Post-growth Rapid Thermal Annealing Research Articles

Interface structures of Al0.85Sc0.15N-on-Si thin films grown by reactive magnetron sputtering upon post-growth cyclic rapid thermal annealing

Al0.85Sc0.15N thin films, about 920 nm thick, have been deposited on the Si (001) substrate by reactive magnetron sputtering at 600 °C. X-ray diffraction and pole-figure measurements revealed [0002]-oriented texture structures of the nitride films without any phase separations before and after cyclic annealing at 600–900 °C for up to 48 min. Cross-sectional studies by transmission electron microscopy and energy dispersive x-ray analysis revealed an intermediate Al0.85Sc0.15N layer of ∼24.6 nm thick with smaller grains and tilted [0002]-orientations compared to its overlayer, i.e., a nucleation layer (NL), on the Si substrate. After annealing, apparent morphological changes have been observed at the near-interface regions, including the NL, the NL/Si interface, and the Si substrate, rather than in the Al0.85Sc0.15N overlay. Undesired oxygen has been observed in the nitride film and its composition increased during post-growth thermal annealing without forming oxides. These observations shed new light on crystal growth and post-growth thermal annealing of AlScN toward their high-performance piezoelectric applications.

Energy Storage Properties of Sol-Gel-Processed SrTiO3 Films.

Dielectric films with a high energy storage density and a large breakdown strength are promising material candidates for pulsed power electrical and electronic applications. Perovskite-type dielectric SrTiO3 (STO) has demonstrated interesting properties desirable for capacitive energy storage, including a high dielectric constant, a wide bandgap and a size-induced paraelectric-to-ferroelectric transition. To pave a way toward large-scale production, STO film capacitors were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by the sol-gel method in this paper, and their electrical properties including the energy storage performance were studied as a function of the annealing temperature in the postgrowth rapid thermal annealing (RTA) process. The appearance of a ferroelectric phase at a high annealing temperature of 750 °C was revealed by X-ray diffraction and electrical characterizations (ferroelectric P-E loop). However, this high dielectric constant phase came at the cost of a low breakdown strength and a large hysteresis loss, which are not desirable for the energy storage application. On the other hand, when the RTA process was performed at a low temperature of 550 °C, a poorly crystallized perovskite phase together with a substantial amount of impurity phases appeared, resulting in a low breakdown strength as well as a very low dielectric constant. It is revealed that the best energy storage performance, which corresponds to a large breakdown strength and a medium dielectric constant, is achieved in STO films annealed at 650 °C, which showed a large energy density of 55 J/cm3 and an outstanding energy efficiency of 94.7% (@ 6.5 MV/cm). These findings lay out the foundation for processing high-quality STO film capacitors via the manufacturing-friendly sol-gel method.

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.

Strain and annealing temperature effects on the optical properties of GaNAs layers grown by molecular beam epitaxy

The ex-situ characterization of GaNAs/GaAs system grown by Molecular Beam Epitaxy with two different GaNAs layer thicknesses and similar nitrogen concentration (%N) is presented to explore the residual strain effect in the optimization of the inclusion of nitrogen on the GaAs host matrix by a rapid thermal annealing (RTA). Raman spectroscopy was used to study the effects of post-growth RTA on the alloy´s crystallinity and on %N by analyzing mode´s intensities and frequencies where both an increase of the local vibration mode and lattice disorder with the temperature were noticed. Band structure of the epilayers in the range of 1 to 1.5 eV and their modification with RTA were analyzed by photoreflectance. The E- transition redshifts, indicating increase of nitrogen in the alloy together with a modification of the spectral signature of GaAs by residual strain. Critical points above 1.5 eV were studied by ellipsometry where clear changes in the line shape of the second derivative of the pseudo-dielectric function imaginary part are produced with RTA, making more evident the presence of E+. The E1, E1+Δ1 and E΄0 transitions were also explored, E1 blue-shifts when the annealing has been successfully applied. With this study we show that in the optimization of GaNAs layers properties through RTA processes not only %N should be considered, but also additional parameters as thickness and strain.

Quantum-Dot Based Vertical External-Cavity Surface-Emitting Lasers With High Efficiency

We demonstrate an optically pumped vertical external-cavity surface-emitting laser (VECSEL) based on quantum dots (QDs) with a high optical gain. The photoluminescence (PL) emission of the QDs at the operating wavelength of 1030 nm has been optimized towards a high integrated intensity by using a dot density in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and a narrow PL linewidth of around 30 meV due to an improved size homogeneity. Additionally, post-growth rapid thermal annealing at 650 °C for 45 s was found to reduce the non-radiative recombination centers and further improve the output power efficiency. During this study, the number of QD layers, the heat sink temperature and the heat spreader material were analyzed. The best results with a record slope-efficiency of 35% and a maximum continuous-wave output power of 5.7 W could be achieved with an RTA treated QD based VECSEL containing 24 QD layers in the gain section. The laser was processed with a solder-based flip-chip bonding technique on a diamond heat sink and temperature stabilized at 4 °C. Furthermore, an excellent symmetric beam profile with M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 1.12 was achieved.

Optical studies of InAs/GaAs quantum dot stacks for photovoltaic applications

We have investigated the effect of post growth rapid thermal annealing on self-assembled InAs/GaAs mul tilayer QDs (MQD) overgrown with a combination barrier of InAlGaAs and GaAs for their possible use in photovoltaic device application. The samples were characterized by transmission electron microscopy and photoluminescence measurements. We noticed a thermally induced material interdiffusion between the QDs and the wetting layer in the MQD sample up to a certain annealing temperature. The QD heterostructure exhibited a thermal stability in the emission peak wavelength on annealing up to 700 ◦C . A phenomenological model has been proposed for this stability of the emission peak. The model considers the effect of the strain field, propagating from the underlying QD layer to the upper layers of the multilayer QD and the effect of indium atom gradient in the combination barrier layer due to the presence of a quaternary InAlGaAs layer.

1 eV GaAsSbN–based solar cells for efficient multi-junction design: Enhanced solar cell performance upon annealing

In this work, we demonstrate the beneficial effect of post-growth rapid thermal annealing (RTA) on the performance of ~1 eV GaAsSbN-based solar cells. Different configurations of the dilute nitride material are studied: a bulk quaternary GaAsSbN layer and type-II GaAsSb/GaAsN superlattices (SL) with different period thickness. The RTA treatment leads in both types of structures to an enhanced external quantum efficiency (EQE) and reduced values of the dark saturation current and series resistance. The origin of these improvements is attributed to reduced densities of N and Sb-rich clusters and point defects. Remarkably, all solar cell parameters are substantially improved, this being particularly significant for the short circuit current density (JSC) and power conversion efficiency (PCE), the latter with a relative enhancement as high as 500%. The large increment of JSC is an important step towards current–matching in multi–junction solar cells containing GaAsSbN structures.

Origin of two special Raman modes in dilute nitrides

Two special Raman modes at 197 cm−1 and 255 cm−1 have been observed and discussed for a long time without persuasive conclusion in dilute nitride materials. In this work, using Raman spectroscopy and transmission electron microscopy (TEM), we studied Molecular Beam Epitaxy grown GaAsN/GaAs multiple quantum well samples with nitrogen concentration ranging from 0.6% to 6.1% and undergoing post-growth rapid thermal annealing (RTA) processes in the temperature range of 650 °C–950 °C. We assigned these two modes to the double degeneracy Eg and Ag modes from arsenic crystalline clusters on sample surface according to the Raman measurements and TEM observations. The physical mechanisms involved in the RTA processes at different temperatures were proposed.

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.

Annealing-induced interfacial atomic intermixing in InAs/GaSb type II superlattices

We have investigated the effect of post-growth rapid thermal annealing (RTA) on the interface structure and atomic intermixing of InAs/GaSb type II superlattices (T2SLs). It is found that the mechanism of interfacial atomic interdiffusion of the anion (In/Ga) and the cation (As/Sb) is different. The activation energies of 0.62 eV and 0.27 eV are calculated to describe the In/Ga and As/Sb interdiffusion. In T2SLs, RTA will promote As/Sb intermixing across the interfaces between 450 and 480 °C, while In/Ga intermixing will be activated at 500 °C annealing. This demonstrates that the appropriate In/Ga intermixing is important to control the deterioration of the interfacial quality and the formation of dislocations, which is very crucial for the device performance.

On the Growth, Percolation and Wetting of Silver Thin Films Grown by Atmospheric-Plasma Enhanced Spatial Atomic Layer Deposition

Silver (Ag) thin-films have attracted interest for applications in sensing1, light management2, imaging beyond the diffraction limit3, interlayer in tri-layer transparent conductors (TCs)4 and even as possible replacement for Cu interconnects5. For tri-layer TCs, a narrow optimum exists between optical transparency and electrical conductivity6. The former decreases exponentially whereas the latter increases with thickness. The ability to precisely control both thickness and electrical percolation is crucial in tri-layer TCs, for which ultra-thin and uniform, yet conductive Ag layers are required over large areas. Here, Atomic Layer Deposition (ALD) can fulfill the requirements for precise thickness control and conformal step coverage over large area substrates. In this work we studied the deposition of Ag by spatial-ALD on molybdenum (Mo) layers serving as a growth model substrate. Mo-layers were specifically selected for testing the suitability of spatial-ALD to fabricate MoO3/Ag/MoO3 tri-layer TCs for organic-based solar cells. Since Mo-films are known to have high surface energy (3 Jm-2), Ag is expected to wet a Mo surface. However, the presence of native MoO3 on the surface is likely to reduce the surface energy resulting in poor wetting. Therefore, an H2/N2 plasma pre-treatment was introduced before Ag deposition in order to reduce the native MoO3 and promote coalescence. Ultrathin Ag films were deposited at 120 °C using Ag(fod)(PEt3) as the precursor and an H2/N2 plasma generated by a surface dielectric barrier discharge (SDBD) source as the co-reactant. Ag(fod)(PEt3) was evaporated into the deposition chamber using Ar as carrier gas flowing at 100 sccm, at the same temperature as that of the reactor. An H2/N2 co-reactant gas mixture was composed from 800 sccm H2 and 600 sccm N2. Silicon substrates with sputter-deposited Mo were used as a model substrate for Ag growth, and compared with the growth on bare silicon substrates. Top view and cross-sectional SEM images of the Ag layers show that a more compact and closed Ag layer was achieved on Mo upon introduction of the plasma pre-treatment (Fig. 1). Spectroscopic ellipsometry measurements demonstrate that the use of an H2/N2 plasma pre-treatment significantly enhances the wetting, and therefore, the coalescence of Ag owing to the high surface energy of the Mo-layers (Fig. 2). Furthermore, the better wetting and interfacial quality of the Ag deposited on plasma pre-treated Mo, compared to Ag on Si was probed by post-growth rapid thermal annealing experiments and XRD stress measurements. It was found that at 450 °C the Ag films de-wetted from Si, resulting in discontinuous films whereas the films deposited on plasma pre-treated Mo remained unchanged. Since both Ag films show similar stress content, the de-wetting most likely originates from interfacial properties rather than stress phenomena. We can therefore conclude that a plasma pre-treatment of Mo layers promotes the Ag wetting. This leads to earlier film coalescence compared to that of Ag films grown on Si, as demonstrated by both morphological and optical inspections of the Ag thin films. Thus, upon plasma pre-treatment ALD deposited MoO3 layers can act as wetting layers for subsequent Ag spatial-ALD in tri-layer TCs. This can open up alternative pathways for the fabrication of TCs using ALD. Figure 1

Platinum/yttrium iron garnet inverted structures for spin current transport

30-80 nm thick yttrium iron garnet (YIG) films are grown by pulsed laser deposition on a 5 nm thick sputtered Pt atop gadolinium gallium garnet substrate (GGG) (110). Upon post-growth rapid thermal annealing, single crystal YIG(110) emerges as if it were epitaxially grown on GGG(110) despite the presence of the intermediate Pt film. The YIG surface shows atomic steps with the root-mean-square roughness of 0.12 nm on flat terraces. Both Pt/YIG and GGG/Pt interfaces are atomically sharp. The resulting YIG(110) films show clear in-plane uniaxial magnetic anisotropy with a well-defined easy axis along 〈001〉 and a peak-to-peak ferromagnetic resonance linewidth of 7.5 Oe at 9.32 GHz, similar to YIG epitaxially grown on GGG. Both spin Hall magnetoresistance and longitudinal spin Seebeck effects in the inverted bilayers indicate excellent Pt/YIG interface quality.

Effect of rapid thermal annealing on InP1−xBix grown by molecular beam epitaxy

The effect of post-growth rapid thermal annealing on structural and optical properties of InP1−xBix thin films was investigated. InPBi shows good thermal stability up to 500 °C and a modest improvement in photoluminescence (PL) intensity with an unchanged PL spectral feature. Bismuth outdiffusion from InPBi and strain relaxation are observed at about 600 °C. The InPBi sample annealed at 800 °C shows an unexpected PL spectrum with different energy transitions.

Ordering of low-density Ge quantum dot on patterned Si substrate

We present a study on the growth of a low-density ordered Ge quantum dot (QD) on a nanohole patterned Si (0 0 1) substrate with periods in the order of microns by molecular beam epitaxy. Ordered Ge QDs with different periods were realized, the largest period being 15 µm. From the height-profile evolution of the QD with Ge deposition, it was found that the nanohole filling started at the initial Ge deposition, indicating an immediate nucleation and growth of QDs inside the nanoholes. Such a phenomenon is attributed to a lower surface chemical potential (SCP) inside the nanoholes, which is supported by calculated results on SCP evolution with growth and verified by the observation that the Ge adatoms around the nanoholes exhibited a higher probability of being incorporated into the nanoholes. In this scenario, low density ordered Ge QDs with any large periods could be achieved. The optical properties of the Ge QDs showed a remarkable improvement after a post-growth rapid thermal annealing treatment.

Enhancing optical characteristics of InAs/InGaAsSb quantum dot structures with long-excited state emission at 1.31 μm.

In this study, the optical properties of InAs quantum dots (QDs) with various strain-reducing layers (SRLs) of GaAsSb and InGaAsSb are characterized using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The room-temperature PL results for the InAs/InGaAsSb QDs revealed stronger emission intensities than InAs QDs capped with an GaAs(1-x)Sb(x) (x = 20%)SRL, although both samples were grown under the same Sb flux during the molecular beam epitaxy process. The InAs/InGaAsSb QDs showed a significant elongation of emission wavelengths to 1450 and 1310 nm for the ground and first-excited state at room temperature. The energy band alignment of the InAs QD heterostructures was found tailoring from type II to type I as the GaAsSb SRL was replaced by InGaAsSb layer, which improved the radiative efficiency and was verified by power-dependent PL and TRPL measurements. Post-growth rapid thermal annealing was applied on the InAs/InGaAsSb QDs to further enhance the QD quality and PL emission efficiency. The greatly improved PL intensity, reduced linewidth, shortened radiative lifetime, with increasing annealing temperature were demonstrated, and InAs/InGaAsSb QDs exhibited enhanced optical characteristics for long-wavelength emission applications.

Identification of an isolated arsenic antisite defect in GaAsBi

Optically detected magnetic resonance and photoluminescence spectroscopy are employed to study grown-in defects in GaAs0.985Bi0.015 epilayers grown by molecular beam epitaxy. The dominant paramagnetic defect is identified as an isolated arsenic antisite, AsGa, with an electron g-factor of 2.03 ± 0.01 and an isotropic hyperfine interaction constant A = (900 ± 20) × 10−4 cm−1. The defect is found to be preferably incorporated during the growth at the lowest growth temperature of 270 °C, but its formation can be suppressed upon increasing growth temperature to 315 °C. The AsGa concentration is also reduced after post-growth rapid thermal annealing at 600 °C.

Spectral broadening due to post-growth annealing of a long-wave InGaAs/GaAs quantum dot infrared photodetector with a quaternary barrier layer

We present the effect of post-growth rapid thermal annealing on multilayer, uncoupled In0.50Ga0.50As/GaAs quantum dot infrared photodetectors (QDIPs) with a combination of quaternary In0.21Al0.21Ga0.58As and GaAs capping. Long wave photoresponse (~0.11eV or 10.2μm) with narrow spectral width (14%) is obtained from as-grown QDIPs. The spectral width increases to 45% for QDIP annealed at 800°C. It is likely that a large inter-diffusion effect because of annealing changes the composition of the quantum dots and capping layer, thus increasing the QDIP spectral width. Dark current as well as noise current is decreased for 650°C annealed QDIP compared to as-grown detector. Passivation of as-grown defect by post growth annealing predicted the improvement of dark current. Such improvement and broadening of characteristics may render such QDIPs suitable for use in broadband infrared imaging applications.

Tuning in spectral response due to rapid thermal annealing on dot-in-a-well infrared photodetectors

Here we investigated the effect of post-growth rapid thermal annealing (RTA) on dot-in-a-well infrared photodetectors. In a photoluminescence (PL) study, we initially observed a small red shift in the ground-state PL peak upon annealing to 650°C but then saw the usual blue shift as the annealing temperature increased. We also observed increases in the dark current as the annealing temperature increased up to 700°C but a sudden decrease in the dark current at 750°C. Activation energy is calculated using temperature-dependent PL and dark current measurements. The photoresponse peak was observed at 6.41μm for the as-grown device. As we increased the annealing temperature to 800°C, the peak response shifted to 9.52μm. Thus, we achieved wavelength tunability of peak photoresponse using the post-growth annealing technique.

Rapid thermal annealing and photoluminescence of type-II GaSb single monolayer quantum dot stacks

The effects of rapid thermal annealing on the photoluminescence emission obtained from ten-layer stacks of GaSb/GaAs type-II single monolayer quantum dots and Stranski–Krastanow grown quantum rings have been studied and interpreted. Post-growth rapid thermal annealing was performed with proximity capping at temperatures from 550 °C to 800 °C, resulting in an increase in photoluminescence emission intensity and a blue shift in peak energy in both types of stacks, together with changes in the activation energy for thermal quenching. This behaviour originates from Sb–As intermixing and changes in morphology of the nanostructures formed using the two different growth mechanisms.

Influence of post-growth rapid thermal annealing on the transport and lasing characteristics of terahertz quantum-cascade lasers

We investigate the effect of post-growth rapid thermal annealing (RTA) on the transport and lasing characteristics of terahertz quantum-cascade lasers (THz QCLs) operating in a frequency range between 4.88 and 4.94 THz. The emission frequencies are blue shifted by about 80 GHz after RTA, which is attributed to a shift of the gain maximum to higher frequencies due to composition grading at the interfaces between the quantum wells and barriers of the annealed wafer pieces. The optical output power of the annealed THz QCLs is reduced, which is explained by a broadening of the levels due to the annealing process.