Anthony Birdwell

Electrical Transport Properties of Polycrystalline Monolayer Molybdenum Disulfide

Semiconducting MoS2 monolayers have shown many promising electrical properties, and the inevitable polycrystallinity in synthetic, large-area films renders understanding the effect of structural defects, such as grain boundaries (GBs, or line-defects in two-dimensional materials), essential. In this work, we first examine the role of GBs in the electrical-transport properties of MoS2 monolayers with varying line-defect densities. We reveal a systematic degradation of electrical characteristics as the line-defect density increases. The two common MoS2 GB types and their specific roles are further examined, and we find that only tilt GBs have a considerable effect on the MoS2 electrical properties. By examining the electronic states and sources of disorder using temperature-dependent transport studies, we adopt the Anderson model for disordered systems to explain the observed transport behaviors in different temperature regimes. Our results elucidate the roles played by GBs in different scenarios and give insights into their underlying scattering mechanisms.

Factors influencing the capacitance–voltage characteristics measured by the scanning capacitance microscope

A scanning capacitance microscope (SCM) can measure the local capacitance–voltage (C–V) characteristics of a metal-oxide-semiconductor structure formed by the SCM probe tip and a doped semiconductor sample. A common realization of the SCM depends on a parallel atomic force microscope, which includes a laser focused on the end of the cantilever to monitor the position of the probe tip. In this configuration, it is found that the stray light from the laser can dramatically affect the measured C–V curve. The difference between the SCM C–V curves measured in this high stray light condition and those measured in the true dark condition are shown and discussed. Also discussed is the distortion of the measured C–V curves caused by the SCM method of measuring the differential capacitance using a capacitance-modulating ac voltage and a lock-in amplifier. After reducing and accounting for these effects, the SCM C–V curves show markedly different behavior from that of conventional one-dimensional C–V curves. The mea...

Excitonic transitions in β-FeSi2 epitaxial films and single crystals

Photoreflectance spectra were obtained from an epitaxial film and a bulk single crystal of β-FeSi2 at low temperatures (T⩽180 K). A model based on the results of low-temperature absorption [M. Rebien et al., Appl. Phys. Lett. 74, 970 (1999)] was used to describe the main features of the spectra. In agreement with the absorption results, transitions corresponding to the ground state and first excited state of the free exciton were observed in both the epitaxial film and single crystal. However, additional subband gap features are revealed in the photoreflectance spectra of the thin film. It is suggested that these may be related to impurity transitions or an impurity transition plus a bound exciton resonance. From the analysis of the spectra taken on the thin film, over a temperature range of 12–180 K, we extract a free exciton binding energy of (0.009±0.002) eV and a direct energy gap at T=0 K of (0.934±0.002) eV.

Composition and carrier-concentration dependence of the electronic structure of InyGa1−yAs1−xNx films with nitrogen mole fraction of less than 0.012

The electronic structure of Si-doped InyGa1−yAs1−xNx films on GaAs substrates, grown by nitrogen-plasma-assisted molecular-beam epitaxy, was examined by photoreflectance (PR) spectroscopy at temperatures between 20 and 300K. The films were approximately 0.5μm thick and had nitrogen mole fraction between x=0.0014 and x=0.012, measured indirectly by a secondary-ion-mass spectrometry calibration; indium mole fraction between y=0.052 and y=0.075, measured by electron-dispersive x-ray spectroscopy; and carrier concentration between 2×1016 and 1.1×1018cm−3, measured by Hall effect. Three critical-point transitions were identified by PR: the fundamental band gap (highest valence band to the lowest conduction band); the spin-orbit split valence band to the lowest conduction band; and the highest valence band to a nitrogen impurity band (above the lowest conduction band). The measured critical-point energies were described by a band anticrossing (BAC) model with the addition of a Burstein-Moss band-filling term. T...

Nitrogen-activated bowing of dilute InyGa1-yAs1-xNx based on photoreflectance studies

The dependence of the fundamental band gap and higher-lying critical-point energies of dilute-nitrogen Ga1−yInyAs1−xNx epilayers on nitrogen mole fraction (x), for x⩽0.0125, and temperature, from 20 to 300 K, was investigated by photoreflectance spectroscopy. The band gap, EG, was found to decrease with increasing x in a highly nonlinear manner. The bowing parameter (the second-order parameter b in a quadratic expression for the dependence of EG on x) was found to become less negative with increasing x; the value of b changed from −50 eV, at very low nitrogen fraction, to −20 eV, at x>0.01. These results strongly suggest that nitrogen-related impurity levels arise within the band gap of dilute-nitrogen Ga1−yInyAs1−xNx alloys.The dependence of the fundamental band gap and higher-lying critical-point energies of dilute-nitrogen Ga1−yInyAs1−xNx epilayers on nitrogen mole fraction (x), for x⩽0.0125, and temperature, from 20 to 300 K, was investigated by photoreflectance spectroscopy. The band gap, EG, was found to decrease with increasing x in a highly nonlinear manner. The bowing parameter (the second-order parameter b in a quadratic expression for the dependence of EG on x) was found to become less negative with increasing x; the value of b changed from −50 eV, at very low nitrogen fraction, to −20 eV, at x>0.01. These results strongly suggest that nitrogen-related impurity levels arise within the band gap of dilute-nitrogen Ga1−yInyAs1−xNx alloys.

Evidence for an indirect gap in β−FeSi2 epilayers by photoreflectance spectroscopy

Photoreflectance spectra obtained from epitaxial films of semiconducting β−FeSi2 exhibit complex line shapes resulting from a variety of optical transitions. While we have previously established a direct gap at 0.934±0.002eV at 75K, we find an additional weak structure at a lower energy. We attribute the origin of this spectral feature to indirect transitions assisted by the emission of a phonon. From our analysis, we determine an indirect gap energy of 0.823±0.002eV at 75K.

Composite substrate for large-format HgCdTe IRFPA

Research on silicon based composite substrates is being conducted at the Army Research Laboratory. These substrates can be used to deposit HgCdTe alloys to fabricate large-format infrared photodetector arrays. Traditionally, composite structures are fabricated by growing CdZnTe buffer layers on Si substrates using molecular beam epitaxy process. Recently, we have demonstrated that composite structures using CdSeTe can also be used. The CdSeTe compound offers better surface morphology and control of composition. In this work we present our results on the Si-based substrate technology and its application in the use of substrate material for LWIR HgCdTe detector development. In this paper we also present our study of molecular beam epitaxy and characteristics of CdSexTe1-x ternary films on Si. A detailed study of the alloy composition and lattice structures were investigated. In general, we find that the crystalline quality of CdSeTe films on Si is superior to CdZnTe on Si. Best CdSeTe/Si samples had EPD as low as 1.4x105 cm-2. This study also discusses a comparison of cation versus anion mixing in chalcogenide compounds. Results of LWIR detectors on CdTe/Si are also presented as a precursor and rational for a need of better lattice-matched substrates other than the conventional CdZnTe/Si substrates.

Optical Investigations of Semiconducting FeSi2 using Photoreflectance Spectroscopy

Photoreflectance (PR) spectra obtained from epitaxial films of semiconducting β‐FeSi2 exhibit complex lineshapes resulting from a variety of optical transitions. The main features of the PR spectra can be described by transitions involving the ground and first excited state of the free exciton, in agreement with low temperature absorption measurements. However, additional lower energy (below band gap) resonances and the effects of stress, not observed in single crystal β‐FeSi2, are seen in the PR spectra of thin epitaxial films. The effect of tensile stress, resulting from thermal contraction of the epitaxial film on the Si substrate, is manifest as a substantial redshift of the main spectral feature. The assignment of tensile stress is confirmed by XRD and Raman measurements. In addition to the main PR spectral feature, the appearance of a replica is observed at higher temperatures. The shape of the replica is found to be almost identical to that of the main spectral feature, shifted down in energy by 72 meV, and is attributed to the assistance of an LO phonon.