Post-growth Thermal Annealing Research Articles

Post-growth tuning of detachable Ge membranes adhesion strength via porous Ge transformation

Single crystal germanium (Ge) membranes have recently gained increasing interest for lightweight and low-cost solar cells and flexible optoelectronic devices. These membranes achieved similar material quality as bulk Ge substrate. However, the control of the membrane detachment is still challenging. In this work, we explore post-growth engineering of the adhesion strength of a Ge membrane on a porous germanium (PGe) substrate by inducing morphological transformations in the separation layer through Thermal Budget (TB) control. Indeed, the pillars formed through PGe sintering during epitaxy are found to evolve with post-growth thermal annealing. Scanning electron microscopy (SEM) based analysis of the residue of the post-detachment broken pillars has been performed showing that the pillar's diameter and density can be tuned by thermal annealing. Depending on the post-growth annealing temperature, the membrane adhesion strength can be successively tailored from 0.5 to up to 3.5 MPa while ensuring 100 % detachment yield. The experimental results have been correlated with Finite Element Modeling (FEM) considering realistic pillar distribution revealing that pillar size and density are the dominant factors influencing the membrane adhesion strength.

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.

Controlled nano-roughening of the GaN surface by post-growth thermal annealing

GaN-based semiconductor structures have been widely used in photonic, electronic, and optoelectronic devices. However, due to the lack of lattice-matched substrates for GaN, several schemes related to material growth and device structure, including surface roughening, have been proposed to improve the efficiency of GaN-based devices. In our work, we developed an experimental approach to achieving a controllable roughening of the GaN epilayer surface by thermal post-annealing. The degree of surface roughness, i.e., nanometric modification, is controlled by the number of annealing cycles. The samples were annealed at 1200 °C under N2 ambient. Under this condition, the first stage of GaN decomposition occurs. This annealing process led to the formation of Ga-rich GaN nanoparticles at the film surface with no significant change in the dislocation density. Furthermore, this treatment greatly decreases the compressive stress degree, resulting in a considerable improvement in the optical properties of the GaN samples.

An efficiently excited Eu3+ luminescent site formed in Eu,O-codoped GaN

For the development of III-nitride-semiconductor-based monolithic micro-light-emitting diode (LED) displays, Eu,O-codoped GaN (GaN:Eu,O) is a promising material candidate for the red LEDs. The luminescence efficiency of Eu-related emission strongly depends on the local atomic structure of Eu ions. Our previous research has revealed that post-growth thermal annealing is an effective method for reconfiguring luminescent sites, leading to a significant increase in light output. We observed the preferential formation of a site with a peak at ∼2.004 eV by the annealing process. In this study, we demonstrate that it is a previously unidentified independent site (OMVPE-X) using combined excitation–emission spectroscopy and time-resolved photoluminescence measurements. In addition, we perform excitation power-dependent photoluminescence measurements and show that this OMVPE-X site dominates the emission at a low excitation power region despite its small relative abundance, suggesting a high excitation efficiency. Most importantly, applying our annealing technique to an LED exhibits a reasonably increased electroluminescence intensity associated with OMVPE-X, confirming that this site has a high excitation efficiency also under current injection. These results demonstrate the importance of OMVPE-X as a notable luminescent site for brighter and more efficient GaN:Eu,O-based LEDs.

193 nm laser annealing on p-GaN with enhanced hole concentration and wall-plug-efficiency in deep ultraviolet LED

The high acceptor ionization energy and, thus, low carrier concentration in p-type III-nitride have widely been recognized as the bottleneck preventing the development of high-efficiency light-emitting-diodes (LEDs). In this contribution, the influences of 193 nm pulsed laser annealing on the structure, electrical, and optical properties of p-type GaN were systematically analyzed. The hole concentration of p-GaN first increases and then decreases with increasing laser fluence, regardless of post-growth thermal annealing. The effective dopant activation due to laser annealing can be attributed to the dissociation of Mg–H complexes during the treatment. Laser-annealed p-GaN was utilized in a 280 nm deep ultraviolet LED. A maximum of 1.47 times higher wall-plug-efficiency enhancement factor was obtained compared to that without laser annealing, demonstrating that 193 nm laser annealing plays a decisive role in the boosting of quantum efficiency of optoelectronic devices.

Si–Sn codoped n-GaN film sputtering grown on an amorphous glass substrate

DC-pulse magnetron sputtering was utilized to deposit a 300 nm-thick n-type GaN thin film that was co-doped with Si–Sn onto an amorphous glass substrate with a ZnO buffer layer. The deposited thin films were then subjected to post-growth thermal annealing at temperatures of 300 °C, 400 °C, or 500 °C to enhance their crystal quality. Hall measurements revealed that the film annealed at 500 °C had the lowest thin-film resistance of 0.82 Ω cm and the highest carrier concentration of 3.84 × 1019 cm−3. The thin film surface was studied using atomic force microscopy; the film annealed at 500 °C had an average grain size and surface roughness of 25.3 and 2.37 nm, respectively. Furthermore, the x-ray diffraction measurements revealed a preferential (002) crystal orientation and hexagonal wurtzite crystal structure at 2θ ≈ 34.5°. The thin film had a full width at half maximum value of 0.387°, it was also found to be very narrow. Compositional analysis of the films was conducted with x-ray photoelectron spectroscopy and verified that both Si and Sn were doped into the GaN film utilizing covalent bonding with N atoms. Finally, the film annealed at 500 °C had a high optical transmittance of 82.9% at 400–800 nm, a high figure of merit factor of 490.3 × 10−3 Ω−1, and low contact resistance of 567 Ω; these excellent optoelectronic properties were attributed to the film’s high electron concentration and indicate that the material is feasible for application in transparent optoelectronic devices.

Enhanced light output of Eu, O-codoped GaN caused by reconfiguration of luminescent sites during post-growth thermal annealing

Luminescence efficiency of Eu-related emission from Eu, O-codoped GaN (GaN:Eu, O) strongly depends on the local structure of Eu ions. Growth at relatively low temperature (∼960 °C) not only enables high Eu doping concentration but also elevates Eu-clustering due to its low diffusion coefficient, which results in formation of a large number of inefficient luminescent sites. We have studied the impact of post-growth thermal annealing at high temperatures on elimination of Eu clusters by photoluminescence measurements. These clarify that thermal annealing at high temperatures induces changes in the structural conformation and converts inefficient luminescent sites to efficient ones. As a result, the sample annealed at 1100 °C shows increased luminescence efficiency with a maximum of 5.1 times that of the as-grown sample. Post-growth thermal annealing offers a way to improve the efficiency of GaN:Eu, O further for practical application in III-nitride-based monolithic three-primary colors' light-emitting diodes.

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.

Raman and Photoluminescence Spectroscopic Studies on Structural Disorder in Oxygen Deficient Gd2Ti2O7‐δ Single Crystals

AbstractRaman and photoluminescence spectroscopic studies on oxygen vacancy‐induced structural disorder in Gd2Ti2O7‐δ single crystals grown by optical floating zone technique under argon atmosphere are reported. The oxygen vacancies in Gd2Ti2O7‐δ wafers decrease with thermal annealing in air atmosphere. The full width at half maximum of X‐ray diffraction rocking curve decreases from 245 to 157 arcsec and the optical transmittance increases from 23% to 87% (at 1000 nm) upon post growth thermal annealing. Raman spectroscopic studies reveal a monotonic increase in intensity of O–Gd–O (Eg) and Ti–O (A1g) stretching modes with thermal annealing. Since these modes are associated with modulation of oxygen x parameter which is sensitive to Ti–O octahedron distortion, the increase in Raman intensity indicates an improvement in structural ordering of oxygen sub‐lattice in Gd2Ti2O7‐δ. Moreover, the photoluminescence studies also corroborate the Raman analysis in terms of reduction of structural defects associated with oxygen vacancies as a function of thermal annealing. This study demonstrates the effectiveness of using Raman spectroscopy to probe the structural disorder in Gd2Ti2O7‐δ crystals.

A highly stable, nanotube-enhanced, CMOS-MEMS thermal emitter for mid-IR gas sensing

The gas sensor market is growing fast, driven by many socioeconomic and industrial factors. Mid-infrared (MIR) gas sensors offer excellent performance for an increasing number of sensing applications in healthcare, smart homes, and the automotive sector. Having access to low-cost, miniaturized, energy efficient light sources is of critical importance for the monolithic integration of MIR sensors. Here, we present an on-chip broadband thermal MIR source fabricated by combining a complementary metal oxide semiconductor (CMOS) micro-hotplate with a dielectric-encapsulated carbon nanotube (CNT) blackbody layer. The micro-hotplate was used during fabrication as a micro-reactor to facilitate high temperature (>700 ^{circ }C) growth of the CNT layer and also for post-growth thermal annealing. We demonstrate, for the first time, stable extended operation in air of devices with a dielectric-encapsulated CNT layer at heater temperatures above 600 ^{circ }C. The demonstrated devices exhibit almost unitary emissivity across the entire MIR spectrum, offering an ideal solution for low-cost, highly-integrated MIR spectroscopy for the Internet of Things.

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.

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.

Thermal Annealing Effects on Naturally Contacted Monolayer MoS2

Transition metal dichalcogenides such as MoS2, which can be produced in monolayer form, have attracted attention because of their interesting and potentially useful electrical and optical properties. These properties often depend sensitively on material properties such as defect density and crystallinity. Herein, the effects of postgrowth annealing on monolayer MoS2 grown using a novel chemical vapor deposition process are investigated. In this process bulk molybdenum patterns serve as the nucleation site and source material for high‐quality MoS2 material growth. After postgrowth thermal annealing, the photoluminescence is found to blueshift and become more uniform up to an annealing temperature of 300 °C. At higher temperatures, isolated monolayers begin to crack along the grain boundaries, which leads to variations in luminescence, whereas after annealing temperature of 200 °C, material anchored to the molybdenum patterns is found to easily ablate.

Study of Phase Transition in MOCVD Grown Ga2O3 from κ to β Phase by Ex Situ and In Situ Annealing

We report the post-growth thermal annealing and the subsequent phase transition of Ga2O3 grown on c-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). We demonstrated the post-growth thermal annealing at temperatures higher than 900 °C under N2 ambience, by either in situ or ex situ thermal annealing, can induce phase transition from nominally metastable κ- to thermodynamically stable β-phase. This was analyzed by structural characterizations such as high-resolution scanning transmission electron microscopy and x-ray diffraction. The highly resistive as-grown Ga2O3 epitaxial layer becomes conductive after annealing at 1000 °C. Furthermore, we demonstrate that in situ annealing can lead to a crack-free β-Ga2O3.

Defect properties of solar cells with layers of GaP based dilute nitrides grown by molecular beam epitaxy

Dilute nitrides lattice-matched to GaP were studied to explore the possibilities to improve their properties by additional indium or arsenic content in the GaPN alloy for further utilization in solar cells. Admittance spectroscopy shows that intrinsic layers of GaPNAs and InP/GaPN grown by molecular-beam epitaxy have unintentional background silicon donor doping. Deep-level transient spectroscopy allowed us to reveal several defect levels. In GaPNAs, two defect levels were detected at Ec − 0.58 eV and Ev + 0.44 eV, with respective concentrations of 4 × 1015 cm−3 and 2 × 1015 cm−3. After thermal annealing, these could be reduced by a factor of two and by more than one order of magnitude, respectively, leading to an increase of external quantum efficiency and open-circuit voltage of solar cells. The InP/GaPN layer exhibits a defect level at Ec − 0.44 eV (with a concentration of 2 × 1014 cm−3), which is of similar nature as the one at Ec − 0.58 eV in GaPNAs. Furthermore, unlike in GaPNAs, defect levels close to midgap were also detected in the InP/GaPN layer. These non-radiative recombination centers lead to poorer photoelectric properties of solar cells based on InP/GaPN as compared to those based on GaPNAs. Therefore, the introduction of arsenic in the compound and post-growth thermal annealing allowed us to reduce the defect concentrations in dilute nitrides and improve photoelectrical properties for photovoltaic applications.

Effects of thermal annealing on localization and strain in core/multishell GaAs/GaNAs/GaAs nanowires

Core/shell nanowire (NW) heterostructures based on III-V semiconductors and related alloys are attractive for optoelectronic and photonic applications owing to the ability to modify their electronic structure via bandgap and strain engineering. Post-growth thermal annealing of such NWs is often involved during device fabrication and can also be used to improve their optical and transport properties. However, effects of such annealing on alloy disorder and strain in core/shell NWs are not fully understood. In this work we investigate these effects in novel core/shell/shell GaAs/GaNAs/GaAs NWs grown by molecular beam epitaxy on (111) Si substrates. By employing polarization-resolved photoluminescence measurements, we show that annealing (i) improves overall alloy uniformity due to suppressed long-range fluctuations in the N composition; (ii) reduces local strain within N clusters acting as quantum dot emitters; and (iii) leads to partial relaxation of the global strain caused by the lattice mismatch between GaNAs and GaAs. Our results, therefore, underline applicability of such treatment for improving optical quality of NWs from highly-mismatched alloys. They also call for caution when using ex-situ annealing in strain-engineered NW heterostructures.

Investigation of the effect of different thicknesses and thermal annealing on the optical properties of GaAs0.1P0.89N0.01 alloys grown on GaP substrates

This work investigates the effect of the thickness of the epitaxial layer (100 nm and 1 μm) on the optical properties of quaternary GaAs0.1P0.89N0.01 alloys. Furthermore, the effect of rapid thermal annealing (RTA) on their properties has been studied using the Photoluminescence (PL) technique. Increasing the thickness of the epilayer led to an enhancement of the PL intensity as well as the energy bandgap, which was shifted to higher energy (from 1.82 eV in 100 nm to 1.94 eV in 1 μm layer). However, the 1.94 eV bandgap energy is not ideal for solar cells based materials grown on GaP substrates. Post-growth thermal annealing by rapid thermal annealing (RTA) for both samples resulted in an enhancement in the optical properties as observed by a decrease of the Full Width at Half Maximum (FWHM) and an increase of the PL intensity. Therefore, all results obtained in this study indicate that GaAs0.1P0.89N0.01 with 100 nm epilayer thick is better choice to fabricate good efficiency solar cells based materials on GaP substrates as compared to 1 μm sample.

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.

Microstructure and ferromagnetism of heavily Mn doped SiGe thin flims**Project supported by the National Key R & D Program of China (Grant No. 2017YFB0405702) and the National Natural Science Foundation of China (Grant No. 51671137).

Heavily Mn-doped SiGe thin films were grown by radio frequency magnetron sputtering and then treated by post-growth thermal annealing. Structural characterizations reveal the coexistence of Mn-diluted SiGe crystals and Mn-rich nanoclusters in the annealed films. Magnetic measurements indicate the ferromagnetic ordering of the annealed samples above room temperature . The data suggest that the ferromagnetism is probably mainly contributed by the Ge-rich nanoclusters and partially contributed by the tensile-strained Mn-diluted SiGe crystals. The results may be useful for room temperature spintronic applications based on group IV semiconductors.