Dependence of the activation energies on the well width in CdTe/ZnTe strained single quantum wells

Photoluminescence (PL) measurements on the CdTe/ZnTe strained single quantum wells grown by using the molecular beam epitaxy technique showed that the sharp excitonic peaks corresponding to the transition from the first electronic subband to the first heavy-hole (E₁–HH₁) were shifted to lower energy with increasing well width. The (E₁–HH₁) interband transitions were calculated by using an envelope function approximation taking into account the strain effects, and the values were in reasonable agreement with those obtained from the (E₁–HH₁) excitonic transitions of the PL spectra. The activation energies of the confined electrons in the CdTe quantum well were obtained from the temperature-dependent PL spectra, and their values increased with increasing CdTe well width due to the quantum confinement effect. The present results can help to improve the understanding of the activation energies dependent on the CdTe well width in CdTe/ZnTe single quantum wells.     

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.     

Interband transitions and electronic properties of InAs/GaAs quantum dots embedded in AlxGa1−xAs/GaAs modulation-doped heterostructures

Reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and double-crystal X-ray curves showed that high-quality InAs quantum dot (QD) arrays inserted into GaAs barriers were embedded in an Al_0.3Ga_0.7As/GaAs heterostructure. The temperature-dependent photoluminescence (PL) spectra of the InAs/GaAs QDs showed that the exciton peak corresponding interband transition from the ground electronic subband to the ground heavy-hole subband (E₁-HH₁) was dominantly observed and that the peak position and the full width at half maximum corresponding to the interband transitions of the PL spectrum were dependent on the temperature. The activation energy of the electrons confined in the InAs/GaAs QDs was 115 meV. The electronic subband energy and the energy wave function of the Al_0.3Ga_0.7As/GaAs heterostructures were calculated by using a self-consistent method. The electronic subband energies in the InAs/GaAs QDs were calculated by using a three-dimensional spatial plane wave method, and the value of the calculated (E₁-HH₁) transition in the InAs/GaAs QDs was in reasonable agreement with that obtained from the PL measurement.     

Dependence of the interband transitions on the in mole fraction and the applied electric field in InxGa1−xAs/In0.52Al0.48As multiple quantum wells

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural properties and excitonic transitions in In_xGa_1−xAs/In_0.52Al_0.48As multiple quantum wells (MQWs) for x = 0.54, 0.57 and 0.60. TEM images showed that high-quality 11-period In_xGa_1−xAs/In_0.52Al_0.48As MQWs had high-quality heterointerfaces. The results for the PC spectra at 300 K showed that the peaks corresponding to the excitonic transitions from the ground state electronic sub-band to the ground state heavy-hole band (E₁-HH₁) and the ground state electronic sub-band to the ground state light-hole band (E₁-LH₁) became closer to each other with decreasing In mole fraction and that E₁-HH₁ and E₁-LH₁ excitonic peaks shifted to longer wavelength with increasing applied electric field. The calculated values of the E₁-HH₁ interband transition energies were in qualitative agreement with those obtained form the PC measurements with and without applied electric field. These results can be helpful in understanding potential applications of In_xGa_1−xAs/In_yAl_1−yAs MQWs dependent on In mole fraction and applied electric field in long-wavelength optoelectronic devices.     

Effect of strain and nonparabolicity on interband transition energies of InAs/GaAs coupled double quantum dots

Strained potential profiles and electronic subband energies of InAs/GaAs coupled double quantum dots (DQDs) were calculated by using a three-dimensional finite-difference method (FDM) taking into account shape-based strain and nonparabolic effects. The interband transition energies from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) in the InAs/GaAs DQDs, as determined from the FDM calculations taking into account strain and nonparabolic effects, were in reasonable agreement with the experimental peaks corresponding to the (E₁-HH₁) interband transition energies at several temperatures, as determined from the temperature-dependent photoluminescence spectra.     

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm⁻¹. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm⁻¹, while the ZnTe LO phonon response completely disappears. The 167 cm⁻¹ mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm⁻¹ mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.     

Exciton in wurtzite GaN/AlxGa1−xN coupled quantum dots

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al_xGa_1−xN vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al_xGa_1−xN coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al_xGa_1−xN strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L^AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs.     

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe self-assembled quantum-dot arrays

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe quantum-dot (QD) arrays grown by molecular beam epitaxy were investigated. Transmission electron microscopy showed that vertically stacked self-assembled CdSe QD arrays were embedded in the ZnSe barriers. The results for the photoluminescence (PL) data at 18 K demonstrated clearly that the transition behavior from uncoupled to coupled peaks depended on the ZnSe barrier thickness. The temperature-dependent PL measurements showed that the activation energy of the electrons confined in the CdSe QDs increased dramatically with decreasing ZnSe spacer layer thickness due to the strong coupling between CdSe/ZnSe QD arrays. The present observations can help improve understanding of the dependence of the coupling behavior and activation energy in CdSe/ZnSe QDs on the spacer layer thickness.     

Influence of quantum dot density on excitonic transport and recombination in CdZnTe/ZnTe QD structures

We report on dynamics of excitons in Cd_xZn_1−xTe/ZnTe quantum dots (QDs) and present information of excitonic transport and recombination. Due to different growth methods, samples with different QD's densities were obtained. Time-resolved measurements reveal three decay mechanisms: (i) radiative recombination of excitons in the individual QDs; (ii) thermally activated escape of excitons and (iii) escape due to tunneling (hopping). In the high QD-density samples the hopping (r_HB=2700 ns⁻¹) is two orders of magnitude more efficient than in the low QD-density samples (r_HB=33 ns⁻¹). Radiative recombination rates are similar in both types of samples, r_R=1-1.3 ns⁻¹. Due to the good radiative to nonradiative recombination ratio, the low-density QDs can be a potential source of entangled photon pairs.     

Size-dependent carrier dynamics in self-assembled CdTe/ZnTe quantum dots

We investigate size-dependent carrier dynamics in self-assembled CdTe/ZnTe quantum dots (QDs) grown using molecular beam epitaxy and atomic layer epitaxy. Photoluminescence (PL) spectra show that the excitonic peak corresponding to transitions from the ground electronic subband to ground heavy-hole band in CdTe/ZnTe QDs shifts to a lower energy with increasing ZnTe buffer thicknesses. This shift of the PL peak can be attributed to size variation of the CdTe QDs. In particular, carrier dynamics in CdTe QDs grown on various ZnTe buffer layer thicknesses is studied using time-resolved PL measurements. As a result, the decay time of CdTe QDs is shown to increase with increasing ZnTe buffer layer thicknesses due to the reduction of the exciton oscillator strength in the larger QDs.     

Influence of the thickness of CdTe and ZnTe layers on cathodoluminescence spectra of strained CdTe/ZnTe superlattices with quantum-dot layers

The influence of the thickness of ZnTe barrier layers on the cathodoluminescence spectra of strained CdTe/ZnTe superlattices containing layers of quantum dots with an average lateral size of approximately 3 nm has been investigated. In samples with thick barrier layers (30, 15 nm), the cathodoluminescence spectra of quantum dots exhibit one band with a maximum at E = 2.03 eV. It has been revealed that, at a barrier layer thickness of ∼3 nm, the luminescence band is split. However, at a ZnTe layer thickness of 1.5 nm, the luminescence spectrum also contains one band. The experimental results have been interpreted with allowance made for the influence of elastic biaxial strains on the energy states of light and heavy holes in the CdTe and ZnTe layers. For the CdTe/ZnTe heterostructure with quantum dots in which the thickness of the deposited CdTe layer is 1.5 monolayers and the thickness of the barrier layer is 100 monolayers, the cathodoluminescence spectrum contains 2LO-phonon replicas. This effect has been explained by the resonance between two-phonon LO states and the difference between the energy states in the electronic spectrum of wetting layer fragments.     

Electronic states and vibration spectra of CdTe/ZnTe quantum dot superlattices

Electronic and vibrational states in CdTe/ZnTe quantum dot superlattices are studied using optical spectroscopy techniques (photoluminescence in a wide temperature range, IR reflection, and Raman scattering). The effect of the ZnTe barrier layer thickness on the luminescence spectra of the structures is discussed. The luminescence from electronically coupled islands is assumed to be due to spatially indirect excitons because of the specific features of the CdTe/ZnTe heterostructure band structure. A combination of quantum-dot vibrational modes, which has not been observed earlier, is detected in the Raman spectra. Analysis of the lattice IR reflection spectra shows that, in the case of large barrier thicknesses between the quantum-dot planes, elastic stresses are concentrated in the Zn_1?xCd_xTe layers, whereas in structures with lower barrier thicknesses the elastic-strain distribution exhibits a more complicated pattern.     

Dependence of optical gain and interband transitions on the CdTe well width and temperature for CdTe/ZnTe single quantum wells

Optical gains and interband transition energies for CdTe/ZnTe single quantum wells (SQWs) with different CdTe well widths were investigated. Photoluminescence (PL) spectra for CdTe/ZnTe SQWs at various temperatures were experimentally obtained, and the corresponding optical gains were calculated by using an interacting pair Green’s function and by using an energy space integrated function. The peak energies in the gain spectra that take the Coulomb interaction between the electron and the hole into account were in qualitatively reasonable agreement with those determined from the PL spectra.     

Size and temperature effects on electric properties of CdTe/ZnTe quantum rings

The electronic properties of CdTe/ZnTe quantum rings (QRs) are investigated as functions of size and temperature using an eight-band strain-dependent k·p Hamiltonian. The size effects of diameter and height on the strain distributions around the QRs are studied. We find that the interband transition energy, defined as the energy difference between the ground electronic and the ground heavy-hole subbands, increases with the increasing QR inner diameter regardless of the temperature, while the interband energy decreases with the increasing QR height. This is attributed to the reduction of subband energies in both the conduction and the valence bands due to the strain effects. Our model, in the framework of the finite element method and the theory of elasticity of solids, shows a good agreement with the temperature-dependent photoluminescence measurement of the interband transition energies.     

Reflectance difference spectroscopy during CdTe/ZnTe interface formation

We have investigated the first stages of epitaxial growth of CdTe on ZnTe and ZnTe on CdTe with reflectance difference (RD) spectroscopy. Spectroscopic RD data show strong optical anisotropy responses at the critical points of the bulk dielectric function at the E₀, E₁ and E₁+Δ₁ interband transitions of ZnTe and CdTe, respectively, which indicate that anisotropic in-plane strain occurs during epitaxial growth. Kinetic RD data taken at the E₁ transition of the respective material exhibit with an accuracy of 1 ML the onset of the formation of misfit dislocations for these material systems.     

Optical spectroscopy of CdTe/ZnTe quantum-dot superlattices

Studies of CdTe/ZnTe quantum-dot superlattices (self-assembled quantum-dot multilayers) have been carried out by optical spectroscopy methods in a wide range of temperatures. It has been shown that the ZnTe spacer layer thickness affects the properties of these quantum-dot superlattices due to changes in the elastic strain distribution pattern. An additional luminescence band appearing in the spectrum of the structure with the thinnest ZnTe spacer layer exhibits an anomalous shift of the peak position and an unusual behavior of integral intensity with the temperature increase. We assume that the spectrum of CdTe/ZnTe quantum-dot superlattices with the thinnest ZnTe spacer is caused by two kinds of excitonic states—spatially indirect and spatially direct.     

Exciton photoluminescence, photoconductivity and absorption in GaSe0.9Te0.1 alloy crystals

The photoluminescence, photoconductivity and absorption in GaSe_0.9Te_0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe_0.9Te_0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach-Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of E_A=E_V+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions.     

Manifestation of magnetically induced spatial dispersion in the cubic semiconductors ZnTe, CdTe, and GaAs

Nonreciprocal birefringence due to magnetically induced spatial dispersion was observed in the T _d-class cubic semiconductors ZnTe, CdTe, and GaAs near the fundamental absorption edge. The dispersion of the parameters A and g, describing the contributions from terms of the type B _ik_j to the diagonal and off-diagonal components of the permittivity tensor ε _ij(ω,B,k), is determined for ZnTe and CdTe. Analysis of the dispersion and anisotropy of the nonreciprocal birefringence shows that in ZnTe, CdTe, and GaAs, in contrast to magnetic semiconductors of the type Cd_1−x Mn_xTe, it is due excitonic mechanisms. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 514–519 (10 April 1999)     

Donor bound excitons in wurtzite InGaN quantum dots: Effects of built-in electric fields

Within the framework of the effective-mass approximation and variational approach, we present calculations of the bound exciton binding energy, due to an ionized donor, in wurtzite In_xGa_1−xN/GaN strained quantum dots (QDs), considering three-dimensional confinement of the electron and hole in the QDs and the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Our results show that the position of the ionized donor, the strong built-in electric field, and the structural parameters of the QDs have a strong influence on the donor binding energy. The variation of this energy versus position of the donor ion is in double figures of milli-electron volt. Realistic cases, including the donor in the QD and in the surrounding barriers, are considered.     

Temperature dependence of the photoluminescence of CdTe/ZnTe quantum-dot superlattices

The temperature dependence of the luminescence of CdTe/ZnTe quantum-dot superlattices (self-assembled quantum-dot multilayers) with ZnTe spacers of various thicknesses was studied. Luminescence quenching observed to occur with increasing temperature is shown to depend substantially on the thickness of the ZnTe spacer. Particular attention is focused on the temperature dependence of the luminescence of a structure with the smallest ZnTe layer thickness, containing clusters of regularly arranged quantum dots. The luminescence line of tunneling-coupled quantum dots appearing in this structure exhibits an unusual temperature dependence, more specifically, an anomalously large shift of the peak position and fast luminescence quenching with increasing temperature.