含氮局部侷限能階之GaAsN/GaAs量子井的電子放射特性:照光的影響

Abstract

本論文為探討含有氮局部侷限能階(N-related localized states)之GaAsN/GaAs量子井(QW)其電子放射特性經照光後的影響以及外加偏壓後的螢光特性分析。首先，從電流-電壓(I-V)量測中發現，經照光後產生一個額外的電流貢獻，此光電流發生的位置恰好對應至C-V曲線中的電容平台，說明此光電流是由GaAsN量子井及氮局部侷限能階所貢獻的，藉由選擇激發光源能量的探討，更加確認此光電流的產生來源。 接著，我們利用光電容量測來探討照光對GaAsN量子井載子放射特性的影響。當光源能量激發QW能階所產生的電容抬升，是由於照光後電子、電洞不等量地逃離量子井，淨電荷暫留於空乏區內，導致空乏區的變化。而在頻譜中發現氮局部侷限能階的存在會拉長量子井對光源能量的反應範圍。此外，我們也探討照光對於GaAsN量子井之電子放射速率的影響，在導納頻譜量測(G-F)中，照光後會使電子放射速率明顯增加，進一步在變換光源能量的量測中，發現載子放射速率可以被我們調變。藉由光電容頻譜量測之結果，再配合蕭特基空乏區理論(Schottky depletion theory)，我們可以得知由於各種能量之激發光所產生的淨電荷量不同，導致空乏區、費米能階位置的改變，進而影響載子的穿隧放射速率。同時，在深層能階暫態頻譜(DLTS)中，除了觀察到照光下電子放射速率增加的現象外，在低溫範圍還產生一個定值∆C的訊號，經分析此訊號為氮局部侷限能階之載子發生穿隧放射(tunneling emission)所產生。 最後，我們分析在外加偏壓下含氮局部侷限能階之GaAsN量子井的螢光特性。於低溫時，隨逆向偏壓的增加，由於QW能階的穿隧能障較小的緣故，GaAsN量子井螢光強度下降量大於氮局部侷限能階的螢光強度下降量；當溫度升高時，氮局部侷限能階螢光強度的下降量轉為較大。此結果是由於載子從氮局部侷限能階躍遷至QW的傳輸現象發生所導致。除此之外，我們由光激發I-V量測發現，氮局部侷限能階貢獻光電流的位置隨溫度升高往較小的逆向偏壓移動，此現象也同樣為載子傳輸現象所導致。因此，在GaAsN量子井中，氮局部侷限能階可以產生光電流的貢獻並延伸光電容的能量反應範圍，並且經由不同光源能量的激發可以調變載子放射速率的快慢，即說明氮局部侷限能階的存在是適合應用於光電元件的設計及其特性調變。

This study elucidates the electron emission properties of GaAsN/GaAs quantum well (QW) containing N-related localized states under illumination, and also analyzes the photoluminescence (PL) under applied reverse bias. Initially, according to the current-voltage (I-V) measurement under illumination, two stages of current rise are observed, and the bias range of these current rises corresponds to the voltage of electron escaping from the GaAsN QW and N-related localized states. We apply photocurrent measurement to confirm that the observed current rises are due to the GaAsN QW electron states and N-related localized states. Furthermore, photocapacitance measurement is utilized to investigate the electron emission properties of GaAsN QW containing N-related localized states under illumination. Both GaAsN QW electron states and N-related localized states can contribute the photocapacitance due to the photon-generated electrons escaping into bottom GaAs,which results in a decrease of depletion width of the bottom GaAs. Capacitance-frequency (C-F) measurement was performed to obtain electron emission rate of GaAsN QW electron states under different incident photon energies. According to Schottky depletion theory, the modulation of electron emission rate with photon energy is due to the modulation of Fermi level position. Moreover, we also examine the influence of illuminance on electron emission rate of N-related localized states by deep-level transient spectroscopy (DLTS) measurements. In addition to the increase of electron emission rate of N-related localized states, illumination can induce the tunneling emission of N-related localized states. Finally, we analyze the PL emission under different applied reverse bias. In these measurements, at low temperature, the decrease of PL intensity with increasing reverse bias for the GaAsN QW emission is larger than the decrease of PL intensity for N-related localized states emission, which is attributed to the smaller tunneling barrier for the GaAsN QW electron states. As temperature increases, the carriers on N-related localized states have sufficient thermal energy to transfer to GaAsN QW electron states. Thus, the different PL quenching rate is due to an additional escape path for electrons on the N-related localized states. Furthermore, this carrier transfer phenomenon between GaAsN QW electron states and N-related localized states can be observed from the temperature-dependent I-V measurement under illumination. The bias of the current rise corresponding to N-related localized states is decreased with increasing temperature. This result indicates that the contribution of photo-generated electrons on N-related localized states occurs at the shallower electron states with increasing temperature, and is consistent with the carrier transfer phenomenon for the N-related localized states. Thus, the application of N-related localized states can provide current path for photo-generated electron-hole pairs and extend the energy response range.