Chromium – Niobium Co-Doped Vanadium Dioxide Films : Large Temperature Coefficient of Resistance and Practically No Thermal Hysteresis of the Metal – Insulator Transition *

基 盤 技 術 Vanadium oxides exhibit a temperature-induced metal–insulator transition (MIT) with a discontinuous change in electrical conductivity of several orders of magnitude. The MIT is the first-order structural transition and is also accompanied by a marked change in the optical transmittance in the infrared (IR) region. Among the various oxides of vanadium, vanadium dioxide (VO2) is the most interesting from an application perspective because its MIT occurs at around 340 K, which is above room temperature. An MIT above room temperature is useful in a variety of functional devices, such as electrical switches, gas sensors, smart windows, uncooled bolometers, or thermal memories. Among these devices, VO2-based uncooled bolometers that detect far-IR radiation have been actively studied and developed for several decades. One measure of the suitability of a material for use in Chromium–Niobium Co-Doped Vanadium Dioxide Films: Large Temperature Coefficient of Resistance and Practically No Thermal Hysteresis of the Metal–Insulator Transition*


INTRODUCTION
＊ AIP advances 6, 055012 (2016) より転載。 a bolometer is its temperature coefficient of resistance (TCR), which is defined as | (1/ρ)(dρ/dT) |, where ρ is the resistivity or resistance and T is the temperature of the material.][12] However, VO 2 shows a large thermal hysteresis in the ρ-T curve across the MIT.The hysteretic behavior indicates the coexistence of two phases over a finite temperature range due to superheating and supercooling effects, which is a characteristic of the first-order transition.
The thermal hysteresis in the ρ-T curve results in poor measurement reproducibility in IR sensing.
Consequently, thermal hysteresis has to be minimized to realize high-sensitivity uncooled bolometers based on VO 2 .

Doping of VO 2 with metal ions has been employed
as a means of suppressing its thermal hysteresis; 13,14) however, doping with metal ions also gives rise to a reduction in the TCR of VO 2 .We previously conducted a systematic study of the TCR and thermal hysteresis in VO 2 doped with Cr or with Nb, and we found that there is a correlation between the TCR and thermal hysteresis, which is independent of the doping element 15) .Our findings implied that a high TCR and the absence of thermal hysteresis were difficult to achieve simultaneously in single-element doped VO 2 .
However, Soltani et al. reported that co-doping of VO 2 with Ti and W suppresses thermal hysteresis more effectively than does doping with W alone 16) .In their study, they simultaneously achieved a practical absence of thermal hysteresis and a TCR of 5.12%/K at room temperature in V 0.866 W 0.014 Ti 0.12 O 2 films.This TCR is larger than that of conventional uncooled bolometer materials.However, it is still challenging to achieve the high TCR values in excess of 10%/K that are required for high-sensitivity uncooled bolometers.
In this study, we explored the possibility of obtaining high TCR values with no thermal hysteresis by codoping VO 2 films with Cr and Nb.Note that V, Cr, and Nb ions are tetravalent (4+), trivalent (3+), and pentavalent (5+), respectively, and that their effective radii are 0.058, 0.062, and 0.064 nm, respectively.We previously reported that Nb doping is effective in reducing the thermal hysteresis of VO 2 ; 15) however, it also causes a rapid decrease in its TCR.In contrast, the decrease in the TCR of VO 2 on doping with Cr is moderate, but Cr doping is less effective than Nb doping in reducing the thermal hysteresis.These differing effects on the TCR and thermal hysteresis might be due to the differences in the valence states and/or ionic radii of the Cr and Nb ions 15) .Because From ρ-T measurements on the V 1−x−y Cr x Nb y O 2 films, we derived a TCR and thermal-hysteresis-width (ΔT MI ) diagram for the films.This diagram revealed that doping conditions of x ≳ y and x + y ≥ 0.1 are suitable for producing films that show no thermal hysteresis while retaining a large TCR.We also succeeded in producing a large TCR of 11.9%/K and practically no thermal hysteresis in V 0.90 Cr 0.06 Nb 0.04 O 2 films fabricated on TiO 2 -buffered SiO 2 /Si substrates at process temperatures below 670 K. as targets.The doping range for Cr was x = 0-0.12and that of Nb was y = 0-0.09.We have previously

EXPERIMENTAL
described the detailed conditions for growth of such films on Al 2 O 3 15) or TiO 2 /SiO 2 /Si substrates 17) .The film thickness was set at 70-110 nm, as confirmed by using a surface profiler.Note that there were no significant differences in the structural or electronic properties of the films within this thickness range.The resistivity of the films was measured by conventional four-probe methods using Ti/Au electrodes.Transport properties were examined by using a physical property measurement system (PPMS; Quantum Design), and the temperature sweep rate was set to 0.3 K/min.The results of single-element doping 16) are also plotted for comparison.It is well known that hole doping by lower-valence elements such as Cr 3+ or Al 3+ raises the T MI , whereas electron doping with higher-valence elements such as Nb 5+ or W 6+ lowers the T MI 15,[18][19][20] .
These tendencies are maintained in co-doped VO 2 .
As seen in the top panels in  and to lattice deformation and/or defects 21,22) .As shown in the middle panels in Fig. 2, for singleelement doping with Cr or Nb, the maximum TCR decreased monotonically with increasing dopant content, with Nb doping having the greater effect.
In contrast to single-element doping, co-doped V 0.95- The bottom panels in Fig. 2 show that for singleelement doping, ΔT MI also decreases monotonically with increasing dopant content and that it is reduced more efficiently by doping with Nb than with Cr.
Note that the ΔT MI is defined as the difference in the temperatures at which a film has a given resistivity (ρ MI ) during the heating and cooling phases.For this study, we choose ρ MI as the value of ρ at the temperature of the TCR peak in the heating process 15) .In contrast To further explore the optimal composition of the doped films, we examined the dependence of TCR and ΔT MI on x and y for V For uncooled bolometer applications, VO 2 films should be integrated onto Si platforms through a back-end-of-line (BEOL) process.We previously reported that TiO 2 buffer layers permit the fabrication of VO 2 films that show a sharp MIT on SiO 2 /Si (100) substrates at process temperatures below 670 K, which is compatible with a BEOL process 17) .By using the TiO 2 -buffer technique, we deposited V 1− x−y Cr x Nb y O 2 films on SiO 2 /Si (100) substrates to realize both large TCR values and an absence of thermal hysteresis.Fig. 4 shows the ρ-T curves for the Cr and Nb dopants are effective in maintaining large TCR values and in reducing the thermal hysteresis, respectively, co-doping with Cr and Nb might give rise to a combination of the desirable effects of the two individual ions.

Fig. 1 (
Fig. 1(a) and Fig. 1(b) show the ρ-T curves for the V 0.95−x Cr x Nb 0.05 O 2 and V 0.95−y Cr 0.05 Nb y O 2 films on Al 2 O 3 substrates with 0 ≤ x ≤ 0.12 and 0 ≤ y ≤ 0.08, respectively.As a reference, the ρ-T curve for a nondoped VO 2 film is also shown in Fig. 1(b).A systematic change in T MI with doping was observed.Here, T MI is defined as the halfway point between the temperatures of the two peaks in the TCR for the heating and cooling processes, respectively.addition to the change in T MI , the doping affected the values of the TCR and ΔT MI .The co-doped films showed a broadening of the MIT, and the change in ρ across the MIT for the co-doped films was smaller than that for the nondoped film.These behaviors caused a decrease in the TCR of the co-doped films.Moreover, the codoped films had a smaller ΔT MI compared with the nondoped film.

Fig. 2 (
Fig. 2(a) and Fig. 2(b) show the dependence of T MI , the maximum TCR, and ΔT MI on the total dopant content x + y for the V 0.95−x Cr x Nb 0.05 O 2 and V 0.95− y Cr 0.05 Nb y O 2 films on Al 2 O 3 substrates, respectively.The results of single-element doping16) are also plotted

Fig. 2 ,
Fig. 1 Temperature dependence of the resistivity of (a) V 0.95−x Cr x Nb 0.05 O 2 and (b) V 0.95−y Cr 0.05 Nb y O 2 films.As a reference, the ρ-T curve for the nondoped VO 2 film (dashed line) is plotted in (b).

Fig. 2
Fig. 2 Cr content (x) and Nb content (y) dependence of T MI , TCR, and ΔT MI for (a) V 0.95−x Cr x Nb 0.05 O 2 and (b) V 0.95−y Cr 0.05 Nb y O 2 films.An increase in the TCR in the V 0.95−x Cr x Nb 0.05 O 2 (x = 0.02, 0.05, and 0.08) films with respect to that of the V 0.95 Nb 0.05 O 2 (x = 0) film is highlighted.

Fig. 2 (
Fig. 2(b)].These results suggest that the presence of Cr dopant with the condition x ≳ y is essential for obtaining large TCR values in V 1−x−y Cr x Nb y O 2 films.

3 . 3 .
1−x−y Cr x Nb y O 2 films on Al 2 O 3 substrates and we derived the diagram for the TCR and ΔT MI of the V 1−x−y Cr x Nb y O 2 films shown in Fig.As x and y increase, the TCR value decreases monotonically.Relatively large TCR values were obtained near the line x = y.Furthermore, the TCR contour lines/domains are asymmetric with respect to this line.This asymmetry indicates that a condition of x ≳ y is suitable for obtaining a large TCR for V 1−x−y Cr x Nb y O 2 films, as mentioned earlier.In contrast to the TCR, ΔT MI does not show a clear trend with x and y.However, the diagram confirms that a practical absence of thermal hysteresis can be obtained for co-doped V 1−x−y Cr x Nb y O 2 films with x + y ≥ 0.1.Therefore, because the total dopant content x + y should be as small as possible to obtain a large TCR, the optimal composition can be expected to be near the line x + y = 0.1 with the condition x ≳ y, as shown by the solid circle in Fig.In fact, among the films that showed practically no thermal hysteresis, the V 0.90 Cr 0.06 Nb 0.04 O 2 film showed the best TCR of 16.2%/K.Next, we will briefly discuss the effects of Cr and Nb co-doping on the MIT of VO 2 .One of the important effects of Cr and Nb co-doping is that of charge compensation.Because Cr and Nb ions are trivalent and pentavalent, respectively, Cr and Nb co-doping of VO 2 has less overall effect on the change in the valence state of V 4+ ions than does single-element doping.Therefore, any inhomogeneity of carrier concentration that results in a spatial variation in T MI should be reduced in co-doped VO 2 films.As a result, broadening of the MIT is suppressed, leading to an improvement in the TCR of the co-doped films.However, because of the different ionic radii of V, Cr, and Nb ions, Cr and Nb co-doping still induces lattice deformations and defects in VO 2 .We previously reported that Cr doping suppresses the lattice changes in VO 2 across the MIT that originate from a structural phase transition from a high-temperature tetragonal phase to a low-temperature monoclinic phase15) .The suppression of this lattice change in the doped films might be the cause of the decrease in the TCR and ΔT MI .Because different doping elements induce different low-temperature monoclinic phases in VO 218,19,23) , the lattice change across the MIT in the co-doped films is expected to be more complicated than that in the single-element doped films.To gain a better understanding of the effects of co-doping on the TCR and ΔT MI , detailed investigations of the structural properties of the co-doped VO 2 films are required, and will be a subject of a further study.

Fig. 3 Fig. 4
Fig. 3 TCR and ΔT MI diagram for V 1−x−y Cr x Nb y O 2 films as a function of the Cr and Nb contents.Values of ΔT MI are divided into three categories: ΔT MI ≤ 0.6 K (filled circles), 0.6 K < ΔT MI ≤ 1.0 K (open circles), and ΔT MI > 1.0 K (crosses).The TCR values are classified into three regions: >10%/K, >15%/K, and >30%/K to the TCR value, the x + y dependence of ΔT MI for both V 0.95 − x Cr x Nb 0.05 O 2 and V 0.95 − y Cr 0.05 Nb y O 2 films approached that of V 1−y Nb y O 2 films.This result x Nb y O 2 films, the conditions x ≳ y and x + y ≥ 0.1 are essential for obtaining large TCR values in excess of 10%/K and a near absence of thermal hysteresis (ΔT MI ≤ 0.6 K), respectively.As seen in Fig.2(a), large TCR values of 16.7%/K with a ΔT MI ≈ 0.9 K or 13.6%/K with ΔT MI ≈ 0.6 K were attained with V 0.90 Cr 0.05 Nb 0.05 O 2 (x = 0.05, y = 0.05) and V 0.87 Cr 0.08 Nb 0.05 O 2 (x = 0.08, y = 0.05) films, respectively.