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AlInP tunnel diode by gas source molecular beam epitaxy
Journal of Crystal Growth 209 (2000) 459}462
Be redistribution in GaInP and growth of GaInP/AlInP tunnel diode by gas source molecular beam epitaxy Wei Li!,*, Jari Likonen", Jouko Haapamaa!, Markus Pessa! !Optoelectronics Research Center, Tampere University of Technology, P.O. Box 692, 33101 Tampere, Finland "Technical Research Center of Finland, Chemical Technology, P.O. Box 1404, 02044 VTT, Finland
Abstract The redistribution of Be during growth of GaInP layers by gas source molecular beam epitaxy has been studied using secondary ion mass spectrometry for the "rst time. Apparent Be di!usion occurs at doping level over 4]1019 cm~3 at growth temperature of 5003C. At lower temperature the Be di!usion pro"le exhibits a signi"cant increase of Be concentration and reduced di!usion. In contrast to Zn behavior in metalorganic vapor-phase epitaxy, no enhancement of Be redistribution in both GaInP and GaAs is observed by nearby highly n-type doped layers. Based on these results, a p`}n` GaInP tunnel diode with a high conductance of 15 mA/cm2 at 1.7 mV has been achieved. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 71.55.Eq; 66.30.Jt; 68.55.Ln; 81.15.Hi Keywords: Di!usion; Beryllium; GaInP
1. Introduction A GaInP/GaAs tandem solar cell consisting of a thin GaInP top cell and a GaAs bottom cell has a signi"cantly higher conversion e$ciency than conventional single-junction solar cells. One of the critical issues in the growth of such a two-terminal monolithic tandem cell is a highly doped p`}n` GaInP tunnel junction for the inter-cell connection which connects top and bottom cells without electrical and optical losses. Zn is the conventional
* Corresponding author. Tel.: #358-3-365-2452; fax: #3583-365-3400. E-mail address: [email protected]." (W. Li)
dopants used in metal-organic chemical vapor deposition (MOCVD). For doping levels in the 1019 cm~3 range, concentration-dependent di!usion as well as the enhanced di!usion by nearby highly n-type layers have been reported to be a serious problem [1]. Recently, the use of MBE to grow III}V solar cells for space photovoltaocs is receiving renewed interest due to the advent of production MBE systems [2] and a higher p-type doping e$cency and less severe di!usion of the p-type dopant (Be). So, the study of Be di!usion in GaInP is of prime importance. Unfortunately, there is no data available up to now. In this paper, we have investigated the concentration-dependent Be di!usion in GaInP as well as the e!ects of growth temperature, nearby highly n-type doped layers by
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 9 8 - 9
460
W. Li et al. / Journal of Crystal Growth 209 (2000) 459}462
studying secondary ion mass spectrometry (SIMS) depth pro"les. Based on these results, a p`}n` GaInP tunnel diode with a high conductance of 15 mA/cm2 at 1.7 mV has been achieved.
2. Experimental procedure All studied layers were grown in a gas source molecular beam epitaxy (GSMBE) system. Elemental Ga and In were used to produce the group-III beam #uxes, while As and P are produced from 2 2 cracked AsH and PH . SIMS measurements were 3 3 provided by VG Ionex IX70S instrument. O` ions 2 were used as primary ions and the primary ion energy used was 5 kV.
3. Results and discussion Fig. 1 shows the SIMS analysis of several pulse doped samples grown at 5003C and 4503C, respectively. The Be pulses were 10 nm wide and were separated by undoped regions 200 nm wide. The Be pulses were intended to give doped regions having Be concentrations of 1, 4, 6, and 8]1019 cm~3. The measured Be densities are reasonably close to what would have been expected. At 5003C, the width of 1]1019 cm~3 doped layer at half peak height is about 13 nm. When Be doping increases to
Fig. 1. SIMS analysis of a structure containing 4 Be pulses in GaInP layer grown at 5003C and 4503C, respectively. The intended doping levels are indicated.
4]1019 cm~3, there is slight widening for the peak, and apparent rapid di!usion above this concentration. For doping with 8]1019 cm~3 the pulse has spread out into a region about 69 nm wide while the peak concentration is almost the same as for the lower doping. The abrupt di!usion front pro"le and enhanced di!usion without an increase in the peak concentration with increasing Be #ux are the typical features of concentration-dependent di!usion process. At lower growth temperature (4503C), the Be di!usion pro"les exhibit quite similar trend with Be doping level less than 6]1019 cm~3, and a signi"cant increase of Be concentration and reduced di!usion above this concentration. These results show that at very high doping levels there is an extreme dependence of dopant behavior on dopant #ux and on the growth temperature, which can be explained based on interstitial}substitutional di!usion model [3}5]. The Be di!usion process in GaInP is believed to be dominated by the highly mobile Be interstitials in chemical equilibrium with the substitutional Be. These fast di!using Be interstitials transport Be atoms from high- to low-concentration regions, and most of them convert into substitutional through kick-out mechanism and/or recombining with vacancies. The loss of Be interstitials in the high-concentration region can be supplemented from Be substitutionals via the chemical reactions with point defects. Hence, if there is a su$cient gradient and supply of Be interstitials, di!usion will continue. With increasing doping concentration above certain value, the excess Be interstitials incorporated are free to move resulting rapid Be di!usion as observed in our case. At lower growth temperature, the concentration-dependent Be di!usion will be retarded to some extent since the equilibrium concentration of interstitial Be is reduced. In addition, excesses of phosphorus at low temperature cause an increase of III group vacancies which further suppress the formation and di!usion of Be interstitials. In order to study the Be di!usion in a real tunnel junction, we have grown a structure which is the same as the previous one except adjoining 10 nm highly Si-doped GaInP layers (5]1019 cm~3). The SIMS results are shown in Fig. 2. Signi"cantly reduced Be di!usion was observed even for the layer with the highest Be doping concentration.
W. Li et al. / Journal of Crystal Growth 209 (2000) 459}462
461
Fig. 2. SIMS depth pro"le of a stacked GaInP pn-junction structure grown at 4503C. The intended Be doping levels are indicated. In comparison, a similar structure without adjoining Si-doped layers is shown.
Fig. 3. SIMS depth pro"le of a stacked GaAs pn-junction structure grown at 5003C. The intended Be doping levels are indicated. In comparison, a similar structure without adjoining Si-doped layers is shown.
This result is markedly di!erent from that for Zn doping in MOCVD in which Zn di!usion is enhanced by nearby highly n-type layer [6]. In addition, we have also grown a similar structure for GaAs at 5003C, in which the Be concentration were varied to 1]1020 cm~3 (Fig. 3). It can be seen that sharp Be pro"les are obtained on both sides of the doping peaks. No broadening is observed for the layers with adjacent Si-doped layers indicating the same mechanism responsible for the Be di!usion in MBE. In consideration of Both Be and Zn having a valence of 2, the di!erent di!usion behavior could be related to the the di!erent growth process. It is well known that MOCVD growth is near thermodynamic equilibrium while MBE is mainly a kinetic process at lower temperature. The #ow of self-interstitial defects from highly n-type layer into p` region in MOCVD could be negligible in the MBE growth. In addition, it should be noted that the formation of depletion zone due to p}n junction will result in an increase of acceptor atoms in substitutional positions based on substitutional}interstitial reaction mecahnism. Moreover, the motion of the positively charged Be interstitials are inhibited by the built-in electric "eld. Therefore, Be redistribution should be suppressed. Based on the results above, a p`}n` GaInP/ AlInP tunnel diode was grown. The diode were
Fig. 4. I}< characteristics of a p`}n`GaInP/AlInP tunnel diode.
fabricated by alloying Ti/Pt/Au contacts to the p` GaAs layer and etching photolithographically de"ned mesa structures. The n-type ohmic contact was obtained through Ni/Au/Ge/Au metallization. Fig. 4 shows the I}< characteristics of a GaInP/AlInP tunnel diode. The highest conductance achieved was 15 mA/cm2 at 1.7 mV. This conductance is su$cient for the GaInP/GaAs tandem cell operating under one-sun air-mass-zero illumination.
462
W. Li et al. / Journal of Crystal Growth 209 (2000) 459}462
4. Conclusions
References
Apparent Be di!usion has been found at doping level over 4]1019 cm~3 in GaInP layers grown by GSMBE at 5003C. At lower temperature the Be di!usion pro"le exhibits a signi"cant increase of Be concentration and reduced di!usion. In contrast to Zn behavior in metalorganic vapor-phase epitaxy, no enhancement of Be redistribution in both GaInP and GaAs is observed by nearby highly n-type doped layers. These results are explained based on interstitial}substitutional di!usion model. A p`}n` GaInP tunnel diode with a high conductance of 15 mA/cm2 at 1.7 mV has been achieved.
[1] T. Takamoto, M. Yumaguchi, E. Ikeda, T. Agui, H. Kurita, M. Al-Jassim, J. Appl. Phys. 85 (1999) 1481. [2] J. Lammasniemi, A.B. Kazantsev, R. Jaakkola, R. Aho, T. MaK kelaK , M. Pessa, A. Ovtchinnikov, H. Asonen, A. Rodden, K. Bogus, second World Conference on Photovoltaic Solar Energy Conversion, Vienna, 6}10 July 1998, p. 3520. [3] S.N.G. Chu, R.A. Logan, M. Geva, N.T. Ha, J. Appl. Phys. 78 (1995) 3001. [4] S. Yu, T.Y. Tan, U. GoK sele, J. Appl. Phys. 69 (1991) 3547. [5] T. Mozume, K. Hosomi, J. Cryst. Growth 175 (1997) 1223. [6] D.G. Deppe, Appl. Phys. Lett. 56 (1990) 370.
Be redistribution in GaInP and growth of GaInP/AlInP tunnel diode by gas source molecular beam epitaxy Wei Li!,*, Jari Likonen", Jouko Haapamaa!, Markus Pessa! !Optoelectronics Research Center, Tampere University of Technology, P.O. Box 692, 33101 Tampere, Finland "Technical Research Center of Finland, Chemical Technology, P.O. Box 1404, 02044 VTT, Finland
Abstract The redistribution of Be during growth of GaInP layers by gas source molecular beam epitaxy has been studied using secondary ion mass spectrometry for the "rst time. Apparent Be di!usion occurs at doping level over 4]1019 cm~3 at growth temperature of 5003C. At lower temperature the Be di!usion pro"le exhibits a signi"cant increase of Be concentration and reduced di!usion. In contrast to Zn behavior in metalorganic vapor-phase epitaxy, no enhancement of Be redistribution in both GaInP and GaAs is observed by nearby highly n-type doped layers. Based on these results, a p`}n` GaInP tunnel diode with a high conductance of 15 mA/cm2 at 1.7 mV has been achieved. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 71.55.Eq; 66.30.Jt; 68.55.Ln; 81.15.Hi Keywords: Di!usion; Beryllium; GaInP
1. Introduction A GaInP/GaAs tandem solar cell consisting of a thin GaInP top cell and a GaAs bottom cell has a signi"cantly higher conversion e$ciency than conventional single-junction solar cells. One of the critical issues in the growth of such a two-terminal monolithic tandem cell is a highly doped p`}n` GaInP tunnel junction for the inter-cell connection which connects top and bottom cells without electrical and optical losses. Zn is the conventional
* Corresponding author. Tel.: #358-3-365-2452; fax: #3583-365-3400. E-mail address: [email protected]." (W. Li)
dopants used in metal-organic chemical vapor deposition (MOCVD). For doping levels in the 1019 cm~3 range, concentration-dependent di!usion as well as the enhanced di!usion by nearby highly n-type layers have been reported to be a serious problem [1]. Recently, the use of MBE to grow III}V solar cells for space photovoltaocs is receiving renewed interest due to the advent of production MBE systems [2] and a higher p-type doping e$cency and less severe di!usion of the p-type dopant (Be). So, the study of Be di!usion in GaInP is of prime importance. Unfortunately, there is no data available up to now. In this paper, we have investigated the concentration-dependent Be di!usion in GaInP as well as the e!ects of growth temperature, nearby highly n-type doped layers by
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 9 8 - 9
460
W. Li et al. / Journal of Crystal Growth 209 (2000) 459}462
studying secondary ion mass spectrometry (SIMS) depth pro"les. Based on these results, a p`}n` GaInP tunnel diode with a high conductance of 15 mA/cm2 at 1.7 mV has been achieved.
2. Experimental procedure All studied layers were grown in a gas source molecular beam epitaxy (GSMBE) system. Elemental Ga and In were used to produce the group-III beam #uxes, while As and P are produced from 2 2 cracked AsH and PH . SIMS measurements were 3 3 provided by VG Ionex IX70S instrument. O` ions 2 were used as primary ions and the primary ion energy used was 5 kV.
3. Results and discussion Fig. 1 shows the SIMS analysis of several pulse doped samples grown at 5003C and 4503C, respectively. The Be pulses were 10 nm wide and were separated by undoped regions 200 nm wide. The Be pulses were intended to give doped regions having Be concentrations of 1, 4, 6, and 8]1019 cm~3. The measured Be densities are reasonably close to what would have been expected. At 5003C, the width of 1]1019 cm~3 doped layer at half peak height is about 13 nm. When Be doping increases to
Fig. 1. SIMS analysis of a structure containing 4 Be pulses in GaInP layer grown at 5003C and 4503C, respectively. The intended doping levels are indicated.
4]1019 cm~3, there is slight widening for the peak, and apparent rapid di!usion above this concentration. For doping with 8]1019 cm~3 the pulse has spread out into a region about 69 nm wide while the peak concentration is almost the same as for the lower doping. The abrupt di!usion front pro"le and enhanced di!usion without an increase in the peak concentration with increasing Be #ux are the typical features of concentration-dependent di!usion process. At lower growth temperature (4503C), the Be di!usion pro"les exhibit quite similar trend with Be doping level less than 6]1019 cm~3, and a signi"cant increase of Be concentration and reduced di!usion above this concentration. These results show that at very high doping levels there is an extreme dependence of dopant behavior on dopant #ux and on the growth temperature, which can be explained based on interstitial}substitutional di!usion model [3}5]. The Be di!usion process in GaInP is believed to be dominated by the highly mobile Be interstitials in chemical equilibrium with the substitutional Be. These fast di!using Be interstitials transport Be atoms from high- to low-concentration regions, and most of them convert into substitutional through kick-out mechanism and/or recombining with vacancies. The loss of Be interstitials in the high-concentration region can be supplemented from Be substitutionals via the chemical reactions with point defects. Hence, if there is a su$cient gradient and supply of Be interstitials, di!usion will continue. With increasing doping concentration above certain value, the excess Be interstitials incorporated are free to move resulting rapid Be di!usion as observed in our case. At lower growth temperature, the concentration-dependent Be di!usion will be retarded to some extent since the equilibrium concentration of interstitial Be is reduced. In addition, excesses of phosphorus at low temperature cause an increase of III group vacancies which further suppress the formation and di!usion of Be interstitials. In order to study the Be di!usion in a real tunnel junction, we have grown a structure which is the same as the previous one except adjoining 10 nm highly Si-doped GaInP layers (5]1019 cm~3). The SIMS results are shown in Fig. 2. Signi"cantly reduced Be di!usion was observed even for the layer with the highest Be doping concentration.
W. Li et al. / Journal of Crystal Growth 209 (2000) 459}462
461
Fig. 2. SIMS depth pro"le of a stacked GaInP pn-junction structure grown at 4503C. The intended Be doping levels are indicated. In comparison, a similar structure without adjoining Si-doped layers is shown.
Fig. 3. SIMS depth pro"le of a stacked GaAs pn-junction structure grown at 5003C. The intended Be doping levels are indicated. In comparison, a similar structure without adjoining Si-doped layers is shown.
This result is markedly di!erent from that for Zn doping in MOCVD in which Zn di!usion is enhanced by nearby highly n-type layer [6]. In addition, we have also grown a similar structure for GaAs at 5003C, in which the Be concentration were varied to 1]1020 cm~3 (Fig. 3). It can be seen that sharp Be pro"les are obtained on both sides of the doping peaks. No broadening is observed for the layers with adjacent Si-doped layers indicating the same mechanism responsible for the Be di!usion in MBE. In consideration of Both Be and Zn having a valence of 2, the di!erent di!usion behavior could be related to the the di!erent growth process. It is well known that MOCVD growth is near thermodynamic equilibrium while MBE is mainly a kinetic process at lower temperature. The #ow of self-interstitial defects from highly n-type layer into p` region in MOCVD could be negligible in the MBE growth. In addition, it should be noted that the formation of depletion zone due to p}n junction will result in an increase of acceptor atoms in substitutional positions based on substitutional}interstitial reaction mecahnism. Moreover, the motion of the positively charged Be interstitials are inhibited by the built-in electric "eld. Therefore, Be redistribution should be suppressed. Based on the results above, a p`}n` GaInP/ AlInP tunnel diode was grown. The diode were
Fig. 4. I}< characteristics of a p`}n`GaInP/AlInP tunnel diode.
fabricated by alloying Ti/Pt/Au contacts to the p` GaAs layer and etching photolithographically de"ned mesa structures. The n-type ohmic contact was obtained through Ni/Au/Ge/Au metallization. Fig. 4 shows the I}< characteristics of a GaInP/AlInP tunnel diode. The highest conductance achieved was 15 mA/cm2 at 1.7 mV. This conductance is su$cient for the GaInP/GaAs tandem cell operating under one-sun air-mass-zero illumination.
462
W. Li et al. / Journal of Crystal Growth 209 (2000) 459}462
4. Conclusions
References
Apparent Be di!usion has been found at doping level over 4]1019 cm~3 in GaInP layers grown by GSMBE at 5003C. At lower temperature the Be di!usion pro"le exhibits a signi"cant increase of Be concentration and reduced di!usion. In contrast to Zn behavior in metalorganic vapor-phase epitaxy, no enhancement of Be redistribution in both GaInP and GaAs is observed by nearby highly n-type doped layers. These results are explained based on interstitial}substitutional di!usion model. A p`}n` GaInP tunnel diode with a high conductance of 15 mA/cm2 at 1.7 mV has been achieved.
[1] T. Takamoto, M. Yumaguchi, E. Ikeda, T. Agui, H. Kurita, M. Al-Jassim, J. Appl. Phys. 85 (1999) 1481. [2] J. Lammasniemi, A.B. Kazantsev, R. Jaakkola, R. Aho, T. MaK kelaK , M. Pessa, A. Ovtchinnikov, H. Asonen, A. Rodden, K. Bogus, second World Conference on Photovoltaic Solar Energy Conversion, Vienna, 6}10 July 1998, p. 3520. [3] S.N.G. Chu, R.A. Logan, M. Geva, N.T. Ha, J. Appl. Phys. 78 (1995) 3001. [4] S. Yu, T.Y. Tan, U. GoK sele, J. Appl. Phys. 69 (1991) 3547. [5] T. Mozume, K. Hosomi, J. Cryst. Growth 175 (1997) 1223. [6] D.G. Deppe, Appl. Phys. Lett. 56 (1990) 370.