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High p-conductivity (0.7 Ω−1 cm−1) was achieved in high-Al content AlGaN via Mg doping and compositional grading. A clear transition between the valence band and impurity band conduction mechanisms was observed. The transition temperature depended strongly on the compositional gradient and to some degree on the Mg doping level. A model is proposed to explain the role of the polarization field in enhancing the conductivity in Mg-doped graded AlGaN films and the transition between the two conduction types. This study offers a viable path to technologically useful p-conductivity in AlGaN.

We report here an optically pumped deep UV edge emitting laser with AlGaN multiple quantum wells (MQWs) active region grown on AlN substrate by low pressure organometallic vapor phase epitaxy (LP-OMVPE) in a high-temperature reactor. The 21 period Al0.53Ga0.47N/Al0.7Ga0.3N MQWs laser structure was optically pumped using 193 nm deep UV excimer laser source. A laser peak was achieved from the cleaved facets at 280.3 nm with linewidth of 0.08 nm at room temperature with threshold power density of 320 kW/cm2. The emission is completely TE polarized and the side mode suppression ratio (SMSR) is measured to be around 14 dB at 450 kW/cm2.

Record-low p-type resistivities of 9.7 and 37 Ω cm were achieved in Al0.7Ga0.3N and Al0.8Ga0.2N films, respectively, grown on single-crystal AlN substrate by metalorganic chemical vapor deposition. A two-band conduction model was introduced to explain the anomalous thermal behavior of resistivity and the Hall coefficient. Relatively heavy Mg doping (5 × 1019 cm−3), in conjunction with compensation control, enabled the formation of an impurity band exhibiting a shallow activation energy of ∼30 meV for a wide temperature range. Valence band conduction associated with a large Mg ionization energy was dominant above 500 K. The apparently anomalous results deviating from the classical semiconductor physics were attributed to fundamentally different Hall scattering factors for impurity and valence band conduction. This work demonstrates the utility of impurity band conduction to achieve technologically relevant p-type conductivity in Al-rich AlGaN.

We report a kV class, low ON-resistance, vertical GaN junction barrier Schottky (JBS) diode with selective-area p-regions formed via Mg implantation followed by high-temperature, ultra-high pressure (UHP) post-implantation activation anneal. The JBS has an ideality factor of 1.03, a turn-on voltage of 0.75 V, and a specific differential ON-resistance of 0.6 mΩ·cm2. The breakdown voltage of the JBS diode is 915 V, corresponding to a maximum electric field of 3.3 MV cm−1. These results underline that high-performance GaN JBS can be realized using Mg implantation and high-temperature UHP post-activation anneal.

A two-band transport model is proposed to explain electrical conduction in graded aluminum gallium nitride layers, where the free hole conduction in the valence band is favored at high temperatures and hopping conduction in the impurity band dominates at low temperatures. The model simultaneously explains the significantly lowered activation energy for p-type conduction (∼10 meV), a nearly constant sheet conductivity at lower temperatures (200–330 K), and the anomalous reversal of the Hall coefficient caused by the negative sign of the Hall scattering factor in the hopping conduction process. A comparison between the uniform and graded samples suggests that compositional grading significantly enhances the probability of phonon-assisted hopping transitions between the Mg atoms.

Compositionally graded AlxGa1-xN epilayers were coherently grown on AlN single crystal substrates by MOCVD, and polarization-induced doping was determined by contactless sheet resistance and capacitance-voltage measurements on unintentionally doped, n-type graded layers; and by temperature-dependent Hall effect measurements on a Mg doped, p-type graded layer. The room-temperature resistivity, hole concentration, and hole mobility of the p-type layer graded from x= 1.0 to x= 0.36 were 1.2 Ωcm, 4.5x1018 cm-3, and 1.2 cm2/Vs, respectively, which represents a significant improvement over traditional, thermally-activated Mg doping in AlGaN alloys.

Aluminum nitride (AlN) single crystal boules were grown by physical vapor transport (PVT). Diameter expansion during boule growth, without the introduction of low angle grain boundaries (LAGB) around the boule periphery, was confirmed by crossed polarizer imaging, synchrotron white beam x-ray topography (SWBXT), and synchrotron monochromatic beam x-ray topography (SMBXT). The densities of basal plane dislocations (BPD) and threading edge dislocations (TED) averaged from high-magnification topographs of five regions of a high-quality substrate were 0 cm-2 and 992 cm-2, respectively. Substrates fabricated from AlN boules possessed excellent surface finishes suitable for epitaxy.

In this study, the Raman biaxial stress coefficients KII and strain-free phonon frequencies ω0 have been determined for the E2 (low), E2 (high), and A1 (LO) phonon modes of aluminum nitride, AlN, using both experimental and theoretical approaches. The E2 (high) mode of AlN is recommended for the residual stress analysis of AlN due to its high sensitivity and the largest signal-to-noise ratio among the studied modes. The E2 (high) Raman biaxial stress coefficient of −3.8 cm−1/GPa and strain-free phonon frequency of 656.68 cm−1 were then applied to perform both macroscopic and microscopic stress mappings. For macroscopic stress evaluation, the spatial variation of residual stress was measured across an AlN-on-Si wafer prepared by sputter deposition. A cross-wafer variation in residual stress of ∼150 MPa was observed regardless of the average stress state of the film. Microscopic stress evaluation was performed on AlN piezoelectric micromachined ultrasonic transducers (pMUTs) with submicrometer spatial resolution. These measurements were used to assess the effect of device fabrication on residual stress distribution in an individual pMUT and the effect of residual stress on the resonance frequency. At ∼20 μm directly outside the outer edge of the pMUT electrode, a large lateral spatial variation in residual stress of ∼100 MPa was measured, highlighting the impact of metallization structures on residual stress in the AlN film.

Understanding the band bending at the interface of GaN/dielectric under different surface treatment conditions is critically important for device design, device performance, and device reliability. The effects of ultraviolet/ozone (UV/O3) treatment of the GaN surface on the energy band bending of atomic-layer-deposition (ALD) Al2O3 coated Ga-polar GaN were studied. The UV/O3 treatment and post-ALD anneal can be used to effectively vary the band bending, the valence band offset, conduction band offset, and the interface dipole at the Al2O3/GaN interfaces. The UV/O3 treatment increases the surface energy of the Ga-polar GaN, improves the uniformity of Al2O3 deposition, and changes the amount of trapped charges in the ALD layer. The positively charged surface states formed by the UV/O3 treatment-induced surface factors externally screen the effect of polarization charges in the GaN, in effect, determining the eventual energy band bending at the Al2O3/GaN interfaces. An optimal UV/O3 treatment condition also ...

We demonstrate Al rich AlGaN based APDs grown on AlN substrates capable of high sensitivity at room temperature with ambient lighting rejection showcasing the advantage over Si and Ge based detectors. APDs are operated in linear gain region with maximum gain exceeding 1100.

The interaction between the itinerant carriers, lattice dynamics, and defects is a problem of long-standing fundamental interest for developing quantum theory of transport. Here, we study this interaction in the compositionally and strain-graded AlGaN heterostructures grown on AlN substrates. The results provide direct evidence that a dimensional crossover (2D–3D) occurs with increasing temperature as the dephasing scattering events reduce the coherence length. These heterostructures show a robust polarization-induced 3D electron gas and a metallic-like behavior down to liquid helium temperature. Using magnetoresistance measurements, we analyze the evolution of the interaction between charge carriers, lattice dynamics, and defects as a function of temperature. A negative longitudinal magnetoresistance emerges at low temperatures, in line with the theory of weak localization. A weak localization fit to near zero-field magneto-conductance indicates a coherence length that is larger than the elastic mean free path and film thickness ( l φ > t > l e l), suggesting a 2D weak localization in a three-dimensional electron gas. Our observations allow for a clear and detailed picture of two distinct localization mechanisms that affect carrier transport at low temperature.

We demonstrate large area (25 000 μm2) Al-rich AlGaN-based avalanche photodiodes (APDs) grown on single crystal AlN substrates operating with differential (the difference in photocurrent and dark current) signal gain of 100 000 at 90 pW ( 280 nm, establishing solar blindness of the device.

The influence of the polarization field on the emission properties of the AlGaN-based quantum structures grown on AlN substrates was investigated as a function of well width, barrier width, and barrier height. A thin AlGaN well and a thin AlN barrier design reduced the polarization field to ∼0.5 MV/cm, resulting in an ultralow laser threshold of 3 kW/cm2 in an optically pumped configuration. These experimental results were used to validate the simulation. In the next step, a structure with Al0.7Ga0.3N barriers was designed to support carrier injection with a minimal loss in optical performance. This structure showed a threshold of 7 kW/cm2 under optical pumping and an estimated threshold current of 8 kA/cm2 for the electric injection.

Wide bandgap nitride semiconductors have attracted interest for next-generation devices in applications ranging from optoelectronics to power electronics. Device heterostructures are commonly grown on thermally- and lattice-mismatched substrates, which results in a large density of defects, incurring performance and lifetime limitations. Substrate technology remains a critical issue for the improvement of nitride devices. Performance gains are enabled by lattice-matched, native substrates, which are experiencing steadily increasing demand. Aluminum nitride (AlN) possesses material properties, such as a wide, direct bandgap (6.1 eV), a close lattice match with high Al composition AlGaN epilayers, high thermal conductivity, and a high critical electric field, that make it an excellent substrate for UV optoelectronics and power electronics.

Understanding the band bending at the interface of GaN/dielectric under different surface treatment conditions is critically important for device design, device performance, and device reliability. The effects of ultraviolet/ozone (UV/O

Threading dislocations in aluminum nitride boules grown by physical vapor transport method were systematically studied via synchrotron x-ray topography (white beam and monochromatic) in conjunction with ray tracing simulations. Two major types of threading dislocations were observed in the c-axis-grown boules: threading screw dislocations (TSDs) and threading edge dislocations (TEDs) with Burgers vectors along the [0001] and \(\langle 11\bar{2}0 \rangle \) directions, respectively. TSDs were typically observed in the middle of the boule while TEDs were commonly observed to aggregate into arrays along the \( \langle 1\bar{1}00 \rangle \) and \( \langle 11\bar{2}0 \rangle \) directions in various parts of the boule on basal plane oriented wafers. By comparison with ray tracing simulations, the absolute Burgers vectors of both TSDs and TEDs in the arrays could be unambiguously determined. TEDs comprise over 90 % of all threading dislocations observed. The relationships between TED arrays and low angle grain boundaries and their possible formation mechanisms are discussed.

As development of GaN and SiC material systems for applications in power switching progresses, the ultrawide-bandgap (UWBG) materials, with bandgaps greater than GaN, are gaining increased attention, in pursuit of improved power device performance. Substrate technology is critical to improve this field. High-quality native substrates, possessing low lattice and thermal expansion mismatches to epitaxial active layers, enable epitaxial growth with low densities of extended defects, leading to improved device performance and lifetime. Lateral and vertical power switching devices have been demonstrated in the emerging AlGaN alloy system [1]. Aluminum nitride (AlN) possesses a close lattice match to high Al mole fraction AlGaN alloys and a high thermal conductivity, which make it an excellent substrate for UWBG AlGaN-based devices. Physical vapor transport (PVT) growth has emerged as the most promising technique for the production of large, high-quality AlN single crystal substrates. Seeded growth technology developed by HexaTech enables iterative expansion of single crystal size, while suppressing low angle grain boundary (LAGB) formation, a primary source of grown-in dislocations. X-ray diffraction (XRD) is well suited to accurately and non-destructively probe the extended defect structure, composition, and strain of single crystals and epitaxial films. A variety of x-ray techniques were used throughout AlN substrate processing, from evaluation of as-grown crystals, through substrate polishing, to analysis of epitaxial films grown on AlN. High-resolution x-ray rocking curves (HRXRRC) and x-ray topography (XRT) were used to determine the primary dislocation types in AlN bulk crystals and their likely formation mechanisms [2, 3]. Synchrotron-based XRT images demonstrated that the expanded diameter of freestanding AlN boules contained low densities of threading dislocations (4 cm-3), and no basal plane dislocations or LAGBs [4]. Furthermore, XRT images acquired on a commercial diffractometer were used for quality control of AlN seed crystals. The advent of a new generation of x-ray area detectors, capable of a wide dynamic range and rapid data acquisition, opened the possibility for using XRT in production. HRXRRC and surface-sensitive reciprocal space maps were used to monitor sub-surface damage removal during substrate polishing [5]. These results, together with atomic force microscopy and defect selective etching images, demonstrated that chemo-mechanically polished AlN surfaces were free of plastic damage [4]. Polished AlN surfaces were also studied by x-ray reflectivity, yielding long-range data about surface roughness and density. Finally, epitaxial AlGaN films and heterostructures deposited on AlN substrates were studied by a combination of these x-ray techniques, providing essential information on composition, thickness, relaxation, and heterostructure design. XRD has become a valuable metrology tool in the development of AlN technology. This talk will review the implementation of complementary x-ray based measurements in the processing of AlN, from boule growth to AlGaN-based devices. References [1] R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. R. Dickerson, A. J. Fischer, A. G. Baca, and E. A. Douglas, ECS J. Solid State Sci. and Technol., 6(2), Q3061 (2017). [2] R. Dalmau, B. Moody, J. Xie, R. Collazo, and Z. Sitar, Phys. Status Solidi (c), 208, 1545 (2011). [3] B. Raghothamachar, Y. Yang, R. Dalmau, B. Moody, S. Craft, R. Schlesser, M. Dudley, and Z. Sitar, Mater. Sci. Forum, 740-742, 91 (2013). [4] R. Dalmau, H. S. Craft, J. Britt, E. Paisley, B. Moody, J. Guo, Y. Ji, B. Raghothamachar, M. Dudley, and Raoul Schlesser, Mater. Sci. Forum, 924, 923 (2018). [5] M. Bobea, J. Tweedie, I. Bryan, Z. Bryan, A. Rice, R. Dalmau, J. Xie, R. Collazo, and Z. Sitar, J. Appl. Phys., 113, 123508 (2013).

We investigated two types of threading dislocations in high Al-composition Al0.55Ga0.45N/AlN multiple quantum well (MQW) structures grown on AlN substrate for electrically injected deep ultraviolet light-emitting diodes (LEDs). The surface morphology and defects electrical characteristics of the MQW LED structures were examined via conductive atomic force microscopy (CAFM). We found that the disparity between photoluminescence (PL) and electroluminescence (EL) spectra in terms of light emission output and wavelength shift are attributed to the existence of the surface hillocks, especially to the ones that have open-core dislocations. The open-core dislocations form current leakage paths through their defect states and the composition inhomogeneity (i.e., Ga rich) at the dislocation sites are responsible for the light emission at longer wavelengths.

A systematic study on the density and distribution of extended defects in a typical single crystal AlN boule grown by the physical vapor transport (PVT) method has been carried out in order to gain a detailed understanding of the formation of defects such as dislocations and low angle grain boundaries (LAGBs). Boule surface studies reveal that LAGBs are nucleated during initial stages of growth and propagate to the end of growth. Basal plane dislocations (BPDs) are generated during growth due to thermal gradient stresses. Higher BPD densities are found near the LAGBs at the boule edges due to additional stresses from constrained growth. Threading edge dislocations (TEDs) are typically replicated from the seed, and LAGBs composed of arrays of threading dislocation walls are formed to accommodate the c-axis rotation between different groups of threading screw dislocation (TSD) mediated growth centers.

Sub-300 nm optically pumped ultraviolet lasers were realized on low-defect-density (0001) AlN substrates fabricated from single crystalline AlN boules. The AlxGa1–xN/AlyGa1–yN hetero-structures were grown by metal-organic vapor phase epitaxy near atmospheric pressure. The high structural quality of the pseudomorphically deposited films was confirmed by X-ray reciprocal space mappings and time-resolved photoluminescence (PL) studies of the multiple quantum well emission. The initial PL-decay times for a sample emitting at 267 nm were 0.87 ns and 1.14 ns for T = 295 K and T = 14 K, respectively. Laser resonators with a length of about 1 mm were formed by cleaving the AlN crystal to obtain m-plane mirror facets. Lasing was demonstrated for various wavelengths between 267 and 291 nm with the threshold power density as low as 126 kW/cm2 for the shortest attempted wavelength. The laser emission was TE polarized for all emission wavelengths. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.

Deep ultraviolet (UV) light-emitting diodes (LEDs) at a wavelength of 226 nm based on AlGaN/AlN multiple quantum wells using p-type Si as both the hole supplier and the reflective layer are demonstrated. In addition to the description of the hole transport mechanism that allows hole injection from p-type Si into the wide bandgap device, the details of the LED structure which take advantage of the p-type Si layer as a reflective layer to enhance light extraction efficiency (LEE) are elaborated. Fabricated LEDs were characterized both electrically and optically. Owing to the efficient hole injection and enhanced LEE using the p-type Si nanomembranes (NMs), an optical output power of 225 μW was observed at 20 mA continuous current operation (equivalent current density of 15 A/cm2) without external thermal management. The corresponding external quantum efficiency is 0.2%, higher than any UV LEDs with emission wavelength below 230 nm in the continuous current drive mode. The study demonstrates that adopting p-type Si NMs as both the hole injector and the reflective mirror can enable high-performance UV LEDs with emission wavelengths, output power levels, and efficiencies that were previously inaccessible using conventional p-i-n structures.

In recent years, InGaN-based LED's emitting in the blue and green spectral regions have emerged into widespread use. However, due to fundamental material issues, achieving yellow and red emission from InGaN-based structures has presented a unique challenge. We have developed conditions for the MOCVD growth of a series of InGaN-based MQW devices that emit over a wide spectral range (420nm - 600nm). It has been found that the emission properties of these monochromatic devices depend critically on the growth rate, growth temperature, QW width, barrier growth conditions, and p-type capping layer growth conditions. Additionally, we have demonstrated white electroluminescence through the proper mixing of complementary colors.

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).

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.

The recombination dynamics of optical transitions as well as strain effects in AlInGaN/In0.08Ga0.92N quantum wells (QWs) were studied. QW emission energy, photoluminescence decay behavior, photoluminescence emission line shape, and nonradiative recombination behavior were found to be strong functions of strain as well as localization. The degree of carrier localization was inferred by modeling several aspects of optical behavior obtained from variable temperature time-resolved photoluminescence experiments. According to the modeling results, the degree of localization was found to be a minimum for unstrained QWs and increased as either tensile or compressive strain increased, indicating that InGaN QW microstructure is a function of the lattice-mismatch-induced strain experienced during deposition.

The effect of hydrogen on the incorporation of nitrogen in GaAs1−yNy grown by metalorganic chemical vapor deposition (MOCVD) is reported. Nitrogen content as high as y=0.081 has been achieved when the use of H2 is completely avoided in the MOCVD growth of GaAs1−yNy. When H2 is added to the growth ambient, the value of y in GaAs1−yNy decreases as the relative percent of H2 in the carrier gas increases. We will report on the properties of these GaAsN films and discuss the nature of the effect that H2 has on modulating the N content in these films.

We report on the effects of the piezoelectric field and well width on the transition energy and intensity for InGaN quantum well structures with GaN or AlInGaN quaternary barriers. It was found that the emission energy of compressively strained GaN/In0.08Ga0.92N quantum wells exhibits a strong well width dependence not accounted for by quantum confinement subband energy shifting alone. However, for unstrained quantum well layers with quaternary barriers, no emission energy dependence on width was observed due to the elimination of the piezoelectric field, which was measured to be at least 0.6 MV/cm for the strained quantum wells. Furthermore, the unstrained quantum wells demonstrated a higher intensity than their strained counterparts for all quantum well widths investigated. The current data will help clarify the origin of emission in InGaN quantum wells.

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.

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.

The reliability and output power of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) fabricated on AlN substrates prepared by hydride vapor phase epitaxy are reported. TEM analysis revealed that dislocation density in LED layers, except the p-GaN layer, was below 106 cm-2. DUV-LEDs emitting at 261 nm exhibited an output power of 10.8 mW at 150 mA. The lifetime of these LEDs was estimated to be over 10,000 h for cw operation at 50 mA. No significant acceleration of output power decay at higher operation currents was observed. The estimated lifetime at the operation current of 150 mA was over 5,000 h.

To unambiguously distinguish lasing from super luminescence, key elements of lasing such as longitudinal cavity modes with narrow line-width, polarized emission, and elliptically shaped far-field pattern, need to be demonstrated at the same time. Here, we show transverse electric polarized lasing at 280.8 nm and 263.9 nm for AlGaN based multi-quantum-wells and double heterojunction structures fabricated on single crystalline AlN substrates. An elliptically shaped far-field pattern was recorded when pumped above threshold. With cavities shorter than 200 μm, well-defined, equally spaced longitudinal modes with line widths as narrow as 0.014 nm were observed. The low threshold pumping density of 84 kW/cm2 suggests that the electrically pumped sub-300 nm ultraviolet laser diodes are imminent.

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.

The authors present optical and electrical data for long wavelength (573–601nm) InGaN∕GaN multiple quantum well light emitting diodes (LEDs) grown by metal organic chemical vapor deposition. These results are achieved by optimizing the active layer growth temperature and the quantum well width. Also, the p-GaN is grown at low temperature to avoid the disintegration of the InGaN quantum wells with high InN content. A redshift is observed for both the green and yellow LEDs upon decreasing the injection current at low current regime. In the case of the yellow LED, this shift is enough to push emission into the amber (601nm).

We report here high-reflectivity multilayer DBR mirrors in 200–240 nm spectral region. YSZ (yttria-stabilized zirconia), HfO 2 and SiO 2 films were all prepared by direct RF magnetron sputtering. The film annealing conditions were systematically varied to obtain layers with minimal optical absorption. Combining these results, high reflectivity YSZ/SiO 2 and HfO 2 /SiO 2 DBRs were designed and realized with reflectance greater than 99.9%.