Managing disorder in random GaAs/AlGaAs superlattices

The artificial random Gaussian-type potential built in the GaAs/AlGaAs superlattices grown by molecular beam epitaxy was explored by various methods. The effect of the intentional disorder was shown to dominate intrinsic superlattice imperfections and its impact on the electronic properties was found to be in good agreement with the theoretical predictions. It was demonstrated that the modern state of the molecular beam epitaxy allows for a growth of the superstructured materials with well-defined disorder strength.     

Domain boundary instability in weakly coupled GaAs/AlGaAs superlattices

Self-sustained oscillations of the current with a frequency ranging from 0.7 to 3.6 MHz have been detected in weakly coupled GaAs/AlGaAs superlattice at 4.2 K. A study of the static and dynamic characteristics of the structure showed that the spontaneous oscillations arise in the local region of the superlattice, restricted by a size of the domain boundary expansion. The oscillations arise in the negative differential conductivity regions due to the periodic coupling and decoupling of subbands in adjacent quantum wells, forming the expanded domain boundary. We suggest that the spatio-temporal oscillations of the domain boundary should be considered as oscillations of an ensemble of several strongly phase-coupled oscillators. Each oscillator is a couple of two adjacent quantum wells, which operates as a single resonant tunneling diode.     

Mean free path of ballistic electrons in GaAs/AlGaAs superlattices

The mean free path of ballistic electrons in GaAs/AlGaAs superlattices was measured using the technique of hot electron spectroscopy in magnetic fields perpendicular to the growth direction. We utilize the fact that the total effective path of an injected hot electron is a function of the applied magnetic field. For a superlattice with 6.5 nm GaAs wells and 2.5 nm GaAlAs barriers we measure a mean free path of 80 nm. The experimental results of a ten-period SL sample are compared to a fully three-dimensional calculation of the transmission including interface roughness with island sizes of 10 nm. We demonstrate that the observed mfp is limited due to interface roughness scattering for temperatures up to 50 K.     

Mutual synchronization of two coupled self-oscillators based on GaAs/AlGaAs superlattices

The interaction of self-oscillators based on 30-period weakly coupled GaAs/AlGaAs superlattices is studied. The action of one self-oscillator on the other was observed for a constant bias voltage in the absence of generation of self-sustained oscillations in one of the oscillators. It is shown that induced oscillations in a forced oscillator appear due to excitation of oscillations in the system of coupled oscillators forming the electric-field domain wall at the frequency of one of the higher harmonics of a forcing oscillation.     

Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 623–628 (25 April 1997)     

Correlation between Si diffusion and Si-induced disordering in AlGaAs/GaAs superlattices

Compositional disordering of AlGaAs superlattices induced by Si ion implantation and subsequent annealing has been studied by secondary ion mass spectrometry (SIMS). Distinct correlation is found between the induced disordering and the rapid Si diffusion which occurs above the critical concentration of about 3×10¹⁸ cm⁻³. The annealing-condition dependence of the disordering suggests that AlGa intermixing is induced by the vacancy flow enhanced by the Si_IIISi_V pair movement which causes the rapid Si diffusion. SIMS depth profiles of the heat treated superlattices co-doped with Si and Be do not show any appreciable Si diffusion and induced disordering. This is well-explained by the formation of SiBe pairs which prevents that of Si_IIISi_V pairs.     

Photoluminescence studies of GaAs/AlAs short period superlattices

Low-temperature pressure-dependent photoluminescence measurements on short-period GaAs/AlAs superlattice structures are presented. Measurements show that the lowest energy conduction-band states are in the AlAs layers and the highest energy valence-band states are located in the GaAs layers. This result is supported by the following three experimental observations: (1) the observed pressure coefficient for the conduction-band to valence-band transition energy is negative, (2) the magnetic mass of this transition is “heavy”, and (3) the band-to-band absorption coefficient appears to be small. These experimental observations are in agreement with predictions of tight-binding calculations.     

Resonant tunneling transport through GaAs/AlGaAs superlattices in strong tilted magnetic field

An analysis is presented of the transverse resonant tunneling transport through GaAs/AlGaAs superlattices due to tunneling between Landau levels in quantum wells in a strong tilted magnetic field. A high tunneling rate is demonstrated between Landau levels with Δn ≠ 0 in a magnetic field with a nonzero in-plane component. This leads to substantial broadening and shift of the tunneling resonance and significant changes in the current-voltage characteristics of superlattices. The predicted behavior of the current-voltage characteristics of superlattices in tilted magnetic fields is demonstrated experimentally.     

Bloch oscillations in GaAs/AlGaAs superlattices after excitation well above the bandgap

Bloch oscillations of wave packets in semiconductor superlattices are observed by THz-emission spectroscopy after pulsed photo-excitation well above the fundamental band-gap. We show that Bloch oscillations are induced by excitation of states in both the first and the second electron miniband.     

Terahertz injection electroluminescence in multiperiod quantum-cascade AlGaAs/GaAs structures

Terahertz electroluminescence in the range ≈1.5 THz was observed in a quantum-cascade GaAs/AlGaAs structure containing 40 periods of tunnel-coupled wells. The luminescence is caused by the spatially indirect optical electron transitions between the ground states of neighboring quantum wells.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions. Fiz. Tverd. Tela (St. Petersburg) 40, 833–836 (May 1998)     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Effect of the coherence of free electron-hole pairs on excitonic absorption in GaAs/AlGaAs superlattices

The effect of photoexcited free carriers on the absorption spectra dynamics of GaAs/Al_xGa_1−x As superlattices is investigated experimentally by the pump-probe method. A sharp change in the shift of the excitonic resonance energy from the low-to the high-energy direction is found to occur at the moment that the electromagnetic radiation of the pump and probe beams overlap in the case of band-band excitation. This phenomenon is explained in a model of scattering of high-energy electron-hole pairs. The dephasing time of free high-energy particles is experimentally estimated to be several tens of femtoseconds. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 62–67 (10 January 1998)     

超长波GaAs/AlGaAs量子阱红外探测器光电流谱特性研究

研究了响应波长在15μm附近的超长波GaAs/AlGaAs量子阱红外探测器在不同外加偏压下的光电流谱特性.光电流谱上的两个主要由于阱宽随机涨落而呈现为高斯线形的响应峰被分别指认为量子阱基态E0到第一激发态E1和第三激发态E3的跃迁.跃迁峰随着器件上外加偏压的增大而出现线性红移现象，认为这种变化起源于激发态与基态对量子阱结构中势变化敏感性的不同，采用传输矩阵方法并考虑到电子交互作用修正进行的理论计算在定量上解释了实验结果. 关键词： 量子阱红外探测器 超长波 光电流 传输矩阵     

Spectroscopic studies of real space indirect symmetric GaAs/AlAs short period superlattices

The band structure and the optical transitions of symmetric ultra-short period (GaAs) _m /(AlAs) _n superlattices (n=m is the number of monolayers) have been studied using different spectroscopic techniques, namely photoacoustic spectroscopy, reflectivity and luminescence, photoluminescence excitation and high excitation intensity time-resolved luminescence. Direct observation of the transition from type I to type II energy band alignment is reported for superlattices whose configuration consists of more than 12 monolayers (i.e. m=n>12). The radiative recombination processes associated with the real space indirect transitions have been investigated as a function of the density of photogenerated electron-hole pairs, revealing an unusual density-dependent behaviour. A tentative interpretation in terms of a condensed electron-hole state at the indirect gap is given, which accounts for the long decay time of the type II luminescence at high excitation rate.     

Electro-optical phenomena accompanying electron and hole heating in superlattices and quantum wells GaAs/AlGaAs and Ge/GeSi

New electro-optical phenomena in quantum-well structures, i.e. modulation of the light absorption and birefringence due to carrier heating in a strong electric field, have been investigated. The effects have revealed different features in the three types of structures under investigation, namely: (1) well-dopedn-type GaAs/AlGaAs multiple quantum wells, (2) barrier-dopedn-type GaAs/AlGaAs superlattices and (3) barrier-dopedp-type Ge/GeSi multiple quantum wells. Possible mechanisms of the phenomena have been discussed.     

量子阱中电子自旋注入及弛豫的飞秒光谱研究

采用飞秒脉冲的饱和吸收光谱方法研究了GaAs/AlGaAs多量子阱中电子自旋的注入和 弛豫特性，测得电子自旋极化弛豫时间为80±10ps.说明了电子自旋 轨道耦合相互作用引 起局域磁场的随机化，是导致电子的自旋极化弛豫的主要机理. 关键词： 自旋电子学 半导体量子阱 飞秒激光光谱 自旋 轨道耦合     

Cyclotron masses in InGaAs/GaAs superlattices and InGaAs/AlAs superlattices

Cyclotron resonance (CR) measurements have been carried out to evaluate the effective mass of electron in (InGaAs)_n/(GaAs)_nsuperlattices (SLs) and (InGaAs)_n/(AlAs)_nSLs. To clarify the dependence of cyclotron mass on the monolayer numbern , we measured CR signals using pulsed high-magnetic fields up to 150 T and a far-infrared laser. We found clear cyclotron resonances in the transmission of 10.6 μ m at 75 T at room temperature in (InGaAs)_n/(GaAs)_nSLs and little dependence on the monolayer number n in the SLs. However, for (InGaAs)_n/(AlAs)_nSLs, a large dependence of cyclotron mass on the monolayer number n was observed. We consider that these dependencies are related to the difference between the barrier height in the SLs and the influence of nonparabolicity on the conduction subbands in the SLs.