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Improved ground-state modulation characteristics in 1.3 μm InAs/GaAs quantum dot lasers by rapid thermal annealing.

Zhao H, Yoon SF, Ngo CY, Wang R - Nanoscale Res Lett (2011)

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore. zhao0097@e.ntu.edu.sg.

ABSTRACT
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.

No MeSH data available.


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The lasing spectrum of the same annealed QD laser measured at 320 mA.
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Figure 4: The lasing spectrum of the same annealed QD laser measured at 320 mA.

Mentions: Figure 3 shows the frequency response for the 600°C annealed device with cavity length of 1 mm, at 10°C. The Ith of the 600°C annealed device is approximately 42.6 mA. The lasing spectrum of the 600°C annealed device at 8.1 × Ith (320 mA) is shown in Figure 4. Since the bandwidth saturates at current greater than 320 mA, the maximum bandwidth obtained is 8.18 GHz. After annealing, 18% improvement in modulation bandwidth has been achieved. It is worth mentioning that no ES lasing has been observed in the annealed laser for all bias current levels. Therefore, the maximum bandwidth of 8.18 GHz for the 600°C annealed device is purely from GS lasing. Comparing Figure 2a and Figure 4, the difference in the GS wavelength might be due to the carrier induced bandgap shrinkage [14] and the thermal induced emission wavelength shift [15-17]. At a fixed current injection above threshold, a higher I/Ith ratio is resultant in the annealed lasers (8.1 × Ith) than in the as-grown lasers (5.9 × Ith) due to the reduction of threshold current after annealing [13]. The thermal effects may become more dominant in affecting the wavelength due to annealing. The reasons of this wavelength shift are currently under investigation. Figure 5 shows the measured bandwidth as function of normalized bias current (I-Ith)1/2 measured at 10°C for the: as-grown QD laser GS emission indicated with solid circles, annealed QD laser GS emission indicated with solid squares, and as-grown QD laser ES emission indicated with hollow circles. The slope deduced from the bandwidth vs. (I-Ith)1/2 (also known as modulation efficiency, ηMOD) is approximately 0.33 GHz/mA1/2 for the as-grown device at low bias. Note that there is a sudden increase of the ηMOD when ES lasing starts to dominate. GS lasing dominates in the 600°C annealed device throughout the measured bias current range, while ES lasing has been completely suppressed, even at higher bias currents. The ηMOD for the 600°C annealed device is approximately 0.48 GHz/mA1/2. Note that the ηMOD has been increased by approximately 45% after the 600°C RTA process. Shi et al. [3] have shown that fast inter-level relaxation is favorable for the GS lasing, and leads to higher QD GS capture efficiency. When the inter-level relaxation becomes slower, carriers captured into the ES cannot relax to the GS immediately and continues to occupy the ES. The ES lasing will occur when the population inversion for the transition between the ES electrons and the GS holes is satisfied. Higher ηMOD in the 600°C annealed laser indicates lower wetting layer carrier occupation probability and higher GS capture efficiency, which may be due to faster inter-level relaxation induced by RTA [9].


Improved ground-state modulation characteristics in 1.3 μm InAs/GaAs quantum dot lasers by rapid thermal annealing.

Zhao H, Yoon SF, Ngo CY, Wang R - Nanoscale Res Lett (2011)

The lasing spectrum of the same annealed QD laser measured at 320 mA.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211475&req=5

Figure 4: The lasing spectrum of the same annealed QD laser measured at 320 mA.
Mentions: Figure 3 shows the frequency response for the 600°C annealed device with cavity length of 1 mm, at 10°C. The Ith of the 600°C annealed device is approximately 42.6 mA. The lasing spectrum of the 600°C annealed device at 8.1 × Ith (320 mA) is shown in Figure 4. Since the bandwidth saturates at current greater than 320 mA, the maximum bandwidth obtained is 8.18 GHz. After annealing, 18% improvement in modulation bandwidth has been achieved. It is worth mentioning that no ES lasing has been observed in the annealed laser for all bias current levels. Therefore, the maximum bandwidth of 8.18 GHz for the 600°C annealed device is purely from GS lasing. Comparing Figure 2a and Figure 4, the difference in the GS wavelength might be due to the carrier induced bandgap shrinkage [14] and the thermal induced emission wavelength shift [15-17]. At a fixed current injection above threshold, a higher I/Ith ratio is resultant in the annealed lasers (8.1 × Ith) than in the as-grown lasers (5.9 × Ith) due to the reduction of threshold current after annealing [13]. The thermal effects may become more dominant in affecting the wavelength due to annealing. The reasons of this wavelength shift are currently under investigation. Figure 5 shows the measured bandwidth as function of normalized bias current (I-Ith)1/2 measured at 10°C for the: as-grown QD laser GS emission indicated with solid circles, annealed QD laser GS emission indicated with solid squares, and as-grown QD laser ES emission indicated with hollow circles. The slope deduced from the bandwidth vs. (I-Ith)1/2 (also known as modulation efficiency, ηMOD) is approximately 0.33 GHz/mA1/2 for the as-grown device at low bias. Note that there is a sudden increase of the ηMOD when ES lasing starts to dominate. GS lasing dominates in the 600°C annealed device throughout the measured bias current range, while ES lasing has been completely suppressed, even at higher bias currents. The ηMOD for the 600°C annealed device is approximately 0.48 GHz/mA1/2. Note that the ηMOD has been increased by approximately 45% after the 600°C RTA process. Shi et al. [3] have shown that fast inter-level relaxation is favorable for the GS lasing, and leads to higher QD GS capture efficiency. When the inter-level relaxation becomes slower, carriers captured into the ES cannot relax to the GS immediately and continues to occupy the ES. The ES lasing will occur when the population inversion for the transition between the ES electrons and the GS holes is satisfied. Higher ηMOD in the 600°C annealed laser indicates lower wetting layer carrier occupation probability and higher GS capture efficiency, which may be due to faster inter-level relaxation induced by RTA [9].

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore. zhao0097@e.ntu.edu.sg.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus