Bulk GaN Layers Research Articles

Determination of the leakage current transport mechanisms in quasi-vertical GaN–on–Si Schottky barrier diodes (SBDs) at low and high reverse biases and varied temperatures

Temperature-dependent current–voltage characteristics was measured to investigate leakage conduction mechanisms in GaN-on-Si quasi-vertical Schottky barrier diode. At low reverse bias, thermionic emission is dominated. At voltage range from −1 to −20 V, variable range hopping (VRH) is main leakage current mechanism at low temperatures (298–373 K), while at high temperatures (498–573 K), electrons gain enough energy and emit into the trapped states, Frenkel–Poole emission becomes dominant. At a relatively high voltage range, the increased electric field promotes the electron hopping along the threading dislocation in the bulk GaN layers, and VRH becomes the main current mechanism at whole measured temperature range (298–573 K).

Hydride Vapor‐Phase Epitaxy Reactor for Bulk GaN Growth

An hydride vapor‐phase epitaxy reactor for the growth of bulk GaN crystals with a diameter of 50 mm is developed. Growth rate nonuniformity of 1% is achieved using an axisymmetric vertical gas injector with stagnation point flow. Chemically resistant refractory materials are used instead of quartz in the reactor hot zone. High‐capacity external gallium precursor sources are developed for the nonstop growth of the bulk GaN layers. A load‐lock vacuum chamber and a dry in situ growth chamber cleaning are implemented to improve the growth process reproducibility. Freestanding GaN crystals with a diameter of 50 mm are grown with the reactor.

Leakage mechanism of quasi-vertical GaN Schottky barrier diodes with ultra-low turn-on voltage

The leakage mechanism of quasi-vertical GaN Schottky barrier diodes (SBDs) with ultra-low turn-on voltage has been investigated. By using a tungsten anode, the turn-on voltage is 0.39 V and the average breakdown electric field is above 1 MV cm−1. Under low reverse bias, the thermionic emission is dominated. When the reverse bias increases to a certain value, the increased electric field promotes the electron hopping along the threading dislocation in the bulk GaN layers, and variable range hopping (VRH) becomes the main leakage current mechanism. The leakage difference between tungsten-anode and nickel-anode SBDs is reduced to one order of magnitude due to the VRH mechanism.

Free-standing 2-inch bulk GaN crystal fabrication by HVPE using a carbon buffer layer

A free-standing bulk gallium nitride layer with a thickness of 365 μm and a diameter of 50 mm was obtained by hydride vapor phase epitaxy on a sapphire substrate with a carbon buffer layer. The carbon buffer layer was deposited by thermal decomposition of methane in situ in the same process with the growth of a bulk GaN layer. The bulk GaN layer grown on the carbon buffer layer self-separated from the sapphire substrate during the cooling after the growth. The dislocation density was 8 · 106 cm−2. The (0002) X-Ray rocking curve full width at half maximum was 164 arcsec.

Freestanding patterned polycrystalline GaN substrate by a straightforward and affordable technique

A new process for producing a freestanding patterned polycrystalline GaN substrate by applying a straightforward and affordable technique is presented here. Such substrate was fabricated by depositing ~ 50 µm thick bulk GaN layer on porous Si/Si substrate by e-beam evaporator with successive ammonia annealing to improve the material quality of the GaN layer. The GaN layer was then separated from the porous Si/Si substrate by immersing in an acidic solution. The surface of the freestanding patterned polycrystalline GaN substrate that was in contact with the porous Si/Si substrate consisted of cubic-like structures, as inherited from the porous Si/Si substrate. Although the cubic-like structures were almost uniformly distributed on the surface, they were formed in various heights due to irregular degree of symmetry of the porous Si/Si substrate. X-ray diffraction results suggested that β-Ga2O3 inclusions are inside the freestanding patterned polycrystalline GaN substrate but not on its surface. This was supported by micro-photoluminecsence (PL) measurement, whereby only the GaN PL signals were observed. Furthermore, Raman spectroscopy revealed a small amount of compressive stress (0.23 GPa), suggesting that the substrate was almost relaxed.

Stress-engineered growth of homoepitaxial GaN crystals using hydride vapor phase epitaxy.

We report the growth of a 3.5 mm-thick bulk GaN layer using a stress-engineered homoepitaxy method without any external processes. We employ a gradient V/III ratio during the growth, which enables a 3.5 mm-thick bulk GaN layer with a smooth surface and high crystal quality to be obtained. For a constant V/III ratio of 10, the bulk GaN layer has a flat surface; however, microcracks emerge in the GaN layer. For a constant V/III ratio of 38, the bulk GaN layer has a rough surface, without microcracks. On the other hand, by decreasing the V/III ratio from 38 to 10, the structural properties of the GaN layers are successfully controlled. The higher V/III ratio in the initial growth stage leads to a rough surface, and reduced stress and dislocation density in the bulk GaN layers, while the lower V/III ratio in the second stage of the growth provides an opposite trend, confirmed by Raman spectroscopy and X-ray measurements. We expect that this study will offer a new opportunity to achieve the growth of high-crystallinity bulk GaN without ex situ and complicated processes.

Porosity-induced relaxation of strains in GaN layers studied by means of micro-indentation and optical spectroscopy

We report the fabrication of porous GaN nanostructures using UV-assisted electroless etching of bulk GaN layer grown on c-plane sapphire substrate in a solution consisting of HF:CH3OH:H2O2. The morphology of the porous GaN nanostructures was characterized for different etching intervals using high resolution scanning electron microscopy. The geometry and size of resultant pores do not appear to be affected by the etching time; however, the pore density was augmented for longer etching time. Micro-indentation tests were carried out to quantify the indentation modulus for different porous GaN nanostructures. Our results reveal a relationship between the elastic properties and the porosity kinetics, i.e., a decrease of the elastic modulus was observed with increasing porosity. The photoluminescence (PL) and Raman measurements carried out at room temperature for the etched samples having a high degree of porosity revealed a strong enhancement in intensity. Also, the peak of the PL wavelength was shifted towards a lower energy. The high intensity of PL was correlated to an increase of scattered photons within the porous media and to the reduction of the dislocation density.

Study of Substrate Induced Deep Level Defects in Bulk GaN Layers Grown by Molecular Beam Epitaxy Using Deep Level Transient Spectroscopy

In this paper we have investigated the substrate-induced deep level defects in bulk GaN layers grown onp-silicon by molecular beam epitaxy. Representative deep level transient spectroscopy (DLTS) performed on Au-GaN/Si/Al devices displayed only one electron trap E1at 0.23 eV below the conduction band. Owing to out-diffusion mechanism; silicon diffuses into GaN layer from Si substrate maintained at 1050°C, E1level is therefore, attributed to the silicon-related defect. This argument is supported by growth of SiC on Si substrate maintained at 1050°C in MBE chamber using fullerene as a single evaporation source.

Growth and characterization of free‐standing zinc‐blende GaN layers and substrates

AbstractWe have investigated the growth of bulk, free‐standing zinc‐blende (cubic) GaN layers by plasma‐assisted molecular beam epitaxy (PA‐MBE). The PA‐MBE technique was used for bulk crystal growth and GaN layers up to 100 µm in thickness were produced. We have established that the best structural properties of free‐standing zinc‐blende GaN can be achieved with initiation under Ga‐rich conditions, but without Ga droplet formation. The procedure to produce free‐standing bulk zinc‐blende substrates from thick GaN layers grown on GaAs substrates has been developed. We have demonstrated the scalability of the process by growing free‐standing GaN layers up to 3 in. in diameter. Growth of free‐standing bulk GaN layers has allowed us to refine the values for the basic parameters of zinc‐blende GaN. We have demonstrated also that the PA‐MBE process we have developed has allowed us to achieve free‐standing AlxGa1−xN wafers.

Molecular beam epitaxy as a method for the growth of freestanding zinc-blende (cubic) GaN layers and substrates

The authors have studied the growth of bulk, freestanding zinc-blende (cubic) GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE). They have established that the best structural properties of freestanding zinc-blende GaN can be achieved with initiation under Ga-rich conditions but without Ga droplet formation. It is difficult to initiate the growth of zinc-blende GaN, but it is even more difficult to sustain the growth of the pure zinc-blende polytype in thick layers without any wurtzite inclusions. In order to grow high quality freestanding cubic GaN layers, it is necessary to maintain the same growth conditions for about 1week. The best quality zinc-blende phase GaN was achieved in the first 10μm of the GaN layers. The authors have produced zinc-blende GaN substrates from our thick bulk GaN layers and they used the side previously attached to the GaAs substrate as the episide of these zinc-blende GaN substrates. They have demonstrated the scalability of the process by growing zinc-blende GaN layers on 2 and 3in. diameter wafers. The growth of freestanding bulk GaN layers has allowed them to refine the value for the lattice parameter of zinc-blende GaN as 4.510±0.005Å. They have demonstrated that the PA-MBE process developed has allowed them to grow freestanding AlxGa1−xN wafers.

Investigation of low-temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with Simple Parallel Conduction Extraction Method

The electrical conduction mechanisms in various highly resistive GaN layers of Al x Ga1−x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal–insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln (ρ) is proportional to T −1/4 for the insulating sample and proportional to T −1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.

Vapor phase growth of GaN using GaN powder sources and thermogravimetric investigations of the evaporating behaviour of the source material

AbstractThe growth of GaN from the vapor phase is a promising technique for producing both bulk GaN crystals and GaN layers. For establishing a growth method from the vapor phase the source material and reactor setup are of great importance. Highly pure and self synthesized GaN powder was chosen as source material. The evaporation behaviour of the GaN powder was studied by means of thermogravimetry (TG). A vertical growth reactor was set up according to the results of numerical simulations of the temperature distributions and flow patterns. Freely nucleated GaN platelets of some millimetres in length were grown. Furthermore, thin GaN layers were deposited directly on a sapphire substrate. This nucleation layer was successfully overgrown by low pressure solution growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydride vapour phase epitaxy growth and characterization of thick GaN using a vertical HVPE reactor

Growth of 2-inch diameter bulk GaN layers with a thickness up to 2 mm is demonstrated in a vertical hydride vapour phase growth reactor. Morphology, dislocations, optical and electrical properties of the material have been investigated using atomic force microscopy, optical microscopy, decorative etching in hot H 3PO 4, Hall measurements and low-temperature photoluminescence. Atomic force microscopy reveals a two-dimensional step flow growth mode with step bunching for layers with a thickness of 250 μm. As the growth proceeds, the morphology is changed to a hill and valley structure. The EPD was determined to 5×10 5 cm −2 for a 2 mm thick layer. The Hall mobility and the carrier concentration were determined. For a 1.7 mm thick layer at 300 K the mobility and the carrier concentration is 520 cm 2/V s and about 4×10 17 cm −3, respectively. Low-temperature photoluminescence spectra measured on a 350 μm thick freestanding layer show the DBE line at 3.4707 eV with a full-width half-maximum of 1 meV, confirming a stress free GaN layer.

Bulk GaN layers grown on oxidized silicon by vapor-phase epitaxy in a hydride-chloride system

Bulk GaN layers with a thickness of up to 1.5 mm and a lateral size of 50 mm were grown by hydride-chloride vapor-phase epitaxy (HVPE). The best samples show a half-width (FWHM) of the X-ray rocking curve of ωθ=27′, a charge carrier density of ∼8 × 1019 cm−3, and a mobility of ∼50 cm2/(V s). The photoluminescence spectra of the obtained epitaxial GaN layers exhibit an edge emission band at 348 eV. The HVPE layers are characterized by a high mechanical strength: HV = 14 GPa.

Luminescence of GaN nanocolumns obtained by photon-assisted anodic etching

GaN nanocolumns with transverse dimensions of about 50 nm were obtained by illumination-assisted anodic etching of epilayers grown by metalorganic chemical vapor deposition on sapphire substrates. The photoluminescence spectroscopy characterization shows that the as-grown bulk GaN layers suffer from compressive biaxial strain of 0.5 GPa. The majority of nanocolumns are fully relaxed from strain, and the room-temperature luminescence is free excitonic. The high quality of the columnar nanostructures evidenced by the enhanced intensity of the exciton luminescence and by the decrease of the yellow luminescence is explained by the peculiarities of the anodic etching processing.

Bulk large-area GaN layers

Mirror-smooth, transparent bulk GaN layers with an area of 2×3 cm2 and a thickness of up to 1 mm were grown by hydrochloride vapor-phase epitaxy. Cracking of the material was eliminated by using a two-stage growth process; separation from a substrate was provided by a low-temperature buffer layer of preset thickness. For the best samples, FWHM of the X-ray rocking curve was ωθ=3.5′ and the dislocation density amounted to 107–108 cm−2.

High-temperature annealing of bulk GaN layers

The influence of high-temperature (1010°C) annealing in a flow of gaseous ammonia on the properties of bulk GaN layers, grown by chloride-hydride VPE and then separated from SiO2 substrates, was studied using atomic force microscopy. The bulk (∼360-μm-thick) epitaxial GaN layers were synthesized in two steps: a first stage of nucleation and growth at a low temperature (530°C) followed by epitaxy at a high temperature (970°C). It was found that the annealing increases the nanorelief height and activates the donor-acceptor recombination on the surface of a GaN layer grown at the lower temperature and decreases the intensity of photoluminescence from the layer grown at the higher temperature.

Gas source molecular-beam epitaxy growth of GaN/GaP superlattices and GaN layers on GaP(1 1 1)A substrates

GaN/GaP short-period superlattices were grown on GaP(1 1 1)A substrates by gas source molecular-beam epitaxy. Despite the large lattice-mismatch between GaN and GaP, reflection high-energy electron diffraction patterns showed the quick switch from GaN (GaP) streaks to GaP (GaN) streaks just after the corresponding switch of the growth. This indicates two-dimensional layer-by-layer growth resulting from the relaxation of lattice strain at the initial stage of the hetero-epitaxy. This phenomenon is considered to be due to the use of (1 1 1)A-oriented substrates. Bulk GaN layers were also grown on GaP(1 1 1)A substrates and the flat-surface GaN layers were obtained because of the two-dimensional layer-by-layer growth starting from the interface between GaN and GaP (1 1 1)A substrates. Strong near-band-edge photoluminescence with very weak deep-level emission was observed. GaP(1 1 1)A substrate is considered to be promising for the growth of GaN.

Drift mobility of electrons in AlGaN/GaN MOSHFET

The dependence of electron mobility μn on sheet electron concentration in the channel has been measured for the first time in AlGaN/GaN MOSHFETs. The maximum value of μn ≃ 1400 cm2/Vs observed at ns = 7 × 1012 cm-2, is very close to the value derived from Hall measurements made on the same wafer. As the gate-bias voltage VG approaches the threshold value VT (VG → VT), the measured value of μn is ~100 cm2/Vs. This is very close to the value of electron mobility in a bulk GaN layer. The mechanism controlling the mobility against sheet-carrier density dependence is discussed.

Microstructure of bulk GaN layer grown on sapphire substrates with amorphous buffer

The structure of bulk GaN layers grown on (0001) sapphire substrates by vapor-phase epitaxy has been studied by x-ray diffraction and transmission electron microscopy (TEM). It is found that these layers contain grown-in and screw dislocations. The dislocation density decreases away from the interface. The effect of an amorphous buffer layer on the formation of the initial GaN layer and, thus, on the degree of perfection of gallium nitride layers is elucidated. A model of generating grown-in dislocations and the relaxation mechanism of misfit stresses are proposed.