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Samples of Nickel-Zinc ferrite, Ni1−xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), were synthesized by a solid state reaction method. The morphology of four of the samples (x = 0.0, 0.4, 0.6 and 1.0) was observed with 3D X-ray imaging. The distortion caused by the increase of zinc in the ferrite structure and the respective movement of the trivalent (Fe) and divalent (Ni) ions from the tetrahedral- to the octahedral-sites cause some vacancies which act as positron traps. Doppler broadening of annihilation radiation experiments have been performed with a variable energy monoenergetic positron beam. The positron results as a function of the energy/depth implantation are presented in this work.

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

Cadmium zinc telluride (CZT) possesses excellent material properties for a wide range of applications where room temperature operability, durability, and high efficiency are required. However, because CZT is a challenging material to produce in useful quantities, the growth and fabrication costs have remained high, creating an economic challenge for vendors. While the traveling heater method (THM) is the predominant means of commercial CZT crystal growth, the vertical Bridgman method (VB) is an attractive alternative due to its relatively fast growth rate. However, VB grown CZT has yet to compete with THM grown CZT, particularly in terms of charge collection efficiencies, where the charge collection efficiency is characterized by the single carrier electron mobility lifetime (μτe) product. Despite efforts to overcome this discrepancy, the μτe product in VB grown CZT has remained an order of magnitude lower than THM. Eliminating this difference would bring VB one step closer to outpacing THM in terms of economic feasibility. This paper discusses the development of a unique technique that combines the advantages of both growth methods to better understand this discrepancy and the underlying mechanisms behind it. CZT ingots were grown from melt via VB with highly off-stoichiometric concentrations of tellurium (Te). Melt mixing via accelerated crucible rotation (ACRT) was applied to compensate for any negative effects associated with off-stoichiometry, i.e. flux inclusions. CZT material has been produced at growth rates commensurate with VB (one ingot/week) and with charge collection efficiencies commensurate with THM (mid 10−2 cm2/V) in long bars typical of commercial applications.

Co-doping with divalent elements is known to improve the light yield and decay times of some cerium-activated scintillators, despite the stabilization of tetravalent Ce4+, previously believed to be non-luminescent, from the Ce3+ state. Ce4+ stabilization is a charge compensation mechanism which results from divalent ion substitution of a 3 + site. To elucidate the underlying mechanisms, which remain poorly understood, we have grown three Ce,Ca:YAG crystals with different amounts of calcium co-dopants by the Czochralski method and characterized their scintillation and defect properties. Calcium co-doping reduces the decay times and stabilizes the formation of Ce4+ as expected. Interestingly, X-ray fluorescence analysis reveals a decreased concentration of cerium within the YAG crystal for the sample doped with high levels of Ca, which could contribute to the observation that scintillator properties are improved only for low levels of Ca co-doping. Additionally, positron annihilation spectroscopy reveals an increase in the concentration of vacancies with increasing Ca concentration, while thermoluminescence is observed to show no detectable signal. Furthermore, room temperature photoluminescence of the Ce 4f to 5d1 transition demonstrates decreased emission with increasing Ca co-doping. These data suggest that Ca co-doping decreases the decay time by creating defects with non-radiative decay pathways.

We report polarization dependent photoluminescence studies on unintentionally-, Mg-, and Ca-doped β-Ga2O3 bulk crystals grown by the Czochralski method. In particular, we observe a wavelength shift of the highest-energy UV emission which is dependent on the pump photon energy and polarization. For 240 nm (5.17 eV) excitation almost no shift of the UV emission is observed between E||b and E||c, while a shift of the UV emission centroid is clearly observed for 266 nm (4.66 eV), a photon energy lying between the band absorption onsets for the two polarizations. These results are consistent with UV emission originating from transitions between conduction band electrons and two differentially-populated self-trapped hole (STH) states. Calcuations based on hybrid and self-interaction-corrected density functional theories further validate that the polarization dependence is consistent with the relative stability of two STHs. This observation implies that the STHs form primarily at the oxygen atoms involved in the original photon absorption event, thus providing the connection between incident polarization and emission wavelength. The data imposes a lower bound on the energy separation between the self-trapped hole states of ~70–160 meV, which is supported by the calculations.

Knowledge of the origin of deep levels and their impact on electrical properties is critical for device applications of \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. By annealing under an oxygen (${\mathrm{O}}_{2}$) atmosphere, the resistivity in shallow-donor (zirconium) doped \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$:$\mathrm{Zr}$ single crystals is found to increase by more than 10 orders of magnitude to (7 \ifmmode\pm\else\textpm\fi{} 4) \ifmmode\times\else\texttimes\fi{} 10 \ensuremath{\Omega} cm, which is comparable to the resistivity achieved by iron ($\mathrm{Fe}$) acceptor doping of (5 \ifmmode\pm\else\textpm\fi{} 3) \ifmmode\times\else\texttimes\fi{} 10 \ensuremath{\Omega} cm. We combine thermoelectric effect spectroscopy and positron annihilation spectroscopy (PAS), which are sensitive to deep levels and concentration of open-volume defects, with modeling of the electrical properties, to study these strongly compensated crystals. We find the compensating level in the ${\mathrm{O}}_{2}$-annealed \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$:$\mathrm{Zr}$ sample to be located at (0.727 \ifmmode\pm\else\textpm\fi{} 0.021) eV (E2*) below the conduction band, which correlates with a vacancy signal from PAS data. The defect is most likely the relaxed split $\mathrm{Ga}$ vacancy ${V}_{\mathrm{Ga}}^{i}$, rather than a simple gallium vacancy, considering theoretical predictions of a small energy barrier to relax. We observe that, due to the unique nature of these vacancies and anisotropy in the monoclinic lattice, the Doppler-broadening parameter is rather small compared with other wide-gap compounds, and in such a case the positron diffusion length is a suitable parameter to estimate the open-volume defect concentration.

Performing deep level transient spectroscopy (DLTS) on Schottky diodes, we investigated defect levels below the conduction band minima (Ec) in Czochralski (CZ) grown unintentionally-doped (UID) and vertical gradient freeze (VGF)-grown Zr-doped beta-Ga2O3 crystals. In UID crystals with an electron concentration of 10^17 cm-3, we observe levels at 0.18 eV and 0.46 eV in addition to the previously reported 0.86 (E2) and 1.03 eV (E3) levels. For 10^18 cm-3 Zr-doped Ga2O3, signatures at 0.30 eV (E15) and 0.71 eV (E16) are present. For the highest Zr doping of 5*10^18 cm-3, we observe only one signature at 0.59 eV. Electric field-enhanced emission rates are demonstrated via increasing the reverse bias during measurement. The 0.86 eV signature in the UID sample displays phonon-assisted tunneling enhanced thermal emission and is consistent with the widely reported E2 (FeGa) defect. The 0.71 eV (E16) signature in the lower-Zr-doped crystal also exhibits phonon-assisted tunneling emission enhancement. Taking into account that the high doping in the Zr-doped diodes also increases the electric field, we propose that the 0.59 eV signature in the highest Zr-doped sample likely corresponds to the 0.71 eV signature in lower-doped samples. Our analysis highlights the importance of testing for and reporting on field-enhanced emission especially the electric field present during DLTS and other characterization experiments on beta-Ga2O3 along with the standard emission energy, cross-section, and lambda-corrected trap density. This is important because of the intended use of beta-Ga2O3 in high-field devices and the many orders of magnitude of possible doping., 18 pages, 3 figures

We report seminal advances in fabrication and understanding of group V (As) doped thin-film polycrystalline CdTe-based solar cells. The devices are fabricated using a novel approach, by sublimating layers of CdSeTe and CdSeTe:As. This new method allowed us to achieve minority carrier lifetime of over 1 µs, carrier concentration of more than 5×l015cc−1 and external radiative efficiency of over 2 % in a device configuration. We find an increase in open-circuit voltage when comparing As-doped, Cu-doped and undoped devices. The choice of CdSeTe instead of a CdTe-only absorber has been explained using first-principle density functional theory model. A SCAPS device model is used to analyze the potential causes for lower open-circuit voltage.

In-situ doping of CdTe/CdSeTe films with arsenic is performed using pre-doped source material prepared by high pressure Bridgman (HPB) melt growth. Arsenic level in the source material is varied from 1018-1020 cm−3. Correspondingly, dopant incorporation in the films prepared using vapor transport deposition (VTD) varied from 3×1017 cm−3-1×1018cm−3. In this range, dopant activation is found to inversely correlate with the dopant incorporation, suggesting compensation. The results from this study indicate film properties can be adjusted by source material.

Cu doping of CdTe has enabled reasonable device performance, but has some significant drawbacks, including limited achievable carrier concentration and high mobility in the CdTe films. Group V elements, on the other hand, are expected to facilitate higher carrier concentration and expected to have low mobility. Consequently, incorporate of As in CdTe is of great interest to the CdTe community. Here, we deposited CdTe from the CdTe:As feedstock to complete devices. We demonstrate that the As diffuses through the CdTe absorber layer and accumulates at the front and back interfaces with Cl activation. Current density-voltage and capacitance-voltage measurements indicate that the devices have low carrier concentration, even though the concentration of As is high. These results indicate that the process employed here was insufficient to activate the As doping.

This paper reports on the modulation of Schottky barrier heights (SBH) on three different orientations of $\beta$-Ga$_2$O$_3$ by insertion of an ultra-thin SiO$_2$ dielectric interlayer at the metal-semiconductor junction, which can potentially lower the Fermi-level pinning (FLP) effect due to metal-induced gap states (MIGS). Pt and Ni metal-semiconductor (MS) and metal-interlayer-semiconductor (MIS) Schottky barrier diodes were fabricated on bulk n-type doped $\beta$-Ga$_2$O$_3$ single crystal substrates along the (010), (-201) and (100) orientations and were characterized by room temperature current-voltage (I-V) and capacitance-voltage (C-V) measurements. Pt MIS diodes exhibited 0.53 eV and 0.37 eV increment in SBH along the (010) and (-201) orientations respectively as compared to their respective MS counterparts. The highest SBH of 1.81 eV was achieved on the (010)-oriented MIS SBD using Pt metal. The MIS SBDs on (100)-oriented substrates exhibited a dramatic increment ($>$1.5$\times$) in SBH as well as reduction in reverse leakage current. The use of thin dielectric interlayers can be an efficient experimental method to modulate SBH of metal/Ga$_2$O$_3$ junctions., Comment: 8 pages, 10 figures, 2 tables

N type conductivity of \b{eta}-Ga2O3 grown from the melt is typically achieved using Sn and Si. In this paper, we experimentally and computationally investigate Hf doping of \b{eta}-Ga2O3 single crystals using UV-Vis-NIR absorption and Hall Effect measurements and hybrid functional calculations. Unintentionally-doped and Hf-doped samples with a nominal concentration of 0.5at% were grown from the melt using vertical gradient freeze (VGF) and Czochralski method in mixed Ar+O2 atmosphere. We demonstrate Hf dopants, predicted to incorporate on the octahedral GaII site as a shallow donor, achieve degenerate doping in \b{eta}-Ga2O3 with a measured electron concentration 2 x 10^19 cm^-3 , mobility 80-65 cm^2 /Vs, and resistivity down to 5 mOhm-cm in our samples. The concentration of Hf was measured to be 1.3 x 10^19 atoms/cm^3 using glow discharge mass spectroscopy (GDMS) on doped samples, confirming Hf to be the cause of n-type conductivity (electron concentration ~2 x 10^19 cm-3).

We present a combination of X-ray photoelectron spectroscopy (XPS) and time-resolved photoluminescence (TRPL) to probe the details of interface formation between CdTe and alumina deposited by atomic layer deposition (ALD). Alumina ALD using water as the oxygen source causes the elimination of Te oxides that are initially present on air-exposed CdTe surfaces. TRPL on the resulting CdTe interface indicates some degree of passivation. On the other hand, postgrowth treatment of Al2O3/CdTe structures with CdCl2 and oxygen causes regrowth of Te oxides. Some of these structures show improved lifetimes, thereby pointing toward a critical role for Te oxides in interfacial CdTe passivation by Al2O3. Direct measurement of band positions with XPS indicates that the passivation is caused primarily by chemical rather than field effect mechanisms.

By using positron annihilation spectroscopy methods, we have experimentally demonstrated the creation of isolated zinc vacancy concentrations >1020 cm−3 in chemical vapor transport (CVT)-grown ZnO bulk single crystals. X-ray diffraction ω-rocking curve (XRC) shows the good quality of ZnO single crystal with (110) orientation. The depth analysis of Auger electron spectroscopy indicates the atomic concentrations of Zn and O are almost stoichiometric and constant throughout the measurement. Boltzmann statistics are applied to calculate the zinc vacancy formation energies (Ef) of ~1.3–1.52 eV in the sub-surface micron region. We have also applied Fick’s 2nd law to calculate the zinc diffusion coefficient to be ~1.07 × 10−14 cm2/s at 1100 °C. The zinc vacancies began annealing out at 300 °C and, by heating in the air, were completely annealed out at 700 °C.

Research is described here that is directed toward obtaining p-type ZnO crystals either by doping or by creating native defects. Theoretically, zinc vacancies are shallow acceptors that should allow for p-type conduction. Bulk ZnO crystals can be grown by chemical vapor transport (CVT), melt solidification, and hydrothermal methods. Here we have explored annealing processes with the goal of creating zinc vacancies in bulk ZnO crystals. Positron annihilation spectra reveal the reproducible formation of high concentrations (>1020 cm−3) of isolated zinc vacancies (VZn) in oxygen-annealed, CVT-grown ZnO crystals in the ∼100–150 nm near-surface. Melt- and hydrothermally grown samples, in contrast, show insignificant levels of zinc vacancy creation. Photoluminescence (PL) emission spectra indicate a VZn(0/-1) acceptor level at ∼ 155–165 meV; red PL (∼1.7 eV) emission, related to the VZn(-1/-2) level, was also observed. Infrared absorption spectroscopy reveals the presence of a zinc vacancy complex with a hole binding energy range of 420–450 meV – and with a continuum suggesting a p-type region. XPS measurements support the deficiency of Zn after oxygen annealing the CVT-grown ZnO single crystal.

A 4MV Van de Graaff accelerator was used to induce the nuclear reaction 12 C(d,n) 13 N in order to produce an intense beam of positrons. The graphite target was heated so the radioactive 13 N would desorb from the bulk into the vacuum. This radioactive gas is frozen onto a cryogenic freezer where it decays to produce an antiparticle beam of positrons. This high current beam is then guided into a superconducting magnet with field strength up to 7 Tesla where the positrons will be stored in a newly designed Micro-Penning-Malmberg trap. Several source geometries have been experimented on and found a maximum antimatter beam with a positron flux of greater than 0.55 ± 0.03 × 10 6 e + s −1 was achieved. This beam was produced using a solid rare gas moderator composed of krypton (Kr) at a temperature of 25 ± 5 K. Due to geometric restrictions on this set up and other loss mechanisms, 10 7 –10 8 e + s −1 of the total number of positrons are lost. Simulations and preliminary experiments suggest a new geometry, currently under testing, will produce a beam of 10 7 e + s −1 or more. The setup and preliminary results for the new geometry will be discussed as well.

In this Letter, we investigate the atomic and electronic structure of a Hf-doped beta-gallium oxide (β-Ga2O3) single crystal using high resolution scanning transmission electron microscopy imaging and electron energy loss spectroscopy. Ultraviolet-visible (UV-Vis)-near-infrared absorption measurements and density functional theory calculations are performed to further connect the nanoscale observation to the macroscale properties arising from the atomic structure. The Hf-doped sample was grown from the melt with a nominal Hf concentration of 0.5 at. %. We show that the Hf dopants prefer to occupy octahedral over tetrahedral sites by 0.68 eV and have some resistance to form precipitates due to a repulsive interaction of 0.17 eV between Hf atoms on neighboring sites. Also, the presence of Hf atoms on either tetrahedral or octahedral sites do not significantly affect the crystal structure of β-Ga2O3. Finally, the bandgap values of the Hf doped β-Ga2O3 obtained by electron energy loss spectroscopy and UV-Vis-spectroscopy were Eg = 4.83 ± 0.1 and 4.75 ± 0.02 eV, respectively, similar to the values reported for unintentionally doped β-Ga2O3 crystals. All these results make Hf an excellent dopant candidate for β-Ga2O3.

In this study, the formation and character of gallium vacancies (VGa) and their complexes in near surface and bulk regions of single crystal β-Ga2O3 were explored using unintentionally doped single crystals grown by the Czochralski method. As-grown and O2 annealed (up to 1550 °C) samples were investigated using positron annihilation spectroscopy (PAS) to study the top 0.05–6 μm, and also current–voltage measurements and infrared (IR) spectroscopy, with hydrogenated samples to probe VGa, to study the bulk. After annealing in O2 > 1000 °C, the β-Ga2O3 resistivity begins increasing, up to ∼109 Ω cm for 1550 °C treatment, with the top 0.5 mm being many orders of magnitude more resistive. PAS measurements of the top 6 μm (S values) and very near surface 200 nm (diffusion length, L) indicate differential behavior as a function of peak annealing temperature. At least four temperature regimes of behavior are described. VGa are present in the bulk after growth, but considerable changes occur upon annealing at a temperature ≈1000 °C, where L and S decrease simultaneously, suggesting an increasing defect concentration (L) but a decreasing defect volume (S). Annealing at a temperature ≈1400 °C increases S again, showing an increasing volume concentration of VGa, with IR absorption showing a large signature of VGa-2H, indicative of increased VGa formation that was not present when annealing at a temperature ≈1000 °C. These results suggest that defect changes from annealing in oxygen are depth dependent, and that VGa configuration may not be the same near the oxygen-exposed surface of the sample and in the bulk.

In this study, LiAlO2 crystals doped with rare-earth elements and Ti were produced by the CZ method and spectroscopic and neutron detection properties were investigated. Photoluminescence revealed no clear luminescent activation of LiAlO2 by the rare-earth dopants though some interesting luminescence was observed from secondary phases within the crystal. Gamma-ray pulse height spectra collected using a 137Cs source exhibited only a Compton edge for the crystals. Neutron modeling using Monte Carlo N-Particle Transport Code revealed most neutrons used in the detection setup are thermalized, and while using natural lithium in the crystal growth, which contains 7.6% 6Li, a 10 mm O by 10 mm sample of LiAlO2 has a 70.7% intrinsic thermal neutron capture efficiency. Furthermore, the pulse height spectra collected using a 241Am-Be neutron source demonstrated a distinct neutron peak.

The chemical vapor deposition (CVD) process can produce single or poly-crystalline diamond samples of high purity or with controlled doping concentrations. The defect type in the CVD diamonds can be changed by heating the samples. Controlling the defect type can be used to create devices for quantum diamond switches that could be used in radiation sensors and quantum information technology. Eight samples of CVD diamonds were analyzed with Doppler broadening of positron annihilation radiation (DBAR) before and after annealing in high vacuum with an electron gun. Between temperatures of 1700 - 1850 K, nitrogen was liberated from the diamond sample. At these high temperatures, the surface was graphitized and a change in the color and transparency of the diamond was observed. Some of the samples were analyzed with DBAR during periods with and without light. The defect properties were observed to change depending on the time exposure to the positron beam and were then regenerated by exposure to light. The DBAR data is compared to photoluminescence data and a time varying defect state is discussed for detector and optical grade type II CVD diamonds.

Shallow acceptors in single crystal CdTe induced by Group V (P, As, and Sb) doping are identified by thermoelectric effect spectroscopy (TEES). Understanding defects in Group V doped CdTe is critical to improving and controlling the acceptor density and lifetime in the absorber material which is an important factor determining the open circuit voltage and efficiency in CdTe solar cells.

CdTe thin-film photovoltaics have demonstrated some of the lowest costs of electricity generation owing to its low material cost and ease of manufacturing. However, the full potential of polycrystalline CdTe photovoltaics can only be realized if the open-circuit voltage can be increased beyond 1 V Open-circuit voltage ~850-900 mV has been consistently observed for state-of-the-art polycrystalline CdTe solar cells. Open-circuit voltage of over 1V has been demonstrated for single crystal CdTe devices by doping with Group V elements. Therefore, this study is aimed at understanding behavior of polycrystalline CdTe devices with arsenic doping, its activation and process and performance optimization in order to overcome current voltage limitations in CdTe solar cells.

CdTe-based photovoltaic device performance can be improved by increasing both hole density and carrier lifetime simultaneously. In this work, we discuss sources and thin film processes to achieve incorporation and high hole density in conjunction with lifetime using group-V dopants.

Calcium and magnesium doped beta-phase gallium oxide (β-Ga2O3) single crystals were Czochralski grown in an iridium crucible and subsequently annealed in hydrogen at ~800° C. IR spectroscopy was used to investigate the roles of Mg and Ca dopants as well as H and Ir impurities. An IR peak at 3492 cm-1 was previously assigned to an O-H bondstretching mode of a MgH complex in hydrogen-annealed Ga2O3:Mg and an IR peak at 3541 cm-1 is tentatively assigned to a similar mode in a CaH complex for H annealed Ga2O3:Ca. An IR peak at 5148 cm-1 is attributed to the presence of Ir4+ when magnesium is present. Excitation with several different wavelengths was used to confirm the position of the Ir level within the band gap. Our results suggest that isolated substitutional Ca dopants are less effective as deep acceptors than Mg. Polarization dependence of the iridium IR spectra is also discussed.

After coal fly ash activation with alkaline NaOH solution by the hydrothermal method, it was possible to synthesize the zeolites type Cancrinite and Analcime. Slow positron beam experiments have been performed on two set of samples. One in which the NaOH concentration was varied and another one in which the synthesis temperature range was varied. The mineralogical composition was determined by X ray diffraction, the morphology of one of the zeolite samples as compared with the coal fly ash was observed by 3D X-ray imaging. Positron results suggest that increasing the NaOH concentration produces an increase in open volume and/or defects while increasing the temperature in the sample preparation diminishes it.

Recently, the device performance of Cadmium Telluride (CdTe) solar cells have improved significantly. However, little progress has been made in understanding the defected structures responsible for carrier trapping, which have limited further cell efficiency improvements. In order to better understand the underlying electrical compensation mechanism, an undoped CdTe bulk crystal boule was grown from melt with Te-rich stoichiometry using the Vertical Bridgman technique, where the growth process was controlled to maximize net acceptor density. The samples prepared from the boule indicate low resistivity of 100–500 Ω·cm, and Hall measurements provided net acceptor density of (1–2) $\times \,{\text 10^{15}}\,{\text{cm}^{- 3}}$ , which are comparable to relevant values reported from the CdTe films after CdCl2 treatment. Infrared microscopy suggests there are significant amounts of secondary phases (SP's) present in the samples, where the SP's are believed to be Te inclusions and precipitates. The point defects are characterized by thermoelectric effect spectroscopy (TEES), where we observed six TEES current peaks associated with the defect levels ranging from 110 to 700 mV within the bandgap.

Advancing CdTe solar cell efficiency requires improving the open-circuit voltage $\rm{(V_{{\rm{OC}}})}$ above 900 mV. This requires long carrier lifetime, high hole density, and high-quality interfaces, where the interface recombination velocity is less than about 104 cm/s. Using CdTe single crystals as a model system, we report on CdTe/CdS electrical and structural interface properties in devices that produce open-circuit voltage exceeding 950 mV.

The last decade in advanced microelectronics has shown great interest in 3-D architectures, which was paved by multi-wafer alignment technologies. However, many limitations remain in the fabrication of ultratall stacks as the alignment becomes more challenging and very costly. In this paper, a new cost-effective alignment technique was employed using a set of sapphire rods in through-wafer holes. Cross-sectional analysis, edge profilometry, and electron transmission tests showed $\sim 2~\mu \text{m}$ alignment tolerances over 1 cm and $\sim 4~\mu \text{m}$ over 10-cm tall stacks. An off-angle gold sputtering method was developed to fully coat vias of 5:1 aspect ratio before bonding. Also, a new “Stamping” technique is introduced to coat the vias to a desired height where necessary. In this paper, parallel microtubes with the aspect ratios of 1000:1 were formed by aligning $\sim 200$ wafers, each including 20 000 gold-coated vias for storing charged particles. [2016-0049]

Currently, Fe doping in the ~10^18 cm-3 range is the most widely-available method for producing semi-insulating single crystalline beta-Ga2O3 substrates. Red luminescence features have been reported from multiple types of Ga2O3 samples including Fe-doped -Ga2O3, and attributed to Fe or N at O. Herein, however, we demonstrate that the high-intensity red luminescence from Fe-doped beta-Ga2O3 commercial substrates consisting of two sharp peaks at 689 nm and 697 nm superimposed on a broader peak centered at 710 nm originates from Cr impurities present at a concentration near 2 ppm. The red emission exhibits two-fold symmetry, peaks in intensity for excitation near absorption edge, seems to compete with Ga2O3 emission at higher excitation energy and appears to be intensified in the presence of Fe. Based on polarized absorption, luminescence observations and Tanabe-Sugano diagram analysis, we propose a resonant energy transfer of photogenerated carriers in beta-Ga2O3 matrix to octahedrally-coordinated Cr3+ to give red luminescence, possibly also sensitized by Fe3+., Comment: 13 pages, 3 figures

The presence of second phase defects, particularly flux inclusions of tellurium rich composition, are of great concern for charge collection efficiency in cadmium zinc telluride (CZT) and cadmium telluride (CT) material intended for applications such as radiation detection. These inclusions can distort applied electric field lines within the detector as well as act as trapping centers for charge carriers. Reduction and/or elimination of these inclusions is required to achieve appropriate charge collection efficiencies, especially in detectors of thicknesses greater than 5 mm. These so-called flux inclusions are understood to form as a consequence of constitutional undercooling at the crystal growth interface. In this study, a forced melt convection technique was applied in Vertical Bridgman (VB) melt growth of CZT without reducing imposed growth rates of ∼2 mm/hr. Several rotation profiles were tested while adjusting the melt composition from 51.62 to 61.75 atomic percent (at%) Te where the Te concentration was initially increased to improve overall material purity. With forced melt convection, the best inclusion distributions were achieved with highly Te rich melt compositions, far beyond the stoichiometric composition range for the CZT system. Average inclusion diameters were reduced to 2 µm while inclusions greater than 5 µm were essentially eliminated. Composition analyses of these ingots revealed near equilibrium concentrations of Te, even with ingots grown from Te concentrations as high as 61.75 at% Te. In this paper, a recipe for the reduction of inclusions in CZT melt growth is put forward and the implications of this method on our understanding of inclusion formation are discussed.

Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components.

Nd:YAG is a well-known laser material typically grown from the melt using the Czochralski (CZ) method, and like many other doped materials grown from the melt, it suffers from dopant segregation, giving an average Nd concentration in the crystal (Nd%) far less than that in the melt, with Nd variation along the growth axis. Here, the Nd segregation issue is addressed by the successful implementation of the Accelerated Crucible Rotation Technique (ACRT) with CZ. Compared to conventional CZ, ACRT provided improved Nd uniformity, higher Nd incorporation, and control of Nd% along the growth axis, even achieving higher Nd% in the crystal than the initial Nd from the overall melt. ACRT also affects the solid/liquid interface shape and thus the radial segregation. It is thus shown that ACRT and the mixing it provides is promising for achieving unconventional dopant profiles in CZ crystals.

Gallium oxide is a promising semiconductor for its potential as a material in the field of power electronics. The effects of iridium impurities on undoped, Mg-doped, and Ca-doped gallium oxides were investigated with IR spectroscopy. In undoped and Ca-doped β-Ga2O3, IR peaks at 3313, 3450, and 3500 cm−1 are tentatively assigned to O–H bond-stretching modes of IrH complexes. Mg-, Ca-, and Fe-doped samples show an Ir4+ electronic transition feature at 5148 cm−1. By measuring the strength of this feature vs photoexcitation, the Ir3+/4+ donor level was determined to lie 2.2–2.3 eV below the conduction band minimum. Ga2O3:Mg also has a range of sidebands between 5100 and 5200 cm−1, attributed to IrMg pairs. Polarized IR measurements show that the 5248 cm−1 peak is anisotropic, weakest for light polarized along the c axis, consistent with Lenyk et al. [J. Appl. Phys. 125, 045703 (2019)].

Arsenic dopant was incorporated in CdTe absorber layers in high-efficiency CdTe cells using feedstock doped with 1018cc-1for sublimation of films. The goal of the work was to develop a feasible method for creating a hole density equal to, or greater than that currently achievable using copper doping using a CuCl treatment. Doping with arsenic resulted in a modest increase in open-circuit voltage (V OC ) and a large improvement in fill-factor and conversion efficiency when compared with copper-doped devices with similar structure. All experiments were performed in the presence of cadmium overpressure to encourage dopant activation in tellurium vacancy sites. Arsenic incorporation was measured using Secondary Ion Mass Spectrometry (SIMS) at 4E +16 atoms/cc-1,about four times greater incorporation than previously seen by the authors. Using a CdSeTe/CdTe graded absorber and arsenic doping, a conversion efficiency of 16.79% was achieved.

We have used Doppler-broadening of the positron-electron annihilation radiation technique and VASP calculations to verify the previously reported vacancy formation enthalpy Hvf in polycrystalline depleted uranium. Experimentally we have confirmed a Hvf of (1.6 ± 0.2) eV. VASP calculations using GGA and LDA approximations gave vacancy formation enthalpies values of 1.98 eV and 2.22 eV respectively. We found residual oxygen in the sample diminished these values by 50% or more. Our new experimental and theoretical data supports the notion that oxygen impurities in the sample are responsible for lower values of vacancy formation enthalpies. Measured and calculated vacancy formation enthalpies, as well as the obtained oxygen migration enthalpy of (0.6 ± 0.1) eV, are compared and discussed with values reported in the literature. Fil: Lund, K. R.. Washington State University; Estados Unidos Fil: Lynn, K. G.. Washington State University; Estados Unidos Fil: Weber, M. H.. Washington State University; Estados Unidos Fil: Macchi, Carlos Eugenio. Universidad Nacional del Centro de la Provincia de Buenos Aires. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; Argentina Fil: Somoza, Alberto Horacio. Universidad Nacional del Centro de la Provincia de Buenos Aires. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; Argentina Fil: Juan, Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina Fil: Okuniewski, M. A.. Idaho National Laboratory; Estados Unidos

FeNi, with the chemically-ordered L10 tetragonal structure, is a promising material for next-generation rare-earth-free permanent magnets. Due to the extremely low atomic Fe and Ni mobility below the critical chemical order/disorder temperature of 320 °C, no conventional metallurgical methods are able to induce its formation. Diffusion rates could be enhanced with the creation of excessive vacancies. High-energy mechanical alloying was employed to produce a nanocrystalline Fe–50 at.%Ni alloy. The high energy mechanical milling is expected to change the defect characteristics that would be used to study the effect that mechanical alloying has on the defect concentrations. X-ray diffraction revealed that over the entirety of the milling period, the Fe and Ni powders formed an fcc solid solution. The Williamson–Hall equation and Scherrer equation revealed that reductions in grain sizes were caused during alloying. The initial increase in internal strain, and by extension dislocations, was followed by a decrease in strain with further alloying, which was due to the combination effect of creation of dislocation from mechanical deformation and annihilation of dislocations at increasing grain boundaries. Doppler broadening positron annihilation spectroscopy (DB-PAS) showed that the overall number of open-volume defects decreased with increased milling time. Finally, changes of relative permeability, for the mechanically alloyed FeNi phase, were explained by the pinning effect of dislocations.

Zinc Tellurite (ZnTeO 3 ) crystals were grown for the first time using a modified Bridgman method with a 2.5 kHz radio frequency (RF) furnace. Single crystal growth of ZnTeO 3 was hindered by many complicating factors, such as the evaporation of TeO 2 above 700 °C and the formation of more than one phase during crystal growth. While there were several successful runs that produced ZnTeO 3 single crystals, it was found that large (≥10 cm 3 ) single ZnTeO 3 crystals resulted when the crucible was exposed to a steeper vertical thermal gradient and when the temperature of the melt was raised to at least 860 °C. The results of powder X-ray diffraction (XRD) patterns were in accordance with the X-ray powder diffraction file (PDF) for ZnTeO 3 . Some optical, electrical and structural properties of ZnTeO 3 single crystals were reported in this paper.

CdTe materials tend to grow in a Te-rich stoichiometry due to the higher vapor pressure of Cd over Te, making the material p-type. Growing the crystal in an extra Cd environment helps to restore the stoichiometry and changes the material from p-type to semi-insulating or n-type conductivity. The amount of Cd and Te vacancies is expected to change with variations of the stoichiometry. To better understand the effect of stoichiometry on intrinsic defects, we grew a CdTe crystal boule in a Cd-rich conditions using the Vertical Bridgman technique. The electrical properties and defect structures of the resulting sample were compared to a Te-rich sample grown under similar conditions. The point defects were characterized by thermoelectric effect spectroscopy. Analysis suggests that five types of defects were common to both growths whereas two types of defects are associated with the growth stoichiometry.

ZnS-based materials have a long history of use as x-ray luminescent materials. ZnS was one of the first discovered scintillators and is reported to have one of the highest scintillator efficiencies. The use of ZnS for high energy luminescence has been thus far limited to thin powder screens, such as ZnS:Ag which is used for detecting alpha radiation, due to opacity to its scintillation light, primarily due to scattering. ZnS in bulk form (chemical vapor deposited, powder processed, and single crystal) has high transmission and low scattering compared to powder screens. In this paper, the performance of single crystalline ZnS is evaluated for low energy x-ray (

Efficient p-type doping in CdTe has remained a critical challenge for decades, limiting the performance of CdTe-based semiconductor devices. Arsenic is a promising p-type dopant; however, reproducible doping with high concentration is difficult and carrier lifetime is low. We systematically studied defect structures in As-doped CdTe using high-purity single crystal wafers to investigate the mechanisms that limit p-type doping. Two As-doped CdTe with varying acceptor density and two undoped CdTe were grown in Cd-rich and Te-rich environments. The defect structures were investigated by thermoelectric-effect spectroscopy (TEES), and first-principles calculations were used for identifying and assigning the experimentally observed defects. Measurements revealed activation of As is very low in both As-doped samples with very short lifetimes indicating strong compensation and the presence of significant carrier trapping defects. Defect studies suggest two acceptors and one donor level were introduced by As doping with activation energies at ~88 meV, ~293 meV and ~377 meV. In particular, the peak shown at ~162 K in the TEES spectra is very prominent in both As-doped samples, indicating a signature of AX-center donors. The AX-centers are believed to be responsible for most of the compensation because of their low formation energy and very prominent peak intensity in TEES spectra.

Amorphous Fe32Ni52Zr3B13, prepared by rapid solidification, undergoes crystallization during high-energy mechanical milling. The resulting structure consists of face-centered cubic (fcc) FeNi and Zr3Ni20B6 nanocrystallites. Structural evolution and defect analysis of as-solidified Fe32Ni52Zr3B13 ribbons with different milling times is investigated. From the differential thermal analysis (DTA) curve of amorphous ribbons, exothermic peaks were observed at 415 °C and 475 °C corresponding to the crystallization of fcc Zr3Ni20B6 phase and fcc FeNi phase, respectively. However, high-energy mechanical milling induces the formation of FeNi within the first 2 h of mechanical milling. Further milling induces the crystallization of Zr3Ni20B6. Doppler broadening positron annihilation spectroscopy (DBPAS) was used to investigate vacancy-type defects. The milling-induced crystallization appears to be related to enhanced vacancy-type defect concentrations allowing growth of pre-existing Fe(Ni) nuclei. The milling and enhanced vacancy concentration also de-stabilizes the glass, leading to decreased crystallization temperatures for both phases, and ultimately eliminating the glass transition altogether.

A unique approach for the fabrication of long-aspect ratio microtubes is presented for an antimatter trap. Conventionally, non-neutral antimatter is stored using a Penning-Malmberg trap, a single tube with aspect ratios being of the order of less than 10:1. Parallel microtubes with aspect ratios of 1000:1 have the potential to store many orders of magnitude more. The silicon industry has paved the way to microelectromechanical systems technologies which have been utilised in this research. Standard processes such as photolithography, deep reactive ion etching, sputtering and thermo-compression bonding were all used; however, unique methods of these processes were developed to overcome many engineering challenges and realise successful trapping.