Peng Hailin's research group has made important progress in the research of two-dimensional semiconductor ultra-thin single-grating media

As integrated circuit chips move towards sub-3nm technology nodes, the critical dimensions in transistors continue to shrink, bringing higher switching speeds and integration, but also lead to short-channel effects, which seriously affect transistor performance, and Moore's Law is approaching the physical limit, which makes the industry urgently need to develop new materials and new architectures
.
Atomic-thick, highly mobile two-dimensional semiconductors will replace conventional silicon germanium as an alternative channel material, while the equivalent oxide thickness (EOT) of the gate medium will need to be reduced to less than
0.
5 nanometers.
How to effectively integrate high-mobility two-dimensional semiconductors with high dielectric constant gate media and limit miniaturization (EOT < 0.
5 nanometers) is an important challenge
in the field of electronics.
Recently, Professor Peng Hailin of the School of Chemistry and Molecular Engineering of Peking University established a UV light-assisted intercalation oxidation method for high-mobility two-dimensional semiconductor Bi_{2 O2}Se, which realized the controllable preparation of a new natural oxide single grating medium β-Bi_2SeO5, with a dielectric constant of up to₂₂and excellent
insulation properties.
The gate dielectriclayer EOT of the two-dimensional Bi_{2 O2}Se/Bi₂SeO₅ base top-gate field effect transistor can be reduced to 0.
41 nanometers, breaking through the bottleneck
of ultra-thin gate media integration of two-dimensional electronic devices 。 The research results are entitled "A single-crystalline native dielectric for two-dimensional semiconductors with an equivalent oxide thickness below 0.
5 nm".
Published online in Nature Electronics on September 15, 2022
.

The critical size of the transistors in integrated circuit chips is constantly shrinking
under the impetus of Moore's Law.
When the length of the semiconductor channel in the transistor is less than 6 times the eigenlength (λ), short-channel effects such as threshold voltage drift and leakage current increase occur, which restricts the further shrinkage
of the transistor to the sub-3nm technology node.
The industry plans to replace conventional silicon germanium as a channel material with a high mobility two-dimensional semiconductor material with atomic thickness, and at the same time reduce the equivalent oxide thickness (EOT) of the gate medium to less than 0.
5 nanometers to reduce the feature length to suppress the short channel effect
.

However, the effective integration and limiting of high-mobility two-dimensional semiconductors with gate media with high dielectric constants is an important challenge
in the field of electronics.
At present, the gate medium used in commercial silicon-based integrated circuits is hafnium oxide (HfO2) prepared by atomic layer deposition (ALD), which is difficult to be uniformly deposited on the surface of van der Waals without suspension bonds in two-dimensional semiconductors, and it is impossible to form a continuous film
.
The gate medium compatible with two-dimensional semiconductors, such as hexagonal boron nitride (hBN), calcium fluoride (CaF₂), HfO2 composite of organic buffer layers, etc.
, EOT can only be reduced to 0.
9 nanometers due to low dielectric constant and insufficient insulation.

Therefore, there is an urgent need for the field of 2D electronics to develop a gate medium of sub-0.
5 nm EOT compatible with 2D semiconductors
.

Peng Hailin's research group developed a new high-mobility two-dimensional semiconductor Bi_2O2Se (Nature Nanotechnology 2017,₁₂, 530) in the early stage, and conducted in-depth research
on its oxidation conversion and natural oxide dielectric layer.
It was first found that by thermal oxidation (Nature Electronics 2020, 3, 473) or plasma oxidation (Nano Letters 2020, 20, 7469), a two-dimensional Bi 2 O2 Se surface can form a natural oxide gate medium Bi 2 SeO₅ with high dielectric constant (₂₁to₂₂) and good insulation properties (polycrystalline or amorphous phase), and based on the two-dimensional Bi₂O 2 Se/Bi₂SeO₅ to construct high-performance field-effect transistor devices and logic gates (Acc.
Mater.
Res.
2021, 2, 842-853), the EOT of its gate medium can be reduced to 0.
9 nm level
.

Recently, Peng Hailin's research group was inspired by thezipper-shaped layered structure of two-dimensional semiconductor Bi 2 O2 Se, and established the ultraviolet light-assisted intercalation oxidation method of Bi 2 O2 Se (Figure 1a) by drawing on the two-dimensional material intercalation chemistry, keeping its Bi-O layered skeleton structure unchanged, and the two-dimensional semiconductor Bi_2O2 Se is converted in situ to a single crystal oxide gate medium_β-Bi2SeO5
.
This is anotherphase
of the_Bi2O2Se natural oxide after the polycrystalline α-Bi2 SeO5 dielectriclayer prepared by the previous thermal oxidation operation.
This UV-assisted intercalation oxidation method is compatible with UV lithography and enables region-selective oxidation of wafer-level samples with the help of a special lithography mask plate (Figure 1b
).
Two-dimensional Bi_2O2Se can be oxidized layer by layer by layer controlled intercalation to form a high-quality semiconductor/dielectric layer interface with flat atomic level and lattice matching (Figure 1c).

The single crystal β-Bi 2 SeO₅ can be used as an ideal gate medium
for two-dimensional Bi_{2 O2}Se-based transistors.
Scanning microwave impedance microscopy characterization and capacitance-voltage measurements show that the single crystal β-Bi2_SeO5 has an extra-surface dielectric constant of up to_{22 and}is not affected by thickness
.
In addition to having a high-quality interface and high dielectric constant, β-Bi 2 SeO_5-gatemedia also has excellent insulation due to its layered single crystal structure.

The two-dimensional Bi_{2 O2}Se/Bi₂SeO5 top-gate field-effect transistors constructed of in situ intercalation oxidation show excellent electrical properties: room temperature mobility up to 427^cm2/Vs, hysteresis as low as 20 to 60 mV, and sub-threshold swing (SS) below₆₅ mV/dec.
Close to the theoretical value of 60 mV/dec
.
What's more, the gate media β-Bi 2 SeO₅ is as thin as 3 layers (_2.
3nm) and EOT as low as 0.
41 nm, and the leakage current at 1 V gate voltage is still less than 0.
015A/^cm2, meeting the industry's low-power device gate media requirements
.

The single gate medium β-Bi 2 SeO 5 can be reduced to the sub-0.
5 nm level that meets the industry's requirements for low-power devices while the leakage current meets the industry's low power device requirements, compared to various gate media such as commercial HfO₂, hBN,_CaF2, thermal oxide polycrystalline α-Bi₂SeO ₅ , which has advantages in terms of equivalent thickness and insulation (Figure 1d
).
This research result makes up for the shortcomings of two-dimensional semiconductors in the integration of ultra-thin gate media, and is of great significance
to the development of two-dimensional electronic devices.

Fig.
Fig.
Preparation and properties
of single crystal natural oxide gate mediumβ-Bi2_SeO5.
(a) Two-dimensional_Bi2O2Se UV assisted intercalation oxidation into single grating mediumβ-Bi2_SeO5 schematic diagram; (b) Selective preparation of two-dimensional Bi2O2 Se/_{Bi2 SeO5}heterojunctions at the wafer level; (c) Two-dimensional Bi₂O 2Se / Bi₂SeO₅ heterojunction interface structure; (d) The equivalent thicknessof β-Bi2_SeO5 (EOT: equivalent oxide layer thickness; ECT: equivalent capacitance thickness) and insulation compared to other gate media

Hailin Peng is the corresponding author of the paper, and the first author is Zhang Yichi
, a doctoral student in the School of Chemistry and Molecular Engineering of Peking University.
Other key collaborators include Professor Lai Keji in the Department of Physics at the University of Texas at Austin and Professor Peng Gao of the School of Physics at Peking University
.
Each issue of Nature Electronics selects papers and invites authors to write research briefings, which are published online at the same time as research papers, so that readers can better understand the important results
。 Peng Hailin and Zhang Yichi, on behalf of the authors, published a brief introduction to the research entitled "Producing ultrathin monocrystalline native oxide dielectrics for 2D transistors", which briefly introduced the background, process, discovery and significance
of the research.

The research work has been funded by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Beijing National Research Center for Molecular Sciences, the Tencent Foundation and other institutions, and supported by the Laboratory of Molecular Materials and Nanomachining (MMNL) Instrument Platform of the School of Chemistry and Molecular Engineering of Peking University
.