Epitaxial Growth of Bandgap Tunable ZnSnN₂ Films on (0001) Al₂O₃ Substrates by Using a ZnO Buffer

Abstract

Growth of ZnSnN2 films on (0001) Al2O3 substrates is performed by plasma-assisted molecular-beam epitaxy by changing the growth temperatures from 350 to 650 °C. Single crystal ZnSnN2 films have been grown by using ZnO buffer while the film grown without the ZnO buffer has shown amorphous-like disordered characteristics addressed by no observation of any diffraction from reflection high-energy electron diffraction. All the grown crystalline ZnSnN2 films with ZnO buffer show a pseudowurtzite structure without the formation of an orthorhombic structure. Epitaxial relationships between Al2O3 substrate, ZnO buffer, and ZnSnN2 film are determined to be [112̅0] ZnSnN2//[112̅0] ZnO//[101̅0] Al2O3 and [0001] ZnSnN2//[0001] ZnO//[0001] Al2O3. The bandgaps of ZnSnN2 films could be tuned from 1.85 to 2.15 eV, simply by increasing the growth temperatures from 350 to 650 °C. The carrier concentrations and carrier mobilities were investigated and compared. Since the growth of single crystal ZnO films has been reported on various kinds of cheap and large size substrates, our results can expand the method to grow single crystal ZnSnN2 films, which is needed to fabricate new ZnSnN2-based devices.

ZnSnN2 in Real Space and k‐Space: Lattice Constants, Dislocation Density, and Optical Band Gap

“…In fact, samples A and B have FWHM below 0.05°, matching that of single crystal ZnO. These values are an order of magnitude lower of what has been reported in the literature so far, for example, 0.52° for ZnSnN 2 grown on ZnO buffer, employing PAMBE, [ 10 ] or 0.38° for ZnGeN 2 grown on GaN buffer by MBE. [ 24 ] Notably, the use of lattice‐matching substrates or buffer layers reduces dislocation densities as compared to the films grown on lattice‐mismatched substrates such as, Al 2 O 3 (0001).…”

“…[ 12 ] Le et al., demonstrated epitaxially grown wurtzite films by plasma‐assisted molecular beam epitaxy (PAMBE) at 450 and 550 °C, using ZnO buffer layers. [ 10 ] Thus, even though the initial demonstrations of the ZnSnN 2 epitaxy have been made, there is a room for explorations.…”

“…According to calculations, the most energetically favorable structure for ZnSnN 2 is the orthorhombic Pna2 1 , although the orthorhombic Pmc2 1 structure has also been identified as a possible structure for ZnSnN 2 . [ 8 ] In addition to the Pna2 1 , ZnSnN 2 can also be synthesized in the hexagonal P6 3 mc and monoclinic structures, [ 9–11 ] depending on the growth conditions as well as the substrate lattice‐matching. Notably, the temperature‐window for single crystalline growth of ZnSnN 2 is relatively narrow.…”

Carrier Tuning in ZnSnN2 by Forming Amorphous and Microcrystalline Phases

“…Furthermore, the Zn–IV–N 2 compounds, such as ZnSnN 2 , ZnGeN 2 , and ZnSiN 2 , have a similar lattice constant, so that the direct bandgap could be tuned from 1.7 to 4.5 eV by forming alloys with each other. − This enables the coverage of the entire visible region of the solar spectrum, which is an obvious advantage for a photovoltaic absorber material. At present, the crystalline ZTN (c-ZTN) thin films can be fabricated by various techniques. − However, this material suffers from the difficulty in carrier control, because the low formation energy of donor-like intrinsic defects leads to a high n-type background carrier concentration of 10 19 –10 21 cm –3 . ,,− It hinders the p-type doping of ZTN and subsequently results in the lack of p–n homojunction. Even in a p–n heterojunction composed of ZTN and other p-type material, the thin space-charge region caused by a high electron concentration cannot ensure the efficient absorption of sunlight.…”

Band Gap-Tunable (Mg, Zn)SnN2 Earth-Abundant Alloys with a Wurtzite Structure

“…Le et al demonstrated the growth of ZnSnN 2 singlecrystalline epilayers on ZnO-buffered c-Al 2 O 3 . 22 The ZnObuffer method may be useful for epitaxially integrating (Mg,Zn)SnN 2 into III-nitride-based optoelectronic devices.…”