The chemical, physical, and mechanical characteristics of nickel-based superalloys are reviewed with emphasis on the use of this class of materials within turbine engines. The role of major and minor alloying additions in multicomponent commercial cast and wrought superalloys is discussed. Microstructural stability and phases observed during processing and in subsequent elevated-temperature service are summarized. Processing paths and recent advances in processing are addressed. Mechanical properties and deformation mechanisms are reviewed, including tensile properties, creep, fatigue, and cyclic crack growth. I. Introduction N ICKEL-BASED superalloys are an unusual class of metallic materials with an exceptional combination of hightemperature strength, toughness, and resistance to degradation in corrosive or oxidizing environments. These materials are widely used in aircraft and power-generation turbines, rocket engines, and other challenging environments, including nuclear power and chemical processing plants. Intensive alloy and process development activities during the past few decades have resulted in alloys that can tolerate average temperatures of 1050 ◦ C with occasional excursions (or local hot spots near airfoil tips) to temperatures as high as 1200 ◦ C, 1 which is approximately 90% of the melting point of the material. The underlying aspects of microstructure and composition that result in these exceptional properties are briefly reviewed here. Major classes of superalloys that are utilized in gas-turbine engines and the corresponding processes for their production are outlined along with characteristic mechanical and physical properties.

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Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure and Properties

Solid-state electronics

Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2∕3 for ne≤4.2×1019 cm−3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4−8.4×1019 cm−3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widen...

Carrier concentration dependence of band gap shift in n-type ZnO:Al films

A Caughey–Thomas-like mobility model with temperature and composition dependent coefficients is used in this work to describe the dependence of electron and hole mobilities on temperature, doping concentration, and alloy composition. Appropriate parameter sets are given for a large number of III–V binary and ternary compounds, including: GaAs, InP, InAs, AlAs, GaP, Al0.3Ga0.7As, In0.52Al0.48As, In0.53Ga0.47As, and In0.49Ga0.51P. Additionally, physically justifiable interpolation schemes are suggested to find the mobilities of various ternary and quaternary compounds (such as AlxGa1−xAs, In1−xGaxP, In1−xGaxAs, In1−xAlxAs, and In1−xGaxAsyP1−y) in the entire range of composition. The models are compared with numerous measured Hall data in the literature and very good agreement is observed. The limitations of the present model are also discussed. The results of this work should be extremely useful in device simulation packages, which are currently lacking a reliable mobility model for the above materials.

Empirical low-field mobility model for III-V compounds applicable in device simulation codes

GaAs/AlxGa(1−x)As quantum well lasers have been demonstrated via organometallic chemical vapor deposition on relaxed graded Ge/GexSi(1−x) virtual substrates on Si. A number of GaAs/Ge/Si integration issues including Ge autodoping behavior in GaAs, reduced critical thickness due to thermal expansion mismatch, and complications with mirror facet cleaving have been overcome. Despite unoptimized laser structures with high series resistance and large threshold current densities, surface threading dislocation densities for GaAs/AlGaAs lasers on Si substrates as low as 2×106 cm−2 permitted continuous room-temperature lasing at a wavelength of 858 nm. The laser structures are uncoated edge-emitting broad-area devices with differential quantum efficiencies of 0.24 and threshold current densities of 577 A/cm2. Identical devices grown on commercial GaAs substrates showed similar behavior. This comparative data agrees with previous measurements of near-bulk minority carrier lifetimes in GaAs grown on Ge/GeSi/Si substrates.

Monolithic integration of room-temperature cw GaAs/AlGaAs lasers on Si substrates via relaxed graded GeSi buffer layers

The current-voltage characteristics of Au/low-doped n-GaAs Schottky diodes were determined at various temperatures in the range of 77-300 K, The estimated zero-bias barrier height and the ideality factor assuming thermionic emission (TE) show a temperature dependence of these parameters. While the ideality factor was found to show the $T_o$ effect, the zero-bias barrier height was found to exhibit two different trends in the temperature ranges of 77-160 K and 160-300 K, The variation in the Bat-band barrier height with temperature was found to be - (4.7 +/- 0.2) $210^4$ $eVK^1$, approximately equal to that of the energy band gap, The value of the Richardson constant, A**, was found to be $0.27 A cm^-^2 K^-^2$ after considering the temperature dependence of the barrier height. The estimated value of this constant suggested the possibility of an interfacial oxide between the metal and the semiconductor. Investigations suggested the possibility of a thermionic field- emission-dominated current transport with a higher characteristic energy than that predicted by the theory, The observed variation in the zero-bias barrier height and the ideality factor could be explained in terms of barrier height inhomogenities in the Schottky diode.

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Anomalous current transport in Au/low-doped n-GaAs Schottky barrier diodes at low temperatures

Silane (SiH4) was used as an n-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium (TMGa) and arsine (AsH3) as source materials. The electron carrier concentrations and silicon (Si) incorporation efficiency are studied by using Hall effect, electrochemical capacitance voltage profiler and low temperature photoluminescence (LTPL) spectroscopy. The influence of growth parameters, such as SiH4 mole fraction, growth temperature, TMGa and AsH3 mole fractions on the Si incorporation efficiency have been studied. The electron concentration increases with increasing SIH4 mole fraction, growth temperature, and decreases with increasing TMGa and AsH3 mole fractions. The decrease in electron concentration with increasing TMGa can be explained by vacancy control model. The PL experiments were carried out as a function of electron concentration (10(17) - 1.5 x 10(18) cm(-3)). The PL main peak shifts to higher energy and the full width at half maximum (FWHM) increases with increasing electron concentrations. We have obtained an empirical relation for FWHM of PL, Delta E(n) (eV) = 1.4 x 10(-8) n(1/3). We also obtained an empirical relation for the band gap shrinkage, Delta E-g in Si-doped GaAs as a function of electron concentration. The value of Delta E-g (eV) = -2.75 x 10(-8) n(1/3), indicates a significant band gap shrinkage at high doping levels. These relations are considered to provide a useful tool to determine the electron concentration in Si-doped GaAs by low temperature PL measurement. The electron concentration decreases with increasing TMGa and AsH3 mole fractions and the main peak shifts to the lower energy side. The peak shifts towards the lower energy side with increasing TMGa variation can also be explained by vacancy control model. (C) 1999 Elsevier Science S.A. All rights reserved.

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Si incorporation and Burstein-Moss shift in n-type GaAs

Dimethylzinc (DMZn) was used as a p-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium and arsine (AsH3) as source materials. The hole carrier concentrations and zinc (Zn) incorporation efficiency are studied by using the Hall effect, electrochemical capacitance voltage profiler and photoluminescence (PL) spectroscopy. The influence of growth parameters such as DMZn mole fraction, growth temperature, and AsH3 mole fraction on the Zn incorporation have been studied. The hole concentration increases with increasing DMZn and AsH3 mole fraction and decreases with increasing growth temperature. This can be explained by vacancy control model. The PL experiments were carried out as a function of hole concentration (1017–1.5×1020 cm−3). The main peak shifted to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentrations. We have obtained an empirical relation for FWHM of PL, ΔE(p)(eV)=1.15×10−8p1/3. We also obtained an empiric...

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Zn incorporation and band gap shrinkage in p-type GaAs

Photoluminescence (PL) spectroscopy has been used to study the silicon incorporation in polar GaAs on nonpolar Ge substrates. Shifts of PL spectra towards higher energy with growth temperature, trimethylgallium (TMGa) and arsine $(AsH_3)$ mole fractions were observed. The full width at half maximum increases with increasing growth temperature, $AsH_3$ and TMGa mole fractions. The peak at 1.49 eV has been attributed to band-to-acceptor transition involving residual carbon. The PL peak energy shifts towards higher energy with increasing growth temperature due to the increase in electron concentration. A vacancy control model may explain the PL shift towards higher energy with increasing AsH3 mole fraction. The PL peak shifts towards higher energy with increasing TMGa mole fraction. The experimental results about the growth temperature, trimethylgallium, and arsine mole fractions on silicon- doped GaAs on GaAs were presented for comparison. The outdiffusion of Ge into the GaAs epitaxial layer was hardly to be seen from the secondary ion mass spectroscopy result.

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Photoluminescence studies on Si-doped GaAs/Ge

Dimethylzinc (DMZn) was used as a p-type dopant in GaAs grown by low pressure metalorganic chemical vapor deposition
(MOCVD). The influence of growth parameters, such as, DMZn mole fractions, growth temperature, trimethylgallium (TMGa)
mole fractions, substrate surfaces on the Zn incorporation have been studied. The surface morphology of the layers was measured by scanning electron microscopy (SEM). The hole concentrations and zinc (Zn) incorporation efficiency are studied by using Hall effect, electrochemical capacitance voltage (ECV) profiler, and low temperature photoluminescence (LTPL) spectroscopy as functions of hole concentration $(10^{17}-1.5x10^{20} cm^{-3})$ and experimental temperatures (4.2–300 K). The hole concentration increases with increasing DMZn and TMGa mole fractions and decreases linearly with increasing growth temperature. The main PL peak shifted to lower energy and the full width at half maximum (FWHM) increased with increasing hole concentration. An empirical relation for FWHM, $\Delta Ep$, band gap, Eg, and band gap shrinkage, $\Delta Eg$ in Zn doped GaAs as a function of hole concentration were obtained. These relations are considered a useful tool to determine the hole concentration in Zn doped GaAs by low temperature PL measurement. The hole concentration increases with increasing TMGa mole fraction and the main peak is shifted to lower energy side.

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P. Modak

Papers

Anomalous current transport in Au/low-doped n-GaAs Schottky barrier diodes at low temperatures

Si incorporation and Burstein-Moss shift in n-type GaAs

Zn incorporation and band gap shrinkage in p-type GaAs

Photoluminescence studies on Si-doped GaAs/Ge

Low temperature photoluminescence properties of Zn-doped GaAs