以電漿輔助分子束磊晶技術成長III-族氮化物半導體之研究

Investigation of Growth of III-Nitride Alloys by Plasma-Assisted Molecular Beam Epitaxy

楊偉臣(Yang. Wei Chen)

指導教授 ： 鄭克勇

國立清華大學/電機資訊學院/電子工程研究所/博士(2016年)

摘要

在過去二十幾年來，Ⅲ-族氮化物材料在光電元件及高功率元件的應用上擁有相當傑出的特性。然而，目前有許多元件的應用仍無法經由傳統的Ⅲ-族氮化物薄膜成長技術來實現，包括：高電洞濃度基極之異質接面雙極性電晶體，垂直式高崩潰功率元件，以及長波長發光二極體等等。其原因來自於Ⅲ-族氮化物薄膜目前所存在的三大主要的問題: (1) 由於沒有晶格匹配的基板可供Ⅲ-族氮化物成長，而產生許多的缺陷並且限制了材料的品質， (2) 因為氮化銦鎵相分離的問題而造成銦含量的浮動及低銦含量的成長， (3) 較高的鎂活化能使得p型Ⅲ-族氮化物薄膜的電洞濃度偏低。
本論文提出利用分子束磊晶成長系統，成長高品質Ⅲ-族氮化物材料之研究，其目的在於改善與解決目前Ⅲ-族氮化物迫切遇到之三大問題。(1) 發展高溫成長之改良式兩階段成長方式，成功成長出銦含量可高達30%之高亮度綠光氮化銦鎵量子井，並且於高Ⅲ/V比例的成長條件下循著液態磊晶機制，發現氮化銦鎵量子井的銦含量會隨著成長溫度的增加而升高。(2) 發展利用奈米孔洞圖案化之氮化鎵模板，來減少氮化鎵磊晶層之缺陷密度，並且在高解析X光繞射、光激發螢光以及穿透式電子顯微鏡剖面分析確認得知，藉由理想化之奈米孔洞深度與覆蓋率，可成功的將缺陷密度由原本的108~9 cm-2下降至約107 cm-2。(3) 研究在高溫成長條件下，對p型氮化鎵之鎂含量及活化的影響，發現Ⅲ/V比例及鎂/鎵比例對於p型氮化鎵的電特性有極大的影響。雖然研究發現在較低Ⅲ/V比例及較高鎂/鎵比例成長條件下，電洞濃度可高達7.5×1018 cm-3，但過多鎂所形成的缺陷會大幅度的降低電洞移動率。因此，研究發現在理想化的成長條件下，成長適中電洞濃度為1.7×1018 cm-3 之p型氮化鎵，除了可得到5%之高鎂活化率，也可得到最低電阻率為0.56 Ω-cm，及最高電洞移動率為6.42 cm2/V-s之理想電特性。此外，我們也發現，在高溫成長下利用分子束調變的成長方法，並無法增加鎂的摻入量，其結果與在低成長溫度下結果相反。

關鍵字

分子束磊晶 ； 氮化鎵

並列摘要

Over the past two decades, III-nitride materials system boasts outstanding properties of optoelectronic devices and high-power devices. However, many of these applications are currently unavailable using current III-nitride thin film growth techniques, including heterojunction bipolar transistors (HBTs) with a base layer of high hole concentration, vertical power devices with high breakdown voltages, green wavelength light-emitter diodes (LEDs). This is due to three mainly problems: (1) there is no lattice-matched substrate for III-nitride growth, creating high density defects in the grown material, (2) the issue of phase separation in InGaN alloys, which results in a fluctuated In composition and low In incorporation, and (3) high ionization energy of Mg leads to a low hole concentrations in GaN.
In this thesis, III-nitride materials grown by plasma-assisted molecular beam epitaxy are investigated. The objectives of this study are to improve and solve several pressing issues associated with the growth of III-nitride materials; (1) An improved two-step growth method under high growth temperature is successfully developed to increase the In content of the InGaN/GaN SQW to ~30 % while maintaining a strong luminescence intensity in the green spectral range. The In composition in InGaN/GaN SQW grown under group-III-rich condition increases with increasing growth temperature following the growth mechanism of liquid phase epitaxy. (2) A nano-hole patterned GaN substrate template has been developed to reduce the threading dislocation (TD) density in GaN epilayers. The grown layers are analyzed by high resolution x-ray diffraction, photoluminescence, and cross-section transmission electron microscopy. It is confirmed that the TD density of GaN epilayers has been successfully reduced from 109 down to 107 cm2. (3) The influence of growth conditions on the incorporation and activation of Mg in GaN at high growth temperature is studied. It is found that the electrical activated Mg in GaN is highly sensitive both to the III/V flux ratio and Mg/Ga flux ratios. Although the highest hole concentration achieved under a low III/V flux ratio and a high Mg/Ga flux ratio reaches 7.5×1018 cm-3, the hole mobility is suffered due to the formation of defects by the excess Mg. Therefore, to further improve the electrical properties of Mg doped GaN, we found that a maximum Mg activation of ~5 % can be achieved at the optimized growth condition. The lowest resistivity of 0.56 Ω-cm is achieved, which is associated with a high hole mobility of 6.42 cm2/V-s and a moderately high hole concentration of 1.7×1018 cm-3. In addition, we show that modulated beam growth methods do not enhance Mg incorporation at high growth temperatures in contrast to those grown at a low temperature of 500 °C

並列關鍵字

Molecular Beam Epitaxy ； Gallium nitride

參考文獻

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