利用有機金屬化學氣相沉積法 研究砷化鎵於矽基板上成核成長之技術

Abstract

由於鎵原子與砷原子的高度表面擴散性，我們了解若於高溫下直接成長砷化鎵於矽基板上，其表面型態易於趨向三維成長。當島狀結構開始結合時，高密度之低移動性與固定的差排將被產生，最終形成高密度穿透差排的砷化鎵磊晶層。藉由精準控制有機金屬化學氣相沉積法之初始成長階段，將可以達到平坦表面之砷化鎵成核層於矽基板上。本研究論文主要探討，以適當的成長條件，利用有機金屬化學氣相沉積法，直接成長砷化鎵於矽基板上。於實驗起初，我們將對矽基板進行仔細的化學與熱處理，並適度調控其中的成核成長速率與五三比，以求得到較為平坦的砷化鎵成核層。如此，開發平坦砷化鎵/矽的結構，將被視為極有潛力發展作為如砷化銦、砷化銦鎵、砷化銻與鍺等，高遷移率通道材料之人造基板。 本研究已成功發展出直接成長砷化鎵/矽結構之技術。我們使用(001)面偏60切面朝[110]方向之矽基板，並且於實驗起初使用氟化氫溶液清除基板上之氧化層，然後再以40 sccm之砷化氫氣體於高溫650C進行退火反應。這樣的退火技術，有助於形成完全砷原子終止的覆蓋層之矽基板表面；此為形成砷化鎵磊晶層之重要關鍵。藉由控制成長溫度在400C與五三比為18.4之情況，砷化鎵成核層可控制在約10nm的厚度。在此成長條件，我們可以得到RMS值約0.66nm，於矽基板上的平坦砷化鎵成核層。並且透過TEM影像，顯示在砷化鎵/矽結構的界面處，存在許多90錯位差排及少量的60錯位差排；此為形成平坦表面的成核磊晶層之所需要件。

It was realized that a direct, high-temperature growth of GaAs on Si substrates led to a 3D initial growth morphology aided by the high rate of surface diffusion of the Ga and As adatoms. When those islands later coalesce, a high density of immobile and pinned dislocations are produced, resulting in a final GaAs film with high threading dislocation density. A careful control at initial stage of epitaxy growth is desirable to obtain a smooth surface morphology of GaAs nucleation layer on Si substrate. The scope of this thesis includes the investigation of optimum growth conditions for direct integration of GaAs on Si using Metal-Organic Chemical Vapor Deposition method. By careful Si surface chemical and thermal treatment, combining with tuning the nucleation growth temperature and V/III ratios, a very smooth surface of GaAs nucleation layer can be obtained. This smooth surface GaAs/Si integration is considered as a potential artificial substrate for further high mobility channel materials like InAs, InGaAs, InSb, Ge to be continuously grown.

In this thesis, we successfully investigated the conditions for GaAs/Si direct integration. The Si (001) with 6 off-cut angle toward [110] direction was used. The substrate was first cleaned in HF solution to strip out the thin oxide and subsequently exposed to AsH3 flux of 40 sccm during the high temperature 6500C annealing process. This annealing technique can help to form a fully-As-terminated Si surface which is very important for GaAs continuous film formation. The GaAs nucleation layer with thickness of ~10 nm was then grown with temperature of 400C and V/III ratio of 18.4. This growth condition gave us very low roughness surface of GaAs nucleation layer on Si with the RMS value about 0.66 nm. The TEM images of GaAs/Si interface show mostly desirable 90 misfit dislocation and fewer 60 misfit that contribute to smooth surface morphology, a very desirable requirement for a nucleation layer.