Degradation Of Crystal Quality Research Articles

Preparation and Investigation of Micro-Transfer-Printable Single-Crystalline InP Coupons for Heterogeneous Integration of III-V on Si

New requirements for high-frequency applications in wireless communication and sensor technologies need III-V compound semiconductors such as indium phosphide (InP) to complement silicon (Si)-based technologies. This study establishes the basis for a new approach to heterogeneous integration of III-V on Si aimed at the transfer of single-crystalline InP coupons on Si via micro-transfer printing (μTP). The InP coupons will act as high-quality virtual substrates that allow selective homo-epitaxy. We present the chemical-mechanical polishing-based preparation and structural characterization of µm-thin (001) InP platelets, starting from high-quality 4-inch bulk crystals and micro-patterning into transferable coupons of several hundred µm2. The obtained InP platelets exhibit the desired thickness—below 10 ± 1 µm—and low surface roughness—<0.3 nm—on both sides, meeting the precondition for µTP and epitaxy. X-ray rocking curve measurements provide accurate spatial maps of the total strain, which indicate small strain variations in the µm-thin InP sample. Rocking curve mappings of the (0 0 4) reflection reveal half-widths below 16 arcsec in the majority of the sample area after thinning that is similar to commercially available InP bulk substrates. Pole figure measurements show no evidence of stress-induced micro-twinning or stacking faults. Overall, minor indications of crystal quality degradation in the product platelets, compared with the bulk samples, were detected.

Out‐of‐Plane Bi2O2Se Nanoflakes for Sensitive Gate‐Tunable Phototransistors

AbstractPhototransistors based on 2D materials exhibit gate‐voltage‐tunable performance, by which the key device parameters such as the photoresponsivity and photoconductive gain could easily surpass the photodetectors without the photomultiplication effects. Phototransistors with the light‐absorption material of 2D Bi2O2Se are rising due to its desirable material properties, such as small effective electron mass, high carrier mobility, and ultrafast intrinsic non‐equilibrium carrier recombination. The in‐plane growth of Bi2O2Se is prevailing on mica substrate, where the interfacial electrostatic interaction exists in between. Crystal quality degradation caused by the organic contaminants and corrosive acids, nevertheless, is unavoidable during transfer procedures to other substrates. In this work, different out‐of‐plane growth modes of Bi2O2Se are achieved, more specifically, whisker‐like, square‐ and rectangular‐shaped flakes are obtained with optimized growth parameters. The pristine interfaces are preserved in the phototransistors with the clean transfer of Bi2O2Se nanoflakes and the electrodes. A photoresponsivity of over 105 A W−1 and a record‐high response speed of 0.32 ms are achieved for the phototransistors fabricated based on the out‐of‐plane Bi2O2Se nanoflakes. Furthermore, trap state occupation with photoinduced carriers is modulated with gate voltages and explored under cryogenic conditions for improving the device performance.

Synthesis and Characteristics of Transferrable Single‐Crystalline AlN Nanomembranes

AbstractSingle‐crystalline inorganic semiconductor nanomembranes (NMs) have attracted great attention over the last decade, which poses great advantages to complex device integration. Applications in heterogeneous electronics and flexible electronics have been demonstrated with various semiconductor nanomembranes. Single‐crystalline aluminum nitride (AlN), as an ultrawide‐bandgap semiconductor with great potential in applications such as high‐power electronics has not been demonstrated in its NM forms. This very first report demonstrates the creation, transfer‐printing, and characteristics of the high‐quality single‐crystalline AlN NMs. This work successfully transfers the AlN NMs onto various foreign substrates. The crystalline quality of the NMs has been characterized by a broad range of techniques before and after the transfer‐printing and no degradation in crystal quality has been observed. Interestingly, a partial relaxation of the tensile stress has been observed when comparing the original as‐grown AlN epi and the transferred AlN NMs. In addition, the transferred AlN NMs exhibits the presence of piezoelectricity at the nanoscale, as confirmed by piezoelectric force microscopy. This work also comments on the advantages and the challenges of the approach. Potentially, the novel approach opens a viable path for the development of the AlN‐based heterogeneous integration and future novel electronics and optoelectronics.

2D Ti3C2 nanoflakes anchored ZnO photodetector with substantially improved deep-ultraviolet photoresponse and on/off ratio

Deep ultraviolet photodetectors (deep-UV PDs) are promising for multiple applications due to their high detecting accuracy and sensitivity in the absence of background sunlight disturbance. By alloying, the bandgap of ZnO can be extended for deep-UV optoelectronics. However, it is limited by the solubility and the crystal quality degradation for a high-component alloy, which hinders the practical applications. In this work, we simply use the 2D Ti3C2 nanoflakes to anchor on ZnO surface to enhance the deep-UV band absorption. Moreover, van der Waals heterojunctions formed at the dispersed Ti3C2/ZnO interfaces create surface depletion and passivation, which suppresses the dark current and enhances the photocurrent simultaneously. As a result, the Ti3C2 nanoflakes anchored ZnO PD shows enhanced photoresponse in the entire UV region. Particularly, in the deep-UV range this enhancement is much more significant, with a peak light to dark ratio, EQE, responsivity and detectivity of 1.64 × 104, 128.3 mA/W, 66.3% and 5.7 × 1011 Jones respectively achieved at 240 nm light illumination, which is enhanced by 284, 72, 74 and 55 times respectively compared to the pristine ZnO PD. This work provides a newly non-alloyed strategy to broaden ZnO-based deep-UV, high-performance photodetection applications.

Effect of Particles of Irregular Size on the Microstructure and Structural Color of Self-Assembled Colloidal Crystals.

Self-assembled colloidal crystals can exhibit structural colors, a phenomenon of intense reflection within a range of wavelengths caused by constructive interference. Such diffraction effects are most intense for highly uniform crystals; however, in practice, colloidal crystals may include particles of irregular size, which can reduce the quality of the crystal. Despite its importance in realizing high-quality structural colors, a quantitative relationship between particles of irregular size, crystal quality, and the resultant structural color response remains unclear. This study systematically and quantitatively investigates the effect of adding particles of irregular size on the microstructural quality and structural color reflectivity of colloidal crystals formed by evaporative self-assembly via experiment and simulation. We examine two sizes of irregular particles─those which are 1.9 times larger and 0.4 times smaller than the host crystal. We find that small irregular particles are more detrimental to surface crystal quality and structural color reflectivity than large irregular particles. When incorporated with 10% volume fraction of irregularly sized particles, the reflectivity of crystal films with large (small) irregularly sized particles decreases by 18.4 ± 5.6% (27.5 ± 5.8%), and a measure of surface crystal quality derived from Fourier analysis of scanning electron microscopy images reduces by 40.0 ± 4.5% (48.8 ± 6.0%). By modeling colloidal films incorporated with irregular particles via molecular dynamics simulation and computing the reflection spectra of the modeled crystals via the finite-difference time-domain method, we find that the peak reflectivity of the assembled structures increases monotonically with overall crystallinity, and that overall crystallinity is correlated with the volume fraction of incorporated irregular particles. The quantitative relationships developed in this study can be applied to predict the level of irregularly sized particles that can be tolerated in colloidal films before significant degradation in crystal quality and reflectivity occurs.

Optimizing the thermoelectric properties of thermally evaporated AgSnO2 thin films by post growth annealing process

In this manuscript, we have studied the thermoelectric properties of Silver Tin Oxide thin films deposited by thermal evaporation method. Tube furnace was used to deposit thin films on silicon (100) substrate by evaporating a mixture of Silver (Ag) and Tin (Sn) having (1:1) ratio. The experimental conditions were set as: distance from source to substrate (5 cm), Evaporation temperature inside the tube furnace (1000 °C), flow rate of oxygen gas (120sccm) and evaporation time for deposition was set as 40 min. Furthermore, diffusion of oxygen and mobility of charge carriers was controlled by annealing the samples at various temperatures after successful growth. We have observed that the value of Seebeck coefficient was increased from 242 to 630 µV/oC by increasing the annealing temperature from 600 to 900 ° C. This behavior is due the fact that increasing annealing temperature causes the mobility of charge carriers increased due to which simultaneous enhancement in Seebeck coefficient and electrical conductivity is observed. The enhancement in the carrier mobility was demonstrated by the fact that extra oxygen atoms resulted in the generation of grain boundaries defects which block the slow moving carriers and only fast moving electrons are able to reach cold end of sample. XRD and Raman spectroscopy results confirmed the hexagonal structure of AgSnO2. XRD data further confirmed the degradation of crystal quality due to increase in annealing temperature. The morphology of as grown and annealed samples was observed with the help of SEM.

Crystal quality degradation in MoTe2 monolayers by a thermal annealing and its suppression by hexagonal boron nitride encapsulation

Two dimensional transition metal dichalcogenides (TMDs) have attracted great research interests owing to a crossover to a direct gap upon reduction of the layer number down to a monolayer (1L) and exciton formation at room temperature based on extraordinarily large binding energy. In particular, a semiconducting molybdenum ditelluride (MoTe2) emits photons in the near-infrared region around 1.1 eV. Furthermore, it has a strong spin-orbit coupling to result in the longer spin and valley relaxation, providing a path to practical Si-based optoelectronic device applications. However, the 1L-MoTe2 crystal has some drawbacks such as surface oxidation within several days owing to its poor structural stability and crystal defect formation by a thermal annealing at a low temperature of 200 °C. It is reported that encapsulation by hexagonal boron nitride (hBN) prevents the oxidation of 1L-MoTe2 in air. There are few reports on effects of a thermal annealing on structural stability in hBN-encapsulated 1L-MoTe2. In this study, we investigate the effects of a thermal annealing on the crystal quality of hBN-encapsulated 1L-MoTe2 by a photoluminescence (PL) measurement at low temperatures. We demonstrate that hBN encapsulation is effective for suppression of crystal defect formation in 1L-MoTe2, improving thermal stability.Bulk hBN crystals were grown using a temperature-gradient method at a high pressure (4.0-5.5GPa) and high temperature (1500-1750 °C). Monolayer MoTe2 and multilayered hBN flakes were exfoliated from bulk crystals and were prepared on a polydimethylsiloxane (PDMS) sheet. We fabricated two types of sample structures on a 100-nm-thick SiO2/Si (100) substrate by a standard dry transfer technique using these flakes. The first sample comprised 1L-MoTe2 directly deposited on the SiO2/Si substrate. The second sample comprised 1L-MoTe2 encapsulated by hBN. The fabricated sample structures were thermally annealed at temperatures ranging from 100 to 200 °C for 15min in an atmosphere.Figures 1(a) and 1(b) show the PL spectra at 25K of the SiO2-supported and hBN-encapslated 1L-MoTe2, respectively. These PL spectra are reproduced using multi-Voigt functions to evaluate PL peak intensities in each spectrum. PL emissions attributed to neutral exciton (X0), charged exciton (T), and defect-bound excitons (XB1, XB2) are observed in both samples of SiO2-supported and hBN-encapsulated 1L-MoTe2. We identify these MoTe2 crystals as monolayers from the X0 PL peak position of 1.19 eV. Figure 1(c) shows the trion spectral weight (I T /I tot) plotted as a function of the annealing temperature for the SiO2-supported and hBN-encapsulated 1L-MoTe2. For both sample structures, the I T /I tot values decreases by the thermal annealing at 200℃, implying the decrease in excess carriers in 1L-MoTe2. The excess carriers should originate from impurities and residues adsorbed on the 1L-MoTe2. Thus, the thermal annealing makes it possible to reduce the impurities and residues, realizing high-quality sample structures. Figure 1(d) shows the PL intensity ratio (I XB1 + I XB2)/I X0 plotted as a function of the annealing temperature. Here, I XB1, I XB2, and I X0 are PL intensities of XB1, XB2, and X0 peaks, respectively. For the SiO2-supported 1L-MoTe2, the (I XB1 + I XB2)/I X0 value increases by increasing the annealing temperature while it is kept to be almost constant for the hBN-encapsulated 1L-MoTe2. It is reported that the XB1 and XB2 peaks are attributed to the formation of Te vacancies by a thermal annealing and adsorption of O2 and H2O molecules at the crystal defect sites, respectively. Therefore, an increase in the (I XB1 + I XB2)/I X0 value of the SiO2-supported samples should result from the crystal defect formation in 1L-MoTe2, indicating a large density of non-radiative recombination centers at 200 °C. In contrast, for the hBN-encapsulated samples, the Te vacancy formation and adsorption of O2 and H2O molecules at the crystal defect sites should be avoided owing to the top hBN acting as the capping layer to suppress Te atom desorption.In summary, we investigated the effect of a thermal annealing on a PL spectrum for SiO2-supported and hBN-encapsulated samples. It was found that the thermal annealing at 200 °C were able to remove residues and impurities on the 1L-MoTe2. For the SiO2-supported samples, an increase in the (I XB1 +I XB2 )/I X0 value was seen by the thermal annealing, indicating crystal defect formation due to desorption of Te atoms. In contrast, for the hBN-encapsulated samples, it was kept to be almost constant even after the thermal annealing at 200 °C. It was therefore concluded that the hBN encapsulation should improve the thermal stability in 1L-MoTe2. This work was partly supported by JSPS KAKENHI Grant Numbers JP25107003, JP25107004, JP15K21722, JP26248061, JP25107002, JP16H00982, and JP20H00354 and the CREST (JPMJCR15F3), JST. Figure 1

Photoluminescence and Raman Scattering of GaAs1-xBix Alloy

Photoluminescence (PL) and Raman spectra of GaAs1-xBix samples grown at different rates (0.09 to 0.5 µm/h) were investigated. The PL peak wavelength initially redshifted with the increase of growth rate and reached the longest wavelength (1158 nm) for sample grown at 0.23 µm/h. This is followed by PL peak wavelength blueshift for higher growth rates. The Raman data show peaks at 162, 228, 270, and 295 cm-1 which can be attributed to GaAs like phonons. GaBi like vibrational modes were also observed at 183 and 213 cm-1. However, the intensity of Bi induced phonons is significantly weaker compared to GaAs due to low concentration of Bi compared to As and thin GaAs1-xBix epilayer. The PL data and GaAs transverse optical (TO) to longitudinal optical (LO) phonons intensity ratio indicate that Bi concentration is highly dependent on the growth rate and the highest Bi concentration was obtained by sample grown at 0.23 µm/h. It is found that the full-width-at-half-maximum (FWHM) of GaAs LO mode increases significantly for samples grown at high growth rates suggesting crystal quality degradation due to lack of surfactant effects.

Catalytic properties of the composite of La-doped ZnO nanorods and Ag2CrO4 nanoparticles

To study the application of ZnO in photocatalysis, a composite of La-doped ZnO nanorods and Ag2CrO4 nanoparticles was prepared by hydrothermal and precipitation methods. At a fixed concentration, the volume of Ag+ precursor has a significant effect on the composition and structure of the composite. With the increase in the Ag+ source volume, the amount of Ag2CrO4 nanoparticles generated in the composite increases. When the volume of the Ag+ source exceeds some critical value, although the amount of Ag2CrO4 nanoparticles in the composite continues to increase, the quality of the crystals is lower. Both the content and crystal quality of Ag2CrO4 nanoparticles affect the photocatalytic properties of the composite. The introduction of additional oxygen vacancy defects in nanorods leads to the formation of more, and larger, Ag2CrO4 nanoparticles in the composite without degradation of crystal quality. Then, two dyes (methyl orange and methylene blue) were selected to examine the photocatalytic properties of the samples. The improvement of photocatalytic performance of the composite samples is realized by the separation of electron–hole pairs in both ZnO and Ag2CrO4 crystals.

Structural and Magnetic Properties of Fe-Doped GaN by Sol-Gel Technique

We report the effects of Fe doping on GaN with different concentrations synthesized by the sol-gel method. Crystalline quality and phase purity are characterized by X-ray diffraction. There is no deductible second phase up to 3% Fe-doped GaN and higher dopant concentration shows degradation in crystal quality with other phase formation. The morphological studies showed a walnut-shaped structure with different sizes up to 3% Fe dopant and the charged surface was observed in 5% due to agglomeration of Fe atoms in GaN, which is consistent with the XRD. The magnetic measurement revealed that up to 3% Fe-doped GaN were ferromagnetic at room temperature, whereas for 5% Fe-doped GaN, magnetization may arise due to dopant cluster; thus, lower percentage of Fe in GaN is suitable for DMS.

Enhancement of superconducting state properties of Fe0.994Ni0.007Te0.66Se0.34 single crystal with increasing pressure: a correlation with pressure-induced crystallinity degradation

Detailed investigations of Ni substituted Fe0.994Ni0.007Te0.66Se0.34 and unsubstituted Fe0.99Te0.66Se0.34 crystals were performed at ambient pressure and under hydrostatic pressure in order to understand the complex relationship between structure, magnetism, and superconductivity in iron selenides. The aim of the investigations was to determine the structural and superconducting state properties and study their evolution under pressure, up to 11 kbar. A comparison of Fe0.99Te0.66Se0.34, a material with much less developed phase separation, with FeTe0.5Se0.5, a material exhibiting pronounced nanoscale phase separation, was also performed. Under ambient pressure, a weakening of superconducting state properties was observed in the Fe0.994Ni0.007Te0.66Se0.34 crystal, with disorder introduced by Ni substitution, as compared with those in Fe0.99Te0.66Se0.34. For Fe0.994Ni0.007Te0.66Se0.34, the x-ray diffraction studies have revealed a degradation of crystal quality under applied elevated pressure. Superconducting state properties of the single phase Fe0.99Te0.66Se0.34 crystal, such as upper and lower critical fields, were found to be poorer, at both ambient and hydrostatic pressure, than those observed for FeTe0.5Se0.5 crystals exhibiting pronounced nanoscale phase separation. Comprehensive studies of the impact of pressure on crystal structure and on superconducting state properties confirm that enhancement of superconductivity correlates with the appearance of mosaicity under pressure. The above observation leads to the expectation of a pressure-induced increase in inhomogeneity, leading to the enhancement of superconducting properties in other sulpho-iron seleno-tellurides of anti-PbO-type structure.

InGaAs photo field-effect-transistors (PhotoFETs) on half-inch Si wafer using layer transfer technology

InGaAs photo field-effect transistors (PhotoFETs) integrated on a half-inch Si wafer have been demonstrated by means of layer transfer technology. According to the Raman measurement and scanning transmission electron microscopy, the InGaAs layer was transferred onto the half-inch Si wafer without degradation of crystal quality or introduction of strain. The optical performance of InGaAs photoFETs on Si, operating in shortwave infrared (SWIR) region was also investigated. The photoFETs on Si wafer show a responsivity of ∼3 A W−1 at SWIR of 1550 nm with investigated range of bias. These results demonstrate InGaAs layer transfer technology is suitable for SWIR detectors on CMOS compatible platforms for monolithic integration.

Effect of Carbon Doping Level on Static and Dynamic Properties of AlGaN/GaN Heterostructures Grown on Silicon

The impact of the extrinsic carbon doping level was investigated with the aim of finding the optimal level for GaN-on-Si HFETs. A tradeoff between the crystal quality degradation by carbon doping and dynamic properties of HFET structures was observed indicating the role of vertical dislocations in the dynamic performance of the carbon-doped GaN buffer.

Understanding arsenic incorporation in CdTe with atom probe tomography

Overcoming the open circuit voltage deficiency in Cadmium Telluride (CdTe) photovoltaics may be achieved by increasing p-type doping while maintaining or increasing minority carrier lifetimes. Here, routes to higher doping efficiency using arsenic are explored through an atomic scale understanding of dopant incorporation limits and activation in molecular beam epitaxy grown CdTe layers. Atom probe tomography reveals spatial segregation into nanometer scale clusters containing > 60 at% As for samples with arsenic incorporation levels greater than 7–8 × 1017 cm−3. The presence of arsenic clusters was accompanied by crystal quality degradation, particularly the introduction of arsenic-enriched extended defects. Post-growth annealing treatments are shown to increase the size of the As precipitates and the amount of As within the precipitates.

Yellow-red light-emitting diodes using periodic Ga-flow interruption during deposition of InGaN well.

We report a possible way to extend the emission wavelength of InyGa1-yN/InxGa1-xN quantum-well (QW) light-emitting diodes (LEDs) to the yellow-red spectral range with little degradation of the radiative efficiency. The InyGa1-yN well with high indium (In) content (HI-InyGa1-yN) was realized by periodic Ga-flow interruption (Ga-FI). The In contents of the HI-InyGa1-yN well and the InxGa1-xN barrier were changed to manipulate the emission wavelength of the LEDs. An In0.34Ga0.66N/In0.1Ga0.9N-QW LED, grown by continuous growth mode (C-LED), was prepared as a reference. The photoluminescence (PL) wavelengths of the HI-InyGa1-yN/InxGa1-xN QW LEDs were changed from 556 to 597 nm. The PL intensity of the HI-InyGa1-yN/InxGa1-xN LED with a peak wavelength of 563 nm was 2.7 times stronger than that of the C-LED (λ = 565 nm). The luminescence intensity for the HI-InyGa1-yN/InxGa1-xN QW LED emitting at 597 nm was stronger than those of the other LED samples with shorter wavelengths. Considering the previous works on degradation in crystal quality and increase in the quantum-confined Stark effect with increasing In content in InGaN, the approach in this work is very promising for yellow-red InGaN LEDs.

The effect of gaseous ammonia treatment of AM-6 crystallized in the presence of tetramethylammonium cations on their removal from the crystal pores, and the crystal quality and structure of AM-6

Vanadosilicate AM-6 products with different crystal quality (i.e., disorder, or average length of the V-O-V chains in the AM-6 framework), as determined from the FWHM of ∼868 cm−1 Raman band, but with identical crystallinity/long range order and purity, as determined from the XRD data, were hydrothermally synthesized from reaction mixtures with different molar compositions at 448–503 K using tetramethylammonium (TMA+) ions. The combined SEM, EDX, XRD, Raman, TGA, XPS, and nitrogen adsorption isotherm data showed that by treating these as-synthesized materials in gaseous ammonia at 673 K it is possible to completely remove TMA+ ions from the AM-6 pores without adversely affecting product crystallinity and the micropore structure of AM-6, and with only minimal degradation of crystal quality of the resulting products. However, the success in a complete removal of TMA+ ions without affecting other crystalochemical characteristics of the product depended on its initial crystal quality. TMA+ ions could not be completely removed from the lowest crystal quality product (FWHM = 20.3 cm−1) without completely decomposing the V-O-V chains. The intermediate crystal quality product (FWHM = 15.0 cm−1) could be successfully ammonia-treated only at 673 K to remove TMA+ ions, but higher temperatures resulted in a complete decomposition of the V-O-V chains. The highest crystal quality product (FWHM = 12.7 cm−1) retained the integrity of V-O-V chains as well as the long range order and the micropore structure when ammonia-treated up to 748 K.

Growth, structural and optical properties of orderly SnO2 microhemispheres on patterned sapphire substrates

One-dimensional nanoscaled materials, such as nanotubes, nanowires and nanobelts, have attracted a great deal of attention in recent years because of their unique electronic, optical, and mechanical properties. Their potential applications are found in next generation devices, functional materials, and sensors. A material of particular interest is stannic oxide (SnO2), which is a novel oxide semiconductor material for ultraviolet and blue luminescence devices due to its wide band gap of 3.6 eV at room temperature. SnO2 can also be widely used in many fields, such as gas sensors, optoelectronic devices, and transparent conductive glass, because of its high optical transparency in the visible range, low resistivity, and higher chemical and physical stability. In recent years, one-dimensional nanostructures of SnO2 materials, such as nanobelts, nanotubes, and nanowires, have been reported. However, the preparations of orderly SnO2 micro/nanostructures have been rarely reported. In this paper, orderly SnO2 microhemispheres with different sizes are grown on patterned sapphire substrates by a traditional chemical vapor deposition method without using any catalyst. The patterned sapphire substrates are cleaned by using a standard sapphire wafer cleaning procedure. High-purity metallic Sn powders (99.99%) and oxygen gas are used as Sn and oxygen sources, respectively. The flow rate of high-purity Ar carrier gas is controlled at 200 sccm, and the oxygen reactant gas with a flow rate of 100 sccm is introduced into the system. In the growth process, the whole system is kept at 1000℃ for 30 min. The surface morphologies, structural and optical properties of the SnO2 microhemispheres are investigated by the field emission scanning electron microscope (HITACHI S4800), the X-ray diffraction with a Cu Kup radiation (0.15418 nm), the optical absorption spectroscope (UV-3600 UV-VIS-NIR, Shimadzu), and the photoluminescence spectroscope with an excitation source of He-Cd laser (=325 nm) to identify the As related acceptor emission, respectively. These results show that the diameters of SnO2 microhemispheres become larger, and the crystal quality is degraded with the increase of Sn powder mass. The special selective growth of SnO2 microhemisphere on a patterned sapphire substrate is found. In addition, we also find that the optical band gaps of the samples A-D are all redshifted with the increase of Sn powder mass. The shrinkage of Eg in the absorption spectrum should be partly attributed to the degradation of crystal quality because of excess Sn sources. This growth method of SnO2 microhemisphere provides a feasible and effective way of preparing the high density, orderly arrangement of SnO2 micro/nanostructures.

Relaxation rate and polarization charge density model for AlN/Al$_{x}$Ga$_{1 - x}$N/AlN heterostructures

This work describes the strain-relaxation--dependent carrier concentration ($n_{s})$ profile model using spontaneous and piezoelectric polarization for AlN/Al$_{x}$Ga$_{1 - x}$N/AlN HEMTs in all mole fraction ($x)$ interpolations. As $x$ varies, the Aluminum Gallium Nitride (AlGaN) channel shows strain relaxation with the Aluminum Nitride (AlN) barrier. The degree of relaxation is modeled from AlN to GaN regions in the channel. It shows that the AlN barrier and buffer relaxation and strain recovery occurs due to the gradual crystal quality degradation from barrier/buffer to the channel interface. These combination devices show less drain current degradation with temperature variation from 300 K to 573 K. This model shows a good agreement with experimental data with a carrier density of $n_{s}$ = 2.8 $\times $ 10$^{13}$/cm$^{2}$.

(Invited) Germanium Enrichment for Planar-, Fin- and Nanowire-Channel MOSFETs Made on SOI

1. Introduction Depleted body devices such as FinFETs and Ultra-Thin Body and Buried oxide Fully Depleted Silicon-On-Insulator (UTBB FDSOI) MOSFETs have emerged as architectures of choice for technology nodes where conventional planar bulk devices do not provide enough immunity against short channel effects and variability. Offering unsurpassed electrostatic control of the channel, NanoWire FETs (NWFETs)-based architectures are considered as promising successors for future nodes. In all these depleted-body architectures, the control of active power is a strong requirement. Along with supply voltage scaling, it imposes design-level power management strategies, parasitic capacitance reduction and it calls for the use of high-mobility channel materials. For p-type channels, compressively strained silicon germanium alloys (SiGe) are attractive candidates. When depleted-body devices are fabricated on SOI, local germanium enrichment can convert Si to SiGe. This fabrication sequence starts with the growth of an epitaxial SiGe layer, followed by oxidation. During the latter, the surface of the SiGe layer is converted to silicon dioxide while Ge is segregated to the lower part of the stack, which gradually becomes more Ge-rich. This paper discusses the fabrication of SiGe channel for UTBBSOI MOSFETs, FinFETs and NWFETs. It reviews the key points associated to each architecture and critical aspects related to SiGe thermal budget, critical thickness, growth rate control and crystal quality. Results highlight the interest of NWFETs for future technology nodes. 2. SiGe channel UTBB FDSOI MOSFETs The Ge enrichment technique was initially developed for the planar FDSOI architecture and in that context, it was shown that high quality uniform SiGe films can be fabricated by alternating oxidizing and diffusion steps. For a SiGe/Si initial stack of given thickness and composition, the enrichment thermal budget needs to be significantly different when it is performed on SOI or on tensile SOI (sSOI). In the last case, the film initial tensile strain slows Ge diffusion (Fig. 1-a), which can lead to Ge pile up and defect formation if process temperature is too high with respect to the peak concentration (Fig. 1-b). Defect-free SiGe layers can be obtained on sSOI by using longer diffusion steps and reducing process temperature (Fig. 1-c). 3. SiGe-channel FinFET Although Ge enrichment can produce fully-strained layers, the patterning of SiGe blanket film into finite-dimension active areas yields elastic stress relaxation. This feature is particularly attractive for the fabrication of SiGe channel FinFETs where channel configuration results in longitudinal uniaxial compression, which is most favorable to hole transport. However, this processing sequence involves the elaboration of a thick blanket SiGe layer (through Ge enrichment and epitaxial thickening). Even with optimized processing, for a targeted Fin height (SiGe thickness) of 35 nm, significant crystal quality degradation is observed by X-Ray Reciprocal Space Mapping for {113} (RSM) when the Ge fraction approaches 50% (Fig. 2-a), in agreement with literature. To overcome this limit, Ge enrichment of already patterned Si Fins was investigated. This sequence relieves the limits set by SiGe critical thickness in the vertical direction, but it also constricts the pre-enrichment SiGe growth conditions. If SiGe growth rate is strongly dependent on crystal orientation, the final Fin profile is degraded (Fig. 2-b). In contrast, Fin profile can be maintained (Fig. 2-c) when uniform SiGe thickness can be achieved on Fin sidewalls (Fig. 2-d). RSM shows that Fins with up to 50% Ge fraction exhibit no significant relaxation (Fig. 2-e). This approach is expected to be most beneficial for tall Fins. 4. SiGe-channel NWFETs W-gate NWFETs were fabricated in a gate first scheme (Fig. 3-a). This configuration combines the respective benefits of SiGe channel FDSOI MOSFETs and FinFETs in terms of SiGe crystal stability [5] and uniaxial strain. With this and excellent electrostatic integrity, significant mobility gains are demonstrated down to a gate length of 14nm (Fig. 3-b), paving the way for future energy-efficient systems. 5. Acknowledgment Part of these results was obtained in a Joint Development Program between IBM, STMicroelectronics and LETI CEA. Figure 1

Growth of Metamorphic InGaAs on GaAs (111)A: Counteracting Lattice Mismatch by Inserting a Thin InAs Interlayer

We have successfully grown high quality InxGa1–xAs metamorphic layer on GaAs (111)A using molecular beam epitaxy. Inserting a thin 3.0–7.1 monolayer (ML) InAs interlayer between the In0.25Ga0.75As and GaAs allowed the formation of a nearly lattice-relaxed In0.25Ga0.75As with a very flat upper surface. However, when the thickness of the inserted InAs is thinner or thicker than these values, we observed degradation of crystal quality and/or surface morphology. We also revealed this technique to be applicable to the formation of a high quality metamorphic InxGa1–xAs layer with a range of In compositions (0.25 ≤ x ≤ 0.78) on GaAs (111)A. Cross-sectional scanning transmission electron microscope studies revealed that misfit dislocations formed only at the interface of InAs and GaAs, not at the interface of In0.25Ga0.75As and InAs. From the dislocation density analysis, it is suggested that the dislocation density was decreased by growing In0.25Ga0.75As on InAs, which effectively contribute the strain relaxatio...