M. Brandt

ZnO metal-semiconductor field-effect transistors with Ag-Schottky gates

Metal-semiconductor field-effect transistors (MESFETs) were fabricated by reactive dc sputtering of Ag-Schottky gate contacts on ZnO thin-film channels grown by pulsed-laser deposition on sapphire. The n-type conductivity (normally on) of typical MESFETs is tunable over 8 decades in a voltage range of 2.5V with an off voltage of −1.5V and very low off-current density in the range of 10−6A∕cm2. Channel mobilities of up to 27cm2∕Vs have been achieved.

Deep acceptor states in ZnO single crystals

The authors report the observation of both acceptor- and donorlike defects in ZnO by deep level transient spectroscopy. The observation is facilitated by using a p-n junction allowing the injection of holes and electrons. The junction is realized by implanting a n-conducting ZnO wafer grown by pressurized melt growth with nitrogen ions. The authors found the commonly observed donorlike defects E1 and E3 and two acceptorlike defects A2 and A3, as well as a broad acceptorlike defect band. The thermal activation energies of A2 and A3, were determined to be about 150 and 280meV, respectively.

Resistive hysteresis and interface charge coupling in BaTiO3-ZnO heterostructures

We report on temperature, time, and voltage dependent resistive hysteresis measurements of BaTiO3-ZnO heterostructures grown on (001) Si substrates by pulsed laser deposition. We observe a diodelike behavior and cycling-voltage dependent hysteresis formation under forward bias. We explain these effects with depletion layer formation between the ZnO and BaTiO3 layers, an additional barrier due to the spontaneous polarization of ZnO and the ferroelectric nature of BaTiO3. The disappearance of the resistive hysteresis above the ferroelectric-paraelectric phase transition temperature of BaTiO3 conformed that the hysteresis is related to the ferroelectricity of BaTiO3. Time dependent resistance measurements reveal memory effects.

Control of interface abruptness of polar MgZnO/ZnO quantum wells grown by pulsed laser deposition

A strong quantum confined Stark effect (QCSE) was observed in wedge shaped MgZnO/ZnO quantum wells (QWs) grown by pulsed laser deposition. A reduced laser fluence of 1.8 J/cm2 was used. Reference samples grown at higher standard fluence 2.4 J/cm2 showed only a negligible QCSE. Using off-axis deposition without substrate rotation, a constant composition of the barriers was maintained while varying the well width in a wedge shaped QW. A redshift of the QW luminescence with increasing QW thickness up to 230 meV below the ZnO emission was found, accompanied by an increase in the exciton lifetime from 0.3 ns up to 4.2 μs.

High electron mobility of phosphorous-doped homoepitaxial ZnO thin films grown by pulsed-laser deposition

The transport properties of phosphorous-doped ZnO thin films, grown by pulsed-laser deposition on thermally pretreated hydrothermally grown ZnO single-crystal substrates, are reported. The ZnO:P thin films show very good morphological and structural properties as confirmed by atomic force microscopy (AFM), high resolution x-ray diffraction, and Rutherford backscattering (RBS) channeling. Steps of height c/2 are visible in AFM investigations for all samples. For an oxygen partial pressure of 0.1 mbar, two-dimensional growth was found. RBS channeling of a ZnO:P film shows a minimum yield of 0.034 which is comparable to that of an annealed substrate (0.033). Hall effect measurements revealed that all films are n-type for the present growth conditions. Peak mobilities of 800 cm2/Vs have been observed around 70 K, in line with the high structural quality of the samples. Room-temperature mobility in ZnO:P is up to 170 cm2/Vs.

Ferroelectric thin film field-effect transistors based on ZnO/BaTiO3 heterostructures

The authors have grown epitaxial ZnO/BaTiO3 (BTO) heterostructures by pulsed laser deposition on lattice matched Nb-doped SrTiO3 substrates. Epitaxial growth of the BTO layers has been confirmed by x-ray diffraction. The electrical properties of ZnO/BTO heterostructures have been investigated by current-voltage and capacitance-voltage measurements, showing that the BTO layers are highly insulating (leakage current density jl<10−9 A/cm2 at 5 V). The structures were processed into field-effect transistors, and their output and transfer properties have been determined. A large memory effect of the source-drain current on the previously applied “programm” gate voltage (−7 or +20 V) has been observed. It is reproducible in repeated switching cycles, showing the suitability of the structure as a nonvolatile memory device.

Defect properties of ZnO and ZnO:P microwires

We report on the defect properties of nominally undoped and phosphorus-doped ZnO microwires grown by carbothermal vapor phase transport. Cathodoluminescence measurements show very narrow (≈300 μeV), donorlike transitions in the UV spectral range. A recombination-line at 3.356 eV, previously assigned to phosphorus acceptors, is observed in our undoped ZnO. Thus the correlation of this recombination process and possible acceptor doping can be excluded. Hall effect measurements confirmed these findings and revealed n-type conductivity in both ZnO and high quality ZnO:P microwires.

Stable p-type ZnO:P nanowire/n-type ZnO:Ga film junctions, reproducibly grown by two-step pulsed laser deposition

ZnO p-n junctions were grown by two-step pulsed laser deposition (PLD) on a-plane sapphire substrates using a Ga-doped ZnO thin film as n-type conducting material. On top of these n-type films, phosphorous-doped ZnO (ZnO:P) nanowires were prepared by high-pressure PLD. Rectifying I-V curves with threshold voltage of about 3.2 V and a forward/reverse current ratio of 100 at ±3.5 V were measured reproducibly on these junctions. There are three independent indications for reproducible and about 1 year stable p-type conductivity of the ZnO:P wires: (1) Low-temperature cathodoluminescence of single ZnO:P nanowires exhibits phosphorus acceptor-related peaks: (A0,X), (e,A0), and donor-acceptor pair [B. Q. Cao et al., Nanotechnology 18, 455707 (2007)], (2) bottom-gate field effect transistors using undoped (n-type) ZnO and ZnO:P wires showed opposite transfer characteristics [B. Q. Cao et al., Phys. Status Solidi (RRL) 2, 37 (2008)], and (3) the rectifying I-V characteristics of the ZnO:P nanowire/ZnO:Ga-film junctions as shown here.ZnO p-n junctions were grown by two-step pulsed laser deposition (PLD) on a-plane sapphire substrates using a Ga-doped ZnO thin film as n-type conducting material. On top of these n-type films, phosphorous-doped ZnO (ZnO:P) nanowires were prepared by high-pressure PLD. Rectifying I-V curves with threshold voltage of about 3.2 V and a forward/reverse current ratio of 100 at ±3.5 V were measured reproducibly on these junctions. There are three independent indications for reproducible and about 1 year stable p-type conductivity of the ZnO:P wires: (1) Low-temperature cathodoluminescence of single ZnO:P nanowires exhibits phosphorus acceptor-related peaks: (A0,X), (e,A0), and donor-acceptor pair [B. Q. Cao et al., Nanotechnology 18, 455707 (2007)], (2) bottom-gate field effect transistors using undoped (n-type) ZnO and ZnO:P wires showed opposite transfer characteristics [B. Q. Cao et al., Phys. Status Solidi (RRL) 2, 37 (2008)], and (3) the rectifying I-V characteristics of the ZnO:P nanowire/ZnO:Ga-film jun...

Electrical properties of ZnO–BaTiO3–ZnO heterostructures with asymmetric interface charge distribution

We report on capacitance-voltage, current-voltage, Sawyer–Tower, and transient current switching measurements for a ZnO–BaTiO3–ZnO heterostructure deposited on (001) silicon by using pulsed laser deposition. The triple-layer structure reveals asymmetric capacitance- and current-voltage hysteresis and cycling-voltage dependent Sawyer–Tower polarization drift. We explain our findings by coupling of the ferroelectric (BaTiO3) and piezoelectric (ZnO) interface charges and parallel polarization orientation of the ZnO layers causing asymmetric space charge region formation under positive and negative bias. The transient current characteristics suggest use of this structure as nonvolatile memory device.

Oxide Thin Film Heterostructures on Large Area, with Flexible Doping, Low Dislocation Density, and Abrupt Interfaces: Grown by Pulsed Laser Deposition

Advanced Pulsed Laser Deposition (PLD) processes allow the growth of oxide thin film heterostructures on large area substrates up to 4-inch diameter, with flexible and controlled doping, low dislocation density, and abrupt interfaces. These PLD processes are discussed and their capabilities demonstrated using selected results of structural, electrical, and optical characterization of superconducting (YBa 2Cu 3O 7−δ), semiconducting (ZnO-based), and ferroelectric (BaTiO 3-based) and dielectric (wide-gap oxide) thin films and multilayers. Regarding the homogeneity on large area of structure and electrical properties, flexibility of doping, and state-of-the-art electronic and optical performance, the comparably simple PLD processes are now advantageous or at least fully competitive to Metal Organic Chemical Vapor Deposition or Molecular Beam Epitaxy. In particular, the high flexibility connected with high film quality makes PLD a more and more widespread growth technique in oxide research.