Structural, Optical, and Electronic Properties of Cu-Doped TiNO Grown by Ammonothermal Atomic Layer Deposition

Copper-doped titanium oxynitride (TiNO) thin films were grown by atomic layer deposition (ALD) using the TiCl₄ precursor, NH₃, and O₂ at 420 °C. Forming gas was used to reduce the background oxygen concentration and to transfer the copper atoms in an ALD chamber prior to the growth initiation of Cu-doped TiNO. Such forming gas-mediated Cu-doping of TiNO films had a pronounced effect on their resistivity, which dropped from 484 ± 8 to 202 ± 4 μΩ cm, and also on the resistance temperature coefficient (TCR), which decreased from 1000 to 150 ppm °C^-1. We explored physical mechanisms causing this reduction by performing comparative analysis of atomic force microscopy, X-ray photoemission spectroscopy, X-ray diffraction, optical spectra, low-temperature transport, and Hall measurement data for the samples grown with and without forming gas doping. The difference in the oxygen concentration between the films did not exceed 6%. Copper segregated to the TiNO surface where its concentration reached 0.72%, but its penetration depth was less than 10 nm. Pronounced effects of the copper doping by forming gas included the TiNO film crystallite average size decrease from 57-59 to 32-34 nm, considerably finer surface granularity, electron concentration increase from 2.2(3) × 10²² to 3.5(1) × 10²² cm^-3, and the electron mobility improvement from 0.56(4) to 0.92(2) cm² V^-1 s^-1. The DC resistivity versus temperature R(T) measurements from 4.2 to 300 K showed a Cu-induced phase transition from a disordered to semimetallic state. The resistivity of Cu-doped TiNO films decreased with the temperature increase at low temperatures and reached the minimum near T = 50 K revealing signatures of the quantum interference effects similar to 2D Cu thin films, and then, semimetallic behavior was observed at higher temperatures. In TiNO films grown without forming gas, the resistivity decreased with the temperature increase as R(T) = - 1.88T^0.6 + 604 μΩ cm with no semimetallic behavior observed. The medium range resistivity and low TCR of Cu-doped TiNO make this material an attractive choice for improved matching resistors in RF analog circuits and Si complementary metal-oxide-semiconductor integrated circuits.