HgTe Layers Research Articles

The growth and characterization of HgTe and HgCdTe using methylalylltelluride

HgTe and HgCdTe layers have been grown by organometallic vapor phase epitaxy at low temperature by using methylallyltelluride (MATe), dimethylcadmium (DMCd) and elemental mercury. Use of MATe enabled the growth of layers in the 250–320 °C range, which is 50 °C lower than the growth temperature when diisopropyltelluride (DIPTe) is used, for the same growth rate. The layers were characterized by double crystal x-ray diffraction, Fourier-transform infrared (FTIR) spectroscopy, and by low temperature Hall measurements. Growth below 340 °C resulted in featureless HgTe layers. HgTe layers, grown on CdTe, are misoriented with respect to the substrate by about 60–150 arc s. This tilting was not observed when CdZnTe substrates, to which there is a better lattice match, were used. The high quality of low temperature grown HgTe is demonstrated by the very narrow (27 arc s) full width at half-maximum (FWHM) of x-ray diffraction for layers grown on CdZnTe substrates. HgCdTe layers, grown at 320 °C, showed well resolved interference fringes even for thin layers indicating the presence of an abrupt interface with the substrate. As grown layers were n-type, with the best mobility of 40 000 cm2 /V s at 40 K for a layer with composition x of 0.23.

The growth of CdHgTe on GaAs and fabrication of high-quality photodiodes

The use of metal organic vapor phase epitaxy (MOVPE) for the growth of device quality CdHgTe (MCT) on GaAs has been studied extensively. This is the first report of diode formation by mercury diffusion in the MOVPE grown material. It is also the first report of medium-wavelength infrared (MWIR) photodiodes made in the material. The effect of altering the cadmium to tellurium alkyl ratios on the growth of a CdTe buffer on (100) and (111)GaAs was investigated. For every alkyl ratio used, it was possible to grow (111) or (100)CdTe on (100)GaAs. (100)CdTe/(100)GaAs samples exhibited very sharp, well-defined photoluminescence spectra and x-ray rocking curve full width half-maximum (FWHM) peaks of <90 arc s were found for layers of 6–10 μm thick. Generally the (111)CdTe/(111)GaAs layers were dominated by twins, but the growth of untwinned (111)CdTe and MCT is reported. CdTe, high x MCT, and combinations of CdTe and HgTe layers were used as buffers for (100)MCT growth to reduce the hillock density. The combinations of CdTe and HgTe layers gave rise to the lowest hillock density (at best 30–100/cm2) over a large area (25×25 mm2). FWHMs of 80–90 arc s are routinely achieved for (100)MCT layers. Linear diode arrays have been fabricated with 77 K R0A values typically of 3×103 Ω cm2 for 7.5 μm cutoff. The detectivity of a 5.5 μm cutoff diode was 1.5×1011 cm Hz1/2 W−1 at 77 K, and 1.3×1010 cm Hz1/2 W−1 at 193 K. This is comparable to the performance obtained with the best traveling heater method devices.

Large-area HgTe–CdTe superlattices and Hg1−xCdxTe multilayers on GaAs and sapphire substrates grown by low-temperature metalorganic chemical vapor deposition

Specular HgTe–CdTe superlattice epilayers have been obtained on two 2-in. diam GaAs or sapphire wafers per growth run using a horizontal metalorganic chemical vapor deposition (MOCVD) reactor in which the pyrolysis of the organometallics is induced by a cracking susceptor suspended above the substrates. This enables growth of superlattices below 300 °C using the standard precursors dimethyl cadmium, diethyl telluride, and elemental mercury. Annealing the superlattices at 350 °C converts them to homogeneous Hg1−xCdxTe. Compositional profiles obtained by Rutherford backscattering, sputter Auger depth profiling, and e-beam analysis of ultramicrotomed thin cross sections, were used to study interdiffusion of the CdTe and HgTe layers during growth and annealing. Arrays of metal–semiconductor field effect transistors (MESFETS) and photoconductive detectors with room temperature peak responsivity between 1.3 and 1.6 μm have been prepared from incompletely interdiffused material grown on semi-insulating GaAs without thick buffer layers. Device processing used mesa technology and a KI:I2:HBr etchant developed for its selectivity with respect to Hg1−xCdxTe . Capping layers of CdTe and HgTe deposited at low temperatures have been investigated as dielectric and ohmic contacts, respectively. Electrical analyses by Hall and C–V measurements are presented. Room temperature electron mobilities up to 27 500 cm2 /V s and resistivities of 1–2 Ω cm have been observed for the HgTe layers. Information on crystallinity and defect structure of the epilayers was obtained by selected area electron channeling patterns, high-resolution transmission electron microscopy (HRTEM), Rutherford backscattering spectroscopy (RBS) and four-circle, double-crystal, and triple-crystal x-ray diffraction. Compositional profiles in the 100 cm2 deposition area were obtained by wavelength dispersive x-ray analysis (WDX), Fourier transform infrared (FTIR) spectrometry and inductively coupled plasma atomic emission spectrometry (ICPAES).

Low-temperature growth of HgTe and HgCdTe using methylallyltelluride

HgTe and HgCdTe (MCT) layers have been grown by organometallic vapor phase epitaxy at low temperature by using methylallyltelluride (MATe), dimethylcadmium (DMCd), and elemental mercury. Use of MATe enabled the growth of layers in the 250–320 °C temperature range, which is 50 °C lower than the growth temperature when diisopropyltelluride is used as the tellurium alkyl, for the same growth rate. The layers were characterized by optical microscopy, double crystal x-ray diffraction, and Fourier transform infrared spectroscopy. Growth at 320 °C resulted in featureless surfaces for both HgTe and HgCdTe layers. The high quality of HgTe layers grown at 320 °C is demonstrated by the very narrow full width at half maximum of x-ray diffraction (29 arcsec), which is comparable to that of the CdTeZn substrates used in this study. MCT layers grown at 320 °C showed sharp interference fringes even for very thin layers, indicating the presence of a very sharp interface with the substrate.

A new fast-response Hg vapor source for HgCdTe molecular-beam epitaxy growth

A new technique is reported for producing a highly stable, fast-response Hg vapor source for high-quality HgTe and HgCdTe growth. The design principles of the Hg furnace are based on the fact that for choked flow conditions across an orifice the gas flow on the low-pressure side is directly proportional to the pressure on the upstream side of the orifice. Measurements show that this system has a response time of 10 s, a dynamic range of 10−6 to 10−3 Torr, and a long-term stability that exceeds our ability to measure with a conventional ion gauge. Additional measurements show a constant flux output for reservoir temperature changes of 20 °C, a stability of 0.1% over >3 h, and setpoint repeatability of 0.1%. Results on HgTe and homogeneous HgCdTe layer growth are presented.

HgTe/CdTe superlattice band calculation with a transfer matrix method

A transfer matrix method has been applied to study the HgTe/CdTe superlattice system by varying layer thicknesses, valence-band offset, and strain systematically. The energy gap can be understood in terms of a midzone energy separation mainly depending on HgTe layer thickness, and a bandwidth mainly depending on CdTe layer thickness. Strain causes a simple displacement of bands by different amounts for different bands and sometimes causes an interesting band reversal. In the presence of an external magnetic field, optical transition probability and selection rules between Landau levels are obtained.

Selective Area Deposition of Passivants, Insulators, and Epitaxial Films of II-VI Compound Semiconductors

ABSTRACTII-VI semiconductor surface passivants, insulators, and epitaxial films have been deposited onto selective surface areas by employing a new masking and lift-off technique. The II-VI layers were grown by either conventional or photoassisted molecular beam epitaxy (MBE). CdTe has been selectively deposited onto HgCdTe epitaxial layers as a surface passivant. Selective-area deposition of ZnS has been used in metal-insulator-semiconductor (MIS) structures. Low resistance ohmic contacts to p-type CdTe:As have also been realized through the use of selectively-placed thin films of the semi-metal HgTe followed by a thermal evaporation of In. Epitaxial layers of HgTe, HgCdTe, and HgTe-CdTe superlattices have also been grown in selective areas on CdZnTe substrates, exhibiting specular morphologies and double-crystal x-ray diffraction rocking curves (DCXD) with full widths at half maximum (FWHMs) as narrow as 140 arcseconds.

The sources and behaviour of impurities in LPE-grown (Cd,Hg)Te layers on CdTe(111) substrates

The sources of background impurities in LPE layers of (Cd,Hg)Te grown on CdTe(111) substrates are investigated using Spark-source mass spectrometry (SSMS) and secondary-ion mass spectrometry (SIMS). The principal sources of impurities were (i) the compounds HgTe and CdTe used in the LPE solution synthesis, (ii) the graphite sliding boat system and (iii) the CdTe substrate. The main impurities and their typical levels in the LPE layers were found to be Li (0.2 ppma), Na (0.3-1.0 ppma), Si (0.1-0.3 ppma), Cl (0.3-0.4 ppma) and K (0.03-0.06 ppma). To reduce the levels various changes in procedure were adopted such as cleaning of the graphite boat parts in aqua regia, the use of elemental starting materials (Hg, Cd, Te), substrate purification and the use of in-situ wash melts. The effect of Hg saturated isothermal annealing on the impurity concentrations and distribution has been studied, revealing rapid surface segregation of the Group IA impurities. The values of the distribution coefficients for several impurities have been calculated from the analytical results, these values being generally in good agreement with other published work.

Excimer laser‐assisted metalorganic vapor phase epitaxy of CdTe and HgTe on (100) GaAs

The growth of CdTe and HgTe by excimer laser‐assisted metalorganic vapor phase epitaxy on (100) GaAs at 165 °C is reported. HgTe layers have been grown on a 4000‐Å buffer layer of CdTe on GaAs with the use of dimethylmercury and a new Hg‐containing source material, divinylmercury. A linear dependence of CdTe growth rate on laser power and reactant partial pressure has been observed using in situ time‐resolved reflectivity. The photodissociation mechanism for diethyltellurium (DETe) has been investigated using laser induced fluorescence. Results indicate that irradiation of DETe with 248‐nm photons produces ground‐state Te atoms by a single‐photon process.

Highly uniform, large‐area HgCdTe layers on CdTe and CdTeSe substrates

We report here on the growth of Hg0.8Cd0.2Te on lattice matched substrates of CdTe0.96Se0.04, by organometallic vapor phase epitaxy. Layer properties are compared with those for comparable layers grown on CdTe substrates. It is shown that the use of lattice matched substrates results in active layers of improved structural quality, as evidenced by the results of double‐crystal x‐ray diffraction measurements. A compositional uniformity of better than ±0.005 (in the Cd composition) was obtained over 1×2 cm area substrates and the thickness uniformity was better than ±0.7 μm for 12‐μm‐thick layers. These layers were deposited by the direct reaction of mercury, dimethylcadmium, and diisopropyltelluride, without the interdiffusion of separate layers of HgTe and CdTe. Both n‐ and p‐type layers have been grown, by suitable adjustment of the growth process. Mobility values in excess of 6×104 cm2/V s were obtained in n‐type material (x=0.23), with an electron concentration below 1×1015 cm−3. With p‐type layers, mobility values of 680 cm2/V s were obtained, with a hole concentration of 5×1016 cm−3.

Effect of annealing on the optical properties of HgTe-CdTe superlattices

HgTe-CdTe superlattices of high crystalline quality, and with negligible interdiffusion, have been successfully grown at low substrate temperatures (150–170 °C) by molecular beam epitaxy. Optical spectra exhibiting multiple steplike absorption edge features have been measured on as-grown and thermally annealed samples. The data provide valuable information on the superlattice subband structure when coupled with a suitable model. These results then can be used to obtain semiquantitative measures of layer interdiffusion along with such quantities as valence-band offset and strain. To observe subband structure in the optical data, it is necessary to fabricate samples having homogeneous HgTe and CdTe layer thicknesses in the superlattice growth direction. Homogeneity of layers was achieved by carefully controlling elemental source fluxes and shutter times during the epitaxial growth process. Substantial changes in the subband-related absorption structure could be induced by annealing the superlattice samples at temperatures just slightly above the growth temperature. An observed shift of the fundamental absorption edge to higher energy, as well as an apparent broadening of the subband absorption structure upon annealing, can be explained by interdiffusion of HgTe and CdTe layers.

Organometallic epitaxy of HgCdTe on CdTeSe substrates with high compositional uniformity

We report here on the growth of Hg0.8Cd0.2Te on lattice-matched substrates of CdTe0.96Se0.04 by organometallic vapor phase epitaxy. Results are compared with those for comparable layers grown on CdTe substrates. It is shown that the use of lattice-matched substrates results in active layers of improved structural quality, as evidenced by the results of double-crystal x-ray diffraction measurements. A compositional uniformity of better than ±0.005 (in the Cd composition) was obtained over 1 cm ×2 cm area substrates, corresponding to a standard deviation of 0.0024. The thickness uniformity was better than ±0.7 μm for 12-μm-thick layers. These layers were deposited by the direct alloy growth, without the interdiffusion of separate layers of HgTe and CdTe. Both n- and p-type layers have been grown by suitable modification to the growth process. Mobility values in excess of 6×105 cm2/V s were obtained in n-type material (x=0.2), with an electron concentration below 1×1015 cm−3. With p-type layers, mobility values of 400 cm2/V s were obtained with a hole concentration of 1×1017 cm−3.

Photo-stimulated II–VI crystal growth: A study of low temperature epitaxy

A comparison is made with low temperature epitaxial growth of II–VI compounds using photo-MOVPE and alternatives such as MBE and pyrolytic MOVPE. A major attraction for photo-MOVPE is the potential for selective area deposition which could work in the substrate range of 200 to 300°C. However, at temperatures below 200°C, the growth rates are low ( ≈0.1 μm/h) and in this paper reasons for this are sought. The growth rate of HgTe decreases with an activation energy of 24.3 kcal/mol, however, with increased UV intensity the low temperature growth rate could be enhanced. CdTe photo-MOVPE growth rates are reduced at low temperatures to avoid the onset of homogeneous CdTe formation. This process has been modelled and can explain experimental conditions of growth rate and temperature where epitaxial growth will occur. The quality of epitaxial layers has been assessed by double crystal X-ray diffraction. HgTe layers are of very high quality and rocking curve widths approximately two times the theoretical half width have been measured. CdTe and CMT layers are more variable and an unusual feature of tilt for homo-epitaxy has been seen. However, the best CdTe layers had rocking curve widths within a factor of two of theoretical and were similar in quality to HgTe photo-MOVPE.

The growth of CdHgTe by metalorganic chemical vapour deposition for optical communication devices

MOCVD has been used for the first time to grow CdHgTe by the interdiffused multilayer process for 1.3–1.55 μm photodiodes. The growth has been optimised to give uniform material with a smooth surface morphology. The use of a HgTe layer as the p-type Ohmic contact has resulted in photodiodes having a very low series resistance (30–35 Ω). The epitaxial diodes have also exhibited good thermal stability. Other parameters are comparable with those obtained for bulk grown devices.

I n s i t u spectroscopic ellipsometry during molecular-beam epitaxy of cadmium mercury telluride

To improve the control of the growth, a II–VI molecular-beam epitaxy chamber has been equipped with an in situ spectroscopic ellipsometer. Using this characterization technique we checked the interdiffusion between CdTe and HgTe. A thin CdTe layer was deposited on a HgTe layer. The initial abrupt interface (less than 1 nm) evolved during successive annealings. Spectroscopic measurements were performed at each step. A modeling permits the evaluation of the thickness of a graded composition layer resulting from the interdiffusion at 260 °C.

Far-infrared analysis of alloy structure in HgTe–CdTe superlattices

Far-infrared reflectivity measurements have been made on eight molecular-beam epitaxy (MBE)-grown HgTe–CdTe superlattices from two sources. The spectra at 300, 77, and 6 K display the phonon modes of pure HgTe, pure CdTe, and alloy Hg1−xCdxTe . Analysis shows that two of these microstructures contain layers of pure HgTe and CdTe separated by abrupt or graded alloy interfaces, whereas the others are more nearly HgTe–Hg1−xCdxTe structures with x=0.65 to 0.90. Details of the alloy models are discussed. Our analysis appears to be a first test of long-wavelength superlattice dielectric response theory.

HgTe–CdTe superlattices and Hg1−xCdxTe grown by low-temperature metalorganic chemical vapor deposition

We report low-temperature metalorganic chemical deposition of Hg1−xCdxTe and HgTe–CdTe superlattices using a thermal precracking technique. The success of the precracking technique depends on decomposing the incoming metalorganic reactants at a high temperature while allowing the epitaxial growth to occur at a lower temperature. The precracking technique enables one to grow Hg1−xCdxTe and HgTe–CdTe superlattices at temperatures of 200 to 250 °C. The Hg1−xCdxTe epilayers show infrared transmission with sharp cutoffs. The as-grown Hg0.7Cd0.3Te is n type and has a room-temperature mobility of 12 200 cm2/V s with carrier concentration of 2.7×1017 cm−3 and 77 K mobility of 27 000 cm2/V s with carrier concentration of 1.0×1017 cm−3. For the HgTe–CdTe superlattices, HgTe layer thicknesses as small as 60 Å were obtained. All the samples examined using transmission electron microscopy had sharp HgTe–CdTe interfaces. The HgTe–CdTe superlattice structures also exhibited sharp infrared transmission cutoffs. In addition, compared to the band gap of Hg1−xCdxTe alloy with the same composition, a narrowing of the superlattice band gap was observed as predicted by theoretical studies.

Preferential chemical reactions during noble gas ion sputtering of Hg 1− xCd xTe and its oxides

Sputter profiling of Hg 1− x Cd x Te structures to determine in-depth compositional profiles, as e.g. at anodic oxide/substrate interfaces, is a nontrivial method, as demonstrated previously by several groups. In the present work we use Auger spectroscopy in combination with Ne + bombardment to determine the properties of the anodic oxide, the oxide/substrate interface, and the substrate adjacent to the interface. The effects of ion bombardment on the composition of the oxides and substrate material are analyzed by means of pseudoternary phase diagrams, yielding evidence for preferential removal of Hg, through chemical decomposition reactions and preferential sputtering. The amount of preferential sputtering of Hg is estimated by monitoring the surface to bulk concentration ratio in sputter equilibrium. It is found that preferential sputtering is more pronounced in the oxides than in the substrate. Using this information together with recent results from studies of decomposition reactions under noble gas bombardment of native oxides on HgTe and CdTe, it appears plausible to assume the composition of the anodic oxides to be mixtures of HgTeO 3 and CdTeO 3 in about the same proportions as the Hg to Cd ratios of the growth substrates. The interface region contains species of a composition not found in the oxide or in the substrate, assigned as an interfacial HgTe layer. Below this layer, of width of about 50 Å, no deviations in composition of the substrate material are found with respect to the original substrate material.

Quantitative theory of optical properties of HgTe-CdTe superlattices

We have performed full-scale calculations of the optical properties of (001) HgTe-CdTe superlattices, of period ∼60 Å. We present the superlattice band gaps and oscillator strengths for the principal transitions as a function of HgTe layer width, in good agreement with experiment. The valence-band offset is 370 meV. Our results highlight the importance of the degree of localization of interface states in determining the optical properties of the superlattice.

Experimental relation between cut-off wavelength and HgTe layer thickness for HgTe-CdTe superlattices

An experimental curve of room-temperature cut-off wavelength versus HgTe layer thickness for HgTe-CdTe superlattices is presented for the first time, along with an experimental equation relating the two. The equation is used to show that the variation in the cut-off wavelength for HgTe-CdTe superlattices is less than for HgCdTe alloy when the cut-off wavelength is greater than 2.0 μm.