MCT Based Photovoltiac Detectors

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3. Fundamental HgCdTe Properties

HgCdTe ternary alloy is a nearly ideal IR detector material system. Its position is

conditioned by three key features

• tailorable energy band gap over the 1 – 30 µm range,

• large optical coefficients that enable high quantum efficiency and

• favorable inherent recombination mechanisms that lead to high operating

temperature.

Table 1 (attached at Annexure A) summarizes various material properties of Hg1− xCd xTe.

Table 2 (attached as Annexure B) compares important parameters of HgCdTe with other narrow

gap semiconductors used in IR detector fabrication.

3.1 Energy Band Diagram

The electrical and optical properties of Hg1− xCd xTe are determined by the energy gap

structure in the vicinity of the Ґ-point of the Brillouin zone. The energy gap of this compound at

4.2 K ranges from −0.300 eV for semi-metallic HgTe, goes through zero at about x = 0.15 and

opens up to 1.648 eV for CdTe. The band structure of Hg1− xCd xTe is shown in Figure 2.

Figure 2. The bandgap structure of Hg1 – xCdxTe near the Γ-point for three different values of

the Eg.

3.2 Intrinsic Carrier Concentrations and Effective Masses of Electrons and Holes

The intrinsic carrier concentration ni (cm-3) of HgCdTe is given by



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3.5 Optical Properties

Optical absorption coefficient data for several alloy compositions of Hg1− xCd xTe versus

wavelength are plotted in Figure 3. We can see that the absorption strength generally decreases

as the gap becomes smaller.

Figure 3. Optical absorption coefficient plotted versus wavelength

3.6 Lattice Constants

The use of CdTe for passivation is very promising since it has high resistivity, is lattice

matched, and is chemically compatible to Hg1−xCd xTe. Barriers can be found in both the

conduction and valence band. The comparison of lattice constants of various materials is shown

in Figure 4.

Figure 4. Comparison of lattice constants of various semiconductors vs bandgap energy and

wavelength



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3.7 Generation – Recombination Processes

The generation processes which compete against the recombination processes directly affect

the performance of photodetectors, setting up a steady-state concentration of carriers in

semiconductors subjected to thermal and optical excitation and, frequently determining thekinetics of photo generated signals.

3.7.1 Radiative Processes

In HgCdTe, the internal radiative processes although of fundamental nature, do not limit the

ultimate performance of IR detectors.

3.7.2 Auger Processes

Auger mechanisms dominate generation and recombination processes in Hg1−xCd xTe at near

room temperatures. The Auger generation and recombination rates are strongly dependent on

temperature via dependence of carrier concentration and intrinsic time on temperature.

Therefore, cooling is a natural and a very effective way to suppress Auger processes.

3.7.3 Carrier Lifetimes

The total carrier lifetime τ total can be calculated from the independent lifetimes attributed to

each recombination mechanism. The sum of all contributions is given by

4. Operation of HgCdTe p-n Junction Photodiode[4]

The photovoltaic p-n junction photodiode is representative of high performance HgCdTe

IR detector technology. Figure 5 illustrates the basic operation of a simple p-n homojunction

detector under illuminated zero-bias and reverse bias conditions. Incident photons with energy

greater than the material bandgap (E ph>Eg) generate electron-hole pairs in the material on either

side of the p-n junction.

Carriers generated within a diffusion length of the space-charge region diffuse to the

electrical junction and are separated by the strong electric field. The electron and hole minority

carrier diffusion lengths Ln and L p (cm) are given by

(Eq. 4)



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where τ n and τ p are the electron and hole minority carrier recombination lifetimes(s), and Dn and

D p are the electron and hole minority carrier diffusion coefficients (cm2s-1).

Figure 5. Schematic of structure and energy band diagram of a HgCdTe p-n homo-junction

photodetector device at zero-bias (inset) and under reverse bias and illumination.

Figure 6. Schematic representation of current-voltage characteristics of photodiode

The optically excited minority carriers that are injected into the opposite side of the

junction by the E field become majority carriers. By this process, a photocurrent density J ph is

generated by the incident radiation, increasing the dark saturation current value as illustrated in

Figure 6.

The total current density J total (A/cm2) in the p-n junction photodiode is given by[3]

(Eq. 5)



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J total(V bias) = J dark (V bias) -J ph(φ) (Eq.6)

where J dark depends on V bias, and J ph depends on the incident photon flux density (ph/cm2s). In a

simple photovoltaic detector with no gain, the photocurrent is equivalent to[3]

I ph = J ph A = nqφ A (Eq. 7)

4.1 Detector Structure

Most commonly used photovoltaic HgCdTe photodiodes are fabricated in a homojunction

configuration with a highly doped n+ cap layer in contact with a lightly doped, narrow bandgap

p-type absorber (base) layer, or in a heterojunction configuration with wide bandgap (denoted by

capitalized letter) P+ cap layer in contact with a lightly doped, narrow bandgap n-type absorber

layer, as shown in Figure 7.

In these structures, the absorber layer determines the dark current and photo current of

the device. The base layer doping in both structures must be below1016 cm-3 to avoid tunneling

current contribution, and the doping

Figure 7. Schematic of energy band diagrams of commonly used unbiased (a) n-on-p

homojunction and (b) P-on-n heterojunction photodiodes under backside illumination

values used are typically ~5×1015 cm-3 and ~5×1014 cm-3 for p-type and n-type layers,

respectively.



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Figure 8. HgCdTe DLHJ structure on Si substrate for FPA integration

Double layer heterojunction (DLHJ) architectures, shown in Figure 8, have been

developed for high performance LWIR detectors due to low thermal leakage currents[5].

4.2 Current Contribution Mechanisms

The diffusion current I diff (A) is described by the basic diode equation,

and n po and pno are the equilibrium electron and hole concentrations (cm-3) on the p- and n-sides

of the junction, respectively. These mechanisms, several of which are illustrated in Figure 9,

include generation-recombination (G-R) in the depletion region, band-to-band (BTB) tunneling,

trap-assisted tunneling (TAT), and surface recombination. The total dark current can be

expressed as the sum of all contributions

Materials with narrow bandgaps are especially susceptible to thermal G-R processes that

dominate the dark current at high temperatures. The dark current contribution to overall current

is illustrated in Figure 10.

where



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Figure 9. Schematic of structure and energy band diagram of a HgCdTep-n junction DLPH

photodetector device under reverse bias and illumination.

Figure 10 - Dark current contribution mechanisms in reverse-biased p-n junction

Band-to-band tunneling occurs when electrons at a given energy level tunnel across the

bandgap from the valence band on one side of a junction to the conduction band on the other to

fill an unoccupied state at the same energy level. Band-to-band tunneling exhibits a strong

dependence on E g and E field , where the field strength depends on V bias and the doping

concentration of the junction. Trap assisted tunneling is caused by carriers tunneling within the

depletion region via intermediate trap states created by impurities or defects. The tunneling

occurs in a two-step process involving one thermal transition from the valence band to the trap

state and a second transition from the trap state to the conduction band. Because the required

transition energies are smaller, trap-assisted tunneling occurs at lower fields than band-to-band



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extremely small change of lattice constant with composition makes it possible to grow high

quality layers and heterostructures. HgCdTe can be used for detectors operated at various modes,

and can be optimized for operation at the extremely wide range of the IR spectrum (1 – 30 µm)

and at temperatures ranging from that of liquid helium to room temperature.

7. Limitations of HgCdTe Detectors

Thermal G-R processes, namely intrinsic Auger generation, dominate the dark current

density in narrow bandgap materials, fundamentally limiting the detector performance at higher

temperatures. Thus, high performance detectors are typically operated at cryogenic temperatures

(77 K or lower). These cooling systems are extremely bulky, require high input power, and are

prohibitively costly to implement. Increasing the detector operating temperature to a range where

the cooling requirements can be met by TE coolers (193 K-253 K) is advantageous as thesesystems are smaller, simpler, and less expensive to integrate

At the processing level, achieving stable, well-controlled, extrinsic p-type doping

required for p-n junction PV structures by ex situ ion implantation or in situ doping via molecular

beam epitaxy (MBE) poses a significant challenge for HgCdTe detector technology. Ex situ

doping techniques (implantation, in-diffusion) cannot easily provide the level of control over

doping profiles necessary for abrupt junction, multi-layer structures and can introduce material

damage, reducing the carrier mobilities and minority carrier lifetimes.

8. Conclusion

HgCdTe is an important material for high performance IR photodetectors and continues

to outperform competing materials, achieving near theoretical performance limits. Properties

such as the high absorption coefficient, high carrier mobility, and lattice matched bandgap

tunability make HgCdTe an attractive option for IR detector applications. However, extrinsic and

intrinsic G-R processes such as SRH, radiative, and Auger mechanisms are major factors which

affect detector performance in narrow bandgap materials and must be taken into account.



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References

[1]Donald A. Neaman “ Semiconductor Physics and Devices” 3 rd Edition, 2003, McGrawHill,

ISBN 0-07-232107-5

[2]Antoni Rogalski, “HgCdTe Infrared Detector Material: History, Status And Outlook”, Reports

on Progress in Physics, Pages from 2269 to 2331, Rep. Prog. Phys. 68 (2005) 2267 – 2336,

Published 22 August 2005, Institute Of Physics Publishing.

[3][Online] Available at https://en.wikipedia.org/wiki/Infrared_detector.

[4]Anne M. Itsuno, “Bandgap-Engineered HgCdTe Infrared DetectorStructures for Reduced

Cooling Requirements”, 2012, The University of Michigan

[5]P. Capper and J. Garland, editors. Mercury Cadmium Telluride, Growth, Properties and

Applications. Wiley, 2011.

[6] Antoni Rogalski, “Infrared Detectors”, 2nd Edition, CRC Press, ISBN 978-1-4200-7671-4

MCT Based Photovoltiac Detectors

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