Schottky photodiodes

Schottky photodiodes can consist of an undoped semiconductor layer (usually n type) on bulk material and a metal layer deposited on top to form a Schottky barrier. An example is shown in Fig. 9.68. 

Wang, S.Y. and Bloom, D.M. 1983.100 GHz bandwidth planar GaAs Schottky photodiode. Electron. Lett. 19(14) 554-555. 

E. Ozbay, M. S. Mam, B. Onat, M. Gokkavas, O. Aytur, G. Tiittle, E. Towe, R. Henderson et al.. Fabrication of high-speed resonant cavity enhanced Schottky photodiodes. IEEE Photonics Technol. Lett 9(5), 672-674 (1997)... 

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. 

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. 

Photodetectors operate by carrier transport across a semiconductor junction. A wide variety of these photodiodes are available, such as Schottky diodes, phototransistors, and avalanche photodetectors. Typical photodetector materials are gallium arsenic phosphide and gallium phosphide, which are produced by MOCVD or MBE. 

Other types of photodiodes not discussed here include the Schottky barrier, pn homojunction, and heterojunction. 

The first equation is realized at the LKB while the second one is carried out at the LPTF. A first titanium-sapphire laser excites the hydrogen transition. A laser diode (power of 50 mW) is injected by the LD/Rb standard and frequency doubled in a LiBsOs (LBO) crystal placed in a ring cavity. The generated UV beam is frequency compared to the frequency sum (made also in a LBO crystal) of the 750 and 809 nm radiations produced by a second titanium-sapphire laser and a laser diode. A part of the 809 nm source is sent via one fiber to the LPTF. There, a 809 nm local laser diode is phase locked to the one at LKB. A frequency sum of this 809 nm laser diode and of an intermediate CO2 laser in an AgGaS2 crystal produces a wave at 750 nm. This wave is used to phase lock, with a frequency shift S, a laser diode at 750 nm which is sent back to the LKB by the second optical fiber. This 750 nm laser diode is frequency shifted by lyfCOo) + S with respect to the one at 809 nm. In such a way, the two equations are simultaneously satisfied and all the frequency countings are performed in the LKB. Finally, the residual difference between the two titanium-sapphire lasers is measured with a fast photodiode or a Schottky diode. 

Photodetector — Device used to detect photons. After a long period having only thermal photodetectors, quantum photodetectors based on photocurrent were developed and are used quite widely in applications such as photographic meters, flame detectors and lighting control. In the late 1950s the p-i-n photodiode, simply referred to as photodiode, was developed and now is one of the most common photodiodes. There are several types of photodetectors, the most adequate depending on the specific application, like photoconductors, p-i-n photodiodes, Schottky-barrier photodiodes, charge-coupled... 

A similarly high Voc for ITO/PPV/Al photovoltaic devices also was observed by other groups. Jenekhe et al. [63, 64] report the observation of a quantum efficiency IPCE of 5% in ITO/PPV/Al photodiodes and of a power conversion efficiency of approximately 0.1% under low light intensities of 1 mW/cm. The typical film thickness of their devices was varied between 100 to 600 nm. The open circuit voltage of these devices, as defined with respect to the ITO electrode, was measured as 1.2 V. The high open circuit voltage was explained by the formation of a Schottky barrier at the Al/PPV interface. The predicted band bending due the PPV/Al interface formation was verified by XPS measurements [65, 66]. 

LEDs are made from boron-doped 4H-SiC. Three colour displays have been demonstrated. SiC ultraviolet photodetectors made from p-n junction and Schottky barrier diodes can be used up to temperatures of 700 K and are expected to be radiation tolerant. These photodiodes are more sensitive than their silicon counterparts. 

The second photon effect of general utility is the photovoltaic effect. Unlike the photoconductive effect, it requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. Although it is possible to have an extrinsic photovoltaic effect, see Ryvkin [2.32], almost all practical photovoltaic detectors employ the intrinsic photoeffect. Usually this occurs at a simple p — n junction. However, other structures employed include those of an avalanche, p—i — n, Schottky barrier and heterojunction photodiode. There is also a photovoltaic effect occuring in the bulk. Each will be discussed, with emphasis on the p—n junction photoeffect. 

The above discussion has been confined to the photovoltaic effect arising at a p—n junction in a semiconductor due to photoexcitation of carriers on either side of the junction. Among the related areas of interest are a) avalanche photodiodes, b) p—i—n photodiodes, c) Schottky barrier photodiodes, d) heterojunction photodiodes, and e) bulk photovoltaic effect Each of these will be discussed briefly. 

Schottky Barrier Photodiode. A photoeffect similar to that obtained in a p—n junction can be found at a Schottky barrier, formed at a metal-semiconductor interface. As with a p—n junction, a metal-semiconductor interface when properly made provides a potential barrier which causes separation of photoexcited holes and electrons, thereby giving rise to a short circuit photocurrent and an open circuit photovoltage. In most instances the metal is in the form of a very thin film which is semitransparent to the incident radiation. Photoexcitation can occur within the semiconductor or over the potential... 

Not all semiconductors can be prepared in both n- and p-types. Schottky barrier photodiodes are of special interest in those materials in which p — n junctions cannot be formed. They also find application as UV and visible radiation detectors, especially for laser receivers where their high frequency response (in the gigahertz range in many cases) is of particular usefulness. See Ahlstrom and Gartner [2.41], Schneider [2.42] and Sharpless [2.43] for more detailed descriptions. 

Any other form of internally generated noise must depend upon bias. Since they add (quadratically) to Johnson noise, all other types of noise are referred to as excess noise. Three principal forms of excess noise exist. One amenable to analysis which is found in photoconductors is generation-recombination or g — r noise. A second, also amenable to analysis, which is found in photodiodes, i.e., p - n junctions and Schottky barrier diodes, is referred to as shot noise of diffusing carriers, or simply as shot noise. The third form of excess noise, not amenable to exact analysis, is called l//(one over/) noise because it exhibits a 1// power law spectrum to a close approximation. It has also been called flicker noise, a term carried over from a similar power law form of noise in vacuum tubes. 

Photodiodes typically have an absorption region where a strong electric field is applied so that the photogenerated carriers can be swept out to produce a current or voltage signal in an external circuit. Common types of photodiodes are pn junction photodiodes, p-intrinsic- (PIN) photodiodes, Schottky junction photodiodes, and avalanche photodiodes (APDs). 

For material with Kttle carrier trapping, the response time is determined by carrier lifetime. As device dimensions get smaller, however, the response time can be altered by the nature of the metal contact and the method of biasing. With nonaUoyed contacts to the MSM device, Schottky barriers can be formed at the metal semiconductor interfaces, and the depletion region extends across the length of the photoconductor. In this case, the device operates more or less like a photodiode with r replaced by t , and its MB value becomes 1/t (see Eq. (9.19)). 

Hwang, K.C., Li, S.S., and Kao, Y.C. 1991. A novel high-speed dual wavelength InAlAs/InGaAs graded superlattice Schottky barrier photodiode for 0.8 and 1.3 /xm detection. Proceeding ofSPIE1371 128-137. 

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