Chemical Vapor Deposition Epitaxy Research Articles

Simulation study of a novel GaN on Si Quasi‐Vertical Reverse‐Conducting Insulated Gate Bipolar Transistor

Abstract In this paper, a novel GaN on Si quasi-vertical reverse conducting insulated gate bipolar transistor(RC-IGBT) with a new NPN structure is proposed and simulated by Silvaco TCAD. The distinctive feature of this structure lies in fabricating the original bottom P-Collector on the wafer surface. In comparison to conventional IGBT devices with PNPN structures, this NPN structure overcomes two major challenges: the difficulty in forming ohmic contacts due to the activation difficulty of the bottom P-GaN and the necessity of creating backside through-holes for electrode formation. Simultaneously, the N-GaN in the emitter and reverse-conducting regions can be selectively regrown through Metal-Organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE), avoiding the etching effect of top-layer N-GaN on P-GaN during the fabrication of P-type electrodes. This approach demonstrates a high level of process feasibility. In this paper, Silvaco TCAD is used to simulate the static and dynamic characteristics of this novel quasi-vertical RC-IGBT device. The simulation results reveal that the device has a high saturation current(Ion,sat) density of 18224.67A/cm2 at Vge=14V,a threshold voltage (Vth) of 5V,a breakdown voltage of 828V,a latch-up voltage of 800V ,and a switching speed of nanoseconds. In addition, the selection of device parameters is studied in this paper.

Valency-based structural properties of gamma-sheet of boron clusters.

Boron cluster sheets are two-dimensional boron atom-based formations called borophene. They are similar to the two-dimensional sheet known as graphene, which is composed of carbon atoms arranged in a hexagonal lattice. The unique electrical, mechanical, and thermal properties of borophene make it a sought-after substance for a variety of uses, such as catalysis, energy storage, and electronics. There are two ways to manufacture borophene: chemical vapor deposition and molecular beam epitaxy. Vertex-edge valency-based topological descriptors are a great example of a molecular descriptor that provides information on the connection of atoms in a molecule. These descriptions are based on the notion that a node's value in a molecular network is the sum of the valency of those atoms that are directly connected to that node. In this article, we discussed some novel vertex-edge (ve) and edge-vertex (ev) topological descriptors and found their formulations for the boron cluster or borophene sheets. Also, we show the numerical and graphical comparison of these descriptors in this article.

Broadband and ultrafast photodetector based on PtSe2 synthesized on hBN using molecular beam epitaxy

Two-dimensional (2D) van der Waals materials are potential candidates for high-performance optoelectrical devices owing to the bandgap modulation, high on/off ratio, and ultra-stability. However, the challenge of synthesizing large-area, uniform, and high-quality 2D van der Waals materials should be addressed to manufacture large-scale and uniform optoelectrical devices using these materials. Although several previous studies investigated synthesis methods using chemical vapor deposition and molecular beam epitaxy (MBE), there are certain limitations such as the non-uniformity, lots of grain boundaries, and low carrier mobility. Here, by studying a method for synthesizing PtSe2 on hBN buffer layer using MBE, we fabricated a PtSe2-based broadband photodetector. The high-quality PtSe2 synthesized on hBN was evaluated using Raman spectroscopy and scanning transmission electron spectroscopy. These revealed that the synthesized PtSe2 had a high quality and small grain boundary compared with PtSe2 synthesized on SiO2. Owing to the reduction in the carrier scattering and carrier recombination because of the high-quality and small grain boundaries, the photodetector using PtSe2 synthesized on hBN displayed optoelectrical properties that were significantly better than those of PtSe2 synthesized on SiO2. Therefore, using hBN as a buffer layer for PtSe2 growth, a remarkable performance of complex optoelectrical devices could be achieved.

Controllable nitrogen doping of MOCVD Ga2O3 using NH3

We report on the controllable nitrogen doping of β-Ga2O3 as a deep acceptor dopant using ammonia diluted in nitrogen (NH3/N2) as a source of active nitrogen in the metal organic chemical vapor deposition epitaxy. The effects of the NH3/N2 flow rate and substrate temperature on the incorporation efficiency, reproducibility, and controllability of N doping into Ga2O3 were studied using secondary ion mass spectrometry measurements. With the increase in the NH3/N2 molar flow rate from ∼2 × 10−8 to ∼2 × 10−6 mol/min, the N impurities incorporated into the β-Ga2O3 increased linearly from ∼1 × 1018 to ∼2 × 1020 cm−3. At low substrate temperatures (<800 °C), hydrogen was incorporated into the film accompanying nitrogen with comparable concentrations. Despite this, the current–voltage measurements showed that the N and H co-doped films were resistive with a measured resistance of >70 MΩ for a film grown with [N] ≈ [H] of ∼8 × 1018 cm−3. X-ray on-axis (020) and off-axis (111) rocking curve ω-scans and atomic force microscopy measurements show no influence of NH3/N2 dopant on the structural and surface quality of the films. However, the presence of H promoted the growth of (110) and (1¯10) facets elongated along the [001] direction. At high growth temperatures (≥950 °C), the H concentration in the films was reduced by nearly ∼10×, but with a slight increase in the concentration of N. The results show that controllable and repeatable nitrogen doping into β-Ga2O3 can be achieved using ammonia to obtain deep acceptor doping or compensation needed for device engineering in β-Ga2O3-based power electronic devices.

High-Performance P- and N-Type SiGe/Si Strained Super-Lattice FinFET and CMOS Inverter: Comparison of Si and SiGe FinFET.

This research presents the optimization and proposal of P- and N-type 3-stacked Si0.8Ge0.2/Si strained super-lattice FinFETs (SL FinFET) using Low-Pressure Chemical Vapor Deposition (LPCVD) epitaxy. Three device structures, Si FinFET, Si0.8Ge0.2 FinFET, and Si0.8Ge0.2/Si SL FinFET, were comprehensively compared with HfO2 = 4 nm/TiN = 80 nm. The strained effect was analyzed using Raman spectrum and X-ray diffraction reciprocal space mapping (RSM). The results show that Si0.8Ge0.2/Si SL FinFET exhibited the lowest average subthreshold slope (SSavg) of 88 mV/dec, the highest maximum transconductance (Gm, max) of 375.2 μS/μm, and the highest ON-OFF current ratio (ION/IOFF), approximately 106 at VOV = 0.5 V due to the strained effect. Furthermore, with the super-lattice FinFETs as complementary metal-oxide-semiconductor (CMOS) inverters, a maximum gain of 91 v/v was achieved by varying the supply voltage from 0.6 V to 1.2 V. The simulation of a Si0.8Ge0.2/Si super-lattice FinFET with the state of the art was also investigated. The proposed Si0.8Ge0.2/Si strained SL FinFET is fully compatible with the CMOS technology platform, showing promising flexibility for extending CMOS scaling.

Hybrid tunnel junction enabled independent junction control of cascaded InGaN blue/green micro-light-emitting diodes.

We demonstrate vertical integration of nitride-based blue/green micro-light-emitting diodes (µLEDs) stacks with independent junctions control using hybrid tunnel junction (TJ). The hybrid TJ was gown by metal organic chemical vapor deposition (p + GaN) and molecular-beam epitaxy (n + GaN). Uniform blue, green and blue/green emission can be generated from different junction diodes. The peak external quantum efficiency (EQE) of the TJ blue µLEDs and green µLEDs with indium tin oxide contact is 30% and 12%, respectively. The carrier transportation between different junction diodes was discussed. This work suggests a promising approach for vertical µLEDs integration to enhance the output power of single LEDs chip and monolithic µLEDs with different emission colors with independent junction control.

Investigation of Arsenic Transient Enhanced Diffusion From Emitter Process at 550 °C Si:As RP-CVD Epitaxy Using Disilane Precursor

Reduced Pressure Chemical Vapor Deposition (RP-CVD) Epitaxy bellow 600 °C is studied for 3D sequential integration [1] where cyclic deposition is needed. Another approach can be useful with Si epitaxy at temperature bellow 600 °C : the deposition of amorphous Si on dielectric during non-selective epitaxy combined with solid phase epitaxy regrowth (SPER) to extend the monocrystalline area. Indeed, using temperature above 600 °C during non-selective epitaxy, a polycrystalline Si film is formed on dielectric. This technique has been used in heterojunction bipolar transistor for the non-selective base deposition [2] and can also be used to ensure a fully monocrystalline emitter.Si:As process has been developed at 550 °C using disilane (Si2H6) as Si precursor and Arsine (AsH3) for As precursor. At this temperature, disilane is mandatory to ensure a sufficient growth rate. The disilane process will be compared to a process of reference, using silane (SiH4) precursor at 620 °C. The As dopant concentration targeted is between 1x20 at.cm-3 and 5x20 at.cm-3. Finally, the dopant diffusion is controlled on non-productive Si(100) wafer (NPW), without pattern, by SIMS for both processes before and after thermal budget (720 °C, 1 h + high temperature Spike anneal) to simulate the thermal budget undergone by the emitter.Thanks to the ASTAR system [3] from NanoMEGAS, the crystal orientation and phase maps of the emitter are extracted for both processes (Fig 1.a). ASTAR system is based on the Automated Crystal Orientation Mappings technique used on a Transmission Electron Microscope (ACOM-TEM). With silane precursor, a poly-emitter is generated whereas the emitter is fully monocrystalline with disilane. The amorphous deposition coupled with SPER ensure a fully monocrystalline emitter. SIMS analysis have been performed and show abnormal diffusion of As with disilane process. To understand the causes of the diffusion, both Si:As processes have been reproduced on NPW by full sheet epitaxy. The abnormal diffusion is still observed with the disilane process (Fig 1.b). Compared to diffusion length extracted from [4] (19nm) the diffusion of As is enhanced. Moreover, As enhanced diffusion occurs for low temperature thermal budget (Fig. 1.c).Historically, defects generated from implantation are known to enhance the diffusion length of the dopant. This phenomenon is called Transient Enhanced Diffusion (TED) and it has been studied and observed from dopant implantation [5]. The implantations generate a supersaturation of interstitials defects (such as auto-interstitial) which enhance the dopant diffusion for low temperature thermal budgets. This is a transient effect because the enhanced diffusion occurs in a short time [5]. In our case, several anneal at 720 °C were applied on the disilane Si:As layer from 1 h to 24 h (Fig. 1.d). Most of the diffusion occurs after the annealing at 720 °C, 1 h. These results highlight the transient effect of the As diffusion using disilane process. With the disilane process, TED of As occurs. Indeed, enhanced diffusion of Arsenic appears only for low temperature annealing and the effect is also transient. Compared to implantation which generates defects during the bombardment, low temperature epitaxy generates defects when the growth rate is too high. The molecules from the gas phase are adsorbed on the surface and break down on atoms (called adatom). The adatoms diffuse on the surface to find a suitable site. During low temperature epitaxy, when the growth rate is too high, the adatom don’t have enough time to diffuse on a suitable site and generate defects.Finally, dopant profile analysis will be completed by atomic force microscopy (AFM) and sheet resistance measurement. The AFM will provide an evolution of the surface topography with the annealing. The sheet resistance, coupling with SIMS, will help to extract the deactivation of dopant. The causes of the TED from disilane process at 550 °C will be discussed and experimental results will be provided.[1] L. Brunet et al., IEEE International Electron Devices Meeting (IEDM), 2018[2] Holger Rücker and Bernd Heinemann, Semicond. Sci. Technology, 2018[3] A. Valery, Doctoral dissertation. Université Grenoble Alpes, 2017[4] S. W. Jones, "Diffusion in silicon". IC Knowledge LLC, 2008[5] S. C. Jain, et al., Journal of applied physics, 2002 Figure 1

Molecular Beam Epitaxy of in2Se3 Thin Films

In2Se3 is a polymorphous metal-chalcogenide system1 where each phase exhibits dramatically different functional properties. α-In2Se3 is a two-dimensional, ferroelectric semiconductor2,3 and has shown promise in low-power, neuromorphic electronic devices4,5. β-In2Se3 is a two-dimensional, centrosymmetric semiconductor and has shown promise in photodetectors6. γ-In2Se3 is a three-dimensional, defect- wurtzite wide bandgap semiconductor that has shown promise as a precursor layer for CuInSe2 solar cells7. Considered together, the bandgaps of these materials range from 1.3eV to 2.5eV8. Understanding how to synthesize phase-pure thin-films of In2Se3 is of interest to harness the properties of these materials for applications in electronic devices.α-In2Se3 is thermodynamically stable below 200 oC and transitions to the 2D, centrosymmetric β-In2Se3 phase upon heating above 200 oC1. While the β to α-In2Se3 phase transition is achievable in bulk synthesis via slow cooling2, the β to α-In2Se3 phase transition does not occur as readily in thin-films. Recent studies on thin-film synthesis of In2Se3 have demonstrated β- and/or γ-In2Se3 films at temperatures above the thermodynamic stability range of α-In2Se3 (200 oC). Two reports from the last two years demonstrate large-area synthesis of β-In2Se3 thin-films metal-organic chemical vapor deposition and molecular beam epitaxy (MBE) on sapphire at temperatures greater than 300 oC6,9. Another study reported MBE synthesis of metastable, strained, mixed phase β/γ-In2Se3 films on Si(111) substrates at 180 oC10. One study from 2018 claims deposition of large-area α-In2Se3 thin-films on graphene at a substrate temperature of 250 oC via molecular beam epitaxy; however, it is difficult to distinguish the α from β phase by the Raman spectra and TEM micrograph along the c-axis.In this study we demonstrate thin-film synthesis of In2Se3 over a wide array of synthesis conditions via molecular beam epitaxy. We systematically investigate the effects of substrate temperature, cooling rate after deposition, substrate surface, Se/In flux ratio, and precursor type on the structure and properties of In2Se3 films. We characterize the crystalline phase, stoichiometry, and crystal morphology via Raman Spectroscopy, X-ray Photoelectron Spectroscopy and Atomic Force Microscopy, respectively.(1) Okamoto, H. In-Se ( Indium-Selenium ). J. Phase Equilibria 2004, 25 (2).(2) Küpers, M.; Konze, P. M.; Meledin, A.; Mayer, J.; Englert, U.; Wuttig, M.; Dronskowski, R. Controlled Crystal Growth of Indium Selenide, In2Se3, and the Crystal Structures of α-In2Se3. Inorg. Chem. 2018, 57, 11775–11781.(3) Xiao, J.; Zhu, H.; Wang, Y.; Feng, W.; Hu, Y.; Dasgupta, A.; Han, Y.; Wang, Y.; Muller, D. A.; Martin, L. W.; et al. Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking. Phys. Rev. Lett. 2018, 120 (22), 227601.(4) Tang, B.; Tang, B.; Hussain, S.; Xu, R.; Cheng, Z.; Liao, J.; Chen, Q. Novel Type of Synaptic Transistors Based on a Ferroelectric Semiconductor Channel. ACS Appl. Mater. Interfaces 2020, 12 (22), 24920–24928.(5) Wang, L.; Liao, W.; Wong, S. L.; Yu, Z. G.; Li, S.; Lim, Y. F.; Feng, X.; Tan, W. C.; Huang, X.; Chen, L.; et al. Artificial Synapses Based on Multiterminal Memtransistors for Neuromorphic Application. Adv. Funct. Mater. 2019, 29 (25), 1–10.(6) Claro, M. S.; Grzonka, J.; Nicoara, N.; Ferreira, P. J.; Sadewasser, S. Wafer-Scale Fabrication of 2D β-In2Se3 Photodetectors. Adv. Opt. Mater. 2020, 2001034, 1–9.(7) Ashok, A.; Regmi, G.; Velumani, S. Growth of In2Se3 Thin Films Prepared by the Pneumatic Spray Pyrolysis Method for Thin Film Solar Cells Applications. 2020 17th Int. Conf. Electr. Eng. Comput. Sci. Autom. Control. CCE 2020 2020, 2–7.(8) Li, J.; Li, H.; Niu, X.; Wang, Z. Low-Dimensional In2Se3 Compounds: From Material Preparations to Device Applications. ACS Nano 2021, 15 (12), 18683–18707.(9) Zhang, X.; Lee, S.; Bansal, A.; Zhang, F.; Terrones, M.; Jackson, T. N.; Redwing, J. M. Epitaxial Growth of Few-Layer β-In2Se3 Thin Films by Metalorganic Chemical Vapor Deposition. J. Cryst. Growth 2020, 533 (December 2019), 125471.(10) Shen, Y. F.; Yin, X. B.; Xu, C. F.; He, J.; Li, J. Y.; Li, H. D.; Zhu, X. H.; Niu, X. Bin. Growth and Structural Characteristics of Metastable β-In2Se3 Thin Films on H-Terminated Si(111) Substrates by Molecular Beam Epitaxy. Chinese Phys. B 2020, 29 (5).

First-principles study of the effect of point defects (Hi-VAl) on the magnetic and photocatalytic properties of monolayer AlN: Be/Mg/Ca

Single-layer AlN is obtained by chemical vapor deposition and molecular beam epitaxy; therefore, Hi impurities will inevitably remain in AlN in a vacuum environment. Hi impurities are often overlooked and VAl is experimentally challenging. Given these problems, the effects of point defects (Hi-VAl) on the photocatalytic and magnetic properties of monolayer AlN: Be/Mg/Ca were studied using first-principles calculations. Results showed that monolayer Al34HiBeN36 had relatively high stability, relatively strong carrier activity, relatively strong visible light effect, long life, relatively difficult carrier recombination, and relatively good oxidation–reduction properties. The results provide a theoretical reference for designing novel two-dimensional photocatalysts.

Progress in III-Nitride Tunnel Junctions for Optoelectronic Devices

The recent progress in III-nitride tunnel junction (TJ) contacts for optoelectronic devices is summarized in this review paper. Due to the rapid developments of various III-nitride based optoelectronic devices for different emerging applications, TJs are of interest for these devices to achieve more efficient device performance. This review paper first briefly describes the working principles of TJs and the advantages of employing TJs in optoelectronic devices. The significant advancements in III-nitride TJs realized by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are addressed, as well as their TJ device performances. Lastly, the critical challenges for employing III-nitride TJs in wide bandgap ultraviolet (UV) light emitters are discussed for future investigations.

(Invited, Digital Presentation) Ultrathin Lateral Heterostructures Based on Two-Dimensional Semiconductors

Low-dimensional (2D) materials, including carbon nanotubes, graphene, boron nitrides, and transition metal dichalcogenides (TMDs), have provided a platform to explore novel physics at the nano-scale. In addition to the fascinating properties of low-dimensional materials themselves, they allow exploring novel superstructures, such as heterojunctions, heterostacks, and superlattices, which give even broader possibilities. We are working on low-dimensional superstructures fabricated by (1) crystal growth with metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), and (2) stacking each component with the full-dry-transfer-based manipulation technique[1]-[6]. This presentation will focus on our recent works on low-dimensional superstructures, such as 2D ultrathin lateral superlattices. For example, we have successfully realized MoS2/WS2 2D lateral superlattices with a periodicity of down to one-atom-thick by MOCVD with an automatic valve control system. Also, we have observed characteristic PL arising probably from 1D junction structures. More details on the fabrication and optical properties of these superstructures will be addressed in this talk.[1] Y. Murai, et. al., ACS Nano 2021, doi.org/10.1021/ascnano.1c04584 [2] T. Hotta, et. al., ACS Nano 2021, 51:1370-1377.[3] T. Hotta, et al., Phys. Rev. B 2020, 102:115424.[4] S. Zhao et al., Phys. Rev. Lett. 2020, 124:106101.[5] Y. Uchiyama et. al., npj 2D Mater. App. 2019, 3:26.[6] M. Okada, et. al., ACS Nano 2018, 12:2498-2505.

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices.

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano‐electronics and nano‐optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed‐laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up‐to‐date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up‐to‐date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

High thermal conductivity and thermal boundary conductance of homoepitaxially grown gallium nitride (GaN) thin films

Gallium nitride (GaN) has emerged as a quintessential wide band-gap semiconductor for an array of high-power and high-frequency electronic devices. The phonon thermal resistances that arise in GaN thin films can result in detrimental performances in these applications. In this work, we report on the thermal conductivity of submicrometer and micrometer thick homoepitaxial GaN films grown via two different techniques (metal-organic chemical vapor deposition and molecular beam epitaxy) and measured via two different techniques (time domain thermoreflectance and steady-state thermoreflectance). When unintentionally doped, these homoepitaxial GaN films possess higher thermal conductivities than other heteroepitaxially grown GaN films of equivalent thicknesses reported in the literature. When doped, the thermal conductivities of the GaN films decrease substantially due to phonon-dopant scattering, which reveals that the major source of phonon thermal resistance in homoepitaxially grown GaN films can arise from doping. Our temperature-dependent thermal conductivity measurements reveal that below 200 K, scattering with the defects and GaN/GaN interface limits the thermal transport of the unintentionally doped homoepitaxial GaN films. Further, we demonstrate the ability to achieve the highest reported thermal boundary conductance at metal/GaN interfaces through in situ deposition of aluminum in ultrahigh vacuum during molecular beam epitaxy growth of the GaN films. Our results inform the development of low thermal resistance GaN films and interfaces by furthering the understanding of phonon scattering processes that impact the thermal transport in homoepitaxially grown GaN.

Antimony segregation in an InAs/InAs1−xSbx superlattice grown by metalorganic chemical vapor deposition

Metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are two versatile growth techniques that can readily produce multilayer structures with atomic-level precision control, which have found broad applications in technology. However, compared to MBE, MOCVD growth involves the surface reaction of metal-organic precursor compounds, which changes during film deposition. Consequently, a thorough investigation on the chemical profile layer-by-layer is critical for optimizing MOCVD film performance. Here, we examine Sb segregation in an MOCVD-grown InAs/InAs1−xSbx superlattice by analyzing composition and lattice strain at atomic resolution using scanning transmission electron microscopy and compare with the previously reported MBE growth results. Our findings show a different Sb profile along the growth direction in MOCVD, with the segregation coefficient being higher at the InAsSb-on-InAs interface (0.807 ± 0.021) than at the InAs-on-InAsSb interface (0.695 ± 0.009), giving rise to asymmetric composition and lattice strain profiles unlike those obtained with MBE. Furthermore, we obtain direct evidence of Sb clusters with size of ∼1–3 nm and Sb ordering within the InAs1−xSbx layer, which is largely absent in the reported MBE growth. These findings demonstrate the concurrent interplay between surface segregation, surface reconstruction, and surface reaction that is unique to MOCVD growth with broad implications on preparing Sb-containing quantum materials.

Ge doping of β-Ga2O3 by MOCVD

We report on the Ge doping of Ga2O3 using metalorganic chemical vapor deposition (MOCVD) epitaxy. The effects of the GeH4/N2 flow rate, substrate temperature, VI/III ratio, type of Ga precursor, and MOCVD reactor geometry on the incorporation efficiency of Ge into Ga2O3 were explored. The Ge concentration incorporated into the films was quantified using Hall and secondary ion mass spectroscopy measurements. The increase in the GeH4/N2 flow rate, decrease in the substrate temperature, and increase in the VI/III ratio increase the amount of Ge incorporated into Ga2O3. The incorporation of Ge into the lattice of Ga2O3 was found to be strongly dependent on the substrate temperature, i.e., lowering the growth temperature leads to a higher doping concentration. Films with a free carrier concentration ranging from ∼2 × 1016 to ∼3 × 1020 cm−3 and corresponding mobilities ranging from ∼140 to ∼38 cm2/Vs were realized. The incorporation of Ge into the films was also found to be strongly dependent on the metalorganic precursor type used for the growth of the Ga2O3 film. We found that it was more challenging to dope Ga2O3 with Ge using trimethylgallium rather than triethylgallium as a source for Ga. Additionally, we found that Ge doping has a strong memory effect dependent on the reactor geometry. The result highlights the challenges in achieving controllable Ge doping for n-type conductivity despite all the positive indicators from theoretical studies that suggest that Ge is a suitable dopant candidate for Ga2O3 similar to Si and Sn.

Hydrogen-Mediated CVD Epitaxy of Graphene on SiC: Implications for Microelectronic Applications

Despite the large body of literature reporting on the growth of graphene (Gr) on 6H–SiC(0001) by chemical vapor deposition (CVD), some important issues have not yet been solved, and full-wafer-scale epitaxy of Gr remains challenging, hampering applications in microelectronics. With this study, we shed light on the generic mechanism which produces the coexistence of two different types of Gr domains: Gr on hydrogen (H-Gr) and Gr on buffer layer ((6 × 6) Gr), whose proportion can be carefully controlled by tuning the H2 flow rate. We show for the first time that the growth of Gr by CVD under a H2/Ar flow rate proceeds in two stages. First, the nucleation of free-standing epitaxial Gr on hydrogen (H-Gr) occurs; then, H-atoms eventually desorb from either step edges or defects. This gives rise, for a H2 flow rate below a critical value, to the formation of (6 × 6) Gr domains. The front of H-desorption progresses proportionally to the reduction of H2. Using the robust and generic X-ray photoelectron spectroscopy (XPS) analysis, we realistically quantify the proportions of H-Gr and (6 × 6) Gr domains of a Gr film synthesized under any experimental conditions. Scanning tunneling microscopy supports the XPS measurements. From these results, we can deduce that the H-assisted CVD growth of Gr developed here is a unique method to grow fully free-standing H-Gr in contrast to the method consisting of H-intercalation below (6 × 6) Gr epitaxial layer. These results are of crucial importance for future applications of Gr/SiC(0001) in nano- and microelectronics and in particular for field-effect transistors, for which maximization of mobility is mandatory. This work also provides the groundwork for the use of Gr as an optimal template layer for van der Waals homo- and heteroepitaxy for optoelectronic applications.

Two‐dimensional magnetic transition metal chalcogenides

AbstractThe field of two‐dimensional (2D) magnets has expanded rapidly during the past several years since the first demonstration of intrinsic 2D magnetism in atomically thin CrI3 and Cr2Ge2Te6 in 2017. 2D transition metal chalcogenides (TMCs), a class of strongly correlated materials, have exhibited a wide variety of novel electronic and optical properties, and more recently magnetism. Here, we review recent experimental progress achieved in the growth of 2D magnetic TMC materials using chemical vapor deposition and molecular beam epitaxy methods. Outstanding examples include the demonstration of room temperature intrinsic and extrinsic ferromagnetism in monolayer VSe2, MnSe2, Cr3Te4, V‐doped WSe2, and so on. A brief discussion on the origin of the exotic magnetic properties and emergent phenomena is also presented. Finally, we summarize the remaining challenges and future perspective on the development of 2D magnetic materials for next‐generation spintronic devices.

Structural and optical properties of self-assembled AlN nanowires grown on SiO2/Si substrates by molecular beam epitaxy

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO2/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO2 interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20–30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1–2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy.

Mg acceptor doping in MOCVD (010) β -Ga2O3

In this Letter, in situ Mg doping in β-Ga2O3 was demonstrated via metalorganic chemical vapor deposition (MOCVD) epitaxy. The electrical insulating property of the Mg acceptors in β-Ga2O3 was found to be intrinsically activated in the as-grown Mg-doped β-Ga2O3 thin films. Growth conditions for MOCVD β-Ga2O3 were further explored and optimized at a lower growth temperature regime, leading to a better confinement of the Mg-doping profile. Detailed analysis of Mg diffusion characteristics revealed a diffusion barrier energy Ebarrier ∼ 0.9 eV for Mg in MOCVD β-Ga2O3, which is likely related to an interstitial-assisted process. Surface morphologies and electron transport were characterized on samples grown with different growth temperatures and Mg doping levels. The MOCVD growth method demonstrated its feasibility to grow semi-insulating Mg-doped β-Ga2O3 epilayers with controllable Mg incorporation while maintaining good material quality and smooth surface morphology. From capacitance-voltage charge profiling, it is verified that the Mg-doped buffer layer grown at the substrate-epilayer interface could effectively compensate the charge accumulation at the interface. The in situ acceptor doping of Mg in MOCVD β-Ga2O3 will provide versatility for designing β-Ga2O3 power devices.

Electrical Characterization of Gallium Nitride Thin Films Synthesized By Electrochemical Deposition

Group III-nitride semiconductor materials show promise in a variety of electronics applications, including diodes, transistors, LEDs and photovoltaic applications. The new generation of semiconductors rely heavily on nanotechnology and advances in nanomaterials. Group III-nitride semiconductor materials such as GaN, AlN and InN are promising materials since their properties are ideal for use in optoelectronic devices, high power and high temperature electronic devices. These materials have wide band gaps between 1.9 and 6.3 eV and, for electronic applications, these materials are usually synthesised by means of chemical vapour deposition or molecular beam epitaxy. These techniques are expensive, therefore we investigated room temperature electrochemical growth to synthesize GaN. The advantages of electrochemical deposition are the controllability of the thickness and surface morphology by varying the deposition parameters, cost-effectiveness, the minimum waste generation, long bath lifetime and easy set-up. In this study, room temperature electrochemical deposition has been used to synthesise GaN nitride on Si substrate. Results: The XRD spectrum of GaN thin films grown under the different conditions are shown in Figure 1. Sample a (1 mA/cm2 for 30 min) and b(3 mA/cm2 for 30 min) were prepared at room temperature. The XRD analysis showed the presence of hexagonal and cubic structure with crystallite sizes calculated from Scherer’s formula were 130 and 140 nm From SEM images, however, the results varied with growth condition. Good quality Schottky diodes were fabricated on the GaN thin films and DLTS measurements were performed. Electron traps with an activation energy of 0.48 and 0.47 eV, were observed in sample a and b, respectively. Figure 1