1:30PM, S1 *Invited
"Applications of Synchrotron White Beam X-Ray Topography to the Non-Destructive Characterization of Defect Structures in Semiconductor Crystals:" WEIMIN SI, Michael Dudley, Dept. of Materials Science and Engineering, SUNY at Stony Brook, NY 11794-2275
Applications of Synchrotron White Beam X-ray Topography (SWBXT) in the non-destructive characterization of defect structures (growth and process-induced) in semiconductor crystals are reviewed. The basic principles of the technique, including descriptions of the transmission, reflection, and grazing Bragg-Laue geometries, as well as the fundamentals of contrast theory, will be briefly reviewed. The following applications will be presented and discussed: dislocation generation following RTP processing in Si wafers; interfacial dislocations and slip bands in SiGe strained layers; distribution of dislocations in MBE grown InGaAs strained multilayers; basal plane and superscrew dislocations in SiC wafers; polytype identification and investigation of their depth distribution in SiC epilayers; twin distribution in ZnTe bulk crystals and CdTe homoepitaxial buffer layers; solid-liquid interface shape during bulk crystal growth of InGaAs, GaInSb, and InP crystals; minute strain due to bonding process and electrode deposition in Quartz resonators. Methodologies for analyzing dislocations, twins, stacking faults, precipitates and other crystalline defects will be reviewed.
"Synchrotron White Beam X-ray Topography Characterization of Structural Defects in Liquid Encapsulated Kyropoulos Grown InP Single Crystals:" H. CHUNG, W. Si, M. Dudley, Department of Materials Science and Engineering, SUNY at Stony Brook, NY 11794; A. Anselmo, D.F. Bliss, Rome Laboratory, Hanscom AFB, MA 01731, V. Prasad, Department of Mechanical Engineering, SUNY at Stony Brook, NY 11794
Structural defects in Liquid Encapsulated Kyropoulos (LEK) grown InP single crystals have been studied using Synchrotron White Beam X-ray Topography (SWBXT). Structural defects in both longitudinal and perpendicular cut wafers were systematically studied in order to reveal defect microstructure development in different stages of crystal growth. Slip bands initiated from high stress concentration neck regions were observed to propagate into the interior of the longitudinal cut wafers. Transmission X-ray topographs recorded from perpendicular cut wafers revealed dislocations of well defined four-fold symmetric distributions. In order to investigate the formation of this four-fold symmetric pattern, a thermal stress model composed of uniformly distributed, radially compressive hoop stress was introduced. Stress calculations revealed a four-fold thermal stress distribution. Maximum stresses were concentrated in the <100> directions while minimum stresses were induced in the <110> directions. The calculated results are consistent with our experimental observations. In addition, our studies revealed that the distribution and density of dislocations and precipitates changes with the different stages of crystal growth. High density, uniformity distributed dislocations and precipitates were observed in the initial and final stages of crystal growth. Other defects such as small angle tilt boundaries, precipitates and twins were also observed. The formation of these structural defects will be discussed.
"Spatial Characterization of Cadmium Zinc Telluride Radiation Detector Crystals:" H.YOON, J.M. Van Scyoc, M.S. Goorsky, J.C. Lund, M.S. Schieber, R.B. James, F.P. Doty, University of California, 405 Hilgard Avenue, Los Angeles, CA 90095-1595; Sandia National Laboratories, PO Box 5800, Livermore, CA 94551-0969; Digirad, San Diego, CA 92121-2410
Cadmium Zinc Telluride (Cd1-xZnxTe or CZT) is a semiconductor that has experienced rapid development since its first use as a nuclear radiation detector in 1992. However, CZT detectors, particularly as they are made larger, are still limited by the poor transport properties of the holes and the non-uniformities of the material. Large crystals are required not only for fabricating large volume detectors with high sensitivity to high energy gamma photons, but also for large area imaging detectors, which are becoming an increasingly important application of these devices.
Two properties that have received considerable attention are the inhomogeneity of the zinc fraction (x) and plastic deformation across the wafer. Observed changes in the alloy composition of several atomic percent lead to changes in the bandgap and to degraded spectral response. In this study spatially mapped materials characterizations were combined with spatially mapped detector characterizations to study the variations in CZT materials properties and their relation to device performance. Triple axis x-ray diffraction was used to measure the absolute lattice parameter, and thus zinc composition (+/-x= 0.002), and the mosaicity over the active area of several CZT crystals. Room temperature photoluminescence mapping also provided a similar measure of the relative zinc composition for the same crystals. Finally, the gamma-ray (Am241, 60 keV photons) spectral responses of detectors fabricated from these crystals were measured and correlated with the changes in the materials properties.
"Characterization of Layer-Thickness Variations in Distributed Bragg Structures:" D.H. CHRISTENSEN, J.R. Hill, R.K. Hickernell, R.S. Rai, Optoelectronics Division, NIST, Boulder, CO 80303; K. Matney, M.S. Goorsky, University of California, Dept. of MS & E, Los Angeles, Los Angeles, CA 90024
Variations of epitaxial layer-thicknesses from uniform periodicity in semiconductor Bragg-reflectors and microcavities are investigated experimentally and theoretically. Characterization of these structures has received considerable attention recently, due to their technological importance in assessing and optimizing growth uniformity, and as components in vertical-cavity devices. We report the results of reflectance spectroscopy (RS) and high- resolution x-ray diffractometry (HRXRD) measurements and their corresponding simulations made on a number of growth runs which include both random and systematic variations from perfect periodicity. In this study, reflectors peaked at 980 nm and composed of AlAs/GaAs pairs are investigated. We measure and accurately model: 1) RS over the 500-2000 nm range, and 2) HRXRD using 004 and 002 reflections. The specimens were also characterized using TEM and triple-axis reciprocal-space maps. The features of the RS signature most sensitive to linear growth variations are the RS minima on either side of the Bragg peak and growth rates which increase or decrease in time are easily distinguishable using these features. The RS signatures for random variations have feature changes which occur a few fringes away from the Bragg peak and are thus distinguishable from linear variations. Similar signatures exist in the satellite peaks of the HRXRD rocking curves, where, in this case, peak lineshapes are distorted or broadened in a manner unique to to the variation present The low order peaks are useful for average thickness measurements, however, small period variations requires the analysis of the higher order peaks, >10, and thus HRXRD is needed. When higher order satellite peaks are analyzed, we find that random increasing, and decreasing linear variations are again distinguishable. Measurement and modeling results like these are important since the manufacturing yield, reproducibility, and optimized performance of vertical-cavity devices depends critically on layer-thickness control and uniformity. In conclusion, these measurement methods and structures are found to be powerful non-destructive techniques applicable to the evaluation and control of epitaxial growth, and they serve as baseline calibrations for in-situ growth measurements.
"Microwave Modulated Photoluminescence in Doped GaAs:" C.E. INGLEFIELD, M.C. DeLong, P.C. Taylor, Department of Physics, University of Utah, Salt Lake City, UT 84112
Microwave modulated photoluminescence (MMPL) is a spectroscopy wherein a sample is placed in the electric field maximum of a microwave cavity and is simultaneously subjected to continuous optical pumping and chopped microwave electric fields. In this work MMPL has been performed on GaAs epilayers with n- and p-type doping ranging from 1015 to 1020 cm-3 as part of an ongoing project to develop MMPL as a spectroscopy. As reported previously,[l] the MMPL interaction can be separated into two categories, thermal (where the PL is modulated through lattice heating) and non-thermal (where lattice heating is not the source of the signal). These two cases may be experimentally distinguished, and have previously been used to qualitatively categorize samples. The interaction is predominately non-thermal, or faster, in the lightly doped samples and thermal in the heavily doped samples. These observations are consistent with previous observations that the thermal component dominates in `inhomogeneous' materials, e.g. materials with internal electric fields. MMPL may be a useful tool for probing internal electric fields in inhomogeneous semiconductors. A model for the quantitative conditions under which the interaction should be thermal or non-thermal will be presented.
[1) M. C. DeLong, I. Viohl, W. D. Ohlsen, P. C. Taylor, F. P. Dablowski, K. Meehan, J. E. Williams, and M. Hopkinson, Spectroscopic Characterization Techniques for Semiconductor Technology IV, edited by Orest Glembocki [Proc. SPlE 1678, 221 (1992)]
"Optical Characterization of AlN Buffer Layers in GaN on Sapphire Heteroepitaxy:" CHRISTIAN WETZEL, E.E. Haller, R. Jeanloz, J Ager III, Lawrence Berkeley National Laboratory and University of California, MS 2-200, Berkeley, CA 94720; R. Held, Department of Chemical Engineering, Minnesota University, Minneapolis, MN 55455; I. Akasaki, H. Amano, Department of Electrical and Electronic Engineering, Meijo University, Nagoya, Japan
Epitaxy of high quality GaN still suffers from the lack of a lattice matched substrate material. Incorporation of an intermediate-buffer layer, typically deposited at lower temperatures, has proven to significantly improve crystalline quality and device performance. However, optical characterization of the buffer layer has not been feasible up to now due to its small thickness typically lying below 50A. Here we present a method to positively register the presence of an AlN buffer layer and to assess its structural quality in an easy-to-apply, purely optical approach. From a combination of Raman scattering and infrared spectroscopy in the optical phonon region, we identify the individual layers of the heterostructure. We demonstrate the methods versatility with GaN films deposited on various substrates comprising sapphire and Si.
Work supported by US DOE under Contract DE-AC03-76SF00098. C.W. thanks the Deutsche Forschungsgemeinschaft for a grant.
"Non-Destructive 4" Wafer Characterization of In-, Ga- and Al-Based III-V Materials Prepared by MOCVD Turbo Disk Technology:" R.A. STALL, R. Zawadzki, Z.C. Feng, D. Collins, E. Armour, C. Beckham, N. Schumaker, EMCORE Corporation, 394 Elizabeth Ave., Somerset, NJ 08873
Various III-V compound semiconductor materials and structures have many applications for electronic and optoelectronic devices in a wide wavelength range covering ultraviolet (UV), visible, near infrared (IR), middle IR to far-IR. Large size epitaxial growth of these materials with high quality and high uniformity are in great demand. EMCORE Corporation has developed and applied an advanced Turbo Disk technology, with a vertical growth configuration and a high speed rotating disk, for the metalorganic chemical vapor deposition (MOCVD) of the large size and multiple wafer growth of various semiconductor, ferroelectric, oxides and superconductor materials. We have established a series of techniques, including x-ray diffraction (XRD), Hall effects, sheet resistivity (SR), scanning electron microscopy (SEM), Nomarski micrograph (NM), optical transmission and reflectance, photoluminescence (PL), Raman scattering and Fourier transform infrared (FTIR) spectroscopy, to characterize these thin film materials in large size wafer scale. In this study, we demonstrate non-destructive materials characterization on 4" wafer size epitaxial films of binary InSb, ternary InGaAs and AlGaAs, and quaternary GaAlInP and GaAlAsP grown on GaAs or InP substrates by MOCVD Turbo Disk technology. We illustrate a series of mapping distributions of the film thickness, sheet resistivity, surface morphology, XRD peaks and PL spectra. These data show that the grown materials are of high crystalline quality and uniformity. For example, uniformities of our epitaxial film thickness, major PL band peak wavelength, intensity, and width are better than 1- 5%, respectively. For some techniques without automated mapping abilities, we employed multiple point measurements to obtain information over the entire wafers. These wafer scale material characterizations were tightly combined with the epitaxial growth processes and help to improve uniformity of the large scale wafer epitaxy. Additional instrumental arrangements and requirements are discussed.
"Micro-Raman Characterization of Cobalt Salicide Process:" TAKAHIRO KIMURA, Junichi Watanabe, Tomoji Nakamura, Fujitsu Laboratories Ltd. 10-1 Morinosato-Wakamiya, Atsugi, 243-01, Japan
The cobalt salicide process is thought to be a key technique to realize deep sub-micron CMOS devices . The salicide process is composed of two techniques which must be precisely controlled within the device scale, that is, a solid state selective reaction between a metal and the Si substrate, and a selective etching of the unnecessary part. Therefore in order to control the cobalt salicide process and to make it more reliable, it is desirable to establish a non-destructive characterization technique with sub-micron resolution. Our purpose in this study is to set a foundation for a micro-Raman characterization methods of the very thin cobalt salicide process and hopefully to show its usefulness.
Raman spectrum features of CoSi2 was first investigated. Since the Raman peaks for CoSi ate well known but the Raman peaks for CoSi2, the actual phase used in devices, is uncertain. The CoSi2 mono-phase sample was prepared by annealing a 180 nm thick cobalt film deposited on Si at 770deg.C far 60s under an Ar gas ambient in a RTA system. A cobalt film was deposited by magnetron sputtering on a chemically cleaned Si(100) substrate. The CoSi2 phase was confirmed by X-ray diffraction measurement. Raman measurements were performed with a back scattering geometry using a micro-Raman system (Renishaw system 2000). An Ar ion laser of 488 nm wavelength with a power of less than 5 mW was focused an the sample 16 a diameter of about 0.7 um. The observed Raman spectrum showed a weak but definite peak at 325 cm-1, and an also weak and somewhat broader peak at 725 cm-1, with an overall broad background. It is known that CoSi2 has a CaF2 type crystal structure and so it has one Raman active phonon mode. Accordingly the peak at 325 cm-l was ascribed to the one phonon mode of CoSi2 and the peak at 725 cm-1 possibly originates from a higher-order phonon mode.
Next, Raman spectrum changes due to silicidation of 10 nm Co on a Si substrate were observed and compared with the X-ray diffraction measurements. Cobalt deposited samples were annealed at 400deg.C, 425deg.C, 450deg.C, 500deg.C, 600deg.C, and 700deg.C for 30s. Raman measurements were performed in the same manner described above. The sample annealed at 400deg.C showed a weak Raman peak at 206 cm-1 which is known to originate from CoSi2. Up to the annealing temperature of 550deg.C, the 206 cm-1 peak intensity grew gradually. At 600deg.C annealing, in the spectrum, the 206 cm-1 peak became weak and in turn 325 cm-1 and 725 cm-1 of CoSi2 peaks appeared. At 700deg.C, both CoSi2 peaks became more intense. The cobalt silicidation process observed in the Raman spectrum change agreed qualitatively with the results of X-ray diffraction measurements. However, below the annealing temperature of 450 deg.C, the signals of Co, CoSi2 and CoSi was difficult to resolve in X-ray diffraction pattern because of severe overlapping. On the other hand, in the Raman spectrum it was easy to identify the CoSi peak. For the CoSi2 phase, on the contrary, the signal was clearly resolved in the X-ray measurement but overlapped with the background of the Si substrate in the Raman measurement.
Finally, in order to demonstrate applicability to microscopic regions, Raman spectra were measured across a 2 um wide line of cobalt silicide at 0.4 um intervals. The spatial resolution of our micro-Raman system was better than 0.4 um which was validated by other experiments. The line was formed on the active region of a LOCOS structure using the salicide process with a RTA temperature of 425deg.C. The observed Raman spectra revealed that the CoSi was formed uniformly on the fine and the washing-out process of unreacted Co and Co2Si was successful.
 K. Goto, A. Fushida, J. Watanabi, T. Sukegawa, K. Kawamura, T. Yamazaki,
and T.Sugii, IEDM Tech Dig., 449 (1995).  H. Ying, Z. Wang, D. B. Aldrich.
D. E. Sayers, and R.J. Nemanich, Mat. Res. Soc. S .Proc.320 335 (1994).
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