Punam Murkute

Effect of time varying phosphorus implantation on optoelectronics properties of RF sputtered ZnO thin-films

ZnO has potential application in the field of short wavelength devices like LED’s, laser diodes, UV detectors etc, because of its wide band gap (3.34 eV) and high exciton binding energy (60 meV). ZnO possess N-type conductivity due to presence of defects arising from oxygen and zinc interstitial vacancies. In order to achieve P-type or intrinsic carrier concentration an implantation study is preferred. In this report, we have varied phosphorous implantation time and studied its effect on optical as well structural properties of RF sputtered ZnO thin-films. Implantation was carried out using Plasma Immersion ion implantation technique for 10 and 20 s. These films were further annealed at 900°C for 10 s in oxygen ambient to activate phosphorous dopants. Low temperature photoluminescence (PL) spectra measured two distinct peaks at 3.32 and 3.199 eV for 20 s implanted sample annealed at 900°C. Temperature dependent PL measurement shows slightly blue shift in peak position from 18 K to 300 K. 3.199 eV peak can be attributed to donoracceptor pair (DAP) emission and 3.32 eV peak corresponds to conduction-band-to-acceptor (eA0) transition. High resolution x-ray diffraction revels dominant (002) peak from all samples. Increasing implantation time resulted in low peak intensity suggesting a formation of implantation related defects. Compression in C-axis with implantation time indicates incorporation of phosphorus in the formed film. Improvement in surface quality was observed from 20 s implanted sample which annealed at 900°C.

Temperature-dependent phosphorous dopant activation in ZnO thin film deposited using plasma immersion ion implantation

High band gap (3.34 eV) and large exciton binding energy (60 meV) at room temperature facilitates ZnO as a useful candidate for optoelectronics devices. Presence of zinc interstitial and oxygen vacancies results in n-type ZnO film. Phosphorus implantation was carried out using plasma immersion ion implantation technique (2kV, 900W) for constant duration (50 s) on RF sputtered ZnO thin films (Sample A). For dopant activation, sample A was subjected to Rapid Thermal Annealing (RTA) at 700, 800, 900 and 1000°C for 10 s in Oxygen ambient (Sample B, C, D, E). Low temperature (18 K) photoluminescence measurement demonstrated strong donor bound exciton peak for sample A. Dominant donor to acceptor pair peak (DAP) was observed for sample D at around 3.22 eV with linewidth of 131.3 meV. High resolution x-ray diffraction measurement demonstrated (001) and (002) peaks for sample A. (002) peak with high intensity was observed from all annealed samples. Incorporation of phosphorus in ZnO films leads to peak shift towards higher 2θ angle indicate tensile strain in implanted samples. Scanning electron microscopy images reveals improvement in grain size distribution along with reduction of implantation related defects. Raman spectra measured A1(LO) peak at around 576 cm-1 for sample A. Low intensity E2 (high) peak was observed for sample D indicating formation of (PZn+2VZn) complexes. From room temperature Hall measurement, sample D measured 1.17 x 1018 cm -3 carrier concentration with low resistivity of 0.464 Ω.

Improvement in grain size and crystallinity of sputtered ZnO thin film with optimized annealing ambient

ZnO is gaining substantial interest day by day because of its wide bandgap (3.4 eV) and large exciton binding energy (60 meV) due to which lasing emission is possible from ZnO based materials even above room temperature. Here we are reporting the influence of growth temperature and annealing ambient on photoluminescence properties, crystalline size and surface morphology of ZnO thin films deposited on Si substrates at 200°C by RF sputtering. Achieved thickness is 198 nm as confirmed by Profilometer. Grown samples were further rapid thermal annealed at 800°C in Ar, N2, O2, and in vacuum ambient. The as-grown sample did not exhibit any near band edge emission peak due to presence of deep level defects. Low temperature (18 K) photoluminescence spectra exhibited strong emission peak around 3.32 eV when the as-grown sample was annealed at 800° C in oxygen ambient which indicates defects state passivation. A lowest full width half maximum (FWHM) of 73.85meV was achieved for sample annealed in O2 ambient .Sample annealed in vacuum showed peak with highest intensity at 3.25eV, which corresponds to donor-bound-acceptor (DAP). High resolution Xray diffraction measurement exhibited a dominant <002> peak. Atomic Force Microscopy also revealed surface roughness of 7.72 nm for sample annealed in O2 ambient.

Enhancement of photoluminescence in RF sputtered ZnMgO thin films by optimizing annealing temperature

Zn 0.85Mg 0.15O a promising material for the future in the area of the optoelectronic devices due to the flexibility of changing bandgap. The impact of thermal annealing on Zn 0.85Mg 0.15O thin films grown by RF sputtering on intrinsic Si substrate by RF sputtering at constant temperature 400°C. During deposition gas flow 80% Argon and 20% oxygen was used. The samples were rapid thermal annealed at 900°C (20 sec) and 950°C for 20 and 30 sec to yield samples A, B and C, respectively. Low temperature photoluminescence (PL) measurements show presence of violet emission around 3.1 eV in as-grown sample due to the presence of zinc interstitial defects. Near-band-edge emission was found at around 3.65 eV for sample A. However, for sample B this peak was redshifted and found around 3.63 eV but with much higher intensity. Further increase on annealing time (30 sec) sample was further red-shifted (sample C). On comparing with sample a, sample B showed 3 times enhancement in PL intensity and 30 times enhancement compared to as grown sample. X-ray diffraction measurements confirmed the growth of highly c-axis oriented <002> Zn 0.85Mg 0.15O thin films for all samples. Uniform lattice constant (a= 0.29 and c= 0.51 nm) was achieved for all annealed samples. The <002> peak for all annealed samples shows higher intensity in comparison with the as-grown. A slight shift in the peak was observed which is due to presence of strain. For sample B surface roughness were measured 6.34nm.

Enhancement in optical and structural properties of Zn0.85Mg0.15O nanorods over thin films synthesized by hydrothermal chemical treatment

We are reporting an enhancement in optical properties by changing the structure of Zn0.85Mg0.15O thin films through formation of crystalline hexagonal nanorods. Zn0.85Mg0.15O thin films were deposited on Si (100) substrate using dielectric sputter followed by annealing in oxygen ambient at temperatures of 700, 800 and 900° C for 10 seconds to reduce oxygen vacancies defects. Deposited thin film annealed at 900 °C (sample A) measured highest peak intensity and it was subjected to controlled the hydrothermal bath conditioning for forming hexagonal nanorods. Four samples were dipped in 2 different solutions with variable molar ratio of zinc nitrate hexahydrate and hexamethylentetramine for 2 and 3 hours, respectively. Samples processed in solution 1 (1:1) ratio for 2 and 3 hours were named B and C and those in solution 2 (2:1) were D and E, respectively. Photoluminescence measurement at 18K demonstrates exciton near-band-edge (NBE) emission peak at 3.61eV from Zn0.85Mg0.15O sample A whereas other samples exhibited slight blue shift along with bimodal peaks. The other peak observed at lower energy 3.43eV corresponds to transitions due to presence of ZnO phase in Zn0.85Mg0.15O. All samples compared to sample A exhibited more than 10 times increment in peak intensities with sample B producing the highest (~ 20 times). Nanorods formation was confirmed using crosssectional SEM imaging. X-ray diffraction measurements revealed that all Zn0.85Mg0.15O samples had (002) preferred crystal orientation with peak position at 34.7°. All nanorods samples measured lower reflectance compared to sample A, indicating high absorption in nanorods due to high scattering of light at the nanorods surface.

Influence of oxygen partial pressure on optical and structural properties of RF sputtered ZnO thin films

In this paper we report a detailed investigation of ZnO thin film properties deposited on Si<100> substrate at 400°C using RF sputtering. To reduce oxygen induced vacancies and interstitial defects in samples, variable oxygen flow rate during deposition followed by post growth annealing in oxygen ambient were carried out. Four samples were deposited under constant temperature condition but with variable oxygen partial pressure of 0%, 20%, 50% and 80% in Argon and Oxygen mixture, namely sample S1, S2 , S3 and S4 respectively. Deposited films were further annealed at 700, 800, 900 and 1000°C in oxygen ambient for 10s. Photoluminescence (PL) measurements carried at low temperature (18K) demonstrated near band edge emission peak of ZnO at 3.37eV. Increment in PL intensity was observed with increasing annealing temperature and a particular sample S4 annealed at 900 measured narrowest full width half maxima (FWHM) of ~0.1272eV. Defects peaks observed at lower energies were suppressed with increasing oxygen flow and post growth annealing, indicating improvement in film quality. From HRXRD measurement it was observed S4 sample annealed at 900°C has the highest peak intensity and narrowest FWHM compared to other samples, demonstrating the best crystalline property of annealed film at 900°C. Highest XRD peak intensity measured at 34.53° corresponds to (002) crystal orientation reveals that the films were highly caxis oriented. AFM results show increase in grain size with increasing oxygen flow and annealing temperature which ensures improvement in morphological properties of the film.