| 其他摘要 | 硅基光电集成技术作为实现未来超高速超低损耗光互连技术的重要基础，其已成为当前信息技术发展的重要前沿研究领域，而目前实现硅基光电集成及硅基片上、片间光互连最为关键的问题是优质硅基光源的制备。在各种结构的激光器中，量子点激光器具有阈值电流密度低、特征温度高和光增益高等优点，尤其是对缺陷的敏感度低，因此研制硅基量子点激光器件在硅基光源的制备方面有着重要的意义和广阔的应用前景。本论文工作以实现高质量的硅基量子点激光器为目的，开展了对Si基、Ge基GaAs外延层和InAs自组织量子点材料的分子束外延(MBE)生长制备及材料物理性质的研究，并对硅基量子点激光器的结构设计、材料生长和器件制备工艺进行了初步研究，取得了以下主要结果：1. 采用两步生长法进行Si基GaAs材料的MBE外延生长研究，探究了其初始生长阶段的基本机制，并获得了表面光洁、缺陷较少的Si基GaAs外延层。通过研究发现：（1）Si基GaAs薄膜的生长模式为：在最初的生长阶段，在衬底表面GaAs生长呈岛状结构，而后由小岛逐渐融合成大岛并向二维平面生长转变，最终形成GaAs连续薄膜。这与在RHEED中观察到的由点状图形到线状图形的变化相一致，而图形转变时的生长厚度约为200 nm。（2）在300℃条件下生长低温成核层后的样品表面起伏小、缺陷少。温度偏低时，由于表面输运变弱，使得初始GaAs岛重叠生长，造成生长后样品表面起伏较大；温度偏高时，初始GaAs成岛密度降低、尺寸变大，使得后续融合不完全，造成表面缺陷较多。（3）在Ⅴ/Ⅲ比为14的条件下生长低温成核层后的样品表面起伏小、缺陷少。Ⅴ/Ⅲ比偏低时，初始GaAs成岛尺寸较大，造成样品表面起伏较大；Ⅴ/Ⅲ比偏高时，初始成岛时会形成分立岛，在局部没有岛的地方留下孔洞，使得后续生长不易融合，造成表面缺陷较多。（4）初始GaAs岛的融合离不开生长厚度的积累，而过厚的低温成核层将导致初始GaAs岛沿生长方向上的积累，使得在衬底平面内的融合过程不完全，造成表面粗糙度变大，其主要原因是低温下成核层中的原子迁移能力相对偏弱。（5）通过研究迁移率增强外延MEE生长方式和退火对低温成核层的影响，发现MEE生长方式对降低表面起伏起着积极作用，而高温退火会对初始成核层中的GaAs岛起到重构甚至是破坏作用，不利于后续融合过程。2. 在传统两步生长法基础上提出四步生长法，即在低温成核层和高温外延层之间加入低温缓冲层和中温缓冲层，通过此方法生长出的Si基1微米厚GaAs外延层在强光下依旧光亮如镜，表面平整（起伏在±5 nm之内）且无缺陷；通过DCXRD（ω-scan）测试出的半高宽只有210.6 arcsec，成晶质量相比于两步法有较大的提高。（1）通过对比两步法与只加入低温缓冲层生长后的样品表面形貌，发现加入低温缓冲层可以有效地减少表面缺陷并降低表面起伏。其原因在于低温缓冲层既可以保护初始成核层中的GaAs岛，又在一定程度上促进了GaAs岛的融合。（2）通过对比四步生长法与只加入低温缓冲层生长后的样品表面形貌，发现中温缓冲层进一步促进了生长模式由三维岛状向二维平面生长的转变，对提高表面平整度起着重要作用。3. 在四步法制备的高质量Si基GaAs缓冲层基础上，生长了InGaAs/GaAs位错阻挡层。对In组分、各层的厚度和周期数等结构参数进行了设计，以控制材料中应力的积累，实现了对GaAs/Si界面形成的大部分穿透型失配位错的有效阻挡。4. 在优化的Si基GaAs缓冲层、InGaAs/GaAs位错阻挡层基础上，设计并生长了硅基InAs/GaAs量子点激光器材料，研究了材料的结构和光学特性；初步探索了硅基量子点激光器的制备工艺，并制备出原型器件。其中，激光器有源区的量子点密度约为2.1×1010 cm-2，室温光致发光谱(PL)波长约为1.22 μm，实现了室温电致发光波长约为1.26 μm的激光器件。5. 为实现Ge/Si虚拟衬底上GaAs外延层及量子点激光器材料的高质量生长，对Ge基GaAs材料的MBE生长工艺进行了研究，采用三步生长法，通过降低初始生长温度，抑制界面互扩散，制备出表面平整且无反相畴错的1微米厚Ge基GaAs外延层。并在此基础上上生长了InAs自组织量子点材料，量子点的面密度约为4.5×1010 cm-2，室温光致发光谱（PL）波长1.30 μm。; The optoelectronic integration technique based on Si has become one of the most important research frontiers in the development of modern information technology in order to realize ultra-high speed and ultra-low loss optical interconnection, and the most crucial problem is how to successfully fabricate light emitting materials and laser devices based on Si. Among various kinds of lasers, quantum dot (QD) lasers have such advantages as low threshold current density, insensitive to defects, high characteristic temperature, high optical gain, and so on. Therefore, there will be important significance and broad application prospects to develop QD epitaxial materials and laser devices on Si. Aiming at fabricating QD lasers on Si, in this dissertation, we concentrate on the epitaxy of GaAs films and self-assembly InAs QDs on Si and Ge by MBE. And the physical properties of these materials are also studied. In addition, the structural design and the fabrication of QD lasers on Si have also been studied preliminarily. The related work and results are summarized as follows: 1. The epitaxy of GaAs films on Si by MBE was studied using two-step growth method in order to explore the optimized conditions for mirror-like GaAs/Si surface with less defects. And the basic mechanism of the initial growth was also studied. (1) It was found that GaAs islands were formed at the initial growth stage and then gradually coalesced to a planar GaAs/Si surface, which was corresponding to the image transformation from spotty pattern to streaky pattern observed by RHEED. And the critical thickness for transformation was about 200 nm. (2) The surface morphology of GaAs films on Si with nucleation layer grown at different temperatures were investigated. It was found that the most optimized temperature for the growth of nucleation layer was 300℃. When the temperature was lower, initial GaAs islands would be overlapped resulting in larger surface undulation and when it was higher, the density of initial GaAs islands would be lower resulting in surface defects due to the incomplete coalescence process. (3) The surface morphology of GaAs films on Si with nucleation layer grown at different Ⅴ/Ⅲ ratios were investigated. It was found that the most optimized Ⅴ/Ⅲ ratio for the growth of nucleation layer was 14. When the value was lower, bigger initial GaAs islands would be formed resulting in larger surface undulation and when it was higher, the surface defects would be more due to the low-density initial GaAs islands. (4) The initial GaAs islands coalesced with the increase of thickness. But if the nucleation layer was too thick, the surface RMS value would be higher, because the migration of Ga atoms was inactive at low temperature which would result in large surface undulation. (5) Growing the nucleation layer by MEE method was helpful to reduce the value of surface undulation. And annealing the nucleation layer would rearrange or even destroy the initial GaAs islands. 2. A new method named as four-step growth was proposed by successively inserting a low temperature (LT) and an intermediate temperature (IT) GaAs buffer layer. A 1 μm thick GaAs epilayer on Si with mirror-like surface, less defects and reduced roughness was obtained while the full width at half maximum (FWHM) value of the GaAs (004) peak from double crystal x-ray diffraction ω-scan was just 210.6 arcsec. (1) The surface morphology of GaAs films on Si with inserting the LT buffer layer or not were investigated. It was found that the LT buffer could effectively reduce surface defects and roughness due to its protection for the initial GaAs islands. (2) The surface morphology of GaAs films on Si with inserting the IT buffer layer or not were investigated. It was found that the IT buffer layer played an important role in promoting the growth transformation from 3D to 2D mode further. 3. Based on the high-quality GaAs epilayer on Si grown by four-step method, the dislocation blocking layer was grown. Through the reasonable design of growth parameters, the threading dislocations originated from the GaAs/Si interface were effectively blocked.4. Based on the optimized GaAs buffer and dislocation blocking layer, the structure of a QD laser on Si was desgined and grown by MBE in one run. The density and wavelength at room temperature of these QDs were 2.1×1010 cm-2 and 1.22 μm, respectively. In addition, the fabrication of QD laser devices on Si was studied preliminarily. The EL light emitting wavelength at about 1.26 μm was realized.5. In order to grow high-quality GaAs buffer and QD laser materials on Ge/Si vitual substrates, the growth of GaAs epilayer on Ge was studied. Through optimizing the growth temperature and Ⅴ/Ⅲ ratio, a 1 μm thick GaAs epilayer on Ge with mirror-like surface, less defects and reduced roughness was obtained. On this basis, self-assembly InAs QDs with a density of 4.5×1010 cm-2 and PL light wavelength of 1.30 μm were also grown on Ge successfully. |
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