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/AlAs super-flat interfaces in GaAs/AlAs and GaAs/(GaAs) (AlAs) quantum wells grown on (4 1 1)A GaAs substrates by MBE

Effectively atomically flat GaAs/AlAs interfaces over a macroscopic area (“super-flat interfaces”) have been realized in GaAs/AlAs and GaAs/(GaAs) (AlAs) quantum wells (QWs) grown on (4 1 1)A GaAs substrates by molecular beam epitaxy (MBE). A single and very sharp photoluminescence (PL) peak was observed at 4.2 K from each GaAs/AlAs or GaAs/(GaAs) (AlAs) QW grown on (4 1 1)A GaAs substrate. The full-width at half-maximum (FWHM) of a PL peak for GaAs/AlAs QW with a well width ( ) of 4.2 nm was 4.7 meV and that for GaAs/(GaAs) (AlAs) QW with a smaller well width of 2.8 nm (3.9 nm) was 7.6 meV (4.6 meV), which are as narrow as that for an individual splitted peak for conventional GaAs/AlAs QWs grown on (1 0 0) GaAs substrates with growth interruption. Furthermore, only one sharp peak was observed for each GaAs/(GaAs) (AlAs) QW on the (4 1 1)A GaAs substrate over the whole area of the wafer (7 7 mm ), in contrast with two- or three-splitted peaks reported for each GaAs/AlAs QW grown on the (1 0 0) GaAs substrate with growth interruption. These results indicate that GaAs/AlAs super-flat interfaces have been realized in GaAs/AlAs and GaAs/(GaAs) (AlAs) QWs grown on the (4 1 1)A GaAs substrates.     

Raman study of the segregation of GaAs/AlAs heterostructures grown by molecular beam epitaxy

The Raman spectra of the optical confined phonons in the GaAs/AlAs ultra-thin layer superlattices grown with different growth conditions were used to determine the compositional profiles and to study the process of segregation at the heterointerfaces. A modified kinetic model was developed in order to calculate the compositional profiles in the samples under investigation. The comparison between the experimentally obtained compositional profiles and those calculated by the kinetic model allowed us to determine the parameters characterizing the segregation. It was shown that the increase of pressure of As acts equivalently to the decrease of the growth temperature, resulting in a more abrupt compositional profile.     

Impurity cyclotron resonance in InGaAs/GaAs superlattice and InGaAs/AlAs superlattice grown on GaAs substrates

Various temperature measurements of cyclotron resonance (CR) under pulsed ultra-high magnetic field up to 160 T were carried out in InGaAs/GaAs superlattice (SL) and InGaAs/AlAs SL samples grown by molecular beam epitaxy on GaAs substrates. Clear free-electron CR and impurity CR signals were observed in transmission of CO₂ laser with wavelength of 10.6 μm. A binding energy of impurities in these SLs was roughly estimated based on the experiment as result, and we found it was smaller than the previous experimental result of GaAs/AlAs SLs and theoretical calculation with a simple model.     

SiO/SiO2超晶格结构界面发光的研究

利用热蒸发技术在硅衬底上制备了层厚不同的SiO/SiO2超晶格样品.对其光致发光谱进行研究发现,随着SiO/SiO2超晶格中SiO层厚度的增加,发光峰在400～600 nm之间移动.研究表明,样品的发光中心来自于SiO/SiO2界面处的缺陷发光(界面态发光).即在样品沉积的过程中,在SiO/SiO2的界面处由于晶格的不连续性,会形成大量的Si-O悬挂键,这些悬挂键本身相互结合可以形成一定数量的缺陷,同时由于O原子容易脱离Si原子的束缚而产生扩散,因此,这些悬挂键可以与扩散的O原子结合,随着SiO层厚度的增加,在SiO/SiO2的界面处先后出现WOB(O3<≡Si-O-O·),NOV(O3≡Si-Si≡O3),E'中心(O≡Si·),NBOHC(O3≡Si-O·)等缺陷,这些缺陷在SiO层厚度增大的过程中对发光先后起到主导作用,从而使得发光峰产生红移.     

Magneto-oscillation of mid-gap photoluminescence in AlAs:Yb/GaAs superlattices

On AlAs:Yb/GaAs superlattice samples, we measured photoluminescence (PL) spectra including their temperature dependence, magnetic field dependence and resistance up to 25 T. In case of selective excitation of well layers, two broad band PLs were observed in additional to the exception of intra-4f PL from Yb. These peaks show oscillatory behavior similar to that of two-dimensional electron system. From the periods of the oscillation, the electron densities are estimated of the order of which are cannot be archived by usual photoexcitation. It was found that the electron density shows a linear dependence on the excitation energy. To explain such distinctive phenomena, we proposed a new model where Yb ions form hole traps in AlAs.     

Far-infrared absorption studies of Be acceptors in δ-doped GaAs/AlAs multiple quantum wells

We report a far-infrared absorption study of internal transitions of shallow Be acceptors in both bulk GaAs and a series of δ-doped GaAs/AlAs multiple quantum well samples with well thicknesses of 20, 15 and 10 nm. Low temperature far-infrared absorption measurements clearly show three principal absorption lines due to transitions of Beacceptor states from the ground state to the first three odd-parity excited states, respectively. Using a variational principle, the 2p-1s transition energies of quantum confined Be acceptors are calculated as a function of the well width. It is found that the theoretical calculation of the 2p_z → 1s transitions is in good agreement with the D-like line experimental data.     

Structure and photoluminescence study of type-II GaAs quantum wires and dots grown on nano-faceted (3 1 1)A surface

(3 1 1)A GaAs/AlAs corrugated superlattices (CSLs) and satellite (3 1 1)B and (1 0 0) SLs were studied using Raman spectroscopy, high-resolution transmittance electron microscopy (HRTEM) and photoluminescence (PL). The thickness of GaAs layers was varied from 1 monolayer (ML) to 10 ML, the thickness of AlAs barriers was 10 ML in (3 1 1) direction. The strongest modification of the Raman spectra is found for the case of partial (<1 nm) GaAs filling of the AlAs surface. The calculated and experimental Raman spectra demonstrated a good agreement for both complete (1 nm) and partial (<1 nm) GaAs filling of the AlAs surface. According to Raman and HRTEM data, in the case of partial filling of (3 1 1)A AlAs surface, GaAs forms quantum well wires of finite length (quantum dots). A drastic difference of PL from grown side-by-side (3 1 1)A and (3 1 1)B SLs was observed. A strong room temperature PL in the green–yellow spectral region was observed in GaAs/AlAs (3 1 1)A CSLs containing GaAs type-II quantum dots.     

Selectivity control of Stark-ladder transitions in asymmetric double-well GaAs/AlAs superlattices by barrier sequence modulation

We have investigated field-induced features of optical transitions in asymmetric double-well superlattices (ADW-SLs) with and without barrier sequence modulation by low-temperature photocurrent (PC) spectroscopy. The difference between the heavy-hole confinement energies in the wide and narrow wells results in two types of Stark-ladder transitions, so that four ±1st-order spatially indirect transitions are expected to exist in the ADW-SL. The oscillator strength of these indirect transitions is affected by the strength and the direction of coupling between the wide and narrow wells. The introduction of the barrier sequence modulation into the ADW-SL realizes the control of these coupling mechanisms. This technique is a new method to modulate the oscillator strength of the indirect Stark-ladder transitions.     

高组分稀磁半导体Cd1-xMnxTe/CdTe超晶格的荧光谱研究

报道用分子束外延(MBE)技术生长的x=0.4,0.8的高组分稀磁半导体Cd1-xMnxTe/CdTe超晶格低温和室温荧光谱研究结果.基态激子跃迁能级荧光谱实验结果显示高组分超晶格中具有高量子效率和高质量光发射.对激子能级随温度的变化进行了详细研究,给出激子跃迁能量的温度系数.激子能级线型的展宽随温度变化关系可用激子-纵向光学声子耦合模型解释.与光调制反射谱实验结果进行了比较.     

Structural, optical and intraband absorption properties of vertically aligned In0.32Ga0.68As/GaAs quantum dots superlattices

The self-organization growth of In_0.32Ga_0.68As/GaAs quantum dots (QDs) superlattices is investigated by molecular beam epitaxy. It is found that high growth temperature and low growth rate are favorable for the formation of perfect vertically aligned QDs superlattices. The aspect ratio (height versus diameter) of QD increases from 0.16 to 0.23 with increase number of bi-layer. We propose that this shape change play a significant role to improve the uniformity of QDs superlattices. Features in the variable temperature photoluminescence characteristics indicate the high uniformity of the QDs. Strong infrared absorption in the 8–12 μm was observed. Our results suggest the promising applications of QDs in normal sensitive infrared photodetectors.     

Cyclotron resonance in (GaAs)n/(AlAs)n superlattices under ultra-high magnetic fields

We present a review on recent study of the type I to type II transition in short-period superlattices (SLs) of GaAs/AlAs by means of cyclotron resonance (CR) in pulsed high magnetic fields. The behavior of CR varies depending on the thickness of the GaAs and AlAs layers. In CR of (GaAs)_n/(AlAs)_n, the resonance peak at the X minima was observed in the type II regime for n smaller than 14, whereas the resonance at the Γ point was observed for n>15. We estimated electron masses on X and Γ point in the SLs by using the empirical sp³ tight-binding method including second-nearest-neighbor interaction. These calculations have shown good agreement with the experimental results. Moreover, it was found that the angular dependence of the CR peak position does not obey the simple cosine dependence due to the subband mixing in high magnetic fields. From the angular dependence in the SLs, the longitudinal and transverses electron masses of AlAs at the X point were deduced to be m_t=0.21m₀ and m_l=1.04m₀, respectively.     

A theoretical study on the electronic structure of GaAs/AlAs and AlAs/GaAs core/shell nanoparticles

Results of theoretical investigation on the structural and electronic properties of GaAs/AlAs and AlAs/GaAs core/shell nanoparticles are presented. We have considered relaxed structures of essentially spherical parts of the zinc-blende crystal structure. The electronic properties and the total energy were calculated using density-functional tight-binding method. Our results include the charge distribution, density of states (DOSs), electronic energy levels (in particular HOMO and LUMO), HOMO-LUMO gap, excitation spectra and their variation with shell thickness for both GaAs/AlAs and AlAs/GaAs core/shell systems.     

GaAs/AlAs量子阱中受主束缚能和光致发光

从实验和理论上,研究了量子限制效应对GaAs/AlAs多量子阱中受主对重空穴束缚能的影响。实验中所用的样品是通过分子束外延生长的一系列GaAs/AlAs多量子阱,量子阱宽度为3~20nm,并且在量子阱中央进行了浅受主Be原子的δ-掺杂。在4,20,40,80,120K不同温度下,分别对上述样品进行了光致发光谱测量,观察到了受主束缚激子从基态到激发态的两空穴跃迁,并且从实验上测得了在不同量子阱宽度下受主的束缚能。理论上应用量子力学中的变分原理,数值计算了受主对重空穴束缚能随量子阱宽度的变化关系,比较发现,理论计算和实验结果符合地较好。     

Dependence on temperature of homogeneous broadening of InGaAs/InAs/GaAs quantum dot fundamental transitions

We report systematic temperature-dependent measurements of photoluminescence spectra in self-assembled InGaAs/InAs/GaAs quantum dots (QDs). We have studied the rise in temperature of the ground-state homogeneous linewidth.A theoretical model is presented and accounts for the phonon-assisted broadening of this transition in individual QD. We have estimated the homogeneous linewidth of an individual QD from PL spectra of self-organized InAs/GaAs QDs by isolating the PL of each individual QD and fitting the narrow line associated with self-organized QDs through a Lorentzian convoluted by a Gaussian. We have observed a strong exciton–LO–phonon coupling (γ_LO) which becomes the dominating contribution to the linewidth above the temperature of 45 K. We have also derived the activation energy (ΔE) of the exciton–LO–phonon coupling, zero temperature linewidth (Γ₀) and the exciton-LA-phonon coupling parameter (γ_Ac). We report that our values are close to the values found in the literature for single InGaAs QD and InAs QD.     

The effect of layer’s thickness on the optical response of superlattices

We apply the Theory of Finite Periodic Systems to study the photoluminescent behavior of superlattices. Using this formalism, where intra-subband energy levels are neatly identified, we study the optical response for different barrier and well thicknesses, and show that this quantity is very sensitive to small changes in the superlattice parameters. Varying the barrier thickness (for fixed well-width), or the well-width (for constant barrier thickness), the intensities of the relevant PL-peaks change drastically. We make clear also that this highly accurate formalism can be used to design optimal superlattice-optoelectronic devices.     

Tunability of optical gain (SWIR region) in type-II In0.70Ga0.30As/GaAs0.40Sb0.60 nano-heterostructure under high pressure

The interest in applying an external pressure on a nano-heterostructure is to attempt to extract more information about the electronic structure through distortion of the electronic structure. This paper reports the tunability of the optical gain under the high pressure effect in M-shaped type-II In_0.70Ga_0.30As/GaAs_0.40Sb_0.60 symmetric lasing nano-heterostructure designed for SWIR generation. In order to simulate the optical gain, the heterostructure has been modeled with the help of six band k.p method. The 6×6 diagonalized k.p Hamiltonian has been solved to evaluate the valence sub-bands (i.e. light and heavy hole energies); and then optical matrix elements and optical gain within TE (Transverse Electric) mode has been calculated. For the injected carrier density of 5×10¹²/cm², the optimized optical gain within TE mode is as high as ~9000/cm at the wavelength of ~1.95 µm, thus providing a very important alternative material system for the generation of SWIR wavelength region. The application of very high pressure (2, 5 and 8 GPa) on the structure along [110] direction shows that the gain as well as lasing wavelength both approach to higher values. Thus, the structure can be tuned externally by the application of high pressure within the SWIR region.     

Oscillator strength of type-II light-hole exciton in InxGa1−xAs/GaAs strained single quantum wells

We have investigated the excitonic properties of In_0.15Ga_0.85As/GaAs strained single quantum wells by using photoreflectance spectroscopy and a variational calculation method. We clearly detected the photoreflectance signal of the type-II light-hole exciton, which consists of an electron confined in the InGaAs layer and a light hole located in the thick GaAs layer, in addition to the type-I heavy-hole exciton confined in the InGaAs layer. The calculated results of the overlap integral of the envelope function in the type-II light-hole exciton predict that the oscillator strength is remarkably enhanced with decreasing the InGaAs-layer thickness. This is demonstrated by the layer-thickness dependence of the photoreflectance intensity of the type-II light-hole exciton.     

温度对Al0.5Ga0.5As/AlAs分布布喇格反射器的反射谱影响

采用光学传输矩阵理论对Al0.5Ga0.5As/AlAs材料分布布喇格反射器(DBR)进行理论研究,分析了－10℃到100℃的范围内,温度变化对不同DBR结构的反射光谱影响.结果表明：随着温度的升高,传统20周期DBR的反射光谱向长波长方向移动,速率约0.05 nm/℃,其中由线热膨胀系数带来的影响小于0.001 nm/℃.当传统DBR的周期数增大时,温度对DBR光谱反射率的影响在减小,同时DBR的反射谱峰值波长发生红移.为了降低温度对DBR反射光谱的影响,提出一种新型的复式DBR结构.分析指出：该复式DBR比传统DBR有更大的反射光谱半峰宽,基本能覆盖同温度的AlGaInP LED电致发光光谱,这对提高LED的出光效率有现实意义.