GaAs/AlAs/GaAs量子結構的導納頻譜分析

Abstract

在這次的論文中,我們主要是利用導納頻譜分析(Admittance spectroscopy)來研究以MBE成長的GaAs/AlAs/GaAs量子結構.其中之一的 AlAs層是設計長在空乏區內,另外一個則是長在空乏區外,並與一沒量子結 構的參考樣品比較.我們並對以上樣品做了電流量測(I-V),電容量測(C- V),深層能階暫態頻譜(DLTS)與熱激電流(TSC)的量測. 電流分析 上,AlAs層在內的樣品其I-V曲線皆比P-I-N 結構的電流要來的大,推論這 多餘的電流應該是由AlAs層所造成的,而AlAs層在外的樣品,其I-V曲線在 順向偏壓下則被AlAs層影響,而有被壓制的現象.而在電容的量測上,AlAs 層在內的樣品顯示在負偏壓下,仍有缺陷未被空乏而可以調變電容.而在另 外一片樣品則可以看到在負偏壓到3V時,因為空乏區的加寬,使得AlAs層的 缺陷被空乏而無法調變電容,所以此時不論高低頻都會量到相同的電容值. 在導納頻譜的分析上,我們從AlAs層在內的樣品的量測中,觀查到一個和先 前成長的GaAs P-I-N 結構所量到的同一個能階,因此此一缺陷與AlAs的結 構沒有關係.根據推論,此缺陷可能為材料中的本質缺陷(native defect), 或是在MBE的長晶過程中,雜質混入磊晶層所造成的.因此這種AlAs層在空 乏區的結構由導納頻譜分析是量不到的. 在AlAs層在外的樣品中觀察 到兩個缺陷能階:119A和119B.119A這個能階在阿瑞尼斯圖上由斜率可以得 是175 meV,捕捉截面積為1.26 x 10^-17 cm ^-2.根據理論,GaAs的Gama band 和X band之間的band offset也約為170 meV左右.而另一方面,雖然 利用有限量子井的模型來估算X band量子井的subband的能量,是可以得到 符合119A的這個能階,但是在subband中,除了這個能階之外還有其他能量 的能階,而我們並沒有量到這些能階,所以我們 無法確定量到的能階是屬 於這個能階. 而另外一個在高溫時才量的到的能階(119B)其活化能 為0.52eV,捕捉截面積為1.56 x 10^-14 cm^2.而能量大小剛好等於GaAs和 AlAs的valence band offset,推測這是由於電洞受熱激發,而越過valence band的位能障而造成的,為了進一步證實,我們接著做熱激電流實驗來分 析.而由熱激電流實驗的曲線中可以確實有載子受熱激發的現象出現.將實 驗後的曲線和經由模擬的曲線做一比較,得出載子受熱激發的能階深度 為0.6 eV,捕捉截面積1.5 x 10^-15 cm^2,和原本經由導納頻普分析在低 頻高溫所量到的能階十分接近,所以確認了我們原本的假設. The trnsport properties of GaAs/AlAs/GaAs quantum structures werestudied by admittance spectroscopy in this thesis. The samples were grown by molecular bean epitaxy (MBE), and their properties were com-pared with another normally grown GaAs P-I-N structure. Beside the admittance spectroscopy, the current- voltage (I-V), capacitance-voltage(C-V), deep level transient spectroscopy (DLTS), and thermal stimulated current measurment (TSC) were also performed on these samples. By I-V measurement, the current of the sample with AlAs layer growninside the depletion was larger than the normally grown P- I-N sample in both forward and reverse bias. The leakage current should be contributedby the AlAs layer inside the depletion region. By admittance spectroscopy, one level was obtained on the sample withAlAs layer inside the depletion. The level was also observed in normally grown P-I-N sample. Therefore, this level is nothing to do with the AlAsquantum structure, and is commonly observed in all our P-I-N GaAs structure. In the sample with AlAs layer outside the depletion region, a level wasobserved with its activation energy of 175meV. From the theoretical analysis,there exits a band offset about 170 meV between Gama band of GaAs and X bandof AlAs. Whether the observed level at 175 meV is the result of this band offset is uncertain at this point. Further work is needed in order to comfirmthis. In the high temperture; we observed another level at 522 meV. This energyis close to that of the band offset of valence band between GaAs and AlAs. In order to confirm that the level is due to the thermal excition of holes overthis valence barrier, thermal stimulate current was performed and a similarlevel (E/sub a = 0.6 eV, caperture cross section = 1.5 x 10^ -15 cm^ 2)was found. This result indicates that admittance spectroscopy is useful formeasuring the band offset between the GaAs and AlAs.