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Resonant tunneling in Si–SiGe superlattices on relaxed buffer substrates
Applied Surface Science 224 (2004) 377–381
Resonant tunneling in Si–SiGe superlattices on relaxed buffer substrates S. Tsujinoa,*, S. Mentesea, L. Diehla, E. Mu¨llera, B. Haasa, D. Ba¨chlea, S. Stutza, D. Gru¨tzmachera, Y. Campidellib, O. Kermarrecb, D. Bensahelb a
Laboratory for Microtechnology and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland b ST Microelectronics, F-38926 Crolles Cedex, France
Abstract P-type Si–Si0.2Ge0.8 superlattices prepared on Si0.5Ge0.5-relaxed buffer substrates are promising structures for the development of SiGe quantum cascade lasers. To explore the resonant tunneling in this system, we studied the vertical transport in Si–Si0.2Ge0.8 superlattices. Low-temperature molecular beam epitaxy enables growth of highly uniform and relatively thick (0.5 mm) superlattices with strain symmetrized design. The sample with 8.3-nm thick Si0.2Ge0.8 quantum wells and 5-nm thick Si barriers exhibited a series of sharp resonant tunneling peaks and negative differential conductance. A signature of electric field domain formation was also found. By reducing the Si-barrier thickness to 3 nm and also reducing the quantum well thickness to 5 nm, only a single peak was observed, but the resonant tunneling peak is about a factor of 2 enhanced compared to the sample with thicker barriers. # 2003 Elsevier B.V. All rights reserved. PACS: 73.20.Dx; 73.40.Gk. Keywords: Silicon–germanium; Resonant tunneling; Superlattice; Quantum cascade lasers
P-type Si–SiGe superlattices using quantum cascade structures are one of the most promising paths towards Si-based light emitter/laser devices [1–3]. Using molecular beam epitaxy of pseudomorphic Si–SiGe quantum wells on Si substrate, the first midinfrared electroluminescence devices on Si were realized [1]. However, this approach is limited in the number of cascades which can be deposited. To surmount the restrictions, we currently focus on the growth of SiGe quantum well/superlattices with 80% germanium concentration on 50% SiGe relaxed buffer substrates. In this system, by designing the active layer structures * Corresponding author. E-mail address: [email protected] (S. Tsujino).
with the average germanium concentration equal to that of the substrate, the crystal growth is strain symmetrized. Further, the large valence band discontinuity of 500 meV in this system is advantageous for the design. We recently observed a relatively narrow midinfrared intersubband optical absorption in Si0.2Ge0.8 quantum wells [4] and midinfrared intersubband electroluminescence from Si–Si0.2Ge0.8 quantum cascade structure with bound to continuum design [5]. Lowtemperature molecular beam epitaxy with strain-symmetrized growth enables thick, 1–2 mm, active layers with up to 30 cascades for an emission frequency of 170 meV without creating additional dislocations. In the quantum cascade structures, the resonant tunneling into the excited states (upper subband)
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.08.092
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S. Tsujino et al. / Applied Surface Science 224 (2004) 377–381
and the efficient extraction of the carriers from the lower subband, e.g. by miniband transport are the key ingredients to achieve the population inversion. In addition, it is crucial to know the role of the light hole subbands on the current transport and the light emission in our p-type superlattices. To further gain insights into the quality of the growth, the resonant
tunneling and miniband transport, as well as into the role of the light hole subbands, we explored the vertical transport in a regular superlattice consisting of Si barriers and Si0.2Ge0.8 quantum wells grown on Si0.5Ge0.5 relaxed buffer layers. Two Si–Si0.2Ge0.8 superlattices were prepared by molecular beam epitaxy on (1 0 0) Si0.5Ge0.5 relaxed
Fig. 1. High-resolution cross sectional TEM picture of sample-A: strain compensated Si–SiGe superlattice with 30 periods of 5-nm thick Si barriers and 8.3-nm wide Si0.2Ge0.8 quantum wells prepared on (1 0 0) Si0.5Ge0.5 relaxed buffer substrate by molecular beam epitaxy at low temperature.
S. Tsujino et al. / Applied Surface Science 224 (2004) 377–381
buffer substrates at a growth temperature of 300 8C. Sample-A contains 30 periods of 8.3-nm wide Si0.2Ge0.8 quantum wells separated by 5-nm thick Si barriers. Sample-B has 30 periods of 5-nm wide Si0.2Ge0.8 quantum wells and 3-nm thick Si barriers. The superlattice layers are nominally undoped and sandwiched by highly p-doped Si0.5Ge0.5 contact layers. We note that since the germanium concentration is high in these quantum wells, it is important to lower the substrate temperature to suppress islanding of the germanium atoms during growth. Fig. 1 shows the cross section of the sample-A taken by high-resolution TEM. Despite the high germanium concentration, of 80%, highly planar growth with flat interfaces is realized. X-ray diffraction measurement of the samples confirmed the designed germanium concentration and the layer sequences. In the experiment, the samples are processed into mesa-diodes with diameters ranging from 300 to 6 mm. The vertical transport is measured at low temperatures. Fig. 2 shows the I–Vand the differential conductance of sample-A measured at 77 K. The results for the devices with 100 mm diameter, Fig. 2a, and also with
379
10 mm diameter, Fig. 2b, are shown. For the 100 mm device, a series of conductance peaks and some sharp negative differential conductance peaks are observed, indicating uniform well and barrier widths and abrupt interfaces. Compared to the 100 mm device, the 10 mm device shows peaks split into small shoulders and peaks, and several new negative differential conductance peaks are seen. We compared the experimental conductance peak positions with the calculated subband energies, Fig. 2c, using the effective masses and the valence band discontinuities for the strained layers on Si0.5Ge0.5 substrate. The parameters are evaluated following ref. [7–9]. The calculation predicts that the hole subbands are formed in the order of HH1, HH2, LH1, HH3, LH2, . . ., where HHn (LHn) denotes the nth heavy hole (light hole) subband, respectively. From that, the observed conductance peaks/bands are ascribed to the resonant tunneling from the ground state heavy hole subband HH1 to other subbands as indicated in the Fig. 2a and b for two devices. When we compared the calculated energy separation with the single period bias Vs, given by the total applied bias divided by
Fig. 2. The I–V and the differential conductance of sample-A for (a) 1 100 mm mesa-diode and (b) 1 10 mm mesa-diode, measured at 77 K. (c) Calculated subband structure of sample-A. (d) Enlarged view of the I–V trance of 1 10 mm mesa device around 1.5 V.
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the number of periods (upper x-coordinate in Fig. 2b and c), the calculated values tend to be higher than Vs. To check the validity of the peak assignment, we repeated the measurement for several devices. We found that the observed resonant bands, denoted in Fig. 2a and b are reproduced, although the exact shape of the peaks including the number and the exact position of the sharp negative differential conductance peaks varies from sample to sample. A part of the I–V trace of the 10 mm device around 1.5 V is plotted in Fig. 2d. Within this band, we observed four peaks with regular spacings. The period between the peaks indicated by arrows was found to be 0.18 V. Similar periodic structures were also observed in other devices with the period ranging from 0.12 to 0.3 V (not shown). The observation of such saw-tooth I–V profiles indicates that the applied voltage is not uniform in the sample and electric field domains are formed, as is typically observed in doped superlattices [6]. Although the sample is not doped intentionally, the holes can be transferred from the highly doped Si0.5Ge0.5 contact layers, since the ground subband in the Si0.2Ge0.8 quantum wells is located 0.2 eV below the valence band edge of Si0.5Ge0.5. We note that the period 0.18 V of the saw-tooth profile in Fig. 2d agrees well to the calculated energy
separation of 0.182 eV between HH1 and LH2 states. Therefore it is likely that, when the bias voltage is around 1.5 V, there exist two or more domains: in one domain HH1 and LH1 are in resonance, and in the other domain at higher electric field, HH1 and LH2 are brought into resonance one quantum well to the other as the bias voltage is increased by the corresponding energy separation. For a more quantitative analysis, further consideration of the kinetics of the domain formation and the proper inclusion of the band-mixing effect between the heavy hole and the light hole will be necessary. Fig. 3 shows the I–V of sample-B measured at 77 K for the devices from 60 to 6 mm in diameter. In this sample, the energy separation between HH1 and HH2 is calculated to be 0.111 eV, with LH1 positioned close to HH2 at 0.103 eV (Fig. 3a). Fig. 3 shows that a single peak with sharp negative differential conductance is observed. The resonance bias divided by the number of quantum wells varies between 0.1 and 0.12 V for different devices, but agrees well with the calculated resonance energy between HH1 and HH2 and/or LH1, within 10 meV. For the 60 mm device, a sudden increase of the current is observed at large biases above 7.5 V. Compared to the calculation, this is ascribed to the onset of the resonance with the LH2
Fig. 3. (a) Calculated subband structure of sample-B. Also miniband width D is shown when D is finite. (b) The I–V and the differential conductance of sample-B with different mesa sizes from 60 to 6 mm.
S. Tsujino et al. / Applied Surface Science 224 (2004) 377–381
miniband. No signature of the electrical field domain formation is seen, probably due to the smaller hole concentration, since ground state subband is shifted to higher energies compared to sample-A. For quantum cascade lasers, increasing the resonant tunneling current density is important to achieve sufficient optical gain. Increasing it by using thin barriers may be a straightforward way, however, that will also increase the current through non-resonant channels. We compared the current density of these two samples, and found that the exponential background current density of sample-B is about a factor 50 higher than that of sample-A. But comparatively, the intensity of the resonant tunneling peak is still stronger: the peak-valley ratio of the resonant tunneling peak of the HH1–HH2/LH1 at 3 V is equal to 2.1 for the 6 mm device. This is about a factor of 2 larger than that of sample-A at similar bias voltage, i.e. the current through the resonant tunneling channel is more enhanced than the background.
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Acknowledgements The authors acknowledge H. Sigg for helpful discussion and to C.V. Falub for his help for the X-ray diffraction measurement. The work has been partially supported by Swiss National Foundation and the European community within the SiGeNET project. References [1] [2] [3] [4] [5] [6]
G. Dehlinger, et al., Science 290 (2000) 2277. I. Bormann, et al., Appl. Phys. Lett. 80 (2002) 2260. S.A. Lynch, et al., Apply. Phys. Lett. 81 (2002) 1543. L. Diehl, et al., Appl. Phys. Lett. 80 (2002) 3274. L. Diehl, et al., Appl. Phys. Lett. 81 (2002) 4700. L. Esaki, L.L. Chang, Phys. Rev. Lett. 33 (1974) 495, for review, see H.T. Grahn (Ed.), Semiconductor Superlattices, World scientific, Singapore, 1995. [7] C.G. Van de Walle, Phys. Rev. B39 (1989) 1871. [8] R. People, S.K. Sputz, Phys. Rev. B41 (1990) 8431. [9] M.M. Rieger, P. Vogl, Phys. Rev. B48 (1993) 14276.
Resonant tunneling in Si–SiGe superlattices on relaxed buffer substrates S. Tsujinoa,*, S. Mentesea, L. Diehla, E. Mu¨llera, B. Haasa, D. Ba¨chlea, S. Stutza, D. Gru¨tzmachera, Y. Campidellib, O. Kermarrecb, D. Bensahelb a
Laboratory for Microtechnology and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland b ST Microelectronics, F-38926 Crolles Cedex, France
Abstract P-type Si–Si0.2Ge0.8 superlattices prepared on Si0.5Ge0.5-relaxed buffer substrates are promising structures for the development of SiGe quantum cascade lasers. To explore the resonant tunneling in this system, we studied the vertical transport in Si–Si0.2Ge0.8 superlattices. Low-temperature molecular beam epitaxy enables growth of highly uniform and relatively thick (0.5 mm) superlattices with strain symmetrized design. The sample with 8.3-nm thick Si0.2Ge0.8 quantum wells and 5-nm thick Si barriers exhibited a series of sharp resonant tunneling peaks and negative differential conductance. A signature of electric field domain formation was also found. By reducing the Si-barrier thickness to 3 nm and also reducing the quantum well thickness to 5 nm, only a single peak was observed, but the resonant tunneling peak is about a factor of 2 enhanced compared to the sample with thicker barriers. # 2003 Elsevier B.V. All rights reserved. PACS: 73.20.Dx; 73.40.Gk. Keywords: Silicon–germanium; Resonant tunneling; Superlattice; Quantum cascade lasers
P-type Si–SiGe superlattices using quantum cascade structures are one of the most promising paths towards Si-based light emitter/laser devices [1–3]. Using molecular beam epitaxy of pseudomorphic Si–SiGe quantum wells on Si substrate, the first midinfrared electroluminescence devices on Si were realized [1]. However, this approach is limited in the number of cascades which can be deposited. To surmount the restrictions, we currently focus on the growth of SiGe quantum well/superlattices with 80% germanium concentration on 50% SiGe relaxed buffer substrates. In this system, by designing the active layer structures * Corresponding author. E-mail address: [email protected] (S. Tsujino).
with the average germanium concentration equal to that of the substrate, the crystal growth is strain symmetrized. Further, the large valence band discontinuity of 500 meV in this system is advantageous for the design. We recently observed a relatively narrow midinfrared intersubband optical absorption in Si0.2Ge0.8 quantum wells [4] and midinfrared intersubband electroluminescence from Si–Si0.2Ge0.8 quantum cascade structure with bound to continuum design [5]. Lowtemperature molecular beam epitaxy with strain-symmetrized growth enables thick, 1–2 mm, active layers with up to 30 cascades for an emission frequency of 170 meV without creating additional dislocations. In the quantum cascade structures, the resonant tunneling into the excited states (upper subband)
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.08.092
378
S. Tsujino et al. / Applied Surface Science 224 (2004) 377–381
and the efficient extraction of the carriers from the lower subband, e.g. by miniband transport are the key ingredients to achieve the population inversion. In addition, it is crucial to know the role of the light hole subbands on the current transport and the light emission in our p-type superlattices. To further gain insights into the quality of the growth, the resonant
tunneling and miniband transport, as well as into the role of the light hole subbands, we explored the vertical transport in a regular superlattice consisting of Si barriers and Si0.2Ge0.8 quantum wells grown on Si0.5Ge0.5 relaxed buffer layers. Two Si–Si0.2Ge0.8 superlattices were prepared by molecular beam epitaxy on (1 0 0) Si0.5Ge0.5 relaxed
Fig. 1. High-resolution cross sectional TEM picture of sample-A: strain compensated Si–SiGe superlattice with 30 periods of 5-nm thick Si barriers and 8.3-nm wide Si0.2Ge0.8 quantum wells prepared on (1 0 0) Si0.5Ge0.5 relaxed buffer substrate by molecular beam epitaxy at low temperature.
S. Tsujino et al. / Applied Surface Science 224 (2004) 377–381
buffer substrates at a growth temperature of 300 8C. Sample-A contains 30 periods of 8.3-nm wide Si0.2Ge0.8 quantum wells separated by 5-nm thick Si barriers. Sample-B has 30 periods of 5-nm wide Si0.2Ge0.8 quantum wells and 3-nm thick Si barriers. The superlattice layers are nominally undoped and sandwiched by highly p-doped Si0.5Ge0.5 contact layers. We note that since the germanium concentration is high in these quantum wells, it is important to lower the substrate temperature to suppress islanding of the germanium atoms during growth. Fig. 1 shows the cross section of the sample-A taken by high-resolution TEM. Despite the high germanium concentration, of 80%, highly planar growth with flat interfaces is realized. X-ray diffraction measurement of the samples confirmed the designed germanium concentration and the layer sequences. In the experiment, the samples are processed into mesa-diodes with diameters ranging from 300 to 6 mm. The vertical transport is measured at low temperatures. Fig. 2 shows the I–Vand the differential conductance of sample-A measured at 77 K. The results for the devices with 100 mm diameter, Fig. 2a, and also with
379
10 mm diameter, Fig. 2b, are shown. For the 100 mm device, a series of conductance peaks and some sharp negative differential conductance peaks are observed, indicating uniform well and barrier widths and abrupt interfaces. Compared to the 100 mm device, the 10 mm device shows peaks split into small shoulders and peaks, and several new negative differential conductance peaks are seen. We compared the experimental conductance peak positions with the calculated subband energies, Fig. 2c, using the effective masses and the valence band discontinuities for the strained layers on Si0.5Ge0.5 substrate. The parameters are evaluated following ref. [7–9]. The calculation predicts that the hole subbands are formed in the order of HH1, HH2, LH1, HH3, LH2, . . ., where HHn (LHn) denotes the nth heavy hole (light hole) subband, respectively. From that, the observed conductance peaks/bands are ascribed to the resonant tunneling from the ground state heavy hole subband HH1 to other subbands as indicated in the Fig. 2a and b for two devices. When we compared the calculated energy separation with the single period bias Vs, given by the total applied bias divided by
Fig. 2. The I–V and the differential conductance of sample-A for (a) 1 100 mm mesa-diode and (b) 1 10 mm mesa-diode, measured at 77 K. (c) Calculated subband structure of sample-A. (d) Enlarged view of the I–V trance of 1 10 mm mesa device around 1.5 V.
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the number of periods (upper x-coordinate in Fig. 2b and c), the calculated values tend to be higher than Vs. To check the validity of the peak assignment, we repeated the measurement for several devices. We found that the observed resonant bands, denoted in Fig. 2a and b are reproduced, although the exact shape of the peaks including the number and the exact position of the sharp negative differential conductance peaks varies from sample to sample. A part of the I–V trace of the 10 mm device around 1.5 V is plotted in Fig. 2d. Within this band, we observed four peaks with regular spacings. The period between the peaks indicated by arrows was found to be 0.18 V. Similar periodic structures were also observed in other devices with the period ranging from 0.12 to 0.3 V (not shown). The observation of such saw-tooth I–V profiles indicates that the applied voltage is not uniform in the sample and electric field domains are formed, as is typically observed in doped superlattices [6]. Although the sample is not doped intentionally, the holes can be transferred from the highly doped Si0.5Ge0.5 contact layers, since the ground subband in the Si0.2Ge0.8 quantum wells is located 0.2 eV below the valence band edge of Si0.5Ge0.5. We note that the period 0.18 V of the saw-tooth profile in Fig. 2d agrees well to the calculated energy
separation of 0.182 eV between HH1 and LH2 states. Therefore it is likely that, when the bias voltage is around 1.5 V, there exist two or more domains: in one domain HH1 and LH1 are in resonance, and in the other domain at higher electric field, HH1 and LH2 are brought into resonance one quantum well to the other as the bias voltage is increased by the corresponding energy separation. For a more quantitative analysis, further consideration of the kinetics of the domain formation and the proper inclusion of the band-mixing effect between the heavy hole and the light hole will be necessary. Fig. 3 shows the I–V of sample-B measured at 77 K for the devices from 60 to 6 mm in diameter. In this sample, the energy separation between HH1 and HH2 is calculated to be 0.111 eV, with LH1 positioned close to HH2 at 0.103 eV (Fig. 3a). Fig. 3 shows that a single peak with sharp negative differential conductance is observed. The resonance bias divided by the number of quantum wells varies between 0.1 and 0.12 V for different devices, but agrees well with the calculated resonance energy between HH1 and HH2 and/or LH1, within 10 meV. For the 60 mm device, a sudden increase of the current is observed at large biases above 7.5 V. Compared to the calculation, this is ascribed to the onset of the resonance with the LH2
Fig. 3. (a) Calculated subband structure of sample-B. Also miniband width D is shown when D is finite. (b) The I–V and the differential conductance of sample-B with different mesa sizes from 60 to 6 mm.
S. Tsujino et al. / Applied Surface Science 224 (2004) 377–381
miniband. No signature of the electrical field domain formation is seen, probably due to the smaller hole concentration, since ground state subband is shifted to higher energies compared to sample-A. For quantum cascade lasers, increasing the resonant tunneling current density is important to achieve sufficient optical gain. Increasing it by using thin barriers may be a straightforward way, however, that will also increase the current through non-resonant channels. We compared the current density of these two samples, and found that the exponential background current density of sample-B is about a factor 50 higher than that of sample-A. But comparatively, the intensity of the resonant tunneling peak is still stronger: the peak-valley ratio of the resonant tunneling peak of the HH1–HH2/LH1 at 3 V is equal to 2.1 for the 6 mm device. This is about a factor of 2 larger than that of sample-A at similar bias voltage, i.e. the current through the resonant tunneling channel is more enhanced than the background.
381
Acknowledgements The authors acknowledge H. Sigg for helpful discussion and to C.V. Falub for his help for the X-ray diffraction measurement. The work has been partially supported by Swiss National Foundation and the European community within the SiGeNET project. References [1] [2] [3] [4] [5] [6]
G. Dehlinger, et al., Science 290 (2000) 2277. I. Bormann, et al., Appl. Phys. Lett. 80 (2002) 2260. S.A. Lynch, et al., Apply. Phys. Lett. 81 (2002) 1543. L. Diehl, et al., Appl. Phys. Lett. 80 (2002) 3274. L. Diehl, et al., Appl. Phys. Lett. 81 (2002) 4700. L. Esaki, L.L. Chang, Phys. Rev. Lett. 33 (1974) 495, for review, see H.T. Grahn (Ed.), Semiconductor Superlattices, World scientific, Singapore, 1995. [7] C.G. Van de Walle, Phys. Rev. B39 (1989) 1871. [8] R. People, S.K. Sputz, Phys. Rev. B41 (1990) 8431. [9] M.M. Rieger, P. Vogl, Phys. Rev. B48 (1993) 14276.