Silicon-based Rare-earth Doped Oxide Semiconductor Thin Film Electroluminescent Devices

It is well known that the low luminescence efficiency of silicon (Si) due to its indirect bandgap in nature hinders the development of Si-based optoelectronic integration. In the past two decades, a variety of stratergies have been presented to achieve Si-based light-emitting devices (LEDs). The electroluminescence (EL) from Si-based rare-earth (RE) doped semiconductor devices has received intensive attention. The RE-related luminescences feature narrow linewidths, wide spectral range and weak influence from external environment. Oxide semiconductors have the advantages of high RE solubility and the natural oxygen inclusion which is favorable for optical transition in RE3+ions. Moveover, the preparative processes for oxide semiconductor film are compatible with Si integrated circuit manufacturation. In this context, if the EL from Si-based RE-doped oxide semiconductor thin film devices is achieved, it will offer a new strategy to develop Si-based optoelectronic devices. In this dissertation, the EL performances and related physical mechanisms for Si-based LEDs based on RE-doped ZnO or TiO2films have been intensively investigated. The primary achievements are described as follows.(1) The LEDs based on the ZnO:Er/p+-Si heterostructures have been prepared, in which the Er-doped ZnO films are deposited by radio frequency (RF) sputtering. At a bias voltage not less than6V, the Er-related～1.54μm EL can be enabled, together with the ultraviolet emission arisen from near-band-edge recombination and visible emissions related to defects in the ZnO host. The～1.54μm EL from the ZnO:Er(0.9%)/p+-Si heterostructured device is stronger than the device using ZnO:Er(1.7%) film, showing the Er-concentration quenching effect. The Er-related～1.54μm EL is triggered by transfer of the energy released from the defect-assisted indirect recombination in the ZnO host to the incorporated Er3+ions.(2) The LEDs based on the MgxZn1-xO/ZnO:RE heterostructures on Si substrates have been achieved by RF sputtering. Remarkable red, green and blue EL are realized from the devices using the ZnO:Eu, ZnO:Er and ZnO:Tm films as the light-emitting layers, respectively. The threshold voltages are～5V. Moreover, the infrared (IR)(～0.91μm,～1.09μm and～1.54μm) EL can be achieved by using the ZnO:Nd and ZnO:Er films, respectively. The holes injected into the MgxZn1-xO barrier layer are accelerated by the electric field thus becoming the’hot holes’, then they enter into the ZnO:RE film to directly impact-excite the RE3+ions, leading to the characteristic emissions. The strategy of spatially separating the hot-carrier generation and impact excitation of RE3+ions is viable to develop low-voltage driven EL devices based on the RE-doped semiconductor films.(3) The LEDs based on the TiO2:Er/p+-Si heterostructures have been prepared, in which the Er-doped TiO2films are deposited by RF sputtering. The Er-related visible (-522,553,564and663nm) and IR (～1.54μm) EL can be enabled at a bias voltage not less than5.5V. The existence of sufficient oxygen vacancies in TiO2host is critical for the Er-related EL. The energy transfer from the recombination of the trapped-excitons related to oxygen vacancies in TiO2host to Er3+ions triggers the Er-related EL.(4) The LEDs based on the TiO2:(Al,Er)/p+-Si heterostructures have been prepared, in which the (Al,Er) co-doped TiO2films are deposited by RF sputtering. In comparison with the TiO2:Er/p+-Si heterostructured device, the TiO2:(Al,Er)/p+-Si heterostructured device features the Er-related EL with the substantially suppressed visible emissions and the enhanced～1.54μm IR emission. The Al co-doping is proved not to substantially affect the amounts of the oxygen vacancies in TiO2and the Er3+ions doped into TiO2grains. That is, the quantities of both the sensitizers and activators in the energy transfer from TiO2to Er3+ions are almost consistent. Thus the above-mentioned engineering of Er-related EL is tentatively ascribed to the modification of the crystal field around Er3+ions by the Al co-doping in TiO2.(5) The LEDs based on the TiO2:Nd/p+-Si heterostructures have been demonstrated, in which the Nd-doped TiO2film are deposited by RF sputtering. The Nd-related IR EL (-0.91,1.09and1.37μm) can be enabled at a bias voltage not less than5V. The device using the TiO2:Nd(1.1%) film of single anatase phase exhibits considerably weak Nd-related EL accompanied with relatively stronger visible emissions from the TiO2host itself. While, the device with the TiO2:Nd(2.0%) film in which the TiO2host coexists with anatase and rutile phases features only pronounced Nd-related EL. It is proved the Nd3+ions in rutile TiO2are not luminescent, quite other than those in anatase TiO2. The anatase/rutile interface states are believed to be the effective mediators in the energy transfer from anatase TiO2to Nd3+ions, leading to the pronounced Nd-related IR EL.