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1. Increased versatility for carrier profiling of semiconductors by scanning frequency comb microscopy (SFCM)

   https://doi.org/10.1109/WMED.2017.7916928  

   Birch, Timothy ; Hagmann, Mark J. ( April 2017 , Increased versatility for carrier profiling of semiconductors by scanning frequency comb microscopy (SFCM))

   We are developing a new method for the carrier profiling of semiconductors that shows promise for nm-resolution which is required at the new sub-10 nm lithography nodes. A modelocked ultrafast laser focused on the tunneling junction of a scanning tunneling microscope (STM) generates a regular sequence of pulses of minority carriers in the semiconductor. Each pulse of carriers has a width equal to the laser pulse width (e.g. 15 fs). In the frequency domain, this is a microwave frequency comb (MFC) with hundreds of measurable harmonics at integer multiples of the laser pulse repetition frequency (e.g. 74 MHz). After themore »minority carriers diverge rapidly into the semiconductor as a Coulomb explosion, the pulses become broader and decay, so that the MFC has less power with a spectrum limited to the first few harmonics. The frequency-dependent attenuation of the MFC is determined by the resistivity of the semiconductor at the tunneling junction so SFCM is closely related to Scanning Spreading Resistance Microscopy (SSRM). Harmonics of the MFC are measured with high speed, and high accuracy because the signal-to-noise ratio is approximately 25 dB due to their extremely narrow (sub-Hz) linewidth. Now we superimpose a low-frequency signal (e.g. 10 Hz) on either the applied bias or the voltage that is applied to the piezoelectric actuators of the STM to cause sidebands at each harmonic of the MFC which are less affected by the artifacts.« less
2. Carrier transport in polycrystalline silicon at high optical injection: transient photoconductance vs. numerical modeling

   Anyanwu, Uchechi ; Harris, Christian ; Semichaevsky, Andrey ( June 2017 , 44th IEEE-PVSC)

   This paper discusses our experimental results obtained for several p-type polycrystalline Si wafers using transient photoconductance decay method in combination with a numerical model that accounts for the carrier density dependence of recombination lifetime. Using peak illumination intensities of up to 65 Suns, we are able to estimate the recombination rates for the various carrier recombination mechanisms separately. The model with such realistic recombination parameters produces a realistic diffusion length of minority carriers in Si at high optical injection. Our approach can be applied to the characterization of semiconductor materials for concentrator photovoltaics as well as to design of optoelectronicmore »devices.« less
3. Defect Manipulation to Control ZnO Micro-/Nanowire-Metal Contacts

   https://doi.org/DOI: 10.1021/acs.nanolett.8b02892  

   J.W. Cox, G.M. Foster ( January 2018 , Nano letters)

   Surface states that induce depletion regions are commonly believed to control the transport of charged carriers through semiconductor nanowires. However, direct, localized optical, and electrical measurements of ZnO nanowires show that native point defects inside the nanowire bulk and created at metal−semiconductor interfaces are electrically active and play a dominant role electronically, altering the semiconductor doping, the carrier density along the wire length, and the injection of charge into the wire. We used depth-resolved cathodoluminescence spectroscopy to measure the densities of multiple point defects inside ZnO nanowires, substitutional Cu on Zn sites, zinc vacancy, and oxygen vacancy defects, showing thatmore »their densities varied strongly both radially and lengthwise for tapered wires. These defect profiles and their variation with wire diameter produce trap-assisted tunneling and acceptor trapping of free carriers, the balance of which determines the low contact resistivity (2.6 × 10−3 Ω·cm−2) ohmic, Schottky (Φ ≥ 0.35 eV) or blocking nature of Pt contacts to a single nano/microwire. We show how these defects can now be manipulated by ion beam methods and nanowire design, opening new avenues to control nanowire charge injection and transport.« less
4. Photons and charges from colloidal doped semiconductor quantum dots

   https://doi.org/10.1039/C9TC05150C  

   Qiao, Tian ; Parobek, David ; Son, Dong Hee ( December 2019 , Journal of Materials Chemistry C)

   The utility of colloidal semiconductor quantum dots as a source of photons and charge carriers for photonic and photovoltaic applications has created a large field of research focused on tailoring and broadening their functionality beyond what an exciton can provide. One approach towards expanding the range of characteristics of photons and charge carriers from quantum dots is through doping impurity ions ( e.g. Mn 2+ , Cu + , and Yb 3+ ) in the host quantum dots. In addition to the progress in synthesis enabling fine control of the structure of the doped quantum dots, a mechanistic understanding ofmore »the underlying processes correlated with the structure has been crucial in revealing the full potential of the doped quantum dots as the source of photons and charge carriers. In this review, we discuss the recent progress made in gaining microscopic understanding of the photophysical pathways that give rise to unique dopant-related luminescence and the generation of energetic hot electrons via exciton-to-hot electron upconversion.« less
5. Ultrafast photoinduced band splitting and carrier dynamics in chiral tellurium nanosheets

   https://doi.org/10.1038/s41467-020-17766-5  

   Jnawali, Giriraj ; Xiang, Yuan ; Linser, Samuel M. ; Shojaei, Iraj Abbasian ; Wang, Ruoxing ; Qiu, Gang ; Lian, Chao ; Wong, Bryan M. ; Wu, Wenzhuo ; Ye, Peide D. ; et al ( August 2020 , Nature Communications)

   Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-splitH₄andH₅and the degenerateH₆valence bands (VB) and the lowest degenerateH₆conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of theH₆CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriersmore »decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.

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