High Temperature Epitaxial Growth Research Articles

Selective laser melting of 2507 duplex stainless steel: Effect of energy density on microstructure and corrosion resistance

Duplex stainless steel (DSS) is widely used in the marine, petroleum, chemical, automotive, and other fields owing to its excellent mechanical properties and corrosion resistance. However, the research on duplex stainless steel prepared by additive manufacturing is still limited. In this paper, high-density 2507 DSS was successfully prepared by selective laser melting (SLM) additive manufacturing. The effects of energy density on the formability, phase composition, microstructure and corrosion properties of SLM 2507 DSS were investigated. The results showed that with the decrease of the energy density, the density of the specimen increases first and then decreases, and the density achieves 99.18% with the energy density of 190.5 J/mm3. The types of phases are not affected by the energy density, i.e., all 2507 DSS samples prepared by SLM showed a ferrite phase. The YOZ (parallel to the building direction) plane of the SLM 2507 DSS samples showed predominantly columnar grains attributed to the high temperature gradient and epitaxial growth characteristics. With the increase of energy density, the average grain size decreases slightly from 16.97 μm to 15.78 μm, the KAM value decreases slightly from 1.15 to 1.05, and the low angle grain boundaries (LAGBs) increase significantly from 68.4% to 74.8%. The SLM 2507 DSS sample exhibited excellent corrosion resistance. The self-corrosion potential of the sample is 136 mV and the self-corrosion current density is 2.066 × 10−8 A/cm2 at the maximum density. This investigation provides a new approach for the preparation of super duplex stainless steel, which can provide a theoretical basis and guidance for industrialized application.

Shortwave Infrared Photodetection with oCVD Polythiophene.

Shortwave infrared (SWIR, λ = 1-3µm) photodetectors typically use compound semiconductors that are fabricated using high-temperature epitaxial growth and require active cooling. New technologies that overcome these constraints are the focus of intensive current research. Herein, oxidative chemical vapor deposition (oCVD) is used for the first time to create a room temperature, vapor-phase deposited SWIR photoconductive detector with a unique tangled wire film morphology that detects nW-level photons emitted from a 500°C cavity blackbody radiator-a rarity for polymer systems. A new, window-based process that greatly simplifies device fabrication is used to construct doped polythiophene-based SWIR sensors. The detectors feature an 8.97 kΩ dark resistance and are limited by 1/f noise. They feature an external quantum efficiency (gain-external quantum efficiency) product of 395% and have a measured specific detectivity (D*) of 106 Jones, with the potential to reach D* = 1010 Jones after 1/f noise is minimized. Still, the measured D* is only a factor of 102 lower than a typical microbolometer and after optimization, the newly described oCVD polymer-based IR detectors will be in a category competitive with commercially available, room temperature lead salt photoconductors and within reach of room temperature photodiodes.

All-solution-processed perovskite/gallium nitride particles hybrid visible-blind ultraviolet photodetectors

Last decades have witnessed the rapid development of ultraviolet (UV) photodetectors in diversity of applications. The III-nitride semiconductor and metal halide perovskite have both performed promising UV-sensing optoelectronic properties. However, they are still suffering from either the high temperature epitaxial-growth or low photocurrent generated in UV range. In this work, we demonstrate an innovative MAPbCl3/GaN particle hybrid device with all-solution-processed deposition methods. Comparing to the control MAPbCl3 photoconductors, the photo-sensing ability of the hybrid device with the optimal concentration of GaN particles is more than one order of magnitude enhanced, and report a responsivity of 86 mA W−1, a detectivity of 3.1 × 1011 Jones and a rise/fall time of 1.1/10.7 ms at 360 nm. The photocurrent increment could be attributed to the enhanced UV absorption of GaN particles and facilitated charge separation and photoconductive gain at MAPbCl3/GaN heterojunction. This work paves a pathway towards the large-scale low-cost UV photodetectors in versatile applications.

Growth time experiments highlight potential for single crystal diamond film optoelectronics

High pressure high temperature epitaxial growth experiments highlight the importance of pretreatment and point to silicon integration as a route for optoelectronic applications.

Theoretical analysis of induction heating in high-temperature epitaxial growth system

The temperature uniformity and heating efficiency in a high-temperature epitaxial growth system were investigated by modeling and simulating. The finite element method (FEM) was used to calculate the distribution of magnetic field and temperature field in the reactor of growth system. The simulation results showed that due to the skin effect and heat conduction in the conventional susceptor, the temperature distribution of the wafer on the susceptor is not uniform. However, the temperature uniformity of the wafer can be greatly improved by adding an air gap below the wafer. The existence of air gap effectively reduced the temperature on the center of the wafer, and the effect of its radius on temperature uniformity was studied. By calculating different frequencies and coil currents, the temperature and heating rate in the reactor with modified susceptor were investigated. The results indicated that higher temperature and faster heating rate can be obtained by increasing heating frequency and coil current. However, both lower and higher frequency will bring worse temperature uniformity.

Tailoring Perovskite Thin Films to Rival Single Crystals

High temperature epitaxial growth has been a long-standing requirement for forming semiconductor thin films with exceptional optoelectronic properties. In this issue of Joule , Brenes and coworkers have demonstrated that atmospheric and photoinduced post-treatments of solution-processed polycrystalline metal halide perovskites can improve the diffusion length, carrier lifetime, and surface recombination velocity to values previously only achievable by single crystal materials.

Photovoltage field-effect transistors.

The detection of infrared radiation enables night vision, health monitoring, optical communications and three-dimensional object recognition. Silicon is widely used in modern electronics, but its electronic bandgap prevents the detection of light at wavelengths longer than about 1,100 nanometres. It is therefore of interest to extend the performance of silicon photodetectors into the infrared spectrum, beyond the bandgap of silicon. Here we demonstrate a photovoltage field-effect transistor that uses silicon for charge transport, but is also sensitive to infrared light owing to the use of a quantum dot light absorber. The photovoltage generated at the interface between the silicon and the quantum dot, combined with the high transconductance provided by the silicon device, leads to high gain (more than 104 electrons per photon at 1,500 nanometres), fast time response (less than 10 microseconds) and a widely tunable spectral response. Our photovoltage field-effect transistor has a responsivity that is five orders of magnitude higher at a wavelength of 1,500 nanometres than that of previous infrared-sensitized silicon detectors. The sensitization is achieved using a room-temperature solution process and does not rely on traditional high-temperature epitaxial growth of semiconductors (such as is used for germanium and III-V semiconductors). Our results show that colloidal quantum dots can be used as an efficient platform for silicon-based infrared detection, competitive with state-of-the-art epitaxial semiconductors.

Nucleation of two-dimensional islands on Si (111) during high-temperature epitaxial growth

The process of two-dimensional island nucleation at the surface of ultra large Si (111) during hightemperature epitaxial growth is studied by in situ ultrahigh-vacuum reflection electron microscopy. The critical terrace size D crit, at which a two-dimensional island is nucleated in the center, is measured in the temperature range 900–1180°C at different silicon fluxes onto the surface. It is found that the parameter D crit 2 is a power function of the frequency of island nucleation, with the exponent χ = 0.9 ± 0.05 in the entire temperature range under study. It is established that the kinetics of nucleus formation is defined by the diffusion of adsorbed silicon atoms at temperatures of up to 1180°C and the minimum critical nucleus size corresponds to 12 silicon atoms.

Graphene Epitaxial Growth on SiC(0001) for Resistance Standards

A well-controlled technique for high-temperature epitaxial growth on 6H-SiC(0001) substrates is shown to allow the development of monolayer graphene that exhibits promise for precise metrological applications. Face-to-face and face-to-graphite annealing in a graphite-lined furnace at 1200 °C-2000 °C with a 101-kPa Ar background gas lowers the rates of SiC decomposition and Si sublimation/diffusion and thus provides a means to control the rate of graphene layer development. We studied a wide range of growth temperatures and times and describe the resulting sample surface morphology changes and graphene layer structures. The experimental results are compared to a kinetic model based on two diffusion processes: Si vapor diffusion in the Ar-filled gap and atomic diffusion through graphitic surface layers.

Two-step growth of high quality Bi2Te3 thin films on Al2O3 (0001) by molecular beam epitaxy

Large-area topological insulator Bi2Te3 thin films were grown on Al2O3 (0001) using a two-temperature step molecular beam epitaxy growth process. By depositing a low temperature nucleation layer to serve as a template for high temperature epitaxial film growth, a high quality terrace-step surface morphology with a significant reduction in three-dimensional defect structures was achieved. X-ray diffraction measurements indicate that high crystalline quality Bi2Te3 layers were grown incoherently by van der Waals epitaxy using this technique. Angle resolved photoemission spectroscopy measurements verified the integrity of this growth method by confirming the presence of metallic surface states on cleaved two-step Bi2Te3 samples.

Vertical Blocking Voltage Improvement of GaN HEMT Structures on n-SiC by Pre-Epitaxial Substrate Implantation

To overcome the use of expensive semi-insulating SiC substrates, we developed a method to increase the vertical blocking strength of n-SiC by argon implantation prior to the high temperature epitaxial growth (1150°C) of the (Al)GaN layers. We superimpose 4 argon implantation energies (30 keV + 60 keV + 180 keV + 360 keV) to a total maximal concentration of 1·1018 atoms/cm3 and observed a 90 V - 100 V increase of the vertical blocking strength by this method. Implantation profile and dose dependencies are studied showing that the results could be related to an increased substrate resistance achieved by implantation. HRXRD analysis shows that the defect density in the GaN buffer is not increased by substrate implantation.

Achieve high-quality GaN films on La0.3Sr1.7AlTaO6 (LSAT) substrates by low-temperature molecular beam epitaxy

The growth of c-plane GaN films on La0.3Sr1.7AlTaO6 (LSAT) (111) substrates has been carried out by molecular beam epitaxy (MBE) at various substrate temperatures. GaN grown at 750 °C shows poor crystallinity with an up to 8 nm thick interim layer between LSAT and GaN, mainly due to the severe interfacial reaction between the substrate and the film. On the contrary, an atomically abrupt interface has been observed when GaN was grown at a lower temperature of 500 °C. This low-temperature grown GaN exhibits high crystallinity with a very smooth surface and strong band-edge emission, which we attribute to the well-controlled oxygen atom evaporation from LSAT substrate surface thanks to the low temperature growth. On the one hand, it avoids the severe interfacial reaction that takes place in the case of high-temperature epitaxial growth; on the other hand, it conserves the crystalline structure of the substrate surface, and consequently takes good advantage of the small lattice mismatch between GaN and LSAT. Considering the advantages of LSAT over sapphire, and the device-ready film quality of GaN achieved on LSAT, this work exhibits a promising future for high-performance light emitting diodes on this unconventional substrate.

Vapor-phase epitaxial growth of GaN films using Ga2O vapor and NH3

High-temperature epitaxial growth (>1200°C) of GaN using Ga2O vapor and NH3 was performed to increase the crystal growth rate and improve crystal qualities such as surface morphology, crystallinity and oxygen incorporation. Results showed that the growth rate linearly increased with the increasing partial pressure of Ga2O, PGa2O, while the surface morphology and crystallinity degraded, and oxygen concentration in the epilayers increased. When the growth temperature increased, smooth GaN epilayers without the degradation of crystallinity could be grown even at high PGa2O, i.e. a high growth rate. In addition, the oxygen concentration decreased as the growth temperature was increased.

Growth temperature dependence of strain in a GaN epilayer, grown on a c-plane sapphire substrate

Despite the large mismatch in the lattice constants and thermal-expansion coefficients between the epilayer and the sapphire substrate, GaN epilayer is commonly grown on a c-plane sapphire substrate due to the lack of other suitable substrates [1–3]. The GaN epilayer on a c-plane sapphire substrate is widely grown with the use of the twostep metalorganic chemical vapor deposition (MOCVD), employing an AlN [4] or a low-temperature GaN buffer layer [5]. It was reported that a biaxial strain and a hydrostatic strain could coexist in the GaN epilayer [6, 7]. In GaN, the biaxial stress is due to thermal mismatch stress and growth [6–10]. The growth stress can be caused by lattice mismatch, island coalescence, grain growth, or surface stress [8–10]. Some in-situ measurements on the growth stress have been reported during MOCVD of GaN [8–10]. The internal hydrostatic strain was shown to have been introduced by the presence of point defects, which can be compressive or expansive depending on their size [6, 7]. A study was reported on a high-resolution X-ray diffraction (HRXRD) strain–stress analysis of a GaN/sapphire heterostructure grown through molecular beam epitaxy (MBE), particularly in the deformation state, depending on the relative content of N in the Ga1-xNx buffer layer by Harutyunyan et al. [11]. In the authors previous study [12], the strain analysis of a GaN epilayer with different growth times on a c-plane sapphire substrate via a two-step growth, using low-pressure, metalorganic chemical vapor deposition was conducted on the basis of precise measurement of the lattice parameters, using HRXRD. In this continuing study, the growth temperature dependence of the residual strain in a GaN epilayer at room temperature on a c-plane sapphire was studied. The final growth temperature in a fixed two-step MOCVD was varied from 850 to 1,050 C. The cand a-lattice parameters were measured using HRXRD, followed by the out-of-plane and in-plane strains. We have extracted the levels of biaxial and hydrostatic components of strain in the GaN films on sapphire. An undoped GaN epilayer was grown on a c-plane sapphire substrate in a horizontal MOCVD reactor at a low pressure of 300 Torr, as described in the authors’ previous articles [12, 13]. We employed a two-step growth method of GaN—consisting of the deposition of the low-temperature GaN buffer layer and high-temperature GaN epitaxial growth. The growth temperature of the high-temperature GaN was varied from 850 to 1,050 C so that the temperature dependence of the strain of the GaN epilayer induced after the two-step MOCVD growth process could be studied. The V/III ratios were 2,740 and 1,370 for buffer growth and the main high-temperature growth, respectively [12]. The growth rate was fixed at 0.07 lm/min; as such, the thickness of the GaN layer that was grown for 20 min was estimated to be 1.5 lm. The c-lattice and a-lattice parameters of the GaN epilayers were measured with a x-2h scan of the wurtzite GaN epilayer through triple-axis, HRXRD, to obtain precise measurements (PANalytical X’Pert PRO MRD), as described in the authors’ previous article [12]. S. I. Cho K. Chang Department of Chemical Engineering, University of Seoul, 90 Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, South Korea

Thin-film polycrystalline-silicon solar cells on high-temperature glass based on aluminum-induced crystallization of amorphous silicon

AbstractEfficient thin-film polycrystalline-silicon (pc-Si) solar cells on foreign substrates could lower the price of photovoltaic electricity. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening at high temperatures seems a good way to obtain efficient pc-Si solar cells. Due to its transparency and low cost, glass is well suited as substrate for pc-Si cells. However, most glass substrates do not withstand temperatures around 1000°C that are needed for high-temperature epitaxial growth. In this paper we investigate the use of experimental transparent glass-ceramics with high strain-point temperatures as substrates for pc-Si solar cells. AIC seed layers made on these substrates showed in-plane grain sizes up to 16 μm. Columnar growth was observed on these seed layers during high-temperature epitaxy. First pc-Si solar cells made on glass-ceramic substrates in substrate configuration showed efficiencies up to 4.5%, fill factors up to 75% and open-circuit voltage (Voc) values up to 536 mV. This is the highest Voc reported for pc-Si solar cells on glass and the best cell efficiency reported for cells made by AIC on glass.

Review of SiGe HBTs on SOI

This paper reviews progress in SiGe Heterojunction Bipolar Transistors (HBT) on Silicon-On-Insulator (SOI) technology. SiGe HBTs on SOI are attractive for mixed signal radio frequency (RF) applications and have been of increasing research interest due to their compatibility with SOI CMOS (Complementary Metal Oxide Semiconductor) technology. In bipolar technology, the use of SOI substrate eliminates parasitic substrate transistors and associated latch-up, and has the ability to reduce crosstalk, particularly when combined with buried groundplanes (GP). Various technological SOI bipolar concepts are reviewed with special emphasis on the state-of-the-art SOI SiGe HBT devices in vertical and lateral design. More in depth results are shown from a UK consortium advanced RF platform technology, which includes SOI SiGe HBTs. Bonded wafer technology was developed to allow incorporation of buried silicide layers both above and below the buried oxide. New electrical and noise characterisation results pointed to reduced 1/ f noise in these devices compared to bulk counterparts. The lower noise is purported to arise from strain relief of the device structure due to the elasticity of the buried oxide layer during the high temperature epitaxial layer growth. The novel concept of the silicide SOI (SSOI) SiGe HBT technology developed for targeting a reduction in collector resistance, as well as for suppressing the crosstalk, is outlined. The buried tungsten silicide layers were found to have negligible impact on junction leakage. Further to vertical SiGe HBTs on SOI, the challenges of fabricating a lateral SOI SiGe HBT structure are presented.

Vapor phase techniques for the fabrication of homoepitaxial layers of silicon carbide: process modeling and characterization

High temperature epitaxial growth processes for SiC bulk and thin films are reviewed from an academic point of view using heat and mass transfer modeling and simulation. The objective is to show that this modeling approach could provide further information to fabrication and characterization for the improvement of the knowledge of the growth history and to quantify the different phenomena leading to growth. Recent results of our integrated research program on SiC taking into account the fabrication, process modeling and characterization will be presented.

Nonpolar (11&1macr;0)a-Plane Gallium Nitride Thin Films Grown on (1&1macr;02)r-Plane Sapphire: Heteroepitaxy and Lateral Overgrowth

Nonpolar (110) a-plane GaN films with planar surfaces were grown on (102) r-plane sapphire substrates via metalorganic chemical vapor deposition by depositing a low temperature nucleation layer prior to the high temperature epitaxial growth. The film/substrate epitaxial relationship was defined by X-ray diffraction measurements to be [0001]GaN ∥ [101]sapphire and [100]GaN ∥ [110]sapphire while the polarity of the GaN c-axis was confirmed using convergent beam electron diffraction. The film morphology and microstructure were studied using atomic force microscopy and transmission electron microscopy (TEM), respectively. Threading dislocations (TDs), which dominated the heteroepitaxial a-plane GaN microstructure (density ∼2.6 × 1010 cm—2), were reduced via lateral epitaxial overgrowth from mask stripe openings. The overgrowth from stripes aligned along [100] was dislocation-free, as observed using TEM and cathodoluminescence, and dependent upon crystal polarity. For all other stripe orientations, TDs were observed to propagate into the overgrown regions.

Structural characterization of nonpolar (112̄0) a-plane GaN thin films grown on (11̄02) r-plane sapphire

In this letter we describe the structural characteristics of nonpolar (112̄0) a-plane GaN thin films grown on (11̄02) r-plane sapphire substrates via metalorganic chemical vapor deposition. Planar growth surfaces have been achieved and the potential for device-quality layers realized by depositing a low temperature nucleation layer prior to high temperature epitaxial growth. The in-plane orientation of the GaN with respect to the r-plane sapphire substrate was confirmed to be [0001]GaN‖[1̄101]sapphire and [1̄100]GaN‖[112̄0]sapphire. This relationship is explicitly defined since the polarity of the a-GaN films was determined using convergent beam electron diffraction. Threading dislocations and stacking faults, observed in plan-view and cross-sectional transmission electron microscope images, dominated the a-GaN microstructure with densities of 2.6×1010 cm−2 and 3.8×105 cm−1, respectively. Submicron pits and crystallographic terraces were observed on the optically specular a-GaN surface with atomic force microscopy.

Acceptor diffusion and segregation in (AlxGa1−x)0.5In0.5P heterostructures

Acceptor segregation is investigated as a function of compositional difference, Δx, between adjacent layers in (AlxGa1−x)0.5In0.5P heterostructures. Magnesium, Zn, Be, and Mn acceptor species are all shown to segregate out of the high band gap Al-rich (AlxGa1−x)0.5In0.5P layers and into the low band gap Al-poor (AlxGa1−x)0.5In0.5P layers during high temperature epitaxial growth of such heterostructures. The observed acceptor segregation appears to be independent of growth method or dopant incorporation method (metalorganic chemical vapor deposition, gas source molecular-beam epitaxy, or ion implantation), and increases with increasing compositional difference between adjacent (AlxGa1−x)0.5In0.5P layers. A theoretical model is developed to describe acceptor segregation based on charge separation and the resulting electric field across the heterointerface, and the resulting acceptor segregation is shown to vary as (mh AL*/mh CL*)3/2 exp(ΔEV/kT) where mh AL* and mh CL* are the hole effective masses in the active layer and confining layer, and ΔEV is the valence band offset. Comparison between experimentally measured and theoretically predicted acceptor segregation ratios gives excellent agreement for (AlxGa1−x)0.5In0.5P heterostructures over the range of compositional differences from Δx=0.12 to Δx=0.93.