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

光导半导体已经被广泛研究用于需要超快光开关的应用。太赫兹频率光电导体必须具有高载流子迁移率和短载流子复合时间以获得最佳效率。然而，这两个要求很难同时优化，因为材料中的短载流子寿命通常需要某种缺陷，这也会降低载流子迁移率。量子光电导体（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传输和载流子复合。我们的合作重点是使用过渡金属（TM）掺杂的InGaAs/InAlAs异质结构晶格匹配的InP。InGaAs系统与用于驱动开关的电信频率激光器兼容。在之前的DFG项目中，我们首先关注Fe和Rh的使用。在这次合作中，首次展示了使用分子束外延的TM掺杂半导体光电导开关。我们对TM掺杂InGaAs的合作研究表明，Fe，Rh和Ru在InGaAs中起深受主作用。这样的深能级可以补偿在通过MBE外延制备的InGaAs中发现的自然n型导电性。我们最近表明，Rh，像铁，在高浓度下形成集群。这种Fe或Rh团簇不再是简单的点缺陷，深受主的补偿效应饱和。同时，我们还发现，Ru似乎作为一个孤立的深受主纳入高得多的Ru浓度，可能是由于在外延过程中较低的表面迁移率。这种性质导致更高的深受主浓度在InGaAs中使用Ru比与其他过渡金属，并允许我们完全补偿在InGaAs中使用Ru的n型背景。因此，尽管掺杂有Fe或Rh的QPC结构已经显示出优异的室温霍尔迁移率和亚皮秒寿命，但是Fe和Rh的使用与由于成簇而导致的超低暗电导率不一致。因此，剩余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，以及我们希望在拟议的扩展项目中探索的解决方案，是将联合收割机高浓度的簇状Rh或Fe与非簇状Ru相结合，以同时实现超短寿命、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THz发射器和接收器的超快、TM掺杂光电导体。该项目延期的目标是深入研究双掺杂方法，并实现TM掺杂InGaAs材料系统作为THz天线应用的全部潜力。

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.