CN115104190A - 微米尺度的发光二极管 - Google Patents
微米尺度的发光二极管 Download PDFInfo
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Abstract
纳米线发光二极管(LED)可操作用于光的自发发射,相对于常规LED以显著减小的电流密度进行并且具有非常窄的线宽。
Description
相关的美国申请
本申请要求于2020年2月18日提交的序列号62/978168由Xianhe Liu等人的题目为“微米尺度超稳定运行的氮化镓绿色发光二极管(Micrometer Scale in GaN GreenLight Emitting Diodes with Ultra-Stable Operation)”的美国临时申请的优先权,通过引用将其全部内容合并于此。
背景技术
尺寸在微米量级上的高效高亮度发光二极管(LED)高度期望用于广泛的应用,包括虚拟/混合/增强现实、超高分辨率移动显示器、以及生物医学感测和成像,仅举几例。对此,在过去的十年中,基于氮化镓(GaN)的微LED的发展已经吸引了显著的兴趣。然而,迄今为止,使用常规的有机或无机材料实现微米尺度的高效LED仍然是具有挑战性的。
虽然GaN基大面积蓝光量子阱LED可以表现出高效率发射,但是随着器件尺寸减小,效率急剧下降,这很大程度上受到表面复合和由自顶向下蚀刻引起的不良p-型传导的限制。此外,为了实现绿色发射,在量子阱有源区域中需要相对高的铟(In)成分,这增加了缺陷和位错的形成并且增加了相分离,从而导致弱的和宽的发射并且因此导致不良的器件效率和颜色质量。
由于应变诱导的极化场,InGaN基量子阱LED的性能还严重遭受量子受限的斯塔克效应(Stark effect),特别是在绿色光谱中,这导致不稳定的操作,诸如随着电流增加而发射波长的显著偏移。另一方面,有机LED遭受差的稳定性、低亮度、以及随着尺寸减小而显著降低的效率。
发明内容
本发明公开了微米尺度纳米线发光二极管(LED),其可操作用于以显著减小的电流密度并且以非常窄的线宽自发发射光。在实施例中,每一条纳米线具有操作为光子带隙的二维光学腔,该光子带隙导致或修正(影响或改变;例如增强或放大)自发发射(并且因此可以被称为弱光学腔)。在实施例中,电流密度为至少小于每平方厘米十千安培(10kA/cm2)的数量级,且光谱线宽被测量为约四纳米。
因而,根据本发明的实施例实现了微米尺度的高效率LED,提供了稳定的操作和高的颜色质量,并且大部分没有缺陷和位错。
在阅读以下在各个附图中示出的实施例的详细说明之后,本领域普通技术人员将认识到本发明的各个实施例的这些和其他目的和优点。
附图说明
并入本说明书中并构成本说明书的一部分且其中类似标号描绘类似元件的附图示出本公开的实施例,并且与详细描述一起用于解释本公开的原理。附图不一定按比例绘制。
图1A示出了在根据本发明的实施例中的纳米线阵列的俯视图或截面图。
图1B示出了根据本发明的实施例中的纳米线阵列的示例的能带图。
图2A示出了根据本发明的实施例中的纳米线发光二极管(LED)结构。
图2B示出了根据本发明的实施例中的纳米线阵列。
图2C示出了根据本发明的实施例中的纳米线阵列的光致发光光谱的示例。
图3A示出了根据本发明的实施例中的微尺度LED器件的示例。
图3B示出了根据本发明的实施例中的微尺度LED器件的示例的电流-电压特性。
图4A示出了在根据本发明的实施例中在变化的注入电流下测量的纳米线LED的电致发光(EL)光谱。
图4B示出了在根据本发明的实施例中的相对外量子效率与注入电流密度相比。
图5A示出了在根据本发明的实施例中的纳米线LED的发射特性的示例。
图5B示出了在根据本发明的实施例中的纳米线LED的EL光谱的半峰全宽的变化的示例。
图6示出了在根据本发明的实施例中的纳米线LED的EL强度的角度分布。
具体实施方式
现在将详细参考本公开的各种实施例,其示例在附图中示出。虽然结合这些实施例进行描述,但是应理解的是,其并不旨在将本公开限于这些实施例。相反,本公开旨在覆盖可包括在由所附权利要求限定的本公开的精神和范围内的替换、修改和等同物。此外,在本公开的以下详细描述中,阐述了许多具体细节以便提供对本公开的透彻理解。然而,应当理解,本公开可以在没有这些具体细节的情况下实施。在其他实例中,未详细描述众所周知的方法、过程、组件和电路,以免不必要地模糊本公开的各方面。
这些图不一定按比例绘制,并且仅示出了所描绘的器件和结构的部分、以及形成那些结构的各个层。为了简化讨论和说明,可以仅描述一个或两个器件或结构,尽管实际上可以存在或形成多于一个或两个器件或结构。而且,虽然讨论了某些元件、组件和层,但是根据本发明的实施例不限于这些元件、组件和层。例如,除了所讨论的那些之外,还可存在其他元件、组件、层等。
以下详细描述的一些部分是根据用于制造如在此公开的那些器件的程序和其他操作表示来呈现的。这些描述和表示是器件制造领域的技术人员用来将其工作的实质最有效地传达给本领域的其他技术人员的手段。在本申请中,程序、操作等被设想为产生期望结果的步骤或指令的自相一致的序列。描述为单独块的操作可以被组合并在同一处理步骤中(即,在同一时间间隔中,在前一个处理步骤之后且在下一个处理步骤之前)执行。此外,可以以与下面描述的顺序不同的顺序执行操作。此外,制造工艺和步骤可以与本文所讨论的工艺和步骤一起执行;即,在本文所示和所述的步骤之前、之间和/或之后可以存在多个工艺步骤。重要地,根据本发明的实施例可以结合这些其他(或许传统的)过程和步骤实现而不显著扰乱它们。一般而言,根据本发明的实施例可以替换常规工艺的部分而不显著影响外围工艺和步骤。
根据所公开的发明的实施例实现了具有Ⅲ族氮化物纳米线并且具有高效率、高颜色质量以及高稳定操作的微米尺度发光二极管(LED或微型LED或纳米线LED)。由于有效的表面应变弛豫,这样的纳米结构很大程度上没有缺陷和位错。
所公开的纳米线LED采用竖直p-i-n配置(其中,层夹在p掺杂区和n掺杂区之间),该配置可以显著简化器件制造工艺。通过改变嵌入在纳米线结构中的量子点中的铟成分,可以在几乎整个可见光谱(尤其包括绿色光谱)上调谐发射波长。
通过在如在此公开的器件有源区域处采用核-壳结构(core-shell structure),可以在很大程度上抑制表面复合(纳米尺度LED和微尺度LED的效率的主要限制因素)。显著地,通过在InGaN纳米线光子晶体中采用可缩放带-边缘模式,所公开的纳米线结构提供高度稳定和有效的光致发光发射,其中不存在在纤锌矿铟镓氮(InGaN)结构中常见的Varshni和量子受限斯塔克效应(quantum-confined Stark effect)。
在此公开了光子纳米线隧道结表面发射LED,这些光子纳米线隧道结表面发射LED被设计成在该光子带结构的伽马(Γ)点处运行。在实施例中,器件有源区域具有大约三平方微米(μm2)的面积大小。在实施例中,电致发光(EL)光谱表现出大约四纳米(nm)的非常窄的线宽,其比在相同波长范围内操作的常规InGaN量子阱(盘或点)的线宽小接近五至十倍。
显著地,所公开的器件显示出高度稳定的自发发射(与受激发射相反)。随着电流密度增加,实际上不存在发射峰的变化,这表明不存在量子受限的斯塔克效应。在实施例中,外量子效率(EQE)随着电流增加而表现出急剧上升并且在大约每平方厘米五安培(A/cm2)处达到最大值。在室温超过200A/cm2的注入电流密度下测量相对较小(大约30%)的效率下降。这种小尺寸、超稳定的LED非常适合于近眼显示器应用。
首先描述所公开的光子纳米线LED的光学设计。图1A示出了在根据本发明的实施例中的纳米线阵列102的俯视图或截面图。纳米线阵列102包括多个纳米晶体或纳米线,示例为纳米线104。(每一条纳米线是纳米晶体,并且纳米晶体阵列包括纳米线阵列,因此这些术语在本文中可以互换使用。)每一条纳米线104具有六边形形状;即,它们各自具有六边形的横截面。阵列102包括多行纳米线,每行包括多条纳米线。
在所示出的实施例中,纳米线阵列102被布置在三角形晶格中,该三角形晶格还可以被称为六边形晶格。纳米线104的横向尺寸和晶格常数(节距(pitch))分别表示为d和a。在实施例中,d等于298nm并且a等于280nm。每一条纳米线104的直径可以从大约100nm变化直到晶格常数的尺寸。阵列102中的纳米线104的尺寸、间距和表面形态被精确地控制。纳米线104呈现均匀长度、平滑侧壁和高(深度与宽度)纵横比。由于有效的应变弛豫,纳米线104的纳米结构没有缺陷和位错。
图1B示出了对于根据本发明的实施例中的纳米线阵列的示例,使用二维(2D)有限元方法计算的能带图。在示例中,所述纳米线阵列的d等于298nm,a等于280nm。
在根据本公开的实施例中,InGaN光子纳米线LED被设计成在第四带光子带结构(图中标记为120的曲线)的Γ点处工作,其中面内波矢量为零。照此,总波矢量沿着光子纳米线阵列的垂直方向(垂直于阵列所位于的衬底的平面),这导致直接表面发射。此外,群速度在Γ点显著降低,导致光场和活性介质的交互时间长。因此可实现在对应波长处的强谐振,这可导致显著减小的光谱线宽。
在实施例中,第四带的Γ点的归一化频率约为0.504a/λ(其中,a是晶格常数,并且λ是波长),其对应于约555nm的波长,其中晶格常数等于280nm。因为发射很大程度上受光子纳米线的光学共振支配,而不是受半导体有源介质本身支配,所以此类LED的光发射被预期为高度稳定的并且随温度和注入电流相对不变。此外,这种LED的峰值发射波长随着量子阱中的Ⅲ族(例如,In)掺杂的变化是局部不变的,导致LED晶圆(例如,单片器件)以恒定的峰值波长发射光,尽管跨晶圆/阵列的掺杂变化较小,诸如在用于外延生长的制造工艺(诸如金属-有机化学气相沉积(MOCVD)或分子束外延(MBE)工艺)中可能发生的变化。这些特征可以进一步消除对LED波长分档(binning)的需要,这是昂贵的后道工艺操作,该工艺操作增加了LED晶圆的制造成本并且降低了LED晶圆的价值。
在实施例中,选择性区域外延(SAE)技术用于InGaN光子纳米线LED结构生长。在实施例中,使用配备有射频等离子体辅助氮源的MBE系统在蓝宝石衬底上的n+-GaN模板上执行生长。
一般而言,纳米线104包括第一半导体区、第二半导体区、以及异质结构,所述异质结构设置在第一半导体区与第二半导体区之间并且被耦合至所述第一半导体区和所述第二半导体区,其中所述第一半导体区包括n掺杂GaN,并且所述第二半导体区包括p掺杂GaN。
图2A示出了根据本发明的实施例中的纳米线LED结构200。LED结构200表示图1A的纳米线104的结构。
在实施例中,每一条纳米线200(104)的光学腔沿着x轴和y轴而不是z轴,但是自发光发射沿着z轴(其中x轴和y轴平行于器件衬底的平面,并且z轴垂直于该平面)。因此,当光学腔在x和y方向上时,自发光发射沿纳米线的纵轴在不同方向上:z方向。所公开的x轴和y轴光学腔配置在本文中被称为二维(2D)光学腔。相反,典型的光学腔(对于激光器)仅沿着z轴(垂直于器件衬底)并且光的受激发射沿着该相同的轴,并且因此被称为一维(1D)光学腔。
由所公开的纳米线阵列提供的腔效应用于实现如刚刚描述的更有方向性的发射并且还用于实现较窄的光谱线宽。光谱线宽被测量为约四nm,即在该波长范围内,与传统InGaN量子阱LED相比,几乎小5至10倍。
所公开的2D光学腔可以被称为弱腔,因为自发发射在该腔中被增强(或放大),但未实现受激发射(在强光学腔中,可以实现受激发射)。选择纳米线设计参数(例如,直径和晶格常数)以在接近但不精确地在纳米线阵列的光子带边缘处的状态中运行。通过在该状态附近运行,实现弱腔效应。重要的是要注意,与强腔的操作窗口相比,弱腔的操作窗口相对较大。如本文所使用的,“在接近但不(精确地)在光子带边缘处运行”或“作为修正(或影响或改变)自发发射的光子带隙运行”等也意味着“作为导致增强的或放大的自发发射但不导致受激发射的光子带隙运行”。更具体地,例如,对于InGaN,自发发射典型地在绿色波长范围中显示出非常宽的光谱(例如,在30至50nm范围内的半峰全宽(FWHM)),并且发射方向通常是随机的;然而,利用根据在此公开的实施例实现的2D光子晶体效果,自发发射被修改成使得线宽更窄并且发射是更定向的,如以上所讨论的。
在图2A的实施例中,LED结构200包括n+-GaN层202、多个(例如,六个)垂直对准的InGaN/AlGaN量子点或盘(QD)204、p+-(Al)GaN包覆层206、p++-GaN/n++-GaN隧道结208、n-GaN层210以及n++-GaN接触层212。在实施例中,n+-GaN层202的厚度约为450nm,p+-(A1)GaN包覆层206的厚度约为60nm,以及n-GaN层210的厚度约为60nm。在实施例中,n型掺杂剂是硅(Si),并且p型掺杂剂是镁(Mg)。
在实施例中,量子点有源区域204(InGaN/AlGaN量子点或盘的集合)包括InGaN和AlGaN的交替或交错层。例如,InGaN层(可以称为核心层)与AlGaN层(可以称为壳或势垒层)相邻,并且在量子点有源区域204中重复该图案。在量子点有源区域204的生长期间,使用AlGaN势垒而不是GaN势垒促进围绕有源区域的富含Al的AlGaN壳结构的形成,这可以显著减少表面复合。在实施例中,平均Al组成为约5%。
在SAE生长工艺之前,衬底被图案化为具有开口以促进高度规则的纳米线阵列的形成。更具体地,在蓝宝石上GaN衬底(图3A的衬底302)上沉积薄(大约10nm)钛(Ti)层作为生长掩模。电子束光刻和反应离子蚀刻技术可用于限定Ti掩模上的开口图案。仅在开口中形成纳米线,在Ti掩模层上没有发生外延。
图2B中示出了所得纳米线阵列250。纳米线阵列250在位置和尺寸上都表现出非常高的均匀性。通过仔细控制纳米线之间的间距和晶格常数,可见光谱的选定色谱中的强共振。在图2B的实施例中,比例条252的长度代表500nm。特别令人感兴趣的是,从具有280nm的晶格常数和约20nm的间距的InGaN光子纳米线阵列的光致发光(PL)中观察绿色光谱,如图2C中所示。
还使用通过SAE生长的光子纳米线阵列制造微尺度LED。微尺度LED300的实施例在图3A中示出。
用于制造微尺度LED 300的工艺的实施例如下。通过等离子体增强化学气相沉积来沉积二氧化硅(SiO2)层304(例如,300nm厚)以用于表面钝化与隔离。执行光刻和湿化学蚀刻以在SiO2层304中产生开口,所述开口限定用于电流注入的器件有源区域。通过电子束蒸发沉积金属堆叠(例如,五nm的Ti层和五nm的金(Au)层)以形成接触垫306。随后,通过溅射沉积透明导电氧化物层308(例如,180nm氧化铟锡(ITO)层)。还沉积金属堆叠(例如,五nm的Ti层和五nm的Au层)以形成n接触金属310。然后可以执行退火(例如,在氮周围环境下在400℃下持续一分钟)。然后,通过电子束蒸发沉积金属层以形成接触垫312,以便于电探测和测量。
微尺度LED 300的电流-电压(I-V)特性在图3B中示出。微尺度LED300具有大约四伏的导通电压,具有可忽略的小的反向偏压泄漏。在大约7伏特下电流密度可以容易地达到100A/cm2,而没有I-V特性的任何劣化。可以通过优化掺杂和制造工艺进一步改善电性能。
针对绿色光谱测量了所公开的InGaN光子纳米线LED的示例的输出特性。针对从0.5A/cm2变化到超过200A/cm2的电流密度测量EL光谱,所述电流密度至少比每平方厘米十千安培(kA/cm2)小一个数量级。测量结果示于图4A中。在该示例中,发射光谱在约548nm处展现显著的峰值发射。
光谱线宽被测量为约4nm,这比常规InGaN量子阱LED在该波长范围内的光谱线宽小接近五倍至十倍。此外,随着电流的增加,发射峰没有示出任何明显的偏移或变宽。在该波长范围内,在任何常规平面InGaN量子阱LED中没有测量这种不同的发射特性。
相对EQE(定义为积分EL强度除以电流密度)示于图4B中。相对EQE显示出随着注入电流密度的急剧增加,并且在约5A/cm2达到最大值。EQE随注入电流的急剧上升表明非常小的肖克莱里德霍尔(Shockley-Read-Hall)复合系数,这归因于在所公开的纳米线中显著降低的缺陷形成,并且还归因于通过使用芯-壳纳米线内点有源区域来抑制非发射表面复合。效率下降是适度的,在大于200A/cm2的电流密度下,EQE仅下降约30%。这种适度效率下降还表明,几乎无缺陷的InGaN纳米线中俄歇(Auger)复合系数较小。
在图5A中针对绿光的波长示出了所公开的InGaN光子纳米线LED的发射特性的示例。在此示例中,随着注入电流密度从0.5A/cm2增加到211A/cm2,峰值位置在大约548nm处保持极其稳定。明显地,光谱线宽随着注入电流几乎不变。
例如,在图5B中示出了绿光的EL光谱的FWHM的变化。FWHM仅在3nm和约3.7nm之间的范围内,因为在室温下注入电流密度从0.5A/cm2增加到211A/cm2,而没有任何主动冷却。为了比较,绿色波长范围内的传统InGaN量子阱光发射器严重遭量子受限斯塔克效应,其随着电流增加在发射中显示显著的蓝移(blue-shift),伴随有由于带填充效应导致的大的光谱加宽。
所公开的InGaN光子纳米线LED的非寻常稳定性归因于InGaN纳米线中点结构的减小的应变分布,并且更重要的是,在光子带结构的Γ点处的强谐振,该强谐振在很大程度上支配发射特性并且仅由光子纳米线的几何形状确定。所公开的通过MBE生长的InGaN光子纳米线即使在恶劣的操作条件下也是极其稳定的。这种能够在不使用任何主动冷却的情况下操作的超稳定小尺寸LED对于近眼显示器应用是高度有用的。
通过用安装在旋转台上的光纤收集EL发射来研究发射的远场角分布。在该研究中,光纤与LED之间的距离为1英寸。通过在543nm至553nm的光谱范围内积分,计算每个发射/收集角度下的EL强度。图6示出了本实施例的EL强度的角度分布。可见,发射主要沿着竖直方向分布,具有大约十度的发散角。此类无光学、高度定向发射与在此公开的InGaN光子纳米线结构的Γ点处的表面发射模式直接相关,这可以极大地简化设计并且降低下一代超高分辨率显示器件和系统的成本。
总之,根据本发明的实施例提供了利用InGaN光子纳米线的微尺度LED,尤其包括但不限于微尺度绿色LED。通过利用光子带结构的独特共振特性,这种微尺度器件可以表现出不同的发射特性,包括比常规InGaN量子阱LED的光谱线宽窄五到十倍的光谱线宽、不存在通常在该波长范围中的量子阱器件中看到的量子受限斯塔克效应的超稳定操作、以及高定向发射。此外,微米尺寸LED在高注入电流下表现出小的效率下降。本文公开的实施例提供了一种用于实现下一代显示器的高效率、高亮度的光发射器以及用于在新兴的虚拟/混合/增强现实设备和系统中的应用的新方法。
尽管已经用结构特征和/或方法动作专用的语言描述了本主题,但可以理解,本公开中定义的主题不必限于上述具体特征或动作。相反,上述具体特征和动作是作为实现本公开的示例形式来公开的。
因此描述了根据本发明的实施例。虽然已经在具体实施方式中描述了本公开,但是本发明不应被解释为受这些实施方式的限制,而是根据以下权利要求进行解释。
Claims (21)
1.一种纳米线,包括:
第一半导体区;
第二半导体区;以及
异质结构,所述异质结构设置在所述第一半导体区和所述第二半导体区之间并且被耦合到所述第一半导体区和所述第二半导体区;
其中所述纳米线可操作用于光的自发发射,并且其中所述纳米线具有作为修改所述自发发射的光子带隙工作的二维光学腔。
2.根据权利要求1所述的纳米线,能够在至少小于每平方厘米十千安培(10kA/cm2)的数量级的电流密度下操作。
3.根据权利要求2所述的纳米线,其中所述电流密度在0.2kA/cm2以及更小的量级。
4.根据权利要求1所述的纳米线,其中所述第一半导体区包括n掺杂氮化镓,并且其中所述第二半导体区包括p掺杂氮化镓。
5.根据权利要求1所述的纳米线,其中所述异质结构包括量子盘,所述量子盘包括铝镓氮和铟镓氮。
6.根据权利要求1所述的纳米线,其中所述异质结构包括壳层和芯层,并且其中所述芯层与所述壳层交错。
7.根据权利要求1所述的纳米线,其中所述纳米线具有六边形的横截面。
8.根据权利要求1所述的纳米线,其中所述第一半导体区、所述第二半导体区和所述异质结构包括:n+氮化镓(n+-GaN)层、多个垂直对准的铟GaN/铝GaN(InGaN/AlGaN)量子点、p+-AlGaN包覆层、p++-GaN/n++-GaN隧道结、n-GaN层和n++-GaN接触层。
9.根据权利要求8所述的纳米线,其中所述n+-GaN层具有大约450纳米(nm)的厚度,所述p+-AlGaN包覆层具有大约60nm的厚度,并且所述n-GaN层具有大约60nm的厚度。
10.根据权利要求1所述的纳米线,其特征在于,具有小于或等于约四纳米的线宽的电致发光光谱。
11.根据权利要求1所述的纳米线,其特征在于,峰值发射波长不随温度变化。
12.根据权利要求1所述的纳米线,其特征在于,峰值发射波长不随电流密度变化。
13.根据权利要求1所述的纳米线,能操作用于具有520-560纳米范围内的波长的光的自发发射。
14.一种器件,包括:
衬底;以及
表面发射发光二极管(LED),所述表面发射发光二极管被耦合至所述衬底并且包括纳米线阵列,所述纳米线阵列包括多条纳米线,其中所述多条纳米线中的每一条纳米线能操作用于生成与所述衬底正交的波矢量,其中所述每一条纳米线的电致发光光谱具有小于或等于约四纳米(nm)的线宽。
15.根据权利要求14所述的器件,其中所述纳米线能操作用于以比每平方厘米十千安培小至少一个数量级的电流密度发射具有在520-560nm范围内的波长的受激发射光。
16.根据权利要求14所述的器件,其中所述多条纳米线中的每一条纳米线包括:n+氮化镓(n+-GaN)层、多个垂直对准的铟GaN/铝GaN(InGaN/AlGaN)量子点、p+-AlGaN包覆层、p++-GaN/n++-GaN隧道结、n-GaN层以及n++-GaN接触层。
17.根据权利要求16所述的器件,其特征在于,峰值发射波长不随所述量子点中的铟掺杂量变化。
18.根据权利要求14所述的器件,其中所述多条纳米线中的每一条纳米线具有六边形的横截面,并且其中所述纳米线以三角形晶格布置在所述阵列中。
19.根据权利要求18所述的器件,其中所述三角形晶格具有298nm的横向尺寸和280-300nm的晶格常数,并且其中所述多条纳米线中的每一条纳米线具有在100nm与所述晶格常数之间的范围内的直径。
20.根据权利要求14所述的器件,其特征在于,峰值发射波长不随温度变化。
21.根据权利要求14所述的器件,其特征在于,峰值发射波长不随电流密度变化。
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