Š. Haščík

Preparation of nickel oxide thin films for gas sensors applications

Abstract Nickel oxide (NiO) thin films were prepared by dc reactive magnetron sputtering from a nickel metal target in an Ar+O2 mixed atmosphere in two sputtering modes. The oxygen content in the gas mixture varied from 15% to 45%. The films prepared in the oxide-sputtering mode were amorphous while the films in metal-sputtering mode exhibited polycrystalline (fcc) NiO phase. In this case TEM observations showed a dense fine-grained structure with the grain size in the range 4–10 nm and AFM micrograph showed a rough surface with RMS=2.21 nm. We have found that good NiO stoichiometric films are obtainable with a polycrystalline (fcc) structure at 40% oxygen content in the metal-sputtering mode.

Characterization of NiO thin films deposited by reactive sputtering

Abstract Nickel oxide (NiO) thin films were deposited by dc reactive magnetron sputtering Ni in an Ar + O2 mixed atmosphere at room temperature on unheated Si substrates. The oxygen content in the gas was varied from 10 to 50% and its effect on the deposition rate, structural, composition and electrical properties of the films were studied using thickness measurement, X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectrometry and resistivity measurement. A correlation between process parameters and film properties was established.

The improved performance of GaAs micromachined power sensor microsystem

Thermal effects in a 2-μm thick GaAs cantilever beam of the power sensor microsystem are investigated. The increased thermal sensitivity of the microsystem to the thermal conductance changes of the ambient atmosphere is evaluated by the experiment and simulation. A bimetallic effect in the microsystem cantilever beam is studied using both the microscopic laser optical interferometry and laser optical reflectance measurement. The cantilever beam deflections induced by the differential thermal expansion of the cantilever layers are found to be linear with the power dissipated in the microsystem MESFET-heater. The key microsystem transfer characteristics based on the bimetallic effect are obtained.

Schottky-barrier normally off GaN/InAlN/AlN/GaN HEMT with selectively etched access region

A Schottky-barrier normally off InAlN-based high-electron-mobility transistor (HEMT) with selectively etched access regions, high off-state breakdown, and low gate leakage is presented. Metal-organic chemical vapor deposition-grown 1-nm InAlN/1-nm AlN barrier stack is capped with a 2-nm-thick undoped GaN creating a negative polarization charge at a GaN/InAlN heterojunction. Consequently, the gate effective barrier height is increased, and the gate leakage as well as the equilibrium carrier concentration in the channel is decreased. After removal of the GaN cap at access regions by using a highly selective dry process, the extrinsic channel becomes populated by carriers. Normally off HEMTs with 8-μm source-to-drain distance and 1.8-μm -long symmetrically placed gate showed a source drain current of about 140 mA/mm. The HEMT gate leakage at a drain voltage of 200 V and grounded gate is below 10-7 A/mm with a three-terminal device breakdown of 255 V. The passivated InAlN surface potential has been calculated to be 1.45 V; significant drain current increase is predicted for even lower potential.

Impact of GaN cap on charges in Al2O3/(GaN/)AlGaN/GaN metal-oxide-semiconductor heterostructures analyzed by means of capacitance measurements and simulations

Oxide/semiconductor interface trap density (Dit) and net charge of Al2O3/(GaN)/AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor (MOS-HEMT) structures with and without GaN cap were comparatively analyzed using comprehensive capacitance measurements and simulations. Dit distribution was determined in full band gap of the barrier using combination of three complementary capacitance techniques. A remarkably higher Dit (∼5–8 × 1012 eV−1 cm−2) was found at trap energies ranging from EC-0.5 to 1 eV for structure with GaN cap compared to that (Dit ∼ 2–3 × 1012 eV−1 cm−2) where the GaN cap was selectively etched away. Dit distributions were then used for simulation of capacitance-voltage characteristics. A good agreement between experimental and simulated capacitance-voltage characteristics affected by interface traps suggests (i) that very high Dit (>1013 eV−1 cm−2) close to the barrier conduction band edge hampers accumulation of free electron in the barrier layer and (ii) the higher Dit cen...

Bulk and interface trapping in the gate dielectric of GaN based metal-oxide-semiconductor high-electron-mobility transistors

The trapping phenomena in GaN metal-oxide-semiconductor high-electron mobility transistor structures with 10 and 20-nm thick Al2O3 gate dielectric grown by metal-organic chemical vapor deposition were deeply investigated using comprehensive capacitance-voltage measurements. By controlling the interface traps population, substantial electron trapping in the dielectric bulk was identified. Separation between the trapping process and the interface traps emission allowed us to determine distribution of interface trap density in a wide energy range. Temperature dependence of the trapping process indicates thermionic field emission of electrons from the gate into traps with a sheet density of ∼1013 cm−2, located a few nm below the gate.

Thermal actuation of a GaAs cantilever beam

The design and performance of a thermally-actuated GaAs cantilever beam are presented. To actuate the cantilever beam an integrated GaAs MESFET heater was used. A comprehensive cantilever thermal actuation performance is analysed by experiment and simulation. The thermally-actuated GaAs cantilever shows excellent linearity and a high electro-mechanical conversion efficiency (?E = 0.802??m?mW-1). The method of two-dimensional numerical modelling and simulation is proposed to study its thermal and thermo-mechanical properties. The measured and simulated thermo-mechanical and electro-mechanical conversion characteristics are compared, indicating the viability of the simulation method used for further micromechanical analysis.

ZrO2/(Al)GaN metal–oxide–semiconductor structures: characterization and application

We investigate the properties of high-k dielectric insulators on GaN. 22 nm thick ZrO2 is deposited on N- or Ga-polarity GaN. Al/ZrO2/GaN metal–oxide–semiconductor and reference Au/Ni/GaN Schottky contact structures are characterized using current–voltage and capacitance–voltage methods. If compared with Schottky contacts, metal–oxide–semiconductor structures show leakage current substantially reduced for the N-polarity GaN while comparable values were obtained for the Ga-polarity GaN. The oxide relative permittivity is found to be in the range of 20–30. Light-assisted capacitance–voltage method shows density of interface states Dit ~ 1 × 1012 cm−2 eV−1 for Ga-polarity and ~2 × 1012 cm−2 eV−1 for N-polarity GaN/ZrO2. Finally, we prepared AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistor proving minimal transconductance deterioration and small threshold voltage shift. It is suggested that ZrO2 deposition optimization may further reduce the leakage current.

Adjustment of threshold voltage in AlN/AlGaN/GaN high-electron mobility transistors by plasma oxidation and Al2O3 atomic layer deposition overgrowth

We discuss possibilities of adjustment of a threshold voltage VT in normally off GaN high-electron mobility transistors (HEMTs) without compromising a maximal drain current IDSmax. Techniques of a low power plasma or thermal oxidation of 2-nm thick AlN cap over 3-nm thick AlGaN barrier are developed and calibrated for a thorough oxidation of the cap with a minimal density of surface donors at the inherent oxide-semiconductor interface. It has been shown that while a thermal oxidation technique leads to the channel and/or interface degradation, low density of surface donors and scalability of VT with additionally overgrown Al2O3 may be obtained for plasma oxidized HEMTs. With 10-nm thick Al2O3 deposited at 100 °C by atomic-layer deposition, we obtained VT of 1.6 V and IDSmax of 0.48 A/mm at a gate voltage of VGS = 8 V. Density of surface donors was estimated to be about 1.2 × 1013 cm−2, leaving most of the negative polarization charge at the semiconductor surface uncompensated. Further reduction of surface...

The fabrication of thin GaAs cantilever beams for power sensor microsystem using RIE

A method of fabricating two independent thin GaAs cantilever beams using a bulk micromachining technology based on RIE has been developed. The principle of the method is outlined and results of a three-dimensional patterning of the cantilever beams successfully applied in a power sensor microsystem development are described.