TUC / DIODE / Partners

Partners & Industry connections

Technische Universität Chemnitz (TUC)
(http://www.tu-chemnitz.de/physik/HLPH)

The Semiconductor Physics group established by Prof. Dr. D. R. T Zahn in 1993 is part of the Institut für Physik with 16 professors and around 50 academic staff currently training app. 70 PhD students. The Semiconductor Physics group has published more than 200 papers with special emphasis on semiconductor interface formation studied by spectroscopical techniques. Since two years the internal programme is thus defined as "SPEctroscopic Characterisation of Interface Formation Involving Carbon Species" (SPECIFICS). Here organic molecules and their interface formation with inorganic semiconductor substrates and metals play the dominant role.

Role of the team:
Within the DIODE network, the Chemnitz group concentrates on optical spectroscopy ( Raman, infrared (IR), photoluminescence (PL) ) performed predominantly during the deposition process of organic molecules on inorganic substrates and the subsequent metallisation. In addition, electrical transport measurements are applied to study simple device like structures. The expertise in electron spectroscopy and synchrotron techniques (>10 years at BESSY) can be offered to the specialised teams e.g. for support during beamtimes.

Expertise and competence:
The Chemnitz group is currently the only one worldwide possessing the facilities and expertise for performing optical spectroscopy during molecular beam deposition (MBD) in UHV. The potential of Raman monitoring of growth processes and photoluminescence (PL) was well illustrated for epitaxial growth, e.g. II-VI/III-V heterostructures. The results provide information on e.g. interface chemistry, strain evolution in substrate and layer, and crystallinity as well as doping effects. Organic molecules are extremely interesting as a subject of such investigations since their complex multi-atomic internal structure gives rise to a large number of detectable vibrational modes. Results obtained so far clearly indicate that shifts and intensity variations of these modes together with the detection of external (phonon) modes provide even more information than previously achieved for purely inorganic systems. Inorganic/organic heterostructures are metallised after their optical characterisation for in situ electrical studies by I-V and C-V techniques. The results supply input for the device simulations in Rome and can complement the more sophisticated measurements in Braunschweig. Joint efforts of the network like characterisation experiments at large scale facilities (BESSY,....) and..... are planned and organised by Dr. T. U. Kampen of the Semiconductor Physics group. The experimental investigations are supported by the theory group in Chemnitz, in particular Dr. R. Scholz who calculates the relevant Raman and IR spectra in close collaboration with the Paderborn group.

Universidad Autonoma de Madrid (UAM):
(http://www.uam.es/departamentos/ciencias/fismateriac/especifica/Nuevas_mic/)
(http://www.uam.es/departamentos/ciencias/fisicateoricamateria/especifica/)

The Madrid Surface Theory group was established in 1975 and has been mostly devoted to the analysis of electronic surface properties. It belongs to the Department of Condensed Matter Theory of which Prof. F. Flores has been the head for more than 20 years. It has a total of 12 professors and 20 academic staff currently training 14 PhD students with special facilities for computation due to around 10 funded projects. The Laboratory of New Microscopies, coordinated by Prof. A. Baro was established in 1984 and is part of the Department of Condensed Matter Physics with a total of 16 professors and 25 academic staff currently training 20 PhD students. It has special experimental facilities for Scanning Probe Techniques like STM and STS.

Role of the team:
The Madrid node in the DIODE network will model the GaAs/organic molecule interaction, including charge transfer, band offsets and Fermi level pinning. The initial stages of growth of the organic films deposited on the GaAs substrate will be analysed by STM and STS.

Expertise and competence:
The theory group has broad experience in the analysis of Schottky-barrier heights and semiconductor band-offsets. The Induced Density of Interface States (or the virtual gap states) model was first introduced by this group and is, nowadays, widely accepted as the explanation of the Schottky barrier formation for ideal (defect-free) metal-semiconductor interfaces. The group has also developed simulation packages for the analysis of chemical and electrical interactions at the interface of two different inorganic bulk systems, with or without the inclusion of passivating monolayers between the two materials. These interactions modify the electronic charge densities near the interface and determine the band line-ups relating the Fermi level of the metal to the extrinsic charge neutrality level of the semiconductor. The LCAO method used in these calculations is a hybrid between empirical tight-binding parametrization and LDA-based approaches. The application of our simulation package to organic-inorganic interfaces needs detailed input from the Paderborn SCC-DFTB simulation results.
The Laboratory of New Microscopies has a recognised expertise on STM, STS and Scanning Probe Microscopies. In the last two years, a STM working in a UHV chamber has been fully dedicated to the study of organic/semiconductor and organic/metal interfaces. In particular, C60/Si(111), C60/Si(100)-H, C60/Au(111), BEDT-TTF/Au(111) and CAT1/Au(111) have already been carefully analysed and their properties characterised, e.g. growth and bonding to the interface.
The calculations of the electronic density of states at the semiconductor-organic and organic-metal interfaces, for the clean or passivated semiconductor, is needed for predicting the Schottky barrier height of the contact. These barrier heights will be compared directly with the measurements of the Chemnitz group, and they are a necessary prerequisite for the Rome device simulations.

Technische Universität Braunschweig (TUB)
(http://www.tu-braunschweig.de/ihf)

The Institut für Hochfrequenztechnik (IHF) of the TU Braunschweig has been working on organic semiconductors for about eight years. These activities have led to the construction of a multi-chamber UHV organic molecular beam deposition (OMBD) system for organic device processing in the group of Prof. Dr.-Ing. W. Kowalsky. In this system, chambers for organic layer deposition (12 effusion cells, RHEED, mass spectrometer), metallisation, sputtering, and substrate pretreatment are connected by a vacuum transfer system, so that devices are produced within a single vacuum process. Avoiding an exposure to atmosphere between the various device preparation steps results in a better performance and longer operation lifetime. This processing technology yields molecularly defined layer interfaces and therefore, is indispensable for the fabrication of reproducible devices and allows to straightforwardly compare experimental characteristics with theoretical predictions.

Role of the team:
Within the DIODE network the IHF will be occupied with the optimisation of inorganic/organic/metal diode fabrication and the electrical and RF characterisation. The IHF's technological experience will be introduced to the network, while results of the network participants will be employed in the optimization process. IHF measurement results can be related to the detailed examinations of the physics of contacts and will be a basis for device simulations.

Expertise and competence:
From the research and development of organic LEDs and organic matrix displays, vast experience in the structuring of organic devices exists at the IHF. Various analytical measurement technologies, which are essential for the research on organic devices, e.g. atomic force microscopy (AFM), UV photoelectron spectroscopy, TSC, X-Ray diffractometry (XRD), photoluminescence (PL), time-domain spectroscopy (TDS) and other optical measurement systems are routinely employed. In the course of a project of several years, inorganic/organic heterostructure diodes, which are comparable with the subject of the DIODE network, were examined at the IHF. Experience in preparation and electrical and RF characterisation was gained. InP/Organic microwave diodes for detector and mixer applications exhibiting low forward voltages and therefore, high conversion efficiencies at low power levels, were presented. Very well defined interfaces achieved in the OMBD process were essential. Several measurement technologies were established, e.g. IV, CV, impedance spectroscopy (vectorial NWA), and new techniques were developed to generate equivalent circuit parameters.

Trinity College Dublin TCD
(http://www.tcd.ie/Physics/People/Iggy.McGovern/index.html)

The general expertise of TCD is adsorption on semiconductor surfaces, in particular using synchrotron radiation techniques. Active in this area since 1974, the group leader, Prof. I. T. McGovern is the author/co-author of 5 invited reviews and over 70 peer-reviewed research papers. He is a member of 2 Editorial Boards (Journal of Physics: Condensed Matter and Probe Microscopy) and is a reviewer/auditor of the Large Scale Facilities programmes of the European Commission. He also has considerable experience of Network participation (2 HCM Networks, in one as coordinator) with expertise in the training of young researchers.

Competence and role of the team:
The group has a long tradition in the application of surface science to the problem of metal-semiconductor interface formation; early contributions dealt with ideal metal-layered semiconductor interactions probed by soft X-ray photoelectron spectroscopy (XPS), the role of interface alloying and the influence of surface photovoltage. More recently, the focus has been on small molecule adsorption on semiconductor surfaces, with particular reference to adsorbate geometry via X-ray standing wave, photoelectron diffraction and scanning tunneling microscopy (STM). The group is currently working on the adsorption of organic molecules on semiconductor surfaces, again from the point of view of adsorbate structure. This represents a natural extension of its recent work. However, it is also a most appropriate area for the present proposed network.

Universita degli Studi di Roma "Tor Vergata" (ROME)
(http://diana.eln.uniroma2.it/optolab.html)

The Rome device modelling group was established in 1990. It is part of the Electronic Engineering Department with a total of 19 professors and around 30 academic staff currently training 10 PhD students. The device modelling group is headed by Prof. P. Lugli.

Role of the team:
Modelling of the device performance of organic-inorganic heterostructures.

Expertise and competence:
Based on self-consistent tight-binding models, drift-diffusion schemes and transfer-matrix techniques, the Rome group will develop device simulators for the >em>organic-inorganic Schottky diodes. Concerning inorganic semiconductor heterostructures, the group's ability to model the device characterisitics with an atomistic resolution has resulted in a world-leading expertise in the understanding of the device performance under realistic electronic circuit conditions. In order to achieve the corresponding understanding for inorganic-organic Schottky diodes, the modelling will need detailed input both from the Paderborn and Madrid simulation results, as well as from electron spectroscopy and electrical measurements of the band offsets, reduced masses, mobilites, etc.
The Rome simulations will result in the calculation of device-relevant quantities like deep-level trapping, charge redistribution and image charges, self-consistent band bending, and tunneling currents. This will help to develop a microscopic interpretation of equivalent circuits of the IV and CV characteristics both for DC and AC operation, so that these simulations present a crucial step for device optimization.
Rome is the only group in the network performing simulations of the full device under operating conditions. This requires detailed input from the Madrid and Paderborn theory groups and from measurements of the band offsets. The Rome results can be used for the comparison with measured IV, CV, and DLTS characteristics.

University of Wales Aberystwyth (UWA)
(http://www.aber.ac.uk/~dphwww/)

The Materials Physics group at UWA was established in 1996 and forms an important part of the research activity of the Department of Physics, graded 4 in the last UK Research Assessment Exercise. The UWA group brings to the network a particular expertise in the techniques of electron and X-ray spectroscopy, using both laboratory-based and synchrotron radiation-based techniques. The group is led by Prof. N. Greaves, a leading authority in X-ray techniques such as EXAFS, XRD and SAXS. The University of Wales and the UK research councils have recognised the potential of this group by funding a new laboratory suite, containing X-ray and optical techniques for semiconductor characterisation, a new facility in NMR and a new beamline at the Daresbury laboratory. This provides a stimulating training environment in a group whose continued success will be aided by an effective interaction with other more established groups within a structured and focused network such as that provided by DIODE.

Role of the team and competence:
The group will contribute to studies of the fundamental properties of key surfaces and interfaces, having proven experience in, for example, GaAs surface processing, GaAs-semiconductor heterojunction formation and GaAs Schottky diode characterisation. Recent relevant work includes studies into the passivation of III-V surfaces using S-containing etchants, concentrating on the interdependence of surface composition, structure and electronic states. The formation of ordered, passivated surfaces using a low-cost uncomplicated method has obvious attraction in the quest for substrates for thin film growth. Other recent work includes the growth of semiconductor heterojunctions involving III-V substrates and large band gap II-VI materials. This area has involved the measurement and modification of interface band line-ups, the in-situ monitoring of epitaxial growth (MBE and MOVPE) and the formation of metal contacts to wide gap semiconductors. All these areas are directly relevant to the aims of the DIODE network and build on existing successful collaborative links. In both heterojunction and contact formation, the interface potential barriers are intimately dependent on the energy and density of interface states, which are in turn sensitive to changes in atomic structure and bonding. The challenge, both experimental and theoretical, is to provide detailed, independent information in each task to enable the overall objective to be realised. The spectroscopic studies of the UWA group will provide an important input into the distribution of chemical species and electronic states at the GaAs/organic and the organic/metal interfaces and will allow direct monitoring of the organic semiconductor and metal layer growth. This will contribute to the knowledge base that will allow the control of the electrical properties of the GaAs/organic/metal diode structure.

Universität-Gesamthochschule Paderborn (UGHP)
(http://www.phys.uni-paderborn.de/groups/frauenheim/)

The density-functional methods used in the Paderborn theory group were developed in Chemnitz (1993- March 1998) and since April 1998 in Paderborn. Now in Paderborn, the group headed by Prof. Th. Frauenheim is part of the Institute of Theoretical Physics and the Department of Physics, with a total of 14 professors and around 60 academic staff currently training 35 PhD students. The group is involved in various DFG projects und uses special computation facilities partially sponsored by Siemens-Nixdorf.

Role of the team:
The Paderborn team in the DIODE network will perform atomistic modelling of geometric, electronic and vibrational properties of free and adsorbed organic molecules.

Expertise and competence:
The tight-binding approach to density functional theory developed in Chemnitz and Paderborn is a highly efficient scheme combining close to ab initio precision with an outstanding numerical performance. After the recent inclusion of self- consistent charge transfer (SCCT) , this density-functional tight-binding (DFTB) method has become competitive both with more sophisticated density-functional schemes and with established quantum-chemical methods. On simple workstations, the DFTB scheme has been applied successfully to inorganic and organic systems with about 500 atoms. Parallelized codes can be run on the corresponding architectures with about 2000 atoms. Organic molecules forming semiconducting crystals and epitaxial layers shall be investigated with a self-consistent charge density functional tight- binding (SCC-DFTB) calculation scheme. This includes the theoretical analysis of the isolated molecules, their bulk crystal structures, and their interaction with an underlying organic or inorganic substrate. Concerning inorganic semiconductor substrates, surface properties like reconstructions and local charge densites have already been studied in some detail, so that they can be used immediately for the investigation of adsorbed molecules. As the surface reactivity of semiconductors is known to depend crucially on surface reconstructions and passivations, the existence of these previous atomistic surface calculations can be considered a key input for the studies of inorganic-organic interfaces. The results shall provide technologically relevant information concerning good material pairings for organic film deposition on (passivated) substrates.
The existing expertise in the separate calculation of inorganic semiconductors and large organic molecules shall be integrated into simulations handling the interaction of these two material classes on the same footing.

External research linkages:

The group will be the liaison node for the significant non-European connection. The University of Princeton (USA) is host to the Advanced Technology Center for Photonics and Optoelectronic Materials (ATC/POEM), which has pioneered the UHV growth of organic thin films. In the network this connection will enable young researchers to experience the latest developments in a rapidly advancing field: Young researchers from the network will visit Prof. A. Kahn's laboratory, at the ATC/POEM, and these visits will be reciprocated by personnel from Princeton visiting laboratories of the DIODE network.

Industry connections

Multidisciplinarity is a natural ingredient of the DIODE network. It does not only involve experimental and theoretical physics, but industrial input is also an integral part of the programme, as the aim is to produce an improved practical device.

Freiberger Compound Materials (FCM)
(http://www.fcm-germany.com/)

Freiberger Compound Materials (FCM) will actively participate in the DIODE network by supplying materials and advice as well as monitoring the progress and offering training in the network workshops. FCM, one of the leading suppliers of GaAs wafer material, is located approx. 40 km from Chemnitz.

SynTec GmbH Wolfen
(http://www.synthon.de)

SynTec GmbH Wolfen, a chemical company situated in Sachsen-Anhalt, is involved in the research and development of organic molecules. SynTec will provide novel materials for the hybrid structures, and the research effort will in turn drive the manufacture of customised organic molecules with optimum properties for improving the device performance.

United Monolithic Semiconductors (UMS)
(http://www.ums-gaas.com)

United Monolithic Semiconductors (UMS) is a leading microwave diode manufacturer, a joint venture company of Daimler-Benz (D) and Thomson CSF (F). The GaAs company will carefully monitor the progress of the DIODE network through consulting and participation in the workshops.