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Temperature-dependent terahertz radiation from the surfaces of narrow-gap semiconductors illuminated by femtosecond laser pulses
ARTICLE IN PRESS Physica B 403 (2008) 3786–3788
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Temperature-dependent terahertz radiation from the surfaces of narrow-gap semiconductors illuminated by femtosecond laser pulses G. Molis , R. Adomavicˇius, A. Krotkus Semiconductor Physics Institute, A. Gostauto 11, 01800, Vilnius, Lithuania
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 February 2008 Accepted 7 July 2008
Temperature-dependent increase of the terahertz (THz) electric field emitted from the surfaces of optically pumped narrow-gap semiconductors InAs, InSb, and CdxHg1xTe is presented. In the case of Cd0.2Hg0.8Te increase up to 15–17 times has been observed, when cooling the sample from the room temperature to close to liquid-helium temperatures, and THz emission from this material becomes comparable to that of p-InAs emitter. This effect was explained in terms of the increased photoexcited electron excess energy due to the positive temperature coefficient for energy bandgap of CdxHg1xTe, as well as by weaker surface field screening and carrier–carrier scattering. Temperature-dependent modification of the shape of THz pulses emitted from InSb surfaces has been observed and attributed to plasma oscillation of the cold electrons. & 2008 Elsevier B.V. All rights reserved.
Keywords: Terahertz CdHgTe InSb InAs THz Temperature
1. Introduction A majority of semiconductor surfaces, when illuminated by femtosecond laser beams, radiate short electromagnetic transients with characteristic spectra reaching far into terahertz (THz) frequency range [1]. Because some of such surfaces are quite efficient THz emitters, they can be considered as an alternative to more common photoconductive antennae sources of pulsed THzradiation for time-domain spectroscopy systems of this spectral range. Physical mechanisms leading to this radiation can be divided into two groups: (i) ultrafast current surge effects caused by the special separation of photoexcited electrons and holes in the built-in surface electric field or by their different propagation towards the bulk of the materials (so-called photo-Dember effect) and (ii) nonlinear optical effects due to the second-order (optical rectification—OR) or the third-order (electric-field-induced optical rectification—EFIOR) nonlinear susceptibilities [2]. When illuminated by femtosecond Ti:sapphire laser pulses (photon energy of 1.5–1.6 eV), most efficiently THz transients are radiated from the surfaces of narrow-gap semiconductors, especially InAs. At room temperature, because of a combined action of current surge and EFIOR effects, the best THz emitters are moderately doped p-type InAs crystals [3]. Carrier separation effect and ultrafast current surge should be more effective and THz pulse emission should be stronger when the energy bandgap of the semiconductor is narrower, electrons
are photoexcited in the conduction band with larger excess energies, and propagate with larger velocities. However, such semiconductors as InSb or CdxHg1xTe, which have smaller than InAs energy bandgaps, are much poorer THz emitters than the later material. In the case of InSb, the main reason for reduced THz emission is the photoexcited electron transfer to subsidiary, low mobility valleys of the conduction band [4,5], whereas the cause of a relatively weak THz radiation observed from the photoexcited Hg-rich CdxHg1xTe compounds [6] remains unclear. In this work, we will present the results of the investigation of THz pulse emission from various narrow-gap semiconductors at low temperatures. Lowering the lattice temperature affects carrier density and their scattering rates in the materials with a small energy bandgap much more than in other semiconductors; thus, one could expect a stronger impact on their emissive characteristics too. Significant enhancement of THz pulse amplitudes has been experimentally evidenced for all investigated semiconductors, but the strongest increase was found in the case of CdxHg1xTe, which, at liquid-helium temperatures, radiates THz pulses with the amplitude comparable to the signals generated from InAs surfaces. These experimental observations were discussed in terms of different physical mechanisms leading to the THz emission from femtosecond laser-excited semiconductors surfaces.
2. Experimental Corresponding author. Tel./fax: +370 5 261 68 21.
E-mail address: gediminas@pfi.lt (G. Molis). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.07.005
In our experiment, mode-locked Ti:Sapphire laser (Mira, Coherent) pulses with 150 fs duration, repetition rate of 76 MHz and
ARTICLE IN PRESS G. Molis et al. / Physica B 403 (2008) 3786–3788
central wavelength of 820 nm were used as the pump exciting the samples and to gate a photoconductive antenna detector manufactured on a low-temperature-grown, annealed GaAs layer with an electron trapping time of 300 fs. The angle of incidence of the pump laser beam on the sample was close to 451. The pump beam has a maximum average power of about 150 mW. The samples were placed in a cryostat (Oxford Instruments) with quartz and Teflon windows transparent in the optical and THz spectral ranges, respectively. Transient waveforms of the THz radiation were obtained by measuring dc photocurrent as a function of the time delay between the pump and gate optical pulses. The samples investigated were single crystals of n- and p-type InAs (the electron and, respectively, the hole density at room temperature is 2 1016 cm3), n-type InSb (the electron density at the liquid-nitrogen temperature is 2 1014 cm3, the electron mobility is 7,00,000 cm2/V s at the same temperature) and Cd0.3Hg0.7Te as well as epitaxial layer of Cd0.2Hg0.8Te grown on CdTe substrate. Crystalline (111) planes were illuminated in case of InAs and InSb; for both CdxHg1xTe samples the surfaces were of (1 0 0) symmetry. The measurements were performed in the temperature range from 15 to 300 K. THz pulses radiated from the surfaces of various narrow-gap semiconductors at room temperature are compared in Fig. 1. For the same experimental conditions corresponding to the average laser pulse fluency of 0.017 mJ/cm2, the strongest THz emission was observed from p-type InAs surface. Emission from n-type InAs is approximately two times weaker, whereas InSb and CdxHg1xTe samples radiate THz pulses with amplitudes that hardly reach 10% of the amplitudes achieved when using p-InAs crystal. The situation changes drastically when the temperature of the samples is lowered. Temperature dependences of THz electrical field amplitude radiated from two differently doped InAs crystals and two samples of CdxHg1xTe with different material composition are presented in Fig. 2. Amplitudes of THz pulses radiated by both InAs crystals at cryogenic temperatures increase, with respect to their room temperature values, by approximately two times for p-type InAs and by 20% for n-type InAs. For CdxHg1xTe samples this increase is substantially larger, reaching 15–17 times. THz pulse emission from InSb surfaces also significantly increases at low temperatures. Experimental results obtained on this material are summarized in Fig. 3. Lowering the temperature leads not only to the change of amplitude of the observed transient but also to the significant modification of its shape.
3787
Fig. 2. THz electrical field, radiated from various semiconductor surfaces, amplitude dependence on the temperature.
Fig. 3. (a) THz pulses transients shape radiated from the surface of InSb in different temperatures. (b) THz electric field amplitude and plasma oscillation frequency change due to the temperature change.
Fig. 1. THz pulses radiated from the surfaces of various narrow-gap semiconductors at room temperature.
At lower than 150 K temperatures, fast initial transient (its peakto-valley amplitude increases by nine times when the temperature goes down from 300 to 15 K) is followed by a slower oscillation. Decreasing temperature results in increase of the amplitude of this oscillation and in decrease of its frequency.
ARTICLE IN PRESS 3788
G. Molis et al. / Physica B 403 (2008) 3786–3788
3. Discussion The differences in low-temperature behavior of THz emission from various narrow-gap semiconductors can have several causes. E.g., in the case of InAs illumination by Ti:sapphire laser pulses creates in the main conduction band G valley electrons with very large excess energies of 1.03 eV. At least half of these electrons move towards the bulk of the crystal at fairly high group velocities of 2 108 cm/s, leaving less mobile photoexcited holes far behind. Moreover, such quasi-ballistic electron motion will continue for even longer durations than the characteristic electron scattering by longitudinal optical (LO) phonons (the prevailing hot electron scattering mechanism) time of 100 fs, because, due to the peculiarities of non-parabolic conduction band structure of InAs, LO-phonon scattering will proceed most efficiently at small angles and will have little effect on electron momenta [7]. Spatial separation of these quasi-ballistically propagating electrons and the holes staying close to their excitation points will lead to the THz pulse generation due to two effects [8]. First of all, this separation will result in an ultrafast surface current surge that will induce fast changing electromagnetic transient emission. Secondly, large surface electric fields that will appear due to separation of current carriers created during the rising part of the optical pulse will interact with the later parts of this pulse and cause THz pulse emission due to the EFIOR effect. The contribution of the EFIOR effect will be stronger in p-type InAs because of the built-in electric field in the surface inversion layers typical for this material. Photoexcited electron excess energy will decrease with decreasing temperature due to the increase of the energy bandgap, but this should have only a small effect on the group velocity of the electrons in strongly non-parabolic conduction band of InAs. Because photoexcited electrons remain hot during the THz pulse generation, decreasing temperature will have little effect also to their scattering rates. On the other hand, the strength of the built-in surface field will depend on the characteristics of cold, equilibrium carriers and will increase due to reduction of the Debye screening length and the enhancement of the energy bandgap. Therefore, a slight increase of THz pulse amplitudes radiated from InAs surfaces at lower temperatures could be considered as an additional proof of the EFIOR-related mechanism of this radiation. In both InAs crystals, the doping concentration was larger than the intrinsic carrier density at all temperatures, at which the experiments were performed. The situation was different for InSb, where intrinsic conduction was dominating at temperatures higher than 150 K. In this material, the reduction of the carrier density can lead to a stronger increase of the built-in electric field strength. Because, as it is in the case of InAs, the enhancement of THz emission from InSb at low temperatures is difficult to explain by current surge effect, our experimental observations evidence on a significant EFIOR effect contribution also in this material. Another feature typical for InSb is relatively low-frequency oscillation following the THz pulse as shown in Fig. 3a. At lowtemperatures amplitude of this oscillation increases and its frequency decreases. Because the temperature range corresponding to the most significant changes in the oscillation frequency coincides with the temperatures, where the intrinsic carrier density in the sample starts to exceed its doping level, one could assume that the slow features of the emitted transients are caused by the plasma oscillation of the electrons in the sample. This assumption is supported by the fact that plasma frequency
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi estimated as oP ¼ e2 n=s o mn (n is the electron density, eseo is the dielectric permittivity of the crystal, and mn is the electron effective mass) has similar values as the frequencies measured experimentally at low temperatures. Most drastic changes of emitted THz pulse amplitude at low temperatures were observed on the CdxHg1xTe samples. In contrast to InAs or InSb, Hg-rich CdxHg1xTe alloys show anomalous positive-energy bandgap dependence on the temperature [9], therefore, electron excess energies at low temperature will be higher in these alloys than in the room temperature. Moreover, carrier concentration, e.g., in Cd0.2Hg0.8Te, will decrease by lowering the temperature by almost two orders of magnitude, which will reduce the effects of surface field screening and carrier–carrier scattering on the efficiency of the current surge mechanism of THz emission. Significant growth of THz signal at low temperatures evidences that in CdxHg1xTe, similarly to InAs, electrons are excited by Ti:sapphire laser quanta into the main G valley of the conduction band and the subsidiary conduction band valleys in these alloys are lying higher in energy than 1.5 eV from the top of the valence band. At this time there is no sufficient experimental data confirming the occurrence of EFIOR effect in this material. Recent experiments on THz emission from germanium crystals [10] evidence the universal character of this effect in resonantly photoexcited semiconductors; therefore its contribution to THz pulse generation from CdxHg1xTe surfaces at low temperatures cannot be excluded.
4. Conclusions In conclusion, temperature dependences of THz radiation from the surfaces of several narrow-gap semiconductors illuminated by femtosecond laser pulses were measured and interpreted in terms of various physical mechanisms causing this effect. THz pulse amplitudes in all investigated semiconductors have increased with decreasing temperature; this increase was the smallest (1.2–2 times) in InAs crystals and the largest (15–17 times) in CdxHg1xTe. The observed significant increase of THz emission from CdxHg1xTe at low temperatures was explained by a combined effect of increasing photoelectron excess energy and decreasing electron density. It also evidences that the subsidiary, high effective mass valleys of Hg-rich compositions of these alloys are at the energies at least 1.3 eV above the conduction band minimum.
References [1] X.C. Zhang, B.B. Hu, J.T. Darrow, D.H. Auston, Appl. Phys. Lett. 56 (1990) 1011. [2] A. Krotkus, R. Adomavicˇius, V. Malevich, in: R.E. Miles, X.–C. Zhang, H. Eisele, A. Krotkus (Eds.), Terahertz Frequency Detection and Identification of Materials and Objects, Springer, Berlin, 2006, p. 3. [3] R. Adomavicˇius, A. Urbanowicz, G. Molis, A. Krotkus, Appl. Phys. Lett. 85 (2004) 2463. [4] P. Gu, M. Tani, Sh. Kono, K. Sakai, X.-C. Zhang, J. Appl. Phys. 91 (2002) 5533. [5] R. Adomavicˇius, G. Molis, A. Krotkus, V. Sirutkaitis, Appl. Phys. Lett. 87 (2005) 261101. [6] A. Krotkus, R. Adomavicˇius, G. Molis, A. Urbanowicz, H. Eusebe, J. Appl. Phys. 96 (2004) 4006. [7] W.P. Dumke, Phys. Rev. 167 (1968) 783. [8] A. Krotkus, R. Adomavicˇius, G. Molis, V.L. Malevich, J. Nanoelectr. Optoelectron. 2 (2007) 108. [9] R. Dornhaus, G. Nimtz, B. Schlicht, Narrow-Gap Semiconductors, Springer, Berlin, Heidelberg, New York, Tokyo, 1983. [10] A. Urbanowicz, A. Krotkus, R. Adomavicˇius, V.L. Malevich, Physica B. 398 (2007) 99.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Temperature-dependent terahertz radiation from the surfaces of narrow-gap semiconductors illuminated by femtosecond laser pulses G. Molis , R. Adomavicˇius, A. Krotkus Semiconductor Physics Institute, A. Gostauto 11, 01800, Vilnius, Lithuania
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 February 2008 Accepted 7 July 2008
Temperature-dependent increase of the terahertz (THz) electric field emitted from the surfaces of optically pumped narrow-gap semiconductors InAs, InSb, and CdxHg1xTe is presented. In the case of Cd0.2Hg0.8Te increase up to 15–17 times has been observed, when cooling the sample from the room temperature to close to liquid-helium temperatures, and THz emission from this material becomes comparable to that of p-InAs emitter. This effect was explained in terms of the increased photoexcited electron excess energy due to the positive temperature coefficient for energy bandgap of CdxHg1xTe, as well as by weaker surface field screening and carrier–carrier scattering. Temperature-dependent modification of the shape of THz pulses emitted from InSb surfaces has been observed and attributed to plasma oscillation of the cold electrons. & 2008 Elsevier B.V. All rights reserved.
Keywords: Terahertz CdHgTe InSb InAs THz Temperature
1. Introduction A majority of semiconductor surfaces, when illuminated by femtosecond laser beams, radiate short electromagnetic transients with characteristic spectra reaching far into terahertz (THz) frequency range [1]. Because some of such surfaces are quite efficient THz emitters, they can be considered as an alternative to more common photoconductive antennae sources of pulsed THzradiation for time-domain spectroscopy systems of this spectral range. Physical mechanisms leading to this radiation can be divided into two groups: (i) ultrafast current surge effects caused by the special separation of photoexcited electrons and holes in the built-in surface electric field or by their different propagation towards the bulk of the materials (so-called photo-Dember effect) and (ii) nonlinear optical effects due to the second-order (optical rectification—OR) or the third-order (electric-field-induced optical rectification—EFIOR) nonlinear susceptibilities [2]. When illuminated by femtosecond Ti:sapphire laser pulses (photon energy of 1.5–1.6 eV), most efficiently THz transients are radiated from the surfaces of narrow-gap semiconductors, especially InAs. At room temperature, because of a combined action of current surge and EFIOR effects, the best THz emitters are moderately doped p-type InAs crystals [3]. Carrier separation effect and ultrafast current surge should be more effective and THz pulse emission should be stronger when the energy bandgap of the semiconductor is narrower, electrons
are photoexcited in the conduction band with larger excess energies, and propagate with larger velocities. However, such semiconductors as InSb or CdxHg1xTe, which have smaller than InAs energy bandgaps, are much poorer THz emitters than the later material. In the case of InSb, the main reason for reduced THz emission is the photoexcited electron transfer to subsidiary, low mobility valleys of the conduction band [4,5], whereas the cause of a relatively weak THz radiation observed from the photoexcited Hg-rich CdxHg1xTe compounds [6] remains unclear. In this work, we will present the results of the investigation of THz pulse emission from various narrow-gap semiconductors at low temperatures. Lowering the lattice temperature affects carrier density and their scattering rates in the materials with a small energy bandgap much more than in other semiconductors; thus, one could expect a stronger impact on their emissive characteristics too. Significant enhancement of THz pulse amplitudes has been experimentally evidenced for all investigated semiconductors, but the strongest increase was found in the case of CdxHg1xTe, which, at liquid-helium temperatures, radiates THz pulses with the amplitude comparable to the signals generated from InAs surfaces. These experimental observations were discussed in terms of different physical mechanisms leading to the THz emission from femtosecond laser-excited semiconductors surfaces.
2. Experimental Corresponding author. Tel./fax: +370 5 261 68 21.
E-mail address: gediminas@pfi.lt (G. Molis). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.07.005
In our experiment, mode-locked Ti:Sapphire laser (Mira, Coherent) pulses with 150 fs duration, repetition rate of 76 MHz and
ARTICLE IN PRESS G. Molis et al. / Physica B 403 (2008) 3786–3788
central wavelength of 820 nm were used as the pump exciting the samples and to gate a photoconductive antenna detector manufactured on a low-temperature-grown, annealed GaAs layer with an electron trapping time of 300 fs. The angle of incidence of the pump laser beam on the sample was close to 451. The pump beam has a maximum average power of about 150 mW. The samples were placed in a cryostat (Oxford Instruments) with quartz and Teflon windows transparent in the optical and THz spectral ranges, respectively. Transient waveforms of the THz radiation were obtained by measuring dc photocurrent as a function of the time delay between the pump and gate optical pulses. The samples investigated were single crystals of n- and p-type InAs (the electron and, respectively, the hole density at room temperature is 2 1016 cm3), n-type InSb (the electron density at the liquid-nitrogen temperature is 2 1014 cm3, the electron mobility is 7,00,000 cm2/V s at the same temperature) and Cd0.3Hg0.7Te as well as epitaxial layer of Cd0.2Hg0.8Te grown on CdTe substrate. Crystalline (111) planes were illuminated in case of InAs and InSb; for both CdxHg1xTe samples the surfaces were of (1 0 0) symmetry. The measurements were performed in the temperature range from 15 to 300 K. THz pulses radiated from the surfaces of various narrow-gap semiconductors at room temperature are compared in Fig. 1. For the same experimental conditions corresponding to the average laser pulse fluency of 0.017 mJ/cm2, the strongest THz emission was observed from p-type InAs surface. Emission from n-type InAs is approximately two times weaker, whereas InSb and CdxHg1xTe samples radiate THz pulses with amplitudes that hardly reach 10% of the amplitudes achieved when using p-InAs crystal. The situation changes drastically when the temperature of the samples is lowered. Temperature dependences of THz electrical field amplitude radiated from two differently doped InAs crystals and two samples of CdxHg1xTe with different material composition are presented in Fig. 2. Amplitudes of THz pulses radiated by both InAs crystals at cryogenic temperatures increase, with respect to their room temperature values, by approximately two times for p-type InAs and by 20% for n-type InAs. For CdxHg1xTe samples this increase is substantially larger, reaching 15–17 times. THz pulse emission from InSb surfaces also significantly increases at low temperatures. Experimental results obtained on this material are summarized in Fig. 3. Lowering the temperature leads not only to the change of amplitude of the observed transient but also to the significant modification of its shape.
3787
Fig. 2. THz electrical field, radiated from various semiconductor surfaces, amplitude dependence on the temperature.
Fig. 3. (a) THz pulses transients shape radiated from the surface of InSb in different temperatures. (b) THz electric field amplitude and plasma oscillation frequency change due to the temperature change.
Fig. 1. THz pulses radiated from the surfaces of various narrow-gap semiconductors at room temperature.
At lower than 150 K temperatures, fast initial transient (its peakto-valley amplitude increases by nine times when the temperature goes down from 300 to 15 K) is followed by a slower oscillation. Decreasing temperature results in increase of the amplitude of this oscillation and in decrease of its frequency.
ARTICLE IN PRESS 3788
G. Molis et al. / Physica B 403 (2008) 3786–3788
3. Discussion The differences in low-temperature behavior of THz emission from various narrow-gap semiconductors can have several causes. E.g., in the case of InAs illumination by Ti:sapphire laser pulses creates in the main conduction band G valley electrons with very large excess energies of 1.03 eV. At least half of these electrons move towards the bulk of the crystal at fairly high group velocities of 2 108 cm/s, leaving less mobile photoexcited holes far behind. Moreover, such quasi-ballistic electron motion will continue for even longer durations than the characteristic electron scattering by longitudinal optical (LO) phonons (the prevailing hot electron scattering mechanism) time of 100 fs, because, due to the peculiarities of non-parabolic conduction band structure of InAs, LO-phonon scattering will proceed most efficiently at small angles and will have little effect on electron momenta [7]. Spatial separation of these quasi-ballistically propagating electrons and the holes staying close to their excitation points will lead to the THz pulse generation due to two effects [8]. First of all, this separation will result in an ultrafast surface current surge that will induce fast changing electromagnetic transient emission. Secondly, large surface electric fields that will appear due to separation of current carriers created during the rising part of the optical pulse will interact with the later parts of this pulse and cause THz pulse emission due to the EFIOR effect. The contribution of the EFIOR effect will be stronger in p-type InAs because of the built-in electric field in the surface inversion layers typical for this material. Photoexcited electron excess energy will decrease with decreasing temperature due to the increase of the energy bandgap, but this should have only a small effect on the group velocity of the electrons in strongly non-parabolic conduction band of InAs. Because photoexcited electrons remain hot during the THz pulse generation, decreasing temperature will have little effect also to their scattering rates. On the other hand, the strength of the built-in surface field will depend on the characteristics of cold, equilibrium carriers and will increase due to reduction of the Debye screening length and the enhancement of the energy bandgap. Therefore, a slight increase of THz pulse amplitudes radiated from InAs surfaces at lower temperatures could be considered as an additional proof of the EFIOR-related mechanism of this radiation. In both InAs crystals, the doping concentration was larger than the intrinsic carrier density at all temperatures, at which the experiments were performed. The situation was different for InSb, where intrinsic conduction was dominating at temperatures higher than 150 K. In this material, the reduction of the carrier density can lead to a stronger increase of the built-in electric field strength. Because, as it is in the case of InAs, the enhancement of THz emission from InSb at low temperatures is difficult to explain by current surge effect, our experimental observations evidence on a significant EFIOR effect contribution also in this material. Another feature typical for InSb is relatively low-frequency oscillation following the THz pulse as shown in Fig. 3a. At lowtemperatures amplitude of this oscillation increases and its frequency decreases. Because the temperature range corresponding to the most significant changes in the oscillation frequency coincides with the temperatures, where the intrinsic carrier density in the sample starts to exceed its doping level, one could assume that the slow features of the emitted transients are caused by the plasma oscillation of the electrons in the sample. This assumption is supported by the fact that plasma frequency
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi estimated as oP ¼ e2 n=s o mn (n is the electron density, eseo is the dielectric permittivity of the crystal, and mn is the electron effective mass) has similar values as the frequencies measured experimentally at low temperatures. Most drastic changes of emitted THz pulse amplitude at low temperatures were observed on the CdxHg1xTe samples. In contrast to InAs or InSb, Hg-rich CdxHg1xTe alloys show anomalous positive-energy bandgap dependence on the temperature [9], therefore, electron excess energies at low temperature will be higher in these alloys than in the room temperature. Moreover, carrier concentration, e.g., in Cd0.2Hg0.8Te, will decrease by lowering the temperature by almost two orders of magnitude, which will reduce the effects of surface field screening and carrier–carrier scattering on the efficiency of the current surge mechanism of THz emission. Significant growth of THz signal at low temperatures evidences that in CdxHg1xTe, similarly to InAs, electrons are excited by Ti:sapphire laser quanta into the main G valley of the conduction band and the subsidiary conduction band valleys in these alloys are lying higher in energy than 1.5 eV from the top of the valence band. At this time there is no sufficient experimental data confirming the occurrence of EFIOR effect in this material. Recent experiments on THz emission from germanium crystals [10] evidence the universal character of this effect in resonantly photoexcited semiconductors; therefore its contribution to THz pulse generation from CdxHg1xTe surfaces at low temperatures cannot be excluded.
4. Conclusions In conclusion, temperature dependences of THz radiation from the surfaces of several narrow-gap semiconductors illuminated by femtosecond laser pulses were measured and interpreted in terms of various physical mechanisms causing this effect. THz pulse amplitudes in all investigated semiconductors have increased with decreasing temperature; this increase was the smallest (1.2–2 times) in InAs crystals and the largest (15–17 times) in CdxHg1xTe. The observed significant increase of THz emission from CdxHg1xTe at low temperatures was explained by a combined effect of increasing photoelectron excess energy and decreasing electron density. It also evidences that the subsidiary, high effective mass valleys of Hg-rich compositions of these alloys are at the energies at least 1.3 eV above the conduction band minimum.
References [1] X.C. Zhang, B.B. Hu, J.T. Darrow, D.H. Auston, Appl. Phys. Lett. 56 (1990) 1011. [2] A. Krotkus, R. Adomavicˇius, V. Malevich, in: R.E. Miles, X.–C. Zhang, H. Eisele, A. Krotkus (Eds.), Terahertz Frequency Detection and Identification of Materials and Objects, Springer, Berlin, 2006, p. 3. [3] R. Adomavicˇius, A. Urbanowicz, G. Molis, A. Krotkus, Appl. Phys. Lett. 85 (2004) 2463. [4] P. Gu, M. Tani, Sh. Kono, K. Sakai, X.-C. Zhang, J. Appl. Phys. 91 (2002) 5533. [5] R. Adomavicˇius, G. Molis, A. Krotkus, V. Sirutkaitis, Appl. Phys. Lett. 87 (2005) 261101. [6] A. Krotkus, R. Adomavicˇius, G. Molis, A. Urbanowicz, H. Eusebe, J. Appl. Phys. 96 (2004) 4006. [7] W.P. Dumke, Phys. Rev. 167 (1968) 783. [8] A. Krotkus, R. Adomavicˇius, G. Molis, V.L. Malevich, J. Nanoelectr. Optoelectron. 2 (2007) 108. [9] R. Dornhaus, G. Nimtz, B. Schlicht, Narrow-Gap Semiconductors, Springer, Berlin, Heidelberg, New York, Tokyo, 1983. [10] A. Urbanowicz, A. Krotkus, R. Adomavicˇius, V.L. Malevich, Physica B. 398 (2007) 99.