# Study of the Photo-Response of Doped GaAs with Aging
## 3 Results and Discussion
This band corresponds to what is called a native defect, which has been reported by several authors for the case of GaAs [15; 16] as V\({}_{\text{As}}\)-I\({}_{\text{As}}\), where the letter I represents impurities. The formation of Ga and As oxides slowly over many years due to the presence of oxygen, adding defects to the surface, as suggested by [PERSON] [17] and [PERSON] [18]. In the same, it also shows the red PL spectrum obtained by [PERSON] six years later, and one can see that the defect zone (DF) has expanded and shifted towards lower energies, now centering at 1.324 eV. This means that new defects have been formed in greater quantity. For this work, we measured the blue spectrum of PL 26 years after the undoped GaAs sample was grown. The defect peak (DO) has shifted towards 1.244 eV and has grown significantly and broadened. In Figure 2, we show the PL spectra for different undoped and doped GaAs samples at 13 K temperature taken in 2021. The influence of doping in 26-year-old samples is clear. The undoped sample in magenta shows the maximum peak in the defect zone at 1.335 eV. It can be distinguished in the spectrum at 13 K of this sample that the excitonic peak is higher than that of defects, contrary to the blue spectrum at 300 K in Figure 1. The temperature destroys the electron-hole bond of the exciton. We already explained in another article [19] that this peak of defects is due to the formation of vacancies, especially for the As vacancies. The blue spectrum in Figure 2, which corresponds to the sample doped with 1 \(\times\) 10\({}^{16}\) atoms cm\({}^{-3}\), shows this same defect peak at 1.324 eV, but is more diminished with structure. For the green line of the 1 \(\times\) 10\({}^{17}\) atoms cm\({}^{-3}\) sample, that peak is even lower than 1 \(\times\) 10\({}^{16}\) atoms cm\({}^{-3}\) and then this peak disappears for a higher doping level. The excitonic peak is close to the bandgap at 1.512 eV in the blue spectrum with 1 \(\times\) 10\({}^{16}\) atoms cm\({}^{-3}\). In addition to the excitonic peak near the energy gap, other peaks appear at 1.49, 1.45 and 1.41 eV, corresponding to the transitions that involve the bands and/or impurity levels. These latter peaks widen with increasing concentration of the impurities, until they overlap within a single peak for higher concentrations, as seen for the red and black spectra in Figure 2. In the undoped GaAs, this peak is not resolved, indicating a low crystallinity of the sample. As the doping in  
Figure 2: Photoluminescence spectra for different doped GaAs samples at 13 K temperature taken in 2021.  
the semiconductor is increased, the excitonic peak resolves, i.e., the crystallinity improves. This is because the impurities occupy Ga sites and As sites and do not allow the formation of vacancies of those elements, as was also shown in reference [19]. With the increase in impurity, crystallinity is once again lost, due to the substitutional defects that generate shallow energy levels and not deep ones such as vacancies. Figure 3 shows the PL spectra of the defect zone for the GaAs:Sn \(1\times 10^{17}\) atoms cm\({}^{-3}\) sample taken in 2001 and 2021 at low temperatures. It is clearly seen in this figure that the spectrum of the defect zone is similar at both measurement times and that the structure is lost as the temperature increases. This similarity indicates that the type of defect is the same in 2001 and 2021. However, the spectra are shifted towards lower energies for the sample that has been aging longer. We propose a hypothesis that since the defect is of the vacancy type, what could be changing is its charge state. It is also noted in Figure 3 that the maximum of the peaks at each measurement time does not change with temperature, typical of a donor-acceptor transition which is not affected by the change in the energy gap with temperature. This donor-acceptor pair must be between an impurity and the As vacancy, which forms a complex I-VAs as reported by several authors [15; 16].