Abbey Heinlein

University of Michigan
Electrical Engineering

Project Title: Electrochemical Fabrication of Nanowire Arrays
Advisor: Prof. Tim Sands

Introduction

LEDs produce light much more efficiently than the incandescent and flourescent bulbs commonly used today. Unfortunately, the lack of a white LED has kept this form of lighting from becoming a viable option for everyday use. In order to produce a white LED, GaN, which emits blue and green wavelengths of light, must be layered with (In,Ga)N, which emits red and yellow wavelengths. However, when GaN and (In,Ga)N are layered together in a planar structure, dislocations form and prevent an efficient LED from being created. It has been hypothesized that creating an array of nanoscaled wires with a base of GaN and an increasing concentration of In in the upper parts of the wires would alleviate the stresses that cause dislocations and allow an efficient white LED to be created.

In order to create the nanowire array, a template of porous anodic alumina (PAA) is used either as a pattern for deposition of GaN or as a mask for etching of GaN. PAA is a naturally occurring result of anodizing an aluminum thin film--as the anodization is carried out, pores form in the aluminum oxide layer from the surface down to the underlying substrate. If the PAA film is fabricated on top of a layer of GaN, nickel can be deposited in the pores as a shield on the GaN. The PAA can then be etched away, along with a portion of the GaN in areas not covered by the nickel. Removing the nickel would then leave an array of GaN nanodots to be used as a base for growing (In,Ga)N nanowires.

Project Objectives

  • Standardize 2-step anodization procedure for PAA film
  • Produce 100nm layer of PAA--thin enough to allow deposition of another material in pores
  • Use PAA template to create nanowires and/or nanodots

Approach

  • 2-step anodization:
    1. Partially anodize sample in 0.3M oxalic acid at 4&degC for a set time ranging from 15-29 minutes
    2. Strip PAA layer using a solution of chromium (VI) oxide and phosphoric acid at 55&degC
    3. Complete anodization on remaining aluminum layer
    4. Remove barrier layer/widen pores using phosphoric acid at room temperature
  • Check PAA thickness using SEM
  • When correct (~200-300nm) PAA thickness is reached, use PAA as a pattern to deposit platinum (or other substance) for nanowires

Findings

  • Determined that a first anodization of 26-27 minutes creates a 200-300 nm PAA film
  • Found that, due to small (~100nm) variations in aluminum film thickness between samples, reliably fabricating PAA films less than 300nm requires measurement of aluminum thickness for each individual sample prior to anodization
  • Deposited platinum in a 300nm PAA film to create platinum nanowires

Future Work

  • Do additional experiments on rate of anodization
    • Determine standard rate of anodization (nm/min)
    • Test this on samples: measure Al thickness, find appropriate first anodization time from standardized rate, carry out anodizations, measure to determine if correct thickness was reached
  • Create GaN or Si nanodots to serve as a base for GaN/InN pillars, using the following process (see Figure 2 below):
    • Two-step anodization of aluminum layer deposited on GaN thin film with sapphire substrate
    • Deposit nickel in pores of PAA
    • Remove PAA layer, leaving dots of nickel on GaN layer
    • Reactive ion etching removes a portion of GaN film where Ni dots aren't present
    • Remove Ni dots, leaving GaN nanodots on a GaN film

Figure 1: Top view of PAA sample
Figure 2: Side view of PAA sample with
platinum nanowires