Multichamber Ultrahigh Vacuum Research Articles

Magnetization reversal in Fe(001) films grown by magnetic field assisted molecular beam epitaxy

We studied the influence of a magnetic field (MF) on epitaxial growth and magnetic properties of Fe(001) films deposited on MgO(001). Thanks to modular sample holders and a specialized manipulator in our multi-chamber ultrahigh vacuum system, the films could be deposited and annealed in an in-plane MF of 100 mT. In situ scanning tunnelling microscopy showed that MF had a strong influence on the film morphology, and, in particular, on the structure of surface steps. The magnetic properties were studied ex situ using magneto-optic Kerr effect (MOKE) magnetometry and microscopy. We showed that the moderate in-plane magnetic field applied during growth has the visible impact on the magnetic properties. The observed angular dependence of the MOKE loops and domain structures were discussed based on a magnetization reversal model. In particular we found that magnetization reversal occurs via 90° domains and the reversal differs for the no-field and in-field grown samples, in correlation with the film morphology.

Design and performance of an ultra-high vacuum spin-polarized scanning tunneling microscope operating at 30 mK and in a vector magnetic field.

We describe the design and performance of a scanning tunneling microscope (STM) that operates at a base temperature of 30 mK in a vector magnetic field. The cryogenics is based on an ultra-high vacuum (UHV) top-loading wet dilution refrigerator that contains a vector magnet allowing for fields up to 9 T perpendicular and 4 T parallel to the sample. The STM is placed in a multi-chamber UHV system, which allows in situ preparation and exchange of samples and tips. The entire system rests on a 150-ton concrete block suspended by pneumatic isolators, which is housed in an acoustically isolated and electromagnetically shielded laboratory optimized for extremely low noise scanning probe measurements. We demonstrate the overall performance by illustrating atomic resolution and quasiparticle interference imaging and detail the vibrational noise of both the laboratory and microscope. We also determine the electron temperature via measurement of the superconducting gap of Re(0001) and illustrate magnetic field-dependent measurements of the spin excitations of individual Fe atoms on Pt(111). Finally, we demonstrate spin resolution by imaging the magnetic structure of the Fe double layer on W(110).

Growth and characterization of stable SrO-terminated SrTiO3 surfaces

A simple technique for the growth of SrO-terminated SrTiO3 surfaces is reported. High quality SrTiO3 epitaxial films were grown by reflection high energy electron diffraction assisted pulsed laser deposition on suitably prepared NdGaO3 (110) substrates. The surface properties, analyzed within a growth/characterization multichamber ultrahigh vacuum system by photoemission spectroscopy performed on the core-level spectra of Sr and Ti, low energy electron diffraction-, scanning tunneling-, and atomic force microscopy, are fully consistent with a single Sr oxide termination. The availability of such high quality SrO-terminated SrTiO3 surfaces is of major importance for the controlled growth of oxide epilayers and interfaces.

Cryogenic variable temperature ultrahigh vacuum scanning tunneling microscope

A cryogenic variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has been developed. This design utilizes a novel vibration isolation that provides an active thermal link to the cooling source without the standard tradeoff of compromising mechanical isolation. A welded bellows serves as the basis for the vibration isolation system. This bellows houses a heat exchanger which uses helium gas to form a thermal link between the STM and the cryogenic. This STM is fully integrated into a multichamber UHV STM system now in place in our laboratory and atomic resolution operation is demonstrated from 11 to 300 K.

Epitaxial CrN(001) Grown and Analyzed In situ by AES After (1) Deposition and (2) Ar+ Sputter Etching

In situ Auger electron spectroscopy (AES) spectra were obtained on single-crystal CrN(001) layers grown on MgO(001)1 × 1 in a multichamber ultrahigh vacuum (UHV) system. A turbomolecular-pumped magnetron sputter deposition chamber was attached to a Physical Electronics model 660 scanning Auger system to allow spectra to be collected on the as-deposited film without exposure to air. The spectra show that the film surfaces are free of O and C. Spectra are also presented following sputter etching with a 3 keV Ar+ beam at an incident current density of 0.3 mA cm−2. The film was sputtered until the N/Cr peak height ratio reached equilibrium, then spectra were acquired for the after sputter condition. AES spectra were collected for both the as-deposited and the Ar+ sputter etched films using primary beam energies of 3, 5, 10, and 20 keV. The as-deposited bulk film composition was analyzed by Rutherford backscattering spectroscopy (RBS) and found to be stoichiometric with a N/Cr ratio of 1.02 ± 0.02.

Epitaxial VN(001) Grown and Analyzed In situ by AES After (1) Deposition and (2) Ar+ Sputter Etching

In situ Auger electron spectroscopy (AES) spectra were obtained on single-crystal VN(001) layers grown on MgO(001)1 × 1 in a multichamber ultrahigh vacuum (UHV) system. A turbomolecular-pumped magnetron sputter deposition chamber was attached to a Physical Electronics model 660 scanning Auger system to allow spectra to be collected on the as-deposited film without exposure to air. The spectra show that the film surfaces are free of O and C. Spectra are also presented following sputter etching with a 3 keV Ar+ beam at an incident current density of 0.3 mA cm−2. The film was sputtered until the N/V peak height ratio reached equilibrium, then spectra were acquired for the after sputter condition. AES spectra were collected for both the as-deposited and Ar+ sputter etched films using primary beam energies of 3, 5, 10, and 20 keV. The as-deposited bulk film composition was analyzed by Rutherford backscattering spectroscopy (RBS) and found to be stoichiometric with a N/V ratio of 1.04 ± 0.02.

Epitaxial TiN(001) Grown and Analyzed In situ by AES After (1) Deposition and (2) Ar+ Sputter Etching

In situ Auger electron spectroscopy (AES) spectra were obtained on single-crystal TiN(001) layers grown on MgO(001)1 × 1 in a multichamber ultrahigh vacuum (UHV) system. A turbomolecular-pumped magnetron sputter deposition chamber was attached to a Physical Electronics model 660 scanning Auger system to allow spectra to be collected on the as-deposited film without exposure to air. The spectra show that the film surfaces are free of O and C. Spectra are also presented following sputter etching with a 3 keV Ar+ beam at an incident current density of 0.3 mA cm−2. The film was sputtered until the N/Ti peak height ratio reached equilibrium, then spectra were acquired for the after sputter condition. AES spectra were collected for both the as-deposited and the Ar+ sputter etched films using primary beam energies of 3, 5, 10, and 20 keV. The as-deposited bulk film composition was analyzed by Rutherford backscattering spectroscopy (RBS) and found to be stoichiometric with a N/Ti ratio of 1.02 ± 0.02.

Epitaxial ScN(001) Grown and Analyzed In situ by AES After (1) Deposition and (2) Ar+ Sputter Etching

In situ Auger electron spectroscopy (AES) spectra were obtained on single-crystal ScN(001) layers grown on MgO(001) 1 × 1 in a multichamber ultrahigh vacuum (UHV) system. A turbomolecular-pumped magnetron sputter deposition chamber was attached to a Physical Electronics model 660 scanning Auger system to allow spectra to be collected on the as-deposited film without exposure to air. The spectra show that the film surfaces are free of O and C. Spectra are also presented following sputter etching with a 3 keV Ar+ beam at an incident current density of 0.3 mA cm−2. The film was sputtered until the N/Sc peak height ratio reached equilibrium, then spectra were acquired for the after sputter condition. AES spectra were collected for both the as-deposited and the Ar+ sputter etched films using primary beam energies of 3, 5, 10, and 20 keV. The as-deposited bulk film composition was analyzed by Rutherford backscattering spectroscopy (RBS) and found to be stoichiometric with a N/Sc ratio of 1.06 ± 0.03.

In situ preparation of the semiconductor–superconductor interface: Nb contacts on InAs quantum wells

In this article we describe an in situ, low damage, process sequence designed to provide a more reliable, higher quality metal (superconductor)–semiconductor interface. We produce discrete Nb contacts on an InAs quantum well (125 A) that is embedded within thick (∼3000 A) Al0.5Ga0.5Sb barrier layers by utilizing a multichamber ultrahigh vacuum system, in which two Varian GenII, Molecular Beam Epitaxy chambers are directly connected to a high vacuum etch station. The principal process steps include (1) a short, “selective” thermal Cl2 etch of the top barrier layer to expose and define the InAs contact region, (2) a post-etch anneal (under As2 flux) to desorb residual etch products (i.e., InCl3) and restore a clean and well-ordered InAs (2×4) surface, (3) a Sb capping/passivation layer on the treated surface, (4) atmospheric transfer of the capped sample to a remote Nb deposition chamber, and (5) a post-anneal (in vacuum) to desorb the Sb cap immediately followed by Nb deposition. The priority throughout is...

Structural properties of Ga2O3(Gd2O3)–GaAs interfaces

Ga 2 O 3 (Gd 2 O 3 )–GaAs heterostructures in situ fabricated using a multichamber ultrahigh vacuum (molecular beam epitaxy) system were studied by x-ray reflectivity measurement and high-resolution transmission electron microscopy. The oxide–GaAs interfaces were found to be very smooth with the roughness no more than 1 nm. Moreover, an interfacial roughness as small as one atomic layer of GaAs (0.33 nm) was observed using x-ray reflectivity.

Wafer-transfer robot for use in ultrahigh vacuum

A polar coordinate wafer-transfer robot has been developed that enables thin-film manufacturing to be done in an ultrahigh vacuum. This robot features a frictionless spring structure that operates in a vacuum and is magnetically driven by a motor in the atmosphere (Nikkei Mechanical 1993.5.17). The spring arms are made of sheet springs and rigid plates and were optimized by simulation using the finite element method. All moving parts in the vacuum environment have a minimum of two bearings. As a result, a repeat position precision of 0.2 mm or less was able to be achieved in the atmosphere. Also, a maximum transfer distance of 800 mm and a transfer speed of 400 mm/s were achieved at a pressure of 10−7 Pa. After operation in a clean room at atmospheric pressure, only three particles above 0.1 μm in size were observed. Using this wafer-transfer robot, the area needed for film manufacturing in a multichamber ultrahigh vacuum can be reduced about 30% compared with using a conventional linear-magnet–coupling wafer-transfer robot.

Versatile sample handling system for scanning tunneling microscopy studies of molecular beam epitaxy

Abstract : The ongoing development of short-period semiconductor superlattices for electronic and optoelectronic applications requires atomic-scale control of epitaxial growth, especially at the interfaces. Given this requirement, there is a critical need for in situ characterization on the atomic scale as provided by scanning tunneling microscopy (STM). Here we describe a sample handling system designed to integrate a modified commercial STM into a multichamber ultrahigh vacuum (UHV) molecular beam epitaxy (MBE) facility. The system uses a simple, yet versatile, sample holder design that enables quick and easy sample transfers between multiple chambers, including two Riber MBE and two surface characterization chambers interconnected by Riber UHV ModuTrac (Trademark).

Properties of Gate-Quality SiO2 Films Prepared by Electron Cyclotron Resonance Chemical Vapour Deposition in an Ultrahigh Vacuum Processing System

ABSTRACTWe have prepared thin SiO2 layers on Si(100) wafers by electron cyclotron resonance chemical vapour deposition (ECR-CVD) in a multi-chamber ultra-high vacuum (UHV) processing system. The oxides were characterized in-situ by single wavelength ellipsometry (SWE) and x-ray photoelectron spectroscopy (XPS) and ex-situ by Fourier transform infra-red spectroscopy (FTIR), spectroscopic ellipsometry (SE) and capacitance-voltage (CV) electrical measurements. Films deposited at higher pressures, low powers and low silane flow rates had excellent physical and electrical properties. Films deposited at 400 °C had better physical properties than those of thermal oxides grown in dry oxygen at 700 °C. A 1 minute anneal at 950 °C reduced the fast interface state density from 1.2×1011 to 7×1010 eV−1cm−2

Multi-chamber Ultrahigh Vacuum Scanning Tunneling Microscope System for Investigating Processed GaAs Surfaces and Observation of Argon-Ion-Bombarded GaAs Surfaces

A multi-chamber ultrahigh vacuum scanning tunneling microscopy (UHV-STM) system has been constructed for investigations of processed GaAs surfaces. This system comprises five UHV chambers connected by UHV tunnels. A specially designed STM unit which can be used with a 23-mm-diameter molybdenum block, is installed in an STM-chamber for surface observations. A molecular beam epitaxy (MBE) chamber for epitaxial growth of GaAs layers and a process-chamber for argon ion-bombardment are included in the five chambers. First, the surface of the MBE-grown layer on a GaAs (001) wafer was observed with STM. The sample was then transferred to the process-chamber and bombarded with argon ions. Many defects, which seemed to be caused by the ion-bombardment, were found in an STM image taken with the ion-bombarded sample.

Scanning tunneling microscopy of molecular-beam epitaxially grown GaAs (001) surfaces

A multichamber ultrahigh vacuum (UHV) scanning tunneling microscope (STM) system which includes a molecular-beam epitaxy (MBE) growth chamber, as well as a STM chamber, has been constructed for the investigation of processed GaAs surfaces. We observed MBE-grown GaAs surfaces using this system. Samples were grown on GaAs (001) surfaces and cooled to certain temperatures in an As4 flux before being transferred into an UHV. STM images of c(4×4), 2×4, and mixed c(4×2) and 2×2 structures were obtained for the samples cooled to 330, 570, and 470 °C in an As4 flux, respectively. The mixed c(4×2) and 2×2 structure seems to be formed by desorption of As atoms from the c(4×4) surface in an UHV. Also, a new reconstructed structure which has a 2.8-nm periodicity along both the [110] and [11̄0] directions was observed. It seems to be a local structure which has a larger arsenic dimer density than that of the c(4×4) structure.

Surface etching and roughening in integrated processing of thermal oxides

A multichamber ultrahigh vacuum processing and analysis system has been used to study integrated thermal oxide processing, in which the final precleaning process and the thermal oxidation process are integrated by employing transfer of the wafers through ultraclean, inert ambients (purified, dry N2 and then ultrahigh vacuum). Al-gate metal oxide semiconductor capacitors show high breakdown fields (∼12 MV/cm) when a thin oxide passivation layer is present prior to oxidation, but low breakdown fields (< 6MV/cm) when the Si surface is initially oxygen-free. This contrasting behavior is caused by the etching of Si surfaces which occurs at elevated temperature in the presence of trace concentrations (∼100 ppb) of oxygen (e.g., 2Si+O2→2SiO↑ ), leading to surface roughening and then field enhancement at asperities in the structure. Oxide surface passivation prevents etching and assures the dielectric integrity of the structure.