2014年12月19日星期五

Vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry for metallic contamination analysis on Ge wafers

Ge substrates are recently being reconsidered as a candidate material for the replacement of Si substrates in advanced semiconductor devices. The reintroduction of this material requires reengineering of the standard IC processing steps. In this paper, we present the extension of the methodology of vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry (VPD–DC–TXRF) for metallic contamination analysis towards Ge substrates. A first step that asked for adaptation was the collection chemistry as the Ge wafers surface is not hydrophobic after the VPD treatment. The contact angle could be significantly increased using a concentrated HCl solution. This chemistry has been proved to perform well in the collection of metals from intentionally contaminated Ge wafers. A second step that needed optimization was the matrix removal method as a sample preparation step prior to the TXRF analysis. First, the upper limits of TXRF on Ge containing solutions have been characterized. The accuracy of TXRF is found to be acceptable for Ge contents lower than 1×1014 atoms (250 ppb in 50 μL) but decreases systematically with higher Ge contents. Fortunately, Ge can be volatilized at low temperatures as GeCl4 by the addition of HCl. The parameters within this method have been investigated with respect to the removal of Ge and the recovery of metal traces. Finally, the full VPD–DC–TXRF method has been applied on intentionally contaminated Ge wafers and proved to be very accurate.

Keywords

  • Vapor phase decomposition–droplet collection
  • Metallic contamination
  • Ge wafer
  • TXRF matrix effects

2014年12月1日星期一

Plastically deformed Ge-crystal wafers as elements for neutron focusing monochromator

Plastically deformed Ge-crystal wafers that have the cylindrical shape with a large curvature were characterized by neutron diffraction. The box-type rocking curve of Bragg reflection with the angular width ofΓbox≃2° in FWHM, which is observable in the monochromatic neutron diffraction, results in an enhancement in the angle-integrated intensity (Iθ). Besides, Iθ efficiently increases by stacking such Ge wafers. In the course of white neutron diffraction, the reflected-beam width near the focus point becomes sharper than the initial beam width. Further, the dependence of the horizontal beam width on the distance between the sample and detector is quantitatively explained by taking account of the large Γbox, the small mosaic spread of η≃0.1°, and the thickness of the wafers. On the basis of these characterizations, use of plastically deformed Ge wafers as elements for high-luminance neutron monochromator is proposed.

Keywords

2014年11月6日星期四

Vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry for metallic contamination analysis on Ge wafers

Ge substrates are recently being reconsidered as a candidate material for the replacement of Si substrates in advanced semiconductor devices. The reintroduction of this material requires reengineering of the standard IC processing steps. In this paper, we present the extension of the methodology of vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry (VPD–DC–TXRF) for metallic contamination analysis towards Ge substrates. A first step that asked for adaptation was the collection chemistry as the Ge wafers surface is not hydrophobic after the VPD treatment. The contact angle could be significantly increased using a concentrated HCl solution. This chemistry has been proved to perform well in the collection of metals from intentionally contaminated Ge wafers. A second step that needed optimization was the matrix removal method as a sample preparation step prior to the TXRF analysis. First, the upper limits of TXRF on Ge containing solutions have been characterized. The accuracy of TXRF is found to be acceptable for Ge contents lower than 1×1014 atoms (250 ppb in 50 μL) but decreases systematically with higher Ge contents. Fortunately, Ge can be volatilized at low temperatures as GeCl4 by the addition of HCl. The parameters within this method have been investigated with respect to the removal of Ge and the recovery of metal traces. Finally, the full VPD–DC–TXRF method has been applied on intentionally contaminated Ge wafers and proved to be very accurate.

Keywords

  • Vapor phase decomposition–droplet collection
  • Metallic contamination
  • Ge wafer
  • TXRF matrix effects

2014年10月12日星期日

Plastically deformed Ge-crystal wafers as elements for neutron focusing monochromator

Plastically deformed Ge-crystal wafers that have the cylindrical shape with a large curvature were characterized by neutron diffraction. The box-type rocking curve of Bragg reflection with the angular width ofΓbox≃2° in FWHM, which is observable in the monochromatic neutron diffraction, results in an enhancement in the angle-integrated intensity (Iθ). Besides, Iθ efficiently increases by stacking such Ge wafers. In the course of white neutron diffraction, the reflected-beam width near the focus point becomes sharper than the initial beam width. Further, the dependence of the horizontal beam width on the distance between the sample and detector is quantitatively explained by taking account of the large Γbox, the small mosaic spread of η≃0.1°, and the thickness of the wafers. On the basis of these characterizations, use of plastically deformed Ge wafers as elements for high-luminance neutron monochromator is proposed.

Keywords

  • Plastically deformed Ge wafer
  • Neutron monochromator crystal
  • Neutron beam focus
  •  
  • Source:Sciencedirect
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2014年9月17日星期三

Electrical and Structural Characterization of Germanium Nanowires


Abstract
Germanium nanowires were grown by thermal evaporation of germanium powder associated with the well known vapor-liquidsolid mechanism (VLS). The nanowires were investigated by x-ray diffraction (XRD), Raman spectroscopy and field emission gun scanning electron microscopy (FEG-SEM). Through the fabrication of a germanium nanowires-based device we have studied the electronic transport properties of these samples. The transport measurements revealed semiconductor – like features, characterized by the decrease of the resistance as the temperature decreases. The variable range hopping (VRH) was identified as the main transport mechanism in a large temperature range (77 K < T < 400 K) thus giving consistent support to the mechanisms underlying the observed semiconducting character.


1. Introduction
The nanoscience has been the fundamental basis for the technological advances aimed at miniaturization and development of optoelectronic devices. This potential to develop new devices is directly related to the unique characteristics presented by the nanostructures. The nanoscale size (< 100 nm) leads to different and sometimes surprising mechanical, optical and eletronic features. In fact the properties exhibited by the nanostructures are different from those found in three-dimensional materials, and deeply influenced by quantum mechanics. Among the
various typ†es of nanostructures, nanowires and nanotubes are the most widely used for the development of new devices because they can have metallic or semiconducting properties, can be flexible or rigid, and also can selectively conduct heat .Among the various nanostructures studied today, those based on germanium can be very interesting from a
technological standpoint: they are semiconducting with a small and indirect gap (0,67 eV, easily doped with Boron (p type) and phosphorus (type n) thus enabling the development of nanodevices based on pn junctions (nanoleds,solar nanocells) [3,4,5]. Some successful routes for Germanium nanowires synthesis are known; however, they are still difficult to be implemented, have no morphology control and also involve high costs. Thus, to contribute to the  development of a new simple route for the germanium samples synthesis, structural characteristics of these nanostructures synthesized by the vapor-liquid-solid method were studied. Combining X-ray diffraction (XRD),Raman spectroscopy measurements and field emission gun scanning electron microscopy (FEG-SEM), it was possible to characterize the sample's structure, composition, single crystalline character and vibrational spectrum of
the samples. Additionally, electron transport mechanisms were investigated: the temperature dependent resistivity measurements unambiguously showed that samples presented the expected semiconducting behavior. Also, the variable range hopping was identified as the mechanism governing the electron transport in Ge nanowires. This characteristic was attributed to the disorder which leads to localized states.

2. Experiment
wafer were grown by a thermal evaporation process of germanium powder, in association with the well-known vapor-liquid-solid mechanism (VLS) [6]. The device (Fig.1) was fabricated as following: metallic electrodes were defined by using conventional lithographic techniques with Ti electrodes (50nm thick) over an oxidized n+ Si wafer (500 nm layer, n-type). Gold nanoparticles (10 nm) were deposited above titanium electrodes
in order to nucleate the growth of the germanium nanowires. The catalyst (impurity) in the liquid phase is introduced in order to direct and confine the crystal growth in a specific orientation and within a restricted area. The germanium powder (Aldrich, purity> 99.99%) was placed in a quartz crucible and positioned in the central region of the tube furnace. The synthesis temperature was adjusted to 950 °C (heating rate of 32 °C/min) and stands for 20 minutes. The substrates with a previously deposited Au catalyst layer and Ti electrodes were positioned in a cooler region of the furnace (550 ° C <T <650 ° C) favoring the precipitation of the germanium hot steam driven by a carrier gas (Argon, White Martins, purity> 99.998 %) with a flow of 30 sccm.




Figure 
Figure 1. Schematic diagram of the Ge nanowires device fabricated with Ti electrodes and Au catalyst layer.The as-grown Ge nanowires were analyzed by field emission gun scanning electron microscopy (FEG-SEM,Zeiss Supra 35), x-ray diffraction (XRD, Rigaku diffractometer model DMAX 2500PC, 40 kV, 150 mA, all measurements were performed using aluminum sample holders) with a Cu Ka radiation and Raman spectroscopy (Triple grating Jobin Yvon Spectrometer T64000 equipped with a liquid nitrogen cooled charge coupled device).The Raman measurements were performed in backscattering geometry configuration and the samples were excited by a 514.5 nm line of an Ar+ laser at room temperature. The used experimental settings present a spectral resolution about 2 cm-1. The transport measurements were carried out at different temperatures from 77K to 400K using a electrometer (Keithley 6517) and a closed-cycle helium cryostat (Janis CCS 400H) working at a pressure lower
than 5x10-7 Torr. 


3. Results and Discussion
For a first investigation of the as-synthesized samples XRD measurements were performed 







Figure 2. The structural data of the Ge nanowires: (a) The x-ray pattern of the nanowires grown on silicon substrates. (b) The Raman spectrum obtained from one Ge nanowire at room temperature showing the expected 300 cm-1 phonon mode which is related to the diamond structure of germanium phase. The 521 cm-1 peak corresponds to the Si substrate used to hold the sample. (c) The FEG-SEM image of the germanium nanowires grown on silicon substrates and catalyzed by gold nanoparticles.The close agreement between the experimental data and the PDF 4-545 is an indication that the as-synthesized material is composed of atoms arranged in a germanium diamond-like structure (space group Fd-3m). The XRD
pattern also shows that the samples probably have a preferred direction of growth in the (111) direction which is an evidence of a single crystalline character [Fig. 2 (a)]. We also performed Raman measurements in order to study the sample’s composition, structure and crystalline quality. The Raman spectrum of a single germanium nanowire is depicted in Fig. 2 (b). It is observed from the spectrum a single and sharp peak at 300 cm-1 which is commonly
associated [7] to the germanium diamond structure, thus confirming the composition of the samples in agreement with the x-ray diffractogram shown in Fig. 2 (a). The presence of narrow and well-defined peaks points to a single crystalline characteristic of the samples which also agrees with the presence of a preferential growth direction (111) plane as obtained from the XRD data Fig. 2 (a). Additional data were obtained from FEG-SEM measurements, as
shown in the inset of Fig. 2 (b). This analysis revealed that the nanowires have a great uniformity of diameters and lengths of several tens of micrometers Fig. 2 (c).



Figure 3 depicts the temperature-dependent resistance measurement revealing a semiconducting character(resistance decreases as the temperature increases).





Figure 3. Temperature-dependent resistivity measurement revealing a semiconducting character dominated by the variable range hopping instead of the expected simple thermal excitation law for a semiconductor.However, the experimental curve does not follow the expected simple thermal excitation law for a semiconductor [8]. The observed behavior was well fitted to the variable range hopping (VRH) conduction mechanism due to Mott [9]. In this model, phonons are required to conserve energy during a hop from site to site: the higher phonon density at a higher temperature increases the hopping rate and thereby decreases the resistivity.

The VRH mechanism is described by where T0 = 5,7α  3/kBN(EF), N(EF) is the density of states at the Fermi level and α-1 the localization length. In this mechanism the conduction is governed by carriers with a small extra energy (kBT) in the vicinity of the Fermi level
where the density of states remains almost a constant (m=1/4, considering a three-dimensional system). This condition is fulfilled when the temperature is sufficiently small or when the energy states are uniformly distributed.The agreement between theoretical and experimental curves (Fig. 3) confirms that the VRH process governs the transport in all temperature range measured (100 to 400K) providing us with a localization length of 42.3 nm. This
length is in full agreement with the Bohr radius of germanium (~24 nm) and it is smaller than the cross section of the sample, also in agreement with the three-dimensional character of the sample.

The question here is: where do the localized states leading to hopping come from? The surface of germanium nanowires are naturally covered by a thin oxide layer (mostly GeOx as follows from the X-ray data) during the synthesis process due to the presence of unavoidable oxygen molecules. The interface Ge/GeO2 is disordered due to its nature and then it induces a disordered potential that should affect the conduction of electrons inside the Ge nanowires. In fact, a small surface disorder could dominate nanowire bulk properties as the sizes of samples are scaled down (by controlling the Fermi level and screening out electrons from surface, for instance). Such disorder produces interface states (surface levels) which could also act as charge traps. When the system temperature is large enough to produce free electrons which begin to increase the conductivity of the samples, the presence of a disordered potential will make difference. Any electron subjected to a random potential is not able to move freely
through the system if either the potential fluctuations exceed a critical value or the electron energy is lower than the characteristic value of the potential fluctuations. Taking into account the motion of charges in that disordered potential the conduction mechanism should change its character from a simply activated process to another one like the observed VRH. It should be notice that some authors have also observed VRH as the dominating transport mechanism in Ge films [10].


4. Conclusions
In summary, we have studied electronic properties of the Ge nanowires and the influence of the disorder on the electron transport. Such a disorder produces surface levels which could also act as charge traps leading the semiconducting Ge nanowires to present the variable range hopping conduction mechanism

Keywords:Germanium nanowires,ge wafer,Ge nanowires

Source:Sciencedirect

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2014年8月29日星期五

Analysis of the surface state of epi-ready Ge wafers

Highlights
► Ge epi-ready wafers from two different vendors studied using X-ray spectroscopy. ► Oxide layer on all the Ge wafer surfaces formed by GeO and GeO2; layer thickness depending on wafer vendor. ► Probable presence of chlorine at the wafer surfaces. ► Wafer surfaces from one of the vendors contaminated by carbonates.

Keywords
Germanium wafers; III–V solar cells; Photoelectron spectroscopy


The surface state of Ge epi-ready wafers (such as those used on III–V multijunction solar cells) supplied by two different vendors has been studied using X-ray photoemission spectroscopy. Our experimental results show that the oxide layer on the wafer surface is formed by GeO and GeO2. This oxide layer thickness differs among wafers coming from different suppliers. Besides, several contaminants appear on the wafer surfaces, carbon and probably chlorine being common to every wafer, irrespective of its origin. Wafers from one of the vendors show the presence of carbonates at their surfaces. On such wafers, traces of potassium seem to be present too.