Design of a Unipolar Barrier for a Nanocrystal-Based Short-Wave Infrared Photodiode

TLDR: In this paper, a unipolar barrier was designed from another layer of colloidal quantum dots (CQDs) with a wider band gap to reduce the dark current injection and enhance the signal-to-noise ratio.

Abstract: Nanocrystals are promising materials for the design of low-cost, infrared (IR) detectors. Here we focus on HgTe colloidal quantum dots (CQDs) as an active material for detection in the extended short-wave infrared (2.5 μm as cutoff wavelength). In this paper, we propose a strategy to enhance the performances of previously reported photodiodes. In particular, we integrate in this diode a unipolar barrier, whose role is to reduce the dark current injection and subsequently enhance the signal-to-noise ratio. We demonstrate that such unipolar barrier can be designed from another layer of HgTe CQDs with a wider band gap. Using a combination of IR spectroscopy and photoemission, we show that the barrier is resonant with the absorbing layer valence band, while presenting a clear offset with the conduction band. The combination of contacts with improved design and use of a unipolar barrier allows us to reach a detectivity as high as 3 × 108 Jones at room temperature with 3 dB cut off frequency above 10 kHz.

Figures

Figure 2 a. Responsivity for a photodiode with the following structure ITO/ZnO/HgTe (ambipolar -4000 cm-1)/MoO3/Au as a function of the applied bias over the diode for different thicknesses of the MoO3 layer. The inset plot is the relative evolution of the responsivity as a function of the MoO3 thickness for a 50 mV bias. b. Responsivity for a photodiode with the following structure of transparent conductive layer/n-type conduction layer/HgTe (ambipolar – 4000 cm-1)/MoO3/Au as a function of the applied bias over the diode for different couples of transparent conductive layer (ITO and FTO) and n-type extraction layer (ZnO and TiO2). The thickness of the MoO3 layer is 10 nm. The inset plot is the relative evolution of the responsivity for the different considered couples of transparent conductive layer and n-type extraction layer, under 50 mV bias.

Figure 3 a. Infrared spectrum for dodecanethiol quenched HgTe CQDs with band edge at 4000 cm-1. b. Transfer curve (drain current as a function of gate bias) for dodecanethiol quenched HgTe CQDs with band edge at 4000 cm-1. c Infrared spectrum for oleic acid quenched HgTe CQDs with band edge at 4000 cm-1. d. Transfer curve (drain current as a function of gate bias) for oleic acid quenched HgTe CQDs with band edge at 4000 cm-1. Note that for both transistor measurements, the long ligands have been exchanged for ethanedithiol. All transistor curves are acquired at room temperature under a 400 mV bias.

Figure 5 a. Responsivity as a function of the applied bias for different thicknesses of the HgTe 6000 cm1 p-type layer used as electron blocking layer for a FTO/TiO2/HgTe (ambipolar - 4000 cm-1)/HgTe (p type - 6000 cm-1)/MoO3/Au photodiode. The inset reports the detectivity value of this diode, at 0 V and room temperature, for different thicknesses of the HgTe 6000 cm-1 p-type layer used as electron blocking layer. b. Responsivity as a function of the applied bias for ITO/TiO2/HgTe (ambipolar -4000 cm-1)/Au and for FTO/TiO2/HgTe (ambipolar - 4000 cm-1)/HgTe (p-type - 6000 cm-1)/MoO3/Au photodiode. c. Detectivity as a function of the applied bias for ITO/TiO2/HgTe (ambipolar - 4000 cm-1)/Au and for FTO/TiO2/HgTe (ambipolar - 4000 cm-1)/HgTe (p-type - 6000 cm-1)/MoO3/Au photodiode. d Bode diagram for the device of the second generation giving the frequency dependence of the photocurrent. The inset is a time trace of the photocurrent as the light source (here a 1.55 µm laser diode) is turned on and off.

Figure 4 a, b and c are respectively transfer curves for electrolyte gated thin films of HgTe CQDs with band edge at 7000 cm-1, 4000 cm-1 and 700 cm-1. All transistor curves are acquired at room temperature under a 400 mV bias. d. Ratio of the electron and hole mobilities (µe-/µh+) for thin films of HgTe with different band edges. When material presents only p-type conduction, we set the ratio µe-/µh+ to 10-2. e. Band alignment of HgTe nanocrystals with different values of band edge energy. In particular, we highlight the weak valence band offset between 4000 cm-1 and 6000 cm-1 band edge energy materials.

Figure 1 a. Infrared spectrum of HgTe nanocrystals with a band-edge energy at 4000 cm-1. The background is a TEM image of HgTe nanocrystals. b. Scheme of electrically active photodiode. c. Energy

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Design of Unipolar Barrier for Nanocrystal Based Short
Wave Infrared Photodiode
Amardeep Manikrao Jagtap, Bertille Martinez, Nicolas Goubet, Audrey Chu,
Clément Livache, Charlie Gréboval, Julien Ramade, Dylan Amelot, Paul
Trousset, Amaury Triboulin, et al.
To cite this version:
Amardeep Manikrao Jagtap, Bertille Martinez, Nicolas Goubet, Audrey Chu, Clément Livache, et al..
Design of Unipolar Barrier for Nanocrystal Based Short Wave Infrared Photodiode. ACS photonics,
American Chemical Society„ 2018, �10.1021/acsphotonics.8b01032�. �hal-01908221�

Design of Unipolar Barrier for Nanocrystal Based Short Wave Infrared
Photodiode
Amardeep Jagtap
1
, Bertille Martinez
1,2
, Nicolas Goubet
1,2
, Audrey Chu
1
, Clément
Livache
1,2
, Charlie Gréboval
1
, Julien Ramade
1,2
, Dylan Amelot
1
, Paul Trousset
1
,
Amaury Triboulin
1
, Sandrine Ithurria
2
, Mathieu G. Silly
3
, Benoit Dubertret
2
, Emmanuel
Lhuillier
1*
1
Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75005
Paris, France.
2
Laboratoire de Physique et d’Etude des Matériaux, ESPCI-Paris, PSL Research
University, Sorbonne Université Univ Paris 06, CNRS UMR 8213, 10 rue Vauquelin
75005 Paris, France
3
Synchrotron-SOLEIL, Saint-Aubin, BP48, F91192 Gif sur Yvette Cedex, France
* To whom correspondence should be sent: el@insp.upmc.fr
Abstract: Nanocrystals are promising materials for the design of low cost infrared detectors. Here we
focus on HgTe colloidal quantum dots (CQDs) as an active material for detection in the extended short-
wave infrared (2.5 µm as cut-off wavelength). In this paper, we propose a strategy to enhance the
performances of previously reported photodiodes. In particular we integrate in this diode an unipolar
barrier which role is to prevent the dark current injection to enhance the signal to noise ratio. We
demonstrate that such unipolar barrier can be designed from another layer of HgTe CQDs with a wider
band gap. Using a combination of IR spectroscopy and photoemission, we show that the barrier is
resonant with the absorbing layer valence band, while presenting a clear offset with the conduction
band. The combination of contacts with improved design and use of unipolar barrier allows us to reach
a detectivity as high as 3·10
8
Jones at room temperature with 3 dB cut off frequency above 10 kHz.
Keywords: nanocrystals, infrared photodetection, short wave infrared, photodiode

INTRODUCTION
Colloidal nanocrystals are promising materials for the design of low-cost optoelectronic devices.
Beyond their bright luminescence properties, which are used in displays and light emitting diodes
(LEDs), their use for infrared detection have generated a lot of interest over the recent years
1
especially
for two applications: solar cells
2
(ie at wavelengths below 1 µm) and thermal imaging (for wavelengths
above 3 µm).
3
The intermediate range of wavelengths from 1 to 3 µm is called short wave infrared range (SWIR) and
finds applications such as imaging of biological tissue sections,
4,5
telecommunications, night-glow
assisted night vision
6
and active imaging.
7
InGaAs is the leading technology in this range of
wavelengths. Performances reached by InGaAs detectors are undoubtedly impressive with high
quantum efficiency around 80 % and low dark current densities (≈10
-9
A·cm
-2
). However, InGaAs
technology suffers from two main issues, which are its high fabrication cost and the lack of wavelength
tunability. In this sense, nanocrystals may offer an interesting low-cost platform especially if they can
reach wavelengths where InGaAs is not effective.
Here we typically target the extended SWIR for detection up to 2.5 µm (4000 cm
-1
or 0.5 eV). Mercury
chalcogenides are certainly the most mature material to achieve absorption and photoconduction in
this range of wavelengths. There is nevertheless a limited number of reports based on mercury
chalcogenides photodiodes, which are generally focused either on shorter
8,9
or longer wavelengths.
10–
12
Our group has recently reported photodetection at this wavelength (2.5 µm) using HgTe
nanocrystals.
13
In addition to report an encapsulation strategy to obtain stable in air operability, we
have also pointed that photodiodes can be an interesting way to enhance the device detectivity,
compared to photoconductive devices.
10
The reported level of performances (quantum efficiency
below 1% and D*≈2·10
7
Jones at room temperature) was however too limited. The proposed diode
was relying on a Schottky junction with the following structure: ITO/TiO
2
/HgTe/Au, a scheme of its
energetic profile is shown in Figure 1c. This structure was nevertheless suffering from two major flaws:
(i) energetically unoptimized contact design, which limited the extraction of photocharged carriers and
(ii) a too high dark current, as shown in Figure 1c.
To further enhance the performances of this device, not only we improve the charge extraction by
optimizing the contact design, but we propose to introduce an unipolar barrier to reduce the electronic
dark current. While the concept of unipolar barrier has been extensively investigated for infrared
photodetection based on III-V epitaxially grown semiconductors,
14–16
it has only been marginally
applied to colloidal nanocrystal based solar cells to funnel the carriers in a graded band gap structure,
17
or in p-n junctions.
18
Here, we integrate this unipolar barrier to reduce the dark current of our device
and increase the detectivity, see Figure 1b and d. We report HgTe nanocrystal-based photodiodes with
detectivity reaching 3·10
8
Jones for 2.5 µm detection at room temperature. This is an order of
magnitude larger than the value reported for previous diodes at the same wavelength.
DISCUSSION
Two families of nanocrystal materials are potentially interesting to achieve absorption in the SWIR:
lead and mercury chalcogenides. Lead chalcogenide nanocrystals have been extensively used to
harvest the near infrared
2
(from 800 nm to 1 µm) part of the solar spectrum in quantum dots based
solar cells. However, in the extended SWIR (>2.5 µm), they are difficult to integrate
19,20
into

photodiodes with a vertical geometry. Indeed, large nanoparticles are required to reduce the
confinement energy and achieve band edge energy around 0.5 eV, and such nanoparticles tend to have
a poor colloidal stability. This leads to CQD films with a low quality (ie with pinholes) and results in
electrical shortcuts. To build a detector based on CQDs in the extended SWIR, narrower band gap
materials are required and mercury chalcogenides compounds offer the most mature alternative.
21–24
In the following we will focus on HgTe as active material to address the extended SWIR range. HgTe
CQDs have been synthetized using the procedure developed by Keuleyan et al.
25
Nanoparticles have a
tetrapodic shape with a size around 8 nm, see transmission electron microscopy (TEM) image in Figure
1a, and they present an optical band edge at 4000 cm
-1
(0.5 eV or 2.5 µm), see Figure 1a. It was
previously demonstrated that the material is an ambipolar conductor (i.e. it conducts both holes and
electrons), see supporting information figure S1 and S3-5. Its work function is equal to 4.7 ± 0.1 eV and
it presents an Urbach energy, which describes the gap trap distribution, of 35 meV.
13,26
Figure 1 a. Infrared spectrum of HgTe nanocrystals with a band-edge energy at 4000 cm
-1.
The
background
is
a TEM image of HgTe nanocrystals. b. Scheme of electrically active photodiode. c. Energy

band profile of the first generation of short wave infrared photodiode with a structure ITO/TiO
2
/HgTe
(ambipolar – 4000 cm
-1
)/Au. d. Energy band profile of the second generation of short wave infrared
photodiode with a structure FTO/TiO
2
/HgTe (ambipolar – 4000 cm
-1
)/HgTe (p type – 6000 cm
-
1
)/MoO
3
/Au. The relative position of the band is determined assuming a 4.7 eV work function for ITO,
27
4.4 eV work function for FTO,
28
5.1 eV for gold and 3.2 eV band gap for anatase TiO
2
29
with HOMO and
LUMO respectively at 7.3 eV and 4.1 eV. Band alignment for MoO
3
(figure S1) and values for HgTe
(figure S6) are the one determined in the supporting information.
Jagtap et al
13
proposed a first generation of HgTe based photodiode operating in the extended SWIR
based on a ITO/TiO
2
/HgTe/Au structure, see Figure 1c. While oxides layers are processed in air, all the
following steps of the device fabrication are conducted in air-free conditions to avoid the substantial
increase of dark conductance observed when ligand-exchanged HgTe CQD films are exposed to
air.
13,30,31
On top of the device, a thick Poly(methyl-methacrylate) (PMMA; water repellant) and Poly
(vinyl alcohol) (PVA; O
2
repellant) are deposited at low temperature (room temperature and annealed
at 50 ˚C) to encapsulate and obtain air-stable performances.
13
The I-V curve rectifying behavior is the result of the TiO
2
layer, which is used as electron extractor and
hole blocking layer (see Figure 1c). The responsivity of this structure (a few µA.W
-1
around 0 V) is
actually very low, which is the signature of a poor photocharge extraction. Optimizing the electron and
hole extraction is required to increase device performances. In the first section of this paper we discuss
how both hole and electron extractions can be improved by designing better contacts.
We first screen the effect of adding a hole extraction layer between the HgTe CQD layer and the gold
contact. We chose to deposit a MoO
3
layer (a scheme of the device is given in Figure 1b). The effect of
this layer on the device responsivity is shown in Figure 2a. In presence of this layer, we observe an
increase of the responsivity up to a factor two. The optimal thickness is in the range from 10 to 20 nm,
see the inset of Figure 2a.
Similarly, we screen different strategies to prepare the electron extraction layer. Two transparent
conductive oxides have been tested (indium tin oxide: ITO and fluorine doped tin oxide: FTO) and two
electron transport layers have been tested (ZnO and TiO
2
), see Figure 2b. Out of these four
combinations, the combination of FTO and TiO
2
is the one leading to the highest responsivity with a
factor two improvement compared to the former generation, see the inset of Figure 2b. This
improvement comes from the better band alignment between FTO and the top of the conduction band
of TiO
2
, see Figure 1c-d.

Frequently asked questions

Q1. What are the contributions in "Design of unipolar barrier for nanocrystal based short wave infrared photodiode" ?

In this paper, the authors propose a strategy to enhance the performances of previously reported photodiodes. The authors demonstrate that such unipolar barrier can be designed from another layer of HgTe CQDs with a wider band gap. Using a combination of IR spectroscopy and photoemission, the authors show that the barrier is resonant with the absorbing layer valence band, while presenting a clear offset with the conduction band. 

Q2. How do the authors reduce the confinement energy of large nanoparticles?

large nanoparticles are required to reduce the confinement energy and achieve band edge energy around 0.5 eV, and such nanoparticles tend to have a poor colloidal stability. 

Q3. What are the main requirements for a detector in the extended SWIR?

To build a detector based on CQDs in the extended SWIR, narrower band gap materials are required and mercury chalcogenides compounds offer the most mature alternative. 

Q4. What is the mature material to achieve absorption and photoconduction in this range?

Mercury chalcogenides are certainly the most mature material to achieve absorption and photoconduction in this range of wavelengths. 

Q5. What materials are potentially interesting to achieve absorption in the SWIR?

Two families of nanocrystal materials are potentially interesting to achieve absorption in the SWIR: lead and mercury chalcogenides. 

Q6. What is the common use of lead chalcogenide nanocrystals?

Lead chalcogenide nanocrystals have been extensively used to harvest the near infrared2 (from 800 nm to 1 µm) part of the solar spectrum in quantum dots based solar cells. 

Q7. How is the internal quantum efficiency of the HgTe layer determined?

The internal quantum efficiency, (ηint %) = ΔIPh/(eφ), where ΔI Ph is the photocurrent at 0 V bias, e is the electronic charge and Φ is the photon flux (photon/seconds), is enhanced by ≈ 26-fold as compared to the photovoltaic device without p-type HgTe layer. 

Q8. What is the work function of the ambipolar photodiode?

Its work function is equal to 4.7 ± 0.1 eV and it presents an Urbach energy, which describes the gap trap distribution, of 35 meV.13,26band profile of the first generation of short wave infrared photodiode with a structure ITO/TiO2/HgTe (ambipolar – 4000 cm-1)/Au. d. Energy band profile of the second generation of short wave infrared photodiode with a structure FTO/TiO2/HgTe (ambipolar – 4000 cm-1)/HgTe (p type – 6000 cm1)/MoO3/Au. 

Q9. What are the performance characteristics of InGaAs detectors?

Performances reached by InGaAs detectors are undoubtedly impressive with high quantum efficiency around 80 % and low dark current densities (≈10-9 A·cm-2). 

Q10. What is the ambipolar conductivity of the HgTe CQD?

While oxides layers are processed in air, all the following steps of the device fabrication are conducted in air-free conditions to avoid the substantial increase of dark conductance observed when ligand-exchanged HgTe CQD films are exposed to air. 

Q11. What is the sensitivity of the structure of the HgTe CQD?

The responsivity of this structure (a few µA.W-1 around 0 V) is actually very low, which is the signature of a poor photocharge extraction. 

Q12. What is the Fermi level for the smallest band gap?

The Fermi level is in the bottom part of the band gap for the widest band gap materials (7000 cm-1 ≈0.87 eV and 6000 cm-1 ≈0.75 eV band edge energy); in the upper part of the band gap, but still close to the middle for intermediate band gap (4000 cm-1 ≈0.5 eV and 2700 cm-1 ≈ 0.33 eV band edge energy); while for the smallest band gap, the Fermi level is found to be within the conduction band. 

Q13. What is the effect of the field effect transistor?

This field effect transistor configuration allows (i) air operability of the device, (ii) low operating biases and (iii) gating of thick films. 

Q14. What is the i-V curve rectifying behavior of the TiO2 layer?

13The I-V curve rectifying behavior is the result of the TiO2 layer, which is used as electron extractor and hole blocking layer (see Figure 1c). 

Q15. What is the ambipolar conductor of the HgTe CQD?

It was previously demonstrated that the material is an ambipolar conductor (i.e. it conducts both holes and electrons), see supporting information figure S1 and S3-5. 

Q16. What are the main flaws of the structure?

This structure was nevertheless suffering from two major flaws: (i) energetically unoptimized contact design, which limited the extraction of photocharged carriers and (ii) a too high dark current, as shown in Figure 1c. 

Q17. How much does the unipolar barrier need to be offset?

In other words, the unipolar barrier valence band needs to be aligned with the absorbing layer valence band while the conduction band needs to be offset, by an amount >> 

Q18. What is the valence band offset for the HgTe CQDs?

As a result, using a layer of HgTe CQDs with a 6000 cm-1 (≈0.75 eV) band edge energy appears as a suitable path for the design of an unipolar barrier.