脈衝雷射沉積非極性A面氧化鋅於R面藍寶石基板之磊晶研究 - 表面形貌、應變，以及結晶性之演變

Abstract

氧化鋅(ZnO)為寬直接能隙半導體，並且具有高達60meV的激子束縛能，為極具潛力的固態光源材料之一。然而，由於其纖鋅礦結構存在的自發極化場，致使一般沿著C面成長的磊晶薄膜會因為量子侷限史塔克效應(Quantum Confined Stark Effect, QCSE)而影響其光學性質，為了解決此QCSE效應，各種沿著非極性面磊晶成長的研究因而於近十年來備受重視。 本研究以脈衝雷射蒸鍍法(pulsed laser deposition, PLD)於R面藍寶石基板磊晶成長A面ZnO，主要探討不同成長溫度沉積之ZnO薄膜形成的過程，並以臨場反射式高能電子繞射儀(reflection high-energy electron diffraction, RHEED)、原子力顯微鏡(atomic force microscope, AFM)、高解析X光繞射儀(high-resolution X-ray diffractometry, HRXRD)以及穿透式電子顯微鏡(transmission electron microscope, TEM)對其成長演變的過程進行了完整的研究，並且明顯的觀察到溫度以及鋁摻雜對其磊晶行為的影響。不同成長模式所產生的結晶行為，包含錯配(misfit)補償機制、應力、以及界面結構，也都進行了系統性的分析。 成長溫度的影響方面，在初始成長階段，高溫沉積的ZnO呈現階梯狀表面形貌，而低溫沉積的ZnO則為島狀形貌為主；隨著厚度的增加，不論沉積溫度高低，其表面皆會演變為條紋狀表面形貌，且結晶性會隨著厚度增加而有所改善。 在晶格錯配度較大的ZnO[1-100]方向上，高溫環境下沉積的ZnO傾向以平均間距1.3-2.2 nm的a型差排補償與基板間的錯配應力，而低溫製程環境下，TEM晶格影像卻觀察到氧ZnO傾向生成平均間距為2.8-3.5 nm之錯配差排對。儘管此方向之錯配應力可藉由晶域式磊晶(domain matching epitaxy, DME)成長機制所生成的高密度錯配差排補償，但是低溫製程的ZnO仍存在著額外的殘留張應力，而高溫製程的ZnO則於表面尚未出現異向性時，存在著些微的殘留壓應力。 於晶格匹配度較小的ZnO c軸方向，不論ZnO是以高溫或低溫沉積，其晶格皆因錯配應力的影響而存在殘留壓應變。然而，相較於高溫ZnO由於較少的差排以及較完美的界面接合所呈現的完全應變狀態，低溫ZnO有部份的錯配應力藉由較多的錯配差排以及較不完美的界面接合所補償。 而在降溫過程中，可以清楚看到RHEED圖型在降溫過程中存在著明顯的變化，此現象可能與薄膜與基板間的熱錯配有關，較快的降溫速率也的確對磊晶膜的結晶特性有影響，尤其在鋁摻雜2%的ZnO磊晶膜更是顯著。

As a wide direct bandgap wurtzite semiconductor, zinc oxide (ZnO) is an attractive material for potential applications in optoelectronic devices. However, the built-in electrostatic field due to piezoelectric polarization in wurtzite structure makes the lower efficiency of carrier recombination which degrades the optical emitting properties. In order to avoid the so-called quantum confined Stark effect, nonpolar films without polarity along the growth direction is required. Hence, nonpolar ZnO film growth in a-plane and m-plane has been intensively studied in recent years. A systematic study of pulse laser deposited (PLD) a-plane ZnO grown on r-plane sapphire at different temperature has been done by using in-situ reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), high-resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM). The significant effects of growth temperature and Al-doping on the growth characteristics were observed. The misfit accommodation including strain evolution and interface structure at various growth stagehave been carefully characterized. For initial growth in PLD, ZnO grown at 750°C (HT-ZnO) shows step morphology, while ZnO grown at 450°C (LT-ZnO) exhibits island growth mode. For thick films, both HT- and LT-ZnO surfaces develop into stripe morphology. The crystallinity is shown to be improved with film thickness for both HT-ZnO and LT-ZnO. Along ZnO[1-100] of large lattice mismatch, TEM examination shows that a-type misfit dislocations are spaced in a distance of 1.3 - 2.2 nm on HT-ZnO/sapphire interface, whereas dislocation pairs in spacing of 2.8 - 3.5 nm are observed for the LT-ZnO/sapphire one. Also, tensile strains are present in LT-ZnO films in ZnO[1-100] direction, but residual compressive strains are observed for 10 nm and 50 nm HT-ZnO films. For smaller lattice mismatch along ZnO c-axis direction, reciprocal space maps of XRD show that HT-ZnO is nearly fully strained without much relaxation and has a highly coherent interface with sapphire, in contrast with partial relaxation in LT-ZnO. Finally, it is observed that the streaky RHEED patterns obtained at growth temperature may evolve to spotty RHEED patterns during cooling. It may be related with the thermal mismatch between ZnO and sapphire. High cooling rate can result in crystallinity deterioration, which is apparent for Al-doped ZnO epitaxial films.