Photon detector, infrared

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

Since it is a thermal and not a photon detector, there is no band gap, and no specific cut-off frequency. The device is a broadband detector capable of operation from the NIR to the long-wavelength mid-infrared. 

The convenient photon detectors discussed in the previous section cannot be used to measure infrared radiation because photons of these frequencies lack the energy to cause photoemission of electrons as a consequence, thermal detectors must be used. Unfortunately, the performance characteristics of thermal detectors are much inferior to those of phototubes, photomultiplier tubes, silicon diodes, and photovoltaic cells. 

A photon detector produces a current or voltage as a result of the emission of electrons from a photosensitive surface when struck by photons. A heat detector consists of a darkened surface to absorb infrared radiation and produce a temperature increase. A thermal transducer produces an electrical signal whose magnitude is related to the temperature and thus the intensity of the infrared radiation. 

The ability to obtain single-photon counting using methods such as avalanche photon detectors and negative electron affinity photocathode photomultiphers has thus far been limited to the visible and infrared regions. The vertical QCD which utihzes a triple-well quantum (Q) dot system of the type illustrated in Fig. 9 offers a novel approach to sense THz radiation. Here, the detector is first primed into active-mode by tunnel injection into the top Q-dot (QDl) of the SES followed by an IR pulse that puts the electron into the middle Q-dot (QD2) of the THz-RDC. This electron will remain in the QD2 until a THz photon induces the electron s transition to QD3. Finally, an IR photon ejects the electron from QD3 thus resetting the detector. Since the electron injection into the QCD system and ejection from the detector are quick transitions, only the middle Q-dot (QD2) will be occupied for a significant period of time. Consequently, to successfully read-out the state of our triple Q-dot system one must be able to differentiate between the two possible states (1) if THz photons are present, the electron will quickly be ejected from the entire QCD system and no electrons will be present in QD2 or (2) if no THz photon is present, QD2 will remain occupied by an electron. 

Because many atmospheric constituents absorb efficiently in the infrared spectral region, the development of an IR multichannel spectrometer was important to complement our near IR spectrometers. Originally, the pyroelectric vidicon (16) was selected as the IR image devices. This imager is a thermal rather than photon detector, and as such detects only variations... 

S. Komiyama, O. Astafiev, V. Antonov, T. Kutsuwa, H. Hirai, A single-photon detector in the far-infrared range. Nature, 403 (2000), 405 07. 

Photographic plates are extremely sensitive photon detectors in this same region. The speed of data acquisition is, of course, much lower owing to the processing time required for the plate. Most photographic plates are insensitive in the infrared region, but infrared phosphors extend their range somewhat into the infrared. 

R. W. Boyd, Photon bunching and the photon-noise-limited performance of infrared detectors. Infrared Phys. 22(3), 157-162 (1982). ISSN 0020-0891. doi 10.1016/0020-0891(82)90034-3. http //www.sciencedirectcom/science/article/pii/0020089182900343... 

The topics included here are limited to the usual types of noise in the common types of infrared photon detectors. Noise in thermal detectors, such as temperature noise in bolometers, is not included. Noise associated with the avalanche process is omitted. The detailed noise theory of phototransistors, an extension of shot noise in photodiodes, is not included. Modulation noise, an example of which arises from conductivity modulation by means of carrier trapping in slow surface states, is not included. Pattern noise, due to the... 

Because the performance of infrared detectors is limited by noise, it is important to be able to specify a signal-to-noise ratio in response to incident radiant power. An area-independent figure of merit is D ( dee-star ) defined as the rms signal-to-noise ratio in a 1 Hz bandwidth per unit rms incident radiant power per square root of detector area. D can be defined in response to a monochromatic radiation source or in response to a black body source. In the former case it is known as the spectral D, symbolized by Df X, f, 1) where A is the source wavelength,/is the modulation frequency, and 1 represents the 1 Hz bandwidth. Similarly, the black body D is symbolized by Z> (T,/1), where T is the temperature of the reference black body, usually 500 K. Unless otherwise stated, it is assumed that the detector Held of view is hemispherical 2n ster). The units of D are cm Hz Vwatt. The relationship between )J measured at the wavelength of peak response and D" (500 K) for an ideal photon detector is illustrated in Fig. 2.14. For an ideal thermal detector, Df = D (T) at all wavelengths and temperatures. 

These are the second and third conditions on the material and design of an infrared photon detector they are partly interrelated insofar as they involve common parameters. When conditions 1-3 are satisfied the only detector parameter on which Z)J depends is q. One generally also wants to achieve the highest possible value of DJ (BLIP) by having the quantum efficiency near its maximum possible value i.e.,... 

Summarizing and simplifying (4.6) and (4.16-18), we have four conditions on the material and design of an infrared photon detector ... 

Condition 1 is not as readily satisfied by doped Si as it is by the alloy semiconductors used in intrinsic photon detectors. The various known dopants can satisfy condition 1 at a number of different wavelengths in the infrared, but none of them is well matched to the important 8-14 pm interval, and possible dopants for the 3-5 pm interval are relatively scarce see Fig. 4.15 in Appendix F. Thus there is a need for better dopants to satisfy condition 1 more closely for some important detection wavelengths, but the prospects for success of a search for new dopants are uncertain. It remains to be seen whether new doping methods can be developed to satisfy condition 1 better. 

We have formulated the theory of infrared photon detectors in terms of four conditions which a detector must satisfy to.achieve nearly BLIP performance. These conditions are convenient for intercomparing the materials and effects used in photon detectors. 

These materials include compounds and alloys formed between elements of groups IV and VI of the periodic table. The properties of the narrow-gap semiconductors of this class which are useful in infrared photon detectors have been reviewed in detail very recently by Harman and Meingailis [4.3], so that we will only summarize here those properties needed in this chapter. The alloy systems Pbi j.Sn Te, Pbi jjSnjj Se and Pb, Ge Te have all been studied as detector materials, but Pbj Sn Te has received the most emphasis. Accordingly, we will review the properties only of Pbj j(Sn Te in this appendix the other materials are analogous. 

The materials in this class which are used in infrared photon detectors include the compounds InSb and InAs and the alloy system Hg,, Cd,jTe. The Hgj Cd Te alloys are the most important of these materials for detectors, and their properties shall be reviewed here. But InSb and InAs are analogous to those Hgi j Cd, Te alloys which have comparable energy gaps. Since the most recent comprehensive review of Hgj, Cd,jTe was published several years ago [4.40], we shall take this opportunity to update it, although still emphasizing the properties needed in this chapter. 

The more notable and widely used infrared detectors can be divided into three basic classes which are indicative of the primary effect produced by the photon-detector interaction, i.e., thermal, photoconductive, photovoltaic, and photoemissive. Chapters 3, 4, and 5 offer a detailed treatment of each of these important processes. 

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