量子井紅外線偵測器之研究

Abstract

本篇論文之目的在針對傳統n型量子井紅外線偵測器的缺點進行改善。這裡針對兩個主要的研究課題來進行研究：正向入射吸收及低暗電流之結構設計。此外，我們也利用自發應變誘導組成的量子點結構取代傳統的量子井結構，研製量子點偵測器。 在正向入射吸收的研究中，我們採用高應變砷化銦鎵/砷化鎵量子井的結構來進行元件設計。這種結構具有較強的能帶混成效應，在實驗上已被證實能夠吸收正向入射之紅外線。我們將砷化銦鎵/砷化鎵量子井結構置於傳統之量子井紅外線偵測器中，實驗發現確實得到高正向入射之響應。利用改變不同的量子井雜質濃度，我們發現高雜質濃度對正向入射吸收有增強的作用。其正向入射吸收率可以到達一般橫向吸收的0.4倍。 利用這個結果我們繼續研製高正向入射吸收之中波段偵測器。利用高雜質之砷化銦鎵/砷化鋁鎵之量子井結構，我們得到在4.5微米具有高正向入射之響應之偵測器。利用這個結果搭配上述之砷化銦鎵/砷化鎵量子井結構，我們成功的得到正向入射響應的雙波段偵測器。而利用砷化銦鎵/砷化鋁鎵之量子井結構的調整，我們設計出兩種不同調變形式的雙波段偵測器。 在降低暗電流的部份，我們提出了一個創新的非均勻量子井組成，將傳統相同的多重量子井改變成具有不同位障寬度及雜質濃度的量子井組合。實驗發現暗電流可降低十倍左右，而同時其光響應並不受到影響。這有效地提升了元件的背景限制溫度到77K以上。而元件的偵測度更提升了兩倍。我們利用元件的電場分布計算，成功的解釋這種結構的操作原理，並且解釋其溫度效應。 對於量子點紅外線偵測器的研究，我們成功的製作出寬頻的長波段偵測器。並且利用不同的磊晶條件得到在不同波段吸收的量子點。利用原子力顯微鏡的觀察我們調整適當的雜質濃度。這個元件的表現已達到世界上其他實驗結果的水準。量子點偵測器具有較高的光增益，但是其暗電流與響應度的表現目前仍無法與量子井結構比擬，我們利用其暗電流之分析了解其高暗電流之原因並且提出一可降低暗電流與提高響應度之新結構。

The purpose of this dissertation is to solve two major problems of quantum well infrared photodetectors(QWIPs). The conventional n-type QWIPs normally suffer from higher dark currents compared to the HgCdTe detectors. Because of the selection rule for the inter-subband transitions, they can not absorb light with electric field polarized parallel to the quantum wells. Besides, the novel self-assembled quantum dot structure was used to fabricate quantum dot infrared photodetectors. In the studies of normal incident absorption, the highly strained InGaAs/GaAs quantum wells were used. With the narrow bandgap structure, the band mixing effect is enhanced, and the TE mode absorption has been observed experimentally. In our experiment, the InGaAs/GaAs QWIPs showed large normal incident response comparable to the samples with surface gratings. The TE to TM mode reponsivity ratio is about 0.4. By changing the doping concentration of the quantum wells, the doping effect of the TE mode absorption was found. The high doping wells is needed for the TE mode absorption. Using this concept, the InGaAs/AlGaAs MWIR QWIPs with large TE mode response was demonstrated. Combining a LWIR structure with a MWIR structure, the normal incident two color QWIPs were fabricated. Compared to the conventional MWIR QWIPs, the InGaAs/AlGaAs structure gives more flexibility of the MWIR QWIPs. Two different structures were demonstrated to show different tuning behavior of the two color QWIPs. In the studies of low dark current QWIPs, a new non-uniform structure was proposed. The conventional uniform structure is replaced by a non-uniform structure with different doping concentration and barrier width for the wells. The dark current was observed to be lower by an order of magnitude while the responsivity of the new devices remained the same. The BLIP temperature was enhanced by 6 degrees compared to the uniform structure and was higher than 77K. Besides, the detectivity was twice of the uniform sample. These characteristics are explained by the non-uniform electric field distribution in the devices. A local high electric field region causes impact ionization resulting in a higher gain. The electric field distribution is calculated by a self-consistent model. The temperature effect is also explained by this model. Board band LWIR detectors were successfully fabricated by the self assembled InAs quantum dots. Using different growth conditions, different absorption spectra were obtained. The density and the shape of the dots were investigated by AFM. Optimal doping concentration was determined. The performance of the device is still worse than the QWIPs. By the dark current analysis, the origin of high dark current and low responsivity were suggested, and a new design of QDIP was proposed to eliminate the problems.