Growth of hole-conducting Cu2O & CuO semiconductors using CVD for next-generation electronic devices: Thin Films Transistors, Memristors, and Gas Sensors
Semiconducting oxides with visible-range transparency and high electrical conductivity have tremendous potential for transparent CMOS devices. Oxide semiconducting materials are a good choice for emerging transparent electronics due to their stability, good transparency, wide bandgap, and low processing temperature. However, oxide semiconductors arelagging in electrical properties compared to single-crystal silicon. Still, they give excellent competition to amorphous silicon with low cost of production and eco-friendly nature. Transparent electronic applications are limited by the lack of availability of p-type oxide semiconductors with adequate performance. In digital electronics, a p-type oxide transistor is a key component of CMOS devices, but due to the unavailability of a p-type oxide transistor semiconductor with performance comparable to n-type, it's challenging to develop a high-performance transparent CMOS device. So, p-type oxides are the primary culprit and bottleneck in achieving high performance in their devices. Although much effort has been put into P-N junction devices such as solar cells, LEDs, and CMOS electronics, their performance is still limited. The only applications that can be accomplished to date are based on n-type oxide semiconductors. If high-performance p-type oxide semiconductors can be synthesized, it will usher in a new age of next-generation transparent electronics devices that will impact many aspects of our everyday life. The main reason for this discrepancy lies in the difference in the low mobility of holes in p-type oxide semiconductors compared to the high electron mobility of n-type oxide semiconductors, as the effective mass of electrons is lower than the effective mass of holes. There is a vast list of n-type oxide semiconductors with low effective mass; however, there are relatively few p-type oxide semiconductor materials. Unfortunately, none have a comparable effective mass. Cu2O is a rare transition-metal oxide with a bandgap of 2.2 eV and one of the few oxides that show p-type conductivity with high Hall mobility. Unlike other p-type semiconductor metal-oxides, Cu2O has the high hole mobility needed for transparent electronics. Unfortunately, the thin-film deposition of pure Cu2O is not trivial, especially with physical vapour deposition (PVD). Pure phase Cu2O is formed in a narrow pressure-temperature window, only under precise oxygen potential. Therefore, we have deposited Cu2O using CVD. For device-grade films, chemical vapour deposition (CVD) is superior, as it allows a more robust control of deposition parameters, leading to better uniformity, topology control, and step coverage over large areas. CVD also gives the freedom to control the supersaturation, so crystallinity, morphology, and grain size can be engineered, which plays a significant role in device performance. As previous literature states, theoretically, Cu2O has the potential to show good mobility, but unfortunately, the performance of the reported device is neither remarkable nor consistence. Therefore, to achieve a better performance in this work, we fabricated the TFTs using CVD-grown Cu2O with high Hall mobility on four dielectrics. We have also investigated the origin of poor device characteristics in conventional reposted Cu2O-TFTs. We have also proposed a systematic approach to passivating these interface traps, improving the field-effect mobility, subthreshold swing, threshold voltage, and enhanced gate-bias-voltage stressing stability. The bottleneck of efficient implementation of CMOS data handing is challenging due to the ‘memory wall’. A memristor with tunable resistance is an ideal building block for storing memory. As a memristor is an emerging fourth electronic element, lots of work must be done in the material process engineering domain. Here, we have proposed CVD deposited resistive switching layers memristor. In this work, the device stack of the memristor contains an intrinsic defective layer of cupric oxide (CuO) and cuprous oxide (Cu2O) sandwiched between electrodes. These Cu2O and CuO layers were deposited at four different temperatures (300C, 400C, 500C, and 600C). Electrical and material characterizations illustrate that grains or corresponding grain boundaries play a vital role in controlling the switching behavior. Overall, we demonstrate good consistency in device parameters such as reproducibility, endurance, and retention data for the film deposited at 600C. Further, several micro and nanostructures of Cu2O have been utilized in gas sensing of oxidizing and reducing gases. However, large-area Cu2O films are needed for the mass production of sensors, which was a challenge. In this work, we also report a chemiresistive gas sensor based on pure-phase Cu2O deposited by chemical vapour deposition (CVD). The sensing results of Cu2O films have been explained from the standpoint of roughness, morphology, and unpassivated bonds present on the surface of films. At an operating temperature of 200℃, the sensor is highly sensitive to ammonia. The device's response time (𝛕response) and recovery time (𝛕recovery) were found to be decent for practical applications. Unlike competing techniques for Cu2O deposition, Cu2O from chemical vapour deposition leads to more repeatable, stable, and reproducible sensors.