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Chemical states and optical properties of thermally evaporated Ge–Te and Ge–Sb–Te amorphous thin films
Applied Surface Science 258 (2012) 7406–7411
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Chemical states and optical properties of thermally evaporated Ge–Te and Ge–Sb–Te amorphous thin films S. Kumar, D. Singh, S. Shandhu, R. Thangaraj ∗ Semiconductor Laboratory, Department of Physics, Guru Nanak Dev University Amritsar, India
a r t i c l e
i n f o
Article history: Received 17 November 2011 Received in revised form 23 March 2012 Accepted 9 April 2012 Available online 21 April 2012 Keywords: Chalcogenide Amorphous semiconductors Chemical state XPS
a b s t r a c t Thin amorphous films of Ge22 Sb22 Te56 and Ge50 Te50 have been prepared from their respective polycrystalline bulk on glass substrates by thermal evaporation technique. The amorphous nature of the films was checked with X-ray diffraction studies. Amorphous-to-crystalline transition of the films has been induced by thermal annealing and the structural phases have been identified by X-ray diffraction. The phase transformation temperature of the films was evaluated by temperature dependent sheet resistance measurement. The chemical structure of the amorphous films has been investigated using X-ray photoelectron spectroscopy and the role of Sb in phase change Ge22 Sb22 Te56 film is discussed. Survey and core level (Ge 3d, Te 3d, Te 4d, Sb 3p, Sb 3d, O 1s, C 1s) band spectra has been recorded and analyzed. For optical studies, the transmittance and the reflectance spectra were measured over the wavelength ranges 400–2500 nm using UV–vis–NIR spectroscopy. The optical band gap, refractive index and extinction coefficient are also presented for thermally evaporated amorphous thin films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Chalcogenide glasses, which are typical amorphous semiconductors, have been attracting much attention in the fields of electronics as well as infrared optics because of such peculiar properties as electrical conductivity, IR transmission, and photoinduced changes [1–3]. The year 1955 may be considered as the origin of the field of amorphous materials. It was in 1955 when Kolomiets and his collaborators have shown that amorphous chalcogenide glasses are semiconductors [4]. Generally, they are ptype semiconductors and applicable to such optical, electrical and opto-electrical devices as image sensors, non-volatile memories, photoresist and optical data storage, etc. [4–6]. Also, chalcogenide semiconductors have shown interesting electrical effects involving a sudden change in resistance; they are grouped as switching properties. In a chalcogenide glass memory device, the memory action is the result of a reversible structural change between an amorphous state, which is of high resistance, and a small-grain crystalline state, which is of low resistance [7]. The most commonly used phase-change materials are chalcogenide-based GeTe–Sb2 Te3 and pseudo-binary systems such as Ge1 Sb4 Te7 , Ge1 Sb2 Te4 , and Ge2 Sb2 Te5 . Among, Ge2 Sb2 Te5 is one of the most important phase change materials, since its good particular performance had led
∗ Corresponding author. Tel.: +91 183 2258802/3165; fax: +91 183 2258820. E-mail addresses: [email protected], [email protected] (R. Thangaraj). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.050
to promising applications with respect to digital video disk random access memory (DVD-RW) and Compact Disc ReWritable (CD-RW) [6,8,9]. There has been much recent research attempting to prove the feasibility of non-volatile memory devices based on these phase-change materials [10]. Most of this research has focused mainly on device fabrication, material formation and the macroscopic (electrical, optical and structural) properties of such materials. However, the electronic structure and chemical state to resolve the phase-change mechanism are not fully understood. The study of electronic structure and chemical state can offer the new materials and fabrication conditions, allowing for effective performance of memory devices. X-ray photoelectron spectroscopy (XPS) is a useful tool for the analysis of electronic structures and chemical states [11–13]. It is noted that, the addition of antimony to the Ge–Te binary system increase the practicality of commercial-scale production by easing the requirement for precision in composition of the deposited film without deleterious effects in the speed or stability [5,14]. On the contrary, if antimony is substituted for germanium, it helps to relax the interatomic stresses between Te and Ge and allowing the formation of a more symmetrical (fcc) structure [14]. The objective of present work is to compare the chemical environment and optical properties of thermally evaporated Ge–Sb–Te and Ge–Te systems, and to investigate the role of Sb in phase change Ge–Sb–Te amorphous system. This paper reports the chemical states, optical properties and phase transition of thermally evaporated amorphous thin films of Ge–Te and Ge–Sb–Te by means of X-ray diffraction (XRD), XPS, UV–vis–NIR spectroscopy and temperature dependent sheet resistance measurements.
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2. Experimental The poly-crystalline bulks of Ge22 Sb22 Te56 (GST) and Ge50 Te50 (GT) were synthesized from 99.999% pure elemental Ge, Sb and Te sealed in quartz ampoule (length ∼10 cm, internal diameter ∼6 mm) evacuated to ∼10−5 Torr, heated in a furnace at 1273 K with constant shaking. After 48 h, the furnace was switched off with the ampoule inside, to make the poly-crystalline ingot for the deposition of highly stoichiometric films utilizing the sharp melting points of the crystalline form of the materials. The as-prepared bulk samples of above compositions were used for depositing thin films on cleaned glass slides by thermal evaporation technique using HINDHIVAC coating unit (model no. 12A4D). The deposition was carried out at pressure ∼10−5 Torr and thickness of the films was kept ∼250 nm. The deposited thin films were kept inside vacuum chamber for 24 h for attaining metastable equilibrium as suggested by Abkowitz [15]. The actual composition of the as-deposited films was checked by electron probe micro-analyzer (EPMA) (CAMECA SX 100) with 15 keV electrons having beam size 10 m using a wavelength-dispersive spectrometer (WDS) and found close to the original composition with ±4–5% variation. XPS was carried out to examine the chemical environment of thermally evaporated thin films. In XPS, we performed the survey scan from binding energy (B.E.) ranging from 0 to 800 eV and core level band spectra of Ge 3d, Sb 3p, Sb 3d, Te 3d, Te 4d, O 1s and C 1s. The satisfactory levels of the signals were obtained in the survey scan. The X-ray photoelectron spectrum was recorded in an ESCA instrument (VSW) using Al K␣ radiation at 2 × 10−9 Torr base vacuum. To remove surface contamination the samples were sputtered with Ar+ ions with energy of 0.5 keV and current density of Ar+ approximately 10 A/cm2 having pit size 1 cm2 . The B.E. scale was calibrated by assigning the main C 1s peak at 285.0 eV. Post-deposited Al electrodes in a square geometry (5.0 mm) were used for temperature dependent sheet resistance measurements, carried out in the existing vacuum. The annealing of films at phase transition temperatures for 5 min was performed using the substrate heater with a temperature controller under the existing vacuum conditions (∼10−5 mbar). The amorphous nature/crystalline phases were identified using XRD (Bruker D8 focus) with Cu as the anode material, operated at 35 mA and 40 kV. The optical studies of amorphous films was done with transmittance (T) and reflectance (R) data measured at room temperature using UV–vis–NIR spectrophotometer (PerkinElmer 950) in the wavelength range 400–2500 nm. The thickness of films was measured using surface profiler (KLA Tencor P15).
Fig. 1. X-ray scan of as-deposited and thermally annealed GST and GT films along with respective 2 value.
temperature of the thermally evaporated GST and GT films at the heating rate 2 ◦ C/min. The sheet resistance of as-deposited GST and GT films is 148 M/ and 0.85 M/, respectively and these corresponds to the resistivity () values of 3700 cm and 22 cm, respectively. The sheet resistance decreases continuously in the lower temperature range until an abrupt drop appears at the temperature of 150 ◦ C and 180 ◦ C for GST and GT films, respectively (Fig. 2), which means that the phase transition of amorphous to crystal phase occurred and the temperature is called crystallization temperature (Tc ). The obtained Tc of the films is in good agreement with the values reported in the literature [17–19]. The reduction at the sheet resistance upon the phase transition is more than four orders of magnitude.
3. Results and discussion 3.1. X-ray diffraction studies Fig. 1 shows the XRD scan for as-deposited and thermally annealed GST and GT films. Absence of any sharp peak in the X-ray scan of the films confirms the amorphous nature of the as-deposited films. The GST and GT films are annealed at their respective phase transition temperature, which was evaluated from temperature dependent sheet resistance measurement discussed in the next section. The diffraction peaks for crystallized GT film are indexed with GeTe (47–1079) phases (JCPDS database, 1997) and for crystallized GST film; the fcc phases of Ge2 Sb2 Te5 are identified [16] as shown in Fig. 1. 3.2. Sheet resistance measurements The structural phase transition of the films is studied by using XRD and temperature dependent sheet resistance (Rs ) measurements. Fig. 2 shows the relationship between Rs with annealing
Fig. 2. Relationship between sheet resistance (Rs ) and annealing temperature of the as-deposited GeTe and Ge2 Sb2 Te5 films.
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Fig. 3. Survey scans of thermally evaporated thin films. Fig. 4. Core level spectra of Ge 3d and Te 4d.
3.3. X-ray photoelectron spectroscopy (XPS) 3.3.1. Survey scan XPS is a surface analysis technique with about 65% of the signal originating from the outermost ∼30 A˚ of the surface [20]. The binding states of the films were examined by using XPS analysis. Fig. 3 shows survey scan of GST and GT films from the binding energy ranging from 0 to 800 eV. The satisfactory level of the signals for Ge, Sb, Te and Ge and Te in GST and GT films, respectively are obtained in the survey scan. The films are contaminated with O during transportation of the films from deposition chamber to the XPS analysis chamber. As binding energies of the O 1s (531.8 eV) peak and Sb 3d (528.2 eV) peak are approximately same, we cannot find separate signal of O 1s peak in GST films while O 1s peak is detected in GT films. The rise in the Te 4d, Te 3d peak intensity of GT films is found due to increase in the Te % on the surface. Also, Ge 3p and Ge auger lines are also observed in the GT spectra (Fig. 3). Furthermore, C 1s can be detected due to surface contamination with hydrocarbon after transportation in air and was used as calibration of the data. The XPS spectra of the alloy films consist of two species; one is an oxidized species coming from the surface, and the other is a metallic species coming from underneath the alloy surface. Doublet of Te 3d spectra in GT film and Sb, Te doublet of 3d spectra in GST film are identified using the reference spectra in the handbook [21] (Fig. 3). 3.3.2. Core level band spectra The chemical environment of the elements present in the films has been investigated by taking the core level band spectra of Ge 3d, Te 3d, Te 4d, Sb 3p, Sb 3d and C 1s as shown in Figs. 4–6. All results were recalculated to a binding energy of C 1s, which is 285.0 eV. Fig. 4 shows the core level spectra of Ge 3d and Te 4d in the binding energy range 25–50 eV for amorphous GST and GT films. Peak at 29.4 eV is identified as Ge 3d due to Ge Ge homopolar bond in GT film [21] and due to the existence of native oxide, the oxidized peak of GeO2 (32.4 eV) [22] at higher binding energy appeared (Fig. 4). But, the Ge 3d core level spectra of GST film, the first peak appear at 31.5 eV and identified as due to the Ge Te bond [22]. A broad second peak at 33.7 eV is due to formation of Sb2 Te3 [22] along with oxidized peak of Ge. This means that in the GST film, wrong homopolar bonds i.e. Ge Ge (29.4 eV) and Sb Sb (31.7 eV) not formed. The Te 4d peaks in the GT spectrum are appeared at 40.1 eV which is of Te Te bond [21] but a negative shift (0.8 eV) is found in the Te 4d spectrum of GST film (Fig. 4) and the negative
shift may due to formation of Te Ge or Te Sb bonds, which again suggest that the incorporation of Sb in GST suppress the formation of wrong homopolar bonds. Fig. 5 shows the core level spectra of Te 3d in the binding energy range 565–590 eV for amorphous GST and GT films. The Te 3d peaks in the GT spectrum are identified as Te 3d3/2 (573.2 eV) and Te 3d5/2 (583.7 eV) which are at higher binding energy of pure Te 3d3/2 (572.9 eV) and Te 3d5/2 (583.3 eV) [8] due to the formation of Ge Te bonds (Fig. 5). From XPS spectrum of Te 3d for GST, the doublet (Te 3d3/2 572.7 eV and Te 3d5/2 583.1 eV) appears at lower binding energy than for GT film and also small negative binding energy shift than of pure Te spectrum [21], that is due to change in the chemical environment on Sb incorporation in the GST system. Oxide of Te also has been identified as shown in Fig. 5. The core level spectra of Sb (3p & 3d) for GST also have been investigated and local bonding of Sb with other elements (Ge and Te) is discussed (Fig. 6). In the spectra of Sb 3p, the doublet occurs at Sb 3p3/2 (769.3 eV) and Sb 3p1/2 (815.5 eV), which is at higher
Fig. 5. Core level spectra of Te 3d.
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Fig. 7. Spectral variation of the transmittance (T) and reflectance (R) (in the inset) for amorphous films.
minima of the transmission spectra confirms the optical homogeneity of the deposited films. Fig. 6. Core level spectra of Sb 3p and Sb 3d of GST film.
value (positive shift 2.9 eV) than the elementary Sb 3p spectra (Sb 3p3/2 766.4 eV, Sb 3p1/2 812.6 eV) [21]. The core level spectrum for Sb 3d is also shown in the Fig. 6. The doublet of Sb 3d spectra in GST also shifted positive as core level spectrum of elemental Sb 3d occurs at Sb 3d5/2 (528.2 eV) and Sb 3d3/2 (537.6 eV) [21] as shown in the Fig. 6. Also the peaks of Sb2 O5 [22] are also identified from the spectra (Fig. 6). The positive binding energy shift from both Sb (3d and 3p) of GST spectra, it is evaluated that the Sb in GST do not form wrong homopolar Sb Sb bond, but rather form bond with other elements of GST and also the role of Sb in GST system is to suppresses the formation of wrong homopolar bond which likely to be formed in GT system. 3.4. Optical properties Fig. 7 shows the transmittance spectra (T) and inset of the Fig. 7 shows the reflection spectra (R) as functions of wavelength (400–2500 nm) for the as-deposited amorphous films. Both the films are transparent in a wide spectral range (mainly in NIR region) as shown by the T and R measurements, indicating a fairly smooth surface and relatively good homogeneity. In the weak absorption region, interference fringes were apparent, indicating that the interfaces, air/film and film/glass were flat and parallel. The observed strange absorption edge extended from 600 nm up to nearly 1200 nm, where the interference effects were suppressed almost completely due to a well-defined band edge (Fig. 7). The GT films are more transparent than GST films, as incorporation of Sb decrease the transparency and red shift of the fringes is observed for GST film. Such a shift is a consequence of the decrease of the corresponding optical band gap in GST films as calculated in the next section (Fig. 8). Also, an increase in the amplitude of the optical interference fringes, resulting from Sb-incorporation, can be seen in Fig. 7, this clearly reflects an increase in the linear refractive index of the GST films (inset of Fig. 9) and GST films are more reflective than GT films (inset of Fig. 7) as Sb increase the reflectivity of the films. Moreover, the appearance of interference maxima of the reflectance spectra at the same wavelength positions of the
3.4.1. Optical band gap and optical constants Optical energy gap (Eg ) and optical constants of amorphous GST and GT thin films was also investigated. The optical absorption (˛) was computed from experimental measured values of R and T according to the Eq. (1). T = (1 − R)2 exp(−˛d)
(1)
where d is the thickness of the film in centimeter [23,24]. Three distinct regions have been generally recognized in absorption edge spectrum of an amorphous semiconductor. They are [25,26] (1) the weak-absorption tail (˛ < 1 cm−1 ) which originates from defects and impurities; (2) the exponential-edge region (1 < ˛ < 104 cm−1 ) which is strongly related to the randomness of structure; and (3) the high-absorption region (˛ > 104 cm−1 ) which
Fig. 8. The variation of (˛h)1/2 vs h for amorphous GT and GST films and inset shows the variation of absorption coefficient (˛) with photon energy.
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along with hetropolar bonds but in GST film less formation of homopolar bond is observed and it is concluded that the role of Sb is to suppress the formation of homopolar bonds. The positive binding energy shift of Sb (3p and 3d) spectra of GST film, further confirm that the Sb not in the elemental form, form bond with Ge and Te and also inhibit the formation of Ge Ge, Te Te bond formation. From the optical studies it was found that the band gap decreases while reflectivity and refractive index increase for GST than GT films. The GST film was more absorptive than GT film for the whole studied wavelength range. The sheet resistance and amorphous-to-crystalline phase transition temperature has been decreased on Sb incorporation in the amorphous GT system. Acknowledgements
Fig. 9. Spectral variation of the extinction coefficient (k) and refractive index (n) (in the inset) of the amorphous films.
The authors are grateful to CSIR for the sanctioning of SRF (NET) (09/254(0175)/2008-EMR-1) to one of the authors and for funding a project (03/1140/09/EMR-11). The authors also wish to thank Dr. T. Shripathi (Scientist G) and Mr. U. Deshpande, UGC-DAE Consortium for Scientific Research Indore, India for providing access to the XPS and UV–vis–NIR facilities. References
determines the Eg . In the high absorption region, the expression relating the ˛ to Eg as described in the Eq. (2). ˛hv = B(hv − Eg )
m
(2)
where B is the quality factor depending on the transition probability, h is the photon energy, and the power term m is an appropriate selected index, which depends on the nature of the electronic transition and can be assumed to have a value of either 1/2 or 2 for the direct transition and indirect transition, respectively. Many papers have shown m = 2 acceptable for amorphous chalcogenide semiconductors, including Ge, GeTe, and GeSbTe [27]. Therefore, the value of Eg is obtained from the intercept on the energy axis of the plot of (˛h)1/2 vs h, as shown in Fig. 8. The optical band gap (Eg ) of GT films is approximately 0.81 eV (Fig. 8) which decreases for GST film to 0.72 eV [8,28] (Fig. 8). The decrease in the band gap with increasing Sb composition is also reported [29]. The optical constants, the refractive index (n) and extinction coefficient (k) of the thin films can be calculated from their reflectance (R) and transmittance (T) spectra using simple approximations [30]. If the substrate is non-absorbing and its n is smaller than that of the transparent film, the R, gives the thin films refractive as described in Eq. (3) [24]. R=
(n − 1)2 (n + 1)2
(3)
The absorption coefficient (˛) is related to the k by [23]: ˛=
4k
(4)
where is the free space wavelength. Thus, the n and k can be calculated from the measured values of R and T. It is noted that films has high values of refractive index, which are useful in the industry of reflectors [31]. The n and k values are higher in GST than GT films as shown in Fig. 9. At 800 nm, the optical constants have values for GST (n = 6.63 and k = 1.85) and for GT (n = 6.50 and k = 1.29). The values obtained for n and k are in accordance with the reported values [32–34] for both the systems. 4. Summary The optical properties, chemical states and phase transition of thermally evaporated thin amorphous GST and GT films has been evaluated and discussed. In GT film, the wrong homopolar formed
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Chemical states and optical properties of thermally evaporated Ge–Te and Ge–Sb–Te amorphous thin films S. Kumar, D. Singh, S. Shandhu, R. Thangaraj ∗ Semiconductor Laboratory, Department of Physics, Guru Nanak Dev University Amritsar, India
a r t i c l e
i n f o
Article history: Received 17 November 2011 Received in revised form 23 March 2012 Accepted 9 April 2012 Available online 21 April 2012 Keywords: Chalcogenide Amorphous semiconductors Chemical state XPS
a b s t r a c t Thin amorphous films of Ge22 Sb22 Te56 and Ge50 Te50 have been prepared from their respective polycrystalline bulk on glass substrates by thermal evaporation technique. The amorphous nature of the films was checked with X-ray diffraction studies. Amorphous-to-crystalline transition of the films has been induced by thermal annealing and the structural phases have been identified by X-ray diffraction. The phase transformation temperature of the films was evaluated by temperature dependent sheet resistance measurement. The chemical structure of the amorphous films has been investigated using X-ray photoelectron spectroscopy and the role of Sb in phase change Ge22 Sb22 Te56 film is discussed. Survey and core level (Ge 3d, Te 3d, Te 4d, Sb 3p, Sb 3d, O 1s, C 1s) band spectra has been recorded and analyzed. For optical studies, the transmittance and the reflectance spectra were measured over the wavelength ranges 400–2500 nm using UV–vis–NIR spectroscopy. The optical band gap, refractive index and extinction coefficient are also presented for thermally evaporated amorphous thin films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Chalcogenide glasses, which are typical amorphous semiconductors, have been attracting much attention in the fields of electronics as well as infrared optics because of such peculiar properties as electrical conductivity, IR transmission, and photoinduced changes [1–3]. The year 1955 may be considered as the origin of the field of amorphous materials. It was in 1955 when Kolomiets and his collaborators have shown that amorphous chalcogenide glasses are semiconductors [4]. Generally, they are ptype semiconductors and applicable to such optical, electrical and opto-electrical devices as image sensors, non-volatile memories, photoresist and optical data storage, etc. [4–6]. Also, chalcogenide semiconductors have shown interesting electrical effects involving a sudden change in resistance; they are grouped as switching properties. In a chalcogenide glass memory device, the memory action is the result of a reversible structural change between an amorphous state, which is of high resistance, and a small-grain crystalline state, which is of low resistance [7]. The most commonly used phase-change materials are chalcogenide-based GeTe–Sb2 Te3 and pseudo-binary systems such as Ge1 Sb4 Te7 , Ge1 Sb2 Te4 , and Ge2 Sb2 Te5 . Among, Ge2 Sb2 Te5 is one of the most important phase change materials, since its good particular performance had led
∗ Corresponding author. Tel.: +91 183 2258802/3165; fax: +91 183 2258820. E-mail addresses: [email protected], [email protected] (R. Thangaraj). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.050
to promising applications with respect to digital video disk random access memory (DVD-RW) and Compact Disc ReWritable (CD-RW) [6,8,9]. There has been much recent research attempting to prove the feasibility of non-volatile memory devices based on these phase-change materials [10]. Most of this research has focused mainly on device fabrication, material formation and the macroscopic (electrical, optical and structural) properties of such materials. However, the electronic structure and chemical state to resolve the phase-change mechanism are not fully understood. The study of electronic structure and chemical state can offer the new materials and fabrication conditions, allowing for effective performance of memory devices. X-ray photoelectron spectroscopy (XPS) is a useful tool for the analysis of electronic structures and chemical states [11–13]. It is noted that, the addition of antimony to the Ge–Te binary system increase the practicality of commercial-scale production by easing the requirement for precision in composition of the deposited film without deleterious effects in the speed or stability [5,14]. On the contrary, if antimony is substituted for germanium, it helps to relax the interatomic stresses between Te and Ge and allowing the formation of a more symmetrical (fcc) structure [14]. The objective of present work is to compare the chemical environment and optical properties of thermally evaporated Ge–Sb–Te and Ge–Te systems, and to investigate the role of Sb in phase change Ge–Sb–Te amorphous system. This paper reports the chemical states, optical properties and phase transition of thermally evaporated amorphous thin films of Ge–Te and Ge–Sb–Te by means of X-ray diffraction (XRD), XPS, UV–vis–NIR spectroscopy and temperature dependent sheet resistance measurements.
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2. Experimental The poly-crystalline bulks of Ge22 Sb22 Te56 (GST) and Ge50 Te50 (GT) were synthesized from 99.999% pure elemental Ge, Sb and Te sealed in quartz ampoule (length ∼10 cm, internal diameter ∼6 mm) evacuated to ∼10−5 Torr, heated in a furnace at 1273 K with constant shaking. After 48 h, the furnace was switched off with the ampoule inside, to make the poly-crystalline ingot for the deposition of highly stoichiometric films utilizing the sharp melting points of the crystalline form of the materials. The as-prepared bulk samples of above compositions were used for depositing thin films on cleaned glass slides by thermal evaporation technique using HINDHIVAC coating unit (model no. 12A4D). The deposition was carried out at pressure ∼10−5 Torr and thickness of the films was kept ∼250 nm. The deposited thin films were kept inside vacuum chamber for 24 h for attaining metastable equilibrium as suggested by Abkowitz [15]. The actual composition of the as-deposited films was checked by electron probe micro-analyzer (EPMA) (CAMECA SX 100) with 15 keV electrons having beam size 10 m using a wavelength-dispersive spectrometer (WDS) and found close to the original composition with ±4–5% variation. XPS was carried out to examine the chemical environment of thermally evaporated thin films. In XPS, we performed the survey scan from binding energy (B.E.) ranging from 0 to 800 eV and core level band spectra of Ge 3d, Sb 3p, Sb 3d, Te 3d, Te 4d, O 1s and C 1s. The satisfactory levels of the signals were obtained in the survey scan. The X-ray photoelectron spectrum was recorded in an ESCA instrument (VSW) using Al K␣ radiation at 2 × 10−9 Torr base vacuum. To remove surface contamination the samples were sputtered with Ar+ ions with energy of 0.5 keV and current density of Ar+ approximately 10 A/cm2 having pit size 1 cm2 . The B.E. scale was calibrated by assigning the main C 1s peak at 285.0 eV. Post-deposited Al electrodes in a square geometry (5.0 mm) were used for temperature dependent sheet resistance measurements, carried out in the existing vacuum. The annealing of films at phase transition temperatures for 5 min was performed using the substrate heater with a temperature controller under the existing vacuum conditions (∼10−5 mbar). The amorphous nature/crystalline phases were identified using XRD (Bruker D8 focus) with Cu as the anode material, operated at 35 mA and 40 kV. The optical studies of amorphous films was done with transmittance (T) and reflectance (R) data measured at room temperature using UV–vis–NIR spectrophotometer (PerkinElmer 950) in the wavelength range 400–2500 nm. The thickness of films was measured using surface profiler (KLA Tencor P15).
Fig. 1. X-ray scan of as-deposited and thermally annealed GST and GT films along with respective 2 value.
temperature of the thermally evaporated GST and GT films at the heating rate 2 ◦ C/min. The sheet resistance of as-deposited GST and GT films is 148 M/ and 0.85 M/, respectively and these corresponds to the resistivity () values of 3700 cm and 22 cm, respectively. The sheet resistance decreases continuously in the lower temperature range until an abrupt drop appears at the temperature of 150 ◦ C and 180 ◦ C for GST and GT films, respectively (Fig. 2), which means that the phase transition of amorphous to crystal phase occurred and the temperature is called crystallization temperature (Tc ). The obtained Tc of the films is in good agreement with the values reported in the literature [17–19]. The reduction at the sheet resistance upon the phase transition is more than four orders of magnitude.
3. Results and discussion 3.1. X-ray diffraction studies Fig. 1 shows the XRD scan for as-deposited and thermally annealed GST and GT films. Absence of any sharp peak in the X-ray scan of the films confirms the amorphous nature of the as-deposited films. The GST and GT films are annealed at their respective phase transition temperature, which was evaluated from temperature dependent sheet resistance measurement discussed in the next section. The diffraction peaks for crystallized GT film are indexed with GeTe (47–1079) phases (JCPDS database, 1997) and for crystallized GST film; the fcc phases of Ge2 Sb2 Te5 are identified [16] as shown in Fig. 1. 3.2. Sheet resistance measurements The structural phase transition of the films is studied by using XRD and temperature dependent sheet resistance (Rs ) measurements. Fig. 2 shows the relationship between Rs with annealing
Fig. 2. Relationship between sheet resistance (Rs ) and annealing temperature of the as-deposited GeTe and Ge2 Sb2 Te5 films.
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Fig. 3. Survey scans of thermally evaporated thin films. Fig. 4. Core level spectra of Ge 3d and Te 4d.
3.3. X-ray photoelectron spectroscopy (XPS) 3.3.1. Survey scan XPS is a surface analysis technique with about 65% of the signal originating from the outermost ∼30 A˚ of the surface [20]. The binding states of the films were examined by using XPS analysis. Fig. 3 shows survey scan of GST and GT films from the binding energy ranging from 0 to 800 eV. The satisfactory level of the signals for Ge, Sb, Te and Ge and Te in GST and GT films, respectively are obtained in the survey scan. The films are contaminated with O during transportation of the films from deposition chamber to the XPS analysis chamber. As binding energies of the O 1s (531.8 eV) peak and Sb 3d (528.2 eV) peak are approximately same, we cannot find separate signal of O 1s peak in GST films while O 1s peak is detected in GT films. The rise in the Te 4d, Te 3d peak intensity of GT films is found due to increase in the Te % on the surface. Also, Ge 3p and Ge auger lines are also observed in the GT spectra (Fig. 3). Furthermore, C 1s can be detected due to surface contamination with hydrocarbon after transportation in air and was used as calibration of the data. The XPS spectra of the alloy films consist of two species; one is an oxidized species coming from the surface, and the other is a metallic species coming from underneath the alloy surface. Doublet of Te 3d spectra in GT film and Sb, Te doublet of 3d spectra in GST film are identified using the reference spectra in the handbook [21] (Fig. 3). 3.3.2. Core level band spectra The chemical environment of the elements present in the films has been investigated by taking the core level band spectra of Ge 3d, Te 3d, Te 4d, Sb 3p, Sb 3d and C 1s as shown in Figs. 4–6. All results were recalculated to a binding energy of C 1s, which is 285.0 eV. Fig. 4 shows the core level spectra of Ge 3d and Te 4d in the binding energy range 25–50 eV for amorphous GST and GT films. Peak at 29.4 eV is identified as Ge 3d due to Ge Ge homopolar bond in GT film [21] and due to the existence of native oxide, the oxidized peak of GeO2 (32.4 eV) [22] at higher binding energy appeared (Fig. 4). But, the Ge 3d core level spectra of GST film, the first peak appear at 31.5 eV and identified as due to the Ge Te bond [22]. A broad second peak at 33.7 eV is due to formation of Sb2 Te3 [22] along with oxidized peak of Ge. This means that in the GST film, wrong homopolar bonds i.e. Ge Ge (29.4 eV) and Sb Sb (31.7 eV) not formed. The Te 4d peaks in the GT spectrum are appeared at 40.1 eV which is of Te Te bond [21] but a negative shift (0.8 eV) is found in the Te 4d spectrum of GST film (Fig. 4) and the negative
shift may due to formation of Te Ge or Te Sb bonds, which again suggest that the incorporation of Sb in GST suppress the formation of wrong homopolar bonds. Fig. 5 shows the core level spectra of Te 3d in the binding energy range 565–590 eV for amorphous GST and GT films. The Te 3d peaks in the GT spectrum are identified as Te 3d3/2 (573.2 eV) and Te 3d5/2 (583.7 eV) which are at higher binding energy of pure Te 3d3/2 (572.9 eV) and Te 3d5/2 (583.3 eV) [8] due to the formation of Ge Te bonds (Fig. 5). From XPS spectrum of Te 3d for GST, the doublet (Te 3d3/2 572.7 eV and Te 3d5/2 583.1 eV) appears at lower binding energy than for GT film and also small negative binding energy shift than of pure Te spectrum [21], that is due to change in the chemical environment on Sb incorporation in the GST system. Oxide of Te also has been identified as shown in Fig. 5. The core level spectra of Sb (3p & 3d) for GST also have been investigated and local bonding of Sb with other elements (Ge and Te) is discussed (Fig. 6). In the spectra of Sb 3p, the doublet occurs at Sb 3p3/2 (769.3 eV) and Sb 3p1/2 (815.5 eV), which is at higher
Fig. 5. Core level spectra of Te 3d.
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Fig. 7. Spectral variation of the transmittance (T) and reflectance (R) (in the inset) for amorphous films.
minima of the transmission spectra confirms the optical homogeneity of the deposited films. Fig. 6. Core level spectra of Sb 3p and Sb 3d of GST film.
value (positive shift 2.9 eV) than the elementary Sb 3p spectra (Sb 3p3/2 766.4 eV, Sb 3p1/2 812.6 eV) [21]. The core level spectrum for Sb 3d is also shown in the Fig. 6. The doublet of Sb 3d spectra in GST also shifted positive as core level spectrum of elemental Sb 3d occurs at Sb 3d5/2 (528.2 eV) and Sb 3d3/2 (537.6 eV) [21] as shown in the Fig. 6. Also the peaks of Sb2 O5 [22] are also identified from the spectra (Fig. 6). The positive binding energy shift from both Sb (3d and 3p) of GST spectra, it is evaluated that the Sb in GST do not form wrong homopolar Sb Sb bond, but rather form bond with other elements of GST and also the role of Sb in GST system is to suppresses the formation of wrong homopolar bond which likely to be formed in GT system. 3.4. Optical properties Fig. 7 shows the transmittance spectra (T) and inset of the Fig. 7 shows the reflection spectra (R) as functions of wavelength (400–2500 nm) for the as-deposited amorphous films. Both the films are transparent in a wide spectral range (mainly in NIR region) as shown by the T and R measurements, indicating a fairly smooth surface and relatively good homogeneity. In the weak absorption region, interference fringes were apparent, indicating that the interfaces, air/film and film/glass were flat and parallel. The observed strange absorption edge extended from 600 nm up to nearly 1200 nm, where the interference effects were suppressed almost completely due to a well-defined band edge (Fig. 7). The GT films are more transparent than GST films, as incorporation of Sb decrease the transparency and red shift of the fringes is observed for GST film. Such a shift is a consequence of the decrease of the corresponding optical band gap in GST films as calculated in the next section (Fig. 8). Also, an increase in the amplitude of the optical interference fringes, resulting from Sb-incorporation, can be seen in Fig. 7, this clearly reflects an increase in the linear refractive index of the GST films (inset of Fig. 9) and GST films are more reflective than GT films (inset of Fig. 7) as Sb increase the reflectivity of the films. Moreover, the appearance of interference maxima of the reflectance spectra at the same wavelength positions of the
3.4.1. Optical band gap and optical constants Optical energy gap (Eg ) and optical constants of amorphous GST and GT thin films was also investigated. The optical absorption (˛) was computed from experimental measured values of R and T according to the Eq. (1). T = (1 − R)2 exp(−˛d)
(1)
where d is the thickness of the film in centimeter [23,24]. Three distinct regions have been generally recognized in absorption edge spectrum of an amorphous semiconductor. They are [25,26] (1) the weak-absorption tail (˛ < 1 cm−1 ) which originates from defects and impurities; (2) the exponential-edge region (1 < ˛ < 104 cm−1 ) which is strongly related to the randomness of structure; and (3) the high-absorption region (˛ > 104 cm−1 ) which
Fig. 8. The variation of (˛h)1/2 vs h for amorphous GT and GST films and inset shows the variation of absorption coefficient (˛) with photon energy.
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along with hetropolar bonds but in GST film less formation of homopolar bond is observed and it is concluded that the role of Sb is to suppress the formation of homopolar bonds. The positive binding energy shift of Sb (3p and 3d) spectra of GST film, further confirm that the Sb not in the elemental form, form bond with Ge and Te and also inhibit the formation of Ge Ge, Te Te bond formation. From the optical studies it was found that the band gap decreases while reflectivity and refractive index increase for GST than GT films. The GST film was more absorptive than GT film for the whole studied wavelength range. The sheet resistance and amorphous-to-crystalline phase transition temperature has been decreased on Sb incorporation in the amorphous GT system. Acknowledgements
Fig. 9. Spectral variation of the extinction coefficient (k) and refractive index (n) (in the inset) of the amorphous films.
The authors are grateful to CSIR for the sanctioning of SRF (NET) (09/254(0175)/2008-EMR-1) to one of the authors and for funding a project (03/1140/09/EMR-11). The authors also wish to thank Dr. T. Shripathi (Scientist G) and Mr. U. Deshpande, UGC-DAE Consortium for Scientific Research Indore, India for providing access to the XPS and UV–vis–NIR facilities. References
determines the Eg . In the high absorption region, the expression relating the ˛ to Eg as described in the Eq. (2). ˛hv = B(hv − Eg )
m
(2)
where B is the quality factor depending on the transition probability, h is the photon energy, and the power term m is an appropriate selected index, which depends on the nature of the electronic transition and can be assumed to have a value of either 1/2 or 2 for the direct transition and indirect transition, respectively. Many papers have shown m = 2 acceptable for amorphous chalcogenide semiconductors, including Ge, GeTe, and GeSbTe [27]. Therefore, the value of Eg is obtained from the intercept on the energy axis of the plot of (˛h)1/2 vs h, as shown in Fig. 8. The optical band gap (Eg ) of GT films is approximately 0.81 eV (Fig. 8) which decreases for GST film to 0.72 eV [8,28] (Fig. 8). The decrease in the band gap with increasing Sb composition is also reported [29]. The optical constants, the refractive index (n) and extinction coefficient (k) of the thin films can be calculated from their reflectance (R) and transmittance (T) spectra using simple approximations [30]. If the substrate is non-absorbing and its n is smaller than that of the transparent film, the R, gives the thin films refractive as described in Eq. (3) [24]. R=
(n − 1)2 (n + 1)2
(3)
The absorption coefficient (˛) is related to the k by [23]: ˛=
4k
(4)
where is the free space wavelength. Thus, the n and k can be calculated from the measured values of R and T. It is noted that films has high values of refractive index, which are useful in the industry of reflectors [31]. The n and k values are higher in GST than GT films as shown in Fig. 9. At 800 nm, the optical constants have values for GST (n = 6.63 and k = 1.85) and for GT (n = 6.50 and k = 1.29). The values obtained for n and k are in accordance with the reported values [32–34] for both the systems. 4. Summary The optical properties, chemical states and phase transition of thermally evaporated thin amorphous GST and GT films has been evaluated and discussed. In GT film, the wrong homopolar formed
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