Abstract - Garnet-type Sm3Fe5O(12), SmIG, (001) films with thicknesses between 170 and 1450 A, were grown epitaxially on Gd3Ga5O(12), GGG, (001) substrates. Strained and relaxed films were obtained with a critical thickness of 602 A, as confirmed by x-ray diffraction (XRD). Moreover, dielectric measurements (Q-V curves) revealed that hysteresis loops were clearly observed on the films with thicknesses below 790A. Thickness dependency of remnant charge amount Qs and d(Qs/t)/dV(V=0) revealed that they remained the same below 602 A, indicating that the crystal structure of strained films were tetragonally-distorted. On the other hand, wurtite-type ZnO (0001) films were grown epitaxially on GGG (111) and (001) substrates. X-ray diffraction revealed that the out-of-plane and in-plane epitaxial relations of ZnO films on the GGG (111) substrates [ZnO/GGG (111)] were the [0001]ZnO || [001]GGG and [10-10]ZnO || [1-12]GGG±21°, while ZnO films on the GGG (001) substrates [ZnO/GGG (001)] showed the out-of-plane and in-plane epitaxial relations with the [0001]ZnO || [001]GGG and [10-10]ZnO || [100]GGG, [010]GGG.

I. Research backgrounds

Rare earth (Re) irons-based garnets (Re3Fe5O(12): ReIG) have received much attention for their magnetic and magneto-optical properties. Thus far, practical applications such as bubble memories and microwave devices have been reported on garnet materials1,2. Recently, it was found interesting magneto-electric (ME) effects on garnets. The ME effects can control magnetization and electric polarizations by applying external fields such as magnetic and electric fields. Moreover, it is known that garnets show a magneto-dielectric (MD) effect that can control dielectric constants in the host by applying magnetic fields 3. Crystal symmetry breaking based on tri-color superlattices demonstrated the ME effect in garnets 4.

A garnet structure belongs to the cubic centro-symmetric space group (Ia-3d). A unit cell is composed of 160 atoms. A chemical formula can be described as {Re3}[M2]O(M3)TO(12), where Re is a rare earth ion with a dodecahedral coordination. MO and MT are occupied by Fe ions with octahedral and tetragonal coordination, respectively. In particular, Re ions occupy 24c Wyckoff sites that are known as special crystallographic positions, while Fe ions occupy both 16a octahedral sites and 24d tetragonal sites. On the other hand, the oxygen ions occupy 96h sites. ReIG exhibits ferrimagnetic behaviors with a Curie temperature of 550 K at room temperature. Moreover, ME and MD effect is observed in the vicinity of 4 and 150K, respectively (3, 5). Therefore, it is suggested that ReIG is candidate for one of multiferroic materials, which is expected to be promising for memory devices such as field-effect transistors with four-states.

II. Introduction

This work is devoted to two experiments: One experiment is focused on electric polarizations of ReIG induced by breaking crystal symmetry (research-A). The other is epitaxial film growth of ReIG on non-garnet substrates (research-B).

For research A, it is indispensable for realizing crystal symmetry breaking of ReIG with a cubic-type in order to obtain spontaneous polarizations from ReIG. J. H. Haeni et al reported SrTiO3 films showed ferroelectricity at room temperature (RT) 6, which is ascribed to interface strains caused by lattice mismatches between films and substrates. For example, SrTiO3 films, which were observed ferroelectricity at RT, were grown using DyScO3 substrates with an orientation of (110) (a = 5.440 A, b = 5.713 A, and c = 7.887 A). In addition, ferroelectric SrTiO3 films could be realized using LSAT substrates (a = 3.869A) that are known as (LaAlO3)(0.29)×(SrAl(0.5)Ta(0.5)O3)(0.71). These reports indicate that the centro-symmetry of SrTiO3 breaks by the interface strains between the films and substrates. In this work, Sm3Fe5O(12) (SmIG) films are grown on Gd3Ga5O(12) (GGG) substrates (SmIG/GGG). It is expected that this heteroepitaxial film growth introduces interface strains between the films and substrates, and breaks crystal symmetry of the films. From a theoretical calculation, a critical thickness of SmIG/GGG system was estimated as 602 A 7.

Research B is aiming at fabricating ReIG epitaxial films on non-garnet substrates. However, it is difficult to achieve heteroepitaxial growth of garnets using non-garnet substrates at the present time. As a consequence, we cannot obtain ReIG films with better magnetic and optical properties. Therefore, this study is focused on epitaxial growth of oxide materials on garnet substrates as a first step. Thereafter, we develop to film growth of ReIG on garnet substrates with oxide buffers. Finally, epitaxial film growth of ReIG is carried out using non-garnet substrates. J. Haisma et al reported heteroepitaxy of GaAs and InP films on garnet substrates on the basis of higher-ordered epitaxy (8, 9). Epitaxial film growth of ZnO on garnet substrates also results from the higher-ordered epitaxy.

III. Experimental

The (111) and (001) planes of GGG substrate (Surface-Net GmbH; Germany) were mechano -chemically polished, and then thermally annealed at 1250 °C for 2.5 h for obtaining atomically flat surfaces. The height scales of both atomic force microscopy (AFM) images revealed that step and terrace structures at atomic scale were clearly observed on both GGG substrates. Step heights of GGG (111) and (001) substrates were 0.19 and 0.33 nm, respectively. AFM (Seiko SPI-3800) was used to observe the surface morphologies of the substrates and the films. Crystal quality and lattice parameters were investigated by X-ray diffraction (XRD: Philips X'-Pert) equipped with a double-crystal monochromator and an analyzer crystal. SmIG and ZnO films were grown on GGG substrates by a PLD apparatus with a background pressure of 1.4×10(-5) Pa. The beam out of an ArF excimer laser with a wavelength of 193 nm (Lambda Physik) was focused to produce an energy density of 4 J/cm2 on a SmIG (99.9%) and a ZnO target (99.999%) at a repetition rate of 3 Hz. The distance between the target and the substrate was fixed to 5 cm during film growth.

III-A. Physical properties of the SmIG films

The SmIG films were grown on the GGG (001) substrates at a temperature of 750 °C in an oxygen atmosphere of 1.0×10(-1) Pa. The dielectric properties of obtained films were measured by a ferroelectric characteristic evaluation (FCE) system.

III-B. Heteroepitaxy

The ZnO films were gown at a temperature of 650 °C in an oxygen atmosphere of 1.2×10(-3) Pa. Thicknesses of ZnO films on the GGG (111) and (001) substrates were in the range from 80 to 150 nm.

IV. Results and discussions

IV-A. Physical properties of the SmIG films

The SmIG films were successfully grown on GGG substrates with thicknesses from 390 to 1240 A. The strained SmIG films were obtained blow the critical thickness of 602 A, as confirmed by XRD. From AFM observations, step and terrace structures were observed on the SmIG films with thicknesses blow 602 A. Over the critical thickness, step and terrace structures disappeared owing to a lattice relaxation.

The thickness (t) dependence of dielectric properties on SmIG films was measured. Figures 1 (a) and (b) showed Q-V and Q/t-V characteristics, respectively. The remnant charge amounts (Qs and d(Qs/t)/dV (V=0) were proportional to a remnant polarization (Ps) and dielectric constant (ε), respectively. Ps and ε showed the same values in all films below thicknesses of 602 A, while the values of Ps and ε of the samples with thicknesses of 790 A and 620 A decreased. It was thought that the lattice relaxations resulted in breaking of crystal symmetry in the SmIG films.

VI-B. Heteroepitaxy

Figures 2(a) and 2(b) show the XRD patterns of ZnO/GGG (111) and ZnO/GGG (001), respectively. From 2θ/ω scans, we only found a diffraction peak of the (0002) plane of ZnO on both samples, revealing film growth of ZnO with the c-axis orientation.

Figure 3 (a) and (b) showed X-ray pole figure measurements for ZnO/GGG (111) and ZnO/GGG (001) as measured by the (10-11) plane of ZnO. The pole figures showed that ZnO films with the c-axis direction possessed two types of in-plane rotation domains, which were identified with the 12 diffraction peaks. In the case of ZnO/GGG (111), the 12 peaks in the pole figure derived from the (10-11) plane of ZnO were two types of in-plane orientations. The rotation angle between these domains was 42°. The two domains were asymmetrically rotated with angles of ±21° with respect to the [1-12](GGG) direction and the [10-10](ZnO) direction. The in-plane epitaxial relations of ZnO/GGG (111) were determined as [10-10](ZnO) || [1-12](GGG)±21°.While in the case of ZnO/GGG (001), 12 peaks derived from the ZnO (10-11) were also observed in the pole figure. The two types of in-plane orientations were measured with a rotation angle of 30° around the surface normal. The in-plane epitaxial relations of ZnO/GGG (001) were identified as [10-10](ZnO) || [100](GGG) and [10-10](ZnO) || [010](GGG).

V. Summary

V-A. Physical properties of SmIG films

We introduced the epitaxial lattice strains in the SmIG films. From the dielectric properties, Qs and d(Qs/t)/V(V=0) were dependent on film thickness. We found the supplementary dielectric properties in the SmIG films.

V-B. Heteroepitaxy

ZnO films were grown epitaxially on GGG (111) and (001) substrates. The epitaxial relation of out-of-plane and in-plane epitaxial relations of ZnO films on the GGG (111) substrates [ZnO/GGG (111)] were the [0001]ZnO || [001]GGG and [10-10]ZnO || [1-12]GGG±21°, while ZnO films on the GGG (001) substrates [ZnO/GGG (001)] showed the out-of-plane and in-plane epitaxial relations with the [0001]ZnO || [001]GGG and [10-10]ZnO || [100]GGG, [010]GGG.

Fig. 1. The dielectric properties of SmIG films. (a) Q-V characteristics. (b) Q/t-V characteristics

Fig. 2. 2θ/ω scans of (a) ZnO/GGG (111), (b) ZnO/GGG (001)

Fig. 3. X-ray pole figure measurements for (a) ZnO/GGG (111) and (b) ZnO/GGG (001).

本論文は、「Epitaxial growth and physical properties of Garnet-type Sm3Fe5O(12) films and Wurzite-type ZnO films (ガーネット型Sm3Fe5O(12)薄膜およびウルツ鉱型ZnO薄膜のエピタキシャル成長とその物性)」と題し、全6章からなる。希土類ガーネット型フェライトは、室温でフェリ磁性を示し、大きなVerdet定数を有することから光アイソレータなどとして利用されている。本論文では、希土類ガーネット型フェライトであるSm3Fe5O(12) (SmIG) 薄膜化することでバルクとは異なる特性発現を誘発し、当該材料が利用されているデバイスに新たな機能を付与し、新たなデバイスへの適応を提案している。そのための基礎技術として2つの研究項目に実施した。一つは、基板との格子ミスマッチを活用した高品質な歪SmIG薄膜を形成し、歪誘起の新規物性を発現させること、もう1つは、ガーネット型材料(SmIG)と非ガーネット材料とのエピタキシャル成長を実現し、薄膜デバイス化への道を開く事である。

第1章は「Introduction」あり、現在の電気産業のキーワードである多機能化が物質科学においても求められておりSmIG薄膜にエピタキシャル歪みを印加することによる新たな磁気・誘電特性の発現、スピンFET等の新規デバイス構想について説明するとともに、希土類ガーネット型フェライトRe3Fe5O(12) (ReIG)と非ガーネット:ウルツ鉱型ZnOの特性について説明し、実験計画の概要を示す。

第2章は「Proposed devices and materials」であり、SmIG薄膜がマルチフェロイック物質として機能する場合に考えられるデバイスであるFeRAMをベースとしたマルチフェロイックRAMやスピンFET等の新規デバイスについて説明し、希土類ガーネット、ZnOの基礎特性について示す。

第3章は「Experimental」であり、本論文で使用した成膜装置であるパルスレーザー堆積(PLD)装置、構造解析に利用したX線回折(XRD)装置、電気・誘電特性測定に利用したFerroelectric characteristic evaluation (FCE)装置、表面構造評価に利用したAtomic force microscopy (AFM)装置、磁気光学装置測定に利用したMagnetic circular dichroism (MCD)装置について説明する。

第4章は「ガーネット(GGG)基板上への希土類ガーネット型Sm3Fe5O(12) (SmIG) 薄膜のエピタキシャル成長」である。GGG基板との格子歪、SmIG薄膜の臨界膜厚評価。さらにSmIG薄膜の結晶構造評価、表面形状評価、磁気光学特性評価、面内方向での誘電特性評価、面内方向での誘電特性の磁場依存性の評価についての実験結果と考察を示す。

第5章は「ガーネット型基板(GGG)上への非ガーネット型結晶であるWurzite型ZnO 薄膜のエピタキシャル成長」である。GGG(111)、(001)面にエピタキシャル成長したZnO薄膜の結晶構造解析、結晶性、Photoluminescence (PL)特性についての実験結果と面方位による比較、考察を示す。

第6章は「General conclusions」であり、得られた研究結果のまとめと今後の展望について示す。

以上要するに、本論文は磁気誘電光学特性に優れた希土類ガーネット型フェライトのエピタキシャル薄膜結晶を合成し、格子歪誘起の特異な物性(誘電・磁気・光学特性)を発現させるとともに、デバイス応用の鍵となるガーネット型フェライト上での非ガーネット型結晶(ZnO)のエピタキシャル結晶成長に初めて成功したものであり、電子工学および酸化物オプトエレクトロニクス工学の発展のために寄与するところが少なくない。よって本論文は博士(工学)の学位請求論文として合格と認められる。