Selective Molecular Beam Epitaxy Growth Research Articles

Preparation of Si substrates for monolithic integration of III−V quantum dots by selective MBE growth

A new method for nanopit formation by molecular-beam epitaxy (MBE) on different silicon substrates has been investigated. The dependence of the shape and depth of the nanopits on substrate orientation and doping type has been studied. The samples with an array of nanopits with 25 nm depth and (2-6)·108 cm−2 density for subsequent monolithic selective growth of quantum dots (QD) have been created.

Selective growth of InP on localized areas of silicon (100) substrate by molecular beam epitaxy

AbstractTo use III‐V compound semiconductors as channel materials in the future advanced MISFETs with high performance, it is important to acheive the growth of III‐V‐on‐Insulator structures on Si substrates. As a first step to accomplish this, we propose the selective formation of InP only on localized areas of Si (100) substrate by molecular beam epitaxy (MBE). It was found that InP was selectively grown only on bare Si window areas, while not grown on native silicon oxide areas at elevated substrtate temperatures (500°C). It was also found that by decreasing the window size (0.7×0.7 μm2) only single grain of InP was grown on each bare Si window area. This indicates that the selective MBE growth of InP on Si substrate is possible. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent Progress and Surface-Related Key Issues in III-V Semiconductor Nanoelectronics

Recent trends toward a “ubiquitous network society” have opened up new horizons for III-V nanoelectronics based on nanostructures which will have much wider application areas than they do today. In this paper, recent progresses and issues of III-V nanoelectronics are reviewed, focusing on achievements by the author’s group. As for nanostructure fabrication, a combination of top-down and bottom-up approaches, instead of the traditional top-down approach applied to multi-layer epitaxial wafers is required to form high-density nanostructure networks. As such, a selective molecular beam epitaxy growth of nanowire networks is presented and discussed. Large-scale integration of quantum nanodevices requires a new combination of system architecture and hardware architecture, such as the hexagonal (BDD) quantum logic circuit approach. The remaining key issue is the removal of surface states and Fermi level pinning from surfaces of nanostructures. Use of a silicon interface control layer (Si ICL) is shown to be promising for this purpose.

Selective MBE growth of hexagonal networks of trapezoidal and triangular GaAs nanowires on patterned (1 1 1)B substrates

As a combination of novel hardware architecture and novel system architecture for future ultrahigh-density III–V nanodevice LSIs, the authors’ group has recently proposed a hexagonal binary decision diagram (BDD) quantum circuit approach where gate-controlled path switching BDD node devices for a single or few electrons are laid out on a hexagonal nanowire network to realize a logic function. In this paper, attempts are made to establish a method to grow highly dense hexagonal nanowire networks for future BDD circuits by selective molecular beam epitaxy (MBE) on (1 1 1)B substrates. The (1 1 1)B orientation is suitable for BDD architecture because of the basic three-fold symmetry of the BDD node device. The growth experiments showed complex evolution of the cross-sectional structures, and it was explained in terms of kinetics determining facet boundaries. Straight arrays of triangular nanowires with 60 nm base width as well as hexagonal arrays of trapezoidal nanowires with a node density of 7.5×10 6 cm −2 were successfully grown with the aid of computer simulation. The result shows feasibility of growing high-density hexagonal networks of GaAs nanowires with precise control of the shape and size.

Selective molecular beam epitaxy growth of size- and position-controlled GaN∕AlGaN nanowires on nonplanar (0001) substrates and its growth mechanism

Fundamental growth properties were investigated for the size-controlled selective MBE growth of AlGaN∕GaN nanowires on the GaN (0001) prepatterned substrates both experimentally and theoretically. The lateral size of the present GaN nanowire was determined by two facet boundaries formed within AlGaN barrier layers. From the series of wire growth experiments, the growth selectivity and the measured angle of the facet boundary strongly depended on the Al composition and the initial crystalline facets of the mesa patterned templates. The experimental evolution of the cross-sectional structures was well reproduced by a computer simulation based on the phenomenological growth model where the slope angle dependence of lifetime of adatoms was taken into account. The lateral width of present nanowires could be kinetically controlled by the growth conditions and the supply thickness of AlGaN layers.

Growth kinetics and theoretical modeling of selective molecular beam epitaxy for growth of GaAs nanowires on nonplanar (001) and (111)B substrates

The growth kinetics involved in the selective molecular beam epitaxy growth of GaAs quantum wires (QWRs) on mesa-patterned substrates is investigated in detail experimentally, and an attempt is made to model the growth theoretically, using a phenomenological continuum model. Experimentally, ⟨−110⟩-oriented QWRs were grown on (001) and (113)A substrates, and ⟨−1−12⟩-oriented QWRs were grown on (111)B substrates. From a detailed investigation of the growth profiles, it was found that the lateral wire width is determined by facet boundaries (FBs) within AlGaAs layers separating growth regions on top facets from those on side facets of mesa structures. Evolution of FBs during growth was complicated. For computer simulation, measured growth rates of various facets were fitted into a theoretical formula to determine the dependence of a lifetime of adatoms on the slope angle of the growing surface. The continuum model using the slope angle dependent lifetime reproduced the details of the experimentally observed growth profiles very well for growth on (001), (113)A, and (111)B substrates, including the complex evolution of facet boundaries

Cross-Sectional Evolution and Its Mechanism during Selective Molecular Beam Epitaxy Growth of GaAs Quantum Wires on (111)B Substrates

The mechanism of the cross-sectional evolution during the selective growth of GaAs quantum wires (QWRs) on (111)B-patterned substrates was studied in detail both experimentally and theoretically. For this purpose, growth experiments were carried out on <-1-12>-oriented wires by systematically changing growth conditions and pattern sizes. A detailed investigation on cross sections of wires has shown that the lateral wire width is determined by facet boundaries (FBs) within AlGaAs layers separating growth regions on top facets from those on side facets of mesa structures. FBs were found to be planar or curved, depending on initial pattern sizes and growth conditions. Computer simulation based on a phenomenological growth model was attempted, taking account of the facet-angle-dependent lifetime of adatoms. The simulation well reproduced the experimentally observed growth features including the evolution of FBs, indicating that the cross sections of wires grown on (111)B-patterned substrates are kinetically controlled by the pattern sizes and growth conditions.

Study on ECR dry etching and selective MBE growth of AlGaN/GaN for fabrication of quantum nanostructures on GaN (0 0 0 1) substrates

This paper attempts to form AlGaN/GaN quantum wire (QWR) network structures on patterned GaN (0 0 0 1) substrates by selective molecular beam epitaxy (MBE) growth. Substrate patterns were prepared along 〈 1 1 2 ¯ 0 〉 - and 〈 1 1 ¯ 0 0 〉 -directions by electron cyclotron resonance assisted reactive-ion beam etching (ECR-RIBE) process. Selective growth was possible for both directions in the case of GaN growth, but only in the 〈 1 1 2 ¯ 0 〉 -direction in the case of AlGaN growth. A hexagonal QWR network was successfully grown on a hexagonal mesa pattern by combining the 〈 1 1 2 ¯ 0 〉 -direction and two other equivalent directions. AFM observation confirmed excellent surface morphology of the grown network. A clear cathodoluminescence (CL) peak coming from the embedded AlGaN/GaN QWR structure was clearly identified.

Growth kinetics and modeling of selective molecular beam epitaxial growth of GaAs ridge quantum wires on pre-patterned nonplanar substrates

The growth kinetics involved in the selective molecular beam epitaxial growth of GaAs ridge QWRs is investigated in detail experimentally and an attempt is made to model the growth theoretically. For this purpose, detailed experiments were carried out on the growth of 〈1̄10〉-oriented AlGaAs–GaAs ridge quantum wires on mesa-patterned (001) GaAs substrates. A phenomenological modeling was done based on the continuum approximation including parameters such as group III adatom lifetime, diffusion constant and migration length. Computer simulation using the resultant model well reproduces the experimentally observed growth features such as the cross-sectional structure of the ridge wire and its temporal evolution, its temperature dependence and evolution of facet boundary planes. The simple phenomenological model developed here seems to be very useful for design and precise control of the growth process.

Selective Molecular Beam Epitaxy Growth of GaAs Hexagonal Nanowire Networks on (111)B Patterned Substrates

In view of applications to hexagonal binary decision diagram (BDD) quantum circuits, the growth of hexagonal GaAs nanowire networks was attempted by selective molecular beam epitaxy (MBE) on (111)B prepatterned substrates. The basic feasibility of <112>-oriented straight nanowires was first investigated. GaAs nanowires were selectively formed on the top (111)B plane of AlGaAs mesa structures with high uniformity. The lateral width of the wires was determined by two facet boundary planes separating the growth regions on the top and the side facets of the mesa structures. The angle of the boundary planes remained constant during growth, resulting in the wire width being precisely controlled by the thickness of the AlGaAs barrier layer. Then, GaAs/AlGaAs hexagonal nanowire networks were grown on the patterned substrates consisting of three equivalent <112>-oriented wires for a (111)B plane. The results of detailed structural and optical studies showed that highly uniform and smoothly connected hexagonal nanowire networks having threefold symmetry were successfully fabricated by the present selective MBE growth technique.

Characterization and control of surfaces and interfaces for III–V nanoelectronics

Recent progress and issues of characterization and control of surfaces and interfaces as related to III–V nanoelectronics are reviewed. After a brief general review of III–V nanotechnology, a novel hexagonal binary decision diagram (BDD) quantum logic circuit approach is introduced where hexagonal nanowire networks are controlled by nanoscale Schottky gates. For controlled formation of high-density nanostruc-tures, selective molecular beam epitaxy growth is described together with a brief description of a cathodoluminescence-based characterization of buried heterointerfaces. As the key processing issue, problems associated with nanostructure surfaces and Schottky gates are discussed, introducing a recent scanned probe study and attempts to remove Fermi level pinning by a silicon interface control layer.

Characterization and Optimization of Atomic Hydrogen Cleaning of InP Surface for Selective Molecular Beam Epitaxial Growth of InGaAs Quantum Structure Arrays

An oxide removal process for InP surface using atomic hydrogen cleaning was characterized and optimized for use in the selective molecular beam epitaxial (MBE) growth of InGaAs quantum structures. In-situ X-ray photoelectron spectroscopy (XPS), reflection high energy electron diffraction (RHEED), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used. Detailed XPS measurements clarified the effects of atomic hydrogen on the planar InP surfaces. The cleaning efficiently removed oxygen. However, the P/In atomic ratio on the surface depended sensitively on the cleaning procedure. Under the optimum low-temperature atomic hydrogen cleaning, the surface stoichiometry could be maintained and realized an atomically flat InGaAs/InP interface. This has led to the formation of uniform InGaAs ridge quantum wires (QWRs) with good optical properties.

Formation of device-oriented InGaAs coupled quantum structures by selective MBE growth on patterned InP substrates

Attempts were made to realize device-oriented InGaAs coupled quantum structures by selective molecular beam epitaxy (MBE) on specially designed patterned InP substrates. At first, selective MBE growth of individual quantum wires (QWRs) and quantum dots (QDs) were established. By combining these growths, InGaAs/InAlAs QWR–QD–QWR coupled structures and QWR Y-branch couplers were successfully formed. The QWR–QD–QWR coupled structure realized a double-barrier potential profile needed for a single-electron transistor. A QWR honeycomb network consisting of the QWR Y-branch couplers seems promising for constructing quantum device networks and quantum/classical interconnections.

Vertical barrier layer formation during selective MBE growth of InGaAs ridge quantum wires on InP patterned substrates

Transmission electron microscope (TEM) observations and energy dispersive X-ray (EDX) analyses were carried out to investigate the microscopic structure and local variation of alloy composition in InGaAs/InAlAs ridge quantum wires (QWRs) grown by selective molecular beam epitaxy (MBE) on patterned InP substrates. In the QWR structure, formation of the vertical barrier layer having a width of 5–15 nm, which bisects whole InGaAs/InAlAs QWR structure including the QWR itself, was observed for the first time. Particularly, marked reduction of In composition down to xIn=0.45 was observed for the InGaAs layer grown at 500°C. The formation of the vertical barrier layer and temperature dependence of its composition can be explained in terms of a curvature-induced capillarity and a free energy of alloy mixing competing with the adatom diffusion effect.

Selective molecular beam epitaxy growth of quantum wire–dot coupled structures with novel high index facets for InGaAs single electron transistor arrays

For the InGaAs wire–dot–wire coupled structure arrays formed by selective molecular beam epitaxy (MBE) growth method on a specially designed patterned InP substrates, applicability to high density InGaAs single electron transistor (SET) arrays were investigated from the viewpoint of reproducibility of the selective MBE growth, size controllability and integration level. Appearance of the crater-like structure becomes a problem for reproducible formation of the InGaAs quantum wire–dot coupled structure. However, such a crater structure can be removed by appropriately adjusting growth condition and pattern sizes. The InGaAs wire size, the InGaAs dot size and the lateral width of the potential barrier were found to be controlled by the growth condition and the pattern geometry and can be reduced down to decananometer range. Highly integrated SET circuits with the device density larger than 109cm−2 appear to be realizable using the present selective MBE method.

Formation of Two-Dimensional Arrays of InP-Based InGaAs Quantum Dots on Patterned Substrates by Selective Molecular Beam Epitaxy

Two-dimensional (2D) arrays of InP-based In0.53Ga0.47As quantum dots are successfully formed using two different approaches of selective molecular beam epitaxial (MBE) growth. One is to introduce modulations into wire-arrays by growing wires on intentionally mis-oriented mesa-stripes based on our recent study on the effect of mis-orientation on the wire growth. The other is to carry out selective MBE growth on a 2D array of <100>-oriented square-mesa pedestals. Both types of InGaAs dot arrays show fairly strong photoluminescence (PL) and cathodoluminescence (CL) emissions, indicating the reasonably good crystal quality of the dots. Both approaches are found to be useful for constructing highly integrated networks of single electron devices.

Fabrication of InGaAs Quantum Wires and Dots by Selective Molecular Beam Epitaxial Growth on Various Mesa-Patterned (001)InP Substrates

Systematic growth experiments were undertaken for the successful formation of InGaAs/InAlAs quantum wires and dots on patterned InP substrates by selective molecular beam epitaxy (MBE). Multi layer test structures consisting of InGaAs and InAlAs alternate layers were formed on 7 different types of mesa patterns including stripe- and square-shaped mesas oriented along the [110], [110] and [100] directions. The growth led to the formation of sidewalls consisting of a variety of facets. Although these sidewalls were often rough or not uniform, the growth on the stripe-mesas along the [110] and [100] directions, and the growth on the square-mesas with (201) sidewall facets were found to be promising since they resulted in very smooth sidewall facets, cross-sectional and lateral uniformity, and good growth selectivity. Using these results, InGaAs quantum wires and dots were successfully formed.

Surface passivation of In0.53Ga0.47As ridge quantum wires using silicon interface control layers

Surface state effects in In0.53Ga0.47As one-dimensional quantum wires and the effectiveness of the Si interface control layer (Si ICL)-based passivation technique are investigated using photoluminescence (PL) as the probe. Scanning electron microscope and x-ray photoelectron spectroscopy measurements were made to characterize the structure and the interface properties. The In0.53Ga0.47As quantum wires embedded in In0.52Al0.48As barrier material were fabricated by selective molecular beam epitaxy growth on patterned InP substrates. Unpassivated near-surface quantum wires showed an exponential decrease of PL intensity with reduction of surface-to-well distance, tws, similarly to the near-surface quantum wells. By applying the Si ICL-based passivation, a nearly complete recovery of PL intensity was achieved with an observed maximum recovery factor of 250 for the InGaAs quantum wire directly passivated with SiO2/Si ICL (tws=0). The mechanism for the PL recovery is explained in terms of suppression of surface states by passivation.

Fabrication of InP-based InGaAs ridge quantum wires utilizing selective molecular beam epitaxial growth on (311)A facets

InP-based InGaAs/InAlAs ridge quantum wires were successfully fabricated by our new approach using selective molecular beam epitaxy (MBE). As the starting structures, array of InGaAs ridge structures composed of smooth (311)A facets were formed by MBE on mesa-patterned InP substrates. Prior to actual fabrication of the wires, MBE growth characteristics of In0.53Ga0.47As and In0.52Al0.48As layers on the starting structure were studied in detail. The results of growth experiments were then successfully applied to the fabrication of InGaAs ridge quantum wires with high spatial uniformity. Low temperature cathodoluminescence spectrum measured in response to spot excitation of wire region showed a strong light emission whose analysis indicated that it originates from InGaAs ridge quantum wire itself. In photoluminescence measurements, the emission from the wire had strong intensity even at room temperature, indicating that the wire crystal possesses excellent bulk and interface quality, and are largely free from nonradiative recombination centers.

Selective molecular beam epitaxial growth for multifunction microwave monolithic and opto-electronic integrated circuits

Multifunction microwave monolithic integrated circuits (MMICs) and opto-electronic integrated circuits (OEICs) on both GaAs and InP based substrates have been demonstrated by a solid source molecular beam epitaxial (MBE) selective regrowth process. Typical MBE regrown 0.25 μm pseudomorphic high electron mobility transistors (PHEMTs) have demonstrated an extrinsic transconductance, g m , of over 560 mS/mm with an extrinsic current gain cut-off frequency of 70 GHz with good multifunction MMIC circuit yield (⪢ 70%). For MBE selectively regrown, lattice matched 0.15 μm InAlAs/InGaAs/InP high electron mobility transistor (HEMTs), an extrinsic g m of 1000 mS/mm with a current gain cut-off frequency of approximately 200 GHz have been achieved. Using the small area (e.g., 20 μm or less in dimension) MBE selective regrowth to provide two-dimensional strain relaxation, a large lattice mismatched In 0.53Ga 0.47As MIN photodetector has been demonstrated on GaAs substrates with an operational frequency up to 50 GHz.