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Photoreflectance study of the interdiffusion effects in the InGaAsP-based quantum well laser structures
Available online at www.sciencedirect.com
Physica E 17 (2003) 602 – 603 www.elsevier.com/locate/physe
Photore ectance study of the interdi"usion e"ects in the InGaAsP-based quantum well laser structures R. Kudrawieca;∗ , G. Sek ; a , K. Ryczkoa , W. Rudno-Rudzi0 nskia , J. Misiewicza , J. Wojcikb , B.J. Robinsonb , D.A. Thompsonb , P. Mascherb a Institute b Centre
of Physics, Wroclaw University of Technology, Wybrze˙ze Wyspia nskiego 27, 50-370 Wroclaw, Poland for Electrophotonic Materials and Devices, Department of Engineering Physics, McMaster University, Hamilton, Ont., Canada L8S 4L7
Abstract Photore ectance spectroscopy has been used to study interdi"usion e"ects of dielectric-capped, rapid-thermal-annealed InGaAsP-based quantum well laser structures grown by gas source molecular beam epitaxy. Post-growth modi9cation-induced changes of the quantum well shape in uence its energy levels. For the processed laser structures a blue shift of ground and excited state transitions has been observed. It has been found that the energy di"erence between the two lowest heavy hole levels decreases approximately linearly with the blue shift of the ground state transition. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 73.63.Hs; 78:67: − n; 66.10.Cb Keywords: Quantum wells; Photore ectance; Interdi"usion
Samples used in this study were grown by gas source molecular beam epitaxy on n-doped (1 0 0) InP substrates. The active region is composed of three compressively strained 5 nm thick quantum wells (QWs) of In0:76 Ga0:24 As0:85 P0:15 , separated by 10 nm In0:76 Ga0:24 As0:52 P0:48 barriers lattice-matched to InP. This ‘partial’ laser structure is completed with 80 nm upper and lower cladding regions of a quaternary layer (In0:76 Ga0:24 As0:39 P0:61 ) doped with, respectively, 5 × 1017 cm−3 Be and Si and capped with undoped InP, In0:53 Ga0:47 As and InP layers. Dielectric 9lms were deposited by electron cyclotron resonance plasma-enhanced chemical vapour deposition ∗ Corresponding author. Tel.: +48-71-320-2579; fax: +48-71328-3696. E-mail address: [email protected] (R. Kudrawiec).
(ECR-PECVD). These structures were annealed under a temperature of 780◦ C for 60 s. The photore ectance (PR) measurements were performed at room temperature in the so-called bright con9guration. The technical details of the structure, deposition system, and PR experiment have been described in Refs. [1,2]. PR spectra of the laser structures in the range of the QW transitions are shown in Fig. 1. The transition energies have been obtained from the 9tting procedure according to the 9rst derivative Gaussian line shape, the most appropriate shape in the case of con9ned state transitions at room temperature. Fig. 1(a) shows PR spectra of laser structures without dielectric cap for as-grown (i) and as-grown annealed (ii) samples. Results obtained for SiOx Ny capped and annealed laser structures are presented in Fig. 1(b). Curves (iii) – (vii) represent PR spectra recorded for laser structures
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00882-2
R. Kudrawiec et al. / Physica E 17 (2003) 602 – 603 x5
(ii)
x5
120
without cap
100
(b)
x2
(iii)
Photoreflectance [a. u.]
(a)
SiOXNY cap x2
(iv) (v)
x2
(vi)
x2
0.75
0.85
SiO2 cap
0.90
as-grown annealed
20
SiOxNy capped and annealed
0
10
20
30
40
50
60
70
Fig. 2. Energy di"erence between the second and the 9rst heavy hole levels versus the blue shift of the ground state transition.
(c)
0.80
as-grown 40
Blue shift [meV]
x2 x2
60
SiO2 capped and annealed
(viii) (ix)
80
0
x2
(vii)
EHH2-EHH1 [meV]
(i)
603
0.95
1.00
Energy [eV] Fig. 1. RT PR spectra for samples without dielectric cap (a), with SiOx Ny cap (b), and with SiO2 cap deposited under a di"erent microwave power of the ECR-PECVD process (c).
with di"erent stoichiometry of the dielectric cap [3]. Fig. 1(c) shows two selected PR spectra recorded for laser structures capped by the same SiO2 9lm and annealed. In this case, the 9lm has been deposited under various microwave power (600 and 800 W for curves (viii) and (ix), respectively) of the ECR-PECVD process [2]. In all PR spectra presented in Fig. 1 three features are clearly seen. In order to assign them to speci9c transitions, the SchrNodringer equation had to be solved for the structure of the as-grown sample. The 9rst resonance at 0:790 eV (1HH-1C) is associated with transition between the 9rst heavy hole and 9rst electron subbands. The second resonance at 0:888 eV (2HH-1C) is associated with transition between the second heavy hole and the 9rst electron subbands, and the third feature at 0:910 eV (1LH-1C) is related to transition between the 9rst light hole and the 9rst electron subbands. Generally, the 2HH-1C transition is forbidden, but in this case the electric 9eld of the junction breaks the selection rules making possible
its observation. The same transitions shifted to higher energy are observed for the post-growth modi9ed samples (curves from (ii) – (ix) in Fig. 1). In this work we focus on the heavy hole transitions. The di"erence between energies of the second and 9rst heavy hole transitions gives a di"erence between energies of the heavy hole levels in QW. It is known that the di"erence should decrease with an increase of the width and/or the decrease of the depth of the QW. The degree of the QW intermixing, due to interdi"usion e"ects, is directly associated with the blue shift of the ground state transition. It is expected that with the increase of the ground state blue shift the QW pro9le should change from narrow and deep to broad and shallow parabolic-like shape. According to this the di"erence between the heavy hole levels should decrease. Such a behaviour is observed in Fig. 2. It has been found that with an increase of the blue shift, the di"erence between the second and the 9rst heavy hole transitions decreases for all post-growth modi9ed laser structures. This conclusion is expected, but it has not been observed so far experimentally for InGaAsP-based QW. To date, investigations of similar structures have mainly used the photoluminescence technique that detects only the ground state transition. References [1] M. Boudreau, et al., Appl. Phys. Lett. 63 (1993) 3014. [2] R. Kudrawiec, et al., Acta Phys. Pol. A 102 (2002) 649. [3] J.F. Hazell, et al., Semicond. Sci. Technol. 16 (2001) 986.
Physica E 17 (2003) 602 – 603 www.elsevier.com/locate/physe
Photore ectance study of the interdi"usion e"ects in the InGaAsP-based quantum well laser structures R. Kudrawieca;∗ , G. Sek ; a , K. Ryczkoa , W. Rudno-Rudzi0 nskia , J. Misiewicza , J. Wojcikb , B.J. Robinsonb , D.A. Thompsonb , P. Mascherb a Institute b Centre
of Physics, Wroclaw University of Technology, Wybrze˙ze Wyspia nskiego 27, 50-370 Wroclaw, Poland for Electrophotonic Materials and Devices, Department of Engineering Physics, McMaster University, Hamilton, Ont., Canada L8S 4L7
Abstract Photore ectance spectroscopy has been used to study interdi"usion e"ects of dielectric-capped, rapid-thermal-annealed InGaAsP-based quantum well laser structures grown by gas source molecular beam epitaxy. Post-growth modi9cation-induced changes of the quantum well shape in uence its energy levels. For the processed laser structures a blue shift of ground and excited state transitions has been observed. It has been found that the energy di"erence between the two lowest heavy hole levels decreases approximately linearly with the blue shift of the ground state transition. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 73.63.Hs; 78:67: − n; 66.10.Cb Keywords: Quantum wells; Photore ectance; Interdi"usion
Samples used in this study were grown by gas source molecular beam epitaxy on n-doped (1 0 0) InP substrates. The active region is composed of three compressively strained 5 nm thick quantum wells (QWs) of In0:76 Ga0:24 As0:85 P0:15 , separated by 10 nm In0:76 Ga0:24 As0:52 P0:48 barriers lattice-matched to InP. This ‘partial’ laser structure is completed with 80 nm upper and lower cladding regions of a quaternary layer (In0:76 Ga0:24 As0:39 P0:61 ) doped with, respectively, 5 × 1017 cm−3 Be and Si and capped with undoped InP, In0:53 Ga0:47 As and InP layers. Dielectric 9lms were deposited by electron cyclotron resonance plasma-enhanced chemical vapour deposition ∗ Corresponding author. Tel.: +48-71-320-2579; fax: +48-71328-3696. E-mail address: [email protected] (R. Kudrawiec).
(ECR-PECVD). These structures were annealed under a temperature of 780◦ C for 60 s. The photore ectance (PR) measurements were performed at room temperature in the so-called bright con9guration. The technical details of the structure, deposition system, and PR experiment have been described in Refs. [1,2]. PR spectra of the laser structures in the range of the QW transitions are shown in Fig. 1. The transition energies have been obtained from the 9tting procedure according to the 9rst derivative Gaussian line shape, the most appropriate shape in the case of con9ned state transitions at room temperature. Fig. 1(a) shows PR spectra of laser structures without dielectric cap for as-grown (i) and as-grown annealed (ii) samples. Results obtained for SiOx Ny capped and annealed laser structures are presented in Fig. 1(b). Curves (iii) – (vii) represent PR spectra recorded for laser structures
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00882-2
R. Kudrawiec et al. / Physica E 17 (2003) 602 – 603 x5
(ii)
x5
120
without cap
100
(b)
x2
(iii)
Photoreflectance [a. u.]
(a)
SiOXNY cap x2
(iv) (v)
x2
(vi)
x2
0.75
0.85
SiO2 cap
0.90
as-grown annealed
20
SiOxNy capped and annealed
0
10
20
30
40
50
60
70
Fig. 2. Energy di"erence between the second and the 9rst heavy hole levels versus the blue shift of the ground state transition.
(c)
0.80
as-grown 40
Blue shift [meV]
x2 x2
60
SiO2 capped and annealed
(viii) (ix)
80
0
x2
(vii)
EHH2-EHH1 [meV]
(i)
603
0.95
1.00
Energy [eV] Fig. 1. RT PR spectra for samples without dielectric cap (a), with SiOx Ny cap (b), and with SiO2 cap deposited under a di"erent microwave power of the ECR-PECVD process (c).
with di"erent stoichiometry of the dielectric cap [3]. Fig. 1(c) shows two selected PR spectra recorded for laser structures capped by the same SiO2 9lm and annealed. In this case, the 9lm has been deposited under various microwave power (600 and 800 W for curves (viii) and (ix), respectively) of the ECR-PECVD process [2]. In all PR spectra presented in Fig. 1 three features are clearly seen. In order to assign them to speci9c transitions, the SchrNodringer equation had to be solved for the structure of the as-grown sample. The 9rst resonance at 0:790 eV (1HH-1C) is associated with transition between the 9rst heavy hole and 9rst electron subbands. The second resonance at 0:888 eV (2HH-1C) is associated with transition between the second heavy hole and the 9rst electron subbands, and the third feature at 0:910 eV (1LH-1C) is related to transition between the 9rst light hole and the 9rst electron subbands. Generally, the 2HH-1C transition is forbidden, but in this case the electric 9eld of the junction breaks the selection rules making possible
its observation. The same transitions shifted to higher energy are observed for the post-growth modi9ed samples (curves from (ii) – (ix) in Fig. 1). In this work we focus on the heavy hole transitions. The di"erence between energies of the second and 9rst heavy hole transitions gives a di"erence between energies of the heavy hole levels in QW. It is known that the di"erence should decrease with an increase of the width and/or the decrease of the depth of the QW. The degree of the QW intermixing, due to interdi"usion e"ects, is directly associated with the blue shift of the ground state transition. It is expected that with the increase of the ground state blue shift the QW pro9le should change from narrow and deep to broad and shallow parabolic-like shape. According to this the di"erence between the heavy hole levels should decrease. Such a behaviour is observed in Fig. 2. It has been found that with an increase of the blue shift, the di"erence between the second and the 9rst heavy hole transitions decreases for all post-growth modi9ed laser structures. This conclusion is expected, but it has not been observed so far experimentally for InGaAsP-based QW. To date, investigations of similar structures have mainly used the photoluminescence technique that detects only the ground state transition. References [1] M. Boudreau, et al., Appl. Phys. Lett. 63 (1993) 3014. [2] R. Kudrawiec, et al., Acta Phys. Pol. A 102 (2002) 649. [3] J.F. Hazell, et al., Semicond. Sci. Technol. 16 (2001) 986.