GaN Samples Research Articles

Preparation and Characterization of Mg-Doped GaN Nanowires by Au-Catalyzed Magnetron Sputtering Deposition

Mg-doped GaN nanowires have been successfully synthesized on Si (111) substrates by magnetron sputtering deposition through ammoniating Ga2O3/Au thin films at 900 °C for 15 min. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and photoluminescence (PL) spectrum were carried out to characterize microstructure, morphology, and optical property of GaN sample. The results demonstrate that the nanowires are single-crystal Mg-doped GaN with hexagonal wurtzite structure and high crystalline quality, which have the size of 40 nm in diameter and several tens of microns in length and good emission property. The growth procedure mainly follows the VLS mechanism, and Au plays an important role as catalyst, and more defect energy is formed due to metallic Au and thus promote the growth of GaN nanowires.

Blue inorganic light emitting diode on flexible polyimide substrate using laser lift-off process.

The fabrication process for the blue GaN inorganic light emitting diode (ILED) on flexible polyimide (PI) substrate by laser lift off (LLO) method was demonstrated. The GaN epi-structure was grown on patterned sapphire wafer. GaN samples were temporary bonded with polyimide substrate by flexible silver epoxy. Separation of the whole GaN LED film from GaN/sapphire wafer was accomplished using a single KrF excimer (248 nm) laser pulse directed through the transparent sapphire wafer. Device fabrication was carried out on both rigid silicon and flexible polyimide substrate, and I-V performance for both devices was measured. The optimized LLO process for the whole GaN LED film transfer would be applicable in flexible LED applications without compromising electrical properties.

Optical properties of ultra-thin (< 30 nm) GaN layers on c-sapphire substrates with different initial growth conditions measured by surface-plasmon enhanced Raman scattering.

We have carried out surface-plasmon enhanced Raman spectroscopy (SERS) on 30 nm-thick GaN samples grown at various temperatures, in order to investigate the properties of ultra thin GaN films on sapphire. We found that the properties, such as the strain and the free-carrier density of the thin layers, were sensitively affected by the growth temperatures. Our results show that SERS, by selectively enhancing the Raman signal near the surface, can be a very useful technique to investigate the optical properties of ultra-thin GaN films and their initial growth mode.

Effect of Basal-plane Stacking Faults on X-ray Diffraction of Non-polar (1120) a-plane GaN Films Grown on (1102) r-plane Sapphire Substrates

We report the effect of basal-plane stacking faults (BSFs) on X-ray diffraction (XRD) of non-polar (1120) a-plane GaN films with different SiNx interlayers. Complete SiNx coverage and increased three-dimensional (3D) to two-dimensional (2D) transition stages substantially reduce BSF density. It was revealed that the Si-doping profile in the Sidoped GaN layer was unaffected by the introduction of a SiNx interlayer. The smallest in-plane anisotropy of the (1120) XRD ω-scan widths was found in the sample with multiple SiNx layers, and this finding can be attributed to the relatively isotropic GaN mosaic resulting from the increase in the 3D-2D growth step. Williamson-Hall (WH) analysis of the (h0h0) series of diffractions was employed to determine the c-axis lateral coherence length (LCL) and to estimate the mosaic tilt. The c-axis LCLs obtained from WH analyses of the present study’s representative a-plane GaN samples were well correlated with the BSFrelated results from both the off-axis XRD ω-scan and transmission electron microscopy (TEM). Based on WH and TEM analyses, the trends in BSF densities were very similar, even though the BSF densities extracted from LCLs indicated that the values were reduced by a factor of about twenty.

Spectroscopic study of semipolar (112¯2)-HVPE GaN exhibiting high oxygen incorporation

Spatially resolved luminescence and Raman spectroscopy investigations are applied to a series of (112¯2)-GaN samples grown by hydride vapor phase epitaxy (HVPE) grown over an initial layer deposited by metal organic vapor phase epitaxy on patterned sapphire substrates. Whereas these two differently grown GaN layers are crystallographically homogeneous, they differ largely in their doping level due to high unintentional oxygen uptake in the HVPE layer. This high doping shows up in luminescence spectra, which can be explained by a free-electron recombination band for which an analytical model considering the Burstein-Moss shift, conduction band tailing, and the bandgap renormalization is included. Secondary ion mass spectrometry, Raman spectroscopy, and Hall measurements concordantly determine the electron density to be above 1019 cm−3. In addition, the strain state is assessed by Raman spectroscopy and compared to a finite element analysis.

Porous GaN for Gas Sensing Application

Porous wide bandgap semiconductors have been widely studied in the last decade due to their unique properties compared to the bulk crystals. GaN received attention from the researchers as an ideal material to fabricate chemical sensing devices due to its excellent properties such as high thermal, mechanical and chemical stabilities, large band gap and high breakdown voltage. In this work, porous GaN was prepared by ultraviolet (UV) assisted electroless chemical etching method. The samples used in this study were commercial n-GaN grown on sapphire (Al2O3) substrates. The samples were initially cleaned in 1:20 NH4OH:H2O, followed by second cleaning in 1:50 HF:H2O and final cleaning in 3:1 HCl: HNO3and these samples were etched in HF:H2O2:CH3OH under UV illumination for 60 minutes. The structural properties was characterized using Scanning Electron Microscope (SEM). Hydrogen sensor was subsequently fabricated by depositing Pd Schottky contact onto the porous GaN sample. The effect of sensing dilute H2gas with different concentration which is 1% and 2% H2in a N2gas ambient was analyzed. The Schottky barrier height of the gas sensor samples was reduced upon exposure to gas. The porous GaN resulted better sensitivity compared to the as grown GaN sample in H2gas sensing.

HVPE로 성장된 GaN의 용융 KOH/NaOH 습식화학에칭

Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea*UNIMO Photron, Seoul 137-063 Korea(Received June 10, 2014)(Revised July 7, 2014)(Accepted July 18, 2014)Abstract The hydride vapor phase epitaxy (HVPE) grown GaN samples to precisely measure the surface characteristicswas applied to a molten KOH/NaOH wet chemical etching. The etching rate by molten KOH/NaOH wet chemical etchingmethod was slower than that by conventional etching methods, such as phosphoric and sulfuric acid etching, which may bedue to the formation of insoluble coating layer. Therefore, the molten KOH/NaOH wet chemical etching is a betterefficient method for the evaluation of etch pits density. The grown GaN single crystals were characterized by using X-raydiffraction (XRD) and X-ray rocking curve (XRC). The etching characteristics and surface morphologies were studied byscanning electron microscopy (SEM). From etching results, the optimum etching condition that the etch pits were wellindependently separated in space and clearly showed their shape, was 410

Deep Level Centers in Carbon-Doped High Resistivity GaN

The deep levels of carbon doped high resistivity (HR) GaN samples grown by metal-organic chemical vapor deposition (MOCVD) has been investigated using thermally stimulated current (TSC) spectroscopy and high temperature (HT) Hall measurement. Two different thickness of 100 and 300 nm were used to be compared. It was found that four distinct deep levels by TSC and one deep level by HT Hall measurement were observed in both samples, which means great help for the decrease of leakage current and lifetime limitations of device utilizing the structure. The activation energy of these levels was calculated and their possible origins were also proposed. The low temperature traps, might be related to VN, 0.50 and 0.52eV related to incorporate a high level carbon, 0.57eV related to VGa, 0.59eV related to CGaor NGa, 0.91 and 0.97eV related to interstitial N1.

Effect of porosity on GaN for hydrogen gas sensing

Porous wide bandgap semiconductors have been widely studied in the last decade due to their unique properties compared to the bulk crystals. The high surface area, shift of bandgap, luminescence intensity enhancement and efficient photoresponse when porosity is formed can be tailored to fabricate new sensing devices. In this work, porous GaN was prepared by ultraviolet (UV) assisted electroless chemical etching method. The commercial Si doped n-type GaN film grown on two inches diameter sapphire (0001) substrate with GaN thickness of 5.5 μm was used in this study. The wafer was then cleaved into few pieces, and these samples were etched in HF:H2O2:CH3OH under UV illumination for 60 minutes. The structural properties was characterized using Scanning Electron Microscope (SEM) and Atomic Force Microscopy (AFM). Hydrogen sensor was subsequently fabricated by depositing Pt Schottky contact onto the porous GaN sample. The effect of sensing dilute H2 gas with different concentration which is 1% and 2% H2 in a N2 gas ambient was analyzed. The Schottky barrier height of the gas sensor samples was reduced upon exposure to gas. The porous GaN resulted better sensitivity compared to the as grown GaN sample in H2 gas sensing

Synchrotron-based XPS studies of AlGaN and GaN surface chemistry and its relationship to ion sensor behaviour

Abstract Soft X-ray photoelectron spectroscopy was used to investigate the fundamental surface chemistry of both AlGaN and GaN surfaces in the context of understanding the behaviour of AlGaN/GaN heterostructures as chemical field-effect transistor (CHEMFET) ion sensors. AlGaN and GaN samples were subjected to different methods of oxide growth (native oxide and thermally grown oxide) and chemical treatment conditions. Our investigations indicate that the etching of the oxide layer is more pronounced with AlGaN compared to GaN. Also, we observed that chloride ions have a greater tendency to attach to the GaN surface relative to the AlGaN surface. Furthermore, chloride ions are comparatively more prevalent on surfaces treated with 5% HCl acid solution. The concentration of chloride ions is even higher on the HCl treated native oxide surface resulting in a very clear deconvolution of the Cl 2p 1/2 and Cl 2p 3/2 peaks. For GaN and AlGaN surfaces, a linear response (e.g. source-drain current) is typically seen with variation in pH of buffered solutions with constant reference electrode voltage at the surface gate; however, an inverted bath-tub type response (e.g. a maximum at neutral pH and lower values at pH values away from neutral) and a general tendency to negative charge selectivity has been also widely reported. We have shown that our XPS investigations are consistent with the different sensor response reported in the literature for these CHEMFET devices and may help to explain the differing response of these materials.

Broadband measurements of the refractive indices of bulk Gallium Nitride

New measurements of the index of refraction for free-standing samples of gallium nitride are reported. A simple dispersive prism technique is used to obtain the birefringent indices from 500 to 5100 nm, covering most of the transparency range of this wide band gap semiconductor. Millimeter thick samples prepared by both ammonothermal growth and hydride vapor phase epitaxy are found to have nearly identical refractive indices. The observed dispersion fits well to a two-pole Sellmeier equation with an estimated overall accuracy of ± 0.002. Our results are found to be in good agreement with previous visible and near-IR measurements on thin-film GaN samples, however we observed significantly less dispersion in the mid-IR. Moderate heating of the samples also provided a new determination of dn/dT.

MOVPE growth of GaN on Si substrate with 3C-SiC buffer layer

We investigated the effect of the thickness of a 3C-SiC buffer layer on the growth of GaN on a Si substrate. GaN samples with thicknesses of 2.0 and 4.5 µm were grown by metal organic vapor phase epitaxy. Islands were observed at the initial growth of the GaN samples, and they became larger and then coalesced with each other with increasing growth time. The crystalline quality of the GaN samples was affected by the thickness of the 3C-SiC buffer layer and was improved by increasing their thickness. This indicates that the crystalline quality trend for thick GaN growth is different from that for initial GaN growth. Moreover, the GaN sample with a 100-nm-thick SiC buffer layer had a low initial curvature, and the crystalline qualities on GaN(0004) and planes were 389 and 460 arcsec for 4.5-µm-thick GaN growth, respectively.

Photo-Electroless-Etching of Wide Band Gap Material for Flexible Solid-State Devices

We report on photo electroless chemical (PEC) etching of III-Nitride material for thin layer lift-off for flexible solid-state light emitting devices. We investigate three different structures: 30 µm unintentionally doped GaN on sapphire, 30 µm n-type GaN on sapphire, and 200 nm InGaN on 4 µm unintentionally doped GaN on sapphire with different indium composition. Initially the three structures were etched as-grown for the same duration and the same conditions in a HF/CH3OH/H2O2 solution under UV illumination. Scanning electron microscopy (SEM) measurements performed on the etched samples reveals porous structure and high density nanowire on the surface, as well as the formation of V-shaped pits. The etch rate in each structure were compared. Optical study were performed using photoluminescence (PL) and Raman spectroscopies to analyze PL peak shift and phonon frequency shift compared to the as-grown, which we attributed to the compressive strain relaxation and localized impurity formed in the crystalline structure after PEC etching. We also fabricated microcolumns of diameters ranging from 5 to 100-µm via inductively coupled plasma (ICP) etching process in the InGaN on GaN samples. Dry etching of micropillars exposes unintentionally doped GaN to the wet etching. We study the selectivity of GaN over InGaN in the PEC etching process. SEM images reveal lateral etching and porous GaN layer formation under the InGaN after wet etching. We compare etching process in the InGaN structure with and without microcolumns. PEC etching is sensitive to band bending in the surface of the semiconductor and electrolyte, when InGaN surface is in contact with HF solution, oxidation and reduction reaction happens at slow rate and leads to the porous structure in the surface. However, when the GaN under the InGaN is exposed, the etching in this layer is preferred due to the low resistivity and the confinement of the photogenerated minority in the upward band bending depleted region near the contact surface between GaN and electrolyte.Undercut InGaN microdisks were lifted off for optical study via flexible polydimethylsiloxane (PDMS) material. Microdisks were then transferred on to a Si wafer for PL measurements. We also performed Raman study on the microdisks and observed a shift in the phonon frequency attributed to the relaxation of the compressive strain with respect to the as-grown. InGaN microdisks with higher indium composition were transferred on a blue LED for optical pumping. Optical measurements on transferred microdisks revealed typical prominent visible peaks. These observations confirm the promising application of this technique to selectively etch III-V semiconductor. The lift-off process is a compelling technique for next generation transferred substrate devices and III-Nitride micro phosphor fabrication.

Characterization of in-depth cavity distribution after thermal annealing of helium-implanted silicon and gallium nitride

Single-crystalline silicon wafers covered with sacrificial oxide layer and epitaxially grown gallium nitride layers were implanted with high-fluence helium ions (2–6×1016cm−2) at energies of 20–30keV. Thermal annealings at 650–1000°C, 1h were performed on the Si samples and rapid thermal annealings at 600–1000°C, 120s under N2 were performed on the GaN samples. The as-implanted samples and the near-surface cavity distributions of the annealed samples were investigated with variable angle spectroscopic ellipsometry. In-depth defect profiles and cavity profiles can be best described with multiple independent effective medium sublayers of varying ratio of single-crystal/void. The number of sublayers was chosen to maximize the fit quality without a high parameter cross-correlation. The dependence of the implantation fluence, oxide layer thickness and annealing temperature on the cavity distribution was separately investigated. The ellipsometric fitted distributions were compared and cross-checked with analyses of transmission electron micrographs where the average surface cavity was determined sublayer by sublayer. The in-depth profiles were also compared with simulations of He and vacancy distributions.

Characterization of undoped and Si‐doped bulk GaN fabricated by hydride vapor phase epitaxy

AbstractIn this study, the optical, structural, electrical and thermoelectric properties of undoped and Si‐doped bulk GaN samples are investigated. The room temperature PL near band emission peaks of undoped and Si‐doped bulk GaN samples are 3.414 eV and 3.402 eV. The undoped and Si‐doped bulk GaN samples’ PL spectra have different defect emission bands which are green band and red band with the maximum at 2.4 eV and 2.0 eV, respectively. At low temperature, the PL spectrum of undoped bulk GaN has well‐defined excitonic emission lines, A0X, D0X, FXA (n = 1, 2), while the PL spectrum of Si‐doped bulk GaN has just broad emission peaks at the similar energy level. The low temperature PL spectra of both undoped and Si‐doped samples clearly show the first, second and third longitudinal‐optical phonon (1 LO, 2 LO and 3 LO) replicas. There are no donor‐acceptor pairs and electron‐acceptors’ emission in the PL spectra of the samples. The temperature dependence PL spectra of undoped GaN samples shows that the intensity ratio of the FXA to D0X decreases with the increase of the temperature. The full width at half maximum (FWHM) of D0X emission is about 4.7 meV. The carrier density of the undoped and Si‐doped bulk GaN are 4.58 × 1015 and 2.41 × 1018 cm–‐3, and the mobility of the undoped and Si‐doped bulk GaN are 1050 and 286 cm2V–1s–1. The FWHM of (0002) X‐ray diffraction rocking curve are 54 arcsec and 95 arcsec for the undoped and Si‐doped bulk GaN samples. The power factors of the undoped GaN sample and the Si‐doped GaN sample are 0.315 × 10–4W/mK2 and 0.35495 × 10–4 W/mK2, respectively. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Comparison of thermoelectric properties of GaN and ZnO samples

AbstractIn this paper, room temperature thermoelectric (TE) properties of wide bandgap thin film GaN and bulk ZnO are studied. Bulk GaN is also incorporated with epitaxy films to make comparison. GaN and ZnO materials have superior electrical performance and chemical stability at high temperatures and are currently found in many commercial applications, such as, photovoltaic, solid‐state lighting, and gas sensors. Since there are not many semiconductor materials that can operate effectively at high temperatures, wide bandgap materials like GaN and ZnO would be a promising solution for high temperatures thermoelectric power generation. In order to understand their TE properties, we systematically compared and characterized TE behaviour of GaN and ZnO thin films with a function of doping concentrations. The common trend of a decrease in Seebeck coefficients with the increase of carrier concentration for thin film and bulk GaN is observed, however, reverse TE trend for bulk ZnO, were observed. The most commonly observed TE trend is attributed to Mott‐Jones relation to simple transport models while inverse trend could be suggested as hopping conductance. Moreover, the Seebeck coefficients of ZnO samples are found to be larger than those of thin film and bulk GaN samples in the similar carrier density. Highest power factors, 2.6×10–4 W/mK2and 0.65×10–4 W/mK2, and Seebeck coefficients, 478 µV/K and 481 μV/K were measured for thin film GaN and bulk ZnO, respectively. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Insulating gallium oxide layer produced by thermal oxidation of gallium‐polar GaN

AbstractThe benefits of dry oxidation of n ‐GaN for the fabrication of metal‐oxide‐semiconductor structures are reported. GaN thin films grown on sapphire by MOCVD were thermally oxidized for 30, 45 and 60 minutes in a pure oxygen atmosphere at 850 °C to produce thin, smooth GaOx layers. The GaN sample oxidized for 30 minutes had the best properties. Its surface roughness (0.595 nm) as measured by atomic force microscopy (AFM) was the lowest. Capacitance‐voltage measurements showed it had the best saturation in accumulation region and the sharpest transition from accumulation to depletion regions. Under gate voltage sweep, capacitance‐voltage hysteresis was completely absent. The interface trap density was minimum (Dit = 2.75×1010 cm–2eV–1) for sample oxidized for 30 mins. These results demonstrate a high quality GaOx layer is beneficial for GaN MOSFETs. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of growth temperature on defects in epitaxial GaN film grown by plasma assisted molecular beam epitaxy

We report the effect of growth temperature on defect states of GaN epitaxial layers grown on 3.5 μm thick GaN epi-layer on sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The GaN samples grown at three different substrate temperatures at 730, 740 and 750 °C were characterized using atomic force microscopy and photoluminescence spectroscopy. The atomic force microscopy images of these samples show the presence of small surface and large hexagonal pits on the GaN film surfaces. The surface defect density of high temperature grown sample is smaller (4.0 × 108 cm−2 at 750 °C) than that of the low temperature grown sample (1.1 × 109 cm−2 at 730 °C). A correlation between growth temperature and concentration of deep centre defect states from photoluminescence spectra is also presented. The GaN film grown at 750 °C exhibits the lowest defect concentration which confirms that the growth temperature strongly influences the surface morphology and affects the optical properties of the GaN epitaxial films.

Sulphide passivation of GaN based Schottky diodes

Wet chemical passivation of n-GaN surface was carried out by dipping GaN samples in ammonium sulphide diluted in aqueous and alcoholic solvent base solutions. Photoluminescence (PL) investigations indicated that sulphide solution effectively led to the reduction of GaN surface states. Increased band edge PL peak showed that S2− ions are more active in alcohol based solvents. X-ray photoelectron spectroscopy revealed reduction in surface oxides by introduction of sulphide species. Ni/n-GaN Schottky barrier diodes were fabricated on passivated surfaces. Remarkable improvement in the Schottky barrier height (0.98 eV for passivated diodes as compared to 0.75 eV for untreated diodes) has been observed.

Influences of post-heat treatment on microstructures, optical and magnetic properties of unintentionally doped GaN epilayers implanted with Mn ions

In this study, GaN:Mn thin films are fabricated by implementing Mn ions into the undoped GaN material. The effects of annealing temperature on microstructures, optical and magnetic properties of the thin films are investigated. The Raman spectra measured from Mn-implanted GaN samples at different annealing temperatures show that new phonon modes, which are related to macroscopic disorder or vacancy-related defects caused by Mn-ion implantation and the local vibrational mode of Mn atoms in the (Ga, Mn)N, are created. The results of photoluminescence measurement show that new peaks appear at 2.16, 2.53, and 2.92 eV. Among these, the new emission around 2.16 eV, besides some contributions from optical transitions from the conduction band or shallow donor to a deep acceptor, cannot exclude the contribution from optical transitions of free electrons in the conduction band to Mn acceptor level. The Hall test shows that the annealed samples are of n type. Ferromagnetism is observed in the Mn doped GaN thin film at 300 K and found to be sensitive to the density of holes that mediate the Mn-Mn magnetic exchange interaction in this Mn-related impurity band.