Infrared Detector Characterization

Optical radiation is regarded as the radiation over the range from vacuum ultraviolet to the submillimeter wavelength (25 nm to 3000 pm). The terahertz (THz) region of the electromagnetic spectrum (Fig. 2.1) is often described as the final unexplored area of the spectrum and still presents a challenge for both electronic and photonic technologies. It is frequently treated as the spectral region within the frequency range v и 0.1-10 THz (Л » 3 mm-30 pm) and is partly overlapping with the loosely treated submillimeter (sub-mm) wavelength band z/«0.1—3 THz (2«3 mm-100 pm).

Classification of Infrared Detectors

The majority of optical detectors can be classified into two broad categories: photon detectors (also called quantum detectors) and thermal detectors.

Photon Detectors

In photon detectors, the radiation is absorbed within the material by interaction with electrons either bound to lattice atoms, to impurity atoms, or with free electrons. The observed electrical output signal results from the changed electronic energy distribution. The fundamental optical excitation processes in semiconductors are illustrated in Fig. 2.2. In quantum wells [Fig. 2.2(b)] the intersubband absorption takes place between the

The electromagnetic spectrum (after Ref. [1])

FIGURE 2.1 The electromagnetic spectrum (after Ref. [1]).

energy levels of a quantum well associated with the conduction band (n-doped) or the valence band (p-doped). In the case of a type-II InAs/ GaSb superlattice [Fig. 2.2(c)], the superlattice bandgap is determined by the energy difference between the electron miniband E, and the first heavy-hole state HH, at the Brillouin zone center. A consequence of the type-II band alignment is spatial separation of electrons and holes.

Relative response of infrared detectors is plotted as a function of wavelength, with a vertical scale of either W"1 or photon 1 (Fig. 2.3). The photon detectors show a selective wavelength dependence of response per unit incident radiation power. Their response is proportional to the rate of arrival of photons, as the energy per photon is inversely proportional to the wavelength. In consequence, the spectral response increases linearly with increasing wavelength [Fig. 2.3(a)], until the cut-off wavelength is reached, which is determined by the detector material. The cut-off

Optical excitation processes in (a) bulk semiconductors, (b) quantum wells, and (c) type-II InAs/GaSb superlattices

FIGURE 2.2 Optical excitation processes in (a) bulk semiconductors, (b) quantum wells, and (c) type-II InAs/GaSb superlattices.

Relative spectral response for a photon and thermal detector for (a) constant incident radiant power and (b) photon flux, respectively

FIGURE 2.3 Relative spectral response for a photon and thermal detector for (a) constant incident radiant power and (b) photon flux, respectively.

wavelength is usually specified as the long-wavelength point at which the detector responsivity falls to 50% of the peak responsivity.

Thermal detectors tend to be spectrally flat in the first case (their response is proportional to the energy absorbed), so that they exhibit a flat spectral response [Fig 2.3(a)], whereas photon detectors are generally flat in the second case [Fig. 2.3(b)].

Photon detectors exhibit both good signal-to-noise performance and a very fast response. But, to achieve this, the photon IR detectors may require cryogenic cooling. This is necessary to prevent the thermal generation of charge carriers. The thermal transitions compete with the optical ones, making non-cooled devices very noisy.

Depending on the nature of the interaction, the class of photon detectors is further sub-divided into different types (Table 2.1). The most important are intrinsic detectors, extrinsic detectors, photoemissive detectors (Schottky barriers), and quantum well detectors [2]. Different types of detectors are briefly characterized in Table 2.2 [3].

There is a fundamental relationship between the temperature of the background viewed by the detector and the lower temperature at which the detector must operate to achieve background-limited performance (BLIP). FIgCdTe photodetectors, with a cut-off wavelength of 12.4 pm, operate at 77 K. One can scale the results of this example to other temperatures and cut-off wavelengths by noting that, for a given level of detector performance, Tc« constant [4]; i.e., the longer the Ac, the lower T is, when their product remains roughly constant. This relationship holds because quantities that determine detector performance vary mainly as an exponential of EexclkT=hdkTXc> where Eexc is the excitation energy, к is Boltzmann’s constant, h is Planck’s constant, and c is the velocity of light.

Detect

or Type

Advantages

Disadvantages

Thermal (thermopile, bolometers, pyroelectric)

Light, rugged, reliable, and low cost Room temperature operation

Low detectivity at high frequency Slow response (ms order)

Photon

Intrinsic

IV-VI

(PbS, PbSe, PbSnTe)

Easier to prepare More stable materials

Very high thermal expansion coefficient Large permittivity

  • 11-VI
  • (HgCdTe)

Easy bandgap tailoring Well-developed theory and experiment.

Multicolor detectors

Non-uniformity over large area High cost in growth and processing Surface instability

III-V

(InGaAs, InAs, InSb, InAsSb)

Good material and dopants Advanced technology Possible monolithic integration

Heteroepitaxy with large lattice mismatch Long-wavelength cut-off limited to 7 pm for InAsSb at 77 К

Extrinsic

(Si:Ga, Si:As, Ge:Cu, Ge:Hg)

Very-long-wavelength operation Relatively simple technology

High thermal generation Extremely-low-temperature operation

Free carriers (PtSi, Pt2Si, IrSi)

Low cost, high yields

Large and close-packed 2-D arrays

Low quantum efficiency Low-temperature operation

Quantum wells

Type I

(GaAs/AlGaAs, InGaAs/ AlGaAs))

Matured material growth Good uniformity over large area Multicolor detectors

High thermal generation Complicated design and growth

Type II

(InAs/GaSb, InAs/InAsSb)

Low Auger recombination rate Easy wavelength control Multicolor detectors

Complicated design and growth Sensitive to the interfaces

Quantum dots

InAs/GaAs, InGaAs/InGaP, Ge/Si

Normal incidence of light Low thermal generation

Complicated design and growth

Mode of Operation

Schematic of Detector

Operation and Properties

Photoconductor

This is essentially a radiation-sensitive resistor, generally a semiconductor, in either thin-film or bulk form. A photon may release an electron-hole pair or an impurity-bound charge carrier, thereby increasing the electrical conductivity. In almost all cases, the change in conductivity is measured by means of electrodes attached to the sample. For low-resistance material, the photoconductor is usually operated in a constant current circuit. For high-resistance photoconductors, a constant voltage circuit is preferred and the signal is detected as a change in current in the bias circuit.

Blocked-impurity- band (BIB) detector

The active region of a BIB detector structure, usually based on epitaxially grown n-type material, is sandwiched between a higher-doped degenerate substrate electrode and an undoped blocking layer. Doping of the active layer is high enough for the onset of an impurity band to display a high quantum efficiency for impurity ionization (in the case of Si:As BIB, the active layer is doped to «5Х1017 cm'3). The device exhibits a diode-like characteristic, except that photoexcitation of electrons takes place between the donor impurity and the conduction band. The heavily doped n-type IR-active layer has a small concentration of negatively charged compensating acceptor impurities. In the absence of an applied bias, charge neutrality requires an equal concentration of ionized donors. Whereas the negative charges are fixed at acceptor sites, the positive charges associated with ionized donor sites (I)' charges) are mobile and can propagate through the IR-active layer via the mechanism of hopping between occupied (D°) and vacant (D+) neighboring sites. A positive bias to the transparent contact creates a field that drives the pre-existing D+ charges towards the substrate, whereas the undoped blocking layer prevents the injection of new D+ charges. A region depleted of I)' charges is therefore created, with a width depending on the applied bias and on the compensating acceptor concentration.

(Continued)

Mode of Operation

p-n junction photodiode

Operation and Properties

This is the most widely used photovoltaic detector but is rather rarely used as a THz detector. Photons with energy greater than the energy gap create electron-hole pairs in the material on both sides of the junction. By diffusion, the electrons and holes generated within a diffusion length from the junction reach the space-charge region, where they are separated by the strong electric field; minority carriers become majority carriers on the other side. This way, a photocurrent is generated, causing a change in voltage across the open-circuit cell or a current to flow in the short-circuited case. The limiting noise level of photodiodes can ideally be Jl times lower than that of the photoconductor, due to the absence of recombination noise. Response times are generally limited by device capacitance and detector-circuit resistance.

nBn detector

The nBn detector consists of a narrow-gap n-type absorber layer (AL), a thin wide-gap barrier layer (BL), and a narrow-gap n-type contact layer (CL). The thin wide-gap BL presents a large barrier in the conduction band that eliminates electron flow. Current through the nBn detector relies on transport of mobile holes through drift and diffusion in the BL between the two n-type narrow-gap regions. Effectively, the nBn detector is designed to reduce the dark current (generation- recombination current originating within the depletion layer) and noise, without impeding the photocurrent (signal). In particular, the barrier serves to reduce the surface leakage current. The nBn detector operates as a unipolar unity-gain detector and its design can be described as being a hybrid between a photoconductor and a photodiode.

(Continued)

Mode of Operation

Schematic of Detector

Operation and Properties

Metal-insulator-

semiconductor

(MIS)

photodiode

The MIS device consists of a metal gate separated from a semiconductor surface by an insulator (I). By applying a negative voltage to the metal electrode, electrons are repelled from the I-S interface, creating a depletion region. When incident photons create hole-electron pairs, the minority carriers drift away to the depletion region and the volume of the depletion region shrinks. The total amount of charges that a photogate can collect is defined as its well capacity. The total well capacity is decided by the gate bias, the insulator thickness, the area of the electrodes, and the background doping of the semiconductor. Numerous such photogates with proper clocking sequence form a charge-coupled device (CCD) imaging array.

Schottky barrier photodiode

Schottky barrier photodiodes reveal some advantages over p-n junction photodiodes: fabrication simplicity (deposition of metal barrier on n(p)-semiconductor), absence of high-temperature diffusion processes, and high speed of response. Since it is a majority carrier device, minority carrier storage and removal problems do not exist, and therefore higher bandwidths can be expected.

The thermionic emission process in the Schottky barrier is much more efficient than the diffusion process and, therefore, for a given built-in voltage, the saturation current in a Schottky diode is several orders of magnitude higher than in the p-n junction.

The detector temperature of low-background operation can be approximated as

The general trend is illustrated in Fig. 2.4 for six high-performance detector materials suitable for low-background applications: Si, InGaAs, InSb, HgCdTe photodiodes, and Si:X (X = As and Sb) blocked-impurity-band (BIB) detectors, and extrinsic Ge:Ga unstressed and stressed detectors. Terahertz photoconductors are operated in extrinsic mode.

The most widely used photovoltaic detector is the p-n junction, where a strong internal electric field exists across the junction, even in the absence of radiation. Photons incident on the junction produce free hole-electron pairs that are separated by the internal electric field across the junction, causing a change in voltage across the open-circuit cell or causing a current to flow in the short-circuited case. Due to the absence of recombination noise, the noise level of the limiting p-n junction can ideally be Jl times lower than that of the photoconductor.

Photoconductors that utilize excitation of an electron from the valence to the conduction band are called intrinsic detectors, whereas those which

Operating temperatures for low-background material systems with their spectral band of greatest sensitivity

FIGURE 2.4 Operating temperatures for low-background material systems with their spectral band of greatest sensitivity. The dashed line indicates the trend toward lower operating temperature for longer-wavelength detection.

operate by exciting electrons into the conduction band or holes into the valence band from impurity states within the band (impurity-bound states in energy gap, quantum wells, or quantum dots), are called extrinsic detectors. A key difference between intrinsic and extrinsic detectors is that extrinsic detectors require considerable cooling to achieve high sensitivity at a given spectral response cut-off, in comparison with intrinsic detectors. Low-temperature operation is associated with longer-wavelength sensitivity to suppress noise due to thermally induced transitions between close- lying energy levels. Intrinsic detectors are most commonly used at the short wavelengths, below 20 pm. In the longer-wavelength region, the photoconductors are operated in extrinsic mode. One advantage of photoconductors is their current gain, which is equal to the recombination time divided by the majority carrier transit time. This current gain leads to higher responsiv- ity than is possible with non-avalanching photovoltaic detectors. However, a series problem of photoconductors operated at low temperature is nonuniformity of the detector element due to recombination mechanisms at the electrical contacts and its dependence on electrical bias.

More recently, interfacial work-function internal photoemission detectors, quantum well, and quantum dot detectors, which can be included in extrinsic photoconductors, have been proposed, especially for IR and THz spectral bands [2,5]. The very fast time response of quantum-well semiconductor detectors and quantum dot semiconductor detectors makes them attractive for heterodyne detection.

Thermal Detectors

The second class of detectors is thermal detectors. In a thermal detector, shown schematically in Fig. 2.5, the incident radiation is absorbed to change the material temperature, and the resultant change in some physical property is used to generate an electrical output. The detector is suspended on lags, which are connected to the heat sink. The signal does not depend upon the photonic nature of the incident radiation. Thus, thermal effects are generally wavelength independent [Fig. 2.3(a)]; the signal depends upon the radiant power (or its rate of change) but not upon its spectral content. Since the radiation can be absorbed in a black surface coating, the spectral response can be very broad. Attention is directed toward three approaches that have found the greatest utility in infrared technology, namely bolometers, pyroelectric detectors, and thermoelectric effects. The thermopile is one of the oldest IR detectors, and is a collection of thermocouples, connected in series, to achieve greater temperature

Schematic diagram of a thermal detector

FIGURE 2.5 Schematic diagram of a thermal detector.

sensitivity. In pyroelectric detectors, a change in the internal electrical polarization is measured, whereas, in the case of thermistor bolometers, a change in the electrical resistance is measured. For a long time, thermopiles were slow, insensitive, bulky, and costly devices. But with developments in semiconductor technology, thermopiles can be optimized for specific applications. Recently, thanks to conventional complementary metal-oxide semiconductor (CMOS) processes, the on-chip circuitry technology of thermopiles has opened the door to mass production.

Usually, a bolometer is a thin, blackened flake or slab, the impedance of which is highly temperature dependent. Bolometers may be divided into several types. The types most commonly used are the metal, the thermistor, and the semiconductor bolometers. A fourth type is the superconducting bolometer. This bolometer operates on a conductivity transition, in which the resistance changes dramatically over the transition temperature range. Fig. 2.6 shows schematically the temperature dependence of resistance of the different types of bolometers.

Many types of thermal detectors are operated in the wide spectral range of electromagnetic radiation. The operation principles of thermal detectors are briefly described in Table 2.3.

Temperature dependence of resistance of three bolometer material types

FIGURE 2.6 Temperature dependence of resistance of three bolometer material types.

The microbolometer detectors are now produced in larger volumes than all other IR array technologies combined. At present, VOA. microbolometer arrays are clearly the most used technology for uncooled detectors. VOx is the winner in the battle between the amorphous silicon bolometers and the barium strontium titanate (BST) ferroelectric detectors.

The high cost of cryogenically cooled imagers, of around US$ 50,000, restricts their installation to critical military applications, involving operations in complete darkness. The commercial systems (microbolometer imagers, radiometers, and ferroelectric imagers) are derived from military systems that are too costly for widespread use. Imaging radiometers employ linear thermoelectric arrays operating in the snapshot mode; they are less costly than the TV-rate imaging radiometers that employ microbolometer arrays. As the volume of production increases, the cost of commercial systems will inevitably decrease. The current market price for a low-cost thermal imager is generally below US$1000. Recently, the first thermal imaging smartphone was launched [7].

 
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