1.4.1 Al-doped ZnO thin films
In recent years, ZnO materials have attracted many attentions due to its unique features, inclusive of a wide band gap (3.3 eV), a low threshold for optical pumping, a larger exciton binding energy (60 meV), high transparency (>80%) in the visible wavelength region, and high conductivity with group-Ⅲ elements [7,8,9]. These characteristics make ZnO films useful in optoelectronic devices, such as light-emitting diodes (LEDs), transparent conductors, and photovoltaic devices.
Al-doped ZnO (AZO) films have better stability, conductivity and transparency than other dopants of group-Ⅲ elements due to the close covalent bond length of Al-O (0.192 nm) to that of Zn-O (0.197 nm) [10]. Furthermore, AZO films are prepared by many deposition techniques, such as sputtering, the sol-gel process, pulsed laser deposition (PLD), chemical vapor deposition (CVD), etc.
S. Mridha et al. deposited AZO thin films by the sol-gel spin coating technique for different Al concentration [10]. As the Al doping level was increased, the film became more transparent. The film showed highest carrier concentration and lowest resistivity for 1-2% Al concentration. K. K. Kim et al. proposed AZO thin films deposited by RF magnetron sputtering and annealed by rapid-thermal annealing (RTA) [9]. With the increase of annealing temperature, carrier concentration and electron mobility increased until 900 ℃ due to the activation of Al, and then degraded due to the out diffusion of dopants or the decomposition of the films.
1.4.2 Nano-crystalline Silicon Quantum Dot Solar Cells
Nano-crystalline Si (nc-Si) thin films have some different features to a-Si, micro-crystalline Si and single-crystallince Si, such as an enlargement of the effective
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band gap and efficient emission in the visible range at room temperature [11]. Thus, nc-Si QD materials have attracted extensive studies for potential applications in the fields of optoelectronics, semiconductor memories, and photovoltaics [12].
For Si precipitation from Si-rich layers, in order to form nc-Si QDs, high temperature annealing is necessary, shown as Fig. 1.7(a). In general, nc-Si QDs are embedded in the Si-based materials, such as SiO2, Si3N4, and SiC. The Si precipitation mechanism can be expressed as [2,4]:
Si(O, N, C)x→ �x2� Si�O2, N4/3, C� + �1 −x2� Si. (2.4)
To control the size of silicon quantum dots more precisely, the multilayer structure stacked Si-based materials and their corresponding Si-rich layer (SiOx, Si1-xCx or SiNx) periodically is proposed, shown as Fig. 1.7(b). By adjusting annealing conditions, including the annealing temperature and time, nc-Si QDs are precipitated from Si-rich layers. The size of nc-Si QDs is close to the thickness of Si-rich layers.
Fig. 1.7 The formation of nc-Si QDs in (a) a single Si-rich layer and (b) a multilayer structure.
The nc-Si QDs embedded in different materials with different bandgaps will affect the tunneling properties. Fig. 1.8 shows the energy band diagram of crystalline-silicon (c-Si) and its carbide, nitride and oxide [3,4,5]. Materials with smaller barrier height have higher tunneling probability and longer decay length. In order to overlap the
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wave functions to produce electrical conductivity, the distance between nc-Si QDs embedded in different materials must be controlled effectively. For instance, dots in a SiO2 matrix have to be separated by no more than 1-2 nm, while they in a SiC matrix are separated by more than 4nm [5].
Fig. 1.8 The energy band diagram of c-Si and its carbide, nitride and oxide [3,4,5].
X. J. Hao et al. fabricated nc-Si QDs in the SRO/SiO2 multilayer films by using the sputtering method [12]. SRO layers with different O/Si ratio were grown by a co-sputtering technique. Increasing O/Si ratio caused Si QD size decrease and the absorption edge blue-shift, which was coincident with the quantum confinement. D.
Song et al. prepared nc-Si QDs embedded in the SiC matrix by magnetron sputtering [11]. Different annealing temperatures effected the formation of Si and SiC nanocrystals. When the annealing temperature was higher (＞800℃), the more SiC nanocrystals formed. T. W. Kim et al. reported nc-Si QDs embedded in silicon nitride by plasma-enhanced chemical vapor deposition (PECVD) using SiH4 and NH3 as precursors [13]. The flow of SiH4 was fixed, while the flow of NH3 was varied.
Increasing the flow of NH3 caused the photoluminescence peak blue-shift and the nc-Si QD size decrease.
M. A. Green et al. demonstrated the conversion efficiency of the n-type nc-Si
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QDs/p-type c-Si heterojunction device was 10.58% [14]. However, the conversion efficiency is lower than the theoretical calculation, because of a large power loss coming from low efficiency of carrier collection [3].
1.4.3 p-type ZnO thin films
For subsequent applications, the deposition of ZnO p-n homojunctions is needful.
Due to the natural defects of oxygen vacancies and zinc interstitials, ZnO thin films generally show n-type conductivity [15,16,17]. It is difficult to obtain p-type ZnO.
Therefore, researches in p-type ZnO have attracted a lot of attentions in recent years.
Some studies have found acceptor dopants such as nitrogen (N), arsenic (As), and phosphorus (P) can cause ZnO to have p-type conductivity [15]. Compared with the deep acceptors of As and P, N is easier to contribute to p-type conductivity.
M. L. Tu et al. fabricated nitrogen-doped p-type ZnO (ZnO:N) using ZnO target by radio-frequency (RF) magnetron sputtering with an Ar sputtering gas mixed with various flow levels of N2 [15]. The undoped ZnO thin films was n-type but was converted to p-type, when the N2 was flowed and the acceptor-type N dopant entrapped into ZnO thin films. The type of conductivity was resolved by the ratio of N2(molecular):N(atomic). C. Wang et al. deposited ZnO:N using pure Zn target by DC reactive magnetron sputtering [16]. The sputtering gas was the mixture of Ar-N2-O2. The structure and electrical properties of the films were influenced by the post-annealing treatment and the ratio of N2-to-O2. M. Dutta et al. prepared Al-N codoped p-type ZnO thin films on n-Si wafers by sol-gel fabrication [18]. The heterojunction showed a good rectifying I-V characteristic.
Otherwise, the working pressure and the annealing process are key points to the formation of p-type ZnO. D. K. Hwang et al. proposed phosphrous (P)-doped p-type
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ZnO grown by radio-frequency magnetron sputtering [19]. The films were grown at working pressure in a range of 20 to 1 mTorr and annealed at 800 ℃ for 3 min under ambient nitrogen by rapid thermal annealing (RTA) process. The P-doped ZnO films grown at a low working pressure had high crystallinity and low native defects. The annealing process converted the as-grown P-doped ZnO films with a semi-insulating property into P-doped p-type ZnO films.