The control of size and areal density of InAs self-assembled quantum dots in selective area molecular beam epitaxy on GaAs (0 0 1) surface

The growth of InAs quantum dots (QDs) on GaAs (0 0 1) substrates by selective area molecular beam epitaxy (SA-MBE) with dielectric mask is investigated. The GaAs polycrystals on the mask, which is formed during growth due to low GaAs selectivity between dielectric mask and epitaxial region in MBE, strongly affect the distribution of InAs QDs on the neighbouring epitaxial regions. It is found that the GaAs polycrystalline regions strongly absorb indium during QD growth, confirmed by microscopic and optical studies. GaAs polycrystalline deposit can be reduced under low growth rate and high-temperature growth conditions. Almost no reduction in QD areal density is observed when there is minimal polycrystalline coverage of the mask.     

Heteroepitaxial growth of InAs on GaAs(0 0 1) by in situ STM located inside MBE growth chamber

The growth of InAs on GaAs(0 0 1) is of great interest primarily due to the self-assembly of arrays of quantum dots (QDs) with excellent opto-electronic properties. However, a basic understanding of their spontaneous formation is lacking. Advanced experimental methods are required to probe these nanostructures dynamically in order to elucidate their growth mechanism. Scanning tunneling microscopy (STM) has been successfully applied to many GaAs-based materials grown by molecular beam epitaxy (MBE). Typical STM–MBE experiments involve quenching the sample and transferring it to a remote STM chamber under arsenic-free ultra-high vacuum. In the case of GaAs-based materials grown at substrate temperatures of 400–600 °C, operating the STM at room temperature ensures that the surface is essentially static on the time scale of STM imaging. To attempt dynamic experiments requires a system in which STM and MBE are incorporated into one unit in order to scan in situ during growth. Here, we discuss in situ STM results from just such a system, covering both QDs and the dynamics of the wetting layer.     

(Ga,Mn)As on patterned GaAs(0 0 1) substrates: Growth and magnetotransport

A new type of (Ga,Mn)As microstructures with laterally confined electronic and magnetic properties has been realized by growing (Ga,Mn)As films on -oriented ridge structures with (1 1 3)A sidewalls and (0 0 1) top layers prepared on GaAs(0 0 1) substrates. The temperature- and field-dependent magnetotransport data of the overgrown structures are compared with those obtained from planar reference samples revealing the coexistence of electronic and magnetic properties specific for (0 0 1) and (1 1 3)A (Ga,Mn)As on a single sample.     

In situ atomic layer deposition and synchrotron-radiation photoemission study of Al2O3 on pristine n-GaAs(0 0 1)-4 × 6 surface

This work presents the in situ reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and synchrotron-radiation photoemission studies for the morphological and interfacial chemical characterization of in situ atomic layer deposited (ALD) Al₂O₃ on pristine molecular beam epitaxy (MBE) grown Ga-rich n-GaAs (1 0 0)-4 × 6 surface. Both the RHEED pattern and STM image demonstrated that the first cycle of ALD-Al₂O₃ process reacted immediately with the GaAs surface. As revealed by in situ synchrotron-radiation photoemission studies, two types of surface As atoms that have excess in charge in the clean surface served as reaction sites with TMA. Two oxidized states were then induced in the As 3d core-level spectra with chemical shifts of +660 meV and +1.03 eV, respectively.     

H2S molecular beam passivation of Ge(0 0 1)

A fundamental issue regarding the introduction of high-mobility Ge channels in CMOS circuits is the electrical passivation of the interface with the high-k gate dielectric. In this paper, we investigate the passivation of p-Ge(0 0 1) using molecular H₂S. The modification of the semiconductor surface is monitored in situ by RHEED and the interface is characterized by XPS analyses. MOS capacitors are fabricated to extract interface state density, and finally we demonstrate the efficiency of the passivation scheme using a combination with an ultra thin Al interlayer.     

Atomic hydrogen assisted molecular beam epitaxy on patterned GaAs (311)A substrates: Formation of highly uniform quantum-dot arrays

The idea of combining self-organized growth with growth on patterned substrates to produce new types of nanostructures in a controlled manner is realized in atomic hydrogen assisted molecular beam epitaxy (MBE) on patterned GaAs (311)A substrates. In conventional MBE on patterned substrates mesa stripes along [01 ] develop a fast growing sidewall to form quasi-planar lateral quantum wires having a smooth, convex curved surface profile. In atomic hydrogen assisted MBE, the surface naturally develops quasiperiodic one-dimenional step arrays by step bunching along [ 33], i.e., perpendicular to the wire direction with a lateral periodicity around 40 nm. The step array is maintained over the curved sidewall without displacement. Thus, a dense array of dotlike nanostructures is realized with precise control of the position on the substrate surface. High uniformity of the dot array is revealed in micro-photoluminescence spectroscopy with the emission dominated by one single sharp line.     

Epitaxy of BaTiO3 thin film on Si(0 0 1) using a SrTiO3 buffer layer for non-volatile memory application

In this study we report the epitaxial growth of BaTiO₃ films on Si(0 0 1) substrate buffered by 5 nm-thick SrTiO₃ layer using both MBE and PLD techniques. The BaTiO₃ films demonstrate single crystalline, (0 0 1)-oriented texture and atomically flat surface on SrTiO₃/Si template. The electrical characterizations of the BaTiO₃ films using MFIS structures show that samples grown by MBE with limited oxygen pressure during the growth exhibit typical dielectric behavior despite post deposition annealing process employed. A ferroelectric BaTiO₃ layer is obtained using PLD method, which permits much higher oxygen pressure. The C-V curve shows a memory window of 0.75 V which thus enable BaTiO₃ possibly being applied to the non-volatile memory application.     

Direct epitaxial growth of InP based heterostructures on SrTiO3/Si(0 0 1) crystalline templates

A study of the structural and optical properties of an InAsP/InP quantum well heterostructure grown on a crystalline SrTiO₃ (STO)/Si(0 0 1) template is presented. The mismatch between InP and STO is fully accommodated by an array of geometric dislocations confined at the heterointerface. As a consequence, InP takes its bulk lattice parameter as soon as growth begins, and does not contain threading dislocations related to plastic relaxation. It contains twins related to the initially three-dimensional growth. Despite these twins, photoluminescence from the quantum well is detected at room temperature, showing that STO/Si(0 0 1) templates have an interesting potential for the monolithic integration of III-V semiconductors on silicon.     

Epitaxial growth of LaAlO3 on Si(0 0 1) using interface engineering

In this work, the potentiality of molecular beam epitaxy techniques to prepare epitaxial lanthanum aluminate (LaAlO₃) films on Si(0 0 1) is explored. We first demonstrate that the direct growth of LaAlO₃ on Si(0 0 1) is impossible : amorphous layers are obtained at temperatures below 600 °C whereas crystalline layers can be grown at higher temperatures but interfacial reactions leading to silicate formation occur. An interface engineering strategy is then developed to avoid these reactions. SrO and SrTiO₃ have been studied as buffer for the subsequent growth of LaAlO₃. Only partial LaAlO₃ epitaxy is obtained on SrO whereas high quality layers are achieved on SrTiO₃. However both SrO and SrTiO₃ appear to be unstable with respect of Si at the growth temperature of LaAlO₃ (700 °C). This leads to the formation of relatively thick amorphous interfacial layers. Despite their instability at high temperature, these processes could be used for the fabrication of twins-free LaAlO₃ templates on Si, and for the fabrication of complex oxide/Si heterostructures for various applications.     

Lateral growth of GaAs on patterned {-1-1-1}B substrates for the fabrication of nano wires using metalorganic molecular beam epitaxy

We have grown GaAs nano wire structures (45 × 20 nm²) buried in AIAs layers by lateral metalorganic molecular beam epitaxy on the terraced sidewalls of mesa-grooved (-1-1-1)B substrates. The growth of GaAs occurred primarily on the sidewall of the mesa-grooves and not on the (-1-1-1)B surface for arsenic pressures greater than 2.0 × 10⁻³ Pa at a substrate temperature of 480°C. An (0-1-1) facet formation during the lateral epitaxy at the intersection region between the bottom (-1-1-1)B surface and the (1-2-2)A sidewall has been directly observed by real-time scanning microprobe reflection high-energy electron diffraction. The growth rate on the (0111) facet was estimated from the variation of its width with growth time.     

Low threading dislocation density Ge deposited on Si (1 0 0) using RPCVD

Epitaxial Ge layer growth of low threading dislocation density (TDD) and low surface roughness on Si (1 0 0) surface is investigated using a single wafer reduced pressure chemical vapor deposition (RPCVD) system. Thin seed Ge layer is deposited at 300 °C at first to form two-dimensional Ge surface followed by thick Ge growth at 550 °C. Root mean square of roughness (RMS) of ∼0.45 nm is achieved. As-deposited Ge layers show high TDD of e.g. ∼4 × 10⁸ cm⁻² for a 4.7 μm thick Ge layer thickness. The TDD is decreasing with increasing Ge thickness. By applying a postannealing process at 800 °C, the TDD is decreased by one order of magnitude. By introducing several cycle of annealing during the Ge growth interrupting the Ge deposition, TDD as low as ∼7 × 10⁵ cm⁻² is achieved for 4.7 μm Ge thick layer. Surface roughness of the Ge sample with the cyclic annealing process is in the same level as without annealing process (RMS of ∼0.44 nm). The Ge layers are tensile strained as a result of a higher thermal expansion coefficient of Ge compared to Si in the cooling process down to room temperature. Enhanced Si diffusion was observed for annealed Ge samples. Direct band-to-band luminescence of the Ge layer grown on Si is demonstrated.     

Epitaxial lateral overgrowth of InP by liquid-phase epitaxy on InP (0 0 1), (1 1 1)A,B and (1 1 0) surfaces

The epitaxial lateral overgrowth (ELO) was performed on {0 0 1}, {1 1 1}A,B and {1 1 0} oriented InP by liquid-phase epitaxy at constant growth temperature (450–650°C). According to the observations of cross-sectional shape, the orientation dependence of the vertical growth rate was determined to be {1 1 0}>{1 1 1}A,B>{1 0 0} under the present experimental conditions. The etch pit density in the ELO layer was lower than on openings. In PL mapping observations, PL properties were improved on the ELO layer.     

Highly ordered C60 films on epitaxial Fe/MgO(0 0 1) surfaces for organic spintronics

Hybrid interfaces between ferromagnetic surfaces and carbon-based molecules play an important role in organic spintronics. The fabrication of devices with well defined interfaces remains challenging, however, hampering microscopic understanding of their operation mechanisms. We have studied the crystallinity and molecular ordering of C₆₀ films on epitaxial Fe/MgO(0 0 1) surfaces, using X-ray diffraction and scanning tunneling microscopy (STM). Both techniques confirm that fcc molecular C₆₀ films with a (1 1 1)-texture can be fabricated on epitaxial bcc-Fe(0 0 1) surfaces at elevated growth temperatures (100–130 °C). STM measurements show that C₆₀ monolayers deposited at 130 °C are highly ordered, exhibiting quasi-hexagonal arrangements on the Fe(0 0 1) surface oriented along the [1 0 0] and [0 1 0] directions. The mismatch between the surface lattice of the monolayer and the bulk fcc C₆₀ lattice prevents epitaxial overgrowth of multilayers.     

Selective incorporation of Si along step edges during delta-doping on MOVPE-grown GaAs (001) vicinal surfaces

We investigated the delta-doping (δ-doping) of Si using SiH₄ on MOVPE-grown GaAs (001) vicinal surfaces to explore the possibility of selective incorporation of Si along atomic steps, and to demonstrate doping quantum wires by the combination of multiatomic steps and wire-like doping. It was found that the doping density on vicinal surfaces was enhanced as the misorientation angle was increased, which suggested the enhanced decomposition of SiH₄ and the selective incorporation of Si at step edges. It was also found that this selective incorporation could be enhanced by annealing the surface prior to the δ-doping, which resulted from the reduced incorporation of Si at the terrace regions. Anisotropic electron transport properties which are expected from the wire-like incorporation along step edges are also discussed.     

Epitaxial growth of CdTe on (211) silicon mesas formed by deep reactive ion etching

By patterning a (211) Si substrate wafer into mesas and depositing CdTe onto this substrate by molecular beam epitaxy (MBE), we achieved the removal of nearly all threading dislocations from the epilayer. Faceting of mesa surfaces is observed and characterized. Deposition of CdTe on mesa sidewalls nucleates stacking faults along the (111) planes, which result in nonradiative carrier recombination. The density of these stacking faults can be reduced if care is taken to align the molecular beams from the effusion cells with particular crystallographic directions of the substrate.     

Effects of carbon coverage on Ge quantum dots formation on Si(100) using C-Si reaction and transition of Ge growth mode

Effects of carbon (C) coverage on C-mediated Ge quantum dots (QDs) formation on a Si(100) substrate changing a state of surface reconstruction were investigated by using solid-source molecular beam epitaxy. For C=0–2.0 monolayers (MLs), the Ge QD scaled down and its density increased with C coverage. In addition, growth mode of Ge QDs changed from Volmer-Weber (VW) mode without a wetting layer to Stranski-Krastanov (SK) mode with the wetting layer for C=0.50–0.75 ML. This transition was induced by decrease in interfacial energy between Ge and Si surface due to the formation of C-Ge bonds near the Ge/Si interface. For C≥2.5 MLs, the Ge QD enlarged slightly and its density decreased with increasing C coverage, and he Ge growth mode went back to the VW mode. The Raman spectroscopy and X-ray photoelectron spectroscopy revealed the formation of a mixture of amorphous C and nano-crystalline graphite on the Si surface. Thus, the formation of a large amount of C–C (sp²) bonds induced the growth transition of QDs from the SK mode to the VW mode due to the decrease in surface energy of C.     

Si_(1-x)Ge_x/Si多层异质外延结构的研究

对制作的 Ｓｉ１－ｘＧｅｘ／Ｓｉ多层异质外延结构进行了研究。并对其做了反射高能电子衍射（ＲＨＥＥＤ）、Ｘ射线衍射（ＸＲＤ）和扩展电阻（ＳＲ）等测量,给出了利用这种结构研制出的异质结双极晶体管（ＨＢＴ）的输出特性曲线。     

Seeded growth of GaN at high N2 pressure on (0 0 0 1) polar surfaces of GaN single crystalline substrates

GaN single crystals obtained by the high nitrogen pressure solution method without an intentional seeding, show strong growth anisotropy which results in their platelet shape. The attempts to enhance the growth into (0 0 0 1) directions by the increase of the integral supercooling in the solution, often lead to the growth instabilities on both Ga-polar and N-polar (0 0 0 1) surfaces. This can be avoided only by the precise control of the growth conditions at the crystallization front on the particular surface.The results of the seeded growth on both Ga- and N-polar (0 0 0 1) surfaces in configuration enabling such a control is reported. It is shown that dominating mechanisms of the unstable growth such as the cellular growth or edge nucleation can be suppressed. Differences in nucleation and growth in dependence on surface polarity are discussed.     

Temperature-controlled self-organized InP nanostructures grown on GaAs(1 0 0) substrate by MOCVD

The self-organized InP nanostructures grown on GaAs(0 0 1) substrates by metalorganic vapor deposition were examined in detail using atomic force microscopy. By properly selecting growth temperature, three kinds of nanostructure, islands, pits and ripples were formed. For growth temperature of 400–450 °C, the surface morphologies were governed by islands; but, for the growth temperature of 500 °C, the formation of surface ripples instead of islands was presumably due to the combination effect of temperature-controlled surface kinetics and strain effect. On the other hand, the observation of enhanced growth of pits upon a high-temperature annealing (at 685 °C for 90 s) indicated that the strained InP epitaxial film would be morphologically stabilized by taking the form of pits formation.     

Photoelectron spectroscopy (XPS) and photoelectron diffraction (XPD) studies on the system hafnium silicide and hafnium oxide on Si(1 0 0)

Continuous down-scaling of silicon based transistors results in device lengths of less than 100 nm. This requires a reduction of the gate dielectric thickness to less than 15Å which is not possible for SiO₂ due to an increasing leakage current. One of the most promising candidates for a replacement material for the gate dielectric is HfO₂ [Wilk GD, Wallace RM, Anthony JM. J Appl Phys 2001; 89:5243].In this work we applied X-ray photoelectron spectroscopy (XPS) and photoelectron diffraction measurements in order to study the interface of hafnium oxide to Si(1 0 0). The high resolution measurements were performed with synchrotron radiation at beamlines 5 and 11 at DELTA (Dortmund). For the first time, photoelectron diffraction patterns for this system were recorded. The spectral resolution allowed to separate different spectral components.The preparation of hafnium oxide films on Si(1 0 0) was performed by evaporation of hafnium at a partial oxygen background pressure of . Three different spectral components were observed in the hafnium 4f photoemission signal by high resolution XPS. The photoelectron signals with binding energies shift of 3.1 and 1.2 eV with respect to signal of hafnium silicide were assigned to hafnium dioxide and hafnium silicate, respectively. The corresponding high-resolution diffraction patterns result from different local environments for each component. The experimental patterns are compared with simulations for a model structure of hafnium silicide.