In this section, the distribution of electron and hole traps in the depletion region of GaAsN grown by CBE will be dressed using DLTS and related methods.
The DLTS spectrum of Fig. 2(a) shows an electron trap (E2) at 0.69 eV below the CBM of GaAs. After rapid thermal annealing at 720°C for 2 min, E2 disappears completely whereas a new electron trap (E3) appears at 0.34 eV below the CBM. From the Arrhenius plots of Fig. 2(d), the capture cross sections of E2 and E3 are calculated to be aE2 = 8.1 x 10-15 cm2 and
aE3 = 7.5 x 10-18 cm2, respectively. Based on previous results about native defects in n-type GaAs, E2 and E3 are independent of N and considered to be identical to EL2 and EL3, respectively (Reddy et al., 1996). In order to focus only on N-related lattice defects, these two energy levels will be excluded from the DLTS spectra of N-containing n-type GaAsN. The addition
Fig. 2. DLTS spectra of (a) N free as grown and annealed GaAs, (b) as grown n-type GaAs0.998N0.002, (c) annealed n-type GaAs0.998N0.002, and (d) Arrhenius plots of DLTS spectra.
of a small atomic fraction of N to GaAs leads to the record of a new electron trap (E1), at an average activation energy 0.3 eV below the CBM of GaAsN. The DLTS spectra of as grown and annealed и-type GaAso.998No. oo2 are given in Figs. 2. (b) and (c), respectively. The activation energies (Eel) and the capture cross sections (aE1) of E1 for N varying GaAsN samples are given Fig. 3 (a) and (b), respectively. The fluctuation of Ee1 from one sample to another can be explained by the effect of Poole-Frenkel emission, where the thermal emission from E1 is affected by the electric field (Johnston and Kurtz, 2006). As illustrated in Fig. 3(c), with increasing the filling pulse duration, the DLTS peak height of E1 saturates
Fig. 3. Nitrogen dependence of (a) thermal activation energy, (b) capture cross section, and (d) adjusted density of E1 in as grown and annealed GaAsN samples. The large capture cross section is confirmed with (c) the filling pulse width dependence of the DLTS peak height of E1. (e) Density profiling of E1 in the bulk of GaAsN films, and (f) DLTS spectrum of undoped p-type GaAsN grown by CBE.
rapidly. This behavior is explained by the large value of oE1, compared with that of E2, E3, and other native defects in GaAs. The adjusted densities of E1 (NEi) in as grown and annealed samples are plotted in Fig. 3(d). NE1 increases considerably with increasing [N] in the film and persists to post thermal annealing. This indicates that E1 is a N-related and a stable electron trap.
The defect center E1 was not observed previously in N free GaAs grown by CBE despite the existence of N species in the chemical composition of the As source. This can be explained through three possible scenarios. First, the absence of tensile strain in GaAs prevents the formation of E1. Second, the N atom in the atomic structure of E1 comes from the N source. Finally, the N atom comes from the N and As compound sources and in presence of tensile strain E1 can be formed. The tensile strain was reported in most theoretical and experimental studies. As given in Fig. 3(e) and using the ICTS, this idea is supported by the uniform distribution of NE1 in the bulk of GaAsN. This indicates that E1 is formed during growth to compensate for the tensile strain in the GaAsN films caused by the small atomic size of N compared with that of As.
Furthermore, the properties of E1 are identical to that of the famous electron traps reported by Johnston et Kurtz (Johnston and Kurtz, 2006) and Krispin et al. (Krispin et al., 2003) in MOCVD and MBE grown и-type GaAsN, respectively. As illustrated in Fig. 3(d), the densities of these traps are approximately similar to NE1 despite the large difference in the density of residual impurities between the three growth methods. Therefore, the atomic structure of E1 may be free from impurities. Furthermore, by carrying out DLTS measurements for minority carriers in undoped p-type film, E1 was also observed. This indicates clearly that E1 is independent of doping atoms (see Fig. 3(f)).