The Schottky contact behavior of Silver (Ag) with parts of semiconductor materials, such as GaAs and GaN, greatly limits its application for electrodes in electronic devices. Here, a giant photosensitivity is observed in Co-doped amorphous carbon (a-C:Co) films deposited on n-type low-resistance GaAs substrates through a Schottky contact between GaAs and Ag. We ascribe the giant photosensitivity to the turn-on voltage difference in the series-opposing connected a-C:Co/GaAs photosensitive diode and GaAs/Ag Schottky junction with and without light illumination, and also to the surface plasmon resonance absorption of Co nanoparticles.
© 2015 Optical Society of America
Amorphous carbon (a-C) thin films have a complex electronic structure owing to their mixture of graphite-like sp2 and diamond-like sp3 bonds . Recently, a number of new properties, including humidity-dependent ammonia sensitivity , field emission , positive magneto-resistance effect [4,5], metal-insulator transition , and nonvolatile resistive switching memory , have been found in a-C-related materials.
Photovoltaics and photoconductivity have been investigated in the a-C/Si system [8–15]. These materials are expected to have similar photovoltaic properties to Si, and can be used as alternative photovoltaic materials because they are cheaper than Si [8–10]. Moreover, a-C exhibits a low degree of photoconductivity, with white light illumination at room temperature (RT) . Its photoconductivity has been enhanced to a magnitude of 5-20 with illumination power of 100 mW/cm2 at RT [12,13]. Using iron doping, Wan et.al  achieved a high degree of photoconductivity, about 170-220, with illumination power of 20 mW/cm2 at RT. They ascribed that degree of photoconductivity to the formation of a p-n junction between the p-type a-C:Fe film and n-type Si substrate. J. Gao et.al recently reported giant photosensitivity of 105 at 100 K in an a-C:Co/Si p-n heterostructure . In this paper, we are the first to report the giant photosensitivity of a-C:Co/GaAs/Ag heterojunctions.
The a-C:Co films are deposited on Si-doped low-resistivity (0.1 Ω cm) n-type GaAs (L-GaAs) (100) substrates using pulsed laser deposition (PLD). A KrF excimer laser (Lambda Physik 201) is operated with 390 mJ/pulse energy at a frequency of 5 Hz. Co pieces are embedded on the surface of a-C target. The purity of both the Co and carbon target is better than 99.9%. Before deposition, the substrates were ultrasonically cleaned in acetone, ethanol, and then deionized water. The chamber used for PLD is pumped to 5 × 10−4 mbar, and the temperature of the substrate is raised to 480°C. The deposited films are then annealed maintaining the growth temperature for half an hour. The films are about 40 nm thick, which is verified by a Vecco Dektak 6M stylus surface profilometer. The nominal Co atomic fraction is roughly 10% . A LabRam HR800 Raman and photoluminescence (PL) microscope (excitation laser, Ar+488 nm) is used for the visible Raman and PL spectroscopy. Silver electrodes are obtained through thermal evaporation by covering the a-C film and GaAs substrate with a mask. The electro-optical properties of a-C:Co/L-GaAs/Ag heterojunctions are measured using a two-probe method with a Keithley 2400 SourceMeter. The intensity of the illuminating laser diode with a wavelength of 650 nm is 45 mW/cm2. The laser spot sizes used for photoconductivity measurements are all limited to a diameter of 6 mm through a slit.
3. Results and discussion
The amorphous carbon feature of the deposited a-C:Co films is confirmed by the Raman spectra in Fig. 1(a), where the D peak is centered at 1362 cm−1, and G peak at 1595 cm−1, with an intensity ratio of ID/IG≈1.0. In accordance with previous statistical and experimental results, we speculate that a-C:Co films have polymer-like carbon features with up to 45% sp3 and a band gap of about 1.0~1.5 eV . From the PL spectra in Fig. 1(b), the L-GaAs single crystal has a band gap of 1.43 eV. The a-C:Co/L-GaAs thin film shows a wide peak with maximum value of 1.26 eV and a sharp peak located at 1.43 eV. The wide peak may come from the intra-sp2-cluster PL emission of amorphous carbon film .
The dark and light current-voltage (I-V) curves of the heterojunctions obtained at RT are displayed in Fig. 2. Insert (a) is a schematic illustration of the experiment. Good rectifying behavior, with dark turn-on voltage (corresponds to the voltage when current versus applied forward bias voltage begins to increase sharply from nearly zero, which is obtained from linear extrapolation intersecting with the voltage axis, as marked in Fig. 2 insert (b)) equal to 0.45 V, can be detected. With light illumination, the measured current increases slowly at first, and then increases sharply after reaching a voltage value of = 0.25 V. When voltage reaches 0.45 V, both light and dark I-V curves begin to increase linearly, and are nearly parallel to each other until 3.0 V, which means that photocurrent achieves nearly the maximum value at 0.45 V. Above 3 V, with an increase in voltage, the difference between light and dark currents reduces because of the gradual increase in the conduction current and saturation of light current. It is interesting that dark and light currents almost overlap, resulting in no photoconductivity under reverse bias voltage. The Ag electrode works at forward bias voltage, whereas Ag/a-C:Co works at reverse bias voltage.
Photosensitivity is one of the most important characteristics for photosensitive materials, which is defined as σ/σ0 or RD/RL, where σ and σ0 are photoconductivity and dark conductivity, RD and RL are dark resistance and light resistance, respectively . Figure 3(a) shows the variation in photosensitivity with forward bias voltage at RT. The maximum value of 652 is obtained at a bias voltage of 0.3 V. Photosensitivity then decays exponentially. The insert in Fig. 3(a) shows the periodic changes in dark and light resistance, which are about 2.1 × 106 Ω and 3.3 × 103 Ω, respectively. Remarkably, even when photosensitivity is as low as 1.06 at 8 V forward bias voltage, we can clearly see the behavior of the resistance switch, which means that the heterojunctions are suitable for application in photon-controlled resistance switches.
We also measured photosensitivity at low temperatures. Figure 3(b) shows the photosensitivity at various temperatures. Giant photosensitivity of 1.3 × 104 was observed with a forward bias voltage of 0.3 V at 200 K. The periodic changes in RD and RL at different temperatures are also shown in the insert in Fig. 3(b). The fluctuation of RD increases when it exceeds 10 MΩ at temperatures below 200 K, and cannot be measured accurately.
PbS, PbSe, and PbTe are usually used for infrared detection. CdS is a kind of visible photoelectric material, but the peak of its spectral response limits it to green light (the band gap is 2.4 eV) . In addition, one of the common shortcomings of these photosensitive materials is their nonlinear sensitivity-power relationship. Furthermore, they contain the heavy metals Pb and Cd, which are environmental pollutants. The photon energy (wavelength) and power dependence of the photosensitivity of the a-C:Co/L-GaAs/Ag heterojunctions are shown in Fig. 4. In Fig. 4(a), photosensitivity monotonically decreases with an increase in photon energy at the measured energy scale. These data were obtained at 0.3 V forward bias voltage at RT using several lasers with different wavelengths at the same power after conversion. A clear linear photosensitivity-power relationship can be observed in Fig. 4(b). Our results show that the a-C:Co/L-GaAs/Ag heterojunctions are applicable for visible, particularly red, and even infrared photosensitive resistance, watt meters, and other related devices. These heterojunctions may thus partially displace Pb- and Cd-containing photosensitive resistance in future.
To understand the physical mechanisms of the foregoing experimental phenomena, we first measured the contact characteristics between Ag and a-C:Co film, as shown in Fig. 5(a). The configuration of the prepared film was as follows. The a-C:Co film was first deposited on a glass substrate. Then, two Ag electrodes were evaporated on the film. The I-V curves exhibited a clear linear relationship, which confirms the characteristics of ohmic contact between Ag and the a-C:Co film. The light and dark curves nearly overlap. Our results indicate that the a-C:Co film itself displays no photoconductivity behavior. Next, we measured the contact characteristics between Ag and L-GaAs. The I-V curves in Fig. 5(b) confirm that a Schottky junction formed between Ag and L-GaAs [18, 19]. In addition, the light and dark I-V curves show negligible photoconductivity behavior.
We then measured the contact characteristics between Ag/a-C:Co and L-GaAs. A weak p-n contact was generated, as can be deduced from Fig. 6(a). The a-C:Co film is a p-type semiconductor, which can form a p-n junction with n-type Si [13–15,20,21]. We also measured the photoconductivity properties of a-C:Co/L-GaAs, as shown in Fig. 6(b). As can be seen from Fig. 6(b), the photosensitivity values were 1.04 and 1.95 when the laser was directed onto the L-GaAs substrate and a-C:Co film, respectively. These results show that photoconductivity is produced through the formation of an a-C:Co/L-GaAs p-n junction.
Based on the foregoing results, we drew an equivalent circuit diagram of a-C:Co/L-GaAs/Ag heterojunctions, which is shown in Fig. 7(a).The voltage source, Rc, a-C:Co/L-GaAs p-n junction, and Ag/L-GaAs Schottky junction form a circuit in series, where Rc is the resistance of a-C:Co film with resistivity of 1-5 KΩ cm. The a-C:Co/L-GaAs p-n junction exhibits the characteristics of a photosensitive diode .
In Fig. 7(a), the applied voltage is the sum of u1, u2, and u3:
The current-voltage relationship of the Schottky junction, photosensitive diode, and Rc can be expressed by Eqs. (2)-(4):
In the foregoing equations, Is, q, k, and T are the reverse saturation current of the Schottky junction, electronic charge, Boltzmann constant, and absolute temperature, respectively. I0, Isc, and Voc are the reverse saturation current, short-circuit current, and open-circuit voltage of the photosensitive diode, respectively.
Figure 7(b) depicts the measured I-V curves at a low applied voltage, in dark and under light illumination, of a-C:Co/GaAs/Ag heterojunctions, and Fig. 7(c) and 7(d) the curves fitted by formulas 1-4. It can be seen that they are qualitatively consistent with the experimental data, thus also confirming our hypothesis that the a-C:Co/L-GaAs photosensitive diode connects with the L-GaAs/Ag Schottky junction in a series-opposing structure.
When forward bias voltage is applied to a-C:Co/L-GaAs/Ag heterojunctions, as shown in Fig. 7(a), the a-C:Co/L-GaAs photosensitive diode and Ag/L-GaAs Schottky junction work at reverse and forward bias voltage (because of the series-opposing structure), respectively. Dark current is very small in a-C:Co/L-GaAs photosensitive diode at reverse bias voltage. However, when light shines onto the diode, photon-generated hole and electron pairs are generated at the interface of the junction, and drift to the a-C:Co and GaAs sides under the actions of the built-in electric field and applied electric field, as shown in Fig. 7(a). These carriers transmit to the Ag/L-GaAs Schottky junction, thus reducing the thickness of the Schottky junction barrier, and then induce diminution of turn-on voltage from 0.45 V to 0.25 V. The Schottky junction is turned-off (turned-on) at 0.3 V forward voltage without (with) light illumination, which may be part of the sources of the huge photosensitivity observed in a-C:Co/GaAs/Ag heterojunctions at 0.3 V forward bias voltage at RT.
The a-C:Co/L-GaAs/Ag heterojunctions have the property of conventional semiconductor in the dark as shown in Fig. 7(b) and 7(c). Figure 7(b) shows that even when the applied bias voltage is zero, the current is not zero under light illumination but takes a positive value, which originates from the drift of photon-generated hole and electron pairs under the actions of the built-in electric field of an a-C:Co/GaAs photosensitive diode. When reverse bias voltage is applied to the a-C:Co/L-GaAs/Ag heterojunctions, the a-C:Co/L-GaAs photosensitive diode works at forward bias voltage. The total current decreases to zero when photon-generated current is equal to the opposite conduction current, as shown in Fig. 7(d). One of the common properties of photosensitive diodes is that they work at reverse bias voltages because the photoelectric effect is very weak at forward bias voltage, which explains the lack of photoconductivity at reverse bias voltage for a-C:Co/L-GaAs/Ag heterojunctions. The measured photocurrent at forward bias voltage is qualitatively coincident but quantitatively different with the theoretical value. Because of doping and crystal defects, there are a large number of recombination centers and trap centers in the a-C:Co film, which affect the photon-generated carrier concentration. During the process of theoretical calculation, it’s hard to take these factors into consideration. This could be the main cause of the difference.
Photocurrent enhancement through localized Schottky effects was also reported in oxide semiconductor SnO2 nanowires and single ZnO nanowire by Au-nanoparticles decoration [21,22]. The giant photoconductivity was attributed to the surface plasmon resonance absorption of Co nanoparticles because of the Co doping in a-C/Si heterojunction . Since our samples were deposited at almost the same conditions with the above samples, we believe that Co nanoparticles improve the light absorption and thus enhance the photoconductivity of a-C:Co/L-GaAs/Ag heterojunctions, especially at low temperature.
4. Summary and outlook
A series-opposing connected a-C:Co/L-GaAs photosensitive diode and Ag/L-GaAs Schottky junction display giant photosensitivity, especially at low temperatures. Considering its excellent photoconductive properties, besides giant photosensitivity, wide visible spectral response and linear photosensitivity-power relationship, this system can be used for photosensitive resistance-related devices, such as fire alarms and watt meters.
This research was supported by funding from the National; Natural Science Foundation of China (NSFC) (U1332205, 11274153, 11404169), the Natural Science Foundation of Jiangsu Province (grant no. BK20140450), and Huaian science and technology (industry) project (grant no. HAG2014043).
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