InGaN/GaN多量子阱的结构及其光学特性的研究
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摘要
近年来,随着科学技术的不断进步,在半导体材料方面取得的成果也越来越明显,特别是AIN, GaN, InN及其合金化合物等III-V族氮化物半导体材料,更是得到了快速的发展。其中,GaN材料作为III-V族化合物半导体材料的代表,由于其拥有着优良的物理、化学性质和十分广泛的应用前景,逐渐成为研究的热点。GaN及其合金化合物都是直接带隙半导体材料并且带隙宽度的变化范围很大,根据组分的不同,带隙可从InN的0.7eV变到AIN的6.28eV,对应的波长覆盖了从近红外到紫外极为宽广的光谱范围。另外,GaN材料硬度高,化学性质稳定,不溶于一般的酸碱,耐高温和强辐射,因此成为半导体光电子器件的主要候选材料。在光电器件方面,随着对III族氮化物材料和器件的研究与开发工作的不断深入,InGaN超高亮度蓝光、绿光LED技术已经实现商品化,蓝光LED已经成为目前世界各大公司和研究机构的重点开发项目。在微电子器件方面,AlGaN/GaN异质结界面产生的二维电子气在室温下迁移率已经达到了很高的水平,这使得AlGaN/GaN高电子迁移率晶体管(HEMT)器件以及场效应晶体管(HFET)都得到很大的发展。在探测器方面,对于GaN基紫外探测器以及p-n结光伏探测器的研究都达到了很高的水平,器件的指标、性能均实现了历史性突破,使得Ⅲ族氮化物的研究保持了持续稳定的发展。
在光电子器件方面,目前已经进入蓝光的时代,InGaN合金作为一个重要的材料,已经被广泛应用于蓝光LED和LD等器件的制作。因此,对于InGaN多量子阱结构发光机理的研究在进一步发展此类光电器件的过程中显得尤为重要。目前普遍接受的观点是In组分的非均一分布产生了富In区局域态,其很强的限制作用阻止了载流子向材料中非辐射复合中心的转移,而GaN基LED很高的发光效率就是由于载流子在这些局域态中较高的辐射效率造成的。然而,由于GaN和蓝宝石衬底之间存在着较大的晶格失配(13%),因此在InGaN外延层中也会存在高密度的位错,再加上GaN外延层中存在着一些天然缺陷,以及生长过程中引入的非故意掺杂,这些都可以作为非辐射复合中心降低载流子的复合效率。同时,大的晶格失配会导致在外延层中出现很强的极化电场,而且GaN本身也存在很强的自发极化,这些因素也会导致发光效率的下降。另外,在电致发光中,经常会出现大电流或者低温下效率的下降问题,该问题的出现是由于大的内建电场以及外加偏压、载流子在量子阱中的分布不均匀、以及器件自热等因素造成的。尽管如此,基于InGaN/GaN多量子阱的LED器件却经常显示非常高的量子效率。这个现象就是由于InGaN中自组装形成的富In团簇造成的。而InGaN/GaN多量子阱LED的发光特性就是由所有这些因素共同的竞争结果支配的,并且在不同的条件下,这些因素所占的比重有所不同。因此,深入研究InGaN/GaN多量子阱LED内部发光机理对深化理论模型以及器件的发展都具有非常重要的意义。本论文利用光谱分析手段,对具有多量子阱结构样品的光学行为进行了系统的研究,通过探索温度、激发功率、电流强度等外界条件对样品发光特性的影响,来深入了解器件的结构以及载流子内部动力学机制。具体包括以下内容：
(1)研究了样品中存在的由于内建电场引起的斯塔克效应对光致发光(PL)的影响。通过低温6K下PL峰位和半高宽随激发功率的变化,发现小功率下光生载流子对斯塔克效应的屏蔽占主导,而在大功率下局域态高能级的载流子填充效应起主要作用。而在高温300K时,非辐射复合中心被热激活,在小功率范围内,缺陷相关的非辐射复合是载流子的主要复合机制,而随着功率的增加,斯塔克效应的屏蔽和局域态的能级填充逐渐开始起作用。
(2)研究了PL峰位和半高宽随温度反常的S形和W形变化行为,以及不同激发功率对该行为的影响。在小功率下,样品的发光峰峰位随温度升高表现出明显的S形变化(红移-蓝移-红移),而伴随着半高宽的W形(减小-增加-减小-缓慢增加-快速增加)变化,该现象说明了载流子在温度变化过程中所经历的传输机制的转变,即随温度升高,局域的载流子由非热化的随机分布到吸收能量以后的热化分布,最后到常规的载流子完全热化。
随着激发功率的增加,PL峰位的S形温度变化行为逐渐变为倒V形,而半高宽的W形行为则逐渐减弱,最后变为随温度单调上升,这说明大的激发功率产生了更多的光生载流子,因此量子阱中的局域态被大量载流子占据,局域效果不明显。同时半高宽的W形变化行为中最开始的下降趋势随着激发功率增大先增强后减弱,这归因于带尾态中随着能级升高指数式增加的态密度分布。
(3)对Varshni公式进行了修正,加入局域态的影响,并利用修正后的Varshni公式对不同功率下峰位随温度的变化曲线进行拟合,结合S形变化的温度拐点,进一步证实了大的激发功率会削弱局域效果的作用。
(4)研究了LED器件的内量子效率随激发功率的变化。首先利用幂指数函数对不同温度下积分PL强度随激发功率的变化曲线进行拟合,根据拟合出来的指数因子判断出低温下在整个功率范围内,载流子所经历的都是辐射复合过程,而在室温下,小功率范围内非辐射复合占主导,只有在大功率范围内辐射复合才开始起作用。而内量子效率的变化则是由非辐射复合、辐射复合、斯塔克效应以及局域态的能级填充等因素的共同作用决定的。
(5)仔细的研究了InGaN/GaN多量子阱中载流子的传输和复合机制。在中等温度范围以及小功率下观察到了PL谱中分别位于2.42和2.66eV位置处的两个发光峰(分别标记为PD和PM),这两个峰分别来自于InGaN量子阱中相分离的量子点和InGaN母体的发光。这两个分离的相在高分辨率透射电子显微镜(HRTEM)图片中可以清楚的分辨出来。在随温度的变化过程中,PD峰和PM峰强度的变化行为反映了载流子在量子阱中的传输过程。即温度从6K升高时,载流子首先会吸收热能从InGaN母体向富In区量子点转移,然后随着温度的继续升高,载流子从量子点中溢出的过程更加显著。另外,InGaN母体发光峰的S形温度变化行为反映了InGaN母体中存在着轻微的In组分波动,而量子点相关发光峰峰位随温度升高而单调降低的行为则说明了量子点内载流子很强的弛豫过程。
(6)利用时间分辨光致发光(TRPL)的手段进一步研究了载流子的传输机制。在TRPL谱中,明显的观察到两个衰减成分,利用双指数函数对TRPL谱进行拟合,分别得到大约几个纳秒和几十纳秒的衰减时间,这归因于载流子的逃逸和载流子的有效复合过程,包括辐射复合、非辐射复合和可能的载流子的扩展传输机制。快速衰减时间随着温度升高而单调下降的现象说明热能加剧了载流子的逃逸,而PM峰和PD峰对应的慢速衰减时间则显示了不同的变化情况,这是由于InGaN母体和富In区量子点对载流子不同的俘获效率造成的。在温度升高的过程中,一开始母体对载流子的俘获效果减弱而量子点的增强,然而温度继续升高至室温时母体中载流子的溢出变得越来越显著,并且非辐射复合的增强也会导致载流子寿命的变化。因此,载流子的寿命是由逃逸速率、俘获效率、辐射寿命、非辐射寿命以及跃迁几率等因素共同决定的。InGaN母体和富In区局域态所对应的载流子衰减时间随温度的变化行为很好的反映了量子阱中光生载流子的传输以及复合机制。
(7)研究了在电注入情况下LED器件的光电特性及其内部物理学机制。从不同温度下的电流电压(I＝V)变化结果发现,样品中电流的传输机制主要是隧道电流的方式,并且外加偏压在大电流下显著增大,特别是在低温情况下尤为明显。外加偏压对样品的发光特性同样会有影响。电致发光(EL)谱的测量结果发现,在低温小电流范围内,EL峰位出现蓝移伴随半高宽的展宽,这个现象是由于低温下载流子的泄露严重,InGaN母体载流子数目的减少导致屏蔽效果的不显著,而具有对载流子较强限制作用的局域态和类量子点的低能级填充效果则相对增强。EL强度和外量子效率(EQE)随温度的变化进一步证明了载流子泄露在低温整个电流范围内发生,而在高温下只在大电流下发生。另外,不同电流强度下EL谱随温度的反常变化行为证明了在电致发光中,载流子的复合效率不仅受到量子阱内载流子的输运、斯塔克效应、非辐射复合等因素的影响,同样还会受到注入载流子在量子阱之间的传输以及分布情况的共同作用。
因此,为了得到高量子效率的InGaN/GaN多量子阱蓝光LED,可以提出不同的方法来克服非辐射复合,减小量子阱中内建电场的影响,增加局域态的深度来抑制载流子向扩展态的逃逸,平衡电子和空穴在量子阱中的空间分布,减少载流子泄露等因素。
Recently, great achievements have been made in the field of semiconductor materials with the progress of science and technology, especially for the III-V nitride semiconductor materials, such as AlN, GaN, InN and their alloys, which have got rapid development. As representative of III-V compound semiconductor materials, GaN has been the focus of intense research due to the excellent physical and chemical properties and broad application prospects. GaN and their alloys have direct energy gaps, covering a wavelength range from the infrared (InN,-0.7eV) to the ultraviolet (AlN,～6.0eV). Moreover, GaN can work under those conditions such as strong acid, alkali and radialization due to its high degree of hardness and stable chemical properties. As a result, it is an important material for optoelectronic devices. In the field of optoelectronic devices, InGaN-based ultra-high brightness blue and green LEDs have been commoditized, and blue LED has become the key development project of the major companies and research institutions around the world. In the field of microelectronic devices, great development has been made for AlGaN/GaN high electron mobility transistor (HEMT) and field effect transistor (HFET) due to the large electron mobility of the two dimensional electron gas (2DEG) generated in the AlGaN/GaN heterojunction interface. In the field of detector devices, deep research has been made for the GaN-based ultraviolet detector and p-n junction photovoltaic detector, which make the performance of the devices up to a high level.
The development of optoelectronic devices has come to the era of blue light. InGaN alloy has been widely used for the fabrication of blue LED and LD devices. Thus, the study of InGaN multiple quantum well structure emitting mechanism is essential for further development of such optoelectronic devices. The most accepted point of view so far is that the inhomogeneous distribution of In component leads to the In-rich localized states, which prevent the transfer of the carriers to the nonradiative recombination centers in the material. However, high density of dislocations usually appears in the InGaN epitaxial layer due to the large lattice mismatch between GaN and sapphire substrate. Together with the natural defect in the GaN epitaxial layer and the unintentional dopant during the growth process, nonradiative recombination centers are usually formed in the materials, reducing the recombination efficiency. Meanwhile, the strong polarization field induced by large lattice mismatch in the epitaxial layer and the spontaneous polarization in GaN will also reduce the recombination efficiency. In addition, the so-called efficiency droop usually appears in the electroluminescence, which resulted from the large build-in electric field and applied bias, the non-uniform distribution of the carriers among the quantum wells and the self-heating of the devices under high current. Even so, the InGaN/GaN multiple quantum well LEDs usually exhibit extremely high quantum efficiency, which resulted from the effect of In-rich localized states. The luminescence of the devices is governed by the competition of the above mentioned factors at different conditions. Thus, the study of InGaN multiple quantum well structure emitting mechanism is important for both the luminescence theory and the development of such devices. In this paper, we study the optical behavior of the InGaN MQWs LED in detail by measuring the spectrum of the sample at different temperatures, excitation powers and current densities, to seek the carrier dynamic mechanics in the devices. The main conclusions of the dissertation are listed below.
(1) We have studied the influence of Stark effect resulting from the built-in electric field on the photoluminescence (PL). It can be inferred from the excitation power dependences of PL peak energy and full width at half maximum (FWHM) that with increasing the excitation power at6K, the luminescence process undergoes a mechanism converting from the screening of Stark effect to the high energetic band filling of the localized states. While at300K, nonradiative recombination centers are thermally activated, and defect-related nonradiative recombination dominates the process in the low excitation power range.
(2) We have studied the anomalous temperature behavior of the peak energy and linewidth at different excitation powers. At low excitation power, the S-and W-shaped temperature dependences of the emission energy and linewidth reflect the conversion of the carrier transferring mechanisms from nonthermalized to thermalized distribution of localized carriers, and finally to the regular thermalization of the carriers.
The disappearance of the S-and W-shaped temperature dependences with increasing excitation power, is attributed to the reduced localization effect. The initial decreasing in the W-shaped temperature dependent linewidth strengthens first and then weakens with increasing excitation power, which we attribute to the exponentially increased density of states with energy in the band tail.
(3) We fitted the temperature-indueced blueshit of the peak energy by the modified Varshni empirical law, in consideration of the influence of the localized states. The results indicate that the increasing excitation power can reduce the localization effect.
(4) We have studied the excitation power dependence of the internal quantum efficiency. The excitation power dependence of the emission intensity shows that the emission process of the MQWs is dominated by the radiative recombination at low temperature, and by nonradiative recombination at room temperature within low excitation range. The conclusion is that the internal quantum efficiency is determined by nonradiative recombination, radiative recombination, Stark effect and band filling of localized states.
(5) We have studied the carrier transfer and recombination mechanism in the InGaN/GaN MQWs in detail. Two InGaN-related emission peaks observed in the full PL spectrum at moderate temperatures and low excitation powers are assigned to the QDs and the InGaN matrix, due to a strong phase separation confirmed by HRTEM. With increasing temperature, the intensity behavior of the two peaks is attributed to that with increasing temperature the transfer of the photo-generated carriers from the InGaN matrix into the QDs first is enhanced below-100K due to the increased mobility in the low temperature range, and then is gradually suppressed up to300K due to the reduced nonradiative lifetime. In addition, the S-shaped temperature behavior of the InGaN matrix-related emission peak energy reflects a slight composition fluctuation in the InGaN matrix, while the monotonic decreasing of the QDs-related peak energy with increasing temperature is ascribed to the relaxation process via hopping of the carriers inside the QDs.
(6) We have studied the carrier transfer and recombination mechanism in the InGaN/GaN MQWs by time-resolved photoluminescence (TRPL). All the curves for both the peaks show multiple-exponential decay, including a relatively faster decay (a few nanoseconds) in the early stage and a slower decay (several tens nanoseconds) in the extended range, which are due to the carrier escape into either higher or lower levels and the effective decay times, including the mechanisms of radiative recombination, nonradiative recombination, and possibly extended carrier transport. We make the conclusion that the carrier lifetime is determined by escape rate, capture rate, radiative recombination lifetime, nonradiative recombination lifetime and transition probability. The TRPL measurement results explain the carrier transfer and recombination mechanism in the InGaN/GaN MQWs more clearly.
(7) We have measured the current-voltage (I-V) characteristics and electroluminescence (EL) spectra of the InGaN/GaN MQWs LED. From the I-V curves, we deduced that the major carrier transfer mechanism is tunneling current and the forward bias to get a certain current is significantly increased when the current is increased, especially at lower temperatures.The anomalous behavior of the EL spectra at low temperatures is attributed to the electron leakage resulting from the large forward bias since the electron leakage results in the failure of Coulomb screening effect and the relative enhancement of the low-energetic localized state filling. The current dependences of the integrated EL intensity and EQE further show that the electron leakage occurs in the whole current range at low temperatures, whereas only in high current range at high temperatures. The temperature dependences of the EL intensity at different current densities indicate that the EL efficiency is determined by not only carrier transfer, Stark effect, nonradiative recombination within the quantum well, but also carrier transport and distribution among the quantum wells.
Thus, to achieve a high-quantum efficiency of InGaN-based LED, it is essential to overcome the nonradiative recombination, weaken the built-in electric field, increase the depth of the localized states and balance the distribution of electrons and holes in the MQWs region to reduce the electron leakage.
在光电子器件方面,目前已经进入蓝光的时代,InGaN合金作为一个重要的材料,已经被广泛应用于蓝光LED和LD等器件的制作。因此,对于InGaN多量子阱结构发光机理的研究在进一步发展此类光电器件的过程中显得尤为重要。目前普遍接受的观点是In组分的非均一分布产生了富In区局域态,其很强的限制作用阻止了载流子向材料中非辐射复合中心的转移,而GaN基LED很高的发光效率就是由于载流子在这些局域态中较高的辐射效率造成的。然而,由于GaN和蓝宝石衬底之间存在着较大的晶格失配(13%),因此在InGaN外延层中也会存在高密度的位错,再加上GaN外延层中存在着一些天然缺陷,以及生长过程中引入的非故意掺杂,这些都可以作为非辐射复合中心降低载流子的复合效率。同时,大的晶格失配会导致在外延层中出现很强的极化电场,而且GaN本身也存在很强的自发极化,这些因素也会导致发光效率的下降。另外,在电致发光中,经常会出现大电流或者低温下效率的下降问题,该问题的出现是由于大的内建电场以及外加偏压、载流子在量子阱中的分布不均匀、以及器件自热等因素造成的。尽管如此,基于InGaN/GaN多量子阱的LED器件却经常显示非常高的量子效率。这个现象就是由于InGaN中自组装形成的富In团簇造成的。而InGaN/GaN多量子阱LED的发光特性就是由所有这些因素共同的竞争结果支配的,并且在不同的条件下,这些因素所占的比重有所不同。因此,深入研究InGaN/GaN多量子阱LED内部发光机理对深化理论模型以及器件的发展都具有非常重要的意义。本论文利用光谱分析手段,对具有多量子阱结构样品的光学行为进行了系统的研究,通过探索温度、激发功率、电流强度等外界条件对样品发光特性的影响,来深入了解器件的结构以及载流子内部动力学机制。具体包括以下内容：
(1)研究了样品中存在的由于内建电场引起的斯塔克效应对光致发光(PL)的影响。通过低温6K下PL峰位和半高宽随激发功率的变化,发现小功率下光生载流子对斯塔克效应的屏蔽占主导,而在大功率下局域态高能级的载流子填充效应起主要作用。而在高温300K时,非辐射复合中心被热激活,在小功率范围内,缺陷相关的非辐射复合是载流子的主要复合机制,而随着功率的增加,斯塔克效应的屏蔽和局域态的能级填充逐渐开始起作用。
(2)研究了PL峰位和半高宽随温度反常的S形和W形变化行为,以及不同激发功率对该行为的影响。在小功率下,样品的发光峰峰位随温度升高表现出明显的S形变化(红移-蓝移-红移),而伴随着半高宽的W形(减小-增加-减小-缓慢增加-快速增加)变化,该现象说明了载流子在温度变化过程中所经历的传输机制的转变,即随温度升高,局域的载流子由非热化的随机分布到吸收能量以后的热化分布,最后到常规的载流子完全热化。
随着激发功率的增加,PL峰位的S形温度变化行为逐渐变为倒V形,而半高宽的W形行为则逐渐减弱,最后变为随温度单调上升,这说明大的激发功率产生了更多的光生载流子,因此量子阱中的局域态被大量载流子占据,局域效果不明显。同时半高宽的W形变化行为中最开始的下降趋势随着激发功率增大先增强后减弱,这归因于带尾态中随着能级升高指数式增加的态密度分布。
(3)对Varshni公式进行了修正,加入局域态的影响,并利用修正后的Varshni公式对不同功率下峰位随温度的变化曲线进行拟合,结合S形变化的温度拐点,进一步证实了大的激发功率会削弱局域效果的作用。
(4)研究了LED器件的内量子效率随激发功率的变化。首先利用幂指数函数对不同温度下积分PL强度随激发功率的变化曲线进行拟合,根据拟合出来的指数因子判断出低温下在整个功率范围内,载流子所经历的都是辐射复合过程,而在室温下,小功率范围内非辐射复合占主导,只有在大功率范围内辐射复合才开始起作用。而内量子效率的变化则是由非辐射复合、辐射复合、斯塔克效应以及局域态的能级填充等因素的共同作用决定的。
(5)仔细的研究了InGaN/GaN多量子阱中载流子的传输和复合机制。在中等温度范围以及小功率下观察到了PL谱中分别位于2.42和2.66eV位置处的两个发光峰(分别标记为PD和PM),这两个峰分别来自于InGaN量子阱中相分离的量子点和InGaN母体的发光。这两个分离的相在高分辨率透射电子显微镜(HRTEM)图片中可以清楚的分辨出来。在随温度的变化过程中,PD峰和PM峰强度的变化行为反映了载流子在量子阱中的传输过程。即温度从6K升高时,载流子首先会吸收热能从InGaN母体向富In区量子点转移,然后随着温度的继续升高,载流子从量子点中溢出的过程更加显著。另外,InGaN母体发光峰的S形温度变化行为反映了InGaN母体中存在着轻微的In组分波动,而量子点相关发光峰峰位随温度升高而单调降低的行为则说明了量子点内载流子很强的弛豫过程。
(6)利用时间分辨光致发光(TRPL)的手段进一步研究了载流子的传输机制。在TRPL谱中,明显的观察到两个衰减成分,利用双指数函数对TRPL谱进行拟合,分别得到大约几个纳秒和几十纳秒的衰减时间,这归因于载流子的逃逸和载流子的有效复合过程,包括辐射复合、非辐射复合和可能的载流子的扩展传输机制。快速衰减时间随着温度升高而单调下降的现象说明热能加剧了载流子的逃逸,而PM峰和PD峰对应的慢速衰减时间则显示了不同的变化情况,这是由于InGaN母体和富In区量子点对载流子不同的俘获效率造成的。在温度升高的过程中,一开始母体对载流子的俘获效果减弱而量子点的增强,然而温度继续升高至室温时母体中载流子的溢出变得越来越显著,并且非辐射复合的增强也会导致载流子寿命的变化。因此,载流子的寿命是由逃逸速率、俘获效率、辐射寿命、非辐射寿命以及跃迁几率等因素共同决定的。InGaN母体和富In区局域态所对应的载流子衰减时间随温度的变化行为很好的反映了量子阱中光生载流子的传输以及复合机制。
(7)研究了在电注入情况下LED器件的光电特性及其内部物理学机制。从不同温度下的电流电压(I＝V)变化结果发现,样品中电流的传输机制主要是隧道电流的方式,并且外加偏压在大电流下显著增大,特别是在低温情况下尤为明显。外加偏压对样品的发光特性同样会有影响。电致发光(EL)谱的测量结果发现,在低温小电流范围内,EL峰位出现蓝移伴随半高宽的展宽,这个现象是由于低温下载流子的泄露严重,InGaN母体载流子数目的减少导致屏蔽效果的不显著,而具有对载流子较强限制作用的局域态和类量子点的低能级填充效果则相对增强。EL强度和外量子效率(EQE)随温度的变化进一步证明了载流子泄露在低温整个电流范围内发生,而在高温下只在大电流下发生。另外,不同电流强度下EL谱随温度的反常变化行为证明了在电致发光中,载流子的复合效率不仅受到量子阱内载流子的输运、斯塔克效应、非辐射复合等因素的影响,同样还会受到注入载流子在量子阱之间的传输以及分布情况的共同作用。
因此,为了得到高量子效率的InGaN/GaN多量子阱蓝光LED,可以提出不同的方法来克服非辐射复合,减小量子阱中内建电场的影响,增加局域态的深度来抑制载流子向扩展态的逃逸,平衡电子和空穴在量子阱中的空间分布,减少载流子泄露等因素。
Recently, great achievements have been made in the field of semiconductor materials with the progress of science and technology, especially for the III-V nitride semiconductor materials, such as AlN, GaN, InN and their alloys, which have got rapid development. As representative of III-V compound semiconductor materials, GaN has been the focus of intense research due to the excellent physical and chemical properties and broad application prospects. GaN and their alloys have direct energy gaps, covering a wavelength range from the infrared (InN,-0.7eV) to the ultraviolet (AlN,～6.0eV). Moreover, GaN can work under those conditions such as strong acid, alkali and radialization due to its high degree of hardness and stable chemical properties. As a result, it is an important material for optoelectronic devices. In the field of optoelectronic devices, InGaN-based ultra-high brightness blue and green LEDs have been commoditized, and blue LED has become the key development project of the major companies and research institutions around the world. In the field of microelectronic devices, great development has been made for AlGaN/GaN high electron mobility transistor (HEMT) and field effect transistor (HFET) due to the large electron mobility of the two dimensional electron gas (2DEG) generated in the AlGaN/GaN heterojunction interface. In the field of detector devices, deep research has been made for the GaN-based ultraviolet detector and p-n junction photovoltaic detector, which make the performance of the devices up to a high level.
The development of optoelectronic devices has come to the era of blue light. InGaN alloy has been widely used for the fabrication of blue LED and LD devices. Thus, the study of InGaN multiple quantum well structure emitting mechanism is essential for further development of such optoelectronic devices. The most accepted point of view so far is that the inhomogeneous distribution of In component leads to the In-rich localized states, which prevent the transfer of the carriers to the nonradiative recombination centers in the material. However, high density of dislocations usually appears in the InGaN epitaxial layer due to the large lattice mismatch between GaN and sapphire substrate. Together with the natural defect in the GaN epitaxial layer and the unintentional dopant during the growth process, nonradiative recombination centers are usually formed in the materials, reducing the recombination efficiency. Meanwhile, the strong polarization field induced by large lattice mismatch in the epitaxial layer and the spontaneous polarization in GaN will also reduce the recombination efficiency. In addition, the so-called efficiency droop usually appears in the electroluminescence, which resulted from the large build-in electric field and applied bias, the non-uniform distribution of the carriers among the quantum wells and the self-heating of the devices under high current. Even so, the InGaN/GaN multiple quantum well LEDs usually exhibit extremely high quantum efficiency, which resulted from the effect of In-rich localized states. The luminescence of the devices is governed by the competition of the above mentioned factors at different conditions. Thus, the study of InGaN multiple quantum well structure emitting mechanism is important for both the luminescence theory and the development of such devices. In this paper, we study the optical behavior of the InGaN MQWs LED in detail by measuring the spectrum of the sample at different temperatures, excitation powers and current densities, to seek the carrier dynamic mechanics in the devices. The main conclusions of the dissertation are listed below.
(1) We have studied the influence of Stark effect resulting from the built-in electric field on the photoluminescence (PL). It can be inferred from the excitation power dependences of PL peak energy and full width at half maximum (FWHM) that with increasing the excitation power at6K, the luminescence process undergoes a mechanism converting from the screening of Stark effect to the high energetic band filling of the localized states. While at300K, nonradiative recombination centers are thermally activated, and defect-related nonradiative recombination dominates the process in the low excitation power range.
(2) We have studied the anomalous temperature behavior of the peak energy and linewidth at different excitation powers. At low excitation power, the S-and W-shaped temperature dependences of the emission energy and linewidth reflect the conversion of the carrier transferring mechanisms from nonthermalized to thermalized distribution of localized carriers, and finally to the regular thermalization of the carriers.
The disappearance of the S-and W-shaped temperature dependences with increasing excitation power, is attributed to the reduced localization effect. The initial decreasing in the W-shaped temperature dependent linewidth strengthens first and then weakens with increasing excitation power, which we attribute to the exponentially increased density of states with energy in the band tail.
(3) We fitted the temperature-indueced blueshit of the peak energy by the modified Varshni empirical law, in consideration of the influence of the localized states. The results indicate that the increasing excitation power can reduce the localization effect.
(4) We have studied the excitation power dependence of the internal quantum efficiency. The excitation power dependence of the emission intensity shows that the emission process of the MQWs is dominated by the radiative recombination at low temperature, and by nonradiative recombination at room temperature within low excitation range. The conclusion is that the internal quantum efficiency is determined by nonradiative recombination, radiative recombination, Stark effect and band filling of localized states.
(5) We have studied the carrier transfer and recombination mechanism in the InGaN/GaN MQWs in detail. Two InGaN-related emission peaks observed in the full PL spectrum at moderate temperatures and low excitation powers are assigned to the QDs and the InGaN matrix, due to a strong phase separation confirmed by HRTEM. With increasing temperature, the intensity behavior of the two peaks is attributed to that with increasing temperature the transfer of the photo-generated carriers from the InGaN matrix into the QDs first is enhanced below-100K due to the increased mobility in the low temperature range, and then is gradually suppressed up to300K due to the reduced nonradiative lifetime. In addition, the S-shaped temperature behavior of the InGaN matrix-related emission peak energy reflects a slight composition fluctuation in the InGaN matrix, while the monotonic decreasing of the QDs-related peak energy with increasing temperature is ascribed to the relaxation process via hopping of the carriers inside the QDs.
(6) We have studied the carrier transfer and recombination mechanism in the InGaN/GaN MQWs by time-resolved photoluminescence (TRPL). All the curves for both the peaks show multiple-exponential decay, including a relatively faster decay (a few nanoseconds) in the early stage and a slower decay (several tens nanoseconds) in the extended range, which are due to the carrier escape into either higher or lower levels and the effective decay times, including the mechanisms of radiative recombination, nonradiative recombination, and possibly extended carrier transport. We make the conclusion that the carrier lifetime is determined by escape rate, capture rate, radiative recombination lifetime, nonradiative recombination lifetime and transition probability. The TRPL measurement results explain the carrier transfer and recombination mechanism in the InGaN/GaN MQWs more clearly.
(7) We have measured the current-voltage (I-V) characteristics and electroluminescence (EL) spectra of the InGaN/GaN MQWs LED. From the I-V curves, we deduced that the major carrier transfer mechanism is tunneling current and the forward bias to get a certain current is significantly increased when the current is increased, especially at lower temperatures.The anomalous behavior of the EL spectra at low temperatures is attributed to the electron leakage resulting from the large forward bias since the electron leakage results in the failure of Coulomb screening effect and the relative enhancement of the low-energetic localized state filling. The current dependences of the integrated EL intensity and EQE further show that the electron leakage occurs in the whole current range at low temperatures, whereas only in high current range at high temperatures. The temperature dependences of the EL intensity at different current densities indicate that the EL efficiency is determined by not only carrier transfer, Stark effect, nonradiative recombination within the quantum well, but also carrier transport and distribution among the quantum wells.
Thus, to achieve a high-quantum efficiency of InGaN-based LED, it is essential to overcome the nonradiative recombination, weaken the built-in electric field, increase the depth of the localized states and balance the distribution of electrons and holes in the MQWs region to reduce the electron leakage.
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