Far-infrared magneto-absorption in pure germanium under photo-excitation

Far-infrared cyclotron resonance observation revealed that the hole cyclotron masses in germanium depend on magnetic field owing to the influence of the split-off valence band. The Zeeman spectrum of excitons was also observed at lower temperatures, and it is shown that for this spectrum the magneto-Stark effect resulting from thermal motion of excitons brings about an asymmetric line-broadening.     

Optical orientation of donor-bound excitons in nanosized InP/InGaP islands

Spin splitting of optically active and inactive excitons in nanosized n-InP/InGaP islands has been revealed. Optically inactive states become manifest in polarized-luminescence spectra as a result of excitons being bound to neutral donors (or of the formation of the trion, a negatively charged exciton) in InP islands. The exchange-splitting energies of the optically active and inactive states have been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1745–1752 (September 1998)     

Far-infrared absorption by excitons in silicon

We report on the first observation of 1s to 2p absorption by excitons in pure silicon. A group of strong lines at 10.2, 11.4 and 12.0 meV is observed along with a continuum extending to higher frequency. A theoretical model for the excitonic absorption accounts well for the observed structure.     

Strain-narrowing of excitons in germanium

We have observed a narrowing of the luminescence line from excitons in germanium with the application of a uniaxial stress along the [111] crystallographic axis. In zero stress the line is broadened by the two split excitons and the mass-reversal effect, while at our stress of 15 $\text{kg}\text{mm}{}^2$ only one band is occupied. We find that the high stress density of states varies nearly as the square root of energy.     

Creation of spin-oriented 2P excitons in CdS by two-photon magneto-absorption

We report experimental results on the creation of 2P excitons in CdS by the two-photon magneto-absorption process involving a simultaneous absorption of visible and infrared photons. For a magnetic field parallel to the c axis and particularly for a visible beam propagating along the c axis, spin-oriented 2P (A) excitons are created from the A (Γ₉) valence band. We explain these results on the basis of the main contribution of the 1SΓ₅ (A) exciton intermediate state.     

Far-infrared recombination radiation from impact-ionized donors in germanium

We studied the far-infrared emission spectrum resulting from recombination of an electron with an ionized impurity of As and Sb in germanium under impact ionization at liquid, helium temperatures. The emission peaks at the position corresponding to the transition from the 2p _± excited state to the ground state. This observation indicates that recombination occurs through the capture by the excited states of the donor impurity, which is consistent with the cascade trap model. The intensity of emission radiation is of the order of 10⁻⁷ watts for the excitation power of about one watt, which implies a dominant process of recombination to be accompanied by phonon emission.     

Diffusion of excitons and electron-hole drops in germanium

Diffusion of excitons and electron-hole drops is investigated in pure germanium, using a time-resolved cyclotron resonance method. The diffusion coefficient of excitons at 4.2 K is obtained to be ≈ 1000 cm²/sec. For electron-hole drops, when excitation is not so high, it is expected to be lower than ≈ 500 cm²/sec at 1.6 K.     

Far-infrared laser oscillation due to cyclotron emission in p-type germanium

Far-infrared laser oscillation due to cyclotron emission in the light hole band of p-type germanium was observed under crossed electric and magnetic fields. The wavelength is inversely proportional to magnetic fields with a cyclotron mass ofm _c =0.048m _c . Numerical calculations based on Luttinger Hamiltonian show that mixing of wavefunctions between the light and heavy hole bands causes population inversion betweenn=0 andn=1 light-hole Landau levels. It is also shown that non-equidistant energy spacing of light-hole Landau levels is essential to yield net amplification.     

Microwave magneto-absorption in Bi and the NENP model

A new type of quantum oscillations of the microwave magneto-absorption in bismuth is presented. As an application a support of the non-ellipsoidal non-parabolic (NENP) Cohen model is given.     

Far-infrared absorption and near-infrared excitation spectroscopy of isoelectronic bound excitons in Si:Be+C

Intense photoluminescence from the axial isoelectronic centre formed by a pair of Be atoms occupying a Si vacancy is often accompanied by a weaker set of lines lying at higher energy. A correlation between the intensity of these lines and the concentration of C in the sample led to the suggestion that the new centre is a Be-Be centre perturbed by a nearest-neighbour C. Our measurements reveal strong similarities between the excited states of both systems, supporting this model.     

Photoluminescence of germanium near the screening ionization limit of excitons

Photoluminescence spectra from highly excited germanium at 25–30 K are presented. The density of free electrons and holes is determined from induced free carrier absorption and achieved values up to 10¹⁷ cm^-3. At such densities the electron-hole system is pumped to well above the screening ionization limit of excitons. The band-to-band luminescence. however, is strongly Coulomb enhanced. The spectral shape of the measured luminescence fits well with calculations based on a Yukawa interaction potential.     

Exciton and magneto-absorption in tellurium

Infrared magneto-absorption has been investigated in tellurium crystals in magnetic fields up to 45 koe at liquid helium temperatures. When the magnetic field was applied parallel or perpendicular to the c-axis, oscillatory absorptions with many transmission minima involving the exciton absorption were observed. The results were analysed taking into account the complicated structure of the valence band.     

Microwave magneto-absorption resonances in metals

Microwave magneto-absorption resonances are predicted in metals when, in the frame of a semiclassical theory, scatterers are considered. Experimental evidence is found in bismuth at the frequency $\text{? = 22.5}\text{GHz}$ and the temperature T = 1.3 K.     

Interaction between free carriers,free excitons,and electron-hole drops in germanium

Under steady-state conditions, we have observed an increase of photo-conductivity in Ge at low temperature simultaneously with radiation from electron-hole drops. A set of rate equations for a 3-phase system of carriers, free excitons and drops are used to interpret the results. Values of the exciton-recombination coefficient and the lifetimes of the three phases are determined.     

Theory of donor-bound multi-exciton complexes in Germanium

We report calculations of ground and excited state energies for multi- exciton complexes bound to phosphorus, arsenic, and bismuth donors in germanium. Band structure effects are included in a self consistent density functional calculation. Theoretical recombination spectra derived from our results agree very well with existing data. The results imply the existence of a kinetic bottleneck to the growth of large complexes.     

Saturation of interband magneto-absorption in InSb

The saturation of interband magneto-absorption in InSb has been observed using low power incident radiation from a c.w. CO laser. The measured absorption lineshape and saturation characteristic for a pair of nearly isolated Landau levels is compared to theory (ibid) to yield information on the coherent electron-hole dephasing time and the carrier relaxation time both of which are of fundamental importance to the electro-optic device potential of semiconductors.     

Far-infrared photoconductivity spectra of quenched-in acceptors in germanium: Chemical effects on the SA1 acceptors

Far-I.R. photoconductivity reveals the existence of at least four different kinds of SA₁ acceptors in germanium quenched from T > 800 each acceptor producing two or more hydrogenic excitation line series. One acceptor is correlated with the presence of Ni, another one with strong Cu contamination. The two other acceptors, which we call ‘normal’ and ‘shifted’, occur in samples quenched without intentional doping. The experiments indicate that the SA₁ acceptors in germanium are impurity complexes involving fast diffusing elements. We discuss a model according to which the SA₁ acceptors would be interstitial-substitutional pairs of transition metals. In particular, we propose to assign the ‘normal’ SA₁ acceptors to Fe_I-Cu_S pairs.