Exciton Binding Research Articles

Radiative and nonradiative recombination kinetics of indirect bound excitons studied bytime-resolved level anticrossing experiments

Time-resolved investigation of the level anticrossing effect in exciton emission is first usedto determine separately the radiative and nonradiative indirect bound exciton lifetimes in asemiconductor crystal using GaSe as an example. A theoretical treatment of theexperimental level anticrossing signal is carried out for different instances of theindirect bound exciton lifetime. The indirect bound exciton radiative lifetimeτr = 4.8 µs is found to be about five times shorter than the nonradiative lifetimeτ0 = 19 µs and about two orders of magnitude longer than the radiative lifetimeof direct bound excitons in the same compound. Within the two-levelscheme, the value of the indirect bound exciton spin relaxation timeT1 is assessedto be 100 µs.

Ordered, uniform-sized ZnO nanolaser arrays

Ordered ZnO nanorod arrays with almost uniform rod size have been grown perpendicularly on GaN∕Al2O3 substrates by a controlled vapor phase transport growth method. The ZnO nanorods are [0001] oriented single crystals with diameter of 200nm and length of 4.7μm, with a rod-to-rod spacing of 500nm. Photoluminescence spectra of the rod arrays indicate that the rods are of high crystal quality: very strong, well-separated bound and free exciton emission in the ultraviolet (UV) region are resolved at low temperature. Time resolved microphotoluminescence measurements are performed on single nanorods standing on the substrate which demonstrates lasing behavior with multiple UV lasing modes. Under quasistationary excitation lasing is observed up to room temperature. The lasing emission peaks are sharp, with a linewidth about 0.1nm, and have a fast decay time of ∼8ps. These high crystal quality nanorod arrays may be promising candidates for UV nanolaser devices.

Quantum-Confined Stark Effect in Ge/SiGe Quantum Wells on Si for Optical Modulators

We present observations of quantum confinement and quantum-confined Stark effect (QCSE) electroabsorption in Ge quantum wells with SiGe barriers grown on Si substrates, in good agreement with theoretical calculations. Though Ge is an indirect gap semiconductor, the resulting effects are at least as clear and strong as seen in typical III-V quantum well structures at similar wavelengths. We also demonstrate that the effect can be seen over the C-band around 1.55-mum wavelength in structures heated to 90degC, similar to the operating temperature of silicon electronic chips. The physics of the effects are discussed, including the effects of strain, electron and hole confinement, and exciton binding, and the reasons why the effects should be observable at all in such an indirect gap material. This effect is very promising for practical high-speed, low-power optical modulators fabricated compatible with mainstream silicon electronic integrated circuits

Influence of the self-buffer layer on ZnO film grown by atmospheric metal organic chemical vapor deposition

ZnO films with and without a self-buffer layer were grown on c-plane sapphire substrates by atmospheric metal organic chemical vapor deposition. The influence of the buffer layer thickness, annealing temperature and annealing time on ZnO films has been investigated. The full width at half maximum of the ω-rocking curve of the optimized self-buffer layer sample is only 395 arc sec. Its surface is composed of regular columnar hexagons. After the buffer layer was introduced, the A1 longitudinal mode peak at 576 cm−1, related to the defects, disappears in Raman spectra. For the photoluminescence, besides the strong donor binding exciton peak at 3.3564 eV, an ionized donor binding exciton and a free exciton peak is respectively observed at 3.3673 and 3.3756 eV at the high-energy side in the spectrum of the sample with the buffer layer.

Pressure dependence of the near-band-edge photoluminescence from ZnO microrods at low temperature

The temperature and pressure dependences of band-edge photoluminescence from ZnO microrods have been investigated. The energy separation between the free exciton (FX) and its first order phonon replica (FX-1LO) decreases at a rate of kBT with increasing temperature. The intensity ratio of the FX-1LO to the bound exciton (BX) emission is found to decrease slightly with increasing pressure. All of the exciton emission peaks show a blue shift with increasing pressure. The pressure coefficient of the FX transition, longitudinal optical (LO) phonon energy, and binding energy of BX are estimated to be 21.4, 0.5, and 0.9 meV/GPa, respectively.

Interband Optical Transitions due to Donor Bound Excitons inWurtzite InGaN Strained Coupled Quantum Dots: Strong Built-in ElectricField Effects

Considering the three-dimensional confinement of the electrons and holesand the strong built-in electric field (BEF) in the wurtziteInGaN strained coupled quantum dots (QDs), the positively chargeddonor bound exciton states and interband optical transitions areinvestigated theoretically by means of a variational method. Ourcalculations indicate that the emission wavelengths sensitivelydepend on the donor position, the strong BEF, and the structureparameters of the QD system.

Properties of Excitons Bound to Neutral Donors in GaAs-AlxGa1−xAsQuantum-Well Wires

In the effective-mass approximation, using asimple two-parameter wave function and a one-dimensional (1D)equivalent potential model, we calculate variationally the bindingenergy of an exciton bound to a neutral donor (D0,X) in finiteGaAs-AlxGa1−xAs quantum well wires (QWWs). At the wirewidth of 25 Å, the binding energy has a peak value, whichis also at the position of the peak of the exciton bindingenergy, and the center-of-mass wave functions of excitons reachesthe most centralized distribution. In addition, the changingtendency of the average interparticle distance as the wire widthis reverse to that of the binding energy.

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 .

Exciton-Phonon Interaction in CuAlS2 Powders

High-resolution photoluminescence (PL) measurement was carried out for copper aluminum disulfide (CuAlS2) powder at 12 K. Several sharp PL lines were observed in the range from 3.580 to 3.320 eV. The emission peaks at photon energies from 3.566 to 3.459 eV were attributed to free-exciton (FE) and bound-excitons (BE). The several weak emissions at below 3.476 eV were clarified to be phonon replicas (PR) by Raman scattering and in the viewpoint of exciton-phonon interaction. We observed the one, two and three-phonon replicas related to E(LO, TO) and B2(LO, TO) vibrational modes in chalcopyrite structure. It was suggested that the strong interaction between excitons and optical phonons took place in obtained CuAlS2 powder.

Optical and morphological features of bulk and homoepitaxial ZnO

ZnO substrate crystals from two different sources, and epitaxial layers have been studied by SEM, AFM, photoluminescence ( T = 2 – 135 K ) and EPR. Although fabricated by the same growth principle, i.e. the hydrothermal technique, the substrates differ in terms of purity and structural quality. In the PL spectra of all samples the dominating emission originates from the donor bound exciton (BE) recombination positioned at about 3361 meV. The temperature dependence of the spectra confirms the assignment of the free exciton emission in the purest sample, the line at 3376 meV evolves into a broad peak at higher temperatures, probably including both A and B excitons. Another FE-related emission appears as a shoulder on the high-energy side of FE A,B above 40 K. It is expected and associated with the crystal-field split-off counterpart of the valence band. Free-exciton related emission in the less pure sample can only be seen if the temperature is above 45 K. At T = 135 K all bound excitons are quenched and the spectrum in both samples consists of the free exciton no-phonon lines and their replicas. However, the emission from the pure samples is several orders of magnitude stronger than that from the other sample, which indicates strong non-radiative quenching of the excitons in the latter sample. The EPR measurements reveal a possible scenario of impurity re-arrangement, e.g. annealing at 950 ∘C may dissociate existing complexes and release Fe as isolated ions. The AFM and SEM investigations of an epilayer grown by MOCVD on one of the studied substrates have indicated growth instabilities and structural irregularities, thus pointing to the need for substrate quality and epitaxial process optimization.

Electron-hole binding in a quantum-dot lattice: Excitonic oscillator strength

We study the excitonic oscillator strength and energies arising from the binding of an electron and a hole interacting through an attractive potential in a tunnel-coupled quantum-dot lattice. The effect of interdot tunneling of the electron and the hole and their attraction on the exciton oscillator strength and exciton binding is evaluated in one-dimensional (1D) and two-dimensional (2D) lattices. For short-range interaction, we find that close packing of the quantum dots into a 2D lattice can result in a nearly abrupt loss of electron-hole binding and the oscillator strength in contrast with a 1D lattice, where the effect is gradual. Numerical application includes general electron-hole attraction in 1D lattices and on-site plus nearest-neighbor attraction in 2D lattices. The time-dependent behavior of the oscillator strength is also examined for 1D on-site attraction.

Microscopic Spatial Distribution of Bound Excitons in High-Quality ZnO

The surface morphology of the ZnO layers is dominated by a distinct hexagonal domain structure. While the laterally integrated cathodoluminescence spectrum shows intense and narrow I8 luminescence, a distinct emission line at spectral position of I0/I1 emerges in the local spectra taken at domain boundaries. In contrast, no I0/I1 emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Monochromatic images of the I8 peak wavelength directly visualize the strain relaxation across the domains towards their very center, where a drop in quantum efficiency indicates enhanced defect concentration.

Intensity Ratio of the Doublet Signature of Excitons Bound to 3C-SiC Stacking Faults in a 4H-SiC Matrix

In 4H-SiC, 3C stacking fault (SF) behaves like a finite thickness type II quantum well. As a consequence, it can bind two excitons per well. We show in this work that, as the SF thickness increases, the relative intensity of the two transitions changes. This comes from a change in the wave functions overlap between the electron trapped in the well and the holes trapped neighbouring parts of the 4H-SiC matrix.

Shallow donor generation in ZnO by remote hydrogen plasma

We investigate the effects of a remote hydrogen-plasma treatment on Hall parameters as well as on the bound exciton (BEx) photoluminescence (PL) for a variety of ZnO single crystals: bulk air-annealed, Li-doped, and epitaxially grown on sapphire. We present transport and spectroscopic evidence in favor of the hypothesis that hydrogen behaves as a shallow donor rather than a compensating center in ZnO. Specifically, we show that H-plasma-induced increases in I4 luminescence (photon energy: ∼3.363 eV at 4 K) correlate with increases in free-carrier concentrations from Hall-effect measurements.

Properties of Excitons Bound to Neutral Donors in GaAsQuantum-Well Wires

In the effective mass approximation, the binding energy of anexciton bound to a neutral donor (D0,X) is calculatedvariationally for rectangular GaAs quantum-well wires (QWWs) by using athree-parameter wavefunction. The Coulomb interaction terms are treatedexactly in their full three-dimensional forms throughout thecalculation, especially in the case of (D0,X),which is a more physically realistic procedure than those employed inprevious calculations which used effective one-dimensional potential[Chin. Phys. Lett. 21 (2004) 919]. Our treatment is unique in the useof a two-dimensional Fourier expansion of the Coulomb potential, whichremoves the numerical difficulty with the 1/rsingularity and considerably reduces the computational effort. Inaddition, we also calculate the binding energies of (D0,X) whenthe donor is in different positions (at the centre, boundary andcorner). The average interparticle distances in the square QWWs arediscussed.

Simulated Annealing for Ground State Energy of Ionized Donor Bound Excitons in Semiconductors**The project supported by Liu Hui Applied Mathematics Center of Nankai University and the 985 Education Development Plan of Tianjin University

We present a global optimization method, called the simulated annealing, to the ground state energies of excitons. The proposed method does not require the partial derivatives with respect to each variational parameter or solving an eigenequation, so the present method is simpler in software programming than the variational method, and overcomes the major difficulties. The ground state energies of ionized-donor-bound excitons (D+, X) have been calculated variationally for all values of effective electron-to-hole mass ratio σ. They are compared with those obtained by the variational method. The results obtained demonstrate that the proposed method is simple, accurate, and has more advantages than the traditional methods in calculation.

Excitonic recombination processes in GaAs grown by close-space vapour transport

Epitaxial GaAs layers were grown using the close-space vapour transport. From deep level transient spectroscopy measurements, the native EL2 donor has been observed in all of the layers with deposition temperature-dependent concentration. On the GaAs samples, also performed are photoluminescence experiments in the temperature range 10–300 K. Two peculiar features were revealed: (i) the radiative recombination in GaAs layers is increasingly dominated by bound–exciton transitions, (ii) the excitonic luminescence is found to be very sensitive to the growth conditions. A study of the near-band-edge photoluminescence as a function of power excitation and temperature has been done in an attempt to elucidate the origin of the enhanced bound–exciton luminescence.

Scaling of excitons in carbon nanotubes.

Light emission from carbon nanotubes is expected to be dominated by excitonic recombination. Here we calculate the properties of excitons in nanotubes embedded in a dielectric, for a wide range of tube radii and dielectric environments. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength.

Molecular beam epitaxy of high-quality ZnO using hydrogen peroxide as an oxidant

ZnO layers were grown on (0 0 0 1) GaN/sapphire substrates by molecular beam epitaxy (MBE) with the use of hydrogen peroxide as an oxidant. Single-crystal ZnO films were found to grow at substrate temperatures from 400°C to 650°C, whereas textured ZnO layers were prepared at the temperatures below 400°C. Structural and optical properties as well as surface morphology of the single-crystal ZnO films were investigated as a function of growth parameters. The single-crystal material showed strong bound-exciton (BE) emission with a full-width at half-maximum down to 4.7 meV. The increase in both substrate temperature and Zn flux was found to result in higher crystal perfection and stronger BE emission. The data obtained prove the feasibility of the peroxide-assisted MBE for the growth of high-quality ZnO layers.

Properties of the Bound Excitons Associated to the 3838Å Line in 4H-SiC and the 4182Å Line in 6H-SiC

Properties of the bound excitons associated to the 3838A line in 4H-SiC and the 4182A line in 6H-SiC