Wurtzite GaN Structure Research Articles

Angular correlation studies on 172Lu(172Yb) in GaN and measurements at low temperatures

For optoelectronic devices semiconductors with large band gap doped with rare earth are used. Doping is generally performed during growth but for more structured doping the ion implantation technique is preferable. The perturbed γγ-angular correlation technique is an ideal tool to study the behavior of semiconductors after implantation. An adequate rare earth isotope for such investigations of semiconductors is 172Yb. The temperature dependence of the hyperfine fields for 172Lu(172Yb) in GaN has been analysed. The total electric field gradient (EFG) at the site of this probe is a superposition of the lattice EFG due to the GaN wurtzite structure and the EFG due to the 4f shell of the rare earth probe itself. The latter is strongly temperature dependent and opposed to the lattice EFG which in contrast is nearly constant since the lattice parameters change only negligibly with temperature. At elevated temperatures the sublevels of the 4f shell, split by the crystal electric field, are equally populated. But at low temperatures the lowest level is occupied preferentially. Sign and magnitude of the EFG caused by the 4f shell can be calculated (Tomala et al, J Magn Magn Mater 89(1–2):143, 1990) and the result compared to the experimental data.

Propagation of Spontaneous Emission in Birefringent m-Axis Oriented Semipolar (1122) (Al,In,Ga)N Waveguide Structures

We report on the observation of rotation of spontaneous emission polarization resulting in polarization tilted with respect to transverse electric (TE) mode, in m-axis oriented semipolar (1122) (Al,In,Ga)N waveguide structures. The principle polarizations were aligned parallel and perpendicular to the c-axis, i.e., at an angle of ∼30 and ∼120°, respectively, with respect to TE mode. This phenomenon was attributed to birefringence of uniaxial GaN wurtzite structure. This birefringence property is an important consequence for m-axis oriented (1122) laser diode structures, since it is expected to reduce the optical gain by a factor of ∼0.5–0.75.

A Systematic Investigation of the Stability Field of Spinel-type Gallium Oxonitrides

Over the last years, the synthesis and characterization of new spinel-type gallium oxonitrides were in the focus of several working groups. Our systematic investigation of the formation of the gallium oxonitride spinel phases was done under different experimental conditions and with various educts already used in former studies. The experiments took place under high-pressure/high-temperature conditions in a multianvil apparatus, starting from the end members wurtzite-structured GaN and β -Ga2O3. Beside the variation of the molar ratio of the educts (w-GaN:β -Ga2O3 = 9 :1 - 1: 9), different pressures (1 - 11.5 GPa), temperatures (500 - 1350 °C), and heating protocols were studied to localize the formation area of the spinel-type gallium oxonitrides. From these results, initial conditions of 2.5 - 11.5 GPa at temperatures between 1000 - 1300 ◦C can be stated as leading to the formation of cubic, spinel-type gallium oxonitride phases. The temperature stability of the gallium oxonitride spinel phase as well as new results of the transformation of β -Ga2O3 into α-Ga2O3 are reported.

Theory of high field carrier transport and impact ionization in wurtzite GaN. Part I: A full band Monte Carlo model

High field electron and hole transport in wurtzite phase GaN is studied using an ensemble Monte Carlo method. The model includes the details of the full band structure derived from nonlocal empirical pseudopotential calculations. The nonpolar carrier-phonon interaction is treated within the framework of the rigid pseudoion approximation using ab initio techniques to determine the phonon dispersion relation. The calculated carrier-phonon scattering rates are consistent with the electronic structure and the phonon dispersion relation thus removing adjustable parameters such as deformation potential coefficients. The impact ionization transition rate is computed based on the calculated electronic structure and the corresponding wave-vector dependent dielectric function. The complex band structure of wurtzite GaN requires the inclusion of band-to-band tunneling effects that are critical at high electric fields. The electric-field-induced interband transitions are investigated by the direct solution of the time dependent multiband Schrödinger equation. The multiband description of the transport predicts a considerable increase in the impact ionization coefficients compared to the case in which tunneling is not considered. In the second part of this work it will be shown that the proposed numerical model correctly predicts the carrier multiplication gain and breakdown voltage of a variety of GaN avalanche photodetectors that have been recently fabricated by several research groups.

Effects of annealing temperature on amorphous GaN films formed on Si(1 1 1) by pulsed laser deposition

GaN thin films were grown on Si(1 1 1) substrates using a pulsed KrF excimer laser-deposition system, and post-annealing was examined to improve the films' quality. In order to investigate the effect of thermal annealing temperature on the crystalline quality, optical properties and surface morphology of the samples, after deposition, the samples were subsequently annealed at different temperatures in ammonia (NH3) ambience for 15 min. The annealed films were characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), Raman spectra and photoluminescence spectra. The measured results show that annealing temperature plays an important role in improving the quality of GaN films, polycrystalline wurtzite structure GaN thin films with c-axis preferred orientation were successfully obtained by annealing, and the optimum annealing temperature is 1100 °C.

Non-Uniform Strain Field in a Wurtzite GaN Cylinder under Compression and the Related End Friction Effect on Quantum Behavior of Valence-Bands

Strain has a considerable effect on the band structure where conduction and valence bands are shifted and split apart. As a result, the knowledge of strain distributions is essential for the analysis of the optoelectronic properties of the semiconductors. In this paper, an analytical solution for the non-uniform strain field within a finite, transversely isotropic cylinder of wurtzite GaN under compression with end frictions is derived. The method follows Lekhnitskii's stress function approach to decouple the equations of equilibrium. Numerical results reveal that all of the strain components, including the axial, radial, circumferential and shear strains, are non-uniform in both distribution pattern and magnitude. In addition, by employing the effective-mass Hamiltonian, the effect of strain on the valence-band structure of wurtzite GaN is studied. It is found that strain can induce band splitting and alter the shape of constant energy surfaces of the heavy-hole, light-hole and split-off bands of wurtzite GaN.

Preparation of GaN Film and Investigation of Vibrational Spectrum of GaN by DFT

Using Ga_2O_3 as Ga source,the GaN films are successfully deposited on Si(111)substrates by two-step method of sol-gel and high nitridation temperature technique.The as-prepared films were confirmed as single crystalline GaN with wurtzite structure by X-ray diffraction(XRD).The dimension of GaN crystal particulates is smaller than 100nm under the observation of scanning electron micro- scope(SEM).Moreover,E_1(TO)vibrational mode of GaN are surveyed in IR spectrum of the GaN films.Vibrational frequency of GaN small clusters is calculated using density functional theory(DFT). The results show that the vibrational frequencies of the Ga reached clusters are close to the peaks of the phonon vibration modes of the wurtzite structure GaN,and 2200cm~(-1)is an intensive vibrational frequency of N-N bond stretch in N reached GaN.FTIR of the sample are analyzed in further.

Spin-orbit coupling in bulk ZnO and GaN

Using group theory and Kane-type k⋅p model together with the Löwdin partition method, we derive the expressions for the spin-orbit coupling of electrons and holes, including the linear-k Rashba term due to the intrinsic structure inversion asymmetry and the cubic-k Dresselhaus term due to the bulk inversion asymmetry in wurtzite semiconductors. The coefficients of the electron and hole Dresselhaus terms of ZnO and GaN in wurtzite structure and GaN in zinc-blende structure are calculated using the nearest-neighbor sp3 and sp3s∗ tight-binding models, respectively.

Room Temperature Ferromagnetism and Optical Tunability in Cu Doped GaN Nanowires

GaN nanowires doped with 2at.% and 6at.% Cu ions are synthesized by chemical vapour deposition method. Structural and compositional analyses demonstrate that the as-grown nanowires are of single crystal wurtzite GaN structure. Magnetic characterizations reveal that the doped GaN nanowires exhibit room temperature ferromagnetism. The measured saturation magnetic moments are 0.37μB and 0.47μB per Cu atom at 300K for Cu 2at.% and 6at.%, respectively. The photoluminescence spectra show that Cu dopant can tune the band gap of the GaN, which leads to a red shift of band-edge emission with increasing dopant concentration.

X-ray absorption in GaGdN: A study of local structure

In this study, we report on the incorporation of dilute Gd amounts into GaN films grown by molecular beam epitaxy. A combination of x-ray fluorescence with x-ray absorption spectroscopic techniques enabled us to examine not only the distribution of rare earth atoms in the GaN matrix but also the short-range structural order. Our results show Gd atoms in a trivalent state with tetrahedral coordination, thus substituting Ga in the wurtzite GaN structure.

Effect of Growth Temperature and Mn Incorporation on GaN:Mn Thin Films Grown by Plasma-Assisted MOCVD

In this paper, the growth of GaN:Mn thin films by plasma-assisted metalorganic chemical vapor deposition (PAMOCVD) method is reported. The method used in this study, utilizes a microwave cavity as a cracking cell to produce nitrogen radicals, which in turn reduce the growth temperature. Trimethylgallium (TMGa), nitrogen (N2) and cyclopentadienyl manganese tricarbonyl (CpMnT) were used as a source of Ga, N and Mn, respectively, while hydrogen gas was used as a carrier gas for both TMGa and CpMnT. The effect of growth temperature and Mn incorporation on structural properties and surface morphology of GaN:Mn films are presented. The growth of GaN:Mn thin films were conducted at varied growth temperature in range of 625 o C to 700 o C and the Mn/Ga molar fraction in the range of 0.2 to 0.5. Energy dispersive of X-ray (EDX) and X-ray diffraction (XRD) methods were used to analyze atomic composition and crystal structure of the grown films, respectively. The surface morphology was then characterized using both atomic force microscopy (AFM) and scanning electron microscopy (SEM) images. A systematic XRD analysis reveal that maximum Mn incorporation that still produces single phase GaN:Mn (0002) is 6.4 % and 3.2 % for the film grown at 650 o C and 700 o C, respectively. The lattice constant and full width at half maximum (FWHM) of the single phase films depend on the Mn concentration. The decrease in lattice constant accompanied by the increase in FWHM is due to incorporation of substitutional Mn on the Ga sub-lattice. The maximum values of doped Mn atoms incorporated in the wurtzite structure of GaN:Mn as substitutional atoms on Ga sub-lattice are 2.0 % and 2.5 % at 650 o C and 700 o C, respectively. AFM and SEM images show that the film grown at lower growth temperature and Mn concentration has a better surface than that of film grown at higher growth temperature and Mn concentration.

Growth and characterization of III-N bulk crystals

Hexagonal BN crystals were grown from solution by application of a thermal gradient. The solvent used to dissolve the source was optimized by changing the ratio of components to have the lowest melting point. The investigation of adding a third component demonstrated further reduction of the melting point with BN as an additive. A solution was created with enhanced properties allowing the growth of BN. BN was grown on a PBN seed at T g = 900 °C and P = 0.2 MPa for approximately 65 h. The BN crystals were found to be embedded in a solvent matrix, as determined by EDS. In addition, GaN crystals were grown in a modified solution at T g = 800 °C and P = 0.2 MPa. Raman spectroscopy verified wurtzite GaN structure with good crystallinity. The successful growth of BN and GaN from solution suggests this to be a method of choice for growth of the III-Ns, and may prove to be a viable alternative to current costly wafer production techniques.

Growth of GaN nanorods prepared by ammoniating Ga 2O 3/ZnO films on Si substrates and their properties: Structure, morphology, chemical state and photoluminescence

Large quantities of GaN nanorods have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating the Ga 2O 3/ZnO films at 950 °C in a quartz tube. The GaN nanorods are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), a field-emission transmission electron microscope (FETEM), a Fourier transform infrared (FTIR) spectrophotometer and a fluorescence spectrophotometer. The results show that the nanorods have pure hexagonal GaN wurtzite structure, with lengths of about several micrometers and diameters of about 200 nm. The clear lattice fringes demonstrate that the synthesized nanorods are single-crystal GaN. The FTIR spectrum further confirms that the GaN is obtained under this condition. The representative photoluminescence spectrum at room temperature exhibited a strong emission peak at 375.7 nm and two weak emission peaks at 436.2 and 475.5 nm. The GaN nanorods show a very good emission property, which will have a good advantage for applications in laser device using one-dimensional structures. Finally, the growth mechanism is also briefly discussed.

Effect of ammoniating temperature on morphologic and optical properties of GaN nanostructured materials

GaN nanostructured materials have been obtained on Si(111) substrates by ammoniating the Ga 2O 3/ZnO films at different temperature in a quartz tube. X-ray diffraction (XRD), Scanning electron microscope (SEM), and photoluminescence (PL) are used to analyze the structure, morphology and optical properties of GaN nanostructured films. The results show that their properties are investigated particularly as a function of ammoniating temperature. The optimally ammoniating temperature of Ga 2O 3 layer is 950 °C for the growth of GaN nanorods. These nanorods are pure hexagonal GaN wurtzite structure with lengths of about several micrometers and diameters of about 200 nm, which is conducive to the application of nanodevices. Finally, the growth mechanism is also briefly discussed.

Growth and characterization of GaN nanorods through ammoniating process by magnetron sputtering on Si(111) substrates

GaN nanorods have been successfully synthesized on Si(111) substrates by magnetron sputtering through ammoniating Ga2O3/ZnO films at 950 °C in a quartz tube. The GaN nanorods are characterized by X-ray diffraction, scanning electron microscopy, field-emission transmission electron microscopy, X-ray photoelectron spectroscopy and fluorescence spectrophotometry. The results show that the nanorods have a pure hexagonal GaN wurtzite structure with lengths of about several micrometers and diameters of about 200 nm, and the growth direction of the GaN nanorods is parallel to the (101) plane. The photoluminescence spectrum indicates that the nanorods have a good emission property. Finally, the growth mechanism is also briefly discussed.

Synthesis of one-dimensional GaN nanorods on Si(111) substrates by magnetron sputtering

GaN nanorods have been successfully synthesized on Si(111) substrates by magnetron sputtering through ammoniating the Ga 2 O 3 /ZnO films at 950 °C in a quartz tube. The GaN nanorods have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), field-emission transmission electron microscope (FETEM), Fourier transform infrared (FTIR) system and fluorescence spectrophotometer. The results show that the nanorods are pure hexagonal GaN wurtzite structure with lengths of about several micrometers and diameters ranging from 100 nm to 750 nm, and the growth direction of GaN nanorods is perpendicular to (101) plane. The photoluminescence (PL) spectrum indicates that the good emission property for the nanorods. Finally, the growth mechanism is also briefly discussed.

Electronic structure of diluted magnetic semiconductorsGa1−xMnxN and Ga1−xCrxN

We have undertaken a study of the diluted magnetic semiconductorsGa1−xMnxN and Ga1−xCrxN with x = 0.0625,0.125, using the all-electron linearized augmented plane wave method (LAPW) for differentconfigurations of Mn as well as Cr. We study four possible geometries of the impurityatoms in the wurtzite GaN structure to predict the energetically most favourable structurewithin a 32-atom supercell and conclude that the near-neighbour geometry has the lowestenergy. The on-site magnetic moment at the transition metal site shows insignificantvariation with distance. We have also analysed two configurations in which the localmoments on the impurity atoms are parallel and anti-parallel. The density ofstates as well as band structure indicate a half-metallic state for all the systems.Tc has also been estimated for these systems and shows that for near-neighbour substitution of dopantsTc is above room temperature.

Coupled optoelectronic modeling and simulation of nanowire lasers

A self-consistent, coupled optoelectronic simulation approach for studying microcavity nanowire lasers is presented. To focus the discussion, we use GaN nanowires to illustrate the methodology. The comprehensive model solves, simultaneously and self-consistently, the carrier transport equations and the photon rate equations. The basic physical model takes into account both bulk and surface dark carrier recombination processes. It includes stimulated emission, the anisotropic optical gain typical of the wurtzite GaN structure, the modified spontaneous emission, and its coupling into the lasing modes as a consequence of microcavity effects. The model further incorporates band-gap shrinkage effects due to band renormalization; it includes the complex dispersion and reflectivity relations of the guided modes, and the effects of multiple lateral and longitudinal lasing and nonlasing optical modes. We provide a detailed discussion of the influence of other important effects on the operation of optically pumped GaN nanowire lasers. Finally, we want to point out that the method is sufficiently general that it can be used to study any class of nanowire lasers.

Meta-GGA calculation of the electronic structure of group III–V nitrides

The ground state electronic structure of wurtzite AlN, GaN, and InN has been calculated using full-relativistic all-electron full-potential linearized-augmented plane-wave method. Several DFT exchange-correlation functionals, including the recently proposed non-empirical meta-generalised gradient approximation (Meta-GGA) have been used. The role played by relativistic effects and meta-GGA functional on the band structures and the density of states is discussed. We find that the meta-GGA improves the accuracy of the structural properties as well as the energies of the semicore d states in both GaN and InN. This new functional slightly outperforms the local density approximation (LDA) and the generalized gradient approximation (GGA) overall as to lattice parameters, bulk modulus, and valence band widths. We find also that the meta-GGA induced modifications of the band structure are significant, but limited to the valence band states, while leaving all other features identical to LDA and GGA calculations. Finally, our results show that GaN and InN present anomalous dependence of the valence band splitting versus pressure.

Preferential substitution of Fe on physically equivalent Ga sites in GaN

The EPR spectra of Fe 3 + in high-quality thick freestanding hydride vapor phase grown GaN have been studied in the X- and Q-band. A complex resonance pattern with numerous lines of different intensities provided by three different defects is observed for these nearly stress-free iron-doped samples. The dominating part is due to isolated Fe 3 + on Ga sites and can be well described with a spin Hamiltonian (SH) with S = 5 2 considering the two magnetically inequivalent Ga sites with C 3 v symmetry in the wurtzite structure of GaN. Aside from the displacement of their magnetic axis the two Ga sites are physically equivalent. However, contrary to the expectation, we observed different Fe populations of both sites that varied from a ratio 1:4 to a ratio of nearly 1:1 in different crystals. In addition and in contrast to previously published data we determined nearly isotropic g-values and larger values for the axial fine-structure parameters D , a – F and much larger value for the cubic fine-structure parameter a.