Broadband Photodetectors Based on c-Axis Tilted Layered Cobalt Oxide Thin Films

Laser-induced voltage (LIV) effects in c-axis tilted thin films of YBa2Cu3O7- (YBCO) and La1-xCaxMnO3 (LCMO) have been extensively studied in the past decades due to their potential applications in photodetectors [1-9]. Compared to the commonly used photodiodes-based photodetectors, this new type of photodetectors has the advantage of broad spectrum response ranging from ultra-violet (UV) to infrared (IR). Other advantages of this type of detectors consist in that they can be operated without cryogenic cooling and bias voltage.


Introduction
Laser-induced voltage (LIV) effects in c-axis tilted thin films of YBa 2 Cu 3 O 7- (YBCO) and La 1-x Ca x MnO 3 (LCMO) have been extensively studied in the past decades due to their potential applications in photodetectors [1][2][3][4][5][6][7][8][9].Compared to the commonly used photodiodes-based photodetectors, this new type of photodetectors has the advantage of broad spectrum response ranging from ultra-violet (UV) to infrared (IR).Other advantages of this type of detectors consist in that they can be operated without cryogenic cooling and bias voltage.
The origin of the LIV signal was explained as result of the transverse thermoelectric effect which becomes effective when the c axis of YBCO or LCMO films are tilted by an angle  with respect to the film surface normal [8].The induced voltage can be quantitatively described by the equation Where T is the temperature difference between the top and bottom of the film, which is generated by heating the film surface due to the absorption of the incident laser radiation; S=S ab -S c is the difference of the seebeck coefficient in the ab-plane and along the c-axis of the film;  is the tilted angle of the film with respect to the surface normal; d is the thickness of the tilted film and  is the laser spot diameter [1].According to this equation, searching for materials having large anisotropy in seebeck coefficient, that is large S, is a key factor for developing this new type photodetectors.
Recently, layered cobalt oxides including Na x CoO 2 , Ca 3 Co 4 O 9 , Bi 2 Sr 2 Co 2 O y and etc. have attracted great attention as promising thermoelectric materials due to their good thermoelectric performance as well as the good thermal stability, lack of sensitivity to the air and non-toxicity [10][11][12][13][14].The crystal structure of these cobalt oxides consists of the conducting CoO 2 layer and the insulating Na, Ca 2 CoO 3 or Bi 2 Sr 2 O 4 layer, which are alternately stacked along the c-axis.This layered structure results in a large anisotropy of the seebeck coefficient in the ab-plane and along the c-axis of the film, realizing ∆S =S ab −S c of tens of V/K, which is about several times larger than that of YBCO (~ 10 V/K) and hundreds times larger than that of LCMO (~ 0.22 V/K ) [3,4].

LIV measurements
Fig. 2 presents the schematic illustration of the LIV measurements.Two indium or gold electrodes with the diameter of ~ 0.4 mm were symmetrically deposited on the film surface along the inclined direction and they were separated by 4 mm.To prevent the generation of any electric contact effect, the electrodes were always kept in the dark.A XeCl excimer pulsed laser (λ=308 nm, t p ~20 ns) and an Nd:YAG pulsed laser (λ=532 and 1064 nm, t p ~25 ps) were used as the light sources.The incident laser beam, adjusting to 2 mm in diameter using an aperture, was directed perpendicular to the film surface at the middle position between the two electrodes.The induced lateral voltage signals were recorded using a digital oscilloscope of 500 MHz bandwidth terminated into 1 M (Tektronix, TDS 3052).To reduce the influence of the RC measurement circuit on the response time, a 2 Ω load resistance is connected parallel with the tilted film while keeping other experimental conditions unchanged.As shown in Fig. 4, the rise time is dramatically decreased from 100 ns shown in Fig. 2a to about 6 ns and the FWHM is also decreased from 470 ns to be about 20 ns.It should be mentioned here that the response time has a pronounced dependence on the pulse width of the incident lasers.The smaller pulse width usually leads to a faster response time.
For example, the FWHM of the induced voltage signal is reduced to 1-2 ns when the same film is irradiated by the Nd:YAG picosecond laser pulse.The nanosecond-scale response of the tilted Bi 2 Sr 2 Co 2 O y thin film to different laser pulses ranging from UV to NIR reveals that it has a potential application in broadband photodetectors with fast response.

Dependence of the peak voltage of the LIV response on the laser energy, tilted angle and film thickness
Fig. 5a shows the dependence of the peak value of the open-circuit voltage signal (V p ) in the tilted Bi 2 Sr 2 Co 2 O y thin film on the 308 nm laser energy on the film surface.A linear dependence is obtained for laser energy below the destruction limit of the film.The dependence of the LIV signal of the tilted Bi 2 Sr 2 Co 2 O y film on the tilted angle  is also investigated and an almost linear relationship between the V p value and  is obtained, seen in Fig. 5b.This Usin2 dependence again demonstrates that the LIV effect in the caxis tilted layered cobalt oxides thin films mainly originates from the transverse thermoelectric effect since all other light-induced effects do not shown such tilt angle dependence.
Fig. 5c presents V p as a function of the film thickness under 308 nm laser radiation.The V P increases with the decrease of film thickness and reaches a maximum value when the film thickness is about 100 nm, and then V p turns to a reduction with further decreasing film thickness from 100 nm to 60 nm.This is inconsistent with Eq. ( 1) where V p increases monotonically with decreasing d.Similar V p -d dependence was observed in the LIV measurements for c-axis tilted YBCO and LCMO films.An improved equation based on the plane heat source and cascade power net model was proposed to explain this abnormal behavior.Calculations based on this improved equation revealed that V p was no more monotonic variation with the thickness d and there existed an optimum thickness corresponding to a maximum peak of the induced signal [9].The responsivity is calculated to be about 11 V/mJ.It is much larger than the responsivity of YBCO, LCMO and other two cobalt oxide thin films [1-4, 18, 19].This might be due to the Na x CoO 2 film has better crystal quality which corresponds to a larger ΔS, and the exact reason needs to be clarified in our future work.

Conclusion
In conclusion, the performed investigations of the LIV effect in c-axis tilted Na x CoO

Fig. 1a -
Fig.1a-cpresents the x-ray diffraction (XRD) -2 scans of the c-axis tilted cobalt oxide thin films on 10 o tilted substrates.The offset angle  is the angle between the c-axis direction and the substrate surface-normal direction and it is set as 10 o (See the inset of Fig.1a).Apart from the substrate peak, all peaks in these patterns can be indexed to the (00l) diffractions of the corresponding layered cobalt oxides, indicating that phase-pure c-axis tilted Na x CoO 2 , Ca 3 Co 4 O 9 and Bi 2 Sr 2 Co 2 O y thin films are obtained and the tilted angle is 10 o .

Fig. 1 .
Fig. 1.XRD -2 scans for the c-axis tilted (a) Na x CoO 2 , (b) Bi 2 Sr 2 Co 2 O y and (c) Ca 3 Co 4 O 9 thin film on 10 o tilted single crystal substrates.The inset of Fig. 1a is the sketch map of the XRD -2 measurement and the offset angle  is set as 10 o .

3. Results and discussion 3 . 1
Fig.3a-cpresents the response of a 10 o tilted Bi 2 Sr 2 Co 2 O y thin film (~ 100 nm) to the laser pulses of 308 nm, 532 nm and 1064 nm, respectively.Large open-circuit signals with the response time in the order of several hundred nanoseconds are detected for these three wavelengths.The voltage responsivity is calculated to be about 440 mV/mJ, 348 mV/mJ and 65 mV/mJ for 308 nm, 532 nm and 1064 nm pulsed radiations respectively.The stronger LIV signals obtained for the UV 308 nm and visible 532 nm lasers than that for the NIR 1064 nm laser can be explained by considering the different optical absorption and penetration depth of the radiations with different wavelength in Bi 2 Sr 2 Co 2 O y film.The higher absorption and smaller penetration depth of 308 nm and 532 nm radiation in comparison with these of the 1064 nm radiation lead to a larger T and thus a larger induced voltage.

Fig. 3 .Fig. 4 .
Fig. 3. LIV signals of a 10 o c-axis tilted Bi 2 Sr 2 Co 2 O y thin film (~ 100 nm) when its surface is irradiated by the pulsed lasers with different wavelength of (a) 308 nm, (b) 532 nm and (c) 1064 nm.The input impedance of an oscilloscope is 1 MΩ and the laser energy on the sample is about 2.5 mJ.

Fig. 5 .
Fig. 5. Dependence of the peak value of the induced open-circuit voltage (V p ) on the (a) laser energy on the film surface, (b) tilted angle  and (c) film thickness.Solid lines are guide for eyes.

3. 3 Fig. 6 .Fig. 7 .
Fig. 6.A typical LIV signal of a 10 o c-axis tilted Ca 3 Co 4 O 9 thin film (~ 100 nm) under the 308 nm laser radiation.The laser energy on the sample is about 1 mJ.

. Sample preparation and LIV measurements 2.1 Fabrication of c-aixs tilted Na x CoO 2 , Ca 3 Co 4 O 9 and Bi 2 Sr 2 Co 2 O y thin films The
c-axis inclined Na x CoO 2 thin film was obtained by epitaxially growing a layer of Na x CoO 2 (x~0.7) on a tilted Al 2 O 3 (0001) single crystal substrate by topotaxially converting an epitaxial CoO film to Na x CoO 2 with annealing in Na vapor.A CoO film was first epitaxial grown on the tilted Al 2 O 3 (0001) by pulsed laser deposition.The CoO film was then sealed in an alumina crucible with NaHCO 3 powder and heated to 700-750°C for 60 min to form the Na x CoO 2 film.The c-axis tilted Ca 3 Co 4 O 9 and Bi 2 Sr 2 Co 2 O y thin films can be grown on the tilted LaAlO 3 (001) or Al 2 O 3 (0001) substrates with the pulsed laser deposition (PLD) or chemical solution deposition (CSD) methods.The detailed PLD and CSD fabrication parameters can be found in Ref.15-17.Transport measurements on these films reveal that they have the room temperature seebeck coefficient comparable to that of the single crystals, suggesting good quality of these films.
2 , Bi 2 Sr 2 Co 2 O y and Ca 3 Co 4 O 9 thin films at room temperature show that large open-circuit lateral voltage signals with the rise time in the order of several nanoseconds are measured when the surface of these films is irradiated by different laser sources with the wavelength ranging from UV to NIR.The obtained results suggest that c-axis tilted cobalt oxide thin films can be used for broadband photodetectors with fast response.