Análisis de Markov

The electrical characterization of the devices has been carried out with air-pressure homemade probe (see Figure 3.19). This probe station takes advantage of the reproducible geometry the devices fabricated on pre-patterned electrodes to make electrical contact by two fixed probes made of copper (or platinum) wires. A PCB board, consisting in three electrodes, two of approximately 7 mm x 5 mm and another one of approximately 15 mm x 10 mm, is used to make electrical contact between the probes and the electronics and between the substrate (used as gate) and the electronic equipment. The sample is placed in the biggest electrode (electrical contact with the substrate is done with silver conductive paint) while the two probes lie on the sample pads. The electronics consist of two source-meter source- measure units with voltage range from 20 mV to 200 V and current range from 10 nA (resolution of 500 fA) to 1 A (resolution of 50 μA).

Figure 3.19. Picture of the whole setup for air-pressure device characterization.

1.8.1 Photoresponse in optoelectronic devices

Optoelectronics devices are electronic devices which can interact with light by either absorbing or emitting radiation from the ultraviolet (UV) to the near-infrared (NIR) region of the electromagnetic spectrum. Optoelectronics brings together optics and electronics within a single device, which allows for the manipulation of light, the manipulation of electrical current, and their interaction. In this way, devices can be designed to allow for the transformation of light into current and vice versa. Semiconductors are characterized by two types of mobile carriers, electrons in the conduction band and holes in the valence band. The concentration (n) of electrons in the conduction bands and the concentration (p) of holes in the valence band control the electrical conductivity ρ of semiconductors. The electron-hole separation mechanisms cover photoconduction and photovoltaic effect. In this chapter we are going to focus our attention about the physical effect (photoconduction and photovoltaic) that produce the photoresponse in optoelectronic devices.

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1.8.2 Photoconductive effect

In this process when the energy of an incident photon, is greater than the energy gap Eg, free

electron-hole pairs are produced in the crystal. These electrons and holes serves as the carriers of electrical current (see Figure 3.20). In semiconductor materials the electrons in the valence band are promoted to the conduction band by absorbtion of the incident photons. The electron-hole pairs generated by the incident light, in addition to those created thermally, increase the conductivity.

Figure 3.20. Schematic band structure of a photoconductive effect.

Photoconductive detectors are based on semiconducting materials that alter their characteristics when light energy impinges on them. In dark conditions a small current can flow through the semiconductor (dark current, Idark) due to the thermal effect. When the

photodetector is upon illumination with a light wavelength smaller than the cut off wavelength, the light absorption generates electron-hole pairs which are separated by a drain- source voltage. Thanks to the applied drain-source voltage, the electrons drift in one direction and the holes in the opposite one, resulting in a net current (Ilight) that increases the material

conductance. Photodetectors are characterized by certain key parameters as:

Responsivity (R): This is defined as the ratio between the photocurrent generated by light absorption and the power of incident light, giving an estimation of the sensitivity of the photodetector.

𝑅 =

𝐼𝑝ℎ

𝑃_𝑒𝑓𝑓

=

𝐼_𝑝ℎ

𝑃_𝑑 𝐴_{𝑑𝑒𝑣𝑖𝑐𝑒}

Where Peff is defined as the power density of the incident light multiplied by the device area

(Peff = Pd · Adevice).This responsivity may be affected by photogating which is a particular case of

the photoconductive effect. When there are charge traps in localized states of the band structure of the material, they can act as local gate modulating the conductance. These

147 localized trap states reside for long times and are usually located at defects or at the surface of the material.58 _{This effect becomes particularly important at the nanoscale.}

External quantum efficiency: The external quantum efficiency (EQE) is the ratio between the number of charge carriers responsible for the photocurrent (ne) and the total number of

incident photons. It can be related to the responsivity:

𝐸𝑄𝐸 =

𝑛

𝑛

= 𝑅

ℎ𝑐

𝑒𝜆

Time response: is defined as the time needed for a photodetector to switch on (changing from dark current to illumination current) and off (changing from illumination current to dark current) when the incident light is switched on and off. From an experimental point of view, it is the time required for the photodetector to increase from 10% to 90% of the final output level. A good photodetector should have a small time response.

1.8.3 Photovoltaic effect

The photovoltaic effect was discovered in 1839 by Becquerel,59 _{however, it was not until 1883}

that Charles Fritts developed the first solar cell, based on selenium.60 _{In 1954 Chaplin et al.}

from Bell Laboratories found that silicon doped with certain impurities was very sensitive to light.61 _{The photovoltaic effect describes the generation of voltage when a device is exposed to}

light by the separation of photogenerated electron-hole pairs due to an internal electric field. The collection of light-generated carriers by the p-n junction diode or Schottky barriers causes a movement of electrons to the n-type side and holes to the p-type side of the junction. Experimentally, the power generated by a device is calculated from its current versus voltage (I-V) curve, an example of which is shown in figure 3.21. The power generated is:

𝑃_𝑜𝑢𝑡

= 𝐼_𝑚 𝑉_𝑚

Where Pout is the maximum power point and is given by the product of Im and Vm (see Figure

3.21), alternatively, Pout can be expressed as:

𝑃_𝑜𝑢𝑡

= 𝑉_𝑜𝑐 𝐼_𝑠𝑐𝐹𝐹

where Isc is short circuit current and is defined as the current produced with zero bias voltage.

Voc is the open circuit voltage and is defined as the applied bias at which no current flows since

58 _{M. Buscema, J. O. Island, D. J. Groenendijk, S. I. Blanter, G. A. Steele, H. S. van der Zant and A. Castellanos-Gomez,}

Chemical Society Reviews, 2015, 44, 3691-3718.

59 _{A. E. Becquerel, Memoire sur les effects d’electriques produits sous l’influence des rayons solaires. Comptes} Rendus de L’Academie des Sciences, 1839, 9, 561.

60 _{C. E. Fritts, Am. J. Sci., 1883, 26, 465-472.}

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the photogenerated current is equal to the dark current. Finally the fill factor (FF) describes how well the maximum power rectangle fills the area of the I-V curve.

Figure 3.21. Current-voltage characteristic of the photovoltaic effect in a p-n junction. The internal electric field causes a zero voltage current (Isc) and voltage at zero current (Voc). Im and Vm are current and voltage values for which the device reaches the maximum output power.