Hydride Vapor Phase Epitaxy Method Research Articles

Reduction of dislocation density in epitaxial GaN layers by overgrowth of defect-related etch pits

GaN templates grown by the metal organic chemical vapor deposition method were etched in a defect-selective molten salts eutectic and were subsequently overgrown by a GaN layer using the hydride vapor phase epitaxy (HVPE) method. Optimized conditions of etching and of HVPE growth processes resulted in a significant reduction of the dislocations density (DD). Local areas virtually free of dislocations were obtained on ∼50% of the surface, while the average DD was reduced from 3×109 cm−2 in the template to about 2×107 cm−2 in the HVPE-grown GaN layer. A model has been developed to explain the mechanism of reduction of the DD during the overgrowth process. The model was confirmed by the photoetching of cleaved layers.

Effects of temperature and carrier gas flow amount on the formation of GaN nanorods by the HVPE method

We fabricated one-dimensional GaN nanorods on AlN/Si (1 1 1) substrates at various temperatures, and carrier gas flow amount, using the hydride vapor phase epitaxy (HVPE) method. An AlN buffer layer of 50 nm thickness was deposited by RF sputtering for 25 min. Stalagmite-like GaN nanorods formed at a growth temperature of 650 °C. The diameters and lengths of GaN nanorods increase with growth time, whereas the density of nanorods decreases. And we performed the experiments by changing the carrier gas flow amount at a growth temperature of 650 °C and HCl:NH 3 flow ratio of 1:40. GaN nanorods, with an average diameter of 50 nm, were obtained at a carrier gas flow amount of 1340 sccm. The shape, structures, and optical characteristics of the nanorods were investigated by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence.

Effects of temperature and HCl:NH 3 flow ratio on the growth of GaN nanorods

We studied the formation of GaN nanorods on AlN-covered Si(1 1 1) by the hydride vapor phase epitaxy method. The most well-formed GaN nanorods were obtained at an HCl:NH 3 gas flow ratio of 1:40 and a growth temperature of 650 °C. A high density of straight nanorods with diameters of about 350 nm formed uniformly over the entire substrate. The synthesis and structural properties of the nanorods were investigated by X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS). The obtained XRD patterns indicate that the GaN nanorods are preferentially oriented, with c-axes perpendicular to the substrate. EDS measurements of the nanorods indicate that they are composed of Ga and N with a ratio of 1.00:0.99.

Increase in the growth rate of GaN single crystals grown by gallium hydride vapor phase epitaxy method

AbstractA dependence of Gallium hydride (GaHx: x = 1, 2, 3) production on the growth rate of GaN crystals grown by the gallium hydride vapor phase epitaxy (GaH‐VPE) method was investigated. By this method, GaN crystals are synthesized in a reaction between GaHx and NH3. The growth rate of GaN crystals has been as low as 20 μm/h at a maximum because the poor reactivity of H2 gas with Ga metal limits the production of GaHx. In this study, the interfacial area between Ga metal and H2 gas was enlarged to increase the GaHx synthesis, which thereby enabled an increase in the growth rate to 68 μm/h. We showed clearly that the growth rate of GaN by the GaH‐VPE method is limited by the efficiency of the reaction between the Ga metal and H2 gas. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of growth temperature on the crystallinity of GaN in gallium hydride vapor phase epitaxy method

AbstractHigh temperature growth was investigated as a way of improving the crystallinity and surface morphology of the gallium hydride vapor phase epitaxy (GaH‐VPE) crystal growth method. In this method, gallium hydride (GaHx: x=1, 2, 3) is characteristically used as a Ga source to grow GaN crystals. We previously confirmed that the growth rate of GaN increased with GaHx supply and could attain almost the same rate as that of the hydride vapor phase epitaxy (HVPE) method. However, an improvement of the surface morphology and crystallinity of GaN crystals was required. In the present study, the dependence of growth temperature on the crystallinity and the surface morphology of grown crystals was investigated, resulting in both a flat surface and an improvement in crystallinity at temperatures higher than 1150 °C. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation related defect states in GaN irradiated with 1 MeV electron‐beam

AbstractUndoped GaN layers with thickness of 278 µm were grown by hydride vapor phase epitaxy method. The samples with different threading dislocation densities of 9.0x106 cm–2 and 7.2x106 cm–2 were irradiated by electron‐beam with the energy of 1 MeV and dose of 1x1015 cm–2. The defect states of the samples after electron beam irradiation were characterized by deep level transient spectroscopy measurement. After the electron‐beam irradiation, the the defects appeared to states with the activation energies of 0.61 eV, 0.30 eV, and 0.57 eV. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of a SAG‐InGaN/AlGaN LED by mixed‐source HVPE with a multi‐sliding boat system

Abstract— The selective area growth (SAG) of a InGaN/AlGaN light‐emitting diode (LED) is performed by using mixed‐source hydride vapor‐phase epitaxy (HVPE) with a multi‐sliding boat system. The SAG‐InGaN/AlGaN LED consists of a Si‐doped AlGaN cladding layer, an InGaN active layer, a Mg‐doped AlGaN cladding layer, and a Mg‐doped GaN capping layer. The carrier concentration of the n‐type AlxGa1−xN (x ∼ 16%) cladding layer depends on the amount of poly‐Si placed in the Al‐Ga source. The carrier concentration is varied from 2.0 × 1016 to 1.1 × 1017 cm−3. Electroluminescence (EL) characteristics show an emission peak wavelength at 426 nm with a full width at half‐maximum (FWHM) of approximately 0.47 eV at 20 mA. It was found that the mixed‐source HVPE method with a multi‐sliding boat system is a candidate growth method for III‐nitride LEDs.

Synchrotron X-ray topography and electrical characterization of epitaxial GaAs p–i–n structures

Results from synchrotron X-ray topography and electrical characterization of thick epitaxial GaAs p–i–n structures suitable for manufacturing of radiation detectors are reported. The structures under study have been grown with hydride vapor phase epitaxy method. A comprehensive set of large-area transmission, large-area back-reflection and transmission section topographs are analyzed. The X-ray topography results are compared with the dark current density of the detector diodes. The X-ray topographs show the defect structure in the samples and provide important information for epitaxial process optimization.

Bulk growth of gallium nitride: challenges and difficulties

AbstractThe present status of the GaN bulk growth by High Pressure Solution (HPS) method and combination of HPS and Hydride Vapor Phase Epitaxy (HVPE) methods is reviewed. Up to now the spontaneous high pressure solution growth of GaN results in crystals having habit of hexagonal platelets of surface area of 3 cm2 or needles with length up to 1 cm. Recently, the platelets and needles have been used as seeds for the HVPE growth. On the other hand, the LPE technique under pressure with pressure‐grown GaN (hp‐GaN), GaN/sapphire template, patterned GaN/sapphire template and free standing HVPE GaN as seeds has been examined and developed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetoluminescence Studies of GaN:Fe

We report on magneto-optical studies on iron doped GaN crystals grown using hydride vapor phase epitaxy method on bulk GaN substrate. The investigated samples showed an intensive 1.3 eV luminescence band, characteristic of Fe(d) center in GaN. A high quality of the investigated samples allowed us to observe a well-resolved fine structure of intracenter transitions between T1(G) and A1(S) states, consisting of four sharp no-phonon lines. All the observed no-phonon lines showed pronounced splittings in magnetic field. From the analysis of the magneto-optical data, the structure of split T1(G) multiplet in the magnetic field applied along c-axis of GaN crystals was established.

Characterization of AlGaN/InGaN/AlGaN heterostructure with selective area growth of Te‐doped AlGaN cladding layer grown by mixed‐source HVPE

The selective area growth (SAG) of AlGaN/InGaN/AlGaN light-emitting diodes (LEDs) is performed by mixed-source hydride vapor phase epitaxy (HVPE). The structure is grown on a n-GaN templated (0001) sapphire substrate. The SAG-double heterostructure (DH) is consisted of a Te-doped AlGaN cladding layer, an InGaN active layer, a Mg-doped AlGaN cladding layer, and a Mg-doped GaN capping layer. All of the epitaxial layers of LED structure are grown consecutively with a multi-sliding boat system. Room-temperature electroluminescence (EL) characteristics show an emission peak wavelength of 400 nm with a full width at half-maximum (FWHM) of approximately 0.38 eV (at 20 mA). We find that the mixed-source HVPE method with multi-sliding boat system is possible to be one of the growth methods of III-nitride LEDs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of InGaN layer on GaN templated Al 2 O 3 (0001) and Si (111) substrates by mixed‐source HVPE

The InGaN layers on GaN templated sapphire (0001) and Si (111) substrates are grown by mixed-source hydride vapor phase epitaxy (HVPE) method. As a new attempt in obtaining an InGaN layers, the growth of the thick InGaN layer is performed by putting small amount of Ga into the In source. The InGaN layer is compounded from chemical reaction between a NH3 and an Indium-gallium chloride formed by HCl flown over metallic In mixed with Ga. The InGaN layer is analyzed by X-ray photoelectron spectroscopy (XPS) to characterize the InGaN ternary crystal alloy. The optical property of the selective area growth (SAG) of the InGaN layer is investigated by the photoluminescence (PL) spectrum and the cathodoluminescence (CL) images. Indium compositions are estimated to be in the range 3-10%. In order to obtain the GaN layer on GaN templated Si (111) substrates, an InGaN layer is used as an intermediate layer. The PL and CL of the InGaN intermediate layer are measured at 300K. We find that the InGaN intermediate layer is possible to be one of the growth methods of thick GaN layer on Si substrates by HVPE method. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of AlGaN, Te‐doped GaN and Mg‐doped GaN grown by hydride vapor phase epitaxy

The AlxGa1–xN, Te-doped GaN and Mg-doped GaN layers on GaN/Al2O3 substrates are grown by mixed-source hydride vapor phase epitaxy (HVPE) method. The metallic Ga mixed with Al is used as group III source material to get the AlxGa1–xN layers. The values of the compositions x of the AlxGa1–xN layers characterized by X-ray diffraction (XRD) measurements are 0.6% ∼ 80% at the various temperatures of the source zone. The metallic Ga mixed with Te (or Mg) is used as source material for n-type (or p-type) doping. The electron concentrations of the Te-doped GaN layers are varied from 1.8 × 1017 to 8.3 × 1018/cm3. The hole concentrations of the Mg-doped GaN layers are varied from 1.5 × 1016 to 3.2 × 1016/cm3. We find that the mixed-source HVPE method is suitable to get a thick AlGaN layer with an arbitrary composition, a Te-doped GaN layer with a high n-type concentration and a Mg-doped GaN layer with p-type concentration. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of SAG‐AlGaN/InGaN/AlGaN LEDs by mixed‐source HVPE with multi‐sliding boat system

The selective area growth (SAG) of AlGaN/InGaN/AlGaN light-emitting diodes (LEDs) is performed by mixed-source hydride vapor phase epitaxy (HVPE). In order to obtain the SAG-AlGaN/InGaN/AlGaN heterostructure, a special graphite fixture to use in HVPE is designed. First, an n-type AlGaN layer is grown at 1090 °C on a GaN templated (0001) sapphire substrate with a patterned SAG-structure of a silicon oxide (SiO2). On this selectively grown n-type AlGaN layer, a nominally undoped-InGaN layer is grown using an In-Ga mixed metallic source at 990 °C. After the growth of InGaN layer, Mg-doped AlGaN and Mg-doped GaN layers are grown as a cladding and capping layers at 1090 °C and 1050 °C, respectively. All of the epitaxial layers of LED structure are grown consecutively with a multi-sliding boat system. Room-temperature electroluminescence (EL) characteristics show an emission peak wavelength of 415 nm with a full width at half-maximum (FWHM) of approximately 0.37 eV. We find that the mixed-source HVPE method with multi-sliding boat system is possible to be one of the growth methods of III-nitride LEDs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High power ultraviolet light emitting diodes based on GaN∕AlGaN quantum wells produced by molecular beam epitaxy

In this paper, we report on the growth by molecular beam epitaxy and fabrication of high power nitride-based ultraviolet light emitting diodes emitting in the spectral range between 340 and 350nm. The devices were grown on (0001) sapphire substrates via plasma-assisted molecular beam epitaxy. The growth of the light emitting diode (LED) structures was preceded by detailed materials studies of the bottom n-AlGaN contact layer, as well as the GaN∕AlGaN multiple quantum well (MQW) active region. Specifically, kinetic conditions were identified for the growth of the thick n-AlGaN films to be both smooth and to have fewer defects at the surface. Transmission-electron microscopy studies on identical GaN∕AlGaN MQWs showed good quality and well-defined interfaces between wells and barriers. Large area mesa devices (800×800μm2) were fabricated and were designed for backside light extraction. The LEDs were flip-chip bonded onto a Si submount for better heat sinking. For devices emitting at 340nm, the measured differential on-series resistance is 3Ω with electroluminescence spectrum full width at half maximum of 18nm. The output power under dc bias saturates at 0.5mW, while under pulsed operation it saturates at approximately 700mA to a value of 3mW, suggesting that thermal heating limits the efficiency of these devices. The output power of the investigated devices was found to be equivalent with those produced by the metal-organic chemical vapor deposition and hydride vapor-phase epitaxy methods. The devices emitting at 350nm were investigated under dc operation and the output power saturates at 4.5mW under 200mA drive current.

CFD and reaction computational analysis of the growth of GaN by HVPE method

GaCl synthesis reaction during hydride vapor phase epitaxy (HVPE) growth of GaN in horizontal flow reactor has been analyzed using computerized fluid dynamics (CFD) and molecular estimates of the reaction rates. Finite element code FIDAP (commercially available from Fluent Inc.) [Fidap User Manual, Fluent Inc. [1]] has been used to obtain the flow pattern in the GaCl synthesis section of the HVPE reactor. As in the typical HVPE design, it was assumed that the flow pattern is weekly dependent on the temperature distribution in the reactor, i.e. the system in this part is virtually isothermal. The HCl+Ga(l) surface reaction rate has been estimated using ideal gas approximation for HCl impingement flux and predefined sticking coefficient for HCl adsorption on liquid Ga surface. In parallel, Ga evaporation process and transport has been accounted in the simulations. Using Ga evaporation rates, the total flux of transporting Ga has been obtained. The flux includes GaCl resulting from the surface reaction rate, and the Ga evaporation flux. The GaCl synthesis rate was obtained in function of the pressure, flow velocity and geometry of the reactor. The HCl to GaCl conversion degree in function of the above parameters was obtained for the several selected values of the sticking coefficient of HCl on liquid Ga surface. It was shown that for the typical design of the reactor, for the sticking coefficient below 0.01, the conversion rates depend on the sticking coefficient. For higher values of the coefficient the GaCl synthesis reaction is diffusion controlled and independent on the surface reaction kinetics. The calculations were made for the temperatures range of 800–1000 °C for which the Ga/GaCl ratio was determined. It was shown that, even in case of relatively low values of sticking coefficient for HCl, such as 0.01, gallium transport is dominated by GaCl for temperatures up to 1300 K. These results suggest that for the temperatures up to 1500 K Ga vapor transport by direct evaporation is inefficient, i.e. the sublimation growth of GaN is inefficient in this temperature range.

Accurate dependence of gallium nitride thermal conductivity on dislocation density

The authors experimentally find that the thermal conductivity of gallium nitride depends critically on dislocation density using the 3-omega technique. For GaN with dislocation densities lower than 106cm−2, the thermal conductivity is independent with dislocation density. The thermal conductivity decreases with a logarithmic dependence for material with dislocation densities in the range of 107–1010cm−2. These results are in agreement with theoretical predictions. This study indicates that the hydride vapor phase epitaxy method offers an attractive route for the formation of semi-insulating gallium nitride with optimal thermal conductivity values around 230W∕mK and very low dislocation density near 5×104cm−2.

Optical characteristics of GaN single crystals grown by the HVPE: Effects of thermal annealing and N 2 plasma treatment

Single-crystalline 350-μm-thick GaN were grown on 2 in (0 0 0 1) sapphire substrate by the hydride vapor phase epitaxy method. The free-standing bulk-like GaN crystals were obtained by delaminating from the sapphire substrate by the laser-induced lift-off technique. Mechanical polishing of the GaN crystals to remove the damage layer on their surfaces resulted in deteriorating optical properties. In order to recover the optical damages induced by the mechanical polishing, thermal annealing and N 2 plasma treatment were adopted. As the annealing temperature was increased up to 900 °C, the surface morphology and photoluminescence property of the polished GaN crystals improved, resulting from removal of the polishing-induced damage layer on the surface. Moreover, the N 2 plasma treatment on the mechanically polished samples effectively removed the polishing-induced damage, leading to remarkably improve the surface morphology and photoluminescence.

Enhanced internal quantum efficiency and light extraction efficiency from textured GaN∕AlGaN quantum wells grown by molecular beam epitaxy

Ga N ∕ Al 0.2 Ga 0.8 N multiple quantum wells (MQWs) were grown by molecular beam epitaxy on randomly textured and atomically smooth (0001) GaN templates. Smooth and textured GaN templates were deposited on (0001) sapphire substrates by varying the III∕V ratio and the substrate temperature during growth by the hydride vapor-phase epitaxy method. We find that the MQWs replicate the texture of the GaN template, which was found to have a Gaussian distribution. The peak photoluminescence intensity from the textured MQWs is always higher than from the smooth MQWs and for GaN (7nm)∕Al0.2Ga0.8N (8nm) MQWs, it is 700 times higher than that from similarly produced MQWs on smooth GaN templates. This result is attributed partly to the enhancement in light extraction efficiency and partly to the enhancement in internal quantum efficiency. The origin of the increase in internal quantum efficiency is partly due to the reduction of the quantum-confined Stark effect, since the polarization vector intersects the quantum well (QW) planes at angles smaller than 90°, and partly due to the charge redistribution in the QWs caused by the polarization component parallel to the planes of the QWs.

Growth of thick AlGaN by mixed-source hydride vapor phase epitaxy

Growth of a thick AlGaN layer on GaN/Al 2O 3 substrate is performed by mixed-source hydride vapor phase epitaxy (HVPE) method. The AlGaN material is compounded from chemical reaction between a NH 3 and an aluminum-gallium chloride formed by HCl which is flown over metallic Ga mixed with Al. The AlGaN epitaxial layer is analyzed by Auger-electron spectroscopy (AES) and X-ray diffraction (XRD) measurement to characterize the AlGaN. Al concentration is estimated to be in the range of 0.5–6%. We suggest that the metallic Ga mixed with Al can be used as a group III source material in the HVPE growth of the AlGaN layer.