Due to the wide bandgap and other key materials properties of 4H-SiC, SiC MOSFETs offer 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 has progressed rapidly since the market release of Cree’s 1200V 4H-SiC power MOSFET in 2011. This is due to continued advancements in SiC substrate quality, epitaxial growth capabilities, and device processing. For example, high-quality epitaxial growth of thick, low-doped SiC has enabled the fabrication of SiC MOSFETs capable of blocking extremely high voltages (up to 15kV); while dopant control for thin highly-doped epitaxial layers has helped enable low on-resistance 900V SiC MOSFET production. Device design and processing improvements have resulted in lower MOSFET specific on-resistance for each successive device generation. SiC MOSFETs have been shown to have a long device lifetime, based on the results of accelerated lifetime testing, such as high-temperature reverse-bias (HTRB) stress and time-dependent dielectric breakdown (TDDB).

Interfacial dislocations (IDs) and half-loop arrays (HLAs) present in the epilayers of 4H-SiC crystal are known to have a deleterious effect on device performance. Synchrotron X-ray Topography studies carried out on n-type 4H-SiC offcut wafers before and after epitaxial growth show that in many cases BPD segments in the substrate are responsible for creating IDs and HLAs during CVD growth. This paper reviews the behaviors of BPDs in the substrate during the epitaxial growth in different cases: (1) screw-oriented BPD segments intersecting the surface replicate directly through the interface during the epitaxial growth and take part in stress relaxation process by creating IDs and HLAs (Matthews-Blakeslee model [1] ); (2) non-screw oriented BPD half loop intersecting the surface glides towards and replicates through the interface, while the intersection points convert to threading edge dislocations (TEDs) and pin the half loop, leaving straight screw segments in the epilayer and then create IDs and HLAs; (3) edge oriented short BPD segments well below the surface get dragged towards the interface during epitaxial growth, leaving two long screw segments in their wake, some of which replicate through the interface and create IDs and HLAs. The driving force for the BPDs to glide toward the interface is thermal stress and driving force for the relaxation process to occur is the lattice parameter difference at growth temperature which results from the doping concentration difference between the substrate and epilayer.

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

The thermal conductivity, bulk modulus, thermal expansion and heat capacity for bulk β-Ga2O3 are calculated from lattice dynamics using both a quasi-harmonic approximation and an anharmonic force-constant approach involving a solution of the linearized Boltzmann transport equation. The role of anharmonicity in β-Ga2O3 is determined to be small, which leads to the weak volume dependence of the calculated thermal conductivity. The negligible anharmonic contribution to the overall thermal conductivity is consistent with both thermal expansion measurements and also with comparisons between the quasi-harmonic and anharmonic methods. Phonon-mode-dependent Grüneisen parameters are found to be weakly dependent on temperature. Negative values of the mode Grüneisen parameters are found for certain low energy optical modes, but their net effect on the overall thermal expansion is insignificant. Bulk modulus as well as heat capacity are also given and found to be in agreement with experimental results.

We study effects of deposition temperature on growth mode and surface morphology of hetero-epitaxial (ZnO)x(InN)1-x (ZION) films on ZnO templates. ZION films deposited at low temperature of RT-250oC grow two dimensionally, whereas ZION films deposited at high temperature of 350-450oC grow three dimensionally. Growth mode is changed from two-dimensional growth mode to three-dimensional one, because the critical thickness where film strain begin to relax decreases with increasing the deposition temperature. At high deposition temperatures, the number of point defects in ZION films decreases because migration of adatoms on the growing surface is enhanced. The strain energy in ZION films increases with increasing the deposition temperature, since the strain energy is not released by point defects. Therefore, lattice relaxation for the higher deposition temperature begins 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) and scintillation measurements. In this work, we combine X-ray-induced luminescence (XRIL) spectroscopy [Review of Scientific Instruments 83, 103112 (2012)] with PL and standard scintillation measurements to give insight into the scintillation properties of un-doped ZnO single crystals. XRIL revealed that ZnO luminescence proportionally increases with X-ray power and exhibits excellent linearity - indicating the possibility of developing radiation detectors with good energy resolution. By coupling ZnO crystals to fast photomultiplier tubes and monitoring 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 metallic β-Nb2N thin films on hexagonal SiC substrates. When the N/Nb flux ratios are greater than one, the most critical parameter for high-quality β-Nb2N is the substrate temperature. The X-ray diffraction (XRD) of films grown between 775 °C and 850 °C demonstrates pure β-Nb2N phase formation which was also confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy 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. This suggests that β-Nb2N can be used for high-quality metal/semiconductor heterostructures that cannot be fabricated at present.

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

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

A new device-first low-temperature bonded gallium nitride (GaN)-on-diamond high-electronic mobility transistor (HEMT) technology with state-of-the-art, radio frequency (RF) power performance is described. In this process, the devices were first fabricated on a GaN-on-silicon carbide (SiC) epitaxial wafer and were subsequently separated from the SiC and bonded onto a high-thermal-conductivity diamond substrate. Thermal measurements showed that the GaN-on-diamond devices maintained equivalent or lower junction temperatures than their GaN-on-SiC counterparts while delivering more than three-times higher RF power within the same active area. Such results demonstrate that the GaN device transfer process is capable of preserving intrinsic transistor electrical performance while taking advantage of the excellent thermal properties of diamond substrates. Preliminary step-stress and room-temperature, steady-state life testing shows that the low-temperature bonded GaN-on-diamond device has no inherently reliability limiting factor. GaN-on-diamond is ideally suited to wideband electronic warfare (EW) power amplifiers as they are the most thermally challenging due to continuous wave (CW) operation and the reduced power-added efficiency obtained with ultra-wide bandwidth circuit implementations.

A modified design of the transmission line model test structure uses the simple calculation of specific contact resistance, ρc, based on a two contact linear pattern but without the requirement of a mesa etch. This modified structure uses a linear TLM with semicircular terminations at each end. The function of the semicircular terminations is to confine the fringing fields at the ends of the linear TLM contacts. Simple analytical equations for determining ρc have been developed on the basis of the modified linear TLM pattern. These calculations have shown good agreement with a finite element model (FEM) of the modified TLM test structure using typical parameters for metal/ SiC contacts.

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

Mg, the only effective p-type dopant for nitrides, is well-studied in thin films due to the important role the impurity plays in light emitting diodes and high power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here we evaluate the material quality and point defects in GaN grown by hydride vapor phase epitaxy (HVPE) using metallic Mg as the doping source. The crystal quality is typical of commercially grown HVPE substrates, and the photoluminescence measurements reveal distinctively sharp excitonic and shallow-donor shallow-acceptor features. Secondary ion mass spectroscopy indicates total Mg concentrations between 7x1016 and 6x1018 cm-3 in the four separate samples studied but, more significantly, photoluminescence and electron paramagnetic resonance spectroscopy show that the Mg is incorporated as a shallow acceptor.

Bandgap energies of the group III-V quaternary alloy semiconductor, cubic AlxGa1−xNyAs1−y, were calculated by means of the dielectric method. While only GaN and GaAs are considered to be direct transition type among the four constituent binary compounds of this quaternary alloy system, the calculation results show that the bandgap energy range covered in the direct transition regime of this alloy system was further extended to the higher energy side of GaN as well as to the lower energy sides of GaAs. The extension to the higher energy side was attributed to the larger direct bandgap of AlN. On the other hand, the extension to the lower energy side was caused by the 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 devices for both industrial and defense applications but near-junction thermal barriers severely limit the inherent capability of high-quality GaN materials. Recent “embedded cooling” efforts, funded by Defense Advanced Research Projects Agency Microsystems Technology Office (DARPA-MTO), have focused on reduction of this near-junction thermal resistance, through the use of diamond substrates and efficient removal of the dissipated power with convective and evaporative microfluidics. An overview of the accomplishments of the DARPA Near-Junction Thermal Transport (NJTT) program and recent results from the 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 a 3-5× increase in power handling capability per transistor unit area, while use of microfluidic cooling has enabled heat fluxes of 30 kW/cm2 at the transistor level and 1 kW/cm2 at the die-level, for a 3-6× improvement in the total RF output power of GaN power amplifiers. These demonstrations provide near-term validation of the large improvement in output power gained through embedded cooling and confirm the potential for well above a 6× improvement in GaN power amplifier output power to the electrical, rather than thermal, limits of GaN.

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