Many of our every day products, such as mobile phones, eye glasses, and flat-panel displays contain thin films deposited by cathode evaporation. Up to now, this requires bulky and costly vacuum coaters. Researchers at FBH successfully transferred this process to atmospheric pressure. Thus, costs go down and new coating applications can be opend up. The technology is to be commercialized in near future by founding a spin-off company.
Research News 2012
The FBH develops normally-off GaN transistors based on a p-GaN gate technology. Switching transistors with 150 mm gate width have been realized showing an on-state resistance RON = 100 mΩ and a maximum pulse current of 75 A for 300 V blocking strength. Using a double-pulse switching set-up from TU Berlin with inductive load and a time resolution < 5 ns lead to excellent results when analyzing the switching properties of the FBH transistors.
Current results from the InP-HBT high frequency circuits fabricated within the "HiTeK" project prove the capability of the developed technology platform for heterogeneous integrated circuits at terahertz frequencies (0.1 - 1 THz)
Narrow bandwidth red-emitting diode lasers will be an alternative for commonly used He-Ne-Lasers in metrology and Raman spectroscopy in the future. Just recently, scientists from the FBH succeeded to integrate DBR surface gratings in waveguide structures for diode laser emitting in wavelength range 630 nm…640 nm and achieved promising results.
At FBH, work has been performed aiming at higher output power levels of mm-wave power amplifiers. To achieve this, multifinger transistors in FBH's Transferred-Substrate InP HBT technology were developed as 4-, 6-, and 8- finger transistors with transit frequency (fmax) values of 323, 283, and 256 GHz, respectively. The realized multi-finger HBTs are suited for power amplifier applications in the mm-wave range around 100 GHz.
The improvement of HVPE grown Al0.45Ga0.55N crystal quality is a current research topic at FBH. For that purpose, the sapphire substrate is patterned with parallel trenches which then are laterally overgrown by material growing from the top of the ridges.
The application fields of ultraviolet (UV) emitters in the UV-B range are manifold: ranging from material processing to medical applications. After successful optimization of the fabrication technology we managed to increase the optical power of the devices into the milliwatt range, which is comparable or even better than international record values for this wavelength range.
An ideal diode laser would deliver high output power in a diffraction limited beam, with all emission contained within a narrow, stable spectral window. Each improvement in this regard enables new applications. Recent work at the Ferdinand-Braun-Institut has enabled significant progress leading to an output power of over 50 W from a single power amplifier under quasi-continuous–wave measurement conditions.
At FBH a GaN-based H-bridge class-D PA for the 900 MHz band has been developed. The demonstrator achieved a maximum efficiency of 50% and a saturated output power of 8 W.
GaN-based high-voltage switching transistors enable particularly efficient power converters. Due to lower losses per switching cycle, GaN-based converters can operate at higher frequencies than converters with Si-based switches. However, the dynamic on-state resistance is increased immediately after biasing the drain terminal at several hundreds of volts at off-state. Recently developed GaN buffer compositions using AlGaN or iron-doped GaN are significantly reducing this problem: They feature electrical blocking strengths comparable to carbon-doped GaN buffer compositions with low doping concentrations and allow for transistors with a significantly reduced dynamic on-state resistance.
A growing variety of applications for detectors of UV radiation depend on highly reliable and long-term stable detection systems. In a joint project of FBH, IKZ, and sglux highly efficient and reliable polytype 4H-SiC p-n photodiodes have been fabricated on n-type substrates. Aging tests demonstrate their robustness against UV radiation and confirm that they are suitable for high irradiance UV applications.
Tapered lasers show a high market potential for industrial applications. However, for many applications it is necessary to transport the beam using single-mode fibers. As tapered lasers feature a complex beam profile, the commonly used beam propagation factor (M²) is insufficient to predict the coupling efficiency into a fiber. At the FBH, it could be shown that a more in-depth description of the beam profile using the Wigner distribution function allows to precisely predict the coupling efficiency.
Increasingly, sapphire substrate with a larger diameter are used for GaN-LEDs leading to a wafer bow during the growth processes of several 100 µm. This affects the temperature of the wafer during epitaxy. Inhomogeneous wafer temperature, however, leads to inhomogenous wavelengths and thus reduced yield of chips. At FBH, an advanced temperature sensor has been tested in order to measure the temperature of the wafer surface - which is indispensable for an exact and reproducible adjustment of the emission wavelength of the devices.
The most power-hungry components in mobile base stations are microwave power amplifiers. Situation becomes even worse for modern modulation techniques such as WiMAX and LTE, because then they need to be operated in a low efficient back-off region most of the time. One of the most promising ways to improve back-off efficiency is to vary the supply voltage of the microwave amplifier in an appropriate way (envelope tracking). For this purpose, high-speed and efficient supply modulators are required, which are being investigated at FBH - results have currently been awarded the best paper prize at GeMIC 2012.
At FBH, the efficiency of top-emitting UV-LEDs for sensor applications with emission wavelengths between 360 and 380 nm could be significantly increased by optimizing the epitaxial structure. Optimizations include a single quantum well structure as active region as well as adjusting the AlGaN blocking layers underneath and on top of the active region.
FBH has developed narrow linewidth, high-power DFB diode lasers for quantum optics experiments on ensembles of ultra-cold potassium atoms. An improved design of the grating layers has been implemented resulting in better grating and device performance of 767 nm DFB lasers. Recent results show a comparable electro-optical performance to those obtained from 780 nm.
Various radar and satellite systems operate in the frequency range from 8 to 12 GHz. In this X-band range, amplifiers are commonly built as monolithic circuits, as critical tolerances and parasitic properties can be reduced through monolithic realization. A respective GaN MMIC process is available at FBH whose performance has been further improved by reducing the gate length of the GaN transistors to 0.25 μm.
In cooperation with its spin-off company BeMiTec FBH develops highly efficient power transistors for output power levels of more than 100 W in the frequency range between 1 and 3 GHz.
Their excellent material properties qualify InP-based transistors as key components for systems requiring high operating frequencies. At FBH, a transferred substrate technology has been established to optimize high frequency and power performance of InP-HBTs. The linear device set-up eliminates dominant transistor parasitics and relaxes design trade-offs.
FBH has developed micro-integrated master-oscillator power-amplifier (MOPA) laser and extended cavity diode laser (ECDL) modules for experiments on Rubidium Bose-Einstein condensates on board a sound rocket to be launched in 2013. Both module types feature excellent performance parameters and further omit any moveable parts so that the high requirements on mechanical stability for space applications can be met.
In the framework of the project Low-power Wireless Sensor Network with Localization, a low phase-noise oscillator was developed at FBH. It generates the radar signal in localization blocks of e.g. collision warning systems.
At FBH, a powerful analytics tool have been put into operation. The system is suitable for characterization of degradation processes and has been expanded with different systems to analyze structural and optical properties of semiconductor lasers and chips. Centerpiece of the tool is an Ultra+ high resolution scanning electron microscope with a thermal field emission electron gun for high-resolution imaging of surfaces.
GaN-based laser diodes are currently available on the market only for a limited number of wavelengths. FBH has now together with partners started to develop laser diodes with customized wavelengths for use in atom spectroscopy. The current focus is on the mercury lines at 404.7 nm and 435.9 nm. For such applications, the lasing wavelength needs to be precisely adjusted and the lasers have to show a low threshold current. Therefore, ridge waveguide laser diodes with a small ridge width of 1.5 µm and a resonator length of 600 µm have been fabricated.
THe FBH developed a compact laser module with more than 500 mW output power at 636 nm wavelength. The module emits a nearly diffraction-limited, collimated beam with a radiance of more than 19 MW/cm²/sr. This corresponds to a luminance of more than 27 TCd/m², which is more than 10,000 times brighter than the luminance of the sun (1.6 GCd/m²) – and a new record value for red-emitting diode lasers.
Passive elements such as inductors, capacitors and transmission lines take significant influence on mm-wave circuits based on high-end CMOS processes. Hence, the electrical behavior of these elements must be included already in the basic design steps using EM simulation. Respective investigations have been carried out in detail at FBH.