Photoluminescence (PL) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers grown by organometallic chemical-vapor deposition. Both undoped and Mg-doped GaN films were investigated. Samples were studied from three laboratories to obtain general trends and behavior. The ODMR experiments on the undoped films reveal resonances from both effective-mass and deep-donor states. These two features appear in all of the undoped GaN films and are most likely associated with an intrinsic point defect or defects. The same two donor resonances and a Mg-related quasideep acceptor resonance are found from the ODMR experiments on the Mg-doped samples. The energy level of the deep-donor state is found from ODMR spectral studies to be \ensuremath{\sim}1 eV below the conduction-band minimum. The PL experiments provide evidence for shallow acceptors in the undoped films and in a GaN sample lightly doped by residual Mg in the growth reactor. Models are proposed that describe the capture and recombination among these shallow- and deep-donor and acceptor states. The assignments of the magnetic resonance features are compared with the latest theoretical calculations of defect states in GaN.

We fabricated a 0.25 μm gate length AlGaN/GaN heterostructure field effect transistor (HFET) with a maximum extrinsic transconductance of 27 mS/mm (at room temperature) limited by the source series resistance. The device exhibited an excellent pinch-off and a low parasitic output conductance in the saturation regime. We measured the cutoff frequency fT and the maximum oscillation frequency fmax as 11 and 35 GHz, respectively. These values are superior to the highest reported values for field effect transistors based on other wide band-gap semiconductors such as SiC. These results demonstrate an excellent potential of AlGaN/GaN HFETs for microwave and millimeter wave applications.

Availability of optoelectronic components operating in the U V-Visible part of the spectrum opens several exciting and important system applications. Solid state ultraviolet and blue-green lasers can increase the optical data storage density of CDROM/WORM and magneto-optical disks by a factor of four. They are also ideally suited for environmental pollutant identification and monitoring. On the other hand, solid state ultraviolet detectors that do not respond to visible or IR radiation are highly desirable for various commercial systems. These include medical imaging, industrial boiler systems, fire/flame safeguard systems around oil and gas installations and several military applications. A key requirement for these ultraviolet laser and sensor devices is the availability of a semiconductor material system with high quality controlled doping and fabrication technology. AlxGa1−xN and InxGa1−xN for which the direct bandgap can be tailored from the visible to the deep UV is such a material system. Ours and several other research groups (nationally and internationally) have been developing AlxGa1−xN materials and processing technologies over the past several years. Recently, by employing innovative approaches, significant advances have been made in heteroepitaxy of AlxGa1−xN on sapphire substrates. Also, controlled n and p-type doping has been achieved. Several high performance devices that form the basis of exciting future research have been demonstrated. These include high responsivity visible blind ultraviolet sensors, basic transistor structures and high power blue light emitting diodes. These pave the way for future research leading to exciting products such as blue-green lasers and UV-imaging arrays. The demonstrated transistor structures are foundation for building AlxGa1−xN -GaN based high power, high frequency and high temperature electronic components. In this paper, we will summarize some of our recent work and reflect on the potential and the issues in AlxGa1−xN-InxGa1−xN based device development.

We have fabricated photoconductive and photovoltaic ultraviolet sensors from GaN single layers and pn-junctions. These sensors exhibit a sharp long wavelength cut-off in responsivity at the bandgap (365 nm). The active layers (GaN) were deposited using low pressure MOCVD. The p-type doping was accomplished using Mg as the dopant. Photoconductive, and schottky barrier detectors were then fabricated using photolithography, reactive ion etching and contact metallizations. These processing techniques were developed specific to the A1xGa1-xN material system. We will discuss growth, fabrication and characterization details for these various device types. The measured values of device parameters will be contrasted with those estimated from active layer material characterization.

We report the observation of room temperature violet (415 nm) stimulated emission in the vertical cavity mode from photopumped GaN/In0.25Ga0.75N heterojunctions. The InGaN/GaN heterojunction was deposited over sapphire substrates using low-pressure metalorganic chemical vapor deposition and was of high enough optical quality to achieve room-temperature stimulated emission. The observed emission intensity was found to be a nonlinear function of incident optical pump power density. At threshold we observe a clear line narrowing of the output optical signal from 20 to 1.5 nm full width at half-maximum.

We report the first low pressure (76 Torr) metalorganic chemical vapor deposition of AlxGa1−xN using trimethylamine alane (TMAAl) as the aluminum source. AlxGa1−xN epilayers deposited using TMAAl exhibited an excellent surface morphology and very strong room temperature photoluminescence. For AlN layers (using TMAAl as the aluminum precursor) we obtained a total carbon contamination level as low as 1017 cm−3.

Optically detected magnetic resonance has been observed from GaN. Two magnetic resonances have been detected on the 2.2 eV-deep photoluminescence band. The first resonance is sharp [full width at half-maximum (FWHM) ∼2.2 mT] with g∥=1.9515±0.0002 and g⊥=1.9485±0.0002 and is assigned to conduction electrons, in agreement with recent electron paramagnetic resonance (EPR) studies of similar samples. The second feature, which has not been seen by EPR, is much broader (FWHM∼13 mT) with g∥=1.989±0.001 and g⊥=1.992±0.001. These parameters indicate a deep state. A tentative assignment is made to a deep state associated with the N vacancy.

Recently several research groups, including ours, have reported on the deposition of extremely high quality single crystal GaN layers over sapphire substrates. One of the keys to obtaining the high quality was the use of a thin AlN or GaN buffer layer between the sapphire substrate and the grown film. In this communication, we discuss the crystallinity and the influence of the buffer layer in controlling the crystalline, optical, and the electrical properties of the GaN depositions. We also compare the use of GaN and AlN as the buffer layer material. Our results indicate that the buffer layer thickness and the total film thickness are the key factors controlling the electrical, optical, and crystalline properties of the GaN depositions over sapphire substrates.

Single crystal GaN thin films resulting from various deposition techniques are usually dominated by residual donors. To date, the true nature of this donor conduction is not known. Nitrogen vacancies, residual oxygen, and growth defects are cited as potential causes for the residual n-type conduction. In this communication we present the first systematic study of near conduction band edge states in n-type GaN samples deposited over basal plane sapphire substrates using low pressure metal organic chemical vapor deposition. Electron spin resonance, low temperature photoluminescence, and Van der Pauw–Hall measurements were used as the basis for our study. We concluded that the residual n-type conduction in GaN results from a band of delocalized donors.

In this letter we report the fabrication and characterization of a metal semiconductor field effect transistor (MESFET) based on single crystal GaN. The GaN layer was deposited over sapphire substrate using low pressure metalorganic chemical vapor deposition. MESFET devices were fabricated on isolated mesas using TiAu for the source and drain ohmic contacts and silver for the gate Schottky. For devices with a gate length of 4 μm (channel opening, i.e., source to drain separation of 10 μm), a transconductance of 23 mS/mm was obtained at −1 V gate bias. Complete pinch-off was observed for a gate potential of −12 V.

Recently, using low pressure metalorganic chemical vapor deposition we grew extremely high quality single crystal GaN films over basal plane sapphire substrates. Optimization of the buffer layers and the total film thickness played a key role in determining the electrical, optical, crystalline and surface qualities. In this communication we present the surface characterization results for these high quality GaN layers. Reflection high energy electron diffraction, low energy electron diffraction (LEED), and electron energy loss spectroscopy data are presented for clean GaN surfaces. The cleaning procedure was developed using Auger electron spectroscopy analysis. These electron diffraction data indicate the grown surface to be excellent single crystal GaN with a wurtzite structure. To the best of our knowledge ours is the first reported LEED data for single crystal GaN.

In this letter we report the fabrication of GaN/AlN short-period superlattices using switched atomic layer epitaxy. Superlattice structures with GaN well thicknesses ranging from 2.6 to 20.8 Å (with AlN barrier thicknesses of 2.5, 7.5, and 15 Å) were deposited over basal plane sapphire and characterized for their structure, crystallinity, and optical properties. Cross-sectional transmission electron micrographs indicate GaN/AlN interfaces and the superlattice surfaces are atomically smooth. The structures exhibit strong room temperature photoluminescence and a sharp absorption edge indicating a high optical quality.

In this letter we report the fabrication and dc characterization of a high electron mobility transistor (HEMT) based on a n-GaN-Al0.14Ga0.86N heterojunction. The conduction in our low pressure metalorganic chemical vapor deposited heterostructure is dominated by two-dimensional electron gas at the heterostructure interface. HEMT devices were fabricated on ion-implant isolated mesas using Ti/Au for the source drain ohmic and TiW for the gate Schottky. For a device with a 4 μm gate length (10 μm channel opening, i.e., source-drain separation), a transconductance of 28 mS/mm at 300 K and 46 mS/mm at 77 K was obtained at +0.5 V gate bias. Complete pinchoff was observed for a −6 V gate bias.

In this letter we report the fabrication and characterization of Schottky barrier photodetectors on p-type GaN films. These films were grown over basal plane sapphire substrates using low pressure metalorganic chemical vapor deposition and magnesium as the p-type dopant. The current-voltage and capacitance-voltage characteristics were measured for Ti/Au Schottky barriers for a film with a p doping of 7×1017 cm−3. We measured a 1.5 V forward turn on and a 3 V reverse breakdown. The zero bias responsivity of a detector with 1 mm2 area was measured to be 0.13 A/W. For these photovoltaic detectors, the photoresponse was nearly constant from 200 to 365 nm and fell sharply by several orders of magnitude for wavelengths above 365 nm.