Extrinsic Photoconductive Detectors

Specially doped Si is the major material in this category. Some data have now been published in the open literature on doped Si as an extrinsic photoconductive infrared detector material. Properties of dopants, not previously well known, have been measured and seem to provide additional possibilities for doped Si as an effective detector material. 

Sclav [8.77] has reported an extensive study of extrinsic photoconductive Si detectors for the 3-5 pm and 8-14 pm infrared wavelength ranges. His study included indium, sulfur, and thallium as dopants for 3-5 pm detection and aluminum, gallium, bismuth, and magnesium as dopants for 8-14 pm. 

Scott and Schmit [8.78] have reported a careful study of the infrared properties of thallium-doped Si, doped as highly as 5 x 10 atoms/cm thallium acts as an acceptor. They measured an infrared ionization energy of0.246 eV, corresponding to a long-wavelength cutoff = 5.0 pm, well suited for 3-5 pm detection. They also estimated a peak optical cross section of 2.6 x 10 cm for thallium in Si. Later work by Brotherton and Gill [8.79] verified the 0.24 eV ionization energy of thallium by means of thermal emission rate measurements. 

Vydyanath and co-workers [8.80] have reported the development of selenium-doped Si for 3-5 pm detection selenium acts as a donor. The ionization energy of selenium was found from photoconductivity measurements to be 0.3 eV, corresponding to = 4.1 pm, They found the maximum solubility of selenium in Si to be slightly under 10 atoms/cm.  

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The basic theory of photovoltaic and photoconductive detectors shall be presented in Section 4.1 in a unified form convenient for intercomparison of the two effects and of the various detector materials. Then Sections 4.2,4.3, and 4.4 shall cover photovoltaic, intrinsic photoconductive, and extrinsic photoconductive detectors, respectively, each of these sections including first a subsection in which the general theory of Section 4.1 is specialized to that class of detector, and then a subsection in which specific materials suitable for that class of detector are evaluated in terms of the theory. Finally in Section 4.5 we will draw some conclusions about the status and prospects of photovoltaic and photoconductive infrared detectors. Symbols used in this chapter which are not defined in the text are defined in Table 4.1. 

Little data have been published very recently in the open literature on doped Si as an extrinsic photoconductive detector [4.32]. 

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

Impurity photoconductivity (extrinsic photoconductivity) is a type of absorption measurement where the detector is the sample itself. Classical photoconductivity occurs when the absorption of an electron or of a hole takes place between a discrete state and a continuum, where it can contribute to the electrical conductivity. When the final state of a discrete transition is separated from the continuum by an energy comparable to k T at the measurement temperature, the electron or the hole in this state can be thermally ionized in the continuum and give rise to photoconductivity at the energy of the discrete transition. This two-step process, which is temperature-dependent, is known as photo-thermal ionization spectroscopy (PTIS) and is discussed in more detail later in the section on extrinsic photoconductors. 

The performance of the radiation detectors depends on their intrinsic properties, temperature and external conditions of use. They can be compared by using a factor of merit D, known as the detectivity, equal to the inverse of the NEP for a detector with unit area used with an electrical band-width A/ of 1 Hz and expressed in cm Hz1/2 W-1. When a value of D is indicated for a thermal detector, it is considered to be independent of the radiation frequency and the time modulation frequency is assumed to be adapted to the intrinsic time constant t, of the detector. For a photoconductive detector, D peaks at a radiation frequency very close to the band gap for an intrinsic detector or to the ionization energy of the relevant centre for an extrinsic detector and decreases steadily at lower energies. 

The left side of (4.88) is plotted vs N — N in Fig. 4.10 for several possible operating temperatures we have used the values of B and NJg given in Appendix F, as well as t = 5 x 10 cm to be consistent with condition 4 below. By comparing these curves with that for Hg gCd jTe in Fig. 4.10 and with the abscissa of Fig. 4.1, we see that this extrinsic Si photoconductive detector requires considerably lower operating temperatures than the intrinsic photoconductor for comparable performance. This result is a well-known disadvantage of an extrinsic photoconductor [4.31]. The curves for a p-type example would lie... 

Condition 4 provides the highest possible BLIP detectivity by requiring that the quantum efficiency approach its maximum value of unity. This condition is easily met by relatively thin photovoltaic and intrinsic photoconductive detectors. However, it is a major problem for extrinsic Si photoconductors, because limited maximum values of dopant concentrations and absorption cross sections give rather low absorption coefficients, requiring undesirably thick detectors for high quantum efficiencies. 

Fig. 6.9. Monolithic IR focal plane using extrinsic photoconductivity in silicon. The CCD material can be grown epitaxially on the IR detector material. A longitudinal detector bias is used with L, being the interelectrode spacing for the photoconductor...

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. 

Consider first the simple extrinsic photoconductor. Here the sample is a semiconductor containing a single impurity level, the source of the free electrons (or holes) present in the sample. Thus the fluctuation in the number of the free carriers arises from the fluctuation in the generation and recombination rates through that level. If it is assumed that the temperature is so low that very few of the extrinsic centers are thermally ionized (which is valid for most extrinsic cooled photoconductive infrared detectors), then the short circuit g—r noise current and the open circuit g — r noise voltage which appear only in the presence of a bias current Ig, are given by... 

Single crystals of Hg,, Cd Te are grown by several different methods [4.42]. Almost regardless of the growth method or composition x in the above range, undoped ( pure ) crystals which are n-type at low temperatures have an extrinsic electron concentration near 10 cm" this is a relatively low carrier concentration for a semiconductor, and it is one of the major reasons for the success of Hg, j.Cd Te as a photoconductive infrared detector material. However, undoped crystals often turn out p-type with or have... 

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