Nanocrystalline diamond thin film integration in AlGaN/GaN high electron mobility transistors and 4H-SiC heterojunction diodes

The extremely high thermal conductivity and mechanical hardness of diamond would make it the natural choice for device substrates when large area wafer production becomes possible. Until this milestone is achieved, people could utilize nanocrystalline diamond (NCD) thin films grown by chemical vapor deposition (CVD). A topside thermal contact could be pivotal for providing stable device characteristics in the high power, high temperature, and high switching frequency device operating regime that next-generation power converter circuits will mandate. This work explores thermal and electrical benefits offered by NCD films to wide bandgap semiconductor devices. Reduction of self-heating effects by integrating NCD thin films near the device channel of AlGaN/GaN high electron mobility transistors (HEMTs) is presented. The NCD layers provide a high thermal conductivity path for the reduction of hot electron dispersion, a phenomenon caused by self-heating and detrimental to the continuous operation of GaN devices in power switching circuits. Recent advances in diamond doping have made it possible to think of this material as a very wide bandgap semiconductor (5.5 eV for ideal diamond). A few unique properties, such as negative electron affinity (χ = -0.2 eV for H-terminated diamond), make this material very interesting. Using H-terminated NCD, a heterojunction with 4H-SiC has been developed. Undoped and B-doped NCD were deposited on both n- and p- 4H-SiC epilayers. Different metals were studied to provide an Ohmic contact to the NCD layer. I-V measurements on p+ NCD / n- 4H-SiC p-n junctions indicated Schottky rectifying behavior with a turn-on voltage of around 0.2 V. The current increased over 8 orders of magnitude with an ideality factor of 1.17 at 30 °C. Ideal energy-band diagrams suggested a possible conduction mechanism for electron transport from the SiC conduction band to either the valence band or Boron acceptor level of the NCD film. Cathodoluminescence and thermally stimulated current methods were employed to study the deep level assisted conduction in this heterojunction. Applications as a simultaneous UV-transparent optical and Schottky electrical contact to 4H-SiC are discussed

Fabrication and characterization at high temperature of AlGaN/GaN enhancement HEMTs

Enhancement-mode (E-mode) high electron mobility transistors (HEMTs) based on a standard AlGaN/GaN heterostructure have been fabricated using two different methods: 19F implantation and fluorine-based plasma treatment. The need of a thermal annealing after both treatments has been proven in order to restore the ID and gm levels. DC characterization at high temperature has demonstrated that ID and gm decrease reversibly due to the reduction of the electron mobility and the drift velocity. Pulsed measurements (state period and variable pulse width) have been performed to study the self-heating effects

Quantifying pulsed laser induced damage to grapheme

As an emerging optical material, graphene’s ultrafast dynamics are often probed using pulsed lasers yet the region in which optical damage takes place is largely uncharted. Here, femtosecond laser pulses induced localized damage in single-layer graphene on sapphire. Raman spatial mapping, SEM, and AFM microscopy quantified the damage. The resulting size of the damaged area has a linear correlation with the optical fluence. These results demonstrate local modification of sp2-carbon bonding structures with optical pulse fluences as low as 14 mJ/cm2, an order-of-magnitude lower than measured and theoretical ablation thresholds

Thermal effects in Ni/Au and Mo/Au gate metallization AlGaN/GaN HEMT's reliability

AlGaN/GaN high electron mobility transistors (HEMT) are key devices for the next generation of high-power, high-frequency and high-temperature electronics applications. Although significant progress has been recently achieved [1], stability and reliability are still some of the main issues under investigation, particularly at high temperatures [2-3]. Taking into account that the gate contact metallization is one of the weakest points in AlGaN/GaN HEMTs, the reliability of Ni, Mo, Pt and refractory metal gates is crucial [4-6]. This work has been focused on the thermal stress and reliability assessment of AlGaN/GaN HEMTs. After an unbiased storage at 350 o C for 2000 hours, devices with Ni/Au gates exhibited detrimental IDS-VDS degradation in pulsed mode. In contrast, devices with Mo/Au gates showed no degradation after similar storage conditions. Further capacitance-voltage characterization as a function of temperature and frequency revealed two distinct trap-related effects in both kinds of devices. At low frequency (< 1MHz), increased capacitance near the threshold voltage was present at high temperatures and more pronounced for the Ni/Au gate HEMT and as the frequency is lower. Such an anomalous “bump” has been previously related to H-related surface polar charges [7]. This anomalous behavior in the C-V characteristics was also observed in Mo/Au gate HEMTs after 1000 h at a calculated channel temperatures of around from 250 o C (T2) up to 320 ºC (T4), under a DC bias (VDS= 25 V, IDS= 420 mA/mm) (DC-life test). The devices showed a higher “bump” as the channel temperature is higher (Fig. 1). At 1 MHz, the higher C-V curve slope of the Ni/Au gated HEMTs indicated higher trap density than Mo/Au metallization (Fig. 2). These results highlight that temperature is an acceleration factor in the device degradation, in good agreement with [3]. Interface state density analysis is being performed in order to estimate the trap density and activation energy

Structural and morphological studies on wet-etched InAlGaN barrier HEMT structures

The quaternary nitride-based high electron mobility transistor (HEMT) has been recently a focus of interest because of the possibility to grow lattice-matched barrier to GaN and tune the barrier bandgap at the same time