Larry Burton Rowland

Temperature-dependent emission intensity and energy shift in InGaN/GaN multiple-quantum-well light-emitting diodes

Temperature-dependent electroluminescence(EL) of InGaN/GaN multiple-quantum-welllight-emitting diodes(LEDs) has been investigated to illustrate the role of localization effects in carrier capture and recombination. The devices have identical structure but with varying indium content in the active region. A large redshift of the emission peak with decreasing temperature is observed in the UV and blue LEDs over the temperature range of 77–200 K, accompanying a pronounced decrease of EL intensity. This redshift reflects carrier relaxation into lower energy localized states and the change in carrier recombination dynamics at low temperatures. In contrast, the peak energy of the green LEDs exhibits a smaller temperature-induced shift, and the emission intensity increases monotonically with decreasing temperature down to 5 K. Based on a rate equation analysis, we find that the densities of the localized states in the green LEDs are more than two orders of magnitude higher than that in the UVLED.

700-V asymmetrical 4H-SiC gate turn-off thyristors (GTO's)

Silicon Carbide (4H-SiC), asymmetrical gate turn-off thyristors (GTOs) were fabricated and tested with respect to forward voltage drop (V/sub F/), forward blocking voltage, and turn-off characteristics. Devices were tested from room temperature to 350/spl deg/C in the dc mode. Forward blocking voltages ranged from 600-800 V at room temperature for the devices tested. V/sub F/ of a typical device at 350/spl deg/C was 4.8 V at a current density of 500 A/cm/sup 2/. Turn-off time was less than 1 /spl mu/s. Although no beveling or advanced edge termination techniques were used, the blocking voltage represented approximately 50% of the theoretical value when tested in an air ambient. Also, four GTO cells were connected in parallel to demonstrate 600-V, 1.4 A (800 A/cm/sup 2/) performance.

Demonstration of ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes

We report ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes. An external quantum efficiency of 83% (187 mA/W) at 278 nm, corresponding to unity gain after reach-through was achieved. A gain higher than 1000 was demonstrated without edge breakdown.

Critical Materials, Device Design, Performance and Reliability Issues in 4H-SiC Power Umosfet Structures

The long-term reliability of gate insulator under high field stress of either polarity presents a constraint on the highest electric field that can be tolerated in a 4H-SiC UMOSFET under on or off condition. A realistic performance projection of 41H-SiC UMOSFET structures based on electric field in the gate insulator (1.5 MV/cm under on-condition and 3 MV/cm under offcondition) consistent with long-term reliability of insulator is provided for the breakdown voltage in the range of 600 to 1500 V. The use of P + polysilicon gate allows us to use a higher field of 3 MV/cm in the insulator under off-condition and leads to a higher breakdown voltage as the Fowler Nordheim (FN) injection from the gate electrode is reduced. FN injection data is presented for p type 4H-SiC MOS capacitor under inversion at room temperature and at 325°C. It is concluded that the insulator reliability, and not the SiC, is the limiting factor and therefore the high temperature operation of these devices may not be practical.

Vapor-phase epitaxial growth on porous 6H–SiC analyzed by Raman scattering

SiC vapor-phase epitaxy on porous silicon carbide (PSC) substrates formed by electrochemical anodization is reported. Raman scattering indicates that the polytype of the optically smooth SiC grown on PSC formed in both p-type and n-type 6H substrates is 6H. The Raman scattering selection rules in these films are the same as those observed in the bulk substrate and epilayers grown on bulk, indicating high crystalline quality. The formation of epitaxial 6H–SiC on porous 6H–SiC may open up new possibilities for dielectric device isolation, fabrication, and epitaxial lift-off.

Properties of epitaxial GaN on refractory metal substrates

The authors demonstrate here that GaN films with good surface morphology and structural, optical, and electronic properties can be grown on metallic titanium carbide substrates. X-ray rocking curve and Raman scattering measurements confirmed the high crystalline quality of the wurtzite structure film. Variable temperature photoluminescence measurements of sharp and intense emission lines provided insights into the nature of the recombination processes, the carrier background type, and the carrier concentration. The high quality of the interface and substrate Ohmic contacts was verified. The ability to grow high-quality films on metallic substrates provides the means for advanced vertical and high-power and/or high-temperature device fabrication.

1700V, 5.5mOhm-cm2 4H-SiC DMOSFET with Stable 225°C Operation

We report a 1700V, 5.5mΩ-cm2 4H-SiC DMOSFET capable of 225°C operation. The specific on-resistance of the DMOSFET designed for 1200V applications is 8.8mΩ-cm2 at 225°C, an increase of only 60% compared to the room temperature value. The low specific on-resistance at high temperatures enables a smaller die size for high temperature operation. Under a negative gate bias temperature stress (BTS) at VGS=-15 V at 225°C for 20 minutes, the devices show a threshold voltage shift of ΔVTH=-0.25 V demonstrating one of the key device reliability requirements for high temperature operation.

Influence of Buffer Layer on DC and RF Performance of 4H SiC MESFET

DC and RF characteristics are compared for power 4H-SiC MESFETs fabricated on semi-insulating substrates, with and without a low-doped p-type buffer layer. Output admittance and gate contact thermal spectroscopy measurements are performed to analyze carrier trapping phenomena at the substrate-channel interface. Measurements indicate trapping of free carriers by vanadium acceptor at the channel-substrate interface in devices without the buffer. In the case of devices with the buffer, neither the frequency dispersion of the output or gate admittance nor the gate capacitance transient was observed. Measurements of S-parameters are performed on the MESFETs in the frequency range 0.045-5 GHz. Measurement results are used to build MESFET equivalent circuit.

4H-SiC Power Devices: Comparative Overview of UMOS, DMOS, and GTO Device Structures

Silicon Carbide (SiC) is an emerging semiconductor material which has been widely predicted to be superior to both Si and GaAs in the area of power electronic switching devices [1]. This paper presents an overview of SiC power devices and concludes that MOS Turn-Off Thyristor (MTO TM ) is one of the most promising near term SiC switching device given its high power potential, ease of turn-off, 500°C operation and resulting reduction in cooling requirements. It is further concluded that in order to take advantage of SiC power devices, high temperature packages and components with double sided attachment need to be developed along with the SiC power devices.

Defects limiting performance of devices fabricated on GaN/metal heterostructure

Ni Schottky barrier contacts were processed to characterize quality and suitability of GaN films grown on (111) face of titanium carbide metallic substrates for vertical device application. We found that defects such as voids (pores) in the GaN film strongly influence the optical and electrical properties of the epitaxial layers. Micro-Raman studies showed that these voids have a high concentration of free carriers. Schottky barrier contacts placed on the regions with high defect density are characterized by high leakage current. Barrier height of Schottky contacts containing smaller number of defects were typically around 0.72eV.