Gary E. Bulman

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.

Optical studies of GaN and GaN/AlGaN heterostructures on SiC substrates

We present the results of spectroscopic studies on GaN based epitaxial materials on SiC substrates by metalorganic chemical vapor deposition. A variety of techniques has been used to study the optical properties of GaN epilayers and GaN/AlGaN heterostructures. Sharp spectral structures associated with the intrinsic free excitons were observed by photoluminescence and reflectance measurements from GaN based materials grown on SiC substrates. The residual strain was found to have a strong influence in determining the energies of exciton transitions. Picosecond relaxation studies of exciton decay dynamics suggest that an AlGaN cladding layer with a small mole fraction of AlN can be relatively effective in enhancing the radiative recombination rate for excitons by reducing the density of dislocations and suppressing surface recombination velocity in the GaN active layer for the GaN/AlGaN heterostructure samples.

Mechanism of efficient ultraviolet lasing in GaN/AlGaN separate-confinement heterostructures

We report the results of an experimental study on efficient laser action in an optically pumped GaN/AlGaN separate-confinement heterostructure (SCH) in the temperature range of 10–300 K. The lasing threshold was measured to be as low as 15 kW/cm2 at 10 K and 105 kW/cm2 at room temperature. Strongly polarized (TE:TM⩾300:1) lasing peaks were observed over the wavelength range of 358–367 nm. We found high-finesse lasing modes that originate from self-formed microcavities in the AlGaN and GaN layers. Through analysis of the relative shift between spontaneous emission and lasing peaks, combined with the temperature dependence of the lasing threshold, we conclude that exciton–exciton scattering is the dominant gain mechanism leading to low-threshold ultraviolet lasing in the GaN/AlGaN SCH over the entire temperature range studied. Based on our results, we discuss possibilities for the development of ultraviolet laser diodes with a GaN active medium.

Photoelectrochemical etching of GaN

A photoelectrochemical etching process for n-type GaN using KOH solution and broad-area Hg arc lamp illumination is described. Etch rates as high as 320 nm/min are obtained. The etch rate is investigated as a function of light intensity for stirred and unstirred solutions. Preliminary results on etching of Al 0.1 Ga 0.9 N layers are reported.

Dry and Wet Etching for Group III – Nitrides

The group-III nitrides have become versatile semiconductors for short wavelength emitters, high temperature microwave transistors, photodetectors, and field emission tips. The processing of these materials is significant due to the unusually high bond energies that they possess. The dry and wet etching methods developed for these materials over the last few years are reviewed. High etch rates and highly anisotropic profiles obtained by inductively-coupled-plasma reactive ion etching are presented. Photoenhanced wet etching provides an alternative path to obtaining high etch rates without ion-induced damage. This method is shown to be suitable for device fabrication as well as for the estimation of dislocation densities in n-GaN. This has the potential of developing into a method for rapid evaluation of materials.

Dynamics of photoexcited carriers in AlxGa1−xN/GaN double heterostructures

We present results of a time-resolved photoluminescence study of the dynamics of photoexcited carriers in AlxGa1−xN/GaN double heterostructures (DHs). The carrier dynamics including generation, diffusion, spontaneous recombination, and nonradiative relaxation were studied by examining the time decay of photoluminescence associated with the spontaneous recombination from the samples. The temporal evolution of the luminescence from the GaN active layers of the DH samples was found to be governed by a carrier–diffusion dominated capture process. The determination of the capture time for the carriers drift and diffusion into the GaN active region, in addition to the effective lifetimes of the spontaneous recombination for carriers in the AlGaN cladding layers and the GaN active region, allows an estimation of the diffusion constants for the minority carriers in the AlxGa1−xN cladding layers of the DHs. Our results yield a diffusion constant of 2.6 cm2/s for Al0.03Ga0.97N and 1.5 cm2/s for Al0.1Ga0.9N at 10 K.

Nitride lasers on SiC substrates

Laser diode (LD) structures were fabricated by metal-organic chemical vapor deposition (MOCVD) from the AlN-InN-GaN system on single crystal 6H-SiC substrates. A conducting buffer layer was developed for these devices which uses an AlGaN buffer layer and provides a 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 threshold current density obtained for lasing was 7.1 kA/cm/sup 2/ in a 4-well structure.

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.

High Heat Flux, High Temperature Cooling of Electronics With Thermoelectric Devices

Single couple PbTe/TAGS-85 and thin-film superlattice Bi₂Te₃-based thermoelectric couples operated in a downhill active cooling mode, where the cold side temperature (T_c) is above that of the heat sink (T_h), are shown to be able to pump heat loads of 204 W/cm² (T_c ~ 300^°C) and 329 W/cm² (T_c ~ 150^°C), respectively, using a 25 ^°C heat sink temperature. Such TE devices would be useful in active cooling of wide bandgap RF electronics operating ~ 300^°C and ~ 150^°C, respectively.