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Influence of number of quantum dot layers on possibility of achieving lasing threshold in vertical cavity surface emitting lasers

Łukasz Piskorski , Michał Wasiak , Robert P. Sarzała

Technical University of Łódź, Institute of Physics, Wólczańska 219, Łódź 93005, Poland

Abstract

Arsenide quantum dots have attracted a lot of attention since late 1990s as potential light sources for the second telecommunication window (1310 nm). For this application the most suitable construction type is a Vertical Cavity Surface Emitting Laser (VCSEL). Unfortunately VCSELs require high material gain, but quantum dots suffer form the gain saturation. Its level depends on the density of the dots and their uniformity, and is much lower than gain in quantum well structures. This is the reason why in a VCSEL one has to use many quantum-dot layers. Since such a layer should be placed at the anti-node of the standing wave, the number of the layers is limited by the resonator length. Usually at one anti-node a group of three layers is placed. The resonator length is a very important factor, because it has to be p-doped, so as to provide holes for the active layers. It obviously significantly increases the absorption. Despite all those drawbacks it is possible to fabricate a quantum-dot VCSEL, but only few groups make structures for InGaAs/GaAs VCSELs.

In this paper we want to analyse what is the minimal number of quantum-dot layers in a continuous-wave (CW) room-temperature (RT) VCSEL. In our simulations we model structures based on description presented in [1, 2]. We modify number of the active layers and the resonator length. The temperature distribution is determined by assumption that the applied voltage is 7 V, which is the threshold value in the actual laser. For each resonator and active region design we calculate the required material gain and compare it with the saturation level. Value of the latter parameter can be tuned by changing the density of the dots or their uniformity.

In the figure we present the required material gain versus number of 3-layer groups of quantum dots for different resonator lengths. Points which are over or near the saturation line represent structures which do not give enough gain for a VCSEL (unless the dot density or the uniformity are better than assumed by us). The figure suggests that the active region should contain at least 9 layers of dots (3 groups of 3 layers), which agrees with the fact that the actual devices contains usually 5 or 3 group, sometimes with additional two single layers.

The authors would like to acknowledge support from the Polish Ministry of Science and Higher Education (MNiSzW), grant No 85/SIN/2006/02.

References

[1] H.C. Yu, J.S. Wang, Y.K. Su, S.J. Chang, F.I. Lai, Y.H. Chang, H.C. Kuo, C.P. Sung, H.P.D. Yang, K.F. Lin, J.M Wang, J.Y. Chi, R.S. Hsiao, and S. Mikhrin, 1.3-um InAs-InGaAs quantum-dot vertical-cavity surface-emitting laser with fully doped DBRs grown by MBE, IEEE Photonics Technology Letters, vol. 18, p. 418, (2006)
[2] Y.H. Chang, P.C. Peng, W.K. Tsai, Gray Lin, FangI Lai, R.S. Hsiao, H.P. Yang, H.C. Yu, K.F. Lin, J.Y. Chi, S.C. Wang, and H.C. Kuo, Single-mode monolithic quantum-dot VCSEL in 1.3 um with sidemode suppression ratio over 30 dB, IEEE Photonics Technology Letters, vol. 18, p. 847, (2006)

Figure 1. Relations between the number of 3-layers quantum-dot groups and material gain necessary for the lasing threshold. The non-integer value means that there are two additional layers near the DBRs.

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