**4.1.2 Nature of the electron trap E1**

496 Solar Cells – New Aspects and Solutions

of a small atomic fraction of *N* to GaAs leads to the record of a new electron trap (*E*1), at an average activation energy 0.3 eV below the CBM of GaAsN. The DLTS spectra of as grown and annealed *n*-type GaAs0.998N0.002 are given in Figs. 2. (b) and (c), respectively. The activation energies (*E*E1) and the capture cross sections (E1) of *E*1 for *N* varying GaAsN samples are given Fig. 3 (a) and (b), respectively. The fluctuation of *E*E1 from one sample to another can be explained by the effect of PooleFrenkel emission, where the thermal emission from *E*1 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 *E*1 saturates

Fig. 3. Nitrogen dependence of (a) thermal activation energy, (b) capture cross section, and (d) adjusted density of *E*1 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 *E*1. (e) Density profiling of *E*1 in the bulk of GaAsN films, and (f) DLTS spectrum

**10-14**

**10<sup>15</sup>**

**0.0**

**(f)**

**-0.6**

**-0.3**

 **DLTS signal (arb. unit)**

**10<sup>16</sup>**

*NE***1**

(**cm-3**)

**10<sup>17</sup>**

**(d)**

**10-13**

**Capt. cross sect.** *E***1 (cm2)**

**10-12**

**10-11**

**(b)**

**0.1 0.2 0.3 0.4**

**[N](%)**

**0.00 0.12 0.24 0.36 0.48**

**[N] (%)**

*E***1**

*p***-type GaAsN**

**135 180 225 270**

**Temperature (K)**

*E***2**

 MBE [6] MOCVD [5] CBE (annealed) CBE (as grown)

**As grown Annealed** 

**0.0 0.1 0.2 0.3 0.4**

**10-2 10-1 <sup>10</sup><sup>0</sup> 0.40**

**60 80 100 120 140**

of undoped *p*-type GaAsN grown by CBE.

**Depth (nm)**

**[N] = 0.036 %**

**Peak saturation**

**Filling pulse width tp (s)**

**[N] = 0.212 %**

**[N](%)**

**As grown Annealed**

**(a)**

**CBM**

**0.9 1.0 1.1 1.2 1.3 1.4**

**0.45**

**1**

**N**

*E***1 profiling** (**x 1015cm-3**)

**10**

**(e)**

**0.50**

**Peak height of** 

*E***1 (a. u.)**

**0.55**

**0.60**

**(c)**

**Activation energy of E1 (eV)**

Two methods are used to verify whether *E*1 is a recombination center or not. The first method is indirect, in which the activation energy of deep levels is correlated with that of the reverse bias current in the depletion region of *n*-type GaAsN Schottky junction and *n*+- GaAs/*p*-GaAsN heterojunction. These two device structures are selected because the current is due mainly to electrons. The second method is direct, in which DC-DLTS is used to show the behavior of the electron traps in simultaneous injection of majority and minority carriers in the depletion region.
