Photoluminescence of Layered Semiconductor Materials for Emission-Color Conversion of Blue Micro Light-Emitting Diode (µLED)

: A micro light- emitting diode (μLED ) is a key device for the future of advanced information. Owing to expand its application widely, the concept of the emission-color conversion using layered semiconductors as a color converter is proposed. In addition, it is demonstrated that layered semic onductors were transferred directly onto μLE D chips, and the emission-color conversion is realized. The layered GaS 1 − x Se x alloy, whose energy bandgap can be controlled by tuning the S and Se compositions, was selected as a color converter. The photoluminescence (PL) measurements using a blue LED as an excitation source revealed that GaS 0.65 Se 0.35 and GaSe can show green and red luminescence with center energies of 2.34 and 1.94 eV, respectively. The emission color of gallium nitride (GaN)- based blue μLEDs c overed with GaS 0.65 Se 0.35 and GaSe thin films were clearly converted to green and red, respectively. Furthermore, the emission color could be controlled by changing the film thickness. Thus, these results suggest the possibility of emission-color conversio n of blue μLED chips utilizing layered materials.


Introduction
Micro light-emitting diodes (μLEDs) based on gallium nitride (GaN) material systems have received much attention in novel optoelectronic technologies such as next-generation display and wireless communication for the internet of things as well as in interdisciplinary fusion such as biomedical applications [1][2][3]. In particular, a head-mounted display for virtual reality, augmented reality and these kinds desire full-color light-emitting devices have been expected as specific application examples. This is because of their properties including high brightness, high response speed, long device lifetime, low power operation, and the realization of microfabrication. Therefore, μLEDs are expected to be used as a key device underpinning the future of advanced information society. To achieve the above-mentioned applications, the emission color of GaN-based μLEDs is converted from blue or near-ultraviolet light to visible light, such as green, yellow and red by several methods, as shown in Figure 1a. In any case, the emission-color conversion of GaN-based μLEDs achieves utilizing principles of fluorescence or photoluminescence (PL). Directly coated phosphors or colloidal quantum dots (QDs) can be realized the emission-color conversion from blue to various visible colors [4][5][6][7][8][9]. These color converters should be at least several micrometres (μm) thick because of the realization of high light absorption and high-efficiency luminescence. In addition, II-VI based quantum wells (QW) on glass substrates are also investigated as the color converter [10,11]. Although QW structures with reflectors can be designed below 5 μm thick, the carrier substrate is approximately 200 μm. Ideally, the thickness of the color converter should be thinner and ultimately negligible for any further miniaturization and integration expanding to the application for other products.
On the other hand, layered materials are also expected for optoelectronic device applications on a micro-and nano-scale. The thickness can be controlled by exfoliating from the bulk form, and the thin film can be arranged directly onto other materials through van der Waals forces by the transfer method. In addition, their characteristics of luminescence exhibit high potential of them being used as photonic materials [12][13][14][15]. Therefore, a combination of GaN-based μLEDs and layered materials is one of the candidate methods for realizing full-color μLEDs miniaturized in all dimensions, as shown in Figure 1b. However, this approach of color conversion has not yet been demonstrated. Note that the luminescence characteristics of layered materials are generally achieved via the microscopic PL measurement using a solid-state laser as an excitation source, and the luminescence is detected using an optical detector through an optical microscope and a spectrometer. In particular, the high visibility of luminescence after converting GaN-based μLEDs is important in this approach for any application. Thus, it is an important issue to demonstrate the emission-color conversion of GaNbased μLEDs using layered materials. Transition metal dichalcogenides (TMDCs) such as molybdenum and tungsten dichalcogenides (MoX2 and WX2, X: S or Se) have been extensively studied for luminescence characteristics in layered materials, and its nanosheet has excellent light absorption characteristics compared to other optoelectronic materials. However, the light absorption efficiency is about 20% at most, therefore, it is difficult to realize PL by exciting at the output level of the LED. In addition, there are few investigated nanosheet materials with green luminescence. In this study, gallium sulfur selenium (GaS1−xSex) alloys showing green and red luminescence were utilized for the first demonstration of emission-color conversion combining with GaN-based μLEDs and layered materials. We report the color conversion from blue to green and red lights for GaN-based μLEDs using semiconducting GaS1−xSex alloys as a color converter of layered material. First, the compositions of GaS1−xSex alloy were designed to obtain green and red luminescence. Next, PL measurement of GaS1−xSex films was performed using a shell-type blue LED to determine whether color conversion could be achieved at the output level of an LED light. Then, GaS1−xSex films were exfoliated and transferred onto the device chips, and the emission-color conversions of GaN-based μLEDs from blue to cyan, green and red were realized. Schematic diagram of our proposal for emission-color conversion of GaN-based μLEDs from blue to green and red. Additionally, layered materials can be directly placed on GaN-based μLEDs. The thickness of layered materials is below 100 μm in this work. For future work, the thickness expects to be ultimately reduced to the order of a few tens nm.

Materials and Method
Vapor-grown bulk GaSSe and GaSe alloys were prepared for color converters of green and red luminescence, respectively. For the GaSSe alloy, the S and Se compositions were estimated by using the lattice constant determined through X-ray diffraction (XRD) and Vegard's law ( Figure S1). The PL was obtained using a microscopic PL system based on HORIBA LabRAM HR Evolution. A commercially available shell-typed LED with blue emission at a center wavelength of 470 nm was used as an excitation source for PL measurements. The blue light of the LED was incident on the samples from the back side through the microscope glass. The PL signal was detected by a chargecoupled device through an optical microscope and a spectrometer with a grating of 1800 lines/mm.
Flip-chip-bonded blue μLED chips (130 × 240 μm 2 ) were purchased from the Epistar Corporation. Typical characteristics of μLED operation include a turn-on voltage of 2.5 V and external quantum efficiency of more than 25% at a current density of 300 A/cm 2 ( Figure S2). GaSSe and GaSe thin films with the thickness below 100 μm were obtained by mechanical exfoliation onto μLED chips. The devices were operated with a probe station equipped with tungsten needles connected to a source measure unit (2450, Keithley, Solonm, OH, USA) at room temperature for electroluminescence (EL) measurements. To observe color conversion from blue to green and red, the emission light was monitored using a fiber optic spectrometer (Ocean Optics, FLAME-S, Amersham, UK).

Compositions of Layered GaS1−xSex Alloy for Green and Red Luminescenes
Layered GaS1−xSex alloys categorizing into III-VI materials show the energy bandgap bowing between 1.98 and 2.58 eV with the tuning of composition x, as shown in Figure 2a. This characteristic was estimated by fitting a quadratic function using the reported bandgap energy of compositions x (0%, 25%, 50%, 75% and 100%) [16]. When the composition is controlled between 25% and 48%, greencolor luminescence with bandgap energy between 2.40 and 2.26 eV can be realized, respectively. In addition, red-color luminescence can be obtained with the composition x = 100%, which represents pure GaSe. Therefore, we adopted layered GaS1−xSex alloys as the emission-color converters of GaNbased blue μLEDs. In this study, the compositions of S and Se for GaS1−xSex for green luminescence were set to 65% and 35%, respectively. XRD profiles of GaS0.65Se0.35 and GaSe bulk films are indicated the orientation along [001] direction. ( Figure S1). Additionally, no phase separation was obtained in GaS0.65Se0.35 bulk film. Figure 2b shows the PL spectra of GaS0.65Se0.35 and GaSe bulk films. Peak energies of 2.36 and 1.94 eV were achieved from GaS0.65Se0.35 and GaSe, respectively. The full-width half-maximums (FWHMs) for PL spectra of GaS0.65Se0.35 and GaSe were 129 and 110 meV, respectively. The large FWHMs were affected through phonon scattering because both direct and indirect transitions can occur in the band structures of these materials [17]. Additionally, for GaS0.65Se0.35, the occurrence of microscopic composition fluctuations that are Ga-S and Ga-Se condensations, resulting in the distribution of density of states near the band edge. This usually occurs for other disordered compound alloys, such as AlInGaN and ZnMgO [18,19]. On the other hand, background noise was detected in the PL spectrum of GaSe because the excitation efficiency using blue light is lower than that of GaS0.65Se0.35 owing to the large energy difference. The inset in Figure 2b clearly shows green and red PL images obtained through the irradiation of blue light. Therefore, color conversion from blue to green and red light was achieved using GaS0.65Se0.35 and GaSe, respectively, suggesting that they are candidate materials for the emission-color converters of GaN-based blue μLEDs.

Demonstration of the Emission-Color Conversion of Blue μLED Utilizing GaS0.65Se0.35 and GaS Thin Films
We conducted the emission-color conversion of the blue μLED chip through GaS0.65Se0.35 and GaSe thin films. Figure 3 shows the EL spectra of the μLED before and after the color conversion. The inset photographs show the EL images of the μLED before and after the color conversion. A luminescence peak was detected at 2.70 eV from the μLED before color conversion. Color-converted peaks were obtained between 2.32 and 2.60 eV by utilizing GaS0.65Se0.35 thin film with different layer thicknesses. The energy of the luminescence peak from the GaS0.65Se0.35 thin film was shifted toward the low-energy side with an increase in the thickness due to the increase in each layer interaction [20,21]. The low efficiency of color conversion was estimated to be below 1%. Weak luminescence at 1.88 eV was detected from color-converted μLED through a GaSe thin film. However, the excitation efficiency was low, as with the PL measurement shown in Figure 2b, leading to low efficiency of color conversion below 1%. This can be improved by designing a stacking-layer structure with stepped band alignment, such as double heterostructure with wide-bandgap barrier layers, which can enhance high-energy absorption. On the other hand, a part of the spectrum included the μLED peak at 2.70 eV as the excitation source owing to the spread and scattering of the blue light of the μLED. For applications of display and optogenetics, the improvement of light directivity is necessary using an integrated microlens. These results suggest the possibility of emission-color conversion of blue μLED chips utilizing layered materials. In this study, the thicknesses of layered GaSSe alloys as color converters were below 100 μm, which are slightly thinner than phosphors in the previous method. For future work, the thickness expects to be ultimately reduced to the order of few tens nm by using a superlattice consisting of monolayer sheets.

Conclusions
Layered materials have several advantages over other color converters, such as direct formation onto μLED chips and thin formation by designing a stacked-layer structure. This paper proposed the emission-color conversion of GaN-based μLEDs from blue to green and red emissions by utilizing layered materials, and presented its proof of concept. Layered GaS1−xSex alloys can show energy bandgap bowing within the visible light range and its luminescence. The PL of GaS0.65Se0.35 and GaSe can be achieved using blue light as the excitation source with the output level of the LED. Emissioncolor conversions were realized from blue to cyan, green, and red by directly utilizing the arranged GaS0.65Se0.35 and GaSe thin films onto GaN-based μLEDs.
Supplementary Materials: The following are available online at www.mdpi.com/2079-6412/10/10/985/s1, Figure  S1: symmetrical XRD profiles of GaS0.65Se0.35 and GaSe bulk. Figure   Acknowledgments: A part of this work was carried out using the analysis facilities at the Global Aqua Innovation Center (AICS) at Shinshu University.

Conflicts of Interest:
The authors declare no conflict of interest.