Processing, Fabrication and Characterization of Advanced 

Target Sensors Using Mercury Cadmium Telluride (MCT)

by F. Semendy, G. Brill, P. Wijewarnasuriya, Y. Chen, J. Sun, J. Bickford, 

and N. Bambha

ARL-TR-5292 September 2010

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Army Research Laboratory

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ARL-TR-5292 September 2010

Processing, Fabrication and Characterization of Advanced 

Target Sensors Using Mercury Cadmium Telluride (MCT)

F. Semendy, G. Brill, P. Wijewarnasuriya, Y. Chen, J. Sun, J. Bickford, 

and N. Bambha

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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Processing, Fabrication and Characterization of Advanced Target Sensors Using 

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14. ABSTRACT

In this report the growth, processing, fabrication, and characterization of HgCdTe (or mercury cadmium telluride [MCT])

infrared (IR) detectors for advanced target detection are reported. The material was grown by solid source molecular beam 

epitaxy (MBE). Grown materials were characterized prior to processing to obtain information on carrier concentration, 

mobility and resistivity. Inductive coupled plasma (ICP) etching and metallization as required by the design of the sensors at 

different levels of processing were carried out using either AZ 5214 or AZ 9245 as photoresist masks. For the first time spin 

coating and lithography parameters were optimized with AZ9245 as the photoresist mask to achieve smooth surface and 

vertical side walls. An etch-through process of MCT and Si with a combined thickness of about 510 μm deep was carried out 

by ICP etching first by Ar/H2 gases followed by covering the sample by a thick AZ 9245 photoresist mask and Bosch process 

using SF6 , O2 and C4F8 to detach the device from the base carrier silicon substrate. To understand the nature of the etching 

and to protect the surface of the detector, the wafers were periodically withdrawn and thickness of the photoresist measured. 

A twenty-step process finally yielded the desired ring type detectors that were used for electrical characterization and testing 

for target detection. The results of extensive steps involved in processing, device fabrication, material, and device 

characterizations are reported.

15. SUBJECT TERMS

MCT, processing, lithography, photodetector, sensors

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Fred Semendy

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(301) 394-4627

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iii

Contents

List of Figures iv

List of Tables iv

1. Introduction 1

2. Experimental 2

3. Results and Discussion 9

4. Conclusions 12

5. References 13

List of Symbols, Abbreviations, and Acronyms 14

Distribution List 15
iv

List of Figures

Figure 1. A slab of IR photodetector with a volume of l.w.d with the signal measured across 

the load resistor RL.....................................................................................................................1

Figure 2. (a) Depiction of the ICP system, and (b) HgCdTe/CdTe/Si wafer on silcon carrier 

wafer thickness of the material and the surface of the devices..................................................4

Figure 3. (a) HdCdTe/CdTe/Si wafer with PR, (b) after exposure and developing a pattern, 

and (c) after ICP dry etching of HgCdTe/CdTe/Si with Ar/H2 gas system...............................5

Figure 4. Two different PR thickness before and after etching the PR HgCdTe/CdTe/Si. ............6

Figure 5. Five different PR thickness measurements of PR before and in between, after 

etching the silicon substrate......................................................................................................7

Figure 6. (a) Etched HdCdTe/CdTe/Si wafer with PR, (b) DRIE ICP etching, and (c) after 

ICP-DRIE dry etched sample by BOSCH process. ...................................................................7

Figure 7. Percentage change in the thickness of the PR during and after the etch with respect 

to the original thickness of the PR. ............................................................................................8

Figure 8. Nomarski microscope pictures of part of the fuze device, (a) fabricated device 

before etching, and (b) same part after etching.........................................................................8

Figure 9. Nomarski microscope pictures of fabricated device, (a) before, and (b) after dry 

etching........................................................................................................................................9

Figure 10. Fabricated devises, (a) 1-in size fuze detector, (b) test detectors from a 3-in wafer, 

and (c) another fuze detector......................................................................................................9

Figure 11. Blackbody flood illumination measurement illustration. ............................................10

Figure 12. Example detector response versus bias plot (line illustrates linear variation in 

response to applied bias. ..........................................................................................................11

List of Tables

Table 1. Representative blackbody test results for device comparison. .......................................11
1

1. Introduction

Infrared (IR) photodetectors are of great importance for military and civilian applications such as 

thermal imaging, heat seeking devices and proximity detection. IR detectors are divided into two 

categories: thermal and photon detectors (1, 2). Thermal detectors absorb incident radiation, 

increasing device temperature. This change in temperature, in turn, changes device resistance in 

the case of bolometric detectors and changes internal electric polarization in the case of 

pyroelectric detectors. Thermal detectors are inherently slow because the speed with which they 

change temperature is limited by their material’s specific heat and density. Although they can 

operate at room temperature (RT), these detectors are typically thermoelectrically (TE) cooled 

for thermal stability (3, 4). In photon detectors, photon absorption causes generation of electronhole pairs that give rise to an observable electrical signal. Photon detectors can be further 

divided into: (1) intrinsic direct band gap materials like HgCdTe (or mercury cadmium telluride 

[MCT]); (2) extrinsic type like Si (degenerately doped with In, Ga, Bi, As, P, or Sb) and Ge 

(doped with Au, Hg, Cd, Be, or Ga); or (3) photoemissive such as metal silicides and negative 

electron affinity materials. Photoconductive and photovoltaic phenomena are the two widely 

used modes for photo detection. Photon detectors are highly sensitive to radiation and operate 

only in a particular wavelength depending upon the bandgap of the material. The 

photoconductor, which is a sensor of choice in many applications, is designed in such a way that 

the electrons dominate the conductivity due to their high mobility, and holes play a secondary 

role. The operation of the photodiode is shown in figure 1. Photons of energy greater than the 

band gap energy Eg are absorbed to produce electron hole pairs. This in turn will change the 

electrical conductivity. 

Figure 1. A slab of IR photodetector with a volume

of l.w.d with the signal measured across the

load resistor RL.

The change in conductivity is measured by means of electrodes attached to the detector. A bias 

voltage is applied to sense the change in the conductivity (5). The first results of 

photoconductivity in Hg1–xCdxTe were reported by Lawson et al. (6). A comparison of detector 
2

performance using both one-dimensional and two-dimensional models have been reported (7, 8).

For low resistance material, such as Hg1–xCdxTe, the photoconductor is usually operated in a 

constant current circuit and the signal is detected as a charge in voltage developed across the 

load.

Infrared Hg1–xCdxTe MCT detectors can be designed properly to detect the presence of moving 

targets, or hot objects. These detectors can generally discriminate against various types of

radiation. Such detection systems are generally known as fuze systems, which activates the 

mechanism in the vicinity of the target (9). In the past, passive systems have depended upon the 

hot exhaust of an aircraft, or exhaust of nozzles of aircraft and missiles, or molecular emission 

from hot gases. In many cases the fuzes can be prone to premature triggering on exhaust plumes. 

Such false triggering can be alleviated by proper design of the fuze. A high quality of design 

would include an optics and detector cells sensitive to radiation from the skin of the target due to 

kinetic heating, and insensitive to any other radiation. The fuze detector system can be designed 

in different shapes and forms. For example a circular type fuze has a central hole at the center 

for mounting and holding and with certain dimensions. Special attention must be paid to the 

opening in the Hg1–xCdxTe material and provisions must be made for attaching wires to the 

detector. A properly designed, processed, and fabricated Hg1–xCdxTe detector with proper optics 

will perform as a good proximity fuze and was the focus of this work.

2. Experimental

Hg1–xCdxTe material was grown by molecular beam epitaxy (MBE) using 3-in Si substrates. 

Although, a large lattice mismatch exists (19%), growth on Si substrates has the advantage of 

being able to fabricate numerous dies per wafer. This is especially advantageous when the 

device is large, as was the case for devices discussed in this report. To overcome the lattice 

mismatch issue, extensive research has gone into developing buffer layer technology that 

transitions from the Si lattice parameter to the HgCdTe lattice parameter while still maintaining 

high quality crystalline material. In summary, a thin ZnTe buffer layer is deposited by MBE and 

annealed followed by a thicker CdTe layer. During CdTe MBE growth, the growth is interrupted 

several times while the sample is annealed under an appropriate material flux. It has been 

determined recently that this annealing process is critical for obtaining high quality CdTe/Si 

material (10). Once growth is completed, the CdTe/Si material is known as a composite 

substrate. HgCdTe is then nucleated on this composite substrate using an MBE system capable 

of depositing Hg compounds. For the devices fabricated in this study, the HgCdTe layer 

thickness varied between 2 and 6 m and had a cut-off wavelength varying between 2.5 and 

5.0 m measured at room temperature. All layers were characterized by Fourier Transform 

Infrared Spectroscopy (FTIR) prior to device fabrication. Additionally, some layers had 

representative pieces cleaved off from the larger 3-in sample for Hall measurements. 
3

The lithographic processing of the Hg1–xCdxTe 3-in wafers must be carried out very carefully. 

There are a number of problems that arise when processing Hg1–xCdxTe/Si. First, group II–VI

materials are soft and highly sensitive to temperature. Second, after all the processing and 

metallization is completed, the sensor material (Hg1–xCdxTe) and the substrate (silicon) must be 

dry etched away to separate the fabricated detector from the rest of the unprocessed material. 

Each of these steps raises special and unique problems and challenges due to the fact that the 

sensor material is soft and highly temperature sensitive. The lithographic process includes a 

number of baking at certain temperatures for a duration of time. This becomes a challenge with 

a thick photoresist (PR). A thick PR is required for the dry etching of the Hg1–xCdxTe/Si. Since 

this type of deep etch is generally not performed for Hg1–xCdxTe/Si planar devices, new 

processes must be explored, tried and developed through experimentation. Second, dry inductive 

coupled plasma (ICP) etching needs to be done twice with a set of conditions to etch MCT and 

CdTe the buffer layer and another set of conditions to etch through the silicon substrate. The 

first etching is performed with a certain amount of argon and hydrogen gases at a pre-decided 

pressure, RF chuck power and ICP power to etch through the Hg1–xCdxTe/CdTe material. This 

etching is followed by a second ICP deep reactive ion etch (ICP-DRIE) also known as Bosch 

process using SF6, O2, andC4F8. The Bosch process is composed of multiple steps of alteration 

between etching and deposition cycle. First, the silicon is etched in almost isotropic mode in 

short-time etch step with SF6 gas and platen power. Then, a polymer layer, which serves to 

protect sidewall during the etching, is deposited on the silicon and mask with C4F8 gas. In the 

following step, the polymer layer is removed and the underneath silicon is subsequently etched 

away. There are several key process parameters in Bosch process, such as coil power platen 

power, gas flow rate, pressure, and duration time of etch/deposition process cycle. The platen 

power is employed to vary the substrate potential while the coil power is inductively coupled,

which is the high power supply and is used to generate the plasma; the chamber pressure is 

normally controlled by varying the automatic pressure control valve and a high value indicates 

high pressure (11, 12). The controlling parameters can affect the resulting profile, the etch rate, 

and selectivity. The DRIE process affects the device’s performance seriously; extreme measures 

must be taken to protect the surface by monitoring the surface by retrieving and measuring the 

photoresist thickness at least a number of times before the end point is reached. In plasma 

etching, micro-loading (13), which is present during etching and must be considered for the

feature size of the elements the device under consideration. Micro-loading can be described as 

the interdependency between the etch rate and the aspect ratio. Usually the wider trenches etch 

faster than the narrow trenches. In the case of etch through, as in our case, it is necessary to 

consider an over etch time in order to compensate this difference of etch rate in wider and 

narrow trenches. Undesirable effects occur in the wide trenches over etched for a long time. But 

in our case this effect was minimal due to the optimized process we developed in our lab.

The HgCdTe wafers were cleaned in HF to remove the native oxide and used after further 

cleaning in acetone and isopropyl alcohol (IPA) and blow dried in acetone. It was spin coated

with AZ 5214 and baked at 75 °C for 30 min and exposed with appropriate mask for alignment
4

and mesa etch. The pattern was developed and surface cleaned for any hanging bonds by a 

descumming step using March RIE system. Wet etching technique was used to open up MCT as 

defined by the the mask using Br2 in 100 ml lactic acid for about 60 s. The PR was removed 

using acetone and IPA and blow drying with N2. This was followed for in-situ CdTe contact hole 

lithography using standard procedures using AZ5214 PR. Holes were created by a wet etching 

technique using acid-dichromate solution. Test samples were cleaned in acid solution to remove 

native oxide and leaded in to an ebeam evaporator. Gold was deposited to a thickness of 3000 Ǻ 

at a pressure of 1×10–6torr. The test sample was cleaned in acetone to remove all of the 

remaining PR. Next, as part of the design of the device, a passivation layer of CdTe was grown 

on top of the processed wafer. Using photolithography and AZ 5214, patterns were laid on top 

of the device and contact holes were opened up when exposed PR was developed, which was 

followed by another image by photo-lithography using the same PR. After proper preparation, 

Ti/Au in the amount of 200/3000 Ǻ were deposited at a pressure of 1×10–6 torr in the e-beam 

evaporator. After proper cleaning of the wafer it was prepared again using standard 

photolithography using AZ 5214 PR with the soft bake done at 75 °C for 45 min and after 

exposure a hard bake at 90 °C for 3 min. After developing the pattern for support metal, the 

wafer was descummed and acid dipped prior to loading on to the e-beam evaporator. At a 

pressure of 1×10–6 torr support metals Ti/Au (300, 9000 Ǻ) were deposited with careful 

deposition of Au in three steps with a waiting time of 45 min in between to avoid any over 

heating of detector material. Once the metallization was complete, samples were retrieved and 

PR removed using the conventional techniques and inspected. Processed wafers, which appeared 

without any imperfections with the fuze detector pattern, were chosen for further steps of 

fabrication. Figure 2 shows (a) the etch ICP etch system and (b) the etching material 

HgCdTe/CdTe/Si mounted on a carrier Si wafer. In figure 2 (b) the rings indicate the surface 

depiction of the device and the red circles indicate position of the thickness measurement of the 

PR before and after etching.

Figure 2. (a) Depiction of the ICP system, and (b) HgCdTe/CdTe/Si wafer on silcon carrier wafer 

thickness of the material and the surface of the devices.
5

The next important step in fabricating the device is to vertically dry etch MCT/CdTe/Si as part of 

the etch through and separate the device from the mother wafer. Since the entire thickness of the 

MCT film had to be removed, standard PR such as AZ5214 cannot be used because of the 

selectivity issues. There will be substantial erosion of the resist due to the damage that can be 

caused by the etching gases. Hence we substitute AZ 9245 as the PR of choice. It is a thick PR 

and will provide high resolution, superior aspect ratio and wide focus, exposure latitude, and 

good sidewall profiles. Patterns can be developed in inorganic based potassium hydroxide. 

However, AZ 9245 has never been used with MCT for a large scale and almost 3-h dry etch 

processing. Hence we found it a challenge to optimize the etch steps and procedures for II–VI 

MCT. In our newly developed proprietary process, a typical processing of the MCT etching 

includes spinning AZ 9245 at 1000 rpm and baking at 80 °C for 60 min followed by leaving the 

wafer at room temperature for 60 min to absorb the moisture. The wafer was aligned and photoexposed using a Karl Susse MA-6; the wafer was developed in AZ 400 K developer, which was 

followed by baking the sample at 90 °C for 15 min on a hot plate. The sample was left at RT for 

another 2 h before processing. The 3-in wafer was then mounted on a 4-in carrier Si wafer using 

cool grease. For it to stick to the 4-in wafer surface it was heated at 50 °C for 1 min. Oxford 

Plasmalab system 100 ICP-RIE system was used to etch the MCT wafer. It has a 13.56 MHz 

driven parallel plate reactor. To etch the wafer we used Ar/H2 (45 sccm/45 sccm) gas system 

with 10 mT pressure, ICP power of 800 W, and RF power of 50 W. Care was taken to measure 

the five point (as depicted in figure 2 (b) by the red circle) thickness of the PR before and after 

etch. The material etching is with the PR mask and is indicated in figure 3.

(a) (b) (c) 

Figure 3. (a) HdCdTe/CdTe/Si wafer with PR, (b) after exposure and developing a pattern,

and (c) after ICP dry etching of HgCdTe/CdTe/Si with Ar/H2 gas system.

An additional step of measuring the thickness midway during etching was also included as part 

of the monitoring of PR thickness after etch stop and retrieving the sample. The measured 

thickness of PR is shown in figure 4.

 plasma

 PR

 CdTe

 MCT

 CdTe

 Silicon
6

Figure 4. Two different PR thickness before and after etching the PR HgCdTe/CdTe/Si.

The blue cylindrical illustrations indicate the thickness measurements of PR before etching and 

the red are after etching. Calculations indicate a markedly reduced thickness of the PR of about 

80% between the two states. The MCT 3-in wafer was detached from the carrier 4-in silicon 

wafer and cleaned thoroughly with acetone and IPA and removed of all PR with a final stop of 

RIE O2 descumming etch for 90 s to remove the PR at low RF power. The optical patterning 

was repeated with the AZ 9245 deposited at 1000 rpm for 40 s. The wafer was soft baked at 

80 °C for 60 min followed by exposure with the mask. The pattern was developed using AZ 

400 K developer and cleaned carefully using deionized water and blow dried using nitrogen. 

The wafer was then baked at 90 °C for 15 min on a hot plate and left at room temperature for 

60 min. The wafer was mounted a 4-in silicon wafer for ICP-DRIE etch through to separate the 

fuze detectors. As part of the plan to make sure that there is enough PR to protect the MCT 

surface, the wafer was periodically withdrawn and thickness of the PR was carried out about five 

times. The etch rate for the silicon using Bosch process is about 3 μm/min, which will give a 

calculated time to etch a 500-μm silicon wafer about 2.8 h (~10000 s). The measurement of the 

thickness of PR at five different positions as indicated in figure 2 (b) at five different times are 

recorded and the graphical representation is shown in figure 5.
7

Figure 5. Five different PR thickness measurements of PR before and in between, after etching 

the silicon substrate.

In figure 6 the ICP-DRIE etch using Bosch process is depicted. The full etch through of the 

silicon wafer took about 10000 s for completion. 

(a) (b) (c)

Figure 6. (a) Etched HdCdTe/CdTe/Si wafer with PR, (b) DRIE ICP etching, and (c) after ICP-DRIE dry etched 

sample by BOSCH process.

Figure 7 illustrates the percentage change in the thickness of the PR during and after the etch 

with respect to the original thickness. One can observe that a drastic reduction in the thickness 

occurs between the second and third measurements. The larger difference is due to the time 

difference in measurements. However, after the next measurement the thickness difference does

not show a major change. This may possibly be due to back filling and saturation.

 PR

 CdTe

 MCT

 CdTe

 Silicon

 plasma
8

Figure 7. Percentage change in the thickness of the PR during and after the etch with respect to the original 

thickness of the PR.

In our fabrication of devices and subsequent steps, we were extremely careful in pursuing the

step-by-step processing of lithography and dry etching. This extreme care was necessary and has 

produced quality undamaged surface as can be observed in the surfaces of the fabricated devices, 

as seen by the microscopic pictures in figures 8 (a), (b) and 9 (a), (b). This certainly indicates

that the process developed in the U.S. Army Research Laboratory (ARL) is a desirable procedure 

in protecting the surface of the MCT while processing and dry etching test samples for an

extended period of time. This newly developed procedure enabled us to fabricate MCT/CdTe/Si 

based thick photo detectors.

Figure 8. Nomarski microscope pictures of part of the fuze device, (a) fabricated device before 

etching, and (b) same part after etching. 
9

Figure 9. Nomarski microscope pictures of fabricated device, (a) before, and (b) after dry etching.

3. Results and Discussion

The pictures of the fabricated fuze devices are given below in figure 10. Figure 10 (a), (b), and 

(c) show detectors fabricated form a single 3-in wafer using the ARL developed procedure. 

Yellow rings indicate metal and grey colored parts indicate the sensing material. 

Figure 10. Fabricated devises, (a) 1-in size fuze detector, (b) test detectors from a 3-in wafer, and (c) another fuze 

detector.

The fuze devices have detector active areas, bond pads, and openings at the center to 

accommodate lens system. In a final assembly, the detector fuze and the wires will be 

encapsulated to protect them from damages. 

Test devices consisted of both square and tactical ring geometries. Several different devices 

were tested and compared across various growth and fabrication iterations to understand their
10

impact. Measurements were taken using a calibrated blackbody source at a specific temperature 

T, set a given distance away from the detectors d, and apertured with a circular opening of 

diameter D as shown in figure 11. Given that the blackbody source used has a nearly spectrally 

flat emissivity  of 0.99, the full-spectrum Incidence E in W/m2shining on the detector is given 

by equation 1, where  is Stefan-Boltzmann’s constant equal to approximately 5.67e-8 

W/(m2.K

4

). This full-spectrum Incidence is used to characterize the detectors because the 

relative spectral response of each detector was not accounted for.

Figure 11. Blackbody flood illumination measurement illustration.

 

 

  





















   

2

2

2

4 4

2

2

Incidence

d D

D

E TBlackbody TAmbient 

(1)

Photoconductors exhibit a change in resistance that is linearly proportional to their incident 

power, which is the product of incidence E and device area A. This linear response continues 

from negligible incident power up to the detector’s saturation limit. This limit was not exceeded 

during testing and all data presented here is from within the linear incident power region. Each

photoconductor’s change in resistance is easily compared with other similar sized and shaped 

devices if the incident power is accounted for as shown in equation 2 (represented in W–1).

















 















R E A

R

R E A

R R

dark dark

light dark 1 1

(2)

The change in resistance is divided by the device’s dark resistance in order to normalize each 

device’s change in resistance versus bias. This ratio is used because any given device’s change 

in resistance is linearly proportional to its bias voltage up to its saturation bias. By staying below 

this saturation level the slope of this linear proportion can be described by this ratio of change of 

resistance over dark resistance as shown in figure 12.
11

Figure 12. Example detector response versus bias plot (line illustrates linear 

variation in response to applied bias.

This figure of merit is invariant for a given device geometry and size and varies only with device 

material and contact characteristics. In this way duplicate device geometries and sizes in 

separate runs can be used to compare growth and fabrication variations consistently. Deriving a 

device geometry and size invariant figure of merit is therefore beyond the scope of this study. 

Select device results are shown in table 1.

Table 1. Representative blackbody test results for device comparison. 

Device Rdark () Area (m2) Incidence (W/m2) Response (W–1)

HgCdTe fuze #2 

250 m×250 m

101.0 6.25E-8 35.541 1.332E+3

HgCdTe fuze #5 

250 m×250 m

327.96 6.25E-8 2.897 5.811E+3

HgCdTe fuze #7 

250 m×250 m

197.61 6.25E-8 2.897 2.168E+3

HgCdTe fuze #11 

250 m×250 m

78.28 6.25E-8 2.897 2.661E+2

HgCdTe fuze #14 

250 m×250 m

4864 6.25E-8 2.897 –

HgCdTe fuze #15 

250 m×250 m

10058 6.25E-8 2.897 –

HgCdTe fuze #19 

250 m×250 m

contacts 

nonlinear – – –
12

Although the data shows considerable measurement variance, table 1 shows an improvement in 

response figure of merit with higher dark resistance. This was the motivation to purposefully 

increase the HgCdTe resistivity. The resistance was improved by increasing the Cd composition, 

which yielded a higher bandgap. This higher bandgap increased the resistance by having less 

free carriers at room temperature. In addition to increasing the resistance, the higher bandgap 

also decreased the absorption cutoff wavelength causing the responsivity to decrease. In 

addition, the higher bandgap also made electrical contact to the devices much more resistive. 

The highest Cd composition devices tested (fuze #19) had nonlinear contacts, whose 

measurement results could not be fairly compared. Some batches of HgCdTe suffered from 

unintentional doping, as well, which lowered device resistance. In future revisions the Cd 

composition should be optimized, along with contact resistivity, and the unintentional doping 

lowered as much as possible to create the best possible device candidate material.

4. Conclusions

We reported the growth, processing, fabrication and characterization of HgCdTe IR detectors for 

advanced target detection. The material was grown by solid source MBE. Grown materials 

were characterized prior to processing to obtain information on carrier concentration, mobility 

and resistivity. ICP etching and metallization as required by the design of the sensors at different 

levels were carried out using either AZ 5214 or AZ 9245 as PR masks. For the first time spin 

coating and lithography parameters were optimized with AZ9245 as the PR mask to achieve 

smooth surface and vertical side walls. An etch-through process of MCT and Si with a 

combined thickness of about 510-μm deep was carried out by ICP etching first by Ar/H2 gases 

followed by a deposition and photolithography with AZ 9245 PR mask, and a DRIE using SF6, 

O2, and C4F8 gas chemistry by the Bosch etch process to detach the device from the base carrier 

silicon substrate. To understand the nature of the etching and to protect the surface of the 

detector, the wafers were periodically withdrawn and thickness of the PR measured. A twentystep process finally yielded the desired ring type as well as other test detectors that were used for 

electrical characterization and testing for target detection. The data shows considerable 

measurement variance, an improvement in response figure of merit with higher dark resistance. 

This was the motivation to purposefully increase the HgCdTe resistivity. The resistance was 

made to improve by increasing the Cd composition, which yielded a higher bandgap. This 

higher bandgap increased the resistance by having less free carriers at RT. In addition to 

increasing the resistance, the higher bandgap also decreased the absorption cutoff wavelength 

causing the responsivity to decrease. In addition, the higher bandgap also made electrical contact 

to the devices much more resistive. The highest Cd composition devices tested had nonlinear 

contacts, whose measurement results could not be fairly compared. Some batches of HgCdTe 

suffered from unintentional doping as well, which lowered device resistance. In future revisions 

the Cd composition should be optimized, along with contact resistivity, and the unintentional 

doping lowered as much as possible to create the best possible device candidate material.
13

5. References

1. Longand, D.; Schmit, J. L. Mercury-cadmium Telluride and Closely Related Alloys. In 

Semiconductors and Semimetals; Willardson, R. K., Beer, A. C. Eds.; Vol 5, Academic 

Press: New York, 1970, pp 175–255.

2. Michael, E.; Razeghi, M. Opto-electronics rev 1998, 6 (1), 11–23.

3. Rogalskiand, A.; Piotrowski, J. J. Intrinsic Infrared detectors, Prog. Quant. Electr. 1988, 12,

87–289.

4. Singh, R.; Mittal, V. Defense Sci. Jnl 2003, 53 (3), 281–324.

5. Ritter, E. S. Electron Process in Photodetectors. In Photoconductivity Conference;

Backeridge, R., Russel, B., Hahn, E., Eds.; Atlantic City, 1954, Wiley: New York, 1956, 

pp 215–66.

6. Lawson, W. D.; Neilson, S.; Putley, E. H.; Young, A. S. Preparation and Properties of HgTe 

and Mixed Crystals of HgTe-CdTe. J. Phys. Chem. Solids. 1959, 9, 325–329.

7. Kolodny; Kidron, I. Two Dimensional Effects in Intrinsic Photoconductive Detectors. 

Infrared Physics 1982, 22, 9–22.

8. Smith, E.P.G; Musca, C. A.; Farone, L. Two Dimensional Modeling of HgCdTe 

Photoconductive Detectors. Infrared Phys. and Technol. 2000, 41, 175–186.

9. Http://www.fas.org/man/dod-101/navy/docs/fun/part14.htm (Accessed 07/26/2010).

10. Chen, Y.; Farrell, S.; Brill, G.; Wijewarnasuriya, P.; Dhar, N. J. Crystal Growth 2008, 310

(24), 5303–5307.

11. McAuley, S. A.; Ashraf, H.; Atabo, L.; Chambers, A.; Hall, S.; Hopkins, J.; Nicholls, G. 

2001, 34, 2769–2774.

12. Pendharkar, S. V.; Wolfe, J. C.; Ramprasad, H. R.; Chau, Y. L.; Licon, D. L.; Morgan, M.

D.; Horne, W. E.; Tiberio, R. C.; Randall, J. N. Jnl. Vac. Sci, & Tech, B 1995, 13,

2588–2592.

13. Kuhl, K.; Vogel; Schaber, U.; Scafflik, R.; Hillerich, B. SPIE 1998, 3511, 97–105.
14

List of Symbols, Abbreviations, and Acronyms

ARL U.S. Army Research Laboratory

FTIR Fourier Transform Infrared Spectroscopy

ICP inductive coupled plasma

ICP-DRIE ICP deep reactive ion etch

IPA isopropyl alcohol

IR infrared

MBE molecular beam epitaxy

MCT mercury cadmium telluride

PR photoresist

RT room temperature

TE thermoelectrically
NO. OF NO. OF

COPIES ORGANIZATION COPIES ORGANIZATION

15

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