No-phonon Transition Research Articles

Emission characteristics of self-assembled strained Ge1−xSnx islands for sources in the optical communication region

Self-assembled strained Ge1−xSnx islands on Si (100) have been grown at a low temperature using molecular beam epitaxy. The in-built strain and fraction of Sn in the islands have been estimated using x-ray photoelectron spectroscopy and high resolution x-ray diffraction study of grown samples. No-phonon assisted transition in the optical communication wavelength range of 1.4–1.8 μm has been observed in the Ge1−xSnx island samples. The direct band gap transition intensity is found to increase with a growth in Sn concentration, with this increase in intensity sustained up to a temperature of 130 K in Ge1−xSnx islands. The observed electroluminescence in p-i-n devices fabricated on Ge1−xSnx island samples above a threshold bias of 4 V makes them attractive for future Si based optical devices.

Optically enabled magnetic resonance study ofAs75andSb121inSi28

The electron and nuclear spins of donor impurities in enriched $^{28}\mathrm{Si}$ have great potential as long-lived qubits for a silicon-based quantum information technology. The ability to resolve the hyperfine-split neutral donor ground-state levels in the near-infrared donor bound exciton transitions of the ubiquitous phosphorus impurity in highly isotopically enriched $^{28}\mathrm{Si}$ has led to new methods of hyperpolarizing and measuring the donor electron and nuclear spins. This has resulted in optically assisted magnetic resonance methods that have permitted the measurement of remarkably long nuclear coherence times for both the neutral and ionized phosphorus donor in very lightly doped and highly enriched $^{28}\mathrm{Si}$. Other shallow donors such as arsenic, antimony, and bismuth offer the potential of larger hyperfine couplings and nuclear spins as compared to phosphorus. Here, we investigate whether donor bound exciton transitions can be used to initialize and read out the nuclear spins of arsenic and antimony in $^{28}\mathrm{Si}$. The projective readout of the electron and nuclear spins is demonstrated for both $^{75}\mathrm{As}$ and $^{121}\mathrm{Sb}$, and these optical transitions can strongly hyperpolarize the nuclear spin of $^{75}\mathrm{As}$. Only a small nuclear hyperpolarization is achieved for $^{121}\mathrm{Sb}$, likely due to the relative weakness of the no-phonon transition of the Sb donor bound exciton. Optically assisted EPR and NMR is demonstrated for $^{75}\mathrm{As}$, including Hahn echo coherence time measurements of the six NMR transitions.

Dynamics of donor bound excitons in ZnO

Comprehensive time-resolved photoluminescence measurements are performed on shallow neutral donor bound excitons (D0Xs) in bulk ZnO. It is found that transients of the no-phonon D0X transitions (I6-I9 lines) are largely affected by excitation conditions and change from a bi-exponential decay with characteristic fast (τf) and slow (τs) time constants under above-bandgap excitation to a single exponential one, determined by τs, under two-photon excitation. The slow decay also dominates transients of longitudinal optical phonon-assisted and two-electron-satellite D0X transitions, and is attributed to “bulk” D0X lifetime. The fast component is tentatively suggested to represent effects of surface recombination.

Excitation intensity dependence of photoluminescence spectra of SiGe quantum dots grown on prepatterned Si substrates: Evidence for biexcitonic transition

The pumping intensity ($I$) dependence of the photoluminescence (PL) spectra of perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phonon-assisted recombination and no-phonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear $I$ dependence of the PL intensity. At approximately $I\ensuremath{\ge}3\phantom{\rule{0.28em}{0ex}}\text{W}\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, additional bands with a nearly quadratic $I$ dependence appear in the PL spectra, resulting from biexcitonic transitions. These emerging PL contributions shift the composite no-phonon PL band of the SiGe quantum dots to higher energies. The experimentally obtained energies of the no-phonon transitions are in good agreement with the exciton and biexciton energies calculated using the envelope function approximation and the configuration interaction method.

Photoluminescence of deep defects involving transition metals in Si: New insights from highly enriched 28Si

Deep luminescence centers in Si associated with transition metals have been studied for decades, both as markers for these deleterious contaminants, as well as for the possibility of efficient Si-based light emission. They are among the most ubiquitous luminescence centers observed in Si, and have served as testbeds for elucidating the physics of isoelectronic bound excitons, and for testing ab-initio calculations of defect properties. The greatly improved spectral resolution resulting from the elimination of inhomogeneous isotope broadening in the recently available highly enriched 28Si enabled the extension of the established technique of isotope shifts to the measurement of isotopic fingerprints, which reveal not only the presence of a given element in a luminescence center, but also the number of atoms of that element. This has resulted in many surprises regarding the actual constituents of what were thought to be well-understood deep luminescence centers. Here we summarize the available information for four families of centers containing either four or five atoms chosen from (Li, Cu, Ag, Au, Pt). The no-phonon transition energies, their isotope shifts, and the local vibrational mode energies presented here for these deep centers should prove useful for the still-needed theoretical explanations of their formation, stability and properties.

Photoluminescence measurements of zero-phonon optical transitions in silicon nanocrystals

Optical transitions in silicon nanocrystals with different surface passivations were probed at low temperatures on a single-particle level. A type of quasidirect recombination process, different from the quantum-confined exciton transition, is identified. The luminescence spectra have different emission energies, but the contribution of a no-phonon transition is significantly higher than expected from the quantum-confinement model. Its relative strength was found to be temperature dependent, suggesting spatial localization of excitons as a possible origin.

Optical properties of triplet states of excitons bound to interstitial-carbon interstitial-oxygen defects in silicon

Observation of photoluminescence from spin triplet states of excitons bound to interstitial carbon-oxygen complexes (C${}_{\mathrm{i}}$-O${}_{\mathrm{i}}$) in silicon is reported. New luminescence peak labeled as C${}_{\mathrm{T}}$ line emerges at the energy $2.64\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ below the well-known luminescence from the no-phonon transition of a C${}_{\mathrm{i}}$-O${}_{\mathrm{i}}$ singlet state situating at $790\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ (C line). Observations of local vibrational modes associated with C${}_{\mathrm{T}}$ line and the temperature dependence of the relative intensity between C${}_{\mathrm{T}}$ and C lines lead to unambiguous identification of C${}_{\mathrm{T}}$ line as the no-phonon line from C${}_{\mathrm{i}}$-O${}_{\mathrm{i}}$ defects. In addition, the host silicon isotope shift of C${}_{\mathrm{T}}$ line is equal to that of C line, indicating that C${}_{\mathrm{T}}$ line is no-phonon luminescence as well. Furthermore, our photoluminescence measurements carried out in magnetic field show that C${}_{\mathrm{T}}$ line is associated with an isotropic spin triplet state due to quenching of orbital angular momentum of the hole composing the bound exciton.

Photoluminescence of Selectively Grown Epitaxial SiGe:C/Si layers

Photoluminescence of strained Si1-x-yGexCy alloy layer and Si1-x-yGexCy /Si1-xGex structures selectively grown by RT-CVD is investigated. Spectroscopic PL at low temperature (14K) and integrated PL at room temperature were performed. We report the effect of C atoms on SiGe photoluminescence spectra features, especially intensity ratio between the no-phonon (NP) and transverse-optical (TO) transitions. Using dedicated Si1-x-yGexCy /Si1-xGex structures, influence on SiGe PL spectra of C atoms supposed in non-substitutional positions is reported.

Exciton-phonon interactions observed in blue emission band in Te-delta-doped ZnSe

Photoluminescence (PL) in the blue wavelength region originating from a no-phonon (NP) transition at 2.734 eV and longitudinal optical (LO) phonon sidebands of Te isoelectronic centers (ICs) were clearly resolved after thermal annealing by δ-doping of Te in ZnSe layers. Broad luminescence (conventionally called the S1 band) had been previously observed in this region (i.e., around 2.65 eV). The PL intensities of the NP line and the phonon replicas followed a Poisson distribution with a mean phonon number (Huang–Rhys factor) of S∼1.1. This indicates that the broadening of the 2.65 eV emission band is due to the superposition of the NP lines and their LO phonon replicas originating from the Te ICs with slight variations in their transition energies. The origin of the luminescence is discussed in relation to linearly polarized PL measurements.

Energy spectrum and structure of thin pseudomorphic InAs quantum wells in an AlAs matrix: Photoluminescence spectra and band-structure calculations

The energy spectrum of thin pseudomorphic InAs quantum wells (QWs) in an AlAs matrix has been studied by steady-state and transient photoluminescence (PL) and calculation. It has been found that the PL spectra of the QWs consist of intense lines related to a no-phonon excitonic transition accompanied by its phonon replicas. The band alignment in the QWs is shown to be of type I, with the lowest conduction band states belonging to the indirect ${X}_{XY}$ minimum.

Effects of Si nanocluster size and carrier–Er interaction distance on the efficiency of energy transfer

The effects of Si nanocluster (Si-nc) size and spacing from Er 3+ ions were investigated through studies made on appropriate configurations of multilayers obtained by reactive magnetron sputtering at 650 °C and subsequently annealed at 900 °C. Si-nc larger than about 5 nm appear ineffective for resonant excitation of Er because of the resulting weak confinement responsible for negligible direct radiative recombination. This direct no-phonon transition probability is closely correlated to the energy transfer rate, both decreasing when Si-nc size increases. For large Si-nc having a bandgap lower than 1.26 eV, the energy transfer to the upper levels (second, third, etc.) of Er 3+ is no more possible, leading to the observed abrupt decrease of the 1.54-μm emission. The latter is governed by the distance separating Er ions from their Si-nc sensitizers, whose behavior was well described by an exponentially decreasing exchange interaction. The characteristic interaction distance was found to be dependent on the amorphous or crystalline nature of Si-nc, and it appears as small as 0.4±0.1 nm for the former (amorphous) and of some nanometers for the crystallized Si-nc.

Isotopic mass dependence of the lattice parameter in silicon determined by measurement of strain-induced splitting of impurity bound exciton transitions

The strain-induced splitting of the impurity bound exciton (BE) transitions in epitaxial layers of isotopically enriched 28Si grown on silicon substrates of natural isotopic composition has been studied using high-resolution photoluminescence (PL) spectroscopy. The slight difference in lattice parameter between the 28Si epitaxial layer and the natural silicon substrate induces a biaxial strain in the epitaxial layer, which can be detected with remarkable sensitivity using low-temperature PL. Measurement of the splitting of the BE transitions in these epitaxial layers of 28Si provides us a method for determining the isotopic mass dependence of the lattice parameter in silicon with unprecedented precision. The level of precision achieved is attributed to the fact that the BE no-phonon transitions in isotopically enriched silicon are much sharper than in natural silicon. We find that scaled to an isotopic mass difference (Δ M) of 1 amu, the relative difference in lattice parameter (|Δ a/ a|) for silicon is 3.3 × 1 0 - 5 .

Improvement of the direct optical transition probability in Si by wavefunction engineering

A model for improving the direct photon emission probability from electron-hole interaction in silicon is presented. Electron and hole wavefunctions are engineered for optimum overlap in SiGeC heterostructures by proper definition of alloy compositions, potentials and layer thickness dimensions. The direct no-phonon transition probability is increased by a factor of at least 10, compared to the non-optimized direct transition probability. The calibrated structure corresponds to a type II interband structure where photons are emitted with an energy lower than the bandgap energy of the constituent layers.

SiGe Light-Emitting Diodes and Their Characteristics in the Region of Band-to-Band Transitions

Light-emitting diodes (LEDs) based on single crystal SiGe with the Ge content of 5.2% were fabricated using a planar technology. Their electroluminescence (EL) parameters were studied over a wide range of measured currents (up to 11 A) and temperatures (80 - 300) K. The integrated EL intensity at a fixed current increased approximately two times with temperature increasing from 80 to 200 K and changed insignificantly in the temperature range of 200 – 300 K. The analysis of the EL spectra shows that the recombination involving excitons is the dominant mechanism of radiative recombination at both no-phonon and phonon-assisted transitions in SiGe LEDs not only at low temperature but at room temperature, too. The linear dependence of the integrated EL intensity on the current and the exponential decay of the integrated EL intensity confirm this conclusion. The room temperature internal quantum efficiency of EL in the region of band-to-band transitions is estimated to be 0.5%. A sublinear current dependence of the integrated EL intensity and a fast decay of the integrated EL intensity after the diode turn-off were observed at room temperature and currents > 2.5 A. The effect is associated with the appearance of an additional (Auger) mechanism of non-radiative recombination parallel to Shockley-Read-Hall recombination.

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.

Excitons in Si nanocrystals: Confinement and migration effects

A detailed analysis of the strong room-temperature photoluminescence (PL) signal of size controlled nc-Si is reported. The size control of nc-Si is realized by evaporation of ${\mathrm{S}\mathrm{i}\mathrm{O}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ superlattices and subsequent thermally induced phase separation. By this method the synthesis of completely ${\mathrm{SiO}}_{2}$ passivated Si nanocrystals with a controlled size is demonstrated. A strong blueshift of the photoluminescence signal from 1.3 to 1.65 eV with decreasing crystal size is observed. Resonant photoluminescence measurements prove the breakdown of the k-conservation rule for nc-Si by showing an increase in the no-phonon transition probability with decreasing crystal size. A no-phonon to phonon assisted transition probability ratio above 1 is detected at 4.5 K. These results confirm quantum confinement as the origin of the investigated luminescence signal. The size dependence of the different luminescence properties and the very high no-phonon transition probability indicate a lower confinement barrier compared to other systems containing nc-Si and additional migration effects of the excitons between the nanocrystals. A separation of quantum confinement and migration effects on the PL signal is possible due to the very narrow size distribution of the nc-Si and detailed time and temperature dependent investigations of the photoluminescence.

Optoelectronic properties of thick SiGe layers grown as small mesas by low pressure chemical vapor deposition

Arrays of Si0.80Ge0.20/Si(001) square mesas were epitaxially grown by low pressure chemical vapor deposition to optimize the light emission in the near infrared range. To study the influence of mesa size on light emission the current–voltage characteristics, the spectral photocurrent, and the electroluminescence of p-i-n structures were measured. While the plastic relaxation has a strong influence on the electroluminescence spectra, the current–voltage characteristics are only slightly changed. At low temperatures, a tunneling current was observed and its possible location is discussed. Due to the high SiGe thickness, both the contributions of the no-phonon and transversal optical phonon-assisted transitions to the photocurrent spectra could be observed. Direct evidence of the higher band gap of relaxed SiGe was obtained from electroluminescence studies.

Efficient intraband optical transitions in Si nanocrystals

Efficient intraband optical transitions in the infrared range are predicted in n-type Si nanocrystals using tight-binding calculations of no-phonon and one-phonon-assisted processes taking into account each vibrational mode. New types of transitions are reported, with no equivalence in direct gap semiconductors, between the confined states in the six valleys of the conduction band. They involve phonons with wave vectors either at the center or at the edge of the Brillouin zone. The efficiency of the main no-phonon and phonon-assisted transitions is comparable to the one in III-V semiconductor quantum dots.

Enhancement of photoluminescence by microdisk formation from Si/Ge/Si single quantum wells

A significant enhancement of photoluminescence (PL) intensity is observed in microdisks of 0.5 and 1 μm diam, which have been fabricated from Si/Ge/Si single quantum wells (SQWs) grown by molecular-beam epitaxy. The three major PL peaks found at 0.972, 0.957, and 0.920 eV are identified as a no-phonon transition of localized exciton, associated transverse-acoustical, and transverse-optical phonon replicas in Si, respectively. It is suggested that the formation of microdisks from the Si/Ge/Si SQWs enhances the intrinsic PL transitions significantly by suppressing the impurity-related ones.

Synthesis and size control of Si nanocrystals by SiO/SiO2 superlattices and Er doping

ABSTRACTThe synthesis of nc-Si by reactive evaporation of SiO and subsequent thermal induced phase separation is reported. The size control of nc-Si is realized by evaporation of SiO/SiO2 superlattices. By this method an independent control of crystal size and density is possible. The phase separation of SiO into SiO2 and nc-Si in the limit of ultrathin layers is investigated. Different steps of this phase separation are characterized by photoluminescence, infrared absorption and transmission electron microscopy measurements. The strong room temperature luminescence of nc-Si shows a strong blueshift of the photoluminescence signal from 850 to 750 nm with decreasing crystal size. Several size dependent properties of this luminescence signal, like decreasing radiative lifetime and increasing no-phonon transition properties with decreasing crystal size are in good agreement with the quantum confinement model. Er doping of the nc-Si shows an enhancement of the Er luminescence at 1.54 μm by a factor of 5000 compared to doped SiO2 layers. The decreasing transfer time for the nc-Si to Er transition with decreasing crystal size can be understood as additional proof of increasing recombination probability within the nc-Si for decreasing crystal size.