Molecular Beam Epitaxy Chamber Research Articles

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.

A molecular beam epitaxy facility for in situ neutron scattering

A molecular beam epitaxy (MBE) facility has been built to enable in situ neutron scattering measurements during growth of epitaxial layers. While retaining the full capabilities of a research MBE chamber, this facility has been optimized for polarized neutron reflectometry measurements. Optimization includes a compact lightweight portable design, a neutron window, controllable magnetic field, deposition across a large 76 mm diameter sample with exceptional flux uniformity, and sample temperatures continuously controllable from 38 to 1375 K. A load lock chamber allows for sample insertion, storage of up to 4 samples, and docking with other facilities. The design and performance of this chamber are described here.

Heteroepitaxial growth of a rotated AlInSb layer mediated by an InSb bi‐layer on a Si(111) substrate

AbstractTwo‐step growth of an AlInSb film mediated by an InSb bi‐layer on a Si(111) substrate was performed in a molecular beam epitaxy (MBE) chamber. The growth was observed by in situ reflection high energy electron diffraction. After the deposition, the samples were characterized by X‐ray diffraction (XRD). It is confirmed that the grown AlInSb films were rotated by 30° with respect to the Si substrates. The lattice mismatch between the AlInSb and the Si substrate was reduced by the rotation. Using this method, epitaxial AlyIn(1‐y)Sb layers which have the composition ratio y of 0.22‐0.65 can be achieved without thick buffer layers. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Unintentional aluminum incorporation related to the introduction of nitrogen gas during the plasma-assisted molecular beam epitaxy

We examine unintentional incorporation of Al during the growth of molecular beam epitaxy (MBE) related to N gas introduction. In spite of the closed shutter of Al cell, we observe Al incorporation in the epitaxial layer with a concentration up to 1×10 18 cm −3. The concentration depends on the N 2 gas flow rate and Al source temperature. The concentration is suppressed by the reduction of the Al cell temperature. Shutter control of the N cell, As beam equivalent pressure and the operation of the RF plasma power have no impact on that. The introduction of the large amount of N can modify the beam dispersion of Al in the MBE chamber, which will cause the extrinsic Al incorporation. The unintentional impurity incorporation can induce material deteriorations. We thus suggest that the growth should be carried out with the decreased Al cell temperature and the N gas flow rate as low as possible.

Ultralow resistance in situ Ohmic contacts to InGaAs/InP

We report a sharp reduction in the resistivity of Ohmic contacts using in situ deposition of molybdenum (Mo) contacts onto n-type In0.53Ga0.47As grown on InP. The contacts were formed by evaporating Mo onto the wafer using an electron beam evaporator connected to a molecular beam epitaxy chamber under ultrahigh vacuum. Transmission line measurements showed specific contact resistivities of 0.5±0.3 Ω μm2 (2.90 Ω μm), 0.9±0.4 Ω μm2 (4.3 Ω μm), and 1.3±0.4 Ω μm2 (4.7 Ω μm) for Mo on abrupt InAs/InGaAs heterojunctions, graded InAs/InGaAs, and InGaAs films, respectively. These low resistances meet the requirements for terahertz transistors.

High-quality III–V semiconductor MBE growth on Ge/Si virtual substrates for metal-oxide-semiconductor device fabrication

We describe the molecular-beam epitaxial (MBE) growth and fabrication of III–V metal-oxide-semiconductor (MOS) devices on Ge/Si virtual substrates. We show that high-temperature in-situ H 2 annealing in the chemical-vapor deposition system changes the Ge surface configuration and produces a surface with predominantly double-step-layer conditions, which is crucial for the growth of single-domain GaAs. In addition, the surface morphology of III–V on Ge/Si improved significantly with an annealing treatment of the Ge surface carried out under high arsenic background pressure in the MBE chamber. This facilitates uniform As-monolayer formation on the entire Ge surface. Low-temperature migration-enhanced epitaxy (MEE) and low-temperature conventional GaAs growth not only enhance the growth of single-domain GaAs without Ge outdiffusion but also produce a sufficiently smooth surface for high-k dielectric deposition, achieving low leakage current. A 300-nm-thick GaAs buffer layer was grown, followed by a 10 nm growth of In 0.2Ga 0.8As high-mobility channel layer. A 7–8-nm-thick Al 2O 3 layer was deposited ex-situ by atomic-layer deposition (ALD). We verify the quality of III–V growth using transmission electron microscopy (TEM), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and photoluminescence (PL) measurement. The C–V characteristics show unpinning of the Fermi level, which is a necessary condition for gate voltage control of the drain current. This work suggests this materials combination is a promising candidate for the realization of advanced, nonclassical complementary-MOS and optoelectronic devices on Si substrates.

GaAs high‐k dielectric metal‐insulator‐semiconductor structure having silicon interface control layer

AbstractA pinning‐free high‐k dielectric metal‐insulator‐semiconductor (MIS) gate stack was realized on GaAs surfaces by using a Si interface control layer (Si ICL). The MIS structure has a SiNx/Si ICL ultrathin double layer inserted between GaAs and HfO2. Si ICL was grown epitaxially on a Ga‐stabilized GaAs surface by molecular beam epitaxy (MBE). The ultrathin SiNx buffer film was formed by in situ partial nitridation of Si ICL in the MBE chamber. An X‐ray photoelectron spectroscopy (XPS) analysis indicated that the ultrathin SiNx/Si ICL structure is chemically stable against air‐exposure, preventing the Si ICL/GaAs hetero‐interface from oxidation, although the ultrathin SiNx film itself partially turns into SiOxNy. Using this feature, a high‐k MIS capacitor was formed by ex situ deposition of HfO2 on the SiOxNy/Si ICL/GaAs structure. The capacitance‐voltage (C‐V) analysis of the high‐k MIS sample after rapid thermal annealing at 500 °C for 30 sec indicated that the MIS interface is completely free from Fermi level pinning with a minimum interface state density below 1011 cm–2eV–1. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the unintentional incorporation of Al during the molecular beam epitaxial growth of GaInNAs quantum well

AbstractWe report the effect of unintentional Al incorporation during the growth of GaInNAs/GaAs quantum well (QW) by molecular beam epitaxy (MBE). The effect of extrinsic Al is investigated by secondary ion mass spectrometry. QW grown directly on AlAs layer shows unintentionally incorporated Al with its concentration as high as 1×1018 cm–3, resulting in degraded PL intensity. In order to prevent the Al incorporation, we grow QW without heating up Al source after leaving the MBE chamber within As atmosphere for a long period. The concentration of the extrinsic Al becomes lower than 2×1016 cm–3, leading to a strong PL intensity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure and peculiarities of the (8 × n)-type Si(001) surface prepared in a molecular-beam epitaxy chamber: A scanning tunneling microscopy study

A clean Si(001) surface thermally purified in an ultrahigh vacuum molecular-beam epitaxy chamber has been investigated by means of scanning tunneling microscopy. The morphological peculiarities of the Si(001) surface have been explored in detail. The classification of the surface structure elements has been carried out, the dimensions of the elements have been measured, and the relative heights of the surface relief have been determined. A reconstruction of the Si(001) surface prepared in the molecular-beam epitaxy chamber has been found to be (8 x n). A model of the Si(001)-(8 x n) surface structure is proposed.

In‐situ X‐ray diffraction during epitaxial growth of ZnSe‐based heterostructures on (001)GaAs

AbstractWe developed a real‐time in‐situ X‐ray (RIX) diffraction technique for the detailed layer characterisation during epitaxial deposition of ZnSe‐based alloys on (001)GaAs substrates. A conventional molecular beam epitaxy chamber was equipped with a Cu X‐ray tube. The 113 reflection of the deposited structure is detected in static geometry by a CCD and is immediately analyzed in terms of strain, composition, thickness and strain relaxation behavior of the deposited layer. The in‐situ monitoring of ZnCdSe layers reveals a delay of heteroepitaxial growth start on GaAs, a gradual layer degradation due to dislocation formation, and a critical thickness for plastic relaxation which depends on the Cd content, consistent with an energy‐balance model. ZnCrSe alloy layers can be deposited with Cr content up to 2% at low substrate temperature TS = 210 °C. The layers appear to be epitaxial alloys with good quality, but a broadening of the Bragg peaks without a significant plastic relaxation is observed. Epitaxial growth and layer properties are studied as function of deposition conditions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of ultrathin SiNx∕Si interface control double layer on (001) and (111) GaAs surfaces for ex situ deposition of high-k dielectrics

In order to realize pinning-free high-k dielectric metal-insulator-semiconductor (MIS) gate stack on (001) and (111)B oriented GaAs surfaces using the Si interface control layer (Si ICL) concept, formation of a SiNx∕Si ICL double layer was investigated as a chemically stable structure on (001) and (111)B surfaces which allows ex situ deposition of HfO2 high-k dielectric films without losing the benefit of Si ICL. First, Si ICLs grown by molecular beam epitaxy (MBE) on (001) and (111)B GaAs surfaces with various initial surface reconstructions were investigated in detail by reflection high energy electron diffraction and x-ray photoelectron spectroscopy (XPS) investigations at each step of the interface formation. Large shifts of the surface Fermi level position toward unpinning were observed after Si ICL growth on appropriately formed Ga-stabilized surfaces. It was found that Si layers grow epitaxially with Si–Ga bonds at the Si∕GaAs interface and Si–As termination on top, suggesting surfactant roles played by As atoms. Then, an ultrathin SiNx buffer film was formed on the Si ICL by its in situ partial nitridation in the MBE chamber. An XPS analysis of the resultant SiNx∕Si ICL double layer formed on (001) and (111)B surface indicated that the structure is chemically stable against air exposure on both surfaces in the sense that it prevents the host GaAs surface from subcutaneous oxidation, although SiNx film itself partially turns into SiOxNy. Finally, high-k MIS capacitors were formed by ex situ deposition of HfO2 on the SiNx∕Si ICL/GaAs structure after transferring the sample through air. The capacitance-voltage (C-V) analysis indicated that the MIS interface is completely pinning-free with a minimum interface state density in the range of low 1011cm−2eV−1.

Annealing condition optimization and electrical characterization of amorphous LaAlO3∕GaAs metal-oxide-semiconductor capacitors

The electrical properties of amorphous LaAlO3∕GaAs metal-oxide-semiconductor capacitors fabricated using molecular-beam deposition are investigated. The surface was protected during sample transfer between III-V and oxide molecular beam epitaxy chambers by a thick arsenic-capping layer. Amorphous LaAlO3 was deposited on c(4×4) and (2×4) reconstructed (100) GaAs surfaces. An annealing method, a low temperature-short time rapid thermal annealing (RTA) followed by a high temperature RTA, was developed, yielding extremely small hysteresis (∼30mV), frequency dispersion (∼60mV), and interfacial trap density (mid-1010eV−1cm−2).

Monitoring of ZnCdSe layer properties by in situ x-ray diffraction during heteroepitaxy on (001)GaAs substrates

The authors demonstrate in situ high-resolution x-ray diffraction applied during heteroepitaxy on (001)GaAs for instant layer characterization. The current thickness, composition, strain, and relaxation dynamics of pseudomorphic layers are precisely determined from qz scans at the (113) reflection measured at a molecular beam epitaxy chamber with a conventional x-ray tube in static geometry. A simple fitting routine enables real-time in situ x-ray diffraction analysis of layers as thin as 20nm. Critical thicknesses for dislocation formation and plastic relaxation of ZnCdSe layers versus Cd content are determined. The strong influence of substrate temperature on heteroepitaxial nucleation process, deposition rate, composition, and strain relaxation dynamics of ZnCdSe on GaAs is also studied.

Recovery of Dry Etching–Induced Damage in n-GaN by Nitrogen Plasma Treatment at Growth Temperature

The n-GaN films grown by metal-organic chemical vapor deposition (MOCVD) are etched in an inductively coupled plasma (ICP) reactor chamber. Atomic-force microscopy (AFM) characterization shows an increase of the surface roughness after the etching. Furthermore, Hall measurement and photoluminescence (PL) spectra highlight deterioration of the electrical and optical properties, respectively. Attempts to recover the damage are carried out by nitrogen plasma treatments accompanied by thermal annealing at the growth temperature in a molecular-beam epitaxy (MBE) chamber. Improved electrical and optical properties compared with those of the as-etched GaN, evidenced in both Hall and PL measurements, show a pronounced decrease of the damage introduced by the ICP etching.

Evaluation of sapphire substrate heating behaviour using GaN band-gap thermometry

The recent development of a commercial band-gap thermometry system for wide band-gap materials such as GaN (the k-Space Associates’ “ BandiT TM ”) has allowed a systematic study of the relationship between pyrometric or thermocouple temperature-monitoring methods and the directly determined layer temperature. The wide band gap of GaN gives a weak and sample-dependent coupling to radiant heat sources, and it is common in molecular beam epitaxy (MBE) to use a sputtered Mo coating on the rear of the sapphire substrate to improve the efficiency and consistency of heat transfer. We have investigated the role of this backing layer and the use of a PBN diffuser in an MBE chamber, but similar results are expected to be obtained from band-gap thermometry in a metal-organic vapour phase epitaxy (MOVPE) system. The wide range spectrometer used for the band-edge determination can also be employed in a pyrometric mode, at wavelengths both within the band gap of GaN and above it. The latter is insensitive to thickness oscillations, and is less affected by the presence or absence of a Mo backing layer. Results are presented comparing all these measurement techniques, and analysed to show that the Mo backing does not significantly increase the power coupling into the epitaxial layer, although it does improve the accuracy of pyrometric measurements.

Real-time in-situ flux monitoring by wavelength-modulated atomic absorption spectroscopy in molecular beam epitaxy: Application to Ga flux measurement

A flux monitoring set-up is described, based on wavelength-modulated atomic absorption spectroscopy (WMAAS) in-situ real-time measurements. It is particularly suited for element III fluxes monitoring during the growth of III–V semiconductors and heterostructures inside a molecular beam epitaxy (MBE) chamber. The tunable source is a blue-violet laser diode mounted in an external cavity, followed by an interferometer required to actively stabilize the emitted wavelength. Measurements have been carried out for the Ga element at 403.3 nm, with a growth rate in the range 0.027–1.5 ML/s on GaAs. The WMAAS signal shows a linear dependence on the growth rate over the whole range studied here. The uncertainty and low flux limit are measured to be 0.01 ML/s at most.

Molecular Beam Epitaxy of Lithium Niobate Multifunctional Materials Using a Chloride Refractory Metal Chemistry

A new chloride based chemistry that bypasses many of the traditional pitfalls of Lithium Niobate (LN) epitaxy including the need for electron beam evaporation of refractory metals, and use of complex metal organic precursors prone to pre-reaction is detailed. Methods of dealing with the normally corrosive chloride based chemistry in a Molecular Beam Epitaxy (MBE) chamber are detailed. These methods include careful opening procedures to minimize moisture uptake and a highly effective Li-getter pump that forms a natural desiccant material, LiCl, enhancing water pumping and minimizing corrosive HCl production. The present state of heteroepitaxy of LN on semiconductors is presented with novel optoelectronic and electronic device concepts presented as motivation for development of this multifunctional material - semiconductor system. Future multifunctional applications offer much promise but also strongly hinge on the limits of film quality that can be achieved.

Combined molecular beam epitaxy low temperature scanning tunneling microscopy system: Enabling atomic scale characterization of semiconductor surfaces and interfaces

A molecular beam epitaxy and low temperature scanning tunneling microscopy chamber have been integrated to characterize both compound and elemental semiconductor surfaces and interfaces. The integration of these two commercially available systems has been achieved using a custom designed sample transfer mechanism. The MBE growth chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ±0.5°C. In addition, the microscope can operate at temperatures as low as 4K and perform ballistic electron emission microscopy measurements. The chamber that houses the microscope includes a preparation chamber with an evaporation source for metals. The entire STM chamber also rests on an active vibration isolation table, while still maintaining an all ultrahigh vacuum connection to the MBE system.

Development and performance of the nanoworkbench: A four tip STM for conductivity measurements down to submicrometer scales

A multiple-tip ultrahigh vacuum (UHV) scanning tunneling microscope (MTSTM) with a scanning electron microscope (SEM) for imaging and molecular-beam epitaxy growth capabilities has been developed. This instrument (nanoworkbench) is used to perform four-point probe conductivity measurements at μm spatial dimension. The system is composed of four chambers, the multiple-tip STM∕SEM chamber, a surface analysis and preparation chamber, a molecular-beam epitaxy chamber, and a load–lock chamber for fast transfer of samples and probes. The four chambers are interconnected by a unique transfer system based on a sample box with integrated heating and temperature-measuring capabilities. We demonstrate the operation and the performance of the nanoworkbench with STM imaging on graphite and with four-point-probe conductivity measurements on a silicon-on-insulator (SOI) crystal. The creation of a local FET, whose dimension and localization are, respectively, determined by the spacing between the probes and their position on the SOI surface, is demonstrated.

Ｌ２１構造を有するＣｏ２Ｃｒ１−ｘＦｅｘＡｌ薄膜の結晶構造と磁気特性

We successfully grew Co2Cr1-xFexAl full Heusler films (0 ≤ x ≤ 1) with the L21 structure onto GaAs(001) substrates by achieving stoichiometry in an ultrahigh-vacuum molecular beam epitaxy chamber. The films develop epitaxial crystallinity with the relationship of Co2Cr1-xFexAl(001)‹110›||GaAs(001)‹110›, which induces very strong uniaxial magnetocrystalline anisotropy, except for x = 0. For x = 1, in particular, the film is almost a single phase and its magnetic moment per formula unit exhibits Slater-Pauling behavior. Films of this type were used to fabricate magnetic tunnel junctions with 8.8% and 4.9% tunnel magnetoresistance (TMR) ratios at room temperature for x = 1 with an Al-O tunnel barrier and x = 0.6 with a MgO barrier, respectively. The TMR ratio can be further enhanced by both realizing an L21 single phase and eliminating lattice distortion.