藉由光電容研究深層缺陷能階與砷化銦量子點之載子交互作用

Abstract

本論文主要是藉由光性及電性的量測，包括光激發螢光頻譜(PL)、電容電壓(C-V)、導納頻譜(C-F & G/f-f)、深層能階暫態頻譜儀(DLTS)的量測，來探討在InAs/InGaAs這種quantum dot-in-well (DWELL) 結構中，其量子能階，缺陷能階和電子放射機制做探討。樣品為完美InAs量子點成長2.2 ML（無缺陷）、InAs量子點成長2.2 ML（有缺陷）、應力鬆弛InAs量子點成長3.3 ML（有缺陷）。在厚度2.2 ML之InAs量子點樣品中導納頻譜量測分析，其量子點能帶結構受到應力影響而形成極薄的能障，使得量子點中的基態電子熱放射至第一激發態能階後穿隧至砷化鎵導帶，其放射時間常數低於微秒等級；而3.3 ML之InAs量子點因受到應力鬆弛導致產生兩群量子點可分為利用生成錯為排差來達到應力釋放的低能量量子點與藉由將銦原子往外擴散而使所承受應力減輕的高能量量子點。透過PL量測在110 K到160K之間PL積分強度增加現象，配合兩群量子點隨溫度變化的特性可知高能階量子點中的載子透過兩群量子點中的量子井傳輸至低能階的量子點中，導納頻譜量測分析顯示在78 K～140 K內發現載子躍遷速率有一轉折，可以證明是由載子轉移所致。 最後透過光激發下的電性量測觀察量子點樣品的內部性質。在不同的光能量激發下之光電容量測，其光電容變化來源分為量子點跟缺陷能階的交互作用及缺陷能階兩部分。當光激發能量低於1.3 eV時，量子點能階與缺陷能階中產生光電子與光電洞。光電子的放射速率達到微秒等以下，但光電洞的放射速率達到數秒等，導致量子點電容的平台抬升產生光電容變化。而這些光電容的變化可以觀察到量子點中電子填充效應以及電洞佔據在缺陷能階中產生的壓降所造成的電容電壓曲線之變化。當光激發能量大於1.3 eV時，砷化鎵中的深層缺陷開始吸收產生光電子與光電洞。光電子被放射至砷化鎵導帶上留下正電荷於深層缺陷能階，藉由量子點周圍空乏載子所形成的能障擋住了深層缺陷所放射的光電子，使得此區如山谷狀的能帶結構，隨著光電子濃度增加造成一個壓降於此區，在定電壓下，量子點能帶結構必須往上提產生正向電壓抵銷來平衡電壓，而此物理模型也利用理論模擬方式得到驗證。比較氮砷化鎵量子井樣品，利用其量子井無電洞侷限的特性以及將此樣品熱退火處理後證明此深層能階確實是造成大量的光電容變化來源。

The on optical and electrical properties of post-growth InAs /InGaAs dot-in-well structures grown by molecular beam epitaxy on GaAs(100) were studied by current-voltage measurement (I-V), capacitance-voltage (C-V) profiling, bias-dependent deep level transient spectroscopy (DLTS) and photoluminescence (PL) measurements. For a perfect 2.2 ML InAs QD sample (SH332), C-V profile shows two accumulation peaks at the 77 K. We determine activivation energy of 57 meV according to the PL spectra and admittance spectroscopy measurement. Quanlity of this quantum structure is good since no defects are observed by DLTS. Two quantum peaks of C-V profile are probably originated from the ground and the first excited states of QD, respectively. The electrons in the ground are excited to the excited state of the QD then tunnel out of the potential well. This emission time of the electrons from the ground to excited state is about 106 sec at 77 K. For a 2.2 ML InAs QD sample (TR502), the emission time of the electrons is also the same with perfect InAs QD sample. However, the top GaAs layer has defect with concentration of about 1015 cm3 by low temperature grown. As the InAs deposition exceeds of 3 ML, strain in the InAs QD is relaxed, and the bimodal QDs strat to form at the same time. The existence of two types of QDs in the strain-realxed QDs system: a low energy QD family whose strain is relaxed by the generation of misfit dislocations, and a high energy QD family whose strain is mainly relieved by indium outdiffusion. The effect of interdot carrier transfer on temperature dependent PL is investigated. The integrated-PL intensity of low energy QDs shows two regimes (i) an unusual increment begins about 110 K (ii) and then drops rapidly above 160 K. The full width half maximum (FWHM) of the high energy QDs first decreases about 110 K and reaches a minimum value at about 200 K. The phenomenon can be attributed to that the carrier transfers between the bimodal QDs from the high to the low energy QDs through the InGaAs quantum well. Accordingly the carrier emission time determined by G-F measurement exhibits a V-shape versus the similar temperature dependence (78 K~140 K) due to carrier transfer between bimodal QDs in 3.3 ML sample. Based on G-F data analysis, the mechanism of carrier emission in a large electric field is likely phonon-assisted tunneling when temperature increased. Furthermore, we investigate the carrier interaction between QD and defect states by electrical measurements under illumination. Under the illumination less than 1.3 eV, the photo-capacitance produces origins that the photo-holes trapping into the deep defect level and the photo-electrons fill up at the shallow energy level. The enhance photo-capacitance casues by the trapped holes in the deep defect level and emitted electron from the QD state to bottom GaAs conduction band. Under the illumination of 1.3 eV, the large capacitance produces, suggesting an existence of potential drop at the vally of top GaAs conduction band. At the constant bias, trapped holes and emitted electrons into the valley would produce a potential drop at the valley region near QD. In order to the applied bias balance, the Fermi-level at QD region must drop to pin the QD energy level. Hence, the QD plateau can be found at the small reverse bias under the illumination of 1.3 eV. These photo-capacitance phemonenons also can be verified by theory simulation. Compairsion with InAs QD and GaAsN QW samples, photo-holes trap into deep defect level indeed due to the property of no comfinement in hole states of the GaAsN QW. After thermal annealing 700 ℃, PL spectra show the transitions of QW state enhance and deep defect level to electron state of QW lower and photo-capacitance decreases, suggesting deep defect removed by thermal annealing. Therefore, the sourse of the photo-capacitance is caused by photo-carrier interation between the quantum state and deep defect level.