High Performance Crystalline Semiconductors on Inexpensive Flexible Substrates
Roll-to-roll (R2R) manufacturing is now being employed for semiconductors for electronics, lighting and photovoltaic applications because of its intrinsic benefits that include long deposition runs, use of large-area webs, and higher throughput, all of which can lead to lower manufacturing costs. In general, the performance of roll-to-roll manufactured products in these industries is substantially inferior to their crystalline counterparts. For example, the carrier mobility values of amorphous silicon and organic semiconductors used in flexible electronics is about 1 – 10 cm2/Vs compared to about 100 cm2/Vs of polysilicon and about 500 cm2/Vs of single-crystalline Si. As a result of the low carrier mobility, key performance metrics such as switching speed of thin film transistors (TFTs) fabricated with amorphous Si and organic semiconductors are far below that of TFTs made with crystalline silicon. Similarly, Organic Light Emitting Diodes (OLEDs) and organic solar cells via roll-to-roll manufacturing suffer from inferior performance and reliability when compared to devices made with crystalline materials. Polymer substrates are especially prone to thermomechanical deformation and distortion during device fabrication due to their high coefficient of thermal expansion (18-20 ppm/°C) and maximum temperature of use less than 200°C. The limited temperature capability further restricts the quality of coatings that can be deposited on polymer substrates since generally, higher quality crystalline coatings require higher deposition temperatures. Additionally, due to their higher absorption in the visible range, optical transmission of typical polymer substrates is inferior, limiting their use in optical applications. Table I shows advantages and limitations of R2R manufactured materials in comparison to traditional, discrete, wafer-based manufactured materials.
|Feature||Discrete Wafer-based Manufacturing||R2R Manufacturing|
|Substrate||Single crystal Wafer||Polymer|
Comparison of features of current R2R Manufactured Products and Discrete Wafer-based Products
High Performance Flexible Electronics
Photograph of high performance Si thin film transistor on flexible metal substrate 
Our group has developed technologies to fabricate TFTs of R2R-processed crystalline silicon and crystalline germanium on flexible metal and flexible glass, respectively [1,2]. These technologies are based on creation of intermediate crystalline layers on the flexible glass and metal substrates by ion beam assisted deposition . Field effect mobility of 200 cm2/Vs has been demonstrated on silicon TFTs on flexible substrate , which is more than 100 times better than that achieved in amorphous silicon-based TFTs. Saturation current levels above 50 μA/μm, which are orders-of-magnitude higher than that of conventional flexible TFTs have been achieved in our crystalline silicon TFTs on flexible substrate . Crystalline GaAs has been grown by R2R processing on flexible substrates with mobility exceeding 1200 cm2/Vs in n-type and above 100 cm2/Vs in p-type films with carrier concentration control over 1016-1020 cm-3. Such high mobility crystalline GaAs is currently under development for high efficiency solar cells and flexible electronics. In essence, our technology of combining the high performance of crystalline materials with the advantages of large-area, flexibility and inexpensive substrates provide a unique solution to R2R-manufactured products.
Raman Spectra of single-crystalline-like silicon on metal substrates showing optical quality comparable with that of silicon wafer
Mobility values above 200 cm2/Vs achieved in single-crystalline-like silicon on metal substrates
(Left) Family curves of Ids-Vds for a typical Si-TFT using single-crystal-like n-type Si epitaxially grown on a flexible metal substrate.
Y. Gao, M. Asadirad, Y. Yao, P. Dutta, E. Galstyan, S. Shervin, K. Lee, S. Pouladi, S. Sun, Y. Li, M. Rathi, J. Ryou, and V. Selvamanickam, “High-performance flexible thin-film transistors based on single-crystal-like silicon epitaxially grown on metal tape by roll-to-roll continuous deposition process”, ACS Appl. Mat. & Interfaces 8, 29565-29572 (2016)
M. Asadirad, Y. Gao, P. Dutta, S. Shervin, S. Sun, S. Ravipati, S. H. Kim, Y. Yao, K. H. Lee, A. P. Litvinchuk, V. Selvamanickam, J. H. Ryou, “High-Performance Flexible Thin-Film Transistors Based on Single-Crystal-Like Germanium on Glass”, Adv. Electron. Mater. 1600041 (2016)
V. Selvamanickam, S. Sambandam, A. Sundaram, S. Lee, A. Rar, X. Xiong, A. Alemu, C. Boney, A. Freundlich, “Germanium films with strong in-plane and out-of-plane texture on flexible, randomly textured metal substrates”, J. Crystal Growth 311, 4553 (2009).