Epitaxial Template Research Articles

Nanoimprint lithography patterned GaAs templates for site-controlled InAs quantum dots

We report on the development of a soft UV-nanoimprint lithography (UV-NIL) based process to fabricate nanopatterned templates for epitaxy of site-controlled Stranski–Krastanov grown quantum dots. Surface cleaning and chemical native oxide removal processes are analyzed in detail. The quality of the surface is studied by means of scanning electron microscopy, atomic force microscopy and photoluminescence analysis. To evaluate the suitability of the process for fabricating high optical quality site-controlled quantum dots, we have investigated the photoluminescence signal of the near-surface quantum well and overgrown quantum dots. We present the successful growth of optically active, site-controlled InAs quantum dots, both in the ensemble and single-dot regime, by molecular beam epitaxy on UV-NIL patterned GaAs surfaces using only chemical cleaning prior to the growth.

(Invited) High-Mobility Ge on Insulator (GOI) by SiGe Mixing-Triggered Rapid-Melting-Growth

High-quality orientation-controlled Ge on insulator (GOI) structures are essential to realize high-performance thin-film transistors (TFTs) and epitaxial templates for multifunctional 3-dimensional large-scale integrated circuits (3D-LSIs). We have investigated the Si-seeding rapid-melting process and demonstrated formation of giant Ge stripes with (100), (110), and (111) orientations on Si (100), (110), and (111) substrates, respectively, covered with SiO2 films. We revealed that crystallization is triggered by Si-Ge mixing in the seeding regions in this process. Based on this mechanism, we have proposed a novel technique to realize orientation-controlled Ge layers on transparent insulating substrates by using Si artificial micro-seeds with (100) and (111)-orientations. This achieved epitaxial growth of single crystalline (100) and (111)-oriented Ge stripes on quartz substrates. The transmission electron microscopy observations revealed no-defects in the laterally grown Ge regions. The Ge layers showed a high hole mobility exceeding 1100 cm2/Vs owing to the high crystallinity. This novel SiGe mixing-triggered growth technique opens up the possibility of the next-generation TFTs and multifunctional 3D-LSIs.

Demonstration of negative differential resistance in GaN/AlN resonant tunneling diodes at room temperature

GaN/AlN resonant tunneling diodes (RTD) were grown by metal-organic chemical vapor deposition (MOCVD) and negative differential resistance with peak-to-valley ratios as high as 2.15 at room temperature was demonstrated. Effect of material quality on RTDs’ performance was investigated by growing RTD structures on AlN, GaN, and lateral epitaxial overgrowth GaN templates. Our results reveal that negative differential resistance characteristics of RTDs are very sensitive to material quality (such as surface roughness) and MOCVD is a suitable technique for III-nitride-based quantum devices.

Measuring the Structure of Epitaxially Assembled Block Copolymer Domains with Soft X-ray Diffraction

The size, shape, and interface structure of poly(styrene-b-methyl methacrylate) block copolymer domains assembled on an epitaxial template are characterized with soft X-ray diffraction. The domain size and shape are deformed when the dimensions of the epitaxial template are incommensurate with the equilibrium dimensions of the block copolymer, producing sidewall angles in the range of 1−2° (±0.2°). The average width of the copolymer interface is (4.9 ± 0.1) nm. Comparison with mean-field theoretic predictions for the structure of block copolymer interfaces suggests a low-frequency variance in the copolymer interface position of 1.2 nm2, or a low-frequency line-edge roughness of approximately 3 nm.

Growth of Epitaxial (110) 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 Thin Films on r-Plane Sapphire Substrates by RF Magnetron Sputtering

The role of substrate temperature and substrate surface geometry in determining the crystal structure and crystallinity of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) thin films grown on r-plane sapphire substrates is examined. A 30-nm-thick amorphous PMN-PT seed layer deposited at 250°C and subjected to rapid thermal annealing at 850°C results in the formation of an epitaxial (110) perovskite PMN-PT growth template that can be used for subsequent growth of single-crystal (110) perovskite PMN-PT films at elevated temperature. The data show that single-crystal perovskite is promoted when the films nucleate with the \( \langle \overline{1} 11 \rangle \) PMN-PT direction parallel to the \( \langle 0\bar{2}21 \rangle \) Al2O3 direction.

Freestanding high quality GaN substrate by associated GaN nanorods self-separated hydride vapor-phase epitaxy

This work proposes a method for fabricating 2 in. freestanding GaN substrates of high crystallographic quality and low residual strain. Arrays of GaN nanorods with sidewalls coated with silicon dioxide (SiO2) were randomly arranged on the sapphire substrate as a growth template for subsequent hydride vapor-phase epitaxy (HVPE). The passivation of the sidewalls coated with SiO2 prevents the coalescence of GaN grains in spaces between the rods, causing them to grow preferentially on the top of individual rods. The proposed method significantly improves GaN crystal quality and results in self-separation from the underlying host sapphire substrate due to the relaxation of thermal strains in the HVPE cooling-down process.

Tuning the termination of the SrTiO3(110) surface by Ar+ sputtering

We report a scanning tunneling microscopy study on the SrTiO3(110) surface treated with Ar+ sputtering followed by annealing. Two types of termination coexist on the surface, which are spatially identified as the 4×1 reconstructed SrTiO layer and the O layer covered by Ti-rich oxide clusters, respectively. The relative areal ratio of the two types is tuned by sputtering dose reproducibly, and monophased surface with either SrTiO or O termination is obtained. The surface is stable at temperatures up to 1100 °C and under oxygen partial pressures from 6×10−5 mbar to ultra high vacuum, providing us a flexible epitaxial growth template.

3C-SiC Films Grown on Si(111) Substrates as a Template for Graphene Epitaxy

High quality (111) oriented SiC films were grown Si(111 substrates using a low temperature halogen-based process. The goal of this work was to produce SiC films with a high crystal quality and very low surface roughness for subsequent epitaxial growth of graphene films. SiC films with narrow x-ray diffraction peak widths and surface roughnesses less than 1nm were grown while avoiding common processing defects such as voids at the substrate-film interface.

3C-SiC Films Grown on Si(111) Substrates as a Template for Graphene Epitaxy.

not Available.

Structural characterization of two-step growth of epitaxial ZnO films on sapphire substrates at low temperatures

We have investigated two-step growth of high-quality epitaxial ZnO films, where the first layer—the buffer layer (nucleation layer template)—is grown at a low temperature (230–290 °C) to induce a smooth (two-dimensional) growth. This is followed by growth at a moderate temperature ∼430 °C to form high-quality smooth ZnO layers for device structures. It was possible to reduce the growth temperature to 250–290 °C and obtain a smooth epitaxial template layer on sapphire (0 0 0 1) substrates with surface roughness less than 1 nm. After the high-temperature growth, the film surface undulations (roughness) increased to about 2 nm, but it is still quite smooth. The calculation of c and a lattice parameters by high-resolution x-ray diffraction shows that the a lattice parameter is fully relaxed at the growth temperatures but the c lattice parameter is dependent on the defect concentration in the growing film. A decoupling between a and c lattice parameters of the films is observed, which leads to abnormal Poisson's ratios ranging from 0.08 to 0.54. The decoupling of the lattice parameters is analysed based on growth characteristics and the presence of strain and defects in the grown films. We present our detailed studies on the nature of epitaxy, defects and interfaces by using comprehensive x-ray diffraction and high-resolution TEM studies.

Controllable Epitaxial Crystallization and Reversible Oriented Patterning of Two-Dimensional Colloidal Crystals

We demonstrate a reliable and highly efficient epitaxial templating approach for the formation of two-dimensional (2D) colloidal crystals. By applying an alternating electric field (AEF), one-dimensional colloidal lines are used as an epitaxial template to site specifically initiate 2D colloidal crystallization and control the orientation of the 2D colloidal crystals. The kinetics of the crystallization and structure ordering is precisely and conveniently manipulated by the external AEF. The well-defined artificial linear defects are embedded inside the 2D colloidal crystals by means of heteroepitaxy, whereas the unwanted existing defects are controllably relaxed via an electrically induced annealing process. This epitaxial templating approach is fast, reversible, and amenable for large-area oriented patterning of colloidal crystals, providing a new way for the creation of novel materials and devices with functions and properties that can be reversibly changed, such as electrically tunable photonic waveguides and e-papers.

Growth of one-dimensional III–V structures on Si nanowires and pre-treated planar Si surfaces

This paper presents a technique that allows the growth of epitaxial GaAs nanowires that are aligned to the crystal directions of the Si substrate. Low-pressure chemical vapor deposition (LP-CVD) grown Si nanowires were used as templates for molecular beam epitaxy (MBE) growth. The growth direction of the nanowires aligns with the [1 1 1] direction perpendicular to the Si substrate. After deposition of 200 nm GaAs in a solid source MBE system, we observed the formation of GaAs whiskers perpendicular to the {1 1 2} sidewalls of Si nanowires. By TEM analysis, the crystal structure of the GaAs nanostructures could be identified as wurtzite, where the growth axis of the wires was in the [0 0 0 1] direction. The whiskers are of good crystalline quality as no dislocations or stacking faults were observed in large areas by TEM, which makes these structures potential candidates for III–V integration on Si. In parallel to these experiments, we found a growth technique that allows GaAs nanowires to grow along the [1 1 1] direction of the planar Si [1 1 2] substrates. III–V nanowires with comparable geometry but higher density could be realized.

Synthesis and microstructure of vertically aligned ZnO nanowires grown by high-pressure-assisted pulsed-laser deposition

The microstructure and growth behavior for vertically aligned Zinc oxide (ZnO) nanowires, synthesized on a ZnO thin film template by pulsed-laser deposition (PLD), is reported. The nanowire growth proceeds without any metal catalyst for nucleation, although an epitaxial ZnO thin film template is necessary in order to achieve uniform alignment. Nanowire growth at argon or oxygen background pressures of 500-mTorr results in nanowire diameters as small as 50–90 nm, with diameters largely determined by growth pressure and temperature. Room temperature photoluminescence show both near-band-edge and deep-level emission. The deep-level emission is believed caused by oxygen vancancies formed during growth.

Growth of highly tensile-strained Ge on relaxed InxGa1−xAs by metal-organic chemical vapor deposition

Highly tensile-strained Ge thin films and quantum dots have the potential to be implemented for high mobility metal-oxide-semiconductor field-effect transistor channels and long-wavelength optoelectronic devices. To obtain large tensile strain, Ge has to be epitaxially grown on a material with a larger lattice constant. We report on the growth of tensile-strained Ge on relaxed InxGa1−xAs epitaxial templates by metal-organic chemical vapor deposition. To investigate the methods to achieve high quality Ge epitaxy on III–V semiconductor surfaces, we studied Ge growth on GaAs with variable surface stoichiometry by employing different surface preparation processes. Surfaces with high Ga-to-As ratio are found to be necessary to initiate defect-free Ge epitaxy on GaAs. With proper surface preparation, tensile-strained Ge was grown on relaxed InxGa1−xAs with a range of In content. Low growth temperatures between 350 and 500 °C suppress misfit dislocation formation and strain relaxation. Planar Ge thin films with tensile strain as high as 0.5% were fabricated on relaxed In0.11Ga0.89As. For relatively high In-content (x>0.2) InxGa1−xAs templates, we observed an islanded growth morphology forming tensile-strained Ge quantum dots. Tensile strain as high as 1.37% was measured in these Ge quantum dots grown on In0.21Ga0.79As. The ability to grow these structures will enable us to further study the electronic and optoelectronic properties of tensile-strained Ge.

Future technology pathways of terrestrial III–V multijunction solar cells for concentrator photovoltaic systems

Future terrestrial concentrator cells will likely feature four or more junctions. The better division of the solar spectrum and the lower current densities in these new multijunction cells reduce the resistive power loss ( I 2 R) and provide a significant advantage in achieving higher efficiencies of 45–50%. The component subcells of these concentrator cells will likely utilize new technology pathways such as highly metamorphic materials, inverted crystal growth, direct-wafer bonding, and their combinations to achieve the desired bandgaps while maintaining excellent device material quality for optimal solar energy conversion. Here, we report preliminary results of two technical approaches: (1) metamorphic ∼1 eV GaInAs subcells in conjunction with an inverted growth approach and (2) multijunction cells on wafer-bonded, layer-transferred epitaxial templates.

Growth and properties of BiFeO 3 thin films deposited on LaNiO 3-buffered SrTiO 3(0 0 1) and (1 1 1) substrates by PLD

BiFeO 3 (BFO) films have been epitaxially grown on SrTiO 3(0 0 1) and SrTiO 3(1 1 1) substrates with a LaNiO 3 conductive layer as a template using pulsed laser deposition. X-ray diffraction θ–2 θ and Φ scanning revealed that the epitaxial relationship is BFO[1 0 0]//LNO[1 0 0]//STO[1 0 0]; BFO(0 0 1)//LNO(0 0 1)//STO(0 0 1) and BFO [ 11 2 ¯ ] / / LNO [ 11 2 ¯ ] / / STO [ 11 2 ¯ ] ; BFO(1 1 1)//LNO(1 1 1)//STO(1 1 1) on SrTiO 3(0 0 1) and SrTiO 3(1 1 1) substrates, respectively. The epitaxial relationship was also confirmed by ex situ reflective high-energy electron reflection (RHEED). From P– E measurements, both samples showed typical ferroelectric hysteresis loops, indicating good electrical properties. Furthermore, it was found that the remanent polarization (2 P r) under the same applied electrical field of the epitaxial BFO(1 1 1) thin film on the LNO-coated (1 1 1) STO substrate is much larger than that of the BFO(1 0 0) film on the LNO-coated STO(1 0 0) substrate. The larger polarization of BFO(1 1 1) films may be caused by that the easy polarization axis lays close to [1 1 1] direction, indicating the anisotropic ferroelectric property for BFO films. Thus, with the epitaxial LNO electrode template layers, the epitaxial BFO films exhibit high-quality structure and good electrical properties.

Ga In P ∕ Ga As dual junction solar cells on Ge∕Si epitaxial templates

Large area, crack-free GaInP∕GaAs double junction solar cells were grown by metal organic chemical vapor deposition on Ge∕Si templates fabricated using wafer bonding and ion implantation induced layer transfer. Photovoltaic performance of these devices was comparable to those grown on bulk epi-ready Ge, demonstrating the feasibility of alternative substrates fabricated via wafer bonding and layer transfer for growth of active devices on lattice mismatched substrates.

Epitaxial assembly and ordering of two-dimensional colloidal crystals

An epitaxial assembly method is described to direct the crystallization of two-dimensional colloidal crystals under an alternating electric field (AEF). By using a colloidal line as an epitaxial template, the colloidal crystals with a predefined orientation have been assembled at specified position of an electrode. The epitaxial correlation between the colloidal crystals and the template can be tuned by varying the frequency of the AEF. By control of the template and the frequency, well-defined linear defects have been incorporated into the colloidal crystals, whereas the unwanted defects can be in situ eliminated through a template-guided annealing process.

InGaN/GaN multi-quantum well and LED growth on wafer-bonded sapphire-on-polycrystalline AlN substrates by metalorganic chemical vapor deposition

We report growth of InGaN/GaN multi-quantum well (MQW) and LED structures on a novel composite substrate designed to eliminate the coefficient of thermal expansion (CTE) mismatch problems which impact GaN growth on bulk sapphire. To form the composite substrate, a thin sapphire layer is wafer-bonded to a polycrystalline aluminum nitride (P-AlN) support substrate. The sapphire layer provides the epitaxial template for the growth; however, the thermo-mechanical properties of the composite substrate are determined by the P-AlN. Using these substrates, thermal stresses associated with temperature changes during growth should be reduced an order of magnitude compared to films grown on bulk sapphire, based on published CTE data. In order to test the suitability of the substrates for GaN LED growth, test structures were grown by metalorganic chemical vapor deposition (MOCVD) using standard process conditions for GaN growth on sapphire. Bulk sapphire substrates were included as control samples in all growth runs. In situ reflectance monitoring was used to compare the growth dynamics for the different substrates. The material quality of the films as judged by X-ray diffraction (XRD), photoluminescence and transmission electron microscopy (TEM) was similar for the composite substrate and the sapphire control samples. Electroluminescence was obtained from the LED structure grown on a P-AlN composite substrate, with a similar peak wavelength and peak width to the control samples. XRD and Raman spectroscopy results confirm that the residual strain in GaN films grown on the composite substrates is dramatically reduced compared to growth on bulk sapphire substrates.

Use of ZnO thin films as sacrificial templates for metal organic vapor phase epitaxy and chemical lift-off of GaN

Continued development of GaN-based light emitting diodes is being hampered by constraints imposed by current non-native substrates. ZnO is a promising alternative substrate but it decomposes under the conditions used in conventional GaN metal organic vapor phase epitaxy (MOVPE). In this work, GaN was grown on ZnO∕c-Al2O3 using low temperature/pressure MOVPE with N2 as a carrier and dimethylhydrazine as a N source. Characterization confirmed the epitaxial growth of GaN. The GaN was lifted-off the c-Al2O3 by chemically etching away the ZnO underlayer. This approach opens up the way for bonding of the GaN onto a support of choice.