Results and Discussion
4.1 The influence of illumination .1 Experiment
In this section, there are three subjects to be discussed: (1) The influence of illumination under atmosphere. (2) The influence of illumination under vacuum. (3) The different wavelength of illumination.
We set up an experiment to measure I-V characteristics by using a HP4156C semiconductor parameter analyzer. First, the steady state of ZnZrO TFTs is measured repeatedly under dark at atmosphere, this is the standard electrical characteristics of ZnZrO TFTs. Second, the VD-ID and VG-ID in saturation regime (VD=30V) are measured after illuminating 10 sec. and 30 min. at atmosphere. Then
the light turned off and measured electrical characteristics of ZnZrO TFTs after illumination under dark with different time, including 5min, 1hr and 4hr.
Next, compare the electrical characteristic of ZnZrO TFTs at vacuum and atmosphere in the same measured conditions. In the last subject, the VD-ID and VG-ID in saturation regime (VD=30V) are measured by different wavelength of illumination.
4.1.2 The influence of illumination under atmosphere
Fig. 4.2 shows the VG-ID transfer characteristics of ZnZrO TFTs after illuminating 10 sec. and 30 min. under atmosphere. It shows that the characteristics of ZnZrO TFTs were influenced by illuminated time. The on-current (Ion) and off-current (Ioff) were also increased with illumination time. It is apparent that the electrical characteristics of ZnZrO TFTs transfer to conductor after 30min of illumination. It shows that the ZnZrO TFTs is very photo-sensitive to illumination time. Especially, the on/off ratio of ZnZrO TFTs varies significantly with the increase of illumination time. This is photoconductivity phenomenon that show to be entirely due to desorption of chemically adsorbed oxygen atoms from the surfaces of the porous sample [4.1]. This indicates that the light of intensity near the fundamental absorption edge forms electron-hole pairs of which the hole is captured by the oxygen on ZnO surface, converting the oxygen to physically adsorbed as time increasing [4.2,4.3].
Fig. 4.3 shows the VG-ID transfer characteristics of ZnZrO TFTs illuminate by 30min and then hold on for 5min, 1hr and 4hr under dark environment at atmosphere. It shows that the characteristics of ZnZrO TFTs were influenced by relaxation time, which recovers to standard electrical characteristics of ZnZrO TFTs. The on-current (Ion) and off-current (Ioff) were decreased with relaxation time.
It is apparent that the electrical characteristics of ZnZrO TFTs due to the conductor transfer to transistor after 4hr of relaxation under dark and closer to the standard.
This indicates that the oxygen is chemisorbed to ZnO surface at vacancy sites, forming O2
as increase as relaxation time increasing and resulting in a surface charge depletion layer thus leading to a reduction of the electrical conductivity and the I-V can be transferred to transistor characteristics [4.4].
Fig. 4.4 shows the transformation of drain current with illumination of 15min. It shows that the drain current of ZnZrO bulk was influenced by illumination time and relaxation time. The plot also represents that the conductivity of ZnZrO bulk was influenced by illumination time and relaxation time. Fig. 4.5 shows the physical model of effect of oxygen on the conductance of ZnO film and the interaction with light [4.1]. The increase of conductivity under illumination indicates capture of holes at grain boundaries, which are negatively charged in darkness, and a corresponding number of excess electrons are left in the conduction band. The decrease of conductivity under dark indicates that oxygen in air caused an appreciable decrease in the steady state photoconductivity of the ZnO film [4.5].
4.1.3 The influence of illumination under vacuum
Fig.4.6 shows the VG-ID transfer characteristics of ZnZrO TFTs after illuminating by 10s and 30min under vacuum. It also shows that the characteristics of ZnZrO TFTs were influenced by illumination time. Compare to the plot in Fig. 4.2, the VG-ID transfer characteristics of ZnZrO TFTs showed similar trends. The drain current of ZnZrO TFTs at vacuum was larger than that at atmosphere, where higher conductivity obtained under illumination continued for 30min even after extinguishing the light. In our previous study, we have investigated the photoconductivity effect of the ZnO to realize how the oxygen will influence the
performance of the ZnZrO TFTs.
Fig. 4.7 shows the VG-ID transfer characteristics of ZnZrO TFTs illuminate by 30min and then hold on for 5min, 1hr and 4hr under dark environment at vacuum.
Especially, the electrical characteristics of ZnZrO TFTs illuminate by 30min after 4hr relaxation at vacuum still show a conductor characteristic. Compare to the plot in Fig. 4.3, the recovery of the device at vacuum is less than the device at atmosphere. It is a contrary result to illumination data for the ZnZrO TFTs at atmosphere and the ZnZrO TFTs at vacuum.
Consequently, the ZnZrO TFTs is more photo-sensitive to illumination at vacuum than at atmosphere, cause to the physically adsorbed oxygen easily desorbs and the electron remains to increase the conductivity. In addition, we can infer that the devices of ZnZrO active channel layers were fabricated by the sol-gel spin-coating method may have much porous. Through this pores, oxygen gas may diffuse rather freely into the interior part of the ZnZrO film from the surface. It indicates that the result obtained in this study is that the oxygen reacts with the electron under atmosphere is more than under vacuum.
4.1.4 The different wavelength of illumination
Fig. 4.8 shows the VG-ID transfer characteristics of ZnZrO TFTs under different wavelength of illumination. The on-current (Ion) and off-current (Ioff) were increased with the light of wavelength reduce. The general trend as wavelength decrease is a decrease in the on/off ratio of ZnZrO TFTs. When the transfer characteristics are measured with incident monochromatic light and as wavelength decreases, several changes in the electrical properties of the ZnZrO TFTs are verified as can be seen in Fig. 4.8. It is well-known the energy of the photons (less than 2 eV) is too low to be absorbed by the ZnZrO film that has an optical bandgap
around 3.2 eV [4.6], it is apparent that the ZnZrO TFTs is very photo-sensitive to illumination and also represents the ZnZrO films have traps, cause to emerge evidently the phenomenon.
This reason of a factor among them is the same to photodesorption as previously mentioned, which is that the light of intensity near the fundamental absorption edge forms electron-hole pairs of which the hole is captured by the oxygen on ZnO surface, converting the oxygen to physically adsorbed as decrease as wavelength. Then the conductivity obviously increased that the electron concentration increases in bulk. In other words, we inferred that the light with energy higher than the fundamental absorption band of the ZnZrO film is illuminated lead to a rise of photogenerated hole-electron pairs as the wavelength reducing [4.7,4.8].
4.2 The influence of ambient gas