Plasma-assisted molecular beam epitaxy of SnO(001) films: Metastability, hole transport properties, Seebeck coefficient, and effective hole mass

Abstract

Transparent conducting or semiconducting oxides are an important class of materials for (transparent) optoelectronic applications and -by virtue of their wide band gaps -for power electronics. While most of these oxides can be doped n-type only with room-temperature electron mobilities on the order of 100 cm 2 /Vs, p-type oxides are needed for the realization of pn-junction devices but typically suffer from exessively low (< <1 cm 2 /Vs) hole mobilities. Tin monoxide (SnO) is one of the few p-type oxides with a higher hole mobility yet is currently lacking a well-established understanding of its hole transport properties. Moreover, growth of SnO is complicated by its metastability with respect to SnO 2 and Sn, requiring epitaxy for the realization of single crystalline material typically required for high-end applications. Here, we give a comprehensive account on the epitaxial growth of SnO, its (meta)stability, and its thermoelectric transport properties in the context of the present literature. Textured and single-crystalline, unintentionally-doped p-type SnO(001) films are grown on Al 2 O 3 (00.1) and Y 2 O 3 -stabilized ZrO 2 (001), respectively, by plasma-assisted molecular beam epitaxy and the epitaxial relations are determined. The metastability of this semiconducting oxide is addressed theoretically through an equilibrium phase diagram. Experimentally, the related SnO growth window is rapidly determined by an in-situ growth kinetics study as function of Sn-to-O-plasma flux ratio and growth temperature. The presence of secondary Sn and SnO x (1 < x ≤ 2) phases is comprehensively studied by x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy, indicating the presence of Sn 3 O 4 or Sn as major secondary phases, as well as a fully oxidized SnO 2 film surface. The hole transport properties, Seebeck coefficient, and density-of-states effective mass are determined and critically discussed in the context of the present literature on SnO, considering its strongly anisotropic effective hole mass: Hall measurements of our films reveal room temperature hole concentrations and mobilities in the range of 2•10 18 to 10 19 cm −3 and 1.0 to 6.0 cm 2 /Vs, respectively, with consistently higher mobility in the single-crystalline films. Temperature-dependent Hall measurements of the single-crystalline films closest to stoichiometric, phase-pure SnO indicate non-degenerate band transport by free holes (rather than hopping transport) with dominant polar optical phonon scattering at room temperature. Taking into account the impact of acceptor band formation and the apparent activation of the hole concentration by 40-53 meV, we assign tin vacancies rather than their complexes with hydrogen as the unintentional acceptor. The room temperature Seebeck coefficient in our films confirms p-type conductivity by band transport. Its combination with the hole concentration allows us to experimentally estimate the density of states effective hole mass to be in the range of 1 to 8 times ...

SnO/$β$-Ga2O3 vertical $pn$ heterojunction diodes

“…The results of symmetric, out-of-plane 2Θ − ω Xray diffraction (XRD) using Cu-Kα radiation and bulksensitive Raman spectroscopy measurements using an excitation wavelength of 473 nm, as described in Ref. 25, are shown in Fig. 1 and confirm the presence of (001)-oriented SnO in all samples.…”

“…In addition, we conducted Hall measurements in the van-der-Pauw geometry on the reference layers A015 and A016 for which we expected similar p to those of G015 and G016: A sheet resistance of 184 kΩ and 46 kΩ was extracted with p=2.0 × 10 18 cm −3 and 1.8 × 10 19 cm −3 as well as µ H =1.6 and 0.8 cm 2 /Vs cm 2 for A015 and A016, respectively. All measured hole concentrations are well below the critical value (p Mott ≈ 9 × 10 19 cm −3 ) 25 for the Mott transition; the hole mobilities are below those of single crystalline films due scattering from rotational-domain or grain boundaries in A015/A016, 25 or G016, respectively. In the following we assume the net donor and acceptor concentration N D and N A in the Ga 2 O 3 and SnO to be equal to the measured n and p, respectively.…”

“…1 and confirm the presence of (001)-oriented SnO in all samples. In contrast to A015 and A016 with defined in-plane epitaxial relation 25 , the film of G016 shows a random rotational mosaicity as shown in the supplementary material. 35 In-situ thickness measurements using laser reflectometry 36 on the reference samples A015 and A016 indicate a total SnO thickness of 200 nm and 170 nm, respectively.…”

“…The layers are described as a combination of SnO and Sn (based on Raman and X-ray photoelectron spectroscopy measurements) whose resistance between two Ti contacts were determined. In fact, SnO is a p-type oxide with a two orders of magnitude higher hole mobility than NiO 24,25 and E g ≈ 0.7 eV. 26 Addressing the metastability of SnO with respect to Sn and SnO 2 we have determined the growth window for the formation of stoichiometric SnO by plasma-assisted molecular beam epitaxy (MBE) in our previous study: SnO was grown by a controlled Sn/O-plasma flux ratio at temperatures ≤400 • C , resulting in p-type conductivity with Hall hole concentrations (p H ) and Hall mobilites ranging from 10 18 to 10 19 cm −3 and 1 to 6.0 cm 2 /Vs, respectively.…”

“…26 Addressing the metastability of SnO with respect to Sn and SnO 2 we have determined the growth window for the formation of stoichiometric SnO by plasma-assisted molecular beam epitaxy (MBE) in our previous study: SnO was grown by a controlled Sn/O-plasma flux ratio at temperatures ≤400 • C , resulting in p-type conductivity with Hall hole concentrations (p H ) and Hall mobilites ranging from 10 18 to 10 19 cm −3 and 1 to 6.0 cm 2 /Vs, respectively. The phase was stable under rapid thermal annealing (RTA) in different atmospheres up 300 • C and transformed into n-type Sn 3 O 4 and SnO 2 at 400 • C. 25 In this letter, we report the fabrication and characteristics of vertical pn-heterojunction diodes consisting 36 A room-temperature Hall-measurement as described in Ref. 30 on a reference piece prepared the same way and from the same Ga 2 O 3 wafer indicates an electron concentration (n) of 2.0 × 10 17 cm −3 , taking into account a Hall factor of 1.6.…”