Long intersubband relaxation times in <i>n</i>-type germanium quantum wells

Ortolani, Michele; Stehr, D.; Wagner, Martin; Helm, M.; Pizzi, Giovanni; Virgilio, Michele; Grosso, Giuseppe; Capellini, Giovanni; Seta, M. De

doi:10.1063/1.3662394

Cited by 26 publications

(36 citation statements)

References 18 publications

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“…Nevertheless, it can be stated that the pump-probe data are consistent with the steady-state spectroscopy data, in the sense that there is no contradiction in assuming that the same single-electron scattering mechanisms govern plasmon decay and hot electron relaxation after optical pumping. The n-Ge energy relaxation in the mid-IR range seems to be a much slower process than singleelectron inelastic scattering and therefore at odds with group III-V semiconductors, because of the lack of polar optical phonons, as already observed in n-Ge quantumwell systems 63 .…”

Section: Time-resolved Spectroscopymentioning

confidence: 98%

Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics

et al. 2016

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Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the 10 17 to 10 19 cm −3 range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 -4800 cm −1 range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in ntype germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature.The recent push towards applications of spectroscopy for chemical and biological sensing in the mid-infrared (mid-IR)1-8 has prompted the need for conducting thin films displaying values of the complex dielectric functionǫ(ω) = ǫ ′ (ω) + iǫ ′′ (ω) that can be tailored to meet the needs of novel electromagnetic designs exploiting the concepts of metamaterials, transformation optics and plasmonics 9 . In the design of metamaterials, where subwavelength sized conducting elements are embedded in dielectric matrices, if the values of ǫ ′ of the metal and the dielectric are of the same order, but have opposite sign, the geometric filling fractions of the metal and dielectric can be readily tuned to achieve subwavelengthresolution focusing of radiation 10 . Such requirement is met by silver for wavelengths λ around 400 nm. The same condition cannot be achieved in the IR range by using elemental metals, however, because metals possess an extremely high negative value of ǫ ′ not equaled, in

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Section: Time-resolved Spectroscopymentioning

confidence: 98%

Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics

et al. 2016

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“…Recently, ntype Ge/Si-Ge heterostructures have been identified as a promising material system to realize CMOS-compatible light emitters leveraging on intersubband (ISB) transitions in the THz range and operating at (or close to) room * michele.virgilio@unipi.it temperature [9][10][11]. In particular, the recent observation of ISB-transition signatures in the 4-12 THz range, featuring long nonradiative lifetimes of tens of picoseconds at a relatively high temperature (approximately 100 K) [8,[12][13][14][15], suggests that this material system could enable the fabrication of a Si-Ge-based THz QCL. Besides the great advantage of compatibility with the standard CMOS technology, the use of Ge/Si-Ge could also potentially overcome severe limitations of the present THz-QCL technology based on III-V compounds: (i) the weaker electron-phonon interaction in nonpolar semiconductors could increase the device operating temperature, currently limited to approximately 200 K [16]; (ii) the absence of a spectral range of forbidden propagation (Reststrahlen band) in group-IV semiconductors enables larger emission bandwidth for THz QCLs, well above the current limit of approximately 5 THz [17]; (iii) the absence of a strong frequency dependence of the refractive index in group-IV materials could provide the ideal support for THz frequency combs without dispersion compensation [18][19][20][21].…”

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confidence: 99%

“…After having first observed ISBT in decoupled Ge/Si-Ge multi-quantum-well (MCQW) heterostructures [12][13][14][15]24,25], in this work, we perform the subsequent logical step toward a QCL design, unambiguously demonstrating electronic wavefunction tunneling between adjacent n-type strain-compensated Ge/Si 0.2 Ge 0.8 ACQWs, grown by chemical vapor deposition (CVD). Using FTIR spectroscopy coupled with numerical calculations [13] as a probe for the interwell coupling, we extract relevant information on dipole matrix elements, barrier height, and level broadening due to interface scattering [12,13,[24][25][26].…”

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confidence: 99%

Control of Electron-State Coupling in Asymmetric Ge/Si−Ge Quantum Wells

et al. 2019

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“…However, the low radiative rates for THz emission from ISBTs implies that significant numbers of electrons have to be excited from level 1 to level 3 by the optical pump [16][17][18][19]. This translates into (1) the need for a high-power pulsed pump in the THz range, provided here by a free-electron-laser (FEL) [20,21]; (2) heavy n-doping of the ground state of the ACQWs; and (3) a high dipole moment of the 1→3 ISBT to enhance pump efficiency.…”

Section: Introductionmentioning

confidence: 99%

Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models

et al. 2019

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n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron–phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1→3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.

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Long intersubband relaxation times in n-type germanium quantum wells

Cited by 26 publications

References 18 publications

Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics

Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics

Control of Electron-State Coupling in Asymmetric Ge/Si−Ge Quantum Wells

Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models

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