High-temperature Effusion Cell Research Articles

Low-loss superconducting titanium nitride grown using plasma-assisted molecular beam epitaxy

Titanium nitride (TiN) is a known superconducting material that is attractive for use as passive components in superconducting circuits for both conventional and quantum information devices. In contrast to conventional synthesis techniques, here, plasma-assisted molecular beam epitaxy is reported to produce high-quality TiN on bare silicon wafers. Using a rf-plasma source to crack the nitrogen molecules and a conventional high-temperature effusion cell for titanium, TiN growth is completed under nitrogen-rich conditions. The growth and nucleation is monitored in situ, while the structure and composition are characterized using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and scanning transmission electron microscopy. The stoichiometric TiN (111) films sit on an amorphous nitride layer with low impurity concentrations. The films superconduct with Tc=5.4 K, and coplanar waveguide resonators are fabricated with a small center width of 6 μm that demonstrate single-photon quality factors approaching 1M and high-power quality factors over 5M without observing saturation.

Growth and characterization of β-Ga2O3 thin films by molecular beam epitaxy for deep-UV photodetectors

The growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O3) substrates. The compound source provides oxidized gallium molecules in addition to oxygen when heated from an iridium crucible in a high temperature effusion cell enabling a lower heat of formation for the growth of Ga2O3, resulting in a more efficient growth process. This source also enabled the growth of crystalline β-Ga2O3 without the need for additional oxygen. The influence of the substrate temperatures on the crystal structure and quality, chemical bonding, surface morphology, and optical properties has been systematically evaluated by x-ray diffraction, scanning transmission electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, spectroscopic ellipsometry, and UV-vis spectroscopy. Under optimized growth conditions, all films exhibited pure 2¯01 oriented β-Ga2O3 thin films with six-fold rotational symmetry when grown on a sapphire substrate. The thin films demonstrated significant absorption in the deep-ultraviolet (UV) region with an optical bandgap around 5.0 eV and a refractive index of 1.9. A deep-UV photodetector fabricated on the high quality β-Ga2O3 thin film exhibits high resistance and small dark current (4.25 nA) with expected photoresponse for 254 nm UV light irradiation suggesting that the material grown using the compound source is a potential candidate for deep-ultraviolet photodetectors.

Comparison of beryllium oxide and pyrolytic graphite crucibles for boron doped silicon epitaxy

This article reports on the comparison of beryllium oxide and pyrolytic graphite as crucible liners in a high-temperature effusion cell used for boron doping in silicon grown by molecular beam epitaxy. Secondary ion mass spectroscopy analysis indicates decomposition of the beryllium oxide liner, leading to significant incorporation of beryllium and oxygen in the grown films. The resulting films are of poor crystal quality with rough surfaces and broad x-ray diffraction peaks. Alternatively, the use of pyrolytic graphite crucible liners results in higher quality films.

Scanning tunneling microscopy study of the interfacial bonding structures of Ga 2O and In 2O/In 0.53Ga 0.47As(0 0 1)

Ga 2O and In 2O oxides were deposited on In 0.53Ga 0.47As(0 0 1) − (4 × 2) surface by a high temperature effusion cell to investigate the interfacial bonding geometries and electronic structures by scanning tunneling microscopy/spectroscopy (STM/STS). At low coverage, Ga 2O molecules bond to the As atoms at the edge of the rows and preexisting Ga 2O on the surface. Annealing the Ga 2O/In 0.53Ga 0.47As(0 0 1) − (4 × 2) to 340 °C results in formation of slightly ordered islands running in the [ 1 ¯ 1 0] direction and rectangle shape flat islands on the surface. At high coverage with 340 °C post-deposition annealing (PDA), Ga 2O oxides form disordered structures with the large flat terraces on the surface. Conversely, at high coverage with 380 °C PDA, In 2O on In 0.53Ga 0.47As(0 0 1) − (4 × 2) forms ordered structures running in the [1 1 0] direction. STS results show that Ga 2O oxide does not passivate the interface nor unpin the In 0.53Ga 0.47As(0 0 1) − (4 × 2) surface consistent with its inability to form monolayer ordered islands on the surface; conversely, In 2O/In 0.53Ga 0.47As(0 0 1) − (4 × 2) has an ordered monolayer coverage and is unpinned.

Praseodymium silicide formation at the Pr 2O 3/Si interface

The interface and layer structure of praseodymium (Pr) oxide layers grown on Si(0 0 1) from a high-temperature effusion cell are studied using grazing incidence X-ray diffraction. Due to the interdiffusion of praseodymium and silicon atoms, Pr silicide forms in the layers. We find that Pr silicide is the favorable structure under oxygen deficient growth conditions in the Pr oxide layer. To avoid the silicidation, additional oxygen must be supplied. The formation of Pr silicide is suppressed for layers grown with an oxygen partial pressure of 10 −7 mbar at a substrate temperature of 700 °C.

N incorporation and electronic structure in N-doped TiO 2(1 1 0) rutile

We describe the growth and properties of well-defined epitaxial TiO 2− x N x rutile for the first time. A mixed beam of atomic N and O radicals was prepared in an electron cyclotron resonance plasma source and Ti was supplied from a high-temperature effusion cell or an electron beam evaporator, depending on the required flux. A very high degree of structural quality is generally observed for films grown under optimized anion-rich conditions. N substitutes for O in the lattice, but only at the ∼1 at.% level, and is present as N 3−. Epitaxial growth of TiO 2 and TiO 2− x N x rutile prepared under anion-rich conditions is accompanied by Ti indiffusion, leading to interstitial Ti (Ti i ), which is a shallow donor in rutile. Our data strongly suggest that Ti i donor electrons compensate holes associated with substitutional N 2− (i.e., Ti(III) + N 2− → Ti(IV) + N 3−), leading to highly resistive or weakly n-type, but not p-type material. Ti 2p core-level line shape analysis reveals hybridization of N and Ti, as expected for substitutional N. Ti–N hybridized states fall in the gap just above the VBM, and extend the optical absorption well into the visible.

Growth, structure, and morphology of TiO2 films deposited by molecular beam epitaxy in pure ozone ambients

TiO2 films were grown using a reactive molecular beam epitaxy system equipped with high-temperature effusion cells as sources for Ti and an ozone distillation system as a source for O. The growth mode, characterized in-situ by reflection high-energy electron diffraction (RHEED), as well as the phase assemblage, structural quality, and surface morphology, characterized ex-situ by X-ray diffraction and atomic force microscopy (AFM), depended on the choice of substrate, growth temperature, and ozone flux. Films deposited on (100) surfaces of SrTiO3, (La0.27Sr0.73)(Al0.65Ta0.35)O3, and LaAlO3 grew as (001)-oriented anatase. Both RHEED and AFM indicated that smoother surfaces were observed for those grown at higher ozone fluxes. Moreover, while RHEED patterns indicated that anatase films grown at higher temperatures were smoother, AFM images showed presence of large inclusions in these films.

Role of Ga2O3 template thickness and gadolinium mole fraction in GdxGa0.4−xO0.6/Ga2O3 gate dielectric stacks on GaAs

Amorphous GdxGa0.4−xO0.6/Ga2O3 gate dielectric stacks have been grown onto the GaAs(001) surface by molecular beam epitaxy using high temperature effusion cells. The Ga2O3 template thickness and the Gd mole percent have been systematically varied from 73 to 0 Å and from 8.8 to 22 at. % (0.088⩽x⩽0.22), respectively. Oxide/n-type GaAs samples have been characterized by high-frequency capacitance–voltage measurements. Optimum gate oxide stack and oxide/GaAs interface properties are obtained with a Ga2O3 template thickness of 9–11 Å and a minimum Gd mole percent of 15–17 at. %. While gate oxide films with thicker Ga2O3 templates and/or lower Gd mole fraction show kinks in capacitance–voltage measurements attributed to charge trapping in the oxide, thinner Ga2O3 templates lead to strong stretch-out of capacitance–voltage curves indicating severely degraded oxide/GaAs interface properties.

Growth and physical properties of Ga2O3 thin films on GaAs(001) substrate by molecular-beam epitaxy

We report effusive evaporation of Ga2O3 thin films on GaAs(001) substrates in a production-type molecular-beam epitaxy system. A polycrystalline Ga2O3 charge heated in a high-temperature effusion cell is used as the evaporation source. The Ga2O3–GaAs structures are characterized by atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), ellipsometry, and transmission electron microscopy (TEM). The Ga2O3 films are amorphous and stoichiometric by transmission electron diffraction and RBS, respectively. Under optimal growth conditions, the Ga2O3 film surface has a typical roughness of 2–3 Å as revealed by AFM, while the Ga2O3–GaAs interface is atomically abrupt as confirmed by the cross-sectional TEM. Such amorphous and stoichiometric Ga2O3 oxide paves the way for GaAs gate dielectrics applications.

Growth and characterization of DyP/GaAs and DyAs/GaAs-based heterostructures and superlattices

Details of the structural and electrical properties of epitaxial DyP/GaAs and DyAs/GaAs is reported. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01%. DyAs, on the other hand, has a mismatch of nearly 2.4%. Both DyP and DyAs have been grown by solid source MBE using custom designed group V thermal cracker cells and group III high-temperature effusion cells. High-quality DyP and DyAs epilayers, as determined by XRD, TEM, and AFM analysis, were obtained for growth temperatures ranging from 500°C to 600°C with growth rates between 0.5 and 0.7 μm/h. The DyP epilayers are n-type with measured electron concentrations of the order of 3×10 20 to 4×10 20 cm −3, with room temperature mobilities of 250–300 cm 2/V s, and with a barrier height of 0.75 eV to GaAs. The DyAs epilayers are also n-type with concentration of 1×10 21 to 2×10 21 cm −3, with mobilities between 25 and 40 cm 2/V s. DyP is stable in air with no apparent oxidation taking place, even after months of ambient exposure to untreated air.

Epitaxial growth and characterization of DyP/GaAs, DyAs/GaAs, and GaAs/DyP/GaAs heterostructures

There is a significant interest in the area of improving high temperature stable contacts to III-V semiconductors. Two attractive material systems that offer promise in this area are dysprosium phosphide/gallium arsenide (DyP/GaAs) and dysprosium arsenide/gallium arsenide (DyAs/GaAs). Details of epitaxial growth of DyP/GaAs and DyAs/GaAs by molecular beam epitaxy (MBE), and their characterization by x-ray diffraction, transmission electron microscopy, atomic force microscopy, Auger electron spectroscopy, Hall measurements, and high temperature current-voltage measurements is reported. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01% and is stable in air with no sign of oxidation, even after months of ambient exposure. Both DyP and DyAs have been grown by solid source MBE using custom designed group V thermal cracker cells and group III high temperature effusion cells. High quality DyP and DyAs epilayer were consistently obtained for growth temperatures ranging from 500 to 600°C with growth rates between 0.5 and 0.7 µm/h. DyP epilayers are n-type with electron concentrations of 3 × 1020 to 4 × 1020 cm−3, room temperature mobilities of 250 to 300 cm2/V·s, and a barrier height of 0.81 eV to GaAs. DyAs epilayers are also n-type with carrier concentrations of 1 × 1021 to 2 × 1021 cm−3, and mobilities between 25 and 40 cm2/V·s.

Epitaxial dysprosium phosphide grown by gas-source and solid-source MBE on gallium arsenide substrates

We report the first known study of the growth of epitaxial dysprosium phosphide (DyP) grown on gallium arsenide (GaAs). DyP is lattice matched to GaAs, with the room-temperature mismatch being less than 0.01%. We have grown DyP on GaAs by gas-source and by solid-source molecular beam epitaxy using custom-designed group V thermal cracker cells and group III high temperature effusion cells. X-ray diffraction results show the DyP epilayer to be (001) single crystal on GaAs(001) substrate. Electrical and optical measurements performed to date are inconclusive as to whether DyP is a semi-metal or a semiconductor with a small band gap. The undoped films are n-type with measured electron concentrations on the order of 5 × 10 19–6 × 10 20cm −3 with mobilities of 1–10 cm 2/V · s. DyP GaAs is stable in air with no apparent oxidation taking place, even after months of exposure to ambient untreated air. Material and surface science properties measured for DyP GaAs include Hall measurements, 2ϑ and double-crystal X-ray diffraction spectra and photothermal deflection spectroscopy.

Hard boron oxide thin-film deposition using electron cyclotron resonance microwave plasmas

Hard boron suboxide thin films were deposited in an electron cyclotron resonance (ECR) microwave plasma system at substrate temperatures below 300 °C. A high-temperature effusion cell, operated at 2200°–2250 °C, was used for injection of boron downstream of an Ar/O2 ECR plasma. B ion bombardment is estimated to have been up to 6% of the total boron flux, and Ar ion bombardment is estimated to have contributed ∼100 eV/deposited atom. Boron suboxide films with oxygen concentrations of 11% exhibited hardnesses up to 30 GPa, equal to sapphire and near that of pure boron. The hardness/modulus ratio was 0.1, significantly better than that of sapphire (0.067) or solid boron (0.074), indicating these films may be of interest for a variety of tribological applications.

Elemental boron-doped p+-SiGe layers grown by molecular beam epitaxy for infrared detector applications

SiGe/Si heterojunction internal photoemission (HIP) detectors have been fabricated utilizing molecular beam epitaxy of p+-SiGe layers on p−-Si substrates. Elemental boron from a high-temperature effusion cell was used as the dopant source during molecular beam epitaxy (MBE) growth, and high doping concentrations (≳5×1020 cm−3) have been achieved. Strong infrared absorption, mainly by free-carrier absorption, was observed for the degenerately doped SiGe layers. The use of elemental boron as the dopant source allows a low MBE growth temperature (350 °C), resulting in improved crystalline quality and smooth surface morphology of the Si0.7Ge0.3 layers. Nearly ideal thermionic emission dark current characteristics have been obtained. Photoresponse of the HIP detectors in the long-wavelength infrared regime has been demonstrated.

Growth of boron nitride films by gas molecular-beam epitaxy

Boron nitride is a thermally stable and chemically inert material with a wide bandgap. As such, it has potential optoelectronic applications as windows, deep ultraviolet (UV) detectors, or UV light-emitting devices. Since it has a very high surface energy and a lattice parameter which is very similar to diamond, it may also prove to be an excellent substrate material for heteroepitaxial thin film growth of this material. This paper reports the preliminary results of the growth of BN thin films using gas source molecular beam epitaxy. The B source evaporated elemental B produced in a new high temperature effusion cell. The nitrogen source consisted of N ions as well as activated N atoms and molecules obtained from the decomposition of purified N2 by passing it through an electron cyclotron resonance plasma unit designed specifically for molecular-beam epitaxy (MBE) systems. A graded GaN–BN layer deposited on a monocrystalline β-SiC (100) film was used as the substrate for growth of each BN film. Transmission electron microscopy of the latter films showed them to be amorphous; however reflection high-energy electron diffraction (RHEED) patterns obtained during deposition indicated the presence of some microcrystallinity.

Boron doping of Si molecular-beam epitaxy layers: A new high-temperature effusion cell

The high-temperature effusion cell (HTEC) presented in this paper is designed for materials requiring evaporation temperatures above 1000 °C. It employs a novel concept of charge heating, indirectly, through a graphite crucible, by electron bombardment combined with thermal radiation. The cell is particularly well suited to molecular-beam epitaxy (MBE) systems and we have used it as a source of B in the MBE growth and doping of Si, obtaining the resulting B concentration NB between 1015/cm3 and 5×1019/cm3. Some characteristics of Si:B layers thus fabricated are presented and it is shown that NB is: (i) an exponential function of Tcell ; (ii) inversely proportional to the growth rate; and (iii) does not depend on the substrate temperature. We conclude from (ii) and (iii) that B is totally incorporated in the crystal when coevaporated with Si. Because of the total incorporation of B and the absence of its surface segregation very abrupt doping profiles can be obtained. NB gradients as steep as one decade per 100 Å have been obtained. A comparison of the secondary ion mass spectroscopy (SIMS) data and electrical measurements results indicates that B impurities are all electrically active. The values of resistivity ρ, and the Hall mobility μ, at room temperature are both bulklike and both ρ(NB) and μ(NB) follow the standard bulk Si dependences. The MBE layers grown without intentional doping are n-type, and have an electron concentration of 5×1013/cm3 and μ=1450 cm2/V s at room temperature, like pure n-type Si.

The Influence of Thickness and Wavelength on the Mechanical Properties of a Compositionally Modulated Ceramic Thin Film

AbstractThe hardness and modulus of thin (<1 μm) ceramic films have been investigated using the differential mechanical properties microprobe (DMPM). The DMPM consists of a computer controlled ultra-low load indentation system with a special system that allows the elastic stiffness of the contact to be determined continuously during the indentation process. The films were deposited in a molecular beam epitaxy (MBE) system utilizing unique high temperature effusion cells. Two specimens have been studied. The first is a wedge shaped layer of amorphous Al2O3 on a silicon substrate. The thickness of the layer varies from 0 to 990 nm at the two extreme sides of a 3 in. silicon wafer. The effect of layer thickness on the hardness and modulus as measured with the DMPM has been quantified for this system. The second specimen is made up of 38 repetitions of alternating layers of crystalline TiO2 and amorphous Al2O3. The wavelength of the composition modulation and the thickness of the film varies uniformly from 0 to 38 nm and from 0 to 1460 nm respectively across the 3 in. diameter of the silicon substrate.

Infrared Absorption Spectra of B2O3, B2O2, and BO2 in Solid Argon Matrices

The infrared absorption spectra of B2O3, B2O2, and BO2 isolated in solid argon matrices have been investigated in the region 350–4000 cm—1. The visible absorption spectrum of matrix-isolated BO2 was observed in the region 4000–5500 Å. The gaseous species were generated in a high-temperature effusion cell and trapped under conditions of moderate to high dilution in solid argon matrices at approximately 4°K. Bands were found at 1955, 1921, 1899, 1323, and 1276 cm—1 in the infrared spectra of B210O2, B10B11O2, B211O2, and B10O2, and B11O2, respectively. For both B210O3 and B211O3 infrared spectra of the most dilute matrices show seven distinct bands in the region 450–2100 cm—1. Six of these can be readily assigned as fundamentals and their relative intensities explained if the known ``V'' structure of B2O3 is supposed to have a larger apex angle than that determined by electron diffraction, and the correlation of the normal vibrations and selection rules with those for the linear symmetric (D∞h) model is considered. With additional help from the measured B10–B11 isotope shifts and force constant calculations the following complete assignment was obtained (all frequencies in cm—1): B210O3: 2128(A1),733(A1),536(A1);[172](A1),or [260](A1);[476](A2) 2128(B1),1242(B1),471(B1);493(B2);B211O3: 2060(A1),729(A1),518(A1),[172](A1), or [259](A1);[456](A2),2060(B1),1239(B1),454(B1);477(B2);where the bracketed frequencies were not observed directly. Thermal functions have been computed for B2O3 and compared with the available calorimetric data. Structural parameters and a complete vibrational assignment have been estimated for B2O2. The infrared and green bands observed for BO2 are in agreement with the high-resolution results of Johns. Thermal functions for B2O2 and BO2 have also been computed.