利用飛秒級激發-探測技術研究超導體與半導體之超快動力行為

Abstract

利用自行建立之飛秒級被動式鎖模鈦-藍寶石雷射配合激發-探測系統，針對不同氧含量的釔鋇銅氧高溫超導薄膜及一系列砷離子佈植砷化鎵半導體，我們已成功的量測反射率變化ΔR/R 的瞬態時間弛緩現象。 將不同氧含量釔鋇銅氧超導體薄膜；調變探測能量由1.45eV 至 1.65eV 來量測微分反射率之瞬時變化，實驗結果發現載子-聲子耦合強度會隨氧含量減低而變小；且氧含量 6.68的超導薄膜其在銅氧面上的費米能階約處於比 Cu d9/d10 能帶高 1.5eV 的位置。 對各種佈植劑量或退火處理之砷離子佈植砷化鎵半導體的實驗顯示，載子生命期會隨佈植劑量減少或退火溫度升高兩增長。我們並嘗試利用退火溫度的改變會造成能帶結構的不同，來解釋瞬時折射率和反射率變化在退火溫度為500∼600℃時有正負轉換的情形。

We have set up a home-made femtosecond passively mode-locked Ti:sapphire laser and a room-temperature pump-probe measurement system to measure the transient reflectivity of the YBCO superconductor thin film with different oxygen contents and arsenic-ion-implanted GaAs thin films with different implantation and annealing conditions. The dependence of transient differential reflectivity (ΔR/R) on probing photon energy and oxygen contents of oxygen stoichiometric YBCO films was determined by using femtosecond pump-probe measurement at probing energy ranging from 1.45 eV to 1.65 eV. Experimental results reveal that the strength of carrier-phonon coupling decreases with decreasing oxygen content, and the Fermi level for the partially oxygen-deficient YBCO, film with x=6.68 is located at least 1.5 eV above the Cu d9/d10 band in the CuO2 plane. The carrier lifetime of arsenic-ion-implanted GaAs demonstrated was found to increase as the dosage decreases or annealing temperature increases from 200℃ up to 800℃. A sign reversal of the reflecive index change at annealing temperatures 500∼-600℃ was observed, this phenomenon can be partially explained by the shrinkage of bandgap energy during implantation and recovery during thermal annealing.