FIELD ION MICROSCOPY OF THE II-VI-SEMICONDUCTORS ZnO AND CdSe

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FIELD ION MICROSCOPY OF THE II-VI-SEMICONDUCTORS ZnO AND CdSe

D. Hoffmann, W. Drachsel, J. Block, A. Stepanova

To cite this version:

D. Hoffmann, W. Drachsel, J. Block, A. Stepanova. FIELD ION MICROSCOPY OF THE II-VI-

SEMICONDUCTORS ZnO AND CdSe. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-59-C7-

64. �10.1051/jphyscol:1986711�. �jpa-00225901�

FIELD ION MICROSCOPY OF THE II-VI-SEMICONDUCTORS ZnO AND CdSe

D. HOFFMANN, W. DRACHSEL, J.H. BLOCK a n d A. STEPANOVA*

F r i t z - H a b e r - I n s t i t u t d e r Max-Planck-Gesellschaft, F a r a d a y w e g 4-6, 0-1000 B e r l i n 33, F.R.G.

Abstract

-

^2110- and CdSe-field i o n imaqes with ordered s t r u c t u r e s a r e obtained f o r t h e two o r i e n t a t i o n s /OX1 / a d /CCOi/ usinq w o n a s imaqinq qas. The imaqes show t h e w l a r faces (0001 ) and (COOT) r e s t x c t i v e l y , and f o r t h e /0001/-oriented CdSe t h e . ~ r i s m faces. The obtained c o n t r a s t between t h e imaqes of the/0801/- and t h e / m i / - o r i e n t a t i o n is very stronq f o r CdSe.

Measurements of t h e f i e l d evaporation r a t e a s a function of t h e t i p voltaqe a r e i n d i c a t i n q t h a t t h e charqe t r a n s f e r model is relevant. The increase of e v a w r a t i o n r a t e due t o t h e r e a c t i v e hydroqen q a s , which is added t o t h e i n e r t imaqina qas araon, may be i n t e r p r e t e d as araunent f o r hydroqen i n t e r a c t i o n .

Imaqinq of semiconductor surfaces i n t h e f i e l d ion microscove was described f o r t h e elemental semiconductors S i by Miiller and Tsona /1/ and Ge _by Arthur /2/ andl Ernst /3/. The f i r s t f i e l d ion imaqes of ordered Si-surfaces a r e r e w r t e d by Melmed and S t e i n /4/ i n 1975. Soon t h e r e a f t e r Ge-field ion imwes were obtained by Ernst and Block / 5 / .

The application of FIM t o c o m u n d semiconductors has been c a r r i e d o u t f o r S i c by Kudo e t a l . /6/ and f o r GaAs arid Gap a s III-V-semiconductors by Ohno e t a l . /7/, Sakurai, Sakata e t a l . /8/ and o t h e r authors.

W e now present a FIM study of t h e II-VI-semiconductors ZnO and CdSe. F i e l d i o n images with ordered surfaces are obtained usinq A r as imaainq qas. The current-vol- taqe c h a r a c t e r i s t i c s a r e measured and f i e l d evaporation e w r i m e n t s r e w r t e d . 11. EXPERIMENTAL

A s s t a r t i n q material we used s i n g l e c r y s t a l s f o r ZnO /9/ and whiskers f o r CdSe /lo/. The specimen were adhered by s i l v e r p a s t e t o a Mo-wire, which was spotwelded t o a Mo-loop. The sharp emitters of ZnO and CdSe were prepared by dipping t h e speci- men i n t o a s o l u t i o n of H2S04: H20 (1:4) and i n t o 5 % Br i n methanol ( a t room tem- perature) r e s w x t i v e l y .

It is possible t o determine t h e o r i e n t a t i o n of t h e ZnO-crystals by usinq etchinq techniques / 9 / . The o r i e n t a t i o n of t h e CdSe-whiskers was found by comparison of t h e FEM images with those k n m from t h e l i t e r a t u r e /11/

.

F i e l d ion imaqing was done i n an usual f i e l d i o n ~ i c r o s c o p e a t t h e temperature of l i q u i d nitroqen usinq R r and Y2 as imaqinq ?as=.

*on leave from the Institut of Crystallography of Academy of Sciences of the USSR. Moscow, USSR

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986711

JOURNAL DE PHYSIQUE

The 11-VI-semiconductors ZnO and CdSe crystallize in the wurtzite-structure, which is related to the hexagonale structure as illustrated in Fig. 1.

Fig. 1

-

Hexagonal wurtzite-structure of ZnO and CdSe.

c-axis

In the wurtzite-structure the lattice sites are alternatively occupied by metal- and non-metal-atoms so the bonds have partially ionic character. For examle, for the (0001) plane the first layer is occupied by only metal atoms (Group I1 element) and the second layer by only non-metal-atoms (Grow VI element), for the (0007) polar face it is opposite.

The field ion images of the clean surfaces of ZnO and CdSe with ordered structures are shown in Figs. 2 (top) and 2 (bottom) respectively. The imaqes are obtained for both orientations for /0007/ (left) and for /0001/ (riqht)

.

Ar was used as imaging 9s.

All field ion images-shm the central ( 0001 )

-

,respectively ( 0007) -polar faces. In the case of the /0001/-oriented CdSe one obtains the prism faces in the ion imaqe with the expected six-fold syrranetry. For this imaqe a very high field strength was necessary, which finally damaged the tip.

The field ion images for the /0001/-orientation are different from those for the /000i/-orientation. An explanation for this contrast, which is very strong for CdSe, is the model for preferential field ionization at the non-metal atoms as described by Ohno et al. / 7 / . The (0001 )

-

and (OOOi)-~lanes are polarized ones but in the

( 10iO ) , (Olio )

. . .

planes there are an equal number of metal and non-metal atoms in the same layer. The current-voltaqe-characteristics for CdSe in Fiq. 3 shms quali- tatively the same behaviour as found for metals. That is, the log I vs. loq U plot consists essentially of two nearly straight lines, connected by a transition region /I/. Comparing the current-voltage-characteristics for Ar and H2 as imaging gases,

one obtains for H2 more than one order in magnitude lawer currents. One possible explanation for thls is the hydrogen adsorption, which may cover empty surface states and therefore decrease the ionization probability.

According to theoretical considerations, the evaporation rates k depend on the electric field strength F in the case of the image force b e 1 by

kr oc exp ( c - F ) (1)

in the case of the charge transfer model,

right: for the /0001/-orientation at 14.4 kV

JOURNAL DE PHYSIQUE

Fig. 3

-

Current-voltage-characteristics for 21-10 (left) and CdSe (right) with Ar(0)

and H (8 ) as imaging gases; pk = 5.4 x 1 inbar, pH2 = 6.1 r 1

o - ~

mbar: the arrows 2

mark the best image voltage (BIV) and the field evaporation voltage (FEV).

The field evaporation rates of the polar faces of ZnO and CdSe were measured after the method of Brandon /12/. A s shown in Fig. 4 the field evaporation measurements for the (0001 )- and (000i)-faces of CdSe show the behaviour of (2) indicatinq that the charge transfer model is relevant.

For 2110, usinq Ar as inert imaqinq qas and H as reactive qas, we found that (at constant argon partial pressure) the evaporazion rate increases with increasinq hy- droqen ~artial pressure (see Fiq. 5). This behaviour is similar to that of Si, which is discussed by Sakata and Block /13/.

Financial support by the Deutsche Forschunqsqemeinschaft (SEB 6/81) is qratefully ackncwledqed.

Fig. 5

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Field evaporation rates as a function of the hydrogen partial pressure.

0'

1o0- .

-

+

e

₀

m a-!

>j

a\

n ^a ?a-1

w - L .EL

1uZ

la3

ZnO (000710

%=

2 . 2 ~ 1 0 1

mbar

U,,p=13.0 kV

/

P

/@

. . _/ ^/' Y'

/

*/

./

ir6 1b-5

I

bb

^ib-3

P, / mbar

2

JOURNAL D E PHYSIQUE

REFERENCES

Miiller, E.W., arid Tsong, T.T., i n : F i e l d I o n Microscopy, American E l s e v i e r , New York, 1969, p.120.

Arthur, J., J. Phys. Chem. S o l i d s 25 (1964) 583.

E r n s t , L., Surf. S c i . 32 (1972) 3 8 r

Melmed, A. J. , and ~ t e i r R. J. , Surf. S c i

.

^{49 ( 1975 ) 645.}

E r n s t , L., and Block, J.H., Surf. S c i . 49 n 9 7 5 ) 293.

Kudo, J., Nakamura, S., and Kuroda, T. ,xp. J. Appl. Phys. 14 (1975) 52.

Ohno, Y., Nakamura, S., Adachi, T., and Kuroda, T., Surf. sZ. ^{69 (1977) 521.}

S a k u r a i , T., S a k a t a , T., and

Jim,

A., Jap. Appl. Phys. 22 (1 98J) L775.

Heiland, G., Kunstmann, P., and P f i s t e r , H., Z e i t s c h r i f t Physik

-

176 ( 1963) 485.

/lo/ Givargizov, E.I., in: C u r r e n t Topics i n M a t e r i a l Science, 1 (1978) 79.

/11/ F i s c h e r , R.

,

and Neumann, H. , i n : Feldemission a u s ~albleitzm, F o r t s c h r i t t e d e r Physik 14 (1966) 603.

/12/ rando on,^.^., P h i l . Mag. 14 (1966) 803.

/13/ S a k a t a , T., arid Block, J . H ~ Surf. S c i .

116

(1982) L183.