CdTe Epilayers Research Articles

Hydrogenation and annealing behaviors of deep levels in strained CdTe(111)/GaAs(100) heterostructures

Photo-induced-current transient spectroscopy measurements on CdTe/GaAs strained heterostructures grown by molecular beam epitaxy were carried out to investigate the hydrogenation and the annealing behaviors of the deep levels due to the crystal defects. The six hole-traps of the as-grown CdTe epilayer were observed. As the growth temperature and the rapid thermal annealing (RTA) temperature increased, the intensities of the H5 and the H6 peaks due to the crystal defects decreased, and that of the peak related to the shallow impurities increased. After annealing, the H5 and the H6 deep levels did not disappear in CdTe/GaAs heterostructures. These results indicate that the H5 and the H6 deep levels are related to the crystal defects due to the lattice mismatch. While the H6 signal was remarkably affected by the growth temperature of the CdTe epilayer, it was independent of the thickness of the CdTe. These results indicate that the crystallinity of a CdTe epilayer grown on GaAs(100) is improved by RTA, and that the improvement of the CdTe crystallinity is considered to be due to the elimination of the defects which were formed at heterointerface during the growth of the CdTe epilayer.

Strain effects in CdTe/Si heterostructures

Photoluminescence (PL) and spectroscopic ellipsometry measurements on CdTe/Si strained heterostructures grown by molecular beam epitaxy were carried out to investigate the effect of the strain and the dependence of the strain on the Si tilted substrates. The results of the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and the bound-exciton peak for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value. When rapid thermal annealing (RTA) was performed at 55 °C, the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and that at 1.574 eV for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value. Spectroscopic ellipsometry measurements showed that the spectrum of the dielectric constant of the CdTe epilayer grown on the Si (100) 8° tilted substrate is similar to that of the CdTe bulk. These results indicate that the strains in the CdTe layers grown on Si substrates are strongly dependent on the Si substrate orientation and that the crystallinity of the CdTe epitaxial layer grown on the Si substrate is remarkably improved by RTA.

Materials science communication (100) or (111) heteroepitaxy of cdte layers on (100) gaas substrates by organometallic vapor phase epitaxy

Systematic studies have been conducted on the growth mechanism of (100) or (111) CdTe on (100) GaAs substrates. No native oxide was observed between the CdTe epilayer and GaAs substrate as proved by high resolution transmission electron microscopy. The orientation of CdTe layer grown on (100) GaAs is only related to the growth conditions. Under normal growth conditions, (100) CdTe is preferred on (100) GaAs substrate under Cd-rich growth conditions. Under Te-stabilized growth conditions, only (100) was observed if no substrate prebake step was used. However, (111) layers were formed if the substrates were baked to high temperature before growth. To confirm only (100) growth, atomic layer epitaxy was used as an initial nucleation step. The crystalline quality of (100) layer can be further improved using thermal cycle growth.

Effect of thermal annealing on the structural and optical properties of CdTe (111)/GaAs (100) heterostructures

Reflection high-energy electron diffraction (RHEED), double-crystal X-ray rocking curve (DCRC), and photoluminescence (PL) measurements were performed to investigate the effect of thermal annealing on the structural and the optical properties of CdTe (111) epilayers grown on GaAs (100) substrates by molecular beam epitaxy (MBE) at low temperature. The results of the RHEED patterns showed that the oxidized layer on the GaAs substrate was removed in a Te atmosphere, and that the 20-Å CdTe layer was grown by three-dimensional process. When the rapid thermal annealing (RTA) was performed at 500°C for 14 s, the FWHM of the DCRC for the CdTe layer had the smallest value. After the RTA process, the luminescence intensity of the exciton remarkably increased, and the peak at 1.476 eV was dominant. As the RTA temperature increased, the luminescence intensity of the exciton peak related to neutral acceptors ( A°, X) increased. The temperature dependence of the spectra showed that the ( A°, X) peak originated from the recombination of the excitons bound in high-density defects. The excitation power intensity dependence of the PL spectra showed that the peaks of the transitions due to donor-acceptor pairs shifted to larger energies. These results indicate that the structural and the optical properties of the CdTe epilayers grown on GaAs (100) are improved by RTA, and that the RTA process is very useful for the growth of Hg xCd 1−xTe on CdTe/GaAs heterostructures.

The morphology of CdTe deposited by organometallic vapor phase epitaxy: The effect of substrate misorientation

Substrate misorientation and growth temperature influence the morphology of CdTe epilayers grown by organometallic vapor phase homoepitaxy. These effects were investigated by using CdTe{100} and CdTe{100} misoriented by 2, 4, 6, and 8° toward 〈111〉Te as substrates for growth in the temperature range from 337 to 425°C. Low angle pyramidal facets appeared on films grown on the CdTe(100) surface. The number density of these pyramidal facets decreased to zero as the substrate misorientation angle increased to 4°. At higher misorientation angles, low angle protrusions, resembling fish scales, appeared on the surface. When the temperature was increased, facet size decreased but facet density increased. The film morphology at the high misorientations, however, improved remarkably with increasing temperature. Cross-sectional transmission electron microscopy provided evidence that both the faceted CdTe films and films with a mirror-like finish were epitaxial single crystals with no planar defects. Schwoebel barriers are suggested as the reason for the faceting of the surface grown on CdTe{100}.

Deep levels in In-doped CdTe epitaxial films grown on p-CdTe substrates

Deep-level transient spectroscopy measurements have been carried out to investigate the behavior of the deep levels existing in In-doped n-CdTe grown by molecular beam epitaxy on nominally undoped p-CdTe (211) B-orientation substrates. One electron-trap of the as-grown In-doped CdTe epilayer was observed, and the trap originated from complexes of Cd vacancies and In impurities. After the In-doped CdTe epilayer was annealed, one new trap at E c − 0.49 eV was observed, and the deep level was related to the Te vacancies or the Cd interstitials. These results indicate that the electron deep trap in In-doped CdTe epilayers is affected remarkably by annealing and that the variation of the position of the deep level due to thermal treatment is a significant problem for electronic devices.

High quality large-area CdTe(211)B on Si(211) grown by molecular beam epitaxy

We describe the growth of high quality CdTe(211)B layers by molecular beam epitaxy on nominal Si(211) substrates. Prior to CdTe deposition, thin ZnTe(211)B buffer layers were grown to preserve the homo-orientation. Large-area CdTe(211)B layers were routinely obtained by optimizing the growth parameters. From x-ray diffraction, we observed the presence of twin-free CdTe(211)B layers. One 8 μm thick CdTe epilayer had a near-surface etch pit density of 1.5×105 cm−2, which surpassed the best value reported for CdTe(211)B grown on GaAs(211)B, GaAs/Si(211), or Si(211) substrates.

Structural study of (100) CdTe epilayers grown by MOVPE on ZnTe buffered and unbuffered (100) GaAs

We report on the structural assessment of metalorganic vapour phase epitaxy grown (100)-oriented CdTe epilayers on both (100)GaAs and (100) ZnTe GaAs . Ion channelling Rutherford backscattering spectrometry and cathodoluminescence (CL) measurements are used to study the effect of inserting a proper ZnTe buffer layer [G. Leo et al., J. Vac. Sci. and Technol. B 14 (1996) 1739] between CdTe and GaAs. The insertion of a ZnTe buffer layer improves the surface crystalline and optical quality of the CdTe: CL images show that non-radiative recombination regions, associated with extended defects, strongly decrease when a ZnTe buffer layer is used. Also, enhanced excitonic emissions are observed in the case of CdTe/ZnTe/GaAs samples.

Rapid thermal annealing effects in CdTe (111) thin films grown on Si (100) substrates by molecular beam epitaxy

Photoluminescence (PL) measurements have been carried out to investigate rapid thermal annealing (RTA) effects in CdTe (111) epilayers grown on Si (100) by molecular beam epitaxy, The full width at half maximum (FWHM) of the bound exciton peak of the PL spectrum for the as-grown CdTe layer was 20 meV. When RTA was performed at 400°C, the bound exciton peak was resolved into two bound exciton peaks due to neutral acceptors ( A°, X) and to neutral donors (D°, X) and one free exciton peak; the FWHM of the ( A°, X) peak was as small as 3.7 meV. When RTA was carried out at 600°C, the relative intensity ratio between the luminescences related to the defects and (A°, X) in the as-grown and the annealed CdTe layers decreased by a factor of 15. The activation energy of the ( A°, X) peak as obtained from temperature-dependent PL measurements was 18.7 meV. The bound exciton peak for the CdTe, which was rapidly thermally annealed at 400°C, shifted by 7.5 meV in comparison with that for the CdTe bulk, and the strain obtained from the PL peak shift was −0.0286, These results indicate that the crystallinity of the CdTe epilayers grown on Si is improved by RTA and that the RTA CdTe films grown on Si can be used for applications as buffer layers for the growth of Hg xTd 1−xTe.

The Reduction of The Defect Density in CdTe Buffer Layers for The Growth of HgCdTe Infrared Photodiodes on Si (211) Substrates

CdTe epilayers were grown by molecular beam epitaxy on As-passivated nominal (211) Si substrates using thin interfacial ZnTe layers. By using thin recrystallized (initially amorphous) ZnTe buffei layers, we utilized migration enhanced epitaxy (MEE) in the ZnTe layer and overcome the tendency toward three dimensional nucleation. The threading dislocation densities in 8–9 tm thick CdTe films deposited on the recrystallized amorphous ZnTe films were in the range of 2 to 5 × 105 cm−2. In addition to the reduction of threading dislocation density, the interface between the ZnTe layers and the Si substrate is much smoother and the microtwin density is an order of magnitude lower than in regular MEE growth. In order to understand the initial nucleation mechanism of the ZnTe on the As precursor Si surface, we also grew ZnTe epilayers on Te precursor treated Si substrates. The growth mode, microtwin density, and threading dislocation density are compared for films grown on Si substrates with different surface precursors and grown by different growth methods.

The Reduction of The Defect Density in CdTe Buffer Layers for The Growth of HgCdTe Infrared Photodiodes on Si (211) Substrates

CdTe epilayers were grown by molecular beam epitaxy on As-passivated nominal (211) Si substrates using thin interfacial ZnTe layers. By using thin recrystallized (initially amorphous) ZnTe buffer layers, we utilized migration enhanced epitaxy (MEE) in the ZnTe layer and overcome the tendency toward three dimensional nucleation. The threading dislocation densities in 8–9 μm thick CdTe films deposited on the recrystallized amorphous ZnTe films were in the range of 2 to 5 × 105cm−2. In addition to the reduction of threading dislocation density, the interface between the ZnTe layers and the Si substrate is much smoother and the microtwin density is an order of magnitude lower than in regular MEE growth. In order to understand the initial nucleation mechanism of the ZnTe on the As precursor Si surface, we also grew ZnTe epilayers on Te precursor treated Si substrates. The growth mode, microtwin density, and threading dislocation density are compared for films grown on Si substrates with different surface precursors and grown by different growth methods.

Doping and Characterization of Wide-Gap II-VI Epilayers

In this paper we review the properties of n-type doped ZnSe and CdTe epilayers grown by molecular beam epitaxy on (100) GaAs substrates. Recent results of photoluminescence, transport measurements, secondary ion mass spectroscopy and deep-level transient spectroscopy are discussed. A major emphasis is placed on the effect of different dopant species and the role of the deviation from stoichiometry on the doped epitaxial layers. Since deep defect states play an important role in determining the properties of the doped materials, considerable attention is directed towards characterization and identification of deep-lying defect states, both native and introduced by dopants. In particular, in the case of ZnSe the deep-level transient spectroscopy results clarify why Cl is superior to Ga as an effective n-type dopant. They provide strong evidence that chlorine — unlike Ga — does not introduce by itself any detectable deep defects into the ZnSe lattice. In the case of CdTe, we focus on the influence of the deviation from stoichiometric growth conditions in the molecular beam epitaxy process and on the properties of In doped layers. We discuss resistivity, Mn diffusivity and the presence of various deep defects in layers grown at different Cd/Te flux ratios.

Growth and characterization of heteroepitaxial CdTe and ZnTe on Ge(001) buffer layers

The microstructure of heteroepitaxial CdTe (001) and ZnTe(001) epilayers grown on Ge(001) buffer layers by molecular beam epitaxy has been characterized using electron microscopy. Apart from occasional {111} stacking faults originating at the interfacial region, the prevailing defects present in both systems are identified by high-resolution imaging as perfect Lomer edge dislocations with Burgers vectors of the type a/2〈110〉 parallel to the interface plane, which are indicative of well-relaxed material. Double-crystal rocking-curve measurements using Ge(001) buffer layers give full-width-at-half-maximum values of 210 arc-sec for a 7.5μm thick ZnTe film and 125 arc-sec for a 12μm thick CdTe film. Use of the Ge buffer layers on Si(001) substrates represents a valuable precursor for eventual growth of mercury cadmium telluride since this allows the substrate orientation to be maintained. The buffer layer also permits a substantial reduction of the in situ annealing temperature needed for substrate oxide removal.

Annealing effects of -implanted CdTe epilayers

The annealing effects of As-ion-implanted (211)B CdTe films grown by molecular beam epitaxy have been studied by resonant Raman scattering and photoluminescence spectroscopy. The Raman intensity ratio of 2LO to 1LO phonon modes and the normalized bound-exciton emission intensity indicate that better removal of the implantation-induced damage is obtained with an increase of the annealing temperature beyond , whereas the lattice perfection drops sharply when is higher than . With an increase of in the studied range, more and more As atoms occupy the Te sites as acceptors and the samples behave in a p-type manner with a decreasing compensation coefficient and an increasing hole concentration.

Direct growth of CdTe on (100), (211), and (111) Si by metalorganic chemical vapor deposition

CdTe epilayers were grown directly on (100), (211), and (111) silicon substrates by metalorganic chemical vapor deposition (MOCVD). The crystallinity and the growth orientation of the CdTe film were dependent on the surface treatment of the Si substrate. The surface treatment consisted of exposure of the Si surface to diethyltelluride (DETe) at temperatures over 600°C prior to CdTe growth. Direct growth of CdTe on (100) Si produced polycrystalline films whereas (lll)B single crystals grew when Si was exposed to DETe prior to CdTe growth. On (211) Si, single crystal films with (133)A orientation was obtained when grown directly; but produced films with (211)A orientation when the Si surface was exposed to DETe. On the other hand, only (lll)A CdTe films were possible on (111) Si, both with and without Te source exposure, although twinning was increased after exposure. The results indicate that the exposure to a Te-source changes the initial growth stage significantly, except for the growth on (111) Si. We propose a model in which a Te atom replaces a Si atom that is bound to two Si atoms.

Rapid Thermal Annealing Effects in CdTe (111) Thin Films Grown on GaAs (100) Substrates

Photoluminescence (PL) and double crystal X-ray rocking curve (DCRC) measurements have been carried out to investigate the rapid thermal annealing (RTA) effects in CdTe (111) epilayers grown on GaAs (100) by molecular beam epitaxy. The result of the X-ray diffraction showed that the orientation of the grown CdTe films was the (111) orientation. The full width at half maximum (FWHM) of the DCRC for as-grown CdTe layer was 400 arcs. When RTA was performed at 550° C, the FWHM of the DCRC for the CdTe layer decreased 265 arcs. The results of the PL spectra showed that the luminescence intensity of a bound exciton ( D0, X) for the CdTe/GaAs annealed at 550° C was raised by as much as about 53 times in comparison to the as-grown CdTe. The relative intensity ratio between ( D0, X) and the luminescence related to the defects for the as-grown and the annealed CdTe decreased by a factor of 7.8. These results indicate that the crystallinity of the CdTe epilayers grown on GaAs (100) is improved by RTA and that the RTA CdTe films grown on Si can be used for applications as buffer layers for the growth of Hgx Cd1-x Te.

Photon assisted growth of nitrogen-doped CdTe and the effects of hydrogen incorporation during growth

Nitrogen doping in CdTe epilayers grown by photo-assisted molecular beam epitaxy was demonstrated using an rf plasma source. The effect of the presence of atomic hydrogen during growth of undoped and nitrogen-doped CdTe was investigated. The layers were characterized using photoluminescence spectros-copy (PL), Hall effect, secondary ion mass spectroscopy (SIMS), Fourier transform infrared spectroscopy, and atomic force microscopy. PL confirmed the incorporation of nitrogen as acceptors. While p-type carrier concentrations greater than 1018 cm−3 were easily obtained, SIMS measurements indicated that nitrogen was concentrated near the undoped-doped and epilayer-substrate interfaces which complicates interpretation of activation efficiency. Hydrogen incorporation was found to be enhanced by the presence of nitrogen. Infrared absorption measurements strongly suggested the formation of N-H complexes. Hall measurements indicated that complexes are formed which are donor-like in nature. The presence of atomic hydrogen during growth radically changed the low temperature photoluminescence in both undoped and nitrogen-doped layers. Exciton-related luminescence was quenched at low temperature. Nitrogenrelated donor-acceptor pair luminescence was also absent from the N-doped hydrogenated layers, consistent with complex formation. Copper (a cation-site acceptor) donor-acceptor pair luminescence appeared to be enhanced by hydrogenation.

Influence of a ZnTe buffer layer on the structural quality of CdTe epilayers grown on (100)GaAs by metalorganic vapor phase epitaxy

The influence of a ZnTe buffer layer on the structural quality of CdTe epilayers grown on (100)GaAs substrates by metalorganic vapor phase epitaxy has been investigated by both x-ray diffraction and ion channeling Rutherford backscattering spectrometry measurements. Single-crystal (100) oriented CdTe epilayers of good structural quality have been obtained after inserting a ZnTe buffer layer of a thickness ranging between 300 and 500 nm. The influence of the buffer layer thickness on the crystalline quality and the morphology of the CdTe epilayer has been related to the defect distribution and the surface roughness of the ZnTe buffer layer. The crystalline quality and the surface strain have been thus studied as a function of the CdTe thickness on samples having optimal ZnTe layer thickness. The initial compressive mismatch between CdTe and ZnTe, f=−5.8%, appears to be almost fully relaxed for a CdTe thickness around 200 nm. A residual compressive in-plane strain (about −0.02%), independent of the CdTe epilayer thickness, has been found above 300 nm which can be ascribed to thermal strain and indicates a complete relaxation of the lattice misfit at the growth temperature.

Effect of the post-As+-lmplantation thermal treatment on MBE HgCdTe optical properties

HgCdTe epilayers were grown by molecular beam epitaxy. A series of As+-implanted CdTe and HgCdTe epilayers annealed under different temperatures were investigated by photoluminescence spectroscopy. More As+ ions can occupy the Te sublattice after the samples were annealed at 450°C, and the acceptor level of As+ on the Te sublattice for HgCdTe material (x ≈ 0.39) is 31.5 meV. The Raman spectrum study indicates a recovery of the crystalline perfection after the post-As+-implantation thermal treatment.

(111)A CdTe rotation growth on (111) Si with low growth rate by metalorganic chemical vapor deposition

(111)A CdTe epilayers have been grown directly on (111) silicon substrates with the low growth rate of less than 0.5 μm/h by metalorganic chemical vapor deposition (MOCVD). The full width at half maximum (FWHM) of the x-ray rocking curve of a 1 μm thick epilayer is 100 arcsec, which is much better than the value for (111)B CdTe/(100)Si. The low growth rate effects are deduced from the classical capillarity theory. The nucleation stages were examined by atomic force microscopy (AFM), which showed that the nuclei were less in number when growing with the low growth rate. The geometrical advantages of (111)A CdTe/(111) Si for annihilating the dislocations are also discussed.