Two dimensional homoepitaxial growth of high quality ZnO epilayers was achieved by chemical vapor deposition
techniques without a buffer layer. We report on the optical and structural properties of these epilayers with particular
focus on the polarity of the surface of the substrate. Photoluminescence spectra exhibit strong dependence of the bound
exciton recombinations on the termination of the substrate. This is particularly pronounced in the large variety of
transition lines in the O-face terminated sample with values for the full width at half maximum as low as 80μeV. Cross-sectional
micro Raman spectroscopy and high resolution transmission electron microscopy reveal the presence of strain
in the epilayer grown on O-face ZnO by a shift of the non-polar E2(high) mode and a variation in the lattice constant
ratio. Still, the crystal quality of the films is further increased compared to the substrate, which is shown be a half-width
of 17" of the XRD rocking curve in both epilayers on Zn-face and O-face terminated ZnO substrate.
We have investigated the morphology, crystalline quality, the transport and electronic properties of homoepitaxial ZnO
and ZnO:P thin films grown by pulsed-laser deposition. Atomic surface steps are visible for growth temperatures of
650°C and higher. The unit cell volume of undoped thin films is smaller than that of the hydrothermal substrates.
Phosphorous doping increases the unit cell volume such that a perfect lattice match is achieved for a nominal
phosphorous content of 0.01 wt.%. Undoped thin films have a net doping concentration below 1015 cm-3, whereas the
phosphorous doping increases the free electron concentration at room temperature to 1017 cm-3 and above. Temperature
dependent Hall effect measurements show that interstitial zinc with a thermal activation energy of 34 meV is a dominant
donor in homoepitaxial ZnO:P thin films. The Hall mobility of such samples is similar to ZnO single crystals grown by
seeded chemical vapor transport. Low temperature photoluminescence measurements reveal recombination of free
excitons and excitons bound to interstitial zinc and excitons bound to neutral and ionized aluminum donors. Defect
related deep luminescence is not observed for undoped homoepitaxial thin films. In contrast phosphorous doping
introduces two broad recombination bands centered at 2.9 eV and 1.9 eV.
Plasma-assisted epitaxy has been demonstrated as one possible process for the ZnO growth, which is processed at low
temperatures below 400oC using oxygen-plasma excited by radio-frequency power at 13.56MHz. This paper is focused
on the plasma-assisted epitaxial growth of ZnO layer concerned with shallow acceptor doping using N2+O2 gas plasma
and high-quality undoped-ZnO growth using a novel buffer layer consisted of very thin (below 1nm-thick) strained Ti2O3
on A-sapphire. Donor-acceptor pair emission was clearly observed at 3.273eV in low temperature photoluminescence
spectrum of nitrogen-doped PAE-ZnO layer grown in Zn-rich supply condition and the activation energy of the shallow
acceptor was determined about 132meV. The doping feature of nitrogen related with the oxygen vacancy is also
expected by the photoluminescence study. High-quality ZnO layer with smooth surface including fine hexagonal
pyramids was grown on the buffer layer at the growth temperature as low as 340oC with the photoluminescence spectrum
dominated by free-exciton emissions at 10K. RHEED observations indicated the ZnO on the thin buffer layer was
epitaxially grown toward c-axis with an in-plane relation ship of [-12-10]ZnO//[1-104]Al2O3 without any rotational
Molecular-beam epitaxial growth far from thermal equilibrium allows us to overcome the standard solubility limit
and to alloy ZnO with CdO in strict wurtzite phase up to mole fractions of several 10%. In this way, a band-gap
range extending from 3.3 eV down to 2.3 eV can be covered. Strong improvement of the crystalline quality
indicated by a rocking curve width of only 45 arc sec is achieved when growing the ternary on ZnO substrates.
Despite very low growth temperatures (~150 °C), layer-by-layer growth indicated and controlled by RHEED
oscillations is accomplished. This enables us the fabrication of atomically smooth heterointerfaces and well-defined quantum well structures exhibiting prominent band-gap related light emission in the whole composition
range. Post-growth annealing increases the radiative efficiency up to two orders of magnitude and demonstrates
thermal stability of the structures with respect to phase separation even up to temperatures of about 500°C.
Low-energy shifts of the photoluminescence features reaching the order of 1 eV as well as a dramatic increase of
the lifetime from the sub-ns to the 100-μs time-scale uncover the presence of huge polarization-induced electric
fields of some 108 V/m in ZnCdO/ZnO single quantum well structures. Carrier injection by moderate optical
excitation in the 10 kW/cm2 screens these fields and recovers practically the bare quantum-confined energy
transitions. On appropriately designed structures, laser action from the UV down to the green wavelength
range is observed under optical pumping. The threshold at low temperature is only 60 kW/cm2 and increases
only moderately up to room temperatures. All these findings make ZnO-based heterostructures a promising
alternative to group-III-nitrides for opto-electronic applications in the short-wavelength range.
Thin films of ZnO and MgxZn1-xO were epitaxially grown on Zn-polar ZnO substrates by plasma assisted
molecular beam epitaxy. The miscut of c-plane ZnO substrates toward the [1-100] axis direction leads to a flat substrate
surface with straight step edges. The growth mode of epitaxial ZnO films significantly depended on the growth
temperature, and a substrate temperature over 800°C was needed for flat film surfaces with monolayer-height steps.
Photoluminescence (PL) peak originating from the n = 2 state of A-free excitons was observed at 12 K for the ZnO films
grown under stoichiometric and O-rich growth conditions. MgxZn1-xO films were also fabricated under Zn-rich
conditions. The film surface exhibited a step-and-terrace structure. The effective PL lifetime of Mg0.08Zn0.92O film was as
long as 1.9 ns, which is the highest value ever reported, presumably due to a high purity level of the film.
Strong coupling between the exciton and cavity modes were demonstrated in a bulk ZnO-based hybrid microcavity The
hybrid microcavity consisted of a λ-thick bulk ZnO cavity layer sandwiched between a 29 pair Al0.5Ga0.5N/GaN bottom
distributed Bragg reflector (DBR) and an 8 pair SiO2/Si3N4 top DBR grown by molecular beam epitaxy, metalorganic
chemical vapor deposition, and ultra high vacuum plasma-enhanced chemical vapor deposition, respectively. All layer
interfaces were sharp and optical reflectivity measurements were performed to characterize the DBRs. The anti-crossing
behavior in the polarton dispersion, which indicates the system is in the strong coupling regime was observed in angle-resolved
photoluminescence measurements at room temperature, and a vacuum Rabi splitting of ~50 meV in the ZnO-based
hybrid microcavity was obtained.
An optically pumped ZnO distributed feedback laser operating at 383 nm has been designed, fabricated and
characterized. Single mode operation was observed for a wide temperature range between 10 and 270 K. In order to
avoid technologically difficult etching of ZnO, a 3rd order diffraction grating was dry-etched into an additional 120 nm-thick
Si3N4 layer deposited on the ZnO active region. The spectral linewidth of the laser emission was 0.4 nm, whereas
an optical pump threshold intensity of 0.12 MW/cm2 and a peak output power of 14 mW were seen. The temperature
tuning coefficient of the ZnO refractive index was determined from wavelength vs. temperature measurements; a value
of 9 × 10-5 K-1 was found, in good agreement with literature values.
We report on ZnO wafer bonded III-nitride light emitting diodes (LEDs). ZnO wafer was selectively etched to form a hexagonal pyramid shape. For enhancing light extraction and heat conductivity, two types of LED structures were demonstrated: one is having patterned GaN at the wafer bonded interface between GaN and ZnO, the other is the LED having no sapphire. The experimental results of electrical and optical characteristics indicate the high potential of these types of LEDs for solid state lighting sources.
After our recent successful demonstration of high brightness white light emitting diodes (HB-LEDs)
based on high temperature grown n-ZnO nanowires on different p-type semiconductors, we present
here LEDs fabricated on n-ZnO nano-wires and p-type organic semiconductors. By employing a low
temperature chemical growth (≤ 90 °C) approach for ZnO synthesis combined together with organic
p-type semiconductors, we demonstrate high quality LEDs fabricated on a variety of different
substrates. The substrates include transparent glass, plastic, and conventional Si. Different multi-layers
of p-type organic semiconductors with or without electron blocking layers have been
demonstrated and characterized. The investigated p-type organic semiconductors include
PEDOT:PSS, which was used as a anode in combination with other p-type polymers. Some of the
heterojunction diodes also contain an electron blocking polymer sandwiched between the p-type
polymer and the n-ZnO nano-wire. The insertion of electron blocking layer is necessary to engineer
the device for the desired emission. Structural and electrical results will be presented. The
preliminary I-V characteristics of the organic-inorganic hybrid heterojunction diodes show good
rectifying properties. Finally we also present our findings on the origin of the green luminescence
band which is responsible of the white light emission in ZnO is discussed.
Vertically aligned ZnO nanowires have been successfully synthesized on c-cut sapphire substrates by a catalyst-free
nanoparticle-assisted pulsed-laser ablation deposition (NAPLD) in Ar and N2 background gases. In NAPLD, the
nanoparticles formed in a background gas by laser ablation are used as a starting material for the growth of the
nanowires. The surface density of the nanowires can be controlled by varying the density of nanoparticles, which are
accomplished by changing the energy of the ablation laser, the repetition rate of the laser and so on. When single ZnO
nanowire synthesized in a N2 background gas was excited by 355 nm laser-pulse with a pulse-width of 8 ns, stimulated
emission was clearly observed, indicating high quality of the nanowire. These nanowires were used as building blocks
for an ultraviolet light emitting diode with a structure of n-ZnO/ZnO nanowire/p-GaN.
We report on the epitaxy of vertically aligned ZnO nanowires (NWs), the collective integration technology of these
nanowires and their optical and electrical characterizations. ZnO based nanowires are grown mainly on sapphire
substrates by metal-organic vapour phase epitaxy (MOVPE). Photoluminescence spectra at 4 K exhibit strong excitonic
peaks at around 380 nm without green luminescence band, showing the low deep radiative defect density. Technological
processes have been developed both for mineral and organic integrations of the as-grown nanowires. Photoconducting
properties in the UV-visible range have been investigated through collective electrical contacts. The electrical transport
properties of vertically integrated single nanowires have also been investigated by current sensing AFM measurements.
A comparison of the PL spectra at 300 K of the as-grown and integrated nanowires has shown no significant impact of
the integration process on the crystal quality of the nanowires.
Phosphorous-doped ZnO (ZnO:P) nanowires were prepared by a high-pressure pulsed laser deposition process. To
extend the size range of available wires, μm-thick ZnO:P microwires were grown additionally by a direct carbothermal
deposition process. Low-temperature cathodoluminescence of single ZnO:P nanowires grown by both processes exhibit
characteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission ((A0, X), 3.356 eV), free-electron
to neutral-acceptor emission ((e, A0), 3.314 eV), and donor-to-acceptor pair emission (DAP, ~3.24 and ~3.04
eV). This proves that stable phosphorus acceptor levels have been induced into the ZnO:P nano- and microwires. From
the quantitative evaluation of the spectroscopic features we deduct an acceptor binding energy of 122 meV. The ZnO:P
microwires were used as channels in bottom-gate field effect transistors (FET) built on Si substrates with SiO2 gate
oxide. The electrical FET-characteristics of several wires show reproducibly clear qualitative indication for p-type
conductivity for variation of gate voltage. This behavior is opposite to that of nominally undoped, n-type conducting
wires investigated for comparison. The p-type conductivity was found to be stable over more than six months.
ZnO is a wide bandgap semiconductor that exhibits properties that are near-ideal for light-emitting diodes, but presents
materials challenges that must be overcome in order to achieve highly efficient light emission. The most significant
issue with ZnO is p-type doping. Related materials issues include understanding electron-hole transport across pn
junctions, as well as understanding and minimizing leakage current paths within the bulk and on the surface. In this
paper, the formation and properties of phosphorus-doped Zn1-xMgxO films, ZnO-based pn homojunctions and
heterojunctions is discussed. The behavior of phosphorus within the ZnO and ZnMgO matrices will be described.
Comparisons with other acceptor dopants will be made. Discussion will include stability of transport properties,
stabilization of surfaces, and device characteristics.
The ability to externally control the properties of magnetic materials would be highly desirable both from fundamental
and technological point of views. In this respect, dilute magnetic semiconductor (DMS), in which a fraction of atoms of
the nonmagnetic semiconductor host is replaced by magnetic ions, have recently attracted broad interest for their
potential application in spintronics. In this work, we focused on transition metal (TM) (Co, Mn and Cu) doped Zinc
oxide (ZnO) because room temperature ferromagnetism was both theoretically predicted and experimentally observed.
However, the origin of such ferromagnetism, in particular whether it is a signature of a true DMS behaviour (long range
magnetic interaction between the doping ions) or it arises from the formation of secondary phases, segregation or
clustering is still under debate. Measuring the dependence of the magnetic properties on the carrier concentration can
clarify the underlying physics. The samples were characterized by resistivity, Hall effect, magnetoresistance, Seebeck
effect, synchrotron X-ray adsorption spectra (XAS) and magnetic dichroism (XMD) while modulating the carrier density
by electric field. The insulating-gate field-effect transistor structures are realized in ZnO/Strontium Titanate (SrTiO3)
heterostructures by pulsed laser deposition. These devices offers the capability to modulate the carrier density of a probe
accessible (light, AFM tip, ...) channel, by more than 5 orders of magnitude (from ≈1015 to ≈1020 e-/cm3, estimated by
Hall effect measurements under FE). The Co and Mn films measured by DC SQUID magnetometer result ferromagnetic
and anomalous Hall effect was observed at low temperature but nor ferromagnetic nor antiferromagnetic signal was
detectable in the XMD spectra. Cu doped films are insulating and nonmagnetic. Photo Emission Electron Microscopy (x-PEEM) and magnetic force microscopy (MFM) showed that the sample are homogeneus and no clustering of TM were
detected. A large effect of the magnetic ions, strongly dependent on the carrier concentration, was observed on the
transport properties and this effect according can be explained by a giant s-d exchange leading to spin splitting of the s-type
conduction band. Since the filling of such band can be modified by field effect a electric field control of the spin
polarization can be achieved.
The effect of plasma-induced ion damage on the optical properties of ZnO films grown by plasma-assisted molecular
beam epitaxy on a-sapphire substrates and GaN(0001)/c-sapphire templates prepared has been studied using steady-state
and time-resolved photoluminescence. We observed that the deflecting the ions produced by the RF oxygen plasma
away from substrate results in improved excitonic emission and modification of the defect-related PL spectrum. The
intensity of the near-band-edge lines in the photoluminescence spectra from the layers grown with the ion deflection was
found to increase by factors 7 to 20 for the layers grown on GaN(0001)/c-sapphire at a plasma power of 350 W and by 3
to 4 times for ZnO grown on a-sapphire substrates at a plasma power of 265 W as compared to the controls grown
without the ion deflection. The yellow-green spectral range was dominated by different defect bands in the films grown
with and without ion deflection. The effect of RF power on peak positions of the defect band was studied for the films
grown without ion deflection. For the ZnO films grown on a-plane sapphire substrates, time-resolved photoluminescence
showed a significant increase in luminescence decay times both at RT and 89 K. However, for ZnO on GaN(0001)/csapphire
substrates, virtually no improvement in decay time was found at 89 K with only a moderate increase in decay
constant at room temperature.
ZnO nanostructures were synthesised by Metal Organic Chemical Vapor Deposition growth on Si (100) and c-Al2O3
substrates coated with a 5nm thick layer of Au. The Au coated substrates were annealed in air prior to deposition of
ZnO so as to promote formation of Au nanodroplets. The development of the nanodroplets was studied as a function
of annealing duration and temperature. Under optimised conditions, a relatively homogeneous distribution of regular
Au nanodroplets was obtained. Using the Au nanodroplets as a catalyst, MOCVD growth of ZnO nanostructures was
studied. Scanning electron microscopy revealed nanostructures with various forms including commonly observed
structures such as nanorods, nanoneedles and nanotubes. Some novel nanostructures were also observed, however,
which resembled twist pastries and bevelled-multifaceted table legs.