1. Introduction

For the past few years, InGaAsN has attracted considerable research efforts for realizing GaAs-based 1.3-μm quantum well (QW) lasers [1–15]. So far, both InGaAsN Fabry-Perot edge-emitting lasers [1–12] and vertical cavity surface emitting lasers (VCSELs) [13] have been realized. For edge-emitting 1.3-μm InGaAsN lasers, Tansu et al. [4] reported the lowest transparency current density (J_tr) of 75–80 A/cm² from structures grown by metal organic chemical vapor deposition (MOCVD); Wang et al. [11] recently reported J_tr of 84 A/cm² from 1.3-μm InGaAsN lasers, grown using molecular beam epitaxy (MBE). More recently, 10-Gb/s transmission using floor free InGaAsN triple-QW (TQW) ridge waveguide (RWG) lasers has been successfully demonstrated [12]. Undoubtedly, InGaAsN 1.3-μm QW lasers present excellent potential for telecommunication application.

However, characteristic temperature (T₀) values of the high-performance 1.3-μm InGaAsN single-QW (SQW) lasers are only in the range of 70–110 K [4, 10–11], which is much lower than that of optimized 1200 nm InGaAs SQW lasers, ~200 K [10]. Multiple-QW (MQW) structures would be favorable for the high-gain application in VCSELs [6], for highspeed applications in edge-emitting devices [6, 10], as well as for the reduction of temperature sensitivity [1, 10]. On the other hand, Bour et al. [16] pointed out that narrow stripe RWG lasers could benefit from more efficient lateral heat dissipation, and thus, improvement of high temperature operation could be expected. Therefore, study of InGaAsN MQW narrow stripe RWG lasers would be of great interest for their practical application. However, up to now, majority of InGaAsN laser reports are based on broad area InGaAsN laser results with SQW structure [4, 10–11], while fewer results are reported on high-performance narrow RWG (≤ 4 μm) lasers with MQW structures [2–3, 12].

We have previously fabricated high-power InGaAsN TQW wide stripe (100 μm) lasers [9] using pulsed anodic oxidation (PAO). In our recent work, significant improved T₀ has been observed as the ridge width narrows to 4 μm [15]. In this paper, we report high-performance 1.3-μm range InGaAsN TQW narrow ridge (4 μm) lasers fabricated using PAO. For comparison, InGaAsN 4-μm SQW lasers were also fabricated and studied. In order to reduce the stripe width to smaller dimensions while minimizing the effects of lateral current spreading, the ridge height “h” of the RWG lasers has to be carefully optimized. We have previously reported the “h” effect on J_th of RWG lasers [8]. Here, we will demonstrate that “h” also affects T₀ of the RWG lasers.

2. Experimental procedure

InGaAsN TQW and SQW laser structures were both grown using MOCVD from IQE (Europe) Ltd. The schematic band diagram of InGaAsN TQW laser structure is shown in the inset of Fig. 1(a). The details of InGaAsN TQW laser structure were reported in Ref 8. InGaAsN SQW laser structure is identical with that of TQW structure except for the active region, which contains an SQW of GaAs (20 nm)/In_0.35Ga_0.65As_0.985N_0.015 (6.4 nm)/GaAs (20 nm). InGaAsN TQW and SQW RWG lasers with contact ridge width (w) of 4 μm and “h” of 1.23 μm have been fabricated using PAO as described in Refs. 7 and 8. “h” value of the InGaAsN RWG lasers are important for the device performance, which will be explained in detail in Section III. Subsequent to fabrication, individual laser chips were cleaved at different cavity length (L) for testing without facet coating under continuous wave (CW) operation. Laser output power vs. injection current (P-I) characteristics of the InGaAsN TQW and SQW lasers were measured in the temperature range (20–100 °C).

3. Results and discussion

Figure 1(a) shows the CW P-I characteristic at 20 °C of a P-side-down bonded InGaAsN TQW laser (4 × 1600 μm²) fabricated using PAO. The lasing wavelength from InGaAsN TQW structure is 1297 nm [9]. The upper inset shows a scanning electron microscope (SEM) cross sectional image of the fabricated InGaAsN RWG laser using PAO. The active region width, which was used for J_th estimation, was determined to be 5 μm according to the SEM image. The I_th was ~67.5 mA, corresponding to a J_th of 848 A/cm . Maximum output power 145 mW/facet (290 mW for both facets) was obtained from this device. Figure 1(b) plots ln(J_th) against the inverse of cavity length (1/L) of the InGaAsN TQW RWG lasers. J_tr of the InGaAsN TQW lasers was calculated to be 389 A/cm² (130 A/cm²/well) using Eq. (1) [17].

where α_i is the internal optical loss, η_i, is the internal quantum efficiency, Γ is the optical confinement factor, and g_o is the material gain. R=0.32, is the optical power reflection coefficient at the two cleaved facets. Γ_g0 was calculated to be 20.42 cm^-1 from the slope of the guidance line in Fig. 1(b); α_i and η_i can be derived from the relationship between the reciprocal of external quantum efficiency (η_d) and L for the same batch of InGaAsN TQW lasers as shown in the inset of Fig. 1(b) using Eq. (2) and found to be 9.6 cm^-1 and 93.6 %, respectively.

 

Fig. 1. (a) CW P-I characteristic of a P-side-down bonded InGaAsN TQW laser (4 × 1600 (μm²). The upper inset shows the SEM cross sectional image of the InGaAsN laser fabricated with PAO. The lower inset shows the schematic band diagram of InGaAsN TQW laser structure. (b) ln (J_th) as function of 1/L from a batch of InGaAsN TQW 4-μm RWG lasers. J_tr was determined to be 389 A/cm². The inset shows 1/η_d as a function of L. η_i and α_i were determined to be 93.6 % and 9.6 cm^-1, respectively.

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Figure 2 shows the temperature-dependent CW P-I characteristics (20~100 °C) from an unbonded and uncoated InGaAsN TQW RWG laser (4 × 1500 μm²). Testing was carried out up to 100 °C, limited by our equipment. Temperature-dependent P-I characteristics (20~100 °C) were also measured from InGaAsN TQW RWG lasers with different L. The inset of Fig. 2 shows the plot of ln(I_th) vs. T of InGaAsN TQW RWG lasers with L of 500, 1000, and 1500 μm, respectively. The dots denote the experimental data and the lines are used for eye guidance. Using Eq. (3), the T₀ (in the range of 20–80 °C) of each laser has been calculated to be 143.5 K (L=500 μm), 153.7 K (L=1000 μm), and 157.2 K (L=1500 μm), respectively.

 

Fig. 2. Temperature-dependent (20-100 °C) CW P-I characteristics of an InGaAsN TQW laser (4 × 1500 μm²). The inset shows the ln(I_th) as a function of device temperature from InGaAsN TQW 4-nm RWG lasers with different cavity length L of 500, 1000, and 1500 μm, respectively. T₀ was calculated to be 143.5 K, 153.7 K, and 157.2 K, respectively, in the linear region (20–80 °C).

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It can be found that T₀ is lower for the InGaAsN laser with shorter L. By assuming that J_th, J_tr, act, exponentially increase with T, while η_d, g_o, current injection efficiency (η_inj) exponentially decrease with T, T₀ can be expressed as a function of the physical parameters as following Eq. (4) [5].

where T_tr, T_ηinj, T_go, and T_αi are characteristic temperatures of J_tr, η_inj, g₀, and α_i, respectively. From above equation, it can be seen that when the L is shorter the T₀ is lower due to the higher threshold gain (Γ∙g_th), which agrees well with our experimentally measured T₀.

For the QW laser, comparison of J_th and T₀ is important, as these parameters are typically of practical interest for such devices. Here, we compare the J_th and T₀ of our InGaAsN TQW RWG lasers with similar works as well as devices fabricated using conventional method on the same wafer as ours. Ha et al. [2] reported pulsed operation of an InGaAsN RWG (20 × 770 μm², TQW) laser emitting at 1.315 μm with J_th of 1.31 kA/cm² and relatively low T₀ of 65 K; Kovsh et al. [3] have reported high-power (200 mW) single mode operation under pulsed mode from an InGaAsN SQW RWG laser (2.7 × 1000 μm², λ=1.285 μm). However, their T₀ is only ~84 K. Tansu et al. [5] also reported J_th of 505 A/cm² from InGaAsN TQW lasers (broad area, L=1000 μm) emitting at 1290 nm, with T₀ of 110 K. Using the same wafer as ours as presented in this work, conventional SiO² confined InGaAsN RWG lasers (2 × 400 μm²) presented J_th of ~1.875 kA/cm² and T₀ of 135 K; broad area lasers (50 × 1200 μm²) presented a J_th of ~1.1 kA/cm² [7]. Compared with the above-mentioned published data [2–3, 5], and results from devices fabricated using conventional SiO₂ confinement on the same wafer as ours [7], we have shown that InGaAsN TQW narrow RWG (4-μm) lasers fabricated using PAO showed better or comparable performance with high output power of 290 mW (both facets), low J_tr of 389 A/cm² (130 A/cm²/well), and high T₀ of 157.2 K. Overall, in terms of output power, J_tr and T₀, our results are among the best for InGaAsN narrow RWG TQW lasers in the 1.29~1.30 μm wavelength regime ever reported.

There are several possible reasons for the high-performance of InGaAsN TQW RWG lasers presented here. The primary reason might be due to the benefit of TQW laser structures [1], which would contribute to the high T₀ in this study. For comparison, InGaAsN SQW 4-μm RWG lasers were also fabricated using PAO. Figure 3 shows temperature-dependent (20–80 °C) CW P-I characteristics of an unbonded and uncoated InGaAsN SQW RWG laser (4 × 500 μm²) fabricated using PAO. The inset (left) shows the typical lasing spectrum of the laser at room temperature (RT), which is cantered at 1247 nm. I_th of this InGaAsN SQW RWG laser is as low as 15.7 mA. However, InGaAsN SQW lasers have shown strong temperature dependence of I_th. The inset (right) shows the ln(I_th) vs. temperature in the range of 20–80 °C from InGaAsN SQW lasers with L of 500 and 1500 μm, respectively, as well as InGaAsN TQW laser with L= 500 μm for comparison. T₀ was 62.4 K for the InGaAsN SQW laser with L=500 μm; T₀ for the SQW laser with L=1500 μm is a little higher, 78 K, but is still much lower than that from InGaAsN TQW lasers (L=500 and 1500 μm), which is 143.5 K and 157.2 K, respectively. Our work implies that with the MQW structure, T₀ value of InGaAsN lasers could be greatly improved. Fehse et al. [1] have also noted the stronger temperature-dependent I_th of InGaAsN SQW lasers than that of InGaAsN TQW lasers. They believed increasing the number of QWs could reduce the relative contribution of the Auger recombination current. Tansu et al. [10] also pointed out that the suppression of heavy-hole leakage in the InGaAsN MQW structures will lead to a reduction in the temperature sensitivity of the J_th.

 

Fig. 3. Temperature-dependent (20–80 °C) CW P-I characteristics of an unbonded InGaAsN SQW 4-μm RWG laser fabricated with PAO. The inset (left) shows the lasing spectrum from the InGaAsN SQW RWG laser. The right inset shows the comparison of ln (I_th) as a function of device temperature (20–80 °C) from InGaAsN SQW laser, L=500 μm and 1500 μm, with T₀ of 62.4 and 78 K, respectively, as well as InGaAsN TQW laser, L=500 μm, with T₀ of 143.5 K.

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The proper choosing of the ridge height, “h”, of InGaAsN TQW RWG lasers, might be another reason for our high-performance lasers. “h” effects on the J_th and η_d of RWG lasers have been reported in detail in Ref 8. In our subsequent study, we also observed that “h” has also noticeably effects onT₀. Figure 4 shows the plot of ln(I_th) vs. T of InGaAs RWG lasers (L=1100 μm, w=50 μm) with different “h”. For simplicity, only the lasers with “h” of 0.39, 1.23, and 1.77 μm are shown here. The inset shows all the measured T₀ as a function of “h”. It can be seen that the InGaAs laser with the optimized “h” [8] has the highest T₀ of all the lasers, which is in consistent with our previous study for the lowest J_th and highest η_d [8]. We believe that when the ridge is shallow, the large lateral spreading current will cause the lower T₀ in the cases of h= 0.39 and 0.80 μm; when the ridge is high (below QW active region), the heavy carrier losses at the sidewall becomes large and caused the low T₀ in the case of h=1.55 and 1.77 μm.

 

Fig. 4. ln (I_th) as a function of device temperature (20–80 °C) from InGaAs SQW 50-μm RWG lasers with ridge height (h) of 0.39, 1.23, and 1.77 μm, respectively. The inset shows T₀ as a function of h of 0.39, 0.80, 1.23, 1.55, 1.77 μm, respectively, in the region of (20–80 °C).

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The incorporation of GaAsP strain compensation layers would be another possible reason for the high-performance of the InGaAsN TQW lasers. It was pointed that strain-compensated layers can enhance device performance by allowing higher indium concentration (for longer wavelength) and lower nitrogen concentration (for improving luminescence efficiency) to be used in the active region [4]. The GaAsP layers used in this work were close to the InGaAsN layers and had higher band gap than GaAs, the utilization of larger band gap barrier materials will lead to a suppression of thermionic carrier leakage, which will in turn lead to a reduction in the temperature sensitivity of the threshold-current density of the lasers, in particular, at high temperature operation [4, 14]. The tensile strain in the GaAsP also reduced the overall stress of the active region (strain compensation). Therefore, stress-related defects were reduced in the active region. Furthermore, compared with GaAs as barrier layer, the material gain of the InGaAsN structure using GaAsP as barrier can be increased by more than 40% for standard device operating temperatures, which can greatly reduced temperature sensitivity [14].

4. Conclusions

In conclusion, InGaAsN TQW and SQW narrow RWG (4 μm) lasers have been fabricated using PAO. High output power of 290 mW (both facets), low J_tr of 389 A/cm² (130 A/cm²/well) as well as high T₀ of 157.2 K were obtained from InGaAsN TQW laser. Possible reasons for the high-performance of InGaAsN TQW lasers have been discussed. Extremely low I_th of 15.7 mA was obtained from an InGaAsN SQW RWG laser (4 × 500 μm²). However, I_th of InGaAsN SQW lasers showed strong temperature dependence in our study. Our work implied that with optimized MQW InGaAsN laser structure, T₀ could be further improved.