Spectral selective mid-infrared detector on a silicon
platform
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Wang, Jianfei et al. “Spectral Selective Mid-infrared Detector on
a Silicon Platform.” IEEE, 2009. 235–237. © Copyright 2009
IEEE
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http://dx.doi.org/10.1109/GROUP4.2009.5338376
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FB7
12:00 PM – 12:15 PM
Spectral Selective Mid-infrared Detector on a Silicon
Platform
Jianfei Wang, Juejun Hu, Xiaochen Sun, Piotr Becla, Anuradha M. Agarwal, and Lionel C.
Kimerling
Microphotonics Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
wangjf05@mit.edu, anu@mit.edu
Abstract: We report the design, fabrication, and characterization of spectral selective
mid-infrared PbTe photodetector pixels monolithically integrated on a silicon platform.
We demonstrate spectral selectivity with a peak responsivity of 65.4V/W.
1. Introduction
Recently, people show increasing interest in spectral engineered mid-infrared (mid-IR) photodetectors due
to their promising applications in chem-bio sensing and hyperspectral imaging.[1] Spectrally selective
detection has been demonstrated by two typical structures: monolithic integration of narrow band filters
and broadband infrared detectors,[2][3] and resonant-cavity-enhanced (RCE) photodetectors.[4-7] For the first
type of detectors, incident IR light is modulated by filters and then absorbed by photosensitive layers, such
as polycrystalline PbSe[2] or epitaxial PbTe films.[3] For the second type, photoactive layers, such as
PbTe,[4][5] Pb1-xEuxSe,[6] or HgCdTe,[7] are sandwiched between two one-dimensional photonic crystals (1dPC), and thus a resonant cavity structure can be formed, providing highly wavelength-selective detection
and high quantum efficiency even with much thinner absorber layer. Reduced bulk thermal generation and
recombination noise, and higher operating temperatures can be achieved simulaneously. However, previous
structures rely on either polycrystalline PbSe films, which needs high temperature sensitization process,
[2][8][9]
or expensive growth techniques such as molecular-beam epitaxy (MBE), [4-7] which lack the
capability of being monolithically integrated with Si read-out integrated circuit (Si ROIC) due to backside
illumination and substrate limitation. A low cost spectral engineered mid-IR photodetector technology that
can be monolithically integrated on a Si platform still remains to be explored.
Our work leverages on such an RCE photodetection concept, using polycrystalline or amorphous films
grown on a Si platform, thereby significantly reducing the cost and improving device robustness. Highly
reflective 1d-PC and mid-IR photoconductive PbTe film without any sensitization process have been
demonstrated separately in our initial work.[10] In this paper we report the design, fabrication and
characterization of a mid-infrared photodetector with spectral selectivity and a peak responsivity of
65.4V/W.
2. Design and fabrication
The design of cavity structure is based on transfer matrix method (TMM) and experimental data of optical
properties of single layer films.[10][11] The cavity mode is designed to be at 3.7 μm. 4 periods is chosen for
both bottom 1d-PC and top 1d-PC, which gives a Q value of 124.2.
Film deposition of Ge, As2S3, and PbTe is taken in the same chamber kept at room temperature. 2”
diameter poly-Ge with Cu backing plate of 99.999% purity from Williams Advanced Materials is used as
sputtering target, As2S3 bulk from Amorphous Materials Inc. and PbTe bulk of 99.999% purity from Alfa
Aesar Inc. are used as the source material for thermal evaporation. Both the sputtering and the thermal
evaporation are carried out at a base pressure of 5×10-7 Torr. Oxide coated Si wafers (6” Si wafers with 3
μm oxide, Silicon Quest International) are used as starting substrates. The substrates are held on a rotating
substrate holder, at room temperature, throughout the depositions. Film deposition rate is monitored in realtime through a quartz crystal sensor and is maintained at 1.3 Å/s for Ge, and ~10 Å/s for As2S3 and PbTe.
Figure 1 shows the process flow to fabricate pixels. The pixel dimensions are defined by the width and
distance of Sn metal contacts. The alignment is done using a Karl Suss Model MJB-3 Mask Aligner with a
350W Hg bulb. Negative photoresist is exposed to 320 nm UV light and developed to make openings for
film deposition, which is followed by lift-off process in an ultrasonic bath.[12] Two lift-off steps are used to
pattern top 1d-PC and Sn contacts separately. A 30 μm x 30 μm pixel is shown in Figure 2 to demonstrate
the success of the above fabrication process.
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Fig. 1. Process flow to fabricate PbTe mid-IR pixels on Si platform. Step 1: Bottom 1d-PC and PbTe photoactive layer deposition.
Step 2: Top 1d-PC deposition and patterning. Step 3: Sn deposition and patterning.
Fig. 2. Top-down view of a fabricated 30 μm
x 30 μm pixel, demonstrating the success of
the fabrication process.
Fig. 3. Transmittance and reflectance spectra of the cavity structure. Photonic
band gap is clearly seen in the range of 3.0 μm ~ 4.4 μm, and the cavity mode
is shown in the reflectance spectrum at designed wavelength of 3.7 μm.
3. Experimental results and discussion
Profilometry measurements performed on single-layer Ge, As2S3 and PbTe films yield excellent uniformity
across an entire substrate with thickness variations < 3%. X-ray diffraction analysis on both Ge and As2S3
films confirms their amorphous nature, while the PbTe film features polycrystalline, face-centered-cubic
(FCC) structure. Hall measurement performed at room temperature indicates the PbTe film is p-type, and
has a Hall mobility of 64 cm2/V/s and carrier concentration of 1.0E17 cm-3.
Both transmittance and reflectance spectra are measured using a Nicolet Magna 860 Fourier Transform
Infrared Spectrometer (FTIR) for the cavity structure, which is shown in Figure 3. In both spectra, the
photonic band gap is clearly seen in the range of 3 μm ~ 4.4 μm. It is interesting to notice that the cavity
mode is not shown in the transmittance spectrum, but shown in the reflectance spectrum at designed
wavelength of 3.7 μm, indicating this portion of light is absorbed by the PbTe layer. The cavity mode has a
full width at half maximum (FWHM) of 64 nm, which corresponds to a quality factor of 58. The agreement
between experiment and design indicates excellent film thickness control and uniformity. However, Figure
3 also indicates a limited quantum efficiency of only 6% due to the present strongly under coupled cavity
design. Through cavity design optimization to achieve critical coupling, at least one order of magnitude
improvement in quantum efficiency and responsivity could be expected.
Glow bar light monochromatized through a sodium chloride prism is modulated at 10.3 Hz for the
photoconductivity measurement. Due to the large bandwidth of the monochromator compared to the
FWHM of the cavity mode shown in Figure 3, an external long pass filter is used to block all the light
below 3 μm. Figure 4 shows the infrared responsivity of a 300 μm x 300 μm pixel at 1 mA bias current and
at -60 oC cooled by a thermoelectric cooler (TEC). The peak responsivity is 65.4 V/W and locates at 3.9
μm, which agrees quite well with the design and the result in Figure 3. The peak in Figure 4 is very broad
compared to the FWHM value of the cavity mode obtained by FTIR. This is because the fairly broad
bandwidth of the incident light when the monochromator slit is tuned to be wider to enhance the incident
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236
optical flux. With higher power IR light source and monochromator having better resolution, the
responsivity peak could be located more accurately and the measured responsivity peak will become
narrower.
Responsivity (V/W)
80
70
60
300 μm x 300 μm pixel
o
-60 C, 1 mA
50
40
30
20
10
0
1
2
3
4
Wavelength (μm)
5
6
Fig. 5. Infrared responsivity of a 300 μm x 300 μm pixel at 1 mA bias current and at -60 oC cooled by a thermoelectric cooler (TEC).
The peak responsivity is 65.4 V/W and locates at 3.9 μm, which agrees quite well with the design and the result in Figure 3. The
responsivity peak is very broad compared to the FWHM value of the cavity mode obtained by FTIR. This is because the fairly broad
bandwidth of the monochromator.
4. Conclusion
Spectral engineered mid-IR photoconductive pixels on Si platform have been designed, fabricated, and
characterized. Monolithic integration of one-dimensional photonic crystal and thermally evaporated PbTe
film on a Si platform has been demonstrated. The cavity mode around 3.7 μm is demonstrated by both
FTIR and photoconductivity measurements.
Acknowledgements
This work is supported by DSO National Laboratories, Singapore. The authors would like to thank
Microsystems Technology Laboratories (MTL) at MIT for photolithography facilities. The authors
acknowledge the Center for Materials Science and Engineering (CMSE) at MIT for characterization
facilities.
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