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Modification of Cd1−xMnxTe crystal surface layers by nanopulsed laser irradiation
Applied Surface Science 254 (2007) 993–996 www.elsevier.com/locate/apsusc
Modification of Cd1 xMnxTe crystal surface layers by nanopulsed laser irradiation E.I. Gatskevich a, G.D. Ivlev a,*, A.I. Rarenko b, A.I. Savchuk b, V.N. Strebegev b, Z.I. Zakharuk b a
Institute of Electronics, National Academy of Sciences, Minsk, Belarus b Chernivtsi National University, Chernivtsi, Ukraine
Received 7 June 2007; received in revised form 23 July 2007; accepted 7 August 2007 Available online 14 August 2007
Abstract The surface modification of Cd1 xMnxTe (x = 0–0.3) crystal wafers under pulsed laser irradiation has been studied. The samples were irradiated by a Q-switched ruby laser with pulse duration of 80 ns. Optical diagnostics of laser-induced thermal processes were carried out by means of timeresolved reflectivity measurements at wavelengths 0.53 and 1.06 mm. Laser irradiation energy density, E varied in the range of 0.1–0.6 J/cm2. Morphology of irradiated surface was studied using scanning electron microscopy. The energy density whereby the sample surface starts to melt, depends on Mn content and is equal to 0.12–0.14 J/cm2 for x 0.2, in the case of x = 0.3 this value is about 0.35 J/cm2. The higher Mn content leads to higher melt duration. The morphology of laser irradiated surface changes from a weakly modified surface to a single crystal strained one, with an increase in E. Under irradiation with E in the range of 0.21–0.25 J/cm2, the oriented filamentary crystallization is observed. The Te inclusions on the surface are revealed after the irradiation of samples with small content of Mn. # 2007 Elsevier B.V. All rights reserved. Keywords: CdMnTe; Laser irradiation; Phase transformation; Reflectivity; Scanning electron microscopy
1. Introduction
2. Experimental
Pulsed laser processing is an effective method of purposeful changes in physical properties of submicron surface layers of semiconductor crystals. This method is of interest for many researchers dealing with manufacturing of various electronic devices, in particular, based on AIIBVI semiconductors. Among these materials, CdTe is extensively studied. Phase transformations induced by nanosecond laser irradiation in the surface layers of CdTe were studied [1,2]. The surface morphology and structure changes in CdZnTe and CdMnTe solid solutions irradiated by ruby laser millisecond pulses were studied [3] using scanning electron microscopy (SEM). The goal of this work is to study the effects of nanopulsed laser irradiation on the surface of Cd1 xMnxTe crystals under experimental conditions [1,2].
The samples in question (thickness 1 mm, square = 0.3– 0.5 cm2) were cut from single crystals of Cd1 xMnxTe (x = 0; 0.02; 0.04; 0.08; 0.1; 0.2 and 0.3) grown by Bridgman technique and were mechanically and chemically polished as described in Ref. [3]. The crystals were irradiated by a Q-switched ruby laser (l = 0.69 mm) with pulse duration of 80 ns. The laser optical system ensured high irradiation homogeneity. The irradiated area was about 2.5 mm in diameter. The spatial inhomogeneity of the laser spot energy distribution was not more than 5%. Optical diagnostics of laser-induced thermal processes were carried out by means of time-resolved reflectivity (TRR) measurements at wavelengths l1 = 0.53 and l2 = 1.06 mm. Laser irradiation energy density varied in the range of 0.1–0.6 J/cm2. The probing beam was focused to the central part of the heated zone into a spot of approximately 1 mm in diameter. The angle of incidence was 408. The probing beam was polarized in the incidence plane (p-polarization). Morphology of irradiated surface was studied by scanning microscopy method in the regime of secondary electrons at accelerating voltage 30 kV.
* Corresponding author. Tel.: +375 172 81 35 14; fax: +375 172 83 91 51. E-mail address: [email protected] (G.D. Ivlev). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.019
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E.I. Gatskevich et al. / Applied Surface Science 254 (2007) 993–996
3. Results and discussion As it is known [4], CdTe melting is the phase transition of ‘‘semiconductor–semiconductor’’ type, whereas, for example, molten Si is a typical liquid metal. Under the conditions of laser-induced melting of these semiconductors and during the following solidification of the molten layer, the qualitative difference [1] in dynamics of reflectivity R(t) of CdTe and Si takes place. The behavior of R(t) is similar at l1 and l2 for Si, but it is different in the case of CdTe. This discrepancy is mainly due to the result in considerable (about by a factor of 10) difference in absorptivities, a1 (at l1) and a2 (at l2) in forming melt. It should be noted that photon energy hn1 equal to 2.33 eV (probing beam with l1), exceeds the bandgap Eg = 1.5 eV of CdTe (T = 300 K), whereas hn2 = 1.17 eV < Eg. Relation hn2 < Eg < hn1 holds true for investigated solid solutions also Cd1 xMnxTe. At the values of x 0.3 Eg 2 eV [5]. According to experimental data obtained, the character of R(t) dependencies is similar for the samples with x = 0–0.2 with increase in E. The melting of nanosized surface layer of materials is reached at the energy density E > 0.1 J/cm2 and results in different dynamics of R at l1 and l2, respectively. Thus, at l1, R(t) has two maximums at E = 0.12 J/cm2 (Fig. 1a), this is not observed at l2. The increase in R-value is more pronounced at l2. The discrepancy in R(t) dynamics is likely to be connected with different conditions of interference at reflection of probing radiation from multi-layer system arising under the liquid phase formation. At higher values of E, non-
monotonic changes in R are observed at l2 (Fig. 1b). The changes in R at l1 are weakly expressed due to not so considerable changes in complex refractivity at l1 at the formation of the liquid layer. The periods of time between the maximum of R(t) at l1 and l2 differ; t1 > t2. The t1 value determines more exactly the duration of laser-induced phase transformations in connection with a considerable smaller penetration of probing radiation with l1 into the samples. The increase in energy density up to 0.25 J/cm2 leads to a more complicated behavior of sample reflectivity at l2 as a result of deeper melting of the irradiated material and the change in interference conditions in the multi-layer system. As follows from experimental data, the increase in the content of Mn from sample to sample leads to longer duration of phase transformations (compare Fig. 1c and d). For instance, in the cases x = 0.02 and 0.2 the difference between the value of t1 is about 200 ns (E = 0.25 J/cm2). The difference in t2 value for different x increases from some tens to some hundreds nanoseconds when E value increases up to 0.35 J/cm2 (Fig. 2). This dependence of the duration of phase transformations on Mn content can be explained by the changes in thermophysical parameters of the solid solution with x increase. At the energy density of 0.35 J/cm2 maximal t2 value reaches nearly 2 ms, i.e. it exceeds the laser pulse duration more than by an order of magnitude. According to SEM data, the surface fragments with different morphology are observed under variable conditions of laser irradiation and Mn contents. At small energy density, SEM
Fig. 1. Time evolution of the reflectivity for l1 = 0.53 mm (top) and l2 = 1.06 mm (bottom) of Cd1 xMnxTe under laser irradiation with energy densities (a) E = 0.12 J/cm2, (b) 0.14 J/cm2 and (c and d) 0.25 J/cm2. The time scale is (a and b) 100 ns/div and (c and d) 200 ns/div. Straight sweep traces correspond to R = 0.
E.I. Gatskevich et al. / Applied Surface Science 254 (2007) 993–996
Fig. 2. The time between maximums in reflectivity transients vs. irradiation energy density at 1.06 mm for different Mn contents.
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images show weak changes in microstructure. The action of laser pulses with E > 0.21 J/cm2 leads to oriented filamentary crystallization in separate parts of irradiated area (Fig. 3b). The further increase in irradiation energy density results in the appearance of ripple (knobby) microstructures along with filamentary structures (Fig. 3c). At E = 0.35 J/cm2, we observed the ripple structures (Fig. 3d); the white spots point to Te inclusions, arising due to evaporation of volatile component Cd. The considerable discrepancy in the experimental results for the samples with the highest content of Mn (x = 0.3) should be noted. The threshold energy density, at which the melting of the surface of Cd0.7Mn0.3Te takes place, exceeds 0.35 J/cm2. Action of laser pulse upon the sample in question, does not lead to any changes in morphology of crystal surface and we did not observe any changes in R(t) for both wavelengths of probing radiation. At higher values of energy density (E = 0.4 and 0.6 J/ cm2), morphology of irradiated surface (Fig. 3e and f) is evidenced of the formation of strained single crystal surface as the result of the laser-induced phase transformations.
Fig. 3. SEM images of the Cd1 xMnxTe surface (a) before irradiation and laser-modified at (b) x = 0.1, E = 0.21 J/cm2; (c) x = 0.1, E = 0.25 J/cm2; (d) x = 0.1, E = 0.35 J/cm2; (e) x = 0.3, E = 0.28 J/cm2; (f) x = 0.3, E = 0.6 J/cm2.
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E.I. Gatskevich et al. / Applied Surface Science 254 (2007) 993–996
It is of interest to compare nanosecond and millisecond [3] regimes of laser irradiation of Cd1 xMnxTe crystals. The major difference is the presence of an ordered network of microcracks on the surface after laser irradiation with millisecond pulses. The crack formation is brought about thermoelastic stresses arising due to large depth of heating and large temperature gradients in the samples. We did not observe the crack formation in nanosecond regime of laser irradiation. The absence of this effect points to the advantage of nanosecond regime for laser modification of CdMnTe surface. 4. Conclusions The modification of Cd1 xMnxTe by nanopulsed ruby laser irradiation was studied by means of the in situ optical diagnostics and SEM analysis. The optical diagnostics were carried out at two wavelengths of probing radiation with photon energy higher and lower than the bandgap. From the in situ measurements the energy density Em at which the sample surface starts to melt was determined. This value depends on
Mn content and Em = 0.12–0.14 J/cm2 for x < 0.2 and increases up to 0.35 J/cm2 for x = 0.3. The morphology of laser-irradiated surface changes from a weakly modified surface to the single crystal strained one with an increase in E. In some parts of irradiated areas we observed the oriented filamentary crystallization at the energy density 0.21–0.25 J/cm2. The Te inclusions on the surface are revealed after irradiation of samples with small content of Mn. References ˇ erny´, P. Prˇikryl, V. Cha´b, O. [1] E.I. Gatskevich, G.D. Ivlev, S.P. Zhvavyi, R. C Cibulka, Proc. SPIE 5449 (2003) 10. ˇ erny´, V. Cha´b, O. Cibulka, Appl. [2] E. Gatskevich, G. Ivlev, P. Prˇikryl, R. C Surf. Sci. 248 (2005) 259. [3] O.V. Galochkin, S.G. Dremluzhenko, Y.D. Zakharuk, A.I. Rarenko, E.V. Rybak, V.M. Strebezhev, Phys. Chem. Solid State 4 (2003) 669 (in Ukrainian). [4] A.R. Regel’, V.M. Glazov, Physical Properties of Electronic Melts, Nauka, Moscow, 1980, p. 296 (in Russian). [5] V.F. Aguekian, Soros Educ. J. 8 (2004) 85 (in Russian).
Modification of Cd1 xMnxTe crystal surface layers by nanopulsed laser irradiation E.I. Gatskevich a, G.D. Ivlev a,*, A.I. Rarenko b, A.I. Savchuk b, V.N. Strebegev b, Z.I. Zakharuk b a
Institute of Electronics, National Academy of Sciences, Minsk, Belarus b Chernivtsi National University, Chernivtsi, Ukraine
Received 7 June 2007; received in revised form 23 July 2007; accepted 7 August 2007 Available online 14 August 2007
Abstract The surface modification of Cd1 xMnxTe (x = 0–0.3) crystal wafers under pulsed laser irradiation has been studied. The samples were irradiated by a Q-switched ruby laser with pulse duration of 80 ns. Optical diagnostics of laser-induced thermal processes were carried out by means of timeresolved reflectivity measurements at wavelengths 0.53 and 1.06 mm. Laser irradiation energy density, E varied in the range of 0.1–0.6 J/cm2. Morphology of irradiated surface was studied using scanning electron microscopy. The energy density whereby the sample surface starts to melt, depends on Mn content and is equal to 0.12–0.14 J/cm2 for x 0.2, in the case of x = 0.3 this value is about 0.35 J/cm2. The higher Mn content leads to higher melt duration. The morphology of laser irradiated surface changes from a weakly modified surface to a single crystal strained one, with an increase in E. Under irradiation with E in the range of 0.21–0.25 J/cm2, the oriented filamentary crystallization is observed. The Te inclusions on the surface are revealed after the irradiation of samples with small content of Mn. # 2007 Elsevier B.V. All rights reserved. Keywords: CdMnTe; Laser irradiation; Phase transformation; Reflectivity; Scanning electron microscopy
1. Introduction
2. Experimental
Pulsed laser processing is an effective method of purposeful changes in physical properties of submicron surface layers of semiconductor crystals. This method is of interest for many researchers dealing with manufacturing of various electronic devices, in particular, based on AIIBVI semiconductors. Among these materials, CdTe is extensively studied. Phase transformations induced by nanosecond laser irradiation in the surface layers of CdTe were studied [1,2]. The surface morphology and structure changes in CdZnTe and CdMnTe solid solutions irradiated by ruby laser millisecond pulses were studied [3] using scanning electron microscopy (SEM). The goal of this work is to study the effects of nanopulsed laser irradiation on the surface of Cd1 xMnxTe crystals under experimental conditions [1,2].
The samples in question (thickness 1 mm, square = 0.3– 0.5 cm2) were cut from single crystals of Cd1 xMnxTe (x = 0; 0.02; 0.04; 0.08; 0.1; 0.2 and 0.3) grown by Bridgman technique and were mechanically and chemically polished as described in Ref. [3]. The crystals were irradiated by a Q-switched ruby laser (l = 0.69 mm) with pulse duration of 80 ns. The laser optical system ensured high irradiation homogeneity. The irradiated area was about 2.5 mm in diameter. The spatial inhomogeneity of the laser spot energy distribution was not more than 5%. Optical diagnostics of laser-induced thermal processes were carried out by means of time-resolved reflectivity (TRR) measurements at wavelengths l1 = 0.53 and l2 = 1.06 mm. Laser irradiation energy density varied in the range of 0.1–0.6 J/cm2. The probing beam was focused to the central part of the heated zone into a spot of approximately 1 mm in diameter. The angle of incidence was 408. The probing beam was polarized in the incidence plane (p-polarization). Morphology of irradiated surface was studied by scanning microscopy method in the regime of secondary electrons at accelerating voltage 30 kV.
* Corresponding author. Tel.: +375 172 81 35 14; fax: +375 172 83 91 51. E-mail address: [email protected] (G.D. Ivlev). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.019
994
E.I. Gatskevich et al. / Applied Surface Science 254 (2007) 993–996
3. Results and discussion As it is known [4], CdTe melting is the phase transition of ‘‘semiconductor–semiconductor’’ type, whereas, for example, molten Si is a typical liquid metal. Under the conditions of laser-induced melting of these semiconductors and during the following solidification of the molten layer, the qualitative difference [1] in dynamics of reflectivity R(t) of CdTe and Si takes place. The behavior of R(t) is similar at l1 and l2 for Si, but it is different in the case of CdTe. This discrepancy is mainly due to the result in considerable (about by a factor of 10) difference in absorptivities, a1 (at l1) and a2 (at l2) in forming melt. It should be noted that photon energy hn1 equal to 2.33 eV (probing beam with l1), exceeds the bandgap Eg = 1.5 eV of CdTe (T = 300 K), whereas hn2 = 1.17 eV < Eg. Relation hn2 < Eg < hn1 holds true for investigated solid solutions also Cd1 xMnxTe. At the values of x 0.3 Eg 2 eV [5]. According to experimental data obtained, the character of R(t) dependencies is similar for the samples with x = 0–0.2 with increase in E. The melting of nanosized surface layer of materials is reached at the energy density E > 0.1 J/cm2 and results in different dynamics of R at l1 and l2, respectively. Thus, at l1, R(t) has two maximums at E = 0.12 J/cm2 (Fig. 1a), this is not observed at l2. The increase in R-value is more pronounced at l2. The discrepancy in R(t) dynamics is likely to be connected with different conditions of interference at reflection of probing radiation from multi-layer system arising under the liquid phase formation. At higher values of E, non-
monotonic changes in R are observed at l2 (Fig. 1b). The changes in R at l1 are weakly expressed due to not so considerable changes in complex refractivity at l1 at the formation of the liquid layer. The periods of time between the maximum of R(t) at l1 and l2 differ; t1 > t2. The t1 value determines more exactly the duration of laser-induced phase transformations in connection with a considerable smaller penetration of probing radiation with l1 into the samples. The increase in energy density up to 0.25 J/cm2 leads to a more complicated behavior of sample reflectivity at l2 as a result of deeper melting of the irradiated material and the change in interference conditions in the multi-layer system. As follows from experimental data, the increase in the content of Mn from sample to sample leads to longer duration of phase transformations (compare Fig. 1c and d). For instance, in the cases x = 0.02 and 0.2 the difference between the value of t1 is about 200 ns (E = 0.25 J/cm2). The difference in t2 value for different x increases from some tens to some hundreds nanoseconds when E value increases up to 0.35 J/cm2 (Fig. 2). This dependence of the duration of phase transformations on Mn content can be explained by the changes in thermophysical parameters of the solid solution with x increase. At the energy density of 0.35 J/cm2 maximal t2 value reaches nearly 2 ms, i.e. it exceeds the laser pulse duration more than by an order of magnitude. According to SEM data, the surface fragments with different morphology are observed under variable conditions of laser irradiation and Mn contents. At small energy density, SEM
Fig. 1. Time evolution of the reflectivity for l1 = 0.53 mm (top) and l2 = 1.06 mm (bottom) of Cd1 xMnxTe under laser irradiation with energy densities (a) E = 0.12 J/cm2, (b) 0.14 J/cm2 and (c and d) 0.25 J/cm2. The time scale is (a and b) 100 ns/div and (c and d) 200 ns/div. Straight sweep traces correspond to R = 0.
E.I. Gatskevich et al. / Applied Surface Science 254 (2007) 993–996
Fig. 2. The time between maximums in reflectivity transients vs. irradiation energy density at 1.06 mm for different Mn contents.
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images show weak changes in microstructure. The action of laser pulses with E > 0.21 J/cm2 leads to oriented filamentary crystallization in separate parts of irradiated area (Fig. 3b). The further increase in irradiation energy density results in the appearance of ripple (knobby) microstructures along with filamentary structures (Fig. 3c). At E = 0.35 J/cm2, we observed the ripple structures (Fig. 3d); the white spots point to Te inclusions, arising due to evaporation of volatile component Cd. The considerable discrepancy in the experimental results for the samples with the highest content of Mn (x = 0.3) should be noted. The threshold energy density, at which the melting of the surface of Cd0.7Mn0.3Te takes place, exceeds 0.35 J/cm2. Action of laser pulse upon the sample in question, does not lead to any changes in morphology of crystal surface and we did not observe any changes in R(t) for both wavelengths of probing radiation. At higher values of energy density (E = 0.4 and 0.6 J/ cm2), morphology of irradiated surface (Fig. 3e and f) is evidenced of the formation of strained single crystal surface as the result of the laser-induced phase transformations.
Fig. 3. SEM images of the Cd1 xMnxTe surface (a) before irradiation and laser-modified at (b) x = 0.1, E = 0.21 J/cm2; (c) x = 0.1, E = 0.25 J/cm2; (d) x = 0.1, E = 0.35 J/cm2; (e) x = 0.3, E = 0.28 J/cm2; (f) x = 0.3, E = 0.6 J/cm2.
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E.I. Gatskevich et al. / Applied Surface Science 254 (2007) 993–996
It is of interest to compare nanosecond and millisecond [3] regimes of laser irradiation of Cd1 xMnxTe crystals. The major difference is the presence of an ordered network of microcracks on the surface after laser irradiation with millisecond pulses. The crack formation is brought about thermoelastic stresses arising due to large depth of heating and large temperature gradients in the samples. We did not observe the crack formation in nanosecond regime of laser irradiation. The absence of this effect points to the advantage of nanosecond regime for laser modification of CdMnTe surface. 4. Conclusions The modification of Cd1 xMnxTe by nanopulsed ruby laser irradiation was studied by means of the in situ optical diagnostics and SEM analysis. The optical diagnostics were carried out at two wavelengths of probing radiation with photon energy higher and lower than the bandgap. From the in situ measurements the energy density Em at which the sample surface starts to melt was determined. This value depends on
Mn content and Em = 0.12–0.14 J/cm2 for x < 0.2 and increases up to 0.35 J/cm2 for x = 0.3. The morphology of laser-irradiated surface changes from a weakly modified surface to the single crystal strained one with an increase in E. In some parts of irradiated areas we observed the oriented filamentary crystallization at the energy density 0.21–0.25 J/cm2. The Te inclusions on the surface are revealed after irradiation of samples with small content of Mn. References ˇ erny´, P. Prˇikryl, V. Cha´b, O. [1] E.I. Gatskevich, G.D. Ivlev, S.P. Zhvavyi, R. C Cibulka, Proc. SPIE 5449 (2003) 10. ˇ erny´, V. Cha´b, O. Cibulka, Appl. [2] E. Gatskevich, G. Ivlev, P. Prˇikryl, R. C Surf. Sci. 248 (2005) 259. [3] O.V. Galochkin, S.G. Dremluzhenko, Y.D. Zakharuk, A.I. Rarenko, E.V. Rybak, V.M. Strebezhev, Phys. Chem. Solid State 4 (2003) 669 (in Ukrainian). [4] A.R. Regel’, V.M. Glazov, Physical Properties of Electronic Melts, Nauka, Moscow, 1980, p. 296 (in Russian). [5] V.F. Aguekian, Soros Educ. J. 8 (2004) 85 (in Russian).