Bruce Claflin

Hall carrier density and magnetoresistance measurements in thin film vanadium dioxide across the metal-insulator transition

Temperature-dependent magnetotransport measurements in magnetic fields of up to 12 T were performed on thin-film vanadium dioxide (VO 2 ) across the metal-insulator transition (MIT). The Hall carrier density increases by 4 orders of magnitude at the MIT and accounts almost entirely for the resistance change. The Hall mobility varies little across the MIT and remains low, ~0.1 cm 2 /V sec. Electrons are found to be the major carriers on both sides of the MIT. Small positive magnetoresistance in the semiconducting phase is measured.

Deep traps in AlGaN/GaN heterostructures studied by deep level transient spectroscopy: Effect of carbon concentration in GaN buffer layers

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed DLTS measurements on the two SBDs show that (i) different trap species are seen in the two SBDs: electron traps Ax (0.9 eV), A1 (0.99 eV), and A2 (1.2 eV), and a holelike trap H1 (1.24 eV) in the low-[C] SBD; and electron traps A1, A2, and A3 (∼1.3 eV), and a holelike trap H2 (>1.3 eV) in the high-[C] SBD; (ii) for...

Persistent Photoconductivity Studies in Nanostructured ZnO UV Sensors

The phenomenon of persistent photoconductivity is elusive and has not been addressed to an extent to attract attention both in micro and nanoscale devices due to unavailability of clear material systems and device configurations capable of providing comprehensive information. In this work, we have employed a nanostructured (nanowire diameter 30–65 nm and 5 μm in length) ZnO-based metal–semiconductor–metal photoconductor device in order to study the origin of persistent photoconductivity. The current–voltage measurements were carried with and without UV illumination under different oxygen levels. The photoresponse measurements indicated a persistent conductivity trend for depleted oxygen conditions. The persistent conductivity phenomenon is explained on the theoretical model that proposes the change of a neutral anion vacancy to a charged state.

High-Quality, Melt-Grown ZnO Single Crystals

High-quality, melt-grown ZnO crystals are reported. The reflection and emission spectra of the melt-grown samples are compared with the same spectra from high-quality, vapor-grown ZnO crystals. We isolate the reflection and emission spectra predominantly related to the intrinsic properties associated with the wurtzite structure of the crystals. The quality of the crystals is reflected in the spectral reproduction of the intrinsic properties of the crystals. Both the ground state and the n=2 state of the free excitons associated with the A, B, and C valence bands of the crystals are spectrally observed in reflection. Assuming a hydrogenic character for the free excitons, the binding energy of these excitons associated with all three valence bands was determined. For the intrinsic emission spectra, attention was focused on the A-band free excitons and related optical parameters. Both the reflection and emission spectra for the melt-grown material compared very closely with the same spectra observed from hig...

Origin of Conductive Surface Layer in Annealed ZnO

The highly conductive surface layers found in nearly all as-grown or annealed bulk ZnO wafers are studied by temperature-dependent Hall-effect and secondary-ion mass spectroscopy (SIMS) measurements. In this work, we have used annealing in N2 at 900°C, and forming gas (5% H2 in N2) at 600°C, to cause a large enough surface conduction that SIMS measurements can be reliably employed. The increased near-surface donor density, as determined from two-layer Hall-effect modeling, is consistent with an increased near-surface concentration of Al, Ga, and In atoms, resulting from diffusion. There is no evidence for participation of any donors involving H.

Model for thickness dependence of mobility and concentration in highly conductive zinc oxide

Abstract. The dependences of the 294 and 10 K mobility μ and volume carrier concentration n on thickness (d=25 to 147 nm) are examined in aluminum-doped zinc oxide (AZO). Two AZO layers are grown at each thickness, one with and one without a 20-nm-thick ZnON buffer layer. Plots of the 10 K sheet concentration ns versus d for buffered (B) and unbuffered (UB) samples give straight lines of similar slope, n=8.36×1020 and 8.32×1020  cm−3, but different x-axis intercepts, δd=−4 and +13  nm, respectively. Plots of ns versus d at 294 K produce substantially the same results. Plots of μ versus d can be well fitted with the equation μ(d)=μ(∞)/[1+d*/(d−δd)], where d* is the thickness for which μ(∞) is reduced by a factor 2. For the B and UB samples, d*=7 and 23 nm, respectively, showing the efficacy of the ZnON buffer. Finally, from n and μ(∞) we can use degenerate electron scattering theory to calculate bulk donor and acceptor concentrations of 1.23×1021  cm−3 and 1.95×1020  cm−3, respectively, and Drude theory to predict a plasmonic resonance at 1.34 μm. The latter is confirmed by reflectance measurements.

Electron irradiation induced deep centers in hydrothermally grown ZnO

An n-type hydrothermally grown ZnO sample becomes semi-insulating (ρ~108 Ω cm) after 1-MeV electron-irradiation. Deep traps produced by the irradiation were studied by thermally stimulated current spectroscopy. The dominant trap in the as-grown sample has an activation energy of 0.24 eV and is possibly related to LiZn acceptors. However, the electron irradiation introduces a new trap with an activation energy of 0.15 eV, and other traps of energy 0.30 and 0.80 eV, respectively. From a comparison of these results with positron annihilation experiments and density functional theory, we conclude that the 0.15-eV trap may be related to VZn.

Electron and Hole Traps in N-Doped ZnO Grown on p-Type Si by Metalorganic Chemical Vapor Deposition

Electron and hole traps in N-doped ZnO were investigated using a structure of n+-ZnO:Al/i-ZnO/ZnO:N grown on a p-Si substrate by metalorganic chemical vapor deposition (for growth of the ZnO:N layer) and sputtering deposition (for growth of the i-ZnO and n+-ZnO:Al layers). Current-voltage and capacitance-voltage characteristics measured at temperatures from 200 to 400 K show that the structure is an abrupt n+−p diode with very low leakage currents. By using deep level transient spectroscopy, two hole traps, H3 (0.35 eV) and H4 (0.48 eV), are found in the p-Si substrate, while one electron trap E3 (0.29 eV) and one hole trap H5 (0.9 eV) are observed in the thin ZnO:N layer. Similarities to traps reported in the literature are discussed.

Effects of annealing in N2 ambient on traps and persistent conduction in hydrothermally grown ZnO

Thermally stimulated current (TSC) spectroscopy and temperature–dependent dark current (DC) measurements have been applied to study traps and photoinduced persistent surface conduction in two hydrothermally grown bulk ZnO samples, as-grown, and annealed at 600°C in N2 ambient for 30min, respectively. The as-grown sample had a room-temperature (RT) resistivity of 1.6×103Ωcm, mobility of 2.1×102cm2∕Vs, and carrier concentration of 1.8×1013cm−3, while the annealed sample was highly resistive, with RT resistivity of 3.6×106Ωcm, mobility of 4.4cm2∕Vs, and carrier concentration of 3.9×1011cm−3. The as-grown sample showed strong conduction at low temperatures, which has been shown to be due to near-surface carriers in other studies. The annealed sample did not demonstrate this phenomenon. The dominant trap in the as-grown sample had an activation energy of 0.16eV, was strongest near the surface, and is possibly related to VZn. In the annealed sample, however, the dominant trap had an activation energy of 0.22eV,...

Surface traps in vapor-phase-grown bulk ZnO studied by deep level transient spectroscopy

Deep level transient spectroscopy, current-voltage, and capacitance-voltage measurements are used to study interface traps in metal-on-bulk-ZnO Schottky barrier diodes (SBDs). c-axis-oriented ZnO samples were cut from two different vapor-phase-grown crystals, and Au- and Pd-SBDs were formed on their (0001) surfaces after remote oxygen-plasma treatment. As compared to Au-SBDs, the Pd-SBDs demonstrated higher reverse-bias leakage current and forward-bias current evidently due to higher carrier concentrations, which might have been caused by hydrogen in-diffusion through the thin Pd metal. The dominant traps included the well-known bulk traps E3 (0.27 eV) and E4 (0.49 eV). In addition, a surface-related trap, Es (0.49 eV), is observed but only in the Pd-SBDs, not in the Au-SBDs. Trap Es is located at depths less than about 95 nm and shows an electron capture behavior indicative of extended defects. A possible correspondence between trap Es and the well-known 2.45 eV green band is suggested by depth-resolved ...