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Controllable growth of dielectric/semiconductor integrated films LI YangRong†, ZHU Jun, LUO WenBo, LIU XingZhao & ZHANG WanLi State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China

thin film, integration, semiconductor, dielectrics, pulsed laser deposition

Multi-component oxide functional materials have been widely applied to electronic devices such as resistors, capacitors, inductors, microwave circuits and many other passive electrical devices, because they exhibit ferroelectricity, piezoelectricity, pyroelectricity, high-k dielectricity, magnetism, magnetic-electricity, as well as electro-optic, acoustic-optic and nonlinear optic properties. With their rapid development, more than 10 kinds of functional material systems have been formed. The quantity of passive devices is usually about six times as large as that of active devices. Since these passive devices are made with bulk materials, they take up large volume proportion (60%―80%) of the whole electrical system. So the fabrication of electronic thin films and chip-type devices is of great importance to minimize the electrical system. Semiconductor materials, in which special electrical

properties can be found during the carriertransportation, are the fundamental materials of the microelectronic and optoelectronic industries. The main semiconductor material systems are now made up only by Si, GaAs, GaN, SiC and InP since the discovery of Ge in 1946. As we can see, the types of semiconductor are still very limited. The research interest is mostly focused on how to get large area epitaxal wafer with high uniformity and low defect density. Nowadays, the development of traditional semiconductor materials is hard to meet the need of microelectrical technology. Recently, electrical thin films and chip-type devices are developing quickly because of the evolution trend of Received July 1, 2008; accepted February 18, 2009 doi: 10.1007/s11434-009-0218-z † Corresponding author (email: [email protected]) Supported by the State Key Development Program for Basic Research of China (Grant No. 61363Z01) and National Natural Science Foundation of China (Grant No. 50772019)

Citation: Li Y R, Zhu J, Luo W B, et al. Controllable growth of dielectric/semiconductor integrated films. Chinese Sci Bull, 2009, 54: 2681―2687, doi: 10.1007/s11434-009-0218-z

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Currently, electronic information systems are developing quickly towards further miniaturization and monolithic integration so as to realize smaller volume, higher velocity and lower power consumption. For this purpose, the integration of all sorts of active devices (mainly fabricated by semiconductors) with passive devices (fabricated by functional materials) is particularly important and impendent. Therefore, it is necessary to integrate multifunctional oxide dielectrics possessing electric, magnetic, acoustic, optical and thermal properties characterized by spontaneous polarization with semiconductors bearing the characters of carrier transportation to form artificial structures via deposition of solid films. This kind of integrated films may have two characters, i.e., the all-in-one multifunction and modulation of electromagnetic properties by hetero-interface. This makes it possible to realize monolithic integration of detecting, processing, transmission, executing and storing of electronic information. Meanwhile, possible integrated coupling effects will be pursued instead of exploring the limited physical properties of the related materials. In this paper, we put forward a new direction of developing electronic devices with higher performances, and demonstrate some results concerning our recent research on the interface-controllable integrated growth of dielectrics and GaN. Recent progresses of the related research in the world are also reviewed.

miniaturization and monolithic integration in electronic information systems. Therefore, it is especially important and impendent to realize the integration of multifunctional oxides possessing electric, magnetic, acoustic, optical and thermal properties with semiconductors bearing the carrier transportation characters. The integrated artificial structure (monolayer, multilayer or even superlattice) can be fabricated by depositing solid films alternately. The integrated films with all-in-one multifunction properties make it possible to realize the integration of dielectric passive devices and semiconductor active devices. This integration can accelerate the development of electronic systems towards miniaturization and monolithic. On the other hand, the large polarization of oxide and the interface strain caused by lattice mismatch can be used to modulate the transport properties (such as mobility and carrier density) of semiconductors. It is possible to enhance the electrical properties of semiconductors through interface coupling effect. In addition, the interface strain can be expected to enhance the performance of the oxide materials. As mentioned above, the integration of dielectric and semiconductor owns momentous significance both in science exploration and device application. With the rapid development of the deposition technology, e.g. the reactive molecular beam epitaxy, oxide films can be made to epitaxially grow on Si, GaAs, SiGe and other semiconductor wafers. Most research reports concerning the integration of dielectrics with Si only solved problems such as interfacial reaction and random crystalline orientation. GaN, the third generation semiconductor, exhibits characters of wide band gap, high broken-down field, high thermal conductivity, high carrier mobility and hard radiation. GaN-based materials are now applied to microelectronic and electrooptic devices. However, there are few reports about the fabrication of oxide functional materials on GaN wafers. In this report, the growth technique and interface characters of integrated dielectric and GaN integrated films are discussed. As we all know, oxide functional material and nitride semiconductor are totally two different materials. It will cause many problems when the two materials are integrated together. The issues can be divided into two aspects: the first is how to obtain integrated films by controlling the growth and the second is how to obtain the enhanced properties. Oxide films are usually deposited at high temperature under oxygen atmosphere, while semiconductor materi2682

als are fabricated at relatively low temperature with high vacuum. The deposition conditions demanded differ greatly. Furthermore, the lattice mismatch between oxide film and GaN is larger than 10%. The growth technology and interface character of oxide/GaN integrated films are quite different from that of the single phase material. So it is important to find out a fabrication method to make these two materials grow cooperatively. In addition, the integrated film not only makes multifunction exist in one chip, but also may cause some coupling effects through the interface of oxide/GaN. For example, the spontaneous polarization of functional oxide materials can affect the carrier transportation character in the channel of semiconductor. Additionally, the interface strain caused by lattice mismatch will also impact the polarization properties and the transport performance of oxide films. That is to say the properties such as the electric, magnetic, acoustic, optical behaviors of semiconductors and oxides can be modulated or even coupled through interface effect. Some new physical effects might be observed if the interface between dielectric and semiconductor could be well controlled. Because the growth technique and performance of integrated dielectric and semiconductor films are of great scientific importance and application value, intensive research attention has been focused on this issue by many institutes such as DOE and DARPA. A theoretical prediction made by Ahn announced that the integration of oxide and semiconductor would result in some new effects and devices[1]. In 2005, researchers of Michigan University declared that the polarization in dielectric/semiconductor heterostructure would impact the carrier mobility intensively in theory[2]. The experiment of this integration mainly concerned about the alternate materials for SiO2 used in Si-based MOSFET as gate dielectric material. Amorphous LaAlO3 and CaZrO3[3,4] or epitaxial SrTiO3 or BaTiO3[5,6] were deposited on Si for this purpose. The growth kinetics of pervoskite oxide deposited on the second generation semiconductor GaAs[7] was studied by the Motorola Company. ZnO/ BaTiO3/ZnO sandwich structure was fabricated and a "pinning" of the ferroelectric polarization in the BaTiO3 layer by the cladding ZnO layers[8] was observed. Oxide functional materials, such as YMnO3[9,10] and BiFeO3[11] have been fabricated on GaN wafers in recent years. Polycrystalline PZT[12,13] was deposited on AlGaN/GaN and the carrier mobility of modulated structures was controlled by the switched polarization. However, in-

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Figure 1 Schematic drawing of the LMBE systems

A STO single crystal target and a TiO2 ceramic target were employed in the experiments. The oxide thin films were deposited on 2 μm thick GaN (0002) layers epitaxially deposited on C-cut sapphire. The base pressure of the deposition chamber was maintained at 4.5×10−5 Pa and the substrate temperatures varied from 500℃ to 700℃. X-ray diffraction (XRD) was performed using Bede D1 system equipped with a four-circle X-ray diffractometer with copper Kα radiation. The XRD θ-2θ, Ф and ω scans were carried out to determine the in-plane, out-of-plane orientation and mosaic spread of STO films, respectively. The interface structure was characterized by HRTEM (JEOL JEM 2010). Perovskite oxide SrTiO3 is a typical dielectric material with cubic lattice. The (111) plane of cubic crystal has the similar atomic registry to the hexagon wurtzite GaN (0002) plane. So the (111) plane is considered to be

Figure 2 RHEED pattern along the azimuth of (a) GaN [11-20]; (b) GaN [10-10] before STO deposition; (c) GaN [11-20] and (d) GaN [10-10] after STO deposition, respectively.

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the lowest energy growth plane for STO deposited on c-plane GaN. A possible epitaxial relationship is (111) [1-10]STO//(0001)[11-20]GaN, which gives a lattice mismatch about 13%. The lattice mismatch is relatively larger than that epitaxial growth could be obtained[14]. The biaxial stress existing at the interface would affect the microstructure of the oxide films on GaN. Proper buffer layer should be inserted to decrease the stress caused by the lattice mismatch. In general, the insulator buffer layer must meet the following requirements. The lattice parameter of buffer layer should be similar to both substrate and the subsequent deposited film. The buffer layer must be thermally stable so that the interface diffuse and reaction would be minimized. The dielectric constant should be large enough to decrease the electric field applied to the buffer layer. Rutile TiO2 (cassiterite structure, tetragonal a=4.59 Å, c=2.96 Å) shows small lattice mismatch, high thermal stability and large value of dielectric constant, which can satisfy the above-mentioned requirements. The effect of TiO2 buffer layer on the microstructure of STO was studied by comparing the differences between the directly deposited case and the rutile TiO2 buffered case to illustrate the integrated growth of perovskite oxide and wurtzite GaN with great lattice mismatch. The lattice parameter of cubic STO is 0.397 nm and a-axis parameter of hexagon GaN is 0.319 nm. 50-nmthick STO films were grown on GaN (0002) at 500℃, 600℃, and 700℃, respectively. The whole deposition process was in situ monitored by RHEED. The RHEED patterns along GaN [11-20] and [10-10] directions prior to STO deposition are shown in Figure 2(a) and 2(b).

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stead of increasing, the mobility was decreased by the polarization due to the interface layer with high electron trap between the oxide and nitride. In brief, the study of dielectric and semiconductor integration is of great significance in the view of science and technology, but the research work is just at the initial stage. Presented in this paper are some research results about the integrated dielectric/semiconductor films fabricated by laser molecular beam epitaxy (L-MBE). The oxide films were deposited by LMBE using Lambda Physic Compex 201 KrF excimer laser. Reflective high energy electron diffraction (RHEED) was equipped to in situ monitor the growth mode and the interface characters. The incident electron beams with high energy and low incident angle were projected on the film surface. The diffraction patterns were taken by a CCD camera and recorded by a computer. The schematic drawing of our LMBE system is shown in Figure 1.

The diffraction patterns of GaN have clear bright streaks, indicating the GaN surface was smooth without any formation of amorphous layers. As the deposition of STO started at 700℃, STO patterns were observed a few seconds after the RHEED patterns of GaN disappeared. This indicates that there may be an interface layer between the oxide and semiconductors. RHEED patterns of the [1-10] of STO direction along GaN[11-20] were observed, and patterns along the [11-2] of STO direction were observed along GaN[10-10] direction, as shown in Figure 2(c) and 2(d), respectively. These results indicated that the in-plane epitaxial relationships were [1-10]STO// [11-20] GaN and [11-2] STO // [10-10] GaN. The STO surface was slightly rough indicated by the diffraction spots, suggesting an island growth mode. The STO diffraction patterns show six-fold rotation, demonstrating that the epitaxial (111) STO has two twin variants related by a 180° in-plane rotation. When growth temperature decreased to 600℃, the same phenomenon was observed. However, no obvious RHEED pattern of STO was observed at 500℃, demonstrating that epitaxial STO thin films could not be obtained at this temperature. XRD characterization was performed systematically to analyze the epitaxial characters of STO films. As shown in Figure 2, STO films were (111) orientated grown on GaN at 600℃ and 700℃. When the substrate temperature decreased to 500℃, no obvious diffraction peak related to STO was observed, implying that the STO films did not crystallize at this temperature. This is identical to the observations by RHEED. The XRD Φ scan and ω scan were performed to characterize the crystalline quality of cubic STO films grown at 700 ℃ . The full width at half maximum (FWHM) of ω-scan curves of STO (111) shown in the inset of Figure 3(b) was 1°, indicating a mosaic structure in the films. Ф-scans curves of STO (110) deposited at 700℃ and GaN (10-13) are shown in Figure 3(c), respectively. As it can be seen, the STO (101) peaks and GaN (10-13) peaks locate at the same position, suggesting that the epitaxial relationship is [1-10] STO// [11-20] GaN. Both curves show six-fold symmetry, indicating the STO (111) plane with twin-domain structure separated by 180°. According to the above-obtained results, the epitaxial relationships between the STO, GaN are derived as follows: (111) [1-10] SrTiO3 // (0001) [11-20] GaN. To illustrate the phenomenon observed by RHEED and 2684

Figure 3 (a) XRD θ-2θ spectrum of STO directly deposited on GaN at different substrate temperatures; (b) rocking curve and (c) Φ-scan curve of STO (111) deposited at 700℃.

XRD, the nearly coincidence site lattice (NCSL) theories[15] can be applied to describe a two-dimensional interface between STO and GaN. The relationships are illustrated in Figure 4. In order to keep the best atomic registry, epitaxial STO (111) films form a twin structure separated by 180° as implied by the six-fold symmetric RHEED patterns and Ф-scan spectrum. The lattice mismatch with this alignment is estimated to be 13.3% if we compare the

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ARTICLES Figure 4 on GaN.

Schematic drawing of the epitaxial relationships of STO

Figure 5 RHEED patterns of the samples from the (a) GaN [11-20], (b) STO [11-2] directly deposited on GaN, (c) TiO2 [100], (d) STO [11-2] on TiO2 buffered GaN, respectively.

Figure 6

θ-2θ map of STO/TiO2/GaN/Al2O3 deposited at 500℃

Figure 7 Φ-scan curves of STO (101), GaN(103) and Al2O3(10-13) reflection.

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periodicity in the STO [1-10] direction ( 2 aSTO = 5.521 nm) and the double of that in the GaN[11-20] direction (2aGaN=6.372 nm).The large mismatch can degrade the crystal quality of the STO film. Epitaxial films with large lattice mismatch usually form mosaic structure consisted by slightly misoriented sub-grains. These sub-grains can enlarge the FWHM of the rocking-curves of STO (111), as shown in the inset of Figure 3(b). Although higher deposition temperature can improve the crystalline properties, high growing temperature may result in interfacial reaction. Thus, some proper buffer layers should be used to obtain highly epitaxial STO films. TiO2 is widely used as buffer layer as it has many advantages such as high thermal and chemical stability. TiO2 has three polymorphic forms: rutile, anatase and brookite. TiO2 in the rutile phase has a tetragonal structure with lattice parameters a= 0.4584 nm, c=0.2953 nm. Rutile TiO2 has been grown epitaxially on c-plane sapphire, GaN and AlGaN due to the small lattice mismatch between cTiO2 and aGaN. Furthermore, STO has a similar lattice parameter and the same Ti-O6 octahedron as TiO2. Thus, TiO2 can serve as a proper seed layer or template layer for STO. Phase pure rutile films are desirable for the STO/GaN integration. As shown in Figure 5, the crystallization temperature of STO was reduced by inserting TiO2 buffer layer. The growth mode of STO was also changed as revealed by the differences in the RHEED patterns. The growth mode of STO films deposited on GaN directly was the island mode indicated by the spot-like diffraction pattern. However, several Bragg-reflection spots superposed on sharp streak diffraction patterns was observed, suggesting that the STO film deposited on TiO2 buffered GaN was grown in the Stranski-Krastanov growth mode. Therefore, it is estimated that TiO2 buffer layer can greatly enhance the crystallization properties. XRD characterizations were systematically carried out to investigate the microstructure of STO films. The outof-plane of STO kept the (111) orientation the same as the directly deposited STO film, as shown in Figure 6. Nevertheless, the FWHM of the STO (111) rocking curve was decreased by the TiO2 layer, suggesting the enhanced out-of-plane orientation of STO films. The in-plane epitaxial property of the STO/GaN heterostructure was studied by Φ scans. Both STO and GaN show six-fold symmetry as can be seen in Figure 7.

Figure 8 (a) HRTEM cross-section image and (b) SAED pattern of the multilayer film of STO/TiO2/GaN/Al2O3

Figure 9 Schematic drawing of epitaxial relationships of STO on TiO2 buffered GaN.

As we know, the STO (111) plane is triple symmetric. The six-fold symmetric property resulted from the twinned structure separated 180° to form the best atomic registry to hexagon GaN. The FWHM of Φ scan curves of STO films fabricated on TiO2 buffered GaN was much smaller than that of the directly deposited one shown in Figure 3. The improved crystalline property resulted from the TiO2 layer, although the thickness of TiO2 was very thin as indicated by the weak intensity of XRD peak. HRTEM was carried out in order to study the interface characters of the heterostructure, as shown in Figure 8. The interface was sharp and the thickness of TiO2 was estimated to be 2 nm. SAED pattern reveals that STO films are epitaxial grown on GaN, which is consistent with the RHEED and XRD results. The epitaxial relationship of the heterostructure was estimated to be (111) [1-10] STO // (100) [001] TiO2 // (0002) [11-20] GaN. 1

lecular-beam deposition on silicon. Appl Phys Lett, 2006, 88: 112907 4

Qiu X X, Liu H W, Fang F, et al. Interfacial properties of high-k di-

ence, 2004, 303: 488―491

electric CaZrOx films deposited by pulsed laser deposition. Appl Phys

Wu Y R, Singh J. Polar heterostructure for multifunction de-

Lett 2006, 88(18): 182907

vices:theoretical Studies. IEEE Transactions On Electron Devices, 3

The authors would like to thank Prof. C. Z. Jiang of the Department of Physics of Wuhan University for HRTEM characterization

Ahn C H, Rabe K M, Triscone J M. Ferroelectricity at the nanoscale:local polarization in oxide thin films and heterostructures. Sci-

2

The NCSL theory[15] was applied to describe the epitaxial relationship and the two-dimensional orientation relationship is illustrated schematically in Figure 9. The lattice mismatch between GaN and STO was calculated to be 13.3%. The lattice mismatch was reduced to 1.3% by inserting TiO2 layer, thus the STO films could be epitaxially grown at 500℃. Except for the reduced lattice mismatch, the TiO2 layer can serve as the initial seed layer for STO growth as both of them have the same Ti-O octahedron[16, 17]. Additionally, the integration of ZnO and AlN semiconductor films with STO was also studied. It was found that the orientation of hexagon ZnO could be controlled by the orientation of STO substrates. ZnO films deposited on non-polar STO (001) substrates showed multi-domain structure while those fabricated on polar (110) and (111) substrates exhibited single domain structure[15,18]. Pure cubic AlN was a cubic on the cubic epitaxially deposited on STO (001) due to the small lattice mismatch[19]. In summery, research status on the integration of dielectric oxide and semiconductor are discussed. It is pointed out that the all-in-one multifunction properties and modulated performance can be expected in the integrated films. It makes a new direction of realizing monolithic integration of active devices and passive devices. In order to realize the integration of the two kinds of materials, special buffer layer should be applied to make the hetero-interface under control. Based on the integration of STO and GaN, it is found that STO can be epitaxially grown on both bare and TiO2 buffered GaN by LMBE. Although the epitaxial relationships are the same in both cases, the inserting of 2 nm TiO2 layer can greatly enhance the quality of STO at relatively lower deposition temperature. These results are helpful to understanding the growth and structural control of dielectric/semiconductor integrated films.

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Goncharova L V, Starodub D G, Garfunkel E, et al. Interface structure

2005, 52: 284―293

and thermal stability of epitaxial SrTiO3 thin films on Si (001). J Appl

Edge L F, Schlom D G, Sivasubramani P, et al. Electrical characteri-

Phys 2006, 100(01): 014912

zation of amorphous lanthanum aluminate thin films grown by mo-

2686

6

Vaithyanathan V, Lettieri J, Tian W, et al. C-axis oriented epitaxial Ba-

www.scichina.com | csb.scichina.com | www.springerlink.com

8

9 10 11 12

13

14

2005, 484: 154―159 Zhu J, Wei X H, Zhang Y, et al. Study on interfacial strain behavior of

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TiO3 films on (001) Si. J Appl Phys, 2006, 100: 024108 Liang Y, Kulik J, Eschrich T C, et al. Hetero-epitaxy of perovskite oxides on GaAs(001) by molecular beam epitaxy Appl Phys Lett, 2004, 85: 1217―1219 Mbenkum B N, Ashkenov N, Schubert M, et al. Temperature-dependent dielectric and electro-optic properties of a ZnO-BaTiO3-ZnO heterostucture grown by pulsed-laser-deposition. Appl Phys Lett, 2005, 86: 091904 Posadas A, Yau J-B, Ahn C H, et al. Epitaxial growth of multiferroic YMnO3 on GaN. Appl Phys Lett, 2005, 87(17), 171915 Chye Y, Liu T, Li D, et al. Molecular beam epitaxy of YmnO3 on c-plane GaN. Appl Phys Lett, 2006, 88: 132903 Tian W, Vaithyanathan V, Schlom D G, et al. Epitaxial integration of (0001) BiFeO3 with (0001) GaN. Appl Phys Lett, 2007, 90: 172908 Shen B, Li W P, Someya T, et al. Influence of ferroelectric polarization on the properties of two-dimensional electron gas in Pb(Zr0.53Ti0.47)O3/ AlxGa1-x/GaN structures. Jpn J Appl Phys, 2002, 41: 2528―2530 Cao W, Bhaskar S, Li J, et al. Interfacial nanochemistry and electrical properties of Pb(Zr0.3Ti0.7)O3 films on GaN/sapphire. Thin Solid Films

functional oxide heterostructures. J Appl Phys, 2006, 100: 104106 15

Wei X H, Li Y R, Zhu J, et al. Epitaxial properties of ZnO thin films on SrTiO3 substrates grown by laser molecular beam epitaxy. Appl Phys Lett, 2007, 90: 151918

16

Yamada T, Wakiya N, Shinozaki K, et al. Epitaxial growth of SrTiO3 films on CeO2/yttria-stabilized zirconia/Si(001) with TiO2 atomic layer by pulsed-laser deposition. Appl Phys Lett, 2003, 83: 4815―4817

17

Yang S Y, Zhan Q, Yang P L, et al. Capacitance-voltage characteristics of BiFeO3/SrTiO3/GaN heteroepitaxial structures. Appl Phys Lett, 2007, 91: 022909

18

Wei X H, Li Y R, Jie W J, et al. Heteroepitaxial growth of ZnO on perovskite surfaces. J Phys D: Appl Phys, 2007, 40: 7502―7507

19

Zhu J, Zhao D, Luo W B, et al. Epitaxial growth of cubic AlN films on SrTiO3(100) substrates by pulsed laser deposition. Crystal Growth, 2008, 310: 731―737

INORGANIC NONMETALLIC MATERIAL

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