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Diamond is recognized as one of the most promising wide bandgap materials for advanced electronic applications. However, for many practical uses, hybrid diamond growth combining metal electrodes is often demanded. Here, we present the influence of thin metal (Ni, Ir, Au) layers on diamond growth by microwave plasma chemical vapor deposition (MWCVD) employing two different concepts. In the first concept, a flat substrate (GaN) was initially coated with a thin metal layer, then exposed to the diamond MWCVD process. In the second concept, the thin diamond film was firstly formed, then it was overcoated with the metal layer and finally, once again exposed to the diamond MWCVD. It should be mentioned that this concept allows the implementation of the metal electrode into the diamond bulk. It was confirmed that the Ni thin films (15 nm) hinder the formation of diamond crystals resulting in the formation of an amorphous carbon layer. Contrary to this finding, the Ir layer resulted in a successful overgrowth by the fully closed diamond film. However, by employing concept 2 (ie hybrid diamond/metal/diamond composite), the thin Ir layer was found to be unstable and transferred into the isolated clusters, which were overgrown by the diamond film. Using the Au/Ir (30/15 nm) bilayer system stabilized the metallization and no diamond growth was observed on the metal layer.

In this contribution, we describe the design, technology, and physical parameters of antenna-coupled microbolometer, used for broadband detection of terahertz electromagnetic spectrum. This microbolometer features an application of La0.67Sr0.33MnO3 layer grown on multilayered material stack ensuring lattice matching of the sensing LSMO layer to silicon. By virtue of bulk micromachining of silicon-on-insulator substrates, the sensing structure is built on thin suspended membrane to provide weak thermal link, increasing thermal response, thus sensitivity. Additionally, it helps suppressing excitation of in-plane guided surface modes that will otherwise deteriorate the antenna radiation diagram and affect the spectral responsivity of the sensor. Finally, the operation of sensor is demonstrated using a molecular laser setup at 762 GHz (and also 1.4 THz) emission line. The parameters of the said microbolometers are analyzed in terms of response and time constant. Optimal working temperature of the detectors is about 65 °C.

The last few decades faced on the fabrication of advanced engineering materials involving also different composites. Here, we report on the fabrication of few-layer molybdenum disulfide on top of thin polycrystalline diamond substrates with a high specific surface area. In the method, pre-deposited molybdenum coatings were sulfurized in a one-zone furnace at ambient pressure. As-prepared MoS2 layers were characterized by several techniques including grazing-incidence wide-angle X-ray scattering, atomic force microscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. We found out that the initial thickness of Mo films determined the final c-axis crystallographic orientation of MoS2 layer as previously observed on other substrates. Even though it is well-known that Mo diffuses into diamond at elevated temperatures, the competing sulfurization applied effectively suppressed the diffusion and a chemical reaction between molybdenum and diamond. In particular, a Mo2C layer does not form at the interface between the Mo film and diamond substrate. The combination of diamond high specific surface area along with a controllable layer orientation might be attractive for applications, such as water splitting or water disinfection.

We have prepared high temperature stable Schottky MOS diodes on AlGaN/GaN heterostructure with Ir-Al oxide based gate oxide layer and analyzed current transport mechanism in these diodes up to very high temperatures. The thermionic emission analysis of I-V curves gave high ideality factor in between 2 and 3 decreasing with increasing temperature. Simulation of the thermionic transport with the barrier height extracted from the measured curves and ideality factor unity gave the simulated current densities many orders of magnitude higher than experimentally measured. We explained experimental data by a bias dependent barrier height as a result of thin dielectric layer and a bias dependent charge localized in interface traps between the semiconductor and this dielectric layer. The measured data are then explained by the thin dielectric layer at the interface between (GaN)AlGaN and the metal electrode. Increasing forward voltage and moving Fermi level towards conduction band the charge in the traps becomes negative and increases the effective barrier height for electrons moving from the semiconductor into the metal. By this mechanism also a linear increase of reverse current with bias can be explained. The opposite is true for the reverse voltage.

We present technological issues in the deposition of diamond films on gallium nitride (GaN) membranes. Many wrinkles and thicker diamond layers were observed at the membrane center and poor quality diamond outside the membrane area. The deflection of the membranes was analyzed by a bulging method using white light interferometry. The membrane bending is discussed in the terms of temperature gradient and mismatch of thermal expansion coefficients of materials.

We present the feasibility in fabricating membranes and cantilevers made of diamond grown on Si/SiO2 substrates by femtosecond laser ablation. In the ablation process, we generated nano- and microstructures on the membrane surface. Such laser-induced periodic surface structures (LIPSS) are useful in tailoring the surface chemistry. In combination with wet or reactive ion etching, smooth membranes were generated.

We present an anomal current decrease for Mg contact on semi-insulating (SI) GaAs. The explanation of the observed effect is discussed within the light of the observed transport characteristics and XPS analysis.

Diamond has become an attractive material not only for fundamental research but also for industrial uses due to the extraordinary combination of its intrinsic properties. Its chemical vapor deposition is routinely done over the world on a large variety of substrates. However, the diamond high surface energy hinders its growth on foreign substrates. Therefore, optimizing the (pre-) growth technological step, known as nucleation or seeding step, is crucial to enhance the initial formation of diamond nuclei. Here, we present ultrasonic seeding and polymer-nanodiamond composite as promising nucleation techniques suitable for various substrates. The ultrasonic seeding is shown as a universal nucleation platform for nearly any substrate of complex and 3D-shape geometry. The complementary nucleation from polymer-nanodiamond is useful for the nucleation of soft and brittle materials.

We present technological issues in the deposition of diamond films on the GaN membranes. Many wrinkles and the thicker diamond layer were observed at the membrane centre and poor quality diamond outside the membrane area. The deflection of the membranes were analyzed by a bulging method using white light interferometry. The membrane bending was discussed in the terms of temperature gradient and mismatch of thermal expansion coefficients of materials. Optimized technological procedure was proposed to obtain both-side diamond-coated GaN membranes with minimized effect of wrinkling and induced residual stress.

The effect of silicon oxynitride (SiON) layer on the thermally-induced stress of diamond-coated AlGaN/GaN heterostructures was studied by Raman and SIMS spectroscopy. Diamond films (0.8 or 2.8 µm in thickness) were grown by MWCVD on selected areas of AlGaN/GaN/Si substrates with SiON interlayer. The stress in diamond became tensile for the thinner strip in the range of 0 ÷ 0.5 GPa and compressive for the thicker strip in the range of −0.6 ÷ −0.1 GPa both measured at temperatures ranging from 50 to 400 °C. The applied SiON interlayer positively influenced the induced stress (Δstress decreased by about 0.14 GPa compared to the sample without SiON) independently on the thickness of the diamond film. SIMS depth profiling was applied to analyse the influence of diamond CVD on the AlGaN/GaN interface of SiON passivated samples. As observed, the hydrogen and carbon atoms were trapped in the SiON which acted as a stop layer.

We report on thermal and mechanical analysis of uncooled antenna-coupled La0.67Sr0.33MnO3 microbolometer made on circular SOI (Silicon On Insulator) membrane with no limitation in its active area (circular membrane with diameter up to 2 500 μm). A simple method how to investigate the thermal conversion efficiency (thermal resistance value - Rth) is introduced. Thermal analysis is supported by the ANSYS modelling and simulation. It is found that Rth and thermal time constant (τ) of our LSMO microbolometer (bolometer sensitivity and time response) can be tuned by the SOI membrane thickness. Rth value as high as 188 K/mW and τ value as low as 0.88 ms are estimated from the thermal simulation for SOI membrane with total thickness of 300 nm (SiO2-200 nm, Si-100 nm). Genesis of the induced mechanical stress changes after main processing steps is found and evaluated to explain the mechanical stability of the LSMO based MEMS microbolometer.

We present a comparative study of the symmetric metal-SI GaAs-metal (M-S-M) diodes with full-area contacts on both device sides, in order to demonstrate the effect of contact metal work function in a straightforward way. We compare the conventional high work function Pt contact versus the less explored low work function Mg contact. The Pt-S-Pt, Mg-S-Mg and mixed Mg-S-Pt structures are characterized by the current-voltage measurements, and individual Pt-S and Mg-S contacts are investigated by the X-ray photoelectron spectroscopy (XPS). The transport measurements of Mg-S-Pt structure show a significant current decrease at low bias while the Mg-S-Mg structure shows saturation current at high voltages more than an order of magnitude lower with respect to the Pt-S-Pt reference. The phenomena observed in Mg-containing samples are explained by the presence of insulating MgO layer at the M-S interface, instead of the elementary Mg, as confirmed by the XPS analysis. Alternative explanations of the influence of MgO layer on the effective resistance of the structures are presented. The reported findings have potential applications in M-S-M sensors and radiation detectors based on SI GaAs.

We report on the investigation of low temperature (300 °C) hydrogen plasma treatment influence on the AlGaN/GaN heterostructures. This issue was raised in the frame of study on processes related to hybrid integration of diamond with GaN-based devices. At the same time, the capabilities of thin SiNx covering were investigated. The samples were exposed to low pressure hydrogen plasma ignited in the linear plasma system at low temperature. We analyze the surface morphology of samples by scanning electron microscopy while microstructural changes down to AlGaN/GaN interface were studied using secondary ion mass spectrometry. The sheet resistance, monitored using circular transmission line measurements, increases more than 3.5 times after 60 min treatment. The basic transport properties of the fabricated circular high electron mobility transistors after H2 plasma treatment were analyzed. The sheet resistance increasing was attributed to the decrease of effective mobility. Whilst, the observed Schottky barrier lowering indicates necessity of gate contact protection.

We demonstrate that diaphragms for sensor applications can be fabricated by laser ablation in a~variety of substrates such as ceramics, glass, sapphire or SiC. However, ablation can cause pinholes in membranes made of SiC, Si and metals. Our experiments indicate that pinhole defects in the ablated membranes are affected by ripple structures related to the polarization of the laser. From our simulation results on light propagation in Laser-Induced Periodic Surface Structures (LIPSS) we find out that they are acting as a slot waveguide in SiC material. The results further show that field intensity is enhanced inside LIPSS and spreads out at surface distortions promoting the formation of pinholes. The membrane corner area is most vulnerable for pinhole formation. Pinholes funnel laser radiation into the bulk material causing structural damage and stress in the membrane. We show that a~polarization flipping technique inhibits the formation of pin holes caused by LIPSS.

In this study, we present a complex methodology for evaluation of the thermally induced stress in patterned diamond microstructures. The diamond strips (2 mm in width and 0.78 or 2.8 μm in thickness) were selectively grown on AlGaN/GaN heterostructures. The stress was evaluated from the Raman shift of the diamond peak position within the temperature range from 50 to 400 °C. The shift was measured at two positions, i.e. at center and edge of the strip. The methodology for stress evaluation is based on the appropriate temperature correction of measured Raman spectra. We observed that for temperature increase from 50 to 400 °C the difference between stresses evaluated at the center and edge of diamond strip (Δstress) decreases from 0.27 to 0.18 GPa and from 0.32 to 0.1 GPa for the thinner and thicker diamond films, respectively. Experimental data were compared with FEM simulations. The simulations fitted well to experimental data and confirmed that the stress difference between the center and edge of diamond strip was caused by thermal stress component. As the temperature approaches the value close to the diamond deposition condition (~700 °C), the stress becomes homogeneous and equal to the intrinsic stress induced after the diamond growth. Keywords: Thermally induced stress, Raman spectroscopy, Polycrystalline diamond film, GaN

AlGaN/GaN-based high electron mobility transistors (HEMTs), Schottky diodes and/or resistors have been presented as sensing devices for mechanical or chemical sensors operating in extreme conditions. Creation of appropriate diaphragms and/or cantilevers out of substrate materials is necessary for further improvement of sensing properties of such MEMS sensors. The sensitivity of the AlGaN/GaN based MEMS pressure sensor can be modified by membrane thickness. In case of SiC as substrate material of the epitaxial AlGaN/GaN heterostructure layers, we applied laser ablation technique for micromachining of the membranes. We were able to verify the feasibility of this process by fabrication of micromechanical membrane structures also in bulk 3C-SiC, borosilicate glass, sapphire and Al2O3 ceramic substrates by femtosecond laser (520 nm) ablation. On a 350 µm-thick 4H-SiC substrate we produced an array of 275 µm deep and 1000---3000 µm in diameter blind holes without damaging the 2 µm GaN layer at the back side. Our experiments indicate that pinhole defects in the ablated membranes are affected by ripple structures related to the polarization of the laser. We developed an ablation technique inhibiting the formation of pin holes caused by laser induced periodic surface structures (LIPSS).

We report on thermal analysis of uncooled antenna-coupled La 0.67 Sr 0.33 MnO 3 microbolometer made on circular SOI membrane with no limitation in its active area (circular membrane with diameter up to 2 500 μm). A simple method how to investigate the thermal conversion efficiency (thermal resistance value) is introduced. Thermal analysis is supported by the ANSYS modelling and simulation. It is found that R th of our LSMO bolometer (bolometer sensitivity) can be tuned by the SOI membrane thickness. R th of the bolometer as high as 164°C/mW is estimated from the thermal simulation for SOI membrane with the thickness of 300 nm.

Titanium dioxide (TiO2) is in the focus of research for applications in gas sensors, solar cells photo catalysis, electronics and batteries. We demonstrate a simple confined laser plasma ablation method providing simultaneously laser induced periodic surface structures (LIPSS) in the micro-and nanometer range, covered by nano-textured black TiO 2 . In addition we utilize the LIPSS formation and etalon effects of the confined ablation method to fabricate hollow silicon (Si) needles 10 μm in diameter and 60 μm in length as well as Si-metal-box structures with submicron wall thickness in combination with deep reactive ion etching (DRIE). The gentle field modulation by the ablation etalon reveals some aspects of the LIPSS formation dynamics. Our results on Si and metals show, high spatial frequency LIPSS (HSFL) are first generated parallel to the laser polarization followed by transformation into pinholes and LIPSS perpendicular to the laser polarization.

In this work, we present technological issues in the deposition of polycrystalline diamond films as a backside heat spreader for GaN membranes. We compared three nucleation techniques, including standard ultrasonic seeding by diamond powder (DP) in deionized water, drop-off technique with DP in solution and polymer based nucleation by polyvinyl alcohol (PVA) polymer composite containing fine grained DP. The diamond growth was performed in a hot filament or microwave plasma chemical vapor deposition system. We found that standard nucleation techniques (ultrasonic seeding or immersing in diamond powder solution) resulted in damaging or cracking of GaN membranes, or low nucleation efficiency in the Z-depth of the structures. Implementation of a novel nucleation technique with PVA polymer composite containing diamond powder led to coating of both bottom and top parts of the membrane including side walls. The coverage homogeneity was evaluated by scanning electron microscopy and Raman spectroscopy. The obtained results indicate that the efficiency (and/or nucleation density) of the polymer-based technique is comparable with standard ultrasonic seeding and could be used in preference for nucleation of soft and fragile substrates.

In this article, we present an application of GaN/SiC heterostructures for Surface Acoustic Wave (SAW) hydrogen sensors with enhanced sensitivity. We demonstrate that the mass-loading sensitivity of such sensors can be increased by using shorter acoustic wavelengths and by confining the acoustic wave energy in an epitaxial waveguide. The effect of different Rayleigh-like modes on the mass-loading sensitivity is also discussed. The SAW wavelength-dependent phase and group velocity and electromechanical coupling constants were determined from electrical measurements. The outcome was used to design a microwave SAW delay-line oscillator. Its sensing characteristics are presented. The oscillator frequency shift was larger than 60 kHz after exposure to 1000 ppm H 2 /N 2 . The sensor reaction time was about 12 s for the same sensing conditions at room temperature.

This article reports on how an additional interfacial nickel oxide (NiO) influenced the hydrogen sensory performance of a Pt/AlGaN/GaN Schottky gate diode of a circular HEMT device. The oxide was formed by the oxidation of a thin Ni layer on top of the Pt/AlGaN gate interface at a high temperature. We propose that the oxide should provide a thermally stable interface, and its nanocrystalline surface morphology should be a favourable platform for the subsequent deposition of a catalytic Pt absorption layer on top. We analyzed the electrical properties of this interface and studied the sensory performance of such Schottky gate diode sensors to hydrogen versus the varied thickness of the NiO layer. It is shown that the parameters of the gate interface (barrier height, ideality factor) and the hydrogen sensory performance (sensitivity, time response) can be tuned with the thickness of the NiO interfacial layer. The diode exhibited a 60-fold increase in sensitivity for an optimized 20 nm thick oxide interlayer compared with that of a conventional Pt/AlGaN/GaN gate diode sensor. This can be explained considering that hydrogen dissociated more efficiently at the nanocrystalline surface of the Pt/NiO stack. However, the transient characteristics of the sensors with NiO showed a longer response time due to a longer diffusion path for hydrogen. We show that the optimal operation temperature of the sensor shifted considerably from 250 °C (reference Pt/AlGaN/GaN sensor) to as low as 50 °C (sensor with the NiO layer). This temperature shift is desirable for battery-powered hydrogen sensors.

La0.67Sr0.33MnO3 (LSMO) thin films have been grown by a pulsed laser deposition (PLD) on Bi4Ti3O12(BTO)/CeO2/YSZ buffered silicon-on-insulator (SOI) substrates. We compare the properties of these films with results of other authors. We analyse structural properties of LSMO/BTO/CeO2/YSZ/SOI multilayer structure prepared using PLD. Electrical measurements have shown that the temperature corresponding to maximum of resistance derivative (operating temperature of a microbolometer) is about 330 K (well above room temperature) and the highest resistivity of metal–insulator transition is at temperature (TP) above 400 K. Temperature coefficient of the resistance (TCR) has achieved values of 3.4% K−1 at 325 K for some LSMO films. Transmission electron microscopy analysis has confirmed epitaxial growth of all the layers and showed a mosaic character of the LSMO films due to strain relaxation.

Diamond-on-GaN heterostructure is a promising hybrid material system with a variety of applications such as heat spreaders, high-temperature, and high-power devices. From a practical point of view, polycrystalline diamond is shown to be an attractive solution to the limited use of the expensive monocrystalline diamond substrates. Here we present selective area diamond deposition on AlGaN/GaN layers focusing on the elimination of surface and metal contact damage (Ni, NiO, Ir, and IrO2) and suppressing the spontaneous nucleation of diamond. Metal contacts are important for further applications such as diamond-coated GaN based electronic devices. The growth of diamond films was performed by microwave chemical vapor deposition in different gas mixtures with the addition of CO2 or N2 to CH4/H2. Adding CO2 resulted in polycrystalline (PCD), while adding N2 led to the formation of nanocrystalline diamond film (NCD). The diamond deposition was carried out using selective area nucleation in a three-layer sandwich structure (polymer/seeding layer/polymer), which avoided damage to the electrode and GaN surfaces from ultrasonic seeding by diamond particles. No protective layer was used on the GaN surface before diamond deposition, i.e., diamond films were grown directly on the AlGaN/GaN heterostructures. In the case of NCD deposition, no surface damaging of GaN was observed. For both NCD and PCD films, increasing methane from 1% to 2% in H2 minimized the surface damage of GaN films.

In this article we demonstrate the high sensitivity AlGaN/GaN circular HEMT (C-HEMT) hydrogen gas sensor with new gate interfacial Pt/NiO layer. The wide band-gap III-nitride semiconductor heterostructure allows the sensor operation at elevated temperatures. Likewise, the C-HEMT sensing device is easy to prepare because the MESA insulation step can be omitted. Moreover, the I-V characteristics of ring gate diodes with a dominant thermionic emission of electrons can be easly achieved by elimination of tunneling currents induced on the MESA-etched edges. The Pt/NiO stacked gate absorption layer has nanocrystalline structure, what increases the surface-to-volume ratio. Consequently, the hydrogen gas is more efficiently dissociated at low temperature. Comparing to reference Pt/AlGaN/GaN diode sensor, the optimum operation temperature decreases from 250 oC towards 50oC and the hydrogen detection efficiency is enhanced about 10 times. This is desirable for battery-powered sensors with low current consumption. On the other hand, the fabricated sensor shows longer reaction and regeneration time constants. This is due to longer diffusion path that hydrogen atoms must overcome to reach the AlGaN semiconductor surface.

A circular high electron mobility transistor (C-HEMT) prepared on the AlGaN/GaN membrane surface has been investigated and its potential for pressure sensing has been already demonstrated. The key issue in the design process of such heterostructure based MEMS sensors is the stress engineering. This way we can scale the sensor performance, measured pressure range and sensitivity. Especially, the knowledge of the exact value of the residual stress in membranes (caused by deposition process) helps us to optimize the sensing devices. In this work, the residual stress determination method in gallium nitride circular shaped membrane is reported. It is shown that resonant frequency method using Laser Doppler Vibrometry (LDV) for membrane vibration measurement seems to be an appropriate technique to determine the residual stress in micro-scale membranes. Circularly shaped AlGaN/GaN micro-membranes are excited by acoustic short time pulse. The decay oscillating motion of the membrane is recorded by oscilloscope. By FFT spectral analysis of the signals the resonance frequencies are obtained. For the sample studied, the natural frequency mode resonance peak is used to define the residual stress level. To verify the observed stress in investigated membranes, prestressed modal analysis in finite element method (FEM) code ANSYS is performed. The stress extracted from the measured frequency is taken as an initial stress state of the modelled membrane. Experimentally obtained shock spectra are compared with that computed by FEM simulation.

In this work we demonstrate the fabrication process and electrical characterization of GaN/SiC heterostructure-based surface acoustic wave filters for highly-sensitive hydrogen sensors. In the acoustic propagation path, palladium metal is used as a hydrogen sensing layer. The influence of this layer on SAW velocity is measured. The impact of second-order effects on SAW filter properties is discussed. Two different interdigitated transducer (IDT) topologies are investigated in order to achieve the low insertion loss and high attenuation of triple transit echo (TTE). Finally, the sensory performance of designed delay line SAW oscillator is demonstrated

We report on a high temperature forming of iridium oxides (IrO 2 ) gates of circular AlGaN/GaN high electron mobility transistors (C-HEMTs) to be predetermined for high temperature applications. IrO 2 gate interfacial layer is formed by high temperature oxidation ( T = 500–800 °C, for 1 min) of 15 nm thick Ir gate contact layer. A comprehensive microstructural and electrical characterization of the IrO 2 gates is carried out to explain the improved transport properties and thermal stability of the gate interfaces. It is found that the transformation of Ir gate layer into its crystalline IrO 2 phase at oxidation temperatures of 700 °C and 800 °C provides the high barrier gate interface with prevailing thermionic emission transport mechanism. Accelerated reliability tests are used to confirm C-HEMT thermally stable performance deduced from both the gate interface and 2DEG channel stability. The introduced AlGaN/GaN C-HEMTs with high temperature grown IrO 2 gates seem to be very attractive for both the high temperature operated electronic and sensor devices.

We present a non-conventional bulk micromachining process of SiC substrate directed to fabrication of MEMS sensors based on III-nitrides (III-N) material systems for harsh environment. They consist of AlGaN/GaN/SiC high electron mobility transistors (HEMTs). The key issue is here the creation of appropriate diaphragms necessary to verify the multi-sensing properties of such MEMS sensors. The backside of the SiC substrate must be selectively removed in the defined areas. Considering the extremely high chemical stability of SiC, for the deep patterning we propose a method which uses femtosecond (FS) laser ablation combined with deep reactive ion etching (DRIE). The different writing strategies in order to improve the texturing of SiC are also discussed. Electrical parameters comparison of the fabricated devices on both Si and SiC substrates were performed. Four times higher values of maxima of drain current for HEMTs on AlGaN/GaN/SiC were measured.

The impact of 15 nm thick Schottky gate contact layer based on conductive Ir oxides grown by thermal oxidation in O 2 ambient at 500 °C for 1 and 10 min, respectively, on performance of circular high electron mobility transistor (C-HEMT) has been investigated. Besides the effects of the gate barrier height increase (∼0.32 eV) and gate leakage current decrease (2–4 orders) with the time of oxidation, the shift of both threshold voltage (∼1 V) and peak of transconductance (∼2 V) of the C-HEMT device towards the zero gate voltage has been introduced. X-ray diffraction and depth profiles of secondary ion mass spectroscopy are employed to analyze the microstructure and composition of the gate contacts. They revealed the mixture of Ir and Ir oxides (IrO, IrO 2 ) that have not been formed in the whole gate contact layer thickness. All results confirmed the oxidation proceeding from the top of the Ir gate contact layer up to the interface. A promise of Ir oxides based gate contact layers for design of thermally stable HEMT devices with an enhancement mode of operation could be available.

The issue of gate metallization stability on AlGaN/GaN heterostructure during the diamond deposition process has been studied. Among tested Ni, Ir, NiO and IrO 2 materials the iridium-based has the most promising characteristic to be used. The diamond growth in focused microwave plasma system on tran­sistors with Ir and IrO 2 Schottky contact metallization has been demonstrated and discussed.

The III-Nitrides, especially AlGaN/GaN devices can be widely used in micro-electro-mechanical sensors thanks to their excellent mechanical and electric properties. In this work, we investigate influence of the applied mechanical load on the source-drain current of AlGaN/GaN C-HEMTs located on the clamped edge of the cantilever beam and the membrane, respectively. A linear dependence of the output current on the calculated strain is observed.

AlGaN/GaN based sensors should be integrated into micro-electro-mechanical-systems (MEMS) and microfluidic devices used in biotechnology. The creation of appropriate diaphragms, surface structures or a combination of them is important for the fabrication of devices required in biotechnology and interdisciplinary research. Laser ablation as a direct mask writing procedure and deep reactive ion etching were used for nanostructuring of silicon nanopillars from the opposite site of the sensor device.

The work reports on a study of the symmetric metal/SI GaAs/metal (M-S-M) diodes in order to demonstrate the effect of contact metal work function. We compare the high work function Pt contact versus the low work function Mg contact. The Pt-S-Pt, Mg-S-Mg and Mg-S-Pt structures are characterized by the current-voltage measurements. The Mg-S-Pt structure show a significant current decrease at low bias while the Mg-S-Mg structure shows saturation current at high voltages more than an order of magnitude lower with respect to the Pt-S-Pt reference. The phenomena observed in Mg-containing samples are explained by the presence of insulating MgO layer at the M-S interface. The reported findings have potential applications in sensors based on SI GaAs.

Graphical abstractDisplay Omitted Highlights? We designed a pressure sensor based on circular high electron mobility transistor. ? The 1.9µm thick AlGaN/GaN membrane is fabricated by reactive ion etching of the Si. ? The membrane tensile residual stress is about 280-300MPa. ? The maximal membrane deflection about 2.75µm is for the applied pressure of 1MPa. A new design concept of pressure sensors based on circular high electron mobility transistor (C-HEMT) sensing devices integrated on circular, ring and/or sequential ring AlGaN/GaN membrane structures is introduced for the first time. The micromachining process technology of 1.9µm thick AlGaN/GaN membrane structures patterned on Silicon substrate is experimentally verified. The mechanical properties of the membranes are investigated by two different experimental methods and simulation. They reveal the membrane tensile residual stress of the value about 280-300MPa. A non-linear behavior of maximal deflection with the applied pressure is predicted by the ANSYS simulation. The maximal membrane deflection about 2.75µm is estimated for the applied pressure of 1MPa. The functionality of the integrated membrane-based C-HEMT sensing device is successfully verified to be applied for pressure sensing.

We present a new approach in design of high temperature stable AlGaN/GaN HEMT gate absorption layers for hydrogen sensing at elevated temperature. To suppress undesirable chemical interaction of Pt on the gate interface, a thin conductive IrO2 oxide layer (t ∼ 10 nm) is formed by high temperature oxidation (T= 800 ̊C for 1min in O2 ambience) before Pt catalytic metal deposition. The comparative study of the hydrogen detection on AlGaN/GaN HEMT device with the Pt and Pt-IrO2 based gate absorption layers is shown. The sensitivity of the Pt gate Schottky diode is significantly enhanced with the insertion of IrO2 gate interfacial layer. The proposed Pt-IrO2 gate based diode exhibits a maximum sensing response value of 70%/ppm at 100 ̊C under a 0.1% H2/N2 gas. As comparing with the conventional Pt diode it is increased by more than 12 times. Additionally, the operating temperature for the maximum sensing response is decreased from 150 ̊C to 100 ̊C with the insertion of IrO2 interfacial layer. Moreover, the Schottky gate barrier height lowering at is found to be 254 meV and 100 meV for the IrO2 and Pt gate based diodes, respectively.

The effect of fluorine interface redistribution on dc and microwave performances of SF"6 plasma-treated AlGaN/GaN high-electron mobility transistors (HEMTs) was investigated. Selective SF"6 plasma treatment of the AlGaN/GaN HEMT gate interface yielded increases in the current gain cut-off frequency (f"T) and maximum frequency of oscillation (f"m"a"x) of almost 60%. Annealing induced fluorine interface redistribution showed a low impact on the electron drift mobility and a negligible impact on the peak transconductance of the HEMTs. A large impact of the fluorine interface redistribution was observed for the threshold voltage and sheet carrier concentration of two-dimensional electron gas (2DEG). Consequently, it led to a decrease in the f"T and f"m"a"x values, but the values were still higher than those of conventional reference HEMTs.

This work presents the process technology and high frequency analysis of AlGaN/GaN based SAW (Surface Acoustic Wave) – HEMT (High Electron Mobility Transistor) devices to be applied for chemical gas sensors operating in GHz range. For that purpose, the interdigital transducers (IDTs) of SAW device with submicrometer finger width and spacing were designed and to improve the sensor detection sensitivity. The sensor signal processing electronics containing microwave SAW oscillator was designed, simulated and successfully tested. The automated measurement system for SAW oscillator frequency measurement was built. An integrated SAW-HEMT device for multifunctional, high sensitive and remote gas sensing in the local ambient is also demonstrated.

Ohmic contacts to p-GaN Using Au/Ni-Mg-O Metallization Electrical characteristics and elemental depth profiles of ohmic contacts to p-GaN using Au/Ni-Mg-Ox metallization have been investigated. The objective was to examine the possibilities of increasing the charge carrier concentration in the surface region of GaN by adding Mg, thus of a p-type dopant into the Au/NiOx metallization structure. For this purpose, a Ni-Mg-Ox layer with a low concentration of Mg was deposited on p-GaN by dc reactive magnetron sputtering. The top Au layer was deposited in a similar way. The fabricated contact structures were annealed in N2. When the Ni-Mg layer in the Au/Ni-Mg-Ox/p-GaN structure was deposited in an atmosphere with a low concentration of oxygen (0.2 at%), the structure exhibited a low resistance ohmic nature. The contact resistance was lower than in the case of a Au/Ni-Ox/p-GaN structure without the Mg dopant in the metallic layer. An increase in the concentration of oxygen in the working atmosphere resulted in higher values of the contact resistance of the Au/Ni-Mg-Ox/p-GaN structure. In our opinion the ohmic nature of the contact structure is related to the existence of a metal/p-NiO/p-GaN scheme. The measured values of the contact resistance in the Au/Ni-Mg-Ox/p-GaN structure in comparison with the Au/Ni-Ox/p-GaN structure are caused by an increased charge carrier concentration in the surface region of p-GaN, which is a consequence of Mg diffusion from the Ni-Mg-Ox layer.

This work present a new approach to design and fabrication of AlGaN/GaN based SAW-HEMT structures to be applied for chemical gas sensors operating in harsh environment. A direct on-chip integrated compatibility in the process technology of surface acoustic wave (SAW) structure and high electron mobility transistor (HEMT) was demonstrated. It enabled to integrate two different principles of gas sensing based on both SAW and HEMT. The functionality of two types of SAW structures and HEMT device was investigated. The measured amplitude characteristics of SAW structures revealed excitation at the centre peak frequency about 1.35 GHz and 1.7 GHz, respectively with a both good quality factor and stop-band rejection ratio. The detection electronic based on SAW oscillator was subsequently designed and simulated. It proved the expected function and properties.

The micromachining technology and an electro-thermo-mechanical performance analysis of GaAs based micromachined thermal converter (MTC) device to be designed for metal oxide gas sensors are presented. MTC device introduced exhibits a low power consumption due to by high thermal resistance values ( R th = 15–21 K/mW), uniform temperature distribution, a fast temperature time response ( τ ∼ 1.5–1.8 ms), and good mechanical integrity and thermal stability. It is also fully compatible with the GaAs HEMT based signal-processing and controlling electronics to be monolithically integrated with the gas sensors. The both a simple analytical and three-dimensional thermal modeling of the MTC device was performed. It supported the high electro-thermal conversion efficiency and fast temperature time response of the MEMS device as evaluated by the experiments.

This work describes the design, simulation, fabrication and characterization of a TiN/Pt microheater prepared on a GaAs micromechanical structure as a prospective device for micro-electro-mechanical system (MEMS) sensor arrays. Electro-thermal simulation was employed to verify the properties of the designed microstructure, which confirmed achievement of the operating temperatures in the range from 470 to 600 K with a heating power less than 25 mW. The average temperature gradient in the active area does not exceed 0.6 K/μm. Fabrication of GaAs suspended membranes was demonstrated, realized in two steps by combination of surface and bulk micromachining. Development and characterization of a microheater on a GaAs membrane is described. The mechanical stability of the heated multilayer membrane structure was tested and satisfactory mechanical stability of the hotplate was confirmed. The power consumption at an operating temperature of approximately 550 K is about 30 mW which is in good agreement with the value of about 22 mW obtained from electro-thermal simulation. The achieved thermal resistance value is 8.43 K/mW.