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Thin porous AgBr films were prepared by physical vapor deposition under high vacuum conditions. By varying only one basic parameter of the deposition ...
J O U R N A L O F M AT E R I A L S S C I E N C E : M AT E R I A L S I N E L E C T RO N I C S 1 4 ( 2 0 0 3 ) 7 7 3 ± 7 7 4

Microstructure determined anisotropy in the properties of vacuum-deposited AgBr thin ®lms R. GEORGIEVA, D. KARASHANOVA, N. STARBOV Central Laboratory of Photoprocesses ``Acad.J.Malinowski'', Bulgarian Academy of Sciences, Acad. Georgy Bonchev str, bl. 109, 1113 So®a, Bulgaria E-mail: [email protected] Thin porous AgBr ®lms were prepared by physical vapor deposition under high vacuum conditions. By varying only one basic parameter of the deposition technique ± the vapor incidence angle, ®lms with different microstructures could be obtained. The presence of microstructural inhomogeneity and anisotropy in the microhardness and dark ionic conductivity of the ®lms was revealed. # 2003 Kluwer Academic Publishers

1. Introduction

Our previous investigations demonstrate that the microstructure of vacuum-deposited thin AgBr ®lms is strongly in¯uenced by the vapor incidence angle [1]. It is shown that at high incidence angles the free volume of the deposited ®lms increases, thus leading to a decrease in ®lm microhardness and dark ionic conductivity. In this paper, we report the existence of anisotropy in the structure-related properties of AgBr thin ®lms.

2. Experimental details

The sample preparation was performed by the physical vapor deposition method in a standard vacuum unit, at a background pressure of about 5610 4 Pa. The precleaned glass substrates were positioned so that the angle between the substrate normal and the vapor beam a, was 0 , 15 , 30 , 40 , 60 or 80 . The surface morphologies of the AgBr ®lms were followed using a Phillips SEM 515 scanning electron microscope. The mechanical and electrical properties of all ®lms were measured in two mutually perpendicular directions. One coincided with the projection of the vapor-beam direction on the sample surface, and the other was perpendicular to that direction, with both measurement directions chosen with respect to the ®lm surface plane only. The plane determined by the vapor-beam direction and the substrate normal will be called the ``vapor incidence plane''. In the text and graphs, we use the de®nitions ``parallel'' and ``perpendicular'' to that plane, marking corresponding indices with ``jj'' and `` ? ''. The microhardness of the AgBr ®lms was evaluated by the Knoop prism indentation method [2], each value being averaged from 10 indentation measurements. As mentioned above, the indentations are produced parallel (Mhjj ) and perpendicular (Mh? ) to the vapor incidence plane. The electrical transport in the ®lms is studied by measuring the d.c.-conductivities parallel and perpendicular to the vapor incidence plane (sjj and s? ), using a 0957±4522

# 2003 Kluwer Academic Publishers

four-arm Wheatstone bridge. More experimental details are available in a previous paper [1].

3. Results and discussion

As shown by Eneva and Malinowski [3], AgBr ®lms grown on the same substrates are polycrystalline, with a preferred orientation along the [1 1 1] zone axis. In addition, these ®lms are grown in the form of individual columns, most of them running through the entire ®lm thickness, thus leading to a granular top surface [4]. The columns are separated by low-density material and at higher a values pores are formed that increases the free volume of the ®lm [1]. As demonstrated by scanning electron micrographs of the top surface of 1000 nm thick AgBr ®lms deposited at angles a ˆ 0 and a ˆ 80 (Fig. 1), the oblique deposition leads to a larger mean grain size. Also, a column-shaped anisotropy is observed when a increases. Nearly isotropic at normal vapor incidence (Fig. 1(a)), the grains at a ˆ 80 (Fig. 1(b)) are strongly elongated and closely packed in the direction perpendicular to the vapor incidence plane. Such a column-shape anisotropy was predicted earlier for obliquely deposited thin ®lms [5]. Fig. 2 presents the mean Mhjj and Mh? values of 1000 nm thick AgBr ®lms, as a function of a. It is evident that the microhardness decreases considerably when the vapor incidence angle increases from 0 to 80 . These results are similar to those obtained earlier for thin ZrO2 [6] and GeS2 [7] ®lms, and are due to the increase in the free volume of the ®lm upon going from low to high vapor incidence angles. It is also clearly seen that for all incidence angles Mhjj is greater than Mh? . This in-plane anisotropy is directly related to the different packing density in both directions and to the operating principles of the Knoop indentation method. In contrast to measurements in the direction normal to the vapor incidence plane, the long edge of the Knoop prism should deform and push away more densely packed material if 773

Figure 3 Dark ionic conductivity per unit area sjj and s? of AgBr ®lms versus vapor incidence angle a at t ˆ 30  C.

Figure 1 Scanning electron micrographs of the top surfaces of 1000 nm thick AgBr ®lms deposited at angles: (a) a ˆ 0 , tilt angle 0 and (b) a ˆ 80 , tilt angle 40 . The arrow indicates the vapor beam direction.

between the separate columns increase. In addition, it is seen that the values of sjj are always lower than those of s? . This difference is due also to the microstructure anisotropy and the asymmetrical column cross-section at large incidence angles. Thus, one could suppose that the overall electrical transport should be a combination of ionic motion within AgBr columns and ion transfer along and through the contacts between the individual columns, thus forming a speci®c electrical network. Hence, the current carriers should pass through a longer pathway between the electrodes in a direction parallel to the vapor incidence plane than in a direction normal to that plane. As a result, sjj is lower than s? .

4. Conclusions

Figure 2 Microhardness values Mhjj and Mh? for AgBr ®lms versus vapor incidence angle a.

the indentation is oriented in a parallel direction. Therefore, at a 4 0 , the Mhjj values are higher than those for Mh? . It has been shown that the dark electrical conductivity in the bulk as well as in the subsurface region of AgBr thin ®lms is purely ionic, with interstitial silver ions as the main charge carriers [8]. For the samples studied in the present paper, a typical room-temperature dark electrical conductivity of polycrystalline silver bromide ®lms deposited at a ˆ 0 is s ˆ 1:4610 4 O 1 cm 1 . Fig. 3 presents sjj and s? , as a function of a, using for better comparison and simplicity the conductivity values per unit area. It is seen that when a increases, the electrical conductivity decreases. This means that the surface conductivity of the polycrystalline AgBr is much lower than the bulk conductivity, since at higher a the amount of low-density material and the free volume 774

The present results demonstrate that by applying oblique physical vapor deposition, it is possible to obtain AgBr thin ®lms with a columnar structure, which can intentionally be changed drastically. On increasing the vapor incidence angle a, the column's cross-sections are shown to elongate in the direction normal to the vapor incidence plane. It is demonstrated that the mechanical and electrical properties have measurable in-plane anisotropy, which is sensitive to the vapor incidence angle.

References 1. 2. 3. 4. 5. 6. 7. 8.

R . G E O R G I E VA , D . K A R A S H A N OVA and N . S TA R B OV, Vacuum 69 (2003) 327. F. K N O O P, C . G . P E T E R and B . E M E R S O N , Natl. Bur. Stand. 23 (1939) 39. J . E N E VA and J . M A L I N O W S K I , J. Phot. Sci. 22 (6) (1974) 273. J . A S S A and K . S TA R B OVA , J. Imag. Technol. 11 (1985) 180. A . G . D I R K S and H . J . L E A M Y, Thin Solid Films 47 (1977) 219. M. L E V I C H KOVA , V. M A N KO V, N. S TA R B OV, D. K A R A S H A N OVA , B . M E D N I K A R OV and K . S TA R B OVA , Surf. Coat. Technol. 141 (2001) 70. K . S TA R B OVA , V. M A N KOV, J . D I KOVA and N . S TA R B OV, Vacuum 53 (1999) 441. N . S TA R B O V, A . B U R O F F and J . M A L I N O W S K I , Phys. Status Solidi A 38 (1976) 161.

Received 1 September 2002 and accepted 12 January 2003