Excitons In Silicon Research Articles

Stimulated Terahertz Emission in a System of Excitons in Photoexcited Silicon

Stimulated terahertz emission owing to population inversion between exciton states in silicon crystals at intense interband photoexcitation is detected. The terahertz gain spectrum exhibits 13.7- and 15.5-meV lines, the gains of which reach 0.5 and 1 cm-1, respectively. The 13.7-meV line is due to the population inversion between highly excited states and the ground state of free excitons. The 15.5-meV line can be associated with the population inversion between the two-exciton and biexciton states. The terahertz gain values make it possible to expect the possibility of creating a new type of a terahertz laser on transitions between free excitons in silicon under interband photoexcitation.

Electron nuclear double resonance with donor-bound excitons in silicon

We present Auger-electron-detected magnetic resonance (AEDMR) experiments on phosphorus donors in silicon, where the selective optical generation of donor-bound excitons is used for the electrical detection of the electron spin state. Because of the long dephasing times of the electron spins in isotopically purified $^{28}$Si, weak microwave fields are sufficient, which allow to realize broadband AEDMR in a commercial ESR resonator. Implementing Auger-electron-detected ENDOR, we further demonstrate the optically-assisted control of the nuclear spin under conditions where the hyperfine splitting is not resolved in the optical spectrum. Compared to previous studies, this significantly relaxes the requirements on the sample and the experimental setup, e.g. with respect to strain, isotopic purity and temperature. We show AEDMR of phosphorus donors in silicon with natural isotope composition, and discuss the feasibility of ENDOR measurements also in this system.

Hybrid optical-electrical detection of donor electron spins with bound excitons in silicon.

Electrical detection of spins is an essential tool for understanding the dynamics of spins, with applications ranging from optoelectronics and spintronics, to quantum information processing. For electron spins bound to donors in silicon, bulk electrically detected magnetic resonance has relied on coupling to spin readout partnerssuchas paramagnetic defects or conduction electrons, which fundamentally limits spin coherence times. Here we demonstrate electrical detection of donor electron spin resonance in an ensemble by transport through a silicon device, using optically driven donor-bound exciton transitions. We measure electron spin Rabi oscillations, and obtain long electron spin coherence times, limited only by the donor concentration. We also experimentally address critical issues such as non-resonant excitation, strain, and electric fields, laying the foundations for realizing a single-spin readout method with relaxed magnetic field and temperature requirements compared with spin-dependent tunnelling, enabling donor-based technologies such as quantum sensing.

Excitonic Contribution of Nanosilicon on Light Emitting Properties

A phenomenological model is developed by integrating the effect of excitonic energy states, localized surface states and quantum confinement (QC) to obtain an analytical expression for the room temperature photoluminescence (PL) intensity. We calculate the binding energy of strongly confined excitons in silicon (Si) quantum dots (QD) having sizes 1 to 7.75 nm to examine its contribution on optical band gap and electronic properties. The band gap with excitonic contribution is found to decrease as much as 0.23 eV for the smallest dot. The effect of exciton states explains almost accurately the experimental PL data. Our model provides the mechanism for controlling the PL intensity through fitting parameters. Huge excitonic effects, which depend strongly on QD size and shape, characterize the optical spectra. The results for the size dependence of the optical band gap, the PL intensity, and oscillator strength are presented the role excitonic effects on optical and electronic properties are discussed.

Radiative recombination in phosphorus-doped CVD diamond

Recently published data have shown that state-of-the-art phosphorus-doped homoepitaxial CVD diamond layers exhibit 'perfect' bound exciton cathodoluminescence (CL) spectra with a triplet of narrow nophonon (NP) transitions accompanied by four well resolved wavevector-conserving phonon replicas. Here, we complement these experimental data demonstrating in particular that this superior quality of the CL spectra is highly reproducible for doping concentration from below 10 17 cm -3 to 10 19 cm -3 . In a comparison with shallow bound excitons in silicon we address the radiative quantum efficiency of the exciton luminescence and find that P-bound exciton emission in diamond is less efficient than that from B-bound excitons. Also discussed are the different intensity ratios of NP-to-TO-phonon transitions for P- and B-bound excitons. Finally the rich finestructure in the exciton spectra is considered and compared to similar finestructure appearing in other near-bandedge bound exciton luminescence spectra in diamond.

On the ultimate quantum efficiency of band-edge electroluminescence in silicon barrier structures

The ultimate quantum efficiency of electroluminescence in silicon diodes and p-i-n structures at room temperature is calculated. It is shown that the internal quantum yield of electroluminescence is about 10% and is implemented at optimal doping levels for the n-and p-type regions of silicon diodes, ∼1015 and 5×1016 cm−3, respectively. With a decrease in the Shockley-Read-Hall lifetimes of electrons and holes, the internal quantum yield of electroluminescence in silicon barrier structures drops. The physical processes related to the effect of excitons in silicon has much in common with those in electroluminescence, photoluminescence, and photoconversion. It is shown that only electroluminescent p-i-n structures are promising for use in silicon integrated circuits.

Two - Color Mid-Infrared Spectroscopy of Isoelectronic Centers in Silicon

AbstractOne of the open questions in semiconductor physics is the origin of the small splittings of the excited states of bound excitons in silicon. A free electron laser as a tunable source of the mid-infrared radiation (MIR) can be used to investigate such splittings of the excited states of optical centers created by transition metal dopants in silicon. In the current study, the photoluminescence from silver and copper doped silicon is investigated by two color spectroscopy in the visible and the MIR. It is shown the PL due recombination of exciton bound to Ag and Cu is quenched upon application of the MIR beam. The time-resolved photoluminescence measurements and the quenching effects of these bands are presented. By scanning the wavelength of the free-electron laser ionization spectra of relevant traps involved in photoluminescence are obtained. The formation and dissociation of the bound excitons, and the small splittings of the effective-mass excited states are discussed. The applied experimental method allows correlation of DLTS data on trapping centers to specific channels of radiative recombination. It can be applied for spectroscopic analysis in materials science of semicondutors.

Dielectric Confinement Effects on the Impurity and Exciton Binding Energies of Silicon Dots Covered with a Silicon Dioxide Layer

The binding energies of hydrogenic impurities and excitons in silicon (Si) dots modelled by a sphere covered with a silicon dioxide (SiO2) layer and embedded in various dielectric media, are calculated as a function of the sphere size and the thickness of the oxide. The strong-confinement limit was considered, so that all of the electrostatic interactions are treated as first-order perturbations considering the confined free-electron state as the unperturbed state. A recently proposed size correction for the dielectric constant of the Si dot is also taken into account. These calculations demonstrate the importance of the image-charge effect due to dielectric discontinuity. In addition, it is shown that the binding energies of the impurities and the excitons inside the silicon dots are greatly affected by alteration of the thickness of the oxides or the surrounding dielectric media.

Core excitons in silicon and silicon oxides

Using a newly developed reflectometer we have measured the reflectivity at the silicon K edge for different silicon compounds. Combining these measurements with silicon L edge reflectivity measurements carried out on a different reflectometer we determined the binding energy of core excitons at the L edge. The results show that it is possible to carry out reflectivity measurements with the resolution necessary for the determination of the exciton parameters. Comparison with literature values shows that a rigid Kramers–Kronig analysis is not needed when the angle of incidence is well below the critical angle of total external reflection.

Nonequilibrium carrier and exciton kinetics in silicon and germanium crystals

Asymptotic small-parameter expansion is applied to the solutions for singularly perturbed systems in order to examine the kinetics of laser excitation for nonequilibrium carriers and excitons in silicon and germanium crystals, which incorporates nonequilibrium carrier recombination, linear recombination, and exciton formation and ionization. Asymptotic evaluations can be used that apply throughout the range in pulse length in order to provide indirect and rough estimates of the probabilities for various recombination and ionization processes.

Ultrahigh-resolution photoluminescence studies of excitons bound to boron in silicon under uniaxial stress.

The effects of uniaxial stress on the no-phonon photoluminescence spectrum of the boron bound exciton in silicon have been investigated using ultrahigh-resolution Fourier-transform spectroscopy. Under stresses of 0 to 25 MPa, in the 〈001〉, 〈110〉, and 〈111〉 directions, up to 25 components with linewidths of less than 10 \ensuremath{\mu}eV have been resolved. A theoretical model, extending the shell model, has been developed to interpret the splittings in the bound-exciton energy levels. This includes terms expressing electron-stress, hole-stress, electron-hole, hole-hole, and valley-orbit interactions, with spherical and anisotropic corrections. The model fits the experimental data very closely, and gives values for the magnitude of the interactions.

Phonon wind on excitons in silicon

We observed experimentally the strong effect of nonequilibrium phonons on the intensity of the luminescence of free and bound excitons, created by optical illumination of a Si surface immersed in liquid helium ( T = 1.8 K). The polarity of the phonon-induced luminescence signal strongly depends on the perfection of the surface. Two factors were included in the theoretical explanation of this effect: (1) the phonon-induced dissociation of bound excitons, and (2) the drag of free excitons by phonon flux (phonon-wind effect) which shifts the exciton cloud to the surface, where exciton recombination is strong. This effect is used to study spectral features of phonon propagation in Si and a-Si:H films.

Optical detection of microwave-induced impact ionization of bound excitons in silicon.

In this paper we present the first experimental evidence for the cause of the nonresonant background signals commonly observed in optically detected magnetic resonance (ODMR) experiments in silicon. By making a direct comparison of bound-exciton (BE) photoluminescence (PL) intensity changes in an electric field with microwave-field-induced similar PL changes in n-type Si, we show that the main physical mechanism causing the ODMR background signal at lower microwave frequencies is impact ionization of free and bound excitons. We show that the efficiency of the microwave-induced impact ionization of bound excitons (BE's) is very strong, and it causes complete quenching of PL intensity for BE's associated with shallow donors. As a result the PL intensity can be enhanced by more than 150% for isoelectronic BE's. The change in PL intensity with microwave power shows a characteristic threshold, which was typically 20 mW at 9 GHz microwave frequency in our case.

Detailed ground- and excited-state spectroscopy of indirect free excitons.

Low-noise, high-resolution, far-infrared absorption spectra of free excitons in silicon are presented as a function of temperature. A complex thermalizing structure is revealed, resolving for the first time the electron-hole exchange splitting of an indirect-free-exciton ground state. A remarkable simplification of the spectrum is achieved with uniaxial stress. The relative intensities of the transitions suggest that the existing model for the free-exciton excited-state structure may be flawed, and an alternate scheme is proposed.

Electric-field-induced quenching of shallow and deep bound excitons in silicon.

A comparative study of photoluminescence (PL) quenching of shallow bound excitons (BE's) associated with shallow donors and acceptors and of deep isoelectronic bound excitons (IBE's) in Si in a weak electric field at liquid-He temperatures is reported. In both cases the photoluminescence is quenched due to impact ionization by field-accelerated hot carriers. A comparison between n- and p-type samples showed no significant difference in quenching rate for the shallow donor- and acceptor-related BE's; the quenching starts around 50 V/cm, and both electrons and holes are released as free carriers in the impact process. The IBE's show a completely different behavior and a different impact mechanism. IBE's first show an increase in PL intensity with applied electric field and are difficult to completely quench before the electrical breakdown of the sample around 400 V/cm. In this case it is concluded that only one carrier is released from the center in the impact process. By impact ionizing the shallow donor- and acceptor-related BE's and the free excitons, an efficient recombination channel for free carriers is removed, and the carrier lifetime is drastically increased. This opens up new recombination channels, such as free-to-bound transitions, which are not seen in the absence of an electric field.

A zinc-related isoelectronic bound exciton in silicon

Photoluminescence (PL), photoluminescence excitation (PLE), and photoluminescence lifetime measurements are reported for a newly-discovered optical centre in a silicon sample diffused with iron and zinc. The PL spectrum consists of two thermalizing no-phonon lines, each with sharp low energy local mode phonon replicas in addition to a broad multi-phonon sideband. The no-phonon lines, at 1059.93±0.05 and 1075.12±0.05meV, are attributed, respectively, to transitions from the spin triplet and singlet states of an isoelectronic bound exciton (IBE). Other states of the IBE are observed at 1108.4±0.5, 1110.0±0.5 and 1133.2±0.5meV in the PLE spectrum, which also shows sharp local mode phonon replicas. The energy level structure is consistent with IBE states formed by a hole and an electron subject to a large ground state valley-orbit splitting, and bound at the centre by ≈82meV.

Luminescence in silicon in electric and magnetic fields

The low temperature photoluminescence of free excitons, electron-hole droplets and bound excitons in silicon is investigated as a function of electric and magnetic fields. The luminescence is quenched due to impact ionization by field-accelerated electrons and holes.

Thallium‐belated isoelectronic bound excitons in silicon. A bistable defect at low temperatures

AbstractDetailed investigations are reported on the luminescence spectra of thallium related isoelectronic excitons in silicon, which yield a complete picture of this most unusual luminescence system. The luminescent defect exists in two different configurations, which are able to transform into each other in the temperature range from 10 to 20 K. The stress and Zeeman experiments reveal that the low temperature configuration has trigonal symmetry, whereas the high temperature form of the center is aligned along the [100] directions. Time resolved measurements of the luminescence lines lead to a detailed model for the mechanism of reorientation. In the defect ground state the trigonal symmetry is lowest in energy. The capture of a hole or an exciton changes the relative energies in such a way that now the [100] configuration is energetically favoured.

The far infrared absorption spectra of bound excitons in silicon : M. L. W. Thewalt, D. Labrie and T. Timusk. Solid St. Commun.53 (12) 1049 (1985)

The far infrared absorption spectra of bound excitons in silicon : M. L. W. Thewalt, D. Labrie and T. Timusk. Solid St. Commun.53 (12) 1049 (1985)

The far infrared absorption spectra of bound excitons in silicon

The first application of Fourier transform infrared spectroscopy to the measurement of the odd-parity excited states of a bound exciton is described. These results have reversed the prevailing model for this particular center, and allowed the even-parity excited states to be identified as well. All of these excited states can be put into excellent correspondence with acceptor states, providing the best existing confirmation of the pseudo-donor/pseudo-acceptor model of isoelectronic bound exciton states. Possible refinements of the technique are discussed, along with some suggested areas for further studies.