Improving the light output efficiency of InP-based LEDs using photonic crystals

LEDs are widely used in all walks of life today, whether in display, lighting, or communications. However, in common light-emitting devices, the light output efficiency is greatly limited due to the transmission mode generated by total internal reflection. In recent years, many methods have been proposed to improve the light output efficiency of LEDs, such as surface roughening and resonant cavity method. These methods have been observed to improve the light output efficiency to varying degrees.

Since photonic crystals have a photon bandgap similar to the electronic bandgap in semiconductors, they are expected to become a new generation of optical semiconductors and a hot topic of research. Due to the existence of photon bandgap in photonic crystals, photons of certain frequencies cannot pass through. This property can be used to realize a variety of optoelectronic devices with excellent properties, such as photonic crystal lasers, photonic crystal large-angle curved waveguides, etc.

This paper mainly studies the use of photonic crystal bandgap structure to improve the light extraction efficiency of LEDs. The finite-difference time-domain method is used to theoretically analyze the effect of changes in lattice constants on the change in the bandgap position and the corresponding effect on the light extraction efficiency of LEDs. The photonic crystal structure is prepared using semiconductor process methods, and the effect of photonic crystals on improving the light extraction efficiency of LEDs is experimentally verified.

2 Theoretical analysis and experimental preparation

2.1 Band structure analysis

The FDTD method is mainly used to calculate the propagation and scattering of electromagnetic waves in the photonic crystal in the time domain. By analyzing the distribution of electromagnetic fields at different positions at the same time and the distribution of electromagnetic fields at the same position at different times, the purpose of analyzing the electromagnetic field propagation process inside the photonic crystal is achieved. The FDTD method can not only analyze the band structure of the photonic crystal, but also obtain the distribution of electromagnetic fields at different positions at the same time or at the same position at different times. We used the two-dimensional FDTD method to obtain the band structure of the photonic crystal, and also analyzed the changes in the position of the photonic crystal band when the lattice constant was changed. Figure 1 shows the photonic crystal band structure of the TE mode of a two-dimensional infinite photonic crystal when a=600nm, r/a=0.13. From the figure, it can be found that the normalized frequency range is 0.121~0.128. The photon state density is zero in this frequency range. The calculation also found that there is no band gap in the band structure of the TM polarization mode.

2.2 Microfabrication of photonic crystals

There are many ways to make photonic crystal structures on semiconductor materials, but the processing technology compatible with semiconductor technology has advantages in realizing optoelectronic integration. Due to the improvement of micro-machining technology, the fabrication of devices with fine structures such as photonic crystals has become more and more precise. There are many methods for processing photonic crystals. We mainly use the method of generating patterns using electron beam exposure and transferring patterns using reactive ion beam etching (RIE).

First, we tried to use only PMMA glue as a mask to directly transfer the pattern from the glue to the InP material to make a photonic crystal structure. EBL was used to define the photonic crystal pattern. In order to increase the selectivity of the mask and the material in the etching process, the wafer was baked in a high temperature environment for more than 2 hours after development to make the PMMA mask harder.

InP material is directly etched using the RIE system. A mixed gas of CH4+H2+Ar is used in the etching, and the pattern after etching is shown in Figure 2. As can be seen from the figure, due to the poor selectivity of the PMMA mask, the pattern after etching has been deformed. According to the theoretical design and the EBL definition, the pattern is a structure in which air holes are formed on the material. The result after etching is a conical material column, indicating that the side etching during the etching process is more serious, causing adjacent holes to be interconnected, forming a conical columnar structure. Even so, the uniformity of the pattern etched under this process remains good, which also shows that by optimizing the process and improving the selectivity of the mask, the desired pattern can be obtained.

FIG3 shows typical steps of making photonic crystals by using semiconductor micromachining technology, with SiO2 added as a mask based on the above process.

First, PECVD technology is used to deposit SiO2 on the material, with a deposition temperature of 300℃ and a film thickness of 180nm; after cleaning, a 200nm thick PMMA glue is coated on the SiO2 using a glue spreader; then the pattern is defined on the PMMA glue using electron beam exposure technology; after exposure, the chip is placed in a developer for 10s; it is taken out and cleaned, and then placed in a constant temperature oven to bake the hard film at 150℃ for 30min. The pattern is transferred from PMMA to SiO2 using a reactive ion etching device (RIE model: MPI2500). During the etching process, a CF4+O2 mixed gas is used, with an oxygen volume percentage of 5%. Before etching, the reaction chamber is first evacuated and then filled with a mixed gas with a gas flow rate of 80sccm, maintaining a gas pressure of 113332Pa, turning on the RF power, setting the RF power to 150W, and starting the etching process. PMMA is not removed after etching, and the pattern observed by atomic force microscopy (AFM) is shown in Figure 4. After the pattern is transferred to the SiO2 layer, the SiO2 layer can be used as a new mask to etch the InP layer.

After the pattern is transferred to SiO2, the InP material is directly etched using the RIE system. CH4+H2+Ar mixed gas is used in the etching.

The pattern after etching is shown in Figure 5. As can be seen from the figure, after adding the SiO2 mask, using RIE etching, good photonic crystal patterns can be produced under appropriate process parameters.

3 Testing and Analysis

Since most of the photonic crystal LEDs currently realized are based on photoluminescence structures, the test methods and means are also focused on how to achieve laser pumping of LEDs. Since the microcavity structure area in photonic crystal LEDs is only tens of square microns, or even a few square microns, how to obtain a small pump spot and aim the spot at the microcavity area without irradiating other areas (to prevent the heat generated by non-radiative recombination from causing a substantial increase in the temperature of the device and causing the device performance to deteriorate) puts high demands on the design of the optical path. The basic structure of the test system used in the experiment is shown in Figure 6. The PL spectrum test of the photonic crystal LED was completed using this test system.

According to the order of the optical path, the laser pumping and signal detection system of the photonic crystal active device mainly includes the following structures and components: As shown in Figure 6, the pumping beam emitted from a laser (wavelength 532mm, power 100mW) is reflected by a dielectric film reflector and enters the next reflector. At this reflector, the pumping laser beam and the visible light emitted from the visible light source pass through the next partial reflector and are focused by the microscope objective lens on the surface of the photonic crystal sample. The focused spot can be as small as about 5μm×5μm. The fluorescence or laser beam excited by the pumping light in the photonic crystal sample is collected by the microscope objective lens and becomes parallel light transmission. After passing through multiple reflectors, it can be collected and entered into the spectrum analyzer for analysis. The signal generated by the spectrum analyzer is phase-locked and amplified and then enters the microcomputer for signal processing to obtain the emission spectrum of the device. In order to align the focused spot of the pumping light source with the microcavity defect area of ​​the sample, an additional visible light beam is used to illuminate the device, and a visible light broadband reflector is used to couple the visible light reflected from the sample surface into the CCD observation system, so as to observe the surface structure of the sample.

FIG7 shows the improvement of LED light extraction efficiency by photonic crystal structure when r/a=0.13 is fixed and different lattice constants a=525, 550, 600nm. The three curves in the figure correspond to the PL spectra of the pump spot aimed at the complete photonic crystal area and the area without any process (i.e., InP/InGaAsP quantum well epitaxial wafer) under the same laser pump test conditions. From the test results in the figure, it can be found that under the same test conditions, the light output power of different areas is different, that is, the light output efficiency is different. In the wavelength range of 1400-1600nm, the light output power of the area containing the photonic crystal is significantly higher than that of the area without the photonic crystal structure. It can be seen that the introduction of the photonic crystal structure can significantly improve the light extraction efficiency of the light-emitting device. Since the photonic band gap of the photonic crystal structure is related to the photon frequency, the improvement of the device efficiency by the photonic crystal structure is also related to the light emission wavelength of the device, and the improvement of the light emission efficiency at different wavelengths is different. This phenomenon can be found in the figure. The area where the light extraction efficiency is most significantly improved is near the wavelength of 1400-1600nm. The shorter the wavelength, the more obvious the improvement in light extraction efficiency. For example, in Figure 7 (a), when the wavelength is near 1600nm, the light extraction efficiency of the device is improved by about 10%. As the wavelength decreases, the light extraction efficiency increases. When the wavelength is at 1550nm, the light extraction efficiency is improved by 26%; when the wavelength is 1450nm, the improvement is 90%; when the wavelength is near 1400nm, the light extraction efficiency can be improved by more than 60%. It can be seen that the peak of the light extraction efficiency improvement is near the wavelength of 1450nm. According to theoretical analysis, the introduction of photonic crystal structure can not only introduce a photonic bandgap, suppress the spontaneous emission of some frequencies and improve the internal efficiency of the device, but also, when the device light-emitting frequency is above the photonic crystal bandgap, that is, the radiation mode region, due to the effect of the photonic crystal, some modes belonging to the transmission mode in the flat-structured light-emitting device can be converted into the radiation mode in the photonic crystal light-emitting device, so that more light modes are radiated into the air. This is the basic principle of photonic crystals to improve the light output efficiency of light-emitting devices.

Figure 7 (b) shows the situation that the photonic crystal structure improves the light extraction efficiency of the light-emitting device when the lattice constant a=550nm, r/a=0.13. Similar to the results shown in Figure 7 (a), the light extraction efficiency of the photonic crystal structure LED has been significantly improved compared with the device without this process. When the luminous wavelength of the device is near 1600nm, the light extraction efficiency is improved by about 33%; when the luminous wavelength is near 1550nm, the light extraction efficiency is improved by about 38%; when it is 1450nm, it corresponds to an increase of about 100%; and when it is 1400nm, it is 93%. When the lattice constant increases to a=600nm, the light extraction efficiency improvement ratio continues to increase, as shown in Figure (c). This is mainly caused by the change of the photonic crystal structure parameters. According to the characteristics of the photonic crystal band structure, when the lattice constant increases, the band gap position redshifts to a lower frequency, that is, the long wavelength direction, which is beneficial for the In2GaAsP quantum well structure light-emitting device designed in the experiment to have the emission wavelength in the leakage mode region in the band, thereby converting more transmission modes into leakage modes and improving the light extraction efficiency.

4 Conclusion

Using EBL to expose PMMA glue can generate good photonic crystal structure graphics. Without special curing treatment, if only PMMA glue is used as a mask, the side etching during etching will be serious, which will destroy the integrity of the pattern. After adding SiO2 mask, using RIE etching, good photonic crystal graphics can be produced under appropriate process parameters. The photonic crystal structure can significantly improve the light output efficiency of LEDs, and the improvement effect of light output efficiency is related to the lattice constant of the photonic crystal. Within a certain range, as the lattice constant increases, the improvement of light output efficiency increases accordingly. With the continuous deepening of research work, photonic crystals will definitely make a difference in the production of high-efficiency LEDs.