Molecular Beam Epitaxy Process Research Articles

Three-dimensional self-ordering in an InGaAs/GaAs multilayered quantum dot structure investigated by x-ray diffuse scattering

X-ray diffuse scattering from a 17-fold InGaAs/GaAs quantum dot (QD) multilayer grown by an advanced molecular beam epitaxy process was investigated. The QDs in each layer form extended chains that are oriented along the planar direction. A prominent three-dimensional satellite structure in the x-ray diffuse scattering is found, which is caused by a strong lateral chain–chain correlation and vertical correlation of the lateral chain positions in the multilayer. In contrast, the dot–dot correlation within the chains are comparatively small. Numerical x-ray scattering simulations have been carried out. A comparison with corresponding experimental intensity patterns provides information on the QD size and QD spatial distribution.

The diffusion mechanism in the formation of GaAs and AlGaAs nanowhiskers during the process of molecular-beam epitaxy

The formation of GaAs and AlGaAs nanowhiskers using molecular-beam epitaxy on GaAs (111)B surfaces activated with Au is theoretically and experimentally studied. It is experimentally shown that nanowhiskers whose length exceeds the effective thickness of the deposited GaAs by an order of magnitude can be grown. It is found that the experimental dependences of the nanowhisker length L on its diameter D can differ radically from those observed in the case of a vapor-liquid-solid growth mechanism. The L(D) dependences obtained in this study are decreasing functions of D. The above effects are related to the existence of the diffusion transport of atoms from the surface towards the tips of the whiskers, which leads to a considerable increase in the growth rate of thin whiskers. A theoretical model of the formation of nanowhiskers in the process of molecular-beam epitaxy is developed. The model provides a unified description of the vapor-liquid-solid and diffusion growth mechanisms and qualitatively explains the experimental results obtained.

Self-assembled quantum-dot molecules by molecular-beam epitaxy

Self-assembled InAs quantum-dot (QD) molecules having high dot density and aligned dot set structure, which is defined by nanotemplates, were realized by thin capping and regrowth technique in a molecular-beam epitaxy process. Thin capping of GaAs on InAs QDs leads to the creation of nanoholes having a camel-like nanostructure due to anisotropic strain fields along the [11¯0] crystallographic direction and anisotropic surface diffusion accompanying the QD collapse. Regrowth of InAs QDs on the nanohole templates initially results in the formation of QDs with good size uniformity in the middle of features with the shape of propeller blades. This takes place at the regrowth thickness of 0.6 monolayer (ML). The strain at propellers’ edge starts to play its role, creating sets of quantum dots surrounding the initial and centered dots at the regrowth thickness of 1.2 ML. The elongated configuration of propellers’ blades defines the pattern of QD sets having five to six dots on each side. The dot density of the QD molecules is 3×1010cm−2, one order of magnitude higher than that of initial dot density (2×109cm−2).

Comments on the Article “The Universal Dynamics of Tumor Growth” by A. Brú et al.

We read with great interest the article published by Bru et al. titled “The Universal Dynamics of Tumor Growth” (Biophysical Journal 85:2948–2961). Therein, a number of in vitro growing cell line colonies and developing in vivo developing tumors are analyzed. Bru et al. infer that their results indicate a common growth dynamics that is compatible with the molecular beam epitaxy (MBE) process. The authors achieved this conclusion based on the following reasoning. First, a fractal analysis of the contours was performed. Once the universality class was identified by means of that analysis, a stochastic equation sharing the same universality class was introduced: the linear MBE model. Then, experimental assays to test the features imposed by that model were carried out. Herein, we demonstrate that the scaling analysis performed by the authors is inconsistent and consequently misleading. Moreover, we also show that the experimental evidence provided to check the characteristics imposed by the linear MBE equation do not point exclusively to that model. Finally, we indicate the source of such inconsistent analysis, namely, an effect due to the geometry of tumors, and how to fix it.

Towards High-Quality AlN/SiC Hetero-Interface by Controlling Initial Processes in Molecular-Beam Epitaxy

Towards High-Quality AlN/SiC Hetero-Interface by Controlling Initial Processes in Molecular-Beam Epitaxy

Physical structure of molecular-beam epitaxy growth defects in HgCdTe and their impact on two-color detector performance

The flexible nature of molecular-beam epitaxy (MBE) growth is beneficial for HgCdTe infrared-detector design and allows for tailored growths at lower costs and larger focal-plane array (FPA) formats. Control of growth dynamics gives the MBE process a distinct advantage in the production of multicolor devices, although opportunities for device improvement still exist. Growth defects can inhibit pixel performance and reduce the operability in FPAs, so it is important to understand and evaluate their properties and impact on detector performance. The object of this paper is to understand and correlate the effects of macrodefects on two-color detector performance. We observed the location of single-crystal and polycrystalline regions on planar and cross-sectioned surfaces of two-color device structures when void defects were viewed by scanning electron microscopy (SEM). Compositional analysis via energy dispersive x-ray analysis (EDXA) of voids in the cross section showed elevated Te and reduced Hg when compared to defect-free growth areas. The second portion of this study examined the correlation of macrodefects with pixel operability and diode current-voltage (I–V) characteristics in mid-wavelength infrared (MWIR)/MWIR (M/M) and long wavelength infrared (LWIR)/LWIR (L/L) two-color devices. The probability of diode failure when a void is present is 98% for M/M and 100% for L/L. Voids in two-color detectors also impact diodes neighboring their location; the impact is higher for L/L detectors than M/M detectors. All void-containing diodes showed early breakdown in the I–V characteristics in one or both bands. High dislocation densities were observed surrounding voids; the high density spread further from the void for L/L detectors compared to M/M detectors.

Charge modification in InAs/Al xGa 1− xSb HEMT structures

Intrinsic sheet charge levels in unintentionally doped AlSb/InAs/AlSb quantum wells are reproducibly ∼10 12/cm 2. While these levels are suitable for depletion-mode operation, realizing enhancement-mode devices ultimately depends on the ability to significantly reduce sheet charge. In an effort to limit carrier accumulation in the InAs quantum well, we have studied the impact of the addition of a p-type, Be, doping plane between the channel and the cap and the modification of defect densities present in the films. Molecular beam epitaxy (MBE) process conditions including substrate temperature, Be-doping density, and doping plane spacing above the quantum well were varied during growth of InAs–Al x Ga 1− x Sb high electron mobility transistors (HEMTs) structures. Utilizing a 2 3-full factorial statistical experimental design plus eight additional runs, the ranges for Be-doping (0–2e12/cm 2), substrate temperature (350–420°C), and separation above the quantum well (60–80 Å) were examined. Relationships between MBE process conditions and transport properties in Be-doped InAs–Al x Ga 1− x Sb HEMT structures were established. Among the process parameters investigated, substrate temperature during Be-doping had the most significant impact on 2DEG concentration. Substrate temperature and sheet charge exhibited an inversely proportional relationship, where increasing substrate temperature resulted in decreasing sheet charge values. The lowest charge with corresponding mobility was 6.6×10 10 at 6000 cm 2/V/s.

Synthesis and microstructural manipulation of ceramics by electrophoretic deposition

Critical issues of electrophoretic deposition (EPD) and its potential for fabricating high-performance ceramics are reviewed. Deposition kinetics under constant-current and constant-voltage conditions are discussed. The process of submonolayer formation of mono-sized silica particles as a function of deposition time during EPD has been discussed and is shown to have a remarkable similarity with the atomic thin-film growth (10000 times smaller scale) process of Molecular Beam Epitaxial (MBE) process. Ceramic coatings by EPD are described and strategies to avoid cracking are outlined. Discussion on avoiding cracking using the Reaction Bonded Aluminium Oxide (RBAO) process is also described. The versatility of the EPD process towards fabrication of a wide spectrum of composite microstructures is discussed together with references. Potential use of EPD in the fabrication of Micro-Solid Oxide Fuel Cell (μSOFC) has also been discussed.

Ge nanocrystals in MOS-memory structures produced by molecular-beam epitaxy and rapid-thermal processing

ABSTRACTA method of forming a sheet of Ge nanocrystals in a SiO2layer based on molecular beam epitaxy (MBE) and rapid thermal processing (RTP) is presented. The method takes advantage of the very high precision by which a very thin Ge layer can be deposited by MBE. With proper choice of process parameters the nanocrystal size can be varied between ∼3 and ∼8 nm and the area-density between ∼1×1011and ∼1×1012dots/cm2. The tunneling oxide thickness is determined by the thickness of a thermally grown SiO2layer, and is typically 4 nm. C-V measurements of MOS capacitors reveal hole and electron injection from the substrate into the nanocrystals. Memory windows of about 0.2 and 0.5 V for gate-voltage sweeps of 3 and 6 V, respectively, are achieved.

Anomalous dielectric and optical properties in perovskite-type artificial superlattices

Perovskite-type BaTiO3/SrTiO3 (BTO/STO) artificial superlattices were fabricated by molecular beam epitaxy process and their dielectric properties and refractive indices were measured. A large leakage current was observed in the films on Nb-doped STO substrate. Dielectric permittivity was therefore measured using planer interdigital electrodes. Fine planer electrodes were necessary to reduce the penetration of electric flux into the substrate. Interdigital electrodes were formed by electron-beam lithography. Dielectric permittivity of superlattices along the film surface was determined using electromagnetic field analysis. It was found that the dielectric permittivity of the BTO/STO superlattice with the period of 10 unit cells showed a maximum value above 30,000, which was almost independent from frequency up to 110 MHz. The refractive index of superlattices measured with a spectroscopic ellipsometer also showed the highest value when the period was 10 unit cells. This indicated that the structure of superlattices affected not only the ionic polarization but also the electric polarization. The origin of anomalous properties observed in superlattices may be interpreted by the strains induced into the film.

Methods of High-Energy Chemistry in the Technology of Wide-Gap Chalcogenide Semiconductors

The application of radical beams in the technology of wide-gap semiconductors is discussed. Radical-beam gettering epitaxy, a technique based on the annealing of II–VI (III–V) crystals in activated nonmetal vapor, is considered. This technique offers the possibility of growing both n- and p-type II–VI and III–V epilayers. The use of radical beams in conjunction with molecular-beam epitaxy and metalorganic vapor-phase epitaxy processes makes it possible to produce p-type materials via doping with acceptor impurities in the form of excited molecules or atoms. The reduced growth temperature allows one to fabricate heterostructures with atomically sharp junctions owing to the absence of interdiffusion.

Dielectric and Optical Properties of Perovskite-type Artificial Superlattices

ABSTRACTBaTiO3/SrTiO3, BaTiO3/BaZrO3, and SrZrO3/SrTiO3 artificial superlattices were fabricated on SrTiO3 substrates by the molecular beam epitaxy (MBE) process. The stacking periodicity of each layer was varied from 1 unit cell to 40 unit cells, and the total thickness was fixed at 80 unit cells. In-situ reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD) clearly shows the formation of the superlattice structure. Their lattice parameters obtained from the reciprocal space mapping measurement indicated that the lattice distortion was dependent on the stacking periodicity. The capacitance and the complex admittance of the superlattices were measured with interdigital electrodes by an impedance analyzer up to 110 MHz. It was found that the dielectric permittivity changed with the superlattice periodicity. The Q-V measurement results showed clear hysteresis curves and this suggested that ferroelectricity was induced into the SrZrO3/SrTiO3 superlattices despite a combination between paraelectrics/paraelectrics.

Effect of Chemical Composition upon Mechanical Properties of Thin Layered Mono-crystal SiC

To make thin layered mono-crystal silicon carbide on silicon substrate, the hetero molecular beam epitaxial growth process is promising, in which both silicon and carbon must be made incident as their evaporated molecular states to silicon substrate. As there are some methods of evaporating the material, there exist several combinations that will considerably affect both the chemical and mechanical properties of newly deposited layers. The present study attempts to clarify the relationship between these chemical and mechanical properties and determine the guidelines for the combination and operating conditions in order to obtain the desired surface properties through a series of tests and evaluations.

Design of a film surface roughness-minimizing molecular beam epitaxy process by reduced-order modeling of epitaxial growth

Molecular beam epitaxy of germanium was used along with kinetic Monte Carlo simulations to study time-varying processing parameters and their effect on surface morphology. Epitaxial Ge films were deposited on highly oriented Ge(001) substrates, with reflection high-energy electron diffraction as a real-time sensor. The Monte Carlo simulations were used to model the growth process, and physical parameters were determined during growth under time-varying flux. A reduced version of the simulations was generated, enabling the application on an optimization algorithm. Temperature profiles were then computed that minimize surface roughness subject to various experimental constraints. The final roughness after two layers of growth was reduced to 0.32, compared to 0.36 at the maximum growth temperature. The study presented here is an initial demonstration of a general approach that could also be used to optimize properties in other materials and deposition processes.

Dielectric properties of BaTiO3/SrTiO3 superlattices measured with interdigital electrodes and electromagnetic field analysis

Artificial BaTiO3(BTO)/SrTiO3(STO) superlattices on STO substrates were fabricated by the molecular beam epitaxy process. The capacitance and the complex admittance of superlattices with interdigital electrode structures were measured at 1–110 MHz. The dielectric properties of the superlattices were evaluated by the electromagnetic analysis. It was found that the dielectric permittivity changed with the structure of superlattices and the highest permittivity of 33 000 was obtained for [(BTO)10/(STO)10]4 superlattice from 1 to 110 MHz.

Ge dots self-assembling: Surfactant mediated growth of Ge on SiGe (118) stress-induced kinetic instabilities

The ordering of islands on naturally or artificially nanostructured surfaces is one of the most recent objectives among actual nanotechnology challenges. We show in this letter that, by a combination of two approaches, i.e., a two-step molecular beam epitaxy (MBE) deposition process and surfactant-mediated growth, we are able to obtain chains of nicely ordered ultrasmall islands of lateral size below 50 nm. The two-step MBE process consists of vicinal Si(001) surface self-patterning by SiGe growth instability and Ge dot ordering by subsequent Ge deposition on a SiGe template layer. The surfactant-mediated growth consists of submonolayer Sb deposition prior to Ge growth, in order to reduce the island size up to 25 nm. The best ordering of Ge islands is obtained when the island size matches the wavelength of the template layer.

117 Surface properties of SiC layer grown by MBE with helicon sputtering molecular beam source

Although there are some attempts to produce monocrystalline silicon carbide (SiC) flat surface, the surface properties such as surface roughness don't satisfy the specifications. This study tries to apply helicon sputtering device to Molecular beam epitaxy (MBE) process to improve these properties. This device was used as molecular beam source for generating of Si molecular beam, where the electric field caused by the helicon coil gave energy to the sputtered Si molecules. The amount of the energy was controlled by the electric power applied to the coil. And high purity acetylene gas was used for carbon (C) molecular beam source. Substrate was monocrystalline (111) Si wafer. According to the increase of the electric power, that is, high energy was given to molecules, the roughness of the surface was improved. Auniformly mirror surface of monocrystalline SiC was produced for the entire substrate and the roughness was 1nm (Ra) order.

Carrier recombination processes in MOVPE and MBE grown 1.3 μm GaInNAs edge emitting lasers

By measuring the spontaneous emission (SE) from metal organic vapour phase epitaxy (MOVPE) grown ∼1.3 μm GaInNAs/GaAs-based lasers during normal operation, we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 to 370 K and compared these with results previously obtained for molecular beam epitaxy (MBE) grown GaInNAs lasers. From the SE measurements we determine how the current, I, close to threshold, varies as a function of carrier density, n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination respectively. We find that at room temperature (RT), defect-related recombination contributes ∼360 A/cm 2 (MBE) and ∼565 A/cm 2 (MOVPE) to the total current density at threshold. Radiative recombination accounts for ∼110 A/cm 2 (MBE) and 195 A/cm 2 (MOVPE) of J th with the remaining ∼180 A/cm 2 (MBE) and 760 A/cm 2 (MOVPE) are due to non-radiative Auger recombination. Our results suggest that a larger threshold carrier density in the MOVPE grown device in comparison to the MBE lasers, can reasonably explain the larger current densities of the different recombination processes at RT. We tentatively associate this with higher optical loss processes in the MOVPE grown material.

Ferromagnet (MnAs)/III‐V Semiconductor Hybrid Structures.

Ferromagnet/semiconductor hybrid structures are attractive and promising as artificial materials for 'semiconductor spintronics', because they can possess magnetic and/or spin-related functions and they have excellent compatibility with semiconductor device structures. We review our studies on the molecular beam epitaxy (MBE) growth of ferromagnet (MnAs)/III–V semiconductor layered heterostructures and nanoscale granular structures, and their magnetic, magneto-transport and magneto-optical properties. We show that, by careful control of MBE growth and processing, some useful magnetic or spin-related functions can be realized at room temperature.

Stoichiometry and absolute atomic concentration profiles obtained by combinedRutherford backscattering spectroscopy and secondary-ion mass spectroscopy:InAs nanocrystals in Si

Rutherford backscattering spectroscopy (RBS) and secondary-ion massspectroscopy (SIMS) were combined to achieve depth profiling calibrated inabsolute atomic concentrations. This method was applied to InAs nanocrystals,grown by molecular beam epitaxy (MBE), buried in a Si matrix. By means ofRBS, with its capability of accessing the buried layers, we determined thedepth-integrated areal densities of As and In. These were used to calibratethe SIMS profiles with their high depth resolution and dynamic range inabsolute atomic concentrations. This allowed us to identify, besides a wellconfined layer of stoichiometric InAs nanocrystals, significant diffusion of Inand As into the Si matrix in despite of their larger atomic radii, and anexcess of As due to its non-reactive deposition on Si from the excess As4flux during the MBE growth. On the basis of these findings, we suggestmeasures to optimize the MBE process for InAs/Si and similar systems.