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A review of metal rich and nitrogen rich (N-rich), low-temperature grown In x Ga 1−x N is provided, focusing on two low-temperature approaches that have resulted in non-phase separated In x Ga 1−x N. The metal modulated epitaxy (MME) and N-rich, low temperature approaches to the reduction of defects in In x Ga 1−x N are described and are capable of growing In x Ga 1−x N throughout the miscibility gap. MME films remain smooth at all thicknesses but show device quality material primarily for x 0.6. Low temperature, N-rich grown films show a critical thickness extend well beyond the theoretical values and results in slower relaxation through the 0.2 x Ga 1−x N substrates are necessary to increase design freedom, as well as improve optoelectronic device performance. Initial results with films up to 800 nm are shown to display evidence of defect annihilation which could be promising for future thick optoelectronic templates and thick devices.

Using metalorganic vapor phase epitaxy, a comprehensive study of B x Ga 1−x N growth on GaN and AlN templates is described. BGaN growth at high-temperature and high-pressure results in rough surfaces and poor boron incorporation efficiency, while growth at low-temperature and low-pressure (750–900 °C and 20 Torr) using nitrogen carrier gas results in improved surface morphology and boron incorporation up to ~7.4% as determined by nuclear reaction analysis. However, further structural analysis by transmission electron microscopy and x-ray pole figures points to severe degradation of the high boron composition films, into a twinned cubic structure with a high density of stacking faults and little or no room temperature photoluminescence emission. Films with 1% TEB flow, the crystal structure becomes dominated by the cubic phase. Only when the TEB flow is zero (pure GaN), does the cubic phase entirely disappear from the x-ray pole figure, suggesting that under these growth conditions even very low boron compositions lead to mixed crystalline phases.

Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density ( $10^{4}-10^{5}$ cm $^{-2}$ ) GaN substrate, and has a 15- $\mu \text{m}$ -thick n-type drift layer with a free carrier concentration of $5\times 10^{15}$ cm $^{-3}$ . The ET structure is formed by $N$ implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p) layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. Simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.

Electrical performance and defect characterization of vertical GaN P-i-N diodes before and after irradiation with 2.5 MeV protons and neutrons is investigated. Devices exhibit increase in specific on-resistance following irradiation with protons and neutrons, indicating displacement damage introduces defects into the p-GaN and n- drift regions of the device that impact on-state device performance. The breakdown voltage of these devices, initially above 1700 V, is observed to decrease only slightly for particle fluence

The AlGaN materials system offers a tunable, ultra-wide bandgap that is exceptionally useful for high-power electronics and deep ultraviolet optoelectronics. Moseley et al. (pp. 723–726) investigate a structural defect known as an open-core threading dislocation or ''nanopipe'' that is particularly detrimental to devices that employ these materials. Furthermore, an AlGaN thin film was synthesized using metal-organic chemical-vapor deposition. Electrical current leakage is detected at a discrete point using a conductive atomic-force microscope (CAFM). However, no physical feature or abnormality at this location was visible by an optical microscope. The AlGaN thin film was then etched in hot phosphoric acid, and the same location that was previously analyzed was revisited with the CAFM. The point that previously exhibited electrical current leakage had been decorated with a 1.1 μm wide hexagonal pit, which identified the site of electrical current leakage as a nanopipe and allows these defects to be easily observed by optical microscopy. Moreover, with this nanopipe identification and quantification strategy, the authors were able to correlate decreasing ultraviolet light-emitting diode optical output power with increasing nanopipe density.

Demonstration of Al0.3Ga0.7N PN diodes grown with breakdown voltages in excess of 1600 V is reported. The total epilayer thickness is 9.1 μm and was grown by metal-organic vapour-phase epitaxy on 1.3-mm-thick sapphire in order to achieve crack-free structures. A junction termination edge structure was employed to control the lateral electric fields. A current density of 3.5 kA/cm2 was achieved under DC forward bias and a reverse leakage current

In this work, we investigate the use of nanoscale polarization engineering to achieve efficient hole injection from metals to ultra-wide band gap AlGaN, and we show that UV-reflective aluminum (Al) layers can be used for hole injection into p-AlGaN. The dependence of tunneling on the work function of the metal was investigated, and it was found that highly reflective Al metal layers can enable efficient hole injection into p-AlGaN, despite the relatively low work function of Al. Efficient tunneling hole injection was confirmed by light emission at 326 nm with on-wafer peak external quantum efficiency and wall-plug efficiency of 2.65% and 1.55%, respectively. A high power density of 83.7 W/cm2 was measured at 1200 kA/cm2. The metal/semiconductor tunnel junction structure demonstrated here could provide significant advantages for efficient and manufacturable device topologies for high power UV emitters.

We present the design, realization, and characterization of optical strong light–matter coupling between intersubband transitions within a semiconductor heterostructures and planar metamaterials in the near-infrared spectral range. The strong light–matter coupling entity consists of a III-nitride intersubband superlattice heterostructure, providing a two-level system with a transition energy of ∼0.8 eV (λ ∼1.55 μm) and a planar “dogbone” metamaterial structure. As the bare metamaterial resonance frequency is varied across the intersubband resonance, a clear anticrossing behavior is observed in the frequency domain. This strongly coupled entity could enable the realization of electrically tunable optical filters, a new class of efficient nonlinear optical materials, or intersubband-based light-emitting diodes.

Indium gallium nitride (InGaN) alloys exhibit substantial potential for high-efficiency photovoltaics. However, theoretical promise still needs to be experimentally realized. This paper presents a detailed theoretical study to provide guidelines to achieve high-efficiency InGaN solar cells. While the efficiency of heterojunction devices is limited to ~11%, homojunction devices can achieve suitable efficiencies, provided that highly p-type-doped InGaN layers and thick, single-phase InGaN films can be grown. Thus, we have developed a novel growth technology that facilitates growth of p-type nitride films with greatly improved hole concentration and growth of InGaN without phase separation, offering promise for future high-efficiency InGaN solar cells.

Analog LiNbO2 memristors are characterized using potentiodynamic electrochemical impedance spectroscopy. It is shown that LiNbO2-based devices exhibit analog memristive and meminductive or memcapacitive behavior depending on the applied frequency. The impedance spectra are fit to a circuit model whose elements correspond to ionic and electronic effects in the bulk and at the electrode-semiconductor interface. By separating out the bulk and interfacial effects, it is shown that the majority of the ionic movement in the cation-based analog memristor is at the interfaces. It is also shown that ionic capacitance results from a drift field-driven phase shift between the ac bias and the ionic motion, whereas the ionic inductance phase shift results from diffusion relaxation. Thus, to maximize resistance changes due to this ionic movement, analog memristors should be fabricated with negligible bulk dimensions.

Ultra violet light emitting diodes (UV LEDs) face critical limitations in both the injection efficiency and light extraction efficiency due to the resistive and absorbing p-type contact layers. In this work, we investigate the design and application of polarization engineered tunnel junctions for ultra-wide bandgap AlGaN (Al mole fraction higher than 50%) materials towards highly efficient UV LEDs. We demonstrate that polarization-induced 3D charge is beneficial in reducing tunneling barriers especially for high composition AlGaN tunnel junctions. The design of graded tunnel junction structures could lead to low tunneling resistance below 10-3 Ohm cm2 and low voltage consumption below 1 V (at 1 kA/cm2) for high composition AlGaN tunnel junctions. Experimental demonstration of 292 nm emission was achieved through non-equilibrium hole injection into wide bandgap materials with bandgap energy larger than 4.7 eV, and detailed modeling of tunnel junctions shows that they can be engineered to have low resistance, and can enable efficient emitters in the UV-C wavelength range., Comment: 7 pages, 7 figures

We discuss the engineering of p-AlGaN cladding layers for achieving efficient tunnel-injected III-Nitride ultraviolet light emitting diodes (UV LEDs) in the UV-A spectral range. We show that capacitance-voltage measurements can be used to estimate the compensation and doping in p-AlGaN layers located between the multi-quantum well region and the tunnel junction layer. By increasing the p-type doping concentration to overcome the background compensation, on-wafer external quantum efficiency and wall-plug efficiency of 3.37% and 1.62% were achieved for tunnel-injected UV LEDs emitting at 325 nm. We also show that interband tunneling hole injection can be used to realize UV LEDs without any acceptor doping. The work discussed here provides new understanding of hole doping and transport in AlGaN-based UV LEDs, and demonstrates the excellent performance of tunnel-injected LEDs for the UV-A wavelength range., Comment: 9 pages, 5 figures

We report on the high efficiency tunnel-injected ultraviolet light emitting diodes (UV LEDs) emitting at 287 nm. Deep UV LED performance has been limited by the severe internal light absorption in the p-type contact layers and low electrical injection efficiency due to poor p-type conduction. In this work, a polarization engineered Al0.65Ga0.35N/In0.2Ga0.8N tunnel junction layer is adopted for non-equilibrium hole injection to replace the conventionally used direct p-type contact. A reverse-graded AlGaN contact layer is further introduced to realize a low resistance contact to the top n-AlGaN layer. This led to the demonstration of a low tunnel junction resistance of 1.9 × 10−3 Ω cm2 obtained at 1 kA/cm2. Light emission at 287 nm with an on-wafer peak external quantum efficiency of 2.8% and a wall-plug efficiency of 1.1% was achieved. The measured power density at 1 kA/cm2 was 54.4 W/cm2, confirming the efficient hole injection through interband tunneling. With the benefits of the minimized internal absor...

We have investigated both elastically and plastically deformed GaN films on lithium gallate, LiGaO2, by molecular beam epitaxy. The in-plane lattice parameters were determined from high resolution X-ray diffraction and indicated two different groups of in-plane lattice parameters, influenced by the a- and b-axis of LiGaO2. The measured in-plane lattice parameters indicate that there exist both compressive and tensile strains of in-plane GaN along the a- and b-axis of LiGaO2, respectively. This anisotropic strain in GaN films forms a slight distortion of the basal-plane hexagonal structure of GaN films, leading to a different critical thickness of 4.0 ± 0.17 and 7.8 ± 0.7 nm along the a- and b-axis of LiGaO2, respectively.

Realization of efficient laser diodes with ultra-violet (UV) emission from ∼260–360 nm would enable many applications including fluorescence-based biological agent detection, sterilization, and portable water purification. While InGaN-based laser diodes are well developed down to ∼370 nm, achieving shorter UV wavelengths requires higher Al-content AlGaN alloys with increasing challenges in achieving p-type doping, strain-management, and low threading-dislocation-density (TDD) AlGaN templates. Given these challenges, few groups have reported AlGaN-based edge-emitting laser diodes (LDs) with emission < 355 nm.[1, 2] Most recently, random lasing via Anderson localization in AlGaN nanowire structures has demonstrated a novel approach to realizing deep-UV laser diodes.[3]

We report on the first demonstration of tunnel-injection-based III-Nitride ultraviolet light emitting diodes (LEDs) with emission wavelength lower than 300 nm. The III-Nitride system could enable a new class of solid state ultraviolet sources that could surpass the efficiency and cost of traditional gas-based ultraviolet bulbs. However, III-Nitride based ultraviolet emitters have significantly lower efficiency (10–100x lower) than visible wavelength emitters, even though the internal quantum efficiency (IQE) of AlGaN emitters is high. The main reason for the lower efficiency in these devices is the high acceptor activation energy, poor hole availability, and high p-contact resistance in p-type AlGaN. Interband tunnel junctions (TJ) can overcome these fundamental limitations by non-equilbrium injection of holes into the LED active region, offering an approach to increase both the electrical injection as well as light extraction efficiency. However, interband PN tunnel junctions become increasingly more resistive as the band gap is increased. We show here that polarization engineering can enable non-equilibrium hole injection into wide bandgap AlGaN with bandgap larger than 4.7 eV (Al composition > 50%), enabling UV LEDs at a wavelength of 295 nm, with a record low differential resistance of 1.6 × 10−3 Ωcm2 at a current density of 1 kA/cm2.

Ultra-violet emitters have several applications in the areas of sensing, water purification, and data storage. While the III-Nitride semiconductor system has the band gap region necessary for ultraviolet emission, achieving efficient ultraviolet solid state emitters remains a challenge due to the low p-type conductivity and high contact resistance in wide band gap AlGaN-based ultra-violet light emitters. In this work, we show that efficient interband tunneling can be used for non-equilibrium injection of holes into ultraviolet emitters. Polarization-engineered tunnel junctions were used to enhance tunneling probability by several orders of magnitude over a PN homojunction, leading to highly efficient tunnel injection of holes to ultraviolet light emitters. This demonstration of efficient interband tunneling introduces a new paradigm for design of ultra-violet light emitting diodes and diode lasers, and enables higher efficiency and lower cost ultra-violet emitters., Comment: 13 pages, 7 figures, Submitted

We report on the realization of a GaN high voltage vertical p-n diode operating at > 3.9 kV breakdown with a specific on-resistance 1.4 kA/cm2. An effective critical electric field of 3.9 MV/cm was estimated for the devices from analysis of the forward and reverse current-voltage characteristics. Furthermore this suggests that the fundamental limit to the GaN critical electric field is significantly greater than previously believed.

We report on tunnel-injected deep ultraviolet light emitting diodes (UV LEDs) configured with a polarization engineered Al0.75Ga0.25N/ In0.2Ga0.8N tunnel junction structure. Tunnel-injected UV LED structure enables n-type contacts for both bottom and top contact layers. However, achieving Ohmic contact to wide bandgap n-AlGaN layers is challenging and typically requires high temperature contact metal annealing. In this work, we adopted a compositionally graded top contact layer for non-alloyed metal contact, and obtained a low contact resistance of Rc=4.8x10-5 Ohm cm2 on n-Al0.75Ga0.25N. We also observed a significant reduction in the forward operation voltage from 30.9 V to 19.2 V at 1 kA/cm2 by increasing the Mg doping concentration from 6.2x1018 cm-3 to 1.5x1019 cm-3. Non-equilibrium hole injection into wide bandgap Al0.75Ga0.25N with Eg>5.2 eV was confirmed by light emission at 257 nm. This work demonstrates the feasibility of tunneling hole injection into deep UV LEDs, and provides a novel structural design towards high power deep-UV emitters.

Electronic quality epitaxial graphene has been selectively grown on silicon carbide capped with a patterned aluminum nitride layer, providing a pathway to produce device structures that avoid lithographic patterning of graphene itself. Patterning of the cap exposes SiC where graphene will grow. Capped areas inhibit graphene growth and withstand graphitization temperatures up to 1420°C under 100 Pa of argon pressure. Graphene Hall bars were fabricated and characterized by scanning Raman spectroscopy, ellipsometry, and transport measurements. Hall-mobility is about 600 cm2/V s and can be further enhanced by fine tuning the argon pressure and improving the quality of SiC surface prior to graphitization.

Metal Modulation Epitaxy (MME) [1] was introduced as a new growth technique wherein only the metal fluxes (Al, Ga, In, Si, and Mg) are modulated in a short periodic fashion in a plasma-assisted MBE system, while maintaining a continuous nitrogen plasma flux. This initially lead to dramatic improvements in grain size and demonstrated hole concentrations in excess of 4.5×1018 cm3 [1, 2, 3] for Mg-doped GaN grown on c-plane sapphire, eventually leading to p-type GaN with hole concentrations of ~1.5×1019 cm−3[4] and >4e19 cm3 [5]. Herein, we report the MME growth and characterization of deeply degenerately doped p-GaN with hole concentrations of ~7.9× 1019 cm3.

Metal Modulated Epitaxy (S. D. Burnham et al., J. Appl. Phys. 104, 024902 (2008) [1]) is extended to include modulation of both the shutters of Ga and Mg, the Mg being delivered from a Veeco corrosive series valved cracker (S. D. Burnham et al., Mater. Res. Soc. Proc. 798, Y8.11 (2003) [2]). The Ga fluxes used are sufficiently large that droplets rapidly form when the Ga shutter opens and are subsequently depleted when the Ga shutter closes. The result is the ability to limit surface faceting while predominantly growing under average N-rich growth conditions and thus, possibly reduce N-vacancy defects. N-vacancy defects are known to result in compensation. This ability to grow higher quality materials under N-rich conditions results in very high hole concentrations and low resistivity p-type materials. Hole concentrations as high as 2×1019 cm–3 have been achieved on c-plane GaN resulting in resistivities as low as 0.38 ohm-cm. The dependence on Ga flux, shutter timing, the corresponding RHEED images for each condition is detailed and clearly show minimization of faceting and crystal quality variations as determined by X-ray diffraction. Quantification of the Mg incorporation and residual impurities such as hydrogen, oxygen, and carbon by SIMS, eliminates co-doping, while temperature dependent hall measurements show reduced activation energies. X-ray diffraction data compares crystalline quality with hole concentration. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The AlGaN alloy system has been the material system of choice for development of solid-state UV emitters and has promising properties for high-voltage power electronics. The performance and reliability of both optical emitters and power devices are critically dependent on the density of threading dislocations. While the binary substrates, GaN and AlN, are becoming commercially available, only a limited range of AlGaN compositions can be grown pseudomorphicly to device-relevant thicknesses of a micron or more. This his restricts the range of heterostructures available for designing advanced devices, and creates a need for a ternary-like pseudo-substrate with a tunable lattice constant. We present a method for fabricating crack-free AlxGa1-xN templates over the entire alloy range with threading dislocation densities (TDD) in the low 108 cm-2 ranges. We employ AlxGa1-xN overgrowth of previously grown AlxGa1-xN/AlN/sapphire templates that have been patterned with parallel trenches on a 2 μm pitch consisting of submicron-wide-mesas.1 Following overgrowth, the remaining threading dislocations are uniformly distributed over 2-inch-wafer-size areas enabling large area devices such as LEDs and PIN diodes to consist of only low TDD material. As a result, laser diodes do not need to be aligned to the underlying trench pattern. Wafer bow is reduced to less than 15 μm and epilayer cracking is essentially eliminated by using 1.3 mm-thick sapphire substrates (three times thicker than typical 2-inch wafers). The utility of the described overgrowth process for various alloy compositions is demonstrated by growing 5 to 10 μm thick Al0.3Ga0.7N, Al0.7Ga0.3N and AlN epilayers on sapphire substrates. While AlN substrates continue to improve, they are still relatively immature, expensive, and small in area compared to GaN substrates. Using the described overgrowth process and a modest growth temperature of 1100 °C, AlN epilayers grown on sapphire have achieved FWHM linewidths < 300 arc seconds for the (10-11) x-ray diffraction reflection and an RMS roughness of 1 Å over a 10 x 10 μm2region. Such x-ray linewidth and surface roughness values are comparable to AlN grown in specialized MOCVD reactors capable of growth in excess of 1300 °C. These low TDD AlGaN layers have been used for various devices. For example, the AlN overgrowth templates are used to demonstrate 2DEG formation in MODFETs formed by Al0.85Ga0.15N / Al0.7Ga0.3N heterostructures. Additionally, UV laser diodes were grown on Al0.3Ga0.7N overgrowth templates that achieved room-temperature, pulsed-current operation with emission at 352 nm and peak output powers > 2.5 mW per facet. Finally, we will also discuss the potential of using >10 μm thick Al0.3Ga0.7N overgrowth templates for PIN diodes. [1.] Allerman, et. al. J. Crystal Growth 38876 (2014). This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

The growth temperature dependence of Si doping efficiency and deep level defect formation was investigated for n-type Al0.7Ga0.3N. It was observed that dopant compensation was greatly reduced with reduced growth temperature. Deep level optical spectroscopy and lighted capacitance-voltage were used to understand the role of acceptor-like deep level defects on doping efficiency. Deep level defects were observed at 2.34 eV, 3.56 eV, and 4.74 eV below the conduction band minimum. The latter two deep levels were identified as the major compensators because the reduction in their concentrations at reduced growth temperature correlated closely with the concomitant increase in free electron concentration. Possible mechanisms for the strong growth temperature dependence of deep level formation are considered, including thermodynamically driven compensating defect formation that can arise for a semiconductor with very large band gap energy, such as Al0.7Ga0.3N.

Current-voltage (IV) characteristics of two AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) with differing densities of open-core threading dislocations (nanopipes) are analyzed. A three-diode circuit is simulated to emulate the forward-bias IV characteristics of the DUV-LEDs, but is only able to accurately model the lower leakage current, lower nanopipe density DUV-LED. It was found that current leakage through the nanopipes in these structures is rectifying, despite nanopipes being previously established as inherently n-type. Using defect-sensitive etching, the nanopipes are revealed to terminate within the p-type GaN capping layer of the DUV-LEDs. The circuit model is modified to account for another p-n junction between the n-type nanopipes and the p-type GaN, and an excellent fit to the forward-bias IV characteristics of the leaky DUV-LED is achieved.

The improvement in light extraction efficiency from reflective scattering structures in AlGaN ultraviolet light-emitting diodes (UVLEDs) emitting at ∼270 nm is shown to be influenced by optical polarization. Three UVLEDs with different reflective scattering structures are investigated and compared to standard UVLEDs without scattering structures. The optical polarization and therefore the direction of light propagation within the various UVLEDs are altered by changes in the quantum well (QW) thickness. The improvement in light extraction efficiency of the UVLEDs with reflective scattering structures increases, compared to the UVLEDs without scattering structures, as the fraction of emitted light propagating parallel to the QW plane increases. Additionally, the light extraction efficiency increases as the average distance to the reflective scattering structures decreases.

Electrical current transport through leakage paths in AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) and their effect on LED performance are investigated. Open-core threading dislocations, or nanopipes, are found to conduct current through nominally insulating Al0.7Ga0.3N layers and limit the performance of DUV-LEDs. A defect-sensitive phosphoric acid etch reveals these open-core threading dislocations in the form of large, micron-scale hexagonal etch pits visible with optical microscopy, while closed-core screw-, edge-, and mixed-type threading dislocations are represented by smaller and more numerous nanometer-scale pits visible by atomic-force microscopy. The electrical and optical performances of DUV-LEDs fabricated on similar Si-doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these nanopipes, despite their small fraction (

We have studied the properties of thick InxGa1−xN films, with indium content ranging from x ∼ 0.22 to 0.67, grown by metal-modulated epitaxy. While the low indium-content films exhibit high density of stacking faults and dislocations, a significant improvement in the crystalline quality and optical properties has been observed starting at x ∼ 0.6. Surprisingly, the InxGa1−xN film with x ∼ 0.67 exhibits high luminescence intensity, low defect density, and uniform full lattice-mismatch strain relaxation. The efficient strain relaxation is shown to be due to a critical thickness close to the monolayer range. These films were grown at low temperatures (∼400 °C) to facilitate indium incorporation and with precursor modulation to enhance surface morphology and metal adlayer diffusion. These findings should contribute to the development of growth techniques for nitride semiconductors under high lattice misfit conditions.

An in situ Auger probe (AP) is employed to monitor changes in growth surface chemistry of the component elements of Terfenol-D, a completely miscible alloy of DyFe2 and TbFe2. The AP's surface sensitivity is quantified by depositing Dy onto a layer of Tb in 0.01 monolayer increments. The AP is able to distinguish surface coverage changes as small as 2% of a monolayer when elements with similar Auger spectra are used. The AP also allowed the identification of oxygen contamination in the system. Additionally, the glancing angle incidence primary electrons result in enhanced surface sensitivity compared to a more traditional Auger electrons spectroscopy configuration.

InN, high indium content InGaN, and Mg-doped InGaN were grown by metal modulated epitaxy (MME). Transient reflection high-energy electron diffraction intensities were analyzed during the growth of InN and found to be similar to that previously reported for GaN and AlN. The x-ray diffraction rocking curve and background electron concentration of InN grown by MME were found to be respectable in comparison to recent reports in literature. InGaN alloys grown by MME were also investigated, and a method for detecting indium surface segregation was demonstrated. It was found that the shutter modulation scheme could be modified to prevent phase separation by indium surface segregation, and a range of single-phase InGaN samples with indium contents throughout the miscibility gap were grown. Using the discovered method of suppressing phase separation, several p-InxGa1 − xN samples were grown with indium contents from x = 0 to 0.22. A maximum hole concentration of 2.4 × 1019 cm−3 was detected by Hall effect characterization, demonstrating feasibility of these p-InGaN layers for use in several device applications.

Highly p-type GaN films with hole concentrations exceeding 6 × 1019 cm−3 grown by metal-modulated epitaxy are electrically characterized. Temperature-dependent Hall effect measurements at cryogenic temperatures reveal minimal carrier freeze-out in highly doped samples, while less heavily doped samples exhibited high resistivity and donor-compensated conductivity as is traditionally observed. Effective activation energies as low as 43 meV were extracted, and a maximum Mg activation efficiency of 52% was found. In addition, the effective activation energy was found to be negatively correlated to the hole concentration. These results indicate the onset of the Mott-Insulator transition leading to impurity band conduction.

The growth of InGaN alloys via Metal-Modulated Epitaxy has been investigated. Transient reflection high-energy electron diffraction intensities for several modulation schemes during the growth of 20% InGaN were analyzed, and signatures associated with the accumulation, consumption, and segregation of excess metal adlayers were identified. A model for shuttered, metal-rich growth of InGaN was then developed, and a mechanism for indium surface segregation was elucidated. It was found that indium surface segregation only occurs after a threshold of excess metal is accumulated, and a method of quantifying this indium surface segregation onset dose is presented. The onset dose of surface segregation was found to be indium-composition dependent and between 1 and 2 monolayers of excess metal. Below this surface threshold off excess metal, metal-rich growth can occur without indium surface segregation. Since at least 2 monolayers of excess metal will accumulate in the case of metal-rich, unshuttered growth of InG...

Electron paramagnetic resonance measurements are used to monitor the passivation and activation of the Mg-related acceptor in GaN doped with different concentrations of Mg, up to 2 × 1020 cm−3. Samples were annealed in either forming gas (H2:N2) or pure N2 between 200 and 900 °C. As expected, the Mg-related EPR signal is reduced by at least a factor of ten during the forming gas treatment; while the pure N2 environment revives the signal. However, the study also shows that reactions between Mg and hydrogen occur at a temperature as low as 525 °C in the 1020 cm−3 Mg doped samples; while in more lightly doped samples, temperatures greater than 700 °C are required to observe changes in the Mg signal intensity. While the observations support the model in which a hydrogen atom ionizes at the Mg impurity and the remaining proton bonds at a near neighbor, the different temperature dependence suggests that hydrogen diffusion is affected by the increased Mg concentration.

The surface kinetics of InGaN alloys grown via metal-modulated epitaxy (MME) are explored in combination with transient reflection high-energy electron diffraction intensities. A method for monitoring and controlling indium segregation in situ is demonstrated. It is found that indium segregation is more accurately associated with the quantity of excess adsorbed metal, rather than the metal-rich growth regime in general. A modified form of MME is developed in which the excess metal dose is managed via shuttered growth, and high-quality InGaN films throughout the miscibility gap are grown.

An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination.

The DNA sequence coding for an analog of bovine growth hormone releasing factor (bGRF), having the methionine at position 27 substituted by leucine, was chemically synthesized with flanking methionine codons. Tandem ligations of this sequence allowed the monomer or fusion proteins of 2, 4, or 8 bGRF units to be expressed in E. coli from a pBR322-based vector. Although the monomer and dimer genes do not generate detectable products by SDS-PAGE, the fusion products of 4 and 8 bGRF accumulate as insoluble aggregates representing over 20% of the cellular protein; there appears to be decreased protease susceptibility as the fusion protein increases in size. The inclusion body material was solubilized in formic acid and octameric bGRF was treated with cyanogen bromide to release the monomer containing the natural amino-terminal tyrosine and an unnatural carboxy-terminal homoserine lactone. The reaction conditions represented a compromise between minimizing a formic acid-dependent modification of bGRF and achieving complete digestion. About 150 g of the bGRF analog were isolated at ≥99% purity using ion exchange and preparative reverse-phase chromatography. Intravenous injection of this peptide into steers stimulates somatotropin release as measured by significant increases in serum concentrations of the hormone.