Orientation-patterned templates GaAs/Ge/GaAs for nonlinear optical devices. II. Investigation of properties

GaAs/Ge/GaAs heterostructures in which GaAs layer lattices are rotated at a right angle in the substrate plane are studied. High quality of heterostructures is confirmed by X-ray diffraction and photoluminescence methods.     

Molecular beam epitaxy growth of GaAs on an offcut Ge substrate

Molecular beam epitaxy growth of GaAs on an offcut Ge (100) substrate has been systemically investigated. A high quality GaAs/Ge interface and GaAs film on Ge have been achieved. High temperature annealing before GaAs deposition is found to be indispensable to avoid anti-phase domains. The quality of the GaAs film is found to strongly depend on the GaAs/Ge interface and the beginning of GaAs deposition. The reason why both high temperature annealing and GaAs growth temperature can affect epitaxial GaAs film quality is discussed. High quality In_0.17Ga_0.83As/GaAs strained quantum wells have also been achieved on a Ge substrate. Samples show flat surface morphology and narrow photoluminescence line width compared with the same structure sample grown on a GaAs substrate. These results indicate a large application potential for III--V compound semiconductor optoelectronic devices on Ge substrates.     

Molecular beam epitaxy growth of quantum devices

The inherent fragility and surface/interface-sensitivity of quantum devices demand fabrication techniques under very clean environment. Here, I briefly introduces several techniques based on molecular beam epitaxy growth on pre-patterned substrates which enable us to directly prepare in-plane nanostructures and heterostructures in ultrahigh vacuum. The molecular beam epitaxy-based fabrication techniques are especially useful in constructing the high-quality devices and circuits for solid-state quantum computing in a scalable way.     

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.     

Molecular beam epitaxy and optical evaluation of AlxGa1-xAs

The epitaxial growth of Al_xGa_1-xAs on GaAs by the molecular beam vaporization method was investigated. The ratio of Al to Ga incorporated into the film was monitored by a mass spectrometer. Observations by reflection of high energy electron diffraction during growth and measurements of optical reflectivity suggest that it is possible to produce high quality Al_xGa_1-xAs single crystal thin films by the molecular beam method. Films grown with x varying from 0.1 to 0.47 were studied.     

Quantitative evaluation of substrate temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy

Combined Hall effect and low-temperature photoluminescence measurements have been used to perform a thorough evaluation of the growth temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy (MBE) over the entire substrate temperature range (400°≦T _s ≦600[°C]) practicable forn-type layer growth. Using a constant As₄ to Ga flux ratio of two, growth below 500°C yieldsn-GaAs: Ge films having electrical and optical properties rapidly deteriorating with decreasingT _s . Growth at 500° ≦T _s ≦600[°C] produces high-qualityn-GaAs: Ge films (N _D /N _A ≈4) with C as well as Ge residual acceptors competing on the available As sites. The amount of Ge atoms on As sites [Ge_As] increases with substrate temperature, whereas simultaneously the amount of C atoms on As sites [C_As] decreases thus leading to the well-establishednonlinear behaviour of the (N _A /N _D vs. 1/T _s plot. Counting the incorporated Ge impurities separately, however, yields alinear behaviour of the ([Ge_As]/[Ge_Ga]) vs. 1/T _s plot which has exactly the same slope as the (P _{As 2}/P _Ga) vs. 1/T _s plot derived from vapour pressure data of As₂ and Ga over solid GaAs surfaces. The important result is, therefore, that the incorporation behaviour of Ge in GaAs during molecular beam epitaxy is directly correlated with theevaporation behaviour of the growing GaAs surface.     

Optical properties of GaInNAs/GaAs prepared by molecular beam epitaxy

Optical properties of GaInNAs are studied with emphasis on photoluminescence (PL) and photoreflectance (PR) using GaInNAs single quantum wells (GaInNAs-SQW) and thick GaInNAs layers lattice-matched to GaAs (LM-GaInNAs) grown by solid-source molecular-beam epitaxy. First, a blue-shift of band-gap energy as a result of a rapid thermal annealing (RTA) has been confirmed by both PL and PR. Temperature-dependent PL and PR spectra were measured. At low temperature, GaInNAs-SQWs exhibited a PL due to the localized exciton (LE), and the localization was strongly reduced by RTA. A very high-quality GaInNAs-SQW exhibited very intense PL with small half-width (10–20 meV for 8–300 K) without LE emission. In LM-GaInNAs, low-temperature PL exhibited both the LE emission and a deep PL band. Temperature-dependent PL showed that LM-GaInNAs contains large number of non-radiative centers. PR spectra of LM-GaInNAs were composed of more than two transitions, and their relative intensity depended on temperature and annealing.     

Molecular beam epitaxy of GaSb on ZnTe/GaAs: Influence of the chemical composition of ZnTe surface

We have studied the molecular beam epitaxy (MBE) of GaSb films on GaAs (0 0 1) substrates by using ZnTe as a new buffer layer. GaSb films were grown on two distinct ZnTe surfaces and the influence of surface chemical composition of ZnTe on the morphological and structural properties of GaSb films has been investigated. Initial 2-dimensional (2D) growth of GaSb films is obtained on Zn-terminated surface consequently smooth morphology and high crystal quality GaSb films are achieved. The thin GaSb film (0.4 μm) grown on Zn-terminated ZnTe surface reveals considerably narrow X-ray diffraction linewidth (113 arcsec) along with small residual strain, which strongly supports the availability of ZnTe buffer for the growth of high-quality GaSb film.     

Molecular beam epitaxy and reconstructed surfaces

In the last years molecular beam epitaxy (MBE) has become a very important method to create new materials. Scattering and channeling of high-energy ions is a mass-dispersive, surface-sensitive crystallographic technique, particularly suited for the investigation of epitaxial systems. The combination of both techniques allows new insight into some of the fundamental processes at interfaces and surfaces. As an example we discuss the influence of substrate reconstruction on epitaxial growth. We show for the Si/Ge and Si/Si systems that the reordering of the reconstruction-induced displacements at the substrate surface, a necessary condition for epitaxial growth, is critically dependent on the type of reconstruction: Deposition of Ge or Si on Si(100)2×1 at room-temperature relieves the reconstruction, whereas Si(111)7×7 appears unaffected. This difference is discussed in terms of structural models for these surfaces. We shall also discuss the implications of these results with respect to MBE and, in particular, to Si homoepitaxial temperatures.     

Improvedp/n junctions in Ge-doped GaAs grown by molecular beam epitaxy

The effects of single-pulse ruby laser irradiation have been investigated in Si samples with disorder layers located at a depth of 2000 Å from the crystal surface and extending up to 8000 Å. This disorder was obtained by implantation with 350 keV N⁺ to a fluence of 2×10¹⁶/cm². Channeling, diffraction and transmission electron microscopy were used to characterize the structure of the irradiated layers. After 1.5 J/cm² irradiation the damaged layer reorders partially, while for about 2.0J/cm² the surface single crystal becomes polycrystalline. At a higher energy density all the material undergoes the transition to single crystal. Calculations based on the liquid model accounts in part for the experimental results.     

Zone–edge optical transitions in GaAs/AlAs lateral superlattices grown on vicinal surfaces by molecular beam epitaxy

We report on new features in the photoluminescence excitation (PLE) spectra and PLE linear polarization spectra of GaAs/AlAs lateral superlattices grown by molecular beam epitaxy (MBE). These lines appear systematically as the tilt angle of the lateral superlattice is varied. They are identified as zone–edge excitonic transitions by comparison between experimental data and detailed numerical calculations of optical transitions including valence-band mixing and tilt effects.     

GaAs/Ge/GaAs Heterostructures for Use in Nonlinear Optics

GaAs/Ge/GaAs heterostructures with sublattice reversed GaAs on Ge epilayers grown via molecular beam epitaxy (MBE) for periodic domain inverted structures are presented. For the first time, such structures are grown in separate MBE machines for GaAs and Ge with atmospheric wafer transfer. The high quality of the heterostructures is confirmed via X-ray diffraction and photoluminescence. It is proposed that the surfactants (Bi, Sb) be used to control the nucleation of GaAs epilayers on a Ge epilayer.     

An interface study of crystalline Fe/Ge multilayers grown by molecular beam epitaxy

Fe/Ge multilayers were grown on single crystal Ge(0 0 1) substrates by molecular beam epitaxy. The structural, electronic and magnetic properties of Fe/Ge have been studied. The analysis shows that Fe grows in a layer-by-layer epitaxial growth mode on Ge(0 0 1) substrates at 150 °C and no intermixing has been observed. Growth of a crystalline Ge film at 150 °C on a single crystal Fe film has been observed. At this temperature Ge films grow by means of the island growth mode according to reflection of high energy electron diffraction patterns. Fe layers of 36 nm thickness, deposited at 150 °C on Ge(0 0 1) substrates, show two magnetization reversal values indicating the growth of Fe in two different crystal orientations. 36 nm thick Fe and Ge layers grown at 150 °C in Ge/Fe/Ge/Fe/Ge(0 0 1) sequence shows ferromagnetic behavior, however, the same structure grown at 200 °C shows paramagnetic behavior.     

Molecular beam epitaxy with synchronization of nucleation

A new MBE technique based on the possibility to maintain undamping RHEED oscillations by periodic increase of the surface supersaturation synchronized with the oscillations is presented. Surface supersaturation may be increased by any way that is suitable for a particular case. The technique has been referred to by the authors as MBE with nucleation synchronization and allows epistructures of any thickness to be grown with in situ control of their properties by RHEED or automatic ellipsometry. It can be applied both to elementary semiconductors and compounds like A_IIIB_V, A_IIB_VI, etc.     

Optical and nonlinear optical characteristics of the Ge and GaAs nanoparticle suspensions prepared by laser ablation

The laser ablation of Ge and GaAs targets placed in water and ethanol was carried out using the fundamental radiation of nanosecond Nd:YLF laser. The results of preparation and the optical and nonlinear optical characterization of the Ge and GaAs nanoparticle suspensions are presented. The considerable shift of the band gap energy of the nanoparticles compared to the bulk semiconductors was observed. The distribution of nanoparticle sizes was estimated in the range of 1.5-10 nm on the basis of the TEM and spectral measurements. The nonlinear refractive indices and nonlinear absorption coefficients of Ge and GaAs nanoparticles were defined by the z-scan technique using second harmonic radiation of picosecond Nd:YAG laser (λ = 532 nm).     

The effect of arsenic vapour species on electrical and optical properties of GaAs grown by molecular beam epitaxy

Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As₂) vs tetrameric (As₄) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As₂ and As₄ sources, respectively, and from a single source providing an adjustable As₂/As₄ flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As₄/As₂ flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As₂ molecular beam species which provide a higher steady-state arsenic surface population.     

Molecular beam epitaxy growth of iodide thin films

Study of two-dimensional(2D)magnetic materials is important for both fundamental research and application.Here we report molecular beam epitaxy growth of iodides,candidates for exhibiting 2D magnetism.Decomposition of CrI₃is utilized to produce stable gaseous I₂flux.Growth of MnI₂,GdI₃,and CrI₂down to monolayer is successful achieved by co-depositing I2 and corresponding metal atoms.The thin films of the three materials are characterized by scanning tunneling microscope and found to be insulators with bandgaps of 4.4 e V,0.6 e V,and 3.0 e V,respectively.The film growth paves the way for further study of magnetic properties at the 2 D limit.     

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.     

Raman study of the segregation of GaAs/AlAs heterostructures grown by molecular beam epitaxy

The Raman spectra of the optical confined phonons in the GaAs/AlAs ultra-thin layer superlattices grown with different growth conditions were used to determine the compositional profiles and to study the process of segregation at the heterointerfaces. A modified kinetic model was developed in order to calculate the compositional profiles in the samples under investigation. The comparison between the experimentally obtained compositional profiles and those calculated by the kinetic model allowed us to determine the parameters characterizing the segregation. It was shown that the increase of pressure of As acts equivalently to the decrease of the growth temperature, resulting in a more abrupt compositional profile.     

Growth of high-crystallinity uniform GaAs nanowire arrays by molecular beam epitaxy

The self-catalyzed growth of Ga As nanowires(NWs) on silicon(Si) is an effective way to achieve integration between group III–V elements and Si. High-crystallinity uniform Ga As NW arrays were grown by solid-source molecular beam epitaxy(MBE). In this paper, we describe systematic experiments which indicate that the substrate treatment is crucial to the highly crystalline and uniform growth of one-dimensional nanomaterials. The influence of natural oxidation time on the crystallinity and uniformity of Ga As NW arrays was investigated and is discussed in detail. The Ga As NW crystallinity and uniformity are maximized after 20 days of natural oxidation time. This work provides a new solution for producing high-crystallinity uniform III–V nanowire arrays on wafer-scale Si substrates. The highly crystalline uniform NW arrays are expected to be useful for NW-based optical interconnects and Si platform optoelectronic devices.