Two-step Metalorganic Chemical Vapor Research Articles

Lateral heterostructures of WS2 and MoS2 monolayers for photo-synaptic transistor

Von Neumann architecture-based computing, while widely successful in personal computers and embedded systems, faces inherent challenges including the von Neumann bottleneck, particularly amidst the ongoing surge of data-intensive tasks. Neuromorphic computing, designed to integrate arithmetic, logic, and memory operations, has emerged as a promising solution for improving energy efficiency and performance. This approach requires the construction of an artificial synaptic device that can simultaneously perform signal processing, learning, and memory operations. We present a photo-synaptic device with 32 analog multi-states by exploiting field-effect transistors based on the lateral heterostructures of two-dimensional (2D) WS2 and MoS2 monolayers, formed through a two-step metal–organic chemical vapor deposition process. These lateral heterostructures offer high photoresponsivity and enhanced efficiency of charge trapping at the interface between the heterostructures and SiO2 due to the presence of the WS2 monolayer with large trap densities. As a result, it enables the photo-synaptic transistor to implement synaptic behaviors of long-term plasticity and high recognition accuracy. To confirm the feasibility of the photo-synapse, we investigated its synaptic characteristics under optical and electrical stimuli, including the retention of excitatory post-synaptic currents, potentiation, habituation, nonlinearity factor, and paired-pulse facilitation. Our findings suggest the potential of versatile 2D material-synapse with a high density of device integration.

Two-step growth of β-Ga2O3 on c-plane sapphire using MOCVD for solar-blind photodetector

In this work, a two-step metal organic chemical vapor deposition (MOCVD) method was applied for growing β-Ga2O3 film on c-plane sapphire. Optimized buffer layer growth temperature (T B) was found at 700 °C and the β-Ga2O3 film with full width at half maximum (FWHM) of 0.66° was achieved. A metal−semiconductor−metal (MSM) solar-blind photodetector (PD) was fabricated based on the β-Ga2O3 film. Ultrahigh responsivity of 1422 A/W @ 254 nm and photo-to-dark current ratio (PDCR) of 106 at 10 V bias were obtained. The detectivity of 2.5 × 1015 Jones proved that the photodetector has outstanding performance in detecting weak signals. Moreover, the photodetector exhibited superior wavelength selectivity with rejection ratio (R 250 nm/R 400 nm) of 105. These results indicate that the two-step method is a promising approach for preparation of high-quality β-Ga2O3 films for high-performance solar-blind photodetectors.

Effect of Two-Step Metal Organic Chemical Vapor Deposition Growth on Quality, Diameter and Density of InAs Nanowires on Si (111) Substrate

High-density (∼ 80/um2) vertical InAs nanowires (NWs) with small diameters (∼ 28 nm) were grown on bare Si (111) substrates by means of two-step metal organic chemical vapor deposition. There are two critical factors in the growth process: (1) a critical nucleation temperature for a specific In molar fraction (approximately 1.69 × 10−5 atm) is the key factor to reduce the size of the nuclei and hence the diameter of the InAs NWs, and (2) a critical V/III ratio during the 2nd step growth will greatly increase the density of the InAs NWs (from 45 μm−2 to 80 μm−2) and at the same time keep the diameter small. The high-resolution transmission electron microscopy and selected area diffraction patterns of InAs NWs grown on Si exhibit a Wurtzite structure and no stacking faults. The observed longitudinal optic peaks in the Raman spectra were explained in terms of the small surface charge region width due to the small NW diameter and the increase of the free electron concentration, which was consistent with the TCAD program simulation of small diameter (< 40 nm) InAs NWs.

Spectrally resolved modal characteristics of leaky-wave-coupled quantum cascade phase-locked laser arrays

The modal characteristics of nonresonant five-element phase-locked arrays of 4.7-μm emitting quantum cascade lasers (QCLs) have been studied using spectrally resolved near- and far-field measurements and correlated with results of device simulation. Devices are fabricated by a two-step metal-organic chemical vapor deposition process and operate predominantly in an in-phase array mode near threshold, although become multimode at higher drive levels. The wide spectral bandwidth of the QCL’s core region is found to be a factor in promoting multispatial-mode operation at high drive levels above threshold. An optimized resonant-array design is identified to allow sole in-phase array-mode operation to high drive levels above threshold, and indicates that for phase-locked laser arrays full spatial coherence to high output powers does not require full temporal coherence.

Morphology-Dependent Stimulated Emission and Field Emission of Ordered CdS Nanostructure Arrays

Highly ordered CdS nanostructure arrays were successfully fabricated via a simple two-step metal-organic chemical vapor deposition process. Both stimulated emission and field-emission measurements were carried out in an attempt to understand the correlation between the morphologies, alignments, and emission performances of five ordered CdS nanostructure array types (well-aligned truncated nanocones, nanorods, cleft nanorods, quasi-aligned nanowires, and nanowires). The 1D CdS nanostructures of various types displayed notable differences in stimulated and field-emission performances. The stimulated emission strongly correlated with the structure alignment: the better the alignment, the lower the threshold. Both of the alignments and aspect ratios greatly affected the field-emission properties; the CdS emitters of higher aspect ratio and better alignment exhibited better field-emission performance. Thus the well-aligned CdS nanorod arrays had the lower threshold for stimulated emission, and quasi-aligned nanowire arrays produced the higher field-emission current and possessed the lower turn-on fields.

Self-Catalytic and Selective Growth of ZnO Nanoneedles by Micro-Contact Printing and CVD

A self-catalytic method to selectively grow ZnO nanoneedles is demonstrated by simply combining a conventional micro-contact printing (mu-CP) and two-step metal-organic chemical vapor deposition (MOCVD) techniques. Self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) were patterned on Si substrates by mu-CP. The patterned structures of the ZnO nanoneedles were realized on the SAMs-patterned Si substrates by two-step MOCVD at high temperature of 600 degrees C via the initial preparation of a thin ZnO seed layer with retaining the patterned morphology at low temperature of 170 degrees C. This combined method of mu-CP and self-catalytic two-step MOCVD should be applicable to fabricate the patterned alignments of metal oxide nanostructures.

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

Single-Mode Emission From Vertical-Cavity Surface-Emitting Lasers With Low-Index Defects

Mode confinement to low-index regions of a two-dimensional square photonic lattice has been studied as a means to realize single-spatial-mode vertical-cavity surface-emitting lasers (VCSELs). A selective etching process with two-step metal-organic chemical vapor deposition is used to fabricate the low-index defect VCSELs. A device with a 9-mum-wide aperture demonstrates 2.9-mW continuous-wave (CW) output power. The near diffraction-limited single-lobe far-field and high-resolution spectral analyses confirm single fundamental defect mode emission. Comparison studies between pulsed and CW operation indicate that thermal effects have a significant impact on the modal performance of the devices. Single-mode emission with more than 8 mW of peak pulsed power has been observed with short pulsewidth operation (0.1 mus, 2%). Improvements in thermal management are required to achieve higher CW single-mode emission in these devices

The two-step chemical vapor deposition of Pd(allyl)Cp as an atom-efficient route to synthesize highly dispersed palladium nanoparticles on carbon nanofibers

Highly dispersed palladium nanoparticles supported on carbon nanofibers, which show high catalytic activity and stability in the hydrogenation of cyclooctene, were synthesized by the two-step metal organic chemical vapor deposition (MOCVD) of allylcyclopentadienylpalladium (Pd(allyl)Cp) as precursor at atmospheric pressure.

Near-diffraction-limited coherent emission from large aperture antiguided vertical-cavity surface-emitting laser arrays

We demonstrate that in-phase mode operation with a near-diffraction-limited beam can be realized in large aperture (up to 100 elements) antiguided vertical-cavity surface-emitting laser (VCSEL) arrays. A selective etching process with two-step metalorganic chemical vapor deposition is used for fabrication of the antiguided VCSEL array structures. Modal discrimination is enhanced by intentionally choosing a GaAs cap thickness so as to introduce suitable loss to array interelement regions. Far field patterns indicate in-phase mode operation from both triangular and rectangular geometry antiguided VCSEL arrays, which is in good agreement with theory.

Review of polarity determination and control of GaN

Polarity issues affecting III-V nitride semiconductors are reviewed with respect to their determination and control. A set of conditions crucial to the polarity control of GaN is provided for each of the following growth techniques; molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and hydride vapor phase epitaxy (HVPE). Although GaN films might have been deposited by identical growth methods using the same buffer layer technologies, there is often a conflict between the resulting polarities achieved by different research groups. In this paper, we present the implications of the conditions used in each of the processes used for two-step metalorganic chemical vapor deposition (MOCVD), demonstrating systematic control of the polarity of GaN films on sapphire substrates. The potential for confusion in polarity control will be explained, taking into account the implications clarified in our studies. The correlation between the polarity and the growth conditions will be discussed in order to provide a mechanism for the determination and control of the crystal polarity during the growth of GaN films.

Systematic analysis and control of low-temperature GaN buffer layers on sapphire substrates

The growth of low-temperature (LT) GaN buffer layers on sapphire substrates was systematically studied using x-ray photoelectron spectroscopy with regards to processes such as substrate treatment and deposition conditions, along with annealing treatments of the GaN buffer layer during two-step metalorganic chemical vapor deposition. Variations observed in the LT-buffer layer depended strongly on both the chemical state of the sapphire surface as a result of the substrate treatment and the subsequent annealing conditions. A 20 nm buffer layer on non-nitrided sapphire evaporated after the formation of islands during the conventional annealing process (N2, H2, and NH3 gas mixture). Adding H2 gas to the annealing ambient enhanced the evaporation and reduced the surface coverage. It was found that AlxGa1−xN was formed at the interface, which has a low evaporation coefficient. In contrast, a buffer layer deposited onto a nitrided sapphire substrate evaporated completely in a layer-by-layer mode. The buffer layer contained domains with N face (−c) polarity that were almost covered with a Ga face (+c) layer. It was found that using Ga-rich conditions (a lower V/III ratio) for the deposition suppressed the formation of the −c domains, even on the nitrided sapphire. High temperature (HT) GaN layers were deposited on these well-defined LT-buffer layers. The influence of the various conditions used in preparing the LT-buffer layer on the HT-GaN layers are discussed in terms of the crystalline quality and the polarity of the HT-GaN layers.

High-power single-mode antiresonant reflecting optical waveguide-type vertical-cavity surface-emitting lasers

Antiresonant reflecting optical waveguide (ARROW) techniques are employed in vertical cavity surface emitting lasers (VCSELs) to achieve high-power single-mode emission. Using the effective-index method and fiber mode approximation, the cold-cavity lateral modal behavior for the circular shaped ARROW VCSEL demonstrates significant reduction of radiation loss from that of a single antiguide, while maintaining strong discrimination against high-order modes. The circular-waveguide is created by selective chemical etching and two-step metal-organic chemical vapor deposition growth, with proton implantation used to confine the current injection to the low-index core region. A single-mode CW power of 7.1 mW has been achieved from an 8 /spl mu/m diameter ARROW device (index step /spl Delta/n = 0.05, emission at /spl lambda//sub 0/ = 980 nm) with a far-field FWHM of 10/spl deg/. Larger aperture (12 /spl mu/m) devices exhibit multimode operation at lower drive currents with a maximum single-mode continuous-wave output power of 4.3 mW.

Modal properties of two-dimensional antiguided vertical-cavity surface-emitting laser arrays

We present a detailed analysis on 2-D (4 /spl times/ 4 square-lattice) antiguided vertical-cavity surface-emitting laser (VCSEL) arrays based on the effective-index model. The calculation shows that the array can operate under 2-D resonant coupling, provided that the resonance condition in both the horizontal and vertical directions is satisfied. Consequently, the resonant-mode edge radiation loss is inversely proportional to the number of array elements along one direction for a N /spl times/ N array. Low-edge-loss out-of-phase and adjacent array modes are found to compete with the in-phase resonant mode. While the 3-D gain overlap is not a significant factor in modal discrimination, the introduction of inter-element loss allows the in-phase mode to exhibit the lowest threshold gain for a wide range of inter-element width, s (/spl Delta//sub 8/ /spl ap/ 0.5 /spl mu/m for 980-nm wavelength devices). The 2-D antiguided array results from shifting the cavity resonance between the element and inter-element regions and is fabricated by selective chemical etching and two-step metalorganic chemical vapor deposition growth. Diffraction-limited in-phase and out-of-phase array mode operation is observed from top-emitting arrays, depending on the inter-element width. Substrate-emitting array structures are investigated as a means to lower heating and increase the coherent output power.

Chemical Vapor Deposition of Sr1−xBaxNb2O6 Thin Films Using Metal Alkoxide Precursors

A novel two-step metalorganic chemical vapor deposition process was used in this study to prepare Sr1−xBaxNb2O6 (SBN) thin films. Two thin layers of single-phase SrNb2O6 and BaNb2O6 were deposited alternately on a silicon substrate, and the solid solution of SBN was obtained by high-temperature annealing. The stoichiometry control of the SrNb2O6 and the BaNb2O6 thin films was achieved through deposition process control, according to the evaporation characteristics of double metal alkoxide. The evaporation behavior of double metal alkoxide precursors SrNb2(1-OC4H9)12 and BaNb2(1-OC4H9)12 was studied, and the results were compared with the evaporation of single alkoxide Nb(1-OC4H9)5.

Growth mode and surface morphology of a GaN film deposited along the N-face polar direction on c-plane sapphire substrate

The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face (−c) polarity, independent of both the V/III ratio and the deposition rate. The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the {101̄0} directions to cover a room among the facets until coalescence. After coalescence, −c GaN growth on a flat hexagonal facet results in a pyramidal hexagonal facet. The growth mode for −c GaN has been discussed with respect to surface structure and migration length of adsorbing precursors, in comparison with Ga-face (+c) GaN.

Simplified-antiresonant reflecting optical waveguide-type vertical-cavity surface-emitting lasers

A two-step metalorganic chemical vapor deposition growth process is used to fabricate antiguided vertical-cavity surface-emitting lasers (VCSELs) incorporating a simplified-antiresonant reflecting optical waveguide (S-ARROW) design. Preliminary results show single-mode cw operation up to 1 mW output power from a 12 μm-diam (λ=930 nm) S-ARROW VCSEL with a large lateral index step (Δn=0.1). Modal discrimination in the S-ARROW-VCSEL is calculated using a fiber-mode approximation and device optimization for high-single-mode powers is discussed.

Nitride Semiconductor Surfaces. Characterization of the Polarity of GaN Semiconductor Films by Coaxial Impact Collision Ion Scattering Spectroscopy. Correlation between GaN Growth Process and the Polar Direction.

Nondestructive determination of the polarity of GaN has been achieved by use of coaxial impact collision ion scattering spectroscopy. GaN films were deposited on c-plane sapphire substrates by a two-step atmospheric pressure metalorganic chemical vapor deposition using GaN buffer layers. The correlation between the samples prepared by interrupting the growth sequence at the various stage and their polarity was systematically investigated. It has been found that the polar direction of GaN growth is influenced by the polarity at the interface prior to the deposition of GaN epitaxial layer. We describe the mechanism of determining the polar direction by addressing our recent research related to substrate nitridation, buffer layer, and annealing of buffer layer.

Analysis of the polar direction of GaN film growth by coaxial impact collision ion scattering spectroscopy

Nondestructive determination of the polarity of GaN has been achieved by the use of coaxial impact-collision ion scattering spectroscopy analysis. The polarity of a GaN film with a smooth surface on non-nitrided c-plane sapphire was identified (0001) (Ga face; +c). GaN films with a 20 nm buffer layer on nitrided sapphire had (0001̄) (N face; −c) polarity and a hexagonal faceted surface. The influence of both the buffer layer and of substrate nitridation on the polarity of wurtzite {0001} GaN films deposited by two-step metal organic chemical vapor deposition (MOCVD) has been investigated. The polarity of the buffer layer on a nitrided sapphire substrate was altered by varying its thickness or the annealing time. It was found that the polarity of the GaN film is determined by the polarity of the annealed buffer layer; MOCVD-GaN films on buffer layers with +c and −c polarity have either +c (smooth surface) or −c (hexagonal facet) polarity, respectively.

Dislocation generation in GaN heteroepitaxy

In this work, we study the microstructural evolution, with particular emphasis on threading dislocation (TD) generation, in the two-step metal-organic chemical vapor deposition (MOCVD) of GaN on sapphire. The MOCVD growths were carried out at atmospheric pressure in a horizontal two-flow reactor. Nominally, 200Å thick nucleation layers (NL) were deposited at temperatures in the range 525–600°C followed by high temperature (HT) growth at 1060–1080°C. Throughout the different stages of growth, the microstructure was studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Two growth conditions were closely studied: brief pre-growth ammonia exposure of the sapphire (`Material A') and extensive pre-growth ammonia exposure of the sapphire (`Material B'). The as-grown Material B NL has a ∼25Å hexagonal GaN wetting layer followed by predominantly (111) oriented cubic GaN. After HT exposure, Material B NL predominantly transforms to hexagonal GaN and has TDs. These TDs propagate into the HT GaN and lead to a TD density of 2×1010 after 1μm of HT growth. Material A NLs, before and after HT exposure, have rough morphologies and a high-degree-of-stacking disorder (predominantly (111) oriented cubic GaN). On Material A NLs, The HT GaN grows by a coarse island mechanism in which the GaN laterally overgrows the NL without generating TDs. Stacking disorder and misorientation between the HT hexagonal GaN and the NL islands is accommodated either by Shockley or Frank partial dislocations or local strain. The majority of TDs are subsequently generated at the coalescence of the HT islands.