Molecular Beam Epitaxy Growth System Research Articles

An ultra-compact, high-throughput molecular beam epitaxy growth system.

We present a miniaturized molecular beam epitaxy (miniMBE) system with an outer diameter of 206 mm, optimized for flexible and high-throughput operation. The three-chamber system, used here for oxide growth, consists of a sample loading chamber, a storage chamber, and a growth chamber. The growth chamber is equipped with eight identical effusion cell ports with linear shutters, one larger port for either a multi-pocket electron beam evaporator or an oxygen plasma source, an integrated cryoshroud, retractable beam-flux monitor or quartz-crystal microbalance, reflection high energy electron diffraction, substrate manipulator, main shutter, and quadrupole mass spectrometer. The system can be combined with ultrahigh vacuum (UHV) end stations on synchrotron and neutron beamlines, or equivalently with other complex surface analysis systems, including low-temperature scanning probe microscopy systems. Substrate handling is compatible with most UHV surface characterization systems, as the miniMBE can accommodate standard surface science sample holders. We introduce the design of the system, and its specific capabilities and operational parameters, and we demonstrate the epitaxial thin film growth of magnetoelectric Cr2O3 on c-plane sapphire and ferrimagnetic Fe3O4 on MgO (001).

Facility for low-temperature spin-polarized-scanning tunneling microscopy studies of magnetic/spintronic materials prepared in situ by nitride molecular beam epitaxy.

Based on the interest in, as well as exciting outlook for, nitride semiconductor based structures with regard to electronic, optoelectronic, and spintronic applications, it is compelling to investigate these systems using the powerful technique of spin-polarized scanning tunneling microscopy (STM), a technique capable of achieving magnetic resolution down to the atomic scale. However, the delicate surfaces of these materials are easily corrupted by in-air transfers, making it unfeasible to study them in stand-alone ultra-high vacuum STM facilities. Therefore, we have carried out the development of a hybrid system including a nitrogen plasma assisted molecular beam epitaxy/pulsed laser epitaxy facility for sample growth combined with a low-temperature, spin-polarized scanning tunneling microscope system. The custom-designed molecular beam epitaxy growth system supports up to eight sources, including up to seven effusion cells plus a radio frequency nitrogen plasma source, for epitaxially growing a variety of materials, such as nitride semiconductors, magnetic materials, and their hetero-structures, and also incorporating in situ reflection high energy electron diffraction. The growth system also enables integration of pulsed laser epitaxy. The STM unit has a modular design, consisting of an upper body and a lower body. The upper body contains the coarse approach mechanism and the scanner unit, while the lower body accepts molecular beam epitaxy grown samples using compression springs and sample skis. The design of the system employs two stages of vibration isolation as well as a layer of acoustic noise isolation in order to reduce noise during STM measurements. This isolation allows the system to effectively acquire STM data in a typical lab space, which during its construction had no special and highly costly elements included, (such as isolated slabs) which would lower the environmental noise. The design further enables tip exchange and tip coating without breaking vacuum, and convenient visual access to the sample and tip inside a superconducting magnet cryostat. A sample/tip handling system is optimized for both the molecular beam epitaxy growth system and the scanning tunneling microscope system. The sample/tip handing system enables in situ STM studies on epitaxially grown samples, and tip exchange in the superconducting magnet cryostat. The hybrid molecular beam epitaxy and low temperature scanning tunneling microscopy system is capable of growing semiconductor-based hetero-structures with controlled accuracy down to a single atomic-layer and imaging them down to atomic resolution.

A portable ultrahigh vacuum organic molecular beam deposition system for in situ x-ray diffraction measurements

A portable UHV molecular beam deposition system has been developed for synthesis, in situ, and real-time x-ray diffraction measurements of organic thin films, multilayers, and superlattices. The system has been optimized for small size, while still incorporating full features necessary to achieve thin film growth under molecular beam epitaxy (MBE) conditions. It can be used independently for thin film growth, or it can be transported and mounted on standard diffractometers. Additionally, it can be docked to a stationary multipurpose MBE growth system for sample transfer, thus permitting more extensive growth and characterization. The design and performance of this system are reported, with emphasis on modifications required to deposit organic materials. To demonstrate the capabilities for real-time x-ray scattering experiments, some preliminary results of a study of epitaxial growth of 3,4,9,10-perylene-tetracarboxylic dianhydride on Ag(111) substrates are given.

Growth and characterization of patterned ZnCdSe structures for application in integrated R-G-B II–VI light-emitting diodes

We report the growth and characterization of patterned ZnCdSe structures on GaAs substrates as our initial attempt to use shadow mask selective area molecular-beam epitaxy (MBE) to integrate II–VI (Zn,Cd,Mg)Se-based red-green-blue (R-G-B) light-emitting diodes (LEDs). Patterned ZnCdSe thick layers and ZnCdSe/ZnSe quantum wells (QWs) were grown on GaAs substrates using a silicon shadow mask mounted on a mask fixture that allows the mask to be placed and removed within the MBE growth system. Excellent pattern definition and good optical properties were obtained. Integration of patterned ZnCdSe/ZnSe QWs having different thickness and Cd composition, therefore different emission wavelengths, on a single GaAs substrate was also achieved. These results will be applied to the (Zn,Cd,Mg)Se material system to integrate R-G-B LEDs on a single InP substrate.

Approaches to understanding MBE growth phenomena

A physical picture of molecular beam epitaxy (MBE) growth is presented in a consistent form taking into consideration the atomistic nature of crystallization phenomena occurring in this process. To this end, the entire MBE growth system directly involved in epitaxial crystallization is divided into three different parts. The crystalline solid phase of the substrate, or the already grown epilayer, is one extreme, the gaseous phase of the intersecting molecular beams is the second extreme and the transition layer in between, where transition from a gas to a crystal occurs, creates the third part of the system. The transition layer, where all processes leading to epitaxy occur, is obviously the most important part of the growth system. Its geometrical form and the processes occurring there depend strongly on the growth conditions chosen. The following approaches have been considered when discussing the attributes of MBE growth processes: (i) thermodynamics, (ii) “surface sites-particle” interaction analysis, and (iii) chemical bond quantum mechanics. Taking as examples: (i) MBE of non-stoichiometric structures, (ii) UHV atomic layer epitaxy, and (iii) self-organization effects in MBE, the importance of the transition layer concept for understanding MBE growth processes has been illustrated.

Influence of mismatch on the defects in relaxed epitaxial InGaAs/GaAs(100) films grown by molecular beam epitaxy

Thick (∼3 μm) films of InxGa1−xAs grown on GaAs(100) substrates, across the whole composition range, have been examined by transmission electron microscopy and double-crystal x-ray diffraction. The results were compared with the observed growth mode of the material determined by in situ reflection high-energy electron diffraction in the molecular beam epitaxy growth system. The quality of the material degraded noticeably for compositions up to x∼0.5 associated with an increased density of dislocations and stacking faults. In contrast, improvements in quality as x approached 1.0 were correlated with the introduction of an increasingly more regular array of edge dislocations.

System related impurity incorporation during the growth of gallium arsenide by molecular beam epitaxy

Manganese, carbon and silicon have been identified as major unintentional impurities present in a particular ultra high vacuum molecular beam epitaxy growth system. The various origins of these impurities are discussed and results are presented on the optical and electrical properties of MBE (100) GaAs layers containing these impurity atoms.