【作者】 陈培良；

【导师】 杨德仁； 马向阳；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2008， 博士

【摘要】 近年来,Ⅱ-Ⅵ族半导体ZnO由于具有3.37eV的宽直接带隙、60meV的高激子束缚能、抗辐射能力强、湿化学腐蚀容易和制备工艺相对简单等优点,引起了人们巨大的研究热情。基于ZnO的发光器件是目前的研究热点,人们期望用ZnO取代GaN作为短波长发光器件的基础材料。然而,ZnO的p型掺杂还没有被彻底解决,因此在实现同质p-n结型的ZnO发光二级管和激光器方面还面临着相当大的困难。为了避开ZnO的p型掺杂的困难,人们在尝试利用异质结和金属—绝缘体—半导体（Metal-insulator-semiconductor,MIS）这两类器件来实现ZnO的电致紫外发光。众所周知,硅是最重要的半导体,是集成电路的基础材料。然而,硅是间接带隙半导体,不能直接用于制备发光器件。因此,将ZnO和Si的各自的优势相结合,形成硅基ZnO发光器件,不仅在理论上有重要的探索意义,而且在硅基光电子领域有良好的应用前景。本文详细地研究了硅基ZnO（MgZnO）薄膜材料及发光器件,在制备ZnO（MgZnO）/Si异质结、硅基ZnO（MgZnO）薄膜MIS器件的基础上,对这两类器件的发光特性及其物理机制进行了系统的研究,取得如下有创新意义的结果:（1）用重掺和轻掺的n型和p型（即n⁺、n^-、p⁺和p^-）硅片,分别与ZnO和MgZnO薄膜形成ZnO/Si和MgZnO/Si异质结,实现了它们的室温电致发光。研究发现:ZnO（MgZnO）/n⁺-Si异质结在正向偏压下,产生源于ZnO（MgZnO）近带边辐射的紫外发光和与缺陷相关的可见发光,而在反向偏压下只有可见发光;ZnO（MgZnO）/n^--Si异质结只有在正向偏压下可以产生紫外发光和可见发光,而在反向偏压下几乎不发光;利用隧穿效应实现了ZnO（MgZnO）/p⁺-Si异质结在反向偏压下的紫外和可见发光,而在正向偏压下只产生可见发光;ZnO（MgZnO）/p^--Si异质结无论在正向偏压还是反向偏压下都不能产生发光。从这些异质结的电流—电压特性和它们在不同偏压下的能带图出发,阐明了上述电致发光现象的物理机制。（2）利用以ZnO（MgZnO）薄膜为半导体层、以SiO_x（x≤2）为绝缘层的硅基MIS器件,在正向偏压下获得了源于ZnO（MgZnO）薄膜的较纯的紫外电致发光,并且通过改变MgZnO薄膜中Mg的含量,实现了紫外发光峰的蓝移。明确指出:适当厚度的绝缘层对于实现较纯的紫外发光至关重要,载流子在SiO_x/ZnO（MgZnO）界面处的积累是紫外发光被增强而可见发光被抑制的根本原因。（3）利用硅基ZnO多晶薄膜MIS器件,实现了室温电抽运ZnO薄膜的随机激光。当正向偏压不断增大时,ZnO薄膜的电致发光由自发辐射转变为随机激光。产生随机激光的原因是:发出的一部分光在薄膜近表面区域平面内传播时会遭到ZnO晶粒的散射,由于光散射的平均自由程小,因此经过多次散射后,一部分被散射的光可以回到起始散射点,形成随机闭环谐振腔。当正向偏压足够大时,在一些随机谐振腔中受激辐射产生的光的增益等于或超过损耗,从而产生激光。此外,还实现了电抽运的MgZnO薄膜的随机激光,所不同的是激光波长发生了蓝移。（4）发现了基于ZnO、MgZnO和Si的MIS器件在高电压下激发N₂微等离子体发光的现象。通过对MIS器件中电子输运特性的分析,指出N₂微等离子体是由于MIS器件在高电压下发射出足够高能量的电子而激发器件表面附近的空气所产生的。（5）利用硅基ZnO（MgZnO）薄膜MIS器件,研究了电场控制下的ZnO（MgZnO）薄膜的光致发光。与MIS器件在零偏压时的ZnO（MgZnO）薄膜的光致发光相比,正向偏压显著增强ZnO（MgZnO）薄膜的紫外发光而抑制其可见发光。在这种情况下,光生电子在SiO_x/ZnO（MgZnO）界面处积累,使得ZnO（MRZnO）薄膜的发光主要集中在近表面区域,并使ZnO（MgZnO）薄膜的带间辐射复合显著增强而参与发光的缺陷数目显著减少。另一方面,反向偏压对ZnO（MgZnO）薄膜的光致发光几乎没有影响。（6）利用硅基ZnO（MgZnO）薄膜MIS器件,通过电场诱导使ZnO（MgZnO）薄膜在恒定的He-Cd激光辐照下产生随机激光。研究表明,当正向电压足够大时,ZnO（MgZnO）薄膜的光致紫外发光的某些波长处出现随机激光。这是由于在较大正向电压下,光生电子在ZnO（MgZnO）薄膜近表面区的强烈积累导致该区域的光致发光增强,从而使得在该区域平面内传播的一部分光在随机谐振腔内振荡并产生出激光。更多还原

【Abstract】 In recent years,enormous research enthusiasm onⅡ-Ⅵgroup semiconductor ZnO has been greatly spurred due to that it has a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV and,moreover,it has distinct advantages over GaN, such as lower material cost,higher radiation hardness,simpler processing of device owing to amenability to chemical wet etching and ease of film deposition.Currently, great efforts have been expended on development of ZnO-based light emitting devices （LEDs）in order that ZnO can be an alternative to GaN as the semiconductor for short-wavelength optoelectronics.Unfortunately,the strategy of p-type doping for ZnO has not been substantially defined,thus leading to considerable difficulties in achieving ZnO-based light emitting and laser diodes of p-n junction.In order to avoid the difficulties in p-type doping of ZnO,the heterojunctions and metal-insulatorsemiconductor （MIS）devices have been employed in attempt to realize the ultraviolet （UV）electroluminescence（EL）from ZnO.It is well known that silicon is the most important semiconductor as the base material for integrated circuits.However,silicon cannot be applied to LEDs due to its intrinsic indirect bandgap.Evidently,the EL from ZnO and its alloy films on silicon substrate will find applications in silicon-based optoelectronics.In this dissertation,the silicon-based ZnO materials and LEDs have been intensively addressed.Based on the fabrication of ZnO（MgZnO）/Si heterojunctions and silicon-based ZnO（MgZnO）MIS devices,the light emission characteristics and related mechanisms for these two kinds of LEDs have been systematically investigated.In the following,the primary achievements in this work are described.（1）Electroluminescent ZnO/Si and MgZnO/Si heterojunctions were fabricated by deposition ZnO and MgZnO films on silicon substrates of n- and p-type with heavy-and light-doping（i.e.,n⁺,n^-,p⁺ and p^-）,respectively.The ZnO（MgZnO）/n⁺-Si heterojunction exhibited UV light characteristics of near-band-edge（NBE）emission from ZnO（MgZnO）and defect-related visible light under the forward bias but only visible light under reverse bias;while the ZnO（MgZnO）/n^--Si heterojunction only emitted relatively weak UV and visible light under the forward bias.On the other hand,the ZnO（MgZnO）/p⁺-Si heterojunction was electroluminescent in the visible region under the forward bias while in both the UV and visible regions under the reverse bias;and the ZnO（MgZnO）/p^--Si heterojunction did not exhibit detectable EL under either forward or reverse bias.Starting from the current-voltage characteristics and the energy-band diagrams under different biases for these heterojunctions,the mechanisms underlying the EL performances as mentioned above have been essentially elucidated.（2）With the silicon-based MIS devices wherein the ZnO or MgZnO film and SiO_x （x≤2）film acted as the semiconductor and insulator respectively,fairly pure UV light emission from ZnO or MgZnO film was achieved and,moreover,the blue-shift of UV light could be realized by adjustment of Mg content in MgZnO. It is definitely pointed out that an appropriately thick SiO_x film is critical for the fairly pure UV light emission,while,the confinement of carriers in the region near the SiO_x/ZnO（MgZnO）interface is the root cause for the enhancement of UV emission and the suppression of visible light.（3）With the silicon-based polycrystalline ZnO film MIS device,the electrically pumped ZnO film random lasing was well demonstrated.Along with the increase of forward bias on the MIS device,the EL of ZnO film transformed from spontaneous emission to random lasing.Furthermore,the random lasing at shorter wavelengths from MgZnO was also realized.The reason for the random lasing is as follows:the in-plane propagation of electroluminescent light within the near-surface region of ZnO film is inevitably subjected to the scattering by the ZnO grain structure,due to the short scattering mean-free path,a part of scattered light will return to the scatterer from which it has been scattered before, thus,forming closed-loop random cavities for the light.With sufficiently high forward bias on the MIS device,in some cavities,the optical gain of stimulated emission will be equal to and even larger than the optical loss,thus leading to random lasing.（4）The luminescence of N₂ microplasma was found in the case of the ZnO,MgZnO and Si based MIS device applied with high forward voltage.Through the in-depth analysis of electron transportation in the MIS device,it is pointed out that the N₂ microplasma was formed by the activation of air around the surface of MIS device by the highly energetic electrons emitted from the MIS device under the high voltage. （5）Taking advantage of the silicon-based ZnO（MgZnO）MIS device applied with different bias,the electric-field-controlled photoluminescence（PL）,i.e., electro-photoluminescence of ZnO（MgZnO）was investigated.Compared with the PL of ZnO（MgZnO）film in the case where there was no bias on the MIS structure,the positive bias with negative voltage applied on silicon substrate significantly enhanced the NBE UV emission while suppressing the defect-related visible emissions.This is due to the following reasons:under the positive bias,the photo-generated electrons accumulated in the region near the SiO_x/ZnO（MgZnO）interface,which,on one hand,made the PL of ZnO （MgZnO）proceed in the near-surface region;on the other hand,enhanced the inter-band radiative recombination and significantly reduced the amount of defects involved in PL.In contrast,the negative bias on the MIS device hardly changes the PL of ZnO film.（6）Taking advantage of the silicon-based ZnO（MgZnO）MIS device on the silicon substrate applied with appropriate forward bias,the random lasing from ZnO （MgZnO）film was induced by the electric field under a constant He-Cd laser illumination.It was found that with sufficiently high forward bias on the MIS device,random lasing occurred at certain wavelengths of photoluminescent UV light of ZnO（MgZnO）film.Such a random lasing action is due to that the strong accumulation of photo-generated electrons in the near-surface region of ZnO （MgZnO）film under the high forward bias significantly enhances the PL therein, in this case,a part of sufficiently strong photoluminescent light propagating in the plane of ZnO（MgZnO）film achieves optical gain and oscillates in the closed-loop random cavity,thus leading to random lasing.更多还原