Electrically Pumped Wavelength-tunable Random Lasing From ZnO-based Films On Si And Related Physical Mechanism

Owing to the wide band gap (～3.37 eV) and large exciton binding energy (～60 meV) at room temperature, ZnO has been considered as one of the most promising materials for ultraviolet (UV) optoelectronics. Meanwhile, ZnO has a considerably large refractive index. In the past decade, random lasing (RL) from ZnO materials has attracted extensive research interests. Subsequent to the essential investigation of the optically pumped RL from ZnO films, the electrically pumped RL actions from ZnO materials have been achieved by different strategies in recent years. It should be mentioned that the bandap of ZnO can be tuned to larger or smaller ones by alloying ZnO with MgO or CdO, respectively. Therefore, in principle, the electrically pumped RL actions from ZnO-based films MgZnO (CdZnO) films can be tuned from ultraviolet (UV) to red regimes. It is well know that silicon, as the utmost important semiconductor, is the base material for microelectronics. However, due to the intrinsic indirect bandgap, silicon is not an efficient light-emitting material. In this context, the realization of silicon-based ZnO random lasers with a wide range of emitting spectra is of significance, which integrates the merits of silicon and ZnO and, moreover, could provide light sources for certain silicon-based photoelectronic devices.In this dissertation, the electrically pumped wavelength-tunable RL from ZnO, MgZnO and CdZnO films on silicon and the related physical mechanism have been intensively investigated. Based on the fabrication of devices based on the heterojunctions and metal-insulator-semiconductor (MIS) structures in which ZnO, MgZnO or CdZnO films act as the light-emitting layers, the light-emission characteristics and the related mechanisms for these devices have been systematically addressed. In the following, the primary achievements in this work are described.(1) The electrically pumped wavelength-tunable ultraviolet RL from the MgxZn1-xO films with different bandgap energies, which act as the semiconductor components in the MIS structures fabricated on Si substrates has been achieved. When the metal (Au herein) gates of the MIS structures are applied with sufficiently high positive voltages, random lasing from the MgxZn1-xO films occurs, featuring a series of narrow spikes in the emitted spectra. Overall, the central wavelength of the random lasing spectrum is tuned from～380 to 356 nm with the increase of x value in MgxZn1-xO from 0 to 0.35. When the x value in MgxZn1-xO is 0.8, the central wavelength of the RL spectrum can be tuned to～285nm. The mechanism for the electrically pumped random lasing has been tentatively elucidated taking into account both the multiple optical scattering and the optical gain proceeding in the MgxZn1-xO films.(2) The electrically pumped simultaneous UV and visible random laser actions from ZnO-CdO interdiffused film has been demonstrated. The interdiffusion between ZnO and CdO films at 700℃forms composition-graded CdxZn1-xO alloy within the ZnO-CdO interdiffused film, which is luminescent in both UV and visible regions. A device based on SiO2/ZnO-CdO/SiO2 double-barrier structure on silicon substrate, where SiO2 acts as the barrier, is constructed for electrical pumping of the ZnO-CdO interdiffused film. As the device is applied with sufficiently high forward bias with negative voltage connecting to the silicon substrate, the UV and visible RL actions simultaneously occur. The mechanism for such electrically pumped random laser actions has been tentatively elucidated.(3) The electrically pumped wavelength-tunable blue RL from the hexagonal CdZnO films with different Cd contents, with central wavelength changing from-490 to 425 nm, has been demonstrated. The devices based on the MIS structures of Au/SiO2/CdZnO on silicon substrates are constructed for electrical pumping of the CdZnO films. The insulator layers of SiO2 onto the CdZnO films in the devices should be annealed at sufficiently low temperature such as 400℃so that the CdZnO films can be kept their integrity in terms of near-band-edge emissions.(4) The compensation between optical gain and light scattering in the electrically pumped RL from ZnO films has been clarified through investigating the electroluminescence of MgxZn1-xO/ZnO (x=0.25 and 1) heterostructured devices. As the active ZnO films are sol-gel derived, both heterostructured devices can be electrically pumped into RL at appropriate forward currents. While the active ZnO films are prepared by sputtering, only the MgO/ZnO heterostructured devices can generate RL pumped with sufficient forward currents. It is believed that the light scattering in the sputtered ZnO films is weaker than that in the sol-gel derived ones. However, the inferior light scattering capability of the sputtered ZnO films can be compensated by the MgO/ZnO-heterostructure-enabled higher optical gain in the electrically pumped RL.(5) The electrically pumped ultraviolet RL from the heterostructures formed by bi-layered MgZnO films on silicon, where Mg0.15Zn0.85O and MgxZn1-xO (x= 0.25 or above) films act as the light-emitting and barrier layers, respectively, has been achieved. While, with a barrier layer composed of a Mg0.20Zn0.80O film, only spontaneous electroluminescence occurs in the heterostructures. It has thus been proved that a large enough conduction-band offset (△Ec) is necessary for the electrically pumped RL from the MgZnO film-based heterostructures. The△Ec required herein is estimated to be～0.17 eV. The mechanism for the results as mentioned above has been tentatively elucidated.(6) The optically and electrically pumped random laser actions in the ZnO films annealed at low and high temperatures have been investigated. While the optically pumped RL threshold for the ZnO film annealed at a low temperature, which features stronger light scattering and larger optical loss, is far higher than that for the ZnO film annealed at a high temperature, the electrically pumped RL threshold currents for the two ZnO films are almost the same with the electrical pumping scheme of metal-insulator-semiconductor structure. It is suggested that if the lasing region within the ZnO film is narrow enough in the case of electrical pumping, the strong light scattering can substantially alleviate the adverse effect of large optical loss on the onset of RL.