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The hot-electron effect on electron transport in nominally undoped ZnO epilayers is studied at electric fields up to 430 ± 50 kV/cm applied with 3 ns voltage pulses. The transient measurements do not demonstrate any change in the electron density at the fields up to 320 kV/cm. The deviation from Ohm's law can be approximated by the linear dependence of the current on the electric field valid at the moderate fields (from 50 to 250 kV/cm). The model calculations based on the Boltzmann kinetic equation are used to demonstrate that the differential mobility at the moderate fields is almost independent of the scattering on the charged point defects. The electron drift velocity is estimated from the experimental values on the differential conductivity and the current density together with the calculated differential mobility as a reference for calibration. The drift velocity reaches 2.7 ± 0.3 × 10 7 cm/s at the field of ∼ 320 kV/cm and approaches the theoretical limit predicted by the known Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

In nonlinear optics, significant effort is concentrated on improving the strength and efficiency of interactions; however, experimentally investigating nonlinear materials is a complex, time-consuming, and costly investment. Moreover, it is often challenging to isolate, study, and optimize material parameters in an experiment due to complexities in the growth process. Recently, epsilon-near-zero materials have received a great deal of attention as promising nonlinear optical materials, but like many up-and-coming materials, the ability to explore and optimize their properties has been challenging. Here, we establish a framework to rapidly evaluate the performance of nonlinear epsilon-near-zero materials for both inter- and intraband effects in silico, requiring only an energy–momentum (E–k) diagram, linear optical properties, and experimental conditions. Measured nonlinear reflection and transmission in gallium-doped zinc oxide films are compared to the numerical framework for both intra- and interband excitation to verify accuracy across wavelength and irradiance while two figures of merit (FoMs) are introduced to quickly evaluate the performance of films without a full numerical framework. This capability is used to predict the performance of highly doped gallium nitride, cadmium oxide, zinc oxide, and indium tin oxide films, and efficient intra- and interband operation conditions are identified. Through this numerical framework and the FoMs, the exploration of unstudied epsilon-near-zero materials is enabled without the need for a nonlinear experiment, thereby accelerating the search for more efficient nonlinear materials and excitation conditions. [ABSTRACT FROM AUTHOR]

Microwave noise is used to study high-electric-field electronic properties of ZnO channels with electron densities in the range from 1017 to 1019 cm–3. The strong source of noise is observed to superimpose onto the standard hot-electron noise governed by the hot-electron energy relaxation. At a given current, the excess noise temperature Δ T n increases with the channel length, and values up to and above 10 000 K are reached. The steep dependence Δ T n ∝ I 12 on the current I approximately holds for the longest channels. The source of noise in question is suppressed in ZnO epilayers at high electron densities and in a ZnO/MgZnO heterostructure with two-dimensional electron gas. The observed results are evaluated and discussed in terms of the self-formation of high field domains. The estimated domain voltage U d increases with the current; the dependence is close to U d ∝ I 6 . The domain self-formation is additionally confirmed by measuring the spectral density of current fluctuations; the usual hot-electron noise turns into shot noise as the current increases. The Fano factor demonstrates an increasing number of nearly ballistic electrons that traverse the self-supporting domain. [ABSTRACT FROM AUTHOR]

We propose an approach to tune the emission of a single semiconductor quantum dot (QD) to couple with a planar cavity using hydrostatic pressure without inducing temperature variation during the process of measurement. Based on this approach, we studied the influence of cavity mode on the single-photon purity of an InAs/GaAs QD. Our measurement demonstrates that the single-photon purity degrades when the QD emission resonates with the cavity mode. This negative influence of the planar cavity is mainly caused by the cavity feeding effect. [ABSTRACT FROM AUTHOR]

We report an experimental study on the optical properties of the self-organized 1.55-μm InAs/ InGaAsP/InP quantum dots (QDs) under hydrostatic pressure up to 9.5 GPa at 10 K. The obtained pressure coefficients of emissions from InGaAsP to InAs QDs are 92 meV/GPa and 76 meV/GPa, respectively. Their photoluminescence intensities are found to decrease significantly with increasing pressure due to the pressure-induced Γ-X mixing of InGaAsP at about 8.5 GPa. The lifetime of excitonic emission from QDs decreases from about 1.15 at zero pressure to about 1.05 ns at 7.41 GPa. The wavelength of QD emission was tuned from 1.55 to 0.9 μm by applying a pressure of 8 GPa, displaying the feasibility for indirectly characterizing the individual InAs/ InGaAsP/InP QDs of 1.55-μm emission (at zero pressure) under high-pressure using silicon avalanche photodiode. [ABSTRACT FROM AUTHOR]

The first-principles calculations based on the density functional theory are applied to investigate the effect of a chemical group coadsorbed on one electrode surface on the electronic transport of a molecular device. We find that the types of the coadsorbed groups and their sites on one electrode surface affect the electronic transport significantly, and the resulting shift of the molecular levels upon coadsorption depends jointly on various effects, such as the electrostatic interaction, shift of the Fermi level of the electrode, the chemical interaction, and so on. Among these factors, the chemical interaction-induced the charge transfer across metal-molecule interface is identified as a determining factor resulting in the variation of transport properties. Our findings suggest that the coadsorption may offer the novel possibility to modify the transport behaviors of a molecular device in a controlled way and can improve/add some particular functionalities or should be avoided in order to keep a stable transport for a molecular device. [ABSTRACT FROM AUTHOR]

We have investigated the temperature and pressure dependences of the copper-related green emission, which show fine structure at low temperature, from tetrapodlike ZnO microrods. The temperature dependence of the green emission energy follows the changes in the band gap from 10–200 K, but deviates from this behavior above 200 K. The pressure dependence of the copper-related green band (25±5 meV/GPa) is similar to that of the band gap of ZnO, and is larger than that reported previously for defect-related green emission in ZnO. [ABSTRACT FROM AUTHOR]

The temperature and pressure dependence of the photoluminescence from ZnS:Mn[sup 2+], ZnS:Cu[sup 2+], and ZnS:Eu[sup 2+] nanocrystals were investigated in the temperature range from 10 to 300 K and under hydrostatic pressure up to 6 GPa at room temperature. The orange emission (590 nm) from the [sup 4]T[sub 1]-[sup 6]A[sub 1] transition of Mn[sup 2+] ions, the green emission (518 nm) from the 4f[sup 6]5d[sup 1]-4f[sup 7] transition of Eu[sup 2+] ions and the blue emission (460 nm) related to the transition from the conduction band of ZnS to the t[sub 2] level of Cu[sup 2+] ions were observed in the Mn-, Eu-, and Cu-doped samples, respectively. It was found that all of these emission bands decrease in intensity with increasing temperature. Among them the intensity of the Mn-orange emission dropped faster. The activation energies were estimated to be 58, 16, and 42 meV for the Mn-orange, Eu-green, and Cu-blue emissions, respectively. A negative pressure coefficient of -26 meV/GPa was obtained for the Mn-orange emission, which agrees with the value calculated from the crystal field theory. The pressure coefficient of the Cu-blue emission was found to be 62 meV/GPa, which is almost same as the value of the band gap of bulk ZnS. However, the pressure coefficient of the Eu-green emission is 23 meV/GPa, which is contrary to the predication by the crystal field theory. The strong interaction between the 4f[sup 6]5d[sup 1] state of the Eu[sup 2+] ions and the conduction band of ZnS may be the origin for the positive pressure coefficient and the small thermal activation energy of Eu-green emission. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The erbium-doped hydrogenated amorphous silicon suboxide films containing amorphous silicon clusters were prepared. The samples exhibited photoluminescence peaks at around 750 nm and 1.54 μm, which could be assigned to the electron-hole recombination in amorphous silicon clusters and the intra-4f transition in Er[sup 3+], respectively. Correlations between the intensities of these two photoluminescence peaks and oxidation and dehydrogenation of the films during annealing were studied. It was found that the oxidation is triggered by dehydrogenation of the films even at low annealing temperatures, which decisively changes the intensities of the two photoluminescence peaks. On the other hand, the increase of Er content in the erbium-doped hydrogenated amorphous silicon suboxide film will enhance Er[sup 3+] emission at 1.54 μm, while quench amorphous silicon cluster emission at 750 nm, such a competitive relationship, was also observed in the erbium-doped silicon nanocrystals embedded in SiO[sub 2] matrix. Moreover, we found that Er[sup 3+] emission is not sensitive to whether silicon clusters are crystalline or amorphous. The amorphous silicon clusters can be as sensitizer on Er[sup 3+] emission as that of silicon nanocrystals. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies photoluminescence and Raman spectra of silicon nanocrystals prepared by Si ion implantation into SiO2 layers on Si substrate at room temperature. Dependence on annealing temperature; Characterized Raman scattering peak of silicon nanocrystals.

Zn-polar BeMgZnO/ZnO heterostructures grown by molecular beam epitaxy on high resistivity GaN templates producing high-density two-dimensional electron gas (2DEG) are investigated. This is motivated by the need to reach plasmon-longitudinal optical (LO) phonon resonance for attaining minimum LO phonon lifetime. Achievement of high 2DEG concentration in MgZnO/ZnO heterostructures requires growth of the MgZnO barrier at relatively low temperatures, which compromises the ternary quality that in turn hinders potential field effect transistor performance. When this ternary is alloyed further with BeO, the sign of strain in the BeMgZnO barrier on ZnO switches from compressive to tensile, making the piezoelectric and spontaneous polarizations to be additive in the BeMgZnO/ZnO heterostructures much like the Ga-polar AlGaN/GaN heterostructures. As a result, a 2DEG concentration of 1.2 x 1013cm-2 is achieved in the Be0.03Mg0.41Zn0.56O/ZnO heterostructure. For comparison, a 2DEG concentration of 7.7 x 1012cm-2 requires 2% Be and 26% Mg in the barrier, whereas the same in the MgZnO/ZnO system would require incorporation of more than 40% Mg into the barrier, which necessitates very low growth temperatures. Our results are consistent with the demands on achieving short LO phonon lifetimes through plasmon-LO phonon resonance for high carrier velocity. [ABSTRACT FROM AUTHOR]

Graphene has been discovered to have two effects on the photoluminescence (PL) properties of graphene/GeSi quantum dot (QD) hybrid structures, which were formed by covering monolayer graphene sheet on the multilayer ordered GeSi QDs sample surfaces. At the excitation of 488 nm laser line, the hybrid structure had a reduced PL intensity, while at the excitation of 325 nm, it had an enhanced PL intensity. The attenuation in PL intensity can be attributed to the transferring of electrons from the conducting band of GeSi QDs to the graphene sheet. The electron transfer mechanism was confirmed by the time resolved PL measurements. For the PL enhancement, a mechanism called surface-plasmon-polariton (SPP) enhanced absorption mechanism is proposed, in which the excitation of SPP in the graphene is suggested. Due to the resonant excitation of SPP by incident light, the absorption of incident light is much enhanced at the surface region, thus leading to more exciton generation and a PL enhancement in the region. The results may be helpful to provide us a way to improve optical properties of low dimensional surface structures. [ABSTRACT FROM AUTHOR]

We have investigated the variations of optical property and electronic structure in heavily Al-doped ZnO (AZO) films during the growth process, which were formed by first creating Zn vacancies in O2-rich atmosphere and second filling the vacancies with Zn atoms in Zn-vapor atmosphere. After the first step, the high-resistance AZO films have the same optical bandgap with nominally undoped ZnO, indicating that negligible variations in the fundamental bandgap happened to the AZO films although Al atom was incorporated into the ZnO lattice. After the second step, once free electrons were brought into the lattice by Zn-filling, the optical transition energy blueshifts due to the band-filling effect. X-ray absorption fine structure measurements suggest that Zn-filling process decreased the unoccupied states of the conduction band, but not raised the conduction band minimum. [ABSTRACT FROM AUTHOR]

The effect of aluminum doping on the growth of ZnO films on c-plane ZnO (0001) single crystal substrates during metalorganic chemical vapor deposition was investigated. It was found that aluminum doping induces a growth mode of three-dimensional columnar growth. X-ray photoemission spectroscopy demonstrates that partial aluminum is segregated to the growth front. A combined experimental contact angle measurements and theoretical first-principle calculations suggest that the surface energy of the films is promoted by aluminum doping. Besides, aluminum doping also tends to decrease the adatoms diffusion mobility. We conclude that aluminum acts as an antisurfactant element during the homoepitaxial growth, and it increases the difficulty in obtaining high quality n-type ZnO films. [ABSTRACT FROM AUTHOR]

High-quality surface tin-doped hexagonal CdS whiskers were synthesized by a well-controlled in situ source exchange chemical vapor deposition route. Under local light excitation, the detected microphotoluminescence from any positions along the length of these whiskers exhibits strong and multi-mode trap-state whispering-gallery (WG) mode emission. With elevating the pumping power, some of these WG modes start lasing at an ultra-low threshold, with lasing wavelength covering a broad range from ∼540 to ∼750 nm. Calculations using a plane-wave model of WG modes show that all these trap-state lasing modes are transverse magnetic polarized, which was well explained by a two-dimensional finite element simulation. The surface doped semiconductor structures have potential applications as low-threshold tunable micro/nanoscale lasers in optical storage, lighting, and optical communications. [ABSTRACT FROM AUTHOR]

Insufficient hole injection is a major impediment to the luminescence efficiency of III-nitride light-emitting diodes (LEDs). In our previous work by Zhang et al. [Appl. Phys. Lett. 97, 062103 (2010)], high-density mobile three-dimensional hole gas is obtained in Mg-doped Al composition graded AlGaN layer with Al composition linearly decreasing from a certain value to 0. In this paper, it is revealed by a theoretical study that the hole injection efficiency in blue-light GaN-based LEDs can be greatly enhanced by using this polarization-doped method. An increase in the electroluminescence intensity and the internal quantum efficiency in polarization-doped GaN-based LEDs is observed, in comparison with a conventional LED. [ABSTRACT FROM AUTHOR]

Molecular devices are constructed based on a molecule connected into both electrodes with different metal materials, and their transport properties are investigated by the first-principles method. The result shows that such devices can generate two asymmetrical Schottky barriers at contacts; the current rectification thus is created. This rectification is also fully rationalized by the calculated transmission spectra and the spatial distribution of the lowest unoccupied molecular orbital and highest occupied molecular orbital states. Our study suggests that it might be a very important way for both electrodes using different materials to realize a molecular rectification. [ABSTRACT FROM AUTHOR]

Polarization-doping via graded AlGaN layer on N-face [formula] GaN has been demonstrated as an inspiring p-type doping method for wide-band-gap nitrides. However, the polarity of III-nitrides grown by metal organic chemical vapor deposition is metal-face typically. In this paper, we show that three-dimensional mobile hole gas induced by polarization can be formed in (0001)-oriented metal-face III-nitride structure. The hole concentration of a Mg-doped AlxGa1-xN layer with graded Al composition from x=0.3 to 0 grown on AlN buffer layer is remarkably enhanced, compared with that of a Mg-doped GaN layer grown under the same conditions. In addition, the hole concentration in the graded AlGaN layer is absence of freezeout as the temperature decreases, indicating that the hole is induced by polarization. This p-type doping method paves a way for achieving high-efficiency in wide-band-gap semiconductor light-emitting devices with p-type doping problem. [ABSTRACT FROM AUTHOR]

The lifetimes of a series of N-related photoluminescence lines (A2–A6) in GaAs1-xNx (x=0.1%) were studied under hydrostatic pressures at ∼30 K. The lifetimes of A5 and A6 were found to increase rapidly with increasing pressure: from 2.1 ns at 0 GPa to more than 20 ns at 0.92 GPa for A5 and from 3.2 ns at 0.63 GPa to 10.8 ns at 0.92 GPa for A6. The lifetime is found to be closely correlated with the binding energy of the N impurity states, which is shown either in the pressure dependence for a given emission line or in the lifetime variation from A2 to A6. [ABSTRACT FROM AUTHOR]

ZnS:Te epilayers with Te concentration from 0.5% to 3.1% were studied by photoluminescence under hydrostatic pressure at 15 K. Two emission bands related to the isolated Te[sub 1] and Te[sub 2] pair isoelectronic centers were observed in the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only the Te[sub 2]-related peak was observed. The pressure coefficients of all the Te[sub 1]-related bands were found to be unexpectedly much larger than that of the ZnS band gap. The pressure coefficients for all the Te[sub 2]-related bands are, however, rather smaller than that of ZnS band gap as usually observed. Analysis based on a Koster-Slater model indicates that an increase of the valence bandwidth with pressure is the main reason for the faster pressure shift of the Te[sub 1] centers, and the huge difference in the pressure behavior of the Te[sub 1] and Te[sub 2] centers is due mainly to the difference in the pressure-induced enhancement of the impurity potential on the Te[sub 1] and Te[sub 2] centers. [ABSTRACT FROM AUTHOR]

We have investigated the interaction between carriers and polar phonons by using Raman scattering spectroscopy in highly conductive Al-doped ZnO films grown by metalorganic chemical vapor deposition. Different from the longitudinal optical phonon-plasmon coupled modes (LOPPCM) observed in nondegenerate ZnO, an A1(LO)-like mode appears at the low frequency side of the uncoupled A1(LO) mode, and it monotonically shifts to higher frequencies and approaches to the uncoupled A1(LO) mode as Al composition increases. Based on line shape calculations, the A1(LO)-like mode is assigned to the large wave-vector LOPPCM arising from nonconserving scattering dominated by the Al impurity-induced Fröhlich mechanism. Benefiting from the nonmonotonic Al composition dependence of the electron density, it is revealed that the LOPPCM depends mainly on the doping level but not the carrier concentration. [ABSTRACT FROM AUTHOR]

Diamond anvil cell (DAC) technique is widely employed for the high pressure measurements in the field of physics, chemistry, and material research. The new designs of DAC and new techniques for applying pressure have been gradually developed. This paper introduces a piezoelectric driving DAC device to achieve tunable high pressure at low temperature down to 20 K by in situ continuous pressure tuning. The tuning range is about 2-4 GPa depending on the applied electric voltage to piezoelectric lead zirconic titanate ceramic stack. The DAC device is well compatible with the cold chamber of a low temperature cryostat due to its small size and the convenient operation.

Periodic density functional theory calculations have been performed to study the most stable structure of the (WO(3))(3) nanocluster deposited on the MgO(001) surface with three kinds of F(S) centers (F(S)(0), F(S)(+), and F(S)(2+)). Our results indicate that the configuration of (WO(3))(3) cluster, including the cyclic conformation and the heights of three W atoms, and the oxidation states are sensitive to the charge state of the F(S) center. It is interesting that the electron-riched F(S) (0) vacancy on the MgO(001) surface can act as a promoting site to enhance the W-W interaction and the W(3)O(3) cyclic conformation is maintained, while the skeleton of cluster becomes flexible when (WO(3))(3) is adsorbed on the electron-deficient vacancy (F(S)(+) and F(S)(2+)). Accordingly, three F(S)-centers exhibit different arrangements of X-ray photoelectron spectra, the scanning tunneling microscopy images, and the vibrational spectra after depositing (WO(3))(3) cluster. Present results reveal that the (WO(3))(3) cluster may be used as a probe to identify the different F(S) centers on the MgO(001) surface.