We demonstrated ferroelectric field effect transistors ͑FFETs͒ with hysteretic I-V characteristics in a modulation-doped field effect transistors ͑MODFET͒ AlGaN/ GaN platform with ferroelectric Pb͑Zr, Ti͒O 3 between a GaN channel and a gate metal. The pinch-off voltage was about 6-7 V comparable to that of conventional Schottky gate MODFET. Counterclockwise hysteresis appeared in the transfer characteristics with a drain current shift of ϳ5 mA for zero gate-to-source voltage. This direction is opposite and much more pronounced than the defect induced clockwise hysteresis in conventional devices, which suggests that the key factor contributing to the counterclockwise hysteresis of the FFET is the ferroelectric switching effect of the lead zirconate titanate gate.
GaN nanorods were grown on c-plane sapphire substrates under N-rich conditions by plasma-assisted molecular-beam epitaxy. Scanning electron microsopy revealed densely packed nanorods of hexagonal cross section with diameters ranging from roughly 40 to 100 nm. Atomic force microscopy indicated that the rods protruded 50 to 75 nm above the average height of the surface. Transmission electron microscopy (TEM) showed that the nanorods were approximately 1.4 micrometers in length but an accurate measurement of the rod separation was difficult to assess. Contrary to expectations for GaN grown under N-rich conditions, a high density of basal plane stacking faults were not revealed in TEM under typical imaging conditions. X-ray diffraction using the (0002), (0004), (0006) reflections yielded c = 0.5188 ± 0.0002 nm and a = 0.3188 ± 0.0004 nm. Low temperature photoluminescence and cathodoluminescence showed broad near-bandgap emission around 3.4 eV that shifted to the blue with reduced temperature in the usual manner, and the presence of a similarly blue-shifting peak near 3.2 eV. The spectra were deconvolved using nine lineshape functions revealing 2 phonon replicas asssociated with the peak near 3.2 eV. Room temperature spectroscopic reflection fited to the standard Aspnes thirdderivative lineshape function yielded a transition energy of 3.407 eV for the A exciton and 3.490 eV for the B+C excitons (not spectrally resolved). Both the x-ray and photoreflectance results indicate that the nanorods are fairly relaxed.
Undoped-and Si-doped GaN epitaxial films were grown on sapphire substrates by metal-organic chemical vapor deposition. Variable angle spectroscopic ellipsometry (VASE) and surface photovoltage (SPV) spectroscopy were used to determine the minority carrier diffusion lengths in the films. Both the reflectivity (R) and absorption coefficient (α) could be calculated from the VASE measurements. In the SPV spectra at room temperature, a strong transition with a threshold at 3.42 eV was observed in both films, while an exciton-related absorption was observed only in the undoped GaN. The minority carrier diffusion lengths were measured to be about 200 nm for the undoped GaN and about 20 nm for the Si-doped GaN. Moreover, a large divergence in the range of 1/α in the undoped GaN was found and attributed to a high surface barrier energy.Gallium nitride (GaN) and its alloys are promising materials for optoelectronics, high-power electronics, and high-temperature electronics due to their physical and electrical properties [1]. Despite high background electron concentrations in GaN films, GaN-based devices have shown good efficiencies [2]. Since the minority carrier transport properties play a significant role in the performance of bipolar devices, measurement of properties such as diffusion length and lifetime are essential to understanding and improving device performance. The diffusion length of minority carriers, L, has been mainly been determined with surface photovoltage (SPV) and electron beam induced current (EBIC) measurements [3,4]. Most previous studies on the SPV of GaN have extracted the diffusion lengths of minority carriers, ignoring the effect of reflectivity, R, and incident light flux, Φ, on the photon energy [5,6]. In this study, L in n-type GaN films was investigated using both variable angle spectroscopic ellipsometry (VASE) and SPV measurements. Moreover, the reflectivity, R(E), and the incident light flux, Φ(E), as a function of the photon energy were considered in the analysis, in contrast with the previous analyses of GaN.Two GaN films were grown on sapphire substrates by metallorganic chemical vapor deposition. One was unintentionally doped (undoped) with a resulting background electron concentration of 1.5 × 10 17 cm -3 , and the other was Si-doped with an electron concentration of 5.8×10 18 cm -3 . The thickness of both GaN layers was about 1.1 µm. In order to obtain refractive index and extinction coefficient, VASE measurements were performed in the range of approximately 2 eV to 6 eV. The incident light was polarized and the incident angle was fixed at 75 degrees. For the SPV measurements, monochromatic light from a Xe lamp was chopped at a frequency of 320 Hz. The sample stage was grounded and the surface photovoltage was measured at room temperature.
Photoreflectance (PR) modulation spectroscopy was performed to investigate surface properties of GaN films grown on sapphire substrates. From the period of the Franz-Keldysh oscillations, the surface electric field across the GaN space charge region was found to be (197 ± 11) kV/cm, which corresponds to a surface state density of 1.0×10 12 cm -2 . A surface barrier height of 0.71 eV was determined by fitting the dependence of the PR intensities on pump beam power density. We suggest that a deep level is formed at 2.68 eV above the GaN valence band edge due to the large density of surface states.
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