Molecular Beam Epitaxy Technique Research Articles

Analysis of the GaSb-p+/GaSb-p/GaSb-n+/GaSb-n structure performances at room temperature, for thermo-photovoltaic applications

In this paper a GaSb-p+/GaSb-p/GaSb-n+/GaSb-n structure destined for thermo-photovoltaic applications at room temperature was fabricated by Molecular Beam Epitaxy (MBE) technique, in a Varian gen II solid-source machine. At direct bias, the J(V) characteristics of this structure in the dark and under light conditions, have been studied by numerical simulation and compared to the experimental measurements. The spectral response (SR) of the device has been also calculated and examined. For simulation we have used AMPS-1D software program to examine the effect of three parameters like the front contact barrier height (ϕb0) (barrier at metal/GaSb-p+ layer interface), the doping concentration (Na) in the GaSb active region and the thickness (d) of this same layer. Our numerical calculations show that the experimental and simulated J(V) characteristics under dark and light conditions are in good agreement when ϕb0 is equal to 0.24 eV and Na of about 1015 cm−3. These later values of ϕb0 and Na with an active zone thickness of 3 μm give the best results for the cell’s external parameters, like the short circuit current (JSC = 19.21 mA/cm2), the open circuit voltage (VOC = 0.23 V), the fill factor (FF = 0.67) and the efficiency (Eff = 3%).

Effect of HfO2 nitridation on structural, optical and electrical properties of GaN films grown on HfO2/Si(100) by laser molecular beam epitaxy

Growth of high quality III-nitride films on high-k oxide/Si substrates is crucial for the fabrication of nitride based metal-oxide-semiconductor devices. Here, we report the effect of nitridation of HfO2 surface on the physical properties of gallium nitride (GaN) films grown on HfO2 (5 nm)/p-Si(100) substrates using laser molecular beam epitaxy (LMBE) technique. The grazing incidence x-ray diffraction (GIXRD) measurements showed the prominent GaN growth along (0002) direction. It is observed that the HfO2 nitridation significantly improves the surface coverage of GaN film with larger grains of size in the range 200 ∼ 250 nm. Room temperature photoluminescence (PL) spectroscopy exhibits a sharp, intense near band-edge emission line at 3.4 eV with negligible defect-related deep bands for the GaN film grown with HfO2 nitridation. Also, the current-voltage characteristics of Au/GaN Schottky contacts show a reduced leakage current for HfO2 nitridation. Thus, the surface nitridation of HfO2 is found vital to improve the quality of GaN films for the development of GaN-on-Si technology.

Work function variation of monolayer MoS2 by nitrogen-doping

Monolayer MoS2 films with substantial sulfur vacancies are obtained using the laser molecular beam epitaxy (L-MBE) technique benefitted by high substrate temperature and ultrahigh vacuum growth conditions. The intrinsic sulfur vacancies present an excellent opportunity for varying the work function of monolayer MoS2 films by nitrogen doping. The in-plane doping of nitrogen atoms on L-MBE-synthesized monolayer MoS2 films is realized on the basis of rapid thermal annealing in a nitrogen environment. The as-grown and nitrogen-doped monolayer MoS2 films are evaluated by using Raman and photoluminescence spectroscopies. In accordance with the X-ray photoelectron spectroscopy results, the ultraviolet photoelectron spectroscopy investigation shows that the work function of the monolayer MoS2 films increases by 0.29 eV after covalent nitrogen doping. Nitrogen doping on monolayer MoS2 is further treated theoretically using first-principles calculations. Based on theoretical calculations and experimental validations, it is illustrated that nitrogen is a promising in-plane heteroatom dopant for work function variation of monolayer MoS2.

High-Temperature Thermoelectricity in LaNiO3-La2CuO4 Heterostructures.

Transition metal oxides exhibit a high potential for application in the field of electronic devices, energy storage, and energy conversion. The ability of building these types of materials by atomic layer-by-layer techniques provides a possibility to design novel systems with favored functionalities. In this study, by means of the atomic layer-by-layer oxide molecular beam epitaxy technique, we designed oxide heterostructures consisting of tetragonal K2NiF4-type insulating La2CuO4 (LCO) and perovskite-type conductive metallic LaNiO3 (LNO) layers with different thicknesses to assess the heterostructure-thermoelectric property-relationship at high temperatures. We observed that the transport properties depend on the constituent layer thickness, interface intermixing, and oxygen-exchange dynamics in the LCO layers, which occurs at high temperatures. As the thickness of the individual layers was reduced, the electrical conductivity decreased and the sign of the Seebeck coefficient changed, revealing the contribution of the individual layers where possible interfacial contributions cannot be ruled out. High-resolution scanning transmission electron microscopy investigations showed that a substitutional solid solution of La2(CuNi)O4 was formed when the thickness of the constituent layers was decreased.

Incorporation of hydrogen into MBE-grown dilute nitride GaInNAsSb layers in a MOCVD growth ambient

In this work, lattice-matched 3-junction and 4-junction solar cells including the dilute nitride subcells were fabricated by a hybrid growth technique of metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). It was found that photocarrier extraction in the MBE grown dilute nitride GaInNAsSb subcells degraded after the overgrowth of the upper subcells by MOCVD. Then, we focused on the effect of annealing for a set of MBE grown GaInNAsSb single junction solar cells annealed in the MOCVD reactor under an arsine flow ambient. It was found that degradation of the carrier extraction due to the MOCVD growth ambient is in a correlation with the selective hydrogen incorporation into the GaInNAsSb layers. We demonstrated that the subsequent annealing in nitrogen ambient promoted the outdiffusion of hydrogen which led to a recovery of the GaInNAsSb solar cell performances.

In Situ STM Observation of Nonmagnetic Impurity Effect in MBE-grown CeCoIn5 Films

Local electronic effects in the vicinity of an impurity provide pivotal insight into the origin of unconventional superconductivity, especially when the materials are located on the edge of magnetic instability. In high-temperature cuprate superconductors, a strong suppression of superconductivity and appearance of low-energy bound states are clearly observed near nonmagnetic impurities. However, whether these features are common to other strongly correlated superconductors has not been established experimentally. Here, we report the {$in$} {$situ$} scanning tunneling microscopy observation of electronic structure around a nonmagnetic Zn impurity in heavy-fermion CeCo(In$_{1-x}$Zn$_x$)$_5$ films, which are epitaxially grown by the state-of-the-art molecular beam epitaxy technique. The films have very wide atomically flat terraces and Zn atoms residing on two different In sites are clearly resolved. Remarkably, no discernible change is observed for the superconducting gap at and around the Zn atoms. Moreover, the local density of states around Zn atoms shows little change inside the $c$-$f$ hybridization gap, which is consistent with calculations for a periodic Anderson model without local magnetic order. These results indicate that no nonsuperconducting region is induced around a Zn impurity and do not support the scenario of antiferromagnetic droplet formation suggested by indirect measurements in Cd-doped CeCoIn$_5$. These results also highlight a significant difference of the impurity effect between cuprates and CeCoIn$_5$, in both of which $d$-wave superconductivity arises from the non-Fermi liquid normal state near antiferromagnetic instabilities.

Design complexity of DPN patterning with Cr3+ and Co2+ metallic ions on Au (111) thin film

We present in this work a fabrication chain of an electrochemical sensor platform based on detection of Cr3+ and Co2+ metallic ions species. Through molecular beam epitaxy (MBE) technique, a high quality Au (111) thin film was firstly fabricated in ultra-high vacuum (UHV) conditions, and used as support for chemical SAMs functionalization with 1,4-dithiothreitol (DTT) spacers. On the formed hybrid surface/linker structures, we employed Dip Pen Nanolithography (DPN) for patterning various templates of Cr3+ and Co2+ metallic ion species. Coated AFM tips with metallic molecular inks were used to fabricate different paths on the Au substrate, offering a unique functional design complexity. By varying the tip speed and/or dwell time, micro-nano-scale templates of both species were patterned in the form of square, double-square, triangle and lines. A full control of molecular ink transport mechanisms during the drawing process was assured, succeeding the writing of Cr3+ and Co2+ metallic ions on Au film substrate. Surface topology analyzed by Lateral Force Microscopy demonstrated the chemical contrast of patterned species, being a clear indicative of DPN tracing paths. Molecular recognition of both Cr3+ and Co2+ metallic ions has been made by potentio-electrochemical impedance spectroscopy (PEIS) measurements. The Nyquist plots were compared and the equivalent circuits obtained in both conformations (Cr3+/DTT/Au, Co2+/DTT/Au) were discussed in the light of experimental parameters.

Study of PV characteristics of AlxGa1-xAs/GaAs photodiodes

Simulation of the structure of multijunction photodiodes has been performed on the assumption that an equal of the number of photons is absorbed in them, and PV characteristics have been stadied AlxGa1-xAs/GaAs photodiodes fabricated by the molecular-beam epitaxy technique. According to the performed measurements and analysis of the results, it has been shown that the efficiency of single-junction photodiodes in converting monochromatic optical radiation at a wavelength of 810 nm reaches 50%. The relationship between the “saturation” currents calculated from the dark I - V characteristic for the diffusion current flow (Shockley) in the space charge region of a photodiode p-n junction and the obtained values for the efficiency of converting optical radiation in the wavelength range of 700 - 900 nm has been shown. When the “saturation” current flow for the diffusion mechanism rises by an order of magnitude, the efficiency value at excitation by monochromatic radiation at 810 nm and 780 nm drops by 25% and 11%, respectively.

Anomalous Hall conductivity and electronic structures of Si-substituted Mn2CoAl epitaxial films

We study anomalous Hall conductivity ($\sigma$$_{\rm AHC}$) and electronic band structures of Si-substituted Mn$_{2}$CoAl (Mn$_{2}$CoAl$_{1-x}$Si$_{x}$). First-principles calculations reveal that the electronic band structure is like a spin-gapless system even after substituting a quaternary element of Si for Al up to $x = $0.2 in Mn$_{2}$CoAl$_{1-x}$Si$_{x}$. This means that the Si substitution enables the Fermi level shift without largely changing the electronic structures in Mn$_{2}$CoAl. By using molecular beam epitaxy (MBE) techniques, Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ epitaxial films can be grown, leading to the systematic control of $x$ (0 $\le$ $x$ $\le$ 0.3). In addition to the electrical conductivity, the values of $\sigma$$_{\rm AHC}$ for the Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ films are similar to those in Mn$_{2}$CoAl films shown in previous reports. We note that a very small $\sigma$$_{\rm AHC}$ of $\sim$ 1.1 S/cm is obtained for $x =$ 0.225 and the sign of $\sigma$$_{\rm AHC}$ is changed from positive to negative at around $x =$ 0.25. We discuss the origin of the sign reversal of $\sigma$$_{\rm AHC}$ as a consequence of the Fermi level shift in MCA. Considering the presence of the structural disorder in the Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ films, we can conclude that the small value and sign reversal of $\sigma$$_{\rm AHC}$ are not related to the characteristics of spin-gapless semiconductors.

Stabilization and enhanced energy gap by Mg doping in ε-phase Ga2O3 thin films

Mg-doped Ga2O3 thin films with different doping concentrations were deposited on sapphire substrates using laser molecular beam epitaxy (L-MBE) technique. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and ultraviolet-visible (UV-vis) absorption spectrum were used to characterize the crystal structure and optical properties of the as-grown films. Compared to pure Ga2O3 thin film, the Mg-doped thin films have transformed from the most stable β-phase into ε-phase. The absorption edge shifted to about 205 nm and the optical bandgap increased to ∼ 6 eV. These properties reveal that Mg-doped Ga2O3 films may have potential applications in the field of deep ultraviolet optoelectronic devices, such as deep ultraviolet photodetectors, short wavelength light emitting devices and so on.

Electrical and Optical Properties of In₂O₃ Thin Films Deposited on Sapphire Substrate.

In2O3 thin films were prepared on c-plane sapphire substrates using laser molecular beam epitaxy technique. The X-ray diffraction (XRD) patterns revealed that the In2O3 thin films were highly oriented along the (111) direction. The intensity of (222) diffraction peaks mainly depend on growth temperature, and the crystallite sizes mainly depend on oxygen pressure. The carrier concentrations exhibit a decrease with increasing growth temperature and oxygen pressure, meantime, the resistivity increase. The red shift of In2O3 thin films respect to that of bulk In2O3 can be explained by defect energy levels formation, the blue shift of In2O3 thin films depends on carrier concentration, can be explained by Burstein-Moss band-filling effect.

Laser Molecular Beam Epitaxy (LMBE) Technique grown GaN p-n junction

Gallium Nitride (GaN) thin films have been prepared using Laser Molecular Beam Epitaxy (LMBE) technique at moderate growth temperature in N2 gas ambience. The structural and optical properties of the films were optimized. A p-n junction diode has been successfully realised using GaN film grown by UHV-LMBE technique and p-type Mg:GaN/Sapphire substrate. Rectifying diode characteristics with low knee voltage of 1.2V were obtained for the prepared p-n junction, highlighting the promising prospects of employing LMBE technique for the realization of GaN based LEDs.

Thermodynamic analysis of Al0.17Ga0.83As/GaAs (001) in annealing process

For matching lattice parameters, AlGaAs alloy is usually grown on a GaAs (001) substrate. The AlGaAs/GaAs multilayer structure has been widely used to manufacture various photoelectric and electronic devices. The practical importance of atomic flat surfaces lies in improving the performances of modern optoelectronic devices based on AlGaAs/GaAs multilayer structure. The influence of temperature on the flatness of the film has not been analyzed in detail, so it is very important to prepare the surface at an atomic level by adjusting annealing temperature. In this paper, 15 ML Al0.17Ga0.83As are deposited on an n-doped GaAs (001) substrate by the molecular beam epitaxy (MBE) technique. We study the effects of various annealing temperatures (520℃, 530℃, 540℃) on the flattening of Al0.17Ga0.83As/GaAs (001) surface under the same condition of arsenic BEP about 1.210-3 Pa, annealing time 60 min and growth rate (0.17 ML/s). The (1000 nm1000 nm) scanning tunneling microscope (STM) images and Fourier transform graphs are obtained to show the evolution of surface morphology. In a temperature range of 520-530℃, island is ripening, the coverage of the island increases, the pit also begins to merge into a larger pit; when the temperature exceeds 530℃, the increasing of ripening rate leads to a big island and the pit turns into terrace, while the coverage of island and the pit gradually decreases. In the annealing process, the area of terrace increases and gradually approaches to 100%. By quantitatively analysing the coverage of pit (island, terrace) and root mean square (RMS) roughness varying with the annealing temperature, a 545℃ (1℃) better annealing temperature is proposed by fitting the curve of RMS roughness variation. At the same time, the film annealing model is analyzed in this paper. Comparing the results in the literature with our experimental data, it is found that the change of annealing temperature can influence the number of active atoms, in which the ratio of annealing atoms contributing to surface flattening () should be proportional to the annealing temperature. According to the experimental results, Al0.17Ga0.83As surface basically presents the flat morphology with 60 min annealing at 540℃ when 0.20 0.25. When the annealing temperature reaches 545℃, we can also speculate that the annealing time is about 55-60 min. This is consistent with our previous conclusion. It should be pointed out that our experiment avoids metallizing the film surface caused by the anti-evaporation of the atoms and the metal gallium atoms climbing on the surface of the film when the annealing temperature is too high. The experimental results are applicable to the Al0.17 Ga0.83As thin film growth and annealing.

A New Approach for Heavy N-Doping Process in Ge Epilayers Using Specific Solid Source

As an indirect band gap material, Germanium (Ge) has low efficiency of radiative recombination in the wavelength area of about 1550 nm. However, the difference between the direct and indirect band gap is very small (～140 meV) and photoluminescence emission ability of Ge could be greatly enhanced by heavy n-doping process. In this work, Ge growth directly on Si (001) substrate by molecular beam epitaxial (MBE) technique and a high n-doping level in Ge was obtained owning to using GaP decomposition source. The GaP solid source produces P2 molecules instead of P4 molecules, which have higher sticking coefficient than that of P4 comparing to the traditional doping method from PH3 gas precursor molecules. The dependence of optical property on Ge film thickness is also studied. The out-diffusion phenomenon of phosphorus dopant has been observed through SIMS profile analysis. An activated phosphorus concentration can be achieved up to more than 2 × 1019 atoms?cm&minus3 confirmed by Hall effect measurement. This result contributes to realization of Ge-on-Si devices for optoelectronic applications.

Ultraviolet electroluminescence from a n-ZnO film/p-GaN heterojunction under both forward and reverse bias

ZnO film was fabricated on p-GaN film using the molecular beam epitaxy technique to form heterojunction light emitting diodes (LEDs).

Dependence of Al incorporation on growth temperature during laser molecular beam epitaxy of AlxGa1−xN epitaxial layers on sapphire (0001)

We report the successful growth of AlxGa1−xN (0 ≤ x ≤ 0.25) epitaxial films on c-plane sapphire substrate by using laser molecular beam epitaxy technique. The role of growth temperature on the Al incorporation and the structural, electronic and optical properties of the AlxGa1−xN epitaxial layers grown in the temperature range 500–700 °C have been systematically studied. The atomic force microscopy analysis shows that the grain size of AlxGa1−xN increases with increase in growth temperature and flat surface epilayers are obtained at ≥ 600 °C. The Al incorporation is confirmed with high resolution x-ray diffraction, x-ray photo electron microscopy and photoluminescence studies. It is observed that the growth temperature plays a critical role in determining the Al composition, which increases with increasing growth temperature. AlxGa1−xN layer with about 23% of Al composition is obtained on sapphire (0001) substrate at a growth temperature of 700 °C, which is about 100–150 °C lower than the conventional molecular beam epitaxy growth.

Frontiers in the Growth of Complex Oxide Thin Films: Past, Present, and Future of Hybrid MBE

AbstractDriven by an ever‐expanding interest in new material systems with new functionality, the growth of atomic‐scale electronic materials by molecular beam epitaxy (MBE) has evolved continuously since the 1950s. Here, a new MBE technique called hybrid‐MBE (hMBE) is reviewed that has been proven a powerful approach for tackling the challenge of growing high‐quality, multicomponent complex oxides, specifically the ABO3 perovskites. The goal of this work is to (1) discuss the development of hMBE in a historical context, (2) review the advantageous surface kinetics and chemistry that enable the self‐regulated growth of ABO3 perovskites, (3) layout the key components and technical challenges associated with hMBE, (4) review the status of the field and the materials that have been successfully grown by hMBE which demonstrate its general applicability, and (5) discuss the future of hMBE in regards to technical innovations and expansion into new material classes, which are aimed at expanding into industrial realm and at tackling new scientific endeavors.

P-type single-crystalline ZnO films obtained by (Na,N) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown by plasma-assisted molecular beam epitaxy technique on c-plane sapphire substrates. The films have been implanted with fixed fluence of 130 keV Na and 90 keV N ions at 460 °C. It is observed that dually-implanted single crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 1.24 × 1016–1.34 × 1017 cm−3, hole mobilities between 0.65 and 8.37 cm2 V−1 s−1, and resistivities in the range of 53.3–80.7 Ω cm by Hall-effect measurements. There are no other secondary phase appearing, with (0 0 2) (c-plane) orientation after ion implantation as identified by the X-ray diffraction pattern. It is obtained that Na and N ions were successfully implanted and activated as acceptors measured by XPS and SIMS results. Also compared to other similar studies, lower amount of Na and N ions make p-type characteristics excellent as others deposited by traditional techniques. It is concluded that Na and N ion implantation and dynamic annealing are essential in forming p-type single-crystalline ZnO films.

Heat Dissipation Schemes in AlInAs/InGaAs/InP Quantum Cascade Lasers Monitored by CCD Thermoreflectance

In this paper, we report on the experimental investigation of the thermal performance of lattice matched AlInAs/InGaAs/InP quantum cascade lasers. Investigated designs include double trench, single mesa, and buried heterostructures, which were grown by combined Molecular Beam Epitaxy (MBE) and Metal Organic Vapor Phase Epitaxy (MOVPE) techniques. The thermal characteristics of lasers are investigated by Charge-Coupled Device CCD thermoreflectance. This method allows for the fast and accurate registration of high-resolution temperature maps of the whole device. We observe different heat dissipation mechanisms for investigated geometries of Quantum Cascade Lasers (QCLs). From the thermal point of view, the preferred design is the buried heterostructure. The buried heterostructures structure and epi-layer down mounting help dissipate the heat generated from active core of the QCL. The experimental results are in very good agreement with theoretical predictions of heat dissipation in various device constructions.

Deposition of Cubic AlN Films on MgO (100) Substrates by Laser Molecular Beam Epitaxy

Cubic AlN (c-AlN) films were deposited on MgO (100) substrates by laser molecular beam epitaxy (LMBE) technique. The crystal structure and surface morphology of deposited films with various laser pulse energy and substrate temperature were investigated. The results indicate that c-AlN films exhibit the (200) preferred orientation, showing a good epitaxial relationship with the substrate. The surface roughness of c-AlN films increases when the laser pulse energy and substrate temperature increase. The film grown at laser pulse energy of 150 mJ and substrate temperature of 700 °C shows the best crystalline quality and relatively smooth surface.