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Positron interactions with overlayers of oxygen on GaAs(100)
Radiation Physics and Chemistry 58 (2000) 655±658
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Positron interactions with overlayers of oxygen on GaAs(100) J.H. Kim a,*, A. Nangia b, A.H. Weiss b a
The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama, 351-0106, Japan b Department of Physics, University of Texas at Arlington, P.O. Box 19059, Arlington, TX 76019, USA
Abstract The positron work-function, re-emission yield, and positronium fraction of an n-doped GaAs(100) surface were measured as a function of oxygen exposure. The energy distribution of positrons observed to be re-emitted indicated that the clean and oxygen exposed n-doped GaAs(100) surfaces had negative positron work-functions. The fraction of incident positrons re-emitted as bare positrons, (Y), was found to increase and the fraction re-emitted as positronium, (fPs), to decrease with increasing oxygen exposure. This suggests that surface modi®ed GaAs may be useful as a contact material in the fabrication of GaAs based FAMs. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction GaAs has been of interest due to its possible utility in ®eld-assisted positron moderator (FAM) (Shan et al., 1994). Positron mobility measurement in semi-insulating (SI) GaAs made using positron lifetime techniques indicated that this material might be suitable for making a high-eciency (010%) ®eld-assisted positron moderator. The FAM, in which the fraction of implanted positrons reaching the surface is enhanced by an applied electric ®eld which imparts to the positrons a drift velocity, was ®rst suggested by Lynn (Lynn and Makee, 1979). A decade later the fabrication of FAMs from epitaxial metal-on-semiconductor systems was proposed for materials, such as Au(Pd)/Si (Shan et al., 1994) and Au/GaAs(SI) (Shan et al., 1996).
* Corresponding author. Fax: +81-48-461-5301. E-mail address: [email protected] (J.H. Kim).
The surface properties of FAM materials, particularly as they relate to the re-emission of positrons into the vacuum, can be expected to be critical factors in the eort to develop FAMs. Therefore, an understanding of the eects of surface conditions on the positron work-function and positron re-emission fraction from GaAs is an essential step in the eorts to develop a ®eld assisted positron moderator. In this paper, the results of positron re-emission yields will be presented along with Ps fraction measurements as a function of the oxygen exposure. 2. Experimental procedures The GaAs(100) sample (n-type, Si doping level 01 1018 cmÿ3) was etched with HF solution (50%) and cleaned by sputtering and annealing cycles. The measurements of the positron work-function, positron re-emission yield, positronium fraction and surface composition of a GaAs(100) surface were performed
0969-806X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 0 ) 0 0 2 3 3 - 4
656
J.H. Kim et al. / Radiation Physics and Chemistry 58 (2000) 655±658
using the UTA-magnetically guided positron beam system (Lei et al., 1989) as a function of oxygen exposure at three dierent positron beam energies (25, 500 and 1000 eV). Approximately 5 104 slow positrons sÿ1 are generated by 20 mCi 22Na source. The energy spectra of re-emitted positrons and gamma rays were measured by varying the grid voltage (Vg) from a few volts below zero to a few volts above zero (Kim et al., 1997). Oxygen exposure was accomplished by back-®lling the chamber with oxygen and maintaining a pressure of 10ÿ6 or 10ÿ5 torr over the exposure time required. Exposure, measured in Langmuirs, L, is determined from the product of pressure and time and quanti®es the number of gas molecules allowed to impact the surface (1 L is de®ned as exposure to 10ÿ6 torr for 1 s at room temperature and corresponds roughly to the exposure needed to form a complete atomic layer if all of the atoms incident remained on the surface).
The positron work-function value was estimated from the energy spectra of re-emitted positrons measured as a function of a retarding grid voltage (Vg). The details of the positron work-function measurement were reported (Kim et al., 1997) and the value of the positron work-function was estimated as
ÿ0.6 eV from clean n-type GaAs(100) surface. Fig. 1 shows the energy spectra of re-emitted positrons from a clean (dotted line) and oxygen (70,000 L) covered GaAs(100) surface (solid line). The work-function is then taken to be dierence between two values (times e) of the peak and the zero points (V0), where positrons just start to be emitted from the sample (ÿ0.1 eV for both cases). The solid curve from the surface exposed to oxygen shows an increase of the positron work-function by 00.1 eV. This change in the positron work function is presumably due to charge transfer to the adsorbed oxygen causing an increase of the surface dipole layer. This would cause the electron work-function to become more positive and the positron workfunction to become more negative by a similar magnitude. The numbers of re-emitted positrons are plotted as a function of oxygen exposure on a GaAs(100) surface in Fig. 2. The yield of re-emitted positrons can be seen to increase with increasing oxygen exposure for both incident beam energies (500 and 1000 eV). The lower yield at higher beam energy can be explained by the fact that positrons are implanted more deeply at higher energies and thus they have a lower probability of diffusing back to the surface and being re-emitted into the vacuum. The increase in the fraction of re-emitted positrons from Ni or Cu as a result of H2S exposure (Murry et al., 1980) was interpreted as the result of the increase in the positron work-function produced by the exposure. Gullikson et al. (1988) review a number of models of positron re-emission, all of which result in a dependence of the slow positron yield on the positron
Fig. 1. The energy spectra of re-emitted positrons from a clean (dotted line) and oxygen covered GaAs(100) surface (solid line) with a 1 keV incident positron beam. The curves were obtained by taking a smoothed negative derivative of the data and normalizing the maximum to unity. The zero points were found to be ÿ0.1 eV for both cases.
Fig. 2. The slow positron yield is plotted as a function of oxygen exposure (in Langmuirs) on a GaAs(100) surface with a incident positron beam energies at 500 eV (open circles) and at 1000 eV (®lled squares). The solid and dash lines are theoretical ®ts to the positron yield data using the model function: y=y0exp[ÿ(ÿb/f+)1/2] with y0=2.1 (at 1000 eV) and 3.5 (at 500 eV) and b = 12 eV for both cases.
3. Results and discussion 3.1. Positron work-function from a clean and oxygen covered n-GaAs(100) surfaces
J.H. Kim et al. / Radiation Physics and Chemistry 58 (2000) 655±658
657
work-function y 0 (ÿf +)1/2 in agreement with their data for Ni and Cu surfaces modi®ed by alkali adsorption. Such a dependence result in a fractional change in yield
in which positron emission is increased in favor of Ps formation.
Dy=y 1=2 Df=f:
3.3. Discussion
1
Referring to Fig. 2 it may be seen that there is an increase in yield of Dy/y= 0 0.5 which is signi®cantly larger than would be predicted from Eq. (1) and the fractional change in the work-function, Df/f= 0 0.2, observed (see Fig. 1). This suggests that the increased yield observed when O was adsorbed on GaAs is not simply the result of the small increase in the magnitude of the positron work-function. 3.2. Positronium fraction measurement Fig. 3 showed the positronium fraction, fPs, from a GaAs(100) surface as a function of oxygen exposure at the incident beam energies at 25, 500 and 1000 eV. The fPs can be seen to decrease with increasing beam energy corresponding to the fact that positrons at the higher energy implanted deeper and became lower its probability of diusing back to the surface to form Ps. A model was proposed in which the probability of Ps formation is enhanced at low incident energy (Baker et al., 1988). It may also be seen in Fig. 3 that fPs decreases with increasing oxygen exposure. Taken together with the observation that positron emission was observed to increase with oxygen exposure this suggests that oxygen adsorption produces a change in the branching ratios for positrons reaching the surface
Three main interactions of thermal positrons with over-layer of oxygen on metal surface were proposed (Baker et al., 1988): (i) re-emitted positrons (ii) formation of positronium and (iii) surface state positrons. We observed that the branching ratios for positron emission, Ps formation and surface state are strongly dependent on the amount of oxygen exposure. That the increase of slow-positron yield (Y) with increasing oxygen exposure was associated with a corresponding decrease in the positronium fraction, fPs, is consistent with the fact that positron work-function becomes more negative which is consistent with the ion-neutralization mechanism. With continued exposure of the sample to oxygen, the fraction of Ps formation decreases, indicating that the surface state positrons are not energetically favorable to form Ps. The sticking coecient of oxygen on the GaAs(100) was estimated from ®ts of the attenuation of the PAES signal versus gas exposure to be on the order of 10ÿ5 which is close to the value (3 10ÿ5) reported by Dorn et al. (1974) for n-type GaAs crystals. Because the low value of the sticking coecient (coverages were estimated to be less then 1 ML of oxygen), therefore the increase of the positron re-emission and the decrease of positronium fraction are associated with the surface but not with the bulk properties.
4. Conclusion
Fig. 3. The positronium fraction (fPs) obtained from a GaAs(100) surface as a function of oxygen exposure at 25 eV (open triangles), with incident positron beams of 500 eV (open circles) and 1000 eV (®lled squares). The fPs can be seen to decreases with higher beam energy and with oxygen exposure.
Measurements were presented of the positron workfunction, positron re-emission yield, positronium fraction and surface composition of a n-doped GaAs(100) surface made as a function of oxygen exposure at three dierent positron beam energies (25, 500 and 1000 eV). The fraction (Y) of incident positrons re-emitted at low energies was found to increase and the fraction reemitted as positronium fraction (fPs) to decrease with increasing oxygen exposure. This suggests that the change in the branching ratios of Ps and re-emitted positron caused by oxygen adsorption is responsible for both eects and that they are not related to bulk diusion or trapping at defect. The increased positron re-emission yield at higher oxygen contamination level suggests that surface modi®ed GaAs may be useful as a contact material in the fabrication of GaAs based FAMs.
658
J.H. Kim et al. / Radiation Physics and Chemistry 58 (2000) 655±658
References Baker, J.A., Touat, M., Coleman, P.G., 1988. Branching ratios for electron-volt positrons at Cu(110) surface. J. Phys. C.: Solid State Phys 21, 4713. Dorn, R., Luth, H., Russell, G.J., 1974. Adsorption of oxygen on clean cleaved (110) gallium-arsenide surfaces. Phys. Rev. B 10, 5049. Gullikson, E.M., Mills Jr, A.P., Murray, C.A., 1988. Dependence of the positron reemission probability on the positron work unction of a metal surface. Phys. Rev. B 38, 1705. Kim, J.H., Nangia, A., Weiss, A.H., 1997. Measurement of positron work-function, positronium fraction and PAES intensities from GaAs(100). Materials Science Forum 255, 638.
Lei, C., Mehl, D., Koymen, A.R., Gotwald, F., Weiss, A., 1989. Apparatus for positron annihilation induced Auger electron spectroscopy. Rev. Sci. Instrum. 60, 3656. Lynn, K.G., Makee, B.T., 1979. Some investigations of moderators for slow positron beam. Appl. Phys. 19, 247. Murry, C.A., Mills Jr, A.P., Rowe, J.E., 1980. Correlations between electron and positron work-functions on copper surfaces. Surf. Sci. 100, 647. Shan, Y.Y., Au, H.L., Ling, C.C., Lee, T.C., Panda, B.K., Fung, S., Beling, C.D., Wang, Y.Y., Weng, H.M., 1994. Semi-insulating GaAs: A possible substrate for a ®eldassisted positron moderator. Appl. Phys. A. 59, 259. Shan, Y.Y., Asoka-Kumar, P., Lynn, K.G., Fung, S., Beling, C.D., 1996. Field eect on positron diusion in semi-insulating GaAs. Phys. Rev. B. 54, 1982.
www.elsevier.com/locate/radphyschem
Positron interactions with overlayers of oxygen on GaAs(100) J.H. Kim a,*, A. Nangia b, A.H. Weiss b a
The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama, 351-0106, Japan b Department of Physics, University of Texas at Arlington, P.O. Box 19059, Arlington, TX 76019, USA
Abstract The positron work-function, re-emission yield, and positronium fraction of an n-doped GaAs(100) surface were measured as a function of oxygen exposure. The energy distribution of positrons observed to be re-emitted indicated that the clean and oxygen exposed n-doped GaAs(100) surfaces had negative positron work-functions. The fraction of incident positrons re-emitted as bare positrons, (Y), was found to increase and the fraction re-emitted as positronium, (fPs), to decrease with increasing oxygen exposure. This suggests that surface modi®ed GaAs may be useful as a contact material in the fabrication of GaAs based FAMs. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction GaAs has been of interest due to its possible utility in ®eld-assisted positron moderator (FAM) (Shan et al., 1994). Positron mobility measurement in semi-insulating (SI) GaAs made using positron lifetime techniques indicated that this material might be suitable for making a high-eciency (010%) ®eld-assisted positron moderator. The FAM, in which the fraction of implanted positrons reaching the surface is enhanced by an applied electric ®eld which imparts to the positrons a drift velocity, was ®rst suggested by Lynn (Lynn and Makee, 1979). A decade later the fabrication of FAMs from epitaxial metal-on-semiconductor systems was proposed for materials, such as Au(Pd)/Si (Shan et al., 1994) and Au/GaAs(SI) (Shan et al., 1996).
* Corresponding author. Fax: +81-48-461-5301. E-mail address: [email protected] (J.H. Kim).
The surface properties of FAM materials, particularly as they relate to the re-emission of positrons into the vacuum, can be expected to be critical factors in the eort to develop FAMs. Therefore, an understanding of the eects of surface conditions on the positron work-function and positron re-emission fraction from GaAs is an essential step in the eorts to develop a ®eld assisted positron moderator. In this paper, the results of positron re-emission yields will be presented along with Ps fraction measurements as a function of the oxygen exposure. 2. Experimental procedures The GaAs(100) sample (n-type, Si doping level 01 1018 cmÿ3) was etched with HF solution (50%) and cleaned by sputtering and annealing cycles. The measurements of the positron work-function, positron re-emission yield, positronium fraction and surface composition of a GaAs(100) surface were performed
0969-806X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 0 ) 0 0 2 3 3 - 4
656
J.H. Kim et al. / Radiation Physics and Chemistry 58 (2000) 655±658
using the UTA-magnetically guided positron beam system (Lei et al., 1989) as a function of oxygen exposure at three dierent positron beam energies (25, 500 and 1000 eV). Approximately 5 104 slow positrons sÿ1 are generated by 20 mCi 22Na source. The energy spectra of re-emitted positrons and gamma rays were measured by varying the grid voltage (Vg) from a few volts below zero to a few volts above zero (Kim et al., 1997). Oxygen exposure was accomplished by back-®lling the chamber with oxygen and maintaining a pressure of 10ÿ6 or 10ÿ5 torr over the exposure time required. Exposure, measured in Langmuirs, L, is determined from the product of pressure and time and quanti®es the number of gas molecules allowed to impact the surface (1 L is de®ned as exposure to 10ÿ6 torr for 1 s at room temperature and corresponds roughly to the exposure needed to form a complete atomic layer if all of the atoms incident remained on the surface).
The positron work-function value was estimated from the energy spectra of re-emitted positrons measured as a function of a retarding grid voltage (Vg). The details of the positron work-function measurement were reported (Kim et al., 1997) and the value of the positron work-function was estimated as
ÿ0.6 eV from clean n-type GaAs(100) surface. Fig. 1 shows the energy spectra of re-emitted positrons from a clean (dotted line) and oxygen (70,000 L) covered GaAs(100) surface (solid line). The work-function is then taken to be dierence between two values (times e) of the peak and the zero points (V0), where positrons just start to be emitted from the sample (ÿ0.1 eV for both cases). The solid curve from the surface exposed to oxygen shows an increase of the positron work-function by 00.1 eV. This change in the positron work function is presumably due to charge transfer to the adsorbed oxygen causing an increase of the surface dipole layer. This would cause the electron work-function to become more positive and the positron workfunction to become more negative by a similar magnitude. The numbers of re-emitted positrons are plotted as a function of oxygen exposure on a GaAs(100) surface in Fig. 2. The yield of re-emitted positrons can be seen to increase with increasing oxygen exposure for both incident beam energies (500 and 1000 eV). The lower yield at higher beam energy can be explained by the fact that positrons are implanted more deeply at higher energies and thus they have a lower probability of diffusing back to the surface and being re-emitted into the vacuum. The increase in the fraction of re-emitted positrons from Ni or Cu as a result of H2S exposure (Murry et al., 1980) was interpreted as the result of the increase in the positron work-function produced by the exposure. Gullikson et al. (1988) review a number of models of positron re-emission, all of which result in a dependence of the slow positron yield on the positron
Fig. 1. The energy spectra of re-emitted positrons from a clean (dotted line) and oxygen covered GaAs(100) surface (solid line) with a 1 keV incident positron beam. The curves were obtained by taking a smoothed negative derivative of the data and normalizing the maximum to unity. The zero points were found to be ÿ0.1 eV for both cases.
Fig. 2. The slow positron yield is plotted as a function of oxygen exposure (in Langmuirs) on a GaAs(100) surface with a incident positron beam energies at 500 eV (open circles) and at 1000 eV (®lled squares). The solid and dash lines are theoretical ®ts to the positron yield data using the model function: y=y0exp[ÿ(ÿb/f+)1/2] with y0=2.1 (at 1000 eV) and 3.5 (at 500 eV) and b = 12 eV for both cases.
3. Results and discussion 3.1. Positron work-function from a clean and oxygen covered n-GaAs(100) surfaces
J.H. Kim et al. / Radiation Physics and Chemistry 58 (2000) 655±658
657
work-function y 0 (ÿf +)1/2 in agreement with their data for Ni and Cu surfaces modi®ed by alkali adsorption. Such a dependence result in a fractional change in yield
in which positron emission is increased in favor of Ps formation.
Dy=y 1=2 Df=f:
3.3. Discussion
1
Referring to Fig. 2 it may be seen that there is an increase in yield of Dy/y= 0 0.5 which is signi®cantly larger than would be predicted from Eq. (1) and the fractional change in the work-function, Df/f= 0 0.2, observed (see Fig. 1). This suggests that the increased yield observed when O was adsorbed on GaAs is not simply the result of the small increase in the magnitude of the positron work-function. 3.2. Positronium fraction measurement Fig. 3 showed the positronium fraction, fPs, from a GaAs(100) surface as a function of oxygen exposure at the incident beam energies at 25, 500 and 1000 eV. The fPs can be seen to decrease with increasing beam energy corresponding to the fact that positrons at the higher energy implanted deeper and became lower its probability of diusing back to the surface to form Ps. A model was proposed in which the probability of Ps formation is enhanced at low incident energy (Baker et al., 1988). It may also be seen in Fig. 3 that fPs decreases with increasing oxygen exposure. Taken together with the observation that positron emission was observed to increase with oxygen exposure this suggests that oxygen adsorption produces a change in the branching ratios for positrons reaching the surface
Three main interactions of thermal positrons with over-layer of oxygen on metal surface were proposed (Baker et al., 1988): (i) re-emitted positrons (ii) formation of positronium and (iii) surface state positrons. We observed that the branching ratios for positron emission, Ps formation and surface state are strongly dependent on the amount of oxygen exposure. That the increase of slow-positron yield (Y) with increasing oxygen exposure was associated with a corresponding decrease in the positronium fraction, fPs, is consistent with the fact that positron work-function becomes more negative which is consistent with the ion-neutralization mechanism. With continued exposure of the sample to oxygen, the fraction of Ps formation decreases, indicating that the surface state positrons are not energetically favorable to form Ps. The sticking coecient of oxygen on the GaAs(100) was estimated from ®ts of the attenuation of the PAES signal versus gas exposure to be on the order of 10ÿ5 which is close to the value (3 10ÿ5) reported by Dorn et al. (1974) for n-type GaAs crystals. Because the low value of the sticking coecient (coverages were estimated to be less then 1 ML of oxygen), therefore the increase of the positron re-emission and the decrease of positronium fraction are associated with the surface but not with the bulk properties.
4. Conclusion
Fig. 3. The positronium fraction (fPs) obtained from a GaAs(100) surface as a function of oxygen exposure at 25 eV (open triangles), with incident positron beams of 500 eV (open circles) and 1000 eV (®lled squares). The fPs can be seen to decreases with higher beam energy and with oxygen exposure.
Measurements were presented of the positron workfunction, positron re-emission yield, positronium fraction and surface composition of a n-doped GaAs(100) surface made as a function of oxygen exposure at three dierent positron beam energies (25, 500 and 1000 eV). The fraction (Y) of incident positrons re-emitted at low energies was found to increase and the fraction reemitted as positronium fraction (fPs) to decrease with increasing oxygen exposure. This suggests that the change in the branching ratios of Ps and re-emitted positron caused by oxygen adsorption is responsible for both eects and that they are not related to bulk diusion or trapping at defect. The increased positron re-emission yield at higher oxygen contamination level suggests that surface modi®ed GaAs may be useful as a contact material in the fabrication of GaAs based FAMs.
658
J.H. Kim et al. / Radiation Physics and Chemistry 58 (2000) 655±658
References Baker, J.A., Touat, M., Coleman, P.G., 1988. Branching ratios for electron-volt positrons at Cu(110) surface. J. Phys. C.: Solid State Phys 21, 4713. Dorn, R., Luth, H., Russell, G.J., 1974. Adsorption of oxygen on clean cleaved (110) gallium-arsenide surfaces. Phys. Rev. B 10, 5049. Gullikson, E.M., Mills Jr, A.P., Murray, C.A., 1988. Dependence of the positron reemission probability on the positron work unction of a metal surface. Phys. Rev. B 38, 1705. Kim, J.H., Nangia, A., Weiss, A.H., 1997. Measurement of positron work-function, positronium fraction and PAES intensities from GaAs(100). Materials Science Forum 255, 638.
Lei, C., Mehl, D., Koymen, A.R., Gotwald, F., Weiss, A., 1989. Apparatus for positron annihilation induced Auger electron spectroscopy. Rev. Sci. Instrum. 60, 3656. Lynn, K.G., Makee, B.T., 1979. Some investigations of moderators for slow positron beam. Appl. Phys. 19, 247. Murry, C.A., Mills Jr, A.P., Rowe, J.E., 1980. Correlations between electron and positron work-functions on copper surfaces. Surf. Sci. 100, 647. Shan, Y.Y., Au, H.L., Ling, C.C., Lee, T.C., Panda, B.K., Fung, S., Beling, C.D., Wang, Y.Y., Weng, H.M., 1994. Semi-insulating GaAs: A possible substrate for a ®eldassisted positron moderator. Appl. Phys. A. 59, 259. Shan, Y.Y., Asoka-Kumar, P., Lynn, K.G., Fung, S., Beling, C.D., 1996. Field eect on positron diusion in semi-insulating GaAs. Phys. Rev. B. 54, 1982.