Variation of Microstructure and Electrical Conductivity of Amorphous Aginsbte and Sbte Films During Crystallization

Materials Transactions, Vol. 48, No. 3 (2007) pp. 610 to 617 #2007 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

Variation of Microstructure and Electrical Conductivity of Amorphous AgInSbTe and SbTe Films during Crystallization

Chien-Chih Chou, Fei-Yi Hung and Truan-Sheng Lui*

Department of Material Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan City 701, Taiwan, R. O. China

Two kinds of chalcogenide films with similar Sb/Te atomic ratios, AgInSbTe (AIST) and SbTe (ST) were deposited on alkali-free glass using the RF sputtering method. The microstructure characteristics of both ST and AIST films were demonstrated to correlate with the electrical properties. TEM observation and GI-XRD profiles revealed that the structures of as-deposited AIST and ST films were amorphous characteristics. The sheet resistance of the as-deposited AIST films was twice as high as that of ST films and the effect of Ag and In additives was proposed. After annealing at 523 K for 1 h, the sheet resistance of both chalcogenide films decreased by four orders of magnitude due to crystallizations and the sheet resistance of crystallized AIST film was still twice as large as that of crystallized ST film. DSC measurement was used to determine the crystallization temperature of AIST films as 476 K and that of ST films as 445 K. The activation energies of crystallization were determined using Kissinger’s method, and 0.94 eV and 0.84 eV were obtained for AIST and ST films, respectively. TEM observations showed that AgSbTe2 phase exists in the AIST film after heat treatment in addition to -Sb phase. No indium compounds were discovered in the AIST film by the energy dispersive spectroscopy (EDS) and XRD measurements. [doi:10.2320/matertrans.48.610]

(Received November 2, 2006; Accepted January 19, 2007; Published February 25, 2007) Keywords: chalcogenide ﬁlm, sheet resistance, amorphous, crystallization

1. Introduction properties for both SbTe and AgInSbTe films in association with the microstructural characteristics before and after heat Thin chalcopyrite films such as Ge-Sb-Te and Ag-In-Sb- treatment. The differences between SbTe and AgInSbTe Te have been used as the recording layer in optical recordable films were discussed. discs. A characteristic of these films is that they are able to be quickly and reversibly transformed from an amorphous state 2. Experimental Procedure to a crystalline state. The optical contrast and the electric properties corresponding to the change of structures are Alkali-free glass sheets in 0.7 mm thickness were de- expected to be significantly different.1–3) Ovonic Unified greased and ultrasonically cleaned in acetone, and then were Memory (OUM) is a kind of nonvolatile memory that utilizes dried with clean compressed air. A 13.56 MHz RF sputtering such reversible characteristics in a GeSbTe chalcogenide system was adopted to deposit films on glass substrates in this alloy material. Such a technology can satisfy the require- study, and the substrate-to-target distance was fixed at ments for non-volatile memory because both the amorphous 52 mm. The coater was pre-pumped to a pressure of less and polycrystalline structures are stable at room temper- than 0.006 Pa. The sputtering pressure was controlled within ature.2–5) In practical applications, OUM exhibits a high/low 0.06 Pa with Ar flow. In the first part of the experiment, AIST resistance ratio of about 10100, depending on its program- films in different thickness were obtained by changing the med states, film structures and material properties.4,5) A key sputtering duration at a sputtering power of 200 W. Film physical property for OUM applications is the sheet resist- thicknesses and sheet resistance were measured by -step ance with dependence on different crystalline structures. equipment and a four-point-probe method, respectively. The The thermoelectric power and electrical resistivity of variation in sheet resistance with film thickness was crystalline SbTe films have been studied. The sheet resistivity analyzed. and Seebeck coefficient of SbTe films can change with the In the next part of the experiment, the same sputtering crystalline state6) and the appearance of the second phase.7) parameters, 100 W sputtering power and 60 s sputtering The SbTe films were also reported to have the potential for duration were used to deposit AIST and ST films using two the fabrication of chalcogenide nonvolatile memory by using separate targets. The film thickness was recognized as the focus ion beam method.5) However, there are few approximate 150 nm and the thickness difference between evidences reported on the relationship between microstruc- ST and AIST specimens was less than 10% due to the similar ture characteristics and its corresponding electrical proper- deposition rate. The chemical compositions were analyzed by ties. The structural change of as-deposited SbTe film caused induction-coupled-plasma spectroscopy and recognized sim- by post-heat-treatment is investigated in terms of crystal- ilar Sb/Te atomic ratio for AIST and ST films (shown in lization temperature and thus, activation energy in the present Table 1). A differential scanning calorimeter (DSC) was used study.6–13) Besides, previous studies have reported that SbTe to analyze the thermal behavior of the AIST and ST films films with Ag and In additives have a higher crystallization with five heating rates 0.083, 0.17, 0.25, 0.33, 0.50 Ks1 from temperature and better stability than the SbTe binary room temperature to 673 K. According to Kissinger’s system.14) Hence, this study aims to investigate the electrical method, the activation energy can be derived from eq. (1) by assuming that the crystallization rate reaches the max- *Corresponding author, E-mail: [email protected] imum value in the transformation process.15,16) Variation of Microstructure and Electrical Conductivity of Amorphous AgInSbTe and SbTe Films during Crystallization 611

Table 1 Chemical compositions of two kinds of chalcogenide ﬁlms deposited using the same sputtering parameters.

Ag In Sb Te 600s AgInSbTe 3.4 at% 5.2 at% 60.8 at% 30.6 at% SbTe — — 68.9 at% 31.1 at% 240s

E 1 ln ¼ C ð1Þ T2 R T 120s where is the heating rate, E is the activation energy, T is the absolute peak temperature in Kelvin, R and C are a constants. 2 60s A straight line was obtained by plotting lnð=T Þ versus Intensity, / a.u ð1=TÞ. The activation energy of crystallization was calculated from the slope of the straight line.15) ST and AIST films were isothermally annealed at different 30s temperatures for 1 h and were naturally cooled down in the furnace. After heat treatment, the sheet resistance data could be plotted with the parameter against annealing temperatures 10s and the transition temperatures for sheet resistance reduction could be recognized for ST and AIST films. The crystalline 10 20 30 40 50 60 70 80 structures of specimens were analyzed using a Grazing- 2 Theta Incident X-ray Diffractometer (GI-XRD), and the microstructural feature was also observed by transmission electron Fig. 1 GI-XRD spectra of as-deposited AIST films deposited with different microscopy (TEM). Morphologies, chemical compositions, durations. lattice images and Fast Fourier Transform (FFT) images were analyzed to examine the relationship between electrical regressive plot, corresponding to infinite thickness, repre- conductivity and extension of crystallization in ST and AIST sents the bulk resistivity of amorphous AIST as 3:11 105 films. cm. The variation in the sheet resistance resulting from film 3. Results and Discussion thickness must be clarified because crystallization of amorphous films must be taken into account in addition to the 3.1 The relationship between sheet resistance and film contribution due to the size effect. According to the eq. (2), thickness the maximum variation in the sheet resistivity could be From X-ray diffraction profiles of AIST films deposited at approximately 5% if the variation in film thickness can be different durations as shown in Fig. 1, the broadened patterns controlled within 10%. However, it is reasonable that the which appear at low diffraction angles prove that all thickness effect is assumed to be negligible in the next specimens possess amorphous structures even though the electrical resistance measurements through crystallization sputtering duration is as long as 600 s. It is a well-known because the film thickness has been controlled to be the same. phenomenon that for most amorphous materials the sheet resistance will actually be higher than the polycrystalline 3.2 Crystallization characteristics of AIST and ST films films.17) However a classical size-effect theory is also DSC analysis of the AIST and ST films at the heating rate considered in this investigation quantitatively to confirm of 0.17 Ks1 are shown in Fig. 3. Exothermal peaks appeared that it tends to decrease with increasing film thickness.17–20) at 476 K in the AIST curve and at 445 K in the ST curve, The theory takes into account surface scatterings in addition representing the crystallization temperatures of AIST and ST to bulk scatterings, and therefore the mobile electrons are films. This result is compatible with other studies21,22) but the scattered by the surface structure when film thickness crystallization temperature of ST film is found to be lower becomes thinner.19,20) The size effect describes an inverse than that for AIST film in the present experiment. In addition, relationship between sheet resistivity s and film thickness t the XRD profiles of ST and AIST films after annealing at as eq. (2). 523 K (as shown in Fig. 5) indicate that the peaks correspond with the crystallization of -Sb phase. The plots are obtained 3 C0 ¼ 1 þ ð2Þ with Kissinger’s method of eq. (1), as shown in Fig. 4. The s 0 8 t activation energies of crystallization for AIST and ST films where 0 is the bulk resistivity, C0 is a material constant were obtained as 0.94 eV and 0.84 eV, respectively. The related to the electron mean free path and the fraction of higher crystallization temperature of AIST films indicates electrons incident on the surface that are scattered.18) The plot that the addition of the elements of Ag and In improved the of sheet resisitivity versus film thickness is shown in stability of the amorphous structure of SbTe films.23) Fig. 2(a), where the sheet resistivity has a inverse function However, the crystallization activation energies of ST and of film thickness. Also, a linear relationship between lnðsÞ AIST films are fairly similar, which reveals that the and lnðt1Þ is recognized in Fig. 2(b). The intercept of the crystallization rate is slightly affected by Ag and In additives. 612 C.-C. Chou, F.-Y. Hung and T.-S. Lui

4000 (a)

445 K cm

Ω 3000 / k s ρ

/ mW 476 K 2000 HF

1000 Heat Flow,

Sheet Resistivity, AIST

0 0 0.05 0.1 0.15 0.2 0.25 0.3 Thickness, t / cm 350 400 450 500 550 600 650 700 4000 Temperature, T / K (b) Fig. 3 Diﬀerential scanning calorimeter measurements of ST and AIST ﬁlms at a heating rate of 0.17 Ks1. cm

Ω 3000 / k

s -8.8 ρ

2000 Slope= -9750 -9.2

1000 -1 ) -9.6 ρ × 2 s = 19.96 (t ) + 311 T 2 /

Sheet Resistivity, R =0.85 α

ln( -10 0 0 40 80 120 160 Slope = -10828 -1 -1 Thickness, t / cm -10.4

Fig. 2 Sheet resistance versus film thickness of AIST films: (a) An inverse :ST relationship between sheet resistance and film thickness; (b) Regressive :AIST plot fitting to the formalism of the size-effect, the slope of the straight line -10.8 determined with the least-squares fitting method of eq. (2) at a maximum 0.0019 0.002 0.0021 0.0022 0.0023 coefficient of determination (R2 ¼ 0:85). The slope is 1:996 104 and the Temperature, T -1 / K-1 intercept on the Y-axis is 3:11 105 cm. Fig. 4 The activation energy obtained using regressive plots of lnð=T2Þ versus 1=T for ST and AIST films, where is the heating rate and T is the peak temperatures from the DSC curves at different . The slope 3.3 Electrical properties of AIST and ST films represents the activation energy E over gas constant R. Isothermal annealing was adopted to demonstrate the change in the electrical properties associated with the variation of microstructure. Figure 5 shows that the sheet Figure 6 shows the GI-XRD spectra of the AIST and ST resistance decreases with an increase of annealing temper- films before and after crystallization, where the broad halo ature for AIST and ST films, where the sheet resistance of as- peak at low diffraction angles reveals that the structures of as- deposited film is 3 104 times larger than the film after deposited specimens are amorphous. The profiles of ST and annealing at 533 K. Furthermore, the sheet resistance of the AIST films after annealing at various temperatures are also amorphous AIST film (as-deposited) is approximately twice shown in Fig. 6(a) and 6(b). The ST film remains amorphous as high as that of the amorphous ST film and the ratio is structure at the annealing temperatures below 403 K. Once maintained for specimens annealed at different temperatures. the annealing temperature rises above 413 K, the -Sb phase The transition temperature from a high resistive state appears and the crystallinity significantly increases with an (amorphous state) to a low one (crystalline state) is about increase in annealing temperature. The same phenomenon is 423 K for AIST film and 413 K for ST film. Hence, it seems occurred in AIST films but the initial crystallization temper- that the higher sheet resistance of as-deposited AIST film is ature is 20 K higher than ST films. In Fig. 6(a), there is only attributed to Ag and In addition. the -Sb phase existing in the crystallized ST films after heat Variation of Microstructure and Electrical Conductivity of Amorphous AgInSbTe and SbTe Films during Crystallization 613

10000 (a) :δ−Sb 1000 Ω

/ k (012) 523 K (110) SR 100 (104) (202)

10 433 K

1 413 K

Sheet Resistance, 0.1 : AIST Intensity, / a.u 403 K : ST 0.01

250 300 350 400 450 500 393 K Annealing Temperature, AT / K

Fig. 5 Plots of sheet resistances versus annealing temperatures for AIST as-deposited and ST films after annealing at a given temperature for 1 h. treatment. The same phase is recognized in the crystallized 20 30 40 50 60 AIST film as shown in Fig. 6(b). However, some previous 2 Theta studies have reported that AgSbTe phase can precipitate in 2 :AgSbTe (b) 2 the SbTe film with silver additives after annealing at a δ− 21–25) : Sb temperature above 423 K. Based on the experimental (200) data, the crystalline structure of AgSbTe2 is cubic and that of (012) -Sb phase is rhombohedral, and the lattice parameters of (220) both phases are similar. Consequently, the diffracted peaks of (110) (222) 523 K (104) (202) -Sb phase are not easy to distinguish from those of AgSbTe2 phase. Another literature has also reported that the In atoms can substitute for Sb sites as a solid solution if the In content 453 K is less than 5 at%.7,21,22) According to the results shown in Fig. 6(b), it shows a good agreement with the mentions that no indium compound is found. In summary, AgSbTe2 and - 433 K Sb phases are expected to be formed in AIST film after annealing at 523 K in this study and it is obvious that the Intensity, / a.u drastic change of sheet resistance from a high value to a low 423 K value is due to crystallization of amorphous films, as shown in Figs. 5 and 6. The electronic transportations in chalcogenide glass is 413 K different from the amorphous semiconductors, which is generally attributed to changes occurring in the concentration as-deposited of so-called valence alternation pairs.26,27) The chalcogenide alloy in the polycrystalline state shows a drastic increase in free electron density, which gives rise to a difference in 20 30 40 50 60 resistivity and reflectivity.4,28) That is the reason why the 2 Theta sheet resistance of AIST films can show a drastic decrease from the amorphous to crystalline state. So far, there were Fig. 6 Grazing incidence X-ray diffraction spectra of (a) ST and (b) AIST two chalcogenide films utilizing such electrical property in films after post-heat-treatment at different temperatures. denotes AgSbTe phase and denotes -Sb phase. OUM applications: (1) Ge-Sb-Te ternary alloy system, in 2 which the sheet resistivity of amorphous film is around 103– 104 times compared to that of crystalline one.4,29) (2) Sb-Te amorphous and crystalline states can be positive to the OUM binary alloy system, in which the sheet resistivity of application because more programmed states (various resis- amorphous film is around 30 times higher than that of tive states) can be achieved for higher capacity of memory crystalline one.5) In the present study, the sheet resistance of device.4) amorphous AIST film is around 104 times compared to that of crystalline film, the value of which is close to the ternary 3.4 TEM observation of microstructure alloy system. Larger difference of sheet resistance between TEM observations were carried out for the as-deposited 614 C.-C. Chou, F.-Y. Hung and T.-S. Lui

+ I + II

Fig. 7 TEM images and SAD patterns of (a) as-deposited ST film and (b) Fig. 8 TEM images of (a) ST film and (b) AIST film after annealing at as-deposited AIST film. 523 K for 1 h.

AIST and ST films as shown in Fig. 7, where a random the AIST film after heat treatment possesses a similar atomic modular patterns and selected area diffraction (SAD) morphology with the crystallized ST film but the size of the patterns refer the as-deposited films to amorphous state. Fine crystallites is smaller. In addition to the amorphous and -Sb crystallites (black contrast) are observed in the images of the phases, a secondary crystallite, denoted T, is also observed in films after annealing at 523 K as shown in Fig. 8, and the Fig. 10(a). The FFT diffraction pattern as shown in AIST film seems to have a larger amount of crystallites than Fig. 10(b) shows the structure of the AgSbTe2 phase with a 25) the ST film. In order to identify this crystalline phase, the Fast cubic structure of a zone axis [100] (dð200Þ ¼ 0:303 nm). It Fourier Transformation (FFT) diffraction patterns using high is worth noting that a discontinuous interface boundary exists resolution lattice images were investigated. The FFT pattern between the adjacent -Sb crystallites and this may indicate of the crystallite denoted D in Fig. 9(a) is shown in Fig. 9(b), that each crystalline -Sb can nucleate and grow independ- which reveals that the crystallite D is -Sb phase with a zone ently. The nucleation and growth of -Sb phase can be easy axis of [121](dð012Þ ¼ 0:31 nm) and rhombohedral structure. because the literature has reported that there is little volume The size of a single -Sb crystallite in the crystallized ST film fraction change associated with the formation of a nucleus for is estimated to be larger than 20 nm. A partial amorphous crystallization, and less strain and surface energy need to be structure could still remain in the crystallized ST film, as overcome when a nucleus grows from the amorphous denoted A in Fig. 9(a). The images of Fig. 8(b) shows that matrix.30–32) The crystallization rates of amorphous AIST Variation of Microstructure and Electrical Conductivity of Amorphous AgInSbTe and SbTe Films during Crystallization 615

D (012)

Fig. 9 High resolution TEM image of ST film after annealing at 523 K for Fig. 10 High resolution TEM image of AIST film after annealing at 523 K 1 h: (a) the lattice image and (b) FFT diffraction pattern. A denotes for 1 h: (a) the lattice image and (b) FFT diffraction pattern. A denotes amorphous phase and D denotes -Sb phase. amorphous phase, D denotes -Sb phase and T denotes AgSbTe2 phase. and ST films can be expected to be small and be close to each TEM observation and GI-XRD analysis showed that no other because the same major phase (-Sb) is formed. The indium compounds precipitated in the crystallized AIST similar activation energy obtained in Fig. 4 shows a good films. EDS analysis as shown in Fig. 11 reveals that the In agreement with this. element dissolves in the amorphous and crystalline areas. The crystallization of amorphous chalcogenide films has Previous study has reported that In atoms usually substitute been proven by other researchers to be a process involving for Sb sites in the lattice of the SbTe material.7) Furthermore, nucleation and growth.16,22,30) The secondary phase precipi- the higher content of Ag element in the crystalline areas, as tated in the early stage can be expected to enhance the shown in Table 2 corresponds to the existence of AgSbTe2 heterogeneous nucleation but the higher crystallization crystallites in the crystalline areas, which agrees with the temperature of the AIST film reveals that crystallization identification from the diffraction pattern. may be more difficult for the movement of Ag and In atoms Several factors have been proven to affect the sheet in the amorphous matrix. It is believed that the formation of a resistance, such as size effect, grain boundary effect and 18) small amount of AgSbTe2 phase slightly affected the additives. The size effect has been proven to be a negligible activation energy and thus, crystallization rate of the factor in this study. The average grain size of two crystalline amorphous AIST film. films can be derived by Scherrer’s formula,33) where 15.4 nm 616 C.-C. Chou, F.-Y. Hung and T.-S. Lui

size is small between ST and AIST films. (a) The influence of additives on the higher resistance of amorphous and crystalline AIST films than ST films can be mainly from the In element. This phenomenon can be explained by the reduction of the mobility of carriers after increasing the In concentration for In2Te3-Sb2Te3 solid solution, which is because In atoms occupy the lattice position of Sb and concentration of antisite defects is reduced.7,34) Concerning of the effect of Ag, the addition of Ag was reported to change the local bonding structure from amorphous to crystalline state, which results in a higher Count, / a.u SbL energy needed for crystallization. It means that the amorphous state will be thermally stabilized by the existence of TeL 14) Ag. However, the appearance of the AgSbTe2 phase in the InL crystalline AIST films did not further change the sheet AgL resistance as compared with the crystalline ST films, which indicates the minor effect of the AgSbTe2 phase on the electrical resistivity. 0 200 400 600 800 1000 1200 1400 Energy, E / keV 4. Conclusions

(b) The structures of the as-deposited AIST and ST films are a single amorphous state and their sheet resistances show an extremely high value. The sheet resistance of the as- deposited films is sensitive to film thickness and decreases with increasing film thickness. The sheet resistance of SbTe films with Ag and In additives is twice as high as that of SbTe films before and after crystallization. For AIST films, the sheet resistance ratio of a high resistive state to a low one is approximately 3 104, which results from a drastic decrease in sheet resistance due to crystallization. The difference in the

Count, / a.u sheet resistance of the AIST film can be high enough to be SbL used in OUM fabrication. TeL After annealing at 523 K, -Sb phase was formed in the InL crystallized SbTe films, while AgbSTe2 and -Sb phases AgL were formed in the crystalline AIST film. No indium compound was precipitated in the AIST films with 5.2 at% In content after crystallization, while the additives can cause 0 200 400 600 800 1000 1200 1400 a higher sheet resistance for the films after crystallization. Energy, E / keV The precipitation of secondary AgSbTe2 phases in AIST film resulted in the production of fine -Sb phases. Fig. 11 Energy dispersive spectroscopy analysis of AIST film after annealing at 523 K for 1 h: (a) the amorphous area of point I in Fig. 8(b), Acknowledgement and (b) the crystalline area of point II in Fig. 8(b). This study was financially supported by the National Table 2 The element contents of the selected point I and point II by EDS Science Council of Taiwan for which we are grateful analysis as shown in Fig. 11. The point I and II were selected from the (Contract No. NSC 95-2221-E-006-118). AIST film after annealing at 523 K for 1 h as shown in Fig. 8(b).

Ag In Sb Te REFERENCES Point I 5.3 at% 10 at% 55.2 at% 29.5 at% 1) T. Ohta: The Joint International Symposium on Optical Memory and Point II 6.7 at% 10.8 at% 50 at% 32.4 at% Optical Data Storage, ed. by S. R. Kubota, R. Katayama and D. G. Stinson, (SPIE, 1999) pp. 188–190. 2) H. Horii, J. H. Yi, B. J. Kuh, Y. H. Ha, Y. T. Kim, K. H. Lee, S. O. Park, U. I. Chung and J. T. Moon: Proc. 15th Symposium on Phase Change and 11.4 nm were obtained for ST and AIST films, Optical Information Storage, ed. by T. Ide, (The Society of Phase respectively. The mobile electron scattering by the grain Change Recording, 2003) pp. 37–41. boundary is also correlated to the mean free path, which is 3) T. Ohta, W. Czubatyi, T. Lowrey and D. Strand: Proc. 15th Symposium 18) on Phase Change Optical Information Storage, ed. by T. Ide, (The affected by the grain size. Consequently, the estimated Society of Phase Change Recording, 2003) pp. 42–51. difference of the sheet resistivity resulted from the grain 4) E. Mytilineou, S. R. Ovshinsky, B. Pashmakov, D. Strand and D. effect can be small because the difference of average grain Jablonski: J. Non-Cryst. Solids 352 (2006) 1991–1994. Variation of Microstructure and Electrical Conductivity of Amorphous AgInSbTe and SbTe Films during Crystallization 617

5) B. Liu, Z. Song, S. Feng and B. Chen: Microelectron. Eng. 82 (2005) 20) R. C. Munoz, G. Vidal, G. Kremer, L. Morgan and C. Arenas: J. Phys. 168–174. -Condes. Matter 11 (1999) 299–L307. 6) V. D. Das and N. Soundararajan: J. Appl. Phys. 65 (1989) 2332–2341. 21) H. Iwasaki, M. Harigaya, O. Nonoyama, Y. Kageyama, M. Takahashi, 7) S. N. Dhar and C. F. Desai: Philos. Mag. Lett. 82 (2002) 581–587. K. Yamada, H. Deguchi and Y. Ide: Jpn. J. Appl. Phys. 32 (1993) 5241– 8) M. J. H. Kemper and P. H. Oosting: J. Appl. Phys. 53 (1982) 6214– 5247. 6217. 22) J. Li and F. Gan: Thin Solid Films 402 (2002) 232–236. 9) I. J. M. M. Raaijmakers, A. H. van Ommen and A. H. Reader: J. Appl. 23) Y. M. Chen and P. C. Kuo: IEEE Trans. Magn. 34 (1998) 432–434. Phys. 65 (1989) 3896–3906. 24) H. A. Zayed, A. M. Ibrahim and L. I. Soliman: Vacuum 47 (1996) 49– 10) E. M. Sanchez, E. F. Prokhorov, J. G. Hernandez and A. M. Galvan: 51. Thin Solid Films 471 (2005) 243–247. 25) D. Lakshminarayana: Thin Solid Films 201 (1991) 91–96. 11) W. Palmer and E. E. Marinero: J. Appl. Phys. 61 (1987) 2294–2300. 26) N. F. Mott and E. A. Davis: Electronic Process in Non-crystalline 12) A. K. Pattanaik and A. Srinivasan: Semicond. Sci. Technol. 19 (2004) Materials, (Clavendon Press, Oxford, 1979). 157–161. 27) A. K. Pattanaik and A. Srinivasan: Semicond. Sci. Tech. 19 (2004) 157. 13) K. Tanaka: Encylopedia of Materials: Science and Technology, 28) S. R. Ovshinsky: Phys. Rev. Lett. 21 (1968) 1450. (Elsevier Science, 2001) pp. 1123–1131. 29) T. J. Park, S. Y. Choi and M. J. Kang: Thin Solid Films (2006) to be 14) H. Tashiro, M. Harigaya, Y. Kageyama, K. Ito, M. Shinotsuka, K. Tani, accepted. A. Watada, N. Yiwata, Y. Nakata and S. Emura: Jpn. J. Appl. Phys. 41 30) S. Fujimori, S. Yagi, H. Yamazaki and N. Funakoshi: J. Appl. Phys. 64 (2002) 3758–3759. (1988) 1000–1004. 15) H. E. Kissinger: Analytical Chemistry 29 (1957) 1702–1706. 31) P. Arun, P. Tyagi, A. G. Vedeshwar and V. K. Paliwal: Physica B 307 16) D. Chiang, T. R. Jeng, D. R. Huang, Y. Y. Chang and C. P. Liu: Jpn. J. (2001) 105–110. Appl. Phys. 38 (1999) 1649–1651. 32) C. R. Barrett, W. D. Nix and A. S. Tetelman: The Principles of 17) K. L. Chopra: Thin Film Phenomena, (McGraw Hill, New York, 1969). Engineering Materials, (Prentice Hall, New Jersey, 1973) pp. 163–170. 18) D. Lakshminarayana and R. R. Desai: J. Mater. Sci.-Mater. Electron. 4 33) B. D. Cullity: Elements of X-ray Diﬀraction, (Addison-Wesley, (1993) 183–186. Philippines, 1978) pp. 102. 19) V. Kuckhermann, G. Thummes and H. H. Mende: J. Phys. F: Met. 34) J. Horak, Z. Stary, P. Lostak and J. Pancir: J. Phys. Chem. Solids 49 Phys. 15 (1985) 153–159. (1988) 191.