Advanced Semiconductor Materials and Devices Group

Our group focused on the Fabrication of Schottky barrier diodes on β-Ga₂O₃ and studied the effect of harsh environments such temperature, gamma-irradiation, swift heavy ion irradiation on electrical performance of these Schottky barrier diodes. β-Ga₂O₃ with superior material properties like ultrawide bandgap ~4.9 eV, high BFOM and JFOM, high thermal and chemical stability emerged as potential candidate for next generation power electronics especially Schottky barrier diodes and field effect transistors. Along with excellent material properties, the availability and ease of growth of high-quality low cost single crystalline wafer up to 6 inch dia. have been demonstrated by melt growth techniques such Czochralski, edge defined film fed growth etc., which further enables the growth of high quality homoepitaxial thin film by various thin film growth methods such as halide vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), metal-oxide chemical vapor deposition (MOCVD). This is a huge advantage over other commercialized wide bandgap semiconductors like SiC and GaN and makes possible future mass production of β-Ga₂O₃ power devices. Controllable n-type conductivity has been also achieved using Si, Sn as dopant in range of 10¹⁶ – 10²⁰ cm^‒3 which enable flexibility for device engineering.

Vertical Schottky diodes on β-Ga₂O₃ by utilizing whole wafer volume and without affecting chip area offered high current density and high breakdown voltage capabilities. For high performance of Schottky diodes on β-Ga₂O₃, apart from high quality wafer; high thermal stable Metal-semiconductor contacts: Ohmic contact and Schottky contact, and repeatability is critical.

In this Direction we are fabricating Schottky diodes on HVPE-grown β-Ga₂O₃ (001) epilayers using various Schottky contacts such as Ni, Pt etc. These Epilayers were grown on high conductive EFG-grown β-Ga₂O₃ substrates. Fabricated SBDs exhibited high rectification ratio of order of 10¹² at room temperature, and we observed breakdown voltage greater than 200 V with high Schottky barrier heights greater than 1 eV and near unity ideality factor.

We reported the temperature endurance capability of high-quality Pt-based Schottky Barrier Diodes (SBDs). The measurements have been performed over a wide temperature range of 80–525 K, and the prepared SBDs showed outstanding thermal stability over repeated cycles of electrical measurements. At higher temperatures, a near unity ideality factor was observed, which was attributed to the dominance of pure thermionic emission current transport mechanism at higher temperatures. SBH (ϕ)C–V calculated from the capacitance-voltage characteristics were found to decrease with increasing temperature, and their temperature coefficient values of −3.74 to −7.29 × 10−4 eV/K were observed, which can be attributed to the narrowing of the bandgap of β-Ga₂O₃ with the increase of temperature. A negligible value of intrinsic carrier concentration below 300 K and a low value of the order of 10−5 – 10−6 cm−3 at room temperature was also calculated from the temperature-dependent C-V characteristics, which indicated the excellent quality of HVPE-grown Si-doped (001) β-Ga₂O₃ epilayers.

We proposed the use of Cu as the alternative of Ni and Pt to fabricate SBDs on β-Ga₂O₃. The fabricated SBDs using Cu as Schottky contact exhibited high performance. A high Schottky barrier height (SBHs) with values greater than 1.0 eV, near-unity ideality factors, and a high rectification ratio (RR) of 10¹² at 300 K were observed. Temperature-dependent current-voltage (I–V–T) and capacitance-voltage (C–V–T) measurements were performed multiple times in the temperature range of 300 K – 500 K. The SBHs (ϕ)_IV calculated from the IVT characteristics initially increased with temperature and then decreased with near-unity ideality factors. The increase in (ϕ)_IV with temperatures up to 410 K indicated the spatial inhomogeneity at the metal-semiconductor interface. The observed decrease in (ϕ)_IV above 410 K showed the enhancement of barrier homogeneity at higher temperatures (⩾410 K). The reduction in (ϕ)_IV above 410 K and decrease in (ϕ)_CV calculated from the CVT characteristics, with increasing temperature, were assigned to the bandgap narrowing of β-Ga₂O₃. The IVT measurements were repeated many times, and the SBDs exhibited excellent thermal stability and showed high SBHs >1.0 eV, a high RR, and near‒unity ideality factors. All these findings were attributed to the formation of a high-work-function copper oxide thin film at the interface between Cu and β-Ga₂O₃. These findings allowed fabricating a Schottky contact with a high and homogeneous barrier and thermal stability using cheap, abundant Cu material, enabling low-cost mass production of future power semiconductor devices based on oxide semiconductors.

References

1.     Hardhyan Sheoran, and Rajendra Singh, Investigation of high-performance Schottky diodes on a Ga₂O₃ epilayer using Cu with high barrier height, high temperature stability and repeatability. Journal of Physics D: Applied Physics, 2023.

2.     Hardhyan Sheoran, Janesh K. Kaushik, and Rajendra Singh, Study of electrical characteristics of high quality Pt SBDs fabricated on HVPE-Grown β-Ga₂O₃ epilayers in a wide temperature range (80–525 K). Materials Science in Semiconductor Processing, 2023.

3.     Hardhyan Sheoran, Vikram Kumar, and Rajendra Singh, A Comprehensive Review on Recent Developments in Ohmic and Schottky Contacts on Ga₂O₃ for Device Applications. ACS Applied Electronic Materials, 2022. (Review Article)

4.     Hardhyan Sheoran, Bhera Ram Tak, N. Manikanthababu, and Rajendra Singh, Temperature-Dependent Electrical Characteristics of Ni/Au Vertical Schottky Barrier Diodes on β-Ga₂O₃ Epilayers. ECS Journal of Solid State Science and Technology, 2020.