The effects of doping concentration and position on characteristics of Al doped SnO2 thin film transistors

Abstract

Atomic layer deposition (ALD) is a well-established chemical vapor deposition method that utilizes the surface mediated reactions of various chemical compounds. Due to the nature of its growth mode, films grown by ALD are usually highly conformal over even the most complex structures and portray excellent thickness and composition control. These aspects led the technique to be vigorously exercised in various fields of research, with semiconductor technologies such as memory and high-k dielectrics being the most prevailing. However, another relatively unexplored potential the ALD technique holds is its possible application to the local doping of thin films. Since ALD is performed on an atomic layer by layer basis according to the input precursor or reactant, doping control can also be achieved at an atomic level. This indicates that different doping levels can be achieved in the deposition direction of the film, and hence local doping can also be achieved. An easy and intuitive approach to explore the potential of this technique is to apply it to thin film transistors (TFTs). Classic amorphous Si based TFTs operate in accumulation–depletion–inversion mode, where the on-current characteristics are governed by the inversion layer formed near the active channel/insulator interface during field application while the off-current characteristics are governed by depletion in the bulkier regions away from the interface. This implies that precise doping in these specific regions will have a distinctive impact on the observed electrical performance of a TFT device. However, the influence of doping in separate parts of an active channel layer is yet to be investigated. This is most likely due to the difficulties encountered in precise doping during the fabrication of these films, as most of them still rely on sputtering or other fast rate deposition schemes. ALD operates on a much slower basis, as mentioned above, and hence allows us to thoroughly examine the consequence of doping endemic to explicit regions of a thin film. Also, unlike conventional metal oxide semiconductor field effect transistors (MOSFETs), the active channel material in TFT structures is usually amorphous or polycrystalline in nature and has thicknesses that usually fall in the finite regime in semiconductor physics. This means that some of the assumptions made to derive classical formulae in MOSFETs are no longer viable, and terms such as accumulation width can no longer be accurately calculated for TFTs. Since local doping by ALD can not only vary doping composition but also doping position, it could provide empirical means to determine the accumulation width in these structures.3 Therefore, the relatively simple ALD system of Al doped SnO2 was selected to probe the features of these concepts. SnO2 is a well-known semiconductor material that is widely used in various fields of applications, with TFTs, sensors, and solar cells being the most notable. In this study, the effects of Al concentration and doping position on electrical and chemical properties of doped SnO2 TFTs were also investigated. The experimental results given here show that the local doping concept mentioned here has great potential to be applied to various fields of research in future, and especially in the field of tuning the transfer curves of TFTs. The electrical results show that with the increase of Al contents from 0 to 1/50 cycle doping ratio, the resistivity increased from 2.5 x 10-1 Ω-cm. The Al(III) acts as an acceptor to Sn(IV), which increases the resistivity of thin film. Since SnO2 has a very low in resistance, is not affected by the flied effect of the channel. As the Al concentration increases, the carrier concentration of the channel is controlled and the switching characteristic appears. However, if the Al concentration becomes too high, the characteristics of the channel will be lost. The depletion region is formed at the doping position, and effect channel area is reduced accordingly. When the doping position is the back channel, the effect channel region is sufficiently secured to exhibit the best device characteristics. When Al doped to the front channel, the back channel effect and the reduction of the effect channel area can be confirmed. When Al doped to the intermediate position, the TFT characteristics are greatly reduced by the depletion region formed across the channel.