Aqueous Synthesized Epitaxial ZnO via Microwave Heating

Frederick F. Lange, University of California-Santa Barbara, DMR 0905254

Aqueous Synthesis of ZnO in Ammonium Solutions

Thermodynamic Calculations show:

• Retrograde solubility controls precipitation and growth

• at room temperature: zinc amine complex, Zn(NH3)4

2+ , creates large ZnO solubility

• at higher temeratures: Zn(NH3)4

2+ + 4(OH-) → Zn(OH)4-2 + 4NH3

enables ZnO synthesis and epitaxy

• rapid heating via microwaves: 1 second @ 90°C creates nano crystallites that epitaxial grow on a GaN substrate

1 µm

610 nm

Time(min)

Temp

Temperature-Time

BioTage® Micowave Reactor

GaN LED Shines Brighter with Aqueous ZnO Electrode

Frederick F. Lange, University of California-Santa Barbara, DMR 0905254

Current (ma)

Ou

t p

ut p

ower

(m

W)

NiAu

ITO

AqueousZnO

93% ImprovementSapphire

n-ZnO

n-GaN

p-pad

n-padp-GaN

p-AlGaN

Active Layer

Grown in 90°C Aqueous Solution

Today, GaN light emitting diodes (LEDs) are the most efficient forms of lighting, but too expensive for lighting homes and buildings. Recent work has shown that aqueous synthesized ZnO produces an inexpensive electrode (right figures) that is twice better than the conventional Ni-Au metal electrode, and slightly better than the very expensive Indium-tin oxide (ITO) shown in the left figure. Aqueous synthesis is also GREEN!

packaged

Notes for two slides Frederick F. Lange, University of California-Santa Barbara, DMR 0905254

Slide 1 (Fundamental) Prior to the start of our NSF program (Aug 15, 2009) thermodynamic calculations showed [1] that ZnO would exhibit retrograde solubility in water containing ammonium., namely, its much more soluble at room temperature, and precipitates at higher temperatures. These calculations lead to the construction of a continuous reactor [2], and the synthesis of epitaxial films, under different conditions, that were used to validate the thermodynamic calculations. The best films were produced under equilibrium conditions.

In this new work, the same thermodynamic calculations suggested that rapid heating would produce nano powder, and nano nuclei on a single crystal substrate, which would quickly provide a ‘seed’ layer for the equilibrium growth of a ZnO film of higher quality.

The microwave experiments consisted of dissolving ZnO powder in a glass container containing an aqueous solution of 0.5M NH3, the heating to different temperatures for different periods. The results for heating to 90°C for only 1 sec are shown; longer heating periods produced thicker films (up to 2 microns), but film thickness was limited by the amount dissolved ZnO.

Nano crystallites (small critical radius), that form epitaxial on the substrate are predicted due to the very rapid heating rate that can be achieved in the microwave.

[1,2] JJ Richardson and FF Lange, “Controlling Low Temperature Aqueous Synthesis of ZnO” Part I and II Crystal Growth & Design, 9 [6] 2570 (2009)

Side 2 (Broader Impact) White GaN LEDs, the light of our life, are the most efficient means of lighting. Currently, lighting consumes about 20% of all electricity consumed, world wide Despite their wide use in traffic lighting, large TV screens, etc, LED lighting is currently too expensive to be used in houses and buildings.

This year, it was demonstrated [3] that ZnO, synthesized in water as an epitaxial film on a commercial LED could be used as a current spreading electrode. It was shown that the current spreading ZnO electrode was twice better than the Ni-Au thin metal electrode used on commercial LEDs. The problem with the Ni-Au electrode is that it blocks about 40% of the light produced by the LED, whereas the ZnO is transparent.

More recently, my student, working with a student and postdoc in our Solid-State Lighting and Energy Center, co-directed by Shuji Nakamura and Steve DenBaars, constructed LEDs using ZnO, ITO (indium-tin oxide) and Ni-Au electrodes As shown in the figure, the ZnO was, as previously reported nearly twice as good as the Ni-Au electrode, but also slightly better than the ITO electrode.

Because ITO is currently used to produce electrical conducting films on glass used in computer, and many other applications, it is being depleted, world-wide, and thus has an enormous cost. Our new information suggests that aqueous synthesized ZnO has a good possibility of replacing indium.

[3] Thompson, D. B.; Richardson, J. J.; DenBaars, S. P.; Lange, F. F.,

Applied Physics Express 2009, 2, (4), 042101.