GaN Structures Research Articles

GaN taper rods: Solid-phase synthesis, crystal defects, and optical properties

Wurtzite GaN taper rods assembled from highly oriented nanoparticles were synthesized using NaNH 2 and the as-synthesized GaOOH prismatic rods as reactants at 600 °C for 5 h. The lengths of the GaN taper rods are in the range of 4–6 μm and the diameters are about 0.5–1.5 μm. It was found that a slow heating rate (1 °C min −1) was beneficial to keeping the one-dimensional (1D) skeleton of GaN, otherwise only GaN nanoparticles were obtained with a quick heating rate (10 °C min −1). Selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) observations revealed that the GaN taper rods assembled from highly oriented nanoparticles and there were crystal defects in the GaN structure. The GaN taper rods displayed luminescence emission in the blue-violet region, which may be related to crystal defects.

COMPARISON OF LOW FIELD ELECTRON TRANSPORT IN SiC AND GaN STRUCTURES FOR HIGH-POWER AND HIGH-TEMPERATURE DEVICE MODELING

Temperature and doping dependencies of electron mobility in SiC and GaN structures have been calculated using an iteravive technique. The following scattering mechanisims, i.e. impurity, polar optical phonon, acoustic phonon, piezoelectric and electron–plasmon are included in the calculation. Ionized imurity scattering has been treated beyond the Born approximation using the phase-shift analysis. It is found that the electron mobility decreases monotonically as the temperature increases from 100 K to 600 K. The low temperature value of electron mobilty increases significantly with increasing doping concentration. The iterative results are in fair agreement with other recent calculations obtained using the relaxation-time approximation and experimental methods.

Electronic structure of GaN codoped with Mn and Cr

We have studied the spin-polarized electronic structure of GaN codoped with transition metal (TM) atoms Mn and Cr, with different possible geometrical sites for the dopants, to determine the most favored geometry. The variations in the band structure, density of states, and TM site magnetic moment are analyzed for these geometries. Parallel and antiparallel spin configurations are energetically compared for all the geometries and it is observed that the parallel spin configuration with near neighbor substitution of TM atoms is the most favorable. Our results indicate that Ga(MnCr)N is a prospective candidate as a spintronics material to work at room temperature.

Effects of interface states and temperature on the C-V behavior of metal/insulator/AlGaN/GaN heterostructure capacitors

The impact of states at the insulator/AlGaN interface on the capacitance-voltage (C-V) characteristics of a metal/insulator/AlGaN/GaN heterostructure (MISH) capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiNx/AlGaN interface state density Dit(E) was increased. We attribute this behavior to the position of the Fermi level at the SiNx/AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300 °C. We analyzed the experimental C-V characteristics from a SiNx/Al2O3/AlGaN/GaN structure measured at room temperature and 300 °C, and extracted a part of Dit(E). The relatively low Dit (∼1011 eV−1 cm−2) in the upper bandgap indicates that the SiNx/Al2O3 bilayer is applicable as a gate insulator and as an AlGaN surface passivant in high-temperature, high-power AlGaN/GaN-based devices.

Phase-dependent distribution of Fe-rich nanocrystals in MOVPE-grown (Ga,Fe)N

We present a detailed transmission electron microscopy study of the structure and distribution of self-organized Fe-rich nanocrystals in (Ga,Fe)N grown via metal-organic vapor phase epitaxy (MOVPE). In samples with a concentration of magnetic ions above the solubility limit— x ⩾ 0.4 % —of Fe in GaN at the growth conditions we can identify three different phases of the Fe-rich enclosures, namely: (a) α -Fe segregated in proximity of the surface, (b) ε -Fe 3N in the volume beneath, though well above the nominal interface between the undoped GaN buffer and the Fe-doped overlayer, and (c) nanocrystals at an intermediate location showing the presence of adjacent α -Fe and ε -Fe 3N. These observations suggest that nitrogen evaporation at the growing surface play an important role in the formation of Fe-rich phases. The occurrence of α -Fe and ε -Fe 3N in the same nanocrystal indicates that the two phases are actually correlated: phase separation may first lead to the precipitation of ε -Fe 3N, which inherits the hexagonal structure of GaN but generates misfit dislocations, and then a further depletion of nitrogen promotes the conversion of ε -Fe 3N into α -Fe.

GaN and ZnO freestanding micromechanical structures on silicon‐on‐insulator substrates

AbstractUsing a combination of selective dry etching techniques, surface micro‐machined GaN and ZnO micromechanical structures are demonstrated on silicon‐on‐insulator (SOI) substrates. The dry releasing technique employs a controlled gas phase pulse etching with non‐plasma xenon difluoride (XeF2), which selectively etches the Si overlayer of SOI, thus undercutting the wide bandgap semiconductor material on top. This method prevents crystal damage of overhanging wide bandgap semiconductor mechanical structures. The mechanical properties of these released microstructures are characterized by micro‐Raman spectroscopy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nitride photocatalyst to generate hydrogen gas from water

AbstractWe propose a new nitride photocatalyst of an n‐n+‐GaN structure to increase H2 gas generation. This structure consists of a thin lightly‐doped n‐type layer on a heavily‐doped n+‐type layer to realize an optimized depletion layer for optical absorption and highly conductive region, respectively. An optimized n‐n+‐GaN structure could generate much more H2 gas rather than a single n‐GaN layer structure by about 1.4 times. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of nitride laser cavities with cleaved facets on transferred laser diodes on GaAs substrates

AbstractSmoothly cleaved facets with high reflectivities have been demonstrated on GaN laser diodes after the devices were transferred onto GaAs substrates. The GaN based laser diode structure was first fabricated by metal organic chemical vapor deposition (MOCVD) on c‐plane sapphire substrates. The samples were then mounted onto thin GaAs substrates using wafer‐bonding technology. Laser lift‐off (LLO) technique was applied to remove the original sapphire substrate and transfer the GaN laser structure onto GaAs substrates. Since the cubic substrates have well‐defined laser cavity cleavage facet, the GaN structures bonded onto the substrates also formed smooth facets after cleavage. The cleaved facets of GaN laser diodes have been characterized using atomic force microscopy (AFM) with less than 2 nm roughness. The present study demonstrated the feasibility of transferring GaN laser structures onto other more appealing substrates for formation of laser cavities. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN-core/SiO x-sheath nanowires

We have fabricated GaN-core/SiO x-sheath nanowires, sheathing the core MgO nanowires by sputtering with Si target. The product has wire-like morphology, regardless of SiO x -sheathing and subsequent annealing. EDX elemental mapping results have coincided with what can be expected for the SiO x -coated GaN nanowires. The core nanowires correspond to a hexagonal GaN structure, whereas the sheath layer is amorphous. Gaussian fitting analysis on the photoluminescence spectra of GaN-core/SiO x -sheath nanowires have exhibited two emission bands peaked at 2.4 eV and 2.9 eV, respectively. We observed that the relative intensity of 2.9 eV-peak to 2.4 eV-peak was increased by the thermal annealing.

Very low sheet resistance and Shubnikov–de-Haas oscillations in two-dimensional electron gases at ultrathin binary AlN∕GaN heterojunctions

Very low sheet resistance two-dimensional electron gases have been obtained at ultrathin AlN∕GaN heterojunctions. By investigating the molecular beam epitaxy growth conditions, the interface roughness at the heterojunction has been reduced, leading to high room-temperature electron mobilities (μ∼1400–1600cm2∕Vs) and high electron sheet densities due to spontaneous and piezoelectric polarization (ns∼1–3×1013cm−2) depending on the AlN thickness. This combination led to a large reduction of the room-temperature sheet resistance of 148Ω∕◻, a new record. The improved transport properties have made it possible to observe Shubnikov–de-Haas oscillations of the magnetoresistance of the two-dimensional electron gas at ultrathin binary AlN∕GaN heterojunctions for the first time. The results are indicative of the vast potential of high-quality AlN∕GaN structures for a variety of device applications.

Magnesium diffusion profile in GaN grown by MOVPE

The diffusion of magnesium has been studied in GaN layers grown on sapphire substrate by atmospheric pressure metalorganic vapor-phase-epitaxy (MOVPE) in a “home-made” reactor. Secondary Ion Mass Spectroscopy (SIMS) was used to visualise the Mg profiles in two kinds of multi-sublayer GaN structures. One structure was grown with a variable flow of Ga precursor (TMG) and the second one with a variable growth temperature. In both cases, the Mg dopant precursor (Cp 2Mg) flow was kept constant. Using the second Fick's law to fit the experimental SIMS data, we have deduced an increasing then a saturating Mg diffusion coefficient versus the Mg concentration. Mg incorporation was found to get higher for lower growth rate, i.e. when TMG flow is reduced. Furthermore, based on the temperature-related behaviour we have found that the activation energy for Mg diffusion coefficient in GaN was 1.9 eV. It is suggested that Mg diffuses via substitutional sites.

Electronic and photoconductive properties of ultrathin InGaN photodetectors

We report on the compositional dependencies of electron transport and photoconductive properties for ultrathin metal-semiconductor-metal photodetectors based on In-rich InxGa1−xN alloys. For a In0.64Ga0.36N∕GaN structure, the rise time close to the RC constant at low fields has been measured along with a transparency of ∼77% and an absorbance of ∼0.2 at a wavelength of 632nm. The electron density profiles and low-field mobilities for different compositions of InGaN have been calculated by numerically solving the Schrödinger and Poisson equations and applying the ensemble Monte Carlo method, respectively. It was demonstrated that in ultrathin InxGa1−xN∕GaN (0.5<x<1) heterostructures, in contrast to bulk InN exhibiting a strong surface electron accumulation, free electrons mostly tend to accumulate at the buried InGaN∕GaN interface. We have also found that the low-field mobility in the InGaN∕GaN heterostructures is strongly limited by the buried interface roughness which causes more than 95% of all scattering events occurred by two-dimensional electron transport under low electric field conditions.

Ferromagnetism in GaN:Gd: A Density Functional Theory Study

First-principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the generalized gradient approximation (GGA) of the density functional theory with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic p-d coupling is found to be over 2 orders of magnitude larger than the s-d exchange coupling. The experimental colossal magnetic moments and room-temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4f spins via p-d coupling involving holes introduced by intrinsic defects such as Ga vacancies.

Intrafacet migration effects in InGaN∕GaN structures grown on triangular GaN ridges studied by submicron beam x-ray diffraction

Synchrotron radiation has been utilized for x-ray diffraction and reciprocal space mapping of InGaN∕GaN multiple-quantum-well (MQW) structures grown on the sidewalls of 10-μm-wide triangular GaN ridges with {1−1.1} facets. Samples were produced by lateral overgrowth through a patterned dielectric mask by using metal-organic vapor-phase epitaxy. Global MQW strain, period, and the tilt of the (00.1) crystallographic planes have been measured across the sidewall facets using a 240nm x-ray beam. Results of this study are interpreted in terms of suppressed intrafacet migration of In and Ga precursors during the MQW growth.

Bonding of GaN structures with Si(100) substrates using sequentially deposited NiAu metal layers

A process has been developed to bond gallium nitride structures grown on sapphire with silicon substrates, using sequentially deposited nickel-gold thin metal layers. Temperature and pressure treatment results in alloying of NiAu with robust interface bond strength of 7.1MPa between GaN and Si. Transmission electron microscopy showed bright and dark regions of varying nickel-gold composition with uniform layer thickness. Micro-Raman and x-ray photoelectron spectroscopy revealed compressive stress relaxation and Ga–Ni and Au–Si alloying phases, respectively. The transferred light emitting diodes exhibit a turn-on voltage of 2.5V and could sustain beyond 300mA, proving a way to have vertical structure light emitting diodes on Si substrates.

Diffuse x-ray scattering from statistically inhomogeneous distributions of threading dislocations beyond the ergodic hypothesis

Diffuse x-ray scattering from threading dislocations in epitaxial structures is simulated numerically by a Monte Carlo method. The method allows one to simulate diffraction curves for dislocation types, where macroscopic approaches fail. That includes dislocation types for which analytical ensemble averaging is not feasible as well as microdiffraction curves from small sample volumes. In the latter case, the degree of statistic fluctuation of characteristic features is determined. The Monte Carlo method makes it possible to correlate quantitatively the widths of the microdiffraction curves to the densities of various dislocation types. The potential of the method has been demonstrated by a quantitative estimation of the density distribution of edge and screw threading dislocations in laterally overgrown epitaxial GaN structures, which is investigated by a full-field microdiffraction imaging technique. Measuring the asymptotic behavior of the microdiffraction curves allows one to conclude on the prevailing type of threading dislocations.

Effect of electron distribution in InN films on infrared reflectance spectrum of longitudinal optical phonon-plasmon interaction region

The infrared reflectance spectra of InN∕GaN structures are analyzed in two energy regions for higher and lower energy branches of the longitudinal optical phonon-plasmon coupled modes. For samples with smaller residual electron density than 5×1017cm−3 and the film thickness of 0.7–4μm, the spectra down to 200 or 250cm−1 shows the existence of the electron accumulation with sheet electron density of 1013cm−2 in the vicinity of the InN∕GaN-interface. The effect of the surface electron accumulation on the spectra is not identified, which is possibly caused by the small mobility of the order of 10cm2∕Vs or less for the surface electrons. The electron density of the inside bulk region is obtained with account of this interface accumulation. However, for samples with higher electron density and thickness of several micrometers a model with uniform electron density well reproduce the experimental spectra. With this spectrum fitting we extract the electron properties inside the bulk region for samples with the thickness of about 700nm or larger.

Electron field emission from nanostructured surfaces of GaN and AlGaN

AbstractThe possibility of high frequency electromagnetic wave generation by field emission based devices has great interest. The wide bandgap materials GaN and AlGaN are very promising for these applications due to low electron affinity and the existence of satellite valleys in conduction band. The results of investigations of the peculiarities of electron field emission from nanostructured surfaces of GaN and AlGaN are presented. Multilayer GaN and AlGaN structures with various levels of layer doping on sapphire and bulk GaN substrates were used as initial wafers. The surface of the upper layers was nanostructured by photoelectrochemical etching in water solution of KOH. Intensive electron field emission into vacuum was observed and explained by low electron affinity and electric field enhancement on surface nanowires. A decrease of the slope in the Fowler‐Nordheim characteristics was revealed. The changing slope suggests a lowering of effective work function. It is caused by electron heating and transfer into an upper satellite valley with lower electron affinity. A theory was developed for the observed phenomena and interpretation of results. It is baseed on electron intervalley transition upon heating and on energy band reconstruction of the surface of the nanowires due to quantum size‐confinement effect. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure and magnetism of Eu-doped GaN: first-principles study based on LDA+U

Electronic and magnetic properties of the rare-earth-doped III-nitride semiconductor Ga1−xEuxN (x = 0.0625) is studied with the full potential (linearized) augmented plane wave method plus the local orbitals method, where we have explored the electronic and magnetic properties of the ferromagnetic Ga1−xEuxN in the zinc-blende phase. The existence of Eu 4f orbitals has demonstrated that the common local density approximation leads to quantitatively and qualitatively wrong results, while the LDA + U method performs much better. In this paper we present only the results related to the LDA + U method. It is found from the calculation of density of states that the Eu f orbitals spin-polarize the host system and they are predominant in this diluted magnetic semiconductor. The magnetic description is achieved by the calculation of spin densities difference contours and the magnetic moment.

Characterization of nanoscale electronic structure in nonpolar GaN using scanning capacitance microscopy

Scanning capacitance microscopy is used to characterize nanoscale, local electronic structure in nonpolar n-type GaN grown in the a-plane orientation using lateral epitaxial overgrowth (LEO). Analysis of the bias dependence of the scanning capacitance image contrast observed reveals the presence of a linear, positively charged feature aligned along the [1¯100] direction, extending from an LEO window region into the adjacent wing region and terminating a few microns into the wing region. Comparison of the scanning capacitance images with cathodoluminescence and transmission electron microscopy data, revealing the presence of line defects aligned along the [1¯100] direction that emerge from the window regions, indicates that this positively charged feature likely corresponds to a partial dislocation at the edge of a stacking fault. The observation of positive dislocation charge is striking in that studies of GaN grown in the polar c-plane orientation have predominantly revealed the presence only of negatively charged or electrically neutral dislocations.