Rudolf Rösch

Planar semipolar (101¯1) GaN on (112¯3) sapphire

We report on the growth of planar semipolar (101¯1) GaN on (112¯3) prepatterned sapphire. This is a method that allows the growth of semipolar oriented (101¯1) GaN on large scale. Using x-ray diffraction only the peaks of the desired (101¯1) plane could be observed. Scanning electron, transmission electron, and atomic force microscopy measurements show an atomically flat surface. Further investigations using photoluminescence spectroscopy show spectra that are dominated by the near band edge emission. The high crystal quality is furthermore confirmed by the small full width at half maximum values of x-ray rocking curve measurements of less than 400 arcsec.

Ultrathin Body InAlN/GaN HEMTs for High-Temperature (600

Lattice matched 0.25-μm gatelength InAlN/GaN high electron mobility transistors are realized in an ultrathin body mesa technology (50-nm AlN nucleation layer/50-nm GaN buffer) on sapphire. At room temperature, the maximum output current density is I_DS=0.4A/mm, the threshold voltage V_th=-1.4 V with an associated subthreshold voltage swing of 73 mV/dec and a leakage current ≈ 1 pA (for W_G=50 μm) and thus a current on/off ratio of 10¹⁰. At 600°C, the maximum drain current, threshold voltage, and transconductance are nearly unchanged. The current on/off ratio is still approximately 10⁶. First 1-MHz class A measurements with ±2.0 V peak-to-peak signal amplitude have resulted in 109-mW/mm output power at V_DS=8.75 V.

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Lattice-matched InAlN/GaN high electron mobility transistors (HEMTs) have been prepared in a silicon-on-insulator (SOI)-like configuration. Here, this implies an ultrathin body 50 nm GaN channel/50 nm AlN nucleation layer material structure on sapphire with the active areas confined by mesa etching, resulting in semi-enhancement mode device characteristics. In contrast to conventional technologies, the device characteristics (maximum drain current, threshold voltage and 1 MHz large signal operation) change only within less than approx. 10% up to 600 degrees C compared to room temperature (RT). The current on/off ratio decreases from 10(10) at RT to 10(6) at 600 degrees C, due to residual defect activation. These first results of ultrathin body GaN-on-sapphire-based materials and device technology may indicate that essential improvements in the temperature-handling capability of electronic device structures beyond what is common at present may be possible with only limited sacrifice of device performance.

) Electronics

In this paper, we present a carefully elaborated layer design for semiconductor disk lasers. Experimental results of devices mounted on simple copper heat spreaders reveal a conversion efficiency of 54% at 13.2 W for 970-nm wavelength laser emission and a differential quantum efficiency of 73%.

GaN-on-insulator technology for high-temperature electronics beyond 400 °C

We present the monolithic design, fabrication and properties of 850nm wavelength AlGaAs-GaAs-based transceiver chips with a stacked layer structure of a VCSEL and a PIN photodetector. Bidirectional data transmission via a single, two-side butt-coupled multimode fiber (MMF) is thus enabled. The approach aims at a miniaturization of transceiver chips in order to ensure compatibility with standard MMFs with core diameters of 50 and 62.5μm used predominantly in premises networks. These chips are supposed to be well suited for low-cost and compact half- and full-duplex interconnection at Gbit/s data rates over distances of a few hundred meters.

Design of Highly Efficient High-Power Optically Pumped Semiconductor Disk Lasers

The combination of microfluidics and optical manipulation offers new possibilities for particle handling and sorting on a single-cell level in the field of biophotonics. We present particle manipulation in microfluidics based on vertical-cavity surface-emitting lasers (VCSELs) which constitute a new low-cost, high beam quality nanostructured laser source for optical trapping, additionally allowing easy formation of small-sized, two-dimensional laser arrays. Single devices as well as densely packed linear VCSEL arrays with a pitch of only 24 μm are fabricated. Microfluidic channels with widths of 50 to 150 μm forming T- and Y-junctions are made of PDMS using common soft-lithography. With a single laser, selected polystyrene particles are trapped in the inlet channel and transferred to the desired outlet branch by moving the chip relatively to the optical trap. In a second approach, a tilted, linear laser array is introduced into the setup, effectively forming an optical lattice. While passing the continuously operating tweezers array, particles are not fully trapped, but deflected by each single laser beam. Therefore, non-mechanical particle separation in microfluidics is achieved. We also show the route to ultra-miniaturization of the system avoiding any external optics. Simulations of an integrated particle deflection and sorting scheme as well as first fabrication steps for the integrated optical trap are presented.

Monolithic integration of VCSELs and PIN photodiodes for bidirectional data communication over standard multimode fibers

We have fabricated arrays of top-emitting VCSELs with pitches down to 24 mum and an only 2 mum wide gap between the devices. For enhanced single-mode emission, the inverted surface relief technique was applied on some devices. The output facets then contain a shallow, circular etch in their center.

Application of vertical-cavity laser-based optical tweezers for particle manipulation in microfluidic channels

We present optically pumped semiconductor disk lasers (OPSDLs) emitting in the 900–1100nm band which are frequency doubled to access the visible spectrum. In particular, we focus on presenting the design, fabrication, and specific characteristics. Fundamental outputs exceeding 20W and visible radiation with powers of 5–10W are achieved. The blue and green emission at wavelengths around 450–470 and 520–540nm yield advantageous gamuts for display applications and can be utilized for stereo projection. Moreover, other accessible wavelengths in this spectral region, e.g. 493 nm, and the good beam quality of these devices enable optical ion trapping experiments.