適合高溫與低偏壓操作之超晶格紅外線偵測器的研發

The study and development of superlattice infrared photodetectors for high temperature and low bias operation

盧仁祥(Jen-Hsiang Lu)

指導教授 ： 管傑雄

國立臺灣大學/電機資訊學院/電子工程學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.02841

摘要

利用量子井紅外線偵測器所製成的大型焦平面陣列熱影像系統已經展現出許多可應用在軍事，醫藥與民生方面的可能。然而，操作在高溫下的量子井紅外線偵測器有一個缺點，就是相當高的暗電流，並且此缺點會使得其在製作熱影像系統時受到限制。在本論文中，我們的目標就是設計出一個具有較低暗電流與較高光電流，且適合在高溫下操作的紅外線偵測器。
如果我們將元件操作在較低的偏壓環境時，可以有效地降低其所產生的電流值。因此，因為超晶格具備適合低偏壓操作的特性，我們選擇超晶格結構做為我們偵測器中的主要作用結構。為了瞭解如何將偵測器結構參數理想化，使之更適合在高溫下操作，我們首先研究的是具有單一電流阻擋層的超晶格紅外線偵測器中的光響應與溫度的相依關係。我們歸納出共有四個因素會影響此相依關係，分別是超晶格中的摻雜濃度，外加偏壓大小，單一電流阻擋層的高度與厚度。摻雜濃度與溫度會影響在超晶格中第一迷你能帶的電子分佈情形，進而影響光響應。外加偏壓大小會影響電子在經過電流阻擋層時的散射行為，而電流阻擋層的高度與厚度則是分別影響穿隧機制與彈道傳遞行為。我們也以這些因素為基礎建立一個簡單的模型來模擬光響應與溫度的相依關係，而得到的結果也與我們的實驗結果吻合。
依據上面所述的因素，我們設計出一個將十五週期的超晶格結構與五十週期的量子井結構相結合的紅外線偵測器結構，來驗證它在高溫操作下的光響應。在此結構中，量子井結構主要是用來降低雜訊電流功率與增加響應的偵測波段。 我們發現超晶格所產生的光電流並沒有因為量子井結構而降低，但暗電流卻有減少。因此，由於暗電流與雜訊增益都降低，最大值的偵測度可以在負低偏壓時出現。此外，此元件即使操作在80 K下也依然可以觀察到光致電壓的響應。與先前的單一電流阻擋層的超晶格紅外線偵測器相比，此結構有較高的光電流與較低的暗電流，並且很適合操作在低偏壓與高溫的環境下。
雖然超晶格結構適合操作在低偏壓下，超晶格的光電流相較之下還是比量子井結構的要低。為了增加超晶格結構的光電流，我們設計了一個雙電流阻擋層的超晶格紅外線偵測器，其結構為超晶格結構夾在兩個不同厚度的電流阻擋層中。鄰近集極的薄電流阻擋層是為了讓電子以彈道傳導經過，因而減少受到散射的機率。而厚電流阻擋層則是為了阻擋一些向後傳遞的電子，因而增加光響應的強度。從實驗結果中，我們發現此元件的光響應的確有增加，所以增加此一厚電流阻擋層的確有助於增加超晶格紅外線偵測器的光響應強度，特別是在低偏壓操作下。若將我們上述的量子井結構與厚電流阻擋層結合，並設計在同一個偵測器結構中，再將週期性金屬光柵金屬做在元件的表面以利吸收正向入射的紅外光，此元件應該是在製作適合高溫操作下的熱影像系統很好的選擇。

關鍵字

紅外線偵測器 ； 超晶格 ； 量子井 ； 光響應與溫度的相依關係 ； 光響應增強 ； 高溫與低偏壓操作

並列摘要

The large scale focal plane array (FPA) imaging systems based on quantum well infrared photodetectors (QWIPs) have shown the potential use for military, medical and civil applications. However, the drawback of QWIPs under high temperature operation is the high dark current. In this dissertation, the aim is to design the superlattice infrared photodetectors (SLIPs) which are suitable for high temperature (80 K) and low bias operation with the enhanced photocurrent and the lower dark current.
The current magnitude can be lowed if we operate the device under low bias range. Due to the low operational bias of SL, we choose superlattice (SL) as the active region of photodetectors. At first, we study the temperature dependence of photoresponse of a single-barrier SLIP whose structure is a 15-period SL with a single barrier to know how to optimize its structure parameters. We conclude that four factors will affect the temperature dependence including doping density in SL, externally applied bias, the single barrier’s thickness and energy height. The doping density and temperature will change electron distribution in the first miniband of SL, and thereby cause the variations of photoresponse. Besides, the scattering during electron transport through the single barrier is expected to increase with the applied bias. The single barrier’s thickness and height influence the ballistic transport behavior and the tunneling mechanism, respectively. We also establish a simple model based on these factors to simulate the temperature dependence of photoresponse. The fitting results show the agreement with the experimental results.
By understanding these factors’ effect, we design an infrared photodetector using the structure of a 15-period SL integrated with a 50-period multiple quantum wells (MQWs) to demonstrate its photoresponse under high temperature and low bias operation. The MQWs are utilized to reduce the noise current power and to add the response range. We find that the photocurrent of this device is not reduced by the additional MQWs but the dark current is. Hence, due to the low noise gain and low dark current, the maximum detectivity (D*) can occur at low negative bias. In addition, the photovoltaic response even appears at 80 K. In comparison with the previous sample, a 15-period SL with a single barrier, this device demonstrates not only the higher photocurrent and lower dark current, but also the better temperature dependence of photoresponse because the whole spectrum increases with temperature. By using the MQWs as a noise filter, this device is more suitable for operation under low bias and high temperature condition.
Because of the lower electron mobility in the miniband, the photocurrent of SL is relatively lower than that of MQWs. Therefore, we design a double-barrier SLIP whose structure is a SL sandwiched between two different thickness barriers to enhance its photocurrent. The thin barrier adjacent to the collector contact is for electrons to traverse ballistically and to reduce the scattering loss of photocurrent, while the thick barrier is to block electrons moving backward and thereby to increase the photocurrent. However, we find that the current-voltage curves of this device are quite asymmetric. We attribute it to the carrier depletion in the SL and the resulting large built-in electric field to worsen its performance. In order to solve this problem, we fabricate the metallic contact on the SL instead of the emitter contact layer to supply electrons immediately. For this new processed device, we observe that its photoresponse is increased and the dark current is lowered. From our results, we find a backward thick barrier is helpful for photoresponse enhancement in SLIPs, especially under low bias operation.
In our future work, we will integrate the above structures such as the thick barrier and MQWs into one device to improve its performance. For the n-type quantum well light coupling problem, we adopt the periodic metal grating layer on the top of the photodetector to absorb the normal incident light. Based on these concepts, we hope that we can fabricate a photodetector array with good performance which can be operated at 80 K and low bias condition.

並列關鍵字

Infrared Photodetectors ； Superlattice ； Quantum Wells ； Temperature Dependence of Photoresponse ； Photoresponse Enhancement ； High Temperature and Low Bias Operation

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