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  1. Abstract

    In this work, we report a simplified method to measure thermal conductivity from the typical Raman thermometry method by employing a much simpler dispersion relationship equation and the Debye function, instead of solving the heat equation. Unlike the typical Raman thermometry method, our new method only requires monitoring of the temperature-dependent Raman mode shifting without considering laser power-dependent Raman mode shifting. Thus, this new calculation method offers a simpler way to calculate the thermal conductivity of materials with great precision. As a model system, theβ-Ga2O3nanomembrane (NM) on a diamond substrate was prepared to measure thermal conductivity ofβ-Ga2O3NMs at different thicknesses (100 nm, 1000 nm, and 4000 nm). Furthermore, the phonon penetration depth was investigated to understand how deep phonons can be dispersed in the sample so as to guide the dimensional design parameter of the device from the thermal management perspective.

  2. Abstract In this paper, transient delayed rise and fall times for beta gallium oxide ( β -Ga 2 O 3 ) nanomembrane (NM) Schottky barrier diodes (SBDs) formed on four different substrates (diamond, Si, sapphire, and polyimide) were measured using a sub-micron second resolution time-resolved electrical measurement system under different temperature conditions. The devices exhibited noticeably less-delayed turn on/turn off transient time when β -Ga 2 O 3 NM SBDs were built on a high thermal conductive (high- k ) substrate. Furthermore, a relationship between the β -Ga 2 O 3 NM thicknesses under different temperature conditions and their transient characteristics were systematically investigated and verified it using a multiphysics simulator. Overall, our results revealed the impact of various substrates with different thermal properties and different β -Ga 2 O 3 NM thicknesses on the performance of β -Ga 2 O 3 NM-based devices. Thus, the high- k substrate integration strategy will help design future β -Ga 2 O 3 -based devices by maximizing heat dissipation from the β -Ga 2 O 3 layer.
    Free, publicly-accessible full text available July 18, 2023
  3. In this study, we demonstrate a tolerant and durable Cr/Ni bilayer metal etch mask that allows us to realize approximately 150:1 etch selectivity to diamond. This result is achieved through the use of a very thin initial Cr layer of <10 nm thickness as part of the bilayer metal mask, which results in five to ten times improved selectivity than thick single metal layer masks or bilayer masks with thicker combinations. A finite element analysis was employed to design and understand the physics and working mechanism of the bilayer metal masks with different thicknesses. Raman spectroscopy and energy-dispersive x-ray spectroscopy on the diamond surface were also performed to investigate the changes in diamond quality before and after the deep diamond etching and found that no noticeable etch damage or defects were formed. Overall, this mask strategy offers a viable way to realize deep diamond etching using a high heat and chemistry tolerant and durable bilayer metal etching mask. It also offers several technological benefits and advantages, including various deposition method options, such as sputtering and physical vapor deposition, that can be used and the total thinness of the bilayer metal mask required given the higher selectivity allows us to realize finemore »diamond etching or high-aspect ratio etching, which is a critical fabrication process for future power, RF, MEMS, and quantum device applications.

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  4. In this paper, we demonstrated large-size free-standing single-crystal β-Ga 2 O 3 NMs fabricated by the hydrogen implantation and lift-off process directly from MOCVD grown β-Ga 2 O 3 epifilms on native substrates. The optimum implantation conditions were simulated with a Monte-Carlo simulation method to obtain a high hydrogen concentration with a narrow ion distribution at the desired depth. Two as grown β-Ga 2 O 3 samples with different orientations ([100] and [001]) were used to successfully create 1.2 μm thick β-Ga 2 O 3 NMs without any physical damage. These β-Ga 2 O 3 NMs were then transfer-printed onto rigid and flexible substrates such as SiC and polyimide substrates. Various material characterization studies were performed to investigate their crystal quality, surface morphologies, optical properties, mechanical properties, and bandgaps before and after the lift-off and revealed that the good material quality was maintained. This result offers several benefits in that the thickness, doping, and size of β-Ga 2 O 3 NMs can be fully controlled. Moreover, more advanced β-Ga 2 O 3 -based NM structures such as (Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 heterostructure NMs can be directly created from their bulk epitaxy substrates;more »thus this study provides a viable route for the realization of high performance β-Ga 2 O 3 NM-based electronics and optoelectronics that can be built on various substrates and platforms.« less
  5. This paper reports the fabrication of β-Ga 2 O 3 nanomembrane (NM) based flexible photodetectors (PDs) and the investigation of their optoelectrical properties under bending conditions. Flexible β-Ga 2 O 3 NM PDs exhibited reliable solar-blind photo-detection under bending conditions. Interestingly, a slight shifting in wavelength of the maximum solar-blind photo-current was observed under the bending condition. To investigate the reason for this peak shifting, the optical properties of β-Ga 2 O 3 NMs under different strain conditions were measured, which revealed changes in the refractive index, extinction coefficient and bandgap of strained β-Ga 2 O 3 NMs due to the presence of nano-sized cracks in the β-Ga 2 O 3 NMs. The results of a multiphysics simulation and a density-functional theory calculation for strained β-Ga 2 O 3 NMs showed that the conduction band minimum and the valence band maximum states were shifted nearly linearly with the applied uniaxial strain, which caused changes in the optical properties of the β-Ga 2 O 3 NM. We also found that nano-gaps in the β-Ga 2 O 3 NM play a crucial role in enhancing the photoresponsivity of the β-Ga 2 O 3 NM PD under bending conditions due to the secondarymore »light absorption caused by reflected light from the nano-gap surfaces. Therefore, this research provides a viable route to realize high-performance flexible photodetectors, which are one of the indispensable components in future flexible sensor systems.« less
  6. Theβ-Ga2O3nanomembrane (NM)/diamond heterostructure is one of the promising ultra-wide bandgap heterostructures that offers numerous complementary advantages from both materials. In this work, we have investigated the thermal properties of theβ-Ga2O3NM/diamond heterostructure with three different thicknesses ofβ-Ga2O3nanomembranes (NMs), namely 100 nm, 1000 nm, and 4000 nm thickβ-Ga2O3NMs using Raman thermometry. The thermal property—temperature relationships of theseβ-Ga2O3NM/diamond heterostructures, such as thermal conductivity and interfacial thermal boundary conductance were determined under different temperature conditions (from 100 K to 500 K with a 40 K interval). The result provides benchmark knowledge about the thermal conductivity ofβ-Ga2O3NMs over a wide temperature range for the design of novelβ-Ga2O3-based power electronics and optoelectronics.