Epitaxial Process Research Articles

Investigation on band alignment of Bi2Se3–PbSe heterojunction

In this Letter, in situ growth of the Bi2Se3–PbSe heterojunction was achieved by the molecular beam epitaxy process. An atomically abrupt heterointerface was observed. The band alignment configuration of the Bi2Se3–PbSe heterojunction was investigated by x-ray photoelectron spectroscopy. A broken (type-III) energy band discontinuity with the valence band offset of 0.4 eV at the heterointerface was determined. This result might broaden the application prospect of Bi2Se3 in the future development of optoelectronics.

Dopant Diffusion Equilibrium Overcoming Impurity Loss of Doped QDs for Multimode Anti‐Counterfeiting and Encryption

AbstractHigh ion diffusivity of Cu+ dopants accelerates outdiffusion in quantum dots (QDs) at high temperatures. This kind of self‐purification gives rise to a sharp decrease in dopant photoluminescence (PL) intensity. Herein, a dopant diffusion equilibrium strategy is developed by introducing extra Cu+ in the epitaxial growth process. By preventing the loss of dopant Cu+, both the decrease of dopant PL and increase of band‐edge PL intensity are successfully suppressed. The coexistence of visible and invisible near‐infrared (NIR) PL bands for Cu‐doped CdSe@CdS QDs demonstrates their novel multimode anti‐counterfeiting/encryption potential. Via printing by combined using of intrinsic and doped QD “inks” with visible and invisible NIR emissions, respectively, the bulk‐sized patterns, texts, and numerals can achieve visible–invisible PL synergetic confidential information multimode anti‐counterfeiting and encryption.

Nanoscale Dopant Profiling of Individual Semiconductor Wires by Capacitance-Voltage Measurement.

Developing nanoscale electrical characterization techniques adapted to three-dimensional (3D) geometry is essential for optimization of the epitaxial structure and doping process of nano- and microwires. In this paper, we demonstrate the assessment of the depletion width as well as the doping profile at the nanoscale of individual microwire core-shell light-emitting devices by capacitance-voltage measurements. A statistical study carried out on single wires shows the consistency of the doping profile values measured for individual microwires compared to assemblies of hundreds of wires processed on the same sample. The robustness of this method is then demonstrated on four epitaxial structures with different growth and doping conditions. Finally, electron-beam-induced current and secondary electron profiles are used to validate the depletion region width and the position in the core-shell structure.

Initial investigations into the MOS interface of freestanding 3C-SiC layers for device applications

This letter reports on initial investigation results on the material quality and device suitability of a homo-epitaxial 3C-SiC growth process. Atomic force microscopy surface investigations revealed root-mean square surface roughness levels of 163.21 nm, which was shown to be caused by pits (35 μm width and 450 nm depth) with a density of 1.09 × 105 cm−2 which had formed during material growth. On wider scan areas, the formation of these were seen to be caused by step bunching, revealing the need for further epitaxial process improvement. X-ray diffraction showed good average crystalline qualities with a full width of half-maximum of 160 arcseconds for the 3C-SiC (002) being lower than for the 3C-on-Si material (210 arcseconds). The analysis of C–V curves then revealed similar interface-trapped charge levels for freestanding 3C-SiC, 3C-SiC on Si and 4H-SiC, with forming gas post-deposition annealed freestanding 3C-SiC devices showing D IT levels of 3.3 × 1011 cm−2 eV−1 at E C−E T = 0.2 eV. The homo-epitaxially grown 3C-SiC material’s suitability for MOS applications could also be confirmed by leakage current measurements.

Comparison of high-order silanes and island formation phenomena during SiGe epitaxy at 500 °C

High-order silane precursors, including trisilane, are candidates for the low-temperature epitaxy process owing to the low energy of Si–Si bond. Higher order silanes are regarded as being more reactive than lower order ones. We compared the SiGe epitaxial growth behaviors of high-order silane precursors on Si substrates in an ultra-high vacuum chemical vapor deposition chamber without a carrier gas. SiGe epitaxial layers with a thickness of 25 nm or more were grown using disilane, trisilane, or tetrasilane precursor at 500 °C. Interestingly, trisilane exhibited more severe island formation than tetrasilane, which has higher reactivity than trisilane, even at lower partial pressure. These islands were not eliminated by lowering pressure but could be suppressed by higher Ge content owing to enhanced surface diffusion.

Ultra-fast growth (up to 100 nm s−1) of heavily doped EuBa2Cu3O7 film with highly aligned BaHfO3 nanocolumn structure

Ultra-fast growth (up to 100 nm s−1) of high temperature superconducting film was demonstrated by using an advanced pulsed laser deposition technique. Highly textured EuBa2Cu3O7−δ (EuBCO) film with 8 mol.% of BaHfO3 (BHO) was deposited on the IBAD-based Hastelloy substrates. Structure characterizations reveal formation of high density of BHO nanocolumns with diameter of ∼5 nm in the film, which is well beyond the expectation. Comparison study on different amount of BHO in EBCO film confirmed that nanocolumn formation strongly depends on the dopant level. The epitaxial growth process of BHO is dominated by ultra-fast self-assembly associated with enhanced diffusion of high flux element and liquid phase. Due to the correlated pinning landscape, a pronounced broad peak appears at B//c in the J c(θ) curves at 30 K 5 T, while a strong pinning force of about 900 GN m−3 at 4.2 K, 10 T (B//c) are achieved.

Molecular dynamics simulation of cubic InxGa(1-x)N layers growth by molecular beam epitaxy

The epitaxial growth of cubic InxGa1-xN layers on GaN (001) buffer substrates is investigated using molecular dynamics simulation. The substrate temperature, flux ratio of In, Ga, and N, In concentration, and thermal annealing post-growth were simulated and studied. The dislocation, the critical thickness, and the incorporation of the hexagonal phase into the cubic structure for InxGa1-xN are discussed in detail. Theoretical model conditions were simulated by Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) free-source code; the simulated growths reproduce the experimental thermodynamic conditions for molecular beam epitaxy process during the nucleation and first layers deposited. We consider the Stillinger-Weber In-Ga-N-system potentials. We apply time-and-position-dependent boundary constraints that vary the ensemble treatments of the near-surface solid-phase and the bulk-like regions of the growing layer. The results demonstrate that the simulations are suitable for reproducing the experimental epitaxial growths of cubic phase InGaN alloys with In concentration with 0 < x < 0.5. The hexagonal inclusion is a maximum of 5% for In < 0.2, indicating a preferential pure cubic structure. For In >0.2, the RMS roughness surface increase due to the inclusion of hexagonal planes parallel (110), (111), and (112) in cubic matrix. The average error for cubic phase percentage in the InGaN layers is <1% for x < 0.3, whereas for x = 0.48, the error is 4.8% compared with experimental results.

Energy conservation for weak solutions of a surface growth model

We are concerned with one-dimensional scalar surface growth model arising from the physical process of molecular epitaxy. The mathematical theory of the surface growth model is known to share a number of striking similarities with the Navier-Stokes equations, including the results regarding existence and uniqueness of solutions. In this paper, we shall investigate an important subject in mathematical physics: the energy conservation for weak solutions of the surface growth model. As an analogue of the Navier-Stokes equations, we find some sufficient integral conditions that guarantee the validity of energy equality. As far as we know, this is the first result in this aspect.

Development of an Epitaxial Growth Technique Using III-V on a Si Platform for Heterogeneous Integration of Membrane Photonic Devices on Si

The rapid increase in total transmission capacity within and between data centers requires the construction of low-cost, high-capacity optical transmitters. Since a tremendous number of transmitters are required, photonic integrated circuits (PICs) using Si photonics technology enabling the integration of various functional devices on a single chip is a promising solution. A limitation of a Si-based PIC is the lack of an efficient light source due to the indirect bandgap of Si; therefore, hybrid integration technology of III-V semiconductor lasers on Si is desirable. The major challenges are that heterogeneous integration of III-V materials on Si induces the formation of dislocation at high process temperature; thus, the epitaxial regrowth process is difficult to apply. This paper reviews the evaluations conducted on our epitaxial growth technique using a directly bonded III-V membrane layer on a Si substrate. This technique enables epitaxial growth without the fundamental difficulties associated with lattice mismatch or anti-phase boundaries. In addition, crystal degradation correlating with the difference in thermal expansion is eliminated by keeping the total III-V layer thickness thinner than ~350 nm. As a result, various III-V photonic-device-fabrication technologies, such as buried regrowth, butt-joint regrowth, and selective area growth, can be applicable on the Si-photonics platform. We demonstrated the growth of indium-gallium-aluminum arsenide (InGaAlAs) multi-quantum wells (MQWs) and fabrication of lasers that exhibit &gt;25 Gbit/s direct modulation with low energy cost. In addition, selective-area growth that enables the full O-band bandgap control of the MQW layer over the 150-nm range was demonstrated. We also fabricated indium-gallium-arsenide phosphide (InGaAsP) based phase modulators integrated with a distributed feedback laser. Therefore, the directly bonded III-V-on-Si substrate platform paves the way to manufacturing hybrid PICs for future data-center networks.

Fabrication and electrical characterization of the Al/n-Si/CZTSe4/Ag heterojunction

This study presents the manufacture of the Cu2ZnSnSe4/n-Si (CZTSe4/Si) heterojunction via the liquid phase epitaxy process. The current-voltage estimation for CZTSe4/Si heterojunction is evaluated at temperatures ranged from 300 K to 420 K in the dark condition. The ideality factor (n) of the present heterojunction was calculated from the logarithmic plot of the current-voltage curve. Also, the series resistance (Rs) and the effective barrier height (φb) for the CZTSe4/Si heterojunction have evaluated and discussed. Also, diode parameters such as ε′, ε'', M′, M″, tan δ, Rs and σac were investigated from the C-V measurements. Negative values of the C, G, ε′, ε'', M″, tan δ, Rs and σac were observed at different frequencies, temperatures and applied bias voltages. Moreover, the unique homogenous structure of the heterojunction reaching to a smart device that switches from negative to positive magnitudes for most dielectric parameters.

Oriented Perovskite Growth Regulation Enables Sensitive Broadband Detection and Imaging of Polarized Photons Covering 300–1050 nm

Photodetectors selective to the polarization empower breakthroughs in sensing technology for target identification. However, the realization of polarization-sensitive photodetectors based on intrinsically anisotropic crystal structure or extrinsically anisotropic device pattern requires complicated epitaxy and etching processes, which limit scalable production and application. Here, solution-processed PEA2 MA4 (Sn0.5 Pb0.5 )5 I16 (PEA= phenylethylammonium, MA= methylammonium) polycrystalline film is probed as photoactive layer toward sensing polarized photon from 300 to 1050nm. The growth of the PEA2 MA4 (Sn0.5 Pb0.5 )5 I16 crystal occurs in confined crystallographic orientation of the (202) facet upon the assistance of NH4 SCN and NH4 Cl, enhancing anisotropic photoelectric properties. Therefore, the photodetector achieves a polarization ratio of 0.41 and dichroism ratio (Imax /Imin ) of 2.4 at 900nm. At 520nm, the Imax /Imin even surpasses the one of the perovskite crystalline films, 1.8 and ≈1.2, respectively. It is worth noting that the superior figure-of-merits possess a response width of 900kHz, Ion /Ioff ratio of ≈3 × 108 , linear dynamic range from 0.15 nW to 12mW, noise current of 8.28 × 10-13 A × Hz-0.5 , and specific detectivity of 1.53 × 1012 Jones, which demonstrate high resolution and high speed for weak signal sensing and imaging. The proof of concept in polarized imaging confirms that the polarization-sensitive photodetector meets the requirements for practical application in target recognition.

Instrument for in situ hard x-ray nanobeam characterization during epitaxial crystallization and materials transformations.

Solid-phase epitaxy (SPE) and other three-dimensional epitaxial crystallization processes pose challenging structural and chemical characterization problems. The concentration of defects, the spatial distribution of elastic strain, and the chemical state of ions each vary with nanoscale characteristic length scales and depend sensitively on the gas environment and elastic boundary conditions during growth. The lateral or three-dimensional propagation of crystalline interfaces in SPE has nanoscale or submicrometer characteristic distances during typical crystallization times. An in situ synchrotron hard x-ray instrument allows these features to be studied during deposition and crystallization using diffraction, resonant scattering, nanobeam and coherent diffraction imaging, and reflectivity. The instrument incorporates a compact deposition system allowing the use of short-working-distance x-ray focusing optics. Layers are deposited using radio-frequency magnetron sputtering and evaporation sources. The deposition system provides control of the gas atmosphere and sample temperature. The sample is positioned using a stable mechanical design to minimize vibration and drift and employs precise translation stages to enable nanobeam experiments. Results of in situ x-ray characterization of the amorphous thin film deposition process for a SrTiO3/BaTiO3 multilayer illustrate implementation of this instrument.

Doped Highly Crystalline Organic Films: Toward High-Performance Organic Electronics.

Today's organic electronic devices, such as the highly successful OLED displays, are based on disordered films, with carrier mobilities orders of magnitude below those of inorganic semiconductors like silicon or GaAs. For organic devices such as diodes and transistors, higher charge carrier mobilities are paramount to achieve high performance. Organic single crystals have been shown to offer these required high mobilities. However, manufacturing and processing of these crystals are complex, rendering their use outside of laboratory‐scale applications negligible. Furthermore, doping cannot be easily integrated into these systems, which is particularly problematic for devices mandating high mobility materials. Here, it is demonstrated for the model system rubrene that highly ordered, doped thin films can be prepared, allowing high‐performance organic devices on almost any substrate. Specifically, triclinic rubrene crystals are created by abrupt heating of amorphous layers and can be electrically doped during the epitaxial growth process to achieve hole or electron conduction. Analysis of the space charge limited current in these films reveals record vertical mobilities of 10.3(49) cm2 V−1 s−1. To demonstrate the performance of this materials system, monolithic pin‐diodes aimed for rectification are built. The f3db of these diodes is over 1 GHz and thus higher than any other organic semiconductor‐based device shown so far. It is believed that this work will pave the way for future high‐performance organic devices based on highly crystalline thin films.

Integrated all-Si traveling-wave optical modulators and photodetectors operating beyond 40 GHz

We present high-speed, traveling-wave (TW) Si Mach–Zehnder modulators and Si sub-bandgap photodetectors (SBPD) monolithically integrated on an Si-only photonics platform without incorporation of Ge epitaxial growth process. Through constructing a detailed equivalent circuit model on the components, we design the device structure and TW electrodes for operating the device with bandwidth beyond 40 GHz. The experimental results show the 3-dB bandwidths of the Si modulator and photodetectors are 35 and 44 GHz, respectively, generally agreeing well with our design. The measured photoresponsivity of the SBPD varies from 0.1 A / W to nearly 1 A / W, depending on the bias voltage. These two components potentially can be utilized for an integrated optical transceiver operating for 50 Gbit / s data transmission.

Optical anisotropies of asymmetric double GaAs (001) quantum wells

In the present work, we were able to identify and characterize a source of in-plane optical anisotropies (IOAs) occurring in asymmetric double quantum wells (DQWs), namely a reduction of the symmetry from ${D}_{2d}$ to ${C}_{2v}$ as imposed by asymmetry along the growth direction. We report on reflectance anisotropy spectroscopy (RAS) of double GaAs quantum well structures coupled by a thin ($&lt;2$ nm) tunneling barrier. Two groups of DQW systems were studied: one where both QWs have the same thickness (symmetric DQW) and another where they have different thicknesses (asymmetric DQW). RAS measures the IOAs arising from the intermixing of the heavy and light holes in the valence band when the symmetry of the DQW system is lowered from ${D}_{2d}$ to ${C}_{2v}$. If the DQW is symmetric, residual IOAs stem from the asymmetry of the QW interfaces, e.g., that associated with Ga segregation into the AlGaAs layer during the epitaxial growth process. In the case of an asymmetric DQW with QWs with different thicknesses, the AlGaAs layers (that are sources of anisotropies) are not distributed symmetrically at both sides of the tunneling barrier. Thus the system loses its inversion symmetry, yielding an increase in the RAS strength. The RAS line shapes were compared with reflectance spectra in order to assess the heavy- and light-hole mixing induced by the symmetry breakdown. The energies of the optical transitions were calculated by numerically solving the one-dimensional Schr\"odinger equation using a finite-difference method. Our results are useful for interpretation of the transitions occurring in both symmetric and asymmetric DQWs.

Lifetime limiting defects in 4H-SiC epitaxial layers: The influence of substrate originated defects

The minority carrier lifetime is a decisive factor to obtain 4H-SiC bipolar devices with a low forward voltage drop at high blocking voltages. The lifetime is directly correlated with the concentration of the so-called Z1/2 deep level and depends on the epitaxial growth process and post-epi processing like ion implantation, annealing, and thermal oxidation. The substrate has so far been attributed a subordinate role for the minority carrier lifetime. In this work, the influence of substrate quality on the minority carrier lifetime after epitaxial growth and post-epi processing is studied on substrates from different manufacturers. The investigation revealed a significant impact of the substrate contamination on the minority carrier lifetime of epitaxial layers and on the efficiency of lifetime enhancement by thermal oxidation. A deep level named SD2 was found in the samples which acts as an additional Shockley-Read-Hall recombination center. The deep level SD2 was traced back to the substrates themselves showing different levels of incorporation of recombination sites from the substrate into the epitaxial layer during the growth. A comparison of the energy levels and electron capture cross sections of the SD2 deep level with defects caused by a tungsten contamination shows a good agreement.

Prominent luminescence of silicon-vacancy defects created in bulk silicon carbide p\u2013n junction diodes

We investigate fluorescent defect centers in 4H silicon carbide p–n junction diodes fabricated via aluminum-ion implantation into an n-type bulk substrate without the use of an epitaxial growth process. At room temperature, electron-irradiated p–n junction diodes exhibit electroluminescence originating from silicon-vacancy defects. For a diode exposed to an electron dose of 1 times 10^{18},{{mathrm{cm}}}^{-2} at 800,{{mathrm{keV}}}, the electroluminescence intensity of these defects is most prominent within a wavelength range of 400–1100,{{mathrm{nm}}}. The commonly observed {{mathrm{D}}}_1 emission was sufficiently suppressed in the electroluminescence spectra of all the fabricated diodes, while it was detected in the photoluminescence measurements. The photoluminescence spectra also displayed emission lines from silicon-vacancy defects.

Fabrication of low stress GaN-on-diamond structure via dual-sided diamond film deposition

The fabrication of GaN-on-diamond structure by the process of diamond growth on GaN technology is becoming more and more attractive for high-power GaN devices. However, due to the high-temperature steps employed during the GaN epitaxial layer transfer process, significant stress accumulates in the GaN-on-diamond structure and induces the formation of cracks or deformations in the GaN layer. To improve the stress characteristics of the GaN layer, in this work, dual-sided diamond film deposition was carried out to accomplish the GaN-on-diamond structure. The surface morphology, interface microstructure and crystal quality of diamond on GaN were analyzed by using scanning electron microscopy, atomic force microscopy, transmission electron microscopy and X-ray diffraction. The results showed that the stress accumulation originating from the GaN epitaxial layer transfer process was effectively alleviated as compared with other reporteds in the literature and that the crystal quality of the GaN was only slightly altered. The stress relaxation was attributed to the low-quality diamond used as the temporary carrier during the GaN transfer process. In addition, a thin SiC transition layer in the interface is also beneficial in alleviating the mismatch between the coefficients of thermal expansion for the GaN and the diamond.

Role of dislocations in nitride laser diodes with different indium content

In this work we investigate the role of threading dislocations in nitride light emitters with different indium composition. We compare the properties of laser diodes grown on the low defect density GaN substrate with their counterparts grown on sapphire substrate in the same epitaxial process. All structures were produced by metalorganic vapour phase epitaxy and emit light in the range 383–477 nm. We observe that intensity of electroluminescence is strong in the whole spectral region for devices grown on GaN, but decreases rapidly for the devices on sapphire and emitting at wavelength shorter than 420 nm. We interpret this behaviour in terms of increasing importance of dislocation related nonradiative recombination for low indium content structures. Our studies show that edge dislocations are the main source of nonradiative recombination. We observe that long wavelength emitting structures are characterized by higher average light intensity in cathodoluminescence and better thermal stability. These findings indicate that diffusion path of carriers in these samples is shorter, limiting the amount of carriers reaching nonradiative recombination centers. According to TEM images only mixed dislocations open into the V-pits, usually above the multi quantum wells thus not influencing directly the emission.

Solid phase recrystallization induced by multi-pulse nanosecond laser annealing

The solid phase recrystallization of partially amorphized SOI structures using a pulsed Ultra-Violet Nanosecond Laser Annealing (UV-NLA) is reported. It is shown that combining UV-NLA process with a heating chuck at low temperature (450 °C) results in the perfect recrystallization of the amorphized portion of the silicon layer. Actually, sub-melt multi-pulse UV-NLA enables obtaining the total crystal recovery and a very high dopant activation rate, comparable to melt laser annealing approaches or high temperature rapid thermal annealing, with a very low overall thermal budget. This approach is a great alternative to traditional solid phase epitaxial regrowth process, which reach it limits, in terms of recrystallization rate, at temperatures lower than 500 °C. The process presented here is thus suitable for integration paths in which the thermal budget must be limited as for example 3D sequential integration. Besides the high activation rate, another benefit of the solid phase recrystallization, compared to an equivalent laser anneal in the melt regime, is the almost unchanged surface roughness. Furthermore, the use of an in-situ metrology based on Time Resolved Reflectometry (TRR) enables the monitoring of the crystalline seed thickness evolution during UV-NLA and the estimation of the recrystallization rate.