G. Biehne

Mean barrier height of Pd Schottky contacts on ZnO thin films

We have investigated the temperature dependence of the barrier height of high-quality Pd Schottky contacts on (0001)-oriented ZnO thin films by temperature-dependent current-voltage and capacitance-voltage (CV) measurements. The films have been grown by pulsed-laser deposition. The effective Schottky barrier height ΦB,eff deduced from the current-voltage measurements was evaluated by considering a Gaussian barrier height distribution with a standard deviation σ around a mean barrier height ΦB,m. We determined ΦB,m=(1.16±0.04)eV which agrees well with the value of 1.14eV determined by CV measurements. The standard deviation is determined to be (134±10)meV.

Lateral homogeneity of Schottky contacts on n-type ZnO

The electrical properties of Schottky contacts (SCs) produced ex situ on n-type ZnO single crystals and epitaxial thin films were investigated. Electron beam induced current imaging was used to study lateral variations of the current induced in the space charge region of the SC. Further, the effective barrier height was determined by current–voltage and capacitance–voltage measurements. Pd contacts prepared on ZnO thin films that had undergone treatment in a plasma-enhanced chemical vapor deposition with nitrous oxide (N2O) as ambient gas are laterally homogeneous with an effective barrier height of (600±30) meV.

Properties of reactively sputtered Ag, Au, Pd, and Pt Schottky contacts on n-type ZnO

Highly rectifying Ag, Au, Pd, and Pt Schottky contacts have been fabricated on heteroepitaxial pulsed-laser deposited ZnO-thin films by reactive sputtering. X-ray photoelectron spectroscopy revealed an oxidation of the Ag, Pd, and Pt contact material; the gold contacts are purely metallic. The necessity of a conductive capping of the oxidized contacts is proven by photocurrent measurements of AgxO contacts. The ideality factors and the effective barrier heights were determined by current-voltage measurements. Capacitance-voltage and temperature dependent current-voltage measurements were furthermore carried out to determine the mean barrier height, the standard deviation and the respective voltage dependencies taking lateral fluctuations of the barrier height into account.

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.

Spin Manipulation in Co-Doped ZnO

We report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom ferromagnetic electrode and Co as a top ferromagnetic electrode prepared by pulsed laser deposition. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al2O3 and tunnelled through the amorphous Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices.

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.

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...

ZnO-based metal-semiconductor field-effect transistors on glass substrates

We investigate the influence of quartz glass and borosilicate glass substrates on the electrical properties of ZnO-based metal-semiconductor field-effect transistors (MESFETs). The n-type ZnO thin-film channels were grown by pulsed-laser deposition and MESFETs were processed by reactive dc sputtering of AgxO-Schottky gate contacts. All devices are in the normally-off state. They exhibit very low off-currents in the range of 10−13A and on/off ratios of maximum 6 decades. The channel mobilities are highest for ZnO on quartz with 1.3cm2/Vs. The glass substrates introduce a compensating effect on the conduction of the ZnO channel resulting in higher on/off-voltages and lower on-current.

Electronic properties of shallow level defects in ZnO grown by pulsed laser deposition

We have used deep level transient spectroscopy (DLTS) to characterise four defects with shallow levels in ZnO grown by pulsed laser deposition (PLD). These defects all have DLTS peaks below 100 K From DLTS measurements and Arrhenius plots we have calculated the energy levels of these defects as 31 meV, 64 meV, 100 meV and 140 meV, respectively, below the conduction band. The 100 meV defect displayed metastable behaviour: Annealing under reverse bias at temperatures of above 130 K introduced it while annealing under zero bias above 110 K removed it. The 64 meV and 140 meV defects exhibited a strong electric field assisted emission, indicating that they may be donors.