GaAsN/GaAs量子井結構中光激發載子的光電容與光電流分析

Abstract

本論文主要為研究GaAsN/GaAs量子井結構中光激發載子的光電容與光電流的 特性。首先，我們為GaAsN量子井中的光激發載子建立一套行為模型，並藉此模型來瞭解GaAsN量子井中光電流與光電容的產生機制。而根據我們所建立的模型，GaAsN量子井中光電流和光電容的產生是與其光激發載子速率、載子復合速率、電子與電洞的放射速率…等密切相關。接下來，利用我們所建立的光激發載子行為模型來探討GaAsN量子井結構在各種不同情況下（包含不同的環境溫度下、不同的外加電場下、樣品經過熱退火後、不同的量子井厚度）其光激發載子的行為。 對於不同的環境溫度，GaAsN QW的PL發光效率隨溫度的提高而下降，此現象即意味著GaAsN QW的載子復合速率隨溫度的提高而減弱。因此，隨著環境溫度的提高，GaAsN QW中會有更多的電子-電洞對貢獻在光電流與光電容的產生上。此外，對於不同厚度的GaAsN QW，其電子-電洞波函數在空間中重疊的程度會隨著厚度的增加而減少，且其電子-電洞波函數在空間中重疊的程度即對應於GaAsN QW的載子復合速率。因此，隨著GaAsN QW厚度的增加，其載子復合速率會隨之下降，因而會有更多的電子-電洞對貢獻在光電流與光電容的產生上。 而由於電子與電洞的放射速率會隨偏壓的增大而提升，因此在較小的外加偏壓時，GaAsN QW中的電子放射速率會隨偏壓的增大而提升，所以GaAsN QW中會有更多的電洞貢獻在其光電容的產生；然而，隨著外加偏壓再增大時，此時GaAsN QW中的電洞放射速率會提升至接近其電子放射速率的程度，因而導致GaAsN QW的光電流隨之提高，且GaAsN QW的光電容亦隨之下降。當GaAsN QW中存在氮相關局部侷限能階(N-related localized states)時，其會抑制GaAsN QW電子能階的穿隧放射特性，進而造成其有著較慢的電子放射速率；而當GaAsN QW經過熱退火後，GaAsN QW中的氮相關局部侷限能階(N-related localized states)會隨之減少，因此GaAsN QW電子能階會恢復為一般高品質量子井的穿隧放射特性，進而造成其有著相當快的電子放射速率。所以，當GaAsN QW經過熱退火後，GaAsN QW中的淨電洞量會隨之增加，因而其有著相對較大的光電容值。 同時，我們也藉由光電容的暫態變化來探討GaAsN QW中光產生載子的反應速率。對於GaAsN QW所產生的光電容，其是由於電洞被侷限在GaAsN QW的電洞能階。因此，在中斷激發光源後，相較於電洞被侷限在缺陷能階所造成的光電容，GaAsN QW所產生之光電容在極短的時間內即會消失。並且，GaAsN QW所產生之光電容在極短時間內即消失之特性是由於GaAsN QW有著相對較快的電洞放射速率。

This study investigates the photo-capacitance and photo-current of light-induced excess carriers in GaAsN/GaAs quantum wells. Initially, we establish a escaping model for light-induced excess carriers in GaAsN quantum wells (QWs), and utilize this escaping model to understand the generation mechanism of photo-capacitance and photo-current in GaAsN QWs. According to our escaping model for light-induced excess carriers, the generation of photo-capacitance and photo-current in GaAsN QWs is correlated to the generation rate of light-induced excess carriers in GaAsN QWs、recombination rate of light-induced excess carriers in GaAsN QWs、electron and hole emission rates of GaAsN QWs, and these behaviors of light-induced excess carriers in GaAsN QWs are different under various conditions. Therefore, we utilize photo-capacitance and photo-current analysis combined with our escaping model to probe the light-induced excess carriers in GaAsN QWs under various conditions. During the increase of temperature, the photoluminescence (PL) efficiency of GaAsN QWs is decreased, and this result is also indicated that the recombination rate of light-induced excess carriers is reduced during increasing temperature. Thus, during increasing temperature, the amount of electron-hole pairs escaping from GaAsN QWs is increased, resulting in the enhancement of photo-capacitance and photo-current in GaAsN QWs. When the thickness of GaAsN QWs is increased, the overlapping of electron and hole wave functions in GaAsN QWs is reduced, and the overlapping of electron and hole wave functions in GaAsN QWs is also correspond to the recombination rate of light-induced excess carriers in GaAsN QWs. Hence, as increasing the thickness of GaAsN QWs, the amount of electron-hole pairs escaping from GaAsN QWs is increased simultaneously, leading to the enhancement of photo-capacitance and photo-current in GaAsN QWs. Furthermore, the electron emission rate of GaAsN QWs electron states is determined by the electric field in the bottom GaAs, which depends on the applied bias. Therefore, as the applied bias increasing, the electron emission rate of GaAsN QWs electron states is also increased, leading to the enhancement for photo-capacitance of GaAsN QWs. However, further increasing the applied bias, the hole emission rate of GaAsN QWs will increase to the extent close to the electron emission rate of GaAsN QWs, resulting in the enhancement of photo-current and the diminution of photo-capacitance. In addition, the presence of N-related localized states effectively suppresses the tunneling emission of GaAsN QW electron states, leading to a slow electron emission rate for GaAsN QWs； thermal annealing can reduce the number of N-related localized states, resulting in a recovery of the tunneling emission of GaAsN QWs electron states, leading to a fast electron emission rate for GaAsN QWs. Therefore, during thermal annealing, the electron emission rate of GaAsN QW is also increased, resulting in the enhancement of photo-capacitance in GaAsN QWs. Moreover, we also use the transient measurement to probe the decay rate of photo-capacitance in GaAsN QWs. According to our results, comparing with the photo-capacitance induced by the defect states, because the photo-capacitance in GaAsN QWs is induced by the holes confined in GaAsN QWs, the decay rate of photo-capacitance in GaAsN QWs is relatively faster than the decay rate of photo-capacitance induced by the defect states, which is attributed to the relatively fast hole emission rate for GaAsN QWs.