Effect Of Post-growth Thermal Annealing Research Articles

Ex-Situ Thermal Treatment Effects on the Temperature Dependent Carriers Dynamics in InAs/InGaAs/GaAs Quantum Dots

The effects of post-growth thermal annealing of InAs QD with the high in-content strain reducing layer (SRL) on the temperature dependent PL properties have been investigated. The as-grown QD have shown an atypical behavior manifested by a sigmoidal emission energy and V-shaped linewidth evolution with temperature. These behaviors have been progressively glossed by subjecting the structure to post growth annealing at 650 °C and 750 °C for 50 s. The results are discussed in the frame of the localized states ensemble model, which reveals that carriers transfer take place by thermal activation to the continuum states of the strain-reducing layer and subsequent redistribution.

The effects of post-growth thermal annealing on the structural and electrical properties of RF-magnetron sputtered ZnO

The effects of two thermal annealing steps on the crystallographic orientation, film stress and crystal domain size in RF-magnetron sputtered ZnO thin films for ultrasonic transducers were investigated. As-grown films contained high levels of compressive stress introduced by the sputtering process, as well as low (002) crystal orientation selectivity. It was shown that residual stress could be eliminated by post-growth annealing at temperatures of 400°C or higher, however, that annealing process also lead to a reduction in the desired (002) crystal orientation selectivity. The use of the relatively low temperature (250°C) in-situ anneal in the sputtering chamber in an oxygen-rich environment was found essential for prompting grain growth and recrystallisation, resulting in (002) textured ZnO films that have high electrical resistivity.

The annealing temperature dependence of anatase TiO2 thin films prepared by the electron-beam evaporation method

In this paper, we report on titanium dioxide (TiO2) thin films deposited by an electron beam evaporation method on quartz glass substrates (15 × 15 × 2 mm3 in size), followed by post-annealing at 300 °C to 600 °C for an annealing time of up to 2 h. The substrate temperature during the film deposition was kept at 150 °C. The effect of post-growth thermal annealing on the structural and optical properties of TiO2 thin films were systematically studied as a function of annealing temperature. We found that the as-deposited TiO2 films are amorphous in structure, while the films started to crystallize into the anatase phase when annealed at temperatures ≥450 °C. An increase in annealing temperature results in a decrease of transmittance percentage and also in optical band gap energy. The refractive indices of the films were evaluated from the measured transmittance spectra using the envelope method. An increase in the refractive index with an increase of annealing temperature was observed.

Phase Transformation and Optical Properties of Annealed Hydrothermally Synthesized ZnO/ZnCr2O4 Nanocomposites

The effect of postgrowth thermal annealing on ZnO/ZnCr2O4 nanocomposites synthesized by hydrothermal method has been thoroughly investigated. XRD data have clearly revealed the transformation of spinel cubic phase of ZnCr2O4 to wurtzite phase ZnO and indicated the incorporation of Cr in ZnO lattice with annealing at high temperatures. Photoluminescence spectra have shown a strong dependence of emission on annealing temperatures. This work demonstrated the unique and simple route to fabricate Cr‐doped ZnO and tuning of the luminescence with annealing temperature. Thus, the work has immense potential for optoelectronic and photovoltaic applications.

Effects of post-growth thermal annealing on room temperature pulsed laser deposited ZnO thin films

ZnO thin films have been elaborated by Pulsed Laser Deposition (PLD) onto glass substrates, at room temperature. Post-growth annealing treatment was applied to the films in air background. Morphology, chemical composition and optical characteristics of ZnO films were evaluated as a function of annealing temperature. With the increase in annealing temperature, the 002 axis peak intensity is improved and optical band gap is shifted to lower values. The PL peak positions shift are associated at stoichiometry change, it is confirmed with the experimental results. The films elaborated with this experimental setup could be used to applications in short wavelength optoelectronic devices, chemical and optical sensors.

Molecular beam epitaxial growth and characterization of nitrogen δ-doped AlGaAs/GaAs quantum wells

The authors carry out δ-doping at the middle of AlGaAs/GaAs quantum wells employing molecular beam epitaxy and varying the nitrogen coverage up to 0.5 monolayers. Transmission electron micrography and x ray diffraction indicate the introduction of a nitrogen δ-doped layer with precisely controlled position and nitrogen coverage. Photoluminescence spectra obtained for the samples show clear redshift of spectral peak positions depending on the amount of nitrogen, suggesting the band structure is modified by the δ-doping. The growth can be carried out at a substrate temperature of 560 °C. The growth temperature, which is high compared with that of standard dilute nitride compounds, could suppress the formation of growth-induced defects, resulting in the weak effect of post-growth thermal annealing on the characteristics of room-temperature photoluminescence.

The influence of post-growth annealing on the optical properties of InAs quantum dot chains grown on pre-patterned GaAs(100)

We report on the effect of post-growth thermal annealing of [011]-, []-, and [010]-oriented quantum dot chains grown by molecular beam epitaxy on GaAs(100) substrates patterned by UV-nanoimprint lithography. We show that the quantum dot chains experience a blueshift of the photoluminescence energy, spectral narrowing, and a reduction of the intersubband energy separation during annealing. The photoluminescence blueshift is more rapid for the quantum dot chains than for self-assembled quantum dots that were used as a reference. Furthermore, we studied polarization resolved photoluminescence and observed that annealing reduces the intrinsic optical anisotropy of the quantum dot chains and the self-assembled quantum dots.

Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure

Effect of post-growth thermal annealing within the temperature range 200 to 430 oC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD) heterostructure was studied. Annealing at lower temperatures (Tann 270 oС) results in an increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the first being caused by excitonic transitions in QDs and the second one being connected with the defect complex including a column II vacancy. The effect is supposed to be caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures (Tann > 270 oС) stimulates a decrease of the QD photoluminescence band intensity and up to 100 meV blue shift of its peak position. The former is explained by generation of extended defects and reduction of the QD density. The blue shift observed at 370-430 oС is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD density. It is found that the energy of excitonic transitions in the wetting layer does not change upon annealing. Lower thermal stability of QDs as compared to that of the wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via vacancies. The presence of column II vacancies in the wetting layer is proved by characteristics of defect-related PL band and its excitation spectra.

Influence of thermal annealing on the electron emission of InAs quantum dots containing a misfit defect state

We have investigated the effect of postgrowth thermal annealing on the electron emission from InAs quantum dots (QDs) containing a misfit-related defect state induced by strain relaxation. Additional carrier depletion in the GaAs bottom layer near the QD, caused by the defect state, can effectively suppress electron tunneling from the QD, leading to the observation of a thermal emission from the QD electron ground state to the GaAs conduction band with a large emission energy of 213 meV, in contrast to defect-free nonrelaxed QDs in which an emission of 58 meV from the QD electron ground state to first excited state is observed. The emission energy is reduced to 193 meV and to 164 meV after annealing at 650 and 700 °C for 1 min, respectively. This emission energy reduction is correlated with the photoluminescence blueshift which is attributed to the interdiffusion of atoms across the QD interface. The electron emission from the QD first excited and ground states is found to be a thermal emission at high temperatures and a tunneling emission at low temperatures. The tunneling energy barrier is found to be comparable to the thermal emission energy, supporting a thermal emission to the GaAs conduction band. This study illustrates a significant effect of a defect state on the electron-emission process in the QDs, suggesting the possibility of modifying the electron emission time of the QDs by purposely introducing a deep defect state.

Annealing of self-assembled InAs/GaAs quantum dots: A stabilizing effect of beryllium doping

We investigated the effects of postgrowth thermal annealing on optical properties of beryllium-doped InAs quantum dot (QD) heterostructures grown by molecular beam epitaxy. Thermal annealing induced a blueshift of up to 200 meV in light emission from an undoped sample, while a sample having GaAs layer heavily doped with beryllium on top of the QD region exhibited a much smaller blueshift. This phenomenon is interpreted as due to suppression of annealing-induced In/Ga interdiffusion.

Post-growth thermal annealing of high N-content GaAsN by MOVPE and its effect on strain relaxation

We report on the further investigation of the effect of post-growth thermal annealing on optical and structural properties of the high N-content GaAs 0.949N 0.051 layer grown on a GaAs(0 0 1) substrate by metalorganic vapor phase epitaxy (MOVPE). Photoluminescence (PL) spectroscopy was performed to measure the energy positions of the near band edge excitonic emission. The high-resolution X-ray diffraction and Raman spectroscopy were conducted to examine the lattice parameters, also the N concentration of the layers annealed at 650 °C with different annealing times. The layer subjected to thermal anneals exhibits an increasing of N incorporation, a strain relaxation and a blue shift of the PL peak energy. For such high N-containing layer, the interstitial N atoms generated in the growth process may replace the As atoms/vacancies on the lattice sites to become more stable substitutional N atoms through the thermal annealing process, which will produce the strain relaxation, in addition to an improvement of the alloy uniformity. Our results suggest the two major effects: (i) the reorganization of N and (ii) the strain relaxation in the GaAsN layer that can be explained the blue shift in the PL peak energy after annealing.

Effect of rapid thermal annealing on the properties of GaNAs thin films grown by molecular beam epitaxy

High-resolution X-ray diffraction (HRXRD) and micro-Raman scattering probe have been used to study the effect of post-growth thermal annealing on GaNAs thin films grown on (0 0 1) GaAs substrates by solid-source molecular beam epitaxy assisted with a nitrogen radio-frequency plasma source. X-ray reciprocal space mapping shows slightly in-plane strain relaxation after annealing, however, the vertical lattice constant measured by HRXRD from the (0 0 4) atoms planes shows a shrinkage, which is contrary to both the relaxation of tensile strain and the decreasing of nitrogen due to out-diffusion. The ratio of Raman intensity between the nitrogen-localized vibration mode and the GaAs-like LO is found to increase as a function of annealing temperature, which provides evidence for the increase of substitutional N As atoms at elevated temperatures. The anneal-induced vertical lattice shrinkage can be explained in term of the redistribution of nitrogen bonding configurations among the substitutional N As, interstitial N–N, and the N–As complex.

Annealing in GaInNAs system

The effect of post-growth thermal annealing on crystallinity and bandgap in GaInNAssystems is systematically investigated. Annealing greatly improves the crystallinity ofGaInNAs and also blue shifts the bandgap. This improved crystallinity is due to theelimination of non-radiative centres, and is not directly related to the blue shift in thebandgap. The blue shift may be closely related to the red shift in the bandgap seen when Nis added to GaInNAs; if so, this would provide a clue to the reasons for the red shift in thebandgap of III–N–V alloys. The unexpected behaviour of the bandgap in GaInNAs isinvestigated by examining the bonds through optical phonons. We have found that thebandgap varies with the strength or length of the GaN bonds in GaInNAs. Annealingchanges atomic positions in a unit cell of GaInNAs and thus blue shifts its bandgap.

Effects of post-growth annealing on the optical properties of self-assembled GaAs/AlGaAs quantum dots

Photoluminescence spectroscopy is used to analyze the effects of post-growth thermal annealing on the electronic properties and capture processes of self-assembled GaAs/AlGaAs quantum dots grown by modified droplet epitaxy. Post-growth annealing induces deep changes in the electronic structure of the quantum dots material, modifying both capture processes and photoluminescence quenching channels. The optical data, together with theoretical models, are used to quantify such structural and electronic modifications in the annealed materials.

Effects of post-growth thermal annealing on the indium aggregated structures in InGaN/GaN quantum wells

Size and distribution of indium-rich quantum dots (QDs) are important parameters for improving photon emission efficiency of InGaN/GaN quantum well (QW) structures. Our results showed that post-growth thermal annealing of such a sample with temperature ranging from 800°C to 900°C led to a better confinement of indium-rich clusters near InGaN QW layers. Transmission electron microcopy (TEM) and energy filter TEM results manifested that the sizes of indium-rich QDs were reduced with increasing annealing temperature. Also, the size homogeneity was improved. Quasi-regular arrays of indium-rich QDs embedded in InGaN QWs were observed in the sample of 900°C annealing. X-ray diffraction also showed the enhancement of InN relative intensity. Photoluminescence measurements revealed blue shifts of photon emission spectral peak, indicating stronger quantum confinement after thermal annealing. However, such a process of regular QD formation disappeared when annealing temperature was increased to 950°C. In this situation, coarsening of indium-rich clusters occurred and their distribution became irregular.

Low-temperature growth of GaAs and AlGaAs by MBE and effects of post-growth thermal annealing

GaAs and Al 0.3Ga 0.7As films were grown by molecular beam epitaxy (MBE) using substrate temperatures between 350 and 620°C. These samples were produced under otherwise identical conditions using either elemental As (tetrameric As) or GaAs (dimeric As) as the arsenic source, and using fixed fluxes of either Si or Sn, corresponding to an atomic dopant concentration of approximately 3 × 10 18 cm −3. The samples were characterized by Hall effect measurements at 300 and 77 K, and by photoluminescence measurements at 300 K. In the as-grown samples, the free carrier concentration, n, the Hall mobility, μ, and the luminescence intensity all decrease as the substrate temperature decreases below 450°C The lower values of n, μ, and luminescence intensity can, however, be improved by thermally annealing the films at 850°C for between 5 and 30 s. After the anneal, values of n, μ, and photoluminescence intensity are comparable to those observed in unannealed films grown at 620°C. For a given growth temperature, the measured electrical and optical properties and the annealing behavior of these samples are very similar, whether Si or Sn is used as the dopant, and whether a tetrameric or dimeric As source is used.

Effect of post-growth annealing on patterned GaAs on silicon

We have investigated the effect of post-growth thermal annealing on the defect surface of patterned GaAs on silicon. Significant improvements in defect structure of the patterned GaAs were observed by transmission electron microscopy (TEM) after short anneal times (15 min) at temperatures as low as 750 °C. At anneal temperatures of 950 °C, long-range single-crystal lateral regrowth of the original polycrystalline GaAs deposited over the amorphous patterning mask was observed. Micro-Raman spectroscopic characterization of the GaAs grown on the oxide/nitride mask corroborated the TEM results by showing a significant improvement in the crystalline quality of patterned GaAs on silicon following post-growth annealing.