Growth of (111)-oriented PbTe films on Si(001) using a BaF2 buffer

Thin Solid Films 358 (2000) 277±282 www.elsevier.com/locate/tsf

Growth of (111)-oriented PbTe ®lms on Si(001) using a BaF2 buffer A. Belenchuk a, A. Fedorov b, H. Huhtinen c, V. Kantser a, R. Laiho c,*, O. Shapoval a, V. Zakhvalinskii a a

Institute of Applied Physics, Kishinev 2028, Moldova Institute of Single Crystals, Kharkov 310001, Ukraine c Wihuri Physical Laboratory, University of Turku, 20014 Turku, Finland b

Received 13 January 1999; received in revised form 24 August 1999; accepted 24 August 1999

Abstract Epitaxial PbTe(111) ®lms are prepared by hot-wall-beam epitaxy on Si(001) substrates using a BaF2(111) buffer layer grown by molecular beam epitaxy. According to re¯ection high energy electron diffraction (RHEED) analysis, the growth of BaF2(111) on Si(001) is twodimensional with twins related to two equivalent epitaxial orientations. The twins cannot be avoided by varying the growth conditions. The PbTe (111) ®lms repeat the twinned structure of the BaF2 buffer, with the PbTe [112Å] axis aligned along one of the Sik110l directions. The surface morphology and the carrier mobility of the PbTe ®lms depend strongly on the growth temperature of the BaF2 buffer layer. These heterostructures exhibit electron mobilities up to 1600 cm 2/V s (300 K) and 24 000 cm 2/V s (77 K) and an excellent ability for the relaxation of thermal strains. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PbTe; Si(001); BaF2; Epitaxial relations; Surface morphology; Twins

1. Introduction A thin buffer layer of ¯uorides (CaF2 or BaF2-on-CaF2) can be used for overcoming the high lattice and thermal expansion mismatches between Si and IV±VI semiconductor ®lms applied on it [1]. So far, ¯uoride buffers have been successfully used only for the growth of (111)-oriented IV± VI ®lms on Si (111) substrates[2]. While growth of (001)oriented ®lms on Si(001) is possible using an (001)-oriented ¯uoride buffer [2,3] or ¯uorides combined with intermediate PbSe layers obtained by LPE [4], or a layer of YbS(001) [5], the quality of the ®lms is not usually good. PbSe(001) ®lms having a thickness more than 0.5 mm tend to crack during cooling to cryogenic temperatures [6]. The cracking originates from an inef®cient strain relaxation mechanism in this orientation. Whereas, in PbSe(111) ®lms the strain is relieved by a dislocation glide in the main {001} k110l glide system, since in this case the {001} planes are inclined to the interface [7]. To realise this epitaxial relation for IV±VI compounds on Si, it is necessary to intervene an appropriate buffer layer having the (111)-orientation, since the IV±VI semiconductors have a preferred (001) growth mode [8]. Since BaF2 grows directly on Si(001) with * Corresponding author. Tel.:1358-2-3335-943; fax: 1358-2-2319-636. E-mail address: reino.laiho@utu.® (R. Laiho)

the (111) growth mode [9,10], it can be used as buffer material for (111)-oriented IV±VI semiconductor layers on Si. In this paper we report on the growth of PbTe(111) ®lms on Si(001) using BaF2(111) as the buffer layer. Investigations of the crystalline structure, the epitaxial relation, the surface morphology and electrical properties of PbTe and BaF2 layers are described. Such information is necessary for a better understanding of this dissimilar heterostructure and for ®nding ways of further improvement of the quality of the ®lms. 2. Experimental details BaF2 buffer layers were grown by molecular beam epitaxy (MBE) on exactly oriented p-type Si(001) substrates of resistivity 5 V cm. Before loading into the MBE chamber the substrates were chemically cleaned and passivated with hydrogen by dipping them into HF-solution. Then the substrates were heated to 8008C for a period of 30 min to desorb the passivation layer. The chamber (base pressure 2 £ 10210 Torr) had two pyrolytic graphite owens containing high-purity monocrystalline BaF2 and CaF2. The deposition of BaF2 was made with a growth rate of 5 nm/min at substrate temperatures between 300 and 8008C. The layer thickness and the deposition rate were monitored using a

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quartz oscillator calibrated by optical ellipsometry. Re¯ection high energy electron diffraction (RHEED) was used to monitor in situ successive stages of the growth. After deposition of the buffer layer the substrates were withdrawn from the MBE chamber to the air and immediately loaded into another apparatus for growth of the PbTe ®lm by hot-wall-beam epitaxy (HWBE). This was made in a vacuum of 3 £ 1028 Torr, using a polycrystalline PbTe source synthesized from elements of 6 N purity and an additional Te source for ®ne control of the carrier concentration. Before initiating the growth, the substrates were thermally cleaned for 30 min at 5008C. Shadow masks were used to get ®lms of one cm 2 in area, at a growth rate of 2 mm/h and a substrate temperature of 4008C. For comparison, a few PbTe ®lms were grown under the same conditions also on Si(111) substrates coated with a BaF2± CaF2 buffer prepared by the technique described in Refs. [1,11]. The thickness of the PbTe layer was determined by Fourier transform infrared transmission (FTIR) measurements. The epitaxial relations were determined from asymmetric X-ray diffraction data obtained with a double-crystal diffractometer using Cu Ka 1 radiation. Surface morphologies of both BaF2 and PbTe ®lms were investigated by atomic force microscopy (AFM) in the air. Electrical properties of the PbTe ®lms were determined by Hall measurements in Van der Paw geometry using the conventional dc technique. The ability for relaxation of thermal strain was evaluated by measuring the Hall mobility of the grown heterostructures before and after accomplishing a few fast temperature cycles between 300 and 77 K.

3. Results 3.1. Growth and structure of the BaF2 buffer layer Well-de®ned (2 £ 2) RHEED patterns, related to (2 £ 1) and (1 £ 2) surface reconstructed domains, were observed from the Si(001) substrates after desorbtion of the hydrogen passivation. It is known that the dipole moment directed perpendicular to the (001) plane of ¯uoride ®lms, and the resulting high surface energy, tends to promote surface faceting and the formation of spotted RHEED patterns [12]. We found, however, that during all stages of the growth of BaF2 on Si(001), starting from a couple of monolayers to the ®nal thickness, the RHEED patterns were streaky and free from 3 D transmission spots. The 2 D heteroepitaxy with the (111) layer orientation of BaF2 on Si(001) was observed over the whole range of the growth temperatures from 300 to 8008C. The best RHEED images, having a minimum amount of diffuse scattering, were obtained at 6008C. In Fig. 1 a typical RHEED image is shown, observed from the BaF2 layer using the [110] azimuth of the Si(001) substrate. The spacing of the diffraction lines corre-

Ê thick BaF2 Fig. 1. RHEED patterns observed from the surface of a 800 A layer grown on Si(001) at 5008C (e-beam is directed along the [110] axis of the Si substrate). The zero-order line is located in the middle of the picture.

sponds to that obtained from a BaF2 ®lm grown on a CaF2(111)/Si(111) heterostructure when the e-beam is directed along the [112Å] axis of BaF2(111). The diffraction patterns consist of double streaks, indicating a superposition of two surface structures. Identical RHEED images could be repeated after every change of the angle by 308. These results con®rm that the BaF2 ®lms grow in the (111) orientation on Si(001), in domains with the [112Å] direction of BaF2 aligned either along the [110]Si or [11Å0]Si directions. The corresponding epitaxial relations are: (111)BaF2// (001)Si, [112Å]BaF2//[110]Si or [112Å]BaF2//[11Å0]Si. It is important to note that even for deposits as thin as one monolayer, the diffraction line spacing differs from that of the substrate and does not change during continuation of the growth. This indicates that BaF2 nucleates directly according to its own lattice parameter, without elastic adaptation to the Si lattice. Furthermore, the nucleation of the two twin domains rotated by 908 with respect to each other occurs during the deposition of the ®rst monolayers. 3.2. Growth and structure of PbTe ®lms A set of PbTe/BaF2(111)/Si(001) structures were deposited under identical conditions on 120 nm thick BaF2 buffers grown at different temperatures. The thickness of the PbTe layer was determined from the spacing of the FTIR interference fringes observed at photon energies below 0.4 eV (Fig. 2) using published values of the refractive indices [13]. Analysis of the u ±2u X-ray diffraction data showed that in all the samples the (111) plane of PbTe is parallel to the (001) plane of Si. The full width at half maximum (FWHM)

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3.3. Surface morphologies of the BaF2 and PbTe ®lms

Fig. 2. FTIR transmission spectrum observed at 300 K from a PbTe ®lm grown at 4008C on BaF2(111)/Si(001).

of the (333) PbTe diffraction peak was found to depend crucially on the growth temperature of the BaF2 buffer layer. For a 4 mm thick ®lm grown at 6008C on a BaF2 buffer, the FWHM was 560 arc s. A much lower value of FWHM…180 arc s) was obtained for the PbTe ®lms evaporated on a BaF2±CaF2/Si(111) layer under the same growth conditions. The broad diffraction peaks and structured rocking curves observed from PbTe ®lms grown on BaF2(111)/ Si(001), indicate the presence of low-angle grain boundaries due to misorientation of the grains. Instead, the PbTe ®lms grown directly on Si(111) were monocrystalline. The epitaxial relation between PbTe and Si was determined by comparison of X-ray re¯ection directions from the (311) plane of (001)-oriented Si and the (331) plane of (111)-oriented PbTe, inclined to the surface of the ®lm and the substrate. Instead of three re¯ections observed during azimuthal rotation of untwinned ®lms grown on a BaF2± CaF2/Si(111) substrate, the {331} planes of PbTe gave twelve re¯ections repeated after every rotation by 308. Therefore, PbTe(111) grown on BaF2(111)/Si(001) consists of four types of twinning domains rotated by 908 about the normal of the surface. All re¯ections from {331} PbTe had almost equal intensity, showing rather equal quantities of all the types of twins present in the ®lms. The directions of the {331} PbTe re¯ections relative to those of the {311} Si re¯ections indicate that the [112Å] direction in PbTe is aligned along one of the k110l Si surface axes, corresponding to the following epitaxial relations: (111)PbTe//(001)Si, [112Å]PbTe//[110]Si, [11Å0]Si, [1Å1Å0]Si and [1Å10]Si. Compared to the RHEED results for the BaF2 buffer, where only two types of epitaxial relations were de®ned, the asymmetric X-ray diffraction from the overlying PbTe ®lm revealed additionally twins rotated by 1808 about those detected by RHEED. However, the RHEED patterns are identical in 1808 rotation of the surface, and the BaF2 layers seem to have the same types of epitaxial relationships as the PbTe ®lm. The 1808 twinning is often observed in k111l growths of ¯uorides [14].

For imaging the BaF2 layer by AFM, part of the sample was covered with a mask during growth of the PbTe ®lm. In Fig. 3a the surface topography of a 220 nm thick BaF2 ®lm grown at 4008C on Si(001) is shown. The image consists of ¯at mosaic blocks having lateral dimensions of several micrometers, separated from each other by distinct boundaries. In the parts of the blocks far from the boundaries, the root-mean-square surface roughness amounts to a few aÊngstrom units. These regions, considered to be atomically ¯at, cover a major part of the surface, which produces streaked RHEED patterns. As shown in Fig. 3b,c, respectively, the boundaries between the blocks consist of 5±6 nm deep and 100±200 nm wide grooves surrounded by ridges along the edges of the blocks. The BaF2 layers grown at 6008C have otherwise similar surface morphology but the lateral dimensions of the blocks reach several tens of micrometers and the depth of the grooves is reduced by a factor of 2±5. The same kind of surface structure of BaF2 on Si(001) has been observed earlier with Nomarsky interference microscopy and was interpreted as cracks in the BaF2 layer [9]. The boundaries seen in our AFM images differ from simple cracks by the presence of ridges near the edges of the blocks and by the fact that the depth of the grooves, not more than a few percent of the layer thickness, is much less than their width. As evident from Fig. 4a the PbTe ®lm grown on BaF2(111)/Si(001) at 6008C has a block structure on the surface, with the grooves between the blocks being 5±10 nm deep. The regions on each side of the grain boundary are ¯at with faint sets of straight lines, (no such lines appear in Fig. 3a taken on a bare BaF2 buffer layer). These lines represent surface height steps aligned along one of the three equivalent k110l directions resulting from dislocation glides in the primary {001}±k110lglide system of lead chalcogenides [7]. High-resolution 2 D AFM images from centre parts of the two adjacent grains shown in Fig. 4a are presented in Fig. 4b,c These AFM images are obtained with the same orientation of the sample relative to the AFM scan lines. The triangles formed by crossing the surface height steps, indicate a 1808 rotation of the epitaxial relationship in the neighbouring grains. A sequence of AFM images scanned along the [110] axis of the Si substrate demonstrates that each grain has the same orientation of the surface triangles. The orientations of adjacent grains are always different, corresponding to rotations of either 180 or 908 around the normal of the surface. Thus the surfaces of both PbTe ®lms and the underlying BaF2 buffers consist of ¯at blocks characterized by epitaxial relations different from the Si(001) substrate. To explain the shape of the boundaries between the blocks, it is necessary to take into account that in addition to different epitaxial relations the grains have small misorientations, as demonstrated by the X-ray rocking curves. This generates local strains and a high density of disloca-

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decreases above the strained boundaries of the blocks. This difference in the growth rates leads to the formation of a groove above the strained boundary of the blocks and ridges above the relaxed regions along the edges of the grains. At the optimal growth temperature of 6008C the misorientation of the grains in BaF2 is smaller and the grooves are lower than in the ®lms grown at other temperatures. The overgrown PbTe layers repeat the structure of the BaF2 buffer completely, but the grooves are signi®cantly deeper due to the much greater thickness of the PbTe ®lm. 3.4. Electrical properties of the PbTe ®lms The PbTe ®lms grown on BaF2(111)/Si(001) as described above have electron Hall concentrations of n ˆ …1:2±1:3† £ 1017 cm 23 and …6±7† £ 1016 cm 23 at 300 and 77 K, respectively, independent of the growth temperature of the BaF2 buffer. The highest values of the free carrier mobility m ˆ 1600 cm 2/V s (300 K) and m ˆ 24 000 cm 2/Vs (77 K) are obtained when the BaF2 buffer is grown at 6008C, proving its crystalline quality. The value of m at 300 K is comparable to 1700 cm 2/V s observed in the ®lms grown on BaF2±CaF2/ Si(111), and in the best PbTe ®lms grown on cleaved BaF2(111) crystals [16]. The value of m at 77 K is still signi®cantly lower than 44 000 cm 2/Vs observed for PbTe ®lms on BaF2±CaF2/Si(111) substrates or bulk BaF2(111) crystals. With the typical temperature dependence of m(T) / T 22.5, the ratio m (77 K)/m (300 K) is theoretically ù 30 in the lead chalcogenide ®lms grown on BaF2±CaF2/Si(111). For the best PbTe ®lms on BaF2/Si(001) this ratio is only 15. Since the reduction of the low-temperature mobility is more pronounced when the average size of the twinned grains in the ®lm is smaller, the scattering of the electrons at twin boundaries is a likely reason for the reduction of m at 77 K. This mechanism is negligible at room temperature because the mean free path of the carriers is much smaller. 3.5. Relaxation of thermal strain

Fig. 3. AFM topographies of the BaF2 buffer layer. (a) Surface of a BaF2 layer grown at 4008C. The white spots are caused by PbTe islands grown under the shadow mask. (b) Image of the boundary between two blocks. (c) Height pro®le taken across the boundary along the line shown in the image (b).

tions relaxing the strain in the vicinity of the grain boundaries. Adding an atom into the strained part of the growing layer gains less energy than its incorporation into the strainreleased part [15]. Due to stress-driven surface diffusion, the growth rate increases locally above the dislocations and

Thermocycling experiments were made by dipping the PbTe/BaF2/Si(001) heterostructure into liquid nitrogen and warming it rapidly back to 300 K. To estimate its ability for the relaxation of strain arising from the high difference of the thermal expansion coef®cients, 2.6 for Si, 19.8 BaF2 and PbTe (10 26 K 21 at 300 K) [1,7]; the Hall mobility, which is closely related with structural perfection of the material, was measured simultaneously. In the samples where the BaF2 buffer was grown at 6008C no change of m, 24 000 cm 2/V s at 77 K and 1600 cm 2/V s at 300 K, could be observed even after a great number of thermocycles. The width of the X-ray rocking curve of the PbTe(333) diffraction peak was practically constant after thermocycling. Instead when the BaF2 layer was grown at a temperature suf®ciently different from 6008C the values of m were clearly reduced and numerous cracks and delaminations

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Fig. 4. AFM topographies of a PbTe ®lm. (a) Surface of a 4-mm thick ®lm grown at 4008C on BaF2(111)/Si(001) imaged around the boundary between two adjacent grains. (b) Region inside the right-upper grain in the image (a). (c) Region inside the left lower grain in the image (a).

were formed, mainly along the edges of the PbTe layer and on the surface of uncoated BaF2. In spite of the mosaic structure, the best ®lms exhibited excellent ability for relaxation of the thermal mismatch strain. AFM data indicate that inside each block the relaxation occurs via glide dislocations in the main {001}±k110l glide system. This is a conventional mechanism of strain relaxation in (111)-oriented lead chalcogenide ®lms. However, the strain at the boundaries between the blocks are relieved only when these blocks have suf®ciently large dimensions and are at the same time slightly misoriented with respect to one another. Evidently, in this case the boundary strain is smaller and the dislocation glides can relax the strain in the large interior part of the blocks.

4. Discussion Our results show that the structure, the surface morphology and the electrical properties of the PbTe ®lms are determined during deposition of a few ®rst monolayers of BaF2. At the initial nucleation stages of BaF2 on Si(001), regions are formed in which the (111) plane of BaF2 is parallel to the Si(001) plane but the BaF2 [112Å] direction is aligned along one of the Sik110l surface directions. This results in the observed four types of domains, distinguished by diverse epitaxial relations with respect to the substrate. Since adjacent twinned domains are slightly misoriented, upon further growth of the layer thickness strain appears at interdomain boundaries. This strain results in stress-driven surface diffu-

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sion of adatoms and local variation of the growth rate, leading to the generation of grooves and ridges along the boundaries between the twinned regions. As a result these regions are delineated with distinct borders, which form the mosaic structure of the surface. Adjustment of the growth temperature does not affect the mode of the epitaxial growth, but allows to increase the dimensions of the blocks. This increases the carrier mobility and improves the relaxation of the strain caused by thermal mismatch. The optimal growth temperature of BaF2(111) on Si(001), 6008C, is very close to that of CaF2(001) on Si(001) [3,4,9]. Seemingly, this temperature provides the maximal diffusion length of the BaF2 molecules on Si(001) before re-evaporation and 3 D growth start to in¯uence. The structure of the PbTe ®lms reproduces fully the twin structure of the BaF2 buffer layer but with 1808 rotation of the lattice around the normal of the surface, in agreement with the well known tendency of (111)-oriented IV±VI compounds to grow in the B-type with respect to the BaF2(111) buffer layer [7]. The presence of the twins leads to deterioration of the surface morphology and to a reduction of the carrier mobility in the PbTe layer, especially at cryogenic temperatures. It is clear that the elimination of twins is the main problem in the fabrication of high quality PbTe(111) ®lms on Si(001) substrates. Since the twin structure is formed at the BaF2(111)/Si(001) interface, a detailed study of this heterosystem is necessary. Certain proposals can be made already at present. A similar type of twinning has been observed in other systems having a large lattice mismatch, such as CdTe(111)/Si(001) [8], CdTe(111)/GaAs(001) [17] and InSb(111) on CaF2(001)/Si(001) [18]. A prevalent requirement of this growth mode is the reduction of the lattice mismatch between the [112Å] direction of an (111)oriented ®lm and the k110l direction of an (001)-oriented substrate. Replacement of CdTe on Si(001) by InSb on CaF2(001)/Si(001) decreases the mismatch from 19 to 3(. In the case of BaF2 on Si(001) the epitaxial relations allow the lattice mismatch to be reduced from 14 to 1.2( at 300 K and close to zero at 6008C. Hence, the appearance of the BaF2(111) orientation on Si(001) is not surprising. All the heterosystems mentioned above show invariably the type of twinning known as double-positioning. Multidomain growth of CdTe on Si can be reduced by cutting the Si substrate along the [110] direction to have a vicinal surface consisting of double height atomic steps and single domain (2 £ 1) reconstruction of the surface [8]. This effect was recently attributed to the formation of an incommensurate interface due to the presence of a Te overlayer whose orientation is controlled by the surface step morphology [19]. BaF2 can form an incommensurate interface with a Ge(111) substrate [20] but the atomic mechanism is not understood up to now. Hence, for the control of twinning

a careful study of atomic structure of BaF2(111)/Si(001) interface is necessary together with studies of the in¯uence of the misorientation of the substrate away from the [110] direction. 5. Conclusions Epitaxial growth of (111)-oriented PbTe ®lms on Si(001) substrates has been realised using an (111)-oriented BaF2 buffer layer. Investigations of the structure, the surface morphology and the electrical properties indicate that heteroepitaxial growth of PbTe(111) ®lms with full relaxation of the thermal mismatch strain is obtained. However, twinning at the BaF2(111)±Si(001) heteroboundary leads to some reduction of the carrier mobility and makes the surface morphology of the overgrown PbTe ®lms worse. For these reasons detailed investigations of the atomic structure of the BaF2(111)/Si(001) interface and the in¯uence of the substrate misorientation on the formation of twins are needed before ®nal conclusions about prospects of PbTe(111)/BaF2(111)/Si(001) heterostructures in device applications are possible.

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