppress the dislocation density and enhance the p-type performance. The influence of pre-purge step on the structural properties of GaN was also investigated. The hole concentration of p-GaN decreases when using a pre-purge step. These results can be explained convincingly using a simple model of impurity incorporation under Ga-free growth condition.

Electrostatic and structural properties of GaN nanorods/nanowires from first principles

The first-principles calculation has revealed that the GaN nanorod has a greatly enhanced dipole moment per area relative to that of a film, which in conjunction with the geometry effect suggests that the top surface of the nanorod has a greater electrostatic attraction for gas-phase Ga and N source species than the film surface during epitaxial growth of GaN. The first-principles molecular-dynamics calculation shows that the average Ga–N bond length of the GaN nanowire decreases with the decrease of the diameter of the nanowire, which demonstrates a surface tension effect.

Heteroepitaxial growth of GaN on atomically flat LiTaO 3 (0 0 0 1) using low-temperature AIN buffer layers

We have grown GaN films on atomically-flat LiTaO 3 substrates by using pulsed laser deposition (PLD), and we then investigated the effect of the use of low-temperature AlN (LT-AlN) buffer layers on the structural properties of GaN. The full-width at half-maximum (FWHM) values for the crystal orientation distribution of the GaN films in the tilt directions were reduced from 0.48° to 0.17°, and those in the twist directions were reduced from 0.40° to 0.17° by the incorporation of AlN buffer layers grown at 580 °C. The surface morphology of GaN has also been improved by the insertion of LT-AlN buffer layers. X-ray reflectivity measurements have revealed that the interfacial layer thickness between LT-AlN and LiTaO 3 is as thin as 1.7 nm, and that the increase in the interfacial layer thickness caused by annealing at up to 700 °C is quite small. These results indicate that the PLD growth of GaN on atomically flat substrates using LT-AlN buffer layers is quite promising for achieving GaN on LiTaO 3.

Annealing behavior of N-bonding configurations in GaN 0.023As 0.977 ternary alloy grown on GaAs (0 0 1) substrate by molecular beam epitaxy

We have investigated by high-resolution X-ray diffraction, secondary-ion-mass-spectroscopy (SIMS) and Raman scattering the effects of rapid thermal annealing on the structural properties of GaN x As 1− x ternary alloys coherently grown on GaAs (0 0 1) substrates by molecular beam epitaxy. X-ray diffraction indicates that the in-plane strain does not relax and the vertical lattice in the GaNAs film tends to shrink, while SIMS reveals the absence of any N concentration change upon annealing. Furthermore, post-growth annealing causes an increase of substitutional N as revealed by the increase of the ratio of Raman scattering intensity between the nitrogen-localized vibration mode and the GaAs-like LO. After performing strain analysis, we conclude with the changes of N configurations in the GaN x As 1− x from the N–N split interstitial to the N–As split interstitial and the N As substitutional at elevated temperatures, the thermal behaviors of N–N, N–As and N As are reported over 700–850 °C.

Raman studies on spintronics materials based on wide bandgap semiconductors

Structural properties of GaN and ZnO layers doped with magnetic impurities wereinvestigated by Raman scattering. Long-range lattice ordering and local atomicarrangement around magnetic impurities were analysed, and their solubility limit wasconsidered.For this study, GaN layers doped with Mn and Cr, and ZnO layers doped with Coand V were prepared by molecular beam epitaxy and pulsed laser deposition,respectively. ZnO layers codoped with Ga and N were also studied for the purposeof p-type activation. These samples were observed using a Raman microprobeusing visible and deep UV lasers for excitation. The main results are as follows:In Ga1−xMnxN layers, a uniform solid solution was formed for Mn concentration up tox = 1–2%. An impuritymode was observed at 585 cm−1 and assigned to a local vibrational mode of Mn substituting the Ga site. Athigher Mn concentrations, rapid deterioration in lattice ordering occurred.Ga1−xCrxN layers showed goodlattice ordering up to x = 3–5%. The samples showed resonance enhancement of LO-phonon signals when excited by aUV laser at 266 nm (4.7 eV). This indicates the photo-injection of free carriers to a dilutedmagnetic semiconductor.ZnO layers codoped with Ga and N showed many impurity modes due to host lattice defects. A strongsignal at 580 cm−1 showed a characteristic broadening at high concentrations of N and Ga.This suggested the formation of complex centres with N or related defects.Zn1−xCoxO and Zn1−xVxO layers formed uniform solid solutions up to , but precipitation of the secondary phase was observed at . These samples presented common defect modes as observed in codoped samples. Ourresult suggests that the impurity modes in ZnO-based materials can be used as asensitive probe of host lattice defects induced by the impurity incorporation process.

Characterization of GaN1−xPx alloys grown by metal-organic chemical vapor deposition

Photoluminescence (PL), X-ray diffraction (XRD) and Raman spectra have been measured to characterize the effects of phosphorus on the optical and structural properties of GaN 1-x P x alloys with x ≤ 15%, grown by means of light-radiation heating, low-pressure metal-organic chemical vapor deposition. The PL spectra of GaN 1-x P x show the largest red shift of 100 meV from the band-edge emission of GaN. XRD rocking curves show that the (0002) peak of GaN 1-x P x shifts to smaller angles with increasing P content. The Raman spectra of GaN 1-x P x , recorded in backscattering geometry, exhibit four new vibrational modes at 256, 314, 377 and 428 cm -1 compared with an undoped GaN sample. The modes at 377 and 428 cm -1 are attributed to gap modes related to the Ga-P bond vibrations, whereas the other two modes at 256 and 314 cm -1 are assigned to phosphorus clusters and disorder-activated scattering, respectively. The frequency of the A 1 (LO) mode is found to decrease with increasing x at a much high rate of approximately 268 cm -1 /x for x < 0.03. This red shift is attributed to the effects of alloying and strain. Furthermore, we have not observed characterizations related to GaP resulted from the phase separation from PL, XRD and Raman spectra of all investigated GaN 1-x P x samples.