Full length articleInterface analysis of TiN/n-GaN Ohmic contacts with high thermal stability
Introduction
The compound semiconductor gallium nitride (GaN) has gained increasing applications in optical and electronic devices because of its superior intrinsic properties, such as direct wide band-gap, high electron mobility, and high sheet carrier density [[1], [2], [3]]. Following the successful commercialization of light-emitting diodes (LEDs) and laser diodes (LDs), GaN-based high electron-mobility transistors (HEMTs) are being widely used in modern communication systems. Moreover, owing to its strong dielectric breakdown field, GaN-based power devices have an enormous potential to provide a much higher performance [4] than those of Si- and silicon carbide- (SiC-) based devices. With the fast developments of GaN-based operational electronic devices, one question stands out: how to form good Ohmic contacts to wide-band-gap GaN and its related III–N compounds. The resistance, uniformity, reproducibility and long-term stability of Ohmic contacts are critical to the performance and reliability of GaN-based devices [5]. In conventional processes, Ohmic contacts were normally realized by directly depositing metals (such as Ti, Ta…) on semiconductors and/or followed by post-annealing at high temperature. The devices with such kind of Ohmic contacts commonly suffered from a serious problem of poor thermal stability [[6], [7], [8], [9], [10], [11], [12]] under high operation temperature. In order to improve the thermal stability of Ohmic contacts, the mechanisms of contact formation must be deeply understood.
Up to date, it has been widely accepted that thermal reaction during formation of Ohmic contact is important to produce TiN phases at the Ti/GaN interface and nitrogen vacancies (VN) in GaN surface layer [[13], [14], [15]]. TiN phase is considered as a diffusion barrier to enhance the thermal stability, and N-vacancies result in a heavily doped n+-GaN region that allows carriers to transport through metal/GaN contacts by tunneling rather than thermionic emission [16]. It is pointed out that the formation of TiN phase at the interface is a key to make good Ohmic contact, not only because of the simultaneously formed donor-like N-vacancies, but also for its low work function [17]. However, it is difficult to control the uniformity and thickness of TiN interlayer through annealing, and also high temperature process is not favorable for thermo-sensitive devices. By using Remote Plasma Enhanced Atomic Layer Deposition (RPEALD) [17], we have successfully grown stoichiometric TiN thin film with high uniformity and purity, which, in this study, could be employed to achieve large and uniform Ohmic contacts with low contact resistance and high thermal stability by directly deposition of TiN thin films at low temperatures. Several kinds of Ohmic contact schemes, including TiN-based and Ti-based, have been designed and prepared to thoroughly investigate the mechanism of Ohmic contact on the wide-band-gap semiconductors. And then the structure and process conditions were optimized by combining I-V characterizations with X-ray photoemission spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry analyses.
Section snippets
Sample preparation
Ti/TiN/Pt/Au (10 nm/40 nm/50 nm/100 nm), TiN/Ti/Pt/Au (10 nm/40 nm/50 nm/100 nm) and TiN/Pt/Au (50 nm/50 nm/100 nm) contacts were fabricated on MOCVD-grown (0001) n-type GaN (Si doped >1 × 1018 cm−3) on (0001) Al2O3 substrates (supplied by MTI Corp.), as shown in Fig. 1. Prior to the deposition of metal layers, GaN (0001) surfaces were cleaned with acetone and isopropanol at 75 °C for 10 min in sequence and then treated with mixed solution of HF, H2O2 and H2O (HF:H2O2:H2O = 1:1:5) at 70 °C for
Results and discussion
Ti/TiN/Pt/Au (10 nm/40 nm/50 nm/100 nm) contact scheme was designed to allow interface reaction occurs for generating more N vacancies due to the formation of TiN phase at the interface. Fig. 2(a) and (b) show the I-V curves and specific contact resistances of the Ti/TiN/Pt/Au samples after each annealing process. The as-deposited Ti/TiN/Pt/Au contact is non-Ohmic, which turns to be Ohmic by annealing at above 400 °C (except 550 °C~600 °C) for 1 min in N2. These facts suggest that Ti/TiN/Pt/Au
Conclusions
Various n-GaN based Ohmic contact schemes, including n-GaN/TiN/Ti/Pt/Au, n-GaN/TiN/Pt/Au and n-GaN/Ti/TiN/Pt/Au, have been studied to understand the mechanism of TiN/n-GaN Ohmic contact and optimized for low resistance and high thermal stability. TiN/n-GaN contact is Ohmic regardless of annealing temperature, while Ti/n-GaN becomes Ohmic contact only after annealing at a temperature above 400 °C in nitrogen. The SHB at the TiN (or Ti)/n-GaN interfaces is a critical factor for the contact
Acknowledgements
This work was funded by the National Natural Science Foundation of China (61674165, 61604167, 61574160, 61704183, 61404159 and 11604366), the Natural Science Foundation of Jiangsu Province (BK20170432, BK20160397 and BK20140394); the National Key R&D Program of China (2016YFB0401803); and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA09020401). F. S. L acknowledges support from the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2017370
References (31)
- et al.
Ohmic contacts to gallium nitride materials
Appl. Surf. Sci.
(2016) - et al.
TiN growth on Si (100) by pulsed laser deposition using homogenized KrF excimer laser beam
Appl. Surf. Sci.
(1999) - et al.
Metal contacts to n-GaN
Appl. Surf. Sci.
(2006) Specific contact resistance using a circular transmission line model
Solid State Electron.
(1980)- et al.
Study on the measurement accuracy of circular transmission line model for low-resistance Ohmic contacts on III–V wide band-gap semiconductors
Curr. Appl. Phys.
(2018) - et al.
Field and thermionic-field emission in Schottky barriers
Solid State Electron.
(1966) - et al.
Characteristics of TiN barrier layer against Cu diffusion
Thin Solid Films
(1999) - et al.
Prospects for LED lighting
Nat. Photonics
(2009) SiC and GaN transistors is there one winner for microwave power applications?
Proc. IEEE
(2002)- et al.
Synchrotron radiation X-ray photoelectron spectroscopy of Ti/Al ohmic contacts to n-type GaN: key role of Al capping layers in interface scavenging reactions
Appl. Phys. Express
(2016)