Showing papers by "Marko J. Tadjer published in 2015"

Ultraviolet detector based on graphene/SiC heterojunction

Abstract: There has been significant research on graphene as a sensor owing to the inherent high sensitivity and surface area associated with two-dimensional (2D) materials. Often, the ability of graphene to form heterojunctions with wide-bandgap semiconductors is overlooked. In this study, we present a detector based on an epitaxial graphene/SiC heterojunction, exploiting the 2D nature of graphene to minimize absorption losses for high-efficiency sensing while simultaneously taking advantage of the epitaxial p–n junction to achieve low reverse leakage. We measured a quantum efficiency above 80% at 4 eV using a graphene/SiC p–n heterojunction with a dark current <1 nA/cm2.

Trapping phenomena in AlGaN and InAlN barrier HEMTs with different geometries

Abstract: Trapping effects were evaluated by means of pulsed measurements under different quiescent biases for GaN/AlGaN/GaN and GaN/InAlN/GaN. It was found that devices with an AlGaN barrier underwent an increase in the on-resistance, and a drain current and transconductance reduction without measurable threshold voltage change, suggesting the location of the traps in the gate-drain access region. In contrast, devices with an InAlN barrier showed a transconductance and a decrease in drain associated with a significant positive shift of threshold voltage, indicating that the traps were likely located under the gate region; as well as an on-resistance degradation probably associated with the presence of surface traps in the gate-drain access region. Furthermore, measurements of drain current transients at different ambient temperatures revealed that the activation energy of electron traps was 0.43 eV and 0.38 eV for AlGaN and InAlN barrier devices, respectively. Experimental and simulation results demonstrated the influence of device geometry on the observed trapping effects, since devices with larger gate lengths and gate-to-drain distance values exhibited less noticeable charge trapping effects.

Aerosol Deposition of Yttrium Iron Garnet for Fabrication of Ferrite-Integrated On-Chip Inductors

Abstract: We have employed aerosol deposition (AD) to deposit 39 µm thick polycrystalline films of yttrium iron garnet at room temperature onto sapphire at a rate of 1‐3 µm/min as an initial investigation of utilizing AD for fabricating ferrite-integrated on-chip inductors. We characterize the structural and magnetic properties of the as-received starting powder, as-deposited film, and a pressed puck formed from the starting powder. Results show that the films are comprised of randomly oriented polycrystalline grains with structural and magnetic properties that closely resemble that of the starting powder. Results from coating a gold single-turn inductor show an increase in inductance of 79% up to ∼300 MHz without affecting the Q-factor. These results demonstrate AD as a promising technique for depositing thick ferrite films at high deposition rates for low-temperature fabrication of ferrite-integrated on-chip inductors.

Simulation of temperature and electric field-dependent barrier traps effects in AlGaN/GaN HEMTs

Abstract: We considered electron trapping and detrapping processes in the AlGaN barrier of AlGaN/GaN high electron mobility transistors as that trapping is the process of electrons directly tunneling from the gate metal into the AlGaN barrier traps while detrapping is electrons emission from the traps into the AlGaN conduction band by phonon-assisted tunneling. By fully coupling the electron emission characteristic time with the device thermal and electrical behavior, simulations were performed to comprehensively analyze the contributions of device bias, accounting for self-heating and electric field dependent electron detrapping. Discussions on the current-transient method for traps characterization based on these simulations and impact of traps-dependent reverse gate current on drain current collapse are presented.

(Invited) Failure Mechanisms in AlGaN/GaN HEMTs Irradiated with 2MeV Protons

Abstract: Gallium nitride high electron mobility transistors (HEMTs) have shown the potential to be highly resistant to radiation damage, making them ideal for use in microwave power amplifiers and DC/DC converters in space-based applications. To investigate the mechanisms of radiation-induced degradation in AlGaN/GaN HEMTs, 2MeV protons were used to simulate the space radiation environment. The initial material quality was intentionally varied by studying HEMTs on sapphire, Si, and SiC substrates. The Hall mobility and 2DEG density was measured on Van der Pauw structures before irradiation and incrementally at each dose, up to up to a fluence of 6x10 14 cm -2 (Figure 1). The decrease in mobility can be attributed to increased carrier scattering in the 2DEG as a result of radiation-induced defects while the decrease in sheet carrier density is attributed to screening of the 2DEG from charged trap formation [1]. The magnitude of change in the 2DEG density is the same for all substrate materials (1x10 12 cm -2 ), though the percent change varies due to different initial values. A representative set of FET I-V curves from the HEMT on SiC before and after irradiation is shown in Figures 2 and 3. It is clear from Figure 2 that the ON-resistance increases and saturation current decreases, following the Hall data, and from the inset of Figure 3 that the OFF-state leakage decreases. TEM imaging was employed to directly probe the mechanisms suggested by the electrical measurements. A new, radiation-induced void at both edges of the gate in the Ni region of the Ni/Au gate metallization was revealed, shown in figure 4c [2]. EDS line scans (Figure 4b,d) indicate that, after radiation, the 20 nm Ni layer diffused up into the Au, and to a lesser extent, into the AlGaN, leaving voids protruding ~300 nm under the gate edges. The mechanism of the void formation shows all features of Ni/Au inter-diffusion through vacancy exchange, known as the Kirkendall effect. The exact process is still under investigation. It is particularly rare that such pronounced diffusion occurs at room temperature. Since the voids only occur at the gate edges, an additional electrochemical or strain-induced driving force is likely present. Therefore the role of the SiN x passivation layer, as well as strain, at the gate edge will be factor in this ongoing research.