In typical SiC semiconductor devices, SiC is thermally oxidized which leads to the formation of insulating amorphous SiO2 (a-SiO2) layers [1]. Unlike silicon, where a stable thermal oxide can be readily grown, the formation of a high-quality oxide layer on SiC is still challenging. Standard thermal oxidation processes often result in many interface defects that can introduce defect levels within the band gap of SiC and act as traps for carriers. By confocal microscope scanning, single photon emitters upon 532-nm illumination were found at the interface with unknown origin [2, 3] where the number of emitters can be reduced but not fully eliminated by post-oxidation treatments [4, 5]. In this study, we focus on the origin of the reported single photon emitters at the 4H-SiC/a-SiO2 interface which exhibit high-energy local vibration modes in the photoluminescence (PL) spectrum that can be associated with carbon clusters with short C-C bonds [2, 3]. We model the interface, introduce defects and calculate their optical properties by means of density functional theory (DFT). The atomistic model of the 4H-SiC/a-SiO2 interface is created by molecular dynamics simulation with several steps to obtain the final starting model. All the first-principles calculations are performed using DFT within the projector augmented wave potential plane-wave method, as implemented in the VASP. The screened hybrid density functional of Heyd, Scuseria, and Ernzerhof (HSE06) is employed to calculate the electronic structure which is able to reproduce the experimental band gap. The single Γ-point scheme is convergent for the k-point sampling of the Brillouin zone (BZ). The excited states were calculated by ΔSCF method. We applied 10.72×12.38 Å2 lateral size and SiC width of 20.26 Å to embed the defects. The thicknesses of the a-SiO2 and vacuum layers are 14.83 Å and 15.00 Å, respectively. The atomistic model of 4H-SiC/a-SiO2 interface is shown in Fig. 1(a). Apart from carbon clusters, the interface transition region also encompasses paramagnetic dangling bonds, Si-Si dimers, and oxygen-related defects. For the C-related defects (Cn, where n is the number of C atoms), the configurations exhibit increased complexity due to structural changes in the interface transition and the diverse combinations of defect atoms. We considered more than 120 defect configurations in our study with n=1…4. These distinct localized structures of defects lead to significant variations in the distribution of defect levels among them. Consequently, the vertical excitation energies also span a wide range (e.g., the vertical excitation energies for C-related defects range from 0.5 to 3.1 eV) [Fig. 1(b)]. Silicon dangling bonds (Sid) and Si-Si dimer defects, on the other hand, are primarily situated on the SiC surface, exhibiting a relatively uniform defect structure. The position of defect levels remains generally constant, resulting in a relatively fixed energy for vertical excitation. Particularly for Si-Si dimer defects, the defect levels are predominantly located near the valence band maximum, with the vertical excitation energy typically around 3.2 eV. Subsequently, we simulated the PL spectrum for carbon-related defects and Si-Si dimer defects that are relatively stable structures. For carbon-related defects, the zero-phonon-line (ZPL) is predominantly distributed in the range of 1.7 to 2.7 eV. As for Si-Si dimer defects, the ZPL peak fall in the deep blue region. These observations align with previous experimental finding on PL centers at the interface with ZPL peaks ranging from 1.5 to 2.5 eV upon illumination with 532-nm and 633-nm laser, respectively [6, 7]. We find that these PL centers can be well explained by carbon-clusters at the interface. We note that we have recently identified carbon-clusters in bulk 4H-SiC [8]. These carbon clusters appear due to incomplete oxidation of SiC at the interface in a complex environment consisting of silicon-oxygen bonds too. The concentration of these PL centers may be reduced to an isolated level resulting in quantum emission at the 4H-SiC/a-SiO2 interface [3] that show sharp high-energy local-vibration-modes (LVMs) between 120 and 200 meV. We find that many of the considered carbon-related emitters (around 100 configurations) exhibit high energy modes. Notably, the carbon-cluster consisting of four carbon atoms produce very similar PL spectrum to the observed one with a ZPL peak at around 2.1 eV [3] so the origin of those quantum emitters is tentatively identified. We note that PL spectra significant vary with the actual environment. Because these carbon clusters are formed during thermal oxidation of SiC which is a stochastic process with different environment, it is principally impossible to form indistinguishable color centers, thus it might be difficult to use them as a resource for quantum technologies. We note that most of these color centers occurs in their neutral charge state with bound exciton excited state [8] and they have singlet ground state. These defects at the interface introduce a myriad of deep levels in the band gap of SiC [Fig. 1(d)]. These defect levels, characterized by their diverse structures, become virtually pervasive throughout the entire band gap. In the context of quantum sensor applications, the broad distribution and varying magnitudes of deep levels from the interface defects can lead to spectral broadening and shifts in the emission peak of the shallow implanted defect qubits, potentially impeding the accurate initialization and readout in quantum sensor applications. Moreover, the defect levels may impact charge transfer processes and may temporal or permanent photostability issues of the quantum sensor. We conclude from this study that the concept of surface termination of 4H-SiC should be radically changed for quantum sensor applications. We acknowledge the support from EU Commission (SPINUS project, Grant No. 101135699), NKFIH Grant No. 2022-2.1.1-NL-2022-00004 (Quantum Information National Laboratory of Hungary). [1] P. Fiorenza, F. Giannazzo, and F. Roccaforte, Energies 12, 2310 (2019). [2] S.-i. Sato, T. Honda, T. Makino, Y. Hijikata, S.-Y. Lee, and T. Ohshima, ACS Photon. 5, 3159 (2018). [3] B. Johnson et al., Phys. Rev. Appl. 12, 044024 (2019). [4] R. Kosugi, W.-J. Cho, K. Fukuda, K. Arai, and S. Suzuki, Journal of Applied Physics 91, 1314 (2002). [5] T. Kobayashi, T. Okuda, K. Tachiki, K. Ito, Y.-I. Matsushita, and T. Kimoto, Appl. Phys. Exp. 13, 091003 (2020). [6] A. Lohrmann et al., Nature Communications 6, 7783 (2015). [7] A. Lohrmann et al., Applied Physics Letters 108, 021107 (2016). [8] P. Li, P. Udvarhelyi, S. Li, B. Huang, and A. Gali, Physical Review B 108, 08201 (2023).