Acceptor Ground State Research Articles

Ab Initio Study of Donor and Acceptor Ground States in the P–B Codoped Si Nanostructures

<i>Ab Initio</i> Study of Donor and Acceptor Ground States in the P–B Codoped Si Nanostructures

Physical, optical and luminescence properties of B2O3-SiO2-Y2O3-CaO glasses with Sm3+ions for visible laser applications

Silicoborate glasses with composition of (BSYCaSm:(55-x)B2O3+ 10SiO2 +25Y2O3 +10CaO + xSm2O3), (where, x= 0.05, 0.10, 0.20, 0.25, 0.30, 0.40, 0.50, mol%) have been synthesized by using the well-known melt quenching technique. The present work deals with physical, optical, photoluminescence, X-ray luminescence and decay time studies of silicoborate glasses. Judd-Ofelt (JO) intensity parameters (Ω2, Ω4 and Ω6) and radiative properties for the important luminescent level of Sm3+ ions were derived by using the absorption spectrum of 0.3mol% Sm2O3 doped glass. The luminescence spectra in the visible region was obtained due to 4G5/2 → 6HJ (5/2, 7/2, 9/2 and 11/2) transition of Sm3+ ion under 401nm excitation. The decay profile for the 4G5/2 level of Sm3+ ions was analyzed and found that for lower concentration (≤ 0.20mol%) it is single exponential in nature whereas for higher concentration (≥ 0.25mol%), it turns in to non-exponential due to the transfer of energy between donor (excited state Sm3+ion) and acceptor (ground state Sm3+ ion). As the concentration of Sm3+ ions increases the decay time of 4G5/2 state decreases. The well-known Inokuti-Hirayama (IH) model was used for fitting the non-exponential decay curves where S = 6 indicates that energy transfer process is of dipole-dipole type. Hence, in the present work, intense transition of 4G5/2 → 6H7/2 (601nm) is found to be suitable for reddish-orange laser emission. CIE chromaticity diagram has been performed for verifying the results of fluorescence in visible laser applications at 601nm.

Optical orientation of electrons in compensated semiconductors

The theory of the optical orientation of charge carriers in compensated III-V semiconductors and quantum wells for the case where electrons are excited to the conduction band from Mn-charged acceptor states is presented. It is shown that, in GaAs/AlGaAs quantum wells, the degree of the spin orientation of conduction-band electrons in this excitation scheme can be as high as 85%. This spin-orientation enhancement results from an increase in the heavy-hole contribution to the acceptor state in the vicinity of the defect center rather than from level splitting caused by quantum confinement. It is shown that the degree of circular polarization of the photoluminescence emitted upon the recombination of electrons thermalized at the bottom of the band with holes occupying the acceptor ground state in a quantum well can exceed 70%.

Dy3+-doped zinc fluorophosphate glasses for white luminescence applications

Dysprosium (Dy3+) ions doped zinc fluorophosphate (PKAZLFDy: P2O5–K2O–Al2O3–ZnF2–LiF–Dy2O3) glasses have been prepared and investigated their spectroscopic properties using absorption, emission and decay measurements. Judd–Ofelt analysis has been carried out to obtain the intensity parameters and in turn predicted radiative properties for the 4F9/2 level of 1.0mol% of Dy2O3 doped glass. Visible luminescence spectra have been obtained due to 4F9/2→6HJ (J=11/2, 13/2, 15/2) transitions of Dy3+ ions under 385nm excitation. The yellow-to-blue luminescence intensity ratios and chromaticity coordinates of Dy3+ ions in these glasses have been analyzed as a function of Dy3+ ion concentration. The decay profiles for the 4F9/2 level exhibit perfectly single exponential at lower concentrations (up to 1.0mol%) and turn into non-exponential for higher concentrations (>1.0mol%) due to energy transfer between donor (excited state Dy3+ ion) and acceptor (ground state Dy3+ ion). The results reveal that these glasses emit bright white light which is suitable for the development of W-LEDs.

Isotope effect on electron paramagnetic resonance of boron acceptors in silicon

The fourfold degeneracy of the boron acceptor ground state in silicon, which is easily lifted by any symmetry breaking perturbation, allows for a strong inhomogeneous broadening of the boron-related electron paramagnetic resonance (EPR) lines, e.g. by a random distribution of local strains. However, since EPR of boron acceptors in externally unstrained silicon was reported for the first time, neither the line shape nor the magnitude of the residual broadening observed in samples with high crystalline purity were compatible with the low concentrations of carbon and oxygen point defects, being the predominant source of random local strain. Adapting a theoretical model which has been applied to understand the acceptor ground state splitting in the absence of a magnetic field as an effect due to the presence of different silicon isotopes, we show that local fluctuations of the valence band edge due to different isotopic configurations in the vicinity of the boron acceptors can quantitatively account for all inhomogeneous broadening effects in high purity Si with a natural isotope composition. Our calculations show that such an isotopic perturbation also leads to a shift in the g-value of different boron-related resonances, which we could verify in our experiments. Further, our results provide an independent test and verification of the valence band offsets between the different Si isotopes determined in previous works.

Fano resonances in the impurity photocurrent spectra of GaAs samples and an InGaAs/GaAsP quantum-well heterostructure doped with shallow acceptors

The photocurrent spectra of p-GaAs samples and a p-InGaAs/GaAsP quantum-well heterostructure doped with shallow acceptors (C, Be, Zn) are investigated. Fano resonances associated with the ground and excited acceptor states are revealed and analyzed. It is found that sharp change in dielectric constant in the GaAs reststrahlen region strongly affects the lineshape of the Fano resonance associated with the acceptor ground state.

Impurity photoconductivity in strained p-InGaAs/GaAsP heterostructures

The impurity-photoconductivity spectrum is observed for strained quantum wells of the p-InGaAs/GaAs solid solution at T= 4.2 K. In addition to the broad photoconductivity band attributed to the transitions from the acceptor ground state to the continuum of the first size-quantization subband, the spectrum exhibits a peak due to the transitions from the ground state to the excited localized acceptor state, a band corresponding to the transitions to the resonance states associated with the second heavy-hole size-quantization subband, and a narrow photoconductivity peak (Fano resonance) in the spectral range corresponding to the optical-phonon energy.

Spectroscopy of Be Acceptor Ground State in GaAs/AlGaAs Heterostructure

Photoluminescence measurements were carried out on Be δ-doped GaAs/Al0.33Ga0.67As heterostructure at 1.6 K in magnetic fields (B) up to 4 T. Luminescence originating from recombination of a two-dimensional electron gas and photoexcited holes localized on Be acceptors was analyzed. The degree of circular polarization (γC) of the luminescence from fully occupied Landau levels was determined as a function of B and the two-dimensional electron gas concentration, ns. At B constant, γC decreased with the increase in ns. The intensity of the optical transition considered was calculated with taking into account sand d-like parts of the acceptor envelope function. It is shown that the presence of the d-like part explains the observed γC(ns) dependence quantitatively. This shows that polarization spectroscopy on acceptor δ-doped heterostructures enables one to test experimentally the contribution of the L > 0 component of the envelope in a shallow acceptor description.

Optical absorption in GaAs quantum wells caused by donor–acceptor pair transitions

Optical absorption related to donor–acceptor pair transitions in GaAs quantum wells wastheoretically investigated. The donor and acceptor ground state wavefunctions and energieswere obtained using variational method in the frames of the effective mass approximation.The absorption coefficient related to acceptor-to-donor transition was calculated. Thespreading of the absorption curve as a result of averaging over impurity distances was takeninto account. The character of the dependence of the absorption coefficient on bothquantum well thickness and impurity concentration was determined. The blueshift in theabsorption spectra was theoretically investigated. Comparison with available experimentaldata was made.

Spectroscopy of excitons and shallow impurities in isotopically enriched silicon—electronic properties beyond the virtual crystal approximation

Recent high resolution spectroscopic studies of excitonic and impurity transitions in high-quality samples of isotopically enriched Si have dramatically expanded our understanding of the effects of isotopic composition on the electronic properties of semiconductors. Prior to these studies on Si, the results for other semiconductors, focusing mainly on the isotopic dependence of the electronic band gap energy E G in the T→0 limit, could all be explained within the virtual crystal approximation (VCA), in which only the average isotopic mass was relevant. Remarkably, not only were the effects of isotopic randomness observable in natural Si (when compared to enriched 28Si), but the random isotopic distribution present in natural Si was found to be the true source of what had been thought of as ‘fundamental’ spectroscopic limits in Si, including the linewidths of bound exciton emission lines and impurity absorption transitions, and the ‘intrinsic’ acceptor ground state splitting. Many of these transitions are far narrower in highly enriched 28Si than in the most perfect natural Si, challenging existing spectroscopic methods, and opening up new possibilities for precision measurements, and for the observation of new physics.

Shallow Acceptor Levels in Ge∕GeSi Heterostructures with Quantum Wells in a Magnetic Field

Shallow acceptors in Ge/GeSi heterostructures with quantum wells are studied theoretically and experimentally in the presence of a magnetic field. It is shown that, in addition to the cyclotron resonance lines, magnetoabsorption spectra reveal transitions from the acceptor ground state to excited states related to Landau levels from the first and second confinement subbands, as well as the resonances caused by ionization of A+ centers.

Magnetic field dependence of the acoustic two‐pulse echo envelope modulation in Si:B at very low temperatures

At temperatures below about 30 mK in Si:B the phase relaxation time T2 of the two-level systems associated with the acceptor ground state can be determined from the decay of the two-pulse echo. It has been shown to depend on the concentration of interacting resonant states if the latter is high enough. This concentration is not only given by the boron concentration but depends also on internal and external stress, and the magnetic field. Under conditions of low concentration T2 becomes long and we observe a modulation of the echo decay. From its dependence on magnetic field strength we can associate this modulation with the interaction of the acceptor hole with the 29Si nuclei (I = 1/2) present within the extended acceptor wave function. The interaction with the central boron nucleus does not seem to contribute to the observed modulation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge manipulation and imaging of the Mn acceptor state in GaAs by cross-sectional scanning tunneling microscopy

An individual Mn acceptor in GaAs is mapped by cross-sectional scanning tunneling microscopy (X-STM) at room temperature and a strongly anisotropic shape of the acceptor state is observed. An acceptor state manifests itself as a cross-like feature which we attribute to a valence hole weakly bound to the Mn ion forming the (Mn 2+3d 5+hole) complex. We propose that the observed anisotropy of the Mn acceptor wavefunction is due to the d-wave present in the acceptor ground state.

On the impurity photoconductivity of uniaxially stressed p‐Ge

Far infrared photoconductivity of stressed p-Ge in the high-frequency spectral range corresponding to transitions from the acceptor ground state to the split-off subband has been observed for the first time. Calculations of photoconductivity spectra of stressed Ge taking into account the resonant states have been made. The calculation results are in good agreement with the experimental data. It has been shown that the transitions into the resonant states are much weaker compared with those into the continuum of the upper subband, and therefore the corresponding spectral lines are difficult to observe in the photoconductivity spectra.

Origin of the residual acceptor ground-state splitting in silicon.

The residual ground-state splitting of acceptors in high-quality silicon has been studied intensely by different experimental techniques for several decades. Recently, photoluminescence studies of isotopically pure silicon revealed the ground-state splitting to result from the random distribution of isotopes in natural silicon. Here we present a new model that explains these surprising experimental results, and discuss the implications for acceptor ground-state splittings observed in other isotopically mixed semiconductors, as well as for the acceptor ground state in semiconductor alloys.

Photoluminescence studies of isotopically enriched silicon

AbstractWe report the first high resolution photoluminescence studies of isotopically pure silicon. New information is obtained on isotopic effects on the indirect band gap energy, phonon energies, and phonon broadenings, which is in good agreement with previous results obtained in germanium and diamond. Remarkably, the line widths of the no‐phonon boron and phosphorus bound exciton transitions in the 28Si sample (99.896% 28Si) are much sharper than in natural Si, revealing new fine structure in the boron bound exciton luminescence. Most surprisingly, the small splittings of the neutral acceptor ground state in natural Si are absent in the photoluminescence spectra of acceptor bound excitons in isotopically purified 28Si, demonstrating conclusively that they result from the randomness of the Si isotopic composition.

Selective magneto‐luminescence spectroscopy of donoacceptor pairs in n‐GaAs

AbstractThe results of selective magneto‐luminescence measurements of donoacceptor pairs in n‐type GaAs are discussed. The experimental data are compared with the calculated energy levels of a hydrogen‐like atom in a magnetic field, enabling us to identify a large number of high angular momentum states of the hydrogen‐like impurity in magnetic fields. It is shown that in addition to intra‐donor transitions, the spin‐flip excitations of donor–acceptor pairs involving heavy and light holes can be observed, yielding information about g‐factors for heavy and light holes. For the pairs with separations smaller than the donor Bohr radius, a strong modification of the SPL spectrum is observed. This effect is discussed in terms of modification of the acceptor ground state induced by the interaction with a neighbouring donor centre.

"Intrinsic" acceptor ground state splitting in silicon: an isotopic effect.

One of the oldest open questions in semiconductor physics is the origin of the small splittings of the neutral acceptor ground state in silicon which lead to a distribution of doublet splittings rather than the fourfold-degenerate state of Gamma(8) symmetry expected in the absence of perturbations. Here we show that these acceptor ground state splittings are absent in the photoluminescence spectra of acceptor bound excitons in isotopically purified 28Si, demonstrating conclusively the surprising result that the splittings previously observed in natural Si result from the randomness of the Si isotopic composition.

Acoustic solitons, phonon echoes, and sound amplification in Si:B at very low temperatures

Phonon echoes, sound amplification, and acoustic solitons are observed in Si:B at temperatures below about 25 mK depending on the density of states of two-level systems associated with the acceptor ground state. We report on the influence of acceptor concentration, external stress and magnetic field and we compare our results with those associated with the two-level systems in glasses.

Localised and resonant states of shallow acceptors in Ge/Ge 1− xSi x multiple-quantum well heterostructures

A new theoretical approach allowing to calculate both the localised and the continuum states of charge carriers in QW heterostructures in the presence of the Coulomb potential has been developed. The method allows to satisfactorily describe the observed photoconductivity spectra of Ge/GeSi heterostructures. The optical transitions from the acceptor ground states to the resonant states have been revealed in the measured far IR photoconductivity spectra.