Sanjay K. Ram

Infrared upconversion in radio frequency magnetron sputtered Er-doped zinc oxide thin films

Upconversion in radio frequency magnetron sputtered Er-doped zinc oxide thin films on Si substrate has been demonstrated using 1550 nm cw laser excitation. As-sputtered thin films did not show any upconversion emission, and annealing was required to optically activate the Er3+-ions. Emissions at 985, 809, and 665–675 nm were observed in annealed thin films, corresponding to transitions from 4I11∕2, 4I9∕2, and 4F9∕2 to the ground state 4I15∕2, respectively. The emission from 4I11∕2 was the dominant one, whereas emission from 4I9∕2 was the weakest. The highest intensity at 985 nm was obtained with 2.4 at. % of Er by annealing the film at 700 °C. Annealing at higher temperatures causes Er to diffuse and segregate to the Si-ZnO interface between the Si substrate and the ZnO film.

Influence of TiO 2 host crystallinity on Er 3+ light emission

This work presents a study of the luminescence properties of Er3+ when included into two different TiO2 hosts: a polycrystalline and an amorphous host. The two host environments were produced by depositing two thin films with different Er3+ concentration using radio-frequency magnetron sputtering. Structural analysis revealed the presence of the rutile and anatase phases in the polycrystalline film. Time-resolved and steady-state photoluminescence measurements evidenced the presence of two distinct local Er3+ environments in the polycrystalline host. For the amorphous TiO2 host, only one Er3+ environment was observed, which differed from the two environments in the polycrystalline host. A method for extracting a fast and slow time-resolved emission spectrum from the two observed local environments in the polycrystalline host is also presented.

Directly patterned TiO2 nanostructures for efficient light harvesting in thin film solar cells

A novel, scalable, and low-cost strategy for fabricating sub-wavelength scale hierarchical nanostructures by direct patterning of TiO2 nanoparticles on glass substrates is reported. Two nanostructural designs of light-trapping back-surface reflectors (BSR) have been fabricated for increasing the photon-harvesting properties of thin-film solar cells: a quasi-periodic nano-crater design and a random nano-bump design. The efficient light-scattering properties of the nano-crater design over a broad wavelength range are demonstrated by the measured haze factor being larger than 40% at wavelengths (~700 nm) near the band edge of amorphous silicon (a-Si:H). The a-Si:H-based n-i-p solar cell fabricated with an only ~200 nm thick absorber layer on the nano-crater BSR shows a short-circuit current density (J sc) of ~16.1 mA cm−2 representing a 28% enhancement compared to the cell deposited on a non-textured flat substrate. Measurements of the external quantum efficiency of the cell fabricated on the quasi-periodic nano-crater surface at long wavelengths, λ > 700 nm, demonstrate an increase of a factor of 5 relative to that of a flat reference solar cell. The theoretical modeling results of optical absorption corroborate well with the experimental findings and are used to identify the volumes of strong optical absorption in the a-Si:H active layer of the textured BSR devices.

Effect of temperature conditions on the emission of ion-induced secondary electrons from MgO films

We report that the vacuum annealing of MgO films at 225°C results in the removal of water based contamination and the secondary electron emission coefficient increases from 0.09 to 0.15. The effective secondary electron emission yield increases from 0.2 to 0.75 at 200 mbar chamber pressure, as temperature increases to 250 °C. The effective secondary electron emission yield at 200, 350 and 800 mbar pressure shows similar trend during the heating as well as cooling of the MgO sample. Thermionic emission, smaller surface band bending and the removal of impurities at high temperature are possible reasons for the increase in secondary electrons of MgO thin films.