Impact of phosphorus doping to multiple-stacked Si quantum dots on electron emission properties

We have fabricated multiple-stacked phosphorous doped Si quantum dots (P-doped Si-QDs) embedded in SiO₂ on n-Si(100) structures and characterized their field electron emission under DC bias application to semitransparent Au top-electrodes. At applied biases of −8 V and over, the electron emission signal with a peak kinetic energy at ~2.0 eV was detected. In addition, we also found that the electron emission was drastically enhanced with an increase in the applied DC bias over −11 V. The applied bias dependence of emission intensities shows that the P-doped Si-QDs is effective to improve electron emission efficiency while undoped Si-QDs stack is suited to low power operation. This indicates that electric field was reduced near the top side of the Si-QDs stack and an increase in electron injection rate from the n-Si(100) to the dots by phosphorus doping plays a role on high efficient electron emission from the Si-QDs stacked structures.     

Electron emission from phosphorus- and boron-doped polycrystallinediamond films

The electron emission from CVD-grown phosphorus (P-) and boron (B)-doped polycrystalline diamond films has been studied. The current density against electric field characteristics of the P-doped film showed low-field emission compared to the B-doped film. From the slope ratio of the Fowler-Nordheim (F-N) characteristics of P- and B-doped films, a ratio of 0.66 for the emission barrier height was obtained. The small ratio might be caused by the n-type semiconducting properties of P-doped diamond films     

Field emission characteristics of CoSi2/TaN-coatedsilicon emitter tips

This work has improved the emission characteristics of Si emitter tips by coating a CoSi₂/TaN bilayer on the tips. The CoSi₂ layer was grown in situ by a reactive chemical-vapor deposition of cyclopentadienyl dicarbonyl cobalt at 650°C. The TaN was then deposited on the CoSi₂ layer at 550°C by a reactive sputtering of Ta with N as a reactive gas. The CoSi₂/TaN-coated emitters showed a lower turn-on voltage and higher emission current than the CoSi₂- or TaN-coated emitters due to the low work function by TaN and the easy transport of electron by CoSi₂ with low resistivity. The long-term emission stability of CoSi₂/TaN-coated Si emitter was as good as TaN-coated emitter     

Field-emission enhancement of Mo-tip field-emitted arrays fabricated by using a redox method

This work has improved the emission characteristics for Mo-tips of field-emitter array (FEA) through a reduction-oxidation (redox) process. The maximum emission current of the 1600 tips array significantly increased from 23.2 /spl mu/A to 2.36 mA with the turn-on voltage decreasing from 70 V to 48 V after the redox treatment. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicate that the main factor improving electron field emission characteristics for the Mo-tips is the increase in enhancement factor (/spl beta/) and, possibly, number of emission sites of Mo films.     

超纳米金刚石薄膜场发射特性的研究

超纳米金刚石(UNCD)是一种全新的纳米材料，具有许多独特性能。介绍了Si微尖和微尖阵列阴极沉积超纳米金刚石薄膜的工艺及其场发射特性。研究发现，适当的成核工艺和微波等离子体化学气相沉积工艺可在Si微尖上沉积一层光滑敷形的金刚石薄膜；沉积后阴极的电压-电流特性、发射电流的稳定性以及工作在氧气环境下的发射特性都获得明显提高。讨论了超纳米晶金刚石薄膜阴极的发射机理。     

Amorphous-silicon-on-glass field emitter arrays

An amorphous-silicon (a-Si) field emitter array (FEA) has been fabricated on a glass substrate and characterized, At first, a 0.3-μm-thick Cr film was deposited on the glass by vacuum evaporation technique and subsequently a 1-μm-thick Si film was deposited on the Cr film by conventional RF sputtering technique with an undoped Si target at room temperature. The sputtered Si film was identified as amorphous from X-ray diffraction patterns and had a resistivity of 3-5 kΩ-cm. The FEA consists of 1-μm-high emitter tips and a gate electrode with 1.8-μm-diameter openings. This a-Si FEA with 5×5 (=25) tips exhibited a threshold voltage of 30 V and an emission current of 2 μA at a gate voltage of 100 V. Structure and emission characteristics are discussed     

High current density Si field emission devices with plasmapassivation and HfC coating

A novel self-aligned process was developed to fabricate gated Si field emission devices. At a gate voltage of 100 V, the emission current from an array of 100 tips increased from 283 to 460 μA and the turn-on voltage decreased from 31 to 21 V after H₂ plasma passivation using an inductively coupled plasma (ICP) source for 2 min. The improvements correspond to a 1.28-eV reduction in the effective work function of the emitters and the instability of the emission current decreased from ±1,25 to ±0.25% after H₂ plasma passivation. Emitter tips were also coated with Mo silicide and HfC. The emission current increased from 230 μA for uncoated emitters to 268 μA for emitters coated with Mo silicide and 389 μA for emitters coated with HfC. The turn-on voltage decreased from 50 to 41 and 25 V while the breakdown voltage increased from 126 to 129 and 143 V when Mo silicide and HfC were used for coating, respectively, which correspond to reductions of 0.95 and 2.23 eV, respectively, in the effective work function of the emitters. Single emitter tips have similar emission characteristics as high-density field emitter arrays, indicating excellent emission uniformity from the arrays     

Si和Si尖上纳米金刚石场电子发射性质的研究

研究了Si和Si尖上纳米金刚石场电子发射性质。纳米金刚石是利用热灯丝CVD法合成的，反应气体是CH4、N2和H2的混合物。实验结果表明Si衬底上的纳米金刚石特别是Si尖上的金刚石与多晶金刚石相比，前者极大地改善了场发射性质。场发射电场阈值和电流与金刚石晶粒度密切相关。文中对其结果进行了讨论。     

粒子轰击对碳纳米管微结构和场发射性能的影响

研究了粒子轰击对碳纳米管膜微结构和场电子发射性能的影响。研究结果表明．采用多种等离子体处理碳纳米管能改变其微结构．并显著提高场发射点密度．从而改善场发射性能。随着粒子轰击时间的延长,发射电流先降低,然后又增加甚至超过原始碳纳米管膜的发射电流。我们认为发射性能的变化归功于不同的发射机理：粒子轰击处理后．电子由纳米管尖端发射逐渐变为主要从管体上的纳米瘤发射,致使发射点密度显著增加。     

Growth of Si and Ge nanostructures on Si substrates using ultrathin SiO/sub 2/ technology

Using scanning reflection electron microscopy and a high-temperature scanning tunneling microscopy (STM), we study the growth processes of Si and Ge nanostructures on Si substrates covered with ultrathin SiO/sub 2/ films. Si windows are formed in the ultrathin SiO/sub 2/ films by irradiating focused electron beams used for SREM or field emission electron beams from STM tips before or during heating samples. Ge nanoislands are grown only at the Si window positions by depositing Ge on the samples and by subsequent annealing of them. Moreover, Ge nanoislands about 7 nm in size and ultrahigh density (>10/sup 12//cm/sup 2/) are grown on the ultrathin SiO/sub 2/ films. These nanoislands can be manipulated by STM when they are separated from Si substrates by the ultrathin SiO/sub 2/ films. Si, Ge, Ge/Si and Si/Ge/Si nanoislands can also be grown on the Si windows by selective growth using Si/sub 2/H/sub 6/ and GeH/sub 4/ gases. These nanoislands are found to be stable on the Si windows during high-temperature annealing. These results indicate that ultrathin SiO/sub 2/ technology is useful for growing Si and Ge nanostructures on given areas.     

金刚石镶嵌非晶碳膜表面微尖对场致发射的增强作用

用微波等离子体化学气相沉积设备,在金属钼衬底上沉积出了表面存在大量微尖的金刚石镶嵌非晶碳膜,分别用扫描电子显微镜（ＳＥＭ）、金相显微镜和Ｘ射线衍射谱（ＸＲＤ）及Ｒａｍ ａｎ谱对样品进行了分析测试,并研究了各样品的场致电子发射特性。结果发现：在笔者的实验范围内,金刚石薄膜表面微尖对场致电子发射具有增强作用,且薄膜表面微尖数目越多,场发射电流密度和发射点密度越高,场发射的发射阈值越低。最后建立了一个二次场增强模型对实验结果进行了解释     

A formation of cobalt silicide on silicon field emitter arrays byelectrical stress

A novel process utilizing electrical stress is proposed for the formation of Co silicide on single crystal silicon (c-Si) FEAs to improve the field emission characteristics. Co silicide FEAs formed by electrical stress (ES) exhibited a significant improvement in turn-on voltage and emission current compared with c-Si FEAs. The improvement mainly comes from the lower effective work function of Co silicide and less blunting of tips during silicidation by electrical stress in an ultra high vacuum (UHV) environment less than 10^-8 torr     

Synthesis,field emission and optical properties of ZnSe nanobelts,nanorods and nanocones by hydrothermal method

ZnSe nanostructures, such as nanobelts, nanorods and nanocones, were successfully synthesized on Zn foils via a hydrothermal method using EDTA as soft template at low temperature. EDTA played a significant role on the morphology of ZnSe nanomaterials. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS) were carried out to characterize the microstructures and chemical compositions of the as-synthesized ZnSe samples. XRD patterns indicated that the as-synthesized ZnSe samples belonged to a cubic zinc blende structure. SEM observation obviously showed that the nanocones had very sharp tips compared to nanorods and nanobelts. The field emission (FE) measurement showed that the as-synthesized ZnSe nanocones had a lower turn-on field of ~1.6 V μm⁻¹ at the current density of 10 μA cm⁻². A high field enhancement factor of ~4514 was achieved for the ZnSe nanocones. The superior field emission properties were probably attributed to the sharp tips of the ZnSe nanocones. Room temperature photoluminescence (PL) spectroscopy of the ZnSe nanostructures showed a wide band emission from blue light to orange light. The as-prepared ZnSe nanomaterials have promising applications in optoelectronic devices. A possible formation mechanism of ZnSe nanobelts, nanorods and nanocones was also proposed and discussed.     

Characterization of thin textured tunnel oxide prepared by thermaloxidation of thin polysilicon film on silicon

In this paper, the characteristics of thin textured tunnel oxide prepared by thermal oxidation of thin polysilicon film on Si substrate (TOPS) are studied. Because of the rapid diffusion of oxygen through the grain boundaries of the thin polysilicon film into the Si substrate and the enhanced oxidation rate at the grain boundaries, the oxidation rate of the TOPS sample is close to that of a normal oxide grown on a (111) Si substrate. Also, a textured Si/SiO₂ interface is obtained. The textured Si/SiO₂ interface results in localized high fields and causes a much higher electron injection rate. The optimum TOPS sample can be obtained by properly oxidizing the stacked α-Si film, independent of the substrate doping level. Also, the optimum TOPS sample exhibits a smaller electron trapping rate and a lower interface state generation rate when compared to the sample from a standard tunnel oxide process. These differences are attributed to a lower bulk electric field and a smaller injection area in the TOPS samples     

Influence of alloy composition,substrate temperature,and doping concentration on electrical properties of Si-doped n-Alx Ga1−x As grown by molecular beam epitaxy

Capacitance and Hall effect measurements in the temperature range 10-300 K were performed to evaluate the deep and shallow level characteristics of Si-doped n-Al_xGa_-xAs layers with 0 × 0.4 grown by molecular beam epitaxy. For alloy compositions × 0.3 the overall trap concentration was found to be less than 10⁻² of the carrier concentration. In this composition range the transport properties of the ternary alloy are comparable to those of n-GaAs:Si except for lower electron mobibities due to alloy scattering. With higher Al content one dominant electron trap determines the overall electrical properties of the material, and in n-Al_0.35Ga_0.65As:Si the deep trap concentration is already of the order of the free-carrier concentration or even higher. For the composition × = 0.35 ± 0.02 the influence of growth temperature and of Si dopant flux intensity on the deep trap concentration, on shallow and deep level activation energy, and on carrier freeze-out behaviour was studied and analyzed in detail. Our admittance measurements clearly revealed that the previously assumed deepening of the shallow level in n-Al_x Ga_1-x As of alloy composition close to the direct-indirect cross-over point does actuallynot exist. In this composition range an increase of the Si dopant flux leads to a reduction of the thermal activation energy for electron emission from shallow levels due to a lowering of the emission barrier by the electric field of the impurities. The increasing doping flux also enhances the concentration of the dominant electron trap strongly, thus indicating a participation of the dopant atoms in the formation of deep donor-type (D,X) centers. These results are in excellent agreement with the model first proposed by Lang et al. for interpretation of deep electron traps in n-Al_x Ga_1-x grown by liquid phase epitaxy.     

Bipolar transistor fabrication using selective epitaxial growth ofP- and B-doped layers in gas-source Si molecular beam epitaxy

Si n-p-n bipolar transistor fabrication using selective epitaxial growth in disilane gas-source Si molecular beam epitaxy (Si-MBE) is discussed. Selective growth of B-doped and P-doped Si was used for the base- and emitter-layer formation, respectively. The growth temperature was 600°C. No ion-implantation process was used. The base ohmic contact was formed using Al selective chemical vapor deposition. The fabricated transistor showed normal emitter-base and base-collector I-V characteristics. The common-emitter characteristics revealed a maximum current gain of 30     

Effect of Si nanoparticles on electronic transport mechanisms in P-doped silicon-rich silicon nitride/c-Si heterojunction devices

We successfully fabricated hetero-junction (H-J) devices from P-doped silicon-rich SiN_x embedded with Si nanoparticles on a p-type crystalline Si substrate at low temperature. High-resolution transmission electron microscopy (HRTEM) analysis indicates that the thin films contain nano-crystallites. The H-J devices showed a good rectification ratio at room temperature. Three distinct regions of temperature dependent J-V characteristics curve can be identified, where different current density variations are indicated. In the low voltage range, the current across the interface of H-J follows an ohmic behavior. In the intermediate range of voltage, the current transport mechanism shows a transition from the phosphorus diffusion to tunneling dominant due to the silicon nanoparticle size and interface of HJ device changed, while the space-charge-limited current (SCLC) dominates the conduction mechanism in the high voltage range and the density of trapping states also affects the electron transport proceeding. At last, the proper size of silicon nanoparticle can reduces the interface charge density of H-J, which is confirmed via the numerical C-V matching technique and we propose a new energy band diagram to fit the HJ device embedded by the silicon nanoparticles.     

Simultaneous STM and UHV electron microscope observation of silicon nanowires extracted from Si(111) surface

A miniaturized scanning tunnelling microscope (STM) was fitted in a side-entry holder of an ultra-high vacuum electron microscope. The clean Si(111)7 x 7 surface was observed by both STM and reflection electron microscopy (REM) at atomic resolution. The tungsten tips were often rounded off upon tip-approach with a constant current, through a gentle touch with the sample surface. The apices of such rounded tips had radii of several tens of granometre with widths of about 3 x 3 nm. Atomically resolved STM of the Si(111)7 x 7 surface was obtainable when an atom or an atomic cluster sits on the tip surface. The rounded tips were used for fabrication of Si nanowires by the touch-and-away operation of the tip. The nanowires grew longer at higher substrate temperature and they reached as long as several tens of nanometre at 700 degrees C. The nanowire had many twins and the (111) twin lamellae were stacked in the direction of the wire axis. In another case, the twin planes were oblique to the wire axis so that the (112) direction was nearly parallel to the wire axis.     

碳纳米球薄膜的场发射特性

利用微波等离子体化学气相沉积法,在Si(100)衬底上制备了碳纳米球薄膜。利用拉曼光谱和场发射扫描电子显微镜研究了薄膜的结构以及表面形貌,表明碳纳米球薄膜是由约2～3μm长、100nm宽的无定形碳纳米片相互缠绕、交织成球状而构成的。在高真空系统中测量了碳纳米球薄膜的场发射特性,结果表明,碳纳米球薄膜具有良好的场发射特性,阈值电场为3.1V/μm,当电场增加到10V/μm时,薄膜的场发射电流密度可达到60.7mA/cm2。通过三区域电场模型合理地解释了碳纳米球薄膜在低电场、中间电场和高电场区域的场发射特性。     

The preparation of oxygen-deficient ZnO nanorod arrays and their enhanced field emission

Vertical and uniform zinc oxide (ZnO) nanorod arrays (NRAs) with sharp tips were fabricated on Zn substrate by a straightforward hydrothermal method without the assistance of seed layer, template or surfactant. Whereafter, the as-synthesized ZnO NRAs were successfully doped with oxygen vacancies by sodium borohydride (NaBH₄) solution reduction, aiming to generate donor energy levels below the conduction band. More importantly, the doped concentration of oxygen vacancies could be effectively controlled by adjusting the reduction temperature, and we have ultimately achieved the purpose of controllable tailoring the energy band structure of ZnO NRAs. As with design, the oxygen-deficient ZnO NRAs present a lower turn-on field of 0.67 V/μm, higher field enhancement factor of 64601 and better field emission stability. Such excellent FE performance of the as-prepared emitter should originate from the optimization of geometry, the efficient electron transport, as well as the decreased work function.