MOCVD GaN Research Articles

Improving Optical and Electrical Characteristics of GaN Films via 3D Island to 2D Growth Mode Transition Using Molecular Beam Epitaxy

Molecular beam epitaxy (MBE) is demonstrated as an excellent growth technique for growing a low-defect GaN channel layer, which is crucial for controlling vertical leakage current and improving breakdown voltage (BV) in GaN-based high-electron mobility transistors (HEMTs). The 3D islands to 2D growth mode transition approach was induced by modulating substrate growth temperature (Tsub), displaying an overall improvement in film quality. A comprehensive investigation was conducted into the effects of Tsub on surface morphologies, crystal quality, and the optical and electrical properties of GaN films. Optimal results were achieved with a strain-relaxed GaN film grown at 690 °C, exhibiting significantly improved surface characteristics (root-mean-square roughness, Rq = 0.3 nm) and impressively reduced edge dislocations. However, the film with the smoothest surface roughness, attributed to the effect of the Ga-rich condition, possessed a high surface pit density, negatively affecting optical and electrical properties. A reduction in defect-related yellow emission further confirmed the enhanced crystalline quality of MBE GaN films. The optimized GaN film demonstrated outstanding electrical properties with a BV of ~1450 V, surpassing that of MOCVD GaN (~1180 V). This research significantly contributes to the advancement of MBE GaN-based high electron mobility transistor (HEMT) applications by ensuring outstanding reliability.

(Invited) Gallium Nitride for Vertical Power Devices: Improving Morphology and Unintentional Impurities for Better Devices

While GaN has been researched for many years as a material for light emitting diodes, lasers, high electron mobility transistors, and other devices, further improvements to material purity are still needed to realize the full potential of this material for vertical device applications. To achieve high quality, high purity GaN for power devices, our research employs ammonia-assisted molecular beam epitaxy (NH3-MBE) and plasma-assisted molecular beam epitaxy (PA-MBE). Past studies of fast growth rates (more than 1 μm/hr) and ultra-low impurity incorporation revealed that both PAMBE and NH3-MBE can achieve low net doping levels, and NH3-MBE as low as 1x1015 cm-3.1 However, This work also indicated that impurity incorporation increased with increasing growth rate.Recent research efforts have focused on further improvements to morphology and unintentional dopants in NH3-MBE-grown homoepitaxial layers with fast growth rates. By optimizing growth conditions for faster growth rates and using indium as a surfactant, devices with drift layers up to 10 μm thick were grown. These layers exhibited smooth surface morphology as seen in AFM scans with RMS surface roughness as low as 0.21 nm for a 2x2 μm2 scan area.2 Adding the In surfactant also led to reduced incorporation of unintentional Si impurities in UID films as shown by SIMS depth profiles for GaN films grown with and without In. The extremely low impurity levels were further confirmed by CV measurements which indicated that the net doping density in the UID GaN layers decreased from 2x1016 cm-3 to 5x1015 cm-3 with the addition of the In surfactant.2 Based on these UID levels and the theoretical critical electric field of GaN (3 MV/cm), drift layers up to 29.5 μm and 7.4 μm can be depleted with a triangular electric field profile for net donor concentrations of 5x1015 cm-3 and 2x1016 cm-3, respectively. Devices with the same net concentrations with drift regions 3 μm thick can hold voltages of 854 V and 717 V, respectively.Based on the previously described studies of low-impurity GaN at different growth rates, we developed vertical p-n GaN power devices grown by NH3 MBE.3 These p-n diodes contained a 0.25 µm n+ region, 4 µm n- region with 3x1015 cm-3 unintentional carriers, 0.4 µm p+ region and 10nm p++ capping layer to improve the ohmic anode contact. The circular diodes were fabricated with Pd/Pt anode contacts and Ti/Au cathode contacts. Device diameters were between 80 and 100 µm. The devices featured a 15 µm field plate made from Ti/Au contacts on top of an Al2O3/Si3N4 dielectric stack.The diodes exhibited outstanding forward J-V behavior with a low specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and a minimum ideality factor of 1.36, which are among the best achieved among similar homoepitaxial GaN p-n diodes.3-5 The best diodes also exhibited a breakdown voltage of > 1 kV (equipment limit 1 kV). The reverse-voltage properties indicate that punch-through was achieved with a peak electric field (EC) >2.6 MV/cm, as confirmed by Silvaco simulations (Fig. 7). The combination of >1 kV breakdown voltage and on-resistance of 0.28 mΩ-cm2 represents one of the best performances of the p-n GaN diodes, achieved with a significantly thinner drift layer compared to comparable MOCVD GaN p-n diodes with drift layers more than 8 µm. Work is ongoing to develop NH3-MBE p-n diodes with indium surfactant-assisted drift layer growth which shows promise to further improve the surface morphology, reduce leakage, and enhance breakdown performance of the vertical devices.

Luminescence Characteristics of the MOCVD GaN Structures with Chemically Etched Surfaces.

Gallium nitride is a wide-direct-bandgap semiconductor suitable for the creation of modern optoelectronic devices and radiation tolerant detectors. However, formation of dislocations is inevitable in MOCVD GaN materials. Dislocations serve as accumulators of point defects within space charge regions covering cores of dislocations. Space charge regions also may act as local volumes of enhanced non-radiative recombination, deteriorating the photoluminescence efficiency. Surface etching has appeared to be an efficient means to increase the photoluminescence yield from MOCVD GaN materials. This work aimed to improve the scintillation characteristics of MOCVD GaN by a wet etching method. An additional blue photo-luminescence (B-PL) band peaking at 2.7-2.9 eV and related to dislocations was discovered. This B-PL band intensity appeared to be dependent on wet etching exposure. The intensity of the B-PL was considerably enhanced when recorded at rather low temperatures. This finding resembles PL thermal quenching of B-PL centers. The mechanisms of scintillation intensity and spectrum variations were examined by coordinating the complementary photo-ionization and PL spectroscopy techniques. Analysis of dislocation etch pits was additionally performed by scanning techniques, such as confocal and atomic force microscopy. It was proved that this blue luminescence band, which peaked at 2.7-2.9 eV, is related to point defects those decorate dislocation cores. It was shown that the intensity of this blue PL band was increased due to enhancement of light extraction efficiency, dependent on the surface area of either single etch-pit or total etched crystal surface.

Mapping of the electrostatic potentials in MOCVD and hybrid GaN tunnel junctions for InGaN/GaN blue emitting light emitting diodes by off-axis electron holography correlated with structural, chemical, and optoelectronic characterization

Off-axis electron holography has been used to measure the width of the depletion region in a series of tunnel junction GaN light emitting diodes that have been prepared using different growth processes for blue emission. The total measured potentials are combinations of the mean inner potential, dopant potential, and piezoelectric contributions. The dopant potential has been unmixed from the mean inner potential such that the width of the tunnel junctions in the different diodes can be measured. The experimental results are then compared to secondary ion mass spectrometry, simulations, and opto-electronic testing. We find that the measured tunnel junction widths are consistent with simulations as well as the current density and voltage characteristics. As such, off-axis electron holography has been demonstrated as a unique technique that can be used to reproducibly measure the electrostatic potentials in tunnel junctions with nm-scale resolution in real III–V device specimens.

Dependence of growth temperature on the electrical properties and microstructure of MBE-grown AlN/GaN resonant tunneling diodes on sapphire

AlN/GaN double-barrier resonant tunnel diodes have been grown by rf-plasma assisted molecular beam epitaxy at temperatures between 760 and 860 °C on metalorganic chemical vapor deposition-grown GaN templates with sapphire substrates. Room temperature negative differential resistance (NDR) was observed for all samples despite the presence of higher densities of threading dislocations in the device layers than in the MOCVD GaN template. The fraction of devices exhibiting NDR and the peak-to-valley current ratio was small for each sample (typically 1%–10% and 1.003–1.1, respectively). A clear trend of increasing peak current density with increasing growth temperature was observed.

An in situ monitored and controlled etch process to suppress Mg memory effects in MOCVD GaN growth on Si substrate

Mg is the most common p-type dopant in III-nitride devices and is becoming increasingly important in the design and development of transistors for power applications. The diffusion of Mg atoms to adjacent layers during growth has been a persistent problem. We report on a simple method involving in situ etching by hydrogen, the most commonly used carrier gas in nitride growth, for suppressing Mg diffusion. This method can be implemented during growth itself without removing the wafer from the chamber and can be controlled by in situ monitoring. A Mg concentration decay rate of 24 nm/dec, is reported for the etched sample compared to 160 nm/dec for the unetched one. An increase in 2DEG mobility in AlGaN/GaN HEMT structures on silicon substrate from 591 cm2 V−1 s−1 for the unetched sample to 1214 cm2 V−1 s−1 for the etched sample was observed. Capacitance–voltage studies to understand the effect of diffused Mg atoms on conduction channel are also reported.

Resistive heater element made of highly durable TaC-coated graphite for high-temperature and highly corrosive processes: application to MOCVD GaN epitaxial growth

Ultra-thick TaC-coated graphite prepared via a novel process was verified for use as a highly durable heater material in high-temperature and highly corrosive process environments such as those for SiC, GaN, and AlN bulk/epitaxial growth. As fundamental data for heater design, the temperature dependence of the electrical resistivity of the ultra-thick TaC layer was assessed. Estimation based on resistivity data indicates that the TaC-coated graphite heater element requires a slightly higher electrical current than that for conventional heaters. A coil-shaped ultra-thick TaC-coated heater for a MOCVD GaN growth system was fabricated using the proposed process. It was found to successfully heat the system to the target temperature for GaN epilayer growth. The quality of the MOCVD-grown GaN epilayers obtained with the TaC-coated heater was confirmed to be almost identical to that of those obtained with a conventional heater in terms of crystallinity, thickness homogeneity, intrinsic defects, impurity incorporation, and contamination.

Pulsed photo-ionization spectroscopy in carbon doped MOCVD GaN epi-layers on Si

GaN with intentionally incorporated carbon to create semi-insulating buffer layers shows improved performance in AlGaN/GaN heterojunction field effect transistors. Moreover, various impurities and crystal imperfections of GaN epi-layers determine a rather rich spectrum of deep levels. Thus, spectroscopy of defects is of paramount importance in optimizing GaN-based devices. In this work, pulsed photo-ionization spectroscopy (PPIS) has been applied to trace the prevailing defects and simultaneously monitor variations of carrier lifetime in metal−organic chemical vapor deposition grown GaN epi-layers on Si substrates of various carbon content concentration. A contactless technique for microwave probed photoconductivity has been employed to measure changes in excess carrier density, in order to evaluate variations in carrier lifetime and reveal the type of photo-activation processes. Six PPI spectral steps have been resolved in samples containing either superlattices (SLs) or AlN/AlGaN stress management layers. Three of these centres, namely, and (CNON)0, have been associated with carbon dopants. Other PPIS peaks have been ascribed to Ga vacancies and metal impurities. It has been deduced from analysis of the PPIS peak broadening factor that rather strong electron−phonon coupling is inherent in photo-ionization of carbon related complexes at room temperature. The concentration of separated defects and cross-sections for photon−electron coupling have been determined using calibration measurements of optical transmittance spectra and correlations between microwave probed photoconductivity peak signals and absorption coefficient.

Comparative study of deep levels in HVPE and MOCVD GaN by combiningO-DLTS and pulsed photo-ionization spectroscopy

Operational characteristics of sensors made of GaN significantly depend on technologically introduced defects acting as rapid traps of excess carriers which reduce charge collection efficiency of detectors. In order to reveal the prevailing defects in HVPE and MOCVD grown GaN, the carrier lifetime and photo-ionization spectra have been simultaneously measured by using microwave probed photo-conductivity transient technique. Several traps ascribed to impurities as well as vacancy and anti-site type defects have been identified in HVPE GaN material samples by combining photo-ionization and electron spin resonance spectroscopy. The optical deep level transient spectroscopy technique has been applied for spectroscopy of the parameters of thermal emission from the traps ascribed to technological defects in the Schottky barrier terrace structures fabricated on MOCVD GaN.

Carrier decay and luminescence characteristics in hadron irradiated MOCVD GaN

Crystalline GaN is a promising material for producing of the radiation hard particle detectors of different types capable to operate in harsh areas of particle accelerators. Moreover, GaN crystals show rather efficient luminescence properties in several spectral bands under excitation by high energy radiation. Thereby, GaN material can be employed for fabrication of a combined device which is able to operate both as scintillating and charge collecting detector. However, the efficiency of such detectors and their functionality has insufficiently been investigated. This work is addressed to study the evolution of the efficiency of photon and hadron induced luminescence. To evaluate the density of excess carriers induced by the high energy protons, a correlation between the microwave probed photoconductivity transients and the proton induced luminescence intensity has been examined using 1.6 MeV protons to produce a nearly homogeneous and rather strong excitation in 2.6 μm thick MOCVD grown GaN epi-layers. To estimate the radiation hardness of such material, the evolution of the photoconductivity transients and of the proton induced photoluminescence characteristics has been studied by in situ measurements of the changes of luminescence intensity and photoconductivity decay rate during the exposure to a proton beam reaching fluences up to 1015 cm-2. The production rate of radiation defects, determined from in situ and post-irradiation examination of the changes of radiative and non-radiative recombination have been examined by combining penetrative hadron (nuclear reactor neutrons and 24 GeV/c protons) irradiations with those of the 1.6 MeV protons. The parameters of the efficiency κP of carrier pair generation by asingle proton of κP = nP/NP ≅ 1.3 × 107 cm-3 per proton and κPApr = 40 carrier pairs per a micrometer of layer depth per proton have been estimated. The production rate of radiation defects is estimated to be KP ≅ 0.6 cm-1 for both penetrative neutrons and for 24 GeV/c protons. The hadronirradiation determines both the creation of the specific radiation defects with rate of KP ≅ 0.6 cm-1 and the modification of the material structure by increasing its disorder. Increase of disorder has been deduced from the observed decrease of value of the stretched-exponent index.

Deposition of GaN films on crystalline rare earth oxides by MOCVD

Epitaxy of GaN films on crystalline rare earth oxide (cREO) buffer layers on Si(111) substrates was performed to explore the possibility of cREOs as a buffer layer for GaN-on-Si epitaxy. MOCVD GaN films were first deposited on Er2O3 and Gd2O3 layers with a MBE GaN capping layer to prevent reduction of the oxide in the H2 environment of an MOCVD reactor. This showed that a multi-layer stack of Er2O3 and Gd2O3 results in a higher quality GaN film than individual cREO layers with the rms roughness improvement of >1nm. MOCVD GaN films were also deposited on bare multilayer stacks of Er2O3 and Gd2O3 by first depositing a low temperature GaN layer grown without the presence of H2 at 700°C followed by a high temperature GaN film at 1030°C. This resulted in smooth films with rms roughness around 1nm and XRD rocking curve FWHM values of 1360″ for the (002) peak. For depositions on both GaN capped and bare oxides, nitridation of the oxides was found with XRD after the wafers went through the MOCVD process. This nitridation process could be impactful for the epitaxy of GaN-on-Si with cREOs by serving as a method of decoupling the epitaxial film from the mechanical constraints of the Si substrate.

Low Dislocation Density and High Mobility GaN Layers for DHFET Channels Grown on High-Temperature AlN/AlGaN Buffer Layer by Ammonia MBE

The growth of AlN buffer layer at extremely high temperature (1100-1150oC) in ammonia MBE STE3N2 system is shown to be the key step to obtain high quality GaN layers for DHFET channels. The buffer layer sequence from c-sapphire substrate involved AlN, AlGaN/AlN superlattice and AlGaN transition layers. TEM study showed gradual decrease of threading dislocation density from (2-4)×1010 cm-2 in AlN to (9-10)×108 cm-2 in the top GaN active layer. The improvement of structural quality resulted in substantial increase in electron mobility up to 600-650 сm2/Vs in a 1.5-μm-thick GaN top layer lightly doped with silicon up to n=(3-5)×1016 cm-3. These results correspond to a good quality MOCVD GaN grown on sapphire and several times better than in conventional MBE. Employing such a GaN layer in a double heterostructure (DH) with the cap AlxGa1-xN barrier layer (x=0.25-0.4) allows to change the electron sheet density, mobility and sheet resistance in a two dimensional electron gas in the range of 1300-1700 cm2/V.s, (1.0-1.8)×1013 cm-2 and 230-400 Ω/, respectively. Application of this technology and DH design for growing on SiC substrates enabled one to manufacture a DHFET with a gate length of 0.5 μm for 0.03-4.0 GHz extra-broadband power amplifiers having Pout=2.5 W, gain 17-25 dB and efficiency 30%.

DLTS and MCTS analysis of the influence of growth pressure on trap generation in MOCVD GaN

We demonstrate with DLTS and MCTS how changing growth pressure during MOCVD growth of GaN affects the majority and minority carrier trap signatures. Results indicate which specific traps are most strongly connected with the increased carbon concentration that lower growth pressure has led to. Carbon concentration is also verified with SIMS measurements. DLTS and MCTS measurements, related to carrier thermalization from traps, made up to a temperature of 450K have found four majority carrier and two minority carrier traps. Using a lower growth pressures, at which greater incorporation of carbon is possible, has lead to larger concentrations of most of the traps; just as others have observed. However, the trap at EC−0.48eV is the least affected by the growth pressure. We relate all the traps found from DLTS and MCTS to those identified by others and mention the constituent atoms that the research community identifies with these traps.

The effect of carbon impurities on lightly doped MOCVD GaN Schottky diodes

Abstract

Selective etching of dislocations in GaN grown by low-pressure solution growth

This work presents an experimental study on the identification and quantification of different types of dislocations in GaN grown by low-pressure solution growth. A reliable defect selective etching procedure in a NaOH-KOH melt is developed and validated using transmission electron microscopy that permits to define groups of etch pits that belong each to dislocations with a specific Burgers vector. This way a comparably fast method is provided for determining the total, the specific dislocation densities and the type of dislocation in a statistically representative way. The results for the solution grown samples are compared to those obtained for MOCVD GaN.

Characterizing the thickness dependence of epitaxial GaN grown over GaN nanocolumns using X-ray diffraction

The crystal quality, as determined by X-ray diffraction, of GaN epitaxial films synthesized by MOCVD overgrowth of oriented GaN nanocolumns on Si(1 1 1) is studied as a function of film thickness and compared with control films grown on sapphire substrate. Samples were characterized using a three-beam X-ray diffraction technique from which depth-dependent rocking-curve widths (for the (0 1 1¯ 3)/(0 1¯ 12¯) crystal plane) are obtained as functions of depth. Through the examination of four samples prepared under conditions spanning key growth parameters it is found that optimizing for overgrowth at higher substrate temperatures can lead to coalesced films with significantly narrower rocking-curve widths. The results indicate that initially disconnected crystal domains stemming from individual columns do merge under the right conditions and that the coalesced crystal improves with increasing thickness. Overgrowth with the substrate temperature at 1000 °C (the highest of the study) resulted in a GaN overgrowth with properties superior to those of the control sample, a standard high-quality 2 μm MOCVD GaN thin-film grown on sapphire substrate.

Dislocation Nucleation in MOCVD GaN Films on Stepped and Unstepped 4H-SiC Substrates

not Available.

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Abstract Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm2 of substrate A and on an area of 1×1 cm2 of substrate B. Two traps labeled B1 ( E c – 0.23 eV ) and B2 ( E c – 0.58 eV ) are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al2O3 substrates by using a HT (>1150°C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4Å. The full width at half maximum (FWHM) for 800nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.

Atomic structure of pyramidal defects in GaN:Mg: Influence of annealing

The atomic structure of the characteristic defects (Mg-rich hexagonal pyramids) in p-doped bulk and MOCVD GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by direct reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities were present inside the defects, confirmed also with positron annihilation. The inside walls of the cavities were covered by GaN of reverse polarity compared to the matrix. Annealing of the MOCVD layers lead to slight increase of the defect size and an increase of the room temperature photoluminescence intensity. Positron annihilation confirms presence of vacancy clusters of different sizes triggered by the Mg doping in as-grown samples and decrease of their concentration upon annealing at 900 and 1000 °C. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)