Metal-insulator-metal tunnel junctions (MIMTJs) are an enabling technology for future electronics including advanced computing, data storage, sensors, etc. MIMTJs are formed by inserting an ultrathin insulating layer, known as the tunnel barrier (TB), between metal electrodes. Devices based on MIMTJs have advantages of enhanced quantum coherent transport, fast speed, small size, and energy efficiency. The performance of MIMTJs depends critically on the thickness and quality of the tunnel barrier. Specifically, the tunneling current, for example, the superconducting critical current in superconductor-insulator-superconductor Josephson junctions (JJs) or the spin tunneling current in ferromagnetic-insulator-ferromagnetic magnetic tunnel junctions (MTJs), decreases exponentially with the TB thickness. This means thinner TBs would enable stronger coherent tunneling in MIMTJs. In addition, the defects in the TBs can degrade the quantum coherence of electrons (spins) of JJs and MTJs, respectively, resulting in decoherence and degraded performance of the MIMTJs. This justifies the urgent need in research and development of ultrathin (subnanometers to 1 nm) pinhole-free and defect-free TBs beyond the current state-of-the-art TBs of larger thickness (>1–2 nm) and high defect concentration made using thermal diffusion of oxygen or physical vapor deposition (PVD) including magnetron sputtering and molecular beam epitaxy. Atomic layer deposition (ALD) can provide a unique resolution to achieving ultrathin and defect-free dielectric TBs for high-performance MIMTJs for future electronics. In this article, a review on their recent effort in the development of in vacuo ALD for the fabrication of ultrathin TBs for JJs and MTJs is presented. A custom-designed system that integrates high-vacuum/ultrahigh-vacuum PVD, ALD, and scanning probe microscopy was established for in vacuo fabrication of MIMTJs and characterization of the electronic properties of ALD TBs including Al₂O₃, MgO, and Al₂MgO₄ on both superconductor metals (Al) and ferromagnetic metals (Fe and FeCoB). Capacitors with ALD dielectric of thickness in the range of 1–5 nm were also constructed for the characterization of the dielectric properties of the ALD TBs. The authors have found that the metal-insulator interface plays a critical role in controlling the quality of the ALD TBs including the tunnel barrier height, dielectric constant, electric breakdown, and uniformity. They have shown that JJs and MTJs with 0.1–1.0 nm thick ALD Al₂O₃ TBs can be obtained with highly promising performance. The result obtained suggests that the in vacuo ALD may provide a unique approach toward MIMTJs with an atomic-scale control of the device structure required for high-performance future electronics.

中文翻译：

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用于金属/绝缘体/金属隧道结的超薄介电膜的真空原子层沉积和电子隧穿特性

金属-绝缘体-金属隧道结（MIMTJ）是未来电子设备的使能技术，包括先进的计算，数据存储，传感器等。MIMTJ是通过在金属电极之间插入称为隧道势垒（TB）的超薄绝缘层而形成的。基于MIMTJ的设备具有增强的量子相干传输，速度快，体积小和能效高的优点。MIMTJ的性能主要取决于隧道势垒的厚度和质量。具体而言，隧穿电流，例如，超导体-绝缘体-超导体约瑟夫森结（JJs）中的超导临界电流或铁磁-绝缘体-铁磁隧道体（MTJs）中的自旋隧穿电流随TB厚度呈指数下降。这意味着更薄的TB将在MIMTJ中实现更强的连贯隧道效应。此外，TB中的缺陷会分别降低JJ和MTJ的电子（自旋）的量子相干性，从而导致MIMTJ的去相干性和性能下降。这证明了在研究和开发超薄（亚纳米级至1 nm）无针孔和无缺陷TB的迫切需要，超越了目前最先进的更大TB（> 1-2 nm）和高缺陷浓度的TB。使用氧气的热扩散或包括磁控溅射和分子束外延的物理气相沉积（PVD）制成。原子层沉积（ALD）可以提供独特的分辨率，以实现用于未来电子产品的高性能MIMTJ的超薄且无缺陷的介质TB。在这篇文章中，介绍了用于JJ和MTJ的超薄TB制造的真空ALD。建立了集成高真空/超高真空PVD，ALD和扫描探针显微镜的定制设计系统，用于真空制造MIMTJ和表征ALD TB的电子特性，包括Al ₂ O ₃，MgO和Al ₂氧化镁₄在超导金属（Al）和铁磁金属（Fe和FeCoB）上都存在。还构造了厚度为1-5 nm的ALD介电常数的电容器，以表征ALD TB的介电特性。作者发现，金属-绝缘体界面在控制ALD TB的质量（包括隧道势垒高度，介电常数，电击穿和均匀性）方面起着至关重要的作用。他们表明，可以以0.1-1.0 nm的ALD Al ₂ O ₃ TB厚度获得JJ和MTJ，并具有很高的前景。获得的结果表明真空 ALD可以通过对未来高性能电子设备所需的设备结构进行原子级控制，从而为MIMTJ提供独特的方法。