A Novel Metal-Rich Anneal for In Vacuo Passivation of High-Aspect-Ratio Mercury Cadmium Telluride Surfaces

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Journal of Electronic Materials ISSN 0361-5235 Journal of Elec MateriDOI 10.1007/s11664-013-2818-2

A Novel Metal-Rich Anneal for In VacuoPassivation of High-Aspect-Ratio MercuryCadmium Telluride Surfaces

Chang-Feng Wan, Thomas Orent,Thomas Myers, Ishwara Bhat, AndyStoltz & Joe Pellegrino

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A Novel Metal-Rich Anneal for In Vacuo Passivationof High-Aspect-Ratio Mercury Cadmium Telluride Surfaces

CHANG-FENG WAN,1,5,6 THOMAS ORENT,1 THOMAS MYERS,2

ISHWARA BHAT,3 ANDY STOLTZ,4 and JOE PELLEGRINO4

1.—VISM Corporation, Dallas, TX, USA. 2.—Texas State University at San Marcos, San Marcos, TX,USA. 3.—Rensselaer Polytechnic Institute, Troy, NY, USA. 4.—U.S. Army, RDECOM, CERDEC,NVESD, Fort Belvoir, VA, USA. 5.—e-mail: [email protected]. 6.—e-mail: [email protected]

A new method for Cd-rich annealing of mercury cadmium telluride (MCT) wasdeveloped based on the observation that the deposition of Cd onto MCT byvacuum evaporation became self-limiting whenever the substrate tempera-ture was above 70�C regardless of the Cd evaporation rate. Preliminaryresults indicated that this new method may be suitable for passivation of high-aspect-ratio MCT surfaces, for passivation at low temperatures, for in vacuooperation, and/or for vacancy annihilation in MCT. Furthermore, the processcan be carried out in the conventional open-tube reactors used for molecularbeam epitaxy, metalorganic chemical vapor deposition, and physical vapordeposition.

Key words: Passivation, HgCdTe, infrared FPA, photodiode, Cd-rich anneal,high aspect ratio

INTRODUCTION

Surface passivation is a critical step during mer-cury cadmium telluride (MCT) photodiode fabrica-tion since poorly passivated surfaces lead to highdepletion or tunneling dark currents and/or exces-sive noise. The conventional CdTe passivation pro-cess1 employs CdTe thin-film deposition followed byan interdiffusion anneal to form a graded-bandgap,or compositionally graded region at the CdTe–MCTinterface which greatly reduces the deleteriouseffects of the interface. However, the conventionalphysical vapor deposition (PVD) and chemical vapordeposition (CVD) techniques used for CdTe deposi-tion are inadequate for state-of-the-art, small-pitch,multicolor focal-plane arrays, since this generationof devices requires conformal coverage of high-aspect-ratio (HAR) surfaces and low-temperatureprocessing. Unconventional methods such as atom-ic-layer deposition (ALD)2,3 have been proposed, butthey require processing temperatures (>250�C)4

that exceed the low-temperature limits imposed bysome of the more advanced device fabrication tech-

niques such as in vacuo processing, which limitstemperatures to less than 100�C to prevent Hg lossfrom freshly dry-etched MCT surfaces.5 A viableprocess needs to meet the following requirements: (1)conformal over HAR surfaces, (2) able to compensatefor Hg losses, (3) compatible with low process tem-peratures, and (4) production-worthy.

Passivation methods that do not use depositedCdTe are attractive because the issue of noncon-formal coverage of HAR surfaces by thin films iseliminated. Annealing in Cd-Hg atmospheres hasbeen explored.6,7 The experimental data suggestedthe formation of a graded-bandgap layer at thesurface which improved the device performance.Since the process does not involve thin-film deposi-tion where gas-phase mass transfer properties areimportant, good conformality to HAR surfaces isexpected. However, the process temperature wasvery high (250�C to 400�C), and the Hg used duringannealing complicates production. Such issues maybe avoided if MCT is annealed in a Cd atmosphereinstead of Hg-Cd; For instance, the processingtemperature may be lowered because Cd may stillattach to Te dangling bonds (TDB) to form a thinCdTe layer on the MCT. Since Cd has a much lowervapor pressure than Hg (Fig. 1), it is vacuum com-(Received December 18, 2012; accepted September 23, 2013)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-013-2818-2� 2013 TMS

Author's personal copy

patible. However, the Cd vapor pressure may be toolow to be utilized effectively. This may haveprompted the use of Cd-Hg in the aforementionedstudy.5 Heating Cd alone to generate a sufficientlyhigh vapor pressure for processing MCT held atlower temperature runs the risk of depositing Cd toform an electrical shunt and/or erode the MCTsurface. Such issues need to be addressed forCd-based processes to become viable for passivatingMCT surfaces.

THEORETICAL

Thermodynamics

It has been proposed that microscopic MCTdefects, including metal vacancies (point defect),surface bandgap states (surface defect), and dislo-cations (line defect), are mainly due to TDB thatresulted from the breaking of Hg–Te bonds, whichare much weaker than Cd–Te bonds, by heat and/ormechanical stress. This notion is supported by thefact that long-wavelength infrared (LWIR) MCTexhibited lower yield strength,8 higher metalvacancy concentration,9 and higher dislocationdensity than mid-wavelength infrared (MWIR) orshort-wavelength infrared (SWIR) MCT. The lattermaterials have fewer Hg–Te bonds and more Cd–Tebonds. Thermodynamic calculations can be used toestimate the effect of the respective chemical bond-ing energies on the population of TDB.

Equations 1 and 2 below describe the interactionsof Cd and Hg, respectively, with TDB:

Cdþ TDB! CdTe; K1 ¼aCdTe

aCd � aTDB

¼ exp �DG1

RT

� �¼ exp �DHCdTe � TDSCdTe � DGTDB

RT

� �;

(1)

Hgþ TDB! HgTe; K2 ¼aHgTe

aHg � aTDB

¼ exp �DG2

RT

� �¼ exp �DHHgTe � TDSHgTe � DGTDB

RT

� �;

(2)

where aCd, aHg, aTDB, aCdTe, and aHgTe are theactivities of Cd, Hg, TDB, Cd–Te bond, and Hg–Tebond in the MCT, respectively; DG1, DG2, DHCdTe,DHHgTe, DSCdTe, DSHgTe, and DGTDB are the Gibb’sfree energy, enthalpy of formation, and entropy offormation for CdTe or HgTe, and TDB, respectively.Assuming that MCT is an ideal solution of HgTeand CdTe, aCdTe = 0.2 and aHgTe = 0.8. DividingEq. 1 by Eq. 2 and noting that aCd and aHg are unityunder Hg- and Cd-rich conditions, and DSCdTe �DSHgTe � 0, the result is

cTDB2

cTDB1

¼ expDHHgTe � DHCdTe

RT

� �

¼ exp�7900þ 24; 100

1:986T

� �¼ exp

16; 200

1:986T

� �:

(3)

Equation 3 describes the ratio of equilibrium TDBconcentrations in Hg-rich versus Cd-rich conditionsand is plotted in Fig. 2. The concentration of TDB inMCT can be many orders of magnitude lower underCd-rich conditions than under a similar Hg-rich con-dition. The lower the temperature, the larger the dif-ference; For example, at 100�C, the TDB concentrationin MCT under a Cd atmosphere is about eight orders ofmagnitude lower than in Hg; at 250�C, the difference isabout six orders of magnitude. This analysis indicatesthat annealing MCT in a Cd atmosphere can achievebetter passivation results than Hg-rich annealing.

0 100 200 300 400 500 600

Temperature (°C)

Vap

or

pre

ssu

re (

torr

)

Cd

Hg

105

103

101

10-1

10-3

10-5

10-7

Fig. 1. Equilibrium partial pressure of Cd and Hg as a function oftemperature.

0 50 100 150 200 250 300

Temperature (°C)

CT

DB(H

g) /

CT

DB(C

d)

1012

1010

108

106

104

Fig. 2. Calculated ratio of tellurium dangling bonds (TDB) underHg-rich versus Cd-rich conditions as a function of temperature.

Wan, Orent, Myers, Bhat, Stoltz, and Pellegrino


Kinetics

During a metal-rich anneal, no deposition canoccur or the MCT surface will shunt and/or deteri-orate. A Langmuir adsorption isotherm9 may beused to model the adsorption of gas molecules:

h ¼ aP

1þ aP; (4)

where h is the fractional coverage of the surface atequilibrium, P is the gas pressure or concentration,and a is a constant that increases with decrease intemperature.

The Langmuir adsorption model should apply tothe Hg-Cd anneal described by An et al.6 No Cd orHg deposition occurred as the metal-rich anneal wasisothermal; i.e., the Cd-Hg vapors were generated atthe anneal temperature. Since no thin-film deposi-tion or gas-phase mass transfer were rate-limitingas the passivation process was governed by the veryslow process of compositional interdiffusion (CID),the primary cause of nonconformal coverage of HARsurfaces does not come into play: Thus Cd has en-ough time to be evenly distributed, and uniformcoverage of the passivation is expected.

The compositional gradient can be modeled byFick’s laws and known diffusion coefficients10–13 forone-sided CID, where the Cd activity at the surfacecan be at supersaturation by imposing the Cd vaporpressure or flux, in contrast to the Cd-poor condi-tions, which can be several orders of magnitudelower, for the case of the CdTe passivation. Hencethe time for forming the passivation could be dras-tically shorter.

EXPERIMENTAL PROCEDURESAND ANALYSIS

Process Development

CdTe films were deposited by thermal evapora-tion from dual-source CdTe:Cd in a Varian 3118thermal evaporation system equipped with a cryo-pump (Fig. 3). The CdTe source was precompound-ed polycrystalline CdTe. The Cd source was in theform of teardrop-shaped shot. The CdTe and Cdsources were held in alumina crucibles and wereheated by resistance heaters, which comprise atantalum envelope and heat baffles. The substrateholder was made of aluminum metal. Heating of thesubstrate was accomplished by two quartz halogenlamps on the backside of the holder. The substratetemperature was monitored with a control thermo-couple (TC) in the holder. A temperature controllermaintained the temperature. Temperature labels(dots) made from liquid crystal by OMEGA wereused to calibrate the substrate temperature and TCtemperature. The dots turned dark when the tem-perature reached the label temperature, which wasobserved through a viewport on the metal bell jar.Two labels, 93�C–121�C–148�C–176�C and 132�C–137�C–142�C–148�C, were employed. When the

labels were in direct contact with the aluminumsubstrate holder, the temperatures at which thelabels turned dark agreed very well with the TCreadings as shown in Fig. 4, indicating that thetemperature labels were consistent. When thetemperature labels were attached to the substratesurface clipped to the holder, their readings weresubstantially lower than the TC readings, indicat-ing that the thermal contact was inadequate. Ruf-fled aluminum foils were placed between thesubstrate holder and the substrates to improve thethermal contact. Then, temperatures were readfrom the labels attached to silicon slices which wereheld on the holder and the TC. Figure 5 shows thecorrelation between the label readings and the TCreadings. The temperature of the silicon slices was�20�C lower than the TC readings with run-to-runreproducibility between 5�C and 10�C.

To start a run, a silicon slice was placed on thesample holder; the vacuum chamber was evacuatedto a base pressure in the mid 10�6 Torr range. Therewas a main shutter that shielded the substrate fromboth CdTe and Cd fluxes, and a secondary shutterthat shielded the substrate from only the CdTe flux.A lone quartz-crystal oscillator was used to monitorthe deposition rate. Films were deposited by ther-mal evaporation from either the CdTe source aloneor both the CdTe and Cd sources. The single-sourcerun was done in standard fashion, simply by heat-ing up the CdTe source to attain the desired evap-oration rate, opening the main shutter to commencedeposition, and closing the shutter when the desiredthickness was read on the thickness monitor. Therewas not enough space to install a second thicknessmonitor for simultaneous monitoring of both sour-ces. The Cd evaporation was started first. When

CdTe Substrate

Heater

Vacuum Pump

Crucible

Heating element

Cd

Heat baffle

Fig. 3. Schematic diagram of the vacuum thermal evaporator withCdTe:Cd dual vapor sources used in this study.

A Novel Metal-Rich Anneal for In Vacuo Passivation of High-Aspect-Ratio Mercury Cadmium Telluride Surfaces


the desired evaporation rate of typically 2 A/s wasreached and stabilized, the CdTe shutter was openedand its evaporation rate was adjusted to achieve acombined rate of 10 A/s, which corresponded to 8 A/sfor CdTe and 2 A/s for Cd. Finally the substrateshutter was opened to commence deposition. Thus,during CdTe:Cd coevaporation, only the CdTe evap-oration rate was adjusted based on the assumptionthat the Cd evaporation rate was stable. Any changesin the Cd flux would result in automatic adjustmentsof the CdTe flux in the opposite direction in order tomaintain the combined rate. When the desired filmthickness (2000 A) was reached, the shutters wereclosed to stop deposition. The Cd:Te ratio was mea-sured using glazing-angle wavelength-dispersivex-ray fluorescence (GA-WDS-XRF) analysis. The

reproducibility of the measurement was found to beless than 0.02 in the Cd:Te ratio. Such coevaporatedfilms are called CdTe:Cd films. Unless otherwisenoted, they were deposited with standard CdTe andCd fluxes (evaporation rates) set at 8 A/s and 2 A/s,respectively.

Figure 6 shows the compositions of the CdTe:Cdfilms deposited at 30�C (no heating of the substrate,dotted line) and 140�C (solid line), as a function ofthe Cd evaporation rate. The films deposited at 30�Chad Cd:Te ratios that increased linearly with the Cdflux from �1.0. This result was consistent with asticking coefficient of 1.0 for both species. x-Raydiffraction (XRD) results showed that the filmscontained CdTe and Cd. The films deposited at140�C, however, exhibited a fixed Cd:Te ratio of1.02 ± 0.004, which was significantly different fromthe results obtained at 30�C. XRD analysis indi-cated that these films were single-phase CdTe pin-ned at the Cd-rich side of the phase boundary.Therefore, such a CdTe film, which was depositedwith excess Cd flux at elevated temperatures, iscalled Cd-rich CdTe. Notice that the Cd:Te ratios ofthe 30�C films deposited with excess Cd are sub-stantially higher. This may be due to the afore-mentioned issues of monitoring and adjustingthe evaporation rates during deposition. Due to theproximity of the two source crucibles, heating theCdTe source, which required very high tempera-ture, can easily heat the Cd source to raise itsevaporation rate. Judging from the data, the Cdevaporation rate may have been increased by up tothreefold due to proximity heating.

It can be seen that, at some temperature between30�C and 140�C, the Cd incorporation into the filmtransitioned from nondiscriminatory to self-limit-ing. To further study the transition, CdTe:Cd filmswere deposited at various substrate temperatures.The Cd:Te ratios of the resultant films were mea-sured and are plotted in Fig. 7, which shows thatthe transition to the self-limiting region occurred at70�C, above which the Cd:Te ratio became constantdespite the extra Cd flux. This showed thatannealing a substrate under Cd-rich conditions inan open-tube system is a distinct possibility.

CdTe films evaporated from a single CdTe source(without the extra Cd flux) had a Cd:Te ratio sig-nificantly less than 1, indicating that a minuteamount of Cd was lost during evaporation. In con-trast, a CdTe film deposited by molecular beamepitaxy (MBE) had negligible Cd loss. Since themost significant difference between a MBE reactorand a thermal evaporator is the shape of the sourcecontainers, it was conjectured that the difference inthe Cd:Te ratio was due to the difference in theheights and openings of the source containers: awider orifice and/or a shorter height allows thegaseous species to be effused over a larger solidangle. The Cd and Te2 molecules generated bycongruent sublimation of CdTe can collide with eachother in the source container, where the gas pres-

90

100

110

120

130

140

150

160

170

180

190

90 110 130 150 170 190

Controller S.P. (°C)

Tem

p.

(°C

)

Fig. 4. Temperatures measured on the substrate holder by liquid–crystal temperature dots versus those measured by thermocouple.Note that different symbols denote different runs of the temperaturemeasurements; same symbols mean that the measurements weremade in the same run (four temperature dots).

80

100

120

140

160

180

200

100 120 140 160 180 200 220

Controller S.P. (°C)

Tem

p. (

°C)

Fig. 5. Temperature measured on the substrate by liquid–crystaltemperature dots versus those measured by thermocouple. Note thatdifferent symbols denote different runs of the temperature mea-surement; same symbols mean that the measurements were madein the same run (four temperature dots).



sure is high. Cd, with much lower molecular weightthan Te2, can be deflected more than Te2, giving riseto a larger effusion solid angle than for Te, allowingCd molecules to spread over a wider area to result ina lower flux than for Te. This hypothesis was testedby increasing the evaporation rate, which causes ahigher scattering rate and hence a lower Cd:Teratio. The experimental results shown in Fig. 8,where the Cd:Te ratio of CdTe films is plotted as afunction of CdTe flux, support this hypothesis.

XRD results showed that Cd-rich CdTe filmsdeposited at 140�C on silicon were highly texturedin the h111i direction; those deposited on (111) MCTsubstrates had XRD double-crystal rocking-curve(DCRC) full-width at half-maximum (FWHM) val-ues of 2� to 3�, indicating that they are single-crystalCdTe. Higher substrate temperatures gave rise to

smaller FWHM values. At substrate temperature of200�C, the FWHM value of the Cd-rich CdTe filmswas approximately 200 arcsec.

Kinetic mechanisms of Cd and Te deposition canbe discerned. Both Cd and Te gas molecules werenonselective in their deposition onto a surface attemperatures up to 70�C, as they bind readily to anysurface site on the substrate. Above 70�C, Te depo-sition is still nonselective, while Cd binds readily toTe sites to form CdTe but not to Cd or other sites onthe surface. Instead, the Cd desorbs to form sub-critical nuclei, which dissipate, diffuse on the sur-face, or diffuse into the substrate. Thus, when aMCT substrate is heated and subjected to excess Cdflux, almost all the TDBs are bound by Cd, withphysisorbed Cd atoms ready to diffuse into the MCTwithout forming Cd solid phase. This constitutes aCd-rich anneal under an excess Cd condition whichpassivates the MCT surface.

Cd-Rich Anneal of MCT

A LWIR HgCdTe wafer, grown by the verticalBridgman method, was provided by Spitfire Semi-conductors, New Zealand with the following speci-fications:

� 77-K cutoff: 9.8 lm to 10.2 lm� Nd: 1.7 9 1014 cm�3

� 77-K mobility: 1 9 105 cm2/V-s� Crystal orientation: (111) ± 20�� Etch pit density (EPD): 1 9 105 cm�2

The 15-mm-diameter wafer was diced into the nine‘‘sister’’ slices shown in Fig. 9. An MBE reactor atTexas State University, San Marcos (TSU-SM) anda horizontal hot-wall metalorganic chemical vapordeposition (MOCVD) reactor at Rensselaer Poly-technic Institute (RPI) were used for the Cd-richannealing runs. The MBE reactor used Cd and

0

1

2

3

1 2 3 4Cd/Te ratio in flux (AU)

Cd

/Te

rati

o in

film 30°C

140°C Cd-rich

140°C Te-rich

Linear (30°C)

Fig. 6. Cd:Te ratio in 30�C and 140�C vacuum-evaporated CdTe:Cdfilms as a function of Cd flux in the vapor source while the CdTe fluxstayed constant at 8 A/s.

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.61000/T (1/K)

Cd/

Te

Rat

io

70 50 3090110200 140 °C

Fig. 7. Cd:Te ratio in vacuum-evaporated CdTe:Cd films as afunction of substrate temperature with CdTe and Cd fluxes of 8 A/sand 2 A/s, respectively.

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 10 20 30Dep Rate (Å/sec)

Cd/

Te

Rat

io

Fig. 8. Cd:Te ratio in CdTe films evaporated from a single CdTe sourceas a function of the deposition rate at substrate temperature of 30�C.



CdTe as vapor sources; the MOCVD reactor used Cdand di-isopropyl tellurium (DiPTe) with hydrogenas carrier gas.

Cd-rich annealing was conducted in the respec-tive reactors according to the following steps:

1. Dip the MCT into diluted Br-ethylene glycolsolution for a short time;

2. Load into the reactor, pump down, and heat theMCT to 140�C;

3. Turn on the Cd flux to deposit 200 A of Cd at2 A/s;

4. Without turning the Cd flux off, turn on the CdTe(MBE) or DiPTe (MOCVD) flux at 8 A/s;

5. Turn off the CdTe (MBE) or DiPTe (MOCVD)flux and the substrate heater;

6. Turn off the Cd vapor flux only when thesubstrate has cooled to below 95�C.

The Te-rich anneal was done in a similar fashion:

1. Dip the MCT into diluted Br-ethylene glycolsolution for a short time;

2. Load into the reactor, pump down, and heat theMCT to 140�C;

3. Deposit 1200 A of CdTe:

� MBE reactor: turn on the CdTe flux at 8 A/s;� MOCVD reactor: turn on both the Cd flux and the

DiPTe flux at 8 A/s.

4. Turn off the CdTe flux and the substrate heater.

It should be noted that process control in theMOCVD reactor was imprecise due to the lack of areal-time thickness monitor and that the use of a Cdmetallorganic vapor as source of Cd may improvethe situation.

Minority-Carrier Lifetime Measurements

Photoconductive response lifetime (PCL) mea-surements were carried out using a standard

apparatus.14 Figure 10a, b shows the PCLs of the‘‘sister’’ MCT slices annealed in the MBE reactor.The peak PCL, which was observed in a CdTe:Cdsample at 100 K, was �15 ls, which is close or equalto the state of the art for MCT having similar cutoffand carrier concentration and represents a fourfoldincrease over the as-received sample, for which themeasured value was 4 ls. This mediocre lifetime(4 ls), which is shorter than expected from the spec-ifications, can be attributed to inadequate surfacepassivation, dislocations, and/or metal vacancies.Since the Cd-rich anneal appeared to have restoredthe MCT to pristine condition, the short, low-tem-perature Cd-rich anneal appeared to have the capa-bilities of (1) surface passivation, (2) dislocationreduction, and/or (3) metal vacancy annihilation.Additional experiments are needed to resolve andquantify these capabilities individually. The PCL

Fig. 9. LWIR MCT samples diced from a 15-mm-diameter verticalBridgman wafer used in the present study.

0

2

4

6

8

10

12

14

16

4 6 8 10 12 14

1000/T (K-1)

Lif

etim

e (µ

s)A

B

C

0

5

10

15

20

As-received LWIRMCT

140ºC standardMBE CdTe

140ºC Cd-richCdTe w/Cd-richanneal in MBE

Ph

oto

con

du

ctiv

e lif

etim

e (µ

s)(a)

(b)

Fig. 10. (a) Photoconductive lifetime (PCL) plots as a function ofreciprocal temperature for samples A (140�C Cd-rich anneal withCd-rich CdTe in MBE), B (140�C CdTe in MBE), and C (as-receivedLWIR MCT). (b) Peak PCLs for the samples shown in (a).



measured on the Te-rich annealed sample (MBE‘‘CdTe cap’’) was 3 ls, which was slightly shorter thanfor the as-received sample. This decrease in the PCLcan be explained by the generation of metal vacanciesdue to the 140�C Te-rich anneal, which took placeduring the standard CdTe deposition.

Figure 11a, b shows the PCL plotted as a function ofreciprocal temperature for the samples annealed inthe MOCVD reactor. Samples annealed with Cd fluxwere quite good, substantiating the MBE results. Theflux control issues appear not to be a very significantproblem, although they may have ruined sample F.Thus, such issues may need to be resolved before useof the MOCVD technique for the Cd-rich annealmethod can be considered production worthy.

It can be seen from Fig. 10a that the PCL of the‘‘MBE CdTe:Cd cap’’ sample suffered a drastic drop

when the sample was cooled to 95 K. This changecannot be explained by Auger recombination orShockley–Read–Hall (SRH) recombination. It wasconjectured that this drop was due to a gradualbuildup of ultraviolet (UV)-induced negative fixedcharge from ambient lighting during the PCLmeasurement that caused the surface to invert andgenerate surface recombination dark current. Thefact that the p-type (Te-rich annealed) sample didnot exhibit such a sharp drop in PCL is consistentwith this explanation.

CONCLUSIONS

A low-temperature, HAR-compatible processbased on a Cd-rich anneal has been developed andtested for passivation of MCT. Preliminary resultsshowed that the PCLs of LWIR HgCdTe samplesincreased drastically compared with reference sam-ples, indicating that surface recombination centersand metal vacancies had been annihilated in a veryshort time (<10 min) at a very low temperature(140�C). The passivation, being rate-limited by CIDinstead of gas-phase diffusion, should exhibit excel-lent conformal coverage capability for HAR surfacessince gas-phase mass transfer would no longer be alimiting factor. Also, this Cd-rich anneal process canbe performed in the open-tube reactors used forthermal evaporation, MBE or MOCVD and is com-patible with in vacuo operation. Because of thesupersaturated amount of Cd instead of its equilib-rium concentration and because the CID is one-sidedinstead of ‘‘two-sided’’, the passivation should besubstantially faster than by CdTe interdiffusion. TheCd-rich anneal process should be suitable for pro-cessing MCT at higher as well as low temperatures.Lastly, the method is deemed novel, as a US patenthas been issued (US patent no. 8,541,256) for theinitial application (no. 13/421,860).

ACKNOWLEDGEMENTS

The authors wish to acknowledge Dr. Leigh AnnFiles for GA-WDS-XRF analysis, Carlos Aramayofor technical assistance, and Dr. Herb Schaake forhelpful discussion. This work is supported in part byUS DoD SBIR Contract No. W909MY-11-C-0027.

REFERENCES

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5. Army SBIR, In-Vacuo Passivation of High Aspect RatioHgCdTe Surfaces—Topic A10-018, SBIR/SITIS, (2010).

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0

2

4

6

8

10

12

4 6 8 10 12 14

1000/ T (K-1)

Lif

etim

e (µ

s)

D

E

F

0

5

10

15

20

140ºC Cd:DiPTew/Ugly surface

140ºC Cd:DiPTeCdTe in MOCVD

140ºC Cd:DiPTeCdTe w/Cd-rich

anneal in MOCVD

Ph

oto

con

du

ctiv

e lif

etim

e (µ

s)

(a)

(b)

Fig. 11. (a) Photoconductive lifetime (PCL) plots as a function ofreciprocal temperature for MCT samples D (140�C Cd-rich annealwith Cd:DiPTe CdTe in MOCVD), E (140�C Cd:DiPTe CdTe inMOCVD), and F (140�C Cd:DiPTe CdTe with ugly surface). (b) PeakPCLs for the three MOCVD samples of (a).



8. H. Alexander and P. Haasen, Solid State Phys. 22, 27 (1968).9. M.A. Kinch, Fundamentals of Infrared Detector Materials

(Bellingham, WA: SPIE, 2007).10. I. Langmuir, J. Am. Chem. Soc. 40, 1361–1402 (1918).11. M.S. Tang and D.A. Stevenson, Appl. Phys. Lett. 50, 1272–

1274 (1987).

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13. K. Zanio and T. Massopust, J. Electron. Mater. 15, 103–109(1986).

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A Novel Metal-Rich Anneal for In Vacuo Passivation of High-Aspect-Ratio Mercury Cadmium Telluride Surfaces