砷化銦/砷化銦鎵Dots in Well量子點結構之電性研究

Abstract

本論文主要探討的是由晶格不匹配之三五族半導體材料所成長之Dots in Well(DWELL)結構(InAs/InGaAs)之光電特性。我們成長三片不同厚度的量子點(InAs)樣品分別為1.98ML、2.31ML及3.30ML，量子井(InGaAs)的厚度都是 60Å。其中1.98ML及2.31ML樣品的波長分別為1240nm與1300nm。2.31ML的峰值強度比1.98ML大，半高寬也比較小，可見2.31ML樣品的量子侷限效應比1.98ML樣品來的好。從C-V量測可以發現這兩片樣品的量子侷限效應都相當不錯，對頻率變化也不會有響應發生。我們也從量子侷限效應最好的2.31ML樣品去估計被侷限的載子濃度約為 ，和我們的量子點濃度 去作比較可以發現，DWELL結構所侷限的載子大部分是由量子井所侷限。此外，DLTS的量測並未發現1.98ML和2.31ML樣品有任何缺陷訊號的存在，所以對於1.98ML和2.31ML的樣品而言磊晶品質是相當不錯的。從3.30ML的PL量測結果可以明顯看到半高寬較其他兩片樣品大了許多，峰值訊號強度也小了許多，且量子點分成兩群，發光波長分別落在1200nm與1300nm附近，我們推測原因是量子點厚度已經超過臨界值而受到晶格鬆弛影響所致。此外，從IV的量測結果可也以明顯看到3.30ML樣品已經有串聯電阻的效應發生，在其他兩片樣品則觀察不到。再從變頻CV量測的結果可以看到3.30ML樣品對頻率變化有明顯的響應，所以我們推測其載子被侷限的效應是被缺陷捕捉所造成的，並非是量子侷限效應。而且我們在量子點/量子井區附近也觀察到載子濃度低於背景濃度且隨著偏壓加大載子濃度便急速上升的現象。為了探究其原因，我們對3.30ML樣品在量子點/量子井區作G-T量測及DLTS量測，分別得到0.20eV及0.60eV兩個不同活化能的缺陷能階，其中活化能0.20eV的缺陷是造成載子從量子點區放射出來所需一時間常數的原因；而活化能為0.6eV的缺陷則是造成當偏壓大到使費米能階低於此缺陷能階時，大量被缺陷捕捉的載子跳至導電帶而使得載子濃度急速上升的現象發生。此外，我們也研究自由載子通過量子點/量子井區的穿越特性，根據小偏壓的C-F量測可以發現三片樣品都存在串聯電阻的效應，所以推論DWELL結構本身就會形成一串聯電阻，然而以量子點濃度 而言，量子點與量子點間將會有高濃度區域的存在可以讓自由載子通過，因此串聯電阻不大。我們也提出了一個等效電路模型來估計此串聯電阻大小，最後發現此串聯電阻大小是隨InAs QD成長厚度增加而增大。

The electric properties of InAs/InGaAs dots in well structures are investigated by current-voltage (I-V), capacitance-voltage (C-V), admittance and deep-level transient spectroscopy. Three samples with different InAs deposition thickness of 1.98, 2.31 and 3.30ML, are grown by molecular beam epitaxy (MBE). The PL data show QD emission wavelength of 1240 and 1300 nm for the 1.98 and 2.31ML samples. From their C-V spectra which are found to be frequency –independent, good carrier confinement in the QD/QW region can been seen for both samples. No defect traps are observed in both samples by DLTS. This result indicates that by increasing the InAs thickness to 2.31ML, good quality InAs/InGaAs Dots in well emission at 1300nm can be achieved. However, further increasing the InAs deposition to 3.30ML, the PL spectra shows two groups of different size QDs, one emits at 1223nm and the other at 1300nm. The PL intensity of the large-size QDs is much weaker than that of the small-size QDs. In contrast to the other two samples, the C-V spectra of the 3.30ML sample show frequency dependent with carrier depletion at high frequency and poor carrier confinement at low frequency. This result suggests that the electrons emit from the QD/QW region with an observable time constant, from C-F measurement, we determine activation energy of 0.20eV. Another trap at 0.60eV is also detected around the QD region by DLTS. Therefore, we conclude that the free electrons are depleted around the QW/QD region probably be these two traps at 0.20eV and 0.60eV. Moreover, we also study the transport property of free electrons through the QD/QW region by analyzing the capacitance spectra at small bias. Based on a simple equivalent circuit which includes the potential distribution across the device, we determine the series resistance for the free electrons in the front Schottky to transverse through the QD/QW region and show an increase of the series resistance with increasing the InAs deposition thickness.