InGaAsN/GaAs量子井之成份波動效應

Abstract

本篇論文主要是藉由光激發螢光頻譜(PL)、穿透式電子顯微鏡(TEM)、電容-電壓(C-V)、導納頻譜、深層能階暫態頻譜(DLTS)等量測方式，探討InGaAsN單一量子井其成份波動效應。我們成長三種不同長晶速率的樣品，分別為4.4 Å/s、2.8 Å/s、0.506 Å/s，實驗結果顯示降低長晶速率惡化量子井的成份波動。從低溫PL的量測結果發現，InGaAsN主峰值旁出現一低能量的小peak，次快的其低能量小peak更明顯，可推知量子井中有局部N-rich的區域存在；而長晶速率最慢的樣品的訊號則是非常弱且寬，對照其TEM圖可看到InGaAsN層變成三維成長，已不具量子井結構。因此，長晶速率最慢的樣品已出現含氮材料常見的相分離問題，摻雜的氮原子有群聚跟分離的情形，長晶速率較快的兩片雖無明顯相分離，但是量子井層依舊存在成份波動效應。 從量子井層的載子濃度分布圖中，在主要載子堆積峰值旁出現一小突起，顯示在量子井的基態下有能態存在，其活化能可能和PL中分裂的InGaAsN激發訊號差值雷同。因此，量子井中的局部區域可能產生一電子能階。再由導納頻譜及DLTS量測結果，組成波動效應於0.506 Å/s樣品量子井層形成較淺連續能階(0~83 meV)及較深的缺陷能階(0.21~0.25 eV)。和其他兩片樣品比較，較深的能階其活化能隨著長晶速率下降，由0.1 eV (4.4 Å/s及2.8 Å/s)增大為0.25 eV(0.506 Å/s)，且隨偏壓變化的趨勢比另外兩片明顯，推測此較深的缺陷能階是成份波動程度太嚴重而導致相分離，因此不只形成深層缺陷更造成一寬能帶。另一淺連續能階，則可能是成份波動效應所引發的晶格膨脹或是侷限能態，所造成的結構缺陷。

Effect of the composition fluctuation in InGaAsN/GaAs single quantum well is investigated by photoluminescence (PL), capacitance-voltage (C-V), admittance spectroscopy, deep-level transient spectroscopy (DLTS) and cross-sectional transmission electron microscope (TEM). Three samples were grown by molecular beam epitaxy (MBE) with different growth rates, 4.4, 2.8 and 0.506 Å/s, respectively. PL studies show that the composition fluctuation causes a splitting of the InGaAsN emission into a main emission and a low-energy bump. Lowering the growth rate degrades the composition fluctuation, as indicated by the broadening of the InGaAsN emission and enhancement of the low-energy bump, suggesting the presence of the local N-rich regions in the well. Further lowering the deposition rate leads to a three-dimensional growth, destroying the size-quantization effect of the well. According to the carrier distribution in the well, the existence of a carrier bump following an accumulation peak suggests the presence of an energy state below the ground state of the well. The activation energy of this energy state is comparable to the energy separation between the InGaAsN emission and the low-energy bump. Therefore, the local N-rich region probably induces an electron state below the ground state of the InGaAsN layer. From admittance and DLTS results, two defect states, one shallow at 0~83 meV and the other deeper at 0.21~0.25 eV, are present in the 0.506 Å/s sample due to composition fluctuation. Decreasing the growth rate is found to increase the activation energy of the deeper energy state from 0.1 eV(4.4 and 2.8 Å/s) to 0.25 eV(0.506 Å/s). Furthermore, the activation energy of the deeper state is more bias-dependent in 0.506 Å/s sample than others, implying that this state broadens into a band as a result of increased composition fluctuation. As to the shallow state, it is probably related to structural defects produced by lattice expansion or localized state due to composition fluctuation.