【作者】 田野；

【导师】 马向阳； 杨德仁；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2012， 博士

【摘要】 ZnO由于具有3.37eV的宽直接带隙,60meV的高激子束缚能等优点,被认为是优异的紫外发光材料。同时,ZnO还具有较高的折射率。因此,近十年来ZnO薄膜的随机激射引起了人们巨大的研究热情。在ZnO薄膜的光抽运随机激射得到充分研究的基础上,近年来人们以多种方式实现了ZnO薄膜的电抽运随机激射。需要指出的是,ZnO与MgO形成的合金半导体MgZnO的禁带宽度比ZnO的更大；而ZnO与CdO形成的合金半导体CdZnO的禁带宽度比ZnO的更小。因此,若能实现MgZnO和CdZnO薄膜的电抽运随机激射,则可使ZnO系薄膜的电抽运随机激射的波长覆盖紫外(甚至深紫外)至可见光区。众所周知,Si作为最重要的半导体被广泛用于微电子器件的制造,然而由于间接带隙的限制,Si不能作为发光器件的活性层。因此,如果能实现Si基ZnO系薄膜的电抽运随机激射,则不仅可发挥ZnO和Si各自的优势,而且还有可能为某些硅基光电子器件提供所需的光源。本文详细地研究了硅基ZnO、MgZnO和CdZnO薄膜的波长可调的电抽运随机激射及其物理机制。在制备以硅基ZnO、MgZnO和CdZnO薄膜为发光层的异质结或金属-绝缘体-半导体(MIS)器件的基础上,对这些器件的发光特性及其发光机制进行了研究,取得如下有创新意义的结果：(1)利用溶胶-凝胶法在硅衬底上制备了MgxZn1-xO (x=0,0.05,0.15,0.2,0.25, 0.35,0.8)薄膜,并以它们为半导体层形成MIS器件。在足够高的正向偏压(硅衬底接负压)下,器件均可产生来自于MgxZn1-xO薄膜的紫外随机激射。当Mg的组分在x=0～0.35间变化时,随着MgxZn1-xO薄膜中Mg含量的增高,随机激射的中心波长从～380nm逐渐蓝移到～356nm；而当Mg的组分增至x=0.8时,器件的随机激射的中心波长移至～285nm。从MgxZn1-xO薄膜的光增益和光多重散射的角度,阐明了上述电抽运随机激射现象的物理机制。(2)利用射频溅射法在硅衬底上制备了ZnO/CdO双层膜结构,通过高温热处理,使双层膜的组分发生互扩散,从而形成Cd和Zn组分渐变的CdxZn1-xO薄膜。利用此薄膜作为发光层,制备了SiO2/ZnO-CdO/SiO2双势垒结构器件,实现了紫外光和可见光并存的电抽运随机激射。研究表明,其紫外及可见光区的随机激射分别来源于SiO2/ZnO界面处附近的ZnO薄膜和其以下的组分渐变的CdxZn1-xO薄膜。(3)利用射频溅射法制备了Cd组分较高的CdZnO薄膜。通过对薄膜进行Ar气氛下不同温度的快速热处理,实现了CdZnO薄膜在可见光区的发光波长从-490nm到～425nm的可调。在硅衬底上制备了以上述薄膜为半导体层的MIS器件。在足够大的正向偏压下,器件在可见光区均产生了随机激射。随着CdZnO薄膜的快速热处理温度的升高,器件的随机激射中心波长从～490nm蓝移～430nm。研究表明,在MIS器件制备过程中,作为绝缘层的SiO2薄膜必须在足够低的温度下制备,才有可能保持CdZnO薄膜的发光性能不发生变化。(4)在硅衬底上制备了基于MgxZn1-xO/ZnO(x=0.25或1)异质结的器件,其中ZnO薄膜作为发光层,分别由溶胶-凝胶法和磁控溅射法制备;MgxZn1-xO薄膜作为势垒层。溶胶-凝胶法制备的ZnO薄膜晶粒取向更多且较为疏松,因而具有更强的光散射能力。当异质结中的ZnO薄膜由溶胶-凝胶法制备时,两种异质结器件在足够大的正向电流下都可产生随机激射；而当异质结中的ZnO薄膜由溅射法制备时,仅有MgO/ZnO异质结器件才在合适的电流下产生随机激射。分析表明：对于ZnO薄膜的电抽运随机激射,其光增益和光散射能力间存在补偿关系。即：光散射能力较强时,随机激射可在光增益较小的情况下产生。(5)在重掺N型硅衬底上制备了以Mg0.15Zn0.85O薄膜作为发光层,MgxZn1-xO (x=0.2,0.25,0.35)薄膜作为势垒层的异质结。在足够大的正向电流下,当势垒层为Mg0.2Zn0.8O薄膜时,异质结的电致发光表现为从可见到紫外光区的自发辐射；而当势垒层为Mg0.25Zn0.75O或Mg0.35Zn0.65O薄膜时,异质结在紫外光区产生随机激射,同时在可见光区产生自发辐射。分析表明,足够大的电子势垒是上述异质结产生随机激射的必要条件。对发光层为Mg0.15Zn0.85O薄膜的异质结来说,产生电抽运随机激射所需的电子势垒较小,大约为0.17eV。(6)研究了不同的ZnO薄膜的光抽运和电抽运随机激射在阈值上的差异。对溅射法制备的ZnO薄膜分别做不同温度的热处理,并在硅衬底上制备以此热处理后的ZnO薄膜作为半导体层的MIS器件。经高温处理后的ZnO薄膜具有较大的光增益,而经低温处理后的ZnO薄膜具有较强的光散射能力。对光抽运随机激射而言,经过高温热处理的ZnO薄膜产生随机激射的阈值较低。而对电抽运随机激射而言,基于两种ZnO薄膜的MIS器件产生随机激射的阈值几乎相同。从光抽运和电抽运两种情况下的载流子分布情况以及对光多重散射情况的分析出发,对此两种情况下激射阈值的差异进行了解释。更多还原

【Abstract】 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.更多还原