More Like this

1. Electric field tuned anisotropic to isotropic thermal transport transition in monolayer borophene without altering its atomic structure

   https://doi.org/10.1039/d0nr03273e  

   Yang, Zhonghua ; Yuan, Kunpeng ; Meng, Jin ; Hu, Ming ( October 2020 , Nanoscale)

   null (Ed.)

   Thermal anisotropy/isotropy is one of the fundamental thermal transport properties of materials and plays a critical role in a wide range of practical applications. Manipulation of anisotropic to isotropic thermal transport or vice versa is in increasing demand. However, almost all the existing approaches for tuning anisotropy or isotropy focus on structure engineering or materials processing, which is time and cost consuming and irreversible, while little progress has been made with an intact, robust, and reversible method. Motivated by the inherent relationship between interatomic interaction mediated phonon transport and electronic charges, we comprehensively investigate the effect of external electric field on thermal transport in two-dimensional (2D) borophene by performing first-principles calculations along with the phonon Boltzmann transport equation. Under external electric field, the lattice thermal conductivity of borophene in both in-plane directions first increases significantly to peak values with the maximum augmentation factor of 2.82, and the intrinsic anisotropy (the ratio of thermal conductivity along two in-plane directions) is boosted to the highest value of 2.13. After that, thermal conductivities drop down steeply and anisotropy exhibits oscillating decay. With the electric field increasing to 0.4 V Å −1 , the thermal conductivity is dramatically suppressed to 1/40 of the original value at no electric field. More interestingly, the anisotropy of the thermal conductivity decreases to the minimum value of 1.25, showing almost isotropic thermal transport. Such abnormal anisotropic to isotropic thermal transport transition stems from the large enhancement and suppression of phonon lifetime at moderate and high strength of electric field, respectively, and acts as an amplifying or reducing factor to the thermal conductivity. We further explain the tunability of phonon lifetime of the dominant acoustic mode by an electron localization function. By comparing the electric field-modulated thermal conductivity of borophene with the dielectric constant, it is found that the screened potential resulting from the redistributed charge density leads to phonon renormalization and the modulation of phonon anharmonicity and anisotropy through electric field. Our study paves the way for robust tuning of anisotropy of phonon transport in materials by applying intact, robust, and reversible external electric field without altering their atomic structure and would have a significant impact on emerging applications, such as thermal management of nanoelectronics and thermoelectric energy conversion. 

   more » « less
2. Net negative contributions of free electrons to the thermal conductivity of NbSe 3 nanowires

   https://doi.org/10.1039/d0cp03484c  

   Pan, Zhiliang ; Yang, Lin ; Tao, Yi ; Zhu, Yanglin ; Xu, Ya-Qiong ; Mao, Zhiqiang ; Li, Deyu ( September 2020 , Physical Chemistry Chemical Physics)

   null (Ed.)

   Understanding transport mechanisms of electrons and phonons, two major energy carriers in solids, are crucial for various engineering applications. It is widely believed that more free electrons in a material should correspond to a higher thermal conductivity; however, free electrons also scatter phonons to lower the lattice thermal conductivity. The net contribution of free electrons has been rarely studied because the effects of electron–phonon (e–ph) interactions on lattice thermal conductivity have not been well investigated. Here an experimental study of e–ph scattering in quasi-one-dimensional NbSe 3 nanowires is reported, taking advantage of the spontaneous free carrier concentration change during charge density wave (CDW) phase transition. Contrary to the common wisdom that more free electrons would lead to a higher thermal conductivity, results show that during the depinning process of the condensed electrons, while the released electrons enhance the electronic thermal conductivity, the overall thermal conductivity decreases due to the escalated e–ph scattering. This study discloses how competing effects of free electrons result in unexpected trends and provides solid experimental data to dissect the contribution of e–ph scattering on lattice thermal conductivity. Lastly, an active thermal switch design is demonstrated based on tuning electron concentration through electric field. 

   more » « less
3. Characterizing Nonlinear Field Dependent Conductivity Layers to Mitigate Electric Field Within (U)WBG Power Modules Under High Frequency, High Slew Rate Square Voltage Pulses

   https://doi.org/10.1109/TPEC60005.2024.10472176  

   Adhikari, Pujan ; Ghassemi, Mona ( February 2024 , 2024 IEEE Texas Power and Energy Conference (TPEC))

   A vast majority of research on (U)WBG power modules has been going on to implement nonlinear resistive field grading material on metal-brazed substrates in reducing the electric field that is maximum at triple points (TPs). However, nearly all investigations have been conducted under either DC or 50/60 HZ sinusoidal AC voltages, even though the actual operation of envisioned (U)WBG power modules involves high-frequency square voltages with high slew rates. It has been validated by several studies that fast rise times of square voltages rapidly degrade the breakdown strength of insulation materials, leading to premature failure. Therefore, this paper introduces a nonlinear resistive field grading material or field-dependent conductivity (FDC) layer around the TP and metal edges to evaluate the electric field mitigation under a high frequency and high slew rate square voltage. The modeling and simulation of both coated and uncoated (U)WBG substrates were performed in COMSOL Multiphysics to assess the electric field reduction with the nonlinear FDC layer. The improvement of reduction in electric field under 100 kHz high slew rate square voltage is compared with that of 60 Hz. The results reveal a significant decrease in field stress at the TP, even under square voltages with fast rise times and high frequencies, when applying a nonlinear FDC coating, as opposed to the uncoated substrate. The influence of switching field (Eb) and nonlinearity coefficient (α) of nonlinear FDC layer is studied under 100 kHz square voltage, and it is concluded that α and Eb should be more than 10 and less than 8 kV/mm, respectively to achieve effective performance of resistive field grading material. 

   more » « less
4. RTD Light Emission around 1550 nm with IQE up to 6% at 300 K

   https://doi.org/10.1109/DRC50226.2020.9135175  

   Brown, E. R. ; Zhang, W-D. ; Fakhimi, P. ; Growden, T. A. ; Berger, P.R. ( June 2020 , IEEE Device Research Conference (DRC), Columbus, OH (June 22-24, 2020))

   Resonant tunneling diodes (RTDs) have come full-circle in the past 10 years after their demonstration in the early 1990s as the fastest room-temperature semiconductor oscillator, displaying experimental results up to 712 GHz and fmax values exceeding 1.0 THz [1]. Now the RTD is once again the preeminent electronic oscillator above 1.0 THz and is being implemented as a coherent source [2] and a self-oscillating mixer [3], amongst other applications. This paper concerns RTD electroluminescence – an effect that has been studied very little in the past 30+ years of RTD development, and not at room temperature. We present experiments and modeling of an n-type In0.53Ga0.47As/AlAs double-barrier RTD operating as a cross-gap light emitter at ~300K. The MBE-growth stack is shown in Fig. 1(a). A 15-μm-diam-mesa device was defined by standard planar processing including a top annular ohmic contact with a 5-μm-diam pinhole in the center to couple out enough of the internal emission for accurate free-space power measurements [4]. The emission spectra have the behavior displayed in Fig. 1(b), parameterized by bias voltage (VB). The long wavelength emission edge is at  = 1684 nm - close to the In0.53Ga0.47As bandgap energy of Ug ≈ 0.75 eV at 300 K. The spectral peaks for VB = 2.8 and 3.0 V both occur around  = 1550 nm (h = 0.75 eV), so blue-shifted relative to the peak of the “ideal”, bulk InGaAs emission spectrum shown in Fig. 1(b) [5]. These results are consistent with the model displayed in Fig. 1(c), whereby the broad emission peak is attributed to the radiative recombination between electrons accumulated on the emitter side, and holes generated on the emitter side by interband tunneling with current density Jinter. The blue-shifted main peak is attributed to the quantum-size effect on the emitter side, which creates a radiative recombination rate RN,2 comparable to the band-edge cross-gap rate RN,1. Further support for this model is provided by the shorter wavelength and weaker emission peak shown in Fig. 1(b) around = 1148 nm. Our quantum mechanical calculations attribute this to radiative recombination RR,3 in the RTD quantum well between the electron ground-state level E1,e, and the hole level E1,h. To further test the model and estimate quantum efficiencies, we conducted optical power measurements using a large-area Ge photodiode located ≈3 mm away from the RTD pinhole, and having spectral response between 800 and 1800 nm with a peak responsivity of ≈0.85 A/W at  =1550 nm. Simultaneous I-V and L-V plots were obtained and are plotted in Fig. 2(a) with positive bias on the top contact (emitter on the bottom). The I-V curve displays a pronounced NDR region having a current peak-to-valley current ratio of 10.7 (typical for In0.53Ga0.47As RTDs). The external quantum efficiency (EQE) was calculated from EQE = e∙IP/(∙IE∙h) where IP is the photodiode dc current and IE the RTD current. The plot of EQE is shown in Fig. 2(b) where we see a very rapid rise with VB, but a maximum value (at VB= 3.0 V) of only ≈2×10-5. To extract the internal quantum efficiency (IQE), we use the expression EQE= c ∙i ∙r ≡ c∙IQE where ci, and r are the optical-coupling, electrical-injection, and radiative recombination efficiencies, respectively [6]. Our separate optical calculations yield c≈3.4×10-4 (limited primarily by the small pinhole) from which we obtain the curve of IQE plotted in Fig. 2(b) (right-hand scale). The maximum value of IQE (again at VB = 3.0 V) is 6.0%. From the implicit definition of IQE in terms of i and r given above, and the fact that the recombination efficiency in In0.53Ga0.47As is likely limited by Auger scattering, this result for IQE suggests that i might be significantly high. To estimate i, we have used the experimental total current of Fig. 2(a), the Kane two-band model of interband tunneling [7] computed in conjunction with a solution to Poisson’s equation across the entire structure, and a rate-equation model of Auger recombination on the emitter side [6] assuming a free-electron density of 2×1018 cm3. We focus on the high-bias regime above VB = 2.5 V of Fig. 2(a) where most of the interband tunneling should occur in the depletion region on the collector side [Jinter,2 in Fig. 1(c)]. And because of the high-quality of the InGaAs/AlAs heterostructure (very few traps or deep levels), most of the holes should reach the emitter side by some combination of drift, diffusion, and tunneling through the valence-band double barriers (Type-I offset) between InGaAs and AlAs. The computed interband current density Jinter is shown in Fig. 3(a) along with the total current density Jtot. At the maximum Jinter (at VB=3.0 V) of 7.4×102 A/cm2, we get i = Jinter/Jtot = 0.18, which is surprisingly high considering there is no p-type doping in the device. When combined with the Auger-limited r of 0.41 and c ≈ 3.4×10-4, we find a model value of IQE = 7.4% in good agreement with experiment. This leads to the model values for EQE plotted in Fig. 2(b) - also in good agreement with experiment. Finally, we address the high Jinter and consider a possible universal nature of the light-emission mechanism. Fig. 3(b) shows the tunneling probability T according to the Kane two-band model in the three materials, In0.53Ga0.47As, GaAs, and GaN, following our observation of a similar electroluminescence mechanism in GaN/AlN RTDs (due to strong polarization field of wurtzite structures) [8]. The expression is Tinter = (2/9)∙exp[(-2 ∙Ug 2 ∙me)/(2h∙P∙E)], where Ug is the bandgap energy, P is the valence-to-conduction-band momentum matrix element, and E is the electric field. Values for the highest calculated internal E fields for the InGaAs and GaN are also shown, indicating that Tinter in those structures approaches values of ~10-5. As shown, a GaAs RTD would require an internal field of ~6×105 V/cm, which is rarely realized in standard GaAs RTDs, perhaps explaining why there have been few if any reports of room-temperature electroluminescence in the GaAs devices. [1] E.R. Brown,et al., Appl. Phys. Lett., vol. 58, 2291, 1991. [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [2] M. Feiginov et al., Appl. Phys. Lett., 99, 233506, 2011. [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [3] Y. Nishida et al., Nature Sci. Reports, 9, 18125, 2019. [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [4] P. Fakhimi, et al., 2019 DRC Conference Digest. [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). 

   more » « less
5. Electric field–dependent phonon spectrum and heat conduction in ferroelectrics

   https://doi.org/10.1126/sciadv.add7194  

   Wooten, Brandi L. ; Iguchi, Ryo ; Tang, Ping ; Kang, Joon Sang ; Uchida, Ken-ichi ; Bauer, Gerrit E. W. ; Heremans, Joseph P. ( February 2023 , Science Advances)

   This article shows experimentally that an external electric field affects the velocity of the longitudinal acoustic phonons (v_LA), thermal conductivity (κ), and diffusivity (D) in a bulk lead zirconium titanate–based ferroelectric. Phonon conduction dominates κ, and the observations are due to changes in the phonon dispersion, not in the phonon scattering. This gives insight into the nature of the thermal fluctuations in ferroelectrics, namely, phonons labeled ferrons that carry heat and polarization. It also opens the way for phonon-based electrically driven all-solid-state heat switches, an enabling technology for solid-state heat engines. A quantitative theoretical model combining piezoelectric strain and phonon anharmonicity explains the field dependence ofv_LA, κ, andDwithout any adjustable parameters, thus connecting thermodynamic equilibrium properties with transport properties. The effect is four times larger than previously reported effects, which were ascribed to field-dependent scattering of phonons.

    

   more » « less