Scattering of electrons by confined interface polar optical phonons in a double-barrier heterostructure

It is shown that capture of the surface (interface) phonons can occur in a double-barrier heterostructure in addition to the capture of bulk polar optical phonons. The strength of interaction of electrons with confined interface phonons becomes lower as the thickness of the phonon well (a semiconductor layer where phonons are captured) is decreased. A new approach is suggested for reducing the scattering of electrons by polar optical phonons in a double-barrier quantum well; this approach is based on separate capture of phonons in narrow phonon wells. The calculated scattering rate with the capture of interface phonons into the GaAs/InAs/GaAs and AlAs/GaAs/AlAs quantum wells taken into account is found to be much lower than the rate obtained in the approximation of scattering of electrons by bulk phonons. A multifold decrease in the rate of electron-phonon scattering in the AlAs/GaAs/AlAs quantum well is obtained by separating this well by the monomolecular InAs layer that is transparent for electrons but acts as a reflecting barrier for polar optical phonons.     

Calculation of photogenerated carrier escape rates from GaAs/AlGa1-xAs quantum wells

We present a new theory for photogenerated carrier escape rates from single quantum wells, as a function of an applied electric field, that includes thermionic emission, direct tunneling, and tunneling via thermal occupation of upper subbands, and compare the results for GaAs/Al_xGa_1-xAs quantum wells with recent experiments. We account for the two dimensional (2D) density of states below the barrier, assume thermal equilibrium of carriers within the well, allow for the possibility of strain in the well and/or barrier, and include the contribution to electron thermionic emission from indirect conduction band minima. Our expressions for thermionic emission reduce, in the limit of large well width, to those derived by assuming a three-dimensional (3D) density of states. The results for electron emission from GaAs/Al_xGa_1-xAs quantum wells with x=0.2 and x=0.4 barriers at room temperature agree well with experiment. For wells with x=0.2 barriers, thermally assisted tunneling overtakes thermionic emission around 40 kV/cm, while for wells with x=0.4 barriers thermionic emission from the L valley conduction band minima dominates for fields less than 70 kV/cm. For holes we show that the escape rates are very sensitive to the in-plane effective masses, and results using simple expressions for the in-plane masses that do not include light/heavy-hole mixing agree poorly with experiment. The agreement with experiment is improved using in-plane masses that include light/heavy-hole mixing, particularly for wells with high barriers. We suggest that agreement with experiment would be improved by using more accurate in-plane hole masses for all of the subbands     

Photonic quantum ring laser of 3D whispering cave mode

A new whispering cave mode (WCM), a three-dimensional (3D) case of Lord Rayleigh's 2D whispering gallery modes, based upon 3D total internal reflection is presented, where GaAs quantum well (QW) near-vertical microdisk resonators exhibit a strong carrier-photon coupling for the QW carriers located in the Rayleigh band, generating 2D-to-1D carrier phase transitions of photonic quantum ring (PQR). It is a ‘photonic’ quantum corral effect (PQCE), whose simulation work produces a tangled web pattern of imminent recombinant carriers in picoseconds timescale. The PQCE is responsible for the ultralow threshold currents below 300 nA for a single mode GaAs PQR laser, and a prototype GaN PQR laser with a threshold near and below 1 μA.     

Damping of bloch oscillations in one-, two-, and three-dimensional quantum-dot superlattices

In the preceding paper by the same authors, the density-matrix formalism was used to derive a quantum kinetic equation describing the damping of Bloch oscillations (BOs) in perfect one-, two-, and three-dimensional quantum-dot superlattices (QDSLs) and the conditions were determined under which the only process of the charge-carrier scattering by phonons in 2D and 3D QDSLs that contributes to the BO damping is the acoustic-phonon scattering within the transverse minibands of the Stark ladder of the carrier states. In this paper, the possibilities of suppressing this remaining scattering channel are analyzed. It is shown that the BO damping time in 2D and 3D QDSLs at room temperature may exceed the oscillation period by a factor of several hundreds and the conditions necessary for such strong suppression of the scattering are revealed. This makes a considerable difference between the QDSLs and the quantum-well superlattices, where, in reality, the BOs damping over a single oscillation period at room temperature.     

On the delta-type doping of GaAs-based heterostructures with manganese compounds

Heterostructures, which incorporate GaAs/InGaAs/GaAs quantum wells and are doped with spatially remote monatomic Mn layers, are formed on GaAs(001) substrate under conditions of multilayer buildup by the method of molecular-beam epitaxy. Combined studies of the obtained samples were performed by the method of secondary-ion mass spectrometry, by measurements of X-ray diffraction, and using a transmission electron microscope. The heterostructures under study with a doping impurity concentration amounting to 0.5 single layers are elastically stressed and demonstrate planar clearly defined interfaces without visible extended or point defects. A method for visualization of the distribution of the manganese concentration in the three-dimensional GaAs matrix in the vicinity of a quantum well is suggested. According to experimental results, there is a probability for manganese diffusion into the GaAs/InGaAs/GaAs quantum well when the critical thickness of the GaAs buffer layer is decreased to a value smaller than 3 nm.     

Electron spin-flip Raman scattering in asymmetric quantum wells: Spin orientation

We present Raman scattering measurements of intrasubband single-particle excitations (SPE) in the two-dimensional gas confined in a GaAs/AlGaAs quantum well. The intrinsic spin-splitting in the conduction subband of the quantum well may be determined from the spin-flip SPE observed in depolarized Raman scattering. From measurements on samples of varying carrier density, we analyse the dependence of the spin-splitting and its anisotropy on electron wavevector. The results of tight binding calculations support our conclusions about the origin of the spin orientation in a quantum well.     

QUANTUM MECHANICAL MODEL AND SIMULATION OF GaAs/AlGaAs QUANTUM WELL INFRARED PHOTO- DETECTOR-Ⅰ OPTICAL ASPECTS

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Photoluminescence characterization technique for resonant-tunneling structures based on a long-period GaAs/AlGaAs superlattice,applicable at different stages of fabrication

A technique is developed for the photoluminescence-spectroscopy characterization of resonant-tunneling structures based on a long-period GaAs/AlGaAs superlattice that can be used for quality evaluation at all the stages of fabrication, including molecular-beam epitaxy, photolithography, and annealing. Factors such as the small energy difference between the quantum confined states in wide quantum wells, which make the photoluminescence characterization of such structures more difficult are taken into account. The long-period multiquantum-well structures are promising for the development of a new kind of solid-state intersub-band-transition devices emitting the narrow band radiation in far infrared. Their potential is essentially based on the fact that the scattering and the decay of carriers in the lower quantum-confined states may or may not involve optical phonons. The technique works at both liquid-helium and room temperature. It helps one optimize the process conditions to fabricate high-quality wide-quantum-well structures with excellent uniformity and desired parameters.     

Growth mode transition processes in a GaAs/lnP system studied by scanning tunneling microscopy

The growth mode transition processes of GaAs heteroepitaxial growth on an InP substrate were investigated using a scanning tunneling microscopy (STM) multi-chamber system equipped with a molecular beam epitaxy facility. During the initial stage, the growth mode transition from two-dimensional (2D)-to three-dimensional (3D)-island growth occurred with more than 2.0 ML of GaAs deposition onto the InP surface. This 3D-island structure gradually became flat upon increasing the amount of GaAs deposition, and finally recovered to a GaAs 2D structure. Furthermore, an extended dislocation line can be observed in the STM image of the recovered GaAs surface.     

Carrier capture and relaxation in narrow quantum wells

In separate confined heterostructure (SCH) lasers, injected electrons and holes thermalize into a quantum well after diffusion through the outer cladding layers. The carriers move towards equilibrium by emitting optical phonons. In narrow quantum wells, as compared to the 1-2 ps required in bulk semiconductors, this phonon emission process can be considerably slowed down due to the 2-D density of states and the nature of the electron-optical phonon interaction. This process has been studied theoretically using a Monte Carlo program which allows us to see the carrier distribution as a function of time. Typical times for carrier relaxation are 10-15 ps for a 50 Å GaAs well with Al_0.30Ga_0.70As barriers and ~5 pS for a 200 Å well. These calculations have been complemented by time-resolved photoluminescence measurements on SCH structures where the relaxation time from a 3D distribution into In_0.20Ga_0.80As/GaAs wells is measured at T=200 K. Carrier relaxation times of 50, 41, 22, and 17 ps are obtained for wells of sizes 30, 40, 50, and 100 Å, respectively. The results show clearly that the use of narrow quantum wells in low threshold lasers will pose a serious limitation to the efficiency and small-signal modulation bandwidth of these devices     

InAs／GaAs亚单层结构的静压光谱研究

在１５Ｋ下测量了ＩｎＡｓ／ＧａＡｓ亚单层结构的静压光致发光，静压范围为０～８ＧＰａ．常压下ＩｎＡｓ层中重空穴激子的发光峰随ＩｎＡｓ层厚的减小向高能移动，同时峰宽变窄，强度减小．其压力行为与ＧａＡｓ基体的基本一致，表明量子阱（线、点）模型仍适用于ＩｎＡｓ／ＧａＡｓ亚单层结构．得到平均厚度为１／３单分子层的样品中由于附加的横向限制效应引起的电子和空穴束缚能的增加分别为２３和４２ｍｅＶ     

Electron-phonon interaction within an unbiased and biased quantumwell

This paper addresses the electron-phonon interaction when a perpendicular electric field is applied across a quantum well (QW). An applied perpendicular electric field redistributes the one-dimensional density of states in position and energy within a QW. The redistribution of the one-dimensional density of states modifies the calculation of the initial and final states in the electron-phonon interaction. The overall result is to increase the scattering rate between the electrons and phonons while adding additional characteristics to the shape of the phonon scattering time     

光泵调制掺杂阶梯量子阱THz激光器瞬态动力学的Monte Carlo模拟

采用系综Monte Carlo（EMC）方法首次对光泵量子阱THz激光器的载流子瞬态动力学进行了分析。提出的器件原型为三能级调制掺杂GaAs/GaxAl(1-x)As系列非对称阶梯量子阱,激射频率为6.1THz。模拟中包括了电子-电子、电子-光学声子和电子-声学声子等散射机制,采用调制掺杂以得到较高电子密度可以忽略电子-电离杂质散射。已报道的研究工作都是在量子阱中掺杂,而对于这种器件原型能否得到电子布居反转,报道的结果也是相互矛盾。器件原型在温度为77K,光泵强度达到一定值时可以得到电子布居反转,所得到的研究结果对相关的实验研究具有一定的指导意义。     

Electrons and phonons in quantum wells

The modulation of electron and polar optical phonon states in an AlGaAs/GaAs/AlGaAs quantum well (QW) with an inserted thin AlAs barrier is considered. The OW width dependence of electron-phonon scattering rates are estimated. The large contribution to the change of the electron subband population, the photovoltaic effect, and the electron mobility in the QW accounts for the resonant intersubband scattering of electrons by interface phonons. The decrease of electron mobility limited by polar optical phonon scattering with increasing carrier concentration in the QW is established. The conditions for the increase of mobility in the QW by inserting the AlAs barrier are found. Fiz. Tekh. Poluprovodn. 33, 1049–1053 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Coupling of electron states in the InAs/GaAs quantum dot molecule

Deep level transient spectroscopy (DLTS) is used to study electron emission from the states in the system of vertically correlated InAs quantum dots in the p-n InAs/GaAs heterostructures, in relation to the thickness of the GaAs spacer between the two layers of InAs quantum dots and to the reverse-bias voltage. It is established that, with the 100 Å GaAs spacer, the InAs/GaAs heterostructure manifests itself as a system of uncoupled quantum dots. The DLTS spectra of such structures exhibit two peaks that are defined by the ground state and the excited state of an individual quantum dot, with energy levels slightly shifted (by 1–2 eV), due to the Stark effect. For the InAs/GaAs heterostructure with two layers of InAs quantum dots separated by the 40 Å GaAs spacer, it is found that the quantum dots are in the molecule-type phase. Hybridization of the electron states of two closely located quantum dots results in the splitting of the levels into bonding and antibonding levels corresponding to the electron ground states and excited states of the 1s ⁺, 1s ^?, 2p ⁺, 2p ^?, and 3d ⁺ types. These states manifest themselves as five peaks in the DLTS spectra. For these quantum states, a large Stark shift of energy levels (10–40 meV) and crossing of the dependences of the energy on the electric field are observed. The structures with vertically correlated quantum dots are grown by molecular beam epitaxy, with self-assembling effects.     

量子限制效应对受主跃迁的影响

通过光致发光光谱,研究了量子限制效应对GaAs体材料中均匀掺杂和一系列GaAs/AlAs多量子阱(阱宽范围从30A到200A)中δ-掺杂浅受主杂质铍(Be)原子带间跃迁的影响.实验中所用的样品是利用分子束外延技术生长的均匀掺Be受主的GaAs外延层和一系列在量子阱的中央进行了浅受主Be原子δ-掺杂的GaAs/AlAs多量子阱.在4.2K的低温下,测量了上述样品的光致发光谱,很清楚地观察到了受主束缚激子从基态1S3/2(Γ6)到两个激发态2S3/2(Γ6)和3S3/2(Γ6)的双空穴跃迁.研究发现,随着量子限制效应的增强,受主跃迁能量会增加.对量子限制效应调节受主杂质问跃迁能量的研究,进一步增强了对受主能态可调性的认识,为太赫兹远红外发光器或激光器的研发提供了一种新的途径.     

Study of the optical and structural properties of multi quantum well structures grown on high-index surfaces

The MOVPE overgrowth of high [0 1¯ 1]-oriented ridges confined at sides by facets related to {n 1 1} crystallographic planes is reported. We studied the influence of the side tilt on the thickness of the AlGaAs and GaAs epitaxial layers grown under the condition of the kinetic growth mode. The multi quantum well (MQW) structures were prepared on the sides of ridges tilted at 54.7°, 45° and 30° to (1 0 0). The sidewall surface morphologies before and after epitaxial growth were evaluated and compared. We observed no tendency towards planarization towards a neighbouring high-index crystallographic plane, such as (2 1 1) and (3 1 1). We also showed that the quantum wells of the MQW structure make a smooth transition over the edge between the top surface and the facet as both AlGaAs and GaAs grew at similar rates on the surfaces.     

Optically detected impurity D and D transitions in GaAs quantum wells

We have carried out a far-infrared magnet-optical study on shallow donor states confined in GaAs quantum wells (QWs), applying a recently developed optical detection technique. We have observed, in addition to cyclotron resonance, the 1s → 2p₊ transition of neutral donors (D⁰), and singlet and triplet transitions of negative donor ions (D⁻); the latter observation verifies the existence of D⁻ ions in well-only doped QWs under optical pumping. This is the first observation of optically detected impurity resonances in confined systems and demonstrates the power and utility of this technique for such studies.     

Electronic transport in n- and p-type modulation-doped GaInNAs/GaAs quantum wells

We present electronic transport in n- and p-type modulation-doped GaInNAs/GaAs quantum well structures. The Hall mobility of electrons in the n-type material decreases dramatically with increasing nitrogen composition. The mobility of 2D holes in p-modulation-doped quantum wells is significantly higher than that of 2D electrons in n-modulation-doped material with similar nitrogen concentration. The mobility of 2D electrons is discussed using a S-matrix model for N-related alloy scattering. The results indicate that the electron mobility is intrinsically limited by scattering from nitrogen complexes. The high mobility of 2D holes is explained in terms of negligible effect of nitrogen on valance band and the absence of scattering with localized nitrogen complexes.