單晶純鍺奈米點陣列製程之研究

Abstract

鍺奈米點本身擁有新穎的電性與光學特性，可應用於電子或光電元件之中，且製程上更相容於矽製程技術，因此，其鍺奈米點之相關製程技術已被廣泛地研究。雖然有許多方法都證明了製作高結晶性鍺奈米點之能力，但是其中仍然存在著嚴重的矽鍺材料相互混合之問題，以及將鍺奈米點製作於平面矽基板上時，容易呈現隨機排列與尺寸分布不均勻等問題，因而影響並限制了元件實際應用之特性。 本篇論文為了解決矽鍺材料之相互混合、鍺奈米點成核位置呈現隨機排列，以及尺寸分布不均勻之問題，因此，研究發展出一個三明治結構，係由二氧化矽覆蓋層/非晶鍺奈米點/凹洞圖形化矽基板所組成，隨後利用脈衝式紫外光準分子雷射進行退火處理。其中藉由寬能隙之二氧化矽覆蓋層作為一個穿透熱絕緣層，以防止鍺奈米點所吸收的熱能經由空氣而散失，並且適當的搭配選擇雷射波長與氧化層，可以有效的避免矽鍺材料之相互混合。另外，所設計之二維凹洞圖形化矽基板用以侷限鍺奈米點所吸收之熱能，以延長鍺奈米點中熱能的持續時間；同時由於矽基板給予鍺奈米點的壓縮應力，可降低鍺奈米點之熔點，因此，進而有助於鍺奈米點之結晶，更可以防止鍺奈米點的表面遷移。隨後透過實驗獲得最佳化雷射照射參數之後，成功的於凹洞矽基板中製作出規則排列且尺寸均勻分布之鍺奈米點，其中面積密度可達3.9×109 cm-2。 最後為了評估此鍺奈米點陣列之結晶品質，分別利用非破壞性的拉曼散射頻譜之光學量測方式與破壞性的高解析度穿透式電子顯微鏡進行分析，於拉曼頻譜量測結果，顯示出一高度對稱且半高寬為4.2 cm-1之結晶鍺訊號，其中峰值位置位於300.7 cm-1，此結果非常接近單晶鍺晶圓之訊號(半高寬為4.0 cm-1，峰值位置位於300.7 cm-1)，並且無任何矽鍺相互混合之峰值訊號(400 cm-1)出現。於高解析度之顯微影像與所對應之選區繞射圖形結果，顯示出清楚之單晶結構且無任何雜質存在。因此，藉由實驗分析結果證實，利用此研究方式處理過之鍺奈米點陣列之結晶品質屬於單晶純鍺之特性。本論文之研究方法將有助於避免矽鍺材料之相互混合，並且能於低溫環境中迅速的製作出單晶純鍺奈米點，所獲得之高密度且尺寸均勻之鍺奈米點陣列，更有機會改善電子或光電元件之效能。

Ge nanodots (NDs) have novel electrical and optical properties, as well as for their compatibility with Si complementary metal - oxide - semiconductor (MOS) technology. Therefore, the fabrication technology of Ge NDs have been investigated extensively. Although these approaches have illustrated the capability of fabricating highly crystalline Ge NDs, a severe problem still exists regarding the intermixing of Si and Ge due to the high temperature of the annealing treatment, which may degrade the performance and decrease the efficiency of the Ge ND devices. In addition, the uniformity of the Ge NDs needs to be further improved for practical applications. In this paper, to solve these problems of the intermixing, uniformity and random nucleation of the Ge NDs. A sandwich structure comprising a SiO2 capping layer, amorphous Ge NDs (a-Ge NDs), and a pit-patterned Silicon (Si) substrate is developed, which is then annealed by utilizing a pulsed ultraviolet (UV) excimer laser. A wide bandgap SiO2 capping layer is used as a transparent thermally isolated layer to prevent thermal loss and Si–Ge intermixing. The two dimensional pit-patterned Si substrate is designed to confine the absorbed laser energy, reduce the melting point, and block the surface migration of the Ge. After optimizing the laser radiation parameters, the NDs exhibit excellent size uniformity and arrangement, and the area density is about 3.9 × 109 cm-2. The degree of crystallinity of the NDs was determined from their Raman spectra and verified using high-resolution transmission electron microscopy (HR-TEM). Raman spectrum shows a highly symmetric Ge transversal optical peak with a full width at half maximum of 4.2 cm-1 at 300.7 cm-1, which is close to that of the original Ge wafer (where the FWHM is 4.0 cm-1and the peak position is at 300.7 cm-1). In addition, it is worth noting that no Si–Ge intermixing signal was observed from the Raman spectrum of the crystallized Ge NDs. The high-resolution transmission electron microscopy image for the Ge NDs and the corresponding selected area electron diffraction pattern shows a clear single crystalline structure without any impurities. Therefore, these results reconfirm that the processed Ge ND is a single crystal without any impurities. In this paper, the novel method is developed for fabricating an array of pure, single crystal Ge NDs at room temperature. Moreover, it have the opportunity to improve the performance of electronic or optoelectronic device.