Tunable Quantum Photoconductor for efficiency-enhanced THz switches (QPC-Switch)

光导半导体在需要超快光开关的应用中得到了广泛的研究。太赫兹频率光导体必须同时具有高载流子迁移率和短载流子复合时间才能获得最佳效率。然而，这两个要求很难同时优化，因为材料中载流子寿命短通常需要某种缺陷，这种缺陷也会降低载流子的迁移率。协作量子光导体（QPC）开关项目的目标是通过明智地使用半导体异质结构来解耦这两个参数的优化，使我们能够分离载流子输运和载流子重组。我们的合作重点是利用过渡金属（TM）掺杂与InP晶格匹配的InGaAs/InAlAs异质结构。InGaAs系统与用于驱动开关的电信频率激光器兼容。在以前的DFG项目中，我们首先关注的是Fe和Rh的使用。分子束外延在掺杂tm半导体光导开关中的应用首次在此次合作中得到证实。我们对tm掺杂InGaAs的合作研究表明，Fe， Rh和Ru作为InGaAs的深层受体。这样的深能级可以补偿由MBE外延制备的InGaAs中的天然n型电导率。我们最近发现，Rh和Fe一样，在浓度升高时形成团簇。这样的铁或铑簇不再是简单的点缺陷，深层受体的补偿作用饱和。同时，我们还发现Ru似乎作为一个孤立的深层受体结合到更高浓度的Ru中，这可能是由于外延过程中较低的表面迁移率。这一特性导致使用Ru的InGaAs中深层受体浓度高于使用其他过渡金属的InGaAs，并允许我们完全补偿使用Ru的InGaAs中的n型背景。因此，尽管掺杂Fe或Rh的QPC结构表现出优异的室温霍尔迁移率和亚皮秒寿命，但由于聚类，Fe和Rh的使用与超低暗电导率不一致。因此，剩下的挑战在于进一步降低背景载流子浓度。我们提出的解决方案，也是我们希望在拟议的扩展项目中探索的，是将高浓度聚集的Rh或Fe与非聚集的Ru结合起来，同时实现超短寿命、高迁移率和低暗载流子浓度。初步结果表明，Rh和Ru的组合可以显著改善用于太赫兹发射器和接收器的超快，tm掺杂光导体。项目延长的目标是深入研究双掺杂方法，充分发挥tm掺杂InGaAs材料体系作为太赫兹天线应用的潜力。

Photoconductive semiconductors have been widely investigated for applications requiring ultrafast optical switching. THz-frequency photoconductors must have both high carrier mobility and short carrier recombination time for optimal efficiency. These two requirements, however, are difficult to simultaneously optimize because short carrier lifetimes in materials usually requires some sort of defect that also degrades the carrier mobility. The goal of the collaborative Quantum Photoconductor (QPC) Switch project has been to decouple the optimization of these two parameters through the judicious use of semiconductor heterostructures, allowing us to separate carrier transport and carrier recombination. We have focused in this collaboration on the use of transition metal (TM) doping of InGaAs/InAlAs heterostructures lattice-matched to InP. The InGaAs system is compatible with telecom-frequency lasers for driving the switch. In prior DFG projects, we have focused first on the use of Fe and Rh. The use of molecular beam epitaxy for TM-doped semiconductor photoconductive switches was first demonstrated within this collaboration. Our collaborative investigation of TM-doped InGaAs has shown that Fe, Rh and Ru act as deep acceptors in InGaAs. Such deep levels can compensate the natural n-type conductivity found in InGaAs prepared epitaxially by MBE. We have recently shown that Rh, like Fe, forms clusters at elevated concentrations. Such Fe- or Rh-clusters are no longer simple point defects and the compensation effect of deep acceptors saturates. At the same time, we have also found that Ru appears to incorporate as an isolated deep acceptor to much higher Ru concentrations, perhaps due to the lower surface mobility during epitaxy. This property leads to higher deep acceptor concentrations in InGaAs using Ru than with the other transition metals, and allows us to completely compensate the n-type background in InGaAs using Ru. Thus, although QPC structures doped with Fe or Rh have shown excellent room-temperature Hall mobilities and sub-picosecond lifetimes, the use of Fe and Rh is not consistent with ultra-low dark conductivity due to clustering. Thus, a remaining challenge lies in reducing further the background carrier concentration. The solution we propose, and that we would like to explore in the proposed extension project, is to combine high-concentration clustered Rh or Fe with non-clustered Ru to achieve simultaneously ultrashort lifetimes, high mobility, and low dark carrier concentration. Preliminary results indicate that the combination of Rh and Ru can result in significant improvements in ultrafast, TM-doped photoconductors for THz emitters and receivers. The goal of the project prolongation is to investigate the dual-doping approach in depth and to realize the full potential of TM-doped InGaAs material system for application as THz antennas.