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High temperature growth of AlN single crystals by physical vapor transport (PVT) was used to produce 2-inch diameter AlN substrates free of macrodefects and with average dislocation densities below 103 cm-2 [1, 2]. In spite of its high structural quality and uniformity, AlN grown by PVT has been characterized by high below-bandgap optical absorption in the ultraviolet C (UV-C) region, due to the presence of several deep-level, impurity-related absorption bands. UV-C light emitting diodes (LEDs) operating in the 260-280 nm range are being actively pursued for applications such as water purification and surface disinfection. These devices typically require light extraction through the substrate, for which high transparency in the emission wavelength range of interest is critical to high device performance. Adoption of AlN substrates for use in UV-C LEDs thus requires low optical absorption coefficients in the 260-280 nm emission range. However, several groups have identified an absorption band around 265 nm, which is associated with the carbon impurity, in these substrates [3]. Reducing this unwanted absorption band is critical to enhancing light extraction in UV-C LEDs grown on AlN substrates. In earlier work, co-doping with silicon was shown to reduce the unwanted 265 nm absorption band [4]. Formation of a carbon-silicon defect complex with an absorption band at 225 nm was demonstrated by optical absorption and photoluminescence measurements together with density functional theory calculations [4]. In this work, we have grown large diameter boules with varying concentrations of impurities in order to demonstrate 2-inch AlN substrates with low 265 nm absorption coefficients. The optical properties of double-side polished, c-plane substrates were measured using a high performance UV/Vis spectrophotometer (PerkinElmer Lambda 850). Transmittance and absolute reflectance spectra were collected using a 150 mm integrating sphere, and absorption coefficients were calculated with an approximation-free method [5] which accurately accounted for all reflectance losses. The absorption coefficients at 265 nm were correlated with impurity concentrations measured by secondary ion mass spectrometry (SIMS) of the three most prevalent electrically-active impurities, carbon, oxygen, and silicon, for various locations throughout the boule volume. Growth and characterization results of UV-C transparent 2-inch AlN substrates grown by PVT will be presented. References [1] R. Dalmau et al., Mater. Sci. Forum 924, 923 (2018). [2] R. Dalmau et al., ECS Trans. 92(7), 113-121 (2019). [3] R. Collazo et al., Appl. Phys. Lett. 100, 191914 (2012); K. Irmscher et al., J. Appl. Phys. 114, 123505 (2013); I. Gamov et al., J. Appl. Phys. 126, 215102 (2019). [4] B. Gaddy et al., Appl. Phys. Lett. 104, 202106 (2014). [5] D. C. Look and J. H. Leach, J. Vac. Sci. Technol. B 34(4), 04J105 (2016).

Deep-level luminescence at 3.3 eV related to the presence of Al vacancies (V Al ) was observed in room temperature photoluminescence (RT-PL) spectra of homoepitaxial AlN layers grown at 1450 °C by hydride vapor-phase epitaxy (HVPE) and cooled to RT in a mixture of H 2 and N 2 with added NH 3 . However, this luminescence disappeared after removing the near surface layer of AlN by polishing. In addition, the deep-level luminescence was not observed when the post-growth cooling of AlN was conducted without NH 3 . Secondary ion mass spectrometry (SIMS) studies revealed that although the point defect density of the interior of the AlN layers remained low, the near surface layer cooled in the presence of NH 3 was contaminated by Si impurities due to both suppression of the surface decomposition by the added NH 3 and volatilization of Si by decomposition of the quartz reactor walls at high temperatures. The deep-level luminescence reappeared after the polished AlN wafers were heated in presence of NH 3 at temperatures above 1400 °C. The surface contamination by Si is thought to generate V Al near the surface by lowering their formation energy due to the Fermi level effect, resulting in deep-level luminescence at 3.3 eV caused by the shallow donor (Si) to V Al transition.

Growth of Si-doped AlN layers by hydride vapor phase epitaxy on AlN/sapphire templates prepared by metalorganic vapor phase epitaxy and bulk AlN substrates prepared by physical vapor transport was investigated using silicon tetrachloride (SiCl4) as a doping gas. On the AlN/sapphire templates, when the SiCl4 supply was low, the incorporation ratio of Si decreased due to the influence of O impurities resulting from the decomposition of the sapphire. In addition, pits were formed on the surfaces, and the pit density increased significantly at Si concentrations exceeding 2 × 1019 cm-3. In contrast, on the bulk AlN substrates, the Si concentration increased linearly with increasing SiCl4 supply, and a pit- and stress-free layer could be grown with a Si concentration as high as 6.5 × 1019 cm-3. Hall effect measurements revealed that this layer exhibited n-type conductivity with a donor activation energy of 253 meV. However, the carrier density at room temperature (RT) was as low as 3.6 × 1013 cm-3 because of the high compensation ratio due to the presence of acceptors. A broad peak centered at 3.3 eV was observed in the RT photoluminescence spectra of the Si-doped AlN layers grown on the bulk AlN substrates, indicating that Al vacancies formed by the Fermi level effect due to Si doping acted as acceptors that compensated for carriers.

Aluminum nitride (AlN) layers doped intentionally with different concentrations of silicon atoms acting as shallow donors were grown by MOVPE on bulk c-plane AlN to minimize dislocations in the doped layers; typical values for the dislocation density in the bulk AlN substrates are less than 104 cm−2. The actual silicon concentration was confirmed by secondary ion mass spectroscopy (SIMS) analysis and ranging from 1017 to 1019 cm−3. In highly resolved low temperature photoluminescence (PL) investigations, we found dominating bandgap-related sharp emission lines with linewidth lower than 500 µeV for undoped samples. We tentatively assign silicon as the shallow donor causing the bound exciton line with 28.5 meV exciton localization energy, for which we found an intensity ratio relative to the free exciton line being linearly dependent on the silicon concentration. For increasing Si content, the respective emission band also broadened asymmetrically and shifted to lower energies as expected from the analogy with other semiconductor systems.

In this study, seeded sublimation growth of aluminum nitride (AlN) on SiC substrates was investigated. Large diameter (15–20 mm) and thick (1–2 mm) AlN layers were demonstrated on Si-face, 3.5° off-axis 6H-SiC (0 0 0 1). A c-axis growth rate of 15–20 μm/h was achieved at 1830 °C, and the surface morphology was highly textured: step features were formed with a single facet on the top of the layer. High-resolution X-ray diffraction (HRXRD), X-ray photoelectron spectroscopy (XPS), and molten KOH/NaOH etching were employed to characterize the AlN layers. The AlN crystals grew highly orientated along the c-axis, however, the impurities of Si (3–6 at%) and C (5.9–8 at%) from the SiC changed the lattice constants of AlN and shifted the AlN (0 0 .2) 2θ value from pure AlN toward SiC. All the growth surfaces had Al-polarity and the dislocation density decreased from 108 to 106 cm−2 as the film thickness increased from 30 μm to 2 mm.

Electrical characteristics of 1.3 µm thick GaN films were investigated with respect to the polar type. Unintentionally doped N-polar GaN grown on nitrided sapphire substrates had a hexagonal facetted morphology and consistently exhibited a high n-type conductivity with a sheet resistance of ∼10 Ω/sq, corresponding to a carrier concentration on the order of 1019 cm–3. In contrast, Ga-polar GaN films grown on low temperature AlN buffer layers showed a high sheet resistance of ∼109 Ω/sq. GaN films of mixed polarity exhibited sheet resistance in-between the two extreme values obtained for polarity pure films. The results of this work show that the existence of N-polar domains (even on the nano-scale) in an otherwise Ga-polar matrix significantly decreases the insulating properties of unintentionally doped GaN films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Oxidized surfaces of aluminum nitride (AlN) epilayers grown on sapphire substrates and of AlN bulk crystals grown by physical vapor transport were studied by x-ray photoelectron spectroscopy (XPS). Analysis of the oxygen core level spectra showed approximately equal contributions from oxygen in two bonding states, which were identified by the binding energies and relative separation of the fitted peaks as OH− and O2−. The oxide on air-exposed AlN surfaces was identified as aluminum oxide hydroxide. Systematic annealing experiments were conducted in vacuum to study the thermal evolution of hydroxide layers, and a dehydration mechanism resulting in the formation of Al2O3 at high temperature was identified.

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...

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.

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 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)

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.

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.

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.

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.

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

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.