Carrier Transport Dynamics in High Speed Black Phosphorus

Jan 25, 2018 - Here, we use high speed black phosphorus photodetectors with sub-40 ps response time to elucidate carrier transport dynamics along its ...
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Carrier Transport Dynamics in High Speed Black Phosphorus Photodetectors Jianbo Gao, Apparao M. Rao, Hongbo Li, Jianbing Zhang, and Ou Chen ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01431 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Carrier Transport Dynamics in High Speed Black Phosphorus Photodetectors Jianbo Gao,

*,§

Apparao M. Rao,

*,§

‡

Hongbo Li, Jianbing Zhang,∥ and Ou Chen

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§

Department of Physics and Astronomy, Clemson University, Clemson 29634, United States School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ∥ School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China # Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States ‡

*

E-mails: [email protected]; [email protected]

A fundamental understanding of carrier transport mechanism is imperative for efficient semiconductor electronics and optoelectronics. Here, we use high speed black phosphorus photodetectors with sub-40 ps response time to elucidate carrier transport dynamics along its armchair and zigzag directions. Here we report a direct observation of carrier transport transition dynamics from phonon scattering transport to multiple trapping and release transport mechanism along the armchair direction, resulting from the relaxation of free carriers above the band edge to the band-tail states. We identified that the suppression of phonon scattering effects, a characteristic by Hall and field effect transistor measurements, is due to carrier transport in band tail states. Along the zigzag direction, only multiple trapping and release transport in band-tail states is observed, which might be due to low carrier mobility.

Key Words: Two dimensional, black phosphorus, photodetector, transport dynamics The most widely studied two-dimensional (2D) materials include the zero-band gap graphene1-2, large band gap transition metal dichalcogenides (TMDCs)3-4, and the insulating hexagonal boron nitride. Black phosphorus (BP), an emerging 2D semiconductor, bridges this energy band gap owing to its thickness-dependent tunable band gap (0.3 eV-2.0 eV). In addition, its carrier mobility (on the order of ~ 103 cm/Vs) bridges the mobility gap between that of TMDCs (~ 102 cm/V) and graphene (104 cm/Vs). These unique features of BP are promising for efficient field effect transistors (FETs)5, solar cells6, photodiodes7, photodetectors8-12, high speed photodetectors13-14 and thermal electronics15-16. However, our fundamental understanding of its carrier transport dynamics is still in its infancy17-20. To date, the FETs and Hall measurements 5, 21-22 provide evidence for a transition from phonon scattering transport (dominated at high temperatures) to thermally activated transport owing to defect scattering (dominated at low temperatures). In the high temperature range (~100 K to room temperature), the carrier mobility exhibits a strong temperature dependence with a ~T –n power law relation, implying band transport mechanism. The theoretical index n value is close to 1.5, which is expected for phonon scattering that depends on phonon density and the thermal velocity of carriers. However, the reported values of index n varies from 0.5 to 0.75, 21, 23, much smaller than that of bulk BP crystals and other 2D materials which range between 1.2~1.422, 24. Therefore, the origin of phonon scattering suppression in BP remains unclear, and scattering by defects or impurities that lead to mobility increases with increasing temperature may play a critical role. In general, it remains challenging to unravel the carrier transport properties in devices since they are altered in the presence of defects/impurities or trap states. An ultrafast probe or high quantum efficiency device (the ratio between electrons collected to photon number), or both, are required to elicit carrier transport properties before significant carrier trapping into trap states and recombination occur. Although ultrafast optical pump-probe approaches with sub-ps time resolution have been used 25 for characterizing solution-based or thin film samples, the probe beam suffers from scattering due to the complex device architectures. Here, we describe a BP based high speed photodetector26-30 with sub-40 ps response and ~100 % external quantum efficiency (EQE) to study the carrier transport dynamics in BP along its armchair and zigzag directions. We discovered direct evidence for a transition from phonon scattering transport above the band edge to multiple trapping and release (MTR) transport in band-tail states. This observation 1

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suggests that the thermal activation energy increases with time as the index n decrease from 1.2 to 0. To the contrary, only MTR transport in band-tail states was observed along the zigzag direction, which may be due to the low carrier mobility along this direction.

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Fig. 1 High speed BP photodetector configuration and typical transient photocurrents along its armchair direction. a, A schematic of the high speed black phosphorus (BP) photodetector. An electric field is imposed on the BP flake, and a pulsed laser (800 nm, 100 fs) illuminates it through the glass substrate to generate photocurrent along the armchair direction, which is collected by a 20 GHz sampling oscilloscope (R). The left inset is a SEM image of the BP photodetector (scale bar = 25 µm). The right inset depicts the lattice structure of BP along the armchair and zigzag directions. b, Raman spectra of the BP flake with the excitation laser polarized along the armchair and zigzag directions to confirm the orientation of the flake. c, Representative temperature-dependent transient photocurrents in 12 2 3 the 78 - 325 K temperature range. The photon flux was 10 /cm and the electric field was 10 V/cm. The inset shows a rise time of 40 ps for the BP photodetector, which is the time interval in which the photocurrent amplitude rises from 10% to 90%. d, Arrhenius photocurrent plots at various times, from the photocurrent peak measured at ~0 ns to 6 ns with a step size of 50 ps.

BP synthesis and the fabrication of a high speed BP based photodetector with ~ 100% EQE is provided in the methods and Supporting Information sections (Figs. S1-6). Design considerations to avoid gain in this study slightly differ from those of time-integrating high-gain photodetectors with much slower response times in the range of microseconds, which are operated under steady-state illumination conditions31. In steady-state devices, gain is achieved through the reinjection of high-mobility holes, which circulate to maintain charge neutrality as long as there are long-lived low-mobility electrons in deep trap states31. In high-speed photodetectors, on the other hand, illumination is via pulsed photons, and only the directly photogenerated charges participate in the observed current transient, with no carrier reinjection through electrodes28-30. The photodetector is comprised of coplanar Au microstrips (Fig. 1a) in which the electrode capacitance and inductance were distributed in the form of transmission lines, leading to a small capacitance and fast response time. Notably, we were able to achieve a system response time of sub-40 ps (Fig. 1c inset) that is limited only by the bandwidth of SMA tab connectors, coaxial cables and sampling oscilloscope. Other groups achieved ~100 ps time resolution to study amorphous semiconductor26-27 and organic semiconductors32-33. The 2

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orientation of the BP flake was confirmed through polarized Raman spectroscopy (Fig. 1b). Consistent with previous observations, three distinct Raman peaks are present at ~365, ~440, and ~470 cm-1, which correspond to the Ag1, B2g, and Ag2 modes5, 21, 23. The relative intensities of the Ag1 and Ag2 peaks in the Raman spectra serve as a good indicator for the orientation of the flake, which is consistent with the three peaks reported by other groups, who used high quality (99.999 % purity) BP flakes34. In addition to a fast system response time, the high EQE (approaching 100%, SI) allowed us to record carrier transport properties before they were significant influenced by recombination and trapping. Because BP is a p type semiconductor with high hole mobility, the measured photocurrent is mainly attributed to hole transport. We first investigated the carrier transport dynamics along the armchair direction. A typical photocurrent response is characterized by a fast rise followed by a temperature dependent decay (Fig. 1c). In the temperature range (78 K to 325 K) used in this study the photocurrent peak increased with decreasing temperature, indicating a phonon scattering mediated transport behavior above the band edge, which is consistent with previous observations5, 21. On the other hand, the photocurrent decreases with decreasing temperature at longer time (for example at 6.0 ns), indicating a thermally activated transport behavior due to defect scattering. To further elucidate the experimentally observed transition of transport behaviors in Fig. 1c, Arrhenius plots of the photocurrent spanning from its peak (0 ns) up to 6.0 ns are illustrated in Fig. 1d, which suggest that two different transport mechanisms are present depending on the temperature range and time. The temperature dependence of the photocurrent was analyzed using the power law relation T-n and an exponential relation of exp(-Ea/kBT) in two different temperature ranges (120 K to 180 K and 180 K to 325 K), where Ea is the thermal activation energy, kB is the Boltzmann constant, and T is the temperature (Figs. 2a,b). The non-zero index n values suggest phonon scattering transport of the carriers above the band edge, while the exponential relationship suggests multiple trapping and release (MTR) transport in the band-tail states, indicating that the activation energy increases with time.

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Fig. 2 Carrier transport dynamics along the armchair direction. a, A schematic models of the transition from

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phonon scattering transport to MTR transport before and after the filling of the band-tail states. The Ea (t) is the activation energy and Ec is the conduction band edge. Here, we use electrons to demonstrate the transport behavior, however, a similar transport behavior also applies for holes, which are the majority carriers in BP. b, c, Dependence of the power law index (n) and thermal activation energy (Ea) on time, derived respectively from the power law -n relation of T and exp(-Ea/kBT) in the temperature range of (b) 180 K to 325 K and (c) 120 K to 180K. The carrier escape frequency v from trap states is determined from a fit to the thermal activation energy dependence on time (blue curves in b and c).

In the high temperature range of 180 K to 325 K, the index n value decreases from 1.2 at the peak photocurrent to zero at 3.25 ns (red curve in Fig. 2b), with a fitting decay constant of 2.6 ns (Fig. S7). This decreasing index n values is indicative of carrier thermalizing from above the band edge states to band tail states. In contrast to other index n values characterized by FETs (n = 0.5)5 and Hall measurements (n = 0.7) 21, the index n values characterized by a BP based photodetector approaches the theoretical value of 1.5, indicating that, at early time the photogenerated carriers exhibit phonon scattering transport behavior above the band edge. Next, they thermalize to the band tail states, which is inferred from the decreasing n values. Therefore, the smaller index n values characterized by FETs and Hall measurements are due to the contributions from band tail states which mask the phonon scattering transport behavior as evidenced in the left panel of Fig. 2a. When carriers are fully trapped into band tail states at ~3.2 ns, they occasionally get de-trapped under thermal activation back to the band edge, then fall into deeper states to exhibit MTR transport, indicated by the increase in thermal activation energy with time (Fig. 2a, right panel). The carrier escape frequency from trap states, which is determined by fitting the thermal activation energy dependence on time (blue curve in Fig. 2b with solid fitting line) is described by the MTR model: Ea (t) = kBT ln (vt)  (1), where v is the carrier escape frequency from the band-tail states, and t represents time from the instance the sample is excited by the laser. The carrier escape frequency of 0.3 ns-1 (Fig. S8) is close to the index n value decay constant of 2.6 ns (0.38 ns-1), because the processes of free carrier relaxation into the band tail and reactivation above the band edge share the similar mechanisms. Similarly, the transition from phonon scattering transport above the band edge to MTR transport can be observed in the low temperature data (Fig. 2c). The extracted carrier escape frequency of the band tail states is 2.5 ns-1 (blue curve in Fig. 2c with solid fitting line; Fig. S9). MTR transport occurs up to 3 ns, after which the hopping-by-tunneling transport mechanism may dominate, as indicated by the constant thermal activation energy of ~13 meV.

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Fig. 3| Carrier decay rate dependence on electrical field along armchair direction. a, b, Dependence of the transient photocurrents on electric field at temperature (a) 300 K and (b) 78 K. The inset in a represents the 2 dependence of the carrier decay rate on electric field, which was used to extract a mobility of 373 cm /Vs. Likewise, 1/2 the inset in b represents the dependence of the carrier decay rate with (electrical field) , which was used to extract a un-trapping rate of 0.3 ns according to the Poole-Frenkel model.

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The above described transport behaviors can be further confirmed by the dependence of photocurrent decay with electric field (Figs. 3a, b). At room temperature, an electric field sweeps free carriers from one side of the electrode to the other side if the transit time is shorter than the carrier lifetime, which is indicated by a faster decay with increasing electric field (Fig. 3a). Therefore, the carrier mobility can be described by the following relation: µ = L2/VτOOOOOOOOOOOOOO(2), where µ is the carrier mobility, L is the electrode spacing, V is the bias, and τ is the carrier transit time. By fitting the data depicted in Fig. 3a inset using equation 2, a carrier mobility of 373 cm2/Vs (Fig. S10) is deduced, which is consistent with the values reported in FET and Hall measurement studies5, 17, 23, 35. It is worth noting that a mobility of ~1000 cm2/Vs can be obtained at 78 K because of the power law relation of T -n. On the other hand, at low temperatures the trapped carriers can be de-trapped from the shallow band tail states due to field-assisted emission, leading to a slower photocurrent decay with increasing electric field (Fig. 3b). According to the Poole-Frenkel model 36, which describes the relationship between the carrier emission rate (escape rate from the band tail states) and the electric field (F), e(F) = e(0) exp (αV1/2).(3), where e(0) is the zero field emission rate, α is a constant that relates the electrical field and the lowering of the trap level, and V is the applied voltage. A zero field emission rate of 3.0 ns-1 (Fig. S11) can be derived as indicated by the solid line in the inset in Fig. 3c. This value is in agreement with the escape frequency of 2.5 ns-1 in the low temperature range (Fig. 2c).

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Fig. 4| Carrier transport dynamics along the zigzag direction. a, Representative temperature-dependent transient photocurrents in the 78 K to 325 K range. Similar to the experimental conditions used for the probing carrier transport 12 2 3 behavior along the armchair direction, the photon flux was 10 /cm and the electric field was 10 V/cm. b, Dependence of the thermal activation energy on time in the temperature range of 120 K to 325 K.

The carrier transport along the zigzag direction shows a thermally activated behavior (Fig. 4 a), which is similar to that along the armchair direction in the low temperature range. The photocurrent peaks decrease slightly with decreasing temperature, while the photocurrent decays faster. This indicates that within a rise time of sub-40 ps, carriers fall into band-tail states and exhibit MTR transport behavior, wherein the thermal activation energy increases with time from 2.5 meV at 0 ns up to 15 meV at 6 ns (Fig. 4 b). The carrier escape frequency of 0.4 ns-1 can be extracted according to Eq.1, as indicated by the solid fitting line in Fig. 4b before 3 ns. The rapid trap falling might be due to low carrier mobility, which is one order lower than that along the armchair direction because the effect mass in the zigzag direction is close to one order of that along the armchair direction5. Moreover, the calculated mobility of ~ 100 cm2/Vs (SI), close to one fourth of armchair direction mobility, is consistent with trap filling effect. However, further experimental and theoretical studies are needed to clarify the role of defect states in term of their chemical origin and distribution.

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Similar to other 2D materials and bulk semiconductors, a general feature of the carrier transport behavior includes phonon scattering and thermal activation due to defect scattering. In this respect, we have elucidated the transport transition dynamics for BP along the armchair and zigzag directions to provide a better foundation for its device applications in solar cells, FETs, and photodetectors. In addition, we demonstrate a nearly 100 % EQE and high speed photodetector, an important application of BP optoelectronics. METHODS Black phosphorus crystal preparation. Black phosphorus bulk crystals were synthesized from red phosphorus powders in a sealed tube with SnI4 (American Elements, electronic grade 99.995%) and Sn ingot (Sigma Aldrich) as promoters. In a typical growth process, 20 mg of SnI4, 40 mg of Sn, and 1 g of red phosphorus was mixed in a silica glass ampoule (15 cm in length and 1.14 cm in diameter) and evacuated to a low pressure (~1×10-5 Torr). Synthesis was carried out in a three-zone Lindberg furnace using 1-inch diameter quartz tube. To facilitate the growth, the empty side of the ampoule was set to 50-75 °C below the growth temperature (~700 °C). The furnace was set to 700 °C (ramp time ~3 hours) and kept at this temperature for 3 hr. Then, the ampoule was cooled d own to 560 °C in 10 hours, followed by natural cooling down to room temperature. During the natural cooling step, dark orange and red fumes, associated with SnI4 and red phosphorus, were formed at the colder end. Finally, large shiny black phosphorus crystals were formed towards the cold end of the ampoule, well separated from Sn-rich Snphosphites, red phosphorus, and SnI4-x deposits. The black phosphorus bulk crystals synthesized following this method is comprised of bundle of axially Zigzag-oriented crystallites. High quantum efficiency and high speed photodetector fabrication and photocurrent characterization. Black phosphorus flakes of ~ 200 nm thickness were mechanically exfoliated from bulk black phosphorus crystals. Then BP were manually picked up using a sharp Tungsten needle (600 nm tip diameter, Cascade Microtech) assisted with the micro-manipulator probe station, and transferred onto top of the pre-patterned Au transmission line electrodes (Fig. S2). Au transmission line electrodes of 100 nm thickness, 25 µm separation, and 2 mm width were thermally deposited under the vacuum of 10-7 Torr. One side of the electrode is biased with a power supply and the other side is connected to a 20 GHz sampling oscilloscope through a coaxial cable. The Au transmission line electrode is connected to a SMA tab connector via silver paint point contact. Next, the whole device was mounted onto a cold finger of an optical cryostat to have a good thermal contact. A 1 KHz repetition rate, 800 nm wavelength laser with 100 fs pulse width illuminated the active black phosphorus flake to generate photocurrent, which is collected by the sampling oscilloscope. Prior to measurements, the devices are not under annealing or other treatments. Photocurrent signal collection is the difference between the signal under laser illumination and without illumination (dark current). Therefore, transport dynamics is independent of sample variation, while 6 BP samples were tested and demonstrate similar transport behaviors. ADDITIONAL INFORMATION Supporting Information Available: Black phosphorus and morphology characterization. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS J.G is thankful to Clemson University for providing start-up grant. We thank Huili Liu, Prof. Paul A. Alivisatos, and Prof. Junqiao Wu at University of California, Berkeley for useful discussions. REFERENCES 6

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Carrier Transport Dynamics in High Speed Black Phosphorus Photodetectors Jianbo Gao,

*,§

Apparao M. Rao,

*,§

‡

Hongbo Li, Jianbing Zhang,∥ and Ou Chen

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Department of Physics and Astronomy, Clemson University, Clemson 29634, United States School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ∥ School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China # Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States ‡

Carrier transport mechanism (right panel) along the armchair direction in a BP high speed photodetector device (left panel).

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ACS Paragon Plus Environment