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

Shufang Wang\* and Guangsheng Fu

*Hebei Key Lab of Optic-Electronic Information and Materials, Hebei University, Baoding PR China* 

### **1. Introduction**

164 Photodetectors

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ISSN 0003-6951

Ultrasensitive hot-electron nanobolometer for Terahertz Astrophysics, *Nature* 

photodetectors at low temperatures, *Applied Physics Letters*, Vol. 76, pp. 206 1-3,

Resonant Photo-Coupler for Charge-Sensitive Infrared Phototransistors, *IEEE* 

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.

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

$$
\Delta U = \frac{l}{2d} \sin(2\alpha) \Delta S \Delta T \tag{1}
$$

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=Sab-Sc 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 NaxCoO2, Ca3Co4O9, Bi2Sr2Co2Oy 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 [10-14]. The crystal structure of these cobalt oxides consists of the conducting CoO2 layer and the insulating Na, Ca2CoO3 or Bi2Sr2O4 layer, which are alternately stacked along the *c*-axis. This layered structure results in a large anisotropy of the

<sup>\*</sup> Corresponding Author

Broadband Photodetectors Based on *c*-Axis Tilted Layered Cobalt Oxide Thin Films 167

**Al2**

**004**

**O3**

**10 20 30 40 50 60 70 80**

**10 20 30 40 50 60**

**10 20 30 40 50 60**

scans for the *c-*axis tilted (a) NaxCoO2, (b) Bi2Sr2Co2Oy and (c) Ca3Co4O9

**2**(**degree**)

thin film on 10o tilted single crystal substrates. The inset of Fig. 1a is the sketch map of the

**2 (degree)**

004

003

LaAlO3(001)

LaAlO3(001)-K

002

measurement and the offset angle is set as 10o.

**0 0 14**

**LaAlO3(002)**

**0 0 16**

006

LaAlO3(002)

LaAlO3(002)-K

005

**0 0 12**

**2 (degree)**

**0 0 10**

**0 0 8**

**LaAlO3(001)**

**LaAlO3(001)-K**

**006**

**detector**

**008**

**0 0 18**

**x-xay source**

**c-ax si**

**n**

**101**

**101**

**102**

**103**

**104**

**Intensity** (**arb. units**)

Fig. 1. XRD

XRD *-*2 *-*2 001

**(c)**

**105**

**106**

**102**

**103**

**104**

**Intensity (arb. units)**

**105**

**0 0 2**

**(b)**

**106**

**107**

**Intensity (arb. units)**

**002**

**(a)**

**Al2**

**0 0 6**

**0 0 4**

**O3**

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]. This fact indicates the layered cobalt oxides might have potential applications in the field of high-sensitive broadband photodetectors. In this chapter, we present our investigation of LIV effects in *c*axis tilted cobalt oxide thin films (NaxCoO2, Ca3Co4O9 and Bi2Sr2Co2Oy) with different pulsed laser sources with wavelength ranging from UV to NIR. The open-circuit voltage signals were detected in these films when their surfaces were irradiated by the laser light. The results demonstrate that the *c*-axis tilted cobalt oxide thin films have great potential applications in the broadband photodetectors.

### **2. Sample preparation and LIV measurements**

### **2.1 Fabrication of** *c***-aixs tilted NaxCoO2, Ca3Co4O9 and Bi2Sr2Co2Oy thin films**

The *c*-axis inclined NaxCoO2 thin film was obtained by epitaxially growing a layer of NaxCoO2 (x~0.7) on a tilted Al2O3 (0001) single crystal substrate by topotaxially converting an epitaxial CoO film to NaxCoO2 with annealing in Na vapor. A CoO film was first epitaxial grown on the tilted Al2O3 (0001) by pulsed laser deposition. The CoO film was then sealed in an alumina crucible with NaHCO3 powder and heated to 700- 750°C for 60 min to form the NaxCoO2 film. The *c*-axis tilted Ca3Co4O9 and Bi2Sr2Co2Oy thin films can be grown on the tilted LaAlO3 (001) or Al2O3 (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.

Fig. 1a-c presents the x-ray diffraction (XRD) *-2* scans of the *c*-axis tilted cobalt oxide thin films on 10o tilted substrates. The offset angle is the angle between the *c*-axis direction and the substrate surface-normal direction and it is set as 10o (See the inset of Fig. 1a). Apart from the substrate peak, all peaks in these patterns can be indexed to the (00*l*) diffractions of the corresponding layered cobalt oxides, indicating that phase-pure *c*-axis tilted NaxCoO2, Ca3Co4O9 and Bi2Sr2Co2Oy thin films are obtained and the tilted angle is 10o.

#### **2.2 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, tp~20 ns) and an Nd:YAG pulsed laser (λ=532 and 1064 nm, tp~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).

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]. This fact indicates the layered cobalt oxides might have potential applications in the field of high-sensitive broadband photodetectors. In this chapter, we present our investigation of LIV effects in *c*axis tilted cobalt oxide thin films (NaxCoO2, Ca3Co4O9 and Bi2Sr2Co2Oy) with different pulsed laser sources with wavelength ranging from UV to NIR. The open-circuit voltage signals were detected in these films when their surfaces were irradiated by the laser light. The results demonstrate that the *c*-axis tilted cobalt oxide thin films have great potential

**2.1 Fabrication of** *c***-aixs tilted NaxCoO2, Ca3Co4O9 and Bi2Sr2Co2Oy thin films** 

The *c*-axis inclined NaxCoO2 thin film was obtained by epitaxially growing a layer of NaxCoO2 (x~0.7) on a tilted Al2O3 (0001) single crystal substrate by topotaxially converting an epitaxial CoO film to NaxCoO2 with annealing in Na vapor. A CoO film was first epitaxial grown on the tilted Al2O3 (0001) by pulsed laser deposition. The CoO film was then sealed in an alumina crucible with NaHCO3 powder and heated to 700- 750°C for 60 min to form the NaxCoO2 film. The *c*-axis tilted Ca3Co4O9 and Bi2Sr2Co2Oy thin films can be grown on the tilted LaAlO3 (001) or Al2O3 (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

> *-2*

films on 10o tilted substrates. The offset angle is the angle between the *c*-axis direction and the substrate surface-normal direction and it is set as 10o (See the inset of Fig. 1a). Apart from the substrate peak, all peaks in these patterns can be indexed to the (00*l*) diffractions of the corresponding layered cobalt oxides, indicating that phase-pure *c*-axis tilted NaxCoO2,

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, tp~20 ns) and an Nd:YAG pulsed laser (λ=532 and 1064 nm, tp~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

Ca3Co4O9 and Bi2Sr2Co2Oy thin films are obtained and the tilted angle is 10o.

scans of the *c*-axis tilted cobalt oxide thin

applications in the broadband photodetectors.

Fig. 1a-c presents the x-ray diffraction (XRD)

films.

**2.2 LIV measurements** 

(Tektronix, TDS 3052).

**2. Sample preparation and LIV measurements** 

Fig. 1. XRD *-*2 scans for the *c-*axis tilted (a) NaxCoO2, (b) Bi2Sr2Co2Oy and (c) Ca3Co4O9 thin film on 10o tilted single crystal substrates. The inset of Fig. 1a is the sketch map of the XRD *-*2measurement and the offset angle is set as 10o.

Broadband Photodetectors Based on *c*-Axis Tilted Layered Cobalt Oxide Thin Films 169

**-1 0 1 2 3**

**Time** (**s**)

**-1 0 1 2 3**

**Time** (**s**)

**Time** (**s**)

Fig. 3. LIV signals of a 10o *c-*axis tilted Bi2Sr2Co2Oy 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

**0.0**

**0.0**

**0.3**

**0.6**

**Voltage (V)**

sample is about 2.5 mJ.

**0.9**

**(b)**

**0.3**

**0.6**

**Voltage (V)**

**0.9**

**1.2**

**(a)**

Fig. 2. Schematic illustration of the LIV measurements.

### **3. Results and discussion**

### **3.1 LIV response of the c-axis tilted Bi2Sr2Co2Oy thin films to different laser pulses**

Fig. 3a-c presents the response of a 10o tilted Bi2Sr2Co2Oy 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 Bi2Sr2Co2Oy 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.

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 Bi2Sr2Co2Oy thin film to different laser pulses ranging from UV to NIR reveals that it has a potential application in broadband photodetectors with fast response.

**3.1 LIV response of the c-axis tilted Bi2Sr2Co2Oy thin films to different laser pulses**  Fig. 3a-c presents the response of a 10o tilted Bi2Sr2Co2Oy 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 Bi2Sr2Co2Oy film. The higher absorption and smaller penetration depth of 308 nm and 532 nm radiation in comparison with these of

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 Bi2Sr2Co2Oy thin film to different laser pulses ranging from UV to NIR reveals that it has

the 1064 nm radiation lead to a larger T and thus a larger induced voltage.

a potential application in broadband photodetectors with fast response.

Fig. 2. Schematic illustration of the LIV measurements.

**3. Results and discussion** 

Fig. 3. LIV signals of a 10o *c-*axis tilted Bi2Sr2Co2Oy 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.

Broadband Photodetectors Based on *c*-Axis Tilted Layered Cobalt Oxide Thin Films 171

**0246**

**Laser energy**(**mJ**)

**0 2 4 6 8 10**

 **(degree)**

**0 100 200 300 400 500**

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

**Film thickness (nm)**

**0**

**0.0**

**0.0**

**0.3**

**0.6**

**Vp (V)**

**0.9**

**1.2**

**(c)**

**0.3**

**V**

**p (V)**

**0.6**

**0.9**

**1.2**

**(b)**

**1**

**Vp (V)**

**2**

**3**

**(a)**

Fig. 4. LIV signal of a 10o *c-*axis tilted Bi2Sr2Co2Oy thin film under the 308 nm radiation after connecting a 2 Ω load resistance in parallel with the tilted film.

#### **3.2 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 (Vp) in the tilted Bi2Sr2Co2Oy 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 Bi2Sr2Co2Oy film on the tilted angle is also investigated and an almost linear relationship between the Vp value and is obtained, seen in Fig. 5b. This Usin2 dependence again demonstrates that the LIV effect in the *c*axis 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 Vp as a function of the film thickness under 308 nm laser radiation. The VP increases with the decrease of film thickness and reaches a maximum value when the film thickness is about 100 nm, and then Vp turns to a reduction with further decreasing film thickness from 100 nm to 60 nm. This is inconsistent with Eq. (1) where Vp increases monotonically with decreasing d. Similar Vp-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 Vp 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].

**0 100 200**

**Time (ns)**

Fig. 4. LIV signal of a 10o *c-*axis tilted Bi2Sr2Co2Oy thin film under the 308 nm radiation after

**3.2 Dependence of the peak voltage of the LIV response on the laser energy, tilted** 

Fig. 5a shows the dependence of the peak value of the open-circuit voltage signal (Vp) in the tilted Bi2Sr2Co2Oy 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 Bi2Sr2Co2Oy film on the tilted angle is also investigated and an almost linear relationship between the Vp value and is obtained, seen in Fig. 5b. This Usin2 dependence again demonstrates that the LIV effect in the *c*axis 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

Fig. 5c presents Vp as a function of the film thickness under 308 nm laser radiation. The VP increases with the decrease of film thickness and reaches a maximum value when the film thickness is about 100 nm, and then Vp turns to a reduction with further decreasing film thickness from 100 nm to 60 nm. This is inconsistent with Eq. (1) where Vp increases monotonically with decreasing d. Similar Vp-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 Vp was no more monotonic variation with the thickness *d* and there existed an optimum thickness

**0**

connecting a 2 Ω load resistance in parallel with the tilted film.

corresponding to a maximum peak of the induced signal [9].

**5**

**10**

**Voltage (mV)**

**angle and film thickness** 

dependence.

**15**

**20**

**25**

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

Broadband Photodetectors Based on *c*-Axis Tilted Layered Cobalt Oxide Thin Films 173

The performed LIV measurements on the *c*-axis tilted NaxCoO2 thin films show similar dependence of Vp on the laser energy, tilted angle and film thickness. However, the *c*-axis tilted NaxCoO2 thin films have a much larger induced voltage signal than that of the Bi2Sr2Co2Oy and Ca3Co4O9 thin films. Fig. 7 illustrates the laser-induced open-circuit voltage signal of the 10o tilted NaxCoO2 film irradiated by the 308 nm pulsed laser with the laser energy on the sample of 1.5 mJ. A giant open-circuit voltage signal with Vp of 16.3 V is observed. 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 NaxCoO2 film has better crystal quality which corresponds to a larger Δ*S*, and

In conclusion, the performed investigations of the LIV effect in *c*-axis tilted NaxCoO2, Bi2Sr2Co2Oy and Ca3Co4O9 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

This work was partially supported by NSFC of China under Grant No. 10904030, SFC of

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the exact reason needs to be clarified in our future work.

films can be used for broadband photodetectors with fast response.

Hebei Province under Grant No. A2009000144 and E 2006001006.

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**4. Conclusion** 

**5. Acknowledgments** 

60, 601(1992)

313, 37(1999)

(2009)

(2009)

**6. References** 

#### **3.3 LIV signal in c-axis Ca3Co4O9 and NaxCoO2 thin films**

Similar results are also observed in the *c*-axis tilted Ca3Co4O9 thin films when their surface is irradiated by the above lasers. Fig. 6 presents a typical laser-induced open-circuit voltage signal of the 10o tilted Ca3Co4O9 thin film (~ 100 nm) under the 308 nm pulsed illumination with the laser energy on the sample of 1 mJ. Both the responsivity (~ 230 mV/mJ) and the response time (the rise time~60 ns and the FWHM~700 ns) of the LIV signal in this tilted Ca3Co4O9 thin film is in the same order as that of the tilted Bi2Sr2Co2Oy thin film with the same thickness.

Fig. 6. A typical LIV signal of a 10o *c-*axis tilted Ca3Co4O9 thin film (~ 100 nm) under the 308 nm laser radiation. The laser energy on the sample is about 1 mJ.

Fig. 7. A typical LIV signal of a 10o *c-*axis tilted NaxCoO2 thin film (~140 nm) under the 308 nm laser radiation. The laser energy on the sample is about 1.5 mJ.

The performed LIV measurements on the *c*-axis tilted NaxCoO2 thin films show similar dependence of Vp on the laser energy, tilted angle and film thickness. However, the *c*-axis tilted NaxCoO2 thin films have a much larger induced voltage signal than that of the Bi2Sr2Co2Oy and Ca3Co4O9 thin films. Fig. 7 illustrates the laser-induced open-circuit voltage signal of the 10o tilted NaxCoO2 film irradiated by the 308 nm pulsed laser with the laser energy on the sample of 1.5 mJ. A giant open-circuit voltage signal with Vp of 16.3 V is observed. 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 NaxCoO2 film has better crystal quality which corresponds to a larger Δ*S*, and the exact reason needs to be clarified in our future work.

### **4. Conclusion**

172 Photodetectors

Similar results are also observed in the *c*-axis tilted Ca3Co4O9 thin films when their surface is irradiated by the above lasers. Fig. 6 presents a typical laser-induced open-circuit voltage signal of the 10o tilted Ca3Co4O9 thin film (~ 100 nm) under the 308 nm pulsed illumination with the laser energy on the sample of 1 mJ. Both the responsivity (~ 230 mV/mJ) and the response time (the rise time~60 ns and the FWHM~700 ns) of the LIV signal in this tilted Ca3Co4O9 thin film is in the same order as that of the tilted Bi2Sr2Co2Oy thin film with the

**024**

**Time** (**s**)

Fig. 6. A typical LIV signal of a 10o *c-*axis tilted Ca3Co4O9 thin film (~ 100 nm) under the 308

**-200 0 200 400**

Fig. 7. A typical LIV signal of a 10o *c-*axis tilted NaxCoO2 thin film (~140 nm) under the 308

**Time (ns)**

**3.3 LIV signal in c-axis Ca3Co4O9 and NaxCoO2 thin films** 

**0.0**

**0**

**5**

**10**

**Voltage (V)**

**15**

**20**

nm laser radiation. The laser energy on the sample is about 1 mJ.

nm laser radiation. The laser energy on the sample is about 1.5 mJ.

**0.1**

**0.2**

**Voltage (V)**

**0.3**

same thickness.

In conclusion, the performed investigations of the LIV effect in *c*-axis tilted NaxCoO2, Bi2Sr2Co2Oy and Ca3Co4O9 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.

### **5. Acknowledgments**

This work was partially supported by NSFC of China under Grant No. 10904030, SFC of Hebei Province under Grant No. A2009000144 and E 2006001006.

### **6. References**


**9** 

*Spain* 

**Geiger-Mode Avalanche Photodiodes in** 

Photodiodes are the simplest but most versatile semiconductor optoelectronic devices. They can be used for direct detection of light, of soft X and gamma rays, and of particles such as electrons or neutrons. For many years, the sensors of choice for most research and industrial applications needing photon counting or timing have been vacuum-based devices such as Photo-Multiplier Tubes, PMT, and Micro-Channel Plates, MCP (Renker, 2004). Although these photodetectors provide good sensitivity, noise and timing characteristics, they still suffer from limitations owing to their large power consumption, high operation voltages and sensitivity to magnetic fields, as well as they are still bulky, fragile and expensive. New approaches to high-sensitivity imagers tend to use CCD cameras coupled with either MCP Image Intensifiers, I-CCDs, or Electron Multipliers, EM-CCDs (Dussault & Hoess, 2004), but

A fully solid-state solution can improve design flexibility, cost, miniaturization, integration density, reliability and signal processing capabilities in photodetectors. In particular, Single-Photon Avalanche Diodes, SPADs, fabricated by conventional planar technology on silicon can be used as particle (Stapels et al., 2007) and photon (Ghioni et al., 2007) detectors with high intrinsic gain and speed. These SPAD are silicon Avalanche PhotoDiodes biased above breakdown. This operation regime, known as Geiger mode, gives excellent single-photon sensitivity thanks to the avalanche caused by impact ionization of the photogenerated carriers (Cova et al., 1996). The number of carriers generated as a result of the absorption of a single photon determines the optical gain of the device, which in the case of SPADs may

The basic concepts concerning the behaviour of G-APDs and the physical processes taking place during their operation will be reviewed next, as well as the main performance

APDs can be obtained by implementing two possible approximations that produce two different structures with differentiated capabilities. On one hand, thin silicon APDs (Lacaita et al., 1989) are devices with a depletion layer of few micrometres and low breakdown voltages. They also present good detection efficiency and time resolution. As in planar

they still have limited performances in extreme time-resolved measurements.

**1.1 Basic concepts for Geiger-mode avalanche photodiodes, G-APDs** 

**1. Introduction** 

be virtually infinite.

parameters and noise sources.

**Standard CMOS Technologies** 

*Electronics Department, University of Barcelona* 

Anna Vilà, Anna Arbat, Eva Vilella and Angel Dieguez

