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Significant interest in compact InAs/InGaAs quantum dot (QD) lasers emitting near 1.3 mkm is caused by the diversity of their applications including non-invasive medicine and ultra-fast data transmission. In such lasers, lasing typically starts at the ground-state (GS) optical transitions of QDs. A further increase in injection may result in the appearance of an additional, short-wavelength spectral line associated with the excited-state (ES) optical transitions of QDs – a simultaneous lasing via QD GS and ES, i.e. multi-state lasing, takes place.
The appearance of the ES-line may sufficiently affect the useful GS component. As injection current exceeds the multi-state lasing threshold, a decrease and even a complete quenching of GS-lasing may take place. As it was shown in [V.V. Korenev et. al, Appl. Phys. Lett. 102, 112101 (2013)], the usage of modulation p-doping has a positive influence on the hole concentration in QDs making GS-lasing quenching less pronounced. However, the influence of the concentration of p-dopant on multi-state lasing in general – and on the GS-lasing quenching in particular – has not been yet studied.
To clarify this question experimentally, a series of InAs/InGaAs QD laser wafers was grown by molecular beam epitaxy. The active region of each sample was comprised of 10 layers of InAs/InGaAs QDs separated by 35 nm-thick GaAs spacers. Each spacer was p-doped into its central part of 10 nm using carbon atoms. Dependent on the sample, the carbon concentration was equal to 0, 3·10^17 cm^(-3), or 5·10^17 cm^(-3).
A series of light-current curves corresponding to the GS component of output power was studied both theoretically and experimentally for “short” (0.5-mm-long) and for “long” (1.0-mm-long) samples. The experiment shows that in case of the short samples, the increase in p-doping level from 0 to 5·10^17 cm^(-3) results in the increase in maximum output power corresponding to the GS of QDs (WGS) from 0.8 to 2.2W, while in case of the longer samples the situation is opposite and WGS decreases from 4.5 to 3.7W correspondingly [V.V. Korenev et. al, Appl. Phys. Lett. 111, 132103 (2017)]. Qualitatively, such a discrepancy can be explained as follows. In case of the short samples, the higher p-doping level results in the faster hole capture into QDs mitigating the competition for the common holes between GS and ES optical transitions, which is an important reason for the GS-lasing quenching.
In longer samples, optical loss is small and GS gain is far from its saturated value. Consequently, the ES energy level is weakly occupied and p-doping is not necessary to apply. However, even a small increment in the internal loss due to the p-dopant may lead to a noticeable decrease in laser`s differential efficiency.
As a result, the higher p-doping level does not necessarily lead to the higher GS power as it was previously expected. However, if the sample is sufficiently short, the usage of modulation p-doping increases GS power. For a given cavity length, there is a certain p-doping level improving GS lasing characteristics.
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