Electrical and Optical Properties of P-Type NiO Films Grown Via Nickelocene Precursor in a Hollow-Cathode Plasma-ALD Reactor

Abstract

Electrical and Optical Properties of P-type NiO Films Grown via Nickelocene Precursor in a Hollow-Cathode Plasma-ALD Reactor In contrast to the relative abundance of as-grown unintentionally doped n-type semiconductor materials, there are only a few alternative alloys showing p-type conduction without needing extra high-temperature doping processes. Among them, CuO, Bi2O3, SnO, and NiO represent the most widely studied metal oxides which exhibit native p-type properties. NiO is of particular interest mainly due to its stability and promising performance as hole-transport layers in emerging solar cell device structures. However, to broaden the NiO application domain towards potential back-end-of-the-line (BEOL) transistor devices as potential p-type channel layers, the transport properties of NiO needs to be improved significantly. While low-temperature thermal, plasma, and ozone-assisted ALD efforts have resulted in p-type NiO films with atomic-level precision and large-area uniformity, the hole mobility is far from being sufficient, typically lower than 1 cm2/Vs.In an attempt to enhance the hole transport properties in low-temperature as-grown NiO layers, in this study we have carried out a systematic study on plasma-enhanced ALD (PEALD) of NiO films on Si and glass substrates using nickelocene (NiCp2) and O2/Ar plasma mixture in two separate reactors: (i) 8” plasma-ALD reactor with a quartz-based inductively-coupled plasma (ICP) source; (ii) 4” plasma-ALD reactor with a stainless steel-based hollow-cathode plasma (HCP) source, equipped with an in-situ ellipsometer. 300 cycle ICP-ALD growth experiments with 300 W O2/Ar plasma at 150 °C substrate temperature were carried out to identify the optimal NiCp2 precursor temperature (50 - 110 °C) and observe the influence of precursor pulse time (0.1 - 1 s). Detailed saturation studies using in-situ ellipsometer monitoring were carried out in the HCP-ALD reactor, scanning for NiCp2 pulse time, O2 plasma exposure time, purge time, and plasma power. Optimal growth conditions were identified as 90 ms NiCp2 pulse / 5 s purge / 10 s O2-plasma at 100 W plasma exposure / 5 s purge. To check the growth linearity and obtain thicker films for materials characterization, 800 cycle runs were conducted to evaluate the substrate temperature impact (100 - 250 °C) on growth-per-cycle (GPC) and film properties. The last growth parameter studied was the plasma gas composition, where several O2/Ar mixtures (20/0, 20/20, 50/0 sccm) were tested at the optimal substrate temperature of 200 °C.The resulting NiO films are characterized for their structural, optical, and electrical properties. Films grown at optimal conditions (200 °C) exhibit refractive index values reaching 2.3, which is in good agreement with reported values for the best polycrystalline NiO films in the literature. NiO films deposited on both Si and glass substrates exhibit polycrystalline single-phase cubic structure (c-NiO). Hall measurements of the as-grown NiO layers on glass substrates, revealed film resistivity values about 40 and 150 ohm-cm for HCP and ICP samples, respectively. Post-deposition annealing within 200 – 400 °C temperature range resulted in a significant reduction of resistivity for more than two orders of magnitude. However, the relatively volatile and unstable Hall voltage values led to unreliable carrier concentration and Hall mobility values, therefore we were not able to confirm the type of transport via Hall measurements.In order to determine the carrier type and extract field-effect mobility values, we have fabricated vertical NiO/n-Si p-n junction diodes as well as NiO/Al2O3/Si thin-film-transistor (TFT) device prototypes to characterize diode I-V behavior and confirm p-type layer structure for NiO, and extract the field-effect carrier mobility, respectively. Our I-V measurement results confirmed p-n junction diode characteristics with decent ON/OFF ratios for forward and reverse diode current values. TFT characterization results and findings will be presented as well.