Investigation of high density two-dimensional electron gas in Zn-polar BeMgZnO/ZnO heterostructures

Abstract

Zn-polar BeMgZnO/ZnO heterostructures grown by molecular beam epitaxy on high resistivity GaN templates producing high-density two-dimensional electron gas (2DEG) are investigated. This is motivated by the need to reach plasmon-longitudinal optical (LO) phonon resonance for attaining minimum LO phonon lifetime. Achievement of high 2DEG concentration in MgZnO/ZnO heterostructures requires growth of the MgZnO barrier at relatively low temperatures, which compromises the ternary quality that in turn hinders potential field effect transistor performance. When this ternary is alloyed further with BeO, the sign of strain in the BeMgZnO barrier on ZnO switches from compressive to tensile, making the piezoelectric and spontaneous polarizations to be additive in the BeMgZnO/ZnO heterostructures much like the Ga-polar AlGaN/GaN heterostructures. As a result, a 2DEG concentration of 1.2 × 1013 cm−2 is achieved in the Be0.03Mg0.41Zn0.56O/ZnO heterostructure. For comparison, a 2DEG concentration of 7.7 × 1012 cm−2 requires 2% Be and 26% Mg in the barrier, whereas the same in the MgZnO/ZnO system would require incorporation of more than 40% Mg into the barrier, which necessitates very low growth temperatures. Our results are consistent with the demands on achieving short LO phonon lifetimes through plasmon-LO phonon resonance for high carrier velocity.

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

“…Incorporation of BeO into MgZnO to form the quaternary BeMgZnO provides the feasibility to tune the extent and sign of strain in the quaternary and hence significantly increases the 2DEG density 8 . The representative results show that the Be 0.02 Mg 0.26 ZnO/ZnO heterostructure results in a 2DEG density close to the desired plasmon-LO phonon resonance electron density (~7×10 12 cm -2 ) 24 .…”

“…As a result, high 2DEG concentration can be achieved with relatively moderate Mg content. Utilizing this approach, high 2DEG densities is observed near the plasmon-LO phonon resonance (~7×10 12 cm -2 ) in BeMgZnO/ZnO heterostructures while the Mg content below is 30% and the Be content is only at 2~3% 8 .…”

Characterization of Ag Schottky Barriers on Be0.02Mg0.26ZnO/ZnO Heterostructures

“…ZnO as a wide bandgap semiconductor (≈3.3 eV at room temperature) with high electron saturation velocity (theoretical 3.5 × 10 7 cm s −1 ), has garnered popularity owing to its potential for applications in thin film transparent transistors, transparent electrodes for solar cells, light emitters, chemical and biological sensors etc . Alloying with other II–VI oxides with higher bandgap (MgO and BeO) paves the way for heterojunction field effect transistors with polarization induced two‐dimensional electron gas (2DEG) . Performance of such devices at high electric field, where electron transport is highly controlled by longitudinal optical (LO) phonons, suffers due to accumulation of non‐equilibrium hot LO phonons owing to strong electron–LO phonon coupling.…”

“…Alternatively, alloying ZnO concurrently with BeO and MgO, provides quaternary BeMgZnO barriers with tensile strain on ZnO, as BeO has lower (2.698 Å) and MgO has higher (theoretical 3.32 Å) in‐plane lattice parameter compared to ZnO (3.25 Å) . As a result, with the two aforementioned types of polarization charges operating in unison, sheet carrier concentrations as high as 1.2 × 10 13 cm −2 have been demonstrated for BeMgZnO/ZnO heterostructures …”

Controlled compensation via non-equilibrium electrons in ZnO

“…The substrates were thermally cleaned at 650 °C for 12 hours prior to loading into the molecular beam epitaxy (MBE) growth chamber. To ensure uniform Zn-polar oriented growth, the surfaces of (0001) GaN templates were pre-exposure to Zn atom beam to prevent the oxidation when growth starting 20 , 21 . Conversely, for O-polar oriented growth, the gallium oxide (Ga 2 O 3 ), obtained by O pre-exposure, between GaN and ZnO was essential.…”