Transmission Electron Microscopy Study of Defect Structure in Epitaxial Sno2 Rutile Thin Film

Journal of the Ceramic Society of Japan 110 [2] 86-91 (2002) Paper

Transmission Electron Microscopy Study of Defect Structure in Epitaxial SnO2 Rutile Thin Film

Toshimasa SUZUKI, Hirotaka WAKABAYASHI, Yuji NISHI and Masayuki FUJIMOTO

Taiyo Yuden Co., Ltd., 5607-2, Nakamuroda, Hayuna-machi, Cunma 370-3347

エピタキシャルSnO2ルチル薄膜中の欠陥構造の透過型電子顕微鏡研究

鈴木利昌・若林博孝・西湯二・藤本正之太陽誘電(株), 370-3347群馬県群馬郡榛名町中室田 5607-2

SnO2 thin film heteroepitaxially grown on a rutile (100) TiO2 single crystal substrate exhibits high-density interfacial misfit dislocations and related planar defects. The misfit dislocation network formed on the het- erointerface consists mainly of two types of partial edge dislocations with Burgers vectors of 1/2[101] and 1/2[110], to fully relieve the lattice mismatch, and the dislocations inevitably involve inclined (101) and ver- tical (010) planar defects extending toward the film surface, respectively. Some of the (101) planar defects form wedge-shaped defects, and frequently accompany a single or multiply stacked (101) nanotwin lamellae. Another (011) type of planar defect originating at fused threading dislocations with 1/2[011] Burgers vectors is also detected by plan-view observation, showing the same crystallographic feature as (101) planar defects observed in the cross-sectional (010) projection. The dominant relaxation mechanism through par tial edge dislocations suggests the low interfacial energy of the defect plane in the rutile structure, hinder ing the control of high crystallinity in rutile oxide thin films. [Received August 7, 2001; Accepted December 5, 2001]

Key-words: SnO2, Defect, Heteroepitaxy,Rutile, Crystallographic shear structure, Misfit dislocation

1. Introduction vectors.9),10) It is of interest that other types of non- Recently advancing thin-film planar technology has stoichiometry-related CSP defects commonly reported in driven an increasing interest in new classes of smaller, high- reduced bulk ceramics or powders, (121), (132), and performance integrated microdevices including oxide (142) ,13)-16) were not observed in low-temperature- materials formed with various elements of the periodic ta processed rutile oxide films, even with oxygen-deficient ble. The oxide materials exhibit various physical properties, compositions, probably due to limited vacancy migration. such as f erroelectric, high-dielectric, piezoelectric, optical, The epitaxial single-crystalline forms, on the other hand, are and superconductive properties, and thus the extensive in- less desirable for gas sensing applications, because of their tegration of such materials based on planar technology will boundary-less structure, but there exist some more attrac provide highly functional devices, where a significant selec tive studies focusing on epitaxial rutile films with the expec- tion of constituent elements and crystallographic structures tation of superior performance based on simplified gas-solid are available. Many obstacles for these recent applications, interactions of single crystal film17) and in-plane valiant however, remain especially in the controllability of oxide structure utilizing the difference between the film and sub thin-film crystal growth.1) Next-generation heteroepitaxial strate symmetry.18) Such epitaxial techniques are naturally multiplayer applications such as artificially structured quan- required for advancing integrated electronic devices and op tum wells and superlattices, and multiplayer optical devices, tical wave guide systems,19),20) and thus the detailed under actually require high controllability of crystal growth and standing of lattice defects closely related to misfit relaxation lattice defects comparable to ordinal semiconductor is essential for acquiring the desired crystallinity. technology.2) The detailed investigations of heteroepitaxial In a previous work, we reported the epitaxial growth, growth and associated defect generation mechanisms are, defect structure and related surface morphology of rutile however, quite few in number compared to those for semi- SnO2 thin films, using transmission electron microscopy, conductor electrical and optical devices, and, in particular, reflection high-energy electron diffraction, and atomic force the basic crystallographic understanding of lattice defects is microscopy.21) The films generated high-density interfacial extremely insufficient. misfit-induced dislocations and related planar defects, SnO2 rutile oxide thin films are typical ionic metal oxides through misfit relaxation. The evolution of the surface mor- exhibiting unique electronic and optical properties and are phology and cross-sectional structure for thinner films sug- attractive for applications to transparent conductive ele- gested that the introduction of such defects occurs at the ctrodes, polarizing filters, gas sensors, and optical early growth stage, due to a large lattice misfit, and is catalyses.3)-7) Poly-nanocrystalline thin films are of sig strongly affected by point defect condensation. Here, we nificant interest especially for application to integrated pla report on a further detailed lattice defect structure compli- nar array gas sensors, and several detailed structural catedly formed in the heteroepitaxially grown SnO2 thin research works have been performed on crystallite size dis films, mainly focusing on planar defects, since the precise tribution and lattice defects governing the sensor analysis and control of the characteristics of planar defects response.8)-10) The nanocrystalline films reportedly includ are essential for realizing future integrated high-perfor- ed undissociated dislocations, {101} crystallographic shear mance devices. plane (CSP) defects9)'10) and {101} twins,11),12)and the CSP defects acted as sinks for nonstoichiometric composition of 2. Experimental procedures oxygen vacancies, depending on their displacement SnO2 thin films were fabricated on (100) TiO2 rutile sin-

86 Toshimasa SUZUKI et al. Journal of the Ceramic Society of Japan 110 [2] 2002 87

gle-crystal substrates by ablating pressed SnO2 powder dis ks, which are of high purity (99.99%), with an ArF excimer laser. The substrates were annealed in a furnace at 1000•Ž under oxygen atomosphere (1 atm) before deposition. Films were grown at a substrate temperature of 500•Ž and under a partial oxygen pressure of 13.3Pa. The deposition rates were adjusted to 0.3-0.5nm/min, which were relatively low growth rates. Crystallographic defect structures of the thin films were evaluated by transmission electron microscopy (Model 002B, Topcon, Tokyo) with a high-resolution charged-cou- pled device (CCD) camera (MegaScan, Gatan Inc., Pleasanton, CA). Thin film specimens for TEM observation were prepared by mechanical polishing and were then milled by low-incident-angle Ar ion milling (precision ion polishing system (PIPS), Gatan Inc., Pleasanton, CA).

3. Results and discussion Figures 1 and 2 show cross-sectional [010] and [001] high-resolution transmission electron microscopy (HR- TEM) images of a SnO2 thin film heteroepitaxially grown on a (100) TiO2 single crystal substrate and the insets show the corresponding selected area electron diffraction pat- Fig. 1. (010)-projected cross-sectional HR-TEM image of SnO2 terns. The selected-area electron diffraction (SAD) pat thin film heteroepitaxially grown on a (100) TiO2 substrate. The in terns, each composed of only two patterns of the rutile set shows a corresponding selected-area electron diffraction pat- structure, SnO2 film and TiO2 substrate, exhibit a well-de- tern. fined feature of heteroepitaxial growth with a crystallographic relationship of [010] (100)SnO2//[010](100)TiO2, despite a largely lattice-mismatched system of SnO2 thin film and TiO2 substrate, where the lattice mismatches of lattice parameters a and c are approximately 3 and 8% at a growth temperature of 500•Ž, respectively (unconstrained lattice parameters of SnO2 and TiO2:aSnO2=bSnO2=0.475nm nO2=0.320nm, aTiO2=bTiO2=0.461nm,, CS and CTiO,=0.297nm at 500•Ž). In both diffractions, the inner diffraction spots corresponding to an epitaxially grown SnO2 film are notably diffused in comparison to the outer sharp diffraction spots of the TiO2 substrate, suggesting a significant deterioration in crystallinity probably due to misfit relaxation. In fact, the calculated out-of-plane lattice parameter of the a-axis was a =0 .4751nm, and in-plane lattice parameters of c- and b-ax- es of SnO2 film were b=0.4735nm and c=0.3154nm, indicating that the biaxial compressive misfit strain of the film is almost fully relaxed in both in-plane directions. In the (010) projection of cross-sectional HR-TEM, high-density interfacial dislocation cores seemingly related to such misfit relaxation and inclined (101) planar defects originating in the plane of the SnO2/TiO2 interface are clearly visible. Some of the planar defects extend through to the uppermost surface, while the others are terminated in the film, occasionally forming overlaid wedge-shaped defects. From the Fig. 2. (001)-projected cross-sectional HR-TEM image of SnO2 closure failure of their Burgers circuits, the Burgers vector thin film heteroepitaxially grown on a (100) TiO2 substrate. The in set shows a corresponding selected-area electron diffraction pat- components of the interfacial dislocations projected on the tern. micrograph of the (010) plane are classified into two types, 1/2[101] of partial type and [001] of complete type. In the (001) projection, the high-density misfit-induced interfacial dislocations are also detected, and perpendicular (010) planar defects extending from the dislocations pene- Figure 3 shows the presence of an array of parallel [010] trate through the film. The (001)-projected components of dislocation lines, corresponding to the dislocations observed their Burgers vectors are 1/2[110] of partial type. in the (010) projection (Fig. 1), although some contrasts The projected components of the Burgers vectors can be due to planar defects and threading dislocations in the film readily determined from HR-TEM images using the above- overlap. mentioned Burgers circuit method, but further characteriza- Figure 4 was obtained under a different two-beam condi tion of out-of-plane components is required for precisely tion of g=[020], where the array observed under the g= defining the defect nature, such as plan-view dark-field [002] condition disappears and a different array of parallel TEM observation using g.b=0 extinction criteria under [001] dislocation lines appears, corresponding to the dislo- two-beam conditions. Figures 3 and 4 show g=[002] and cations observed in the (001) projection (Fig. 2). This g• b [020] plan-view dark-field TEM images of SnO2/TiO2 inter- =0 extinction criteria clearly indicates the pure edge nature faces, where g is the diffraction vector. of these dislocations, and their Burgers vectors are uniquely 88 Transmission Electron Microscopy Study of Defect Structure in Epitaxial SnO2 Rutile Thin Film

Fig. 5. Schematic diagram showing crystallographic relationship between partial misfit dislocations and planar defects; (I) (101) planar defect parallel to the Burgers vector of the terminating partial misfit dislocation (parallel configuration), (II) (101) planar defect not parallel to the Burgers vector (nonparallel configuration), and (III) wedge-shaped planar defects terminated by two partial misfit dislocations.

Fig. 3. Dark-field plan-view TEM image of SnO2 thin film under two-beam condition of g=[002]. An array of parallel [010] disloca tion lines is denoted by A. [001], partial edge dislocations with Burgers vectors of b= 1/2[101], and inclined planar defects. The origin of the for mation of this planar defect would be mainly the introduc-- tion of partial misfit dislocations, although other origins should be considered in order to interpret the high density of planar defects which exceeds the density of partial dislocations. As schematically shown in Fig. 5, the presence of such partial dislocations causes lattice incoherency, in this case, yielding two possible types of inclined planar defects parallel to the Burgers vectors of partial dislocations terminating the planar defects, (101)1/2[101] (parallel configuration, model I), and nonparallel to the Burgers vectors, (101)1/2[101] (nonparallel configuration, model II). Both types of planar defects are observable in Fig. 1 and crystallographically correspond to a sort of so-called "crystallographic shear plane" (CSP), both exhibiting an identical interface structure whether the configuration is parallel or nonparallel. As shown in Fig. 1 and schematically in Fig. 5, a pair of planar defects on the two complementary planes of (101) and (101) disappears at the crossing point, forming wedge-shaped defects in both cases of parallel and nonparallel configurations (Model III in Fig. 5). At the apices Fig. 4. Dark-field plan-view TEM image of SnO2 thin film under of wedge-shaped defects, planar defects were coherently two-beam condition of g=[020]. An array of parallel [001] dislocation lines is denoted by B. ended, and no additional lattice defects resulting form the termination of planar defects were formed (Fig. 1). The total Burgers vectors of the wedge-shaped defects, hence, can be obtained by summing only the two bottom dislocations. identified as 1/2[101] and [001] for [010]-aligned disloca- In contrast to the above-mentioned partial dislocation tions, and 1/2[110] for [001]-aligned dislocations . models, some planar defects originated at b=[100] perfect The [001] Burgers vector of the edge type lies on the in- edge dislocations to form the similar V-shaped defects, and terface, to efficiently serve as a misfit-relaxing dislocation. other V-shaped planar defects were also generated at the co- The Burgers vectors of 1/2[101] and 1/2[110] of partial herent interface without forming any misfit dislocations edge dislocations were inclined with respect to the interface , (Fig. 1). The former case indicates that the perfect edge but the interface components of |1/2[100]| and |1/2 dislocations actually act as dissociated partial dislocations [010]| contribute to misfit relaxation. That is, all these in- with Burgers vectors b=1/2[101] and 1/2[101] (Model IV terfacial dislocations are the so-called "misfit dislocation" in Fig. 6). On the other hand, the coherently inserted and form an orthogonally crossing dislocation network con- V-shaped defects can be interpreted as the formation of zig- sisting of the edge dislocations along the [010] and [001] zag-shaped single planar defects folded at the interface. directions, to homogeneously relax misfit strain between the Figure 6 (V) shows a schematic picture of a folded planar SnO2 film and TiO2 substrate. The planar defects perpen- defect bound by two partial misfit dislocations. Such a dicular and inclined to the interface thus seem to be closely characteristic feature of the (101) planar defects can also be related to misfit relaxation through the partial misfit disloca- distinctly seen in another type of (011) planar defect tions, which inevitably accompany lattice incoherency as a described below. planar defect. Figure 7 shows another typical cross-sectional HR-TEM As shown in Fig. 1, dominant lattice defects observed in image of the (100) projection. Significant amounts of planar the (010) projection are classified into three types: misfit-in- defects are nucleated from the interface, and some defects duced perfect edge dislocations with Burgers vectors of b = exhibit a single and nonperiodically stacked band structures Toshimasa SUZUKI et al. Journal of the Ceramic Society of Japan 110 [2] 2002 89

Fig. 6. Schematic diagram showing crystallographic relationship between partial misfit dislocations and planar defects; (IV) V-shaped planar defect originating at b=[001] misfit dislocation and (V) folded wedge-shaped planar defect.

Fig. 7. (010)-projected cross-sectional HR-TEM image of SnO2 thin film including (101) nanotwin lamellae. The arrows indicate Fig. 8. (a) Close-up image of terminated twin lamella correspond the termination of coherently intergrown nanotwin lamellae. ing to region enclosed by white lines in Fig. 7 and filtered images using diffraction spots of (b) (101) and (c) (101) in a fast Fourier transformed spectrum.

of (110) twin lamellae, not a simple planar feature. Several of these planar defects were independently terminated in the matrix of the rutile structure, without involving any dislocation components. Figure 8 shows digitized images of the typical terminated twin, filtered using a diffraction spot of a fast Fourier transformed spectrum, indicating no presence of lattice defects. Figure 9 shows a possible atomic configuration model of these coherent planar defects, including only tin ions. In the twinning system of [101](101) in the rutile structure, the tin sublattice structure exhibits (101) mirror symmetry, whereas the oxygen sublattice forming octa- hedral coordination around tin ions shows no mirror symmetry.22) The (101) nanotwin with a width of double (101)-plane spacing, 2d101,can be coherently intergrown in the rutile structure, generating no lattice mismatch or dislocations, while the insertion of a (101) nanotwin one-atomic- plane wide yields the shear displacement, crystallographi- Fig. 9. Possible atomic arrangement model of terminated twin cally equivalent to (101) CSP. That is, the observed (101) lamellae two atomic layers wide. planar defects originating at the coherent interface are also attributable to coherently intergrown 2n-atomic-planes-wide nanotwin lamellae (n=1,2,3....), not only V-shaped planar defects folded at the interface. Furthermore, the inter- and the stacked band structure consisting of 2n and/or 2n+ growth of 2n+1-atomic-planes-wide nanotwin lamellae 1-wide nanotwin lamellae would be expected to contribute 90 Transmission Electron Microscopy Study of Defect Structure in Epitaxial SnO2 Rutile Thin Film

to misfit relaxation through partial dislocation bordering the nanotwin at the interface, since 2n+1-wide nanotwin lamel lae and the stacked defects can equivalently serve as (101) CSPs with displacement vectors of R=1/2[101]. In fact, the dispersed thick contrast and band-like feature of the planar defects, not well-defined edge-on single planar contrast , strongly supports the possibility of formation of such nanotwin models. Consequently, planar defects observed in the (010) projection would consist of single (101) planar defects of CSPs, coherently intergrown (101) twin lamellae without partial dislocations, intergrown (110) twin lamellae accompanying lattice displacement of R=1/2[101], and their combination. All of the perpendicular planar defects observed in the (001) projection, on the other hand, can be completely interpreted as misfit-related defects terminated by interfacial partial misfit dislocations. In order to confirm the three-dimensional defect structure of SnO2 thin films, the plan-view HR-TEM observation of SnO2 thin film was carried out. Figure 10 shows the plan view TEM image of SnO2 thin film devoid of the interface region including the misfit dislocation array. The dominant Fig. 10. Plan-view HR-TEM image of SnO2 thin film. defects observed in the in-plane projection are zigzag- shaped edge-on planar defects densely formed in the SnO2 film. These defects mostlylie on (011) and (011) planes , exhibiting the same crystallographic feature as wedge shaped defects observed in the cross-sectional (010) projec tion. With almost no termination, these planar defects are repeatedly folded and extend from threading dislocations of partial types of 1/2[011] and 1/2[011], which act as nucleation sites of zigzag-shaped (011) planar defects. The possible formation mechanism of threading dislocations of this partial type is considered to be the reaction of threading components of partial misfit dislocations of b=1/2[110] and 1/2[101] as follows: 1/2[110]+1/2[101]=1/2[011]. This reaction gives a smaller magnitude of |b|2, and is ener- getically favorable, although the formation of accompanying planar defects increases the lattice energy. The observed threading dislocations were only the partial type of b=1/2 [011], and no other types of threading dislocations were observed. Figure 11 shows a schematic diagram of three geometri- cally classified types of planar defect configurations found in the present study, which are closely related to misfit relaxation through partial pure edge misfit dislocations and threading dislocations, with the exception of coherently intergrown nanotwin lamellae. The well-accepted misfit relaxation model for hetero epitaxial thin films on defect-free single crystal substrates is that of surface nucleation and the subsequent gliding of mis fit dislocation.23) In this system, the crystallographic plane of the (101) planar defect corresponds to the most active slip plane in the rutile structure,22) and thus the lattice relax ation mechanism through the b=1/2[101] partial misfit dis location of parallel configuration can readily be explained as being due to the propagation of the partial dislocations from the free film surface. Such a well-known lattice relaxation Fig. 11. Three-dimensional geometric configuration of observed mechanism based on half-loop propagation from a free sur planar defects, (a) inclined (101) CSPs, occasionally forming wed- face, however, cannot be applied as being responsible for ge-shaped arrangement, (b) (010) CSPs perpendicularly extending the introduction of b=1/2[110] partial misfit dislocations to the film surface, and (c) folded wedge-shaped (011) CSPs. and b=1/2[101] partial misfit dislocations of nonparallel configuration, since their defect planes do not correspond to the glide plane of the misfit dislocations. These dislocations are crystallographically of sessile type and possibly, these planar defects significantly affecting the physical properties types of partial dislocations could be formed by the accumu of the films. This relaxation mechanism presents a striking lation of vacancies at the earliest stage of crystal growth , as contrast to that of generally well-known oxide thin films previously reported by Wakabayashi et al.21) such as perovskite oxide thin films, where the perfect pure In this largely mismatched system of the rutile heteros- edge dislocation serves as dominant misfit-relaxing tructure, partial edge dislocations preferentially contribute components.1) This experimental finding suggests that the to the misfit relaxation, leading to the generation of plentiful formation energy of planar defects in the rutile structure is Toshimasa SUZUKI et al. Journal of the Ceramic Society of Japan 110 [2] 2002 91

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