Supplemental material:
Room-temperature
operation
of
a
Ti
supersaturated
Si-based
infrared
photodetector.
E. García-Hemme1, 2, R. Garcia-Hernansanz1, 2, J. Olea2, 3, D. Pastor1, 2, A. del Prado1, 2 ,
I. Mártil1, 2 and G. González-Díaz1, 2.
1
Dept. de Física Aplicada III (Electricidad y Electrónica), Univ. Complutense de
Madrid, 28040 Madrid, Spain
2
CEI Campus Moncloa, UCM-UPM, 28040 Madrid, Spain
3
Instituto de Energía Solar, E.T.S.I. de Telecomunicación, Univ. Politécnica de Madrid.
28040 Madrid, Spain
These figures and discussions are provided in support of the main data presented
in the manuscript.
Figure S 1 – Spectral noise density as a function of the frequency for the Ti supersaturated Si device
measured at room-temperature. As a reference, pure 1/f1/2 noise dependence has been plotted as the
dashed line.
1
Figure S1 shows the spectral noise density (𝑆𝑛 ) as a function of the frequency
for the Ti supersaturated Si device operating at room temperature. We could observe
two different behaviors of the 𝑆𝑛 as the frequency increases. A first one at lower
frequencies, where the 𝑆𝑛 exhibit an almost 1/f ½-like noise dependence, and a second
region at higher frequencies, where the 𝑆𝑛 shows an almost independent and constant
value with the frequency (white noise dominates). Note that it’s the square of the
voltage (i.e. power) that declines at a 1/f rate. Noise voltage falls at 1/f ½. The corner
frequency (frequency where the class of noise changes) is approximately at 660 Hz.
Regarding the region of 1/f ½-like noise, we could fit the experimental points to a
𝛽
function of the form 𝑆𝑛 = 𝑓𝛼, where 𝛼 and 𝛽 are fitting parameters. From the fitting
performed on data of Fig. S1 (red line), we have obtained values of 𝛼=0.64±0.01 and
𝛽=(7.2±0.1)×10-5. An 𝛼 value different than 0.5 generally corresponds to a wide range
of non-equilibrium driven dynamical systems.1 In fact, this is in accordance with our
measurements since generation-recombination noise, which is a kind of non-equilibrium
process due to our frequency pulsed light experiment, could be a fundamental source of
noise. Further investigations on the different sources of noise are in progress.
Regarding the crystalline quality of the Ti supersaturated Si samples, we have
performed transmission electron microscopy (TEM) images and electron diffraction
(ED) patterns of a sample identically prepared.
2
Figure S 2 – Transmission electron microscopy image of an identical prepared sample, showing
stacking faults defects. Also, electron diffraction pattern is presented showing a crystalline layer
without differences in the pattern with respect to the silicon substrate electron diffraction pattern.
Figure S2 shows a crystalline layer of 360 nm that presents extended defects.
We clearly classify these defects as stacking faults due to the orientation they presents.
This kind of defect has been observed previously in rapid solidification of Si (111), as it
was shown in Ref. 2, 3. In any case, the ED pattern of the implanted and PLM layers
shows no differences with the Si substrate ED pattern, confirming the high degree of
crystallinity of the implanted and PLM layer. This high degree of crystallinity is also
confirmed in the high resolution TEM image (upper-right side of Fig. S 2), showing the
crystalline arrangement.
The presence of these defects has to imply a harmful effect on the
optoelectronics performances presented in the manuscript. The reason is clear: extended
crystalline defects would drastically reduce the 𝜇𝜏 product (mobility × lifetime). Taking
into account that the responsivity in a photoconductive detector is directly related with
the increase of the sheet conductance:
3
Δ𝐺□
= 𝑞𝜇𝑡𝜂𝛼𝜏
𝐼0
Where
Δ𝐺□
𝐼0
is the sheet conductance increase normalized to the light intensity, 𝑞
is the charge carrier, 𝜇 is the mobility, 𝑡 is the thickness of the layer, 𝜂 is the quantum
efficiency, 𝛼 is the absorption coefficient and 𝜏 is the charge carriers lifetime. So, the
responsivity is directly associated with the crystal quality of the sample and its 𝜇𝜏
product. That means that, if we were able to obtain a layer without this extended defect,
our photodetectors performances would be even higher than the ones we have presented
in the manuscript.
In a previous work, we have shown the detrimental effect of extended defects on
the photoresponse (Ref. 4). Samples with a higher degree of extended defects and a nonuniform distribution of the Ti content across the implanted and PLM layer (process
known as cellular breakdown) shows, in fact, a lower photoresponse that samples that
presents a monocrystalline layer without any kind of defects. We have also studied the
possibility that these extended defects can be responsible for optoelectronic response in
silicon because they introduce deep levels similar to those created by Ti atoms. For that
study we have used a control device similar to the one presented here, but with a Si
implantation, showing that the extended defects have a negative effect on the
performances of the photodetector (Ref. 5 - 6).
We have also studied the possible segregation of Ti to the extended defects
during the solidification process (via cellular breakdown). For that study we have
measured qualitatively the homogeneity of the distribution of Ti atoms in the Si lattice
using the Energy Dispersive X-Ray Analysis (EDX) with the microscope working in the
Scanning Transmission Electron Microscopy (STEM) mode.
4
Figure S 3 – STEM image of the Ti supersaturated Si sample along with the EDX scan line and Ti
detection counts profile (light blue). In green we represent the spatial positions of some stacking fault
defects.
Figure S 3 shows a STEM image of a sample identically prepared as well as the
Ti content profile (light blue) obtained by EDX along the yellow scan line. In green
lines we represent some of the stacking fault defects. Notice that the EDX detection
limit for Ti in Si is ~1at% (5×1020 cm-3).
We can observe that the EDX reveal Ti content in almost all the scan line. The
analysis reveals that there no exists an abrupt Ti content variation between the extended
defects (stacking faults) and the rest of the regions of the Ti supersaturated Si layer.
References:
1.-
Kogan, Shulim (1996). Electronic Noise and Fluctuations in Solids. Cambridge
University Press. ISBN 0-521-46034-4.
2.-
A. Cullis, H. Webber, N. Chew, J. Poate, and P. Baeri, Phys. Rev. Lett. 49, 219
(1982)
5
3.-
G. Foti, E. Rimini, W. Tseng, and J. Mayer, Appl. Phys. A: Mater. Sci. Process.
15, 365 (1978).
4.-
E. García-Hemme, R. García-Hernansanz, J. Olea, D. Pastor, A. del Prado, I.
Mártil, and G. González-Díaz, Applied Physics Letters 101, 192101 (2012).
5.-
J. Olea, D. Pastor, A. del Prado, E. García-Hemme, R. García-Hernansanz, I.
Mártil, and G. González-Díaz, Journal of Applied Physics 114, 053110 (2013).
6.-
E. García-Hemme, R. García-Hernansanz, J. Olea, D. Pastor, A. del Prado, I.
Mártil, and G. González-Díaz, Applied Physics Letters 103, 032101 (2013).
6