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Register a new userIdentification by deuterium diffusion of a nitrogen-related deep donor preventing the p-type doping of ZnO
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Deuterium diffusion is investigated in nitrogen-doped homoepitaxial ZnO layers. The samples were grown under slightly Zn-rich growth conditions by plasma-assisted molecular beam epitaxy on m-plane ZnO substrates and have a nitrogen content [N] varied up to 5x1018 at.cm-3 as measured by secondary ion mass spectrometry (SIMS). All were exposed to a radio-frequency deuterium plasma during 1h at room temperature. Deuterium diffusion is observed in all epilayers while its penetration depth decreases as the nitrogen concentration increases. This is a strong evidence of a diffusion mechanism limited by the trapping of deuterium on a nitrogen-related trap. The SIMS profiles are analyzed using a two-trap model including a shallow trap, associated with a fast diffusion, and a deep trap, related to nitrogen. The capture radius of the nitrogen-related trap is determined to be 20 times smaller than the value expected for nitrogen-deuterium pairs formed by coulombic attraction between D+ and nitrogen-related acceptors. The (N2)O deep donor is proposed as the deep trapping site for deuterium and accounts well for the small capture radius and the observed photoluminescence quenching and recovery after deuteration of the ZnO:N epilayers. It is also found that this defect is by far the N-related defect with the highest concentration in the studied samples.
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We report ultrafast transient-grating experiments on heavily p-type InP at 15 K. Our measurement reveals the dynamics and diffusion of photoexcited electrons and holes as a function of their density n in the range 2E16 to 6E17 cm-3. After the first few picoseconds the grating decays primarily due to ambipolar diffusion. While at low density we observe a regime in which the ambipolar diffusion is electron-dominated and increases rapidly with n, at high n it appears to saturate at 34 cm2/s. We present a simple calculation that reproduces the main results of our measurements as well as of previously published measurements that had shown diffusion to be a flat or decreasing function of n. By accounting for effect of density on charge susceptibility we show that, in p-type semiconductors, the regime we observe of increasing ambipolar diffusion is unique to heavy doping and low temperature, where both the holes and electrons are degenerate; in this regime the electronic and ambipolar diffusion are nearly equal. The saturation is identified as a crossover to ambipolar diffusion dominated by the majority carriers, the holes. At short times the transient-grating signal rises gradually. This rise reveals cooling of hot electrons and, at high photocarrier density, allows us to measure ambipolar diffusion of 110 cm2/s in the hot-carrier regime.
Robust control over the carrier type is fundamental for the fabrication of nanocrystal-based optoelectronic devices, such as the p-n homojunction, but effective incorporation of impurities in semiconductor nanocrystals and its characterization is highly challenging due to their small size. Herein, InAs nanocrystals, post-synthetically doped with Cd, serve as a model system for successful p-type doping of originally n-type InAs nanocrystals, as demonstrated in field-effect transistors (FETs). Advanced structural analysis, using atomic resolution electron microscopy and synchrotron X-ray absorption fine structure spectroscopy reveal that Cd impurities reside near and on the nanocrystal surface acting as substitutional p-dopants replacing Indium. Commensurately, Cd-doped InAs FETs exhibited remarkable stability of their hole conduction, mobility, and hysteretic behavior over time when exposed to air, while intrinsic InAs NCs FETs were easily oxidized and their performance quickly declined. Therefore, Cd plays a dual role acting as a p-type dopant, and also protects the nanocrystals from oxidation, as evidenced directly by Xray photoelectron spectroscopy measurements of air-exposed samples of intrinsic and Cd doped InAs NCs films. This study demonstrates robust p-type doping of InAs nanocrystals, setting the stage for implementation of such doped nanocrystal systems in printed electronic devices.
Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution in SrTiO3 single crystals. On the surfaces of Nb- and La-doped SrTiO3, structures with different fluorescence intensities and lifetimes were found that could be related to different concentrations of Ti3+. Furthermore, the inhomogeneous distribution of donors caused a non-uniform conductivity of the surface, which complicates the production of potential electronic devices by the deposition of oxide thin films on top of doped single crystals. Hence, we propose FLIM as a convenient technique (length scale: 1 $mu$m) for characterizing the quality of doped oxide surfaces, which could help to identify appropriate substrate materials.
Scalable substitutional doping of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is a prerequisite to developing next-generation logic and memory devices based on 2D materials. To date, doping efforts are still nascent. Here, we report scalable growth and vanadium (V) doping of 2D WSe2 at front-end-of-line (FEOL) and back-end-of-line (BEOL) compatible temperatures of 800 {deg}C and 400 {deg}C, respectively. A combination of experimental and theoretical studies confirm that vanadium atoms substitutionally replace tungsten in WSe2, which results in p-type doping via the introduction of discrete defect levels that lie close to the valence band maxima. The p-type nature of the V dopants is further verified by constructed field-effect transistors, where hole conduction becomes dominant with increasing vanadium concentration. Hence, our study presents a method to precisely control the density of intentionally introduced impurities, which is indispensable in the production of electronic-grade wafer-scale extrinsic 2D semiconductors.
GeO$_2$ has an $alpha$-quartz-type crystal structure with a very wide fundamental band gap of 6.6 eV and is a good insulator. Here we find that the stable rutile-GeO$_2$ polymorph with a 4.6 eV band gap has a surprisingly low $sim$6.8 eV ionization potential, as predicted from the band alignment using first-principles calculations. Because of the short O$-$O distances in the rutile structure containing cations of small effective ionic radii such as Ge$^{4+}$, the antibonding interaction between O 2p orbitals raises the valence band maximum energy level to an extent that hole doping appears feasible. Experimentally, we report the flux growth of $1.5 times 1.0 times 0.8$ mm$^3$ large rutile GeO$_2$ single crystals and confirm the thermal stability for temperatures up to $1021 pm 10~^circ$C. X-ray fluorescence spectroscopy shows the inclusion of unintentional Mo impurities from the Li$_2$O$-$MoO$_3$ flux, as well as the solubility of Ga in the r-GeO$_2$ lattice as a prospective acceptor dopant. The resistance of the Ga- and Mo-codoped r-GeO$_2$ single crystals is very high at room temperature, but it decreases by 2-3 orders of magnitude upon heating to 300 $^circ$C, which is attributed to thermally-activated p-type conduction.
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