- Email: [email protected]
Laser-induced optical reflection and absorption of GaP
60
LASER-INDUCED Mitsuru OKIGAWA,
Nuclear Instruments and Methods in Physics Research B9 (1985) 60 65 North-Holland. Amsterdam
OPTICAL
REFLECTION
Takeyoshi NAKAYAMA
AND ABSORPTION
OF GaP
and Noriaki ITOH
Department of Crystalline Materials Science, Faculty of Engineering, Nagoya University, Furo - cho, Chikusa - ku, Nagoya 464, Japan
Received 15 June 1984 and in revised form 19 November 1984
Changes in the optical absorption and reflection coefficients of GaP induced by irradiation with 600 ns laser pulses of the wavelengths at the direct and indirect band gaps have been measured. It is found that a persistent increase in the absorption coefficient and a permanent decrease in the reflection coefficient, in addition to transient increases in the absorption and reflection coefficients, are induced, by irradiation at the band gap. The persistent component is found to be evolved with a surprisingly long time constant of about 10 ms. This component is ascribed to laser-induced modification of the surface layers or damage formation. The transient component is ascribed to temperature rise on the basis of time-resolved optical absorption measurements. It is found that the persistent changes are induced by irradiation with the indirect band gap photons at a fluence which induces only a little change in the transient reflectivity. The threshold laser fluence to create the surface modification by the indirect band-gap irradiation is found to be only four times that by direct band-gap irradiation. A delayed reflectivity change is found to be induced by irradiation at the indirect band gap and is ascribed to the modification of surface layers by photons absorbed at the surface layer, which enhances the absorption coefficient. We interpret these experimental results in terms of non-thermal laser-induced atomic processes.
1. Introduction It has been well established that laser annealing is induced by formation of temporarily molten surface layers [1]. Laser-irradiation, however, is known to introduce damage that cannot be annealed completely [2,3]. For compound semiconductors, recent studies by the present authors of laser-induced effects by means of Rutherford backscattering (RBS) channeling experiments have shown that the surface layer of G a P is decomposed into a Ga-rich layer and a P-layer, which is located on the irradiated side, and that such a modification of the surface layer is more pronounced for a 600 ns laser pulse than for a 15 ns laser pulse of the same fluence [4,5]. Such an enormous dependence of the laser-induced effects on the pulse width may be related to the difference in the temperature rise induced by a 15 ns and 600 ns laser pulse. In view of the results of a calculation, which indicated that a 15 ns laser pulse causes a much larger temperature rise than a 600 ns laser pulse of the same fluence, we interpreted that the larger laser-induced surface modification by a 600 ns laser pulse originates from a non-thermal process. Measurements of laser-induced optical reflectivity changes from a surface layer are known to yield information on formation of electron-hole plasma [6,7] of molten phases [8-10] and of a damaged layer [11,12]. Optical absorption due to free electrons has also been measured [13]. An advantage of these optical techniques over the RBS-channeling technique is that the former 0168-583X/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
are capable of yielding information on the time-evolution of the damaged layers. Since such information is useful for understanding the mechanism of laser-induced surface-layer modification, we have undertaken experimental investigations of the surface-layer modification by employing optical techniques. In this paper we report the optical absorption and reflection measurements of GaP irradiated with a 600 ns laser pulse. For studying the laser-induced surface-layer modification, use of 600 ns laser pulses is advantageous, since the surface layer is modified to a greater extent [4,5]. GaP is an indirect semiconductor and hence changing the wavelength may alter the heating rate to some extent. It is shown that both the optical absorption and reflectivity changes have transient and persistent components. The former was ascribed to the temperature rise, the latter to the modification of surface layers.
2. Experimental technique Specimens used in the present investigation are n-type G a P (100) wafers doped with 5 × 10 -17 cm -3 Te. They were polished mechanically and then etched with H C 1 - 2 H N O 3 - H 2 0 solution at 50°C for 15 min. The specimens were placed in air in the optical paths of flash-lamp pumped dye laser, which is used for creating damage, and of a H e - N e laser or of light from a xenon lamp which is used for probing. The dye laser was operated with single-pulse mode with a width of
M. Okigawa et al. / Laser-induced absorption of GaP 600 ns. To obtain the laser-induced change of optical absorption and reflection, H e - N e laser or light from the xenon lamp was transmitted through or reflected from the irradiated surface and detected with a photomultiplier, the output of which was recorded on a storage oscilloscope. A monochromator was placed in the optical path between the specimen and the photomultiplier to obtain time-resolved optical absorption spectra. The dye-laser was incident normally on the specimen through a hole 2 mm O, while the probe light was 30 ° from the surface normal. The spot size of the H e - N e laser at the specimen was 0.5 mm and that of light from the Xe lamp was 2 mm 0 . For the reflection measurements, light emergent 30 ° from the surface normal was detected. The intensity of the dye laser was measured with a photodiode, which was calibrated by using a calorimeter.
3. Experimental results and discusssion 3.1. Effect of irradiation near the direct band gap
61
clearly seen at lower fluences, while the magnitude of the persistent change increases with increasing the laser fluence. In order to clarify the nature of the two components, we measured the laser-induced optical absorption change at various wavelengths using a Xe lamp and the monochromator. The result obtained with a fluence of 1.6 J / c m 2 is shown in fig. 2. In this case the peak is blurred because of the large size of the light beam from the xenon lamp. It is clear that the peak is observed only for shorter wavelengths and is absent for longer wavelengths. The time-resolved optical absorption spectra were derived from fig. 2 and shown in fig. 3. The results indicate that the optical absorbance is not dependent on the wavelength between 700 and 900 nm but is enhanced towards shorter wavelength. This increase towards the shorter wavelength is largest around 1.0 /~s and is absent in the final spectrum, which is independent of wavelength. The reflectivity change induced by a 440 nm laser pulse was also measured using H e - N e laser and the result is shown in fig. 4. It is clear that the reflectivity starts to increase at the start of the laser pulse. Above
First we describe the results of laser-irradiation at 440 nm (2.82 eV), which is close to the direct band gap (2.78 eV). A typical laser-induced change in optical absorption, probed with the He-Ne laser, is shown in fig. 1. It is clear that the optical absorption change consists of a peak slightly delayed in time from the laser pulse and a component that shows a persistent change. In the latter component, an increase in the optical absorption within 3/~s and a slow build up with a time constant of about 10 ms are observed. The peak is more
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62
M. Okigawa et al. / Laser-induced absorption of GaP I
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is above 1.4 J / c m 2. A persistent decrease similar to that found presently has been reported for GaP irradiated with a train of picosecond laser pulses [11,12]. In order to clarify the nature of the persistent change in optical reflectivity and absorbance, we compare, in fig. 5, the change in optical absorbance 3 /~s after the pulse and the final changes of both optical absorbance and reflectivity with the change A X m i n in the m i n i m u m yield of the RBS-channeling obtained previously [5]. It is known that AXminis indicative of the laser-induced persistent surface-layer modification. It is clear that all of these values start changing at nearly the same fluence. Moreover the relative increase in the AXmin is nearly parallel to these changes. Thus it is concluded that the persistent changes in the reflectivity and absorbance originates in the modification of the surface layers that causes the increase in A X m i n . We note further that the reflectivity decreases even through the absorbance increases. Since the increase in the optical absorbance causes the increase in the reflectivity, if the real part of the reflectivity is not changed [15], a substantial change in the material is induced on the surface layer. This conclusion is consistent with the result of RBS-channeling studies, which indicate that the surface layer is
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1.4 J / c m 2, the increase in the reflectivity saturates at about twice of the original reflectivity. Such a saturation for GaAs [10,11,14] and G a P [12] has been ascribed to the formation of the molten phase. We note also that the reflectivity is reduced finally when the laser fluence
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M. Okigawa et aL / Laser- induced absorption of GaP
decomposed into a gallium-rich layer and a phosphorous layer by laser-irradiation [5]. We note also that this final optical absorption change is almost wavelength independent• This result is also consistent with the result of RBS-channeling, since both the Ga-layer and P-layer will show almost wavelength independent optical absorption. As discussed above the increase in the reflectivity has been ascribed to the temperature rise. It is known also that the temperature rise induces a shift in the opticalabsorption vs photon-energy relation to longer wavelength [16,17]. The time-resolved optical absorption spectra shown in fig. 3 is regarded as a superposition of the wavelength-dependent optical absorption change in the wavelength region shorter than 700 nm and wavelength-independent absorption change. The former is indicative of the shift of the optical absorption due to the temperature rise which is known to decrease drastically as the wavelength increases. Since the shift of the absorption edge does not reach beyond 700 nm, as seen in fig. 3, the optical absorption change measured at 800 nm includes only the latter component, while that measured with the H e - N e laser includes both components• Since, as seen from fig. 2, the absorption change measured at 800 nm begins to increase at the start of the laser pulse, we consider that the persistent component begins to be formed at the start of laser irradiation. 3.2. Effect of irradiation near the indirect band gap
A study of the laser-induced surface modification at the indirect band gap is of interest, since the heating effect is less significant because of the low absorption coefficient. Fig. 6 shows the results of the optical reflection and absorption measurements induced by a dyelaser at 536 nm; the probe light is a H e - N e laser. The most significant results are that for a fluence less than 2.7 J / c m 2 the reflectivity change is extremely small while the persistent optical absorption change is evident, and that for all fluences studied there is a delay of about 500 ns in the increase in the reflectivity. The former result indicates that the persistent absorption change is not necessarily associated with the temperature rise. The latter result indicates that the temperature rise does not occur at the beginning of laser irradiation, which is easily understood since the laser is not strongly absorbed. The fact that the increase in the reflectivity and hence the temperature rise occurs with a delay is indicative of surface layer modification that causes the strong optical absorption. The fact that the surface layer is modified by irradiation at the indirect-band-gap irradiation is probably due to the high optical absorption at the surface top layer for the indirect band-gap photons. We note here that the persistent reflectivity change due to the surface-layer modification induced by 536 nm laser continues increasing to a steady value but
63
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the persistent optical absorption change is partially annihilated. The latter behavior is contrary to that observed for irradiation at the direct band gap (see fig. 1). The dependence of the amount of the persistent optical absorption change on the laser wavelength was also measured. Fig. 7 shows the relation between the laser-induced final optical absorption change and the laser fluence obtained at several wavelengths. The
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M. Okigawa et al. / Laser-induced absorption of GaP
64
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threshold laser fluence above which the optical absorption change is observed is plotted as a function of the wavelength by open circles in fig. 8. Fig. 8 includes also the wavelength dependence of the optical absorption coefficient [18], which is scaled logarithmically. A remarkably slow dependence of the threshold laser fluence has been observed also for the laser-induced sputtering of GaP [19], although a sharp dip in the threshold laser fluence at the indirect band gap has been observed for sputtering.
3.3. On the persistent optical absorption and reflection changes The measurements of the optical absorption and reflectivity change induced by a laser pulse of the direct-band-gap and indirect-band-gap energies have revealed that a persistent change in the surface layers which causes an optical absorption increase and a reflectivity decrease, is induced. This persistent change is clearly distinguished from the temperature rise, which induces a transient absorption and reflectivity increases near the optical absorption edge. We summarize the experimental results for the persistent change as follows: (1) For irradiation with a 1.6 J / c m 2 pulse, the persistent optical absorption changes at 882 nm (fig. 2) appear to start to increase immediately after the pulse, even though the delay in melt for a 1.8 J / c m 2 pulse is 200 ns (fig. 4). (2) The persistent change is formed by irradiation by a 2.6 J / c m 2 laser pulse of the indirect-
band-gap energy, while the temperature rise is negligible (fig. 6). (3) The fluence dependence of the persistent change is very similar to that of the change in the channeling minimum yield. Based on the results (3), we conclude that the origin of the persistent change in the optical absorption and reflectivity and that of the channeling minimum yield are the same. The RBS measurements have shown that the surface layer is decomposed into a Ga-rich layer and a P-layer. The P-layer which is on the surface, is considered to give a reduction of the reflectivity and an increase in the optical absorbance, which is consistent with the above conclusion. According to the present results, the persistent change appears to start to be evolved at the start of the laser pulse and to evolve further with a time constant of 14 ms. We refer to the condition that formed intermediately as the precursor state, the nature of which is not yet clear. Since the precursor state induces the change in the real part of the dielectric constants, it involves also a substantial change in the material. The transformation from the precursor state to the final decomposed state may involved oxidation of phosphorous or slow diffusion. This transformation is larger as the fluence is larger. We note also that the similar precursor state is formed by irradiation with indirect-band-gap photons but the precursor state is partially annealed in this case, as observed from optical absorption measurement. Thus whether the precursor is annealed or evolved appears to depend on the condition of irradiation. To reveal the nature of the precursor state is a problem for future investigation. Since the formation of the precursor state occurs even when the reflectivity change due to the temperature rise is small, as in the case of irradiation with indirect-band-gap photons, we conclude that the formation of the precursor state and hence the formation of the decomposed layers is initiated by a non-thermal process. The reason why the formation of a decomposed layer is observed by irradiation with a 600 ns laser pulse, but not to any appreciable extent with a 15 ns laser pulse [14,20-22], has been interpreted [4,5] to be due to the maximum temperature reached by a 600 ns laser pulse being lower and hence the effect of the electronic excitation is more prevalent. We note also that irradiation of semiconductors with a laser pulse with a fluence lower than the melt threshold by a factor more than 10 induces sputtering of atomic species [23,24] and surface damage as detected by LEED patterns [25,26]. The results indicate that the atomic displacement is induced by laser irradiation well below the melt threshold. We consider that the same process may lead to the formation of the precursor state, if the surface layer is heated by laser irradiation. The process may be similar to the photolytic decomposition in alkali halides of which the primary process is known to be the formation of Frenkel pairs by electronic excitation [27]. In
M. Okigawa et al. / Laser-induced absorption of GaP view of the existence of the threshold fluence for sputtering and the surface layer modification, the "photolytic decomposition" of the surface layer of comp o u n d semiconductors appears to occur only under dense electronic excitation. There has been a great deal of discussion of the mechanisms of the non-thermal laser-induced processes [28,29]. Briefly, the mechanisms discussed can be divided into two types: bond weakening [29] and recombination-induced [30,31]. Even though the formation of Frenkel excitons [31] has been suggested for the latter, we consider [30] that the Feibelman-Knotek-type multi-hole states [32] are formed under an electron-hole plasma near the surface, assisted by screening of Coulomb repulsion and an Anderson negative-U interaction [33]. It appears that one of the clues to reveal the mechanism is to investigate the precursor state employing several analytical techniques. In the present paper we have shown the optical absorption and reflection change by irradiation of GaP with a 600 ns laser pulse of direct-band-gap and indirect-band-gap energies. It is shown that in addition to the optical absorbance and reflectivity increases, as usually observed with a shorter laser-pulse irradiation, a persistent optical absorbance increase and reflectivity decrease is induced. For the direct-band-gap irradiation, these changes increase further with a time constant of 10 ms, while those for the indirect-band-gap irradiation are partially annealed. The laser-induced persistent changes were ascribed to a non-thermal effect.
References [1] B.R. Appleton and G.K. Celler (eds), Laser and Electron Beam Interactions with Solids (North-Holland, Amsterdam, 1982). [2] L.C. Kimerling and J.L. Benton, Laser and Electron Beam Processing of Materials, eds. C.W. White and P.S. Peercy (Academic Press, New York, 1980) p. 385. [3] J.A. Golovchenko and T.N.C. Venkatesan, Appl. Phys. Lett. 32 (1978) 147. [4] M. Okigawa, T. Nakayama, K. Takayama and N. Itoh, Solid State Commun. 49 (1984) 347. [5] M. Okigawa, T. Nakayama, K. Morita and N. Itoh, Appl. Phys. Lett. 43 (1983) 1054. [6] L.M. Blinov, V.S. Vavilov and G.N. Galkin, Sov. Phys. Semicond. 1 (1967) 1124. [7] J.M. Lin, H. Kurz and N. Bloembergen, Appl. Phys. Lett. 41 (1982) 643.
65
[8] D.H. Auston, C.M. Surko, T.N.C. Venkatesan, R.E. Slusher and J.A. Golovchenko, Appl. Phys. Lett. 33 (1978) 437. [9] Y.S. Lin and K.S. Wang, Appl. Phys. Lett. 34 (1979) 363. [10] D.H. Auston, J.A. Golovechenko, A.L. Simons, C.M. Surko and T.N.C. Venkatesan, Appl. Phys. Lett. 34 (1979) 777. [11] K. Murakami, M. Kawabe, K. Gamo, S. Namba and Y. Aoyagi, Phys. Lett. 70A, (1979) 332. [12] K. Murakami, K. Garno, S. Namba, M. Kawabe and Y. Aoyagi, Appl. Phys. Lett. 35 (1979) 628. [13] A. Aydinli, H.W. Lo, M.C. Lee and A. Compaan, Phys. Rev. Lett. 46 (1981) 1640. [14] T.N.C. Venkatesan, D.H. Auston, J.A. Golovchenko and C.M. Surko, Appl. Phys. Len. 35 (1979) 88. [15] F.C. Brown, The Physics of Solids (Benjamin, New York, 1967). [16] M.R. Lorenz, G.D. Petit and R.C. Taylor, Phys. Rev. 171 (1968) 876. [17] M.B. Panish and H.C. Casey, Jr., J. Appl. Phys. 40 (1969) 163. [18] J.E. Davey and T. Pankey, J. Appl. Phys. 40 (1969) 212. [19] T. Nakayama, H. Ichikawa and N. Itoh, Surf. Sci. 123 (1982) L693. [20] R. Tsu, J.E. Baglin, G. Lasher and J.C. Tsang, Appl. Phys. Lett. 34 (1979) 153. [21] F.H. Eisen, Laser and Electron Beam Processing of Materials, eds., C.W. White and P.S. Peercy (Academic Press, New York, 1980) p. 309. [22] T. de Jong, Z.L. Wang and F.W. Saris, Phys. Lett. 90A (1982) 147. [23] T. Nakayama, Surf. Sci. 133 (1983) 101. [24] M. Okigawa, T. Nakayama and N. Itoh, Proc. 13th Int. Conf. on Defects in Semiconductors, Coronado (August, 1984) to be published. [25] D.M. Zehner, J.R. Noonan, H.L. Davis, C.W. White and G.W. Ownby, Laser and Electron-Beam Solid Interactions and Material Processing, eds., J.F. Gibbons, L.D. Hess and T.W. Sigmon (North-Holland, Amsterdam, 1981) p. 111. [26] J.M. Moison and M. Bensoussan, J. Vac. Sci. Technol. 21 (1982) 315. [27] N. Itoh, Adv. in Phys. 31 (1982) 491. [28] J.A. Van Vechten, Proc. Meeting of Materials Research Society, Boston (1983) to be published. [29] J.A. Van Vechten, R. Tsu and F.W. Saris, Phys. Lett. 72A (1979) 422. [30] N. Itoh and T. Nakayama, Phys. Lett. 92A (1982) 471. [31] J.A. Van Vecliten, Cohesive Properties of Semiconductors under Laser Irradiation, ed., D. Lande (M. Nijhoff, The Hague, 1983) p. 429. [32] P.J. Feibelman and M.L. Knotek, Phys. Rev. B18 (1978) 6531. [33] P.W. Anderson, Phys. Rev. Lett. 34 (1975) 953.
LASER-INDUCED Mitsuru OKIGAWA,
Nuclear Instruments and Methods in Physics Research B9 (1985) 60 65 North-Holland. Amsterdam
OPTICAL
REFLECTION
Takeyoshi NAKAYAMA
AND ABSORPTION
OF GaP
and Noriaki ITOH
Department of Crystalline Materials Science, Faculty of Engineering, Nagoya University, Furo - cho, Chikusa - ku, Nagoya 464, Japan
Received 15 June 1984 and in revised form 19 November 1984
Changes in the optical absorption and reflection coefficients of GaP induced by irradiation with 600 ns laser pulses of the wavelengths at the direct and indirect band gaps have been measured. It is found that a persistent increase in the absorption coefficient and a permanent decrease in the reflection coefficient, in addition to transient increases in the absorption and reflection coefficients, are induced, by irradiation at the band gap. The persistent component is found to be evolved with a surprisingly long time constant of about 10 ms. This component is ascribed to laser-induced modification of the surface layers or damage formation. The transient component is ascribed to temperature rise on the basis of time-resolved optical absorption measurements. It is found that the persistent changes are induced by irradiation with the indirect band gap photons at a fluence which induces only a little change in the transient reflectivity. The threshold laser fluence to create the surface modification by the indirect band-gap irradiation is found to be only four times that by direct band-gap irradiation. A delayed reflectivity change is found to be induced by irradiation at the indirect band gap and is ascribed to the modification of surface layers by photons absorbed at the surface layer, which enhances the absorption coefficient. We interpret these experimental results in terms of non-thermal laser-induced atomic processes.
1. Introduction It has been well established that laser annealing is induced by formation of temporarily molten surface layers [1]. Laser-irradiation, however, is known to introduce damage that cannot be annealed completely [2,3]. For compound semiconductors, recent studies by the present authors of laser-induced effects by means of Rutherford backscattering (RBS) channeling experiments have shown that the surface layer of G a P is decomposed into a Ga-rich layer and a P-layer, which is located on the irradiated side, and that such a modification of the surface layer is more pronounced for a 600 ns laser pulse than for a 15 ns laser pulse of the same fluence [4,5]. Such an enormous dependence of the laser-induced effects on the pulse width may be related to the difference in the temperature rise induced by a 15 ns and 600 ns laser pulse. In view of the results of a calculation, which indicated that a 15 ns laser pulse causes a much larger temperature rise than a 600 ns laser pulse of the same fluence, we interpreted that the larger laser-induced surface modification by a 600 ns laser pulse originates from a non-thermal process. Measurements of laser-induced optical reflectivity changes from a surface layer are known to yield information on formation of electron-hole plasma [6,7] of molten phases [8-10] and of a damaged layer [11,12]. Optical absorption due to free electrons has also been measured [13]. An advantage of these optical techniques over the RBS-channeling technique is that the former 0168-583X/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
are capable of yielding information on the time-evolution of the damaged layers. Since such information is useful for understanding the mechanism of laser-induced surface-layer modification, we have undertaken experimental investigations of the surface-layer modification by employing optical techniques. In this paper we report the optical absorption and reflection measurements of GaP irradiated with a 600 ns laser pulse. For studying the laser-induced surface-layer modification, use of 600 ns laser pulses is advantageous, since the surface layer is modified to a greater extent [4,5]. GaP is an indirect semiconductor and hence changing the wavelength may alter the heating rate to some extent. It is shown that both the optical absorption and reflectivity changes have transient and persistent components. The former was ascribed to the temperature rise, the latter to the modification of surface layers.
2. Experimental technique Specimens used in the present investigation are n-type G a P (100) wafers doped with 5 × 10 -17 cm -3 Te. They were polished mechanically and then etched with H C 1 - 2 H N O 3 - H 2 0 solution at 50°C for 15 min. The specimens were placed in air in the optical paths of flash-lamp pumped dye laser, which is used for creating damage, and of a H e - N e laser or of light from a xenon lamp which is used for probing. The dye laser was operated with single-pulse mode with a width of
M. Okigawa et al. / Laser-induced absorption of GaP 600 ns. To obtain the laser-induced change of optical absorption and reflection, H e - N e laser or light from the xenon lamp was transmitted through or reflected from the irradiated surface and detected with a photomultiplier, the output of which was recorded on a storage oscilloscope. A monochromator was placed in the optical path between the specimen and the photomultiplier to obtain time-resolved optical absorption spectra. The dye-laser was incident normally on the specimen through a hole 2 mm O, while the probe light was 30 ° from the surface normal. The spot size of the H e - N e laser at the specimen was 0.5 mm and that of light from the Xe lamp was 2 mm 0 . For the reflection measurements, light emergent 30 ° from the surface normal was detected. The intensity of the dye laser was measured with a photodiode, which was calibrated by using a calorimeter.
3. Experimental results and discusssion 3.1. Effect of irradiation near the direct band gap
61
clearly seen at lower fluences, while the magnitude of the persistent change increases with increasing the laser fluence. In order to clarify the nature of the two components, we measured the laser-induced optical absorption change at various wavelengths using a Xe lamp and the monochromator. The result obtained with a fluence of 1.6 J / c m 2 is shown in fig. 2. In this case the peak is blurred because of the large size of the light beam from the xenon lamp. It is clear that the peak is observed only for shorter wavelengths and is absent for longer wavelengths. The time-resolved optical absorption spectra were derived from fig. 2 and shown in fig. 3. The results indicate that the optical absorbance is not dependent on the wavelength between 700 and 900 nm but is enhanced towards shorter wavelength. This increase towards the shorter wavelength is largest around 1.0 /~s and is absent in the final spectrum, which is independent of wavelength. The reflectivity change induced by a 440 nm laser pulse was also measured using H e - N e laser and the result is shown in fig. 4. It is clear that the reflectivity starts to increase at the start of the laser pulse. Above
First we describe the results of laser-irradiation at 440 nm (2.82 eV), which is close to the direct band gap (2.78 eV). A typical laser-induced change in optical absorption, probed with the He-Ne laser, is shown in fig. 1. It is clear that the optical absorption change consists of a peak slightly delayed in time from the laser pulse and a component that shows a persistent change. In the latter component, an increase in the optical absorption within 3/~s and a slow build up with a time constant of about 10 ms are observed. The peak is more
?
'--'~
540 nm
Ul ¢-
~IR0 . 8
10
c-
~o 0.6
Ko f ~
1.8 Jcm -2 O C
7(10 nm
._c
° I
____/
0.4
Ot ¢-
~0.2
J _
i-
'U
826, nm
J~ (J
f
0
882 nm
taS¢ r f t¢
,
; i
0
1
2
time ( p s )
3
i
i
i
integrat of , tase, r pulse
2O406O
time(ms)
Fig. 1. Change in optical density of GaP probed by 633 nm light (He-Ne laser) during and after irradiation with a 440 nm laser pulse having a duration of 0.6/~s and fluences of 1.0, 1.6 and 1.8 J/cm 2. Arrows show the final value (after an hour) of the optical density change. The profile of the laser pulses is shown at the bottom.
0
1
2
3
time (ps) Fig. 2. Change in optical density of GaP probed by Xe lamp light (540-882 nm) during and after irradiation with a 440 nm laser pulse having a duration of 0.6 ~s and a fluenc~ of 1.6 J/cm 2. The integral of the laser pulse is shown at the bottom.
62
M. Okigawa et al. / Laser-induced absorption of GaP I
I
I
"o-O~o~o__.o O.I ps "¢ *'-- o.... o
is above 1.4 J / c m 2. A persistent decrease similar to that found presently has been reported for GaP irradiated with a train of picosecond laser pulses [11,12]. In order to clarify the nature of the persistent change in optical reflectivity and absorbance, we compare, in fig. 5, the change in optical absorbance 3 /~s after the pulse and the final changes of both optical absorbance and reflectivity with the change A X m i n in the m i n i m u m yield of the RBS-channeling obtained previously [5]. It is known that AXminis indicative of the laser-induced persistent surface-layer modification. It is clear that all of these values start changing at nearly the same fluence. Moreover the relative increase in the AXmin is nearly parallel to these changes. Thus it is concluded that the persistent changes in the reflectivity and absorbance originates in the modification of the surface layers that causes the increase in A X m i n . We note further that the reflectivity decreases even through the absorbance increases. Since the increase in the optical absorbance causes the increase in the reflectivity, if the real part of the reflectivity is not changed [15], a substantial change in the material is induced on the surface layer. This conclusion is consistent with the result of RBS-channeling studies, which indicate that the surface layer is
o---o- L
0.2
~0~o ~o
0.3
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8
~,........., I. 0
o 0
i1__11.
20
¢:: 0 .C ¢J
,
,.
\
+\
-
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t -,.....,+ _ _ +
final
I
500
I
700
600
I
•
800
900
+
1.2
wavelength (nm) Fig. 3. Time-resolved optical absorption spectra of GaP after irradiation with a laser pulse having a fluence of 1.6 J/cm 2.
13
'~ 0.8 U
1.4 J / c m 2, the increase in the reflectivity saturates at about twice of the original reflectivity. Such a saturation for GaAs [10,11,14] and G a P [12] has been ascribed to the formation of the molten phase. We note also that the reflectivity is reduced finally when the laser fluence
+
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2
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0.8
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-
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o 0.6
0.2 ,
/
fina[
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0.5
u
"d
0
GoP- 44 n
0 0
I
0 2
3
time (ps) Fig. 4. Change in optical reflectivity of GaP probed by 633 nm fight (He-Ne laser) during and after irradiation with a 440 nm laser pulse having a duration of 0.6 #s and fluences of 0.9, 1.4, 1.8 and 2.3 J/cm 2. The arrows show the final value (after an hour) of the reflectivity change.
i
i
i
i
0.5
1.0
1.5
2.0
2.5
laser flucnce ( J cm -2) Fig. 5. The dependence on the laser fluence of the permanent increase in optical absorbance (crosses) and of reflectivity (triangles) of GaP induced by 440 nm laser pulses with a duration of 0.6 Vs. The dependence of the change in the optical density (open circles) 3 ~s after the pulse and of the minimum RBS-channeling yield (closed circles) AXmin are also shown.
M. Okigawa et aL / Laser- induced absorption of GaP
decomposed into a gallium-rich layer and a phosphorous layer by laser-irradiation [5]. We note also that this final optical absorption change is almost wavelength independent• This result is also consistent with the result of RBS-channeling, since both the Ga-layer and P-layer will show almost wavelength independent optical absorption. As discussed above the increase in the reflectivity has been ascribed to the temperature rise. It is known also that the temperature rise induces a shift in the opticalabsorption vs photon-energy relation to longer wavelength [16,17]. The time-resolved optical absorption spectra shown in fig. 3 is regarded as a superposition of the wavelength-dependent optical absorption change in the wavelength region shorter than 700 nm and wavelength-independent absorption change. The former is indicative of the shift of the optical absorption due to the temperature rise which is known to decrease drastically as the wavelength increases. Since the shift of the absorption edge does not reach beyond 700 nm, as seen in fig. 3, the optical absorption change measured at 800 nm includes only the latter component, while that measured with the H e - N e laser includes both components• Since, as seen from fig. 2, the absorption change measured at 800 nm begins to increase at the start of the laser pulse, we consider that the persistent component begins to be formed at the start of laser irradiation. 3.2. Effect of irradiation near the indirect band gap
A study of the laser-induced surface modification at the indirect band gap is of interest, since the heating effect is less significant because of the low absorption coefficient. Fig. 6 shows the results of the optical reflection and absorption measurements induced by a dyelaser at 536 nm; the probe light is a H e - N e laser. The most significant results are that for a fluence less than 2.7 J / c m 2 the reflectivity change is extremely small while the persistent optical absorption change is evident, and that for all fluences studied there is a delay of about 500 ns in the increase in the reflectivity. The former result indicates that the persistent absorption change is not necessarily associated with the temperature rise. The latter result indicates that the temperature rise does not occur at the beginning of laser irradiation, which is easily understood since the laser is not strongly absorbed. The fact that the increase in the reflectivity and hence the temperature rise occurs with a delay is indicative of surface layer modification that causes the strong optical absorption. The fact that the surface layer is modified by irradiation at the indirect-band-gap irradiation is probably due to the high optical absorption at the surface top layer for the indirect band-gap photons. We note here that the persistent reflectivity change due to the surface-layer modification induced by 536 nm laser continues increasing to a steady value but
63
2
3.2 J.cm-
d
"~" " ~
2"1
""= " "
0
0.8 "o
0.6
3.0 J.cm-2 0
0.4
0.2 O~ C
0
1
2
3
time ( IJs ) Fig. 6. Change in optical reflectivity a) and absorbance b) of GaP during and after irradiation with a 536 nm laser pulse having a duration of 0.6/~s and several fluences as indicated in the figure. Arrows show the final (after an hour) optical density and reflectivity changes.
the persistent optical absorption change is partially annihilated. The latter behavior is contrary to that observed for irradiation at the direct band gap (see fig. 1). The dependence of the amount of the persistent optical absorption change on the laser wavelength was also measured. Fig. 7 shows the relation between the laser-induced final optical absorption change and the laser fluence obtained at several wavelengths. The
+
A
/
1,2
final vatue
~ 1.0 .~ 0.8 K 0
/¢
/d /
.=_ 0.6
0.4
"Y $ 1
//o
0.2
//.
+
0.5
1.0 1.5 2.0 2.5 taser ftuence ( J.cm-2 )
3.0
Fig. 7. Laser-fluence dependence of the final value of the optical density change of GaP induced by a 0.6/~s laser pulse of several wavelengths.
M. Okigawa et al. / Laser-induced absorption of GaP
64
wavetength ( nm ) 600 3.0 i
E
2.5
u
500 .
.
.
.
42(
I
j
GaP~-O.6ps laser / / / /
/ 1 04 I
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2.0 o to
~- 1.5
/
/
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/
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10 3 u t-
.9
////
O t-
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0.5
0
0 .Q
/ / I I II
2.0
g
I I
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10 2
I
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I
2.4
2.6
2.8
3.0
photon energy ( eV )
Fig. 8. Wavelength dependence of the threshold fluence for laser-induced damage (open circles). Fundamental optical absorption spectrum of GaP is also shown (ref. [18]).
threshold laser fluence above which the optical absorption change is observed is plotted as a function of the wavelength by open circles in fig. 8. Fig. 8 includes also the wavelength dependence of the optical absorption coefficient [18], which is scaled logarithmically. A remarkably slow dependence of the threshold laser fluence has been observed also for the laser-induced sputtering of GaP [19], although a sharp dip in the threshold laser fluence at the indirect band gap has been observed for sputtering.
3.3. On the persistent optical absorption and reflection changes The measurements of the optical absorption and reflectivity change induced by a laser pulse of the direct-band-gap and indirect-band-gap energies have revealed that a persistent change in the surface layers which causes an optical absorption increase and a reflectivity decrease, is induced. This persistent change is clearly distinguished from the temperature rise, which induces a transient absorption and reflectivity increases near the optical absorption edge. We summarize the experimental results for the persistent change as follows: (1) For irradiation with a 1.6 J / c m 2 pulse, the persistent optical absorption changes at 882 nm (fig. 2) appear to start to increase immediately after the pulse, even though the delay in melt for a 1.8 J / c m 2 pulse is 200 ns (fig. 4). (2) The persistent change is formed by irradiation by a 2.6 J / c m 2 laser pulse of the indirect-
band-gap energy, while the temperature rise is negligible (fig. 6). (3) The fluence dependence of the persistent change is very similar to that of the change in the channeling minimum yield. Based on the results (3), we conclude that the origin of the persistent change in the optical absorption and reflectivity and that of the channeling minimum yield are the same. The RBS measurements have shown that the surface layer is decomposed into a Ga-rich layer and a P-layer. The P-layer which is on the surface, is considered to give a reduction of the reflectivity and an increase in the optical absorbance, which is consistent with the above conclusion. According to the present results, the persistent change appears to start to be evolved at the start of the laser pulse and to evolve further with a time constant of 14 ms. We refer to the condition that formed intermediately as the precursor state, the nature of which is not yet clear. Since the precursor state induces the change in the real part of the dielectric constants, it involves also a substantial change in the material. The transformation from the precursor state to the final decomposed state may involved oxidation of phosphorous or slow diffusion. This transformation is larger as the fluence is larger. We note also that the similar precursor state is formed by irradiation with indirect-band-gap photons but the precursor state is partially annealed in this case, as observed from optical absorption measurement. Thus whether the precursor is annealed or evolved appears to depend on the condition of irradiation. To reveal the nature of the precursor state is a problem for future investigation. Since the formation of the precursor state occurs even when the reflectivity change due to the temperature rise is small, as in the case of irradiation with indirect-band-gap photons, we conclude that the formation of the precursor state and hence the formation of the decomposed layers is initiated by a non-thermal process. The reason why the formation of a decomposed layer is observed by irradiation with a 600 ns laser pulse, but not to any appreciable extent with a 15 ns laser pulse [14,20-22], has been interpreted [4,5] to be due to the maximum temperature reached by a 600 ns laser pulse being lower and hence the effect of the electronic excitation is more prevalent. We note also that irradiation of semiconductors with a laser pulse with a fluence lower than the melt threshold by a factor more than 10 induces sputtering of atomic species [23,24] and surface damage as detected by LEED patterns [25,26]. The results indicate that the atomic displacement is induced by laser irradiation well below the melt threshold. We consider that the same process may lead to the formation of the precursor state, if the surface layer is heated by laser irradiation. The process may be similar to the photolytic decomposition in alkali halides of which the primary process is known to be the formation of Frenkel pairs by electronic excitation [27]. In
M. Okigawa et al. / Laser-induced absorption of GaP view of the existence of the threshold fluence for sputtering and the surface layer modification, the "photolytic decomposition" of the surface layer of comp o u n d semiconductors appears to occur only under dense electronic excitation. There has been a great deal of discussion of the mechanisms of the non-thermal laser-induced processes [28,29]. Briefly, the mechanisms discussed can be divided into two types: bond weakening [29] and recombination-induced [30,31]. Even though the formation of Frenkel excitons [31] has been suggested for the latter, we consider [30] that the Feibelman-Knotek-type multi-hole states [32] are formed under an electron-hole plasma near the surface, assisted by screening of Coulomb repulsion and an Anderson negative-U interaction [33]. It appears that one of the clues to reveal the mechanism is to investigate the precursor state employing several analytical techniques. In the present paper we have shown the optical absorption and reflection change by irradiation of GaP with a 600 ns laser pulse of direct-band-gap and indirect-band-gap energies. It is shown that in addition to the optical absorbance and reflectivity increases, as usually observed with a shorter laser-pulse irradiation, a persistent optical absorbance increase and reflectivity decrease is induced. For the direct-band-gap irradiation, these changes increase further with a time constant of 10 ms, while those for the indirect-band-gap irradiation are partially annealed. The laser-induced persistent changes were ascribed to a non-thermal effect.
References [1] B.R. Appleton and G.K. Celler (eds), Laser and Electron Beam Interactions with Solids (North-Holland, Amsterdam, 1982). [2] L.C. Kimerling and J.L. Benton, Laser and Electron Beam Processing of Materials, eds. C.W. White and P.S. Peercy (Academic Press, New York, 1980) p. 385. [3] J.A. Golovchenko and T.N.C. Venkatesan, Appl. Phys. Lett. 32 (1978) 147. [4] M. Okigawa, T. Nakayama, K. Takayama and N. Itoh, Solid State Commun. 49 (1984) 347. [5] M. Okigawa, T. Nakayama, K. Morita and N. Itoh, Appl. Phys. Lett. 43 (1983) 1054. [6] L.M. Blinov, V.S. Vavilov and G.N. Galkin, Sov. Phys. Semicond. 1 (1967) 1124. [7] J.M. Lin, H. Kurz and N. Bloembergen, Appl. Phys. Lett. 41 (1982) 643.
65
[8] D.H. Auston, C.M. Surko, T.N.C. Venkatesan, R.E. Slusher and J.A. Golovchenko, Appl. Phys. Lett. 33 (1978) 437. [9] Y.S. Lin and K.S. Wang, Appl. Phys. Lett. 34 (1979) 363. [10] D.H. Auston, J.A. Golovechenko, A.L. Simons, C.M. Surko and T.N.C. Venkatesan, Appl. Phys. Lett. 34 (1979) 777. [11] K. Murakami, M. Kawabe, K. Gamo, S. Namba and Y. Aoyagi, Phys. Lett. 70A, (1979) 332. [12] K. Murakami, K. Garno, S. Namba, M. Kawabe and Y. Aoyagi, Appl. Phys. Lett. 35 (1979) 628. [13] A. Aydinli, H.W. Lo, M.C. Lee and A. Compaan, Phys. Rev. Lett. 46 (1981) 1640. [14] T.N.C. Venkatesan, D.H. Auston, J.A. Golovchenko and C.M. Surko, Appl. Phys. Len. 35 (1979) 88. [15] F.C. Brown, The Physics of Solids (Benjamin, New York, 1967). [16] M.R. Lorenz, G.D. Petit and R.C. Taylor, Phys. Rev. 171 (1968) 876. [17] M.B. Panish and H.C. Casey, Jr., J. Appl. Phys. 40 (1969) 163. [18] J.E. Davey and T. Pankey, J. Appl. Phys. 40 (1969) 212. [19] T. Nakayama, H. Ichikawa and N. Itoh, Surf. Sci. 123 (1982) L693. [20] R. Tsu, J.E. Baglin, G. Lasher and J.C. Tsang, Appl. Phys. Lett. 34 (1979) 153. [21] F.H. Eisen, Laser and Electron Beam Processing of Materials, eds., C.W. White and P.S. Peercy (Academic Press, New York, 1980) p. 309. [22] T. de Jong, Z.L. Wang and F.W. Saris, Phys. Lett. 90A (1982) 147. [23] T. Nakayama, Surf. Sci. 133 (1983) 101. [24] M. Okigawa, T. Nakayama and N. Itoh, Proc. 13th Int. Conf. on Defects in Semiconductors, Coronado (August, 1984) to be published. [25] D.M. Zehner, J.R. Noonan, H.L. Davis, C.W. White and G.W. Ownby, Laser and Electron-Beam Solid Interactions and Material Processing, eds., J.F. Gibbons, L.D. Hess and T.W. Sigmon (North-Holland, Amsterdam, 1981) p. 111. [26] J.M. Moison and M. Bensoussan, J. Vac. Sci. Technol. 21 (1982) 315. [27] N. Itoh, Adv. in Phys. 31 (1982) 491. [28] J.A. Van Vechten, Proc. Meeting of Materials Research Society, Boston (1983) to be published. [29] J.A. Van Vechten, R. Tsu and F.W. Saris, Phys. Lett. 72A (1979) 422. [30] N. Itoh and T. Nakayama, Phys. Lett. 92A (1982) 471. [31] J.A. Van Vecliten, Cohesive Properties of Semiconductors under Laser Irradiation, ed., D. Lande (M. Nijhoff, The Hague, 1983) p. 429. [32] P.J. Feibelman and M.L. Knotek, Phys. Rev. B18 (1978) 6531. [33] P.W. Anderson, Phys. Rev. Lett. 34 (1975) 953.