- Email: [email protected]
GaAs quantum wells grown by molecular beam epitaxy
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Journal of Crystal Growth 281 (2005) 249–254 www.elsevier.com/locate/jcrysgro
Influence of arsenic pressure on photoluminescence and structural properties of GaInNAs/GaAs quantum wells grown by molecular beam epitaxy E.-M. Pavelescua,b,, T. Hakkarainena, V.D.S. Dhakab, N.V. Tkachenkob, T. Jouhtia, H. Lemmetyinenb, M. Pessaa a
Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, FI-33101, Tampere, Finland
b
Received 27 March 2005; accepted 11 April 2005 Available online 26 May 2005 Communicated by S. Hiyamizu
Abstract We have investigated the photoluminescence and structural properties of GaInNAs/GaAs quantum wells grown by molecular beam epitaxy under different arsenic pressures with all the other fluxes kept constant. The best optical properties are achieved when the V/III beam equivalent pressure ratio (V/IIIBEP) is equal to 10. The emission wavelength remains unchanged for the V/IIIBEP ratios between 8 and 12, suggesting that within this range neither the alloy composition nor the nitrogen sticking coefficient is changed. For V/IIIBEPo8, incorporation of nitrogen into the crystal is enhanced and for V/IIIBEP412 incorporation is reduced. Post-growth thermal annealing induces a spectral blue-shift, which decreases as the V/IIIBEP ratio is increased above 12. This phenomenon is likely due to combined effects of Ga/In interdiffusion and a change in the nearest neighbourhood of nitrogen. r 2005 Elsevier B.V. All rights reserved. PACS: 81.15.H; 78.55; 78.66 Keywords: A1. Photoluminescence; A3. Molecular beam epitaxy; A3. Quantum wells; B1. Dilute nitride alloys
Corresponding author. Optoelectronics Research Centre,
Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland. Tel.: +358 3 3115 2675; fax: +358 3 3115 3400. E-mail address: [email protected].fi (E.-M. Pavelescu).
1. Introduction Dilute nitrides, such as GaNxAs1x and Ga1yInyNxAs1x, exhibit large bandgap bowing coefficients, which give rise to a strong fundamental bandgap reduction [1–4]. By alloying N
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.04.025
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with InGaAs, one can control the size and sign of lattice strain in GaInNAs [5,6]. Due to bandgap bowing, the conduction band discontinuity at a GaInNAs/GaAs junction becomes very large, offering exceptionally strong electron confinement in the GaInNAs quantum well (QW) [7]. Therefore, GaInNAs is a promising material for the active regions of 1.3–1.55 mm diode lasers, and is compatible to high-contrast GaAs/AlAs distributed Bragg reflectors used in vertical cavity surface emitting lasers [8–10]. Dilute nitrides are made of binary constituents, the sub-lattices of which possess different crystal structures, and they have an alloy miscibility gap [11]. Therefore, a non-equilibrium growth method, such as molecular beam epitaxy (MBE), is desired for preparing these alloys. Despite considerable efforts made in attempts to grow device-quality GaInNAs by MBE, effects of the V/III beam pressure ratio—an important growth parameter— are only little studied in the literature. In the photoluminescence (PL) measurements of Kondow and Kitatani [12], the emission wavelength exhibited a parabolic behaviour, as the V/III ratio was varied. Egorov et al. [13] observed a PL blueshift when the arsenic pressure was increased. Also, Li et al. [14] found that PL blue-shifted and peak intensity dropped, as the arsenic pressure was increased. Unfortunately, Li et al. grew the GaAs buffer layers at different arsenic pressures, which almost certainly influenced PL and made their results less dependable. In our study, presented here, the GaInNAs QWs were grown using different V/III ratios (by varying the As pressure), while all the other growth parameters were kept constant. We observed that PL and structural properties of both the as-grown and post-growth annealed QWs depended on arsenic pressure. We also found that the blueshift, caused by annealing, decreased when the arsenic pressure was increased.
with an atomic nitrogen RF-plasma source. The layers on either side of the QW were deposited under identical growth conditions at T g ¼ 590 1C, while the QW was grown at 450 1C under various arsenic pressures. Each sample consisted of a 170nm GaAs buffer layer, a 7-nm Ga0.63In0.37N0.013As0.987 QW and a 100-nm GaAs cap layer. The QW growth rate was 1:59 mm=h, whereas the GaAs growth rate was 1 mm=h. No growth interruption was employed. With 5 min before growing the QW, the plasma was ignited and the substrate temperature was ramped down in order to reach 450 1C with 10 s before the QW growth. The target position of the arsenic source valve was changed 15 s before growth of the QW, and was switched back to its previous value immediately after the QW growth. After the QW growth, the substrate temperature was ramped up back to 590 1C with the same rate as that used during the ramping down. The arsenic beam equivalent pressures (BEP) were between 6 108 mbar (corresponding to the valve position of 200 mils of the As source) and 3 107 mbar (for 300 mils). We determined the V/IIIBEP ratio from the equation V=IIIBEP ¼ PAs2 =ðPGa þ PIn Þ,
(1)
where PAs2 , PGa, and PIn are the BEPs of As2, Ga, and In, respectively. The V/IIIBEP ratio was varied from 6 to 14. The V/IIIBEP ratio has the advantage that it is simply the ratio of quantities measured prior to growth. As a result of this simplicity, the growth conditions can be reproduced reliably from one growth to the next or by other MBE machines which have a similar geometry. To have a parameter which is more conveniently related to the growth stoichiometry, and hence to the ‘‘real’’ V/III ratio (comparable among MBE reactors irrespective of their geometry of each source and ion gauge) that is atomic ratio on the growing surface, we have deduced the incident V/III flux ratio (V/IIIincid) by the formula [15]:
2. Experimental procedure V=IIIincid Single-QW GaInNAs/GaAs samples were grown on n-type (1 0 0) GaAs substrates by a 8port VG Semicon V80H MBE reactor, equipped
¼Z
As1 PIn
ZIn
PAs qffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z2 P qffiffiffiffiffiffiffiffiffiffiffiffiffiffi , M In T As2 M Ga T As2 As1 Ga M As T In þ Z M As T Ga 2
Ga
2
ð2Þ
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where ZX is the atomic number of the element X. With our growth parameters the V/IIIincid varied from 4.7 to 10.9. Having the V/IIIincid flux ratio found out, one could estimate the ‘‘real’’ V/III ratio on the growing surface by taking S As2 S III V=IIIincid , where SAs and SIII are the sticking coefficients of As2 and group-III elements, respectively. Considering at 450 1C (thermocouple) S As 0:35 [16] and SIII 1, the ‘‘real’’ V/III ratio estimation ranges from 1.6 to 3.8. That is, an excess of As with respect to group-III atoms exists on the growing surface over the whole range of As pressures used in our experiment. PL was recorded using a sample holder with a closed-cycle He cryostat and the 488-nm line of an Ar+-ion laser for excitation. Time-resolved PL was measured using a femtosecond up-conversion method and a mode-locked Ti:sapphire laser with 50-fs pulses. The overall time resolution of the instrument was 150 fs. X-ray diffraction (XRD) rocking curves were measured by a double-crystal diffractometer. Rapid thermal annealing (RTA) was performed at 610 1C for 40 min (in time steps of 5 min) in dry nitrogen atmosphere using a GaAs proximity cap.
3. Results and discussion The growth temperature of the QW (450 1C) was chosen so that the highest PL intensity and the smallest full-width at half-maximum (FWHM) was achieved. The sticking coefficient of nitrogen is known to be much higher than that of arsenic and is always close to unity [12]. Consequently, the nitrogen concentration is controlled with the RF
0.4
0.3
1330
38
1325
37
1320
36
1315
35
1310
FWHM (meV)
(3)
PL Wavelength (nm)
ZX 0:4Z x þ 0:6, ¼ ZN2 14
power and a proper choice of the plasma flow, while the V/III ratio is controlled by the arsenic flow. The resultant concentration of nitrogen is inversely proportional to the group-III growth rate [13]. Fig. 1 shows room-temperature PL from QWs grown under different arsenic pressures. The strongest emission and the smallest FWHM were obtained at the As valve position 250 mils, corresponding to the V/IIIBEP ratio of 10 (V/ IIIE2.7). The sample grown at the lowest V/ IIIBEP of 6 was optically dead at room temperature. PL dropped and FWHM increased marginally for the V/IIIBEP of 14. When the arsenic BEP was increased far above the optimal value (250 mils valve position), arsenic antisites (AsGa) probably appeared, impairing the PL efficiency [17]. The samples with 8pV/IIIBEPp12 emitted at a fixed wavelength. Therefore, within this range the V/IIIBEP ratio neither affected the alloy composition nor the sticking coefficient. When the As valve position was set to 300 mils, corresponding to the V/IIIBEP of 14, PL blueshifted by about 25 nm, indicating that there was a change in layer structure or composition during sample growth.
PL Intensity (a.u.)
where ZAs1 ; ZIn ; ZGa are the ionization efficiencies of As, In and Ga, respectively; T As2 ; T In ; T Ga are the temperatures (in Kelvin) of the molecular beams of As2, In, and Ga, respectively, taken to be the same as the source temperatures; and M As2 ; M In ; M Ga are the molecular weights of the molecular beams of As2, In, and Ga, respectively. The ionization efficiency with respect to N2 for each element is given by
251
V/IIIBEP = 10
V/IIIBEP = 12
V/IIIBEP = 8
34
1305 8 10 12 14
0.2
V/IIIBEP V/IIIBEP = 14
0.1
900
1000
1100
1200
1300
1400
Wavelength (nm) Fig. 1. PL curves from QWs grown at different V/IIIBEP ratios. The inset shows PL peak wavelength and FWHM vs. V/IIIBEP ratio.
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Poor optical properties of as-grown dilute nitrides in general can be improved by thermal annealing which, however, causes an undesired blue-shift [18]. Fig. 2 illustrates the dependence of 60-K PL (we have chosen this temperature in order to minimize the PL localization phenomena usually seen in GaInNAs-containing structures at very low temperatures) peak intensity on the V/ IIIBEP ratio, before and after RTA. The most remarkable improvement in PL was obtained for the V/IIIBEP ratio of 8. Notably, the PL efficiency of the sample grown under the lowest As pressure remained poor even after annealing. One can see that the blue-shift induced by RTA depends on the arsenic pressure, viz., the blue-shift is less enhanced at high arsenic pressure. If Tg of the QW is below 500 1C, we have quite a large pressure range where no accurate control of the As2 flux is necessary. These observations imply that MBE growth of GaInNAs may be more reproducible from one sample to another than is MBE of GaInAsP for which a precise control over the two group-V fluxes is required [19]. The samples grown with the V/IIIBEP ratios of 6, 10, and 14 were studied by XRD. The X-ray rocking curves for the as-grown samples (Fig. 3) appear to be almost identical. The presence of
110
60 K
90
10 as-grown annealed
80 70
Blue-shift (nm)
PL intensity (a.u.)
100
60
1
50 40 6
8
10 12 V/IIIBEP ratio
14
Fig. 2. V/IIIBEP dependence of PL peak intensity of as-grown (squares) and annealed (dots) QWs and annealing-induced blue-shift.
Reflectivity (a.u.)
E.-M. Pavelescu et al. / Journal of Crystal Growth 281 (2005) 249–254
252
V/III BEP = 6
as-grown annealed
V/III BEP = 10
as-grown annealed
V/III BEP = 14
as-grown annealed
-7500
-5000
-2500
0
2500
Angle θ-2θ (arc sec) Fig. 3. Double-crystal XRD rocking curves measured before and after annealing for QWs grown under different arsenic pressures.
Pendello¨sung fringes indicates that the QW remains strained and the interfaces are abrupt. The main difference between these XRDs is the position of the QW-related diffraction peak on the y–2y axis. This peak shifts closer to the substrate peak when the V/IIIBEP ratio is decreased from 14 to 6. The peak shift shows that the QWs grown with low V/IIIBEP are less strained than the QW grown with optimal V/IIIBEP ( ¼ 10, valve position ¼ 250 mils), while those grown at high V/IIIBEP are more strained. We attribute these phenomena of strain to changes in incorporation efficiency of nitrogen; i.e., nitrogen is incorporated more effectively into the group-V sub-lattice when the V/IIIBEP ratio is low. An increase in nitrogen concentration will also increase the number of non-radiative defects, deteriorating PL intensity. This is exactly what we have observed: the sample grown at the smallest As pressure has very weak PL which, in addition, is the most red-shifted with respect to PL for the V=IIIBEP ¼ 10 sample. Such PL behaviour can be understood as a result of competition between As and N ad-atoms in occupying the anion lattice sites.
ARTICLE IN PRESS E.-M. Pavelescu et al. / Journal of Crystal Growth 281 (2005) 249–254
I PL ðtÞ et=tcap et=tdecay .
(2)
Here, tcap represents a characteristic time for carrier capture by the QW and tdecay is the effective lifetime of a carrier in the QW. Notice that it has been sufficient to use a single exponential in Eq. (2) to describe the carrier capture into the QW. This mono-exponential nature of the PL rise indicates that the direct carrier capture is the major timelimiting step in filling the QW. The carrier decays from the QW can also be described to a good accuracy by a single exponential. The effective capture times were measured to be 3–4 ps for all
Annealed
PL Intensity (a.u.)
The Pendello¨sung fringes, present in all the rocking curves of the as-grown samples (Fig. 3), disappear upon RTA; i.e., the layer structure is deteriorated. The sample grown with V=IIIBEP ¼ 6 shows no significant change in the shape or position of the QW-related XRD feature after annealing. This suggests that the blue-shift is mainly due to a formation of In–N bonds [18,20]. Instead, the samples with the larger V/ IIIBEP ratios show a small, but discernable, systematic shift of the QW-related XRD feature towards the substrate peak (y ¼ 01), as the arsenic pressure is increased. This shift indicates a systematic decrease in QW compressive strain. Such an effect upon annealing can be attributed to atomic interdiffusion at the QW interfaces, likely Ga/In interdiffusion [21], or to partial strain relief (strain relaxation without mass transport). Basically, the effect on QW emission from these two mechanisms is opposite, the former leads to a blueshift, the latter gives rise to a red-shift. The observed dependence of the blue-shift on arsenic pressure is not sufficient to distinguish between these two possibilities. Moreover, an enhanced blue-shift with decreasing arsenic pressure could also be due to an increase in the number of In–N bonds, following an increase of arsenic vacancies [20]. In order to investigate the effects of arsenic pressure on carrier dynamics in the QWs, we measured the time evolution of PL intensity IPL for the samples grown with V/IIIBEP of 10 and 14. The results are shown in Fig. 4. The points denote the experimental data and the full lines represent the best fits to the following expression:
253
V/IIIBEP = 14
As-grown V/IIIBEP = 10
0
50
100
150
200
250
Time (s) Fig. 4. PL decay profiles for the QW samples with different V/ IIIBEP ratios before and after annealing.
the samples, and they were independent of arsenic pressure or annealing. The effective decay times were between 109720 ps (V=IIIBEP ¼ 10) to 126717 ps (V=IIIBEP ¼ 14); a very marginal dependence on arsenic pressure. Although the steady-state PL intensity of the sample with V=IIIBEP ¼ 10 was improved considerably upon annealing, tdecay remained the same (110713 ps) as tdecay for the as-grown sample. However, tdecay for the V=IIIBEP ¼ 14 sample increased upon annealing to 233715 ps. This arsenic-dependent behaviour of tdecay for the V=IIIBEP ¼ 14 case upon annealing could be understood, as judged from the afore-mentioned PL and X-ray studies, as due to a competition of several arsenic-dependent annealing-induced structural and compositional changes in the QW. The observed insensitivity of tdecay upon annealing for a certain arsenic pressure range (from V=IIIBEP ¼ 8 to 12) suggests to us an interesting approach that may be applied to keep tdecay of GaInNAs-based optical switches and saturable absorber mirrors (SESAMs) rather unchanged, while improving their thermal stability and optical efficiency by RTA.
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4. Conclusions GaInNAs/GaAs QW samples were grown under different arsenic pressures by MBE, and photoluminescence (PL) and X-ray diffraction were investigated. The best optical properties (the highest PL intensities) were obtained when the V/ IIIBEP pressure ratio was 10. The emission wavelength remained unchanged for 8pV/ IIIBEPp12, which indicated that the V/III ratio neither affected the alloy composition nor the sticking coefficient of nitrogen. For the lower and higher V/IIIBEP ratios, the results suggest that the N incorporation into the group-V sub-lattice is enhanced or reduced, respectively. The annealinginduced blue-shift decreased when the V/IIIBEP ratio was increased. Based on the steady-state and time-resolved PL observations, and the XRD results, variations in blue-shifts upon RTA probably indicated a combined effect of Ga/In interdiffusion and a change in the nearest neighbours of nitrogen atoms.
Acknowledgement This work was supported, in part, by the Academy of Finland within the TULE/QUEST Project. One of the authors (T. Hakkarainen) acknowledges the support by the Academy via the ATOMISTIC Project.
References [1] J. Toivonen, T. Hakkarainen, M. Sopanen, H. Lipsanen, J. Crystal Growth 221 (2000) 456.
[2] U. Tisch, E. Finkman, J. Salzman, Appl. Phys. Lett. 81 (2002) 463. [3] W.G. Bi, C.W. Tu, Appl. Phys. Lett. 70 (1997) 1608. [4] S.-H. Wei, A. Zunger, Phys. Rev. Lett. 76 (1996) 664. [5] J.F. Geisz, D.J. Friedman, J.M. Olson, S.R. Kurtz, B.M. Keyes, J. Crystal Growth 195 (1998) 401. [6] E.-M. Pavelescu, C.S. Peng, T. Jouhti, W. Li, M. Pessa, M. Dumitrescu, S. Spanulescu, Appl. Phys. Lett. 80 (2002) 3054. [7] W. Shan, W. Walukiewicz, J.W. Ager III, E.E. Haller, J.F. Geisz, D.J. Friedman, J.M. Olson, S.R. Kurtz, Phys. Rev. Lett. 82 (1999) 1221. [8] T. Jouhti, O. Okhotnikov, J. Konttinen, L.A. Gomes, C.S. Peng, S. Karirinne, E.-M. Pavelescu, M. Pessa, NJ Phys. 5 (2003) 84.1. [9] T. Kitatani, K. Nakahara, M. Kondow, K. Uomi, T. Tanaka, Jpn. J. Appl. Phys. 39 (2000) L86. [10] C.S. Peng, N. Laine, J. Konttinen, S. Karirinne, T. Jouhti, M. Pessa, IEE Electron. Lett. 40 (2004) 604. [11] D. Schlenker, T. Miyamoto, Z. Pan, F. Koyama, K. Iga, J. Crystal Growth 196 (1999) 67. [12] M. Kondow, T. Kitatani, Semicond. Sci. Technol. 17 (2002) 746. [13] A. Yu. Egorov, D. Bernklau, B. Borchert, S. Illek, D. Livshits, A. Rucki, M. Schuster, A. Kaschner, A. Hoffmann, Gh. Dumitras, M.C. Amann, H. Riechert, J. Crystal Growth 227–228 (2001) 545. [14] L.H. Li, Z. Pan, W. Zhang, Y.W. Lin, X.Y. Wang, R.H. Wu, J. Crystal Growth 227–228 (2001) 527. [15] J.Y. Tsao, T.M. Brennan, B.E. Hammons, Appl. Phys. Lett. 53 (1988) 288. [16] C.T. Foxon, B.A. Joyce, Surf. Sci. 64 (1977) 293. [17] N.Q. Thinh, I.A. Buyanova, W.M. Chen, H.P. Xin, C.W. Tu, Appl. Phys. Lett. 79 (2001) 3089. [18] E.-M. Pavelescu, T. Jouhti, M. Dumitrescu, P.J. Klar, S. Karirinne, Y. Fedorenko, M. Pessa, Appl. Phys. Lett. 83 (2003) 1497. [19] R.R. Lapierre, B.J. Robinson, D.A. Thompson, J. Appl. Phys. 79 (1996) 3021. [20] P.J. Klar, H. Gru¨ning, J. Koch, S. Scha¨fer, K. Volz, W. Stolz, W. Heimbrodt, A. Kamal Saadi, A. Lindsay, E.P. O’Reilly, Phys. Rev. B 64 (2001) 121203. [21] H.P. Xin, K.L. Kavanagh, C.W. Tu, J. Crystal Growth 208 (2000) 145.
Journal of Crystal Growth 281 (2005) 249–254 www.elsevier.com/locate/jcrysgro
Influence of arsenic pressure on photoluminescence and structural properties of GaInNAs/GaAs quantum wells grown by molecular beam epitaxy E.-M. Pavelescua,b,, T. Hakkarainena, V.D.S. Dhakab, N.V. Tkachenkob, T. Jouhtia, H. Lemmetyinenb, M. Pessaa a
Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, FI-33101, Tampere, Finland
b
Received 27 March 2005; accepted 11 April 2005 Available online 26 May 2005 Communicated by S. Hiyamizu
Abstract We have investigated the photoluminescence and structural properties of GaInNAs/GaAs quantum wells grown by molecular beam epitaxy under different arsenic pressures with all the other fluxes kept constant. The best optical properties are achieved when the V/III beam equivalent pressure ratio (V/IIIBEP) is equal to 10. The emission wavelength remains unchanged for the V/IIIBEP ratios between 8 and 12, suggesting that within this range neither the alloy composition nor the nitrogen sticking coefficient is changed. For V/IIIBEPo8, incorporation of nitrogen into the crystal is enhanced and for V/IIIBEP412 incorporation is reduced. Post-growth thermal annealing induces a spectral blue-shift, which decreases as the V/IIIBEP ratio is increased above 12. This phenomenon is likely due to combined effects of Ga/In interdiffusion and a change in the nearest neighbourhood of nitrogen. r 2005 Elsevier B.V. All rights reserved. PACS: 81.15.H; 78.55; 78.66 Keywords: A1. Photoluminescence; A3. Molecular beam epitaxy; A3. Quantum wells; B1. Dilute nitride alloys
Corresponding author. Optoelectronics Research Centre,
Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland. Tel.: +358 3 3115 2675; fax: +358 3 3115 3400. E-mail address: [email protected].fi (E.-M. Pavelescu).
1. Introduction Dilute nitrides, such as GaNxAs1x and Ga1yInyNxAs1x, exhibit large bandgap bowing coefficients, which give rise to a strong fundamental bandgap reduction [1–4]. By alloying N
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.04.025
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with InGaAs, one can control the size and sign of lattice strain in GaInNAs [5,6]. Due to bandgap bowing, the conduction band discontinuity at a GaInNAs/GaAs junction becomes very large, offering exceptionally strong electron confinement in the GaInNAs quantum well (QW) [7]. Therefore, GaInNAs is a promising material for the active regions of 1.3–1.55 mm diode lasers, and is compatible to high-contrast GaAs/AlAs distributed Bragg reflectors used in vertical cavity surface emitting lasers [8–10]. Dilute nitrides are made of binary constituents, the sub-lattices of which possess different crystal structures, and they have an alloy miscibility gap [11]. Therefore, a non-equilibrium growth method, such as molecular beam epitaxy (MBE), is desired for preparing these alloys. Despite considerable efforts made in attempts to grow device-quality GaInNAs by MBE, effects of the V/III beam pressure ratio—an important growth parameter— are only little studied in the literature. In the photoluminescence (PL) measurements of Kondow and Kitatani [12], the emission wavelength exhibited a parabolic behaviour, as the V/III ratio was varied. Egorov et al. [13] observed a PL blueshift when the arsenic pressure was increased. Also, Li et al. [14] found that PL blue-shifted and peak intensity dropped, as the arsenic pressure was increased. Unfortunately, Li et al. grew the GaAs buffer layers at different arsenic pressures, which almost certainly influenced PL and made their results less dependable. In our study, presented here, the GaInNAs QWs were grown using different V/III ratios (by varying the As pressure), while all the other growth parameters were kept constant. We observed that PL and structural properties of both the as-grown and post-growth annealed QWs depended on arsenic pressure. We also found that the blueshift, caused by annealing, decreased when the arsenic pressure was increased.
with an atomic nitrogen RF-plasma source. The layers on either side of the QW were deposited under identical growth conditions at T g ¼ 590 1C, while the QW was grown at 450 1C under various arsenic pressures. Each sample consisted of a 170nm GaAs buffer layer, a 7-nm Ga0.63In0.37N0.013As0.987 QW and a 100-nm GaAs cap layer. The QW growth rate was 1:59 mm=h, whereas the GaAs growth rate was 1 mm=h. No growth interruption was employed. With 5 min before growing the QW, the plasma was ignited and the substrate temperature was ramped down in order to reach 450 1C with 10 s before the QW growth. The target position of the arsenic source valve was changed 15 s before growth of the QW, and was switched back to its previous value immediately after the QW growth. After the QW growth, the substrate temperature was ramped up back to 590 1C with the same rate as that used during the ramping down. The arsenic beam equivalent pressures (BEP) were between 6 108 mbar (corresponding to the valve position of 200 mils of the As source) and 3 107 mbar (for 300 mils). We determined the V/IIIBEP ratio from the equation V=IIIBEP ¼ PAs2 =ðPGa þ PIn Þ,
(1)
where PAs2 , PGa, and PIn are the BEPs of As2, Ga, and In, respectively. The V/IIIBEP ratio was varied from 6 to 14. The V/IIIBEP ratio has the advantage that it is simply the ratio of quantities measured prior to growth. As a result of this simplicity, the growth conditions can be reproduced reliably from one growth to the next or by other MBE machines which have a similar geometry. To have a parameter which is more conveniently related to the growth stoichiometry, and hence to the ‘‘real’’ V/III ratio (comparable among MBE reactors irrespective of their geometry of each source and ion gauge) that is atomic ratio on the growing surface, we have deduced the incident V/III flux ratio (V/IIIincid) by the formula [15]:
2. Experimental procedure V=IIIincid Single-QW GaInNAs/GaAs samples were grown on n-type (1 0 0) GaAs substrates by a 8port VG Semicon V80H MBE reactor, equipped
¼Z
As1 PIn
ZIn
PAs qffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z2 P qffiffiffiffiffiffiffiffiffiffiffiffiffiffi , M In T As2 M Ga T As2 As1 Ga M As T In þ Z M As T Ga 2
Ga
2
ð2Þ
ARTICLE IN PRESS E.-M. Pavelescu et al. / Journal of Crystal Growth 281 (2005) 249–254
where ZX is the atomic number of the element X. With our growth parameters the V/IIIincid varied from 4.7 to 10.9. Having the V/IIIincid flux ratio found out, one could estimate the ‘‘real’’ V/III ratio on the growing surface by taking S As2 S III V=IIIincid , where SAs and SIII are the sticking coefficients of As2 and group-III elements, respectively. Considering at 450 1C (thermocouple) S As 0:35 [16] and SIII 1, the ‘‘real’’ V/III ratio estimation ranges from 1.6 to 3.8. That is, an excess of As with respect to group-III atoms exists on the growing surface over the whole range of As pressures used in our experiment. PL was recorded using a sample holder with a closed-cycle He cryostat and the 488-nm line of an Ar+-ion laser for excitation. Time-resolved PL was measured using a femtosecond up-conversion method and a mode-locked Ti:sapphire laser with 50-fs pulses. The overall time resolution of the instrument was 150 fs. X-ray diffraction (XRD) rocking curves were measured by a double-crystal diffractometer. Rapid thermal annealing (RTA) was performed at 610 1C for 40 min (in time steps of 5 min) in dry nitrogen atmosphere using a GaAs proximity cap.
3. Results and discussion The growth temperature of the QW (450 1C) was chosen so that the highest PL intensity and the smallest full-width at half-maximum (FWHM) was achieved. The sticking coefficient of nitrogen is known to be much higher than that of arsenic and is always close to unity [12]. Consequently, the nitrogen concentration is controlled with the RF
0.4
0.3
1330
38
1325
37
1320
36
1315
35
1310
FWHM (meV)
(3)
PL Wavelength (nm)
ZX 0:4Z x þ 0:6, ¼ ZN2 14
power and a proper choice of the plasma flow, while the V/III ratio is controlled by the arsenic flow. The resultant concentration of nitrogen is inversely proportional to the group-III growth rate [13]. Fig. 1 shows room-temperature PL from QWs grown under different arsenic pressures. The strongest emission and the smallest FWHM were obtained at the As valve position 250 mils, corresponding to the V/IIIBEP ratio of 10 (V/ IIIE2.7). The sample grown at the lowest V/ IIIBEP of 6 was optically dead at room temperature. PL dropped and FWHM increased marginally for the V/IIIBEP of 14. When the arsenic BEP was increased far above the optimal value (250 mils valve position), arsenic antisites (AsGa) probably appeared, impairing the PL efficiency [17]. The samples with 8pV/IIIBEPp12 emitted at a fixed wavelength. Therefore, within this range the V/IIIBEP ratio neither affected the alloy composition nor the sticking coefficient. When the As valve position was set to 300 mils, corresponding to the V/IIIBEP of 14, PL blueshifted by about 25 nm, indicating that there was a change in layer structure or composition during sample growth.
PL Intensity (a.u.)
where ZAs1 ; ZIn ; ZGa are the ionization efficiencies of As, In and Ga, respectively; T As2 ; T In ; T Ga are the temperatures (in Kelvin) of the molecular beams of As2, In, and Ga, respectively, taken to be the same as the source temperatures; and M As2 ; M In ; M Ga are the molecular weights of the molecular beams of As2, In, and Ga, respectively. The ionization efficiency with respect to N2 for each element is given by
251
V/IIIBEP = 10
V/IIIBEP = 12
V/IIIBEP = 8
34
1305 8 10 12 14
0.2
V/IIIBEP V/IIIBEP = 14
0.1
900
1000
1100
1200
1300
1400
Wavelength (nm) Fig. 1. PL curves from QWs grown at different V/IIIBEP ratios. The inset shows PL peak wavelength and FWHM vs. V/IIIBEP ratio.
ARTICLE IN PRESS
Poor optical properties of as-grown dilute nitrides in general can be improved by thermal annealing which, however, causes an undesired blue-shift [18]. Fig. 2 illustrates the dependence of 60-K PL (we have chosen this temperature in order to minimize the PL localization phenomena usually seen in GaInNAs-containing structures at very low temperatures) peak intensity on the V/ IIIBEP ratio, before and after RTA. The most remarkable improvement in PL was obtained for the V/IIIBEP ratio of 8. Notably, the PL efficiency of the sample grown under the lowest As pressure remained poor even after annealing. One can see that the blue-shift induced by RTA depends on the arsenic pressure, viz., the blue-shift is less enhanced at high arsenic pressure. If Tg of the QW is below 500 1C, we have quite a large pressure range where no accurate control of the As2 flux is necessary. These observations imply that MBE growth of GaInNAs may be more reproducible from one sample to another than is MBE of GaInAsP for which a precise control over the two group-V fluxes is required [19]. The samples grown with the V/IIIBEP ratios of 6, 10, and 14 were studied by XRD. The X-ray rocking curves for the as-grown samples (Fig. 3) appear to be almost identical. The presence of
110
60 K
90
10 as-grown annealed
80 70
Blue-shift (nm)
PL intensity (a.u.)
100
60
1
50 40 6
8
10 12 V/IIIBEP ratio
14
Fig. 2. V/IIIBEP dependence of PL peak intensity of as-grown (squares) and annealed (dots) QWs and annealing-induced blue-shift.
Reflectivity (a.u.)
E.-M. Pavelescu et al. / Journal of Crystal Growth 281 (2005) 249–254
252
V/III BEP = 6
as-grown annealed
V/III BEP = 10
as-grown annealed
V/III BEP = 14
as-grown annealed
-7500
-5000
-2500
0
2500
Angle θ-2θ (arc sec) Fig. 3. Double-crystal XRD rocking curves measured before and after annealing for QWs grown under different arsenic pressures.
Pendello¨sung fringes indicates that the QW remains strained and the interfaces are abrupt. The main difference between these XRDs is the position of the QW-related diffraction peak on the y–2y axis. This peak shifts closer to the substrate peak when the V/IIIBEP ratio is decreased from 14 to 6. The peak shift shows that the QWs grown with low V/IIIBEP are less strained than the QW grown with optimal V/IIIBEP ( ¼ 10, valve position ¼ 250 mils), while those grown at high V/IIIBEP are more strained. We attribute these phenomena of strain to changes in incorporation efficiency of nitrogen; i.e., nitrogen is incorporated more effectively into the group-V sub-lattice when the V/IIIBEP ratio is low. An increase in nitrogen concentration will also increase the number of non-radiative defects, deteriorating PL intensity. This is exactly what we have observed: the sample grown at the smallest As pressure has very weak PL which, in addition, is the most red-shifted with respect to PL for the V=IIIBEP ¼ 10 sample. Such PL behaviour can be understood as a result of competition between As and N ad-atoms in occupying the anion lattice sites.
ARTICLE IN PRESS E.-M. Pavelescu et al. / Journal of Crystal Growth 281 (2005) 249–254
I PL ðtÞ et=tcap et=tdecay .
(2)
Here, tcap represents a characteristic time for carrier capture by the QW and tdecay is the effective lifetime of a carrier in the QW. Notice that it has been sufficient to use a single exponential in Eq. (2) to describe the carrier capture into the QW. This mono-exponential nature of the PL rise indicates that the direct carrier capture is the major timelimiting step in filling the QW. The carrier decays from the QW can also be described to a good accuracy by a single exponential. The effective capture times were measured to be 3–4 ps for all
Annealed
PL Intensity (a.u.)
The Pendello¨sung fringes, present in all the rocking curves of the as-grown samples (Fig. 3), disappear upon RTA; i.e., the layer structure is deteriorated. The sample grown with V=IIIBEP ¼ 6 shows no significant change in the shape or position of the QW-related XRD feature after annealing. This suggests that the blue-shift is mainly due to a formation of In–N bonds [18,20]. Instead, the samples with the larger V/ IIIBEP ratios show a small, but discernable, systematic shift of the QW-related XRD feature towards the substrate peak (y ¼ 01), as the arsenic pressure is increased. This shift indicates a systematic decrease in QW compressive strain. Such an effect upon annealing can be attributed to atomic interdiffusion at the QW interfaces, likely Ga/In interdiffusion [21], or to partial strain relief (strain relaxation without mass transport). Basically, the effect on QW emission from these two mechanisms is opposite, the former leads to a blueshift, the latter gives rise to a red-shift. The observed dependence of the blue-shift on arsenic pressure is not sufficient to distinguish between these two possibilities. Moreover, an enhanced blue-shift with decreasing arsenic pressure could also be due to an increase in the number of In–N bonds, following an increase of arsenic vacancies [20]. In order to investigate the effects of arsenic pressure on carrier dynamics in the QWs, we measured the time evolution of PL intensity IPL for the samples grown with V/IIIBEP of 10 and 14. The results are shown in Fig. 4. The points denote the experimental data and the full lines represent the best fits to the following expression:
253
V/IIIBEP = 14
As-grown V/IIIBEP = 10
0
50
100
150
200
250
Time (s) Fig. 4. PL decay profiles for the QW samples with different V/ IIIBEP ratios before and after annealing.
the samples, and they were independent of arsenic pressure or annealing. The effective decay times were between 109720 ps (V=IIIBEP ¼ 10) to 126717 ps (V=IIIBEP ¼ 14); a very marginal dependence on arsenic pressure. Although the steady-state PL intensity of the sample with V=IIIBEP ¼ 10 was improved considerably upon annealing, tdecay remained the same (110713 ps) as tdecay for the as-grown sample. However, tdecay for the V=IIIBEP ¼ 14 sample increased upon annealing to 233715 ps. This arsenic-dependent behaviour of tdecay for the V=IIIBEP ¼ 14 case upon annealing could be understood, as judged from the afore-mentioned PL and X-ray studies, as due to a competition of several arsenic-dependent annealing-induced structural and compositional changes in the QW. The observed insensitivity of tdecay upon annealing for a certain arsenic pressure range (from V=IIIBEP ¼ 8 to 12) suggests to us an interesting approach that may be applied to keep tdecay of GaInNAs-based optical switches and saturable absorber mirrors (SESAMs) rather unchanged, while improving their thermal stability and optical efficiency by RTA.
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4. Conclusions GaInNAs/GaAs QW samples were grown under different arsenic pressures by MBE, and photoluminescence (PL) and X-ray diffraction were investigated. The best optical properties (the highest PL intensities) were obtained when the V/ IIIBEP pressure ratio was 10. The emission wavelength remained unchanged for 8pV/ IIIBEPp12, which indicated that the V/III ratio neither affected the alloy composition nor the sticking coefficient of nitrogen. For the lower and higher V/IIIBEP ratios, the results suggest that the N incorporation into the group-V sub-lattice is enhanced or reduced, respectively. The annealinginduced blue-shift decreased when the V/IIIBEP ratio was increased. Based on the steady-state and time-resolved PL observations, and the XRD results, variations in blue-shifts upon RTA probably indicated a combined effect of Ga/In interdiffusion and a change in the nearest neighbours of nitrogen atoms.
Acknowledgement This work was supported, in part, by the Academy of Finland within the TULE/QUEST Project. One of the authors (T. Hakkarainen) acknowledges the support by the Academy via the ATOMISTIC Project.
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