**4.2.2 Radiative shallow recombination center H0**

502 Solar Cells – New Aspects and Solutions

ratio NE1/NHP1 is less than 2. If *E*1 is the split interstitial (N-N)As, NHP1 must be at least equals to that of *E*1 and one N atom remains free. This expectation is disapproved by DLTS measurements, where *N*HP1 is largely less than *N*E1. This means that *E*1 contains only one N atom in its atomic structure. Considering the results of last sub-section, *E*1 may be the split interstitial (*N-As*)As formed from one *N* and one *As* in a single *As* site. This result is

The effect of *E*1 on the electrical properties of GaAsN can be evaluated through the calculation of minority carrier lifetime using the SRH model for generationrecombination (Hall, 1952; Shockley & Read, 1952). Such parameter has been estimated to be less than 0.2

> <sup>1</sup> 1 11 0.2 *E thn E E*

*v σ N ns*

Therefore, E1 is considered to be the main cause of short minority carrier in GaAsN. It is required to investigate the formation mechanism of this defect in order to decrease its

Here, we only focus on the hole traps that coexist in all *p*-type GaAsN based Schottky junctions and *n*+-GaAs/*p*-GaAsN heterojunction. The difference between these two structures is the temperature range in which the DLTS measurements can be carried out due to the freeze-out of carriers. The DLTS spectrum of *p*-type GaAsN in the heterojunction is shown in Fig. 8(a). Three hole traps *H*0, *H*2, and *H*5 are observed at 0.052, 0.185, and 0.662 eV above the VBM of GaAsN. Their peak temperatures are 35, 130, and 300 K, respectively. The thermal dependence of emission from the hole traps is plotted as an Arrhenius plot in Fig. 8(b). The activation energy, capture cross section, and density are given in Table 2.

**51**

**Ln**(**N**

Fig. 8. (a) DLTS spectra of p-type GaAsN grown by CBE and (b) their Arrhenius plots.

The hole traps *H*2 and *H*5 were also observed in Schottky junctions and coexist in all samples. *H*2 is a N-related acceptor-like state and was proved to be in good relationship with high background doping in GaAsN films. These properties will be discussed later. The

**v**

**vth.**)

**54**

**57**

**(b)** *H***5**

(17)

**0 4 8 24 28 32**

**1000/T(1/K)**

*H***2**

*H***0**

supported by the theoretical calculation (Zhang et al., 2001).

**4.1.4 Effect of E1 on minority carrier lifetime in GaAsN** 

density and to recover the minority carrier lifetime in GaAs.

**4.2.1 DLTS spectra and properties of hole traps in GaAsN** 

**30 35 150 300**

**Temperature (K)**

*H H***5 2**

**0.0**

**0.5**

**1.0**

**DLTS signal(a.u)**

**1.5**

**2.0**

**(a)** *H***0**

ns as a result of the calculation according to

**4.2 Hole traps in GaAsN grown by CBE** 

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 *H*0 is observed and confirmed by varying the voltage and the duration of the injected pulse.

Fig. 9. (a)DC-DLTS spectra of p-type GaAsN for various second pulse voltage and (b) *H*0 DC-DLTS peak height dependence of second pulse voltage and duration.

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 holerecombination. 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 H0 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 *H*0 does not exhibit an Arrhenius behavior, which excludes the non-radiative recombination process. Its shallow energy level suggests

Investigation of Lattice Defects in GaAsN

**ECBM**

 **EF**

**EVBM**

**EH0**

**Et**

charged bound transition.

range (Baldereschi & Lipari, 1974).

**4.2.3 Deep N-H related acceptor state H2** 

Grown by Chemical Beam Epitaxy Using Deep Level Transient Spectroscopy 505

is suggested to be in relation with *H*0. 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 (*E*t). The transition between free electrons in the CBM and charged *H*0 is called free-to-

**ECBM**

 **EF**

**EVBM**

**(c)** 

**ECBM**

**EF**

**EVBM**

**EH0**

**Et**

**EH0**

**Et**

Fig. 11. Band diagram of radiative recombination through *H*0, where the transition occurs

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 *H*0 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

The ionized acceptor density (*N*A) 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 (*C*j) 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 *n*-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 *N*<sup>A</sup> and *C*j 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 *E*A1 = 130 ± 20 meV (Suzuki et al., 2008). On the other hand, the properties of *H*2 in N-varying GaAsN schottky junctions are cited below: The peak temperature of *H*2 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 *E*H2 from one sample to another depending on N concentration (see Table 3). However, the average of *E*H2 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), *N*H2-adj is in linear dependence with *N* concentration. Therefore, *H*2 is proved to be the N-related hole trap acceptor-like state, which thermal ionization increased *C*j and

between *H*0 and the CBM and/or a donor-like defect (*E*t).

**(a) (b)** 

that *H*0 plays the role of an intermediate center in the recombination process, with the exception that the recombination is quite often radiative.

Fig. 10. (a) Emission rate window dependence of *H*0 peak, (b) temperature dependence of capture cross section and activation energy of *H*0, (c) *PL* spectra of p-type GaAsN at 20 to 50 K, and (d) temperature dependence of the peaks *P*1, *P*2, and *P*3 obtained from the fitting of PL spectra.

Furthermore, the capture cross section of electrons can be estimated using Eq. 13 and by the reduction of the peak height of *H*0, which follows from the injection of minority carriers. By varying the injected pulse voltage at fixed duration, the average capture cross section of electrons n, at a temperature T = 35 K, is estimated to be <sup>n</sup> 3.64 × 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 <sup>n</sup> 3.05 × 10-16 cm2. These two values are nearly identical and indicate that the capture cross section of electrons and holes of *H*0 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 *P*1, *P*2, and *P*3 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*) *P*1 is the result of band-to-band transition, (*ii*) *P*2 is caused by free exciton or related to shallow energy level, and (*iii*) *P*3 originates from the transition between a neutral donor and a neutral acceptor pair (DAP) (Inagaki et al., 2011). Therefore, the peak *P*2

that *H*0 plays the role of an intermediate center in the recombination process, with the

Fig. 10. (a) Emission rate window dependence of *H*0 peak, (b) temperature dependence of capture cross section and activation energy of *H*0, (c) *PL* spectra of p-type GaAsN at 20 to 50 K, and (d) temperature dependence of the peaks *P*1, *P*2, and *P*3 obtained from the fitting of

Furthermore, the capture cross section of electrons can be estimated using Eq. 13 and by the reduction of the peak height of *H*0, which follows from the injection of minority carriers. By varying the injected pulse voltage at fixed duration, the average capture cross section of electrons n, at a temperature T = 35 K, is estimated to be <sup>n</sup> 3.64 × 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 <sup>n</sup> 3.05 × 10-16 cm2. These two values are nearly identical and indicate that the capture cross section of electrons and holes of *H*0 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 *P*1, *P*2, and *P*3 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*) *P*1 is the result of band-to-band transition, (*ii*) *P*2 is caused by free exciton or related to shallow energy level, and (*iii*) *P*3 originates from the transition between a neutral donor and a neutral acceptor pair (DAP) (Inagaki et al., 2011). Therefore, the peak *P*2

**1.31**

**1.32**

**Peak Energy (eV)**

**1.38**

**1.41**

**10-17**

**10-16**

**Capt. cross sect.** *p*

**10-15**

 **(cm2)**

**10-14**

**(b)**

**26 28 30 32 10-18**

**P3 (d)**

**20 30 40 50**

**Temperature (K)**

**P1**

**P2**

**1000/T(1/K)**

exception that the recombination is quite often radiative.

**Peak shift with (a)**

*H***0 Emission rate window**

**25 30 35 40 45 50 55**

**Temperature (K)**

 20k 30k 40k 50k

**1.26 1.33 1.40**

**Photon Energy (eV)**

**P1 P2**

**P3**

**0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4**

**(c)**

**PL intensity (a.u)**

PL spectra.

**DLTS signal (arb. unit)**

is suggested to be in relation with *H*0. 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 (*E*t). The transition between free electrons in the CBM and charged *H*0 is called free-tocharged bound transition.

Fig. 11. Band diagram of radiative recombination through *H*0, where the transition occurs between *H*0 and the CBM and/or a donor-like defect (*E*t).

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 *H*0 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).
