M. Mikulics - Academia.edu (original) (raw)

Papers by M. Mikulics

Journal of Crystal Growth, 2015

ABSTRACT We present a growth of GaN layers doped by cobalt using low pressure metalorganic vapor ... more ABSTRACT We present a growth of GaN layers doped by cobalt using low pressure metalorganic vapor phase epitaxy on c-plane sapphire substrates. The in situ doping of GaN by Co was performed by the decomposition of bis(cyclopentadienyl)cobalt precursor. Three parameters, the temperature and pressure of the deposition and the Ga/Co ratio in the gas phase, influencing cobalt concentration were investigated. The obtained results were confronted with the thermodynamic predictions of Co solubility within GaN lattice and electronic structure calculations of GaN:Co. The magnetic properties of GaN:Co thin films were investigated using superconducting quantum interference device magnetometer. In addition, the layers were characterized by Raman and photoluminescence spectroscopy and atomic force microscopy. The concentration of Co was measured using electron microprobe and depth profile was measured using secondary ion mass spectroscopy. Room temperature ferromagnetic ordering was observed on the Co doped GaN layers.

2007 Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics, 2007

ABSTRACT We present an all room-temperature cw THz spectrometer based on robust and low-cost dipo... more ABSTRACT We present an all room-temperature cw THz spectrometer based on robust and low-cost dipole antennas without fragile interdigitated structures. Very accurate spectroscopic measurements and THz image with signal to noise ratio in amplitude exceeding 20 dB are presented. The overall cost, stability and speed of the presented system is superior to the one of the standard THz pulsed spectrometer based on a Ti: Sapphire laser.

Applied Physics Letters, 2014

The Tenth International Conference on Advanced Semiconductor Devices and Microsystems, 2014

2001 International Symposium on Electron Devices for Microwave and Optoelectronic Applications. EDMO 2001 (Cat. No.01TH8567), 2001

ABSTRACT We discuss and experimentally demonstrate several approaches for output power enhancemen... more ABSTRACT We discuss and experimentally demonstrate several approaches for output power enhancement of metal-semiconductor-metal (MSM) structures of photomixers fabricated using molecular-beam epitaxial (MBE) GaAs grown at low substrate temperature. We focus on the parameters, following from the theory describing output power from a photomixer device. While low carrier lifetime and high photoconductivity was achieved by optimised LT GaAs growth conditions, substantial breakdown voltage increase was attained by improving MSM geometry, by metallic contacts recessing, as well as transferring the thin LT GaAs layer on top of high bandgap and high thermal conductivity substrates. Our results indicate a responsivity enhancement of up, to 40% as compared to the photodetectors with standard surface contacts

ABSTRACT a b s t r a c t Performance of AlGaN/GaN heterostructure field-effect transistors (HFETs... more ABSTRACT a b s t r a c t Performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed gate was investi-gated and compared with non-recessed counterparts. Optimal dry etch conditions by plasma assisted Ar sputtering were found for w6 nm gate recess of a 20 nm thick AlGaN barrier layer. A decrease of the residual strain after the gate recessing (from À0.9 GPa to À0.68 GPa) was evaluated from the photoluminescence measurement. The saturation drain current at the gate voltage V G ¼ 1 V decreased from 1.05 A/mm to 0.85 A/mm after the recessing. The gate voltage for a maximal transconductance (240À250 mS/mm) has shifted from À3 V for non-recessed HFETs to À0.2 V for recessed counterparts. Similarly, the threshold voltage increased after the gate recessing. A decrease of the sheet charge density from 1 Â 10 13 cm À2 to 4 Â 10 12 cm À2 at V G ¼ 0 V has been evaluated from the capacitance measurements. The RF measurements yielded a slight increase of the cut-off frequencies after the gate recessing. All these indicate that the gate recessing is a useful tool to optimize the AlGaN/GaN HFET performance for high-frequency applications as well as for the preparation of normally-off devices.

ABSTRACT We report the subpicosecond Faraday effect, measured in high quality Cd1-xMnxTe (x = 0.1... more ABSTRACT We report the subpicosecond Faraday effect, measured in high quality Cd1-xMnxTe (x = 0.12 and x = 0.09) single crystals at room temperature. Using a femtosecond pump-probe technique, we were able to generate sub-picosecond current pulses by illuminating a free-standing LT-GaAs photoswitch, couple those pulses to the CdMnTe probe crystal using a coplanar transmission line, and, finally, optically sample the temporal evolution of the resulting magnetic transients with subpicosecond resolution and the excellent signal-to-noise ratio. The ultrafast (below 600 fs) Faraday rotation, responsible for the observed magneto-optical effect, has been attributed to the ultrafast spin dynamics of holes in our p-type CdMnTe crystals. The observed femtosecond Faraday effect can be the basis for a development of a magneto-optical sampling system for ultrafast, time-resolved characterization of current transients in novel electronic and spintronic devices.

Optica Applicata

Eight synthetic histidine analogues of oxytocin and vasopressin are subject of investigations. Th... more Eight synthetic histidine analogues of oxytocin and vasopressin are subject of investigations. The spectra of the peptides have been investigated in the terahertz band. The results are obtained in the terahertz time-domain spectroscopy arrangement.

Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 2015

Frontiers and Innovations, 2013

2009 34th International Conference on Infrared, Millimeter, and Terahertz Waves, 2009

ABSTRACT We present simulations and measurements which verify the improved continuous wave THz ou... more ABSTRACT We present simulations and measurements which verify the improved continuous wave THz output power of folded dipole antennas compared to full wavelength dipole antennas.

Proceedings of SPIE - The International Society for Optical Engineering, 2002

We report on fabrication and ultrafast photoresponse of novel, freestanding low-temperature-grown... more We report on fabrication and ultrafast photoresponse of novel, freestanding low-temperature-grown GaAs (LT-GaAs) photoconductive (PC) devices. 1-mum-thick, LT-GaAs single-crystal films were grown by molecular beam epitaxy at the temperature range of 200°C to 250°C. Next, the films were patterned to the desired device sizes, lifted-off from their host substrates, and placed on predetermiend places on either SiO2/Si or MgO wafers. Our freestnding LT-GaAs devices consisted of either approximately 20-mum by 20-mum PC switches, or 150-mum by 150-mum metal-semiconductor-metal (MSM) interdigitated structures with Ti/Au fingers patterned directly on top of the LT-GaAs film. For testing purposes, our devices were integrated with Ti/Au coplanar striplines, fabricated directly on SiO2/Si and MgO substrates. The test structures were illuminated with 100-fs-wide optical pulses and their time-resolved photoresponse was measured with an electro-optic sampling system, characterized by 200-fs time resolution and sub-millivolt sensitivity. Using 810-nm optical excitation, we recorded as narrow as 360-fs-wide electrical signals (1.25 THz, 3-dB bandwidth) for PC switches, resulting in 155 fs carrier lifetime in our freestanding LT-GaAs. For both types of devices, the photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the laser excitation power, below well-defined saturation thresholds. Our freestanding photo-switches are robust and very reproducible. They are best suited for applications in hybrid optoelectronic and ultrafast electronic systems, since they can be placed at virtually any point on a test circuit.

IRMMW-THz 2006 - 31st International Conference on Infrared and Millimeter Waves and 14th International Conference on Terahertz Electronics, 2006

ABSTRACT We drive photoconductive dipole antennas with pulsed and continuous wave excitation. The... more ABSTRACT We drive photoconductive dipole antennas with pulsed and continuous wave excitation. The source resistance in these two cases is dramatically different which causes a significant shift in the radiation pattern of the antenna. Measurements are in a good agreement with simulation results.

Proceedings of SPIE - The International Society for Optical Engineering, 2007

ABSTRACT We present a compact, robust, and transportable fiber-coupled THz system for inline moni... more ABSTRACT We present a compact, robust, and transportable fiber-coupled THz system for inline monitoring of polymeric compounding processes in an industrial environment. The system is built on a 90cm x 90cm large shock absorbing optical bench. A sealed metal box protects the system against dust and mechanical disturbances. A closed loop controller unit is used to ensure optimum coupling of the laser beam into the fiber. In order to build efficient and stable fiber-coupled antennas we glue the fibers directly onto photoconductive switches. Thus, the antenna performance is very stable and it is secured from dust or misalignment by vibrations. We discuss fabrication details and antenna performance. First spectroscopic data obtained with this system is presented.

33rd International Conference on Infrared and Millimeter Waves and the 16th International Conference on Terahertz Electronics, 2008, IRMMW-THz 2008, 2008

ABSTRACT We present a systematic study of annealed ohmic contacts and their effect on the perform... more ABSTRACT We present a systematic study of annealed ohmic contacts and their effect on the performance of THz antennas. Annealing of the ohmic contact causes a strong decrease in the contact resistance and an enhancement of the electric field distribution inside the antenna structure. This doubles the output power of the devices compared to conventional photoconductors with standard Schottky-type metallization fabricated on an identical material.

Journal of the European Optical Society, 2009

In this paper we study the effect of the length of dipole antennas on the spectrum of the radiate... more In this paper we study the effect of the length of dipole antennas on the spectrum of the radiated THz signal in pulse-excited opto-electronic terahertz systems. In particular, we investigate the origin of the commonly observed sharp dips that occur in the spectra of photoconductive dipole antennas, and explain them on the basis of reflections of the excitation current pulse that take place at the ends of the antenna. We develop a hybrid time-domain model for the system and show that the predictions of our model are in good agreement with experimental results.

Conference Proceedings - The 8th International Conference on Advanced Semiconductor Devices and Microsystems, ASDAM 2010, 2010

ABSTRACT GaAs is broadly used in modern electronics. On the other hand, application of GaAs-based... more ABSTRACT GaAs is broadly used in modern electronics. On the other hand, application of GaAs-based devices in high power electronics is complicated due to the substantial excess heat generated during device operation. One possibility to remove the excess heat is to employ substrates with high thermal conductivity. In this contribution we present the growth of GaAs layers by MOVPE on aluminum (111) pseudo-substrates designed for an improved heat management in GaAs electronic circuits. Pseudo-substrates for GaAs deposition were prepared by Al evaporation on (100) GaAs substrate and subsequently heat treated. The GaAs layers exhibit polycrystalline character with high preferential orientation in (100) direction. The roughness of the layers was in the range of 10 to 100 nm and the thickness in the range of 500 - 3000 nm. These layers exhibit extremely low carrier lifetime due to the growth-induced defects and are suitable for fabrication of ultrafast MSM photodetectors.

Opto-electronics Review, 2012

The paper gives the results of refractive indexes for three plastic materials: polyethylene, poly... more The paper gives the results of refractive indexes for three plastic materials: polyethylene, polymethyl methacrylate and poly− vinyl chloride, which are measured and compared in two arrangements-a CW THz photomixer and a pulsed THz spectro− meter.