As-grown Material Research Articles

Surface Analysis of AlGaN Channel Hemts

High electron mobility transistors (HEMTs) with AlGaN channels are excellent candidates for high-power applications due to increased bandgap and electric field as compared to GaN channel HEMTs. An increase in critical electric field is achieved by increasing the Al mole fraction of both the barrier and channel layer of the HEMT to AlyGa1-yN/AlxGa1-xN, where y>x≥0.3. However, the effects of the fabrication process on the surface of high Al content AlGaN has yet to be studied. As AlyGa1-yN/AlxGa1-xN HEMTs gain interest for a variety of applications, unintended modifications to the surface of the AlGaN can have unintended consequences to device performance. X-ray photoelectron spectroscopy (XPS) was performed on Al0.45Ga0.55N/Al0.3Ga0.7N heterostructure. XPS indicated that native surface oxidation of Al0.45Ga0.55N occurs in as-grown material stored at ambient of approximately 8 atomic %. A standard low power microwave O2 ash is a standard process performed to remove organic contamination and descum after photolithography. Though an Al0.45Ga0.55N sample exposed to an O2 ash indicated a slight decrease in C, an increase in the surface oxidation from approximately 8 atomic % (as grown) to approximately 20 atomic % after O2 plasma was observed (Fig. 1). XPS spectra show that after exposure to an O2 ash, oxygen is binding to both Ga and Al. This has substantial implications for device performance and integration schemes for heterostructures with increasing Al content. As the presence of an oxide interlayer can have large implications to Schottky contacts, the effect of an HCl clean to remove the native oxide was investigated. Even after exposure to a 35% HCl dilute in H2O, the O atomic % decreased from 8% to only 6%. Presuming that lower oxygen surface content is desirable, the HCl clean offers a tangible benefit for the surface of Al0.45Ga0.55N/Al0.3Ga0.7N heterostructures. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

Nanomorphology and performance of pure and doped lead selenide for infrared detector

Lead selenide (PbSe) has been studied as a promising material for room temperature midwave infrared detection. We have investigated pure PbSe, as well as tin (Tn) and cadmium (Cd)-doped PbSe, nanocrystalline materials produced using physical vapor transport methods on glass and high-resistivity silicon substrates. The morphologies were investigated by scanning electron microscopy and energy-dispersive x-ray analysis. Pure PbSe layers consisted of nanocrystals that change into cubes and cuboids upon annealing. Cuboids generally grew in [100] orientation and ultimately developed in nanorods. Growth on silicon and glass substrates showed different morphologies of pure PbSe material. Parabolic and elongated morphologies resulted in nanowires on the top of thin layers of PbSe nanofilm, which acted as the substrate. Under low gradient annealing conditions (<20 K/cm), elongated morphologies grew into nanorods. Annealing of these samples resulted in coarse nanomorphologies with higher resistivity. In the case of Tn-doped PbSe, annealing dissolved a Tn-rich phase observed in as-grown films. Cd- and iodine-doped films produced through the addition of Cd selenide and Cd iodide, respectively, showed higher resistivity than similarly treated pure PbSe films. Annealing of as-grown materials in the presence of oxygen or iodine showed increased resistivity and significant changes in optical characteristics.

Correlation of Etch Pits and Dislocations in As-grown and Thermal-Cycle-Annealed HgCdTe(211) Films

This paper reports observations of the different types of etch pits and dislocations present in thick HgCdTe (211) layers grown by molecular beam epitaxy on CdTe/Si (211) composite substrates. Dislocation analysis for as-grown and thermal cycle-annealed samples has been carried out using bright-field transmission electron microscopy. Triangular pits present in as-grown material are associated with a mixture of Frank partials and perfect dislocations, while pits with fish-eye shapes have perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector. The dislocations beneath skew pits are more complex as they have two different crystallographic directions, and are associated with a mixture of Shockley partials and perfect dislocations. Dislocation analysis of samples after thermal cycle annealing (TCA) shows that the majority of dislocations under the etch pits are short segments of perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector while the remainder are Shockley partials. The absence of fish-eye shape pits in TCA samples suggests that they are associated with mobile dislocations that have reacted during annealing, causing the overall etch pit density to be reduced. Very large pits with a density ∼2×103 cm−2 are observed in as-grown and TCA samples. These defects thread from within the CdTe buffer layer into the upper regions of the HgCdTe layers. Their depth in as-grown material is so large that it is not possible to locate and identify the underlying defects.

Dominant Formation of the Carbon Nanocoils at Low Temperature

The single helix type carbon nanocoils were formed under the low temperature (550°C) condition. For the formation of the carbon nanocoils, C2H2 was used as a source gas and SF6 was used as an additive gas under the thermal chemical vapor deposition system. The morphologies of as-grown carbon materials at the low temperature (550°C) and the mass ratios of product/catalyst were investigated according to the ratio of C2H2/ SF6 flow and the injection times of SF6 and C2H2flows. The conversion efficiency from the carbon source gas (C2H2) to the as-grown carbon materials was estimated using the mass ratios of product/catalyst. The sample having the ratio of C2H2/SF6 flow = 50 had the highest conversion efficiency. For the dominant formation of the single helix type CNCs, the optimal ratio of C2H2/SF6 flow was determined to be around 10. As the growth aspect of the single helix type CNCs under the optimal ratio of C2H2/SF6 flow, the formation of the CNCs was understood to be initiated within the reaction time of 5min after the formation of the carbon nanofilaments. The detailed growth mode of the CNCs was proposed.

Preparation of high performance Bi2Sr2Co1.8Ox thermoelectric materials from nanosized precursors

ABSTRACTBi2Sr2Co1.8Ox thermoelectric (TE) materials were prepared by three different synthesis methods producing nanosized precursors: coprecipitation (with ammonium carbonate or oxalic acid) and attrition milling, which were compared with those obtained by the classical solid state method. Microstructure has shown that precursors produced by coprecipitation and attrition milling methods produced nanometric precursors much smaller than the typical sizes produced by the solid state route. The TE properties are in agreement with the microstructural features, leading to lower resistivity in all the samples, compared with the solid state ones, while Seebeck coefficient is practically unchanged in all cases. As a consequence, maximum power factor values of around four times higher than those obtained in the classical solid state method have been determined. Moreover, the highest power factor value at 650°C is higher than the best results obtained in as-grown textured materials produced by the laser floating zone technique.

Correlation of Etch Pits and Dislocations in As-grown and Thermal Cycle-Annealed HgCdTe(211) Films

This paper reports observations of the different types of etch pits and dislocations present in thick HgCdTe (211) layers grown by molecular beam epitaxy on CdTe/Si (211) composite substrates. Dislocation analysis for as-grown and thermal cycle-annealed samples has been carried out using bright-field transmission electron microscopy. Triangular pits present in as-grown material are associated with a mixture of Frank partials and perfect dislocations, while pits with fish-eye shapes have perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector. The dislocations beneath skew pits are more complex as they have two different crystallographic directions, and are associated with a mixture of Shockley partials and perfect dislocations. Dislocation analysis of samples after thermal cycle annealing (TCA) shows that the majority of dislocations under the etch pits are short segments of perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector while the remainder are Shockley partials. The absence of fish-eye shape pits in TCA samples suggests that they are associated with mobile dislocations that have reacted during annealing, causing the overall etch pit density to be reduced. Very large pits with a density ∼2×103 cm−2 are observed in as-grown and TCA samples. These defects thread from within the CdTe buffer layer into the upper regions of the HgCdTe layers. Their depth in as-grown material is so large that it is not possible to locate and identify the underlying defects.

XPS Studies of the Graded Band Gap Al&lt;sub&gt;x&lt;/sub&gt;Ga&lt;sub&gt;1-x&lt;/sub&gt;N Material Grown by MOCVD

To characterize the properties of the as-grown AlxGa1-xN material for producing high property AlxGa1-xN photocathode in ultraviolet (UV) detection, the Ar+ sputtering and X-ray photoelectric spectroscopy (XPS) scan are performed. XPS spectra indicates that although processed with chemical solutions, AlxGa1-xN still contains large amount of carbon and oxide on the surface, which can be completely removed by Ar+ sputtering within few minutes. Ga3d and Al2p curves show that there are other compounds of Ga and Al on the surface but both become very concentrated when sputtering continues. The proportion of Al increases and that of Ga decreases gradually from surface to AlN bulk, which testify the graded band gap profile of the AlxGa1-xN sample. There is always a very slight amount of oxygen in the AlN layer, which is regarded as native element during material growth. At the interface of AlN and sapphire, an abrupt transition appears which can influence the properties of the AlxGa1-xN photocathode when it works with the transmittance mode

Low Temperature Synthesis of the Carbon Micro Materials

The formation of the carbon microcoils could be achieved under the low temperature (550°C). Ni powders were used as the catalyst on the alumina substrate. C2H2 was used as a source gas and a few amount of SF6 gas were used as an additive gas under the thermal chemical vapor deposition system. The surface morphologies of as-grown carbon materials at the low temperature (550°C) were also investigated according to the total pressure and the injection time of SF6 flow. At 80Torr and 10min injection time of SF6 flow, the carbon microcoils were highly developed on the entire surface of the sample. The lower or higher total pressure and the longer injection time of SF6 flow deteriorated the formation of CMCs. The head area of the carbon microcoils was closely investigated and the growth mode for the initiation of the carbon microcoils at the low temperature was suggested.

Anomalous Above-Gap Photoexcitations and Optical Signatures of Localized Charge Puddles in Monolayer Molybdenum Disulfide.

Broadband optoelectronics such as artificial light harvesting technologies necessitate efficient and, ideally, tunable coupling of excited states over a wide range of energies. In monolayer MoS2, a prototypical two-dimensional layered semiconductor, the excited state manifold spans the visible electromagnetic spectrum and is comprised of an interconnected network of excitonic and free-carrier excitations. Here, photoluminescence excitation spectroscopy is used to reveal the energetic and spatial dependence of broadband excited state coupling to the ground-state luminescent excitons of monolayer MoS2. Photoexcitation of the direct band gap excitons is found to strengthen with increasing energy, demonstrating that interexcitonic coupling across the Brillouin zone is more efficient than previously reported, and thus bolstering the import and appeal of these materials for broadband optoelectronic applications. Narrow excitation resonances that are superimposed on the broadband photoexcitation spectrum are identified and coincide with the energetic positions of the higher-energy excitons and the electronic band gap as predicted by first-principles calculations. Identification of such features outlines a facile route to measure the optical and electronic band gaps and thus the exciton binding energy in the more sophisticated device architectures that are necessary for untangling the rich many-body phenomena and complex photophysics of these layered semiconductors. In as-grown materials, the excited states exhibit microscopic spatial variations that are characteristic of local carrier density fluctuations, similar to charge puddling phenomena in graphene. Such variations likely arise from substrate inhomogeneity and demonstrate the possibility to use substrate patterning to tune local carrier density and dynamically control excited states for designer optoelectronics.

Enhanced electrical and thermoelectric properties from textured Bi1.6Pb0.4Ba2Co2Oy/Ag composites

Bi1.6Pb0.4Ba2Co2Oy/3 wt% Ag composites have been textured using the laser floating zone technique. Microstructural observations have shown the formation of relatively high amounts of secondary phases and cracks along the plate-like grains in the as-grown materials. In spite of these microstructural features, electrical properties have not been drastically decreased. Annealing of these as-grown rods has enhanced the grains connectivity, decreased the contents of the secondary phases, and healed the cracks, thereby increasing their electrical performances. It has been deduced that oxygen content in the thermoelectric phase has not been significantly modified through annealing, as demonstrated by the very similar Seebeck values at room temperature in the as-grown and annealed samples. As a consequence of the low electrical resistivity values in the annealed materials, they reached higher power factor values than the as-grown ones. The highest power factor at room temperature is in the range of those reported for single crystals, while at 650 °C, it is the highest reported in Bi–Ba–Co–O polycrystalline family so far.

Modulation of electronic properties of tin oxide nanobelts via thermal control of surface oxygen defects

Nanomaterials made from binary metal oxides are of increasing interest because of their versatility in applications from flexible electronics to portable chemical and biological sensors. Controlling the electrical properties of these materials is the first step in device implementation. Tin dioxide (SnO2) nanobelts (NB) synthesized by the vapor–liquid–solid mechanism have shown much promise in this regard. We explore the modification of devices prepared with single crystalline NBs by thermal annealing in vacuum and oxygen, resulting in a viable field-effect transistor (FET) for numerous applications at ambient temperature. An oxygen annealing step initially increases the device conductance by up to a factor of 105, likely through the modification of the surface defects of the NB, leading to Schottky barrier limited devices. A multi-step annealing procedure leads to further increase of the conductance by approximately 350% and optimization of the electronic properties. The effects of each step is investigated systematically on a single NB. The optimization of the electrical properties of the NBs makes possible the consistent production of channel-limited FETs and control of the device performance. Understanding these improvements on the electrical properties over the as-grown materials provides a pathway to enhance and tailor the functionalities of tin oxide nanostructures for a wide variety of optical, electronic, optoelectronic, and sensing applications that operate at room temperature.

Electronic properties of the residual donor in unintentionally doped β-Ga2O3

Electron paramagnetic resonance was used to study the donor that is responsible for the n-type conductivity in unintentionally doped (UID) β-Ga2O3 substrates. We show that in as-grown materials, the donor requires high temperature annealing to be activated. In partly activated materials with the donor concentration in the 1016 cm−3 range or lower, the donor is found to behave as a negative-U center (often called a DX center) with the negative charge state DX− lying ∼16–20 meV below the neutral charge state d0 (or Ed), which is estimated to be ∼28–29 meV below the conduction band minimum. This corresponds to a donor activation energy of Ea∼44–49 meV. In fully activated materials with the donor spin density close to ∼1 × 1018 cm−3, donor electrons become delocalized, leading to the formation of impurity bands, which reduces the donor activation energy to Ea∼15–17 meV. The results clarify the electronic structure of the dominant donor in UID β-Ga2O3 and explain the large variation in the previously reported donor activation energy.

High-k/GaN interface engineering toward AlGaN/GaN MIS-HEMT with improved Vth stability

Metal–insulator–semiconductor (MIS) capacitor structures were fabricated on AlGaN/GaN two-dimensional electron gas heterostructure material in order to investigate important aspects of the gate module of a corresponding MIS-high electron mobility transistor device. The process sequence started with an initial wet chemical surface treatment of the as-grown semiconductor material followed by an atomic layer deposition of Al2O3 (high-k first). The electrical analysis focused on the gate leakage current as well as on the shift of the threshold voltage (Vth) upon bias stress in the off- and the on-state regions. The high-k first samples showed much better Vth stability compared to lithographically processed samples, in which the high-k deposition was performed after ohmic contact formation and just before the gate electrode metallization. These results reflect a superior quality of the high-k/GaN interface for the processed structures according to the high-k first approach.

Engineering and Localization of Quantum Emitters in Large Hexagonal Boron Nitride Layers.

Hexagonal boron nitride is a wide-band-gap van der Waals material that has recently emerged as a promising platform for quantum photonics experiments. In this work, we study the formation and localization of narrowband quantum emitters in large flakes (up to tens of micrometers wide) of hexagonal boron nitride. The emitters can be activated in as-grown hexagonal boron nitride by electron irradiation or high-temperature annealing, and the emitter formation probability can be increased by ion implantation or focused laser irradiation of the as-grown material. Interestingly, we show that the emitters are always localized at the edges of the flakes, unlike most luminescent point defects in three-dimensional materials. Our results constitute an important step on the roadmap of deploying hexagonal boron nitride in nanophotonics applications.

Preparation of Mn-Zn ferrite nanoparticles and their silica-coated clusters: Magnetic properties and transverse relaxivity

Hydrothermal synthesis of Mn1−xZnxFe2O4 nanoparticles followed by direct encapsulation of the as-grown material into silica is demonstrated as a fast and facile method for preparation of efficient negative contrast agents based on clusters of ferrite crystallites. At first, the hydrothermal procedure is optimized to achieve strictly single-phase magnetic nanoparticles of Mn-Zn ferrites in the compositional range of x≈0.2–0.6 and with the mean size of crystallites ≈10nm. The products are characterized by powder X-ray diffraction, X-ray fluorescence spectroscopy, and SQUID magnetometry, and the composition close to x=0.4 is selected for the preparation of silica-coated clusters with the mean diameter of magnetic cores ≈25nm. Their composite structure is studied by means of transmission electron microscopy combined with detailed image analysis and magnetic measurements in DC fields. The relaxometric studies, performed in the magnetic field of B0=0.5T, reveal high transverse relaxivity (r2(20°C)=450s−1mmol(Me3O4)−1L) with a pronounced temperature dependence, which correlates with the observed temperature dependence of magnetization and is ascribed to a mechanism of transverse relaxation similar to the motional averaging regime.

The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2.

We report on preparation dependent properties observed in monolayer WS2 samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO2, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS2) exhibit distinctly different optical properties than transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following transfer to fresh Si/SiO2 there is a ~50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ~0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS2 for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS2. These results demonstrate a strong sensitivity to sample preparation that is important for both a fundamental understanding of these novel materials as well as reliable reproduction of device properties.

Solar Cell Efficiency and High Temperature Processing of n-type Silicon Grown by the Noncontact Crucible Method

The capital expense (capex) of conventional crystal growth methods is a barrier to sustainable growth of the photovoltaic industry. It is challenging for innovative techniques to displace conventional growth methods due the low dislocation density and high lifetime required for high efficiency devices. One promising innovation in crystal growth is the noncontact crucible method (NOC-Si), which combines aspects of Czochralski (Cz) and conventional casting. This material has the potential to satisfy the dual requirements, with capex likely between that of Cz (high capex) and multicrystalline silicon (mc-Si, low capex). In this contribution, we observe a strong dependence of solar cell efficiency on ingot height, correlated with the evolution of swirl-like defects, for single crystalline n-type silicon grown by the NOC-Si method. We posit that these defects are similar to those observed in Cz, and we explore the response of NOC-Si to high temperature treatments including phosphorous diffusion gettering (PDG) and Tabula Rasa (TR). The highest lifetimes (2033μs for the top of the ingot and 342μs for the bottom of the ingot) are achieved for TR followed by a PDG process comprising a standard plateau and a low temperature anneal. Further improvements can be gained by tailoring the time-temperature profiles of each process. Lifetime analysis after the PDG process indicates the presence of a getterable impurity in the as-grown material, while analysis after TR points to the presence of oxide precipitates especially at the bottom of the ingot. Uniform lifetime degradation is observed after TR which we assign to a presently unknown defect. Future work includes additional TR processing to uncover the nature of this defect, microstructural characterization of suspected oxide precipitates, and optimization of the TR process to achieve the dual goals of high lifetime and spatial homogenization.

Efficient strategy to Cu/Si catalyst into vertically aligned carbon nanotubes with bamboo shape by CVD technique

Bamboo-shaped vertically aligned carbon nanotubes (bs-VACNTs) were fabricated on Cu/Si catalyst by chemical vapour deposition (CVD) technique under the atmospheric pressure. The catalytic material (Cu/Si) played a vital role in attaining bs-VACNTs, which is synthesized by drop cast method in a cost-effective manner. Using this catalytic support, we have achieved the tip growth bs-VACNTs at low temperature with well graphitization. The as-grown carbon material was then characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) analyzer, high-resolution transmission electron microscope (HRTEM) and Raman spectroscopy. XRD technique confirms the formation of hexagonal graphitic carbon planes of carbon nanotubes (CNTs). The surface morphology of the material was characterized by SEM, which clearly infer vertically aligned CNTs. The nature, diameter and crystallinity were noticed by HRTEM and Raman spectroscopy, respectively. Further, we have also studied the electrochemical properties of the bs-VACNTs and it seems to be proved as highly electroconductive when compared to multi-walled carbon nanotubes (MWCNTs).

Temporal Stability of Metal-Chloride-Doped Chemical-Vapour-Deposited Graphene.

Graphene has proven to be a promising material for transparent flexible electronics. In this study, we report the development of a transfer and doping scheme of large-area chemical vapour deposited (CVD) graphene. A technique to transfer the as-grown material onto mechanically flexible and optically transparent polymeric substrates using an ultraviolet adhesive (UVA) is outlined, along with the temporal stability of the sheet resistance and optical transparency following chemical doping with various metal chlorides (Mx Cly The sheet resistance (RS ) and 550 nm optical transparency (%T550 ) of the transferred un-doped graphene was 3.5 kΩ sq(-1) (±0.2 kΩ sq(-1) ) and 84.1 % (±2.9 %), respectively. Doping with AuCl3 showed a notable reduction in RS by some 71.4 % (to 0.93 kΩ sq(-1) ) with a corresponding %T550 of 77.0 %. After 200 h exposure to air at standard temperature and pressure, the increase in RS was found to be negligible (ΔRS AuCl3 =0.06 kΩ sq(-1) ), indicating that, of the considered Mx Cly species, AuCl3 doping offered the highest degree of time stability under ambient conditions. There appears a tendency of increasing RS with time for the remaining metal chlorides studied. We attribute the observed temporal shift to desorption of molecular dopants. We find that desorption was most significant in RhCl3 -doped samples whereas, in contrast, after 200 h in ambient conditions, AuCl3 -doped graphene showed only marginal desorption. The results of this study demonstrate that chemical doping of UVA-transferred graphene is a promising means for enhancing large-area CVD graphene in order to realise a viable platform for next-generation optically transparent and mechanically flexible electronics.

Electrical properties of as-grown and proton-irradiated high purity silicon

The complex permittivity of as-grown and proton-irradiated samples of high purity silicon obtained by the floating zone method was measured as a function of temperature at a few frequencies in microwave spectrum by employing the quasi TE011 and whispering gallery modes excited in the samples under test. The resistivity of the samples was determined from the measured imaginary part of the permittivity. The resistivity was additionally measured at RF frequencies employing capacitive spectroscopy as well as in a standard direct current experiment. The sample of as-grown material had the resistivity of ∼85kΩcm at room temperature. The sample irradiated with 23-MeV protons had the resistivity of ∼500kΩcm at 295K and its behavior was typical of the intrinsic material at room and at elevated temperatures. For the irradiated sample, the extrinsic conductivity region is missing and at temperatures below 250K hopping conductivity occurs. Thermal cycle hysteresis of the resistivity for the sample of as-grown material is observed. After heating and subsequent cooling of the sample, its resistivity decreases and then slowly (∼50h) returns to the initial value.