MOCVD 生长及 TEM 表征 GaAs/Al_xGa_(1-x)As 量子异质结构材料

本文报道 MOCVD 生长 GaAs/Al_xGa_(1-x)As 量子异质结构材料(超晶格、量子阱及量子共振隧穿二极管),采用横断面透射显微术表征了样品的界面结构。实验表明:多量子阱与超晶格的周期性良好,层与层之间界面清晰,采用[100]带轴入射,观察到超晶格的 TEM 卫星衍射斑,测量到量子阱中电子的子能级跃迁吸收。研究了生长工艺和材料结构的关系,分析了影响 RT 器件的因素。     

隧穿型量子效应薄膜材料制备技术

探讨了隧穿型量子效应薄膜材料制备技术,并应用分子束外延方法制备了典型结构外延材料GaAs基共振隧穿二极管,经过器件验证,得到了较好的结果.重点讨论了关键制备技术,包括束流精细控制和间歇式生长方式,主要是为了生长出更接近完美的晶体结构和晶体表面,并分析了测试结果和器件验证结果,最终得出整套隧穿型量子效应薄膜材料制备技术.     

极化效应对AIN／GaN共振隧穿二极管电流特性的影响

本文采用半经验紧束缚能带理论，通过自洽计算薛定谔方程和泊松方程研究了A1N／GaN共振隧穿二极管中极化效应对电流的影响。结果发现，极化效应导致电流曲线发生不对称性，并影响电流的共振电压位置，这与实验报道的结果相一致。并且随着极化电荷的增加，在一定的偏压条件下，只能观测到一个子能级隧穿或者根本没有负微分电阻现象发生。     

内嵌InAs量子点的场效应晶体管特性研究

研究了内嵌InAs量子点的异质结场效应晶体管在室温和低温下的电学特性,获得了量子点影响下器件的输出特性曲线。在室温下,通过分别测试在近红外光照和量子点充电条件下器件的Ⅰ-Ⅴ特性,证明了量子点通过类似纳米悬浮栅的作用,对邻近沟道的二维电子气施加影响。在低温下观察到器件漏电流出现负微分电导现象。这一现象可由2DEG和量子点之间的共振隧穿来解释。这些结果提供了一种新的操作传统场效应晶体管的方法,并有望制成新型量子点存储器。     

极化效应对AlN/GaN共振隧穿二极管电流特性的影响

本文采用半经验紧束缚能带理论,通过自洽计算薛定谔方程和泊松方程研究了AlN/GaN共振隧穿二极管中极化效应对电流的影响.结果发现,极化效应导致电流曲线发生不对称性,并影响电流的共振电压位置,这与实验报道的结果相一致.并且随着极化电荷的增加,在一定的偏压条件下,只能观测到一个子能级隧穿或者根本没有负微分电阻现象发生.     

自组织InAs/GaAs量子点的X射线双晶衍射研究

用X射线双晶衍射方法测定了自组织生长的InAs／GaAs量子点的摇摆曲线，根据Takagi-Taupin方程对曲线进行了拟合。在考虑量子点层晶格失配的情况下，理论曲线和实验曲线符合得很好，从而确定了量子点垂直样品表面的失配度，约为4～6％，这与宏观连续体弹性理论的预测相近。结合电镜、原子力显微镜的观察结果表明对子单层沉积方法获得的量子点层采用化合物构层进行拟合所得结果是合理的。     

超高真空化学气相淀积自组织生长锗量子点

采用超高真空化学气相淀积系统制备了小尺寸、高密度、纵向自对准的Ge量子点．通过TEM和AFM对埋层和上层量子点的形貌和尺寸分布进行了研究，对生长的温度和时间进行了优化．采用硼预淀积的方法得到了尺寸分布小于3％的均匀的圆顶形Ge量子点．采用低温光荧光测量了多层量子点的光学特性．在10K的PL谱可以观察到明显的蓝移现象，表明量子点中较强的量子限制效应．量子点非声子峰的半高宽约为46meV，表明采用UHV／CVD工艺生长的多层量子点具有较窄的尺寸分布．     

新型半导体纳米材料实现高效光化学转化

正记者从中国科学技术大学获悉,该校俞书宏教授课题组与合作者合作,设计了一种"脉冲式轴向外延生长"方法,成功制备了尺寸、结构可调的一维胶体量子点-纳米线分段异质结,利用ZnS纳米线对Cd S量子点的晶面选择性钝化作用,可同时实现量子点表面的有效钝化和光生载流子的有效转移。     

退火条件对Sn量子点生长和红外光学性质的影响

首次采用固相外延生长技术在Si(001)表面直接生长Sn量子点,并应用原子力显微镜(AFM)、X射线衍射(XRD)和同步辐射傅里叶红外光谱(FTIR)研究了退火条件对量子点样品的表面形貌、结晶性和红外光学性质的影响.AFM结果表明,随着退火温度的升高和退火时间的延长,量子点的平均尺寸变大,面密度减小.XRD结果显示,外延得到的Sn量子点为四方结构的β-Sn,与衬底的相对取向为Sn(110)//Si(001).由于β-Sn量子点的尺寸仍较大,同步辐射FTIR谱中没有观察到量子点的特征吸收峰.     

预成核对蓝宝石衬底上生长氮化镓低温层的影响北大核心CSCD

金属有机物化学气相沉积(MOCVD)技术以气相源的热分解反应作为基础,其适合规模化生产,是现今生长半导体材料的主要制备方式。在MOCVD生长GaN的过程中,衬底表面初始条件直接影响到材料成核与生长,因此对于外延生长非常关键。本论文研究了GaN外延生长过程中蓝宝石衬底的表面预成核工艺对GaN低温成核的影响。通过对比未处理样品和高温预通TMGa、高温预通TMGa和NH3预成核以及高温预通TMAl和NH3预成核的样品上生长的低温层退火后的形貌,我们发现高温预成核形成的成核点有利于吸引其周围气相源并入,并降低成核岛的密度。结合光学实时反射率监测气相沉积中晶粒的成核过程,进一步横向比较可发现由于高温时AlN更稳定,预成核的效果更好,对退火以后GaN小岛形貌影响更加显著。X射线衍射表征成核层的晶体质量,发现预成核工艺可将退火后成核层的(002)衍射峰半高宽从1636 arcsec降低到最低1088 arcsec。通过对比分析,我们认为高温预成核工艺的优点可能来源于其可以改善成核初期小岛的晶向。这些研究为进一步提高GaN外延质量提供了新的工艺思路。     

MBE grown InGaN quantum dots and quantum wells: effects of in-plane localization

InGaN/GaN heterostructure samples were grown by molecular beam epitaxy using ammonia as a nitrogen precursor. The growth of InGaN/GaN self-assembled quantum dots was monitored in situ by reflection high energy electron diffraction intensity oscillations. Atomic force microscopy scans showed a very high density of InGaN islands, 1×10¹¹ cm⁻², well above the dislocation density. This could explain the increased radiative efficiency of these samples compared to homogeneous quantum wells. Light emitting diodes (LEDs) with InGaN active layers buried in GaN were realized. Electroluminescence and photocurrent spectra of these LEDs evidence a strong Stokes shift that can be attributed to high localization of carriers in InGaN layers.     

InGaN-based nano-pillar light emitting diodes fabricated by self-assembled ITO nano-dots

InGaN/GaN based nano-pillar light emitting diodes (LEDs) with a diameter of 200-300 nm and a height of 500 nm are fabricated by inductively coupled plasma etching using self-assembled ITO nano-dots as etching mask, which were produced by wet etching of the as-deposited ITO films. The peak PL intensity of the nano-pillar LEDs was significantly higher than that of the as-grown planar LEDs, which can be attributed to the improvement of external quantum efficiency of the nano-pillar LEDs due to the large sidewall of the nano-pillars. We have also demonstrated electrical pumping of the InGaN/GaN based nano-pillar LEDs with a self-aligned TiO2 layer as a passivation of sidewall of the nano-pillars.     

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photoluminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system.     

High-efficiency InGaN/GaN/AlGaN light-emitting diodes with short-period InGaN/GaN superlattice for 530–560 nm range

A new method of forming the active region in high-efficiency InGaN/GaN/AlGaN light-emitting diode (LED) structure for long-wave green range is described. The introduction of a short-period InGaN/GaN superlattice situated immediately under the emitting quantum well and overgrown with GaN layer at reduced temperature leads to a more than tenfold increase in the efficiency of emission. For the proposed LEDs, the maximum quantum efficiency was 12% at 552 nm and 8% at 560 nm.     

Recent Progress in GaN‐Based Light‐Emitting Diodes

In the last few years the GaN‐based white light‐emitting diode (LED) has been remarkable as a commercially available solid‐state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V‐grooved c‐plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light‐emitting efficiency (because of the enhanced reflection from the V‐grooved plane). Furthermore, anomalous tunneling‐assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to ～72%. Finally, a single‐chip phosphor‐free white LED is fabricated, a stable white light is emitted for currents from 20 to 60 mA, which makes the LED chip suitable for lighting applications.     

Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon

We report on the achievement of a new class of nanowire light emitting diodes (LEDs), incorporating InGaN/GaN dot-in-a-wire nanoscale heterostructures grown directly on Si(111) substrates. Strong emission across nearly the entire visible wavelength range can be realized by varying the dot composition. Moreover, we have demonstrated phosphor-free white LEDs by controlling the indium content in the dots in a single epitaxial growth step. Such devices can exhibit relatively high internal quantum efficiency (>20%) and no apparent efficiency droop for current densities up to ~ 200 A cm(-2).     

Investigation of V-defects formation in InGaN/GaN multiple quantum well grown on sapphire

In the growth of InGaN multiple quantum well structure, V-pits has been observed to be initiated at the threading dislocations which propagate to the quantum well layers with high indium composition and substantially thick InGaN well. A set of samples with varying indium well thickness (3-7.6 nm) and composition (10-30%) are grown and characterized by photoluminescence (PL), X-ray diffraction, transmission electron microscopy and atomic force microscopy. The indium content and the layer thicknesses in InGaN/GaN quantum well are determined by high-resolution X-ray diffraction (XRD) and TEM imaging. With indium composition exceeding 10%, strain at the InGaN/GaN interface leads to the generation of V-pits at the interlayers of the MQW. Higher indium composition and increase in thickness of a period (InGaN well plus the GaN barrier) appear to enhance pits generation. With thicker InGaN well and reduction in thickness of GaN to InGaN (or the R ratio), pit density is substantially reduced, but it results in greater inhomogeneity in the distribution of indium in the InGaN well. This leads to a broadened PL emission and affect the PL emission intensity.     

Electrically driven quantum dot/wire/well hybrid light-emitting diodes

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.     

Study on modulating the indium composition in InGaN quantum wells to improve the luminous efficiency of GaN LED

This study primarily used metal-organic chemical vapor deposition to grow gallium nitride (GaN) light-emitting diode (LED) structures with InGaN quantum wells (QWs). During the InGaN QW growing process, an identical concentration of trimethylindium gas was prepared and introduced at different times (Before(B), Middle(M), and After(A)) into the QW structures for an investigation of the variation in GaN LED luminous efficacy. Because of segregation resulting from the different concentrations of In content of the InGaN QWs during the process and because of the stress resulting from lattice mismatch between atoms, the interaction between segregation and stress forms quantum dots (QDs). Under processes with the appropriate parameters, the QDs can improve the luminous efficacy of GaN LEDs. Postprocess LEDs were measured for their electroluminescence, photoluminescence, cathodoluminescence, thermal stability, light output power, and external quantum efficiency. The QW structures were analyzed and observed using high-resolution transmission electron microscopy. The results revealed that the Before (B) LED had the greatest light output power at 46.6 mW, an increase of approximately 15.6%. Thermal annealing was then used to treat the LED at 850 °C, after which the photoluminescence intensity increased by 1.7 times.