In Search of the Next Top Semiconductor: Cuprous Oxide vs. Silicon

When it comes to semiconductor materials, Si and GaN are the big players. However, depending on who you ask, it's always good to keep your options open. A material that keeps popping up in research studies is cuprous oxide (or copper (I) oxide).

 

Thermal conductivity of different materials. Image used courtesy of Plansee

 

Cuprous oxide was one of the earliest materials found to have semiconducting properties. This article explores some benefits and uses of this unique material.

 

What Is Cuprous/Copper Oxide?

Cuprous oxide is a semiconductor known for its variety of bandgaps ranging from 2.0–2.2 eV and its photovoltaic properties. As a p-type oxide of copper with the chemical formula Cu₂O, it can exist as a thin film or a nanoparticle, offering novel physical features useful for fundamental research due to its low costs.

Over the last few decades, researchers have identified various methods for synthesizing cuprous oxide. A few techniques discovered are:

• Thermal oxidation
• Chemical vapor deposition
• Anodic oxidation
• Reactive sputtering
• Electrodeposition
• Plasma evaporation
• Sol–gel-like dip technique

Even with extensive research, a significant hurdle with employing most of these methods is producing a mixture phase of Cu, CuO, and Cu₂O, resulting in the sparse application of cuprous oxide as a semiconductor.

 

Preparing cuprous oxide (Cu₂O) by reactive sputtering ((a) Cu film (100% Ar), (b) Cu2O film (95:5), (c) Cu₂O film (90:10) and (d) Cu2O film (80:20)). Image used courtesy of Dolai et al

 

Despite this shortcoming in applications, it does have potential use in solar cells, transparent electronics, rechargeable lithium battery electrodes, and memristors.

 

Research on Cuprous Oxide's Electronic Properties

Research has been ongoing with cuprous oxide, namely to learn its electronic properties and potential as a semiconductor material.

One research team reported enhancing its optical properties for applications in photocatalysis and sensor devices when it is doped with 12.5% zinc. According to another group, doping it in a 0.34%-concentrated F element reduces its band-gap to 1.91 eV from 1.96 eV. However, it increases its photovoltage and photocurrent density to 0.4457 V and 2.79 mA/cm2, respectively. 

Other notable studies include:

• Cuprous oxide combined with graphene to form a Cu₂O/RGO composite for enhanced lighting application
• Cuprous oxide development of an improved p-type thin-film transistor (TFT) at comparatively lower costs
• The impact annealing methods have on chemical bath deposition prepared cuprous oxide and their morphological and optical properties
• The role ALD Al₂O₃ surface passivation play on p-Type Cuprous oxide TFTs' performance

These serve as indications that there are consistent improvements on cuprous oxide's properties that may extend its range of applications. 

 

Technological Applications

The main applications of cuprous oxide in electronics include supercapacitors, Li-ion batteries, photocatalysis, solar energy conversion, and sensing applications. Though these applications, currently, are limited, they are still worth exploring. 

 

Supercapacitors and Electrodes for Lithium-ion Batteries

Cuprous oxide is suitable for use as an anode material in Li-ion batteries because of its controllable structure, polymorphic forms, and high cycling capacity. 

 

Photocatalysis and Solar Energy Conversion

Due to an abundance of copper and oxygen in nature, appropriate band-gap for visible light absorption, and relatively easy and cheaper fabrication, cuprous oxide is suitable for large-scale solar energy conversion. 

 

Sensing

Cuprous oxide thin films can be used for gas sensing by adsorbing gas molecules on its surface, which consequently causes a noticeable electrical conductivity change. Precisely, the thermal oxidation-synthesized cuprous oxide can sense methane gas at high sensitivity, fast response, and recovery times.

 

Image of solar energy conversion with cuprous oxide. Image used courtesy of Wick and Tilley

 

Comparing Cuprous Oxide to Si and GaN

The table below shows a comparison between the semiconductor properties mentioned above.

 

 

Analyzing the above table confirms that the three semiconductors have different crystal structures resulting from the differences in their unit cells. A high band-gap for GaN implies that it requires higher energy to excite valence electrons into the semiconductor's conduction band. However, cuprous oxide and Si, with relatively lower band-gap require lesser energy, making them the preferred choice if bandgap is the deciding factor.

Overall, cuprous oxide seemed to be at a disadvantage over the other two when considering electron mobility, meaning that electrons move faster in Si and GaN. Despite this setback, with further research, cuprous oxide might have potential, depending on its application. 

 

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What's your reaction to seeing new semiconductor research? At what point does research become "real" to you as an engineer? Share your thoughts in the comments below.