High Performance Amplifier Element Realization via MoS2/GaTe Heterostructures

Abstract 2D layered materials (2DLMs), together with their heterostructures, have been attracting tremendous research interest in recent years because of their unique physical and electrical properties. A variety of circuit elements have been made using mechanically exfoliated 2DLMs recently, including hard drives, detectors, sensors, and complementary metal oxide semiconductor field‐effect transistors. However, 2DLM‐based amplifier circuit elements are rarely studied. Here, the integration of 2DLMs with 3D bulk materials to fabricate vertical junction transistors with current amplification based on a MoS2/GaTe heterostructure is reported. Vertical junction transistors exhibit the typical current amplification characteristics of conventional bulk bipolar junction transistors while having good current transmission coefficients (α ∼ 0.95) and current gain coefficient (β ∼ 7) at room temperature. The devices provide new attractive prospects in the investigation of 2DLM‐based integrated circuits based on amplifier circuits.


Raman spectroscopy
is the Raman spectroscopic measurement of different positions on the Si substrate. Figure S1a shows that the first order peaks 2 1 and 1 correspond to in-plane and out-of-plane vibrations. In this case, these vibrations are located at 383.9 cm -1 and 408.3 cm -1 with a separation of 24.31 cm -1 . The Raman spectrum from individual GaTe has two peaks at 120.6 and 139.8 cm -1 , in agreement with the literature. The respective Raman peaks suggest that the representative vibration modes of both GaTe and MoS 2 indicates the coexistence of two distinct materials within the heterostructure.

Electrical characterization of the FET based on MoS 2 and GaTe
To investigate the transport characteristics of the exfoliated MoS 2 and GaTe, the transistors with the back-gate field effect, based on the exfoliated MoS 2 and GaTe, have been fabricated. The transfer characteristics (I d -V g ) of the FET are shown in Figure S2. The field-effect mobility (µ) was then extracted using equation µ = , where is the top gate capacitance per unit area, L is the channel length, W is the channel width, is the source-drain voltage and is the transconductance (i.e., the slope of transfer curve in a linear region). The electron mobility of the GaTe FET shown in Figure S2a is estimated to be only 0.01 cm 2 /V·s, while that of MoS 2 FET reaches 5.2067 cm 2 /V·s in Figure S2b.

Calculation details
The I-V characteristics have been fitted with the diode equation, To calculate the ideality factor n, As shown in Figure S3a, V T is 0.75 V, and through curve fitting, n=1.654, and ) When V T is 1.1 V in Figure S3b, n=1.178. For MoS 2 /GaTe junction, as shown in Figure   S3c, V T is 0.75 V, and through curve fitting, n=1.117. When V T is 1.1 V in Figure

NPN HBT based on MoS 2 /GaTe/n-Si heterostructure
After removing the native oxide on the exposed Si substrate via additional wet etching, few layers of the GaTe were exfoliated on the n-type silicon substrate, as shown in Figure   S4a. After placing the GaTe on the substrate, the insulating layer used for isolating the emitter and metal electrode from the collector was patterned using e-beam lithography, as shown in Figure S4b. Subsequently, without removing the photoresistance, approximately 6 40 nm of Al 2 O 3 was grown via atomic layer deposition. Next, the photoresist was removed using acetone so that only Al 2 O 3 remained in the trenches and was washed away (with the photoresist) from the other regions, as shown in Figure S4c. With the help of OM, MoS 2 was transferred directionally to the target GaTe flake, as shown in Figure S4d. Finally, electrode patterns were defined by a standard EBL process, as shown in Figure S4e.
Additionally, the 10/60 nm Cr/Au electrodes were deposited via physical vapor deposition, as shown in Figure S4f. To verify this concept, that two-dimensional materials could be used to fabricate the amplifying device, we fabricated a new NPN BTJ by sequentially transferring 7 mechanically exfoliated GaTe and MoS 2 thin films onto a slightly n-doped Si substrate.
The GaTe was located on the gap between MoS 2 and Si, and the schematic of the MoS 2 /GaTe/n-Si heterostructure is shown in Figure S5. Figure S5. Schematic of the NPN HBT based on MoS 2 /GaTe/n-Si heterostructure As shown in Figure S6a, a different I E from 0 nA to 400 nA with a 100-nA step size are applied and the V CB is swept from 0 V to 3 V. In the active region, where the collector-base junction is reverse-biased, I C is approximately equal to the emitter current I E . As V CB decreases, the collector-base junction becomes more forward biased. Hence, the I C sharply decreases. In the saturation region, the collector current slightly depends on the emitter current. The plot of common-base current gain (α) vs. base-collector voltage (V CB ) curves under room temperature is shown in Figure S6b. As shown in Figure S6c, in the active region, the curves exhibit an ascendant trend under the common-emitter configuration, which indicates that the I C increases as the V CE increases. In the saturation 8 region, the V CE becomes small, and the collector-base junction becomes forward biased.
Hence, the I C decreases sharply. In this region, the collector current depends on the base current. The plot of the common-emitter current gain (β) vs. emitter-collector voltage (V CE ) under room temperature results in a curve that is shown in Figure S6d, and the maximum β increases from 0 to 1.4.
The common-emitter current gain (β) of the P-N-P HBT can reach 7 at room temperature, but that of the N-P-N HBT can be only 1.2. The base of the P-N-P HBT is MoS 2 and the emitter is GaTe, while the base of the N-P-N HBT is GaTe and MoS 2 is emitter. The thickness of GaTe flake in the N-P-N HBT is much thicker than the MoS 2 flakes in the P-N-P HBT, which means that the base layer of the N-P-N HBT is much thicker than that of P-N-P HBT. However, the base layer has to be thinner so that fewer carriers (injected from the emitter) will be recombined with the carriers of opposite polarity in the base layer so that the current amplification gain will be enhanced. GaTe, whose impurity concentration is relatively high for the base material, and the electrons transferred from the emitter region will compound with the majority carrier holes in the base region under the amplification condition. Hence, electrons collected in the collector region will decrease greatly. Therefore, the current amplification gain of N-P-N HBT with a thick GaTe base layer will be much lower than the P-N-P HBT with a thin MoS2 base layer. Current gain β is not particularly large in the N-P-N HBT, but β is greater than 9 1, so there are still some amplification effects. In the future, we will choose another kind of p-type 2D material as the base layer in the N-P-N HBT to achieve higher performance.