Thick Hydride Vapor Phase Epitaxy Research Articles

HVPE GaN wafers with improved crystalline and electrical properties

The quest for low cost GaN substrates with optimized crystalline and electrical properties continues to fuel the search for a fast growth method to produce commercial wafers that will allow the fabrication of devices capable of achieving high performance at high power and/or high frequency. Thick films grown by hydride vapor phase epitaxy (HVPE) on Ammono substrates in addition to reproducing the high crystalline quality of those substrates show significant reduction in free carrier concentration. This work presents a detailed spectroscopic, X-ray diffraction, and Raman spectroscopy imaging investigation of thick freestanding HVPE GaN films deposited on HVPE/Ammono-GaN templates. The results demonstrate that they are stress-free, and have a nearly uniform and relatively lower residual background doping, in addition to high crystalline quality. This result is extremely important, because it demonstrates the usefulness of this new type of HVPE-GaN substrate to fabricate highly efficient optoelectronic and electronic devices.

Incorporation of Mg in Free-Standing HVPE GaN Substrates

Mg, the only effective p-type dopant for nitrides, is well studied in thin films due to the important role of the impurity in light-emitting diodes and high-power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here, we demonstrate successful incorporation of Mg into GaN grown by hydride vapor-phase epitaxy (HVPE) using metallic Mg as the doping source. The concentration of Mg obtained from four separate growth runs ranged between 1016 cm−3 and 1019 cm−3. Raman spectroscopy and x-ray diffraction revealed that Mg did not induce stress or perturb the crystalline quality of the HVPE GaN substrates. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies were performed to investigate the types of point defects in the crystals. The near-band-edge excitonic and shallow donor–shallow acceptor radiative recombination processes involving shallow Mg acceptors were prominent in the PL spectrum of a sample doped to 3 × 1018 cm−3, while the EPR signal was also thought to represent a shallow Mg acceptor. Detection of this signal reflects minimization of nonuniform strain obtained in the thick free-standing HVPE GaN compared with heteroepitaxial thin films.

Incorporation of Mg into thick free-standing HVPE GaN

ABSTRACTMg, the only effectivep-type dopant for nitrides, is well-studied in thin films due to the important role the impurity plays in light emitting diodes and high power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here we evaluate the material quality and point defects in GaN grown by hydride vapor phase epitaxy (HVPE) using metallic Mg as the doping source. The crystal quality is typical of commercially grown HVPE substrates, and the photoluminescence measurements reveal distinctively sharp excitonic and shallow-donor shallow-acceptor features. Secondary ion mass spectroscopy indicates total Mg concentrations between 7x1016and 6x1018cm-3in the four separate samples studied but, more significantly, photoluminescence and electron paramagnetic resonance spectroscopy show that the Mg is incorporated as a shallow acceptor.

Improved GaN films with low background doping and low deep trap density grown by hydride vapor phase epitaxy

AbstractElectrical properties and deep electron and hole traps spectra are compared for undoped n‐GaN films grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates at 850 °C (Group 1) and 950 °C (Group 2). Group 1 samples have very low residual donor concentration ((2‐5)×1014 cm‐3), very low density of electron and hole traps, low leakage current of Schottky diodes. Group 2 samples are not very different from the usual thick HVPE samples in terms of predominant electron and hole traps and MCL spectra. But the concentration of both electron and hole traps is quite high and could contribute to rather high reverse leakage current, low photocurrent and pronounced tunneling. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of nano‐column properties on self‐separation of thick GaN layers grown by HVPE

AbstractThe implementation of control over self‐separation of thick GaN flayers grown by Hydride Vapor‐Phase Epitaxy (HVPE) on Metal‐Organic Chemical Vapor Deposition (MOCVD)‐grown templates with patterned nano‐column interlayer is reported. Both numerical simulation and experimental data show that separation of thick HVPE layers is provided by non‐uniform redistribution of the growth stress. It is shown that the geometrical parameters of the nano‐column structure such as fill factor and shape of the columns are most critical for self‐separation effect. By adjusting the nano‐column pattern, successful self‐separation of thick HVPE layers from 2‐inch structured MOCVD GaN/sapphire substrate was demonstrated. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydride vapor phase epitaxial growth of thick GaN layers with improved surface flatness

Thick GaN layers have been grown by hydride vapor phase epitaxy (HVPE) on different GaN templates grown by metalorganic vapor phase epitaxy. Crack formation could be reduced by using a hydrogen/nitrogen carrier gas mixture. We found strongly different HVPE layer quality when templates grown by different groups have been used, although these templates showed very much the same basic properties. Over 100 µm thick crack-free HVPE layers with extremely flat surface could be grown on optimized templates. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Bowing of epitaxial layers grown on bulk GaN substrates

We investigated the bowing of mixed group III nitride epitaxial layers grown by HVPE (Hydride Vapor Phase Epitaxy) and MOVPE (Metalorganic Vapor Phase Epitaxy) on bulk GaN substrates, grown at high pressure (HP). We measured the radius of bowing, lattice-mismatch (by X-Ray Diffraction), aluminum content (by XRD) and density of threading dislocations (by Defect Selective Etching) as a function of the layer thickness. The results section is divided into two parts. In Part I, we present the results for thick HVPE GaN layers mismatched to bulk HP GaN by 0.01–0.02%. We found the critical thickness for its plastic relaxation of about 30 µm, when grown on 60 µm thick HP GaN substrate. In Part II, thin MOVPE AlxGa1–xN layers are characterized. For most of these layers, the bowing is larger than predicted by theory, what suggests not equal elastic properties of AlGaN and HP bulk GaN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Visible Light-Emitting Diodes Grown by Plasma Assisted Molecular Beam Epitaxy on Hydride Vapor-Phase Epitaxy GaN Templates and the Development of Dichromatic (Phosphorless) White LEDs

AbstractMuch of the work on III-Nitride-based LEDs that has been published and applied commercially has been done using metal-organic chemical vapor deposition (MOCVD) as a method of film growth. We report on the growth and fabrication of visible light emitting diodes, by combining hydride vapor-phase epitaxy (HVPE) and rf plasma-assisted MBE (PAMBE) methods. Thick (∼7 μm to 10 μm) HVPE n+-GaN smooth and textured templates, were used as substrates for the growth of LED structures by rf-PAMBE. The active regions of the LED structures, which consist of InGaN/GaN MQWs, were grown using the pulsed nitrogen plasma technique leading to abrupt well and barrier interfaces as confirmed by x-ray diffraction (XRD) measurements. Using this method, we obtained InGaN/GaN MQWs whose room temperature photoluminescence (PL) spectra have a full width at half maximum (FWHM) of 12 nm (105 meV). Visible LEDs on smooth GaN templates emitting in the blue to green were produced with EL spectrum FWHM as narrow as 27 nm. On the other hand, white LEDs without the use of phosphor have been produced utilizing textured MQWs as the active region, a phenomenon we initially attribute to different incorporation of In on the different QW-planes. The growth and fabrication of these devices was preceded by detailed growth and doping studies of the various layers of the LED structure. These include detailed nucleation studies on (0001) sapphire substrates as well as identification of kinetic factors which lead to good crystalline-quality InGaN alloys and InGaN/GaN MQWs.

Fast Proton Damage Effects on the Luminescence Properties of High-Quality GaN

AbstractHigh-quality GaN layers grown by hydride-vapor phase epitaxy (HVPE) and homoepitaxial layers grown thereon by molecular beam epitaxy (MBE) are studied after fast proton irradiation. A radiation-induced decrease of the band edge luminescence is observed. Time-resolved photoluminescence (PL) reveals a significant reduction of the carrier lifetime. A lifetime degradation constant of Kt = 4×10-15cm2/ns is reported for very thick HVPE GaN, and large variations are observed for the different layers. Partial recovery of the carrier lifetime and PL intensity with annealing is observed for thin HVPE GaN but not for the MBE overgrowth layer.

Bragg and Laue x-ray diffraction study of dislocations in thick hydride vapor phase epitaxy GaN films

The dislocation structure of hydride vapor phase epitaxial thick GaN layers grown on sapphire is studied by analysis of the microdistortion tensor components. Symmetrical reflections (including reflections from planes forming a large angle with the basal plane) with two modes of scanning (θ and θ–2θ) in two geometries (Bragg and Laue) are used to obtain the tensor components. The instant connections between the tensor components and major dislocation types are specified. Different types of dislocation distributions have been identified in the thick GaN films grown on sapphire with and without undoped and Si-doped metalorganic chemical vapor deposited templates. Transmission electron microscopy was used to confirm the x-ray results by direct visualization of defect rearrangements.

Interface structure of hydride vapor phase epitaxial GaN grown with high-temperature reactively sputtered AlN buffer

Thick hydride vapor phase epitaxy GaN layers have been grown on a-plane sapphire using high-temperature ion-assisted reactively sputtered AlN as a buffer layer. Transmission electron microscopy and atomic force microscopy were carried out to study the formation of the two interfaces sapphire/AlN and AlN/GaN, and their influence on the microstructure of both the buffer layer and the main GaN layer. It was demonstrated that the high-temperature reactively sputtered buffer layer provides a good alternative for hydride vapor phase epitaxy growth of GaN layers. In particular, the buffer promotes a specific interface ordering mechanism different from that observed on low-temperature buffers.

Doping Dependence Of The Thermal Conductivity Of Hydride Vapor Phase Epitaxy Grownn-GaN/Sapphire (0001) Using A Scanning Thermal Microscope

We have measured the doping concentration dependence of the room temperature thermal conductivity (κ) of two series ofn-GaN/sapphire (0001) fabricated by hydride vapor phase epitaxy (HVPE). In both setsκdecreased linearly with logn, the variation being about a factor two decrease inκfor every decade increase inn.κ≈ 1.95 W/cm-K was obtained for one of the most lightly doped samples (n= 6.9×1016cm−3), higher than the previously reportedκ≈ 1.7-1.8 W/cm-K on lateral epitaxial overgrown material [V.A. Asninet al, Appl. Phys. Lett.75, 1240 (1999)] andκ≈ 1.3 W/cm-K on a thick HVPE sample [E.K. Sichel and J.I. Pankove, J. Phys. Chem. Solids38, 330 (1977)]. The decrease in the lattice component ofκdue to increased phonon scattering from both the impurities and free electrons outweighs the increase in the electronic contribution toκ.

Photoluminescence, Magnetospectroscopy, and Resonant Electronic Raman Studies of Heteroepitaxial Gallium Nitride

Optical spectroscopy, including low and room temperature photoluminescence (PL), reflectance, PL measurements in high magnetic fields up to 12 T, and resonantly-enhanced electronic Raman scattering (RERS) in zero and high magnetic field, has been used to investigate exciton and impurity states and surface recombination in high quality heteroepitaxial GaN grown on sapphire and SiC. Theoretical finite-difference calculations of the donor states as a function of magnetic field have been carried out for comparison, including the effects of anisotropy in the effective mass and dielectric constant. Up to six residual donor species are observed in material grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) from their n=2 and n=3 two-electron satellites observed in PL and by RERS. The donor-related nature of the relevant transitions is confirmed from their magnetic field dependence, and the spectral resolution is improved at high fields. The Si donor level is determined to have a binding energy of about 21 meV from observation of its two-electron satellite in lightly Si-doped HVPE material. The free exciton binding energy is shown to be about 26.4 meV, independent of strain, based on observations of the n=2 free exciton. The room temperature band-edge PL peak is confirmed to be free excitonic in nature, based on its linewidth and on comparison with simple reflectance measurements. Reflectance from the edge of a thick HVPE layer shows clear evidence of A, B, and C excitons obeying the relevant selection rules at both low and room temperature. Surface chemical treatments are shown to have substantial effects on room temperature PL efficiency. Passivation with ammonium or sodium sulfide solutions, in particular, yields increases in PL efficiency by a factor of five to seven over air-exposed surfaces. The passivation effect is stable in air, lasting at least one month.

Doping Dependence of the Thermal Conductivity of Hydride Vapor Phase Epitaxy Grown n-GaN/Sapphire (0001) Using a Scanning Thermal Microscope

AbstractWe have measured the doping concentration dependence of the room temperature thermal conductivity (κ) of two series of n-GaN/sapphire (0001) fabricated by hydride vapor phase epitaxy (HVPE). In both sets n decreased linearly with log n, the variation being about a factor two decrease in κ for every decade increase in n. κ ≈ 1.95 W/cm-K was obtained for one of the most lightly doped samples (n = 6.9×1016 cm−3), higher than the previously reported κ ≈ 1.7-1.8 W/cm-K on lateral epitaxial overgrown material [V.A. Asnin et al, Appl. Phys. Lett. 75, 1240 (1999)] and κ κ 1.3 W/cm-K on a thick HVPE sample [E.K. Sichel and J.I. Pankove, J. Phys. Chem. Solids 38, 330 (1977)]. The decrease in the lattice component of κ due to increased phonon scattering from both the impurities and free electrons outweighs the increase in the electronic contribution to κ.

Optically Pumped GaN-AlGaN Double-Heterostructure Lasers Grown by ECR-GSMBE and HVPE

ABSTRACTWe have observed laser emission with well-defined cavity modes in optically pumped GaN-Al0.1Ga0.9N double-heterostructure (DH) lasers. The laser structures were grown using an electron-cyclotron-resonance nitrogen-discharge source and gas-source molecular beam epitaxy (ECR-GSMBE) on thick (≥10 µm) GaN buffers grown by hydride vapor-phase epitaxy (HVPE) on c-plane sapphire. Transversely pumped cavities using a 337.1 nra nitrogen laser pump source exhibit a threshold pump fluence ranging from 0.15 to 0.3 mJ/cm2 at 77 K, a linear light output above threshold, a lasing wavelength of 358 nm, and an estimated differential quantum efficiency of 1%. The room-temperature threshold is about 1.7 times higher. Longitudinal mode structure has been resolved in a shorter-cavity (23 µn) device at 77 K. The measured mode spacing of 0.56 nm corresponds to a group index of 5.0. Far-field measurements in a plane perpendicular to the plane of the heterostructure indicate a double-lobed pattern for a 1000 Ǻ thick GaN active region, and a single lobe with a FWHM of 60° for a 4000 Ǻ active region. The thick HVPE GaN buffer layer provides for a lattice-matched growth and results in improved nucleation in MBE, as indicated by a high-quality reflection-electron-diffraction pattern of the as-loaded wafers. The surface morphology of the MBE layers on the HVPE buffer shows improved optical smoothness as compared to layers grown directly on sapphire using a low-temperature, MBE-grown GaN buffer. Laser facets were formed either by saw cutting or cleaving of the GaN buffer and epilayer along crystal planes. Details of the material development and laser performance are described.