Chang-Chi Pan

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs.

Effect of external strain on the conductivity of AlGaN/GaN high-electron-mobility transistors

The changes in conductance of the channel of AlGaN/GaN high electron mobility transistor structures during application of both tensile and compressive strain were measured. For fixed Al mole fraction, the changes in conductance were roughly linear over the range up to 2.7x 10 8 N.cm -2 , with coefficients for planar devices of -6.0 +/- 2.5 x 10 -10 S.N -1 .m -2 for tensile strain and +9.5+/-3.5 x10 -10 S.N -1 .m -2 for compressive strain .For mesa-isolated structures, the coefficients were smaller due to the reduced effect of the AlGaN strain, with values of 5.5 +/- 1.1 x10 -13 S.N -1 .m -2 for tensile strain and 4.8 x10 -13 S.N -1 .m -2 for compressive strain. The large changes in conductance demonstrate that simple AlGaN/GaN heterostructures are promising for pressure and strain sensor applications.

AlGaN/GaN HEMT based liquid sensors

Abstract An AlGaN/GaN high electron mobility transistor structure was used for sensing different liquids present in the gate region. The forward current showed significant decreases upon exposure of the gate area to solvents (water, acetone) or acids (HCl). The pH sensitivity is due to changes in net surface charge that affects the relative depletion in the channel of the transistor. The results indicate that nitride-based heterostructures may have application in integrated chemical, gas and fluid monitoring sensors.

Spatial manipulation of nanoacoustic waves with nanoscale spot sizes

Coherent acoustic phonons are generated at terahertz frequencies when semiconductor quantum-well nanostructures are illuminated by femtosecond laser pulses. These phonons-also known as nanoacoustic waves-typically have wavelengths of tens of nanometres, which could prove useful in applications such as non-invasive ultrasonic imaging and sound amplification by the stimulated emission of radiation. However, optical diffraction effects mean that the nanoacoustic waves are produced with spot sizes on the micrometre scale. Near-field optical techniques can produce waves with smaller spot sizes, but they only work near surfaces. Here, we show that a far-field optical technique--which suffers no such restrictions--can be used to spatially manipulate the phonon generation process so that nanoacoustic waves are emitted with lateral dimensions that are much smaller than the laser wavelength. We demonstrate that nanoacoustic waves with wavelengths and spot sizes of the order of 10 nm and 100 nm, respectively, can be generated and detected.

Luminescence efficiency of InGaN multiple-quantum-well ultravioletlight-emitting diodes

The electroluminescence efficiency of In0.06Ga0.94N∕GaN multiple-quantum-well UV light-emitting diodes (LEDs) with emission wavelength of 400nm has been investigated and compared with blue (470nm) LEDs. Based on their injection current-dependent characteristics under dc and pulsed operation, it can be concluded that carrier overflow is the dominant factor that affects the external quantum efficiency of UVLED before thermal effects take over. It is experimentally shown that increasing the number of quantum wells is necessary to alleviate the carrier overflow issue and improve the luminescence efficiency of the UVLEDs.

On the origin of spin loss in GaMnN/InGaN light-emitting diodes

Spin polarization of GaMnN/InGaN light-emitting diodes grown by molecular beam epitaxy is analyzed. In spite of the ferromagnetic behavior of the GaMnN spin injector, the diodes are shown to exhibit very low efficiency of spin injection. Based on resonant optical orientation spectroscopy, the spin loss in the structures is shown to be largely due to fast spin relaxation within the InGaN spin detector, which itself destroys any spin polarization generated by optical spin orientation or electrical spin injection.

Light output improvement of InGaN ultraviolet light-emitting diodes by using wet-etched stripe-patterned sapphire substrates

GaN-based epilayers are grown on wet-etched stripe-patterned sapphire substrates, with stripes along the ⟨11−20⟩sapphire and ⟨1−100⟩sapphire directions, for 400nm ultraviolet light-emitting diodes (LEDs). The effects of the etching depth and stripe orientation on the structural and optical properties of the GaN layer as well as on the LEDs are investigated. Much better material quality and light output power are obtained when the GaN and the LEDs are grown on a 0.9μm deep patterned sapphire substrate with stripes along the ⟨1−100⟩sapphire direction. Stripe-orientation dependent growth modes accounting for the observed experimental results are proposed.

Current–voltage and reverse recovery characteristics of bulk GaN p-i-n rectifiers

p-i-n rectifiers were fabricated on epitaxial layers grown on free-standing GaN substrates. The forward turn-on voltage, VF was ∼5 V at 300 K and displayed a positive temperature coefficient. The specific on-state resistance (RON) was ∼5 mΩ cm2 at 300 K, with an ideality factor of ∼2 and activation energy for low forward current density of ∼1.6 eV. This is consistent with carrier recombination in the space charge region via a midgap deep level. The figure-of-merit, VB2/RON, where VB is the reverse breakdown voltage, was 0.32 MW cm−2. The reverse recovery time was ⩽600 ns at 300 K. The improved forward characteristics relative to previous heteroepitaxial p-i-n GaN rectifiers show the advantages of employing a GaN substrate to make a true vertical transport geometry device.

Two-dimensional nanoultrasonic imaging by using acoustic nanowaves

Two-dimensional ultrasonic imaging is demonstrated by using acoustic nanowaves. With a 14nm acoustic wavelength, both axial and transverse resolutions of a few tens of nanometers are thus achieved. This ultrasonic-based nondestructive technique not only images but also reconstructs the subsurface nanostructures including the depth positions of the buried interfaces. By demonstrating two-dimensional nanoultrasonic scans in depth and transverse (or z-x) axes, we show that acoustic nanowaves can be a promising tool for future subsurface three-dimensional noninvasive imaging with nanometer resolutions.

Optical piezoelectric transducer for nano-ultrasonics

Piezoelectric semiconductor strained layers can be treated as piezoelectric transducers to generate nanometer-wavelength and THz-frequency acoustic waves. The mechanism of nano-acoustic wave (NAW) generation in strained piezoelectric layers, induced by femtosecond optical pulses, can be modeled by a macroscopic elastic continuum theory. The optical absorption change of the strained layers modulated by NAW through quantum-confined Franz-Keldysh (QCFK) effects allows optical detection of the propagating NAW. Based on these piezoelectric-based optical principles, we have designed an optical piezoelectric transducer (OPT) to generate NAW. The optically generated NAW is then applied to one-dimensional (1-D) ultrasonic scan for thickness measurement, which is the first step toward multidimensional nano-ultrasonic imaging. By launching a NAW pulse and resolving the returned acoustic echo signal with femtosecond optical pulses, the thickness of the studied layer can be measured with <1 nm resolution. This nano-structured OPT technique will provide the key toward the realization of nano-ultrasonics, which is analogous to the typical ultrasonic techniques but in a nanometer scale.