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SiGe quantum wells
ARTICLE IN PRESS Physica E 41 (2009) 972–975
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Direct gap related optical transitions in Ge/SiGe quantum wells M. Bonfanti a,, E. Grilli a, M. Guzzi a, D. Chrastina b, G. Isella b, H. von Ka¨nel b, H. Sigg c a
` degli Studi di Milano-Bicocca, Via Cozzi 53, I-20125 Milano, Italy CNISM and L-NESS, Dipartimento di Scienza dei Materiali, Universita CNISM and L-NESS, Dipartimento di Fisica del Politecnico di Milano, Polo di Como, Via Anzani 42, I-22100 Como, Italy c Laboratory for Micro and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland b
a r t i c l e in f o
a b s t r a c t
Available online 28 August 2008
An experimental study of the direct-gap related optical transitions in strain-compensated Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) is presented. These structures are of particular interest due to the proximity of G-type and L-type conduction band states and due to their type I band alignment. The samples were grown by low-energy plasma-enhanced CVD and consist of Ge MQWs with a large numbers of periods and good morphological quality grown onto thick graded Si1x Gex buffer layers. The transmission spectra, which shows clear evidence of excitonic transitions, are studied as a function of temperature in the 5–300 K range. Preliminary results of photocurrent measurements performed on the same structures using metal–semiconductor–metal contact are discussed. & 2008 Elsevier B.V. All rights reserved.
PACS: 73.21.Fg 78.67.De 72.40.þw Keywords: Quantum wells SiGe Absorption Photocurrent
1. Introduction Recent progresses in Si photonics, such as the demonstration of photonic bandgap structures from Si [1], the discussion of gain in Si-nanocrystal [2], and the achievement of the Si Raman laser [3] suggest that Si-based materials could provide a strong support for the technological integration of optical functions in CMOS microelectronics. In this perspective SiGe alloys are of particular interest because they can be epitaxially grown on Si substrates and they are compatible with a large number of standard Si processes and CMOS technology [4]. Moreover, bulk mobilities of SiGe alloys with high Ge concentration are higher than those of Si for both electrons and holes [5], and strain and composition facilitate device design by band gap and band alignment engineering. Finally, SiGe technology allows the integration on Si substrates of high speed electronic devices, such as modulation doped field effect transistors, high electron mobility transistors and heterojunction bipolar transistors [4]. In Ge, unlike in Si, the direct gap at the G-point is not far above the indirect fundamental gap [6] and, moreover, the direct gap energy is in the range of wavelengths used in telecommunications. If it is possible to exploit the direct gap features, it will become possible to get a Si-based structure with optical properties very similar to those of III–V semiconductors. This fact has led to a number of proposals for Si compatible Ge based photonic Corresponding author.
E-mail address: [email protected] (M. Bonfanti). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.08.052
device applications. Photo detectors made from epitaxial Ge layers on Si substrates [7,8], optical modulators based on the Franz– Keldysh effect [9], and optical modulators based on the quantumconfined Stark effect in strained-Ge quantum wells (QWs) [10] have all been demonstrated. However most of the papers devoted to SiGe based heterostructures have studied systems grown on Si substrates using materials with low Ge mole fraction, typically xGe o0:3020:50 [11–16]. This was motivated by the difficulty in the growth of high quality Ge-rich SiGe alloys and heterostructures [4]. Recently, innovative epitaxial growth techniques, such as low-energy plasma-enhanced chemical vapor deposition (LEPECVD) [17] have made possible to grow on Si substrates high Ge content structures of exceptional quality. In this paper, the temperature dependence of transmission spectra as well as photocurrent (PC) spectra of Ge/SiGe multiple quantum wells (MQWs) with Ge-rich barriers are discussed.
2. Samples and experimental set-up Samples growth by LEPECVD started with a buffer graded from Si to Si0:1 Ge0:9 over a thickness of 13 mm and capped with 2 mm Si0:1 Ge0:9 layer on 100 mm Si(100) substrates with resistance above 1 O cm. This forms a fully relaxed virtual substrate (VS) for the nanostructured part of the samples, which consist of a ‘‘strainbalanced’’ structure formed by 50 Ge QWs with Si0:15 Ge0:85 barriers. The nominal well and barrier thickness (14.3 and 19.8 nm, respectively) are such that the mean composition of the nanostructured region is very close to that of the VS, in
ARTICLE IN PRESS M. Bonfanti et al. / Physica E 41 (2009) 972–975
order to get approximately zero net strain. X-Ray diffraction measurements indeed confirm the pseudomorphic growth of the MQWs structure on the VS. VSs having the same design, but without MQWs layers, were used as reference samples for the transmission measurements. The high deposition rate ð426 nm s1 Þ possible with the LEPECVD growth allowed us to get a limited sample growth time. A JASCO FT/IR-800 Fourier transform spectrometer equipped with an InGaAs detector has been used for optical transmission measurements at temperatures ranging from T ¼ 5 to 300 K. The optical density (O.D.) is defined as O.D. ðhnÞ ¼ log10 ½IS ðhnÞ=IR ðhnÞ, where IS ðhnÞ is the transmission spectrum of the sample and IR ðhnÞ that of the reference sample, both measured on samples with mirror polished back surface. The PC measurements were performed at room temperature and at T ¼ 2 K with an halogen light bulb and a Bruker Fourier transform spectrometer on metal–semiconductor–metal (MSM) structures grown on samples having the same nominal structure as those used in the transmission measurements. MSM photodetectors are important devices because of their high electrical bandwidth, high sensitivity and performance, ability to generate ultra-short electrical pulses, simple processing, and compatibility with large-scale planar integrated circuit technology [25–29]. The implementation of the MSM differs from that of other photodetectors in that it has a planar rather than a vertical device structure [30]. This is an advantage in applications where photodetectors must be integrated with amplifiers or light emitters. After removing the native oxide with hydrofluoric acid, 200 nm of Al were deposited on the top of the MQW layers in order to get a MSM structure. The MSM structure used in these experiments consists of 20 interdigitated fingers with length of 300 mm and width of 10 mm. The distance between two adjacent finger is 3 mm and thus the active area between adjacent fingers is 3 300 mm2 (inset in Fig. 4). The room temperature resistance of the MSM device is about 40 O. The PC spectra have been corrected for the spectrum of the halogen light bulb.
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show typical features due to confined excitons, much better defined than those reported for analogous structures [32,33], superimposed to the well-known staircase lineshape typical of the absorption coefficient of type I QWs. The peaks in the transmission spectra can be attributed to dipole allowed transitions between quantum-confined heavy hole (HHn) or light hole (LHn) valence states and conduction states (En) at the G-point; the relaxed Ge direct gap energy at T ¼ 5 K is also reported for reference. The structures present in the low temperature spectra are attributed to HHn–En or to LHn–En transitions on the basis of the calculations performed in Ref. [32] on an analogous structure; these attributions are reported in Fig. 1. The full-width at half-maximum (FWHM) of the HH1–E1 exciton line at T ¼ 5 K is about 7.6 meV, which is comparable with the width of excitonic absorption lines in GaAs/AlGaAs MQWs of similar thickness [24]. The FWHM of the HH1–E1 exciton absorption at room temperature is even narrower than that of analogous MQWs structures with 10 periods [32]. This means that, even if the number of periods is large, the structural uniformity is good and the effects of the interface roughness are limited. The enhanced absorption of the HH2–E2 transition can be justified as being due to an increase of the joint density of states caused by a nearly flat dispersion curve around the G-point [32]. The peak O.D. of the HH1–E1 transition is about 0.17; considering that only the MQWs absorb, an absorption coefficient of about 0:6 104 cm1 and an attenuation per well of about 0.78% can be obtained. These values reasonably compare with those typical of III–V direct gap QWs [31,34]. The HH1–E1 and the LH1–E1 exciton peaks have been also studied as a function of the temperature; the temperature dependence of the energy of the HH1–E1 exciton transition is reported in Fig. 2(a). The red-shift of the peak energy follows the red-shift of the direct energy gap of bulk Ge calculated
3. Optical measurements The transmission spectra of a 50 periods Ge/SiGe MQWs structure measured at T ¼ 300 and 5 K are shown in Fig. 1. They
Fig. 1. Transmission spectra of a 50 period sample measured at 300 K (dashed line) and at 5 K (full line). The arrows indicate the HHn–En and LHn–En transitions; the low temperature Ge direct energy gap is also reported for reference.
Fig. 2. (a) Temperature dependence of the energy peak of the first heavy-hole (full squares) transitions in 5–300 K temperature range; the solid line is the temperature dependence of the Ge direct gap calculated using the Varnish’s law with the parameters of Ref. [35]. (b) Energy difference (triangles) between lighthole and heavy-hole first transitions in the same temperature range.
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M. Bonfanti et al. / Physica E 41 (2009) 972–975
Fig. 3. Temperature dependence of the FWHM of the HH1–E1 exciton transition. The full line is the fit of Eq. (1) to the data with the parameters given in the text.
Fig. 4. Photocurrent spectra measured at T ¼ 300 and 2 K with a bias voltage of 0.2 V; the arrows indicate the HHn–En and LHn–En transitions on the basis of the calculations in Ref. [32]. In the inset, a scheme of the MSM structure (not to scale) is reported.
using the Varnish’s law with the parameters reported in Ref. [35]. Thus the temperature dependence of the MQWs optical gap in the 5–300 K temperature range is not markedly affected by the quantum-confinement effects. The difference between the LH1–E1 and the HH1–E1 exciton peak energy, reported in Fig. 2(b), slightly increases (by about 5 meV) when the temperature increases from 5 K to room temperature. This can be attributed to the thermal strain induced by the mismatch of the thermal expansion coefficients of Si substrate and Ge/SiGe MQWs. A lorentzian fit performed on the low energy part of the HH1–E1 transition provides the linewidth of the HH1–E1 exciton transition from low to room temperature. Increasing the temperature, the FWHM increases, as shown in Fig. 3. The relation
gðTÞ ¼ g0 þ aT þ b
1 e_o=K B T 1
(1)
has been proved to successfully describe the temperature broadening of the absorption and emission lines in III–V quantumconfined nanostructures [18–21]. In Eq. (1), g0 is the FWHM at T ¼ 0 K, the linear term aT is the acoustic phonons contribution, while the last term describes the contribution of longitudinal optical phonons. Data in Fig. 3 have been fitted to Eq. (1); the value _o ¼ 36 meV has been used, corresponding to the energy of degenerate LO and TO Ge phonons at the G-point. A good fit was obtained (see Fig. 3) with the following values of the fitting parameters: a ¼ 0:017 0:002 meV T1 and b ¼ 4:1 2:0 meV. However Kuo et al. [33] observed that the actual contribution to the width of the absorption edge or of the exciton peak at high temperature remains an open question. A detailed analysis of the temperature dependence of the FWHM of the HH1–E1 transition in Ge MQWs of different thickness is in progress in order to clarify this point.
4. Photocurrent measurements The PC spectra measured at room temperature and at T ¼ 2 K with a potential difference between fingers of 0.2 V are reported in Fig. 4. The spectra are characterized by structures whose energy agrees with that of the transmission spectra (see Fig. 1). The exciton contribution is clearly present also in the room temperature spectrum and it is particularly evident for the
Fig. 5. Photocurrent spectra measured T ¼ 2 K with bias voltage from 0.2 to 0.8 V; the spectra are normalized for the HH1-E1 excitonic peak signal. The inset show the decreasing of the FWHM of the excitonic peak and the slight blue-shift with increasing bias voltage.
HH1–E1 transition. In the low temperature PC spectrum peaks are clearly visible also at energies above the Si indirect gap; as before [32], these peaks can be attributed to HHn–En and LHn–En transitions with n ¼ 1; 2 and 3 (Fig. 4). Furthermore, the structure at 1.22 eV may be attributed to direct gap absorption of the Si0:1 Ge0:9 layer. In Fig. 5 PC spectra measured at 2 K with potential difference between the fingers ranging from 0.2 to 0.8 V are reported. The inset shows that the FWHM of the first exciton peak decreases, from 16 to 10 meV, with increasing the electric field. In fact, increasing the electric field, carriers are more efficiently drawn towards the contacts, the screening effect is reduced and the exciton peak sharpens [22]. This carrier density decrease is also supported by the slight blue-shift of about 2 meV, due to the
ARTICLE IN PRESS M. Bonfanti et al. / Physica E 41 (2009) 972–975
electric field, that can tentatively be attributed to bang gap renormalization, as reported in Ref. [23].
5. Conclusions In this paper, transmission and PC spectra of 50 period strainbalanced Ge/SiGe MQWs structure with high Ge content barriers, as well as their temperature dependence, have been studied in detail. Our results demonstrate that Ge MQWs structures show optical properties analogous to direct gap III–V based quantum wells, such as an attenuation per well of about 0.8%. A FWHM of the HH1–E1 exciton peak of 7.6 meV at low temperature reveals good structural uniformity and layer-by-layer reproducibility of the LEPECVD grown structures. Preliminary photocurrent spectra obtained with a MSM structure indicate the potential of Ge QWsbased devices in Si photonics. These devices may represent a next step towards the integration of optoelectronic devices on standard Si microelectronics.
Acknowledgments The authors gratefully acknowledge Stefano Sanguinetti (Universita` di Milano-Bicocca) for useful discussions and the CARIPLO Foundation for financial support through the SIMBAD Project. Anja Weber (PSI) is acknowledged for her help with the device processing. References [1] H. Miyazaki, et al., Phys. Rev. B 67 (2003) 235109.
[2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
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Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Direct gap related optical transitions in Ge/SiGe quantum wells M. Bonfanti a,, E. Grilli a, M. Guzzi a, D. Chrastina b, G. Isella b, H. von Ka¨nel b, H. Sigg c a
` degli Studi di Milano-Bicocca, Via Cozzi 53, I-20125 Milano, Italy CNISM and L-NESS, Dipartimento di Scienza dei Materiali, Universita CNISM and L-NESS, Dipartimento di Fisica del Politecnico di Milano, Polo di Como, Via Anzani 42, I-22100 Como, Italy c Laboratory for Micro and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland b
a r t i c l e in f o
a b s t r a c t
Available online 28 August 2008
An experimental study of the direct-gap related optical transitions in strain-compensated Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) is presented. These structures are of particular interest due to the proximity of G-type and L-type conduction band states and due to their type I band alignment. The samples were grown by low-energy plasma-enhanced CVD and consist of Ge MQWs with a large numbers of periods and good morphological quality grown onto thick graded Si1x Gex buffer layers. The transmission spectra, which shows clear evidence of excitonic transitions, are studied as a function of temperature in the 5–300 K range. Preliminary results of photocurrent measurements performed on the same structures using metal–semiconductor–metal contact are discussed. & 2008 Elsevier B.V. All rights reserved.
PACS: 73.21.Fg 78.67.De 72.40.þw Keywords: Quantum wells SiGe Absorption Photocurrent
1. Introduction Recent progresses in Si photonics, such as the demonstration of photonic bandgap structures from Si [1], the discussion of gain in Si-nanocrystal [2], and the achievement of the Si Raman laser [3] suggest that Si-based materials could provide a strong support for the technological integration of optical functions in CMOS microelectronics. In this perspective SiGe alloys are of particular interest because they can be epitaxially grown on Si substrates and they are compatible with a large number of standard Si processes and CMOS technology [4]. Moreover, bulk mobilities of SiGe alloys with high Ge concentration are higher than those of Si for both electrons and holes [5], and strain and composition facilitate device design by band gap and band alignment engineering. Finally, SiGe technology allows the integration on Si substrates of high speed electronic devices, such as modulation doped field effect transistors, high electron mobility transistors and heterojunction bipolar transistors [4]. In Ge, unlike in Si, the direct gap at the G-point is not far above the indirect fundamental gap [6] and, moreover, the direct gap energy is in the range of wavelengths used in telecommunications. If it is possible to exploit the direct gap features, it will become possible to get a Si-based structure with optical properties very similar to those of III–V semiconductors. This fact has led to a number of proposals for Si compatible Ge based photonic Corresponding author.
E-mail address: [email protected] (M. Bonfanti). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.08.052
device applications. Photo detectors made from epitaxial Ge layers on Si substrates [7,8], optical modulators based on the Franz– Keldysh effect [9], and optical modulators based on the quantumconfined Stark effect in strained-Ge quantum wells (QWs) [10] have all been demonstrated. However most of the papers devoted to SiGe based heterostructures have studied systems grown on Si substrates using materials with low Ge mole fraction, typically xGe o0:3020:50 [11–16]. This was motivated by the difficulty in the growth of high quality Ge-rich SiGe alloys and heterostructures [4]. Recently, innovative epitaxial growth techniques, such as low-energy plasma-enhanced chemical vapor deposition (LEPECVD) [17] have made possible to grow on Si substrates high Ge content structures of exceptional quality. In this paper, the temperature dependence of transmission spectra as well as photocurrent (PC) spectra of Ge/SiGe multiple quantum wells (MQWs) with Ge-rich barriers are discussed.
2. Samples and experimental set-up Samples growth by LEPECVD started with a buffer graded from Si to Si0:1 Ge0:9 over a thickness of 13 mm and capped with 2 mm Si0:1 Ge0:9 layer on 100 mm Si(100) substrates with resistance above 1 O cm. This forms a fully relaxed virtual substrate (VS) for the nanostructured part of the samples, which consist of a ‘‘strainbalanced’’ structure formed by 50 Ge QWs with Si0:15 Ge0:85 barriers. The nominal well and barrier thickness (14.3 and 19.8 nm, respectively) are such that the mean composition of the nanostructured region is very close to that of the VS, in
ARTICLE IN PRESS M. Bonfanti et al. / Physica E 41 (2009) 972–975
order to get approximately zero net strain. X-Ray diffraction measurements indeed confirm the pseudomorphic growth of the MQWs structure on the VS. VSs having the same design, but without MQWs layers, were used as reference samples for the transmission measurements. The high deposition rate ð426 nm s1 Þ possible with the LEPECVD growth allowed us to get a limited sample growth time. A JASCO FT/IR-800 Fourier transform spectrometer equipped with an InGaAs detector has been used for optical transmission measurements at temperatures ranging from T ¼ 5 to 300 K. The optical density (O.D.) is defined as O.D. ðhnÞ ¼ log10 ½IS ðhnÞ=IR ðhnÞ, where IS ðhnÞ is the transmission spectrum of the sample and IR ðhnÞ that of the reference sample, both measured on samples with mirror polished back surface. The PC measurements were performed at room temperature and at T ¼ 2 K with an halogen light bulb and a Bruker Fourier transform spectrometer on metal–semiconductor–metal (MSM) structures grown on samples having the same nominal structure as those used in the transmission measurements. MSM photodetectors are important devices because of their high electrical bandwidth, high sensitivity and performance, ability to generate ultra-short electrical pulses, simple processing, and compatibility with large-scale planar integrated circuit technology [25–29]. The implementation of the MSM differs from that of other photodetectors in that it has a planar rather than a vertical device structure [30]. This is an advantage in applications where photodetectors must be integrated with amplifiers or light emitters. After removing the native oxide with hydrofluoric acid, 200 nm of Al were deposited on the top of the MQW layers in order to get a MSM structure. The MSM structure used in these experiments consists of 20 interdigitated fingers with length of 300 mm and width of 10 mm. The distance between two adjacent finger is 3 mm and thus the active area between adjacent fingers is 3 300 mm2 (inset in Fig. 4). The room temperature resistance of the MSM device is about 40 O. The PC spectra have been corrected for the spectrum of the halogen light bulb.
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show typical features due to confined excitons, much better defined than those reported for analogous structures [32,33], superimposed to the well-known staircase lineshape typical of the absorption coefficient of type I QWs. The peaks in the transmission spectra can be attributed to dipole allowed transitions between quantum-confined heavy hole (HHn) or light hole (LHn) valence states and conduction states (En) at the G-point; the relaxed Ge direct gap energy at T ¼ 5 K is also reported for reference. The structures present in the low temperature spectra are attributed to HHn–En or to LHn–En transitions on the basis of the calculations performed in Ref. [32] on an analogous structure; these attributions are reported in Fig. 1. The full-width at half-maximum (FWHM) of the HH1–E1 exciton line at T ¼ 5 K is about 7.6 meV, which is comparable with the width of excitonic absorption lines in GaAs/AlGaAs MQWs of similar thickness [24]. The FWHM of the HH1–E1 exciton absorption at room temperature is even narrower than that of analogous MQWs structures with 10 periods [32]. This means that, even if the number of periods is large, the structural uniformity is good and the effects of the interface roughness are limited. The enhanced absorption of the HH2–E2 transition can be justified as being due to an increase of the joint density of states caused by a nearly flat dispersion curve around the G-point [32]. The peak O.D. of the HH1–E1 transition is about 0.17; considering that only the MQWs absorb, an absorption coefficient of about 0:6 104 cm1 and an attenuation per well of about 0.78% can be obtained. These values reasonably compare with those typical of III–V direct gap QWs [31,34]. The HH1–E1 and the LH1–E1 exciton peaks have been also studied as a function of the temperature; the temperature dependence of the energy of the HH1–E1 exciton transition is reported in Fig. 2(a). The red-shift of the peak energy follows the red-shift of the direct energy gap of bulk Ge calculated
3. Optical measurements The transmission spectra of a 50 periods Ge/SiGe MQWs structure measured at T ¼ 300 and 5 K are shown in Fig. 1. They
Fig. 1. Transmission spectra of a 50 period sample measured at 300 K (dashed line) and at 5 K (full line). The arrows indicate the HHn–En and LHn–En transitions; the low temperature Ge direct energy gap is also reported for reference.
Fig. 2. (a) Temperature dependence of the energy peak of the first heavy-hole (full squares) transitions in 5–300 K temperature range; the solid line is the temperature dependence of the Ge direct gap calculated using the Varnish’s law with the parameters of Ref. [35]. (b) Energy difference (triangles) between lighthole and heavy-hole first transitions in the same temperature range.
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Fig. 3. Temperature dependence of the FWHM of the HH1–E1 exciton transition. The full line is the fit of Eq. (1) to the data with the parameters given in the text.
Fig. 4. Photocurrent spectra measured at T ¼ 300 and 2 K with a bias voltage of 0.2 V; the arrows indicate the HHn–En and LHn–En transitions on the basis of the calculations in Ref. [32]. In the inset, a scheme of the MSM structure (not to scale) is reported.
using the Varnish’s law with the parameters reported in Ref. [35]. Thus the temperature dependence of the MQWs optical gap in the 5–300 K temperature range is not markedly affected by the quantum-confinement effects. The difference between the LH1–E1 and the HH1–E1 exciton peak energy, reported in Fig. 2(b), slightly increases (by about 5 meV) when the temperature increases from 5 K to room temperature. This can be attributed to the thermal strain induced by the mismatch of the thermal expansion coefficients of Si substrate and Ge/SiGe MQWs. A lorentzian fit performed on the low energy part of the HH1–E1 transition provides the linewidth of the HH1–E1 exciton transition from low to room temperature. Increasing the temperature, the FWHM increases, as shown in Fig. 3. The relation
gðTÞ ¼ g0 þ aT þ b
1 e_o=K B T 1
(1)
has been proved to successfully describe the temperature broadening of the absorption and emission lines in III–V quantumconfined nanostructures [18–21]. In Eq. (1), g0 is the FWHM at T ¼ 0 K, the linear term aT is the acoustic phonons contribution, while the last term describes the contribution of longitudinal optical phonons. Data in Fig. 3 have been fitted to Eq. (1); the value _o ¼ 36 meV has been used, corresponding to the energy of degenerate LO and TO Ge phonons at the G-point. A good fit was obtained (see Fig. 3) with the following values of the fitting parameters: a ¼ 0:017 0:002 meV T1 and b ¼ 4:1 2:0 meV. However Kuo et al. [33] observed that the actual contribution to the width of the absorption edge or of the exciton peak at high temperature remains an open question. A detailed analysis of the temperature dependence of the FWHM of the HH1–E1 transition in Ge MQWs of different thickness is in progress in order to clarify this point.
4. Photocurrent measurements The PC spectra measured at room temperature and at T ¼ 2 K with a potential difference between fingers of 0.2 V are reported in Fig. 4. The spectra are characterized by structures whose energy agrees with that of the transmission spectra (see Fig. 1). The exciton contribution is clearly present also in the room temperature spectrum and it is particularly evident for the
Fig. 5. Photocurrent spectra measured T ¼ 2 K with bias voltage from 0.2 to 0.8 V; the spectra are normalized for the HH1-E1 excitonic peak signal. The inset show the decreasing of the FWHM of the excitonic peak and the slight blue-shift with increasing bias voltage.
HH1–E1 transition. In the low temperature PC spectrum peaks are clearly visible also at energies above the Si indirect gap; as before [32], these peaks can be attributed to HHn–En and LHn–En transitions with n ¼ 1; 2 and 3 (Fig. 4). Furthermore, the structure at 1.22 eV may be attributed to direct gap absorption of the Si0:1 Ge0:9 layer. In Fig. 5 PC spectra measured at 2 K with potential difference between the fingers ranging from 0.2 to 0.8 V are reported. The inset shows that the FWHM of the first exciton peak decreases, from 16 to 10 meV, with increasing the electric field. In fact, increasing the electric field, carriers are more efficiently drawn towards the contacts, the screening effect is reduced and the exciton peak sharpens [22]. This carrier density decrease is also supported by the slight blue-shift of about 2 meV, due to the
ARTICLE IN PRESS M. Bonfanti et al. / Physica E 41 (2009) 972–975
electric field, that can tentatively be attributed to bang gap renormalization, as reported in Ref. [23].
5. Conclusions In this paper, transmission and PC spectra of 50 period strainbalanced Ge/SiGe MQWs structure with high Ge content barriers, as well as their temperature dependence, have been studied in detail. Our results demonstrate that Ge MQWs structures show optical properties analogous to direct gap III–V based quantum wells, such as an attenuation per well of about 0.8%. A FWHM of the HH1–E1 exciton peak of 7.6 meV at low temperature reveals good structural uniformity and layer-by-layer reproducibility of the LEPECVD grown structures. Preliminary photocurrent spectra obtained with a MSM structure indicate the potential of Ge QWsbased devices in Si photonics. These devices may represent a next step towards the integration of optoelectronic devices on standard Si microelectronics.
Acknowledgments The authors gratefully acknowledge Stefano Sanguinetti (Universita` di Milano-Bicocca) for useful discussions and the CARIPLO Foundation for financial support through the SIMBAD Project. Anja Weber (PSI) is acknowledged for her help with the device processing. References [1] H. Miyazaki, et al., Phys. Rev. B 67 (2003) 235109.
[2] [3] [4] [5] [6]
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