4.2.2 Radiative shallow recombination center H0

Подпись: 2nd pulse voltage (V) 0 12 3 4 Fig. 9. (a)DC-DLTS spectra of p-type GaAsN for various second pulse voltage and (b) H0 DC-DLTS peak height dependence of second pulse voltage and duration.

DC-DLTS measurements were carried out to confirm whether there is a recombination center among the hole traps or not. As shown in Figs. 9(a) and (b), the DC-DLTS signal is compared to that of conventional DLTS. A decrease in the DLTS peak height of H0 is observed and confirmed by varying the voltage and the duration of the injected pulse.

The shallow hole trap H0 is observed and reported for the first time owing to the temperature range it was recorded, which cannot be reached with standard DLTS systems. Second, its capture cross section is large enough to capture majority carriers and minority carriers. The reduction in the peak height of H0 is explained by the electron hole- recombination. This implies that H0 is a shallow recombination center in p-type GaAsN grown by CBE and can also play the role of an acceptor state. To verify whether the recombination process via H0 is radiative or not, the temperature dependence of the capture cross section of electrons is obtained by varying the emission rate erw from 1 to 50 s-1. As shown in Fig. 10(a), the peak temperature of H0 shifts to high temperatures with increasing erw. The value of nH0 is obtained from the fitting of the Arrhenius plots for each erw. It is important to note that the fitting errors of activation energy and capture cross section are relatively large owing to the instability of temperature in the range of measurements. As shown in Fig. 10(b), the capture cross section of H0 does not exhibit an Arrhenius behavior, which excludes the non-radiative recombination process. Its shallow energy level suggests

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that H0 plays the role of an intermediate center in the recombination process, with the exception that the recombination is quite often radiative.

Furthermore, the capture cross section of electrons can be estimated using Eq. 13 and by the reduction of the peak height of H0, which follows from the injection of minority carriers. By varying the injected pulse voltage at fixed duration, the average capture cross section of electrons at a temperature T = 35 K, is estimated to be an ~ 3.64 x 10-16 cm2 . However, by varying the width of the injected pulse at fixed pulse voltage, it is estimated, at the same temperature, to be an ~ 3.05 x 10-16 cm2. These two values are nearly identical and indicate that the capture cross section of electrons and holes of H0 are approximately the same. To identify this radiative recombination, photoluminescence (PL) measurements were carried out at low temperature on the same GaAsN sample. The PL spectra at 20, 30, 40, and 50K are shown in Fig. 10 (c). Three different peaks P1, P2, and P3 can be distinguished from PL spectra fitting. The temperature dependence of their energies is plotted in Fig. 10 (d). The three peaks were sufficiently discussed in many N-varying GaAsN samples and they are proposed to be: (i) P1 is the result of band-to-band transition, (ii) P2 is caused by free exciton or related to shallow energy level, and (iii) P3 originates from the transition between a neutral donor and a neutral acceptor pair (DAP) (Inagaki et al., 2011). Therefore, the peak P2

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is suggested to be in relation with H0. The band diagram of such recombination is shown in Fig. 11, where the transition occurs between H0 and the CBM and/or a donor-like defect (Et). The transition between free electrons in the CBM and charged H0 is called free-to – charged bound transition.

Fig. 11. Band diagram of radiative recombination through H0, where the transition occurs between H0 and the CBM and/or a donor-like defect (Et).

Concerning the origin of the acceptor-like hole trap H0, more experiments are required to discuss it. However, considering its density and the distribution of shallow acceptors in GaAs, it can be suggested that H0 is a carbon-related acceptor, where the reported ionization energy from PL and Hall Effect measurements is 0.026 eV and its density in the 10-15 cm-3 range (Baldereschi & Lipari, 1974).

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