On the Etching of II–VI and III–V Compounds

On the Etching of II-VI and III-V Compounds G. A. W o l f f , 1 J. J. Frawley, 2 and J. R. H i e t a n e n 1

Solid State Research and Electronics Division, The Harshaw Chemical Company, Cleveland, Ohio

ABSTRACT

Etch pit shape and orientat ion as obtained from microscope and electron microscope investigations are related to l ight-figure reflection patterns. The usefulness of the l ight-figure technique is demonstrated using I I -VI and II I -V compounds as examples.

Chemica l and phys ica l e tch ing s tudies have p roved to be of impor t ance in m a n y inves t iga t ions of c rys- tals. The d e t e r m i n a t i o n of c rys ta l lographic o r i e n t a - t ion, the type and n u m b e r of dislocations, and the po la r i ty of crysta ls w i thou t a cen te r of s y m m e t r y are common examples . 3 Less k n o w n is the appl ica- t ion of mechan ica l and chemical e tch ing lead ing to the d e t e r m i n a t i o n of chemical bond ing on c lean su r - faces and on surfaces con t a in ing absorbed a toms (2, 3).

Etched surfaces are gene ra l l y inves t iga ted by microscope or by goniomet r ic inspect ion. The l igh t - figure, a l t hough it p rovides in m a n y cases more usefu l i n f o r m a t i o n as far as a n g u l a r and s t r u c t u r a l da ta are concerned, is less f r e q u e n t l y ut i l ized. The v i sua l microscopic and e lec t ron microscopic inves t iga t ions revea l the shape and topography of the etched su r - faces r a the r t h a n the i r more in t r i ca te morpho logy fea tures such as the etch pit, growth, and so lu t ion form. Comple te and de ta i led i n f o r m a t i o n on the morpho logy of crys ta ls can be ob ta ined by c o n v e n - t iona l bu t somet imes tedious goniomet ry . The l igh t - figure t echn ique suppl ies this i n fo rma t ion in a more e legan t and rap id m a n n e r (4) . This me thod has no t been c o m m o n l y applied, however . A possible reason for this is that , in add i t ion to c rys ta l lograph ic factors, k ine t ica l factors also d e t e r m i n e the type of l igh t - f igure observed. This of ten r ende r s the l igh t - figure reflection more complex and difficult to i n t e r - pret . For the usefu l app l ica t ion of this method, therefore , an u n d e r s t a n d i n g of the inf luence of these factors is necessary. A descr ip t ion and e x p l a n a t i o n of the i r inf luence is g iven below. The inves t iga t ion of the e tching of the I I -VI and I I I - V compounds wi l l demons t r a t e an appl ica t ion of the l igh t - f igure tech- n ique and its eva lua t ion . 4

Crystal Etching and Light-Figure Reflections

In genera l , the bo t tom or concave pa r t of an etch pi t and the convex par t s of a g rowing crys ta l are f inal ly b o u n d e d by flat p lanes w h e n the etch or g rowth process has proceeded a sufficient t ime. On the o ther hand , the r ims or convex pa r t s of the etch pits and the concave or h idden par t s of the g rowing crys ta l wi l l a ssume cu rved shapes. The cu rved por - t ions of a c rys ta l wil l more of ten t h a n not be curved

P r e s e n t addres s : Er ie T e c h n o l o g i c a l P r o d u c t s Inc. , Erie , Pa .

~ P r e s e n t addres s : R e n s s e l a e r P o l y t e c h n i c I n s t i t u t e , Troy, N. Y.

Fo r i n f o r m a t i o n on th i s s u b j e c t consu l t r e v i e w , a n d r e l a t ed a r t i - cles in re f (1).

On r e l a t ed i n v e s t i g a t i o n consu l t ref . (5).

cyl indr ica l ly , i.e., curved pa ra l l e l to a c rys ta l lo - graphic d i rec t ion or zone (6) . In the co r respond ing l ight - f igure , i.e., in the pro jec ted f igure r e su l t ing f rom the reflect ion of pa ra l l e l l ight i m p i n g i n g on the crys ta l surface, c rys ta l lographic p lanes wi l l appear as i n t ense spot reflections or as the in t e r sec t ion of l ines. These, in tu rn , r ep re sen t l ight reflections f rom at least two sets of v ic ina l p lanes t i l ted a long spe- cific zonal direct ions. In some cases ref lect ion spots appear alone. This occurs w h e n the surface is composed en t i r e ly of macroscopic flat planes . These points are i l lus t ra ted in Fig. 1.

In some cases the e tching can be p a r t l y n o n p r e f e r - ent ial . In this case ce r ta in por t ions of the crys ta l surface appear polished. These por t ions of the su r - face are gene ra l l y cu rved to var ious degrees and at t imes con ta in n e a r l y flat areas which are for the most pa r t c rys ta l lograph ica l ly insignif icant . Thei r

Fig. 1. Photomicrographs and light-figure patterns of concave and convex etching of CdS. (a) Light-figure patterns of this type are associated with concave etching. The central spot is the reflection from the flat bottomed pits. Each spoke results from the reflection of light from vicinal planes tilted about one zonal axis. Pits with pointed bottoms contain dislocations. (b) Light-figure patterns of this type are associated with a combination of concave and convex etching. The central star-like reflection is the result of concave etching. The extra features of the pattern appear as convex etching increases. (c) Light-figure patterns of this type are associated with nearly total convex etching. Ridges are now discernible; some concave etching is still evidenced by their broken appearance. (d) Light-figure patterns of this type are associated with total convex etching. Ridges appear unbroken. Reflection comes from relatively smooth areas between ridges. The (00.1) plane shown was identi- fied by x-ray as the "cationic" plane (with R.F. Belt).

22

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Vol. 111, No. 1 E T C H I N G I I -VI A N D I I I -V C O M P O U N D S 23

raining small crystal grains larger than about 50~, light-figure studies can be accomplished using a properly constructed microscope. The determination of the effect of grain orientation on grain boundaries is also possible by this technique. The same applies to twin boundary and other surface topography studies (see Fig. 2).

Etching of II-VI Compounds, Wurtzite Structure As has been previously stated, the usefulness of

this method lies in the fact that the light-figure reflection pat tern if correctly interpreted, i.e., if re- duced to zones and planes, can be directly t rans- formed to the stereographic projection of an ideal solution or etch pit form from which, in turn, the

Fig. 2. Photomicrograph of (00"1) of CdS etched in HNOs. Broad light-figure pattern results from the reflection of a large diameter light beam. This pattern is composed of a combination of reflections from small areas within certain portions of the beam. The narrower light-figure line reflections are associated with the respective marked areas of the surface of the photomicrograph.

existence depends largely on the kinetic factors of dissolution rates and only to a minor degree on crys- tallographic considerations. Therefore, care must be taken during light-figure examinations of crystals to distinguish between preferentially and nonprefer- entially etched regions of a crystal.

The light-figure technique of examining crystals has many useful applications. By using light beams of sufficiently small diameters, polycrystalline samples can be scanned and the orientation of various grains of the sample determined. For samples con-

Fig. 4. Negative crystal etch form of CdS (cavity etching). Com- pare with form in Fig. S. Etched by flowing concentrated HNO3 into crystal cavity via two horizontal (inlet and outlet) holes with masked walls; surface protected with nonreactive Apiezon wax.

,...-(00.1)

(O0.l).J"

Fig. 3. Solution form photomicrograph of CdS. (Etched 1 min in concentrated HNO~). Compare with form in Fig. S. (a) Cadmium side. Point of interaction of six ridges represents (00"1). Ridges are zones of slowest attack during dissolution. Each point of the ridge represents a vicinal plane tilted in one direction for all other planes of that zone. (b) Side view. Intersection of horizontal and vertical ridges represent (10"1-). (c) Sulfur side.

/ --~ i0o-~l \ ! \

Fig. 5. Schematic drawing of the solution and etch pit form of CdS and their associated stereographic projection. The partial light-figure pattern of the solution form is given in Fig. ld; that of the etch-pit form is given in Fig. la. A complete light-figure of the etch-pit form exactly corresponds to the stereographic projection.



24 JOURNAL OF THE ELECTROCHEMICAL SOCIETY January 1964

,"-f ~ , "~ I/_11', /A l\

oo

( ] - ~ } h * g t

e . Foa u ! .,.{00. I}

s ~h*t ~ } {I0"0} " ~ - } - - - - lO -O} 2h-t" ( o o . T } ~ "

Fig. 7(o). Stereographic and parallel perspective projections of crystal forms of wurtzite structure for various ratios 7 of cotion to onion contribution to the surface (or interface) free energy. These structures are calculated by the methods given in ref. (7) for first neorest neighbor interactions only. fl is assumed to be unity [see Fig. 7(b) ].

respect ive ideal g rowth or e q u i l i b r i u m fo rm can be deduced. This type of a cor re la t ion is d e m o n s t r a t e d in Fig. 3-6 us ing CdS as an example .

The etch pi t and so lu t ion fo rm of CdS etched in HC1 and HNO~ can be ma tched wi th the e q u i l i b r i u m fo rm whe re :~ --~ 1/3 in Fig. 7a. Vapor and v a c u u m etch p a t t e r n s of CdS revea l (see Fig. 7), however , pa t t e rn s t ha t more n e a r l y ma t ch the condi t ions 1 > ~, > 1/3 in Fig. 7a or 3 > fl > 1 in Fig. 7b. This is also t rue for the observed vapor phase g rowth fo rm (see Fig. 8). The differences in the observed forms a re exp la ined on the basis of b o n d i n g changes tha t occur at the surface in the var ious e tchants . These changes at the surface m a y invo lve surface defor- m a t i o n by a tomic d i sp lacement , fo re ign a tom ad- sorpt ion, or the fo rma t ion of bond chains f rom bond a r rays by the s t r e n g t h e n i n g of bonds w h e n elec- t ronic charge separa t ion w i t h i n these a r rays or ionic cha rac te r is i n t roduced (7) . The ev idence ava i lab le is no t sufficient to d i s t ingu i sh comple te ly b e t w e e n the var ious possibil i t ies. Vapor g rowth forms of crystals , however , are closely re la ted to ideal e q u i l i b r i u m forms; differences in the two be ing associated wi th surface de fo rma t ion or the effects of second or more d i s tan t neighbors�9 Vac- u u m solu t ion or t h e r m a l etch pi t forms yield, in a first approx imat ion , the same resu l t s in regard to the exis tence of s t rong b o n d i n g a r r ays ( " b o n d

g~o.1) S~r ?zojecza~la i~,~;*t,e f ~ l * ~ ~ r g ~ v e rZch

tkl { ;O 'T

~1o o] { ) ]

(oo.i] I ~ o . ; 1 ~

oo.,,

;o.T)

L[IO,O) (OO.T) ~

: , :o~#< l

00.:} Ioo.T) (oo.I)

Fig. 7(b) Stereographic and parallel perspective projections of

Fig. 6. Schematic drawing of the etch-pit form intersecting various surfaces of an arbitrary chosen CdS polyhedron of wurtzite structure. The ideal positive growth form and its supplement, the negative crystal cavity etch (pit) form, and the stereographic projection are also given. The pits shown intersecting the various surfaces are ideal. Only those concave pit-edges are stable which correspond to strong bond chains. Stable edges are denoted in the stereographic projection by lines between points of intersection; e.g., all edges of the pit intersecting (00"1) are stable, while all edges of the pit intersecting (00. i ) are unstable. Stable and unstable edges for pits intersecting other faces can, in a like manner, be determined with the use of the stereographic projection shown. Unstable edges will "wash away" leaving undefined pits if all edges are unstable, as on (00"1-) or poorly defined pits if some but not all the edges are stable, as on (10'1), (10"0), and (10. I-).

crystal forms of wurtzite structure for various ratios of c-directional to a-directional contribution to the surface (or interface) free

strength of "c-bond" energy, fl = , when "y ~ 1.

strength of "a- bond"

cha ins") in crystals. Differences b e t w e e n vapor g rowth and v a c u u m so lu t ion or t h e r m a l etch pit forms are exp la ined on the basis of a difference in the p re fe r red reac t ion pa ths of the two proc- esses. (The respec t ive reac t ion paths in e i ther case can be t r ea ted as quas i - equ i l i b r i a . This type of a t r e a t m e n t is pe rmiss ib le in c rys ta l sur face kinet ics . ) Differences b e t w e e n the a f o r e m e n t i o n e d forms and forms r e su l t ing f rom dissolu t ion p ro -



Vat. 111, No. 1 E T C H I N G I I - V I A N D I I I - V C O M P O U N D S 25

1) ( ~ 0 . - - ~ ' ~ ' / ( 2 / 0 1 l O * l ) ~ ' k - ' ~ "

900"C, 5hr . Zn c~dmiu.n uapor 9~0"C, 5 * . in . i . u a c u u m 900"C, 2 h ~ i n H2S

o.~}" ~ _2 "---~--~

9 0 0 ' C , 7 h~ . i n ~ u s u c [ p o a v a p o r g r o w t h luorm

Fig. 8. Light-figure patterns of vapor phase grown CdS crystals. The polarity was measured vs . piezoelectric (8) and by x-ray techniques (9).

HF HCl

+ -- +

' " ~ :.: I "o" ' (

I t zns

I t Cds

+ -- + - - �9

g ~ r HI

/ znO IX / I

4- 4" -

, . j j " , . - , -~j

Cd$

4- -- 4- - -

Fig. 9. Screw dislocations on (00q) revealed by a concentrated HNO3 etchant containing silver ions. Associated light figure pattern at lower right.

cesses occu r r i ng in f o r e ign med ia , e.g., in ac id so lu - t ions, r e s u l t f r o m the a b s o r p t i o n of f o r e ign a t o m s into su r f ace s i tes in such a w a y t h a t n e w s t rong b o n d i n g a r r a y s a r e c rea ted . The so lu t ion and e tch p i t f o rms of CdS e t ched in HC1 a n d HNO3 can be e x p l a i n e d b y the c r ea t i on of s t r ong b o n d i n g a r r a y s r e s u l t i n g f r o m the a d s o r p t i o n of, for e x a m p l e , w a t e r or h a l i d e an ions on c a d m i u m a t o m s of pos i t i ve charge . In th is case w e l l d e v e l o p e d e t ch p i t s of h e x a g o n a l s h a p e a p p e a r on the "ca t ion ic" (00.1) p lane . I f on the o the r hand , s i l ve r ca t ions a r e a p - p l i ed in t he e t ch ing process , e tch p i t s a r e p r o d u c e d

on the "an ion ic" (0007) p l a n e t h r o u g h the ca t ion a d s o r p t i o n on the n e g a t i v e su l fu r su r f ace a toms (Fig . 9) .

The g r o w t h a n d e tch hab i t s of o t h e r I I - V I c o m - p o u n d s y i e l d s i m i l a r resu l t s . A n u m b e r of o t h e r w u r t z i t e - t y p e I I - V I c o m p o u n d s e t ched in v a r i o u s h y d r o g e n h a l i d e ac ids a r e s h o w n in Fig . 10. The s i m i l a r i t y b e t w e e n the e tch p i t h a b i t of t he v a r i o u s m a t e r i a l s and t h a t of CdS is obvious . W i t h t he e x - cep t ion o f ZnO the ca t ion p l a n e r e m a i n s s t a b l e w h i l e t he an ion p l a n e d i s appea r s . This is in accord w i t h the d i f f e ren t e t ch ing b e h a v i o r of a n t i p o l e p l anes o b - s e r v e d b y m a n y i n v e s t i g a t o r s on I I - V I and I I I - V compounds . H e r e aga in , the v a p o r a n d v a c u u m e tch

+ -- + -

CdSz %

" { 1 0 . 3 )

Fig. 10. Light-figure patterns of wurtzite type crystals etched in hydrogen halide acids. (00-1) is specular, (00.1) is matte. Polarity was measured by piezoelectric techniques. ZnO was Li-doped schematic.

fo rms and t h e v a p o r g r o w t h forms , w h i l e d i f fe ren t f rom each o t h e r (Fig . 11) a r e also d i f fe ren t f r o m the ac id e tch forms. In add i t ion , t he v a c u u m e tch fo rms of CdSe changes a t d i f f e r en t t e m p e r a t u r e s , i n d i c a t i n g a change in con t ro l l i ng p rocess or r e a c - t ion p a t h s w i t h t e m p e r a t u r e s . This t y p e of change can be e x p l a i n e d b y the p r e s e n c e of s e l e n i u m chains , the l e n g t h of w h i c h v a r y w i t h t e m p e r a t u r e , a d s o r b e d on the c r y s t a l sur face . In t he two cases d i f fe ren t spec ies of s e l e n i u m molecu l e s and t h e i r r e l a t i v e c o n c e n t r a t i o n r a d i c a l l y changes t h e r e a c - t ion process .

Etching of II-V1 and III-V Compounds, Sphalerite Structure

The same t y p e of an a n a l y s i s can be c a r r i e d out for s p h a l e r i t e t y p e s t r u c t u r e s of t he g roups I I - V I a n d I I I - V compounds . The t h e o r e t i c a l e q u i l i b r i u m fo rms a re g iven in Fig . 12. The p i t shapes to be e x - p e c t e d on v a r i o u s su r faces of a s p h a l e r i t e t y p e c r y s - t a l a r e g iven in Fig. 13 for t he cases ~/ ~ 3 and 3 > ~, > 1. Some of t he e tch fo rms for s p h a l e r i t e t y p e I I - V I c o m p o u n d s a r e g i v e n in Fig . 14. H e r e aga in , h o w e v e r , t he e x p e r i m e n t a l d a t a a r e no t suffi-



26 JOURNAL OF THE ELECTROCHEMICAL SOCIETY January 1964

uaj,o~ 9~OWZ;* io,~,~,

. . . . �9 . . . . . . . . . . . . . . :, ,~0.~2'7/22':'.% ?;: .....

Fig. 11. Light-figure patterns of vapor and vacuum etched and growth form crystals of some wurtzite type crystals. Polarity was measured by matching the observed pattern with hydrogen halide etch patterns on the same crystal.

( I I I I ~ 1 1 u I } [ h , l : 1 , 4 ~ { I I l )

\, /

Fig. 12. Stereographic and parallel perspective projections of sphalerite type materials for various ratios ~, of cation to anion contribution to the surface (or interracial) free energy. These structures are calculated by the method given in ref. (7) for first nearest neighbor interactions only.

c ien t for a c o m p l e t e ana lys i s to be p e r f o r m e d . Mos t of the g e n e r a l conclus ions m a d e for w u r t z i t e - t y p e s t r u c t u r e s a r e d i r e c t l y app l i cab le , h o w e v e r .

Acknowledgment T h a n k s a r e due to Messrs . W. E. McCa l lum, Jr . ,

and F. D. G a g e for t h e i r a s s i s t ance in th is w o r k ; to Mr. R. D. B r a d f o r d for his efforts in d e v e l o p m e n t of t he l i g h t - f i g u r e mic roscope . The a u t h o r s also w o u l d l ike to t h a n k Dr. D. C. R e y n o l d s of t he A e r o n a u t i c a l R e s e a r c h L a b o r a t o r y , W r i g h t A i r D e v e l o p m e n t Cen - te r , U S A F , Ohio, for his p e r s o n a l i n t e r e s t in th is work .

This p a p e r was s u p p o r t e d in p a r t b y the A i r F o r c e R e s e a r c h Divis ion , A i r R e s e a r c h and D e v e l o p m e n t C o m m a n d , U n i t e d S t a t e s A i r Force , u n d e r C o n t r a c t AF33 (657) -7916.

P ~ c j e ~ r i ~ , r~,, , P,~x rea~

Fig. 13(a). Schematic drawing of the etch-pit form intersecting various surfaces of an arbitrarily shaped sphalerite type crystal. y ~ 3. Etch pits on (1 i i ) are stable.

/! \\

S g c e e o ~ r h ~ c t'o*gr e c~,r t ~ t~e4azgve Er Pee]ec~,e~ re,^r~ }'i_~ learn'

Fig. 13(b). 3 ~ ~, ~ 1. The explanation as given in the caption of Fig. 6. holds here. Etch pits on both tetrahedral planes are stable.

Manuscr ip t rece ived May 1, 1963. This paper was presented at the Boston Meeting, Sept. 16-20, 1962.

A n y discussion of this pape r wi l l appea r in a Discus- sion Section to be publ i shed in the December 1964 J O U R N A L .

REFERENCES 1. H. C. Gatos, in "The Surface Chemis t ry of Metals

and Semiconductors ," p. 381, H. C. Gatos, Editor, John Wi ley & Sons, New York (1960) ; J. W. Faust , Jr., in "Methods of Expe r imen ta l Physics," Vol. 6, p. 147, "Solid Sta te Physics," K. La rk -Horov i t z and V. A. Johnson, Editors, Academic Press, New York (1959).

2. G. A. Wolff and J. D. Broder , Acta Cryst., 10, 848 (1957) ; 12, 313 (1959) ; G. A. Wolff, in "Compound Semiconductors ," Vol. 1, p. 34, R. K. Wi l la rdson and H. L. Goering, Editors, Reinhold Publ i sh ing Corp., New York (1962).

3. W. B. Pearson and G. A. Wolff, Discussions, Faraday Sac., 28, 143 (1959).

4. G. A. Wolff, J. M. Wilbur , Jr. , and J. C. Clark, Z. Elektrochem., 61, 101 (1957).



Vol. 111, No. 1 ETCHING II-u AND III-V COMPOUNDS 27

f lF / 4 ( ' s H B a ~ I

{11~) . ~ > - . . / - - / - - . , , f / - ' - / , iL~ z i

\ \ l , .

l a p /~o

~ _ _

O e P No

" . -L J " - 2 . : '--J-J

ZaTe. ZnSe: CdTr

llO0"C,I-I/~ha I~O0"C~Sm~.n. 7lO'C, ~]2h~t

Fig. 14. Light-figure patterns of sphalerite type crystals etched in hydrogen halide acids. The last three stereograms denote the thermal etch patterns of ZnTe, ZnSe, and CdTe, respectively. Polarity was measured by piezoelectric techniques; this is indicated by -I- and ~ signs the same as in Fig. 8 and 10. The sign denotes the electric charge on the respective tetrahedral plane which appears on expansion of the crystal in the [111]-direction. (111) is specular; (111) is matte.

5. S. B. Austerman, D. A. Berlineourt , and H. H. A. Krueger, J. Appl. Phys., 34, 339 (1963); J. W. Faust, Jr., and A. Sagar, ibid., 32, 331 (1960);

H. C. Gatos and M. C. Lavine, This Journal, 107, 427 (1960); J. AppL Phys., 31, 743 (1960); M. Inoue, I. Teramoto, and S. Takayanagi , ibid., 33, 2578 (1962); M. C. Lavine, H. C. Gatos, and M. C. Finn, This Journal, 108, 974 (1961); A. N. Mariano and R. E. Hanneman, J. Appl. Phys., 34, 384 (1963) ; J. C. Monier and R. Kern, Compt. rend., 241, 69 (1955); Bull. sac. franc. Mineral. Crist., 79, 495 (1956); E. P. Warekois, M. C. Lavine, A. N. Mari- ano, and H. C. Gatos, J. AppL Phys., 33, 690 (1962) ; E. P. Warekois and P. H. Metzger, ibid., 30, 960 (1959); J. G. White and W. C. Roth, ibid., 30, 946 (1959) ; G. A. Wolff, U. S. Army Signal Res. & Dev. Lab., Exploratory Res. Div. E, Publ icat ion No. 5910 (1959): This Journal, 106, 207C (1959); G. A. Wolff and J. R. Hietanen, in Contract AF 33 (616)- 7528 Reports, Thi rd Quar ter ly Progress Report, pp. 51-59, (1961), Fif th Quar ter ly Progress Report, pp.68-80 (1961), ASD-TDR-62-69, Vol. I, pp. 40-53 (1962), ASD-TDR-62-69, Vol. II, pp. 48-61 (1962),

in Contract AF33 (657)-9975 Reports, Second Quar ter ly Report pp. 32-37 (1963), in Contract AF33 (657)-7916 Reports, Third Quar ter ly Report pp. 4-15 (1963); Proc. In terna t ional Symposium on Condensation and Evaporat ion of Solids, Day- ton, Ohio, Sept. 1962; R. Zare, W. R. Cook, and L. R. Shiozawa, Nature, 189, 217 (1961).

6. J. G. Gualtieri , M. J. Katz, and G. A. Wolff, Z. Krist., 114, 9 (1960).

7. G. A. Wolff and J. G. Gualtieri , Amer. Mineral., 47, 562 (1962); G. A. Wolff, Z. Phys. Chem., 31, 1 (1962).

8. D. C. Reynolds and S. J. Czyzak, J. Appl. Phys., 31, 94 (1960).

9. R. F. Belt, ASD-TDR-62-69, Vol. II, pp. 58-61, (1962).

Charge Storage Effects in Tantalum Oxide Films Rudolf Dreiner

Research Laboratory, Sp~ague E~ectric Company, North Adams, Massachusetts

ABSTRACT

The phenomenon of the residual discharge cur ren t following an inverse t ime law, i r es cc b/t , was confirmed for t an ta lum electrolytic capacitors. It was shown that the coefficient b was independent of charging fields up to 2/3 of the format ion field and that the residual current increased with temperature. The activation energy was 2.5 kcal/mole. Fur thermore, it was shown that, if the external resistor was removed and the cell was left with open terminals, the in te rna l residual cur ren t cont inued to flow unt i l the bu i ld -up field modified the flow. If the oxide film were i l luminated dur ing discharging, two different photoeffects were recorded: the t rans ient effects due to wavelengths of 320 m~ to about 600 m~ and the s ta t ionary photocurrent mostly due to wavelengths shorter than 300 m~.

Several models are discussed which lead to an inverse t ime law. Preference was given to the idea that a net positive space charge arises wi th in the oxide dur ing charging; that this space charge causes two in te rna l fields of opposite directions and that the residual discharge current is the difference of two cur- rents flowing in the opposing fields, thus neutra l iz ing the space charge. I r ra - diation dur ing charging and discharging affected the positive t ransient effect and the space charge field.

The p h e n o m e n a of r e s idua l effects, specifically the res idua l d ischarge c u r r e n t and the re s idua l vol tage, are c o m m o n to m a n y types of capacitors. The r e - s idual d ischarge c u r r e n t is the c u r r e n t in excess of the ideal RC discharge c u r r e n t which is re leased f rom a charged capaci tor u n d e r g o i n g a d ischarge t h rough an e x t e r n a l resistor. The res idua l or open

c i rcui t vol tage is the po t en t i a l which appears across the t e r m i n a l s of a capaci tor if the capaci tor has b e e n charged, d ischarged, a n d t h e n lef t w i t h open t e r - mina ls . G ue n t he r s c hu l z e and Betz (1) have descr ibed these effects and more r ecen t ly Lehovec and R e i n h e i m m e r (2) r epor t ed more ex tens ive da ta on the res idua l d ischarge c u r r e n t e m e r g i n g f rom an



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