Introduction
The comprehension of the transport mechanisms in extended state bands or through localized states together with the evaluation of carrier concentration and mobility, deep energy states, and relevant trapping/detrapping mechanisms is of the highest importance for the assessment of semiconductors. We are active for many years in the study of electrical properties of different semiconductors (III-V, II-VI) and wide bandgap materials (SiC, CdTe, CdZnTe, TiO2, ZnO, Ga2O3, etc.), as bulks, nanostructures, low-dimensional structures, complex devices with many homo- and heterojunctions.The characterization is of interest in semiconductors and semi-insulated materials but also near the semiconductor to metal transition (heavy doped materials).
In the following, an overview on our main investigation techniques and a brief description of their utility:
· Current-voltage, capacitance-voltage, TLM investigation, device parameter analysis (10-600 K): junction investigation, transport mechanisms through the junction, net doping profile, contact investigation, measurement of electrical parameters of devices
· Classical Hall and Photo-Hall effect, physical and geometrical magneto-resistance measurements (10-600 K, van der Pauw and Hall bar method): resistivity, majority carrier mobility and carrier concentration, scattering mechanisms, extrinsic energy levels, dopant density and compensation level
· Magneto-transport, quantum magneto-transport (0-12 T; 1.5-300 K): Shubnikov De Haas oscillations, quantum Hall effect, weak localization, metal to insulator transition induced by magnetic field
· AC conductivity, admittance spectroscopy (10-600 K, 50-106 Hz): investigation of shallow and deep levels, dielectric response
· Photoconductivity DC and pulsed: carrier mobility and lifetime, electric field profile, optical band-edge and shallow/deep levels.
In addition, to integrate and support the electrical investigations:
· X-rays and alpha spectroscopy: carrier mobility and lifetime, electric field profile
· Photoluminescence: carrier recombination mechanisms near the band-edge, from excitons and localized states.
· IR and VISA bsorption: optical band-edge and optical transitions near the band gap
· Preparation of ohmic and Schottky contacts by evaporation and subsequent annealing (access to photo-lithographic processes)
Actually, research themes developed in collaboration with researcher of SMFI Department mainly concern: Ga2O3, In2O3 for power electronics, CdTe, CIGS for photovoltaic applications, PANI for memristor.
Research lines developed with external collaborations are:
SiC for electronic/sensoristic applications in harsh environment (high temperature, high power)
A collaboration with the research group of Dr. Roberta Nipoti, at the Institute of Microelectronics and Microsystems - CNR - Bologna (CNR-IMM) is active for a few years and concerns the electrical investigation of Silicon Carbide (SiC), polytype 4H, heavily doped by ion implantation. The research is mainly aimed to project electronic devices based on bipolar junctions: the interest on such devices is due to the fact that, in the fabrication of micro-sensors and applications for information-communication technology (ICT), SiC is preferred to silicon for devices working at high temperatures, sharp thermal excursion, corrosive and harsh environment; SiC is even adequate for a robust power electronics, owing its wide band gap. Electronics based on SiC requires to achieve thin layers of high quality and purity and high controllability of the doping. Our research group is involved in the low temperature electrical characterization – by Hall effect measurements, admittance spectroscopy, current-voltage and capacitance-voltage characteristics – of ion implanted layers prepared in Bologna under varying conditions of the implant process and of the subsequent thermal treatment, necessary for the electrical activation of implanted impurities and the reconstruction of the lattice after the ion bombardment. The spatial selectivity of the ion implantation is of technological interest for the fabrication of electronic devices: an example is the preparation of low resistive contact pads.
Bulk Tellurides for high energy detection (X- and gamma rays, alpha particles) in environmental, medical, and astrophysic monitoring.
X-ray detectors are extremely widespread for medical and environmental applications (microPET systems, characterization of diagnostic beams used in computed tomography, monitoring of background radiation), but also for astrophysics and for security (for example luggage control in the airports). The main goal in this growth area is to obtain detection systems (detector and electronics) with high energy resolution, high spatial and temporal resolution, and operating at room temperature.
In close collaboration with Andrea Zappettini at the Institute of Materials for Electronics and Magnetism - CNR – Parma (IMEM-CNR) and Ezio Caroli at the National Institute for Astrophysics – Bologna (INAF) we are workingonthe study and optimization of bulk detectors based ontellurides (CdTe, CdZnTe) and operating in the 10keV-1MeV energy range.
The problems of purity and homogeneity of these materialsare not yet been overcome.As a consequence, transport properties are poorif comparedto those of silicon, germanium and gallium arsenide. On increasingthe photon energy, the mean absorption depth arises and the chargecollection efficiency dropsas effect of thelow hole mobility and the charge trapping during the drift, with a consequent line broadening.
The main purpose of our work is to study charge carrier transport and collection on changing the material growth parameters, the sample preparation (etching, passivation, contact deposition), the contact geometries, the charge generation method (X-rays, low energy photons, alpha particles). We make use of photoconductivity techniques,transport modeling and charge deficit correction models mainly to put light on the creation of space charge regions which are responsible for the degradation of the spectroscopic features of samples.
DC photoconductivityas a function of photon energy and applied voltage allows to highlights the presence of energy levels in the band-gap and to evaluate the mobility-lifetime product, respectively.The study of current pulses induced in the detector following irradiation with photons or alpha particles (Transient Current Technique) can be fitted to evaluate the two most important transport-related quantities for charge carriers, the mobility and lifetime, and to reconstruct the electric field spatial profile. We are also involved in the design and development of detector read-out electronics.
Nanostructured Oxides and Thin Film Oxides for high power electronics,sensing and optoelectronics
An emerging interest in the applications of wide-bandgap semiconductor exists at present. Devices based on nanoarchitectures such as nanowires, nanotubes and nanorods or nanocrystalline thin filmshave attracted vast and persistent attention for a variety of applications, including high power electronics, detecting ultraviolet (UV) radiation, gas sensing,and detecting chemical and biological molecules. Detectionof UV radiation is important in a number of applications like flamesensing, missile plume detection, space-to-space communication,astronomy and biological research.
Due to thesmall size of the diameter (100 nm or less) of nanocrystals, surface related effects can significantly influence the electronic and optical properties. The major drawback is the strong persistence of the photoconductivity (PPC), which prevents a fastrecovery of the dark current. This effect has been observed not only in nanowires but also in thin film and bulk.There are some controversial opinions about the originof the persistence of the photoconductivity; some authorsattributed it to the slow trapping of photoexcited electrons at surface defects, whereas others assigned the effect to the presence of metastable conductive states relatedto bulk oxygen vacancies. Some further studies are therefore necessary in order to clarify the possible origin of this effect.
In this context, we are working to put light on the presence of surface or bulk levels electrically and/or optically active, to study the transport processes in different regimes of temperature, pressure, gas atmosphere, illumination, electric and magnetic fields. The attention is mainly focused on nanostructures of ZnO and TiO2and thin films of Ga2O3 grown in the Laboratories of IMEM-CNR (Parma).
Contacts
The activity on oxide semiconductors is coordinated by
Prof. Antonella Parisini
Phone +39 0521 905272/5252
E mail antonella.parisni (at) unipr.it
Prof. Maura Pavesi
Phone +39 0521 905237/263
E mail maura.pavesi (at) unipr.it
