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Multi-crystalline silicon as active medium for terahertz intracenter lasers
ARTICLE IN PRESS
Physica B 403 (2008) 535–538 www.elsevier.com/locate/physb
Multi-crystalline silicon as active medium for terahertz intracenter lasers S.G. Pavlova,, H.-W. Hu¨bersa, N.V. Abrosimovb, H. Riemannb, L.V. Gavrilenkoc, A.V. Antonovc a
Institute of Planetary Research, German Aerospace Center, D-12489 Berlin, Germany b Institute for Crystal Growth, D-12489 Berlin, Germany c Institute for Physics of Microstructures, Russian Academy of Sciences, 603950 N. Novgorod, Russia Received 11 July 2007; received in revised form 29 August 2007; accepted 30 August 2007
Abstract Stimulated emission at terahertz frequencies has been obtained from multi-crystalline silicon doped by phosphor under optical excitation by a mid-infrared laser. The silicon samples consist of grains with a characteristic size distribution in the range from 50 to 500 mm. The maximum operation temperature of the laser made from multi-crystalline silicon is 6 K less than that of monocrystalline lasers and the maximum output power is three times less while its laser threshold is only slightly higher and the emission frequency is the same. These effects are attributed to internal strain and enhanced phonon scattering induced by grain boundaries. r 2007 Elsevier B.V. All rights reserved. PACS: 42.55.Rz; 61.72.Mm Keywords: Terahertz; Silicon laser; Multi-crystalline silicon
1. Introduction One of the challenges in silicon photonics is the realization of low-cost photonic devices which are compatible and can be possibly easily integrated in optoelectronic circuits [1]. High-speed optical modulators [2] and highspeed infrared SiGe detectors [3] made from silicon have been demonstrated. The development of a silicon-based laser, however, faces significant problems caused by the indirect bandgap of silicon. So far optically pumped terahertz (THz) intracenter lasers [4] and Raman-type infrared [5,6] and THz [7] lasers have been realized. Some approaches, such as a hybrid-type laser that utilizes a silicon waveguide bonded to AlGaInAs quantum wells [8], adapt the existing AIIIBV infrared lasers to CMOScompatible silicon processing. Another significant amount of recent activities has focused on achieving stimulated Corresponding author. Tel.: +49 30 67055594; fax: +49 30 67055507.
E-mail address: [email protected] (S.G. Pavlov). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.08.221
emission in silicon by a modification of basic material properties, for example with silicon nanocrystals [9–11] and porous silicon [12–14]. The latter approaches demonstrated a significant enhancement of light amplification from silicon due to quantum-confinement effects. Multi-crystalline silicon (mc-Si), a material consisting of multiple small silicon crystals (crystallites), is widely used for integrated circuits, large area imaging sensor arrays and liquid-crystal displays, as well as solar cell fabrication [15]. The commonly used range of small crystallites (grains) is 0.01–1 mm for polycrystalline silicon and rises up to hundreds of micrometers in the case of multi-crystalline silicon. In extreme cases, grains can be as small as hundreds of A˚ngstroms and as large as few millimeters. The major electrical, thermal and optical properties of mc-Si lie between those of bulk crystalline silicon (c-Si) and amorphous silicon (a-Si). Boundaries between separate crystallites induce internal strain in mc-Si. Variations in a grain size can lead to a modification of the principal phonon frequencies in mc-Si [16]. Scattering of phonons
ARTICLE IN PRESS 536
S.G. Pavlov et al. / Physica B 403 (2008) 535–538
and charge carriers on grain boundaries result in a reduced thermal and electrical conductivity of this material compared to monocrystalline silicon [17–19]. All these changes are expected to induce modifications of the performance of the THz-range intracenter silicon laser, which operates on transitions between localized states of group-V donor centers. Resonances between shallow impurity states and intervalley phonons in c-Si determine to a large extent the intracenter population inversion schemes [4]. These donor-phonon resonances are supposed to change when going from monocrystalline to multicrystalline silicon. Strain in the crystal can result in a significant broadening of impurity states, which may cause, in turn, a decrease of population inversion as well as optical cross-section for intracenter transitions. 2. Growth, preparation and characterization of c-Si:P and mc-Si:P
3. Optical characterization of mc-Si and c-Si intracenter lasers Laser thresholds and output power were measured in a cryogenic dipstick which was immersed in liquid helium. This was found to be a compact and reproducible optical
Fig. 1. Micrograph (a Sirtl etch view of the polished sample facet) of the multi-crystalline silicon sample doped with phosphor (top) and an enlarged section (bottom).
c-Si Stokes Emission Intensity (arb. units)
For a direct comparison of the laser performance, both mono- and multi-crystalline silicon crystals have been grown by the Czochralski technique with simultaneous doping by phosphor from the melt. Monocrystalline silicon doped by phosphor with NPE2.3 1015 cm 3 has been prepared in the same way as in Ref. [4]. For the mc-Si growth, specially prepared polycrystalline seeds have been used. The mc-Si crystals had monocrystallite grain dimensions in the range of 50–500 mm (Fig. 1). The ingots have been grown in the /1 0 0S direction. They had phosphor concentrations of NPE1.6 1015 cm 3. The compensation in the crystals was below 1%. The crystals were cut into parallelepipeds with typical dimensions of (7 7) 5 mm3 with a cross-sectional area of (7 7) mm2 in the plane perpendicular to the growth axis. All opposite facets of the samples were optically polished plane parallel to each other with an accuracy of 1 arcmin in order to provide a high-Q resonator on internal reflection modes. The c-Si and mc-Si samples were investigated by Raman spectroscopy. The measurements were performed at room temperature using an optical parametric oscillator operating at 0.55 mm as excitation source and a grating spectrometer with a spectral resolution of 0.1 cm 1. The Raman spectra of c-Si show the Stokes line corresponding to the zone-centered optical phonon at about 521 cm 1. The Stokes line of the mc-Si sample is red-shifted by 0–1.0 cm 1 with respect to Stokes line from the c-Si sample. The exact value depends on the position of the sample where the measurement was done (Fig. 2). This indicates an influence of the grain structure on the phonon spectra in the mc-Si samples.
500
mc-Si pos. 1 mc-Si pos. 2
505
510
515 520 525 Raman Shift (cm-1)
530
535
540
Fig. 2. Raman spectra of the c-Si:P and mc-Si:P crystals, taken at two different positions on the mc-Si sample facet. The Raman shifts have been determined by a Lorentzian fit to the spectra. The Stokes shift of the c-Si sample is 521 cm 1 and the spectral resolution is 0.1 cm 1.
ARTICLE IN PRESS S.G. Pavlov et al. / Physica B 403 (2008) 535–538
537
Laser emission power (arb.units)
100
10-1 g-TA
100 2p0 1s (T2)
10-1 c-Si mc-Si
1s (A1) 10-2
10-2
10-3
10-3
Laser Emission Power (arb.units)
1.0
g-LO
c-Si mc-Si
0.8 0.6 0.4 0.2 0.0 8
10
4
5
10 Pump intensity (W/cm2)
10
6
Fig. 3. Dependence of the THz emission from c-Si (gray curve) and mc-Si (black curve) samples on the pump power (wavelength 10.59 mm, repetition rate 4 Hz, and excitation pulse length at FWHM is 70 ns) measured at a temperature of 8 K. The inset shows a schematic presentation of the four-level Si:P laser mechanism. Straight lines are for optical transitions and curved ones are for intraband (g-LO) and intracenter (g-TA) phonon-assisted relaxation.
scheme for pumping of the silicon laser as well as detection of its emission with a Ge:Ga photoconductive detector integrated in the dipstick. For spectral and temperature measurements, the silicon crystals were mounted on a copper block inside a closed-cycle cryogenic refrigerator and cooled to 8 K. The temperature was controlled with a silicon diode mounted on the copper block. In this case, emission from the silicon samples was detected by a liquidhelium cooled Ge:Ga detector in a separate cryostat. The silicon crystals were optically excited by radiation from a pulsed TEA (transversely excited atmospheric-pressure) CO2 laser operating at wavelengths from 9.2 to 10.7 mm (photon energy 116–135 meV). The pump laser had a spot diameter of 6 mm (1/e2) on the (7 7) mm2 facet of the sample. The emission spectra were measured with a Fourier transform spectrometer with a resolution of 3 GHz (0.1 cm 1). Both active media exhibited stimulated emission above a certain pump power level (Fig. 3). The population mechanism of intracenter silicon lasers is dominated by peculiarities of the relaxation process of photoexcited electrons accompanied by interaction with intervalley phonons of the host lattice. At low lattice temperature, the donor electrons are bound to the phosphor ground state, 1s(A1) (inset in Fig. 3). After being photoexcited by radiation of a CO2 laser (arrow up) into the continuum of states in the conduction band, electrons relax first to the band bottom and then into the highly excited localized states. Each intraband and intracenter relaxation step is accompanied by emission of a phonon. The most important phonons in the case of Si:P as active medium are the intervalley g-LO phonon (frequency 15.3 THz) for intraband relaxation and the transverse acoustic g-TA phonon (2.9 THz) responsible for relaxation between the
10
12 14 16 Heat Sink Temperature (K)
18
20
Fig. 4. Temperature dependence of the c-Si and mc-Si laser output power (pumping conditions are the same as in Fig. 3). The temperature was measured on the copper block which holds the laser sample.
1s(T2) and 1s(A1) excited donor states (curved arrows downwards). Relaxation from the 2p0 state is suppressed because of the absence of an appropriate resonant phonon. The f-TA phonon (4.5 THz) which is most close to the interstate 2p021s(T2) gap, limits the lifetime of the upper laser state, 2p0. Thus, population inversion is formed between the 2p0 and 1s(T2) phosphor excited states. Changes of the energy of intervalley phonons in silicon are expected to be similar to that measured by Raman spectroscopy for the zone-centered optical phonon (Fig. 2). This may affect the impurity-phonon resonances controlling the population inversion in intracenter lasers. The output power from the mc-silicon active medium is about three times less at 4 K in comparison with the monocrystalline material (Fig. 3). The difference in the output power is temperature dependent and increases towards the laser thresholds (Fig. 4). The gain slope of the mc-Si:P laser is about a factor of five less and the laser threshold is slightly higher (Fig. 3). These differences can be attributed partly to the slightly different doping, which is for the c-Si laser a bit closer to the optimal doping concentration (5 1015 cm 3) [4]. The operation frequency of both lasers is 5.424 THz (Fig. 5) what corresponds to the 2p0-1s(T2) phosphor transition [20], and the difference in the laser frequency between c-Si and mc-Si lasers lies within the spectral resolution in our experiments. The operation temperature of the mc-Si:P laser is limited to 10 K compared to 16 K for the monocrystalline Si:P laser (Fig. 4). This pronounced difference can be attributed to the different phonon spectra caused by enhanced phonon scattering on grain boundaries as well as reduced thermal conductivity of the mc-Si medium. 4. Summary We have shown that phosphor-doped multi-crystalline silicon with submillimeter grain size can serve as active
ARTICLE IN PRESS S.G. Pavlov et al. / Physica B 403 (2008) 535–538
538
Frequency (THz) 0
1
2
3
4
5
6
7
(THz)
1.0
5.410
5.419
5.428
5.437
180.9
181.2
1.0
Laser Intensity (arb.units)
0.8
0.8 c-Si:P 0.6 mc-Si:P
0.6
0.4 0.2
0.4 0.0 180.3
180.6 (cm-1)
0.2
0.0 0
50
100
150
200
250
Wavenumber (cm-1) Fig. 5. Emission spectra for c-Si (gray curve) and mc-Si (black curve) laser samples. The pump wavelength is 10.59 mm, the heat sink temperature is 8 K and the spectral resolution is 0.125 cm 1.
medium for intracenter THz silicon lasers. However, the operation temperature is limited to 10 K compared to 16 K for monocrystalline Si:P lasers and the output power is about three times less. Other important operational features, such as the laser scheme and the pump threshold, remain almost unchanged. Acknowledgment This work was supported by the European Commission through the ProFIT programme of the Investitionsbank Berlin (Grant no. 10126728). References [1] A. Barkai, Y. Chetrit, O. Cohen, et al., J. Opt. Netw. 6 (2007) 25. [2] L. Liao, D. Samara-Rubio, M. Morse, A. Liu, D. Hodge, D. Rubin, U. Keil, T. Franck, Opt. Express 13 (2006) 3129. [3] J. Liu, J. Michel, W. Giziewicz, D. Pan, K. Wada, D.D. Cannon, S. Jongthammanurak, D.T. Danielson, L.C. Kimerling, J. Chen, F.O¨. Ilday, F.X. Ka¨rtner, J. Yasaitis, Appl. Phys. Lett. 87 (2005) 103501. [4] H.-W. Hu¨bers, S.G. Pavlov, V.N. Shastin, Semicond. Sci. Technol. 20 (2005) S211. [5] O. Boyaraz, B. Jalali, Opt. Express 12 (2004) 5269.
[6] H. Rong, A. Liu, R. Jones, O. Cohen, D. Hak, R. Nicolaescu, A. Fang, M. Paniccia, Nature 433 (2005) 292. [7] S.G. Pavlov, H.-W. Hu¨bers, J.N. Hovenier, T.O. Klaassen, D.A. Carder, P.J. Phillips, B. Redlich, H. Riemann, R.Kh. Zhukavin, V.N. Shastin, Phys. Rev. Lett. 96 (2006) 037404. [8] H. Park, A. Fang, S. Kodama, J. Bowers, Opt. Express 13 (2005) 9460. [9] L. Tsybeskov, K.L. Moore, D.G. Hall, P.M. Fauchet, Phys. Rev. B 54 (1996) R8361. [10] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408 (2000) 440. [11] S.G. Clouter, P.A. Kossyrev, J. Xu, Nat. Mater. 4 (2005) 887. [12] L.T. Canham, W.Y. Leong, M.I.J. Beale, T.I. Cox, L. Taylor, Appl. Phys. Lett. 61 (1992) 2563. [13] P.M. Fauchet, J. Lumin. 70 (1996) 294. [14] J.-C. Lin, W.-L. Chen, W.-C. Tsai, Opt. Express 14 (2006) 9764. [15] R.A. Street (Ed.), Technology and Applications of Amorphous Silicon, Springer Series in Materials Science, vol. 37, Springer, Berlin, 2000. [16] M. Becker, H. Scheel, S. Christiansen, H.P. Strunk, J. Appl. Phys. 101 (2007) 0163531. [17] I.H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [18] H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. [19] S. Uma, A.D. McConnell, M. Asheghi, K. Kurabayashi, K.E. Goodson, Int. J. Thermophys. 22 (2001) 605. [20] H.-W. Hu¨bers, S.G. Pavlov, M. Greiner-Ba¨r, M.H. Ru¨mmeli, M.F. Kimmitt, R. Kh. Zhukavin, H. Riemann, V.N. Shastin, Phys. Stat. Sol. 233 (2003) 191.
Physica B 403 (2008) 535–538 www.elsevier.com/locate/physb
Multi-crystalline silicon as active medium for terahertz intracenter lasers S.G. Pavlova,, H.-W. Hu¨bersa, N.V. Abrosimovb, H. Riemannb, L.V. Gavrilenkoc, A.V. Antonovc a
Institute of Planetary Research, German Aerospace Center, D-12489 Berlin, Germany b Institute for Crystal Growth, D-12489 Berlin, Germany c Institute for Physics of Microstructures, Russian Academy of Sciences, 603950 N. Novgorod, Russia Received 11 July 2007; received in revised form 29 August 2007; accepted 30 August 2007
Abstract Stimulated emission at terahertz frequencies has been obtained from multi-crystalline silicon doped by phosphor under optical excitation by a mid-infrared laser. The silicon samples consist of grains with a characteristic size distribution in the range from 50 to 500 mm. The maximum operation temperature of the laser made from multi-crystalline silicon is 6 K less than that of monocrystalline lasers and the maximum output power is three times less while its laser threshold is only slightly higher and the emission frequency is the same. These effects are attributed to internal strain and enhanced phonon scattering induced by grain boundaries. r 2007 Elsevier B.V. All rights reserved. PACS: 42.55.Rz; 61.72.Mm Keywords: Terahertz; Silicon laser; Multi-crystalline silicon
1. Introduction One of the challenges in silicon photonics is the realization of low-cost photonic devices which are compatible and can be possibly easily integrated in optoelectronic circuits [1]. High-speed optical modulators [2] and highspeed infrared SiGe detectors [3] made from silicon have been demonstrated. The development of a silicon-based laser, however, faces significant problems caused by the indirect bandgap of silicon. So far optically pumped terahertz (THz) intracenter lasers [4] and Raman-type infrared [5,6] and THz [7] lasers have been realized. Some approaches, such as a hybrid-type laser that utilizes a silicon waveguide bonded to AlGaInAs quantum wells [8], adapt the existing AIIIBV infrared lasers to CMOScompatible silicon processing. Another significant amount of recent activities has focused on achieving stimulated Corresponding author. Tel.: +49 30 67055594; fax: +49 30 67055507.
E-mail address: [email protected] (S.G. Pavlov). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.08.221
emission in silicon by a modification of basic material properties, for example with silicon nanocrystals [9–11] and porous silicon [12–14]. The latter approaches demonstrated a significant enhancement of light amplification from silicon due to quantum-confinement effects. Multi-crystalline silicon (mc-Si), a material consisting of multiple small silicon crystals (crystallites), is widely used for integrated circuits, large area imaging sensor arrays and liquid-crystal displays, as well as solar cell fabrication [15]. The commonly used range of small crystallites (grains) is 0.01–1 mm for polycrystalline silicon and rises up to hundreds of micrometers in the case of multi-crystalline silicon. In extreme cases, grains can be as small as hundreds of A˚ngstroms and as large as few millimeters. The major electrical, thermal and optical properties of mc-Si lie between those of bulk crystalline silicon (c-Si) and amorphous silicon (a-Si). Boundaries between separate crystallites induce internal strain in mc-Si. Variations in a grain size can lead to a modification of the principal phonon frequencies in mc-Si [16]. Scattering of phonons
ARTICLE IN PRESS 536
S.G. Pavlov et al. / Physica B 403 (2008) 535–538
and charge carriers on grain boundaries result in a reduced thermal and electrical conductivity of this material compared to monocrystalline silicon [17–19]. All these changes are expected to induce modifications of the performance of the THz-range intracenter silicon laser, which operates on transitions between localized states of group-V donor centers. Resonances between shallow impurity states and intervalley phonons in c-Si determine to a large extent the intracenter population inversion schemes [4]. These donor-phonon resonances are supposed to change when going from monocrystalline to multicrystalline silicon. Strain in the crystal can result in a significant broadening of impurity states, which may cause, in turn, a decrease of population inversion as well as optical cross-section for intracenter transitions. 2. Growth, preparation and characterization of c-Si:P and mc-Si:P
3. Optical characterization of mc-Si and c-Si intracenter lasers Laser thresholds and output power were measured in a cryogenic dipstick which was immersed in liquid helium. This was found to be a compact and reproducible optical
Fig. 1. Micrograph (a Sirtl etch view of the polished sample facet) of the multi-crystalline silicon sample doped with phosphor (top) and an enlarged section (bottom).
c-Si Stokes Emission Intensity (arb. units)
For a direct comparison of the laser performance, both mono- and multi-crystalline silicon crystals have been grown by the Czochralski technique with simultaneous doping by phosphor from the melt. Monocrystalline silicon doped by phosphor with NPE2.3 1015 cm 3 has been prepared in the same way as in Ref. [4]. For the mc-Si growth, specially prepared polycrystalline seeds have been used. The mc-Si crystals had monocrystallite grain dimensions in the range of 50–500 mm (Fig. 1). The ingots have been grown in the /1 0 0S direction. They had phosphor concentrations of NPE1.6 1015 cm 3. The compensation in the crystals was below 1%. The crystals were cut into parallelepipeds with typical dimensions of (7 7) 5 mm3 with a cross-sectional area of (7 7) mm2 in the plane perpendicular to the growth axis. All opposite facets of the samples were optically polished plane parallel to each other with an accuracy of 1 arcmin in order to provide a high-Q resonator on internal reflection modes. The c-Si and mc-Si samples were investigated by Raman spectroscopy. The measurements were performed at room temperature using an optical parametric oscillator operating at 0.55 mm as excitation source and a grating spectrometer with a spectral resolution of 0.1 cm 1. The Raman spectra of c-Si show the Stokes line corresponding to the zone-centered optical phonon at about 521 cm 1. The Stokes line of the mc-Si sample is red-shifted by 0–1.0 cm 1 with respect to Stokes line from the c-Si sample. The exact value depends on the position of the sample where the measurement was done (Fig. 2). This indicates an influence of the grain structure on the phonon spectra in the mc-Si samples.
500
mc-Si pos. 1 mc-Si pos. 2
505
510
515 520 525 Raman Shift (cm-1)
530
535
540
Fig. 2. Raman spectra of the c-Si:P and mc-Si:P crystals, taken at two different positions on the mc-Si sample facet. The Raman shifts have been determined by a Lorentzian fit to the spectra. The Stokes shift of the c-Si sample is 521 cm 1 and the spectral resolution is 0.1 cm 1.
ARTICLE IN PRESS S.G. Pavlov et al. / Physica B 403 (2008) 535–538
537
Laser emission power (arb.units)
100
10-1 g-TA
100 2p0 1s (T2)
10-1 c-Si mc-Si
1s (A1) 10-2
10-2
10-3
10-3
Laser Emission Power (arb.units)
1.0
g-LO
c-Si mc-Si
0.8 0.6 0.4 0.2 0.0 8
10
4
5
10 Pump intensity (W/cm2)
10
6
Fig. 3. Dependence of the THz emission from c-Si (gray curve) and mc-Si (black curve) samples on the pump power (wavelength 10.59 mm, repetition rate 4 Hz, and excitation pulse length at FWHM is 70 ns) measured at a temperature of 8 K. The inset shows a schematic presentation of the four-level Si:P laser mechanism. Straight lines are for optical transitions and curved ones are for intraband (g-LO) and intracenter (g-TA) phonon-assisted relaxation.
scheme for pumping of the silicon laser as well as detection of its emission with a Ge:Ga photoconductive detector integrated in the dipstick. For spectral and temperature measurements, the silicon crystals were mounted on a copper block inside a closed-cycle cryogenic refrigerator and cooled to 8 K. The temperature was controlled with a silicon diode mounted on the copper block. In this case, emission from the silicon samples was detected by a liquidhelium cooled Ge:Ga detector in a separate cryostat. The silicon crystals were optically excited by radiation from a pulsed TEA (transversely excited atmospheric-pressure) CO2 laser operating at wavelengths from 9.2 to 10.7 mm (photon energy 116–135 meV). The pump laser had a spot diameter of 6 mm (1/e2) on the (7 7) mm2 facet of the sample. The emission spectra were measured with a Fourier transform spectrometer with a resolution of 3 GHz (0.1 cm 1). Both active media exhibited stimulated emission above a certain pump power level (Fig. 3). The population mechanism of intracenter silicon lasers is dominated by peculiarities of the relaxation process of photoexcited electrons accompanied by interaction with intervalley phonons of the host lattice. At low lattice temperature, the donor electrons are bound to the phosphor ground state, 1s(A1) (inset in Fig. 3). After being photoexcited by radiation of a CO2 laser (arrow up) into the continuum of states in the conduction band, electrons relax first to the band bottom and then into the highly excited localized states. Each intraband and intracenter relaxation step is accompanied by emission of a phonon. The most important phonons in the case of Si:P as active medium are the intervalley g-LO phonon (frequency 15.3 THz) for intraband relaxation and the transverse acoustic g-TA phonon (2.9 THz) responsible for relaxation between the
10
12 14 16 Heat Sink Temperature (K)
18
20
Fig. 4. Temperature dependence of the c-Si and mc-Si laser output power (pumping conditions are the same as in Fig. 3). The temperature was measured on the copper block which holds the laser sample.
1s(T2) and 1s(A1) excited donor states (curved arrows downwards). Relaxation from the 2p0 state is suppressed because of the absence of an appropriate resonant phonon. The f-TA phonon (4.5 THz) which is most close to the interstate 2p021s(T2) gap, limits the lifetime of the upper laser state, 2p0. Thus, population inversion is formed between the 2p0 and 1s(T2) phosphor excited states. Changes of the energy of intervalley phonons in silicon are expected to be similar to that measured by Raman spectroscopy for the zone-centered optical phonon (Fig. 2). This may affect the impurity-phonon resonances controlling the population inversion in intracenter lasers. The output power from the mc-silicon active medium is about three times less at 4 K in comparison with the monocrystalline material (Fig. 3). The difference in the output power is temperature dependent and increases towards the laser thresholds (Fig. 4). The gain slope of the mc-Si:P laser is about a factor of five less and the laser threshold is slightly higher (Fig. 3). These differences can be attributed partly to the slightly different doping, which is for the c-Si laser a bit closer to the optimal doping concentration (5 1015 cm 3) [4]. The operation frequency of both lasers is 5.424 THz (Fig. 5) what corresponds to the 2p0-1s(T2) phosphor transition [20], and the difference in the laser frequency between c-Si and mc-Si lasers lies within the spectral resolution in our experiments. The operation temperature of the mc-Si:P laser is limited to 10 K compared to 16 K for the monocrystalline Si:P laser (Fig. 4). This pronounced difference can be attributed to the different phonon spectra caused by enhanced phonon scattering on grain boundaries as well as reduced thermal conductivity of the mc-Si medium. 4. Summary We have shown that phosphor-doped multi-crystalline silicon with submillimeter grain size can serve as active
ARTICLE IN PRESS S.G. Pavlov et al. / Physica B 403 (2008) 535–538
538
Frequency (THz) 0
1
2
3
4
5
6
7
(THz)
1.0
5.410
5.419
5.428
5.437
180.9
181.2
1.0
Laser Intensity (arb.units)
0.8
0.8 c-Si:P 0.6 mc-Si:P
0.6
0.4 0.2
0.4 0.0 180.3
180.6 (cm-1)
0.2
0.0 0
50
100
150
200
250
Wavenumber (cm-1) Fig. 5. Emission spectra for c-Si (gray curve) and mc-Si (black curve) laser samples. The pump wavelength is 10.59 mm, the heat sink temperature is 8 K and the spectral resolution is 0.125 cm 1.
medium for intracenter THz silicon lasers. However, the operation temperature is limited to 10 K compared to 16 K for monocrystalline Si:P lasers and the output power is about three times less. Other important operational features, such as the laser scheme and the pump threshold, remain almost unchanged. Acknowledgment This work was supported by the European Commission through the ProFIT programme of the Investitionsbank Berlin (Grant no. 10126728). References [1] A. Barkai, Y. Chetrit, O. Cohen, et al., J. Opt. Netw. 6 (2007) 25. [2] L. Liao, D. Samara-Rubio, M. Morse, A. Liu, D. Hodge, D. Rubin, U. Keil, T. Franck, Opt. Express 13 (2006) 3129. [3] J. Liu, J. Michel, W. Giziewicz, D. Pan, K. Wada, D.D. Cannon, S. Jongthammanurak, D.T. Danielson, L.C. Kimerling, J. Chen, F.O¨. Ilday, F.X. Ka¨rtner, J. Yasaitis, Appl. Phys. Lett. 87 (2005) 103501. [4] H.-W. Hu¨bers, S.G. Pavlov, V.N. Shastin, Semicond. Sci. Technol. 20 (2005) S211. [5] O. Boyaraz, B. Jalali, Opt. Express 12 (2004) 5269.
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