Migration-enhanced Molecular Beam Epitaxy Research Articles

Migration-Enhanced Epitaxial Growth of InAs/GaAs Short-Period Superlattices for THz Generation

The low-temperature-grown InGaAs (LT-InGaAs) photoconductive antenna has received great attention for the development of highly compact and integrated cheap THz sources. However, the performance of the LT-InGaAs photoconductive antenna is limited by its low resistivity and mobility. The generated radiated power is much weaker compared to the low-temperature-grown GaAs-based photoconductive antennas. This is mainly caused by the low abundance of excess As in LT-InGaAs with the conventional growth mode, which inevitably gives rise to the formation of As precipitate and alloy scattering after annealing. In this paper, the migration-enhanced molecular beam epitaxy technique is developed to grow high-quality (InAs)m/(GaAs)n short-period superlattices with a sharp interface instead of InGaAs on InP substrate. The improved electron mobility and resistivity at room temperature (RT) are found to be 843 cm2/(V·s) and 1648 ohm/sq, respectively, for the (InAs)m/(GaAs)n short-period superlattice. The band-edge photo-excited carrier lifetime is determined to be ~1.2 ps at RT. The calculated photocurrent intensity, obtained by solving the Maxwell wave equation and the coupled drift–diffusion/Poisson equation using the finite element method, is in good agreement with previously reported results. This work may provide a new approach for the material growth towards high-performance THz photoconductive antennas with high radiation power.

Temperature-dependent carrier dynamics of InP/InGaP quantum structures grown at various growth temperatures using migration-enhanced epitaxy

The optical properties of InP/InGaP quantum structures (QSs) grown using migration-enhanced molecular beam epitaxy at growth temperatures ranging from 440 to 520 °C were investigated using temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopies. As the growth temperature increased from 440 to 520 °C, the PL peak energies originating from quantum dots and quantum dashes were blue-shifted, which is attributed to the intermixing of InP and Ga in the InGaP wetting layer. The structural and luminescence properties of InP/InGaP QSs strongly depend on the growth temperatures. The sample grown at 460 °C exhibited the strongest PL intensity at room temperature. In addition, as the temperature increased from 10 to 220 K, the PL decay time of the sample grown at 460 °C increased up to 100 K and then, gradually decreased up to 220 K. The PL decay times of the sample grown at 520 °C increased up to 60 K and then, rapidly decreased up to 120 K. The strongest PL intensity at room temperature and the longest decay time at 100 K for the sample grown at 460 °C indicate that the temperature of 460 °C is optimal for the formation of the InP/InGaP QSs.

Luminescence properties of InP/InGaP quantum structures grown by using a migration-enhanced epitaxy at different growth temperatures

The optical properties of InP/InGaP quantum structures (QSs) grown by using a migrationenhanced molecular beam epitaxy method have been investigated using temperature (T)-dependent photoluminescence (PL) and time-resolved PL. InP QSs were grown by varying the growth temperature from 440 °C to 520 °C. InP/InGaP QS samples grown at temperatures of 440 °C - 480 °C show typical characteristics of a QS, such as rapid bandgap shrinkage at high T and enhanced PL lifetime at low T while the sample grown at 520 °C exhibits the properties of bulk InP. The growth temperature is found to determine the formation of the InP/InGaP QSs; thus, it significantly affects the structural and the optical properties of the InP/InGaP QSs. The best luminescence properties are demonstrated by the sample grown at 460 °C, indicating an optimum growth temperature of 460 °C.

Photoluminescence Studies of InP/InGaP Quantum Structures Grown by a Migration Enhanced Molecular Beam Epitaxy

InP/InGaP quantum structures (QSs) grown on GaAs substrates by a migration-enhanced molecular beam epitaxy method were studied as a function of growth temperature (T) using photoluminescence (PL) and emissionwavelength-dependent time-resolved PL (TRPL). The growth T were varied from 440℃ to 520℃ for the formation of InP/InGaP QSs. As growth T increases from 440℃ to 520℃, the PL peak position is blue-shifted, the PL intensity increases except for the sample grown at 520℃, and the PL decay becomes fast at 10 K. Emission-wavelengthdependent TRPL results of all QS samples show that the decay times at 10 K are slightly changed, exhibiting the longest time around at the PL peak, while at high T, the decay times increase rapidly with increasing wavelength, indicating carrier relaxation from smaller QSs to larger QSs via wetting layer/barrier. InP/InGaP QS sample grown at 460℃ shows the strongest PL intensity at 300 K and the longest decay time at 10 K, signifying the optimum growth T of 460℃.

Optical Properties of InP/InGaP Quantum Structures Grown by a Migration Enhanced Epitaxy with Different Growth Cycles

InP/InGaP quantum structures (QSs) were grown on GaAs (001) substrates by a migrationenhanced molecular beam epitaxy method. Temperature-dependent photoluminescence (PL) and emission wavelength-dependent time-resolved PL (TRPL) were performed to investigate the optical properties of InP/InGaP QSs as a function of migration enhanced epitaxy (MEE) growth cycles from 2 to 8. One cycle for the growth of InP QS consists of 2-s In and 2-s P supply with an interruption time of 10 s after each source supply. As the MEE growth cycle increases from 2 to 8, the PL peak is redshifted and exhibited different (larger, comparable, or smaller) bandgap shrinkages with increasing temperature compared to that of bulk InP. The PL decay becomes faster with increasing MEE cycles while the PL decay time increases with increasing emission wavelength. These PL and TRPL results are attributed to the different QS density and size/shape caused by the MEE repetition cycles. Therefore, the size and density of InP QSs can be controlled by changing the MEE growth cycles.

Effects of Growth Parameters on the Structural and Optical Properties of InP/InGaP Quantum Structures for 808-nm-Wavelength Emissions

InP/InGaP quantum structures with 808-nm-wavelength emissions were grown on semi-insulating GaAs (100) substrates via migration-enhanced molecular beam epitaxy. The effects of the growth conditions on the structural and optical properties of the InP/InGaP quantum structures were investigated. The scanning electron microscopy and atomic force microscopy images showed that the two-dimensional InP/InGaP quantum structures were transited to one-dimensional structures with an increasing repetition cycle. The photoluminescence spectra showed that the optical properties of the InP/InGaP quantum structures were significantly affected by various migration-enhanced epitaxy repetition numbers and growth temperatures. These results can help improve understanding of the effects of growth parameters on the structural and optical properties of InP/InGaP quantum structures for 808-nm-wavelength emissions.

Temperature dependence of the excitonic energy band gap in In(Ga)As nanostructures

We analyzed the temperature-dependent variation of the peak energies in the photoluminescence of InAs/GaAs quantum dots and InAs/In0.2Ga0.8As/GaAs quantum dots in an asymmetric well. The InAs dots were grown by using migration-enhanced molecular beam epitaxy. The peak emission energies decreased monotonously with increasing temperature over the range of measurements from 13 K to 225 K. The temperature dependence of the peak energy was successfully fitted with two analytic models, namely, the phenomenological Varshni equation and the semi-empirical Fan equation based on electron-phonon statistics, respectively. The physical meaning of the Varshni coefficients is discussed in terms of the phonon energy and the strain in the nanostructures.

Migration Enhanced Molecular-Beam Epitaxy of III-V Quantum Dot Arrays Using Non-Lithographic AAO Template

We report a highly effective method on selective growth of ordered arrays of III-V quantum dots (QDs) on GaAs(100) substrate by migration enhanced molecular-beam epitaxy (ME-MBE) using anodic aluminum oxide (AAO) template. ME-MBE method can enhance the migration and evaporation of group III adatoms on AAO template resulting in highly selective growth with no visible nucleation on the surface of AAO. The shadowing and blocking effects commonly experienced with conventional molecular-beam epitaxy (MBE) are significantly reduced. Uniform arrays of III-V QDs with mean diameter of 60 nm and periodicity of 100 nm are obtained. Our results show that ME-MBE method has potential applications for selective growth of semiconductor hetero- nanostructures.

Temperature-dependent energy band gap variation in self-organized InAs quantum dots

We investigated the temperature-dependent variation of the photoluminescence emission energy of self-organized InAs/GaAs quantum dots (QDs) grown by conventional Stranski-Krastanov (SK) molecular beam epitaxy and migration-enhanced molecular beam epitaxy (MEMBE) and that of MEMBE InAs QDs in a symmetric and an asymmetric In0.2Ga0.8As/GaAs well. The temperature-dependent energy variation of each QD is analyzed in low and high temperature regions, including a sigmoidal behavior of conventional SK quantum dots with the well-known Varshni and semi-empirical Fan models.

Role of magnesium in band gap engineering of sub-monolayer type-II ZnTe quantum dots embedded in ZnSe

Modification of the bandgap of sub-monolayer type-II ZnTe quantum dots (QDs), by means of direct incorporation of magnesium in the QDs, is reported. Nitrogen co-doped QDs embedded in a ZnSe matrix have been grown by a migration-enhanced molecular beam epitaxy technique. Incorporation of Mg in the ZnTe QDs decreases the valence band discontinuity, leading to reduced localization of the holes, which results in a higher electrical conductivity in the samples as deduced from the Hall effect measurements. The type-II alignment of the bands in the QDs is supported by intensity dependent and time-resolved photoluminescence measurements. Hall effect measurements indicate that the material has p-type conductivity with mid-1015 carriers/cm3 and hole mobilities in the 5–50 cm2/V·s range.

Carrier transfer and redistribution dynamics in vertically aligned stacked In0.5Ga0.5As quantum dots with different GaAs spacer thicknesses

We have investigated optical and structural properties of three-stacked InGaAs quantum dot (QD) structure with GaAs spacer thicknesses of 22, 35, and 88 nm (denoted by QD22, QD35, and QD88, respectively) grown by migration-enhanced molecular beam epitaxy. From temperature-dependent photoluminescence (PL) analysis, it is found that thermal carrier redistribution between vertically adjacent QD layers plays an important role as the thickness of GaAs spacer is reduced from 88 to 22 nm. Although the QD sizes of upper layers are quite similar to those of the first bottom layer, the QDs of the upper layers appear to emit at higher energies probably due to different alloy compositions caused by the strain-induced intermixing effect between InGaAs QDs and GaAs barriers with stacking. Especially for QD22 sample, we observed thermally assisted carriers transfer among vertically adjacent QD layers with increasing temperature by using time-resolved PL measurements, which is in good agreement with the temperature dependence of integrated PL intensity and peak energy position.

Effect of Rapid Thermal Annealing on the Electrical Properties of GaAs Schottky Diodes Embedded with Self-Assembled InAs Quantum Dots

After rapid thermal annealing (RTA), deep levels were found to be generated in Au/GaAs Schottky diodes embedded with InAs quantum dots grown by migration enhanced molecular beam epitaxy (MEMBE). From the corner frequency of the 1/f2 part of the low-frequency noise specrtral density, the locations of the deep levels were estimated to be 0.58, 0.61, and 0.66 eV below the conduction band edge for the samples without quantum dot layer, with quantum dot layer and capping layer thickness of 0.8 microm, and with quantum dot layer and the capping layer thickness of 0.4 microm, respectively. RTA also lowered the Schottky barrier height.

Effect of symmetric and asymmetric In0.2Ga0.8As wells on the structural and optical properties of InAs quantum dots grown by migration enhanced molecular beam epitaxy for the application to a 1.3 μm laser diode

In this article, we present an in-depth study of the effects of the structural and optical properties of InAs “dots in an In0.2Ga0.8As well” (DWELL) and InAs “dots in an asymmetric In0.2Ga0.8As well” (asym. DWELL) grown by migration-enhanced molecular beam epitaxy. The energy spacing (ΔE1) between the ground-state and the first-excited-state transitions increases from 66 meV for the DWELL to 73 meV for the asym. DWELL. These results are consistent with ΔE1 measured by photoluminescence excitation and the values of activation energy fitting. The photoluminescence linewidth of the asym. DWELL (40 meV) is narrower than that of the DWELL (70 meV), which shows superior uniformity in the former over the latter. The trends of the properties of the DWELL and the asym. DWELL deduced from the structural properties are in good agreement with those from the optical properties. From the results, it is strongly supported that the asym. DWELL is more suitable for application to long wavelength optical communication than the DWELL counterpart.

Comparison of structural and optical properties of InAs quantum dots grown by migration-enhanced molecular-beam epitaxy and conventional molecular-beam epitaxy

We strongly support Guryanov’s speculation—that a thinner wetting layer is expected with quantum dots (QDs) grown by migration-enhanced epitaxy—with structural and optical measurements. InAs QDs grown by migration-enhanced molecular-beam epitaxy showed a larger size, lower density, ∼40% enhanced uniformity, ∼2 times larger aspect ratio, and a measurement temperature insensitivity of the photoluminescence linewidth compared to QDs grown by conventional molecular-beam epitaxy. The thickness of the wetting layer for the migration-enhanced epitaxial InAs QD (2.1nm) was thinner than that of the counterpart (4.0nm).

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], \([0\overline {11} ]\), and [011] directions leads to the formation of several groups of quantum dots differing in geometric dimensions and electronic structure.

Initial stages of growth of GaAs on silicon (211) substrates by migration-enhanced molecular beam epitaxy

The nucleation of GaAs films grown by the alternate deposition of gallium and arsenic monolayers on Si(211) and (100) substrates has been studied in situ using Auger and energy-loss spectroscopies, and low-energy electron diffraction. Films were observed to become continuous at much smaller thicknesses when grown on (211), as compared to (100), substrates. This implies that the nucleation site density is much greater on the (211) surface, as expected from the very high step density for this orientation. The observed growth was consistent with a layer-by-layer growth mode.

High-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well lasers on Si substrates

A high-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well laser on Si substrates has been demonstrated for the first time by a hybrid growth of migration-enhanced molecular beam epitaxy followed by metalorganic vapor phase epitaxy. The quantum well laser showed an output power of more than 400 mW per facet under pulsed conditions. A room-temperature threshold current of 300 mA was obtained with a differential quantum efficiency of 40% without facet coating. The threshold current density was 550 A/cm2 for a cavity length of 500 μm. These results show the highest peak power reported to date for low-threshold lasers on Si substrates. The full width at half maximum of the far-field pattern parallel to the junction was 6°. Threshold current densities as low as 250 A/cm2 were obtained for lasers on GaAs substrates.

Transient-mode liquid phase epitaxial growth of GaAs on GaAs-coated Si substrates prepared by migration-enhanced molecular beam epitaxy

Planar oxide-maskless growth of GaAs was demonstrated by transient-mode liquid phase epitaxy (TMLPE) on GaAs-coated Si substrates that were prepared by migration-enhanced molecular beam epitaxy (MEMBE). In TMLPE, the cool substrate was brought into contact with hot melts for a short time. A GaAs layer as thick as 30 μm was grown in 10 s. The etch pits observed in TMLPE-grown layers became longer in one direction and decreased in density with increasing the TMLPE epilayer thickness. The density of etch pits in a 20 μm thick layer was approximately 5 × 10 6 cm -2. Strong bandgap emission elliptically polarized with a major axis perpendicular to the surface was observed at about 910 nm, while deep-level emission from the TMLPE/MEMBE GaAs interface was detected at 980 nm. The photoluminescence intensity divided by the carrier concentration of the TMLPE-grown layer was about 270 times larger than that of the MEMBE-grown layer used as a substrate.

High-Power AlGaAs/GaAs DH Stripe Laser Diodes on GaAs-on-Si Prepared by Migration-Enhanced Molecular Beam Epitaxy

We report on high-power low-threshold AlGaAs/GaAs double heterostructure (DH) stripe laser diodes on Si substrates by a hybrid growth of migration-enhanced molecular beam epitaxy (MEMBE) and metalorganic chemical vapor deposition (MOCVD). MEMBE-grown GaAs-on-Si had excellent surface morphology and showed much lower dislocation density than that of normal two-step MBE-grown or in situ annealed layers. The MEMBE growth and material characterization are described in detail. The 6-µm-wide oxide stripe lasers showed the highest peak power of up to 184 mW per facet reported to date, room-temperature pulsed threshold currents as low as 150 mA, and differential quantum efficiencies as high as 30% without mirror facet coating. The intrinsic threshold current density has been estimated to be about 2 kA/cm2 when taking current spreading and lateral diffusion effects into account. For comparison, the room-temperature pulsed threshold current density of 1.1 kA/cm2 was obtained for the broad-area DH laser diode fabricated on GaAs substrates in the same epitaxial deposition.

Growth and characterization of GaAs layers on Si substrates by migration-enhanced molecular beam epitaxy

We first report on migration-enhanced molecular beam epitaxial (MEMBE) growth and characterization of the GaAs layer on Si substrates (GaAs/Si). Excellent surface morphology GaAs layers were successfully grown on (100)Si substrates misoriented 4° toward the [110] direction. The MEMBE growth method is described, and material properties are compared with those of normal two-step MBE-grown or in situ annealed layers. Micrographs of cross-section view transmission electron microscopy (TEM) and scanning surface electron microscopy (SEM) of MEMBE-grown GaAs/Si showed dislocation densities of 1×107 cm−2, over ten times lower than those of normal two-step MBE-grown or in situ annealed layers. AlGaAs/GaAs double heterostructures have been successfully grown on MEMBE GaAs/Si by both metalorganic chemical vapor deposition and liquid phase epitaxy.