Two-dimensional heterostructures based on ZnO

The multiple quantum wells (MQW) Mg_0.27Zn_0.73O/ZnO have been grown by the pulsed laser deposition method with different well widths L _w . The interface roughness of quantum wells was inherited from the bottom one and did not exceed 1 nm. We observed the quantum confinement effect showing up in the blue shift of the exciton peak in the low temperature (8 K) photoluminescence and absorption spectra at well width reduction. The exciton binding energy of the two-dimensional structures Mg_0.27Zn_0.73O/ZnO was two times higher in comparison with the bulk ZnO. It has been established that Einstein’s characteristic temperature Θ _E sharp increase with reduction of well width L _w up to L _w =2.6 nm. It has been revealed that the discontinuity ratio of conduction and valence bands in the heterostructure Mg_0.27Zn_0.73O/ZnO is 0.65/0.35. We demonstrated the abrupt increase in quantum efficiency at a reduction of the well width that allowed us to observe the optically excited stimulated emission in ZnO quantum wells with the excitation threshold of ∼210 kW/cm².     

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.     

The quantum confinement effect observed in the multiple quantum wells Mg0.27Zn0.73O/ZnO

The multiple quantum wells (MQW) Mg_0.27Zn_0.73O/ZnO have been grown by pulsed laser deposition method with different well width L _w . The optical and structural characteristics of MQW Mg_0.27Zn_0.73O/ZnO have been investigated. The quantum confinement effect showing up in the blue shift of exciton peak in low temperature (8 K) photoluminescence spectra at well width reduction has been studied. It is established that intensity exciton peak I _ex and Einstein’s characteristic temperature Θ _E increase at reduction of well width L _w . It is revealed that the discontinuity ratio of conduction and a valence bands in heterostructure Mg_0.27Zn_0.73O/ZnO is 0.65/0.35 that corresponds to the literature.     

Photoluminescence studies of GaAs/GaAlAs multiple quantum well heterostructures

The photoluminescence excited by He:Ne and Nd:YAG lasers of GaAs/Ga_0.75Al_0.25As multiple quantum well heterostructures grown by MBE was measured as a function of temperature from 4.2 K up to room temperature and for different pumping powers at constant temperature. The excitonic transitions associated with carriers confined in the quantum wells as well as other transitions associated with impurities either already present in the substrates or introduced into the samples during growth are identified in the spectra and fully characterized. From Arrhenius plots of the photoluminescence peak integrated intensities versus inverse temperature, activation energies are estimated for acceptor defects in the samples as well as for quantum well related excitonic transitions. Photoluminescence polarization experiments demonstrate a dramatic manifestation of the selection rules governing heavy hole and light hole optical transitions in quantum wells.     

Gated spin relaxation in (1 1 0)-oriented quantum wells

Growth, photoluminescence characterisation and time-resolved optical measurements of electron spin dynamics in (1 1 0)-oriented GaAs/AlGaAs quantum wells are described. Conditions are given for MBE growth of good-quality quantum wells, judged by the width of low-temperature excitonic photoluminescence. At 170 K the electron spin relaxation rate in (1 1 0)-oriented wells shows a 100-fold reduction compared to equivalent (1 0 0)-oriented wells and also a 10-fold increase with applied electric field from 20 to 80 kV cm⁻¹. There is evidence for similar dramatic effects at 300 K. Spin relaxation is field independent below 20 kV cm⁻¹ reflecting quantum well asymmetry. The results indicate the achievability of voltage-gateable quantum well spin memory time longer than 10 ns at room temperature simultaneously with high electron mobility.     

Evidence for barrier-to-well injection of carriers in high quality ZnO/Zn0.9Mg0.1O multiple quantum wells grown on (111) Si

High quality ZnO/Zn_0.9Mg_0.1O multiple quantum wells were grown on (111) Si employing epitaxial Lu₂O₃ buffer layer and a thin ZnO nucleation layer. The linewidth of the low temperature ZnO well emission is only 17 meV. The effective recombination lifetime of emission from ZnO wells is 183 ps. Temperature-dependent photoluminescence is investigated to reveal the behavior of the carriers in the multiple quantum wells. Evidence for the barrier-to-well injection is indicated from the thermal quenching of both the barrier and well emissions. Carrier localization and thermal delocalization are observed in ZnMgO cap/barriers. The depth of the local potential well is determined to be ∼7 meV$\sim 7\text{meV}$.     

Structure and optical properties of multi-quantum wells grown on Si(111) substrates

A series of ZnO/Zn_1−xMg_xO multi-quantum wells (MQWs) were grown on Si(111) substrates by pulsed laser deposition (PLD), with different well widths and depths. Transmission electron microscopy (TEM) analysis revealed that all the samples exhibited a good periodic structure with clear interfaces. Moreover, temperature- dependent behavior of excitonic photoluminescence (PL) was investigated in order to investigate the mechanism of the carrier dynamics. In the temperature range studied, the luminescence was dominated by localized exciton (LE) emission, which was significantly blueshifted with respect to the ZnO single layer, due to the well-known quantum-size effect. The blueshift values increased with elevating Mg compositions in barrier layers or decreasing well thickness, being consistent with the calculated results. The relevant activation energies were deduced, varying in a range of 6.2 to 22 meV.     

Growth and characterization of A-plane ZnO and ZnCoO based heterostructures

Non polar ZnO and (Zn, Co)O layers were successfully grown on (11̄02) sapphire (R-plane sapphire). The growth process was shown to directly influence the surface morphology as well as the strain state in (112̄0) ZnO (A-plane ZnO). The dominant defect lines seen in photoluminescence were due to basal stacking faults as demonstrated by means of selective photoluminescence and transmission electron microscopy. We present a novel method for growing high quality A-plane ZnO by inserting a (Zn, Co)O thin buffer layer, which strongly reduced the surface roughness. Finally (Zn, Mg)O/ZnO quantum well structures were grown on such a buffer layer. These quantum wells exhibited no intrinsic quantum confined Stark effect. PACS 81.05.Dz; 81.15.Hi; 78.67.Hc; 68.65.Fg     

m面蓝宝石上ZnO/ZnMgO多量子阱的制备及发光特性研究

利用等离子体辅助分子束外延设备(P-MBE)在m面的蓝宝石(m-Al₂O₃)衬底上制备了ZnO/Zn_0.85Mg_0.15O多量子阱.反射式高能电子衍射谱(RHEED)图样的原位观察表明,多量子阱结构是以二维模式生长的.从光致发光谱中可以看到ZnO/Zn_0.85Mg_0.15O多量子阱在室温仍具有明显的量子限域效应.在290 K时阱宽为3 nm的ZnO/Zn_{0.85关键词： 等离子体辅助分子束外延 ZnO多量子阱 光致发光}     

Polaron effects on excitons in parabolic quantum wells: fractional-dimension variational approach

Polaron effects on excitons in parabolic quantum wells are studied theoretically by using a variational approach with the so-called fractional dimension model. The numerical results for the exciton binding energies and longitudinal-optical phonon contributions in GaAs/Al_0.3Ga_0.7As parabolic quantum well structures are obtained as functions of the well width. It is shown that the exciton binding energies are obviously reduced by the electron (hole)-phonon interaction and the polaron effects are un-negligible. The results demonstrate that the fractional-dimension variational theory is effectual in the investigations of excitonic polaron problems in parabolic quantum wells.     

Excitonic effects in diluted magnetic semiconductor nanostructures

Excitonic properties and the dynamics are reported in quantum dots (QDs) and quantum wells (QW) of diluted magnetic semiconductors. Transient spectroscopies of photoluminescence and nonlinear-optical absorption and emission have been made on these quantum nanostructures. The Cd_1−x Mn_xSe QDs show the excitonic magnetic polaron effect with an increased binding energy. The quantum wells of the Cd_1−x Mn_xTe/ZnTe system display fast energy and dephasing relaxations of the free and localized excitons as well as the tunneling process of carriers and excitons in the QWs depending on the barrier widths. The observed dynamics and the enhanced excitonic effects are the inherent properties of the diluted magnetic nanostructures. Fiz. Tverd. Tela (St. Petersburg) 40, 846–848 (May 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Low-temperature photoluminescence in SiGe single quantum wells

1-x Ge_x single quantum wells (x=0.19) grown by rapid thermal chemical vapor deposition at 625 °C. A well-resolved strong excitonic luminescence with TO-phonon and no-phonon transitions with a full width at half-maximum as low as 6 meV is observed for a quantum well of 98 Å. The photoluminescence emission shows a significant blue shift and a broadening with excitation intensity. The results are analysed in terms of localization of photoinduced charge carriers at the heterointerfaces. Received: 11 September 1996/Accepted: 15 August 1997     

The optical properties of high quality ZnO/ZnMgO quantum wells on Si(111) substrates

The ZnO/Zn_0.85Mg_0.15O multiple quantum wells (MQWs) were fabricated on Si(111) substrates by plasma-assisted molecular beam epitaxy (P-MBE) using ZnMgO as buffer layers. The RHEED images indicated that the MQWs were of high quality. The free exciton (FE) emission line originated from the well region and its phonon replicas (FE-1LO, FE-2LO and FE-3LO) were observed in the 86 K photoluminescence (PL) spectrum. Blueshift of the FE line in the MQW sample as compared to that in the ZnO bulk was found at temperatures as high as room temperature. Time-resolved PL study on the FE line showed a fast lifetime of 140 ps. The high quality of the MQW structure was revealed by the observation of the quantum-confinement-induced blueshift and shortened lifetime of the FE emission.     

Electric-field-induced energy spectra and dispersions of ZnO/MgxZn1−xO MQWs

The energy spectra and dispersion relations of carriers in the presence of an electric field applied along the growth direction in ZnO/Mg_xZn_1−xO multiple quantum wells (MQW) are calculated using the asymptotic transfer method (ATM) on the basis of the quasistationary state approximation. The energy spectra of the carriers induce some quasi-bound levels under electric fields. The dispersion relations for the energy of the ground state and lower excitation states still have parabolic shapes for both the electrons and the heavy holes in the presence of a moderate electric field. Our results also reveal that the number of energy levels increases with increasing number of ZnO quantum wells and that the energies increase with both increasing Mg composition x and electric field strength.     

WELL-WIDTH DEPENDENCE OF THE EXCITON LIFETIME IN GaAs/AlGaAs QUANTUM WELLS

The dependence of the excitonic lifetime on the well width has been studied in conventional GaAs/AlGaAs quantum wells. Two clearly different variations of the measured excitonic lifetime have been observed. For wide well widths, we find a nearly linear decrease of the lifetime with decreasing well width. However, when the well is further decreased, a saturation and even increase of the lifetime with decreeing well width are observed. The experimental data are compared with the theory of radiative excitonic recombination, and show that well width dependence of the measured photoluminescence lifetime can be attributed mainly to the change of the excitonic effective volume and the overlap integral as well.     

Carbon incorporation in GaAs/Al0.2Ga0.8As triple quantum wells and its effect on laser performance

We report an investigation of the interface quality of the Al_0.2Ga_0.8As/GaAs triple quantum wells (QWs) grown on the GaAs substrates 0° and 6° off (100) towards 〈111〉A at a high CO environment, using the photoluminescence technique. The direct correlation between the quantum well quality and the performance of lasers which contain such quantum wells as an active region is also reported. It is found that impurity-related photoluminescence is observed only in the sample grown on the exact (100) GaAs substrate but not in the tilted one, as confirmed by temperature dependence results. The full width at half maximum (FWHM) of the intrinsic luminescence is as high as 9.0 meV in the 0° tilted samples and decreases to 3.10 meV in the samples misoriented 6°, indicating a remarkable difference in their interface quality. The impurities incorporated into the interfaces of the QWs are carbon, incorporation of which becomes unobservable by photoluminescence when the quantum wells are grown on substrates misoriented by 6° degrees. The threshold current and quantum efficiency of the laser devices with Al_0.2Ga_0.8As/GaAs quantum wells as their active region are found to be directly related to the interfacial quality of the quantum wells.     

Kinetics of indirect photoluminescence in GaAs/AlxGa1−x As double quantum wells in a random potential with a large amplitude

The kinetics of indirect photoluminescence of GaAs/Al_xGa_1−x As double quantum wells, characterized by a random potential with a large amplitude (the linewidth of the indirect photoluminescence is comparable to the binding energy of an indirect exciton) in magnetic fields B≤12 T at low temperatures T≥1.3 K is investigated. It is found that the indirect-recombination time increases with the magnetic field and decreases with increasing temperature. It is shown that the kinetics of indirect photoluminescence corresponds to single-exciton recombination in the presence of a random potential in the plane of the double quantum wells. The variation of the nonradiative recombination time is discussed in terms of the variation of the transport of indirect excitons to nonradiative recombination centers, and the variation of the radiative recombination time is discussed in terms of the variation of the population of optically active excitonic states and the localization radius of indirect excitons. The photoluminescence kinetics of indirect excitons, which is observed in the studied GaAs/Al_xGa_1−x As double quantum wells for which the random potential has a large amplitude, is qualitatively different from the photoluminescence kinetics of indirect excitons in AlAs/GaAs wells and GaAs/Al_xGa_1−x As double quantum wells with a random potential having a small amplitude. The temporal evolution of the photoluminescence spectra in the direct and indirect regimes is studied. It is shown that the evolution of the photoluminescence spectra corresponds to excitonic recombination in a random potential. Zh. éksp. Teor. Fiz. 115, 1890–1905 (May 1999)     

Optical properties of GaNAs/GaAs triple quantum well structures

Optical properties of the GaNAs/GaAs triple quantum well structures were characterized by using photoreflectance and photoluminescence spectroscopy at different temperatures. The excitonic interband transitions of the triple quantum well systems were observed in the spectral range above hν=E_g(GaN_xAs_1−x). A matrix transfer algorithm was used to match the GaN_xAs_1−x/GaAs boundary conditions and calculate the triple quantum well subband energies numerically for theoretical comparison. The internal electric field in the system was extracted from Franz-Keldysh oscillations in the photoreflectance spectra. The influences of the annealing treatment on the transition energy and the internal electric field are also analyzed.     

The optical properties of ZnO/ZnMgO single quantum well grown by P-MBE

In this paper, ZnO/Zn_0.9Mg_0.1O single quantum well (SQW) structures were fabricated on c-plane sapphire (Al₂O₃) substrate by plasma-assisted molecular beam epitaxy (P-MBE). The photoluminescence (PL) peak of the SQW shifted from 3.31 to 3.37 eV as the well layer thickness was decreased from 6 to 2 nm. The spectral linewidth increases with temperature due to the scattering of excitons with acoustic and optical phonons. The transition energy of the localized exciton in the ZnO/Mg_0.1Zn_0.9O SQW with well width of 3 nm was found to be about 3.407 eV at 80 K, consistent with theoretical calculation. The first subband energies in the conduction and valence band were calculated to be 49 and 11 meV, respectively.     

Excitonic polarons in quasi-two-dimensional structures

A theory of excitonic polarons in semiconductor quantum wells is presented. Using a unitary transformation, we have diagonalized the exciton-phonon interaction operator in a quasi-two-dimensional system partially and then calculated the ground-state energy of an excitonic polaron. We have numerically evaluated the energy gap shift and effective mass of an excitonic polaron. We have numerically evaluated the energy gap shift and effective mass of an excitonic polaron in GaAs-Al _x Ga_1–x As systems. The results obtained here indicate that the polaronic effect is significant in the case of the light hole excitons in quantum wells of small well widths.