Research
FBH research: 07.05.2012
- Fig. 1: Single-quantum well, RW DFB diode laser with 1.5 mm chip length emitting at 767 nm: power, voltage, and conversion efficiency vs. injection current at a mount temperature of 20°C.
- Fig. 2: Emission spectrum vs. injection current of the same diode laser. Individual spectra are normalized to their peaks so that the color coding displays the side mode suppression ratio (range: 0 … 40 dB).
- Fig. 3: FWHM linewidth and intrinsic linewidth (FWHM) of the same diode laser determined by means of a self-delayed heterodyne linewidth measurement. The intrinsic linewidth is inferred from the white noise floor of the frequency noise power spectrum.
FBH has developed narrow linewidth, high-power distributed feedback (DFB) diode lasers for quantum optics experiments on ensembles of ultra-cold potassium atoms. Based on its latest results on the realization of DFB lasers [1] for Rubidium Bose-Einstein condensation (BEC) and atom interferometry applications, FBH has now developed DFB lasers that are suited for the corresponding experiments on potassium. This development was carried out within the framework of a project supported by the German Space Agency DLR. Ultimately, these activities aim at testing the equivalence principle by comparing the free fall of rubidium and potassium ultra-cold atomic ensembles by means of an atom interferometric measurement in space [2].
The development of narrow linewidth, high-power DFB diode lasers becomes a more and more challenging task in terms of laser design and fabrication technology as the emission wavelength is shifted from 780 nm towards shorter wavelength. This is due to increased absorption in the overgrown gratings with decreasing wavelengths. An improved design of the grating layers has been implemented and resulted in better grating and device performance.
The recent results obtained at FBH for single-quantum well, ridge waveguide DFB diode lasers emitting at 767 nm clearly show an electro-optical performance that is comparable to the performance of the DFB diode lasers for 780 nm. With a 1.5 mm long chip an output power of more than 150 mW can be reached at an injection current beyond 250 mA, see Fig. 1. The slope efficiency corresponds to 0.68 W/A and is hence comparable to 0.6 A/W … 0.8 A/W recently reached with DFB lasers for 780 nm. Depending on the actual chip temperature the wavelength can be current-tuned continuously by more than one nanometer with single mode emission being maintained, see Fig. 2 . The side-mode suppression ratio reaches 40 dB at injection current settings beyond 150 mA. A self-delayed heterodyne linewidth measurement reveals a short term linewidth (10 µs) of about 1 MHz full-width-at-half-maximum (FWHM) and an intrinsic linewidth of a few 10 kHz FWHM at large injection current settings, see Fig. 3. The instrinsic linewidth is deduced from the white noise floor of the frequency noise power spectrum and hence excludes technical noise, mostly flicker noise of the current source.
This work is supported by the German Space Agency DLR with funds provided by the Federal Ministry of Economics and Technology (BMWi) under grant number 50WM0940.
Publications
[1] T.-P. Nguyen, M. Schiemangk, S. Spießberger, H. Wenzel, A. Wicht, A. Peters, G. Erbert, G. Tränkle, "Optimization of 780 nm DFB diode lasers for high-power narrow linewidth emission", accepted for publication in Appl. Phys. B
[2] T. van Zoest, et al., "Bose-Einstein Condensation in Microgravity", Science, 328, 1540-1543 (2010).
FBH research: 23.04.2012
- Fig. 1: Chip photo of a two-stage X-Band GaN power amplifier MMIC
- Fig. 2: Measurement data of the two-stage X-Band GaN power amplifier MMIC from Fig. 1: Output power, gain, and PAE of the final stage as a function of input power at 10 GHz
Power amplifiers (PA) are key components of any communication, radar and satellite system. As the last element in the transmitter chain before the antenna they dominate the overall properties. The most important figures of merit are output power Pmax and power-added efficiency PAE (PAE: Power Added Efficiency). The PAE indicates how much of the consumed power is actually available for the application and how much is dissipated into (thermal) losses. High efficiency is a key issue in view of environment (CO2 emission) as well as system performance.
The combination of high power, high efficiency and high operating frequency is a problem for most common semiconductor technologies, which is due to physical constraints. In this regard, gallium nitride (GaN) outperforms most of its competitors as it offers both high breakdown electric fields and high electron mobility. This makes it the ideal choice for microwave power amplifiers. It allows realizing PAs with previously unattainable values for output power and PAE.
Various radar and satellite systems operate in the X-band, the frequency range from 8 to 12 GHz. In this frequency range, amplifiers are commonly built as monolithic circuits (MMICs, Monolithic Microwave Integrated Circuits), because through monolithic realization critical tolerances and parasitic properties can be reduced. A GaN MMIC process is available at FBH which also serves this purpose. Recently, the performance of this process has been further improved by reducing the gate length of the GaN transistors to 0.25 μm. Devices achieve efficiencies of 50% and more at 10 GHz in deep-AB operation.
Fig. 1 presents a recently designed power amplifier MMIC realized by using this process. Because the GaN semiconductor layers are grown on silicon carbide (SiC), the substrate is transparent and the metal structure of the circuit seems to float. In order to achieve high gain, a two-stage design is employed. As can be seen from the measurement data in Fig. 2, this circuit reaches a maximum output power Pmax of 11 W at 10 GHz. The maximum linear gain is approximately 25 dB and the efficiency of the final stage (PAE) almost 40%. These values can be enhanced further by appropriate circuit optimization. Work on this is ongoing.
Publication
Erhan Ersoy, Chafik Meliani, Serguei Chevtchenko, Paul Kurpas, Mathias Matalla, and Wolfgang Heinrich, "A High-Gain X-Band GaN-MMIC Power Amplifier", presented at 7th German Microwave Conference (GeMiC), Ilmenau, Germany, on 12-14 March 2012.
FBH research: 11.04.2012
- Fig. 1: Mounted GaN microwave power transistor of 100-W-class for L- and S-band applications
- Fig. 2: Focused-Ion-Beam (FIB) cross section through Source connected field plate structure
- Fig. 3: Properties of microwave power transistors of 50-W-class in dependence on operation voltage (28 V, 40 V and 50 V). a) Scaling of output power with increasing operation voltage. b) Power added efficiency (PAE) in dependence on operation voltage
Highly efficient GaN- based microwave power transistors are implemented into modern communication and radar systems to an ever increasing degree. Often complex microwave systems are only possible if compact and highly efficient devices can be integrated within a small volume. In cooperation with its spin-off company Berlin Microwave Technologies (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. Well adjusted optimizing efforts in materials technology (epitaxy), device processing, general device design including layout as well as chip mounting techniques into a suitable microwave package are key for the development of such devices. Consequently our activities led to promising devices which are combining high power density and high absolute power levels with high efficiency. The devices are mounted into a microwave package according to fig. 1 and are available for implementation into microwave systems.
Decisive for the development of transistors showing high output power and efficiency at the same time has been the careful optimization of field plate technology along with the introduction of ballasting resistors between the individual power cells to avoid parasitic oscillations of the packaged power bars. The intention of field plates (see fig. 2) is to influence distribution and maximum intensity of the electric field in internal devices regions such that for all targeted device operation conditions the field maximum always stays below critical field levels. This is a prerequisite for an increase of device operation voltage (for example to 50 V) and for a linear scaling of output power with operation voltage as shown in fig. 3a). The high power added efficiency of close to 70% obtained along with these optimizations qualifies these devices for implementation in microwave amplifier systems operating at 40 V and above. The high operation voltage in turn leads to a higher level of optimum input and output impedance for power matching conditions and therefore enables either very broadband amplifier systems or, by combining multiple power transistors, very powerful microwave amplifiers with power levels above 200 W.
FBH research: 30.03.2012
- Fig. 1: FIB cross-section of a TS-HBT with emitter (e), base (b) and collector (c)
- Fig. 2: Extrapolated fT and fmax of a TS-HBT
- Fig. 3: S-parameter measurements of a 90 GHz power amplifier in TS technology
Research on high-speed transistors is driven by applications for imaging and wide band communications. Recent technical advances of InP-based transistors with several hundred gigahertz (GHz) operating frequencies together with their outstanding material properties qualify them as key components in such systems.
At FBH, a transferred substrate (TS) technology has been established to optimize high frequency and power performance of InP heterojunction bipolar transistors (HBT). The 3" wafer-level process enables lithographic access to both the front- and backside of the HBT aligned to each other. The resulting linear device set-up in Fig. 1 eliminates dominant transistor parasitics and relaxes design trade-offs. The essential step for gaining access to both sides of the epitaxial structure is to completely remove the supporting substrate. Therefore, a robust adhesive wafer bonding procedure via benzocyclobutene (BCB) has been developed. It yields a homogenous, crack- and void-free composite matrix of transistors transferred on a wafer-level scale.
The optimized device topology manifests in excellent HBT performance. Transistors with 2× 0.8×5 μm2 emitter area, as depicted in Fig. 2, feature fT = 376 GHz and fmax = 385 GHz at breakdown voltages BVCEO > 4.5 V. They combine high frequency performance with saturated output power Pout > 14.2 dBm @77 GHz and an inherently good matching to 50 Ω. The highly scalable device architecture is capable to even further increase high frequency as well as power performance in the future. Power amplifiers have been designed and realized in TS technology for 90 GHz operation. Their S-parameter measurements shown in Fig. 3 confirm a good agreement with modeling.
Currently, the innovative transistor set-up is utilized in an ongoing project to integrate InP-based circuits ontop of BiCMOS wafers heterogeneously.
Publication
T. Al-Sawaf, C. Meliani, W. Heinrich, and T. Krämer, "W-Band Amplifier with 8 dB Gain Based on InP- HBT Transferred-Substrate Technology", Proc. German Microwave Conference, Ilmenau, Germany, 12–14 March 2012.
FBH research: 13.03.2012
- Extended cavity diode laser for precision quantum optics experiments on board a sounding rocket
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. The MOPA concept is based on optimizing a low power distributed feedback (DFB) or distributed Bragg reflector (DBR) for stable narrow linewidth emission and amplification of its radiation by means of a separate power amplifier chip. The ECDL concept uses optical feedback from an external optical grating. This way, large cavity lengths can be realized which provide a reduction of the frequency noise linear spectral density by 1 to 2 orders of magnitude with regard to monolithic lasers.
Hybrid micro-integration technology is used to integrate laser chips, optics, and electronics on an aluminum nitride ceramic body that takes a volume of only 8 x 2.5 x 1.5 cm3. This amounts to a reduction of the form factor by 3 orders of magnitude with respect to commercial systems. Both module types further omit any moveable parts so that the requirements on mechanical stability for space applications can be met. These MOPA modules have already successfully passed vibration test at 8 gRMS that simulate the mechanical stress of a sounding rocket launch. Further tests are pending.
The MOPA modules provide an optical power in excess of 1 W at 780.24 nm. Depending on the laser chip used as a master oscillator, a short term (10 µs) emission linewidth below 1 MHz and an intrinsic linewidth as small as a few 10 kHz can be realized. The ECDL provides an output power of typically 50 mW with a short term linewidth of well below 100 kHz and an intrinsic linewidth of a few kHz only. It is hence suited for applications with the most stringent requirements on spectral stability. Coarse frequency tuning is realized by thermally tuning the optical grating which provides a tuning range of approximately 80 GHz.
MOPA and ECDL technology can be transferred to other wavelengths in the 650 nm to 1100 nm wavelength range. It is also considered for the realization of lasers for portable optical clocks.
Publications:
Ch. Kürbis, A. Kohfeldt, E. Luvsandamdin, M. Schiemangk, S. Spießberger, A. Wicht, A. Peters, G. Erbert, G. Tränkle, "Mikrointegrierte Lasersysteme für die höchstauflösende Atomspektroskopie und die kohärente Nachrichtenübertragung im Weltraum", 60. Deutscher Luft- und Raumfahrtkongress, Bremen, Germany, Sep 27-29 (2011).
FBH Research: 23.02.2012
- Fig 1. Two versions of a voltage controlled oscillator (VCO) for 24 GHz in 130 nm CMOS technology.
- Fig. 2. Single sideband phase noise of both VCO versions vs. carrier offset frequency.
Just imagine two robot welders, together assembling a car body with high speed and high force. If they crash, the damage will be expensive: Not only the robots will be damaged, but also the assembly line will have to stop. In near future, such pitfalls can be avoided using two tiny localization blocks placed on each robot’s arm. These localization blocks are able to measure the distance to each other and to provide early warning of collisions.
The localization block is based on radar principle at 24 GHz and its core is a voltage controlled oscillator (VCO) generating the radar signal. For high localization accuracy in the range of a fraction of an inch it is necessary to have a radar signal with low phase noise. This is due to the fact that phase noise directly determines the localization accuracy. In the framework of the BMBF project LoWiLo (Low-power Wireless Sensor Network with Localization), such a 24 GHz low phase-noise VCO was developed at FBH [1]. Based on an advanced 130 nm CMOS technology, some versions of cross-coupled VCO were designed and characterized.
The two versions differ in the choice of the frequency-determining spiral inductor with respect to its quality factor and in the way the varactor is coupled, see Fig. 1. Fig. 2 presents the phase noise spectrum of both versions of the VCO. Clearly, version B exhibits a phase noise lower than that of version A by 10 dB at 100 kHz offset, which translates into enhanced localization accuracy.
Publication:
[1] Hossain, M.; Kravets, A.; Pursche, U.; Meliani, C.; Heinrich, W.: "A Low Voltage 24 GHz VCO in 130nm CMOS For Localisation Purposes in Sensor Networks," paper to be presented at German Microwave Conference 2012 in Ilmenau (Session 12, 13 March 2012).
FBH research: 13.02.2012
- Fig. 1: Analytics tool for the characterization of degradation processes.
- Fig. 2: High-resolution image of the vertical and lateral structure of a laser with integrated Bragg grating.
- Fig. 3: 20 µm thick HVPE-GaN layer on structured sapphire substrate; (a) CL image (defect density:1,3 x 108 cm-2), (b) surface structure with regular growth terraces and pits where dislocations terminate.
In the framework of the EFRE project "Application center for high frequency technologies" the Ferdinand-Braun-Institut recently installed a materials analytics tool for characterization of degradation processes in semiconductor devices (Fig. 1). The centerpiece of the tool is an Ultra+ high resolution scanning electron microscope (SEM) with a thermal field emission electron gun from Carl Zeiss NTS which is capable of high-resolution imaging of surfaces. During installation, a lateral resolution of 1.0 nm was demonstrated.
The SEM is equipped with several systems for the analysis of structural and optical properties of semiconductor layers and devices. Among these is an energy-dispersive X-ray spectrometer (EDXS) from Bruker Nano with an energy resolution of 125 eV for quantitative determination of composition of compound materials as well as a system for detection of electron-induced currents (EBIC) allowing for failure analysis of transistors and semiconductor laser diodes.
Moreover, a cathodoluminescence system from Gatan is attached to the microscope for characterization of the optical properties of laser chips in the temperature range from 80 K to 300 K before and after life testing. The Mono-CL4-system provides fast spectral mappings in the wavelength range of 200 nm to 1100 nm. This allows also for high spatial resolution to detect areas where material properties have changed due to the high electrical and optical load during operation.
Some examples illustrate the capabilities of the new tool that considerably improves the equipment potential for characterization of semiconductor layer structures and devices at FBH. Fig. 2 shows the vertical and lateral structure of a laser with an integrated Bragg grating for wavelength stabilization with a high resolution. The contrasts are caused by the different material composition of the etched grating and the waveguide layers showing self organization during growth in the second epitaxial step. The new SEM enables an improved material contrast together with a very good lateral resolution. In Fig. 3(a) 20 µm thick HVPE-GaN layer is depicted which was grown on a structured sapphire substrate. Fig. 3(a) is a cathodoluminescence (CL) image with dark spots at dislocation positions (defect density 1,3 x 108 cm-2) and Fig. 3(b) shows the surface structure with regular growth terraces and small pits where the dislocations terminate at the surface.
FBH research: 31.01.2012
- Fig. 1: Current-light power characteristic of a 44x nm RW laser diode with a ridge width of 1.5 µm under CW operation.
- Fig. 2: Threshold current density versus ridge width for laser diodes with two different etching depth of the ridge.
Laser diodes based on GaN are currently available on the market only for a limited number of specific wavelengths. FBH, together with TU Berlin und the company eagleyard Photonics, has 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. The laser diodes will be operated in an external cavity, which involves a diffraction grating to precisely adjust the lasing wavelength. As required for the desired spectroscopic applications, the lasers have to show a low threshold current. Therefore, ridge waveguide (RW) laser diodes with a small ridge width of 1.5 µm and a resonator length of 600 µm have been fabricated. The narrow ridge is also essential to assure an optimum beam quality. Threshold currents as low as 40 mA have been obtained for devices emitting around 41x nm. The threshold voltage and slope efficiency in pulsed operation were 7.5 V and 0.5 W/A, respectively. Under continuous wave (CW) operation, an output power of 40 mW has been reached as shown in Fig. 1 for a device emitting at 440 nm.
A systematic study of numerous laser diodes has shown that the lasing threshold very much depends on the geometry of the ridge waveguide. The almost square-sectioned ridge waveguide is formed by etching several hundreds of nanometers deep into the semiconductor surface. Its purpose is to laterally confine both the optical mode and the vertical current path. Fig. 2 shows the threshold current density as a function of the ridge width for two batches of laser diodes whose etching depth of the ridge differs only by 175 nm. Although the impact of the ridge depth on the threshold vanishes when the ridge width increases, narrow ridge lasers exhibit more than a factor of two higher threshold current densities in case of shallow etched ridges. Systematic near field and far field measurements along with two-dimensional electro-optical simulations of the devices have been started in collaboration with the NUSOD institute. The anti-guiding effect originating from the high carrier density during lasing, the optical absorption in the region of the lateral mode tails, and the lateral current spreading are considered. Although a comprehensive explanation of the effect still needs to be found, a huge current spreading in the layer structure seems to be unlikely. Rather, the weakening of the mode confinement by the anti-guiding effect is currently favored.
Publications:
L. Redaelli, J. Piprek, M. Martens, H. Wenzel, C. Netzel, A. Linke, Y. V. Flores, S. Einfeldt, M. Kneissl and G. Tränkle, "Effect of ridge waveguide etch depth on laser threshold of InGaN MQW laser diodes", Proc. SPIE, to be published in 2012.
C. Netzel, S. Hatami, V. Hoffmann, T. Wernicke, A. Knauer, M. Kneissl and M. Weyers
"GaInN quantum well design and measurement conditions affecting the emission energy S-shape", phys. stat. sol. (c), vol. 8, no. 7-8, pp. 2151-2153 (2011).
FBH research: 18.01.2012
- Red-emitting laser module for display applications
Red-emitting diode lasers are needed as compact light sources not only for displays, but also for medical treatment and sensing applications. An important property required for these applications is high radiance (i.e. brightness). In the framework of the InnoProfile initiative „Hybride Diodenlasersysteme“, the FBH succeeded in developing 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. Perceived by a human eye, this corresponds to a luminance of more than 27 TCd/m² [1], 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.
This result was made possible by the development of red-emitting tapered lasers and their subsequent mounting on CVD-diamond heat spreaders with two contacts. This allows optimal heat extraction from the chip and individual currents through the ridge-waveguide and taper sections of the laser. Additionally, the radiation of the chip was formed by micro-lenses mounted inside the module. The shaped beam could be coupled into an optical fiber featuring a small aperture of only 16 µm with an efficiency of more than 75%. Using this fiber, the laser light can then be guided to its point of use, e.g. a projector head of a laser display system.
Publication:
[1] G. Blume, C. Kaspari, D. Feise, A. Sahm, B. Sumpf, B. Eppich, and K. Paschke, “Tapered diode laser modules at 638 nm with efficient fiber coupling “, IEEE Phot. Technol. Lett. (submitted 05.01.2012).
FBH research: 13.01.2012
- Fig. 1: Multiplier circuit (left) with different passive elements and ports replacing the active elements, ports and metal stack are emphasized on the right side.
- Fig. 2: S parameter S21 as a function of frequency; results for the multiplier circuit from measurements compared to EM simulations and circuit simulator (SpectreRF) using the manufacturer's data
Due to the rapid advances in the miniaturization of semiconductor processes (according to Moore’s law), high-end CMOS circuits can be used today in a frequency range far beyond 10 GHz. In contrary to low-frequency and digital circuits, their functionality is no longer defined primarily by the active elements, i.e., the transistors, but the passive elements such as inductors, capacitors and transmission lines take significant influence. Hence the electrical behavior of these elements must be included already in the basic design steps.
The CMOS processes realize the passive elements through a stack of metal layers separated by dielectrics on top of the semiconductor substrate with the active elements. During circuit design, it is important to use accurate models for the passive structures in the circuit simulator (eg. SpectreRF). It is also indispensable to check the circuit functions in a final step in this way. In this case, a simulation of the entire circuit must be carried out using 3D EM simulation. Active elements are not considered in the EM simulation and are replaced by internal ports. In a later step, the designer implements both the EM simulated data and the properties of the active elements in the circuit simulator in order to obtain an accurate description of the circuit. An example of such a circuit is the multiplier shown in fig. 1.The large ratios of cell sizes and the high number of cells is a challenging task for the simulation, resulting in a mesh with typical 25 million cells. Fig. 2 shows measurement data together with simulation results of the standard models, as provided by the foundry, as well as the results of the in-house EM simulation. As one can see, the results of the standard models deviate considerably from the measurements and using 3D EM simulation leads to a significantly better fit with the measured data.