退火工艺对含硅量子点SiC_x薄膜光谱特性的影响

采用磁控共溅射法并结合微波退火和快速光热退火工艺,在不同退火温度下制备了含硅量子点的富硅SiC_x薄膜。采用掠入射X射线衍射(GIXRD)、拉曼光谱和光致发光(PL)光谱技术对薄膜进行了表征,研究了退火工艺对薄膜中硅量子点数量、尺寸、晶化率以及发光峰的影响。结果表明:与快速光热退火相比,微波退火不但能降低硅量子点的形成温度(降低200℃),而且还能降低β-SiC量子点的形成温度(降低100℃);在相同退火温度下,微波退火制备的硅量子点的数量更多、晶化率更高、光致发光峰更强;采用1000℃温度的微波退火样品的硅量子点数量最多、尺寸最大(5.26nm)、晶化率最高(74.25%)、发光峰最强,说明微波退火能凝析出高质量的硅量子点。     

包含硅量子点的富硅SiNx 薄膜结构与发光特性

采用等离子体增强化学气相沉积法, 以NH₃与SiH₄为反应气体, n型单晶硅为衬底, 低温(220 ℃)沉积了富硅氮化硅(SiN_x)薄膜. 在N₂氛围中, 于500–1100 ℃ 范围内对样品进行了热退火处理. 采用Raman 光谱技术分析了薄膜内硅量子点的结晶情况, 结果表明, 当退火温度低于950 ℃时, 样品的晶化率低于18%, 而当退火温度升为1100 ℃, 晶化率增加至53%, 说明大部分硅量子点都由非晶态转变为晶态. 实验通过Fourier 变换红外吸收(FTIR)光谱检测了样品中各键的键合结构演变, 发现Si–N键和Si–H键随退火温度升高向高波数方向移动, 说明了薄膜内近化学计量比的氮化硅逐渐形成. 实验还通过光致发光(PL)光谱分析了各样品的发光特性, 发现各样品中均有5个发光峰, 讨论了它们的发光来源, 结合Raman光谱与FTIR光谱表明波长位于500–560 nm的绿光来源于硅量子点, 其他峰则来源于薄膜内的缺陷态. 研究了硅量子点的分布和尺寸对发光带移动的影响, 并根据PL峰位计算了硅量子点的尺寸, 其大小为1.6–3 nm, 具有良好的限域效应. 这些结果有助于制备尺寸不同的硅量子点和基于硅量子点光电器件的实现. 关键词： 硅量子点 氮化硅薄膜 光致发光 Fourier 变换红外吸收     

退火及溅射气氛对氮化硅薄膜光致发光的影响

利用射频磁控反应溅射方法制备富硅的氮化硅薄膜。衬底材料为抛光的硅片,靶材为硅靶,在Ar-N₂气环境下,通过改变两种气体的组分比来改变样品成分,并在高纯N₂气氛下对其进行高温退火处理。用X射线光电子能谱(XPS)和X射线衍射(XRD)对样品进行了表征,并测试了样品的光致发光谱 (PL)。实验结果表明：X射线光电子能谱中出现了Si—N键合结构,同时还有少量的Si—O键生成,通过计算得出Si/N比值约为1.51,制备出了富硅的氮化硅薄膜;薄膜未经退火前,在可见光区域没有观察到明显的光致发光峰,经过高温退火后,XRD中新出现的衍射峰证实了纳米硅团簇的生成,PL图谱中在可见光区域出现了光致发光峰的蓝移现象,结合XRD结果,用纳米晶的量子限域效应对上述现象进行了合理解释。     

纳米Si-SiO_x和Si-SiN_x复合薄膜的低温制备及其发光特性

用等离子体增强化学气相沉积法在低温 (低于 5 0℃ )衬底上沉积Si SiOx 和Si SiNx 复合薄膜 ,可得到平均颗粒尺寸小至 3nm的高密度 (最高可达 4 0× 10 1 2 cm- 2 )纳米硅复合薄膜 .5 0 0℃快速退火后 ,这种复合薄膜显现出优异的可见光全波段光致发光特性 .通过比较相同条件下所制备的纳米Si SiOx 和Si SiNx 复合薄膜的光致发光效率 ,发现纳米Si SiNx 具有更为优异的光致发光效率 ,这一点在可见光短波区表现得尤为显著     

用硅离子注入方法制备的纳米硅的拉曼散射研究

在直角散射配置下测量了纳米硅样品的拉曼散射谱及其退火温度的关系。结果表明,在８００℃以下退火的样品只观察到单晶硅衬底的光学声子模,在９００℃以上退火,才观察到纳米硅的特征拉曼散射峰。在１２００℃下退火后,纳米硅的特征拉曼散射峰消失,观察到类似于非晶硅的光学声子特征峰,可能表示纳米硅不能承受这样的高温热退火。这些结果进一步证实了光致发光谱的结果。     

生长温度对ZnO薄膜晶体质量和发光特性的影响

采用激光脉冲沉积法在Si(100)衬底上生长ZnO薄膜,衬底温度分别为室温,200℃,300℃,400℃和500℃.用X射线衍射仪、拉曼光谱、扫描电子显微镜对薄膜的微结构进行了测量,并测量了室温下薄膜的光致发光特性.结果表明,300℃时.ZnO具有最佳择优取向,随着衬底温度升高.衍射峰半峰全宽减小,薄膜晶粒尺寸增大,400℃时,薄膜具有各向等大的品粒尺寸.同时拉曼谱结果显示,薄膜内部的缺陷随衬底温度变化无明显差别,应力表现为张应力,400℃时应力最小,紫外发光峰在衬底温度为400℃时最强,而黄绿光带最弱.在减少薄膜缺陷,提高择优长向和晶粒尺寸的同时.使晶粒横向尺寸和纵向尺寸尽可能相同,可极大提高薄膜的发光特性.     

脉冲功率对含硅量子点SiCx薄膜物相结构及光谱特性的影响

基于硅量子点（Si-QDs）的全硅叠层太阳电池被认为是最有潜质的高效太阳电池之一。目前所报道的硅量子点薄膜存在硅量子点数密度低、缺陷多等问题,限制了硅量子点太阳电池的光电转换效率。微波退火(microwave annealing, MWA)被认为是一种有益于制备纳米结构材料的方法。微波退火的非热效应可以降低形核能,改善薄膜的微结构和光电性能。因此,采用磁控共溅射技术并结合微波退火工艺,在不同的脉冲功率下制备了含硅量子点SiC_x薄膜;采用掠入射X射线衍射(GIXRD)、拉曼(Raman)光谱、紫外-可见-近红外分光光度计和光致发光(PL)光谱表征薄膜的物相结构及光谱特性;研究不同脉冲功率对硅量子点数密度和性能的影响,进而改进磁控共溅射工艺,制备硅量子点数密度较高和性能良好的薄膜。样品的GIXRD谱和Raman谱均显示其中存在硅量子点,其强度先增大后减小;通过谢乐(Scherrer)公式估算出硅量子点尺寸呈现先增大后减小的规律,脉冲功率为80 W时尺寸达到最大(8.0 nm）。在Raman光谱中还观察到中心位于511 cm-1处出现硅量子点Si-Si横向光学振动模式的拉曼峰,其强度也呈现先增大后减小的趋势;对拉曼光谱做最佳高斯(Gauss)分峰拟合,得出薄膜的晶化率均高于62.58%,脉冲功率为80 W时制备的薄膜具有最高的晶化率(79.29%)。上述分析表明薄膜中均有硅量子点的形成,且数量先增加后减小,脉冲功率为80 W时硅量子点数量最多。通过测量样品的透射率T、反射率R等光学参数,利用Tauc公式估算出薄膜的光学带隙,发现带隙值随溅射功率的增加先减小后增大,在脉冲功率为80 W时最小(1.72 eV)。硅量子点尺寸与光学带隙成反比,说明薄膜中的硅量子点具有良好的量子尺寸效应。通过PL光谱分析样品的发光特性,对其做最佳高斯拟合,发现样品中均有6个发光峰。结合Raman光谱的分析结果,可以得出波长位于463~624 nm的发光峰源于硅量子点的作用;而波长位于408和430 nm的发光峰则源于薄膜内部的缺陷态,峰位没有偏移,但强度有变化。根据发光峰对应的波长可计算其能带分布,从而确定缺陷态类型：408 nm的发光峰归因于≡Si°→E_v电子辐射跃迁,430 nm的发光峰则归因于≡Si°→≡Si-Si≡的缺陷态发光。还研究了硅量子点的尺寸对发光峰移动的影响。结果表明,随硅量子点尺寸变小（大）,发光峰蓝移（红移）。综上,溅射功率为80 W时制备的含硅量子点SiC_x薄膜性能最佳。研究结果为硅量子点太阳电池的后续研究奠定了基础。     

镶嵌有纳米硅的SiN_x薄膜光致发光的温度依赖特性研究

采用对靶磁控溅射法在单晶硅衬底上沉积镶嵌有纳米硅的氮化硅薄膜,然后在形成气体FG(10%H2,90%N2)气氛中进行450℃常规热退火50min。通过荧光光谱仪测得的稳态/瞬态光致发光(PL)谱研究了镶嵌有纳米硅的氮化硅(SiNx)薄膜样品光致发光特性。结果表明,样品的发光过程可以归因于纳米硅的量子限制效应发光和与缺陷相关的发光。随着激发光能量的增加,PL谱峰位发生蓝移,表明较小粒度的纳米硅发光比例增加;温度的降低会抑制非辐射复合过程,提高辐射复合几率,因此发光寿命延长,发光强度呈指数增加;随着探测波长的减小,样品的发光寿命则明显缩短,表明纳米硅的量子限制效应发光对温度有很强的依赖性。     

Sb掺杂Si_3N_4基Si量子点薄膜的制备与结构

基于多靶射频磁控溅射技术,结合快速光热退火后处理制备了Sb掺杂Si_3N_4基Si量子点(SiQDs)薄膜。采用透射电镜、掠入射X射线衍射、拉曼光谱和光致发光光谱等手段对薄膜的微结构和发光特性进行了研究,分析了Sb掺杂对Si-QDs薄膜的微结构和发光特性的影响规律.结果表明,Sb掺杂表现出明显的诱导晶化作用.掺杂的Sb有助于Si原子在Si_3N_4基质中的扩散并形成Si-QDs.随着Sb掺杂量的增加,Si-QDs的尺寸逐渐增大,薄膜的结晶率X_c有效提高,其PL谱峰随之增强,谱峰的半高峰宽逐渐变窄;由于Si-QDs尺寸的增加还导致PL发光谱峰位产生红移.     

C~ 注入a-SiN_x∶H薄膜的微结构及光发射的研究

常温下对低压化学气相沉积制备的纳米硅镶嵌结构的a-SiNx∶H薄膜进行低能量高剂量的C 注入后,在800~1 200℃高温进行常规退火处理。X射线光电子能谱(XPS)及X射线光电子衍射(XRD)等实验结果表明,当退火温度由800℃升高到1200℃后,薄膜部分结构由SiCxNy转变成SiNx和SiC的混合结构。低温下利用真空紫外光激发,获得分别来自于SiNx、SiCxNy、SiC的,位于2.95,2.58,2.29 eV的光致发光光谱。随着退火温度的升高,薄膜的结构发生了变化,发光光谱也有相应的改变。     

Black-body emission from nanostructured materials

Photoluminescence (PL) experiments on materials of low thermal conductance can cause black-body emission from the sample even at low intensities of laser excitation. This thermal emission may be misinterpreted in terms of quantum emission. Although the quantum origin of most radiative emissions in nanostructured materials such as porous silicon is well established, we show in this paper that SiC nanoparticles and mechanically milled Si do exhibit thermal emission at typical excitation intensities for PL measurements provided the samples are under vacuum. An Si membrane was also investigated and the fact that it did not emit black-body radiation is explained with a simple analysis of the heating in materials of reduced dimensionality.     

硅铁合金中硅的间接测定

通过对样品的Ｘ光物相分析,探明硅铁合金的成分,经分光光度法测定铁的含量。从而间接地得到非单质硅的含量。本方法快速准确,为硅铁中有效硅的含量分析提供了一条捷径。     

Amorphous Si/insulator multilayers grown by vacuum deposition and electron cyclotron resonance plasma treatment

a-Si/insulator multilayers have been deposited on (0 0 1) Si by electron gun Si evaporation and periodic electron cyclotron resonance plasma oxidation or nitridation. Exposure to an O or N plasma resulted in the formation of a thin SiO₂ and SiN_x layer whose thickness was self-limited and controlled by process parameters. For thin-layer (2 nm) Si/SiO₂ and Si/SiN_x multilayers no visible photoluminescence (PL) was observed in most samples, although all exhibited weak “blue” PL. For the nitride multilayers, annealing at 750°C or 850°C induced visible PL that varied in peak energy with Si layer thickness. Depth profiling of a-Si caps on thin insulating layers revealed no detectable contamination for the SiN_x layers, but substantial O contamination for the SiO₂ films.     

Formation of silicon oxide nanowires directly from Au/Si and Pd–Au/Si substrates

Amorphous silicon oxide (SiO_x) nanowires were directly grown by thermal processing of Si substrates. Au and Pd–Au thin films with thicknesses of 3 nm deposited on Si (0 0 1) substrates were used as catalysts for the growth of nanowires. High-yield synthesis of SiO_x nanowires was achieved by a simple heating process (1000–1150 °C) in an Ar ambient atmosphere without introducing any additional Si source materials. The as-synthesized products were characterized by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy measurements. The SiO_x nanowires with lengths of a few and tens of micrometers had an amorphous crystal structure. The solid–liquid–solid model of nanowire formation was shown to be valid.     

Field-induced spin-orientational phase transitions in Néel ferrimagnets. I

The breakdown of the aligned states of Néel ferrimagnets in an applied magnetic field is considered. The behaviour of isotropic two-and three-sublattice ferrimagnets is treated in the framework of the mean-field approximation. A phenomenological method also limited by the mean-field approximation is used to analyse the behaviour of the anisotropic ferrimagnet. Particular emphasis is placed on the vicinity of the compensation temperature. Approximate analytical expressions which are valid near the compensation temperature are given for the phase-boundary positions, typical magnetic fields, and temperature ranges of the phase diagrams.     

Electrical properties of purified solar grade silicon substrates using a combination of porous silicon and SiCl4

This work investigates the photo-thermal treatment of solar grade (SG) silicon to reduce impurities to a low level suitable for high efficiency low-cost solar cells application. It describes experiment carried out by using a tungsten lamps furnace (rapid thermal processing, RTP) to purify solar grade silicon wafers using a combination of porous silicon (PS) and silicon tetrachloride. This process enables to attract the impurities towards the porous layer where they react with SiCl₄ to form metallic chlorides. The gettering effect was studied using the Hall Effect and the Van Der Pauw methods to measure the resistivity, the majority carrier concentration and mobility. We have obtained a significant improvement of the majority carrier mobility after such thermo-chemical treatment. The gettering efficiency is also evaluated by the relative increase of the minority carrier diffusion length L, measured by the light beam induced current (LBIC) technique.     

Properties of the SiO2/SiC interface investigated by angle resolved studies of the Si 2p and Si 1s levels and the Si KLL Auger transitions

Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL_2,3L_2,3 Auger transitions from SiO₂/SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered √3×√3 reconstructed 4H-SiC(0 0 0 1) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 Å. The angle resolved data show that two oxidation states only, Si⁺¹ and Si⁺⁴, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra.The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si₂O) at the interface with a thickness ranging from 2.5 to 4 Å. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 Å.The SiO₂ chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO₂ shift is found to be fairly constant for oxides less than about 10 Å thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 Å and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined.The relative final state relaxation energy ΔR(2p) is determined from the modified Auger parameter. This yields a value of ΔR(2p)=−1.7 eV and implies, for SiO₂/SiC, a “true” chemical shift in the Si 2p level of only ≈0.4 eV for oxide layers of up to 10 Å thick.     

Investigation of rapid thermal oxide/ silicon nitride passivation stack of n+ emitter

To anticipate the initial phosphorus diffusion parameters of silicon solar cells process fabrication, we report in this paper an overview of our experiments on silicon n⁺-emitters passivation by means of rapid thermal silicon oxide/silicon nitride stack. The process-induced changes have been evaluated and explained. We found that 900 °C and 80 s were the appropriate process parameters to grow 10 nm silicon oxide. Investigation of the effect of this oxidation on n⁺ multicrystalline silicon emitters revealed a large decrease (more than 25%) of the sheet resistance and around 12% increase of the junction depth. The experiments also revealed that the passivation effect of the optimal silicon oxide/silicon nitride stack is efficient only for higher emitter quality. In addition, we found that this stack reduces the surface reflection more than the optimal single silicon nitride layer.     

含次环己基有机硅单体及二聚体的NMR谱与结构

含次环已基有机硅单体及其二聚体业已成功地合成^[1],本文研究了这类新单体和二聚体的¹H-NMR、¹³C-NMR、和²⁹Si-NMR谱,从而确定了它们的结构。     

Understanding and control of the erbium non-radiative de-excitation processes in silicon

Erbium doping of silicon has recently emerged as a promising method to tailor the optical properties of Si towards the achievement of a light emission at 1.54 μm. In this paper we will review our recent work on this subject. In particular a detailed investigation of the non-radiative processes, competing with the radiative emission of Er in Si will be presented. Among these processes, an Auger de-excitation with the energy released to free carriers will be demonstrated to be extremely efficient, with an Auger coefficient C_A4.4×10⁻¹³ cm³/s. Moreover, at temperatures above 100 K a phonon-assisted back-transfer decay process, characterized by an activation energy of 0.15 eV is seen to set in. This understanding of the physical properties competing with the radiative light emission allowed us to control them and obtain efficient room temperature luminescence. Two examples will be reported. It will be shown that by exciting Er within the depletion region of reverse biased p⁺–n⁺ Si diodes in the breakdown regime it is possible to avoid Auger quenching and to achieve high efficiency. Moreover, at the switch off of the diode, when the depletion region shrinks, the excited Er ions become suddenly embedded within the neutral heavily doped region of the device. In this region Auger de-excitation with free carriers sets in and quenches rapidly the luminescence. This allows to modulate the light signal at frequencies as high as a few MHz. Furthermore, the introduction of Er within Si nanocrystals is demonstrated to be a promising way to eliminate back-transfer processes by a widening of the bandgap while maintaining the full advantage of the efficient electron-hole mediated excitation present in Si. These data are presented and future perspective discussed.