- Email: [email protected]
Si quantum dot photodetectors
Infrared Physics & Technology 44 (2003) 513–516 www.elsevier.com/locate/infrared
Photoconductivity of Ge/Si quantum dot photodetectors N. Rappaport a, E. Finkman a, P. Boucaud b, S. Sauvage b, T. Brunhes b, V. Le Thanh b, D. Bouchier b, S.E. Schacham c,* a
Department of Electrical Engineering and Solid State Institute, Technion, Haifa 32000, Israel Institut dElectronique Fondamentale UMR CNRS 8622, Universit e Paris XI, 91405 Orsay, France Department of Electrical and Electronic Engineering, College of Judea and Samaria, Ariel 44837, Israel b
c
Abstract Various structures of self-assembled Ge/Si quantum dot infrared photodetectors were implemented and investigated. The electronic structure of the QDIPs was studied by electrical and optical techniques including I–V characteristics, dark current, photoconductivity, photoluminescence, and photo-induced infrared absorption. The photoconductive spectra consist of a broad multi-peak, composed of peaks ranging from 70 to 220 meV. Their relative intensity changes with bias. Comparative dark current measurements were performed. Dark current limits the performance of this first generation of Ge/Si QDIPs. It is plausible that direct doping in the dot layer is a viable way of reducing the dark current. 2003 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled Ge/Si quantum dot infrared photodetectors (QDIPs) were designed and realized. The major advantage of Ge/Si over other QDIP structures is the possibility of integration with silicon technology. This should allow for an easy incorporation of such detector matrices in silicon read-out electronics. The monolithic integration with Si-based circuits is expected to avoid problems encountered for large-area infrared focal plane arrays like the thermal mismatch between III–V and IV–IV materials. In addition, relaxation of absorption selection rules in quantum dots allows for detection of normal incidence radiation, avoiding coupling gratings that are needed in QWIPs.
*
Corresponding author.
The large difference between the energy gap of Si and Ge manifests itself almost entirely as a valence band discontinuity. Hence, in the case of pseudomorphic SiGe grown on Si, a significant valence band offset exists between both strained and relaxed Ge on Si. The existence of this valence band offset enables the realization of p-type devices either for quantum dots or quantum wells. As shown in Fig. 1, the discontinuity can change from type I to type II depending on the composition. Si-based valence band intersubband photodetectors using SiGe quantum wells were implemented in past years [1–3]. Some GeSi QDIPs were studied as well in the last few years [4–7].
2. Crystal growth and characterization Mesa structure detectors were implemented with Ge/Si self assembled dots grown by chemical
1350-4495/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1350-4495(03)00173-7
514
N. Rappaport et al. / Infrared Physics & Technology 44 (2003) 513–516
Fig. 1. Band offsets of Si1 x Gex strained layers on Si substrates.
vapor deposition. Several QDIP structures were studied, with 10–15 periods of Ge quantum dots grown on Si layers, with various concentrations of delta p-doping, either in the Si barriers or in the Ge dots. The growth parameters of the investigated structures are presented in Table 1. The dots, when grown, are in a cone shape. Following the overgrowth of the Si cap layer, a lens-shape form is obtained due to interdiffusion, with a height of about 6–7 nm, a diameter of 90–100 nm, and a composition of about 50% [8]. The dots are stacked in a self-organized pattern, one above the other. The electronic structure of the QDIPs was investigated by a combination of electrical and optical techniques, namely by measurements of I–V characteristics, dark current, photoconductivity, photoluminescence, and photo-induced infrared absorption. Here we present in particular the photoconductivity results. Fig. 2 shows the photoconductive spectra of intraband transitions in sample A206 for several applied voltages, taken
Fig. 2. Normalized (signal divided by bias voltage) photocurrent spectral response of sample A206 for various voltage biases.
at 18 K. Both P and S polarizations are presented. The spectral response consists of a broad multipeak. At low bias voltages, two peaks are resolved around 115 and 155 meV. A weaker peak is noticeable around 220 meV. With increasing bias, a strong peak appears around 90 meV, and a shoulder around 70 meV. At the largest biases, the normalized intensity (signal divided by bias) decreases with bias, in other words––the signal reaches saturation. Photoconductive spectra for sample A410, taken at 13 K, are presented in Fig. 3. The main difference between this sample and the previous one is that here the delta doping concentration is 2 orders of magnitude larger. Here the 90 meV is dominant at all biases with the 70 meV as a second line, while the 115, 155 and 220 meV transitions appear as shoulders. Similar spectra were measured for sample A411, taken at 18 K, as seen in Fig. 4. This sample is identical to A410, only that the doping is in the dot layer rather than in the barrier. No saturation of signal intensity is observed in both A410 and A411 when
Table 1 Sample growth parameters Sample A206 A410 A411
Growth temperature (C) 600–620 600–620 600–620
Dot concentration (cm 2 ) 9
2–3 · 10 2–3 · 109 2–3 · 109
d-doing (cm 3 )
Doping position
No. of periods
5 · 1016 5 · 1018 5 · 1018
In barrier In barrier In dots
10 15 15
N. Rappaport et al. / Infrared Physics & Technology 44 (2003) 513–516
515 220meV 155meV ? 115meV 85meV
155meV ?
WL
QD
Fig. 5. Tentative schematic energy level scheme in the Ge0:5 Si0:5 /Si QD structure. Fig. 3. Normalized photocurrent spectral response of sample A410 for various voltage biases.
Fig. 4. Normalized photocurrent spectral response of sample A411 for various voltage biases.
the bias is increased. The signal of A411, however, is noisier when compared to that of A410. Dark current measurements were performed on detectors of the different structures as a function of temperature, with a cold radiation shield attached. They were compared to the low temperature IV characteristics of the same detectors taken without radiation shield. This enables us to determine the background limited temperature (T BLIP) of the detectors. It was typically as low as 25 K. 3. Analysis and discussion Tentative interpretation for the origin of the photoconductive peaks is presented in Fig. 5. The
possible transitions corresponding to the measured lines are marked on the figure. The assignment is corroborated by photoluminescence results, and by 8 band k Æ p calculations (to be published). Peak intensities of the photoconductive spectra increase superlinearly with bias at low biases, and saturate at high-applied voltage for sample A206, as seen in Fig. 2. This saturation of the signal is most probably due to exhaustion of all available holes from the dots. The saturation was avoided by higher doping, as evidenced in the other two samples. The superlinear increase of all the observed peaks indicates that the photoconductive signal is generated by bound-to-bound transitions following by tunneling. Dark current limits the performance of this first generation of Ge/Si QDIPs. Comparing the signal and the dark currents measured on samples A410 and A411 it is clear that the latter performance is superior to the former. This may indicate that direct doping in the dot layer is a viable way of reducing the dark current. It seems that delta doping in the barriers creates internal electric fields that facilitate the thermally assisted emission of holes from the dots to increase the dark current. References [1] J.S. Park, R.P.G. Karunasiri, K.L. Wang, Appl. Phys. Lett. 61 (1992) 681. [2] R. People, J.C. Bean, S.K. Sputz, C.G. Bethea, L.J. Peticolas, Thin Solid Films 222 (1992) 120. [3] P. Kruck, M. Helm, T. Fromherz, G. Bauer, J.F. Nutzel, G. Abstreiter, Appl. Phys. Lett. 69 (1996) 3372. [4] N. Rappaport, E. Finkman, T. Brunhes, P. Boucaud, S. Sauvage, N. Yam, V. Le Thanh, D. Bouchier, Appl. Phys. Lett. 77 (2000) 3224.
516
N. Rappaport et al. / Infrared Physics & Technology 44 (2003) 513–516
[5] P. Boucaud, T. Brunhes, S. Sauvage, N. Yam, V. Le Thanh, D. Bouchier, N. Rappaport, E. Finkman, Phys. Status Solidi B 224 (1) (2001) 233. [6] C. Miesner, O. Rothig, K. Brunner, G. Abstreiter, Physica E 7 (2000) 146.
[7] T. Fromherz, W. Mac, C. Miesner, K. Brunner, G. Bauer, G. Abstreiter, Appl. Phys. Lett. 80 (2002) 2093. [8] G. Patriarche, I. Sagnes, P. Boucaud, V. Le Thanh, D. Bouchier, C. Hernandez, Y. Campidelli, D. Bensahel, Appl. Phys. Lett. 77 (2000) 370.
Photoconductivity of Ge/Si quantum dot photodetectors N. Rappaport a, E. Finkman a, P. Boucaud b, S. Sauvage b, T. Brunhes b, V. Le Thanh b, D. Bouchier b, S.E. Schacham c,* a
Department of Electrical Engineering and Solid State Institute, Technion, Haifa 32000, Israel Institut dElectronique Fondamentale UMR CNRS 8622, Universit e Paris XI, 91405 Orsay, France Department of Electrical and Electronic Engineering, College of Judea and Samaria, Ariel 44837, Israel b
c
Abstract Various structures of self-assembled Ge/Si quantum dot infrared photodetectors were implemented and investigated. The electronic structure of the QDIPs was studied by electrical and optical techniques including I–V characteristics, dark current, photoconductivity, photoluminescence, and photo-induced infrared absorption. The photoconductive spectra consist of a broad multi-peak, composed of peaks ranging from 70 to 220 meV. Their relative intensity changes with bias. Comparative dark current measurements were performed. Dark current limits the performance of this first generation of Ge/Si QDIPs. It is plausible that direct doping in the dot layer is a viable way of reducing the dark current. 2003 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled Ge/Si quantum dot infrared photodetectors (QDIPs) were designed and realized. The major advantage of Ge/Si over other QDIP structures is the possibility of integration with silicon technology. This should allow for an easy incorporation of such detector matrices in silicon read-out electronics. The monolithic integration with Si-based circuits is expected to avoid problems encountered for large-area infrared focal plane arrays like the thermal mismatch between III–V and IV–IV materials. In addition, relaxation of absorption selection rules in quantum dots allows for detection of normal incidence radiation, avoiding coupling gratings that are needed in QWIPs.
*
Corresponding author.
The large difference between the energy gap of Si and Ge manifests itself almost entirely as a valence band discontinuity. Hence, in the case of pseudomorphic SiGe grown on Si, a significant valence band offset exists between both strained and relaxed Ge on Si. The existence of this valence band offset enables the realization of p-type devices either for quantum dots or quantum wells. As shown in Fig. 1, the discontinuity can change from type I to type II depending on the composition. Si-based valence band intersubband photodetectors using SiGe quantum wells were implemented in past years [1–3]. Some GeSi QDIPs were studied as well in the last few years [4–7].
2. Crystal growth and characterization Mesa structure detectors were implemented with Ge/Si self assembled dots grown by chemical
1350-4495/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1350-4495(03)00173-7
514
N. Rappaport et al. / Infrared Physics & Technology 44 (2003) 513–516
Fig. 1. Band offsets of Si1 x Gex strained layers on Si substrates.
vapor deposition. Several QDIP structures were studied, with 10–15 periods of Ge quantum dots grown on Si layers, with various concentrations of delta p-doping, either in the Si barriers or in the Ge dots. The growth parameters of the investigated structures are presented in Table 1. The dots, when grown, are in a cone shape. Following the overgrowth of the Si cap layer, a lens-shape form is obtained due to interdiffusion, with a height of about 6–7 nm, a diameter of 90–100 nm, and a composition of about 50% [8]. The dots are stacked in a self-organized pattern, one above the other. The electronic structure of the QDIPs was investigated by a combination of electrical and optical techniques, namely by measurements of I–V characteristics, dark current, photoconductivity, photoluminescence, and photo-induced infrared absorption. Here we present in particular the photoconductivity results. Fig. 2 shows the photoconductive spectra of intraband transitions in sample A206 for several applied voltages, taken
Fig. 2. Normalized (signal divided by bias voltage) photocurrent spectral response of sample A206 for various voltage biases.
at 18 K. Both P and S polarizations are presented. The spectral response consists of a broad multipeak. At low bias voltages, two peaks are resolved around 115 and 155 meV. A weaker peak is noticeable around 220 meV. With increasing bias, a strong peak appears around 90 meV, and a shoulder around 70 meV. At the largest biases, the normalized intensity (signal divided by bias) decreases with bias, in other words––the signal reaches saturation. Photoconductive spectra for sample A410, taken at 13 K, are presented in Fig. 3. The main difference between this sample and the previous one is that here the delta doping concentration is 2 orders of magnitude larger. Here the 90 meV is dominant at all biases with the 70 meV as a second line, while the 115, 155 and 220 meV transitions appear as shoulders. Similar spectra were measured for sample A411, taken at 18 K, as seen in Fig. 4. This sample is identical to A410, only that the doping is in the dot layer rather than in the barrier. No saturation of signal intensity is observed in both A410 and A411 when
Table 1 Sample growth parameters Sample A206 A410 A411
Growth temperature (C) 600–620 600–620 600–620
Dot concentration (cm 2 ) 9
2–3 · 10 2–3 · 109 2–3 · 109
d-doing (cm 3 )
Doping position
No. of periods
5 · 1016 5 · 1018 5 · 1018
In barrier In barrier In dots
10 15 15
N. Rappaport et al. / Infrared Physics & Technology 44 (2003) 513–516
515 220meV 155meV ? 115meV 85meV
155meV ?
WL
QD
Fig. 5. Tentative schematic energy level scheme in the Ge0:5 Si0:5 /Si QD structure. Fig. 3. Normalized photocurrent spectral response of sample A410 for various voltage biases.
Fig. 4. Normalized photocurrent spectral response of sample A411 for various voltage biases.
the bias is increased. The signal of A411, however, is noisier when compared to that of A410. Dark current measurements were performed on detectors of the different structures as a function of temperature, with a cold radiation shield attached. They were compared to the low temperature IV characteristics of the same detectors taken without radiation shield. This enables us to determine the background limited temperature (T BLIP) of the detectors. It was typically as low as 25 K. 3. Analysis and discussion Tentative interpretation for the origin of the photoconductive peaks is presented in Fig. 5. The
possible transitions corresponding to the measured lines are marked on the figure. The assignment is corroborated by photoluminescence results, and by 8 band k Æ p calculations (to be published). Peak intensities of the photoconductive spectra increase superlinearly with bias at low biases, and saturate at high-applied voltage for sample A206, as seen in Fig. 2. This saturation of the signal is most probably due to exhaustion of all available holes from the dots. The saturation was avoided by higher doping, as evidenced in the other two samples. The superlinear increase of all the observed peaks indicates that the photoconductive signal is generated by bound-to-bound transitions following by tunneling. Dark current limits the performance of this first generation of Ge/Si QDIPs. Comparing the signal and the dark currents measured on samples A410 and A411 it is clear that the latter performance is superior to the former. This may indicate that direct doping in the dot layer is a viable way of reducing the dark current. It seems that delta doping in the barriers creates internal electric fields that facilitate the thermally assisted emission of holes from the dots to increase the dark current. References [1] J.S. Park, R.P.G. Karunasiri, K.L. Wang, Appl. Phys. Lett. 61 (1992) 681. [2] R. People, J.C. Bean, S.K. Sputz, C.G. Bethea, L.J. Peticolas, Thin Solid Films 222 (1992) 120. [3] P. Kruck, M. Helm, T. Fromherz, G. Bauer, J.F. Nutzel, G. Abstreiter, Appl. Phys. Lett. 69 (1996) 3372. [4] N. Rappaport, E. Finkman, T. Brunhes, P. Boucaud, S. Sauvage, N. Yam, V. Le Thanh, D. Bouchier, Appl. Phys. Lett. 77 (2000) 3224.
516
N. Rappaport et al. / Infrared Physics & Technology 44 (2003) 513–516
[5] P. Boucaud, T. Brunhes, S. Sauvage, N. Yam, V. Le Thanh, D. Bouchier, N. Rappaport, E. Finkman, Phys. Status Solidi B 224 (1) (2001) 233. [6] C. Miesner, O. Rothig, K. Brunner, G. Abstreiter, Physica E 7 (2000) 146.
[7] T. Fromherz, W. Mac, C. Miesner, K. Brunner, G. Bauer, G. Abstreiter, Appl. Phys. Lett. 80 (2002) 2093. [8] G. Patriarche, I. Sagnes, P. Boucaud, V. Le Thanh, D. Bouchier, C. Hernandez, Y. Campidelli, D. Bensahel, Appl. Phys. Lett. 77 (2000) 370.