- Email: [email protected]
Unintentional aluminum incorporation related to the introduction of nitrogen gas during the plasma-assisted molecular beam epitaxy
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 1646–1649
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Unintentional aluminum incorporation related to the introduction of nitrogen gas during the plasma-assisted molecular beam epitaxy F. Ishikawa , S.D. Wu, M. Kato, M. Uchiyama, K. Higashi, M. Kondow Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e in f o
a b s t r a c t
Available online 19 November 2008
We examine unintentional incorporation of Al during the growth of molecular beam epitaxy (MBE) related to N gas introduction. In spite of the closed shutter of Al cell, we observe Al incorporation in the epitaxial layer with a concentration up to 1 1018 cm 3. The concentration depends on the N2 gas flow rate and Al source temperature. The concentration is suppressed by the reduction of the Al cell temperature. Shutter control of the N cell, As beam equivalent pressure and the operation of the RF plasma power have no impact on that. The introduction of the large amount of N can modify the beam dispersion of Al in the MBE chamber, which will cause the extrinsic Al incorporation. The unintentional impurity incorporation can induce material deteriorations. We thus suggest that the growth should be carried out with the decreased Al cell temperature and the N gas flow rate as low as possible. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.72.S– 68.65.Fg 78.55.Cr 78.66.Fd 78.67.De 81.15.Hi Keywords: A3. Molecular beam epitaxy B1. Nitrides B2 Semiconducting III–V materials B3. Laser dioses B3. Optical fiber devices
1. Introduction Plasma-assisted molecular beam epitaxy (MBE) have been employed for the growth of nitride and dilute nitride semiconductors. Because of the feasibility of low growth temperature, MBE is suitable for the growth of metastable dilute nitride semiconductors such as GaInNAs [1,2]. The growth of GaInNAs has progressed, recently materializing commercial 1.3 mm laser diodes [3]. However, there are yet remaining problems to obtain ideal high-quality crystals. Plasma-induced material deterioration is one of the remaining controversial issues. There have been reports of the suppression of such deteriorations by the reduction of undesirable ion species, or the exploration of favorable plasma conditions [4–6]. On the other hand, we have carried out a study focusing on the unintentional incorporation of Al during the growth [7,8]. Similar results have been reported for the growth of GaInNAs lasers, not only for the growth of MBE but also for metal-organic vapor phase epitaxy, crucially degrading the laser performance [9–11]. In this report, we investigate the origin of the unintentional incorporation of Al during the plasma-assisted MBE growth of dilute nitride semiconductors. Secondary ion mass spectrometry (SIMS) study is carried out for the distributions of N and Al within
Corresponding author. Tel.: +81 6 6879 7767; fax: +81 6 6879 7753.
E-mail address: [email protected] (F. Ishikawa). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.11.042
the epitaxial layer. From the understanding of this study, we suggest the origin, and approach for the suppression of the unintentional Al incorporation.
2. Experimental procedure Samples were grown on n-GaAs (0 0 1) substrate by ANELVA620 molecular beam epitaxy. A schematic configuration of the equipped cells in the MBE system is shown in Fig. 1. The MBE have six evaporation cell ports. The vacuum line connected to a turbomolecular pump (TMP) and an ion pump (IP) is located on the chamber wall between the cells of As and Si. A self-made RF plasma source was used to produce active N species. Elemental Ga and Al were supplied from conventional Knudsen effusion cells. As was supplied with a VEECO 500 V-As valved cracker cell operating at As2 mode. N2 gas was introduced through Millipore WaferpureTM purifier. A mass flow controller (MFC) was used to adjust N2 gas flow rate. A variable leak valve (VLV) was attached between the MFC and the plasma cell, which was used just for opening and shutting the gas line. A standard shaft-rotating shutter was used to control N flux. We grew samples having GaNAs/GaAs/AlAs stacked structure at 450 1C. Prior to the growth of the structure, a 200 nm GaAs buffer layer was grown at 580 1C. On the buffer, we grew 1-mm-thick AlAs layer to make the growth situation similar to the growth of laser structure. The growth was carried out without growth interruption throughout the structure.
ARTICLE IN PRESS F. Ishikawa et al. / Journal of Crystal Growth 311 (2009) 1646–1649
1647
(xvi)
Substrate Vacuum (TMP and IP)
GaAs250 nm GaAs
250 nm
GaNAs nm GaNAs83 GaAs83 nm GaAs GaNAs83 nm GaNAs
83 nm 83 nm 83 nm
(xiii) (xii) (xi) (x) (ix)
830 nm
GaAs830 nm GaAs
(i) AlAs1000 nm AlAs As
Si
1000 nm
GaAsbuffer GaAs buffer200 nm 200 nm GaAs(001) sub. Al
In
(i) Grow GaAs (ii) Open variable leak valve N
Ga
(iii) Set N2 flow rate at 0.12 sccm (iv) Open N shutter (v) Close N shutter
Fig. 1. Schematic diagram of the cells configurations in the MBE system.
(vi) Increase As BEP (vii) Decrease As BEP (viii) Turn on RF power, set at 300 W (ix) Set N2 flow rate at 0.020 sccm (x) Open N shutter (xi) Close N shutter (xii) Open N shutter (xiii) Close N shutter (xiv) Turn off RF power (xv) Set N2 flow at 0 sccm, Close VLV (xvi) Close Ga shutter and finish the growth Fig. 2. Schematic of the sample structure with the list of growth procedures. Every procedure has its thickness of 83 nm except procedure (i) of 166 nm.
(ix)
(vii) (vi)
(iii)
2 Pressure ( x 10–5 Torr)
The growth rates of GaAs and AlAs were fixed at 1.0 mm/h. After the growth of the AlAs layer, the shutter of the Al cell was closed and simultaneously the shutter of Ga cell was opened. Then, the Al source temperature was kept at the growth temperature of 1020 1C until the end of the full structure growth. The growth of GaNAs with plasma-assisted MBE requires many operations. We grew the first sample shown in Fig. 2 with procedures summarized there. We here assigned GaNAs layer for the layer grown with the bright mode N plasma with its shutter opened [4]. There are 16 procedures within the structure, labeled as (i)–(xvi). We carried out the listed operations at the beginning of every procedure. We kept the condition for 5 min, corresponding to the growth of 83 nm for each layer. Exceptionally, procedure (i) was kept for 10 min growing 166 nm to avoid the effect of the bottom AlAs layer, namely, effect of Al diffusion as reported by Sundgren et al. [12]. Those thicknesses were chosen to resolve the effect of each operation on the concentrations of the investigating elements with SIMS measurement. The second and third samples were grown with identical procedures with the first one skipping the procedures from (iv) to (vii). Those were grown at different Al cell temperatures of 1020 and 750 1C of standard idling temperature corresponding to its flux of 1 ML/s and o1 10 3 ML/s at the growth front, respectively. SIMS measurements were carried out on the samples for the concentrations of N and Al. The detection limit of the measurement was about 3 1016 cm 3 for N, and about 1 1015 cm 3 for Al. The details of the procedures listed in Fig. 2 are the followings: (i) we grow GaAs for the first. At this procedure, the cell shutters of N and Al are closed and N2 gas flow rate is 0 sccm. We (ii) open the VLV, and (iii) set N2 gas flow rate at 0.12 sccm to supply sufficient amount of N2 gas which would enable the succeeding plasma ignition. We (iv) open and (v) close N shutter to examine those effect. To examine the effect of As flux, we increase and decrease its beam equivalent pressure (BEP) at procedures (vi) and (vii) as shown in Fig. 3. The As2 BEP of 3.5 10 6 Torr at procedure (vii) is standard growth condition for the growth of GaAs at 1.0 ML/s in the system. We (viii) turn on the RF-power of the plasma cell and adjust it at 300 W, where N plasma ignites at dark mode. (ix) N2 flow rate is decreased to 0.02 sccm to make the
Total
N2
1
(xv) As2
0 0
500 Depth (nm)
1000
Fig. 3. Partial pressures of N2 and As2 estimated from total background pressure.
plasma transferred to bright mode. [4] At procedures (x)–(xiii), we open and close the shutter of the N plasma cell, and repeat that twice to certainly examine its effect. (xiv) The RF-power is turned off and then the plasma is extinguished. (xv) The N2 flow rate is set to zero and VLV is closed. (xvi) The shutter of Ga cell is closed to finish the growth.
ARTICLE IN PRESS 1648
F. Ishikawa et al. / Journal of Crystal Growth 311 (2009) 1646–1649
3. Results and discussion
(xv)
Fig. 4 shows SIMS depth profile of N and Al for the sample shown in Fig. 2. N concentration is preferably controlled by the applied power, the flow rate, resulting plasma mode transfer, and its shutter operation. Al is incorporated in spite of the closed shutter of Al cell. When the VLV is opened at procedure (ii), the Al profile shows a peak. The largest concentration of 1.0 1018 cm 3 is observed between the procedures (iii) and (viii). The concentration decreases to 2 1017 cm 3 at procedure (ix) keeping the amount until procedure (xv). Then, the concentration dropped to 3 1016 cm 3. The above results cannot be due to the failure of the shutter control since the Al concentration is suppressed at procedures (i) and (xv) where N is not introduced. Those results cannot be related to the diffusion or segregation from the AlAs layer as mentioned by Sundgren et al. [12]. Since the incorporation of Al is not detected in the GaAs layer at procedure (i) locating just above the AlAs layer. Consequently, the Al concentration is solely depends on N2 flow rate. The plasma power, mode, shutter conditions and As flux within the parameters varied in this study have no impact on that. A peak observed at procedure (ii) probably results from the residual N2 gas remaining at the gas line between VLV and MFC. It was evacuated in about 1 min corresponding to the 16 nm GaAs growth, which agrees well with the peak widths. Since the variation of As BEP has no impact on the concentration of extrinsic Al, the phenomena observed is considered to be due to the specific characteristics of N2 under our conventional growth conditions. 3.2. Al incorporation for varied Al cell temperature To investigate the origin of the incorporated Al, we carry out a similar experiment for two samples grown with different Al cell temperatures. We prepare two samples grown with the same procedures of the sample shown in Fig. 2, skipping procedures between (iv) and (vii). Fig. 5 shows the Al concentration for those samples obtained by SIMS. As clearly seen in the figure, the sample grown at the Al cell temperature of 1020 1C shows similar Al concentration profile with Fig. 4. A peak observed at the depth around 100 nm, corresponding to the procedure (ix), is due to the mechanical failure of MFC. When we set the flow rate value to zero, it occasionally shows spiky pressure increase, inducing the observed peak.
1022
Concentration (cm–3)
(xii) (xiii)
(i)
(x)
(vii)
(v)
(viii)
(vi)
(iv) (iii)
N
1016
(xv) (ii)
Al
0
(iii)
1018 TAl = 1020°C
TAl = 750°C 1016 0
500 Depth (nm)
1000
Fig. 5. Depth profile of Al for the samples grown at the Al cell temperature (TAl) of 1020 and 750 1C. Those were grown with the sequences (i)–(xvi) skipping the procedures between (iv) and (vii) shown in Fig. 1. The representative numbers of the procedure are mentioned in the figure.
In contrast, the sample grown at the Al cell temperature of 750 1C shows no Al incorporation corresponding to the N gas flow rate throughout the structure. Namely, at the common Al cell idling temperature, the unintentional incorporation could be efficiently suppressed. The above results predict the unintentionally incorporated Al can be originated from the Al beam sublimated from the Al source metal. The Al remaining on the chamber wall, cell wall or shutter is hard to be its origin because of the dependence of the cell temperature. Since the Al cell is not neighboring to the N cell as seen in Fig. 1, the observed results could not result from those configurations. We presume that the introduction of N modify the beam dispersion of Al, because of the large amount of N2 molecules which dominates the background pressure of the chamber as seen in Fig. 3. Then, the Al that usually does not impinge on the growth front would be carried on there, e.g., the beam escaped from the shutter or the beam usually blocked by the shutter. The incorporation of Al will induce material deteriorations of the epitaxial layers [7,8]. The introduction of N can also enhance the concentration of the other undesirable impurities such as C an O, possibly originated from the constituents of N gas or residuals in the chamber [7]. Those would be harmful to the epitaxial layers, resulting in the poorer devise performance [10,11]. Hence, we suggest here that the growth of dilute nitride, especially of active layer such as quantum well, should be carried out with the Al cell temperature and the N gas flow rate as low as possible within permissible range.
4. Summary
(xiv)
1018
(ii) (ix)
(xi) (ix)
1020
Al concentration (cm–3)
3.1. Al incorporation at al cell temp. at 1020 1C
500 Depth (nm)
1000
Fig. 4. SIMS depth profile of N and Al for the sample shown in Fig. 1.
We have investigated the unintentional incorporation of Al during the MBE growth related to N gas introduction. In spite of the closed shutter of Al cell, we observed Al incorporation in the epitaxial layer. The concentration depends on the N2 gas flow rate and Al source temperature. The concentration was thus suppressed by the reduction of the Al cell temperature. Shutter control of the N cell, As BEP and the operation of the RF plasma power had no impact on that. The introduction of large amount of N was considered to modify the beam dispersion of Al in the MBE chamber. The unintentional impurity incorporation can induce material deteriorations. Hence, dilute nitride semiconductors
ARTICLE IN PRESS F. Ishikawa et al. / Journal of Crystal Growth 311 (2009) 1646–1649
should be grown with the decreased Al cell temperature and the N flow rate. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grantin-Aid for Scientific Research (B) from Japan Society for the Promotion of Science (JSPS) and an Industrial Technology Research Grant Program in 2006 from NEDO, Japan. References [1] J.S. Harris Jr., R. Kudrawiec, H.B. Yuen, S.R. Bank, H.P. Bae, M.A. Wistey, D. Jackrel, E.R. Pickett, T. Sarmiento, L.L. Goddard, V. Lordi, T. Gugov, Phys. Status Solidi (b) 244 (2007) 2707. [2] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. 35 (1996) 1273.
1649
[3] J. Jewell, L. Graham, M. Crom, K. Maranovski, J. Smith, T. Franning, M. Schnoes, Phys. Status Solidi (c) 5 (2008) 2951. [4] V.A. Grant, R.P. Campion, C.T. Foxon, W. Lu, S. Chao, E.C. Larkins, Semicond. Sci. Technol. 22 (2007) 15. [5] M.A. Wistey, S.R. Bank, H.B. Yuen, H. Bae, J.S. Harris Jr., J. Crystal Growth 278 (2005) 229. [6] J. Miguel-Sa´nchez, A´. Guzma´n, E. Munoz, Appl. Phys. Lett. 85 (2004) 1940. [7] S.D. Wu, M. Kato, M. Uchiyama, K. Higashi, F. Ishikawa, M. Kondow, Appl. Phys. Express 1 (2008) 035004. [8] S.D. Wu, M. Kato, M. Uchiyama, K. Higashi, F. Ishikawa, M. Kondow, Phys. Status Solidi (c) 9 (2008) 2736. [9] K. Adachi, K. Nakahara, J. Kasai, T. Kitatani, T. Tsuchiya, M. Aoki, M. Kondow, Electron. Lett. 42 (2006) 1354. [10] S. Sato, Y. Osawa, T. Saitoh, Jpn. J. Appl. Phys. 36 (1997) 2671. [11] T. Takeuchi, Y.-L. Chang, M. Leary, D. Mars, Y.K. Song, S.D. Roh, H.-C. Luan, L.-M. Mantese, A. Tandon, R. Twist, S. Belov, D. Bour, M. Tan, Jpn. J. Appl. Phys. 43 (2004) 1260. [12] P. Sundgren, C. Asplund, K. Baskar, M. Hammar, Appl. Phys. Lett. 82 (2003) 2431.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Unintentional aluminum incorporation related to the introduction of nitrogen gas during the plasma-assisted molecular beam epitaxy F. Ishikawa , S.D. Wu, M. Kato, M. Uchiyama, K. Higashi, M. Kondow Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e in f o
a b s t r a c t
Available online 19 November 2008
We examine unintentional incorporation of Al during the growth of molecular beam epitaxy (MBE) related to N gas introduction. In spite of the closed shutter of Al cell, we observe Al incorporation in the epitaxial layer with a concentration up to 1 1018 cm 3. The concentration depends on the N2 gas flow rate and Al source temperature. The concentration is suppressed by the reduction of the Al cell temperature. Shutter control of the N cell, As beam equivalent pressure and the operation of the RF plasma power have no impact on that. The introduction of the large amount of N can modify the beam dispersion of Al in the MBE chamber, which will cause the extrinsic Al incorporation. The unintentional impurity incorporation can induce material deteriorations. We thus suggest that the growth should be carried out with the decreased Al cell temperature and the N gas flow rate as low as possible. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.72.S– 68.65.Fg 78.55.Cr 78.66.Fd 78.67.De 81.15.Hi Keywords: A3. Molecular beam epitaxy B1. Nitrides B2 Semiconducting III–V materials B3. Laser dioses B3. Optical fiber devices
1. Introduction Plasma-assisted molecular beam epitaxy (MBE) have been employed for the growth of nitride and dilute nitride semiconductors. Because of the feasibility of low growth temperature, MBE is suitable for the growth of metastable dilute nitride semiconductors such as GaInNAs [1,2]. The growth of GaInNAs has progressed, recently materializing commercial 1.3 mm laser diodes [3]. However, there are yet remaining problems to obtain ideal high-quality crystals. Plasma-induced material deterioration is one of the remaining controversial issues. There have been reports of the suppression of such deteriorations by the reduction of undesirable ion species, or the exploration of favorable plasma conditions [4–6]. On the other hand, we have carried out a study focusing on the unintentional incorporation of Al during the growth [7,8]. Similar results have been reported for the growth of GaInNAs lasers, not only for the growth of MBE but also for metal-organic vapor phase epitaxy, crucially degrading the laser performance [9–11]. In this report, we investigate the origin of the unintentional incorporation of Al during the plasma-assisted MBE growth of dilute nitride semiconductors. Secondary ion mass spectrometry (SIMS) study is carried out for the distributions of N and Al within
Corresponding author. Tel.: +81 6 6879 7767; fax: +81 6 6879 7753.
E-mail address: [email protected] (F. Ishikawa). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.11.042
the epitaxial layer. From the understanding of this study, we suggest the origin, and approach for the suppression of the unintentional Al incorporation.
2. Experimental procedure Samples were grown on n-GaAs (0 0 1) substrate by ANELVA620 molecular beam epitaxy. A schematic configuration of the equipped cells in the MBE system is shown in Fig. 1. The MBE have six evaporation cell ports. The vacuum line connected to a turbomolecular pump (TMP) and an ion pump (IP) is located on the chamber wall between the cells of As and Si. A self-made RF plasma source was used to produce active N species. Elemental Ga and Al were supplied from conventional Knudsen effusion cells. As was supplied with a VEECO 500 V-As valved cracker cell operating at As2 mode. N2 gas was introduced through Millipore WaferpureTM purifier. A mass flow controller (MFC) was used to adjust N2 gas flow rate. A variable leak valve (VLV) was attached between the MFC and the plasma cell, which was used just for opening and shutting the gas line. A standard shaft-rotating shutter was used to control N flux. We grew samples having GaNAs/GaAs/AlAs stacked structure at 450 1C. Prior to the growth of the structure, a 200 nm GaAs buffer layer was grown at 580 1C. On the buffer, we grew 1-mm-thick AlAs layer to make the growth situation similar to the growth of laser structure. The growth was carried out without growth interruption throughout the structure.
ARTICLE IN PRESS F. Ishikawa et al. / Journal of Crystal Growth 311 (2009) 1646–1649
1647
(xvi)
Substrate Vacuum (TMP and IP)
GaAs250 nm GaAs
250 nm
GaNAs nm GaNAs83 GaAs83 nm GaAs GaNAs83 nm GaNAs
83 nm 83 nm 83 nm
(xiii) (xii) (xi) (x) (ix)
830 nm
GaAs830 nm GaAs
(i) AlAs1000 nm AlAs As
Si
1000 nm
GaAsbuffer GaAs buffer200 nm 200 nm GaAs(001) sub. Al
In
(i) Grow GaAs (ii) Open variable leak valve N
Ga
(iii) Set N2 flow rate at 0.12 sccm (iv) Open N shutter (v) Close N shutter
Fig. 1. Schematic diagram of the cells configurations in the MBE system.
(vi) Increase As BEP (vii) Decrease As BEP (viii) Turn on RF power, set at 300 W (ix) Set N2 flow rate at 0.020 sccm (x) Open N shutter (xi) Close N shutter (xii) Open N shutter (xiii) Close N shutter (xiv) Turn off RF power (xv) Set N2 flow at 0 sccm, Close VLV (xvi) Close Ga shutter and finish the growth Fig. 2. Schematic of the sample structure with the list of growth procedures. Every procedure has its thickness of 83 nm except procedure (i) of 166 nm.
(ix)
(vii) (vi)
(iii)
2 Pressure ( x 10–5 Torr)
The growth rates of GaAs and AlAs were fixed at 1.0 mm/h. After the growth of the AlAs layer, the shutter of the Al cell was closed and simultaneously the shutter of Ga cell was opened. Then, the Al source temperature was kept at the growth temperature of 1020 1C until the end of the full structure growth. The growth of GaNAs with plasma-assisted MBE requires many operations. We grew the first sample shown in Fig. 2 with procedures summarized there. We here assigned GaNAs layer for the layer grown with the bright mode N plasma with its shutter opened [4]. There are 16 procedures within the structure, labeled as (i)–(xvi). We carried out the listed operations at the beginning of every procedure. We kept the condition for 5 min, corresponding to the growth of 83 nm for each layer. Exceptionally, procedure (i) was kept for 10 min growing 166 nm to avoid the effect of the bottom AlAs layer, namely, effect of Al diffusion as reported by Sundgren et al. [12]. Those thicknesses were chosen to resolve the effect of each operation on the concentrations of the investigating elements with SIMS measurement. The second and third samples were grown with identical procedures with the first one skipping the procedures from (iv) to (vii). Those were grown at different Al cell temperatures of 1020 and 750 1C of standard idling temperature corresponding to its flux of 1 ML/s and o1 10 3 ML/s at the growth front, respectively. SIMS measurements were carried out on the samples for the concentrations of N and Al. The detection limit of the measurement was about 3 1016 cm 3 for N, and about 1 1015 cm 3 for Al. The details of the procedures listed in Fig. 2 are the followings: (i) we grow GaAs for the first. At this procedure, the cell shutters of N and Al are closed and N2 gas flow rate is 0 sccm. We (ii) open the VLV, and (iii) set N2 gas flow rate at 0.12 sccm to supply sufficient amount of N2 gas which would enable the succeeding plasma ignition. We (iv) open and (v) close N shutter to examine those effect. To examine the effect of As flux, we increase and decrease its beam equivalent pressure (BEP) at procedures (vi) and (vii) as shown in Fig. 3. The As2 BEP of 3.5 10 6 Torr at procedure (vii) is standard growth condition for the growth of GaAs at 1.0 ML/s in the system. We (viii) turn on the RF-power of the plasma cell and adjust it at 300 W, where N plasma ignites at dark mode. (ix) N2 flow rate is decreased to 0.02 sccm to make the
Total
N2
1
(xv) As2
0 0
500 Depth (nm)
1000
Fig. 3. Partial pressures of N2 and As2 estimated from total background pressure.
plasma transferred to bright mode. [4] At procedures (x)–(xiii), we open and close the shutter of the N plasma cell, and repeat that twice to certainly examine its effect. (xiv) The RF-power is turned off and then the plasma is extinguished. (xv) The N2 flow rate is set to zero and VLV is closed. (xvi) The shutter of Ga cell is closed to finish the growth.
ARTICLE IN PRESS 1648
F. Ishikawa et al. / Journal of Crystal Growth 311 (2009) 1646–1649
3. Results and discussion
(xv)
Fig. 4 shows SIMS depth profile of N and Al for the sample shown in Fig. 2. N concentration is preferably controlled by the applied power, the flow rate, resulting plasma mode transfer, and its shutter operation. Al is incorporated in spite of the closed shutter of Al cell. When the VLV is opened at procedure (ii), the Al profile shows a peak. The largest concentration of 1.0 1018 cm 3 is observed between the procedures (iii) and (viii). The concentration decreases to 2 1017 cm 3 at procedure (ix) keeping the amount until procedure (xv). Then, the concentration dropped to 3 1016 cm 3. The above results cannot be due to the failure of the shutter control since the Al concentration is suppressed at procedures (i) and (xv) where N is not introduced. Those results cannot be related to the diffusion or segregation from the AlAs layer as mentioned by Sundgren et al. [12]. Since the incorporation of Al is not detected in the GaAs layer at procedure (i) locating just above the AlAs layer. Consequently, the Al concentration is solely depends on N2 flow rate. The plasma power, mode, shutter conditions and As flux within the parameters varied in this study have no impact on that. A peak observed at procedure (ii) probably results from the residual N2 gas remaining at the gas line between VLV and MFC. It was evacuated in about 1 min corresponding to the 16 nm GaAs growth, which agrees well with the peak widths. Since the variation of As BEP has no impact on the concentration of extrinsic Al, the phenomena observed is considered to be due to the specific characteristics of N2 under our conventional growth conditions. 3.2. Al incorporation for varied Al cell temperature To investigate the origin of the incorporated Al, we carry out a similar experiment for two samples grown with different Al cell temperatures. We prepare two samples grown with the same procedures of the sample shown in Fig. 2, skipping procedures between (iv) and (vii). Fig. 5 shows the Al concentration for those samples obtained by SIMS. As clearly seen in the figure, the sample grown at the Al cell temperature of 1020 1C shows similar Al concentration profile with Fig. 4. A peak observed at the depth around 100 nm, corresponding to the procedure (ix), is due to the mechanical failure of MFC. When we set the flow rate value to zero, it occasionally shows spiky pressure increase, inducing the observed peak.
1022
Concentration (cm–3)
(xii) (xiii)
(i)
(x)
(vii)
(v)
(viii)
(vi)
(iv) (iii)
N
1016
(xv) (ii)
Al
0
(iii)
1018 TAl = 1020°C
TAl = 750°C 1016 0
500 Depth (nm)
1000
Fig. 5. Depth profile of Al for the samples grown at the Al cell temperature (TAl) of 1020 and 750 1C. Those were grown with the sequences (i)–(xvi) skipping the procedures between (iv) and (vii) shown in Fig. 1. The representative numbers of the procedure are mentioned in the figure.
In contrast, the sample grown at the Al cell temperature of 750 1C shows no Al incorporation corresponding to the N gas flow rate throughout the structure. Namely, at the common Al cell idling temperature, the unintentional incorporation could be efficiently suppressed. The above results predict the unintentionally incorporated Al can be originated from the Al beam sublimated from the Al source metal. The Al remaining on the chamber wall, cell wall or shutter is hard to be its origin because of the dependence of the cell temperature. Since the Al cell is not neighboring to the N cell as seen in Fig. 1, the observed results could not result from those configurations. We presume that the introduction of N modify the beam dispersion of Al, because of the large amount of N2 molecules which dominates the background pressure of the chamber as seen in Fig. 3. Then, the Al that usually does not impinge on the growth front would be carried on there, e.g., the beam escaped from the shutter or the beam usually blocked by the shutter. The incorporation of Al will induce material deteriorations of the epitaxial layers [7,8]. The introduction of N can also enhance the concentration of the other undesirable impurities such as C an O, possibly originated from the constituents of N gas or residuals in the chamber [7]. Those would be harmful to the epitaxial layers, resulting in the poorer devise performance [10,11]. Hence, we suggest here that the growth of dilute nitride, especially of active layer such as quantum well, should be carried out with the Al cell temperature and the N gas flow rate as low as possible within permissible range.
4. Summary
(xiv)
1018
(ii) (ix)
(xi) (ix)
1020
Al concentration (cm–3)
3.1. Al incorporation at al cell temp. at 1020 1C
500 Depth (nm)
1000
Fig. 4. SIMS depth profile of N and Al for the sample shown in Fig. 1.
We have investigated the unintentional incorporation of Al during the MBE growth related to N gas introduction. In spite of the closed shutter of Al cell, we observed Al incorporation in the epitaxial layer. The concentration depends on the N2 gas flow rate and Al source temperature. The concentration was thus suppressed by the reduction of the Al cell temperature. Shutter control of the N cell, As BEP and the operation of the RF plasma power had no impact on that. The introduction of large amount of N was considered to modify the beam dispersion of Al in the MBE chamber. The unintentional impurity incorporation can induce material deteriorations. Hence, dilute nitride semiconductors
ARTICLE IN PRESS F. Ishikawa et al. / Journal of Crystal Growth 311 (2009) 1646–1649
should be grown with the decreased Al cell temperature and the N flow rate. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grantin-Aid for Scientific Research (B) from Japan Society for the Promotion of Science (JSPS) and an Industrial Technology Research Grant Program in 2006 from NEDO, Japan. References [1] J.S. Harris Jr., R. Kudrawiec, H.B. Yuen, S.R. Bank, H.P. Bae, M.A. Wistey, D. Jackrel, E.R. Pickett, T. Sarmiento, L.L. Goddard, V. Lordi, T. Gugov, Phys. Status Solidi (b) 244 (2007) 2707. [2] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. 35 (1996) 1273.
1649
[3] J. Jewell, L. Graham, M. Crom, K. Maranovski, J. Smith, T. Franning, M. Schnoes, Phys. Status Solidi (c) 5 (2008) 2951. [4] V.A. Grant, R.P. Campion, C.T. Foxon, W. Lu, S. Chao, E.C. Larkins, Semicond. Sci. Technol. 22 (2007) 15. [5] M.A. Wistey, S.R. Bank, H.B. Yuen, H. Bae, J.S. Harris Jr., J. Crystal Growth 278 (2005) 229. [6] J. Miguel-Sa´nchez, A´. Guzma´n, E. Munoz, Appl. Phys. Lett. 85 (2004) 1940. [7] S.D. Wu, M. Kato, M. Uchiyama, K. Higashi, F. Ishikawa, M. Kondow, Appl. Phys. Express 1 (2008) 035004. [8] S.D. Wu, M. Kato, M. Uchiyama, K. Higashi, F. Ishikawa, M. Kondow, Phys. Status Solidi (c) 9 (2008) 2736. [9] K. Adachi, K. Nakahara, J. Kasai, T. Kitatani, T. Tsuchiya, M. Aoki, M. Kondow, Electron. Lett. 42 (2006) 1354. [10] S. Sato, Y. Osawa, T. Saitoh, Jpn. J. Appl. Phys. 36 (1997) 2671. [11] T. Takeuchi, Y.-L. Chang, M. Leary, D. Mars, Y.K. Song, S.D. Roh, H.-C. Luan, L.-M. Mantese, A. Tandon, R. Twist, S. Belov, D. Bour, M. Tan, Jpn. J. Appl. Phys. 43 (2004) 1260. [12] P. Sundgren, C. Asplund, K. Baskar, M. Hammar, Appl. Phys. Lett. 82 (2003) 2431.