Photoluminescence of InAs quantum dots grown on disoriented GaAs substrates

The photoluminescence spectra in an external magnetic field of an ensemble of InAs quantum dots grown by molecular beam epitaxy on a (001) GaAs substrate with a disorientation in the [010] direction are studied. A redistribution of the photoexcited carriers among different groups of dots under the influence of the magnetic field is observed. The concentration of quantum dots is determined by analyzing the data. Fiz. Tekh. Poluprovodn. 33, 1084–1087 (September 1999)     

Built-in electric fields in InAs quantum dots grown on (N11) GaAs substrates

We report on the optical properties and carrier kinetics of a set of InAs self-assembled quantum dots on (N11)A/B GaAs substrates by means of cw and time-resolved PL. The cw-PL spectra show a blue shift of the PL band on different (N11) QD structures when increasing the carrier photoinjection. This is attributed to a photoinduced screening of the quantum confined Stark shift of the QD optical transition due to a large built-in electric field. The presence of an internal electric field also induces intrinsic optical non-linearity in time-resolved measurements. The analysis of the recombination kinetics shows that the carrier screening occurs inside the QDs, thus demonstrating the intrinsic nature of the built-in field. The dependence of the internal field on the substrate orientation and termination agrees with the presence of piezoelectric field and permanent dipole moment inside the QDs.     

InAs/GaAs quantum dots grown on different GaAs substrates with graded InxGa1-xAs strain-reducing layer

Self-assembled InAs quantum dots with graded composition strain-reducing layer (SRL) grown on exact substrates were studied. It is shown that a graded InxGa1-xAs SRL leads to growth quality improvement, emission efficiency enhancement, and wavelength blueshift. Samples grown on 2° misoriented substrates with different In contents in graded InxGa1-xAs SRL were also investigated, and emission efficiency enhancement and wavelength blueshift were found when graded SRL was introduced and when the change rate of In content in graded InxGa1-xAs SRL was enlarged.     

Wannier-Stark states in a superlattice of InAs/GaAs quantum dots

Electron and hole emission from states of a ten-layer system of tunneling-coupled vertically correlated InAs/GaAs quantum dots (QDs) is studied experimentally by capacitance—voltage measurements and deep-level transient spectroscopy. The thickness of GaAs interlayers separating sheets of InAs QDs was ≈3 nm, as determined from transmission electron microscope images. It is found that the periodic multimo-dal DLTS spectrum of this structure exhibits a pronounced linear shift as the reverse-bias voltage U _r applied to the structure is varied. The observed behavior is a manifestation of the Wannier—Stark effect in the InAs/GaAs superlattice, where the presence of an external electric field leads to the suppression of coupling between the wave functions of electron states forming the miniband and to the appearance of a series of discrete levels called Wannier—Stark ladder states.     

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.     

Recombination emission from InAs quantum dots grown on vicinal GaAs surfaces

Photoluminescence (PL) spectra of InAs/GaAs heteroepitaxial structures with quantum dots (QDs) have been studied. The structures were grown by submonolayer migration-enhanced epitaxy on vicinal substrates with the amount of deposited InAs close to the critical value of 1.8 monolayer (ML). The origin and evolution of the structure of PL spectra were studied in relation to the direction and angle of misorientation, temperature, and power density and spectrum of the exciting radiation. A blue shift and narrowing of the PL band with increasing misorientation angle was established experimentally. The fact that QDs become smaller and more uniform in size is explained in terms of a lateral confinement of QDs on terraces with account taken of the step bunching effect. The temperature dependences of the positions and full widths at half-maximum (FWHM) of PL bands are fundamentally different for isolated and associated QDs. The exciton ground states contribute to all low-temperature spectral components. The excited exciton state contributes to the recombination emission from QDs, as evidenced by the temperature dependence of the integrated intensity of the PL bands. A quantitative estimate is given of the electronic structure of different families of InAs QDs grown on GaAs substrates misoriented by 7° in the [001] direction.     

InAs/InGaSb photodetectors grown on GaAs bonded substrates

The results of wafer fusion between GaAs and InP followed by transfer of an InGaAs film from the InP to GaAs substrate are presented in this paper. This technique of film transfer allowed the subsequent growth of epitaxial materials with approximately 7% lattice mismatch. Type-II InAs/GaInSb superlattices photodetectors of different designs have been grown by molecular beam epitaxy (MBE) on the alternative InGaAs/GaAs substrate and on standard GaSb substrates. Comparison between photodetectors grown on the two different substrates with nearly identical superlattice periods showed a shift in the cut-off wavelength. The superlattices grown on the alternative substrates were found to have uniform layers, with broader x-ray linewidths than superlattices grown on GaSb substrates.     

The transition from thermodynamically to kinetically controlled formation of quantum dots in an InAs/GaAs(100) system

The results of experimental and theoretical studies of quantum dot formation in an InAs/GaAs(100) system in the case of a subcritical width of the deposited InAs layer (1.5–1.6 monolayers) are presented. It is shown that, in the subcritical range of InAs thicknesses (smaller than 1.6 monolayers), regardless of the deposition rate, the density of quantum dots increases and their size decreases in response to an increase in surface temperature. In the overcritical range of InAs thicknesses (more than 1.8 monolayers), the density of quantum dots increases and their size decreases in response to a decrease in temperature and an increase in the deposition rate. The observed behavior of quantum dot morphology is attributed to the transition from a thermodynamically to kinetically controlled regime of quantum dot formation near the critical thickness.     

Theoretical study of InAs/GaAs quantum dots grown on [11k] substrates in the presence of a magnetic field

Eight-band k.p theory including strain and piezoelectric effects are employed to calculate the strain distribution and electron and hole energy levels of InAs/GaAs quantum dots grown on [11k] substrates in the presence of an external magnetic field. Height of the dot determines how the increasing of k influences isotropic part of the strain tensor while biaxial part of the strain tensor is always reduced with increasing k. Because of the reduced symmetry of high index surfaces, influence of piezoelectric effect on the electronic structure becomes more dominant with increasing k. Electron energy levels are influenced by the isotropic part of the strain compared to the hole energy levels, where strong heavy hole–light-hole mixing is observed. For the dots grown on the [11k] surfaces magnetic field has smaller influence on the electron and hole energy levels as compared to the referent case.     

Molecular beam epitaxial growth of site-controlled InAs quantum dots on pre-patterned GaAs substrates

Ex situ electron-beam lithography followed by conventional wet etching has been used to pattern small holes 60–150 nm wide, 13 nm deep in GaAs substrates. These holes act as preferential nucleation sites for InAs dot growth during subsequent overgrowth. By varying either the InAs deposition amount or the thickness of a GaAs buffer layer, the occupancy over the patterned sites can be controlled. Comparison with growth on a planar substrate shows that preferential nucleation occurs due to a reduction in the apparent critical thickness above the pattern site; the magnitude of this reduction is dependent on the dimensions of the initial pattern.     

Formation of InAs quantum dots in a GaAs matrix during growth on misoriented substrates

The formation of InAs quantum dots grown by submonolayer migration-enhanced molecular-beam epitaxy on GaAs(100) surfaces with various misorientation angles and directions is investigated. It is shown for the deposition of 2 monolayers (ML) of InAs that increasing the misorientation angle above 3° along the [010], , and [011] directions leads to the formation of several groups of quantum dots differing in geometric dimensions and electronic structure. Fiz. Tekh. Poluprovodn. 32, 95–100 (January 1998)     

Metastable population of self-organized InAs/GaAs quantum dots

In the present work, we report on the investigation of a p-n heterostructure with InAs/GaAs quantum dots (QD) by capacitance-voltage and deep level transient spectroscopy. We have observed controllable and reversible metastable population of the energy states of quantum dots and interface in the structure containing one plane of InAs QDs as a function of temperature of isochronous annealing as well as under bias-on-bias-off cooling conditions and white light illumination. This effect was attributed to the change in the Fermi level position due to the hole capture on self-trapped defects similar to the DX center in GaAs after isochronous annealing and white light illumination.     

Long-wavelength emission from single InAs quantum dots layer grown on porous GaAs substrate

In this paper, we present the growth and photoluminescence (PL) results of InAs quantum dots (QDs) on a p-type porous GaAs (001) substrate. It has been shown that critical layer thickness of InAs overgrowth on porous GaAs has been enhanced compared to that deposited on nominal GaAs. Using porous GaAs substrate, growth interruption and depositing 10 atomic monolayer (ML) In_0.4Ga_0.6As on InAs QDs, photoluminescence measured at 10 K exhibits an emission at 0.739 eV (∼1.67 μm) with an ultranarrow full width at half maximum (FWHM) of 16 meV. This emission represents the longer wavelength obtained up to date to our knowledge and has been attributed to the radiative transition in the InAs QDs.     

基于石墨烯/铟砷量子点/砷化镓异质结新型光电探测器

研究了一种石墨烯/铟砷量子点/砷化镓界面形成的异质结探测器的暗电流特性以及光电响应性质.虽然石墨烯具有很高的电子迁移率,但受限于较低的光子吸收率,使其在光电探测领域的应用受到了限制.而半导体量子点具有量子效率高,光吸收能力强等独特优点.于是利用石墨烯-砷化铟量子点-砷化镓异质结结构制备了一种新型光电探测器.并对该探测器的响应率、I-V特性曲线、暗电流特性、探测率、开关比等关键性能进行了研究.其在637 nm入射光情况下的响应率、探测率以及开关比可分别达到为17. 0 m A/W、2. 3×10~(10)cm Hz~(1/2)W~(-1)和1×10~3.而当入射光为近红外波段的940纳米时,响应率进一步增加到了207 m A/W.同时,还证实了该器件的暗电流、肖特基势垒高度和理想因子对温度的都具有较高的依赖性都较强.     

Structural and photoluminescence properties of low-temperature GaAs grown on GaAs(100) and GaAs(111)A substrates

Undoped, uniformly Si-doped, and δ-Si-doped GaAs layers grown by molecular-beam epitaxy on (100)- and (111)A-oriented GaAs substrates at a temperature of 230°C are studied. The As₄ pressure is varied. The surface roughness of the sample is established by atomic-force microscopy; the crystal quality, by X-ray diffraction measurements; and the energy levels of different defects, by photoluminescence spectroscopy at a temperature of 79 K. It is shown that the crystal structure is more imperfect in the case of GaAs(111)A substrates. The effect of the As₄ flux during growth on the structure of low-temperature GaAs grown on different types of substrates is shown as well.     

Optical properties of GaAs/AlAs superlattices grown on (311)A GaAs surfaces

We present a temperature-dependence photoluminescence of (GaAs)₅/(AlAs)₅ superlattice grown on (311)A-oriented semi-insulating substrate by molecular beam epitaxy. The temperature dependence reveals an anomalous decrease of the PL width, which is explained in terms of phonon-assisted thermal activation of localized excitons.     

Modified fermi-level pinning of the (100) GaAs surface through InAs quantum dots in different stages of overgrowth

The presence of InAs quantum dots on a {100} GaAs surface results in a pronounced increase of the Fermi level pinning energy. Using room-temperature photo-reflectance measurements combined with atomic force microscopy, we find that the presence of the quantum dots results in the Fermi level being pinned approximately ∼250 meV deeper in the bandgap, an effect which is reversed by either removing or overgrowing the dots. Both overgrowth and complete etching of quantum dots results in the disappearance of the polar InAs facets; we explain the change in Fermi level in terms of such facets. We also discuss the phase delay for the InAs related feature of the photoreflectance experiment as a detrapping of photo-generated electron-hole pairs in the dots.     

Pulsed laser annealing of self-organized InAs/GaAs quantum dots

The effect of pulsed laser annealing (PLA), using an excimer laser, on the luminescence efficiency of self-organized InAs/GaAs and In_0.4Ga_0.6As/GaAs quantum dots has been investigated. It is found that such annealing can enhance both the peak and integrated photoluminescence (PL) efficiency of the dots, up to a factor of 5–10 compared to as-grown samples, without any spectral shift of the luminescence spectrum. The improved luminescence is attributed to the annealing of nonradiative point and extended defects in and around the dots.     

Photoelectric properties of GaAs/InAs heterostructures with quantum dots

The capacitive photovoltage and photoconductivity spectra of GaAs/InAs heterostructures with quantum dots is discussed. For these structures, which were fabricated by metallorganic gas-phase epitaxy, the photosensitivity spectrum has a sawtoothed shape in the wavelength range where absorption by the quantum dots takes place, which is characteristic of a δ-function-like density of states function. The spectra also exhibit photosensitivity bands associated with the formation of single-layer InAs quantum wells in the structure. An expression is obtained for the absorption coefficient of an ensemble of quantum dots with a prespecified size distribution. It is shown that the energy distribution of the joint density of states, the surface density of quantum dots, and the effective cross section for trapping a photon can all be determined by analyzing the photosensitivity spectrum based on this assumption. Fiz. Tekh. Poluprovodn. 31, 1100–1105 (September 1997)     

Spatial ordering of self-organized InGaAs/AlGaAs quantum disks on GaAs (311)B substrates

We report the control of self-organization of In_xGa_1−xAs/AlGaAs quantum disks on GaAs (311)B surfaces using a novel technique based upon lithography-defined SiN dot arrays. A strained InGaAs island array selectively grown using the SiN dots provides periodic strain field. When the pitch of lateral ordering corresponds with the period of the strain field, self-organized quantum disks stacked on the InGaAs islands are precisely arranged just as the buried SiN dot array. The spacing of the array element is 250–300 nm (x = 0.3) and around 150 nm (x = 0.4). Vertical alignment by strain is achieved at a very thick (95 nm) separating layer. Characterization using atomic force microscopy reveals the size-fluctuation of disk is dramatically improved with spatial ordering.