High-quality Epitaxial Layers Research Articles

3C-SiC on Si Hetero-Epitaxial Growth for Electronic and Biomedical Applications

The growth of cubic silicon carbide on silicon, namely 3C-SiC/Si, has been extensively studied at the University of South Florida over the past decade and numerous electronic and biomedical applications explored using this material system. The key step to 3C-SiC devices is the growth of high-quality epitaxial layers of 3C-SiC. In order to improve the manufacturability of future 3CSiC devices, a simplified 3C-SiC growth process on 50 and 100 mm Si (100) substrates has been developed in a low-pressure horizontal hot-wall chemical vapor deposition (CVD) reactor. A simplified growth process consists of a single thermal ramp to the growth temperature followed by the 3C-SiC growth. The 3C-SiC epitaxial layers were characterized via optical microscopy, secondary electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and secondary ion mass spectrometry (SIMS). Examples of biomedical devices realized with this material system are also introduced, including neural probes and in-vitro recording devices, as well as myoglobin and glucose biosensors.

Structural and band alignment properties of Al2O3 on epitaxial Ge grown on (100), (110), and (111)A GaAs substrates by molecular beam epitaxy

Structural and band alignment properties of atomic layer Al2O3 oxide film deposited on crystallographically oriented epitaxial Ge grown in-situ on (100), (110), and (111)A GaAs substrates using two separate molecular beam epitaxy chambers were investigated using cross-sectional transmission microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). High-resolution triple axis x-ray measurement demonstrated pseudomorphic and high-quality Ge epitaxial layer on crystallographically oriented GaAs substrates. The cross-sectional TEM exhibited a sharp interface between the Ge epilayer and each orientation of the GaAs substrate as well as the Al2O3 film and the Ge epilayer. The extracted valence band offset, ΔEv, values of Al2O3 relative to (100), (110), and (111) Ge orientations using XPS measurement were 3.17 eV, 3.34 eV, and 3.10 eV, respectively. Using XPS data, variations in ΔEv related to the crystallographic orientation were ΔEV(110)Ge>ΔEV(100)Ge≥ΔEV(111)Ge and the conduction band offset, ΔEc, related to the crystallographic orientation was ΔEc(111)Ge>ΔEc(110)Ge>ΔEc(100)Ge using the measured ΔEv, bandgap of Al2O3 in each orientation, and well-known Ge bandgap of 0.67 eV. These band offset parameters are important for future application of Ge-based p- and n-channel metal-oxide field-effect transistor design.

Structural, morphological, and band alignment properties of GaAs/Ge/GaAs heterostructures on (100), (110), and (111)A GaAs substrates

Structural, morphological, and band offset properties of GaAs/Ge/GaAs heterostructures grown in situ on (100), (110), and (111)A GaAs substrates using two separate molecular beam epitaxy chambers, connected via vacuum transfer chamber, were investigated. Reflection high energy electron diffraction (RHEED) studies in all cases exhibited a streaky reconstructed surface pattern for Ge. Sharp RHEED patterns from the surface of GaAs on epitaxial Ge/(111)A GaAs and Ge/(110)GaAs demonstrated a superior interface quality than on Ge/(100)GaAs. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of Ge about 0.2–0.3 nm. High-resolution triple axis x-ray rocking curves demonstrate a high-quality Ge epitaxial layer as well as GaAs/Ge/GaAs heterostructures by observing Pendellösung oscillations. Valence band offset, ΔEv, have been derived from x-ray photoelectron spectroscopy (XPS) data on GaAs/Ge/GaAs interfaces for three crystallographic orientations. The ΔEv values for epitaxial GaAs layers grown on Ge and Ge layers grown on (100), (110), and (111)A GaAs substrates are 0.23, 0.26, 0.31 eV (upper GaAs/Ge interface) and 0.42, 0.57, 0.61 eV (bottom Ge/GaAs interface), respectively. Using XPS data obtained from these heterostructures, variations in band discontinuities related to the crystallographic orientation have been observed and established a band offset relation of ΔEV(111)Ga>ΔEV(110)>ΔEV(100)As in both upper and lower interfaces.

Study of threading dislocation density reduction in AlGaN epilayers by Monte Carlo simulation of high-resolution reciprocal-space maps of a two-layer system

High-resolution X-ray diffraction in coplanar and noncoplanar geometries has been used to investigate the influence of an SiNxnano-mask in the reduction of the threading dislocation (TD) density of high-quality AlGaN epitaxial layers grown on sapphire substrates. Our developed model, based on a Monte Carlo method, was applied to the simulation of the reciprocal-space maps of a two-layer system. Good agreement was found between the simulation and the experimental data, leading to an accurate determination of the dislocation densities as a function of the overgrowth layer thickness. The efficiency of the SiNxnano-mask was defined as the ratio of the TD densities in the AlGaN layers below and above the mask. A significant improvement in the AlGaN layer quality was achieved by increasing the overgrowth layer thickness, and a TD density reduction scaling law was established.

In situ grown Ge in an arsenic-free environment for GaAs/Ge/GaAs heterostructures on off-oriented (100) GaAs substrates using molecular beam epitaxy

High-quality epitaxial Ge layers for GaAs/Ge/GaAs heterostructures were grown in situ in an arsenic-free environment on (100) off-oriented GaAs substrates using two separate molecular beam epitaxy (MBE) chambers, connected via vacuum transfer chamber. The structural, morphological, and band offset properties of these heterostructures are investigated. Reflection high energy electron diffraction studies exhibited (2 × 2) Ge surface reconstruction after the growth at 450 °C and also revealed a smooth surface for the growth of GaAs on Ge. High-resolution triple crystal x-ray rocking curve demonstrated high-quality Ge epilayer as well as GaAs/Ge/(001)GaAs heterostructures by observing Pendellösung oscillations and that the Ge epilayer is pseudomorphic. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of ∼0.45 nm and room temperature photoluminescence spectroscopy exhibited direct bandgap emission at 1583 nm. Dynamic secondary ion mass spectrometry depth profiles of Ga, As, and Ge display a low value of Ga, As, and Ge intermixing at the Ge/GaAs interface and a transition between Ge/GaAs of less than 15 nm. The valence band offset at the upper GaAs/Ge-(2 × 2) and bottom Ge/(001)GaAs-(2 × 4) heterointerface of GaAs/Ge/GaAs double heterostructure is about 0.20 eV and 0.40 eV, respectively. Thus, the high-quality heterointerface and band offset for carrier confinement in MBE grown GaAs/Ge/GaAs heterostructures offer a promising candidate for Ge-based p-channel high-hole mobility quantum well field effect transistors.

Revisiting the Hydrothermal growth of YAG

As new compositions and configurations of single crystals are specified for optical applications the crystal growth community must continue to optimize and explore new options for developing such materials. New crystal growth technology and a growing understanding of certain chemical and physical phenomena sometimes implore us to reconsider our perceptions even about the most studied systems. Thus, the hydrothermal growth of YAG was reexamined after nearly 40 years of inactivity and met with promising new results. While the hydrothermal growth of YAG at 500°C was previously met with slow growth rates or poor crystal quality we have found that growth at 600–630°C yields good quality single crystals at rates of about 0.5mm/side/week on (100) seeds. In the current study, high quality epitaxial layers of transition metal and rare earth doped YAG have been grown from 0.1 to 2.75mm in thickness. This has enabled several more advanced proof of concept designed growth experiments including the growth of co-doped Ca2+,Cr4+,Nd:YAG for self Q-switching, and the growth of Sm:YAG around Nd:YAG to potentially suppress amplified spontaneous emission. The interface quality of the substrate and the new growth was investigated and application of a very narrow thermal gradient at the early stages of growth has proven to greatly reduce optical scattering at the interface. The morphology of hydrothermally-grown YAG crystals is also discussed and several intermediate forms are occasionally observed.

Epitaxial Growth of 4H-SiС Using Si<sub>2</sub>(CH<sub>3</sub>)<sub>6</sub>+Si<sub>2</sub>Cl<sub>6</sub>+C<sub>3</sub>H<sub>8</sub>+H<sub>2</sub> System by Atmospheric Pressure Hot CVD Method

The chlorinated precursor is recently focused for high growth rate and high quality epitaxial layer. In the previous studies, the addition of chlorinated species from Si2Cl6 in the gas phase eliminated simultaneous Si nucleation which interferes with epitaxy. In this work, the characterization of epitaxial layers grown with chlorinated species is focused. High growth rate of 30 μm/h was achieved by using Si2(CH3)6 and Si2Cl6 as chlorinated precursors. We concluded that high growth rate was achieved by using HMDS and HCDS as the precursor of SiC at growth temperature of 1600 °C.

Uniform and position-controlled InAs nanowires on2′′ Si substrates for transistor applications

This study presents a novel approach for indirect integration of InAs nanowires on2′′ Si substrates. We have investigated and developed epitaxial growth of InAs nanowires on2′′ Si substrates via the introduction of a thin yet high-quality InAs epitaxial layer grown bymetalorganic vapor phase epitaxy. We demonstrate well-aligned nanowire growth includingprecise position and diameter control across the full wafer using very thin epitaxial layers (<300 nm). Statistical analysis results performed on the grown nanowires across the2′′ wafer size verifies our full control on the grown nanowire with 100% growth yield. From thecrystallographic viewpoint, these InAs nanowires are predominantly of wurtzite structure.Furthermore, we show one possible device application of the aforementioned structure invertical wrap-gated field-effect transistor geometry. The vertically aligned InAs nanowiresare utilized as transistor channels and the InAs epitaxial layer is employed as the sourcecontact. A high uniformity of the device characteristics for numerous transistorsis further presented and RF characterization of these devices demonstrates anft of 9.8 GHz.

Volume production of high quality SiC substrates and epitaxial layers: Defect trends and device applications

We review the progress of silicon carbide (SiC) bulk growth by the sublimation method, highlighting recent advances at Dow Corning, which resulted in the commercial release of 100mm n-type 4H-SiC wafers with median micropipe densities (MPD) in production wafers <0.1cm−2 and the demonstration of micropipe free material over a full 100mm diameter. Investigations by Synchrotron White Beam X-ray Topography (SWBXRT) and molten KOH etch pit analysis of 100mm wafers demonstrate threading screw dislocation densities <500cm−2. Additional results indicate the positive impact of maintaining thermo-mechanical stress levels in the growing crystal below the critical resolved shear stress on reducing basal plane dislocation densities to values as low as ∼300–400cm−2 in 100mm crystals. We summarize the steps of systematic quality improvements on increasing wafer diameter, utilizing numerical simulations of the SiC growth system as a critical tool to guide this process. For the economical production of SiC epitaxy, a 10×100mm wafer platform has been established in a warm-wall planetary chemical vapor deposition (CVD) reactor. The combined improvements in the epitaxy process, pre-epi wafer surface preparation and the underlying substrate quality itself have led to a reduction of the device killer defect density from 8cm−2 to 1.5cm−2 on a volume product like 100mm 4° off-axis 6.5μm epi-wafers. Dow Corning production epi-wafers routinely show Schottky diode yields above 90% at a die size of 2mm×2mm. Additionally, 50–100μm thick epitaxy on 76mm 4° off-axis wafers with morphological defect densities of 2–6cm−2, a surface roughness (RMS) ≤1nm as measured by atomic force microscopy (AFM), and carrier lifetimes consistently in the range of 2–3μs has been demonstrated.

Germanium Nanowire Metal–Oxide–Semiconductor Field-Effect Transistor Fabricated by Complementary-Metal–Oxide–Semiconductor-Compatible Process

This work presents a complementary metal-oxide-semiconductor-compatible top-down fabrication of Ge nanowires along with their integration into pMOSFETs with "HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /TaN" high-k/metal gate stacks. Lateral Ge wires down to 14 nm in diameter are achieved using a two-step dry etch process on a high-quality epitaxial Ge layer. To improve the interface quality between the Ge nanowire and the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , thermally grown GeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and epitaxial-Si shells are used as interlayers. Devices with a GeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> shell demonstrated excellent I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> ratios (>; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> ), whereas the epitaxial-Si shell was found to improve the field-effect mobility of the holes in Ge nanowires to 254 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .

Crystal growth and characterization of RbTi1−x−yYbxNbyOPO4/RbTiOPO4 (001) non-linear optical epitaxial layers

In this paper we report the growth of RbTi1−x−yYbxNbyOPO4/RbTiOPO4 epitaxial layers by the liquid phase epitaxy method and discuss their structural and optical characterization. The layers were grown on (001) oriented substrates because this plane contains the phase-matching direction for Yb3+ type-II second-harmonic generation. In RbTi0.971Yb0.015Nb0.014OPO4/RbTiOPO4 epitaxial layers we obtained enough contrast of refractive indices (Δnz = 0.007 at 632.8 nm) to guide the light in TM polarization. The maximum Yb3+ concentration obtained in these layers was 1.43 × 1020 atoms per cm3, which may be suitable for laser action. The lattice mismatch between the epitaxial layers and the substrates was low enough to obtain high quality epitaxial layers. Confinement of the light at 532 and 632.8 nm was demonstrated and there was good agreement between theoretical calculations, dark-mode spectra and modes of propagation.

The phototransistor action of a reverse-biased Au/CaF2[3–7 ML]/n-Si(1 1 1) structure

Metal–ultra-thin insulator–semiconductor (MIS) tunnel structures with a high-quality ultra-thin epitaxial fluoride layer have been fabricated on n-Si wafers. Under the reverse bias, these structures have been experimentally shown to amplify photocurrent. They can therefore be treated as MIS tunnel emitter bipolar phototransistors. The gain value reaches 102–103, and is due to the asymmetry between conduction-band and valence-band tunneling. The importance of the valence-band component has been elucidated. The experimental data have been well reproduced by simulations considering the conservation of the transverse wave vector of tunneling particles. The current multiplication effect was observed within the voltage range up to the structure overload. After the insulator damage, the structure ceased to exhibit gain rendering a photodiode.

Exploring Waveguiding Properties of Heavily Doped $ \hbox{Yb}^{3+}:\hbox{KLu}(\hbox{WO}_{4})_{2}$ Epitaxial Layers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> High-quality epitaxial layers of heavily doped <formula formulatype="inline"><tex Notation="TeX">$\hbox{Yb:KLu}( \hbox{WO}_{4})_{2}$</tex></formula> grown on <formula formulatype="inline"><tex Notation="TeX">$\hbox{KLu}(\hbox{WO}_{4})_{2}$</tex> </formula> substrates by liquid phase epitaxy were characterized in terms of lattice mismatch and refractive indexes of the layer and the substrate. The effective refractive indexes of the guided modes that these structures can support were modeled and measured experimentally for the transverse electric (TE) and transverse magnetic (TM) mode excitation by dark m-lines spectroscopy. The upper limit for optical losses in these waveguides corresponding only to scattering processes was estimated to be <formula formulatype="inline"><tex Notation="TeX">$\sim$</tex></formula>1.5 dB/cm at <formula formulatype="inline"><tex Notation="TeX">$\lambda = 632.8\ \hbox{nm}$</tex></formula>. These results show that, apart for highly efficient thin-disk laser applications, these epilayers can be used as waveguide devices at wavelengths different from those used to excite <formula formulatype="inline"><tex Notation="TeX">$\hbox{Yb}^{3+}$</tex></formula>. </para>

Effect of a surface pre-treatment on graphene growth using a SiC substrate

This study reports surface pre-treatment techniques for the formation of a high-quality graphene layer on a SiC surface. It is demonstrated that silicon passivation of SiC surface using a silane flow and subsequent sacrificial oxidation can significantly improve the surface condition of a graphene layer on SiC by ensuring much fewer carbon dumps and wrinkles, reducing the electrical resistance, and providing smoother surface roughness and a larger domain size. The effect of in situ cleaning by a SF 6 treatment before graphitization was also studied. It was found that in situ cleaning using SF 6 gas can be a simple and effective means of improving the quality of a graphene layer grown on SiC. The results of this study suggest that a surface treatment before graphitization is the key to synthesize high-quality epitaxial graphene layer.

Epitaxial Growth of Lattice Matched KY1−x−yGdxLuy(WO4)2 Thin Films on KY(WO4)2 Substrates for Waveguiding Applications

The main purpose of this work was the determination of the composition of a KY 1-x-y Gd x Lu y (WO 4 ) 2 layer grown by liquid phase epitaxy (LPE) technique on KY(WO 4 ) 2 substrate having enough refracave index contrast and appropriate lattice mismatch (f hkl ) to act as planar waveguides. To achieve this we have grown KY 1-x-y Gd x Lu y (WO 4 ) 2 single crystals by the flux method and characterized them in terms of chemical composition, structure and refractive indices. A high quality epitaxial layer of KY 0.58 Gd 0.22 Lu 0 . 20 (WO 4 ) 2 with a thickness of 10 μm was grown on a b oriented KY(WO 4 ) 2 substrate by LPE. At λ = 632.8 nm this planar waveguide can support five and four guided modes along the N g and N m directions, respectively and five TM modes along N p direction.

Correlation between YBa2Cu3O7 nuclei density and the grain orientation of the CeO2 buffered Ni–W template of the second-generation superconducting wire

The prime goal of the second-generation superconducting wire technology is to grow high-quality epitaxial layers of YBa2Cu3O7 (YBCO) superconductor using high-rate deposition on low-cost, kilometer-long substrates. We analyze the influence of Ni–W RABiTS™ substrate grain misalignment on nucleation of epitaxial YBCO during metal-organic ex situ processing. Electron backscatter diffraction orientation maps are correlated with YBCO nuclei density obtained from scanning-electron microscopy. A critical Ni–W grain tilt misorientation angle of 8.5° was identified above which the YBCO nuclei density was observed to be extremely low, approaching zero. A proposed model explains the reduction in nuclei density as being due to the absence of (001) substrate terraces wide enough to accommodate the critical size for YBCO nuclei. This study emphasizes the strong effect of the out-of-plane tilt of substrate grains on superconducting properties of YBCO layers produced by metal-organic deposition.

Surface Passivation Using Silane for Epitaxial Growth of Graphene on SiC substrate

We report surface pre-treatment technique for high quality graphene layer formation on SiC surface. It has been demonstrated that silicon passivation of SiC surface using silane flow and subsequent sacrificial oxidation can significantly improve surface condition of the graphene layer on SiC with much fewer carbon dumps and wrinkles, reduced electrical resistance, smoother surface roughness of ~ 2.7 Aå and larger domain size of ~380 Aå. Our results suggest that surface treatment before graphitization is the key to synthesize high quality epitaxial graphene layer.

Low-temperature growth of GaAs with high quality by metalorganic vapor phase epitaxy

Growth of GaAs by MOVPE (Metal-Organic Vapor-Phase Epitaxy) at a much lower temperature than conventional conditions of 650°C required to produce highly resistive buffer layers and molecular-layer-level abrupt heterojunctions and doped-layer interfaces. It has been widely recognized that growth at 550∼600°C causes poor morphological surfaces on the grown layers. This paper describes a fundamental improvement of the low-temperature growth of GaAs, resulting in smooth surfaces with very low impurity concentrations. Excellent GaAs layers can be grown at 500°C by increasing partial pressure of AsH3 during growth. The method has been applied to high quality epitaxial layers for electronic device production.

Surface Morphology Improvement and Repeatable Doping Characterization of 4H-SiC Epitaxy Grown on 4° Off-Axis 4H-SiC Wafers

4H Silicon Carbide (4H-SiC) has a great potential for low-loss power devices due to its superior electrical properties. However, the increase in demand for the power devices requires high quality SiC substrates and epitaxial layers. Mercury probe Capacitance Voltage (Hg CV) measurement is a well known procedure to characterize epi layers grown on SiC substrates, due to its non-destructive technique. However, careful calibration of the tool is very important for repeatable and accurate measurements. Here we present very close repeatability of Hg CV within 2.4% (standard deviation 0.7%), between different Solid State Measurements (SSM) setups compared with Ni Schottky (NiS) CV. In addition to growing uniformly doped epi layers, high surface quality of the epi layer is also needed for improved device performance. Improved process conditions resulted in a smooth epi with a surface roughness Ra 1.2 nm for a 6 µm thick epi layer. Molten Potassium Hydroxide (KOH) etching analysis also revealed a significant correlation between the surface roughness and epi defects.

Structure dependence of epitaxial Pr2O3∕Si(001) on oxygen pressure during growth

The authors study the structure dependence of epitaxial Pr2O3 layers on Si(001) on the partial pressure of oxygen during growth using reflection high-energy electron diffraction and x-ray reflectivity. The oxygen is necessary to suppress the formation of silicide, but it also inhibits the nucleation of the crystalline oxide phase. High oxygen partial pressure during growth causes the layers to be polycrystalline or even amorphous with hexagonal contributions. It also increases silicate formation at the interface between oxide layer and Si substrate. Thus, control of oxygen during growth is very important to obtain high quality epitaxial Pr2O3 layers on Si.