MOCVD Grown Hgcdte Heterostructures for Medium Wave Infrared Detectors

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Article MOCVD Grown HgCdTe Heterostructures for Medium Wave Infrared Detectors

Waldemar Gawron 1,2, Jan Sobieski 1, Tetiana Manyk 2 , Małgorzata Kopytko 2, Paweł Madejczyk 2,* and Jarosław Rutkowski 2

1 Vigo System S.A., 129/133 Poznanska Str., 05-850 Ozarow Mazowiecki, Poland; [email protected] (W.G.); [email protected] (J.S.) 2 Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland; [email protected] (T.M.); [email protected] (M.K.); [email protected] (J.R.) * Correspondence: [email protected]

Abstract: This paper presents the current status of medium-wave infrared (MWIR) detectors at the Military University of Technology’s Institute of Applied Physics and VIGO System S.A. The metal– organic chemical vapor deposition (MOCVD) technique is a very convenient tool for the deposition of HgCdTe epilayers, with a wide range of compositions, used for uncooled infrared detectors. Good compositional and thickness uniformity was achieved on epilayers grown on 2-in-diameter, low-cost (100) GaAs wafers. Most growth was performed on substrates, which were misoriented from (100) by between 2◦ and 4◦ in order to minimize growth defects. The large lattice mismatch between GaAs and HgCdTe required the usage of a CdTe buffer layer. The CdTe (111) B buffer layer growth was enforced by suitable nucleation procedure, based on (100) GaAs substrate annealing in a Te-rich atmosphere prior to the buffer deposition. Secondary-ion mass spectrometry (SIMS) showed that ethyl iodide (EI) and tris(dimethylamino)arsenic (TDMAAs) were stable donor and acceptor dopants, Citation: Gawron, W.; Sobieski, J.; Manyk, T.; Kopytko, M.; Madejczyk, respectively. Fully doped (111) HgCdTe heterostructures were grown in order to investigate the P.; Rutkowski, J. MOCVD Grown devices’ performance in the 3–5 µm infrared band. The uniqueness of the presented technology HgCdTe Heterostructures for manifests in a lack of the necessity of time-consuming and troublesome ex situ annealing. Medium Wave Infrared Detectors. Coatings 2021, 11, 611. https:// Keywords: MOCVD; HgCdTe growth; MWIR photodiodes; infrared detectors doi.org/10.3390/coatings11050611

Academic Editor: Massimo Longo 1. Introduction Received: 21 April 2021 Many branches of science and industry require detectors operating in the medium- Accepted: 19 May 2021 wave infrared (MWIR) band (3–5 µm), where the atmosphere is mostly transparent. This Published: 20 May 2021 region is typically exploited for thermal imaging, understood as the detection and processing of slight temperature differences in many types of devices and objects, as well as in Publisher’s Note: MDPI stays neutral an environment. Thermal imaging is often used in astronomy and astrophysics to survey with regard to jurisdictional claims in distant galaxies, whose near-light speed has shifted their emission spectra from the visible published maps and institutional afﬁl- and ultraviolet to the MWIR region. iations. The progress in infrared (IR) detector technology has been associated mainly with photon detectors, among which photodiodes are typically the most sensitive devices. They are characterized both by a high signal-to-noise ratio and a fast response [1]. In spite of other competitive technologies and materials, mercury cadmium telluride (Hg1−xCdxTe) is Copyright: © 2021 by the authors. still the main material for infrared detectors [2]. Licensee MDPI, Basel, Switzerland. Conventional HgCdTe IR photodetectors need to be cooled to the temperature of This article is an open access article liquid nitrogen (77 K) in order to reduce the noise and leakage currents resulting from the distributed under the terms and thermal generation processes. Increasing the operating temperature without detectivity conditions of the Creative Commons Attribution (CC BY) license (https:// deterioration in so-called high-operating-temperature (HOT) detectors has been the subject creativecommons.org/licenses/by/ of research by many scientiﬁc centers, and can be realized in a different ways [3]. Among 4.0/). them, we can list: a suppression of Auger thermal generation in non-equilibrium devices;

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an optical immersion; multiple passes of IR radiation; magnetoconcentration detectors; Dember detectors; barrier detectors; and alternative materials such as superlattices and cascade infrared detectors [4,5]. In general, there are two main epitaxial growth techniques applied to HgCdTe: liquid-phase epitaxy (LPE); and vapor-phase epitaxy (VPE). LPE is the most technologically mature method, and has been used widely for industrial production for many years. The more advanced VPE techniques, such as molecular- beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD), allow the construction of more complex device structures, with good lateral homogeneity and abrupt composition and doping proﬁles. Historically, the best HgCdTe detectors have been grown on bulk CdZnTe substrates. However, bulk CdZnTe substrates present many technical and cost-related challenges that justify the search for a viable alternate substrate for HgCdTe growth by MOCVD—such as GaAs, for example. GaAs is an easily available, inexpensive, and high-quality substrate material, but the lattice mismatch between GaAs and the HgCdTe layer requires the use of an additional CdTe buffer layer. Modern advances in the MOCVD of HgCdTe have created the opportunity to realize novel detector designs through multilayer in situ growth, with complete ﬂexibility in the choice of the alloy compositions and the doping concentrations [6]. This allows for the construction of complex HgCdTe heterostructure-based IR detectors dedicated to HOT conditions. In this paper, we present MOCVD-grown (111) HgCdTe heterostructures for medium-wave infrared detectors operating above 200 K. The uniqueness of the presented technology manifests in a lack of the necessity of time-consuming and troublesome ex situ annealing.

2. Materials and Methods 2.1. MOCVD-Grown HgCdTe The HgCdTe epitaxial growth was carried out in the horizontal reactor of an AIX-200 Aixtron MOCVD (AIXTRON, Aachen, Germany) unit. The system operated in the laminar flow regime with process pressures from 50 to 1000 mbar, using a butterfly valve for pressure control. The reactor pressure of 500 mbar was used for all successful growth runs. The hydrogen was used as a carrier gas. Dimethylcadmium (DMCd) and diisopropyl telluride (DIPTe) were used as the precursors. Ethyl iodide (EI) and tris(dimethylamino)arsenic (TDMAAs) were used as donor and acceptor dopant sources, respectively. The DMCd and EI were delivered through one channel, while DIPTe and TDMAAs were delivered through another channel over the quartz container, where the elementary mercury was being held. Aixtron’s gas foil rotation technique was applied for better composition uniformity. There were two temperature zones in the reactor: the mercury source zone; and the growth zone with the graphite susceptor. High-temperature annealing was used before each growth run for the reactor and the substrate cleaning. The gas delivery system was additionally equipped with Piezocon ultrasonic precursor concentration monitors. The usage of Piezo- cons contributed to a better repeatability of the growth processes. Adaptation of the EpiEye reflectometer (ORS Ltd., St Asaph, UK) allowed for in situ monitoring of the thickness and the surface morphology of the growing layer. Laser radiation, incident and reflected from the growing layer, with a length of 650 nm, was passed through the hole in the quartz liner, allowing us to obtain reliable interferograms. Figure1 presents a fragment of the precursors’ delivery installation, together with the horizontal reactor cell scheme with an internal mercury source of the Aixtron AIX 200 MOCVD, designed for HgCdTe deposition. Growth was carried out on 2-inch, epi-ready, semi-insulating (100) GaAs substrates, oriented 2◦ off toward the nearest <110>. Typically, a 3–4-µm-thick CdTe layer was used as a buffer layer, reducing the stress caused by the crystal lattice misfit between the GaAs substrate and the HgCdTe epitaxial layer structure [7]. The buffer also prevented gallium diffusion from the substrate to the HgCdTe layer. The GaAs substrate was annealing in a Te-rich atmosphere prior to the growth of the CdTe buffer. The interdiffused multilayer process (IMP) technique was applied for HgCdTe deposition [8]. The HgCdTe was grown at 350 ◦C, with the mercury source kept at a temperature of 160–220 ◦C. The II:VI mole ratio was kept in the range of 1.5–5 during the CdTe cycles of the IMP process. Acceptor Coatings 2021, 11, 611 3 of 13

and donor doping were examined over the wide range of the compositions, and doping levels of 5 × 1014–5 × 1017 cm−3 were obtained. In order to reduce the mercury vacancies’ concentration, and to increase the uniformity of the HgCdTe heterostructures, the in Coatings 2021, 11, 611 situ annealing was performed under mercury-rich conditions. However, the obtained3 of 13

heterostructures were not annealed ex situ [9].

FigureFigure 1. 1.A A fragment fragment ofof thethe precursors’precursors’ deliverydelivery installation,installation, together with the the horizontal horizontal reactor reactor cellcell scheme scheme with with an an internal internal mercury mercury source source of of the the Aixtron Aixtron AIX AIX 200 200 MOCVD, MOCVD, designed designed for for HgCdTe HgCdTe deposition. deposition.

TheGrowth crystalline was carried quality out of on the 2-inch, HgCdTe epi-ready, films depends semi-insulating on the MOCVD (100) GaAs system substrates, purity, theoriented substrate 2° off quality, toward the the nucleation, nearest and<110>. the Typi growthcally, conditions. a 3–4-µm-thick The growth CdTe oflayer the HgCdTewas used epilayersas a buffer with layer, the (111)reducing B orientation the stress has caused some by advantages, the crystalsuch lattice as misfit a lack between of large macrode- the GaAs fects,substrate high growthand the rate,HgCdTe lower epitaxial consumption layer struct of theure precursors, [7]. The buffer and effective also prevented n-type doping gallium withdiffusion iodine from up to the 10 substrate18 cm−3. However,to the HgCdTe the (111) layer. B layersThe GaAs exhibit substrate some drawbacks, was annealing which in a affectTe-rich the atmosphere performance prior of the to devices-namely,the growth of th relativelye CdTe buffer. rough The surface interdiffused morphology, multilayer high (>10process15 cm (IMP)−3) concentration technique was of applied residual for donor HgCd defects,Te deposition and less [8]. efficientThe HgCdTe p-type was doping grown withat 350 arsenic °C, with in comparison the mercury with source (100) kept HgCdTe. at a temperature Most of our of research, 160–220 and °C. all The of theII:VI work mole presentedratio was inkept this in paper, the range concerns of 1.5–5 (111) during HgCdTe; the theCdTe selected cycles growth of the IMP parameters process. are Acceptor listed inand Table donor1. doping were examined over the wide range of the compositions, and doping levels of 5 × 1014–5 × 1017 cm−3 were obtained. In order to reduce the mercury vacancies’ Tableconcentration, 1. Typical growth and to parameters increase the of the uniformity (111) HgCdTe of the layers. HgCdTe heterostructures, the in situ annealing was performed under mercury-rich conditions. However, the obtained hetero- Prior-to-Growth Annealing 600 ◦C for 7 min structures were not annealed ex situ [9]. NucleationThe crystalline Condition quality of the HgCdTeTe films flush depends for (111) growthon the orientationMOCVD system purity, the substrate quality, the nucleation, andCdTe the buffer growth conditions. The HgCdTe growth of the Growth Pressure HgCdTe epilayers with the (111) B orientation950 mbar has some advantages, such 500 as mbar a lack of large macrodefects, high growth rate, lower consumption of the precursors, and effective n- Susceptor Temperature 350 ◦C 18 −3 type doping with iodine up to 10 cm . However, the (111) B◦ layers exhibit some drawbacks,Mercury which Zone affect Temperature the performance of the devices-namely,(210–220) relativelyC rough surface morphology, II:VIhigh Ratio(>1015 cm−3) concentration1.03 during of residual buffer growth donor and defects, 1.5 during and less CdTe efficient IMP cycles p-type dopingBuffer with Thicknessarsenic in comparison with (100) HgCdTe.3–4 µ mMost of CdTe of our research, and all of the workWafer presented Rotation Ratein this paper, concerns (111) HgCdTe;35–60 the rpm selected growth parameters are listed in Table 1. upper 600 sccm HgTe H Gas Flow Rates lower 180 sccm 2 reactor gas artery (111) HgCdTe upper 1200 sccm CdTe lower 1200 sccm Susceptor temperature 350 ◦C In Situ Annealing Mercury zone temperature (180–220) ◦C Time 15–40 min

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Table 1. Typical growth parameters of the (111) HgCdTe layers.

Prior-to-Growth Annealing 600 °C for 7 min Nucleation Condition Te flush for (111) growth orientation CdTe buffer HgCdTe Growth Pressure 950 mbar 500 mbar Susceptor Temperature 350 °C Mercury Zone Temperature (210–220) °C II:VI Ratio 1.03 during buffer growth and 1.5 during CdTe IMP cycles Buffer Thickness 3–4 µm of CdTe Wafer Rotation Rate 35–60 rpm upper 600 sccm HgTe H2 Gas Flow Rates lower 180 sccm reactor gas artery (111) HgCdTe upper 1200 sccm CdTe lower 1200 sccm Coatings 2021, 11, 611 Susceptor temperature 350 °C 4 of 13 In Situ Annealing Mercury zone temperature (180–220) °C Time 15–40 min

2.2.2.2. Photodiode Design MOCVD-grownMOCVD-grown HgCdTeHgCdTe heterostructuresheterostructures areare designeddesigned forfor infraredinfrared photodiode construction.struction. In In this this section, section, the the fundamental fundamental rules rules of ofinfrared infrared photodiode photodiode design design will will be dis- be discussed.cussed. A classic A classic N+/p/P N++/p/P structure+ structure was enriched was enriched with a transient with a transient P layer inserted P layer insertedbetween + betweenthe absorber the absorberand the highly and the doped highly P+ doped region. P Thisregion. additional This additional P layer helped P layer to helped shape toan + shapeinterface an between interface the between p and the the P p+, andpreventing the P , the preventing formation the of formationunfavorable of potential unfavorable bar- + + potentialriers; then, barriers; a photodiode then, a photodiodetook the form took of thethe formN+/p/P/P of the+ structure. N /p/P/P Furthermore,structure. Further-in order + more,to improve in order the toelectrical improve contact the electrical properties contact of the properties metallization of the of metallization the P+ layer, the of the struc- P + + layer,ture was the upgraded structure was with upgraded the P+/n+ withtunneling the P junction./n tunneling Finally, junction. the resulting Finally, structure the resulting took + + + structurethe form of took the the N+ form/p/P/P of+/n the+. An N example/p/P/P of/n the. An HOT example MWIR of HgCdTe the HOT photodiode MWIR HgCdTe archi- photodiodetecture is presented architecture in Figure is presented 2. in Figure2.

FigureFigure 2.2. TheThe generalgeneral ideaidea ofof aa HOTHOT MWIRMWIR HgCdTeHgCdTe photodiodephotodiode withwith gradedgraded interfaces.interfaces.

TheThe schematic immersion immersion lens lens on on the the top top wa wass formed formed from from GaAs GaAs substrate. substrate. The The im- immersionmersion lens lens increases increases the the optical optical area area of of the the detector detector by by approximately approximately 10 times.times. UnderUnder thethe lens, there there is is a a3-µm 3-µm CdTe CdTe buffer buffer layer. layer. Below Below the lens, the lens, there there is a heavily is a heavily donor-doped donor- dopedcontact contactN+ layer, N with+ layer, the withcomposition the composition x = 0.48. However,x = 0.48. this However, composition this composition of the N+ layer of thecan Nbe+ changed,layer can in be the changed, range from in thex = 0.38 range to fromx = 0.57,x = depending 0.38 to x = on 0.57, the dependingcut-on wavelength on the cut-on(λCUT-ON wavelength) of the detector. (λCUT-ON For) technologica of the detector.l reasons, For technological the thickness reasons, of the N the+ layer thickness was ex- of thetended N+ layerto 10 wasµm. extendedNext, there to is 10 a µmedium-dopedm. Next, there p-type is a medium-doped absorber region p-type with absorbera doping regionlevel higher with athan doping the donor-like level higher background than the donor-like concentration, background which concentration,is typically 3 × which1015 cm is−3 typically 3 × 1015 cm−3 in size. The absorber composition corresponds to the required cut- off wavelength of the detector, and for MWIR devices has a value in the range of 0.27–0.36. The thickness of the absorber should be greater than the minority carrier diffusion length, and is typically about 3 µm. The parameters of the next transient P layer were calculated individually for each device, in order to prevent the formation of a potential barrier, taking into account the interdiffusion processes during growth. The composition of the P+ layer should be signiﬁcantly larger than that of the absorber, and the doping should be as high as possible in order to attain low concentrations of the minority carriers and low resistance. Our MOCVD system enabled us to achieve an acceptor concentration of around 1017 cm−3. The thickness of the P+ layer was about 1 µm, in order to minimize the in-diffusion of the electrons from the metallic contact. Finally, the n+ layer, with a composition lower than that of the absorber and donor concentration, at about 1018 cm−3, ﬁnishes the structure. The thickness of the n+ layer is typically about 1 µm, so as to achieve low resistance. Following the rules described above, the obtained HgCdTe heterostructures were subjected to the standard processing procedure, which includes, among others: photolithography masking; chemical etching; metallization; cutting; and assembling. The circular photodiodes with mesa diameters ranging from 100 to 900 µm were fabricated Coatings 2021, 11, 611 5 of 13

in size. The absorber composition corresponds to the required cut-off wavelength of the detector, and for MWIR devices has a value in the range of 0.27–0.36. The thickness of the absorber should be greater than the minority carrier diffusion length, and is typically about 3 µm. The parameters of the next transient P layer were calculated individually for each device, in order to prevent the formation of a potential barrier, taking into account the interdiffusion processes during growth. The composition of the P+ layer should be significantly larger than that of the absorber, and the doping should be as high as possible in order to attain low concentrations of the minority carriers and low resistance. Our MOCVD system enabled us to achieve an acceptor concentration of around 1017 cm−3. The thickness of the P+ layer was about 1 µm, in order to minimize the in-diffusion of the electrons from the metallic contact. Finally, the n+ layer, with a composition lower than that of the absorber and donor concentration, at about 1018 cm−3, finishes the structure. The thickness of the n+ layer is typically about 1 µm, so as to achieve low resistance. Following the rules described above, the obtained HgCdTe heterostructures were Coatings 2021, 11, 611 subjected to the standard processing procedure, which includes, among others: photoli-5 of 13 thography masking; chemical etching; metallization; cutting; and assembling. The circular photodiodes with mesa diameters ranging from 100 to 900 µm were fabricated and mountedand mounted on the thermoelectric on the thermoelectric coolers. The coolers. structures The were structures prepared were to preparedbe back-side to beillu- back- minatedside (irradiated illuminated from (irradiated the substrate from the side, substrate as was illustrated side, as was in illustratedFigure 2). The in Figure photodi-2). The odesphotodiodes were not passivated, were not and passivated, no anti–reflection and no anti–reﬂection coating was coatingapplied. was applied.

2.3. Surface2.3. Surface Characterization Characterization FigureFigure 3a presents3a presents an atomic an atomic force force micros microscopycopy (AFM) (AFM) image image of (100) of (100) GaAs GaAs oriented oriented 2° off2◦ towardoff toward the nearest the nearest <110> <110> epi-ready epi-ready substrate, substrate, as typically as typically used used in our in experiments. our experiments. AlthoughAlthough the substrate the substrate is optically is optically perfect, perfect, its surface its surface is not completely is not completely smooth if smooth we con- if we siderconsider the micro the scale. micro The scale. AFM The micrograph AFM micrograph shows the showstypical the small-grain typical small-grain texture visible texture on thevisible surface on theof the surface best commercially of the best commercially available, epi-ready available, epi-readyGaAs wafer. GaAs The wafer. surface The state surface of thestate substrate of the substratedirectly conditions directly conditions the crystalline the crystalline quality of quality the on-growing of the on-growing material. material.Fig- ure 3bFigure presents3b presents an AFM an image AFM imageof MOCVD-grown of MOCVD-grown CdTe (111) CdTe B (111) buffer B bufferon (100) on GaAs (100) GaAs2° ◦ → <110>2 → substrate.<110> substrate. The (111) The B (111)growth B growth was enfo wasrced enforced via a suitable via a suitable nucleation nucleation procedure, procedure, basedbased on GaAs on GaAs substrate substrate annealing annealing in a inTe-r aich Te-rich atmosphere atmosphere prior prior to the to growth the growth of the of the CdTeCdTe buffer. buffer. The Thebuffer buffer is 3 µm is 3 µthick,m thick, and andits surface its surface roughness roughness is typically is typically about about 10 nm; 10 nm; its growthits growth on misoriented on misoriented (100) (100) GaAs GaAs substrat substrateses contributes contributes to the to reduction the reduction of the of twin the twin domaindomain sizes sizes [10,11]. [10 ,11Visible]. Visible mounds mounds and andunev unevennessenness result result from from the existence the existence of small of small amountsamounts of the of (100) the (100) phase phase and andfast fastthree-dimensional three-dimensional (111) (111) growth growth [12]. [ 12].

(a) (b)

FigureFigure 3. Examples 3. Examples of AFM of AFM micrographs: micrographs: (a) GaAs(100) (a) GaAs(100) 2° → 2<110>◦ → <110> substrate; substrate; (b) MOCVD-grown (b) MOCVD-grown CdTeCdTe (111) (111) B buffer B buffer layer. layer.

Figure4 presents the surface morphology of a MOCVD-grown (111) HgCdTe heterostructure, taken with a Nomarski microscope. The heterostructure is a 20-µm-thick MWIR photodiode, and its surface roughness is typically about 40 nm. The rough surface of the CdTe buffer is enhanced and reﬂected at the on-growing HgCdTe surface. Much effort has been made in order to optimize the growth parameters and to improve the surface quality of the (111) HgCdTe, but the results are still unsatisfactory. On the other hand, the (111) HgCdTe layers were devoid of the troublesome hillocks typical of (100) HgCdTe. A high surface roughness reﬂects poor crystalline quality, but nevertheless, the (111) HgCdTe heterostructures presented in this paper are good materials for high-performance HOT IR detectors, which is supported by our previous research [13]. Coatings 2021, 11, 611 6 of 13

Figure 4 presents the surface morphology of a MOCVD-grown (111) HgCdTe heterostructure, taken with a Nomarski microscope. The heterostructure is a 20-µm-thick MWIR photodiode, and its surface roughness is typically about 40 nm. The rough surface of the CdTe buffer is enhanced and reflected at the on-growing HgCdTe surface. Much effort has been made in order to optimize the growth parameters and to improve the surface quality of the (111) HgCdTe, but the results are still unsatisfactory. On the other hand, the (111) HgCdTe layers were devoid of the troublesome hillocks typical of (100) HgCdTe. Coatings 2021, 11, 611 A high surface roughness reflects poor crystalline quality, but nevertheless, the 6(111) of 13 HgCdTe heterostructures presented in this paper are good materials for high-performance HOT IR detectors, which is supported by our previous research [13].

FigureFigure 4.4. SurfaceSurface morphology of a MOCVD-grown (111) HgCdTe heterosturcture obtained with with a a NomarskiNomarski microscope.microscope.

2.4.2.4. Heterostructure CharacterizationCharacterization EightEight heterostructures of type type N N++/p/P/P/p/P/P+/n++ were/n+ were investigated investigated during during this study. this study. They Theydiffered differed in the in composition the composition and the and thickness the thickness of the ofN+ the and N absorber+ and absorber regions. regions. The details The detailsof the chosen of the chosenN+/p/P/P N+/n/p/P/P+ heterostructures+/n+ heterostructures are presented are in presented Table 2. in Table2.

TableTable 2.2.The The parametersparameters ofof thethe chosenchosen NN++/p/P/P+/n++/n heterostructures.+ heterostructures.

CompositionComposition (x (x)) Thickness Thickness [µm] [µm] No No + + NN Region+ Region Absorber Absorber N N +RegionRegion Absorber Absorber #241 0.389 0.319 10.01 4.32 #241 0.389 0.319 10.01 4.32 #465#465 0.392 0.392 0.286 0.286 9.54 9.54 3.95 3.95 #716#716 0.57 0.57 0.271 0.271 9.76 9.76 4.17 4.17 #773#773 0.48 0.48 0.296 0.296 9.88 9.88 4.24 4.24 #858 0.499 0.359 9.79 4.31 #858 0.499 0.359 9.79 4.31 #871 0.515 0.338 9.55 4.40 #871#892 0.515 0.471 0.338 0.320 9.55 9.48 4.40 3.75 #892#962 0.471 0.479 0.320 0.337 9.48 10.52 3.75 5.62 #962 0.479 0.337 10.52 5.62 Figure5 presents the scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) imageFigure of the 5 Npresents+/p/P/P the+ /nscanning+ #962 electron HgCdTe mi heterostructurecroscopy (SEM, cleavage. Hitachi, Tokyo, The sample Japan) was im- + + + brokenage of the along N /p/P/P the growth/n #962 direction. HgCdTe This heterostructure allowed us to specifycleavage. the The individual sample componentwas broken layers,along the and growth to measure direction. their This thickness. allowed The us obtainedto specify structure the individual was compared component with layers, the designedand to measure one (Figure their 2thickness.). The measured The obtained thicknesses structure were was given compared with 0.01 withµm the resolution. designed However,one (Figure because 2). The of measured the blurred thicknesses boundaries were between given the with layers, 0.01 the µm practical resolution. measurement However, precisionbecause of was the assumedblurred boundaries at about 1%, between and this the was layers, sufﬁcient the practical for our measurement experiments. preci- The gradedsion was interfaces assumed between at about the 1%, individual and this layerswas sufficient were also for hardly our experiments. observed. The The measured graded thicknessinterfaces ofbetween the presented the individu MWIRal heterostructurelayers were also was hardly 20.72 observed.µm. The measured thickness Figureof the 6presented presents MWIR the N + heterostructure/p/P/P+/n+ HgCdTe was 20.72 heterostructure µm. cleavage, taken with an optical microscope at ×1000 magniﬁcation. The thickness of the CdTe buffer layer was about 2.5 µm. The total thickness of the HgCdTe structure was estimated at 20.6 µm, and was comparable with the result obtained using the SEM method. The P transition layer is barely visible, due to the interdiffusion process during growth, and because the image resolution is too low. An inset with a diagram of the individual component layers was added to the photo at the bottom for better visualization. This method is less precise than the SEM measurements, but both are prompt and useful, and give better insight into the device performance. Coatings 2021, 11, 611 7 of 13

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Figure 5. SEM image of the MOCVD-grown N+/p/P/P+/n+ #962 HgCdTe heterostructure cleavage.

Figure 6 presents the N+/p/P/P+/n+ HgCdTe heterostructure cleavage, taken with an optical microscope at ×1000 magnification. The thickness of the CdTe buffer layer was about 2.5 µm. The total thickness of the HgCdTe structure was estimated at 20.6 µm, and was comparable with the result obtained using the SEM method. The P transition layer is barely visible, due to the interdiffusion process during growth, and because the image resolution is too low. An inset with a diagram of the individual component layers was added to the photo at the bottom for better visualization. This method is less precise than the SEM measurements, but both are prompt and useful, and give better insight into the + + + FiguredeviceFigure 5. 5.performance. SEMSEM imageimage ofof thethe MOCVD-grownMOCVD-grown N N+/p/P/P/p/P/P /n+/n #962+ #962 HgCdTe HgCdTe heterostructure heterostructure cleavage. cleavage.

+ + + FigureFigure 6.6. The N+/p/P/P/p/P/P/n+/n HgCdTe+ HgCdTe heterostructure heterostructure cleavage cleavage taken taken with with an optical an optical microscope. microscope.

Figure7 7presents presents the the secondary-ion secondary-ion mass mass spectroscopy spectroscopy (SIMS, (SIMS, Cameca, Cameca, Gennevilliers, Gennevil- + + + ++ + France)liers, France) profile profile of the of N the/p/P/P N /p/P/P/n/nHg Hg1−1x−xCdCdxxTeTe heterostructureheterostructure withwith thethe absorbingabsorbing regionregion compositioncompositionx x= = 0.32.0.32. TheThe greengreen line line represents represents iodine iodine counts: counts: 10 105 5and and 2 2× ×10 1044 inin thethe nn+ and N+ regions,regions, respectively, respectively, which correspond to thethe designeddesigned concentrationconcentration levels:levels: 10101818 cm−33 andand 2 2 ×× 10101717 cmcm−3−, 3respectively., respectively. In In general, general, abrupt abrupt transitions transitions were were observed inin thethe iodineiodine profile.profile. AsAs aa positive consequence,consequence, therethere waswas nono iodineiodine presencepresence inin thethe adjacentadjacent layers, especially in the absorber region. The pink line represents the arsenic-stepped profile, corresponding to abruptly increasing doses of the arsenic in successive layers: p, P, and P+. WeFigure did 6. not The observe N+/p/P/P any+/n+ HgCdTe undesirable heterostructure arsenic diffusion cleavage totaken the with adjacent an optical layers, microscope. suggesting that arsenic is a stable dopant. The mercury, cadmium, and tellurium counts enabled us toFigure calculate 7 presents a composition the secondary-ion (x) profile (the mass light spectroscopy blue line) of(SIMS, the whole Cameca, Hg 1Gennevil-−xCdxTe heterostructure.liers, France) profile We can of seethe that N+/p/P/P the narrower+/n+ Hg1 gap−xCd absorberxTe heterostructure with the composition with the xabsorbing= 0.32 is surroundedregion composition by wider xenergy = 0.32. gapThe Ngreen+ and line P+ layersrepresents with iodine compositions counts: of 10 around5 and 2 x× =10 0.464 in ,the as assumedn+ and N in+ regions, the heterostructure respectively, project. which Duecorrespond to the interdiffusive to the designed processes concentration in the HgCdTe levels: alloy1018 cm during−3 and growth, 2 × 1017 a cm composition−3, respectively. profile In of general, the interfaces abrupt between transitions the particular were observed layers isin notthe abrupt.iodine profile. Systematic As a studies positive of consequence, the SIMS results, there supported was no iodine by the presence theoretical in calculations,the adjacent

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layers, especially in the absorber region. The pink line represents the arsenic-stepped profile, corresponding to abruptly increasing doses of the arsenic in successive layers: p, P, and P+. We did not observe any undesirable arsenic diffusion to the adjacent layers, suggesting that arsenic is a stable dopant. The mercury, cadmium, and tellurium counts enabled us to calculate a composition (x) profile (the light blue line) of the whole Hg1−xCdxTe heterostructure. We can see that the narrower gap absorber with the composition x = 0.32 is surrounded by wider energy gap N+ and P+ layers with compositions of around x = 0.46, Coatings 2021, 11, 611 as assumed in the heterostructure project. Due to the interdiffusive processes8 of 13 in the HgCdTe alloy during growth, a composition profile of the interfaces between the particular layers is not abrupt. Systematic studies of the SIMS results, supported by the theoret- icalallow calculations, us to make allow the corrections us to make in the programmedcorrections in growth the programmed recipes, and growth to obtain recipes, the and toassumed obtain proﬁlesthe assumed in the realprofiles HgCdTe in the heterostructures. real HgCdTe heterostructures.

+ + + + ++ FigureFigure 7. The SIMSSIMS proﬁle profile of of the the N N/p/P/P/p/P/P/n/n Hg11−−xxCdCdxxTeTe #892 heterostructureheterostructure with with an absorb-an absorbing regioning region composition composition of ofx =x 0.32.= 0.32.

AnotherAnother characterizationcharacterization method method we we used used was was IR transmission IR transmission measurement, measurement, which which waswas performed performed using using a a PerkinElmer PerkinElmer Fourier-transform Fourier-transform infraredinfrared (FTIR, (FTIR, PerkinElmer, PerkinElmer, Wal- Waltham, MA, USA) spectrometer. Figure8 presents the transmittance characteristics tham, USA) spectrometer. Figure 8 presents the transmittance characteristics of particular of particular fragments of the #773 N+/p/P/P+/n+ structure, after selective etching. The + + + fragmentsselected area of ofthe the #773 sample N /p/P/P was covered/n structure, with a after photoresist selective ﬁlm etching. deposited The during selected the area of thephotolithography sample was covered process. Aswith a result, a photoresist the part of film the sampledeposited surface during intended the photolithography for etching process.was exposed. As a result, The selective the part etching of the was sample performed surface in intended a solution for of Bretching in HBr was diluted exposed in . The selectivedeionized etching water (50:50:1 was performed Br:HBr:H2). in The a greensolution line (ofta )Br represents in HBr thediluted transmission in deionized of the water + (50:50:1total heterostructure Br:HBr:H2). The just aftergreen growth, line (ta where) represents the narrow-gap the transmission n top layer of the mainly total deter-heterostruc- turemines just its shape.after growth, Additionally, where the the clear narrow-gap interference fringesn+ top werelayer useful mainly inthe determines estimation its of shape. the actual thickness of the measured sample. The red line (tb) was obtained after 4-µm-deep Additionally, the clear interference+ fringes+ were useful in the estimation of the actual chemical etching. As a result, the n and P layers were removed. The slope of the tb curve thickness of the measured sample. The red line (tb) was obtained after 4-µm-deep chemical depends on the absorber transmission. This allows us to determine the composition of + + etching.the absorber, As a whichresult, in the this n exemplary and P layers structure were takes removed. a value The of x =slope 0.302. of After the t removingb curve depends onanother the absorber 7 µm of thetransmission. material (11 Thisµm in allows total), theus to N+ determinelayer was exposed, the composition and the blue of the curve absorber, whichtc represents in this its exemplary transmission structure spectral takes dependence. a value Inof thisx = 0.302. way, the After composition removing (x) another of the 7 µm + ofobtained the material N layer (11 was µm determined in total), the at N the+ layer value was of x =exposed, 0.492 (whereas and the the blue predicted curve valuetc represents + itswas transmissionx = 0.48). The spectral N layer dependence. composition determinesIn this way, the theλCUT-ON compositionvalue of ( thex) of photodiode the obtained N+ laywhener was it is determined back-side illuminated. at the value In of general,x = 0.492 the (whereas IR transmission the predicted measurement value resultswas x = 0.48). provide valuable feedback for MOCVD growers. The N+ layer composition determines the λCUT-ON value of the photodiode when it is back- side illuminated. In general, the IR transmission measurement results provide valuable feedback for MOCVD growers. CoatingsCoatings 20212021, 11, 11, 611, 611 9 of9 of13 13

+ + + FigureFigure 8. Transmittance 8. Transmittance characteristics characteristics of of particular particular fragments fragments of of the the #773 #773 N N+/p/P/P/p/P/P+/n+ structure/n structure afterafter selective selective etching. etching. 2.5. Device Characterization 2.5. Device Characterization The obtained MOCVD-grown (111) HgCdTe photodiodes were characterized using The obtained MOCVD-grown (111) HgCdTe photodiodes were characterized using the available measurement methods. The spectral characteristics were measured using a the available measurement methods. The spectral characteristics were measured using a PerkinElmer Spectrum 2000 FT–IR Spectrometer. The absolute values of current sensitivity PerkinElmer Spectrum 2000 FT–IR Spectrometer. The absolute values of current sensitiv- Ri [A/W] were obtained using a supporting measurement set, which includes a lock-in ity Ri [A/W] were obtained using a supporting measurement set, which includes a lock-in analyzer, a monochromator, and a chopped 1000 K blackbody-calibrated silicon carbide rod. analyzer,The current–voltage a monochromator, characteristics and a chopped of the photodiodes1000 K blackbody-calibrated were measured usingsilicon a carbide Keithley rod.2400 The SourceMeter current–voltage (Keithley, characteristics Cleverland, OH,of the USA), photodiodes controlled viawere the measured LabView applicationusing a Keithleyfor automation. 2400 SourceMeter (Keithley, Cleverland, USA), controlled via the LabView ap- plication for automation. 3. Results and Discussion 3. ResultsIn this and section, Discussion we describe the exemplary results of MWIR photovoltaic detectors fabricatedIn this section, with MOCVD-grown we describe the N +exemplar/p/P/P+y/n results+ heterostructures. of MWIR photovoltaic Figure9 presents detectors the fabricatedJ(V) characteristics with MOCVD-grown of MWIR HgCdTeN+/p/P/P photodiodes+/n+ heterostructures. fabricated Figure with 9 #858 presents structure the J(V) mea- characteristicssured at the of temperatures MWIR HgCdTeT = photodiodes 230 K and T fabricated= 300 K. Thewith absorber #858 structure composition measured of thisat thestructure temperatures was designed T = 230 K at and the T value = 300x K.= 0.359The absorber in order composition to enable the of MWIR this structure operation. was The designedcircular at mesa the value diameter x = 0.359 of the in photodiode order to enable was 820the MWIRµm. The operation. obtained TheJ(V) circular characteristics mesa diameterfollow aof classic the photodiode Shockley was equation 820 µm. with The the obtained parameter J(V)β characteristics= 1.1 and shunt follow resistance a classicR b 7 2 7 2 Shockley(RbA = equation 5 × 10 Ω with·cm the): parameter β = 1.1 and shunt resistance Rb (RbA = 5 × 10 Ω·cm ): U = qU/βkT − − J Js e ⁄β 1 𝑈 (1) 𝐽 = 𝐽𝑒 −1− Rb A (1) 𝑅𝐴 where q is the elementary charge, U is the voltage, k is Boltzman’s constant, A is detector’s wherearea. q is the elementary charge, U is the voltage, k is Boltzman’s constant, A is detector’s area. A value of the β coefficient, close to the unity, proves the predominant share of diffusion current in the total junction current. The determined value of the R0A product was 160 Ωcm2, consistent with the value determined on the basis of the saturation current determined from the fitting (Js = 0.136 mA/cm2) for temperature T = 230 K. A high detector series resistance value will be a matter for future processing improvements.

CoatingsCoatings 20212021, 11, 11, 611, 611 1010 of of13 13

FigureFigure 9. 9.ExampleExample of of the the current–voltage current–voltage characterist characteristicsics of of an an MWIR MWIR HgCdTe HgCdTe photodiode, measuredmeas- uredat different at different temperatures. temperatures.

FigureA value 10 presents of the β thecoefficient, spectral characteri close to thestics unity, of MWIR proves HgCdTe the predominant photodiodes shareof dif- of ferentdiffusion architectures current inmeasured the total at junction the temperature current. The T = determined230 K. The detectors value of were the R measured0A product 2 withoutwas 160 biasingΩcm , voltage. consistent The with presented the value #773, determined #241, and on #858 the basis structures of the saturationdiffer mainly current in 2 theirdetermined N+ the absorber from the compositions. fitting (Js = 0.136 The mA/cm composition) for temperature (x) of the NT += layer 230 K. determines A high detector the cut-onseries wavelength resistance value (λCUT-ON will) beof athe matter detector. for future For example, processing structure improvements. #858 contains an N+ layer withFigure composition 10 presents x = the0.499, spectral and the characteristics λCUT-ON is equal of to MWIR 2.1 µm; HgCdTe this is the photodiodes shortest cut- of ondifferent wavelength architectures seen in Figure measured 10. atOn the the temperature other hand,T structure= 230 K. The #241 detectors contains were the N measured+ layer without biasing voltage. The presented #773, #241, and #858 structures differ mainly in with the lowest composition (x = 0.389) and the λCUT-ON is equal to 2.7 µm; this is the longest + + cut-ontheir Nwavelengththe absorber of the compositions. presented characteri The compositionstics. Analogously, (x) of the Nthe layercut-off determines wavelength the cut-on wavelength (λ ) of the detector. For example, structure #858 contains an N+ (λCUT-OFF) of the detectorsCUT-ON can vary as a result of the absorbers’ composition changes. layer with composition x = 0.499, and the λCUT-ON is equal to 2.1 µm; this is the shortest Hence, we can adjust the shape of the spectral characteristics to the needs of the recipient + bycut-on appropriate wavelength shaping seen of the in Figure energy 10 gap. On profile the other of the hand, heterostructure. structure #241 The contains maximal the cur- N layer with the lowest composition (x = 0.389) and the λCUT-ON is equal to 2.7 µm; this is rent sensitivity values, Ri(λpeak), of the presented photodiodes are comparable, and are the longest cut-on wavelength of the presented characteristics. Analogously, the cut-off above 1.7 A/W at 230 K. wavelength (λCUT-OFF) of the detectors can vary as a result of the absorbers’ composition changes. Hence, we can adjust the shape of the spectral characteristics to the needs of the recipient by appropriate shaping of the energy gap profile of the heterostructure. The maximal current sensitivity values, Ri(λpeak), of the presented photodiodes are comparable, and are above 1.7 A/W at 230 K. Table3 presents the parameters of exemplary MWIR photodiodes, obtained on the basis of MOCVD-grown HgCdTe heterostructures. The detectivity (D *) of all presented MWIR photodiodes at 230 K was above 1010 cm·Hz1/2/W (and 1011 cm·Hz1/2/W with an optical immersion), and was determined by background-limited photodetector (BLIP) condition, which confirms the device quality of MOCVD-grown (111) HgCdTe material.

Coatings 2021, 11, 611 11 of 13 Coatings 2021, 11, 611 11 of 13

FigureFigure 10. 10. SpectralSpectral characteristics characteristics of of MWIR MWIR HgCdTe HgCdTe ph photodiodesotodiodes of different architecturesarchitectures measuredmeas- uredat T at= T 230 = 230 K. K.

Table 3. The parameters of the chosen MWIR photodiodes. Table 3 presents the parameters of exemplary MWIR photodiodes, obtained on the basis of MOCVD-grown HgCdTePhotodiode heterostructures. Parameters at TemperatureThe detectivityT = 230(D K*) of all presented MWIR photodiodes at 230 K was above 1010 cm⋅Hz1/2/W (and 1011 cm⋅Hz1/2/W with an op- D *(λ ) D *(λ ) λ λ R AR (λ ) peak peak ticalNo immersion),10%CUT-ON and was10%CUT-OFF determined by0 background-limitedi peak Non photodetector Immersed (BLIP)Immersed condition, which confirms the device quality of MOCVD-grown (111) HgCdTe material. µm µm Ωcm2 A/W cm·Hz1/2/W cm·Hz1/2/W Table#716 3. The parameters 1.8 of the 5.7 chosen MWIR 0.7 photodiodes. 1.9 1.23 × 1010 1.23 × 1011 #465 2.85 5.2 1.1 2.0 1.62 × 1010 1.62 × 1011 #773 2.2Photodiode 4.75 Parameters 2.4 at 1.75 Temperature2.09 T× =10 23010 K 2.09 × 1011 10 11 #241 2.7 4.35 1.8 1.78 D 1.84* (λ×peak10) D 1.84* (λ×peak10) No λ10%CUT-ON λ10%CUT-OFF R0A Ri (λpeak) 10 11 #962 2.18 3.98 9 2.1 Non 4.86Immersed× 10 Immersed4.86 × 10 #871 2.0 3.9 6.5 2.2 4.32 × 1010 4.32 × 1011 µm µm Ωcm2 A/W cm⋅Hz1/2/W cm⋅Hz1/2/W #858 2.1 3.6 160 1.7 1.66 × 1011 1.66 × 1012 #716 1.8 5.7 0.7 1.9 1.23 × 1010 1.23 × 1011 #465 2.85 5.2 1.1 2.0 1.62 × 1010 1.62 × 1011 Figure 11 shows a comparison of the R A product of MWIR HgCdTe detectors, as a #773 2.2 4.75 2.4 0 1.75 2.09 × 1010 2.09 × 1011 function of the 50% cut-off wavelength (λ ), with theoretical calculations. Using #241 2.7 4.35 1.850%CUT-OFF 1.78 1.84 × 1010 1.84 × 1011 numerical simulations of I–V characteristics on the SimuApsys platform (Crosslight Inc., 10 11 #962 2.18 3.98 + 9 + 2.1+ 4.86 × 10 4.86 × 10 Vancouver, BC, Canada)) for a given N /p/P/P /n heterostructure, the values of the R0A 10 11 product#871 as a function 2.0 of cut-off 3.9 wavelength 6.5 were calculated 2.2 at 4.32 temperatures × 10 of 4.32 170, × 200,10 230, 11 12 and#858 280 K. The numerical2.1 model 3.6 implemented 160 in the 1.7 APSYS platform1.66 × 10 incorporates 1.66 × HgCdTe10 generation–recombination mechanisms: radiative, Auger, and Shockley–Read–Hall (SRH). Figure 11 shows a comparison of the R0A product of MWIR HgCdTe detectors, as a The R0A product determined experimentally was consistent with the theoretical values functionfor most of of the the 50% samples. cut-off The wavelength dominant (λ contribution50%CUT-OFF), with to thetheoretical dark current calculations. was from Using SRH numericalrecombination. simulations The reason of I–V for characteristics the location ofon somethe SimuApsys experimental platform points (Crosslight slightly below Inc., or Vancouver,above the theoreticalCanada)) for curve a given was N a+ different/p/P/P+/n value+ heterostructure, of the SRH lifetimethe values compared of the R0 toA theprod- one uctadopted as a function in the calculations. of cut-off wavelength Some samples were were calculated characterized at temperatures by much lower of 170, values 200, of230, the andR0 A280product K. The in relationnumerical to themodel theoretical implemente calculations.d in the This APSYS is probably platform due incorporates to improper HgCdTeprocessing generation–recombination and an increase in the sharemechanisms: of leakage radiative, currents Auger, in the and total Shockley–Read– current ﬂowing Hallthrough (SRH). the The junction. R0A product determined experimentally was consistent with the theoretical values for most of the samples. The dominant contribution to the dark current was from SRH recombination. The reason for the location of some experimental points slightly below or above the theoretical curve was a different value of the SRH lifetime compared

Coatings 2021, 11, 611 12 of 13

to the one adopted in the calculations. Some samples were characterized by much lower values of the R0A product in relation to the theoretical calculations. This is probably due Coatings 2021, 11, 611 to improper processing and an increase in the share of leakage currents in the total current12 of 13 flowing through the junction.

Figure 11. R0A data at different temperatures for MWIR HgCdTe photodiodes. Single points Figure 11. R0A data at different temperatures for MWIR HgCdTe photodiodes. Single +points repre-+ + represent experimental values. Curves represent calculated R0A values for analyzed N /p/P/P /n sent experimental values. Curves represent calculated R0A values for analyzed N+/p/P/P+/n+ heter- heterostructures. ostructures. 4. Conclusions 4. ConclusionsThe MOCVD technique, with a wide range of composition and donor/acceptor doping,The is a MOCVD very convenient technique, tool with for thea wide deposition range of of composition HgCdTe epilayers and donor/acceptor used in uncooled doping,infrared is a very detectors. convenient The uniquetool for advantage the deposition of the of MOCVD HgCdTe growth epilayers method used in for uncooled HgCdTe infraredheterostructures detectors. is The the unique lack of advantage the necessity of the of time-consuming MOCVD growth and method unsuitable for HgCdTe ex situ heterostructurespost-growth annealing is the lack in sealedof the necessity quartz ampoules of time-consuming in the presence and unsuitable of mercury ex vapors. situ post- The + + + growthSIMS profile annealing of the in Nsealed/p/P/P quartz/n ampoulesHg1−xCd inx Tethe heterostructure presence of mercury showed vapors. that EIThe and SIMS TD- profileMAAs of are the stable N+/p/P/P donor+/n and+ Hg acceptor1−xCdxTe heterostructure dopants, respectively. showed Fully that EI doped and (111)TDMAAs HgCdTe are stableheterostructures donor and wereacceptor grown dopants, in order respecti to investigatevely. Fully the devices’doped (111) performance HgCdTe inheterostruc- the 3–5 µm turesinfrared were band. grown The in detectivity order to investigateD * of all presented the devices’ MWIR performance photodiodes in the at 3–5 230 µm K was infrared above 10 1/2 11 1/2 band.10 cm The·Hz detectivity/W (and D 10 * ofcm all· Hzpresented/W with MWIR an optical photodiodes immersion), at 230 and K was determinedabove 1010 cmby⋅Hz BLIP1/2/W condition, (and 10 confirming11 cm⋅Hz1/2 the/W devicewith an quality optical of MOCVD-grownimmersion), and (111) was HgCdTe determined material. by BLIP condition, confirming the device quality of MOCVD-grown (111) HgCdTe material. Author Contributions: Conceptualization, W.G. and P.M.; methodology, W.G.; investigation, J.S. and AuthorT.M.; writing—original Contributions: Conceptualization, draft preparation, P.M.; W.G. writing—review and P.M.; meth andodology, editing, W.G.; J.R.; investigation, supervision, M.K.; J.S. andfunding T.M.; acquisition, writing—original J.R. All draft authors preparation, have read andP.M.; agreed writing—review to the published and editing, version J.R.; of the supervision, manuscript. M.K.;Funding: fundingThe acquisition, writing of theJ.R. paper All authors has been have partially read and achieved agreed with to the the published financial supportversion of Thethe manuscript.National Centre for Research and Development (Poland)—Grant No. Mazowsze/0090/19-00; and Funding:the National The Sciencewriting Center of the (Poland)—Grantpaper has been partially No. UMO-2017/27/B/ST7/01507. achieved with the financial support of The NationalInstitutional Centre Review for Research Board Statement: and DevelopmenNot applicable.t (Poland)—Grant No. Mazowsze/0090/19-00; and the National Science Center (Poland)—Grant No. UMO-2017/27/B/ST7/01507. Informed Consent Statement: Not applicable. Institutional Review Board Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author.

Acknowledgments: We would like to thank Cezary Kobyłecki from VIGO System S.A. for SEM measurements. Conﬂicts of Interest: The authors declare no conﬂict of interest. Coatings 2021, 11, 611 13 of 13

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