雙能位障超晶格紅外線偵測器的研發

Abstract

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性。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。 我們設計了雙能障超晶格紅外線偵測器結構並分析其暗電流機制。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。因為超晶格具備適合低偏壓操作的特性，故我們選擇超晶格結構做為我們偵測器中的主要作用結構。為了瞭解如何將暗電流有效抑制使之更適合在高溫下操作，我們歸納出三種主要的暗電流機制，其中正偏壓時暗電流會急速上升而低偏壓時則不會有此現象，我們也以這些機制為基礎建立解決方案，即將原本沉積在底部射極金屬接觸層改成沉積在超晶格上改善了暗電流過大的問題，而實驗結果也證明有效抑制了暗電流。雖然超晶格結構適合操作在低偏壓下，但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，我們採用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 (0.17V )，且在100 K時達到1.1×109 cm Hz1/2/W 之偵測率。而我們更進一步分析了光電子在第二能帶的增強穿透現象，在低偏壓時因為光電子的增強穿透現象使得整體的光響應有效提升。 依據上面結果，我們最後設計出一個將十五週期的超晶格結構與三十週期的量子井結構相結合在輔以雙能障的概念之紅外線偵測器結構，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110 K溫度下也依然可以觀察到光響應。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。 根據前面的研究結果，超晶格被包含在兩厚能位障的週期結構，此紅外線超晶格與量子井集合的偵測器其光響應範圍橫跨7~16 μm，並有電壓調變光響應的特性。此元件受摻雜遷移影響而有內建電場存在，所以可以操作在光致電壓模式。極低偏壓下，內建電場將主導整個能帶。開路響應有兩個波峰，此兩個波峰所對應的電子能態運動方向相反，因此在低偏壓下，施加同樣強度但不同極性之電場，有大小相異的響應。我們提出受內建電場影響的能帶圖，理論計算後得到相對應的兩個能態，及其能態運動相反的原因。 最後若將我們將週期性金屬結構做在元件的表面以利吸收正向入射的紅外光，此元件應該是製作適合高溫操作下的熱影像系統之單一影像細胞很好的選擇。

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, we describe the design of 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. Therefore, we design a double-barrier SLIP whose structure is a SL sandwichedbetween two different thickness barriers to enhance its photocurrent. The current magnitude can be lowed if we operate the device under low bias range. Due to thelow operational bias of SL, we choose superlattice (SL) as the active region of photodetectors.Because of the lower electron mobility in the miniband, the photocurrent of SL is relatively lowerthan that of MQWs. The thin barrier adjacent to thecollector 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 increasethe 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 degrade 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. The detectivity (0.17 V) at 100K is 1.1×109 cm Hz1/2/W. We further analyze that there exist photoelectron resonance in second miniband for photoresponse enhancement. By understanding effect of these factors, we design an infrared photodetector using the structure of a 15-period SL integrated with a 30-period multiple quantum wells (MQWs) and thick barrier. We study its photoresponse under high temperature and low bias operation. The MQWs are utilized to reduce the noise current power. 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. By using the MQWs as a noise filter, this device is more suitable for operation under low bias and high temperature condition. From above experimental results, a new structure of superlattices bounded by two thick barriers was studied. The detection wavelength of this infrared photodetector was around 7~16μm and this detector shows the voltage-tunable phenomenon. Build-in electric field exists in the device because of dopant migration. As a consequence, this detector can be operated in photovoltaic mode. Under ultra low bias, build-in electric field dominates the energy band. Two peaks can be obtained in the open-circuit photoresponse. Photoelectrons in the two energy states which are corresponding to the two peaks transport to different directions. Hence, responsivity between two peaks is different under same amplitude but different bias polarity. Finally, We presented the energy band diagram affected by build-in electric field in detail. In future work, the n-type quantum well light coupling problem, we adopt the periodic hole array and 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 high temperature and low bias condition.