On the thermal donors in silicon

It is shown that the series of progressively shallower ground states observed for the thermal donors found after annealing Czochralski silicon at ≈ 450°C can be explained by assuming that successive steps in the agglomeration of oxygen introduce a repulsive potential which the Rydberg electrons experience and that the volume of this repulsive potential grows with each agglomeration step.     

A DLTS study on plasma-hydrogenated n-type high-resistivity magnetic Cz silicon

N-type high resistivity (HR) magnetic Czochralski (MCz) silicon wafers are hydrogenated via direct hydrogen plasma exposures at 270 °C. After the hydrogenation, isochronal annealing and isothermal annealing are carried out up to 450 °C. Deep Level Transient Spectroscopy (DLTS) measurement is used to study the deep levels created in the hydrogenated MCz silicon samples. Up to 9 kinds of deep levels are detected. The diffusion-like concentration profiles suggest a clear role of hydrogen in the formation of some defects. A considerable amount of oxygen thermal donors (OTD) is also observed in the hydrogenated samples even after annealing. The formation of the deep levels suggests that care should be taken for some application using the MCz silicon as a HR material.     

轴向磁场对硅单晶Czochralski生长过程的影响

利用有限元方法对炉内的传递过程进行了全局数值模拟,假定熔体和气相中的流动都为准稳态轴对称层流,熔体为不可压缩流体,Cz炉外壁温度维持恒定,模拟磁场强度范围为(0～0.3)T,研究了用Czochralski(Cz)法生长单晶硅轴向磁场对熔体流动和氧传输过程的影响.结果表明:轴向磁场可有效地抑制熔体内的流动,但增大加热器功率和结晶界面处晶体内的轴向温度梯度;对于常规Cz炉,轴向磁场可增大结晶界面平均氧浓度,而对于具有气体导板的Cz炉,则会减小结晶界面平均氧浓度.     

Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

Abstract

The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength but higher ductility than the other cases; this was attributed to the presence of reverted austenite. Homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling resulted in optimum tensile properties. A substantial increase in fusion zone toughness was observed after homogenisation+solutionising+aged condition due to a decrease in the content of austenite content compared to the directly aged condition. The reduction in microsegregation by diffusion of alloying elements from cell boundaries to the cell during homogenisation treatment is responsible for the decrease in austenite content.     

Formation of a silicate layer between lead oxide and a silicon-wafer surface during heat treatment

PbO thin films were deposited on a silicon substrate by plasma-enhanced chemical vapour deposition (PECVD) using Pb(C₂H₅)₄ and oxygen at 250°C. The interdiffusion reaction phenomena between the PbO thin film and the silicon substrate during heat treatments were investigated in a horizontal furnace in the temperature range between 350 and 650°C under a nitrogen ambient for 1 h. The PECVD PbO film deposited on the silicon substrate at 250°C, was amorphous and contained carbon-related contaminants which could almost be removed by heat treatment at 350°C. The PbO on the silicon substrate initially participated in the interdiffusion reaction in the temperature range between 400 and 450°C. This produced a silicate layer containing lead components. The lead content in the film varied with the depth of film and heat-treatment temperature. Metallic lead was observed as a cluster in the specimen heated at 550°C. This cluster was produced by the agglomeration of metallic lead originating from PbO decomposition. The oxygen source for silicate formation was not ambient oxygen coming from the decomposition of Pb–O bonding. The metallic lead clusters dissolved as weakly bound metallic lead or as an unbound nanosized metal particle in the silicate layer at 650°C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Non-equilibrium intergranular segregation in ultra low carbon steel

Abstract

Impurity segregation to grain boundaries in ultra low carbon steel was investigated by Auger electron spectroscopy and SEM during isothermal annealing at 900°C and continuous cooling. The results of isothermal annealing at 900°C show that a concentration peak appears at different times for phosphorus, sulphur, and boron, which is contrary to the equilibrium segregation theory of McLean. The phenomena could be satisfactorily explained by the non-equilibrium segregation theory based on the impurity–vacancy complex mechanism. Under continuous cooling, the segregation concentration at the grain boundary largely depends on the cooling rate. At a low cooling rate the concentration of phosphorus and boron at the grain boundary is higher than that of sulphur, while at the higher cooling rate the concentration of sulphur is higher.     

Infrared spectroscopic study of thermal donors in Czochralski-grown silicon developed at 450°C

Characteristics of thermal donors developed upon annealing Czochralski-grown silicon crystals at 450°C are investigated by means of infrared spectroscopy. It is shown that the thermal donors are divalent and that at least six kinds of thermal donors having similar structures are developed in the annealed crystals. The ionization energies of each kind of thermal donors in neutral and singly ionized states are determined.     

硅单晶Czochralski法生长全局数值模拟I．传热与流动特性

利用有限元方法对炉内的动量和热量传递过程进行了全局数值模拟，研究了硅单晶Czochral—ski(Cz)法生长时的总体传热和流动特性假定熔体和气相中的流动都为准稳态轴对称层流，熔体为不可压缩流体，Cz炉外壁温度维持恒定，模拟结果表明：熔体流型及炉内传热特性与Marangoni效应密切相关，设置在晶体和坩埚间的气体导板能降低加热器的功率并改变熔体流型．     

Directional solidification of Ni base superalloy IN738LC to improve creep properties

Abstract

The high Cr, Ni base superalloy IN738LC has been directionally solidified on both laboratory and industrial scales using Bridgman and liquid metal cooling (LMC) methods respectively. In the Bridgman experiments, cylindrical rods were grown using a graphite chill with temperature gradient G = 5·0 K mm^-1 and a water cooled copper chill with G = 8·5 K mm^-1, and a wide range of withdrawal rates of R = 60, 120, 240, 600, and 1200 mm h^-1. In the LMC rigs, several turbine blades were grown using a wide range of withdrawal rates of R = 120, 225, 330, 420, and 630 mm h^-1. Grain and dendritic structures in both cylindrical and turbine blade specimens were evaluated in longitudinal and transverse directions. Dendritic segregation of rods was determined with SEM as a function of processing parameters. Some specimens were given a two stage heat treatment followed by tension tests at 25 and 650°C and creep tests at 152 MPa and 982°C, 340 MPa and 850°C, and 586 MPa and 760°C. It was shown that at R = 600 mm h^-1 with water cooled copper chill, directionally solidified rods with a well orientated dendritic structure and better segregation pattern gives higher tensile properties at 25°C and creep properties at 340 MPa and 850°C. Tension and creep tests of turbine blades showed that although the yield and tensile strength of directionally solidified specimens are in the range of conventionally cast ones, the creep properties of the blades have been significantly improved using the LMC process.     

Precipitate recognition and recombination strength in annealed Czochralski silicon wafers

Abstract

Two experimental methods have been used to reveal the presence of precipitates and to evaluate their recombination strength in Czochralski grown silicon wafers. One method uses scanning infrared microscopy, the other light beam induced current mapping. These two non-destructive techniques have been associated with Fourier transform infrared spectroscopy and with minority carrier diffusion length measurements. After two step annealing, scanning infrared microscopy reveals the presence of precipitates, while light beam induced current mapping displays a ringlike distribution of recombination centres and the minority carrier diffusion length decreases drastically to 2 μm. Copper contamination does not modify these observations even when precipitates are formed. Phosphorus diffusion near the surface decreases the size of the precipitates revealed by scanning infrared microscopy but does not suppress the ringlike light beam induced current maps, and the minority carrier diffusion length increases slightly. It is concluded that the recombination strength of precipitates does not depend on metal decoration but more probably on interfacial states or on associated extended defects.     

Thermoelectric properties of silicon at high pressures in the region of the semiconductor-metal transition

The thermo emf in Czochralski grown silicon single crystals annealed at 450°C was experimentally studied in a range of pressures up to 16 GPa in a chamber with synthetic diamond anvils. There is a correlation between the curves of thermo emf versus pressure, the semiconductor-metal transition pressure, and the mechanical properties (microhardness, compressibility) of samples with various oxygen content. The values of thermo emf in the high-pressure metallic phases have been determined.     

Investigation of ammonium diuranate calcination with high-temperature X-ray diffraction

The thermal decomposition of ammonium diuranate (ADU) in air is investigated using in situ high-temperature X-ray diffraction (HT-XRD), thermogravimetry and differential thermal analysis. Data have been collected in the temperature range from 30 to 1000 °C, allowing the observation of phase transformation and the assessment of the energy changes involved in the calcination of ADU. The starting material 2UO₃·NH₃·3H₂O undergoes a process involving several endothermic and exothermic reactions. In situ HT-XRD shows that amorphous UO₃ is obtained after achieving complete dehydration at 300 °C, and denitration at about 450 °C. After cooling from heat treatment at 600 °C, a crystalline UO₃ phase appears, as displayed by ex situ XRD. The self-reduction of UO₃ into orthorhombic U₃O₈ takes place at about 600 °C, but a long heat treatment or higher temperature is required to stabilise the structure of U₃O₈ at room temperature. U₃O₈ remains stable in air up to 850 °C. Above this temperature, oxygen losses lead to the formation of U₃O_8?x , as demonstrated by subtle changes in the diffraction pattern and by a mass loss recorded by TGA.     

Loss of oxygen and carbon during donor formation in CZ-silicon

Due to a lack of proper understanding about the formation mechanism of oxygen related donors during the transition temperature range (465–540°C) which exhibits the transition of TDs to NDs, an attempt has been made to study their behaviour in the present context. We have found the formation and diffusion of molecule like oxygen at low temperature annealing of silicon and observed that second order kinetics of oxygen diffusion holds good. A relatively low value of 0·6 eV has been estimated to be the activation energy for the diffusion of oxygen in silicon which is supposed to be due to the hydrogen passivation.     

Oxidation behaviour of TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings with Al2O3 topcoat deposited on γ-TiAl alloys

Abstract

TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl specimens using magnetron sputtering techniques. The nitride layers were manufactured by unbalanced magnetron sputtering (UBM) and high power impulse magnetron sputtering (HIPIMS). The CrAlYN/CrN coatings had an oxy-nitride overcoat. On some of the coated samples an additional alumina topcoat was deposited. The oxidation behaviour of the different coatings was investigated at 750 and 850°C performing quasi-isothermal oxidation tests in laboratory air. Mass change data were measured during exposure up to failure or the maximum exposure length of 2500 h. When exposed to air at 750°C, the Ti-based nitride films exhibited higher oxidation resistance than the Ti – 45Al –8Nb substrate material. The alumina topcoat enhanced the oxidation protection of this coating system, acting as diffusion barrier to oxygen penetration. At 850°C, the TiAlYN/CrN films exhibited poor stability and rapidly oxidised, and therefore were not applicable for long-term protective coatings on γ-TiAl alloys. The beneficial effect of the additional Al₂O₃ layer was less pronounced at this exposure temperature. The Cr-based nitride films exhibited high oxidation resistance during exposure at 850°C. HIPIMS deposition improved the oxidation behaviour of the CrAlYN/CrN nanoscale multilayer coatings in comparison to UBM coatings. For these coatings, the decomposed and partially oxidised nitride films were an effective barrier to oxygen inward diffusion. The alumina topcoat did not significantly increase the oxidation resistance of the γ-TiAl alloy coated with Cr-based nitride films.     

Comparison between furnace and rapid thermal annealed iridium silicide Schottky barrier diodes

Abstract

Iridium silicide Schottky barrier diodes fabricated by low temperature rapid thermal annealing (R TA) either in vacuum or in an argon atmosphere are investigated. A comparison with furnace annealed diodes is made in terms of their electrical characteristics. An ideality index close to unity has been obtained. Measured values of Schottky barrier height were close to that reported for Ir₁Si₁/n type silicon <111> diodes. An increase in series resistance with annealing time is detected for diodes fabricated in the gas atmosphere by either furnace annealing or RTA: this effect seems to be related to oxygen diffusion through the iridium layer. The effect of atmosphere control on the grown material is also significant: silicidation and a phase change from Ir₁ Si₁ to Ir₁ Si_1-75 occur at lower temperatures when the reaction atmospheres contain a lower oxygen concentration. Complete silicidation to Ir₁Si₁ is obtainedfor all the 400°C RTA samples, but temperatures above 450°C are required for open tube furnace argon annealed samples to obtain the same reaction. A low barrier silicide phase appears only for RTA in vacuum at 450°C. It seems that RTA in vacuum at low temperature (400°C) is the most promising methodfor diode fabrication.

MST/3334     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

Early stages of oxygen aggregation and thermal donors in silicon annealed under hydrostatic pressure

Thermal donor formation in Czochralski-grown silicon heat treated at T=450 °C under hydrostatic pressure is investigated by means of optical and electrical measurements. It has been shown that oxygen agglomeration processes in stressed Cz-Si lead to an enhanced formation of the well-known thermal double donors. This effect is believed to be due to increasing oxygen diffusivity under stress. Some important differences between the formation processes in Cz-Si annealed under normal conditions and high hydrostatic pressure are discussed.     

The strengthening mechanism of a nickel-based alloy after laser shock processing at high temperatures

Abstract

We investigated the strengthening mechanism of laser shock processing (LSP) at high temperatures in the K417 nickel-based alloy. Using a laser-induced shock wave, residual compressive stresses and nanocrystals with a length of 30–200 nm and a thickness of 1 μm are produced on the surface of the nickel-based alloy K417. When the K417 alloy is subjected to heat treatment at 900 °C after LSP, most of the residual compressive stress relaxes while the microhardness retains good thermal stability; the nanocrystalline surface has not obviously grown after the 900 °C per 10 h heat treatment, which shows a comparatively good thermal stability. There are several reasons for the good thermal stability of the nanocrystalline surface, such as the low value of cold hardening of LSP, extreme high-density defects and the grain boundary pinning of an impure element. The results of the vibration fatigue experiments show that the fatigue strength of K417 alloy is enhanced and improved from 110 to 285 MPa after LSP. After the 900 °C per 10 h heat treatment, the fatigue strength is 225 MPa; the heat treatment has not significantly reduced the reinforcement effect. The feature of the LSP strengthening mechanism of nickel-based alloy at a high temperature is the co-working effect of the nanocrystalline surface and the residual compressive stress after thermal relaxation.     

Effect of Preannealing on Substitutional Gold Concentration in Silicon

Data are presented on the effect of long-term high-temperature heat treatment (750°C, 200 h; 950°C, 50 h; and 1050°C, 50 h) preceding Au diffusion on the acceptor behavior of Au in n-type Czochralski silicon. Detailed studies of Au diffusion reveal that the impurity gettering process depends strongly on the heat treatment conditions. The results are interpreted in terms of oxygen precipitation in Si.     

Low temperature oxygen dissolution in titanium

In situ resistivity measurements, X-ray diffraction, ⁴He⁺ megaelectronvolt backscattering and Auger electron spectrometry were used to investigate the effect of oxygen diffusion on the electrical properties of a thin titanium film deposited onto silicon and heated at temperatures below 500°C. The annealing was performed in a vacuum of 10^-5 Pa and in a hot purified helium furnace. The vacuum-annealed samples show a sharp increase in resistivity around 300°C. The increase is not due to silicon diffusion but is attributed to oxygen contamination. The presence of oxygen deforms the hexagonal structure of the titanium; the bond length along the c axis increases proportionally to the resistivity of the film. Annealing at temperatures higher than 500°C promotes silicide formation. The oxygen contained in the titanium film is segregated towards the outermost surface.