- Email: [email protected]
Hydrogen molecules in GaAs
ARTICLE IN PRESS
Physica B 340–342 (2003) 329–332
Hydrogen molecules in GaAs E.V. Lavrova,b,*, J. Webera a
ITTP, Technische Universitat . Dresden, Mommsenstr. 13, 01062 Dresden, Germany b IRE RAS, 101999 Moscow, Russia
Abstract GaAs samples treated in a hydrogen plasma have been studied by Raman spectroscopy. In addition to the known Raman line at 3912 cm1 of H2 trapped at the interstitial TGa site surrounded by Ga neighbors, two new Raman signals at 4043 and 4112 cm1 have been observed at room temperature. The 4043 cm1 line is assigned to H2 trapped at the interstitial TAs site with As closest neighbors and the 4112 cm1 line is associated with H2 trapped in voids formed by the hydrogen plasma. Para-H2 trapped at the interstitial TGa site is shown to be unstable against irradiation with the band-gap light at room temperature and can be observed only at temperatures below 120 K: r 2003 Elsevier B.V. All rights reserved. PACS: 61.72.Ji; 78.30.Fs Keywords: Hydrogen; GaAs; Raman scattering
1. Introduction GaAs is a compound semiconductor with two types of interstitial sites: one with Ga closest neighbors ðTGa Þ; the other one with As atoms nearby ðTAs Þ: All theories predict that TGa is the most favorable site for H2 [1–4] and, therefore, the only known Raman signal of H2 in GaAs at 3912 cm1 is assigned to the molecule trapped at TGa [5]. Here we report on the Raman scattering study of GaAs treated in an hydrogen (H) plasma. Two new Raman signals at 4043 and 4112 cm1 are detected. The 4043 cm1 line is identified as H2 at the TAs site, whereas the line at 4112 cm1 is *Corresponding author. Tel.:+49-351-463-33637; fax:+49351-463-37060. E-mail address: [email protected] (E.V. Lavrov).
assigned to the molecule trapped in voids formed by the plasma treatment. We present the annealing kinetics in the dark of H2 at TGa performed in the temperature range from room temperature (RT) to 100 C: It is shown that the Raman signal of para-H2 in GaAs is affected by the band-gap light. The appropriate Raman line can be observed only at temperatures below 120 K:
2. Experimental GaAs samples used in this study were n-type ð1 0 0Þ-wafers with resistivity of 0:06 O cm grown by the vertical gradient freeze (VGF) technique. For hydrogenation, the samples were exposed for 15 min–4 h to a hydrogen and/or deuterium (D) DC plasma in a parallel plate system, with a plate
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.081
ARTICLE IN PRESS E.V. Lavrov, J. Weber / Physica B 340–342 (2003) 329–332
330
voltage of 1000 V: The temperature of the sample block was varied from RT to 250 C: Raman measurements were performed with the 413:1 nm line of a Krþ laser for excitation. A detailed description of the Raman and plasma setups can be found in Ref. [5].
3. Results and discussion 3.1. New H2 signals Fig. 1 shows the Raman spectra measured at RT for GaAs samples treated with H and/or D plasma. During the treatment, the samples were lying on the cathode of the plasma system. The intense Raman lines labeled by H2 ; HD, and D2 dominate in the spectra. These lines positioned at 3912, 3429, and 2827 cm1 were previously assigned to H2 trapped at the TGa site [5]. In the case of H-treated sample, two additional lines at B4112 and 4043 cm1 are seen in the spectrum with FWHMs of B70 and 12 cm1 ; respectively. The 4112 and 4043 cm1 lines are replaced by two lines with lower frequencies at 2967 and 2920 cm1 ; when using a pure D plasma instead of H. Finally, using a mixture of 50% H2 and 50%
H2O
HD
3602
GaAs:D 3541
H2
*
GaAs:H
3000
4112
GaAs:H:D 4043
2920
Intensity (arb. units)
2967
D2
3500
Raman shift (cm−1)
4000
Fig. 1. Raman spectra of a GaAs sample measured at 350 K in the spectral region characteristic for the hydrogen molecule stretch vibrational modes after exposure to D (top), D and H (middle), and H (bottom) plasma at RT for 30 min: The line labeled as H2 O originates from the atmospheric water.
D2 for the plasma, these lines plus additional ones at 3541 and 3602 cm1 appear in the spectrum. The frequencies of the stretch modes of the free H2 ; HD, and D2 are 4161, 3632, and 2994 cm1 ; respectively. As can be seen from Fig. 1, for each isotope combination the frequencies of the new modes are located between the frequencies of the corresponding hydrogen isotopes trapped at the TGa site and the free molecule. This strongly indicates that the new Raman signals originate from other forms of H2 in GaAs. The energy difference between the TAs and TGa sites most of the theories find small, tens of meV [1,2], which implies that even at RT the fraction of H2 located at the TAs can be as much as a few percent of the total H2 concentration. Within the error bars of the calculation methods local vibrational modes of the H2 trapped at the two different interstitial sites are rather close to each other as well [3,4]. All these findings point to the interstitial H2 trapped at the TAs site as the most likely candidate for the 4043 cm1 line. This line is much weaker than the Raman line of the H2 at the TGa site but is still clearly detectable indicating that the energy difference (DE) between two configurations is not too high. Each isotope of the molecule gives rise to the Raman line with FWHM close to the appropriate Raman line of H2 at the TGa site. From the relative intensities of the 4043 and 3912 cm1 lines shown in Fig. 1 the Boltzmann exponent gives DE ¼ 60 meV: The other new H2 Raman line at 4112 cm1 is much broader than the interstitial Raman H2 signals and is positioned close to the signal of the free H2 at 4161 cm1 : Based on this, we tentatively assign the 4112 cm1 line to a hydrogen molecule trapped in voids formed by plasma treatment, which from TEM studies are known to exist in our samples [6]. 3.2. H2 at the TGa site 3.2.1. Annealing kinetics The decrease of the Raman signals of H2 ; HD and D2 as a function of the annealing time at 100 C is shown in Fig. 2. The spectra are measured just after the plasma treatment and after
ARTICLE IN PRESS E.V. Lavrov, J. Weber / Physica B 340–342 (2003) 329–332 D2:TGa
H2:TGa
HD:TGa
Intensity (arb. units)
ortho para
(a) (b) (c) (d)
2800
2850
3400
3450
Wave numbers (cm−1)
3900
3950
Fig. 2. Raman spectra of a GaAs sample measured at 95 K after exposure to hydrogen and deuterium plasma at 250 C for 3 h: (a) just after the plasma treatment and after annealing at 100 C (b) for 1 min; (c) 2 min; and (d) 3 min: Spectra are offset vertically for clarity.
annealing for 1, 2, and 3 min: As seen from the figure, the decay rates of the molecule within the noise level depend neither on the isotope nor on the nuclear spin state of H2 : The activation energy and prefactor of the annealing rate were determined from the annealing kinetics of the H2 Raman signal at RT, 65 C and 100 C: This time dependence can be fitted by t ¼ t0 expðEa =kTÞ with t0 in the range 1:4 1012 – 1:4 1010 s and the activation energy Ea ¼ 920765 meV: From SIMS data we know that all H2 after the plasma treatment are located in a thin layer close to the sample surface. An explanation of our annealing data presented in Fig. 2 could be a diffusion of the molecules into the bulk of the sample. The probing depth of our laser is 10 nm: We can, therefore, estimate the diffusion constant of H2 at the TGa site; D ¼ D0 expðEa =kTÞ; where D0 is in the region 3–0:03 cm2 s1 and Ea ¼ 920765 meV: Note that the value of Ea agrees reasonably well with the predicted value of the H2 diffusion barrier, which is in between 0.9 and 1 eV [1,3]. On the microscopic scale, the diffusion process happens in two steps: first the molecule jumps from the TGa site to the neighboring TAs site and then to the next TGa site. The upper limit for D0 one can estimate from D0 ¼ l 2 =t; where l is the distance between the neighboring TGa and TAs
331
sites and t ¼ l=v%T ; the time needed to make a jump of the molecule with the mean thermal velocity v%T : ( and at RT tE1013 s: Thus, For GaAs, l ¼ 2:46 A we get that D0 should not be larger than 4 103 cm2 s1 : In Si the experimental value of the diffusion constant at around 50 C was found to be DH2 ¼ 2:6 104 expð780 meV=kTÞ cm2 s1 [7]. In GaAs the value of D0 determined from the annealing experiments is at least 10 times larger than expected and is, therefore, not consistent with the microscopic model of the diffusion process. In order to explain the annealing data in GaAs, we consider a dissociation process rather than diffusion of the molecule into the bulk of the sample. GaAs is an ionic crystal. The molecule at the hexagonal H interstitial site through which the diffusion takes place is affected by a strong local electrical field. This is different from the case of Si, which is a covalent crystal and therefore the energy surface at the H site is flat resulting in much weaker local field compared to GaAs. This explanation, however, has a difficulty. One may expect that the dissociation rate of the molecule should depend on the molecular mass as m1=2 via the mass-dependent thermal velocity of H2 : But it follows from Fig. 2 that within the error bars all hydrogen isotopes anneal out with the same time constant. 3.2.2. Stability with respect to the band-gap light Fig. 3 shows the Raman spectra of H2 at the TGa site measured in the temperature range 100–170 K: At 100 K the Raman line is split by B8 cm1 into two components with a ratio 3:1. The two split-off components have been previously identified as ortho- and para-H2 [5]. It follows from the figure that para-H2 is not stable with respect to the bandgap illumination at elevated temperatures and appears in the Raman spectra only at temperatures below 120 K: The same phenomenon was recently observed for the interstitial H2 in Si [8]. It was also shown that para-H2 in Si disappears from the Raman spectra after prolonged storage at RT in the dark. This phenomenon was tentatively explained by a higher diffusivity of H2 in the J ¼ 0 state (para-H2 ) compared to that of the J ¼ 1 state (ortho-H2 ). It was suggested that the analogous
ARTICLE IN PRESS 332
E.V. Lavrov, J. Weber / Physica B 340–342 (2003) 329–332
4. Conclusions GaAs treated with hydrogen plasma has been studied by means of Raman spectroscopy. In addition to the known signal at 3912 cm1 originating from H2 at the TGa site, two new H2 Raman lines at 4043 and 4112 cm1 have been observed at RT. The 4043-cm1 line is assigned to H2 trapped at the TAs site. The energy difference between the molecule located at the TAs and TGa sites was found to be 60 meV: We assign the 4112 cm1 line to H2 trapped in the voids formed by the hydrogen plasma treatment. Para-H2 at the TGa site is shown to be unstable against irradiation with the band-gap light at elevated temperatures and appears in the Raman spectra only at temperatures below 120 K: 1
Acknowledgements Fig. 3. Raman spectra of a GaAs sample measured after exposure to hydrogen plasma at RT for 30 min: Spectra are offset vertically for clarity.
behavior should be valid also for interstitial H2 in GaAs [8]. Fig. 3 demonstrates that with respect to the band-gap illumination it is indeed so. Contrary to that, the annealing data presented in Fig. 2 unambiguously show that both orthoand para-species anneal out at the same rate, which seems to rule out the suggestion of different diffusivities of para- and ortho-H2 : But as pointed out in the previous section, the annealing kinetics of H2 in GaAs is unlikely to be determined by the diffusion and, therefore, one cannot directly compare the annealing data for H2 in Si and GaAs.
We thank J.R. Botha (University of Port Elisabeth) for fruitful discussions. E.V. Lavrov acknowledges the Russian Foundation for Basic Research (Grant No. 02-02-16030).
References [1] [2] [3] [4] [5] [6] [7] [8]
L. Pavesi, P. Giannozzi, Phys. Rev. B 46 (1992) 4621. S.J. Breuer, et al., Phys. Rev. B 53 (1996) 16289. Y. Okamoto, et al., Phys. Rev. B 56 (1997) 10016. C.G. Van de Walle, Phys. Rev. Lett. 80 (1998) 2177. J. Vetterh.offer, et al., Phys. Rev. Lett. 77 (1996) 5409. J.R. Botha, et al., Proc. Microsc. Soc. A 27 (1997) 37. V.P. Markevich, M. Suezawa, J. Appl. Phys. 83 (1998) 2988. E.V. Lavrov, J. Weber, Phys. Rev. Lett. 89 (2002) 215501.
Physica B 340–342 (2003) 329–332
Hydrogen molecules in GaAs E.V. Lavrova,b,*, J. Webera a
ITTP, Technische Universitat . Dresden, Mommsenstr. 13, 01062 Dresden, Germany b IRE RAS, 101999 Moscow, Russia
Abstract GaAs samples treated in a hydrogen plasma have been studied by Raman spectroscopy. In addition to the known Raman line at 3912 cm1 of H2 trapped at the interstitial TGa site surrounded by Ga neighbors, two new Raman signals at 4043 and 4112 cm1 have been observed at room temperature. The 4043 cm1 line is assigned to H2 trapped at the interstitial TAs site with As closest neighbors and the 4112 cm1 line is associated with H2 trapped in voids formed by the hydrogen plasma. Para-H2 trapped at the interstitial TGa site is shown to be unstable against irradiation with the band-gap light at room temperature and can be observed only at temperatures below 120 K: r 2003 Elsevier B.V. All rights reserved. PACS: 61.72.Ji; 78.30.Fs Keywords: Hydrogen; GaAs; Raman scattering
1. Introduction GaAs is a compound semiconductor with two types of interstitial sites: one with Ga closest neighbors ðTGa Þ; the other one with As atoms nearby ðTAs Þ: All theories predict that TGa is the most favorable site for H2 [1–4] and, therefore, the only known Raman signal of H2 in GaAs at 3912 cm1 is assigned to the molecule trapped at TGa [5]. Here we report on the Raman scattering study of GaAs treated in an hydrogen (H) plasma. Two new Raman signals at 4043 and 4112 cm1 are detected. The 4043 cm1 line is identified as H2 at the TAs site, whereas the line at 4112 cm1 is *Corresponding author. Tel.:+49-351-463-33637; fax:+49351-463-37060. E-mail address: [email protected] (E.V. Lavrov).
assigned to the molecule trapped in voids formed by the plasma treatment. We present the annealing kinetics in the dark of H2 at TGa performed in the temperature range from room temperature (RT) to 100 C: It is shown that the Raman signal of para-H2 in GaAs is affected by the band-gap light. The appropriate Raman line can be observed only at temperatures below 120 K:
2. Experimental GaAs samples used in this study were n-type ð1 0 0Þ-wafers with resistivity of 0:06 O cm grown by the vertical gradient freeze (VGF) technique. For hydrogenation, the samples were exposed for 15 min–4 h to a hydrogen and/or deuterium (D) DC plasma in a parallel plate system, with a plate
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.081
ARTICLE IN PRESS E.V. Lavrov, J. Weber / Physica B 340–342 (2003) 329–332
330
voltage of 1000 V: The temperature of the sample block was varied from RT to 250 C: Raman measurements were performed with the 413:1 nm line of a Krþ laser for excitation. A detailed description of the Raman and plasma setups can be found in Ref. [5].
3. Results and discussion 3.1. New H2 signals Fig. 1 shows the Raman spectra measured at RT for GaAs samples treated with H and/or D plasma. During the treatment, the samples were lying on the cathode of the plasma system. The intense Raman lines labeled by H2 ; HD, and D2 dominate in the spectra. These lines positioned at 3912, 3429, and 2827 cm1 were previously assigned to H2 trapped at the TGa site [5]. In the case of H-treated sample, two additional lines at B4112 and 4043 cm1 are seen in the spectrum with FWHMs of B70 and 12 cm1 ; respectively. The 4112 and 4043 cm1 lines are replaced by two lines with lower frequencies at 2967 and 2920 cm1 ; when using a pure D plasma instead of H. Finally, using a mixture of 50% H2 and 50%
H2O
HD
3602
GaAs:D 3541
H2
*
GaAs:H
3000
4112
GaAs:H:D 4043
2920
Intensity (arb. units)
2967
D2
3500
Raman shift (cm−1)
4000
Fig. 1. Raman spectra of a GaAs sample measured at 350 K in the spectral region characteristic for the hydrogen molecule stretch vibrational modes after exposure to D (top), D and H (middle), and H (bottom) plasma at RT for 30 min: The line labeled as H2 O originates from the atmospheric water.
D2 for the plasma, these lines plus additional ones at 3541 and 3602 cm1 appear in the spectrum. The frequencies of the stretch modes of the free H2 ; HD, and D2 are 4161, 3632, and 2994 cm1 ; respectively. As can be seen from Fig. 1, for each isotope combination the frequencies of the new modes are located between the frequencies of the corresponding hydrogen isotopes trapped at the TGa site and the free molecule. This strongly indicates that the new Raman signals originate from other forms of H2 in GaAs. The energy difference between the TAs and TGa sites most of the theories find small, tens of meV [1,2], which implies that even at RT the fraction of H2 located at the TAs can be as much as a few percent of the total H2 concentration. Within the error bars of the calculation methods local vibrational modes of the H2 trapped at the two different interstitial sites are rather close to each other as well [3,4]. All these findings point to the interstitial H2 trapped at the TAs site as the most likely candidate for the 4043 cm1 line. This line is much weaker than the Raman line of the H2 at the TGa site but is still clearly detectable indicating that the energy difference (DE) between two configurations is not too high. Each isotope of the molecule gives rise to the Raman line with FWHM close to the appropriate Raman line of H2 at the TGa site. From the relative intensities of the 4043 and 3912 cm1 lines shown in Fig. 1 the Boltzmann exponent gives DE ¼ 60 meV: The other new H2 Raman line at 4112 cm1 is much broader than the interstitial Raman H2 signals and is positioned close to the signal of the free H2 at 4161 cm1 : Based on this, we tentatively assign the 4112 cm1 line to a hydrogen molecule trapped in voids formed by plasma treatment, which from TEM studies are known to exist in our samples [6]. 3.2. H2 at the TGa site 3.2.1. Annealing kinetics The decrease of the Raman signals of H2 ; HD and D2 as a function of the annealing time at 100 C is shown in Fig. 2. The spectra are measured just after the plasma treatment and after
ARTICLE IN PRESS E.V. Lavrov, J. Weber / Physica B 340–342 (2003) 329–332 D2:TGa
H2:TGa
HD:TGa
Intensity (arb. units)
ortho para
(a) (b) (c) (d)
2800
2850
3400
3450
Wave numbers (cm−1)
3900
3950
Fig. 2. Raman spectra of a GaAs sample measured at 95 K after exposure to hydrogen and deuterium plasma at 250 C for 3 h: (a) just after the plasma treatment and after annealing at 100 C (b) for 1 min; (c) 2 min; and (d) 3 min: Spectra are offset vertically for clarity.
annealing for 1, 2, and 3 min: As seen from the figure, the decay rates of the molecule within the noise level depend neither on the isotope nor on the nuclear spin state of H2 : The activation energy and prefactor of the annealing rate were determined from the annealing kinetics of the H2 Raman signal at RT, 65 C and 100 C: This time dependence can be fitted by t ¼ t0 expðEa =kTÞ with t0 in the range 1:4 1012 – 1:4 1010 s and the activation energy Ea ¼ 920765 meV: From SIMS data we know that all H2 after the plasma treatment are located in a thin layer close to the sample surface. An explanation of our annealing data presented in Fig. 2 could be a diffusion of the molecules into the bulk of the sample. The probing depth of our laser is 10 nm: We can, therefore, estimate the diffusion constant of H2 at the TGa site; D ¼ D0 expðEa =kTÞ; where D0 is in the region 3–0:03 cm2 s1 and Ea ¼ 920765 meV: Note that the value of Ea agrees reasonably well with the predicted value of the H2 diffusion barrier, which is in between 0.9 and 1 eV [1,3]. On the microscopic scale, the diffusion process happens in two steps: first the molecule jumps from the TGa site to the neighboring TAs site and then to the next TGa site. The upper limit for D0 one can estimate from D0 ¼ l 2 =t; where l is the distance between the neighboring TGa and TAs
331
sites and t ¼ l=v%T ; the time needed to make a jump of the molecule with the mean thermal velocity v%T : ( and at RT tE1013 s: Thus, For GaAs, l ¼ 2:46 A we get that D0 should not be larger than 4 103 cm2 s1 : In Si the experimental value of the diffusion constant at around 50 C was found to be DH2 ¼ 2:6 104 expð780 meV=kTÞ cm2 s1 [7]. In GaAs the value of D0 determined from the annealing experiments is at least 10 times larger than expected and is, therefore, not consistent with the microscopic model of the diffusion process. In order to explain the annealing data in GaAs, we consider a dissociation process rather than diffusion of the molecule into the bulk of the sample. GaAs is an ionic crystal. The molecule at the hexagonal H interstitial site through which the diffusion takes place is affected by a strong local electrical field. This is different from the case of Si, which is a covalent crystal and therefore the energy surface at the H site is flat resulting in much weaker local field compared to GaAs. This explanation, however, has a difficulty. One may expect that the dissociation rate of the molecule should depend on the molecular mass as m1=2 via the mass-dependent thermal velocity of H2 : But it follows from Fig. 2 that within the error bars all hydrogen isotopes anneal out with the same time constant. 3.2.2. Stability with respect to the band-gap light Fig. 3 shows the Raman spectra of H2 at the TGa site measured in the temperature range 100–170 K: At 100 K the Raman line is split by B8 cm1 into two components with a ratio 3:1. The two split-off components have been previously identified as ortho- and para-H2 [5]. It follows from the figure that para-H2 is not stable with respect to the bandgap illumination at elevated temperatures and appears in the Raman spectra only at temperatures below 120 K: The same phenomenon was recently observed for the interstitial H2 in Si [8]. It was also shown that para-H2 in Si disappears from the Raman spectra after prolonged storage at RT in the dark. This phenomenon was tentatively explained by a higher diffusivity of H2 in the J ¼ 0 state (para-H2 ) compared to that of the J ¼ 1 state (ortho-H2 ). It was suggested that the analogous
ARTICLE IN PRESS 332
E.V. Lavrov, J. Weber / Physica B 340–342 (2003) 329–332
4. Conclusions GaAs treated with hydrogen plasma has been studied by means of Raman spectroscopy. In addition to the known signal at 3912 cm1 originating from H2 at the TGa site, two new H2 Raman lines at 4043 and 4112 cm1 have been observed at RT. The 4043-cm1 line is assigned to H2 trapped at the TAs site. The energy difference between the molecule located at the TAs and TGa sites was found to be 60 meV: We assign the 4112 cm1 line to H2 trapped in the voids formed by the hydrogen plasma treatment. Para-H2 at the TGa site is shown to be unstable against irradiation with the band-gap light at elevated temperatures and appears in the Raman spectra only at temperatures below 120 K: 1
Acknowledgements Fig. 3. Raman spectra of a GaAs sample measured after exposure to hydrogen plasma at RT for 30 min: Spectra are offset vertically for clarity.
behavior should be valid also for interstitial H2 in GaAs [8]. Fig. 3 demonstrates that with respect to the band-gap illumination it is indeed so. Contrary to that, the annealing data presented in Fig. 2 unambiguously show that both orthoand para-species anneal out at the same rate, which seems to rule out the suggestion of different diffusivities of para- and ortho-H2 : But as pointed out in the previous section, the annealing kinetics of H2 in GaAs is unlikely to be determined by the diffusion and, therefore, one cannot directly compare the annealing data for H2 in Si and GaAs.
We thank J.R. Botha (University of Port Elisabeth) for fruitful discussions. E.V. Lavrov acknowledges the Russian Foundation for Basic Research (Grant No. 02-02-16030).
References [1] [2] [3] [4] [5] [6] [7] [8]
L. Pavesi, P. Giannozzi, Phys. Rev. B 46 (1992) 4621. S.J. Breuer, et al., Phys. Rev. B 53 (1996) 16289. Y. Okamoto, et al., Phys. Rev. B 56 (1997) 10016. C.G. Van de Walle, Phys. Rev. Lett. 80 (1998) 2177. J. Vetterh.offer, et al., Phys. Rev. Lett. 77 (1996) 5409. J.R. Botha, et al., Proc. Microsc. Soc. A 27 (1997) 37. V.P. Markevich, M. Suezawa, J. Appl. Phys. 83 (1998) 2988. E.V. Lavrov, J. Weber, Phys. Rev. Lett. 89 (2002) 215501.