MOCVD异质外延硅基ZnO和SiC薄膜及其特性研究

【作者】 姚然；

【导师】 傅竹西；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2007， 博士

【摘要】 SiC是一种宽禁带半导体材料，其具有优秀的电学、化学和导热性能，因此SiC器件经常被应用于高温、高电场、高辐射这些应用领域。单晶SiC薄膜生长一直是研究的重点。同质外延单晶SiC薄膜较为容易实现，但是SiC很昂贵。Si衬底具有十分优秀的性能而且比较廉价，但是异质外延SiC／Si比较困难，主要由于晶格失配和热膨胀失配等因素造成的。本论文就SiC／Si薄膜的制备展开了研究。ZnO是一种Ⅱ-Ⅵ族直接带隙的宽禁带半导体材料，其室温下的禁带宽度为3.36eV，由于其具有紫外发光的特性，近来受到研究者的广泛关注。ZnO在高效率光散发设备和其它光学方面有应用前景，所以要进行高质量的ZnO薄膜制备的研究。由于晶格失配，ZnO／Si的异质结质量不高，现在主要应用过渡层或表面处理的方法进行改善。本论文围绕ZnO薄膜的MOCVD异质外延这个课题开展了研究，通过ZnO薄膜生长条件的探索以及Si衬底上各种过渡层生长ZnO薄膜的研究，最终利用3C-SiC作为过渡层，在Si(111)衬底上实现了单晶ZnO薄膜的异质外延。主要的研究工作及结果如下：1．高质量单晶SiC薄膜的制备利用低压高温MOCVD系统，成功的在Si(111)和Si(100)基片上外延出了具有高质量的SiC薄膜。通过XRD和微区拉曼测量表明外延的SiC薄膜为3C-SiC。SiC／Si薄膜具有良好的结晶质量，SiC(111)X射线摇摆半宽仅为0.3度，这在国内报道中属于领先水平。2．RF预处理对ZnO／Si生长的影响用MOCVD设备生长ZnO／Si薄膜，除了对衬底进行常规的化学清洗以外，在生长前进行Ar RF的预处理，是氩离子对硅表面进行一定的破坏，处理能量从0～150W进行了梯度变化，再以同样的生长条件进行原位生长。对于样品我们分别作了XRD、PL、AFM测量，发现Ar-RF预处理对薄膜结晶有很大影响，未作处理的样品一般呈多晶态，而处理后的样品在一定能量范围内晶格取向有显著提高，但随预处理能量达到一定限值后取向性被破坏。预处理对于发光也有很大的影响，在一定能量范围内发光强度只随处理能量加大缓慢衰减，但在高能量状态下，发光明显减弱，峰位也随之变化可见氩离子轰击硅表面形成了缺陷，这些缺陷在生长中顺延在ZnO部分，并且这些缺陷是发光淬灭中心，随能量的增加而增加。3．缓冲层生长ZnO薄膜利用直流溅射，先在Si衬底上溅射一层ZnO多晶薄膜，通过对直流溅射时间的控制，可以得到不同厚度的ZnO缓冲层。再利用MOCVD设备生长高质量的ZnO薄膜。通过研究发现，直流溅射ZnO薄膜的厚度对于最终的薄膜质量有很大的影响。随着缓冲层的引入，双晶衍射XRD的摇摆半宽有显著下降，并且随着最终ZnO薄膜质量上升，光致发光也有显著的提升。可见缓冲层的引入对ZnO／Si薄膜的质量和发光强度有很大的贡献。另外我们试验了用SiC作为过渡层的生长方法，在Si基片上外延出高质量的ZnO薄膜。测量了样品的XRD和摇摆曲线，以及室温下的PL谱。实验结果表明，SiC过渡层的引入的确大大提高了ZnO薄膜的质量和发光性能，并实现了实现Si上制备ZnO单晶薄膜这一前沿课题。4．等离子辅助生长ZnO薄膜，实现N掺杂利用改造的等离子辅助生长设备，进行了ZnO薄膜的等离子辅助生长。X射线衍射谱表明利用此方法生长的ZnO薄膜具有良好的结晶质量。为了验证裂解氮气是否能够进行ZnO的N掺杂，对样品进行了二次离子质谱的测量，发现N在ZnO薄膜中的浓度很高，实现了高浓度的N掺杂，但是由于表面电阻率高，无法进行Hall测量，ZnO薄膜的导电类型无法进行判断。更多还原

【Abstract】 Silicon carbide is a wide gap semiconductor which exhibits outstanding electronic properties associated with chemical and thermal stability. The SiC-based devices ability to function at high temperature, high power and high radiation conditions will enable large performance enhancements to a wide variety of systems and applications. It attracted more attention for the preparation of single crystal SiC films, which makes easy for homo-epitaxy. However, the SiC crystal wafer is too expensive. Si is a powerful substrate for its low cost. But hetero-epitaxy growth of SiC film directly on Si substrate is very difficult because of the large lattice mismatch and mismatch of thermal expansion coefficients. The present paper has launched the research on the SiC/Si thin film preparation.ZnO is a II -VI compound semi-conductor material with a wide direct band gap of 3.36eV at room temperature. Recently, it has attracted great attention due to its ultraviolet luminescence. For the use of ZnO in high-efficiency light emitting devices and other optical applications, high quality single crystal ZnO films are required. But it is difficult to get high quality ZnO films on the Si substrates because of the large lattice mismatch. Using the buffer layer or substrate pretreatment is a way to get high quality ZnO films. This thesis focus on hetero-epitaxy of ZnO films by MOCVD, researching on the growth condition and using some kinds of buffer layers to grow ZnO films on Si substrates, the single crystal ZnO films on Si substrates using a 3C-SiC buffer layers. The major works and conclusions as follows:1. Single crystal SiC films were successfully prepared on Si substrates.Poly-crystal SiC films were successfully prepared on Si（111） and Si （100） substrates by low pressure metal-organic chemical vapor deposition （LP-MOCVD）. The structure of the films has been investigated by X-ray diffraction （XRD） and micro-Raman spectra. 3C-SiC films were grown on Si（111） substrates. The FWHM of SiC（111） rocking curve is only 0.3 arcdegree.2. Effect on Radio Frequency Plasma Pretreatment on ZnO/Si Thin FilmZnO films have been deposited on Si （111） substrates by low-pressure MOCVD. Si （111） substrates were pretreated by RF plasma before growth. The power of RF plasma was arranged from 0W to 110W. Then ZnO thin films were grown in-situ. The great effect of RF plasma pretreatment on the structure and luminescence properties was investigated by X-ray diffraction （XRD）, atomic force microscope （AFM） and photoluminescence （PL） spectrum at room temperature. The quality and surface roughness of ZnO thin films using pretreatument are better. The orientation of ZnO films was degraded when the pretreatment energy achieved the certain limiting value. The UV emission intensity decreased because the argon ion bombardment silicon surface has formed the crystal defect which makes quenching of luminescence in growth.3. Using buffer layers to grow high quality ZnO films on Si substratesDifferent buffer layers such as 3C-SiC, DC sputtered ZnO films have been employed, Buffer layers prepared on Si substrates by DC reactive sputtering system is reported in this paper. The thickness of buffer layer depends on the sputtering time. Then the high quality ZnO thin films were grown on the Si substrates by low pressure metal-organic chemical vapor deposition （LP-MOCVD） with sputtering buffer layers. The effect of sputtering buffer layer thickness on the structure and luminescence properties was investigated. The full width at half maximum （FWHM） of rocking curve of ZnO （002） peak was reduced by using sputtering buffer layer and annealing. And the intensity of luminescence was increased. High quality ZnO thin films were deposited on Si（111） substrates by LP-MOCVD with double connected, using SiC buffer layer. The effect of SiC buffer layer on the structure and luminescence properties has been investigated by X-ray diffraction （XRD）, photoluminescence （PL） spectra at room temperature. The full-width at half maximum （FWHM） of rocking curve of ZnO （002） reflection is reduced by using SiC buffer layer. And the intensity of luminescence of ZnO films is also increased. In addition, with the SiC buffer layer, green luminescence is appeared which was considered to be from the electron transition from conduction band bottom to the O_Zn level formed in the band gap. With the optimized condition for the SiC buffer layer and ZnO film growth, the single crystal ZnO films were successfully prepared on Si（111） substrates.4. Nitrogen doping in ZnO films by plasma-assisted MOCVDNitrogen doped ZnO films are grown by the plasma-assisted metal-organic chemical vapor deposition（MOCVD） on Si. X-ray diffraction spectra show that they are strongly c-oriented while the N-doped sample is high crystal quality. Secondary-ion mass Spectrometry （SIMS） analysis confirmed the presence of N, The N concentration is rather high, and this means that the values are very uncertain. But p-type ZnO films were not obtained.更多还原