Regular articleSynthesis of highly transparent titanium-indium-tin oxide conductive film at low temperatures for application in near-infrared devices
Introduction
Near infrared light-emitting diodes (NIR-LEDs) are widely used as emitters for remote controls, photo couplers, vehicle sensors, and closed-circuit televisions (CCTVs) [1], [2]. In recent times, their applications are being extended to optical sensors in wearable devices, small vehicles, time of flight (ToF) sensors, and flying drones [3], [4]. To meet the demands of these applications, small NIR-LEDs that can deliver a high power output at large injection currents are required. Generally, multiple quantum wells (MQWs), window layers, distributed Bragg reflectors (DBRs), and textured surfaces are used to improve the output power in NIR-LEDs [5], [6], [7], [8]. However, the sharp decrease in the surface emitting area of these devices due to a reduction in the chip size is not an aspect investigated by conventional research [9].
On the basis of previous studies, it may be assumed that using indium tin oxide (ITO) films is an effective approach to improve the output power of small NIR-LEDs [10]. However, the high annealing temperature of ITO films is a clear limiting factor in NIR-LED application owing to the relatively low epitaxial growth temperature of the latter (below 680 °C). Post annealing at temperatures higher than 650 °C would adversely affect the epitaxial structure. This limiting factor must be seriously considered for NIR-LEDs with a peak wavelength greater than 1,000 nm as the epitaxial growth temperature gradually decreases with an increase in peak wavelength.
In this investigation, to improve the output power of 940 nm InxGa1–xAs IR-LED systems, an ~2 nm-thick titanium buffer was inserted below the ITO film in the device assembly. Upon the inclusion of this titanium buffer, it was found that the transparency and resistivity of the ITO film improved remarkably even when annealed at low temperatures in the range of 450–550 °C. Especially, the optical characteristics of the Ti-containing ITO (TITO) films obtained at 550 °C were very similar to those of conventional ITO annealed at 650 °C. This is because Ti atoms could successfully re-activate the inactive nucleation sites in ITO films. As an extension of these observations, one can assume that the use of optimized TITO films is highly effective in improving the output power of NIR-LEDs with longer wavelengths.
Section snippets
Materials and methods
To fabricate the required samples, epitaxial 940 nm IR-LEDs were initially prepared. n- and p-doped Al0.1Ga0.9As were used as the n- and p-confinement layers, respectively. A 6 μm-thick Al0.15Ga0.8As window layer was used to improve the current-spreading effect. The conventional active region consisted of five pairs of MQWs, each with 4 nm-thick In0.08Ga0.92As wells and 10 nm-thick GaAs barriers. Fig. 1 shows structural schematic and compositional information of 940 nm infrared-light emitting
Results and discussion
The conductivity and transparency of ITO films depend strongly on the post-annealing process as their crystallization is activated during external thermal treatment [11]. Therefore, post-annealing processing is critical for ITO films. However, the high temperatures involved in post annealing pose specific limitations for some devices grown at lower temperatures. Fig. 2 show optical images (1000 × 1000 μm2) of the surfaces of a 940 nm In0.09Ga0.91As epi-wafer after post-annealing treatment at
Conclusion
In this study, ITO films were modified and optimized for application in NIR-LEDs with relatively low growth temperatures. Upon inserting a 2 nm-thick Ti barrier between the top layer of the LED and the ITO, the transmittance of device increased remarkably when compared to devices containing conventional ITO films. TITO films with a transparency of 98% were obtained after post-annealing at 550 °C (much lower than the epitaxial growth temperature of 680 °C). SEM, XRD, TEM analyses indicated that
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03012069)
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