K. Doverspike

Role of localized and extended electronic states in InGaN/GaN quantum wells under high injection, inferred from near-field optical microscopy

We report on spatially resolved optical measurements of high-quality InGaN/GaN multiple quantum wells under conditions of direct high optical injection (>1019 cm−3) using near-field optical microscopy in the collection mode. The spectral dependence of the spatial distribution of the photoluminescence indicates that the range of In-composition fluctuations reaches the 100-nm lateral scale. The spectra are dependent on the carrier injection level and reveal significant state filling effect. We sketch tentative conclusions about the In-cluster size distribution in terms of contributions to the radiative processes that involve localized and extended states, respectively, in the regime of electron–hole (e–h) pair densities at which present diode lasers operate.

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.

Chapter 7 SiC-Based UV Photodiodes and Light-Emitting Diodes

Publisher Summary This chapter discusses silicon carbide or Sic-based UV photodiodes and light-emitting diodes (LEDs). Sic has been investigated as a short-wavelength optoelectronic material since the early years of semiconductor development. As a result of the 3.0-eV bandgap of 6H.SiC, any color LED in the visible spectrum can be achieved in this material. Development of 6H-SiC blue LEDs was for many years a major research effort at Siemens Research Laboratory, various labs in the former Soviet Union, and a number of Japanese labs, in particular Sanyo Electric Co., Ltd. Several of these groups released blue LEDs as products. High-temperature Sic ultra violet (UV) photodiodes may have a significant impact in many application areas. Improvement in combustion control is anticipated with the ability to sense flames in aircraft engines, building boiler systems, and industrial processes. Air quality monitoring and UV dosimetry for industrial processes are other important application areas. However, 6H-Sic is an excellent substrate for heteroepitaxial growth of III-nitrides, and Crees line of G. SiC LEDs is based on nitride structures grown on 6H-Sic.

InGaN/GaN lasers grown on SiC

Single crystal thin films with compositions from the AlN-InN- GaN system were grown via metal-organic chemical vapor deposition (MOCVD) on single crystal 6H-SiC substrates. Blue light emitting (LED) and laser diode (LD) structures were fabricated. A conducting buffer layer was developed which uses an AlGaN buffer layer which provides a conduction path between SiC and the active device region. This conducting buffer layer was utilized in both the LEDs and the LDs. The external quantum efficiency of the LEDs was 3% at 20 mA (3.6V) with a peak emission wavelength of 430 nm. Violet and blue LDs were fabricated which consisted of an 8-well InGaN/GaN multiple quantum well (MQW) active region in a separate confinement heterostructure (SCH) design. The devices lased at room temperature under pulsed and continuous wave operation with an emission wavelength of 404-435 nm. The lowest pulsed operation threshold current density obtained for lasing under was 10.4 kA/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%.