xCdxTe Surface Research Articles

Peculiarities of admittance in MOS structures based on MBE-grown MCT layers

Admittance of MOS structures based on Hg1−xCdxTe layers grown by molecular beam epitaxy on semi-insulating GaAs substrates is studied. Effect of anomalous generation on the Hg1−xCdxTe surface in a strong electric field (≈105 V/cm) is found. It is shown that the density of surface states of the MCT — SiO2 interface depends weakly on the presence of the graded energy-gap layer and the type of semiconductor admittance.

Effects of a-Si:H resist vacuum-lithography processing on HgCdTe

A vacuum-compatible process for carrying out lithography on Hg1−xCdxTe and CdTe films was previously demonstrated. It was shown that hydrogenated amorphous silicon (a-Si:H) could be used as a dry resist by projecting a pattern onto its surface using excimer laser irradiation and then developing that pattern by hydrogen plasma etching. Pattern transfer to an underlying Hg1−xCdxTe film was then carried out via Ar/H2 plasma etching in an electron cyclotron resonance (ECR) reactor. Despite the successful demonstration of pattern transfer, the possibility of inducing harmful effects in the Hg1−xCdxTe film due to this vacuum lithography procedure had not been explored. Here we present structural and surface compositional analyses of Hg1−xCdxTe films at key stages of the a-Si:H vacuum lithography procedure. X-ray diffraction double crystal rocking curves taken before and after a-Si:H deposition and after development etching were identical, indicating that bulk structural changes in the Hg1−xCdxTe film are not induced by these processes. Cross-section transmission electron microscopy studies show that laser-induced heating in the 350 nm thick a-Si:H overlayer is not sufficient to cause structural damage in the underlying Hg1−xCdxTe surface. In vacuo surface analysis via Auger electron spectroscopy and ion scattering spectroscopy suggest that the hydrogen plasma development process produces Hg-deficient surfaces but does not introduce C contamination. However, after ECR plasma etching into the Hg1−xCdxTe film, the measured x value is much closer to that of the bulk.

Characterization of physical-chemical drivers of Hg1−xGdxTe etch rate using time of flight—Secondary ion mass spectrometry and optical interferometry

Time of flight-secondary ion mass spectrometry (TOF-SIMS) is a Hg1−xCdxTe surface diagnostic tool with unprecedented analysis capabilities, including analyzing a 0.5-µm diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. The present investigation demonstrates the power of TOF-SIMS when coupled with optical interferometry in understanding process reproducibility and uniformity critical to the fabrication of Hg1−xCdxTe detector arrays at RVS. Previous published works and unpublished studies at RVS have shown that geometry and fluid dynamics influence the lateral uniformity of surface chemistry, topography, and etch rates. By combining a set of photolithographically delineated features having various relative areas of photoresist-coated and uncoated regions in varying proximity to each other with various wet etching chemistries, we have exploited TOF-SIMS interrogation along with optical interferometry to investigate physical-chemical drivers of etch rate variation with window geometry orientation with respect to vertical gravity etchant fluid draining direction and proximity to other structures. This study has given us the ability to deconvolve two important etch rate drivers (depletion of etchant species and cross-contamination of etched windows) and elucidate their roles in enhancing and diminishing etch rates for features having far and close proximities to neighboring structures, respectively. This information allows a more judicious optimization of processing technology.

An improved method for Hg1−xCdxTe surface chemistry characterization

Hg1−xCdxTe surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg1−xCdxTe surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-µm-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg1−xCdxTe processing steps including photolithography, bromine etching, and photoresist stripping.

Raman characterization of ion beam etched Hg 1− xCd xTe surface

We report Raman scattering (RS) measurements at 300 K from Hg 1− x Cd x Te ( x∼0.48), which was etched for a few minutes by an Ar + beam. Features of HgTe-like transverse optical (TO 2) mode at 120±1 cm −1 and longitudinal optical (LO 2) mode at 140±1 cm −1 are identified. We identified a feature observed at 135±1 cm −1 as clustering mode, which represents the integrity of the crystal lattice. We measured the Raman scattering on a beveled surface in line-scanning manner; the results showed that ion beam induced damage extends to a depth of more than 1 μm, while the ion beam’s penetrating depth is no more than 10 nm.

Real-time diagnostics of II–VI molecular beam epitaxy by spectral ellipsometry

Spectral ellipsometry has been applied to in situ diagnostics of Hg1−xCdxTe molecular beam epitaxial growth. Spectral ellipsometry provides surface, composition, and film thickness properties of thin film electronic materials in real-time during growth and processing. This study is discussed in two parts. In the first part, the surface state properties of Hg1−xCdxTe and the substrate material Cd1−xZnxTe were studied under ultrahigh vacuum (UHV) conditions. By measuring the spectral ellipsometric quantities Ψ and Δ versus temperature under UHV conditions, the presence and desorption temperatures of surface species have been determined. Tellurium stabilized Cd1−xZnxTe surfaces were not observed to exhibit any apparent surface changes during heating under these conditions. Oxidized Cd1−xZnxTe surfaces, however, exhibit discontinuities in Ψ and Δ indica-tive of desorption of a surface species in the 300 °C range. Tellurium stabilized Hg1−xCdxTe surfaces also exhibit discontinuities in Ψ and Δ occurring at about 177 °C. In the second part of the study, dielectric functions at the growth temperature, 180 °C, were determined for Hg1−xCdxTe alloys and Cd0.96Zn0.04Te. Film compositions were monitored in situ and in real-time during Hg1−xCdxTe epitaxial growth based on this knowledge of growth temperature dielectric func-tions. An experimental analysis of measurement factors illustrates that compositional sensitivity for in situ Hg1−xCdxTe measurement is limited by uncertainties in the angle and plane of inci-dence. Simulations show an ultimate multiwavelength sensitivity of significantly better than ±0.0008 is possible for in situ composition measurements.

Oxide interfacial layers in Au ohmic contacts to p-type Hg1−xCdxTe

Annealed Au contacts to p-type Hg1−xCdxTe with thin interfacial oxide layers exhibit ohmic behavior. These interfacial layers have been produced by plasma oxidizing the Hg1−xCdxTe surface prior to Au evaporation or as a result of electroless Au deposition from AuCl3 which also produces an interfacial layer. We believe this ohmic behavior is primarily a result of the low interface state density at the interfacial layer/Hg1−xCdxTe interface and in addition, a 100 °C anneal promotes a further reduction in the interface state density and thus lowered the contact resistance. In comparison, as-deposited and annealed Au contacts without a thin interfacial layer were rectifying with a large barrier height.

The characterization of anodic fluoride films on Hg1−xCdxTe and their interfaces

A novel anodic fluoridization process for forming native fluoride films on Hg1−xCdxTe is described. As in the case of the anodic oxide, the anodic fluoride grows in two steps: a dissolution–precipitation step is followed by a bulk growth. Anodic fluorides free of oxygen are grown from nonaqueous solutions. The presence of water causes the growth of films containing both fluorine and oxygen, whereas a low hydroxyl ion concentration causes the growth of anodic oxide alone. However, its electrical behavior is different from that of anodic oxide grown from fluoride‐free baths. The results of Auger electron spectroscopy analysis indicate that the anodic fluoride is composed of cadmium fluoride in a matrix of tellurium, cadmium, and mercury. The fluoride is homogeneous and has a relatively abrupt dielectric–semiconductor interfacial transition, free of contaminants. Metal–insulator–semiconductor devices containing a fluoride film and evaporated ZnS have been fabricated and characterized. By choosing an appropriate ratio of fluoride to hydroxyl ions in the anodization bath the band bending at the n‐type Hg1−xCdxTe surface can be adjusted from near flatband conditions to strong accumulation. Anodization from a nonaqueous solution results in a p‐type Hg1−xCdxTe surface that is slightly depleted due to a fixed charge density of ∼5×1010 cm−2. There is only a negligible density of both slow and fast surface states. Low and reproducible surface state densities are achieved by a short dissolution–precipitation step. Such interfaces are stable up to 105 °C.

Local stoichiometry and atomic interdiffusion during reactive metal/mercury–cadmium–telluride junction formation

We summarize synchrotron radiation photoemission studies of Ag, Ge, and Sm overlayers on Hg1−xCdxTe(110) surfaces. These metals exhibit widely different interface reactivity with Hg1−xCdxTe and yield a range of different interface morphologies. To assess the relative importance of the microscopic driving forces that determine the local composition at the interface and in the semiconductor near-surface region we present a systematic comparison of our data with calculated thermodynamic parameters such as the cation–metal heat of solution, the heat of alloying from Miedema’s semiempirical model, and the metal–telluride formation enthalpy.

Surface-enhanced alloy effects on Hg1-xCdxTe(100) surface states and the site-dependent coherent-potential approximation.

Surface-enhanced alloy effects on Hg1-xCdxTe(100) surface states and the site-dependent coherent-potential approximation.

Effect of surface preparation on Ge overlayer growth on (HgCd)Te

The interactions between thin Ge overlayers and both cleaved and ion-sputtered Hg1−xCdxTe surfaces have been examined using synchrotron radiation. Ge forms an unreactive layer on cleaved substrates of x=0.21 and x=0.28 with only a small loss of Hg (∼20%) from the interface. In contrast, deposition of Ge onto sputtered substrates results in approximately two (x=0.21 material) or three (x=0.28 material) times the Hg loss relative to the clean surface. The difference in behavior of the sputtered and cleaved material is due to sputter-induced defects at the surface. The increased loss of Hg from the sputtered x=0.28 material is a result of a greater number of these defects caused by the weaker Hg–Te bonding and the corresponding increase in the preferential sputtering of Hg from the surface. No difference was observed between sputtered p-type and n-type material. These results are a consequence of GeTe and HgTe having very similar heats of formation; as such, deposition of Ge provides an indication of the reactivity of (HgCd)Te surfaces.

Deposition of the reactive metals Al and In onto sputtered and cleaved Hg1−xCdxTe surfaces

The interactions of the reactive metals Al and In with both cleaved and sputtered p-type Hg1−xCdxTe surfaces have been investigated using synchrotron radiation-induced ultraviolet photoelectron spectroscopy. The sputtered surfaces are depleted of a fraction of their Hg (∼25% and ∼40% for x=0.21 and x=0.28 material, respectively, relative to the Hg found on the corresponding cleaved surfaces) and are more inverted than the cleaved surfaces with the Fermi level higher in the conduction band. During metal deposition, the cleaved and sputtered surfaces behave similarly: in the initial stages, the metal reacts with the HgTe alloy component to form a metal telluride and Hg, which leaves the surface region. At the same time, the inverted surface becomes more degenerate. After the metal has reacted with all the available HgTe within a certain surface region, an unreacted metallic film grows on the surface. Such identical behavior of the two types of surfaces is explained by the large difference in thermodynamic stability between HgTe and the metal tellurides.

Electron beam induced Hg desorption and the electronic structure of the Hg depleted surface of Hg1−xCdxTe

Auger electron spectroscopy (AES), x-ray photoemission spectroscopy (XPS), low energy electron diffraction (LEED), and angle-resolved ultraviolet photoemission spectroscopy (ARPES) were used to study the electron beam induced Hg desorption from a cleaved (110)Hg1−xCdxTe surface and the electronic structure of the Hg depleted surface. Solid state recrystallized Hg1−xCdxTe single crystals were used. It was found that the electron beam heating dominated the electron beam induced Hg desorption on Hg1−xCdxTe. At the electron beam energy used, the electron beam heating extended several thousand angstroms deep. However, the Hg depletion saturated after a few monolayers were depleted of Hg atoms. At the initial stage of Hg loss (only 3%), the surface band bends upward (more p type). The ARPES spectrum showed the loss of some E vs k dispersion after 22% Hg atoms were removed from the surface region, and no dispersion was observed after 43% Hg atoms were removed. These results have important implications on the electronic structure of the surfaces and interfaces of which the stoichiometry is altered.

Surface characterization of Hg0.7Cd0.3Te native oxides

Ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (ELS) were used to characterize the surfaces of Hg0.7Cd0.3Te native oxide films grown by air exposure, ozone treatment, oxygen plasma, and electrochemical anodization. The surfaces of the air-grown, plasma-grown, and anodically grown native oxides were found to all be relatively rich in Cd, while the ozone oxide was rich in Hg. Two ELS absorption edges were detected at ∼2.1 and ∼3.6 eV and are attributed to an oxide trap state and the fundamental band gap of the oxide, respectively. Changes in the ELS and XPS spectra following prolonged electron beam exposure resulted from a loss of Hg from the oxide and incomplete bonding of Te in the oxide. The electron beam induced changes were much more dramatic in the ozone oxide spectrum than for any of the other samples studied. XPS analysis of the interface of an air-grown native oxide which was subsequently covered with a thin layer of photochemical SiO2 demonstrated that the air-grown native oxide was partially, but not completely, reduced by chemical reaction with SiH4 during the deposition of SiO2. A generic energy band diagram of the native oxide Hg0.7Cd0.3Te interface is constructed and discussed in terms of its implications for the passivation of Hg1−xCdxTe surfaces.

HgCdTe-Cr interface chemistry

The microscopic interaction of Hg1−xCdxTe surfaces with refractory metals was examined through synchrotron radiation photoemission studies of HgCdTe cleaved in vacuum and of HgCdTe-Cr junctions formed in situ. The results indicate that strong Te-Cr interdiffusion is accompanied by rapid Hg depletion of the surface layer. In sharp contrast to similar studies of simple and noble metals on HgCdTe, variations in the surface anion-to-cation ratio and the Hg-to-Cd ratio do not affect the interface position of the Fermi level. The strongly inverted character of the near-surface region found after cleaving appears to be preserved during the Hg-Cr exchange reaction.

Solid-state quaternary phase equilibrium diagram for the Hg–Cd–Te–O system

For the Hg–Cd–Te–O system an approximate three-dimensional diagram of quaternary phase equilibrium at room temperature has been constructed for the first time, to aid in the study of Hg1−xCdxTe surface oxides. Thermodynamic calculations based on standard Gibbs energies of formation were performed to determine which phases are mutually stable. Stability relationships were checked by reaction experiments in mixtures of bulk reference compounds, using Raman spectroscopy for analysis. Elemental Hg is shown to coexist in equilibrium with all of the oxide and telluride phases. Thus, it is likely to be present whenever oxidation is incomplete. CdTeO3 is shown to be stable with respect to the other oxides and tellurides. Several instabilities are indicated, in which an oxide can react with the Hg1–xCdxTe to release elemental Hg or Te, or HgTe.

Coupled subband-phonon resonances in the far-infrared reflection from Hg 1− xCd xTe surfaces

Subband resonances coupled to the lattice polarization are observed in the far-infrared reflection from surfaces of Hg 1− x Cd x Te. A substantial enhancement of signal amplitude occurs for frequencies just above ω LO. At such a frequency the resonances show a large splitting of the collective-mode energy Ẽ nm and the single-particle mode energy E nm.

Composition of native oxides and etched surfaces on Hg1−xCdxTe

Hg1−xCdxTe surfaces have been investigated by XPS and ellipsometry after spray etching or polish etching with Br solutions, and after the growth of native oxide films. Etching depletes the surface Cd, and enriches either Hg or Te depending on the etch method. Air‐grown native oxides after etching range up to 25 A in thickness and are essentially TeO2. This thickness increases with increasing surface Te concentration, which depends on the etch. Native oxides 700 A thick were grown by anodizing. Results are consistent with CdTeO3 as the major constituent. Stripping anodic oxides with HCl reveals a ∠100‐A Te‐rich layer between the anodic oxide and substrate. A new way of comparing Hg1−xCdxTe native oxide films, the TeO2 stoichiometry plot, is introduced.

5018. XPS investigation of the oxidation of Hg 1- xCd xTe surfaces : S P Kowalczyk and J T Cheung, J Vac Sci Technol, 18 (2), 1981, 944–948

5018. XPS investigation of the oxidation of Hg 1- xCd xTe surfaces : S P Kowalczyk and J T Cheung, J Vac Sci Technol, 18 (2), 1981, 944–948

Stability of an atomically clean Hg 1− xCd xTe surface in vacuum and under O 2 exposure

We report studies of the stability of single crystal surfaces of Hg 1- x Cd x Te with respect to loss of Hg. Ultraviolet photoemission spectroscopy (UPS) and X-ray-induced photoemission (XPS) were used to monitor the electronic structure and atomic composition of a cleaved surface under ultrahigh vacuum (UHV) ( p ≅ 5 x 10 -10 Torr) and under oxygen exposures. These experiments showed the surface to be remarkably stable in UHV for extended periods of time at room temperature. However, changes in the surface concentrations can be induced during exposure to excited oxygen or during ion bombardment. The importance and relevance of these findings to the question of the stability of the material are discussed.