John Adam Edmond

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 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.

InGaN/GaN MQW SCH lasers grown on SiC

Laser diode structures were fabricated by metal-organic chemical vapor deposition (MOCVD) from the AlN-InN-GaN system on single crystal 6H-SiC substrates. An AlGaN conducting buffer layer was developed for these devices, which provides a vertical conduction path between SiC substrate and the active device region. Violet and blue multiple quantum well (MQW) separate confinement heterojunction (SCH) LDs were fabricated having InGaN wells and GaN barriers. The lowest pulsed operation room temperature threshold current density obtained for lasing was 7.1 kA/cm2 in a 4-well structure. Lasing has also been obtained in these same devices at duty cycles up to 75%.

Gain characteristics of InGaN/GaN quantum well diode lasers

We report on gain spectra of InGaN/AlGaN MQW SCH diode lasers, grown and fabricated on 6H-SiC substrates. The active medium was composed of 8 QWs with a nominal indium composition of x=0.1. The devices were index guided ridge waveguide structures with a typical cavity length of 500 /spl mu/m and a mesa width of 5 /spl mu/m. The gain studies are based on the analysis of the spontaneous emission spectra of the devices, collected normal to the resonator axis as a function of injection, while making use of fundamental connections between spontaneous emission, stimulated emission, and absorption. An example of the gain/absorption spectra in the InGaN MQW diode laser at room temperature is shown.