鋁酸鑭上之半極性氧化鋅磊晶薄膜-成長與結構分析

Abstract

纖鋅礦結構之氧化鋅(ZnO)是直接寬能隙的半導體，具有優越的光電性質，然而在c方向具有之極性通常造成元件發光性質劣化，而半極性之薄膜則可以改善其發光特性。 本論文研究利用雷射脈衝沉積製程(Pulsed laser deposition, PLD)成功地在(112)鋁酸鑭(LaAlO3, LAO)緩衝層上成長出半極性(11-2-2) ZnO磊晶薄膜，並透過高解析X光繞射(High-resolution X-ray diffraction, HRXRD)、穿透式電子顯微鏡(Transmission electron microscopy, TEM)、原子力顯微鏡(Atomic force microscopy, AFM)及光激發螢光量測(Photoluminescence, PL)等觀察與分析之結果探討(11-2-2) ZnO的磊晶關係、微觀結構特徵及成長機制。其主要結果如下： 1.在(112) LSAT ((LaAlO3)0.29(Sr2AlTaO6)0.35)基板上以PLD製作出單晶品質的LAO緩衝層，LAO並沒有相變化造成的雙晶結構特徵，且其呈現陽離子的非等計量比。 2.在晶體方向上，在(112) LAO/LSAT上(11-2-2) ZnO以[1-100]ZnO // [1-10]LAO/LSAT的 關係磊晶成長，並且其極性軸[000-1]指向偏向試片表面。 3.從XRD分析，1.6 micron厚的(11-2-2) ZnO有不錯的結晶品質，如：搖擺曲線的半高寬約0.1°以及較大的側向相干長度。 4.ZnO表面特徵以沿著[1-100]ZnO方向的條紋以及隨意分佈的微小凹坑為主，此條紋巨觀上由一對晶面構成；而微小凹坑對應到貫穿式差排在試片表面終結的位置。 5.10 K低溫下的PL光譜主要發光峰為neutral donor bound exciton，其半高寬小於10 meV，而到了室溫，其主要發光為free exciton和free electron to neutral acceptor以及其縱向光聲子的輔助躍遷發光。 6.(11-2-2) ZnO/(112) LAO緩衝層界面的錯配應力釋放，在[11-23]方向上主要是以b = 1/6[11 3]的錯配差排(misfit dislocation, MD)為主，但存在b = 1/6[11-20]的MDs， 且其造成(11-2-2)ZnO晶面和(112)LAO/LSAT晶面有些微傾斜。在[1-100]ZnO方向則是以b = 1/3[1-210]的MD為主。 7.(11-2-2) ZnO的基面疊差密度< 10^4 cm^-1，但仍存在沿著[000-1]方向延伸的六角柱狀之面缺陷，柱面為{10-10}的面缺陷，其特徵與反轉晶界相似。(11-2-2) ZnO中的差排幾乎為完美差排，其布格向量有b = a、c和a+c，差排密度在1.6 micro厚的薄膜表面約1×10^9 cm^-2。 8.透過改變LAO退火條件改變表面結構，在具有平整(112)LAO面的(112) LAO單晶基板上可以成長 (10-10)ZnO面的ZnO薄膜，而(11-2-2) ZnO薄膜則成長在具有鋸齒狀之LAO單晶基板上，鋸齒面是由原子尺度的(001)LAO和(110)LAO晶面所構成。 9.透過改變LAO退火條件，亦證實(11-2-1)半極性和(13-40)非極性 ZnO亦可成長在(114) LAO基板上，且這兩種ZnO和LAO的方向關係分別與(11-2-2)和(10-10) ZnO在(112) LAO上的方向關係一致。 此外，本研究進一步討論所觀察的MDs成因、差排密度隨著膜厚增加而減少的特徵與機制，以及ZnO與(112) LAO界面結構的原子排列模型對形成半極性(11-2-2)和非極性(10-10) ZnO的影響。

Wurtzite ZnO, a direct wide bandgap semiconductor, is promising for optoelectronic applications. However, the polarization along [0001]ZnO usually degrades luminescent properties. To reduce the polarization effect, the growth of ZnO along semipolar directions is desirable. In this study, growth of semipolar (11-2-2) ZnO thin films has been sucessfully demonstrated on (112) LaAlO3-buffered LSAT substrate by pulsed laser deposition. We investigated the structural characteristics and the growth mechanism of (11-2-2) ZnO by using high resolution X-ray diffraction, transmission electron microscopy, atomic force microscopy, and photoluminescence (PL). The main results are listed below. 1. High quality LAO buffer is ahchieved on (112) LSAT ((LaAlO3)0.29(Sr2AlTaO6)0.35) substrates by PLD. The LAO buffer layer has a twin-free structure, implying the phase transformation from cube to rhombohedron at 813 K during cooling does not occur, and is cation nonstoichiometric (La/Al atomic ratio = 1.13). 2. The epitaxial relationship of (11-2-2) ZnO on (112) LAO/LSAT is [1-100]ZnO // [1-10]LAO/LSAT, and the polar axis [000-1]ZnO points to the surface. 3. The ZnO films show good crystalline quality, such as narrrow X-ray diffraction rocking curves (near 0.1°), and large lateral coherent length (over 1 micron). 4. [1-100]ZnO-oriented stripes and pits are the main characteristics of (11-2-2) ZnO surface morphology. The stripes are constituted of two facets, and the pits are corresponding to the termination positions of threading dislocations at the surface. 5. In low temperature (10 K) PL spectra, the main peak with a FWHM value below 10 meV is neutral-donor-bound exciton recombination. For room temperature PL spectra, the near-band-edge emissions mainly consist of free exciton recombination, free electron to neutral acceptor transition, and their longitudinal-optical-phonon-assisted transitions. 6. The misfit strain of (11-2-2) ZnO on (112) LAO buffer along [11-23]ZnO is relaxed by the formation of misfit dislocations (MDs) with b = 1/6[11-23]ZnO and b = 1/6[11-20]ZnO. The latter causes slight tilt of (11-2-2)ZnO plane toward (110)LAO from (112)LAO plane. Along [1-100]ZnO, MDs with b = 1/3[1-210]ZnO are responsible for the misfit strain relaxation. 7. The basal stacking faults in (11-2-2) ZnO films are rarely found (the density is below 10^4 cm^-1), while planar defects bound by {10-10}ZnO and extending along [000-1]ZnO are observed and resemble inversion domain boundary. Only perfect dislocations with b = a、c and a + c are shown, and the density near the surface of 1.6 micron ZnO films is 1 ×10^9 cm^-2. 8. Growth of nonpolar (10-10) and semipolar (11-2-2) ZnO on (112) LAO substrates can be obtained by annealing the substrate surface in vacuum and oxygen atmosphere prior to ZnO deposition, respectively. It is shown that (10-10) ZnO is grown on atomic flat (112)LAO plane, whereas (11-2-2) ZnO is grown on faceted (112) LAO surface with (001)LAO and (110)LAO facets in atomic scale. 9. By using the growth conditions of (11-2-2) and (10-10) ZnO, (11-2-1) and (13-40) ZnO can be achieved on (114) LAO substrates, respectively. In addition, it is found that (11-2-2) and (11-2-1) have the same orientation relationships with LAO ( (000-1)ZnO // (110)LAO and [1-100]ZnO // [1-10]LAO ). We further discuss the origin of MDs and the mechanism for the reduction of dislocation density with increasing film thickness. The atomic configurations of (11-2-2) ZnO/LAO and (10-10) ZnO/LAO interfaces are also modeled to account for their epitaxial relationships.