Yu-Liang Hong

Intense terahertz emission from a-plane InN surface

We report a significant enhancement in terahertz emission from the indium nitride (InN) films grown along the a axis (a-plane InN), relative to the InN films grown along the c axis. The primary radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a axis, which effectively enhances the geometrical coupling of the radiation out of semiconductor. In addition, azimuthal angle dependence measurement shows that the p-polarized terahertz output consists of a large angularly independent component and a weak component with a distinctive fourfold rotation symmetry.

Highly Sensitive pH Sensing Using an Indium Nitride Ion-Sensitive Field-Effect Transistor

We demonstrated an ultrathin (~10 nm) ifndium nitride (InN) ion-sensitive field-effect transistor (ISFET) for pH sensing. The native indium oxide formed on the InN surface functions as a chemical binding layer with a high pH sensitivity, while the strong surface electron accumulation of InN along with the ultrathin conduction channel results in a large ion-induced surface potential to current transconductance. The ultrathin InN ISFETs were characterized to show a gate sensitivity of 58.3 mV/pH in the pH range of 2-12, a current variation ratio of 4.0%/pH, a resolution of less than 0.03 pH, and a response time of less than 10 s.

Carrier dynamics in InN nanorod arrays

A fast initial decay was observed from InN nanorods with ~30 nm diameter, the lifetime of which is much shorter than the carrier cooling time, demonstrating the substantial surface-associated influence on carrier relaxation in nanorods.

Terahertz emission mechanism of magnesium doped indium nitride

We report carrier concentration-dependence of terahertz emission from magnesium doped indium nitride (InN:Mg) films. Near the critical concentration (nc∼1×1018 cm−3), the competition between two emission mechanisms determines the polarity of terahertz emission. InN:Mg with n>nc exhibits enhanced positive polarity terahertz emission compared to the undoped InN, which is due to the reduced screening of the photo-Dember field. For InN:Mg with n<nc, the polarity of terahertz signal changes to negative, indicating the dominant contribution of the surface electric field due to the large downward surface band bending within the surface layer extending over the optical absorption depth.

Effects of (NH4)2Sx treatment on indium nitride surfaces

Indium nitride (InN) surfaces treated with ammonium sulfide [(NH4)2Sx] are investigated using Hall effect measurement, x-ray photoelectron spectroscopy (XPS), and scanning Kelvin probe microscopy (SKPM). Upon the (NH4)2Sx treatment, the sheet carrier density is reduced by (0.8–0.9)×1013 cm−2, leading to an increase in the sheet resistance. By numerically solving the Poisson’s equation, the associated upward shift of the surface band bending is derived to be 0.3 eV. XPS characterization shows, on the (NH4)2Sx treated InN surface, the formation of native oxide is effectively suppressed and a covalently bonded sulfur layer with surface In atoms is formed. This surface In–S dipole layer results in an increase in the electron affinity, thus giving rise to a lower surface bending shift (0.2 eV) observed in XPS. The electron affinity increase of 0.1 eV can be deduced, which is consistent with the result obtained by SKPM. Thus, the (NH4)2Sx treatment has been demonstrated to be an effective method for reducing th...

Investigation on −c-InN and a-InN:Mg field effect transistors under electrolyte gate bias

The electrical properties of N-polar undoped InN and nonpolar a-InN:Mg ion sensitive field effect transistors (ISFETs) have been investigated by electrolyte-gate-biased current-voltage (IDS-VGS) measurements. IDS-VGS characteristics reveal that the a-InN:Mg ISFETs have a large (∼52%) current variation ratio at a gate bias of 0.3 V with respect to the unbiased one, which is higher than that from the undoped InN ISFETs (∼18% and <0.1% for 10-nm and 1-μm-thick −c-InN epilayers, respectively). The a-InN:Mg ISFETs can also function as a pH sensor with a sensitivity of 56.5 mV/pH and a response time less than 10 s.

Direct probe of the built-in electric field of Mg-doped a-plane wurtzite InN surfaces with time-resolved electric-field-induced second harmonic generation

The exceptionally large electron affinity of InN causes the pinning of surface Fermi level well above the conduction band minimum. This unique electronic property leads to the electron accumulation at InN surfaces and a large built-in electric field in the topmost few nanometers of InN surfaces. In this letter, we demonstrate that this surface electric field can be unambiguously determined and monitored in a-plane wurtzite InN surface via time-resolved electric-field-induced second harmonic generation. This finding makes it possible to directly probe and characterize the surface electronic properties of Mg-doped InN with an all-optical technique in ambient environment.

Resonant raman scattering and dispersion of polar optical and acoustic phonons in hexagonal inn

It is shown that a study of the dependence of impurity-related resonant first-order Raman scattering on the frequency of excitation light makes it possible to observe the dispersion of polar optical and acoustic branches of vibrational spectrum in hexagonal InN within a wide range of wave vectors. It is established that the wave vectors of excited phonons are uniquely related to the energy of excitation photon. Frequencies of longitudinal optical phonons E1(LO) and A1(LO) in hexagonal InN were measured in the range of excitation-photon energies from 2.81 to 1.17 eV and the frequencies of longitudinal acoustic phonons were measured in the range 2.81–1.83 eV of excitation-photon energies. The obtained dependences made it possible to extrapolate the dispersion of phonons A1(LO) and E1(LO) to as far as the point Γ in the Brillouin zone and estimate the center-band energies of these phonons (these energies have not been uniquely determined so far).

Background and Photoexcited Carrier Dependence of Terahertz Radiation from Mg-Doped Nonpolar Indium Nitride Films

We report terahertz (THz) generation from Mg-doped nonpolar (a-plane) InN (a-InN:Mg). While the amplitude and polarity of the THz field from Mg-doped polar (c-plane) InN depend on the background carrier density, the p-polarized THz field from a-InN:Mg has background carrier-insensitive intensity and polarity, which can be attributed to carrier transport in a polarization-induced in-plane electric field. A small but apparent azimuthal angle dependence of the THz field from a-InN:Mg shows the additional contribution of the second-order nonlinear optical effect. Meanwhile, in this study, we did not observe the contribution of the intrinsic in-plane electric field which is significant for high stacking fault density nonpolar InN.

Ultrafast E1(LO) phonon and plasma dynamics in a-plane wurtzite InN

Coherent phonon spectroscopy of an a-plane wurtzite InN epitaxial thin film is demonstrated with time-resolved second-harmonic generation technique. Coherent E1(LO) phonons are launched via a transient electric field screening process in the near surface region of the a-plane wurtzite InN. Because of the macroscopic electric dipole interaction between coherent E1(LO) phonons and photoexcited plasmons, two hybridized vibration modes, phononlike (L−) and plasmonlike (L+) E1(LO) phonon-plasmon coupling modes, are identified. The investigation of these coupling modes allows us to understand the ultrafast carrier and phonon dynamics in a-plane wurtzite InN.