Semiconductors

• Analysis of interface layers by spectroscopic ellipsometry
  Authors: T.J. Kim, J.J. Yoon, Y.D. Kim, D.E. Aspnes, M.V. Klein, D.S. Ko, Y.W. Kim, V.C. Elarde, J.J. Coleman
  APPLIED SURFACE SCIENCE 255(3) 640-642 (2008)
  Abstract: We investigate the relative validity of the Bruggeman effective-medium approximation and several alloy models to describe interfaces in the analysis of spectroscopic ellipsometric data of laminar samples, using data obtained on an AlxGa1 As-x multilayer sample fabricated specifically for this purpose. The investigation highlights the types of errors that result from the use of inappropriate models. Optimum results are obtained with the alloy model where the graded-composition regions are approximated with multilayer stacks.
  Email: ydkim@khu.ac.kr
  
  
• Doping concentration control of SiGe layers by spectroscopic ellipsometry
  Authors: Fursenko O, Bauer J, Zaumseil P, Yamamoto Y, Tillack B
  THIN SOLID FILMS 517 (1) 259-261 (2008)
  DOI: 10.1016/j.tsf.2008.08.009
  Abstract: In this paper spectroscopic ellipsometry (SE) was applied for thickness and composition characterization of C and B doped SiGe layers grown on Si (100) by RPCVD at 600 degrees C. We present the effect of B doping (up to 1.2x10(21) cm(-3)) and C incorporation (up to 0.7%) on optical constants dispersions of Si1-xGex (0x<0.3) layers in the energy range 1.5-5.2 eV and their impact on thickness evaluation. We observed the changes of the positions of critical points and their broadening and found systematic monotonic changes for C incorporation in SiGe and nonmonotonic behaviour for B doped SiGe. The correlation between SE results and active boron concentration in the case of B incorporation was observed. The decoupling of the effect of C (or B) and Ge incorporation by SE was performed that can be applied in multi-dimensional lookup models for thickness and composition estimation and thus used for nondestructive characterization of SiGe-based materials.
  Email: fursenko@ihp-microelectronics.com
  
  
• Characterization of chromium silicide thin layer formed on amorphous silicon films
  Authors: Caputo D, de Cesare G, Ceccarelli M, Nascetti A, Tucci M, Meda L, Losurdo M, Bruno G
  JOURNAL OF NON-CRYSTALLINE SOLIDS 354 (19-25) 2171-2175 (2008)
  DOI: 10.1016/j.jnoncrysol.2007.10.040
  Abstract: A detailed investigation of the compositional, optical and electrical properties of a chromium silicide layer grown at room temperature on top of doped amorphous silicon films is presented. The formation of the layer is promoted only when phosphorous atoms are present in the film. The deposition of a very thin n-type doped layer (around 5 nm) on top of a p-type doped film has allowed us to achieve the chromium silicide formation also on p-type material without changing its doping properties. Angle resolved X-ray photoelectron spectroscopy measurements demonstrate the presence of chromium-oxide, chromium silicide and metallic chromium in similar percentages for both p- and n-type doped layers: From the ellipsometric analysis, the refractive index spectra have been extracted, and the layer thickness has been estimated to be 5 nm for both p- and n-type doped layers. From planar conductivity measurements, we have found that the chromium silicide promotes an activation energy reduction from 0.24 eV down to 0.017 eV for the n-type layer and from 0.36 eV down to 0.14 eV for the p-type film.
  Email: mailto:Characterization%20of%20chromium%20silicide%20thin%20layer%20formed%20on%20amorphous%20silicon%20films
  
  
• Oxidation of SiC investigated by ellipsometry and Rutherford backscattering spectrometry
  Authors: Szilagyi E, Petrik P, Lohner T, Koos AA, Fried M, Battistig G
  JOURNAL OF APPLIED PHYSICS 104 (1) 014903 (2008)
  DOI: 10.1063/1.2949268
  Abstract: Oxidation of SiC was performed in Ar-O2 mixture of atmospheric pressure at 1100 degrees C and compared with that of Si. The partial pressure of O2 varied from 100 to 1000 mbar, while the oxidation time ranged from 0.5 to 45 h. The thickness of the oxide films was determined by spectroscopic ellipsometry and Rutherford backscattering spectrometry. The time and the pressure dependence of the oxidation kinetics of SiC are well described by the modified Deal-Grove model. In the diffusion-limited region, even for the faster case, the oxidation kinetics of the C-terminated face of SiC is not clearly limited by oxygen indiffusion, as for pure silicon. To interpret the ellipsometry spectra, two models of possible structure were used. In the case of the one-layer model, for layer thicknesses above 30 nm, the refractive index of the oxide layers is identical to that of thermally oxidized Si, and it increases rapidly with decreasing thickness below about 15 nm. This increase is significantly larger for C-terminated than for Si-terminated faces, and this difference can be explained by a transition layer introduced into the two-layer model. This model contains a pure SiO2 layer and a transition layer modeled by a mixture of 50 % SiO2 and 50 % SiC. The transition layer is thicker on the C-terminated surface than on the Si-terminated one. The thickness ratio of the transition layers is slightly larger than the surface roughness ratio on the two different sides determined by atomic force microscopy. The density of the oxide films, which can be determined from the backscattering and spectroscopic ellipsometry spectra, decreases with decreasing thickness below about 30 nm. For thicker films, the density of the oxide is equal to the bulk density of SiO2.
  
  
• Band gap and effective electron mass of cubic InN
  Authors: Schley P, Napierala C, Goldhahn R, Gobsch G, Schormann J, As DJ, Lischka K, Feneberg M, Thonke K, Fuchs F, Bechstedt F.
  PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS 5 (6) 2342-2344 (2008)
  Abstract: We succeeded in growing single crystalline c-InN films on 3C-SiC substrate with a c-Ga-N buffer layer by MBE. Spectroscopic ellipsometry is applied in order to determine the complex dielectric function for cubic InN from mid-infrared into the visible spectral region. The high electron densities above 10(19) cm(3) cause pronounced Bursteiu-Moss shifts at the gap. Taking into account the non-parabolicily and the filling of the conduction band, data analysis yields renormalized band edges between 0.430 and 0.455 eV. Including carrier-induced band-gap renormalization we estimate a zero-density band gap of similar to 0.596 eV for c-InN with a corresponding effective electron mass of 0,041 m(0) at the Gamma point of the Brillouin zone.
  
  
• Investigation of energy band gap and optical properties of cubic CdS epilayers
  Authors: Kim DJ, Yu YM, Lee JW, Choi YD
  APPLIED SURFACE SCIENCE 254 (22) 7522-7526 (2008)
  DOI: 10.1016/j.apsusc.2008.06.008
  Abstract: High quality cubic CdS epilayers were grown on GaAs ( 1 0 0) substrates by the hot-wall epitaxy method. The crystal structure of the grown epilayers was confirmed to be the cubic structure by X-ray diffraction patterns. The optical properties of the epilayers were investigated in a wide photon energy range between 2.0 and 8.5 eV using spectroscopic ellipsometry ( SE) and were studied in the transmittance spectra at a wavelength range of 400-700 nm at room temperature. The data obtained by SE were analyzed to find the critical points of the pseudodielectric function spectra, [epsilon(E)] = [epsilon(1)(E)] + i[epsilon(2)(E)], such as E-0, E-1, E-2, E-0', and E-1' structures. In addition, the optical properties related to the pseudodielectric function of CdS, such as the absorption coefficient alpha(E), were investigated. All the critical point structures were observed, for the first time, at 300 K by ellipsometric measurements for the cubic CdS epilayers. Also, the energy band gap was determined by the transmittance spectra of the free-standing film, and the results were compared with the E-0 structure obtained by SE measurement.
  
  
• Ellipsometric investigation of porous silicon layers for the design of a DBR
  Authors: Bardaoui A, Boudaya L, Abdellaoui T, Ben Sedrine N, Lajnef M, Chtourou R.
  EUROPEAN PHYSICAL JOURNAL-APPLIED PHYSICS 43 (1) 87-91 (2008)
  DOI: 10.1051/epjap:2008126
  Abstract: Porous silicon layers (PSL) were fabricated by electrochemical etching and investigated by spectroscopic ellipsometry (SE) in the energy range 0.6-5 eV. Within the effective medium approximation (EMA) and through an optical model consisting of a mixture of void and crystalline silicon (cSi), we were able to determine the porosity (void concentration) and the thicknesses of the PSL. The PSL were divided into several sublayers in order to obtain the best agreement between measured and simulated spectra. Once the etching parameters have been controlled and by choosing the appropriate conditions, it was possible to design a distributed Bragg reflector (DBR) with a high reflectivity band centered at 800 nm. This DBR consists on stacks of alternate PSL having two different refractive indices.
  
  
• Ellipsometric detection of GaAs(001) surface hydrogenation in H-2 atmosphere
  Author(s): Vasev AV
  SURFACE SCIENCE 602 (11) 1933-1937 (2008)
  DOI: 10.1016/j.susc.2008.03.022
  Abstract: Optical properties of MBE-grown GaAs(0 0 1) surfaces have been studied by spectroscopic ellipsometry under dynamic conditions of ramp heating and cooling after desorption of passivating As-cap-layer with low pressure H-2 atmosphere (14 Torr) applied to the surface. The temperature dependence of GaAs pseudo-dielectric function with atomically smooth (0 0 1) surface carrying the fixed Ga-rich (4 x 2)reconstruction was obtained for the temperature range of 160-600 degrees C. It is shown ellipsometrically that GaAs(0 0 1) heating in the molecular hydrogen atmosphere results in the formation of hydrogenated layer on the surface.
  
  
• Optical properties of (GeTe, Sb2Te3) pseudobinary thin films studied with spectroscopic ellipsometry
  Authors: Park JW, Baek SH, Kang TD, Lee H, Kang YS, Lee TY, Suh DS, Kim KJ, Kim CK, Khang YH, Da Silva JLF, Wei SH
  APPLIED PHYSICS LETTERS 93 (2) 021914 (2008)
  DOI: 10.1063/1.2959818
  Abstract: The authors measure the dielectric functions of (GeTe, Sb2Te3) pseudobinary thin films by using spectroscopic ellipsometry. By using standard critical point model, they obtained the optical transition (critical point) energies of the amorphous (crystalline) thin films. The optical (indirect band) gap energies of the amorphous (crystalline) phase are estimated from the linear extrapolation of the absorption coefficients. The band structure calculations show that GeTe, Ge2Sb2Te5, and Ge1Sb2Te4 have indirect gap whereas Ge1Sb4Te7 and Sb2Te3 have direct gap. The measured indirect band gap energies match well with electronic band structure calculation.
  
  
• Optical properties and phase change transition in Ge2Sb2Te5 flash evaporated thin films studied by temperature dependent spectroscopic ellipsometry
  Authors: Orava J, Wagner T, Sik J, Prikryl J, Frumar M, Benes L
  JOURNAL OF APPLIED PHYSICS 104 (4) 043523 (2008)
  DOI: 10.1063/1.2970069
  Abstract: We studied the optical properties of as-prepared (amorphous) and thermally crystallized (fcc) flash evaporated Ge2Sb2Te5 thin films using variable angle spectroscopic ellipsometry in the photon energy range 0.54-4.13 eV. We employed Tauc-Lorentz (TL) model and Cody-Lorentz (CL) model for amorphous phase and TL model with one additional Gaussian oscillator for fcc phase data analysis. The amorphous phase has optical bandgap energy E-g(opt)=0.65 eV (TL) or 0.63 eV (CL) slightly dependent on used model. The Urbach edge of amorphous thin film was found to be similar to 70 meV. Both models behave very similarly and accurately fit to the experimental data at energies above I eV. The CL model is more accurate in describing dielectric function in the absorption onset region. The thickness decreases similar to 7% toward fcc phase. The bandgap energy of fcc phase is significantly lower than amorphous phase, E-g(opt)=0.53 eV. The temperature dependent ellipsometry revealed crystallization in the range 130-150 degrees C. The bandgap energy of amorphous phase possesses temperature redshift -0.57 meV/K (30-110 degrees C). The crystalline phase has more complex bandgap energy shift, first +0.62 meV/K (150-180 degrees C) followed by -0.29 meV/K (190-220 degrees C). The optical properties (refractive index, extinction coefficient, and optical bandgap energy) of as-prepared and fcc flash evaporated Ge2Sb2Te5 thin films are very similar to those values previously reported for sputtered thin films.
  
  
• High-Temperature Complex Refractive Index of Phase Change Recording Medium of Ge2Sb2Te5 Determined Using Phase Change Static Tester and Spectroscopic Ellipsometer
  Authors: Xue Zhe Li, Joong Kyu Choi, Young Sup Byun, Sang Youl Kim, Kyung Soo Sim, and Soo Kyung Kim
  DOI: 10.1143/JJAP.47.5477
  JAPANESE JOURNAL OF APPLIED PHYSICS 47 5477-5481 (2008)
  Abstract. We investigated the high-temperature optical properties of the optical phase change material Ge2Sb2Te5 (GST) using an in situ ellipsometer equipped with a convetional heating chamber and using a spectroscopic ellipsometer combined with a phase change static tester. In addition, we compared the results for the high-temperature complex refractive indices of GST thin films determined using a spectroscopic ellipsometer after the samples were crystallized by the in situ and a phase-change random access memory (PRAM) methods. The ellipsometric constants and the RMS value of the surface roughness of the GST samples were measured by in situ ellipsometry and atomic force microscopy (AFM), respectively. Results showed wide variations at high-temperatures when the GST samples were crystallized by the in situ method. The complex refractive indices of GST thin films were consistent at mid-range temperatures up to 370 °C for the two methods. In contrast, they showed a large variation at a higher temperature of ∼450 °C for the in situ method and remained stable in the entire temperature range for the PRAM method. This indicates that the optical properties of GST thin film are affected by the film structure and protective layer for the in situ method but not for the PRAM method at high-temperatures. It is concluded that the combined use of the spectroscopic ellipsometer with the phase change static tester gives us more precise and consistent results not affected by either the film structure nor the protective material.