Room Temperature Terahertz Quantum Cascade Lasers on Silicon Substrates

电磁波谱的太赫兹部分有许多潜在的应用，包括肿瘤学（皮肤癌成像）、安全成像、太赫兹带宽光子学、生产监控和天文学。英国一直是太赫兹研究的先驱国家之一，同时也是该技术的开发利用的许多公司，包括TeraView， QMC Instruments和Thruvision。目前，大多数商用成像和光谱系统使用昂贵的飞秒激光器和光导天线，这从根本上限制了功率输出到微瓦级。如果要开发并行、快速、高性能成像和/或光谱系统，几乎上述所有应用都需要输出功率超过10mw的室温光源。虽然硅和锗中电子和空穴的带间复合由于半导体的间接带隙而效率低下，但子带间跃迁为低能量辐射（如太赫兹频率）的激光提供了另一种途径。以量子级联激光器形式的子带间单极激光器已被证明使用III-V材料。在太赫兹频率下，功率高达248mw (10k)，但由于极性光学声子散射和随着温度升高而相关的子带间寿命减少，这种器件只能在低温下工作。先前的工作已经在p型Si/SiGe量子级联激光器上进行，但由于价带的非抛物性和有效质量大（0.3至0.4 m_0）， 10 cm^-1以上的显著增益很难设计。在本提案中，我们建议使用纯Ge量子阱设计和l -谷电子进行在Si衬底上生长的n型Si基量子级联激光器的首次实验演示。我们证明，在Ge/SiGe系统中，0.118 m_0的低效率和较长的非极性寿命可能提供接近4 K时GaAs太赫兹量子级联激光器所显示的值的增益，也可能允许300 K的工作。此外，与砷化镓器件相比，廉价和成熟的可用Si工艺技术将使太赫兹量子级联激光器的成本至少降低x100。这种器件可以进一步发展成垂直腔发射器（即vcsel），用于并行成像应用或与硅光子学集成，以实现太赫兹带宽电信。最后，我们提出了具有宽带可调性、更高输出功率和比太赫兹量子级联激光器更高工作温度的光泵浦结构。该计划将利兹大学的建模和设计工具集与华威大学的CVD生长专业知识结合格拉斯哥大学SiGe设备的制造和测量专业知识，提供国际领先的研究。我们有许多工业合作伙伴（AdvanceSis， Kelvin Nanotechnology和TeraView），他们为这项研究提供了直接的开发途径。成功的室温量子级联激光器是许多太赫兹应用新市场的使能技术，包括肿瘤学（皮肤癌成像）、安全成像、生产监控、蛋白质组学、药物发现和天文学。

The THz part of the electromagnetic spectrum has a number of potential applications which include oncology (skin cancer imaging), security imaging, THz bandwidth photonics, production monitoring and astronomy. The U.K. has been one of the pioneering countries in THz research but also in the exploitation of the technology with a number of companies including TeraView, QMC Instruments and Thruvision. At present most commercial imaging and spectroscopy systems use expensive femtosecond lasers with photoconductive antenna which fundamentally limits the power output to the microWatt level. Virtually all the applications referenced above require room temperature sources with over 10 mW of output power if parallel, fast, high performance imaging and/or spectroscopy systems are to be developed.While interband recombination of electrons and holes in Si and Ge are inefficient due to the indirect bandgap of the semiconductors, intersubband transitions provide an alternative path to a laser for low energy radiation such as THz frequencies. Intersubband unipolar lasers in the form of quantum cascade lasers have been demonstrated using III-V materials. Powers up to 248 mW at 10 K have been demonstrated at THz frequencies but due to polar optical phonon scattering and the associated reduction in intersubband lifetimes as the temperature is increased, such devices only operate at cryogenic temperatures. Previous work has been undertaken on p-type Si/SiGe quantum cascade lasers but due to large non-parabolicity and large effective mass (0.3 to 0.4 m_0) in the valence band, significant gain above 10 cm^-1 is difficult to engineer.In this proposal, we propose to use pure Ge quantum well designs and L-valley electrons for the first experimental demonstration of a n-type Si-based quantum cascade laser grown on top of a Si substrate. We demonstrate that the low effective of 0.118 m_0 and long non-polar lifetimes in the Ge/SiGe system potentially provide gain close to values demonstrated in GaAs THz quantum cascade lasers at 4 K and also potentially allow 300 K operation. Further the cheap and mature available Si process technology will allow at least a x100 reduction in the cost of THz quantum cascade lasers compared to GaAs devices. Such devices could be further developed into vertical cavity emitters (i.e. VCSELs) for parallel imaging applications or integrated with Si photonics to allow THz bandwidth telecoms. Finally we propose optically pumped structures which have the potential for broadband tunability, higher output powers and higher operating temperatures than THz quantum cascade lasers.This programme has brought together the modelling and design toolsets at Leeds University with the CVD growth expertise at Warwick University combined with the fabrication and measurement expertise of SiGe devices at Glasgow University to deliver internationally leading research. We have a number of industrial partners (AdvanceSis, Kelvin Nanotechnology and TeraView) who provide direct exploitation paths for the research. Successful room temperature quantum cascade lasers are an enabling technology for many new markets for THz applications including oncology (skin cancer imaging), security imaging, production monitoring, proteomics, drug discovery and astronomy.