Power oscillators can contribute significantly to the miniaturization of future generators used in the fields of microwave heating and plasma technology by combining signal generation and amplification in a single small circuit. For the first time, FBH has developed a power oscillator with mechanical frequency tuning capability inside the 2.4 GHz ISM band. Compared to established technologies space requirements can be reduced to a tenth.
Research News 2014
For Ka-band broadband satellite communication systems highly sophisticated transceiver MMICs are required, offering high linearity and efficiency for transmitters as well as low noise for receivers. Recently, the FBH has developed a reliable and scalable technology for the fabrication of GaN-based MMICs, enabling transistors with short gate lengths ranging from 100 nm to 200 nm. Two different epi-structurs have been systematically compared.
Flip-chip connections offer shorter interconnection paths as compared to wire bonding, thus enabling higher bandwidth. At the FBH, only recently miniaturized waveguide structures with highest precision have been fabricated on RF substrates , featuring -1 dB bandwidth at 250 GHz. Small-signal RF measurements of back-to-back flip-chip transitions showed an insertion loss below 1.0 dB per interconnect, and a return loss of more than 1 dB from DC up to 250 GHz. These figures are so far the best reported flip-chip mounts in this frequency range.
At the FBH, a GaN-based high-power diode laser system with an optical output power of 400 mW and narrowband emission at 445 nm has been realized for the first time. By using an external cavity, the institute manged to reduce the spectral width to 20 pm (with a side-mode suppression of more than four orders of magnitude). Thus, the system is well-suited as pump source for nonlinear frequency conversion into the deep ultraviolet (DUV) spectral region.
InP double hetero bipolar transistors (DHBTs) offer the highest output powers available for integrated circuits in the range from 100 GHz to 500 GHz. High output power requires a low thermal resistance in order to keep self-heating of the device at a reasonable level. In order to improve this, the InP-HBT-Transferred-Substrate process at FBH was complemented by an integrated diamond heatsink.
GaN-based lasers emitting in the blue and violet spectral range still show – compared to lasers in the infrared spectral range – a lower optical output power and a lower efficiency. For the first time, the emission of blue-violet laser diodes has been measured with high spatial resolution to improve the laser parameters. The results are now used to further optimize the layer structure.
By using a new heterostructure design and carefully optimizing the corresponding semiconductor structures FBH and TU Berlin succeeded to fabricate UV LEDs with a dominant emission wavelength <240 nm in the framework of their Joint Lab.
Outer space offers optimum conditions for quantum optical experiments, since zero gravity is ideally suited to meet the stringent requirements on measurement accuracy and precision. However, until now, one of the main technological limitations was the lack of a space-qualified technology for the lasers. At the FBH, a compact and very robust platform based on GaAs diode lasers has been developed closing this technology gap.
Digital power amplifier (PA) concepts are highly attractive, especially when it comes to optimizing wireless infrastructure. Processing all signals digitally up to the final stage would simplify the system set-up, reducing both space requirements and energy consumption. Recently, NEC (Japan) and FBH proposed and verified a new digital transmitter architecture. For the first time, the Doherty concept was proven for digital signals, successfully transferring it to the "digital world".
GaN-based high-voltage power switching transistors enable efficient power converters with increased power density. The FBH has now successfully transferred its 600 V technology for normally-off power switching transistors from the GaN-on-SiC platform to 4" GaN-on-Si wafers - an essential precondition for competitive manufacturing costs. DC measurements demonstrated comparable characteristic values.
At FBH nanosecond-pulsed laser radiation (pulse length < 30 ns) with a wavelength of 355 nm is successfully used to scribe the material followed by cleaving. At 355 nm GaN absorbs, decomposes and ablates which allows for efficient laser micromachining. Processing parameters, such as pulse energy, pulse repetition frequency (PRF), and scan velocity were optimized to avoid damage to the laser active region and obtain best device performance.
The FBH developed fiber coupled pump modules for high repetition rate chirped pulse amplification thin disk lasers of the Joule class. The diode laser pump modules are designed to emit 6 kW, 1 ms pulses with 200 Hz repetition rate from a 1.9 mm fiber.
For the characterization of integrated circuits probes are used which deteriorate the measurement result – especially in higher frequency ranges. To correct these parasitic effects a calibration process must be applied. Current electromagnetic research at the FBH revealed some of the so far unknown parasitic effects leading to errors in calibration.
Thick AlN template layers with low dislocation density and smooth surfaces are required for fabrication of high-efficiency light emitting diodes (LEDs) operating in the ultra-violet spectral range. One of the methods leading to the necessary reduction of the threading dislocation density is epitaxial lateral overgrowth (ELO) - the FBH achieved an effective defect reduction in AlN templates by three orders of magnitude.
Compact semiconductor laser emitting in the UV-C spectral region ≤ 280 nm are attractive for applications like e.g. medical diagnostics and material processing. However, the realization of such lasers based on the material system AlGaN is very challenging. In the framework of the Joint Lab GaN Optoelectronics, FBH in collaboration with TU Berlin successfully demonstrated the shortest wavelength under optical pumping at around 237 nm ever reported for this material.
The FBH has developed a diode laser system emitting light at two closely neighboring wavelengths around 785 nm. This light source is ideally suited for Shifted Excitation Raman Difference Spectroscopy (SERDS) and allows free selection of the excitation power without requiring spectral re-calibration of the Raman measurement system.
The frequency limits of the semiconductor device technologies are continuously increased to meet growing demands. The FBH with its state-of-the-art equipment for network and spectrum analysis is well prepared for the challenge of characterizing such devices up to the terahertz range.
New 600 V devices with low on-state restance have been fabricated at the FBH using a novel layout that was optimized for more efficiently using the semiconductor area. This GaN transistor requires less chip area and shows improved switching efficency as compared to similarly rated 600 V Si-based super junction MOSFETs.
Epitaxial growth of GaN/AlGaN layers for light emitters in the ultraviolet spectral range has increasingly attracted interest in recent years. To improve the internal quantum efficiency of such LEDs, epitaxial layers with reduced dislocation densities are required. The FBH successfully fabricates patterned sapphire substrates and uses them for epitaxial lateral overgrowth (ELO), thus reducing strain and dislocation density in GaN/AlGaN layers.
The spectral emission width of semiconductor lasers can be significantly reduced from typically a few nanometers down to the femtometer scale by implementing Bragg gratings into the resonator. These gratings can be realized cost-efficiently by etching V-shaped grooves into the semiconductor surface. However, their reflectivity could be predicted only insufficiently with simulations. Now, the FBH successfully expanded its simulation tools in a way that reflection spectra of V-shaped grooves can be calculated, too.
The FBH currently develops AlGaN-based UV light emitting diodes (LED) and laser diodes for the wavelength range 200 – 350 nm in close cooperation with TU Berlin. Lately, we demonstrated LED operation down to 245 nm and stimulated emission from optically pumped laser structures down to 237 nm.
In order to enhance the lifetime of currently available UV-B LEDs, extended stress tests of UV-B LEDs have been started at FBH to study the relevant degradation mechanisms in these devices - with promising results.
Chip-based phase modulators allow to integrate phase control and modulation into hybrid laser and spectroscopy modules, leading to very compact and robust systems. III-V compound semiconductors are highly compatible with existing electronics, which makes phase modulators based on GaAs/AlGaAs double heterostructures a particularly attractive option.
The FBH has started several activities in the field of emitters and detectors at THz frequencies using GaN HEMTs. This frequency range offers a variety of applications in biology, medicine, and industry.
In many applications, UV radiation is monitored in a certain wavelength range by photodetectors (PD). We have extended our studies on UV photodetectors to devices that are only sensitive in the lower UV-C range with cut-off wavelengths below 220 nm. The FBH sucessfully increased their responsivity by using defect-reduced templates with stripe pattern.