Heteroepitaxy on nanoscale substrates: Application to solid -state lighting

Thin film heterostructures are limited by a maximum critical thickness before introduction of extended defects. One-dimensional form factors like nanowires/nanorods, due to the possibility of lateral elastic relaxation, can tolerate much larger lattice mismatch than their thin film counterparts. The present document begins with the description of some modeling work employing solution thermodynamics and finite element analysis as a motivation to the nanoheteroepitaxy approach to achieve a monolithic phosphor-free white light emitting diode (LED). A nanorod with a pointed tip morphology has been shown to be required for pushing the emission wavelengths from the InN-GaN system to longer values (red). The process developed to synthesize diameter controlled GaN pyramidal tipped nanorods without the use of any catalysts has also been developed in the course of the present research work. The same template-based process along with optical lithography techniques has been demonstrated to yield controlled nanorod diameter variation on the same substrate. Due to the large surface-area-to-volume ratio of the synthesized nanorods, it is required to ascertain that the nanorods are not devoid of charge carriers due to the surface depletion effect. Electrical characterization of the nanorods in the form of single and multiple GaN nanorod Schottky and p-n junctions diodes employing conductive atomic force microscopy has also been performed and described in the document. Finally, cathodoluminescence spectra from (In,Ga)N nanorods have been used to show the potential of the nanorod form to incorporate higher InN mole fractions as compared to thin film counterparts. Along with applications in solid-state lighting, the pointed tip morphology of the nanorods, resulting in a very high field enhancement factor, are contenders as field emitters. Coupled with such a high field enhancement factor, the incorporation of (Al,Ga)N on the nanorod tip helps to reduce the effective surface work function resulting in a significant reduction in the turn-on field from (Al,Ga)N/GaN nanorod heterostructures as compared to GaN nanorods. Such an approach circumvents the doping problem of (Al,Ga)N, still utilizing its low electron affinity. Results from vacuum field emission experiments from (Al,Ga)N/GaN nanorod heterostructures and their analysis have also been presented in the document

Field emission from GaN and (Al,Ga)N/GaN nanorod heterostructures

Vacuum field emission from GaN and (Al,Ga)N/GaN nanorods with pyramidal tips has been measured. The turn-on fields, defined at a current density of 0.1 mu A/cm(2), were found to be 38.7 and 19.3 V/mu m, for unintentionally doped GaN and (Al,Ga)N/GaN nanorods, respectively. The 5 nm (Al,Ga)N layer reduced the electron affinity at the surface, thereby lowering the turn-on field and increasing the current density. The nanostructures exhibit a field enhancement factor of approximately 65 and the work function of the (Al,Ga)N/GaN nanorod heterostructure was estimated to be 2.1 eV. The stability of the emission characteristics and the simple fabrication method suggest that intentionally doped and optimized (Al,Ga)N/GaN nanorod heterostructures may prove suitable for field-emission device

High-Reflectivity Al-Pt Nanostructured Ohmic Contact to p-GaN

The effect of nanoscale Pt islands on the electrical characteristics of contacts to p-type gallium nitride (GaN) has been investigated to explore the feasibility for the flip-chip configuration light-emitting diodes (LEDs) using an Al-based reflector. An as-deposited Al contact to p-GaN with a net hole concentration of 3 × 1017 cm−3 was rectifying. However, an Al contact with nanoscale Pt islands at the interface exhibited ohmic behavior. A specific contact resistivity of 2.1 × 10−3 Ω · cm2 and a reflectance of 84% at 460 nm were measured for the Al contact with nanoscale Pt islands. Current–voltage temperature measurements revealed a Schottky barrier height reduction from 0.80 eV for the Al contact to 0.58 eV for the Al contact with nanoscale Pt islands. The barrier height reduction may be attributed to electric field enhancement and the enhanced tunneling due to the presence of the nanoscale Pt islands. This will offer an additional silver-free option for the p-type ohmic contact in flip-chip configuration LEDs. Theory suggests that the ohmic contact characteristics may be improved further with smaller Pt islands that will enhance tunneling across the interface with the GaN and in the vicinity of the Pt–Al interface

Nanopatterned contacts to GaN

The effect of nanoscale patterning using a self-organized porous anodic alumina (PAA) mask on the electrical properties of ohmic and Schottky contacts to n-GaN was investigated with the aim of evaluating this approach as a method for reducing the specific contact resistance of ohmic contacts to GaN. The electrical characteristics of contacts to these nanopatterned GaN samples were compared with contacts to planar, chemically prepared ( as-grown ) GaN samples and reactive ion etched (RIE) GaN films without any patterning. The specific contact resistivities to unintentionally doped n-GaN using a Ti/Al bilayer metallization were determined to be 7.4 x 10(-3) Omega cm(2) for the RIE sample and 7.0 x 10(-4) Omega cm(2) for the PAA patterned sample. Schottky metal contacts with Pt and Ni were prepared on the three samples to validate the effects of RIE and nanopatterning on electrical behavior. The effective barrier height was decreased and the reverse current was increased significantly in the PAA patterned sample. The radius of curvature of the nanoscale corrugation in the patterned interface was smaller than the depletion width. The reduction of the depletion width at sharp corners enhanced the local tunneling current, reducing the specific contact resistivity and decreasing the effective barrier height. These results suggest that nanopatterning with PAA on GaN can significantly lower the contact resistance

Selection of parameters (indicators) for soil quality index using eigenvectors obtained through principal component analysis.

Selection of parameters (indicators) for soil quality index using eigenvectors obtained through principal component analysis.</p

Heatmap of variables showing Pearson’s correlation coefficient.

Heatmap of variables showing Pearson’s correlation coefficient.</p

Soil quality index scores under different land use classes.

Soil quality index scores under different land use classes.</p

Proportion and eigenvalues of the selected indicators for calculation of weighted additive soil quality index.

Proportion and eigenvalues of the selected indicators for calculation of weighted additive soil quality index.</p