Deep N-H related acceptor state H2

The ionized acceptor density (NA) is found to be in good linear dependence with N concentration in p-type GaAsN samples (see Fig. 12 (a)). As given in Figs. 12(b) and (c), the junction capacitance (Cj) showed a N-related sigmoid behavior with temperature in the range 70 to 100 K. This behavior has not yet been observed in GaAs and и-type GaAsN grown by CBE. It was recorded at 20 K in silicon p-n junction and explained by the ionization of a shallow energy level (Katsuhata, 1978; 1983). Hence, the N dependence of NA and Cj is explained by the thermal ionization of a N-related acceptor-like state. The thermal ionization energy of this energy level was estimated in the temperature range 70 to 100 K to be between 0.1 and 0.2 eV. It is in conformity with the theoretical calculations, which suggested the existence of a N-related hole trap acceptor-like defect with an activation energy within 0.03 and 0.18eV above the VBM of GaAsN (Janotti et al., 2003; Suzuki et al., 2008). Experimentally, a deep acceptor level, A2, was confirmed in CBE grown undoped GaAsN with ionization energies of EA1 = 130 ± 20 meV (Suzuki et al., 2008). On the other hand, the properties of H2 in N-varying GaAsN schottky junctions are cited below: The peak temperature of H2 is within the temperature range of increase of Cj. This means that the electric field at this temperature range fellows the same behavior of Cj and depends on N concentration. Hence, the emission of carriers from the charged traps is affect by the Poole – Frenkel emission (Johnston and Kurtz, 2006). This is confirmed by the fluctuation of EH2 from one sample to another depending on N concentration (see Table 3). However, the average of EH2 is within the energy range of the acceptor level obtained from theoretical prediction and identical to EA2 (Suzuki et al., 2008; Janotti et al., 2003). Furthermore, as given in Fig. 12 (d), NH2-adj is in linear dependence with N concentration. Therefore, H2 is proved to be the N-related hole trap acceptor-like state, which thermal ionization increased Cj and

drops the depletion region width. The contribution of H2 in the background doping of p – type GaAsN films grown by CBE can be evaluated from the ratio between the real NH2-Cj calculated from the change of Cj and NA at room temperature (Bouzazi et al., 2010). As shown in Fig. 12 (e) and f), This ratio comes closer to the unit for a N concentration greater than 0.2%. Thus, H2 is the main cause of the high background doping in p-type GaAsN.

4 –

><  O’2 CM X 0

0.1 0.2 0.3 0.4 0.5 [N] (%)

0.1 0.2 0.3 0.4 0.5 [N] (%)

Fig. 12. N dependence of (a) NA, (c) amplitude of Cj after thermal ionization of H2, and (d) NH2_adj. (b) Sigmoid increase of Cj between 70 and 100 K for two different GaAsN samples. N dependence of (e) NH2.q and (f) NH2.q /NA.

To investigate the origin of H2, it is worth remembering some previous results about carrier concentration and the density of residual impurities in undoped GaAsN grown by CBE, obtained using Hall Effect, Fourier transform infra-red (FTIR), and second ion mass spectroscopy (SIMS) measurements. On one hand, under lower Ga flow rates (TEGa), NA and NH2 sowed a rapid saturation with [N], despite the increase of [N] (see Fig. 13 (a)). This means that the atomic structure of H2 depends on other atoms, either than N. In addition, the densities of C and O was found to be less than free hole concentration, which excludes these two atoms from the origin of H2. On the other hand, using SIMS measurements, the ratio [H]TEGa = o. o2/[H]TEGa = 0.1 was evaluated to be ~0.6 (Sato et al., 2008). Furthermore, the free hole concentration at room temperature showed a linear increase with the density of N – H bonds (Nishimura et al., 2006). This means that NA depends strongly on [H] and the saturation of NA under lower TEGa can be explained by the desorption of H from the growth surface, since the growth rate in our films was found to be in linear dependence with TEGa. Hence, the structure of H2 is related to the N-H bond. However, the N-H bond may not be the exact structure of H2 because the slope of the linear relationship between NA and [N-H] increased with increasing growth temperature (TG є [400, 430] °C). This indicates that Na is determined by both the number of N-H and another unknown defect, which concentration increased with increasing TG. The binding energy of this unknown defect can be determined from Arrhenius plot. Furthermore, the formation energy of (N-H-VGa)-2 was found to be lower than (N-VGa)-3, (H-VGa)-2, and isolated VGa-3 (Janotti et al., 2003). This means that the unknown defect may be VGa. These predictions were experimentally supported using positron annihilation spectroscopy results (Toivonen et al., 2003). Hence, H2 may be related to the N-H-VGa structure.

[M](%J TEGa(sccm) Fig. 13. N dependence of (a) NA and (b) TEGa flow rate dependence of growth rate and N concentration at a growth temperature of 420 °C.

[N](%)	Eh2 (eV)	OH2(cm2)	NH2-adj(cm-3)	Emax (V/cm)	NH2-Cj(cm-3)	Nrn-Cj/ Na
0.12	0.210	2.8 x 10-14	2.64 x 1015	6.2 x 104	2.23 x 1015	0.36
0.20	0.150	6.3 x 10-16	3.08 x 1015	1.4 x 105	2.88 x 1016	0.91
0.34	0.138	6.3 x 10-16	5.20 x 1015	2.1 x 105	6.56 x 1016	0.93
0.51	0.103	1.3 x 10-17	9.12 x 1015	2.3 x 105	7.55 x 1016	0.87

Table 3. Summary of EH2, aH2, NH2-adj, Emax, NH2-est, and the ratio NH2.q/NA for CBE grown undoped p-type GaAsN Schottky junctions.

5. Conclusion

Three defect centers, related to the optoelectronic properties of GaAsN, were identified and characterized using DLTS and some related methods:

– The first defect is a N-related non-radiative recombination center (£1), located approximately 0.33 eV below the CBM of GaAsN grown by CBE. £1 is a stable defect and exhibits a large capture cross section at room temperature. Its activation energy comes closer to the midgap by increasing the N concentration in the film, which increases the recombination rates of carriers. In addition, the density profiling of E1 was found to be uniformly distributed, which may indicates that this defect was formed during growth to compensate for the tensile strain caused by N. Using the SRH model for generation-recombination, the lifetime of electrons to £1 was evaluated to be ~0.2 ns. Therefore, £1 is suggested to be the main cause of poor minority carrier lifetime in GaAsN films. Its origin was suggested to be the split interstitial formed from one N and one As in a single arsenic site.

– The second defect is a radiative shallow hole trap acceptor-like state (H0), at 0.052 eV above the VBM of GaAsN. it was observed and reported for the first time in GaAs and GaAsN films. The radiative recombination process through H0 is confirmed to be a free – to-charged bound transition. This defect may be in relationship with carrier density and the optical properties of the film.

– The last defect is a N-related hole trap acceptor-like state (H2) located approximately 0.15 eV above the VBM of GaAsN. It was newly observed in GaAsN films. H2 was proved to have a good relationship with the carrier density and to be the main origin of high background doping in GaAsN films, essentially for relatively high N concentration. H2 is strongly suggested to be related with N-H bond. Its formation limits the junction depth and minimizes the contribution of the depletion region in the quantum efficiency of GaAsN based solar cells.

In conclusion, the results obtained in this study are very useful for scientific understanding of defects in III-V-N materials and to improve GaAsN and InGaAsN qualities for realizing high efficiency multi-junction solar cells.

[1] Qu iescent reverse bias