Category Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development

Deep N-H related acceptor state H2

The ionized acceptor density (NA) 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 (Cj) 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 и-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 NA and Cj 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...

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4.2.2 Radiative shallow recombination center H0

Подпись: 2nd pulse voltage (V) 0 12 3 4 Fig. 9. (a)DC-DLTS spectra of p-type GaAsN for various second pulse voltage and (b) H0 DC-DLTS peak height dependence of second pulse voltage and duration.

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 H0 is observed and confirmed by varying the voltage and the duration of the injected pulse.

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 hole- recombination...

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4.2 Hole traps in GaAsN grown by CBE

4.2.1 DLTS spectra and properties of hole traps in GaAsN

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 H0, H2, and H5 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.

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Fig. 8...

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Effect of E1 on minority carrier lifetime in GaAsN

The effect of £1 on the electrical properties of GaAsN can be evaluated through the calculation of minority carrier lifetime using the SRH model for generation-recombination (Hall, 1952; Shockley & Read, 1952). Such parameter has been estimated to be less than 0.2 ns as a result of the calculation according to

*E1 =( VthnOE1NE1 11 < 0.2 nS (17)

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 density and to recover the minority carrier lifetime in GaAs.

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Effect of H implantation on lattice defects in GaAsN

GaAsN films were treated by H implantation. This experiment was used because H bounds strongly to N in GaAsN films to form N-H complexes (Suzuki et al., 2008; Amore & Filippone, 2005). H ions with multi-energy from 10 to 48 keV were implanted into GaAsN layers with peaks concentration of 5 x 1018 (GaAsNHD1) and 1 x 1019 atom/ cm3 (GaAsNHD2),

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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...

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Dependence of Nei to As source flow rates

The objective of this experiment is to clarify whether the density of E1 is sensitive to the As atom or not. The MMHy was supplied to 9.0 sccm and the TDMAAs was varied between and 0.7 and 1.5 sccm. As shown in Fig. 6. (a), increasing TDMMAs drops the N concentration in the film and tends to saturate for a flow rate higher than 1 sccm. For two emission rate erw = {100, 10} s-1 and filling pulse tp = {0.1, 5} ms values, the DLTS spectra are normalized on the junction capacitance to exclude the effect of various carrier densities in the samples. The same N-related recombination center E1 was observed in all samples. The TDMAAs flow rate dependence of the DLTS peak height of E1 for two settings of measurement parameters is given in Fig. 6(b). A peaking behavior at approximately TDMAAs = 0...

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Possible origin of the N-related recombination center E1

It is worth remembering that the atomic structure of E1 may be free from impurities and doping atoms owing to the difference in the density of residual impurities in GaAsN grown with MOCVD, MBE, and CBE. Furthermore, the uniform distribution of NE1 in the bulk of GaAsN indicates that E1 is formed during growth to compensate for the tensile strain caused by the small atomic size of N compared with that of As. Therefore, the origin of E1 has high probability to depend only on the atoms of the alloy (N, As, Ga). To confirm these expectations, the origin of E1 is investigated qualitatively by considering the results of two different experiments: (i) the dependence of NE1 to the As source flow rate and (ii) the effect of H implantation on the distribution of lattice defects in и-type GaAsN.

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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...

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