Truly bulk ZnGa2O4 single crystals were obtained directly from the melt. High melting point of 1900 ± 20 °C and highly incongruent evaporation of the Zn- and Ga-containing species impose restrictions ...on growth conditions. The obtained crystals are characterized by a stoichiometric or near-stoichiometric composition with a normal spinel structure at room temperature and by a narrow full width at half maximum of the rocking curve of the 400 peak of (100)-oriented samples of 23 arcsec. ZnGa2O4 is a single crystalline spinel phase with the Ga/Zn atomic ratio up to about 2.17. Melt-grown ZnGa2O4 single crystals are thermally stable up to 1100 and 700 °C when subjected to annealing for 10 h in oxidizing and reducing atmospheres, respectively. The obtained ZnGa2O4 single crystals were either electrical insulators or n-type semiconductors/degenerate semiconductors depending on growth conditions and starting material composition. The as-grown semiconducting crystals had the resistivity, free electron concentration, and maximum Hall mobility of 0.002–0.1 Ωcm, 3 × 1018–9 × 1019 cm−3, and 107 cm2 V−1 s−1, respectively. The semiconducting crystals could be switched into the electrically insulating state by annealing in the presence of oxygen at temperatures ≥700 °C for at least several hours. The optical absorption edge is steep and originates at 275 nm, followed by full transparency in the visible and near infrared spectral regions. The optical bandgap gathered from the absorption coefficient is direct with a value of about 4.6 eV, close to that of β-Ga2O3. Additionally, with a lattice constant of a = 8.3336 Å, ZnGa2O4 may serve as a good lattice-matched substrate for magnetic Fe-based spinel films.
SnO2 is a semiconductor with a wide optical bandgap (3.5 eV), which makes it an attractive transparent semiconducting oxide (TSO) for electronic and opto‐electronic applications. At elevated ...temperatures it is, however, much more unstable than other TSOs (such as ZnO, Ga2O3, or In2O3). This leads to a rapid decomposition even under very high oxygen pressures. Our experiments showed that stoichiometric SnO2 does not melt up to 2100 °C, in contradiction to earlier published data. Bulk SnO2 single crystals, that could provide substrates for epitaxial growth, have not been reported so far. Hereby we report on truly bulk SnO2 single crystals of 1 inch diameter grown by physical vapor transport (PVT). The most volatile species during SnO2 decomposition is, in addition to oxygen, SnO, which is stable in the gas phase at high temperature and reacts again with oxygen at lower temperatures to form SnO2. We identified a relatively narrow temperature window, temperature gradients and a ratio of SnO/O2 for providing the best conditions for SnO2 single crystal growth. X‐ray powder diffraction (XRD) proved the single SnO2 phase. Moreover, by selecting a suitable SnO/O2 ratio it was possible to obtain either n‐type conductivity with electron concentrations up to 2 × 1018 cm−3 and electron mobilities up to 200 cm2 V−1 s−1, or insulating behavior. The crystals exhibited an optical absorption edge located at 330–355 nm, depending on the crystal orientation, and a good transparency over visible and near infrared (NIR) spectra.
Mirrors made of silicon have been proposed for use in future cryogenic gravitational-wave detectors, which will be significantly more sensitive than current room-temperature detectors. These mirrors ...are planned to have diameters of ≈50 cm and a mass of ≈200 kg. While single-crystalline float-zone silicon meets the requirements of low optical absorption and low mechanical loss, the production of this type of material is restricted to sizes much smaller than required. Here we present studies of silicon produced by directional solidification. This material can be grown as quasi-monocrystalline ingots in sizes larger than currently required. We present measurements of a low room-temperature and cryogenic mechanical loss comparable with float-zone silicon. While the optical absorption of our test sample is significantly higher than required, the low mechanical loss motivates research into further absorption reduction in the future. While it is unclear if material pure enough for the transmissive detector input mirrors can be achieved, an absorption level suitable for the highly reflective coated end mirrors seems realistic. Together with the potential to produce samples much larger than ≈50 cm, this material may be of great benefit for realizing silicon-based gravitational-wave detectors.
The growth of bulkx β-Ga2O3 single crystals by the Czochralski method is reported and discussed in terms of crucial growth conditions and correlated with basic electrical and optical properties of ...the obtained crystals. β-Ga2O3 crystals have a tendency to a spiral formation due to free carrier absorption in the near infrared (NIR) wavelength range, which hampers radiative heat transfer through the growing crystal. Moderate or low free electron concentrations (<5×1017cm−3) lead to cylindrical crystals with a high crystallized fraction (g≥0.5). The use of a CO2-containing growth atmosphere provides oxygen partial pressures between 0.8 and 4.4×10−2bar that is sufficient to obtain cylindrical and semiconducting crystals. Doping with Sn increases the free electron concentration in the crystals to high values (~1019cm−3) that lead to an immediate spiral formation, while doping with Mg (>6wtppm) provides insulating crystals with reduced probability of the spiral formation. The estimated Mg equilibrium segregation coefficient across the liquid–solid interface is 0.10–0.12. Annealing of undoped crystals in an oxidizing atmosphere at temperatures ≥1200°C for 20h decreases the bulk free electron concentration by about one order of magnitude, while the crystal surface becomes insulating. However, Mg:β-Ga2O3 crystals are insensitive to annealing in both oxygen- and hydrogen-containing atmospheres. The transmittance spectra showed a steep absorption edge at 260nm and virtually full transparency in the visible and NIR wavelength range for low and moderate free electron concentrations. We also demonstrated the possibility of growing 2in. diameter β-Ga2O3 single crystals by the Czochralski method. The good crystal quality is evidenced by rocking curve FWHM values of below 50". We noted that most dislocations propagate parallel to (100) plane. Further, we also provide thermal properties of the crystals as a function of temperature.
•Bulk β-Ga2O3 single crystals were grown from the melt by the Czochralski method.•Two inch diameter Czochralski-grown β-Ga2O3 single crystal has been demonstrated.•Growth conditions have been found to be critical for the growth stability and properties.•Crystal properties upon annealing and crystal quality have been discussed.
We present a new approach for scaling-up the growth of β-Ga2O3 single crystals grown from the melt by the Czochralski method, which has also a direct application to other melt-growth techniques ...involving a noble metal crucible. Experimental and theoretical results point to melt thermodynamics as the crucial factor in increasing the volume of a growing crystal. In particular, the formation of metallic gallium in the liquid phase in large melt volumes causes problems with crystal growth and eutectic or intermetallic phase formation with the noble metal crucible. The larger crystals to be grown the higher oxygen concentration is required. The minimum oxygen concentration ranges from about 8 to 100 vol.% for 2 to 4 inch diameter cylindrical crystals, challenging the use of iridium crucibles in a combination with such high oxygen concentrations. A specific way of oxygen delivery to a growth furnace with the iridium crucible allows to minimize the formation of metallic gallium in the melt and thus obtaining large crystal volumes while decreasing the probability of the eutectic formation.
•Bulk β-Ga2O3 crystals grown the Czochralski method are doped with Cr, Ce, and Al.•Segregation phenomena of the dopants are analyzed.•Effective segregation coefficients for the dopants are ...determined.•An impact of the dopants on optical properties of β-Ga2O3 crystals is investigated.
We experimentally evaluated segregation of Cr, Ce and Al in bulk β-Ga2O3 single crystals grown by the Czochralski method, as well as the impact of these dopants on optical properties. The segregation of Cr and Ce and their incorporation into the β-Ga2O3 crystal structure strongly depends on O2 concentration in the growth atmosphere which has a noticeable impact on decomposition of Ga2O3 and Cr2O3, as well as on the charge state of Cr and Ce. Effective segregation coefficients for Cr are in the range of 3.1–1.5 at 7–24 vol% O2, while for Ce they are roughly below 0.01 at 1.5–34 vol% O2. The effective segregation coefficient for Al is 1.1 at 1.5–21 vol% O2. Both dopants Ce and Al have a thermodynamically stabilizing effect on β-Ga2O3 crystal growth by supressing decomposition. While Ce has no impact on the optical transmittance in the ultraviolet and visible regions, in Cr doped crystals we observe three absorption bands due to Cr3+ on octahedral Ga sites, one in the ultraviolet merging with the band edge absorption of β-Ga2O3 and two in the visible spectrum, for which we estimate the absorption cross sections. Al doping also does not induce dopant related absorption bands but clearly shifts the absorption edge as one expects for a solid-solution crystal Ga2(1−x)Al2xO3 still in the monoclinic phase. For the highest doping concentration (Ga1.9Al0.1O3) we estimate an increase of the energy gap by 0.11 eV.
•Bulk β-Ga2O3 single crystals are grown the Czochralski method.•The crystals are doped with a number of mono-, di-, tri-, and tetravalent ions.•Incorporation of the dopants into bulk crystals is ...investigated.•Impact of dopants on growth stability and physical properties of crystals is studied.
The present report relates to a systematic study of dopant incorporation into bulk β-Ga2O3 single crystals grown by the Czochralski method, and their impact on growth stability, crystal appearance (growth habit), electrical properties, and transmittance of the obtained crystals. At very similar growth conditions, the dopant incorporation is driven mainly by ionic radii difference between dopant and Ga3+ ion and by thermal stability of the dopant during crystal growth. Good growth stability was achieved with Li1+, Mg2+, Co2+, Ni2+, Ce3+, Al3+, and Ge4+ doping, as that resulted in lowering or entirely compensating the free electron concentration (ne), and, in some cases, presence of additional oxygen through a dopant oxide/carbonate decomposition that is added to the starting material.
Undoped crystals had the ne of 2.5 × 1016–2 × 1018 cm−3 with the Hall mobility of 80–152 cm2 V−1 s−1. The ne within that range was also achieved by doping the melt with Li1+, Cu1+, Cr3+, Ce3+, and Ge4+. The two former (Li, Cu) and the latter (Ge) dopants entirely evaporate during or even before growth due to very high partial pressures, but at the same time they leave in the melt extra oxygen that affect to some extent (depending on its initial concentration) the ne. Therefore, we provide a new tool to control the free electron concentration at low levels (ne = 1016–1017 cm−3) by doping a Ga2O3 starting material with thermally unstable oxides or carbonates (such as GeO2 or Li2CO3) that undergo thermal decomposition at high temperatures with entirely evaporated cations and released in the melt an extra oxygen (dopant acting as an additional oxygen source). Si4+ and Sn4+ increase the ne to 2.5 × 1018–1019 cm−3, consistent with previous studies. At such high ne, the Hall mobility drops to values of 50–84 cm2 V−1 s−1. Divalent ions (Mg2+, Co2+, Ni2+) and trivalent Al3+ made the crystals electrically insulating. We also empirically showed that the underlying conductivity of undoped β-Ga2O3 crystals is caused by residual solid impurities, mainly by Si4+ and hydrogen, the latter could be easily removed by annealing.
The transmittance near the absorption edge is not affected by the dopants at studied concentrations, except Cr3+, Co2+, and Ni2+ that introduce an extra absorption in the UV and blue spectral regions, and Al3+ that slightly shifts the absorption edge towards shorter wavelengths.
Properties of a highly compensated high-purity germanium Palleti, Pradeep Chandra; Seyidov, Palvan; Gybin, Alexander ...
Journal of materials science. Materials in electronics,
2024/1, Letnik:
35, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The electrical and optical properties of a compensated high-purity germanium (HPGe) single crystal was investigated using various characterization techniques. Aluminium, boron, and phosphorus were ...the major residual shallow-level impurities identified by photothermal ionization spectroscopy (PTIS). Hall effect measurements performed at low temperatures (77 K) along the growth length reveal that the crystal is
p
-type at the top and bottom, while
n
-type in the middle part with a net carrier concentration in the range of 10
10
–10
11
cm
−3
. The obtained very high resistivity (2.3 ⨯ 10
8
Ω·cm) at 91 K in the temperature-dependent Hall measurements (TDH) at the bottom part of the crystal indicates a high level of compensation (84%) of charge carriers by deep-level impurities or defects with an activation energy near 0.195 eV. The highest hole mobility
(
μ
p
=
46700
c
m
2
/
V
·
s
)
at 77 K was obtained at the top part, which is moderately compensated. The carrier lifetimes of the grown HPGe crystals were measured using microwave-detected photoconductivity (MDP) at room temperature. The average carrier lifetime decreases from the top to bottom part of the crystal from 130 to 60 µs, which is less than that of the usually observed values in HPGe crystals due to a strong compensation.
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