BY 4.0 license Open Access Published by De Gruyter October 18, 2021

Waveguide Schottky photodetector with tunable barrier based on Ti3C2T x /p-Si van der Waals heterojunction

Changming Yang, Shiyu Qin, Yan Zuo, Yang Shi, Tong Bie, Ming Shao and Yu Yu
From the journal Nanophotonics

Abstract

MXene, a new advanced two-dimensional material, has attracted great attention in energy storage, transparent electrodes, and electromagnetic shielding due to its high conductivity, high specific surface area, and hydrophilic surface. Given the solution-processability and tunable work function, MXene also holds great potential for wide-range photodetection and integrated optics. Here, we demonstrate a waveguide integrated Schottky photodetector based on Ti3C2T x /Si van der Waals heterojunction. Specifically, the barrier of the Schottky photodetector can be adjusted by using simple surface treatment. The work function of the Ti3C2T x is reduced from 4.66 to 4.43 eV after vacuum annealing, and the barrier height of Ti3C2T x /p-Si Schottky junction is correspondingly increased from 0.64 to 0.72 eV, leading to 215 nm working wavelength blue-shift. The photodetector exhibits working wavelength tunability in short-wavelength infrared regions due to the engineered Schottky barrier. To our best knowledge, this is the first demonstration of utilizing MXene in waveguide-integrated photodetection, showing the potential applications for various scenarios thanks to the flexible working wavelength range induced by the tunable barrier.

1 Introduction

MXenes are a new class of two-dimensional transition metal carbides, carbonitrides, and nitrides, which have drawn a great amount of interest due to their impressive electronic, optical, and mechanical properties [1], [2], [3]. Among the experimentally synthesized over 20 different MXenes, Ti3C2T x is the most studied one and thus better understood [4], where T represents the surface-terminated functional groups, such as –OH, –F, and =O. Owing to the high electrical conductivity, broadband nonlinear optical response, high photothermal conversion efficiency, large saturable absorption, good chemical stabilities, and environment friendly, the Ti3C2T x has been explored in many applications, such as transparent electrodes, electromagnetic interference shielding, photocatalysts, mode-locking lasers, and surface-enhanced Raman scattering [5], [6], [7], [8], [9], [10], [11], [12].

Besides, the other promising characteristics of Ti3C2T x , including hydrophilic surface, intense surface plasmon excitations in short-wavelength infrared (SWIR) regions, and tunable work function, make Ti3C2T x a potential candidate for on-chip photodetection. First, the hydrophilic surface allows Ti3C2T x to be easily transferred and grown by complementary metal-oxide-semiconductor compatible manufacturing processes [13], [14], [15], [16]. Second, the real part of permittivity of Ti3C2T x becomes negative in SWIR regions, indicating the onset of free carrier oscillations [17]. Previous work has experimentally confirmed that surface plasmon polaritons could be excited in Ti3C2T x in SWIR regions by electron energy loss spectroscopic microscopy [18]. Third, the work function of Ti3C2T x can be adjusted using different synthetic methods or surface treatment methods [19, 20]. In contrast with graphene [21], the work function adjustment of Ti3C2T x is nonvolatile and wide-range, and it strongly depends on the surface-terminated function groups. Compared to a bare surface, the =O termination always increases the work function of MXenes, while –OH decreases it and –F has either trend depending on the specific material [22]. Since the Schottky barrier depends on the difference in the work function of the two materials in contact, a flexible and suitable barrier height can be obtained by engineering the work function of Ti3C2T x . Previous reports showed several MXene based photodetection applications, in which MXene is either used as electrodes [23, 24] or used to improve the responsivity of the detector by surface plasmon effect [25, 26]. However, there is no report on the wide response range tunable photodetector taking advantage of the modification work function of MXene, especially on the silicon photonic platform.

Here, an on-chip Schottky SWIR photodetector is constructed by forming MXene–Si van der Waals heterojunction. The Ti3C2T x is spray deposited onto the p-Si waveguide to form a good Schottky contact. We achieved 0.23 eV of the work function of Ti3C2T x adjustment by thermal annealing. The experimental results show that the measured responsivities at telecom wavelengths show a variation of an order of magnitude after thermal annealing. Moreover, the work function of the Ti3C2T x is reduced from 0.23 eV after vacuum annealing, leading to a 215 nm working wavelength blue-shift. Consequently, the photodetector demonstrates wide working wavelength tunability in SWIR regions. Our work highlights that MXenes are prospective in on-chip optical communication, wide-spectrum photodetection, and on-chip optical sensing applications.

2 Results and discussion

2.1 Device structure and working principle

The proposed waveguide Schottky photodetector is schematically depicted in Figure 1a. It contains a p doping silicon waveguide with a ridge on one side, a Ti3C2T x film, and two gold electrodes. The grating couplers are used to coupler SWIR light into/out of the silicon waveguide, with fundamental quasi-transverse electric (TE0) mode. When propagating to the region of the Schottky junction, the light is absorbed by Ti3C2T x and the photocurrent is produced under reversed bias. The p doping region is used to form a Schottky contact of low barrier height with Ti3C2T x , while the p++ doping region is used to form an ohmic contact with the electrode. The whole silicon device is fabricated on a silicon-on-insulator (SOI) wafer with 2 μm buried oxide and 220 nm top silicon, through deep UV lithography. The cross-section of the proposed photodetector is shown in Figure 1b. In order to facilitate the transfer of MXene to the waveguide, the width of silicon waveguide W 1 is tapered from 500 nm to 10 μm through an adiabatic tapering process. The widths of p doping region W 2 and p++ doping region W 3 in the 90 nm-height slab layer are 20 and 10 μm, respectively. Firstly, we use a customized mask and UV lithography to transfer the required pattern to the photoresist, and then the synthesized MXene colloidal solution was sprayed onto the whole chip. Finally, acetone is used to remove the redundant photoresist and MXene to obtain the required MXene film with a feature size of 60 μm × 40 μm. The film thickness is measured to be 130 nm by step profiler and the length of the Schottky junction is 60 μm, ensuring the light being fully absorbed. The microscope image of the fabricated device is shown in Figure 1c.

Figure 1: 
Structures of proposed waveguide Schottky photodetector.
(a) The whole structure is based on an SOI chip with a p doping silicon rib waveguide. The Ti3C2T
x
 film is deposited on the side of the doped waveguide without a ridge layer and covers the whole doped silicon waveguide and part of the gold electrode. (b) The cross-section of the proposed photodetector. (c) Microscope image of the proposed photodetector.

Figure 1:

Structures of proposed waveguide Schottky photodetector.

(a) The whole structure is based on an SOI chip with a p doping silicon rib waveguide. The Ti3C2T x film is deposited on the side of the doped waveguide without a ridge layer and covers the whole doped silicon waveguide and part of the gold electrode. (b) The cross-section of the proposed photodetector. (c) Microscope image of the proposed photodetector.

The principle of the proposed waveguide Schottky photodetector is based on the internal photoemission (IPE) process [27]. The energy band diagram of the equilibrium state Schottky junction of Ti3C2T x /p-Si heterostructure is shown in Figure 2a. According to a previous report, the Schottky barrier energy for the Ti3C2T x /p-Si contact ( Φ B ) is calculated by the following equation [28]:

(1) Φ B = S [ E g ( W m χ ) ] + B

where S and B are parameters depending on the distribution and density of the surface states. B is a constant, and the value of S varies from 0 to 1. The surface function groups like –OH, –F, and =O on the MXene easily form abundant surface states at the silicon layer, which is strongly influenced by the materials synthesis, film deposition, and post thermal annealing process. Thus, the change of work function value is not identical to the variation of the Schottky barrier height. W m is the work function of the Ti3C2T x , χ is the electron affinity of silicon, and E g is the bandgap of silicon. The obtained Schottky barrier is usually lower than the bandgap of silicon, allowing IPE of holes from Ti3C2T x to p-silicon upon the absorption of infrared photons with energy hv exceeding the barrier at the interface. The IPE process can be divided into three steps: photoexcitation, transport, and emission. Firstly, an electron in a state below W m absorbs the energy of an incident photon in the silicon waveguide, raising it to a state above W m . Correspondingly, a hot hole is raised from a state above W m to a state below W m (photoexcitation step). Then the hot hole is headed toward the interface (transport step). If the hot hole gains sufficient energy to overcome the barrier, it can be emitted into the silicon and finally collected as photocurrent (emission step). In the ideal case, as the electron affinity of silicon χ and the bandgap of silicon E g remain constant, the barrier Φ B depends on the work function W m . Meanwhile, the Φ B determines the internal quantum efficiency and the cutoff wavelength of the Schottky photodetector [29]. The responsivity R of Schottky photodetector can be expressed by

(2) R = q A 2 h v ( 1 Φ B h v ) 2

where q is the elemental charge, h is Planck’s constant, v is the frequency of incident photons, and A is the optical absorptance. Using the equation, we can get the ideal responsivity curve under the different barrier Φ B , as depicted in Figure 2b. As Φ B increases, the overall responsivity decreases and the cutoff wavelength is blue-shifted. According to Equation (1), the Schottky barrier between MXene and Si will be modified, depending on the work function of MXene. Actually, the computed result in Figure 2b is calculated by Equation (2) under the ideal case that the external quantum efficiency is 100%, and some issues including surface states are not taken into consideration. Therefore, the computed responsivities in Figure 2b are much larger than the responsivities of the actual device. Recent studies show that the work function of synthesized Ti3C2T x can be effectively changed by modifying the surface termination moieties such as –OH, –F, and =O, through thermal annealing [30]. Therefore, thermal annealing is employed to adjust the Schottky barrier of Ti3C2T x /p-Si.

Figure 2: 
(a) The energy band diagram of the Ti3C2T
x
/p-Si Schottky junction upon illumination. Here, E
vac, E

c
, and E

v
 stand for the vacuum level, the bottom of the conduction band, and the top of the valence band, respectively. E

F
 corresponds to the Fermi energy in the silicon at thermal equilibrium. 




Φ
B




${{\Phi}}_{B}$



 is the barrier height between Ti3C2T
x
 and p-Si, W

m
 is the work function of Ti3C2T
x
, χ is the electron affinity of p-Si, and E

g
 is the bandgap of p-Si. (b) The computed responsivity of the ideal Ti3C2T
x
/p-Si Schottky photodetector with a different barrier 




Φ
B




${{\Phi}}_{B}$



.

Figure 2:

(a) The energy band diagram of the Ti3C2T x /p-Si Schottky junction upon illumination. Here, E vac, E c , and E v stand for the vacuum level, the bottom of the conduction band, and the top of the valence band, respectively. E F corresponds to the Fermi energy in the silicon at thermal equilibrium. Φ B is the barrier height between Ti3C2T x and p-Si, W m is the work function of Ti3C2T x , χ is the electron affinity of p-Si, and E g is the bandgap of p-Si. (b) The computed responsivity of the ideal Ti3C2T x /p-Si Schottky photodetector with a different barrier Φ B .

2.2 Electrical characterizations of the tunable barrier

Since Ti3C2T x shows the metallic electrical conductivity and tunable work function, we first check the change of the work function of Ti3C2T x under thermal annealing post treatment. The Ti3C2T x film is spray deposited on the SOI substrate in a large area, followed by the vacuum thermal annealing in a tube furnace at 300 °C for 2 h. As depicted in Figure 3, the Kelvin probe is used to measure the work functions at different positions of Ti3C2T x film. We measured the work functions of 100 different positions of the Ti3C2T x film before and after annealing. From the average value of the work functions at different positions, the work function of the Ti3C2T x film is shifted from 4.66 to 4.43 eV after the annealing treatment. The decreased work function is probably due to the removing –F, and =O functional groups with high dipole moment by the vacuum annealing, as pointed out by the theoretical predictions and previous experiment results [31, 32].

Figure 3: 
Work functions at different positions of Ti3C2T
x
 film.

Figure 3:

Work functions at different positions of Ti3C2T x film.

Next, we measured the current–voltage curves of Ti3C2T x /p-Si heterostructure under different temperatures. As depicted in Figure 4a, both forward and reversed currents are increased with temperature, which is consistent with the predication from a Schottky junction obeying the thermionic emission model [33]. As most practical Schottky diodes show deviations from ideal thermionic emission behavior, the current–voltage relationship can be expressed as

(3) I = I s [ exp ( q V D ) n k T 1 ]

where n is the ideality factor, q is the elemental charge, V D is the voltage applied across the diode, k is the Boltzmann constant and T is the absolute temperature. The ideal factor of the diode is a key parameter to measure the quality of the Schottky junction. The closer it is to 1, the better the Schottky junction performance of the Ti3C2T x /p-Si device. I s can be expressed by

(4) I s = A eff A T 2 exp ( q Φ B k T )

where A eff is the effective area of the Schottky diode, A* is the Richardson constant and it is 32 A cm−2 K−2 for p-Si substrates. Using the current–voltage data, the ideal factors and Schottky barriers height of the Schottky contact can be easily extracted by exploiting the modified Norde method [34]. Accordingly, the tunable barrier is tested by measuring the current–voltage characteristics of the proposed photodetector before and after vacuum annealing at 300 °C for 2 h. As shown in Figure 4b, the dark current after the annealing treatment is overall reduced. According to the fitting method, the ideal factors and Schottky barriers height before and after annealing are 3.1/0.64 eV and 2.6/0.72 eV, respectively, at 20 °C. The decreased current and ideal factor after annealing imply that the defect of Ti3C2T x and p-Si interface is removed and the Schottky junction performance becomes better, and the increased barrier height implies that the barrier of Ti3C2T x /p-Si Schottky junction can be adjusted by annealing.

Figure 4: 
(a) The temperature-dependent current–voltage measurements of Ti3C2T
x
/p-Si Schottky junction in the dark. (b) The current–voltage characteristics of the Schottky photodetector in the dark before and after vacuum annealing.

Figure 4:

(a) The temperature-dependent current–voltage measurements of Ti3C2T x /p-Si Schottky junction in the dark. (b) The current–voltage characteristics of the Schottky photodetector in the dark before and after vacuum annealing.

2.3 Optical characterizations of the tunable barrier

Since the barrier determines the internal quantum efficiency of the Schottky photodetector, we further verify the tunability of the barrier from the responsivity test. In order to calculate the responsivity accurately, the loss spectrum of the grating coupler is first characterized by an optical spectrum analyzer (AQ6370, Yokogawa, Japan), and it is used to normalize the input optical power of different wavelengths. Compared with the absorption of MXene, the absorption of the p-doping Si with a low doping concentration is very weak in the wavelength range from 1530 to 1570 nm. Therefore, we exclude the absorption of p doping Si. Due to the bandwidth limitation of the grating coupler, we measure the photocurrent of the photodetector at five wavelengths by a sourcemeter (2634B, Keithley, USA), equally spaced from 1530 to 1570 nm. Figure 5a and b show the measured current-voltage characteristics of the photodetector upon 1550 nm light illumination, before and after vacuum annealing. The photocurrent increases with the increase of input optical power, and the increment of photocurrent after vacuum annealing is significantly smaller. In other words, the Schottky barrier height after annealing is higher, leading to a decrease in the responsivity of the photodetector. To obtain the responsivities, we plot the photocurrent values at the reversed bias of 1, 2, and 3 V as a function of normalized optical power, as shown in Figure 5c and d. The responsivities of photodetector without thermal annealing are 2.1, 16.4, and 35 μA W−1 for 1, 2, and 3 V, respectively. After vacuum annealing, the responsivities at the reversed bias of 1 and 2 V are 0.18 and 0.55 μA W−1, respectively. Due to the low on/off ratio at the high reversed bias, the responsivity at the reversed bias of 3 V is hard to be measured.

Figure 5: 
Current–voltage measurements under 1550 nm illumination (a) before and (b) after vacuum annealing. And photocurrent as a function of the optical power at different reversed biases (c) before and (d) after vacuum annealing.

Figure 5:

Current–voltage measurements under 1550 nm illumination (a) before and (b) after vacuum annealing. And photocurrent as a function of the optical power at different reversed biases (c) before and (d) after vacuum annealing.

Furthermore, the responsivities at the other four wavelengths are also measured, as shown in Figure 6. The red mark and the green mark indicate the responsivity values obtained before and after thermal annealing. However, since the input optical power in the waveguide is partially scattered into the air or the substrate when entering the Schottky junction area, the actual responsivity of the photodetector may be even higher. As the barrier height of the Ti3C2T x /p-Si Schottky junction is shifted from 0.64 to 0.72 eV after thermal annealing, the measured responsivities at telecom wavelengths show a variation of an order of magnitude, which is consistent with the theoretical expectation.

Figure 6: 
Comparison of responsivity before and after vacuum annealing at five operation wavelengths.

Figure 6:

Comparison of responsivity before and after vacuum annealing at five operation wavelengths.

2.4 Discussion

The results above indicate that the barrier of the proposed waveguide integrated Ti3C2T x /p-Si Schottky photodetector can be modified by adjusting the functional group of Ti3C2T x through vacuum annealing. Though the responsivity of the demonstrated photodetector is ∼40 μA W−1, some effective methods can be utilized to improve its performance. Firstly, exciting the surface plasmons of the Ti3C2T x /Si interface is a feasible method to improve the internal quantum efficiency and responsivity, such as doping gold particles in the MXene film. The hot electrons or holes in the MXene film typically acquire low transmission probability due to considerable momentum mismatch and poor overlap of the electron wavefunctions between MXene and silicon. Therefore, exciting the surface plasmons is a common way to enhance light absorption and increase the transmission probability of hot carriers by concentrating optical electromagnetic energy at the boundary. On the other hand, adopting other grow methods such as chemical vapor deposition or using other kinds of MXene such as Mo2CT x to construct a planar and compact film is also a promising way. Due to the fabrication limitation, the synthesized Ti3C2T x contains clastic films of different sizes and the film grown by spray coating is unfavorable for hot carriers to transit into silicon. It can be expected to achieve a high responsivity and wide-range tunable barrier by applying these methods in future work.

3 Conclusions

In summary, we propose and demonstrate a waveguide integrated Ti3C2T x /p-Si Schottky photodetector with a tunable barrier by modifying the work function of Ti3C2T x . Using the vacuum annealing method, the work function of Ti3C2T x film can be shifted from 4.66 to 4.32 eV. Correspondingly, the barrier of Ti3C2T x /p-Si Schottky photodetector is adjusted from 0.64 to 0.72 eV, leading to a 215 nm blue-shift of working wavelength. The measured responsivities at telecom wavelengths show a variation, which further validates the increased barrier. This work brings MXene to waveguide-integrated photodetection fields and shows the potential applications for various scenarios thanks to the flexible working wavelength range induced by the tunable barrier.


Corresponding author: Ming Shao and Yu Yu, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China, E-mail: (M. Shao), (Y. Yu)
Changming Yang and Shiyu Qin have contributed equally to this work.

Funding source: National Key Research and Development Program of China10.13039/501100012166

Award Identifier / Grant number: 2019YFB1803801

Funding source: National Natural Science Foundation of China10.13039/501100001809

Award Identifier / Grant number: 61775073

Award Identifier / Grant number: 61922034

Funding source: Key Research and Development Program of Hubei Province

Award Identifier / Grant number: 2020BAA011

Funding source: HUST Academic Frontier Youth Team

Award Identifier / Grant number: 2018QYTD08

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This work is funded by the National Key Research and Development Program of China (2019YFB1803801), National Natural Science Foundation of China (61775073, 61922034), Key Research and Development Program of Hubei Province (2020BAA011), and program for HUST Academic Frontier Youth Team (2018QYTD08).

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2021-07-28
Accepted: 2021-10-02
Published Online: 2021-10-18

© 2021 Changming Yang et al., published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.