Narrow ridge waveguide high power single mode 1.3-μm InAs/InGaAs ten-layer quantum dot lasers
© to the authors 2007
Received: 18 April 2007
Accepted: 23 May 2007
Published: 14 June 2007
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© to the authors 2007
Received: 18 April 2007
Accepted: 23 May 2007
Published: 14 June 2007
Ten-layer InAs/In0.15Ga0.85As quantum dot (QD) laser structures have been grown using molecular beam epitaxy (MBE) on GaAs (001) substrate. Using the pulsed anodic oxidation technique, narrow (2 μm) ridge waveguide (RWG) InAs QD lasers have been fabricated. Under continuous wave operation, the InAs QD laser (2 × 2,000 μm2) delivered total output power of up to 272.6 mW at 10 °C at 1.3 μm. Under pulsed operation, where the device heating is greatly minimized, the InAs QD laser (2 × 2,000 μm2) delivered extremely high output power (both facets) of up to 1.22 W at 20 °C, at high external differential quantum efficiency of 96%. Far field pattern measurement of the 2-μm RWG InAs QD lasers showed single lateral mode operation.
High-performance GaAs-based quantum dot (QD) lasers are of great interest due to their potential applications in advanced optical fiber communication systems [1–9]. The reduced density of states arising from the three-dimensional confinement of carriers give QDs the advantages to be able to achieve low threshold current density and high differential gain [2, 5, 6, 10]. High power, high efficiency, and temperature insensitivity have been reported for InAs QD lasers [3, 5, 6]. However, the laser performance is commonly restrained by the intrinsically low surface density (NQD) of a single-layer QD structure . As the achievable optical gain, which is limited by saturated gain (Gsat), in a single-layer QD is proportional to the surface density, i.e., Gsat ∝ NQD, the finite NQD of the order of 1010 cm−2 in a self-assembled single-layer QD structure directly limits the available optical gain in the ground state (GS) [7, 8]. This leads to undesirable excited state (ES) lasing at high current and/or high temperature .
Over the last decade, it has been shown that utilization of multiple QD layers is an effective way to prevent gain saturation [3, 5–7, 9, 11–15]. Ideally, the saturation gain  and maximum output power increase  following increase in the number of QD layers. However in practice, the high strain accumulated in the multiple-layer QD active region generates defects formation, leading to degradation in the threshold current (I th ) and internal quantum efficiency (η i ) [6, 14]. This limits the number of stacking layers that can be incorporated into the QD active region. So far, laser structures comprising three to five QD active layers have been reported [5, 9, 11–13, 16, 17]. However, there have been relatively few reports [3, 6, 14, 15] on QD lasers emitting at 1.3 μm or above, with the number of QD active layers exceeding five.
Furthermore, single mode laser operation [12, 13, 18–20] is desirable for better device to fiber coupling efficiency in optical fiber communication systems. This could be achieved using narrow ridge waveguide (RWG) laser structure [12, 13, 15, 18–21]. There have been many studies of RWG structure in InGaAsN/GaAs QW and In(Ga)As/GaAs QD systems, where light emission at 1.3 μm is realized. High power single mode operation has been achieved in InGaAsN/GaAs QW lasers, where high performance in terms of light output, beam quality and high-temperature operation have been demonstrated [18–20]. Comparatively, fewer works have been reported on single mode operation in high performance In(Ga)As/GaAs QD lasers [12, 13]. It is commonly known that as the ridge width narrows, the sidewall condition plays an important role in the laser performance, where sidewall scattering/recombination  tends to degrade the laser performance. Undesirable lateral current spreading resulting from sidewall effects have been investigated for improving the laser structure design [22–24]. Moreover, the small lasing volume in narrow RWG lasers may increase the optical losses as result of process related scattering. Such effects may increase the threshold current density and limit high temperature operation . A key factor to achieve single mode emission is narrow ridge width of the QD laser structure. To obtain strong index guiding and to suppress current spreading, careful balance between etch depth and ridge width should be accomplished . Our previous works [26, 27] have shown that by optimizing the pulsed anodic oxidation (PAO) process after sidewall etching, high-performance RWG lasers with reduced lateral current spreading could be achieved.
While we have previously demonstrated  low transparency current density and high temperature characteristic ten-layer InAs broad area QD lasers, this paper reports the characteristics of ten-layer narrow ridge width (2 μm) InAs QD lasers. We will show results from devices with high output power of 272.6 mW (both facets) operated in continuous wave (CW) mode under GS lasing at 1.3 μm emission. Devices of dimension 2 × 2,000 μm² operated under pulsed mode (pulse width = 1 μs, duty cycle = 1%) showed extremely high output power of up to 610 mW per facet. The narrow RWG InAs QD lasers also emit in single lateral mode.
The wafer was processed into 2 μm wide RWG lasers by standard wet chemical etching using a solution of H3PO4:H2O2:H2O (1:1:5). Good control of the etch depth is necessary to achieve single lateral mode operation, since the refractive index step between the ridge and trench region is determined by the etch depth. Through optimization of the ridge height [27, 29], the entire p-doped layers above the QD active region outside the ridge was etched before the pulsed anodic oxidation (PAO) process. A 200 nm-thick oxide layer was formed by PAO, whose experimental setup is described in Ref. . Subsequently, p-type ohmic contact layers (Ti/Au, 50/300 nm) were deposited by electron beam evaporation, while n-type ohmic contact layers (Ni/Ge/Au/Ni/Au, 5/20/100/25/300 nm) were deposited on the backside of the substrate following lapping down to ∼100 μm. All samples were annealed at 410 °C for 3 min in N2 ambient. Finally, the wafers were cleaved into laser bars of different cavity lengths (550–3,000 μm), whereas, the ridge width was kept constant for all the laser devices at w = 2 μm. The output power (P) versus injection current (I) (P I) characteristics were measured under CW operation at 10 °C. To minimize device heating, the InAs QD lasers were also tested under pulsed operation (pulse width = 1 μs, duty cycle = 1%) at 20 °C. The far field patterns (parallel to the junction plane) of the InAs QD lasers were measured under the above mentioned pulsed conditions at 20 °C.
The unstable switching between modes causes intensity noise, resulting in degradation in the laser performance . Furthermore, mode hopping is expected to be more pronounced in narrow ridge structures where the cross-sectional area is relatively small. More detailed investigation on the mode hopping behavior is warranted to further study this effect. Nevertheless, our observations from operating the device in CW mode suggest the presence of a significant heating effect.
Ouyang et al.  has reported narrow RWG InAs QD lasers with ridge width of 8 μm, and observed that lasers with deep-mesa geometry exhibited superior characteristics compared with shallow-mesa devices. Under pulsed operation (500 ns, 5 kHz), the HR/uncoated InAs QD laser of dimension 8 × 1,500 μm2 showed high external differential efficiency of 50% and low threshold current density of ∼130 A/cm2 at moderate output power ∼6 mW. Compared with this report, our results show that the ten-layer InAs QD lasers fabricated using PAO were able to deliver comparable, and in some cases better performance with near ideal external differential efficiency of 96% and extremely high output power of 610 mW/facet under pulsed operation. Furthermore, the devices also exhibit single lateral mode emission.
The output powerP and external differential quantum efficiencyη d of our ten-layer InAs narrow RWG QD lasers are among the highest values in the 1.29–1.30 μm wavelength range ever reported. The high device performance is attributed to the high quality QD laser structure and optimized self-aligned PAO method compared with conventional SiO2confinement. The better passivation of the sidewalls by the native oxide formed by the PAO process could contribute to the reduction in nonradiative centers between the sidewall and oxide layer. This is particularly critical in narrow RWG devices such as the ones investigated in this study. These factors are believed to have contributed significantly to the high performance observed in our narrow RWG devices.
In summary, narrow RWG lasers based on ten-layer InAs/InGaAs QD active region have been fabricated and characterized. Devices fabricated using an optimized PAO process exhibited GS lasing at high total output power of 272.6 mW at ∼1.3 μm under CW operation. Extremely high single lateral mode output power of 610 mW/facet was achieved in pulsed operation with minimal power saturation under high current injection. High slope efficiency of 0.46 W A−1per facet, near ideal external differential quantum efficiency of 96% and low lateral beam divergence of 4° have been achieved in the devices.
This research is partially sponsored by A*STAR under the ONFIG-II program SERC Grant No. 042 108 0098. The authors would also like to acknowledge the assistance of Dr Tong Cunzhu for his useful inputs to this research.