Mahaveer K. Jain

Synthesis of Cu2O from CuO thin films: Optical and electrical properties

Hole conducting, optically transparent Cu2O thin films on glass substrates have been synthesized by vacuum annealing (5×10−6 mbar at 700 K for 1 hour) of magnetron sputtered (at 300 K) CuO thin films. The Cu2O thin films are p-type and show enhanced properties: grain size (54.7 nm), optical transmission 72% (at 600 nm) and Hall mobility 51 cm2/Vs. The bulk and surface Valence band spectra of Cu2O and CuO thin films are studied by temperature dependent Hall effect and Ultra violet photo electron Spectroscopy (UPS). CuO thin films show a significant band bending downwards (due to higher hole concentration) than Cu2O thin films.

Growth of InN thin films by modified activated reactive evaporation

Indium nitride films have been grown using modified activated reactive evaporation (MARE). The films were grown on glass and silicon substrates at room temperatures, i.e. without any intentional substrate heating. In this technique, the substrates were kept on the cathode instead of the grounded electrode and hence subjected to low energy nitrogen ion bombardment leading to highly c-axis oriented films. The photoluminescence (PL) and Raman spectrum shows significant improvement in the quality of the films compared with conventional activated reactive evaporation. The band gap measured from the room temperature PL was found to be 1.9 eV. Very high growth rates can be achieved in the MARE growth technique. The modification in the activated reactive evaporation technique may have a large impact on the growth of various compounds such as metal oxides.

Electron-electron interactions based metal-insulator transition in Ga doped ZnO thin films

We report on the charge carrier transport mechanisms of undoped and Ga doped (2 wt. % and 4 wt. %) ZnO thin films grown by pulsed dc magnetron sputtering technique. Temperature dependent resistivity measurements showed typical semiconducting behaviour for undoped ZnO thin films where as Ga doped ZnO thin films showed metallic nature at higher temperatures and insulating nature with a metal to insulator transition at lower temperatures. The observed transition temperatures are 91 K and 140 K for 2 wt. % and 4 wt. % Ga doped ZnO films respectively. The observed metal insulator transition is attributed to the electron-electron interactions at low temperatures. The variations in the transition temperatures are explained based on the disorderness induced in the system due to the doping effect. ZnO doped with 4 wt. % Ga showed the lowest resistivity of 5.7 × 10−4 Ω cm with a carrier concentration of 1.2 × 1021/cm3. Undoped and doped ZnO thin films are about 90% transparent in the visible region. Blue shift is observed in the absorption edge with the effect of doping and it is explained based on B-M shift. The Fermi level measured from valance band spectroscopy showed a shift of +0.6 eV for 2 wt. % Ga doped ZnO thin film and +0.7 eV for 4 wt. % Ga doped ZnO thin film compared to the Fermi edge of undoped ZnO thin films. This ascertains the movement of Fermi level in to the conduction band with the effect of doping.

Effect of polyethylene glycol additive in sol on the humidity sensing properties of a TiO2 thin film

We describe the effect of polyethylene glycol (PEG) addition to the precursor solution on the humidity sensing properties of a TiO2 thin film. Thin films of TiO2 were prepared by sol–gel and spin coating techniques. An increase in humidity sensitivity has been observed after the addition of PEG. This increase in sensitivity is due to smaller crystallites and pores created during the combustion of the polymer. The response and recovery time of the sensor were about 10 and 176 s, respectively. An equivalent circuit has been proposed and fitted well with the experimental data. We demonstrate that a controlled amount of polymer addition leads to the sensor being highly sensitive in the lower humidity (<40% RH) region.

Carrier transport in InxGa1−xN thin films grown by modified activated reactive evaporation

In the present work, we report the temperature dependent carrier transport properties of InxGa1−xN thin films in the entire composition range grown by modified activated reactive evaporation. The carrier transport in these degenerate semiconductors is controlled by impurity band conduction. A transition from metallic to semiconducting type resistivity was observed for indium rich films. The semiconducting behavior originates from electron–electron interaction and weak localization, whereas higher temperature scattering contributes to the metallic type resistivity. A transition of resistivity behavior from the quantum phenomena to the classical Arrhenius approach was observed for x = 0.12 film.

Structure of melt-quenched AgIn 3 Te 5

Polycrystalline AgIn 3 Te 5 synthesized by melt-quench technique has been analyzed using proton induced X-ray emission (PIXE), X-ray diffraction (XRD), and selected area electron diffraction. PIXE analysis yielded the content of Ag, In, and Te, respectively, to be 9.76%, 31.18%, and 59.05% by weight. Structure refinement was carried out considering those space groups from I- and P-type tetragonal systems which possess 4 symmetry and preserve the anion sublattice arrangement of the chalcopyrite structure (space group: I 4 2 d ) as well. The results showed that AgIn 3 Te 5 synthesized by melt-quench method crystallizes with P-type tetragonal structure (space group: P 4 2 c ; unit-cell parameters a = 6.2443(8) and c = 12.5058(4) A), the presence of which was corroborated by selected area electron diffraction studies.

Stability Studies on Nitrogen Doped p-ZnO (NZO) Thin Films Grown by Reactive Magnetron Sputtering

Nitrogen doped ZnO (NZO) thin films, at different N2 flow rates have been deposited on glass substrates by pulsed DC reactive magnetron sputtering technique. The effect of N2 flow rate (1.0 sccm - 3.0 sccm) on the structural, optical, electrical and chemical state of N has been studied. With the effect of N2 flow rate: the crystallinity of the films decreased, tensile stress is developed, optical transmittance decreased (80% to 60%), conductivity decreased till 1.5 sccm and films were n-type conducting. At 2.0 sccm and 2.5 sccm of N2 flow rates, NZO thin films showed p-type conductivity. The changes in the magnitude and type of conductivity have a direct relation with the changes observed in N-chemical state in ZnO lattice. p- NZO thin films are electrically unstable; this instability has been explained based on the changes occurred in the N chemical states, resulting from the stress release in NZO lattice.

Gate recess structure engineering using silicon-nitride-assisted process for increased breakdown voltage in pseudomorphic HEMTs

We report the fabrication of pseudomorphic high electron mobility transistors (pHEMTs) with engineered recess structure of any width of choice, by a single lithography and etching step with the help of silicon-nitride-assisted process. In this process, a silicon nitride layer is deposited prior to gate lithography. First, the silicon nitride is etched by buffered hydrofluoric acid (BHF) in the gate opening and then selective recessing is performed. The recess base width can be engineered by varying etch time of silicon nitride in BHF. The base width increases linearly with etch time as shown by SEM. We demonstrate that the top photoresist gate opening that decides the gate length is unaffected by any duration of silicon nitride etch time. Thereby, we have engineered the distance from gate edge to n+-GaAs (Lgn+) which decides the gate-to-drain breakdown voltage (BVgd). With this method, BVgd?increased from 12 to 20?V as a function of Lgn+. The electric field distribution across the recess structure has been simulated to interpret this result. Since the high BVgd?of pHEMT is essential for power applications as well as switch applications, this method can be easily adopted even though the corresponding reduction in transconductance and unit current gain cut-off frequency (ft) is only marginal from 375 to 350 mS mm?1?and from 39 to 31?GHz, respectively.

GROWTH OF INDIUM-RICH NANOCRYSTALLINE INDIUM GALLIUM NITRIDE THIN FILMS BY MODIFIED ACTIVATED REACTIVE EVAPORATION

Indium-rich InxGa1-xN thin films were prepared on glass substrates by a mixed source modified activated reactive evaporation technique. All the films exhibit hexagonal wurtzite structure preferentially oriented along the c-axis. The band gap values obtained through Urbach fitting of the absorption edge were found to be in good agreement with the values obtained from photoluminescence spectra. The decrease in band gap below 1.9 eV (i.e., for pure InN) for indium-rich films is mainly due to the compensation of Burstein–Moss shift due to gallium incorporation into the lattice which is further confirmed from the carrier concentration measurements.

Room Temperature Growth Of Nano‐crystalline InN Films On Flexible Substrates By Modified Activated Reactive Evaporation

Nano‐crystalline c‐axis orientated indium nitride (InN) thin films were prepared on amorphous polycarbonate, polyimide and glass substrates by modified activated reactive evaporation (MARE) method without any intentional heating of the substrate. The films show strong visible photoluminescence (PL) peak at ∼1.95 eV indicating the band edge transition which is in good agreement with the optical absorption. The shift in the band gap from reported value is mainly due to Burstein‐Moss shift and presence of residual oxygen. InN film grown on inexpensive flexible substrates at room temperature opens opportunity for large scale device applications like solar cells and displays.