CaCu3Ti4O12 陶瓷松弛损耗机理研究

CaCu₃Ti₄O₁₂介电损耗较大且损耗机理尚不明确, 因此限制了其应用.本文采用固相法和共沉淀法合成CaCu₃Ti₄O₁₂陶瓷, 利用宽带介电温谱研究在交流小信号作用下, 双Schottky势垒耗尽层边缘深陷阱的电子松弛过程、 载流子松弛过程以及CaCu₃Ti₄O₁₂陶瓷的介电损耗性能. 研究发现, 在低频下以跳跃电导和直流电导的响应为主, 而高频下主要为深陷阱能级的松弛过程所致, 特别是活化能为0.12 eV的深陷阱浓度, 这是决定CaCu₃Ti₄O₁₂陶瓷高频区介电损耗的重要因素.降低直流电导, 有利于降低低频区介电损耗; 而高频区介电损耗的降低, 需要降低深陷阱浓度或增大晶粒尺寸. 共沉淀法制备的CaCu₃Ti₄O₁₂陶瓷, 有效降低直流电导及控制深陷阱浓度, 介电损耗降低明显. 关键词： 3Ti₄O₁₂陶瓷')" href="#">CaCu₃Ti₄O₁₂陶瓷 介电损耗 松弛过程 Schottky势垒     

Dielectric spectroscopy studies of ZnO single crystal

Dielectric relaxation and charge transport induced by electron hopping in ZnO single crystal are measured by using a novocontrol broadband dielectric spectrometer. Typical Debye-like dielectric relaxation originating from electronic hopping between electronic traps and conductive band in surface Schottky barrier region is observed for ZnO single crystal-Au electrode system. However, after insulation of ZnO single crystal by heat treatment in rich oxygen atmosphere, dielectric relaxation and alternating current conductance are observed simultaneously in the dielectric spectra, implying that dielectric relaxation and charge transport can be induced simultaneously by electronic hopping at high temperature in an ordered system. The intrinsic correlation between local dielectric relaxation and long range charge transport offers us a new method to explore complicated dielectrics.     

Photoresponse and trap characteristics of transparent AZO-gated AlGaN/GaN HEMT

AZO-gated and Ni/Au-gated AlGaN/GaN HEMTs are fabricated successfully,and an excellent transparency of AZOgated electrode is achieved.After a negative gate bias stress acts on two kinds of the devices,their photoresponse characteristics are investigated by using laser sources with different wavelengths.The effect of photoresponse on AZO-gated electrode device is more obvious than on Ni/Au-gated electrodes device.The electrons are trapped in the AlGaN barrier of AZO-gated HEMT after it has experienced negative gate bias stress,and then the electrons can be excited effectively after it has been illuminated by the light with certain wavelengths.Furthermore,the trap state density D_T and the time constantτ_T of the AZO-gated Schottky contact are extracted by fitting the measured parallel conductance in a frequency range from10 kHz to 10 MHz.The constants of the trap range from about 0.35 μs to 20.35 μs,and the trap state density increased from1.93×l0~(13)eV 1·cm~2 at an energy of 0.33 eV to 3.07×10~(11) eV~1·cm~2 at an energy of 0.40 eV.Moreover,the capacitance and conductance measurements are used to characterize the trapping effects under different illumination conditions in AZO-gated HEMTs.Reduced deep trap states' density is confirmed under the illumination of short wavelength light.     

Swift heavy ion induced capacitance and dielectric properties of Ni/n-GaAs Schottky diode

The in-situ capacitance and dielectric properties of 25 MeV C⁴⁺ ion irradiated Ni/n-GaAs Schottky barrier diode (SBD) were studied at 100 kHz in the fluence range 5 × 10¹⁰ – 5 × 10¹³ ions/cm². The investigation shows reduction in capacitance and charge density with increase in ion fluence. Consequent changes were observed in other related parameters like conductance, dielectric constant, dielectric loss, loss tangent and electrical modulus. The results were interpreted in terms of generation of swift heavy ion induced acceptor trap states by electronic energy loss mechanism. Besides, the switch over characteristics of depletion to inversion regions in the CV plot reveals minority carrier recombination centers also. The dispersion and relaxation peaks observed in bias dependent dielectric plots were ascribed to the polarization and relaxation mechanism due to the interfacial trap states. The traps and recombination centers were found to alter the barrier characteristics of the fabricated SBD depending upon the ion fluence.     

Near-surface and bulk deep trap states in AlxGa1-xAs/GaAs

We have measured the field-effect deep-level transient spectra of Al_xGa_1-xAs/GaAs (where x=0.385) at different reverse-bias fields to probe the near-surface deep trap and bulk deep trap states. In the temperature range 77 to 380 K and for a reverse-bias field -1 to -5 V/cm, four major deep traps were identified. The results of our investigation indicate a distinct effect on the deep-level spectra. Three of the deep trap states E1, E2 and E3 showed definite peak enhancement with the applied reverse-bias field and were identified as bulk deep trap states. The fourth deep trap state E4 was a very weak deep trap state and it showed a decrease of the peak height with the applied reverse-bias field. It was labeled as a near-surface deep trap state. PACS 73.60.Fw     

ZnO压敏陶瓷的介电谱

在-160℃-200℃温度范围内、0.1 Hz-0.1 MHz频率范围内测量了 ZnO压敏陶瓷的介电频谱, 发现可以采用电导率谱低频端的类直流特性来表征晶界Schottky势垒的电子输运过程, 获得的Schottky势垒高度为0.77 eV. 基于背靠背双Schottky势垒模型, 提出当存在直流偏压时, 势垒高度将随直流偏压线性增大. 基于此势垒模型计算了ZnO压敏陶瓷单晶界的直流偏压大小, 进而计算出晶粒平均尺寸为6.8 μm, 该理论值与通过扫描电子显微镜断面照片获得的测量值的偏差在5%以内. 可见采用介电谱不但可以获得势垒高度实现电气性能的表征, 还能获得晶粒尺寸实现显微结构的表征.     

直流老化对CaCu3Ti4O12陶瓷介电性能的影响

在电场为3.5 kV/cm的条件下, 对CaCu₃Ti₄O₁₂陶瓷进行了60 h的直流老化, 研究了老化过程对CaCu₃Ti₄O₁₂陶瓷介电性能和电气特性的影响. J-E特性测试结果表明, 直流老化导致CaCu₃Ti₄O₁₂陶瓷击穿场强、非线性系数和势垒高度明显降低. 介电性能测试结果表明, 低频介电常数和介电损耗明显增大, 并且介电损耗随频率的变化遵从Debye弛豫理论, 可分解为直流电导损耗和弛豫损耗, 直流老化主要导致了电导损耗的增加. 在低温233 K, 介电损耗谱中出现两个弛豫峰, 其活化能分别为0.10, 0.50 eV, 认为对应着晶粒和畴界的弛豫过程, 且不随直流老化而变化. 通过电模量谱对CaCu₃Ti₄O₁₂陶瓷的弛豫过程进行了表征, 发现直流老化导致的界面空间电荷在外施交变电场的作用下符合Maxwell-Wagner极化效应, 并在低频区形成新的弛豫峰. 在高温323-473 K的阻抗谱中, 晶界弛豫峰在直流老化后明显向高频移动, 其对应的活化能从1.23 eV 下降到0.72 eV, 晶界阻抗值下降了约两个数量级. 最后, 建立了CaCu₃Ti₄O₁₂陶瓷的阻容电路模型, 分析了介电弛豫过程与电性能之间的关联.     

Physical meaning of conductivity spectra for ZnO ceramics

With the help of broadband dielectric spectroscopy in a wide temperature and frequency range, the conductivity spectra of ZnO polycrystalline ceramics are measured and the direct-current-like (DC-like) conductivity and relaxation polarization conductivity are observed successively along the frequency axis. According to the classical Debye theory and Cole-Cole equation, the physical meanings of the two conductivities are discussed. It is found that the DC-like conductivity corresponds to electron transportation over the Schottky barrier at the grainboundary. The relaxation polarization conductivity corresponds to electronic trap relaxation of intrinsic point defects (zinc interstitial and oxygen vacancy). When in the high frequency region, the relaxation conductivity obeys the universal law with the index n equal to the index α in the Cole-Cole equation as an indictor of disorder degree.     

ZnO压敏陶瓷介电损耗的温度谱研究

利用Novocontrol宽频介电谱仪在-100—20 ℃温度范围内测量了ZnO-Bi₂O₃系压敏陶瓷的介电频谱,其频率范围为10^-2—10⁶ Hz. 研究表明： ZnO压敏陶瓷特征损耗峰的活化能分别为0.26和0.36 eV,结合实验条件、理论计算结果及其他现象的分析排除了特征损耗峰源于阴极电子注入、夹层极化和偶极子转向极化的可能.热刺激电流（TSC）谱共出现三个峰,其中高温峰对应于TSC实验加压过程引入的热离子极化,而中温峰和低温峰对应于介电损耗峰. 在分析的基础上,提出了ZnO压敏陶瓷的特征损耗峰起源于耗尽层内本征缺陷的电子弛豫过程. 关键词： ZnO压敏陶瓷 本征缺陷 介电谱 热刺激电流     

Electrical conductivity of NaCl ¦ graphite interface

It was observed that a steady d. c. current flows through the cell graphite ¦NaCl¦ graphite even at voltages very small compared to the decomposition potential difference. The activation enthalpy of the small signal conductance (1·20±0·05) eV differs from the enthalpy of the bulk conductivity. The interface conductance is neither in simple relation to the conductivity of the crystal, nor to the partial conductivities of anions and cations. The nonlinearity of the cell and the independence of interface conductance from some properties of the bulk can be explained by the finite rate of electrochemical reactions, ensuring the charge transfer across the interfaces.     

单层有机器件的电子传输特性的数值模拟

在漂移扩散模型的基础上建立了单层有机器件的模型,包括了电荷注入、传输、空间电荷效应和陷阱的影响.电荷注入考虑了热电子发射电流和隧道电流.模拟得到的结果和文献中报道的实验测试数据一致.模拟研究了各个因素对器件J-V曲线的影响,电流和器件长度成反比,电流随着空穴注入势垒的减小而增加.电子注入势垒从1.7 eV减少到0.5 eV时,电流随着电子注入势垒的减小而减小,这主要是因为有机材料中电子迁移率太小,电子注入电流的增加可以忽略,而电子注入势垒的减小使内建势增加,在同样的电压下,场强 关键词： 有机器件 传输特性 数值模拟     

巨介电常数氧化物CaCu3Ti4O12的介电和阻抗特性

采用固相烧结法合成了单相巨介电常数氧化物CaCu₃Ti₄O₁₂（CCTO）.用阻抗分析仪分析了10—420 K温度范围内的介电频谱和阻抗谱特性,并结合ZVIEW软件进行了模拟.结果表明：温度高于室温时,频谱出现两个明显的弛豫台阶,低频弛豫介电常数随温度升高而显著增大,表现出热离子极化特点;温度低于室温时,频谱表现出类德拜弛豫,且高、低平台介电常数值基本不随温度变化,表现出界面极化特点和较好的温度稳定性.频谱中依次出现的介电弛豫对应于阻抗谱中 关键词： 3Ti₄O₁₂')" href="#">CaCu₃Ti₄O₁₂ 介电频谱 阻抗谱 Cole-Cole半圆弧     

Double‐layer channel structure based ZnO thin‐film transistor grown by atomic layer deposition

A double channel structure has been used by depositing a thin amorphous‐AlZnO (a‐AZO) layer grown by atomic layer deposition between a ZnO channel and a gate dielectric to enhance the electrical stability. The effect of the a‐AZO layer on the electrical stability of a‐AZO/ZnO thin‐film transistors (TFTs) has been investigated under positive gate bias and temperature stress test. The use of the a‐AZO layer with 5 nm thickness resulted in enhanced subthreshold swing and decreased V_th shift under positive gate bias/temperature stress. In addition, the falling rate of the oxide TFT using a‐AZO/ ZnO double channel had a larger value (0.35 eV/V) than that of pure ZnO TFT (0.24 eV/V). These results suggest that the interface trap density between dielectric and channel was reduced by inserting a‐AZO layer at the interface between the channel and the gate insulator, compared with pure ZnO channel. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetics of Relaxation of Stored Light Sums in Silver Chloride

Experimental investigation of the time and temperature dependences of the process of relaxation of excited silver chloride has been carried out by the method of a photostimulated burst of luminescence. It is found that the relaxation process can be divided into two components, the rates of which differ by an order of magnitude. The activation energies of these processes of 0.03 and 0.06 eV correspond to the thermal ionization energies of electrons from shallow traps. Thermally activated relocalization of charges from shallow to deep traps in the process of relaxation has been revealed. The depth of a corresponding trap appeared equal to 0.17 eV. A model of a crystal phosphor has been suggested that contains five levels in the forbidden band: a hole center of recombination (luminescence), a deep electron trap, two shallow electron traps located at a depth of 0.03 and 0.06 eV from the bottom of the conductivity band, and a hole trap located at a height of 0.17 eV from the valence band top. Within the framework of this model, we can qualitatively explain the time and temperature dependences of stored light sums. We have shown the possibility of applying the method of a photostimulated burst of luminescence to investigation of the processes of relaxation of electron excitations in crystals to establish the relaxation mechanisms.     

Mechanisms of asymmetric leakage current in Pt/Ba0.6Sr0.4TiO3/Nb-SrTiO3 capacitor

Mechanisms of leakage current have been investigated in the capacitor consisting of a Ba_0.6Sr_0.4TiO₃ thin film, a Pt top electrode, and a Nb-doped SrTiO₃ (STON) bottom electrode. The leakage current shows asymmetric behavior for different bias voltage. For the Pt electrode negatively biased, the leakage current can be explained by modified Schottky emission mechanism, and the barrier height is obtained as 0.44 eV. For the Pt electrode positively biased, the leakage current shows a space-charge-limited current behavior. The trap in dielectric film is regarded as deep traps, and the density of trapped carrier is estimated as about 3.2×10²³/m³. PACS 77     

The electronic and chemical structure of the a-B(3)CO(0.5):H(y)-to-metal interface from photoemission spectroscopy: implications for Schottky barrier heights

The electronic and chemical structure of the metal-to-semiconductor interface was studied by photoemission spectroscopy for evaporated Cr, Ti, Al and Cu overlayers on sputter-cleaned as-deposited and thermally treated thin films of amorphous hydrogenated boron carbide (a-B(x)C:H(y)) grown by plasma-enhanced chemical vapor deposition. The films were found to contain ～10% oxygen in the bulk and to have approximate bulk stoichiometries of a-B(3)CO(0.5):H(y). Measured work functions of 4.7/4.5?eV and valence band maxima to Fermi level energy gaps of 0.80/0.66?eV for the films (as-deposited/thermally treated) led to predicted Schottky barrier heights of 1.0/0.7?eV for Cr, 1.2/0.9?eV for Ti, 1.2/0.9?eV for Al, and 0.9/0.6?eV for Cu. The Cr interface was found to contain a thick partial metal oxide layer, dominated by the wide-bandgap semiconductor Cr(2)O(3), expected to lead to an increased Schottky barrier at the junction and the formation of a space-charge region in the a-B(3)CO(0.5):H (y) layer. Analysis of the Ti interface revealed a thick layer of metal oxide, comprising metallic TiO and Ti (2)O (3), expected to decrease the barrier height. A thinner, insulating Al(2)O(3) layer was observed at the Al-to-a-B(3)CO(0.5):H(y) interface, expected to lead to tunnel junction behavior. Finally, no metal oxides or other new chemical species were evident at the Cu-to-a-B(3)CO(0.5):H(y) interface in either the core level or valence band photoemission spectra, wherein characteristic metallic Cu features were observed at very thin overlayer coverages. These results highlight the importance of thin-film bulk oxygen content on the metal-to-semiconductor junction character as well as the use of Cu as a potential Ohmic contact material for amorphous hydrogenated boron carbide semiconductor devices such as high-efficiency direct-conversion solid-state neutron detectors.     

Phase transitions in CuBiP2Se6 crystals

Investigation results of dielectric (20?Hz–1?MHz) properties of layered CuBiP₂Se₆ crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at T _c?=?136?K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473?K (0.21?eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1?K (0.045?eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics.     

直流老化及热处理对ZnO压敏陶瓷缺陷结构的影响

在电场为3.2 kV/cm, 电流密度为50 mA/cm²条件下对ZnO压敏陶瓷进行了115 h的直流老化, 研究了直流老化对ZnO压敏陶瓷电气性能及缺陷结构的影响. 发现直流老化115 h 后ZnO压敏陶瓷的电位梯度、非线性系数分别从2845 V/cm, 38.3下降到51.6 V/cm, 1.1, 介电损耗中的缺陷松弛峰被增大的直流电导掩盖, 电模量中只观察到一个缺陷松弛峰, 低频区交流电导率急剧增大并且相应的电导活化能从0.84 eV下降到只有0.083 eV. 通过对直流老化后的ZnO压敏陶瓷在800 ℃进行12 h 的热处理, 发现其电气性能和介电性能都得到了良好的恢复并有一定的增强, 电位梯度、非线性系数恢复到3085 V/cm, 50.8, 电导活化能上升到0.88 eV. 另外, 其本征氧空位缺陷松弛峰也得到了一定的抑制. 因此, 认为热处理过程中氧在晶界处的扩散作用对ZnO压敏陶瓷的直流老化恢复起到了关键作用. 关键词： ZnO压敏陶瓷 介电性能 直流老化 热处理     

Electrical properties and conduction mechanism of an organic-modified Au/NiPc/n-InP Schottky barrier diode

The effect of nickel phthalocyanine (NiPc) organic interlayer on the electronic parameters of Au/n-InP Schottky contacts has been investigated using current–voltage (I–V) and capacitance–voltage (C–V) measurements. Measurements showed that the barrier heights and ideality factors are 0.58 eV (I–V), 0.69 eV(C–V) and 1.32 for Au/n-InP Schottky contact and 0.80 eV (I–V), 1.12 eV (C–V) and 1.73 for Au/NiPc/n-InP Schottky contact, respectively. Experimental results show that the interfacial layer of NiPc increases the effective barrier height by the influence of the space charge region of the Au/n-InP Schottky junction. Further, Cheung’s and modified Norde functions are used to extract the barrier height, series resistance and ideality factors. The discrepancy between barrier heights estimated from I–V to C–V methods is also explained. Moreover, the energy distribution of interface state density is determined from the forward bias I–V data. Results show that the interface states and series resistance play an important role on electrical properties of the structures studied. The reverse leakage current conduction mechanism is investigated. Results reveal that the Schottky conduction mechanism is found to be dominant in the Au/n-InP Schottky contact. However, in the case of Au/NiPc/n-InP Schottky contact, the Schottky conduction mechanism is found to be dominant in the higher bias region, while Poole–Frenkel conduction is found to be dominant in the lower bias region.     

Emergence and visualization of an interface state during contact formation with a single molecule

Contact formation dynamics and electronic perturbations arising from the interaction of a metallic probe and a single molecule (1,3 cyclohexadiene) bound on the Si (100) surface are examined using a series of plane wave, density functional theory calculations. The approach of the probe induces a relaxation of the molecule that ultimately leads to the formation of an interface state due to a specific interaction between the probe apex atom and the C=C bond of the molecule. The calculated interface state is located 0.2 eV above the Fermi energy, in agreement with low temperature scanning tunneling spectroscopy local density of states data (0.35 eV), and is responsible for the contrast observed in low bias empty-state STM images.