In recent years, a novel InGaAs well-cluster composite (WCC) quantum-confined structure has been demonstrated that the special structure has excellent optical properties, which are important for the realization of ultra-wide tunable lasers and synchronous dual-wavelength lasers. The WCC structure is based on the self-fit migration of indium atoms caused by the indium-rich cluster (IRC) effect, which are typically regarded as defects to be avoided for the conventional InGaAs quantum-well structure. Therefore, its special optical characteristics remain neglected. The formation mechanism of this WCC structure is based on the migration of indium atoms under high strain background. The strain will gradually accumulate with the continuous deposition of InGaAs material thickness. In order to relax the high strain in the InGaAs layer, indium atoms would automatically migrate along the material growth direction and form IRCs after the In_xGa_1-xAs is grown to exceed the critical thickness on the GaAs. Therefore, how to effectively determine the critical thickness of indium atom migration is of great significance for the study of WCC structures. However, there is little research on the critical thickness of the WCC structure. The traditional measurement methods on quantum well thickness make it difficult to obtain the thickness fluctuations at different positions. Furthermore, it is not possible to accurately evaluate the critical thickness of indium atom migration in the asymmetric In_xGa_1-xAs WCC structure. Therefore, the critical thickness of indium atom migration is investigated by collecting spontaneous emission (SE) spectra from different positions in the WCC structure.

First, in order to study the critical thickness of indium atom self-fit migration in the IRC effect, an asymmetrical InGaAs WCC quantum confinement structure is grown on a GaAs substrate. Because IRCs generally occur in highly strained InGaAs/GaAs systems, the active layer used In_0.17Ga_0.83As/GaAs/GaAs_0.92P_0.08. The thickness of the In_0.17Ga_0.83As layer is designed to be 10 nm because an InGaAs layer thinner than 10 nm is insufficient to obtain the IRC effect. Second, in order to measure SE spectra, the sample is processed to obtain a 3.0 mm×1.5 mm configuration. The device is vertically pumped from a fiber-coupled 808 nm pulsed laser at room temperature. The pump beam is focused into a 0.2 mm diameter spot. The fiber coupler is used to collect the SE spectra emitted from the corresponding pumping region from the bottom of the WCC structure. The SE spectra from different positions of the WCC structure are measured by moving the sample. The SE spectra exhibit typical bimodal characteristics. The formation mechanism is that the self-fit migration of the indium atoms in the WCC structure would reduce the indium content in the corresponding InGaAs regions, consequently generating normal and indium-deficient In_xGa_1-xAs regions. The spectra with dual peaks come from the superposition of spectra emitted from the normal In_0.17Ga_0.83As layer and indium-deficient In_0.12Ga_0.88As layer with different band gaps. The intensity fluctuation of the dual peaks mainly depends on the thickness fluctuation of the two materials. Third, the critical thickness can be evaluated by comparing the intensity of dual peaks.

The self-fit migration of indium atoms leads to the formation of both normal In_0.17Ga_0.83As and indium-deficient In_0.12Ga_0.88As regions in the WCC structure. The bimodal configuration in the spontaneous emission spectra is a remarkable feature of the IRC effect taking place in the InGaAs-based WCC structure. The SE intensity mainly depends on the In_xGa_1-xAs material thickness L and the peak wavelength λ. Based on the dual peaks in SE spectra from different positions of the WCC structure, the intensity ratio of the dual peaks can be calculated, with a maximum intensity ratio of 1.2115 and a minimum value of 0.5968. The thickness of the In_0.17Ga_0.83As layer corresponds to 4.6 nm and 6.4 nm, respectively (Fig. 3). Due to the migration of indium atoms occurring after the thickness of the In_0.17Ga_0.83As layer reaches the critical thickness, the material within the critical thickness is normal In_0.17Ga_0.83As material. This means that as long as the growth thickness of the In_0.17Ga_0.83As layer does not exceed 4.6 nm, indium atoms will not migrate. This is because the strain accumulation is not sufficient to generate the IRC effect. In summary, the critical thickness for self-fit migration of indium atoms can be evaluated as approximately 4.6 nm. Finally, in order to illustrate the accuracy of this conclusion, the spontaneous emission spectrum of a 4 nm thick In_0.17Ga_0.83As/GaAs compressively strained quantum well is collected under the same injected carrier density. It is found that there is only one peak in the spectra (Fig. 4). The result indicates that indium atoms do not migrate to form IRCs in the 4 nm thick In_0.17Ga_0.83As/GaAs material. Although there is strain accumulation in the 4 nm thick In_0.17Ga_0.83As material, it is not enough to produce the IRC effect. Therefore, the bimodal configuration in spectra disappears. This is consistent with the experimental results, which demonstrate the relative accuracy of the conclusion.

In this paper, the critical thickness of indium atom migration in InGaAs asymmetric WCC quantum confinement structures is calculated by measuring the spontaneous emission spectra emitted from the different positions of the WCC structure. The SE spectra emitted from different pump regions are measured by focusing the pump beam on the local surface of the WCC sample. By analyzing the bimodal intensity and ratio of the SE spectra, the normal In_0.17Ga_0.83As layer thickness fluctuation of 4.6-6.4 nm is obtained. Furthermore, the critical thickness for the migration of indium atoms is determined to be approximately 4.6 nm. This research content has important value for the development and application of In_xGa_1-xAs asymmetric WCC quantum confinement structures.