The ferroelectric effect has been demonstrated on an AlGaN/GaN heterostructure field effect transistor using a Pb(Zr0.52Ti0.48)O3 layer deposited by radio-frequency magnetron sputtering. The device with a metal-ferroelectric-metal-semiconductor (MFMS) structure was fabricated with a Schottky contact placed between ferroelectric PZT and AlGaN/GaN 2-dimensional electron gas (2DEG) channel. The Schottky contact performs as a bottom electrode of the ferroelectric PZT and also as a barrier layer to prevent interaction at the interface between PZT and GaN. X-ray diffraction revealed the formation of (111)-oriented perovskite phase PZT on gate patterned AlGaN/GaN heterostructures. Transfer characteristics of the double-gate ferroelectric field effect transistor was determined by measuring its source-drain current as the gate bias applied on the top electrode was swept from -15 V to 15 V and then back to -15 V with a voltage step of 0.1 V. Ferroelectric behavior was observed in the plot of source-drain current versus gate voltage that showed a large counterclockwise hysteresis with a 50 % current modulation at zero gate bias.
We report a comparison of various gate dielectrics including SiO2, Si3N4, ZrO2 and Pb(Zr, Ti)O3
(PZT) on AlGaN/GaN heterojunction field-effect transistors, deposited by PECVD, MBE, and
sputtering respectively. In terms of I-V characteristics, maximum drain-source current could be
enhanced under positive gate voltage and the reverse leakage current level decreases by orders of
magnitude. In terms of DC measurements, very thin SiO2 layers can improve performance, which
may be due to the passivation effect to remove surface states. No significant difference exists
between control and the Si3N4 and ZrO2 samples. Slightly reduction in transconduction is observed
on the sample with PZT probably because of the much thicker layer was utilized. The thickness of
insulator layers examined from C-V measurements reveals a better crystal quality can be obtained
by PECVD deposition. While the RF S-parameters measurements shows the PZT gate dielectric
brings the highest cut-off frequency or the lowest gate capacitance confirmed also by C-V data,
which makes it a better candidate for microwave applications.
Lead zirconate titanate PbZr52Ti48O3 (PZT) layers were deposited on ZnO layers by rf-sputtering at varying substrate temperatures. The effect of annealing on PZT crystal properties has been studied by X-ray diffraction and atomic force
microscopy. It is shown that the annealing in oxygen ambient has significant effect on the quality of the deposited PZT
layers. The optimum growth temperature has been found to be 650 C.
The conduction band offset of n-ZnO/n-6H-SiC heterostructures prepared by rf-sputtered ZnO on commercial n-type
6H-SiC substrates has been measured. Temperature dependent current-voltage characteristics, photocapacitance, and
deep level transient spectroscopy measurements led to conduction band offsets of 1.25 eV, 1.1 eV, and 1.22 eV,
Electrical properties of n-ZnO/n-GaN isotype heterostructures prepared by rf-sputtering of ZnO films on GaN layers
which in turn grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n-
ZnO/n-GaN diodes exhibited highly rectifying characteristics with forward and reverse currents being ~1.43x10-2 A/cm2
and ~2.4x10-4 A/cm2, respectively, at ±5 V. From the Arrhenius plot built representing the temperature dependent
current-voltage characteristics (I-V-T) an activation energy 0.125 eV was derived for the reverse bias leakage current
path, and 0.62 eV for the band offset from forward bias measurements. From electron-beam induced current
measurements and depending on excitation conditions the minority carrier diffusion length in ZnO was estimated in the
range 0.125-0.175 &mgr;m. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073
Epitaxial growth of ZrO2 has been achieved on MOCVD-grown GaN(0001) templates by
oxides molecular beam epitaxy using reactive H2O2 for oxygen and organometallic
source for Zr. Utilizing a low temperature buffer layer followed by high temperature
insitu annealing and high-temperature growth, monoclinic (100)-oriented ZrO2 thin films
were obtained. The full width at half maximum of ZrO2 (100) rocking curve was 0.4 arc
degree for 30-nm-thick films and the rms roughness for a 5&mgr;m by 5 &mgr;m AFM scan was 4 Å. The employment of epitaxial ZrO2 layer in the AlGaN/GaN heterojunction field effect
trasnsistor as a gate dielectric has resulted in the increase of the saturation-current density
and pinch-off voltage as well as in near symmetrical gate-drain I-V behavior.
Ferroelectric field effect transistors (FFETs) with hysteretic I-V characteristics were attained with 25 nm thick Pb(Zr0.52Ti0.48)O3 (PZT)/Si3N4 gated AlGaN/GaN heterostructure. The PZT films used in the gate of the device was deposited by magnetron rf-sputtering at the substrate temperature of 700 oC. Increasing the PZT deposition temperature from that in previous device structures from 600 oC to 700 oC we obtained much improved device performance in terms of the IV characteristics inclusive of hysteretic behavior. The pinch-off voltage was about 7 V in FFET device compared to 6 V in a the control (conventional) AlGaN/GaN device. Counterclockwise hysteresis appeared in the transfer characteristic curve of a FFET with a maximal drain current shift of about 10 mA at the gate-to-source voltage of -6 V.