G. Benndorf

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.

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.

Structural characterization of a-plane Zn1−xCdxO (0⩽x⩽0.085) thin films grown by metal-organic vapor phase epitaxy

Zn1−xCdxO(112¯0) films have been grown on (011¯2) sapphire (r–plane) substrates by metal-organic vapor phase epitaxy. A 800-nm-thick ZnO buffer, deposited prior to the alloy growth, helps to prevent the formation of pure CdO. A maximum uniform Cd incorporation of 8.5 at. % has been determined by Rutherford backscattering spectrometry. Higher Cd contents lead to the coexistence of Zn1−xCdxO alloys of different compositions within the same film. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 2.93 eV for the highest Cd concentration (8.5 at. %). The lattice deformation, due to Cd incorporation, has been described using a new reference frame in which the lattice distortions are directly related to the a-plane surface structure. Cd introduction does not affect the c lattice parameter but expands the lattice along the two perpendicular directions, [112¯0] and [1¯100], resulting in a quadratic volume increase.

High-quality m-plane GaN thin films deposited on γ-LiAlO2 by ion-beam-assisted molecular-beam epitaxy

GaN(11¯00) thin films are deposited on γ-LiAlO2(100) by low-energy-ion-beam-assisted molecular-beam epitaxy. Structural properties of the epitaxial GaN films are investigated by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. X-ray diffraction measurements give evidence for a high crystalline quality far better than previously reported in literature. Cross-section transmission electron microscopy and atomic force microscopy show an anisotropy in defect structure and surface topography parallel and perpendicular to the GaN c axis. Optical properties are examined by photoluminescence spectroscopy at various temperatures. The spectra exhibit a strong and sharp near-band-gap transition, as well as a donor-acceptor pair transition.

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.

Intense white photoluminescence emission of V-implanted zinc oxide thin films

Pulsed laser deposited ZnO films were implanted with vanadium ions using ion energies between 30 and 250 keV with different fluences yielding vanadium concentrations in the range between 0.8 and 5 at. %. After annealing under oxygen ambient at 800 °C, a broad luminescence band observed by photoluminescence covers nearly the total visible spectral region. This luminescence is a superposition of different bands triggered by the incorporated V and remaining implantation defects. The visual impression of the bright whitish emission of the implanted ZnO has been quantified using the color space map of the Commission internationale de l’Eclairage. Furthermore, the intensity of the white emission strongly increases with increasing V concentration, whereas Ar-implanted reference sample shows only weak white emission.

Origin of the near-band-edge luminescence in MgxZn1−xO alloys

The carrier dynamics of donor-bound and free excitons, localized in the alloy disorder potential, were investigated for MgxZn1−xO (0.08≤x≤0.33) thin films. The measured transients show a fast decrease in the luminescence intensity within the first nanoseconds, followed by a slow, strongly nonexponential decay. Shortly after the excitation pulse, the time-delayed spectra are dominated by the (D0,X) recombination. With increasing time, the free exciton recombination becomes visible on the high-energy side, dominating the spectra at large times after the excitation pulse. By fitting the transients with nonexponential model decay functions, we can deconvolve the luminescence spectra. As expected, the mean decay time of the excitons localized in the alloy disorder potential significantly increases with increasing Mg content.

Luminescence properties of ZnO/Zn1-xCdxO/ZnO double heterostructures

We report on luminescence properties from T=2 K up to room temperature of ZnO/Zn1−xCdxO/ZnO double heterostructures grown by pulsed-laser deposition on a-plane sapphire substrates. Depending on the growth conditions, the spectral position of the Zn1−xCdxO related maximum has been tuned from 3.19 to 3.056 eV, corresponding approximately to Cd contents between 2.1% and 5.6%. Independent of x we observe intense phonon replicas of the photoluminescence (PL) maximum. The quenching of the luminescence intensity indicates the presence of two thermal activation energies, one of them being assigned to the delocalization of excitons from donors. The temperature-dependent PL spectra exhibit the so-called “S-shape” behavior as function of temperature for the Zn1−xCdxO due to the superposition of the usual S-shape, caused by the alloy, and a change in the peak character from donor-bound exciton to free exciton.

Electronic and optical properties of ZnO/(Mg,Zn)O quantum wells with and without a distinct quantum-confined Stark effect

The luminescence properties of polar ZnO/(Mg,Zn)O quantum wells (QWs) are determined, besides confinement effects, by a redshift caused by the Stokes shift and the quantum-confined Stark effect (QCSE). We present a comprehensive study of photoluminescence and optical transmission measurements to separate these two effects. Single QW structures have been prepared by pulsed laser deposition on a-plane sapphire exhibiting excitons with and without a distinct QCSE. The QCSE leads to a redshift of the QW luminescence maximum beneath the free exciton energy in ZnO as well as a change of the dynamics from a single exponential decay function to a non-exponential one, well described by a stretched exponential decay function. The internal electric field was evaluated to 0.66 MV/cm. In the absence of an internal electric field, the Stokes shift was determined to drop from 64 meV down to 24 meV with increasing well width. Additionally, with increasing temperature, the QCSE vanishes as the internal electric field is s...