Determination of threshold values and monitoring of the surface state of semiconductors under pulsed laser irradiation

Transcription

1 Determination of threshold values and monitoring of the surface state of semiconductors under pulsed laser irradiation Volodymyr A. Gnatyuk,, Toru Aoki, Olexandr I. Vlasenko Olena S. Gorodnychenko V.E. Lashkaryov Institute of Semiconductor Physics of National Academy of Sciences of Ukraine, prospekt Nauky, Kyiv 8, Ukraine Tel.: +8 [] Fax: +8 [] Research Institute of Electronics, Shizuoka University -5- Johoku, Hamamatsu -8, Japan Taras Shevchenko Kyiv National University, 6 Volodymyrska str., Kyiv, Ukraine Abstract Detection and dynamics of laser-induced melting in different semiconductor crystals (CdTe, CdHgTe, CdZnTe, InSb, GaAs and SiC) were directly monitored by the in situ timeresolved optical reflectivity technique. The melting and ablation thresholds have been determined for nanosecond pulses of KrF excimer and ruby lasers and features of the laserinduced phase transitions in the surface region of the semiconductors are discussed. The temperature fields in CdTe crystals under laser irradiation are calculated at different laser pulse energy densities. Keywords: Semiconductors, laser irradiation, time-resolved reflectivity, melting threshold. Introduction One of the important problems in pulsed laser processing of semiconductors is concerned with the knowledge of the energy densities of laser pulses at which threshold processes (melting, recrystalization, intensive evaporation, ablation, desorption) in the surface region of a crystal start. Despite the extensive study of laser-induced melting of semiconductors during the last decade, a direct determination of the melting thresholds and in situ methods of observation of the dynamics (kinetics) of phase transitions in a surface layer during nanosecond laser pulse action are under study [-5]. The data on the threshold values of energy density of nanosecond laser pulses and laserinduced processes in semiconductors have principal importance in application of laser procedures for epitaxial recrystallization, local annealing, removal of an oxide film, stress relieving, cleaning of the surface, improving the uniformity in surface layer composition, doping, formation of homo- and hetero-interfaces, contact alloying, etc [-6]. One of a direct and appropriate technique to determine the melting threshold and study of pulsed-laser-induced phase transitions in a semiconductor is the time-resolved reflectivity (TTR) [-]. This method consists in detection and observation of the time evolution of the optical reflectivity of the surface illuminated with a CW probe laser beam under irradiation with a power nanosecond laser pulse. This is an effective method for IV and III V semiconductors because the transformation of these materials from solid to molten state leads to higher optical reflectivity associated with the metallic liquid phase [-6]. The use of TRR for II VI semiconductors has been rather problematic because they, as most molten materials of II VI group, remain semiconductors [7-]. We have used TRR for the study of the dynamics of phase transitions in semiconductors (CdTe, CdHgTe, CdZnTe, InSb, GaAs and SiC) irradiated with nanosecond pulses of KrF excimer and ruby lasers, the threshold energy densities have been experimentally determined and theoretically calculated and features of laser-induced transformation of the structure in the surface region of semiconductors are discussed. -8

2 . Experimental details The pulsed radiation source was a ruby ( = 69 nm) or KrF excimer laser ( = 8 nm) emitting single pulses of = ns duration (FHWHM). The energy density of an incident laser pulse was varied in the wide range both below and above the melting threshold of the semiconductors. The in situ detection and dynamics of phase transitions in the surface region of the semiconductors were monitored by the TRR technique using a CW red ( = 6 nm) or green ( = 5 nm) laser as a probe beam. The TRR experimental setup is shown in Fig.. Amplifier Nanosecond laser pulse High speed photodetector Filter Diaphragm CW probe laser beam Sample P ~ Pa Oscilloscope Vacuum chamber T ~ K Fig.. Schematic diagram of the apparatus for time-resolved optical reflectivity investigation. The reflected beam was detected by a high-speed photodetector and after amplifying the signal was traced by a storage oscilloscope. A diaphragm and optical filters were located just in the front of a photodetector to cut power pulsed laser radiation diffusely reflected from the sample surface (Fig. ). The subjects for investigation are the semiconductors which have been widely used as materials for radiation sensors (CdTe, CdHgTe, CdZnTe, InSb, GaAs and SiC). The samples were subjected to various preliminary surface treatments such as polishing by chemical etching, mechanical polishing, cleaning and etching by laser irradiation, and also samples after storage under natural conditions were used.. Results and discussion Irradiation of semiconductor crystals with laser pulses in the order of increasing energy density allowed us to determine the threshold value J m at which the transient reflection coefficient R(t) was sharply peaked. A jump in R(t), observed at the laser pulse energy density J = J m, was associated with melting of a thin (of the order of the laser radiation absorption depth) surface layer. No changes in R(t) were observed at J < J m. Figure shows TRR for a CW red probe laser beam measured for the CdTe crystals under irradiation with a KrF laser pulse at different energy densities and at the same value of J but at different time scales. Study of the time evolution of the reflectivity revealed the features associated with a critical dependence of the transient reflection coefficient R(t) on J. The melting threshold lay in the range J m = 8-5 mj/cm for different CdTe crystals, depending on the state of their surface (polished, fresh etched surface or after storage in natural conditions) [9-]. As seen from Fig. a, showing the dynamics of the reflection coefficient at different laser pulse energy densities, irradiation with J J m results in an increase in R with following decaying with time. The maximum of R(t) is independent of energy density that is evidence of formation of a molten surface layer with higher reflectivity and it is not caused by a -9

3 temperature increase in R. The contribution of the long-time component to the total relaxation of R(t) increases with rising J (Fig. a, curves -). This is attributed to melting and following crystallization of a deeper surface layer when laser pulse energy density increases []. J = 98 mj/cm J mj/cm μs J = 8 mj/cm μs J = 6 mj/cm μs J = 5 mj/cm μs 6 6 Time Fig.. Oscillograms of the amplified photodetector signal showing the dynamics of the reflection coefficient R(t) of a red probe laser beam from the CdTe crystal surface under irradiation with KrF excimer laser pulses of different energy densities J and at different time scales (J mj/cm ). The reflection coefficient relaxation occurred during the time much longer than the laser pulse action and it could be attributed to deterioration of the crystallinity, amorphization and following crystallization of the surface layer [, ]. The laser-induced formation of a thin surface amorphous Te film in CdTe crystals was observed also under ruby laser irradiation []. Probably the reflection coefficient relaxation occurred during several seconds because R(t) in the oscillograms did not decay to the initial value (Fig. b). Variations of the reflection coefficient of CdTe under heating and melting are very small [7]. The reflection coefficient is incremented only by -% under laser pulse action that is substantially less than in the case of the TRR measurements of IV and III V compounds which were transformed from semiconductors to metallic liquids under laser-induced melting [-6]. Another difficulty in the TRR measurements was that the depth of absorption of KrF excimer laser radiation in CdTe is very shallow is ~ 9 nm and the depth of absorption of a red (He-Ne, λ = 6 nm) probe laser radiation is ~.6 μm. Thus, it was difficult to monitor of a very thin (~ - nm) surface layer which was melted under radiation with a KrF excimer laser pulse because. Therefore we also used a probe laser with a shorter wavelength. Figure a shows the dynamics of TRR of CdTe crystals under irradiation with a KrF laser pulse with using a CW green (λ = 5 nm) probe laser beam. In this case the changes in the reflected beam were about 6-7%. The character of the relaxation of R(t) is similar to TRR measured with a red probe laser (Fig. a). A sharp increase in R(t) under pulsed laser irradiation means melting of a thin surface layer of CdTe. The irradiation of CdTe surface caused an increase in the steady-state reflection coefficient (Fig. b). This was a reason that R(t) did not decay to the initial value R (Fig., a). It was shown that irradiation of the CdTe crystals with nanosecond laser pulses of energy density J J m resulted in improvement of surface smoothness because of melting of irregularities and morphology asperities [, ]. Irradiation of CdTe crystals with KrF laser pulses of energy density higher 5 mj/cm modified the TRR dynamics. The reflection coefficient R(t) increases and falls to a value lower than initial one (Fig., curve ). This is due to an increase in roughness of the CdTe surface because of boiling, evaporation and ablation of an overheated surface layer and following solidification of the disordering region. In this case, crystallization can occur from both the bulk and surface sides [7, 8]. The value J a 5 mj/cm is considered as the ablation threshold of CdTe at irradiation with nanosecond pulses of KrF excimer laser. -5

4 Reflection coefficient (arb. units). J = 5 mj/cm J = mj/cm. J = 6 mj/cm. J = 5 mj/cm Time ( s) 5 5 Laser energy density (mj/cm ) Fig.. Dynamics of the reflection coefficient R(t) and steady state reflection coefficient of a green probe laser beam from the CdTe crystal surface after irradiation with KrF excimer laser pulses of different energy densities. Dynamics of the reflection coefficient R(t) of SiC crystal irradiated with KrF laser pulses of energy densities J > J m and J > J a is shown in Fig.. A jump in R(t) is due to the beginning of melting of the surface, however the decrease and the following increase in R up to the second maximum is associated with the interference of reflections of the probe beam from the surface and from the moving solid-liquid interface (Fig.a) [7, 8] Fig.. Dynamics of the reflection coefficient R(t) of a green probe laser beam from the SiC crystal surface under irradiation with KrF excimer laser pulses of energy density higher than the melting and ablation threshold. The TRR measurements were also performed for CdTe, CdHgTe, CdZnTe, GaAs, InSb and SiC semiconductors and a ruby laser as a pulsed laser source was also used. The following threshold values have been obtained [8-]. For CdTe, Cd x Zn -x Te and Cd x Hg -x Te with x =.8- J m 5 mj/cm, J a 5 mj/cm (KrF excimer laser pulse) and J m mj/cm (ruby laser pulse). In the case of the solid solutions the melting thresholds vary by -% depending on x. For SiC J m 58 mj/cm, J a 65 mj/cm (KrF excimer laser pulse). For InSb J m mj/cm (KrF excimer laser pulse), J m mj/cm (ruby laser pulse). For GaAs J m mj/cm (ruby laser pulse). The experimental data are in good agreement with the calculations of the temperature of the semiconductors subjected to laser pulse irradiation. The time distributions of the surface temperature in CdTe crystals under irradiation with KrF excimer and ruby laser pulses of different energy densities are shown in Fig

5 Temperature (K) Temperature (K) 5 5 J (mj/cm ) T m 5 5 J (mj/cm ) T m 5 Laser pulse Fig. 5. Calculated time dependence of the temperature of the CdTe crystal surface under irradiation with KrF excimer and ruby laser pulses of different energy densities. 5 Laser pulse The simulation was made by solving the time-dependent heat flow equation [,, 9]. Curves correspond to the energy densities J m of KrF excimer and ruby laser pulses when the melting of a surface layer of CdTe is attained.. Conclusion The TRR is an effective technique for the in situ study of the dynamics of phase transitions in the surface region of semiconductors subjected to nanosecond laser irradiation and it can be advantageously used for determination of the threshold energy densities. References. L.A. Golovan. B.A. Markov P.K. Kashkarov, V.Yu. Timoshenko. Solid State Commun. 998, 8, No., pp S.P. Zhvavyi, G.L. Zykov. Appl. Surf. Sci. 6, 5, issue, pp V.Yu. Timoshenko, Th. Dittrich, F. Koch. Appl. Phys. Lett., 77, No. 9, pp V.Yu. Timoshenko, Th. Dittrich, I. Sieber, J. Rappich, B.V. Kamenev. P.K. Kashkarov. Phys. Stat. Sol.., 8, issue, pp G. Ivlev, E. Gatskevich, V. Chab, J. Stuchlik, V. Vorlicek, J. Kocka. Appl. Phys. Lett. 999, 75, No., pp G.D. Ivlev, E.I. Gatskevych. Semicond., 7, No. 5, pp E. Gatskevich, G. Ivlev, P. Prˇikryl, R. Cˇerny, V. Chab, O. Cibulka. Appl. Surf. Sci. 5, 8, issues -, pp A. Baidullaeva, A.I. Vlasenko, E.I. Gatskevich, V.A. Gnatyuk, G.D. Ivlev, P.E. Mozol. Nanosystems, Nanomaterials, Nanotechnologies. 8, 6, No., pp (in Russian). 9. V.A. Gnatyuk, T. Aoki, O.S. Gorodnychenko, Y. Hatanaka. Appl. Phys. Lett., 8, No. 8, pp V.A. Gnatyuk, T. Aoki, Y. Nakanishi, Y. Hatanaka. Surf. Sci., 5, pp V.A. Gnatyuk, T. Aoki, Y. Hatanaka. IEEE Trans. Nucl. Sci., 5, No. 5, pp V.A. Gnatyuk, T. Aoki, Y. Hatanaka. Appl. Phys. Lett. 6, 88, pp. -.. A. Baidullaeva, B.K. Dauletmuratov, A.I. Vlasenko, V.A. Gnatyuk, P.E. Mozol. Semicond. 99, 7, No., pp V.A. Gnatyuk, O.S. Gorodnychenko. Semicond., 7, No., pp A. Medvid, L. Fedorenko, B. Korbutjak, S. Kryluk, M. Yusupov, A. Mychko. Radiat. Meas. 7,, issues -5, pp A.D. Compaan, I. Matulionis, S. Nakade. Opt. Laser. Eng.,, issue, pp K. Zanio. Cadmium telluride. In: Semiconductors and Semimetals. 978,, New York, Academic Press. -5