Formation mechanism of Zn2SiO4 crystal and amorphous SiO2 in ZnO/Si system

In our recent study Xu et al (2002 Chem. Phys. Lett. 364 57–63), a phase transformation from the hexagonal to the tetragonal structure in the annealed ZnO films on silicon was studied by atomic force microscopy. Cathodoluminescence (CL) and glancing-angle x-ray diffraction analysis of the ZnO films indicated that such a transformation is due to the generation of a tetragonal zinc silicate. In order to identify the formation mechanism of the zinc silicate and the bottom broadening of the UV band, a depth profile secondary ion mass spectroscopy experiment was carried out. The results show that vast atomic diffusion between the ZnO film and the silicon substrate occurred due to the annealing temperature. Such interdiffusion can create not only a mixed crystal of ZnO and Zn2SiO4, but also an amorphous silicon dioxide (a-SiO2) in a deep range from the surface to the interface of the ZnO/Si system. The a-SiO2 is most probably the source of the 453 nm blue band hidden in the tail of the 390 nm UV band, since the blue band agrees with the CL spectra of the amorphous quartz glass and the thermally oxidized silicon. ZnO film has been widely studied for a variety of applications in piezoelectric acoustic wave devices [1, 2], varistors [3, 4], optical waveguides [5], substrates or buffer layers for the growth of GaN [6, 7], or as a material for light-emitting diodes [8]. In addition, ZnO deposited on silicate glass has been widely used as a transparent conducting oxide buffer in the construction of semiconductor film solar cells [9]. A ZnO/Si heterojunction was also investigated as a candidate for a mono-junction solar cell [10]. Under such conditions, it is necessary to 4 Author to whom any correspondence should be addressed. 0953-8984/03/400607+07$30.00 © 2003 IOP Publishing Ltd Printed in the UK L607 L608 Letter to the Editor carry out studies of ZnO/Si or ZnO/silicate glasses, which are the important parts of thinfilm solar cells. In our recent study [11], an analysis of atomic force microscopy (AFM), cathodoluminescence (CL) and glancing-angle x-ray diffraction (GXRD) of ZnO films on Si annealed at different temperatures was carried out. The results show that the crystal quality of the film was improved with increasing the annealing temperature, while the hexagonal phase of the ZnO film was transformed into a mixed phase including a hexagonal and a tetragonal phase when annealing at a temperature approaching or higher than 800 ◦C. On increasing the temperature continuously above 800 ◦C, such a mixed phase changed into a tetragonal structure. The light emission of the sample also changed from the intrinsic emission of ZnO dominating to the emission of zinc silicate dominating. However, the above analysis did not give a reasonable explanation for the broadening at the low energy side of the bottom tail in the UV band. In order to identify the formation mechanism of the zinc silicate and to find the reason for such broadening, a depth profile secondary ion mass spectroscopy (DSIMS) analysis was used in this work. Combining DSIMS, GXRD and further CL analysis of the amorphous quartz glass and the thermally oxidized silicon, an amorphous silicon dioxide (a-SiO2) was found to be created in the ZnO film. The samples in this work are the same as those used in the previous work [11]. A reactive DC sputtering method was chosen to grow the ZnO film on silicon. The target was a zinc disc with 4N purity. Before inserting it into the vacuum chamber, the Si(100) substrate was treated by a 5% HF solution for 3 min to remove silicon oxide on the Si surface. The growth procedure progressed in two steps: first, sputtering a Zn buffer layer with a thickness of about 50 Å at room temperature in pure Ar gas; then sputtering a ZnO film with thickness of about 2000 Å at 400 ◦C in a gas mixture of 50% Ar + 50% O2. The pressure of the chamber during deposition was 2× 10−2 Torr. The sputtering power was 15 W. After sputtering, four samples were separated from the main one. Sample 1 is the as-grown sample, the other three were annealed in ambient air for 1 h at different temperatures 600 ◦C (sample 2), 800 ◦C (sample 3) and 950 ◦C (sample 4). In order to obtain a detailed picture of the atomic diffusion present in the ZnO/Si system, we investigated a depth profile of four elements (P, Si, O and Zn) in samples 1 (as-grown) and 3 (800 ◦C, 1 h annealing) using DSIMS, as shown in figure 1. The reason the element P was chosen is that the silicon substrate is P-doped. Discussions from figure 1 are as follows: (i) Distribution of P element: although sample 1 is as-grown, the growth temperature was as high as 400 ◦C, which was enough to cause a segregation effect of P diffusing from the substrate to the ZnO surface and forming a P peak near the surface. DSIMS counts for the P peak are1.4× 102. For sample 3 annealed at 800 ◦C, P was more concentrated near the surface region with the peak value 2 × 102. Since the DSIMS yields in figure 1 are in exponential units, such a segregation effect could be seen to be very strong. However, since the P content in the ZnO/Si system is very small due to the carrier concentration in the silicon substrate being only 1.2 × 1016 cm−3, the influence of P and its compounds on the luminescence is fairly weak. On the other hand, the influence of P on the electric transport in the ZnO/Si heterojunction should not be neglected, which needs further study. (ii) Distribution of Si and the formation of SiO2: as was shown in figure 1, the Si curve in the substrate is lower than that in the film. This is because the DSIMS yields in the film are greater than those in the substrate. Si exists as a monoelement in the substrate but as a compound in the film. Compared to sample 3, although the diffusion of Si to the film surface was found in sample 1, the silicon component in the near-surface region (0– 0.07 μm) is 2.5 times lower than that in the far-to-surface region (0.08–1.5 μm). This Letter to the Editor L609 10 10 10 10 10 10 10 0.0 0.1 0.2 0.3 10 10 10 10 10 10 10 O