Molecular Beam Epitaxy Chamber Research Articles

Straining Nanomembranes via Highly Mismatched Heteroepitaxial Growth: InAs Islands on Compliant Si Substrates

Freestanding, edge-supported silicon nanomembranes are defined by selective underetching of patterned silicon-on-insulator substrates. The membranes are afterward introduced into a molecular beam epitaxy chamber and overgrown with InAs, resulting in the formation of InAs islands on flat areas and at the top of the Si nanomembranes. A detailed analysis of sample morphology, island structure, and strain is carried out. Scanning electron microscopy shows that the membrane stays intact during overgrowth. Atomic force microscopy reveals a lower island density on top of the freestanding membranes, denoting a modified wetting or diffusivity in these areas. An observed bending of the membrane indicates a strain transfer from the InAs islands to the compliant substrate. X-ray diffraction and finite-element modeling indicate a nonuniform strain state of the island ensemble grown on the freestanding membrane. A simulation of the bending of the nanomembranes indicates that the islands at the center of the freestanding area are highly strained, whereas islands on the border tend to be fully relaxed. Finally, continuum elasticity calculations suggest that for a sufficiently thin membrane InAs could transfer enough strain to the membrane to allow coherent epitaxial growth, something not possible on bulk substrates.

In situ grown Ge in an arsenic-free environment for GaAs/Ge/GaAs heterostructures on off-oriented (100) GaAs substrates using molecular beam epitaxy

High-quality epitaxial Ge layers for GaAs/Ge/GaAs heterostructures were grown in situ in an arsenic-free environment on (100) off-oriented GaAs substrates using two separate molecular beam epitaxy (MBE) chambers, connected via vacuum transfer chamber. The structural, morphological, and band offset properties of these heterostructures are investigated. Reflection high energy electron diffraction studies exhibited (2 × 2) Ge surface reconstruction after the growth at 450 °C and also revealed a smooth surface for the growth of GaAs on Ge. High-resolution triple crystal x-ray rocking curve demonstrated high-quality Ge epilayer as well as GaAs/Ge/(001)GaAs heterostructures by observing Pendellösung oscillations and that the Ge epilayer is pseudomorphic. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of ∼0.45 nm and room temperature photoluminescence spectroscopy exhibited direct bandgap emission at 1583 nm. Dynamic secondary ion mass spectrometry depth profiles of Ga, As, and Ge display a low value of Ga, As, and Ge intermixing at the Ge/GaAs interface and a transition between Ge/GaAs of less than 15 nm. The valence band offset at the upper GaAs/Ge-(2 × 2) and bottom Ge/(001)GaAs-(2 × 4) heterointerface of GaAs/Ge/GaAs double heterostructure is about 0.20 eV and 0.40 eV, respectively. Thus, the high-quality heterointerface and band offset for carrier confinement in MBE grown GaAs/Ge/GaAs heterostructures offer a promising candidate for Ge-based p-channel high-hole mobility quantum well field effect transistors.

Germanium nanowire growth controlled by surface diffusion effects

Germanium nanowires (NWs) were grown onto Ge(111) substrates by the vapor-liquid-solid process using gold droplets. The growth was carried out in a molecular beam epitaxy chamber at substrate temperatures between 370 °C and 510 °C. The resulting nanowire growth rate turns out to be highly dependent on the substrate temperature exhibiting the maximum at T = 430 °C. The temperature dependence of growth rate can be attributed to surface diffusion both along the substrate and nanowire sidewalls. Analyzing the diffusive material transport yields a diffusion length of 126 nm at a substrate temperature of 430 °C.

Application of hydrogenation to low-temperature cleaning of the Si(001) surface in the processes of molecular-beam epitaxy: Investigation by scanning tunneling microscopy, reflected high-energy electron diffraction, and high resolution transmission electron microscopy

Structural properties of the clean Si(001) surface obtained as a result of low-temperature (470–650 °C) pre-growth annealings of silicon wafers in a molecular-beam epitaxy chamber have been investigated. To decrease the cleaning temperature, a silicon surface was hydrogenated in the process of a preliminary chemical treatment in HF and NH4F aqueous solutions. It has been shown that smooth surfaces composed of wide terraces separated by monoatomic steps can be obtained by dehydrogenation at the temperatures ≳600 °C, whereas clean surfaces obtained at the temperatures <600 °C are rough. It has been found that there exists a dependence of structural properties of clean surfaces on the temperature of hydrogen thermal desorption and the process of the preliminary chemical treatment. The frequency of detachment/attachment of Si dimers from/to the steps and effect of the Ehrlich-Schwoebel barrier on ad-dimer migration across steps have been found to be the most probable factors determining a degree of the resultant surface roughness.

Growth of epitaxial oxides on silicon using atomic layer deposition: Crystallization and annealing of TiO2 on SrTiO3-buffered Si(001)

Epitaxial anatase titanium dioxide (TiO2) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer without any amorphous SiOx layer at the STO–Si interface. Four unit cells of STO grown by molecular beam epitaxy (MBE) serve as the surface template for ALD growth. To preserve the quality of the MBE-grown STO, the samples were transferred in situ from the MBE chamber to the ALD chamber. The growth of TiO2 was achieved using titanium isopropoxide and water as the coreactants at a substrate temperature of 250 °C. In situ x-ray photoelectron spectroscopy analysis revealed that the ALD process did not induce Si–O bonding at the STO–Si interface. Slight improvement in crystallinity of the TiO2 film was achieved through in situ annealing under vacuum (10−9 Torr) at 450–600 °C. However, the amount of Si–O bonding increased following annealing at temperatures greater than 250 °C. X-ray diffraction revealed that TiO2 films annealed at a temperature of 250 °C in vacuum (10−9 Torr) for 1 h were the anatase phase and well crystallized. The results indicate that careful consideration of growth temperature and annealing conditions may allow epitaxial oxide films to be grown by ALD on STO-buffered Si(001) substrates without formation of an amorphous SiOx layer.

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO2) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO2 was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225–250°C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600°C in vacuum (10−7Pa) for 1–2h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO2 by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO2 growth. X-ray diffraction revealed that the TiO2 films were anatase with only the (004) reflection present at 2θ=38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ=37.9°). Anatase TiO2 films up to 100nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.

Development of an in situ magnetoelastic magneto-optical Kerr effect magnetometer

Reported here is the development and implementation of an integrated in situ magnetoelastic measurement setup with a MOKE magnetometer, repositionable electromagnet, and sample transfer/straining device. The former were used within a molecular beam epitaxial vacuum growth chamber. Consequently the magnetostriction constants for both Cr capped and uncapped Fe/GaAs(100) films were acquired without film oxidization occurring. Samples were bent in a four point bending geometry to produce a quantifiable tensile mechanical strain on the films during magnetoelastic measurements. In addition, a laser measurement system was developed to confirm the induced strain in the samples.

結晶成長条件下におけるGaAs(001)表面構造

Surface sturctures of GaAs(001) under growth conditions have been investigated using in situ X-ray diffraction. Theatomic arrangements of GaAs(001) surface stabilized at an elevated temperature under As pressure were quantitativelydetermined in a molecular-beam epitaxy chamber integrated with an X-ray diffractometer. With the help of high angularresolution of synchrotron radiation, disordered sructures appearing in the transition from (2×4) to other phases wereclarified. Energy tunability of synchrotron X-rays allowed for element-sepcific analysis of the c (4×4) struture,providing direct evidence for Ga-As heterodimer formation.

ChemInform Abstract: Nanosynthesis Routes to New Tetrahedral Crystalline Solids: Silicon‐Like Si3AlP.

AbstractThe new monocrystalline semiconductor Si3AlP is synthesized on Si substrates involving thermally activated reactions between P(SiH3)3 and Al atoms generated from a Knudsen cell in a gas source molecular beam epitaxy chamber at 500 °C and 10‐5 Torr.

Revelation of Topological Surface States in Bi2Se3 Thin Films by In Situ Al Passivation

Topological insulators (TIs) are extraordinary materials that possess massless, Dirac-like topological surface states in which backscattering is prohibited due to the strong spin-orbit coupling. However, there have been reports on degradation of topological surface states in ambient conditions, which presents a great challenge for probing the original topological surface states after TI materials are prepared. Here, we show that in situ Al passivation inside a molecular beam epitaxy (MBE) chamber could inhibit the degradation process and reveal the pristine topological surface states. Dual evidence from Shubnikov-de Hass (SdH) oscillations and weak antilocalization (WAL) effect, originated from the π Berry phase, suggests that the helically spin-polarized surface states are well preserved by the proposed in situ Al passivation. In contrast, we show the degradation of surface states for the unpassivated control samples, in which the 2D carrier density is increased 39.2% due to ambient n-doping, the SdH oscillations are completely absent, and a large deviation from WAL is observed.

Molecular beam epitaxy growth of BaTiO3 thin films and crucial impact of oxygen content conditions on the electrical characteristics

In this study, high quality crystalline BaTiO3 films were fabricated in different oxygen content conditions on Nb-doped SrTiO3 (001) substrates by molecular beam epitaxy (MBE). BaTiO3 epitaxial film presents a critical thickness of 6.4nm and is almost entirely relaxed on SrTiO3 with a thickness of ~50nm. The electrical properties of 50nm strain-free BaTiO3 films were investigated by both macro- and microscopic measurements. Their electrical characteristics were found to be strongly influenced by different oxygen content conditions used during the preparation or annealing of the samples. Limited molecular oxygen partial pressure in a MBE chamber probably leads to a great amount of oxygen vacancies in the oxide film and results in rectification behavior of the oxygen-deficient BaTiO3-x/SrTiO3:Nb interface. Several approaches such as using atomic oxygen ambiance during the growth, annealing under elevated oxygen pressure were employed in order to decrease the oxygen vacancy density and these approaches eventually permit obtaining BaTiO3 films with good ferroelectric characteristics.

Growth of Few-Layer Graphene on Sapphire Substrates by Directly Depositing Carbon Atoms

Few-layer graphene (FLG) is successfully grown on sapphire substrates by directly depositing carbon atoms at the substrate temperature of 1300°C in a molecular beam epitaxy chamber. The reflection high energy diffraction, Raman spectroscopy and near-edge x-ray absorption fine structure are used to characterize the sample, which confirm the formation of graphene layers. The mean domain size of FLG is around 29.2 nm and the layer number is about 2–3. The results demonstrate that the grown FLG displays a turbostratic stacking structure similar to that of the FLG produced by annealing C-terminated α-SiC surface.

Dielectric response of AlP by in-situ ellipsometry

We present and analyze pseudodielectric function data < ε> = < ε 1> + i< ε 2> of AlP from 0.75 to 5.05 eV. The sample is a 1.0 μm thick AlP film grown on (001) GaAs by molecular beam epitaxy (MBE). Spectroscopic ellipsometric data were obtained before removing the sample from the MBE chamber to avoid oxidation and related artifacts. Analysis of interference oscillations and corrections for overlayer effects with a multilayer parametric model yield the closest representation to date of the intrinsic bulk dielectric response ε of AlP. From this analysis we obtain the energies of the E 0′ and E 1 critical points of AlP.

A versatile metal-halide vapor chemistry for the epitaxial growth of metallic, insulating and semiconducting films

A metal-chloride-based chemistry that bypasses many traditional pitfalls present in the epitaxy of refractory-metal-containing films is described. Methods of dealing with the corrosive chloride-based chemistry in a Molecular Beam Epitaxy (MBE) chamber are detailed. The present state of refractory metal oxide thin film heteroepitaxy is presented along with an explanation of the deposition chemistry involved. To demonstrate the chemical diversity of this technology, oxidation-limited conditions in a vacuum environment are used to grow single crystal epitaxial films of; lithium niobate (LiNbO 3), a wide bandgap ferroelectric; lithium niobite (LiNbO 2), a semiconducting lithium-metal-oxide; and epitaxial metals niobium and iron. Additionally, the chemistry can easily be applied to other chemical vapor deposition and atomic layer epitaxial growth methods. The novel LiNbO 2 thin films are confirmed to have a 2.0 eV bandgap, and several are found to have unusually high p-type conductivities of up to 2500 S/cm.

Substrate wobble compensation for in situ spectroscopic ellipsometry measurements

Spectroscopic ellipsometry (SE) is an optical characterization technique that has been implemented on molecular beam epitaxy chambers for in situ characterization and growth control. SE measurements require collecting light reflected obliquely from the substrate. As the substrate is usually rotating during growth (to promote uniformity), acquiring accurate in situ SE data typically requires a substrate manipulator with low wobble (preferably &amp;lt;0.1°). To overcome the low wobble manipulator requirement, the authors have designed and tested a return path SE configuration that compensates for the effects of substrate wobble. The prototype wobble compensation system demonstrated the near elimination of beam precession in the outgoing ellipsometer beam. The accuracy of the SE data acquired in the return path configuration during substrate rotation was also studied.

Phase transition between (2 × 1) and c(8 × 8) reconstructions observed on the Si(001) surface around 600°C

The Si(001) surface subjected to different treatments in ultrahigh vacuum molecular beam epitaxy chamber for SiO$_2$ film decomposition has been in situ investigated by reflected high energy electron diffraction (RHEED) and high resolution scanning tunnelling microscopy (STM). A transition between (2 x 1) and (4 x 4) RHEED patterns was observed. The (4 x 4) pattern arose at T <~600C during sample posttreatment cooling. The reconstruction was observed to be reversible. The c(8 x 8) structure was revealed by STM at room temperature on the same samples. The (4 x 4) patterns have been evidenced to be a manifestation of the c(8 x 8) surface structure in RHEED. The phase transition appearance has been found to depend on thermal treatment conditions and sample cooling rate.

Excitation Intensity Driven PL Shifts of SiGe Islands on Patterned and Planar Si(001) Substrates: Evidence for Ge-rich Dots in Islands

For randomly nucleated SiGe/Si(001) islands, a significantly stronger blue-shift of the PL spectra as a function of the excitation intensity is observed when compared to islands grown on patterned substrates side by side within the same run in a solid source molecular beam epitaxy chamber. We ascribe this different PL behavior to the much larger inhomogeneity of the Ge distribution in islands on planar substrates when compared to islands grown on pit-patterned ones, as observed previously. 3D band-structure calculations show that Ge-rich inclusions of approximately 5 nm diameter at the apex of the islands can account for the observed differences in the PL spectra. The existence of such inclusions can be regarded as a quantum dot in an island and is in agreement with recent nano-tomography experiments.

TEM analysis of the container effect of Au‐based catalyst droplets during vapour‐liquid‐solid growth of axial ZnTe/CdTe nanowires

AbstractAxial heterostructure nanowires (NWs) of ZnTe/CdTe were grown by vapour‐liquid‐solid growth realized in a molecular beam epitaxial chamber. By alternative supply of Zn or Cd and constant Te the heterostructure was generated. The liquid phase is provided by a Au‐based eutectic droplet which stays at the tip of the NW during the entire growth. For structural and chemical characterization by TEM the NWs were harvested from the substrate and transferred to a holey carbon film. The NWs exhibit an expansion of the diameter correlated with the interface region between ZnTe and CdTe. Idiomorphic growth of the CdTe is evident from electron diffraction experiments. The growth rate of CdTe appears to be smaller compared to that of ZnTe at the same temperature. Both, quantitative high‐resolution TEM and energy dispersive X‐ray spectroscopy line scans reveal a smeared ZnTe/CdTe interface along about 200 nm. The smearing is due to both, the liquid catalyst which buffers the supply of Cd instead of Zn at the liquid/solid interface and to the strain which is induced by the lattice mismatch. It forces the system to consume the remnant Zn for the NW growth in favour of Cd. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Defects of Ge quantum dot arrays on the Si(001) surface

The defects of Ge quantum dot arrays formed at moderate temperatures on the Si(0 0 1) surface have been investigated by the ultra high vacuum scanning tunnelling microscope integrated with the molecular beam epitaxy chamber. A preliminary classification of the defects has been carried out. Morphological peculiarities of the defects have been studied. The surface densities of defects of different types have been determined.

Epitaxial Growth of Graphene on 6H-SiC (0001) by Thermal Annealing

An epitaxial graphene (EG) layer is successfully grown on a Si-terminated 6H-SiC (0001) substrate by the method of thermal annealing in an ultrahigh vacuum molecular beam epitaxy chamber. The structure and morphology of the EG sample are characterized by reflection high energy diffraction (RHEED), Raman spectroscopy and atomic force microscopy (AFM). Graphene diffraction streaks can be seen in RHEED. The G and 2D peaks of graphene are clearly observed in the Raman spectrum. The AFM results show that the graphene nominal thickness is about 4−10 layers.