We report materials characterization on optically pumped InGaN/GaN lasers by farfield and spectral measurements. Through a comparison between measured and calculated farfield data for an In0.15Ga0.85N/In0.05Ga0.95N multi quantum well laser structure with AIGaN cladding layers, we could extract important information about the optical confinement in the transverse direction. The analysis of Fourier-transformed emission spectra of our devices allowed us to make qualitative statements about the material quality in terms of surface pits and cracks. We believe that optical pumping with these supplementary techniques is an important and powerful tool which helps to overcome critical material quality requirements in gallium-nitride.

AlGaN photodiode detectors are grown on (0001) sapphire by RF atomic nitrogen plasma molecular beam epitaxy. Both individual detectors and 1 × 10 element arrays are fabricated. The individual detectors have active areas of 0.5 mm2, 1.0 mm2, and 2.0 mm2. Individual elements in the l × 10 detector arrays range in size from 250×250 μm to 450×450 μm. The detectors are fabricated using a chlorine-based reactive ion etch (RIE) and refractory metal ohmic contacts. At room temperature, GaN p-i-n photovoltaic detectors show peak responsivity at 360 nm as high as 0.198 A/W, corresponding to an internal quantum efficiency of 85%. These detectors also exhibit five orders of magnitude of rejection for radiation longer than 500 nm. The electrical and spectral characteristics of these detectors are examined at elevated temperatures. The short wavelength UV responsivity remains fairly constant at elevated temperatures, while the peak responsivity actually increases with increasing temperature. The smooth surface morphology of heavily doped p-type material grown by MBE makes possible diode structures with a p-type bottom layer. The effect of the spectrally broader p-type material in the photodiode responsivity will be discussed.

High quality AJGaN/GaN heterostructures have been successfully deposited on both nand p-type SiC substrates. Heterostructure field effect transistors fabricated using these layers exhibited high channel current density (1.71 A/mm), well behaved pinch-off characteristics, and excellent extrinsic transconductance (Gm = 229 mS/mm). There is negligible channel current degradation up to a source to drain bias of 20 V as opposed to devices grown on sapphire substrates. The 0.25 μm gate-length devices fabricated on the heterostructures grown on p-type SiC has allowed us to extract a cutoff frequency of 53 GHz. The cutoff frequency showed little deterioration with increasing drain bias voltage. These results demonstrate for the first time the high frequency and high power operation potential of the heterostructure field effect transistors based on AlGaN grown on SiC.

Ni/Si-based ohmic contact scheme for GaN, based on the solid-phase regrowth (SPR) mechanism have been developed. Using Mg and Si as dopant species, ohmic contacts with a resistivity of ∼1*10-3Ωcm2 to p-GaN (p≈3*1017 cm-3) and n-GaN (n≈2*1017cm-3), respectively, have been obtained. SIMS, XRD, and RBS analysis show in as-deposited contacts, an initial reaction at GaN/Ni interface, leading to the formation of an Ni-Ga-N layer. The ohmic behavior of contacts, observed after annealing at 400°C, is accompanied by structural transformations in the contact region: i) the decomposition of Ni-Ga-N layer and ii) the growth of NiSi compound.

We investigated the electrical properties and interfacial reactions of the Si/Ti based ohmic contacts to n-GaN grown by metal organic chemical vapor deposition and the electrical properties were related to the material reactions. The perfect ohmic characteristics were obtained after annealing at 700 °C for 3 min under N2 ambient, and Ti silicide was formed in Ti-Si based contact systems. The lowest value for the specific contact resistance of 3.86×10-6 Ωcm2 was obtained for Au(1000 Å)/Ni(400 Å)/Ti(400 Å)/Si(1460 Å)/Ti(150 Å) after annealing at 900 °C for 3min. It could be concluded from the material analyses that the ohmic characteristics of Ti-Si based contact systems were due to the low barrier height by the formation of Ti silicides with a low work function

A variety of metal contacts to n-GaN is investigated, including Al, Au, Ce, Ti, Cr, Mg, Sb, W, and Mo, which were deposited using DC magnetron plasma sputtering onto MBE grown GaN. The contacts were characterized by current-voltage analysis and contact resistances were measured by the circular geometry transmission line method. The effects on contact resistance and Schottky barrier properties by surface sputtering treatments, GaN doping, and post-deposition annealing are reported.

In this paper we report on the formation of ohmic contacts to n- AlxGa1-xN alloys. The films were produced by plasma-assisted MBE and doped n- type with silicon at doping levels between 1018 to 1019 cm-3. Contacts were formed by sequential deposition of 200 Å of Ti and 2000 Å of Al and the contact resistivities were determined from TLM measurements. For low Al- content (x<. 10) the I-V characteristics are linear with contact resistivities of between 10-4 to 10-5 cm2. The contacts become progressively non-ohmic at Al concentrations greater than 10%. There results are consistent with the Schottky limit being applicable to these alloys and thus the Ti/Al contact forms Schottky barriers with higher barrier height as the conduction band of the alloy moves towards the vacuum level.

We report here the characterization of metalorganic chemical vapor deposition grown GaN layer on sapphire substrate with (0001) orientation (c-face). The 2.4-μm-thick GaN layer on sapphire substrate showed the specular surface morphology. The full width at half-maximum of X-ray double crystal rocking curve was 214 arcsec. Photoluminescence measurement at 4.2 K showed the emissions from the two free excitons (FEA and FEB), acceptor and donor bound excitons (ABE and I2) and the first excited state of FEA. The etch pit density (EPD) was measured using molten KOH etching was 2∼4×108 cm-2 . The electron mobilities from Hall effect measurement were 585 cm2/V·s for the electron concentration 1.1×1017 cm-3 at 300 K, and 1217 cm2/V·s for 2.4×1016 cm-3 at 77 K, respectively. The measured mobility values are much higher than the previously reported values. The fabricated Pt/Ti/Au Schottky diode on GaN layer revealed the ideality factor 1.04 and the barrier height 0.9 eV which was calculated from forward current-voltage characteristics. The reverse current was as low as 1 mA for the reverse voltage 100 V. The barrier height was also determined to be 1.4 eV from the capacitance-voltage measurement. The electron-beam induced current observations were performed on the fabricated GaN Schottky diode. The dark spot was observed and the dark spot density was estimated to be 2×108 cm-2, which was comparable to the EPD measured using molten KOH etching. GaN metal semiconductor field effect transistors (MESFETs) with 3-μm gate-length and 15-μm gate-width have also been fabricated on high-quality GaN single crystalline layer. Maximum transconductance 25 mS/mm and the drain-source current 210 mA/mm have been achieved for the GaN MESFET.

Metal-insulator-semiconductor (MIS) capacitors have been fabricated on n-type GaN (0001) films using thermally grown Ga2O3, remote plasma enhanced chemical vapor deposited (RPECVD) SiO2, and molecular beam epitaxy (MBE) AIN as the gate insulator and Al as the gate electrode. Each GaN epitaxial layer was grown by organometallic chemical vapor deposition (OMCVD) on a 6H-SiC(0001) substrate on which was previously deposited a 1000Å buffer layer of AIN. Nitrogen-free polycrystalline films of Ga2O3 were grown on the GaN. Capacitancevoltage measurements of capacitors fabricated from this oxide showed distinct depletion and accumulation regions with significant leakage. The AIN and SiO2 capacitors demonstrated better electrical characteristics than the Ga2O3 because of lower leakage. The RPECVD SiO2/GaN heterostructures, in particular, showed good agreement with the curves calculated for an ideal oxide and a small amount of hysteresis.

We report on the fabrication and characterisation gallium nitride light emitting diodes (LEDs) grown by molecular beam epitaxy on (0001) oriented sapphire and (111)B GaAs substrates. The current voltage characteristics of the devices grown on sapphire show turn on voltages of between 4 and 5V with large on-series resistance of 600Ω; for corresponding devices grown on GaAs these parameters are between 6 and 7V and 150 Ω, respectively.

Room temperature electroluminescence (EL) spectra from the GaN LEDs ,grown on sapphire substrates, show a dominant emission at 3.2 eV (397nm) with a full width half maximum (FWHM) of 335 meV which is attributed to free electron to acceptor transitions (e, A−Mg). A broad low intensity deep level emission is also observed centred at 2.4 eV (506nm). The peak of the EL from the devices grown on GaAs is at 3.1eV rather than 3.2eV. The differences between the two sets of devices are probably caused by the different device geometry.

Preliminary results show that an “annealing” effect caused by electrical stressing resulted in an improvement of the EL spectra. The stressed samples show an increase in the near band edge emission intensity, a 20meV reduction in the FWHM and a significant reduction in the intensity of the deep level emission. The devices have a large 1/f noise contribution which does not appear to change after electrical stressing.

The valence subband structures, density-of-states, and optical gain of (0001) wurtzite (WZ) InxGa1-xN/GaN quantum wells (QWs) are studied using a numerical approach without analytical approximations. We used the effective-mass parameters of GaN and InN derived using the Empirical Pseudopotential Method. By varying the well width and mole fraction of In in the well material, the effects of quantum confinement and compressive strain are studied. A narrower well width and a higher In mole fraction in the well lead to TE enhancement and TM suppression of the optical gain. From the relationship between the optical gain and the radiative current density, we obtain the transparent current density for a single QW to be 200 A/cm2. Further, we analyze the InxGa1-xN/GaN/AlGaN separate confinement heterostructure multiple-QW laser structure. It is shown that a suitable combination of well width and number of QWs should be selected in optimizing the threshold current density in such MQW lasers

The optical gain of a BGaN ternary system lattice-matched to 6H-SiC was estimated. The parameters used in this estimation, such as effective mass and a bandgap energy, were estimated according to Harrison's theory. The optical gain was estimated using the density matrix theory with intraband relaxation broadening. The transparency carrier density of BGaN is slightly only smaller than that of GaN. It may be possible to fabricate a BGaN-based semiconductor laser operating in the UV spectral region.

GaN is an attractive wide bandgap semiconductor material for optoelectronic device applications at blue and UV wavelengths. The breakdown behavior of interdigital MSM type of AlGaN UV photodetectors (PDs) are reported here. The A1GaN was deposited using MOCVD technique. The MSM devices were fabricated using undoped AlGaN (Al composition is 0.13) epitaxial layer on sapphire substrate. Au/Ti metallization was patterned as metal electrodes on AlGaN. Very high UV responsivity of the devices was achieved at DC bias voltage in the range of 60–140V depending on the device dimensions. The breakdown voltage exhibits a negative temperature coefficient. The relationship between the breakdown voltage and device dimensions has been investigated using both of I-V, C-V characteristics. It has been found that annealing of the contact modifies the device characteristics, in particular, the breakdown behaviors.

Opportunity of creation of several optoelectronic devices such as optron, photoreceiver, switch device, IR-to-visible signal transformer and others based on i-n-GaN light emitting diodes (LED) is shown. Technology and properties of GaN LED in relation to the desired device properties are discussed. A discussion of issues relating to electrical and optical positive feed back between the device elements is also included.

InGaN multi-quantum-well (MQW) structure laser diodes with A10.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN substrate were demonstrated to have an estimated lifetime of more than 10,000 hours under continuous-wave operation at 20°C. Under operation at a high temperature of 50°C, the lifetime was longer than 1,000 hours. The activation energy of the lifetime was estimated to be 0.5 eV. With the operating current increasing above the threshold, a self-pulsation with a high frequency of 3.5 GHz was observed. The relative intensity noise (RIN) less than -145 dB/Hz was obtained even at the 6% optical feedback using the high-frequency modulation of 600 MHz. The threshold carrier density of the InGaN MQW-structure LDs was estimated to be 3 × 1019/cm3 using a carrier lifetime of 1.8 ns.

Advantages of applying III-V nitride materials for short wavelength light-emitting devices despite their extremely high dislocation density are discussed from the viewpoint of dislocation motion. There are also difficulties proper to these materials, which make it difficult to fabricate laser diodes. We present recent works to realize high performance laser diodes. We introduce the nitrogen ambient metalorganic chemical vapor deposition (MOCVD) growth which realizes the highly p-typed GaN films without any post-treatments. Some of our results respecting the room temperature pulsed operation of the conventional laser diode and the advanced inner stripe (IS) laser diode with InGaN based multi-quantum-well (MQW) grown by MOCVD are reported.

Single crystal thin films with compositions from the A1N-InN-GaN system were grown via metal-organic chemical vapor deposition (MOCVD) on single crystal 6H-SiC substrates. Blue light emitting (LED) and laser diode (LD) structures were fabricated. The conducting buffer layer LEDs employed an AlGaN buffer layer which provides a conduction path between SiC and the active device region. The external quantum efficiency of the LEDs was 3% at 20 mA- 3.6V and peak emission wavelength of 430 nm. Violet and blue LDs were fabricated and consisted of an 8-well InGaN/GaN multiple quantum well (MQW) active region in a separate confinement heterostructure (SCH) design. Lasing was obtained both on structures using an insulating buffer layer, and also on structures using a conducting buffer layer. The resulting lasers operated at room temperature using pulsed and continuous wave operation with an emission wavelength of 404-435 rim. The lowest threshold current density obtained for lasing was 11 kA/cm2.

This paper reports the events at NCSU leading up to and including those of June 5, 1997 which produced the first demonstration of a nitride laser diode on silicon carbide – and the very first nitride laser demonstration outside of Japan. All of the laser diode samples tested at NCSU were designed, grown, and fabricated into cleaved cavity test structures at Cree Research. Laser testing at NCSU consisted of spectral emission versus current measurements, light output power versus current (L-I) measurement, and light output polarization measurements versus current. The first successful laser on silicon carbide emitted at 402.6 nm. Subsequently, lasers displaying outputs ranging from 402.6 to 430.2 have been successfully tested at NCSU.

We report the crystal growth and the characteristics of InGaN multiple quantum well (MQW) laser diodes grown on a 6H-SiC substrate using a low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). We discuss the buffer layer, the control of InGaN and AlGaN alloy composition, the magnesium doping of GaN and AlGaN, and the characteristics of the MQW structure. We also demonstrate the room-temperature pulsed operation of the laser diode. The threshold voltage was reduced to 15 V by improving the p-contact resistance. The threshold current was reduced to 500 mA by changing the MQW structure and employing high reflection coating.

Blue-emitting nitride laser diodes have been fabricated on a-plane sapphire (1120). The active region is composed of 10 In.18Ga.82N quantum wells, which were grown by MOCVD at atmospheric and low pressure in a modified two-flow Thomas-Swan Ltd. horizontal reactor. The chemical precursors used were trimethylgallium (TMGa), trimethylindium (TMIn), trimethylaluminum (TMAI), ammonia, and disilane. The n- and p-contacts were formed by depositing Ti/Al/Ni/Au and Ni/Au/Ni/Au, respectively. Diode wafers were thinned to less than 50 μm before they were cleaved along the sapphire r-plane (1120).Lasers show TE polarization, spectral line narrowing, and far field interference patterns above the lasing threshold. The laser emission spectra peak at 410–420 nm. Under pulsed operation at room temperature, the lowest observed threshold current density was 15 kA/cm2 with threshold voltages ranging from 50–90 V. Differential efficiencies are as high as 7% with maximum output powers greater than 50 mW. Near and far field mode patterns are presented. Structures are gain-guided devices with each device occupying a mesa with a width of 125 μm. Device widths range from 3 to 20 μm, with lengths of 500 to 1200 μm.