Evgeni M. Kaidashev

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.

Raman scattering in ZnO thin films doped with Fe, Sb, al, Ga, and Li

Polarized micro-Raman measurements were performed to study the phonon modes of Fe, Sb, Al, Ga, and Li doped ZnO thin films, grown by pulsed-laser deposition on c-plane sapphire substrates. Additional modes at about 277, 511, 583, and 644 cm−1, recently assigned to N incorporation [A. Kaschner et al., Appl. Phys. Lett. 80, 1909 (2002)], were observed for Fe, Sb, and Al doped films, intentionally grown without N. The mode at 277 cm−1 occurs also for Ga doped films. These modes thus cannot be related directly to N incorporation. Instead, we suggest host lattice defects as their origin. Further additional modes at 531, 631, and 720 cm−1 seem specific for the Sb, Ga, and Fe dopants, respectively. Li doped ZnO did not reveal additional modes.

Infrared dielectric functions and phonon modes of high-quality ZnO films

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

Dielectric functions (1 to 5 eV) of wurtzite MgxZn1−xO (x⩽0.29) thin films

The optical dielectric functions for polarization perpendicular and parallel to the c-axis (optical axis) of pulsed-laser-deposition grown wurtzite MgxZn1−xO (0⩽x⩽0.29) thin films have been determined at room temperature using ellipsometry for photon energies from 1 to 5 eV. The dielectric functions reveal strong excitonic contributions for all Mg concentrations x. The band gap energies (E0A=3.369 eV for ZnO to 4.101 eV for x=0.29) show a remarkable blueshift. The exciton binding energy (61 meV for ZnO) decreases to approximately 50 meV for x≈0.17 and increases to approximately 58 meV for x=0.29. In contrast to ZnO, the MgxZn1−xO alloys are found uniaxial negative below the band gap energy, opposite to previously reported results.

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.

Infrared dielectric functions and phonon modes of wurtzite MgxZn1-xO (x≤0.2)

Infrared dielectric function spectra and phonon modes with polarization parallel and perpendicular to the c axis of high quality, highly relaxed, and single crystalline wurtzite MgxZn1−xO films with 0⩽x⩽0.2 prepared by pulsed-laser deposition on c-plane sapphire substrates were obtained from infrared spectroscopic ellipsometry (380–1200 cm−1) and Raman scattering studies. A two-mode behavior is found for the modes with E1 symmetry, a lattice mode and an impurity-type mode are obtained for the A1 symmetry phonons. Model dielectric function spectra will become useful for future infrared ellipsometry analysis of complex MgxZn1−xO-based heterostructures.

Electron paramagnetic resonance in transition metal-doped ZnO nanowires

The wide-band-gap zinc oxide-based diluted magnetic semiconductors currently attract considerable attention due to their possible use in spintronic devices. In this work, we studied ZnO nanowire samples synthesized on 10×10 mm2 a-plane sapphire substrates by high-pressure pulsed laser deposition. The samples were characterized by scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) in the X-band (≃9.3 GHz) from T=4 to 300 K. According to the SEM pictures, the nanowires exhibit a length of about 1 μm and are aligned perpendicular to the substrate surface. The structures have a hexagonal cross section and their diameter ranges from 60 nm up to 150 nm. For the lowest nominal concentrations of xMn=3 at. % and xCo=5 at. %, we detect the anisotropic EPR spectra of isolated Mn2+ (3d5, S6) and Co2+ (3d7, F4), respectively, on Zn sites. The detection of the well-resolved anisotropic spectra proves a coherent crystallographic orientation of the nanowires. The linewidth was larger than the be...

Incorporation and electrical activity of group V acceptors in ZnO thin films

We have investigated the incorporation of the group V elements N, P, and Sb into ZnO with the intention to produce p‐type ZnO. We used pulsed laser deposition for sample growth and sapphire as substrates. The free carrier concentration of as‐deposited ZnO:P decreases with increasing oxygen partial pressure. After rapid thermal annealing the carrier concentration of ZnO:P is about 1017 cm−3 independent of the carrier concentration before the annealing. This is explained by the diffusion of Al from the sapphire substrates into the thin films and can be strongly reduced by the insertion of a MgO buffer layer. As‐deposited codoped ZnO:(Ga,N) samples have lower carrier concentrations as nominally undoped samples if the Ga target concentration is in the ppm range. In photoluminescence measurements we found a donor‐acceptor pair related transition for a sample grown using a ZnO target containing 5% Li3N and for an annealed sample containing about 5% Sb.

Optical and structural properties of ZnO nanorods grown by pulsed laser deposition without a catalyst

Pulsed laser deposition without a catalyst is used to grow ZnO nanorods less than 10 nm in diameter. The structure of the rods is studied by Raman scattering during excitation in the visible and UV regions. The temperature dependences of exciton spectra and the behavior of green luminescence are investigated in the temperature range 10–280 K. At room temperature, the luminescence intensity of the ZnO nanorods in the exciton region is higher than the green luminescence intensity by a factor of 7.8.

Growth and Characterization of ZnO Nano- and Microstructures

Publisher Summary This chapter deals with growth and characterization of zinc oxide (ZnO) nano- and microstructures. ZnO exhibits favorable properties such as a high exciton binding energy of 60 meV, unproblematic wet-chemical etching, radiation hardness, and the availability of substrates for homoepitaxy. Other advantages of ZnO include its biocompatibility, a high piezoelectric constant, and large optical gain. This fuels high device-related expectations not only for optoelectronics but also for biomedical and biosensor applications. ZnO is already in everyday use as UV blockers in sun tan lotions, in surface acoustic wave devices (SAW), as gas sensors, in high-field varistors, and as a catalyst, to name only a few of its well-established applications. The chapter illustrates that the ZnO nanowires are grown by high-pressure PLD on sapphire substrates covered with gold colloidal particles as nucleation sites. It is also found that the aspect ratio varies linearly with the deposition time.