新穎性氮化鋁與氮化銦鋁三族氮化物半導體磊晶成長之研究

Abstract

三族氮化物半導體涵蓋AlN、GaN、InN及其合金，為高頻、高功率、光電元件的關鍵材料，而磊晶成長攸關元件特性至鉅，本論文利用有機金屬化學氣相沉積系統(MOCVD)及材料分析技術，先行探討磊晶成長氮化鋁(AlN)於矽(Si)基板之基本特徵，進而研究新的富鋁之氮化銦鋁(Al-rich AlInN)薄膜於氮化鎵(GaN)基板之磊晶成長，最後一部分則是磊晶成長富銦之氮化銦鋁(In-rich InAlN)薄膜於Si基板之研究。 由於GaN與Si之間存在著晶格失合以及熱膨脹係數差異大，常常導致形成過多的差排缺陷和薄膜的龜裂，還有在界面處發生Ga與Si回熔等問題，因此解決之道是使用AlN作為GaN與Si之間的緩衝層。然而成長AlN薄膜於Si基板首要面臨的是，如何避免生成非晶氮化物(SiNx)的中間層，因為SiNx會降低薄膜的磊晶品質。因此本研究的第一部份是成長AlN於2吋Si(111)基板，藉由調控前驅物的預流、壓力、溫度、V/III流量比等製程參數，探討SiNx形成的機制以及對薄膜品質的影響。從X光繞射、電子顯微鏡、原子力顯微鏡等之觀察與分析，進而得知在860 °C、低壓、低V/III流量比以及使用三甲基鋁(TMAl)預流等條件，可以成長表面平整、結晶度高並且無SiNx在界面生成的AlN薄膜於Si基板上；而當薄膜成長溫度超過1000 °C，即使先通入TMAl預流的狀況下，仍無法避免在界面處形成SiNx的中間層。 第二部份的研究是在1.5吋的單晶GaN基板上，直接成長Al-rich之AlInN薄膜。AlInN具有寬能隙0.7 ~ 6.2 eV之特徵，且自發極化很強，運用於高功率、高頻元件有很大的發展潛力。關於AlInN的研究大多成長在GaN/sapphire上，其中Al0.83In0.17N薄膜具有晶格匹配於GaN之特質，但仍受到熱應力及藍寶石基板的影響，而不易得知其基本特性。由於三甲基銦(TMIn)對溫度的高敏感性，因此本研究藉由溫度的控制改變其薄膜組成，結果顯示在780 °C下成長Al-rich之薄膜，Al/In組成比例約為20，具有表面平整、成分均勻、結晶品質佳，與GaN有良好的契合關係。然而，由於MOCVD成長之GaN緩衝層造成殘留之Ga摻入而形成AlInGaN四元薄膜，其中Ga約佔0.3。 第三部份挑戰成長單相In-rich InAlN於Si基板，因為AlN緩衝層跟Si之間的晶格失合較大，且需要在高溫成長才可得到較高結晶品質，而InAlN與Si有較小的晶格失合，屬於低溫成長；因此尋找介於AlN和InN兩個分別在高溫與低溫極端條件下，成長In-rich InAlN薄膜的最佳參數。此部份的實驗藉由TMAl與TMIn流量的控制，在500 °C成長In含量為漸進式增加的InAlN薄膜，平均組成約為In0.7Al0.3N，膜厚約600 nm，橫截面電子顯微鏡觀察界面沒有In-Si與Al-Si共晶回熔的現象，且沒有In相分離及SiNx界面層形成之情況。

III-nitride semiconductors including AlN, GaN, InN, and their alloys are critical materials for high-frequency, high-power, and optoelectronic devices in which film quality of epitaxial growth significant affects the device performance. In this study, the formation of AlN film on Si(111), Al-rich AlInN film on free-standing GaN substrate, and In-rich InAlN film on Si(110) and (111) were carried out in an metal organic chemical vapor deposition (MOCVD) system. AlN is generally used as a buffer layer to reduce the high density defect and strain induced by lattice mismatch and thermal expansion difference between GaN and Si(111). Also, AlN buffer layer is capable of preventing the meltback etching of Si by Ga. An important challenge for the growth of III-nitrides on Si is to eliminate SiNx formation at the AlN/Si interface and clarify the mechanism for the formation of SiNx interlayer. Here, AlN growth on 2-inch Si(111) wafer was carried out by varying deposition parameters of precursor flow, pressure, temperature, V/III ratio. For deposition at 860 °C at low pressure with low V/III flow ratio, the high quality AlN thin film without the SiNx interlayer can be grown on Si using trimethylaluminum(TMAl) pre-flow based on the results of X-ray diffraction, scanning electron microscopy, transmission electron microscopy(TEM), and atomic force microscopy. The SiNx interlayer can still be formed at the AlN/Si interface even though TMAl preflow has been applied onto the Si surface at 1000 °C. In the second part of this thesis, Al-rich AlInN epilayer growth has been explored on GaN. AlInN is a newly developed III-nitride for many promising applications due to its band gap being able to be tuned in a wide range of 0.70 ~ 6.14 eV with high spontaneous polarization. Most of AlInN/GaN studies are done on films grown on sapphire substrate which has strong effects on the film properties, and among them, lattice-matched Al1-xInxN/GaN with x close to 0.17. Nevertheless, the Al1-xInxN/GaN properties and device performance will suffer from the effects of sapphire due to the high defect density of heterepitaxy and thermal stress. In the second part of this thesis, a high-quality Al-rich AlInN epilayer was deposited on 1.5-inch GaN substrate at 780 °C with uniform composition by using TMAl and TMIn precursors. The composition analyses show that the film is AlInGaN with Al/In ratio of about 20, and Ga incorpation in the film can be about 0.3 due to residual Ga in MOCVD. The AlInGaN has a good coherent relationship with GaN. The third part of this work is focused on the growth of single phase In-rich InAlN on Si(110) and (111) substrates which cam have better lattice mismatch with Si. In order to get high quality AlN film on Si, it is necessary to grow at high temperature. However, In-rich InAlN film prefers to form under low growth temperature due to desorption of In from the surface. InAlN film was grown by seeking an appropriate temperature within two drastic conditions of AlN and InN. By controlling TMAl and TMIn flow rate and optimizing growth temperature at 500 °C, a graded InAlN film with increasing In was grown on Si substrate with average composition of In0.7Al0.3N in 600 nm film thickness. From the TEM result, it shows a sharp interface between InAlN and Si with no SiNx interlayer and without In-Si and Al-Si meltback.