Session T23: Semiconductors: Thermodynamic & Transport Properties (Experimental)

Hall Effect Measured Using a Waveguide Tee

We describe a simple microwave apparatus to measure the Hall effect in semiconductor wafers. The advantage of this technique is that it does not require contacts on the sample, unlike the Van der Pauw method.\footnote{L. J. van der Pauw, Philips Research Reports \textbf{13}, 1 (1958)} Our method consists of placing the semiconductor wafer into a slot cut in an X-band waveguide tee and placing the tee in the center of an electromagnet. The next step is to inject power into two arms of the tee and to balance the output so that no power comes out of the third arm of the tee at zero magnetic field. Application of a nonzero magnetic field gives a Hall signal that is linear in the magnetic field and which reverses phase when the magnetic field is reversed. We use a network analyzer to measure the ratio of the Hall signal to the input power. This method yields the semiconductor mobility in the wafer, which we can compare for calibration purposes with mobility data from our Van der Pauw measurements. This talk presents data for silicon and germanium samples doped with boron or phosphorus. Preliminary measurements on doped III-V semiconductor samples will also be presented.   

Compositional Distribution in Semiconductor Ternary Quantum Dots and Its Effects on Their Optoelectronic Properties

We present a systematic theoretical and computational analysis of compositional distribution in semiconductor ternary quantum dots (TQDs) and the resulting effects on the TQDs' electronic band structure. The analysis is based on a hierarchical modeling approach that combines first-principles density functional theory calculations and classical Monte Carlo simulations with a continuum model of species transport in spherical nanocrystals. In many cases of TQD composition, the model predicts the formation of core/shell-like structures characterized by the formation of concentration boundary layers near the nanocrystal surfaces. A systematic analysis over the size-composition parameter space generates a database of transport properties that is used to design post-growth thermal annealing processes to establish thermodynamically stable compositional distributions in TQDs. We explore the impact of compositional distribution on the TQDs' electronic band gaps and find that TQDs with thermodynamically stable compositional distributions allow for precise band-gap tuning. Our findings lead to a proposal for an efficient one-step TQD synthesis method followed by annealing to promote self assembly of the thermodynamically stable configuration, for optimal optoelectronic function in devices.   

Weak localization and low temperature transport in MoS$_2$ flakes

With the recent identification of the indirect to direct bandgap transition for monolayer MoS$_2$ [1] and the use of MoS$_2$ in field-effect transistors [2,3], this material has attracted recent interest in the physics and nanotechnology communities. We report studies of transport in MoS$_2$ at low temperature from 1K up to 70K, characterized by Hall mobility and weak localization. We find that the mobility at T$=$400mK in this few-layer MoS$_2$ flake varies from 50cm$^2$/Vs to 300cm$^2$/Vs as electron density varies from 6x10$^{12}$ cm$^{-2}$ to 1.2x10$^{13}$ cm$^{-2}$ via the back gate bias. Additionally, we find that the mobility decreases with increasing temperature as a power law with a characteristic exponent of 1.6 at a carrier concentration of 1.2x10$^{13}$ cm$^{-2}$. Magneto-transport measurements reveal weak localization in this MoS$_2$ sample up to temperatures as high as 10K. The phase coherence length in MoS$_2$ is estimated to be about 40nm at 1K for a carrier concentration of 1.2x10$^{13}$ cm$^{-2}$.\\[4pt] [1] K. F. Mak et al. \textbf{PRL}, 105, 136805 (2010)\\[0pt] [2] B. Radisavljevic et al. \textbf{Nature Nano}, 6, 147 (2011)\\[0pt] [3] H. Liu et al, \textbf{IEEE EDL}, 33, 546 (2012).   

Ionic-Liquid Gated Few-layer MoS$_2$ Field-Effect Transistors

We report the electrical characterization of ionic-liquid-gated bilayer and few-layer MoS$_2$ field-effect transistors. The extrinsic mobility of our ionic-liquid-gated devices exceeds 70 cm$^{2}$V$^{-1}$S$^{-1}$ at 250 K, which is 1-2 orders of magnitude higher than that measured in the Si back-gate configuration (without ionic liquid). These devices also show ambipolar behavior with a high ON-OFF current ratio of \textgreater\ 10$^{7}$ for electrons and \textgreater\ 10$^6$ for holes, and a near ideal subthreshold swing (SS) of $\sim$ 50 mV/decade at 250 K for the electron channel. More significantly, we show that the mobility increases from $\sim$ 100 cm$^{2}$V$^{-1}$S$^{-1}$ at 180 K to $\sim$ 220 cm$^{2}$V$^{-1}$S$^{-1}$ at 77K as the temperature decreases following a $\mu $ $\sim$ T$^{-\gamma}$ dependence with $\gamma \approx $ 1, indicating that the intrinsic phonon-limited mobility can be achieved in few-layer MoS$_{2}$ FETs. We attribute the enhanced device performance to the drastic reduction of the Schottky barrier width (thus higher tunneling efficiency) via highly efficient band bending at the MoS$_{2}$/metal interface afforded by the extremely large electrical double layer capacitance of the ionic liquid.   

Phonon-Limited Electron Transport in Back-Gated Few-layer MoSe$_2$ Field- Effect Transistors

The ultrathin body of monolayer (and few-layer) semiconducting transition-metal-dichalcogenides (TMDs) in conjunction with their highly desirable surface properties makes them excellent candidates for the ultimate downscaling of digital electronics. We have fabricated field effect transistors (FETs) of mechanically exfoliated few-atomic- layer-thick MoSe$_{2}$, a member of the semiconducting TMD family; and measured their device characteristics as a function of temperature. We find that the field-effect mobility of the devices increases with the applied back-gate voltage, which can be attributed to the Schottky barrier reduction via band bending at the contacts. In the limit of high back-gate voltages, the mobility increases from $\sim$ 135 cm$^{2}$/V.s at room temperature to over 300 cm$^{\mathrm{2}}$/V.s at 200 K following the power law of $\mu $ $\sim$ T$^{-2.1}$, indicating that the mobility is chiefly limited by phonon scattering rather than charged impurity scattering. We attribute the high mobility and its temperature dependence to the extremely low density of defects and/or impurities in the starting MoSe$_{2}$ crystals as verified by low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements.   

Electrical Transport Properties of Liquid Phase Exfoliated MoS$_{2}$ Thin Films

In this presentation we will report the electrical transport properties of thin films consisting of liquid phase exfoliated MoS$_{2}$ flakes. The D.C electrical transport properties will be discussed in the light of 2D VRH model. Our preliminary investigations on the A.C transport properties on these materials indicate similar found in disordered semiconductors. These results will be discussed based different existing charge transport mechanisms under the application of an A.C field.   

Electrical transport and contact characteristics of single layer MoS$_{2}$ devices

MoS$_{2}$ and related metal dichalcogenides (MoSe$_{2}$, WS$_{2}$, WSe$_{2})$ are layered two dimensional materials with analogous structure to graphene. The monolayer MoS$_{2}$, where the Mo layer is sandwiched between two sulfur layers, is a semiconductor with a direct band gap (1.8 eV) at valley K and K' points. These materials are of significant technological interest for nanoscale electronic devices with high on off ratio, opto-electronics, and gas sensing. Also, due to giant spin-orbit coupling and spin splitting ($\sim$ 150 meV) in the valence band of monolayer MoS$_{2}$, monolayer MoS$_{2}$ has great potential for fascinating spin behavior, including the intrinsic spin Hall effect. Towards investigating spin transport in monolayer MoS$_{2}$, we have investigated ferromagnetic metal contacts on monolayer MoS$_{2}$. Through transport measurements, we are able to determine the Schottky barrier height between the Co contact electrodes and monolayer MoS$_{2}$ with characteristic temperature dependence.   

Investigating the thermal stability of electron transport properties in modulation-doped semiconductor heterostructure systems

Since the 1950s, materials scientists have pursued the fabrication of solid-state heterostructure (HS) devices of sufficient purity to replicate electron interference effects originally observed in vacuum. The ultimate goal of HS engineering is to create a semiconductor ``billiard table'' in which electrons travel ballistically in a 2-D plane---that is, with scattering events minimized such that the electron's mean free path exceeds the device size. For the past two decades, the modulation-doped (MD) HS architecture has yielded devices supporting very high electron mobilities. In this architecture, ionized dopants are spatially removed from the plane of the electrons, such that their influence on electron trajectories is felt through presumably negligible small-angle scattering events. However, we observe that thermally induced charge redistribution in the doped layers of AlGaAs/GaAs and GaInAs/InP MD heterostructures significantly alters electron transport dynamics as measured by magnetoconductance fluctuations. This result demonstrates that small-angle scattering plays a far larger role than expected in influencing conduction properties.   

High temperature conductivity measurement of La and Sb doped BaSnO3 thin films

We have recently found that doped BaSnO$_3$ (BSO) system offers great potential for scientific investigations as well as technical applications due to its transparency, high mobility and chemical stability. We investigated the temperature dependent conductivity in two differently n type doped BSO, La doped BaSnO$_3$ (BLSO) and Sb doped BaSnO$_3$ (BSSO), at high temperatures in O$_2$ and Ar atmosphere. Firstly, by switching gas atmosphere, we have measured the diffusion constant of oxygen atoms in BSO thin films from the time dependent conductivity measurement much lower than those of other oxides exhibiting its stable oxygen stoichiometry. Secondly, although both BLSO and BSSO are n typed doped, slight different behavior in temperature dependent conductivity was found; while the BLSO thin films showed expected results that the conductivity decreased as increasing temperature, the BSSO films displayed increasing conductivity as the temperature increased above 500C. In that high temperature region the BLSO and BSSO films also showed different behavior when the gas atmosphere was exchanged between O$_2$ and Ar. We will present possible explanations for the observation of the different behavior of BLSO and BSSO in high temperature region by taking into consideration the role of the dopant site and threading dislocations in conductivity of BSO system.   

Time-Resolved Electroabsorption Measurement of Electron Velocity in InGaN Heterostructures due to Internal Electric Fields

Carrier transport was measured in c-plane, $p$-down, $n$-GaN/$i$-In$_{\mathrm{1-x}}$Ga$_{\mathrm{x}}$N/$p$-GaN solar cell heterostructures using a time-resolved electroabsorption pump-probe technique with sub-picosecond resolution. Large built-in electric fields are present in the InGaN region associated with the termination of large polarization at hetero-interfaces. The change in transmission of a probe beam (tuned for maximum sensitivity to changes in the band edge) due to the transport of photogenerated carriers under the built-in field is monitored to determine the electron transit time and average electron velocity. Time-domain THz measurements indicate the direction of electron transport is dominated by drift towards the $n$-GaN. Samples with a 200-nm In$_{0.13}$Ga$_{0.87}$N layer show a change in signal rise time with carrier density. At the lowest injection level, an $\sim$ 1.5-ps rise time is observed, which corresponds to an average electron velocity of 6.7x10$^{6}$~cm/s for an average distance of travel of 100~nm in an internal field of $\sim$ 135~kV/cm. This velocity is significantly smaller than in GaN with a similar field, which may be indicative of transport through compositional inhomogeneities.   

Measurement of the phonon mean free path spectra and the universality in the high temperature limit

Here, we use broadband frequency domain thermoreflectance (BB-FDTR) to measure thermal conductivity accumulation functions ($k_{\mathrm{accum}})$ of Si, GaAs, GaN, AlN, and SiC at temperatures of 80 K, 150 K, 300 K, and 400 K and show that they collapse to a universal accumulation function ($k_{\mathrm{univ}})$ in the high temperature limit. BB-FDTR is a novel technique developed to measure the spectral contributions of phonons to bulk thermal conductivity as a function of phonon MFP i.e., $k_{\mathrm{accum}}$. BB-FDTR uses a heterodyne approach allowing for continuous resolution of the phonon MFP spectrum spanning two orders of magnitude (0.3 - 8 $\mu $m in Si at $T \quad =$ 300 K). Results in Si and GaAs compare favorably to numerical predictions (Esfarjani, et al., PRB, 2011) (Luo et al., arXiv, 2012) and show that phonons with long MFPs (\textgreater 1 $\mu $m) contribute significantly to the bulk thermal conductivity at $T \quad =$ 300 K. Next, we present a method to predict $k_{\mathrm{accum}}$ as the temperature of the material approaches its Debye temperature. Using the measured spectra at $T \quad =$ 400 K and assuming Umklapp scattering as the dominant scattering mechanism, $k_{\mathrm{univ}}$ was found to exist in GaAs, GaN, and Si after normalizing the phonon MFP. The existence of $k_{\mathrm{univ}}$ suggests that the phonon MFP spectrum is a universal feature of matter in the high temperature limit, and can be used to predict $k_{\mathrm{accum}}$ for any crystalline semiconductor near its Debye temperature.   

Thermometry and Refrigeration using Quantum Dots

The 2D electron gas in GaAs/AlGaAs heterostructures has diverse applications at cryogenic temperatures, but is heated by unintended noise in the measurement set up. Our work involves the fabrication of a quantum dot refrigerator (QDR) which can cool the gas to below the ambient lattice temperature [1]. Lithographically defined gates define three quantum dots tunnel-coupled to an enclosed, macroscopic reservoir of electrons 100 $\mathrm{\mu m^2}$ in area. Energy selective transport of electrons via the discrete energy levels of two quantum dots through the electron reservoir modifies its Fermi-Dirac distribution, thus cooling it. The third quantum dot (the `thermometer') probes the temperature of the reservoir being cooled by monitoring the current flowing through an adjacent quantum point contact. We have demonstrated measuring electronic temperatures in the range 100 mK to 300 mK, with an estimated error of about 10\%. We have also investigated the variation in electron temperature as a function of the energies of the entrance and exit dots. Our results are consistent with cooling an area of 64$\mathrm{\mu m^2}$ by 30 mK, starting from 150mK, and agree qualitatively with theory [2].\\ {[1]} Prance e. a. Phys. Rev. Lett. 102 146602 \\ {[2]} Edwards e. a. Phys. Rev. B 52 5714   

Structures, electronic and magnetic properties of transition metal doped MoS2 intercalation compounds

Molybdenum disulfide (MoS2) has recently attracted much attention due to its potential applications in high efficiency hydrogen storage, catalysts, and nanoelectronic devices. While intrinsic MoS2 bulk is a well-known diamagnetic material, zigzag nanoribbons of MoS2 have been predicted by density functional theory (DFT) to be metallic and ferromagnetic. The effects of transition metal (TM) doping on the magnetic properties of MoS2 appear to be a very interesting issue. In this work, we have synthesized a series of TM (Co,Ni,Cu) doped MoS2 intercalation compounds by an exfoliation/restacking method with different TM concentration (0.01-10 at. \%) and annealed at various temperatures (300-1000K). Raman spectra and x-ray diffraction (XRD) data show that the synthesized TM-MoS2 intercalation compounds are in 2H-MoS2 structure with average size $\sim$100 nm. The average distance between MoS2 host layers strongly depends on the TM concentration. XANES and EXAFS reveal that TM atoms are located on tetrahedral sites between the MoS2 sheets with valence number +1. A series of DFT simulations indicate that Co-MoS2 may exhibit half-metallic ferromagnetic states while ferromagnetism is absent in Cu-MoS2 and Ni-MoS2. Experimental data obtained from magnetic measurements will also be presented.   

NRG study of the transmission phase shift through a two-level quantum dot with Kondo correlations

The transmission phase shift through a Kondo quantum dot has been predicted to take the universal value $\pi/2$ in the center of the Kondo valley\footnote{U.~Gerland \emph{et al.}, Phys.~Rev.~Lett., 84, 3710 (2000).}. Several experimental studies using a quantum dot embedded in an Aharonov-Bohm ring have aimed to check this prediction, which was finally verified in \footnote{M.~Zaffalon \emph{et al.}, Phys.~Rev.~Lett., 100, 226601 (2008).}. A recent experiment\footnote{S.~Takada \emph{et al.}, to be published (2012).} has obtained particularly clean results for the transmission phase shift by eliminating the effect of backscattering. We provide a Numerical Renormalization Group study of a two-level quantum that shows very good qualitative agreement with these new experimental results. The effect of the second level, with width different from the first, is crucial for accounting for some of the observed experimental details.   

Transmission phase shift across a Kondo correlated quantum dot

We report on measurements of the transmission phase across a quantum dot embedded in an original two-path interferometer both in the strong and weak Kondo regime. The Kondo effect is a well known many-body phenomenon, which is characterized by a single energy scale, the Kondo temperature $T_{\mathrm{K}}$. In the strong Kondo regime at low temperatures ($T$/$T_{\mathrm{K}}$ \textless 1) we found that the transmission phase is locked to $\pi $/2 in the Kondo valley when the single level spacing $\delta $is significantly larger than the level broadening $\Gamma $. When $\Gamma $ is relatively large, on the other hand, the phase smoothly shifts by $\pi $ across two peaks on both ends of the Kondo valley without showing any plateau. As the temperature is increased exceeding $T_{\mathrm{K}}$, the Kondo correlation becomes lifted and then the phase shift looks similar to that in the Coulomb blockade regime, where the phase evolves $\pi $ across a Coulomb peak followed by a $\pi $-phase lapse in the Coulomb valley. In such a weak Kondo regime ($T$/$T_{\mathrm{K}}$ \textgreater 1) we observed asymmetric phase evolution about the valley center, which is linked to the orbital parity relation between the levels of interest.   

Scanning probe microscopy measurements of charge in PbS quantum dot (sub)monolayers

Nanocrystal quantum dots (NQDs) are of intense interest because their optical and electronic properties can be tuned by altering the dot size and material. Transport in arrays of NQDs is generally dominated by disorder, and strongly influenced by the immediate environment. Fully understanding transport through arrays of NQDs would allow the design of improved devices, such as LEDs, photodetectors and lasers. The goal of our study is to use electrostatic force microscopy techniques to study charge transport in (sub)monolayers of NQDs. These 2D PbS NQD arrays are achieved by spin-coating the NQDs between lithographically patterned electrodes, and the measurements take place in a custom-built nitrogen environment cell for our AFM.