Molecular Beam Epitaxy Growth Of HgCdTe Research Articles

Investigation of Substrate Effects on Interface Strain and Defect Generation in MBE-Grown HgCdTe

Si, Ge, and GaAs have been extensively investigated as alternative substrates for molecular-beam epitaxy (MBE) growth of HgCdTe and, at present, are widely used for HgCdTe-based infrared focal-plane arrays. However, the problem of high dislocation density in HgCdTe layers grown on these lattice-mismatched substrates has yet to be resolved. In this work, we investigated another alternative substrate, GaSb, which has a significantly smaller lattice mismatch with HgCdTe in comparison with Si, Ge, and GaAs, and is readily available as large-area, epiready wafers at much lower cost in comparison with lattice-matched CdZnTe substrates. The resultant stress due to lattice and thermal mismatch between the HgCdTe epilayer and various substrates has been calculated in this work using the elasticity matrix, and the corresponding stress distribution simulated using ANSYS. The simulated structures were matched by experimental samples involving MBE growth of HgCdTe on GaAs, GaSb, and CdZnTe substrates, and were characterized via reflection high-energy electron diffraction and x-ray diffraction analysis, followed by etch pit density (EPD) analysis. In comparison with other alternative substrates, GaSb is shown to have lower interface stress and lower EPD, rendering it an interesting and promising alternative substrate material for HgCdTe epitaxy.

Evaluation of HgCdTe on GaAs Grown by Molecular Beam Epitaxy for High-Operating-Temperature Infrared Detector Applications

Molecular beam epitaxy (MBE) growth of HgCdTe (MCT) on alternative substrates enables production of both cheaper and more versatile (third-generation) infrared (IR) detectors. After rapid progress in the development of MBE-grown MCT on GaAs in recent years, the question of whether the considerable benefits of this material system are also applicable to high-operating-temperature (HOT) applications demands attention. In this paper, we present a mid-wavelength-IR 640 × 512 pixel, 15-μm-pitch focal-plane array with operability of 99.71% at operating temperature of 120 K and low dark current density. In the second part of the paper, MBE growth of short-wavelength IR material with Cd fraction of up to 0.8 is investigated as the basis for future evaluation of the material for low-light-level imaging HOT applications.

Performance and reproducibility enhancement of HgCdTe molecular beam epitaxy growth on CdZnTe substrates using interfacial HgTe∕CdTe superlattice layers

Interfacial layers including HgTe∕CdTe superlattices (SLs) were introduced during the molecular-beam epitaxy growth of HgCdTe on CdZnTe (211)B substrates. Transmission-electron-microscopic observations show that the SLs smooth out the substrates’ surface roughness during growth, and can also bend or block threading dislocations in a way that prevents their propagation from the substrate into the functional HgCdTe epilayers. An average etch pit density value in the low-105cm−2 range was reproducibly achieved in long wavelength HgCdTe samples, with the best value being 4×104cm−2. Photoconductive decay lifetime measurements give values approaching theoretical limits, as determined by the intrinsic radiative and Auger recombination mechanisms. The use of such interfacial layers thus leads to enhanced growth yields and material properties.

MBE growth of HgCdTe on silicon substrates for large format MWIR focal plane arrays

HgCdTe p-on-n double layer heterojunctions (DLHJs) for mid-wave infrared (MWIR) detector applications have been grown on 100 mm (4 inch) diameter (211) silicon substrates by molecular beam epitaxy (MBE). The structural quality of these films is excellent, as demonstrated by x-ray rocking curves with full widths at half maximum (FWHMs) of 80–100 arcsec, and etch pit densities from 1 106 to 7 106 cm−2. Morphological defect densities for these layers are generally less than 1000 cm−2. Improving Hg flux coverage of the wafer during growth can reduce void defects near the edges of the wafers. Improved tellurium source designs have resulted in better temporal flux stability and a reduction of the center to edge x-value variation from 9% to only 2%. Photovoltaic MWIR detectors have been fabricated from some of these 100mm wafers, and the devices show performance at 140 K which is comparable to other MWIR detectors grown on bulk CdZnTe substrates by MBE and by liquid phase epitaxy.

Energetics of molecular-beam epitaxy models

A Green’s function method is used to calculate the removal energies of constituent atoms from various unreconstructed semiconductor surfaces. An efficient difference equation approach within the second-neighbor tight-binding model is used. For a compound AB, binding energies for the A and B atoms on the (111), (1̄1̄1̄), (100), and (110) surfaces are calculated. Energy to remove an atom from the nearly full surface, Ec (where the removed atom leaves behind a surface vacancy), and from the nearly empty surface, Ed (where the removed atom was isolated on the surface), is obtained. Results are presented for Si, GaAs, CdTe, and HgTe. The surface sublimation energies are shown to depend on surface coverage and do not exhibit a simple linear relationship to the number of bonds broken, as is often assumed in modeling growth by molecular-beam epitaxy (MBE). Although the anion and cation extraction energies depend on surface coverage and orientation, when averaged over a double layer, they always sum to the bulk cohesive energy. Moreover, Ec−Ed can be positive, implying effective attractive in-plane surface interactions, or negative, implying effective repulsive interactions. Ec−Ed tends to be positive for covalent and narrow-gap semiconductors, and negative for wide-gap and more ionic semiconductors. Surface sublimation energies are important input parameters for the modeling of MBE growth; their importance is demonstrated using a simple thermodynamic growth model and results are shown to explain anomalies found in MBE growth of HgCdTe.

On the concept of “pumping speed” in gettered and cryopumped systems: T A Giorgi and C Pisani,Vacuum, 16 (12), Dec 1966, 669–675

<dm:abstracts xmlns:dm="http://www.elsevier.com/xml/dm/dtd"><ce:abstract xmlns:ce="http://www.elsevier.com/xml/common/dtd" view="all" class="author" id="aep-abstract-id7"><ce:section-title>Publisher Summary</ce:section-title><ce:abstract-sec view="all" id="aep-abstract-sec-id8"><ce:simple-para id="fsabs008" view="all">This chapter focuses on the current status of HgCdTe molecular-beam epitaxy (MBE) growth technology as practiced in laboratory. The chapter addresses the following topics: a summary of the ‘hardware’ aspects of MBE growth technology, including the role of process sensors, a discussion of the key parameters of the epitaxial growth process and associated thin-film material properties, and a discussion of current state-of-the-art results in device application areas that have been particularly well-served by the evolution of HgCdTe MBE technology. Significant progress has been achieved in HgCdTe MBE growth technology that has enabled the synthesis of complicated multilayer infrared (IR) detector device structures. The development and integration of in situ sensors for monitoring and control of the sensitive HgCdTe MBE growth process, has significantly advanced the capabilities and consistency of the technology, culminating in demonstrations of excellent device performance in a wide variety of IR device architectures such as multispectral detectors, near-IR avalanche photodiodes (APD), high-performance medium wavelength infrared (MWIR) detectors, and megapixel arrays on Silicon (Si) substrates. Cryopumps have found widespread use in MBE systems designed for the growth of HgCdTe alloys. The epitaxial growth of device-quality HgCdTe by MBE is extremely sensitive to fundamental growth parameters such as source fluxes and substrate temperature. MBE growth of HgCdTe generally utilizes Hg, CdTe, and Te as source materials. HgCdTe alloys, because of their unique energy band structure, can be designed to provide low-excess-noise carrier multiplication. MBE growth technology, besides allowing for the integration of sensors that can be used to precisely control the process, also offers the added benefit of excellent lateral uniformity in HgCdTe epitaxial material quality.</ce:simple-para></ce:abstract-sec></ce:abstract></dm:abstracts>