Optical Properties of GaAs in Si nano-trench grown by Gas Source Molecular Beam Epitaxay

我們利用掃描式電子顯微鏡(SEM）、陰極螢光(CL)及拉曼(Raman)光譜量測以氣態源分子束磊晶法成長在平面矽基板及有圖樣的矽基板上的砷化鎵。 從SEM的觀測，我們可以看到300奈米厚的砷化鎵成長於平面矽基板時，呈現連續山狀的樣貌，並且有方向的小縫隙分佈其間，砷化鎵的覆蓋率超過95％。而成長於有圖樣的矽機板的砷化鎵，幾乎全部都長入矽奈米溝渠中，藉由氫等離子體的幫助，成功地達成選擇性成長，在溝渠寬度小於200奈米之後，才有眼形的小塊堆積在溝渠邊緣的二氧化矽上。 砷化鎵成長於平面矽基板及成長於奈米矽溝渠內的室溫(RT)和低溫(LT)的CL光譜也被量測。無論是成長於平面矽基板還是成長於奈米矽溝渠內，室溫陰極螢光的波峰相較於同質磊晶的砷化鎵皆呈現相同程度約三倍的變寬，以及30 meV左右的藍移。我們認為波峰變寬主要歸因於各個被拉張或擠壓的結晶為了達到費米能階一致而造成的能帶變形，以致於原有的能階數減少，在電子束產生大量載子的同時，也同時填滿能階到更高/低能階，造成放光時波峰的變寬。而大程度的藍移則歸咎於成長時的自動摻雜，或所謂的Burstein-Moss shift。成長於矽奈米溝渠內的砷化鎵LTCL光譜在溝渠寬度介於90奈米至140奈米間的波形呈現雙峰態。較低能量的波段被歸因於深層載子與碳受體的復合，而較高能量的波段則被歸因於施體與受體的復合。在溝渠寬度大於140奈米之後，其放光波形接近成長於平面矽機板的砷化鎵，藉由高斯(Gaussian)分佈擬合波峰，可以得到三至四個波峰。而溝渠邊緣的二氧化矽由於缺陷結構放光，共有三個波段。常溫時為1.9 eV的放光，歸因於非橋接氧電洞中心；2.2 eV的放光，歸因於氧空洞與空隙氧分子間電子電洞的復合；2.7 eV的放光，歸因於矽孤對電子與電洞的復合。在LTCL的觀察中，2.2 eV及2.7 eV這兩種放光會縮減，起因於降溫時對這兩種缺陷結構的消減。 常溫的拉曼光譜也觀察到不尋常的現象：在所有的試片中我們均量測到強烈的原本禁帶的橫向聲子模態。我們認為此一現象是源於晶體中大量的微雙晶缺陷，此種缺陷會把原本(001)面轉成{122}面。另一現象是在溝渠寬度小於100 nm以後，我們使用Lorentz分佈擬合的實驗值在橫向聲子與縱向聲子模態之間有多一「表面聲子」模態，起因於較大的表面積/體積比。We have utilized scanning electron microscopy (SEM), cathodoluminescence (CL) spectroscopy and Raman spectroscopy to investigate heteroepitaxial GaAs on planar Si and patterned Si wafer samples grown by gas source molecular beam epitaxy (GSMBE) system. With the observation of SEM image of the sample top view, the hydrogen-plasma -assisted-grown-300-nm-thick-GaAs on Si planar substrate formed a successional mountain-like “perforated film”. This structure composed by GaAs covers more than 95% area of the planar Si substrate. The filling ratio of GaAs in the trenches is estimated to be nearly 100%. GaAs in the trenches break to become nanowires with lengths varying from hundreds nanometer to several micron. When the trench widths decrease less than 200 nm, the epi-GaAs overflow the trench onto the SiO2 sidewall and form eye-shaped islands whose dimension is about 500 nm. Room temperature CL (RTCL) and low temperature CL (LTCL) are also performed at trenches whose widths vary from 50 nm to 500 nm. The RTCL GaAs peaks of all trenches are about 3 times broadened and the same blue shifted about 30 meV regardless of the trench width. Meanwhile, the energies of SiO2 peaks remain unchanged. This phenomenon indicates that the blue shift and the broadening are due to the same mechanism. We attribute the blue shift to the Burstein-Moss shift. The broadening is attributed to the Fermi-level-consistency of grains induced band banding. The LTCL GaAs peaks reveal a two-peak feature when trench width between 90 nm and 140 nm. The low energy band is attributed to deep level carrier to carbon acceptor and the high energy band is attributed to donor to acceptor recombination, respectively. When trench width is larger than 140 nm, the peak form is like GaAs on planar (001) Si. With the assistance of Gaussian fitting, there are three or four bands. The origins of three SiO2 RTCL peaks are also surveyed. The 1.9 eV peak is attributed to the NBOHC; the 2.2 eV peak is attributed to the (VO;(O2)i) structure and the 2.7 eV peak is attributed to E’ center. From the observation of LTCL, we find out that the elimination of 2.7 eV and 2.2 eV peaks, which represents the elimination of these defects The RT Raman spectra of GaAs on planar Si (001) substrates are measured in z(XX)z’ and z(xx)z’ configurations. Regardless of the growth condition, all samples reveal a strong originally forbidden transverse optical (TO) phonon mode. The RT Raman spectra of GaAs in variant trench widths are measured in z(YY)z’ configuration. Each of the epi-GaAs in Si nanotrenches also reveals a strong TO phonon mode and the longitudinal optical (LO) phonon mode broadens. Furthermore, there is an additional peak between the TO and the LO peak while trench width is under 100 nm, which is attributed to the surface optical (SO) phonon mode. The SO mode is measured because of the large surface-to-volume ratio