Showing papers by "David P. Norton published in 2007"
TL;DR: In this paper, the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation and exhibits the anomalous Hall effect.
Abstract: Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and exhibits the anomalous Hall effect. ZnO is expected to be one of the most promising materials for room-temperature polarized light emission; but to date, we have been unable to detect the optical spin polarization in ZnO. The short spin relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or the use of additional stressor layers to create a larger spin splitting in order to get a polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches
143 citations
TL;DR: In this article, a depletion mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering.
Abstract: Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range from −5.5to−6.5V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ∼105. The maximum field effect mobility in the channel was ∼4.5cm2V−1s−1, lower than the Hall mobility of ∼17cm2V−1s−1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures make these devices suitable for applications requiring flexible substrates.
136 citations
TL;DR: In this article, the adsorption of dodecanethiol on zinc-and oxygen-terminated ZnO surfaces was investigated and the results indicated a higher surface coverage of the thiol on the zincterminated surface.
Abstract: We have investigated the adsorption of dodecanethiol on zinc- and oxygen-terminated ZnO surfaces. Strong enthalpic adsorption is demonstrated by the stability of sulfur on both ZnO surfaces for temperatures up to 400°C. The minimal presence of the S 2p3∕2 170eV peak suggests absorption of the sulfur as an unoxidized thiol. The results indicate a higher surface coverage of the thiol on the zinc-terminated surface. Evidence from reflection high energy electron diffraction measurements for the surface ordering after thiol treatment of the oxygen-terminated ZnO surface suggests that the dodecanethiol molecules can adsorb in a highly ordered manner. These results further open the possibility for biofunctionalization of ZnO for biosensing applications.
110 citations
TL;DR: In this article, the authors review experimental results on transition-metal doping of ZnO and the current state of theories for ferromagnetic second phases in the material and conclude that the spin device appears to have no advantage relative to the conventional charge-control electronic analog.
Abstract: ZnO is an attractive candidate for spintronics studies because of its potential for exhibiting high Curie temperatures and the relative lack of ferromagnetic second phases in the material. In this paper, we review experimental results on transition-metal (TM) doping of ZnO and the current state of theories for ferromagnetism. It is important to re-examine some of the earlier concepts for spintronics devices, such as the spin field-effect transistor, to account for the presence of the strong magnetic field that has deleterious effects. In some of these cases, the spin device appears to have no advantage relative to the conventional charge-control electronic analog. We have been unable to detect optical spin polarization in ZnO.
87 citations
73 citations
TL;DR: In this paper, the SnO 2 -coated ZnO nanorods on c-plane sapphire substrates were synthesized by pulsed laser deposition and tested by depositing Ti/Au Ohmic contacts on a random array of the nanorod and measuring the current at fixed voltage.
49 citations
TL;DR: In this article, the annealing temperature and time on Ag catalyst size and density for subsequent growth of ZnO nanorods by catalyst-driven molecular beam epitaxy (MBE).
47 citations
TL;DR: In this paper, the magnetic and transport properties of Mn-doped ZnO thin-film with P doping were examined and it was shown that the films are ferromagnetic with an inverse correlation between magnetization and electron density as controlled by P doping.
Abstract: The magnetic and transport properties of Mn-doped ZnO thin films co-doped with P are examined. Superconducting quantum interference device magnetometry measurements indicate that the films are ferromagnetic with an inverse correlation between magnetization and electron density as controlled by P doping. In particular, under conditions where the acceptor dopants are activated leading to a decrease in free-electron density, magnetization is enhanced. The result is consistent with hole-mediated ferromagnetism in Mn-doped ZnO, in which bound acceptors mediate the ferromagnetic ordering. Increasing the electron density decreases the acceptor concentration, thus quenching the ferromagnetic exchange. This result is important in understanding ferromagnetism in transition metal doped semiconductors for spintronic devices.
42 citations
TL;DR: In this article, the thermal stability of CdZnO ∕ ZnO multi-quantum-well (MQW) structures was studied using rapid thermal annealing in nitrogen from 300 to 750°C.
Abstract: The thermal stability of CdZnO∕ZnO multi-quantum-well (MQW) structures was studied using rapid thermal annealing in nitrogen from 300to750°C. Photoluminescence (PL) emission from the MQWs was studied while varying the annealing temperature and time. For 15min annealings, the PL center wavelength showed a 7nm reduction for temperatures up to 650°C. Above 650°C, the wavelength changed rapidly, with a 50nm reduction at 750°C. Annealing at 700°C for up to 20min produced a systematic reduction in PL wavelength up to 39nm. The data suggest that CdZnO∕ZnO MQWs are relatively stable for nitrogen annealing below 650°C for times up to 15min.
35 citations
TL;DR: The transport properties of as-deposited and rapid thermal annealed phosphorus-doped ZnO films grown by pulsed laser deposition are reported in this paper, with evidence for P segregation in the higher phosphorus concentrations.
Abstract: The transport properties of as-deposited and rapid thermal annealed phosphorus-doped ZnO films grown by pulsed laser deposition are reported. As-grown ZnO:P samples showed n-type characteristics, presumably due to the formation of antisite PZn defects. Rapid thermal annealing yielded a carrier-type conversion from n- to p-type for the ZnO:P films grown at ∼700 °C; samples grown at substantially lower or higher temperatures tended to remain n-type even after the thermal annealing process. The properties and behavior of the n-to-p conversion are most consistent with the formation of PZn‐2VZn as the active acceptor state. Variable magnetic field Hall measurements confirmed the p-type behavior. Phosphorus doping concentrations in the range of 0.5−1.0 at. % were considered, with evidence for P segregation in the higher phosphorus concentrations.
30 citations
TL;DR: In this article, the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy was investigated and it was shown that energy transfer between the tail states is facilitated by tunneling of localized excitons.
Abstract: Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content. © 2007 American Institute of Physics.
27 Apr 2007
TL;DR: In this paper, a review of wet etch of three important materials, namely ZnO, GaN and SiC, is presented, including defect decoration, polarity and polytype identification by producing characteristic pits or hillocks.
Abstract: Wide bandgap semiconductors have many properties that make them attractive for high power, high temperature device applications. In this paper we review wet etching of three important materials, namely ZnO, GaN and SiC. While ZnO is readily etched in many acid solutions including HNO3/HCl and HF/HNO3, and in the nonacid acetyleacetone, the group III nitrides and SiC are very difficult to wet etch and generally dry etching is used. Various etchants for GaN and SiC have been investigated, including aqueous mineral acid and base solutions, and molten salts. Wet etches have a variety of applications to wide bandgap semiconductor technology, including defect decoration, polarity and polytype (for SiC) identification by producing characteristic pits or hillocks, and device fabrication on smooth surfaces. Electrochemical etching is successful at room temperature in some situations for GaN and SiC. In addition, photo-assisted wet etching produces similar rates independent of crystal polarity.
TL;DR: In this paper, the properties of Ta-Ge-O(O)N as a diffusion barrier for Cu on silicon have been investigated and the results indicate that Ta-O fails after annealing at 500°C for 1h.
Abstract: The properties of Ta–Ge–(O)N as a diffusion barrier for Cu on silicon have been investigated. Ta–Ge–(O)N was deposited on single crystal p‐Si(001) by reactive sputtering. This was followed by in situ deposition of Cu. Diffusion barrier tests were conducted by subsequent annealing of individual samples in Ar atmosphere at higher temperature. The films were characterized by x-ray diffraction, Auger electron spectroscopy, and four-point probe. The results indicate that Ta–Ge–(O)N fails after annealing at 500°C for 1h compared to Ta(O)N which fails after annealing at 400°C for 1h indicating better diffusion barrier properties.
TL;DR: In this paper, the authors studied the vibrational modes in n-type and p-type ZnMgO films doped with P (Mg composition of 7at.%) grown by pulsed laser deposition on sapphire.
Abstract: Vibrational modes were studied in n-type and p-type ZnMgO films doped with P (Mg composition of 7at.%) grown by pulsed laser deposition on sapphire. The characteristic phonon frequencies were deduced from the analysis of IR reflectance measured by Fourier-transform spectroscopy. From comparison with similarly grown ZnO (P) films, Mg incorporation reduced the frequency of TO phonons by 14.5cm−1 and introduced two Mg related modes near 530 and 969cm−1. The first is likely to belong to the local vibrational mode of substitutional Mg, and the second is tentatively attributed to strongly lattice relaxed off-center Mg atoms. In addition, it was found that Mg incorporation triggers the formation of two phonon bands with characteristic frequencies of 501 and 634cm−1 that most likely belong to lattice defects.
TL;DR: In this paper, a self-powered wireless hydrogen sensor node has been designed and developed from a system level approach using multi-source energy harvesting circuitry such as scavenged or reclaimed energy from light emitting and vibrational sources as the source of power for commercial low power microcontrollers, amplifiers, and RF transmitters.
Abstract: A self-powered wireless hydrogen sensor node has been designed and developed from a system level approach. By using multi-source energy harvesting circuitry such as scavenged or “reclaimed” energy from light emitting and vibrational sources as the source of power for commercial low power microcontrollers, amplifiers, and RF transmitters, the sensor node is capable of conditioning and deciphering the output of hydrogen sensitive ZnO nanorods sensors. Upon the detection of a discernible amount of hydrogen, the system will ‘wake’ from an idle state to create a wireless data communication link to relay the detection of hydrogen to a central monitoring station. Two modes of operation were designed for the use of hydrogen detection. The first mode would sense for the presence of hydrogen above a set threshold, and alert a central monitoring station of the detection of significant levels of hydrogen. In the second mode of operation, actual hydrogen concentrations starting as low as 10 ppm are relayed to the receiver to track the amount of hydrogen present.
TL;DR: Ohmic contact formation on p-type Mg-doped CuCrO2 layers grown by pulsed-laser deposition was investigated in this article, where the currentvoltage characteristics from Ti∕Au contacts showed back-to-back Schottky behavior, achieving a specific contact resistance of ∼1×10−4Ωcm2.
Abstract: Ohmic contact formation on p-type Mg-doped CuCrO2 layers grown by pulsed-laser deposition was investigated. While the current-voltage characteristics from Ti∕Au contacts showed back-to-back Schottky behavior, a specific contact resistance of ∼1×10−4Ωcm2 was achieved by using Ni instead of Ti. The contact resistivity was fairly independent of measurement temperature, suggesting that tunneling is the dominant transport mechanism. The contact resistance remained practically constant upon annealing in the 100–400°C range. Above 500°C, the morphology became rough and the contact showed rectifying behavior. This degradation resulted from both the out-diffusion of oxygen and the in-diffusion of Ni and Au in CuCrO2.
TL;DR: In this article, Au contacts were deposited on bulk, n-type single-crystal ZnO at either 77 K or 300 K and the difference in contact behavior were stable to anneal temperatures of ∼300 K.
TL;DR: By monitoring the X-ray energy dependence of the various luminescence peaks, this work has determined the local environment of the sites where these peaks originate.
Abstract: We have measured the time-resolved, X-ray excited optical luminescence spectra from two types of Mg{sub x}Zn{sub (1-x)}O core-shell, heterostructured nanowires: type I, with a small x, wurtzite core, encased in a larger x, wurtzite sheath; and type II, with a wurtzite core (x {approx} 0), encased in a rock-salt sheath (x > 0.62). By monitoring the X-ray energy dependence of the various luminescence peaks, we have determined the local environment of the sites where these peaks originate.
TL;DR: In this paper, the synthesis and properties of CuAl 2 O 4 thin films have been examined using reactive direct current magnetron sputter using a CUAl 2 target, and as-deposited films were amorphous.
TL;DR: In this paper, the deuterium concentration is saturated at 6×1018cm−3 throughout the entire 1.2μm epi-ZnO thickness at 250°C.
Abstract: Hydrogen is found to diffuse rapidly into ZnO from either a molecular (D2) gas source or from a D2 plasma at temperatures in the range of 100–200°C. In samples deuterated from a plasma excited at 13.56MHz, the incorporated deuterium concentration is saturated at 6×1018cm−3 throughout the entire 1.2μm epi-ZnO thickness at 250°C. This deuterium is stable against reverse bias application at 25°C for 24h. By sharp contrast, deuterium incorporated from exposure to D2 gas at 100°C can be entirely removed by reverse (negative) bias application in a diode structure using a Pt contact on the n-type ZnO. These results are consistent with the state of the deuterium in the ZnO being a function of both the concentration and the flux during incorporation. At low concentrations, the deuterium is likely to be in a positively charged atomic form or bound in defects that are readily dissociated by an electric field, while at higher concentrations the deuterium may be in neutral states such as D2 or OH complexes.
TL;DR: In this paper, the authors investigated the deposition of amorphous indium-zinc oxide (IZO) films by cosputtering from In 2 O 3 and ZnO targets near room temperature as a function of power, process pressure and oxygen partial pressures in the sputtering ambient.
Abstract: The deposition of amorphous indium-zinc oxide (IZO) films by cosputtering from In 2 O 3 and ZnO targets near room temperature was investigated as a function of power, process pressure and oxygen partial pressures in the sputtering ambient. The resistivity of the films with In/Zn ratio between 0.3 and 0.6 could be controlled between 10 -3 and 10 3 Ω cm by varying the oxygen partial pressure. The corresponding electron mobilities were 5-20 cm 2 V -1 s -1 . The optical transmittance of the IZO films was >70% in all cases. Ohmic contact resistances in the range of 3-8 × 10 -5 Ω cm were obtained with both Ni/Au and Ti/Au deposited by electron beam evaporation. ZnO films deposited under the same conditions always showed evidence of polycrystallinity, while the InZnO films remained amorphous.
TL;DR: In this article, the electrical properties, microcathodoluminescence spectra, and persistent photoconductivity observed for as-grown and annealed ZnO(P) and Zn 0.93 Mg 0.07 O (P) films grown by pulsed laser deposition on sapphire substrates are reported.
Abstract: The electrical properties, microcathodoluminescence spectra, and persistent photoconductivity observed for as-grown and annealed ZnO(P) and Zn 0.93 Mg 0.07 O(P) films grown by pulsed laser deposition on sapphire substrates are reported. As-grown ZnO(P) had n-type conductivity with the Fermi level pinned near E c -0.035 eV. Annealing in Ar at progressively higher temperatures of 850, 900, and 950°C shifted the Fermi level first to E c -0.07 eV and then to E v + (0.1-0.13) eV, the latter level being most likely the acceptor level of P. In ZnMgO(P) the residual donors and the P acceptors are slightly deeper than in ZnO, respectively, 0.08 and 0.19 eV. The persistent photoconductivity observed was due to centers located above E v + 0.5 eV and having a barrier for capture of electrons of 0.45 eV. These centers are tentatively associated with off-center P atoms located on Zn sites.
TL;DR: In this article, the saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone alone alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role.
Abstract: ZnO is a very promising material for spintronics applications, with many groups reporting room temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during PLD, we find an inverse correlation between magnetization and electron density as controlled by Sn doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for the ferromagnetism include the bound magnetic polaron model or exchange is mediated by carriers in a spin-spilt impurity band derived from extended donor orbitals. Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors.
TL;DR: The dry etching characteristics of bulk, single-crystal zinc-oxide (ZnO) and rf-sputtered indium-zincoxide (IZO) films have been investigated using an inductively coupled high-density plasma with different plasma chemistries as discussed by the authors.
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TL;DR: In this article, the authors review the recent results on developing functional nanowires in the ZnMgO systems and show some applications in hydrogen gas sensing, pH sensing, transparent transistors and UV detectors.
Abstract: ZnO has properties well-suited to UV/visible light emitters, transparent thin film electronics and a variety of gas and chemical sensor applications. There has been extensive interest in recent times in synthesis of ZnO nanowires by a number of methods using both catalyst and catalyst-free approaches. In this paper we review our recent results on developing functional nanowires in the ZnMgO systems and show some applications in hydrogen gas sensing, pH sensing, transparent transistors and UV detectors. In terms of sensors, the main selling points are large surface-to-volume ratio for improved detection sensitivity and also low power requirements.
Abstract: The effect of cryogenic temperatures during metal deposition on the contact properties of Pd, Pt, Ti, and Ni on bulk single-crystal n-type ZnO has been investigated. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. By sharp contrast, both Pd and Pt contacts showed rectifying characteristics after deposition with barrier heights between 0.37 eV and 0.69 eV. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300 °C, showing an increase in barrier height along with a decrease in ideality factor with increasing annealing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however, low temperature contacts demonstrated some peeling/cracking for Pt and Pd.
TL;DR: An (NH4) 2 Ce(N03) 6 /HNO 3 /H 2 O solution was used to obtain wet etch rates at 25°C in the range 500-1000 A min -1 for CuCrO 2 grown on sapphire substrates by pulsed laser deposition.
Abstract: An (NH4) 2 Ce(N03) 6 (/HNO 3 /H 2 O solution was used to obtain wet etch rates at 25°C in the range 500-1000 A min -1 for CuCrO 2 grown on sapphire substrates by pulsed laser deposition. The etching was reaction-limited, with an activation energy of 11.9 kCal mol -1 . Under these conditions, the etching of ZnO grown in a similar fashion was much faster (∼5 μm min -1 ), providing highly selective removal of ZnO from CuCrO 2 . In addition, the conventional etchants for ZnO (HCl, HNO 3 , H 3 PO 4 ) did not etch the CuCrO 2 . Simple photoresist masking can be used with the (NH4)2Ce(NO 3 ) 6 /HNO 3 /H20 mixtures to allow patterning of device structures.
TL;DR: In this paper, the contact characteristics on bulk single-crystal n-type ZnO of an Ir/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 200-1000°C (N 2 ambient).
Abstract: The contact characteristics on bulk single-crystal n-type ZnO of an Ir/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 200-1000°C (N 2 ambient). The contacts exhibited ohmic behavior for all temperatures and show a minimum specific contact resistivity of 3.6 X 10 -5 Ω cm 2 after a 1000°C anneal. The contacts transition to rectifying behavior after annealing above 1100°C, coincident with a degraded surface morphology including agglomeration of Ir to the surface and heavy intermixing of the Ir and Au. The Ir contacts exhibit higher thermal stability but poorer specific contact resistivity than conventional Ti/Au metal stacks on bulk n-type ZnO. The contacts showed very little change in resistance after extended aging (30 days) at 350°C. Annealing under O 2 ambient led to an increase in contact resistivity by orders of magnitude.