The photocurrent suppressed by light-induced excess electrons in GaAsN/GaAs quantum well：Analysis of RC time constant of the equivalent circuit
|關鍵字:||分子束磊晶;GaAsN/GaAs量子井;光電流;光電流抑制;RC時間常數;MBE;GaAsN/GaAs quantum well;Photocurrent;Photocurrent suppression;RC time constant|
|摘要:||藉由本論文的探討中可以很明確的了解GaAsN/GaAs量子井結構在照光後的載子分佈情形、電場變化情形、內建壓降產生的機制、以及光電流抑制效應的原理。首先，我們藉由GaAsN量子井樣品在光激發螢光下的光譜圖以及TEM分析得知，熱退火改善了原本GaAsN量子井中嚴重的N原子成分波動效應，因此減少了N原子群聚的數量，同時提升了GaAsN量子井對電子的侷限能力。我們也在熱退火的GaAsN量子井樣品中觀察到有較明顯的光電容抬升，根據我們所建立的GaAsN量子井照光模型，認為在照射能量為1.32eV的激發光源下，整個系統將分成Schottky band (蕭基接面至GaAsN量子井之間的能帶結構) 跟Quantum band (GaAsN量子井附近的能帶結構)，光激發產生載子(電子)受到外加電場影響而暫存於GaAsN量子井內使跨在Quantum band的壓降上升，為了維持整個系統的電壓平衡，必須使跨在Schottky band的壓降降低，因此造成在Schottky band形成的光電流值降低，我們稱此現象為「抑制光電流的情形」。此情形在低溫下改變I-V量測掃動速率的實驗中可以很明顯的看出，掃動速率越慢，抑制光電流的情形將更為明顯。我們也藉由不同環境溫度下的量測以及模擬得知，升溫的過程即是將樣品整體電阻值降低，Quantum band上的壓降生成速率提升，抑制光電流的情形將更加訊速。另一方面，我們利用照射1.22eV的激發光源在改變掃動速率的I-V圖中發現，GaAsN量子井形成的光電流值亦有明顯的下降，推測為電子暫存於GaAsN量子井能階，造成GaAsN量子井對光源的吸收率及載子產生率降低所引致的另一種抑制光電流的情形。|
This study elucidates the carrier redistribution, electric field variation, formation mechanism of interior potential drop, and photocurrent suppression of GaAsN/GaAs quantum well (QW) under illumination. Initially, according to the photoluminescence (PL) and TEM analysis, 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, thermal annealing enhances QW confinement of electrons. We apply capacitance-voltage (C-V) measurement to confirm that the sample after thermal annealing is resulting in a large rise of photocapacitance under illumination. Accroding to our model for light induced excess electrons in QW, we consider that the energy band can be divided into the Schottky band (The energy band between Schottky contact and QW) and the quantum band (The energy band around QW) under the illuminating energy of 1.32eV. Light induced excess electrons cause a temporary existence of net negative charge in QW when illumination achieving the steady-state, resulting in a rise of potential drop across the quantum band. In order to maintain the balance of potential drop across the energy band, the potential drop across the Schottky band must eventually decline, which decreases photocurrent from the top GaAs layer, the so-called “Photocurrent suppression”. This phenomenon can be observed in the various sweep rate of current-voltage (I-V) measurement at low temperature. If the sweep rate is slow, then the photocurrent suppression becomes significant. Furthermore, we analyze the various sweep rate of I-V measurements under different temperature. In these experiments, at low temperature, the rate of photocurrent suppression is slow, which is attributed to smaller charging current to the QW. As temperature increases, the rate of photocurrent suppression becomes faster because of increasing charging current. Thus, the different rate of photocurrent suppression is due to the different resistance of the equivalent circuit. The resistance is decreased with increasing temperature. On the other hand, according to the various sweep rate of I-V measurements under the illuminating energy of 1.22eV, we’ve also observed that the photocurrent suppression not only for top GaAs layer, but also for QW. This result indicates that energy states of QW are gradually occupied by electrons. These elecrtons work to decrease absorption and carrier generation, resulting in the decrease of photocurrent from GaAsN/GaAs QW.