High-output-power and high-temperature operation of blue GaN-based vertical-cavity surface-emitting laser

GaN-based vertical-cavity surface-emitting lasers (VCSELs) represent an attractive alternative to conventional LEDs and edge-emitting lasers because of their helpful properties. These include limited catastrophic optical damage (COD), ready integration with two-dimensional arrays, minimal effect of temperature on the lasing wavelength, and circular output beams with limited divergence. Recently, several research groups have achieved current-injected GaN-based VCSELs^1–8) and reported output power values of approximately 1,^7,9–12) 3 mW¹³⁾ and 4.2 mW¹⁴⁾ under continuous-wave (CW) operation. Very recently, our own group has demonstrated a blue VCSEL with a high output power of 6 mW under CW operation, as a result of reducing both the internal loss and reflectivity of the front cavity mirror.¹⁵⁾ However, the maximum output power was clearly limited by thermal rollover, and thus, reducing the thermal resistance (R_th) is also very important to enhance the output power. Our previous VCSEL¹⁵⁾ basically consisted of a 42-pair AlInN/GaN distributed Bragg reflector (DBR), a 4.5λ cavity, and a 10.5-pair SiO₂/Nb₂O₅ DBR. Considering the relatively low thermal conductivity of DBR materials including AlInN^16,17) and the very high thermal conductivity of the main cavity material GaN,^18,19) a long-cavity structure is highly expected to be effective for reducing R_th.^20–22) Mei et al.²²⁾ theoretically analyzed the dependence of R_th on cavity length in a GaN-based VCSEL with AlInN/GaN DBR by cylindrical model simulation and predicted that the R_th of this type of VCSEL with a long cavity can be decreased by 43%. In this Letter, we report the improved output power (15.7 mW) of a blue GaN-based VCSEL upon introducing a long-cavity (10λ) structure. To evaluate the impact of this structure, we also fabricated 5λ-cavity VCSELs and compared the I–L characteristics of the two types of devices over wide ranges of temperature and R_th.

A schematic of the fabricated VCSEL structure is shown in Fig. 1 and the details of the structure and fabrication process are provided in Ref. 15. The GaN-based device was grown on a GaN substrate by metalorganic vapor phase epitaxy. The basic structures, having 5λ- or 10λ-cavity constructions, consisted of a 42/41-pair AlInN/GaN DBR, a 5λ/10λ cavity, and a 10.5-pair SiO₂/Nb₂O₅ DBR. The reflectivity of the AlInN/GaN DBR mirror in the VCSEL with the 10λ-cavity structure was reduced in advance to compensate for the loss of slope efficiency resulting from the long-cavity structure.¹⁵⁾ The epitaxial structure between the DBRs was composed of an n-GaN layer, a 5-pair GaInN (3 nm)/GaN (4 nm) multiple quantum well, a p-AlGaN electron blocking layer, and a p-GaN layer. The 10λ-cavity structure was obtained solely by changing the n-GaN thickness from 660 to 1570 nm. The VCSEL also incorporated a SiO₂-buried structure to reduce the internal loss.¹⁵⁾ This structure was formed by etching an area of the p-GaN layer having a diameter of 8 µm to a depth of 20 nm by reactive ion etching, after which a 20 nm SiO₂ thin film was deposited and lifted off by a self-aligned process. Both the central current injection and the SiO₂-buried regions were subsequently covered with a 20 nm indium tin oxide (ITO) contact layer. Finally, a Nb₂O₅ spacer layer and a 10.5-pair SiO₂/Nb₂O₅ DBR were deposited on the ITO to form the VCSEL cavity. After forming the n- and p-electrodes, the GaN substrate was polished and the back surface of the substrate was coated with SiO₂/Nb₂O₅ dielectric multilayers as an antireflection (AR) coating. Herein, we define the substrate side as the front side (as shown in Fig. 1). The reflectivity of the 10.5-pair SiO₂/Nb₂O₅ DBR was calculated to be 99.98% at 445 nm. The reflectivity values of the AlInN/GaN DBR were determined by calculating the ratio between the front and the rear slope efficiency values under pulsed conditions,^15,23) with a calculated value of 99.98% for the reflectivity of the 10.5-pair SiO₂/Nb₂O₅ DBR. The measured reflectivity values of the 41-pair and 42-pair AlInN/GaN DBRs were 99.1 and 99.3%, respectively. VCSEL chips with an 8 µm aperture were subsequently mounted on a Cu heat sink block with a p-side-down (flip-chip bonding) configuration and operated under CW conditions.

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Figure 2(a) shows the I–L/I–V characteristics of the fabricated VCSEL with a 5λ-cavity structure in the case-temperature range from 20 to 100 °C. These data confirm that the device exhibited high-temperature lasing operation up to 100 °C. The threshold current (I_th), threshold current density, and threshold voltage at 20 °C were 4.2 mA, 8.4 kA/cm², and 5.1 V, respectively. The value of I_th was slightly higher than that in our previous report¹⁵⁾ because the front mirror loss was higher owing to the lower reflectivity of the front mirror. To achieve high output power and efficiency, we reduced the reflectivity of the front mirror even though the threshold current was increased.¹⁵⁾ In Fig. 2(a), the maximum output power was 8.2 mW at an operating current of 20 mA, owing to a high slope efficiency (η_s) of 0.71 W/A and a differential quantum efficiency (η_d) of 25.4%. The maximum output power of the device was limited by thermal rollover, and both the operating current at rollover and the maximum output power were decreased at high heat-sink temperatures. Thus, a low R_th was expected to allow operation at a relatively high current at rollover, giving an elevated output power over a wide temperature range.

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Figure 2(b) summarizes the I–L/I–V characteristics of the VCSEL with the 10λ-cavity structure over the temperature range from 20 to 110 °C. It is apparent that both the maximum output power and the operating current at rollover were dramatically enhanced by introducing the 10λ-cavity structure. An increased output power of 15.7 mW together with a high η_d of 31% and an η_s of 0.87 W/A were achieved at 20 °C. The threshold current, threshold current density, and threshold voltage at 20 °C were 4.5 mA, 9.0 kA/cm², and 5.1 V, respectively. Compared with the 5λ-cavity device, the operating current at rollover was significantly enhanced from 20 to 29 mA at 20 °C, likely due to a reduction in R_th, while the slope efficiency was also enhanced by reducing the reflectivity of the front cavity mirror.¹⁵⁾ These improvements resulted in an approximately doubled light output power at 20 °C. The maximum wall-plug efficiency at 20 °C was estimated to be 8.9% for the 10λ-cavity VCSEL and this device exhibited a higher lasing temperature up to 110 °C, at which an output power of 2.7 mW was obtained [as shown in Fig. 2(b)]. To the best of our knowledge, the output power of 15.7 mW, the differential quantum efficiency of 31%, and the operating temperature of 110 °C obtained from this unit are the highest values yet reported for a GaN-based VCSEL under CW operation.

Figures 3(a) and 3(b) show the emission spectra of the fabricated VCSELs with 5λ- and 10λ-cavity structures at various current injection levels of 0.9I_th, 0.95I_th, I_th, and 1.05I_th, respectively. The peak intensities were markedly increased above the threshold in both Figs. 3(a) and 3(b). The lasing wavelengths with a narrow linewidth of 0.08 nm were 443.5 and 440.1 nm for the VCSELs with 5λ- and 10λ-cavity structures under CW operation at 20 °C, respectively.

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The effect of reducing R_th by introducing a long-cavity structure was evaluated by assessing the R_th of the device, which is typically determined experimentally using the equation

$$\begin{equation} R_{\text{th}} = \frac{\Delta T}{\Delta P_{\text{diss}}} = \frac{\Delta\lambda/\Delta P_{\text{diss}}}{\Delta\lambda/\Delta T_{\text{hs}}}, \end{equation} \tag{ 1 }$$

following the measurement of variations in the wavelength shift with changes in the dissipated power (Δλ/ΔP_diss) and with changes in the heat-sink temperature (Δλ/ΔT_hs).^21–24) The reported Δλ/ΔT_hs values for GaN-based VCSELs range from 0.012 to 0.0185 nm/K.^21,22,24,25) In this work, Δλ/ΔT_hs values were acquired under pulsed operation to prevent the thermal effect, and the results obtained for VCSELs with 5λ- and 10λ-cavity structures were 0.0142 and 0.0146 nm/K, respectively. These are comparable to the literature values. The Δλ/ΔP_diss values determined for the VCSELs having 5λ- and 10λ-cavity structures were 0.015 and 0.0103 nm/mW, respectively. Using Eq. (1), the R_th values were calculated to be 1100 and 710 K/W for the 5λ- and 10λ-cavity VCSELs, respectively. This 65% improvement indicates that the operating current at rollover in Figs. 2(a) and 2(b) was enhanced by a factor of approximately 1.5. As an example, the operating current at rollover was increased from 20 to 29 mA at 20 °C to give a ratio of 1.45. These results confirm that reducing R_th leads to a higher output power. Compared with the results of the theoretical analysis of the R_th values in Ref. 22, the impact of the R_th reduction was quite reasonable, indicating that a thicker n-GaN layer essentially improves the thermal dissipation in our VCSEL structure.²²⁾

The internal temperatures at rollover ($T_{\text{i}}^{\text{rollover}}$) were estimated to be 159 and 161 °C for the 5λ- and 10λ-cavity VCSELs using the respective R_th and dissipated power values at rollover. These similar $T_{\text{i}}^{\text{rollover}}$ values also suggest that the reduced R_th was the primary reason for the higher output power observed for the 10λ-cavity VCSEL. Thus, both $T_{\text{i}}^{\text{rollover}}$ and R_th are important with regard to establishing the maximum lasing temperature ($T_{\text{hs}}^{\text{max}}$). Under the lasing condition, this temperature can be expressed as

$$\begin{align} &T_{\text{hs}}^{\max} < T_{\text{i}}^{\text{rollover}} - R_{\text{th}} \times P_{\text{diss}}^{\text{threshold}}\\&\qquad\approx T_{\text{i}}^{\text{rollover}} - R_{\text{th}}\times I_{\text{th}}\times V_{\text{th}}, \end{align} \tag{ 2 }$$

where $P_{\text{diss}}^{\text{threshold}}$ is the dissipated power at the threshold, I_th is the threshold current, and V_th is the threshold voltage. Thus, the temperature dependence of I_th is also important in determining $T_{\text{hs}}^{\text{max}}$. Figure 4 shows plots of the effects of temperature on the threshold currents of the fabricated VCSELs. These data demonstrate that, over the temperature range from 293 to 333 K, the temperature dependence of I_th was minimal, and a relatively high characteristic temperature (T₀ = 316 K) was obtained in the case of the 10λ-cavity VCSEL. In contrast, I_th rapidly increased with increasing temperature above 330 K. This tendency led to a significant internal temperature increase in the VCSELs below the threshold within the high-temperature region, resulting in the maximum lasing temperature of 110 °C observed for our device. The threshold current in VCSELs is governed by the mismatch between the wavelength of the cavity mode and the peak gain. Therefore, temperature-induced detuning of the lasing mode should be optimized in accordance with the requirements of each application.²⁶⁾

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In conclusion, we have demonstrated the highest output power values (15.7 mW at 20 °C and 2.7 mW at 110 °C) yet obtained from a blue GaN-based VCSEL, as a result of introducing a long-cavity (10λ) structure and reducing the reflectivity (to 99.1%) of the front cavity mirror. Compared with a 5λ-cavity VCSEL, the thermal resistance was reduced from 1100 to 710 K/W. An analysis of thermal resistance revealed a high internal temperature of 434 K at the rollover point in these devices. This improvement, together with the adjusted reflectivity of the front mirror, led to superior performance, including a high output power of 15.7 mW, a differential efficiency of 31%, and a wall-plug efficiency of 8.9% at 20 °C, as well as a maximum CW operational temperature of 110 °C together with a significant output power of 2.7 mW. These results mark a major step toward establishing high-power GaN-based VCSELs.

Acknowledgments