A non-destructive method for extracting the true signatures of intersubband energy level states inside a single triangular quantum well for AlGaN/GaN heterostructure is presented. Herein, we report experiments showing light interaction with high-mobility two-dimensional electron gas (2DEG) in Al0.3Ga0.7N/AlN/GaN based HEMT structure using low-temperature photoluminescence spectroscopy. An interband transition from two-dimensional electron gas (2DEG) subbands to the valence band is identified. These observed transitions were confirmed by comparing the PL spectra of the as grown and top barrier layer etched samples. The luminance peak related to 2DEG disappeared when the top AlGaN barrier layer was removed using reactive ion etching (RIE) system. Furthermore, the emission peak data are also supported by a calculation based on a self-consistent solution of one-dimensional Poisson and Schrodinger equations. The three PL spectra peaks corresponding to the interband transitions from 2DEG subbands to the valence band were reported at 3.269, 3.356 and 3.438 eV respectively. The corresponding intersubband energy states inside the quantum well were extracted (simulated) with 87 (91) and 178 (186) meV energy spacing between E0 & E1 and E0 & E2 respectively. The temperature and excitation power-dependent PL measurement makes it easy to identify the transitions from the 2DEG subbands to valence bands.

This paper presents the computational study of the combined metamaterial driven inter-sub band transition phenomenon in GaN HEMT which is possible mode to extend its operating frequency well beyond its present cutoff frequency to THz band. It has been reported that the device dimension and geometry act as terahertz (THz) metamaterial and its couple with two-dimensional electron gas (2DEG) inside triangular quantum well of HEMT. Semiconductor device modeling has been used to extract intersubband (ISB) resonance phenomena by solving Schrodinger- Poisson equations self-consistently and k.p method. Terahertz response of entire GaN HEMT device has been modeled using Finite difference frequency domain electromagnetic simulation for 0.3 to 3.0 THz region. The combination of external electrical field dependent tunable ISBT and metamaterial driven enhancement of electric field distribution inside channel due to THz illumination clearly explain the GaN HEMT device behavior at THz frequency

A new and versatile mechanism for electrical tuning of intersubband transitions (ISBT) in GaN High Electron Mobility Transistor (HEMT) device at room temperature is presented. In present study, experimental demonstration is provided which clearly discriminate ISBT from any other transitions induced by deep level traps, defects, etc. in 100 nm GaN HEMT device at room temperature. The strong interaction of light with two-dimensional electron gas (2DEG) inside asymmetrical triangular quantum well of GaN HEMT is investigated. The resultant ISBT of the carriers can be explained through pinning of the fermi level inside the quantum well by applying an electrical field along growth direction through gate. Presently intersubband (ISB) based devices are operated at cryogenic temperature to minimize the thermal lattice vibration. The inherent advantage of conduction band tuning through external bias in HEMT structure as demonstrated in this works can leads to room temperature device operation feasibility.

In the present work, undoped AlGaN/AlN/GaN heterostructure has been simulated and physics based device model has been developed for GaN high-electron-mobility transistors (HEMT) based Terahertz applications. The heterostructure properties like electron mobility, 2DEG concentration, polarization charge concentration, conduction band energy have been extracted and compared with our experimental results using different semiconductor characterization tools. Further, these 2DEG carriers are confirmed within the triangular quantum well generated at the interface between AlGaN and GaN. We have computed self-consistent solution of the Schrödinger and Poisson equations in order to calculate the quantized energy levels of conduction and valence bands. Based on extracted quantized energy levels, inter-sub band emission and absorption phenomenon was studied on AlGaN/AlN/GaN based HEMT for different Al compositions and thickness of AlGaN barrier layer. In addition to it inter-sub band transition energies, gain, emission, absorption and potential has been extracted and showed good agreement with reported literature results. The current state of art for 100 nm GaN HEMT devices is up to 100 GHz cut off frequency. The present study shows potential of same 100 nm GaN based HEMTs application beyond their cut off frequency limit.

Amongst various gas sensor structures developed so far probably the least investigated one is the Metal- (Active) Insulator-Metal (MIM) structure. The vertical electron transport mechanism of this structure offers high response with fast response and recovery for reducing gases like H2 and methane compared to the conventional devices with planar configurations. In this paper we report on the comparative performances of MIM sensors based on nanocrystalline-nanoporous ZnO as the active sensing layer derived by sol-gel and by the UV assisted electrochemical anodization method respectively. The sensor structures based on Pd-Ag (26 %)/ZnO/Zn were investigated in presence of five methane concentrations (0.01, 0.05, 0.1, 0.5 and 1.0 %) and in the temperature range 150 °C to 300 °C. The electrochemically developed ZnO showed superior performance in terms of operating temperature, response magnitude, response time and recovery time. Moreover, it showed response to much lower concentrations (like 0.01 % and 0.05 %) of methane that could not be detected by the sol-gel derived sensors. The possible reason for the superior performance of the electrochemically grown ZnO compared to the sol-gel derived one was shown to be the significantly lower grain size and appreciably high porosity. But, in spite of its promising performance the electrochemically developed MIM sensor has no choice of selection of the substrate and so it is not compatible to the standard IC technology. Therefore, it has limited applications to develop the stand-alone sensor devices. On the other hand, sol-gel derived MIM device is based on Si substrate and so, it is IC compatible and is suitable for the integrated sensor platform.

Conventional pre-treatment process for saw damage removal before texturization of monocrystalline silicon wafers is by higher concentration (6–10%) caustic etch at 50–60°C. In this paper a novel low cost approach for this pre-treatment of surface texture by a new composition of hot sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) solution is reported for industrial large area, high efficiency, single crystalline silicon solar cells. The moderate silicon etching rate of hot NaOH–NaOCl solution generates a better control on removal of damaged surface. This new damage etching process also helps in the formation of optimized pyramidal structure on silicon wafer during texturization. This process is highly suitable for thin starting raw wafers with thicknesses in 160–200μm range used by most of cell manufacturing industries. Substantial reduction of yield loss due to breakage of wafers is achieved by using this modified process. Optimized recipe of this surface texturization process is ascertained by the Scanning Electron Microscopic (SEM) study of front textured surface on non-metallized and metallized areas. Also reflectivity, cell dark and illuminated voltage–current characteristic measurements validate the superiority of this process to the existing one, which finally leads to low cost, improved quality solar cells for any monocrystalline PV industry.

Sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) solution (1:1 ratio by volume) based texturization process at 80–82°C is an easy, low cost and comparatively new and convenient option for fabrication of any multicrystalline silicon (mC-Si) solar cell. In the present study atomic force microscope is used to observe the intragrain surface in a miniscule area (3μm×3μm) of NaOH–NaOCl textured surface by two and three dimensional analysis, roughness analysis and section analysis. The r.m.s value of the surface parameter of 7.0nm ascertains the smoothness of the textured surface and further the surface reflectivity is minimized to 4–6% in the 500–1000nm wavelength range by a proper silicon nitride anti-reflection coating. Comparing with the standard HF–HNO3–CH3COOH acid textured cell, the NaOH–NaOCl textured cell shows a comparatively lower value of series resistance of 7.17mΩ, higher value of shunt resistance of 18.4Ω to yield a fill factor of 0.766 leading to more than 15% cell efficiency in the industrial cell processing line. This AFM study yields different surface roughness parameters for the NaOH–NaOCl textured wafers which can be used as a reference standard for optimized texturing.

In this work the process for the realization of Oxidised Macro Porous Silicon (OMPS) layer as a material for thermal isolation has been studied. Macro Porous silicon layers are created by anodisation of P-type (10–20 Ω-cm) silicon with HF and DMF (Dimethylformamide), which are then followed by thermal oxidation in order to find a compromise between higher thermal isolation and good mechanical stability. The morphology of the samples are studied by FESEM. A simple model for determining the thermal conductivity (TC) of the OMPS layer has also been formulated which shows that the TC of OMPS layer are two to three order less than crystalline silicon. Heat distribution of a microheater over the OMPS layer has been simulated by using finite element analysis with ANSYS software which shows the higher degree of thermal isolation and better mechanical strength with OMPS layer compared to conventional methods.

In the present paper, intragain surface morphology of multicrystalline silicon (mC-Si) wafer surface of area 3μm×3μm polished by the acid-based solution comprising of hydrofluoric (HF), nitric (HNO3) and acetic (CH3COOH) acids and new alkaline solution containing sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) has been studied using an atomic force microscope (AFM). From the roughness and section analysis study of the intergrain surface by the AFM, it is revealed that the NaOH–NaOCl polishing process is quite superior to the existing acid polishing one. Quantitative measurements indicate better smoothness of polished silicon surface after the NaOH–NaOCl treatment as compared with acid polishing. Also process cost per wafer involved in the NaOH–NaOCl polishing process is far lower than that by the acid polishing process along with additional advantageous features of high productivity, environment friendliness and safety. All these factors finally contribute to make the NaOH–NaOCl solution a better polisher for mC-Si surface.

Nanocrystalline-nanoporous ZnO thin films were prepared by an electrochemical anodization method, and the films were tested as methane sensors. It was found that Pd-Ag catalytic contacts showed better sensing performance compared to other noble metal contacts like Pt and Rh. The methane sensing temperature could be reduced to as low as by sensitizing nanocrystalline ZnO thin films with Pd, deposited by chemical method. The sensing mechanism has been discussed briefly.

The nanoporous ZnO thin film was prepared by anodic oxidation of high purity Zn sheet in absence and in presence of UV radiation and a methane gas sensor was fabricated using an interesting metal-insulator-metal (MIM) device configuration, Pt/ZnO/Zn. The undoped ZnO film deposited in presence of UV radiation showed superior material properties and improved gas sensing characteristics for methane. The V-I characteristics of the sensor device were recorded in absence and in presence of low concentrations of methane at different temperatures. The response, its variations with temperature and the time of response were calculated. With platinum as catalytic contact the response of the order of 0.78 +/- 5% and the time of response of 12 +/- 0.5% sec were obtained with 1% methane in nitrogen carrier gas at an optimum temperature of 240 degrees C. The performance of the sensor in presence of 1% methane in synthetic air was also studied and a decrease in the response was observed. The sensor was also tested with 1% hydrogen in nitrogen and a selectivity for methane was obtained at higher temperature. A cursory stability study of the sensor was carried out and a stable operation for about 16 hours could be achieved. A possible mechanism for methane sensing was proposed to understand the response characteristics of the ZnO-based MIM device for gas sensors.

Nanoporous ZnO thin films were deposited by electrochemical anodization of high purity Zn at room temperature using Pt counter electrode, calomel reference electrode and oxalic acid as the electrolyte. The crystallinity and the surface morphology were studied by X-ray diffraction (XRD) and Field emission scanning electron microscope (FESEM). The variation in molar concentration of oxalic acid during anodization had significant effect on the crystal size and the pore size particularly in the presence of UV light. An increase in room temperature band gap from 3.25 to 3.87 eV of ZnO film grown in 0.3 M oxalic acid indicates a quantum confinement effect and it was further confirmed by a blue shift of the photoluminescence (PL) spectra. A possible mechanism of the anodization and the photoetching in the presence of UV light of the ZnO film have been suggested.

Undoped nanocrystalline n-ZnO thin films were deposited on SiO2-coated p-Si substrates by a sol–gel method to fabricate ZnO-based resistive sensors for methane detection. The sensor performance was studied with three different catalytic noble metal contacts to ZnO, i.e. palladium–silver (26%), rhodium and platinum. The response magnitude, response time and recovery time were studied in the temperature range of 50–350 °C and at methane concentrations of 0.1%, 0.5% and 1%. Pd–Ag and Rh contacts were found to give lower optimum operating temperature (250 °C) than Pt contact (300 °C). Although Rh contact showed a higher response (83.6%) than Pd–Ag (74.3%) and Pt (69.3%) contacts, Pd–Ag contact was found to exhibit the shortest response and recovery time amongst the three catalytic contacts to ZnO.

High purity Zn was electrochemically anodized to grow nanocrystalline and nanoporous ZnO thin film, which was subsequently surface modified by Pd using electroless chemical dipping method. Two different device configurations e.g., a planar structure and a metal-active insulator-metal (MIM) sandwich structure, both working in Schottky barrier mode were designed, fabricated and tested as a hydrogen sensor. A Pd-Ag alloy was used as catalytic electrode contact for both the planar and MIM (metal-insulator-metal) configurations. Nitrogen and synthetic air were used as carrier gases for hydrogen in separate experiments. A dynamic response of similar to 65, response time of 2.2 s and a recovery time of 21.8 s for the planar structures and the corresponding values of similar to 62, 1.8 s and similar to 14 s were recorded for the MIM structures at an optimum temperature and voltage of 50 degrees C and 0.5 V. The sensing operation for 16 h showed an excellent stability.

Nanocrystalline ZnO based sensor using micromachined silicon substrate has been reported for efficient detection of methane as opposed to conventional SnO2 based micromachined sensors for its higher compatibility to silicon IC technology and greater response. A suitably designed nickel microheater has been fabricated on to the micromachined Si platform. The optimum temperature for highest response magnitude and lowest response time were found to be 250 °C although relatively high (76.6%) response is obtained even at as low as 150 °C. Our study showed quite high response magnitude (87.3%), appreciably fast response time (8.3 s) and recovery time (17.8 s) to 1.0% methane at 250 °C. The sensor showed appreciably fast response (14.3 s) and recovery time (28.7 s) at 150 °C. The power consumption at an operating temperature of 250 °C was 120 mW and at 150 °C is only ∼70 mW. Moreover, this type of sensor was found to give fairly appreciable response for lower methane concentrations (0.01%) also. For higher methane concentrations (>0.5%) response is detectable even at 100 °C where the power consumption is only ∼40 mW.

The functional characteristics of the planar resistive and MIM (metal-insulator-metal) sensors using electrochemically grown nanocrystalline–nanoporous ZnO thin films and surface modified by dipping in an aqueous solution of PdCl2 were investigated for methane sensing. It was found that the operating temperature was substantially reduced to 70 °C and 100 °C for the two different configurations, respectively, after this interesting and somewhat novel surface modification step. A high purity Zn was anodized to produce ZnO thin films using a Pt cathode, a calomel reference electrode and a 0.3 M oxalic acid electrolyte. Pd–Ag (26%) was used as the catalytic metal contact to ZnO to fabricate a resistive and an MIM configuration. The response of the order of ∼48, a response time of ∼4.5 s and a recovery time of ∼22.7 s were obtained for the planar resistive structures, while the MIM structures showed a response of the order of ∼32, a response time ∼2.7 s and a recovery time of ∼16 s. The sensors were studied in the presence of 1% methane in nitrogen and in synthetic air in separate experiments. The performance was somewhat reduced in synthetic air for both the sensor structures while maintaining the optimum operating temperature the same. Both the sensors were stable in 1% methane in nitrogen as well as in 1% methane in synthetic air.

Pd–Ag/ZnO/Zn and Rh/ZnO/Zn MIM (metal–insulator–metal) gas sensors were fabricated using nanoporous ZnO thin films, obtained by an electrochemical deposition method in the absence and presence of UV light. A high-purity Zn anode, a Pt cathode, a calomel reference electrode and a 0.3-M oxalic acid electrolyte were used for deposition. Pd–Ag (26%) and Rh were used separately as the catalytic metal electrodes to fabricate the two different types of MIM configurations. A gas response of the order of 3.85±2, a response time of 5±0.5s and a recovery time of 16±0.5s were obtained with the Pd–Ag contact, while the Rh contact showed a response of the order of 4.82±2, a response time of 24±0.5s and a recovery time of 72±0.5s, at the optimum temperature of 220°C, which is the lowest temperature so far reported for metal oxide sensors to sense 1% methane in a N2 carrier gas. The undoped zinc oxide thin films grown by UV-assisted electrochemical anodization of high-purity Zn demonstrated a better performance for methane sensing. The experiments were repeated in synthetic air and a somewhat reduced performance was observed. The selectivity in the presence of hydrogen and the stability of the sensors were studied.

Detection and estimation of methane gas for continuous environmental monitoring is of growing importance for prevention of explosions inside coalmines and asphyxiations in cramped localities and green house warning. For continuous monitoring in field locations, low power and sometimes low temperature operation of methane sensors are essential. This could be achieved by employing MEMS structure for low power operations and nanocrystalline Zinc oxide for low temperature activation. The present communication reports the design, fabrication, characterization and laboratory testing of MEMS based nano ZnO methane sensors. Power consumption range 100-150 mW and temperature range of 150-200degC have been achieved with moderately high (Gt50%) sensitivity and response time (les15 sec). Further improvement is expected through optimization.

Undoped nanocrystalline n-ZnO thin films were deposited by a novel galvanic technique at room temperature on p-Si 〈100 〉 substrates to fabricate ZnO–Si heterojunctions. The I–V characteristics were studied at different temperatures with two different metallic contacts e.g., gold and palladium–silver (26%), in air and in presence of different concentrations (0.1, 0.5 and 1%) of methane gas. A shift in I–V characteristics in presence of methane was observed. The sensitivity and response time were studied at different temperatures (30 through 350 °C). Pd–Ag (26%) catalytic contacts showed much improved sensor performance.