Characterisation of defects in p-GaN by admittance spectroscopy

Mg-doped GaN films have been grown on (0 0 0 1) sapphire using metal organic vapour phase epitaxy. Use of different buffer layer strategies caused the threading dislocation density (TDD) in the GaN to be either approximately 2×10⁹ cm⁻² or 1×10¹⁰ cm⁻². Frequency-dependent capacitance and conductance measurements at temperatures up to 450 K have been used to study the electronic states associated with the Mg doping, and to determine how these are affected by the TDD. Admittance spectroscopy of the films finds a single impurity-related acceptor level with an activation energy of 160±10 meV for [Mg] of about 1×10¹⁹ cm⁻³, and 120±10 eV as the Mg precursor flux decreased. This level is thought to be associated with the Mg acceptor state. The TDD has no discernible effect on the trap detected by admittance spectroscopy. We compare these results with cathodoluminescence measurements reported in the literature, which reveal that most threading dislocations are non-radiative recombination centres, and discuss possible reasons why our admittance spectroscopy have not detected electrically active defects associated with threading dislocations.     

Study of silicon-organic interfaces by admittance spectroscopy

An admittance spectroscopy technique has been developed for the interfaces between organic monolayers and silicon. The present work involves the development of an effective equivalent circuit to represent the silicon/organic-monolayer system, and the development of a parameter extraction procedure, which yields the monolayer capacitance and the monolayer thickness, the flat-band voltage, the silicon doping density, the silicon surface potential, the interface trap density, the interface trap capture cross-section and the interface trap energy. This technique was applied to three types of silicon/organic-monolayer system.     

Optical properties of dye-doped zirconia films by spectroscopic ellipsometry

The optical properties of zirconia films doped with rhodamine 6G and oxazine 725 by the sol-gel process were investigated using spectroscopic ellipsometry (SE). Accurate refractive index n and the extinction coefficient k were determined using a three-oscillator classical Lorentz model in the wavelength range of 300-800 nm. The derived refractive index of dye-doped films exhibited anomalous dispersion in the absorption region. Wavelength tunable output lasing action yellow and near-infrared wavelength region was achieved by DFB configuration using zirconia films doped with R6G and oxazine 725.     

富勒烯掺杂NPB空穴传输层的有机电致发光器件

报道了不同掺杂浓度NPB：C₆₀（富勒烯）作为空穴传输层对有机电致发光器件性能的影响.采用真空热蒸镀方法,制作了ITO/ NPB：C₆₀ （x % ）/Alq₃/LiF/Mg:Ag结构的四种有机电致发光器件.当NPB：C₆₀的掺杂浓度是15％时,器件的启亮电压是4 V,最大亮度是11000 cd/m².然而,当NPB：C₆₀的掺杂浓度是20％时,器件的最大亮度降     

Fast optical recording in dye-doped polymer-dispersed liquid-crystal films

We describe a fast optical recording material based on a dye-doped polymer-dispersed liquid-crystal film. A holographic grating is written in this film with a single Q -switched Nd:YAG laser pulse that has a duration of ~6 ns . Such a grating is due to the reorientation effect of the liquid-crystal molecules through interaction with the photoinduced adsorption of the doped azo dyes. Experimental results indicate that the grating thus formed is permanent but electrically switchable.     

Energy levels in doped SiGe quantum well studied by admittance spectroscopy

SiGe/Si quantum wells (QWs) with different Boron doping concentrations were grown by molecular beam epitaxy (MBE) on p-type Si(1 0 0) substrate. The activation energies of the heavily holes in ground states of QWs, which correspond to the energy differences between the heavy hole ground states and Si valence band, were measured by admittance spectroscopy. It is found that the activation energy in a heavily doped QW increases with doping concentration, which can be understood by the band alignment changes due to the doping in the QWs. Also, it is found that the activation energy in a QW with a doping concentration of 2 × 10²⁰ cm⁻³ becomes larger after annealing at a temperature of 685 °C, which is attributed to more Boron atoms activation in the QW by annealing.     

Dynamic studies of holographic gratings in dye-doped liquid-crystal films

The dynamic behavior of a holographic grating induced in a homeotropically aligned dye-doped liquid-crystal film is investigated. In the presence of an applied dc voltage, photoexcited azo dyes induce a photorefractive grating and then diffuse and are adsorbed onto cell substrates. The reorientation of liquid crystals as a result of adsorbed dyes leads to a phase grating that is phase shifted 90 degrees from the photorefractive grating. Competition of these two gratings induces two-beam coupling of the writing beams, initially transferring energy from beam 1 to beam 2 and then, after a pause, from beam 2 to beam 1.     

六方InN薄膜的载流子输运特性研究

关键词：     

Carrier transport characteristics in PbSrSe thin films grown by molecular beam epitaxy

The dark-current characteristics of PbSrSe thin films grown by molecular beam epitaxy on BaF₂ substrates with Sr composition from 0.066 to 0.276 have been measured systematically under different temperatures from 77 K to 300 K. The carrier-transport characteristics have been explained on the basis of a grain-boundary barrier model. The barrier height is found to be strongly related to the Sr composition. The different conductance behavior among the PbSrSe thin films is due to the variation of the grain-boundary barrier. Both the experimental barrier height, determined from the temperature-dependent conductance, and the theoretical results, deduced from the Poisson equation, reveal that the barrier height decreases with increasing applied bias. Furthermore, the success in explaining the observed negative-capacitance phenomenon gives further evidence that the accumulation of electrons at the grain boundaries plays a key role in the carrier transport of the PbSrSe thin films. PACS 72.80.Jc; 73.61.Le; 73.50.Bk     

利用NPB/MoO3/NpB作为空穴传输层的低驱动电压的有机发光器件

通过引入(NPB/MoO3)x/NPB作为空穴传输层,获得了低驱动电压的有机电致发光器件(OLEDs),(NPB/MoO3)x为多层结构(x为0,1和2).通过对比发现,在相同亮度下,x=1对应的器件具有最低的工作电压.这是由于在NPB和MoO3之间产生了电荷转移复合物(charge transfer,CT),这将会降低器件的空穴注入势垒,从而降低其工作电压,文中所研究器件为基于8-羟基喹啉铝(tris(8-hydroxyquino-line)aluminum,Alq3)的绿光器件.与x=0时的普通器件相比,在亮度为1 000 cd·m-2时,x=1时的工作电压降低了 0.8 V.     

Highly-efficient solution-processed phosphorescent multi-layer organic light-emitting diodes investigated by electromodulation spectroscopy

We report on electromodulation (EM) spectroscopy studies of phosphorescent multi-layer organic light-emitting diodes (OLEDs) that are processed from solution. Compared to conventional single-layer OLEDs, they comprise an additional layer of a crosslinkable, oxetane-functionalized triphenylamine-dimer (XTPD) that is inserted between the PEDOT:PSS anode and the emissive layer. Devices with optimized stack architecture feature reduced operating voltages and reach a current efficiency approaching 40 cd/A—twice as much as the corresponding single-layer device. Using EM measurements, we quantify the electric field in the XTPD layer and the emissive layer of such a multi-layer OLED and also measure the average electric field in a single-layer reference device. By comparing the dependence of the internal field on the applied voltage for devices with and without the XTPD layer, we find that in the device containing the XTPD layer there is an increased accumulation of electrons at the anode side of the emissive layer. This accumulation enhances the recombination probability and supports the injection of holes into the emissive layer which explains the observed efficiency improvement and reduction in operating voltage compared to conventional single-layer OLEDs.     

Femtosecond photon echo in dye-doped polymer films at room temperature

The signals of primary and stimulated femtosecond photon echo in polymer films doped with dye (phthalocyanine) molecules have been experimentally investigated at room temperature. A femtosecond echo spectrometer for these echo experiments is described. The decay curves of echo signals with increasing time intervals between excitation femtosecond pulses are obtained and blue shifts of the spectra of femtosecond echo signals with respect to the spectrum of excitation pulses are revealed. The possibilities of using the studied doped polymer films as recording media for high-temperature echo processors and coolants in optical refrigerators are analyzed.     

Laser-induced reorientation effect and ripple structure in dye-doped liquid-crystal films

The effects of light-induced reorientation on a homeotropical dye-doped liquid crystal (DDLC) cell are discussed. The photoexcited azo dye Methyl Red (MR) is diffused and adsorbed onto the substrate, thus forming a ripple structure. The adsorbed dye and the laser-induced ripple structure then reorient the liquid-crystal molecules and induce a holographic grating. Initially, the liquid-crystal directors are reoriented primarily by the adsorbed dye. However, given a sufficiently large ripple groove amplitude, the torque imposed by the ripple grooves overcomes that which is due to the adsorbed dyes, and the liquid crystals are realigned along the groove direction.     

Shallow levels in virgin hydrothermally grown n-type ZnO studied by thermal admittance spectroscopy

Three n-type single crystal hydrothermally grown ZnO samples with resistivities of 5.1±0.6, 15±2 and 220±20 Ω cm, respectively, have been electrically characterized using thermal admittance spectroscopy (TAS). The presence of three main donors: two shallow ones D₁ and D₂ and a deeper one D₃ with activation energies of ∼30, ∼50 and ∼290 meV, respectively, are detected. In addition, the TAS spectra reveal the presence of a fourth level, D_X, with a peak amplitude in the conductance spectra that decreases with the temperature occurrence. It is shown that this anomalous behavior is consistent with D_X being a negative-U defect of donor-type. An activation energy of ∼80 meV for the ++/+ transition, a capture cross section equal to ∼3×10⁻¹⁷ cm² and an energy barrier for atomic reconfiguration of ∼0.25 eV, respectively, deduced according to the assignment of D_X to a negative-U defect. A tentative assignment of the D_X defect with oxygen vacancies is discussed.     

Mechanism of ion transport in YSZ-dispersed polymer nanocomposite films: an analysis by vibrational spectroscopy approach

We report a novel and interesting finding on nanoceramic filler-induced symmetry lowering of ClO ₄ ^? anion from \( {T_d} \leftrightarrow {C_{2\nu }} \) in polymer nanocomposite films, based on poly(ethylene oxide)₈–LiClO₄?+?x wt.% nano-Yttria-stabilized zirconia (YSZ) in sharp contrast to the lowering pattern from \( {T_{\text{d}}} \leftrightarrow {C_{{3}\nu }} \) in the polymer–salt complex (undispersed) film. This is evidenced by a systematic absence of E ₁ mode in YSZ-dispersed nanocomposite films. Based on the experimental evidences from Fourier transform infrared, conductivity, and transmission electron microscopy analysis, an ion conduction mechanism has been proposed to probe the interaction prevailing among various components of the nanocomposite film. An explanation for the concentration dependence of ion transport phenomena has been proposed in terms of filler–ion–polymer interaction. The central idea of the proposed ion transport mechanism lies in a practically feasible Lewis acid–base type interaction between dipolar nanofiller having Lewis acid characteristics with the anions (ClO ₄ ^? ) and ether oxygen sites of the host polymer acting as the Lewis base.     

Hole transport in solid xenon

The hole mobility μ_η in solid Xe has been investigated by drift mobility techniques in the temperature range from 110K to 160K. μ_η = 1.8 × 10⁻²cm²Sec⁻¹V⁻¹ at 160K and has a temperature dependence of the form μ_η ∞ T^−1.6. It is concluded that a non-adiabatic hopping transport with a small polaron binding energy and small intermolecular overlap is the most likely transport mechanism.     

染料掺杂液晶可调谐光纤荧光光源的研究

本文提出了染料掺杂液晶填充空心光纤构造荧光可调谐光源.基于染料分子能级结构理论分析B4400荧光光谱依赖温度的变化特性,采用脉宽8 ns,波长为532 nm YAG倍频脉冲激光器抽运,向列相液晶作基体,实验分析染料B4400掺杂液晶填充空心光纤荧光光谱选择性荧光放大规律及温度调谐特性.结果表明:通过控制染料浓度可控制荧光输出功率水平;当温度升高时,中心波长发生红移,中心波长调谐范围为590—605 nm;荧光谱宽呈单调展宽,调制范围为228—236 nm;染料掺杂液晶填充空心光纤荧光光源可实现一定范围内的温度调谐.     

Investigating the charge relaxation in semiconductor heterostructures with quantum wells by means of admittance spectroscopy

Charge is evaluated as a function of temperature, based on the analysis and nmathematical processing of experimental capacitance-voltage characteristics obtained in frequency range of 1 kHz to 1 MHz and temperature range of 320 to 10 K for samples of InGaAs/GaAs heterostructures with quantum wells (QWs) and ultrathin InAs/GaAs wetting layers specially prepared for admittance measurements. It is found that the charge in the QW determined by the apparent concentration profiles from experimental C-V characteristics monotonically increases compared with the charge in quantum wells.     

Tailoring of random lasing characteristics in dye-doped nematic liquid crystals

Amplified spontaneous emission and random lasing are investigated in random systems with dye-doped nematic liquid crystals. And that temporal stability of random lasing is analyzed. The influence of pumping polarization as well as the multiple scattering and reflection between boundaries on the emission behavior and the formation of coherent feedback is investigated in detail. For freely suspended samples, certain emission wavelength can be obtained by changing the pump wavelength. This feature is useful in making wavelength-tunable lasers. Moreover, as the pumping thickness of wedge sample increases, the emission spectrum is red shifted and the average spacing of adjacent spikes decreases. This property can be applied in laser mode selection, i.e., the number of modes within certain wavelength range can be chosen.     

Dynamics in thin polymer films by dielectric spectroscopy

After a brief review on the recent developments of the dielectric studies in thin polymer films, our recent results on dynamics in thin films of poly(methyl methacrylate) (PMMA) and polyisoprene (PIP) are shown. For PMMA, the tacticity effect on dynamics in thin films has been investigated and the disappearance of this effect was found below a critical thickness. For PIP, the motion of entire polymer chains, namely, the normal mode, has been investigated. The dielectric-loss spectrum of the normal mode is much more sensitive to the decrease in film thickness than that due to the -process. The broadening of dielectric-loss spectra of the normal mode is observed for film thicknesses below about 150 nm, while the position of the loss peak does not change in the thickness range down to about 50 nm. Anomalous increase in dielectric loss between the -process and the normal mode was observed, which is consistent with the recent report on the existence of an additional relaxation process.Received: 1 January 2003, Published online: 14 October 2003PACS: 64.70.Pf Glass transitions - 68.60.-p Physical properties of thin films, nonelectronic