Phone numbersMid-IR Physics and Devices :: Quantum Dot Infrared Photodetectors
Infrared detectors are widely used in applications in defence technology as well as civilian applications such as in meteorology, medical imaging, solid state spectrometry, chemical analysis etc. The detectors which are mostly investigated are the ones operating in the wavelength regions of 1-3µm, 3-5µm and 8-14µm (atmospheric windows). Especially the 8-14µm window can be useful for imaging as the temperature of the human body is around 300K, corresponding to a wavelength of thermal radiation of around 10µm.
Intraband quantum dot infrared photodetectors (QDIPs) have attracted considerable attention due to the potentially beneficial characteristics which arise from the three-dimensional confinement provided by the QDs, such as the intrinsic capability of normal incidence detection and potentially low dark currents.
Thereafter good performance at higher temperatures for these devices is expected. Additionally, it has been shown that the spectral response of a QDIP can vary from single to potentially three-colour behaviour, which in turn can be voltage dependent.
The research in Sheffield involves the design and fabrication of InAs/InxGa1-xAs QDIPs based on a dots-in-a-well structure (DWELL), resulting in a two-colour spectral behaviour corresponding to transitions from the QD ground state to a state in the well and to the continuum as seen in Figure 1.
DWELL conduction band structure and the electron transitions involved.
We have achieved a tuning of the transition energy of 20% by increasing the number of InAs ML deposited during growth in a controllable way, in addition to post-growth wavelength tunability with applied bias voltage via the intraband Stark effect (Figure 2).
Figure 1: Normalised 10K photoresponse for 2.2(black) and 2.9(red) ML samples at -1V
Figure 2: Normalised photoresponse for a QDIP with 2.9MLs of InAs dots at 0V (blue), +1V (red) and -1V (black)
Finally we have demonstrated the good performance of these devices with responsivities (R) in the order of ~1A/W at ±1V and 10K, and detectivities (D*) in the order of 1012cmHz1/2W-1.
This research is led by Luke Wilson in collaboration with:
John P.R. David & Chris Groves, Department of Electrical and Electronic Engineering, The University of Sheffield, and
B.Aslan & H.C.Liu, National Research Council, Canada.
Funding was provided by:
Electro-Magnetic Remote Sensing Defence Technology Centre (EMRS DTC) established by the UK Ministry of Defence
Engineering and Physical Sciences Research Council (EPSRC)
Mid-IR Physics and Devices :: Polarons
Recently it has been demonstrated that in quantum dots (QDs) the strong coupling of the electron and phonon results in the formation of coherent admixtures of the two, known as polarons [1].
We have performed detailed investigations of polaron relaxation processes in n-type InAs quantum dots using energy, polarisation and temperature dependent far-infrared pump-probe spectroscopy. A range of samples has been studied, providing energy dependent polaron decay times over a wide spectral region (Fig. 1) and allowing the identification of the polaron decay channel (Fig. 2). In addition, the lifting of the degeneracy of the p-like first excited state in InAs quantum dots has enabled us to selectively measure polaron decay times from the lower (p-) or higher (p+) energy state using orthogonal linear polarisations.
Figure 1: Energy dependence of the polaron decay time over a wide energy range obtained measure by pump-probe spectroscopy.
Figure 2: Identification of the polaron decay mechanism from studies of the temperature dependence of the polaron decay time.For excitation into p- states between 40meV and 52meV the polaron lifetime exhibits a monotonic increase from 20ps to 65ps, which is explained by considering decay to acoustic phonons due to anharmonicity of the lattice forces (Fig. 1) [2]. By analysing the temperature dependence of the polaron lifetime in this energy range the main polaron decay mechanism has been identified as the cubic overtone decay channel involving two equal energy longitudinal acoustic (LA) phonons (Fig. 2). Above ~52meV, the polaron decay time plateaus, arising from the increase of the density of states for each of the two final state LA phonons at energies higher than ~25meV and the opening of additional 2LO phonon decay channels.
For excitation into the p+ state, polarons can decay either directly to the ground state or via p- through the emission of low energy acoustic phonons. As shown in Fig. 3 Pump-probe measurements at the same excitation energy but for different polarisations (excitation to either p+ or p-) show a reduced decay time from the p+ state resulting from the additional parallel decay channel. We have studied several samples with different p+ to p- splittings and find that the transfer time from p+ to p- deduced from the experimental data using combined rate equations decreases from 35ps for the sample with 5.7meV splitting to 15ps for the sample with 3.7meV splitting, in excellent agreement with our calculated acoustic phonon assisted scattering times (Fig. 4).
Figure 3: Polarisation dependence of the absorption and pump-probe decay time for different p+ / p- excited state separations.
Figure 4: Calculated acoustic phonon assisted scattering times, in excellent agreement with the experimental results.Work in this area involves collaboration with Professors G Bastard and R Ferreira, ENS, Paris; Dr A Andreev, Surrey; Dr M Sadowski, Grenoble; and the staff at FELIX, Nieuwegein. A presentation of this work has been given at ICPS (~1.5MB, .pdf).
| [1] | S. Hameau, Y. Guldner, O. Verzelen, R. Ferreira, G. Bastard, J. Zeman, A. Lemaitre and J. M. Gerard, Phys. Rev. Lett. 83, 4152 (1999) |
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| [2] | X-Q. Li, H. Nakayama and Y. Arakawa, Phys. Rev. B 59, 5069 (1999) |
