Advances in the deposition of metallic thin films are discussed. The ALD growth of ultrathin Ir films is analyzed by
transmission electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, and optical and
electrical measurements. The morphology of iridium metallic layers is assessed based on Ir/ Al2O3 nanolaminate films.
High resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements show sharp
interfaces and pure Ir layers in the nanolaminates. The iridium films as polycrystalline. Excellent thickness control, high
uniformity and low roughness of ALD films are demonstrated. Four point probe measurements of the resistivity of Ir
coatings with various thicknesses have been performed and proved conductive layers with an Ir film thickness of ca. 10
nm. The optical properties of the Ir films deposited by ALD are similar to those of the bulk Ir. Thin iridum layers
deposited on high aspect ratio linear gratings have been successfully used as electrodes in the electrochemical deposition
of gold nanoparticles and gold layers. The gold deposition evolves through the formation of gold islands with ca. 40 nm
diameters that coalesce after ca. 60 seconds deposition. The density of the gold islands within the grating pattern is much
lower than on the flat region of the substrate. The combination of ALD with electrochemical deposition allows the
diversification of conductive layers on complex nanostructured surfaces.
During the deposition of Pentacene on a Si-SiO2 gate structure with Au bottom contacts for source and drain, the film growth was monitored with simultaneous in situ macro Raman spectroscopy and drain current measurements of the OFET device. The deposition of the active layer was carried out under UHV conditions at a growth rate of 0.65 Å/min. The purpose of the in situ characterization was to determine the minimum nominal thickness of the Pentacene layer required for efficient charge transport through the OFET circuit. At a thickness around 1.5 nm nominal coverage, the first percolation paths through the first organic monolayer develop, resulting in a sharp rise of the drain current. Up to a nominal film thickness of 30 nm, a subsequent slower increase of the drain current can be observed, revealing that the percolation of the first monolayer continues on a slower pace up to rather thick organic layers. These in situ measurements were complemented by ex situ isothermal deep level transient spectroscopy (charge QTS).
Potentiometry with a Kelvin probe atomic force microscope is used to investigate the contact resistances of pentacene OFETs, so that the injection of the charges at the source contact and their extraction at the drain contact can be distinguished from the influence of trap
states on the charge transport through the accumulation channel. The
samples consist of Au bottom contacts on a SiO2 gate dielectric with a channel length of L=10- 15 μm and a channel
width of W=100 μm. The gate oxide is first treated by an
oxygen plasma before depositing about 30 nm of pentacene under high
vacuum conditions. The output characteristics are measured as a
function of temperature in an evacuated cryostat, revealing
temperature-activated hole transport. The potentiometry measurements
are performed ex situ under atmospheric conditions after storing
the samples in air for several weeks. At room temperature, the
pentacene OFETs are dominated by the resistance at the injection
contact, so that the mobility in the channel region as deduced from
potentiometry is about one order of magnitude higher than the value
obtained from the output characteristics. The measurements are
interpreted with microscopic model calculations for the
temperature-activated currents.
The understanding of metal/organic interfaces is fundamental for achieving high quality contacts in organic based field effect transistors,OFETs. Here the formation of In and Ag interfaces on 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA)and N,N dimethy l3,4,9,10 perylene tetracarboximide (DiMe-PTCDI)is investigated by in situ Raman spectroscopy. Upon metal deposition a significant enhancement of the Raman signals arising
from internal vibrational modes is observed for these two perylene derivatives,clearly indicating the presence of surface enhanced Raman scattering (SERS). Molecules in direct contact with the metal are involved in a weak dynamical charge transfer in the electronic ground state, resulting in a breakdown of the Raman selection rules for the free molecule. Both metals show very weak interactions with the organic thin films. This finding is opposite to previous suggestions of covalent bond formation between In and O atoms in PTCDA. The Raman
scattering of molecules close to the metal interface is enhanced by coupling to the plasmon resonance of small metal clusters,indicating a high roughness of the metallic film. The enhancement factors can be used to extract information on diffusion of the metal atoms into the organic film and on the morphology of the contact layer.
Raman spectroscopy is capable of distinguishing neutral and charged states of organic molecules due to its sensitivity for charge induced changes in the molecular geometry and the bond strength.Combined with in situ electrical measurements it provides a powerful tool for characterizing charged molecules in the channel of organic field effect transistors OFETs. The active 3nm C60 layer in an OFET structure with bottom gate con figuration was characterized by in situ Raman spectroscopy using the 514.5 nm (2.41 eV)Ar+ laser line as a function of the drain source (Vds) voltage. The Raman spectra show pronounced changes upon application of a drain source Vds voltage. The experimental findings are compared to the vibrational spectra calculated for molecules under external bias fields. Complementary to the Raman characterization,the Id-Vds and Id-Vgs characteristics were recorded and the mobility value of 0.1 cm2/V s was derived from these measurements.
Raman spectroscopy which provides valuable information on the structural parameters of QDs was used for monitoring of the lateral oxidation of InAs/AlAs QD structures and study of the phonon properties of InAs QDs in aluminium oxide matrix. Optical phonons of InAs QDs were found to be affected by both strain and confinement. Raman spectra measured from non-oxidized area reveal asymmetric lineshape of LO phonons in InAs QDs and demonstrate its low-frequency shift with increasing excitation energy that is explained by QD size distribution and phonon confinement in smaller-size dots. Raman spectra taken from oxidized area show an increase of the LO peak intensity and the shift of the phonon line position towards higher frequency. The first effect is explained by formation of wide bandgap aluminium oxide matrix that leads to the shift of confined electronic states in InAs QDs closer to the resonance with the laser excitation energy. The latter is caused by increasing mechanical strain in InAs QDs due to a shrinkage of the aluminium oxide layers. At the boundaries of oxidized/non-oxidized areas the presence of amorphous and crystalline As clusters is evident.
Raman spectroscopy which provides valuable information on the structural parameters of QDs was used for monitoring of the lateral oxidation of InAs/AlAs QD structures and study of the phonon properties of InAs QDs in aluminium oxide matrix. Optical phonons of InAs QD's were found to be affected by both strain and confinement. Raman spectra measured from non-oxidized area reveal asymmetric lineshape of LO phonons in InAs QDs and demonstrate its low-frequency shift with increasing excitation energy that is explained by QD size distribution and phonon confinement in smaller-size dots. Raman spectra taken from oxidized area show an increase of the LO peak intensity and the shift of the phonon line position towards higher frequency. The first effect is explained by formation of wide bandgap aluminium oxide matrix that leads to the shift of confined electronic states in InAs QDs closer to the resonance with the laser excitation energy. The latter is caused by increasing mechanical strain in InAs QDs due to a shrinkage of the aluminium oxide layers. At the boundaries of oxidized/non-oxidized areas the presence of amorphous and crystalline As clusters is evident.
Two different versions of hotwall--epitaxy reactors were used to grow ZnSe and ZnTe layers. In the first type of hotwall epitaxy reactor Zn and Se were evaporated seperately from elemental sources. By changing the Zn/Se ratio in the gas phase we could alter the growth direction of the ZnSe layers from 111 to 100 Raman spectroscopy proved the existence of a Ga2Se3 layer at the GaAs/ZnSe interface. In the other HWE reactor stoichiometric ZnSe and ZnTe were used as source materials. Photoluminescence and Xray rocking curves proved the high quality of the epilayers. The rocking curves showed a full width at half maximum of 75 arcseconds for ZnTe.
Thin low doped GaAs layers were deposited on Si(100) substrates using molecular beam epitaxy (MBE). The samples were investigated by nondestructive Far Infrared (FIR) Fourier Transform Spectroscopy. A new evaluation method was employed in order to obtain the phonon parameters the layer thicknesses as well as the densities and mobilities of the carriers. This method was successfully applied for layer thicknesses as low as O. lprn and carrier concentrations of about 2 . 1O''6cm3.
Boron doped diamond films were deposited onto (100) orientated Si
substrates using a thermal filament CVD method. Boron trioxide was
used as a doping source and samples with boron to carbon (B/C)
ratios of 0, 10, 100, and 1000 ppm were prepared for the optical
measurements. The infrared (IR) results reveal the formation of an
ultrathin SiC layer at the interface between Si and diamond.
Furthermore, the IR data confirm the resistivity data obtained
from electrical measurements. Raman spectroscopy was used to probe
the quality and homogeneity of the diamond films. Upon increasing
B/C ratio the diamond phonon line shifts to lower frequency and is
also broadened revealing a softening of the diamond. In addition,
the optical absorption was found to increase strongly with
increasing B/c ratio.
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