COMMUNICATION Heterojunction Bipolar Transistor www.advelectronicmat.de 2D Material-Based Vertical Double Heterojunction Bipolar Transistors with High Current Amplification Geonyeop Lee, Stephen J. Pearton, Fan Ren, and Jihyun Kim* been applied to various types of semicon- The heterojunction bipolar transistor (HBT) differs from the classical homo- ductor devices, such as lasers, solar cells, junction bipolar junction transistor in that each emitter-base-collector layer high electron mobility transistors, and het- [3–6] is composed of a different semiconductor material. 2D material (2DM)- erojunction bipolar transistors (HBTs). Notably, with a bipolar junction transistor, based heterojunctions have attracted attention because of their wide range which is a three-terminal transistor fabri- of fundamental physical and electrical properties. Moreover, strain-free cated by connecting two P–N homojunc- heterostructures formed by van der Waals interaction allows true bandgap tion diodes, there is a trade-off between engineering regardless of the lattice constant mismatch. These characteristics the current gain and high-frequency ability [3,7] make it possible to fabricate high-performance heterojunction devices such because of these problems. In sharp contrast, HBTs realized using the hetero- as HBTs, which have been difficult to implement in conventional epitaxy. structure can avoid these trade-offs and Herein, NPN double HBTs (DHBTs) are constructed from vertically stacked improve device performance.[8] HBTs, due 2DMs (n-MoS2/p-WSe2/n-MoS2) using dry transfer technique. The forma- to their high power efficiency, uniformity tion of the two P–N junctions, base-emitter, and base-collector junctions, of threshold voltage, and low 1/f noise in DHBTs, was experimentally observed. These NPN DHBTs composed characteristics, have been widely used in of 2DMs showed excellent electrical characteristics with highly amplified high power amplifiers and high frequency switching devices.[3] It is challenging to current modulation. These results are expected to extend the application field realize high-quality hetero-interfaces in of heterojunction electronic devices based on various 2DMs. heterojunction devices, including HBTs owing to the various growth limitations involving mitigation of diffusion of both Heterostructures are widely employed in semiconducting dopants and lattice elements. If present, these constraints con- devices to take advantage of band-structure engineering tribute to performance degradation or even the complete loss of effects that lead to significantly improved carrier confinement the benefits of incorporating the heterojunction.[3] For instance, and injection.[1] This leads to a wide variety of device perfor- III–V compound semiconductors such as GaAs/AlGaAs and mance advantages in photonic and electronic devices and over GaN/AlGaN used in conventional heterojunction-based devices their homojunction analogs, including higher gains in tran- require high vacuum and high-cost growth equipment such as sistors or brighter light outputs from the photonic devices.[2] metalorganic vapor deposition (MOCVD) and molecular beam This approach is also advantageous for mitigating the prob- epitaxy (MBE). This growth is difficult to achieve when the dif- lems related to the reduction of carrier mobility as the doping ference of the lattice constants is too significant. Additionally, concentration increases in homojunctions by separating the obstacles such as dislocation defects, strain caused by lattice carriers from the dopants and also in situations where efficient mismatch, cross-contamination, and inter-diffusion are difficult bipolar doping is difficult. As a result, heterostructures have to overcome. These problems cause device performance dete- rioration in the form of increased leakage current, a decrease G. Lee, Prof. J. Kim of breakdown voltage, and an increase of recombination rate in Department of Chemical and Biological Engineering HBT devices.[1–3] Korea University 2D materials (2DMs) have been studied in various fields over Seoul 02841, Korea the past decade because of their excellent electrical, thermal, E-mail: [email protected] and mechanical properties.[9–13] In particular, heterostructures Prof. S. J. Pearton Department of Materials Science and Engineering based on 2DMs have attracted interest because of their weak University of Florida interlayer bonding, quantum effect, and tunneling, which Gainesville, FL 32611, USA are differentiated from conventional 3D bulk materials.[10,14] Prof. F. Ren Weak van der Waals interactions of 2DMs can not only easily Department of Chemical Engineering separate each layer, but can also layer materials regardless of University of Florida lattice mismatch.[11] Additionally, because 2DMs have a sharp Gainesville, FL 32611, USA interface and no dangling bonds, heterostructures using 2DMs The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.201800745. can solve problems such as atomic diffusion and disloca- tion propagation, which have been regarded as limitations of DOI: 10.1002/aelm.201800745 existing 3D bulk materials.[11,12] Selection of 2DMs provides Adv. Electron. Mater. 2019, 5, 1800745 1800745 (1 of 7) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advelectronicmat.de Figure 1. Optical microscope images a–d) showing the process of fabricating an NPN DHBT using the exfoliated MoS2 and WSe2 flakes in the order MoS2/WSe2/MoS2. d) Definition of Ti/Au electrodes for MoS2 and Pt/Au electrodes for WSe2 flakes. Note that two electrodes were defined on each flake to confirm the Ohmic contact. Scale bars of all figures represent 10 µm. latitude in achieving specific electrical and optical properties common-base mode. In particular, the high current gain (β) because each material has a different energy bandgap, elec- obtained through the Gummel plot was comparable to those of tron affinity, and carrier mobility, thereby allowing the design commercialized III–V thin film semiconductor HBTs. of desired devices to be fine-tuned without being limited by Figure 1 shows optical microscope images that depict the lattice constants.[11,12,15] Roy et al. demonstrated the nega- fabrication process of DHBTs based on 2D materials via van der tive differential resistive characteristics of the Esaki diode in Waals epitaxy (dry transfer technique). The bottom n-type MoS2 a hetero WSe2-MoS2 junction and analyzed its electrical and flake was first transferred onto the SiO2/Si (300 nm/525 µm) [16] optical properties. Liu et al. fabricated P–N diodes composed substrate (Figure 1a). Then, the p-type WSe2 and n-type of black phosphorus and MoS2 and proved that diode charac- MoS2 flakes were stacked vertically onto the previously trans- teristics can be changed according to the thickness of black ferred flake, using a micromanipulator, in the order NPN [17] phosphorus. Although studies on heterostructures based on (Figure 1b,c). This procedure separated the top MoS2 from the various 2DMs have been carried out, the research is still limited bottom MoS2 flakes with a WSe2 layer. A Ti/Au electrode and to fundamental structures, such as a single P–N junction diode. Pd/Au electrode were defined for the MoS2 (emitter or collector) Even though three-terminal active devices, like HBTs and junc- and WSe2 (base), respectively, using standard electron-beam tion field-effect transistors, are more useful than two-terminal lithography, electron-beam evaporation, and lift-off processes devices from the point of view of the multiple applications (Figure 1d). Figure 2a shows a schematic of the fabricated NPN ranging from signal applications to the design of digital logic, DHBT, which was fabricated by stacking two MoS2 (n-type) [13] those have not yet been widely studied. flakes and a WSe2 (p-type) flake vertically, in the order NPN, This study demonstrates the highly amplified current modu- using a drytransfer technique. Here, the top or bottom MoS2 lation of a double HBT (DHBT), composed of vertically stacked acts as an emitter (collector) or a collector (emitter), respec- 2DMs. MoS2, used as an n-type emitter and collector, was stacked tively, and WSe2 in the middle layer is used as a base material. vertically with WSe2, a p-type base, to fabricate an NPN DHBT Figure 2b,c shows the AFM images and height profile of at room temperature and atmospheric pressure. MoS2 and one of the fabricated devices (shown in Figure 1). The green, WSe2 are extensively used as 2D semiconductor materials. They blue, and red dotted lines in Figure 2b indicate the bottom have excellent electrical properties of electron or hole mobility MoS2, WSe2, and top MoS2 flakes, respectively. The 2D flakes and possess a tunable bandgap depending on their thickness, were ≈9, ≈8, ≈10 nm thick, respectively. 2DM flakes of similar which enables easy bandgap engineering for the HBTs. In addi- thickness were used for other samples, and the maximum tion, several researchers have reported a WSe2–MoS2 type II thickness did not exceed 50 nm. A cross-sectional TEM image P–N junction, which has a potential for HBTs.[16,18] The NPN was obtained using another fabricated DHBT sample to con- structure was selected because it has been reported that junc- firm the quality and structure of the heterostructure vertically tion barrier for the electron is slightly lower than that for the stacked in MoS2/WSe2/MoS2. A TEM image (Figure 2d) and [19] hole carrier in WSe2-MoS2 junction . Lateral type HBTs fab- the corresponding energy-dispersive X-ray microscopy (EDX) ricated with 2DMs have been reported by Lin et al. but vertical mapping images (Figure S1, Supporting Information) show structure can reduce the recombination loss in the base region the well-formed heterostructure without strain or bubbles. more efficiently than the lateral type, making very thin 2D mate- This indicates that van der Waals epitaxy can prevent the lat- rials even more advantageous.[1,3,7,20] Double heterojunction tice mismatch that occurs because of the 5.53% larger lattice [21] structure was adopted because it can help avoid the difficulty constant of WSe2 compared to that of MoS2.