Formation of GaAsN/GaN cluster nanostructures on the surface of GaAs by the implantation of low-energy nitrogen ions

A nitride nanolayer fabricated on a GaAs (100) surface by implanting ions N ₂ ⁺ (E _i = 1.5 keV) has been studied by high-resolution photoelectron spectroscopy with the use of synchrotron radiation. It has been found that, apart from the dominant GaN wide-gap semiconductor phase, an additional phase of the GaAs_{1 ? x} N _x narrow-gap solid solution (x < 0.10) is present in the nitride layer. It has been shown that the nitride layer created by ion implantation is a nanostructure with an attribute of a system of quantum dots, since it consists of nanoclusters of the narrow-gap semiconductor in the wide-gap matrix.     

Effect of thickness on the microstructure of GaN films on Al2O3 (0001) by laser molecular beam epitaxy

Heteroepitaxial GaN films are grown on sapphire (0001) substrates using laser molecular beam epitaxy. The growth processes are in-situ monitored by reflection high energy electron diffraction. It is revealed that the growth mode of GaN transformed from three-dimensional (3D) island mode to two-dimensional (2D) layer-by-layer mode with the increase of thickness. This paper investigates the interfacial strain relaxation of GaN films by analysing their diffraction patterns. Calculation shows that the strain is completely relaxed when the thickness reaches 15 nm. The surface morphology evolution indicates that island merging and reduction of the island-edge barrier provide an effective way to make GaN films follow a 2D layer-by-layer growth mode. The 110-nm GaN films with a 2D growth mode have smooth regular hexagonal shapes. The X-ray diffraction indicates that thickness has a significant effect on the crystallized quality of GaN thin films.     

Formation and Shape Transition of Nanostructures on Si(100) surfaces after MeV Sb Implantation

We have studied the formation of nanostructures on Si(100) surfaces after 1.5 MeV Sb implantation. Scanning Probe Microscopy has been utilized to investigate the ion implanted surfaces. We observe the formation of nanostructures after a fluence of 1×10¹³ ions/cm². These surface structures are elliptical in shape with an eccentricity of 0.86 and their major and minor axes having dimensions of about 11.6 nm and 23.0 nm, respectively. The area of the nanostructure is 210 nm²at this fluence. Although the nanostructures remain of elliptical shape, their area increase with increasing fluence. However, after a fluence of 5×10¹⁴ ions/cm² a transition in shape of nanostructures is observed. Nanostructures become approximately circular with an eccentricity of 0.19 and a diameter of about 30.1 nm. At this fluence we also observe a large increase in the area of the nanostructures to 726 nm². Surface morphology and surface roughness of the ion implanted surfaces has also been discussed.     

Small-angle neutron scattering investigation of the nanostructure of ferritic-martensitic 12%-chromium steels

The nanostructure (nanoparticle distribution) of ferritic-martensitic 12%-chromium steels EK-181 (Fe-12Cr-2W-V-Ta-B) and ChS-139 (Fe-12Cr-2W-V-Ta-B-Nb-Mo) subjected to different modes of mechanical and heat treatments and neutron irradiation has been investigated using small-angle neutron scattering. The samples have been studied in the initial state and after neutron irradiation (IVV-2M reactor) at a temperature of 80°C with fluences of 10¹⁸, 10¹⁹, and 5 × 10¹⁹ cm^?2 (E ≥ 0.1 MeV). The nanostructure of the steels is characterized by precipitations of nanoparticles with two characteristic sizes of 1.0–1.5 and 7–8 nm. The dependence of the nanostructure parameters on the composition of the steels and on the conditions of heat treatment and irradiation has been discussed.     

梯度掺杂与均匀掺杂GaN光电阴极的对比研究

为了提高负电子亲和势(NEA)GaN光电阴极的量子效率,利用金属有机化合物化学气相淀积(MOCVD)外延生长了梯度掺杂反射式GaN光电阴极,其掺杂浓度由体内到表面依次为1×10¹⁸ cm^-3,4×10¹⁷ cm^-3,2×10¹⁷ cm^-3和6×10¹⁶ cm^-3,每个掺杂浓度区域的厚度约为45 nm,总的厚度为180 nm.在超高真 关键词： NEA GaN光电阴极 梯度掺杂 量子效率 能带结构     

ITON Schottky contacts for GaN based UV photodetectors

Lateral Schottky ultraviolet detectors were fabricated in GaN using indium-tin-oxynitride (ITON) as a contact metal. The GaN semiconductor material was grown on 2 in. sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The Schottky contact has been realized using ITON that has been deposited using sputter techniques. I-V characteristics have been measured with and without UV illumination. The device shows photo-to-dark current ratios of 10³ at −1 V bias. The spectral responsivity of the UV detectors has been determined. The high spectral responsivity of more than 30 A/W at 240 nm is explained by a high internal gain caused by generation-recombination centers at the ITON/GaN interface. Persistent photocurrent effect has been observed in UV light (on-off) switching operation, time constant and electron capture coefficient of the transition has been determined.     

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C-SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar⁺ ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2-4 keV energy and fluence ranging from 3 × 10¹³ ions/cm² to 1 × 10¹⁸ ions/cm². Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N₂⁺ ions and 7.2 × 10¹⁷ ions/cm² are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar⁺ ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.     

Electrodeposition from ZnO nano-rods to nano-sheets with only zinc nitrate electrolyte and its photoluminescence

This very paper focuses on the synthesis of ZnO nano-structures by means of electro-chemical-deposition process. The crystalline structure and morphologies of the prepared ZnO were characterized with X-ray diffraction and scanning electronic microscopy, respectively. It is found that in case of low Zn(NO₃)₂·6H₂O electrolyte concentration the fast growth mode in the c-axis direction leaded to the formation of 1D nanostructure of ZnO. On the other hand, at high concentration, this fast growth mode was restricted because the absorbed NO₃⁻ on (0 0 0 1) plane would bond with Zn²⁺ ions, which, therefore, resulted in the formation of 2D nanostructure of ZnO. Room temperature photoluminescence performances of different ZnO structures were also investigated. A blue shift of 15 nm for ZnO nano-sheets has been found as the shapes of ZnO evolved from nano-rods to nano-sheets.     

Initial stages of misfit stress relaxation through the formation of prismatic dislocation loops in GaN–Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> composite nanostructures

The initial stages of misfit stress relaxation through the formation of rectangular prismatic dislocation loops in model composite nanostructures have been considered. The nanostructures are either spherical or cylindrical GaN shells grown on solid or hollow β-Ga²O₃ cores or planar thin GaN films on β-Ga²O₃ substrates. Three characteristic configurations of prismatic dislocation loops, namely, square loops, loops elongated along the GaN/Ga²O₃ interface, and loops elongated along the normal to the GaN/Ga²O₃ interface, have been analyzed. The generation of prismatic dislocation loops from the interface into the bulk of the GaN shell (film), from the free surface into the GaN shell (film), and from the interface into the β-Ga²O₃ core (substrate) has been investigated. It has been shown that, for the minimum known estimate of the lattice misfit (2.6%) in some of the considered nanostructures, no any prismatic dislocation loops can be generated. If the generation of prismatic dislocation loops is possible, then in all the considered nanostructures, the energetically more favorable case corresponds to prismatic dislocation loops elongated along the GaN/Ga²O₃ interfaces, and the more preferred generation of prismatic dislocation loops occurs from the GaN free surface. The GaN/Ga²O₃ nanostructures that are the most and least resistant to the formation of prismatic dislocation loops have been determined. It has been found that, among the considered nanostructures, the planar two-layer GaN/Ga²O₃ plate is the most resistant to the generation of prismatic dislocation loops, which is explained by the action of an alternative mechanism for the relaxation of misfit stresses due to the bending of the plate. The least resistant nanostructure is the planar three-layer GaN/Ga²O₃/GaN plate, in which GaN films have an identical thickness and which itself as a whole does not undergo bending. The critical thicknesses of the GaN shells (films), which must be exceeded to ensure the growth of these shells (films) so as to avoid the formation of prismatic dislocation loops, have been calculated for all the studied nanostructures and three known estimates of the lattice misfits (2.6, 4.7, and 10.1%).     

Effects of thermal annealing on the properties of GaN MSM UV photodetectors

Metal-semiconductor-metal-structured GaN ultraviolet photodetectors have been fabricated on sapphire substrates by metalorganic chemical vapor deposition. The properties of GaN photodetectors have been improved through thermal annealing. With a 3 V bias, the very low dark current is about 200 pA, the maximum responsivity of 0.19 A/W is achieved at 362 nm, and the corresponding detectivity is 1.2×10¹¹ cm Hz^1/2/W. The physical mechanism of the effects of thermal annealing also has been studied.     

Investigating gate metal induced reduction of surface donor density in GaN/AlGaN/GaN heterostructure by electroreflectance spectroscopy

The internal field of GaN/AlGaN/GaN heterostructure on Si-substrate was investigated by varying the thickness of an undoped-GaN capping layer using electroreflectance spectroscopy. The four samples investigated are AlGaN/GaN heterostructure without a GaN cap layer (reference sample) and three other samples with GaN/AlGaN/GaN heterostructures in which the different thickness of GaN cap layer (2.7 nm, 7.5 nm, and 12.4 nm) has been considered. The sheet carrier density (n_s) of a two-dimensional electron gas has decreased significantly from 4.66 × 10¹² cm⁻² to 2.15 × 10¹² cm⁻² upon deposition of a GaN capping layer (12.4 nm) over the reference structure. Through the analysis of internal fields in each GaN capping and AlGaN barrier layers, it has been concluded that the undiminished surface donor states (n_s) of a reference structure and the reduced n_s caused by the Au gate metal are approximately 5.66 × 10¹² cm⁻² and 1.08 × 10¹² cm⁻², respectively.     

Electric field gradients at 151Eu sites in GaN

The electric field gradients at Eu sites in GaN have been investigated in conversion electron Mössbauer spectroscopy (CEMS) in which ¹⁵¹Eu probe ions were implanted into an undoped GaN layer grown on a sapphire substrate. The sample was implanted with 120 keV ¹⁵¹Eu ions to a fluence of 1 × 10¹⁵, and annealed at 1,200 K. CEMS spectra of the ¹⁵¹Eu 21.6 keV transition were collected, of the GaN sample as well as of a Si sample implanted with overlapping profiles of ¹⁵¹Eu and O. The GaN spectra were fitted with two symmetric doublets, D1 and D2, with isomer shifts and quadrupole splittings of δ?=??0.27 mm/s (relative to Eu₂O₃), ΔE _Q?= 0.85 (3) mm/s; and δ?=?? 0.22 mm/s, ΔE _Q?= 2.90 (5) mm/s, respectively. D1 is attributed to Eu at substitutional Ga lattice sites; D2 to Eu at or near substitutional sites but with extensive lattice damage. The splittings of D1 and D2 correspond to quadrupole coupling frequency of 15 (2) and 50 (4) MHz, consistent with measurements of ⁶⁹Ga, ⁷¹Ga and ¹¹¹In in GaN.     

Current-voltage characteristics of Au/GaN/GaAs structure

Au/GaN/n-GaAs structure has been fabricated by the electrochemically anodic nitridation method for providing an evidence of achievement of stable electronic passivation of n-doped GaAs surface. The change of the electronic properties of the GaAs surface induced by the nitridation process has been studied by means of current-voltage (I-V) characterizations on Schottky barrier diodes (SBDs) shaped on gallium nitride/gallium arsenide structure. Au/GaN/n-GaAs Schottky diode that showed rectifying behavior with an ideality factor value of 2.06 and barrier height value of 0.73 eV obeys a metal-interfacial layer-semiconductor (MIS) configuration rather than an ideal Schottky diode due to the existence of GaN at the Au/GaAs interfacial layer. The formation of the GaN interfacial layer for the stable passivation of gallium arsenide surface is investigated through calculation of the interface state density N_ss with and without taking into account the series resistance R_s. While the interface state density calculated without taking into account R_s has increased exponentially with bias from 2.2×10¹² cm⁻² eV⁻¹ in (E_c−0.48) eV to 3.85×10¹² cm⁻² eV⁻¹ in (E_c−0.32) eV of n-GaAs, the N_ss obtained taking into account the series resistance has remained constant with a value of 2.2×10¹² cm⁻² eV⁻¹ in the same interval. This has been attributed to the passivation of the n-doped GaAs surface with the formation of the GaN interfacial layer.     

Fabrication of a three-dimensional complex carbon nanoneedle from carbon nanowalls

A three-dimensional complex carbon nanoneedle has been fabricated from carbon nanowalls by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibits novel three-dimensional complex nanostructure, the center of which is a carbon nanoneedle, and many carbon nanowalls growing from the needle. The density of unique nanostructure emitters was about 5 × 10⁷/cm². The I-V characteristic addressed an emission current density of 314 mA/cm² at the electric field of 2.5 V/μm.     

The structure of crystallographic damage in GaN formed during rare earth ion implantation with and without an ultrathin AlN capping layer

The crystallographic nature of the damage created in GaN implanted by rare earth ions at 300 keV and room temperature has been investigated by transmission electron microscopy versus the fluence, from 7×10¹³ to 2×10¹⁶ at/cm², using Er, Eu or Tm ions. The density of point defect clusters was seen to increase with the fluence. From about 3×10¹⁵ at/cm², a highly disordered ‘nanocrystalline layer’ (NL) appears on the GaN surface. Its structure exhibits a mixture of voids and misoriented nanocrystallites. Basal stacking faults (BSFs) of I₁, E and I₂ types have been noticed from the lowest fluence, they are I₁ in the majority. Their density increases and saturates when the NL is observed. Many prismatic stacking faults (PSFs) with Drum atomic configuration have been identified. The I₁ BSFs are shown to propagate easily through GaN by folding from basal to prismatic planes thanks to the PSFs.When implanting through a 10 nm AlN cap, the NL threshold goes up to about 3×10¹⁶ at/cm². The AlN cap plays a protective role against the dissociation of the GaN up to the highest fluences. The flat surface after implantation and the absence of SFs in the AlN cap indicate its high resistance to the damage formation.     

Effects of high-temperature annealing on magnetic properties of V-doped GaN thin films grown by MOCVD

Metal organic chemical vapor deposition (MOCVD) has been used to grow vanadium-doped GaN (GaN:V) on c-sapphire substrate using VCl₄ as the V source. The as-grown GaN:V exhibited a saturated magnetic moment (M_s) of 0.28 emu/cm³ at room temperature. Upon high-temperature annealing treatment at 1100 °C for 7 min under N₂ ambient, the M_s of the GaN:V increased by 39.28% to 0.39 emu/cm³. We found that rapid thermal annealing leads to a remarkable increase in surface roughness of the V-doped GaN as well as the electron concentration. The annealing also leads to a significant increase in the Curie temperature (T_C), we have identified Curie temperatures about 350 K concluded from the difference between the field-cooled and zero-field-cooled magnetizations. Structure characterization by x-ray diffraction indicated that the ferromagnetic properties are not a result of secondary magnetic phases.     

Characterization of a-plane orientation ZnO film grown on GaN/Sapphire template by pulsed laser deposition

In this study, the authors have investigated the structural and optical properties of ZnO layer grown by pulsed laser deposition on GaN/r-plane sapphire. X-ray diffraction results demonstrate the ZnO film to be highly preferentially deposited at a-axis orientation; the different rocking curve values along the two orthogonal directions indicate the low C_2v symmetry in the growth a-plane ZnO. From free stress to large tensile stress (about 1.34 × 10⁹ Pa) distribution along the growth direction of ZnO is revealed by visible Raman mapping spectra. The enhanced significantly high-order longitudinal-optical (LO) phonon modes up to 4th and no TO phonons have been observed in Raman spectrum under UV 325 nm by resonance conditions; an intense and broad disorder activated surface phonon mode is also observed, resulting from the increased disorder on the film surface with stripe-like growth features. Low-temperature photoluminescence measurements reveal that the band-edge emission of ZnO is dominated by neutral donor-bound exciton and free electrons to neutral acceptor emissions. Interfacial microstructure of ZnO/GaN has been examined by transmission electron microscopy, with the epitaxial relationship () ZnO//() GaN. All these results indicated that GaN template played an important role in the growth of ZnO film, with full advantage of small lattice mismatch.     

Synthesis and characterization of graphene-based nanocomposites with potential use for biomedical applications

In this paper, GaN nanoparticles were synthesized from the complex Ga(H₂NCONH₂)₆Cl₃ in the flow of NH₃ at a mild temperature (350 °C). Further purification was performed by the ethanol-thermal method. The ethanol-thermal method also prompted the GaN nanoparticles to grow into an anisotropic morphology. XRD patterns reveal that GaN nanoparticles have crystallized in a hexagonal wurtzite structure. TEM observation shows that the average size of the as-prepared nanoparticles is about 5–10 nm. The photoluminescence spectrum exhibits a broad green emission band with a peak at 510 nm. It can be known from the first-principle theoretic simulation by the TDDFT method that this fluorescence emission band is attributed to the hydride defects of V _N-H on the surface of GaN nanoparticles.     

Effect of growth pressure on the morphology evolution and doping characteristics in nonpolar a-plane GaN

Nonpolar a-plane GaN layers grown on r-plane sapphire substrates were examined by using a two-step growth process. The higher initial growth pressure for the nucleation layer resulted in the improved crystalline quality with lower density of both threading dislocations and basal stacking faults. This was attributed to the higher degree of initial roughening and recovery time via a growth mode transition from three-dimensional (3D) to quasi two-dimensional (2D) lateral growth. Using Hall-effect measurements, the overgrown Si doped GaN layers grown with higher initial growth pressure were found to have higher mobility. The scattering mechanism due to the dislocations was dominant especially at low temperature (<200 K) for the lower initial growth pressure, which was insignificant for the higher initial growth pressure. The temperature-dependent Hall-effect measurements for the Mg doped GaN with a higher initial growth pressure yielded the activation energy and the acceptor concentration to be 128 meV and 1.2 × 10¹⁹ cm⁻³, respectively, corresponding to about 3.6% of activation at room temperature. Two-step growth scheme with a higher initial growth pressure is suggested as a potential method to improve the performance of nonpolar a-plane GaN based devices.     

The effect of sub-band gap photon illumination on the properties of GaN layers grown on Si(111) by MBE

Nanostructured GaN layers are fabricated by laser-induced etching processes based on heterostructure of n-type GaN/AlN/Si grown on n-type Si(111) substrate. The effect of varying laser power density on the morphology of GaN nanostructure layer is observed. The formation of pores over the structure varies in size and shape. The etched samples exhibit dramatic increase in photoluminescence intensity compared to the as-grown samples. The Raman spectra also display strong band at 522 cm⁻¹ for the Si(111) substrate and a small band at 301 cm⁻¹ because of the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 and 568 cm⁻¹ due to E2 (low) and E2 (high), respectively. Surface morphology and structural properties of nanostructures are characterized using scanning electron microscopy and X-ray diffraction. Photoluminance measurement is also taken at room temperature by using He–Cd laser (λ = 325 nm). Raman scattering is investigated using Ar⁺ Laser (λ = 514 nm).