We studied electron transport and microwave noise in Zn-polar BeMgZnO/ZnO and O-polar ZnO/MgZnO heterostructures with 2-dimensional electron gas (2DEG) grown on c-sapphire substrates by molecular beam epitaxy. In a short-pulse (<5 ns) high-field experiment, the electron drift velocity reached 1.2E7 cm/s at an electric field up to 200 kV/cm. Pulsed microwave hot-electron noise temperature measurements near 10 GHz in O- polar channels indicate that the hot electron temperature is controlled by hot LO phonons, which increase electron temperature, whereas the presence of excess noise (over "thermal" hot-electron noise) in the Zn- polar channels suggests some inhomogeneity of BeMgZnO barriers.
We report the direct current (DC) and microwave performance of BeMgZnO/ZnO heterostructure field effect transistors (HFETs) on sapphire substrates. The devices fabricated using Al2O3 as the gate dielectric with a gate length of 1.5 um and a gate width of 75 um exhibited a pinch-off voltage of −4.0 V and a maximum peak transconductance of 63 mS/mm. A current gain cutoff frequency fT of 5.0 GHz was achieved, highest among ZnO-based FETs. The corresponding electron velocity of above 1E7 cm/s estimated based on the gate transit time inches closer to the theoretical peak velocity in ZnO (3.5E7 cm/s). This value is significantly higher than the previously reported values in ZnO-based HFETs, which is attributed to the two-dimensional electron gas (2DEG) concentration at or near the resonance of longitudinal optical (LO)-phonon and plasmon frequencies as well as the improved quality of the heterostructure owing to optimized ZnO buffer growth and BeO and MgO alloying in the barrier. To probe the high-frequency response of the HFETs, extrinsic and intrinsic parameters of the small-signal equivalent circuit for the BeMgZnO/ZnO HFETs were investigated using the hybrid extraction method.
Accumulation of non-equilibrium hot longitudinal optical (LO) phonons limits the electron drift velocity for electronic devices operating under high electric field. Ultrafast decay of hot phonons can take place via plasmon-LO phonon resonance, which leads to fast electron energy relaxation and hence high electron drift velocity and optimum operation of the devices. This need motivates us to create heterostructures with 2DEG density close to the plasmon-LO phonon resonance region. Through incorporating a few percent of Be into the BeMgZnO barrier to switch the strain sign in the barrier from compressive to tensile, we have achieved 2DEG densities over a wide range in Zn-polar BeMgZnO/ZnO heterostructures with moderate Mg content (below 30%) grown by molecular beam epitaxy. We have obtained electron mobility of 250 cm2/Vs at room temperature (293 K) and 1800 cm2/Vs at 13 K in Be0.02Mg0.26ZnO/ZnO heterostructures. Via capacitance-voltage (CV) spectroscopy, we have explored the depth profiles of the apparent carrier density of samples grown under different conditions. The correlations between electrical properties and MBE growth parameters of Zn-polar BeMgZnO/ZnO heterostructures are discussed.
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