Exciton Photoluminescence Line Research Articles

Chemically Tailored Growth of 2D Semiconductors via Hybrid Metal-Organic Chemical Vapor Deposition.

Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) are an exciting platform for excitonic physics and next-generation electronics, creating a strong demand to understand their growth, doping, and heterostructures. Despite significant progress in solid-source (SS-) and metal-organic chemical vapor deposition (MOCVD), further optimization is necessary to grow highly crystalline 2D TMDCs with controlled doping. Here, we report a hybrid MOCVD growth method that combines liquid-phase metal precursor deposition and vapor-phase organo-chalcogen delivery to leverage the advantages of both MOCVD and SS-CVD. Using our hybrid approach, we demonstrate WS2 growth with tunable morphologies─from separated single-crystal domains to continuous monolayer films─on a variety of substrates, including sapphire, SiO2, and Au. These WS2 films exhibit narrow neutral exciton photoluminescence line widths down to 27-28 meV and room-temperature mobility up to 34-36 cm2 V-1 s-1. Through simple modifications to the liquid precursor composition, we demonstrate the growth of V-doped WS2, MoxW1-xS2 alloys, and in-plane WS2-MoS2 heterostructures. This work presents an efficient approach for addressing a variety of TMDC synthesis needs on a laboratory scale.

Piezoelectric Modulation of Excitonic Properties in Monolayer WSe2 under Strong Dielectric Screening.

We investigate the interaction of excitons in monolayer WSe2 with the piezoelectric field of surface acoustic wave (SAW) at room temperature using photoluminescence (PL) spectroscopy and report a large in-plane exciton polarizability of 8.43 ± 0.18 × 10-6 Dm/V. Such large polarizability arises due to the strong dielectric screening from the piezoelectric substrate. In addition, we show that the exciton-piezoelectric field interaction and population distribution between neutral excitons and trions can be optically manipulated by controlling the field screening using photogenerated free carriers. Finally, we model the broadening of the exciton PL line width and report that the interaction is dominated by type-II band edge modulation, because of the in-plane electric field in the system. The results help understand the interaction of excitons in monolayer transition-metal dichalcogenides that will aid in controlled manipulation of excitonic properties for applications in sensing, detection, and on-chip communication.

Dipolar Biexcitons in Lateral Traps in Si/SiGe/Si Heterostructures

Si/Si1–xGex/Si heterostructures with large-scale (micrometer-size) lateral potential fluctuations at the upper SiGe/Si-cap heterointerface are grown. These potential fluctuations are caused by partial strain relaxation in the SiGe layer. Low-temperature photoluminescence (PL) spectra show that these fluctuations form lateral traps where photoexcited nonequilibrium charge carriers are accumulated and bind into dipolar excitons, which ultimately recombine. At temperatures below 6 K, a new narrow line with a width considerably less than that of the dipolar exciton PL line emerges in the spectra as the level of excitation increases. It is shown that this line is associated with the recombination of dipolar biexcitons in large-scale traps.

Photoluminescence of Molecular Beam Epitaxy-Grown Mercury Cadmium Telluride: Comparison of HgCdTe/GaAs and HgCdTe/Si Technologies

Properties of HgCdTe films grown by molecular beam epitaxy on GaAs and Si substrates have been studied by performing variable-temperature photoluminescence (PL) measurements. A substantial difference in defect structure between films grown on GaAs (013) and Si (013) substrates was revealed. HgCdTe/GaAs films were mostly free of defect-related energy levels within the bandgap, which was confirmed by PL and carrier lifetime measurements. By contrast, the properties of HgCdTe/Si films are affected by uncontrolled point defects. These could not be always associated with typical “intrinsic” HgCdTe defects, such as mercury vacancies, so consideration of other defects, possibly inherent in HgCdTe/Si structures, was required. The post-growth annealing was found to have a positive effect on the defect structure by reducing the full-widths at half-maximum of excitonic PL lines for both types of films and lowering the concentration of defects specific to HgCdTe/Si.

Compensation of the exciton-ion exchange interaction in a quantum dot by application of a magnetic field

The exchange interaction between band carriers and electrons from the d-shell of a single magnetic ion embedded in a semiconductor quantum dot leads to the splitting of exciton-ion states which allows to optically read the ion spin state. The influence of this interaction on ion states can be described in terms of an effective magnetic field induced by the exciton. It means that the exchange interaction can be partially compensated by the application of a magnetic field along the growth axis. We study this effect for CdTe/ZnTe quantum dots with a single Mn2+ ion. The in-plane component added to the perpendicular magnetic field induces mixing of different ion spin states, resulting in multiple anticrossings of the excitonic photoluminescence lines of the dot. This enables detailed investigation of the compensation.

Effect of spin polarization of excitons on the energy spectra of GaAs/AlGaAs heterostructures

The evolution of the energy position of the maximum of the free exciton photoluminescence line in high-quality GaAs/AlGaAs heterostructures under optical excitation by short laser pulses of high density 5 · 1014–2 · 1018 cm−3 is considered. The influence of screening of the Coulomb interaction and interexciton exchange interaction are discussed. These effects give corrections to the exciton peak energy position of opposite signs. The second effect is sensitive to the exciton spin orientation and is manifested as splitting of the energy peaks of excitons with angular momentum projections +1 and −1. The value of splitting is proportional to the density of excitons and to the degree of their spin polarization and reaches 1.5 meV.

Luminescence from ZnO nanoparticles/SiO 2 fabricated by ion implantation and thermal oxidation

Annealing temperature dependence of photoluminescence (PL) of SiO 2 implanted with Zn + ions of 60 keV to 1.0×10 17 ions/cm 2 is evaluated at room temperature. In as-implanted state, no PL is observed because implanted Zn atoms form Zn metallic nanoparticles (NPs). After the annealing in oxygen gas at 700 °C for 1 h, most of Zn NPs transform to ZnO NPs. A strong exciton PL line is observed at 375 nm, in addition to the defect band centered at ∼500 nm. After the oxygen annealing at 800 °C for 1 h, the defect band and the exciton line become much stronger and much weaker, respectively. After the oxygen annealing at 900 °C for 1 h, ZnO NPs transform to Zn 2SiO 4 phase. From thermodynamics consideration, the increase of the defect PL band after the 800 °C annealing is ascribed to defect formation via partial transformation of ZnO NPs to Zn 2SiO 4 phase.

Field Effect of Photoluminescence from Excitons Bound to Nitrogen Atom Pairs in GaAs

Field effect of photoluminescence due to excitons bound to nitrogen atom pairs in GaAs has been investigated for uniformly doped and atomic-layer-doped samples grown on (001) GaAs substrates. The intensities of excitonic photoluminescence lines due to distant nitrogen atom pairs decrease much more rapidly than those from closer pairs when the electric field is increased. In addition, photoluminescence due to the nearest neighbor pairs in atomic-layer-doped samples exhibits much more stable characteristics than that of uniformly doped samples against an applied electric field. This stability is observed only when the electric field is applied in either the [110] or [110] direction. This anomalous field effect can be explained by considering the electron trapping process to the isoelectric N traps modulated by the electric field.

Dynamic band-structure modulation of quantum wells by surface acoustic waves

The dynamical modulation of the band structure of GaAs quantum wells by a surface acoustic wave (SAW) is investigated using photoluminescence (PL) spectroscopy. The strain field of the SAW modulates the excitonic transitions, leading to a splitting and to a polarization anisotropy of the excitonic PL lines. The oscillator strength of the split line gives direct information about the spatial distribution of carriers in the potential modulation induced by the SAW.

Photoluminescence characterization of defects created in electron-irradiated silicon at elevated temperatures

Photoluminescence (PL) spectroscopy is employed to investigate radiative defects created in Si during electron-irradiation at elevated temperatures. The use of high temperature during electron irradiation has been found to affect considerably the defect formation process. The effect critically depends on the temperature of the irradiation as well as doping of the samples. For carbon-lean Si wafers high temperature electron irradiation stimulates the formation of extended defects, such as dislocations and precipitates. For carbon-rich Si wafers the increase of irradiation temperature up to 300°C enhances the formation of the known carbon-related defects. In addition, several new excitonic PL lines were observed after electron irradiation at T≥450°C. The dominant new PL center gives rise to a BE PL emission at 0.961 eV. The electronic structure of the 0.961 eV defect is discussed based on temperature-dependent and magneto-optical studies.

Effect of high-temperature electron irradiation on the formation of radiative defects in silicon

Defect formation processes in silicon caused by electron irradiation performed at elevated temperatures are studied in detail using photoluminescence (PL) spectroscopy. The use of high temperature during electron irradiation has been found to affect considerably the defect formation process. In particular, several new unknown excitonic PL lines were discovered in carbon-rich Si wafers subjected to electron irradiation at temperatures higher than 450°C. The dominant new luminescent center gives rise to a bound exciton PL emission at 0.961eV. The center is shown to be efficiently created by electron irradiation at temperatures from 450°C up to 600°C. The electronic structure of the 0.961eV PL center can be described as a pseudodonor case, where the hole is strongly bound at a level 187meV above the valence band, while the electron is a effective-mass-like particle weakly bound by ≈21meV in the BE state.

Study on bound exciton dynamics in CdS crystals at acoustic driving

A variety of ps transient and cw optical techniques including photoluminescence (PL), Raman scattering and optical absorption spectroscopy has been applied to elucidate acoustically driven bound exciton dynamics. The lifetime of the I 1 bound excitons in CdS platelets is found to be initially enhanced with increasing driving amplitude whereas a drop in the lifetime is achieved at sufficiently high drivings. The line shape of the I 1 exciton PL lines experiences remarkable broadening in the region of the lifetime decrease and this is accompanied by a reduced light emission in the acoustic phonon wing of the I 1 exciton PL line. Additionally, an enhancement of the E 1 LO phonon resonant Raman line and an appearance of the A 1 LO phonon Raman line, normally forbidden in the employed scattering geometry, are achieved by increasing the driving amplitude. The detailed analysis of the observations is made within the framework of internal electric fields generated by the driving. It is shown that the effects presented can be reasonably well understood by taking into account both the long-range electric fields and nonuniform microfields accompanying acoustic driving. It is shown that the microfields likely arise from variations in the local-crystal environment at the exciton site imposed by the driving.

Effect of substrate misorientation on quantum-dot size distribution in the InAs/GaAs system

A study is reported of the effect of (001)GaAs substrate misorientation in the [010] direction on the distribution of MBE-grown self-assembled InAs/GaAs quantum dots in size and position in the GaAs matrix. Temperature-induced narrowing of the exciton photoluminescence line of a quantum-dot ensemble caused by redistribution of photoexcited carriers among dots of different size has been observed.

Raman and photoluminescence studies of biaxial strain in GaN epitaxial layers grown on 6H–SiC

The effect of biaxial strain on optical phonons in high-quality GaN epitaxial layers grown on 6H–SiC substrates by metal organic chemical vapor deposition has been studied. The deformation potential constants for the E2(1), A1(TO), E1(TO), and E2(2) optical phonon modes in hexagonal GaN have been obtained. A method for calculating strain in hexagonal GaN layers from Raman data alone is suggested. A comparative analysis of the strain measured by x-ray diffraction and Raman spectroscopy shows that these data agree well. It is found that the biaxial stress of 1 GPa results in a shift of the excitonic photoluminescence lines of 20±3 meV.

Ionized donor bound excitons in GaN

The temperature and excitation power dependence of a bound exciton photoluminescence line S with a localization energy Q=11.5 meV has been studied in undoped and moderately Mg-doped wurtzite GaN of high resistivity. The data provide strong evidence that line S is due to recombination of excitons bound to ionized shallow donors. The consistency of this assignment with theoretical predictions is demonstrated.

Photoluminescence excitation spectroscopy of GaN thin layers as a function of temperature

We report on photoluminescence and photoluminescence excitation experiments performed on hexagonal GaN layers grown on a Sapphire substrate. Information about extrinsic and intrinsic optical properties have been obtained. We show that, at low temperature, the fundamental A excitons are preferentially involved in the relaxation towards the neutral donor bound exciton photoluminescence line, while electron-hole pairs rather participate in the relaxation towards D0−A0 emission and the yellow band. The relaxation from the A exciton towards the yellow band and D0−A0 emission is made easier by temperature. The band structure of the GaN layers has been determined from temperature dependent photoluminescence excitation spectroscopy: A and C excitons and A continuum band gap have been identified up to 210K.

Electronic and structural properties of GaN grown by hydride vapor phase epitaxy

The electronic and structural properties of GaN were investigated for heteroepitaxial layers grown by hydride vapor phase epitaxy. Uniform film nucleation on the sapphire substrates was facilitated by a GaCl pretreatment. The films were all unintentionally doped n type. Variable temperature Hall effect measurements reveal electron concentrations as low as 2×1017 cm−3 and electron mobilities as high as 460 cm2/V s at 300 K. The films exhibit bound exciton photoluminescence lines with a full width at half-maximum (FWHM) of 2.42 meV at 2 K. Transmission electron microscopy studies of the GaN/sapphire interface reveal a ∼200 nm thick, highly defective GaN layer consisting predominantly of stacking faults. The excellent quality of these GaN films is attributed to this ‘‘auto-buffer’’ layer which enables growth of GaN cells with a dislocation density of ∼3×108 cm−2 after ∼12 μm of film growth.

Dynamics of doubly resonant Raman scattering and resonant luminescence in ultrathin InAs/GaAs quantum wells

We have studied the transient behavior of a sharp emission line, which emerges close to the heavy-hole (hh) exciton photoluminescence line in a 1.6-monolayer-thick InAs layer embedded in a GaAs matrix. The sharp emission line appears under resonant excitation of the light-hole (lh) exciton transition, whose energy separation from the hh exciton transition amounts to one GaAs LO phonon. We have measured the decay time of the sharp emission line in addition to the lifetime of the hh and the lh excitons, using time-correlated single-photon counting and two-wavelength picosecond time-resolved pump–probe transmission techniques. We find that the decay time of the sharp emission line is equal to the lh exciton lifetime for excitation on resonance but that it strongly decreases for excitation off resonance. We conclude that, whereas the sharp line originates from doubly resonant Raman scattering for off-resonant excitation, it is predominantly due to resonant luminescence for resonant excitation. We derive a lh exciton dephasing time of 40 ps, which suggests that the length scale of potential fluctuations from disorder is larger than the lh exciton Bohr radius.

Zeeman splitting and g factor of heavy-hole excitons in InxGa1-xAs/GaAs quantum wells.

Zeeman splittings have been investigated for n=1 heavy-hole excitons ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs strained-layer quantum wells at 1.8 K and at magnetic fields up to 6 T applied parallel to the growth axis (001). The sign and magnitude of the splitting to precision of \ifmmode\pm\else\textpm\fi{}2 \ensuremath{\mu}eV were obtained from the shift in the exciton photoluminescence line upon switching between right and left circularly polarized detection. Measurements have been made for well widths between 3 and 12 nm and for x values of 0.075 and 0.11. Linear field dependence is observed below 1.5 T giving exciton g factors. For higher fields in narrow wells the dependence becomes nonlinear. We have calculated the g factor using an eight-band k\ensuremath{\cdot}p model and obtain satisfactory agreement with experiment by using the Luttinger parameter \ensuremath{\kappa} for ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As as a variable parameter. From this we obtain an empirical concentration dependence \ensuremath{\kappa}\ensuremath{\approxeq}7.68x+1.1(1-x)-4.0x(1-x).

Exciton Dynamics in Ultrathin InAs/GaAs Quantum-Wells

AbstractWe have studied the transient behaviour of a sharp emission line, which emerges close to the heavy-hole (hh) exciton photoluminescence line in a 1.6 monolayer thick InAs layer embedded in a GaAs matrix, when the light-hole (lh) exciton transition is excited. We find, that the decaytime of the sharp line is equal to the lh-exciton lifetime for excitation on resonance, but it strongly decreases for excitation off resonance. Since in our structure the energy separation between the hhand lh-exciton transitions amount to one GaAs LO phonon we conclude, that the sharp line originates from double resonant Raman scattering for off resonant excitation, whereas it is predominantly due to resonant luminescence for excitation on resonance. The strong decrease of the intensity of the sharp line due to a temperature rise from 4 to 18K proves the existence of a well confined lh-exciton state. In addition, we performed carrier capture measurements on this structure. We find a constant capture time of 20ps at the lh-exciton transition, whereas the capture time decreases step-like from 55ps to 22ps within the hh-exciton transition. This provides clear evidence for a two-step capture process, where the lh-level acts as an intermediate capture state.