雪崩热电子注入研究富氮SiO_xN_y纳米级薄膜的陷阱特性

采用雪崩热电子注入技术研究了富氮 Si Ox Ny 纳米级薄膜的陷阱特性。观察到该薄膜存在着受主型电子陷阱 ,随着注入的增长、界面上产生的这种陷阱将起主导作用 ,其密度大过施主型界面电子陷阱。揭示出界面陷阱密度在禁带中分布 ,其密度随雪崩注入剂量增加而增大 ,禁带上半部增大得尤其显著。指出雪崩注入过程中在 Si/ PECVD Si Ox Ny 界面上产生两种性质不同的电子陷阱 ,并给出它们在禁带中的位置及密度大小关系。支持了界面陷阱来源于悬挂键的物理模型 ,由于本实验的重要结果可用该理论模型圆满地解析。给出 PECVD形成纳米级薄膜的优化工艺条件 ,该条件成膜的界面特性良好、耐压范围高、抗雪崩注入能力及其他电子特性也较好     

激光诱导化学气相沉积法制备纳米晶硅的制备参数和退火工艺的研究

用CO2红外激光诱志化学气相沉积的方法制备纳米Si，激光强度越大，则SiH4受热温度越高，纳米Si的成核率越高，纳米Si核的密度越大，每一个核生长所吸收的Si原子数目越少，从而所得的纳米Si粒小而均匀。当激光强度减小到一个低限阈值，则SiH4温度太低，不能裂解。SiH4的流速越快，则纳米Si成核后生长期越短，纳米Si粒也小而均匀。当SiH4流速快到一个高限阈值，则SiH4受热时间太短，升不到裂解所需的高温。以上2个产生纳米Si的阈值正相关。纳米Si制取后退火脱H，3440cm^-1光谱带红移并增强，2150cm^-1光谱带形状变化，1100cm^-1光谱带形状变化，1100cm^-1光谱带由低频处蓝移而来并展宽。这都表明含O键在纳米Si很大的表面上出现。为了减轻含O键出现，纳米Si应在Ar气氛中而不是在空气中，从低于300℃的温度开始退火。     

a-Si:(Cl,H)薄膜的PESIS研究

利用 Mg Ka X射线作为激发源,对 a-Si:(Cl,H)薄膜进行了 PESIS研究.样品是采用辉光放电方法在SiCl_4-SiH_4-H_2混合气体中,当石墨衬底温度为350℃时生长的.实验发现,薄膜中的Cl只起补偿 Si悬挂键的作用,不起掺杂作用.对Si 2p光电子峰进行分解,得到Cl引起Si 2p能级的化学位移为 1.1± 0.1eV.根据原子电负性的考虑,认为在 a-Si:(Cl,H)薄膜中,只存在 Si-Cl单键结合,不存在 Si-Cl_2、Si-Cl_3和 Si-Cl_4的结合形式.对Cl 2p光电子峰的分析结果表明,在这种薄膜中,Cl只有一种化学形态,即Si-Cl结合态,不存在分子Cl_2.     

H_2O在Si(100)-(2×1)表面上的吸附和氧化

利用AES以及HeI(21.2eV)和同步辐射(138.5eV)光电子谱研究了H_2O在Si(100)-(2 ×1)表面上的室温吸附、高温退火效应以及开始氧化的过程.AES的测量结果表明,H_2O在这种表面上的粘附系数很大,在350K下,当曝汽量约为2L时就达到饱和,其覆盖率θ=1/2.价带区域的光电子谱出现三个由于H_2O吸附而引起的特征峰,低于价带顶分别为6.2、7.2和11.5eV.350K下的 H_2O吸附导致Si 2p能级的结合能增加0.8±0.1eV,相当于一个Si原子同一个氧原子键合的情况.在 640K下退火后,出现Si 2p的第二个化学位移峰,其位置比体内Si 2p 峰约低 1.8eV,表明这时的 H_2O已经完全离解,一部分Si原子同两个氧原子键合(可能是桥键方式). 在 870K下退火,得到Si 2p的四个化学位移峰,说明氧已经贯穿进Si的体内,形成SiO_x(x=1、2、3和4).当退火温度进一步提高到T(?)970K时,恢复Si的清洁表面,表明氧被完全脱附.     

PECVD形成纳米级薄膜界面陷阱的物理模型

采用雪崩热电子注入技术研究了纳米级富氮 Si Ox Ny 薄膜界面陷阱的物理模型。证实了 PECVDSi Ox Ny 薄膜中界面陷阱来源于悬挂键的物理模型。观察到该纳米膜内存在着受主型电子陷阱 ,随着注入的增长 ,界面上产生的这种陷阱将起主导作用。发现到 Dit随雪崩热电子注入剂量增加而增大 ,禁带上半部 Dit的增大较下半部显著。指出了雪崩注入过程中在 Si Ox Ny 界面上产生两种性质不同的电子陷阱 ,并给出它们能级位置及密度大小关系。揭示出 PECVD法形成的这种纳米膜与快速热氮化制备的薄膜中、氮氧含量不同、界面陷阱特性变化不一样 ,并从薄膜氮化机制予以物理解析。给出了 PECVD形成纳米级薄膜的优化工艺条件 ,该工艺条件制成膜的界面陷阱及其它物理电学特性都比较好     

硅中铂杂质能级的电子辐照效应

观测到掺铂硅中 E_c-0.23eV能级的 DLTS信号因电子辐照而衰减,而E_v+0.32eV能级则对电子辐照不敏感.这一实验结果使我们认为,掺铂硅中的两个能级不是与同一个深中心有关的.     

钴掺杂Mg2Si磁性和光学性质的第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法研究了本征Mg2Si及钴(Co)掺杂Mg2Si体系的晶体结构、自旋态密度、磁性和光学性质.结果表明,Co替Mg(CoMg)缺陷的形成能为负,可以形成稳定的缺陷.从自旋态密度可以看出,本征Mg2Si为无磁性半导体;向Mg2Si体系掺入Co后,体系的磁矩由于Co-3d态和Si-3p态杂化(pd杂化)诱导产生,且体系呈明显的半金属特性.超胞中Co的磁矩为0.53 μB.从吸收光谱可以看出,Co掺杂Mg2Si的主吸收峰强度略小于本征Mg2Si,但吸收跨度则明显大于本征Mg2Si.本征Mg2Si对于能量小于1.55 eV(对应波长为800 nm)的光子几乎不吸收,而掺杂体系还存在着较大的吸收,说明Co元素的掺杂显著地改善了Mg2Si对低能(红外)光子的吸收.计算结果为Mg2Si基自旋电子器件和光电子器件的设计和应用提供了理论依据.     

Si(111)面氟吸附的研究

本文先用集团模型和电荷自治的EHT方法研究了Ⅶ族元素F在 Si(111)面的化学吸附,利用能量极小原理确定了化学吸附位置;在此基础上,采用薄片(SLAB)模型和EHT经验紧束缚方法对吸附体系的电子结构作了计算.计算结果表明,F可以吸附在Si(111)面的顶位和三度空位上,但以顶位为更稳状态.顶位吸附时,F与表面的距离为1.53A,吸附能为4.7eV;三度位吸附时,F与表面的距离为-2.2A,吸附能为3.1eV.吸附后,F对总态密度和能带的贡献主要局域在真空能级下-17eV～-19eV之间.此外,顶位吸附使原清洁表面悬键引起的表面态消失.     

硅纳米管结构和电子性质的第一性原理研究

基于密度泛函理论的第一性原理方法,在广义梯度近似下,研究了(5,0)和(5,5)硅纳米管结构和电子性质。计算结果表明:(5,0)管硅原子相邻键长波动范围为0.068 nm,大于(5,5)管的0.006 nm;通过对(5,0)管的分波态密度进行分析发现,其3s电子和2p电子能量分布在-13～3 eV,但2p电子集中分布在能量较高的-6～3 eV,出现了明显的sp3轨道杂化。同时对(5,0)和(5,5)硅纳米管最高占据轨道和最低未占据轨道的能隙进行了分析,发现两种管导电性能与结构的手性相关,锯齿型(5,0)管能带交叠具有明显的金属性,而扶手型(5,5)管能隙为0.151 eV是半导体纳米管。     

Si基纳米结构的电子性质

各种Si基纳米发光材料在Si基光电子器件及其全Si光电子集成技术中具有潜在的应用前景,从理论和实验上对其电子结构进行研究,有助于我们深化对其发光机制的认识与理解。本文主要从量子限制效应发光这一角度,着重介绍了Si纳米晶粒、Ge/Si量子点,SiO2/Si超晶格和超小尺寸Si纳米团簇等不同Si基纳米结构的电子性质以及它们与发光特性之间的关系。还讨论了介质镶嵌和表面钝化对其电子结构的影响。     

Quantum-confinement effect in silicon-on-insulator films implanted with high doses of hydrogen ions

280-nm-thick silicon-on-insulator films are implanted with high doses of hydrogen with the energy 24 keV and the dose 5 × 10¹⁷ cm^?2. Peaks corresponding to optical phonons localized in the silicon nanocrystals 1.9?C2.5 nm in size are observed in the Raman spectra. The fraction of the nanocrystal phase is ??10%. A photoluminescence band with a peak at about 1.62 eV is detected. The intensity of the 1.62 eV band nonmonotonically depends on the measurement temperature in the range from 88 to 300 K. An increase in the radiative recombination intensity at temperatures <150 K is interpreted in the context of a two-level model for the energy of strongly localized electrons and holes. The activation energy of photoluminescence enhancement is 12.4 meV and corresponds to the energy of splitting of the excited state of charge carriers localized in the silicon nanocrystals.     

The energy levels of palladium in silicon

A study is undertaken of the energy levels of palladium in silicon. It is shown that electronically active palladium exists in silicon in the form of two independent species. The first, designated Pd_I, is amphoteric and exhibits an acceptor level at 0.22±0.01 eV below the conduction band edge, as well as a donor level at 0.33±0.01 eV above the valence band edge. The second species, designated Pd_II, exhibits an acceptor level at 0.32±0.1 eV above the valence band edge. The ratio of Pd_I to Pd_II which is incorporated into the silicon varies from 40 to 5 for diffusion temperatures from 900 to 1200°C respectively.     

用HREELS和UPS研究多孔硅的电子结构

使用高分辨电子能量损失谱(HREELS)和紫外光电子能谱(UPS)研究了新腐蚀的多孔硅(PS)样品的电子结构。实验结果表明,从HREELS谱中能量损失阚值测得的PS的能隙移到2．9eV,与文献报道的光激发谱(PLE)结果相近。UPS结果表明PS的费米能级到价带顶的距离不同于单晶Si。结合HREELS和UPS结果可以初步得出PS与Si界面的能带排列。     

Specific features of photoelectric and optical properties of amorphous hydrogenated silicon films produced by plasmochemical deposition from monosilane–hydrogen mixture

Photoelectric and optical properties of amorphous hydrogenated silicon films produced by plasmochemical deposition from a monosilane-hydrogen mixture have been studied at a fraction of hydrogen in the mixture that corresponds to the onset of formation of a nanocrystalline phase in the structure of the films obtained. A behavior untypical of amorphous hydrogenated silicon films is observed for the photoconductivity and the spectral dependence of the absorption coefficient. The temperature dependences of the photoconductivity in the films under study are found to vary with the energy of incident photons. At a photon energy of 1.3 eV, temperature quenching of photoconductivity is observed. Prolonged illumination of the films led to a certain decrease in the absorption coefficient at photon energies in the range 1.2–1.5 eV. The results obtained are attributed to the possible presence of silicon nanocrystals in the structure of the films and to the influence of these nanocrystals on their photoelectric and optical properties.     

利用X_α—重叠球方法研究半导体的局域电子结构——Ⅱ.共价半导体金刚石和硅的轨道能级

本文利用X_α—重叠球和电荷分割方法计算了完整的共价半导体金刚石和硅的能谱.它们的禁带宽度(3a—3t_2)分别为5.2和2.0eV.价带宽度(3t_2—1a_1)分别为16.46和11.42eV.除了硅的禁带宽度数据偏大外,其他计算数据与实验和某些理论结果基本相符.文中还讨论了禁带内出现1t_1能级的原因以及与实验能谱的比较.     

Electrical and photoelectric properties of Si-based metal–insulator–semiconductor structures with Au nanoparticles at the insulator–semiconductor interface

It is shown that the formation of Au nanoparticles at the insulator–silicon interface in structures with a high density of surface states results in a shift of the Fermi-level pinning energy at this interface towards the valence-band ceiling in silicon and in increasing the surface-state density at energies close to the Fermi level. In this case, a band with a peak at 0.85 eV arises on the photosensivity curves of the capacitor photovoltage, which is explained by the photoemission of electrons from the formed Au-nanoparticle electron states near the valence-band ceiling in silicon.     

Special features of the electrical conductivity in doped α-Si:H films with silicon nanocrystals

The electrical properties of undoped and phosphorus-doped α-Si:H films with Si nanocrystals are studied. The silicon nanocrystals are formed by a solid-solid phase transition resulting from the nanosecond effect of a XeCl excimer laser on an amorphous film. The formation of the nanocrystals in the undoped films is accompanied by an increase in the electrical conductivity by two to three orders of magnitude and a simultaneous decrease in the effective activation energy of the conductivity from 0.7 to 0.14 eV. The nanocrystal sizes range from 2 to 10 nm for various laser treatment modes and are determined from Raman scattering data and high-resolution electron microscopy. The temperature dependence of the Fermi level is obtained by calculating the energies of the localized states of electrons and holes in the nanocrystals. It is shown that, as the temperature decreases, the Fermi level tends to the energy of the states in the Si nanocrystals for a wide concentration range of the dopant. The Fermi level’s location close to the states in the nanocrystals is a consequence of the fact that these states are multicharged. It is found that phosphorus effectively transforms into an electrically active state during laser treatment of the doped amorphous Si films, which is an important consideration in the fabrication of shallow p-n junctions and contacts for amorphous Si films.     

Cathodoluminescence and photoluminescence of amorphous silicon oxynitride

The cathodoluminescence and photoluminescence of amorphous silicon oxynitride (a-SiO_xN_y) films with different compositions were measured for the first time in the near infrared to visible–ultraviolet range. The red band with energy at 1.8–1.9 eV, blue band with energy 2.7 eV, ultraviolet bands with energies 3.1, 3.4–3.6, 4.4–4.7, and 5.4 eV were observed in a-SiO_xN_y. The 1.8–1.9 eV band is attributed to the oxygen and nitrogen atoms with unpaired electrons and the 2.7 eV band is attributed to twofold coordinated silicon atoms with two electrons. The 5.4 eV shoulder is due to the peroxy radicals and other ultraviolet bands are supposed to be due to the Si–Si bonds.     

Mechanisms of transport and injection of carriers into a porous silicon: Electroluminescence in electrolytes

A model describing the transport and injection of carriers into a heterophase system (silicon substrate)/(silicon nanocrystals)/(electrolyte) under excitation of electroluminescence is proposed. The main fraction of current passes from the electrolyte directly into a substrate by-passing the nanocrystals. Electromechanical processes at the electrolyte-substrate interface produce the electroactive particles, which inject one or both types of carriers in macro-and nanocrystals, respectively. For the bipolar injection, the nanocrystals play the role of catalyst in the exothermal reaction of charge exchange between the electroactive particles, which possess the opposite charges. In this case, the particles transfer the energy to nanocrystals, which they accumulated in the process of their formation. A fraction of this energy is released by the radiative recombination. The resulting efficient visible electroluminescence is weakly dependent on the doping level and conduction type of the initial silicon.     

Characteristic photoluminescence band in Si-implanted SiO2 grown on Si wafer

A possible mechanism for the photoemission from Si nanocrystals in an amorphous SiO₂ matrix fabricated by ion implantation is reported. We have measured the implantation dose dependence as well as the oxidation effect of the photoluminescence behavior of Si nanocrystals in SiO₂ layers fabricated by ion implantation and a subsequent annealing step. After annealing, a photoluminescence band, peaking just below the 1.7 eV was observed. The peak energy of the photoluminescence was found to be affected by the dose of implanted Si ions, but to be independent of annealing time and excitation photon energy. We also present experimental results of an oxidation induced continuous peak energy shift of the photoluminescence peak, up to around 1.8 eV. This peak energy, however, was found to return to its previous position with re-annealing. These results indicate that, whilst the excitation photons are absorbed by Si nanocrystals, the emission is not simply due to electron–hole recombination inside the Si nanocrystals, but is related to the presence of defects, most likely located at the interface between the Si nanocrystals and the SiO₂, for which the characteristic energy levels are affected by cluster–cluster interactions or the roughness of the interface.