•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.
We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase ...(1 compound), including binary (β-Ga
2
O
3
, In
2
O
3
, ZnO, SnO
2
), ternary (ZnSnO
3
, BaSnO
3
, MgGa
2
O
4
, ZnGa
2
O
4
), and quaternary (Zn
1−x
Mg
x
Ga
2
O
4
, InGaZnO
4
) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (
n
e
) between 5 × 10
15
and 5 × 10
20
cm
−3
and Hall electron mobilities (
μ
H
) up to 240 cm
2
V
−1
s
−1
. The widest range of
n
e
values was achieved for β-Ga
2
O
3
and In
2
O
3
. The most electrically conducting bulk crystals are InGaZnO
4
and ZnSnO
3
with
n
e
> 10
20
cm
−3
and
μ
H
> 100 cm
2
V
−1
s
−1
. The highest
μ
H
values > 200 cm
2
V
−1
s
−1
were measured for SnO
2
, followed by BaSnO
3
and In
2
O
3
single crystals. In
2
O
3
, ZnO, ZnSnO
3
, and InGaZnO
4
crystals were always conducting, while others could be turned into electrical insulators.
Graphic abstract
β-Ga2O3 is an emerging ultra-wide bandgap (4.9 eV) oxide semiconductor that additionally scintillates under gamma excitation. A unique combination of transparency in the UV/visible spectrum, ...semiconducting, and scintillation properties makes that compound interesting for fundamental studies of underlying physics and design of novel devices, in particular compact detectors for gamma radiation. Undoped, and singly (Ce, Si, Al), doubly (Ce + Si, Ce + Al), and triply (Ce + Al + Si) doped bulk β-Ga2O3 single crystals were grown by the Czochralski method under very similar conditions and systematically studied in terms of electrical and optical properties that were correlated with scintillation light yield under gamma excitation. A wide spectrum of doping enabled to control the free electron concentration in semiconducting β-Ga2O3 crystals within almost three orders of magnitude (7 × 1015–6 × 1018 cm−3) with the Hall mobility approaching 150 cm2 V−1s−1. The maximum of light yield under gamma excitation was recorded for undoped and Ce-doped β-Ga2O3 single crystals having the free electron concentration of mid 1016 cm−3. The light yield significantly decreases for both electrically insulating and highly conducting (Si-doped) crystals. None of the dopants (Ce, Si, Al) introduces any absorption bands in the spectral region of light emission (340–410 nm) under gamma excitation. The dopants in quest do not affect the structure of neither cathodoluminescence (CL) nor radioluminescence (RL) emissions, but modify their absolute intensity. A double-band structure of RL spectra corresponds to UV and blue emissions observed in CL spectra that are assigned to self-trapped excitons.
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•Doped semiconducting β-Ga2O3 was grown and studied for gamma radiation detection.•The crystals are fully transparent to scintillation emissions at 340–410 nm.•Maximum of light yield was found at free electron concentration of mid 1016 cm−3.•Luminescence spectra point to self-trapped excitons in scintillation mechanism.
In the course of development of transparent semiconducting oxides (TSOs) we compare the growth and basic physical properties bulk single crystals of ultra-wide bandgap (UWBG) TSOs, namely β-Ga2O3 and ...Ga-based spinels MgGa2O4, ZnGa2O4, and Zn1-xMgxGa2O4. High melting points of the materials of about 1800 -1930 °C and their thermal instability, including incongruent decomposition of Ga-based spinels, require additional tools to obtain large crystal volume of high structural quality that can be used for electronic and optoelectronic devices. Bulk β-Ga2O3 single crystals were grown by the Czochralski method with a diameter up to 2 inch, while the Ga-based spinel single crystals either by the Czochralski, Kyropoulos-like, or vertical gradient freeze / Bridgman methods with a volume of several to over a dozen cm3. The UWBG TSOs discussed here have optical bandgaps of about 4.6 - 5 eV and great transparency in the UV / visible spectrum. The materials can be obtained as electrical insulators, n-type semiconductors, or n-type degenerate semiconductors. The free electron concentration (ne) of bulk β-Ga2O3 crystals can be tuned within three orders of magnitude 1016 - 1019 cm−3 with a maximum Hall electron mobility (μ) of 160 cm2V−1s−1, that gradually decreases with ne. In the case of the bulk Ga-based spinel crystals with no intentional doping, the maximum of ne and μ increase with decreasing the Mg content in the compound and reach values of about 1020 cm−3 and about 100 cm2V−1s−1 (at ne > 1019 cm−3), respectively, for pure ZnGa2O4.
Amyotrophic lateral sclerosis (ALS) has substantial heritability, in part shared with fronto-temporal dementia (FTD). We show that ALS heritability is enriched in splicing variants and in binding ...sites of 6 RNA-binding proteins including TDP-43 and FUS. A transcriptome wide association study (TWAS) identified 6 loci associated with ALS, including in NUP50 encoding for the nucleopore basket protein NUP50. Independently, rare variants in NUP50 were associated with ALS risk (P = 3.71.10
; odds ratio = 3.29; 95%CI, 1.37 to 7.87) in a cohort of 9,390 ALS/FTD patients and 4,594 controls. Cells from one patient carrying a NUP50 frameshift mutation displayed a decreased level of NUP50. Loss of NUP50 leads to death of cultured neurons, and motor defects in Drosophila and zebrafish. Thus, our study identifies alterations in splicing in neurons as critical in ALS and provides genetic evidence linking nuclear pore defects to ALS.
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