A. Knauer

Advances in group III-nitride-based deep UV light-emitting diode technology

The field of AlGaInN ultraviolet UV light-emitting diodes (LEDs) is reviewed, with a summary of the state-of-the-art in device performance and enumeration of applications. Performance-limiting factors for high-efficiency UV LEDs are identified and recent advances in the development of deep UV emitters are presented.

Optical polarization characteristics of ultraviolet (In)(Al)GaN multiple quantum well light emitting diodes

The polarization of the in-plane electroluminescence of (0001) orientated (In)(Al)GaN multiple quantum well light emitting diodes in the ultraviolet-A and ultraviolet-B spectral range has been investigated. The intensity for transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with decreasing emission wavelength. This effect is attributed to rearrangement of the valence bands at the Γ-point of the Brillouin zone with changing aluminum and indium mole fractions in the (In)(Al)GaN quantum wells. For shorter wavelength the crystal-field split-off hole band moves closer to the conduction band relative to the heavy and light hole bands and as a consequence the transverse-magnetic polarized emission becomes more dominant for deep ultraviolet light emitting diodes.

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0  0) and on semipolar (1 0  2), (1 1  2), (1 0  1) as well as (2 0  1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0  1) < (1 1  2) = (0 0 0 1) < (2 0  1) < (1 0  0) = (1 0  2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0  0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0  0) and the semipolar (1 0  2) and (2 0  1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1  2) and (1 0  1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

Normally-off high-voltage p-GaN gate GaN HFET with carbon-doped buffer

Normally-off GaN transistors for power applications in p-type GaN gate technology with a modified carbon-doped GaN buffer are presented. A combination of an AlGaN back-barrier with the carbon-doped buffer prevents early off-state punch-through. Simultaneously, the on-state resistance could be kept low and the threshold voltage with 1.1 V high enough for secure normally-off operation. 1000 V breakdown strength has been obtained for devices with 6 μm gate-drain spacing. The resulting breakdown scaling slope is 170 V/μm gate-drain distance. The on-state resistance is 7.4 Ωmm. The resulting VBr-to-RONA ratio (1000 V, 0.62 mΩcm2) is beyond so far reported ratios for normally-off GaN transistors. Modifications of the p-type GaN layer have shown to additionally increase the threshold voltage by 0.4 V without paying a price in the on-state resistance of the device.

Mounting of high power laser diodes on boron nitride heat sinks using an optimized Au/Sn metallurgy

High power diode lasers have become more and more important to industrial and medical applications. In contrast to low power applications, long cavity lasers or laser bars are used in this field and mounting quality influences considerably laser performance and life time. In this paper we focus on the solder metallurgy and stress-induced laser behavior after mounting. The laser chips have been bonded fluxless epi-side down on translucent cubic boron nitride (T-cBN) using Au/Sn solder. The laser behavior has been tested with different chip metallizations preserving the eutectic solder composition or forming the Au rich /spl zeta/-phase during reflow. The resulting additional stress in the lasing region has been independently indicated by polarization measurements of the emitted light. A metallization scheme has been developed which forms the highly melting /spl zeta/-phase during soldering within a wide process window. This procedure yields better results then using eutectic Au/Sn which has a higher hardness than the /spl zeta/-phase. Laser diodes up to a cavity length of 2000 /spl mu/m and an aperture of 200 /spl mu/m have successfully been mounted on T-cBN. State of the art laser data, excellent thermal stability, high yield and reliability have been obtained.

High-Power Monolithic Two-Mode DFB Laser Diodes for the Generation of THz Radiation

We have devolved 1064 nm high-power monolithic distributed feedback lasers which operate simultaneously on two longitudinal modes. These modes correspond to the fundamental and first-order lateral mode and arise from a 7.5 mum width ridge waveguide supporting both of them. They are further stabilized by a first-order grating built into an InGaP/GaAs/InGaP multilayer structure. The threshold current of the laser is 66 mA, the slope efficiency is 0.5 W/A, and an output power of ~500 mW is reached. Detailed investigations of the intensity distribution of lateral and vertical far fields and the spectral behavior are shown. The longitudinal mode spacing at 260 mW is 0.56 nm corresponding to approximately 150 GHz. THz generation is demonstrated by mixing the two-line laser emission in a LT-GaAsSb photomixer.

Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates

The performance characteristics of optically pumped laser heterostructures emitting in the UV-C spectral range between 272 and 279 nm are investigated. The laser heterostructures were grown by metal-organic vapor phase epitaxy on (0001) planar AlN/sapphire, epitaxially laterally overgrown (ELO) AlN/sapphire, and bulk AlN substrates with threading dislocation densities ranging from 2×1010 to 104 cm-2. We found that the defect density strongly affects the laser performance. The lowest pulse threshold energy density of 50 mJ/cm2 under resonant optical pumping condition was obtained for an AlGaN multiple quantum well laser grown pseudomorphically on low defect density bulk AlN substrate. Lasing was also observed for AlGaN MQW heterostructures grown on ELO AlN/sapphire templates. The laser emission in all lasers was TE polarized. However, no lasing was observed for heterostructures grown on high defect density AlN/sapphire.

High-power tensile-strained GaAsP-AlGaAs quantum-well lasers emitting between 715 and 790 nm

Tensile-strained GaAsP quantum wells embedded in AlGaAs large optical cavity structures were studied. In the wavelength range between 715 and 790 nm, very high output power and excellent conversion efficiencies of broad-area lasers have been obtained.

High-power 980-nm DFB RW lasers with a narrow vertical far field

We compare 980-nm distributed-feedback ridge-waveguide lasers having cavity lengths of 1.5 and 3 mm. The maximum single-mode output powers are 500 and 700 mW, respectively. The full-width at half-maximum of the vertical far-field profile is only 22/spl deg/ due to a superlarge optical cavity.

Effect of the barrier composition on the polarization fields in near UV InGaN light emitting diodes

The electroluminescence from near ultraviolet (UV) light emitting diodes containing InGaN multiple quantum wells (MQWs) with GaN, AlGaN, and InAlGaN barriers was investigated. Based on band-structure calculations the observed wavelength shift in the peak emission with increasing injection current is attributed to the screening of the polarization fields and to band gap renormalization. InGaN MQWs with almost zero net polarization have been realized. No blueshift in the emission spectra of these devices was observed over the entire current range.