V. V. Rumyantsev

Study of lifetimes and photoconductivity relaxation in heterostructures with HgxCd1 − xTe/CdyHg1 − yTe quantum wells

Carrier lifetimes in the continuum of the quantum well of a HgxCd1 − xTe/CdyHg1 − yTe hetero-structure were studied by terahertz pump-probe spectroscopy. It is found that the relaxation duration of the transmission signal is ∼65 ps and is independent of the pump power. Such rapid relaxation in these structures is most likely determined by the interaction of holes with acoustic phonons due to a high density of states in the valence band and a larger effective mass compared with electrons. By the obtained data, the times of the interband nonradiative recombination of holes are determined. In this publication, we report the results of numerical calculation of the energy spectrum of the model structure, in which the possibility of obtaining population inversion at specified concentrations of nonequilibrium carriers is analyzed.

Efficient long wavelength interband photoluminescence from HgCdTe epitaxial films at wavelengths up to 26 μm

Photoluminescence (PL) and photoconductivity (PC) studies of Hg1−xCdxTe (0.19 ≤ x ≤ 0.23) epitaxial films are presented. Interband PL is observed at wavelengths from 26 to 6 μm and in the temperature range 18 K–200 K. The PL line full width at half maximum is about 6 meV (4kT) at 18 K and approaches its theoretical limit of 1.8kT at higher temperatures. Carrier recombination process is also investigated by time resolved studies of PL and PC at pulsed excitation. Radiative transitions are shown to be the dominating mechanism of carrier recombination at high excitation levels.

Terahertz spectroscopy of quantum-well narrow-bandgap HgTe/CdTe-based heterostructures

The energy spectra of quantum-well narrow-bandgap Hg1 − yCdyTe/CdxHg1 − xTe heterostructures have been studied. The dependences of the effective cyclotron mass on the density (in classical magnetic fields) and the transition energy (in quantizing fields) have been obtained from the cyclotron resonance measurements. These dependences confirm the near-linear dispersion law for the electrons with small mass at the band bottom (the minimum cyclotron mass measured is 0.003 m0). The interband photoconductivity of the CdHgTe-based structures with the long-wavelength photoresponse edge lower than 6 meV has been demonstrated.

Time resolved photoluminescence spectroscopy of narrow gap Hg1−xCdxTe/CdyHg1−yTe quantum well heterostructures

Photoluminescence (PL) spectra and kinetics of narrow gap Hg1−xCdxTe/CdyHg1−yTe quantum well (QW) heterostructures grown by molecular beam epitaxy technique are studied. Interband PL spectra are observed from 18 K up to the room temperature. Time resolved studies reveal an additional PL line with slow kinetics (7 μs at 18 K) related to deep defect states in barrier layers. These states act as traps counteracting carrier injection into QWs. The decay time of PL signal from QW layers is about 5 μs showing that gain can be achieved at wavelengths 10–20 μm by placing such QWs in HgCdTe structures with waveguides.

Long wavelength stimulated emission up to 9.5 μm from HgCdTe quantum well heterostructures

Stimulated emission from waveguide HgCdTe structures with several quantum wells inside waveguide core is demonstrated at wavelengths up to 9.5 μm. Photoluminescence line narrowing down to kT energy, as well as superlinear rise in its intensity evidence the onset of the stimulated emission, which takes place under optical pumping with intensity as small as ∼0.1 kW/cm2 at 18 K and 1 kW/cm2 at 80 K. One can conclude that HgCdTe structures potential for long-wavelength lasers is not exhausted.

Long-wavelength injection lasers based on Pb1–x Sn x Se alloys and their use in solid-state spectroscopy

Diffusion lasers based on Pb1–xSnxSe alloys for a wide spectral range (7–40 μm) are developed. The emission spectra of these lasers in the temperature range of 18–80 K are investigated. It is shown that the laser-emission wavelength can be widely tuned by varying the operation temperature, which allows a spectral range of 7–26 THz to be covered. The possibility of using these lasers for the spectroscopy of solids, and, in particular, for the magnetooptical spectroscopy of narrow-gap semiconductor structures based on HgCdTe, is demonstrated.

Specific features of the spectra and relaxation kinetics of long-wavelength photoconductivity in narrow-gap HgCdTe epitaxial films and heterostructures with quantum wells

The spectra and relaxation kinetics of interband photoconductivity are investigated in narrow-gap Hg1 − xCdxTe epitaxial films with x = 0.19–0.23 and in structures with HgCdTe-based quantum wells (QWs), having an interband-transition energy in the range of 30–90 meV, grown by molecular-beam epitaxy on GaAs (013) substrates. A long-wavelength sensitivity band caused by impurities or defects is found in the spectra of the structures with quantum wells in addition to the interband photoconductivity. It is shown that the lifetimes of nonequilibrium carriers in the structures with QWs is less than in bulk samples at the same optical-transition energy. From the measured carrier lifetimes, the ampere-watt responsivity and the equivalent noise power for a film with x = 0.19 at a wavelength of 19 μm are estimated. When investigating the relaxation kinetics of the photoconductivity at 4.2 K in high excitation regime, it is revealed that radiative recombination is dominant over other mechanisms of nonequilibrium-carrier recombination.

Features of impurity-photoconductivity relaxation in boron-doped silicon

A series of studies of the impurity-photoconductivity relaxation in Si:B is carried out under pulse optical excitation by a narrow-band tunable radiation source in low and “heating” (10–500 V/cm) electric fields. It is shown that the dependence of the carrier-capture time in a band on the applied electric field is nonmonotonic and, in high fields (>75 V/cm), the capture time decreases with increasing field intensity, which is related to initiating the relaxation processes with optical-phonon emission within the band. The dependence of the relaxation rate for the carriers on the excitation-radiation wavelength is investigated, and a decrease in the carrier-capture time in the band is revealed in the vicinity of the Breit-Wigner-Fano resonances caused by direct capture at an impurity with optical-phonon emission.

Long wavelength superluminescence from narrow gap HgCdTe epilayer at 100 K

Experimental evidence of long wavelength superluminescence (SL), i.e., amplification of spontaneous emission, in narrow gap HgCdTe bulk epitaxial film at 100 K is reported. Photoluminescence line narrowing is observed at 8.4 μm as pump power increases. However, plasmonic contribution to dielectric function is shown to be detrimental for light confinement at high pumping intensities, limiting the SL line intensity growth. The design of the structures optimal for obtaining stimulated emission in 10–36 μm range is further discussed.

Kinetics of terahertz photoconductivity in p-Ge under impurity breakdown conditions

Relaxation times of impurity photoconductivity in p-Ge samples excited by a nanosecond narrow-band source of terahertz radiation are studied at various bias voltages. It is shown that the relaxation time in prebreakdown fields increases with the applied electric field and decreases as the impurity breakdown field is exceeded. Nonmonotonic photoconductivity kinetics is observed in the studied samples differing by acceptor concentrations and degrees of compensation when approaching the impurity breakdown field.