Post-growth Annealing Treatment Research Articles

Negative magnetopolarization in thermally annealed self-assembled quantum dots

A negative magnetic dispersion of the degree of circular polarization is reported for the photoluminescence of self-assembled ZnCdSe quantum dots (QDs) after a postgrowth thermal annealing (TA) treatment. Multiband calculations in the framework of the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model reveal that the phenomenon is explicable considering, apart from the changes in the confinement and composition of the QDs due to TA-induced Zn-Cd interdiffusion, the presence of extra charges of one kind within the structure.

Post-growth annealing treatment effects on properties of Na-doped CuInS 2 thin films

Structural and optical properties of Na-doped CuInS 2 thin films grown by double source thermal evaporation method were studied. The films were annealed from 250 to 500 °C in a vacuum after evaporation. X-ray diffraction pattern indicated that there are traces of Cu and In 6S 7, which disappeared on annealing above 350 °C. Good quality CuInS 2:Na 0.3% films were obtained on annealing at 500 °C. Furthermore, we found that the absorption coefficient of Na-doped CuInS 2 thin films reached 1.5 × 10 5 cm −1. The change in band gap of the doped samples annealed in the temperatures from 250 to 500 °C was in the range 0.038–0.105 eV.

P -type conduction in unintentional carbon-doped ZnO thin films

p -type conduction has been observed in unintentional carbon-doped ZnO thin films grown by metal organic chemical vapor deposition through postgrowth annealing treatment. The existence of carbon, which has been verified by secondary ion mass spectrometry and x-ray photoelectron spectroscopy, was predicted to immobilize the oxygen in the interstitial site in ZnO thin films after annealing. Using first principles calculations, the formation of carbon-oxygen cluster defect in ZnO was found to be favorable and acts as a shallow acceptor. The cryogenic photoluminescence of the p-type carbon-doped ZnO thin films shows an additional peak located at 3.3564eV, which was attributed to the acceptor (carbon-oxygen cluster defect) bound exciton emission.

Pulsed Laser Deposition of High-Quality Manganite Thin Films at Low Background Pressures with in-situ Reflection High Energy Electron Diffraction

We have developed a methodological approach which allows the deposition of high quality fully oxidized manganite thin films at low oxygen background pressure (10-4 - 10-3 mbar) by PLD. In such oxygen conditions the thickness of the deposited layer can be controlled at the level of a single unit cell by the use in situ of a conventional Reflection High Energy Electron Diffraction diagnostic. We show that by decreasing the laser fluence it is possible to improve the oxidation process in thin films under low background oxygen pressure. Films deposited at low laser fluence (about 0.2 J/cm2) show a two-dimensional growth mode and possess very good transport properties without the necessity of any further post-growth annealing treatment. A physical model, based on the plume-background interaction as a primary mechanism of film oxidation during growth, is proposed to explain the experimental findings.

High-quality in situ manganite thin films by pulsed laser deposition at low background pressures

We show that by decreasing the laser fluence it is possible to improve the oxidation process in manganite thin films under low background oxygen pressure, allowing the in situ use of conventional Reflection High Energy Electron Diffraction diagnostic. Films deposited at low fluence (corresponding to a deposition rate per pulse lower than 10-2 unit cells per laser shot) show a two-dimensional growth mode and possess very good transport properties without the necessity of any further post-growth annealing treatment. A physical model, based on the plume-background interaction as a primary mechanism of film oxidation during growth, is proposed to explain the experimental findings.

Effects of postgrowth annealing treatment on the photoluminescence of zinc oxide nanorods

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.

Influence of postdeposition annealing on the structural and optical properties of cosputtered Mn doped ZnO thin films

The influence of postgrowth annealing on the structural and optical properties of rf cosputtered Mn doped ZnO thin films deposited on glass substrate at room temperature has been investigated. All as deposited Zn1−xMnxO films are highly textured, with the c axis of the wurtzite structure along the growth direction. The as grown films are in a state of compressive stress and a reduction in stress with postgrowth annealing treatment are observed. The band gap of Mn doped ZnO films (3.34eV) is slightly larger than the pure ZnO film (3.30eV) and is found to decrease with an increase in annealing temperature for all the samples. The optical dispersion of refractive index with photon energy in Zn1−xMnxO films with varying x and different annealing temperature is studied in the light of single oscillator and Pikhtin-Yas’kov [Sov. Phys. Semicond. 15, 81 (1981)] model, respectively.

Oxygen induced modifications in the growth of ultrathin iron films on Cu 3Au( [formula omitted

Codeposition of oxygen and Fe on Cu 3Au(0 0 1) has been investigated in order to look at the modifications of the growth regime induced by the chemical additive. Oxygen deeply modifies the growth modes both at relatively low temperature (150 K) and at room temperature (RT). The bilayer growth mode active at earlier stages of growth in absence of the surfactant element is suppressed. We concentrated our investigation on RT deposition. A relatively ordered, pseudomorphic iron film has been found only at low thickness (up to 4 ML). This film is covered with an oxygen layer, whose coverage and order depends on the post-growth annealing treatment. The annealed oxygen layer has been used as a surfactant for further Fe deposition cycles. During deposition, part of the oxygen remains trapped into the film. Restoration of the chemisorbed layer is obtained after annealing at moderate temperature (700 K). As the film thickness increases as a result of successive depositions, the film is first characterized by coexistence of pseudomorphic and bcc patches. The pseudomorphic patches seem stabilized not only by presence of the oxygen, but also by Cu and Au impurities in the film. For relatively large thickness (above 25 ML) bcc domains become majority.

Structural Characterization of SiF4, SiH4 and H2 Hot-Wire-Grown Microcrystalline Silicon Thin Films with Large Grains

ABSTRACTIn this paper, we present a comprehensive study of microcrystalline silicon thin film samples deposited by a novel growth process intended to maximize their grain size and crystal volume fraction. Using Atomic Force Microscopy, Raman spectroscopy, and x ray diffraction the structural properties of these samples were characterized qualitatively and quantitatively. Samples were grown using a Hot-Wire Chemical Vapor Deposition process with or without a post-growth hot-wire annealing treatment. During Hot-Wire Chemical Vapor Deposition, SiF4 is used along with SiH4 and H2 to grow the thin films. After growth, some samples received an annealing treatment with only SiF4 and H2 present. These samples were compared to each other in order to determine the deposition conditions that maximize grain size. Large microcrystalline grains were found to be aggregates of much smaller crystallites whose size is nearly independent of deposition type and post-annealing treatment. Thin films deposited using the deposition process with SiF4 partial flow rate of 2 sccm and post-growth annealing treatment had the largest aggregate grains ∼.5 µm and relatively high crystal volume fraction.

Molecular Beam Epitaxial Growth and Characterization of the Vertically Aligned InAs Quantum Dots Embedded in Al0.5Ga0.5As

In this paper we describe structural and optical properties of vertically aligned InAs quantum dots (QDs) embedded in Al0.5Ga0.5As. These aligned QDs were grown at 520°C in the Stranski-Krastanow growth mode of molecular beam epitaxy. Measurements by reflection high-energy electron diffraction and atomic force microscopy showed that both the size and density of these QDs increased with the number of stacking periods. The temperature dependence of the wavelength-integrated photoluminescence (PL) intensity revealed that the intense emission from excitons, which dominated the PL spectra at low temperatures, was easily thermally quenched by the large dots in the upper layers. To improve these unfavorable characteristics, we employed a size- and density-controlled growth procedure for the QDs in the upper layers, and succeeded in increasing the PL stability at high temperatures. We also studied a postgrowth annealing treatment for these aligned QD-structures, and found that it is effective to increase the PL intensity when the annealing is performed at around 570°C. By combining the controlled growth procedure with the postgrowth annealing treatment, we realized a strong excitonic emission of which the quenching temperature associated with an activation energy of 0.65 eV was as high as ∼300 K.

Thin films of CdTe/CdS grown by MOCVD for photovoltaics

A doped CdTe/CdS photovoltaic structure has been grown onto ITO coated glass substrates by metal organic chemical vapour deposition (MOCVD). CdS is nucleated from dimethylcadmium and ditertiarybutylsulphide at 290°C followed by CdTe growth from DMCd and diisopropyltelluride at 300–310°C. The grain size of the CdTe is approximately the same as the underlying CdS layer and with a strongly prefered (1 1 1) orientation. This has been achieved without post growth annealing or chemical treatment. Similarly, doping of both the CdS and CdTe layer have been achieved during growth to form a n–p junction. The dopant precursors are n-butylchloride for the CdS layer and dimethylaminoarsenic for the CdTe layer. Preliminary photovoltaic devices show good rectification, with an open circuit voltage of 0.14 V, and fill factors of 35–50%.

Extended X-ray absorption fine structure study of defects in Cl doped ZnSe

Local structures around the dopant in heavily Cl doped ZnSe films were investigated by extended X-ray absorption structure (EXAFS) analysis. Measurements were performed for a carrier saturated sample (with N d − N a = 3 × 10 19cm −3) without and with post-growth annealing treatment. In addition to the bond identified as the normally substituted Cl atoms at Se sites, the bond with the length of 0.28 nm was observed for the non-annealed sample. In the annealed sample, only a bond of 0.29 nm was observed. We consider the 0.28–0.29 nm-bond corresponds to the ClZn vacancy complex, which is formed by heavy doping or thermal treatment.

Optical and electro-optical investigation of low-temperature grown GaAs

We report on the effects of growth temperature, arsenic source and post growth annealing treatment on the optical properties of low temperature grown GaAs. We have determined the absorption coefficient over a wide spectral range of energies between 0.8 and 2.0 eV. Special interest was dedicated to the shape of the band edge absorption for the different growth conditions and post growth treatments. The band edge is sharper for samples grown with an As 4 source than for samples grown using As 2. However, the band edge becomes sharper due to the annealing process for all samples. A sharp band edge is essential for a high-contrast electro-optical modulator. We also measured the electro-absorption of a metal-semiconductor-metal structure based on annealed LT-GaAs and achieved a contrast ratio of 1:1.35 corresponding to an absorption change of 2300 cm −1.

Recipe to minimize Te precipitation in CdTe and (Cd,Zn)Te crystals

Using a quasichemical approach, the total native defect concentration and the minimum deviation in stoichiometry have been calculated in CdTe crystals as a function of the Cd pressure at various temperatures. With this knowledge, CdTe and (Cd,Zn)Te wafers have been subjected to postgrowth step annealing treatment under conditions such that the crystals are in equilibrium with a Cd or (Cd,Zn) vapor corresponding to the minimum in deviation from stoichiometry at each annealing temperature. The step annealed CdTe and (Cd,Zn)Te wafers have been examined under infrared microscopy and have shown significant reduction in the concentration of Te precipitates, whereas the unannealed wafers have had numerous Te precipitates distributed throughout the bulk. HgCdTe epitaxial films have been grown on the step annealed CdTe and (Cd,Zn)Te wafers as well as on unannealed wafers from the same boule. Examination of the cross sections of the epitaxial films indicates appearance of Te precipitates in films grown on unannealed substrates, whereas no Te precipitation was evident in films grown on the annealed substrates leading to the inference that the occurrence of Te precipitates in the (Hg,Cd)Te films is possibly related to the presence of Te precipitates in the substrates. Thermal migration of Te under a temperature gradient during step annealing is suggested as a possible mechanism in the elimination of larger size Te precipitates whereas the extremely fine precipitates (<1 μm) appear to need in-diffusion of metal vapor for their elimination.

Photoluminescence measurements in Ge-doped p-type Ga0.60Al0.40As

Results of photoluminescence and Hall effect measurements of p-type Ge-doped Ga0.60Al0.40As grown by liquid phase epitaxy are reported. The effective segregation coefficient for Ge for growth at 785 °C is estimated to be ∼2×10−3. The photoluminescence spectra at 5.5 K are characterized by two edge emission bands at ∼1.91 and ∼1.88 eV and a broadband at ∼1.55 eV. The edge emission bands are identified to be donor-acceptor pair recombination bands involving the same donor but two different acceptors. The ionization energy of the donor is estimated to be 50–60 meV and the acceptor ionization energies are estimated to be ∼60 and ∼100 meV for the 1.91- and 1.88-eV bands, respectively. The deep acceptor is believed to involve a background impurity, most likely C or Si. It is suggested that the 1.55-eV band arises from a next-nearest neighbor complex consisting of Ge on an arsenic site and an As vacancy. Post-growth annealing treatment at 830 °C is found to decrease the photoluminescence intensity suggesting the presence of annealing induced nonradiative centers.

Growth and Characterization of Spinel Single Crystals for Substrate Use in Integrated Electronics

An extensive study has been made on the growth and characterization of single-crystal magnesium aluminate spinel. This material has received attention as a superior substrate for epitaxial silicon-integrated electronic devices. Large spinel single crystals within the compositional range MgO:Al2O3–MgO:3Al2O3, have been successfully grown by flame fusion using a modern three-tube post-mix-type Verneuil burner. Substrate wafers of (111), (100), and (110) orientations were prepared for silicon epitaxy. Surface preparation of the substrates was studied, including mechanical polishing, hydrogen annealing, and chemical etching. The composition, lattice parameter, crystalline perfection, mechanical hardness, thermal stability, and dielectric properties of the spinel single crystals have been characterized by various techniques in an effort to obtain a basic understanding for substrate use of this material system. Some trends in the dependence of these properties on composition were observed. In particular, the thermal stability increases with decreasing Al content in the spinel system. It was found that the spinel crystals with alumina content up to MgO:2.5Al2O3 are thermally stable and exhibit no cracking and/or exsolution behavior to at least 1200°C, which is the temperature commonly used for fabricating many silicon devices. However, the ease of growth and the mechanical stability decrease with decreasing Al content in spinel. A post-growth annealing treatment enhances the mechanical stability and prevents formation of cracks during cutting. Substrates within the compositional range of MgO:1.5Al2O3–MgO:2.5Al2O3 are suitable for silicon epitaxy, and the choice of the optimum substrate composition depends on the specific conditions for processing of the silicon devices.