Showing papers on "Epitaxy published in 1989"

Structural properties of As‐rich GaAs grown by molecular beam epitaxy at low temperatures

Abstract: GaAs layers grown by molecular beam epitaxy (MBE) at substrate temperatures between 200 and 300 °C were studied using transmission electron microscopy (TEM), x‐ray diffraction, and electron paramagnetic resonance (EPR) techniques. High‐resolution TEM cross‐sectional images showed a high degree of crystalline perfection of these layers. For a layer grown at 200 °C and unannealed, x‐ray diffraction revealed a 0.1% increase in the lattice parameter in comparison with bulk GaAs. For the same layer, EPR detected arsenic antisite defects with a concentration as high as 5×1018 cm−3. This is the first observation of antisite defects in MBE‐grown GaAs. These results are related to off‐stoichiometric, strongly As‐rich growth, possible only at such low temperatures. These findings are of relevance to the specific electrical properties of low‐temperature MBE‐grown GaAs layers.

Blue light emitting diode formed in silicon carbide

Abstract: The present invention comprises a light emitting diode formed in silicon carbide and that emits visible light having a wavelength of between about 465-470 nanometers, or between about 455-460 nanometers, or between about 424-428 nanometers. The diode comprises a substrate of alpha silicon carbide having a first conductivity type and a first epitaxial layer of alpha silicon carbide upon the substrate having the same conductivity type as the substrate. A second epitaxial layer of alpha silicon carbide is upon the first epitaxial layer, has the opposite conductivity type from the first layer, and forms a p-n junction with the first epitaxial layer. In preferred embodiments, the first and second epitaxial layers have carrier concentrations sufficiently different from one another so that the amount of hole current and electron current that flow across the junction under biased conditions are different from one another and so that the majority of recombination events takes place in the desired epitaxial layer.

Surface segregation of third-column atoms in group III-V arsenide compounds: Ternary alloys and heterostructures

Abstract: We report a systematic study of the segregation of third-column elements involved in group III-V arsenide structures to their (100) surface. The surface composition is obtained by in situ electron spectroscopies, on special structures built by molecular-beam epitaxy. In ternary alloys, an important surface enrichment in one of the third-column components is most often observed, leading to a near-binary surface. Heterojunctions between two given binary materials A and B are abrupt or not in composition, depending on the growth sequence (A grown on B or B grown on A): for one sequence, the top monolayer of the base material is gradually distributed in the growing overlayer. All these behaviors can be summarized by tendencies to surface segregation following IngGa\ensuremath{\ge}Al, and by segregation efficiencies that are either near zero or near unity depending on the way structures are built. The application of standard models to the segregation isotherms for ternary alloys yields segregation energies of 0.1--0.2 eV. The physical origin for the segregation process and its consequences on interface roughness along the growth axis are discussed.

Growth of cubic phase gallium nitride by modified molecular‐beam epitaxy

Abstract: Gallium nitride is a compound semiconductor with a wide direct band gap (3.45 eV) and a large saturated electron drift velocity. Nearly all single‐crystal thin films grown to date have been wurtzite (hexagonal) structure. Cubic GaN has the potential for higher saturated electron drift velocity and somewhat lower band gap. These properties could increase its applicability for high‐frequency devices (such as impact ionization avalanche transit time diodes) as well as short‐wavelength light emitting diodes and semiconductor lasers. This paper reports the growth of cubic phase single‐crystal thin‐film GaN using a modified molecular‐beam epitaxy technique. A standard effusion cell was used for gallium, but to activate nitrogen gas prior to deposition, a microwave glow discharge was used. Auger electron spectroscopy showed a nominally stoichiometric GaN film. Transmission electron microscopy with selected area diffraction indicated the crystal structure to be zinc blende.

Oxidation studies of SiGe

Abstract: We have studied the kinetics and mechanism of oxidation of SiGe alloys deposited epitaxially onto Si substrates by low‐temperature chemical vapor deposition. Ge is shown to enhance oxidation rates by a factor of about 3 in the linear regime, and to be completely rejected from the oxide so that it piles up at the SiO2/SiGe interface. We demonstrate that Ge plays a purely catalytic role, i.e., it enhances the reaction rate while remaining unchanged itself. Electrical properties of the oxides formed under these conditions are presented, as well as microstructures of the oxide/substrate, Ge‐enriched/SiGe substrate, and SiGe/Si substrate interfaces, and x‐ray photoemission studies of the early stages of oxidation. Possible mechanisms are discussed and compared with oxidation of pure silicon.

Silicon surface passivation by hydrogen termination: A comparative study of preparation methods

Abstract: Clean silicon surfaces having low carbon and oxygen contamination are necessary for good epitaxial overgrowth. Methods for low‐temperature preparation of clean surfaces are needed to fully enable low‐temperature fabrication processes on silicon. In this paper silicon surfaces are compared for residue and chemical passivation after (i) hydrogen termination by various low‐temperature, wet‐chemical techniques, (ii) cleaving in ultrahigh vacuum (UHV), and (iii) ion sputtering. Surface residue was characterized with x‐ray photoelectron spectroscopy (XPS) and small‐spot Auger electron spectroscopy (AES). Low‐energy electron diffraction (LEED) was also used. Evaluations of passivation were done by exposing the cleaned surfaces to various environments, e.g., UHV, N2 gas, and room air. We have obtained especially promising results with a technique whereby the wafer surface oxide is etched using an HF‐alcohol reagent in a flowing nitrogen atmosphere at room temperature while the wafer is spinning, i.e., a spin etch...

Control of silicon network structure in plasma deposition

Abstract: Controllability of the Si network structure ranging from amorphous to microcrystalline, polycrystalline, or epitaxial growth has been achieved in conventional glow discharge of silane by controlling the extent of hydrogen ‘etching’ during film growth. The concept of film formation as a balance between deposition and ‘etching’ of the growing surface is discussed. Since energetically unfavorable configurations are preferentially eliminated, ‘etching’ reduces the temperature required to remove weak or strained bonds and produces device quality a-Si:H. It is also responsible for achieving low temperature crystallinity.

Molecular Beam Epitaxy

Abstract: MBE is a versatile technique for growing thin epitaxial structures made of semiconductors, metals or insulators [7.1] (see also the definition given in Sect. 6.1.3). What distinguishes MBE from previous vacuum deposition techniques is its significantly more precise control of the beam fluxes and growth conditions. Because of vacuum deposition, MBE is carried out under conditions far from thermodynamic equilibrium and is governed mainly by the kinetics of the surface processes occurring when the impinging beams react with the outermost atomic layers of the substrate crystal. This is in contrast to other epitaxial growth techniques, such as LPE or atmospheric pressure VPE, which proceed at conditions near thermodynamic equilibrium and are most frequently controlled by diffusion processes occurring in the crystallizing phase surrounding the substrate crystal.

Ultrahigh doping of GaAs by carbon during metalorganic molecular beam epitaxy

Abstract: Recent advances in heterostructure bipolar transistor technology have created a need for p‐type doping at levels ≥1020 cm−3. Furthermore, such levels may eliminate the need for alloying during ohmic contact formation. We have achieved p‐type doping levels as high as 5×1020 cm−3 using an unconventional dopant, C, derived from the gaseous source chemical, trimethylgallium (TMG), during metalorganic molecular beam epitaxial (MOMBE) growth of GaAs. We have controllably achieved doping levels between 1019 and 5×1020 cm−3 by diluting the TMG flow with another metalorganic, triethylgallium (TEG). By utilizing the so‐called δ‐doping or atomic planar doping method we have also been able to grow C‐doped spikes with hole concentrations as high as 7×1019 cm−3, with a full width at half maximum of ∼50 A at 300 K. This doping level is the highest yet reported for planar doping, and the narrow width indicates that the C atoms are restricted to one or two atomic planes. By switching out the TMG, and switching in the TEG ...

Stoichiometry‐related defects in GaAs grown by molecular‐beam epitaxy at low temperatures

Abstract: GaAs layers grown by molecular‐beam epitaxy (MBE) at very low substrate temperatures have gained considerable interest as buffer layers for GaAs metal–semiconductor field effect transistors (MESFET’s) due to high resistivity and excellent device isolation. However, the structure and the electronic properties of such layers have not yet been investigated in detail. We have studied unannealed low temperature (LT) MBE layers grown at 200 °C using transmission electron microscopy (TEM), analytical TEM, x‐ray diffraction, the Hall effect, and electron paramagnetic resonance (EPR) techniques. TEM data indicated large arsenic‐rich deviations from stoichiometry of ∼1–1.5 at. %. X‐ray rocking curves showed a uniform increase of 0.1% in all directions of lattice parameters compared to semi‐insulating GaAs substrate. The Hall effect and thermally induced changes of photo‐EPR measurements revealed the presence of an acceptor level at an energy of ∼0.3 eV above the valence band. This acceptor level has been tentatively attributed to a gallium vacancy defect.

Fabrication of heteroepitaxial YBa2Cu3O7−x‐PrBa2Cu3O7−x‐YBa2Cu3O7−x Josephson devices grown by laser deposition

Abstract: We have fabricated Josephson weak links from i n s i t u laser‐deposited four‐layer structures of YBa2Cu3O7−x /PrBa2Cu3O7−x /YBa2Cu3O7−x Au. These devices show superconductor/normal‐metal/superconductor‐like current‐voltage characteristics with good areal scaling of both the critical currentsI c and resistancesR j , with I c R j ≊3.5 mV. Constant‐voltage current steps observed in response to 84 GHz mm‐wave radiation and modulation of the dc supercurrent in a transverse magnetic field demonstrate that both the ac and dc Josephson effects occur in these devices.

Epitaxial Lateral Overgrowth of GaAs on a Si Substrate

Abstract: Epitaxial lateral overgrowth (ELO) of GaAs on a Si substrate was successfully achieved by a combination of LPE and MBE. To prevent meltback of the Si substrate by Ga solution, a thin GaAs layer (2.4 µm) was grown by MBE prior to the deposition of SiO2 by P-CVD. Uniform GaAs ELO layers with mirror surface were grown through a line-shaped window cut in SiO2 film on a Si substrate with a GaAs layer. Chemical etching with an RC-1 etchant showed that there were no etch pits observed in ELO layers except in the region just over the line seed. Although cracks are observed in ELO layers due to the difference in thermal expansion coefficients between GaAs and Si, it is concluded that this technique is promising for obtaining dislocation- free GaAs on a Si substrate.

Scanning tunneling microscopy study of low‐temperature epitaxial growth of silicon on Si(111)‐(7×7)

Abstract: Scanning tunneling microscopy is used to investigate nucleation and growth phenomena in the molecular‐beam epitaxial (MBE) growth of silicon on Si(111)‐(7×7) from the submonolayer range up to a few monolayers. At room temperature small amorphous clusters form which grow in locally ordered arrays on the (7×7) lattice. Deposition at a higher substrate temperature produces triangular islands of epitaxial silicon which have preferred step propagation in the [112] direction. Preferred nucleation of Si islands is found to occur along boundaries between (7×7) superstructure translational domains of the substrate. The preferred nucleation which arises from defects in the epilayer accounts for the formation of a second epitaxial layer long before the first layer is completed. A variety of metastable reconstructions which differ from (7×7) are also found in the epitaxial islands and are discussed.

Semiconducting silicide‐silicon heterojunction elaboration by solid phase epitaxy

Abstract: Semiconducting βFeSi2 has been successfully grown on a Si (111) substrate. It has been proven that under ultrahigh vacuum conditions, the solid phase epitaxy temperature can be lowered to ∼800 K, where only the βFeSi2 phase is stabilized. The disilicide formation was monitored in situ by various surface‐sensitive techniques such as low‐energy electron diffraction, Auger electron spectroscopy, and ultraviolet photoelectron spectroscopy. The epitaxial relationships were ascertained by transmission electron diffraction and microscopy including high‐resolution cross‐sectional image. The results show the epitaxy of βFeSi2 (110) and (101) planes parallel to the Si (111) plane. The disilicide‐silicon heterojunction displays an atomically abrupt interface.

Crystallography of YBa 2 Cu 3 O 6+ x thin film-substrate interfaces

Abstract: The epitactic nature of the growth of YBa2Cu3O6+x (YBCO) superconducting thin films on ceramic substrates has been studied using high-resolution electron microscopy (HREM) and selected-area diffraction (SAD) of cross-sectional specimens. The films were grown in situ on (001)-oriented MgO and (001)-oriented Y2O3-stabilized cubic ZrO2 (YSZ) single-crystal substrates by electron beam evaporation. Both of these materials have large lattice misfits with respect to YBCO. Different orientation relationships were observed for films grown on the two types of substrates. These orientation relationships are shown to provide the best matching of the oxygen sublattices across the substrate-film interfaces. A crystalline intermediate layer, 6 nm thick, between the YBCO film and YSZ substrate was observed by HREM and shown by EDS to be a Ba-enriched phase, possibly barium zirconate formed by a reaction. In contrast, the YBCO–MgO interface was found to be sharp and free of any intermediate layers.

Electrical Characteristics of Metal Contacts to Boron-Doped Diamond Epitaxial Film

Abstract: Current-voltage characteristics have been obtained for various metal contacts formed on boron-doped diamond epitaxial film prepared on synthesized Ib diamond by the microwave plasma-assisted chemical vapor deposition method. Ti contacts and W contacts have exhibited good ohmic and Schottky properties, respectively. For the first time, we have fabricated Schottky diodes on boron-doped diamond epitaxial films using these contacts and investigated their properties.

The calibration of the strength of the localized vibrational modes of silicon impurities in epitaxial GaAs revealed by infrared absorption and Raman scattering

Abstract: n‐type silicon‐doped epitaxial layers of gallium arsenide grown by molecular‐beam epitaxy (MBE) or metal‐organo chemical vapor deposition (MOCVD) have been investigated by measurements of the Hall effect and the strengths of the localized vibrational modes (LVM) of silicon impurities using both Fourier transform absorption spectroscopy and Raman scattering at an excitation energy of 3 eV close to the E1 band gap. Lines from Si(Ga) donors, Si(As) acceptors, Si(Ga)‐Si(As) pairs, and Si‐X, a complex of silicon with a native defect, were detected and correlated for the two techniques. The maximum carrier concentration [n] found for samples grown under standard conditions was 5.5×1018 cm−3. At higher doping levels Si‐X becomes dominant and acts as an acceptor, so reducing [n]. An integrated absorption of 1 cm−2 in the Si(Ga) LVM line corresponds to 5.0±4×1016 atoms cm−3: a similar calibration applies to the Si(As) line, but for Si‐X, an absorption of 1 cm−2 corresponds to only 2.7±1.0×1016 defects cm−3. Possib...

Properties of Ga1-xInxN Films Prepared by MOVPE

Abstract: Epitaxial films of the solid solution Ga1-xInxN (up to X=0.42) have been fabricated on (0001) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) at 500°C. The properties of the films have been studied by the reflection high-energy electron diffraction technique, X-ray diffraction, and electrical and optical measurements. The fundamental absorption edge of the film decreases linearly with composition (up to X=0.42) from 3.20 eV to 2.01 eV at room temperature.

High critical currents in epitaxial YBa2Cu3O7−x thin films on silicon with buffer layers

Abstract: As‐deposited superconducting thin films (∼0.1 μm) of YBa2Cu3O7−x have been prepared by pulsed laser deposition on (100) Si with buffer layers of BaTiO3/MgAl2O4. X‐ray diffraction studies reveal that the films grow epitaxially with the c axis preferentially oriented normal to the substrate surface. This is confirmed by ion channeling measurements along the (100) (normal to the surface) and (110) directions of the Si substrate showing a minimum yield of 54% along the (100), and 78% along the (110) axes using 2.8 MeV He++. Preliminary transmission electron microscopy study also supports these results. The as‐deposited films have zero resistance temperatures of 86–87 K, and critical current densities of 6×104 A/cm2 at 77 K and 1.2×105 A/cm2 at 73 K. Our results indicate that the superconducting properties of the films are limited primarily by the quality and degree of epitaxal growth of the buffer layers on the silicon substrate.

Molecular beam epitaxial growth of CdTe and HgCdTe on Si (100)

Abstract: CdTe has been grown on Si(100) by molecular beam epitaxy. Two orientations can be obtained: (111)B CdTe when the CdTe is deposited directly on the Si(100) substrates, and (100)CdTe when an intermediate layer of ZnTe is grown first. The (111)B oriented layers are made of two domains which are rotated by 90°. A layer with only one domain can be grown on Si(100) misoriented by 8°, but the best misorientation for this purpose still needs to be found. These layers were characterized by reflection high‐energy electron diffraction, photoluminescence spectroscopy, scanning electron microscopy, and x‐ray diffraction. Hg1−xCdxTe has also been grown by molecular beam epitaxy on (111)B CdTe on Si(100).

Scanning-tunneling-microscopy study of single-domain Si(001) surfaces grown by molecular-beam epitaxy

Abstract: Initial stages of Si epitaxial growth on vicinal Si(001) substrates were investigated using scanning tunneling microscopy. For a growth temperature of about 750 K it was found that initial growth occurs almost exclusively at one of the two nonequivalent types of step edge. This leads to the formation of a single-domain surface with an array of evenly spaced straight steps with biatomic height. This structure can be preserved by quenching the sample to room temperature.

Gas-Source Molecular Beam Epitaxy

Abstract: The replacement of the elemental sources of conventional MBE with simple compounds, first reported in 1980 [1], was initiated in order to bring the advantages of molecular beam epitaxy to the growth of GaxIn1−xAs1−yPy/InP heterostructures. These advantages center about precision in layer thickness and abruptness in doping and heterojunction interfaces. This replacement of elemental sources was necessary because III–V semiconductors containing P, and particularly As and P simultaneously, are very difficult to grow by conventional MBE. A well controlled and useful beam flux from an effusion cell containing elemental phosphorus is difficult to achieve because of the presence of allotropic forms of solid P, each having a different vapor pressure, and because condensed P vaporizes to yield P4 molecules. The morphological observations of Asahi et al [2] of InP grown with P4, and the studies of the relative incorporation of As and P during MBE of GaAs1−yPy and InAsyP1−y by Foxon et al [3], suggest that P4 has a small accommodation coefficient on the III–V surface. It is possible, of course to thermally crack P4 to P2, and P2 can readily be used for epitaxy of P containing III–V compounds. Its accommodation coefficient is approximately unity [4]. However, the generation of P2 by adding a thermal cracker to a conventional effusion oven does not eliminate the underlying stability problem and has added control problems.

A comparison of the magnetic anisotropy of [001] and [111] oriented Co/Pd Multilayers

Abstract: Superlattices of [001]fcc Co/Pd with varying Co thicknesses from one to eight atomic layers per modulation period were epitaxially grown on NaCl by vapour deposition in UHV. Transmission electron diffraction indicates lattice coherence between the Co and the Pd layers for Co thicknesses up to six atomic layers. If deposited at a substrate temperatureTs=50°C, only the superlattices containing Ci-monolayers show perpendicular magnetization. By raisingTs to 200°C, the perpendicular anisotropy for Co monolayers is increased, and is also observed for Co bilayers. We suggest that this is due tolayer smoothening, which increases Neel's interface anisotropy. For more than 6 atomic layers of Co a loss of coherence is observed atTs=50°C, accompanied by a structure transformation to hcp Co with a (0001)Co∥(111)Pd orientation.

Low-temperature silicon selective deposition and epitaxy on silicon using the thermal decomposition of silane under ultraclean environment

Abstract: An ultraclean hot‐wall low‐pressure chemical vapor deposition (CVD) system was developed and Si films were deposited on single‐crystal Si and SiO2 using ultraclean SiH4 and H2 gases in the temperature range 600–850 °C under an ultraclean environment. As a result of ultraclean processing, an incubation period of Si deposition only on SiO2 was found, and low‐temperature Si selective deposition and epitaxy on Si were achieved without addition of HCl under deposition conditions where only nonselective polycrystalline Si growth could be obtained in conventional CVD systems.

Epitaxial Growth of Organic Thin Films by Organic Molecular Beam Epitaxy

Abstract: Epitaxial growth of an organic ultrathin film has been confirmed for the first time by reflection high-energy electron diffraction (RHEED) during film growth. Copper phthalocyanine (CuPc) films were grown heteroepitaxially on a cleaved face of MoS2 under high vacuum of ca. 4×10-9 torr by a newly developed organic molecular beam epitaxy technique. The RHEED pattern revealed that epitaxially grown CuPc has its own lattice constant, which is completely different from that of MoS2 even at the initial deposition stage.

Ge segregation at Si/Si1−xGex interfaces grown by molecular beam epitaxy

Abstract: The interface quality of Si/Si1−xGex (0.08≤x≤0.33) interfaces grown by molecular beam epitaxy has been studied by means of secondary‐ion mass spectrometry. Ge segregation occurs into the Si capping layers. The segregation is characterized by a 17 nm/dec slope; the total amount of segregated Ge corresponds to a few tenths of a monolayer. The phenomenon is independent of the Ge fraction and does not depend on temperature as long as crystal growth is perfect. A possible explanation is given in terms of a Ge adlayer that is formed during growth as a result of site exchange between subsurface Ge and surface Si atoms. This adlayer is incorporated slowly during further Si growth. The Ge segregation can be suppressed by having an adlayer of Ga on the surface of the growing structure.