A. Rahm

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.

MgxZn1−xO(0⩽x<0.2) nanowire arrays on sapphire grown by high-pressure pulsed-laser deposition

MgxZn1−xO nanowires with Mg-content x from 0 to 0.2 have been grown by high-pressure pulsed-laser deposition (PLD) on gold-covered sapphire single crystals. The PLD process allows for a unique wide-range control of morphology, diameter, and composition of the MgxZn1−xO nanowires. The diameter of single ZnO wires could be varied between about 50 and 3000 nm, and the Mg content x of MgxZn1−xO wire arrays was controlled via the PLD gas pressure. The microscopic homogeneity of Mg content is displayed by cathodoluminescence (CL) imaging of the excitonic peak energy. The fluctuation of CL peak energy between individual wires is about an order of magnitude smaller than the alloy broadening.

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.

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.

Infrared optical properties of MgxZn1−xO thin films (0⩽x⩽1): Long-wavelength optical phonons and dielectric constants

Infrared spectroscopic ellipsometry in the spectral range from ω=360cm−1toω=1500cm−1 and Raman scattering spectroscopy are applied to study the long-wavelength optical phonon modes and dielectric constants of MgxZn1−xO thin films in the composition range 0⩽x⩽1. The samples were grown by pulsed laser deposition on sapphire substrates. X-ray diffraction measurements of the thin film samples reveal the hexagonal wurtzite crystal structure for x⩽0.53 and the cubic rocksalt crystal structure for x⩾0.67. A systematic variation of the phonon mode frequencies with Mg-mole fraction x is found for both hexagonal and cubic MgxZn1−xO thin films. The modified random isodisplacement model matches the observed composition dependence of the phonon mode frequencies for the hexagonal structure thin films [J. Chen and W. Z. Shen, Appl. Phys. Lett. 83, 2154 (2003)], whereas a simple linear approximation scheme is sufficient for the cubic structure part. We observe a discontinuous behavior of the transverse optical phonon mod...

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...

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.

Infrared dielectric function and phonon modes of Mg-rich cubic MgxZn1-xO (x≥0.67) thin films on sapphire (0001)

Infrared dielectric function spectra and phonon modes of single-phase rocksalt-type MgxZn1−xO thin films with 0.67⩽x⩽1 prepared by pulsed-laser deposition on c-plane sapphire substrates were obtained from infrared spectroscopic ellipsometry (360cm−1 to 1500cm−1). A one-mode behavior is found. Phonon mode frequencies, the high-frequency limit of the dielectric function, and phonon mode broadening parameters reflect a considerable and systematic dependence on the Mg content x. X-ray diffraction measurements revealed the single-phase growth and a decreasing lattice constant with increasing x.

Damp Heat Treatment of Cu(In,Ga)Se2 Solar Cells with Different Sodium Content

Long term stability is crucial to maturing any photovoltaic technology. We have studied the influence of sodium, which plays a key role in optimizing the performance of Cu(In,Ga)Se2 (CIGSe) solar cells, on the long-term stability of flexible CIGSe solar cells on polyimide foil. The standardized procedure of damp heat exposure (85% relative humidity at 85 ∘C) was used to simulate aging of the unencapsulated cells in multiple time steps while they were characterized by current-voltage analysis, capacitance-voltage profiling, as well as electroluminescence imaging. By comparing the aging process to cells that were exposed to heat only, it could be confirmed that moisture plays the key role in the degradation process. We found that cells with higher sodium content suffer from a more pronounced degradation. Furthermore, the experimental results indicate the superposition of an enhancing and a deteriorating mechanism during the aging process. We propose an explanation based on the corrosion of the planar contacts of the solar cell.

Latest results of the German joint project “flexible CIGSe thin film solar cells for space applications”

This German joint project is directed towards the development of a flexible Cu(In,Ga)Se2 (CIGSe) thin film solar cell technology on a polyimide (PI) substrate for space applications. A group of partners with an academic and/or industrial background in the field of chalcopyrite based thin film solar cells work together to ingrain space technology in production facilities for terrestrial PV. The three production technologies batch type multi-stage co-evaporation, in-line co-evaporation and roll-to-roll co-evaporation are investigated. So far, a maximum total area solar cell efficiency of 15.5 % has been achieved for lab scale devices (0.5 cm2, AM 1.5, no AR). On a large area, standardized device an active area efficiency of 12.7 % has been achieved (25.9 cm2, AM 1.5, no AR) based on an industrial roll-to-roll production process.