High-Performance Broadband Floating-Base Bipolar Phototransistor

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High-Performance Broadband Floating-Base Bipolar Phototransistor Based on WSe/BP/MoS Heterostructure 2

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Hao Li, Lei Ye, and Jian-Bin Xu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00778 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 31, 2017

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High-Performance Broadband Floating-Base Bipolar Phototransistor Based on WSe2/BP/MoS2 Heterostructure Hao Lia, Lei Yeb,*, Jianbin Xua,* a

Department of Electronic Engineering, Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.

b

School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, People’s Republic of China

* Corresponding author at: Department of Electronic Engineering, Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, New Territories, Hong

Kong,

China.

Fax:

+852-2609-8297;

Tel:

+852-2609-8297;

[email protected]; [email protected]

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ABSTRACT Recently, there are increasing interests in two-dimensional materials, as a result of their outstanding electrical and optical properties and numerous potential applications in optoelectronic devices. Here, we first report on a bipolar phototransistor based on WSe2-BP-MoS2 van der Waals heterostructure, showing its broadband photo-response from visible to the infrared spectral regions. Broadband photoresponsivities for visible (532 nm) and the infrared (1550 nm) light waves reach up to 6.32 A W-1 and 1.12 A W-1, respectively, which are both improved by tens of times in comparison with similar photodiode devices composed of WSe2-BP. The phototransistor also exhibits ultrasensitive shot noise limit specific detectivities which are 1.25×1011 Jones for visible light at wavelength λ=532 nm and 2.21×1010 Jones for the near-infrared light at wavelength λ=1550 nm at room temperature. It is a promising candidate for progressive development of photodetector, with implementation of smaller sensor elements, large sensing area, super-high integration and broadband photoresponse.

KEYWORDS: two-dimensional material, optoelectronic, bipolar phototransistor, amplification capacity, infrared

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Photodetector, to convert light into electrical signals, has obtained broad attention due to its numerous practical applications including in video imaging, optical communications, biomedical imaging, security, night-vision, and motion detection.1-3 Along with the development of the scale and diversity of photodetector’s applications, the need for a photo-detection platform with better performance in terms of photo-responsivity, bandwidth, response-time and noise, is becoming more and more eminent. In the past decade, rapid advances in photodetectors have been realized by the introduction of new materials. Especially recently, the rising stars of two-dimensional (2D) layered structure materials present excellent electronic and optoelectronic properties such as ultrafast carrier dynamics, layer-dependent energy bandgap, tunable optical properties, low power dissipation and excellent transparency. 4-9

The most representative 2D material graphene is considered counterproductive to

high-performance photodetectors, because the absence of a natural energy bandgap in graphene appears as an enormous obstacle on its applicability for high-performance optoelectronic devices and other electronic switches, which therefore draws great attention to turn to other alternative 2D materials such as transition metal dichalcogenides (TMDs, tungsten diselenide WSe2, molybdenum disulfide MoS2) and black phosphorus (BP) that have suitable band gaps.10-13 To date, various high performance 2D materials photodetectors have already been developed to rival commercial III-V photodetectors.14 But to further achieve better performance, an evolution of 2D materials photodetectors on its structure is eagerly awaited. Based on previous works on 2D materials photodetectors,15-20 the mostly reported device 3


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structures are operated either in photoconductive mode or photovoltaic mode. Despite the high achievable gains and the consequent large photo-responsivity, the photoconductive detectors typically exhibit slow response time, low on/off ratio, low quantum efficiency and inevitable high dark current,21,

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because the absence of

internal electric field limits their practical applications. For the alternative photodiode detectors, the existence of built-in electric field results in fast response time and low dark current, showing their potential in smaller and faster optoelectronic devices, which paves the way to find a new pathway to improve their native low photo-gain and the consequent low photo-responsivity. 23-25 Bipolar phototransistors, composed of two opposite photodiodes, present ultrasensitive photoelectric response in weak light intensity scenarios due to its additional internal gain to amplify the weak photo-induced current. In a bipolar phototransistor, the inherent photodiode produces a photocurrent which subsequently is to be amplified by a transistor, achieving a well-rounded performance in terms of gain, bandwidth, responsivity, quantum efficiency and dark current noise. 26-28 Here, we have designed a van der Waals assembled WSe2-BP-MoS2 bipolar phototransistor. Black Phosphorus is an ideal material for the base because of its high mobility and broadband photo-absorption. MoS2 as the emitter region is expected to be highly n-doped to provide a large number of electrons for injection into the base region in the amplification process. Lightly n-doped WSe2 whose Fermi level fluctuates over the middle of its bandgap, is utilized as the collector to form BP-WSe2 photodiode. Resembling a traditional n-p-n bipolar junction transistor (BJT), the 4


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WSe2-BP interface acts as a base-collector (B-C) junction which also converts the incident light into a small photocurrent Iph. The MoS2-BP interface acts as a base-emitter (B-E) junction to block holes’ conduction and control electron injection. This carrier injection process is controlled by a base current in a traditional BJT device and this role is played by the small photocurrent Iph in our device. The proof-of-concept phototransistor demonstrates broadband light absorption, showing its broadband photodetection from visible to near-infrared region. In addition, the photoelectrical amplification behavior of WSe2-BP-MoS2 phototransistor is observed by comparing with the performance of WSe2-BP two-component photodiodes. With the internal electrical gain, the device exhibits enhanced photoresponsivities of 6.32 A W-1 responding to visible light of λ=532 nm, and of 1.12 A W-1 responding to the near-infrared light of λ=1550 nm, which are both improved by tens of times in comparison with the corresponding two-component photodiode devices. The device also exhibits a high shot-noise-limit specific detectivity of 1.25×1011 Jones for visible light of λ=532 nm and a specific detectivity of 2.21×1010 Jones for the near-infrared light of λ=1550 nm at room temperature.

RESULTS AND DISCUSSION We fabricated proof-of-concept floating-base bipolar phototransistors using few layers WSe2, BP and MoS2 in combination, as illustrated schematically in Figure 1a. In Supporting Information, Figure S3 is the Raman spectroscopic measurement of different positions of the device, showing the components of device and the formation 5


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of two van der Waals heterojunction. The AFM measurement of sample thickness is also shown in Figure S4. The adoption of such a floating-base approach favors simple monolithic integration with optical receivers since only two external electrodes are required for the emitter and collector regions. WSe2-BP-MoS2 heterojunction is fabricated by sequentially transferring mechanically exfoliated MoS2, WSe2 and BP thin films onto a 100nm SiO2/Si substrate with BP located on the gap between WSe2 and MoS2 and both sides forming van der Waals vertical junction (more details are in the Supporting Information). Based on the respective semiconductor characteristics of these 2D materials, the WSe2-BP-MoS2 floating-base bipolar junction presents the characteristics of an n-p-n structure, where charge flow is limited by the diffusion of charge carriers across the base region and the energy barrier on the BP-MoS2 interface, leading to very small dark current and the consequent high photo-detectivity for high-performance photo-detection. To investigate the underlying photoelectrical characteristics of the bipolar junctions composed of WSe2-BP-MoS2, photoelectrical measurements of the device under different photo-excitation intensities from a red laser (wavelength of 637 nm) are first performed. Strong photoresponse of the device can be observed from the I-V characteristics under different incident laser powers, as shown in Figure 1b. Moreover, by employing a floating-base approach, the interesting photoelectrical behavior of the phototransistor demonstrates a dependence of photocurrent (Ic) on the applied bias Vce across the collector-emitter (C-E) terminals. At low Vce < 1.5 V, a drastic increase in the photocurrent is observed with increasing Vce, but this gradually 6


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saturates at higher applied voltages. In addition, the Ic demonstrates an enlarging trend with the increase of the input illumination, attributing to the increase of photoinduced carrier concentration.

Figure 1. (a) Schematic of the WSe2/BP/MoS2 heterojunction. (b) I-V characteristics of the WSe2/BP/MoS2 heterojunction under illumination by 637 nm laser with various incident powers. Power legend: Illumination power shined on the junction area.

In order to better interpret I-V characteristics of the device under illumination, the schematic and band diagram of the bipolar junction phototransistor is used to reveal its work mechanism induced by incident light. Based on the previously works29, 30 discussing the band diagrams of MoS2/BP and BP/WSe2 junctions, the band diagram of our device without illumination under Vce= 0V is shown in Figure 2a. For the band diagram shown in Figure 2b, the device is under illumination and Vce=3V. In this case, there is an external voltage applied on the MoS2/BP/WSe2 junction, influencing the band diagram of the junction. Due to ohm’s law, the distribution of the external voltage applied on the MoS2/BP/WSe2 junction depends on the resistances of the MoS2/BP and BP/WSe2 junctions. As the conductivity of a forward-bias BP/MoS2 7


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hetero-junction is over 200 times better than that of a reverse-bias BP/WSe2 hetero-junction8, Vce is mainly located on the BP/WSe2 junction, leading to the band diagram of the device working under illumination in Figure 2b. According to the diagram, the photo-induced electron-hole pairs are separated into holes and electrons by the electric field at base-collector interface, and the photo-induced holes are accumulated in the base region (BP), resulting in an increase in the forward bias across the base-emitter junction. This leads to the potential barrier (emitter-base) lowering to enable a significant number of electrons to be injected from the emitter into the base region. Hence, compared with WSe2-BP photodiode whose band diagram is depicted in Figure 2c, the current of the phototransistor Ice is formed by the superposition of Icb (Iph) and Ibe, achieving the amplification of the photocurrent Iph. At low Vce voltage, the poor efficiency of electron-hole pair separation results in a negligible accumulation of holes at the base-emitter interface, leading to a small Ice. But with the increase of Vce, the separation efficiency of photo-induced hole-electron pairs is improved, until it reaches the saturation, resulting in the corresponding increase and saturation of carrier injection (Ibe) at the B-E interface, and hence the device current Ice increases initially and saturates finally. Figure 2d directly presents the relationships between photocurrent and dark current of the phototransistor at the same Vce by the on/off ratio (illuminated under 637 nm), which is the ratio between photocurrent and dark current. The maximal on/off ratio is about 103 around zero-bias voltage, indicating that the photocurrent is about three orders of magnitude higher than the dark current. More importantly, the on/off ratio of the phototransistor 8


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achieves 4×102 at an applied bias of 1 V, which is better than the previously reported values of BP photodetectors. These results demonstrate the high-performance optoelectronic characteristics and the potential of the device.

Figure 2. (a) The schematic and band diagrams of the WSe2/BP/MoS2 heterojunction in dark, Vce=0V (b) The schematic and band diagrams of the WSe2/BP/MoS2 heterojunction under illumination, Vce=3V (c) The schematic and band diagrams of the WSe2/BP heterojunction under illumination (d) The photo on/off ratio between photocurrent Iph and dark current Idark of the WSe2/BP/MoS2 heterojunction under 637 nm illumination with different intensities. Power legend: Illumination power shined on the junction area. 9


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Broadband photo-response is an essential factor to fulfill the requirement of modern photodetectors, which is now developed vigorously. In this work, the existence of BP renders the device a high-performance ultra-broadband photodetector, covering the spectral ranges from visible to the near-infrared, because of its direct narrow bandgap (0.3 eV) and high light absorption efficiency. Characteristics of broadband photoresponse of the device over the wide range of incident wavelengths is investigated by using multiple laser diodes operating from visible to the near-infrared range (532, 637, and 1,550 nm) under the same incident power of 13.5 nW. The total current Ic of the device operated under illumination for various wavelengths are summarized in Figure 3a. Based on the results, the device shows that Ic is about 90 nA at bias voltage Vce = 3 V for 532 nm wavelength, which is slightly higher than the current for 637 nm wavelength and more than higher than the current for 1550 nm wavelength. The reason is that only BP absorbs in the near-infrared region while BP and WSe2 both absorb in visible region. Despite the comparatively low photocurrent under the infrared illumination, the broadband spectral attribute of the device is clearly observed. The wavelength-dependent performance evaluation of the device is performed by studying its photoresponsivity (R), as plotted in Figure 3b. A high photoresponsivity R = 6.32 A W-1 is achieved under 532 nm illumination, which is three orders of magnitude larger than the previously reported values of BP-based photodetectors at the same wavelength.13, 31 For the near-infrared wavelength of 1550 nm, a high responsivity of 1.12 A W-1 has been achieved. We further estimate the photogains (G) to quantify the photosensitivity of the device. In Figure 3b, the 10


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corresponding photogain is plotted, which can be calculated by equation G=(∆I/e)/(P/hν), where ∆I is the photocurrent, P is the incident power, e is the elementary charge, hv is the incident photon energy. From the results, the photogain in visible region (532 nm) is close to 15, whereas in the near-infrared region (1550 nm) it is close to 1. As summarized in Figure 3b, the photogain shows an increasing trend from 1550 to 532 nm, which can be attributed to the increased photon energy.

Figure 3. (a) I-V characteristics of the WSe2/BP/MoS2 heterojunction device under the dark and 13.5 nW illumination from different wavelength lasers (532 nm, 637 nm, 1550 nm). (b) Wavelength-dependent photoresponsivity and photogain of the WSe2/BP/MoS2 heterojunction device.

The coexistence of high responsivity and high on/off ratio of the device is achieved by introducing BJT-based current amplification to the photoresponse of a photodiode. The further confirmation is carried out by comparing the photoresponse measurements of the WSe2-BP-MoS2 (n-p-n) phototransistor and WSe2-BP (n-p) photodiodes. In order to ensure the accuracy of comparative experiments, we studied 11


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the measured photocurrent density, which is calculated by dividing the photocurrent across the effective adsorption area, rather than directly comparing photocurrent to avoid the possible error caused by the difference in device scale. As shown in Figure 4a, obviously larger photocurrent density of WSe2-BP-MoS2 bipolar phototransistor is demonstrated, which reveals the achievement of ten-fold higher responsivity than those of WSe2-BP photodiodes, illustrating its amplification of sensitivity as a photodetector. The underlying mechanism responsible for such impressive amplification is elucidated by the schematic band diagrams shown in Figure 2b. Under the dark, the band alignment of the device leads to the formation of a potential barrier across the B-E (BP-MoS2) junction. After illumination, the optically generated holes, which are obtained by the separation of photoinduced electron-hole pairs at the B-C interface, are swept to the B-E interface, while the relevant electrons are drifted to the collector region by the electric field. By virtue of the hole accumulation, the potential barrier could be lowered to enable significant electrons injection from the emitter into the base region. When the diffusion length of the injected electrons is much longer than the base width, a substantial current gain could be achieved through normal transistor actions. The base width of the device is about 1.5 µm. The diffusion length of the injected electrons is calculated by Equation 1, = √

(1)

where τ is the electron lifetime and D is the electron diffusion coefficient in BP. Based on previous reports,32, 33 τ is 100 ps and D is 1300 cm2s-1, the calculated electron diffusion length L is about 3.6 µm, which is bigger than the base width, thereby 12


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contributing to an amplification of the photocurrent. Moreover, the device employs MoS2, with a wide-bandgap, as emitter material to provide a larger heterobarrier in the valence band to effectively prevent the reverse injection of holes from the base region, thereby contributing to the improved electron injection efficiency, which results in a remarkable enhancement of responsivity over WSe2-BP photodiode. Figure 4b shows the dark current comparison between the WSe2-BP-MoS2 and WSe2-BP devices, illustrating the much smaller dark current of WSe2-BP-MoS2 device than that of WSe2-BP devices, especially at the same forward bias voltage of 3 V. The existence of the potential barrier across the B-E junction provides a great obstacle to make the charges in the device hard to overcome, leading to the small dark current. Another important parameter to quantify the photoresponse characteristics of the photodetector is the specific detectivity D* which is calculated by the equation, D* = (A∆f)1/2/NEP, where A is the effective area of the photodetector, ∆f is the electrical bandwidth in Hz (fixed as 1 Hz in this case), and NEP is the noise equivalent power. As shot noise is the major contributor to the total noise in our device, D* can be expressed as D*shot-noise-limit = IPh/P(2qAIdark)1/2.1, 2 Based on the measured parameters, the D*shot-noise-limit values of the detector are approximately 1.25×1011 Jones under 532 nm illumination and 2.21×1010 Jones under 1550 nm illumination at room temperature (calculation process can be found in Supporting Information), which is much better than that of the commercially available near-infrared photodetectors at room-temperature. 13


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Shot noise limit detectivity is usually measured to demonstrate the ultimate potential of a photodetector in sensitivity. However, this theoretically predicted value could be of great difference to the practically measured value due to the unavoidable ambient noise. To quantify the practical noise, we have extracted the effective noise current from a series of continuously measured dark current values as shown in Figure S5. The noise current spectrum in Figure S6 indicates that the dominating noise in our device is the 1/f (flicker) noise. Based on the measured parameters, the measured D* values of the detector are approximately 1.01×109 Jones under 532 nm illumination and 1.74×108 Jones under 1550 nm illumination at room temperature. (calculation process of D*shot-noise-limit and D* can be found in Supporting Information). As it’s meaningless to compare theoretical values with measured values, we have classified the recently reported detectivities for different 2D material based photodetectors in Table S1. It’s clearly shown that, although the detectivities of our device are limited by the small active area, they are still very competitive to other high performance 2D material based photodetectors in visible and near infrared region.

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Figure 4. (a) The comparison of photocurrent density between WSe2-BP-MoS2 phototransistor and WSe2-BP photodiode, in which the photocurrent density of WSe2-BP photodiode presents five devices deviation (radiation: 135 nW@532 nm, Vce=3 V). (b) The comparison of dark current between WSe2-BP-MoS2 phototransistor and WSe2-BP photodiode, in which the dark current of WSe2-BP photodiode presents five devices deviation.

Figures 5b and 5c exhibit the photocurrent mapping results of the device measured at a forward bias voltage of 3 V and reverse bias voltage of -3 V, respectively, which are both illuminated under the light of λ=532 nm with the intensity of 54.5 nW. Distinct photo-sensitive areas at both the forward and reverse biases can be observed in the device. At the forward bias, the sharp area of photocurrent mapping is mainly concentrated on the overlapped region of WSe2/BP as shown in Figure 5b, indicating that the photoinduced electron-hole pairs are generated and separated at the WSe2/BP interface region which is in accordance with the operating principle of BJT at forward bias. At the reverse bias, as shown in Figure 5c, the two photo-sensitive areas are observed at the overlapping regions of BP/MoS2 and WSe2/metal. The major 15


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photo-induced current is contributed from the electron-hole pair separation at BP/MoS2 and WSe2/metal interface regions with the same orientation of electron-hole separation at BP/MoS2 and WSe2/metal interfaces.

Figure 5. (a) Optical image of the WSe2/BP/MoS2 heterojunction device (b) Photocurrent mapping of the WSe2/BP/MoS2 heterojunction at a forward bias voltage of 3 V. (c) Photocurrent mapping of the WSe2/BP/MoS2 heterojunction at a reverse bias voltage of -3 V. Scale bar (a-c) 4 µm.

Because the narrow base width of the phototransistor makes the carriers easy to diffuse through, the optimized amplification coefficient β can be achieved by varying the base region (BP) width WB. The amplification factor β is therefore calculated by JPh1/JPh2 (JPh1 is the photocurrent density of BJT; JPh2 is the photocurrent density of BP-WSe2 photodiode), at the same bias voltage. To study the influence, the relationship between β and WB is plotted in Figure 6. From the results, the significant enhancement of β is observed from 0.12 to 9.8 with decreasing WB from 7.3 to 1.5 µm, by comparing different devices with different base width. This phenomenon is consistent with the previous works on bipolar phototransistors. The relationship 16


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between β and WB of the devices can be calculated by equation 2, which is usually used to calculate β for traditional bipolar phototransistors.34,35

1 （WB / 2τ B DP ) + ( DNWB ND / DP LP N A）

β≈

2

(2)

where τB is the minority carrier lifetime in the base, DN and DP are the carrier diffusion coefficients in the base, LP is the diffusion length of holes in the emitter, NA and ND are the acceptor and donor densities in the base and emitter, respectively (More details can be found in the Supporting Information). From Figure 6, the measured results are in good agreement with the calculated data based on the above equation, indicating the amplification mechanism, while the inevitable contact resistance becomes less impactful when the device performance improves. Therefore, the high performance WSe2-BP-MoS2 bipolar phototransistor with its internal amplification of the photocurrent provides an appealing platform for photodetector applications, as it does not need separated amplifiers helpfully to save chip area and reduce system costs and inherent noise.

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Figure 6. The relationship of amplification coefficient β and the base region (BP) width WB for the WSe2-BP-MoS2 phototransistor.

CONCLUSION In conclusion, we have fabricated a bipolar phototransistor via a combination of WSe2, BP and MoS2. This 2D-material phototransistor presents broadband photoresponse from visible to the infrared spectral regions, with the corresponding photoresponsivities of 6.32 A W-1 and 1.12 A W-1, respectively, which are both improved by tens of times in comparison with similar photodiodes composed of WSe2-BP, indicating its unique merit of photocurrent amplification capacity. Moreover, its ultrasensitive shot noise limit specific detectivity is estimated to be 1.25×1011 Jones for visible light at λ=532 nm and 2.21×1010 Jones for the near-infrared light at λ=1550 nm at room temperature. The practically measured specific detectivities of the detector are approximately 1.01×109 Jones under 532 nm illumination and 1.74×108 Jones under 1550 nm illumination at room temperature. The value This generic strategy by combining 2D layered materials into 18


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heterostructures to enable multifunctional devices and components may open a new avenue for high-density monolithic integration of functional devices in a limited area.

MEHTODS Device Fabrication. Bulk MoS2, WSe2 and BP were brought from Sigma-Aldrich and Smart-Element. The few-layer BP, few-layer WSe2 and few-layer MoS2 sheets were mechanically exfoliated using adhesive tape from bulk materials onto the SiO2 substrate, respectively. Subsequently, the few-layer WSe2 sheet was transferred onto one side of the electrodes (fabrication method for electrodes: photolithography and thermal evaporation), and then the few-layer MoS2 sheet was transferred onto the other side of the electrodes and lastly the BP sheet was transferred onto the middle of MoS2 and WSe2 to form WSe2/BP/MoS2 heterojunction. Device Measurement Setup. Devices were measured with a Keithley 4200 semiconductor characterization system. Atomic force microscopy (AFM) image of device was taken in the tapping mode by carrying out on a Nanoscope IIIa Multimode apparatus. Raman spectra of device were obtained using a Raman spectrometer (Renishawin Via Raman microscope, excitation at 514 nm). In photocurrent measurement, the device was placed inside an electrically shielded and optically sealed probe station system (Lakeshore CPX-VF). A 3D adjustable optical fiber was used to guide the 637 nm and 1550 nm lasers with a diameter of 0.2 mm. The photocurrent mapping was obtained by raster-scanning over the device using a 19


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scanning Galvo System (Thorlabs GVS212) with a modulated laser beam. The modulated photocurrent signals were amplified and detected using a lock-in (Signal Recovery model 7270) technique.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant Nos. N_CUHK405/12, AoE/P-02/12, 14207515, 14204616, and CUHK Group Research Scheme, as well as Innovation and Technology Commission ITS/096/14. J. B. Xu would like to thank the National Science Foundation of China for the support, particularly, via Grant No 61229401.

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High-Performance Broadband Floating-Base Bipolar Phototransistor Based on WSe2/BP/MoS2 Heterostructure Hao Lia, Lei Yeb,*, Jianbin Xua,* The schematic of the WSe2/BP/MoS2 bipolar phototransistor and an internal photocurrent gain of 9.8 is achieved by introducing BJT-based current amplification to the photoresponse of WSe2/BP photodiodes.

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