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Effects of MBE growth conditions on carbon contamination in GaAs
World Abstracts on Microelectronics and Reliability
1183
7. SEMICONDUCTOR INTEGRATED CIRCUITS, DEVICES AND MATERIALS Applications of ellipsometry to in situ study of the growth of hydrogenated amorphous films. J. PERRIN and B. DREVILLON. Acta electron. 24 (3), 239 (1981/1982). In order to characterize the deposition mechanisms of hydrogenated amorphous silicon from a silane glow discharge, an in situ spectroscopic automatic ellipsometer has been associated with a multipole reactor. The ellipsometer is described; it is based on the photo-acoustic modulation technique and allows a fast data acquisition rate. From real time ellipsometry at a fixed wavelength on a growing film, the growth rate and the refractive index of the material are determined and some indications are given on the growth process. The dielectric constant spectrum measurement allows a posteriori characterization of the influence of plasma conditions or thermal treatments on the optical properties of the sample without air contamination. X-ray double diffraction by thin monocrystalline layers. C. SCHILLER. Acta electron. 24 (3), 267 (1981/1982). X-ray double diffraction has been used for the assessment of thin epitaxial layers of Gll_xAlxAs: it gives a rapid check of the substitution rate x for thicknesses larger than 0.15 lam, of the radius of curvature, of the lateral and in depth composition homogeneity and of the layer thicknesses from 0.25 to 3 lam. Self-consistent theory of diagonal and off-diagonal disorder in the screened impurity band of doped semiconductors. NORBERTO MAJLIS and ENRIQUE V. ANDA. Solid St. Communs 45 (7), 561 (1983). We have developed a theory of the impurity band in doped semi-conductors, using a tightbinding basis and a self-consistent treatment to handle the configurational averages. New ingredients in the present calculation are: (1) overlap between different impurity orbitals; (2) simultaneous treatment of diagonal and offdiagonal disorder; (3) inclusion of electron-electron interactions through the substitution of the bare Coulomb impurity potential by a Yukawa potential. The density of states and the position of the Fermi level (for T = 0) are calculated for different impurity concentrations. Dielectric/semiconductor interfaces analysis using spectroscopic eUipsometry. D. E. ASPNES and J. B. THEETEN. Acta electron. 24 (3), 217 (1981/1982). Ellipsometric measurements with variable energy of the incident light (from 1.5 to 5.8 eV), a technique named spectroscopic ellipsometry (SE), determine in a non-destructive way the optical response of the interface region between a substrate and a transparent superficial film. The method is demonstrated on the case of native oxides on silicon and gallium arsenide. In the case of silicon, the dielectric function of SiO 2 is already known (with a possible variation in density), and the comparison between SE data from oxide on Si (100), Si (110) and Si (111) and a model assuming an atomically abrupt interface between SiO 2 and Si evidences the presence of an interface region of width do~ = 7 _+2 A, consisting of atomically mixed Si and O of average stoichiometry SiO0.4±0. 2. In the case of gallium arsenide, the dielectric function of its electrochemically grown anodic oxide is not known a priori. It can be determined by using a 3-phase model. It is shown that the remaining discrepancy between SE measurements and the 3-phase model can be interpreted as the presence of an approximately 50 A region with a density deficit (probably due to leaching of As203) between oxide and ambient, the oxide/substrate interface being found very abrupt (markedly lower than 10 A in thickness). Alternating space-charge-limited currents in hydrogenated amorphous silicon. W. DEN BOER and A. F. P. Pop. Solid St. Communs 45 (10), 881 (1983). Space-charge-limited currents in n + - i - n + sandwich samples of hydrogenated amorphous silicon have been investigated under alternating current
MF<
r
(AC) conditions. When the voltage sweeping rate exceeds the release rate of carriers from deep traps, the AC currentvoltage characteristics differ markedly from the direct current (DC) characteristics. This is attributed to the remaining space charge in the samples which acts as a potential barrier and reduces the current. A simple model is proposed to describe the time-dependence of this injection-induced barrier.
Developments in crystal growth from high-temperature solutions. HANS J. SCHEEL. Prog. Crystal Growth Char., 5, 277 (1982). With the technological advances in the past 12 years, and with the high quality of flux-grown crystals and layers, this crystal-growth technique has become increasingly important. The technological parameters which have to be optimized to achieve large structurally perfect crystals, of high purity and free of striations, are: choice of solvent, growth temperature and its control, temperature gradient at and along the growth interface, supersaturation, and hydrodynamics. Top-seeded solution growth and the accelerated crucible rotation technique are the most widely applied techniques for growth of large high-quality crystals. Effects of MBE growth conditions on carbon contamination in GaAs. O. TEJAYADI, Y. L. SUN, J. KLEM, R. FISCHER, M. V. KLEIN and H. MORKOC. Solid St. Communs 46 (3), 251 (1983). The effects of growth and pre-growth conditions on the background concentration of carbon in high quality undoped GaAs layers grown by molecular beam epitaxy have been studied. Characterization of the layers by low temperature photoluminescence indicates that a growth temperature of 580°C minimizes carbon contamination, and extended pregrowth outgassing of the substrate under an As 4 overpressure results in increased carbon concentrations. The carbon incorporation was found to be relatively insensitive to outgassing temperature above 615°C. Contrary to expectations, increasing the As/Ga flux ratio during growth resulted in larger carbon luminescence peaks. Chemical impurities and structural imperfections in semiconductor silicon. Part I. HOWARD R. HUFF. Solid St. Technol., 89 (February 1983). The growth of macroscopic dislocationfree silicon crystals including the incorporation of oxygen is briefly reviewed followed by a discussion of the point-defect dilemma created as a result of the absence of grown-in line defects. Several process-induced point, line and surface defect systems are then illustrated along with specific getter processes for controlling their detrimental electrical effects. IC components and circuits are utilized as diagnostic probes to identify relevant device/circuit parameters that are significantly controlled by specific silicon material/IC processes. Several recommendations are presented for the specification of a silicon wafer consistent with VLSI quality circuitry. These topics will be discussed in this and future instalments. Etching characteristics of polysilicon, SiO 2 and MoSi 2 in NF 3 and SF 6 plasmas. C. S. KORMAN, T. P. CHOW and D. H. BOWER. Solid St. Technol., 115 (January 1983). Doped polysilicon etch rates in excess of 1 micron/min, and a selectivity over oxide of ~20 : 1 have been achieved in an N F 3 plasma at pressures above 200 mtorr; this etching is isotropic. Highly anisotropic etching has been demonstrated for pressures below 150mtorr and has been applied to the patterning of "hyperfine" (1500 A) MoSi z structures using an edge-defined technique. The nature of MoSi 2 etching in NF 3 has been related to the binary structure of the silicide and the large differences in the metal fluoride and silicon fluoride vapour pressures. Differences in the etching characteristics of polysilicon (the degree of loading and uniformity) in NF 3 and SF 6 plasmas have been attributed to both the lower ionization potential and heat of formation of N F 3 as well as the
1183
7. SEMICONDUCTOR INTEGRATED CIRCUITS, DEVICES AND MATERIALS Applications of ellipsometry to in situ study of the growth of hydrogenated amorphous films. J. PERRIN and B. DREVILLON. Acta electron. 24 (3), 239 (1981/1982). In order to characterize the deposition mechanisms of hydrogenated amorphous silicon from a silane glow discharge, an in situ spectroscopic automatic ellipsometer has been associated with a multipole reactor. The ellipsometer is described; it is based on the photo-acoustic modulation technique and allows a fast data acquisition rate. From real time ellipsometry at a fixed wavelength on a growing film, the growth rate and the refractive index of the material are determined and some indications are given on the growth process. The dielectric constant spectrum measurement allows a posteriori characterization of the influence of plasma conditions or thermal treatments on the optical properties of the sample without air contamination. X-ray double diffraction by thin monocrystalline layers. C. SCHILLER. Acta electron. 24 (3), 267 (1981/1982). X-ray double diffraction has been used for the assessment of thin epitaxial layers of Gll_xAlxAs: it gives a rapid check of the substitution rate x for thicknesses larger than 0.15 lam, of the radius of curvature, of the lateral and in depth composition homogeneity and of the layer thicknesses from 0.25 to 3 lam. Self-consistent theory of diagonal and off-diagonal disorder in the screened impurity band of doped semiconductors. NORBERTO MAJLIS and ENRIQUE V. ANDA. Solid St. Communs 45 (7), 561 (1983). We have developed a theory of the impurity band in doped semi-conductors, using a tightbinding basis and a self-consistent treatment to handle the configurational averages. New ingredients in the present calculation are: (1) overlap between different impurity orbitals; (2) simultaneous treatment of diagonal and offdiagonal disorder; (3) inclusion of electron-electron interactions through the substitution of the bare Coulomb impurity potential by a Yukawa potential. The density of states and the position of the Fermi level (for T = 0) are calculated for different impurity concentrations. Dielectric/semiconductor interfaces analysis using spectroscopic eUipsometry. D. E. ASPNES and J. B. THEETEN. Acta electron. 24 (3), 217 (1981/1982). Ellipsometric measurements with variable energy of the incident light (from 1.5 to 5.8 eV), a technique named spectroscopic ellipsometry (SE), determine in a non-destructive way the optical response of the interface region between a substrate and a transparent superficial film. The method is demonstrated on the case of native oxides on silicon and gallium arsenide. In the case of silicon, the dielectric function of SiO 2 is already known (with a possible variation in density), and the comparison between SE data from oxide on Si (100), Si (110) and Si (111) and a model assuming an atomically abrupt interface between SiO 2 and Si evidences the presence of an interface region of width do~ = 7 _+2 A, consisting of atomically mixed Si and O of average stoichiometry SiO0.4±0. 2. In the case of gallium arsenide, the dielectric function of its electrochemically grown anodic oxide is not known a priori. It can be determined by using a 3-phase model. It is shown that the remaining discrepancy between SE measurements and the 3-phase model can be interpreted as the presence of an approximately 50 A region with a density deficit (probably due to leaching of As203) between oxide and ambient, the oxide/substrate interface being found very abrupt (markedly lower than 10 A in thickness). Alternating space-charge-limited currents in hydrogenated amorphous silicon. W. DEN BOER and A. F. P. Pop. Solid St. Communs 45 (10), 881 (1983). Space-charge-limited currents in n + - i - n + sandwich samples of hydrogenated amorphous silicon have been investigated under alternating current
MF<
r
(AC) conditions. When the voltage sweeping rate exceeds the release rate of carriers from deep traps, the AC currentvoltage characteristics differ markedly from the direct current (DC) characteristics. This is attributed to the remaining space charge in the samples which acts as a potential barrier and reduces the current. A simple model is proposed to describe the time-dependence of this injection-induced barrier.
Developments in crystal growth from high-temperature solutions. HANS J. SCHEEL. Prog. Crystal Growth Char., 5, 277 (1982). With the technological advances in the past 12 years, and with the high quality of flux-grown crystals and layers, this crystal-growth technique has become increasingly important. The technological parameters which have to be optimized to achieve large structurally perfect crystals, of high purity and free of striations, are: choice of solvent, growth temperature and its control, temperature gradient at and along the growth interface, supersaturation, and hydrodynamics. Top-seeded solution growth and the accelerated crucible rotation technique are the most widely applied techniques for growth of large high-quality crystals. Effects of MBE growth conditions on carbon contamination in GaAs. O. TEJAYADI, Y. L. SUN, J. KLEM, R. FISCHER, M. V. KLEIN and H. MORKOC. Solid St. Communs 46 (3), 251 (1983). The effects of growth and pre-growth conditions on the background concentration of carbon in high quality undoped GaAs layers grown by molecular beam epitaxy have been studied. Characterization of the layers by low temperature photoluminescence indicates that a growth temperature of 580°C minimizes carbon contamination, and extended pregrowth outgassing of the substrate under an As 4 overpressure results in increased carbon concentrations. The carbon incorporation was found to be relatively insensitive to outgassing temperature above 615°C. Contrary to expectations, increasing the As/Ga flux ratio during growth resulted in larger carbon luminescence peaks. Chemical impurities and structural imperfections in semiconductor silicon. Part I. HOWARD R. HUFF. Solid St. Technol., 89 (February 1983). The growth of macroscopic dislocationfree silicon crystals including the incorporation of oxygen is briefly reviewed followed by a discussion of the point-defect dilemma created as a result of the absence of grown-in line defects. Several process-induced point, line and surface defect systems are then illustrated along with specific getter processes for controlling their detrimental electrical effects. IC components and circuits are utilized as diagnostic probes to identify relevant device/circuit parameters that are significantly controlled by specific silicon material/IC processes. Several recommendations are presented for the specification of a silicon wafer consistent with VLSI quality circuitry. These topics will be discussed in this and future instalments. Etching characteristics of polysilicon, SiO 2 and MoSi 2 in NF 3 and SF 6 plasmas. C. S. KORMAN, T. P. CHOW and D. H. BOWER. Solid St. Technol., 115 (January 1983). Doped polysilicon etch rates in excess of 1 micron/min, and a selectivity over oxide of ~20 : 1 have been achieved in an N F 3 plasma at pressures above 200 mtorr; this etching is isotropic. Highly anisotropic etching has been demonstrated for pressures below 150mtorr and has been applied to the patterning of "hyperfine" (1500 A) MoSi z structures using an edge-defined technique. The nature of MoSi 2 etching in NF 3 has been related to the binary structure of the silicide and the large differences in the metal fluoride and silicon fluoride vapour pressures. Differences in the etching characteristics of polysilicon (the degree of loading and uniformity) in NF 3 and SF 6 plasmas have been attributed to both the lower ionization potential and heat of formation of N F 3 as well as the