We report the first application of sublimation molecular beam epitaxy to grow uniformly and selectively doped Si:Er layers with Er concentration up to 5×10^{18} cm^{-3}. The Hall concentration of electrons is about 10% of total Er contents. The mobility is 300-400 cm^{2} V^{-1} s^{-1} at 300 K. All samples exhibit photoluminescence at 1.537 μm up to 100-140 K.
Monocrystalline thin layers of (Eu,Gd)Te, n-type ferromagnetic semiconductor, were grown by molecular beam epitaxy technique on BaF_2 (111) substrates. Reflection high-energy electron diffraction, X-ray diffraction, and atomic force microscopy characterization proved epitaxial mode of growth and high crystal quality of the layers. Magnetic susceptibility and magnetic resonance measurements showed that in (Eu,Gd)Te layers ferromagnetic transition takes place at about 13 K. Electrical characterization carried out by the Hall effect and resistivity measurements revealed very high electron concentration of 10^{20}~cm^{-3} and sharp maximum of resistivity at transition temperature.
Epitaxial LuBa_{2}Cu_{3}O_{7-δ} films were prepared by flash evaporation MOCVD on LaAlO_{3}, SrTiO_{3}, LaSrGaO_{4} and ZrO_{2}(Y_{2}O_{3}) single crystalline substrates. The highest T_{c} and j_{c} (77 K, 100 Oe) values were 89 K, 2.7×10^{6} A/cm^{2} (LaAlO_{3}) and 88 K, 1.5×10^{6} A/cm^{2} (SrTiO_{3}) respectively. The occurrence of secondary phases inclusions in LuBa_{2}Cu_{3}O_{7-δ} films correlates with the possibility of epitaxial relations with the film matrix or the substrate.
Short period type-II 10 ML InAs/10 ML GaSb superlattices epilayers (λ_{cut-off}=5.4 μm) have been grown on near lattice matched GaSb (001) substrate and on lattice mismatched GaAs (001) substrate, by molecular beam epitaxy system. In the case of growing on GaAs substrate, GaSb buffer layer was grown in order to reduce the lattice mismatch of 7.5% between GaAs substrate and InAs/GaSb superlattices. X-ray diffraction characterization shows a good crystalline quality for both samples, with a full width at half maximum of 190 arcsec and 156 arcsec for the zeroth-order peak of the superlattice grown on GaAs and on GaSb substrate, respectively. The Nomarski microscopy revealed a shiny surface for both samples with a root main square of surface roughness of 9 nm and 11 nm on the case of growing on GaSb and GaAs substrate, respectively.
One-dimensional optical waveguide calculations were performed to study the dependence of waveguide design on confinement factor (Γp) and optical losses (α_i) of nitride laser diodes for emission wavelength ranging from 405 nm to 520 nm. We found that the conventional waveguide design containing GaN waveguide and AlGaN cladding layers known from violet laser diode does not support sufficient confinement of the optical mode for long wavelength devices (λ > 450 nm). We proposed a new design consisting of a thick InGaN waveguide which enhances the confinement. We compared the theoretical predictions with laser diodes grown by plasma assisted molecular beam epitaxy.
We show that post growth annealing of GaMnAs under As capping at temperatures in the range of 180-210ºC leads to significant surface modifications. Depending on GaMnAs layer thickness and composition, we obtain either a smooth continuous reacted (MnAs) surface layer or 3D islands (quantum dots). The surface modifications are due to a solid phase epitaxial process, in which Mn interstitials diffusing to the GaMnAs surface are bound with the As.
Results of characterization of AlGaN/GaN high electron mobility transistor (HEMT) structures grown by plasma-assisted molecular beam epitaxy (PAMBE) are reported. High resolution X-ray diffraction (HRXRD) and X-ray reflectivity (XRR) were applied to show that structural properties of the AlGaN/GaN layers strongly depend on the substrate used for growth. It has been found that an additional 10 μm thick HVPE GaN layer grown on a commercial GaN/sapphire substrate significantly improves structural quality of AlGaN layer. However, the best structural parameters have been obtained for the HEMT sample grown on free-standing HVPE bulk GaN substrate.
Current-voltage (I-V) and capacitance-voltage (C-V) characteristics of photovoltaic, thinfilm p-ZnTe/n-CdTe heterojunctions have been studied in the temperature range of 280-400 K. The p-n junctions were grown by MBE on (100) semi-insulating GaAs substrates. From the analysis of I-V and C-V curves the potential barrier height of the junctions and its temperature dependence are determined. The relatively large value of the temperature coefficient of the potential barrier height (2.5-3.0 × 10^{-3} eV/K) indicates a high density of defects at the p-ZnTe/n-CdTe interface. The presence of interface defects limits the efficiency of the solar energy conversion of these devices.
The superlattice-in-well structures were grown using a cycled submonolayer AlGaAs/GaAs deposition technique. The optical quality of Al-Ga interdiffusion in AlGaAs/GaAs superlattice was investigated by measuring the photoluminescence of samples grown at temperature from 610°C to 630°C. Results show that Al composition can be modulated under some growth temperature or period. Effect of the growth interrupt in the growth process of superlattice on film optical quality is also discussed. Especially, the role played by the period of superlattice in the process of obtaining high quality film material with low composition is investigated in detail.
The influence of InAs coverage on the formation of self-assembled quantum dots grown by molecular-beam epitaxy was investigated by atomic force microscopy and photoluminescence measurements. As the InAs coverage increased from 2.0 to 3.0 monolayers, the quantum dot density decreased from 1.1 × 10^{11} to 1.36 × 10^{10} cm^{-2}. This result could be attributed to the coalescence of neighboring small InAs quantum dots resulting in the formation of much larger InAs quantum dots with lower quantum dot density. Atomic force microscopy results revealed that as the InAs quantum dot coverage increased, the transition of size distribution of InAs quantum dots from single-modal to multimodal occurred. The temperature-dependent photoluminescence spectra showed that the photoluminescence spectra red shifted and the photoluminescence peak intensity decreased as the InAs coverage increased. The thermal activation energy was strongly dependent on the InAs coverage, and for InAs quantum dots with 3.0 ML thick InAs coverage, this energy was estimated to be 147 meV.
In this paper, the critical role played by various types of defects and strain relaxation mechanisms in high-T_{c} thin films is highlighted and illustrated with examples. The defects are essential for providing adequate diffusion channels for oxygen ingress during the cooling step in c-axis thin films. The operation of strain relaxation mechanisms necessitated by the lattice mismatch between film and substrate can impose a compressive or tensile biaxial pressure, which either increases or reduces the critical temperature.
This paper reports on theoretical calculations and fabrication by molecular beam epitaxy of wide-gap II-VI heterostructures emitting in the "true" yellow range (560-600 nm) at room temperature. The active region of the structures comprises CdSe quantum dot active layer embedded into a strained Zn_{1-x}Cd_{x}Se (x=0.2-0.5) quantum well surrounded by a Zn(S,Se)/ZnSe superlattice. Calculations of the CdSe/(Zn,Cd)Se/Zn(S,Se) quantum dot-quantum well luminescence wavelength performed using the envelope-function approximation predict rather narrow range of the total Zn_{1-x}Cd_{x}Se quantum well thicknesses (d ≈ 2-4 nm) reducing efficiently the emission wavelength, while the variation of x (0.2-0.5) has much stronger effect. The calculations are in a reasonable agreement with the experimental data obtained on a series of test heterostructures. The maximum experimentally achieved emission wavelength at 300 K is as high as 600 nm, while the intense room temperature photoluminescence has been observed up to λ =590 nm only. To keep the structure pseudomorphic to GaAs as a whole the tensile-strained surrounding ZnS_{0.17}Se_{0.83}/ZnSe superlattice were introduced to compensate the compressive stress induced by the Zn_{1-x}Cd_{x}Se quantum well. The graded-index waveguide laser heterostructure with a CdSe/Zn_{0.65}Cd_{0.35}Se/Zn(S,Se) quantum dot-quantum well active region emitting at λ =576 nm (T=300 K) with the 77 to 300 K intensity ratio of 2.5 has been demonstrated.
Most of the Fe-nitride phases have been studied in much detail. Nevertheless, there is still a debate about the most efficient, exact and controlled way of obtaining thin films of the desired iron nitride phases. Thin films of iron nitrides were deposited by Molecular Beam Epitaxy in Ultra High Vacuum. By changing the growth parameters we tried to obtain the α"-phase in its purest form. We worked also on iron mononitride, FeN (γ"-FeN) which is known to exist in different phases. The stoichiometry of the samples was determined by means of resonant Rutherford Backscattering Spectroscopy. The samples were studied by room temperature Conversion Electron Mössbauer Spectroscopy. We achieved as much as 24% of pure α"-phase and provide evidence of the existence of γ"-FeN with vacancies and of the transformation of nonmagnetic γ"-phase into magnetic ε-phase after time exposure.
The present paper concerns the elastic-plastic nanodeformation of Te-doped GaSb crystals grown by molecular beam epitaxy on the n-type of GaSb substrate. The conventional analysis of nanoindentation data obtained with sharp triangular (Berkovich) and spherical tip revealed the elastic modulus (E=83.07± 1.78 GPa), hardness (H=5.19±0.25 GPa) and "true hardness" (H_{T}=5.73±0.04 GPa). The registered pop-in event which indicates the elastic-plastic transition in GaSb crystal points towards the corresponding yield strength (σ_{Y}=3.8±0.1 GPa). The origin of incipient plasticity in GaSb crystal is discussed in terms of elastic-plastic deformation energy concept.
We report on reduction of optical losses in n-CdTe/p-ZnTe thin-film solar cells grown by molecular beam epitaxy. The investigated thin-film devices were grown from elemental sources on monocrystalline, semi-insulating, (100)-oriented GaAs substrates. The optical losses have been reduced by a texturing of the device surface and by depositing of a ZnO antireflection coating. Current-voltage and spectral characteristics of the investigated p-ZnTe/n-CdTe solar cells depend significantly on the preparation of the surface of the ZnTe window. We describe a procedure of chemical etching of the ZnTe window leading to surface texturing. A ZnO layer of proper thickness deposited by low-temperature atomic layer deposition technique on the ZnTe surface forms an effective antireflection coating that leads to the reduction of optical losses. Due to reduction of the optical losses we observe increase of the short-circuit current, J_{SC}, by almost 60% and of the energy conversion efficiency by 44%.
The paper reports on plasma-assisted MBE growth of good quality N-face GaN layers directly on c-Al₂O₃ substrates. Growth kinetics under different growth conditions (substrate temperature, Ga to activated nitrogen flux ratio, etc.) during deposition of GaN(0001) and GaN(0001̅) both by the ammonia-based MBE or plasma-assisted MBE was studied. It was found that atomically smooth surface of 1 μm thick GaN(0001̅) films can be achieved by plasma-assisted MBE at the relatively high substrate temperature T_S ≈ 760°C and Ga to activated nitrogen flux ratio F_Ga/F_N* ≈ 1.8.
High-resolution X-ray diffractometer was used to study structural quality, lattice parameters and misfit strain in p-ZnTe/n-CdTe heterojunctions grown by the molecular-beam epitaxy technique on two different (001)-oriented substrates of GaAs and CdTe. The X-ray diffractometer results indicate that the CdTe layers, grown on lattice mismatched GaAs substrate, are partially relaxed, by the formation of misfit dislocations at the interface, and display residual vertical strain of the order of 10^{-4}. The presence of threading dislocations in the layers effectively limits the efficiency of solar energy conversion in the investigated heterojunctions. Homoepitaxially grown CdTe layers, of much better structural quality, display unexpected compressive strain in the layers and the relaxed lattice parameter larger than that of the substrate. Possible reasons for the formation of that unusual strain are discussed.
We present results of deep-level transient spectroscopy investigations of defects in a GaN-based heterostructure of a blue-violet laser diode, grown by plasma-assisted molecular beam epitaxy on a bulk GaN substrate. Three majority-carrier traps, T1 at E_C - 0.28 eV, T2 at E_C - 0.60 eV, and T3 at E_V + 0.33 eV, were revealed in deep-level transient spectra measured under reverse-bias conditions. On the other hand, deep-level transient spectroscopy measurements performed under injection conditions, revealed one minority-carrier trap, T4, with the activation energy of 0.20 eV. The three majority-carrier traps were revealed in the spectra measured under different reverse-bias conditions, suggesting that they are present in various parts of the laser-diode heterostructure. In addition, these traps represent different charge-carrier capture behaviours. The T1 trap, which exhibits logarithmic capture kinetics, is tentatively attributed to electron states of dislocations in the n-type wave-guiding layer of the structure. In contrast, the T2, T3, and T4 traps display exponential capture kinetics and are assigned to point defects.
As the most important material parameter of semiconductor, bandgap is necessary to be investigated to meet the design requirements of the high-performance optoelectronic devices. A new method of is proposed to calibrate the bandgap of antimonide based multi-component alloys with considering the effect of spin-orbit splitting off bands and the doublet degeneracy of valance band on the bandgaps of Sb-containing materials. A correction factor is introduced in the conventional calculation, and the spin-orbit splitting method is proposed. Besides, the In_xGa_{1-x}As_ySb_{1-y} films with different compositions are grown on GaSb substrates by molecular beam epitaxy, and the corresponding bandgaps are obtained by photoluminescence to test the accuracy and reliability of this new method. An error rate analysis reveals that the α calculated by the spin-orbit splitting correction method is decreased to 2%, almost one order of magnitude smaller than the Moon method, which means that the new method can calculate the antimonide multicomponent more accurately with some applicability. This work can give a reasonable interpretation for the reported results and beneficial to tailor the antimonides properties and optoelectronic devices.
We report on growth of GaN nanocolumns by plasma assisted MBE on (111) silicon substrates and on their characterization. The nanocolumns nucleate on the substrate spontaneously without use of any catalyst, probably by the Volmer-Weber mechanism. Transmission electron microscopy analysis shows high crystalline quality of GaN nanocolumns and their good alignment with the c-axis being perpendicular to the substrate. Preliminary results on use of GaN nanocolumns in gas sensor devices are presented.
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