determination of the absorption edges of zns polytypes by contact photography with uv
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DETERMINATION OF THE ABSORPTION EDGES OF ZnS POLYTYPES BYCONTACT PHOTOGRAPHY WITH UVO. Brafman and I. T. Steinberger Citation: Applied Physics Letters 7, 110 (1965); doi: 10.1063/1.1754313 View online: http://dx.doi.org/10.1063/1.1754313 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/7/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Absorption and luminescence of the surface states in ZnS nanoparticles J. Appl. Phys. 82, 3111 (1997); 10.1063/1.366152 Photoacoustic study of twophoton absorption in hexagonal ZnS J. Appl. Phys. 53, 615 (1982); 10.1063/1.329967 Madelung Potentials in the Polytypes of SiC and ZnS J. Chem. Phys. 57, 3578 (1972); 10.1063/1.1678799 TWOPHOTON ABSORPTION IN ZnS Appl. Phys. Lett. 10, 265 (1967); 10.1063/1.1754803 Erratum: Determination of the Absorption Edge of the ZnS Polytypes by Contact Photography with UV Appl. Phys. Lett. 9, 218 (1966); 10.1063/1.1754718
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Volume 7, Number 4 APPLIED PHYSICS LETTERS 15 August 1965
samples can be explained by a modification of the interface structure due to phosphorus. Incorporation of P into the Si-Si02 interface structure is expected to decrease the density of donor states. 7 The considerable increase in ta for n-type specimens is attributed to the lack of boron in these oxide films. The presence of B in the Si02 is thought to facilitate the incorporation of hydroxyl through defect reactions.
We thank]. A. Amick and W.]. Merz for helpful discussion and R. ]. Evans and R. O. Wance for assistance in the experimental work.
I A. G. Revesz, IEEE Trans. Electr. Dev. ED-12, 97 (1965).
"The term surface states is used here in a completely operative manner; it lumps together the effect of work-function difference (between metal electrode and Si), built-in oxide charge, oxide states, and interface states. The term oxide states refers to electronic interactions of the oxide with Si, and the term interface states is related to effects associated with the phase boundary. The work-function difference cannot simply be considered as a correction term as was shown in a separate investigation; it was found that Pt. Cr, AI, and Au result in decreasing positive Si surface potential, in this order, instead of the order: AI, Cr, Au, and Pt as expected on the basis of work-function values.
3 K. Lehovec, A. Slobodskoy, and]. 1. Sprague, Phys. Stat. Sol. 3,447 (1963).
4 K. H. Zaininger and G. Warfield, IEEE Trans. Electr. Dev. Ed-12, (1965).
5P. T. Landsberg,]. Chern. Phys. 23,1079 (1955). 6 R. W. Lee, Phys. Chern. Glass 5, 35 (1964). 7]. ]. Lander and J. Morrison, Ann. N. Y. Acad. Sci. 101, Art.
3,605(1963).
DETERMINATION OF THE ABSORPTION EDGES OF ZnS POLYTYPES BY CONTACT PHOTOGRAPHY WITH UVI
(uv spectroscopy; E)
Diffraction and interference effects hamper the determination of absorption constants in small samples (dimensions of the order of a few microns). The difficulty is particularly great in non visible spectral regions where microscopy is cumbersome. The contact photography method to be described here can be used in many cases, where the usual methods fail.
It seems that these difficulties are the main reason for the fact that the absorption edges of the various ZnS polytypes have not yet been measured. These structures are usually found2 as parallel, narrow strips stacked side by side, the planes separating regions of different structures being perpendicular to the common c axis.3 The width of each region may be less than 1 JL in an extreme case, but in this laboratory crystals were grown with uniform polytype structures as wide as 0.5 mm. The change of crystal structure along the c axis is readily demonstrated by the appearance of birefrigence banding.4
The method developed is based on contact photography in monochromatic light. Diffraction effects were eliminated by using flat crystals making a good contact with the photographic plate. The crystals were illuminated from the exit slit of a Hilger D285
110
O. Brafman and I. T. Steinberger Department of Physics, The Hebrew University
Jerusalem, Israel (Received 22July 1965)
monochromator, the light source being a 500 W xenon lamp. The monochromator was equipped with a 7-54 Corning filter to eliminate visible stray light. The slit width of the monochromator was chosen in such a way that the spectral half-width of the emerging beam was 4 A. In the spectral region of interest (3330 to 3341 A) the radiant flux incident on the crystal was independent of wavelength. A polarizing filter, a mirror and a collimating quartz lens were mounted in the light path. The crystal was placed on an Ilford Ql spectroscopic plate. The grain size of the emulsion was about 1 JL; this figure limited the geometrical resolution obtainable. The crystals used were grown as described in a previous work,5 and included similar traces of metallic impurities.
Enlargements of three contact photographs of a crystal obtained with three different wavelengths appear in Fig. 1. It is seen that with the decrease of A the number of black strips on the plate decreases, i.e., the number of transparent crystal strips decreases as well. There is an exact coincidence between the positions of the colored birefringence bands and the positions of the strips of uniform absorption.
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Volume 7, Number 4 APPLIED PHYSICS LETTERS 15 n .... "' ... ,' .. 1965
For quantitative interpretatiou of the photographs, it was assumed that, except for parallel translations along the A axis, the dependence of the absorption coefficient fL on the wavelength A is the same for all polytypes in the absorption edge region. This assumption has been justified for the sphalerite and wurtzite structures6 •7 and verified bv cross checks for polytypes in the course of the present work. Accordingly, one may choose a convenient value of fL and look for that wavelength An in each
Fig. 1. Contact photographs of the same part of a crystal taken with a) 3400 A; b) 3370 A; c) 3350 A. Enlargement: 50x c axis horizontal.
polytype for which fL has this predetermined value. H
The changes of An with structure are equal to the translations of the fL-\'S-A curves and thus they measure the relative positions of the absorption edges. Au for each poly type was determined by finding, under standard conditions, the minimum wavelength for which the plate beneath the polvtype became blackened. For the direct comparison of the absorption edges of any two different structures, these structures had to be available in regions of approximately equal thicknesses.
The conditions were standardized by using the same exposure time for each wavelength and the same development procedure. As far as possible, all photographs on a crystal were taken side by side on the same plate. The ambient temperature was kept at :woC both during the exposure and during the development. A standard set of irradiation wavelengths. differing from each other by !} A, were used for each crystal. From the set of photographs obtained, that one was selected which was taken with the shortest wavelength suflicient to darken the film beneath the strip in question. This selection yielded the approximate value of Ao. Corrections ~f the value of Ao within the 5-:\ intervals were made by observing the degree of blackening under the microscope.
The incident light dose was chosen in such a way that in a purely cubic (without twinning) region of a crystal 40 fL thick, Au was 3410 A. Comparison of samples of different thicknesses revealed that at this wavelength fL = 60 mm-l for cubic structure. This value is in good accord with those which can be read off the curves published by other authors. 7
With a crystal of different thickness, the standard illumination dose used and the "minimum wavelength for blackening" criterion for Ao implied, of course, that Ao was determined for another value offL . .K evertheless the differences of the Ao values still measure the translations of the fL"VS-A curves with structure change, provided the shape of these curves is independent of the structure. The change of Ao with structure was found to be independent of crystal thickness (in a range between 30 to 200 fL) and thus the assumption that the shapes of the fL-vs-A curves are structure independent, is justified.
Other cross checks on the absorption edge determinations were performed by changing the irradiation does with crystal thickness. It was found that the determinations of the absorption edge positions were consistent within ±2 k
The unit cell dimensions of the various structures were determined by x-ray oscillation photo-
III
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Volume 7, Number 4 APPLIED PHYSICS LETTERS 15 August 1965
graphs taken around the common c axis. Only such regions of uniform optical absorption were considered, whose width was larger than the diameter of the x-ray beam (0.1 mm) and where sharp diffraction spots were obtained without any trace of "smearing out". In other words, one-dimensional stacking disorder often observed in ZnS (ref. 4) was absent in the regions investigated.
Results on the absorption-edge positions ~o (defined as that value of ~ for which J.L(~) = 60 mm- I
)
are summarized in Table I. The value of ~o in cubic ZnS was taken as standard; its value in the twolayer hexagonal structure is in good agreement with ref. 7.
No absorption-edge determination has been reported for the 6-layer structure. This is the first instance where the existence of a 27 -layer polytype in ZnS is established.
A report on the relation of absorption edges of ZnS polytypes to other crystal properties is in preparation. The uv contact photography method as developed here can be utilized for the investigation of certain mineralogical and biological specimens as well.
Table I. Positions of Absorption Edges A.o (see text) in ZnS of Various Structures. *
Structure A.ol., A A.oll,A
Cubic 3410 3410 2-layer (hexagonal) 3345 3315 6-layer 3388 3375 27-layer 3400 3395
*A.ol. is the value of A.o with light polarized in a plane perpendicular to the c axis, A.oll the same with polarization plane parallel toc.
I Sponsored in part by the Israel Council of Research and Development.
2L. W. Strock and V. A. Brophy, Am. Mineralogist40,94 (1955). 3S. P. Keller and G. D. Pettit, Phys. Rev. 115,526 (1959). 'W.J. Merz, Helv. Phys.Acta 31, 625 (1958). 50. Brafman, E. Alexander, B. S. Fraenkel, Z. H. Kalman, and
I. T. Steinberger,]. Appl. Physics 35,1855 (1964). 6 J. A. Beun and G. J. Goldsmith, Helv. Phys. Acta 33, 508 (1960). 7W. W. Piper, P. D. Johnson, and D. T. F. Marple,]. Phys.
Chern. Solids 8, 457 (1959). 8 Since comparisons were made at equal,... values of the differ
ent structures, the reflection losses were also the same and thus could be ignored.
ERRATA
In "Electromagnetic Backscatter from Overdense Plasmas", by Philip J. Wyatt, Appl. Phys. Letters 6, 209 (1965), the subscripts 1 and 2 in Eq. (1) are too large. In Eq. (2), kl and k2 should read Kl and K2'
In "Energy Transfer and H 0 3+ Laser Action in Silicate Glass Coactivated with Yb3+ and Ho3+", by H. W. Gandy, R. J. Ginther, and J. F. Weller, Appl. Phys. Letters 6, 237 (1965), Fig. 2 is inverted. There is also a typographical error in line 8, p. 239, left-hand column. The sentence "The stimulated emission in H 0 3+
occurs at a wavelength of Ho3+ in this glass." should be deleted. In "Preparation of Ferrite Films by Evaporation," by A. Baltz, Appl. Phys. Letters 7, lO (1965), the index
p.; .::renee term under the title should read (electron beam evaporation; E) instead of (electron beam melting; E).
In "Activation Volume and Energy for Self-Diffusion in Aluminum," by B. M .. Butcher, H. Hutto, and A. L. Ruoff, Appl. Phys. Letters 7,34 (1965), the index reference term under the title should read (E) instead of (vapor disposition; superconductivity; sputtering; x-ray diffraction; E).
In "Microwave Emission from n-Type Cadmium Sulphide," by W. H. Haydl and C. F. Quate,Appl. 1.'hys. Letters 7,45 (1965), the index reference term under the title should read V /cm instead of Z/cm.
112
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