DC-DLTS measurements

DC-DLTS is used to confirm the recombination nature of £1 and to characterize the recombination process via this defect center. An unintentionally doped n-type GaAsN layer (~ 1 pm) 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 £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 £1 in the forbidden gap of GaAsN. Hence, £1 is reconfirmed to act as a N-related recombination center. To verify the non­radiative recombination process, the temperature dependence of aE1 is obtained by varying the emission rate window e-rw from 0.5 – 50 s-1. The value of aE1 is obtained from the fitting of the Arrhenius plots for each erw. As shown in Fig. 5(b), the natural logarithmic of <rE1 shows a linear increase with the reciprocal of the temperature. It can be expressed as

ln(^E1)=- £apJkT + b(^) (15)

where £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
electrons at an infinite temperature, respectively. At room temperature, пе1(300 K) is evaluated to ~ 8.89 x 10-12 cm2. Such a value is large enough to shorten the lifetime of electrons in p-type GaAsN. This indicates that £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 aE1, the true energy depth of £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 £1 is localized at £a (£1) = 0.20 + 0.02 eV from the CBM of GaAsN. Furthermore, the average capture cross section of holes ap, at a temperature of T = 175 K, is estimated using Eq. 12 to be np(175 K) ~ 5.01 x 10-18 cm2. The physical parameters of £1 can be summarized in a configuration coordinate diagram (CCD), in which the energy state of £1 is described as a function of lattice configuration (Q). As shown in Fig. 5(c), the CCD of £1 can be presented in three different branches: (г)[0, f. e + f. h]: the charge state of £1 is neutral, with a free electron and a free hole, (и)[-, t. e + f. h]: the electron is trapped and the hole remains free, (ш)[0]: the free hole is captured at the crossed point B and recombined with the already – trapped electron. £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

Подпись: Fig. 5. (a) Reduction of peak height of £1 under minority carrier injection spectra, (b) temperature dependence of aE1 for electrons, and (c) Configuration-coordinate-diagram showing the different charge states of E1 as function of lattice coordinate parameter.

£h~,(N) = £ (N) – (£ (£1) – £„,e) (16)

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