Post-growth Annealing Research Articles

Incorporation and activation of arsenic in single-crystal CdTe layers grown by metalorganic chemical vapor deposition

The electrical properties of arsenic doped CdTe epitaxial layers grown on GaAs (100) substrates by metalorganic chemical vapor deposition with dimethylcadmium and diethyltellurium are investigated. Tris(dimethylamino)arsine was used as the arsenic source. CdTe layers with arsenic concentration from 1.6·1017 cm−3 to 1.5·1018 cm−3 were obtained by varying only diethyltellurium/dimethylcadmium mole ratio from 6.2 to 0.5. The as-grown CdTe:As layers had p-type conductivity with hole concentration about 2·1015 cm−3 within diethyltellurium/dimethylcadmium mole ratio 1–6.2 and n-type at diethyltellurium/dimethylcadmium = 0.5. Post-growth annealing (250–500 °C) were used to increase the activation efficiency of arsenic. The highest hole concentration was 2.1·1017 cm−3 at arsenic concentration of 4.5·1017 cm−3, but the maximum activation efficiency approximately of 85% was achieved at lower concentration levels. The ionization energy of AsTe acceptor determined from temperature-dependent Hall measurements was in the range of 96–112 meV. Low-temperature photoluminescence emission spectra of arsenic doped CdTe layers showed a peak at ~1.51 eV which is associated with AsTe acceptor with ionization energy about 90 meV. Possible mechanisms of arsenic compensation in CdTe by intrinsic defects and the AX-centers formation are discussed.

Control of complex quantum structures in droplet epitaxy

We report the controllable growth of GaAs quantum complexes in droplet molecular-beam epitaxy, and the optical properties of self-assembled AlxGaAs quantum rings embedded in a superlattice. We found that Ga droplets on a GaAs substrate can retain their geometry up to a maximum temperature of 490 °C during post-growth annealing, with an optimal temperature of 320 °C for creating uniform and symmetric droplets. Through controlling only the crystallisation temperature under As4 in the range of 450 °C to 580 °C, we can reliably control diffusion, adsorption and etching rates to produce various GaAs quantum complexes such as quantum dots, dot pairs and nanoholes. AlxGaAs quantum rings are also realised within these temperatures via the adjustment of As beam equivalent pressure. We found that crystallisation using As2 molecules in the place of As4 creates smaller diameter quantum rings at higher density. The photoluminescence of As2 grown AlxGaAs quantum rings embedded in a superlattice shows a dominant emission from the quantum rings at elevated temperatures. This observation reveals the properties of the quantum ring carrier confinement and their potential application as efficient photon emitters.

Influence of annealing on the electrical characteristic of GaSbBi Schottky diodes

The influence of postgrowth thermal annealing on GaSbBi Schottky barrier diodes has been investigated. The effects of the annealing temperature and time on the material quality and electrical characteristics of the diodes have been studied. The I-V characteristics indicated a better ideality factor and less leakage current at the reverse bias, as the annealing temperature increased up to 500 °C for a duration of 30 min. X-ray diffraction and scanning transmission electron microscope measurements were performed to verify that the bismuth composition was unaffected during the annealing process. Energy dispersive x-ray analysis indicated that Sb clustering occurs at high annealing temperatures, resulting in a concomitant degradation in the electrical performance. The optimum electrical characteristics of the diode were obtained with an annealing temperature of 500 °C for 30 min, resulting in an ideality factor of 1.3 being achieved.

Improvements in structural and optical properties of wafer-scale hexagonal boron nitride film by post-growth annealing

Remarkable improvements in both structural and optical properties of wafer-scale hexagonal boron nitride (h-BN) films grown by metal-organic chemical vapor deposition (MOCVD) enabled by high-temperature post-growth annealing is presented. The enhanced crystallinity and homogeneity of the MOCVD-grown h-BN films grown at 1050 °C is attributed to the solid-state atomic rearrangement during the thermal annealing at 1600 °C. In addition, the appearance of the photoluminescence by excitonic transitions as well as enlarged optical band gap were observed for the post-annealed h-BN films as direct consequences of the microstructural improvement. The post-growth annealing is a very promising strategy to overcome limited crystallinity of h-BN films grown by typical MOCVD systems while maintaining their advantage of multiple wafer scalability for practical applications towards two-dimensional electronics and optoelectronics.

Research of effect of heat treatment on microhardness and wearing resistance of anodic oxide aluminum coatings modified by nano diamonds

Anodizing of aluminum and its alloys is widely used in various fields of science and technology. The process of modifying porous anodic aluminum oxide with ultradispersed diamond (UDD) particles to improve the mechanical characteristics of coatings requires additional study. UDD was modified by consistent heat treatment at 40 °C and 120 °C. The results of the UDD surface modification were controlled by IR spectroscopy. The surface state analysis was carried out using the PMT-3 microhardnessmeter, the SolverPro P47 atomic-force microscope (AFM), and the experimental probe-electrometry device. One of the ways to improve the mechanical characteristics of such coatings is the use of ultradispersed diamonds with respective pretreatment of their surface. The article presents the results of a study of the influence of additives of ultradispersed diamonds with different functional surface composition in an acid electrolyte to form coatings of porous anodic alumina on the surface of AMg-2 aluminum alloy substrates by electrochemical oxidation. An increase in the microhardness and wear resistance of anodic oxide coatings formed on aluminum alloy substrates after various post-growth heat treatments is noted. It is shown that using a combined method based on doping anodic alumina in the process of synthesis with modified ultradispersed diamonds and post-growth annealing of the coatings obtained in vacuum at T = 500 °C, it is possible to obtain a composite material that is 2 times higher in hardness and 3 times higher in wear resistance compared to the initial coating. The research results can be used to create a new generation of radiation-resistant heat-removing bases, nano and micromechanical devices, elements of passive and active electronics, high-quality parts for spacecraft and satellites on modern composite materials.

Investigation of metal-insulator-semiconductor diode with alpha-Ga2O3 insulating layer by liquid phase deposition

We report a metal-insulator-semiconductor (MIS) diode with an α-Ga2O3 thin-film insulator layer grown by liquid-phase deposition (LPD). α-Ga2O3 exhibits a high energy bandgap of 4.9–5.3 eV, which can effectively reduce the leakage current density and improve the breakdown voltage of the diode. The α-Ga2O3 thin films are synthesized from GaOOH with LPD. The α-GaOOH crystal is simply obtained by the dissociation of the Ga(OH)3 precursor solution. GaOOH can be transformed into α-Ga2O3 crystal and form a uniform thin film following post-growth annealing. When the α-Ga2O3 thin film is inserted in between Ni and Si to form a Ni/α-Ga2O3/Si MIS diode, the barrier height of the diode increases by 0.4 eV and the on/off ratio by 100-fold from those of the Ni/Si Schottky diode. The Ni/α-Ga2O3/Si MIS diode exhibits a leakage current density of 1.07 × 10−5A/cm2 under −2 V bias. The breakdown voltage of the diode reaches −166 V without the guard ring and other insulation structures. Our results demonstrate that LPD-grown α-Ga2O3 thin films can obtain uniform and dense structure under short deposition time and at an annealing temperature of 400 °C. The uniform insulating layer of α-Ga2O3 has a high potential in enhancing the electrical characteristic of diodes and other power electronic devices.

Influence of annealing on spin pumping in sputtered deposited Co/Pt bilayer thin films

We report on the ferromagnetic resonance study of the Co/Pt thin films as a function of the post growth annealing. We found a significant increase in the spin mixing conductance, Gilbert damping parameter and inhomogeneous linewidth broadening with increase in annealing temperature. This behavior is correlated to the structural degradation of Co layer and in particular formation of CoxPt1−x alloy at the interface, which gives rise to increase in dissipation of spin current at the interface. The results are important for understanding of spin-orbit torque in Co/Pt bilayer structures.

Direct growth of ZnSnO nano-wires by thermal evaporation technique for thermoelectric applications

In this research, we have successfully grown the ZnSnO nano-wires by simple thermal evaporation technique and reported the highest Seebeck coefficient and power factor for first time. The ZTO nano-structures were grown by a simple thermal evaporation method using direct evaporation of Zinc and Tin metal powders under constant oxygen flow rate of 100 sccm in a tube furnace. The post growth annealing was performed in the temperature ranges of 600–900 °C with a step of 100 °C in the programmable muffle furnace. The structural, electrical, morphological and thermoelectric properties of as grown and annealed samples were analyzed by X-Ray Diffraction, Raman spectroscopy, Scanning Electron Microscopy, Hall data and Seebeck effect. The XRD results confirmed the formation of Wurtzite hexagonal structure and the vibrational mode of ZTO at 437 cm−1 is verified by Raman spectroscopy measurements. It was also observed that the crystal structure of ZTO was improved with annealing temperature. SEM micrographs showed the nanowires like morphology of the un-annealed sample but it transformed into nano-crystalline grains by post growth annealing. Seebeck data demonstrated that the values of Seebeck coefficient, electrical conductivity and power factors were found to be increased with annealing temperature because Sn atoms get more thermal energy at high temperature and contribute in the carrier concentration.

Quality improvement of CZTS thin films deposited by spray pyrolysis method using pulsed Nd: YAG laser irradiation

Abstract In this work, thin films of Cu2ZnSnS4 (CZTS) were grown by spray-pyrolysis method and were irradiated by pulsed Nd: YAG laser. Their morphological, chemical, structural and substructural characteristics were investigated using AFM, SEM, EDS, XRD and Raman spectroscopy. Upon laser irradiation, improvement of crystallinity, stoichiometry and surface morphology were observed in the obtained CZTS thin films with the normal distribution of newly-formed particles throughout the plane. It was shown that the laser processing by pulsed Nd: YAG laser is an alternative method to the traditional post-growth thermal annealing and can be used not only for local annealing but also for annealing in various depths in the films.

XMCD study of magnetism and valence state in iron-substituted strontium titanate

Room temperature ferromagnetism was characterized for thin films of SrTi$_{0.6}$Fe$_{0.4}$O$_{3-{\delta}}$ grown by pulsed laser deposition on SrTiO$_{3}$ and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from Fe$^{2+}$ cations, whereas Fe$^{3+}$ and Ti$^{4+}$ did not contribute. Films with the highest magnetic moment (0.8 {\mu}B per Fe) had the highest measured Fe$^{2+}$:Fe${^3+}$ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies ({\delta} = 0.19). Post-growth annealing treatments under oxidizing and reducing conditions demonstrated quenching and partial recovery of magnetism respectively, and a change in Fe valence states. The study elucidates the microscopic origin of magnetism in highly Fe-substituted SrTi$_{1-x}$Fe$_x$O$_{3-{\delta}}$ perovskite oxides and demonstrates that the magnetic moment, which correlates with the relative content of Fe$^{2+}$ and Fe$^{3+}$, can be controlled via the oxygen content, either during growth or by post-growth annealing.

Growth of Mg2GeO4 nano-crystals on Si substrate and modulation of Seebeck coefficient by post growth annealing technique

Abstract We have reported the successful growth of Mg2GeO4 nano-crystals by simple thermal evaporation technique. The Mg and Ge metal powders were evaporated on the Si substrate and kept the oxygen flow rate of 100 sccm. The modulation of structural, morphological, thermoelectric and electrical properties was performed by controlling the thermal energy of carriers using different annealing temperatures. XRD data showed a peak at 61.80 which was related to (212) plane of Mg2GeO4. XRD data further suggested that sample annealed at 700 °C has stable crystal structure while sample annealed at 800 °C posses degraded structure because the presence of highest density of donors defects. This defect concentration causes an increase in the conductivity of annealed samples as evident by the Hall data. This argument was also supported by Raman spectroscopy which showed that sample annealed at 700 °C has strongest Mg2GeO4 Raman peak. SEM images also verified the smooth surface of the sample annealed at 700 °C. The temperature dependent (25–100 °C) Seebeck effect measurements were performed to calculate the Seebeck coefficient of Mg2GeO4 nano-crystals at different measurement temperatures. The highest value of room temperature Seebeck coefficient (397 μV/0C) for the sample annealed at 800 °C is due the high density of carrier concentration.

Approach to Defect-Free Lifetime and High Electron Density in CdTe

Achieving simultaneously high carrier density and lifetime is important for II–VI semiconductor-based applications such as photovoltaics and infrared detectors; however, it is a challenging task. In this work, high purity CdTe single crystals doped with indium (In) were grown by vertical Bridgman melt growth under carefully controlled stoichiometry. Two-photon excitation time-resolved photoluminescence was employed to measure bulk recombination lifetime by eliminating surface recombination effects. By adjusting stoichiometry with post growth annealing, high-net free carrier density approaching 1018 cm−3 was achieved simultaneously with lifetime approaching the radiative limit by suppressing non-radiative recombination centers.

The effect of post-growth rapid thermal annealing on InAs/InGaAs dot-in-a-well structure monolithically grown on Si

The effect of post-growth annealing (PGA) on dot-in-well (DWELL) structures grown on Si substrates has been studied. The photoluminescence (PL) measurements showed that, compared to the DWELL structures directly grown on GaAs, the PGA process induces a distinct difference in the tuning of the emission properties. Then, transmission electron microscopy imaging of the samples revealed that PGA improved the uniformity of quantum dots (QDs) while the size of the QDs increased, in agreement with a corresponding red shift and a decrease of the full width at half maximum in the PL emission spectrum. Finally, energy-dispersive x-ray linescan provided a quantitative analysis of the composition change of DWELL grown on Si in the as-grown, 700 °C annealed, and 800 °C annealed samples. The change in the InL/GaK concentration ratio became gradual between the QDs and surrounding materials after 800 °C annealing. The analysis of the optical properties, morphology evolution, and compositional change of the QDs as a function of the annealing temperature showed good agreement.

Growth of Cu2InO4 thin films on Si substrate by thermal evaporation technique and enhancement of thermoelectric properties by post-growth annealing

Thin films of Cu2InO4 were grown by thermal evaporation on Si substrate using tube furnace. Cu and In metals were evaporated in the tube furnace having pressure 0.2 × 10−2 Torr and the oxygen gas flow rate was set 60 standard cubic centimeters (sccm). The post-growth annealing in oxygen environment was performed at different temperatures ranging from 600 to 800 °C for 1 h in programmable furnace. The X-Ray Diffraction (XRD) data confirmed that as grown sample showed the amorphous nature, but it was converted into crystalline at higher annealing temperatures (700–800 °C). Raman spectroscopy measurements further confirmed the XRD results which showed a peak at 305 cm−1 at higher annealing temperatures, which is related to the Cu2InO4 structure along with Si peak at 521 cm−1. Photoluminescence spectroscopy (PL) data was demonstrated a band to band emission of Cu2InO4 at 1.9 eV and the intensity of this band was enhanced with the annealing temperature. XRD, Raman spectroscopy and PL results suggested that the structure of grown thin films was improved with increasing annealing temperature. After structure verification, In metal contacts were fabricated on all samples to investigate the temperature dependent thermoelectric properties. The Seebeck coefficient of Cu2InO4 thin films was found to be increased with annealing as well as measurement temperature. The reported results were explained according to charge mobility engineering, which was reported earlier by our group. We have also performed Hall measurements to strengthen our argument.

Effect of thermal annealing on the strain and microstructures of in-situ phosphorus-doped [formula omitted] films grown on blanket and patterned silicon wafers

In this study, in-situ phosphorus-doped Si1−xCx layers were epitaxially grown on blanket and patterned Si wafers using reduced pressure chemical vapor deposition (RPCVD). The effect of post-growth annealing on the strain and microstructures of the epilayers was investigated by high resolution X-ray diffraction (HR-XRD), Raman scattering, and high resolution transmission electron microscopy (HR-TEM) analyses. Structural investigation revealed that induced tensile strain became significantly less at temperatures over 900°, as confirmed by Raman scattering measurements of SiC vibration modes. Furthermore, strain relaxation and substantial Csub loss were found in XRD and Raman data of Si:C samples annealed at 1000°, resulting from the generation of defects such as β-SiC precipitates or dislocations in Si1−xCx epilayers. Moreover, our microstructural analyses using TEM showed the formation of the β-SiC precipitates during rapid thermal annealing (RTA) over 900° and an increase in their number and size with annealing temperatures up to 1000°. Finally, we examined the impact of thermal annealing on the local strain and microstructures of patterned Si1−xCx samples with different structures. Our findings will provide greater insight into evaluating the strain states and microstructure of as-grown and annealed P-doped Si1−xCx films on both blanket and patterned Si wafers.

Curvature-controllable and crack-free AlN/sapphire templates fabricated by sputtering and high-temperature annealing

Post-growth high-temperature annealing of sputtered AlN films is a promising approach to realize high-quality AlN templates for deep-ultraviolet light-emitting diode applications. This paper proposes the double-sided sputtering of AlN on double-sided polished sapphire to suppress the wafer bowing caused by high-temperature annealing. Similar to the case of single-sided sputtering, the twist component of the X-ray rocking curve was markedly improved after annealing, yielding rocking curve widths of 256–321 arcsec in (10–12). By varying the backside thickness, the curvature was controlled from −27 km−1 to 29 km−1, while a crack-free surface was maintained on the front side of AlN. The experimentally obtained curvature and in-plane stress agreed well with the calculation based on thermal stress analysis in the multilayer system.

Improvements of surface morphology and electrical transport properties of single-crystalline In2O3(111) thin films by postgrowth annealing

We have grown single-crystalline In2O3 films on yttria-stabilized zirconia (001) and (111) substrates by reactive electron beam evaporation, and investigated the effects of ex situ postgrowth annealing in oxygen atmosphere on the surface morphology and electrical transport properties. For the (111)-oriented film, we observed a flat surface in atomic scale as well as high Hall mobility up to 130 cm2 V−1 s−1 with carrier density of 2.3 × 1019 cm−3 at room temperature. The results indicate that the postgrowth annealing is effective in fabricating high-quality In2O3 film.

Impact of thermal annealing on internal device parameters of GaAs 0.965 Bi 0.035 /GaAs 0.75 P 0.25 quantum well lasers

The impact of post-growth thermal annealing on the internal device parameters such as internal loss ( α i ), internal differential quantum efficiency (η 0 d ) and modal material gain (Γ g 0J ) of a single-quantum well (QW) laser diodes employing GaAs 0.965 Bi 0.035/ GaAs 0.75 P 0.25 active regions and emitting near λ ~980 nm was investigated. Parameter extraction from a conventional cavity length analysis indicates that the internal loss remains unchanged while internal differential quantum efficiencies degrade as the annealing time increases. Also, the variation of the modal material gain with annealing correlates to the corresponding change in the photoluminescence intensity. Comparisons between single- and double-QW devices indicate that the relatively high internal loss originates from the QW active region.

Dissolution of antiphase domain boundaries in GaAs on Si(001) via post-growth annealing

GaAs-on-Si epitaxial crystal quality has historically been limited by a number of growth-related defects. In particular, antiphase domain boundaries (APBs) can nucleate at the GaAs/Si interface and propagate throughout the entire GaAs layer. Still little is known about how thermal processing can affect the APB density in GaAs. In this study, GaAs was grown on nominally on-axis Si(001) by metal–organic chemical vapor deposition. The effect of ex situ post-growth annealing was evaluated for a temperature range of 550–700 °C. It was found that upon annealing the APB density was decreased significantly. The rate of APB density decrease was found to be temperature dependent. At annealing temperatures of 650 °C and above, the APB density was reduced from 0.10 μm−1 to approximately 0.010 μm−1 in less than 10 min. The activation energy for APB dissolution was determined to be 3.8 eV. The mechanism of APB dissolution is discussed.

Impact of Crystal Structure and Particles Shape on the Photoluminescence Intensity of CdSe/CdS Core/Shell Nanocrystals.

To study the influence of the chemical and crystalline composition of core/shell NCs on their photoluminescence (PL) the mean structural profile of a large ensemble of NCs has to be retrieved in atomic resolution. This can be achieved by retrieving the chemical profile of core/shell NCs using anomalous small angle x-ray scattering (ASAXS) in combination with the analysis of powder diffraction data recorded by wide angle x-ray scattering (WAXS). In the current synchrotron based study, we investigate CdSe/CdS core/shell NCs with different core dimensions by recording simultaneously ASAXS and WAXS spectra. The CdS shells are grown epitaxial on nominal spherical CdSe cores with core diameters from around 3.5–5.5 nm. Three different CdSe shell thicknesses are realized by depositing around 4, 6, and 8 monolayers (MLs) of CdSe. We reveal that the epitaxial core/shell structure depicts a chemical sharp interface, even after a post growth annealing step. With increasing NC diameter, however, the CdSe/CdS NCs deviate significantly from a spherical shape. Instead an elliptical particle shape with pronounced surface facets for the larger core/shell NCs is found. In combination with the powder diffraction data we could relate this anisotropic shape to a mixture of crystal phases within the CdSe core. The smallest CdSe cores exhibit a pure hexagonal wurtzite crystal structure, whereas the larger ones also possess a cubic zincblende phase fraction. This mixed crystal phase fractions lead to a non-spherical shell growth with different thicknesses along specific crystallographic directions: The long axes are terminated by basal crystal faces parallel either to the a- or c-axis, the short axes by “tilted” pyramidal planes. By combining these structural data with the measured PL quantum yield values, we can clearly connect the optical output of the NCs to their shape and to their shell thickness. Above 6 ML CdS shell-thickness no further increase of the PL can be observed, but for large aspect ratio values the PL is significantly decreased. The gained understanding of the internal crystal structure on CdSe/CdS NCs is general applicable for a precise tuning of the optical properties of crystalline core/shell NCs.