Buffer Layer Structure Research Articles

Dependence of the HVPE GaN epilayer on GaN buffer layer for GaN direct growth on (001) GaAs substrate

Cubic GaN was grown on (001) GaAs substrates by hydride vapor phase epitaxy (HVPE) with a buffer layer grown by low temperature HVPE. The crystalline structure of the GaN buffer layer is studied using reflection high-energy electron diffraction (RHEED). The GaN layers were grown at 800°C on the buffer layer with various growth temperatures and thicknesses. The optimum thickness of the buffer layer grown at 500°C to obtain a cubic GaN layer was found to be 60 nm in our case. When the buffer layer was grown at 525°C, however, the full width at half-maximum (FWHM) of the X-ray diffraction (XRD) pattern was 21.2 min, which is the narrowest ever reported, as far as we know. Furthermore, it was demonstrated that a single crystalline GaN buffer layer was necessary to obtain high-quality thick cubic GaN.

Growth and Characterization of PbSe and Pb1−xSnxSe Layers on Si (100)

AbstractCrack-free layers of PbSe were grown on Si (100) by a combination of liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE) techniques. The PbSe layer was grown by LPE on Si (100) using a MBE-grown PbSe/BaF2/CaF2 buffer layer structure. Pb1−xSnxSe layers with tin contents in the liquid growth solution equal to 3%, 5%, 6%, 7%, and 10%, respectively, were also grown by LPE on Si (100) substrates using similar buffer layer structures. The LPE-grown PbSe and Pb1−xSnxSe layers were characterized by optical Nomarski microscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Optical Nomarski characterization of the layers revealed their excellent surface morphologies and good growth solution wipe-offs. FTIR transmission experimentsshowed that the absorption edge of the Pb1−xSnxSe layers shifted to lower energies with increasing tin contents. The PbSe epilayers were also lifted-off from the Si substrate by dissolving the MBE-grown BaF2 buffer layer. SEM micrographs of the cleaved edges revealed that the lifted-off layers formed structures suitable for laser fabrication.

Growth and Characterization of Pbse and Pbl-Sn1 Se Layers on Si (100)

AbstractCrack-free layers of PbSe were grown on Si (100) by a combination of liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE) techniques. The PbSe layer was grown by LPE on Si (100) using a MBE-grown PbSe/BaF2/CaF2 buffer layer structure. Pb1−xSnxSe layers with tin contents in the liquid growth solution equal to 3%, 5%, 6%, 7%, and 10%, respectively, were also grown by LPE on Si (100) substrates using similar buffer layer structures. The LPE-grown PbSe and Pb1−xSnxSe layers were characterized by optical Nomarski microscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Optical Nomarski characterization of the layers revealed their excellent surface morphologies and good growth solution wipe-offs. FTIR transmission experiments showed that the absorption edge of the Pb1−xSnxSe layers shifted to lower energies with increasing tin contents. The PbSe epilayers were also lifted-off from the Si substrate by dissolving the MBE-grown BaF2 buffer layer. SEM micrographs of the cleaved edges revealed that the lifted-off layers formed structures suitable for laser fabrication.

Strain Relaxation in Iv-vi Semiconductor Layers Grown on Silicon (100) Substrates

ABSTRACTThe large thermal expansion coefficient mismatch between IV–VI semiconductors and silicon results in significant tensile strain when structures are cooled down following growth at high temperatures. Molecular beam epitaxy (MBE) growth of PbSe on Si (100) at 280°C on BaF2/CaF2 buffer layers results in high crack-density because of this strain. Interestingly, crack- free layers of PbSe can be grown by LPE on Si (100) by using MBE-grown PbSe/BaF2/CaF2 buffer layer structures. Unlike LPE-grown PbSe layers, however, a high crack-density was observed in Pbl-xSnxSe1-yTey layers grown by LPE on Si (100) using similar buffer layer structures. It is hypothesized that the addition of tellurium, which is known to increase IV-VI material hardness, inhibits plastic deformation and thus crack formation becomes the thermal strain relief mechanism. Ternary Pbl-xSnxSe layers grown by LPE exhibited much lower crack densities as compared to the quaternary layers and supported the conclusion that tellurium induced solid solution hardening occurs in the IV-VI materials system.

Strain relaxation and dislocations in SiGe/Si structures

Epitaxially grown lattice mismatched structures are of increasing importance for microelectronic and optoelectronic devices and circuits fabricated using a variety of semiconducting materials. For some applications, the strain in the epitaxial layers is intentionally relieved by the introduction of misfit dislocations, and the film has an in-plane lattice parameter close to that of the bulk material. For example, a fully relaxed Si0.7Ge0.3 layer is required as a buffer layer for high mobility field effect transistors having strained Si or strained SiGe channels. In this case it is necessary to control the growth conditions and the design of the structure so as to minimize the density of dislocations threading through device layers grown on top of the relaxed buffer layer. The need to prevent strain relaxation in thin layers or to control the density and distribution of defects in intentionally relaxed structures has lead to extensive research on strain relaxation mechanisms and on the properties of the defects which are required to relieve the strain. This article will focus on recent work on strain relaxed SiGe/Si structures. Two different relaxation mechanisms occurring at low mismatch strain and low growth temperature will be discussed, and the characteristics of step-graded buffer layer structures will be reviewed. Effects of dislocations on the electronic properties of relaxed SiGe layers will also be discussed.

Processing and transport characteristics of YBCO tape conductors formed by IBAD method

Biaxially aligned YBa 2 Cu 3 O 7− x (YBCO) films were fabricated on random Ni-based alloy tapes with textured YSZ buffer layers deposited by ion-beam-assisted deposition (IBAD). Ar + ion bombardment was found to have significant effects on the crystalline structure of the YSZ buffer layers to align a [100] axis with the substrate normal and a [111] axis with the bombarding beam axis. The alignment mechanism is discussed with the structural properties of the YSZ films based on several proposed models. The azimuthal distribution of the a - and b -axes of the YBCO film on the top of the well-textured YSZ film was restricted to 10.2° FWHM. The high J c value of 1.13×10 6 A cm −2 (77 K, 0 T), the large I c value of 103 A (77 K, 0 T), the clear longitudinal field effect (enhanced by 1.7 at 77 K, 0.5 T), and the low self-field AC losses in accordance with the Norris’ formula for thin strip conductors were observed. All the transporting properties indicated that the biaxially aligned structure is a promising way to avoid percolated structures of superconducting current paths in YBCO conductors.

Parametric study on lattice-matched and pseudomorphic InGaAs/InAlAs/InP modulation-doped heterostructures grown by GSMBE

Modulation-doped In x Ga 1 − x As/In y Al 1 − y As/InP heterostructures have been grown with a diffusion pumped V80H gas source molecular beam epitaxy system. Their electrical properties have been studied with Hall measurements. The effects of spacer layer thickness and buffer layer structure (InAlAs or InAlAs + InP) on lattice-matched and pseudomorphic In x Ga 1 − x As/In y Al 1 − y As/InP 2DEGs and HEMTs characteristics have been investigated in detail. Crystalline and interface qualities can be improved by adding an InP buffer layer before the InAlAs buffer, resulting in an improvement in material electrical properties. A thicker spacer layer will cause a decrease of electron gas density and an increase of electron mobility. Adopting strained channel and Schottky layers can dramatically improve material electrical performance. For InGaAs/InAlAs/InP pseudomorphic 2DEG (P-2DEG) structures, electron mobilities of 11 230 cm 2/V · s at n s of 2.3 × 10 12cm −2 at 300 K and 6.52 × 10 4cm 2/V · s at n s of 2.1 × 10 12cm −2 at 77 K were obtained and μ of 9273 cm 2/V · s at 3.6 × 10 12cm −2 and 4.5 × 10 4cm 2/V · s at n s of 3.1 × 10 12cm −2 for both strained InGaAs channel layers and InAlAs Schottky layers in P-HEMT structures were achieved.

Relaxed Si0.7Ge0.3 buffer layers for high-mobility devices

The minimum epitaxial layer thickness required to produce relaxed, thermally stable, Si0.7Ge0.3 buffer layer structures for high electron- and hole-mobility devices has been determined, using high resolution x-ray diffraction. A 1.4-μm-thick layer, step graded to x=0.35, is sufficiently thick so that the residual strain in a uniform composition Si0.33Ge0.67 layer grown on top of it is essentially independent of thickness or growth temperature of the layer. Such structures are stable when annealed at 750 °C.

Correlation of surface morphology and optical properties of GaN by conventional and selective-area MOCVD

ABSTRACTWe have studied GaN films grown by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on sapphire substrates using different buffer layer structures. Surface morphology was characterized by scanning electron microscopy (SEM). Optical properties were measured using photoluminescence (PL), cathodoluminescence (CL) spectroscopy and catho-luminescence wavelength imaging (CLWI) method. It is found that the hexagonal pit-like defects in morphology are associated with the D-A/e-A transition band in the PL and CL spectra. The same correlation of morphology with optical properties is observed for the GaN films grown by selective area epitaxy (SAE). In addition, the possibility of improving optical quality by SAE is investigated. The SAE depth profile is simulated for the first time, and satisfactory results are obtained.

Spatially indirect photoluminescence from InAs/AlSb heterostructures

We present a Fourier transform photoluminescence study of InAs/AlSb type II heterostructures, spanning the mid infrared 1 μm - 12 μm wavelength range. The investigated samples consist of single-, double-, and multi-quantum wells, grown on GaAs substrates by molecular beam epitaxy, with different buffer layer structures for strain accommodation. The photoluminescence intensity is found to be very sensitive to the detailed structure of the underlying buffer sequence. Calorimetric measurements of the absorbance at 500 mK shows the weak spatially indirect transitions superimposed onto the spatially direct absorbance. Very wide quantum wells (100 nm and 200 nm) exhibit spatially direct as well as spatially indirect pbotoluminescence. Polarization resolved photoluminescence of the spatially indirect transitions allow the contributions of the normal (AlSb on InAs) and the inverted (InAs on AlSb) interface to be separated.

Spatially direct and indirect photoluminescence from InAs/AlSb heterostructures

We present a Fourier-transform photoluminescence study of InAs/AlSb type-II heterostructures, spanning the midinfrared 1--12-\ensuremath{\mu}m wavelength range. The investigated samples consist of single and double quantum wells, grown on GaAs substrates by molecular-beam epitaxy, with different buffer-layer structures for strain accommodation. The photoluminescence intensity is found to be very sensitive to the detailed structure of the underlying buffer sequence. The use of a GaSb buffer or a GaSb/AlSb superlattice in the buffer gives rise to an additional radiative-recombination channel in the near infrared, which competes with radiative recombination in the InAs quantum wells at longer wavelengths. Variations in the spectra observed under visible and infrared excitation provide evidence that the recently observed persistent photo effect is governed by hole capture in the GaSb layers in the buffer sequence. Furthermore, very wide quantum wells (100 and 200 nm) show spatially direct as well as spatially indirect photoluminescence, providing insight into the nature of the transition from the bulklike semiconductor to the type-II heterostructure.

Giant magnetoresistance in soft magnetic NiFeCo/Cu multilayers with various buffer layers

The effect of buffer layers on giant magnetoresistance has been investigated for Ni 66Fe 16Co 18/Cu multilayers with Fe, FeCo, NiFeCo and CoZr buffer layers. The structure and the antiferromagnetic coupling strength of the multilayers were found to depend on the crystal structure of buffer layers. At a Cu layer thickness of 2.2 nm, which correponds to the 2nd peak of MR ratio, the multilayers with NiFeCo and CoZr buffer layers exhibited much smaller saturation fields than those with Fe and FeCo buffer layers.

The growth of various buffer layer structures and their influence on the quality of (CdHg)Te epilayers

The suitability of various buffer layer structures on (100) GaAs for (CdHg)Te growth by organometallic vapour phase epitaxy (OMVPE) was investigated. The preferred epitaxial orientation of (100)GaAs (111)CdTe was found to be unsuitable due to the formation of electrically active defects in the material. An intermediate ZnTe layer was used to select the (100) orientation and (100) CdTe layers were when deposited on this ZnTe layer. The quality of the resultant CdTe buffer was found to critically depend on the thickness of this intermediate ZnTe buffer, with a ZnTe thickness of approximately 500 Å producing the best CdTe buffer. (CdHg)Te epilayers grown on these ZnTe CdTe buffers had improved electrical properties, but still suffered from a poor surface morphology. This surface morphology could be improved by using a lattice matched Cd 0.96Zn 0.04Te alloy as the final buffer layer, but the surface pyramids typical of the (100) orientation could never be completely eliminated.

Growth of InAs-AlSb quantum wells having both high mobilities and high concentrations

Low-temperature mobilities in InAs-AlSb quantum wells depend sensitively on the buffer layer structures. Reflection high energy electron diffraction and x-ray diffraction show that the highest crystalline quality and best InAs transport properties are obtained by a buffer layer sequence GaAs → AlAs → AlSb → GaSb, with a final GaSb layer thickness of at least 1 μm. Using the improved buffer scheme, mobilities exceeding 600,000 cm2/Vs at 10 K are routinely obtained. Modulation δ-doping with tellurium has yielded electron sheet concentrations up to 8 × 1012 cm−2 while maintaining mobilities approaching 100,000 cm2/Vs at low temperatures.

Improved charge control and the frequency performance in InAs/AlSb HFET's

It is shown how the well thickness and the buffer layers influence the charge control properties of InAs/AlSb HFETs (heterostructure field-effect transistors) and how the kink can be eliminated from the drain characteristics of micron-sized HFETs, resulting in low output conductances and well-behaved drain characteristics. It is also demonstrated that a proper buffer layer structure is most beneficial in submicron devices, and results in great improvements in operational range and frequency performance.

Effect of structural parameters on InGaAs/InAlAs 2DEG transport characteristics

The effect of structural parameters on the transport characteristics from 15 to 300 K of molecular beam epitaxy-grown InGaAs/InAlAs two dimensional electron gas structures lattice-matched to InP is determined. The InAlAs buffer layer thickness was varied from 1000 to 10,000A. One sample also incorporated a InGaAs/InAlAs superlattice. The buffer layer thickness and structure had almost no effect on the mobility or sheet density. The InAlAs spacer layer was varied from 25 to 200A. Increases in the InAlAs spacer thickness resulted in a monotonically decreasing sheet density and a peak in the mobility versus spacer thickness at 100A. The highest 77 K mobility was 66,700 cm2/V/sds withND=1.2×1012 cm−2. The effect of illumination and temperature on the sheet concentration in these structures as well as on “bulk” InAlAs:Si was much smaller than in AlxGa1−xAs/GaAs structures or “bulk” AlxGa1−xAs, forx≅0.30, indicating that devices based on this material system will not be characterized by many of the device instabilities observed in the AlGaAs/GaAs system.

Crystallographic and Magnetic Properties of Fe/Au(001) and Fe/Au(111) Superlattice Films

On Au buffer layers with (001) and (111) planes, Fe/Au superlattice films were prepared by ultrahigh vacuum evaporation. Prior to investigation of Fe/Au superlattices, the structure of Au buffer layers, which significantly influences the structure of superlattices, was studied. The epitaxial growth of Fe(001)/Au(001) layers on a Au(001) buffer layer and that of Fe(110)/Au(111) layers on a Au(111) layer were confirmed by X-ray diffraction and reflection high-energy electron diffraction (RHEED). The films with the two stacking orientations have considerably different surface magnetic anisotropies.

Effect of structural parameters on transport characteristics of GaInAs/AlInAs two-dimensional electron gases grown by molecular beam epitaxy

The effect of structural parameters on the transport characteristics from 15 to 300 K of molecular beam epitaxy grown GaInAs/AlInAs two-dimensional electron gas structures lattice matched to InP is determined. The AlInAs buffer layer thickness was varied from 1000 to 10 000 Å. One sample also incorporated a GaInAs/AlInAs superlattice. The AlInAs spacer layer was varied from 25 to 200 Å. The buffer layer thickness and structure has almost no effect on the mobility or sheet density. Increases in the AlInAs spacer thickness resulted in a monotonically decreasing sheet density and a peak in the mobility at 100 Å. The highest 77 K mobility was 66 700 cm2/V s with ND=1.20×1012 cm−2 in the structure with the 100 Å spacer. The effect of illumination and temperature on the sheet concentration in these structures as well as on ‘‘bulk’’ AlInAs:Si is much smaller than in AlGaAs/GaAs structures or ‘‘bulk’’ AlGaAs, indicating that devices based on this material system will not be characterized by freeze-out and persistent photoconductivity.

Preparation and Magnetic Properties of Epitaxial Fe/Au (001) Superlattice Films

bec Fe (001)/Au (001) superlattice films were prepared on Au (001) buffer layers by ultra-high vacuum evaporation. Epitaxial structures were studied by X-ray diffraction and RHEED. The structure of the superlattice films was strongly influenced by the structure of the Au buffer layer. The surface anisotropy energy constant K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> is estimated to be about 0.3 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , considerably differing from that of polycrystalline bec Fe (110)/Au (111) superlattice films.

Electron Microdiffraction Investigation of a GexSi1−x Buffer for Strain-Symmetrised Superlattice Structures

ABSTRACTA candidate GeSi buffer layer structure suitable for strain-symmetrised GeSi/Si superlattice overgrowth is grown by molecular-beam epitaxy and its strain state characterised using microdiffraction in a dedicated scanning transmission electron microscope. The structure consists of five alloy layers of increasing Ge concentration grown on a Si (100) substrate, with all but the last annealed at high temperature. Analysis of higher order Laue zone deficit lines in the microdiffraction patterns acquired from each layer indicate that relaxation is complete in all the layers. Images of the structure in the transmission electron microscope show good crystallinity of the final layer with low concentrations of threading dislocations.