Hong-Mao Lee

InGaN/GaN nanorod array white light-emitting diode

Conventional InGaN/GaN light-emitting diodes based on planar quantum wellstructures do not allow for efficient long-wavelength operation beyond the blue region due to a strong quantum confined Stark effect in lattice-mismatched polar InGaNquantum wells. Here we overcome the limitation by using self-assembled GaNnanorod arrays as strain-free growth templates for thick InGaN nanodisks. In combination with enhanced carrier localization and high crystalline quality, this approach allows us to realize full-color InGaN nanodisk emitters. By tailoring the numbers, positions, and thicknesses of polychromatic nanodisk ensembles embedded vertically in the GaNnanorod p - n junction, we are able to demonstrate natural white (color temperature ∼ 6000 K ) electroluminescence from InGaN/GaN nanorod arrays.

High current density InN∕AlN heterojunction field-effect transistor with a SiNx gate dielectric layer

InN∕AlN metal-insulator-semiconductor heterojunction field-effect transistors with a gate-modulated drain current and a clear pinch-off characteristic have been demonstrated. The devices were fabricated using high-quality InN (26nm)∕AlN (100nm) epifilms grown by plasma-assisted molecular-beam epitaxy on Si (111) substrates. The devices exhibited a current density higher than ∼530mA∕mm with a 5μm gate length. The pinch-off voltage was at ∼−7V with an associated drain leakage current less than 10μA∕mm. The observed high current density may be attributed to the high sheet carrier density due to the large spontaneous polarization difference between InN and AlN.

Immobilization of DNA-Au nanoparticles on aminosilane-functionalized aluminum nitride epitaxial films for surface acoustic wave sensing

A generic method for immobilization of gold nanoparticle bioconjugates onto aluminum nitride (AlN) surfaces using aminosilane molecules as cross-linkers is demonstrated for surface acoustic wave (SAW) sensor applications. Electrostatic interaction between positively charged surface amine groups and negatively charged DNA-Au nanoparticle conjugates allows the self-assembly of a probe nanoparticle monolayer onto functionalized AlN surfaces under physiological conditions. Both 10 and 20 nm Au nanoparticles bound with thiolated oligonucleotides were employed in the detection scheme. We show that Au nanoparticles can play multiple roles in SAW sensing for probe immobilization, signal amplification, and labeling.

Observation of femtosecond carrier thermalization time in indium nitride

Ultrafast carrier thermalization in n-type indium nitride (InN) with an electron concentration of 3.8×1018 cm−3 was investigated by femtosecond transient transmission measurements at room temperature with different wavelengths. An extremely fast carrier external thermalization time on the order of 400 fs was observed, which is much faster than all previous reports. This observed femtosecond thermalization time is consistent with a prediction based on a Coulomb screening effect. Through wavelength dependent and power dependent studies, even with a 400 fs thermalization time, we did not observe any evidence of the existence of the hot phonon effect, which agrees with a recent report that a longitudinal optical phonon lifetime could be shorter than 300 fs in specific InN samples.

Site-Selective Biofunctionalization of Aluminum Nitride Surfaces Using Patterned Organosilane Self-Assembled Monolayers

Surface biochemical functionalization of group-III nitride semiconductors has recently attracted much interest because of their biocompatibility, nontoxicity, and long-term chemical stability under demanding physiochemical conditions for chemical and biological sensing. Among III-nitrides, aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) are particularly important because they are often used as the sensing surfaces for sensors based on field-effect transistor or surface acoustic wave (SAW) sensor structures. To demonstrate the possibility of site-selective biofunctionalization on AlN surfaces, we have fabricated two-dimensional antibody micropatterns on AlN surfaces by using patterned self-assembled monolayer (SAM) templates. Patterned SAM templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl), which were fabricated on AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. Because the patterned SAM templates have different surface properties on the same surface, clear imaging contrast of SAM micropatterns can be observed by field-emission scanning electron microscopy (FE-SEM) operating at a low accelerating voltage in the range of 0.5-1.5 kV. Furthermore, the contrast in surface potential of the binary SAM microstructures was confirmed by selective adsorption of negatively charged colloidal gold nanoparticles (AuNPs). The immobilization of AuNPs was limited on the positively charged amino-terminated regions, while they were scarcely found on the surface regions terminated by methyl groups. In this work, selective immobilization of green fluorescent protein (GFP) antibodies was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal from antibody functionalized regions on the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition resulting from the antibody-EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions on AlN-based bioanalytical microdevices.

Valence band offset and interface stoichiometry at epitaxial Si3N4/Si(111) heterojunctions formed by plasma nitridation

Ultrathin {beta}-Si{sub 3}N{sub 4}(0001) epitaxial films formed by N{sub 2}-plasma nitridation of Si(111) substrates have been studied by photoelectron spectroscopy using synchrotron radiation. The valence band offset at the {beta}-Si{sub 3}N{sub 4}/Si interface was determined by valence-band photoelectron spectra to be 1.8 eV. Furthermore, the Si 2p core-level emissions were analyzed for nitride (Si{sup 4+}) and subnitride (Si{sup 3+} and Si{sup +}) components to characterize the interface stoichiometry. In contrast to the interfaces formed by ammonia thermal nitridation and N{sub 2}-plasma nitridation at room temperature, the interface formed by N{sub 2}-plasma nitridation at high substrate temperature is very close to subnitride free with an abrupt composition transition.

Direct imaging of GaN p-n junction by cross-sectional scanning photoelectron microscopy and spectroscopy

uted by them to the existence of high-density steps and defects at the cleavage surface. In this work, we provide direct evidence for the unpinned nature of cleaved a-plane GaN surfaces. Furthermore, we report on the application of XSPEM/S technique for direct imaging of GaN p-n junction. The sample structure for this study is N-polar p-GaN 1.5 m /n-GaN 1.5 m /AlN 25 nm /Si 3 N 4 /Si111, which was grown by plasma-assisted molecular beam epitaxy PA-MBE on a Si wafer. Details of the growth technique can be found elsewhere. 11 The n- and p-type doping was performed by using high-purity Si and Mg solid cells during the PA-MBE growth process. The Mg concentration as determined by secondary ion mass spectroscopy is 7

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.

Observation of sub-100 femtosecond electron cooling time in InN

We report that the electron cooling time in indium nitride can be as fast as sub-100 femtosecond at low electron concentration (<5×1017/cm3), which is much faster than previous reports. Through investigating the dependence of the measured carrier cooling time on electron density, our study proved the dominant role of the screened Frohlich interaction in the reduction in carrier cooling rate at an electron density higher than 1.8×1018/cm3.