Andreas Weimar

Degradation Analysis of InGaN Laser Diodes

The current status of InGaN-MQW-laser diodes developed at Osram OS is presented. These lasers are grown on n-conducting SiC, enabling a vertical current path, cleaved facets and excellent heat spreading. The temperature rise during cw operation is measured for different mountings. A p-side up mounted diode with thermal resistance of 18 K/W showed 143 h of cw lasing at 1 mW optical power (T = 25 °C). DC and pulsed aging shows current as main degradation reason compared to heat for InGaN-LDs. Photoluminescence spectra of the quantum wells are being compared before and after degradation caused by current.

Advanced technologies for high-efficiency GaInN LEDs for solid state lighting

Solid state lighting has seen a rapid development over the last decade. They compete and even outperform light sources like incandescent bulbs and halogen lamps. LEDs are used in applications where brightness, power consumption, reliability and costs are key parameters as automotive, mobile and display applications. In the future LEDs will also enter the market of general lighting. For all of these new applications highly efficient, scalable and cost efficient technologies are required. These targets can be matched by SiC based flip chip LEDs which enable the design of high current chips with efficiencies of up to 28 lm/W in white solderable packages. An alternative approach is the implementation of thinfilm technology for GaInN. The LED is fabricated by transferring the epilayers with laser lift off from sapphire to a GaAs host substrate. In combination with efficient surface roughening and highly reflective p-mirror metalization an extraction efficiency of 70% and wall plug efficiency of 24% at 460 nm have been shown. The chips showed 16 mW @ 20 mA with a Voltage of 3.2 V. The technology is scalable from small size LEDs to high current Chips and is being transferred to mass production.

Light extraction technologies for high efficiency GaInN-LED devices

Data are presented for an GaInN based thinfilm LED. The LED is fabricated by transferring the epilayers with laser lift off from sapphire to a GaAs host substrate. In combination with efficient surface roughening and highly reflective p-mirror metallisation an extraction efficiency of 70% and wall plug efficiency of 24% at 460nm have been shown. The chips showed 12mW @ 20mA with a Voltage of 3.2V. The technology is scalable from small size LEDs to high current Chips and is being transferred to mass production.

Correlation of strain, wing tilt, dislocation density, and photoluminescence in epitaxial lateral overgrown GaN on SiC substrates

Epitaxial lateral overgrown (ELOG) gallium nitride (GaN) on SiC is being studied as a possible substrate for blue laser diodes. A defect density below 2.2×107cm−2 in the wings, compared to 2×109cm−2 in the windows, was achieved. Interaction of the overgrown GaN with the SiO2 mask causes a few degree wing tilt and a transition region of high defect density between windows and wings. Diminished PL, strong tensile stress, and a defect correlated line at around 3.4eV emerge in this up to two-micron-wide transition region. By changing the mask material from SiO2 to SiN we were able to reduce the wing tilt drastically to below 0.7°. This eliminates the defective transition region and extends the low strain and the low defect density area of the ELOG wings. The methods used to study strain, wing tilt, and threading dislocations in the ELOG samples are microphotoluminescence (μPL), transmission electron microscopy, x–ray diffraction, and scanning electron microscope. We also demonstrate the use of the first momen...

Investigation of low-resistance metal contacts on p-type GaN using the linear and circular transmission line method

In this work we investigated the specific contact resistances of the different metallizations Pt, Pd, and Ni on p-type GaN. Those materials were deposited both by thermal and electron beam evaporation on LED wafer material grown on SiC by MOCVD after using a standard surface treatment. Realizing various annealing steps we were able to achieve results in the low 10 -3 Ω cm 2 range. To determine those values, TLM (transmission line method) patterns were made by photolithography technique. To proof the usability of the TLM measurements on LED wafer material a comparison of the results obtained by linear and circular test structures with different geometries is given. Furthermore, the Pt, Pd and Ni contacts were examined by temperature dependent TLM measurements to get information concerning the current transport mechanism at the p-GaN-metal interface. The experiments showed only a weak temperature dependence of the contact resistances which indicates that mainly the field emission determines the contact resistance.

First European GaN-Based Violet Laser Diode

We report on the realization of room temperature pulsed operation of GaInN multiple quantum well laser diodes. The devices were grown by organometallic vapor phase epitaxy on SiC substrates. Gain guided laser structures with a 8 μm wide resonator show a threshold current density of 17 kA/cm2. For decreasing stripe width the threshold current density increases due to decreasing overlap of electrically pumped area and the lateral extension of the optical wave. The devices were operated at temperatures up to 90 °C with characteristic temperatures of 200 and 290 K for emission wavelengths of 418 and 428 nm, respectively.

Microsecond time scale lateral-mode dynamics in a narrow stripe InGaN laser

We studied the transient lateral mode behavior of a 2.25 µm wide InGaN/GaN-based laser diode. At the beginning of a 9 µs long current pulse, the diode heats up, undergoes a lateral mode transition and finally shows a beam steering due to lateral carrier diffusion.

Optical gain and saturation in nitride-based laser structures

We have performed systematic studies of the optical gain and its saturation in (In, Ga)N/GaN/(Al, Ga)N laser structures that depend on the excitation density and number of quantum wells. The unsaturated gain factor which was obtained by the variable stripe-length method increases with excitation power, i.e., increasing modal gain. The gain factor also increases with a decreasing number of quantum wells, as is shown by the investigation of a series of laser structures with 3, 4, 5, and 10 quantum wells for fixed modal gain. Values up to 40 dB at 300 K were measured. Thermal activation energies obtained by temperature dependent photoluminescence measurements yield information on the influence of nonradiative recombination processes on optical gain saturation.

High-power InGaN LEDs : present status and future prospects

The ThinGaN® technology of OSRAM Opto semiconductors enables high power LEDs with wall plug efficiencies of currently up to 50%, enabling efficacies of > 100lm/W for white and green LEDs. The good scalability of the technology enables devices which deliver high luminous flux. The future limitations regarding efficacy of white LED can be estimated to be 150lm/W for high color rendering. Besides efficiency long term stability and high temperature capability are requirements for market adoption

Laterally Coupled InGaN/GaN DFB Laser Diodes

For group III nitride-based lasers a new approach to realise DFB lasers and arrays is presented. Laterally coupled DFB (LC-DFB) lasers are realised to minimise processing-induced damage and to enable post-processing adjustment of DFB laser parameters according to the material parameters achieved after epitaxial growth. These DFB laser arrays enable one to measure the threshold and the refractive index of electrically pumped DFB lasers over a wavelength emission range of 6 nm. DFB emission is demonstrated up to 70 °C and simultaneously a small temperature shift of the DFB mode. A significantly smaller tuning range is observed compared to optically pumped DFB lasers as well as a significantly red-shifted gain region. Furthermore, the dispersion relation gradient determined, dn eff /dλ, appears significantly steeper (by a factor of six) than in the optically pumped case. These effects are attributed to incompletely screened polarisation fields in electrically pumped InGaN/GaN lasers.