Van der Waals (vdW) interfaces based on 2D materials are promising for optoelectronics, as interlayer transitions between different compounds allow tailoring of the spectral response over a broad range. However, issues such as lattice mismatch or a small misalignment of the constituent layers can drastically suppress electron–photon coupling for these interlayer transitions. Here, we engineered type-II interfaces by assembling atomically thin crystals that have the bottom of the conduction band and the top of the valence band at the Γ point, and thus avoid any momentum mismatch. We found that these van der Waals interfaces exhibit radiative optical transitions irrespective of the lattice constant, the rotational and/or translational alignment of the two layers or whether the constituent materials are direct or indirect gap semiconductors. Being robust and of general validity, our results broaden the scope of future optoelectronics device applications based on two-dimensional materials. Type-II van der Waals interfaces formed by different two-dimensional materials enable robust interlayer optical transitions, regardless of common issues such as lattice constant mismatch, layer misalignment or whether the constituent compounds are direct or indirect band semiconductors.

/pdf/design-of-van-der-waals-interfaces-for-broad-spectrum-1c2jwi2nmj.pdf

Design of van der Waals interfaces for broad-spectrum optoelectronics.

Van der Waals (vdW) materials offer new ways to assemble artificial electronic media with properties controlled at the design stage, by combining atomically defined layers into interfaces and heterostructures. Their potential for optoelectronics stems from the possibility to tailor the spectral response over a broad range by exploiting interlayer transitions between different compounds with an appropriate band-edge alignment. For the interlayer transitions to be radiative, however, a serious challenge comes from details of the materials --such as lattice mismatch or even a small misalignment of the constituent layers-- that can drastically suppress the electron-photon coupling. The problem was evidenced in recent studies of heterostructures of monolayer transition metal dichalcogenides, whose band edges are located at the K-point of reciprocal space. Here we demonstrate experimentally that the solution to the interlayer coupling problem is to engineer type-II interfaces by assembling atomically thin crystals that have the bottom of the conduction band and the top of the valence band at the $\Gamma$-point, thus avoiding any momentum mismatch. We find that this type of vdW interfaces exhibits radiative optical transition irrespective of lattice constant, rotational/translational alignment of the two layers, or whether the constituent materials are direct or indirect gap semiconductors. The result, which is robust and of general validity, drastically broadens the scope of future optoelectronics device applications based on 2D materials.

/pdf/design-of-van-der-waals-interfaces-for-broad-spectrum-cq8b39lfkz.pdf

Design of van der Waals Interfaces for Broad-Spectrum Optoelectronics

Close lattice matching and lattice compatibility with mercury cadmium telluride (MCT) make CdTe and related alloys ideal substrate materials for growth of MCT layers for the purpose of making high-performance second-generation infrared detectors. However, the limitations in the properties of CdTe and the difficulties in its bulk growth have prompted extensive research in the area of alternative substrates. Some basic relevant characteristics of substrates such as sapphire, GaAs, and silicon are compared and the possibilities and problems associated with each material are analyzed in the light of the most recent results in the field.

Substrate issues for the growth of mercury cadmium telluride

Surface passivation has been recognized as a crucial step in the fabrication of mercury cadmium telluride photoconductive as well as photovoltaic detectors. The subject has attracted considerable attention in the past and several reviews existed by 1991. The subject matter, however, received added impetus with the development of techniques like MOCVD and MBE and recently there has been considerable work on MCT passivation using in situ grown II-VI semiconductors. In this report, we have tried to give the present status and identify the issues particularly with reference to the recent work on the subject.

Current status and issues in the surface passivation technology of mercury cadmium telluride infrared detectors

In this paper, we present all the successive steps for realizing dual-band infrared detectors operating in the mid-wavelength infrared (MWIR) band. High crystalline quality HgCdTe multilayer stacks have been grown by molecular beam epitaxy (MBE) on CdZnTe and CdTe/Ge substrates. Material characterization in the light of high-resolution x-ray diffraction (HRXRD) results and dislocation density measurements are exposed in detail. These characterizations show some striking differences between structures grown on the two kinds of substrates. Device processing and readout circuit for 128×128 focal-plane array (FPA) fabrication are described. The electro-optical characteristics of the devices show that devices grown on Ge match those grown on CdZnTe substrates in terms of responsivity, noise measurements, and operability.

Dual-band infrared detectors made on high-quality HgCdTe epilayers grown by molecular beam epitaxy on CdZnTe or CdTe/Ge substrates

Infrared photoconductors fabricated with a CdTe/Hg064Cd036Te abrupt heterostructure grown on a GaAs substrate by molecular beam epitaxy are described here for the first time The growth procedure, device fabrication, and measurement results are described The results show that the devices have relatively high detectivity and uniformity The observed enhancement in device performance attributed essentially to the large reduction in interface states and surface contamination by a subsequent growth of a lattice‐matched semi‐insulating CdTe epilayer for the passivation of HgCdTe surface

Infrared photoconductor fabricated with a molecular beam epitaxially grown CdTe/HgCdTe heterostructure

A semiempirical model is presented that correlates the broadening of the absorption edge with both transitions below the energy gap and with transitions by the Kane band model. This model correctly fits both the absorption and luminescence spectra of narrow‐gap (Hg,Cd)Te samples that have been grown by the traveling heater method as well as by molecular‐beam epitaxy. The accuracy of the band‐gap determination is enhanced by this model.

/pdf/a-new-model-for-the-absorption-coefficient-of-narrow-gap-hg-559otyve50.pdf

A new model for the absorption coefficient of narrow-gap (Hg,Cd)Te that simultaneously considers band tails and band filling

Atomic layer epitaxy of (CdTe)m(ZnTe)n-ZnTe multiquantum wells on (001)GaAs substrate

Hg1-xCdxTe is an important material for infrared device technology. Despite a variety of investigations on molecular beam epitaxially (MBE) grown Hg-based superlattices and modulation-doped structures, little attention has been paid to the study of the photoluminescence properties of the constituent materials. The authors report on a systematic study of infrared photoluminescene on Hg1-xCdxTe layers grown by MBE. The Fourier transform photoluminescence of Hg1-xCdxTe layers with 0.25<x<0.93 grown on (110) and (211) GaAs was investigated in the spectral range from 01 eV to 1.5 eV. Emission lines were observed with a full width at half maximum down to 15 meV. Photoluminescence and transmission properties are compared over the temperature range from 4.2 K to 300 K. The possible influence of Burstein-Moss shift, impurity bands and exponential band tails are discussed. Photoluminescence at high temperatures originates from band-to-band transitions. However, low temperature photoluminescence can be attributed either to impurity bands or to additional states due to band tails.

/pdf/infrared-photoluminescence-on-molecular-beam-epitaxially-4xe4s6trqr.pdf

Infrared photoluminescence on molecular beam epitaxially grown Hg1-xCdxTe layers

An infrared photoconductor fabricated with a HgTe/CdTe superlattice grown on a GaAs substrate by molecular beam epitaxy is described here for the first time. The growth procedure, device fabrication, and measurement results are described. The results show that the device has relatively high uniformity and 1000 K black‐body detectivity 2.4×109 cm Hz1/2 W−1. The photoconductivity decay method was used for determining carrier lifetime of the HgTe/CdTe superlattice, the measured lifetime is 12 μs at 77 K, which is the longest lifetime ever reported for HgTe/CdTe superlattices and we believe that the increase of lifetime is mainly due to the reduction of dimensions.

/pdf/infrared-photoconductor-fabricated-with-hgte-cdte-3qhh86x0x8.pdf

Shixin Yuan

Papers

Infrared photoconductor fabricated with a molecular beam epitaxially grown CdTe/HgCdTe heterostructure

A new model for the absorption coefficient of narrow-gap (Hg,Cd)Te that simultaneously considers band tails and band filling

Atomic layer epitaxy of (CdTe)m(ZnTe)n-ZnTe multiquantum wells on (001)GaAs substrate

Infrared photoluminescence on molecular beam epitaxially grown Hg1-xCdxTe layers

Infrared photoconductor fabricated with HgTe/CdTe superlattice grown by molecular beam epitaxy