离子束增强沉积制备p型氧化锌薄膜及其机理研究

采用离子束增强沉积方法在Si和SiO₂/Si衬底上制备In-N共掺杂ZnO薄膜(INZO)，溅射靶是用ZnO和2 atm% In₂O₃粉体均匀混合并压制而成，在氩离子溅射ZnO靶的同时，氮、氩混合离子束垂直注入沉积的薄膜.实验结果显示INZO薄膜具有(002)的择优取向，并且为p型导电，电阻率最低为0.9Ωcm.薄膜在氮气、氧气气氛下退火，对薄膜的结构和电学特性与成膜和退火条件的关系进行了分析. 关键词： 氧化锌薄膜 p型掺杂 离子束增强沉积     

Characterization of semiconductor nanostructures formed by using ultrathin Si oxide technology

We present a method to form semiconductor nanodots on Si substrates by using ultrathin Si oxide technology and the results on their optical properties. We can form ultra-small semiconductor nanodots with the size of ∼5 nm and ultra-high density of ∼10¹² cm⁻² on Si surfaces covered with ultrathin SiO₂ films of ∼0.3 nm thickness. We focus on photoluminescence and electroluminescence properties of Ge nanodots embedded in Si films. These structures exhibit intense luminescence in the energy region of about 0.8 eV.     

As-doped p-type ZnO films grown on SiO2/Si by radio frequency magnetron sputtering

p-Type ZnO:As films with a hole concentration of 10¹⁶-10¹⁷ cm⁻³ and a mobility of 1.32-6.08 cm²/V s have been deposited on SiO₂/Si substrates by magnetron sputtering. XRD, SEM, Hall measurements are used to investigate the structural and electrical properties of the films. A p-n homojunction comprising an undoped ZnO layer and a ZnO:As layer exhibits a typical rectifying behavior. Our study demonstrates a simple method to fabricate reproducible p-type ZnO film on the SiO₂/Si substrate for the development of ZnO-based optoelectronic devices on Si-based substrates.     

Structural investigations of silicon nanostructures grown by self-organized island formation for photovoltaic applications

The self-organized growth of crystalline silicon nanodots and their structural characteristics are investigated. For the nanodot synthesis, thin amorphous silicon (a-Si) layers with different thicknesses have been deposited onto the ultrathin (2 nm) oxidized (111) surface of Si wafers by electron beam evaporation under ultrahigh vacuum conditions. The solid phase crystallization of the initial layer is induced by a subsequent in situ annealing step at 700 °C, which leads to the dewetting of the initial a-Si layer. This process results in the self-organized formation of highly crystalline Si nanodot islands. Scanning electron microscopy confirms that size, shape, and planar distribution of the nanodots depend on the thickness of the initial a-Si layer. Cross-sectional investigations reveal a single-crystalline structure of the nanodots. This characteristic is observed as long as the thickness of the initial a-Si layer remains under a certain threshold triggering coalescence. The underlying ultra-thin oxide is not structurally affected by the dewetting process. Furthermore, a method for the fabrication of close-packed stacks of nanodots is presented, in which each nanodot is covered by a 2 nm thick SiO₂ shell. The chemical composition of these ensembles exhibits an abrupt Si/SiO₂ interface with a low amount of suboxides. A minority charge carrier lifetime of 18 µs inside of the nanodots is determined.     

ZnO nanoparticle creating in a SiO2/Si structure using the Zn ion implantation with subsequent heat treatment

The distribution profiles of the dopant in the surface layer of a SiO₂/Si structure implanted with Zn and O ions are studied via Rutherford backscattering spectroscopy for He²⁺ ions using the channeling technique. The redistribution of implanted impurities in the Si surface layer during the formation process of zinc oxide (ZnO) nanoparticles is analyzed. The effect of the annealing temperature on the formation process and growth of ZnO nanoparticles is studied. The sample-surface morphology is examined via atomic force microscopy. The optical absorption and photoluminescence of the implanted layers are studied.     

Formation of nanoparticles containing zinc in Si(001) by ion-beam implantation and subsequent annealing

The formation of nanoparticles containing zinc in Si(001) substrates by the implantation of ⁶⁴Zn⁺ ions and subsequent annealing in dry oxygen at 800 and 1000°C for 1 h is studied. The structure of the samples is studied by high-resolution transmission electron microscopy, X-ray diffraction, and photoluminescence spectroscopy. 20-nm zinc nanoparticles located at a depth of about 50 nm are revealed in the as-implanted sample. 10–20-nm pores are observed in the surface layer. Annealing leads to oxidation of the Zn nanoparticles to the Zn₂SiO₄ state. It is shown that the oxidation of Zn nanoparticles begins on their surface and at an annealing temperature of 800°C results in the formation of nanoparticles with the “соre–shell” structure. The X-ray diffraction technique shows simultaneously two Zn and Zn₂SiO₄ phases. ZnO nanoparticles are not formed under the given implantation and annealing conditions.     

Texture control and its impact on the properties of SrBi2Ta2O9 thin films prepared by pulsed laser deposition

SrBi₂Ta₂O₉ (SBT) ferroelectric thin films with different preferred orientations were deposited by pulsed laser deposition (PLD). Several methods have been developed to control the preferred orientation of SBT thin films. For SBT films deposited directly on Pt/TiO₂/SiO₂/Si substrates and in situ crystallized at the deposition temperature, the substrate temperature has a significant impact on the orientation and the remnant polarization (Pr) of the films; a higher substrate temperature benefits the formation of (115) texture and larger grain size. The films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C are (115)-oriented and exhibit 2Pr of 6 μC/cm². (115)- and (200)-predominant films can be formed by using a La_0.85Sr_0.15CoO₃ (LSCO) buffer layer or by annealing amorphous SBT films deposited on Pt/TiO₂/SiO₂/Si substrates at 450 °C using rapid thermal annealing (RTA). These films exhibit good electric properties; 2Pr of the films are up to 12 μC/cm² and 17 μC/cm², respectively. The much larger 2Pr of the films deposited on the LSCO buffer layer and of the films obtained by RTA than 2Pr of the films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C is attributed to a stronger (200) texture. Received: 30 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Laser–MBE growth of high-quality ZnO thin films on Al2O3(0001) and SiO2/Si(100) using the third harmonics of a Nd:YAG laser

High-quality ZnO thin films were grown on single-crystalline Al₂O₃(0001) and amorphous SiO₂/Si(100) substrates at 400–640 °C using laser molecular beam epitaxy. For film growth, the third harmonics of a pulsed Nd:YAG laser were illuminated on a ZnO target. The ZnO films were epitaxially grown on Al₂O₃(0001) with the narrow X-ray diffraction full width at half maximum (FWHM) of 0.04° and the films on SiO₂/Si(100) exhibited a preferred c-axis orientation. Furthermore, the films exhibited excellent optical properties in photoluminescence (PL) measurements with very sharp excitonic and weak deep-level emission peaks. At 15 K, PL FWHM values of the films grown on Al₂O₃(0001) and SiO₂/Si(100) were 3 and 18 meV, respectively. Received: 8 May 2001 / Accepted: 18 September 2001 / Published online: 20 December 2001     

Electroluminescence from nano-crystalline Si/ SiO2 structures embedded in pn junctions

Phosphorous-doped and boron-doped amorphous Si thin films as well as amorphous SiO₂/Si/ SiO₂ sandwiched structures were prepared in a plasma enhanced chemical vapor deposition system. Then, the p–i–n structures containing nano-crystalline Si/ SiO₂ sandwiched structures as the intrinsic layer were prepared in situ followed by thermal annealing. Electroluminescence spectra were measured at room temperature under forward bias, and it is found that the electroluminescence intensity is strongly influenced by the types of substrate. The turn-on voltages can be reduced to 3 V for samples prepared on heavily doped p-type Si (p⁺-Si) substrates and the corresponding electroluminescence intensity is more than two orders of magnitude stronger than that on lightly doped p-type Si (p-Si) and ITO glass substrates. The improvements of light emission can be ascribed to enhanced hole injection and the consequent recombination of electron–hole pairs in the luminescent nanocrystalline Si/ SiO₂ system.     

Nanostructuring Si surface and Si/SiO2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO₂ nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO₂ interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10¹⁰ eV⁻¹ cm⁻² is obtained, indicative of the high quality of the thermal silicon oxide).     

Sensitizing effects of ZnO quantum dots on red-emitting Pr-doped SiO2 phosphor

In this study, red cathodoluminescence (CL) (λ_emission=614 nm) was observed from Pr³⁺ ions in a glassy (amorphous) SiO₂ host. This emission was enhanced considerably when ZnO quantum dots (QDs) were incorporated in the SiO₂:Pr³⁺ suggesting that the ZnO QDs transferred excitation energy to Pr³⁺ ions. That is, ZnO QDs acted to sensitize the Pr³⁺ emission. The sol–gel method was used to prepare ZnO–SiO₂:Pr³⁺ phosphors with different molar ratios of Zn to Si. The effects of the ZnO QDs concentration and the possible mechanisms of energy transfer from ZnO to Pr³⁺ are discussed. In addition, the electronic states and the chemical composition of the ZnO–SiO₂:Pr³⁺ phosphors were analyzed using X-ray photoelectron spectroscopy (XPS).     

Effects of oxygen plasma pre-treatments on the characteristics of n-ZnO/p-Si heterojunction diodes

We examine the effects of the oxygen plasma pre-treatments on the material properties of n-ZnO grown on p-Si and characterize the electrical properties of n-ZnO/p-Si heterojunction diodes. The lattice spacing of ZnO becomes larger when the ZnO thin film is grown on the oxygen plasma pre-treated Si substrate. This might be relevant to the growth of (101) ZnO onto the ultra-thin SiO₂ interfacial layer, which is formed during the oxygen plasma pre-treatment onto the Si substrate. The formation of SiO₂ gives rise to the increase in the donor-like defect Zn interstitial, and the increased grain size improves the carrier mobility. Because of all the above, the differential conductance at the on-state is increased for the n-ZnO/p-Si heterojunction diode.     

Formation and properties of high density Si nanodots

Laser induced crystallization of ultrathin hydrogenated amorphous Si films or amorphous Si-based multilayered structures were used to get high density Si nanodots. The present technique can get size controllable Si nanodots embedded in various dielectric materials with uniform distribution which was revealed by cross-section transmission electron microscopy. Room temperature photoluminescence and electroluminescence were achieved with the emission wavelength in a visible light region both from a-SiN/Si nanodots/a-SiN sandwiched and Si nanodots/SiO₂ multilayered structures. The luminescence was associated with the radiative recombination of generated electron-hole pairs in Si nanodots or the luminescent surface states. The electroluminescence intensity is increased with increasing the injection current implying the bipolar carrier injection plays an important role in enhancing the luminescence efficiency. The formed Si nanodots by the present approach can be applied for many kinds of devices such as high efficient light emitting diodes and solar cells.     

Spectroscopic ellipsometry modeling of ZnO thin films with various O2 partial pressures

ZnO films were deposited on thermally oxidized SiO₂/p-type Si (100) substrates and glass substrates by DC magnetron sputtering using a metal Zn target. Three types of samples were prepared with various O₂/(Ar + O₂) ratios (O₂ partial pressure) of 20%, 50%, and 80%. The properties of these ZnO thin films were investigated using X-ray diffraction (XRD), optical transmittance, atomic force microscopy (AFM), and spectroscopic ellipsometry in the spectral region of 1.7–3.1 eV. The structural and optical properties of ZnO thin films were affected by O₂ partial pressure. Relationships between crystallinity, the ZnO surface roughness layer, and the refractive index (n) were investigated with varying O₂ partial pressure. It was shown that the spectroscopic ellipsometry extracted parameters well represented the ZnO thin film characteristics for different O₂ partial pressures.     

Emulsion Combustion and Flame Spray Synthesis of Zinc Oxide/Silica Particles

Two flame spray methods, emulsion combustion method (ECM) and flame spray pyrolysis (FSP), were compared for synthesis of pure and mixed SiO₂ and ZnO nanoparticles. The effect of silicon precursor was investigated using liquid hexamethyldisiloxane (HMDSO) or SiO₂ sol, while for ZnO zinc acetate (ZA) was used. Gas phase reaction took place when using HMDSO as Si precursor, forming nanoparticles, whereas the SiO₂ sol used as Si source was not evaporated in the flame, creating large aggregates of the sol particles (e.g. 1 m). The FSP of ZA produced ZnO homogeneous nanoparticles. Lower flame temperatures in ECM than in FSP resulted in mixed gas and liquid phase reaction, forming ZnO particles with inhomogeneous sizes. The FSP of HMDSO and ZA led to intimate gas-phase mixing of Zn and Si, suppressing each other's particle growth, forming nanoparticles of 19 nm in BET-equivalent average primary particle diameter. Nucleation of ZnO and SiO₂ occurred independently by ECM of HMDSO and ZA as well as by FSP of the SiO₂ sol and ZA, creating a ZnO and SiO₂ mixture. The reaction of ZnO with SiO₂ was likely to be enhanced by ECM of the SiO₂ sol and ZA where both Zn and Si species were not evaporated completely, resulting in ZnO, -willemite and Zn_1.7SiO₄ mixed phase.     

Synthesis of nanocrystalline α - Zn2SiO4 at ZnO-porous silicon interface: Phase transition study

Thermal annealing induced formation of nanocrystalline Zinc silicate (α-Zn₂SiO₄) at the interface of ZnO-porous silicon (PSi) nanocomposites is reported. The PSi templates were formed by electrochemical anodization of p-type (100) Si and ZnO crystallites were deposited on the PSi surface by a Sol-gel spin coating process. The formation of α-Zn₂SiO₄ is confirmed by glancing angle X-ray diffraction and Fourier transform infrared spectroscopy studies. The presence of intense yellow-green emission also confirms the formation of α-Zn₂SiO₄. The mechanism of silicate phase formation at the ZnO-PSi interface and the origin of various photoluminescence (PL) bands are discussed in view of its potential applications in advanced optoelectronic devices.     

Atomic diffusion in annealed Cu/SiO2/Si（100） system prepared by magnetron sputtering

Cu thin films are deposited on p-type Si (100) substrates by magnetron sputtering at room temperature. The inter-face reaction and atomic diffusion of Cu/SiO2/Si (100) systems are studied by x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). Some significant results can be obtained. The onset temperature of interdiffusion for Cu/SiO2/Si(100) is 350 C. With the annealing temperature increasing, the interdiffusion becomes more apparent. The calculated diffusion activation energy is about 0.91 eV. For the Cu/SiO2/Si (100) systems copper silicides are not formed below an annealing temperature of 350 C. The formation of the copper silicides phase is observed when the annealing temperature arrives at 450 C.     

Spherical aberration corrected STEM studies of Ge nanodots grown on Si(0 0 1) surfaces with an ultrathin SiO2 coverage

Germanium (Ge) nanodots of about 7 nm size and 2 × 10¹² cm⁻² density were formed on slightly oxidized silicon surfaces. The spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM) revealed clearly the size, aspect ratio and interface structures among the nanodots, oxide layers and silicon substrates. In particular, a Ge-rich thin layer underneath SiO₂ layers was found for the first time in these kinds of samples. The elemental distribution through the interface was analyzed by EELS and EDX in the Cs-corrected STEM. The high-resolution Cs-corrected annular dark field (ADF)-STEM image shows clearly the existence of a Ge-rich crystalline layer and its geometry against the oxide layer from the Z-contrast image. A new growth model of the Ge nanodots on slightly oxidized silicon surfaces was proposed.     

Characterization of silicon dioxide deposited by low-temperature CVD using TEOS and ozone by monoenergetic positron beams

Monoenergetic positrons were used as a nondestructive probe for SiO₂ films deposited on Si substrates by atmospheric-pressure chemical vapor deposition using tetraethylortho-silicate (TEOS, Si(OC₂H₅)₄) and O₃. The formation of positronium (Ps) in the SiO₂ films was found from measurements of Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons. A clear correlation between the formation probability of Ps and the concentration of H₂O in the SiO₂ films was established.     

Studies of structure and composition of a Pt-based basic electrode for deposition of PZT ferroelectric films on silicon substrates

In this study it has been found that a TiO₂/Ti double adhesion layer is used for SiO₂/Si substrate and a YSZ layer is used for SiO₂/Si and Si substrates to achieve high residual polarization of PZT layers on platinum. Epitaxial deposition of YSZ on Si(100) provides an atomically smooth platinum layer textured in the [100] direction. The produced PZT films are mostly textured in the [111] direction; their residual polarization is 5–15 μC/cm².