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.     

The improvement of InAs/GaAs quantum dot properties capped by Graphene

InAs self‐assembled quantum dots (QDs) were grown by molecular beam epitaxy on (001) GaAs substrate. Uncapped and capped QDs with GaAs and graphene layers were studied using atomic force microscopy and Raman spectroscopy. Graphene multi‐layer was grown by chemical vapor deposition and transferred on InAs/GaAs QDs. It is well known that the presence of a cap layer modifies the size, shape, and density of the QDs. According to the atomic force microscopy study, in contrast to the GaAs capped sample, which induce a dramatic decrease of the density and height of dots, graphene cap layer sample presents a slight influence on the surface morphology and the density of the islands compared with the uncapped one. The difference shown in the Raman spectra of the samples is due to change of strain and alloy disorder effects on the QDs. Residuals strain and the relaxation coefficients have been investigated. All results confirm the best crystalline quality of the graphene cap layer dots sample relative to the GaAs capped one. So graphene can be used to replace GaAs in capping InAs/GaAs dots. To our knowledge, such study has not been carried out until now. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of semiconductor cap layer on the impurity-free vacancy disordering of the In<Subscript>0.2</Subscript>Ga<Subscript>0.8</Subscript>As/GaAs multiquantum-well structure by dielectric capping layers

The feasibility of normal GaAs, low-temperature-grown GaAs (LT-GaAs) and low-temperature-grown InGaAs (LT-InGaAs) as the capping layers for impurity-free vacancy disordering (IFVD) of the In_0.2Ga_0.8As/GaAs multiquantum-well (MQW) structure has been studied. The normal GaAs, LT-GaAs and LT-InGaAs layers were tested as the outermost capping layer and the intermediate cap layer underneath the SiO₂ or Si₃N₄ capping layer. The degree of quantum-well intermixing (QWI) induced by rapid thermal annealing was estimated by the shift of the photoluminescence (PL) peak energy. It was found that the IFVD of the In_0.2Ga_0.8As/GaAs MQW structure using LT-GaAs (LT-InGaAs) as the outermost capping layer was much smaller (larger) than that using a SiO₂ (Si₃N₄) capping layer. It was also observed that the insertion of the normal GaAs, LT-GaAs and LT-InGaAs cap layers below the SiO₂ or Si₃N₄ capping layer reduces the degree of QWI and the PL intensity after the QWI. A plausible explanation for the influence of normal GaAs, LT-GaAs and LT-InGaAs cap layers for the QWI of the InGaAs/GaAs structure is also discussed. PACS 68.55.Ln; 73.20.Dx; 78.55.-m     

Evidence for Fermi level shift in GaInAs/GaAs quantum wells upon nitrogen incorporation

GaInAs/GaAs and GaInNAs/GaAs quantum well (QW) structures grown by metalorganic vapor phase epitaxy have been studied by contactless electroreflectance spectroscopy. In addition to the N-related redshift of QW transitions, an increase in the electric field in the GaAs cap layer has been observed after the incorporation of nitrogen atoms into the GaInAs QW. This observation is associated with the tendency of the Fermi level shift to a given energy in the GaInNAs QW region due to N-related defects.     

不同盖层对InAs/GaAs量子点结构和光学性质的影响

利用金属有机化合物气相沉积设备生长了不同盖层结构的InAs/GaAs量子点,采用原子力显微镜和光致发光光谱仪对量子点的结构和光学性质进行了研究.量子点层之间的盖层由一个低温层和一个高温层组成.对不同材料结构的低温盖层的对比研究表明,In组分渐变的InGaAs低温盖层有利于改善量子点均匀性、减少结合岛数目、提高光致发光强度;当组分渐变InGaAs低温盖层厚度由6.8 nm增加到12 nm,发光波长从1256.0 nm红移到1314.4 nm.另外,还对不同材料结构的高温盖层进行了对比分析,发现高温盖层采用In组分渐变的InGaAs材料有利于光致发光谱强度的提高. 关键词： 半导体量子点 盖层 组分渐变     

Charge transfer features and ferromagnetic order in semiconductor heterostructures δ-doped with manganese

The temperature and field dependences of the specific magnetization and magnetoresistance in heterostructures with a GaAs/Ga_0.84In_0.16As/GaAs quantum well and a δ-layer of atomic Mn in the barrier layer near the quantum well filled with holes are studied. A change in the resistance and magnetization behavior upon ordering of localized magnetic moments in the cap layer due to a change in the manganese ion distribution topology is detected.     

Study of random telegraph signals in a GaAs/AlxGa1 − xAs single electron transistor

A GaAs/Al_xGa_1 − xAs heterointerface contains charge traps, which can capture and emit a single electron stochastically. We study random telegraph signals (RTSs) from these traps using a single electron tunneling (SET) transistor formed in the heterostructure. The output of the SET transistor switches between two states, corresponding to the capture and emission of a single electron. The occurrence of RTS depends on the variation in the potential modified by negatively biased Schottky gates. These gate voltage dependences indicate the spatial positions of traps. Moreover, we investigate the interaction between trappings.     

The combination of QoS,aggregation and RTS/CTS in Very High Throughput IEEE 802.11ac networks

We consider a single WiFi cell in which multiple mobile stations transmit multiple QoS constrained Application flows to the Access Point. We investigate the efficiency of using RTS/CTS in the face of the QoS constraints, in various Access Categories that are defined in the WiFi standard, in a number of transmitting stations and in various amounts of aggregation. We show that using RTS/CTS is efficient in terms of Throughput, when there are collisions and the time to recover these collisions is within the Delay/Loss constraints of the Application flows. It turns out that RTS/CTS is most efficient in the Video and Voice Access Categories and Delay constrains in the order of several tens of milli-seconds. For smaller Delay constraints it is more efficient without RTS/CTS. Thus, we show the existence of cross-points: In Delay constraints smaller than the cross points it is more efficient without RTS/CTS and beyond the cross-points the opposite is true.     

Probing the spin state of a single electron trap by random telegraph signal

We have studied the random telegraph signal (RTS) generated by a single paramagnetic spin center adjacent to a submicrometer silicon metal-oxide-semiconductor field-effect transistor. An in-plane magnetic field induces a substantial change in the statistics of the RTS. We show that a model using the grand partition theorem can qualitatively explain the change in statistics of the RTS as a function of the applied magnetic field. While the data at high temperatures can be well described by this simple model, quantitative discrepancy increases as the temperature is lowered.     

Time resolved magnetization dynamics of ultrathin Fe(001) films: spin-pumping and two-magnon scattering

The time-resolved magnetic response of ultrathin epitaxial Fe(001) films grown on GaAs(001) and covered by Au, Pd, and Cr capping layers was investigated by time and spatially resolved Kerr effect measurements. The magnetization was excited by an in-plane magnetic field pulse using the transient internal field generated at a Schottky barrier while the wavelength of the excitation (resonant mode) was roughly 4 microm. Each of the three cap layers affected the spin relaxation in a unique way. Au cap layers resulted in the bulk Gilbert damping of the Fe film. Pd cap layers caused an additional Gilbert damping due to spin-pump or spin-sink effects. Cr cap layers lead to a strong extrinsic damping which can be described by two-magnon scattering. In this case the strength of the extrinsic damping can be controlled by a field induced shift of the spin wave manifold with respect to the excited k vector.     

Characterization of aluminium concentration in shallow quantum wells AlxGa1−xAs/GaAs types

We report a reflectance study on series of shallow quantum wells GaAs/Al_xGa_1−xAs types with different aluminium concentration. The observed barrier exciton reflectance line shape depends strongly on the shift in aluminium concentration in the two barriers, with the appropriate choice of the cap layer thickness. This observation was based on the reflectivity line shape analysis of anti-Bragg structures.     

Suppression of indefinite peaks in InAs/GaAs quantum dot spectrum by low temperature capping in the indium-flush method

We have investigated effects of growth temperature of thin GaAs capping layer in the initial stage of indium-flush process using atomic force microscopy and microscopic photoluminescence (μ-PL) methods. The shape of capped InAs quantum dot (QD) and its μ-PL properties are sensitive to the growth temperature of thin GaAs capping layer. In the case of the high temperature cap, the QD shape in initial capping stage is elongated along the [1 1 −0] direction, and μ-PL spectrum shows several peaks accompanied with indefinite peaks. On the other hand, the low temperature case, the QD shape is kept in isotropic and μ-PL spectrum shows distinctive emissions from excitonic states of the QD with suppressed indefinite peaks. These results indicate that the low temperature capping is effective to keep an isotropic shape of QD and suppress indefinite peaks.     

Photoluminescence and lasing properties of InAs/GaAs quantum dots grown by metal-organic chemical vapour deposition

Photoluminescence （PL） and lasing properties of InAs/GaAs quantum dots （QDs） with different growth procedures prepared by metalorganic chemical vapour deposition are studied. PL measurements show that the low growth rate QD sample has a larger PL intensity and a narrower PL line width than the high growth rate sample. During rapid thermal annealing, however, the low growth rate sample shows a greater blueshift of PL peak wavelength. This is caused by the larger InAs layer thickness which results from the larger 2-3 dimensional transition critical layer thickness for the QDs in the low-growth-rate sample. A growth technique including growth interruption and in-situ annealing, named indium flush method, is used during the growth of GaAs cap layer, which can flatten the GaAs surface effectively. Though the method results in a blueshift of PL peak wavelength and a broadening of PL line width, it is essential for the fabrication of room temperature working QD lasers.     

Random telegraph signal noise simulation of decanano MOSFETs subject to atomic scale structure variation

As MOSFETs shrink into the decanano regime it is predicted that random telegraph signals (RTS), resulting from trapping events in defect states near the Si/SiO₂ interface, will significantly affect analogue and digital circuit performance. At these same scales, intrinsic parameter fluctuations introduced by atomic differences between devices will also be significant. In this work, a methodology based on 3D simulation is developed which can correctly model RTS noise in the time and frequency domain in the presence of random discrete dopants. The approach is illustrated with results obtained for 30×30  nm devices. We find that atomicity can significantly increase RTS magnitude in devices with particular doping configurations, and ensemble average RTS effects vary markedly from those predicted on an assumption of continuous doping.     

Invited Review Nanoscale devices fabricated by dynamic ploughing with an atomic force microscope

A review is given on the dynamic ploughing technique and its application on the fabrication of nanoscale semiconductor structures. The vibrating tip of an atomic force microscope is used to dynamically plough furrows into a polymer layer of a few nm thickness on top of the semiconductor surface. Wet-chemical etching transfers the desired line pattern. The resulting grooves of 50–100 nm width form an arrangement of barriers in the electron layer of a conventional modulation-doped GaAs/AlGaAs heterostructure. A new type of heterostructure with a compensating p-type doped cap layer shows an electron enhancement if the cap layer is selectively removed. Etching a groove in these structures enables one to induce a one-dimensional electron system. Both types of structures are used to fabricate various ballistic quantum devices and Coulomb-blockade structures.     

Growth of InGaAs-capped InAs quantum dots characterized by Atomic Force Microscope and Scanning Electron Microscope

Atomic force microscopy (AFM) is typically used to measure the quantum dot shape and density formed by lattice mismatched epitaxial growth such as InAs on GaAs. However, AFM images are distorted when two dots are situated in juxtaposition with a distance less than the AFM tip width. Scanning electron Microscope (SEM) is much better in distinguishing the dot density but not the dot height. Through these measurements of the growth of In_xGa_1-xAs cap layer on InAs quantum dots, it was observed that the InGaAs layer neither covered the InAs quantum dots and wetting layer uniformly nor 100% phase separates into InAs and GaAs grown on InAs quantum dots and wetting layer, respectively.     

Study of δ-doped GaAs layers by micro-Raman spectroscopy on bevelled samples

Si δ-doped GaAs layers were studied by micro-Raman spectroscopy. The spectra were recorded along the bevelled structure using the light of Ar⁺-ion laser (514.5 nm line) with high power density. The observed changes in the Raman spectra are discussed in the sense of coupling present between LO phonon and photoexcited electron–hole plasma and plasma of electrons arising from ionised Si atoms. Plasmon-LO-Phonon modes of the coupling of photoexcited plasma in δ-doped GaAs layers were observed for the first time. The minimal thickness of cap layer was estimated in the range of 10–19 nm depending on the doping concentration.     

Microwave irradiation effects on random telegraph signal in a MOSFET

We report on the change of the characteristic times of the random telegraph signal (RTS) in a MOSFET operated under microwave irradiation up to 40 GHz as the microwave field power is raised. The effect is explained by considering the time dependency of the transition probabilities due to a harmonic voltage generated by the microwave field that couples with the wires connecting the MOSFET. The RTS experimental data are in agreement with the prediction obtained with our model.     

Numerical investigation on threading dislocation bending with InAs/GaAs quantum dots

The threading dislocations (TDs) in GaAs/Si epitaxial layers due to the lattice mismatch seriously degrade the performance of the lasers grown on silicon. The insertion of InAs quantum dots (QDs) acting as dislocation filters is a pretty good alternative to solving this problem. In this paper, a finite element method (FEM) is proposed to calculate the critical condition for InAs/GaAs QDs bending TDs into interfacial misfit dislocations (MDs). Making a comparison of elastic strain energy between the two isolated systems, a reasonable result is obtained. The effect of the cap layer thickness and the base width of QDs on TD bending are studied, and the results show that the bending area ratio of single QD (the bending area divided by the area of the QD base) is evidently affected by the two factors. Moreover, we present a method to evaluate the bending capability of single-layer QDs and multi-layer QDs. For the QD with 24-nm base width and 5-nm cap layer thickness, taking the QD density of 10¹¹ cm^-2 into account, the bending area ratio of single-layer QDs (the area of bending TD divided by the area of QD layer) is about 38.71%. With inserting five-layer InAs QDs, the TD density decreases by 91.35%. The results offer the guidelines for designing the QD dislocation filters and provide an important step towards realizing the photonic integration circuits on silicon.     

AFM nanooxidation process - Technology perspective for mesoscopic structures

We study the technology of local anodic oxidation (LAO) by the AFM tip applied to semiconductor heterostructures with two-dimensional electron gas. The aim is to design mesoscopic rings with persistent current and one subband occupied. For this purpose the need is to oxidize narrow lines that represent energy barriers high enough. Using the electrostatic model, we explain the electric field distribution in the system tip-sample just before LAO starts. We study the influence of the conductivity of the cap layer on LAO and explain the origin of the saddle-like profile lines, observed in the experiment. Using Monte Carlo simulation we show that the carrier redistribution in the system with LAO energy barriers effectively lowers the barrier height. In the experimental part we have grown InGaP/AlGaAs/GaAs heterostructures by organometalic vapor phase epitaxy with an active layer only 31 nm below the surface. We have prepared oxide lines on the heterostructures by LAO and characterized them by the temperature-dependent transport measurement.