Si Heterojunction

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Functional Inorganic Materials and Devices

Facile Fabrication of a Two-dimensional TMD/Si Heterojunction Photodiode by Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition Yonghun Kim, Soyeong Kwon, Eun-Joo Seo, Jae Hyeon Nam, Hye Yeon Jang, Se-Hun Kwon, Jung-Dae Kwon, Dong-Wook Kim, and Byungjin Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12896 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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ACS Applied Materials & Interfaces

Facile Fabrication of a Two-dimensional TMD/Si Heterojunction Photodiode by AtmosphericPressure Plasma-Enhanced Chemical Vapor Deposition Yonghun Kim,1, † Soyeong Kwon,2, † Eun-Joo Seo,1 Jae Hyeon Nam,3 Hye Yeon Jang,3 Se-Hun Kwon,4 Jung-Dae Kwon,1,* Dong-Wook Kim,2,* and Byungjin Cho3,*

1

Materials Center for Energy Convergence, Surface Technology Division, Korea Institute of

Material Science (KIMS), 797 Changwondaero, Sungsan-gu, Changwon, Gyongnam 51508, Republic of Korea 2

Department of Physics, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul

03760, Republic of Korea 3

Department of Advanced Material Engineering, Chungbuk National University, Chungdae-ro 1,

Seowon-gu, Cheongju, Chungbuk 28644, Republic of Korea, E-mail: [email protected] 4

School of Materials Science and Engineering, Pusan National University, 30 Jangjeon-dong

Geumjeong-gu, Busan 46241, Republic of Korea †

Y. Kim and S. Kwon contributed equally to this work.

KEYWORDS: Two-dimensional materials, MoS2, Photodetectors, Heterojunctions, AP-PECVD

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ABSTRACT

A growth technique to directly prepare 2D materials onto conventional semiconductor substrates, enabling low-temperature, high-throughput, and large-area capability, is needed to realize competitive 2D TMD/3D semiconductor heterojunction devices. Therefore, we herein successfully developed an atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique, which could grow MoS2 and WS2 multilayers directly onto PET flexible substrate as well as 4-inch Si substrates at temperatures of 107 A/W.21 Also, WS2 stacked with graphene was used in a vertical heterojunction whose internal and external quantum efficiency could reach up to 85% and 55%, respectively.22 In spite of such remarkable performance, 2D–2D heterojunction formation techniques, usually prepared by stacking several layers obtained from


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chemical vapor deposition (CVD) or exfoliation, suffer from serious concerns about the capability of realizing reliable large-area devices.

As alternative approaches, 2D TMD/three-dimensional (3D) semiconductor hybrid heterojunctions have accelerated the practical applications of 2D TMD materials owing to the well-established

device

physics

and

mature

fabrication

processes

of

conventional

semiconductors.23–32 In artificially designed TMD/Si heterojunction structures, the TMD thin layers play the role of a light-absorbing material with a strong light–matter interaction and the Si substrate offers an abrupt junction to promote the separation of photoexcited carriers near the 2D TMD/3D semiconductor interfaces. Recently, a scalable MoS2/Si p-n heterojunction photodetector with a fast photoresponse speed was reported.25 In addition, a monolayer MoS2/GaAs photodetector system enabled extremely high detectivity.32 However, a very high processing temperature (>500 °C) and transfer processes of as-synthesized 2D films are indispensable, which can hinder practical applications of the heterojunction devices. Therefore, a growth technique to directly prepare 2D materials onto conventional semiconductor substrates, enabling low-temperature, high-throughput, and large-area capability, is needed to realize competitive 2D TMD/3D semiconductor heterojunction devices.

In this study, we successfully developed an atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique, which could grow MoS2 and WS2 multilayers directly onto 4-inch Si substrates at temperatures of 0 for both of the TMD/Si devices, as shown in Figures 4b and 4c. The ratio of |ID(V = −0.5 V)/ID(V = +0.5 V)| at 280 K is 8.2×103 and 1.3×102 for our MoS2/Si and WS2/Si devices, respectively. This suggests that the TMD layers act as n-type semiconductors in the heterojunctions, as reported by others.25–30 The rectifying behaviors of our TMD/Si devices are similar to those of conventional semiconductor p-n junction diodes. In our measurement configuration (Figure 4a), the heterojunction is forward and reverse biased at V < 0 and V > 0, respectively. Near 0 V, the ID–V characteristics are highly asymmetric and nonlinear, indicating a very large shunt resistance (Rsh) (see Figure S5 of Supporting Information). Large Rsh suggests that our heterojunctions have well-defined interfaces with few leakage current paths.36

The reverse current (V > 0) of our heterojunctions clearly depends on the magnitude of the applied voltage, which could be attributed to band-to-band (Zener) and/or trap-mediated tunneling.26,27 In this work, a highly doped p-type Si wafer (1019 cm-3) is used, and hence the narrow depletion region allows the carriers to travel across the heterointerface.37 The avalanche


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multiplication, drift of electrons under a strong field, and resulting ionization of atoms could further increase the reverse current.26,27 It can be noted that |ID| at V ~ 0 in WS2/Si (Figure 4b) is much larger than that in MoS2/Si (Figure 4c). It is known that the electron affinity (χ) and the energy bandgap (Eg) of MoS2 and WS2 are not much different: χMoS2 = 4.2 eV, χWS2 = 4.3 eV, Eg,MoS2 = 1.2 eV, and Eg,WS2 = 1.4 eV.3,4,25–30 This suggests that band bending at the MoS2/Si and WS2/Si interfaces should be similar. The traps at the TMD/Si interface can assist tunneling transport by capturing and releasing the carriers at a low voltage.37 Such a trap-assisted tunneling current becomes large at high T, because the excitation probability of a carrier is determined by the Boltzmann factor.37 This well explains the increase of |ID| in the low and positive V region as T is raised (Figures 4b and 4c). Therefore, the larger reverse current of the WS2/Si junction, compared with that of the MoS2/Si junction, suggests that the WS2/Si junction has more interface trap states, as discussed in the XPS and TEM data (Figures 2a–2d). Figures 5a and 5b show the inverse ideality factor (1/A) of the MoS2/Si and WS2/Si heterojunctions as a function of T, respectively. The ideality factor, A, was estimated from the ID– V curves in Figures 4b and 4c, using the diode equation ࡵ۲ (ࢂ) = ࡵ૙ ቂ‫ ܘܠ܍‬ቀ

ࢗࢂ

ቁ − ૚ቃ,

࡭࢑ࢀ

(1)

where I0 is the reverse saturation current, q is the magnitude of the electron charge, and k is the Boltzmann constant.38 The estimated A (1/A) values of our TMD/Si devices are larger (smaller) than 2 (0.5) throughout the measured temperature range, similar to others’ reports.25,30 It should be noted that the A values clearly depend on the measurement temperature. When the Schockley–Read–Hall recombination via a single trap level in the depletion region is dominant, A is independent of temperature and its value is from 1 to 2.38 In conventional semiconductor p-n


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junctions, recombination processes can determine the temperature dependence of A.39 In our TMD/Si heterojunctions, the defect density at the interface, formed during TMD layer preparation, could be much larger than that in a single-crystal Si wafer. The T dependence of 1/A for the conventional p-n junction diode where the trap-assisted tunneling-mediated interface recombination process is dominant is given by ૚

࢑ࢀ

ࡱ

૙૙ 39 = ࢻ ࡱ ‫ ܐܖ܉ܜ‬ቀ ࢑ࢀ ቁ. ࡭ ૙૙

(2)

The term α indicates the ratio of the depletion widths at the n- and p-type semiconductor layers and E00 is a characteristic tunneling energy. The dotted lines in Figures 5a and 5b are fitting curves using Eq. (2). The measured data well agree with the fitting curves, especially at high T, where the trap-assisted recombination can be active. The deviation between the measured data and fitting results at low T for WS2/Si is more notable, compared with that for MoS2/Si. This can imply the difference in the trap level energies of the two devices, which are involved in the interface recombination processes. α for both of our heterojunctions was >0.5, and hence the depletion region in the Si wafer is wider than that in the TMD layer. The values of E00 were 20 and 26 meV for the MoS2/Si and WS2/Si heterojunctions, respectively. Although our attempts to apply the modelling valid for conventional semiconductor devices may require theoretical justification, the extracted parameters of α and E00 have reasonable values. Figures 6a and 6b show the current–voltage curves of the MoS2/Si and WS2/Si heterojunctions in the dark and under illumination of a green laser, respectively. The light current (IL) at V < 0 is more or less the same as ID, whereas IL at V > 0 is larger than ID. The photocurrent (Iph), defined as Iph ≡ IL − ID, is almost zero at V < 0 and has nonzero values at V > 0. A strong bias polarity dependence of Iph has been reported in TMD/Si heterojunctions, which can provide


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us the clues needed to deduce the interfacial band alignment.25–30 Responsivity (R) of our devices at the wavelength (λ) = 532 nm was estimated: 37 mA/W and 60 mA/W for MoS2/Si and WS2/Si, respectively. From the measured R and noise current level, the minimum detectable optical power (so called, noise-equivalent power) of our MoS2/Si and WS2/Si devices was also estimated to be 6.0×10-7 W and 3.4×10-7 W, respectively. In the case of MoS2-based photodetectors, Wang et al. reported a very large R of ~ 300 mA/W even under zero-bias voltage from their MoS2/Si heterojunction.24 The notable performance could be attributed to the strong light absorption of the relatively thick MoS2 film with the unique vertically standing layered structure. To increase R of our device, modification of the device structure as well as the TMD growth condition optimization can be attempted. R of our device did not show strong wavelength dependence in the visible range (see Figure S6 of Supporting Information). Iph of MoS2/Si sharply converges to ∼0.1 mA at V > 0.5 V, but Iph of WS2/Si keeps increasing as the bias voltage increases. High-density defect states and short minority carrier diffusion length can cause recombination loss of the photogenerated carriers. Applying a reverse bias can enhance separation of the electron–hole pairs by widening the depletion region and detrapping of carriers from defect states in p-n junctions.40 The Iph vs. V data of the MoS2/Si device (Figure 6a) shows that the carrier collection probability is not much affected by the reverse bias. This indicates a long diffusion length of the minority carriers and a low density of interface trap states in the MoS2/Si heterojunction. In contrast, the clear bias dependence of the WS2/Si device suggests that interface traps limit the collection efficiency of photogenerated carriers. In addition, Iph of WS2/Si shows notable hysteresis while the bias voltage is swept, as indicated by the arrows in Figure 6b. Such hysteresis can be attributed to the trapping and detrapping of photogenerated carriers at the interface traps.41


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Log-log plots of Iph at V = 1 V vs. laser power for the TMD/Si heterojunctions are shown in Figure 6c (It should be noted that the hysteresis width of Iph is relatively small at V = 1 V, as shown Figures 6a and 6b). The solid and dotted lines are linearly fitted with the measured data of the MoS2/Si and WS2/Si heterojunctions, respectively. The slopes of the fitted lines are 1.1 in MoS2/Si and 0.98 in WS2/Si, respectively. This shows that the photocurrent is proportional to the incoming photon flux. The slopes reported in many other reports are smaller than unity, because trap-mediated recombination by interface defects and surface adsorbates limits efficient collection of photogenerated carriers.29,30 The linear laser power dependence of Iph suggests that our TMD/Si heterointerfaces have low defect densities, giving rise to very high collection efficiency of the photogenerated carriers. Figure 6d shows the Iph data measured while turning on and off the light source using an optical chopper at various frequencies (f). For comparison of the two kinds of heterojunctions, Iph was normalized by Iph measured at f = 100 Hz. The measurement setup and procedures are described in the Experimental section. While increasing f from 100 to 3900 Hz, Iph decreases by 30% in MoS2/Si and by 45% in WS2/Si, respectively, and hence the ‘-3 dB bandwidth’ of our device is larger than 3900 Hz. The high-speed photodetection capability of our TMD/Si heterojunctions is comparable to those reported in previous works.25 The series resistance of the device itself and the load resistor as well as the parasitic junction capacitance will determine the ultimate time response of the photocurrent. The Iph decrease of the MoS2/Si heterojunction at high f is somewhat less than that of the WS2/Si heterojunction, which may indicate fewer defects at the MoS2/Si interface. Overall, our 2D TMDs/p-Si heterojunction devices exhibit reasonably excellent photoresponse characteristics compared to other nanomaterial-based photodetectors42-44 (see Table S1 of Supporting Information).


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Based on the reported data and experimental results, the band diagrams and transport mechanism of our TMD/Si heterojunctions can be proposed, as illustrated in Figures 7a–7d. Figure 7a shows the band diagrams of the TMD layer and the Si wafer before forming the heterojunctions. Figures 7b, 7c, and 7d provide the band diagrams of the TMD/Si heterojunction at zero bias, forward bias (V < 0), and reverse bias (V > 0), respectively. The band diagrams can be obtained using the work function (W), χ, and Eg of the TMD layers and Si reported in the literature: WMoS2 = WWS2 = 4.7 eV, WSi = 4.9 eV, χSi = 4.05, and Eg,Si = 1.1 eV.27,29 Interfacial band bending at the TMD layer is omitted, because the TMD layers are extremely thin (∼2 nm).25 The potential energy barrier heights (ΦB) for the MoS2/Si and WS2/Si heterojunctions, extracted using the conventional p-n junction diode models, are 0.35 and 0.45 eV, respectively.38 The tunneling barrier (TB), originated from the van der Waals gap and/or a very thin native oxide layer, is included.26 Under forward bias (V < 0), the barrier height is reduced to (ΦB − V) and large ID can be obtained (Figure 7c). In contrast, reverse bias (V > 0) increases the barrier height to (ΦB + V), resulting in very small ID (Figure 7d). All these discussions well explain the experimental results in a qualitative manner. Consideration of electric dipoles, originated from the wave function redistribution at the interface, could provide us with better quantitative information about the interface band structures.45,46 Illumination of light, whose energy is larger than the bandgap energies of TMD and Si, generates electron–hole pairs (EHPs). The internal electric field at the depletion region can separate the EHPs, producing Iph. Under forward bias, EHP separation is not readily allowed owing to the TB and the small electric field at the interface (Figure 7c). In contrast, the large electric field under reverse bias (V > 0) promotes EHP separation and carrier transport through the TB (Figure 7d). This scenario well explains the biaspolarity dependence of Iph.


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CONCLUSION In this study, we suggested a AP-PECVD technique for synthesizing MoS2 and WS2 multilayers directly onto 4-inch Si substrates with low processing temperatures ( 0 ΦB − V

Si

− −

ΦB + V

hν

− − −

−

+

+

+

+ +

+

Figure 7. (a) Band diagrams of the TMD layers and the Si wafers before forming the heterojunctions. (b) At V = 0, a depletion region is formed at the Si surface and a tunneling barrier (TB) is present at the interface. (c) At V < 0, the reduced barrier height (ΦB − V) allows thermally generated carriers (black) to travel across the interface easily, resulting in large ID. (d) At V > 0, the increased barrier height (ΦB + V) results in very small ID. The photogenerated carriers (red) can move through the TB with the help of the large electric field, which produces large Iph.


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Si Heterojunction

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