Atomic layer epitaxy of AlP and (AlP)n(GaP)n superlattice using ethyldimethylamine alane as a new aluminum source

Atomic layer epitaxy (ALE) of AlP was realized using ethyldimethylamine alane (EDMAAl) as a new Al source. Self-limiting growth of AlP took place at one and two monolayers per ALE cycle. Secondary ion mass spectroscopy revealed that the amounts of incorporated impurities (carbon, hydrogen and oxygen) in ALE-grown AlP layers was greatly suppressed by using the new Al source, to nearly the same levels as in high-quality MOVPE-grown layers. We also achieved the successful ALE growth of (AlP)_n(GaP)_n short-period superlattices (SLs), taking advantage of the overlapping temperature windows of ALE-GaP and ALE-AlP. X-ray diffraction measurements showed reasonably good interface abruptness of SLs as low as 3. The PL emission peak from SLs involving Al-containing layers was observed in ALE growth for the first time.     

Interface behavior of Mn/PbTe(111) studied by scanning tunneling microscopy and X-ray photoemission spectroscopy

Mn overlayers growth on PbTe(111) have been investigated by using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). The strong chemical interactions were found during the formation of Mn/PbTe(111) interface. At the initial deposition of Mn, one part of Mn adatoms substitute Pb atoms on the PbTe(111) surface, forming a (√3 × √3)R30° MnTe phase, and the other part of Mn adatoms, together with the kicked-out Pb atoms, nucleate at the boundaries of the MnTe islands, forming loop islands around the MnTe islands as an intermediate state. Finally, they develop into regular 3D Pb capped Mn islands upon further Mn deposition. For Mn growth on the PbTe surface where Pb atoms are almost completely substituted by Mn, the deposited Mn atoms either cooperate into the 3D Pb capped Mn islands promoting the upright growth of the 3D Pb capped Mn islands, or nucleate and grow on the MnTe superstructure areas. Free Pb layer always floats on the top of surface, indicating that Pb layer has smaller surface energy, and Mn adatoms always exchange the positions with the underneath Pb atoms during the growth.     

原子层外延及其进展

近十年来发展起来的原子层外延技术(ALE)实际上是对现有气相外延技术(蒸发、沉积、分子束外延、氯化物外延和MOCVL)的一种改进.它以固体衬底表面的化学反应为基础.因此用ALE方法可以获得精确膜厚、符合化学比、高化学稳定性和结构完整而均匀的化合物薄膜.本综述介绍了ALE的工作原理、特点及其进展概况.指出,ALE方法除了用于研制成功性能优良的太面积薄膜电致发光显示器以外,近年来还在单晶衬底上生长出突变异质结、多量子阱结构和超晶格,从而为研究低维数半导体薄层结构提供了一个媒介.     

Optimal doping control of magnetic semiconductors via subsurfactant epitaxy

"Subsurfactant epitaxy" is established as a conceptually new approach for introducing manganese as a magnetic dopant into germanium. A kinetic pathway is devised in which the subsurface interstitial sites on Ge(100) are first selectively populated with Mn, while lateral diffusion and clustering on or underneath the surface are effectively suppressed. Subsequent Ge deposition as a capping layer produces a novel surfactantlike phenomenon as the interstitial Mn atoms float towards newly defined subsurface sites at the growth front. Furthermore, the Mn atoms that failed to float upwards are uniformly distributed within the Ge capping layer. The resulting doping levels of order 0.25 at. % would normally be considered too low for ferromagnetic ordering, but the Curie temperature exceeds room temperature by a comfortable margin. Subsurfactant epitaxy thus enables superior dopant control in magnetic semiconductors.     

HgTe/CdTe superlattices grown by molecular beam epitaxy

In this paper we present results concerning the optical absorption in HgTe-CdTe superlattices. We confirm the narrowing of the superlattice band-gap (the increase of cut-off wavelength, λ_c) compared to the band gap of the equivalent Hg_1?xCd_xTe alloy. We show also, as predicted by the theory, an increase of the cut-off wavelength of the superlattice when the HgTe layer thickness increases. At 300K, the agreement between theory and experiment is fairly good if we consider the onset of the absorption. The λ_c tail shifting towards shorter wavelengths could be explained by the interdiffusion between HgTe and CdTe layers. At 30K, no important change in the I.R. absorption is noticed for all the superlattices.We present for the first time a superlattice exhibiting an absorption in the 8–12 μm window.We have carried out Hall measurements on several superlattices and present for the first time transport properties on these alternate microstructures. The most important features concern the unexpected and not yet understood very high hole mobilities at 10K.     

Optical characterization of CdTe/ZnSe fractional monolayer structures grown by atomic layer epitaxy

Luminescence properties of CdTe/ZnSe fractional monolayer grown by atomic layer epitaxy have been investigated. To investigate the origin of the highly efficient luminescence, various optical spectroscopic methods such as, photoluminescence (PL), temporal/spatial resolved PL, temperature dependence PL, and excitation power dependence PL have been used. It is found that structural inhomogeneities affect dominant influence on the line width and line shape of luminescence. The luminescence intensity greatly enhanced by the localization of exciton at the disorder induced localized states.     

Molecular beam epitaxy

Molecular beam epitaxy (MBE) is a process for growing thin, epitaxial films of a wide variety of materials, ranging from oxides to semiconductors to metals. It was first applied to the growth of compound semiconductors. That is still the most common usage, in large part because of the high technological value of such materials to the electronics industry. In this process beams of atoms or molecules in an ultra-high vacuum environment are incident upon a heated crystal that has previously been processed to produce a nearly atomically clean surface. The arriving constituent atoms form a crystalline layer in registry with the substrate, i.e., an epitaxial film. These films are remarkable because the composition can be rapidly changed, producing crystalline interfaces that are almost atomically abrupt. Thus, it has been possible to produce a large range of unique structures, including quantum well devices, superlattices, lasers, etc., all of which benefit from the precise control of composition during growth. Because of the cleanliness of the growth environment and because of the precise control over composition, MBE structures closely approximate the idealized models used in solid state theory.

This discussion is intended as an introduction to the concept and the experimental procedures used in MBE growth. The refinement of experimental procedures has been the key to the successful fabrication of electronically significant devices, which in turn has generated the widespread interest in the MBE as a research tool. MBE experiments have provided a wealth of new information bearing on the general mechanisms involved in epitaxial growth, since many of the phenomena initially observed during MBE have since been repeated using other crystal growth processes. We also summarize the general types of layered structures that have contributed to the rapid expansion of interest in MBE and its various offshoots. Finally we consider some of the problems that remain in the growth of heteroepitaxial structures, specifically, the problem of mismatch in lattice constant between layers and between layer and substrate. The discussion is phenomenological, not theoretical; MBE has been primarily an experimental approach based on simple concepts.     

The magnetic properties of Be-codoped GaMnN grown via molecular beam epitaxy

GaMnN and Be-codoped GaMnN were grown via molecular beam epitaxy using a single GaN precursor and their structural and magnetic properties were examined. X-ray diffraction and superconducting quantum interference device (SQUID) measurements revealed that the grown layers are homogeneous without precipitates. The saturation magnetization of GaMnN has increased from ∼4 to ∼16 emu/cm³ via codoping of Be. The d–d exchange interaction between Mn atoms was discussed for the ferromagnetism of GaMnN.     

Scanning tunneling microscopy of InAs/GaSb superlattices with various growth conditions

Cross-sectional scanning tunneling microscopy (STM) is used to characterize InAs/GaSb superlattices, grown by molecular beam epitaxy (MBE). Previous STM studies have found an interface asymmetry, with the interfaces of InAs grown on GaSb being rougher than those of GaSb on InAs. In the present work, a comparison is made between samples grown under various conditions, using elevated growth temperature or using atomic layer epitaxy (ALE). In both cases, the interface asymmetry is found to decrease compared to MBE growth at 380°C. In addition, Sb incorporation in the InAs layers is observed directly in the STM images. It is argued that the additional roughness seen at the InAs on GaSb interfaces arises in part from incorporation of excess Sb into the growing InAs layers. Elevated temperatures reduces this incorporation, and hence produces smoother interfaces.     

Carrier confinement and interband transition properties of InAs/GaAs quantum dots grown by using atomic layer epitaxy

The electrical and the optical properties of InAs/GaAs quantum dots (QDs) grown by using atomic layer epitaxy (ALE) technique were investigated by using capacitance-voltage (C-V) and photoluminescence (PL) measurements. C-V curves showed that the plateaus related to the zero-dimensional carrier confinement effect existed and that the number of electrons occupying the InAs QD was approximately 7. The full width at half maxima of the interband transitions from the ground electronic subband to the ground heavy-hole subband and from the first excited electronic state to the first excited state heavy-hole subband were not significantly affected by the temperature variation, indicative of strong confinement of the carriers occupying the InAs QDs. These results can help improve understanding for applications of InAs/GaAs QDs grown by using ALE in high-efficiency electronic and optoelectronic devices.     

Atomic layer molecular beam epitaxy (Almbe) of III–V compounds: Growth modes and applications

A new development of molecular beam epitaxy (MBE) for III–V compounds is described, based on cyclic perturbation of the growth front at atomic layer level by periodic pulsing, alternating or interrupting the molecular beams. The modification of the growth mechanism caused by this perturbations is discussed and related to periodic changes of surface stoichiometry which induce 2D growth mechanism by enhanced layer nucleation.Under appropriate modulation conditions, an atomic layer by layer growth mode can be achieved. A practical implementention of this mode, that we denote atomic layer MBE (ALMBE), is considered in which only group V beams are pulsed in a specially designed effusion cell. A number of growth applications of ALMBE are presented, including growth of highly mismatched heterostructures and short period superlattices containing two different group V elements such as arsenic and phosphorous.     

Surface micromorphology of CdTe(310) layers grown by molecular beam epitaxy

Surface morphology of CdTe(310) buffer layers grown by molecular beam epitaxy has been investigated by the method of reflection of high-energy electron diffraction. It was established that a clean CdTe(310) surface is atomically flat. Its reconstruction can be described by a unit cell coinciding with the unit cell of the unreconstructed (310) surface. It is determined that Te₂ adsorption in amounts of less than 0.2 monolayers results in the surface reconstruction with the formation of terraces parallel to the (100) plane and are 3/2a long. A system of (100) + (210) facets develops on the CdTe(310) surface with the increase in the Te adsorption layer’s thickness up to 0.3 monolayers and more.     

Long-wavelength IR spectroscopy of ZnTe/CdTe superlattices with quantum dots

This paper reports on the results of investigations of the lattice IR reflection spectra of ZnTe/CdTe multilayer superlattices with CdTe quantum dots grown by molecular-beam epitaxy on a GaAs substrate with a CdTe buffer layer. It is found that the lattice IR reflection spectra of the studied structures exhibit three intense bands associated with vibrational excitations in the GaAs substrate, ZnTe barriers separating the layers with CdTe quantum dots, and the CdTe buffer layer. An analysis of the reflection bands and shifts in the phonon frequencies has revealed internal elastic stresses both in the surface layer of the GaAs substrate and in the ZnTe barriers. It is established that elastic stresses undergo relaxation in the separating ZnTe layers with an increase in their thickness. An additional mode observed in the reflection spectra is explained by manifestations of ZnTe-like vibrations in the ZnCdTe alloy due to interdiffusion of Cd and Zn at the interfaces.     

Graphene growth on h-BN by molecular beam epitaxy

The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.     

Interfacial roughnes and alloy scattering in the InGaAs/InAlAs/InP system grown by ALE and LAMBE

Molecular beam epitaxial growth of In_xGa_1−xAs and In_yAl_1−yAs on Inp has been carried out by atomic layer epitaxy (ALE) and laser assisted molecular beam epitaxy (LAMBE). It is shown that these growth techniques have minimized both alloy clustering and interface roughness in the InGaAs/InAlAs system. Splitting of the PLE spectra indicates a roughness of 2–3 monolayers while transport measurements have placed an upper limit to the roughness at 4 monolayers.     

Ordered-disordered ternary alloys by atomic layer epitaxy

Ga₀₅In₀₅P ternary alloys, lattice-matched to GaAs substrates, were grown by atomic layer epitaxy. The growth proceeded by the deposition of monolayers of In-P-Ga-P in a self-regulated fashion. The ternary alloys were found to have different crystal and bandgap structures depending on the growth conditions. Films deposited on (1 0 0) oriented GaAs substrates have a random (disordered) structure withE _g 1.9 eV. However, the same ternary alloy deposited on misoriented substrates showed a high degree of ordering withE _g 1.76 eV. The ordered structure is in the form of highly strained monolayer superlattices (InP-GaP) oriented along the (1 1 1) direction. The ordered-disordered transition can also be achieved by Se doping to the 10¹⁷ cm^–3 range. We report on the atomic layer epitaxy growth conditions for both ordered and disordered GaInP films. We also discuss several possible quantum well structures based on this ternary alloy.     

Studies of the surface structures formed by the alternated electrodeposition of Cd and Te on the low-index planes of Au: II. STM studies

Scanning tunneling microscopy (STM) studies of the alternated deposition of Te and Cd atomic layers on the low-index planes of Au are presented here. These investigations were designed to gain an understanding of the surface chemistry involved in the deposition of CdTe by electrochemical atomic layer epitaxy (ECALE). ECALE is the electrochemical analog of atomic layer epitaxy (ALE). In the present study, the structures formed by the alternated electrodeposition of Te and Cd atomic layers, at underpotential, were examined by STM on the low-index planes of Au.

Deposition of atomic layers of each element were achieved by underpotential deposition (UPD). Studies of the UPD of Cd by STM were not performed due to Cd's instability in aqueous solutions after withdrawal from solution and loss of potential control. Studies of Te UPD were performed, however, since Te is stable in aqueous media without potential control. Ordered Te structures were observed using STM on all three Au low-index planes. Atomic resolution imaging of Cd UPD on Te atomic layers proved more difficult than imaging simple Te deposits, however, ordered CdTe structures were observed on each Au plane. Higher bias voltages were generally required to image the CdTe deposits. On Au(100) the CdTe deposit appears to adopt a structure equivalent to a (100) slice of bulk CdTe (zinc blende), but contracted 10% due to the lattice missmatch on the Au surfaces. Similar structures were observed on the other two low index planes, as well; although Cd-Cd distances were closer to those found in bulk CdTe.     

Type III - Type I transition and strain-effect in Hg1−xCdxTeCdTe and Hg1−xZnxTeCdTe superlattices

In this paper, the results of Hg_1−xZn_xTeCdTe strained layer superlattices grown by MBE are reported, and compared to Hg_1−xCd_xTeCdTe superlattices. Both Type III and Type I Hg_1−xZn_xTeCdTe superlattices with different strain have been grown on CdTe(111)B/GaAs(100) and CdTe(100)/GaAs(100) substrates and characterized by electron, X-ray diffraction, infrared transmission and Hall measurements. The values of hole mobility between 5×10³ up to 2×10⁴cm²v⁻¹s⁻¹ at T = 23K along (111)B growth orientation and up to 4.9×10⁴cm²v⁻¹s⁻¹ at T = 5K along (100) growth orientation are obtained for Type III superlattices whereas in Type I superlattices, the hole mobility is between 200–300cm²v⁻¹s⁻¹. This drastic change in the hole mobility between Type III and Type I superlattices along with the role of the strain are discussed in this paper.     

Defects in HgCdTe grown by molecular beam epitaxy on GaAs substrates

The Hall effect and photoluminescence measurements combined with annealing and/or ion milling were used to study the electrical and optical properties of HgCdTe films grown by molecular-beam epitaxy on GaAs substrates with ZnTe and CdTe buffer layers. Unintentional donor doping, likely from the substrate, which resulted in residual donor concentration of the order of 10¹⁵ cm^?3, was observed in the films. Also, acceptor states, possibly related to structural defects, were observed.     

Midinfrared photoluminescence from SnTe/PbTe/CdTe double quantum wells grown by molecular beam epitaxy

Molecular beam epitaxial growth and photoluminescence (PL) properties of SnTe/PbTe/CdTe double quantum wells (DQWs) on (1 0 0)-oriented GaAs substrates are reported. These DQWs were consisted of a very thin SnTe/PbTe QW nested in a 10-nm-thick PbTe/CdTe QW. Efficient midinfrared PL was observed from the DQWs at 300 K in agreement with the coherent SnTe/PbTe growth on the thick CdTe barrier layer. The PL peak wavelength of the DQWs was found to increase with the SnTe thickness d by covering a wide range of the 3–5 μm atmospheric window with d≤2.5 monolayer.