O P T O - E L E C T R O N I C S 3 ( 1 9 7 1 ) �9 L E T T E R

Letter Infrared Surface Absorption in Tellurium

Because of possible nonlinear device applica- tions, the optical properties of Te are of con- siderable interest [1]. It has been reported that any surface preparation involving abrasion results in a thin damage layer [2] which is optically absorbing [3-5]. The damage consists of dislocation arrays [3] which may be removed by etching [3, 5] providing that the surface flat- ness requirements are not too stringent. In this letter we show that the amount of 10.6/~ surface absorption in Te depends on the lapping method used, and can be as low as a few per cent per surface.

Suppose an optical beam is incident on a crystal thickness l, bulk absorption coefficient and a surface absorption of/~ per surface. If the power fraction reflected at one interface is r, the fraction of the incident power absorbed by the crystal will be

1 - ( 1 _/~)z exp ( - c d ) a = ( 1 - r ) 1 - r ( 1 _/~)2 exp ( - c d ) (1)

Then

?-r-ra l n k l _ r _ a ] = cd -21n (1-/~) . (2)

N g

The value of r was calculated from the index of refraction and "a" was measured calori- metrically [6] at 10.6 /z for a number of Te crystals. The left hand side of equation 2 was then plotted versus l (see fig. 1), and values of and/3 obtained from the slopes and intercepts, respectively. The results are summarised in table I.

The two polishing methods used were: Method I. An oscillating arm polishing machine with a tin lap and 1 # diamond grit. Method 2. Manually on a silk lap with 0.3 # and finally, 0.05 # alumina grit.

All samples were cut with a wire saw, and then polished extensively to minimise the effect of saw cut damage. As expected, the bulk absorp- tion coefficient e correlates with the boule used, while the surface absorption fl correlates with the polishing method use. Laue X-ray photographs of surfaces prepared by Method 2 were con- sistently sharper than those of surfaces prepared

�9 1971 Chapman and HallLtd.

0.7

0.6

0 ,5

(a )

0.4

,o3

0.2

0.I

(b)

(c)

OI I I I I I 0 2 4 6 8 I0

Z ( m r n )

Figure I Te absorption data for a 10.6/z E ray, propagating at 14 ~ to the c axis. The slope yields the bulk absorption, and the ordinate intercept yields the surface absorption (see table I for the results).

T A B L E I Absorption parameters deduced from fig. 1. The values of tzzo.6 in (a) and (b) agree to within the probable error of the data. The lower value of a ctzo.~ in (c) suggests that boule No. 2 has fewer impurities [7].

Curve Polishing Boule ~10.e /~ (per (fig. 1) method (cm -~) surface

(a) 1 No. 1 0.71 0.10 (b) 2 No, 1 0.56 0.02 (c) 2 No. 2 0.19 0.04

by Method 1, confirming that less damage results from Method 2. Kolb and Laudise [7] have illustrated the manner in which surface damage in Te affects Laue photographs.

Intensities, rather than amplitudes, of multiple reflections have been added in deriving equation 1, hence interference effects are not accounted for. Interference effects cause scatter in the data when the absorption measurements are repeated for slightly different crystal orientations. The

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O P T O - E L E C T R O N I C S 3 (1971) �9 L E T T E R

error bars in fig. 1 show the magnitude of this scatter.

In conclusion, surface absorption in Te is dependent on the polishing technique used, and can be as low as a few per cent per surface at 10.6/~ for careful lapping (i.e., Method 2).

Acknowledgements The technical assistance of H. A. Youngman and J. L. Cinque and discussions with E. R. Nagelberg and R. L. Abrams are gratefully acknowledged.

12January 1971 W. B. GANDRUD

Bell Telephone Laboratories New Jersey 07981, USA

References 1. w.B. GANDRUD and R. L. ABRAMS, Appl. Phys. Letts.

17 (1970) 302-305, and references contained therein. 2. R.S. CALDWELL and H. v. FAN, Phys. Rev. 114 (1959)

664-675. 3. J. S. BLAKEMORE, J . W. S C H U L T Z , and K. c .

NOMURA, J. Appl. Phys. 31 (1960) 2226-2231. 4. J. S. BLAKEMORE a n d K. C. NOMI3RA, Phys. Rev. 127

(1962) 1024-1029. 5. J. H. MCFEE, G. D. BOYD, a n d P. H. S C H M I D T , Appl.

Phys. Letts. 17 (1970) 57-59. 6. w. B. GANDRUD, Appl. Opt. 9 (1970) 1936-1937. 7. E. D. KOLB and R. A. LAUDISE, "The Physics of

Selenium and Tellurium" (Pergamon Press, Oxford, 1969) pp. 213-222.

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