Michael M. Oye

Unpinned metal gate/high-κ GaAs capacitors: Fabrication and characterization

Fabrication of GaAs metal-oxide-semiconductor capacitors (MOSCAPs) with an unpinned interface is reported. The MOSCAP structure consists of a few monolayers of germanium grown in a molecular beam epitaxy (MBE) system in order to terminate an MBE-grown silicon-doped (100) GaAs layer. An ex situ HfO2 high-κ dielectric with an equivalent oxide thickness of 12A was deposited by using a dc magnetron sputtering system. A midgap interface state density (Dit) of 5×1011eV−1cm−2 was measured using the high-frequency conductance technique. A rapid thermal annealing study was performed in order to examine the integrity of the gate stack at different temperatures. In addition, a forming gas anneal at 400°C appears to significantly reduce the midgap Dit revealed by probing the frequency dispersion behavior of the MOSCAPs.

Influence of metal–ligand ratio on benzimidazole based luminescent lanthanide complexes: 3-D network structures and chloride anion binding

Reaction of LnCl3·6H2O with the benzimidazole based ligand 2-(1H-benzoimidazol-2-yl)-4-bromo-6-methoxy-phenol (HL) in EtOH or MeOH under reflux gave dinuclear complexes of formula [Ln2(HL)2Cl6(solv)4] (solv = EtOH or MeOH) with a ligand : metal ratio = 1 : 1 (Ln = Tb(1); Er(2); Gd(3); Yb(4)). A ligand : metal ratio of 2 : 1 under similar conditions gave the mononuclear complexes [Eu(HL)2Cl3(MeOH)] (5) and [Ln(HL)2(H2O)4]·3Cl (Ln = Er(6); Yb(7)). A ligand : metal stoichiometry of 4 : 1 gave complexes [Ln(HL)4H2O]·3Cl (Ln = Nd(8); Gd (9); Eu(10)). The compounds have been structurally characterized by single crystal X-ray diffraction studies and their photophysical properties investigated.

Effective Piezoelectric Response of Substrate-Integrated ZnO Nanowire Array Devices on Galvanized Steel

Harvesting waste energy through electromechanical coupling in practical devices requires combining device design with the development of synthetic strategies for large-area controlled fabrication of active piezoelectric materials. Here, we show a facile route to the large-area fabrication of ZnO nanostructured arrays using commodity galvanized steel as the Zn precursor as well as the substrate. The ZnO nanowires are further integrated within a device construct and the effective piezoelectric response is deduced based on a novel experimental approach involving induction of stress in the nanowires through pressure wave propagation along with phase-selective lock-in detection of the induced current. The robust methodology for measurement of the effective piezoelectric coefficient developed here allows for interrogation of piezoelectric functionality for the entire substrate under bending-type deformation of the ZnO nanowires.

Molecular-beam epitaxy growth of device-compatible GaAs on silicon substrates with thin (∼80nm) Si1−xGex step-graded buffer layers for high-κ III-V metal-oxide-semiconductor field effect transistor applications

The authors report the fabrication of TaN–HfO2–GaAs metal-oxide-semiconductor capacitors on silicon substrates. GaAs was grown by migration-enhanced epitaxy (MEE) on Si substrates using an ∼80-nm-thick Si1−xGex step-graded buffer layer, which was grown by ultrahigh vacuum chemical vapor deposition. The MEE growth temperatures for GaAs were 375 and 400°C, with GaAs layer thicknesses of 15 and 30nm. We observed an optimal MEE growth condition at 400°C using a 30nm GaAs layer. Growth temperatures in excess of 400°C resulted in semiconductor surfaces rougher than 1nm rms, which were unsuitable for the subsequent deposition of a 6.5-nm-thick HfO2 gate dielectric. A minimum GaAs thickness of 30nm was necessary to obtain reasonable capacitance-voltage (C-V) characteristics from the GaAs layers grown on Si substrates. To improve the interface properties between HfO2 and GaAs, a thin 1.5nm Ge interfacial layer was grown by molecular-beam epitaxy in situ after the GaAs growth. The Ge-passivated GaAs samples were th...

Effects of different plasma species (atomic N, metastable N2*, and ions) on the optical properties of dilute nitride materials grown by plasma-assisted molecular-beam epitaxy

This letter studies the effects of atomic N, metastable N2*, and ionic species on the optical properties of dilute nitride materials. Ga0.8In0.2N0.01As0.99 was grown using a 1% N2 in Ar gas mix from an Applied-Epi Unibulb™ rf plasma source. Isonitrogen samples with and without ions were studied using various plasma operating conditions. Optical emission spectrometry was used to characterize relative proportions of different active nitrogen plasma species (atomic N and metastable N2*). Samples grown without ions and with a higher proportion of atomic N resulted in the best overall material quality, although this improvement was observed at high annealing temperatures. At lower annealing temperatures, increased blueshifts were observed for samples grown with a higher proportion of atomic N; however, there was no noticeable influence of ions on blueshift regardless of whether atomic N or metastable N2* was the dominant species present in the plasma. The key implication of this work is that it helps to elucid...

Using beam flux monitor as langmuir probe for plasma-assisted molecular beam epitaxy

We present a simple method for measuring the ion flux from a molecular beam epitaxy (MBE) plasma cell in real time. A Langmuir probe was created by attaching the beam flux monitor to a picoammeter and measuring the current impinging upon the collector or filament wires. This provides a noninvasive, convenient, and direct measure of ion flux at the actual wafer position, yet requires no internal changes to the MBE machine. Quantitative measurements of maximum ion energies and relative ion fluxes are possible. Real-time feedback from this measurement allows rapid optimization of the plasma for the minimum ion flux. This method is applicable to GaN and related materials, but is particularly important for growth of dilute nitrides. This was one of the techniques which led to the longest wavelength GaInNAs(Sb) vertical cavity surface emitting lasers and continuous wave edge emitting lasers on GaAs to date.

Controlled growth of vertical ZnO nanowires on copper substrate

We present an approach for diameter control of vertically aligned ZnO nanowires (NWs) grown directly on copper substrates. Vapor-solid growth was done at 550 °C with solid Zn precursor under Ar/O2 flow, and the resulting nanowires with in situ-controllable diameters ranged between 50 to 500 nm. The nanowires were observed to elongate in tip growth and diameters were directly controlled by varying the oxygen concentration. Direct growth of vertical wires on metal substrates is expected to be useful to construct piezoelectric devices and applications involving sensors and detectors.

Ion damage effects from negative deflector plate voltages during the plasma-assisted molecular-beam epitaxy growth of dilute nitrides

We studied the effects of ion damage on the optical properties of dilute nitrides grown by plasma-assisted molecular-beam epitaxy. A dual-grid retarding field ion energy analyzer was used to measure the ion flux and ion energy distribution at the substrate position from an Applied-EPI UniBuilb™ rf plasma cell. These changes were measured as the negative deflector plate voltage varied from 0 to −800V. The largest ion flux resulted with a −100V setting, while the greatest ion energies occurred with −200V. Deflector plate voltages more negative than −300V resulted in a significant reduction in both the ion flux and ion energy. The damage caused by these ions was determined by measuring the pre- and postanneal photoluminescence properties of Ga0.8In0.2N0.01As0.99 quantum wells. Comparable optical properties were possible with various combinations of ion fluxes and ion energies, which demonstrate how the ion flux and ion energy each impart an individual effect on the sample’s optical properties. An awareness o...

Diffusion mechanisms of indium and nitrogen during the annealing of InGaAs quantum wells with GaNAs barriers and GaAs spacer layers

In this article, we discuss two indium diffusion mechanisms that are present during the rapid thermal annealing of InxGa1−xAs quantum wells (x=0.18, 0.22, and 0.26) with GaNyAs1−y barriers (y=0.6 or 1.2%). Samples were grown with and without a GaAs spacer layer in between the quantum well and barrier. The dominant mechanism is dependent on the amount of thermal energy applied during the annealing process. At low annealing times and temperatures, we have observed that In-Ga intra-diffusion entirely within the quantum well is dominant. For the higher times and temperatures, In-Ga inter-diffusion between the quantum well and barrier becomes dominant. These observations were confirmed by high-resolution x-ray diffraction and the peak emission wavelengths were measured by room-temperature photoluminescence. We have also observed that nitrogen had diffused from the GaNAs barriers into the InGaAs quantum wells in all of our annealed samples. In addition, the commonly observed indium-content dependent diffusion i...

Unintentional Calcium Incorporation in Ga(Al, In, N)As

Unintentional calcium incorporation into GaInNAs causes an acceptor-type impurity, which limits the ability of ∼1eV GaInNAs-based solar cells to collect photogenerated current. Here, the authors focus on better understanding the conditions by which Ca is incorporated into GaInNAs. Various material combinations were grown including GaAs, InGaAs, GaInNAs, and Al(Ga)As. The materials were primarily grown by solid-source molecular-beam epitaxy (MBE) at ∼400 and 580–620°C, with comparisons made to metal-organic chemical vapor deposition (MOCVD)-grown materials where appropriate. Calcium incorporation was measured through secondary ion mass spectrometry. There was no measurable Ca incorporation into MBE-grown GaAs at 580°C, but Ca incorporates into GaAs at low MBE growth temperatures (∼400°C) that are comparable to those typically used for GaInNAs. This suggests that the N species is not solely responsible for the observed Ca incorporation into MBE-grown GaInNAs; but rather, defects associated with the low temp...