砷化銦量子點上覆蓋不同材料之電性研究

Abstract

本論文主要是探討低成長速率InAs量子點、傳統Dots-in-Well結構量子點，在DWELL結構中插入了一層10A In0.14AlAs高位能障，以及覆蓋了In0.14AlAs高位能障但將InGaAs QW拿掉等結構的量子點光電特性異同。五片樣品分別是0.09ML/sec的InAs量子點直接覆蓋上GaAs層、0.26ML/sec的InAs量子點蓋上44A In0.14GaAs量子井而形成的DWELL結構、在DWELL結構中插入10A In0.14AlAs高位能障、再將In0.14GaAs QW拿掉只蓋上10A的In0.14AlAs高位能障以及再將In0.14AlAs高位能障加厚到54A。五片樣品的發光波長分別是1276nm、1306nm、1284nm、1264nm以及1268nm都接近目標的1300nm。我們利用計算面積的方法來分析PL及縱深分佈圖，在低溫下縱深分佈出現兩個峰值分別可以對應PL中量子點基態以及激發態對載子的侷限。其中擁有最簡單結構的低成長速率InAs量子點樣品，自PL和縱深分佈上來看半高寬以及量子點密度以及基態與激發態間的能階差表現最不理想，只要加上In0.14AlAs高位能障這材料，不論是不是需要再加上In0.14GaAs量子井，都可使得量子點密度增加到9x1010 cm-2，ΔEge拉大到99meV以上，並且波長也接近1300nm。此外低長晶速率InAs QDs以及加上10A In0.14AlAs樣品中有無法避免的串聯電阻存在，推測這個串聯電阻的產生是來自於GaAs層的摻雜濃度太低(<6x1016cm-3)，串聯電阻造成的時間常數跟量子結構造成的時間常數剛好在同一個範圍，不小心就可能造成對訊號的誤判，我們應用簡單的等效電路模型修正了串聯電阻對C-V量測所造成的影響，低溫時縱深分佈上第二個載子侷限會對量測頻率有響應，並且估計出第二個載子侷限的活化能值約幾十個meV，加上沒有串聯電阻樣品使用G-T量到活化能分別為35meV和51meV。綜合各點推論出縱深分佈的看到的訊號來自於量子點的基態跟激發態，而載子在量子點中運動的行為是基態跳躍到激發態而後再穿隧到GaAs的價帶上，而這個數十個meV的活化能正是基態跟激發態間的能階差。綜合各片樣品來看，在InAs上面覆蓋適當厚度的In0.14AlAs而不用蓋上In0.14GaAs quantum well是最好的一個結構。

The electrical properties of InAs quantum dots covered by different capping layer are investigated by current-voltage (I-V), capacitance-voltage (C-V), and admittance spectroscopy. Five samples, capped with GaAs, 44A In0.14GaAs quantum well, 10A In0.14AlAs and 44A In0.14GaAs combination layer, 10A In0.14AlAs and 54A In0.14AlAs, are studied. The direct capping GaAs sample is grown at a rate as low as 0.09ML/sec and the others grown at 0.26ML/sec. Similar area ratio of low-temperature concentration depth profile and PL data suggests that the two accumulation peaks in the depth profile are the emission processes due to quantum-dot ground and excited state. Low n-GaAs buffer layer doping may cause series resistance with time constant about 10-6sec, a value close to the emission time for the carrier emitting from the ground state to the excited state of the QD. Base on an equivalent circuit model, we observe the quantum emission from the ground state of the QD after removing the effect of the series resistance. An activation energy about 50meV can be obtained, which is supposed to be the activation energy from the ground state to excited state of the QD. Comparison each sample’s FWHM and ground/excited state area ratio from PL and concentration depth profile suggests that the structure capped with 10A In0.14AlAs without the In0.14GaAs QW is the preferable one.