Post-growth Annealing Treatment Research Articles

Selective Control of Phases and Electronic Structures of Monolayer TaTe2.

Transition metal dichalcogenide (TMDC) films exhibit rich phases and superstructures, which can be controlled by the growth conditions as well as post-growth annealing treatment. Here, the selective growth of monolayer TaTe2 films with different phases as well as superstructures using molecular beam epitaxy (MBE) is reported. Monolayer 1H-TaTe2 and 1T-TaTe2 films can be selectively controlled by varying the growth temperature, and their different electronic structures are revealed through the combination of angle-resolved photoemission spectroscopy measurements (ARPES) and first-principles calculations. Moreover, post-growth annealing of the 1H-TaTe2 film further leads to a transition from a superstructure to a new 2 × 2 superstructure, where two gaps are observed in the electronic structure and persist up to room temperature. First-principles calculations reveal the role of the phonon instability in the formation of superstructures and the effect of local atomic distortions on the modified electronic structures. This work demonstrates the manipulation of the rich phases and superstructures of monolayer TaTe2 films by controlling the growth kinetics and post-growthannealing.

Valence Band Dispersion in Bi-Doped (Ga,Mn)As Epitaxial Layers

The impact of incorporating Bi into (Ga,Mn)As layers on their electronic, band structure, magnetic, and structural properties has been studied. The low-temperature molecular beam epitaxy technique was employed to grow homogeneous (Ga,Mn)(Bi,As) layers with high structural perfection. Post-growth annealing treatment resulted in improved structural and magnetic properties, as well as an increase in the hole concentration in the layers. Modulation photoreflectance spectroscopy confirmed modifications to the valence and split-off band in the (Ga,Mn)(Bi,As) layers. The experimental results are consistent with the valence band model of hole-mediated ferromagnetism in the layers. The (Ga,Mn)(Bi,As) compound combines the properties of (Ga,Mn)As and Ga(Bi,As) ternary compounds, offering the possibility of tailoring the bandgap structure to the requirements of novel device functionalities for future spintronic and photonic applications.

Enhancing the p-type conductivity of pure phase SnO via stoichiometry control and annealing

While n-type transparent conducting oxides (TCOs) have been well developed and widely used in optoelectronic devices, the availability of high-performance p-type TCOs is still scarce which in turn hinders the further advancement of oxide based transparent optoelectronic devices. Unlike the most widely used p-type NiO, Tin monoxide (SnO) has a relatively delocalized valence band, which leads to a high hole mobility of >1 cm2/Vs. In this work, we enhance the p-type conductivity of SnO thin films by tunning the oxide stoichiometry through adjusting the growth conditions and post-growth rapid thermal annealing (RTA) treatment. Specifically, we synthesized nominally undoped Sn-rich SnO (SnO1-δ) thin films by co-sputtering an SnO target and a metallic Sn target and control the film stoichiometry by varying the sputter power of the Sn target so that SnO1-δ films with excess Sn (δ) varying from 0 to ∼5% was incorporated. We find that the addition of dilute amount of excess Sn is an effective approach to regulate the film stoichiometry so that a low p-type resistivity can be achieved after post-growth RTA. All as-grown films are amorphous and after RTA exhibit a polycrystalline tetragonal structure, with p-type resistivity decreases with increasing δ. However, metallic β-Sn clusters are formed for films with δ≳ 0.05. The presence of these β-Sn clusters results in a degraded optical transmittance by ∼10% in the visible range. With δ∼0.03 in the as-grown film, a p-type pure phase SnO film with a low resistivity of ∼0.5 Ω−cm, and a decent visible transparency of ∼60% is achieved after 300 °C RTA in N2. This p-type SnO has a wide band gap of 2.8 eV with a high valence band maximum (VBM) located in the range of 4.6–4.8 eV below the vacuum level, making it suitable for many device applications, particularly as hole transport layers in optoelectronic devices.

Effect of annealing temperature on the interface state density of n-ZnO nanorod/p-Si heterojunction diodes

Abstract The effect of post-growth annealing treatment of zinc oxide (ZnO) nanorods on the electrical properties of their heterojunction diodes (HJDs) is investigated. ZnO nanorods are synthesized by the low-temperature aqueous solution growth technique and annealed at temperatures of 400 and 600°C. The as-grown and annealed nanorods are studied by scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. Electrical characterization of the ZnO/Si heterojunction diode is done by current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. The barrier height (ϕ B), ideality factor (n), doping concentration and density of interface states (N SS) are extracted. All HJDs exhibited a nonlinear behavior with rectification factors of 23, 1,596 and 309 at ±5 V for the as-grown, 400 and 600°C-annealed nanorod HJDs, respectively. Barrier heights of 0.81 and 0.63 V are obtained for HJDs of 400 and 600°C-annealed nanorods, respectively. The energy distribution of the interface state density has been investigated and found to be in the range 0.70 × 1010 to 1.05 × 1012 eV/cm2 below the conduction band from E C = 0.03 to E C = 0.58 eV. The highest density of interface states is observed in HJDs of 600°C-annealed nanorods. Overall improved behavior is observed for the heterojunctions diodes of 400°C-annealed ZnO nanorods.

Realization of high-quality RF sputtered ZnMgO (x=15%) thin films by post-growth annealing treatment

ZnMgO, a direct wide-bandgap material with high exciton binding energy, has received widespread research attention in optoelectronics. However, its inherent n-type doping facilitates defect-related visible emission, which hampers the required luminescence properties in the UV region. Also, ZnMgO requires post-growth treatment which releases accumulated strain energy, modulates defect states, expands crystallite size, and augments ad-atoms interdiffusion, thereby affecting its bandgap. This work reports the influence of post-deposition annealing parameters like temperature and time on RF sputtered Zn0.85Mg0.15O thin films using rapid thermal annealing. The study showed that high temperature annealing aided Mg incorporation in ZnO lattice together with an increase in the grain size due to aggregation of grains. Highest activation energy, 78 meV, and narrowest line width, 34 meV, for D°X peak of 900 °C annealed film was obtained from photoluminescence study. Increasing annealing temperature also showed dissociation of bound exciton peaks into corresponding localized and delocalized exciton states.

Impact of In Situ Annealing Time on CdTe Polycrystalline Film and Device Performance

We studied the impact of in situ post-growth annealing process on cadmium telluride (CdTe) polycrystalline thin film in this work. Samples of different annealing times have been characterized by x-ray diffraction (XRD) and a scanning electron microscope (SEM). The optoelectronic properties of CdTe film were deeply studied with light I–V testing, external quantum efficiency (EQE) and photoluminescence (PL)/time-resolved photoluminescence (TRPL) measurements. It is found that the in situ post-growth annealing treatment has a great effect on cadmium sulfide (CdS)/CdTe intermixing as well as interface pinholes. Elementary correlations between annealing time, film and junction morphology, carrier lifetime and device performance were investigated, and the annealing time turns out to be crucial to CdTe device performance. Solar cells with CdTe polycrystalline thin film annealed in situ at 550°C/550°C for 10 min show the best performance. With the help of precisely controlled annealing time, better CdTe film microstructure and morphology can be realized after in situ post-growth annealing.

Evidence of quantum dot size uniformity in strain-coupled multilayered In(Ga)As/GaAs QDs grown with constant overgrowth percentage

Strain coupled multi-layer quantum dot (QD) structures are limited due to their non-uniform dot size distribution, as-grown defects and dislocations. Even with these limitations, they are subject of interest because of high optical and device efficiency. In this work, we propose a modified growth strategy in which monolayer (ML) deposition of QDs is varied based on reflection high energy electron diffraction (RHEED) pattern to achieve defect free structure with uniform dot size distribution. The overgrowth percentage, i.e. monolayer deposition above critical thickness (minimum thickness required for transition from two-dimensional (2D) to three-dimensional (3D) confined structures) was kept constant in modified strategy. To verify this claim, we have compared it with conventional strain coupled multilayer structure and demonstrated improved optical performance and device characteristics. Photoluminescence spectroscopy exhibited monomodal QD distribution in strain-coupled multi-stacked heterostructure grown by the proposed strategy. Power dependent photoluminescence (PL) measurement confirms existence of single ground state peak. The proposed strategy yielded a defect free structure and better carrier capture rate compared to conventional multilayer QDs. The spectral response observed from proposed quantum dot infrared photodetector (QDIP) exhibited multiple peaks in mid-IR region (3.92µm and 4.54µm), which is one of the essential requirements for fabricating infrared detectors or focal plane arrays. Post-growth thermal annealing treatment at various temperatures (650–800°C) were performed to verify the change in activation energy, ground-state emission peak, and thermal stability. Negligible change in the ground-state PL emission peak (< 2%) and thermal activation energy (< 2%) were observed up to 750°C. The full width at half maximum (FWHM) values of the ground-state peak of the annealed samples exhibited a similar trend. High-temperature thermal stability was achieved probably due to high strain field hindering material interdiffusion and improved quantum confinement realized from uniform QD sizes. The modified growth strategy used in this paper is helpful not only for improved optical characteristics, but also for useful device performances, such as a night vision camera in defense areas, cancer detector in medical applications and fabrication of thermal imagers; focal plane arrays.

Luminescence Enhancement and Enlarged Dirac Point Shift of MoS2/Graphene Hetero-Structure Photodetectors With Postgrowth Annealing Treatment

Enhanced photoluminescence intensities are observed for the MoS2 film prepared by chemical vapor deposition (CVD) with an additional 850 °C postgrowth annealing procedure under sulfur-rich environment. The enlarged Dirac point shift of the MoS2/graphene hetero-structure photodetectors with the postgrowth annealing procedure suggests that more photoexcited electrons will transport to the graphene and effectively higher n-type doped the channel. The direct 2-D crystal hetero-structure growth by using CVD has avoided the complicated procedure of sequential attachment of different 2-D crystals, which is advantageous for the practical application of 2-D crystal heterostructures.

Novel Quaternary Dilute Magnetic Semiconductor (Ga,Mn)(Bi,As): Magnetic and Magneto-Transport Investigations

Magnetic and magneto-transport properties of thin layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor grown by the low-temperature molecular-beam epitaxy technique on GaAs substrates have been investigated. Ferromagnetic Curie temperature and magneto-crystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Post-growth annealing treatment of the layers has been shown to enhance the hole concentration and Curie temperature in the layers. A significant increase in the magnitude of magneto-transport effects caused by incorporation of a small amount of Bi into the (Ga,Mn)As layers, revealed in the planar Hall effect (PHE) measurements, is interpreted as a result of enhanced spin-orbit coupling in the (Ga,Mn)(Bi,As) layers. A two-state behaviour of the planar Hall resistance at zero magnetic field provides its usefulness for applications in non-volatile memory devices.

Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG)

Zinc Oxide (ZnO) provides unique properties owing to its wide bandgap, large resistivity range and possibility to tune the physical properties. The surfactant assisted carbon doping was made possible due to the lowering of surface energy. The ZnO and carbon doped ZnO (C-ZnO) nanowires fabricated by hydrothermal process, Poly Ethylene Glycol (PEG) is used as surfactant in hydrothermal synthesis followed by post growth annealing treatment at 600 °C–700 °C. At 5%–10% of diluted PEG carbon is doped in ZnO. The crystallinity, structural morphology and elemental composition analysis for ZnO and C-ZnO nanowires were carried out using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques respectively. Carbon doping in ZnO nanowires in the presence of different percentage of surfactant is explained by calculating the change in surface energy with respect to change in PEG molecule concentration. It was found that the surface energy per molecule modulates from 3.92 × 10−8 J/m2 to 8.16 × 10−7 J/m2 in the PEG concentration range between 5% and 10%. Our results provides a new theoretical calculations, implemented on real system, to observe the details of PEG-assisted Carbon doping in II-VI semiconductor nanowires.

Cathodoluminescence study of Mg activation in non-polar and semi-polar faces of undoped/Mg-doped GaN core–shell nanorods

Spectrally and spatially resolved cathodoluminescence (CL) measurements were carried out at 80 K on undoped/Mg-doped GaN core–shell nanorods grown by selective area growth metalorganic vapor phase epitaxy in order to investigate locally the optical activity of the Mg dopants. A study of the luminescence emission distribution over the different regions of the nanorods is presented. We have investigated the CL fingerprints of the Mg incorporation into the non-polar lateral prismatic facets and the semi-polar facets of the pyramidal tips. The amount of Mg incorporation/activation was varied by using several Mg/Ga flow ratios and post-growth annealing treatment. For lower Mg/Ga flow ratios, the annealed nanorods clearly display a donor–acceptor pair band emission peaking at 3.26–3.27 eV and up to 4 LO phonon replicas, which can be considered as a reliable indicator of effective p-type Mg doping in the nanorod shell. For higher Mg/Ga flow ratios, a substantial enhancement of the yellow luminescence emission as well as several emission subbands are observed, which suggests an increase of disorder and the presence of defects as a consequence of the excess Mg doping.

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.

Controlling preferred orientation and electrical conductivity of zinc oxide thin films by post growth annealing treatment

We report the microstructural evolution of the preferred orientation and electrical conductivity of zinc oxide (ZnO) thin films prepared by ion beam sputtering. Elastic recoil detection analysis results showed 0.6at% H in as-deposited film which decreased to 0.35at% in air annealed film due to H diffusion. XRD results showed that the preferred orientation can be tuned by selecting annealing conditions. Vacuum annealed films exhibited (100) orientation, whereas air annealed film showed (002) orientation. The annealing conditions caused a dramatic increase in the resistivity of air annealed films (∼106Ωcm), whereas vacuum annealed films showed lower resistivity (∼10−2Ωcm). High resistivity in air annealed film is attributed to the lack of hydrogen interstitials and hydrogen-oxygen vacancy complexes. Raman results supported the XRD results which demonstrated that annealing assisted in recovery of the crystalline disorder in as-deposited films. Air annealed film exhibited the highest optical transmission (89.7%) in the UV–vis region compared to as-deposited and vacuum annealed films (∼85%). Optical bandgap was found to vary between 3.11eV and 3.18eV in as-deposited and annealed films, respectively. The bandgap narrowing is associated with the intrinsic defects which introduced defect states resulting in band tail in ZnO films.

Effect of Misfit Strain in (Ga,Mn)(Bi,As) Epitaxial Layers on Their Magnetic and Magneto-Transport Properties

Effect of misfit strain in the layers of (Ga,Mn)(Bi,As) quaternary diluted magnetic semiconductor, epitaxially grown on either GaAs substrate or (In,Ga)As buffer, on their magnetic and magneto-transport properties has been investigated. High-resolution X-ray diffraction, applied to characterize the structural quality and misfit strain in the layers, proved that the layers were fully strained to the GaAs substrate or (In,Ga)As buffer under compressive or tensile strain, respectively. Ferromagnetic Curie temperature and magneto-crystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Post-growth annealing treatment of the layers has been shown to enhance the hole concentration and the Curie temperature in the layers. (Less)

Structural and optical study of Ce segregation in Ce-doped SiO1.5 thin films

Cerium doped SiO1.5 thin films fabricated by evaporation and containing silicon nanocrystals were investigated by atom probe tomography. The effect of post-growth annealing treatment has been systematically studied to correlate the structural properties obtained by atom probe tomography to the optical properties measured by photoluminescence spectroscopy. The atom probe results demonstrated the formation of Ce-Si rich clusters upon annealing at 900 °C which leads to a drastic decrease of the Ce-related luminescence. At 1100 °C, pure Si nanocrystals and optically active cerium silicate compounds are formed. Consequently, the Ce-related luminescence is found to re-appear at this temperature while no Si-nanocrystal related luminescence is observed for films containing more than 3% Ce.

A new approach to thin-film SnS PV using MOCVD

SnS thin films were deposited by an inline metal organic chemical vapour deposition process using tetramethyltin and diethyldisulfide as precursors. A N2/H2 carrier was used with pre-mixing of the precursors before overhead injection into the deposition chamber. NSG AB soda lime glass was used as the substrate with area of 50 × 50 mm2. The resulting SnS films had calculated band gaps between 1.3 and 1.5 eV. Scanning electron microscopy showed relatively large grains ranging from 0.5 to 1 μm across for a SnS film sample deposited at 556–558 °C. X-ray diffraction confirmed the films to be SnS, but with small concentrations of impure phases such as Sn2S3. Post-growth annealing treatment in a N2 atmosphere at 435 °C using SnCl2/MeOH solution at different molar concentrations only showed changes to the film at 0.05 M. The 0.05 M SnCl2/MeOH treatment was aggressive with blistering and etching occurring.

InAs/InGaP quantum dot solar cells with an AlGaAs interlayer

We studied the growth of InAs/InGaP quantum dot (QD) solar cells with near ideal bandgaps for intermediate band solar cells. Using a solid-source molecular beam epitaxy system, the evolution of InAs QDs grown on an InGaP buffer layer was examined as a function of InAs coverage and growth temperature. QDs were initiated with defective clusters for InAs deposition as small as 0.3 monolayer. The cross diffusion of In atoms and As–P exchange between InAs and the InGaP buffer layer could be responsible for the formation of coherent QDs and large defective clusters. An AlGaAs interlayer was inserted between InAs and the InGaP buffer layer to prevent the surface exchange reactions between InAs and InGaP. As a result, InAs QDs with a density of 1×1011cm−2 have been demonstrated with stronger photoluminescence emission compared to those with the GaAs interlayer. This growth technique was applied to fabricate InAs/InGaP QD solar cells. Post-growth annealing treatment was also investigated to improve the performance of QD solar cells.

Grain size and stoichiometry control over RF-sputtered multiferroic BiFeO3 thin films on silicon substrates

This work reports the morphological and chemical characterization of multiferroic BiFeO3 polycrystalline thin films grown on Si(111) by RF-sputtering. Results are shown for a large set of samples and a wide array of experimental techniques, including imaging (atomic/piezoresponse force microscopy) and spectroscopic (μ-Raman, X-ray photoemission, X-ray diffraction) probes. Through growth and post-growth annealing treatment, a fine control over stoichiometry, grain size, grain orientation, crystal order and surface roughness is achieved. In particular, the grain size can be tailored from nanocrystals to large micrometric plates as a function of the annealing temperature. For the optimal stoichiometric sample, an additional X-ray absorption and magnetic circular dichroism analysis has been carried out, which provides high quality spectra comparable with epitaxial films and further proves the expected strong local antiferromagnetic order.

Exploiting the dynamic Sn diffusion from deformation of FTO to boost the photocurrent performance of hematite photoanodes

Herein, we report on exploiting effective Sn diffusion from the underlying FTO (F:SnO2) substrate during post-growth annealing treatment to boost the photocurrent performance of α-Fe2O3 photoanodes. Conventionally, hematite photoanodes are activated at high temperature (800°C>T>650°C) after initial low temperature annealing. The purpose of such activation is to utilize the diffusion of Sn ions leached from FTO deformation for improving the electronic conductivity and hence the photocurrent response of hematite photoanodes. However, the pre-formed oxide layer (in the first annealing) on the surface of FTO creates obstacle for diffusion of Sn ions during FTO deformation. To overcome this difficulty and to exploit Sn diffusion, we employed a direct single-step annealing of as-grown iron-coated FTO electrodes at 800°C for short duration followed by quenching in air. Such activation greatly improved the water oxidation photocurrent response of hematite by 37% on account of increased Sn diffusion which was confirmed from X-ray photoelectron spectroscopy. Moreover, the onset of photocurrent is shifted cathodically meaning that the water oxidation reaction proceeds at lower applied bias. Higher Sn content increased the electron donor concentration and hence improved the charge transfer kinetics of hematite as studied from EIS and Mott–Schottky measurements. Interestingly, despite higher Sn diffusion, the loss of FTO conductivity was minimal and the structural ordering was highest in one-step-activated hematite compared to conventionally activated hematite photoanodes as studied from XAFS analysis.

Effect of post-growth annealing treatment on interfacial misfit GaSb/GaAs heterostructures

Post-growth annealing treatments in the range 400–600°C are performed on GaSb/GaAs Interfacial Misfit grown samples. Current density–voltage (J–V), Capacitance–voltage (C–V), capacitance–frequency (C–F) and Deep Level Transient Spectroscopy (DLTS) measurements are performed on as-grown and annealed samples. Our studies show that possible defect compensation is observed with the annealing treatments, resulting in a significant improvement in the performances of the devices.