It has long been known that shallow donors such as phosphorous and the other group-V elements, have a hydrogen-like optical spectrum. The main difference is that while the spectrum of atomic hydrogen lies in the visible band, the spectrum of shallow donors in silicon is downshifted to the THz frequency band. This is a direct consequence of the reduced Coulomb attraction seen by the loosely bound electron because the core electrons shield the positive donor atom nucleus, and because the electron is now moving in a dielectric material. While spectroscopy has already revealed much about the energy level structure, very little was known about the temporal dynamics of the system until now. We have used THz pulses from the FELIX free electron laser to probe these hydrogen-like levels. By exploiting the well-known pump-probe technique we have measured the characteristic lifetimes of the excited Rydberg states and found them to be of the order 200 ps. Then, by making subtle changes to the geometry of the pump-probe experimental setup we demonstrate the existence of a THz photon echo. The photon echo is a purely quantum phenomenon with no classical analogue, and it allows us to study the quantum state of the donor electron. We then show, using the photon echo, that it is possible to create a coherent superposition of the ground and excited state of the donor. Measuring the photon echo is important because it can also be used to measure a second important characteristic lifetime of the silicon-donor system, the phase decoherence time.
There is strong interest in the development of sources that emit radiation in the far infrared (1-10 THz) frequency range for applications which include early detection of skin cancer, dental imaging, telecommunications, security scanning, gas sensing, astronomy, molecular spectroscopy, and the possible detection of biological weapons. While a number of THz sources are available, there are at present no compact, efficient, cheap and practical high-power solid-state sources such as light emitting diodes or lasers. Silicon is an excellent candidate for such a THz source since the lack of polar optical phonon scattering makes it an inherently low loss material at these frequencies. Furthermore, since over 97% of all microelectronics is presently silicon based, the realisation of a silicon based emitter/laser could potentially allow integration with conventional silicon-based microelectronics. In this paper THz electroluminescence from a Si/SiGe quantum cascade structure operating significantly above liquid helium temperatures is demonstrated. Fourier transform infrared spectroscopy was performed using step scan spectrometer with a liquid helium cooled Si-bolometer for detection. Spectra are presented demonstrating intersubband electroluminescence at a number of different frequencies. These spectral features agree very well with the theoretically calculated intersubband transitions predicted for the structure.
Terahertz (far-infrared) intersubband electroluminescence is reported in p-type Si/SiGe quantum wells and quantum cascade structures. Surface-normal emission (without the aid of a surface grating) from light hole - heavy hole intersubband transitions has been observed for the first time in a quantum cascade device. Edge-emission measurements have also been performed, which show emission from both heavy hole - heavy hole and light hole - heavy hole transitions, and have allowed demonstration of the polarisation dependence of the emitted power, according to the selection rules for the intersubband interactions. The electroluminescence is visible up to temperatures of ~150K, in the multiple quantum well structures, and >=77K in the quantum cascade structure.
We present experimental and theoretical investigations of the temperature dependence of self-pulsation in CD laser diodes. We use a rate equation model to predict the device dynamic behavior over a large temperature range and identify the role of carrier diffusion. We show experimentally and by calculating that the temperature dependence of the threshold current is driven by the carrier diffusion--particularly at low temperature. We experimentally show that for several temperatures the self-pulsation variation with respect to normalized bias current is highly linear. These results call into question whether pulsations in CD laser structures are undamped relaxation oscillations. Our results also suggest that the highly temperature dependent carrier diffusion does not play a first order role in CD laser diode self- pulsation.
Conference Committee Involvement (4)
Terahertz Emitters, Receivers, and Applications VII
28 August 2016 | San Diego, California, United States
Terahertz Emitters, Receivers, and Applications VI
11 August 2015 | San Diego, California, United States
Terahertz Emitters, Receivers, and Applications V
17 August 2014 | San Diego, California, United States
Terahertz Emitters, Receivers, and Applications IV
25 August 2013 | San Diego, California, United States
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