# Study of the Photo-Response of Doped GaAs with Aging
## 3 Results and Discussion  
PL is a suitable technique for studying radiative electronic transitions. For semiconductors, these transitions are made around the energy gap (Eg), which separates the valence band and the conduction band. If the concentration of the impurities does not exceed the Mott limit, the Fermi level is within the energy gap, separating the states occupied by electrons from those unoccupied. The impurity states are placed within that energy gap close to the conduction band if the impurity is a donor type, and close to the valence band if the impurity is an acceptor type. When there are defects in the crystal lattice, they produce electron traps which are located very deep in energy within the energy gap. Based on the above understanding and by analyzing the photoluminescence spectra of the semiconductors, the highest energy transitions correspond to the transition of the conduction band to the valence band (B-B). The excitonic transitions (X) will be in lower energy and involve the interaction of an electron from the conduction band, but that has been attracted by a hole in the valence band. In the semiconductor industry, the PL width of the excitonic peak is used to test the crystalline quality of the grown solid. A narrower excitonic peak indicates better crystalline quality of the material. At even lower energy, the transitions involving impurity states either from a band to an impurity state or between two impurity states. The least energetic, that is, the furthest from the B-B transition, will be the transitions that involve defect states. In Figure 1, we show in black the photoluminescence results for the undoped GaAs sample used by [PERSON] (2001) in 1995. In 2001, [PERSON] (2001) measured the PL spectrum in that same sample, which was stored for 6 years in the environmental conditions described above. [PERSON]'s spectrum is represented in red. This same sample was employed in 2021 for this study and its spectrum is illustrated in blue. It is evident that there has been a change in the optical response of the undoped GaAs sample over time due to aging, at least within the laser penetration length of the PL technique. For a better understanding of the PL spectra in Figure 1, we have superimposed an orange line at 1.375 eV to separate two spectral zones. The highest energy zone corresponds to the transitions that involve the energy bands and impurities. The zone below 1.375 eV is the zone corresponding to deep trap transitions due to defects. In the impurity zone, excitonic peaks are distinguished near the Eg at 1.424 eV and a second peak around 1.385 eV that corresponds to the donor-acceptor transition of shallow impurities. It can be distinguished that in the blue spectral curve, the excitonic peak broadens and shifts towards higher energies compared to other lower aging spectra. This is due to the deterioration of the crystalline quality of the material over time. On the other hand, the zone below 1.375 eV shows a broad peak that corresponds to the transitions involving deep defects. We have called this peak DT for the spectrum taken in 1995, DF for the one taken in 2001 and DO for the one measured in 2021. In this second area, it can be observed that the peak of deep traps moves towards lower energy values, broadens and increases in intensity with respect to the excitonic peak as the sample ages. This is due to the increasing concentration of defects in the samples as the years go by. Figure 1: Normalized photoluminescence spectra obtained using a 488 nm laser at a temperature of 300 K, taken on different dates for an undoped GaAs sample. The spectrum in black is taken from reference [PERSON] (2001) and the one in red is taken from [PERSON] (2001). In Figure 1, we can observe that in the black spectrum obtained by [PERSON], there is a defects band (DT) centered at 1.356 eV, just after the sample was grown.