Proceedings of the 54th DAE Solid State Physics Symposium (2008)
Interaction of light holes with optical photons in copper sulfide Abhay A. Sagade and Ramphal Sharma* Thin Film and Nanotechnology Laboratory, Department of Physics, Dr. B. A. M. University, Aurangabad-431004. Maharashtra, India.
Email:
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Abstract The interaction between light holes (LH) with optical photons is observed experimentally during detection of ammonia using copper sulfide thin films. The Cu1.4S thin film acts as a sensor element for the detection of ammonia gas. The gas detection experiments are carried out in presence of photons (optical gas sensor). It is observed that the change in absorbance of the sensor is large in the 620-690 nm range, in which the interaction between LH and optical photons occurred. The results are explained using microscopic qualitative model.
values reported here of change in absorbance are measured at 650 nm only. The absorbance in the film is 0.55 and 0.69 after adsorption and desorption of ammonia, respectively. When dry air is flowed over the film, the absorption value is returned to 0.57. The response time of the sensor is 7 s and recovery time is 20 s. These values of optical response are smaller than the earlier report for electrical response [3].
INTRODUCTION The CuxS is well known material because of its optoelectronic properties. Although there are number of reports [1], it is still in interest due to variation in properties depending on x = 1 to 2. The opto-electronic and solar control properties of chemically deposited CuxS thin films are well known. Recently it is also reported [2, 3] that, CuxS could also be used for the detection of the traces of ammonia. The CuxS sensors are working at room temperature. In this letter a new type of optical sensor study is discussed.
Absorbance
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EXPERIMENTAL The thin films of Cu1.4S are deposited using solution growth technique. This film is used for the detection of ammonia at room temperature (303 K). The Perkin Elmer Lambda-25 spectrometer is used for absorption measurement. Copper sulfide film is placed vertically in the path of optical waves. A special arrangement is made for the purging of ammonia gas. The absorbance of the sensor (copper sulfide thin film and layers of ammonia gas) is measured in the 300 to 1100 nm wavelength region at different concentrations of ammonia.
Gas off-1 Gas on Gas off-2
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Fig. 1 The change in absorbance of sensor. To check the persistence of the sensor, 300 ppm of ammonia gas is continuously flowed into the chamber and absorbance of the film is measured (Fig. 2). It is seen that the absorbance is increases with time, but get saturated after 240 s. This means that the sensor will send the signals up to four minutes for the presence of ammonia. The qualitative explanation of the ammonia detection by copper sulfide in presence of photon illumination using time dependent adsorption can be given as follows.
RESULT AND DISCUSSIONS Fig. 1 shows the change in absorbance of film when 200 ppm gas is purged in the chamber. For convenience, the
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Proceedings of the 54th DAE Solid State Physics Symposium (2008) The copper sulfide is the p-type semiconductor with high carrier concentration (1.2 1022 cm-3) and free copper is present at the surface between the grain boundaries [3]. Plausibly, there is a band bending at the surface and since it is a degenerate semiconductor Fermi energy level (EFermi) is located inside the valence band (Fig. 3b). The presence of free copper at the surface is shown by small filled circles. The ammonia will adsorb on the surface of copper sulfide from left, so the region of gas-solid interaction is referred as interface specific region (ISR). This is the area of interest and it has dimensions of the order of ~ 3 – 5 nm. Fig. 3a shows the E(k) diagram in three dimensions. It is well know that, in p-type semiconductors with high carrier concentrations (> 1020 cm-3) the valence band is splitted in to three separate bands known as heavy hole band (VB1), light hole band (VB2) and split-off band (VB3) [4]. The plot in Fig. 2 can be categorized in three parts namely, Urbach absorption (fundamental gap absorption), LH–split-off band absorption and free carrier absorption at lower, intermediate and higher wavelengths, respectively [4]. The change in absorbance in these regions is different when ammonia is adsorbed and when saturation occurs with time. Since there should not be any change in band gap of the material due to ammonia adsorption, the change in absorbance is negligible in the lower wavelength part. It was known that when ammonia is adsorbed on the copper sulfide thin films, the electron transfer takes place between lone pair of nitrogen and free copper at the surface. This process causes annihilation of one electronhole pair (EHP) in the sensor and increases its resistivity. Here the gas-solid interaction takes place in presence of external stimulus i.e. photons. The illumination of semiconducting copper sulfide with optical waves will locally produce one EHP across the band gap and additional absorption will takes place between VB1, VB2 and VB3 (Fig. 3a). Hence there is an increase in absorption in the lower band gap region (700–900 nm). This annihilation and creation of charge carriers in the copper sulfide will cause change in the absorption at higher wavelengths (Fig. 2). The large change in absorbance after ammonia adsorption is occurred in the wavelength range of 620 – 690 nm. The copper sulfide is (74%) transparent to this part of
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Gas off, 60 S, 180 S,
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light hole split-off band absorption
electromagnetic spectrum. Henceforth, in this region there is no EHP generation across the band edge and the change in absorbance is totally due to adsorption of ammonia. The lone pair electrons from nitrogen are run down to the lowest energy band i.e. split-off band (VB3) and hence additional optical absorption will occur in between VB3 and VB2 bands. At the end, the saturation in the absorbance is occurred due to two factors: (i) the (local) photoconductivity of copper sulfide is getting saturated within 270 s and (ii) as time passes the adsorption sites at the ISR are decreases. These two factors are affecting simultaneously and therefore the saturation in the optical absorbance of sensor is observed from 240 s. CONCLUSION The LH-photon interaction was being adsorption of ammonia on copper sulfide. was observed in 620-690 nm range of spectrum. This type of interaction helps sensitivity of the sensor.
probed in the This interaction electromagnetic to increase the
ACKNOLEDGEMENTS Authors are thankful to defense ministry (DRDO) for financial support in form of research project ERIP/ER/03050/75/M/01. REFRENCES 1.
M. T. S. Nair et al., Semicond. Sci. Technol. 4 (1989) 191; R. Blachnik et al., Thermochim. Acta 366 (2001) 47; P. K. Nair et al., J. Phys. D: App. Phys. 24 (1991) 83.
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Galdikas A et al., Sens. Actuators B 67 (2000) 76.
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Abhay A. Sagade et al., Sens. Actuators B 133 (2008) 135.
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P.Y. Yu and M. Cardona, Fundamentals of Semiconductors, Springer-Verlag Berlin, p.299 (1996).
10 S 120 S 240 S Free carrier absorption
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Fig. 2 The persistence of sensor.
Fig. 3 Energy band diagram of sensor.
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