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Study of 980nm GaAs based pumping lasers by photo-voltage spectroscopy
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Physica E 17 (2003) 597 – 599 www.elsevier.com/locate/physe
Study of 980 nm GaAs based pumping lasers by photo-voltage spectroscopy M. Udhayasankara; b;∗ , M. Dellagiovannab , S. Morascab , A. Stellaa; c a European
Schools of Advanced Studies, Collegio Borromeo, Piazza Borromeo, I-27100 Pavia, Italy Optical Technologies Italia S.p.A, Viale Sarca 222, I-20126 Milano, Italy c Department of Physics “A. Volta”, Via Bassi 9, I-27100 Pavia, Italy
b Corning
Abstract Application of photo-voltage spectroscopy (PVS) technique in the study of semiconductor superlattices (SLs) and quantum well (QW) structures is presented. Room temperature (RT) PVS spectra in the vicinity of the active layers of the structures display several interesting features that originate from carrier quantum con5nement. A sharp exciton absorption peak was obtained at RT. The other features namely, splitting between the heavy and light holes, other high quantum con5nement levels have also been observed. The PVS measurements have been compared with that of photoluminescence (PL) measurements. The QW wavelengths by PVS measurements were always higher than that of PL measurements and it is due to quantum con5ned Stark e7ect (QCSE). ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Photo-voltage spectroscopy; GaAs lasers; Quantum wells; Quantum con5ned Stark e7ect
1. Introduction PL spectroscopy is the most widely used method for the characterisation of low dimensional structures (LDSs) such as quantum wells, quantum dots and quantum rings. PL normally provides only information about the transitions between the lowest sublevels [1]. So, complementary methods such as absorption spectroscopy are used in order to obtain complex picture of all transitions in LDSs. Absorption measurements are extremely di=cult in the case of LDSs due to small volume of absorbing material and further, ∗
Corresponding author. European Schools of Advanced Studies, Collegio Borromeo, Piazza Borromeo, I-27100 Pavia, Italy. Tel.: +49-641-9933152; fax: +49-641-9933-109/119. E-mail address: [email protected] (M. Udhayasankar).
the task of etching the substrate material down to 1 m is cumbersome [2]. Other methods yielding similar information are photo-reFectance, excited photoluminescence, photo-conductivity, photo-current and photo-voltage measurements. Among all of them, PVS is a simple technique and can be used to study a broad range of semiconductors including bulk, thin 5lms, hetero-structures and more recently, LDSs [3–5]. In this letter, well-resolved RT photo-voltage spectra of sublevel transitions in the active region of a 980 nm laser device structure are reported. 2. Experimental Samples used in this study are conventional GaAs laser structures with InGaAs–GaAs QW and SLs grown by molecular beam epitaxy technique. These
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00880-9
M. Udhayasankar et al. / Physica E 17 (2003) 597 – 599
laser structures were routinely employed for the fabrication of 980 nm GaAs based pumping lasers at Corning OTI S.p.A, Milan. PVS measurement was performed with Bio-Rad PN4350 system. The junction between transparent electrolyte (ammonium tartrate) and semiconductor was formed on the epilayer side and front-contact con5guration was used. The photo-voltage signal produced between the semiconductor contact and the platinum electrode was detected. The system has been calibrated using a calibration cell in order not to include other features due to the measuring system itself.
PVS signal (arb. units)
598
-10
e1-hh1
e2-hh2 e1-lh1
-5
0 830
870
910
e1-hh2
950
990
Wavelength (nm) 3. Results and discussion PVS measures the open-circuit voltaic response of the sample as a function of wavelength of the incident radiation. This open-circuit photo-voltage is proportional to absorption coe=cient of the material measured, and gives the same absorption pro5le as absorption/transmission measurements. The PVS spectrum of the QW laser structure displays several interesting features that originate from carrier quantum con5nement. Fig. 1 shows PVS spectrum of the laser structure with sharp excitonic peak even at RT. Apart from this, well resolved peaks corresponding to transitions from the 1st conduction band sublevel to the 1st heavy-hole sublevel (e1–hh1), 1st conduction band sublevel to the 2nd heavy-hole sublevel (e1–hh2), 1st conduction band sublevel to the 1st light-hole sublevel (e1–lh1) and 2nd conduction band sublevel to the 2nd heavy-hole sublevel (e2–hh2) are observed. The peak at 977 nm corresponds to e1–hh1 transition (sharp exciton absorption peak) and the peak position at about 945 nm is due to e1–hh2 transition. The later transition at 945 nm is in principle forbidden due to the rectangular quantum well selection rules. The presence of this transition is associated to a non-rectangular quantum well, due to both intrinsic growth conditions and also band bending associated to the built-in electric 5eld of the p–n junction, where the quantum well is located (Stark e7ect). The e1–lh1 transition was found at about 920 nm, associated to the light hole fundamental state and also e2–hh2 was observed at about 860 nm. It is also recognised that the step-function shape related to the usual quantum well is step like density of states. For the PVS spec-
Good quantum well
Bad quantum well
Fig. 1. PVS spectra observed in the active layer of a laser structure.
trum of a bad quantum well (as shown in broken line, in Fig. 1), the excitonic transition is absent. It may be due to alloy and/or interface disordering of the quantum well. The PVS measurements have been compared with that of PL measurements and there exists a linear relationship of QW wavelength and full-width at half maximum values between these two measurement techniques. But the QW wavelengths of PVS measurements were always higher than that of PL measurements and this can be explained by the phenomenon of QCSE. Due to QCSE, there is a shift in the absorption tail to the lower energies (i.e. towards the higher wavelength) in the presence of electric 5eld. In our laser device structures, the active region (QW) exactly lies at the p–n junction of the device structure. Thus the QW experiences a built-in electric 5eld felicitated by the junction and thereby shifting the absorption edge towards the lower energy. This is the reason for obtaining higher QW wavelengths by the PVS measurements. The di7erence in energy between the PL and PVS QW measurements is of the range of 0.25 –0:40 meV. Moreover, in order to observe the excitonic luminescence in the QW by PL measurements, it is also necessary to compensate a little the junction 5eld. This has been accomplished by increasing the excitation power. This excess energy causes a blue shift in the PL measurements when compared to the PVS measurements.
M. Udhayasankar et al. / Physica E 17 (2003) 597 – 599
4. Conclusions Signi5cant RT exciton behaviour and well-resolved RT photo-voltage signals corresponding to the sublevel transitions in 980 nm GaAs based QW pumping laser structures (production oriented) have been observed for the 5rst time. The absorption edge displays a step like behaviour and secondly, splitting between the heavy and light holes has been observed since there is a change in symmetry because of the layered structure. Comparable informations about the QWs were obtained from RT PVS as compared to low temperature PL measurements. The QW wavelengths of PVS measurements were always higher than that of PL measurements and this is due to QCSE. Acknowledgements One of the authors (MUS) very thankfully acknowledges the ESAS in Materials Science, IUSS,
599
University of Pavia, Italy for granting 5nancial assistance to undergo this programme and Corning OTI S.p.A for a stage. References [1] X. He, M. Razeghi, Appl. Phys. Lett. 62 (1993) 618. [2] P. Blood, J. Appl. Phys. 58 (1985) 2288. [3] Y.T. Cheng, Y.S. Huang, D.Y. Lin, K.K. Tiong, F.H. Pollak, K.R. Evans, Appl. Phys. Lett. 79 (2001) 949. [4] N. Ashkenasy, M. Leibovitch, Y. Rosenwaks, Y. Shapira, Mater. Sci. Eng. B 74 (2000) 125. [5] J. Touskova, E. Samochin, J. Tousek, J. Oswald, E. Hulicius, J. Pangrac, K. Melichar, T. Simecek, J. Appl. Phys. 91 (2002) 10103.
Physica E 17 (2003) 597 – 599 www.elsevier.com/locate/physe
Study of 980 nm GaAs based pumping lasers by photo-voltage spectroscopy M. Udhayasankara; b;∗ , M. Dellagiovannab , S. Morascab , A. Stellaa; c a European
Schools of Advanced Studies, Collegio Borromeo, Piazza Borromeo, I-27100 Pavia, Italy Optical Technologies Italia S.p.A, Viale Sarca 222, I-20126 Milano, Italy c Department of Physics “A. Volta”, Via Bassi 9, I-27100 Pavia, Italy
b Corning
Abstract Application of photo-voltage spectroscopy (PVS) technique in the study of semiconductor superlattices (SLs) and quantum well (QW) structures is presented. Room temperature (RT) PVS spectra in the vicinity of the active layers of the structures display several interesting features that originate from carrier quantum con5nement. A sharp exciton absorption peak was obtained at RT. The other features namely, splitting between the heavy and light holes, other high quantum con5nement levels have also been observed. The PVS measurements have been compared with that of photoluminescence (PL) measurements. The QW wavelengths by PVS measurements were always higher than that of PL measurements and it is due to quantum con5ned Stark e7ect (QCSE). ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Photo-voltage spectroscopy; GaAs lasers; Quantum wells; Quantum con5ned Stark e7ect
1. Introduction PL spectroscopy is the most widely used method for the characterisation of low dimensional structures (LDSs) such as quantum wells, quantum dots and quantum rings. PL normally provides only information about the transitions between the lowest sublevels [1]. So, complementary methods such as absorption spectroscopy are used in order to obtain complex picture of all transitions in LDSs. Absorption measurements are extremely di=cult in the case of LDSs due to small volume of absorbing material and further, ∗
Corresponding author. European Schools of Advanced Studies, Collegio Borromeo, Piazza Borromeo, I-27100 Pavia, Italy. Tel.: +49-641-9933152; fax: +49-641-9933-109/119. E-mail address: [email protected] (M. Udhayasankar).
the task of etching the substrate material down to 1 m is cumbersome [2]. Other methods yielding similar information are photo-reFectance, excited photoluminescence, photo-conductivity, photo-current and photo-voltage measurements. Among all of them, PVS is a simple technique and can be used to study a broad range of semiconductors including bulk, thin 5lms, hetero-structures and more recently, LDSs [3–5]. In this letter, well-resolved RT photo-voltage spectra of sublevel transitions in the active region of a 980 nm laser device structure are reported. 2. Experimental Samples used in this study are conventional GaAs laser structures with InGaAs–GaAs QW and SLs grown by molecular beam epitaxy technique. These
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00880-9
M. Udhayasankar et al. / Physica E 17 (2003) 597 – 599
laser structures were routinely employed for the fabrication of 980 nm GaAs based pumping lasers at Corning OTI S.p.A, Milan. PVS measurement was performed with Bio-Rad PN4350 system. The junction between transparent electrolyte (ammonium tartrate) and semiconductor was formed on the epilayer side and front-contact con5guration was used. The photo-voltage signal produced between the semiconductor contact and the platinum electrode was detected. The system has been calibrated using a calibration cell in order not to include other features due to the measuring system itself.
PVS signal (arb. units)
598
-10
e1-hh1
e2-hh2 e1-lh1
-5
0 830
870
910
e1-hh2
950
990
Wavelength (nm) 3. Results and discussion PVS measures the open-circuit voltaic response of the sample as a function of wavelength of the incident radiation. This open-circuit photo-voltage is proportional to absorption coe=cient of the material measured, and gives the same absorption pro5le as absorption/transmission measurements. The PVS spectrum of the QW laser structure displays several interesting features that originate from carrier quantum con5nement. Fig. 1 shows PVS spectrum of the laser structure with sharp excitonic peak even at RT. Apart from this, well resolved peaks corresponding to transitions from the 1st conduction band sublevel to the 1st heavy-hole sublevel (e1–hh1), 1st conduction band sublevel to the 2nd heavy-hole sublevel (e1–hh2), 1st conduction band sublevel to the 1st light-hole sublevel (e1–lh1) and 2nd conduction band sublevel to the 2nd heavy-hole sublevel (e2–hh2) are observed. The peak at 977 nm corresponds to e1–hh1 transition (sharp exciton absorption peak) and the peak position at about 945 nm is due to e1–hh2 transition. The later transition at 945 nm is in principle forbidden due to the rectangular quantum well selection rules. The presence of this transition is associated to a non-rectangular quantum well, due to both intrinsic growth conditions and also band bending associated to the built-in electric 5eld of the p–n junction, where the quantum well is located (Stark e7ect). The e1–lh1 transition was found at about 920 nm, associated to the light hole fundamental state and also e2–hh2 was observed at about 860 nm. It is also recognised that the step-function shape related to the usual quantum well is step like density of states. For the PVS spec-
Good quantum well
Bad quantum well
Fig. 1. PVS spectra observed in the active layer of a laser structure.
trum of a bad quantum well (as shown in broken line, in Fig. 1), the excitonic transition is absent. It may be due to alloy and/or interface disordering of the quantum well. The PVS measurements have been compared with that of PL measurements and there exists a linear relationship of QW wavelength and full-width at half maximum values between these two measurement techniques. But the QW wavelengths of PVS measurements were always higher than that of PL measurements and this can be explained by the phenomenon of QCSE. Due to QCSE, there is a shift in the absorption tail to the lower energies (i.e. towards the higher wavelength) in the presence of electric 5eld. In our laser device structures, the active region (QW) exactly lies at the p–n junction of the device structure. Thus the QW experiences a built-in electric 5eld felicitated by the junction and thereby shifting the absorption edge towards the lower energy. This is the reason for obtaining higher QW wavelengths by the PVS measurements. The di7erence in energy between the PL and PVS QW measurements is of the range of 0.25 –0:40 meV. Moreover, in order to observe the excitonic luminescence in the QW by PL measurements, it is also necessary to compensate a little the junction 5eld. This has been accomplished by increasing the excitation power. This excess energy causes a blue shift in the PL measurements when compared to the PVS measurements.
M. Udhayasankar et al. / Physica E 17 (2003) 597 – 599
4. Conclusions Signi5cant RT exciton behaviour and well-resolved RT photo-voltage signals corresponding to the sublevel transitions in 980 nm GaAs based QW pumping laser structures (production oriented) have been observed for the 5rst time. The absorption edge displays a step like behaviour and secondly, splitting between the heavy and light holes has been observed since there is a change in symmetry because of the layered structure. Comparable informations about the QWs were obtained from RT PVS as compared to low temperature PL measurements. The QW wavelengths of PVS measurements were always higher than that of PL measurements and this is due to QCSE. Acknowledgements One of the authors (MUS) very thankfully acknowledges the ESAS in Materials Science, IUSS,
599
University of Pavia, Italy for granting 5nancial assistance to undergo this programme and Corning OTI S.p.A for a stage. References [1] X. He, M. Razeghi, Appl. Phys. Lett. 62 (1993) 618. [2] P. Blood, J. Appl. Phys. 58 (1985) 2288. [3] Y.T. Cheng, Y.S. Huang, D.Y. Lin, K.K. Tiong, F.H. Pollak, K.R. Evans, Appl. Phys. Lett. 79 (2001) 949. [4] N. Ashkenasy, M. Leibovitch, Y. Rosenwaks, Y. Shapira, Mater. Sci. Eng. B 74 (2000) 125. [5] J. Touskova, E. Samochin, J. Tousek, J. Oswald, E. Hulicius, J. Pangrac, K. Melichar, T. Simecek, J. Appl. Phys. 91 (2002) 10103.