Recently celebrating its 50th anniversary, the Fraunhofer Society is Germany's leading organization of institutes for applied research. The Fraunhofer Institute for Applied Solid State Physics focuses on the investigation of compound semiconductor materials and devices.

The Fraunhofer Institute fr Angewandte Festkrperphysik (IAF, or Institute for Applied Solid State Physics) is one of 47 German research institutes of the Fraunhofer Society, a body that recently celebrated its 50th anniversary. To commemorate the event, IAF s innovative researchin particular, its LEDsare now depicted on a postage stamp. In addition to LED research, the Fraunhofer IAF is one of Europe s leading centers engaged in applied R&D of compound semiconductor materials, devices and ICs. Based in Freiburg in the south of Germany, IAF designs, demonstrates and undertakes small volume production of MMICs, mixed signal/digital circuits and optoelectronic devices based on GaAs, InP, antimonides and group III-nitrides. Among the Institute s stated goals is the development of devices and circuits to meet the requirements of its customer base, which includes the German Ministry of Defense, and its industrial partners. This association with the defense industry, which provides 60% of IAF s $15 million in funding and operating costs, contrasts with the majority of other Fraunhofer Institutes. The German Federal Ministry of Education and Research supplies a further 17%, with the remaining 23% supplied by industrial contracts. Around 160 scientists and engineers (total staff 210) are employed in 86,000 sq.ft. of offices and laboratory space, which includes 8,600 sq.ft. of clean rooms for device and circuit processing and epitaxy. Fraunhofer IAF recently installed a 6 2-inch production MOCVD system for the growth of III-nitrides used to manufacture UV LEDs, laser diodes and FETs. The institute also recently formed EpiNova, a commercial enterprise founded by a small group from its Epitaxy Division [CS 6(5), p. 25]. EpiNova rents laboratory and office space at Fraunhofer IAF, and specializes in the design and MBE growth of customized MESFETs and PHEMTs in small and medium quantities, using substrates sizes up to 6-inch. The formation of EpiNova is just an extension of the more commercial practices recently instituted by the Fraunhofer IAF, says marketing manager Roland Diehl. "The emphasis has now shifted from predominantly institution-funded research to contract research supported by revenue obtained from R&D contracts with the industrial sector and public authorities," he says. "In addition, research has been reorganized under new business units, all of which has helped speed up technology transfer and innovation in hot markets." Diehl believes that by capitalizing on its strength in component development, which includes prototype and small-volume production, Fraunhofer IAF can target markets such as wireless communications, optical telecommunications and interconnects, as well as high-speed data processing, LEDs, IR lasers and detectors, and high-power diode lasers. Some of the institute s recent achievements in these and other fields are presented below. HEMTs for Wireless Communications Research into HEMTs is undertaken by the Institute s Devices and High Frequency Circuits division, which employs GaAs HEMT technology to realize MMICs PAs covering the 160 GHz range for cell phones and communication networks, WLANs, wireless broadcast transmitter systems, commercial LMDS and satellite cross-links. The design route involves the use of compact coplanar devices and circuits for power applications with gate lengths of 0.5 m, 0.3 m or 0.15 m. Device simulation at Fraunhofer IAF has led to PHEMTs employing a double recess and asymmetrically placed gate that achieve a breakdown voltage of 24 V. These devices have a small signal gain of 22 dB at 2 GHz, and more than 1.6 W/mm of saturated output power density and power added efficiency (PAE) of 66% (see ). Metamorphic HEMTs The Institute works also closely with companies such as Infineon, UMS and Alcatel on MHEMT processes. Axel Hlsmann, who heads up the III-V Technology Division, says MHEMTs grown on 6-inch GaAs wafers offer cost advantages for high volume production compared to InP HEMTs. "It is hard to directly compare cost structures as 4-inch InP wafers still have no realistic market price," says Hlsmann. "While MHEMTs require a 1 m thick metamorphic buffer that adds to the production costs, the [6-inch] wafer area is more than twice that of a 4-inch wafer, and circuits employing GaAs MHEMTs are currently cheaper to produce." Although he notes that this balance may change when 4-inch InP wafers become available in sufficient quality, price and quantity. Hlsmann adds that for power applications, GaAs-based MHEMTs also allow more freedom when choosing the lattice constant. "InP substrates offer better thermal conductivity compared to GaAs, lowering the channel temperature for a given power dissipation. But as innovative mounting techniques enter production, such as flip-chip with thermal bumps at the source islands, the improved thermal conductivity afforded by InP assumes less importance," he says. IAF s MHEMT designs are based on a quaternary AlxGa1-x-yInyAs buffer with a linear grading of the group III-components (see ), which maintains a two-dimensional growth mode in thick (1mm) buffer layers. A buffer thickness of 1 m has a sufficiently small composition gradient to obtain a continuous two-dimensional growth mode and results in good material properties for electrical devices, says Hlsmann. MHEMTs with a 0.15 m gate length on such buffer layers have exhibited 13.5 dB of gain at 94 GHz, with an fmax values up to 450 GHz. 40 Gb/s Optical Data Transmission Multiplexer Modules As part of KomNet, a project funded by the German Federal Ministry of Education and Research, Fraunhofer IAF develops a variety of high bit rate circuits for 40 Gb/s optical telecommunication systems. These include multiplexers, modulator drivers, limiting amplifiers, frequency dividers, VCOs, data and clock recovery circuits and demultiplexers. The institute has also recently begun offering packaged ICs and modules, for example, a 2:1 multiplexer module is now offered for operation at 40 Gb/s, realized using an enhancement-depletion PHEMT technology with a gate length of 0.2 m (). The multiplexer module multiplexes two 20 Gb/s signals into one 40 Gb/s signal at the transmitter side of optical network. Hybrid Integrated Photoreceivers Fraunhofer IAF s work on photo-receivers includes multi-mode waveguide photodiodes. These devices use a separate confinement heterostructure in which the optical waveguide is no longer formed by the photodiode absorption layer but by a second cladding layer. This allows the optical waveguide width to be chosen independently of the absorption layer width, eliminating the problem of poor bandwidth that occurs with conventional surface illuminated photodiodes, which require a thick absorption layer and thus suffer from carrier transit time limitations. Multimode waveguide photodiodes are designed to be flip-chip mounted on a front end amplifier. These are grown by MBE and based on a 0.15 m gate length AlGaAs/InGaAs/GaAs process. Using this approach, a 3 dB bandwidth at 41 GHz has been obtained for a photo-diode operating at a wavelength of 1.55 m. Low-Noise QWIPs IAF has patented a novel low noise QWIP for use in infrared cameras operating in the 812 m regime. The device optimizes the potential distributions in a four-zone structure (), which are realized using MBE-grown layers of GaAs, AlGaAs and AlAs. In the Fraunhofer IAF design, the photoexcited carriers traverse a barrier layer (drift zone) and are captured by a narrow QW (capture zone) from where they are transferred across a tunnel barrier (tunneling zone) into the excitation zone of the adjacent period. The responsivity of such a low-noise QWIP under applied bias reveals that a significant photocurrent exists at zero bias, which allows the detector to be operated without a dark current. This small photocurrent gain allows the use of about 4 times higher doping densities compared to conventional QWIPs, increasing the internal quantum efficiency to about 40%. In addition, the amount of noise is considerably reduced as a consequence of the suppression of recombination noise. Fraunhofer IAF s industrial partner, AEG Infrarot Module (AIM) of Heilbronn/Germany has fabricated a low-noise QWIP prototype camera based on IAF s QWIP focal plane arrays with 256 256 pixels, which it says shows great promise for high-performance thermal imaging for use in both military and civilian applications, including medical imaging and environmental monitoring. A low-noise version of the 640 512 focal plane array has also been realized. Excellent temperature resolutions of less than 7 and 13 mK have been achieved in IR cameras with 256 256 and 640 512 QWIP arrays, respectively. High-Power Diode Lasers High power diode lasers are being developed by a joint venture of Fraunhofer IAF with the Fraunhofer Institute for Laser Technology (ILT) in Aachen. ILT is responsible for the micro-channel copper heat sinks and the mounting and testing of the devices, which are available commercially in small quantities. According to Michael Mikulla of the Optoelectronics Division, the direct application of high-power diode lasers to material processing applications such as welding, cutting or surface treatment is becoming more attractive. "The main applications for diodes include direct material processing, for example medical treatment with single arrays, up to and including welding of metal sheets with stacked arrays," he says. The Institute recently transferred its epitaxial growth and fabrication of high power diode lasers to a 3-inch wafer process, resulting in a higher yield and up to eighty 2 mm 10 mm chips per wafer compared to around twenty on a 2-inch wafer. Devices are grown by MBE with a single InGaAs QW constituting the active region. The emission wavelength is adjustable between 880 nm and 1060 nm by varying the indium content. The quantum well is embedded in an 880 nm thick AlGaAs core region with a low Alcontent, and the optical waveguide is formed using AlGaAs cladding layers with a higher Alcontent. Single Broad Area Lasers and Arrays As part of the joint venture, IAF and ILT have mounted single broad area emitters (980 nm) p-side down on copper C-mounts. These devices emit a maximum output power of 9.2 W for a drive current of 10 A, and 50C lifetime tests have shown that under a constant power condition of 1.5 W, devices are very stable for more than 3000 hours, with no sudden COMD-related failure. If a 20% current increase over the lifetime is chosen, the life expectancy extrapolates to at least 20,000 hours, and 50,000 hours at room temperature. Broad area 980 nm arrays with a stripe widths up to 1 cm are also produced. These are mounted p-side down on micro-channel heat sinks at the ILT. A novel method of high power testing, allowing drive currents as high as 360 A, has been used to measure the CW output power. According to Mikulla, an output power of 267 W has been achieved at 330 A drive current. "To the best of our knowledge, this sets a record for a single 1 cm laser bar operating at 980 nm," he says. So far, lifetime testing has validated the performance of the devices for up to 5,000 hours when operated at 60 W of CW output power. Long-Wavelength Lasers Current research at the Fraunhofer Institute on infrared diode lasers is based on both GaInAsSb/AlGaAsSb and GaInAsP systems. The aim is to develop diode lasers for room temperature CW operation at wavelengths beyond 2 m, with the primary focus on spectroscopic and sensing applications, as well as for materials processing. GaInAsSb/AlGaAsSb Lasers GaInAsSb/AlGaAsSb type-I QW lasers are grown by solid-source MBE on n-type substrates and processed into ridge waveguide lasers. shows the band edge profile of a triple-QW large optical cavity laser structure, which contains an active region with three compressively strained 10 nm wide Ga0.70In0.30As0.06Sb0.94 QWs separated by 20 nm thick Al0.28Ga0.72As0.02Sb0.98 barriers and sandwiched between 400 nm thick Al0.28Ga0.72As0.02Sb0.98 separate confinement layers. High efficiency, low-loss type I diode lasers emitting in the 1.8 to 2.3 m wavelength range were achieved with an excellent power conversion efficiency of up to 28% at 1.94 m, demonstrating the potential for high power operation, particularly when mounted p-side down to improve thermal management. IAF is currently working to extend the wavelength and power range, and has recently achieved optically pumped laser emission at 3.65 m based on an InAs/GaSb type II mid-infrared laser. GaInAsP Lasers While GaInAsP lasers are widely used for fiber optic telecommunications at 1.3 and 1.55 m, at IAF this material system is also being examined to extend the long wavelength limit out to 2 m for use in gas sensing, among other applications. Long wavelength lasers for this application are grown on InP substrates by MOCVD, using two compressively strained 12 nm wide Ga0.25In0.75As QWs for the active regions. IAF has fabricated ridge waveguide diodes (32 m 1760 m) that showed CW emission up to 2.09 m at a heat sink temperature of 330 K. With these GaInAsP-based devices, tunable diode laser absorption spectroscopy (TDLAS) for the detection of CO2 at 2.004 m has been demonstrated. AlGaInN-Based Violet LEDs LED research at Fraunhofer concentrates on the development of advanced luminescence converting white LEDs that require efficient violet and UV-emitting LED chips based on AlGaInN. The emission of these LED chips is then converted into blue, green and red secondary emission with a phosphor, adding up to white light with a high color rendering index. AlGaInN-based LEDs are fabricated on 2-inch sapphire substrates using low-pressure MOCVD. For an emission wavelength of 420 nm, enscapsulated LEDs emitting up to 3 mW at a current of 20 mA and supply voltage of 3.7 V have already been realized. To demonstrate the feasibility of luminescence-converting LEDs (known as LUCOLEDs) using a violet LED chip, IAF has fabricated single band LUCOLEDs using an organic dye emitting in the green- yellow spectral range as the converter (). The emission spectrum shown in Figure 6b reveals the broad emission band in the visible spectral range, with only a small primary emission from the short violet chip, which indicates that a large fraction of the pump light is absorbed in the converter. Further work is being directed towards even shorter wavelength LEDs (370 to 410 nm) to optimize the efficiency of conversion, and their integration into LUCOLEDs employing inorganic phosphors for three-band (red-green-blue) conversion.