In the past studies have shown that the addition of Ge and Sn into Si lattice to form SiGeSn enhances its carrier mobility and band-gap properties. Conventionally SiGeSn epitaxial films are grown using Ultra-High Vacuum (UHV) conditions with pressures ranging from 10-8 torr to 10-10 torr which makes high volume manufacturing very expensive. On the contrary, the use of low-pressure CVD processes (vacuum levels of 10-2 torr to 10-4 torr) is economically more viable and yields faster deposition of SiGeSn films. This study outlines the use of a cost-effective Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor to study the impact of substrate temperature and substrate type on the growth and properties of polycrystalline SiGeSn films. The onset of polycrystallinity in the films is attributed to the oxygen-rich PECVD chamber conditions explained using the Volmer-Weber (3D island) mechanism. The properties of the films were characterized using varied techniques to understand the impact of the substrate on film composition, thickness, crystallinity, and strain.

Heteroepitaxial growth of Ge films on Si is necessary for the progress of integrated Si photonics technology. In this work, an in-house assembled plasma enhanced chemical vapor deposition reactor was used to grow high quality epitaxial Ge films on Si (100) substrates. Low economic and thermal budget were accomplished by the avoidance of ultra-high vacuum conditions or high temperature substrate pre-deposition bake for the process. Films were deposited with and without plasma assistance using germane (GeH4) precursor in a single step at process temperatures of 350–385 °C and chamber pressures of 1–10 Torr at various precursor flow rates. Film growth was realized at high ambient chamber pressures (>10−6 Torr) by utilizing a rigorous ex situ substrate cleaning process, closely controlling substrate loading times, chamber pumping and the dead-time prior to the initiation of film growth. Plasma allowed for higher film deposition rates at lower processing temperatures. An epitaxial growth was confirmed by X-Ray diffraction studies, while crystalline quality of the films was verified by X-ray rocking curve, Raman spectroscopy, transmission electron microscopy and infra-red spectroscopy.

Epitaxial films of β-(AlxGa1−x)2O3, β-Ga2O3, and β-(InxGa1−x)2O3 were grown on (001) sapphire substrates via metalorganic chemical vapor deposition (MOCVD). The compositions of the films as determined from energy dispersive x-ray analysis (EDX) and x-ray photoelectron spectroscopy (XPS) results were XAl = 0.57 ± 0.05 and 0.76 ± 0.05 and XIn = 0.12 ± 0.05 and 0.21 ± 0.05. The optical bandgap was found to correspondingly vary between 6.0 ± 0.2 and 3.9 ± 0.1 eV, as a function of composition via XPS and UV–visible spectroscopy (UV–Vis). X-ray diffraction, scanning electron microscopy, and atomic force microscopy revealed the films to be highly-oriented $$ \left( {\bar{2}01} \right) $$-epitaxial films with nanocrystalline domains. Schottky- and MSM-based solar-blind UV photodetectors were fabricated on the films and showed responsivities at 20 V varying from > 104 A/W for the Ga2O3 devices, > 103 A/W for the (AlxGa1−x)2O3 devices and > 102 A/W for the (InxGa1−x)2O3 devices. Modest shifts in wavelength selectivity corresponding with the changes in composition/bandgap were also measured. Time response measurements on Schottky and MSM detectors reveal rise and dwell times on the order of a minute, indicating the presence of photoconductive gain. Noise-equivalent powers were in the fW–pW regime with specific detectivities ($$ D^{*} $$) between 1010 and 1012 Jones. Scanning photocurrent maps display large photocurrent generation at the Schottky interface in the case of a β-Ga2O3 Schottky detector, whereas for an β-(InxGa1−x)2O3 MSM detector the photocurrent generation occurs in the device channel and at the Schottky interface.

In recent years, Ga2O3 solar-blind photodetectors (SBPDs) have received great attention for their potential applications in solar-blind imaging, deep space exploration, confidential space communication, etc. In this work, we demonstrated an ultra-high-performance epsilon-Ga2O3 metal-semiconductor-metal (MSM) SBPD. The fabricated photodetectors exhibited a record-high responsivity and fast decay time of 230 A/W and 24 ms, respectively, compared with MSM-structured Ga2O3 photodetectors reported to date. Additionally, the epsilon-Ga2O3 MSM SBPD presents an ultrahigh detectivity of 1.2 x 10(15) Jones with a low dark current of 23.5 pA under an operation voltage of 6 V, suggesting its strong capability of detecting an ultraweak signal. The high sensitivity and wavelength selectivity of the photodetector were further confirmed by the record-high responsivity rejection ratio (R-250 (nm)/R-400 nm) of 1.2 x 10(5). From the temperature-dependent electrical characteristics in the dark, the thermionic field emission and Poole-Frenkel emission were found to be responsible for the current transport in the low and high electric field regimes, respectively. In addition, the gain mechanism was revealed by the Schottky barrier lowering effect due to the defect states at the interface of the metal contact and Ga2O3 or in the bulk of Ga2O3 based on current transport mechanism and density functional theory calculations. These results facilitate a better understanding of epsilon-Ga2O3 photoelectronic devices and provide possible guidance for promoting their performance in future solar-blind detection applications.

SiGeSn is a promising group IV material to develop the field of silicon photonics. Increasing the tin concentration in the alloy is desired in order to achieve a direct bandgap in the material. This necessitates low temperature growth and proper strain management in the films during growth to prevent tin segregation. In this work, plasma enhanced chemical vapor deposition (PECVD) was used to grow composition graded SiGeSn films at low processing temperatures of 350 degrees C-380 degrees C using a simplified PECVD reactor. Polycrystalline films were deposited using a multi-step approach to prevent tin phase separation at higher Sn concentrations by alleviating the strain arising from the incorporation of Sn in the film lattice. Rutherford Back Scattering measurements indicated that films with Sn content of up to 8% without any Sn phase segregation were achieved. The structural and optical properties of the films were analyzed using X-ray diffraction and Raman spectroscopy. (C) 2020 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.

Germanium on silicon has the potential to enable growth of high quality group IV semiconductors for the field of silicon photonics by being used as virtual buffers. In addition Ge layers can be used in the fabrication of passive components such as waveguides and resonators. A cost-effective technology for the heteroepitaxial growth of Ge on Si will be highly beneficial. In this work, a simplified chemical vapor deposition reactor was assembled in-house to deposit Ge films. Epitaxial Ge film growth on Si (100) substrates was accomplished without requiring ultra-high vacuum conditions or a high temperature substrate pre-deposition bake thereby minimizing the economic and thermal budget of the process. Films were deposited using digermane (Ge2H6) precursor in a single step at a process temperature of 350 degrees C and chamber pressures of 1-10 Torr. Films were achieved at high chamber background pressures (>10(-6) Torr) by implementing a thorough ex situ substrate cleaning process and optimizing times for wafer loading, chamber pumping and initiation of film deposition. Transmission electron microscopy and X-ray diffraction were used to confirm the epitaxial growth and crystalline quality of the obtained films. Optical properties of the films were characterized using Raman spectroscopy, Photoluminescence and Infra-red spectroscopy. (C) 2020 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.

SiGeSn is a promising group IV semiconducting alloy to advance the field of silicon photonics. The bandgap of the alloy can be tuned by varying its' Si and Sn concentrations for developing devices in the near infrared (NIR) range. The growth of the material using a cost-effective process is still challenging due to various obstacles. In this work, a simplified in-house assembled plasma enhanced chemical vapor deposition (PECVD) reactor was used to deposit SiGeSn films. Plasma allows for the use of commercially available precursors (GeH4, Si(2)H(6)and SnCl4) while providing high dissociation and deposition growth rates. Polycrystalline films were deposited at susceptor temperatures in the range of 350 degrees C-450 degrees C to study the effect of process temperature on the Sn segregation and Sn incorporation in the films. Selective area growth (SAG) of SiGeSn films was also achieved by depositing films on patterned silicon substrates. The composition of the films was characterized by Rutherford Back Scattering while the structural and optical properties of the films were analyzed using X-ray diffraction and Raman spectroscopy. Selectively grown films were fabricated into basic test photodiodes and evaluated for electrical performance under NIR illumination.

alpha -phase-dominated Ga2O3 films were heteroepitaxially grown on c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD), followed by the fabrication of ultraviolet (UV) photodetectors (PDs) with a metal-semiconductor-metal (MSM) structure using a Ti/Au metal stack as the electrode in an interdigitated geometry. The PDs possess a record low dark current of 81 fA under a bias voltage of 12 V, with a high sensitivity as confirmed by a record high photo-to-dark-current ratio exceeding 10(7) under 254 nm light illumination. Furthermore, the PDs also exhibit a high responsivity and the photoconductive gain of 11.5 A/W and 55, respectively. Most importantly, it shows an ultrahigh detectivity of 1.0 x 10(15) Jones with a quite fast response of 42 ms, paving the way for advancement of next-generation PD applications.

In this letter we present a high performance epsilon-Ga2O3 solar-blind photodetector (SBPD) with asymmetric Schottky electrodes. The epsilon-Ga2O3 films were heteroepitaxially grown on c-plane sapphire by metal-organic chemical vapor deposition (MOCVD). The SBPDs were fabricated in the form of lateral interdigitated Schottky electrodes composed of a Pt/Ti/Au metal stack and a Ti/Au metal stack. The SBPD shows a rectification ratio of similar to 10(2) with low dark current of 25 pA at V = 6 V. A high photo-to-dark-current ratio of 5.7 x 10(4) under 254 nm light illumination was measured. Furthermore, the SBPD exhibits an ultrahigh detectivity and external quantum efficiency of 4.2 x 10(14) Jones and 4.1 x 10(4) %, respectively. Most importantly, the SBPD possesses an ultrahigh responsivity of 84 A/W with a fast response speed of 100 ms, suggesting the promise of epsilon-Ga2O3 in the future photodetector applications.

In this work, we have compared and modeled the electrical characteristics of two different geometrical structures of unintentionally doped (UID) thin film beta-Ga2O3 based Schottky barrier diodes (SBD). It was observed that a lateral SBD structure can withstand much higher breakdown voltage compared to the vertical SBD structure, but suffers from significantly higher on-resistance, which can be explained considering the differences in their geometrical configuration. The schottky barrier height was determined from both capacitance voltage (C-V) and current density voltage (J-V) characteristics of the SBDs, which matched well among themselves and with the values reported in the literature. The C-V measurement was also used to determine the doping concentration in the beta-Ga2O3 substrate, which agreed well with the manufacturer specifications. (c) 2019 The Electrochemical Society.

The annealing behavior and electrical properties of lead titanate (PTO) and lead zirconate titanate (PZT) thin films deposited by atomic layer deposition (ALD) were investigated. ALD films were deposited on platinized silicon substrates. The composition of the PTO films ranged from Pb-deficient Pb0.73TiO3-x to Pb-rich Pb2.3TiO3-x, including stoichiometric PbTiO3. The PZT films were all Pb-deficient, with Pb/(Zr + Ti) ratios of 0.40-0.75. Stoichiometric PbTiO3 films showed the perovskite structure, and a well-defined, dense microstructure after crystallization at 600 degrees C for 1 min in 2 slpm O-2 in a rapid thermal annealer (RTA). Pb excess PbTiO3 films developed into perovskite PbTiO3 after annealing but the surface microstructure showed a large grained microstructure with significant porosity. The dielectric constant was 140 at 10 kHz and a ferroelectric polarization - electric field curve was observed. A Pb-deficient Pb0.66Zr0.55Ti0.45O3-x film showed a dense and fine-grained microstructure after annealing at 700 degrees C for 1 min in 2 slpm O-2 in a rapid thermal annealer (RTA). The dielectric constant was 100 at 10 kHz. (C) 2018 Elsevier B.V. All rights reserved.

Thermoacoustic devices were fabricated on flexible plastic substrates with few layer graphene films. The graphene films were grown by chemical vapor deposition on nickel and copper foil substrates, then transferred to the plastic substrates. A simple and reliable technique for electrical contact formation to the graphene was developed, using material from the nickel or copper growth substrate. The thermal and thermoacoustic behavior of the graphene thermophones was characterized as a function of the input electrical signal voltage and frequency, at frequencies up to 10 kHz. The average surface temperature of the device was measured under the same conditions using an infrared pyrometer. Accurate sound reproduction was demonstrated with graphene based thermoacoustic devices, including voice and music. The efficiency of the device is limited by the thermal quenching effect of the substrate. (C) 2018 Elsevier B.V. All rights reserved.

A new atomic layer deposition (ALD) tool, named the meter-scale ALD system (MSAS), has been designed, constructed, and tested to apply uniform protective coatings over a substrate 36” in diameter. The novel chamber design utilizes a large substrate to be coated as a wall of the chamber. We discuss conceptual design and implementation of this new tool with potential applications to large astronomical optics, specifically protective coatings for silver telescope mirrors, and other large structures. In this work, trimethylaluminum and water was used to deposit aluminum oxide at a low reaction temperature of 60°C. Growth rates, dependent on precursor pulse times and chamber purge times, show that the two half-reactions occur in a saturated regime, which demonstrates typical characteristics of ideal ALD behavior. Thickness uniformity across a 36” substrate is within 2.5% of the average film thickness. MSAS aluminum oxide deposition process parameters are compared with those of a conventional 4” wafer-scale ALD tool. The results show promising application of transparent robust dielectric films as uniform barriers across large optical components.

ABSTRACT Heteroepitaxial films of GaO were grown on c-plane sapphire (0001). The stable phase β-GaO was grown using the metalorganic chemical vapor deposition technique, regardless of precursor flow rates, at temperatures between 500C and 850C. Metastable α- and ϵ-phases were grown when using the halide vapor phase epitaxy (HVPE) technique, at growth temperatures between 650C and 850C, both separately and in combination. XTEM revealed the better lattice-matched α-phase growing semi-coherently on the substrate, followed by ϵ-GaO. The epitaxial relationship was determined to be [] ϵ-GaO [] α-GaO [] α-AlO. SIMS revealed that epilayers forming the ϵ-phase contain higher concentrations of Cl introduced during HVPE growth. GRAPHICAL ABSTRACT IMPACT STATEMENT This study demonstrates one of the first epitaxial growths of multiple polymorphs of GaO on sapphire (0001) substrates, including its β-, α-, and ϵ-phases. Epitaxial relationship is confirmed through HRTEM.

In this Phase I Program, we demonstrated the feasibility of a new generation of sensitive wide bandgap UV tunable photodiodes, which can detect small, fast pulses of UV lights and support detection of the Cherenkov light in crystals and glasses, scintillation light in neutrino, dark matter, and rare decay experiments. We believe the new tunable UV photodetector can replace the photomultiplier tubes in Cherenkov, neutrino, and dark matter detectors and save on size, weight, and required power. Furthermore, the SBIR team anticipates a variety of commercial applications for the UV photodetectors developed in this program. Solar- and visible-blind UV PDs are widely used in military, industrial, and commercial applications and are needed for applications, from solar observations, UV astronomy, missile tracking, automatization, short-range communication security, as well as environmental and biological research. Efficient large area wide bandgap semiconductor PDs can be used to support the detection of spacecraft based radiation as well. In summary, Ga2O3–based UV photodiodes were fabricated and characterized in this project. β-Ga2O3 layers were grown on sapphire and single crystal bulk Ga2O3 substrates. Tunability of energy bandgap of the Ga2O3 in UVA, UVB, and UVC ranges were accomplished by alloying Ga2O3 with Al2O3 and In2O3. Feasibility of the Ga2O3–based UV photodiodes was tested using MOCVD growth of Ga2O3 and its alloys on sapphire and bulk Ga2O3 substrates, both of which were successfully demonstrated. High photo-to-dark current ratio and very low dark currents are achieved for the UV photodiodes.

A new atomic layer deposition (ALD) tool has been designed, constructed, and tested to apply uniform protective coatings over a substrate 36” in diameter. The new tool, named the meter-scale ALD system (MSAS), employs a novel chamber design which utilizes a large substrate to be coated as a wall of the chamber. Conceptual design and implementation of this new tool are discussed with potential applications to large astronomical telescope optics, specifically protective coatings for silver mirrors, and other future large structures. In this work, aluminum oxide was deposited by thermal ALD using trimethylaluminum and water at a low reaction temperature of 60°C. Thickness uniformity across a 36” substrate is within 2.5% of the average film thickness. MSAS aluminum oxide deposition process parameters are compared with those of a conventional 4” wafer-scale ALD tool. The results show promising application of transparent robust dielectric films as uniform barriers across large optical components as well as multiple wafer assemblies encountered in semiconductor processing.

Atomic Layer Deposition (ALD) is very attractive for producing optical quality thin films, including transparent barrier films on metal-coated astronomical mirrors. To date, ALD of mirror coatings has been limited to relatively small-sized substrates. A new ALD tool has been designed, constructed, and tested to apply uniform protective coatings over a 0.9 m diameter substrate in a 1 m diameter scale deposition plane. The new tool, which we have named the meter scale ALD system (MSAS), employs a unique chamber design that isolates a large substrate surface to be coated by utilizing the substrate as a wall of the reaction chamber. The MSAS is mechanically designed to be rapidly reconfigurable for selective area coating of custom substrates with arbitrary shape, size, and permanent backside hardware attachments. The design, implementation, results, and future applications of this new tool are discussed for coating large-area optical substrates, specifically protective coatings for silver mirrors, and other future large astronomical optics. To demonstrate the potential of this new design, aluminum oxide was deposited by thermal ALD using trimethylaluminum and water at a low reaction temperature of 60 °C. Growth rate and uniformity, which are dependent on precursor pulse times and chamber purge times, show that the two half-reactions occur in a saturated regime, matching typical characteristics of ideal ALD behavior. Aluminum oxide deposition process parameters of the MSAS are compared with those of a conventional 100 mm wafer-scale ALD tool, and saturated ALD growth over the 0.9 m substrate is realized with a simple scaling factor applied to precursor pulse and purge times. This initial test shows that lateral thickness uniformity across a 0.9 m substrate is within 2.5% of the average film thickness, and simple steps to realize 1% uniformity have been identified for next growths. Results show promising application of transparent robust dielectric films as uniform coatings across large optical components scaled to meter-sized substrates.

Atomic layer deposition (ALD) processes were demonstrated for lead-titanate (PbTiO3) and lead-zirconium-titanate (PZT) films. The metal organic precursors were tetraethyl lead Pb(Et)4 reacted with ozone; along with tetrakis ethylmethylamino zirconium and tetrakis dimethylamino titanium reacted with either water or ozone. These precursors were selected based on compatibility with ALD processes for the component oxides. Single oxide films of PbO, ZrO2, and TiO2 were easily deposited using the selected precursors. ALD of the complex oxide films was done by combining the ALD processes for the component oxide films. The compositions of Pb, Zr, and Ti in the films could be predictably controlled by the relative ratio of Pb, Zr, and Ti precursor doses during the ALD process, and controlled composition for both PbTiO3 and PZT films was demonstrated.

Precise control of the heteroepitaxy on a low-cost foreign substrate is often the key to drive the success of fabricating semiconductor devices in scale when a large low-cost native substrate is not available. Here, we successfully synthesized three different phases of Ga2O3 (alpha, beta, and epsilon) films on c-plane sapphire by only tuning the flow rate of HCl along with other precursors in an MOCVD reactor. A 3-fold increase in the growth rate of pure beta-Ga2O3 was achieved by introducing only 5 sccm of HCl flow. With continuously increased HCl flow, a mixture of beta- and epsilon-Ga2O3 was observed, until the Ga2O3 film transformed completely to a pure e-Ga2O3 with a smooth surface and the highest growth rate (similar to 1 mu m/h) at a flow rate of 30 sccm. At 60 sccm, we found that the film tended to have a mixture of a- and epsilon-Ga2O3 with a dominant alpha-Ga2O3, while the growth rate dropped significantly (similar to 0.4 mu m/h). The film became rough as a result of the mixture phases since the growth rate of epsilon-Ga2O3 is much higher than that of alpha-Ga2O3. In this HCl-enhanced MOCVD mode, the Cl impurity concentration was almost identical among the investigated samples. On the basis of our density functional theory calculation, we found that the relative energy between beta-, epsilon-, and alpha-Ga2O3 became smaller, thus inducing the phase change by increasing the HCl flow in the reactor. Thus, it is plausible that the HCl acted as a catalyst during the phase transformation process. Furthermore, we revealed the microstructure and the epitaxial relationship between Ga2O3 with different phases and the c-plane sapphire substrates. Our HCl-enhanced MOCVD approach paves the way to achieving highly controllable heteroepitaxy of Ga2O3 films with different phases for device applications.