High-speed nanowire LED in the blue/green spectral range

通过自由空间或聚合物光纤进行高速光数据通信需要快速、安全、低成本的光源。目前，红色光源（650纳米）通常用于聚合物纤维数据传输，尽管蓝色/绿色光源（< 520纳米）在光纤传输过程中会提供更低的强度损失。GaN/GaInN发光二极管在蓝/绿光谱状态下是可商用的。然而，这些二极管的层堆叠是在纤锌矿晶格的c方向生长的，由于自发极化，在有源InGaN量子阱中导致电子和空穴的明显空间分离。这种分离导致长辐射寿命高达约10ns，并从根本上限制了使用GaN/GaInN发光二极管可实现的数据速率。该项目的关键思想是完全消除自发极化，以提高辐射复合率，并将可实现的调制频率提高到2000 MHz以上。我们的方法是一种由n-GaN核组成的核壳纳米线排列，其后是由有源InGaN量子阱和p-GaN接触层组成的无极化m面壳。该器件是通过在导电n型硅衬底上生长的一系列核壳GaN/InGaN纳米线来实现的。导线之间的自由空间将被填充，透明的顶层将作为设备的阳极。为了在纳米线阵列内和晶圆上提供纳米线的强制性均匀性，应开发自催化和选择性区域外延的组合。一个明显的挑战是限制重组和松弛机制的阐明，为高速光源的设计和技术提供关键信息，用于光数据传输。时间分辨率（< 100 ps）的光学和光电子测量将在高空间分辨率（< 0.3µm）的纳米线光源上进行，以便定位器件在速度、强度和发射谱线宽度等方面的限制。结果将直接纳入设备的设计和技术。同时，将通过散射参数测量和等效电路分析来评估和优化器件的高频电性能。该项目的基本目标是科学开发频率在2000兆赫以上的光源，用于自由空间和聚合物光纤数据传输。

High-speed optical data communication via free space or polymer optical fibre requires fast, safe, and low-cost light sources. At present red light sources (650 nm) are routinely used for polymer fibre data transport although a blue/green light source (< 520 nm) would provide a much lower intensity loss during fibre transmission. GaN/GaInN light emitting diodes emitting in the blue/green spectral regime are commercially available. However, the layer stacks of these diodes are grown in the c-direction of the wurtzite lattice causing a pronounced spatial separation of electrons and holes in the active InGaN quantum well due to spontaneous polarization. This separation results in long radiative lifetimes up to some 10 ns and fundamentally limits the achievable data rate using GaN/GaInN light emitting diodes.The key idea of this project is to completely eliminate the spontaneous polarization in order to increase the radiative recombination rate and to push the achievable modulation frequency up to above 2,000 MHz. Our approach is a core-shell nanowire arrangement consisting of a n-GaN core followed by a polarization-free m-plane shell composed of an active InGaN quantum well and a p-GaN contact layer. The device is realized by an array of core-shell GaN/InGaN nanowires grown on a conductive n-type silicon substrate. The free-space between the wires will be filled-up and a transparent top layer will serve as the anode of the device. A combination of self-catalytic and selective-area epitaxy shall be developed in order to provide the mandatory homogeneity of the nanowires within an array of nanowires and across the wafer.A distinct challenge is the elucidation of limiting recombination and relaxation mechanisms providing key information for the design and technology of a high-speed light source for optical data transmission. Time-resolved (< 100 ps) optical and optoelectronic measurements will be performed on the nanowire light sources with high spatial resolution (< 0.3 µm) in order to localize device limitations with respect to e.g. speed, intensity, and spectral line width of emission. The results will be directly incorporated into the design and technology of the devices. In parallel, the electrical high-frequency performance of the devices will be evaluated and optimized by scattering parameter measurements and equivalent circuit analysis. The fundamental goal of this project is the scientific development of a light source at frequencies above 2,000 MHz for free-space and polymer optical fibre data transmission.