Charge storage and electron/light emission properties of silicon nanocrystals

Monodispersed silicon nanocrystals show novel electrical and optical characteristics of silicon quantum dots, such as single-electron tunneling, ballistic electron transport, visible photoluminescence and high-efficiency electron emission.Single-electron memory effects have been studied using a short-channel MOSFET incorporating Si quantum dots as a floating gate. Surface nitridation of Si nanocrystal memory nodes extends the charge-retention time significantly. Single-electron storage in individual Si dots has been evaluated by Kelvin probe force microscopy.Photoluminescence and electron emission are observed for surface-oxidized silicon nanocrystals. Efficiency of the no-phonon-assisted transition increases with decreasing core Si size. Electron emission efficiency as high as 5% has been achieved for the Si-nanocrystal-based cold electron emitter devices. The non-Maxwellian energy distribution of emitted electrons suggests that the mechanism of electron emission is due to ballistic transport through arrays of surface-oxidized Si nanocrystals. Combined with the ballistic electron emission, the quasi-direct light emission properties can be used for developing Si-based lasers.     

硅衬底InGaN/GaN基蓝光发光二极管droop效应的研究

InGaN/GaN基阱垒结构LED当注入的电流密度较大时, LED的量子效率随注入电流密度增大而下降, 即droop效应.本文在Si (111)衬底上生长了 InGaN/GaN 基蓝光多量子阱结构的LED,通过将实验测量的光电性能曲线与利用ABC模型模拟的结果进行对比, 探讨了droop效应的成因.结果显示:温度下降会阻碍电流扩展和降低空穴浓度, 电子在阱中分布会越来越不平衡,阱中局部区域中因填充了势能越来越高的电子而溢出阱外, 从而使droop效应随着温度的降低在更小的电流密度下出现且更为严重, 不同温度下实验值与俄歇复合模型模拟的结果在高注入时趋势相反.这此结果表明,引起 droop效应的主因不是俄歇非辐射复合而是电子溢出,电子溢出的本质原因是载流子在阱中分布不均衡.     

Morphological properties of AlN and GaN grown by MOVPE on porous Si(111) and Si(111) substrates

Metal Organic Vapour Phase Epitaxy (MOVPE) of AlN and GaN layers at a temperature of 1080 C were performed on porous Si(111) and Si(111) substrates. The thermal stability of porous silicon (PS) is studied versus growth time under AlN and GaN growth conditions. The surface morphology evolution of the annealed PS is revealed by scanning electron microscopy (SEM). Porous Si(111) with low porosity (40%) is more thermally stable than porous Si(100) with relatively high porosity (60%).AlN layers with various thicknesses were grown under the same conditions on the two substrates. Morphological properties of AlN were studied by atomic force microscopy (AFM) and compared taking into account the two different surfaces of the substrates. The two growth kinetics of AlN were found to be different due to the initial surface roughness of the PS substrate. The effect of AlN buffer morphology on the qualities of subsequent GaN layers is discussed. Morphological qualities of GaN layers grown on PS are improved compared to those obtained on porous Si(100) but are still less than those grown on Si substrate.     

电介质/半导体结构样品电子束感生电流瞬态特性

电子束照射下电介质/半导体样品的电子束感生电流(electron beam induced current,EBIC)是其电子显微检测的重要手段.结合数值模拟和实验测量,研究了高能电子束辐照下SiO2/Si薄膜的瞬态EBIC特性.基于Rutherford模型和快二次电子模型研究电子的散射过程,基于电流连续性方程计算电荷的输运、俘获和复合过程,获得了电荷分布、EBIC和透射电流瞬态特性以及束能和束流对它们的影响.结果表明,由于电子散射效应,自由电子密度沿入射方向逐渐减小.由于二次电子出射,净电荷密度呈现近表面为正、内部为负的特性,空间电场在表面附近为正而在样品内部为负,导致一些电子输运到基底以及一些出射二次电子返回表面.SiO2与Si界面处俘获电子导致界面附近负电荷密度高于周围区域.随电子束照射样品内部净电荷密度逐渐降低,带电强度减弱.同时,负电荷逐渐向基底输运,EBIC和样品电流逐渐增大,电场强度逐渐减小.由于样品带电强度较弱,表面出射电流和透射电流随照射基本保持恒定.EBIC、透射电流及表面出射电流均随束流呈现近似正比例关系.对于本文SiO2/Si薄膜,透射电流随束能的升高逐渐增大并接近于束流值,EBIC在束能约15 keV时呈现极大值.     

Ti/4H-SiC肖特基势垒二极管抗辐射特性的研究

本文采用γ射线、高能电子和中子对Ti/4H-SiC肖特基势垒二极管(SBD)的抗辐射特性进行了研究.研究发现对于γ射线和1 MeV电子辐照,-30 V辐照偏压对器件的辐照效应没有明显的影响.经过1 Mrad(Si)的γ射线或者1×l0¹³ n/cm²的中子辐照后,Ti/4H-SiC肖特基接触都没有明显退化;经过3.43×10¹⁴ e/cm²的1 MeV电子辐照后Ti/4H-SiC的势垒高度比辐照前轻微下降,这是由于高能 关键词： 碳化硅 肖特基 辐照 偏压     

Si quantum dot structures and their applications

This paper presents briefly the history of emission study in Si quantum dots (QDs) in the last two decades. Stable light emission of Si QDs and NCs was observed in the spectral ranges: blue, green, orange, red and infrared. These PL bands were attributed to the exciton recombination in Si QDs, to the carrier recombination through defects inside of Si NCs or via oxide related defects at the Si/SiOx interface. The analysis of recombination transitions and the different ways of the emission stimulation in Si QD structures, related to the element variation for the passivation of surface dangling bonds, as well as the plasmon induced emission and rare earth impurity activation, have been presented.The different applications of Si QD structures in quantum electronics, such as: Si QD light emitting diodes, Si QD single union and tandem solar cells, Si QD memory structures, Si QD based one electron devices and double QD structures for spintronics, have been discussed as well. Note the significant worldwide interest directed toward the silicon-based light emission for integrated optoelectronics is related to the complementary metal-oxide semiconductor compatibility and the possibility to be monolithically integrated with very large scale integrated (VLSI) circuits. The different features of poly-, micro- and nanocrystalline silicon for solar cells, that is a mixture of both amorphous and crystalline phases, such as the silicon NCs or QDs embedded in a α-Si:H matrix, as well as the thin film 2-cell or 3-cell tandem solar cells based on Si QD structures have been discussed as well. Silicon NC based structures for non-volatile memory purposes, the recent studies of Si QD base single electron devices and the single electron occupation of QDs as an important component to the measurement and manipulation of spins in quantum information processing have been analyzed as well.     

Photoluminescence of ZnO films prepared by r.f. sputtering on different substrates

Polycrystalline ZnO films with good orientation were deposited on sapphire, quartz, Si and 7059 glass substrates by r.f. magnetron sputtering. A strong UV photoluminescence (PL) peak (located at 356 nm) and a weak blue emission peak (located at 446 nm) were observed at room temperature (RT) for the films deposited on sapphire, quartz and Si substrates when excited with 270 nm light. For the films prepared on Corning 7059 glass, only a strong 446 nm blue emission peak was found, and the PL intensity decreased with increasing oxygen pressure during films deposition. The intensity of the UV emission increased 7 and 14 times, respectively, for the films on sapphire and quartz substrates after high temperature annealing in vacuum. The UV emission originates from the inter-band transition of electrons and the blue emission is due to transition of electrons from the shallow donor level of the oxygen vacancies to the valence band.     

Novel Field Emission Organic Light Emitting Diodes with Dynode

This work presents novel field emission organic light emitting diodes(FEOLEDs) with dynode,in which an organic EL light-emitting layer is used instead of an inorganic phosphor thin film in the field emission display(FED).The proposed FEOLEDs introduce field emission electrons into organic light emitting diodes(OLEDs),which exhibit a higher luminous efficiency than conventional OLED.The field emission electrons emitted from the carbon nanotubes(CNTs) cathode and to be amplified by impact the dynode in vacuum.These field emission electrons are injected into the multi-layer organic materials of OLED to increase the electron density.Additionally,the proposed FEOLED increase the luminance of OLED from 10 820 cd/m2 to 24 782 cd/m2 by raising the current density of OLED from an external electron source.The role of FEOLED is to add the quantity of electrons-holes pairs in OLED,which increase the exciton and further increase the luminous efficiency of OLED.Under the same operating current density,the FEOLED exhibits a higher luminous efficiency than that of OLED.     

Computer simulation of the small-signal admittance and negative resistance of double avalanche region IMPATT diodes

A generalized small-signal computer simulation of double avalanche region (DAR) n ⁺-p-v-n-p ⁺ Si and InP IMPATT diodes has been carried out for different frequencies and current densities taking both drift and diffusion of charge carriers into account. The simulation results show that both symmetrically and asymmetrically doped devices based on Si and InP exhibit discrete negative conductance frequency bands separated by positive conductance frequency bands. The magnitudes of both negative conductance and negative resistance of InP devices are larger than those of Si devices in case of symmetrical and asymmetrical diodes. Further, the negative resistance profiles in the depletion layer of these diodes exhibit a single peak in the middle of the drift layer in contrast to double peaks in double drift region diodes.     

Theoretical analysis of semi/non-polar In GaN/GaN light-emitting diodes grown on silicon substrates

A theoretical study of polar and semi/non-polar In Ga N/Ga N light-emitting diodes(LEDs) with different internal surface polarization charges, which can be grown on Si substrates, is conducted by using APSYS software. In comparison with polar structure LEDs, the semi-polar structure exhibits a higher concentration of electrons and holes and radiative recombination rate, and its reduced built-in polarization field weakens the extent of band bending which causes the shift of peak emission wavelength. So the efficiency droop of semi-polar In Ga N/Ga N LEDs declines obviously and the optical power is significantly improved. In comparison with non-polar structure LEDs, although the concentration of holes and electrons as well as the radiative recombination rate of the semi-polar structure are better in the last two quantum wells(QWs) approaching the p-Ga N side, the uniformity of distribution of carriers and radiative recombination rate for the nonpolar structure is better. So the theoretical analysis indicates that the removal of the internal polarization field in the MQWs active regions for non-polar structure LEDs contributes to the uniform distribution of electrons and holes, and decreases the electron leakage. Thus it enhances the radiative recombination rate, and further improves the IQEs and optical powers, and shows the best photoelectric properties among these three structures.     

Technology aspects of GaN-based diodes for high-field operation

This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.     

Steric and electronic effects in the molecular quenching of porous silicon luminescence

In this review, the effects of molecular exposure on the luminescent centers in porous silicon (PS) are described. The primary focus here is an attempt to understand how a variation in the size and electronic properties of a molecular absorbate can affect the quenching of porous Si light emission from different substrates. Results from our laboratories employing Lewis bases such as amines as quenchers of PS luminescence are stressed, as well as an overview of other relevant approaches using organic solvents, aromatic hydrocarbons, Brønsted acids/bases, and transition metal ions is presented.     

New type Schottky diodes based on the heterostructure of transition metal oxides: p-La2/3Sr1/3VO3/n-TiO2

We make a new type of bipolar Schottky diodes using the p-type La_2/3Sr_1/3VO₃ (LSVO)/n-TiO₂ heterostructure. The p-type LSVO metal thin films are grown on various substrates using radio frequency magnetron co-sputtering deposition. We find that the LSVO film grown on anatase TiO₂ layer produce the lowest resistivity of 0.28 mΩ cm. We discover that the resistivity decreases with decreasing LSVO film thickness for LSVO/TiO₂/Si structures. Hall measurements are performed and the dielectric functions of LSVO films are measured. The effective mass of LSVO/TiO₂/Si is determined to be 2.54 ± 0.05 m₀. The current-voltage curves of the Schottky diodes of p-LSVO/n-TiO₂ is measured and is explained using band alignment diagram. We identify a new type of Schottky diode, where both electrons in n-TiO₂ and holes in p-LSVO can flow under bias.     

On the mechanism of photon emission from thin film tunnel structures

The light emission from Al−Al₂O₃−M planar tunnel diodes was investigated where M=Ag, Au or Cu. Experiments on smooth and periodically corrugated (holographic grating) substrates showed that the emitted light originates from surface plasmons scattered by the surface roughness and, probably, from the bremsstrahlung of tunneling electrons. All the three noble metals show qualitatively similar behaviour and their light emission can be described as the superposition of three independent sources. The quantum efficiency of radiation was found to be ≈10⁻⁵ photon/electron, close to the theoretical limit.

     

Radiation resistance of 4<Emphasis Type="Italic">H</Emphasis>-SiC Schottky diodes under irradiation with 0.9-MeV electrons

Degradation of the parameters of 4H-SiC Schottky diodes after irradiation with 0.9-MeV electrons is studied. A charge-carrier removal rate of 0.07–0.09 cm^–1 is determined. The Schottky diodes under investigation are shown to retain rectifying current-voltage characteristics up to doses of ~10¹⁷ cm^–2. The radiation resistance of SiC Schottky diodes is found to be much greater than that of Si p–i–n-diodes with the same breakdown voltage.     

Photoluminescence of nanostructured cubic-lattice silicon carbide films grown on a silicon wafer

The light-emitting properties of cubic-lattice silicon carbide SiC films grown on Si(100) and Si(111) substrates with VPE at low temperatures (T _gr ∼ 700°C) are discussed. Investigations of the grown films reveal a homogeneous nanocrystalline structure involving only the 3C-SiC phase. When the electron subsystem of the structure is excited by a He-Cd laser emitting at λ_exit = 325 nm, the photoluminescence (PL) spectra contain a rather strong emission band shifted by about 3 eV toward a short-wave spectral region. At low temperatures, the PL integral curve is split into a set of Lorentz components. The relation between these components and the peculiarities of the energy spectrum of electrons in the nanocrystalline grains of the silicon carbide layers is discussed.     

Application of a scanning electron microscope in simulating a beta-emission-induced current

The current induced by emission from a thin ⁶³Ni layer is simulated with allowance for the real spectrum of ejected electrons and their angular distribution in Si and GaN. The calculated results are compared with simulation data obtained for a monoenergetic electron beam perpendicular to the semiconductor detector. For both Si and GaN, the ratio between the currents induced by the SEM beam and β emission from ⁶³Ni is demonstrated to be almost completely independent of the diffusion length, if the electron-beam energy of a scanning electron microscope (SEM) is appropriately selected.     

不同衬底上的ZnO薄膜紫外光致发光

用射频磁控溅射法在蓝宝石、硅和石英衬底上沉积出具有好的择优取向的多晶ZnO薄膜. 在270 nm波长的光激发下室温下可观察到显著的紫外光发射（波长为356 nm）和较弱的蓝光发射（波长为446 nm）. 经高温退火后薄膜的结晶质量显著提高, 在蓝宝石、石英衬底上沉积的薄膜，其积分发光强度分别增加了7倍和14倍.而硅衬底上的膜发光强度增强不太显著.紫外光发射源于电子的带间跃迁，而蓝光发射是由电子从氧空位浅施主能级到价带顶的跃迁引起的. 关键词： ZnO薄膜 射频磁控溅射 紫外发光 退火     

The study of structure and optoelectronic properties of ZnS and ZnO films on porous silicon substrates

ZnS and ZnO films were prepared on porous silicon (PS) substrates with the same porosity by pulsed laser deposition (PLD), and the structural, optical and electrical properties of ZnS and ZnO films on PS were investigated at room temperature by X-ray diffraction (XRD), scanning electron microscope (SEM), optical absorption measurement, photoluminescence (PL) and I–V characteristic studies. The prepared ZnS was obtained in the cubic phase along β-ZnS (1 1 1) orientation which showed a perfect match with the earlier report while ZnO films were obtained in c-axis orientation. There appeared some cracks in the surface of ZnS and ZnO films due to the roughness of PS substrates. Luminescence studies of ZnS/PS and ZnO/PS composites indicated room temperature emission in a broad, intense, visible photoluminescence band, which cover the blue emission to red emission, exhibiting intensively white light emission. Based on the I–V characteristic, ZnS/PS heterojunction exhibited the rectifying junction behavior, while the I–V characteristic of ZnO/PS heterostructure was different from that of the common diode, whose reverse current was not saturated.     

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.