Ge nanocrystals in magnetron sputtered SiO2

Structural and optical properties of Ge nanocrystals in SiO₂ films created by magnetron sputtering and heat treatment have been investigated. The formation of nanocrystals is found to be influenced by the temperature of heat treatment and the Ge concentration in the films. After heat treatment at 1000 °C nanocrystals are present throughout the film, with the exception of a region close to the surface that does not contain nanocrystals. This effect is assigned to oxidation of Ge in this part of the film. The size distribution of the nanocrystals is analyzed by transmission electron microscopy for a range of deposition and heat-treatment parameters. By analyzing the transmission electron microscopy images, it is possible to estimate the fraction of nanoclusters that are crystalline for a given set of growth parameters. This analysis shows that all the nanoclusters are created in the crystalline state. Raman spectroscopy is employed to probe the Ge–Ge bonds. In combination with transmission electron microscopy, this information can be used to distinguish between growth modes such as nucleation or Ostwald ripening. The photoluminescence spectra exhibit a strong broad line at 625 nm, the presence of which is demonstrated to correlate to the presence of Ge nanocrystals. PACS 61.46.+w; 78.55.-m; 78.67.-n     

Large melting-point hysteresis of Ge nanocrystals embedded in SiO2

The melting behavior of Ge nanocrystals embedded within SiO2 is evaluated using in situ transmission electron microscopy. The observed melting-point hysteresis is large (+/-17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.     

Voltage-controlled electroluminescence from SiO2 films containing Ge nanocrystals and its mechanism

Luminescent SiO₂ films containing Ge nanocrystals are fabricated by using Ge ion implantation, and metal–oxide–semiconductor structures employing these films as the active layers show yellow electroluminescence (EL) under both forward and reverse biases. The EL spectra are strongly dependent on the applied voltage, but slightly on the mean size of Ge nanocrystals. When the forward bias increases towards 30 V, the EL spectral peak shifts from 590 nm to 485 nm. It is assumed that the EL originates from the recombination of injected electrons and holes in Ge nanocrystals near the Si/SiO₂ interface, or through luminescent centers in the SiO₂ matrix near the SiO₂/metal interface. The mismatch of the injection amounts between holes and electrons results in the low EL efficiency. Received: 28 February 2000 / Accepted: 28 March 2000 / Published online: 5 July 2000     

Structural investigations of Ge nanoparticles embedded in an amorphous SiO2 matrix

Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO₂ matrix. GeSiO thin films are prepared by two methods, sol?Cgel and radio frequency magnetron sputtering. After the deposition, the sol?Cgel films are annealed in either N₂ (at 1 atm and 800 °C) or H₂ (at 2 atm and 500 °C), and the sputtered films in H₂ (at 2 atm and 500 °C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3?C7 nm size) are observed in sol?Cgel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10?C50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol?Cgel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed.     

Synthesis of crystalline Ge nanoclusters in PE-CVD-deposited SiO2 films

The synthesis of evenly distributed Ge nanoclusters in plasma-enhanced chemical-vapour-deposited (PE-CVD) SiO₂ thin films containing 8 at. % Ge is reported. This is of importance for the application of nanoclusters in semiconductor technology. The average diameter of the Ge nanoclusters can be controlled in the range of 3–6 nm by variation of the annealing parameters. Using a combination of transmission electron microscopy and Raman-scattering spectroscopy, the nanoclusters were shown to be crystalline. However, photoluminescence measurements showed no light emission that could be definitively correlated to the presence of the nanoclusters. PACS 61.82.Rx; 78.67.Bf; 81.07.Bc     

Bi nanocrystals embedded in an amorphous Ge matrix grown by pulsed laser deposition

Received: 1 September 1997/Accepted 8 September 1997     

Structural and optical properties of Ge nanocrystals obtained by hot ion implantation into SiO2 and further ion irradiation

In this work, SiO₂ layers containing Ge nanocrystals (NCs) obtained by the hot implantation approach were submitted to an ion irradiation process with different 2 MeV Si⁺ ion fluences. We have investigated the photoluminescence (PL) behavior and structural properties of the irradiated samples as well as the features of the PL and structural recovery after an additional thermal treatment. We have shown that even with the highest ion bombardment fluence employed (2×10¹⁵ Si/cm²) there is a residual PL emission (12% from the original) and survival of some Ge NCs is still observed by transmission electron microscopy analysis. Even though the final PL and mean diameter of the nanoparticles under ion irradiation are independent of the implantation temperature or annealing time, the PL and structural recovery of the ion-bombarded samples have a memory effect. We have also observed that the lower the ion bombardment fluence, the less efficient is the PL recovery. We have explained such behavior based on current literature data.     

Proton diffusion mechanism in amorphous SiO2

We study proton diffusion in amorphous SiO2 from the atomic scale to the long-range percolative regime. Ab initio molecular dynamics suggest that the dominant atomic process consists in cross-ring interoxygen hopping assisted by network vibrations. A statistical analysis accounting for the disorder in amorphous SiO2 yields relations between transition energies and interoxygen distances for both cross-ring and nearest-neighbor hopping. The percolative regime is then addressed through large-size model systems reproducing these relations. Cross-ring hopping is confirmed as the dominant diffusion mechanism and supported by a good agreement with experiment for the activation energy.     

Formation of Ge nanocrystals with sharp size distribution: structural and optical characterization

We have studied the formation of Ge nanocrystals in a SiO_x matrix using high-resolution electron microscopy, Raman- and luminescence spectroscopy. The room temperature cathodoluminescence centered at 2.2-2.4 eV and the photoluminescence at 2.4 eV are correlated with the crystallization of the as-prepared Ge cluster. However, no peak shift was found in spite of the sharp size distribution of the particles. The data suggest that quantum confinement is not the origin of the luminescence.     

SiO2/Ge:SiO2/SiO2夹层结构红外光发射的起源

我们借助傅立叶变换红外光谱（FT－IR）以及光致激发谱（PLE）,研究SiO2/Ge:SiO2/SiO2夹层结构红外光发射的起源。谱分析表明,该红外光发射并非起源于纳米锗、硅的量子限制效应以及锗、硅的中性氧空位,而与锗的氧化物紧密相关。PLE的结果证实它们来源于GeO色心TⅡ‘→S0的光学跃迁,给出的GeO电子态模型描述了载流子激发和复合的过程。     

Size and aerial density distributions of Ge nanocrystals in a SiO2 layer produced by molecular beam epitaxy and rapid thermal processing

We discuss the distribution of size and aerial density of Ge nanocrystals in a metal-oxide-semiconductor (MOS) memory structure fabricated by molecular beam epitaxy combined with rapid thermal processing; the size and aerial density of Ge nanocrystals are controlled by varying the thickness of the deposited Ge layer and the processing time. Variation of tunnel oxide thickness is demonstrated with the extension of the processing time. The effect of processing time and tunnel oxide thickness on the electrical properties of the MOS structures is investigated by high frequency capacitance–voltage measurements. PACS  61.46.+w; 81.07.-b; 81.07.Bc; 81.07.Ta     

Synthesis of Ge nanocrystals by atom beam sputtering and subsequent rapid thermal annealing

Ge nanocrystals embedded in an SiO₂ matrix were prepared by the atom beam co-sputtering (ABS) method from a composite target of Ge and SiO₂. The as-deposited films were rapid thermally annealed at the temperatures 700 and 800 °C in nitrogen ambience. The structure of the films was evaluated by using X-ray diffraction (XRD) and Raman spectroscopy. XRD results reveal that as-deposited films are amorphous in nature whereas annealed samples show crystalline nature. Raman scattering spectra showed a peak of Ge–Ge vibrational mode shifted downwards to 297 cm^?1, presumably caused by quantum confinement of phonons in the Ge nanocrystals. Rutherford backscattering spectrometry has been used to measure the thickness and Ge composition of the composite films. Size variation of Ge nanocrystals with annealing temperature has been discussed. The advantages of ABS over other methods are highlighted.     

Photo-induced charge transport in SiO2films containing Si nanocrystals

Photo-induced charge transport is reported in metal–insulator–semiconductor structures containing Si nanocrystals produced by ion implantion and annealing. Successive shifts in current–voltage (I–V) and capacitance–voltage (C–V) curves are shown to be induced by ultra-violet (UV) light exposure under no bias. These shifts are shown to be enhanced by the application of a negative bias voltage during illumination. The application of a positive bias voltage during illumination is shown to reverse the direction of the shifts in both the I–V and C–V curves. This behaviour can be explained by charging of the nanocrystals induced by photoionization of electrons and charge movements in the insulator layer.     

Soft X-ray fluorescence and photoluminescence of Si nanocrystals embedded in SiO2

We have used ion-beam mixing to form Si nano-crystals in SiO₂ and SiO₂/Si multilayers, and applied photoluminescence and soft-X-ray emission spectroscopy to study the nanoparticles. Ion-beam mixing followed by heat treatment at 1100 °C for 2 h forms the Si nanocrystals. The ion-beam-mixed sample shows higher PL intensity than that of a Si-implanted SiO₂ film. Photon and electron-excited Si L_2,3 X-ray emission measurements were carried out to confirm the formation of Si nanocrystal in SiO₂ matrix after ion-beam mixing and heat treatment. It is found that Si L_2,3 X-ray emission spectra of ion-beam-mixed Si monolayers in heat-treated SiO₂ films lead to noticeable changes in the spectroscopic fine structure. Received: 20 November 1999 / Accepted: 17 April 2000 / Published online: 5 October 2000     

Influence of the deposition and annealing temperatures on the luminescence of germanium nanocrystals formed in GeO x films and multilayer Ge/SiO2 structures

Physics of the Solid State - The GeO x films and multilayer nanoperiodic Ge/SiO2 structures containing germanium nanocrystals were prepared by physical vapor deposition in vacuum. The properties of...     

锗/多孔硅和锗/氧化硅薄膜光致发光的比较研究

采用磁控溅射技术,以锗为溅射靶,在多孔硅上沉积锗薄膜,沉积时间分别为4,8和12 min,及以锗-二氧化硅复合靶为溅射靶,在n型硅衬底上沉积了含纳米锗颗粒的氧化硅薄膜,锗与总靶的面积比分别为5%,15%,30%.各样品在氮气氛中分别经过300,600及900℃退火30 min.对锗/多孔硅和锗/氧化硅薄膜进行了光致发光谱的对比研究,用红外吸收谱分析了锗/多孔硅的薄膜结构.实验结果显示,锗/多孔硅薄膜的发光峰位于517 nm附近,沉积时间对发光峰的强度有显著影响,锗层越厚峰强越弱.锗/氧化硅薄膜的发光峰位于580 nm附近,锗与总靶的面积比对发光峰的强度影响较大,锗/氧化硅薄膜中的锗含量越高峰强越弱.不同的退火温度对样品的发光峰强及峰位均没有明显影响.可以认为锗/多孔硅的发光峰是由多孔硅与孔间隙中的锗纳米晶粒两者界面的锗相关缺陷引起的,而锗/氧化硅的发光峰来自于二氧化硅的发光中心.     

Sound velocity in amorphous Ge and Si

The low frequency velocity of sound for a Young's modulus mode in sputtered amorphous films of Ge and Si is found to be only ～55?60% of the value expected for the crystalline state. The reduction is one of the largest associated with crystalline-amorphous polymorphs.