Van der Waals heterostructure mid -infrared emitters with electrically controllable polarization states and spectral characteristics Po-Liang Chen1 Tian -Yun Chang1 Pei-Sin Chen1 Alvin Hsien -Yi Chan2 Adzilah

2025-05-06 0 0 1017.32KB 19 页 10玖币

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Van der Waals heterostructure mid-infrared emitters with electrically controllable

polarization states and spectral characteristics

Po-Liang Chen1, Tian-Yun Chang1, Pei-Sin Chen1, Alvin Hsien-Yi Chan2, Adzilah

Shahna Rosyadi2, Yen-Ju Lin1, 3, Pei-Yu Huang4, Jia-Xin Li1, Wei-Qing Li1, Chia-Jui

Hsu1, Neil Na3, Yao-Chang Lee4, Ching-Hwa Ho2, Chang-Hua Liu1, 5, 6*

1. Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan

2. Graduate Institute of Applied Science and Technology, National Taiwan University of Science

and Technology, Taipei 10607, Taiwan

3. Artilux Inc., Zhubei City, Hsinchu 30288, Taiwan

4. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

5. Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

6. College of Semiconductor Research, National Tsing Hua University, Hsinchu 30013, Taiwan

Abstract

Modern infrared (IR) microscopy, communication, and sensing systems demand control of

the spectral characteristics and polarization states of light. Typically, these systems require

the cascading of multiple filters, polarization optics and rotating components to manipulate

light, inevitably increasing their sizes and complexities. Here, we report two-terminal mid-

infrared (mid-IR) emitters with electrically controllable spectral and polarization properties.

Our devices are composed of two back-to-back p-n junctions formed by stacking

anisotropic light-emitting materials, black phosphorus and black arsenic-phosphorus with

MoS2. By controlling the crystallographic orientations and engineering the band profile of

heterostructures, the emissions of two junctions exhibit distinct spectral ranges and

polarization directions; more importantly, these two electroluminescence (EL) units can be

independently activated, depending on the polarity of the applied bias. Furthermore, we

show that when operating our emitter under the polarity-switched pulse mode, its EL

exhibits the characteristics of broad spectral coverage, encompassing the entire first mid-

IR atmospheric window (λ: 3–5 µm), and electrically tuneable spectral shapes. Our results

provide the basis for developing groundbreaking technology in the field of light emitters.

Main text:

The mid-infrared (mid-IR) spectral region is of great scientific and technical interest,

primarily because this range of wavelengths falls within the molecular fingerprint region

and covers two atmospheric windows (λ: 3–5 µm and 8–14 µm)1,2. To date, mid-IR light

emitters have been widely used in diverse mid-IR imaging and spectroscopic systems,

opening enormous opportunities for industrial, environmental, medical, defense and

security sensing applications3-5. Continued efforts further demonstrate that these mid-IR

systems can provide an additional dimension of contrast, such as revealing camouflaged

surfaces or the compositional and functional properties of chemical species, if the

polarization states and spectral characteristics of exploited mid-IR light can be actively

controlled.6 In addition to sensing applications, it is notable that mid-IR light sources are

at the centre of modern free-space optical communication systems, and their information

capacities can be further improved via the polarization- and spectral-encoding of optical

signals7,8. However, despite promising advances, manipulating the spectral and

polarization properties of mid-IR light generally requires the cascade of filters, dispersive

optics and polarization optics together with mechanical moving parts. These mid-IR optical

components are relatively more expensive and less developed than their visible or near-

infrared counterparts. More critically, the requirements of multiple optical and mechanical

elements pose fundamental limitations for realizing miniaturized systems with robust

integration and high-speed operation.

To circumvent these technological obstacles, one possible approach is to develop mid-IR

emitters with electrically controllable polarization and spectral properties. However,

conventional mid-IR emitters generally rely on III–V or II–VI semiconductors1,5.

Emissions from these three-dimensional materials are typically nonpolarized. Wavelength-

tuneable emitters with electric control can in principle be achieved by vertically stacking

two or more electroluminescence (EL) units (i.e., creating tandem structures). These

emitters, operated within the visible region, have been successfully demonstrated using

solution-processed organic polymers9,10. Unfortunately, these wavelength-tuneable

devices require multilayered architectures to engineer the band profiles of heterostructures

and multiple electrodes to independently control each EL unit. Exploiting epitaxial

semiconductors, which generally suffer from lattice and thermal mismatches at

heterointerfaces1,5, to form tandem LEDs is a formidable challenge.

In these respects, exploiting a family of layered van der Waals (vdW) materials might be a

promising alternative route for developing electrically controllable mid-IR sources. One

prominent virtue offered by this material family is that different vdW materials can be

stacked vertically with arbitrary chosen sequences and crystal orientations due to their

weak vdW interactions11-13. Moreover, vdW materials provide a large variety of optical

bandgaps, spanning across the electromagnetic spectrum from ultraviolet to terahertz14,15.

For mid-IR emitter applications, black phosphorus (BP) has gained enormous attention,

because it has a direct and narrow gap (~0.33 eV), low Auger recombination characteristics

and an in-plane anisotropic structure16-22. By leveraging these exotic features, pioneering

works have not only successfully demonstrated BP-based mid-IR LEDs23-25, exhibiting

characteristics of linear polarized emission and high emission efficiency, but have also

shown their great promise for integrated mid-IR silicon photonics and gas sensing

applications23,26. In addition, doping BP with arsenic to form black arsenic phosphorus (b-

AsP) results in shrinkage in the band gap16,20,27. Applications of b-AsP to mid-IR

photodetections with operational wavelengths greater than 8 µm have been demonstrated

very recently28,29. However, the potential applications of using b-AsP on mid-IR emissions

and on electrically tuneable sources remain experimentally unexplored.

In this article, we experimentally demonstrate mid-IR light emitters in the two-terminal

configuration, in which their emission wavelengths and polarization states can vary with

the polarity of the applied bias. The performed bias-switchable responses are made possible

by assembling BP, b-AsP and MoS2 together to create p-n-p junctions, which establish two

distinct EL units, and by arranging their crystal orientations. In addition, our optoelectronic

characterization indicates that the emission intensity of each EL unit can be

electrically modulated with a radio frequency (RF) signal reaching a frequency of 3 MHz.

By leveraging these features, we further demonstrate that applying two polarity-switched

voltage pulse trains to the heterostructure emitter can lead to the generation of mid-IR light,

showing the characteristics of a broad spectral range and tuneable spectral shape.

Materials characterizations

We start by characterizing the basic optoelectronic properties of the mid-IR building blocks

used in our emitters. Figure 1a shows a schematic illustration of layered BP and b-AsP,

which have orthorhombic lattices with puckered honeycomb structures. For b-AsP, we

investigate two crystals with different As/P ratios: b-As0.25P0.75 and b-As0.46P0.54. The

stoichiometries of these alloys are identified by Raman and energy-dispersive X-ray

spectroscopy (EDX) measurements (see Supplementary Section 1). To

examine their luminescence properties, we illuminated λ= 2.4 µm light onto the BP, b-

As0.25P0.75 and b-As0.46P0.54 flakes, obtained by the mechanical exfoliation of synthetic

crystals onto 285 nm SiO2/Si substrates (Fig. 1b-d, inset), and then exploit the home-built

mid-IR spectrometer to analyze their emission spectra (see Methods and Supplementary

Section 2). As shown in Fig. 1b, the resolved photoluminescence (PL) spectrum of BP

displays a fingerprinting peak at λ~3.7 µm; this value is close to the band gap energy of

BP and consistent with previous reports19,23. However, the emission band maximum shifts

to longer wavelengths as the As/P ratio in b-AsxP1-x increases (Fig. 1c-d). The observed PL

shifts combined with our synchrotron-radiation-based Fourier-transform infrared (FTIR)

spectroscopy measurements (Fig. 1e, see Methods) clearly indicate that alloying BP with

As causes a reduction in the band gap. In addition to characterizing PL spectra, we

experimentally verify that the emission of b-AsP shows a linear dichroism feature. Figure

1f displays the results of the polarization-resolved PL experiment performed on a b-

As0.46P0.54 flake, which shows strong anisotropy with preferred linear polarization along the

armchair crystal orientation of b-As0.46P0.54; this phenomenon is associated with the

anisotropic selection rules near the band edge16-22. The degree of PL polarization of b-

As0.46P0.54, defined as 𝜌𝑃𝐿 = (IPL, max− IPL, min)/ (IPL, max+ IPL, min), is determined to be 84%.

Similar anisotropic optical transitions are found from BP and b-As0.25P0.75 flakes (see

Supplementary Section 3), because they all share the same puckered structure. These

results exemplify the possibilities of applying BP and b-AsP to linearly polarized mid-IR

light-emitting diodes (LEDs).

Mid-IR emitter with electrically controllable polarization states

Next, we describe the working principle of an electrically driven mid-IR vdW

heterostructure emitter, in which its linear polarization emission is bias-switchable.

Figure 2a,b presents a schematic and optical image of our proposed emitter, composed of

MoS2 (7.2-nm-thick) between two BP light-emitting layers. The top BP (BPt) and bottom

BP (BPb) have thicknesses of 38 nm and 47 nm, respectively, and their armchair crystal

orientations are orthogonal with each other (see Methods and Supplementary Section 4).

Because BP has a smaller work function than MoS2, the band alignment of the

BPt/MoS2/BPb (top to bottom) heterostructures results in two rectifying junctions

connected back-to-back; this finding is corroborated by our scanning photocurrent

measurements (see Supplementary Section 5). In addition, it is notable that MoS2 exhibits

a large valence band discontinuity (ΔEv) but a nearly zero conduction band offset (ΔEc)

with neighbouring BP30,31. Our finite element simulations, which are shown in Fig. 2c-d

(also see Methods), indicate that such band alignment can cause electrons to flow

unimpeded throughout the emitter, but the movement of holes is blocked by the barrier at

the BPt/MoS2 (BPb/MoS2) interface when applying the positive (negative) bias voltage

across two BP layers with BPb connected to the ground. Therefore, whether the

recombination of electron-hole pairs occurs at BPt or BPb is associated with the polarity of

the applied bias, offering the possibility for the electric control of the polarization direction

of the emitted photons.

To verify our proposed device operation principle, we apply the bias voltage, Vb, onto the

BPt/MoS2/BPb heterostructure emitter to examine its optoelectronic properties. Fig. 2e

shows the measured I-Vb characteristics. The rectification behaviour is observed at both

positive and negative bias regions, which is correlated with the p-n-p junctions of the

heterostructures. Interestingly, we find that such electrical excitation leads to mid-IR EL

generation at both DC bias polarities (Fig. 2f-g). The peak of the measured EL spectrum is

at λ= 3.67 µm, which is close to the PL result shown in Fig. 1b, and the peak wavelength

remains unchanged with the variation of Vb (see Supplementary Section 6), confirming that

the emission originates from the electron-hole recombination in BP and not from black-

body radiation. Moreover, our linear polarization-resolved measurement on EL reveals that

the emission is strongly anisotropic (Fig. 2h). Under a positive (negative) bias, the EL

intensity is maximized at 𝜃 = 90° (𝜃 = 0°), which is aligned with the armchair direction

of BPt (BPb). The calculated degree of EL polarization factors 𝜌𝐸𝐿 = (IEL, max− IEL, min)/(IEL,

max+ IEL, min) are 88% for the positive bias and 87% for the negative bias. These features

evidently confirm that a mid-IR emitter with electrically switchable and linearly polarized

emission can be realized by leveraging the potentials of vdW heterostructures and the

anisotropic properties of BP.

To gain further insight, we calibrate the bias-dependent output powers of the BPt/MoS2/BPb

heterostructure emitter (see Supplementary Section 7 for the calibration). The results

shown in Fig. 2i (black dots) indicates that its emission is initiated at low bias voltage

(|𝑉𝑏|∼ 0.2 V). With the increase in 𝑉𝑏, the output power approaches 0.5 μW based on the

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VanderWaalsheterostructuremid-infraredemitterswithelectricallycontrollablepolarizationstatesandspectralcharacteristicsPo-LiangChen1,Tian-YunChang1,Pei-SinChen1,AlvinHsien-YiChan2,AdzilahShahnaRosyadi2,Yen-JuLin1,3,Pei-YuHuang4,Jia-XinLi1,Wei-QingLi1,Chia-JuiHsu1,NeilNa3,Yao-ChangLee4,Ching-HwaHo2,Ch...

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Van der Waals heterostructure mid -infrared emitters with electrically controllable polarization states and spectral characteristics Po-Liang Chen1 Tian -Yun Chang1 Pei-Sin Chen1 Alvin Hsien -Yi Chan2 Adzilah.pdf

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Van der Waals heterostructure mid -infrared emitters with electrically controllable polarization states and spectral characteristics Po-Liang Chen1 Tian -Yun Chang1 Pei-Sin Chen1 Alvin Hsien -Yi Chan2 Adzilah

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