濕氧化砷化鎵鋁之金氧半電容器其電性傳導與介面能態分析

Abstract

本論文主要探討在濕氧化Al0.98Ga0.02As改變氧化時間下，利用電流電壓(I-V)、電容、電導量測來分析其介面能態與傳導機制，以及計算文獻較少討論的Al0.98Ga0.02As濕氧化後氧化層之介電係數。本論文共使用Al0.98Ga0.02As在不同氧化時間下，所獲得的四片樣品作為實驗比較之用。首先，在未氧化的樣品，其電容電壓的量測結果對頻率幾乎為無響應而近乎一個常數，表示在AlGaAs/GaAs介面並無缺陷存在。其次，由在氧化時間45分鐘的樣品，開始發現介面能態因為施加AC頻率的快慢，以致在電容電壓圖中觀察到其調變，表示在氧化的AlGaAs/GaAs之間有介面能態存在。之後，在60分鐘氧化時間之樣品，其隨AC小訊號調變的影響更為明顯，則表示介面能態密度更為增加，並由電容分析獲得氧化層介電係數為7.82。然而，由電流電壓曲線可得知樣品在此氧化狀況下仍無法有效阻絕電流導通。當氧化時間持續延長為150分鐘的樣品，發現氧化層可以有效地阻絕電流，並由電容分析發現樣品不只存在了介面能態所貢獻的缺陷電容，還發現了有固定電荷的存在(約 9.1x10e11(cm-2))，並且由完整等效電路模型可以獲得樣品的介電係數為7.69。 在導納分析中，我們共利用三種不同的模式來分析介面能態，包括電容-頻率(C-ω)、電導-頻率(G-ω)、以及電導-電壓(G-V)。其結果顯示，在氧化60分鐘之樣品，為單一介面能態分佈(位於0.14ev與0.28ev)。在氧化150分鐘的樣品則 (位於0.19ev~0.42ev)為連續能態分佈，其介面能態密度數量級約為10e11~10e12(cm-2)，並且發現與施加偏壓有關。對於此我們提出一個理論模型來解釋活化能與施加偏壓的關係。最後對於利用150分鐘獲得之全氧化樣品，我們發現其電流傳導可以使用與filed-emission有關的Poole-Frenkel電流機制來解釋，依此獲得介電係數為7.07，並與先前所獲得的結果接近。因此，我們推論對於Al0.98Ga0.02As濕氧化後的氧化層其介電係數範圍為7~8之間。由拍得之SEM圖像，並與R. D. Twesten以相同鋁含量樣品以濕氧化所獲得之圖像比較後，發現氧化層中含有細緻尖晶石相的多晶系的Gallium氧化物顆粒，與在半導體及氧化層之間呈非結晶相的介面，由此推論Al0.98Ga0.02As經濕氧化之後將主要成為多晶系的粒狀γ-Al2O3

The conduction and interface-state properties of oxidized Al0.98Ga0.02As are investigated by varying the oxidation time using current-voltage (I-V), capacitance and conductance measurements. Without oxidation, CV spectra show a constant capacitance without any frequency dispersion, implying defect-free AlGaAs/GaAs interfaces. For 45-min. oxidation, C-V spectra show frequency-dependent characteristic, consistent with the presence of interface states between oxidized-AlGaAs/GaAs. For 60-min. oxidation, the frequency-dependent characteristic becomes more prominent, indicating an increase in the concentration of the interface states. From the capacitance, we determine the relative dielectric constant of 7.82. However, I-V characteristic of this sample shows that the oxidation time is not enough to effectively block the current flow. Effective current blocking is achieved for increasing the oxidation to 150 min. Besides the interface states, we observe a presence of significant fixed charge (about 9.1x10e11(cm-2)) for such oxidized sample. From developed effective circuit model, a relative dielectric constant of 7.69 is determined. In this study, we use three different methods, including capacitance-frequency (C-ω), conductance-frequency (G-ω), and conductance-voltage (G-V), to characterize the interface states. The result show that a single interface state (at 0.14, 0.28ev) for the 60 min. oxidation and continuous interface states (at 0.19~0.42ev) with a concentration about 1011~1012(ev-1-cm-2) for 150 min. oxidation time. We propose a theoretical model to explain the effect of the applied voltage on the observed activation energy of the interface states. Finally, we find that the I-V characteristics of the full oxidized sample, the 150 min. oxidation time, can be explained by Poole-Frenkel conduction mechanism which is based on field emission. From this theory, we obtain a relative dielectric constant of 7.07. This value agrees with the two values previously determined, leading us to conclude that a wet oxidized Al0.98Ga0.02As has a relative dielectric constant 7~8. The structural property of the oxidation layer is studied by SEM which shows a fine-grained cubic spinel phase of polycrystalline alumina and an amorphous phase at the semiconductor/oxide interface. This property is similar to the observation by R. D. Twesten, showing that the oxidized AlGaAs mainly is γ-Al2O3.