Hot Wall Epitaxy Technique Research Articles

High-Quality Bi2Te3 Single Crystalline Films on Flexible Substrates and Bendable Photodetectors**Supported by the National Basic Research Program of China under Grant No 2012CB619200, the National Natural Science Foundation of China under Grant Nos 61290304, 11074265 and 11174307, the Natural Science Foundation of Shanghai under Grant No 16ZR1441200, and the Frontier Science Research Project (Key Programs) of Chinese Academy of Sciences

Recently, great efforts have been made in the fabrication of arbitrary warped devices to satisfy the requirement of wearable and lightweight electronic products. Direct growth of high crystalline quality films on flexible substrates is the most desirable method to fabricate flexible devices owing to the advantage of simple and compatible preparation technology with current semiconductor devices, while it is a very challenging work, and usually amorphous, polycrystalline or discontinuous single crystalline films are achieved. Here we demonstrate the direct growth of high-quality Bi2Te3 single crystalline films on flexible polyimide substrates by the modified hot wall epitaxy technique. Experimental results reveal that adjacent crystallites are coherently coalesced to form a continuous film, although amounts of disoriented crystallites are generated due to fast growth rate. By inserting a quartz filter into the growth tube, the number density of disoriented crystallites is effectively reduced owing to the improved spiral interaction. Furthermore, flexible Bi2 Te3 photoconductors are fabricated and exhibit strong near-infrared photoconductive response under different degrees of bending, which also confirms the obtained flexible films suitable for electronic applications.

Amorphous/epitaxial superlattice for thermoelectric application

An amorphous/epitaxial superlattice system is proposed for application to thermoelectric devices, and the superlattice based on a PbGeTeS system was prepared by the alternate deposition of PbS and GeTe using a hot wall epitaxy technique. The structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray analysis, and it was found that the superlattice consists of an epitaxial PbTe-based layer and a GeS-based amorphous layer by the reconstruction of the constituents. A reduction in thermal conductivity due to the amorphous/epitaxial system was confirmed by a 2ω method. Electrical and thermoelectric properties were measured for the samples.

GROWTH AND MICROSTRUCTURES OF ULTRATHIN Bi2Te3 NANOPLATES BY MODIFIED HOT WALL EPITAXY

Ultrathin Bi 2 Te 3 nanoplates have been grown on an oxidized silicon substrate by a modified hot wall epitaxy (HWE) method, in which a quartz plate with holes was employed. The microstructures and optical properties of Bi 2 Te 3 nanoplates were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscope (AFM) and micro-Raman spectroscopy. The results show that ultrathin Bi 2 Te 3 nanoplates with the thickness of about six quintuple layers (QLs) are obtained, which is difficult for the traditional HWE technique. The Raman vibration mode [Formula: see text] from the nanoplates exhibits an obviously red shift with decreasing thickness. The thickness variation of one nanoplate was obtained by the Raman map derived from the vibration frequency of [Formula: see text] mode and is in good agreement with the AFM result, which indicates that Raman map is an effective method to characterize the thickness difference of ultrathin Bi 2 Te 3 nanoplates.

Injection current and infrared photosensitivity in isotype p-PbTe/p-CdTe heterojunctions

Iso-type p-PbTe/p-CdTe heterojunctions were grown on BaF2 substrates by using the hot-wall epitaxy technique. The growth details are presented. The carrier transport mechanism was investigated by means of the current-voltage measurements. At 77 K the dominant conduction mechanism was found to be the space-charge-limited current. The photovoltaic response in the long-wave infrared region with the half-peak cut-off wavelength ranged from 8.0 to 9.0 m was observed in these heterojunctions for the first time. The possible mechanism of the photoresponse is the internal photoemission of holes from p-PbTe across the heterojunction barrier.

The 60° or 180° twinned Bi-doped PbTe film studied by EBSD

The Bi-doped PbTe film was grown on Si(1 1 1) substrate by using hot wall epitaxy (HWE) technique. The film was characterized by means of scanning electron microscopy, micro-area X-ray diffraction and electron backscatter diffraction (EBSD). The results indicate that the film is dominated by 〈1 1 1〉 orientation. The film consists of two twinned domains, rotated 60° or 180° around the normal to the film surface. It is speculated that the twinned PbTe film results from the deviated triangular grains. The ratio between the grains with two different orientations will decrease with the increase of the film thickness.

AlN/GaN superlattice quality improvement by using multiple superlattice structure

AbstractWe report on an improvement of structural and morphological quality of AlN/GaN superlattices when the multiple superlattice (MSL) structure is used. Such a structure contains strained AlN/GaN simple superlattices with a small number of periods separated by thick GaN layer. Hot wall epitaxy technique (HWE) was used to grow these structures and atomic force microscopy (AFM) as well as X‐ray diffraction measurements were used to obtain the information about their structural and morphological properties. According to AFM measurements, the threading dislocation density and the surface roughness were decreased compare to the simple superlattice. Xray rocking curve (XRC) measurements exhibit narrower peaks, so the full width at half maximum (FWHM) of the 0th superlattice peak was decreased by 37%. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Morphological, electrical and optical properties of 1-(3-hydroxypropylamino)-9,10-anthraquinone films prepared by Hot Wall technique

The films of 1-(3-hydroxypropylamino)-9,10-anthraquinone (1) obtained by Hot Wall Epitaxy technique (HWE) have been studied for their morphological and electrical properties. The films were deposited on to the glass substrate at different temperatures kept at pressure of ∼1.33 x 10 -3 Pa. The powder X-ray diffraction (XRD) spectra show the polycrystalline nature of the films. The crystallanity of the films increases with the increase in film deposition temperature. Scanning electron microscope (SEM) shows that the molecules of 1 undergo uniform aggregation to form blade type crystallites with width 5.2-11.8 μm and length 32.6-90 μm at substrate temperature 348 K. The conduction in these films is ohmic in nature and both the increase in substrate temperature during film deposition and increase in absolute temperature of the films during conductivity measurement resulted in increased conductivity. The band gap lies in the range 1.25-1.44 eV.

Study of thermal and optical properties of the semiconductor CdTe by photopyroelectric spectroscopy

In this study, photopyroelectric spectroscopy (PPES) was used to obtain thermal and optical properties of CdTe thin films in the 300–1200 nm wavelength range. The samples studied were grown on glass or BaF2 substrates using the hot wall epitaxy technique with varying thicknesses. The normalized photopyroelectric signal intensity and its phase were independently measured as a function of wavelength λ and chopping frequency f. Equations of both the intensity and the phase of the PPES signal, taking into account the thermal and the optical characteristics of the pyroelectric detector, were used to fit the experimental results. From the fittings, we have obtained the values of thermal diffusivity coefficient α, thermal conductivity k, optical absorption coefficient, and the optical gap of CdTe.

Growth of anthracene derivative thin films with a π-stacking structure

The main purpose of this study is to grow an organic thin film consisting of aromatic molecules with enhanced π-conjugations. For this purpose, an anthracene derivative, 9,10-bis (methylthio) anthracene, was deposited on a CaF2∕Si(111) substrate using a hot-wall epitaxy technique. The crystal structure of this anthracene derivative consists of molecular stacks with a face-to-face configuration due to the sulfur-sulfur interactions between neighboring molecules, which enhances π-electron conjugation. The hot-wall epitaxy technique made precise control of the growth conditions possible. Atomic force microscopy and x-ray diffraction measurements demonstrated that a polycrystalline thin film with the molecules in the face-to-face configuration was obtained, and that, depending on the growth conditions, the thin-film structure was controllable.

Photoluminescence, Reflectance and Photoreflectance Spectra in CdS Epilayers on Si(111) Substrates

Photoluminescence (PL), reflectance and photoreflectance (PR) spectra were measured on hexagonal CdS films grown directly on hydrogen-terminated Si(111) substrates with 3° off-orientation by hot-wall epitaxy technique. By comparing the PL spectrum to the reflectance and PR spectra in the excitonic energy range, we have unambiguously identified free A- and B-exciton transitions in the CdS films on Si(111) substrates. The peak energies of the A- and B-exciton transitions slightly shift to lower energy by about 3 meV than that of bulk crystal. This peak shifts were caused by a tensile strain parallel to the epilayer surface due to a difference of the thermal expansion coefficients between the grown CdS film and Si substrate. Donor–acceptor pair emission and defect-related “Y” bands as well as free and bound exciton peaks in PL spectra were also studied by means of their temperature (10–250 K) and excitation intensity dependences.

Optical evidences of [formula omitted] quantum dots formed in ZnTe matrix

Thin ZnS 0.78 Te 0.22 layers embedded in ZnTe matrix have been grown on GaAs (1 0 0) substrate by hot-wall epitaxy technique with varying growth time. Their optical properties have been investigated by photoluminescence (PL) measurements at low temperature and by temperature-dependent PL measurements. PL measurements show that the quantum dots (QDs) related peak at lower energy side with a broad full-widths at half-maximum (FWHM) and the two-dimensional (2D) layer related peak at higher energy side with a narrow FWHM are clearly resolved and their energies shift to lower energies as the ZnS 0.78 Te 0.22 growth time increases. It is also found from temperature-dependent PL measurements that the activation energy of QDs is almost two times larger than that of 2D layer. Atomic force microscopy confirms their quantum dot formation.

Roughness of CdTe thin films grown on glass by hot wall epitaxy

Cadmium telluride films were grown on glass substrates using the hot wall epitaxy(HWE) technique. The samples were polycrystalline with a preferential (111)orientation. Scanning electron micrographs reveal a grain size between 0.1 and0.5 µm. The surface morphology of the samples was studied by measuring the roughnessprofile using a stylus profiler. The roughness as a function of growth time andscale size were investigated to determine the growth and roughness exponents,β and α, respectively. From the results we can conclude that the growthsurface has a self-affine character with a roughness exponentα equalto 0.69 ± 0.03 and almost independent of growth time. The growth exponentβ wasequal to 0.38 ± 0.06. These values agree with that determined previously for CdTe(111) films grown onGaAs(100).

Strong luminescence from dislocation-free GaN nanopillars

GaN nanostructures were prepared on Si(111) by a hot-wall epitaxy technique employing the modified two-step growth method. Isolated hexagonal pillar-like GaN nanostructures (GaN nanopillars) with the typical diameter, height, and density of 200–300nm, 0.5–1μm, and 3–4×108cm−2, respectively, are self-organized without intentional pre-processing to the Si substrate. The photoluminescence and cathodoluminescence (CL) measurements show the strong near-band-edge emissions without the yellow band at room temperature. Stronger CL is obtained from the GaN nanopillars in comparison to single-crystalline GaN. The obtained strong CL is related to high crystal quality of the dislocation-free GaN nanopillars.

Cathodoluminescence of polycrystalline GaN grown by a hot wall epitaxy technique

AbstractPolycrystalline GaN nanostructures were grown on Si(111) substrates by a hot wall epitaxy technique using metallic gallium and ammonia sources. Self‐organized pillar‐like GaN nanostructures (GaN nanopillar) were grown without intentional pre‐processing to the Si substrate. It is found that the columnar growth was initiated by the GaN dots of 50 nm in diameter, which act as the low temperature buffer, and it resulted in the high photoluminescence and cathodoluminescence emissions due to the high crystal quality of the GaN nanopillars. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Type-II interband transition of ZnS0.78Te0.22/ZnTe single quantum wells

Optical properties of ZnS0.78Te0.22/ZnTe single quantum wells grown on GaAs (100) substrates by hot wall epitaxy technique with varying the ZnS0.78Te0.22 well width from 0.3 to 1.8 nm were investigated by photoluminescence (PL) measurements at low temperature and by temperature-dependent PL measurements. PL results show the evidence of type-II transition and their peak energy shifts to higher energies as the ZnS0.78Te0.22 well width decreases. In addition, temperature-dependent PL measurements show the increase of the activation energy as the well thickness decreases, indicating the increase of confinement effect. This study makes it possible to introduce proper band diagram for this structure, and can give very useful information on their device applications.

Fluorescent Crystallization of Isomeric Quinoline-Capped Thiophene Oligomer in Thin Films Grown by Hot-Wall Epitaxy

Crystal growth and fluorescent characteristics of a quinoline-capped thiophene oligomer were investigated in thin films grown on the (001) surface of KCl by a hot-wall epitaxy technique. Under controlled substrate and wall temperatures, the deposited film consisted of two types of crystallites: one was epitaxial needles oriented along the 〈100〉KCl and 〈110〉KCl directions, and another was irregular aggregates among them. Fluorescence spectromicroscopy revealed that the former crystals exhibited a red-shifted emission band with respect to the latter. Microscopic Raman spectra for the individual crystallites and vibrational analysis by molecular orbital calculations identified the origin of this fluorescence difference as cis−trans isomerization of the peripheral quinoline groups. In the epitaxial needles, the trans-form molecules were lying parallel to the KCl surface while in the irregular aggregates the cis-form molecules were standing. The detailed crystallization depending on the isomerization was also supported by scanning near-field/confocal scanning laser and transmission electron microscopies.

Characterization of hot wall epitaxy grown 1-(2-methoxy benzyloxy)-8-hydroxy-9,10-anthraquinone films

Organic compounds derived from 9,10-anthraquinone has attracted a great deal of interest in recent years because of their optical and photosemiconducting properties. In the present paper, 1-(2-methoxy benzyloxy)-8-hydroxy-9,10-anthraquinone compound has been synthesized and films grown by hot wall epitaxy technique onto the glass substrates held at different temperatures in a vacuum of 1.3×10 −3 Pa. The experimental conditions are optimized to obtain better crystallinity of the films. These films are characterized by nuclear magnetic resonance (NMR), optical absorption (IR, visible, near-UV), X-ray diffraction and scanning electron microscopy. In addition, the electrical properties of the films are determined in the temperature range 293–403 K. The IR and NMR studies identify the formation of 1-(2-methoxy benzyloxy)-8-hydroxy-9,10-anthraquinone deposits on the glass substrates. A platelet type of crystallites as wide as 2.87 μm are observed in the case of films deposited at 348 K. Observations reveal that the current–voltage characteristics of films show ohmic behaviour of conduction within the investigated field and temperature range. The conduction appears to take place by thermally activated hopping mechanism. The electrical conductivity and carrier concentration of the films increases with the increasing temperature. Analysis of optical absorption measurements indicates that the interband transition energies of films lie in the range from 2.1 to 2.63 eV.

Structural relationship between epitaxially grown para-sexiphenyl and mica (0 0 1) substrates

This study focuses on structural properties of para-sexiphenyl (PSP) epitaxial thin films grown on freshly cleaved mica (0 0 1) substrates. The layers were prepared by hot wall epitaxy (HWE) technique resulting in highly ordered organic structures with a needle-like morphology on the substrates. X-ray diffraction (XRD) pole figure technique, transmission electron diffraction (TED) and atomic force microscopy were used to characterise the epitaxial growth. However, on not perfectly cleaved mica substrates the needle direction can change around 120° when the films are grown on different terraces of mica (0 0 1) which are separated by cleavage steps. This behaviour can be referred to the properties of the monoclinic crystal structure of mica which is described in detail.

Characterization of AgInTe 2 films grown by a hot wall epitaxy technique on KCl substrates

AgInTe 2 films have been grown by a hot wall epitaxy technique onto KCl substrates kept at different temperatures in a vacuum of 1.3×10 −3 Pa. Experimental conditions were optimized to obtain better crystallinity in the films. The films thus prepared were studied for their electrical conductivity, Hall mobility, carrier concentration, as well as structural and optical properties. Observations reveal that the electrical conductivity and carrier concentration of films increases with increasing temperature of the substrate during deposition, while the Hall mobility decreases. The results indicate that the films are p-type, thus indicating holes as dominant charge carriers. The scanning electron micrographs of the films show an increase in grain size with increasing substrate temperature. Analysis of optical absorption studies on the films indicate the band gap energies lie in the range 1.12–1.26 eV.

Structural, optical, and electrical characterization of hot wall epitaxy grown 1-methoxy-8-hydroxy-9,10-anthraquinone films

1-methoxy-8-hydroxy-9,10-anthraquinone compound has been synthesized and its films are grown by the hot wall epitaxy technique onto the glass substrates kept at different temperatures in a vacuum of 10−5 Torr. The experimental conditions are optimized to obtain better crystallinity of the films. The films so prepared have been studied for their structural, optical, and electrical properties. Observations reveal that the crystallinity of the films increases with an increase in substrate temperature. Crystallites as large as 3.30 μm are observed in the case of films deposited at 348 K. Analysis of optical absorption measurements on the films indicate that the interband transition energies lies in the range 1.87–2.02 eV. The conduction in these films is found to be ohmic in nature and appears to take place by thermally activated hopping above intermolecular barriers. The electrical resistivity of films decreases with the increase in temperature, while carrier concentration increases.