Site Selective Doping of Ultrathin Metal Dichalcogenides by Laser-Assisted Reaction
Site Selective Doping of Ultrathin Metal Dichalcogenides by Laser-Assisted Reaction
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DOI：
10.1002/adma.201503945
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发表时间：
2016-01-13
影响因子：
29.4
通讯作者：
Grigoropoulos, Costas P.
中科院分区：
文献类型：
--
作者：
Kim, Eunpa;Ko, Changhyun;Grigoropoulos, Costas P.
最近，化学式为MX 2 （M= Mo和W； X= S， Se和Te）的TMDCs由于其有限的直接带隙、丰富的激子动力学以及与逆对称性破缺相关的谷极化（谷电子学）而引起了人们的广泛关注。这些由范德华相互作用组成的垂直堆叠层组成的层状半导体是硅基电子产品的新兴替代品尽管在电子学和光电子学方面具有潜力，但要过渡到实际应用，还需要可靠和稳定的加工方法更具体地说，控制半导体掺杂对于集成到器件中至关重要。特别是对于使用超薄TMDCs作为器件超薄体的纳米级器件，为了最大限度地减少掺杂的随机波动，保证器件性能的再现性，精确控制掺杂水平的位点特异性掺杂变得至关重要。我们的方法提供了一个系统的方法来解决这个问题，展示了对TMDCs的空间分布和掺杂水平的优越的原位控制。与植入或扩散等传统方法相比，激光掺杂方法已被证明具有优越的器件特性。[28,29]激光辅助掺杂工艺示意图如图1a所示。激光具有两个主要功能：(i)在TMDC材料中产生硫空位和（ii）同时解离掺杂分子。释放出的掺杂分子随后被并入到空位中。磷（PH 3）作为超薄TMDCs的p型掺杂前驱体被引入，其中磷占据硫位点，与其用于硅的n型掺杂形成对比。通过实验和理论建模对PH 3掺杂Si进行了广泛的研究。[30-32]尽管硅和TMDCs存在差异，但本研究采用空位机制是合理的。考虑到TMDCs的解离温度在1200-1400 K范围内，略低于此范围的激光功率足以破坏解离温度为685 K的PH 3分子。图1b为机械剥离的单层和五层二氧化钼薄片的光学图像。图1c中的单层MoS 2薄片厚度约为0.7 nm.[4]图1d所示的光致发光（PL）图取自图1c .[33]所示的激光照射区域激光辅助磷掺杂前后单层MoS 2的PL光谱如图2a所示。激光掺杂区域的峰值PL强度大约比剥离的单层2d过渡金属二硫化物（TMDC）器件的峰值PL强度大一个数量级，表现出特别适合下一代光电和电子器件应用的特殊特性。[1-3]它们是晶体管、[4-6]光电探测器、[7,8]电致发光器件、[9]和传感器的优秀候选材料。[10-14]对于大多数这些应用，需要掺杂来调整自由载流子的类型和密度。然而，之前的研究主要集中在通过吸附分子的电荷转移、静电、[20,21]或物理吸附门控、[22]缺陷工程、[23,24]和生长过程中的取代掺杂等方法掺杂TMDCs在这里，我们报道了一种在磷化氢环境中通过聚焦激光照射广泛可调的、位点特异性的超薄TMDCs （MoS 2和WSe 2）掺杂的通用方法。p型磷掺杂具有局域性和选择性，掺杂水平可广泛调节。
Recently, TMDCs with the general chemical formula of MX 2 (M= Mo and W; X= S, Se, and Te) have attracted much interest owing to their finite direct band gaps, rich excitonic dynamics, and valley polarization (valleytronics) associated with the broken inversion symmetry. These layered semiconductors, composed of vertically stacked layers held together by van der Waals interactions, are emerging alternatives to silicon-based electronics.[26] Despite the potential in electronics and optoelectronics, reliable and stable processing methods are needed for transition to practical applications.[27] More specifically, controlled doping of semiconductors is vital for integration into devices. Especially, for nanoscale devices using ultrathin TMDCs as an ultrathin body in devices, in order to minimize random dopant fluctuation and ensure device performance reproducibility, site-specific doping with precise doping level control becomes essential. Our method provides a systematic approach to this problem, demonstrating superior, in situ control of the spatial distribution and doping level of TMDCs. The laser doping method has been demonstrated to produce superior device characteristics compared with other conventional methods such as implantation or diffusion.[28, 29] A schematic diagram of the laser-assisted doping process is shown in Figure 1a. The laser serves two major functions:(i) creation of sulfur vacancies in the TMDC materials and (ii) simultaneous dissociation of the dopant molecules. The released dopant molecules are then incorporated into the vacancy sites. Phosphine (PH 3) was introduced as a p-type dopant precursor for ultrathin TMDCs where phosphorus occupies sulfur sites, in contrast to its use for n-type doping of silicon. The PH 3 doping of Si has been extensively studied both experimentally and through theoretical modeling.[30–32] Despite differences between silicon and TMDCs, it is reasonable to adopt the vacancy mechanism in the present study. Considering that the dissociation temperature of the TMDCs is in the range of 1200–1400 K, a laser power slightly below this range is enough for breaking the PH 3 molecules whose dissociation temperature is 685 K. Figure 1 b shows the optical image of mechanically exfoliated monolayer and five-layer MoS 2 flakes. The monolayer MoS 2 flake in Figure 1 c is identified with a thickness of≈ 0.7 nm.[4] The photoluminescence (PL) map shown in Figure 1 d is taken from the laser-irradiated region indicated in Figure 1 c.[33] The PL spectra of monolayer MoS 2 before and after the laserassisted phosphorus doping are shown in Figure 2a. The peak PL intensity of the laser-doped area is approximately one order of magnitude greater than that of the as-exfoliated monolayer2D transition metal dichalcogenide (TMDC) devices exhibit exceptional characteristics that are particularly suitable for next generation optoelectronic and electronic device applications.[1–3] They are excellent candidate materials for transistors,[4–6] photodectors,[7, 8] electroluminescent devices,[9] and sensors.[10–14] For most of these applications, doping is needed to tune the free carrier type and density. However, previous efforts have mostly focused on doping TMDCs by means of charge transfer from adsorbed molecules,[15–19] electrostatic,[20, 21] or physisorption gating,[22] defect engineering,[23, 24] and substitutional doping during growth.[25] Here we report a versatile method for widely tunable, site-specific doping of ultrathin TMDCs (MoS 2 and WSe 2) through focused laser irradiation in a phosphine environment. The p-type doping with phosphorus is localized and selective, and the doping level is widely tunable by …