Band-gap Engineering

In doping semiconductors, band-gap engineering in term of analytical model demonstrates how the presence of charge carrier concentrations based on external dopants is so crucial for the operation of electronic devices. The model basically interprets several effects such as excitation/recombination of charge behavior within the space-charge region, interface states at a thin oxide (depleting layer), the creation of the Schottky barrier (band bending) at the interface, and tunneling effect due to highly doped concentration. The operating mechanism begins when (1) the external electric field excited electrons from the donor-related state, instead of the VBM, to the CBM. (2) Or, similarly, electrons can be also excited from the VBM to the acceptor-related state and leave a delocalized hole at the original state, if the impurity state locates much closer to the VB than the CB. Consequently, any occurrence of these two cases results in generation of electron and hole (current), flowing in the direction of the applied electric field depending on their signs (+/-).

For two parts in the contact between metal-semiconductor or semiconductor-semiconductor, so-called “spatial inhomogeneities” were normally introduced by effective Schottky barrier (ɸB) at the interface, which also correlate with noise properties of Schottky diodes and grain boundaries [48]. The combination of the metal (or conductive oxide) electrode and the semiconductor affects the energy of electronic state on the semiconductor side. The different work functions (ϕ) between these two layers create a charge depletion region and band bending on the semiconductor, which hinder the electron transfer across the interface. However, corresponding to the relative position of Fermi levels (EF), electrons (negatively charge carriers) move from the semiconductor (lower work function) to the metal (or conductive oxide) electrode (higher work function) until the Fermi levels align at the thermal equilibrium state. Like an

12

example as shown in Figure 2.3, when ITO transparent electrode and TiO2 are brought into the contact [49], band alignment in a ITO-TiO₂ junction induces electrons moving from one side (TiO2) to the other (ITO), responsible for overcoming the potential (Schottky) barrier (Vs). It indicates that the built-in barrier; 𝑞𝑉� = 𝑞(𝜙�− 𝜙�) and the shift in work function; 𝑞Δ𝜙 = 𝑞(𝜙_�− 𝜒), where 𝜒, 𝜙_� 𝑎𝑛𝑑 𝜙_� are the electron affinity, the work functions of ITO and TiO², respectively. To achieve higher efficiency and yield, this work investigated a simple way to conserve the electrical conductivity of ITO films under post annealing process by applying a reducing H2 gas. As a result, this method could prevent annihilation of oxygen vacancies (free carriers generating the electrical conductivity) from oxygen in the atmosphere. In principle, the increased optical band gap of ITO may occur because the additional donor electrons occupy the energy states above the CB, which could higher the Fermi energy level, i.e., reducing the Schottky barrier at the interface. Therefore, the H2-annealed ITO electrode was assumed to employ significantly improved interfacial charge transport due to decreased internal resistance of the cell.

Figure 2.3 Illustration of band alignment and the Schottky barrier formed at the junction of conducting oxide (ITO) and semiconductor (TiO2) (a) before and (b) after H2-gas reduction [49].

13

Furthermore, improvement of interfacial adhesion has been a matter of intensive study.

The advantages and disadvantages of various common techniques depend on a variety of factors, such as contact area, charge-transport dimensionality, structural similarity, and charge recombination. For instance, poor interfacial contact (TiO2/FTO), encountered in nanoparticulate film-based devices, can be tailored by a UV laser welding [50]. One obvious evidence as electrochemical impedance analysis (Z) proved that the contact resistance decreased proportional to an increase in the power of laser beam upto 0.5 W. It was claimed to be a simple, fast, and adaptable for any other efficiency improvement schemes. However, since a thin region of the interface was locally involved in this technique, the experiment must be very tricky to handle.

The exceeded irradiation might readily damage the TiO2 particles or even degrade the conductivity of the ITO film.

Similarly, in the case of p-n junction, two parts in contact are basically composed of one semiconductor doped with donor impurities (n-type) and the other one doped with acceptor impurities (p-type). Due to high mobility of the carriers (electron and hole), they diffuse toward the opposite directions; electrons move to p-doped side and holes move to n-doped side. At the p-n junction, the energy bands of the semiconductors must bend, also forming a depletion region corresponding to the alignment of Fermi energy level in both p and n-doped layers. This consequence leads to a rectifying behavior in oxide materials, which has been reported in term of

“homojunction” or “heterojunction” diode. Toru et al. [51] have fabricated a ZnO-homojunction diode by using laser phosphorus (P) doping to form a p-type ZnO layer on a typically n-type ZnO substrate. Band-edge emission (λ = 370-380 nm) and broad peak (λ = 400-500 nm) of the ZnO diode was revealed via the electroluminescene spectrum under forward current injection at

14

110 K. The light emission of white-violet color was observed, which is evidently caused by a band-edge component of defect states.

In addition, p-i-n (p-type/intrinsic/n-type) heterojunction diodes [52] commonly offer another approach for improving the rectifying configurations of abrupt junction in ordinary p-n diode [53]. Figure 2.4a displays the structure of transparent p-CuYO2:Ca/i-ZnO/n-ITO diode. It concluded that using the selective materials with their proper band-gap mismatch could lead to a smooth transition as shown in Figure 2.4b. This can develop the injection of electrons from n-ITO to p-CuYO2, while the flow of space-charge-limited current is attributed to the single-carrier injection in the i-ZnO layer. The possible applications of p-i-n diode have been widely used in many branches such as fast switches, photodetectors, and high voltage power electronics [54, 55].

Figure 2.4 (a) Structure and (b) equilibrium energy band diagram of the CuYO2:Ca/ZnO/ITO p-i-n heterojunction diode coated on glass substrate [52].

15