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Impurity-assisted terahertz luminescence in quantum well nanostructures under interband photoexсitation
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St. Petersburg Polytechnical University Journal: Physics and Mathematics 2 (2016) 281–286 www.elsevier.com/locate/spjpm
Impurity-assisted terahertz luminescence in quantum well nanostructures under interband photoexсitation Ivan S. Makhov, Vadim Yu. Panevin∗, Maxim Ya. Vinnichenko, Anton N. Sofronov, Dmitry A. Firsov, Leonid E. Vorobjev Peter the Great St. Petersburg Polytechnic University, 29 Politekhnicheskaya St., St. Petersburg 195251, Russian Federation Available online 17 November 2016
Abstract The paper presents the results of an experimental study of impurity-assisted photoluminescence in the far- (terahertz) and near-infrared spectral ranges in n-GaAs/AlGaAs quantum well structures with different well widths under interband photoexcitation of electron–hole pairs. The optical electron transitions between the first electron subband and donor ground state as well as between excited and ground donor states were revealed in the far-infrared photoluminescence spectra. Observation of these optical electron transitions became possible because of the depopulation of the donor ground state in the quantum well due to the non-equilibrium charge carrier radiative transitions from the donor ground state to the first heavy hole subband. The opportunity to tune the terahertz radiation wavelength in structures with doped quantum wells by changing the quantum well width was demonstrated experimentally. Copyright © 2016, St. Petersburg Polytechnic University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Keywords: Terahertz luminescence; Radiation; Quantum well; Spectrum; Nanostructure; Semiconductor.
Introduction The task of developing effective semiconductor sources of terahertz radiation (the wavelength range of electromagnetic radiation is 30–300 μm) is rather important at present as these devices can be used in diverse areas of science and technology, such as medicine, environmental monitoring, security systems,
∗
Corresponding author. E-mail addresses: [email protected] (I.S. Makhov), [email protected] (V.Yu. Panevin), [email protected] (M.Ya. Vinnichenko), [email protected] (A.N. Sofronov), [email protected] (D.A. Firsov), [email protected] (L.E. Vorobjev).
and computer science (see, for example, Refs. [1–3]). One of the most promising mechanisms for generating terahertz radiation is based on optical transitions of nonequilibrium charge carriers involving impurity states in semiconductors and semiconductor nanostructures. This mechanism is an alternative to the quantum cascade laser [4], since fabricating the latter requires very sophisticated techniques of high-quality growth of semiconductor nanostructures. There are currently several known mechanisms for generating terahertz radiation, based on impurityassisted transitions of charge carriers in semiconductors and semiconductor nanostructures. For example, terahertz radiation was observed during optical transitions of nonequilibrium charge carriers involving
http://dx.doi.org/10.1016/j.spjpm.2016.11.006 2405-7223/Copyright © 2016, St. Petersburg Polytechnic University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) (Peer review under responsibility of St. Petersburg Polytechnic University).
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impurity resonance states under impurity breakdown in electric field in mechanically strained p-Ge [5] and in GaAs/GaAsN:Be microstructures with built-in stresses [6]. Additionally, terahertz radiation was observed from bulk silicon doped with various impurities under intraband optical excitation of charge carriers [7]. Terahertz radiation under interband photoexcitation was observed in doped bulk semiconductors such as GaN [8], GaAs and Ge [9]. There are few studies examining terahertz radiation from nanostructures with doped quantum wells (QWs). For example, terahertz radiation in longitudinal electric fields was observed in GaAs/AlGaAs quantum wells doped with donor [10] and acceptor [11] impurities. Terahertz radiation from nanostructures with doped QWs under interband optical pumping was first described in Ref. [11]. This type of pumping entails the generation of electron–hole pairs that are subsequently trapped in the QW. At low crystal lattice temperatures, donor impurities in the QWs are neutral. Electrons from donor ground states can recombine with nonequilibrium holes, which is usually accompanied by the emission of near-infrared photons. The impurity ground states depopulated as a result of this process can be filled with nonequilibrium electrons from the first subband of size quantization. This can occur with an emission of photons of the terahertz range. This study continues our previous studies on the subject [11] and is dedicated to examining radiation of the terahertz and near-IR ranges in nanostructures with donor-doped QWs of different widths. Samples and experimental procedure Optical studies were carried out for three samples. Two of them were grown by molecular-beam epitaxy on a semi-insulating gallium arsenide substrate and contained doped GaAs/AlGaAs QWs of different widths. The first sample contained 226 periods of GaAs QW 16.1 nm in width, separated by 4.8nm-thick Al0.15 Ga0.85 As barriers. The second sample contained 50 periods of GaAs QWs 30 nm in width, separated by 7-nm-thick Al0.30 Ga0.70 As barriers. Structures with narrow and wide QWs had GaAs cap layers 60 and 20 nm thick, respectively. The QWs in both structures were doped with silicon (acting as a donor) with a surface concentration ns = 3·1010 cm–2 . A semiinsulating GaAs substrate, similar to those on which the nanostructures with doped QWs were grown, was used as the third reference sample. During optical measurements, the samples were mounted into a Janis PTCM-4-7 closed-cycle optical
cryostat that allowed maintaining the sample’s temperature in the range from 4 to 320 K. The optical excitation of nonequilibrium charge carriers in the structures was carried out through a fused-quartz window by a continuous wave radiation of a solid-state diodepumped laser (with the wavelength of λ = 532 nm and the average output power of P = 8 mW). The photoluminescence (PL) spectra in the terahertz spectral range were studied using a Bruker Vertex 80v vacuum Fourier transform infrared spectrometer operating in a step-scan mode. The output window of the optical cryostat was made of polymethylpentene, the entrance window of the spectrometer was made from polyethylene. These materials have a high degree of transparency in the terahertz spectral range. The PL radiation of the sample was collected by an off-axis parabolic mirror of the Fourier spectrometer through a black polyethylene filter that prevented the penetration of scattered pumping radiation into the measurement section of the experimental setup. A liquid helium cooled silicon bolometer, which had a vacuum contact with the spectrometer, was used as a detector of terahertz radiation. The signal of the bolometer photoresponse was measured by an SR830 lock-in amplifier which was synchronized with the pump laser. Laser radiation was modulated by a chopper at a frequency of 87 Hz with a duty cycle of 50%. We used two configurations of the optical system of the Fourier spectrometer to obtain the terahertz PL spectra. The first one consisted of a combination of a 0.5mm-thick polyethylene filter at the entrance of the silicon bolometer and a 6-μm-thick multilayer Mylar beam splitter. This optical configuration allowed to perform measurements in the photon energy range from 4 to 40 meV. The second configuration included a filter of crystalline quartz on the bolometer and a 25-μm-thick Mylar beam splitter; this configuration allowed to increase the optical transmission of the Fourier spectrometer in the photon energy range from 2 to 14 meV. The PL spectra in the near infrared spectral range were measured by a Horiba Jobin Yvon FHR 640 grating monochromator with a 1200 groves/mm holographic grating. The fused silica exit window of the closed-cycle cryostat, was used for measuring the PL spectra in the near-IR range. The PL radiation passed through a red optical filter, transparent in the wavelength range 0.68–2.50 μm and stopping the scattered pump radiation, and was focused by a lens onto the entrance slit of the grating monochromator. The
I.S. Makhov et al. / St. Petersburg Polytechnical University Journal: Physics and Mathematics 2 (2016) 281–286
Fig. 1. Terahertz photoluminescence spectra of the GaAs/AlGaAs sample with narrow QWs (1) and the semi-insulating GaAs substrate (2), measured at 4.2 K.
signal was detected by a silicon CCD array, cooled with liquid nitrogen. Results and discussion The terahertz PL spectra, detected for the structure with narrow doped QWs and a semi-insulating GaAs substrate, are shown in Fig. 1. It can be seen that both spectra exhibit the same emission band in the photon energy range from 18 to 27 meV. It can be therefore concluded that this band is not associated with the presence of doped QWs in the structure under investigation. This band can be caused by residual impurities in the substrate or in bulk layers of the structure with QWs. Carbon is one of such impurities that has a binding energy of 20 meV and can emerge in the process of growing bulk gallium arsenide by the Czochralski method [12] or in the process of growing epitaxial layers by molecular beam epitaxy [13]. The difference in the width of terahertz radiation bands (the band is wider for spectrum 1 in Fig. 1) and the absence of fine band structure in spectrum 1 (see. Fig. 1) are due to the lower spectral resolution of the PL spectrum of the structure with narrow QWs (spectrum 1 in Fig. 1). The fine line structure for spectrum 2 is associated with artifact interference in the black polyethylene filter. The interference period is equal to about 1.86 meV, which corresponds to the actual thickness of polyethylene (about 100 μm). The emission band in the photon energy range of 8–16 meV with a maximum near 12 meV is observed only in the terahertz PL spectrum of the sample with narrow QWs, and, conversely, is not observed in the emission spectrum of the GaAs substrate (see Fig. 1).
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This indicates that the emission band can be caused by the presence of QWs in the nanostructure under investigation. The binding energy of the silicon donor impurity in narrow QWs can be estimated from the theoretical calculation of the QW energy spectrum taking into account the impurity states [14]. According to our estimation, the binding energy of the donor impurity for a 16.1-nm wide QW is, about 10 meV. Therefore, the emission band with a maximum near the photon energy of 12 meV may be associated with radiative transitions of nonequilibrium electrons from the first quantum-confinement subband of electrons e1 to the ground state of the ionized donor impurity 1s (indicated by the arrow in Fig. 1). The spectral position of this emission band is in a good agreement with our estimations for the ionization energy of the impurity in the narrow QW of the studied sample, amounting to about 10 meV. The emission band in question is considerably wide, possibly because of the low resolution of the measured terahertz PL spectrum, as well as its inhomogeneous broadening due to inhomogeneous distribution of a substantial amount of impurities in the QWs. In accordance with the above-described mechanism of radiation, the presence of e1 → 1s transitions, discovered in the terahertz PL spectrum of the sample with narrow QWs (see Fig. 1), should be confirmed by the behavior of interband PL spectra related to radiative recombination of nonequilibrium electrons and holes through the ground donor state in the QW. The experimental PL spectra in the near-IR range for the sample with narrow GaAs/AlGaAs QWs are shown in Fig. 2. The calculated value for the energy of the e1 → hh1 interband transition is marked by the arrow at the bottom in Fig. 2. Usually, the interband PL spectra of doped nanostructures (if the spectra are obtained at low lattice temperature and low pumping levels) exhibit radiation peaks related to either radiative recombination of heavy or light-hole excitons, or to radiative recombination of impurity-bound excitons, or to radiative recombination of electron-hole pairs through impurity states (see, for example, Refs. [15–17]). The position of the emission peak at a photon energy of 1.528 eV in Fig. 2 differs from the calculated value of the e1 → hh1 optical transition energy by 7 meV. We assume that this peak is due to radiative recombination of heavy-hole free excitons formed in the ground subbands (the Xe1 → hh1 transition in Fig. 2). The binding energy of a heavy-hole free exciton in bulk GaAs amounts to about 4.2 meV [18],
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Fig. 2. Near-infrared PL spectra for the sample with narrow GaAs/AlGaAs QWs (T = 4.2 K), measured with different integration times: 1 s (curve 1) and 10 s (2); the scale of the curves on the vertical axis is different. Arrows indicate the positions of the potential optical transitions; the calculated energy for the e1 → hh1 transition is also marked by an arrow; M is the matrix.
and the exciton binding energy for sufficiently narrow QWs must be higher than the bulk energy [15]. We similarly detected radiative transitions of heavy- and light-hole free excitons bound to the corresponding subbands. These optical transitions are also indicated by Xe1 → lh1, Xe2 → hh2, Xe2 → lh2, Xe3 → hh3 arrows in Fig. 2. The emission peak at a photon energy of 1.5257 eV differs in energy from the Xe1 → hh1 transition by 2.1 meV and is associated with radiative recombination of donor bound excitons. According to the data from Ref. [15], the binding energy of a donor bound exciton is 2 meV for 20-nm-wide QWs, which agrees well with our results. A shoulder observed in the PL spectrum near the 1.523 eV photon energy (see Fig. 2), is positioned 12 meV away from the calculated value of the e1 → hh1 radiative transition and is associated with radiative recombination of nonequilibrium electrons and holes via the ground state of the Si donor impurity in narrow QWs of the sample under investigation (the 1s → hh1 transition in Fig. 2). Recall that an emission band with a peak near the photon energy of 12 meV, associated with the radiative transitions of nonequilibrium electrons between the ground electron subband e1 and the ground donor state 1s, was observed in the terahertz PL spectrum of the sample with narrow QWs (see Fig. 1). In addition, the energy of the donor impurity in a 20-nm-thick GaAs/AlGaAs QW amounted to 11.6 meV [15], which is also in a good agreement with our results.
Fig. 3. Terahertz PL spectra of the sample with wide GaAs/AlGaAs QWs, measured at 4.4 K (1) and 10 K (2). Arrows indicate the calculated energies of the optical electron transitions.
A wide emission band in the photon energy range from 1.485 to 1.510 eV, marked as M (matrix) in Fig. 2, may be associated with the band → impurity optical transitions in the bulk layers of the structure. Thus, a line related to radiative recombination of nonequilibrium electrons and holes through the ground donor state (the 1s → hh1 transition in Fig. 2) was detected in the interband PL spectra for the sample with narrow doped QWs. The terahertz PL spectrum of the same sample exhibited an emission band caused by optical transitions of electrons between the first electron subband and the ground donor state (the e1 → 1s transition in Fig. 1). The second sample with QWs had the same QW doping level; however its doped QWs were wider. Increasing the QW’s width should lead to a reducing the binding energy of the donor impurity in the QW [19,20]. This should in turn affect the terahertz PL spectra shifting the emission peak related to optical electron transitions to the donor impurity in the QWs towards the long-wave region. The terahertz PL spectra for the sample with wide doped QWs, measured at different crystal lattice temperatures, are shown in Fig. 3. The second configuration of the optical system of the Fourier spectrometer, described above, was used in these measurements. A PL peak corresponding to the photon energy of 8 meV can be seen in the graph. The position of this peak in the spectrum is in a good agreement with the results of calculation of the QW energy spectrum taking into account the presence of impurity states [10]. This peak may be associated with an optical transition of
I.S. Makhov et al. / St. Petersburg Polytechnical University Journal: Physics and Mathematics 2 (2016) 281–286
Fig. 4. Near-infrared PL spectrum of the structure with wide GaAs/AlGaAs QWs (T = 4.2 K). Arrows indicate the positions of the presumed optical transitions.
electrons from the first quantum-confinement subband to the donor ground state (e1 → 1s, the photon energy of 8.7 meV), as well as with the 2pxy → 1s intracenter transition (6.6 meV). The calculated energies of these transitions are indicated by arrows in Fig. 3. The results we obtained are also in a good agreement with the photoconductivity spectra under excitation by p- or s-polarized light in a structure similar to the one we examined, also containing wide doped QWs [10], where a broad absorption line associated with the 1s → e1 and 1s → 2pxy transitions was also observed. Comparing the terahertz PL spectra obtained for the samples with the narrow and the wide wells one can see that the emission band radiation caused by impurity transitions of nonequilibrium electrons in the QW shifted to longer wavelengths with an increase in the width of the QW, which is precisely what we have expected. It can be seen from comparing the terahertz PL spectra obtained for the sample with wide doped QWs at two temperatures (see Fig. 3) that the intensity of terahertz PL decreases with the temperature increase. This may occur because the probability that an electron is trapped by the ionized donor also decreases. This behavior of terahertz luminescence with increasing temperature has already been observed in bulk semiconductors by the authors of Ref. [21]. To confirm the suggested mechanism of terahertz emission involving impurity states in QWs, we measured the interband PL spectrum (Fig. 4), the same as for the sample with the narrow QWs. The arrows in Fig. 4 indicate the spectral peculiarities which may
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be associated with radiative recombination of heavyhole free excitons bound to the ground electron and hole subbands (the Xe1 → hh1 transition in Fig. 4), as well as with radiative recombination of donor bound excitons (the Si → X transition in Fig. 4). The abovedescribed spectral peculiarities were identified based on the experimental data and the calculation of the energy spectrum, and on comparing the estimations of the impurity and the exciton binding energies in the QWs. The broad emission band in the photon energy range from 1.48 to 1.51 eV, marked as M (matrix), may be associated with the band → impurity optical transitions in the bulk layers of the structure. This band is also observed in the interband PL spectra for the structure with narrow doped QWs (see Fig. 2). The emission line at the photon energy of 1.528 eV can be attributed to radiative recombination of nonequilibrium electrons and holes via the ground impurity state (the 1s → hh1 transition in Fig. 4), since it differs from the calculated value of the e1 → hh1 radiative transition by 8 meV. This result is in a good agreement with the calculated electron energy spectrum taking into account the impurity states [10], as well as with the results of the analysis of terahertz PL spectra of the sample with wide QWs (see Fig. 3). Conclusion The mechanism of terahertz emission under interband photoexcitation of nonequilibrium charge carriers in GaAs/AlGaAs quantum wells of different widths doped by silicon donor impurity is discussed. This mechanism is supported by the experimental results obtained for interband photoluminescence spectra of the investigated samples. These spectra were analyzed in detail and compared with the results of the theoretical calculation we performed, as well as with the data from the literature. It was confirmed that tuning the terahertz radiation wavelength in nanostructures is possible by changing the width of the doped QWs. The study was supported by a RFBR grant no. 16-32-60085, a grant of the President of the Russian Federation for young Candidates of sciences MK6064.2016.2, and by the Ministry of Education and Science of the Russian Federation (state assignment). References [1] K. Humphreys, J.P. Loughran, M. Gradziel, et al., Medical applications of terahertz imaging: A review of current technology and potential applications in biomedical engineering, Conf. Proc. IEEE Eng. Med. Biol. Soc. 1 (2004) 1302–1305.
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[2] C.A. Duque, A. Tiutiunnyk, A.L. Morales, et al., Terahertz – Yesterday, today, and tomorrow, Momento 51E (2016) 103–116. [3] A.A. Goryachuk, M.K. Khodzitsky, M.A. Borovkova, et al., The optical properties and spectral features of malignant skin melanocytes in the terahertz frequency range, J. Phys. Conf. Ser. 741 (2016) 012072;. [4] S. Fathololoumi, E. Dupont, C.W.I. Chan, et al., Terahertz quantum cascade lasers operating up to ∼200 K with optimized oscillator strength and improved injection tunneling, Opt. Express 20 (4) (2012) 3866–3876. [5] Y.P. Gousev, I.V. Altukhov, K.A. Korolev, et al., Widely tunable continuous-wave THz laser, Appl. Phys. Lett. 75 (6) (1999) 757–759. [6] V.A. Shalygin, L.E. Vorobjev, D.A. Firsov, et al., Terahertz luminescence in strained GaAsN:Be layers under strong electric fields, Appl. Phys. Lett. 90 (16) (2007) 161128. [7] K.A. Kovalevsky, R.K. Zhukavin, V.V. Tsyplenkov, et al., Shallow-donor lasers in uniaxially stressed silicon, Semiconductors 47 (2) (2013) 235–241. [8] A.O. Zakhar’in, A.V. Bobylev, A.V. Andrianov, Terahertz emission upon the interband excitation of GaN layers, Semiconductors 46 (9) (2012) 1135–1139. [9] A.V. Andrianov, A.O. Zakhar’in, Y.L. Ivanov, M.S. Kipa, Terahertz impurity luminescence under the interband photoexcitation of semiconductors, JETP Lett. 91 (2) (2010) 96–99. [10] D.A. Firsov, V.A. Shalygin, V.Y. Panevin, et al., Terahertz emission and photoconductivity in n-type GaAs/AlGaAs quantum wells: the role of resonant impurity states, Semiconductors 44 (11) (2010) 1394–1397. [11] D.A. Firsov, L.E. Vorobjev, V.Y. Panevin, et al., Terahertz radiation associated with the impurity electron transition in quantum wells upon optical and electrical pumping, Semiconductors 49 (1) (2015) 28–32.
[12] B. Wakefield, P.A. Leigh, M.H. Lyons, C.R. Elliot, Characterization of semi-insulating liquid encapsulated Czochralski GaAs by cathodoluminescence, Appl. Phys. Lett. 45 (1) (1984) 66–68. [13] R. Dingle, C. Weisbuch, H.L. Stormer, H. Morkoc, et al., Characterization of high purity GaAs grown by molecular beam epitaxy, Appl. Phys. Lett. 40 (6) (1982) 507–509. [14] M. Helm, F.M. Peeters, F. DeRosa, et al., Far-infrared spectroscopy of minibands and confined donors in GaAs/AlxGa(1−x)As superlattices, Phys. Rev. B 43 (17) (1991) 13983–13991. ˇ [15] J. Kundrotas, A. Cerškus , G. Valušis, et al., Impurity-related photoluminescence line shape asymmetry in GaAs/AlAs multiple quantum wells: Fractional-dimensional space approach, J. Appl. Phys. 107 (7) (2010) 093109. ˇ [16] J. Kundrotas, A. Cerškus , V. Nargelien, et al., Enhanced exciton photoluminescence in the selectively Si doped GaAs/AlxGa(1−x)As heterostructures, J. Appl. Phys. 108 (6) (2010) 063522. [17] A. Tabata, A. Levine, A. Marti Ceschin, et al., Optical properties of GaAs/AlGaAs selectively doped quantum well structures, Radiat. Eff. Defects Solids Inc. Plasma Sci. Plasma Technol. 146 (1–4) (1998) 207–214. [18] S.B. Nam, D.C. Reynolds, C.W. Litton, et al., Free-exciton energy spectrum in GaAs, Phys. Rev. B 13 (2) (1976) 761–767. [19] R.L. Greene, K.K. Bajaj, Energy levels of hydrogenic impurity states in GaAs/Ga1−x Alx As quantum well structures, Solid State Commun. 45 (9) (1983) 825–829. [20] S. Chaudhuri, Hydrogenic-impurity ground state in GaAs–Ga1−x Alx As multiple-quantum-well structures, Phys. Rev. B 28 (8) (1983) 4480–4488. [21] S.G. Pavlov, H.-W. Hübers, M.H. Rümmeli, et al., Far-infrared stimulated emission from optically excited bismuth donors in silicon, Appl. Phys. Lett. 80 (25) (2002) 4717–4719.
St. Petersburg Polytechnical University Journal: Physics and Mathematics 2 (2016) 281–286 www.elsevier.com/locate/spjpm
Impurity-assisted terahertz luminescence in quantum well nanostructures under interband photoexсitation Ivan S. Makhov, Vadim Yu. Panevin∗, Maxim Ya. Vinnichenko, Anton N. Sofronov, Dmitry A. Firsov, Leonid E. Vorobjev Peter the Great St. Petersburg Polytechnic University, 29 Politekhnicheskaya St., St. Petersburg 195251, Russian Federation Available online 17 November 2016
Abstract The paper presents the results of an experimental study of impurity-assisted photoluminescence in the far- (terahertz) and near-infrared spectral ranges in n-GaAs/AlGaAs quantum well structures with different well widths under interband photoexcitation of electron–hole pairs. The optical electron transitions between the first electron subband and donor ground state as well as between excited and ground donor states were revealed in the far-infrared photoluminescence spectra. Observation of these optical electron transitions became possible because of the depopulation of the donor ground state in the quantum well due to the non-equilibrium charge carrier radiative transitions from the donor ground state to the first heavy hole subband. The opportunity to tune the terahertz radiation wavelength in structures with doped quantum wells by changing the quantum well width was demonstrated experimentally. Copyright © 2016, St. Petersburg Polytechnic University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Keywords: Terahertz luminescence; Radiation; Quantum well; Spectrum; Nanostructure; Semiconductor.
Introduction The task of developing effective semiconductor sources of terahertz radiation (the wavelength range of electromagnetic radiation is 30–300 μm) is rather important at present as these devices can be used in diverse areas of science and technology, such as medicine, environmental monitoring, security systems,
∗
Corresponding author. E-mail addresses: [email protected] (I.S. Makhov), [email protected] (V.Yu. Panevin), [email protected] (M.Ya. Vinnichenko), [email protected] (A.N. Sofronov), [email protected] (D.A. Firsov), [email protected] (L.E. Vorobjev).
and computer science (see, for example, Refs. [1–3]). One of the most promising mechanisms for generating terahertz radiation is based on optical transitions of nonequilibrium charge carriers involving impurity states in semiconductors and semiconductor nanostructures. This mechanism is an alternative to the quantum cascade laser [4], since fabricating the latter requires very sophisticated techniques of high-quality growth of semiconductor nanostructures. There are currently several known mechanisms for generating terahertz radiation, based on impurityassisted transitions of charge carriers in semiconductors and semiconductor nanostructures. For example, terahertz radiation was observed during optical transitions of nonequilibrium charge carriers involving
http://dx.doi.org/10.1016/j.spjpm.2016.11.006 2405-7223/Copyright © 2016, St. Petersburg Polytechnic University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) (Peer review under responsibility of St. Petersburg Polytechnic University).
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impurity resonance states under impurity breakdown in electric field in mechanically strained p-Ge [5] and in GaAs/GaAsN:Be microstructures with built-in stresses [6]. Additionally, terahertz radiation was observed from bulk silicon doped with various impurities under intraband optical excitation of charge carriers [7]. Terahertz radiation under interband photoexcitation was observed in doped bulk semiconductors such as GaN [8], GaAs and Ge [9]. There are few studies examining terahertz radiation from nanostructures with doped quantum wells (QWs). For example, terahertz radiation in longitudinal electric fields was observed in GaAs/AlGaAs quantum wells doped with donor [10] and acceptor [11] impurities. Terahertz radiation from nanostructures with doped QWs under interband optical pumping was first described in Ref. [11]. This type of pumping entails the generation of electron–hole pairs that are subsequently trapped in the QW. At low crystal lattice temperatures, donor impurities in the QWs are neutral. Electrons from donor ground states can recombine with nonequilibrium holes, which is usually accompanied by the emission of near-infrared photons. The impurity ground states depopulated as a result of this process can be filled with nonequilibrium electrons from the first subband of size quantization. This can occur with an emission of photons of the terahertz range. This study continues our previous studies on the subject [11] and is dedicated to examining radiation of the terahertz and near-IR ranges in nanostructures with donor-doped QWs of different widths. Samples and experimental procedure Optical studies were carried out for three samples. Two of them were grown by molecular-beam epitaxy on a semi-insulating gallium arsenide substrate and contained doped GaAs/AlGaAs QWs of different widths. The first sample contained 226 periods of GaAs QW 16.1 nm in width, separated by 4.8nm-thick Al0.15 Ga0.85 As barriers. The second sample contained 50 periods of GaAs QWs 30 nm in width, separated by 7-nm-thick Al0.30 Ga0.70 As barriers. Structures with narrow and wide QWs had GaAs cap layers 60 and 20 nm thick, respectively. The QWs in both structures were doped with silicon (acting as a donor) with a surface concentration ns = 3·1010 cm–2 . A semiinsulating GaAs substrate, similar to those on which the nanostructures with doped QWs were grown, was used as the third reference sample. During optical measurements, the samples were mounted into a Janis PTCM-4-7 closed-cycle optical
cryostat that allowed maintaining the sample’s temperature in the range from 4 to 320 K. The optical excitation of nonequilibrium charge carriers in the structures was carried out through a fused-quartz window by a continuous wave radiation of a solid-state diodepumped laser (with the wavelength of λ = 532 nm and the average output power of P = 8 mW). The photoluminescence (PL) spectra in the terahertz spectral range were studied using a Bruker Vertex 80v vacuum Fourier transform infrared spectrometer operating in a step-scan mode. The output window of the optical cryostat was made of polymethylpentene, the entrance window of the spectrometer was made from polyethylene. These materials have a high degree of transparency in the terahertz spectral range. The PL radiation of the sample was collected by an off-axis parabolic mirror of the Fourier spectrometer through a black polyethylene filter that prevented the penetration of scattered pumping radiation into the measurement section of the experimental setup. A liquid helium cooled silicon bolometer, which had a vacuum contact with the spectrometer, was used as a detector of terahertz radiation. The signal of the bolometer photoresponse was measured by an SR830 lock-in amplifier which was synchronized with the pump laser. Laser radiation was modulated by a chopper at a frequency of 87 Hz with a duty cycle of 50%. We used two configurations of the optical system of the Fourier spectrometer to obtain the terahertz PL spectra. The first one consisted of a combination of a 0.5mm-thick polyethylene filter at the entrance of the silicon bolometer and a 6-μm-thick multilayer Mylar beam splitter. This optical configuration allowed to perform measurements in the photon energy range from 4 to 40 meV. The second configuration included a filter of crystalline quartz on the bolometer and a 25-μm-thick Mylar beam splitter; this configuration allowed to increase the optical transmission of the Fourier spectrometer in the photon energy range from 2 to 14 meV. The PL spectra in the near infrared spectral range were measured by a Horiba Jobin Yvon FHR 640 grating monochromator with a 1200 groves/mm holographic grating. The fused silica exit window of the closed-cycle cryostat, was used for measuring the PL spectra in the near-IR range. The PL radiation passed through a red optical filter, transparent in the wavelength range 0.68–2.50 μm and stopping the scattered pump radiation, and was focused by a lens onto the entrance slit of the grating monochromator. The
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Fig. 1. Terahertz photoluminescence spectra of the GaAs/AlGaAs sample with narrow QWs (1) and the semi-insulating GaAs substrate (2), measured at 4.2 K.
signal was detected by a silicon CCD array, cooled with liquid nitrogen. Results and discussion The terahertz PL spectra, detected for the structure with narrow doped QWs and a semi-insulating GaAs substrate, are shown in Fig. 1. It can be seen that both spectra exhibit the same emission band in the photon energy range from 18 to 27 meV. It can be therefore concluded that this band is not associated with the presence of doped QWs in the structure under investigation. This band can be caused by residual impurities in the substrate or in bulk layers of the structure with QWs. Carbon is one of such impurities that has a binding energy of 20 meV and can emerge in the process of growing bulk gallium arsenide by the Czochralski method [12] or in the process of growing epitaxial layers by molecular beam epitaxy [13]. The difference in the width of terahertz radiation bands (the band is wider for spectrum 1 in Fig. 1) and the absence of fine band structure in spectrum 1 (see. Fig. 1) are due to the lower spectral resolution of the PL spectrum of the structure with narrow QWs (spectrum 1 in Fig. 1). The fine line structure for spectrum 2 is associated with artifact interference in the black polyethylene filter. The interference period is equal to about 1.86 meV, which corresponds to the actual thickness of polyethylene (about 100 μm). The emission band in the photon energy range of 8–16 meV with a maximum near 12 meV is observed only in the terahertz PL spectrum of the sample with narrow QWs, and, conversely, is not observed in the emission spectrum of the GaAs substrate (see Fig. 1).
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This indicates that the emission band can be caused by the presence of QWs in the nanostructure under investigation. The binding energy of the silicon donor impurity in narrow QWs can be estimated from the theoretical calculation of the QW energy spectrum taking into account the impurity states [14]. According to our estimation, the binding energy of the donor impurity for a 16.1-nm wide QW is, about 10 meV. Therefore, the emission band with a maximum near the photon energy of 12 meV may be associated with radiative transitions of nonequilibrium electrons from the first quantum-confinement subband of electrons e1 to the ground state of the ionized donor impurity 1s (indicated by the arrow in Fig. 1). The spectral position of this emission band is in a good agreement with our estimations for the ionization energy of the impurity in the narrow QW of the studied sample, amounting to about 10 meV. The emission band in question is considerably wide, possibly because of the low resolution of the measured terahertz PL spectrum, as well as its inhomogeneous broadening due to inhomogeneous distribution of a substantial amount of impurities in the QWs. In accordance with the above-described mechanism of radiation, the presence of e1 → 1s transitions, discovered in the terahertz PL spectrum of the sample with narrow QWs (see Fig. 1), should be confirmed by the behavior of interband PL spectra related to radiative recombination of nonequilibrium electrons and holes through the ground donor state in the QW. The experimental PL spectra in the near-IR range for the sample with narrow GaAs/AlGaAs QWs are shown in Fig. 2. The calculated value for the energy of the e1 → hh1 interband transition is marked by the arrow at the bottom in Fig. 2. Usually, the interband PL spectra of doped nanostructures (if the spectra are obtained at low lattice temperature and low pumping levels) exhibit radiation peaks related to either radiative recombination of heavy or light-hole excitons, or to radiative recombination of impurity-bound excitons, or to radiative recombination of electron-hole pairs through impurity states (see, for example, Refs. [15–17]). The position of the emission peak at a photon energy of 1.528 eV in Fig. 2 differs from the calculated value of the e1 → hh1 optical transition energy by 7 meV. We assume that this peak is due to radiative recombination of heavy-hole free excitons formed in the ground subbands (the Xe1 → hh1 transition in Fig. 2). The binding energy of a heavy-hole free exciton in bulk GaAs amounts to about 4.2 meV [18],
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Fig. 2. Near-infrared PL spectra for the sample with narrow GaAs/AlGaAs QWs (T = 4.2 K), measured with different integration times: 1 s (curve 1) and 10 s (2); the scale of the curves on the vertical axis is different. Arrows indicate the positions of the potential optical transitions; the calculated energy for the e1 → hh1 transition is also marked by an arrow; M is the matrix.
and the exciton binding energy for sufficiently narrow QWs must be higher than the bulk energy [15]. We similarly detected radiative transitions of heavy- and light-hole free excitons bound to the corresponding subbands. These optical transitions are also indicated by Xe1 → lh1, Xe2 → hh2, Xe2 → lh2, Xe3 → hh3 arrows in Fig. 2. The emission peak at a photon energy of 1.5257 eV differs in energy from the Xe1 → hh1 transition by 2.1 meV and is associated with radiative recombination of donor bound excitons. According to the data from Ref. [15], the binding energy of a donor bound exciton is 2 meV for 20-nm-wide QWs, which agrees well with our results. A shoulder observed in the PL spectrum near the 1.523 eV photon energy (see Fig. 2), is positioned 12 meV away from the calculated value of the e1 → hh1 radiative transition and is associated with radiative recombination of nonequilibrium electrons and holes via the ground state of the Si donor impurity in narrow QWs of the sample under investigation (the 1s → hh1 transition in Fig. 2). Recall that an emission band with a peak near the photon energy of 12 meV, associated with the radiative transitions of nonequilibrium electrons between the ground electron subband e1 and the ground donor state 1s, was observed in the terahertz PL spectrum of the sample with narrow QWs (see Fig. 1). In addition, the energy of the donor impurity in a 20-nm-thick GaAs/AlGaAs QW amounted to 11.6 meV [15], which is also in a good agreement with our results.
Fig. 3. Terahertz PL spectra of the sample with wide GaAs/AlGaAs QWs, measured at 4.4 K (1) and 10 K (2). Arrows indicate the calculated energies of the optical electron transitions.
A wide emission band in the photon energy range from 1.485 to 1.510 eV, marked as M (matrix) in Fig. 2, may be associated with the band → impurity optical transitions in the bulk layers of the structure. Thus, a line related to radiative recombination of nonequilibrium electrons and holes through the ground donor state (the 1s → hh1 transition in Fig. 2) was detected in the interband PL spectra for the sample with narrow doped QWs. The terahertz PL spectrum of the same sample exhibited an emission band caused by optical transitions of electrons between the first electron subband and the ground donor state (the e1 → 1s transition in Fig. 1). The second sample with QWs had the same QW doping level; however its doped QWs were wider. Increasing the QW’s width should lead to a reducing the binding energy of the donor impurity in the QW [19,20]. This should in turn affect the terahertz PL spectra shifting the emission peak related to optical electron transitions to the donor impurity in the QWs towards the long-wave region. The terahertz PL spectra for the sample with wide doped QWs, measured at different crystal lattice temperatures, are shown in Fig. 3. The second configuration of the optical system of the Fourier spectrometer, described above, was used in these measurements. A PL peak corresponding to the photon energy of 8 meV can be seen in the graph. The position of this peak in the spectrum is in a good agreement with the results of calculation of the QW energy spectrum taking into account the presence of impurity states [10]. This peak may be associated with an optical transition of
I.S. Makhov et al. / St. Petersburg Polytechnical University Journal: Physics and Mathematics 2 (2016) 281–286
Fig. 4. Near-infrared PL spectrum of the structure with wide GaAs/AlGaAs QWs (T = 4.2 K). Arrows indicate the positions of the presumed optical transitions.
electrons from the first quantum-confinement subband to the donor ground state (e1 → 1s, the photon energy of 8.7 meV), as well as with the 2pxy → 1s intracenter transition (6.6 meV). The calculated energies of these transitions are indicated by arrows in Fig. 3. The results we obtained are also in a good agreement with the photoconductivity spectra under excitation by p- or s-polarized light in a structure similar to the one we examined, also containing wide doped QWs [10], where a broad absorption line associated with the 1s → e1 and 1s → 2pxy transitions was also observed. Comparing the terahertz PL spectra obtained for the samples with the narrow and the wide wells one can see that the emission band radiation caused by impurity transitions of nonequilibrium electrons in the QW shifted to longer wavelengths with an increase in the width of the QW, which is precisely what we have expected. It can be seen from comparing the terahertz PL spectra obtained for the sample with wide doped QWs at two temperatures (see Fig. 3) that the intensity of terahertz PL decreases with the temperature increase. This may occur because the probability that an electron is trapped by the ionized donor also decreases. This behavior of terahertz luminescence with increasing temperature has already been observed in bulk semiconductors by the authors of Ref. [21]. To confirm the suggested mechanism of terahertz emission involving impurity states in QWs, we measured the interband PL spectrum (Fig. 4), the same as for the sample with the narrow QWs. The arrows in Fig. 4 indicate the spectral peculiarities which may
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be associated with radiative recombination of heavyhole free excitons bound to the ground electron and hole subbands (the Xe1 → hh1 transition in Fig. 4), as well as with radiative recombination of donor bound excitons (the Si → X transition in Fig. 4). The abovedescribed spectral peculiarities were identified based on the experimental data and the calculation of the energy spectrum, and on comparing the estimations of the impurity and the exciton binding energies in the QWs. The broad emission band in the photon energy range from 1.48 to 1.51 eV, marked as M (matrix), may be associated with the band → impurity optical transitions in the bulk layers of the structure. This band is also observed in the interband PL spectra for the structure with narrow doped QWs (see Fig. 2). The emission line at the photon energy of 1.528 eV can be attributed to radiative recombination of nonequilibrium electrons and holes via the ground impurity state (the 1s → hh1 transition in Fig. 4), since it differs from the calculated value of the e1 → hh1 radiative transition by 8 meV. This result is in a good agreement with the calculated electron energy spectrum taking into account the impurity states [10], as well as with the results of the analysis of terahertz PL spectra of the sample with wide QWs (see Fig. 3). Conclusion The mechanism of terahertz emission under interband photoexcitation of nonequilibrium charge carriers in GaAs/AlGaAs quantum wells of different widths doped by silicon donor impurity is discussed. This mechanism is supported by the experimental results obtained for interband photoluminescence spectra of the investigated samples. These spectra were analyzed in detail and compared with the results of the theoretical calculation we performed, as well as with the data from the literature. It was confirmed that tuning the terahertz radiation wavelength in nanostructures is possible by changing the width of the doped QWs. The study was supported by a RFBR grant no. 16-32-60085, a grant of the President of the Russian Federation for young Candidates of sciences MK6064.2016.2, and by the Ministry of Education and Science of the Russian Federation (state assignment). References [1] K. Humphreys, J.P. Loughran, M. Gradziel, et al., Medical applications of terahertz imaging: A review of current technology and potential applications in biomedical engineering, Conf. Proc. IEEE Eng. Med. Biol. Soc. 1 (2004) 1302–1305.
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