**4.1.2.1 Origin of reverse bias current in GaAsN**

The temperature dependence of the reverse bias current in the depletion region of *n*-type GaAsN Schottky junction and *n*+-GaAs/*p*-GaAsN is shown in Fig. 4(a) for reverse bias voltages of 0.5 and -0.5 V, respectively. At lower temperature, the dark current changes slowly in the two structures, then fellows an Arrhenius type behavior. As shown in Fig. 4(b), the same result is obtained by applying reverse bias voltages of 1 and -1 V. Under these conditions, the reverse bias current *I*d(*T*) can be expressed by

$$I\_d(T) = I\_\pi \exp(-\frac{\Delta E}{kT})\tag{14}$$

where *I*, *E*, *k*, and *T* denote the limit of the high-temperature current, the thermal activation energy of the reverse bias current, the Boltzmann constant, and the temperature, respectively. The I-V characteristics deviate in the two samples from the thermionic emission. This is

Investigation of Lattice Defects in GaAsN

Grown by Chemical Beam Epitaxy Using Deep Level Transient Spectroscopy 499

electrons at an infinite temperature, respectively. At room temperature, E1(300 K) is evaluated to 8.89 × 10-12 cm2. Such a value is large enough to shorten the lifetime of electrons in *p*-type GaAsN. This indicates that *E*1 is a strongly active recombination center at room temperature and the *e-h* recombination process is non-radiative. In addition, from the temperature dependence of E1, the true energy depth of *E*1 can be obtained by subtracting the barrier height for electron capture from the thermal activation energy obtained from the Arrhenius plot. The recombination center *E*1 is localized at *E*a (*E*1) = 0.20 0.02 eV from the CBM of GaAsN. Furthermore, the average capture cross section of holes p, at a temperature of *T* = 175 K, is estimated using Eq. 12 to be p(175 K) 5.01 × 10-18 cm2. The physical parameters of *E*1 can be summarized in a configuration coordinate diagram (CCD), in which the energy state of *E*1 is described as a function of lattice configuration (Q). As shown in Fig. 5(c), the CCD of *E*1 can be presented in three different branches: (*i*)[0, f.e + f.h]: the charge state of *E*1 is neutral, with a free electron and a free hole, (*ii*)[-, t.e + f.h]: the electron is trapped and the hole remains free, (*iii*)[0]: the free hole is captured at the crossed point B and recombined with the alreadytrapped electron. *E*1 losses its charge and becomes neutral. As the recombination process is non-radiative, the lattice relaxation occurs with the emission of multi-phonon. The energy of

multi-phonon emission can be evaluated as function of N concentration according to

**150 200 250**

**Temperature (K)**

**5.0 5.5 6.0 6.5 7.0 7.5**

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

Fig. 5. (a) Reduction of peak height of *E*1 under minority carrier injection spectra, (b) temperature dependence of E1 for electrons, and (c) Configuration-coordinate-diagram showing the different charge states of E1 as function of lattice coordinate parameter.

**2nd pulse voltage (0, -1, -3 V)**

**DC-DLTS**

*E***2**

**0**

**(b)**

**-31.5 -31.0 -30.5 -30.0 -29.5 -29.0 -28.5 -28.0**

*ln(***E1**(**cm2**))

**10**

**20**

**30**

**DLTS signal (x 0.01, arb. unit)**

**40**

**50**

*<sup>E</sup>***<sup>1</sup> (a)**

*E N EN EE E phonon <sup>g</sup> a cap* ,*<sup>e</sup>* ( ) ( ) ( ( 1) ) (16)

(c)

*A* 

*E***cap, e**

*Eth*

*E*(eV)

Lattice coordinate parameter

*iii*

*B* 

*ii*

*i*

explained by the fact that supplying the *p-n* junction under reverse bias conditions decreases the product of excess carriers to less than the square of intrinsic carriers. Hence, the Shockley-Read-Hall (SRH) generation mechanism is activated to increase the product of excess carriers to assure the balance of charge. The generated carriers are swept to the transition regions by the electric field in the depletion region. Therefore, an SRH center, with a thermal activation energy arround 0.3 eV, is the origin of the dark current in the SCR of GaAsN. The thermal activation energies are measured with respect to majority and minority carriers in *n*-type GaAsN schottky junction and *n*+-GaAs/*p*-GaAsN heterojunction, respectively. This corresponds to the conduction band in the two structures. By correlating the conduction mechanism and DLTS measurements, the thermal activation energy of the reverse bias current and the activation energy of the *N*related electron trap *E*1 are typically identical. Therefore, *E*1 is responsible for the generation/recombination current in the depletion region of GaAsN grown by CBE.

Fig. 4. Temperature dependence of dark current under reverse bias voltages of (a) 0.5 and - 0.5V and (b) 1 and -1V in *n*-type GaAsN schottky junction and the *n*+-GaAs/*p*-GaAsN heterojunction, respectively.

### **4.1.2.2 DC-DLTS measurements**

DC-DLTS is used to confirm the recombination nature of *E*1 and to characterize the recombination process via this defect center. An unintentionally doped *n*type GaAsN layer ( 1 m) was grown on a *p*type GaAs by CBE. This structure is not commonly used for DLTS measurements. However, the absence of a *p*-type doping source prevented us to obtaining a *p+-n* junction. Here, the *p*-type substrate is used as source of minority carriers. As shown in Fig. 5(a), the DC-DLTS spectrum is compared with that of the conventional DLTS. A decrease in the peak height of *E*1 is observed by varying the voltage of the second injected pulse and also confirmed by varying its duration. The obvious reason for such reduction is the mechanism of *e-h* recombination at the energy level *E*1 in the forbidden gap of GaAsN. Hence, *E*1 is reconfirmed to act as a *N*-related recombination center. To verify the nonradiative recombination process, the temperature dependence of E1 is obtained by varying the emission rate window *erw* from 0.5 - 50 s-1. The value of E1 is obtained from the fitting of the Arrhenius plots for each *erw*. As shown in Fig. 5(b), the natural logarithmic of E1 shows a linear increase with the reciprocal of the temperature. It can be expressed as

$$\ln \left( \sigma\_{\varepsilon 1} \right) = \text{-} E\_{c\eta, \varepsilon} \left/ kT + \ln \left( \sigma\_{\varepsilon} \right) \right. \tag{15}$$

where *E*cap,e = 0.13 0.02 eV, *k*, *T*, and = 1.38 x 10-9 cm2 denote the barrier height for the capture of electron, the Boltzmann constant, the temperature, and the capture cross section of

explained by the fact that supplying the *p-n* junction under reverse bias conditions decreases the product of excess carriers to less than the square of intrinsic carriers. Hence, the Shockley-Read-Hall (SRH) generation mechanism is activated to increase the product of excess carriers to assure the balance of charge. The generated carriers are swept to the transition regions by the electric field in the depletion region. Therefore, an SRH center, with a thermal activation energy arround 0.3 eV, is the origin of the dark current in the SCR of GaAsN. The thermal activation energies are measured with respect to majority and minority carriers in *n*-type GaAsN schottky junction and *n*+-GaAs/*p*-GaAsN heterojunction, respectively. This corresponds to the conduction band in the two structures. By correlating the conduction mechanism and DLTS measurements, the thermal activation energy of the reverse bias current and the activation energy of the *N*related electron trap *E*1 are typically identical. Therefore, *E*1 is responsible for the

generation/recombination current in the depletion region of GaAsN grown by CBE.

Fig. 4. Temperature dependence of dark current under reverse bias voltages of (a) 0.5 and - 0.5V and (b) 1 and -1V in *n*-type GaAsN schottky junction and the *n*+-GaAs/*p*-GaAsN

DC-DLTS is used to confirm the recombination nature of *E*1 and to characterize the recombination process via this defect center. An unintentionally doped *n*type GaAsN layer ( 1 m) was grown on a *p*type GaAs by CBE. This structure is not commonly used for DLTS measurements. However, the absence of a *p*-type doping source prevented us to obtaining a *p+-n* junction. Here, the *p*-type substrate is used as source of minority carriers. As shown in Fig. 5(a), the DC-DLTS spectrum is compared with that of the conventional DLTS. A decrease in the peak height of *E*1 is observed by varying the voltage of the second injected pulse and also confirmed by varying its duration. The obvious reason for such reduction is the mechanism of *e-h* recombination at the energy level *E*1 in the forbidden gap of GaAsN. Hence, *E*1 is reconfirmed to act as a *N*-related recombination center. To verify the nonradiative recombination process, the temperature dependence of E1 is obtained by varying the emission rate window *erw* from 0.5 - 50 s-1. The value of E1 is obtained from the fitting of the Arrhenius plots for each *erw*. As shown in Fig. 5(b), the natural logarithmic of E1 shows a

linear increase with the reciprocal of the temperature. It can be expressed as

*E cap e* 1 , *E kT*

where *E*cap,e = 0.13 0.02 eV, *k*, *T*, and = 1.38 x 10-9 cm2 denote the barrier height for the capture of electron, the Boltzmann constant, the temperature, and the capture cross section of

ln( )=- ln( ) (15)

**-16.08**

**-15.41**

**Ln**(**|Id(A)|**)

**-14.74**

**(b)**

**3.08 3.22 3.36 3.50**

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

**-GaAs/***p***-type GaAsN**

*E***a= 0.29 eV**

*E***a= 0.30 eV**

**n-type GaAsN**

*n***+**

**3 6 9 12 10-11**

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

**n+**

**n-type GaAsN**

**GaAs/p-GaAsN**

**10-10**

heterojunction, respectively.

**4.1.2.2 DC-DLTS measurements** 

**10-9**

**Reverse Bias current |Id(A)|**

**10-8**

**10-7**

**(a)**

electrons at an infinite temperature, respectively. At room temperature, E1(300 K) is evaluated to 8.89 × 10-12 cm2. Such a value is large enough to shorten the lifetime of electrons in *p*-type GaAsN. This indicates that *E*1 is a strongly active recombination center at room temperature and the *e-h* recombination process is non-radiative. In addition, from the temperature dependence of E1, the true energy depth of *E*1 can be obtained by subtracting the barrier height for electron capture from the thermal activation energy obtained from the Arrhenius plot. The recombination center *E*1 is localized at *E*a (*E*1) = 0.20 0.02 eV from the CBM of GaAsN. Furthermore, the average capture cross section of holes p, at a temperature of *T* = 175 K, is estimated using Eq. 12 to be p(175 K) 5.01 × 10-18 cm2. The physical parameters of *E*1 can be summarized in a configuration coordinate diagram (CCD), in which the energy state of *E*1 is described as a function of lattice configuration (Q). As shown in Fig. 5(c), the CCD of *E*1 can be presented in three different branches: (*i*)[0, f.e + f.h]: the charge state of *E*1 is neutral, with a free electron and a free hole, (*ii*)[-, t.e + f.h]: the electron is trapped and the hole remains free, (*iii*)[0]: the free hole is captured at the crossed point B and recombined with the alreadytrapped electron. *E*1 losses its charge and becomes neutral. As the recombination process is non-radiative, the lattice relaxation occurs with the emission of multi-phonon. The energy of multi-phonon emission can be evaluated as function of N concentration according to

$$E\_{phnm}\text{(N)} = E\_{\text{g}}\text{(N)} - \text{(E}\_{\text{a}}\text{(E1)} - E\_{ap,s}\text{)}\tag{16}$$

Fig. 5. (a) Reduction of peak height of *E*1 under minority carrier injection spectra, (b) temperature dependence of E1 for electrons, and (c) Configuration-coordinate-diagram showing the different charge states of E1 as function of lattice coordinate parameter.

Investigation of Lattice Defects in GaAsN

plot in Fig. 7(d).

implanted GaAsN samples.

**-5**

**0**

**Nt**

(**x 1015 cm-3**)

**5**

**10**

**15**

**20**

**50 100 150 200**

*HP***1**

**Temperature (K)**

**As grown GaAsN(HD1) GaAsN(HD2)**

*EP***<sup>1</sup> (a)**

*E***1**

Grown by Chemical Beam Epitaxy Using Deep Level Transient Spectroscopy 501

respectively. The depth of implantation was thought to be distributed between 110 and 410 *nm* from the surface of GaAsN by calculating the *SRIM* 2003 simulation code (Ziegler, 1985). After implantation, the samples were treated by post thermal annealing at 500 C for 10 min under N2 gas and GaAs cap layers. As plotted in Fig. 7(a) and (b), the crystal quality of GaAsN films after implantation was controlled using XRD curves and C-V measurements. DLTS spectra of implanted samples are shown in Fig. 7(c). After implantation, *E*1 was not observed; however, two new lattice defects appeared. The signature and the density of these traps are summarized in Table 1. The thermal emission from them is plotted as an Arrhenius

GaAsN Traps Ea(eV) (cm2) Nt-adj(cm-3) Possible origin As grown E1 ECM-0.331 5.18 10-15 3.37 1017 (N-As)As Implanted EP1 ECM-0.414 8.20 10-13 5.88 <sup>1017</sup> EL5 in GaAs

Table 1. Summary of Ea, , adjusted Nt-adj, and possible origin of defects in as grown and

Fig. 7. (a)DLTS spectra of as grown and implanted GaAsN and (b) their Arrhenius plots.

The new electron trap (*EP*1) is located approximately 0.41 eV below the CBM of GaAsN. Its properties are identical to that of the native defect *EL*5 in GaAs (Reddy, 1996). Its atomic structure was discussed in many publications, where the common result indicated that *EL*5 is a complex defect free from impurities and dominated by As interstitials, such VGa-Asi or AsGa-VGa (Deenapanray et al., 2000; Yakimova et al., 1993). The second new defect is a hole trap (*HP*1) at approximately an average activation energy 0.11 eV above the VBM of GaAsN. Compared with majority carrier traps in GaAs grown with various techniques, no similar hole trap to *HP*1 was reported. However, in *p*-type GaAsN grown by CBE with around the same *N* concentration, *E*HP1 and HP1 are identical to that of the hole trap *HC*2 in *p*-type GaAsN grown by CBE (bouzazi et al., 2011). This defect was confirmed recently to be an acceptor state in GaAsN films (see § 4.2.3) and to be related to the N-H bond. While the H impurity was provided by implantation, the N atom can originate through two possibilities: First; examining XRD results, the N atom can originate from its ideal site. However, [N] in as grown is much higher than that in implanted samples (1019 cm-3). It is also much higher than NHP1. Second, N atom can be originated from the complete dissociation of *E*1, since the

**49**

**50**

**51**

**Ln.vthn.NC**)

**52**

**53**

**4.5 6.0 7.5 9.0 10.5**

*E***1**

**(b)** *EP***1**

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

*HP***1**

HP1 EVM-0.105 5.42 10-18 1.84 1016 N-H-VGa
