Schottky devices

Light sensors made from a-Si H are either p-i-n or Schottky barrier structures. Unlike crystalline silicon, a p-n jimction is ineffective without the undoped layer, because of the high defect density in doped a-Si H. Illumination creates photoexcited carriers which move to the junction by diffusion or drift in the built-in potential of the depletion layer and are collected by the junction. A photovoltaic sensor (solar cell) operates without an externally applied voltage and collection of the carriers results from the internal field of the junction. When the sensor is operated with a reverse bias, the charge collection generally increases and the main role of the doped layers is to suppress the dark current. A Schottky device replaces the p-type layer with a metal which provides the built-in potential. 

Schottky barriers are metal-semiconductor junctions that have the ability to rectify current, because the work fimction of the metal is greater than that of the semiconductor. The junction thus creates a barrier between the semiconductor and the metal that decreases when the junction is forward biased and vice versa. Conduction in Schottky devices is by majority carriers, principally electrons. In conventional p-n devices reverse conduction is predominately via minority carriers. In p- -junction devices, charge is stored in the junction during forward conduction and has to be removed if the jimction is reverse biased before the diode can switch off. The junction capacitance and the capacitive reactance are voltage dependent. 

The Schottky diode is the most practically useful and affordable solid state detector at present and can be obtained for frequencies up to 325 GHz. In terms of frequency range and noise performance the nearest contender, the Si point-contact diode mixer, has a noise figure of 11-14 dB over the range 60-80 GHz, its maximum operating frequency. Compared with 5-6 dB for a Schottky device... 

It has been recently revealed that it is possible to fabricate all vacuum deposited metal (Pb, Al, In, Sn)/polyaniline/metal Schottky devices [101]. It has been shown that the barrier height and the ideality factors determined are dependent on the work function of the metal used in the fabrication of these devices. The improved ideality factor obtained as 1.2 for an Al/polyaniline/Ag device has been attributed to more intimate contact of the metal with the vacuum deposited polyaniline electrode. 

Schottky devices have recently been fabricated by thermal evaporation of indium on polyaniline, poly(o-anisidine) and poly(aniline-co-or oanisidine), respectively [103]. The values of the rectification ratio, the ideality factor and the barrier height of an indium/poly(o-anisidine) have been experimentally determined as 300,4.41 and 0.4972, respectively. The observed deviation from the Schottky behaviour for these devices seen at higher voltages has been explained in terms of either the Poole-Frenkel effect or due to the presence of a large number of defects containing the trapped charges existing at the indium/poly (aniline-co-or oanisidine) interface. 

Recently, the junction properties of Schottky devices using films of chemically synthesised poly(3-cyclohexylthiophene) and poly(3-w-hexylthiophene) units and metals have also been studied [104]. Electrical properties of the poly(3-cyclohexylthiophene)/metal junctions were compared with those of the poly(3-w-hexylthiophene)/metal junctions (Figure 13.11). Better rectification properties of the poly(3-cyclohexylthiophene)/metal junctions were attributed to the decreased conductivity that perhaps results due to steric hindrance in the thiophene ring. 

Mathews NR (2010) Charge transport in a pulse-electrodeposited SnS/Al Schottky device. Semicond Sci Technol 25 105010 (6 pp)... 

The detection of photon- or chemically induced electronic excitation became possible with metal-insulator-metal (MIM) tunnel junctions as well as with Schottky devices. In this case, excited carriers are detected that have enongh energy to overcome either a tunnel or a Schottky barrier. Therefore, the metal film acts as a substrate for the reaction, as a photon-adsorbing layer, and as an emitter of hot carriers. There have been many experimental attempts to elucidate the nature of hot carriers using the MIM junction structure [1, 36 6]. It was found that hot electrons injected in MIM structures influence the surface reactivity [47-49]. 

L. M. Goldenberk, V. I. Krinichnyi, and I. B. Nazarova, The Schottky device based on doped poly(para-phenylene), Synth. Met. 44 199 (1991). 

Sharma, G. D. Sharma, S. K. Roy, M. S. Photovoltaic properties of Schottky device based on dye sensitized poly (3-phenylazomethine thiophene) thin film. Thin Solid Films 2004, 468, 208-215. 

Pandey et al. [983] described In/CP Schottky devices fabricated via thermal evaporation of In on chemically synthesized P(ANi), poly(o-anisidine) and poly(aniline-co-o-anisidine). In the case of die last copolymer, the rectification ratio, ideality factor and barrier height were found to be 300, 4.41 and 0.497 V respectively, while they were 60, 5.5 and 0.510 V for the P(ANi) device. Bantikassegn and Inganas described [984] a Schottky contact made from poly(3-(4-octylphenyl)-2,2 -bithiophene) (P(TOPT)) in its neutral and PF -doped states and Al metal as the sandwich structure ITO/P(TOPT)/Al. Rectification ratios for the neutral and doped CP were observed to be ca. 5 and 3 orders of magnitude respectively, with diode quality factors (n) being 1.2 and 4.2 respectively. Liu et al. [122] fabricated Schottky diodes from L-B films of poly(3-alkyl-thiophenes) doped with an... 

Table 11-2 shows the built-in potential in metal/MEH-PPV/metal structures measured by either electroabsorption [15] or photocurrenl techniques [37] for a variety of contact metals. The uncertainty in both the work function differences and the built-in potential measurements is about 0.1 eV. For all of the structures except the Pt-Ca and Al-Sm devices there is good agreement between the metal work function difference, AW, and the built-in potential, Vhi. This indicates that for a wide range of metal contacts the Schottky energy barrier between the metal and MEH-PPV is well approximated by the ideal Schottky model and that state chaiging, which pins the Schottky energy barrier, is not significant. A built-in potential smaller than the difference between the contact work functions implies that... 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

MIM or SIM [82-84] diodes to the PPV/A1 interface provides a good qualitative understanding of the device operation in terms of Schottky diodes for high impurity densities (typically 2> 1017 cm-3) and rigid band diodes for low impurity densities (typically<1017 cm-3). Figure 15-14a and b schematically show the two models for the different impurity concentrations. However, these models do not allow a quantitative description of the open circuit voltage or the spectral resolved photocurrent spectrum. The transport properties of single-layer polymer diodes with asymmetric metal electrodes are well described by the double-carrier current flow equation (Eq. (15.4)) where the holes show a field dependent mobility and the electrons of the holes show a temperature-dependent trap distribution. 

M.S. Sze, Physics of Semiconductor Devices, Wiley-lnterscience, New York 1981 B. L. Sliarma (Ed.) Metal-Semiconductor Schottky Barrier Junctions and Their Applications, Plenum Press, New York 1984. 

In this section the electronic structure of metal/polymcr/metal devices is considered. This is the essential starting point to describe the operating characteristics of LEDs. The first section describes internal photoemission measurements of metal/ polymer Schottky energy barriers in device structures. The second section presents measurements of built-in potentials which occur in device structures employing metals with different Schottky energy barriers. The Schottky energy barriers and the diode built-in potential largely determine the electrical characteristics of polymer LEDs. 

A polymer layer al a contact can enhance current How by serving as a transport layer. The transport layer could have an increased carrier mobility or a reduced Schottky barrier. For example, consider an electron-only device made from the two-polymer-layer structure in the top panel of Figure 11-13 but using an electron contact on the left with a 0.5 eV injection barrier and a hole contact on the right with a 1.2 cV injection barrier. For this case the electron current is contact limited and thermionic emission is the dominant injection mechanism for a bias less than about 20 V. The electron density near the electron injecting contact is therefore given by... 

In a MESFET, a Schottky gate contact is used to modulate the source-drain current. As shown in Figure 14-6b, in an //-channel MESFET, two n+ source and drain regions are connected to an //-type channel. The width of the depletion layer, and hence that of the channel, is modulated by the voltage applied to the Schottky gate. In a normally off device (Fig. 14-9 a), the channel is totally depleted at zero gate bias, whereas it is only partially depleted in a normally on device (Fig. 14-9 b). 

Friend et at. studied the influence of electrodes with different work-functions on the performance of PPV photodiodes 143). For ITO/PPV/Mg devices the fully saturated open circuit voltage was 1.2 V and 1.7 V for an ITO/PPV/Ca device. These values for the V c are almost equal to the difference in the work-function of Mg and Ca with respect to 1TO. The open circuit voltage of the ITO/PPV/A1 device observed at 1.2 V, however, is considerably higher than the difference of the work-function between ITO and Al. The Cambridge group references its PPV with a very low dark carrier concentration and consequently the formation of Schottky barriers at the PPV/Al interface is not expected. The mobility of the holes was measured at KT4 cm2 V-1 s l [62] and that for the electrons is expected to be clearly lower. 

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