Vivek Garg

Detection of a high photoresponse at zero bias from a highly conducting ZnO:Ga based UV photodetector

Ga-doped ZnO (GZO) based ultraviolet photodetectors (PDs) were fabricated by dual ion beam sputtering with a metal–semiconductor–metal structure. The room-temperature operable PD demonstrated responsivity of 58 mA W−1 at zero bias, which is 15 times larger than that reported on similar material grown by a different physical vapour deposition process, with internal and external quantum efficiency values of ∼22.5% and 37.4%. The unbiased photodetection is attributed to the tunnelling of electrons due to heavy doping of GZO and built-in electric field due to different barriers at the two metal semiconductor contacts. The asymmetry in the electrodes was investigated by temperature-dependent current–voltage measurements.

Forming-free high-endurance Al/ZnO/Al memristor fabricated by dual ion beam sputtering

We report dual ion beam sputtering fabrication of an Al/ZnO/Al memristor displaying forming-free bipolar resistive switching characteristics with memristive behavior without necessitating any post-processing steps. A nearly amorphous ZnO thin film and an appropriate concentration of oxygen vacancies play a significant role in imparting forming-free, stable, and reliable behavior to memory cells. Besides, sufficient non-lattice oxygen ions in the film play a crucial role in the resistive switching process. The AlOx interface layer is observed to strongly affect the switching mechanism in the memory device by altering the barrier at the Al/ZnO interface. The device shows stable switching behavior for >250 cycles with good retention and stable set/reset voltages.

Plasmon generation in sputtered Ga-doped MgZnO thin films for solar cell applications

The crystalline, electrical, morphological, optical properties and plasmonic behaviour of Ga doped MgZnO (GMZO) thin films grown at different substrate temperatures (200–600 °C) by a dual ion beam sputtering (DIBS) system are investigated. Transmittance value of more than ∼94% in 400–1000 nm region is observed for all GMZO films. The particle plasmon features can be detected in the absorption coefficient spectra of GMZO grown at 500 and 600 °C in the form of a peak at ∼4.37 eV, which corresponds to a plasmon resonance peak of nanoclusters formed in GMZO. The presence of such plasmonic features is confirmed by ultraviolet photoelectron spectroscopy measurements. The values of particle plasmon resonance energy of various nanoclusters are in the range of solar spectrum, and these can easily be tuned and excited at the desirable wavelengths while optimizing the efficiency of solar cells (SCs) by simple alteration of DIBS growth temperature. These nanoclusters are extremely promising to enhance the optical sca...

Localized surface plasmon resonance on Au nanoparticles: tuning and exploitation for performance enhancement in ultrathin photovoltaics

We report a detailed correlation analysis of the size, shape, and distribution of Au nanoparticles (NPs) on fine-tuning of localized surface plasmon resonance and optical absorption cross-section. Experimental analysis of annealing temperature and initial Au layer thickness on NP parameters such as size, interparticle distance, surface coverage, and circularity factor has been studied. The effect of annealing on the morphological, structural, dielectric, and elemental behavior of Au NPs has been reported. Theoretically, we have analyzed the tuning of LSPR and absorption cross-section peaks by varying NP parameters, surrounding medium, and substrate. This report is critical in terms of predicting performance enhancement of ultrathin photovoltaics with varied cell architectures.

Band alignment and photon extraction studies of Na-doped MgZnO/Ga-doped ZnO heterojunction for light-emitter applications

Ultraviolet photoelectron spectroscopy is carried out to measure the energy discontinuity at the interface of p-type Na-doped MgZnO (NMZO)/n-type Ga-doped ZnO (GZO) heterojunction grown by dual ion beam sputtering. The offset values at valence band and conduction band of NMZO/GZO heterojunction are calculated to be 1.93 and −2.36 eV, respectively. The p-type conduction in NMZO film has been confirmed by Hall measurement and band structure. Moreover, the effect of Ar+ ion sputtering on the valence band onset values of NMZO and GZO thin films has been investigated. This asymmetric waveguide structure formed by the lower refractive index of GZO than that of NMZO indicates that easy extraction of photons generated in GZO through the NMZO layer into free space. The asymmetric waveguide structure has potential applications to produce ZnO-based light emitters with high extraction efficiency.

Optoelectronic properties of phosphorus doped p-type ZnO films grown by dual ion beam sputtering

We report highly conductive and stable p-type phosphorus doped ZnO (PZO) thin films fabricated by dual ion beam sputtering and subsequent thermal annealing. Hall measurements established that the annealed PZO films were p-type, which were also confirmed by typical diode-like rectifying current-voltage (I-V) characteristics of the p-PZO/n-Si heterojunction. The maximum hole concentration was evaluated to be 8.62 × 1019 cm−3 with a resistivity of 0.066 Ω cm and a mobility of 1.08 cm2/V s at room temperature. The stability of the p-type conduction was verified by Hall measurement performed again after one year of thin film fabrication resulting in a hole concentration of 3.77 × 1019 cm−3. Spectroscopic ellipsometry was employed to determine the complex dielectric function ( e = e 1 + i e 2 ) of p-type PZO films in the 1.2–6.4 eV energy range by a parameterized semiconductor oscillator model. Room temperature excitonic features were identified and the critical point energy was determined by second order deriv...

Investigation of dual-ion beam sputter-instigated plasmon generation in TCOs: A case study of GZO

The use of the high free-electron concentration in heavily doped semiconductor enables the realization of plasmons. We report a novel approach to generate plasmons in Ga:ZnO (GZO) thin films in the wide spectral range of ∼1.87-10.04 eV. In the grown GZO thin films, dual-ion beam sputtering (DIBS) instigated plasmon is observed because of the formation of different metallic nanoclusters are reported. Moreover, formation of the nanoclusters and generation of plasmons are verified by field emission scanning electron microscope, electron energy loss spectra obtained by ultraviolet photoelectron spectroscopy, and spectroscopic ellipsometry analysis. Moreover, the calculation of valence bulk, valence surface, and particle plasmon resonance energies are performed, and indexing of each plasmon peaks with corresponding plasmon energy peak of the different nanoclusters is carried out. Further, the use of DIBS-instigated plasmon-enhanced GZO can be a novel mean to improve the performance of photovoltaic, photodetector, and sensing devices.

Impact of sputter-instigated plasmonic features in TCO films: for ultrathin photovoltaic applications

The structural and optical properties of Ga-doped ZnO (GZO) and Ga-doped MgZnO (GMZO) individual films are analyzed. Sputter-instigated plasmonic features are observed in individual GZO and GMZO films due to the formation of metal and metal oxide nanoclusters. The plasmon generation is verified by electron energy loss spectra obtained by ultraviolet-photoelectron spectroscopy, spectroscopic ellipsometry, and field-emission scanning-electron microscopy measurements. This is promising in terms of increasing the efficiency of the solar cell by increasing the optical path length in the absorbing layer while keeping the same physical length by light scattering and trapping mechanism.

Band alignment study and plasmon generation at dual ion-beam sputtered Ga:ZnO/ Ga:MgZnO heterojunction interface

A flat band offset at 3 atomic% Ga-doped ZnO (GZO)/1 atomic% Ga-doped Mg0.05Zn0.95O (GMZO) interface is obtained with valence and conduction band offset values of -0.045 and -0.065 eV, respectively. The materials are grown by dual ion-beam sputtering (DIBS) system, and the values of band offsets at the interface are calculated by ultraviolet photoelectron spectroscopy measurement. It is observed that the band offset can be further tuned by suitable band-gap engineering by changing the elemental composition of Mg and Ga in ZnO or by altering DIBS growth parameters. Moreover, generation of plasmons in individual GZO and GMZO films due to the formation of metal and metal oxide nanoclusters are observed. This is promising in terms of increasing the efficiency of the solar cell by increasing optical path length in the absorbing layer by light scattering and trapping mechanism.