Metalorganic Chemical Vapor Deposition Samples Research Articles

Non-alloyed ohmic contacts to (010) β -Ga2O3 with low contact resistance

Low resistance non-alloyed ohmic contacts are realized by a metal-first process on homoepitaxial, heavily n+ doped (010) β-Ga2O3. The resulting contacts have a contact resistance (Rc) as low as 0.23 Ω-mm on an as-grown sample and exhibit nearly linear ohmic behavior even without a post-metallization anneal. The metal-first process was applied to form non-alloyed contacts on n+ (010) β-Ga2O3 grown by metal-organic chemical vapor deposition (MOCVD) as well as suboxide molecular beam epitaxy. Identical contacts fabricated on similar MOCVD samples by conventional liftoff processing exhibit highly rectifying Schottky behavior. Re-processing using the metal-first process after removal of the poor contacts by conventional methods does not improve the contacts; however, addition of a Ga-flux polishing step followed by re-processing using a metal-first process again results in low resistance, nearly linear ohmic contacts. The liftoff process, therefore, does not reliably render nearly linear ohmic behavior in non-alloyed contacts. Furthermore, no interface contamination was detected by x-ray photoelectron spectroscopy. This suggests that during the initial liftoff processing, a detrimental layer may form at the interface, likely modification of the Ga2O3 surface, that is not removable during the contact removal process but that can be removed by Ga-flux polishing.

Electrical properties of vertical GaN Schottky diodes on Ammono-GaN substrate

We report on electrical properties of vertical n-GaN high voltage Schottky diodes (SBDs) grown by two different techniques: Metal Organic Chemical Vapor Deposition (MOCVD) and Hydride Vapor Phase Epitaxy (HVPE) on highly-conductive n-type Ammono-GaN substrate. The thermionic emission (TE) current model has been applied for diodes parameters analysis. The fabricated SBDs exhibited a breakdown voltage of 670 V and 220 V, barrier height of 1.05 eV and 0.92 eV, ideality factor of 1.65 and 1.42 and series resistance of 440 Ω and 12 Ω for HVPE and MOCVD samples, respectively. Finally, we demonstrate preliminary point defects analysis for both types of the samples.

AlN/GaN HEMTs grown by MBE and MOCVD: Impact of Al distribution

Binary AlN/GaN high electron mobility transistors (HEMTs) were grown by plasma‐assisted molecular beam epitaxy (PA‐MBE) as well as metal‐organic chemical vapor deposition (MOCVD) and compared with regard to their structural and electrical properties. The investigated structures differ in Al distribution and composition of the AlN barrier due to characteristic differences of the two growth methods such as growth temperature and interface sharpness. While we observe a nearly pure AlN layer and an abrupt interface for MBE growth, a graded “AlN” barrier with a significant amount of Ga is found for the MOCVD grown structures which is reflected by the electrical properties of the HEMT structures. Si‐implanted ohmic contacts were formed on MBE as well as MOCVD grown structures. The activation anneal step subsequent to implantation at temperatures ∼1100 °C changes the observed Al profiles of MBE structures and damages the active region, whereas MOCVD samples react insensitively and thus were able to be further processed. A maximum drain current of ∼1.46 A mm−1 at a gate source voltage of +3 V is observed for the processed devices. A current‐gain cut‐off frequency of 89 GHz and maximum oscillation frequency of 208 GHz were measured, which demonstrate an excellent small‐signal performance of AlN/GaN devices with 100 nm gate length.

Comparison of module structure of wideband response GaAs photocathode grown by MBE and MOCVD

In order to compare the structures of GaAs photocathodes grown by molecular beam epitaxy (MBE) and metal–organic chemical vapour deposition (MOCVD), four wideband response exponential doping photocathodes were prepared. Their reflectivity and transmittivity were measured by the spectrophotometer, and three thin layer thicknesses were fitted based on the matrix theory in Thin-film Optics. The comparison of the results indicated that for the GaAs photocathode grown by MBE, only one amendatory layer with the low Al component should be added between the Ga1−xAlxAs window layer and the GaAs active layer in order to the higher fitting accuracy. Opposite occurs for the MOCVD samples. In the case of accurately controlling the layered doping concentration, the material grown by MBE is the optimal, while that grown by MOCVD is suit for the exponential doping situation. These results are available for the material growth and the module preparation of the varied doping transmission-mode GaAs photocathodes.

Sequential multiple‐step europium ion implantation and annealing of GaN

AbstractSequential multiple Eu ion implantations at low fluence (1×1013 cm‐2 at 300 keV) and subsequent rapid thermal annealing (RTA) steps (30 s at 1000 °C or 1100 °C) were performed on high quality nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) and medium quality GaN:Mg grown by hydride vapour phase epitaxy (HVPE). Compared to samples implanted in a single step, multiple implantation/annealing shows only marginal structural improvement for the MOCVD samples, but a significant improvement of crystal quality and optical activation of Eu was achieved in the HVPE films. This improvement is attributed to the lower crystalline quality of the starting material, which probably enhances the diffusion of defects and acts to facilitate the annealing of implantation damage and the effective incorporation of the Eu ions in the crystal structure. Optical activation of Eu3+ ions in the HVPE samples was further improved by high temperature and high pressure annealing (HTHP) up to 1400 °C. After HTHP annealing the main room temperature cathodo‐ and photoluminescence line in Mg‐doped samples lies at ∼ 619 nm, characteristic of a known Mg‐related Eu3+ centre, while after RTA treatment the dominant line lies at ∼ 622 nm, typical for undoped GaN:Eu. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of hole traps in n-GaN grown by hydride vapor phase epitaxy, metal organic chemical vapor deposition, and epitaxial lateral overgrowth

Optical deep level spectroscopy (ODLTS) and microcathodoluminescence (MCL) spectra were measured for a large group of n-GaN samples grown via metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth (ELOG), or hydride vapor phase epitaxy (HVPE). In the MOCVD and ELOG samples, the ionization energy of dominant hole traps H1 was dependent on the excitation conditions and was ∼0.9 eV for high injection levels providing saturation of the ODLTS peak magnitude. The trap concentration increased with increasing Si donor concentration and correlated with the yellow band intensity in the MCL spectra. For the HVPE samples, the hole trap spectra were radically different from the MOCVD case: four hole traps—H2, H3, H4, and H5—with activation energies of 0.55, 0.65, 0.85, and 1.2 eV, respectively, were detected. In the MCL spectra, a broad green band that peaked near 2.5 eV was observed in addition to the usual yellow luminescence near 2.3 eV. This green band was attributed to the transitions involving the H4 hole traps. Possible identities of the hole traps detected in the MOCVD/ELOG and HVPE samples are discussed.

10 MeV electrons irradiation effects in variously doped n-GaN

We studied 10 MeV electron irradiation effects in a group of n-GaN films grown by standard metalorganic chemical vapor deposition (MOCVD) and by epitaxial lateral overgrowth (ELOG) techniques. The samples were either undoped or Si-doped, so that the shallow donor concentrations ranged from 1014 cm−3 to 3 × 1018 cm−3. It was found that electron irradiation led to the compensation of n-type conductivity and that the carrier removal rate substantially increased with an increase in the starting donor concentration. For the MOCVD samples, it was observed that the main compensating defect introduced by electrons was a 0.15 eV electron trap detected by admittance spectroscopy. Once the Fermi level crossed the level of these traps two other centers with activation energies of 0.2 and 1 eV were found to contribute to the compensation, so that after high doses, the Fermi level in moderately doped samples was pinned near Ec −1 eV. In ELOG samples the 0.15 eV electron traps were not detected. Instead only the 0.2 and 1 eV traps were introduced by irradiation. The carrier removal rate in the ELOG n-GaN was found to be measurably lower than for MOCVD samples with a similar doping level. The results are compared to previously published data and possible models of compensation are discussed.

Comparative study of trap densities of states in CdTe∕CdS solar cells

Density of deep and shallow states has been investigated in three different kinds of CdTe∕CdS samples, two of which were grown by metal-organic chemical vapor deposition (MOCVD) and one by close-space sublimation (CSS) methods. The MOCVD samples were p doped by As and grown either with or without a ZnO buffer layer between the transparent conductor and CdS layers. Capacitance-voltage, admittance spectroscopy, and quantum efficiency measurements show pronounced effects of As doping and ZnO incorporation. It is found that A centers and vacancies of Cd, usually observed in CSS devices, are absent in the defect spectra of MOCVD samples.

Neutron Radiation Effects in Epitaxially Laterally Overgrown GaN Films

Neutron radiation effects were studied in undoped n-GaN films grown by epitaxial lateral overgrowth (ELOG). The irradiation leads to carrier removal and introduces deep electron traps with activation energy 0.8 eV and 1 eV. After the application of doses exceeding 1017 cm−2, the material becomes semi-insulating n-type, with the Fermi level pinned near the level of the deeper electron trap. These features are similar to those previously observed for neutron irradiated undoped n-GaN prepared by standard metal–organic chemical vapor deposition (MOCVD). However, the average carrier removal rate and the deep center introduction rate in ELOG samples is about five-times lower than in MOCVD samples. Studies of electron beam induced current (EBIC) show that the changes in the concentration of charged centers are a minimum in the low-dislocation-density laterally overgrown regions and radiation-induced damage propagates inside these laterally overgrown areas from their boundary with the high-dislocation-density GaN in the windows of the ELOG mask.

Effect of Visible and UV Illumination on the Water Contact Angle of TiO2 Thin Films with Incorporated Nitrogen

Doping TiO2 with nitrogen is recognized as a procedure to get sensitization of this material with visible light. In the present work, incorporation of nitrogen within the structure of TiO2 thin films has been accomplished by N2+ ion implantation in TiO2 anatase thin films (50 keV ion energy for doses of 3 × 1016, 6 × 1016, and 1.2 × 1017 ions cm-2) and during preparation by metalorganic chemical vapor deposition (MOCVD) using nitrogen as carrier gas. The analysis of the samples by X-ray photoemission spectroscopy (XPS) and for the MOCVD samples also by secondary ion mass spectroscopy (SIMS) has shown that nitrogen, in the form of nitride-like species, (N/Ti ratios of 0.03 and 0.12 for the MOCVD and the implanted samples, respectively) has become effectively incorporated within the structure of TiO2. The water contact angle on the implanted thin films varied from about 80° to around 30° when illuminated with visible light, depending on the ion dose. Similarly, the MOCVD samples showed a sharp decrease in w...

Extended X-ray Absorption Fine Structure Studies of InGaN Epilayers

ABSTRACTThe local structure around In atoms in InGaN epilayers grown by Molecular Beam Epitaxy (MBE) and by Metal-Organic Chemical Vapour Deposition (MOCVD) was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). The averaged In fraction of MOCVD grown samples ranged from 10% to 40% as estimated by Electron Probe Microanalysis (EPMA). The In fraction of MBE grown samples spanned the range from 13% to 96%. The In–N bond length was found to vary slightly with composition, both for MBE and MOCVD grown samples. Moreover, for the same In content, the In-N bond lengths in MOCVD samples were longer than those in MBE grown samples. In contrast, the In-In radial separations in MOCVD and MBE samples were found to be indistinguishable for the same In molar fraction. The In-Ga bond length was observed to deviate fromaveragecation-cation distance predicted by Vegard's law for MBE grown samples which indicates alloy compositional fluctuations.

High-temperature annealing of AlGaN: Stress, structural, and compositional changes

The effect of high-temperature annealing on stress in AlxGa1−xN in different ambients and at different temperatures was studied using ultraviolet micro-Raman spectroscopy. Low (x=0.08) and high (x=0.31 and x=0.34) composition AlGaN, grown by metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE), were compared. Compositional and morphological changes were monitored using Auger electron spectroscopy (AES) and atomic force microscopy (AFM), respectively. The Raman results demonstrate that all samples exhibit maximum stress changes in the compressive direction when annealed in an air ambient. AES confirms this to be due to higher oxygen incorporation after annealing in the air ambient, and shows higher oxygen incorporation in the vicinity of cracks and defects. MOCVD and MBE samples of a similar composition were found to reach the same biaxial stress, despite differences in initial stress and growth temperature. Relaxation of a parabolic intercrack stress profile to homogeneous stress was observed with annealing in all ambients for cracked samples. AFM results on cracked samples show an increase in width of the primary cracks along the 〈21̄1̄0〉 directions, and the formation of secondary cracks along the 〈11̄00〉 directions.

Mg in GaN: the structure of the acceptor and the electrical activity

We investigated GaN and AlGaN grown by metal organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) doped with different Mg concentrations by photoluminescence Hall-effect and SIMS measurements. Optically detected magnetic resonance (ODMR) experiments were used to study the structure of the Mg-acceptors. The ODMR experiments reveal that the spin-density of the hole is equally distributed on the four nearst Nitrogen neighbors. Structural variations observed for Mg in MOCVD material are explained in terms of crystalfield variations caused by potential fluctuations due to the presence of compensating donors. Especially for highly doped MOCVD samples saturation of the free hole concentration and a strong 2.9 eV recombination are observed. Considering that compensating donors like nitrogen vacancies hydrogen and complexes of both form during growth we are able to calculate the experimental determined free hole concentrations. In MOCVD samples codoped with Mg and Si a decrease of the acceptor binding energy with increasing donor concentration was observed. The effect is caused by Coulomb interaction between the ionized acceptors and the free holes. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Compensation mechanism in MOCVD and MBE grown GaN : Mg

We studied GaN grown by metal organic chemical vapour deposition (MOCVD) and molecular beam epitaxy doped with different Mg concentrations by photoluminescence, Hall and SIMS measurements. For the MOCVD samples, due to compensating deep donors a saturation of the hole density versus the Mg concentration is observed. These donors are also related to a 2.9 eV recombination observed in photoluminescence. Assuming that the compensating donors formed during growth processes involve nitrogen vacancies, hydrogen and a complex of both species, we are able to calculate the free hole concentration as a function of the Mg concentration for both growth methods.

Polarization effects and transport in AlGaN/GaN system

The AlGaN/GaN heterostructure is important for both electronic and optoelectronic devices. In addition to having a large band gap, the heterostructure has a strong piezoelectric effect and a large spontaneous polarization. This allows one to incorporate a large electric field (>106 V/cm) and high sheet charge (>1013 cm−2) without doping. Theoretical studies are done to examine how polarization effects controls the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier, impurity scattering, sheet charge density, and phonon scattering influence mobility. Results are compared with experimental studies on samples grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface quality compared to the MOCVD samples. By carefully fitting the experimental data we present results on interface roughness parameters for MBE and MOCVD samples.

Charge control and mobility in AlGaN/GaN transistors: Experimental and theoretical studies

In this article we report on two dimensional sheet charge and mobility in GaN/AlGaN heterostructure field effect transistors. Both experimental and theoretical results are presented. Experimental results are reported on samples grown by metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Theoretical studies are done to examine how spontaneous polarization and piezoelectric effect control the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier and phonon scattering influence mobility. We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface compared to the MOCVD samples. By carefully fitting the experimental data we present results on interface roughness parameters for MBE and MOCVD samples.

Magnetoluminescence and Resonant Electronic Raman Scattering Investigation of Donors and Excitons in Hydride Vpe and Mocvd GaN

AbstractThe donor and exciton states in ultra high-quality heteroepitaxial GaN grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire substrates are investigated using low temperature photoluminescence (PL), reflectance, magnetospectroscopy in fields up to 12 T, and resonant electronic Raman scattering (RERS). The A free exciton is confirmed to have a binding energy of about 26.4 meV, independent of strain in the material. Bound n=2 exciton peaks are distinguished in the PL spectrum by their thermalization and sample dependence. The Si donor is shown to have a binding energy of about 21 meV using Si-doped HVPE samples grown at Epitronics. Up to five additional residual donor species are observed when comparing various HVPE and MOCVD samples. Pronounced temperaturedependence of the two-electron satellites is observed, suggesting the existence of unresolved excited rotator states of the neutral donor-bound exciton. Highly resolved magnetic splitting patterns are observed in the two-electron satellites. A nonperturbative theory of these donor splittings is developed, including anisotropy. Resonant electronic Raman scattering from residual donors is reported, and yields improved linewidths compared to PL.

Residual donor contamination in MOCVD, MOMBE and MBE GaAs studied by far-infrared spectroscopy

Far-infrared photothermal ionisation spectroscopy is used to investigate the nature of the donors present in high-purity epitaxial films of GaAs grown by metal-organic chemical vapour deposition (MOCVD), metal-organic gas source molecular beam epitaxy (MOMBE) and solid source molecular beam epitaxy (MBE). Except for one lower mobility sample, the MOCVD samples showed the donor line X1 to be dominant with >50% relative intensity. The higher mobility samples showed the presence of two relatively closely spaced components to X1. Hydrostatic pressures up to 20 kbar were used to increase the chemical shifts and to vary the absorption strengths of the lines. By this means, and by the simultaneous observation of the same components on the 1s-2p and 1s-3p transitions, it was possible to show that these components were not artefacts of over-absorption but arose from two different donor species of very similar chemical shifts. Consequently, any identification of donor X1 as being solely due to silicon must be treated with caution. Germanium and sulphur accounted for the remaining donor contamination in the MOCVD material, with germanium usually being present at higher levels than sulphur. In contrast, earlier far-infrared and photoluminescence data with samples of different origin have shown germanium to be dominant in MOCVD material. MOMBE growth produced very similar proportions of X1, germanium and sulphur, indicating that the gas sources are responsible for the contamination. The MBE samples grown in reactors of different manufacture showed levels of sulphur donors comparable with the background carbon acceptors as well as traces of germanium, in addition to the silicon deliberately introduced. The results for the highest mobility MBE sample with deliberate silicon doping suggest that the lower energy component of the 'X1' doublet is due to silicon. The most likely candidate for the higher energy component is selenium. An even higher mobility MBE sample which was n-type without deliberate silicon doping showed the higher energy component attributed to selenium.

Space-charge-like scattering in epitaxial GaAs from low temperature and high pressure Hall measurements

The Hall electron mobility in GaAs grown by vapor phase epitaxy (VPE), liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), and metalorganic chemical vapor deposition (MOCVD) has been studied both as a function of temperature (77<T<300 K) and hydrostatic pressure (0–8 kbar). The analysis of the data shows that in LPE and VPE crystals, the mobilities are lowered due to space-charge scattering (μSC∝m*−1/2T−1/2) while in MOCVD and MBE samples, a space-charge-like limited mobility (μUN∝m*−nT−1/2) must be included. The value of the exponent n has been found to be 1 for MBE and 2 for MOCVD grown samples. It has been concluded that increasing impurity gradation in these layers is responsible for higher values of n. The pressure coefficient of electron mass in GaAs is determined to be 5.2×10m−4 m*/kbar . The model of central cell scattering due to carbon acceptors fails to account for the experimental data reported in this paper.

Optical and crystallographic properties and impurity incorporation of GaxIn1−xAs (0.44<x<0.49) grown by liquid phase epitaxy, vapor phase epitaxy, and metal organic chemical vapor deposition

Optical, crystallographic, and transport properties of nominally undoped n-type and Zn doped p-type Gax In1−xAs /InP (0.44<x<0.49) grown by liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), and metal organic chemical vapor deposition (MOCVD) have been studied and related to the different growth methods. Samples grown by LPE show in general much larger luminescence intensities than the VPE samples with similar impurity concentration and less structural and compositional inhomogeneities. Peaks related to free and bound excitons and to different impurities are found in the photoluminescence and absorption spectra of the undoped samples. The binding energy of the exciton is determined to be 2.1±0.1 meV, in agreement with hydrogenic theory. A longitudinal optical (LO) phonon energy of 32±0.5 meV is derived from LO-phonon replica of the exciton line. The dependence of the energy gap at T=2 K from the solid solution composition in the range xGa =45%–49% is determined yielding a bowing parameter of C=0.475 and a gap value of Eg =0.811 eV at optimum lattice match. Data on donor-acceptor pair transitions observed in the photoluminescence spectra are combined with secondary ion mass spectrometry data to identify for the first time different acceptors: C, Zn, and Si. Their binding energies are 13±1, 22±1, and 25±1 meV, respectively. C is the dominant acceptor in the MOCVD samples, but is hardly observable in the LPE and VPE samples. Si and Zn are present in LPE, VPE, and MOCVD samples. The Zn doped p-type samples show a broad donor-Zn-acceptor pair transition band accompanied by a weak LO-phonon replica and a very weak exciton line.