Perpustakaan Digital ITB

Defect-induced optical and electrical properties of ZnO:Ti thin films have been studied comprehensively. Due to its wide band gap and high exciton binding energy at room temperature, ZnO is a strong candidate in optoelectronics applications particularly for fabrication of short-wavelength light-emitting devices. The high excitonic binding energy is one of the excellent features of ZnO compared to other wide band gap semiconductors such as GaN. ZnO also showed the ideal choice for high-performance ultraviolet photodetector due to the non-toxicity, inexpensive manufacturing, and especially high sensitivity in ultraviolet region. The development in ZnO-based photodetector has therefore been motivated by the enlarging of the ozone holes, resulting in the increase of received ultraviolet radiation. ZnO:Ti thin films were grown on Si substrates by DC-unbalanced magnetron sputtering (DC-UBMS) technique. Annealing treatments on the fabricated thin films were performed to further study and modify the presence of defects. Structural and morphological properties of the film were investigated using field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. From the FESEM images and EDX analysis, the ZnO:Ti thin films with Ti concentrations of 1 and 3 at.% were grown more homogeneously in comparison to the pure ZnO. Ti doping played a role to uniformly distribute ZnO elements from the sputtering target to the Si substrate. Transmittance spectra of FTIR showed a peak splitting of Zn-O stretching at ~611 cm−1 related to Ti-modified film morphology. Furthermore, the full width at half maximum (FWHM) of diffraction peaks increases upon Ti doping followed by the decrease in the peak intensity, indicating a reduction in crystalline quality. As the focus of our study, photoconductivity, defect, and excitonic properties of ZnO:Ti thin films were investigated using the combination of optical and electrical characterizations. Excitonic and defect properties of ZnO:Ti were investigated using Spectroscopic ellipsometry (SE) and photoluminescence (PL) spectroscopy. SE measurements in the photon energy from 0.5 eV to 6.5 eV were carried out to investigate the excitonic and mid-gap state transitions. The SE data, amplitude ratio and phase difference, were analyzed using a combination of Drude, Tauc-Lorentz, and Gaussian oscillators. The fitting results show that there is a new state in the ZnO band gap as called mid-gap states located in the region of 1.50-3.35 eV. Upon Ti doping of 1 at.%, the amplitude of mid-gap states decreased from 0.26 to 0.04 while the amplitude of excitonic transitions increased from 0.22 to 0.28, as compared to pure ZnO. This phenomenon is explained due to the weakening of excitonic screening effects. From PL investigation, the pure ZnO film shows both the excitonic and defect-related emission as a contribution from complex zinc and oxygen vacancies (VZn+VO). The decrease in the number of VZn states was observed upon Ti doping, resulting in the increase in the number of VO states as green emission character. Similar to the result of dielectric function analysis, the ZnO thin film with Ti concentration of 1 at.% showed a higher excitonic emission than that of the other concentrations. Temperature-dependent PL spectra show that the enhanced emission is originated from donor-bound exciton promoted by Ti dopant and native VO. This study showed the important role of defects in controlling the optical and excitonic properties of ZnO thin films for future to electronic applications. In order to investigate the photoconductivity properties, current versus voltage (IV) characteristic was measured as the function of illumination energy. ZnO:Tibased photodetectors were fabricated by using metal-semiconductor-metal planar configuration with Ag as the metal contact. Ti doping in the ZnO system reduced the dark current and enhanced a photo-to-dark-current ratio. The result showed the important role of the Ti doping on the improvement of photodetector performance. As an additional study, oxygen defect of ZnO films was tuned by plasma-assisted molecular beam epitaxy (P-MBE) technique with adjusting an oxygen plasma power. From PL measurements, the excitonic and defect-related visible emissions were clearly observed. Defect emission in the orange region (~2 eV) increased by increasing oxygen power which was originated from oxygen interstitial (Oi). The film showed high photoconductivity in ultraviolet region at lower Oi concentration. Interestingly, at higher Oi concentration, the photoconductivity was shifted from ultraviolet to orange region. We suggest that the emission ratio of ultraviolet/visible (UV/Vis) plays an important role in the photoconductivity. This result reveals the importance of defect for tuning the sensitivity of ZnO-based photodetector.