Janusz Kozlowski

Thick-film resistive temperature sensors

This paper discusses thick-film resistive temperature sensors investigated at the Technical University of Wroclaw. The technology and electrical properties of resistance temperature detectors, thermistors, low-temperature thermometers and heating elements are presented. The R(T) curve of chosen air-fireable Ni - P based films agrees with Ni wire. The initially aged thermistors from the system can operate in the range from room temperature up to 673 K. The commercial thick-film resistors modified by the negative TCR drivers (e.g. powder) are fully suitable for low-temperature measurements in the range from 20 to 100 K. The integrated gas sensors need heaters because temperature influences their sensitivity, selectivity and response time. The thick-film compatible system based on commercially available resistive and conductive inks allows continuous long-term electrical heating of the sensor up to 673 K. The admissible operating temperature is much higher for the heaters made from conductive inks; for example fritless platinum heaters are satisfactory up to 1073 K.

MOVPE GaN Grown on Alternative Substrates

The structure, morphology and optical properties of GaN films deposited by metalorganic vapour phase epitaxy (MOVPE) on alternative substrates: ZnO, NdGaO, YSZ (yttria stabilized zirconia). Scanning electron microscopy, X-ray diffraction and photoluminescence were used for the epitaxial layers characterisation. The obtained results have been compared to those of GaN layers grown on c-plane sapphire substrates. It was established that the most important step towards the realisation of device quality GaN material on alternative substrates is the first stage of the growth process.

Influence of crystallographic structure on electrical characteristics of (Al,Ga)N epitaxial layers grown by MOVPE method

The paper presents a study on the correlation between the material structure and electrical characteristics of nitride epitaxial layers grown by metalorganic vapour phase epitaxy (MOVPE) on sapphire substrate. Structural characterisation was performed by means of X-ray diffractometry. Distributions of crystallites lateral sizes as well as the lattice strains inside the blocks were studied. Twist and tilt mosaicities were examined using rocking curves and a specially chosen configuration, where an edge of the sample was illuminated. The grain sizes in the layers were calculated on the basis of peak profile analysis and solution of the Fredholm equations. The electrical properties of the nitride epitaxial layers were determined by impedance spectroscopy method. An attempt was made to correlate the layers of electrical and structural characteristics for better physical interpretation of the results and verification of epitaxial structures usefulness for device application.

Growth of high-quality GaN and AlxGa1-xN layers by an MOVPE technique

AlxGa1-xN layers with 0.02 xGa1-xN layer grown on it were evaluated by high resolution X-ray measurements, their surface morphology was observed with SEM and Nomarski optical microscope. Electrical properties of the layers were determined by C-V measurements performed at 100 kHz and 1 MHz using mercury probes. The aluminum incorporation into the solid phase during the growth process has been studied. As a result high quality AlxGa1-xN/GaN layers for electronic application have been deposited.

Field emission from sintered CrSi2

Abstract Resistive sintered CrSi2/Si has been used as a model material for field electron emission experiments. The measured characteristics of emission current show some discrepancy from the standard plot calculated from the Fowler–Nordheim theory with experimentally determined coefficients. There are indications that electric field penetration into the emitter enhances electron emission. It is suggested that this enhancement is caused by ballistic emission.

Growth and characterisation of GaN epitaxial layers

Layers of monocrystalline GaN were grown at different temperatures by atmospheric pressure metalorganic chemical vapour deposition (MOCVD) method on the sapphire substrates. The high quality of the layers was confirmed by X-ray diffraction and photoluminescence spectra measurements. For comparison purposes, simultaneous growth on a-plane and c-plane sapphire substrates was performed for each of the experiments. The GaN layers grown on either a-plane or c-plane sapphire were oriented with the GaN c-plane (0001) parallel to the substrate. The 325 nm line of He-Cd laser was used as an excitation source for photoluminescence experiments. Photoluminescence spectra showed near band peaks and broad yellow band emission at 77 K.

AP-MOVPE growth and characterization of GaAs1-xNx epilayers

Nitrogen incorporation into GaAs epilayers has received interest especially due to band gap decreasing of resultant material. Small fraction of nitrogen makes GaAs1-xNx useful material for optoelectronic devices grown on GaAs substrates. However, achieving high quality of GaAs1-xNx/GaAs heterostructures requires MBE (Molecular Beam Epitaxy) or LP-MOVPE (low pressure-metalorganic vapour phase epitaxy) technique. In this article the technological parameters and characterization of epilayers with small nitrogen content obtained by AP-MOVPE are presented. Under our growth conditions we achieve GaAs1-xNx/GaAs heterostructures which can be used for photodetector applications. We have focused our research on optical, electrical and structural properties of these material.

Application of GaN laterally overgrown on sapphire

The paper presents the results of work concerning the elaboration of GaN pseudo-bulk technology. The growth mechanism of epitaxial lateral overgrowth (ELO) GaN affected by mask patterns and the MOVPE growth process parameters are presented and discussed. The conditions to obtain full coalescence of isolated stripes leading to pseudo-bulk GaN substrate growth are specified.

X-ray analysis of superionic conductors based on silver sulfate

The basic element of electrochemical SO2/SO3 sensors based on superionic conductors is the quality of applied material and its crystallographic structure. This paper presents structural analysis of different solid state electrolytes fabricated on the basis of Ag2SO4. It was doped with different sulfates. The structures of synthesized materials with different concentrations of sulfates are compared. Ionic conductivity strongly depends on composition of electrolyte and its crystallographic structure. X ray investigations were performed with diffractometer made by Philips using CuK(alpha ) radiation. The influence of dopants on the composition of solid solution of synthesized superionic conductors and on the change of microstructure Ag2SO4 was observed and discussed.

X-ray measurement of tungsten films grown in a nonflowing laser-induced CVD process

A lab scale nonflowing reactor was built to study chemical vapor deposition reactions and for the purpose of minimizing the waste of expensive high purity and toxic gases. The reactor operates as a batch process resulting in a time varying gas composition during the course of deposition. Samples were heated either resistively (thermal CVD) or with focused laser light (laser CVD). X-ray measurements were made on the deposited tungsten samples. Obtained results (tungsten structural parameters) were compared with the same parameters obtained for the tungsten films deposited in a commercial, flowing CVD process.