Two methods are used to verify whether E1 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 и-type GaAsN Schottky junction and и+- 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.
188.8.131.52 Origin of reverse bias current in GaAsN
The temperature dependence of the reverse bias current in the depletion region of и-type GaAsN Schottky junction and ^-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 Id(T) can be expressed by
Id(T) = L exp(-f) (14)
where I„ AE, 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
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 £1 are typically identical. Therefore, £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.