skip to main content


Title: Surface Defect Passivation of Silicon Micropillars
Abstract

Reactive ion etching (RIE) used to fabricate high‐aspect‐ratio (HAR) nano/microstructures is known to damage semiconductor surfaces which enhances surface recombination and limits the conversion efficiency of nanostructured solar cells. Here, defect passivation of ultrathin Al2O3‐coated Si micropillars (MPs) using different surface pretreatment steps is reported. Effects on interface state density are quantified by means of electrochemical impedance spectroscopy which is used to extract quantitative capacitance–voltage and conductance–voltage characteristics from HAR dielectric–semiconductor structures which would otherwise suffer from high gate leakage currents if tested using solid‐state metal–insulator–semiconductor structures. High‐temperature thermal oxidation to form a sacrificial oxide on RIE‐fabricated Si MPs, followed by atomic layer deposition of 4 nm thick Al2O3after removal of the sacrificial layer produces an interface trap density (Dit) as low as 1.5 × 1011cm−2eV−1at the mid‐gap energy of silicon. However, a greatly reduced mid‐gapDit(2 × 1011cm−2eV−1) is possible even with a simple air annealing procedure having a maximum temperature of 400 °C.

 
more » « less
NSF-PAR ID:
10069876
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Advanced Materials Interfaces
Volume:
5
Issue:
20
ISSN:
2196-7350
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    High quality dielectric‐semiconductor interfaces are critical for reliable high‐performance transistors. This paper reports the in situ metal–organic chemical vapor deposition of Al2O3on β‐Ga2O3as a potentially better alternative to the most commonly used atomic layer deposition (ALD). The growth of Al2O3is performed in the same reactor as Ga2O3using trimethylaluminum and O2as precursors without breaking the vacuum at a growth temperature of 600 °C. The fast and slow near interface traps at the Al2O3/β‐Ga2O3interface are identified and quantified using stressed capacitance–voltage (CV) measurements on metal oxide semiconductor capacitor (MOSCAP) structures. The density of shallow and deep level initially filled traps (Dit) are measured using ultraviolet‐assisted CV technique. The average Ditfor the MOSCAP is determined to be 6.4×1011cm−2eV−1. The conduction band offset of the Al2O3/ Ga2O3interface is also determined from CV measurements and found out to be 1.7 eV which is in close agreement with the existing literature reports of ALD Al2O3/Ga2O3interface. The current–voltage characteristics are also analyzed and the average breakdown field is extracted to be approximately 5.8 MV cm−1. This in situ Al2O3dielectric on β‐Ga2O3with improved dielectric properties can enable Ga2O3‐based high‐performance devices.

     
    more » « less
  2. Ever-increasing demands for energy, particularly being environmentally friendly have promoted the transition from fossil fuels to renewable energy.1Lithium-ion batteries (LIBs), arguably the most well-studied energy storage system, have dominated the energy market since their advent in the 1990s.2However, challenging issues regarding safety, supply of lithium, and high price of lithium resources limit the further advancement of LIBs for large-scale energy storage applications.3Therefore, attention is being concentrated on an alternative electrochemical energy storage device that features high safety, low cost, and long cycle life. Rechargeable aqueous zinc-ion batteries (ZIBs) is considered one of the most promising alternative energy storage systems due to the high theoretical energy and power densities where the multiple electrons (Zn2+) . In addition, aqueous ZIBs are safer due to non-flammable electrolyte (e.g., typically aqueous solution) and can be manufactured since they can be assembled in ambient air conditions.4As an essential component in aqueous Zn-based batteries, the Zn metal anode generally suffers from the growth of dendrites, which would affect battery performance in several ways. Second, the led by the loose structure of Zn dendrite may reduce the coulombic efficiency and shorten the battery lifespan.5

    Several approaches were suggested to improve the electrochemical stability of ZIBs, such as implementing an interfacial buffer layer that separates the active Zn from the bulk electrolyte.6However, the and thick thickness of the conventional Zn metal foils remain a critical challenge in this field, which may diminish the energy density of the battery drastically. According to a theretical calculation, the thickness of a Zn metal anode with an areal capacity of 1 mAh cm-2is about 1.7 μm. However, existing extrusion-based fabrication technologies are not capable of downscaling the thickness Zn metal foils below 20 μm.

    Herein, we demonstrate a thickness controllable coating approach to fabricate an ultrathin Zn metal anode as well as a thin dielectric oxide separator. First, a 1.7 μm Zn layer was uniformly thermally evaporated onto a Cu foil. Then, Al2O3, the separator was deposited through sputtering on the Zn layer to a thickness of 10 nm. The inert and high hardness Al2O3layer is expected to lower the polarization and restrain the growth of Zn dendrites. Atomic force microscopy was employed to evaluate the roughness of the surface of the deposited Zn and Al2O3/Zn anode structures. Long-term cycling stability was gauged under the symmetrical cells at 0.5 mA cm-2for 1 mAh cm-2. Then the fabricated Zn anode was paired with MnO2as a full cell for further electrochemical performance testing. To investigate the evolution of the interface between the Zn anode and the electrolyte, a home-developed in-situ optical observation battery cage was employed to record and compare the process of Zn deposition on the anodes of the Al2O3/Zn (demonstrated in this study) and the procured thick Zn anode. The surface morphology of the two Zn anodes after circulation was characterized and compared through scanning electron microscopy. The tunable ultrathin Zn metal anode with enhanced anode stability provides a pathway for future high-energy-density Zn-ion batteries.

    Obama, B., The irreversible momentum of clean energy.Science2017,355(6321), 126-129.

    Goodenough, J. B.; Park, K. S., The Li-ion rechargeable battery: a perspective.J Am Chem Soc2013,135(4), 1167-76.

    Li, C.; Xie, X.; Liang, S.; Zhou, J., Issues and Future Perspective on Zinc Metal Anode for Rechargeable Aqueous Zinc‐ion Batteries.Energy & Environmental Materials2020,3(2), 146-159.

    Jia, H.; Wang, Z.; Tawiah, B.; Wang, Y.; Chan, C.-Y.; Fei, B.; Pan, F., Recent advances in zinc anodes for high-performance aqueous Zn-ion batteries.Nano Energy2020,70.

    Yang, J.; Yin, B.; Sun, Y.; Pan, H.; Sun, W.; Jia, B.; Zhang, S.; Ma, T., Zinc Anode for Mild Aqueous Zinc-Ion Batteries: Challenges, Strategies, and Perspectives.Nanomicro Lett2022,14(1), 42.

    Yang, Q.; Li, Q.; Liu, Z.; Wang, D.; Guo, Y.; Li, X.; Tang, Y.; Li, H.; Dong, B.; Zhi, C., Dendrites in Zn-Based Batteries.Adv Mater2020,32(48), e2001854.

    Acknowledgment

    This work was partially supported by the U.S. National Science Foundation (NSF) Award No. ECCS-1931088. S.L. and H.W.S. acknowledge the support from the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 22011044) by KRISS.

    Figure 1

     

    more » « less
  3. 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
  4. Air‐stable p‐type SnF2:Cs2SnI6with a bandgap of 1.6 eV has been demonstrated as a promising material for Pb‐free halide perovskite solar cells. Crystalline Cs2SnI6phase is obtained with CsI, SnI2, and SnF2salts in gamma‐butyrolactone solvent, but not with dimethyl sulfoxide andN,N‐dimethylformamide solvents. Cs2SnI6is found to be stable for at least 1000 h at 100 °C when dark annealed in nitrogen atmosphere. In this study, Cs2SnI6has been used in a superstrate n–i–p planar device structure enabled by a spin‐coated absorber thickness of ≈2 μm on a chemical bath deposited Zn(O,S) electron transport layer. The best device power conversion efficiency reported here is 5.18% withVOCof 0.81 V, 9.28 mA cm−2JSC, and 68% fill factor. The dark saturation current and diode ideality factor are estimated as 1.5 × 10−3 mA cm−2and 2.18, respectively. The devices exhibit a highVOCdeficit and low short‐circuit current density due to high bulk and interface recombination. Device efficiency can be expected to increase with improvement in material and interface quality, charge transport, and device engineering.

     
    more » « less
  5. Abstract

    Pivotal to functional van der Waals stacked flexible electronic/excitonic/spintronic/thermoelectric chips is the synergy amongst constituent layers. However; the current techniques viz. sequential chemical vapor deposition, micromechanical/wet‐chemical transfer are mostly limited due to diffused interfaces, and metallic remnants/bubbles at the interface. Inter‐layer‐coupled 2+δ‐dimensional materials, as a new class of materials can be significantly suitable for out‐of‐plane carrier transport and hence prompt response in prospective devices. Here, the discovery of the use of exotic electric field ≈106 V cm1(at microwave hot‐spot) and 2 thermomechanical conditions i.e. pressure ≈1 MPa, T ≈ 200 °C (during solvothermal reaction) to realize 2+δ‐dimensional materials is reported. It is found that PzPzchemical bonds form between the component layers, e.g., CB and CN in G‐BN, MoN and MoB in MoS2‐BN hybrid systems as revealed by X‐ray photoelectron spectroscopy. New vibrational peaks in Raman spectra (BC ≈1320 cm–1for the G‐BN system and MoB ≈365 cm–1for the MoS2‐BN system) are recorded. Tunable mid‐gap formation, along with diodic behavior (knee voltage ≈0.7 V, breakdown voltage ≈1.8 V) in the reduced graphene oxide‐reduced BN oxide (RGO‐RBNO) hybrid system is also observed. Band‐gap tuning in MoS2‐BN system is observed. Simulations reveal stacking‐dependent interfacial charge/potential drops, hinting at the feasibility of next‐generation functional devices/sensors.

     
    more » « less