Kenneth Irvine

Electric breakdown in GaN p‐n junctions

Electric breakdown in GaN p‐n junctions was investigated. GaN p+‐p‐n+ structures were grown on 6H–SiC substrates by metalorganic chemical vapor deposition. Mg and Si were used as dopants. Mesa structures were fabricated by reactive ion etching. Capacitance–voltage measurements showed that the p‐n junctions were linearly graded. The impurity gradient in the p‐n junctions ranged from 2×1022 to 2×1023 cm−4. Reverse current–voltage characteristics of the p‐n junctions were studied in the temperature range from 200 to 600 K. The diodes exhibited abrupt breakdown at a reverse voltage of 40–150 V. The breakdown had a microplasmic nature. The strength of the electric breakdown field in the p‐n junctions depended on the impurity gradient and was measured to be from 1.5 to 3 MV/cm. It was found that the breakdown voltage increases with temperature. The temperature coefficient of the breakdown voltage was ∼2×10−2 V/K.

Spontaneous and stimulated emission from photopumped GaN grown on SiC

Photoluminescence of GaN layers grown on 6H–SiC substrates was studied in the temperature range 77–900 K. GaN layers were grown by metalorganic chemical vapor deposition. The temperature dependence of the band gap of GaN was measured throughout the entire temperature range. Edge cavity stimulated emission from photopumped GaN layers was observed in the temperature range 77–450 K. The full width at half‐maximum (FWHM) of the stimulated emission peak was ∼3 nm at 300 K and ∼7 nm at 450 K. Multipass stimulated emission with Fabry–Perot modes was detected from GaN. The FWHM of Fabry–Perot modes was ∼0.2 nm (300 K).

Schottky barriers on n-GaN grown on SiC

Characteristics of Schottky barriers fabricated on n-type GaN were investigated. The barriers were formed by vacuum thermal evaporation of Cr, Au, and Ni. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the barriers were measured in a wide temperature and current density range. Fundamental parameters (barrier height and built-in potential) of the Schottky barriers were determined. The dependence of the barrier ideality factor on doping concentration in GaN was measured. Correlation between the barrier height and metal work function was observed. The electron affinity for GaN was determined using both C-V and I-V characteristics. The current flow mechanism through the barriers is discussed.

AlGaN pn junctions

AlGaN pn homo‐ and heterojunctions were fabricated on silicon carbide substrates by metalorganic chemical vapor deposition. AlN concentration in AlGaN layers ranged from 2 to 8 mol %. Mesa structures were made by reactive ion etching. Electroluminescence (EL) from AlGaN pn junctions was studied. EL peaks associated with near band‐to‐band transition in AlGaN were detected. The minimum wavelength of EL peak of ∼348 nm (hν∼3.56 eV, 300 K) was measured for a p‐Al0.08GaN0.92/n‐Al0.06Ga0.94N heterojunction. The dependence of the photon energy of the edge EL peak on AlN concentration in AlGaN was measured.

Nitride-based emitters on SiC substrates

Single crystal thin films with compositions from the AlN- InN-GaN system were grown via metal-organic chemical vapor deposition on single crystal 6H-SiC substrates. AlGaN containing high and low fractions of Al was grown directly on the SiC for use as a buffer layer. Subsequent epitaxial layers of GaN and AlGaN were doped with Mg and Si to achieve p-type conductivity, respectively. N-type InGaN layers with In compositions up to approximately 50 percent were also achieved. Room temperature photoluminescence on these films exhibited single peaks in the spectral range from the UV to green. Various layers were combined to form light emitting diode (LED) and laser structures. Blue LEDs with both insulating and conductive buffer layers exhibited an external quantum efficiency of 2-3 percent with a forward operating voltage of 3.4-3.7 V. Laser diode structures having a separate confinement heterostructure multiple quantum well configuration were optically and electrically pumped. Photopumping resulted in stimulated emission at 391 nm. Electrically pumped structures resulted in a peak emission at 393 nm and a bandwidth of 12 nm. No lasing was observed.

Recent progress in GaN/SiC LEDs and photopumped lasers

Stimulated emission from optically pumped GaN layers grown on SiC has been reported over a wide temperature range of 77 to 450 K. However, there has been no report of photopumped lasing in a DH structure grown on SiC. This presentation will discuss recent progress in the development of GaN-based blue LEDs at Cree and photopumped lasing results obtained on GaN-AlGaN DH laser structures fabricated on SiC.

Recent progress in AlGaN/GaN laser structures on 6H-SiC

Room temperature hole concentrations of 5 multiplied by 1017 cm-3 and mobilities of 8.4 cm2/V-s have been measured on heavily Mg doped GaN layers grown on SiC. Specific contact resistivities of 0.046 (Omega) -cm2 have been obtained from TLM measurements on ohmic contacts to these layers. Double heterostructures (DH) of GaN/AlxGa1-xN with x equals 0.1 have been grown on n-type 6H-SiC substrates. High quality facets have been fabricated by cleaving these DH structures. Photopumped stimulated emission has been observed in undoped structures at 372 nm at a threshold power density of 72 kW/cm2. An optical gain of 1000 cm-1 was measured in the same samples at 200 kW/cm2.

Recent developments in SiC-based visible emitters

In recent years, there has been rapidly growing interest and progress in the development of blue and pure green light emitting materials for LED and laser applications. Most recent developments have been reported for the direct bandgap materials, ZnSe and GaN. However, a compatible substrate is needed for nitride-based emitters to become commercially viable. Although less efficient, the commercial viability of SiC-based LEDs has already been proven. This paper reports on the most recent advancements in increasing the performance of blue and green light emitting diodes fabricated in homoepitaxial 6H-SiC and the potential for SiC-nitride based LEDs.