Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Photoluminescence dynamics of coupled quantum dots

We have investigated effects of the strain-compensating spacer layer on photoluminescence (PL) properties of excitons in the multilayered self-assembled quantum dots (SAQDs). The PL decay time of excitons in the sample with a thin spacer layer is longer than that with a thick spacer layer. This is attributed to the lowering of the oscillator strengths owing to reduction of the overlap integral between the electrons and holes envelope functions because of spatially extended electron envelope functions in the growth direction. In addition, the PL intensity of the transverse-magnetic mode in the sample with the thinner spacer layer has been found to be enhanced in the polarized PL measurement from the cleaved edge surface of the samples. These results indicate the possibility of QDs connecting along the growth direction via penetration of the electron envelope function.     

Thermal dissociation of excitons in a type-I GaAs/AlAs superlattice studied by time-resolved photoluminescence measurements

In order to investigate thermal properties of excitons, we have performed time-resolved photoluminescence (PL) measurements for a type-I (GaAs)₁₅/(AlAs)₁₅ superlattice. At low temperatures, PL signals show ordinal exponential time decay, whereas at high temperatures, the PL shows power decay. Such change of PL signals is understood by considering thermal dissociation of exciton into account. At low temperatures, recombination of bound excitons generates PL which shows exponential decay. At temperatures higher than the exciton binding energy, correlation between electrons and holes disappears. Recombination of free excitons causes PL which decays by a power function. By means of the least-squares fitting, we evaluate the portion of bound excitons, recombination time of bound and free excitons as functions of temperatures.     

GaAs间隔层厚度对InAs/GaAs量子点分子光学性质的影响

采用光致荧光发射谱(PL)和时间分辨荧光发射谱(TRPL)研究了GaAs间隔层厚度对自组装生长的双层InAs/GaAs量子点分子光学性质的影响.首先,测量低温下改变激发强度的PL谱,底层量子点和顶层量子点的PL强度比值随激发强度发生变化,表明两层量子点之间的耦合作用和层间载流子的转移随着间隔层厚度变大而变弱.接着测量改变温度的PL谱,量子点荧光光谱峰值位置(Emax)、半峰全宽及积分强度随温度发生变化,表明GaAs间隔层厚度直接影响到量子点内载流子的动力学过程和量子点发光的热淬灭过程.最后,TRPL测量发现60mL比40mL间隔层厚度样品的载流子隧穿时间有明显延长.     

Optically detected cyclotron resonance in Ge and Si exchange interaction in impact dissociation

We have investigated optically detected cyclotron resonance (ODCR) and ordinary cyclotron resonance (CR) under the same condition, in Ge and Si, which include both high-purity and doped samples. In impact dissociation processes, which are the origin of ODCR, donorbound excitons have larger cross section for impinging electrons than for holes, and conversely, acceptor- bound excitons have larger cross section for holes than for electrons. Moreover, the ratio of impact dissociation cross section for holes to that for electrons varies with the number of excitons bound to an impurity. These phenomena are understood primarily in terms of exchange interaction between impinging carriers and constituents of bound excitons. In addition, it is found that the relative intensity of hole cyclotron resonance against electron resonance is larger in ODCR than in CR. This is understood in terms of exchange interaction by taking the many valley nature of the conduction band into account.     

亚单层InGaAs量子点-量子阱异质结构的时间分辨光致发光谱

测定了亚单层InGaAs/GaAs量子点-量子阱异质结构在5K下的时间分辨光致发光谱.亚单层量 子点的辐射寿命在500 ps 至 800 ps之间，随量子点尺寸的增大而增大，与量子点中激子的 较小的横向限制能以及激子从小量子点向大量子点的隧穿转移有关.光致发光上升时间强烈 依赖于激发强度密度.在弱激发强度密度下，上升时间为 35 ps，纵光学声子发射为主要的 载流子俘获机理.在强激发强度密度下，上升时间随激发强度密度的增加而减小，俄歇过程 为主要的载流子俘获机理.该结果对理解亚单层量子点器件的工作特性非常有用. 关键词： 亚单层 量子点-量子阱 时间分辨光致发光谱     

Coulomb binding of electrons to multiply charged GaSb/GaAs self-assembled quantum dots

We explore the Coulomb binding of electrons to holes confined to type-II GaSb self-assembled quantum dots. We demonstrate that at low laser power electrons are more weakly bound to holes trapped by the dots than to holes in the wetting layer. On the other hand, at high laser power the hydrogenic binding energy of dot excitons increases by more than a factor of two, and so exceeds that of wetting layer excitons. We attribute this to the strong binding of ‘core’ electrons to dots that are highly charged with holes by optical pumping.     

Enhancement of the photoluminescence intensity of a single InAs/GaAs quantum dot by separate generation of electrons and holes

It is demonstrated that the microphotoluminescence (μPL) spectrum of a single InAs/GaAs self-assembled quantum dot (QD) undergoes considerable changes when the primary laser excitation is complemented with an additional infrared laser. The primary laser, tuned slightly below the GaAs band gap, provides electron-hole pairs in the wetting layer (WL), as well as excess free electrons from ionized shallow acceptors in the GaAs barriers. An additional IR laser with a fixed energy well below the QD ground state transition generates excess free holes from deep levels in GaAs. The excess electron and hole will experience diffusion separately, due to the time separation between the two events of their generation, to eventually become captured into the QD. Although the generation rates of excess carries are much lower than that of the electron-hole pair generation in the WL, they considerably influence the QD emission at low temperatures. The integrated PL intensity increases by several times as compared to single-laser excitation, and the QD exciton spectrum is redistributed in favor of a more neutral charge configuration. The dependence of the observed phenomenon on the powers of the two lasers and the temperature has been studied and is consistent with the model proposed. The concept of dual excitation could be successfully applied to different low-dimensional semiconductor structures in order to manipulate their charge state and emission intensity. The text was submitted by the authors in English.     

The photocurrent of resonant tunneling diode controlled by the charging effects of quantum dots

We experimentally studied the photocurrent of AlAs/GaAs/AlAs double barrier resonant tunneling diode (RTD), which is composed of an InAs layer of self-assembled quantum-dots on top of AlAs barrier layer. It is found that the charging InAs quantum dots can effectively modulate the carrier transport properties of the RTD. Moreover, we also found that the resonant tunneling current through a single energy level of an individual quantum dot is extremely sensitive to the photo-excited holes bound nearby the dot, and the presence of the holes lowers the electrostatic energy of the quantum dot state. In addition, it is also observed that the photocurrent behaves like step way with the individual photon pulse excitation when the illumination is low enough. The experiment results well demonstrated the quantum amplified characteristics of the device.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Acoustically induced dynamic potential dots

Mobile potential dots (dynamic dots, DDs) formed by surface acoustic waves (SAWs) are used to transport photogenerated electrons and holes in GaAs quantum wells (QWs). We investigate the interaction between the transported carriers and microscopic trap centers in the QW plane using spatially and time-resolved photoluminescence (PL) spectroscopy. The carriers recombine at the trap site emitting short (width0.6 ns) light pulses at a repetition rate corresponding to the SAW frequency. The dependence of the PL intensity from the traps on the number of carriers transported per DD n exhibits a well-defined, distinct plateau for n in the range from 5–20, which is attributed to the emission of a well-defined number of photons.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Magneto-optical study of nonmagnetic quantum dots coupled to a magnetic semiconductor quantum well

We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs.     

CdSe/ZnSe复合结构非对称量子阱的发光特性

利用MOCVD技术在GaAs衬底上外延生长了非对称量子阱结构CdSe/ZnSe材料,通过对其稳态变温光谱及变激发功率光谱,研究了其发光特性。稳态光谱表明:在82～141K时,观测到的两个发光峰来源于不同阱层厚度的量子阱激子发光,用对比实验验证了高能侧发光的来源。宽阱发光强度先增加后减小,将其归结为激子隧穿与激子热离化相互竞争的结果。通过Arrhenius拟合,对宽阱激子热激活能进行了计算。82K时变激发功率PL光谱表明:由于激子隧穿的存在,使得窄阱发光峰位不随激发功率变化而变化,宽阱发光峰位随激发功率增加发生了蓝移,并对激子隧穿进行了实验验证。     

Influence of the cap layer thickness on photoluminescence properties in strained InGaAs/GaAs single quantum wells

The influence of the GaAs cap layer thickness on the luminescence properties in strained In_0.20Ga_0.80As/GaAs single quantum well (SQW) structures has been investigated using temperature-dependent photoluminescence (PL) spectroscopy. The luminescence peak is shifted to lower energy as the GaAs cap layer thickness decreases, which demonstrates the effect of the GaAs cap layer thickness on the strain of InGaAs/GaAs single quantum wells (SQW). We find the PL quenching mechanism is the thermal activation of electron hole pairs from the wells into the GaAs cap layer for the samples with thicker GaAs cap layer, while in sample with thinner GaAs cap layer exciton trapping on misfit dislocations is dominated.     

Two-dimensional electron gas in double quantum wells with tilted bands

When a voltage is applied to double quantum wells based on AlGaAs/GaAs heterostructures with contact regions (n-i-n structures), a two-dimensional (2D) electron gas appears in one of the quantum wells. Under illumination which generates electron-hole pairs, the photoexcited holes become localized in a neighboring quantum well and recombine radiatively with the 2D electrons (tunneling recombination through the barrier). The appearance, ground-state energy, and density of the degenerate 2D electron gas are determined from the structure of the Landau levels in the luminescence and luminescence excitation spectra as well as from the oscillations of the radiative recombination intensity in a magnetic field with detection directly at the Fermi level. The electron density is regulated by the voltage between the contact regions and increases with the slope of the bands. For a fixed slope of the bands the 2D-electron density has an upper limit determined by the resonance tunneling of electrons into a neighboring quantum well and subsequent direct recombination with photoexcited holes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 840–845 (10 June 1997)     

The tunneling magnetoresistance in GaMnAs/GaAs/GaMnAs junctions

The tunneling magnetoresistance (TMR) in GaMnAs/GaAs/GaMnAs magnetic tunnel junctions is studied under an extended coherent tunneling approach where both the contributions of the light holes and the heavy holes and their mutual competitions are investigated. It is shown that the TMR ratio can increase with decreasing the barrier strength, which is different from the results in the conventional magnetic tunnel junctions but a good news for the applications. It is also shown that the presence of the pinholes in the thin barrier layer gives a possible explanation of the peak in the barrier thickness dependence of the TMR ratio.     

Investigation of barrier tunneling characteristics of symmetrical double quantum well in In0.25Ga0.75As/GaAs/In0.25Ga0.75As heterostructure

The optical and electrophysical properties of the GaAs/In_0.25Ga_0.75As heterostructure with a symmetric double quantum well have been investigated. The influence of tunneling electrons and holes through an internal barrier of the quantum well on the shift and splitting of the quantum levels is analyzed. The theoretical estimates are compared with the results of the photoluminescence and photoconductivity measurements. The Hall measurements indicate that the barrier strongly affects the mobility of charge carriers.     

Radiation Damage Analysis of Individual Subcells for GaInP/GaAs/Ge Solar Cells Using Photoluminescence Measurements

The radiation damage of three individual subcells for GaInP/GaAs/Ge triple-junction solar cells irradiated with electrons and protons is investigated using photoluminescence(PL) measurements. The PL spectra of each subcell are obtained using different excitation lasers. The PL intensity has a fast degradation after irradiation,and decreases as the displacement damage dose increases. Furthermore, the normalized PL intensity varying with the displacement damage dose is analyzed in detail, and then the lifetime damage coefficients of the recombination centers for GaInP top-cell, GaAs mid-cell and Ge bottom-cell of the triple-junction solar cells are determined from the PL radiative efficiency.     

Emission of photoexcited charge carriers from InAs/GaAs quantum dots grown by gas-phase epitaxy

A model describing the emission of photoexcited electrons and holes from an array of InAs quantum dots into the GaAs matrix is suggested. The analytical expression obtained for the emission efficiency takes into account the thermal emission of charge carriers into the GaAs matrix and two-dimensional states of the InAs wetting layer, tunneling and thermally activated tunneling escape, and electron transitions between the quantum-confinement levels in the conduction band of InAs. The temperature dependences of the photosensitivity in the regions of the ground-state and first excited-state optical transitions in InAs/GaAs quantum dots grown by gas-phase epitaxy are investigated experimentally. A number of quantum dot parameters are determined by fitting the results of a theoretical calculation to the experimental data. Good agreement between the theoretical and experimental results is obtained in this way.