Showing papers by "Stephen J. Pearton published in 2004"

Recent advances in processing of ZnO

Abstract: A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures.

ZnO nanowire growth and devices

Abstract: The large surface area of ZnO nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for micro-lasers or memory arrays. In addition, they might be doped with transition metal (TM) ions to make spin-polarized light sources. To date, most of the work on ZnO nanostructures has focused on the synthesis methods and there have been only a few reports of the electrical characteristics. We review fabrication methods for obtaining device functionality from single ZnO nanorods. A key aspect is the use of sonication to facilitate transfer of the nanorods from the initial substrate on which they are grown to another substrate for device fabrication. Examples of devices fabricated using this method are briefly described, including metal-oxide semiconductor field effect depletion-mode transistors with good saturation behavior, a threshold voltage of ∼−3 V and a maximum transconductance of order 0.3 mS/mm and Pt Schottky diodes with excellent ideality factors of 1.1 at 25 °C and very low (1.5 × 10 −10 A, equivalent to 2.35 A cm −2 , at −10 V) reverse currents. The photoresponse showed only a minor component with long decay times (tens of seconds) thought to originate from surface states. These results show the ability to manipulate the electron transport in nanoscale ZnO devices.

Dilute magnetic semiconducting oxides

Abstract: A review of recent results on transition metal doping of electronic oxides such as ZnO, TiO2, SnO2, BaTiO3, Cu2O, SrTiO3 and KTaO3 is presented. There is interest in achieving ferromagnetism with Curie temperatures above room temperature in such materials for applications in the field of spintronic devices, in which the spin of the carriers is exploited. The incorporation of several atomic per cent of the transition metals without creation of second phases appears possible under optimized synthesis conditions, leading to ferromagnetism. Pulsed laser deposition, reactive sputtering, molecular beam epitaxy and ion implantation have all been used to produce the oxide-based dilute magnetic materials. The mechanism is still under debate, with carrier-induced, double-exchange and bound magnetic polaron formation all potentially playing a role depending on the conductivity type and level in the material.

GaN-based diodes and transistors for chemical, gas, biological and pressure sensing

Abstract: There is renewed emphasis on development of robust solid-state sensors capable of uncooled operation in harsh environments. The sensors should be capable of detecting chemical, gas, biological or radiation releases as well as sending signals to central monitoring locations. We discuss the advances in use of GaN-based solid-state sensors for these applications. AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization-induced two-dimensional electron gas (2DEG). Furthermore, spontaneous and piezoelectric polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors to detect gases, polar liquids and mechanical pressure. AlGaN/GaN HEMT structures have been demonstrated to exhibit large changes in source–drain current upon exposing the gate region to various block co-polymer solutions. Pt-gated GaN Schottky diodes and Sc2O3/AlGaN/GaN metal-oxide semiconductor diodes also show large change in forward currents upon exposure to H2. Of particular interest is detection of ethylene (C2H4), which has strong double bonds and hence is difficult to dissociate at modest temperatures. Apart from combustion gas sensing, the AlGaN/GaN heterostructure devices can be used as sensitive detectors of pressure changes. In addition, large changes in source–drain current of the AlGaN/GaN HEMT sensors can be detected upon adsorption of biological species on the semiconductor surface. Finally, the nitrides provide an ideal platform for fabrication of surface acoustic wave (SAW) devices. The GaN-based devices thus appear promising for a wide range of chemical, biological, combustion gas, polar liquid, strain and high temperature pressure-sensing applications. In addition, the sensors are compatible with high bit-rate wireless communication systems that facilitate their use in remote arrays.

Depletion-mode ZnO nanowire field-effect transistor

Abstract: Single ZnO nanowire metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated using nanowires grown by site selective molecular-beam epitaxy. When measured in the dark at 25°C, he depletion-mode transistors exhibit good saturation behavior, a threshold voltage of ∼−3V, and a maximum transconductance of order 0.3mS∕mm. Under ultraviolet (366nm) illumination, the drain–source current increase by approximately a factor of 5 and the maximum transconductance is ∼5mS∕mm. The channel mobility is estimated to be ∼3cm2∕Vs, which is comparable to that reported for thin film ZnO enhancement mode MOSFETs, and the on∕off ratio was ∼25 in the dark and ∼125 under UV illumination.

p-type behavior in phosphorus-doped (Zn,Mg)O device structures

Abstract: The characteristics of device structures that employ phosphorus-doped (Zn,Mg)O have been examined in a effort to delineate the carrier type behavior in this material. The capacitance–voltage properties of metal/insulator/P-doped (Zn,Mg)O diode structures were measured and found to exhibit a polarity consistent with the P-doped (Zn,Mg)O layer being p type. In addition, thin-film junctions comprising n-type ZnO and P-doped (Zn,Mg)O display asymmetric I–V characteristics that are consistent with the formation of a p–n junction at the interface. Although Hall measurements of the P-doped (Zn,Mg)O thin films yielded an indeterminate Hall sign due to a small carrier mobility, these results are consistent with previous reports that phosphorus can yield an acceptor state and p-type behavior in ZnO materials.

Effects of high-dose Mn implantation into ZnO grown on sapphire

Abstract: ZnO films grown by pulsed-laser deposition on c-plane Al2O3 substrates were annealed at temperatures up to 600 °C to produce n-type carrier concentrations in the range 7.5×1015–1.5×1020 cm−3. After high-dose (3×1016 cm−2) Mn implantation and subsequent annealing at 600 °C, all the films show n-type carrier concentrations in the range 2–5×1020 cm−3 and room temperature hysteresis in magnetization loops. 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 that factors such as crystalline quality and residual defects play a role.

Electrical transport properties of single ZnO nanorods

Abstract: Single ZnO nanorods with diameters of ∼130nm were grown on Au-coated Al2O3 substrates by catalyst-driven molecular beam epitaxy. Individual nanorods were removed from the substrate and placed between Ohmic contact pads and the current–voltage characteristics measured as a function of temperature and gas ambient. In the temperature range from 25to150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089±0.02eV and was insensitive to the ambient used (C2H4,N2O,O2 or 10% H2 in N2). By sharp contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors.

Pt∕ZnO nanowire Schottky diodes

Abstract: Pt Schottky diodes were formed on single ZnO nanowires grown by site-selective molecular-beam epitaxy and then transferred to SiO2-coated Si substrates. The diodes exhibit excellent ideality factors of 1.1 at 25 °C and very low (1.5×10−10A, equivalent to 2.35Acm−2, at −10V) reverse currents. The nanowire diodes show a strong photoresponse, with the current–voltage characteristics becoming ohmic under ultraviolet illumination (366 nm light). The on-off current ratio of the diodes at 0.15∕−5V was ∼6. These results show the ability to manipulate the electron transport in nanoscale ZnO devices.

Pressure-induced changes in the conductivity of AlGaN∕GaN high-electron mobility-transistor membranes

Abstract: AlGaN∕GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source∕drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN∕GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of −(+)6.4×10−2mS∕bar for application of compressive (tensile) strain. The AlGaN∕GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.

“Hidden hydrogen” in as-grown ZnO

Abstract: An O–H stretching line at 3326.3cm−1 was previously assigned to a shallow donor that is introduced into ZnO by H. This infrared line has been found to appear in as-grown ZnO samples when they are annealed near 400 °C without an external source of H, showing that there is an H-containing defect in commercially available ZnO that is not seen by infrared spectroscopy that can be converted into a shallow donor. The interstitial H2 molecule in ZnO is suggested as a candidate for the “hidden” H species.

Single Wall Carbon Nanotubes for p-Type Ohmic Contacts to GaN Light-Emitting Diodes

Abstract: Homogeneous films of pure single wall carbon nanotubes (SWNTs) sufficiently thin to be optically transparent in the visible range of the spectrum were employed as p-Ohmic contacts on GaN-InGaN quantum-well light-emitting diodes. The specific contact resistance of the SWNT films on the p-GaN was 1.1 x 10 -2 Ω cm 2 after annealing at 700°C for 60 s under N 2 , which was a factor of 3 lower than standard Ni/Au contacts on the same p-GaN. The SWNT-contacted LEDs showed bright blue emission centered at 434 nm and demonstrate that the SWNT films provide a new class of electrically conducting, p-type, transparent electrode for use with photonic devices.

Wide bandgap GaN-based semiconductors for spintronics

Abstract: Recent results on achieving ferromagnetism in transition-metal-doped GaN, A1N and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carrier ...

``Hidden'' Hydrogen in As-Grown ZnO

Abstract: An O–H stretching line at 3326.3cm−1 was previously assigned to a shallow donor that is introduced into ZnO by H. This infrared line has been found to appear in as-grown ZnO samples when they are annealed near 400 °C without an external source of H, showing that there is an H-containing defect in commercially available ZnO that is not seen by infrared spectroscopy that can be converted into a shallow donor. The interstitial H2 molecule in ZnO is suggested as a candidate for the “hidden” H species.

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

Abstract: We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 175 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V Inversion of the channel was achieved for gate voltages above 6 V The maximum transconductance was 54 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs

Temperature-dependent characteristics of Pt Schottky contacts on n-type ZnO

Abstract: The Schottky barrier height of Pt contacts on n-type (n∼1016 cm−3) thin film ZnO deposited by pulsed laser deposition was obtained from current–voltage measurements as a function of temperature. The resulting values ranged from 0.61±0.04 eV at 25 °C to 0.46±0.06 eV at 100 °C with saturation current densities of 1.5×10−4 A cm−2 (25 °C) to 6.0×10−2 A cm−2 (100 °C), respectively. The reverse current magnitude was larger than predicted by thermionic emission alone. The measured barrier height for Pt on ZnO is similar to the value reported for both Au and Ag rectifying contacts on this material.

AlGaN/GaN-based metal–oxide–semiconductor diode-based hydrogen gas sensor

Abstract: The characteristics of Sc2O3/AlGaN/GaN metal–oxide–semiconductor (MOS) diodes as hydrogen gas sensors are reported. At 25 °C, a change in forward current of ∼6 mA at a bias of 2 V was obtained in response to a change in ambient from pure N2 to 10% H2/90% N2. This is approximately double the change in forward current obtained in Pt/GaN Schottky diodes measured under the same conditions. The mechanism of the change in forward gate current appears to be formation of a dipole layer at the oxide/GaN interface that screens some of the piezo-induced channel charge. The MOS-diode response time is limited by the mass transport of gas into the test chamber and not by the diffusion of atomic hydrogen through the metal/oxide stack, even at 25 °C. These devices look promising for applications requiring sensitive, long-term stable detection of combustion gases.

Carrier concentration dependence of Ti/Al/Pt/Au contact resistance on n-type ZnO

Abstract: Ti/Al/Pt/Au ohmic contacts on n-type ZnO with a range of carrier concentrations (7.5×1015–1.5×1020 cm−3) show as-deposited specific contact resistances in the range from 3×10−4 to 8×10−7 Ω cm2. Temperature-dependent measurements showed that the dominant transport mechanisms were tunneling in the contacts in the most highly doped films and thermionic emission in the more lightly doped films. After annealing at 200 °C, the lowest specific contact resistance achieved was 2.2×10−8 Ω cm2. However, the contacts show evidence of reactions between the Ti and the ZnO film even for this low annealing temperature, suggesting that applications requiring good thermal stability will need metallurgy with better thermal stability.

AlGaN/GaN HEMT based liquid sensors

Abstract: An AlGaN/GaN high electron mobility transistor structure was used for sensing different liquids present in the gate region. The forward current showed significant decreases upon exposure of the gate area to solvents (water, acetone) or acids (HCl). The pH sensitivity is due to changes in net surface charge that affects the relative depletion in the channel of the transistor. The results indicate that nitride-based heterostructures may have application in integrated chemical, gas and fluid monitoring sensors.

Improved Pt∕Au and W∕Pt∕Au Schottky contacts on n-type ZnO using ozone cleaning

Abstract: UV-ozone cleaning prior to metal deposition of either e-beam Pt contacts or sputtered W contacts on n-type single-crystal ZnO is found to significantly improve their rectifying characteristics. Pt contacts deposited directly on the as-received ZnO surface are Ohmic but show rectifying behavior with ozone cleaning. The Schottky barrier height of these Pt contacts was 0.70eV, with ideality factor of 1.5 and a saturation current density of 6.2×10−6Acm−2. In contrast, the as-deposited W contacts are Ohmic, independent of the use of ozone cleaning. Postdeposition annealing at 700°C produces rectifying behavior with Schottky barrier heights of 0.45eV for control samples and 0.49eV for those cleaned with ozone exposure. The improvement in rectifying properties of both the Pt and W contacts is related to removal of surface carbon contamination from the ZnO.

Shallow donor formation in phosphorus-doped ZnO thin films

Abstract: ZnO is a direct band gap semiconductor that is attractive for a number of applications, including high-temperature electronics, transparent electronics, and blue to UV optoelectronics. The most significant impediment to the exploitation of ZnO is the difficulty in achieving effective p-type carrier doping. Attractive acceptor-state candidates for ZnO are the group-V elements substituted on the oxygen site. In this paper, the doping behavior of phosphorus in as-deposited ZnO crystalline thin films grown by pulsed-laser deposition is reported. The experimental work reported here indicates that phosphorus doping significantly increases the electron carrier density in as-deposited films, making the material heavily n-type. These results indicate the formation of a shallow donor level associated with the phosphorus impurity, resulting from either substitution on the Zn site or formation of a compensating complex. Based on X-ray photoelectron spectroscopy measurements, the phosphorus-dopant ions in the films occupy multiple sites yielding oxidation states of -3 and +5. This would explain the compensated n-type behavior for group-V doping in the II-VI host material.

Properties of highly Cr-doped AlN

Abstract: Cr concentrations of ∼2at.% were incorporated into AlN during growth by molecular beam epitaxy. Under optimized conditions, single-phase, insulating AlCrN is produced whose band gap shows a small (0.1–0.2eV) decrease from the value for undoped AlN (6.2eV), a decrease in a-plane lattice constant and the introduction of two absorption bands at 3 and 5eV into the band gap. This material shows ferromagnetism with a Curie temperature above 300K as judged from the difference in field-cooled and zero-field-cooled magnetization. For nonoptimized growth, second phases of Cr2N and AlxCry are produced in the AlN and the material is conducting (∼1000Ωcm) with activation energy for conduction of 0.19eV and apparent band gap of 5.8eV.

Effect of Mn concentration on the structural, optical, and magnetic properties of GaMnN

Abstract: The room temperature magnetization of GaMnN films grown by molecular beam epitaxy on (0001) sapphire substrates with Mn concentrations varying from 0 to 9 at. % was found to depend on Mn concentration, with a maximum magnetization found at ∼3 at. % Mn. High-resolution x-ray diffraction measurements show that the c-plane lattice constant initially decreases with increasing Mn concentration, then increases when the Mn content increases above ∼3 at. %. This increase is accompanied by a decrease in the full width at half maximum of the rocking curves. Extended x-ray absorption fine structure results indicate that the nonsubstitutional Mn is not present in the form of GaxMny clusters and thus is most likely present in the form of an interstitial. Optical absorption measurements show only a slight increase in the band gap for material with 3 at. % Mn, relative to undoped GaN.

Contacts to p-type ZnMgO

Abstract: Ohmic and Schottky contacts to p-type Zn09Mg01O are reported The lowest specific contact resistivity of 3×10−3 Ω cm2 was obtained for Ti/Au annealed at 600 °C for 30 s Ni/Au was less thermally stable and showed severe degradation of contact morphology at this annealing temperature Both Pt and Ti with Au overlayers showed rectifying characteristics on p-ZnMgO, with barrier heights of ∼055–056 eV and ideality factors of ∼19 Comparison of these results with the same metals on n-type ZnO indicates that high surface state densities play a significant role in determining the effective barrier height

Optimization of two dimensional gratings for very long wavelength quantum well infrared photodetectors

Abstract: We have performed a detailed study of two-dimensional grating coupling for quantum well infrared photodetectors in the very long wavelength spectral region λ∼16-17 μm. Using calculations based on the modal expansion method we quantitatively explain the double peaked responsivity spectrum. By optimizing the grating parameters we achieve a normal incidence responsivity and detectivity which are three times larger than the 45° angle of incidence geometry.

Sensitivity of Pt/ZnO Schottky diode characteristics to hydrogen

Abstract: Pt/ZnO Schottky diodes show changes in forward current of 0.3 mA at a forward bias of 0.5 V or alternatively a change of 50 mV bias at a fixed forward current of 8 mA when 5 ppm of H2 is introduced into a N2 ambient at 25 °C. The rectifying current–voltage (I–V) characteristic shows a nonreversible collapse to Ohmic behavior when as little as 50 ppm of H2 is present in the N2 ambient. At higher temperatures, the recovery is thermally activated with an activation energy of ∼0.25 eV. This suggests that introduction of hydrogen shallow donors into the ZnO is a contributor to the change in current of the diodes.

Sensitivity of Pt/ZnO schottky diode characteristics to hydrogen

Abstract: Pt/ ZnO Schottky diodes show changes in forward current of 0.3 mA at a forward bias of 0.5V or alternatively a change of 50 mV bias at a fixed forward current of 8 mA when 5 ppm of H 2 is introduced into a N 2 ambient at 25°C.The rectifying current-voltage(I-V) characteristic shows a non-reversible collapse to Ohmic behavior when as little as 50 ppm of H 2 is present in the N 2 ambient. At higher temperatures, the recovery is thermally activated with an activation energy of ∼0.25 eV. This suggests that introduction of hydrogen shallow donors into the ZnO is a contributor to the change in current of the diodes.

Thermal degradation of electrical properties and morphology of bulk single-crystal ZnO surfaces

Abstract: Bulk ZnO substrates were rapid thermal annealed in either air or N2 at temperatures up to 500°C. The root-mean-square roughness of the surface as measured by atomic force microscopy begins to increase even after 200°C anneals in N2 or 300°C in air. The Schottky barrier height, ΦB, obtained from Pt∕Au diodes fabricated on these surfaces shows a decrease in effective barrier height upon annealing from 0.35eV in control samples to ∼0.30eV in those annealed at 400°C and an increase in the saturation current density (from 4Acm−2 in the control samples to 17–37Acm−2 in the annealed samples). These results show that the ZnO surface is degraded by quite low annealing temperatures and care must be exercised in designing the thermal budget for processing of ZnO devices. At much higher annealing temperatures (1000°C), the surface completely decomposes to leave metallic Zn droplets.