H. von Wenckstern

High electron mobility of epitaxial ZnO thin films on c-plane sapphire grown by multistep pulsed-laser deposition

A multistep pulsed-laser deposition (PLD) process is presented for epitaxial, nominally undoped ZnO thin films of total thickness of 1 to 2 μm on c-plane sapphire substrates. We obtain reproducibly high electron mobilities from 115 up to 155 cm2/V s at 300 K in a narrow carrier concentration range from 2 to 5×1016 cm−3. The key issue of the multistep PLD process is the insertion of 30-nm-thin ZnO relaxation layers deposited at reduced substrate temperature. The high-mobility samples show atomically flat surface structure with grain size of about 0.5–1 μm, whereas the surfaces of low-mobility films consist of clearly resolved hexagonally faceted columnar grains of only 200-nm size, as shown by atomic force microscopy. Structurally optimized PLD ZnO thin films show narrow high-resolution x-ray diffraction peak widths of the ZnO(0002) ω- and 2Θ-scans as low as 151 and 43 arcsec, respectively, and narrow photoluminescence linewidths of donor-bound excitons of 1.7 meV at 2 K.

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.

Phosphorus acceptor doped ZnO nanowires prepared by pulsed-laser deposition

Phosphorus-doped ZnO (ZnO:P) nanowires were successfully prepared by a novel high-pressure pulsed-laser deposition process using phosphorus pentoxide as the dopant source. Detailed cathodoluminescence studies of single ZnO:P nanowires revealed characteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission (A0, X, 3.356 eV), free-to-neutral-acceptor emission (e, A0, 3.314 eV), and donor-to-acceptor pair emission (DAP, ~3.24 and ~3.04 eV). This means that stable acceptor levels with a binding energy of about 122 meV have been induced in the nanowires by phosphorus doping. Moreover, the induced acceptors are distributed homogeneously along the doped nanowires.

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.

Defects in hydrothermally grown bulk ZnO

Hydrothermally grown bulk ZnO (Tokyo Denpa) was investigated using junction-capacitance spectroscopy on silver oxide Schottky contacts (barrier height of 1.20eV, ideality factor of 1.04). Two main shallow defects, T1 and T2, with thermal activation energies of 13 and 52meV, respectively, were identified. Two closely lying, deep defect levels E3∕E3′ at approximately 320meV below the conduction band were found in higher concentrations (mid-1014cm−3) than the shallow donors. 4K photoluminescence showed dominant emission from excitons bound to three neutral donors, aluminum, hydrogen, and an unassigned impurity, with donor binding energies close to the thermal activation energy of T2.

Strain distribution in bent ZnO microwires

ZnO microwires grown by carbothermal reduction were mechanically bent and the uniaxial stress state was studied with spatially resolved low-temperature photoluminescence. Inhomogeneous (tensile and compressive) stress (up to ±1 GPa) is visualized by the redshift and blueshift of the wire luminescence. Experimentally determined tensile and compressive strain along the c-axis (wire axis) of up to 1.5 %, symmetrically distributed with respect to the central axis (neutral fiber), is achieved, resulting in maximum energetic shifts of ±30 meV. Within these experiments, we are able to precisely determine the direct relation between energetic shift of the free A-exciton energy and strain to (−2.04±0.02) eV.

Temperature-Dependent Properties of Nearly Ideal ZnO Schottky Diodes

The current-voltage characteristics of low-ideality factor (1.09 at 300 K) ZnO Schottky diodes were investigated over the temperature range of 40-423 K. Planar geometry devices were fabricated on the Zn-polar face of a low carrier concentration (n = 1 times 1014 cm-3) hydrothermally grown bulk ZnO single crystal wafer. The current transport was dominated by thermionic emission between 293 and 423 K, provided the ZnO surface was exposed to air. The nearly ideal characteristics of the diodes yielded an experimental Richardson constant of 10 plusmn 6 Amiddotcm-2middotK-2, close to the theoretical value of 32 Amiddotcm-2middotK-2, and two orders of magnitude larger than previously reported values.

High-gain integrated inverters based on ZnO metal-semiconductor field-effect transistor technology

We report on the design and fabrication of ZnO-based integrated inverters consisting of normally-on metal-semiconductor field-effect transistors and AgxO Schottky diodes as level shifters. The inverters show high gain values up to 197 at 3 V operating voltage and low uncertainty levels in the range of 0.13 V. The influence of the level shifter and the channel material/thickness on the performance of the inverters has been investigated. Using Zn0.997Mg0.003O for the channel thin film leads to high reproducibility (90%) of the devices. A logic NOR-gate has been implemented showing the possibility to fabricate a complete logic.

Dielectric function in the spectral range (0.5–8.5)eV of an (Alx Ga1−x)2O3 thin film with continuous composition spread

We determined the dielectric function of the alloy system (Alx Ga1−x)2O3 by spectroscopic ellipsometry in the wide spectral range from 0.5 eV to 8.5 eV and for Al contents ranging from x = 0.11 to x = 0.55. For the composition range x < 0.4, we observe single phase material in the β-modification and for larger Al content also the occurrence of γ-(Al,Ga)2O3. We derived spectra of the refractive index and the absorption coefficient as well as energy parameters of electronic band-band transitions by model analysis of the dielectric function. The dependence of the dielectric functions lineshape and the energy parameters on x is highly continuous, reflecting theoretical expectations. The data presented here provide a basis for a deeper understanding of the electronic properties of this material system and may be useful for device engineering.