The use of coupled reflectors was investigated to improve the device performance of GaAs-based 860 nm vertical-cavity surface-emitting lasers (VCSELs). Here, the combined reflector was fabricated by coupling a metallic reflector (Ag) and a distributed Bragg reflector of an epitaxial structure. The performance parameters of the VCSEL, such as light-emitting efficiency, operational voltage, and power consumption, were dramatically improved by using a combined reflector. As a result, infrared GaAs-based VCSELs with combined reflectors could show superior light-emitting performance and reduced operational threshold voltage characteristics compared to conventional VCSELs.
Vertical-cavity surface-emitting lasers (VCSELs) have been actively developed as the surface emitters for smartphone sensors, data networks, and optical information communications.1–5 In general, VCSELs contain a device structure capable of emitting light in a direction perpendicular to the wafer and multiquantum well heterostructures in the active region.6–8 Their performances could be greatly improved through the introduction of oxide apertures, ion implanted sublayers, and indium tin oxide (ITO).9–16 However, it is still difficult to implement a high-performance GaAs VCSEL with excellent light emission efficiency while minimizing problems such as internal heat, wavelength shift, and Fabry–Perot dip shift.
To improve the emission performance of VCSELs by applying reflectors, various distributed Bragg reflectors (DBRs), including silicon DBRs, composition-grade DBRs, graphene-bubbler DBRs, and self-planar mesa structures, have been investigated.17–21 By increasing the number of pairs of layers in the DBR, the reflectivity of the DBR can be increased, and, thus, the output power of the VCSEL can be improved.22 However, if the number of DBR pairs is excessively increased, more power consumption is required to operate VCSEL owing to its increased driving voltage condition. Next, some studies investigated metal reflectors containing single elements (Au, Al, and Ti) and their alloys.23–25 Generally, Au reflectors were widely applied to make high-performance infrared VCSELs.23,24 However, some research has suggested that Ag might be a better candidate because of its lower cost and higher optical reflectivity in theoretical simulations compared to the case of Au.25–27 Next, some alloy metals could show higher reflectance compared to Au reflectors. However, the application of alloying metals is sometimes limited due to complex processes, such as multistep deposition and thermal treatment processes, for alloy formation.
In this study, a reflector combining a metallic reflector and a DBR was applied to improve the light efficiency of the VCSEL. Several surveys confirmed that Ag-based metal reflectors can effectively suppress self-heating problems in device operation due to their effective heat spreading and thermal conductivity.28,29 Thus, the combined reflector manufactured by combining the metallic layer and the DBR is expected to exhibit high light reflectance and efficient heat dissipation compared to the conventional DBR without the metal layer. In fact, the optical efficiency of VCSELs could be dramatically increased through the use of the combined reflector. Therefore, it could be stressed that the use of a reflective metal carrier bonded to the top DBR is essential to improve the optical efficiency of the 860 nm GaAs VCSEL.
To apply to time of flight (ToF) system composed of an infrared light sensor and an infrared light source, we focused on developing a high-performance infrared VCSEL capable of emitting light with a wavelength of 860 nm. Generally, near-infrared lights with wavelengths around 850 and 950 nm are used as a light source for ToF systems. Figure 1 shows the device schematic and the configuration information of the 860 nm VCSEL chip with the combined reflector (Ag reflector + DBRs). An epitaxial wafer of 860 nm VCSEL was fabricated on an n-type GaAs (100) substrate by a metalorganic chemical vapor deposition (MOCVD) system. The n-type GaAs substrate was lattice matched with n-type and p-type AlxGa1−xAs confinement layers fabricated by varying the composition between Al and Ga in the AlxGa1−xAs materials. Al0.15Ga0.85As with a high refractive index and Al0.85Ga0.15As with a low refractive index were repeatedly grown to fabricate multilayer structures for the top DBR and bottom DBR. The bottom and top DBRs have a doping concentration of approximately 3–4 × 1018 cm−3 and 0.9–1.2 × 1018 cm−3, respectively. The active region consists of 5 nm-thick GaAs quantum wells (QWs) and 12 nm-thick Al0.05Ga0.95 As quantum barriers (QBs). It is located between the top and bottom DBRs. The oxide layer present inside the top DBR is an oxide aperture. This serves to minimize lateral currents spreading out of the cavity and to preclude recombination at the sidewall near the optical cavity. Here, the conventional VCSEL has 20-period top DBRs and 40-period bottom DBRs. On the other hand, in the case of the developed VCSEL, the same 20-period top and 20-period bottom DBRs were used, owing to the reversed structure caused by the wafer bonding process. The 200 nm-thick GaInP etch stop layer (ESL) was inserted between the n-DBR and GaAs substrate. In addition, a 400 nm-thick reflector (Ag) material was deposited on the p-DBR in VCSEL by using an e-beam evaporator. A 5 000 nm-thick eutectic structure (Ti/Au/In/Ti = 100/2 000/2 800/100 nm) deposited on the p-Si wafer substrate was used for wafer bonding. In particular, the Au/In structure with a low melting point can play a role in bonding the Ag/p-DBR and the p-Si substrate and can play a role in dissipating heat due to its high thermal conductivity. The Ti layer was employed to prevent the diffusion of indium into the p-DBR and p-Si. During the wafer-to-wafer bonding process, the p-DBR and the metal reflector were directly bonded by applying a force of 4 500 N for 60 min at 230 °C.
After the wafer-to-wafer bonding process, to define light-emitting regions, the absorbing substrate (GaAs) was selectively eliminated using a mixed solution of H2O2 and NH3 until the appearance of the etch stop layer (GaInP layer). The GaInP ESL was selectively eliminated in an HCl solution for 10 s. Next, the epitaxial structure of the 860 nm VCSEL was sequentially cleaned with acetone and H2SO4/H2O2/de-ionized (DI) solution to eliminate conventional contamination elements such as organic materials and dust on the wafer surface. Lift-off photolithography and a selective etch were carried out for the front side. Then, the Si3N4 antireflection layer with 200 nm thickness was deposited on the top DBR by the PECVD system. Photons emitted upward from the VCSEL were effectively extracted out of the top surface through Si3N4 used as an antireflection layer.30 To make the light-emitting pole and oxide aperture region, top n-DBRs with ring-metal in the 860 nm VCSEL were etched by the inductively coupled plasma (ICP) system introduced with Ar and BCl3 gases. The 4 μm-deep trenched circle line on the light-emitting pole was fabricated after an ICP etching process. Next, the trenched sample was annealed in H2O vapor for 20 min at 400 °C. After thermal treatment, the Al0.98Ga0.02 As layer near the bottom region of the n-DBR is changed to a thin Al2O3 layer (oxide aperture) by thermal oxidation reaction. The emission area and light-emitting pole have 10 and 30 μm diameters, respectively. The trenched circle line was filled with a polyimide material to improve the stable property of the light-emitting poles, and then, it was baked for 4h at 375 °C. Next, Au/Ge/Ni (1 600 nm) was deposited on the top surface by using an electron-beam evaporator, and AuBe/Au (160/300 nm) was then deposited on the bottom using a thermal evaporator. As a result, as shown in Fig. 2, a GaAs-based infrared VCSEL chip with 14 light-emitting poles could be fabricated with the combined reflector.
III. RESULTS AND DISCUSSION
Figure 3 exhibits cross-sectional images for a conventional VCSEL and a developed VCSEL (with a reflective metallic substrate). Generally, a p-DBR and an n-DBR are applied above and below the active region of the VCSEL [Figs. 3(a) and 3(c)]. It is well known that this structure can dramatically improve the light-emitting performance through the cavity effect. Here, the emission area in the pole has a diameter of approximately 18 μm. The trench depth between the lighting poles, which was filled with polyimide, was approximately 4 μm. The diameter of the oxide aperture fabricated in a circular standard shape is 12 μm. The trench depth and the oxide aperture were confirmed via the magnified cross-sectional images [Figs. 3(b) and 3(d)]. It was found that a nearly 30 nm-thick oxide apertures existed between the active region and the top DBR. The developed VCSEL includes three types of reflectors: the top DBR, bottom DBR, and metallic Ag reflector [Fig. 3(c)], which were produced by applying a wafer bonding process. The top and bottom DBRs have the same 20 pairs, which is different from the conventional VCSEL with 20-period top DBRs and 40-period bottom DBRs. As shown in Fig. 3(d), a combined reflector fabricated for the developed VCSEL is observed. In particular, the 400 nm-thick metallic Ag reflector is used for the bottom DBR. Meanwhile, it was confirmed that other functional parts, such as the trench depth and oxide aperture, were manufactured almost identically to the conventional VCSEL.
As shown in Fig. 4, to obtain more detailed information, the reflection performance of the 40-period DBR, metallic Ag reflector, and combined reflector (Ag reflector + 20-period DBR) was investigated using thin film calculation (for simulation) and the UV-near-infrared (NIR) spectrometer (for measurement). As shown in Fig. 4(a), based on simulation calculations, the combined reflector (Ag reflector + 20-period DBR) was expected to exhibit approximately 99.5% reflection, similar to that of the 40-period DBR (99.4%) and higher than that of the metallic reflector (98.3%) for wavelengths in the 830–890 nm range. In fact, as shown in Fig. 4(b), when their reflection performance was measured using UV-NIR spectroscopy, it was clearly confirmed that the combined reflector exhibited the highest reflective performance of 99.8%, which is almost the same as the simulated value (99.5%). Meanwhile, the 40-period DBRs and Ag metallic reflector showed low reflective performance of 98.2% and 95.4%, respectively. Therefore, we can expect that the combined reflector will be most suitable for the output improvement of the laser diode. Figure 4(c) shows the measurement scheme for the reflectors. Figure 4(d) shows a cross-sectional image of a simple structure with the combined reflector. Here, it was confirmed that the DBR and Ag reflector in the combined reflector were well bonded with excellent adhesive properties and a fine interface state. Meanwhile, p-Si and Au/In were used as carriers and eutectic metals, respectively, to improve the heat dissipation performance of the developed VCSEL.
Figure 5(a) shows the light-current (L-I) and the current-voltage (I-V) plots of the conventional and developed VCSEL chips. Compared with the case of the conventional VCSEL (VTh = 1.74 V), the developed VCSEL with the combined reflector (Ag reflector + 20-period DBR) shows a lower threshold voltage performance (VTh = 1.67 V), making it suitable for low-power operation in Fig. 5(b). As shown in the L-I curve, the light output of the conventional case rapidly decreased despite the increase in the injection current after saturation to 325 mW at 120 mA. In contrast, the developed VCSEL with the combined reflector exhibited an improved light output power (400 mW) at a higher injection current state of 150 mA. The improved performance, including the reduced threshold and higher light output, is due to the use of a combined reflector with better reflectivity.
Likewise, the current-voltage (I-V) plots of the VCSELs were also examined. In Fig. 5(c), the developed VCSEL exhibited power-efficient driving characteristics with reduced turn-on voltage (VTh = 1.67 V). The reason for the lower series resistance of the developed VCSEL can be attributed to a thinner p-DBR applied in the developed VCSEL.31,32 In addition, the I-V curve of the developed VCSEL with a combined reflector showed a lower increase rate than that of the conventional VCSEL. At 200 mA, the developed VCSEL with a combined reflector showed a lower voltage (2.0 V) compared with 2.2 V of the conventional one. The output power was expected to be positively influenced by lower resistance. In terms of light output, it can be confirmed that the threshold current (ITh = 7.5 mA) of the developed VCSEL is significantly lower than that of the conventional VCSEL (ITh = 14 mA). Most importantly, the developed VCSEL with a combined reflector shows an increased output power of 400 mW, which is 25% higher than that of a conventional VCSEL (320 mW). In addition, the higher peak point on the highest output power shifted from 120 to 150 mA. In the droop region, after the peak point, the output power of the metal bond VCSEL is three times higher than the conventional case. This implies that the output power can be maintained at a higher current. The low current loss in the droop region of the developed VCSEL is expected to be due to the introduction of a thin p-DBR and a thermally conductive eutectic metal structure.
Figure 6 shows the light-emitting images of the conventional and developed VCSELs depending on the injection current states. The infrared laser array consisting of 14 VCSELs is designed as an infrared emitter for a ToF system. A high-performance infrared laser array (chip size: 350 × 350 μm2) with superior output power is required for the ToF system.33–35 In the range of 20–50 mA, there is an increase in the brightness for both the conventional and developed VCSELs. At high injection current states, including 100 and 150 mA, light emitted by the developed VCSEL becomes significantly brighter than that emitted by the conventional VCSEL. These results demonstrate that the use of a combined reflector is effective in increasing the optical efficiency of the 860 nm VCSEL.
IV. SUMMARY AND CONCLUSIONS
In this study, a reflective metallic substrate was fabricated and investigated to improve the optical efficiency of an 860 nm GaAs VCSEL. The reflective metallic substrate was obtained by combining the Ag metallic reflector in the eutectic substrate and the top DBR in the VCSEL structure. Through UV-NIR spectrometry measurements, it was demonstrated that the combined reflector has a significantly higher reflectivity than a DBR and an Ag metallic reflector. To obtain more information, the L-I-V characteristics and brightness images were investigated for the conventional and developed VCSELs. In the L-I-V characteristics, the developed VCSEL with the combined reflector (Ag reflector + DBRs) showed a lower threshold current (ITh = 7.5 mA) and higher maximum current (150 mA) compared with the conventional VCSEL (ITh = 14 and 120 mA, respectively). This outstanding characteristic is attributed to an increase in the reflectivity caused by the developed combined reflector. In addition, brightness images showed a trend similar to that shown in the L-I curve. This result indicates that the optical efficiency of the 860 nm VCSEL can be effectively increased by introducing a combined reflector (Ag reflector + DBRs).
This research was supported by the “Regional Innovation Strategy (RIS)” through the National Research Foundation of Korea (NRF) funded by the Ministry of Education(MOE) (Grant No. 2022RIS-002). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant Nos. NRF-2016R1A6A1A03012069 and NRF-2018R1D1A1B07050752).
Conflict of Interest
The authors have no conflicts to disclose.
I. K. Jang and H. J. Lee contributed equally to this work.
In-Kyu Jang: Data curation (equal); Formal analysis (equal); Investigation (lead); Methodology (lead); Writing – original draft (equal). Hyung-Joo Lee: Conceptualization (supporting); Data curation (supporting); Formal analysis (equal); Investigation (equal); Writing – original draft (lead); Writing – review & editing (supporting). Dae-Kwang Kim: Data curation (equal); Investigation (equal); Methodology (equal). Lee-Ku Kwac: Conceptualization (supporting); Project administration (supporting); Supervision (equal); Writing – review & editing (equal). Sung Woon Cho: Conceptualization (equal); Formal analysis (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding author upon reasonable request.