Metalorganic Chemical Vapor Phase Epitaxy Research Articles

Reduction in density of interface traps determined by C-V analysis in III-nitride-based MOSHFET structure

An in situ metal-organic chemical vapor phase epitaxy is used to grow a complete AlGaN/GaN metal oxide semiconductor heterojunction field effect transistor (MOSHFET) structure, gated by a gallium oxide (Ga2O3) layer; we observed reduction in the interfacial trap density compared to its version wherein the Ga2O3 was grown ex situ, after breaking the vacuum, all else being the same. A remarkable decrease in the interfacial charge density for in situ MOSHFET structures in the range of 70%–88% for 10–30 nm oxide layer thickness and improvements in other electrical parameters required for high-performing devices were observed.

Room temperature pulsed operation of nitride nanowire-based multi-quantum shell laser diodes by MOVPE

In this study, the room-temperature pulsed operation of a nitride nanowire-based multi-quantum shell laser diode with a thick p-type GaN cap layer grown on a conventional c-face GaN substrate was demonstrated using the metalorganic chemical vapor phase epitaxy method. The threshold current density of the laser was approximately 62 kA cm−2, and the voltage at the threshold current was 14 V. Optical spectral characteristics with two peak wavelengths, 403 and 417 nm, were observed.

Differences in electrical responses and recovery of GaN p+n diodes on sapphire and freestanding GaN subjected to high dose 60Co gamma-ray irradiation

We investigate the effects of high-rate and high total doses of 60Co gamma rays on the current–voltage (IV) characteristics of GaN p+n diodes grown by metal-organic chemical vapor phase epitaxy on Ga-face (0001) sapphire and hydride vapor phase epitaxy freestanding GaN substrates. We show that diodes grown on sapphire undergo more permanent changes upon irradiation at doses up to 3900 kGy than those grown on freestanding GaN. By combining diode and circular transfer length method measurements, we show that the p-type contact interface and adjacent p++ Mg-doped layer are sensitive to irradiation. In initial experiments, diodes grown on sapphire exhibited p-type contacts with Schottky characteristics, while those on freestanding GaN were Ohmic. Serendipitously, we identified and subsequently irradiated a freestanding sample with a pre-irradiation spatial gradient of p-contact Schottky vs Ohmic behavior across the die. This sample allowed the root cause of induced change to be identified as differences in the p++ contacting layer. We show that the p-type contact's pre-irradiation Schottky behavior is predictive of diodes' IV characteristics changing significantly upon gamma-ray irradiation. Further, we observe that the IV curves of diodes on freestanding GaN recover fully over several weeks at room temperature to be indistinguishable from pre-irradiation. IV curves from diodes on sapphire do not fully recover; we thus hypothesize that interactions between radiation-induced point defects and threading dislocations affect the evolution of radiation damage.

X-Ray diffraction analysis and modeling of the depth profile of lattice strains in AlGaN stacks

One of the requirements for the functionality of the AlGaN/GaN high-electron mobility transistors is the presence of a specific residual lattice strain in their active region. This lattice strain can be adjusted by depositing a multilayer stack that consists of individual AlxGa1−xN layers with harmonized [Al]/[Ga] ratio, which produces a controlled depth gradient of the lattice misfit and consequently a controlled gradient of the residual lattice strain. However, the lattice misfit at the interfaces between adjacent layers is partially compensated by misfit dislocations and other microstructure features, e.g., V-shaped interfacial pyramids. In this contribution, a non-destructive method for simultaneous quantitative analysis of remaining lattice strains and density of microstructure defects compensating the lattice misfit is presented and illustrated on the example of AlGaN stacks with different composition profiles, which were deposited using metalorganic chemical vapor phase epitaxy on (111)-oriented Si wafers. This technique is based on the mapping of selected reciprocal lattice points using X-ray diffraction. The lattice strains obtained from the reciprocal space mapping were verified by confocal micro-Raman spectroscopy. The presence of the microstructure defects and the depth gradient of their density were confirmed by transmission electron microscopy. The interplay of the composition profile and the thickness of individual layers on the lattice strain is described by a micromechanical model. From the comparison of the experimental results with the lattice strains, which were predicted for a defect-free multilayer stack, the main strain relaxation mechanisms are concluded, and their impact on the strain relaxation is quantified.

Nitrogen and carbon incorporation in GaNxAs1-x grown in a showerhead MOVPE reactor

The growth of dilute nitrides with metal organic chemical vapor phase epitaxy (MOVPE) presents two major challenges: a low N-incorporation combined with a high impurity density of carbon and hydrogen. For the device implementation the full understanding of the growth of the metastable material and the influence of the parameters on the impurity levels is of high interest. We investigated the N- and C-incorporation in GaNxAs1-x as a reference system in a close coupled showerhead MOVPE reactor to minimize gas-phase reactions for a wide range of growth parameters and precursors. The N-incorporation was always proportional to the N/As-ratio, while the absolute N-molar-flow, growth rate and interaction time show only a minor influence, indicating little gas-phase pre-reactions. Pyrolysis and pre-reactions between radicals on the surface have been taken into account according to the used precursor combination. We identified the group-III-methyl-groups as the major C-source and found a linear correlation between the incorporated N and C. GaN0.01As0.99 could be grown with C-background below 1016 atoms/cm3 for optimized conditions.

Semi-insulating GaN for vertical structures: role of substrate selection and growth pressure

Samples comprising 1.3 μm-thick C-doped semi-insulating (SI) GaN layer sandwiched between two n-GaN layers were grown on sapphire or conductive GaN substrates by metal-organic chemical vapor phase epitaxy at varied reactor pressure between 100 and 20 mbar. Vertical cylindrical resistors with a radius of 50 μm were defined by ~ 1.6 μm-deep mesa etching down to the bottom Si-doped n-GaN layer. X-ray diffraction rocking curves revealed almost invariant crystallographic quality of homo-epitaxial structures grown on the GaN substrate, while dislocations in GaN on sapphire lead to curve broadening and pits formations. C concentration in SI GaN grown on the GaN substrate was found to increase from ~ 1 ✕ 1017 cm-3 to ~ 6 ✕ 1018 cm-3 as the growth pressure decreased from 100 mbar to 20 mbar. However, one order of magnitude lower C concentration and inhomogeneous distribution was found for the sapphire substrate.I–V characterization showed that the electrical strength of SI GaN could be as high as 2.8 MV/cm if grown at 20 mbar on the GaN substrate. In this case the non-destructive break-down voltage exceeds 350 V with a positive T coefficient pointing on impact ionization. Sample grown on the GaN substrate at 20 mbar demonstrated ln(I) ~ V dependence which is typical for a barrier-controlled leakage. Extracted barrier height of 0.41 eV at the n-GaN/SI GaN interface was explained by C-related compensation effect and by a shifting of the Fermi level. On the other hand, apart from SI GaN grown on GaN substrate at 20 mbar, all other structures showed I = f(Vn) dependence indicating a space-charge-limited current conduction mechanism. It is suggested that the optimized SI GaN grown on GaN substrate can be considered as a current blocking layer or as a channel in robust geometry vertical transistors.

Indium Incorporation into InGaN Quantum Wells Grown on GaN Narrow Stripes.

InGaN quantum wells were grown using metalorganic chemical vapor phase epitaxy (vertical and horizontal types of reactors) on stripes made on GaN substrate. The stripe width was 5, 10, 20, 50, and 100 µm and their height was 4 and 1 µm. InGaN wells grown on stripes made in the direction perpendicular to the off-cut had a rough morphology and, therefore, this azimuth of stripes was not further explored. InGaN wells grown on the stripes made in the direction parallel to the GaN substrate off-cut had a step-flow-like morphology. For these samples (grown at low temperatures), we found out that the InGaN growth rate was higher for the narrower stripes. The higher growth rate induces a higher indium incorporation and a longer wavelength emission in photoluminescence measurements. This phenomenon is very clear for the 4 µm high stripes and less pronounced for the shallower 1 µm high stripes. The dependence of the emission wavelength on the stripe width paves a way to multicolor emitters.

Step-flow growth in homoepitaxy of β-Ga2O3 (100)—The influence of the miscut direction and faceting

We present a systematic study on the influence of the miscut orientation on structural and electronic properties in the homoepitaxial growth on off-oriented β-Ga2O3 (100) substrates by metalorganic chemical vapour phase epitaxy. Layers grown on (100) substrates with 6° miscut toward the [001¯] direction show high electron mobilities of about 90 cm2 V−1 s−1 at electron concentrations in the range of 1–2 × 1018 cm−3, while layers grown under identical conditions but with 6° miscut toward the [001] direction exhibit low electron mobilities of around 10 cm2 V−1 s−1. By using high-resolution scanning transmission electron microscopy and atomic force microscopy, we find significant differences in the surface morphologies of the substrates after annealing and of the layers in dependence on their miscut direction. While substrates with miscuts toward [001¯] exhibit monolayer steps terminated by (2¯01) facets, mainly bilayer steps are found for miscuts toward [001]. Epitaxial growth on both substrates occurs in step-flow mode. However, while layers on substrates with a miscut toward [001¯] are free of structural defects, those on substrates with a miscut toward [001] are completely twinned with respect to the substrate and show stacking mismatch boundaries. This twinning is promoted at step edges by transformation of the (001)-B facets into (2¯01) facets. Density functional theory calculations of stoichiometric low index surfaces show that the (2¯01) facet has the lowest surface energy following the (100) surface. We conclude that facet transformation at the step edges is driven by surface energy minimization for the two kinds of crystallographically inequivalent miscut orientations in the monoclinic lattice of β-Ga2O3.

Low-temperature epitaxial technology for flexible optoelectronic devices

Flexible optoelectronic devices need to make or loadphotoelectric conversion films on metal, glass, plastic and other flexiblenon-single crystal substrate. The existing methods for realizing flexibleoptoelectronic devices fall into two categories: deposition of organicoptoelectronic conversion films directly on non-single-crystal substrates ortransferring the epitaxial growth of the inorganic optoelectronic conversion filmfrom a single-crystal substrate to non-single-crystal substrate. The formercannot be used to produce inorganic optoelectronic devices on flexible non-single-crystalsubstrates, and the latter requires a difficult large-area transfer. If theinorganic optoelectronic devices can be extended directly on the flexiblenon-single-crystal substrate, a new technological and researchdirection will be developed for flexible inorganic optoelectronicdevices. Traditional epitaxial growth of inorganic optoelectronic devicesoften requires a high epitaxial growth temperature and a single-crystal substrate with a high softening temperature. The former dissolves thereactants and provides the atomic surface with migration capability, and thelatter provides the lattice arrangement of the inorganic optoelectronicconversion films. It is possible to directly grow the inorganicoptoelectronic thin film on the flexible non-single-crystal substratethrough the coupling of an electromagnetic field to reduce the temperaturerequired by the epitaxial growth. This paper analysis the research status oflow-temperature epitaxy technology, and emphatically introduces the low-temperature epitaxial method proposed by our research team-inductively coupledplasma metal-organic chemical vapor phase epitaxy (ICP-MOVPE)-including thedesign concept of ICP-MOVPE, the simulation of the plasma in the reactionchamber, and the preliminary results of this equipment for the epitaxialgrowth of III-nitride semiconductors.

Electrochemical characterization of GaN surface states

In this work, we present a systematic study of the electrochemical properties of metal-organic chemical vapor deposition and hybrid vapor phase epitaxy grown n-type GaN in aqueous electrolytes. For this purpose, we perform cyclic voltammetry and impedance spectroscopy measurements over a wide range of potentials and frequencies, using a pure aqueous electrolyte and adding two different types of redox couples, as well as applying different surface treatments to the GaN electrodes. For Ga-polar GaN electrodes, the charge transfer to an electrolyte is dominated by surface states, which are not related to dislocations and are independent of the specific growth technique. These surface states can be modified by the surface treatment; they are generated by etching in HCl and are passivated by oxidation. Different surface defect states are present on N-polar GaN electrodes which do not significantly contribute to the charge transfer across the GaN/electrolyte interface.

Metalorganic chemical vapor phase epitaxy of narrow-band distributed Bragg reflectors realized by GaN:Ge modulation doping

We report on metalorganic vapor phase epitaxy (MOVPE) of distributed Bragg reflectors (DBR) applying a periodic modulation of the GaN doping concentration only. The doping modulation changes the refractive index of GaN via the Burstein-Moss-effect. MOVPE growth of highly doped GaN:Ge and modulation of the dopant concentration by at least two orders of magnitude within few nanometers is required to achieve a refractive index contrast of 2–3%. Such modulation characteristic is achieved despite the presence of Ge memory effects and incorporation delay. We realized DBRs with up to 100 layer pairs by combining GaN:Ge with a nominal doping concentration of 1.6×1020cm−3 as low-refractive index material with unintentionally doped GaN as high-refractive index layer. Scanning transmission electron microscope images reveal DBR structures with abrupt interfaces and homogenous layer thicknesses in lateral and vertical direction. Reflectance measurements of DBRs designed for the blue and near UV-spectral region show a narrow stopband with a maximum reflectivity of 85% at 418nm and even 95% at 370nm. InGaN/GaN multi-quantum well structures grown on top of such DBRs exhibit narrow emission spectra with linewidths below 3nm and significantly increased emission intensity.

Elimination of trench defects and V-pits from InGaN/GaN structures

The microstructural evolution of InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor phase epitaxy was studied as a function of the growth temperature of the GaN quantum barriers (QBs). We observed the formation of basal stacking faults (BSFs) in GaN QBs grown at low temperature. The presence of BSFs terminated by stacking mismatch boundaries (SMBs) leads to the opening of the structure at the surface into a V-shaped trench loop. This trench may form above an SMB, thereby terminating the BSF, or above a junction between the SMB and a subsequent BSF. Fewer BSFs and thus fewer trench defects were observed in GaN QBs grown at temperatures higher than 830 °C. Further increase in the growth temperature of the GaN QBs led to the suppression of the threading dislocation opening into V-pits.

Schottky diodes and high electron mobility transistors of 2DEG AlGaN/GaN structures on sapphire substrate

We report Schottky diodes (SDs) and High Electron Mobility Transistors (HEMTs) fabricated of 2DEG AlGaN/GaN structures grown by Metalorganic Chemical Vapour Phase Epitaxy (MOVPE) on sapphire substrate. The SDs and HEMTs were designed intentionally without surface passivation and were successfully fabricated at the Center for Physical Sciences and Technology (CPST), using standard UV photolithography procedures. The performance of Ohmic contacts formed of quaternary Ti/Al/Ni/Au stack was optimized varying the temperature of rapid thermal annealing process. Deposited on the semiconductor metal Ni/Au stack was used to form 0.75 eV height Schottky barriers. The fabricated SDs demonstrated low reverse current and high electric current switching ratio while the HEMTs showed high transconductance and drain saturation currents performance with good transistor channel closing. This work paves a way to develop advanced AlGaN/GaN based HEMT structures as well as new electronic components for operation at high powers and high frequencies.

AlGaN/GaN HEMT structures on ammono bulk GaN substrate

The work shows a successful fabrication of AlGaN/GaN high electron mobility transistor (HEMT) structures on the bulk GaN substrate grown by ammonothermal method providing an ultralow dislocation density of 104 cm−2 and wafers of size up to 2 inches in diameter. The AlGaN layers grown by metalorganic chemical vapor phase epitaxy method demonstrate atomically smooth surface, flat interfaces with reproduced low dislocation density as in the substrate. The test electronic devices—Schottky diodes and transistors—were designed without surface passivation and were successfully fabricated using mask-less laser-based photolithography procedures. The Schottky barrier devices demonstrate exceptionally low reverse currents smaller by a few orders of magnitude in comparison to the Schottky diodes made of AlGaN/GaN HEMT on sapphire substrate.

MOVPE Growth and Characterization of AlInN FET Structures on Si(111)

ABSTRACTIn this work metalorganic chemical vapor phase epitaxy (MOVPE) growth and characterization of AlInN in the whole compositional range and the impact on the development of field effects transistors (FET) structures will be presented. Due to the large difference in the lattice parameters of the binaries AlN and InN the growth of AlInN with high indium concentrations is ambitious, and first the growth conditions for the alloy will be discussed. An experimental phase diagram and corresponding theoretical calculations will be displayed. The critical layer thickness of AlInN on GaN has been experimentally determined. Relaxation of the AlInN-layer has a strong influence on the sample morphology. At indium concentration exceeding 30% an polarization induced hole gas is expected at the AlInN/GaN interface from theoretical calculations, but no p-channel conductivity could be confirmed. The absence of the two dimensional hole gas will be discussed.

GaN ceramics obtained by fusing of nanocrystalline GaN powder at high pressures and temperatures as substrate for growth of GaN epilayers

In this paper, we have grown GaN films by metalorganic chemical vapor-phase epitaxy (MOVPE) on GaN ceramics obtained from nanocrystalline powder of different initial grain sizes. The samples have been investigated by X-ray diffraction (XRD) and photoluminescence (PL). XRD reveals that the MOVPE GaN films are of single-phase wurtzite structure. Also, it has been observed that the PL spectrum is different for the GaN films compared to that for the GaN powder and also depends on the initial grain sizes.

MOVPE growth of high-quality AlN

AlN layers were grown on Si and sapphire substrates in a horizontal and a vertical metalorganic chemical vapour phase epitaxy system. AlN grown in the horizontal reactor is smooth and shows intense band-edge cathodoluminescence but Ga can be found in the layers even after several microns of AlN growth. After cleaning the reactor and using a new Ga-free quartz and graphite system the layer quality is drastically reduced and we observe a rough morphology. In the vertical system pure AlN layers with smooth surfaces can be grown even on a Ga-contaminated susceptor and no Ga is found in the layers. In cathodoluminescence (CL) measurements an enhancement of oxygen or silicon-related luminescence peaks is found for the growth on sapphire or silicon substrates, respectively. A comparison of CL and Raman measurements reveals a strong tensile stress for AlN on Si that corresponds to ∼1 GPa and relaxes at cracks where a wavelength shift of 4 nm is observed. AlN grown on sapphire is found to be under compressive stress at room temperature.

Growth of blue GaN LED structures on 150-mm Si(1 1 1)

Up to 5.4-μm thick GaN on Si light emitting diode (LED) structures were grown by metalorganic chemical vapor phase epitaxy (MOVPE) on 150 mm Si(1 1 1) substrates. In-situ curvature measurements enable monitoring of stress development during growth and the influence of interlayers on strain balancing after cooling. In X-ray diffraction (XRD) ω-scans the GaN (0 0 0 2) reflection is about 380 arcsec and in θ–2 θ measurements the InGaN/GaN MQW interference peaks are well resolved indicating the high quality of the grown structure. In comparison to the growth on 2-in sapphire the wafer curvature after growth is low (>50 m) for the growth on Si and also during MQW growth at low temperatures a homogeneous wafer temperature can be achieved. The standard deviation of the wavelength over the whole 150-mm test wafer (5-mm edge exclusion) is <3.5 nm and reflects the three different heater zones of the MOVPE system used.

Optical properties of nonpolar -plane GaN layers

We have studied optical properties of nonpolar a -plane GaN layers grown on r -plane sapphire by metalorganic chemical vapor deposition and hydride vapor phase epitaxy using different nucleation schemes. Several emission bands, which are not typical for c -plane GaN, are observed in the photoluminescence spectra and their excitation-intensity, temperature, and polarization dependencies are examined. In addition, the spatial distribution of the emissions was examined by cathodoluminescence imaging and relations of the different emissions with particular structural features in the layers are revealed. The results are discussed with emphasis on the origin of the emission line and particular recombination mechanisms.

Selective etching of dislocations in violet-laser diode structures

The 415 nm laser diode structures of (AlGaIn)N were grown using metalorganic chemical vapor-phase epitaxy on bulk GaN substrates obtained using a high-pressure, high-temperature method. These substrates have the lowest dislocation density so far reported of less than 100 cm −2. Defect-selective etching performed on a laser diode structure using molten bases at a temperature of 450 °C revealed dislocations at a density of 10 5 cm −2. As the etching rate is different for every part of the epistructure, it was possible to determine the depth at which the dislocation was created. We found that about 25% of dislocations originated at the lower cladding layer of AlGaN, 60% at the quantum wells and 15% at the electron-blocking layer.