Gallium Nitride Structures Research Articles

TECHNOLOGICAL ASPECTS OF ULTRAVIOLET RADIATION SENSORS FORMATION ON THE BASE OF HETEROSTRUCTURE GaSe-GaN

The technique of GaAs ionic sputtering in the ammonia environment using crossed electrical and magnetic fields for gallium nitride structures formation on the GaSe substrates, with an aim to create sensors of ultraviolet radiation, is proposed. The optimization of reactor linear dimensions resulting in homogenous gallium nitride films production have been performed. The results of X-ray phase analysis of GaSe GaN heterostructures revealed polycrystalline hexagonal gallium nitride film on the samples surface. In particular the (110) plane peak is present at the patterns which corresponds to hexagonal GaN. The cathodoluminescent properties of GaSe GaN heterostructures have been studied.

Enhanced Electrostatic Discharge Reliability in GaN-Based Light-Emitting Diodes by the Electrode Engineering

In this study, the electrode structure of Gallium nitride (GaN)-based light-emitting diodes (LEDs) was investigated to improve their antielectrostatic discharge (anti-ESD) ability. The test results indicated that a strong correlation exists between the ESD immunity level of GaN-based blue LEDs and their electrode structure. The proposed LED design features a long parallel extension of the p- and n-electrode areas to facilitate the uniform distribution of ESD current and enhance the anti-ESD abilities. Compared with the standard reference group, the antireverse and antiforward modes of ESD in the experimental group were, respectively, 29% and 171% more than those in a standard reference group, which could enhance the reverse-mode and forward-mode ESD immunity of GaN-based LEDs. Therefore, using LEDs based on the optimal design of p- and n-electrode areas is an excellent strategy for enhancing anti-ESD ability without altering processing conditions.

Near-field imaging and spectroscopy of locally strained GaN using an IR broadband laser.

Scattering-type scanning near-field optical microscopy (SNOM) offers the possibility to analyze material properties like strain in crystals at the nanoscale. In this paper we introduce a SNOM setup employing a newly developed tunable broadband laser source with a covered spectral range from 9 µm to 16 µm. This setup allows for the first time optical analyses of the crystal structure of gallium nitride (GaN) at the nanometer scale by excitation of a near-field phonon resonance around 14.5 µm. On the example of an artificially induced stress field within a GaN wafer, we present a method for a 2D visualization of small deviations in the crystal structure, which allows for fast qualitative characterizations. Subsequently, the stress levels at chosen points were quantified by recording complex near-field spectra and correlating them with theoretical model calculations. Applied to the cross-section of a heteroepitaxially grown GaN wafer, we finally demonstrate the capability of our setup to analyze the relaxation of the crystal structure along the growth axis with a nanometer spatial resolution.

Large Area Stress Distribution in Crystalline Materials Calculated from Lattice Deformation Identified by Electron Backscatter Diffraction

We report a method to obtain the stress of crystalline materials directly from lattice deformation by Hooke's law. The lattice deformation was calculated using the crystallographic orientations obtained from electron backscatter diffraction (EBSD) technology. The stress distribution over a large area was obtained efficiently and accurately using this method. Wurtzite structure gallium nitride (GaN) crystal was used as the example of a hexagonal crystal system. With this method, the stress distribution of a GaN crystal was obtained. Raman spectroscopy was used to verify the stress distribution. The cause of the stress distribution found in the GaN crystal was discussed from theoretical analysis and EBSD data. Other properties related to lattice deformation, such as piezoelectricity, can also be analyzed by this novel approach based on EBSD data.

Optical waveguiding properties into porous gallium nitride structures investigated by prism coupling technique

In order to modulate the refractive index and the birefringence of Gallium Nitride (GaN), we have developed a chemical etching method to perform porous structures. The aim of this research is to demonstrate that optical properties of GaN can be tuned by controlling the pores density. GaN films are prepared on sapphire by metal organic chemical vapor deposition and the microstructure is characterized by transmission electron microscopy, and scanning electron microscope analysis. Optical waveguide experiment is demonstrated here to determine the key properties as the ordinary (n0) and extraordinary (ne) refractive indices of etched structures. We report here the dispersion of refractive index for porous GaN and compare it to the bulk material. We observe that the refractive index decreases when the porous density p is increased: results obtained at 0.975 μm have shown that the ordinary index n0 is 2.293 for a bulk layer and n0 is 2.285 for a pores density of 20%. This value corresponds to GaN layer with a pore size of 30 nm and inter-distance of 100 nm. The control of the refractive index into GaN is therefore fundamental for the design of active and passive optical devices.

Fabrication Gallium Nitride (GaN) Nanowires by Thermal Chemical Vapor Deposition (TCVD) Technique

In this paper, low-dimensional gallium nitride (GaN) nanowires have been successfully grown on silicon substrate through thermal chemical vapor deposition (TCVD); no metal catalyst was used to assist growth of nanostructure. A high purity of gallium nitride powder was used as a starting material, evaporated at 1150 OC for 2 hour and then annealing at temperature 1000 OC under stable flow of ammonia (NH3) gas in horizontal quartz tube. The morphological investigation and crystalline and orientations growth of GaN nanostructure were carried out by employing scanning electron microscopy (SEM), high resolution X-ray diffractmeter (HRXRD). A room temperature micro-Raman spectrum were employed to study the optical properties and crystalline defects. XRD shows the diffraction peaks located at 2θ= 32.43, 34.57, 36.89, 48.05, 57.83, 63.62, 69.02, and 70.470 corresponding to the (100 ), (002), (101), (102), (110) , (103),(112 ) and (201) plane diffraction of GaN structure. These results revealed that the diffraction peaks can be attributed to hexagonal GaN phase with lattice constant of a = 3.190 A° and c = 5.1890 A°. Here we report on the growth of GaN nanowires on Si (111) substrate by CVD . This technique is much simpler and cheaper than such techniques as MBE, MOCVD and HVPE.

Synthesis of wurtzite GaN thin film via spin coating method

In this research, hexagonal wurtzite structure gallium nitride (GaN) thin film was grown on silicon (Si) substrate by using spin coating deposition method. Simple ethanol-based precursor with the addition of diethanolamine solution was used. High resolution X-ray diffraction results revealed that wurtzite structure GaN thin film with (002) preferred orientation was deposited on Si substrate. Flied-emission scanning electron microscopy and atomic force microscopy results showed that crack free GaN thin film with uniform and dense grains of GaN was formed. Finally, lattice vibrational characterization by p-polarized infrared reflectance technique revealed a strong reststrahlen feature of crystalline wurtzite GaN, and the transverse and longitudinal phonon modes of wurtzite GaN were clearly identified.

Spatial Distribution of Dislocations in Relation to a Substructure in High-Quality GaN Film

The dislocation distribution of high-quality single-crystal gallium nitride (GaN) films grown by the hybrid vapor phase epitaxy was analyzed. This study examined the domain structure of GaN from the dislocation distribution on the macroscale by optical microscopy. The surface structure of GaN consisted of domains with microcolumns as the substructure. The inner domains contained a lower density of dislocations but a large number of these dislocations were observed along the domain boundaries. The existence of a domain boundary structure doubly increased the total dislocation density.

Synthesis, characterization, photoluminescence and field emission properties of novel durian-like gallium nitride microstructures

Durian-like gallium nitride (GaN) microstructures were successfully synthesized on Si substrate by pre-treating Ga metal with aqueous NH3 via catalyst assisted chemical vapor deposition (CVD) method at 1200°C. The as-synthesized product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). XRD and EDX analysis revealed that durian-like GaN is pure and single phase. SEM results showed that the size of the durian-like GaN structures was 20–25μm. GaN microstructures exhibited reasonable field emission properties with the turn-on field of 8.24Vμm−1 (0.01mAcm−2) and threshold field of 10.18Vμm−1 (1mAcm−2) which is sufficient for applications of electron emission devices, field emission displays and vacuum micro-electronic devices. Photoluminescence (PL) properties of durian-like GaN studied at room temperature showed a strong near-band-edge emission at 369.4nm (3.36eV) whereas at low temperature it showed near-band-edge emission at 364.2nm (3.4eV) without yellow band emissions. The photoluminescence properties showed that it has also potential application in light-emitting devices.

Gallium Nitride Based Logpile Photonic Crystals

We demonstrate a nine-layer logpile three-dimensional photonic crystal (3DPC) composed of single crystalline gallium nitride (GaN) nanorods, ∼100 nm in size with lattice constants of 260, 280, and 300 nm with photonic band gap in the visible region. This unique GaN structure is created through a combined approach of a layer-by-layer template fabrication technique and selective metal organic chemical vapor deposition (MOCVD). These GaN 3DPC exhibit a stacking direction band gap characterized by strong optical reflectance between 380 and 500 nm. By introducing a "line-defect" cavity in the fifth (middle) layer of the 3DPC, a localized transmission mode with a quality factor of 25-30 is also observed within the photonic band gap. The realization of a group III nitride 3DPC with uniform features and a band gap at wavelengths in the visible region is an important step toward realizing complete control of the electromagnetic environment for group III nitride based optoelectronic devices.

Synthesis, characterization, growth mechanism, photoluminescence and field emission properties of novel dandelion-like gallium nitride

Dandelion-like gallium nitride (GaN) microstructures were successfully synthesized via Ni catalyst assisted chemical vapor deposition method at 1200°C under NH3 atmosphere by pre-treating precursors with aqueous ammonia. The as-synthesized product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). X-ray diffraction analysis revealed that as-synthesized dandelion-like GaN was pure and has hexagonal wurtzite structure. SEM results showed that the size of the dandelion-like GaN structure was in the range of 30–60μm. Dandelion-like GaN microstructures exhibited reasonable field emission properties with the turn-on field of 9.65Vμm−1 (0.01mAcm−2) and threshold field of 11.35Vμm−1 (1mAcm−2) which is sufficient for applications of electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer. Photoluminescence (PL) measurements of dandelion-like GaN showed a strong near-band-edge emission at 370.2nm (3.35eV) with blue band emission at 450.4nm (2.75eV) and 465.2nm (2.66eV) but with out yellow band emission. The room-temperature photoluminescence properties showed that it has also potential application in light-emitting devices. The tentative growth mechanism for the growth of dandelion-like GaN was also described.

Optical waveguide loss minimized into gallium nitride based structures grown by metal organic vapor phase epitaxy

The waveguide properties are reported for wide bandgap gallium nitride (GaN) structures grown by metal organic vapor phase epitaxy on sapphire using a AlN/GaN short period-superlattice (SPS) buffer layer system. A detailed optical characterization of GaN structures has been performed using the prism coupling technique in order to evaluate its properties and, in particular, the refractive index dispersion and the propagation loss. In order to identify the structural defects in the samples, we performed transmission electron microscopy analysis. The results suggest that AlN/GaN SPS plays a role in acting as a barrier to the propagation of threading dislocations in the active GaN epilayer; above this defective region, the dislocations density is remarkably reduced. The waveguide losses were reduced to a value around 0.65dB/cm at 1.55 μm, corresponding to the best value reported so far for a GaN-based waveguide.

Growth and Characterization of High-Quality GaN Nanowires on PZnO and PGaN by Thermal Evaporation

In the current research, an easy and inexpensive method is used to synthesize highly crystalline gallium nitride (GaN) nanowires (NWs) on two different substrates [i.e., porous zinc oxide (PZnO) and porous gallium nitride (PGaN)] on Si (111) wafer by thermal evaporation without any catalyst. Microstructural studies by scanning electron microscopy and transmission electron microscope measurements reveal the role of the substrates in the nucleation and alignment of the GaN NWs. Further structural and optical characterizations were performed using high-resolution X-ray diffraction, energy-dispersive X-ray spectroscopy, and photoluminescence spectroscopy. Results indicate that the NWs have a single-crystal hexagonal GaN structure and growth direction in the (0001) plane. The quality and density of GaN NWs grown on different substrates are highly dependent on the lattice mismatch between the NWs and their substrates. Results indicate that NWs grown on PGaN have better quality and higher density compared to NWs on PZnO.

Impact of AlN seeding layer growth rate in MOVPE growth of semi‐polar gallium nitride structures on high index silicon

AbstractWe present metal organic vapor phase epitaxy growth of semi‐polar GaN structures on high index silicon surfaces. The crystallographic structure of GaN grown on Si(112), (115), and (117) substrates is investigated by X‐ray analysis and scanning electron microscopy. X‐ray diffraction was performed in Bragg Brentano geometry as well as pole figure measurements. The results demonstrate that the orientation of GaN crystallites on Si is significantly dependent on thickness of the AlN seeding layer and TMAl‐flow rate. We observe that the crystallographic structures of GaN by applying thin AlN seeding layers grown with high TMAl‐flow rate depend on Si surface direction while they are independent for thicker layers. By applying such seeding layer we obtain single crystalline semi‐polar GaN on Si(112), while GaN structures grown with the same growth parameters on Si(117) show four components of GaN(0002).

Strain Dependence of Formation Mechanism of Growth Layer in Molecular Beam Epitaxy of Gallium Nitride

In this study, the epitaxial growth of gallium nitride (GaN) on a GaN substrate is investigated by a molecular dynamics (MD) method. Furthermore, the difference between the surface diffusion of atoms of a strained substrate and an unstrained substrate is examined. From the results of this examination, it is found that the diffusion characteristic in the unstrained case is higher than that in the strained case. Therefore, in the unstrained case, GaN grows layer-by-layer. On the other hand, in the strained case, multiple layers of GaN grow simultaneously. Furthermore, it is also found that the wurtzite structure of GaN differs between the strained case and the unstrained case.

Formation of Planar Arrays of One-Dimensional p−n Heterojunctions Using Surface-Directed Growth of Nanowires and Nanowalls

We report a surface-directed vapor-liquid-solid process for planar growth of one-dimensional heterojunctions of zinc oxide on single crystal gallium nitride (GaN) that enables their hierarchical assembly to light emitting diodes. An individual heterojunction is about 10 μm in length and 80 nm in width and is formed by planar growth of an n-type ZnO nanowire or nanowall on p-type GaN surface using Au catalyst. Our results show that a ZnO nanocrystal at its nucleation site has six possible growth directions that can be engineered and controlled using an intentional blockade of the nanocrystal growth in certain directions owing to similarities in crystal structures of ZnO and GaN. The ZnO nanowalls are formed when nanowires during their planar growth slowly grow in direction normal to the substrate via a self-catalytic process. The crystal structure of these heterojunctions is examined from two different crystallographic perspectives using high resolution transmission electron microscopy. Results indicate abrupt and epitaxial formation of n-p heterojunctions, which are difficult to achieve in thin film growth of these heterojunctions. The collective light emission of micrometer- to millimeter-size arrays of the heterojunctions is demonstrated via a simple design that is scalable to literally any platform size. This technique allows in situ growth and combinations of II-VI and III-V semiconductors and offers their easier integration to photonic and lab-on-chip platforms with applications in energy generation and light detection.

Electronic Structures and Magnetic Properties of GaN Sheets and Nanoribbons

First principles calculations were performed to study the electronic structures of gallium nitride (GaN) sheets and nanoribbons (NRs) in order to understand the influence of defects or edge states on magnetic properties. It is shown that the Ga-defective GaN sheet may be a good candidate for spintronics due to its half-metal property under certain conditions, even if a perfect GaN sheet is a nonmagnetic semiconductor. We investigated both zigzag and armchair GaN NRs with and without edge atoms passivated by H. The H-passivated GaN NRs and bare armchair NRs can be classified as nonmagnetic semiconductors. Band gap gradually decreases with the increase of the width of NRs. A ferromagnetic character occurs in bare zigzag GaN NRs with width of about 1.7 nm (mainly determined by edge Ga and N). Furthermore, we have shown that thin layer GaN NRs could also be ferromagnetic. Magnetic moment does not decrease to zero even up to six layers. Results offer a deeper understanding of the influence of both defects and edge states of GaN sheets and monolayer and multilayer NRs, particularly in terms of their structural and magnetic properties.

Study of Temperature Dependence of Island Formation and Structure of Gallium Nitride in MBE Growth

Homoepitaxial growth on gallium nitride substrate is simulated by molecular dynamics (MD) method. Crystal growth configuration is evaluated qualitatively according to the atomic configuration and the radial distribution function. Moreover, crystal structure is evaluated quantitatively by considering variance of local atomic density as a monitoring index. As a result, we found that the growth layer is formed two-dimensionally and like film formation when the substrate temperature is high. It is found that the dynamics of crystal growth can be understood from not only variance value but also gradient of temporal evolution of variance.

Scattering effects of phonons in two polymorphic structures of gallium nitride

Effects of piezoelectric and longitudinal optical (LO) phonon scatterings on transport of electrons confined in quasi-two-dimensional square wells of wurtzite and zinc-blende structures are compared by using a theory of absorption power derived in the linear response scheme. We find for GaN that the absorption power for both wurtzite and zinc-blende structures is keenly affected by the screening in such a way that the power increases, but the half width decreases as the electron density increases, and the piezoelectric phonon scattering is affected by the screening more than the optical phonon scattering. We also find that the piezoelectric phonon scattering (LO phonon scattering) is dominant at high (low) density and low (high) temperature in the wurtzite structure, whereas the tendency is reverse for the zinc-blende structure.

Novel Epitaxial Nanostructures for the Improvement of InGaN LEDs Efficiency

We demonstrated that the efficiency of an InGaN LED can be improved by using a novel epitaxial nanostructure, namely, the nanostructured semipolar (NSSP) gallium nitride (GaN). The NSSP GaN template was fabricated on a c-plane GaN surface using a standard GaN metal-organic chemical vapor deposition tool on c-plane sapphire substrates. We showed that the surface of NSSP GaN consisted of two semipolar orientations: (10-11) and (11-22). InGaN/GaN multiple quantum wells (MQWs) fabricated on NSSP GaN exhibited negligible quantum-confined Stark effect (QCSE) and a 30% improvement in internal quantum efficiency as compared to planar c-plane InGaN/GaN MQWs. Using time-resolved photoluminescence (PL), a considerable improvement in radiative recombination lifetime was also observed. We fabricated and characterized semipolar InGaN LEDs on NSSP GaN that emitted at 543 nm and showed negligible QCSE. The NSSP GaN structure can also be applied to improve the photon extraction efficiency of InGaN-based LEDs. The surface texturing was performed in situ together with the LED epitaxy without additional ex situ etching processes. The in situ surface texturing improved the PL intensity by a factor of two. An electrical injection LED structure employing in situ surface texturing was also demonstrated.