Improved ground-state modulation characteristics in 1.3 μm InAs/GaAs quantum dot lasers by rapid thermal annealing
© Zhao et al; licensee Springer. 2011
Received: 11 March 2011
Accepted: 16 May 2011
Published: 16 May 2011
We investigated the ground-state (GS) modulation characteristics of 1.3 μm InAs/GaAs quantum dot (QD) lasers that consist of either as-grown or annealed QDs. The choice of annealing conditions was determined from our recently reported results. With reference to the as-grown QD lasers, one obtains approximately 18% improvement in the modulation bandwidth from the annealed QD lasers. In addition, the modulation efficiency of the annealed QD lasers improves by approximately 45% as compared to the as-grown ones. The observed improvements are due to (1) the removal of defects which act as nonradiative recombination centers in the QD structure and (2) the reduction in the Auger-related recombination processes upon annealing.
Quantum dots (QDs) are promising for realizing fast and stable laser sources in fiber optic applications, due to their superior characteristics over conventional quantum well (QW) lasers, such as low threshold current, high internal efficiency, and infinite characteristic temperature [1, 2]. However, the high-speed performance of QDs is generally poorer than that of QWs due to several factors: slow inter-level relaxation of the carriers , finite density of state (DOS), and closely spaced hole energy levels . Slow carrier relaxation rate, in combination with the limited DOS, will lead to early switching from ground-state (GS) lasing to excited-state (ES) lasing at high temperature or at high drive current. This is undesired since ES lasing reduces GS lasing efficiency due to gain saturation of the GS transition. This, due to state filling effect in discrete quantum levels, degrades the high-speed characteristics [2, 5] of the QD lasers. Furthermore, for high-speed modulation, shorter cavity length is favored. Unfortunately, transition from GS to ES lasing occurs earlier in short cavity lasers (≤1 mm) due to the increased cavity loss as compared to long cavity lasers . Thus, to improve the high-speed performance of QD lasers, it is important to delay the onset of ES lasing. It is well-documented that p-doping of the QDs can reduce the effect of gain saturation and thus maintain GS lasing up to higher operating temperatures , and potentially faster relaxation time . On the other hand, rapid thermal annealing (RTA) might result in intermixing of the QDs with the surrounding matrix and tunable inter-level energy. This may lead to faster inter-level relaxation of the carriers , and consequently, suppression of ES lasing and improved high-speed modulation efficiency. However, there exists only a handful of works on the effects of RTA on GS modulation of the annealed p-doped QD lasers [8–10].
In this work, we have investigated the effect of RTA on the GS modulation characteristics of 1.3 μm p-doped InAs/GaAs QD lasers. Both the modulation bandwidth and efficiency were found to increase significantly upon annealing.
The ten-layer self-assembled p-doped InAs/GaAs QD laser structure used in this experiment was grown on GaAs (100) substrate by molecular beam epitaxy (MBE). The structure consists of QD active region sandwiched between two 1.5 μm C- and Si-doped Al0.35Ga0.65As cladding layers. The active layer comprises 2.3 monolayer (ML) of InAs QDs capped by a 5-nm In0.15Ga0.85As layer. A 33-nm GaAs layer is used to separate the two QD layers. P-doping modulation (~16 acceptors per QD) was incorporated into the 10 nm GaAs layer in the middle of each 33 nm-thick spacer between the QD rows . The indium-containing layers were grown at approximately 485°C, while the (Al)GaAs layers were grown at approximately 580°C. The QD samples were capped with 200 nm of SiO2 deposited by plasma-enhanced chemical vapor deposition (PECVD) before annealing. The annealing process was then performed in N2 ambient at 600°C for 15 s using a rapid thermal processor. The choice of annealing conditions was determined from our recently reported results . Subsequently, the as-grown and annealed samples were processed into 4-μm-wide narrow ridge waveguide lasers . The high-speed modulation of the as-cleaved QD lasers was performed under continuous-wave (CW) biasing condition using a vector network analyzer (VNA) and a high-speed photoreceiver . The spontaneous emission intensity of the device under different bias currents was obtained with an integrating sphere. A thermoelectric temperature controller controls the device temperature during measurements.
Results and discussion
In summary, we have investigated the GS modulation characteristics of 1.3 μm InAs/GaAs QD lasers that consist of either as-grown or annealed QDs. Compared to the as-grown QD lasers, the annealed ones exhibit approximately 18% improvement in the modulation bandwidth and approximately 45% in the modulation efficiency. The observed improvements are due to the reduction in the Auger-related recombination processes and the removal of nonradiative recombination centers in the QDs upon annealing.
density of state
molecular beam epitaxy
- PEVCD :
plasma-enhanced chemical vapor deposition
- QD :
- QW :
- RTA :
rapid thermal annealing
- VNA :
vector network analyzer.
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