(In)GaAsN/GaAs量子井中N相關之局部侷限能階的形成機制與電性量測分析

Abstract

本論文主要是探討N相關之局部侷限能階在(In)GaAsN量子井(QW)中的形成機制、確切性質、電子捕捉能階特性、以及其對於量子井之電子放射特性所造成的影響。並且，我們同時也對量子井中光激發載子的特性與傳輸行為以及N相關之局部侷限能階的存在對於量子井中光激發載子的影響作一完整且詳細的探討。由於N相關之局部侷限能階為N原子在QW中群聚所產生的電子能階，因此N相關之局部侷限能階的形成與N原子在QW中的群聚情況息息相關；並且，N原子的群聚效應與N原子的成份波動效應會相互關聯。對於高N原子摻雜濃度的QW而言，其N原子的成份波動效應會較嚴重，因而N原子會發生更多種類的群聚方式並同時增加N原子的群聚數量，進而造成N相關之局部侷限能階的增加，並且使得N相關之局部侷限能階訊號變寬。對於長晶時間較久(或長晶速率較慢)的QW而言，摻雜的N原子有足夠的時間在QW中隨意移動並且發生群聚，因而QW中的N原子成份波動效應與N原子群聚效應會較為嚴重，進而造成其有著大量的N相關之局部侷限能階。而熱退火主要是提供N原子足夠的動能使其可以在晶格中隨意的移動並且重新排列，進而對QW中N原子的分佈情況造成影響。當QW中N原子群聚效應與N原子成份波動效應不嚴重時，熱退火反而會使N原子群聚效應與N原子成份波動效應變得較為嚴重，因而造成QW在熱退火後產生了N相關之局部侷限能階；而當QW中N原子群聚效應與N原子成份波動效應較嚴重時，熱退火則會改善N原子群聚效應與N原子成份波動效應，因而造成N相關之局部侷限能階在熱退火後被減少。此外，當N原子在(In)GaAsN QW中群聚時，其效果就像是在(In)GaAsN QW中夾了大小不一的量子點，並且此N原子群聚量子點會在(In)GaAsN QW價電帶的上方產生量子侷限能階。因此，N相關之局部侷限能階與QW電子能階有著類似的尺寸侷限效應。

而N相關之局部侷限能階不僅只是在光譜中會發光的電子能階，其同時也是在半導體中會捕捉電子的電子捕捉能階。並且，N相關之局部侷限能階可藉由N原子摻雜濃度來控制其捕捉電子的能力。當N相關之局部侷限能階是由數量與N原子群聚種類較多(較少)的N原子群聚量子點所構成時，則其可以捕捉較多(較少)的電子，因而其有著較好(較差)的電子捕捉能力，即對應於較大(較小)的電子捕捉截面積。此外，當N相關之局部侷限能階存在於量子井中時，其會有效地抑制QW電子能階的穿隧放射特性，進而造成QW電子能階訊號有著較長的電子放射時間常數。而熱退火可以減少GaAsN QW中的N相關之局部侷限能階，進而造成GaAsN QW電子能階的穿隧放射特性之恢復。並且，隨著熱退火溫度的提高，N相關之局部侷限能階會大幅地隨之減少，因而GaAsN QW電子能階的電子放射特性會恢復為一般高品質QW的穿隧放射特性。

最後，我們探討量子井結構中光電流與光電容的產生機制，並藉此建立一個分析量子井中光激發載子之特性與傳輸行為的完整模型。根據本論文之探討，GaAsN QW與N相關之局部侷限能階本身除了可在結構中產生電流路徑外，其同時會造成淨正電荷量之暫存，因而造成光電容的抬升。而當N相關之局部侷限能階存在於GaAsN QW中時，其可延伸光電容變化的反應範圍以及增加光電容變化的反應靈敏度；並且，N相關之局部侷限能階同時可為GaAsN QW中的光激發載子提供一個額外的電流路徑，進而造成光電流的增加。此外，GaAsN QW電子能階之電子放射速率(1/電子放射時間常數)可藉由不同波長的入射光來調變，且其調變機制即為底層GaAs空乏區之寬度受到不同波長的入射光所調變。並且，由於GaAsN QW電子能階之電子放射速率在不同波長的入射光下可達到兩倍的變化量，因而GaAsN QW電子能階之電子放射速率en的變化可被應用於辨別入射光之波長。

This dissertation investigates the formation mechanism, exact nature, and electrical properties of the N-related localized states in (In)GaAsN/GaAs quantum well (QW). Furthermore, this dissertation also elucidates the influence of the N-related localized states on electron emission properties of QW and the properties of light-induced excess carriers in GaAsN QW containing N-related localized states. The formation of N-related localized states is associated with the clustering of N atoms and N-composition fluctuation in QW. The high N concentration in QW causes severe N-composition fluctuation, subsequently leading to the (In)GaAsN layer having more different forms of N-atoms clusters, resulting in a considerable amount of N-related localized states in QW. The long growth time (or low growth rate) of (In)GaAsN layer provides the N atoms enough time to segregate and cluster in the (In)GaAsN layer, leading to the degradation of the N-composition fluctuation and the drastic clustering of the N atoms, and thus the number of N-related localized states is increased during elongating the growth time (or lowering the growth rate) of (In)GaAsN layer. Thermal annealing provides sufficient kinetic energy for N atoms to segregate randomly, leading to the reorganization of N atoms after thermal annealing. Hence, when N-composition fluctuation and clustering of N atoms are not severe in QW, thermal annealing degrades the N-composition fluctuation and increases the probability of N atoms clustering, resulting in the formation of N-related localized states after thermal annealing; when N-composition fluctuation and clustering of N atoms are severe in QW, thermal annealing ameliorates the N-composition fluctuation and decreases the amount of N-atoms clusters, resulting in the reduction of N-related localized states after thermal annealing. Moreover, the clusters of N atoms in the (In)GaAsN QW act as N clusters quantum dots, and form quantum confined states above the (In)GaAsN conduction band. Therefore, the emission from the N-related localized states exhibits a similar size-confinement effect to that from the QW.

In addition, the N-related localized states in (In)GaAsN QW are identified as both optical and electrical electron trap states. Besides, the trapping ability (electron capture cross section) of N-related localized states can be modulated by N concentration in the QW. When QW has a high (lower) N concentration, the N-related localized states are formed by the more (fewer) N clusters quantum dots, and thus the N-related localized states can trap more (less) electrons, resulting in the increase (decrease) of electron capture cross section for N-related localized states. Furthermore, the presence of N-related localized states effectively suppresses the tunneling emission of QW electron states, leading to a long electron emission time for QW electron states. Thermal annealing can reduce the number of N-related localized states, resulting in a recovery of the tunneling emission for QW electron states. Increasing the annealing temperature can restore the electron emission behavior of QW to the typical electron tunneling emission for a high-quality QW.

Finally, more than just providing current path for light-induced excess carriers in QW, GaAsN QW and N-related localized states also cause a temporary existence of net positive charge in QW when illumination achieving the steady-state, resulting in a rise of photocapacitance after illumination. Therefore, N-related localized states in GaAsN QW can extend response range and response sensitivity on photocapacitance, and produce an additional current path for light-induced excess carriers in QW. Furthermore, the electron emission rate of GaAsN QW electron states can be modulated by different incident photon energy, which is due to the modulation of depletion width of the bottom GaAs. This result also indicate that GaAsN QW containing N-related localized states is suitable for application of high sensitive photodetector. The variation of electron emission time constant can reach twice under different incident photon energy. Thus, the variation of electron emission time constant can be utilized to distinguish the incident photon energy.