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The influence of impurity profile on ultra-shallow GaAs sidewall tunnel junction characteristics
Applied Surface Science 252 (2006) 7283–7285 www.elsevier.com/locate/apsusc
The influence of impurity profile on ultra-shallow GaAs sidewall tunnel junction characteristics Takeo Ohno a,*, Yutaka Oyama a,b, Jun-ichi Nishizawa b a
Department of Materials Science, Graduate School of Engineering, Tohoku University, Aramaki Aza Aoba 6-6-11-1021, Sendai 980-8579, Japan b Semiconductor Research Institute of Semiconductor Research Foundation, Aramaki Aza Aoba 519-1176, Sendai 980-0845, Japan Received 12 September 2005; accepted 15 February 2006 Available online 3 May 2006
Abstract Dynamic SIMS has been applied to investigate the influence of impurity profiles on the characteristics of ultra-shallow GaAs sidewall tunnel junctions. SIMS depth profile on test-element-group areas on the device chips have shown that the Be profiles pile-up, with concentrations of up to 1020 cm3 at the tunnel junction interfaces. This result illustrates one of the dominant causes why very high peak current densities are achieved. # 2006 Elsevier B.V. All rights reserved. Keywords: Semiconductor; GaAs; Impurity doping; Epitaxial growth; Tunnel junction; Dynamic SIMS
1. Introduction The THz has recently been developed for the fields of medical instruments and information technology etc. The ideal static induction transistor [1] and tunnel injection transit time effect diode [2] are promising semiconductor devices for THz operation. In these devices, the most important device parameter is the performance of the p+n+ tunnel junction. At present, ultrashallow GaAs sidewall tunnel junctions with record peak current densities have been achieved by using molecular layer epitaxy (MLE) [3]. It is considered that impurity dopants have a significant influence on this high performance result. In this paper, secondary ion mass spectroscopy (SIMS) was applied to investigate the influence of impurity profiles on the characteristics of sidewall tunnel junctions.
GaAs sidewall mesas were formed through these windows by using an H2SO4-based solution. An AsH3 treatment was then carried out before the re-growth MLE. During this process, the patterned GaAs was heated to 350 8C for 30–120 min at a pressure of 1.1 101 Pa. After the AsH3 treatment, Be-doped p+-GaAs re-growth was carried out using bismethylcyclopentadienyl-beryllium. Triethylgallium and AsH3 were used as precursors for the epitaxial growth of the GaAs. Fig. 1 shows a schematic drawing of sidewall tunnel junction. The concentration of impurities and their profile were measured by dynamic SIMS analysis. A 1 keV Cs+ primary ion beam was used with a beam current of 10 nA, and the concentrations of BeAs, Te and S secondary ions were measured. 3. Results and discussion
2. Experimental procedure Sidewall tunnel junctions were fabricated by using re-growth MLE. A precise description of MLE is presented elsewhere [4]. Firstly, Te and S co-doped n+-GaAs was grown by MLE on a {0 0 1} SI GaAs. The thickness of the epitaxial layer was 50 nm. Silicon nitride (SiN) was then deposited by chemical vapour process, and SiN windows were opened using a wet etching. * Corresponding author. Tel.: +81 22 795 7329; fax: +81 22 795 7329. E-mail address: [email protected] (T. Ohno). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.131
Fig. 2 shows the current density–voltage characteristics of sidewall tunnel junction. These tunnel junctions have shown record peak current densities of up to 31,000 A/cm2. The carrier concentration can be estimated from the peak voltage. From the results of Hall-effect measurements, the electron concentration in the n+-GaAs was 2 1019 cm3 and the hole concentration in the p+-GaAs was 8 1019 cm3. The peak voltage can be obtained from [5]: jn þ jp ; 3
(1)
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T. Ohno et al. / Applied Surface Science 252 (2006) 7283–7285
Fig. 1. Schematic drawing of ultra-shallow GaAs sidewall tunnel junction. The Ti/Au contact is 100 mm in length.
Fig. 4. Be profiles of sidewall tunnel junctions on a {0 0 1} surface as a function of AsH3 treatment time (tFD). The inset shows the gradient of the Be concentration as a function of SIMS depth.
Fig. 2. Current density–voltage characteristics of an sidewall tunnel junction.
where jn and jp are the degeneracy levels on the n and p side, respectively. The degeneracy of the n(jn) and p(jp) regions can be calculated from the expressions below [5]: jp jn n ¼ Nc F1=2 ; p ¼ Nv F1=2 ; (2) kb T kb T where F 1/2 is the Fermi integral of order 1/2, and Nc and Nv are the effective densities of states in the conduction and valence bands, respectively. Assuming that n = 2 1019 cm3 remains fixed, the hole concentration is estimated to be 2 1020 cm3. There was a difference in the values for the carrier concentration between the peak voltage calculation and the Hall-effect measurements. Fig. 3 shows a SIMS depth profile of sidewall tunnel junction. The application of lateral SIMS was reported by
Criegern et al. [6]. Lateral SIMS is not applicable in our sample because the length in the SIMS crater area has the necessity of at least 50 nm or less. Therefore, while the impurity profile on the sidewall surface cannot be obtained, SIMS were performed for the planar structure on the {0 0 1} surface under identical epitaxial process conditions. From SIMS profiles of a testelement-group area on each of the device chips, it was observed that the Te and S profiles were very flat. On the other hand, the Be profile shows a concentration pile-up, rising up to 1020 cm3 at the p+/n+ interface. It is considered that Be pile-up was not caused by an artifacts at the p+/n+ interface and the sample charge, because the matrix yield is not changed at the interface and the Be profile was not changed regardless on the polarity of the primary and secondary ions. It should be noted that pile-up value corresponds well with the hole concentration estimated from the peak voltage. This result could possibly offer a reason for the very high peak current. The peak-to-valley current ratio (PVCR) was also investigated. When the AsH3 treatment time (tFD) is 30, 60 and 120 min, PVCR becomes 1.8, 3.3 and 4.2, respectively. It can be shown that a long AsH3 treatment prior to re-growth MLE improves the characteristics of sidewall tunnel junction. As reported previously [7], it has been shown that the controlled surface stoichiometry that results from the AsH3 treatment reduces defects related to non-stoichiometric effects. Fig. 4 shows the Be profiles as a function of tFD. The pile-up of Be increases as tFD becomes longer, while the concentration of Be is 9 1019, 1.2 1020 and 1.5 1020 cm3 for tFD = 30, 60 and 120 min, respectively. It was also shown that a long AsH3 treatment enhances the steepness of the Be profile. It is considered that these phenomena influence improvements in the sidewall tunnel junction characteristics. 4. Conclusion
Fig. 3. Impurity profile of p+/n+/SI GaAs on a {0 0 1} surface.
The impurity profiles in ultra-shallow GaAs sidewall tunnel junctions were investigated. High-concentration pile-up of Be at the tunnel junction interface is a possible reason for the extremely high peak current density. The characteristics of tunnel junctions have been improved by introducing an AsH3
T. Ohno et al. / Applied Surface Science 252 (2006) 7283–7285
treatment stage just prior to re-growth MLE; these improvements include the steepness of the Be doping profile, interface pile-up and a reduction in stoichiometry-dependent deep states at the re-growth interface. References [1] J. Nishizawa, in: Proceedings of 11th Conference on Solid State Devices, 1979, Jpn. J. Appl. Phys. 19-1 (Suppl.) (1980) 3.
7285
[2] J. Nishizawa, Y. Watanabe, Sci. Rep. Res. Inst. Tohoku Univ. 10 (2) (1958) 91. [3] Y. Oyama, T. Ohno, K. Tezuka, K. Suto, J. Nishizawa, Appl. Phys. Lett. 81 (2002) 2563–2565. [4] J. Nishizawa, Y. Kokubun, Extended Abstracts of 16th Conference on Solid State Devices and Materials, Kobe, Japan, (1984), p. 1. [5] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley Interscience, New York, 1981. [6] R. von Criegern, F. Jahnel, M. Bianco, R. Lange-Gieseler, J. Vac. Sci. Technol. B 12 (1994) 234–242. [7] J. Nishizawa, Y. Oyama, P. Plotka, H.Sakuraba, Surf. Sci. 348(1996) 105–114.
The influence of impurity profile on ultra-shallow GaAs sidewall tunnel junction characteristics Takeo Ohno a,*, Yutaka Oyama a,b, Jun-ichi Nishizawa b a
Department of Materials Science, Graduate School of Engineering, Tohoku University, Aramaki Aza Aoba 6-6-11-1021, Sendai 980-8579, Japan b Semiconductor Research Institute of Semiconductor Research Foundation, Aramaki Aza Aoba 519-1176, Sendai 980-0845, Japan Received 12 September 2005; accepted 15 February 2006 Available online 3 May 2006
Abstract Dynamic SIMS has been applied to investigate the influence of impurity profiles on the characteristics of ultra-shallow GaAs sidewall tunnel junctions. SIMS depth profile on test-element-group areas on the device chips have shown that the Be profiles pile-up, with concentrations of up to 1020 cm3 at the tunnel junction interfaces. This result illustrates one of the dominant causes why very high peak current densities are achieved. # 2006 Elsevier B.V. All rights reserved. Keywords: Semiconductor; GaAs; Impurity doping; Epitaxial growth; Tunnel junction; Dynamic SIMS
1. Introduction The THz has recently been developed for the fields of medical instruments and information technology etc. The ideal static induction transistor [1] and tunnel injection transit time effect diode [2] are promising semiconductor devices for THz operation. In these devices, the most important device parameter is the performance of the p+n+ tunnel junction. At present, ultrashallow GaAs sidewall tunnel junctions with record peak current densities have been achieved by using molecular layer epitaxy (MLE) [3]. It is considered that impurity dopants have a significant influence on this high performance result. In this paper, secondary ion mass spectroscopy (SIMS) was applied to investigate the influence of impurity profiles on the characteristics of sidewall tunnel junctions.
GaAs sidewall mesas were formed through these windows by using an H2SO4-based solution. An AsH3 treatment was then carried out before the re-growth MLE. During this process, the patterned GaAs was heated to 350 8C for 30–120 min at a pressure of 1.1 101 Pa. After the AsH3 treatment, Be-doped p+-GaAs re-growth was carried out using bismethylcyclopentadienyl-beryllium. Triethylgallium and AsH3 were used as precursors for the epitaxial growth of the GaAs. Fig. 1 shows a schematic drawing of sidewall tunnel junction. The concentration of impurities and their profile were measured by dynamic SIMS analysis. A 1 keV Cs+ primary ion beam was used with a beam current of 10 nA, and the concentrations of BeAs, Te and S secondary ions were measured. 3. Results and discussion
2. Experimental procedure Sidewall tunnel junctions were fabricated by using re-growth MLE. A precise description of MLE is presented elsewhere [4]. Firstly, Te and S co-doped n+-GaAs was grown by MLE on a {0 0 1} SI GaAs. The thickness of the epitaxial layer was 50 nm. Silicon nitride (SiN) was then deposited by chemical vapour process, and SiN windows were opened using a wet etching. * Corresponding author. Tel.: +81 22 795 7329; fax: +81 22 795 7329. E-mail address: [email protected] (T. Ohno). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.131
Fig. 2 shows the current density–voltage characteristics of sidewall tunnel junction. These tunnel junctions have shown record peak current densities of up to 31,000 A/cm2. The carrier concentration can be estimated from the peak voltage. From the results of Hall-effect measurements, the electron concentration in the n+-GaAs was 2 1019 cm3 and the hole concentration in the p+-GaAs was 8 1019 cm3. The peak voltage can be obtained from [5]: jn þ jp ; 3
(1)
7284
T. Ohno et al. / Applied Surface Science 252 (2006) 7283–7285
Fig. 1. Schematic drawing of ultra-shallow GaAs sidewall tunnel junction. The Ti/Au contact is 100 mm in length.
Fig. 4. Be profiles of sidewall tunnel junctions on a {0 0 1} surface as a function of AsH3 treatment time (tFD). The inset shows the gradient of the Be concentration as a function of SIMS depth.
Fig. 2. Current density–voltage characteristics of an sidewall tunnel junction.
where jn and jp are the degeneracy levels on the n and p side, respectively. The degeneracy of the n(jn) and p(jp) regions can be calculated from the expressions below [5]: jp jn n ¼ Nc F1=2 ; p ¼ Nv F1=2 ; (2) kb T kb T where F 1/2 is the Fermi integral of order 1/2, and Nc and Nv are the effective densities of states in the conduction and valence bands, respectively. Assuming that n = 2 1019 cm3 remains fixed, the hole concentration is estimated to be 2 1020 cm3. There was a difference in the values for the carrier concentration between the peak voltage calculation and the Hall-effect measurements. Fig. 3 shows a SIMS depth profile of sidewall tunnel junction. The application of lateral SIMS was reported by
Criegern et al. [6]. Lateral SIMS is not applicable in our sample because the length in the SIMS crater area has the necessity of at least 50 nm or less. Therefore, while the impurity profile on the sidewall surface cannot be obtained, SIMS were performed for the planar structure on the {0 0 1} surface under identical epitaxial process conditions. From SIMS profiles of a testelement-group area on each of the device chips, it was observed that the Te and S profiles were very flat. On the other hand, the Be profile shows a concentration pile-up, rising up to 1020 cm3 at the p+/n+ interface. It is considered that Be pile-up was not caused by an artifacts at the p+/n+ interface and the sample charge, because the matrix yield is not changed at the interface and the Be profile was not changed regardless on the polarity of the primary and secondary ions. It should be noted that pile-up value corresponds well with the hole concentration estimated from the peak voltage. This result could possibly offer a reason for the very high peak current. The peak-to-valley current ratio (PVCR) was also investigated. When the AsH3 treatment time (tFD) is 30, 60 and 120 min, PVCR becomes 1.8, 3.3 and 4.2, respectively. It can be shown that a long AsH3 treatment prior to re-growth MLE improves the characteristics of sidewall tunnel junction. As reported previously [7], it has been shown that the controlled surface stoichiometry that results from the AsH3 treatment reduces defects related to non-stoichiometric effects. Fig. 4 shows the Be profiles as a function of tFD. The pile-up of Be increases as tFD becomes longer, while the concentration of Be is 9 1019, 1.2 1020 and 1.5 1020 cm3 for tFD = 30, 60 and 120 min, respectively. It was also shown that a long AsH3 treatment enhances the steepness of the Be profile. It is considered that these phenomena influence improvements in the sidewall tunnel junction characteristics. 4. Conclusion
Fig. 3. Impurity profile of p+/n+/SI GaAs on a {0 0 1} surface.
The impurity profiles in ultra-shallow GaAs sidewall tunnel junctions were investigated. High-concentration pile-up of Be at the tunnel junction interface is a possible reason for the extremely high peak current density. The characteristics of tunnel junctions have been improved by introducing an AsH3
T. Ohno et al. / Applied Surface Science 252 (2006) 7283–7285
treatment stage just prior to re-growth MLE; these improvements include the steepness of the Be doping profile, interface pile-up and a reduction in stoichiometry-dependent deep states at the re-growth interface. References [1] J. Nishizawa, in: Proceedings of 11th Conference on Solid State Devices, 1979, Jpn. J. Appl. Phys. 19-1 (Suppl.) (1980) 3.
7285
[2] J. Nishizawa, Y. Watanabe, Sci. Rep. Res. Inst. Tohoku Univ. 10 (2) (1958) 91. [3] Y. Oyama, T. Ohno, K. Tezuka, K. Suto, J. Nishizawa, Appl. Phys. Lett. 81 (2002) 2563–2565. [4] J. Nishizawa, Y. Kokubun, Extended Abstracts of 16th Conference on Solid State Devices and Materials, Kobe, Japan, (1984), p. 1. [5] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley Interscience, New York, 1981. [6] R. von Criegern, F. Jahnel, M. Bianco, R. Lange-Gieseler, J. Vac. Sci. Technol. B 12 (1994) 234–242. [7] J. Nishizawa, Y. Oyama, P. Plotka, H.Sakuraba, Surf. Sci. 348(1996) 105–114.