Dalian University of Technology develops new GaN nanowire gas sensor

"Bypass-free circuit" nanowire bridge growth scheme

Micro Gas Detector

The rapid development of information technologies such as artificial intelligence, wearable devices, and the Internet of Things requires the support of a large number of sensors . Services such as big data and cloud computing also require various sensors to collect data in real time to support them. However, current sensors have problems such as low localization, low-end products, weak technological innovation, and backward production processes.

Recently, the team led by Professor Huang Hui from the School of Electronic Science and Technology of Dalian University of Technology invented a leakage-free "nanowire bridge growth technology", which solved the problems of arrangement and assembly, electrode contact and material stability of nanowire devices, and developed a high-reliability, low-power and high-sensitivity GaN nanowire gas sensor. This sensor can be extended to biological detection and stress-strain detection, etc. The relevant research results were published in Nano Express.

Micro-nano sensing has a "hurdle"

In recent years, semiconductor integrated circuit chips (ICs) have developed rapidly, driving the rise of the Internet of Things and artificial intelligence industries. "If ICs are compared to the human brain (processing information), sensors are equivalent to human sensory organs (obtaining information)," Huang Hui told China Science Daily, "ICs and sensors are interdependent."

However, the development speed of sensors, especially micro-nano sensors, lags far behind the development level of IC. Huang Hui believes that micro-nano sensors and sensor chips will be another major industry after the IC industry.

Huang Hui introduced that the smallest sensor currently widely used is the MEMS sensor.

MEMS (micro-electromechanical system) sensor is a new type of sensor manufactured using microelectronics and micromachining technology. Its internal structure is generally at the micron or even nanometer level, and it is an independent intelligent system. Compared with traditional sensors, it has the characteristics of small size, light weight, low cost, low power consumption, high reliability, suitable for mass production, easy integration and intelligentization. At the same time, the characteristic size at the micron level allows it to complete some functions that traditional mechanical sensors cannot achieve.

"Compared with MEMS devices, the scale of semiconductor nanowires is 1,000 times smaller and the area is 1 million times smaller. Therefore, nanowires are the smallest devices and are ideal for micro-nano sensors," said Huang Hui.

Compared with traditional bulk materials and thin film materials, semiconductor nanowires have many unique advantages: large specific surface area can improve the sensitivity of devices, easy deformation can improve the integration ability of materials, and nanoscale light-guiding and conductive channels can make single nanowire photonic devices. In addition, the excellent mechanical properties and flexible and diverse structures of nanowires make them more flexible and can form core-cladding and cross-grid structures.

However, the practical application of nanowire devices still faces a series of problems. Xin Xiangjun, a professor at the School of Electronic Engineering of Beijing University of Posts and Telecommunications, told China Science Daily that the material growth and device preparation of nanowires are separate, requiring steps such as peeling, transfer, alignment and positioning, and coating. The process is complicated and will damage and contaminate the nanowires.

In addition, nanowires are difficult to manipulate and arrange. "Moreover, the contact area between the nanowires and the metal electrodes is very small, so the electrode contact resistance is very large, nearly two orders of magnitude higher than the resistance of the nanowires themselves," said Xin Xiangjun.

Nanowire sensors "grow"

In order to solve a series of problems such as the difficulty in arranging and positioning nanowires and the small contact area of ​​electrodes, HP and the University of California jointly invented a "nanowire bridge growth technology" in 2004. By etching grooves on the SOI substrate, the nanowires grow from one side of the groove and dock with the other side, so that metal electrodes can be prepared on the side terraces of the grooves.

Huang Hui said that this solution of integrating the nanowires and the sidewalls through "growth" avoids the need to prepare metal electrodes on the surface of the nanowires, reducing the electrode contact resistance by two orders of magnitude and the noise by three orders of magnitude. In addition, there is no need to arrange and position the nanowires, which simplifies the preparation process and eliminates surface contamination and damage to the nanowires.

However, HP's nanowire bridge growth scheme has not been popularized because during the growth of nanowires, a layer of polycrystalline film (parasitic deposition layer) is usually deposited at the bottom of the groove, which will generate a large bypass current and greatly degrade the performance of nanowire devices.

To this end, Huang Hui's team first studied the parasitic deposition effect in the bridge growth of nanowires and invented a bridge growth method that combines the airflow shielding effect with the surface passivation effect to solve the parasitic deposition problem. The researchers used a new groove scheme and groove structure to avoid material deposition at the bottom of the groove and achieve bridge growth of nanowires.

Huang Hui told reporters: "By using a GaN buffer layer and adjusting the growth conditions of nanowires, such as airflow, catalyst, temperature gradient, etc., the position, direction, diameter and length of nanowire growth can be changed, and the controllable growth of nanowires can be achieved from GaN nanowires, nanoneedles to micron pillars."

It is reported that GaN material is a third-generation semiconductor with excellent stability and biocompatibility. It is resistant to high temperature, oxidation, acid and alkali corrosion, and is suitable for the detection of liquid and gas samples in harsh environments. "Experiments have shown that corrosion in hydrofluoric acid for 48 hours has no effect on the resistance of GaN nanowires, and its application field is very wide," said Huang Hui.

On this basis, the team developed an integrated nanowire gas sensor - GaN nanowire gas sensor. According to tests, the sensor can work at room temperature, with a resistance change rate of <0.8% in 8 months and a NO2 detection limit of 0.5ppb. It has the characteristics of high stability, low power consumption and high sensitivity.

Xin Xiangjun said that this technology has achieved "leakage-free current" GaN bridge nanowires for the first time, and the developed GaN nanowire gas sensor will promote the development of sensor chips.

Sensor chips are coming

Micro-nano sensors are disruptive technologies with huge innovation and market potential. In recent years, micro-nano sensors have become one of the hot areas for government and social capital investment. "Micro-sodium sensors are closely related to the development of the Internet of Things and 5G, and are widely used in mobile phones, automobiles, medical and consumer fields. Their development prospects are very good," said Xin Xiangjun.

The head of the Department of Electrical and Computer Engineering at the University of Michigan said that sensors used to require three major components: electronic devices, wireless networking systems, and wireless network systems. In the future, sensors and sensor applications will be ubiquitous, and when they are combined into a network, micro-nano sensors can be used to achieve a better sensor network in a very small environment.

"Millions of sensors can be carried in just 1 millimeter. Such devices can provide very tiny chips that can monitor data very accurately, timely and accurately, which will help us play a role in the current different energy systems and power systems," said Khalil Najaf.

Huang Hui said that the team will focus on developing GaN nanowire gas sensors with lower power consumption and smaller size, and try to make them into sensor chips. "The ideal situation is to combine them with integrated circuit chips, so that they can perfectly combine sensing, control and signal processing, and be more widely used."

In this regard, Xin Xiangjun pointed out that sensor chips have good development prospects and huge potential, and are worthy of research and development and promotion. At the same time, he suggested that once sensor chip technology matures, it should be quickly promoted in cooperation with professionals in the industry to seize the initiative.