In this paper, a novel, highly-strained InGaAs/GaAs self-fit well-cluster composite (WCC) quantum structure was investigated. The WCC structure differs from the conventional InGaAs/GaAs quantum-well structure, which exhibits considerable potential for numerous laser applications. Presently, the conventional quantum well exhibits a quasirectangle well structure, wherein each well consists of a fixed amount of indium and a single strain type in the material system. The WCC structure with variable indium content and thickness in an In_xGa_1-xAs/GaAs system can yield remarkable results, thereby facilitating the development of new laser types. This structure is associated with the indium-rich cluster (IRC) effect, wherein the IRCs were typically regarded as defects to be avoided for the conventional InGaAs quantum-well structure; hence, its special optical characteristics remain neglected. The migration of the indium atoms to the WCC structure would reduce the indium content in the corresponding InGaAs regions, consequently generating normal and indium-deficient In_xGa_1-xAs regions with hybrid strain types in the InGaAs material and aid in the production of special polarized spectra with dual peaks. Therefore, it is crucial to reveal the underlying corresponding luminescence mechanism between dual peaks in different polarized spectra and multiple In_xGa_1-xAs materials. This work offers new avenues for the development of new types of devices.

First, the InGaAs-based WCC quantum structure was developed via metal-organic chemical vapor-phase deposition. To generate the IRC effect by sufficient strain accumulation, the In_0.17Ga_0.83As/GaAs/GaAsP_0.08 material system was designed as a periodic gain structure (Fig.1). The thickness of the In_0.17Ga_0.83As layer was designed to be 10 nm, because an InGaAs layer thinner than 10 nm is insufficient to obtain the IRC effect. Second, the luminescence mechanism of the WCC structure was studied by coating the WCC sample at a transmittance of T=99.99% at the dual facets to avoid the end reflection. Third, the polarized photoluminescence (PL) spectra in transverse electric (TE) and transverse magnetic (TM) modes were measured using a linear polarizer under varying carrier densities (N) in the range of3.6×10¹⁷-4.8×10¹⁷ cm^-3. The special bimodal feature was observed in both TE- and TM-polarized PL spectra (Fig.2), which could be associated with the emissions from normal and indium-deficient In_0.12Ga_0.88As regions with varying band gaps. To analyze the different strain types, the lattice constant a(x) of the In_xGa_1-xAs material was obtained. Subsequently, the hybrid strain types in normal and indium-deficient In_xGa_1-xAs were obtained. The compressive and tensile strain occurred in the In_0.17Ga_0.83As and In_0.12Ga_0.88As layers, respectively. Further, according to the transition matrix element theory, the PL spectrum in TE polarization was primarily associated with the electron-hole recombination between the first conduction (C₁) and heavy hole (HH₁) subbands, whereas that in TM polarization was associated with the electron-hole recombination between C₁ and light hole (LH₁) subbands. Moreover, in the compressively-strained In_0.17Ga_0.83As layer, the HH₁ subband lies above the LH₁ subband. Conversely, the HH₁ subband lies below the LH₁ subband in the tensile-strained In_0.12Ga_0.88As layer. Finally, the underlying luminescence mechanism of the polarized spectra with dual peaks was revealed according to the above analysis.

The TE- and TM-polarized PL spectra (Fig.2) reveal the special bimodal features, and they are marked using letters A and B, and C and D, which correspond to 1.27 eV and 1.33 eV and 1.35 eV and 1.31 eV, respectively. GaAs, In_0.17Ga_0.83As, and In_0.12Ga_0.88As yield lattice constant values of 5.65325, 5.72215, and 5.7019 ?, respectively. Accordingly, the In_0.17Ga_0.83As layer is subject to compressive strain such that the HH₁ subband lies above the LH₁ subband. Meanwhile, tensile strain is noted in the In_0.12Ga_0.88As layer such that the HH₁ subband falls below the LH₁ subband. According to the transition matrix element theory, the main peaks, which are marked by A and C in both TE and TM spectral curves (Fig.2), are attributed to the compressively-strained In_0.17Ga_0.83As layer; the corresponding TE photon energy is less than the TM photon energy. Meanwhile, the subpeaks marked by B and D (Fig.2) are attributed to the tensile-strained In_0.12Ga_0.88As region; the corresponding TE photon energy exceeds the TM photon energy. Moreover, the characteristics of the hybrid energy band of the InGaAs self-fit WCC quantum structure are obtained (Fig.3). For the compressively-strained In_0.17Ga_0.83As layer, the energy intervals from C1 to HH₁ and LH₁ bands are 1.27 eV and 1.35 eV, which correspond to the photon energy at peaks A and C, respectively. In the case of the tensile-strained In_0.12Ga_0.88As material, the energy intervals from C₁ to HH₁ and LH₁ bands are 1.33 eV and 1.31 eV, which correspond to the photon energy at peaks B and D, respectively.

In this paper, a novel, highly-strained InGaAs/GaAs self-fit WCC quantum structure is investigated based on the IRC effect. The measured spectra in TE and TM polarizations reveal special features with dual peaks, which are attributed to the combination of different emissions produced by the normal and indium-deficient InGaAs active regions. According to the transition matrix element theory, the corresponding luminescence mechanism between the dual peaks in polarized spectra and InGaAs material with different indium content is revealed. Furthermore, the band characteristics associated with the conduction subband C₁ and valence subbands of heavy holes HH₁ and light holes LH₁ are determined to reveal the underlying luminescence mechanism. The special WCC structure demonstrates a hybrid strain distribution, which indicates the simultaneous existence of compressive and tensile strains. The results of this study can greatly enhance the performance of InGaAs-based WCC-tunable lasers.