Transmission electron microscopy (TEM) was used to study pendeo-epitaxial GaN layers grown on polar and non-polar 4H SiC substrates. The structural quality of the overgrown layers was evaluated using a number of TEM methods. Growth of pendeo-epitaxial layers on polar substrates leads to better structural quality of the overgrown areas, however edge-on dislocations are found at the meeting fronts of two wings. Some misorientation between the "seed" area and wing area was detected by Convergent Beam Electron Diffraction. Growth of pendeo-epitaxial layers on non-polar substrates is more difficult. Two wings on the opposite site of the seed area grow in two different polar directions with different growth rates and wings grown with Ga polarity are 17 times wider than wings grown with N-polarity, making coalescence of these layers difficult. Most dislocations in a wing grown with Ga polarity bend in a direction parallel to the substrate, but some of them also propagate to the sample surface. Stacking faults formed on the c-plane and prismatic plane occasionally were found in the wings. Some misorientation between the wings and seed was detected using Large Angle Convergent Beam Diffraction.

Wide-bandgap gallium nitride (GaN) quantum dot (QD) structure is attractive because it is a zero-dimensional (0-D) confinement structure and has many unique physical characteristics. We have successfully grown self-assembled InGaN QDs structure by metal organic chemical vapor deposition. A high quality GaN/sapphire template with a flat surface and the suitable growth condition including low growth temperature and low V/III ratio were used to grow InGaN QDs structure. The density of InGaN QDs is about 4.5 × 10¹⁰ cm^-2 with an average lateral size of 11.5 nm and an average height of 1.6 nm. The effect of the interruption growth for InGaN QDs structure was systematic studied with the growth temperature of 660°C. The surface morphology and optical property was measured by atomic forced microscopy and various temperature PL, respectively. The results indicated that as increasing interruption time from 30s to 120s, QDs area occupied on the surface above the wetting layer increases from 5.2% to 7.2%, and the In composition decreases from 25% to 21%. The results were discussed by considering the influences of ad-atom desorption and diffusion effect between wetting layer and InGaN QDs structure. Our results suggest that the interruption growth during an optimum time can modify the size of InGaN QDs and extend the emission wavelength to short wavelength, and at the same time improve the QD optical quality. Using this technique was feasible for formation of multi layer InGaN QDs structures and applicable for the fabrication of GaN-based light emitting devices.

In this work the results of high pressure solution growth of GaN on various patterned substrates are presented. The growth on GaN/sapphire substrates patterned in GaN parallel stripes and with Si_xN_y and Mo masks between stripes is studied and analyzed. The results are compared with the growth on patterned substrates without any mask, thus with a bare sapphire between stripes. The usefulness of tungsten and iridium for masking is also determined. The HVPE free standing GaN substrates with high stripes, up to 10 mm, are examined in details. The stripes growth modes are shown and described.

Based on the three-dimension finite element approach, we Investigated the strain field distributions of GaN/AlN self-organized quantum dot. The truncated hexagonal pyramid shaped quantum dots that have been found in experiment was adopted as the physical model in our simulation. The material used in this paper is wurtzite phase structure and there are five independent elastic constants. In dealing with the lattice mismatch, we employed a three-dimension anisotropic pseudo-thermal expansion. We compared the calculated results with that calculated by Green's function theory, which lots of assumption is adopted, and proved the correctness of our results. The strain distributions of the equal strain surface three-dimension contour plots of the six strain components are given. In the final, the anisotropic characteristics of the GaN/AlN quantum dot material is discussed, the results showed that the position of the elastic strain energy density minimum was just on top of the buried quantum dot and have little influence on the thickness of the cap layer. So the anisotropy has no obviously influence on the vertical alignment of post-growth for the next layer of quantum dots. Our model did not adopt the assumptions used in the Green's function approach, so better reliability and accuracy results are expected.

Excitonic carrier dynamics taking place in In_xGa_1-xN/GaN multi-quantum-well systems have been studied by low temperature picosecond time resolved photoluminescence (LT-TRPL), HR-TEM, XPS, Dynamic TOF-SIMS, and quantum mechanical simulation methods. Both time-integrated and time-resolved photoluminescence spectra of In_xGa_1-xN/GaN multi-quantum-wells with different well thickness and Indium composition were measured at 10 K. We assigned the natural radiative lifetime of each sample from the time resolved PL. We observed that the natural radiative lifetime of In In_xGa_1-xN/GaN multi-quantum-wells depends strongly on the well thickness and Indium composition. To support the measured natural radiative lifetimes, excitonic oscillator strengths of the In_xGa_1-xN/GaN multi-quantumwells were calculated by using a 2-D particle-in-a-box model as functions of well thickness and Indium composition. Values of the well thickness and Indium compositions from the HR-TEM and XPS compositional depth profiling were used to achieve more realistic computational results and to corroborate the measured natural radiative lifetimes of In_xGa_1-xN/GaN multi-quantum wells.

We have studied the emission distributions in nonpolar α-plane GaN thick films grown by HVPE using different nucleation schemes. The emission spectra show in addition to the near band edge emission band, also defect related bands due to different structural defects being enhanced/reduced to different extent in samples grown on different templates. Spatially resolved cathodoluminescence imaging reveals the in-plane distributions of the respective emission bands, which allows us to correlate the emissions with particular stacking fault structural defects independently revealed by plan-view transmission electron microscopy. In addition, emission distributions were visualized in vicinity of largescale defects like surface triangle pits, depressions and cracks attributed to prevailing defect formation and/or impurity incorporation.

Growth of GaN under pressure from solution in gallium results in almost dislocation free plate-like crystals but with size limited to app. 1-2 cm (lateral) and 100 μm (thickness) or up to about 1cm long needles. Deposition of GaN by HVPE on the pressure grown seeds allows stable crystallization (in terms of flatness of the crystallization front and uniformity of the new grown material) at a rate of about 100 μm/h on both types of seed crystals. However, in the thick GaN crystals grown on almost dislocation free plate-like substrates quite a high number of dislocations appears if the crystal thickness exceeds certain critical value. Since the critical thickness for defect generation is of the order of 100 μm, almost dislocation free layers (density below 10⁴ cm^-2) thinner than 100 μm can be grown. The most obvious further step is removing the substrate and continuation of the HVPE deposition on the free standing low dislocation density layer of sub-critical thickness. The pressure grown substrates were removed by mechanical polishing or conductivity sensitive electrochemical etching (for strongly n-type substrates). Then the HVPE low dislocation density GaN ¹platelets were used as substrates for the growth of a few mm thick bulk GaN crystals. The crystals were characterized by defect selective etching of both polar (0001) and non-polar (10 ^-10) surfaces to check presence and distribution of structural defects. The X-ray measurements allowed concluding about character of strain and deformation in high pressure GaN-HVPE GaN system.

We report on the 1.5 μm intersubband absorption measured on GaInN multi-quantum wells with AlInN barriers grown by RF plasma assisted molecular beam epitaxy (PAMBE). The intersubband light absorption was demonstrated as a function of the well width (1.3 nm - 3 nm) at the wavelength 1.4μm - 2.5 μm. The use AlInN barriers allowed to achieve strain compensated and crack free structures on GaN substrates. The preformed XRD mapping of a and c lattice constants show that AlInN/GaInN MQWs are fully strained and have up to 7% of indium in the barriers. The replacement of AlGaN by AlInN barriers opens new possibility to grow strain compensated crack free intersubband based devices like electooptical modulators and switches operating at telecommunication wavelengths.

We present a luminescence study of as-grown GaN and GaN:Si samples by means of low voltage cathodoluminescence (CL) at low temperature. It is shown that high spatial resolution CL microscopy allows direct luminescence mapping of threading dislocations in the doped and undoped samples. Comparison of monochromatic CL images acquired near the band gap energy (free and bound excitons) and at lower energies (recombination on defects) reveal the dopant segregation around dislocations.

We report on the structural, electrical, and optical characterization of GaN epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on SiN_x and TiN_x porous templates in order to reduce the density of extended defects. Observations by transmission electron microscopy (TEM) indicate an order of magnitude reduction in the dislocation density in GaN layers grown on TiN_x and SiN_x networks (down to ~10⁸ cm^-2) compared with the control GaN layers. Both SiN_x and TiN_x porous network structures are found to be effective in blocking the threading dislocation from penetrating into the upper layer. Supporting these findings are the results from X-Ray diffraction and low temperature photoluminescence (PL) measurements. The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably for the layers grown with the use of SiN_x and TiN_x layers, which generally suggests the reduction of edge and mixed threading dislocations. In general, further improvement is observed with the addition of a second SiNx layer. The room temperature decay times obtained from biexponential fits to time-resolved photoluminescence (TRPL) data are increased with the inclusion of SiN_x and TiN_x layers. TRPL results suggest that primarily point-defect and impurity-related nonradiative centers are responsible for reducing the lifetime. The carrier lifetime of 1.86 ns measured for a TiN_x network sample is slightly longer than that for a 200 μm-thick high quality freestanding GaN. Results on samples grown by a new technique called crack-assisted lateral overgrowth, which combines in situ deposition of SiN_x mask and conventional lateral overgrowth, are also reported.

Single-crystal ternary wurtzite Al_0.8In_0.2N thin films were grown epitaxially onto lattice-matched (111)-oriented Ti_0.2Zr_0.8N seedlayers. The epilayers were grown onto single-crystal MgO (111) substrates by magnetron sputter epitaxy (MSE) using reactive direct current magnetron sputtering in an N₂ discharge under ultra-high-vacuum conditions. The growth temperatures ranged from 20 to 700 ^oC. Low-energy ion-assisted growth conditions, enhancing the epitaxy, were achieved by applying a negative substrate potential of 15-45 V. Film compositions and lattice parameters were determined using Rutherford Backscattering Spectroscopy (RBS) and High-Resolution X-ray diffraction (XRD), respectively. Cross-sectional High-Resolution Electron Microscopy of the interface regions verified the epitaxy and the crystallinity of the films. XRD ω-rocking scans of the Al_0.8In_0.2N 0002-peak showed full-width-at-half-maximum values of ~2400 arcs, indicating a high structural quality of the films. Opto-electronical properties were studied by cathodoluminescence at temperatures between 5 and 293 K. Luminescence was observed at wavelengths as short as 248 nm, corresponding to an energy of 5.0 eV. These results point towards the feasibility of metastable Al_0.8In_0.2N solid solutions as an active luminous material in opto-electronics. It also shows that MSE-grown Al_0.8In_0.2N can be an excellent choice for lattice-matched GaN heterostructures, with a resulting energy band-gap difference enabling strong charge carrier confinement. In addition to these new and original results, a brief review of the present work on Al(_1-x)In_xN growth at Linkoeping University is presented.

GaN /AlGaN transistors are being developed for a variety of RF electronic devices that will eventually replace GaAs- and silicon-based devices for commercial and military applications. In this paper, we present results from the optimization of the growth conditions for GaN/AlGaN HEMT structures. The HEMT epitaxial layers are grown via MOCVD. We demonstrate that the key to high quality HEMT structures is the ability to grow uniform AlGaN layers. Details of the structural, electrical and optical characteristics of the HEMT epitaxial layers are presented. In addition, we present results on an innovative ICP etching used for HEMT fabrication. This technique allows for low damage device processing and improved reliability.

Nanoporous, micron-size GaN particles with pores between 40 and 100 nm in diameter have been synthesized on boron nitride (BN) substrates using chemical vapor deposition (CVD) techniques. The synthesis process is based on the direct reaction of gallium atoms and ammonia molecules without the presence of intentional metal catalysts. Scanning electron micrographs reveal the formation of GaN nanoporous morphologies ranging from micron-size particles with hexagonal pyramidal prismatic shape to hexagonal platelets. The nanopores are only observed on the (0001) basal plane and aligned orderly along the [0001] crystallographic direction. High-Resolution Transmission Electron Microscopy (HRTEM) confirms that the nanoporous particles are, where analyzed, wurtzite GaN single crystal.

We report on a technique for optimizing transport properties in p- and n-type AlGaN/GaN and GaN/InGaN superlattices. As we show highly conductive heterostructures can be obtained by inserting a graded doped layer, which reduces the barrier height while maintaining high sheet carrier density. For optimized p-type AlGaN/GaN SL, an eight fold reduction of the barrier height and a 1.5 times increase in sheet hole density is obtained compared to typical SL. The optimized structure yields 13 orders of magnitude improvement in vertical conductivity (σ_V) compared to typical SL, and 35 times improvement in lateral conductivity (σ_L) compared to bulk p-GaN. For optimized p-type GaN/InGaN SL, an improvement of more than 10 orders of magnitude in σ_V compared to typical SL is obtained with σ_L better than that of bulk p-InGaN. We also investigate n-type SLs as current spreading layers. A significant improvement in current distribution is obtained for the optimized SLs.

Effects of electron irradiation on GaN and Al_xGa_1-xN doped with acceptor-forming species (Mg, C, Fe, and Mn) were studied by cathodoluminescence and electron beam induced current techniques. Low energy electron beam irradiation was shown to induce a systematic decay of the cathodoluminescence intensity, which is accompanied by increased electronic carrier diffusion length, indicating the increase of carrier lifetime. Temperature-dependent cathodoluminescence measurements allowed to estimate the activation energy for irradiation-induced effects, which was found to be comparable to the ionization energy of the dominant acceptor species. These observations are consistent with trapping of non- equilibrium electrons on deep, non-ionized acceptor levels. In (Al) GaN:Mg and GaN:C electrons are trapped by the ground state of the neutral acceptor atom, while in TM-doped compounds, electron irradiation induced processes appear to involve a more energetically accessible excited states of the acceptors.

Here, we report a direct synthesis approach for obtaining GaN nanowires with control on growth directions: <0001> or c-direction, and <10-10> or a-direction, on amorphous substrates. The direct nitridation of Ga droplets using either dissociated ammonia or N₂/H₂ plasma resulted in GaN nanowires with <0001> growth direction; and the vapor transport of controlled (low) amounts of Ga flux in the presence of dissociated ammonia resulted in GaN nanowires with <10-10> growth direction. In both cases, the resulting GaN nanowires have diameters as small as 20 nm and lengths exceeding one hundred microns. Photoluminescence measurements showed that the bandgap of <10-10> wires blue-shifted by 50 meV from the wires with <0001> direction. Homo-epitaxial growth studies onto the pre-synthesized a-direction GaN nanowires led to belt or ribbon shaped morphologies. Homo-epitaxial growth onto c-direction wires developed micro hexagonal prism morphologies. The island growth morphologies observed on the hundred micron long, sub 30 nm size nanowires suggest that the surface transport of adatoms on c-direction wires exhibit ballistic transport or "one-dimensional" transport with mean distances over several tens of microns.

We have used the techniques of atomic force microscopy (AFM) and conductive AFM (C-AFM) to investigate the morphology and localized current conduction of GaN films grown by molecular beam epitaxy (MBE) on metal organic chemical vapor deposition (MOCVD) templates. The most common type of surface morphology consists of undulating spiral "hillocks" terminated by small pits. A low density of holes are interspersed between these hillocks with typical diameters of ~150 nm and densities on the order of 10⁸ cm^-2. For C-AFM measurements, a Pt-coated AFM tip was brought into contact with the GaN surface to form a microscopic Schottky contact. In reverse bias, C-AFM shows localized current leakage at the centers of approximately 10% of spiral hillocks, which are presumably associated with screw dislocations. Shifts in forward-bias turn-on voltages and changes in the conduction mechanism are observed in these defect regions. Local I-V curves indicate a Frenkel-Poole mechanism for forward conduction on defect regions.

Native Aluminum Nitride (AlN) single-crystal substrates with ultra-low dislocation density are very promising for use in III-nitride epitaxial growth required for ultraviolet (UV) electro-optical applications and high power radio frequency (RF) devices. They offer a better lattice and thermal expansion match to AlGaN alloys, especially those with high Al content, than foreign substrates such as SiC or sapphire. An additional advantage of bulk substrates is the possibility of slicing and preparing surfaces with the desired orientation, such as non-polar and pre-determined, specific misorientations, which will permit the fabrication of devices with specific, special properties. In this paper we present chemical and electrical characterization of the AlN material. Secondary Ion Mass Spectroscopy (SIMS) measurements show that oxygen is the main impurity, with concentrations in the order of mid 10¹⁸ cm^-3. The electrical resistivity of the AlN was measured, giving a lower limit of 10¹²Ω-cm at room temperature. The prepared surface of substrates with different orientations, as well as of homo-epitaxial and hetero-epitaxial layers of AlGaN with different Al:Ga ratios were measured by Atomic Force Microscopy. The observation of atomic steps in the bare substrates and step flow in the epilayers are an indication of the good surface preparation. The crystalline quality of the epilayers was assessed by measuring the full width at half maximum (FWHM) of both symmetric and asymmetric X-ray rocking curves.

Wide bandgap nitrides and oxides have been heralded as a possible platform for future semiconductor spintronics applications based on the inherent compatibility of these materials with existing semiconductors as well as theoretical predictions of room temperature ferromagnetism. Experimental reports of room temperature ferromagnetism in these materials are complicated by disparate crystalline quality and phase purity in these materials, as well as conflicting theoretical predictions as to the nature of ferromagnetic behavior in this system. A complete understanding of these materials, and ultimately intelligent design of spintronic devices, will require an exploration of the relationship between the processing techniques, resulting transition metal atom configuration, defects, and electronic compensation as related to the structure, magnetic, and magneto-optical properties of this material. This work explores the growth and properties of Ga_1-xMn_xN films by metalorganic chemical vapor deposition on cplane sapphire substrates with varying thickness, Mn concentration, and alloying elements. Homogenous Mn incorporation throughout the films was verified with Secondary Ion Mass Spectroscopy (SIMS), and no macroscopic second phases were detected using X-ray diffraction (XRD). SQUID and vibrating sample magnetometry measurements showed an apparent room temperature ferromagnetic hysteresis, whose strength can be altered considerably through annealing and introduction of either Si or Mg during the growth process. Three sets of Raman modes appeared to be sensitive to Mn incorporation. The intensities of a broad band around 300cm^-1 and sharper modes near 669cm^-1 increased with increasing Mn concentration. The rise of the former is attributed to a decrease in long-range lattice ordering for higher Mn concentration. The second mode is due to nitrogen vacancy-related local vibrational modes of the GaN host lattice. Si co-doped Ga_1-xMn_xN results in shallow donor states in GaN suppress the formation of nitrogen vacancies by compensating the p-type deep level defects introduced by substitutional Mn. The formation of a Mn-related midgap impurity band is observed via optical transmission measurement in Ga_1-xMn_xN with strong magnetic signatures, but not for Si co-doped samples. Initial studies on light emitting diodes (LEDs) containing a Mn-doped active region have also been produced. Devices were fabricated with different Mn-doped active layer thicknesses, and I-V characteristics show that the devices become highly resistive as thickness of the Mn-doped active layer increases. The electroluminescence of these devices is dominated by a high suppressed band-edge recombination and a midgap defect-related emission, leading to an orange-colored but weakly emitting LED. These results suggest that traditional theoretical and device approaches akin to those realized in Ga_1-xMn_xN may be difficult to realize in Ga_1-xMn_xN, and exploitation of these materials will require further novel device approaches taking into account the nature of this material.

The zincblende In_xGa_1-xN, Al_xGa_1-xN, and Al_xIn_1-xN alloys are studied by numerical analysis based on first-principles calculations. The results show that the lattice constant of the three alloys obeys the Vegard's law. For In_xGa_1-xN the direct band gap bowing parameter obtained with the equilibrium lattice constant is 1.890 ± 0.097 eV. For Al_xGa_1-xN the direct and indirect bowing parameters of 0.574 ± 0.034 eV and 0.055 ± 0.038 eV are obtained, and there is a direct-indirect crossover near x = 0.56. For Al_xIn_1-xN the direct and indirect bowing parameters of 3.5694 ± 0.028 eV and 0.1953 ± 0.054 eV are obtained, and there is a direct-indirect crossover near x = 0.807

Surface profiles of deep levels in GaN sample grown by metal-organic chemical vapor deposition and by hydride vapor phase epitaxy are measured by differential deep level transient spectroscopy (DDLTS). The concentration of acceptor defects at the surface are expected to be lower than the bulk defect concentration because of the shift in Fermi level at the surface, based on theoretical estimates of defect formation energies and the band bending at the surface from spontaneous polarization. Similarly, donor defects are expected to increase in concentration as the surface is approached. The measured concentration profiles of various traps are found to span the range of behavior, from constant, to increasing or decreasing at the interface. Deep level profiling is therefore seen as an important tool to assist in determining defect composition. Although the behavior is as expected, the change in concentration from bulk to surface, is larger than measured values for the defects with the lowest formation energies, based on a conservative estimate of band bending. The difference may reflect a band bending that is different at the growth temperature than predicted, or a consequence of non-equilibrium growth conditions. As growth proceeds, the defects incorporated at the surface are in a non-equilibrium concentration when covered by subsequent layers, unless there is a mechanism whereby equilibrium defects can be formed, e.g. V_Ga by forming interstitial Ga, or there is enough energy for defect diffusion to take place. Peaks in the defect profile were measured, as would be expected for a donor defect formed at the surface, but with a non-equilibrium concentration in the bulk, driving diffusion toward the surface.

In nitride heterostructures and devices, effects related to (i) polarization induced electric fields (PIEFs) and (ii) spatial segregation of indium leading to exciton/carrier localization are of major significance. However, separate investigation of these effects is not straightforward since they give rise to identical observations, such as a Stokes shift of the luminescence with respect to absorption and a blue shift of luminescence with increasing pump intensity. In this work, we review the usefulness of measurements of the hydrostatic pressure dependence of InGaN luminescence for the verification of the presence of PIEFs in quantum structures and light emitting devices. Additionally, we show that the pressure coefficient is not or only slightly sensitive to the degree of localization in the InGaN alloy. Thus, the variation of the luminescence pressure coefficient in different quantum structures can be almost entirely assigned to changes in the magnitude of internal electric field. Using this knowledge, we demonstrate how the degree of PIEF screening in InGaN based LDs can be evaluated.

In the late 1980's, etched facet lasers were demonstrated at Cornell University using a process based on chemically assisted ion beam etching (CAIBE). These etched facets allowed, for the first time, mirror reflectivities to be obtained that were equal to those of cleaved facets. Over the past few years, BinOptics Corporation has used this proprietary Etched Facet Technology (EFT) in fabricating InP based lasers with a quality equal to those of cleaved facets. Etched facets allow mirrors to be placed on the epitaxial substrate with very high precision. EFT eliminates losses that result from mechanical facet cleaving, allows wafer-scale testing and coating, and enables monolithic integration. BinOptics Corporation has now developed a modified version of its EFT for GaN materials and blue lasers where mechanical cleaving losses can be even more problematic. The relatively high defect density of currently available GaN materials creates an additional yield advantage for EFT: it allows the formation of shorter cavity devices with fewer defects per device. The first etched facet GaN devices are Fabry-Perot type ridge waveguide lasers emitting at 405nm for optical storage applications. However, as demonstrated in InP, it is planned to extend the technology to horizontal-cavity surface-emitting lasers (HCSELs) with integrated monitoring photodetectors (MPDs). A surface-emitting blue laser will allow two-dimensional arrays for high power applications and monolithic integration of additional functions. For example, the integration of a blue HCSEL with a receive detector will enable the creation of a compact optical head.

Surface properties of GaN subjected to reactive ion etching and the impact on device performance have been investigated by surface potential, optical and electrical measurements. Different etching conditions were studied and essentially high power levels and low chamber pressures resulted in higher etch rates accompanying with the roughening of the surface morphology. Surface potential for the as-grown c-plane GaN was found to be in the range of 0.5~0.7 V using Scanning Kevin Probe Microscopy. However, after reactive ion etching at a power level of 300 W, it decreased to 0.1~0.2 V. A nearly linear reduction was observed on c-plane GaN with increasing power. The nonpolar a-plane GaN samples also showed large surface band bending before and after etching. Additionally, the intensity of the near band-edge photoluminescence decreased and the free carrier density increased after etching. These results suggest that the changes in the surface potential may originate from the formation of possible nitrogen vacancies and other surface oriented defects and adsorbates. To recover the etched surface, N₂ plasma, rapid thermal annealing, and etching in wet KOH were performed. For each of these methods, the surface potential was found to increase by 0.1~0.3 V, also the reverse leakage current in Schottky diodes fabricated on treated samples was reduced considerably compared with as-etched samples, which implies a partial-to-complete recovery from the plasma-induced damage.

This paper reports the development of aluminum-gallium nitride (AlGaN or Al_xGa_1-xN) photodiode technology for high-operability 256×256 hybrid Focal Plane Arrays (FPAs) for solar-blind ultraviolet (UV) detection in the 260-280 nm spectral region. These hybrid UV FPAs consist of a 256×256 back-illuminated AlGaN p-i-n photodiode array, operating at zero bias voltage, bump-mounted to a matching 256×256 silicon CMOS readout integrated circuit (ROIC) chip. The unit cell size is 30×30 μm². The photodiode arrays were fabricated from multilayer AlGaN films grown by MOCVD on 2" dia. UV-transparent sapphire substrates. Improvements in AlGaN material growth and device design enabled high quantum efficiency and extremely low leakage current to be achieved in high-operability 256×256 p-i-n photodiode arrays with cuton and cutoff wavelengths of 260 and 280 nm, placing the response in the solar-blind wavelength region (less than about 280 nm) where solar radiation is heavily absorbed by the ozone layer. External quantum efficiencies (at V=0, 270 nm, no antireflection coating) as high as 58% were measured in back-illuminated devices. A number of 256×256 FPAs, with the AlGaN arrays fabricated from films grown at three different facilities, achieved response operabilities as high as 99.8%, response nonuniformities (σ/μ) as low as 2.5%, and zero-bias resistance median values as high as 1×10¹⁶ ohm, corresponding to R0A products of 7×10¹⁰ ohm-cm². Noise Equivalent Irradiance (NEI) data were measured on these FPAs. Median NEI values at 1 Hz are 250-500 photons/pixel-s, with best-element values as low as 90 photons/pixel-s at 1 Hz.

Ferroelectric field effect transistors (FFETs) with hysteretic I-V characteristics were attained with 25 nm thick Pb(Zr_0.52Ti_0.48)O₃ (PZT)/Si₃N₄ gated AlGaN/GaN heterostructure. The PZT films used in the gate of the device was deposited by magnetron rf-sputtering at the substrate temperature of 700 ^oC. Increasing the PZT deposition temperature from that in previous device structures from 600 ^oC to 700 ^oC we obtained much improved device performance in terms of the IV characteristics inclusive of hysteretic behavior. The pinch-off voltage was about 7 V in FFET device compared to 6 V in a the control (conventional) AlGaN/GaN device. Counterclockwise hysteresis appeared in the transfer characteristic curve of a FFET with a maximal drain current shift of about 10 mA at the gate-to-source voltage of -6 V.

We report the electrical and optical properties of deep ultraviolet light emitting diodes (LEDs) based on digital alloy structures (DAS) of AlN/Al_0.08Ga_0.92N grown by gas source molecular beam epitaxy with ammonia on sapphire substrates and AlGaN/sapphire templates. AlGaN/sapphire templates were grown by recently developed stress controlled hydride vapor phase epitaxy (HVPE). For DAS with effective bandgap of 5.1 eV we obtain room temperature electron concentrations up to 1x10¹⁹ cm^-3 and hole concentrations of 1x10¹⁸ cm^-3. Based on these results we prepared double heterostructure (DHS) LEDs operating in the range of 250 to 290 nm. The emission wavelengths were controlled through the effective bandgap of the active region. The possible ways for increase of LED's efficiency are discussed. We observed significant improvement in the room temperature luminescence efficiency (by factor of 100) of AlGaN quantum wells when a transition growth mode is induced by reduced flux of ammonia. We found that active layer grown on HVPE AlGaN/sapphire substrates have higher luminescence efficiency (by factor of 3) than DAS grown on sapphire.

Due to their unique physical properties GaN-based heterostructures show a great promise for spintronics applications. This stimulates the search for GaN-based ferromagnetic semiconductors which can be used for injection of spin polarized carriers in device structures. In this study, magnetic properties of GaN layers implanted with Gd⁺ ions to various doses were investigated. Magnetization curves of samples with Gd content n_Gd = 2x10¹⁷ and 2x10¹⁸ cm^-3 show clear hysteresis, while the samples with n_Gd = 2x10¹⁶ and 2x10¹⁹ cm^-3 exhibit no ferromagnetism. Most likely, the lowest Gd concentration produced magnetization below the detection limit, whereas the absence of ferromagnetism in the sample with the highest Gd content may be resulted from heavy implantation-induced damage. Curie temperatures for samples with Gd contents of 2x10¹⁷ and 2x10¹⁸ cm^-3 were estimated to be larger than 300 K. Saturation magnetizations of 1550 μ_B and 1350 μ_B per Gd-atom were found at 5 K and 300 K, respectively, for the sample with n_Gd=2x10¹⁸ cm^-3.

We have investigated the thermal stability of three composite metals on their contact resistivities and luminous intensities for using as the reflector in flip-chip light-emitting diode (FCLED). The composite metals were simultaneously deposited on n-type GaN without alloy to form n-type Ohmic contact and simplify the process. The investigated composite metals were Ti/Al/Ti/Au (30/500/30/300 nm), Cr/Al/Cr/Au (30/500/30/300 nm) and Cr/Ti/Au (500/30/300 nm), respectively. The specific contact resistivity of Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au on the n-type GaN Ohmic contact were changed from 5.4×10^-4, 6.6×10^-4 and 7.7×10^-4 Ω-cm² to 5.3×10^-4, 4.5×10^-4 and 1.3×10^-4 Ω-cm² respectively after 500 hours thermal stress at 150°C in the air. After 96 hours of thermal stress, the luminous intensities at 20 mA of these three structures were decreased 6.2%, 11.1% and 1.4%, respectively. Therefore, in addition to maintain good n-type ohmic contact and simplify the process, the Cr/Ti/Au composite metal demonstrates good thermal stability as a reflector in FCLED.

In this study, ZnO:Al(AZO) Ni/AZO and NiO_x/AZO films were deposited on p-type GaN films followed by thermal annealing to form Ohmic contacts. After thermal annealing, the resistivities reduced from 5×10^-3 to 4.4×10^-4 Ω-cm, 2.6×10^-3Ω-cm, and 1.1×10^-3Ω-cm for AZO, Ni/AZO, and NiO_x/AZO films, respectively. The Ohmic characteristic could be highly improved after inserting Ni and NiO_x between AZO and p-GaN. Both the Ni/AZO and NiO_x/AZO contacts exhibit Ohmic characteristic after annealed at 800°C in N₂ ambient. The light transmittance of Ni/AZO and NiO_x/AZO films were higher than 80% in the range of 380-700nm after the 800°C -annealing treatment. In addition, we fabricated InGaN/GaN MQW LEDs with a dimension of 1×1mm² using the transparent Ni/AZO and NiO_x/AZO Ohmic contact as a current spreading layer for p-GaN in order to increase the light extrication efficiency. For the LED with Ni/AZO contact, the light output approach to saturation when the injection current was about 400mA. But the light output still doesn't approach to saturation when the injection current was 500mA for the LED with NiO_x/AZO contact. This may be due to that the resistivity of Ni/AZO was higher than that of NiO_x/AZO and exhibit more heavy current clouding effect. The increasing of resistivity may be due to the interdiffusion of Ni into AZO. Comparing to GaN LED with Ni/Au ohmic contact, the light output intensity of LEDs with Ni/AZO and NiO_x/AZO contacts was increased by 41% and 60% at 350mA, respectively.

The enhanced output power with improved lifetime is required for the GaN-based blue-violet laser diode (LD) as a light source for Blu-ray Disc or HD-DVD. In this paper, the output power levels and aging behaviors in GaN-based LDs grown on sapphire substrates were compared in epi-up and epi-down bonding. At low current level, the two bondings show little differences in L-I characteristics. At high current level, however, the epi-up bonding shows a rapidly decreased slope efficiency in L-I characteristics with increasing current injection. On the contrary, the slope efficiency in epi-down bonding is not so much deteriorating as that in epi-up bonding. The differences in junction temperature between epi-up and epi-down bonding are large at higher current levels. The junction temperature of epi-up bonding is about two times higher than that of epi-down bonding, implying efficient heat dissipation in epi-down bonding. At aging test, the epi-down bonding LD shows lower degradation rate at the aging slope than that of epi-up bonding LD. The degradation rate is accelerated by poor heat dissipation in epi-up bonding. Thus, for the higher power and longer lifetime, it is necessary to employ efficient heat dissipation structures such as epi-down bonding for the GaN-based LD on sapphire substrate.