Hailin Peng's research group and collaborators in the School of Chemistry report on ultra-flat graphene wafer transfer and integrated optoelectronic devices

The high carrier mobility, strong light-matter interaction and excellent physical property regulation ability of graphene and other two-dimensional materials have important scientific value and broad application prospects
in the field of high-bandwidth optoelectronic devices.
At present, the development of two-dimensional material integration technology compatible with mainstream semiconductor silicon processes has received widespread attention in the industry, and the first challenge is to efficiently transfer the two-dimensional material from its growth substrate to the
target wafer substrate.
However, traditional polymer-assisted transfer techniques often introduce damage, wrinkles, contamination and doping on the surface of two-dimensional materials, which seriously affects the photoelectric properties and device properties
of two-dimensional materials.
Therefore, the realization of lossless, flat, clean and less doping transfer of wafer-level two-dimensional materials is a key problem
that needs to be solved urgently for the application of two-dimensional materials for integrated optoelectronic devices.

In view of this problem, the Peng Hailin Research Group of the School of Chemistry and Molecular Engineering of Peking University cooperated with Professor Qin Shiqiao and Zhu Mengjian of the National University of Defense Technology to design a gradient surface energy modulation composite transfer medium, which can adjust the surface interface energy during the transfer process to ensure the wafer-level ultra-flat graphene to the target substrate (_SiO2).
/Si, sapphire) dry lamination and non-destructive release, wafer-level non-lossy, clean, less doped uniform ultra-flat graphene film, showing the uniform high mobility device transport properties, observed room temperature quantum Hall effect and fractional quantum Hall effect, and constructed a 4-inch wafer-level graphene thermoelectron light-emitting array device, showing significant radiant thermal effect
in the near-infrared wavelength band.
This transfer method is universal and also suitable for the transfer
of other wafer-level two-dimensional materials, such as boron nitride.
The study, titled "Integrated wafer-scale ultra-flat graphene by gradient surface energy modulation," was published online in Nature Communications 2022, 13, 5410 on September 15
.

The paper points out that the transfer behavior of two-dimensional thin film materials from one surface to another is mainly determined
by the energy difference between different surface interfaces.
The larger the surface energy of the substrate, the better the wettability and stronger adhesion to the two-dimensional film, and the more suitable it is as the "acceptor" of the film transfer; Conversely, the smaller the surface energy of the substrate, the more suitable it is as a "release body" during film transfer
.
Therefore, the author designed and prepared a transfer medium with surface energy gradient distribution [Figure 1, polydimethylsiloxane (PDMS)/PMMA/ice sheet], in which the small molecule layer of the ice flake is adsorbed on the surface of graphene, which effectively reduces the surface energy of graphene, and ensures that the surface energy of graphene to the target substrate is much greater than the surface energy of graphene, thereby achieving good dry bonding; On the other hand, the PDMS polymer film on the upper layer of the transfer medium has the smallest surface energy and can achieve the non-destructive release
of graphene.
In addition, the transfer method also has the following characteristics: PDMS as a support layer can achieve dry lamination of graphene to the target substrate, reducing the interface of water and oxygen doping; The volatile ice flakes as a small molecule buffer layer can effectively avoid the direct contact and residue pollution of the upper PMMA polymer film on the graphene, and obtain a clean graphene surface; The rigidity of the polymer PMMA layer makes graphene maintain ultra-flat properties
after transfer.

Figure 1.
The gradient surface of wafer-level two-dimensional materials can regulate the transfer method

Based on the gradient surface can regulate the transfer of graphene film has the characteristics of non-loss, clean, less doping, ultra-flattened, etc.
, showing excellent physico-chemical properties (Figure 2).

The transferred 4-inch graphene wafer has a completeness of up to 99.
8%, good electrical uniformity, and a standard deviation of only 6% (655 ± 39 Ω/sq)
of the surface resistance in the 4-inch range.
The room temperature carrier mobility of graphene transferred to the SiO2/Si substrate is capable of reaching 10,000^cm2/Vs, and the room-temperature quantum Hall effect as well as the fractional quantum Hall effect (encapsulated by boron nitride, 1.
7 K)
can be observed.
Based on a 4-inch graphene wafer on a SiO₂/Si substrate, a thermoelectron light-emitting array device was successfully constructed that achieves a higher graphene lattice temperature (750 K) at a lower electrical power density (P = 7.
7 kW/^cm2) and exhibits significant radiative thermal effects in the near-infrared band (Figure 3).

Figure 2.
Gradient surfaces can regulate transferred graphene wafers
.
(a) non-destructive transfer to a highly complete 4-inch graphene wafer on a_SiO2/Si substrate; (b) Comparison of the number of folds of ultraflat graphene with rough graphene (in the range of 5 ×5^μm2) and typical atomic force microscopy pictures (inline map); (c) the surface resistance of the 4-inch graphene wafer after transfer; (d) Comparison of the electrical transfer curve of graphene with traditional wet transfer of gradient surface can be controlled; (e) Hall curves of graphene transferred to_SiO2/Si at different temperatures and room temperature quantum Hall effects; (f) Landau sector plot of graphene (boron nitride encapsulated, 1.
7 K) after transfer, exhibiting fractional quantum Hall effect

Figure 3.
Wafer-level graphene thermoelectron light-emitting array devices
.
(a) Graphene thermoelectron luminescence schematic diagram; (b) Thermoelectron luminescence arrays based on 4-inch wafer graphene; (c) Photomicroscopy of graphene thermoelectron luminescence array; (d) Infrared photograph of the device at an electrical power density of 3.
0 kW/^cm2; (e) the radiation spectrum of the device at different electrical power densities; (f) The change of graphene lattice temperature with electrical power density

In addition, the gradient surface energy control transfer method can be used as a general method for the transfer of wafer-level two-dimensional materials (graphene, boron nitride, molybdenum disulfide, etc.
) to industrial wafers, which is expected to lay a technical foundation
for the integration of high-performance optoelectronic devices.

The co-corresponding authors of the paper are Peng Hailin, Qin Shiqiao and Zhu Mengjian
.
The first co-authors are Gao Xin, a doctoral candidate at the Institute of Frontier Interdisciplinary Sciences of Peking University, Zheng Liming, a doctoral graduate of the School of Chemistry of Peking University, Dr.
Luo Fang, a member of the School of Frontier Interdisciplinary Sciences of the National University of Defense Technology, and Qian Jun
, a postdoctoral fellow of the School of Chemistry of Peking University.
Other major collaborators include Professor Liu Zhongfan of the School of Chemistry of Peking University, Lin Li, Distinguished Fellow of the School of Materials Science and Engineering of Peking University, Yin Jianbo and Luzhao Sun of the Beijing Graphene Research Institute, and Professor Gao Guanghui of Changchun University of Technology
.

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 projects, and supported by the Laboratory of Molecular Materials and Nanomachining (MMNL) instrument platform of the School of Chemistry and Molecular Engineering of Peking University
.