Due to the wide bandgap and other key materials properties of 4H-SiC, SiC MOSFETsoffer performance advantages over competing Si-based power devices. For example,SiC can more easily be used to fabricate MOSFETs with very high voltage ratings,and with lower switching losses. Silicon carbide power MOSFET development hasprogressed rapidly since the market release of Cree’s 1200V 4H-SiCpower MOSFET in 2011. This is due to continued advancements in SiC substratequality, epitaxial growth capabilities, and device processing. For example,high-quality epitaxial growth of thick, low-doped SiC has enabled thefabrication of SiC MOSFETs capable of blocking extremely high voltages (up to15kV); while dopant control for thin highly-doped epitaxial layers has helpedenable low on-resistance 900V SiC MOSFET production. Device design andprocessing improvements have resulted in lower MOSFET specific on-resistance foreach successive device generation. SiC MOSFETs have been shown to have a longdevice lifetime, based on the results of accelerated lifetime testing, such ashigh-temperature reverse-bias (HTRB) stress and time-dependent dielectricbreakdown (TDDB).

Interfacial dislocations (IDs) and half-loop arrays (HLAs) present in theepilayers of 4H-SiC crystal are known to have a deleterious effect on deviceperformance. Synchrotron X-ray Topography studies carried out on n-type 4H-SiCoffcut wafers before and after epitaxial growth show that in many cases BPDsegments in the substrate are responsible for creating IDs and HLAs during CVDgrowth. This paper reviews the behaviors of BPDs in the substrate during theepitaxial growth in different cases: (1) screw-oriented BPD segmentsintersecting the surface replicate directly through the interface during theepitaxial growth and take part in stress relaxation process by creating IDs andHLAs (Matthews-Blakeslee model [1] ); (2) non-screw oriented BPD half loopintersecting the surface glides towards and replicates through the interface,while the intersection points convert to threading edge dislocations (TEDs) andpin the half loop, leaving straight screw segments in the epilayer and thencreate IDs and HLAs; (3) edge oriented short BPD segments well below the surfaceget dragged towards the interface during epitaxial growth, leaving two longscrew segments in their wake, some of which replicate through the interface andcreate IDs and HLAs. The driving force for the BPDs to glide toward theinterface is thermal stress and driving force for the relaxation process tooccur is the lattice parameter difference at growth temperature which resultsfrom the doping concentration difference between the substrate and epilayer.

We demonstrated the fabrication of 10 emitters InGaN laser diode array of themaximum output power of 9 W at 420 nm. The device as a whole has thedifferential efficiency of above 1 W/A. The maximum output power is limited to 9W (pulse operation) by catastrophic mirror damage or to around 5 W in CWoperation by thermal roll-over. Larger arrays with stripes width of around 15µm and numbers of emitters up to 20 should enable reaching 20 W, whichis suitable for light engine of desktop projectors and a building block ofcinema theater projectors.

The thermal conductivity, bulk modulus, thermal expansion and heat capacity forbulk β-Ga2O3 are calculated from latticedynamics using both a quasi-harmonic approximation and an anharmonicforce-constant approach involving a solution of the linearized Boltzmanntransport equation. The role of anharmonicity inβ-Ga2O3 is determined to be small, whichleads to the weak volume dependence of the calculated thermal conductivity. Thenegligible anharmonic contribution to the overall thermal conductivity isconsistent with both thermal expansion measurements and also with comparisonsbetween the quasi-harmonic and anharmonic methods. Phonon-mode-dependentGrüneisen parameters are found to be weakly dependent on temperature.Negative values of the mode Grüneisen parameters are found for certainlow energy optical modes, but their net effect on the overall thermal expansionis insignificant. Bulk modulus as well as heat capacity are also given and foundto be in agreement with experimental results.

We study effects of deposition temperature on growth mode and surface morphologyof hetero-epitaxial (ZnO)x(InN)1-x (ZION) films on ZnOtemplates. ZION films deposited at low temperature of RT-250oC growtwo dimensionally, whereas ZION films deposited at high temperature of350-450oC grow three dimensionally. Growth mode is changed fromtwo-dimensional growth mode to three-dimensional one, because the criticalthickness where film strain begin to relax decreases with increasing thedeposition temperature. At high deposition temperatures, the number of pointdefects in ZION films decreases because migration of adatoms on the growingsurface is enhanced. The strain energy in ZION films increases with increasingthe deposition temperature, since the strain energy is not released by pointdefects. Therefore, lattice relaxation for the higher deposition temperaturebegins at the smaller film thickness to release the strain energy. As a result,ZION films with atomically-flat surface were obtained even at RT.

Scintillation properties are often studied by photo-luminescence (PL) andscintillation measurements. In this work, we combine X-ray-induced luminescence(XRIL) spectroscopy [Review of Scientific Instruments 83, 103112 (2012)] with PLand standard scintillation measurements to give insight into the scintillationproperties of un-doped ZnO single crystals. XRIL revealed that ZnO luminescenceproportionally increases with X-ray power and exhibits excellent linearity -indicating the possibility of developing radiation detectors with good energyresolution. By coupling ZnO crystals to fast photomultiplier tubes andmonitoring the anode signal, rise times as fast as 0.9 ns were measured.

RF-plasma MBE was used to epitaxially grow 4 – 100-nm-thick metallicβ-Nb2N thin films on hexagonal SiC substrates. When theN/Nb flux ratios are greater than one, the most critical parameter forhigh-quality β-Nb2N is the substrate temperature. The X-raydiffraction (XRD) of films grown between 775 °C and 850 °Cdemonstrates pure β-Nb2N phase formation which was alsoconfirmed by X-ray photoelectron spectroscopy and transmission electronmicroscopy measurements. Using the (0002) and (21 $\bar 3$1) XRD peaks of a β-Nb2N film grown at 850°C reveals a 0.68% lattice mismatch to the 6H-SiC substrate. Thissuggests that β-Nb2N can be used for high-qualitymetal/semiconductor heterostructures that cannot be fabricated at present.

AIZO (Al and In codoped ZnO) thin films were deposited on soda lime glasssubstrates by the ultrasonic spray pyrolysis (USP) technique. The sprayingsolution was prepared from zinc acetate dihydrate, aluminum acetyl acetonate andindium acetate. Depositions were carried out at three different temperatures,425, 450 and 475 °C. Structural, morphological, optical and electricalproperties were examined with respected to the deposition temperatures. All AIZOfilms grown with (002) preferential orientation confirmed from X-ray diffractionanalysis. Hexagonal nanostructures were observed from Scanning electronmicroscopy (SEM) analysis. Minimum electrical resistivity of 2.52x10-3 Ω-cm was achieved for AIZO films.

Gallium Nitride (GaN) and other III-N semiconductors are rapidly gainingimportance in high power and high frequency electronic applications. III-Nmaterial based devices are fabricated on heterostructures that are usually grownby high vacuum techniques such as metal-organic chemical vapor deposition(MOCVD) or molecular beam epitaxy (MBE). However, in many applications, it isnecessary to regrow thin cap layers of III-N materials during devicefabrication. One such application is regrowth of ohmic contacts to III-Ndevices. Heavily doped n+ GaN, or InGaN grown by MBE orMOCVD is used to obtain low resistance non-alloyed ohmic contacts to GaN baseddevices. However, from a commercial point of view, this becomes difficultbecause of the high cost and lack of availability of ultra high vacuum(∼1x10-10 Torr) techniques in most clean roomfacilities. Reactive sputtering provides a cheaper and more ubiquitousalternative for the growth of thin cap layers on parent MOCVD III-Nheterostructures during device fabrication. In this work, we explore thepossibility of using reactive sputtering as a method to grow III-N materials asohmic contacts to GaN based devices.

A new device-first low-temperature bonded gallium nitride (GaN)-on-diamondhigh-electronic mobility transistor (HEMT) technology with state-of-the-art,radio frequency (RF) power performance is described. In this process, thedevices were first fabricated on a GaN-on-silicon carbide (SiC) epitaxial waferand were subsequently separated from the SiC and bonded onto ahigh-thermal-conductivity diamond substrate. Thermal measurements showed thatthe GaN-on-diamond devices maintained equivalent or lower junction temperaturesthan their GaN-on-SiC counterparts while delivering more than three-times higherRF power within the same active area. Such results demonstrate that the GaNdevice transfer process is capable of preserving intrinsic transistor electricalperformance while taking advantage of the excellent thermal properties ofdiamond substrates. Preliminary step-stress and room-temperature, steady-statelife testing shows that the low-temperature bonded GaN-on-diamond device has noinherently reliability limiting factor. GaN-on-diamond is ideally suited towideband electronic warfare (EW) power amplifiers as they are the most thermallychallenging due to continuous wave (CW) operation and the reduced power-addedefficiency obtained with ultra-wide bandwidth circuit implementations.

A modified design of the transmission line model test structure uses the simplecalculation of specific contact resistance, ρc, based on atwo contact linear pattern but without the requirement of a mesa etch. Thismodified structure uses a linear TLM with semicircular terminations at each end.The function of the semicircular terminations is to confine the fringing fieldsat the ends of the linear TLM contacts. Simple analytical equations fordetermining ρc have been developed on the basis of themodified linear TLM pattern. These calculations have shown good agreement with afinite element model (FEM) of the modified TLM test structure using typicalparameters for metal/ SiC contacts.

Cobalt doped ZnO (ZnO:Co) films with a 1 at% doping rate have been successfullygrown on (100) oriented p type Si substrates by radiofrequency magnetronsputtering. Post annealing treatments at 973 K for various short periods havebeen carried out and structural, optical and electrical properties of the filmshave been investigated. Upon rapid annealing, the dopant distribution in thefilm has been found homogeneous. The annealing improves the (002) texture of thefilm and the mean column width increases with the annealing duration from 60 nmup to 95 nm. The lattice parameter of the ZnO:Co films decreases upon annealingand approaches that of bulk ZnO. The photoluminescence (PL) study reveals thatthe Co2+ ions can be excited directly or through a transfermechanism from the matrix. The PL intensity decreases with the annealing timesuggesting a diffusion process of the dopant impeding theCo2+ emission. At last, the electrical conductivity reachesvalues compatible with potential electroluminescent applications of the ZnO:Cofilms.

Mg, the only effective p-type dopant for nitrides, iswell-studied in thin films due to the important role the impurity plays in lightemitting diodes and high power electronics. However, there are few reports of Mgin thick free-standing GaN substrates. Here we evaluate the material quality andpoint defects in GaN grown by hydride vapor phase epitaxy (HVPE) using metallicMg as the doping source. The crystal quality is typical of commercially grownHVPE substrates, and the photoluminescence measurements reveal distinctivelysharp excitonic and shallow-donor shallow-acceptor features. Secondary ion massspectroscopy indicates total Mg concentrations between 7x1016 and6x1018 cm-3 in the four separate samples studied but,more significantly, photoluminescence and electron paramagnetic resonancespectroscopy show that the Mg is incorporated as a shallow acceptor.

Bandgap energies of the group III-V quaternary alloy semiconductor, cubic AlxGa1−x NyAs1−y , were calculated by means ofthe dielectric method. While only GaN and GaAs are considered to be directtransition type among the four constituent binary compounds of this quaternaryalloy system, the calculation results show that the bandgap energy range coveredin the direct transition regime of this alloy system was further extended to thehigher energy side of GaN as well as to the lower energy sides of GaAs. Theextension to the higher energy side was attributed to the larger direct bandgapof AlN. On the other hand, the extension to the lower energy side was caused bythe large bowing in the bandgap energy between group III nitrides and arsenides.Calculations under lattice matching to Si and GaAs are also presented.

GaN has emerged as the material of choice for advanced power amplifier devicesfor both industrial and defense applications but near-junction thermal barriersseverely limit the inherent capability of high-quality GaN materials. Recent“embedded cooling” efforts, funded by Defense AdvancedResearch Projects Agency Microsystems Technology Office (DARPA-MTO), havefocused on reduction of this near-junction thermal resistance, through the useof diamond substrates and efficient removal of the dissipated power withconvective and evaporative microfluidics. An overview of the accomplishments ofthe DARPA Near-Junction Thermal Transport (NJTT) program and recent results fromthe on-going DARPA Intra-Chip Embedded Cooling (ICECool) program are provided.It is shown that growth or bonding of diamond to GaN epitaxy has enabled a3-5× increase in power handling capability per transistor unit area,while use of microfluidic cooling has enabled heat fluxes of 30kW/cm2 at the transistor level and 1 kW/cm2 at thedie-level, for a 3-6× improvement in the total RF output power of GaNpower amplifiers. These demonstrations provide near-term validation of the largeimprovement in output power gained through embedded cooling and confirm thepotential for well above a 6× improvement in GaN power amplifier outputpower to the electrical, rather than thermal, limits of GaN.

Recombination dynamics of InGaN/GaN multiple quantum wells (MQWs) with differentwell thickness have been studied. From the behaviour of temperature dependentphotoluminescence, we find that the activation energy decreases with the wellthickness increasing. In addition, with temperature changing from 10K to roomtemperature, the “W” shape of full width of half maximumis also thickness related, and it becomes more obvious with the well thicknessincreasing. These results indicate that the dominant recombination dynamicschange from exciton localization to quantum confined stark effect with wellthickness increasing. From our measurement, the InGaN/GaN MQWs with 3nmthickness seems a turning point, which shows the best optimized optical andstructural properties.