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

Nature of the electron trap E1

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.

1.2.1.1 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...

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Lattice defects in GaAsN grown by CBE

In this section, the distribution of electron and hole traps in the depletion region of GaAsN grown by CBE will be dressed using DLTS and related methods.

1.2 Electron traps in GaAsN grown by CBE

1.2.1 DLTS spectra and properties of a N-related electron trap

The DLTS spectrum of Fig. 2(a) shows an electron trap (E2) at 0.69 eV below the CBM of GaAs. After rapid thermal annealing at 720°C for 2 min, E2 disappears completely whereas a new electron trap (E3) appears at 0.34 eV below the CBM. From the Arrhenius plots of Fig. 2(d), the capture cross sections of E2 and E3 are calculated to be aE2 = 8.1 x 10-15 cm2 and

aE3 = 7.5 x 10-18 cm2, respectively...

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Experimental procedure

All GaAsN films were grown by CBE on high conductive n – or p-type GaAs 2° off toward [010] substrate using Triethyl gallium ((C2H5)3Ga, TEGa), Trisdimethylaminoarsenic ([(CH3)2N]3As, TDMAAs), and Monomethylhydrazine (CH3N2H3, MMHy) as Ga, As, and N sources, respectively. The flow rates TEGa = 0.1 sccm and TDMAAs = 1.0 sccm were considered as conventional values. The growth temperatures of 420 °C and 460 °C were used for p-type and n-type GaAsN, respectively. Concerning the doping, p-type GaAsN films are unintentionally doped. The n-type alloys were obtained using a silane (SiH4) source or by growing the films under lower MMHy and high growth temperature.

Three different device structures are used in this study: (i) n- and p-type GaAsN schottky contacts, (ii) n+-GaAs/p-GaAsN/p-GaAs, and...

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Double carrier pulse DLTS

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DC-DLTS is used in asymmetric n+-p or p+-n junctions (Khan et al., 2005). It aims to check whether a trap is a recombination center or not. As shown in Fig. 1, two pulsed biases are applied to the sample, in turn, to inject majority and minority carriers to an electron trap. At the initial state, the junction is under reverse bias, and the energy level ET of the trap is higher than the Fermi level (EFn).When the first pulse voltage is applied to the sample, EFn is higher than ET, which allows the trap to capture electrons. During the second reverse biased pulse, with a duration tip, holes are injected to the SCR from the p-side of the junction. After the junction pulse is turned off, electrons and holes are thermally emitted...

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Isothermal capacitance transient spectroscopy

Подпись: NT (x) Подпись: 2 ND (ggo)2 A2 AC CR (Lp, - Li ) Подпись: (9)

ICTS is used to analyze the profiling of lattice defects in the SCR of the semiconductor. It can be done through three different methods. The first one is obtained by fixing VR and varying Vp to build difference of transients among the SCR of the device. The second method is evaluated by measuring at constant Vp and varying VR. The last option is obtained by varying VR and Vp/ where the profiling analysis is also possible without building difference of transients. Using the first method, the medium trap density Nt(xtj) at a point xtj is given by

where ACij is the amplitude difference of the two capacitance transients.

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Other DLTS related techniques

The isothermal capacitance transient spectroscopy (ICTS) and the double carrier pulse DLTS (DC-DLTS) are two DLTS related methods. They are used to obtain the density profiling of lattice defects and to check whether they act as recombination centers or not, respectively.

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Thermal emission of carriers from deep levels

The emission rates for electrons and holes are given, respectively by

= ^nvthnNc exp I _ Еашкт Et

= ^pvthpN„exp(_ Et kEVbm J (8)

where <3n, Nc, and vthn are the thermal capture cross section, the density of states, and the thermal velocity of holes, respectively. av, Nv, and vthp are the same parameters for holes. ECbm, EVbm, and Et are the energy levels of the conduction band minimum, the valence band maximum, and the trap, respectively.

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Fundamental concept of DLTS

1.1.1 Capacitance transient

To fully understand DLTS, it is worth to have a basic knowledge of capacitance transients arising from the SCR of Schottky contacts or p+-n/n+-p asymmetric junctions. If a pulse voltage is applied to one of these device structures that is originally reverse-biased, the SCR width decreases and the trap centers are filled with carriers (majority or minority depending on the structure). When the junction is returned to reverse bias condition, the traps that remains occupied with carriers are emptied by thermal emission and results in a transient decay. The capacitance transients provide information about these defect centers...

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