CdTe Epilayers Research Articles

Analysis of dislocations and their correlation with dark currents in CdTe/Si heterojunction diode-type x-ray detectors

Distribution and density of dislocations as a function of CdTe epilayer thickness as well as their effect on device dark current were studied using a p-CdTe/n-CdTe/n+-Si heterojunction diode type x-ray detector. A high dislocation density was found in thin devices, which decreased by more than an order of magnitude when the device thickness was increased to 35 μm. First, the dislocation density decreased rapidly with the CdTe thickness but the decrement was slowed down for the CdTe thicknesses above 20 μm. The device dark current showed a strong dependence on dislocation density, which increased with an increase in the dislocation density. A nearly temperature independent dark current was observed in thin devices suggesting that trap-assisted tunneling (TAT) through the dislocation-related current paths was dominant. Dark currents of thicker devices, on the other hand, could be explained due to contributions from both TAT and generation-recombination currents. Furthermore, one possible detector design that helps us to reduce the dark current is also proposed.

Band edge thermometry for the MBE growth of (Hg,Cd)Te-based materials

The material system (Hg,Cd)Te is a prototype system for topological insulators and Dirac semi-metals. Molecular beam epitaxy is the preferred method for the growth of (Hg,Cd)Te epilayer heterostructures and CdTe-HgTe core-shell nanowires. Unfortunately, the high crystalline quality desired for charge transport investigations is only achieved for a very narrow window of the growth temperature. Additionally, the low growth temperature requires a special thermometry. Here, we demonstrate the improvement of the growth of (Hg,Cd)Te epilayer heterostructures and CdTe nanowires by the use of band edge thermometry. We show that even for narrow-gap materials band edge thermometry provides a suitable method to control the temperature directly at the growth front with a high precision and reproducibility, in contrast to a conventionally used thermocouple.

Identifying threading dislocations in CdTe films by reciprocal space mapping and defect decoration etching

We study threading dislocation (TD) density of high-quality cadmium telluride (CdTe) layers grown on a (211) oriented GaAs substrate by molecular beam epitaxy. High-resolution X-ray diffraction was performed to calculate the density of screw-type TDs by measuring the broadening of the asymmetrical (511) Bragg reflections of CdTe epilayers. In addition, total TD densities were determined by the Everson-etching method and were compared with screw TDs. Our results show that the total TD densities in CdTe films were dominated by those with screw character. The screw component TDs are estimated to account for more than 90% of the total TD density. CdTe layers grown at a thickness of less than 3.0 μm typically exhibit the screw TD densities in the 106 cm–2 and 107 cm–2 range. It can be noted that as the nucleation temperature increases, i.e., ≥222 °C, both the area density of TDs with the screw component of the CdTe films and the total TD density are roughly four times larger than those of the epilayer grown at the nucleation temperature of 215 °C. Furthermore, we discuss the influence of the II/VI flux ratio on the density of threading dislocations. The contribution of screw TDs to the total TD density showed a significant decrease in roughly 30% in the case of a high II/VI flux ratio. We further examine the reciprocal space maps in the vicinity of the (422) reflections.

MBE-growth of CdTe on GaSb substrates: A case study on the influence of substrate quality

CdTe epitaxial layers were grown by MBE on GaSb (211)B substrates from two different suppliers in order to determine the influence of as-received substrate quality on the material quality of subsequently-grown CdTe epilayers. It is observed that GaSb substrates with smooth surface and lower degree of surface roughness can lead to CdTe epilayers with superior material quality, as evidenced by a lower dislocation density, lower etch pit density, lower degree of epilayer surface roughness, narrower XRD FWHM, and lower strain. It is concluded that in comparison to substrates with a relatively smooth surface (RMS surface roughness ∼ 0.62 nm), substrates with a high degree of surface roughness (RMS surface roughness ∼ 2.24 nm) lead to effects that generate misfit dislocations during the MBE growth process. This results in approximately an order of magnitude increase in measured etch pit density (from ∼9 × 105 to 7 × 106 cm−2) and the calculated dislocation density (from 6.51 × 105 to 6.87 × 106 cm−2, as determined from X-ray diffraction reciprocal space mapping), indicating that a smooth substrate surface is critical in achieving high quality CdTe epilayers on GaSb.

Dry Etching Characteristics of MOVPE-Grown CdTe Epilayers in CH4, H2, Ar ECR Plasmas

Dry etching characteristics of single crystal (100) CdTe epitaxial layers grown on GaAs substrates were studied using CH4, H2, and Ar as process gases in an electron cyclotron resonance plasma. A smooth and anisotropic etching was obtained with CH4, H2, and Ar. No hydrocarbon polymer was found on the etched surface, which was confirmed by x-ray photoelectron spectroscopy measurement. Etching of the CdTe surface was also possible with H2 and Ar; however, no etching was observed in the absence of H2. Dependence of the etch rate on plasma gas composition and flow rates was studied. Mechanisms of etching with and without CH4 supply were also studied. Etched CdTe layers also showed no deterioration of electrical properties, which was confirmed by photoluminescence measurement at 4.2 K and Hall measurement at 300 K.

Reciprocal space mapping study of CdTe epilayer grown by molecular beam epitaxy on (2 1 1)B GaAs substrate

We examine high quality, single crystal CdTe epilayer grown by molecular beam epitaxy (MBE) on ()B GaAs substrate using both positions and full width at half maximums (FWHMs) of reciprocal lattice points (RLPs). Our results demonstrate that reciprocal space mapping (RSM) is an effective way to study the structural characteristics of the high-index oriented epitaxial thin films having a large lattice mismatch with the substrate. The measurement method is defined first, and then the influence of shear strain () on the position of the () node of epilayer is clarified. It is concluded that the lattice tilting is likely to be related with the lattice mismatch. Nondestructive measurement of the dislocation density is achieved by applying the mosaic crystal model. The screw dislocation density, estimated to be cm−2, was calculated utilizing the broadened peakwidths of the asymmetric RLP of the epilayer lattice.

II-VI Material Integration With Silicon for Detector and PV Applications

ABSTRACTHeteroepitaxial growth of high-quality II-VI-alloy materials on Si substrates is a well-established commercial growth process for infrared (IR) detector devices. However, it has only recently been recognized that these same processes may have important applications for production of high-efficiency photovoltaic devices. This submission reviews the process developments that have enabled effective heteroepitaxy of II-VI alloy materials on lattice-mismatched Si for IR detectors as a foundation to describe recent efforts to apply these insights to the fabrication of multijunction Si/CdZnTe devices with ultimate conversion efficiencies >40%. Reviewed photovoltaic studies include multijunction Si/CdZnTe devices with conversion efficiency of ∼17%, analysis of structural and optoelectrical quality of undoped CdTe epilayer films on Si, and the effect that a Te-rich growth environment has on the structural and optoelectronic quality of both undoped and As-doped heteroepitaxial CdTe.

Growing high-quality ternary CdMnTe epilayers by molecular beam epitaxy on Si substrates and its mechanism

Cd(Mn,Zn)Te-based ternary compound semiconductors with wide band-gaps are important in the detection of radiation and photovoltaic applications. This study characterizes Cd1-xMnxTe epilayers on Si substrates with various Mn compositions grown by molecular beam epitaxy. The surface smoothness, crystallinity and optical quality all are significantly improved with increasing Mn content. The Cd0.61Mn0.39Te epilayer with a thickness of only about 500 nm yields a full width at half maximum of the X-ray rocking curve of 165 arcsec. Photoluminescence spectra at 10 K show that the intensity of defect-related emissions is much lower than that of binary CdTe epilayers, reaching zero from the samples with high Mn content, while the integral intensity of the exciton-related emissions is increased by more than two orders of magnitude. Raman scattering spectra reveal that the intensity of the Te–Te related defect vibration modes falls significantly as the Mn content increase, even disappearing altogether in the samples with high Mn content. This work proposes that incorporating Mn atoms during epitaxial growth can promote the decomposition of Te2 sources, owing to the high sticking coefficient of Mn and the high cohesive energy of the Mn–Te bond, and then reduce the number of Te–Te related stacking fault defects, yielding high-quality CdMnTe epilayers. Our results herein demonstrate that the CdMnTe ternary epilayers are much more promising in terms of material quality than the CdZnTe ternary epilayers.

Characterization of CdTe Growth on GaAs Using Different Etching Techniques

CdTe buffer layers which were grown on (211)B GaAs by molecular beam epitaxy were subjected to two different etch treatments to quantify the crystal quality and dislocation density. The optical properties and thicknesses of the samples were obtained by ex situ spectroscopic ellipsometry. The surface morphologies of the CdTe epilayers were analyzed by atomic force microscopy, scanning electron microscopy, and Nomarski microscopy before and after chemical etching. We compare the triangle- and trapezoid-shaped etch pits due to the Everson and Nakagawa etch solutions, respectively. Measured etch pit density (EPD) values of triangle etch pits were found in the 8 × 107 cm−2 to 2 × 108 cm−2 range, and trapezoid-shaped etch pits were found in the 1 × 107 cm−2 to 7 × 107 cm−2 range for samples with thicknesses <2 μm.

Development of Large-Area Imaging Arrays Using Epitaxially Grown Thick Single Crystal CdTe Layers on Si Substrates

Optimization of CdTe crystal growth by adjusting effective Te/Cd ratio at the growth surface, as well as growth-interrupted annealing together with detector array fabrication techniques were studied to achieve large-area, uniform x-ray spectroscopic imaging arrays. The ${\rm p}\hbox{-}{\rm CdTe}/{\rm n}\hbox{-}{\rm CdTe}/{{\rm n}^ +}\hbox{-}{\rm Si}$ heterojunction diode-type detector arrays were fabricated by using epitaxially grown thick single crystal CdTe layers on ${{\rm n}^ +}\hbox{-}{\rm Si}$ substrates, where pixels were patterned by making deep vertical cuts on p-CdTe side using a dicing saw. The study was first focused on development of 2D ( $8 \times 8$ ) array using $12.6~\hbox{mm} \times 11.4~\hbox{mm}$ size wafers, which confirmed uniform dark current distribution among the pixels. We further demonstrated possibility of making larger arrays by fabricating ( $18 \times 18$ ) array using a $25~\hbox{mm} \times 25~\hbox{mm}$ size uniform single crystal CdTe epilayer grown on Si substrate.

Confocal Micro-PL Mapping of Defects in CdTe Epilayers Grown on Si (211) Substrates with Different Annealing Cycles

We have applied confocal microphotoluminescence (μ-PL) microscopy to investigate the effects of extended defects in CdTe epilayers grown on (211) Si substrates with different numbers of annealing cycles N. Our results show that the PL dark spot density (PL-DSD) decreases while the spatially averaged PL intensity increases with increasing number of annealing cycles. This is consistent with the general trend that an increase in the number of annealing cycles leads to a reduction of the x-ray diffraction linewidth and etch pit density (EPD). However, direct comparison between the etch pits, imaged by scanning electron microscopy, and PL dark spots on the same sample area results in two important observations: (1) the PL-DSD is substantially higher than the EPD, and (2) not all etch pits appear as dark spots in PL maps. These findings suggest that PL mapping is a more sensitive technique for revealing the extended defects that are actually detrimental to photogenerated carriers, in addition to the apparent advantages of being noninvasive and more efficient.

Charge transport properties of p-CdTe/n-CdTe/n+-Si diode-type nuclear radiation detectors based on metalorganic vapor-phase epitaxy-grown epilayers

Charge transport properties of p-CdTe/n-CdTe/n+-Si diode-type nuclear radiation detectors, fabricated by growing p-and n-type CdTe epilayers on (211) n+-Si substrates using metalorganic vapor-phase epitaxy (MOVPE), were studied by analyzing current-voltage characteristics measured at various temperatures. The diode fabricated shows good rectification properties, however, both forward and reverse biased currents deviate from their ideal behavior. The forward current exhibits typical feature of multi-step tunneling at lower biases; however, becomes space charge limited type when the bias is increased. On the other hand, the reverse current exhibits thermally activated tunneling-type current. It was found that trapping centers at the p-CdTe/n-CdTe junction, which were formed due to the growth induced defects, determine the currents of this diode, and hence limit the performance of the nuclear radiation detectors developed.

Real-Time In Situ Monitoring of GaAs (211) Oxide Desorption and CdTe Growth by Spectroscopic Ellipsometry

We describe the growth of CdTe (211)B by molecular beam epitaxy on large-area epiready GaAs (211)B substrates. Prior to CdTe growth, GaAs substrates were thermally cleaned under an As4 flux. Oxide desorption was verified by in situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction. The use of in situ SE played a significant role in the study of CdTe-on-GaAs growth and annealing processes. An effective medium approximation (EMA) was used to model the overlayer thickness variation of CdTe epilayers throughout growth and in situ annealing cycles. A correlation between SE-derived EMA thickness values and surface defect formation mitigation is discussed. All annealed samples (11.5 μm to 13 μm thick) exhibited excellent crystalline quality with average double crystal rocking curve full-width at half-maximum (FWHM) values of ~60 arcsec.

Microstructural Characterization of CdTe(211)B/ZnTe/Si(211) Heterostructures Grown by Molecular Beam Epitaxy

Transmission electron microscopy and small-probe microanalysis have been used to investigate the microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures. Thin ZnTe buffer layers and subsequent thick CdTe layers were grown on Si(211) substrates using molecular beam epitaxy. Many {111}-type stacking faults were found to be present throughout the entire ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Local lattice parameter measurement and elemental profiles indicated that some intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd,Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively.

Effects of a ZnTe buffer layer on structural quality and morphology of CdTe epilayer grown on (001)GaAs by molecular beam epitaxy

The structural properties, crystalline quality and surface morphology of CdTe thin films without and with a ZnTe buffer layer grown on (001)GaAs by molecular beam epitaxy (MBE) have been studied. CdTe thin film directly prepared on GaAs substrate displays (111) orientation with an island growth mode, whereas the CdTe epilayers with a ZnTe buffer are (001)-oriented single-crystalline film with a two-dimensional (2D) growth mode. The morphology and surface root-mean-square (RMS) roughness of CdTe epilayers are also dramatically improved by using a ZnTe buffer. Furthermore, it is suggested that the high-temperature (HT) ZnTe buffer grown at 360 °C is more efficient for enhancing CdTe structural quality than the low-temperature (LT) one at 320 °C. The CdTe epilayer on the HT-ZnTe buffer shows a narrower full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (004) reflection and a smaller RMS roughness.

Plasma-Cleaned InSb (112)B for Large-Area Epitaxy of HgCdTe Sensors

Plasma etching of (112)B InSb to prepare this semiconductor for the heteroepitaxy deposition of CdTe and initial studies of CdTe epilayers grown by molecular beam epitaxy (MBE) on InSb (112)B substrates cleaned with various plasma treatments are presented. X-ray diffraction rocking curve maps of the MBE CdTe epilayers on 3-inch InSb (112)B substrates have full-width at half-maxima (FWHM) values in the range of 20 arcsec to 30 arcsec. An etch pit density analysis of the 3-inch CdTe epilayers reveals a defect density of 1.0 × 107 cm−2 and 7.7 × 105 cm−2 at the center and edge of the wafer, respectively. Evaluation of a standard HgCdTe annealing process suggests that the removal of the InSb substrate is likely to be needed prior to any postgrowth annealing in Hg overpressure. Finally, we present a low-energy helium plasma exposure of wet-etched InSb (112)B substrates that provides a uniform epi-ready surface that is nearly stoichiometric, and free of oxide and residual contaminants.

EFFECT OF SUBSTRATE TEMPERATURE ON THE STRUCTURAL AND THE OPTICAL PROPERTIES OF CdTe (100) EPITAXIAL FILMS GROWN ON GaAs (100) SUBSTRATES

CdTe thin films were grown on GaAs (100) substrates by using molecular beam epitaxy at various temperatures. The results of the X-ray diffraction (XRD) patterns showed that the orientation of the grown CdTe thin films was the (100) orientation. XRD patterns, atomic force microscopy images, high-resolution transmission electron microscopy (HRTEM) images, and photoluminescence spectra showed that the crystallinity of CdTe (100) epilayers grown on GaAs (100) substrates was improved by increasing the substrate temperature. HRTEM images showed that misfit dislocations existed at the CdTe / GaAs heterointerface. These results can help improve understanding of the substrate temperature effect on the structural and the optical properties of CdTe (100)/ GaAs (100) heterostructures.

Characterization of CdTe/n$^{+}$-Si Heterojunction Diodes for Nuclear Radiation Detectors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> CdTe/n<formula formulatype="inline"><tex>$^{+}$</tex></formula>-Si heterojunction diodes were fabricated and characterized for the development of gamma ray detectors. With the careful control of the growth parameters thick single crystal CdTe epilayers of high-crystalline quality were grown directly on the (211) Si substrates in a metalorganic vapor phase epitaxy. The heterojunction diode was fabricated by growing a 5 <formula formulatype="inline"><tex>$\mu$</tex> </formula>m thick n-type CdTe buffer layer on the n<formula formulatype="inline"> <tex>$^{+}$</tex></formula>-Si substrate, followed by the growth of 100 <formula formulatype="inline"><tex>$\mu$</tex></formula>m thick undoped p-CdTe layer. The diode fabricated showed very good rectification property with a low value of the reverse bias leakage current, typically <formula formulatype="inline"> <tex>$1.26 \times 10^{- 7}$</tex></formula> A/cm<formula formulatype="inline"> <tex>$^{2}$</tex></formula> for an applied reverse bias of 60 V. The diode clearly demonstrated the gamma radiation detection capability by resolving energy peaks from the <formula formulatype="inline"><tex>$^{241}$</tex></formula>Am radioisotope during the radiation detection test performed at room temperature. </para>

Dependence of the microstructural properties on the substrate temperature in strained CdTe (1 0 0)/GaAs (1 0 0) heterostructures

CdTe thin films were grown on GaAs (1 0 0) substrates by using molecular beam epitaxy at various temperatures. The bright-field transmission electron microscopy (TEM) images and the high-resolution TEM (HRTEM) images showed that the crystallinity of CdTe epilayers grown on GaAs substrates was improved by increasing the substrate temperature. The result of selected-area electron diffraction pattern (SADP) showed that the orientation of the grown CdTe thin films was the (1 0 0) orientation. The lattice constant the strain, and the stress of the CdTe thin film grown on the GaAs substrate were determined from the SADP result. Based on the SADP and HRTEM results, a possible atomic arrangement for the CdTe/GaAs heterostructure is presented.

Analysis of Defects in HgCdTe and CdTe Epilayers on Si by Dual-Beam FIB

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005