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930 nm Thin Film VCSEL - for Biomedical Applications

Recently, a research team composed of South Korea's Asia University (Ajou University) and RayIR company successfully fabricated a PDMS substrate using double transfer technology combined with surface modification assisted bonding (SMB) Top-emitting 930 nm thin-film vertical-cavity surface-emitting laser (VCSEL). The threshold current of this thin-film VCSEL is as low as 1.08 mA at room temperature. When the injected current is 13.9 mA, its maximum output power is 7.52 mW. The relevant research results were published in the journal Scientific Reports under the title of "Highly efficient thin-film 930 nm VCSEL on PDMS for biomedical applications". The method proposed in this paper is expected to open up technical possibilities for the multifunctional biomedical applications of next-generation VCSELs.

Optoelectronics has been extensively researched and developed in the biomedical industry for applications as diverse as light-based biosensing, photodynamic therapy, fluorescence imaging, and laser surgery. In particular, biocompatible light sources have recently attracted considerable attention in the field of biomedical technology because of their potential to drive the development of next-generation biomedical applications, enabling sensors to obtain real-time physiological information such as blood pressure, calorie consumption, and electrocardiogram (ECG). monitoring information.

Compared with conventional light-emitting diodes (LEDs) and edge-emitting laser diodes (EELs), vertical-cavity surface-emitting lasers (VCSELs) are rapidly becoming an attractive alternative due to their low threshold, low divergent beam size, excellent reliability, and low power consumption. Foreground light source. Furthermore, two-dimensional (2D) laser arrays can be fabricated on a large scale, allowing them to be easily packaged into optical chips such as photonic integrated circuits (PICs). With the technological advancement of multifunctional VCSEL applications, many research works propose the integration of traditional VCSELs with biocompatible polymers such as polyethylene terephthalate (PET) and rigid substrates such as Si and sapphire. However, the realization of biocompatible and efficient thin-film VCSELs is limited by the lack of effective techniques for integrating conventional VCSELs with polymers suitable for biological tissues.

Polydimethylsiloxane (PDMS) belongs to a class of polymeric organosilicon compounds that are expected to be suitable materials for bioelectronic applications due to their biocompatibility and biostability. PDMS is also thermally stable, flexible and lightweight, and has lower fabrication costs compared to other materials used in microdevice fabrication. It has been widely used in bioelectronics such as bio-MEMS, microfluidic systems, and bio-optics, enabling it to mitigate adverse effects such as inflammatory responses to human tissues. PDMS also protects electronic components from mechanical and environmental influences over a wide temperature range. This property enables PDMS materials to be used in the bioelectronics industry to protect micro-optical devices such as semiconductor-based waveguides, optical fibers and lasers.

However, there are still some challenges in realizing biocompatible thin-film VCSELs integrated on PDMS substrates due to the unfavorable characteristics of PDMS in terms of device fabrication and characteristic measurement. PDMS has considerable hydrophobicity over the entire surface, making it difficult to bond with the surface of the hydrophilic III–V epitaxial layer during the bonding process. Furthermore, PDMS tends to swell when combined with some reagents, which interrupts quantitative testing for chemical analysis. Although there have been many successful attempts to change PDMS from hydrophobic to hydrophilic, there are still some limitations, such as chemical instability, large-scale manufacturing process limitations, and difficulty in maintaining hydrophilicity for a long time.

In this paper, the research team successfully fabricated a top-emitting 930 nm thin-film VCSEL on a PDMS substrate using double-transfer technology combined with a surface-modified assisted bonding (SMB) process, enabling it to be used as a biocompatible light source . In order to integrate the thin-film III–V epitaxial layer of VCSEL with the PDMS substrate, the researchers used the double transfer technique to transfer the VCSEL to the heterogeneous carrier substrate twice to maintain the p-on-n polarity of the thin-film VCSEL. Furthermore, they used oxygen plasma combined with organosilane treatment for surface modification-assisted bonding, which did not require any additional bonding medium when bonding the PDMS support to the substrate-removed thin-film VCSEL. We demonstrate that the transfer process for integrating thin-film VCSEL structures onto PDMS substrates does not severely degrade VCSEL performance in terms of light-current-voltage (L–I–V) characteristics and spectra. In particular, we also determined that the top-emitting 930 nm thin-film VCSEL has a low operating threshold current of about 1 mA at room temperature, which indicates that the threshold current of the thin-film VCSEL on PDMS substrate is as low as that of conventional VCSEL on GaAs substrate.

Figure 1 shows the schematic structure of a top-emitting 930 nm thin-film VCSEL transferred onto a flexible PDMS substrate using SMB and double-transfer processes.

Fig.1 Schematic diagram of the top-emitting 930 nm thin-film VCSEL transferred onto a PDMS substrate

Figure 2 shows the epitaxially grown p-on-n structure of a thin-film VCSEL. The researchers used metal-organic chemical vapor deposition (MOCVD) to grow the p-on-n structure of the thin-film VCSEL on the n-type GaAs substrate in a vertical upward sequence. The active region of the VCSEL consists of three GaAsP/InGaAs MQWs sandwiched between two distributed Bragg reflectors (DBRs) consisting of alternating high and low index materials of n-DBR and p-DBR. An etch stop layer is grown on the GaAs buffer layer to protect the VCSEL structure during removal of the GaAs substrate.

Figure 2 p-on-n structure of thin film VCSEL fabricated by MOCVD

Figure 3 shows the fabrication process for transferring thin-film VCSELs to PDMS substrates using SMB and double transfer techniques.

Figure 3 Manufacturing process for transferring thin-film VCSELs to PDMS substrates

Figure 4a shows a photograph of a top-emitting 930 nm thin-film VCSEL integrated on a PDMS substrate, which shows considerable flexibility. Figure 4b and Figure 4c depict the FE-SEM top view and cross-sectional view of the fabricated thin-film VCSEL, respectively.

Figure 4 Top-emitting 930 nm thin-film VCSEL integrated on PDMS substrate

Figure 5a shows the L-I-V characteristics of a top-emitting 930 nm thin-film VCSEL integrated on a PDMS substrate under continuous wave (CW) operation at 25 °C, with a threshold voltage and current of approximately 1.69 V and 1.08 mA, respectively. When the injection current is 13.9 mA, the maximum output power of the thin film VCSEL is 7.52 mW. Figure 5b shows the spectrum of the fabricated thin-film VCSEL with a peak wavelength of 929 nm.

Fig.5 L-I-V characteristics and emission spectra of top-emitting 930 nm thin-film VCSEL

Under the same conditions at 25°C, the researchers compared the L–I–V characteristics of conventional VCSELs on GaAs substrates and thin-film VCSELs on PDMS substrates, and the results are shown in Figure 6.

Figure 6 Comparison of L-I-V characteristics of traditional VCSEL on GaAs substrate and thin-film VCSEL on PDMS substrate

In summary, this paper realizes a biocompatible top-emitting 930 nm thin-film VCSEL transferred onto a PDMS substrate with high flexibility. Double-transfer technology enables the fabricated 930 nm thin-film VCSEL to maintain p-on-n polarity. In addition, the surface modification process exhibits excellent bonding performance to integrate the PDMS carrier with the substrate-removed thin-film VCSEL without any additional materials. When the injection current is 13.9 mA, the maximum output power of 930 nm thin-film VCSEL integrated on PDMS substrate is 7.52 mW. The threshold current and voltage of this thin-film VCSEL are 1.08 mA and 1.64 V, respectively. The approach presented here is expected to open up technological possibilities for multifunctional biomedical applications of next-generation VCSELs.

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