GaAs-based long-wavelength InAs bilayer quantum dots grown by molecular beam epitaxy

Molecular beam epitaxy growth of a bilayer stacked InAs/GaAs quantum dot structure on a pure GaAs matrix has been systemically investigated.The influence of growth temperature and the InAs deposition of both layers on the optical properties and morphologies of the bilayer quantum dot（BQD） structures is discussed.By optimizing the growth parameters,InAs BQD emission at 1.436μm at room temperature with a narrower FWHM of 27 meV was demonstrated.The density of QDs in the second layer is around 9×10⁹ to 1.4×10¹⁰ cm^-2. The BQD structure provides a useful way to extend the emission wavelength of GaAs-based material for quantum functional devices.     

GaAs基长波长InAs耦合量子点材料的分子束外延生长

系统介绍了利用分子束外延方法在纯GaAs材料上生长InAs/GaAs耦合量子点结构。讨论了生长温度和上下两层量子点中InAs的淀积量对于材料发光性质和表面形貌的影响。通过优化生长参数,得到了室温发光波长在1.436μm,FWHM为27meV的耦合量子点材料。第二层量子点的密度在910⁹到1.410¹⁰cm^-2之间。耦合量子点结构为拓展GaAs基材料在量子功能器件应用中的发光波长提供了新的途径。     

Self-organization of quantum dots in multilayer InAs/GaAs and InGaAs/GaAs structures in submonolayer epitaxy

The experimental results of RHEED and scanning tunneling microscopy investigations of multilayer structures of InGaAs/GaAs quantum dots, obtained by submonolayer epitaxy on singular and vicinal GaAS (100) substrates, are reported. The results presented show that spatial ordering of nano-objects exists in multilayer structures for InAs and heteroepitaxial InGaAs layers. Fiz. Tekh. Poluprovodn. 33, 733–737 (June 1999)     

Structural and optical properties of InAs quantum dots grown by molecular beam epitaxy

In this work, we have studied the dependence of the size and luminescence of self-assembled InAs quantum dots (SAQDs) on the growth conditions. The SAQDs were grown on GaAs (1 0 0) substrates by molecular beam epitaxy (MBE). Their structural and optical properties were studied by atomic force microscopy (AFM), and photoluminescence spectroscopy (PL). The growth of the InAs SAQDs was in situ monitored by reflection high-energy electron diffraction (RHEED). The shape and size of the InAs SAQDs were significantly affected by the growth temperature and the arsenic over-pressure. We observe a decrease of the SAQDS density and an increase in their height by increasing the growth temperature, and/or decreasing the arsenic over-pressure. This is accompanied by a remarkable red-shift of the PL emission energy from 1.3 to .     

Effects of growth interruption during the formation of InAs/GaAs self-assembled quantum dots grown by molecular beam epitaxy

We have investigated the structural and optical characteristics of InAs quantum dots (QDs) during growth interruption using conventional solid source molecular beam epitaxy. Size of the QDs decreased and size uniformity improved by the introduction of a growth interruption. The average height of the QDs increased while the average width decreased as the interruption time was extended from 0 to 32 s. At longer than 32 s, the average height became almost saturated, but the standard deviation of the height was decreased. These results showed that the indium adatoms around the bottom of the QDs were moved to the top of them. And, self-limiting behavior occurred when the interruption time was longer than 32 s. Low-temperature photoluminescence (PL) emissions from the capped QD structure were shifted to red-side by increasing the average height. On the other hand, both intensity and symmetry of the PL peak were improved because, the height of the QD became uniform by growth interruption.     

The effect of misorientation of the GaAs substrate on the properties of InAs quantum dots grown by low-temperature molecular beam epitaxy

The structural and optical properties of arrays of InAs quantum dots grown on GaAs substrates at relatively low temperatures (250 and 350°C) and with various degrees of misorientation of the surface are studied. It is shown that low-temperature growth is accompanied by the formation of quantum dot clusters along the dislocation loops on the singular surface and along the steps caused by the surface vicinality on the misoriented surface. The formation of quantum dot clusters leads to the appearance of a new long-wavelength band in the exciton photoluminescence (PL) spectra. It is found that the degrees of polarization of the PL spectral band for clusters of various shapes are different.     

Planar isolated GaAs devices produced by molecular beam epitaxy

This paper reports the fabrication of low parasitic capacitance planar beam-leaded structures by means of a novel new technology. The structure is produced by the simultaneous deposition of single crystal and semi-insulating polycrystalline gallium arsenide utilizing the molecular beam epitaxial process. Schottky barrier millimeter wave mixer diodes fabricated with the polycrystalline isolated material have shown normal dc characteristics and substantially reduced parasitic capacitance. Devices measured in a double-balanced downconverter circuit showed a conversion loss of 5.3 dB at 51.5 GHz and 8.5 dB at 103 GHz. This is in excellent agreement with the theoretical conversion loss predicted from the dc characteristics. These devices exceed the performance of structurally identical devices fabricated on conventional n on n⁺material by about 2 dB. Since layers of virtually any desired doping concentration can be produced, the material is potentially applicable to a wide range of devices.     

Luminescence properties of InAs/GaAs quantum dots prepared by submonolayer migration-stimulated epitaxy

The results of an investigation of the luminescence properties of an ensemble of InAs quantum dots, obtained by submonolayer migration-stimulated epitaxy on singular and vicinal GaAs(100) surfaces, are reported. The largest width at half-height of the photoluminescence line is observed in samples with a 3° disorientation, indicating that the size-variance of the quantum dots is largest in this case. Quasiequilibrium quantum dots are formed either with a long sample holding time in an arsenic flow or with a larger quantity of deposited indium. Fiz. Tekh. Poluprovodn. 31, 912–915 (August 1997)     

Deep-level transient spectroscopy in InAs/GaAs laser structures with vertically coupled quantum dots

Indium arsenide/gallium arsenide structures with vertically coupled quantum dots imbedded in the active zone of a laser diode are investigated by deep-level transient spectroscopy (DLTS), and the capacitance-voltage characteristics are analyzed. The DLTS spectrum was found to undergo significant changes, depending on the temperature of preliminary isochronous annealing of the sample, T _a <T _ac =245 K or T _a >T _ac , and on the cooling conditions, with a bias voltage V _b =0 or with an applied carrier pulse V _f >0. The changes are attributed to the onset of Coulomb interaction of carriers trapped in a quantum dot with point defects localized in the nearest neighborhoods of the quantum dots and also to the formation of a dipole when T _a <T _ac and cooling takes place with V _f >0, or to the absence of a dipole when T _a >T _ac and V _b =0. It is discovered that the tunneling of carriers from the deeper states of defects to the shallower states of quantum dots takes place in the dipole, and the carriers are subsequently emitted from the dots into bands. Fiz. Tekh. Poluprovodn. 31, 1249–1255 (October 1997)     

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.     

Ordered InAs nanodots formed on the patterned GaAs substrate by molecular beam epitaxy

Ordered indium arsenide (InAs) nanodots are formed by molecular beam epitaxy (MBE) on patterned gallium arsenide (GaAs) substrates, which are prepared by implanting manganese (Mn) ions through anodic aluminum oxide (AAO) membranes into the GaAs wafers. Morphology and structure of the patterned GaAs substrate is determined both by the oxygen desorption and the Mn ion diffusion. Suitable patterned GaAs substrates with the same dosage of Mn ions for the following epitaxy can be obtained by controlling the deoxidization As₄ pressures during the oxygen desorption. Images of samples with different Mn ion implantation dosages and different molecular beam epitaxial conditions for the following deposition of InAs nanodots on the patterned GaAs substrates are characterized by atomic force microscopy (AFM). The order of the InAs nanodots is determined both by the AAO membrane and dosage of Mn ions. The density of InAs nanodots has great relation to the pore density of the AAO.     

GaAs MESFET's by molecular beam epitaxy

The noise performance of "T" shaped Ti/W/Au gate GaAs Schottky-barrier field-effect transistors fabricated on channel layers grown by molecular-beam epitaxy (MBE) is reported. The nominal gate length was about 0.7 µm with a total gate width of 250 µm. Typical noise figure and the associated gain were 1.2 and 14 dB at 4 GHz, and 1.9 and 8.5 dB at 12 GHz. To out knowledge these are the best results reported to date on devices fabricated using MBE-grown GaAs. These preliminary results show the promise of MBE for high-quality GaAs FET's.     

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)     

Electron microscopy of GaAs Structures with InAs and as quantum dots

An electron-microscopy study of GaAs structures, grown by molecular-beam epitaxy, containing two coupled layers of InAs semiconductor quantum dots (QDs) overgrown with a thin buffer GaAs layer and a layer of low-temperature-grown gallium arsenide has been performed. In subsequent annealing, an array of As nanoinclusions (metallic QDs) was formed in the low-temperature-grown GaAs layer. The variation in the microstructure of the samples during temperature and annealing conditions was examined. It was found that, at comparatively low annealing temperatures (400–500°C), the formation of the As metallic QDs array weakly depends on whether InAs semiconductor QDs are present in the preceding layers or not. In this case, the As metallic QDs have a characteristic size of about 2–3 nm upon annealing at 400°C and 4–5 nm upon annealing at 500°C for 15 min. Annealing at 600°C for 15 min in the growth setup leads to a coarsening of the As metallic QDs to 8–9 nm and to the formation of groups of such QDs in the area of the low-temperature-grown GaAs which is directly adjacent to the buffer layer separating the InAs semiconductor QDs. A more prolonged annealing at an elevated temperature (760°C) in an atmosphere of hydrogen causes a further increase in the As metallic QDs’ size to 20–25 nm and their spatial displacement into the region between the coupled InAs semiconductor QDs.     

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.     

Electron microscopy of GaAs-based structures with InAs and As quantum dots separated by an AlAs barrier

Electron microscopy studies of GaAs-based structures grown by molecular beam epitaxy and containing arrays of semiconductor InAs quantum dots and metal As quantum dots are performed. The array of InAs quantum dots is formed by the Stranski-Krastanov mechanism and consists of vertically coupled pairs of quantum dots separated by a GaAs spacer 10 nm thick. To separate the arrays of semiconductor and metal quantum dots and to prevent diffusion-induced mixing, the array of InAs quantum dots is overgrown with an AlAs barrier layer 5 or 10 nm thick, after which a GaAs layer is grown at a comparatively low temperature (180°C). The array of As quantum dots is formed in an As-enriched layer of the low-temperature GaAs by means of post-growth annealing at 400–760°C for 15 min. It is established that the AlAs barrier layer has a surface profile corresponding to that of a subbarrier layer with InAs quantum dots. The presence of such a profile causes the formation of V-shaped structural defects upon subsequent overgrowth with the GaAs layer. Besides, it was obtained that AlAs layer is thinned over the InAs quantum dots tops. It is shown that the AlAs barrier layer in the regions between the InAs quantum dots effectively prevents the starting diffusion of excess As at annealing temperatures up to 600°C. However, the concentration of mechanical stresses and the reduced thickness of the AlAs barrier layer near the tops of the InAs quantum dots lead to local barrier breakthroughs and the diffusion of As quantum dots into the region of coupled pairs of InAs quantum dots at higher annealing temperatures.     

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.     

Fabrication of GaAs quantum wire structure using metal organic molecular beam epitaxy

GaAs quantum wires (100*20 nm/sup 2/) buried in AlAs layers have been successfully fabricated using metal organic molecular beam epitaxy (MOMBE) for the first time. The underlying growth mechanism is that, under appropriate As/sub 4/ pressure in MOMBE, GaAs preferentially grows only on the sidewalls of the patterned     

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.     

Local structures around Er atoms doped in GaAs by low-temperature molecular beam epitaxy

Er-doped GaAs samples were grown by molecular beam epitaxy at 400°C and 590°C. The samples were investigated by photoluminescence (PL) and fluorescence extended X-ray absorption fine structure (EXAFS) measurements in order to find the relationship between the local structures around the Er atoms and the PL properties. Er-related PL peaks were observed only for the samples grown at a low temperature (400°C). The fluorescence EXAFS measurements showed the Er atoms were located on tetrahedral interstitial site when the growth temperature was low. On the other hand, when the growth temperature was high (590°C), the Er atoms substituted the Ga site in GaAs lattice.