High-efficiency silicon-compatible photodetectors based on Ge quantum dots
S. Cosentino, Pei Liu, Son T. Le, S. Lee, D. Paine, A. Zaslavsky, D. Pacificia)
School of Engineering, Brown University, Providence, RI 02912, USA
S. Mirabella, M. Miritello, I. Crupi, A. Terrasi
MATIS-IMM-CNR and Dipartimento di Fisica ed Astronomia, Università di Catania,
Catania I-95123, ITALY
Supplementary Information
By using a calibrated silicon reference cell, we determined the spectral distribution of
incident photons per unit time, Pph , on our sample upon illumination with a nearly
monochromatic light source. Our monochromator and lamp set-up is characterized by a
spectral resolution of ~ 3 nanometers. In particular, by measuring the net light current (at
V = 0 V) and the external quantum efficiency (EQE) of the reference cell with our
EQE/IQE measurement system (see Fig. S1), we determined Pph by using the formula:
ref cell
Pph 
( I light
 I dark )
ref cell
q EQE
In order to calculate the spectral responsivity of our a-Ge QDs photodetectors we took
into account the spectrum of our light source, by using the formula:
Rsp    
I
light
hc

 I dark 
qPph   
where the quantity at the numerator is the photocurrent under illumination at a given
voltage. Therefore, Rsp measures the ability of collecting generated carriers under
illumination at a given voltage. In Fig. S2 we show a comparison between the spectral
responsivity of our Ge QDs MIS photodetector (at V = –2 V) and the calibrated silicon
reference cell – these data have also been added to Fig. 2 in the main paper.
To get insight into the role of Ge QDs in the light absorption and to clarify the
mechanism of carrier transport in our Ge QDs photodetector, we calculated the spectral
internal quantum efficiency (IQE), defined as the number of photo-carriers collected at
the device terminals per absorbed photon at a given voltage, defined as follows:
IQE 
I
light
 I dark  / q
1  R  Pph
where R is the reflectance of our device. This quantity can be easily related to the
photogeneration of carriers occurring under light exposure.
The volume fraction of the QDs in the SiO2 layer calculated using the MaxwellGarnett approximation and fit to the reflectance measurements, is around 36% as pointed
out in our paper; this leads to a number density of QDs around 2.5x1019 cm-3.
Considering the Ge dose measured by RBS and the size extracted by TEM analysis, a
value of Ge QDs density of ~ 2x1019 cm-3 was found, in reasonable agreement. The
doping level of the n-type Si substrate was measured using the four-point method, giving
the resistivity of 0.0075 .cm, corresponding to substrate doping of ~ 8x1018 cm-3. Given
the measured doping level, the threshold voltage for inversion in our MIS structure is
calculated to be 115 V, much higher than the typical bias values used in our experiments.
Knowing the incident photon flux and absorption in the QD layer and silicon
substrate, we calculated the number of excited QDs in the SiO2 layer and the generated
carriers in the Si substrate within one diffusion length (about 4.5 μm), assuming all
absorbed light generates excited carriers, without considering recombination loss. We
further assume each QD only absorbs one photon in the process. Our results are shown in
the Fig. S3. The actual number of photons absorbed is determined by both the absorption
fraction in QDs and incident photon numbers at that particular wavelength. It is clear that
a large number of carriers is generated in the Si substrate over a wide spectral range,
while the excitation of QDs is mostly lower than the photogeneration in the Si substrate
except at the small wavelengths. The IQE characteristic curve we have measured, where
the maximum value peaks around 700 nm, indicates that both QD and substrate
absorption play an important role. At this point, the detailed mechanism requires further
study, but a likely mechanism is that holes generated in the QDs and in the Si substrate
within a diffusion length of the interface get trapped at the QD interface states. The
resulting positive charge in the insulator enhances the electron injection from the IZO,
giving rise to a higher current and IQE.
Finally, in Fig. S4, we show IQE versus voltage at different wavelengths, where the
incident photon number at each wavelength is specified in the previous discussion, see
Fig. S1. As the magnitude of the reverse bias is increased, the measured IQE increases
correspondingly. In particular, the IQE is largest at 700 nm, where the maximum value is
reached at –10 V. We should point out that IQE below 100% are also observed, for
example, at low reverse voltages for most wavelengths used, or at higher bias voltages for
wavelength shorter than 450nm or longer than 1000 nm. Overall, our device works best
in the 500–1000 nm wavelength range.
-6
2.5x10
Isc reference cell
(from EQE/IQE system)
-6
2.0x10
Current [A]
-6
1.5x10
-6
1.0x10
-7
5.0x10
0.0
100
EQE; IQE
80
60
40
EQE reference cell (from EQE/IQE system)
IQE reference cell (from EQE/IQE system)
EQE reference cell (calibrated)
20
0
incident photon number
13
1.4x10
13
Photon number [1/s]
1.2x10
13
1.0x10
12
8.0x10
12
6.0x10
12
4.0x10
12
2.0x10
300
400
500
600
700
800
900
1000
1100
1200
Wavelength [nm]
Fig. S1: Top panel: short-circuit current I sc of the silicon reference cell at different
wavelengths; middle panel: comparison between the quantum efficiency curves (external,
EQE, and internal, IQE) of the silicon reference cell obtained from the EQE/IQE
measurement system (dashed curve represents the EQE of the calibrated silicon reference
cell); bottom panel: incident photon number at different wavelengths.
2
Responsivity [A/W]
Ge QDs MIS photodetector (V=-2 volt)
Silicon reference cell
1
0
400
600
800
1000
Wavelength [nm]
Fig. S2: Comparison between the spectral responsivity of the Ge QDs MIS photodetector
(at V = –2 V) and of the silicon reference cell (data added to Fig. 2 of revised paper).
Fig. S3: Top panel: fraction of absorbed light intensity in QDs layer and Silicon layer
(within the minority carrier diffusion length of 4.5 µm); bottom panel: number of excited
QDs in SiO2 layer (assuming single-photon absorption per QD) and photogenerated
carriers in Si substrate.
Fig. S4. Comparison of IQE vs. voltage at different wavelengths.