I. V. Bradley

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).

Stimulated terahertz emission from group-V donors in silicon under intracenter photoexcitation

Frequency-tunable radiation from the free electron laser FELIX was used to excite neutral phosphorus and bismuth donors embedded in bulk monocrystalline silicon. Lasing at terahertz frequencies has been observed at liquid helium temperature while resonant pumping of odd parity impurity states. The threshold was about two orders of magnitude below the value for photoionization pumping. The influence of nonequilibrium intervalley TO phonons on the population of excited Bi impurity states is discussed.

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.

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 .

Photoconductivity of Be-doped GaAs under intense terahertz radiation

Lyman transitions of the Be acceptor in GaAs have been investigated under intense radiation generated by a free electron laser. Photoconductivity was found to be a more sensitive method than transmission for detecting the transitions. Three regimes of photoconductive response were distinguished: at high intensity, the signal saturates; at intermediate intensity, it is proportional to intensity; at low intensity, it is background limited. Spectra were obtained by wavelength scanning and showed excellent reproducibility from run to run. With increasing radiation intensity, the D and C lines, being transitions from the 1s3/2(Γ8+) ground state to the 2p5/2(Γ8−) and 2p5/2(Γ7−) excited states, respectively, exhibit a pronounced splitting and broadening of components. This behavior is attributed to thermal stress induced by localized heating.

Effect of quantum-well confinement on acceptor state lifetime in δ-doped GaAs/AlAs multiple quantum wells

Using far-infrared time-resolved spectroscopy, we have investigated the effect of quantum-well confinement on the lifetime of shallow acceptor states in GaAs/AlAs multiple quantum wells with Be δ-doping at the well center. Low-temperature far-infrared absorption measurements clearly show three principal absorption lines due to transitions of Be acceptor states from the ground state to the first three odd-parity excited states, respectively. It is found that the lifetime of excited states monotonically reduces with decreasing quantum-well width, from 350 ps in bulk to 55 ps in a 100 A well. We suggest that the effect of quantum-well confinement on zone-fold acoustic-phonon modes increases the intra-acceptor scattering rate of acoustic-phonon-assisted relaxation.

Free-electron laser studies of energy transfer mechanisms in semiconductors doped with Transition Series ions

Abstract Shallow levels determine electrical and optical properties of semiconductors. Mid-infrared radiation from a free-electron laser can be used for an effective ionization of shallow impurities, leading to a variety of effects. In contrast to thermal ionization, the optically induced ionization process can be tuned to a particular level by adjusting the wavelength. In this way, different impurity and defect levels can be selectively addressed. The short-pulsed output of the free-electron laser allows the experiments to be performed in a manner, which utilizes its unique characteristics. In this contribution, we show how two-color spectroscopy with a free-electron laser can be used to unravel energy transfer between different centers in semiconductor matrices. In particular, energy storage at shallow centers in silicon and mid-infrared-induced Auger recombination process of long-living optically active centers will be discussed. Specific examples for rare earth- and transition metal-doped silicon and rare earth-doped III–V semiconductors will be presented.

THz intersubband dynamics in p-Si/SiGe quantum well structures

We report time-resolved (ps) studies of the dynamics of intersubband transitions in p-Si/SiGe multi-quantum well structures in the FIR regime, ℏω<ℏωLO, utilizing the Dutch free electron laser, FELIX. The calculated scattering rates 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 . This is found to be approximately constant in the temperature range from 4 to , in good agreement with the predictions of alloy scattering in the Si0.7Ge0.3 wells.

Intersubband lifetimes and free carrier effects in optically pumped far infrared quantum wells laser structures

Electron intersubband lifetimes of 1.8 ps and 2.5 ps were measured in GaAs/AlGaAs asymmetric stepped quantum well samples by using a pump–probe three-beam technique at 4 K to room temperature. At high pump intensities, an absorptive signal of sub-nanosecond lifetime was detected following a normal positive transmissive signal of picosecond lifetime. This absorptive signal is believed to be caused by the absorption from the free electrons pumped from the ground subband to the continuum conduction band in the barrier layers associated with incoherent multi-photon process. The intersubband lifetimes and free carrier effects play an important role in optically pumped multiple-quantum-well far infrared lasers. A three-level electron transition model is built to simulate the process, which agrees very well with the experimental data.