Good Structural Quality Research Articles

InAs0.85Sb0.15 infrared photodiodes grown on GaAs and GaAs-coated Si by molecular beam epitaxy

We report on the first InAs0.85Sb0.15 infrared photodiodes, grown on GaAs and GaAs-coated Si substrates by molecular beam epitaxy. Transmission electron microscopy images reveal a good structural quality. The electrical characteristics of the photodiodes were analyzed using current-voltage, current-temperature, and capacitance-voltage measurements. The spectral response and detector noise were measured at 77 K, resulting in a peak detectivity at 3.8 μm of 1.5×1011 cm Hz1/2/W for InAs0.85Sb0.15/GaAs and 5.0×1010 cm Hz1/2/W for InAs0.85Sb0.15/GaAs/Si.

InAs p-n diodes grown on GaAs and GaAs-coated Si by molecular beam epitaxy

InAs p-n diodes have been grown on GaAs and GaAs-coated Si substrates by molecular beam epitaxy. Transmission electron microscopy cross sections of the epilayers demonstrate a good structural quality. Photodiodes were obtained using a Be (p=5×1016 cm−3) and Si (n=3×1016 cm−3) doping scheme. The diodes exhibited 77 K zero-bias resistance area products of 2200 Ω cm2 for InAs/GaAs and 1500 Ω cm2 for InAs/GaAs/Si. The spectral response of the devices peaked at 2.95 μm with Johnson noise limited detectivities D* of 7.0×1011 cm Hz1/2/W for InAs/GaAs and 5.8×1011 cm Hz1/2/W for InAs/GaAs/Si. These results clearly demonstrate the feasibility of the monolithic integration of InAs infrared detectors and GaAs or Si read-out electronics.

Structural and magnetic properties of diode radio-frequency sputtered Cr/Co multilayers

Structural properties of diode radio-frequency (rf)-sputtered Cr/Co multilayers have been investigated using in situ kinetic ellipsometry, grazing x-ray reflection, x-ray diffraction, and nuclear magnetic resonance. Results have been correlated with magnetic characterizations obtained by SQUID susceptometry. Interdiffusion along ≂10 Å occurs at the ‘‘Cr on Co’’ interface. The Co on Cr interface appears sharper. X-ray diffraction and nuclear magnetic resonance show that Co layers grow with bcc structure when the Co layer thickness dCo is lower than ≂15 Å. In this case, the films are strongly textured with the Cr bcc (100) direction perpendicular to the plane of the substrate. A better structural coherence is observed for the thinner layers. When dCo is thicker than ≂15 Å, a mixture of hcp and fcc Co phases appears by x-ray diffraction and nuclear magnetic resonance. The good structural quality of the films is confirmed by the occurrence of satellite peaks in the x-ray diffraction patterns. The Co magnetic moment extracted from the hysteresis loops measurements is coherent with these changes of the cobalt structure with layer thickness. In plane magnetization measurements show a antiferromagnetic coupling of the Co layers through the Cr layers.

Control of Zn(S, Se) composition using reflection high energy electron diffraction oscillations

Abstract Observation of reflection high energy electron diffraction (RHEED) oscillations during molecular beam epitaxy (MBE) growth of ZnSxSe1−x from elemental selenium and compound ZnS sources is reported. Oscillations are readily observed and may be used to control the sulfur content. In particular, they may be used as feedback during growth to maintain “lattice-matched” composition during growth on GaAs substrates. The RHEED oscillations undergo distinctive changes as various growth parameters (substrate temperature, flux ratio and growth rate) are varied. The growth conditions may be adjusted to obtain a particular form of oscillation, corresponding to a known sulfur content. In general, when growth begins under condition yielding oscillations, the intensity level at which the first oscillation occurs is different from the intensity level of later oscillations. As the ZnS flux changes, these two intensity levels change position relative to one another and, for a particular ZnS flux, merge. For a growth rate of 0.5 μm h−1 and an indicated substrate temperature of 275 °C, if growth is done near the point where the two levels merge, then the resulting ZnSxSe1−x is lattice matched (x ≈ 7%). However, if the substrate temperature is raised to 300 °C, then growth near conditions where the levels merge yields a lower sulfur content (x ≈ 5%). Therefore, because the point at which the levels merge does not occur at the same sulfur composition for various substrate temperatures, the RHEED oscillations are not actually directly sensitive to the amount of strain or to the sulfur content and thus are not directly sensitive to the lattice-matching condition. Confirmation of this is provided by a similar evolution of the RHEED oscillations during simple ZnSe growth, with varying zinc-to-selenium ratio. A possible explanation is that the oscillations are responding to a surface stoichiometry change that occurs during the first monolayer of deposition. Nevertheless, for a given substrate temperature and growth rate, the sulfur content obtained for growth yielding a particular form of oscillation is reproducible. Use of RHEED oscillations to maintain the lattice-matched composition has yielded layers of good structural quality. Layers have been growth as thick as 3.5 μm without misfit dislocations. The minimum full width at half-maximum for a lattice-matched layer, as measured by X-ray rocking curves is 28″.

Molecular beam epitaxial growth of (100) oriented CdTe on Si (100) using BaF2-CaF2 as a buffer

Epitaxial CdTe (100) has been grown on (100) oriented Si by molecular beam epitaxy using BaF2-CaF2 as a buffer. Two-dimensional (2-D) growth of BaF2(100) is obtained using low-temperature thermal cycles during growth. CdTe growth is also 2-D above 270 °C substrate temperature and a 2×1 surface reconstruction indicating a Te-stabilized surface is obtained. The growth is 3-D at lower substrate temperatures. Good structural quality films showing sharp electron channeling patterns and pronounced photoluminescence at 77 K are obtained. The full width at half maximum of the band-edge peak is 12 meV at 77 K.

Growth and characterization of GaAs/Al/GaAs heterostructures

Theoretical and experimental aspects of the growth of GaAs/Al/GaAs heterostructures have been investigated. In these heterostructures the GaAs on top of the buried metal layer is grown by migration-enhanced epitaxy (MEE) at low temperatures (200 and 400 °C) to provide a kinetic barrier to the outdiffusion of Al during superlayer growth. The crystallinity and orientation of the Al film itself deposited on (100) GaAs at ∼0 °C was studied by transmission electron diffraction, dark-field imaging, and x-ray diffraction measurements. It is found that the Al growth is polycrystalline with a grain size ∼60 Å and the preferred growth orientation is (111), which may be textured in plane but oriented out of plane. The quality of the GaAs superlayer grown on top of Al by MEE is very sensitive to the growth temperature. The layer grown at 400 °C has good structural and optical quality, but is accompanied by considerable outdiffusion of Al at the Al-GaAs heterointerface. At 200 °C, where the interface has good structural integrity, the superlayer exhibits twinning and no luminescence is observed.

Electrical and structural assessment of CdTe and CdMnTe layers grown by MBE on InSb substrates

Single layers and multiple quantum wells (MQWs) of CdTe and CdMnTe have been grown by MBE on InSb substrates. These systems have been analysed using double crystal X-ray diffraction (DCXRD). The single layers have been shown to be of good structural quality while the MQW systems yield rocking curves with well resolved diffraction satellites (up to the seventh order) indicating good quality and periodicity. All undoped layers have been found to be n-type with free carrier concentrations in the range 10 14-10 15cm -3 but the densities for CdMnTe layers are generallyless than for CdTe layers grown under identical conditions. When doped with indium, both types of layers have exhibited a saturation in carrier concentration as the In atom concentration was increased beyond 10 18 cm -3 probably as a result of the generation of compensating acceptor states.

Selenization of metallic Cu-In thin films for CuInSe2 solar cells

A new approach for formation of photovoltaic CuInSe2 films by chalcogenization of Cu-In multilayers with a thin selenium layer and subsequent heat treatments is presented. Morphological, compositional, and structural properties of films along with aspects of various processing conditions are discussed. The possibility of obtaining perfectly homogeneous chalcopyrite ternary films was affected by composition deviations from stoichiometry, although films obtained in a modified two-stage heat treatment processing were of good structural and photovoltaic quality. Heterojunctions with (Zn, Cd)S as window material showed efficiencies better than 5%, mainly limited by low open-circuit voltage and poor fill factor. Short-circuit currents were comparable to CuInSe2-based cells fabricated by multiple-source evaporation.

MBE p-type Hg1-xCdxTe grown on the (110) orientation

Reported here for the first time is the successful MBE growth of Hg1-xCdxTe (0.16 < x < 0.41) on the nonpolar (110) face of CdTe substrates. (110) HgCdTe epilayers always show p-type conductivity at 77 K with high hole concentration (1016-1019 cm-3) and good structural quality, as determined by double-crystal X-ray rocking curve (100–180 arc sec). We also report that during the MBE growth process, the rate of Hg vacancy formation on the (110) orientation is greater than on the more conventional (111)B and (100) orientations. The unique material properties of (110) HgCdTe may find application in the fabrication of photovoltaic detectors, light emitting diodes, and multiple quantum wells for nonlinear optics applications.

Growth of epitaxial CdS films on CdTe substrates by chemical vapour deposition

The open-tube chemical vapour deposition has been used to grow epitaxial CdS films on CdTe substrates. The growth has been performed in a horizontal reactor, using polycristalline CdS as source material and hydrogen as transporting gas. The deposition and the source temperatures have been varied in the range (400÷800)°C and (700÷900)°C, respectively. The experimental values of the deposition rate have been compared with those calculated assuming a quasi-equilibrium model. The comparison shows that the growth process can be adequately described by such a model in a small-temperature region around 700°C. At lower or higher temperatures the surface kinetics becomes the limiting mechanism. Films grown at 700°C show also the best morphology. X-ray diffraction and photoluminescence measurements evidence that films grown in these conditions have a good structural quality.

Formation and structure of Fe/Cu(001) interfaces, sandwiches, and superlattices.

We have used medium- and high-energy Auger-electron diffraction and low-energy electron diffraction to probe the structure of the Fe/Cu(001) interface, the Cu/Fe/Cu(001) sandwich, and the Fe/Cu/Fe/Cu(001) superlattice. Metastable epitaxial fcc overlayers of Fe with good structural quality grow on room-temperature Cu(001) for coverages up to five monolayers (ML). Above 5 ML, the structural quality of the overlayer diminishes. The first monolayer of Fe does not adsorb uniformly, but rather forms two-layer deep clusters. Above 1 ML, the film coalesces into a well-ordered fcc overlayer. When the substrate temperature is held at 125 \ifmmode^\circ\else\textdegree\fi{}C or higher during evaporation, Fe atoms in-diffuse and displace Cu atoms at lattice sites. The fcc Fe overlayer formed at room temperature also acts as an excellent template for subsequent Cu overlayer growth. We show that Cu depositions on 8-ML Fe/Cu(001) adsorb uniformly (no clustering) and form a fcc film with the same level of structural quality as the substrate. Further Fe depositions on the Cu overlayer maintain an fcc structure, illustrating the feasibility of generating Fe/Cu superlattices in which ultrathin layers of metastable fcc Fe can be sandwiched between Cu layers of any thickness.

Dual Ion Beam Sputter Deposition of Cdte, Hgte and Hgcdte Films

ABSTRACTExperiments have been performed to assess the feasibility of using ionbeam sputter deposition for the growth of CdTe, HgTe and HgCdTe films. Some simple cryogenically cooled dual-target configurations have been employed in an investigation of epitaxial growth on CdTe substrates. Good-quality epitaxy was achieved for CdTe at temperatures down to 140°C, and for HgTe and HgCdTe at temperatures extending to below 50° C.Based upon compositional and phase analyses, and upon IR absorption measurements, we conclude that, using an excess Hg flux, stoichiometric transfer of the HgCdTe target composition to the substrate is approximately obtained. However, some departure from stoichiometry is produced at higher substrate temperatures (&gt; 150”C) due to thermal re-evaporation of Hg, and under high sputtered Hg fluxes due to selective re-sputtering of HgTe. The good structural quality and excellent compositional uniformity of the films indicate that ion-beam sputter deposition may be suitable for low-temperature processing of IR detector structures.

Identification of tellurium precipitates in cadmium telluride layers grown by molecular beam epitaxy

The low-temperature growth of crystalline layers of CdTe on InSb substrates by molecular beam epitaxy has been investigated using transmission electron microscopy. It has been shown that while layers grown at substrate temperatures of ∼180–200 °C are generally single crystal and of good structural quality, layers grown at temperatures of ∼150 °C, although initially single crystal, rapidly become polycrystalline with increasing layer thickness. The mechanism responsible for this growth breakdown at low temperatures is shown to be the formation of second phase precipitates. These precipitates have been identified using diffraction analysis as the low-pressure, hexagonal phase of elemental Te.

Electrical and structural properties of pulse laser-annealed polycrystalline silicon films

Doped epitaxial films of Si on single-crystal high-resistivity Si substrates have been prepared using ion implantation and Q-switched ruby laser annealing of LPCVD polycrystalline Si layers. Films, doped with B or As in the range 1017to 5 × 1020cm-3were studied by the measurement of their resistivities, Hall mobilities, and doping density profiles. The good film quality achieved permitted the fabrication of p-channel MOS transistors which, through measurements of threshold voltage and transconductance, yielded additional data on the surface mobility and the integrity of the Si-SiO 2 interface. The electrical properties of the films compared favorably with those of similarly doped single-crystal material, and transmission electron microscopy was used to confirm the good structural quality of the epitaxial growth.

Deposition of epitaxial layers by ion beam methods

The possibilities and the limitations of ion beam sputter deposition are discussed in connection with results obtained for the growth of epitaxial semiconductor films. It turns out that homo- and hetero-epitaxial films of good structural quality can be deposited at relatively low substrate temperatures and that effective doping offers no difficulties. On the other hand, some radiation damage caused by energetic particles must be taken into account. More recently, ion beam deposition methods could be realized under UHV conditions using about the same diagnostic equipment as in MBE. It can be expected that in preparing special film structures combinations between molecular and ion beam techniques might become attractive, while ion beam methods also will stand on its own in synthesizing unusual film phases.

Preparation of Langmuir films for electrical studies

The procedure and the apparatus used for the preparation of stearic acid multilayers by the Langmuir-Blodgett technique are described and the precautions required to obtain films of good structural quality are outlined. Smooth transfer of a monomolecular layer from the water surface to a solid substrate is identified as one of the most critical factors in building good films and a new device developed to achieve such transfer is discussed in detail. The procedure and apparatus described in this paper are suitable for multilayers of other substances besides stearic acid. The acid multilayers, sandwiched between metal electrodes, require special treatment in vacuum for obtaining reproducible results. A brief discussion is given of the electrical measurements on stearic acid films and the results obtained are described.

Liquid phase epitaxial growth of GaxIn1−xP

Single crystal epitaxial layers of Gaxln1−xP alloys have been grown by the steady-state liquid phase epitaxial growth technique on (111)B GaAs substrates. The crystal growth process has been studied in detail and the resultant epitaxial layers have been characterized with respect to their structural, electrical and optical properties. Epitaxial layers of good structural quality could be grown only in the composition range x = 0.48 to 0.53, where the lattice parameter is close to that of the GaAs substrate. The band gap of these crystals was in the range 1.86 – 1.92 eV as determined by optical absorption and photoluminescence measurements.