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- Front Matter: Volume 6474
- Optical Properties
- ZnO Doping
- Thin Film Epitaxy and Interfaces
- Thin Film Heterostructures
- ZnO Processing and Contact Preparation
- ZnO Based Diodes and LEDs
- ZnO Materials and Properties
- ZnO Bulk and Photonic Crystals
- ZnO Based Thin Film Devices
- ZnO Based Nanostructures I
- ZnO Based Nanostructures II
- Spintronics and Ferroelectrics
Front Matter: Volume 6474
Front Matter: Volume 6474
Show abstract
This PDF file contains the front matter associated with SPIE Proceedings Volume 6474, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
Optical Properties
New developments in ZnO materials and devices
Show abstract
We will examine a few of the outstanding new ZnO materials and device developments published in 2006. In
the area of bulk crystal growth, high-quality 3-inch ZnO wafers grown by the hydrothermal method have
become available, and Bridgman growth has also been developed. In the thin-film area, excellent ZnO layers
have been grown by liquid-phase epitaxy. Other types of epitaxial material have also shown improvements,
and the quantum Hall effect has now been observed, along with photoluminescence (PL) linewidths as low as
110 &mgr;eV. In the area of impurity characterization, radioactive-tracer methods have been used to make positive
identifications of the PL donor-bound exciton lines I8 and I9, as due to Ga and In, respectively. Our
understanding of the common impurity H has also advanced, because it is now known from both theory and
experiment that interstitial H is not stable at room temperature. The same is true of the native interstitials, ZnI and OI. New results suggest that the common H-related shallow donor is probably multibonded H
substitutional on an O site, and the ZnI-related shallow donor is probably a complex, such as ZnI-NO. In the
important area of p-type ZnO, it has been demonstrated that Li and N co-doped material has a resistivity as
low as 1 &OHgr;-cm and is stable for at least one year. Also, many groups were able to make thin-film and
nanowire or nanorod p-n junction light emitting diodes (LEDs). Another very exciting development was the
creation of an edge-emitting laser diode, from rows of n-ZnO nanocrystals on a p-GaN thin film. Electronic
devices, including transparent transistors, also made great strides, producing record field-effect mobilities.
Analysis of localization dynamics of excitons in ZnO-related quantum wells by Monte-Carlo simulation
Takayuki Makino
Show abstract
A quantitative interpretation of the anomalous temperature behavior is presented to describe the so-called "S-shape dependence"
of the photoluminescence (PL) in ZnO-related quantum well layers by using an appropriate modeling and
simulation. Experimental data for CdZnO/MgZnO and ZnO/MgZnO samples are modeled using Monte Carlo simulations
of the involved relaxation mechanisms and thus providing a realistic picture of the excitonic kinetics. The temperature
dependence of the PL maximum and full-width at half maximum could be simultaneously reproduced with reasonably
good accuracy. Several informations about the distribution of the localization potential wells and identify their hopping
transfers between separated states are deduced. We found that the temperature-dependent PL linewidth and Stokes shift is
in a qualitatively reasonable agreement with the above-mentioned model, with accounting for an additional inhomogeneous
broadening for the case of linewidth for CdZnO quantum wells. The density of localized states used in the simulation for
the CdZnO QW was consistent with the absorption spectrum taken at 5 K.
ZnO Doping
Metalorganic vapor phase epitaxy of ZnO: toward p-type conductivity
Show abstract
The promising II-VI-semiconductor ZnO has achieved strong interest in research in the past years. Especially, epitaxial
growth by metal organic vapor phase epitaxy (MOVPE) is a matter of particular interest due to the large scalability of
MOVPE for commercial mass production and its proven high layer quality for other compound semiconductors. In the
past years tremendous advance has been made in the field of epitaxial growth. However, due to the lack of epiready ZnO
substrates, so far mostly heteroepitaxial growth with a multistep growth process was applied to obtain good surface
morphology and until now not all of the physical properties of such multilayers are fully understood. In this paper we
present recent results of the electrical behavior of such multiple undoped ZnO layers. Despite numerous efforts one big
challenge is the p-type doping of ZnO. Here we present our results to doping experiments with arsenic, nitrogen and as a
new approach simultaneous dual doping of nitrogen and arsenic. Homoepitaxial growth offers a great potential for ZnO
due to some advantages as the absence of thermal and lattice mismatch and potentially low dislocation density. We
present experiments on the thermal treatment of commercial ZnO bulk crystals, which is necessary for subsequent homo-
MOVPE.
Characterization of Ag doped p-type ZnO films
Show abstract
Ag-doped ZnO thin films have been fabricated by pulsed laser deposition. Thermal analysis and X-ray photoelectron spectroscopy (XPS) were systematically investigated to verify the doping mechanism of Ag doped ZnO thin film depending on deposition temperature. The fabricated p-type Ag doped ZnO films shows the hole concentration in the range from 4.9x1016 to 6.0x1017 cm-3.
Thin Film Epitaxy and Interfaces
Current-transport mechanisms of isotype n-ZnO/n-GaN heterostructures
Show abstract
Electrical properties of n-ZnO/n-GaN isotype heterostructures prepared by rf-sputtering of ZnO films on GaN layers
which in turn grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n-
ZnO/n-GaN diodes exhibited highly rectifying characteristics with forward and reverse currents being ~1.43x10-2 A/cm2
and ~2.4x10-4 A/cm2, respectively, at ±5 V. From the Arrhenius plot built representing the temperature dependent
current-voltage characteristics (I-V-T) an activation energy 0.125 eV was derived for the reverse bias leakage current
path, and 0.62 eV for the band offset from forward bias measurements. From electron-beam induced current
measurements and depending on excitation conditions the minority carrier diffusion length in ZnO was estimated in the
range 0.125-0.175 &mgr;m. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073
V.
Electrical characteristics of n-ZnO/n-6H-SiC heterostructures grown by rf-sputtering
Show abstract
The conduction band offset of n-ZnO/n-6H-SiC heterostructures prepared by rf-sputtered ZnO on commercial n-type
6H-SiC substrates has been measured. Temperature dependent current-voltage characteristics, photocapacitance, and
deep level transient spectroscopy measurements led to conduction band offsets of 1.25 eV, 1.1 eV, and 1.22 eV,
respectively.
Thin Film Heterostructures
Thin film growth of ZnO and its relation to substrate properties
S. Lautenschläger,
C. Neumann,
S. Graubner,
et al.
Show abstract
We report on the hetero- and homoepitaxial growth of ZnO thin films by the chemical vapor deposition technique. We
compare the results obtained on sapphire substrates, on GaN-templates on sapphire substrates and on silicon (111)
substrates. Even under optimized growth conditions with the insertion of buffer layers the films tend to grow 3-dimensionally. However, also ZnO substrates, expected to be the best choice, need to be prepared before being used in
the epitaxial growth. After mechanical polishing of the ZnO substrates we employed a high temperature annealing step
which produced atomically flat surfaces and removed all of the surface and subsurface damage. Thereafter, the two
dimensional epitaxial growth was achieved without an additional buffer layer. The substrate had a rocking curve full
width at half maximum of 27" which can be compared with that of the film of 22". The films had superior band edge
luminescence as compared to the substrate for which the green luminescence band was dominating.
P-type nitrogen- and phosphorus-doped ZnO thin films grown by pulsed laser deposition on sapphire substrates
J.-P. Mosnier,
S. Chakrabarti,
B. Doggett,
et al.
Show abstract
Nitrogen- and phosphorus-doped ZnO thin films were grown by pulsed laser deposition using an electron cyclotron
resonance (ECR) nitrogen plasma ion source or a ZnO:P2O5 doped target, as the dopant source, respectively. Both types
of films were grown on sapphire substrates first coated at low temperature with a ZnO buffer layer. For the N-doped
ZnO thin films, temperature-dependent Van der Pauw measurements showed consistent p-type behavior over the
measured temperature range of 200-450 K, with typical room temperature acceptor concentrations and mobilities of 5 x
1015 cm-3 and 5.61 cm2/Vs, respectively. The room-temperature photoluminescence spectrum of a N-doped ZnO thin
film featured a broad near band-edge emission at about 3.1 eV photon energy with a width of 0.5 eV. XPS studies
confirmed the incorporation of nitrogen in the samples. The ZnO:P layers (with phosphorus concentrations of between
0.01 and 1 wt %) typically showed weak n-type conduction in the dark, with a resistivity of 70 &OHgr;.cm, a Hall mobility of
&mgr;n ~ 0.5 cm2V-1s-1 and a carrier concentration of n ~ 3 x 1017 cm-3 at room temperature. After exposure to an incandescent
light source, the samples underwent a change from n- to p-type conduction, with an increase in mobility and a decrease
in concentration for temperatures below 300K. Electrical measurements showed noticeable differences for both types of
doped films when carried out in air or in vacuum. The results are discussed in terms of both the presence of surface
conducting channels and the influence of photoconductive effects.
Formation of two-dimensional electron gas and enhancement of electron mobility by Zn polar ZnMgO/ZnO heterostructures
Show abstract
A two-dimensional electron gas (2DEG) was observed in Zn polar ZnMgO/ZnO (ZnMgO on ZnO) heterostructures
grown by radical source molecular beam epitaxy. Reflection high energy electron diffraction patterns taken during the
growth of the ZnMgO layer remained streaky; x-ray diffraction measurements showed no evidence of phase separation
for up 44 % Mg composition. These results shows that the high quality ZnMgO layers up to 44 % Mg composition were
obtained without phase separation. The electron mobility of the ZnMgO/ZnO heterostructures dramatically increased
with increasing Mg composition and the electron mobility (&mgr;~250 cm2/Vs) at RT reached a value more than twice that
of an undoped ZnO layer (&mgr;~100 cm2/Vs) due to the 2DEG formation. The carrier concentration in turn reached values
as high as ~1x1013 cm-2 and remained nearly constant regardless of Mg composition. Strong confinement of electrons at
the ZnMgO/ZnO interface was confirmed by C-V measurements with a concentration of over 4x1019 cm-3.
Temperature-dependent Hall measurements of ZnMgO/ZnO heterostructures also exhibited properties associated with
well defined heterostructures. The Hall mobility increased monotonically with decreasing temperature, reaching a value
of 2750 cm2/Vs at 4 K. Zn polar "ZnMgO on ZnO" structures are easy to adapt to a top-gate device. These results open
new possibilities for high electron mobility transistors (HEMTs) based upon ZnO based materials.
Advances in nonpolar ZnO homoepitaxy: 1D surface nanostructure and electron transport
Show abstract
The development of epitaxial growth techniques for the fabrication of nanostructures provides advantages for
nanoscale engineering and has yielded many impressive results. ZnO possesses attractive characteristics that include
optical, electric and magnetic properties. This material can be utilized to delineate new phenomena through an
investigation of surface nanostructures and quantum heterostructures. Homoepitaxy in ZnO can generate specific growth
directions in the absence of lattice mismatch at the interface between the film and substrate. Many reports have appeared
in the past year concerning the layer growth of nonpolar ZnO. Nonpolar planes are expected to yield large in-plane
anisotropy in electrical and optical characterizatics. In nonpolar (10-10) growth using laser-MBE, we found that novel in
situ growth techniques allowed for the fabrication of dense arrays of conductive one-dimensional nanostripes with a high
degree of lateral periodicity. Highly anisotropic surface morphologies markedly influenced electron transport of ZnO
single layers and Mg0.12Zn0.88O/ZnO multi-quantum wells (MQWs) with conductivity parallel to the nanostripe arrays being more than one order of magnitude larger than that observed perpendicular to the nanostripe arrays.
ZnO Processing and Contact Preparation
Inductively coupled plasma etching of ZnO
Show abstract
The etching characteristics of ZnO epitaxial layers in Oxford Plasmalab 100 ICP 180 and 380 systems are
investigated. Etch rates are studied as a function of gas composition, ICP power and RF bias power. Surface
profilometry and scanning electron microscopy are used to characterize etch rates and surface morphologies. Highlights
from other recently published results are also discussed.
Etching of ZnO toward the development of ZnO homostructure LEDs
Show abstract
Although ZnO has recently gained much interest as an alternative to the III-Nitride material system, the development of
ZnO based optoelectonic devices is still in its infancy. Significant material breakthroughs in p-type doping of ZnO thin
films and improvements in crystal growth techniques have recently been achieved, making the development of
optoelectonic devices possible. ZnO is known to be an efficient UV-emitting material (~380 nm) at room temperature,
optical UV lasing of ZnO has been achieved, and both homojunction and hybrid heterojunction LEDs have been
demonstrated.
In this paper, processing techniques are explored towards the achievement of a homo-junction ZnO LED. First, a
survey of current ZnO processing methods is presented, followed by the results of our processing research.
Specifically, we have examined etching through an n-ZnO layer to expose and make contact to a p-ZnO layer.
ZnO Based Diodes and LEDs
Surface plasmon mediated emission from metal/ZnO: an example for the fabrication of high brightness top-emitting light emitting diodes
Show abstract
In this study, we have attempted to enhance the forward emission from metal-capped
ZnO mediated by surface plasmon (SP) cross coupling. By using metal alloys and
dielectric grating, it is proposed that energy from ZnO can be resonantly coupled to
metal/ZnO SPs, transferred to metal/grating SPs and then Bragg scattered to the free
space. Although the experimental conditions are not yet been optimized, preliminary
results from Al (30 nm) capped ZnO thin film show the forward/backward emission
intensity ratio can be increased from 0.1 to 0.6 after the introduction of dielectric
grating with periodicity of ~ 800nm on metal side. The ratio is compared favorably
with the bare ZnO emission ratio of 0.5. It is thus believed SP cross coupling can be
used for fabricating high brightness top-emitting light emitting diodes (LEDs).
ZnO Materials and Properties
Photonic properties of ZnO epilayers
Show abstract
The characterization by various experimental techniques of homoepitaxial growth and photonic properties of ZnO
epilayers was exhaustively analyzed. The photonic properties of ZnO as promising material for the realization of
polariton lasers were investigated by angular dependent reflection spectroscopy. The fitting of the polariton dispersion
curve with the experimental results provided us information about the longitudinal-transverse exciton-polariton splitting
and damping constants. In addition, the valence band symmetry was examined by angular resolved magneto-optical
photoluminescence. From our theoretical and experimental results we extracted evidence that the topmost A valence
band possesses Г7 symmetry. Micro-Raman spectroscopy revealed even in homoepitaxially grown samples the existence
of compressive or tensile strain which varied not only in the ZnO layers but also in the templates. In contrast, the
untreated substrates were uniformly strained. Sporadically crystal perturbations culminating in the formation of separated
growth domains were observed. Additionally, resonant Raman scattering was performed, showing a strong enhancement
of the 2E1(LO) mode for resonant excitation of the I8 bound exciton complex. We suggest that the resonant Raman
scattering led to a longer lifetime of the resonantly excited phonon mode due to a strong exciton-phonon interaction.
Studies of interfacial optical and electrical properties on dielectric/ZnO systems
Show abstract
The thermal stability of AlOx and MgOx on ZnO films has been studied by using
photoluminescence, cathodoluminescence and current-voltage measurements. It is
found that the interfaces degrade significantly upon thermal annealing, which are
evident by the reduction of the band-edge emission as well as the increase of
conductance with annealing temperature and duration. By using secondary ion mass
spectroscopy and diffusion model, the dependence of luminescence on thermal
treatment can be well simulated and the degradation of oxide/ZnO can be attributed to
the out-diffusion of Zn into the oxide layer from ZnO. Our studies point out the
importance of developing appropriate diffusion barrier for the fabrication of low
temperature processed ZnO transistors.
ZnO Bulk and Photonic Crystals
State-of-the-art ZnO bulk crystal growth
Show abstract
Zinc oxide (ZnO) crystals can be grown by vapor phase transport, hydrothermal solution growth, and high pressure melt
growth. Three inch size single crystal has been demonstrated by the hydrothermal growth technique. We will emphasis
on the hydrothermal technology of large size ZnO crystal. Material characteristics are discussed and compared to ZnO
fabricated from vapor phase and melt growth. A route to obtain high-quality, single-crystalline ZnO films with
mechanically untouched surfaces is employed to screen the doping of ZnO under the conditions of growth near the
thermodynamic equilibrium. Future trends in ZnO crystal growth technology will be discussed.
Vacancy defect distributions in bulk ZnO crystals
Show abstract
We have used positron annihilation spectroscopy to study vacancy defects in ZnO single crystals grown by various
methods from both commercial and academic sources. The combination of positron lifetime and Doppler broadening
techniques with theoretical calculations provides the means to deduce both the identities and the concentrations of the
vacancies. The annihilation characteristics of the Zn and O vacancies have been determined by studying electronirradiated
ZnO grown by the seeded vapor phase technique. The different ZnO samples were grown with the following
techniques: the hydrothermal growth method, the seeded vapor phase technique, growth from melt (skull melting
technique), and both conventional and contactless chemical vapor transport. We present a comparison of the vacancy
defects and their concentrations in these materials.
ZnO Based Thin Film Devices
Surface acoustic wave devices
C. Müller,
A. Nateprov,
G. Obermeier,
et al.
Show abstract
Starting with an introduction to surface acoustic waves, their generation and detection using interdigital transducers
(IDTs) on piezoelectric materials (e.g. LiNbO3 and ZnO) will be reviewed. Then the application of surface acoustic
waves in electronic devices will be presented. Moreover, recent studies, using the technique of attaching the
material of investigation onto the sound path of the acoustic delay line between the IDTs is discussed.
Growth and characterization of doped ZnO films
Show abstract
We report here the synthesis of ZnO films by the pulsed-laser deposition technique using various novel conditions. The
dopants are As, Ga, Al and N. The films show excellent crystalline quality with atomically smooth surface morphology.
The electrical resistivity was found to be close to 2 x~10-4 ohm-cm and transmittance >85% with both Ga and Al doping.
Doping with As shows several distinct transitions in their electrical resistivity and strong aging effects. On the other
hand, doping with Mn in ZnO reduces the grain size. On the other hand, doping with trivalent Er ions in ZnO films
causes two effects: for high doping (>8 wt%), a substantial enhancement of diagonal piezo-optic effect (up to 3.7*10-13
m2/N at &lgr;=633 nm) was observed due to creation of additional dipole moments at the interface of the film and the
substrate, and higher electrical conductivity with enhanced 1.54 &mgr;m emission was demonstrated at room temperature for
low concentration (<2 wt%) of Er. Furthermore, no quenching effects in emission characteristics at 1.54 &mgr;m were
observed up to 2 wt % of Er-doping in ZnO at room-temperature.
Ga:ZnO based transparent conducting oxides and devices
Show abstract
We report on the synthesis and processing, and structure - property correlations in gallium doped ZnO films grown on (0001) sapphire and glass substrates by pulsed laser deposition. Films with varying microstructure were grown on amorphous glass by changing the pulsed laser deposition parameters, namely temperature and oxygen partial pressure. The results corresponding to these films were compared with those from epitaxial single crystal films grown on (0001) sapphire. It is shown that resistivities and transmittance comparable to epitaxial Zn0.95Ga0.05O films (&rgr;=1.4x10-4&OHgr;-cm, %T>80) can be achieved in the nanocrystalline films (&rgr;=1.8x10-4&OHgr;-cm, %T>80) deposited on glass by carefully controlling the deposition parameters. We have investigated and modeled the conduction mechanisms (carrier generation and carrier transport) in the novel Ga:ZnO films through detailed structural characterization, chemical analysis, and electrical and optical property measurements. The device applications based on these highly conducting and transparent films as electrodes will also be discussed. Our preliminary results have demonstrated that power conversion efficiencies comparable to indium tin oxide (ITO) based organic photovoltaic devices can be achieved using ZnGaO films as the anode.
Scaling and parasitic effects in ZnO transparent thin film transistors
Show abstract
Scaling behaviors of ZnO transparent thin-film transistors (TTFTs) have been studied by fabricating series of
miniaturized ZnO TTFTs having various channel widths and lengths. Mobility of >8 cm2/V.s and on/off ratio of up to
107 are achieved with these TTFTs. Results show that these ZnO TTFTs retain rather well-behaved transistor
characteristics down to the channel length of ~5 &mgr;m, rendering possible high-resolution applications. More apparent
short-channel effects (e.g., lowering of threshold voltages, degradation of the subthreshold slope with the decrease of the
channel length and the increase of the drain voltage, and loss of hard saturation, etc.) are observed when the channel
length is reduced below 5 &mgr;m. Influences of parasitic effects on TFT characteristics are also studied by extracting
parasitic resistance and channel resistance using devices of various dimensions. The ratio of parasitic resistance to
channel resistance at VG = 10 V was increased from 0.04 to 0.36, when the channel length decreased from 20 &mgr;m to 2 &mgr;m.
This indicates that parasitic resistance has substantial influences on device performances (e.g., output drain current,
apparent field effect mobility, etc.) when the channel length is reduced, and better contact techniques may be required.
The characteristics of transparent metal-ZnO contacts and ZnO-based photodiodes
Show abstract
Low resistivity and high transparent ITO, RuOx (1⩽x⩽2) and TiW ohmic contacts to ZnO film
was achieved by RF sputter system and annealing treatment. The transmittance of 450°C-annealing
ITO, 650°C-annealing Ru and 200°C-annealing TiW were measured to be 94, 68 and 61%, with
wavelength of 400 nm, respectively. Moreover, the specific contact resistance of 450°C-annealing
ITO, 650°C-annealing Ru and 200°C-annealing TiW on ZnO films was estimated to be 2.15x10-4,
2.72x10-4 and 2.56x10-4 &OHgr;-cm2 by circular transmission line model (CTLM) method, respectively.
In the study of ZnO-based photodiodes, high quality and vertical well-aligned ZnO nanowires were
selectively grown on ZnO:Ga/glass templates by vapor-liquid-solid method. Ultraviolet (UV)
photodetectors using these vertical ZnO nanowires were also fabricated by spin-on-glass technology.
With 2 V applied bias, it was found that dark current density of the fabricated device was only 3.8x10-9 A/cm2. It was also found that UV-to-visible rejection ratio and quantum efficiency of the
fabricated ZnO nanowire photodetectors were more than 1000 and 12.6%, respectively.
Influence of annealing in oxygen ambient on crystal properties of rf-sputtered PZT layers on ZnO substrates
Show abstract
Lead zirconate titanate PbZr52Ti48O3 (PZT) layers were deposited on ZnO layers by rf-sputtering at varying substrate temperatures. The effect of annealing on PZT crystal properties has been studied by X-ray diffraction and atomic force
microscopy. It is shown that the annealing in oxygen ambient has significant effect on the quality of the deposited PZT
layers. The optimum growth temperature has been found to be 650 C.
ZnO Based Nanostructures I
Metal oxide nanowires for optical gas sensing
Show abstract
SnO2 and ZnO and metal oxide nanowires were synthesized by vapor transport process in a horizontal tube furnace. The
peculiar characteristic of these materials is the emission of visible photoluminescence (PL) when they are excited with
UV light. The visible photoluminescence of tin and zinc oxide nanowires is quenched by nitrogen dioxide at ppm level in
a fast (time scale order of seconds) and reversible way. Besides, the response seems highly selective toward humidity
and other polluting species, such as CO and NH3. We believe that adsorbed gaseous species that create surface states can
quench PL by creating competitive nonradiative paths.
Enhancement of gas response of ZnO micro-nano structured films through plasma treatment
Show abstract
Films of ZnO micro-nano structures were deposited on quartz substrates and subsequently plasma treated in O2, N2 and
CF4. It was found that exposure to oxygen plasma enhanced gas response to ethanol vapor of the ZnO films by a factor
2. The effect of surface plasma treatments on the gas response of the ZnO films was discussed in reference to surface
morphology observed by high-magnification SEM and surface chemical state determined by XPS. SEM observation
revealed that O2 plasma treatment induced less surface roughening than N2 and CF4 plasmas, in agreement with the view
that O2 plasma should reduce preferential sputtering. Deconvolution of the O 1s X-ray photoelectron peak indicated an
increase in the Zn-O bond surface density relatively to O-H bond density for the O2 plasma treated surface, whereas the
O-H bond surface density was increased relatively to the Zn-O bond density for the N2 and CF4 plasma treated films.
The O2 plasma was found to partially clean the surface from hydroxyl groups and to expose more Zn cations, which
might have caused the enhancement of sensor response by increasing the density of active sites for oxidation/reduction
reactions.
Fabrication and characterization of zinc oxide based rib waveguide
Show abstract
In this work we investigate the possibility to use Zinc Oxide (ZnO) thin films, deposited by RF magnetron sputtering, for
the realization of integrated optical structures working at 1550 nm. Structural properties of sputtered zinc oxide thin
films were studied by means of X-ray Diffraction (XRD) measurements, while optical properties were investigated by
spectrophotometry and Spectroscopic Ellipsometry (SE). In particular, ellipsometric measurements allowed to determine
the dispersion law of the ZnO complex refractive index (see manuscript) = n - jk through the multilayer modeling using Tauc-Lorentz
(TL) dispersion model. We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates
from 2.5 to 3.8 Å/s. Conversely, the refractive index exhibits, from UV to near IR, a considerable and almost linear
variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm. In order to realize a waveguide
structure, a 3-&mgr;m-thick ZnO film was deposited onto silicon single crystal substrates, where a 0.5-&mgr;m-thick thermal SiO2
buffer layer was previously realized, acting as lower cladding. Dry and wet chemical etching processes have been
investigated to achieve controllable etching rate and step etching profile, with the aim to realize an optical rib waveguide.
The etched surfaces were inspected using scanning electron microscopy (SEM) and optical microscopy. Moreover, we
carried out the experimental measurements of the fringes pattern and Free Spectral Range (FSR) of an integrated Fabry-
Perot etalon, obtained by cleaving of a single mode rib waveguide.
ZnO Based Nanostructures II
Morphological control of ZnO nanostructures grown on silicon
R. T. Rajendra Kumar,
J. Grabowska,
J. P. Mosnier,
et al.
Show abstract
We report growth of ZnO nanostructures on Au-coated Si substrates using vapor phase transport in the temperature range
from 800°C to 1150°C. Nanostructures grown at 800°C are rod-like with diameters of ~ 200 nm. Growth at higher
temperature shows a more complex behaviour with 2-D structures connecting 1-D nanorods at intermediate temperatures
and 3-D growth at the highest temperatures. Our work indicates that it may be possible to systematically control the
growth mode and morphology of ZnO nanostructures by tuning the growth temperature.
Optical characteristics of ZnO nanowires synthesized by nanoparticle-assisted deposition and their application to sensors
Show abstract
ZnO nano-nails which consist of the nanowire at the top of nanorods have been synthesized by the nanoparticle-assisted
deposition technique. When the nanowire was excited by the third harmonics of a Q-switched Nd:YAG laser, the
ultraviolet stimulated emission was clearly observed from single nanowire, indicating the high crystalinity of the
nanowire. The highly sensitive ultraviolet photo-detectors were successfully fabricated by trapping these nanowires
between electrodes using the dielectrophoresis technique.
Patterned growth of ZnO nanorod by solution chemical method
Show abstract
A simple method has been developed for the controlled patterned growth of the ZnO nanorod arrays with different size and shape on substrate. In order to control the position of the ZnO nanorods, exposed ZnO seed is defined, as orderly aligned arrays, with the assistance of photolithography. This technique hinges on the patterning of the seed layer comprised by ZnO sol-gel precursor. The simple way to create patterned ZnO seed array is to use negative photoresist for ZnO seed coating. The UV exposures were performed though mask patterned various shape. The ZnO arrays are synthesized using solution chemical method at normal atmospheric pressure without any metal catalyst. A simple two-step process is developed for ZnO nanorod on substrate at 90°C. The ZnO seed precutsor is prepared by sol-gel process. The ZnO nanorod is grown by solution chemical method. The ZnO nanorod growth was dependent on the ZnO seed layer. The ZnO nanorods have length of 400~500nm and diameter of 25~50nm. The ZnO nanorod is single crystals with wurtzite and grows along the c axis of the crystal plane. The room temperature photoluminescence measurements have shown ultraviolet peaks 378.3nm (3.27eV) with high intensity.
Carrier relaxation and stimulated emission in ZnO nanorods grown by catalyst-assisted vapor transport on various substrates
Show abstract
ZnO nanorods were grown by catalyst-assisted vapor phase transport on Si(001), GaN(0001)/c-Al2O3, and bulk
ZnO(0001) substrates. Morphology studies as well as X-ray diffraction and transmission electron microscopy showed
that ZnO nanorods grew mostly perpendicular to the GaN(0001) and ZnO(0001) substrate surface, whereas a more
random directional distribution was found for nanorods on Si(001). Integral optical properties of fabricated nanorods
were studied by steady-state photoluminescence and time-resolved photoluminescence. Stimulated emission was
observed from ZnO nanorods on GaN(0001)/c-Al2O3 substrates, most likely due to their vertical orientation. Near-field
scanning optical microscopy was applied to investigate luminescent properties of individual rods. Raman
spectroscopy revealed biaxial compressive strain in the nanorod samples grown on Si(001). Conductive atomic force
microscopy showed that nanorods are electrically isolated from each other. I-V spectra of individual nanorods were
measured.
Fabrication of well-aligned ZnO nanorods by hydrothermal process using GaN epitaxial layer
Show abstract
One dimensional (1-D) ZnO nanorod structure of hexagonal shape was fabricated on epitaxial GaN layer by hydrothermal
method. The growth of GaN epitaxial layer was carried out in a two-flow horizontal MOCVD reactor
maintained at a pressure of 200 torr. Firstly, a 25 nm thick GaN buffer layer was grown at 520 °C. Then 2~3&mgr;m thick
GaN epilayer was deposited at 1070 °C. Trimethylgallium (TMG) and NH3 were used as Ga and N source, and H2 gas
was used as carrier gas. After the deposition of GaN epilayer thin-film, single crystalline ZnO nanorod was fabricated in
aqueous solution. XRD and FE-SEM results showed ZnO nanorod arrays were oriented highly along the (002) plane.
The ZnO nanorod was analyzed to have good quality crystallization by FE-TEM. The SAED pattern has shown that
ZnO nanorod was grown in the direction along (002)-plane. Photoluminescence (PL) has shown that the GaN-ZnO
hetero-structure has shown ultra-violet lasing action at room temperature. Narrow and strong ultra-violet peak was
observed in comparison with PL result from epitaxial GaN layer. The analysis results have proved that aqueous solution
growth method developed in the present work can be a good application for optical electronic device.
Spintronics and Ferroelectrics
Transition-metal- and rare-earth-doped ZnO: a comparison of optical, magnetic, and structural behavior of bulk and thin films
Show abstract
Recent theoretical predictions of ferromagnetic behavior in transition metal (TM)-doped ZnO have focused significant
attention on these materials for use as spintronic materials. Moreover, rare earth (RE) elements in wide bandgap
semiconductors would be useful not only in spintronics but also in optoelectronic applications. This work presents
results obtained from an investigation into the optical, magnetic, and structural properties of transition-metal (TM)-
doped ZnO and rare earth (RE) doped ZnO (TM = Mn, Co, Ni, and Fe; RE = Gd, Eu, and Tb) bulk crystals and thin
films. Properties of TM- and RE-doped ZnO bulk crystals and thin films were studied and compared in order to better
understand the nature of these dopant centers and their effects on the properties of the host crystal. Optical properties
confirm the incorporation of substitutional transition metal ions on cation sites. While most thin film samples show
ferromagnetic behavior, the magnetic response of the bulk crystals varies. This suggests that the magnetic behavior of
TM-doped ZnO is highly dependent on growth conditions, and growth conditions which favor the formation of grain
boundaries and interfaces may be more likely to result in ferromagnetic behavior. Origins of this ferromagnetic behavior
are still under investigation. Defect luminescence observed in the RE-doped samples suggests that these materials may
prove useful in optoelectonic applications as well.
Transport properties of non magnetic and magnetic ZnO thin films under field effect
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The integration of ZnO based high mobility transparent semiconductors with perovskites that exhibit a wide spectrum of
physical properties (superconductivity, ferroelectricity, ferromagnetism, etc.) may lead to a wide variety of new
electronic/optoelectronic devices. Here we present results about the deposition of high crystalline quality Al or Co doped
ZnO films grown by pulsed laser deposition on 110 face of strontium titanate single crystals. Field Effect (FE)
experiment, allowing to change the carrier concentration of the film by more then 4 orders of magnitude (from ≈1015 to
≈1020 e-/cm3, estimated by Hall effect measurements under FE), were employed to investigate transport mechanisms in
depth. In particular we observed a crossover of low temperature magnetoresistance from a negative behaviour in the
accumulation state to a positive one in the depletion state. The measurement of the activation energy as a function of the
Gate potential allowed to get information on the density of states.
Polarization coupling in epitaxial ZnO / BaTiO3 thin film heterostructures on SrTiO3 (100) substrates
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Strong polarization coupling is expected by combining ferroelectric materials with switchable polarization and wurtzite
layers exhibiting a permanent spontaneous polarization. To demonstrate these charge coupling effects, current-voltage,
conductivity-frequency and capacitance-frequency (admittance) characteristics have been measured on epitaxial
heterostructures grown of ferroelectric BaTiO3 (001) films on conducting SrRuO3 layers on SrTiO3 (100) substrates with
oxide SrRuOx, metallic Pt and semiconducting ZnO top electrodes. The electrical measurements show clear indications
for polarization coupling of the ferroelectric perovskite BaTiO3 and the piezoelectric wurtzite ZnO thin films.