Narrow Range Of Substrate Temperatures Research Articles

Selective Area Epitaxy of GaN Nanowires on Si Substrates Using Microsphere Lithography: Experiment and Theory.

GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiOx/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature–Ga/N2 flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N2 flux ratios. A model was developed that explains the selective growth range, which appeared to be highly sensitive to the growth temperature and Ga flux, as well as to the radius and pitch of the patterned pinholes. High crystal quality in the GaN nanowires was confirmed through low-temperature photoluminescence measurements.

Controlled nucleation of monolayer MoSe2 islands on Si (111) by MBE

Van der Waals bonding relaxes the constraints of lattice matching, making two-dimensional (2D) transition metal dichalcogenides attractive in the field of epitaxy. Recently, molecular beam epitaxy (MBE) of MoSe2 was demonstrated on a variety of substrates. Here, the authors use MBE to investigate the early stages of 2D nucleation of MoSe2 grown on Si in pursuit of controlled monolayer island size. The 2D nucleation rate varies by a factor of >2 over a narrow substrate temperature range of 550–560 °C. Above 560 °C, the desorption rate of Se from the surface exceeds the nucleation rate leading to fully suppressed 2D monolayer nucleation. X-ray diffraction confirms (001) oriented MoSe2 on Si (111). Raman spectra are consistent with 1–3 monolayer-thick MoSe2, in agreement with atomic force microscopy measurements of the monolayer height of 2D islands.

Synthesis and electronic properties of Ruddlesden-Popper strontium iridate epitaxial thin films stabilized by control of growth kinetics

We report on the selective fabrication of high-quality Sr$_2$IrO$_4$ and SrIrO$_3$ epitaxial thin films from a single polycrystalline Sr$_2$IrO$_4$ target by pulsed laser deposition. Using a combination of X-ray diffraction and photoemission spectroscopy characterizations, we discover that within a relatively narrow range of substrate temperature, the oxygen partial pressure plays a critical role in the cation stoichiometric ratio of the films, and triggers the stabilization of different Ruddlesden-Popper (RP) phases. Resonant X-ray absorption spectroscopy measurements taken at the Ir $L$-edge and the O $K$-edge demonstrate the presence of strong spin-orbit coupling, and reveal the electronic and orbital structures of both compounds. These results suggest that in addition to the conventional thermodynamics consideration, higher members of the Sr$_{n+1}$Ir$_n$O$_{3n+1}$ series can possibly be achieved by kinetic control away from the thermodynamic limit. These findings offer a new approach to the synthesis of ultra-thin films of the RP series of iridates and can be extended to other complex oxides with layered structure.

Van der Waals epitaxy of Ge films on mica

To date, many materials have been successfully grown on substrates through van der Waals epitaxy without adhering to the constraint of lattice matching as is required for traditional chemical epitaxy. However, for elemental semiconductors such as Ge, this has been challenging and therefore it has not been achieved thus far. In this paper, we report the observation of Ge epitaxially grown on mica at a narrow substrate temperature range around 425 °C. Despite the large lattice mismatch (23%) and the lack of high in-plane symmetry in the mica surface, an epitaxial Ge film with [111] out-of-plane orientation is observed. Crystallinity and electrical properties degrade upon deviation from the ideal growth temperature, as shown by Raman spectroscopy, X-ray diffraction, and Hall effect measurements. X-ray pole figure analysis reveals that there exist multiple rotational domains in the epitaxial Ge film with dominant in-plane orientations between Ge1¯10 and mica[100] of (20n)°, where n = 0, 1, 2, 3, 4, 5. A superlattice area mismatch model was used to account for the likelihood of the in-plane orientation formation and was found to be qualitatively consistent with the observed dominant orientations. Our observation of Ge epitaxy with one out-of-plane growth direction through van der Waals forces is a step toward the growth of single crystal Ge films without the constraint in the lattice and symmetry matches with the substrates.

Analysis of superconducting FeSe thin films deposited by a sputtering technique

Superconducting FeSe thin films have been deposited onto (001)-oriented single-crystalline MgO substrates by conventional radio-frequency magnetron co-sputtering of a FeSe target and a pure Se target. The composition of the films is within the homogeneity range of the tetragonal β phase close to stoichiometry. Films with a minimum residual resistivity and a maximum superconducting transition temperature can be prepared within a narrow substrate temperature range from 480 to 510 °C. In contrast to the equilibrium phase diagram the β phase surprisingly exists in thin films deposited at substrate temperatures as high as 680 °C. The polycrystalline tetragonal films grow with a preferred c-axis texture and a tiny admixture of misaligned grains. The temperature-dependent resistivity reveals a low-temperature linear part and a high-temperature part with a pronounced negative curvature. The origin of the non-Fermi liquid behaviour and the underlying electron scattering mechanisms are still unknown so far.

Growth of BGaAs by molecular-beam epitaxy and the effects of a bismuth surfactant

Boron is potentially useful for strain balancing compressively strained materials such as InGaAs and GaAsBi that are being developed for use in optical and electronic devices. Understanding and improving the incorporation of boron in GaAs is an important first step toward the realization of these strain-balanced systems. Here, we show that the apparent boron incorporation in GaAs, determined from X-ray diffraction measurements, decreases as the substrate temperature is increased, although measurements of the metallurgical concentration of boron remain constant. This implies that boron is incorporating preferentially on non-substitutional sites as growth temperature is increased. The addition of a bismuth surfactant flux not only makes the epilayers smoother, but within a narrow range of substrate temperatures, restores the incorporation of substitutional boron.

Effects of substrate temperature on the growth of InGaAs compositionally graded buffers and on quantum well structures grown above them

We demonstrate that photoluminescence from single quantum well structures grown on indium gallium arsenide (InGaAs) compositionally graded buffers on gallium arsenide substrates can be substantially improved by using very high substrate temperatures during the growth of the graded buffer. Due to the high growth temperature, deepened crosshatches develop on top of the graded buffers as the substrate temperature increased, which have consequences for subsequent quantum well growth. It was also observed that indium incorporation becomes a variable of many factors, and has to be calibrated empirically. Vastly higher arsenic pressure is also necessary to keep the growth front arsenic stable. High quality quantum well structures can only be grown on top of the graded buffers in a narrow range of substrate temperatures around 450°C, much lower than what one would typically use for the pseudomorphic InGaAs growth on the InP substrates.

Characterization of crystalline Pb 0.92La 0.08Zr 0.4Ti 0.6O 3 thin films grown by off-axis radio frequency magnetron sputtering

In this study, we examine the growth conditions for Pb 0.92La 0.08Zr 0.4Ti 0.6O 3 thin films with a linear electro-optical response. Films grown on SrTiO 3(100) substrate show good crystalline quality and surface smoothness, if deposited within a narrow range of substrate temperatures around 530 °C. Such films had a (002) rocking curve full width at half maximum less than 0.2°, and a surface roughness less than 0.4 nm. X-ray photoelectron spectroscopy analysis confirmed the absence of surface impurity phases and revealed a single oxidation state for Pb 2+, Zr 4+, and Ti 4+cations. Good crystalline quality and a smooth film surface are required for the use of these materials in optical device applications.

A Numerical and Experimental Investigation of the Influence of Mass Transfer on HF / H 2 O Vapor‐Phase Etching of Silicon Dioxide

The transport phenomena and heterogeneous chemistry involved in the vapor‐phase etching of silicon dioxide were analyzed numerically and experimentally in the classic stagnation‐flow geometry. The mass‐transfer coefficient was determined numerically by solving the coupled conservation equations for momentum, mass, and energy. The calculations included temperature‐dependent transport properties and multicomponent effects. In the absence of homogeneous chemistry, the rate and order of heterogeneous reactions may be determined through knowledge of and experimental measures of the etch rate. This analysis was used to quantify surface chemistry for the case of silicon dioxide etching by HF vapor. Over a narrow range of substrate temperature, the rate‐limiting step for this process switches from transport to reaction control. The apparent reaction order also changes from first order to very high order over the same range. © 2000 The Electrochemical Society. All rights reserved.

Growth characteristics of vacuum coated thick a-Se films for device applications

There has been a resurgence of interest in the growth characteristics of amorphous Se (a-Se) films due to their recently commercialized applications in x-ray imaging as x-ray photoconductors. Electronic quality a-Se films require the substrate temperature during vacuum deposition to be as high as possible but increasing the substrate temperature leads to amorphous films containing crystalline inclusions and eventually to polycrystalline films which have high conductivity and are therefore not useful as photoconductors. We prepared a variety of thick a-Se films on Al substrates by conventional vacuum deposition in which only the substrate temperature Tsb and boat temperature Tbt were varied. Experiments indicated the appearance of crystalline zones (crystallites) in the amorphous Se film as the substrate temperature was increased. We identified an experimental critical Tbt−Tsb boundary between amorphous films with and without crystalline inclusions. A model has been developed for the critical relationship between Tsb and Tbt, i.e., Tbt=f(Tsb) based on two competing rate mechanisms: the rate of deposition (or material condensation on the substrate) and rate of crystal growth in the film assuming heterogeneous nucleation at favorable substrate sites. Over the narrow substrate temperature range, we approximated the Vogel–Tamman–Fulcher behavior of the crystal growth rate by an Arrhenius rate with an apparent activation energy Ecr that accounts for most crystallization studies on a-Se. The condensation rate was assumed to follow an Arrhenius rate with the activation energy determined by the enthalpy of evaporation, Eev. The Tbt=f(Tsb) model was particularly sensitive to the Ecr/Eev ratio. By using the presently accepted values for Eev and Ecr, it was shown that the experimental results could be well fitted to the Tbt=f(Tsb) model. The results have a wider significance than a-Se and can be easily extended to apply to a number of other chalcogenide semiconductor films.

Spontaneous emission from InAs/GaSb quantum wells grown by molecular beam epitaxy

We have investigated the spontaneous emission from InAs/GaSb quantum wells grown by molecular beam epitaxy. A strong photoluminescence signal was observed only from samples grown in a narrow substrate temperature range around 400 °C with InSb-like interfaces. From an analysis of the photoluminescence energy with the excitation density, we demonstrated the formation of triangular quantum wells for holes in the GaSb barriers close to the interfaces. Finally, the temperature dependence of the photoluminescence peak energy showed a deviation from the InAs band gap evolution at low temperature, which could be related to the type II broken gap band alignment.

The (111) oriented growth of C 60 films on GaAs(100) substrates

We have prepared entirely (111) oriented C 60 single crystal films on GaAs(100) using the vacuum vapor deposition technique with double temperature zone, and studied the structure and morphology of films using X-ray diffraction and scanning electron microscopy. The results show that the highly oriented growth of C 60 films occurred only at a narrow range of substrate temperature, higher or lower substrate temperature results in random orientation of grains. A preliminary explanation for experimental results is given and the growth mechanism of the C 60 single film is discussed.

A Raman Study of Diamond Film Growth on Co‐Cemented Tungsten Carbide

Phase purity and crystallinity of diamond films grown by hot filament chemical vapor deposition on ISO‐grade K10 cemented carbide [94.2 weight percent (w/o) WC‐5.8 w/o Co] were studied by Raman spectroscopy as a function of substrate temperature, gas phase composition, and substrate pretreatments. High‐quality diamond films were grown using 0.5% in a rather narrow range of substrate temperatures (750 to 760°C). In all the deposited coatings, the first‐order Raman band of diamond is detected at 1337 cm−1. This fact indicates that a 2 GPa residual compressive stress is present in the diamond phase. The linewidth of the diamond Raman peak increases with deposition temperature. This effect has been ascribed to a higher density of defects in diamond crystallites. It has been observed that Co removal from the substrate surface by wet chemical etching before deposition is less effective than a careful selection of deposition parameters to reduce the codeposition of nondiamond carbon phases. This finding has been attributed to the fast diffusion of the binder from the bulk to the substrate surface, even for the etched substrates.

THE ORIENTED GROWTH OF C60 FILMS-ON GaAs(100) SUBSTRATE

Entirely (111) oriented C60 single films are grown successfully on GaAs (100) substrate by a physical vapor deposition technique with double temperature zone.The structure and morphology of films are studied by X-ray diffraction and SEM.The results show that the highly oriented growth of C60 films occurs only at narrow range of substrate temperature,higher or lower substrate temperature leads to random orientation of grains.A reasonable explanation for experimental results is given and the growth mechanism of C60 epitaxial films is discussed.

Selective InAs growth by chemical beam epitaxy

Selective growth of InAs was achieved using trimethylindium and cracked arsine on GaAs(100) substrates patterned with a silicon nitride mask. True selectivity can only be achieved in a very narrow range of substrate temperature and V III ratios. Growth optimization for various substrate types and pattern coverage were studied. Excellent surface morphology and electrical properties have been achieved at these optimized growth conditions. Vertical sidewalls can be obtained for selectively grown regions oriented along the [011] direction. Regions oriented in other directions show the growth of well-defined crystallographic facets. Device quality material was grown, as demonstrated by resonant interband tunneling diodes (RITDs) with room-temperature peak-to-valley current ratios (PVCRs) comparable to those obtained from RITDs grown and processed in a conventional manner.

Highly strained InGaAs layers on GaAs grown by molecular beam epitaxy for high electron mobility transistors

We have grown highly strained In x Ga 1− x As layers on GaAs, by molecular beam epitaxy, showing excellent electrical and optical quality for high electron mobility transistors (HEMTs). Over a very narrow range of substrate temperature from 400 to 420°C, the mobility of 6 nm thick In 0.35Ga 0.65As channel HEMTs at room temperature exceeds 6200 cm 2/V · s. Furthermore, the In composition can be increased up to 0.42 without generating misfit dislocations. The peak energy and full width at half maximum of a 77 K photoluminescence spectrum from an In 0.42Ga 0.58As/GaAs (well width = 6 nm) strained single quantum well are as small as 1.082 eV and 9.8 meV, respectively. The highly strained In x Ga 1− x As channel HEMTs (0.3 ≤ x ≤ 0.4) show superior transconductance over conventional In 0.25Ga 0.75As channel HEMTs.

Growth kinetics and electronic characteristics of quantum wires

The fabrication and characterization of narrow quantum wires is studied with a combination of theoretical techniques. Using the compositional integrity as a measure of quality, our earlier work has shown that quantum wires generated by a Monte Carlo simulations of crystal growth attain the highest quality within a narrow range of substrate temperatures that depends upon the beam flux and terrace width. We examine the factors that influence the electronic density of states of such quantum wires within a tight-binding framework and discuss the compositional integrity of the structures in relation to the quantum mechanical integrity. The effects of migrating species with different mobility parameters are then considered by appealing to recent work describing growth on vicinal surfaces with an effective nonlinear diffusion equation.

Epitaxial growth of YBa2Cu3O7−x thin films on (110) SrTiO3 single crystals by activated reactive evaporation

The orientation of YBa2Cu3O7−x thin films grown on (110) planes of SrTiO3 by activated reactive evaporation was investigated by means of reflection high-energy electron diffraction. The orientation of the films depended on the substrate temperature. The films with (110) planes parallel to the substrate surface grew in a narrow range of substrate temperatures around 530 °C, while the films with (103) planes parallel to the surface grew at temperatures above 600 °C. The change of the epitaxial orientation with the substrate temperature is discussed in terms of the temperature dependence of the lattice mismatch between YBa2Cu3O7−x and SrTiO3.

ＹＢａ２Ｃｕ３Ｏ７−ｘ薄膜のエピタキシャル成長

The orientation of YBa2Cu3O7-x thin films grown on (110) planes of SrTiO3 by activated reactive evaporation was investigated by means of reflection high energy electron diffraction. The orientation of the films depended on the substrate temperature. The films with the (110) plane parallel to the substrate surface grew in the narrow range of substrate temperatures around 530°C, while the films with the (103) plane parallel to the surface grew at the temperatures above 600°C. The change of the epitaxial orientation with the substrate temperature was discussed refering the temperature dependence of the lattice mismatch between YBa2Cu3O7-x and SrTiO3.

Ge Heteroepitaxy on (100) Si Substrates by Ion Assisted Deposition

ABSTRACTGe films have been grown by ion-assisted deposition on Si(100) substrates. Initial studies were conducted to determine the growth and morphology of Ge using a modified crucible arrangement and ionization chamber. It was observed that a narrow substrate temperature range existed for the deposition of smooth Ge films without ion assistance. Ionization and acceleration appear to induce additional damage to the film without improving crystalline quality.