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Abstract
A 1.5 µm laser diode has applications in eye-safe light detection and ranging (LiDAR) and optical communications via photonic integrated circuits. Photonic-crystal surface-emitting lasers (PCSELs) have lens-free applications in compact optical systems because of narrow beam divergences (<1 degree). However, the output power has still been less than 1 mW for 1.5 µm PCSELs. For higher output power, one approach is suppression of p-dopant Zn diffusion in the photonic crystal layer. Therefore, n-type doping was used for the upper crystal layer. Moreover, an NPN-type PCSEL structure was proposed to reduce intervalence band absorption in the p-InP layer. Here, we demonstrate a 1.5 µm PCSEL with 100 mW output power, which exceeds previous reported values by two orders of magnitude.
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1. Introduction
Semiconductor lasers are widely used for optoelectronic applications. However, the asymmetric beam patterns can have a divergence [1] that requires complex combinations of lenses for beam reshaping [2]. To address this issue, photonic-crystal surface-emitting lasers (PCSELs) reported by Noda et al. [3–5] were fabricated with two-dimensional photonic crystals (PCs) embedded as in-plane lasing cavities. The PCSELs enable large-area coherent oscillations because of the zero group velocity at the photonic band edge that leads to a narrow beam divergence [3] and, thus, lens-free systems [2]. The PCSEL wavelength range is from the ultraviolet [6,7] to the near-infrared, as well as 1 µm [8], 1.3 µm [9], 1.5 µm [10,11], 2 µm [12], the mid-infrared [13–15], and the terahertz region [16]. In particular, 1.5 µm is used for eye-safe (>1.4 µm) light detection and ranging (LiDAR) [17], and optical communication systems implemented via photonic integrated circuits [18]. We have proposed eye-safe NPN-type PCSEL (NPN-PCSEL) structures.
Zinc (Zn) is frequently used as a p-type dopant in III-V compounds for laser diodes. However, it diffuses rapidly [19], and diffusion from a p-cladding layer to an adjacent active layer displaces the p-n junction which results in diode degradation [20]. When the p-cladding layer is grown on the PC structure via regrowth [ Fig. 1], the Zn diffusion is presumably corrugated along the crystal growth direction because it occurs where the p-cladding layer has contacts inside and on the PC. The portion inside the PC depends on the shape of the PC structure, as well as crystal growth conditions. Thus, precise fabrication control is required relative to the case of regrowth on the planer structure. To avoid this issue, PC structures can be formed underneath multi-quantum wells (MQWs). However, the MQWs surface tends to be rough, depending on the crystal growth conditions, because the corrugated PC structure is formed below it.
Fig. 1. Schematic of Zn diffusion during growth processes for a laser diode on a photonic crystal structure after regrowth with a p-cladding layer.
One way to suppress Zn diffusion has been to grow the n-cladding layer on the PC layer because the diffusion length of the n-type dopant, such as silicon (Si), is generally shorter than that of Zn [21]. We initially examined a PCSEL structure fabricated on a p-InP substrate with a n-cladding layer on the PC (PN-PCSEL). The n-type dopant diffusion was sufficiently suppressed in the PC structure because of a short diffusion length. Furthermore, because the interface between the p-cladding and guiding layers was planer, Zn uniformly diffused from the p-InP cladding layer to the MQWs. The heterogeneous InGaAs/InP interface blocked Zn diffusion [22]. Therefore, we utilized the interface between AlInGaAs guiding and p-InP cladding layers to block Zn diffusion.
However, the PN-PCSEL exhibited non-negligible (α∼80 cm-1) intervalence band absorption (IVBA) when emitted through the substrate, which was consistent with that reported at 1.5 µm [23]. To address this issue, we propose a NPN-PCSEL structure in which the p-InP substrate is replaced with an n-InP substrate at first time, which reduced the optical loss and produced >100 mW output power. This is the highest reported output power of a PCSEL at 1.5 µm [10].
Here, we report the NPN-PCSEL operating at 1.5 µm. The fabrication process is discussed, followed by fundamental lasing characteristics, such as the current-light output (I-L), lasing spectra, and far-field pattern (FFP). Finally, we discuss directions toward higher output powers.
2. Device structure
Figure 2(a) shows a schematic of the device structure; the thickness and doping density of each layer is summarized in Table 1. Initially, a p-InGaAs contacting layer, a p-InP cladding layer, an undoped AlInGaAs guiding layer, a layer of InGaAs/AlInGaAs MQWs designed to emit at 1.55 µm, an n-AlInGaAs guiding layer, an n-InP carrier blocking layer, and an n-AlInGaAs PC layer were successively grown on an n-InP substrate via metal-organic vapor-phase epitaxy (MOCVD). The PC structure was fabricated on top of the n-AlInGaAs layer via electron-beam lithography and dry etching. The PC structure was a square lattice in which each lattice point was associated with an air-hole in the shape of a right-angle isosceles triangle. The lattice constant a (i.e., the period in the x- and y-directions) was 480–490 nm. The air-hole filling factor (i.e., the hole area per unit cell) was 20%. Figure 2(b) shows a top-view scanning electron microscope (SEM) image of the PC layer before regrowth. An n-InP cladding layer and an n-InGaAs contacting layer were grown on the PC via MOCVD; simultaneously, the air-hole was embedded in the PC layer. A cross-sectional SEM image of the air-holes in the PC layer is shown in Fig. 2(c).
Fig. 2. (a) Schematic of the NPN photonic-crystal surface-emitting laser. (b) Top-view scanning electron microscope (SEM) image of the photonic crystal before the regrowth. (c) Cross-sectional SEM image of the photonic crystal layer after regrowth. Air-holes were formed inside the photonic crystal.
Table 1. Thicknesses and doping concentration in the NPN and PN photonic-crystal surface-emitting laser (PCSEL)
To clarify the advantage of the NPN-PCSEL, we also fabricated the PN-PCSEL which is summarized in Table 1. The epitaxial structure including thickness and doping concentration are almost same, except the p-InGaAs contacting layer and doping type of the InP substrate.
After the crystal growth, a 500 µm × 500 µm square mesa structure and an L-shaped mesa were formed via wet etching on the p-InGaAs contacting layer. The n-electrode was formed on the square mesa, while the p-electrode was formed on the p-contacting layer that was exposed between the square and L-shaped mesas. The p-contacting layer was connected to the top of the L-shaped mesa via the p-electrode. The contacting area within the n-electrode was a 200 µm × 200 µm square. A SiN anti-reflecting coating was formed on the backside of the n-InP substrate to suppress light reflection. Finally, the PCSEL was mounted on a patterned substrate where the electrodes and the metal patterns were connected via indium solder. The current was laterally injected into the square mesa structure. Figure 3(a) shows a top-view SEM image of the fabricated NPN-PCSEL. A more detailed fabrication is illustrated in the Appendix A.
Fig. 3. (a) Top-view scanning electron microscope image of the NPN photonic-crystal surface-emitting laser (NPN-PCSEL). (b) Fundamental transverse electric mode intensity, and refractive index profile in the waveguide of the NPN-PCSEL.
The fundamental transverse electric mode of the NPN-PCSEL, and its refractive index profile are plotted in Fig. 3(b). This was a single mode along the direction of the epitaxial growth without overlap with the p- and n-InGaAs contacting layer where interband absorption occurred.
3. Lasing characteristics
In all the measurements, the NPN-PCSEL was driven in a pulsed mode with a laser driver (ILX Lightwave, LDP-3830). The pulse width was 50 ns, the duty cycle was 1%, and the temperature was 25 ○C. For I-L characterization, the peak output power was measured with a thermopile (Ophir, 3A-FS). Lasing spectra were acquired with a spectrum analyzer (Yokogawa, AQ-6370C). FFP was captured with a InGaAs camera (Hamamatsu, C14041-10U) through a screen placed 7.8 cm in front of the PCSEL. The beam pattern was projected on the screen.
Figure 4(a) shows the I-L characteristics of the NPN-PCSEL that had a 485 nm lattice constant. The threshold current was 810 mA (2.0 kA/cm2 current density). The 120 mW peak output power was obtained with a 56 mW/A slope efficiency, which was two orders of magnitude higher than that reported in Ref. [10]. The maximum output power was limited by the laser driver. The lasing spectra for lattice constants ranging over 480–490 nm are shown in Fig. 4(b). The peak emission wavelengths ranged over 1543–1572 nm, which corresponded to the band-edge wavelengths of the PC structure. Figure 4(c) shows the FFP at 1.5 A input; the narrow spot beam had <1○ divergence.
Fig. 4. Lasing characteristics of the NPN photonic-crystal surface-emitting laser operated in pulsed mode. (a) Light output vs. current. (b) Lasing spectra for different lattice constants. (c) Far-field pattern.
4. Discussion
Figure 5(a) depicts the I-L characteristics of the NPN-PCSEL (black line) and the PN-PCSEL (red line). The peak output power of the PN-PCSEL is 10 mW at 3.5A. The NPN-PCSEL output power was one order of magnitude higher than that of the PN-PCSEL. Because of the 1.5 µm absorption by p-InP [23], it was expected that the output of the PN-PCSEL would be absorbed. Therefore, we examined the p-InP absorption constant. The dopant was Zn at a 4.0 × 1018 cm-3 concentration, and the p-InP substrate thickness was 200 µm. Both sides of the substrate were polished, and the transparency of 1.54 µm wavelength through a 200 µm thick p-InP substrate was 0.15, as measured by using a PCSEL and a thermopile (Ophir, 3A-FS). Considering Fabri-Pérot interference, the absorption constant was estimated to be 81.1 cm-1, which agreed well with that reported in Ref. [23] for the same wavelength. This indicated that the thick p-InP was not suitable for eye-safe PCSEL. Whereas the 1.6 µm thick p-InP layer in the NPN-PCSEL structure had a transparency of 0.99. Therefore, it exhibited higher power in eye-safe wavelength range. Note that the IVBA of p-InGaAs contacting layer was ignored because its 1.6 µm absorption coefficient was 39 cm-1 [24]. The I-V characteristics is shown in Appendix B.
Fig. 5. Light outputs vs. current of (a) the NPN and PN photonic-crystal surface-emitting laser. (b) Dependence on the p-contacting layer thickness. (c) Dependence on the material of the p-contacting layer (same layer thicknesses).
As the contacting layer operating at 1.5 µm, InGaAs has widely been used in laser diodes [25] and PCSELs [10,11]. Because the NPN-PCSEL laser beam propagated through the interband-absorbing p-InGaAs contacting layer, we examined the thickness of the contacting layer with respect to the output power. Figure 5(b) shows the dependence of the output power on the p-contacting layer thickness (see Appendix B for the I-V characteristics). The peak output power decreased by a factor of one-third when the thickness increased from 50 nm (black line) to 150 nm (red line). Increasing the p-InGaAs contacting layer roughly reduced the output power to 80 mW. The 1.5 µm absorption constant of InGaAs was estimated as ∼10000 cm-1 [26,27] due to the interband absorption. Hence, the p-InGaAs contacting layer severely affected the NPN-PCSEL output power. The 150 nm thick p-AlInGaAs contacting layer was also examined, as shown in Fig. 5(c). Its band gap was larger than that of the p-InGaAs contacting layer. The p-AlInGaAs peak output power was 73 mW at 3.5 A, while that of p-InGaAs was 40 mW. However, the output power of the 150 nm thick p-AlInGaAs contacting layer was lower than that of the 50 nm thick p-InGaAs layer. Therefore, the interband absorption was not completely eliminated in p-AlInGaAs, which may have been attributed to shrinkage of the bandgap by the p-dopant concentration. Considering the bandgap of the contacting layer, p-InP is a candidate for higher output power [28].
As discussed above, the interband absorption occurs in the p-AlInGaAs contacting layer in which similar composition is utilized in the AlInGaAs guiding layer. To achieve the higher output power, the optical absorption in the AlInGaAs guiding layer should be suppressed. In addition, the carrier blocking layer for electron [29], the air-hole structure such as tear-drop shaped air-hole [4,30], and the effective feedback of the backside emission which reflected at electrode [2] may also contribute the higher output power.
5. Conclusions
An NPN-PCSEL structure operating at 1.5 µm suppressed Zn diffusion and optical absorption by the Zn p-dopant. N-type doping was used in the upper layer of the PC layer, and the p-InP layer thickness was minimized to reduce IVBA. We demonstrated >100 mW output power, which was a significant increase relative to previously reported values. The 1.5 µm NPN-PCSEL could lead to compact optical systems, including eye-safe LiDAR and photonic integrated circuits for optical communications. As the NPN-PCSEL structure is classified into transistor laser which might be beneficial for high-speed modulation [31,32].
Appendix A: The fabrication process of the NPN-PCSEL
Figure 6 shows the fabrication process of the NPN-PCSEL.
Fig. 6. Fabrication procedure for the formation of (a) the mesa structure, (b) the insulating layer, electrodes, and the anti-refracting coating. (c) Mounting the NPN photonic-crystal surface-emitting laser on a patterned substrate, and (d) operation by injecting current.
Appendix B: The I-V characteristics
The I-V characteristics of the PCSELs operated under CW condition at 25 ○C was aquired by using the laser driver (ILX Lightwave, LDX-3525) and digital multimeter (KEITHLEY, 2000 MULTIMETER). Figure 7(a) shows the I-V characteristics of the NPN-PCSEL (black line) and the PN-PCSEL (red line). The higher voltage is observed in the PN-PCSEL. The difference might be caused by the barrier height between the p-electrode and the p-contacting layer. We expect that the Schottky contact is formed in the PN-PCSEL because the p-electrode for p-InP contacting layer is not yet optimized. Figure 7(b) shows the dependence of the I-V characteristics on the p-contacting layer thickness. The turn-on voltage is almost same (around 0.9 V), whereas the series resistances are 1.1 Ω for 150 nm thick and 1.4 Ω for 50 nm thick, respectively. The slight difference of resistance might be caused by the thickness of the contacting layer.
Acknowledgments
The authors express their gratitude to A. Hiruma, H. Toyoda, and E. Tadataka for their warmful encouragement throughout this work.
Disclosures
The authors declare no conflicts of interest associated with this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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