Elimination of Inclusions in (CdZn)Te Substrates by Post-grown Annealing

Post-grown annealing of (211) (CdZn)Te substrates has been used for elimination of Te and Cd inclusions with the objective of improving the yield of inclusion-free substrates for MBE growth of (HgCd)Te. Different annealing temperatures and Cd/Te overpressures were used to find the optimum annealing conditions. Te inclusions were significantly reduced by Cd-rich annealing at temperatures higher than 660°C, together with increasing the infrared transmittance at 10 μm to above 60%. Good crystalline quality was preserved after the annealing. Te-rich annealing at 700°C was found to be the optimum method for elimination of most of the Cd inclusions; infrared transmittance at 10 μm was suppressed by the annealing, however. Final Cd-rich annealing is recommended for infrared transmittance improvement.     

Influence of Cd-rich Annealing on Defects in Te-rich CdZnTe Materials

The influence of Cd-rich annealing at temperatures of 440–900 °C on the defect properties of Te-rich CdZnTe materials was studied. Cd-rich annealing at temperatures above the melting point of Te was confirmed to effectively reduce the size of Te-rich inclusions in the materials. However, dislocation multiplication occurred in the regions near Te-rich inclusions. Etch pit clusters were observed on the surfaces of annealed materials etched with Everson etchant. The etch pit clusters were much larger than the as-grown Te-rich inclusions. The dependence of the cluster size on that of the Te-rich inclusions and the annealing conditions was investigated. The density of etch pits in the normal region increased when the annealing temperature exceeded 750 °C. The mechanisms of the evolution of the Te-rich inclusions and the formation of new defects during the Cd-rich annealing are discussed.     

Thermomigration of tellurium precipitates in CdZnTe crystals grown by vertical bridgman method

Te precipitates in CdZnTe have been characterized by x-ray diffraction at room and higher temperatures. From the x-ray results at room temperature, it has been confirmed that Te precipitates in CdZnTe have the same structural phase as observed in elemental Te under high pressure. The x-ray results at higher temperature indicate that Te precipitates melt around 440°C. CdZnTe samples containing Te precipitates have been annealed at temperatures below and above 440°C with thermal gradient of ∼70°C/cm. Results of the observation with infrared microscope before and after the annealings indicate distinct occurrence of thermomigration of Te precipitates in samples annealed at temperature above 440°C compared with ones annealed at temperature below 440°C. Thermomigration velocity obtained from these results is ∼50 μm/h. The average value for the effective diffusion coefficient of the metallic atoms in Te precipitates calculated by using the thermomigration velocity is ∼3 x 10⁻⁵ cm²/s.     

Thermomigration of Te precipitates and improvement of (Cd,Zn)Te substrate characteristics for the fabrication of LWIR (Hg,Cd)Te photodiodes

(Cd,Zn)Te wafers containing Te precipitates have been annealed under well defined thermodynamic conditions at temperatures below and above the melting of Te. Results of the examination of the wafers with infrared microscopy before and after the anneals indicate a substantial reduction of the Te precipitates in wafers annealed at temperatures in excess of the melting point of Te compared with those annealed at temperatures below the melting point of Te. These results confirm the thermomigration of liquid Te precipitates to be the principally operative mechanism during annealing in the elimination of these precipitates in (Cd,Zn)Te wafers. The occurrence of Te precipitates in (Hg,Cd)Te epitaxial layers grown on (Cd,Zn)Te substrates containing Te precipitates is also explained on the basis of thermomigration of these precipitates during LPE growth from the substrates to the epilayers. Absence of occurrence of Te precipitates in (Hg,Cd)Te epilayers grown on annealed (Cd,Zn)Te substrates with negligible Te precipitates is also confirmed. Usefulness of annealing (Cd,Zn)Te substrates—to eliminate Te precipitates—prior to epilayer growth is confirmed via demonstration of improved long wavelength infrared (Hg,Cd)Te device array performance uniformity in epitaxial layers grown on (Cd,Zn)Te substrates with negligible Te precipitates after annealing.     

Annealing studies of undoped Hg1−xMnx Te bulk crystals at high temperatures

The effect of high temperature annealing treatments in varying mercury atmospheres on Hg_1−xMn_xTe crystals with long wavelength infrared/very long wavelength infrared cut off wavelengths has been studied. The undoped Hg_1−xMn_xTe crystals were grown using the traveling heater method with a tellurium solvent zone, and composition was verified by infrared transmission measurements. The crystals were subjected to annealing temperatures of 500 and 550°C under mercury pressures varying from Hg-rich conditions to Te-rich conditions. The samples were either air cooled or water cooled to room temperature. Hall effect measurements were carried out at 77K at magnetic fields varying from 500 Gauss to 10 kGauss. The hole concentration in the annealed crystals was found to be roughly inversely proportional to the partial pressure of Hg indicating that the material is essentially intrinsic at the anneal temperature. A defect model and a relationship between the mass action constants for the native acceptor defects of HgMnTe are presented.     

The annealing of Cd1−xZnxTe in CdZn vapors

As-grown CdZnTe usually contains defects, such as twins, subgrain boundaries, dislocations, and Te precipitates. It is always important to anneal CdZnTe slices in Cd vapor to eliminate these defects, especially Te precipitates. The exchange of Zn atoms between the slices and the vapor plays an important role during the annealing process. In this paper, the effects of Zn partial pressure on the properties of the annealed slices are studied carefully by measuring the concentration profiles, the infrared (IR) transmission spectra, and the x-ray rocking curves. It was found that a surface layer with different compositions and possibly different structure from the bulk crystal formed during the annealing of CdZnTe samples in the saturated Zn vapor. The accumulation of excess Te in the surface layer helps to increase the IR permeability of the bulk crystal greatly. To improve the crystallization quality, a lower Zn-pressure annealing should be used following the high Zn-pressure annealing. The diffusion of Zn in the bulk crystal has also been analyzed at the temperatures of 700°C and 500°C. Calculations determined that D_Zn (700°C)=4.02 × 10⁻¹² cm²s⁻¹ and D_Zn (500°C)=1.22 × 10⁻¹³ cm²s⁻¹.     

Effect of annealing temperature on structural,electrical and optical properties of spray pyrolytic nanocrystalline CdO thin films

Nanocrystalline CdO thin films were prepared onto a glass substrate at substrate temperature of 300 °C by a spray pyrolysis technique. Grown films were annealed at 250, 350, 450 and 550 °C for 2.5 h and studied by the X-ray diffraction, Hall voltage measurement, UV-spectroscopy, and scanning electron microscope. The X-ray diffraction study confirms the cubic structure of as-deposited and annealed films. The grain size increases whereas the dislocation density decreases with increasing annealing temperature. The Hall measurement confirms that CdO is an n-type semiconductor. The carrier density and mobility increase with increasing annealing temperature up to 450 °C. The temperature dependent dc resistivity of as-deposited film shows metallic behavior from room temperature to 370 K after which it is semiconducting in nature. The metallic behavior completely washed out by annealing the samples at different temperatures. Optical transmittance and band gap energy of the films are found to decrease with increasing annealing temperature and the highest transmittance is found in near infrared region. The refractive index and optical conductivity of the CdO thin films enhanced by annealing. Scanning electron microscopy confirms formation of nano-structured CdO thin films with clear grain boundary.     

Sigmoid-growth curve of lattice-constant against annealing-temperature of Cd1−xZnxTe thin films and its structural,optical and morphological characteristics

Stacked CdTe/Zn/CdTe layers were deposited on glass substrates. The vacuum-evaporated thin films were subsequently annealed in vacuum ambience at various temperatures. Change in lattice-constant of major Cd_1−xZn_xTe planes against temperature was plotted from the XRD results. The graphs followed sigmoid-growth model and were regressed well by standard Boltzmann and Logistic functions. Lattice-constant varied maximum in between 375–400 °C and 425–450 °C, giving two separate growth trends. Optical studies suggested that presence of charge impurities and defects reduced the transmittance and band-gap values of the samples. Such reduction occurred, despite of greater formation of Cd_1−xZn_xTe. Decreasing granularity was however associated with increasing band-gap for samples annealed at 425 and 450 °C. SEM micrographs showed that granularity decreased significantly for samples annealed at higher temperatures. EDX results were further used to co-relate the compositional characteristics with structural and optical features.     

Microstructural Characterization of CdTe Surface Passivation Layers

The microstructure of CdTe (CT) surface passivation layers deposited on HgCdTe (MCT) heterostructures has been evaluated using transmission electron microscopy (TEM). The MCT heterostructures were grown by liquid-phase epitaxy and consisted of thick (approximately 10 μm to 20 μm) n-type MCT layers and thin (approximately 1 μm to 3 μm) p-type MCT layers. The final CT (approximately 0.3 μm to 0.6 μm) capping layers were grown either by hot-wall epitaxy (HWE) or molecular-beam epitaxy (MBE). One of the wafers with the CT layer grown by MBE was also annealed in Hg atmosphere at 250°C for 96 h. The as-deposited CT passivation layers were polycrystalline and columnar. The CT grains were larger and more irregular when deposited by HWE, whereas those deposited by MBE were generally well textured with mostly vertical grain boundaries. Observations and measurements with several TEM abrupt structurally after annealing techniques showed that the CT/MCT interface became considerably more abrupt structurally after annealing, and the crystallinity of the CT layer was also improved.     

Effects of Thermal Annealing on the Formation of Buried <Emphasis Type="Italic">β</Emphasis>-SiC by Ion Implantation

A systematic study of the formation of buried β-SiC structures by carbon ion implantation into Si followed by high-temperature thermal annealing has been carried out. A high fluence of carbon ions (8 × 10¹⁷ atoms/cm²) was implanted at 65 keV energy. Formation of the crystalline β-SiC phase was monitored by Fourier-transform infrared (FTIR) spectroscopy, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) techniques. The implanted samples were annealed at 900°C and 1100°C to observe the effects of annealing temperature on the formation of crystalline β-SiC. Formation of crystalline β-SiC was clearly observed in the sample annealed at 1100°C in a flowing nitrogen environment for a period of 1 h. Graphitic carbon clusters were observed at the implanted carbon profile peak position by XPS depth profile measurements. Various structural defects such as grain boundaries were also visualized in the annealed sample by high-resolution TEM.     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

Dislocation Reduction of HgCdTe/Si Through <Emphasis Type="Italic">Ex Situ</Emphasis> Annealing

Current growth methods of HgCdTe/Cd(Se)Te/Si by molecular-beam epitaxy (MBE) result in a dislocation density of mid 10⁶ cm⁻² to low 10⁷ cm⁻². Although the exact mechanism is unknown, it is well accepted that this high level of dislocation density leads to poorer long-wavelength infrared (LWIR) focal-plane array (FPA) performance, especially in terms of operability. We have conducted a detailed study of ex situ cycle annealing of HgCdTe/Cd(Se)Te/Si material in order to reduce the total number of dislocations present in as-grown material. We have successfully and consistently shown a reduction of one half to one full order of magnitude in the number of dislocations as counted by etch pit density (EPD) methods. Additionally, we have observed a corresponding decrease in x-ray full-width at half-maximum (FWHM) of ex situ annealed HgCdTe/Si layers. Among all parameters studied, the total number of annealing cycles seems to have the greatest impact on dislocation reduction. Currently, we have obtained numerous HgCdTe/Si layers which have EPD values measuring ~1 × 10⁶ cm⁻² after completion of thermal cycle annealing. Preliminary Hall measurements indicate that electrical characteristics of the material can be maintained.     

Study of the Au/In Reaction for Transient Liquid-Phase Bonding and 3D Chip Stacking

The latest three-dimensional (3D) chip-stacking technology requires the repeated stacking of additional layers without remelting the joints that have been formed at lower levels of the stack. This can be achieved by transient liquid-phase (TLP) bonding whereby intermetallic joints can be formed at a lower temperature and withstand subsequent higher-temperature processes. In order to develop a robust low-temperature Au/In TLP bonding process during which all solder is transformed into intermetallic compounds, we studied the Au/In reaction at different temperatures. It was shown that the formation kinetics of intermetallic compounds is diffusion controlled, and that the activation energy of Au/In reaction is temperature dependent, being 0.46 eV and 0.23 eV for temperatures above and below 150°C, respectively. Moreover, a thin Ti layer between Au and In was found to be an effective diffusion barrier at low temperature, while it did not inhibit joint formation at elevated temperatures during flip-chip bonding. This allowed us to control the intermetallic formation during the distinct stages of the TLP bonding process. In addition, a minimal indium thickness of 0.5 μm is required in order to enable TLP bonding. Finally, Au/In TLP joints of ∅40 μm to 60 μm were successfully fabricated at 180°C with very small solder volume (1 μm thickness).     

Effect of post deposition heat treatment on microstructure parameters,optical constants and composition of thermally evaporated CdTe thin films

Thin film microstructure and its properties can be effectively altered with post deposition heat treatments. In this respect, CdTe thin films were deposited on glass substrates at a substrate temperature of 200 °C using thermal evaporation technique, followed by air annealing at different temperatures from 200 to 500 °C. Structural analysis reveals that CdTe thin films have a cubic zincblend structure with two oxide phases related to CdTe₂O₅ and CdTeO₃ at annealing temperature of 400 and 500 °C respectively. Regardless of the annealing temperature, the plane (111) was found to be the preferred orientation for all films. The crystallite size was observed to increase with annealing temperature. All films were found to display higher lattice parameters than the standard, and hence found to carry a compressive stress. Optical measurements suggest high uniformity of films both before and after post deposition heat treatment. Films annealed at 400 °C displayed superior optical properties due to its high refractive index, optical conductivity, relative density and low disorder. Furthermore, according to the compositional measurements, CdTe thin films were found to exhibit Te rich and Cd rich nature at regions near the substrate and center of the film respectively, for all annealing temperatures. However, composition of the regions near the substrate was found to become more Te rich with increasing annealing temperature. The study suggests that changing the annealing temperature as a post deposition treatment affects structural and optical properties of CdTe thin film as well as its composition. According to the observations, films annealed at 400 °C can be concluded to be the best films for photovoltaic applications due to its superior optical and structural properties.     

Reduction of CdZnTe substrate defects and relation to epitaxial HgCdTe quality

We have conducted annealing experiments on CdZnTe wafers to restore stoichiometry, eliminate or reduce second-phase (Cd or Te) inclusions, and investigate effects on the quality of epitaxial HgCdTe grown on the thermally treated substrates. Two categories of second phase features were revealed in these materials. Category 1 has a star-like shape with sixfold symmetry (as seen by infrared transmission microscopy) and a central core consisting of cadmium. These stars were observed only in the more stoichiometric materials (having good infrared transmission characteristics). Category 2 consists of triangular, hexagonal, and irregular shaped tellurium inclusions which are present in the off-stoichiometry materials (which exhibit strong IR absorption). Substrates were annealed at temperatures ranging from 500 to 700°C for one to seven days, in vapor derived from elemental Cd or Cd_1-xZn_x alloy (x = 0.005). These anneals were able to eliminate the excess IR absorption and decrease the apparent sizes of both categories of second-phase features. It was found that pinhole-like morphological defects on the surface of a HgCdTe layer grown by liquid phase epitaxy can be caused by Cd and Te inclusions located within the CdZnTe substrate near the interface. Additionally, measurement and spatial mapping of copper concentration by sputter initiated resonance ionization spectroscopy showed 10 to 100 times higher Cu concentration in the inclusions than in the surrounding matrix areas.     

Thermal stability of heavily tellurium-doped InP grown by metalorganic molecular beam epitaxy

The thermal stability of tellurium in InP has been examined in samples doped with Te up to an electron concentration of 1.4 × 10²⁰ cm⁻³. Annealing was conducted using rapid thermal annealing for a period of one minute at temperatures over the range 650–800°C. Secondary ion mass spectroscopy analysis showed virtually no change in the Te profile before and after annealing, even at the highest annealing temperatures. High resolution x-ray diffraction and Hall measurements revealed a general decrease in the lattice strain and carrier concentration for annealing temperatures above 650°C. No evidence of strain relief was found in the form of cross-hatching or through the formation of a dislocation network as examined by scanning electron microscopy or transmission electron microscopy (TEM). These results are most likely due to the formation of Te clusters, though such clusters could not be seen by crosssectional TEM.     

Annealing experiments in heavily arsenic-doped (Hg,Cd)Te

Arsenic doped molecular beam epitaxy (MBE) (Hg,Cd)Te films were grown on (Cd,Zn)Te substrates. The concentration of arsenic was varied from 5 x 10¹⁸ cm^-3 to 1 x 10²⁰ cm^-3. After the growth, the epitaxial layers were annealed at various partial pressures of Hg within the existence region of (Hg,Cd)Te at temperatures ranging from 400 to 500°C. Hall effect and resistivity measurements were carried out subsequent to the anneals. 77K hole concentration measurements indicate that for concentrations of arsenic <10¹⁹ cm⁻³, most of the arsenic is electrically active acting as acceptors interstitially and/or occupying Te lattice sites at the highest Hg pressures. At lower Hg pressures, particularly at annealing temperatures of 450°C and higher, compensation by arsenic centers acting as donors appears to set in and the hole concentration decreases with decrease in Hg pressure. These results indicate the amphoteric behavior of arsenic and its similarity to the behavior of phosphorus in (Hg,Cd)Te previously inferred by us. A qualitative model which requires the presence of arsenic occupying both interstitial and Te lattice sites along with formation of pairs of arsenic centers is conjectured.     

Effects of Growth Temperature and Postgrowth Annealing on Inhomogeneous Luminescence Characteristics of Green-Emitting InGaN Films

Microscopic photoluminescence was applied to investigate μm-order inhomogeneity of InGaN alloys. Samples had InGaN/GaN multiple-quantum-well structures grown on sapphire substrates at various temperatures, and luminescence was adjusted to be green. Luminescence morphologies of dendritic appearance were observed on as-grown samples. Bright spots luminescing at long wavelengths (green to amber) were formed at high growth temperatures. After annealing at 1000°C, the bright spots disappeared and the dendritic morphology turned into a granular morphology. Because of these μm-order inhomogeneities, it has been suggested that small-scale characterization (sub-μm or smaller) requires special attention in order not to miss effects of μm-order inhomogeneity in InGaN alloys.     

Physico-chemical properties of Ga and In dopants during liquid phase epitaxy of CdxHg1−xTe

This work deals with the study by means of radioactive tracers and autoradiography, as well as measuring of galvanomagnetic properties, of Ga and In doping of epitaxial Cd_xHg_1−xTe layers during their crystallization from a Te-rich melt. Ga and In were introduced in the form of Ga⁷² and In¹¹⁴ master alloys with Te. The effective distribution coefficients of Ga and In during the crystallization of the Cd_xHg_1−xTe solid solutions with x=0.20 to 0.23 were determined by cooling the Te-base melt to 515–470°C. Depending on the concentration of the dopants and the time-temperature conditions of Cd_xHg_1−xTe growth, these ratios for Ga and In were 1.5–2.0 and 1.0–1.5, respectively. The electrical activity of Ga and In was determined after annealing of the Cd_xHg_1−xTe layers in saturated Hg vapor at 270–300°C. In doping of the epitaxial layers to (3–8)×10¹⁴ cm⁻³ with subsequent annealing in saturated Hg vapor at ∼270°C increases the carrier lifetime approximately by a factor of two as compared with the undoped material annealed under the same conditions.     

Growth of Thick Epitaxial CdTe Films by Close Space Sublimation

This paper reports on a detailed study of the development of the close space sublimation method, which has been widely used in the preparation of polycrystalline CdTe/CdS solar cells, as an epitaxial method for the growth of thick CdTe single crystal films over 200 μm on GaAs and Ge substrates for high-energy radiation detectors. The resulting microscopic growth phenomena in the process are also discussed in this paper. High-quality single crystalline CdTe thick films were prepared with x-ray rocking curves full width at half maximum (FWHM) values, which were ∼100 arcsec on Ge substrates and 300 arcsec on GaAs substrates. The quality of thick films on Ge(100) showed a substantial improvement with nucleation in a Te-rich growth environment. No Te inclusions in the CdTe films grown on GaAs(211)B and Ge(100) were observed with IR transmission imaging. Photoluminescence of CdTe/Ge shows a large reduction in the 1.44 eV defect energy bands compared with films grown on GaAs substrates. The film resistivity is on the order of 10¹⁰ Ω cm, and the film displayed some sensitivity to alpha particles.