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Excitation of Si : Er with sub-band-gap energies
Physica B 308–310 (2001) 348–349
Excitation of Si : Er with sub-band-gap energies M.A.J. Klik*, T. Gregorkiewicz Van der Waals Zeeman Institute, University of Amsterdam, Valckenierstraat 65, NL-1018 XE, Amsterdam, The Netherlands
Abstract We have performed photoluminescence excitation spectroscopy on molecular beam epitaxy grown Si : Er with a highpower near-infrared laser pulse. It was found that luminescence from the Er3+ ion, originating from the intra-4f-shell transition 4I13/2-4I15/2 could be induced by photon energies far below the band gap of the host material, but above energy of the 4f-shell transition. r 2001 Elsevier Science B.V. All rights reserved. PACS: 32.80.Hd; 78.55.Ap Keywords: Erbium; Silicon; Sub-band-gap excitation
The excitation mechanisms of rare-earth (RE) ions in semiconductors usually involve excitons or a RErelated energy level in the band gap of the host. The RE-ion can be photo-excited through the conduction band of the host material [1] or directly to the RE related level [2]. It is well known that the Er3+ ion can form a variety of complexes in the Si host material, most probably each with its own level within the band gap. Excitation spectroscopy is a powerful method to investigate these levels and their role in the excitation process. We have performed excitation experiments using a Nd : YAG (second harmonic) pumped optical parametric oscillator (OPO) crystal to produce a 5 ns pulsed laser beam with a wavelength range of 700– 2200 nm and repetition rate of 20 Hz. The maximum pulse energy used in these experiments was 7 mJ at a wavelength of 700 nm. The sample under investigation was an oxygen-rich Si : Er layer of 1.8 mm, grown by molecular beam epitaxy (MBE) at a temperature of 5601C on top of a B-doped Si substrate. Luminescence from the sample was gathered using a single grating monochromator and a near-infrared photomultiplier tube with a flat response from 300 to 1600 nm. The sample was cooled to 10 K in an Oxford Instruments closed-cycle cryostat.
*Corresponding author. Tel.: +31-20-525-5644; fax: +3120-525-5788. E-mail address: [email protected] (M.A.J. Klik).
After excitation of the sample from the backside (substrate) with the OPO beam operating in the near infrared, a PL signal could be detected at 1.54 mm, originating from the intra-4f-shell transition 4 I13/2-4I15/2 of the RE ion Er3+. Fig. 1 shows the normalized photo-luminescence excitation (PLE) spectrum measured at 1539 nm at a temperature of 10 K (gray). The photon flux of the OPO-beam is indicated in black. It is clear from the picture that luminescence can be observed for excitation photon energies between the band-gap energy Eg of the host material and the
Fig. 1. PLE spectrum of Si : Er measured at a temperature of 10 K, using a high power pulsed excitation beam.
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 6 - 2
M.A.J. Klik, T. Gregorkiewicz / Physica B 308–310 (2001) 348–349
Fig. 2. Spectrum of Si : Er measured for four different excitation wavelengths. The PL intensity reduces for excitation energies close to that of the 4I13/2-4I15/2 transition in the 4fshell of Er3+.
energy of the intra-4f-shell transition. The cross section for excitation decreases monotonically with smaller photon energy and vanishes for an energy below this transition. No resonant features are present in the range 1125–2200 nm. Although multi-photon absorption seems an obvious explanation for the observed excitation spectrum, the dependence of PL intensity on OPO-beam power shows a completely linear behavior, indicating other processes to be responsible for the excitation of the RE-ion with subband-gap photon energies. Fig. 2 shows the spectra of the Er intra-4f-shell luminescence for excitation wavelengths from 1310 to 1460 nm. It can be seen that the luminescence intensity decreases when the excitation energy approaches the energy of the intra-4f-shell transition.
349
Fig. 3. PLE spectrum of GaAs : Er measured at 10 K.
At present, the observations cannot be explained by any of the (so far) considered models. For a better understanding of the processes responsible, more accurate measurements will be done on a variety of samples in the near future. For comparison, the PLE spectrum for MBE grown GaAs : Er was obtained using the same experimental setup (Fig. 3). Although both PLE spectra are very similar, the luminescence at 1.54 mm could only be observed for excitation wavelengths below 1000 nm.
References [1] K. Thonke, et al., Semicond. Sci. Technol. 5 (1990) 1124. [2] J. Palm, et al., Phys. Rev. B 54 (1996) 17603.
Excitation of Si : Er with sub-band-gap energies M.A.J. Klik*, T. Gregorkiewicz Van der Waals Zeeman Institute, University of Amsterdam, Valckenierstraat 65, NL-1018 XE, Amsterdam, The Netherlands
Abstract We have performed photoluminescence excitation spectroscopy on molecular beam epitaxy grown Si : Er with a highpower near-infrared laser pulse. It was found that luminescence from the Er3+ ion, originating from the intra-4f-shell transition 4I13/2-4I15/2 could be induced by photon energies far below the band gap of the host material, but above energy of the 4f-shell transition. r 2001 Elsevier Science B.V. All rights reserved. PACS: 32.80.Hd; 78.55.Ap Keywords: Erbium; Silicon; Sub-band-gap excitation
The excitation mechanisms of rare-earth (RE) ions in semiconductors usually involve excitons or a RErelated energy level in the band gap of the host. The RE-ion can be photo-excited through the conduction band of the host material [1] or directly to the RE related level [2]. It is well known that the Er3+ ion can form a variety of complexes in the Si host material, most probably each with its own level within the band gap. Excitation spectroscopy is a powerful method to investigate these levels and their role in the excitation process. We have performed excitation experiments using a Nd : YAG (second harmonic) pumped optical parametric oscillator (OPO) crystal to produce a 5 ns pulsed laser beam with a wavelength range of 700– 2200 nm and repetition rate of 20 Hz. The maximum pulse energy used in these experiments was 7 mJ at a wavelength of 700 nm. The sample under investigation was an oxygen-rich Si : Er layer of 1.8 mm, grown by molecular beam epitaxy (MBE) at a temperature of 5601C on top of a B-doped Si substrate. Luminescence from the sample was gathered using a single grating monochromator and a near-infrared photomultiplier tube with a flat response from 300 to 1600 nm. The sample was cooled to 10 K in an Oxford Instruments closed-cycle cryostat.
*Corresponding author. Tel.: +31-20-525-5644; fax: +3120-525-5788. E-mail address: [email protected] (M.A.J. Klik).
After excitation of the sample from the backside (substrate) with the OPO beam operating in the near infrared, a PL signal could be detected at 1.54 mm, originating from the intra-4f-shell transition 4 I13/2-4I15/2 of the RE ion Er3+. Fig. 1 shows the normalized photo-luminescence excitation (PLE) spectrum measured at 1539 nm at a temperature of 10 K (gray). The photon flux of the OPO-beam is indicated in black. It is clear from the picture that luminescence can be observed for excitation photon energies between the band-gap energy Eg of the host material and the
Fig. 1. PLE spectrum of Si : Er measured at a temperature of 10 K, using a high power pulsed excitation beam.
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 6 - 2
M.A.J. Klik, T. Gregorkiewicz / Physica B 308–310 (2001) 348–349
Fig. 2. Spectrum of Si : Er measured for four different excitation wavelengths. The PL intensity reduces for excitation energies close to that of the 4I13/2-4I15/2 transition in the 4fshell of Er3+.
energy of the intra-4f-shell transition. The cross section for excitation decreases monotonically with smaller photon energy and vanishes for an energy below this transition. No resonant features are present in the range 1125–2200 nm. Although multi-photon absorption seems an obvious explanation for the observed excitation spectrum, the dependence of PL intensity on OPO-beam power shows a completely linear behavior, indicating other processes to be responsible for the excitation of the RE-ion with subband-gap photon energies. Fig. 2 shows the spectra of the Er intra-4f-shell luminescence for excitation wavelengths from 1310 to 1460 nm. It can be seen that the luminescence intensity decreases when the excitation energy approaches the energy of the intra-4f-shell transition.
349
Fig. 3. PLE spectrum of GaAs : Er measured at 10 K.
At present, the observations cannot be explained by any of the (so far) considered models. For a better understanding of the processes responsible, more accurate measurements will be done on a variety of samples in the near future. For comparison, the PLE spectrum for MBE grown GaAs : Er was obtained using the same experimental setup (Fig. 3). Although both PLE spectra are very similar, the luminescence at 1.54 mm could only be observed for excitation wavelengths below 1000 nm.
References [1] K. Thonke, et al., Semicond. Sci. Technol. 5 (1990) 1124. [2] J. Palm, et al., Phys. Rev. B 54 (1996) 17603.