Jehn-Huar Chern

High temperature (400/spl deg/C) performance of ohmic contacts to n-type GaN and GaAs

Stable high temperature ohmic contacts to III-V semiconductor materials are still rather problematic. In this work we report results on thermodynamically stable ohmic contacts to n-type Ga-V semiconductor materials (e.g., GaN and GaAs) that are capable of withstanding prolonged 400/spl deg/C exposure in untreated air ambient.

High-power and high-temperature FET technology

The performance of heterojunction metal-semiconductor-field- effect-transistors (MESFETs) and junction-field-effect- transistors (JFETs) fabricated with different buffers is presented. For the JFET, carbon was chosen as the p-type dopant because of its relative low diffusivity compared to other doping elements. The viability of heterojunction MESFET and JFET devices operating at 400 degrees C have been demonstrated. Two key factors contributing to the reduction of drain leakage currents were the use of a high resistivity, undoped AlAs buffer layers and the gate contacting layers: n-type AlGaAs for the MESFET and p-type AlGaAs for the JFET. A two LT-AL0.3Ga0.7As layer scheme were used for the first time specifically for use in high temperature applications. Even at 400 degrees, C the gate leakage current density for a gate length of 2 micrometers was 9 by 10-7 A/micrometers at Vds equals 3V Vgs equals -7. The high resistance of LT-AlGaAs materials after annealing was responsible for such low gate leakage currents. The p- HEMTs became leaky at high temperature because of the parallel conduction and buffer design. The gate diode performed better when contacted to the undoped AlGaAs layer. DC and high-temperature performance of GaN-based MESFETs and MODFETs were compared. Al0.3Ga0.7N/GaN MODFETs with a gate-length of 2micrometers exhibited high transconductance was 47mS/mm, and dropped by 12 percent of its initial value to 41.4 mS/mm at 350 degrees C. The decrease in transconductance with temperature can be explained by the temperature dependence of the electron mobility. The large conduction band discontinuity in this material system may play an important role in terms of better electron confinement thus resulting in less degradation in transconductance.

High-power 0.98-μm broad-waveguide lasers using novel material system of InGaAs/InGaAsP/AlGaAs

We describe the theoretical design and experimental fabrication of high power 0.98 micrometers strained quantum well lasers employing broad waveguide structure and novel hybrid material system of Al-free InGaAs/InGaAsP active region and AlGaAs cladding layers. The use of AlGaAs cladding, instead of InGaP, provides advantages in flexibility of laser structure design, simple epitaxial growth, improvement of surface morphology and laser performance. In addition to the theoretical study of the new structure, we successfully demonstrate the design by obtaining high performance laser devices. The as-grown InGaAs/InGaAsP/AlGaAs laser material exhibits very high quality with low threshold current density of 200 A/cm2, high internal quantum efficiency of approximately 93 percent, and low internal loss of 1.2 cm-1. For 100 micrometers -wide stripe lasers with cavity length of 800 micrometers , a high slope efficiency of 1.03 W/A, low vertical beam divergence of 36 degrees, high output power of 3.65 W, and very high characteristic temperature coefficient of 250 K are achieved.

Contacts to GaAs, InP, and GaP for high-temperature and high-power applications

There is a significant interest in the area of improving high temperature stable contacts to III-V semiconductors. Two attractive materials that offer promise in this area are dysprosium phosphide (DyP) and dysprosium arsenide (DyAs). This paper reports the electrical characterization of MBE- grown DyP and DyAs on GaAs, GaP, and InP substrates. The characterization methods include Hall and I-V measurements. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01% and is stable in air with no sign of oxidation, even after months of ambient exposure. DyP forms Schottky contacts to n-GaAs, n- and p-GaP, and p- InP with barrier heights of 0.81, 0.9, 0.8 and 0.74 eV, respectively. DyP on n-InP and p-GaAs is found to have ohmic behavior with the specific contact resistance of 1 X 10-4 and 2.9 X 10-5 (Omega) (DOT)cm2, respectively. DyAs also forms Schottky contacts to n-GaAs, p-InP and forms ohmic contacts to n-InP.

High-temperature performance of ohmic contacts to n-type GaN and GaAs

Cu3Ge was studied as an Ohmic contact to n-GaN and n- type GaAs. Specific contact resistance of Cu3Ge to n- type GaAs was found to be sensitive to annealing conditions, doping concentrations, and Ge compositions. After vacuum annealing at 400 degree(s)C for 30 min, Cu3Ge exhibited ohmic behavior to n-type GaN with doping concentrations of approximately 1.0 X 1018 cm-3. Unprotected Cu3Ge ohmic contacts suffered from oxidation when exposed at temperatures higher than 300 degree(s)C. Aging tests at 400 degree(s)C where Cu3Ge covered with TiW and Au was used as ohmic contact to n-type GaAs, and TiPtAu covered with Au to p-type GaAs, revealed unchanged I-V characteristics after 120 hr annealing which showed that this contact was suitable for device application at high temperature. Pseudomorphic HEMT employing protected Cu3Ge ohmic contacts and Ti/Pt/Au gates has achieved peak transconductance of 330 mS/mm at room temperature for 2- micrometers long gate. The I-V characteristic of Pd/Al, covered with TiW/Pt changed from Ohmic to Schottky after aging at 350 degree(s)C for 2 hours. By depositing a very thin Cr layer between Ti/Al and Au layers, contact resistance of Ti/Al contacts remained the same even after an aging of 130 hr at 350 degree(s)C and 130 hr at 400 degree(s)C. However, specific contact resistance increased from 2.4 X 10-6 (Omega) cm2 to 3.8 X 10-6 (Omega) cm2 after annealing at 500 degree(s)C for 2 hr.