硒化鎘鋅磊晶層的高壓拉曼散射光譜研究

Abstract

在本論文中，以微拉曼散射光譜來研究在砷化鎵基板上成長不同鎘含量的硒化鎘鋅磊晶層的光學特性。實驗發現在室溫、大氣壓力下不同鎘含量的硒化鎘鋅磊晶層其縱向光學聲子模頻率和橫向光學聲子模頻率會隨著鎘含量的增加而減少，其強度也有減弱的趨勢。晶格振動模態屬於單模特性。並且利用鑽石高壓砧的技術，在室溫下觀察不同鎘含量樣品的拉曼譜線隨壓力之變化。我們發現在對不同鎘含量的硒化鎘鋅其縱、橫向光學聲子模振動頻率會隨壓力增加而變大，即為有藍位移現象，當壓力加到轉換壓力時，其縱向光學聲子模消失，但橫向光學聲子模仍然存在，此壓力即為半導體由閃鋅礦結構轉變為岩鹽結構的臨界壓力。我們發現硒化鎘鋅磊晶層之相位轉變壓力會隨著鎘含量的增加而減少，此現象歸諸為鎘含量增加而造成的晶格不穩定。最後我們也計算聲子頻率與Grünesien參數隨壓力變化的關係。

In this thesis, Zn1-xCdxSe epilayers of zinc-blende phase were grown on the GaAs substrates were studied by using micro-Raman scattering experiment. At ambient pressure, two peaks labeled as transverse optical (TO) phonon mode and longitudinal optical (LO) phonon mode are observed for each Zn1-xCdxSe epilayers. From the Raman spectra, the vibration frequency decreases and the linewidth increases with the Cd concentration. The lattice vibration mode behavior of Zn1-xCdxSe epilayers belongs to the one-mode behavior. The high pressures were generated by using a diamond anvil cell. For Zn1-xCdxSe epilayers with x = 0.00, 0.06, 0.08, 0.14, 0.25, and 0.32, all Raman peaks blue shifted to higher frequencies when the pressure was increased. As the pressure was increased to the transition pressure, the LO phonon disappeared. The disappearance of the LO phonon in Raman spectra at high pressure is attributed to the semiconductor to metal phase transition. The metallization occurred when the crystal structure transformed from a four-coordinated zinc-blende (ZB) phase into a six-coordinated rock-salt (RS) phase. As x was increased, the semiconductor to metal-transition pressure decreased. The decrease in transition pressure with x implies the decreasing crystal stability with x. Finally, we also calculated the pressure variation of the phonon frequencies and Grünesien parameters of the two optical modes.