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Sublimation method, spontaneously nucleated as well as seeded on SiC substrates, has been employed for growing AlN bulk crystals. For GaN growth, in addition to the sublimation method using sapphire substrates, ammonothermal growth (analogous to the hydrothermal method) on HVPE GaN seeds is also being used. Thick plates/films of AlN and GaN grown by these methods have been characterized by synchrotron white beam X-ray topography (SWBXT) and high resolution X-ray diffraction (HRXRD). Results from a recent set of growth experiments are discussed.

The growth of aluminum nitride (AlN) bulk crystals by sublimation of an AlN source requires elevated temperatures, typically in a range of 1800–2300 °C. These temperature requirements, combined with the chemically aggressive nature of the Al vapor, severely limit the choice of reactor hot-zone materials, and most notably, the selection of reaction crucibles. Aside from refractory elements, potentially promising compound materials include refractory nitrides, carbides, and borides. In this work, TaC crucibles were fabricated using a binderless sintering process and were tested in AlN bulk growth experiments. Elemental analysis of crystals grown in these crucibles revealed extremely low Ta contamination, below the analytical detection limit of 1 ppm by weight and C contamination levels as low as 50 ppm by weight; C contamination likely originated from sources unrelated to the crucible material. Crucibles were re-used in several consecutive growth runs; average crucible lifetimes exceeded 200 h at growth temperatures exceeding 2200 °C.

AlN layers with thickness between 0.1 and 3 mm were grown on on-axis and off-axis (0 0 0 1), Si-face SiC seeds by physical vapor transport (PVT) from an AlN powder source. A two-step deposition process was developed for the growth of thick layers. Cracks formed in the AlN layers due to the thermal expansion mismatch between AlN and SiC were observed to decrease with increase in AlN thickness. AlN grown on on-axis SiC was primarily Al-polar, but contained N-polar inversion domains (IDs) revealed by wet etchingin hot, aqueous phosphoric acid or potassium hydroxide solutions. Regions of opposite polarity on basal plane surfaces were imaged by piezoresponse force microscopy (PFM). IDs were not observed in crystals grown on off-axis seeds. The influence of SiC seed orientation and stability on the polarity of the AlN layers is discussed. r 2005 Elsevier B.V. All rights reserved.

Growth of AlN bulk crystals on on-axis and off-axis (0001), Si-face SiC seeds was performed by physical vapor transport (PVT) using an AlN powder source. AlN layers between 0.1–3 mm thickness were grown on inch-sized seeds. Cracks formed in the AlN layers due to the thermal expansion mismatch between AlN and SiC were observed to decrease with increasing AlN thickness. AlN grown on on-axis SiC was primarily Al-polar, but contained N-polar inversion domains (IDs) revealed buy wet etching in hot, aqueous phosphoric acid or potassium hydroxide solutions. IDs were not observed in crystals grown on off-axis seeds. Using X-ray diffraction and scanning electron microscopy results, the influence of SiC seed orientation on the polarity and morphology of the AlN layers is discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High quality AlN single crystals grown by physical vapour transport and by sublimation of AlN powder were investigated by Raman, photoluminescence (PL) and absorption spectroscopy. Absorption edges of the AlN single crystals varying from 4.1 eV to 5.9 eV as determined by transmission measurements. Near band edge absorption, PL and glow discharge mass spectroscopy identified impurities such as oxygen, silicon, carbon, and boron that contribute to the absorption and emission bands below the bandgap. The absorption coefficients were derived from UV (6 eV) to FIR (60 meV) spectral range. The exact crystal orientation of the samples, and their low carrier density were confirmed by Raman spectroscopy. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of impurities and defects on the optical properties of AlN was investigated. High-quality AlN single crystals of more than 20mm2 size were examined. Different crucible materials and growth procedures were applied to the growth of bulk AlN by physical vapor transport method to vary the defect and the impurity concentrations. The crystalline orientation was investigated by Raman spectroscopy. Glow discharge mass spectrometry was used to determine the trace concentration of the incorporated impurities such as oxygen and carbon. The photoluminescence emission and absorption properties of the crystals revealed bands around 3.5 and 4.3eV at room temperature. Absorption edges ranging between 4.1 and 5.95eV were observed. Since no straight correlation of the oxygen concentration was obtained, a major contribution of oxygen or oxygen-related impurities was ruled out to generate the observed emission and absorption bands in the Ultraviolet spectral range. The carbon-related impurities and intrinsic defec...

We propose that the reduction of the Schwoebel barrier by trace B impurities is responsible for the relatively flat c-plane morphology observed in AlN crystals that are deposited via physical vapor transport in BN crucibles relative to crystals grown in boron-free conditions. The model is supported by molecular statics calculations that predict that B substitutional impurities are energetically preferred at steps on the (0 0 0 1) surface, and that these impurities enhance binding and incorporation of growth species onto steps.

Structural defects in AlN single crystals grown by the sublimation method and GaN single crystals grown by the ammonothermal method are characterized by synchrotron white-beam X-ray topography in conjunction with optical microscopy. AlN platelets are either of (1120) or (0001) type depending on the growth conditions. Dislocation densities of the order of 10 3 cm -2 or lower are observed in some crystals. X-ray topographs reveal the presence of growth sector boundaries, inclusions, and growth dislocations that indicate slight impurity contamination. The 2H crystal structure of GaN single crystals obtained by the ammonothermal method was verified by Laue X-ray pattern analysis. GaN crystals grown are of the order of 1 mm in size and are either (0001) platelets or [0001] prismatic needles. Generally, prismatic needles are characterized by lower degree of mosaicity than (0001) platelets.

AlN bulk single crystals were grown by sublimation of AlN powder at temperatures of 2100–2300°C in an open crucible geometry in a 400 Torr nitrogen atmosphere. Small, single crystalline AlN c -platelets, prepared by vaporization of Al in a nitrogen atmosphere, were used as seeds. Seeded growth occurred preferentially in the crystallographic c -direction, with growth rates exceeding 500 μm/h, while the seed crystals grew only marginally in the c- plane. Transparent, centimeter-sized AlN single crystals were grown within 24 h. Characterization by X-ray diffraction showed that rocking curves around the (0 0 0 2) reflection were very narrow (25 arcsec full-width at half-maximum), thus indicating very high crystalline quality of the material grown on the seeds.

Lateral and vertical power switching devices being intensively developed in GaN and SiC materials are beginning to reach a state of maturity and to displace traditional Si devices in certain applications. The relative advantage of these wide bandgap materials, which stems from intrinsic materials properties, can be compared in terms of an applicable figure of merit (FOM), indicating theoretical limits [1]. The most commonly discussed FOM for power applications is the unipolar FOM [2], based on reduction of conduction losses in the on-state and high voltage blocking in the off-state of vertical devices, VB 2/Ron,sp = εμnEc 3/4, where VB is the off-state avalanche breakdown voltage, Ron,sp is the specific on-state resistance, ε is the semiconductor permittivity, μn is the electron mobility, and Ec is the critical electric field. The unipolar FOM scales as the cube of Ec , which is itself a strong function of the bandgap [3], leading to the key advantage for wide bandgap materials. Possessing bandgaps wider than GaN (3.4 eV), AlGaN alloys are expected to provide improved performance. We have investigated quasi-vertical Schottky barrier diodes, consisting of nominal Al0.8Ga0.2N drift layers grown on high quality AlN single crystal substrates. HexaTech has developed proprietary seeded AlN crystal growth technology based on physical vapor transport (PVT), allowing iterative expansion of single crystal size, while maintaining low dislocation density (3 cm-2), as determined by x-ray diffraction and defect-selective etching. AlN boules were oriented, sliced, lapped, and chemo-mechanically polished (CMP) to produce epi-ready c-plane substrates. Due to the highly insulating nature of the Al-face AlN substrates (resistivity >1013 Ωcm at room temperature), quasi-vertical Schottky diode structures were grown by metal-organic chemical vapor deposition (MOCVD). The device structure consisted of an AlN homoepitaxial layer, followed by a 0.6 µm thick pseudomorphic Si-doped Al0.8Ga0.2N contact layer ([Si] ~2x1019 cm-3), a 1 µm thick low-doped Al0.8Ga0.2N drift layer, and a 0.3 µm Mg-doped AlN cap layer ([Mg] ~2x1018 cm-3), serving as a junction termination extension (JTE). Despite the high activation energy of the Mg dopant in AlN, the Mg acceptors are expected to be ionized under high reverse bias, due to the strong band bending. Following epitaxial growth and dopant activation, circular device mesas were defined by dry etching down to the n-AlGaN contact layer, the JTE region atop the mesa was defined into a series of lateral steps and the Schottky contact area was etched down to the drift layer, Ohmic contacts were deposited and annealed, circular Schottky contacts were deposited, and a blanket polyimide passivation layer was deposited and patterned. Schottky diodes with diameters ranging from 0.3 to 1.0 mm were fabricated. Capacitance-voltage measurements were used to determine the free carrier concentration of the drift region, 2.0x1017 cm-3. The forward and reverse current-voltage characteristics were measured as a function of temperature. The specific on-state resistance was 125 mΩcm2 at room temperature and 190 mΩcm2 at 150 °C, which is partly attributed to high contact resistance. Diodes exhibited a current rectification ratio of ~106. The off-state breakdown voltage was up to 500 V. Breakdown tended to be destructive, likely limited by the polyimide passivation, suggesting improved blocking performance could be achieved with a better dielectric and more developed edge termination strategy. Field calculations based on the measured carrier concentration and breakdown voltage indicated a trapezoidal electric field in the drift region with a 7 MV/cm field at the Schottky junction. This talk will review growth and expansion of high quality AlN single crystals by PVT, and fabrication of epi-ready AlN substrates. Results for high Al composition Schottky diodes grown on these substrates will be presented. Challenges for this emerging technology, including doping limitations and compensation in AlGaN alloys, Ohmic contacts, and device termination will be discussed. [1] T. P. Chow, I. Omura, M. Higashiwaki, H. Kawarada, and V. Pala, IEEE Trans. Electron Devices, 64, 856 (2017). [2] K. Shenai, R. S. Scott, and B. J. Baliga, IEEE Trans. Electron Devices, 36, 1811 (1989). [3] J. L. Hudgins, G. S. Simin, E. Santi, and M. A. Khan, IEEE Trans. Power Electron., 18, 907 (2003).

With the advent of bulk single crystal AlN substrates, AlxGa1-xN epilayers can be grown pseudomorphically, under a compressive strain, with Al composition greater than 70%. Temperature dependent Hall effect measurements showed that Si dopant in AlxGa1-xN has an activation energy ranging from 10 to 25 meV for x < 0.8, while the activation energy increases rapidly for x ≥ 0.85. Planar Ni/Au Schottky diodes fabricated on AlxGa1-xN exhibit good diode rectification. Based on current-voltage (I-V) and capacitance-voltage (C-V) measurement, the effective Schottky barrier height was observed to monotonously increase with the increase of Al%. For a film with an n-type carrier concentration of 1x1018 cm-3 and Al content of 81%, the effective barrier height reached 1.8 eV with an ideality factor of 1.46. However, C-V consistently showed much higher barrier height than I-V, suggesting surface oxides play an important role in Ni/Au Schottky diode on AlGaN. The reverse breakdown voltage was also measured in a vertical structure without edge termination. The obtained high breakdown voltage exceeds the theoretical limit of 4H-SiC at a similar carrier concentration, suggesting great potentials for high power switching application of AlGaN on AlN. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental results on growth morphology in N-polar GaN grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) were analyzed using a thermodynamic supersaturation model for gallium. Smooth N-polar GaN films with 1 nm RMS roughness were always obtained under extremely low Ga supersaturation in the vapor, although the growth conditions were seemingly different. It was found that increasing H2 partial pressure during the GaN growth played a role in significantly lowering the Ga supersaturation, since H2 is a product in the formation of GaN. The degree of Ga supersaturation was also controlled by adjusting the partial pressure of Ga species in the vapor. The surface-diffusion theory dealing with step dynamics described by Burton, Cabrera, and Frank (BCF) was also used to relate Ga supersaturation to the observed smooth N-polar GaN growth. These findings showed that a supersaturation model encompassing all major MOCVD growth parameters can be used to predict the smooth N-polar GaN growth conditions. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The density and distribution of extended defects in AIN single crystals grown by physical vapor transport (PVT) was studied by bright field and polarized light microscopy, defect-selective etching, and high resolution X-ray diffraction (HRXRD). Etch pits associated with dislocation arrays forming low angle grain boundaries (LAGB) were observed and two types of LAGB were identified. Reduction of LAGB in AlN grown under reduced radial temperature gradients, as estimated by simulation of the growth cell, was demonstrated.

MOCVD AlN layers grown in c-direction on PVT bulk AlN substrates are investigated by high-resolution photoluminescence in the energy region of the band gap. The experiments allow for observation of different donor bound exciton lines with a full width at half maximum below 0.5 meV. Assignments are suggested for the substitutional shallow donors silicon and oxygen. The visibility of an excited state of the free exciton with a hole from the highest valence band allows to determine the exciton binding energy to 52.4 meV by use of a simple hydrogen model. The resulting band gap energy is 6.094 eV at 10 K. Comparison with earlier reports on homoepitaxial AlN leads to a new identification of the bands reported there.

As the building blocks of deep UV light emitting diode (LED) technology and high-power electronic devices, AlGaN alloys have attracted considerable attention. In this study, AlGaN films with varying compositions doped with Si were deposited on homoepitaxial AlN layers grown on AlN single crystal substrates. The room temperature resistivity of AlGaN alloys of different compositions grown on AlN and sapphire substrates with a constant Si doping level of 6x1018 cm-3 was compared. AlGaN films grown on AlN substrates consistently exhibited a lower n-type resistivity than those grown on sapphire. An n-type resistivity of 0.1 Ω cm was obtained for an AlGaN film with 80% Al content and a sheet resistance of 235 Ω/sq. for an AlGaN film with 70% Al content. The carrier activation energy as a function of Al content in AlGaN for these n-type films was measured. For compositions below 80% Al, the activation energy was around 15 meV due to impurity potential screening. For higher Al compositions, the carrier concentration was limited by a high compensation ratio, except for AlN, which has activation energy of 250 meV. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

© 2014 American Physical Society. We studied bulk crystals of wurtzite AlN by means of uniaxial pressure-dependent Raman measurements. As a result, we derive the phonon pressure coefficients and deformation potentials for all zone center optical phonon modes. For the A1 and E1 modes, we further experimentally determined the uniaxial pressure dependence of their longitudinal optical-transverse optical (LO-TO) splittings. Our experimental approach delivers new insight into the large variance among previously reported phonon deformation potentials, which are predominantly based on heteroepitaxial growth of AlN and the ball-on-ring technique. Additionally, the measured phonon pressure coefficients are compared to their theoretical counterparts obtained by density functional theory implemented in the siesta package. Generally, we observe a good agreement between the calculated and measured phonon pressure coefficients but some particular Raman modes exhibit significant discrepancies similar to the case of wurtzite GaN and ZnO, clearly motivating the presented uniaxial pressure-dependent Raman measurements on bulk AlN crystals.

The difference in surface energy between the Ga-polar orientation and the N-polar orientation of GaN translates into a completely different behavior for the incorporation of intentional and unintentional impurities. Oxygen is found to be an impurity with higher concentration in N-polar films than in Ga-polar films and is the cause for the high carrier concentration observed in N-polar films. We fabricated a lateral p/n junction in GaN by the simultaneous growth of p- and n-type regions, utilizing the doping selectivity of the two different polar domains, which resulted in an n-type carrier concentration of 1.7x1019 cm–3 with an electron hall mobility of 99 cm2/Vs in the N-polar domains, and a p-type carrier concentration of 1.1x1017 cm–3 with a hole hall mobility of 12 cm2/Vs in the Ga-polar domains. These junctions showed the characteristics that define a p/n junction: current rectification, electroluminescence, and photo-effect. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Despite considerable research in thin-film growth of wide-bandgap group III nitride semiconductors, substrate technology remains a critical issue for the improvement of nitride devices. With applications ranging from high-power electronics to optoelectronics, an increasing number of nitride semiconductor devices are becoming commercially available. Currently, many of these devices are being grown heteroepitaxially on nonnative substrates, leading to a high defect density in the active layers, which limits device performance and lifetime. Aluminum nitride (AlN) is considered a highly desirable candidate as a native substrate material for III-nitride epitaxy, especially for AlGaN devices with high Al concentrations. AlN crystals have been grown by a variety of methods. High-temperature growth of AlN bulk crystals by physical vapor transport (PVT) has emerged as the most promising growth technique to date for production of large, high-quality single crystals. This chapter reviews recent growth and characterization results of AlN bulk crystals grown by PVT and discusses several issues that remain to be addressed for continued development of this technology.

Sublimation growth of AlN was performed on Si-face, 8 ° off-axis 4H-SiC (0001) and 3.5 ° off-axis 6H-SiC (0001) seeds. AlN layers 500 - 900 μm thick and 20 mm in diameter were grown at 1830 °C by consecutive growths and continuous growth. The c-axis growth rate was approximately 8 - 18 μm/hr. On both the 8 ° and 3.5° off-axis SiC substrates, “step” features formed on the AlN surface with uniform terrace width and step density. The step heights and terrace widths increased as the AlN grew thicker. In addition, a single facet of 9 mm × 5 mm formed on the top of the layer grown on the 3.5 ° off-axis SiC. High resolution x-ray diffraction showed the AlN (00.2) 2θ value shifted from pure AlN toward SiC. Approximately 3 – 4 at% of SiC was detected at the surface of the AlN by XPS. Molten KOH/NaOH etching revealed that both samples had Al-polar surface with dislocation densities on the order of 106-107 cm−2. The cross-section etching showed the re-nucleation layer and void defects at the interfaces of the consecutive growths.

The A exciton feature at 6.025 eV is observed directly in the low temperature (1.8 K) reflectance spectra of AlN for the first time, using a bulk AlN single crystal which has a relatively flat m‐face that allows both σ and π configurations. Features involving unresolved B and C excitons are also observed around 6.25 eV from this sample and in three c‐oriented MOCVD AlN films. Transmission measurements on the thickest film reveal a clear but weak A excitonic absorption feature at 6.080 eV. The results are explained by theoretical calculations of exciton energies and oscillator strengths. The calculations suggest that AlGaN light‐emitting devices will emit light mainly in the π polarization for wavelengths shorter than 320 nm.

Growth and optical properties of high quality AlN single crystals grown by physical vapor transport using powder sublimation is presented. Crystallinity, incorporated impurities and their effects on the optical properties are evaluated for different crucible materials and growth environments. A significant reduction of impurity incorporation was achieved using a two‐step growth process in a TaN crucible. Crystal defects and the effect of incorporated impurities on the optical properties have been analyzed by their characteristic photoluminescence and absorption, Raman‐ and glow discharge mass spectroscopy.

For nitride based devices such as LEDs, high power FETs and laser diodes, single crystal substrates of AlN are highly desirable. While the sublimation technique is suitable for growing bulk AlN crystals, appropriate seeds are also necessary for growing large diameter oriented boules. 4H- and 6H-SiC substrates which are readily available commercially can potentially be implemented as seeds for bulk AlN growth. However, issues regarding SiC decomposition at high temperatures, thermal expansion mismatch, single crystal growth, etc. need to be addressed. Towards this end, a series of growth experiments have been carried out in a resistively heated reactor using on and off-axis 4H- and 6H-SiC substrates as seeds for AlN growth from the vapor phase. Several hundred microns thick AlN layers have been grown under different growth conditions. Synchrotron white beam x-ray topography (SWBXT) has been used to map the defect distribution in the grown layers and high resolution triple axis x-ray diffraction (HRTXD) experiments were carried out to record reciprocal space maps from which tilt, mismatch and strain data can be obtained. These results are analyzed with respect to the growth conditions in order to gain a better understanding of this growth process.

Growth of AlN bulk single crystals by sublimation of AlN powder was carried out in a resistively heated reactor at temperatures up to 2300°C. A variety of crucible materials, such as BN, W, Ta, Re, ZrO2, TaN, and TaC, were evaluated. Our studies have shown that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, thus influencing nucleation, coalescence, and crystal morphology. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction (XRD), x-ray topography (XRT), glow discharge mass spectrometry (GDMS), and secondary ion mass spectrometry (SIMS). Average dislocation densities were on the order of 103 cm−3, with extended areas virtually dislocation-free, while high-resolution XRD showed rocking curves as narrow as 7 arcsec.

Co-doping AlN crystals with Si is found to suppress the unwanted 4.7 eV (265 nm) deep ultraviolet absorption associated with isolated carbon acceptors common in materials grown by physical vapor transport. Density functional theory calculations with hybrid functionals demonstrate that silicon forms a stable nearest-neighbor defect complex with carbon. This complex is predicted to absorb at 5.5 eV and emit at or above 4.3 eV. Absorption and photoluminescence measurements of co-doped samples confirm the presence of the predicted CN-SiAl complex absorption and emission peaks and significant reduction of the 4.7 eV absorption. Other sources of deep ultraviolet absorption in AlN are also discussed.

AlN single crystals were grown by two different vapor phase processes: by vaporization of metallic Al in a nitrogen atmosphere, or by sublimation of an AlN source. Growth experiments were carried out under quasi-stagnant flow conditions, with typical flow rates of 100 sccm at reactor pressures ranging from 300 to 700 Torr. Growth temperatures ranged from 1800 to 2300°C. In Al vaporization experiments, the crystal shape and fastest growth direction was found to strongly depend on the growth temperature: at relatively low temperatures (1800-1900°C) long needles were grown, temperatures around 1900-2000°C yielded twinned platelets, while c-platelets were formed at temperatures above 2100°C. These c-plates grew at a rate of 5 mm/hr in the c-plane. When using AlN as a source material, growth rates were considerably slower, however, long-term stability of the Al flux was greatly improved. Seeded growth was demonstrated under these conditions. All grown single crystals were transparent and virtually colorless. Analytical results indicated very high crystalline quality and dislocation densities lower than 104cm-3.

A prominent 2.8 eV emission peak is identified in bulk AlN substrates grown by physical vapor transport. This peak is shown to be related to the carbon concentration in the samples. Density functional theory calculations predict that this emission is caused by a donor-acceptor pair (DAP) recombination between substitutional carbon on the nitrogen site and a nitrogen vacancy. Photoluminescence and photoluminescence-excitation spectroscopy are used to confirm the model and indicate the DAP character of the emission. The interaction between defects provides a pathway to creating ultraviolet-transparent AlN substrates for optoelectronics applications.

A high-resolution X-ray diffraction method with enhanced surface sensitivity has been used to investigate the effects of various polishing steps on the near-surface region of single crystal substrates. The method involves the study of a highly asymmetric reflection, observable under grazing incidence conditions. Analysis of rocking curve measurements and reciprocal space maps (RSMs) revealed subtle structural differences between the polished substrates. For aluminum nitride wafers, damage induced from diamond sawing and mechanical polishing was readily identifiable by on-axis rocking curves, but this method was unable to distinguish between sample surfaces subjected to various degrees of chemical mechanical polishing (CMP). To characterize sufficiently these surfaces, (10.3) RSMs were measured to provide both qualitative and quantitative information about the near-surface region. Two features present in the RSMs were utilized to quantitatively assess the polished wafers: the magnitude of the diffuse scatter in the omega-scans and the elongation of the crystal truncation rod. The method is able to distinguish between different degrees of CMP surface preparation and provides metrics to quantify subsurface damage after this polishing step.

AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were fabricated on AlN substrates. The AlN substrates were prepared by growing thick hydride vapor phase epitaxy (HVPE)-AlN layers on bulk AlN substrates prepared by physical vapor transport (PVT). After growing an LED structure, the PVT-AlN substrates were removed by mechanical polishing. This process allowed the fabrication of DUV-LEDs on HVPE-AlN substrates with high crystalline quality and DUV optical transparency. The DUV-LEDs exhibited a single emission peaking at 268 nm through the HVPE-AlN substrates. The output power as high as 28 mW was obtained at an injection current of 250 mA.

The structural and optical quality of a freestanding AlN substrate prepared from a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a bulk (0001)AlN substrate prepared by physical vapor transport (PVT) were investigated. The prepared HVPE-AlN substrate was crack- and stress-free. High-resolution X-ray diffraction ω-rocking curves of symmetric (0002) and skew-symmetric (1011) reflections had small full widths at half maximum (FWHMs) of 31 and 32 arcsec, respectively. Deep-ultraviolet optical transparency of the HVPE-AlN substrate was higher than that of the PVT-AlN substrate, which was related to lower concentrations of C, O impurities, and Al vacancy.

Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.

A characterization procedure was developed to determine the alloy composition and strain state of AlxGa1−xN/GaN(0

AlN single crystals were grown on m-plane (101¯0) and c-plane (0001¯) AlN seeds under identical growth conditions. The m-plane AlN crystals exhibited substantially lower oxygen incorporation, ∼1018cm−3, than the c-plane crystals, ∼1019cm−3. By investigating optical transmission spectra, m-plane AlN had absorption bands at 4.05 and 4.35eV, while c-plane AlN had an absorption band edge at 4.85eV. These below bandgap absorption bands strongly correlate with the reported transitions related to Al vacancy-impurity complexes, such as the complex of an Al vacancy and two oxygen atoms, (VAl–2ON)1− and the complex of an Al vacancy and one oxygen atom, (VAl–ON)2−, becoming the major cause for the poor, below bandgap optical transparency (α>200cm−1) of these crystals.

We have investigated the growth of AlN layers on bulk AlN substrates by modulated precursor flow epitaxial growth using metal-organic and hydride precursors as well as by conventional metal-organic chemical vapor deposition growth. A nanopit-containing morphology was observed for an AlN layer grown by conventional growth, while atomically smooth and pit-free surface was achieved for an AlN layer created by modulated precursor flow epitaxial growth. For similar growth set-point temperatures, nanopit-free surfaces were observed for AlN layers created by conventional growth on sapphire substrates. This is believed to be due to the difference in the temperature of the growing surface, as evidenced by finite element method thermal profiling and the morphology change of the AlN layer with decreasing temperature observed for growth of AlN on sapphire substrates. The AlN layers grown by modulated precursor flow epitaxial growth on the AlN bulk substrates also have excellent crystalline qualities with narrow x-ray...

A thermodynamic supersaturation model for gallium (Ga) was developed to describe GaN growth characteristics in low-pressure metalorganic chemical vapor deposition. The model takes into account the simplified GaN chemical reaction that occurs at the growth interface, Ga+NH3=GaN+3/2H2. The supersaturation was varied in two ways: (1) by the V/III ratio and (2) by the choice of the diluent gas. Two diluent gases were considered: H2, a commonly used diluent gas, and N2, a reaction inert gas. The choice of the diluent played a role in the degree of Ga supersaturation; since H2 is the product in the GaN formation, the addition of hydrogen significantly lowered the supersaturation. Atomic force microscopy revealed that surface morphology was associated with the different Ga supersaturation and the Burton–Cabrera–Frank model was used to relate it to the observed spiral size and terrace width. In addition to growth morphology, the degree of Ga supersaturation also influenced the carrier compensation level in n-type GaN. Secondary ion mass spectrometry studies identified that the trend of carbon incorporation followed that of the compensation level and inversely that of the Ga supersaturation. For samples intentionally doped with Si, it was found that GaN became semi-insulating when the carbon concentration exceeded that of silicon. In general, it has been shown that two growth processes at seemingly different conditions will be equivalent if the corresponding Ga supersaturation is the same. This finding showed that the supersaturation is a much more universal parameter than any other growth parameter alone.

Differences in surface energy between the Ga-polar orientation and the N-polar orientation of GaN translate into a completely different behavior for the incorporation of intentional and unintentional impurities. Oxygen is found to be an impurity with higher concentrations in N-polar films than in Ga-polar films and is the cause for the high n-type carrier concentration observed in N-polar films. We fabricated a lateral p∕n junction in GaN by the simultaneous growth of p- and n-type regions, utilizing the doping selectivity of the two different polar domains. The N-polar domains were n type with a carrier concentration of 1.7×1019cm−3, predominantly due to high oxygen incorporation, while Ga-polar domains were p type with a carrier concentration of 1.7×1017cm−3. No significant difference in Mg incorporation was observed between the two polar domains. This junction showed the characteristics that define a p∕n junction: current rectification, electroluminescence, and the photovoltaic effect.

The band-edge excitonic properties of AlN are investigated using low-temperature (1.7K) optical reflectance and transmission measurements of samples with various crystal orientations. The A, B, and C excitons are found to have energies of 6.025, 6.243, and 6.257eV in unstrained material, which shift with strain. The results are compared to a calculation of exciton energies and oscillator strengths to yield a crystal-field splitting of −230meV in unstrained AlN, in good agreement with previous ab initio calculations.

Bulk AlN crystal growth by Physical Vapor Transport (PVT) is studied both experimentally and numerically. This paper presents the analysis of heat and mass transport mechanisms in closed and partially open crucible geometries. The heat transfer in the growth system used at North Carolina State University (NCSU) is simulated. The computed temperature profiles are used in the analysis of mass transport in the growth cell to gain understanding of the effect of species exchange between the crucible and environment on the AlN growth rate. The model predictions are in reasonable agreement with observations.