Exciton binding energies in II–VI compound strained layer superlattices

II–VI strained-layer superlattices are very efficient emitters of visible light. The dependence of the luminescence intensity on the excitation power density allows us to characterise the recombination processes involved in the emission. At low temperatures excitonic processes are dominant whereas electron-hole recombinations feature at room temperature. No special evidence of the dual nature of the emission is observed at intermediate temperatures because the optical transitions are broadened by well-width fluctuations. In spite of this we may estimate the exciton binding energy from the temperature dependence of the photoluminescence intensity, as long as the photoluminescence remains excitonic. This is the case for narrow wells in CdS---ZnS superlattices over the temperature range zero to room temperature. The estimated exciton binding energy measured in this way approaches the two dimensional limit but does not exceed it.     

Electron–hole symmetry and spin–orbit splitting in IV–VI asymmetric quantum wells

It is shown that, due to the electron–hole symmetry of the fundamental gap of the lead–salts (PbTe, PbSe and PbS), the Rashba spin splitting in their flat band asymmetric quantum wells is much reduced with the usual equal conduction and valence band-offsets. Different from the III–V case, we find that the important structure inversion asymmetry for the Rashba splitting in IV–VI quantum wells with different left and right barriers is not a material property (i.e., barrier height, effective mass or band gap) but results from the band alignment. This is shown by specific envelope function calculations of the spin-dependent subband structure of Pb_1−xEu_xTe/PbTe/Pb_1−yEu_yTe asymmetric quantum wells (x≠y), based on a simple but accurate four-band kp model for the bulk band structure near the gap, which takes into account band anisotropy, nonparabolicity and multi-valley effects.     

Photoreflectance of GaAs doping superlattices

We have performed room-temperature photoreflectance measurements on two GaAs doping superlattices having considerably different built-in potentials (1.2 eV and 85 meV). The first sample exhibits Franz-Keldysh oscillations, the period of the oscillations corresponding to the . A second dc pump beam has been used to change the electron-hole concentration and hence the built-in field. The spectrum of the second sample displays a number of features corresponding to quantized electron and hole states. There is qualitative agreement between experiment and theoretical calculation based on a two-band tight-binding model. In both samples the dependence of the amplitude of the photoreflectance signal on pump chopping frequency yields the minority carrier lifetime.     

Photoluminescence spectra of doping superlattices

Pulsed and cw photoluminescence measurements have been performed on a variety of GaAs doping superlattices with unequal n- and p- doping levels. At 6 K, samples with a p- to n-type dopant ratio greater than 2 and potential well depths between 150 and 500 meV exhibit a two-peak spectrum over a range of excitation intensities sufficient to generate enough carriers to partially compensate the internal electric potential. The peak separation is dependent on the superlattice parameters and scales with the depth of the potential wells. Measured separations as large as 150 meV are inconsistent with an explanation based on quantized valence or conduction minibands. As the sample temperature is increased to 50 K, the higher energy emission decreases with respect to the low-energy peak, indicating that this emission results from recombination involving a metastable state that can be readily depopulated by phonon interaction. A model is proposed which ascribes the high-energy peak to recombination to acceptor states near the boundary of the p-doped layers.     

Optical investigation of PbTe doping superlattices

We report on a study of the optical properties of HWE-grown PbTe doping superlattices using infrared photoluminescence and -cathodoluminescence. Below-bulk-bandgap luminescence has been observed and is interpreted in the framework of a model giving the excitation dependence of the superlattice potential. Cathodoluminescence is used to investigate the properties of nipi layers.     

Photoreflectance spectroscopy of GaAs doping superlattices

Room temperature photoreflectance of molecular beam epitaxy GaAs doping superlattices was measured. Additional structures corresponding to forbidden transitions (Δn≠0) were observed at very low pump beam intensity. Experimental results show that the dominant modulation mechanism of photoreflectance of doping superlattices is different from that of bulk materials. We suggest that the photoreflectance spectrum of doping superlattices have mainly first derivative functional lineshapes, which is caused by the subband shift in doping superlattices. The experiments are well explained by this mechanism.     

Optical absorption in silicon doping superlattices

The optical absorption beyond the fundamental edge in moderate to large period silicon doping superlattices of various designs is studied theoretically. At 1.3 μm, the largest absorption coefficient found is 0.2 cm⁻¹. Only moderate external biases are required to reach maximum absorption.     

Temperature-dependent luminescence of GaAs doping superlattices

The photon energy of the luminescence between spatial separated n- and p-type doped layers of GaAs doping superlattices shifts with excitation density. At room temperature we observe a shift of the luminescence maximum in samples with high doping concentrations (n=4×10¹⁸cm⁻³), whereas for n=1×10¹⁸cm⁻³ this shift is observed at temperatures below 150K only. Temperature-dependent measurements between 4 and 700K show that the tunability disappears near a critical temperature, which is proportional to the doping concentration. A simple model including thermally activated direct transitions and tunable luminescence describes this temperature dependence.     

Magnetooptical properties of PbTe doping superlattices

Magnetotransmission experiments are performed with PbTe doping superlattices using far infrared laser lines. The data are interpreted with a model dielectric function. The quasi parabolic band gap variation causes a lifetime enhancement of non-equilibrium carriers generated by 300 K background radiation. Both electrons and holes contribute to the observed cyclotron resonances. Dedicated to Professor Karlheinz Seeger on the occasion of his 60th birthday     

Structure and thermoelectric properties of nanocomposite bismuth telluride prepared by melt spinning or by partially alloying with IV–VI compounds

Bismuth telluride samples are compared with respect to the evolution of their thermoelectric material parameters like thermal and electrical conductivity. The Seebeck coefficient is discussed in dependence on the melt spinning fabrication technique. The melt spinner used is only able to produce small thin ribbon shaped specimens, some as thin as 10 μm. This limits melt spinning to mainly production of research specimens for alloys with high critical cooling rate, which are difficult to fabricate with other techniques. Additional parameters are alloying or doping of the base material by comparing the properties as prepared to different annealing conditions. The intrinsic p‐ and n‐doped material was alloyed with up to 0.5% lead telluride by rapidly cooling the bulk material to improve the thermoelectric properties analysed from RT up to about 600 K. A Seebeck coefficient of well above 200 µV/K could be obtained for p‐ and n‐type materials. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tunable absorption and electroluminescence in GaAs doping superlattices

Highly tunable electroluminescence is observed in GaAs doping superlattice (n-i-p-i crystal) at room temperature with peak energies shifted more than 600 meV below the bulk bandgap (λ > 1.55 μm). Peak efficiency is about 2 %. Tunability of the optical absorption spectrum with p-n junction bias is also demonstrated by both photoconductivity and direct transmission measurements. A change of transmission of about 9% is obtained at 0.89 μm wavelength through a 1.95 μm thick n-i-p-i crystal by varying the p-n junction bias between −0.5 V and 0.5 V.     

Anomalous mobility enhancement in Si doping superlattices

Enhanced Hall mobility has been measured in Si doping superlattices with pipi or nini doping profiles grown by molecular beam epitaxy. Although the carrier concentration is large, in the range of 10¹⁸cm⁻³, the mobility of holes in the pipi doping superlattice has the temperature dependence such as that expressed in the T^−2.2 law which is characteristic of high purity bulk crystals. The hole mobility is about twice that of high purity crystals, rising up to 40,000 cm²/V.s at about 30 K. In the case that p-type impurity concentration is comparable to n-type impurity concentration, no mobility enhancement is observed and the mobility is anomalously depressed. The mechanisms for drastical change in the mobility of doping superlattices have not yet been clarified.     

Delta doping of ferromagnetism in antiferromagnetic manganite superlattices

We demonstrate that delta doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AFM) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Theoretical consideration of these additional carriers shows that they cause a local enhancement of ferromagnetic double exchange with respect to AFM superexchange, resulting in local canting of the AFM spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy, and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AFM state as was predicted by de Gennes [Phys. Rev. 118, 141 (1960)] but had remained elusive.     

Hydrogenic donors in doping superlattices: A model calculation

The binding energy and wavefunction of a hydrogenic donor in an effective potential ν(z) = ∣e∣?∣z∣ in a semicondusctor is calculated variationally as a function of the effective electric field ?. Such a potential represents the limit of a doping superlattice in which the donor doping layer is very thin, and it allows the properties of shallow donor states for which the effective potential varies substantially across the wavefunction to be discussed. It is found that substantial variations of the binding energy and wavefunction result for fields which are readily realizable experimentally.     

Phonons and periodons in IV–VI semiconductor superlattices

We have calculated the phonon and periodon dispersion relations in IV–VI semi-conducting bulk PbTe and SnTe and their superlattice structure. The model used here is a one-dimensional lattice which includes harmonic interactions up to second neighbours as well as on-site nonlinear electron-ion interactions at the anion site. We calculate the phonon and periodon dispersion relations in bulk and PbTe-SnTe superlattice for the transverse optic and acoustic modes using the transfer matrix method. Our analysis has predicted correct nature of the folding of acoustic and confinement of optical phonons at various frequency intervals corresponding to pass and stop bands of the superlattices.     

Exchange-correlation energy in the subbands of silicon doping superlattices

The energy subbands in three types (pn⁺p, pnpn, nipi) of Si and GaAs doping superlattices are calculated self-consistently including the exchange-correlation energy given by the density functional method. The results show that the exchange-correlation term is more important in Si than in GaAs in all three cases. For the same doping levels, layer thicknesses and electron concentrations, the shift in the lowest subband energy from the value given by the Hartree Approximation is 20–50% greater in Si than in GaAs.