C. R. Pidgeon

Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures.

Auger recombination in long-wavelength infrared InNxSb1−x alloys

Dilute nitrogen alloys of InSb exhibit strong band gap bowing with increasing nitrogen composition, shifting the absorption edge to longer wavelengths. The conduction band dispersion also has an enhanced nonparabolicity, which suppresses Auger recombination. We have measured Auger lifetimes in alloys with 11 and 15 μm absorption edges using a time-resolved pump-probe technique. We find the lifetimes to be longer at room temperature than equivalent band gap Hg1−yCdyTe alloys at the same quasi-Fermi level separation. The results are explained using a modified k⋅p Hamiltonian which explicitly includes interactions between the conduction band and a higher lying nitrogen-related resonant band.

Band anticrossing in dilute InNxSb1-x

Dilute nitrogen alloys of InSb exhibit extremely strong band gap bowing with nitrogen composition that has been associated with anticrossing between the localized resonant states of the nitrogen within the conduction band and the extended states of the conduction band itself. This also results in the conduction band dispersion having an enhanced nonparabolicity. We have measured the electron effective mass near the anticrossing by cyclotron resonance in InNxSb1−x alloys with absorption edge near 15 μm, using pulsed fields up to 150 T. The results directly demonstrate the band anticrossing and quantitatively confirm the increase of effective mass versus x predicted for InNxSb1−x by a tight binding calculation for low nitrogen concentration (x<0.01).

SUPPRESSION OF AUGER RECOMBINATION IN ARSENIC-RICH INAS1-XSBX STRAINED LAYER SUPERLATTICES

Room-temperature pump-probe transmission experiments have been performed on an arsenic-rich InAs/InAs1-xSbx strained layer superlattice (SLS) above the fundamental absorption edge near 10 mu m, using a ps far-infrared free-electron laser. Measurements show complete bleaching at the excitation frequency, with recovery times which are found to be strongly dependent on the pump photon energy. At high excited carrier densities, corresponding to high photon energy and interband absorption coefficient, the recombination is dominated by Auger processes, A direct comparison with identical measurements on epilayers of InSb, of comparable room-temperature band gap, shows that the Auger processes have been substantially suppressed in the superlattice case as a result of both the quantum confinement and strain splittings in the SLS structure, In the nondegenerate regime, where the Auger lifetime scales as tau(aug)(-1)=C1Ne2, a value of C-1 some 100 times smaller is obtained for the SLS structure. The results have been interpreted in terms of an 8x8 k . p SLS energy band calculation, including the full dispersion for both k in plane and k parallel to the growth direction. This is the strongest example of room-temperature Auger suppression observed to date for these long-wavelength SLS alloy compositions and implies that these SLS materials may be attractive for applications as room-temperature mid-IR diode lasers

Picosecond intersubband dynamics in p-Si/SiGe quantum-well emitter structures

We report time-resolved (ps) studies of the dynamics of intersubband transitions in p-Si/SiGe multiquantum-well structures in the far-infrared (FIR) regime, ℏω<ℏωLO, utilizing the Dutch free electron laser, (entitled FELIX—free electron laser for infrared radiation). The calculated scattering rates for optic and acoustic phonon, and alloy scattering have been included in a rate equation model of the transient FIR intersubband absorption, and show excellent agreement with our degenerate pump-probe spectroscopy measurements where, after an initial rise time determined by the resolution of our measurement, we determine a decay time of ∼10 ps. This is found to be approximately constant in the temperature range from 4 to 100 K, in good agreement with the predictions of alloy scattering in the Si0.7Ge0.3 wells.

Suppression of non-radiative processes in semiconductor mid-infrared emitters and detectors

Abstract We review the methods that have been used for suppressing non-radiative processes in mid-infrared (MIR) semiconductor lasers and detectors. Specifically we discuss the results of techniques that have been used recently to minimise the deleterious effect of Auger recombination processes in interband detectors and (bi-polar) lasers, and of phonon scattering in quantum well photodetectors (QWIPs) and quantum cascade (QC) lasers. After summarising the theory of the suppression of Auger and phonon processes in these devices, sections are devoted to specific III–V, II–VI and lead salt materials systems; further sections are devoted to subband detectors, subband cascade lasers, interband cascade lasers and to non-equilibrium devices from the InSb and HgCdTe systems.

Si:P as a laboratory analogue for hydrogen on high magnetic field white dwarf stars.

Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10(5)u2009T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8u2009T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H(2) analogues, and for investigation of He(2), a bound molecule predicted under extreme field conditions.

Experimental determination of the Rashba coefficient in InSb/InAlSb quantum wells at zero magnetic field and elevated temperatures

We report the optical measurement of the spin dynamics at elevated temperatures and in zero magnetic field for two types of degenerately doped n-InSb quantum wells (QWs), one asymmetric (sample A) and one symmetric (sample B) with regards to the electrostatic potential across the QW. Making use of three directly determined experimental parameters: the spin lifetime, τ(s), the sheet carrier concentration, n, and the electron mobility, μ, we directly extract the zero-field spin splitting. For the asymmetric sample where the Rashba interaction is the dominant source of spin splitting, we deduce a room temperature Rashba parameter of α = 0.09 ± 0.1 eV Å which is in good agreement with calculations and we estimate the Rashba coefficient α(0) (a figure of merit for the ease with which electron spins can be modulated via an electric field). We review the merits/limitations of this approach and the implications of our findings for spintronic devices.

Excite-Probe Determination of the Intersubband Lifetime in Wide Gaas/Algaas Quantum-Wells Using a Far-Infrared Free-Electron Laser

A direct excite-probe semiconductor lifetime determination in the picosecond regime has been made for the first time in the far infrared. We have used an RF-linac-pumped free-electron laser to determine the relaxation rate associated with intersubband absorption in GaAs/AlGaAs quantum wells having a subband separation smaller than the optical phonon energy. The measurement yields a relaxation lifetime of 40+or-5 ps. This is compared with a variety of other results obtained with less direct techniques.

Si/SiGe quantum-cascade emitters for terahertz applications

The quantum cascade laser provides one possible method of realizing high efficiency light emission from indirect band gap materials such as silicon. Strain-symmetrized Si/SiGe samples designed to investigate the intersubband properties of quantum wells are examined. Electroluminescence data from Si/SiGe quantum-cascade staircases demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions are presented. In surface-normal emission only light-hole to heavy-hole electroluminescence is observed at ( wavelength) as predicted by theory. Modulation-doped SiGe quantum well samples are also investigated to determine the underlying physics in the system. Results of picosecond time resolved studies of the dynamics of the intersubband transitions using a free electron laser are presented which show approximately constant relaxation times of below .