Low-pressure Metalorganic Vapor Phase Research Articles

Reduced Defect Densities in Cubic GaN Epilayers with AlGaN/GaN Superlattice Underlayers Grown on (001) GaAs Substrates by Metalorganic Vapor Phase Epitaxy

The effects of AlxGa1-xN/GaN superlattice (SL) insertion on the structural homogeneity, photoluminescence (PL) lifetime (τPL), and defect densities were studied in cubic (c-) GaN epilayers on (001) GaAs substrates grown by low-pressure metalorganic vapor phase epitaxy. Values of the full-width at half maximum (FWHM) of both the (002) X-ray diffraction peak and near-band-edge excitonic PL peak were significantly decreased by the insertion of appropriate short-period AlGaN/GaN SLs between the c-GaN epilayer and the c-GaN template layer prepared on a substrate-decomposition-shielding GaN layer deposited at a low temperature. The density or size of Ga-vacancy (VGa)-related defects in the c-GaN epilayer was significantly reduced. Simultaneously, the value of excitonic PL lifetime at 293 K was improved from approximately 20 ps to 230 ps, indicating a tremendous reduction of the nonradiative defect density.

04/00645 Multivariate prediction of eight kerosene properties employing vapour-phase mid-infrared spectrometry: Gómez-Carracedo, M. P. et al. Fuel, 2003, 82, (10), 1211–1218

GaN layers with 〈0 0 0 1〉 crystallographic orientation, grown by low-pressure metal-organic vapour-phase epitaxy (MOVPE) on c-plane sapphire substrates, were implanted with 200 and 400 keV Sm+, Tm+, Eu+, Tb+ and Ho+ ions at fluencies of 1 × 1015–1 × 1016 cm−2. The composition of the ion-implanted layers and concentration profiles of the implanted atoms were studied by Rutherford Back-Scattering spectrometry (RBS). The profiles were compared to SRIM 2008 simulations. The structural properties of the ion-implanted layers were characterised by RBS-channelling and Raman spectroscopy. Changes in the surface morphology caused by the ion implantation were examined by Atomic Force Microscopy (AFM). A structural analysis showed a high disorder of the atoms close to the amorphised structure at the surface layer above an implantation fluence of 5 × 1015 cm−2 while lower disorder density was observed in the bulk according to the projected range of 400 keV ions. The post-implantation annealing induced significant changes only in the Sm and Eu depth profiles; a diffusion of rare-earths implanted at a fluence of 5 × 1015 cm−2 to the surface was observed. The annealing caused the reconstruction of the surface layer accompanied by surface-roughness enhancement.

Photoluminescence probing of non-radiative channels in hydrogenated In(Ga)As/GaAs quantum dots

We report here on the efficient use of hydrogenation as a tool to investigate the intrinsic properties of non-radiative recombination channels in a low-pressure metal-organic vapor phase epitaxy grown quantum dots (QDs) array emitting around 1.3 μm. It is shown that hydrogenation reduces by a factor of two the flow of charge carriers to non-radiative channels in QDs related to the presence of impurities and further suppresses completely the non-radiative recombinations originating from the stacking faults and dangling bonds. Most importantly, hydrogenation improves the room temperature photoluminescence intensity of the ≈1.3 μm emitting In(Ga)As/GaAs QDs by a factor close to ten.

Hall resistance hysteresis in AlGaN/GaN 2DEG

The Hall resistance of AlGaN/GaN two-dimensional electron gas (2DEG) at low temperatures was measured. The AlGaN/GaN heterostructures were grown by low-pressure metal-organic chemical vapor phase epitaxy on (0 0 0 1) SiC substrate. The electron mobility and electron concentration at 4.2 K are 9540 cm 2/ V s and 6.6×10 12 cm −2 , respectively. When the temperature is lower than 4.5 K the hysteresis of Hall resistance is observed near zero magnetic field. The hysteresis of Hall resistance increases with decreasing temperature. At temperatures higher than 4.5 K , the hysteresis of Hall resistance disappears. From the analysis of the residual Hall resistance dependence on the temperature, the Curie temperature of the Hall resistance hysteresis is calculated to be 4.4 K . In general, the hysteresis implies the possibility of ferromagnetism, but the conformation of the ferromagnetism of AlGaN/GaN heterostructure is still difficult and the detailed physical mechanism is still unclear.

Influence of growth rate on charge transport in GaSb homojunctions prepared by metalorganic vapor phase epitaxy

Dark current–voltage (I–V) characteristic measurement in the temperature range from −190 °C to 65 °C was carried out on GaSb p-n homojunctions prepared by low-pressure metalorganic vapor phase epitaxy. It was shown that the charge transport mechanism in these homojunctions is strongly affected by the growth rate of GaSb epitaxial layers. Samples prepared at higher growth rate (40 nm/min.) exhibit an anomalous low-temperature peak of tunneling current which can be explained by the presence of a narrow band of energies due to high concentration of native defects, probably GaSb antisites. The same defect levels are responsible for the generation–recombination current which dominates in these samples at higher temperatures. On the other hand, quite different behavior was found in the case of slowly grown (20 nm/min) samples. At sufficiently low temperatures, a current maximum near 50 mV of forward voltage points out a band-to-band tunneling as a prevailing transport mechanism. With increasing temperature, however, this maximum disappears as at least one side of the junction becomes nondegenerate. Combination of the tunneling via energy states in the band gap and the thermal current governs the forward I–V characteristics at higher temperatures, whereas the direct tunneling remains dominant in the reverse direction.

Metamorphic GaInP-GaInAs Layers for Photovoltaic Applications

ABSTRACTGaxIn1−xAs and GayIn1−yP layers were grown lattice mismatched to GaAs and Ge by low-pressure metal organic vapor phase epitaxy (LP-MOPVE). These materials are very promising for further increasing the efficiency of monolithic triple-junction solar cells. Different buffer layer structures were realized. Transmission electron microscopy and x-ray diffraction analysis were used to characterize the quality of the crystal. Both linear and step-graded buffers in GaxIn1−xAs were successfully used under an active solar cell structure. GayIn1−yP as buffer material showed a worse performance. Excellent solar cell performance was achieved for lattice mismatched single-, dual- and triple-junction solar cells.

Realization of waveguiding epitaxial GaN layers on Si by low-pressure metalorganic vapor phase epitaxy

Waveguiding GaN epitaxial layers have been grown by low-pressure metalorganic vapor phase epitaxy on Si(111) substrates using AlN/GaN short period-superlattice (SPS) buffer layer systems. The AlN/GaN SPS has been studied by x-ray diffraction where it appears as pseudoternary AlxGa1−xN alloy. Using elastic theory an effective Al content of 44% is calculated. This value is confirmed by the average Al content calculated from the AlN:GaN layer thickness ratio measured in cross-section transmission electron microscopy. The GaN waveguiding properties have been assessed using the prism coupling method. They sensibly improve with the total thickness of the underlying AlN/GaN superlattice as well as if an additional AlN/GaN SPS is grown atop the GaN waveguiding layer.

High‐quality AlN epitaxial films on (0001)‐faced sapphire and 6H‐SiC substrate

This paper presents crystal qualities of high-quality AlN epitaxial films on (0001)-faced sapphire and6H-SiC substrates. The AlN epitaxial films are grown using a low-pressure metal organic vapor phase epitaxy (LP-MOVPE) method. 0.5–1 μm-thick AlN films without any cracks are realized on both substrates. Both of AlN films are found to have similar crystal qualities in spite of large difference in in-plane lattice mismatch between AlN and each substrate. The AlN films have an atomically flat surface with clear atomic steps. Results of X-ray rocking curve (XRC) measurement indicate that both of the AlN films have small tilted mosaicity, however relatively large twisted mosaicity. Dislocation density of the AlN films in its surface region is approximately as low as 1 × 1010 cm−2 and most of dislocations consist of edge-type dislocations. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

MOVPE growth and characterization of high‐N content InGaPN alloy lattice‐matched to GaP

InxGa1−xP1−yNy (x = 17.6%, 0 < y ≤ 8.7%) alloy films have been grown on GaP(001) substrates by low-pressure (60 Torr) metalorganic vapor phase epitaxy (MOVPE). Structural and optical properties of the InGaPN films have been investigated. With incorporation of N, a reduction of compressive strain and a strong red shift in both the photoluminescence (PL) peak energy and the absorption edge of PL excitation (PLE) were clearly observed. The residual strain sufficiently relaxes for films with low N contents (0% ≤ y < 3.4%). Whereas, for higher N concentrations (3.4% < y ≤ 8.7%), the InGaPN layers were coherently grown on the GaP (001) substrates. Lattice-matched InGaPN films with N content as high as 7.4% corresponding to a 10 K absorption wavelength of 610.8 nm (2.03 eV) have been obtained. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

MOVPE growth and n‐type conductivity control of high‐quality Si‐doped Al 0.5 Ga 0.5 N using epitaxial AlN as an underlying layer

Conductivity control of 1-μm-thick Si-doped Al0.5Ga0.5N using an epitaxial AlN underlying layer wasrealized by introducing CH3SiH3 during low-pressure metalorganic vapor phase epitaxy. Cathodoluminescence measurements indicated that luminescence from a deep-level transition was caused by silicon doping. X-ray diffraction measurements showed that fluctuations of the tilt component of the c-axis increased with an increasing the silicon dopant flow rate. The electron concentration increased linearly with increasing the CH3SiH3 flow rate from 3.5 × 1017 cm−3 to 9.3 × 1017 cm−3. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dependences of GaN polarity on the growth temperatures of migration‐enhanced‐epitaxy‐grown AlN in MOVPE

Effects of migration-enhanced-epitaxy-grown AlN (MEE-AlN) on polarities of GaN were investigated in low-pressure metal-organic vapor phase epitaxy (LP-MOVPE). AlN was grown on extensively nitrided sapphire substrate by alternatively supplying trimethyl-aluminium (TMA) and ammonia with 2-second purging in between. It was found that the polarities of GaN depend on the growth temperatures of MEE-AlN. With increase of the growth temperature of the MEE-AlN layer, the Ga-polar ratio in GaN epilayer decreased. At the same growth temperature of MEE-AlN, the formation of Ga-polarity could be promoted by prolonging the TMA pre-flow time in the first cycle of MEE-AlN growth. The experimental results well reveal the kinetic selection process of GaN polarity in the MOVPE growth.

Low‐dislocation density AlGaN layer by air‐bridged lateral epitaxial growth

A high quality AlGaN layer with low dislocation density and low c-axis tilt angle in wing regions was demonstrated by the advanced ELO technique, namely air-bridged lateral epitaxial growth. An underlying GaN seed layer was grooved along the 〈〉GaN direction to the sapphire substrate, whose sidewalls and etched bottoms were covered with silicon nitride masks, and regrowth of AlGaN was carried out by a low-pressure metalorganic vapor phase epitaxy system. Fabrication of air-bridged structures suppresses interference of nucleated poly-crystals on the silicon nitride masks during lateral growth, and the low dislocation density AlGaN layer was realized. The threading dislocation density in the wing regions was reduced to 2 × 107 cm−2 and the c-axis tilt angle was 0.19°.

Effect of the growth sequence on the properties of InGaP/GaAs/InGaP quantum wells grown by LP-MOVPE from group-V metalorganic sources

Lattice-matched, single and multiple InGaP/GaAs/InGaP quantum wells (QWs) were grown at 600°C by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE), with the use of the tertiarybuthylarsine (TBAs) and tertiarybuthylphosphine (TBP) group-V sources. In order to enhance the interface abruptness, different gas switching sequences were exploited during the growth of the interface, and the best results were obtained by inserting a few monolayer-thick GaAsP interlayers (IL), at the direct GaAs-on-InGaP interface. Low-temperature photoluminescence (PL), high resolution X-ray diffraction, transmission electron microscopy and photoreflectance spectroscopy analysis were performed on the grown heterostructures, to correlate the adopted growth sequence with the interface properties and the QW optical transitions.Promising results were obtained, among which: (a) the suppression of the anomalous PL emission at low energy, (b) optical emission from the InGaP/GaAs/InGaP QWs, exhibiting a good correlation with theoretical expectations, (c) direct interface fluctuations within 1nm.

Arsenic incorporation into InGaAsP grown by low-pressure metalorganic vapor phase epitaxy using tertiarybutylarsine and tertiarybutylphosphine in N2 ambient

In x Ga 1−x As y P 1−y epilayers have been grown by low-pressure metalorganic vapor phase epitaxy (LPMOVPE) using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) as group V precursors and nitrogen as the carrier gas. Arsenic incorporation into InxGa1−xAsyP1−y films grown by LPMOVPE as a function of the gas phase composition ratio and V/III ratio has been systematically studied. With optimized growth conditions, the arsenic composition of the epilayers does not change linearly with the TBA source flow. It is observed that the arsenic incorporation becomes saturated when the gas phase composition TBA/(TBA+TBP) increases to 0.4. The incorporation kinetics in MOVPE growth of InxGa1−xAsyP1−y alloy has been analyzed by using an adsorption-trapping model. The As composition (y) of the InxGa1−xAsyP1−y films varies with the TBA gas phase composition ξ=TBA/(TBA+TBP) according to the expression y=2Ns*/aN0(1−e−θβξ). It is demonstrated that with the optimized growth conditions, TBA has a higher incorporation efficiency than TBP in MOVPE growth of InxGa1−xAsyP1−y films.

Size- and shape-controlled GaAs nano-whiskers grown by MOVPE: a growth study

We have investigated the Au-catalyzed GaAs 〈 1 ̄ 1 ̄ 1 ̄ 〉 B whisker growth under low-pressure metal-organic vapour phase epitaxy conditions. By varying the growth temperature we found a maximum in the whisker growth rate at about 450–475°C. With increasing temperature the growth rate decreases due to competing growth at the ( 1 ̄ 1 ̄ 1 ̄ ) substrate surface and at the {1 1 0} whisker side facets, which leads to significant tapering of the whiskers. For low temperatures, the growth rate R in the ln R=f(1/T) -plot results in an Arrhenius activation energy of about 67–75 kJ/mol, a value which is in agreement with activation energies reported for low-temperature planar growth of GaAs from TMG and AsH 3. The Au acts as a local catalyst and as a collector of reactants, enabling a liquid-phase-epitaxy-like growth with high growth rates at the GaAs ( 1 ̄ 1 ̄ 1 ̄ ) B/(Au,Ga) interface.

Room-temperature operation of InGaAs/AlInAs quantum cascade lasers grown by metalorganic vapor phase epitaxy

We report the room-temperature operation of λ≈8.5 μm InGaAs/AlInAs quantum cascade lasers, grown by low-pressure metalorganic vapor phase epitaxy. The necessary control of interfacial abruptness and layer thicknesses was achieved by the use of individually purged vent/run valves and a growth rate of 0.8 μm/h for the active region. Low-temperature threshold current densities of ∼1.5 kA cm−2 and a maximum operating temperature of 290 K have been measured in pulsed operation. These values are comparable with those reported for structures of a similar design grown using molecular beam epitaxy.

Three-dimensionally nucleated growth of gallium nitride by low-pressure metalorganic vapour phase epitaxy

At present, a major part of commercially available group III nitride optoelectronic devices is grown on sapphire substrates by metalorganic vapour phase epitaxy. Besides classical techniques involving selective overgrowth of patterned substrates, GaN epitaxial layers with dislocation densities in the order of 10 7 cm −2 (Appl. Phys. Lett. 78 (2001) 1976; MRS Internet J. Nitride Semicond. Res. 7 (2002) 8) can be fabricated on sapphire by epitaxial overgrowth of three-dimensional (3D) GaN islands. In this work, we prove the independence of this technique from the initial substrate surface state if the island formation is induced by a silane/ammonia treatment. Subsequent layer coalescence can be accelerated by high overgrowth temperatures and high group V/III precursor ratios on the expense of the dislocation density. Once optimised, nucleation conditions are operated for the growth of GaN-on-sapphire, the dislocation density is not further reduced by repeated 3D nucleation steps. With growing epilayer thickness, the intensity of cathodo- and photoluminescence spectra increases while excitonic line widths narrow. The latter is a quantitative measure for the decreasing dislocation density. According to both luminescence experiments, 3D nucleated GaN-on-sapphire layers are under increasing compressive strain for film thicknesses above 3 μm.

Effect of thin GaAs interface layer on InAs quantum dots grown on InGaAs/InP using metalorganic vapor phase epitaxy

InAs self-assembled quantum dots (QDs) embedded within an InGaAs quantum well have been grown on InP substrate by low-pressure metalorganic vapor phase epitaxy. We find out that the underlying InGaAs layer could affect the dot growth dramatically in terms of size distribution and luminescence efficiency. After inserting a thin GaAs interface layer between the underlying InGaAs and the InAs QD layer, improved dot size uniformity and strong room temperature photoluminescence (PL) up to 2 μm were observed. In the case of InAs QDs with no GaAs layer, one has to reduce InAs QD layer thickness in order to obtain room temperature PL. These results suggest that InAs from the underlying InGaAs layer contribute to the InAs QD formation, and cause the InAs QDs to be non-uniform, but a thin GaAs interface layer could effectively block the migration of In atoms from the InGaAs layer toward InAs QDs, and therefore lead to more uniform QD formation with better luminescence efficiency.

Nitride-based 2DEG photodetectors with a large AC responsivity

Nitride-based AlGaN/GaN heterostructure two-dimensional electron gas photodetectors have been successfully fabricated by low-pressure metalorganic vapor phase epitaxy on sapphire substrate. By using such an AlGaN/GaN heterostructure, we could significantly reduce the recombination of photogenerated carriers and thus achieve extremely high photodetector responsivity due to the use of AlGaN/GaN heterostructure. With an incident light wavelength of 270 nm, it was found that the AC responsivity could reach 8.7 × 10 6 A/W.

Optical studies of Ga x In 1– x P/Ga 0.5 In 0.5 P quantum dots

Self-assembled type A GaxIn1–xP/Ga0.5In0.5P quantum dots (QDs) have been investigated using continuous wave and time-resolved photoluminescence (PL) techniques. The QDs were grown by low-pressure metal-organic vapour phase epitaxy using the Stranski–Krastanow growth mode. A substrate misorientation of 15� towards the (111)B plane leads to the formation of type A dots with a height of approximately 5 nm. The novel effect of varying the Ga content of the QDs has been examined and the PL emission energy of the dots is observed to increase from 1.71 eV to 1.90 eV as the Ga content is varied from 0% to 30%. Temperature dependent (5 K to 260 K) time-resolved PL measurements have also been performed in order to study the dot carrier dynamics in more detail. These measurements indicate that the Type II InP/GaInP QDs either become smaller or that the band structure changes to a Type I alignment with the addition of Ga. Both effects, if present, could be attributed to a decrease in the dot strain as Ga is added.