Begining in 1998, workshops dedicated to a special field were incorporated to the programme of the MIDEM Conferences. During the workshop, five to seven invited speakers present papers on the chosen topics from different aspects within their special field, thus offering the audience valuable information. Time for thorough discussions is provided between invited presentations. Conference attendees are encouraged to present their research results in the Conference session dealing with the dedicated topic. Attendance at the workshop is included in the Conference registration fee. For the year 2001, we are pleased to announce a

Selected topics associated with advanced aspects within lasers, light emitting devices, thin-film transistors, optical fiber systems, etc. will be presented, covering the basic physical principles, as well as actual and possible applications of these optelectronic devices and systems.

The workshop is organized by Laboratory of Semiconductor Devices at Faculty of Electrical Engineering.

The programme committee is pleased to announce the following invited speakers, who will give their presentations on the following subjects :

Prof. Dr. Martin Stutzmann, Walter Schottky Institut, Technische Universität München, Germany
The Status of GaN-based LEDs and Laser Diodes

High brightness blue, green, and white light emitting diodes based on InGaN/GaN/AlGaN heterostructures have developed into a mass product, with more and more companies competing with each other in a rapidly expanding market. Full colour displays, traffic lights, car lighting, back lighting for LCD displays, and indoor spot light applications are the main present and emerging applications. Yet, the efficiency of GaN-based LEDs is still limited by poor materials quality due to the lack of a suitable substrate for homoepitaxy and by the problems with p-type doping, in particular in the AlGaN-cladding layers of a typical quantum well LED. Here, more research is necessary before the full potential of solid state lighting can be exploited also for high efficiency applications in office or home illumination. The basic materials problems are even more important in the development of blue laser diodes for data storage. At present, only Nichia Corp. has reported laser diodes emitting in the near UV and exhibiting a reasonable device lifetime of several thousand hours. Most other companies still are fighting severe degradation problems of their laser diodes, which are mainly attributed to defects caused by heteroepitaxy on sapphire or SiC substrates. Also, no real blue laser diodes with an acceptable lifetime have been reported. The purpose of this presentation is to summarize the present status concerning GaN-based LEDs and LDs, to discuss the basic materials physics behind the success and the limitations of these devices, and to point out present attempts and ideas to overcome these limitations in the future.

Prof. Dr. R. E. I. Schropp, Debye Institute, Physics of Devices, Utrecht University, The Netherlands
New Developments in Thin Film Transistor Techology

Thin film transistors (TFTs) are currently used as the switching element in active matrix (AM) liquid crystal displays (LCD), such as the TFT display used in lap top computers. Recently, page-size 2D matrix-addressed image sensors for application in digital copiers and X-ray imaging have been presented. Current trends in research and development are: 1) TFTs on plastic substrates (low T deposition is required); 2) The development of low temperature polysilicon for the pixel TFTs: polysilicon TFTs have a higher field-effect mobility and allow larger drive currents, so that the pixel aperture ratio can be increased and bright, low power LCDs can be obtained even when they have a large size, such as in the notebook and the CRT-monitor replacement market; 3) Integration of row and column drivers on the glass, so that panels can be made much thinner: by supporting both n-channel and p-channel operation, poly-Si TFTs also enable CMOS circuits for display drivers; 4) Addressing of OLEDs (Organic Light Emitting Diodes) by silicon TFTs. For such advanced applications of TFTs a few issues are relevant: (i) fast, defect free deposition of thin silicon films and gate dielectrics, (ii) higher electron mobility, (iii) stability. Whereas a high deposition rate is generally needed to reduce the production cost, for novel high current applications the latter two issues have recently become more essential.

Prof. Dr. Gil Rosenman, Electrical Engineering-Physical Electronics, Tel Aviv University, Israel
Engineered Domain Configurations for Nonlinear Optical Devices

New generation of nonlinear optical devices allows developing coherent light sources in spectral regions where conventional lasers are unavailable. It is based on engineered micrometer-scale ferroelectric domain configurations. Applied quasi-phase-matching technique exploits tensorial properties of ferroelectrics where specifically designed domain structure with alternative direction of spontaneous polarization provides needed variation of a sign of a nonlinear optical coefficient. Ferroelectric LiNbO₃, LiTaO₃, KTP and isomorphic crystals are mainly exploited due to their high nonlinear optical coefficients and stable built-in domain structure. The conventional process of domain grating fabrication consists of application of an electric field to a photolithographically patterned electrode on a polar crystal surface made of a arrays of metallic and insulating strips. Detailed studies showed those fundamental processes of minimization of depolarization field, domain nucleation, anisotropy of domain walls velocity determine quality of tailored domain configurations and strongly affect parameters of optical converters. Highly homogeneous periodic, aperiodic and multiple domain gratings with periods in the range 4-39 µm for diverse quasi-phase-matched nonlinear optical devices in UV, visible and infrared regions have been developed.

Dr. Christian Hanke, Corporate Research Photonics, Infineon Technologies, Munich, Germany
High Power Semiconductor Laser Diodes

In the last decades the output power of semiconductor laser diodes has increased dramatically. Starting from a power range of several milliwatts, which is sufficient for a range of mass applications in the field of optical communication and optical storage systems, now semiconductor laser systems with an output power in the kilowatt range are available. The main advantages of diode lasers are the small volume, the high overall efficiency up to 60%, the availability of a wide spectral range and the high reliability. This combination together with the high output power opens a wide field of applications covering e. g. pumping of solid state lasers and amplifiers, transfer to printing plates, soldering and direct machining. The high optical and thermal power densities in semiconductor laser diodes lead to limitations of the maximum useful output power. The approaches to improve the device performance will be described for single stripe laser diodes with diffraction limited beam quality and for laser arrays with multimode emission. To further increase the output power combinations of several emitters are used and beam combining techniques have been developed. The present status and future developments will be presented.

Dr. Helmut Stiebig, Institut für Photovoltaik, Forschungszentrum Jülich, Germany
Color Aliasing Free Detectors

Color image processing is usually performed with the aid of color filter array (CFA) coated CCD or CMOS sensor arrays. However, color detection with CFA leads to the color moiré or color aliasing effect, which is observed when structures with high spatial frequencies are captured. Furthermore, traditional sensor systems exhibit a rather limited resolution and a low fill factor, because one color pixel is split into several chromatic sub pixels. In order to overcome the color aliasing effect, vertically integrated sensor structures have been proposed, which detect the color information in the depth of the structure. The spectral response, dynamic range and the transient behavior of the vertically integrated sensors based on amorphous and crystalline silicon are determined by the optoelectronic properties of the employed materials and the device design (two, three and four terminal devices). We will discuss different three-color detectors and compare the properties of the structures (e.g. p/n, pin, piiin, nipin) regarding their appropriated applications. Additionally, three-color sensors are limited by the mismatch between the spectral sensitivity of the detector and the human eye. This mismatch leads to a color error. For high quality and low color error imaging applications (e.g. picture archiving) typically multi-spectral technology is applied. In this case the sensor array is covered sequentially with different color filters and the individual spectrum of each color point is reconstructed afterwards. Due to the fact that several images of the same scene are taken sequentially real time imaging is prevented. To overcome this drawback of a multi shot mode we have developed vertically integrated 4 and 6 channel detectors based on amorphous silicon and its alloys, which can be read out with one and two shots, respectively. The purpose of this presentation is to demonstrate the advantages of amorphous silicon based detectors in the field of color sensor technology and to point out the operation of the different device structures to fit the demand of various applications.

Dr. Matjaz Vidmar, Faculty of Electrical Engineering, University of Ljubljana, Slovenia
Optical-fiber Communications: Components and Systems

Optical-fiber communications brought a revolution to communication technology, outperforming other communication systems by several orders of magnitude in transmission capacity, unrepeated and repeated communication range and decreasing installation and operating costs. The optical-fiber revolution started approximately 30 years ago, when technology improvements decreased the optical-fiber loss to less than 20dB/km. The theoretical loss limit for silica (SiO2) based fibers was reached only 10 years later, but the fiber handling and line-terminal technology was far from mature at that time. Even with primitive line-terminal technology, optical fibers immediately outplaced coaxial-cable systems and decreased the importance of microwave and satellite point-to-point radio links. In the last two decades, significant improvements have been made in the line-terminal technology, including narrow-spectrum solid-state lasers, wide-bandwidth modulators, laser optical amplifiers, fast and sensitive photodetectors and last but not least, high speed electronics. Although advanced laboratory experiments are quickly approaching the theoretical capacity offered by the >10 THz bandwidth of a single mode optical fiber, several problems have yet to be solved to make high-capacity systems viable, including linear and nonlinear propagation effects in the optical fiber itself, high performance electro/optical and opto/electric converters, efficient high speed electronics an all-optical signal-processing components. The purpose of this presentation is to summarize the present status of optical-fiber communication technology, to discuss the basic components and the limitations of these devices, and to present the requirements and proposals for future systems.

Dr. Gvido Bratina, Nova Gorica Polytechnic, Slovenia
Organic Semiconductors as Candidates for Advanced Optoelectronic Devices

Organic semiconductors are rapidly emerging as promising candidates for the expansion of the optoelectronic devices to the field of flexible material systems. Relatively weak van der Waals intermolecular bonding allows the fabrication of organic semiconducting layers on a variety of substrates ranging from glass to thin polymer foils. The electronic and optical properties of thin organic semiconducting layers are strongly dependent on their chemical composition and structural parameters. Both features may be varied, making the synthesis of light emitting or light detecting devices with variable operating wavelength straightforward. Typical possible applications include multi-color light emitting diodes and flexible full color displays.
The level of understanding of the processes in the field of organic light emitting devices, however, is analogous to the status in the field of III-V semiconductor-based devices in the early seventies. The material purity is one of major concerns, and depends heavily on the method of preparation. The structural parameters crucially affect the mobility of charge carriers, and are a function of growth protocol as well as of the type of the substrate. Doping of organic semiconductors appears to be extremely uncontrollable. Metal-organic-semiconductor contacts are instrumental to the operation of every device, and as such are a subject of intensive research.
The first part of the lecture will give an overview of the demonstrated organic-semiconductor-based light emitting devices. In the second part the efforts to clarify electronic transport in thin organic-semiconductor layers and metallic contacts will be illustrated.

Contact person for the Workshop on
OPTOELECTRONIC DEVICES AND APPLICATIONS
Dr. Marko Topic, workshop chairperson
Faculty of Electrical Engineering
Trzaska 25
1000 Ljubljana, Slovenia
tel. +386 1 4768470, fax. +386 1 4264630
email: Marko.Topic@fe.uni-lj.si