Photoluminescence investigations of graded,totally relaxed GexSi1-x structures

MBE grown, totally relaxed, graded Ge_xSi_1-x on Si heterostructures were studied by using photoluminescence spectroscopy at 4.2 K. The existence and linewidth of the near band gap bound exciton recombination confirms the high quality of these layers. Due to the relaxation we find dislocation related D-bands in the near infrared region of the photoluminescence spectra. The dislocation band D4 proved to be an effective tool for measuring residual strain in the heteroepitaxial layer because of its large line-shift in the presence of strain. We found tensile strain of 0.4% at the interface between the SiGe layer and the Si substrate.     

Electronic structure of GaAs1−xNx alloys

We have performed first-principles total-energy calculations based on pseudopotential and plane-wave method on ordered GaAs_1−xN_x alloys. We have found that the alloy is metallic at x≈0.25, has an indirect gap at 0.10<x<0.35 and a direct gap for the rest of the composition range within LDA. The alloy becomes metallic because the conduction band at R point is lower than the valence band maximum at Γ point. This discrepancy with previous calculations can be explained in terms of band folding and strong order dependence of the electronic structure of GaAs_1−xN_x alloys.     

Auger lifetimes in ideal InGaSb/InAs superlattices

Quantitative calculations are reported of both band-to band Auger and radiative recombination lifetimes in thin-layered type II In_xGa_1−x Sb/InAs superlattices with energy gaps in the 5–17 μm range, using accurate band structure and numerical techniques. Results for an 11 μm superlattice are compared with similar calculations for bulk HgCdTe and a HgTe/CdTe superlattice having the same energy gap. The results show the n-type Auger rates to be comparable and the p-type rates to be suppressed by three orders of magnitude in some experimentally realizable structures. Thus, well fabricated III–V superlattices appear to be excellent candidates as a new class of infrarer detectors.     

Optical spectra of wide band gap BexZn1−x Se alloys

We report optical measurements (photoluminescence, Raman scattering, and infrared reflectance) of direct band gap and of optical phonon energies of Be_xZn_1−x Se alloys grown by MBE on (001) GaAs substrates for a wide range of Be concentrations. The high band gap of BeSe (5.15 eV) suggests the possibility of using isoternary alloys for ultraviolet optoelectronic applications. Be_xZn_1−x Se has the unique advantage that it can be lattice matched to Si at about x=0.5. We observed a strong linear shift of the Be_xZn_1−x Se direct band gap to higher energies with increasing Be content (to 3.63 eV for x=0.34). Furthermore, optical phonon parameters for the entire range of BeSe content have been obtained. Finally, polarized infrared and Raman spectra revealed local atomic ordering (anti-clustering) effects in the group-II sublattice. Fiz. Tekh. Poluprovodn. 33, 1120–1122 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Photoluminescence of Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te based heterostructures grown by molecular-beam epitaxy

Photoluminescence (PL) of Hg_{1 − x} Cd _x Te-based heterostructures grown by molecular-beam epitaxy (MBE) on GaAs and Si substrates has been studied. It is shown that a pronounced disruption of the long-range order in the crystal lattice is characteristic of structures of this kind. It is demonstrated that the observed disordering is mostly due to the nonequilibrium nature of MBE and can be partly eliminated by postgrowth thermal annealing. Low-temperature spectra of epitaxial layers and structures with wide potential wells are dominated by the recombination peak of an exciton localized in density-of-states tails; the energy of this peak is substantially lower than the energy gap. In quantum-well (QW) structures at low temperatures, the main PL peak is due to carrier recombination between QW levels and the energy of the emitted photon is strictly determined by the effective (with the QW levels taken into account) energy gap.     

Direct gap recombination in germanium at high excitation level and low temperature

In highly excited germanium and at low temperatures, a luminescence is observed at energies above the indirect gap at an energy of 0.880 eV. This luminescence is generally observed in connection with a broadened electron-hole-drop emission line thus indicating an increased density in the plasma state. At the same time, absorption in that energy range is reduced drastically. This luminescence is attributed to direct recombination processes between the Γ₂ conduction band minimum and the Γ₂₅ valence band maximum. The measurements we report concern the dependence of this luminescence on excitation conditions keeping the bath temperature constant at 2K. Line-shape, line-width and energetic position indicate that free carrier band-to-band recombination and no excitonic effects are involved. The dependence of the luminescence intensity on excitation power is well explained by a coupling by Auger processes between the indirect and the direct gap in germanium.     

Hybrid functional calculations on the band gap bowing parameters of InxGa1-xN

The electronic band structures and band gap bowing parameters of In_xGa_1-xN are studied by the firstprinciples method based on the density functional theory. Calculations by employing both the Heyd-ScuseriaErnzerh of hybrid functional（HSE06） and the Perdew-Burke-Ernzerhof（PBE） one are performed. We found that the theoretical band gap bowing parameter is dependent significantly on the calculation method, especially on the exchange-correlation functional employed in the DFT calculations. The band gap of In_xGa_1-xN alloy decreases considerably when the In constituent x increases. It is the interactions of s–s and p–p orbitals between anions and cations that play significant roles in formatting the band gaps bowing. In general, the HSE06 hybrid functional could provide a good alternative to the PBE functional in calculating the band gap bowing parameters.     

Mechanisms of excitation of the f-f emission in silicon codoped with erbium and oxygen

The Er₂O₃ cluster in silicon is discussed as a possible source of the Er-related emission in Si:Er, O. We propose a mechanism that gives a simple explanation of the high efficiency of Er atom excitation in Er-O clusters. The cases of photoluminescence and electroluminescence are considered. In the case of photoluminescence the high efficiency of Er excitation is attributed to the electron state localized at the Er-O cluster. The excitation of f-shell electrons in Er atoms occurs via the Auger recombination of the exciton bound at the Er-O cluster. We calculate the rate of this Auger process and discuss the dependence of the photoluminescence intensity on the carrier concentration. In the case of electroluminescence under reverse bias the impact excitation cross section is enhanced due to resonant scattering of the hot electrons at the quasi-discrete levels formed by the Er-O cluster quantum-well potential. The calculated impact excitation cross section is close to the experimental value. Fiz. Tekh. Poluprovodn. 33, 664–670 (June 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Radiative recombination channels in Si/Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanostructures

Using the solution of the 2D Schrödinger equation, systematic features of distribution of charge carriers in the Si/Si_{1 ? x} Ge _x nanostructures and variations in the efficiency of radiative recombination when pyramidal 2D clusters are transformed into 3D dome clusters with increasing thickness of nanolayers are established. The effect of the composition of the layers on the efficiency of the elastic stress in the structure and, as a consequence, the variation in conduction bands and valence band of the Si_{1 ? x} Ge _x nanostructures is taken into account. On realization of the suggested kinetics model, which describes recombination processes in crystalline structures, saturation of radiation intensity with increasing the pump intensity caused by an increase in the contribution of the Auger recombination is observed. A decrease in the contribution of the nonradiative Auger recombination is attained by decreasing the injection rate of carriers into the clusters, and more precisely, by an increase in the cluster concentration and an increase in the rate of radiative recombination.     

Valence band anticrossing model for GaSb1−xBix and GaP1−xBix using k.p method

The reduction in band gap as well as the increase in spin–orbit splitting energy in GaSb_1−xBi_x and GaP_1−xBi_x are explained by the Valence Band Anticrossing (VBAC) model. This restructuring of the valence band is due to the interaction of the Bi related impurity levels with the extended states of the valence band of the host semiconductor. The band gap reduction in GaSb_1−xBi_x and GaP_1−xBi_x calculated using VBAC model are respectively 40.2 meV and 206 meV/at% Bi. A comparison of the theoretical and experimentally obtained values of band gap in GaSbBi shows good agreement. Valence band structure for GaPBi is obtained by the extrapolation of the parameters used for modeling of the GaSbBi system. The upward movement of the spin–orbit split-off E₊ energy level in GaSbBi by 19.2 meV/at% Bi is also responsible for the suppression of Auger recombination processes making it a potential candidate for near and mid-infrared optoelectronic applications.     

A Theoretical Search for Efficient Dopants in Mg<Subscript>2</Subscript>X (X = Si,Ge, Sn) Thermoelectric Materials

Intentional alloying and doping are well-established ways of improving the thermoelectric properties of Mg₂X (X = Si, Ge, Sn). In this study the results of electronic structure calculations for alloying and impurity dilution are presented. We have adapted the fully self-consistent Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA) to treat various types of chemical disorders and to calculate an electronic band structure with complex energy. In Mg₂Si_1−x Sn _x , as the Sn content increases, the conduction and valence bands near the Fermi energy tend to overlap but do not cross each other. In contrast, in Mg₂Si_1−x Ge _x , an energy gap was detected for 0 ≤ x ≤ 1. Moreover, the site preference of selected impurities (Al, P, Zn, Ga, Ag, Cd, In, Sb) in Mg₂Si is discussed in view of total energy calculations. It was found that In, Cd, Ag, and Zn preferentially occupy the Mg site, whereas in other cases site selectivity markedly depends on impurity amount as well as chemical potentials. The sign of the thermopower in the doped systems is analyzed from the position of the Fermi level with respect to valence/conduction band edges. The Seebeck coefficient was estimated from the simplified Mott formula at standard dopant concentrations, diluted at both crystallographic sites in Mg₂Si.     

Electronic structure of Ge1 − x − ySixSny ternary alloys for multijunction solar cells

Ternary group‐IV alloys have a wide potential for applications in infrared devices and optoelectronics. In connection with photovoltaic applications, they are among the most promising materials for inclusion in the next generation of high‐efficiency multijunction solar cells, because they can be lattice matched to substrates as GaAs and Ge, offering the possibility of a range of band gaps complementary to III–V semiconductors. Apart from the full decoupling of lattice and band structures in Ge_{1 − x − y}Si_xSn_y alloys, experimentally confirmed, they allow preparation in a controllable and large range of compositions, thus enabling to tune their band gap. Recently, optical experiments on ternary alloy‐based films, photodetectors measured the direct absorption edges and probed the compositional dependence of the direct gap. The nature of the fundamental gap of Ge_{1 − x − y}Si_xSn_y alloys is still unknown, as neither experimental data on the indirect edges nor electronic structure calculations are available, as yet. Here, we report a first calculation of the electronic structure of Ge_{1 − x − y}Si_xSn_y ternary alloys, employing a combined tight‐binding and virtual crystal approximation method, which proved to be useful to describe group‐IV semiconductor binary alloys. Our results confirm predictions and experimental indications that a 1eV band gap is indeed attainable with these ternary alloys, as required for the fourth layer plan to be added to present‐day record‐efficiency triple‐junction solar cells, to further increase their efficiency, for example, for satellite applications. When lattice matched to Ge, we find that Ge_{1 − x − y}Si_xSn_y ternary alloys have an indirect gap with a compositional dependence reflecting the presence of two competing minima in the conduction band. Copyright © 2013 John Wiley & Sons, Ltd.     

Light‐Emission Performance of Silicon Nanocrystals Deduced from Single Quantum Dot Spectroscopy

Spectra of individual silicon nanocrystals within porous Si grains are studied by the wide‐field imaging microspectroscopy and their ON – OFF blinking is detected by the confocal single‐photon‐counting microscopy. Observed spectral and blinking properties comprise all features reported before in differently prepared single Si nanocrystals (SiNCs). Former apparently contradictory results are shown to be due to different experimental conditions. When the effect of dark periods (OFF switching) is removed the common ultimate photoluminescence properties of SiO₂ passivated SiNCs are found, namely the quantum efficiency (QE) of about 10–20% up to the pumping rate corresponding to one exciton average excitation per quantum dot. At higher pump rates the QE is slowly decreasing as the 0.7th power of excitation. This is most likely due to Auger recombination which, however, seems to be weakened compared with measurements of nanocrystal assemblies. We conclude that SiNCs may be pumped above one exciton occupancy to yield a higher light emission, being advantageous for applications.     

Modeling of Recombination in HgCdTe

Minority carrier recombination lifetime calculations for narrow-gap semiconductors are of direct practical interest in establishing whether a material’s recombination is extrinsically or intrinsically limited, and therefore in guiding research and development programs regarding material quality improvements. We describe efforts to obtain accurate electronic band structures of HgCdTe alloy-based materials with infrared energy gaps and employ them to evaluate Auger recombination lifetimes. We use a 14-band k · p formalism to compute and optimize electronic band structures, and use the obtained electronic energies and matrix elements directly in the numerical evaluation of Auger and radiative lifetimes.     

Special features of the excitation spectra and kinetics of photoluminescence of the Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:Er/Si structures with relaxed heterolayers

Luminescent properties of heteroepitaxial Si_{1 − x} Ge _x :Er/Si structures with relaxed heterolayers are studied. The results of combined studies of the excitation spectra and kinetics of photoluminescence (PL) are used to single out the components providing the largest contribution to the PL signal of the Si_{1 − x} Ge _x :Er/Si structures in the wavelength region of 1.54 μm. It is shown that relaxation of elastic stresses in the Si_{1 − x} Ge _x :Er heterolayer affects only slightly the kinetic characteristics of erbium luminescence and manifests itself in insignificant contribution of the defects and defect-impurity complexes to the luminescent response of the Si_{1 − x} Ge _x :Er/Si structures. In the excitation spectra of the erbium PL, special features related to the possibility of the rare-earth impurity excitation at energies lower than the band gap of the Si_{1 − x} Ge _x solid solution are revealed. It is shown that a peak the width of which depends on the band gap of the solid solution and the extent of its relaxation is observed in the excitation spectra of the erbium-related PL in the Si_{1 − x} Ge _x :Er/Si structures in the wavelength region of 1040–1050 nm. The observed specific features are accounted for by involvement of intermediate levels in the band gap of the Si_{1 − x} Ge _x :Er solid solution in the process of excitation of an Er³⁺ ion.     

Charge transport in HgCdTe-based n +-p photodiodes

It is shown that dark currents measured at 77 K in Hg_1−x Cd_xTe (x⋍0.21) homojunctions can be adequately described by the balance equations with allowance made for the two main charge-transport mechanisms, i.e., tunneling assisted by traps in the band gap and recombination via these traps; the above homojunction may find application in microphotoelectronics in the infrared spectral range of 8–12 μm. Other charge-transport mechanisms are included in the consideration as additive terms. A comparison between the experimental current-voltage characteristics and dynamic resistance of HgCdTe diodes with the results of calculations was carried out. A good agreement was obtained between experimental data and the results of calculations, in which the donor and acceptor concentrations in the n and p regions of diodes, the concentration of traps and the position of their levels in the band gap, and the lifetimes of charge carriers for recombination via these traps were used as adjustable parameters. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 7, 2001, pp. 835–840. Original Russian Text Copyright ? 2001 by Gumenjuk-Sichevskaja, Sizov, Ovsyuk, Vasil’ev, Esaev.     

Excitonic photoluminescence of silicon quantum-well structures

The exciton binding energy and the energies of radiative excitonic transitions in the separate SiO_x-Si-SiO_x quantum wells are calculated in the effective-mass approximation with the quadratic dispersion relation. Along with the real finite offsets of the bands in such quantum well structures, the effect of dielectric enhancement of the exciton binding energy due polarization of the heterointerfaces is taken into account. In addition, the dependence of the zero-phonon radiative excitonic recombination time on the width of the SiO_x-Si-SiO_x quantum well is calculated. This dependence exhibits unsteady (oscillating) behavior, which is caused by the indirect band gap of the silicon material. It is shown that the theoretically calculated energies of the radiative excitonic transitions in the SiO₂-Si-SiO₂ quantum wells match the experimentally determined energies for the quantum wells whose widths are larger than 1.5 nm. Good agreement between the theoretically calculated and experimental spectral dependences of photoluminescence in the SiO₂-Si-SiO₂ quantum wells is attained.     

Structural and luminescent properties of ZnO:P films produced by annealing ZnP<Subscript>2</Subscript> wafers in atomic oxygen

The epitaxial ZnO:P films are fabricated by annealing the ZnP₂ wafers in atomic oxygen (oxygen radicals). The properties of the films are studied by X-ray diffraction analysis, atomic force microscopy, Auger spectroscopy, and photoluminescence measurements. In the X-ray diffraction spectra of the samples, the (002) peak is observed, suggesting that the ZnO:P films are oriented along the c axis. The Auger spectroscopy data show that the ZnO films contain phosphorus. The low-temperature photoluminescence spectrum observed for the ZnO: P films exhibits a 3.356 eV peak presumably corresponding to excitons bound at neutral acceptors and a peak at 3.306 eV (free electron)-acceptor transitions associated with the P_O level. The phosphorus related acceptor level is localized at 128 meV above the top of the valence band. The origin of the 3.312 eV band related to recombination at donor-acceptor pairs is discussed.     

Recombination in small-gap Pb1−xSnxTe

Instrinsic and extrinsic recombination mechanisms in small-gap Pb_1−xSn_xTe are investigated by means of stationary and non-stationary laser-excited photoeffects, carried out under conditions with weak as well as strong deviation from equilibrium. A straightforward analysis of radiative recombination within the Kane type two-band model is given and supported by absorption measurements. This analysis is combined with a treatment of Auger recombination based on the parameters determined from the photoeffects. The conclusion is drawn that at high carrier concentrations (equilibrium or non-equilibrium) strong competition between radiative and Auger recombination takes place. The consequences for the threshold current of double hetero-junction lasers are outlined.