Tactile sensors utilizing optical signals offer advantages over electrical signals, such as ease of analysis, visualization, and low crosstalk. Piezophototronic LED arrays have been explored, but their integration with the Si-based semiconductor industry remains challenging due to Si's indirect bandgap and resulting poor photoelectric performance. Previous research using piezoelectric nanowire (NW) LED arrays based on materials like ZnO, CdS, and GaN has shown promise, but these materials suffer from defect emissions that hinder signal acquisition and processing. Quantum dots (QDs) offer advantages such as narrow emission spectra and tunable colors. This research focuses on overcoming the challenges associated with Si-based pressure sensing by utilizing the flexoelectric effect, which allows for strain-induced polarization in centrosymmetric semiconductors like Si. The study aims to develop a high-density Si-based QLED array using micropillars, exploiting the flexoelectric effect to modulate the electroluminescence performance and enable pressure sensing applications.

Extensive research has been conducted on tactile sensors using electrical and optical signals. Optical signals are preferred for their ease of analysis and visualization. Piezophototronic effects have been explored using various materials such as ZnO, CdS, and GaN, which form piezoelectric nanowire LED arrays. However, these materials suffer from defects that interfere with optical signal processing. Quantum dots (QDs) are attractive alternatives due to their advantages in terms of narrow emission spectra, tunable color, and high stability. The challenge lies in integrating pressure sensing with the dominant silicon-based semiconductor industry, as Si's centrosymmetric structure prevents piezoelectricity. The flexoelectric effect, which is not limited by lattice symmetry, provides a potential pathway to overcome this limitation. While Si's theoretical flexoelectric coefficient is low, experimental results have shown much higher values, making it promising for pressure sensing applications. This study builds upon this understanding, focusing on the flexoelectronic effect in Si and its impact on carrier transport.

The fabrication process involved several steps. First, p-Si micropillars were created using inductively coupled plasma (ICP) etching after patterning a photoresist mask. The space between the micropillars was filled with photoresist (SU8 1040) to ensure device flatness, and then reactive ion etching (RIE) exposed the micropillar tips. A layer of NiO was deposited by magnetron sputtering to act as an electron barrier layer. CdSe/ZnS QDs were then spin-coated as the light-emitting layer and annealed. A TPBi electron transport layer was deposited by thermal evaporation. Finally, patterned ITO (top electrode) and Ag (bottom electrode) layers were deposited by magnetron sputtering. The resulting Si-based QLED array was characterized using field-emission scanning electron microscopy (SEM), and its electroluminescence (EL) was measured using an Ocean Optics spectrometer. Current-voltage (I-V) and current-time (I-t) characteristics were measured using a semiconductor test system. Pressure was applied to the micropillar array using a dynamometer and three-dimensional stages to study the effect of strain on electroluminescence.

The fabricated Si micropillars effectively concentrated stress, enhancing pressure sensitivity. The QLED array exhibited uniform light emission with minimal crosstalk between pixels. The EL intensity increased with increasing bias voltage, and the peak emission wavelengths matched those of the QDs used. Under applied pressure, the current increased significantly, with the EL density enhanced by approximately 600% under a pressure of ~13.8 GPa. This enhancement is attributed to both the piezoresistive and flexoelectronic effects. The asymmetry observed in the I-V curves under pressure supports the significant contribution of the flexoelectronic effect.

The results demonstrate the successful integration of pressure sensing with Si-based QLED technology, opening new avenues for tactile sensors. The observed increase in current and EL intensity under pressure can be attributed to the combined effects of piezoresistivity and the flexoelectronic effect. The asymmetry in the I-V curves under pressure strongly suggests a significant contribution from the flexoelectronic effect. This finding is highly significant as it successfully leverages the flexoelectric effect in Si, enabling efficient pressure-sensitive optical devices that are fully compatible with existing Si-based semiconductor fabrication processes. This compatibility is crucial for large-scale manufacturing and integration.

This study successfully demonstrated a high-density, Si-based QLED array for pressure sensing. The integration of Si micropillars with QDs and the utilization of the flexoelectronic effect enable significant modulation of light emission under pressure. The compatibility of this technology with the existing Si-based semiconductor industry paves the way for large-scale integration and widespread applications in electronic skin, human-machine interfaces, and tactile sensing. Future research can explore optimizing the device structure and QD materials for improved performance and expanded functionalities.

The current study focuses on a specific device design and QD material. Further investigation is needed to explore the impact of variations in micropillar dimensions, QD types, and device architecture on the overall performance and sensitivity. The pressure applied in this study was relatively high; future research could explore lower pressure ranges to enhance the applicability in real-world scenarios. A more in-depth analysis of the relative contributions of piezoresistive and flexoelectronic effects would further refine the understanding of the underlying mechanisms.