Charge transport in ZnO based thin-film transistors
7.2 Temperature dependent field-effect measurements
Temperature dependent field-effect measurements were performed on inverted staggered ZnO TFTs fabricated on Si/SiO2 substrate using Al as S-D contacts, as shown in Figure 7.1. Thin films of ZnO were deposited at 400°C using the spray pyrolysis method discussed in Chapter 4. Electrical characterisation was
performed in high vacuum (10-6 mbar) and in the dark using a Keithley 4200 semiconductor parameter analyser. The device temperature was regulated by a temperature controller with a precision of ±1 K. Before each measurement, the TFTs were placed on the heated chuck, set at the desired measurement temperature, for 15 min in order to allow for thermal equilibrium to be reached. At each temperature the drain current (ID) was recorded as a function of the gate current (VG) across the temperatures range of 100-400 K. Measurements were obtained for several ZnO TFTs with channel width (W) of 1 mm and channel length (L) in the range 70-100 μm.
7.2.1. Temperature dependent current-voltage characteristics
The transfer characteristics for a ZnO TFT measured in the linear operating regime (VD = 2 V) at different temperatures are shown in Figure 7.1. It can be seen that as temperature increases the conductivity of the ZnO film increases.
-20 -10 0 10 20 30 40 50
10-10 10-9 10-8 10-7 10-6 10-5 10-4
ZnO Si
Al Al
SiO2
I D(A)
415 K
101 K VD= 2 V
VG(V)
Figure 7.1: The transition of the transfer characteristics for ZnO TFT with W/L = 1000/70 m, depending on various temperatures measured. There is a consistent shift in these characteristics to the negative direction with increasing temperature from 100K to 415K. The VT of pristine ZnO TFTs is decreased by approximately 20 V, i.e. from 31.5 V to 11.5 V. It is caused by increasing of thermally activated electrons with rise in measured temperature. The inset shows the device structure used for these measurements.
The switch-on voltage also is found to shift towards more negative gate voltages. From these measurements it can be concluded that ID is thermally activated as it increases by one order of magnitude. The only exception is the small drop observed at T > 400°K. The latter is most likely attributed to band like transport and will be discussed in the next section. The increase in conductivity with temperature can be accounted for by rise in carriers contributed from the donor states. These thermally activated electrons then reduce the threshold voltage of the device.
7.2.2. Temperature dependence of electron mobility in ZnO transistors
In order to understand the temperature dependence of the average electron mobility (μave) i.e. the average value between free and trapped carriers as discussed in Chapter 3, transfer characteristics were measured in the linear operating regime (i.e. VD = 2 V where the in-plane electric field is much smaller than the applied VG, which results in an approximately uniform density of charge carriers in the active channel) at different temperatures. Figure 7.2 displays the ln(μave) versus 1000/T for various gate voltages. These characteristics are clearly indicative of thermally activated electron transport having two different conduction regimes. For temperatures > 400 K (region C) a fall in the electron mobility is observed marking the onset of band like transport. The latter implies that electrons above the mobility edge are transported in the delocalized states forming the conduction band. As temperature increases phonon scattering becomes dominant and the electron mobility reduces.For lower temperatures the average electron mobility reduces and exhibit two different regimes: regime A for T < 200 K and Regime B for 200 K< T < 400 K. The latter observation possibly suggests two different competing mechanisms with different activation energies. In both temperature regimes electron transport is dominated by recurrent trapping into localised trap states below the mobility edge followed by thermal activation into delocalised states above the mobility edge, as discussed in Chapter 3. In order to better understand and interpret this data the Multiple Trap and Release (MTR) model has been employed. These results are discussed next.
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Figure 7.2: Arrhenius plots of the average electron mobility at different gate voltages measured from a ZnO transistor. The dashed lines demarcate the different transport regimes. A and B correspond to thermally activated transport under different mechanisms and region C corresponds to band like transport. Inset is a magnified view of region C.
7.2.3. Meyer Neldel rule and multiple trapping
The variable range hopping (VRH) model predicts a linear relationship between ln(μ) and T1/4 (11-13). Thus, the Arrhenius relationship shown in Figure 7.3 excludes the possibility of a classic hopping type transport in our sample. These Arrhenius fits for two regions are separately shown in Figure 7.3(a-b). Upon behaviour is related to both the spatial distribution of defects in the active layer and to the density of sub-gap states, it has often been used to quantify the quality of the
semiconductor layer in TFTs (31, 32). From our measurements, the high values of kTo are most likely associated with large trap density and the high degree of disorder within the ZnO layer. In the lower temperatures regime (A; < 200 K) no such values could be extracted since the linear fits did not intersect at a common point [Figure 7.3(b)]. The latter observation most likely suggests that a different charge transport process may be occurring in this part of the temperature region.
0 1 2 3 4 5
Figure 7.3: Arrhenius plots of the average electron mobility of ZnO TFT at various gate voltages for; (a) high temperature region B, and (b) low temperature region A. Linear fits are intersecting at a single point only for region B from which the Meyer-Neldel energy (kTo) can be estimated.
From the slopes of the Arrhenius plots, the activation energy (Ea) for the electron mobility was calculated. These data are displayed in Figure 7.4(a). This evolution of Ea with VG, for both temperature regimes, reflects the distribution of DOS in the sub-band gap region. Specifically, the decreasing trend of Ea indicates that as VG increases the energy distance between the filled trap states and the band/mobility edge reduces due to shifting of the Fermi level (27). The derived activation energies of 25-61 meV and 65-151 meV correspond to the shallow donors as well as the defect states in ZnO. Both values are in agreement with those reported literature for ZnO prepared by different deposition methods (33-36).
Finally, the activation energy can be fitted using Equation 3.10, hence providing further confirmation on the validity of the trapping model for describing electron transport in ZnO transistors fabricated by spray pyrolysis.
Further evidence of the trap states controlled transport can be obtained from the evolution of VT versus T shown in Figure 7.4(b). In this figure, VT
increases linearly with temperature all the way from 31.5 V to 11.5V. The temperature coefficient (KVT) calculated from this set of data is found to be approximately -60mV/ K. This relatively high value suggests that the variation of VT with temperature is the result of a thermally activated transport process and may be explained in terms of trapping and thermal activation of free electrons.
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Figure 7.4: (a) Activation energy (Ea) of electron transport as a function of VG in both low (region A; < 200°K) and high (region B; > 200°K) temperature regimes. Inset shows the same data fitted using Eq. 3.10 using a value for VT = 12 V. (b) VT and Von as a function of measuring temperature.
At higher temperatures, more electrons can escape from the localized states, because of the increased emission rates and/or shorter trapping. This is manifested in smaller value for VT i.e. the potential at which electron accumulation begins. Almost similar temperature dependence is observed for Von. It is worth noting that the measured temperature dependences of μ and VT are similar to those observed for poly-Si TFTs and commonly explained by invoking trapping and thermal activation of free carriers (37).