利用原子層沉積技術應用於氧化鋅奈米柱陣列之研究

本論文研究利用原子層沉積技術(Atomic Layer Deposition, ALD)應用於氧化鋅奈米柱陣列，同時應用在光電元件上。本論文可以分為三個不同主題，第一個主題研究氧化鋅之核殼型(core-shell)結構之光學特性，首先利用水熱法(Hydrothermal synthesis)成長氧化鋅奈米柱陣列(Nanorod arrays, NRAs)，再利用原子層沉積技術在氧化鋅奈米柱陣列表面進行鍍膜。藉由光激發光(photoluminescence, PL)量測氧化鋅/氧化鋁以及氧化鋅/氧化鋅兩種核殼型結構的發光頻譜以及強度，發現氧化鋅表面會有向上的能帶彎曲(upward surface band bending)，原因可能為表面吸附帶負電荷的氧離子，此能帶彎曲造成光激發的電子電洞之波函數分離，電洞會聚集在氧化鋅之表面，使得氧化鋅的近帶隙(near-band-edge, NBE)發光強度下降，在鍍上氧化鋁薄膜後，表面的氧離子可提供做氧化鋁薄膜中氧的來源，換而言之，表面累積的負電荷可在鍍膜過程中被消耗，電子電洞之波函數可以有效的重疊使得近帶隙發光強度上升。光激發光對氧化鋅/氧化鋅核殼型結構的量測也驗證了此向上的能帶彎曲，在鍍上氧化鋅薄膜後電洞會聚集在表面，因而電子電洞會在此氧化鋅薄膜內躍遷，我們可以藉由螢光頻譜中可見光波段的改變驗證這個結果。第二個主題則是對氧化鋅奈米柱陣列摻雜氧化鎂之研究，在氧化鋅奈米柱陣列表面利用原子層沉積技術鍍上氧化鎂薄膜，再利用爐管退火，使鎂原子擴散進入氧化鋅中。隨著退火溫度增加，從螢光頻譜中我們可以觀察到氧化鋅鎂合金的近帶隙發光有一藍位移(blueshift)之現象，之稱為能隙工程(bandgap engineering)。同時，在中間退火溫度時我們觀察到載子侷限(carrier localization)的效應，由於鎂原子分布不均勻，原子濃度由奈米柱外層向內層遞減，使得氧化鋅鎂奈米柱外層有較高的能隙能量，而最高溫退火時鎂的分佈則比較均勻，內外層的能隙能量也較接近。從變溫的光激發光實驗中，我們發現最高溫退火之樣品有較明顯的熱淬減現象(thermal quenching)，驗證了內外層能隙能量差距大時會有載子侷限之效應。第三個主題研究氧化鋅奈米柱陣列在發光二極體(light emitting diode, LED)方面之應用，由於奈米柱陣列有較高的表面積對體積比(surface-to-volume ratio)，光會比較容易自二極體界面放出，同時能有比較好的散熱，有利於奈米柱陣列應用在發光二極體，同時由於穩定且高品質之p型氧化鋅較難取得，此處利用p型氮化鎵做為p型材料做出氧化鋅/氮化鎵異質介面(heterojunction)的發光二極體，同時研究熱退火對二極體發光的影響。二極體在逆偏下發光來自電子的穿隧效應(tunneling)，我們從電激發光(electroluminescence, EL)頻譜中發現熱退火會使得氧化鋅/氮化鎵介面模糊，同時在介面處產生介面能帶(interfacial state)，沒有熱退火處理時，由於氧化鋅與氮化鎵有很大的能帶差異(band offset)，使得在小的逆偏偏壓之下電子穿隧就會發生，同時順偏之下電流很小也沒有光從二極體放出，在此主題中我們也對電激發光頻譜的機制做了詳細的解釋。

關鍵字

氧化鋅；奈米柱陣列；原子層沉積；發光二極體

並列摘要

In this thesis, it can be divided into three different topics. The first topic is the influence of different shell layer on optical properties of ZnO nanorod arrays. The second topic is about magnesium doping to ZnO NRAs. The third topic is effect of heat treatment on ZnO NRAs based light-emitting-diodes. In Chapter 2, we report that, after coated with Al2O3 shell, surface band bending of ZnO NRAs was lowered while causing strong wave function overlap around surface. In consequence, near-band-edge (NBE) emission was enhanced. On the other hand, visible luminescence band in photoluminescence (PL) spectra changed after coated with ZnO shell. The luminescence mechanism was investigated. The result indicated that green luminescence involved V_O^∙ centers trapping photo-generated holes then recombined with electrons from conduction band. And yellow luminescence was attributed to defect state in the “bulk” of ZnO core. Chapter 3 describes a simple way to dope ZnO NRAs with magnesium. MgZnO nanorod arrays were successfully fabricated through annealing ZnO/MgO core-shell structure. Near-band-edge emission in PL spectra blushifted as annealing temperature increased. Carrier localization in the coaxial structure is reported. Due to difference in core/shell bandgap energy, carrier could be confined in the core of nanorod and non-radiative recombination caused by surface state effect would be suppressed. We verified carrier localization through examining thermal quenching. In chapter 4, ZnO film/ZnO nanorod arrays/GaN layer heterojuction LEDs were fabricated and effect of heat treatment was investigated. Light emitting under reverse bias was observed and resulted from electron tunneling. Rapid-thermal-annealing (RTA) treatment “blur” ZnO/GaN interface and respond for formation of deep-level states at the interface. Visible luminescence in electroluminescence (EL) was observed with RTA treatment. Characteristics of type Ⅱ band alignment were shown when no heat treatment was conducted. I-V curve shows large tunneling current at small applied reverse bias. Tunneling of electrons from valence band of ZnO to conduction band of ZnO was proposed, and EL mechanisms were investigated in detail.

並列關鍵字

ZnO ； nanorod arrays ； atomic layer deposition ； light-emitting diode

參考文獻

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利用原子層沉積技術應用於氧化鋅奈米柱陣列之研究

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