GaN and related nitride semiconductors have attracted great attention in view of their wide applications in photonics and high temperature & high power electronic devices. Among other issues, reduction of defect densities by forming these interfaces at lower temperature and on novel substrates has been the motivation for several researchers. In the present study ion-induced conversion of Si (111) surface into silicon nitride at room temperature is optimized and used as substrate for the growth of Ga films. These Ga films are again nitrided by optimal N+ ion bombardment. Experiments have been performed in-situ in an ultra high vacuum chamber equipped with a Ga source and X-ray photoelectron spectrometer (XPS) at base pressure of 2×10-10 torr. The energy dependence of the nitridation is carefully performed at constant flux. The results clearly demonstrate the Si-N bond formation after a energy of 2 keV and the formation of GaN layer after 800eV of ion bombardment on Si (111) 7×7 surface and Ga adsorbed silicon nitride surface, respectively. The FWHM and chemical shifts in the core-level spectra of Si(2p), Ga(2p) and N(1s) have been analyzed to probe the interface reactions. The results demonstrate a possible novel and low temperature approach towards the integration of III-nitride & silicon technologies, since silicon nitride bonds can act as barriers to dislocation propagation.

Deep Ultra Violet (UV) emitters are of particular interest for applications including, but not limited to, biological detection and sterilization. Within the III-Nitride material system, Aluminum Gallium Nitride (AlxGa1-xN) alloys are the most promising for UV device fabrication due to the wide, direct band gap. The growth of high quality AlxGa1-xN alloys via Metal Organic Vapor Phase Epitaxy (MOVPE) is challenging due to large sticking coefficient of the Al species compared to that of Ga and also the high reactivity of Al precursors. As a result, films are often characterized by large dislocation densities, cracks, and poor conductivity. Digital alloy growth, or Short Period Superlattices (SPS), consisting of layers of binary or ternary alloys with a period thickness of a few monolayers has been shown to be a viable means of growing high quality ternary alloys via Metal Organic Vapor Phase Epitaxy (MOVPE). In certain materials, such as AlGaInP, the electronic properties of digitally grown alloys differ considerably from the equivalent random alloy. Specifically, the bandgap has been shown to differ significantly from the equivalent random alloy. As a result, digital alloy growth presents the potential to further engineer material properties. However, the influence of digital growth on the electronic properties of III-Nitride alloys has not been extensively characterized. This study focuses on Aluminum Gallium Nitride (AlxGa1-xN) alloys grown using a digital technique via MOVPE. The influence of the growth technique over a wide range of compositions is reported along with the electronic properties.

Low dislocation density epitaxial layers of AlxGa1-xN can be grown pseudomorphically on c-face AlN substrates prepared from high quality, bulk crystals. Here, we will report on initial characterization results from deep ultraviolet (UV) light emitting diodes (LEDs) which have been fabricated and packaged from these structures. As reported previously, pseudomorphic growth and atomically smooth surfaces can be achieved for a full LED device structure with an emission wavelength between 250 nm and 280 nm.

A benefit of pseudomorphic growth is the ability to run the devices at high input powers and current densities. The high aluminum content AlxGa1-xN (x∼70%) epitaxial layer can be doped n-type to obtain sheet resistances < 200 Ohms/sq/μm due to the low dislocation density. Bulk crystal growth allows for the ability to fabricate substrates of both polar and non-polar orientations. Non-polar substrates are of particular interest for nitride growth because they eliminate electric field due to spontaneous polarization and piezoelectric effects which limit device performance. Initial studies of epitaxial growth on non-polar substrates will also be presented.

Selective area growth (SAG) of a-plane GaN grown on r-plane sapphire with a stripe orientation along <1-100> was investigated. The key technology of facet-control is optimizing the growth temperature and the reactor pressure. Our experiments reveal that the growth temperature determined facet form: in samples grown at 1000 °C, the structure consists of {11-22}and (000-1); with increasing growth temperature to 1050 °C, the area of {11-22} facet gradually decreases, and two new planes, (0001) and {11-20} facets form; eventually, in samples grown at 1000 oC, the {11-22} facet completely disappears, (0001) and {11-20} facet continue to increase to form a rectangle cross-section. The reactor pressure determines the ratio of the lateral growth rate and the vertical growth rate: with reactor pressure decreasing from 500 torr to 100 torr, the rectangle structure gradually decreases the height and increases the width, and the volume nearly keeps constant.

We present the structural and surface characterization of the alloy formation of scandium gallium nitride ScxGa1-xN(001)/MgO(001) grown by radio-frequency molecular beam epitaxy over the Sc range of x = 0-100%. In-plane diffraction measurements show a clear face-centered cubic surface structure with single-crystalline epitaxial type of growth mode for all x; a diffuse/distinct transition in the surface structure occurs at near x = 0.5. This is consistent with out-of-plane diffraction measurements which show a linear variation of perpendicular lattice constant for x = 0 to 0.5, after which the out-of-plane lattice parameter becomes approximately constant. The x = 0.5 transition is interpreted as being related to the cross-over from zinc-blende to rock-salt structure.

High electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are promising for high power, high speed, and high temperature operation. Especially, AlGaN-GaN HEMTs grown on semi-insulating (SI) SiC substrates are the most promising for both military and commercial applications. High performance characteristics from these devices are possible in part due to the presence of high two-dimensional electron gas charge sheet density maintaining a high Hall mobility at the AlGaN barrier-GaN buffer hetero-interface and in part due to high thermal conductivity of the SiC substrates. However, long-term reliability of these devices still remains a major concern because of the large number of traps and defects present both in the bulk as well as at the surface leading to undesirable characteristics including current collapse. We report on the study of traps and defects in two MOCVD-grown structures: Al0.27Ga0.73N HEMTs on SI SiC substrates and Al0.27Ga0.73N Schottky diodes on conducting SiC substrates. Our HEMT structures consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer show a charge sheet density of ∼1013/cm2 and a Hall mobility of ∼1500cm2/V·sec. Deep level transient spectroscopy (DLTS) was employed to study traps in AlGaN Schottky diodes and HEMTs fabricated with different Schottky contacts consisting of Pt/Au and Ni/Au. Focused ion beam was employed to prepare both cross-sectional and plan view TEM samples for defect analysis using a high resolution TEM.

Ball milling of ammonothermally synthesized GaN powders was performed in an ethanol solution for a variety of durations, resulting in average particle sizes of nanometer. The ball milled powders showed an obviously brightened color and improved dispersability, indicating reduced levels of aggregation. X-ray diffraction (XRD) peaks of the ball milled GaN powders were significantly broadened compared to those of the as-synthesized powders. The broadening of the XRD peaks was partially attributed to the reduction in the average particle size, which was confirmed through SEM analyses. On the other hand, rare earth doping of commercial GaN powders was also achieved through a ball mill assisted solid state reaction process. Rare earth salts were mixed with GaN powder by ball milling. The as-milled powders were heat treated under different conditions to facilitate the dopant diffusion. Luminescence properties of the rare earth doped GaN powders at near infrared range were investigated and the results were discussed.

For light emitting diodes (LEDs) to be used for general lighting, high efficiencies would need to be retained at high injection levels to meet the intensity and efficiency requirements. In this regard, it is imperative to overcome the observed drop in LED efficiency at high injection levels beyond that would be expected from junction temperature. The suggested genesis of efficiency degradation includes electron overflow or spillover, also suggested to be aided by polarization induced electric field, Auger recombination, current crowding, and elevated junction temperature. Setting the junction temperature aside, the degree to which or even whether each of these mechanisms plays a role is still under debate. We have undertaken a series of experiments to isolate, whenever possible, the aforementioned processes in an effort to determine the causes of efficiency loss at high injection levels. By using 1μs pulsed electrical injection with 0.1% duty cycle, we were able to minimize the effect of the junction temperature. By changing the design of the multiple quantum well region as well as by employing or not employing electron blocking layers, we demonstrated the important role that electron overflow plays on efficiency. Furthermore, by also exploring the same on non polar surfaces and observing any lack of dispersion in terms of the effect of the electron blocking layer we can conclude that the polarization induced field does not seem to play a major role. LEDs on non polar surface with no notable efficiency degradation, up to current densities of about 2250 Acm-2 used for measurements, have been obtained which seems to imply that Auger recombination up to these injection levels is not of major importance, at least in the structures investigated. The effect of current crowding on efficiency droop was investigated by comparing semitransparent Ni/Au p-contacts and transparent conducting oxide contacts (Ga-doped ZnO). Because the latter showed notably reduced efficiency degradation at high injection levels, we can conclude that current crowding plays a role as well.

Ultrathin (∼10 nm) InN ion sensitive field effect transistors (ISFETs) are functionalized by immobilized label-free oligonucleotide probes with 3-mercaptopropyltrimethoxysilane (MPTMS) through molecular vapor deposition (MVD) technique. This layer on the InN surface serves the function of selectively detecting the hybridization of complementary deoxyribonucleic acid (DNA). Using MVD technique to perform the gas-phase silanization of MPTMS provided a time-saving and simple method to reach 68° water contact angle after 1.5 h treatment. High resolution X-ray photoelectron spectroscopy (HRXPS) was employed to analyze the surface characteristics after functionalization. Modified probes DNA were covalently bonded to MPTMS-covered gate surface of InN ISFETs. And further hybridized with complementary DNA For a 12-mer oligonucleotide probe, a significant drain-source current decrease (∼ 6 μA) was observed for the hybridization with complementary DNA solution of 100 nM. In contrast, the noncomplementary DNA with single-base mismatch did not show obvious current changes. Functionalized ultrathin InN ISFETs for DNA sequence detection demonstrate the promise of biological sensing and genetic diagnosis applications.

The Mg doped AlN/AlxGa1-xN (0.03 ≤ x ≤ 0.05) short period superlattices (SPSLs) were grown by gas source molecular beam epitaxy on (0001) sapphire substrates. The average AlN mole fraction is ∼ 0.7 and the hole concentration is ∼ 7×1017 cm-3. Contacts formed to the SPSLs using Ni/Au bilayer are found to have specific contact resistance ∼ 5×10-5 Ωcm2 near room temperature and to show weak temperature dependence attributed to activation of Mg acceptors in the AlN barriers of SPSLs. These p-SPSLs are attractive for fabrication of transparent low resistive ohmic contacts for deep UV LEDs.

N-polar and Ga-polar GaN grown on c-plane sapphire by a metal-organic chemical vapor deposition (MOCVD) system were used to fabricate platinum deposited Schottky contacts for hydrogen sensing at room temperature. Wurtzite GaN is a polar material. Along the c-axis, there are N-face (N-polar) or Ga-face (Ga-polar) orientations on the GaN surface. The Ohmic contacts were formed by lift-off of e-beam deposited Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (1200 Å). The contacts were annealed at 850°C for 45 s under a flowing N2 ambient. Isolation was achieved with 2000 Å plasma enhanced chemical vapor deposited SiNx formed at 300°C. A 100 Å of Pt was deposited by e-beam evaporation to form Schottky contacts. After exposure to hydrogen, Ga-polar GaN Schottky showed 10% of current change, while the N-polar GaN Schottky contacts became fully Ohmic. The N-polar GaN Schottky diodes showed stronger and faster response to 4% hydrogen than that of Ga-polar GaN Schottky diodes. The abrupt current increase from N-polar GaN Schottky exposure to hydrogen was attributed to the high reactivity of the N-face surface termination. The surface termination dominates the sensitivity and response time of the hydrogen sensors made of GaN Schottky diodes. Current-voltage characteristics and the real-time detection of the sensor for hydrogen were investigated. These results demonstrate that the surface termination is crucial in the performance of hydrogen sensors made of GaN Schottky diodes.

We report selective area epitaxy of InGaN/GaN micron-scale stripes and rings on patterned (0001) AlN/sapphire. The objective is to elevate indium incorporation for achieving blue and green emission on semi-polar crystal facets. In each case, GaN structures were first produced, and the InGaN quantum wells (QWs) were subsequently grown. The pyramidal InGaN/GaN stripe along the <11-20> direction has uniform CL emission at 500 nm on the smooth {1-101} sidewall and at 550 nm on the narrow ridge. In InGaN/GaN triangular rings, the structures reveal smooth inner and outer sidewall facets falling into a single type of {1-101} planes. All these {1-101} sidewall facets demonstrate similar CL spectra which appear to be the superposition of two peaks at positions 500 nm and 460 nm. Spatially matched striations are observed in the CL intensity images and surface morphologies of the {1-101} sidewall facets. InGaN/GaN hexagonal rings are comprised of {11-22} and {21-33} facets on inner sidewalls, and {1-101} facets on outer sidewalls. Distinct CL spectra with peak wavelengths as long as 500 nm are observed for these diverse sidewall facets of the hexagonal rings.

We report on microstructure analysis of aluminum nitride (AlN) epilayers by transmission electron microscopy (TEM). AlN epilayer samples were grown on sapphire substrates by metal organic chemical vapor deposition. Cross section and plan view images were taken by TEM to investigate the threading dislocations. Edge type threading dislocations dominate the total dislocation density. The threading dislocations are greatly reduced by inserting an intermediate layer prior to the growth of high temperature AlN epilayer. The dislocations further reduce with increasing thickness. Our results correlate with the dislocation density estimated from x-ray diffraction analysis.

Non-polar cubic AlGaN/GaN HFETs were grown by plasma assisted MBE on 3C-SiC substrates. Both normally-on and normally-off HFETs were fabricated using contact lithography. Our devices have a gate length of 2 μm, a gate width of 25 μm, and source-to-drain spacing of 8 μm. For the source and drain contacts the Al0.36Ga0.64N top layer was removed by reactive ion etching (RIE) with SiCl4 and Ti/Al/Ni/Au ohmic contacts were thermally evaporated. The gate metal was Pd/Ni/Au. At room temperature the DC-characteristics clearly demonstrate enhancement and depletion mode operation with threshold voltages of +0.7 V and −8.0 V, respectively. A transconductance of about 5 mS/mm was measured at a drain source voltage of 10 V for our cubic AlGaN/GaN HFETs, which is comparable to that observed in non-polar a-plane devices. From capacity voltage measurements a 2D carrier concentration of about 7×1012 cm-2 is estimated. The influence of source and drain contact resistance, leakage current through the gate contact and parallel conductivity in the underlaying GaN buffer are discussed.

Photoelectrochemical properties of Ga- and N-face GaN grown by hydride vapor phase epitaxy (HVPE) were investigated. The properties were also compared with Ga-face GaN grown by metal-organic vapor phase epitaxy (MOVPE). The flatband potentials were in order of Ga-face GaN grown by MOVPE < N-face GaN < Ga-face GaN. The highest photocurrent density at zero bias was obtained from the N-face GaN. The photocurrent density was over 3 times larger than that of Ga-face GaN.

We have demonstrated enhancement-mode n-channel gallium nitride (GaN) MOSFETs on Si (111) substrates with high-temperature operation up to 300 °C. The GaN MOSFETs have good normally-off operation with the threshold voltages of +2.7 V. The MOSFET exhibits good output characteristics from room temperature to 300 °C. The leakage current at 300°C is less than 100 pA/mm at the drain-to-source voltage of 0.1 V. The on-state resistance of MOSFET at 300°C is about 1.5 times as high as that at room temperature. These results indicate that GaN MOSFET is suitable for high-temperature operation compared with AlGaN/GaN HFET.

We have characterized the surface charge on a variety of GaN samples using two surface potential techniques, conventional Kelvin probe and scanning Kelvin probe microscope (SKPM). Kelvin probe was primarily used to measure the change in surface potential under UV illumination, otherwise known as the surface photovoltage (SPV). Due to band bending near the semiconductor surface of about 1 eV in dark conditions, the SPV signal for n-type GaN typically reaches 0.5 to 0.6 eV upon switching on UV light. This value can slowly decrease by up to 0.3 eV during UV illumination in air ambient for 2-3 hours. We report that samples with many hours of ambient UV exposure do not show this slow decrease during SPV measurements, consistent with the UV-induced growth of a thicker surface oxide that limits charge transfer. In addition to prolonged UV exposure, the surface contact potential was also manipulated by local charge injection. In this procedure, the surface is charged using a metallized atomic force microscope tip which is scanned in contact with the sample. Subsequent SKPM measurements indicate an increase or decrease in the surface contact potential for the charged region, depending on the applied voltage polarity. Measurements of the discharge behavior in dark for these regions show a logarithmic time behavior, similar to the decay behavior during our observations of SPV transients after switching off the light. As expected, illumination of the surface increases the discharge rate and restores the charged area to its original state.

High crystal quality crack-free AlN on sapphire was grown by low pressure metal organic vapor phase epitaxy (MOVPE). Growth experiments combine two recent approaches: the ammonia pulse-flow method and ammonia continuous-flow growth mode by varying the V/III ratio. The detailed aspects of MOVPE, employing the periodic multilayer approach at low, intermediate, and high temperatures are described. This method yields significant reduction of screw dislocation density and provides very smooth surface for thin AlN layers.

Over the past few decades, there has been considerable research and advancement in surface acoustic wave (SAW) technology. At present, SAW devices have been highly successful as frequency band pass filters for the mobile telecommunications and electronics industries. In addition to their inherent frequency selectivity, SAW devices are also highly sensitive to surface perturbations. This sensitivity, along with a relative ease of manufacture, makes SAW devices ideally suited for many sensing applications including mass, pressure, temperature, and biosensors. In the area of biosensing, surface plasmon resonance (SPR) and quartz crystal microbalances (QCM) are still in the forefront of research and development, but advancement in SAW sensors could prove to have significant advantages over these technologies. This study investigates the advantages of using aluminum nitride (AlN) as a material for SAW sensors. AlN retains its piezoelectric properties at relatively high temperatures when compared to more common piezoelectric materials such as lead zirconium titanate (PZT), lithium tantalate (LiTaO3) and zinc oxide (ZnO). AlN is also a very robust material making it suitable for biosensing applications where the sensing target is selectively absorbed by an active layer on the device which may attack the piezoelectric layer. AlN thin films of different thicknesses have been deposited on Si substrates by DC reactive sputtering. Rayleigh-wave SAW devices have been fabricated by the deposition of platinum contacts and interdigital transducers (IDTs) onto AlN thin films using standard photolithographic processes. Experiments have been conducted to measure Rayleigh velocities, resonant frequencies, and insertion loss. Experimental results are compared to theoretical calculations.

The influence of the growth temperature on the phase stability and composition of single-phase In1-xGaxN epilayers has been studied. The In1-xGaxN epilayers were grown by high-pressure Chemical Vapor Deposition with nominally composition of x = 0.6 at a reactor pressure of 15 bar at various growth temperatures. The layers were analyzed by x-ray diffraction, optical transmission spectroscopy, atomic force microscopy, and Raman spectroscopy. The results showed that a growth temperature of 925°C led to the best single phase InGaN layers with the smoothest surface and smallest grain areas