Periodic Reporting for period 2 - TRAPS-2D (Understanding The Role of the defects to Accomplish high Performance and Stable Two Dimensional Devices)
Période du rapport: 2022-01-01 au 2022-12-31
In the last decades, semiconductor chips have had to increase the performance while reducing their size to accomplish the market requirements according to the Moore’s Law. However, to support more scaled process technologies, other alternatives must be addressed.
In this regard, two-dimensional materials (2D), led by the graphene discovery in 2004, have attracted tremendous attention due to their promising electrical properties. Thanks to their low dimensionality, these thin materials present an optimal electrostatic control of the channel, high electrical mobility, flexibility and extremely sensitive capabilities to the changes in their surroundings. In this regard, the integration of 2D materials with standard silicon technology could seem particularly interesting to achieve a more interconnected society, accessibility to internet of things and clean energy transition thanks to the improvement of the energy storage technologies and the transparent features which some of these materials present. However, there are still technological limitations regarding the two-dimensional material integration in semiconductor fabrication flow: i) most studies that explore TMDs obtain films using methods that are not scalable, such as mechanical exfoliation or methods that are not CMOS compatible like synthesis at high temperatures. ii) the devices rarely accomplish the promising theoretical properties for these 2D materials. Defects and impurities inducing Fermi level pinning at the metal interfaces, Schottky-barrier formation, current hysteresis or Coulomb scattering are some of the reliability issues that the fabricated 2D devices present.
In this regard, two-dimensional materials (2D), led by the graphene discovery in 2004, have attracted tremendous attention due to their promising electrical properties. Thanks to their low dimensionality, these thin materials present an optimal electrostatic control of the channel, high electrical mobility, flexibility and extremely sensitive capabilities to the changes in their surroundings. In this regard, the integration of 2D materials with standard silicon technology could seem particularly interesting to achieve a more interconnected society, accessibility to internet of things and clean energy transition thanks to the improvement of the energy storage technologies and the transparent features which some of these materials present. However, there are still technological limitations regarding the two-dimensional material integration in semiconductor fabrication flow: i) most studies that explore TMDs obtain films using methods that are not scalable, such as mechanical exfoliation or methods that are not CMOS compatible like synthesis at high temperatures. ii) the devices rarely accomplish the promising theoretical properties for these 2D materials. Defects and impurities inducing Fermi level pinning at the metal interfaces, Schottky-barrier formation, current hysteresis or Coulomb scattering are some of the reliability issues that the fabricated 2D devices present.
Here we summarize the up-to-date performed work to fulfill the planned objectives:
- Objective 1: shed light into the defect implications on state-of-the-art 2D semiconductor materials.
o Initially we have focused on the proper fabrication of these state-of-the art 2D semiconductor materials (MoS2, WS2 and Graphene). The fabrication of MoS2 devices have been optimized by using a one-step synthesis process through the chemical vapor deposition technique (Figure 1). However, for the fabrication of WS2, a novel two-step process has been developed to improve the sample homogeneity (Figure 2). Graphene has explored in this project due to its electrical and thermal properties, its sensing capability, and its potential integration with other 2D materials.
o Once materials were synthesized, the next step was the corroboration of the material properties. To test whether the fabricated material fits with the expected stoichiometry, different rounds of structural characterization were carried out (Raman, XPS and AFM techniques).
o Then, the processing of the fabricated layered materials to perform devices has been addressed. Several lithography approaches have been optimized for these layered materials (Figure 3).
o Electrical evaluation of these fabricated materials has allowed us to shed light on the defect implications on these novel materials/devices. Analysis of the contact/material interfaces and electrical parameter extraction gave us an idea about the potential application of these materials and the possible path to improve their performance (Figure 4).
- Objective 2: learn to control the negative effects of defects to achieve specific operations and performance enhancement in fabricated devices (Defect-engineering).
o MoS2 processed devices present a behavior highly dependent on the light and pressure conditions due to the high presence of defects at the metal/semiconductor interface. In our international collaboration we have already exploited these defect implications in MoS2/metal diodes to demonstrate pressure/light sensors (Figure 5).
o Regarding graphene, due to its capability to detect changes in the surrounding, we are simultaneously exploring several application paths for our fabricated devices using graphene as a biosensor and as a gas sensor.
- Objective 3: Address the compatibility of the TMD-fabrication with standard Si CMOS fabrication process.
o In this regard, we have fabricated 200mm-diameter MoS2 wafers at low temperature (370ºC) using the ALD equipment recently installed at the host institution. This fabrication technique allowed us to synthesize the semiconductor layers in a lower temperature than in CVD accomplishing the thermal budget requirements for the Si-CMOS integration (450ºC). Examples of MoS2 samples fabricated via ALD with different thicknesses are shown in Figure 6.
- Objective 1: shed light into the defect implications on state-of-the-art 2D semiconductor materials.
o Initially we have focused on the proper fabrication of these state-of-the art 2D semiconductor materials (MoS2, WS2 and Graphene). The fabrication of MoS2 devices have been optimized by using a one-step synthesis process through the chemical vapor deposition technique (Figure 1). However, for the fabrication of WS2, a novel two-step process has been developed to improve the sample homogeneity (Figure 2). Graphene has explored in this project due to its electrical and thermal properties, its sensing capability, and its potential integration with other 2D materials.
o Once materials were synthesized, the next step was the corroboration of the material properties. To test whether the fabricated material fits with the expected stoichiometry, different rounds of structural characterization were carried out (Raman, XPS and AFM techniques).
o Then, the processing of the fabricated layered materials to perform devices has been addressed. Several lithography approaches have been optimized for these layered materials (Figure 3).
o Electrical evaluation of these fabricated materials has allowed us to shed light on the defect implications on these novel materials/devices. Analysis of the contact/material interfaces and electrical parameter extraction gave us an idea about the potential application of these materials and the possible path to improve their performance (Figure 4).
- Objective 2: learn to control the negative effects of defects to achieve specific operations and performance enhancement in fabricated devices (Defect-engineering).
o MoS2 processed devices present a behavior highly dependent on the light and pressure conditions due to the high presence of defects at the metal/semiconductor interface. In our international collaboration we have already exploited these defect implications in MoS2/metal diodes to demonstrate pressure/light sensors (Figure 5).
o Regarding graphene, due to its capability to detect changes in the surrounding, we are simultaneously exploring several application paths for our fabricated devices using graphene as a biosensor and as a gas sensor.
- Objective 3: Address the compatibility of the TMD-fabrication with standard Si CMOS fabrication process.
o In this regard, we have fabricated 200mm-diameter MoS2 wafers at low temperature (370ºC) using the ALD equipment recently installed at the host institution. This fabrication technique allowed us to synthesize the semiconductor layers in a lower temperature than in CVD accomplishing the thermal budget requirements for the Si-CMOS integration (450ºC). Examples of MoS2 samples fabricated via ALD with different thicknesses are shown in Figure 6.
The ground-braking aspect of this proposal resides in the “defect-engineering” approach and the potential CMOS co-integration. These novel aspects consist in analyzing the defects/traps which affect the performance and stability of the fabricated 2D devices trying either to reduce their influence on the device operation or to use them to implement new applications compatible with CMOS standard technology.
The extensive results of the electrical characterization have opened the door to work on two lines to progress beyond the state-of-the-art:
- On the one hand, due to the high density of defects and Schottky barrier formations that the fabricates devices have demonstrated at the interfaces, we have worked to improve these interfaces. For that, we have implemented solutions using alternatives metals and interfacial layers to perform more stable and reliable electronic devices.
- On the other hand, we have demonstrated that the Schottky barriers formed, far from being only a disturbance source, they are the origin of the ambient-dependent operation that these devices present. We have taken advantage of this influence to design ambient, pressure and light sensors with our as-synthesized materials. In this regard, we are also exploring the potential application of graphene-based devices as biosensors to detect bio-interactions in terms of charge density changes.
In research terms, we have published several of these advances in international and indexed journals. Moreover, as mentioned, these advances in novel semiconductors materials have direct implications on the society, proposing alternative materials and fabrication approaches which would alleviate the chip shortage or improve the waste disposal.
Knowledge dissemination, communication and scientific outreach have been developed:
- “Science week”: For one week we have explained our research actions to secondary school students in collaboration with the host institution. In both editions, 2021 and 2022, we have explained the advances in the 2D materials and their implications in the society.
- European Research Night 2022: We have participated with one stand explaining the advances of the electronic with 2D materials.
- Marie Curie Newsletter publication. A contribution in the special issue of December 2021 has been elaborated describing our advances in the project and the contingency plan during the pandemic.
- Twitter: TRAPS-2D account has posted the advances in the action to communicate to the generic public.
- Local Media TV documentary: in the framework of our advances in 2D materials an interview in a TV has recently been published in the reginal channel.
- Local Media newspaper: News about the advances of the group in 2D research, including this action, has been recently published in a local newspaper.
- Participation in several national and international conferences (SISPAD, EUROSOI, ARQUS).
The extensive results of the electrical characterization have opened the door to work on two lines to progress beyond the state-of-the-art:
- On the one hand, due to the high density of defects and Schottky barrier formations that the fabricates devices have demonstrated at the interfaces, we have worked to improve these interfaces. For that, we have implemented solutions using alternatives metals and interfacial layers to perform more stable and reliable electronic devices.
- On the other hand, we have demonstrated that the Schottky barriers formed, far from being only a disturbance source, they are the origin of the ambient-dependent operation that these devices present. We have taken advantage of this influence to design ambient, pressure and light sensors with our as-synthesized materials. In this regard, we are also exploring the potential application of graphene-based devices as biosensors to detect bio-interactions in terms of charge density changes.
In research terms, we have published several of these advances in international and indexed journals. Moreover, as mentioned, these advances in novel semiconductors materials have direct implications on the society, proposing alternative materials and fabrication approaches which would alleviate the chip shortage or improve the waste disposal.
Knowledge dissemination, communication and scientific outreach have been developed:
- “Science week”: For one week we have explained our research actions to secondary school students in collaboration with the host institution. In both editions, 2021 and 2022, we have explained the advances in the 2D materials and their implications in the society.
- European Research Night 2022: We have participated with one stand explaining the advances of the electronic with 2D materials.
- Marie Curie Newsletter publication. A contribution in the special issue of December 2021 has been elaborated describing our advances in the project and the contingency plan during the pandemic.
- Twitter: TRAPS-2D account has posted the advances in the action to communicate to the generic public.
- Local Media TV documentary: in the framework of our advances in 2D materials an interview in a TV has recently been published in the reginal channel.
- Local Media newspaper: News about the advances of the group in 2D research, including this action, has been recently published in a local newspaper.
- Participation in several national and international conferences (SISPAD, EUROSOI, ARQUS).