| Summary: | 碩士 === 國立臺灣大學 === 電子工程學研究所 === 99 === 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.
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