Low-Temperature RF Plasma Treatment Effect on Junctionless Pd-Al2O3-Ingaas Misfet Operation
Authors: Nazarov Alexei , Gomeniuk Yuri V. , Gomeniuk Y Y. , Okholin Pavel N , Nazarova Tamara M. , Djara Vladimir , Cherkaoui Karim , Hurley Paul K. 
Published: February 2020

The junctionless (JL) device concept for silicon-on-insulator MOSFETs was introduced by Colinge et al. in 2010 [1], demonstrating considerable gains in terms of process simplicity when compared to conventional inversion-mode MOSFETs. The objective of this work is to implement the JL device concept in an In0.53Ga0.47As channel, where the SiO2 insulator in [1] is replaced by a wider bandgap p-type In0.52Al0.48As barrier layer and to investigate the effect of low-temperature RF hydrogen plasma treatment on the main electrical parameters of such devices. The JL device architecture is particularly well suited to III-V channel materials. Firstly, the high doping concentration (Nd) present in the channel of a JL MOSFET is less problematic for In0.53Ga0.47As than it is for Si. Indeed, the bulk electron mobility in Si is ~100 cm2/V·s at Nd = 1×1019 cm-3, while in In0.53Ga0.47As the bulk mobility is ~4,000 cm2/V·s at a similar Nd level. Moreover, the JL architecture circumvents the difficulties associated with the implantation or regrowth techniques generally used to form the source/drain (S/D) regions of III-V inversion-mode MOSFETs. However in order to form a good Ohmic S/D contacts it is necessary to anneal the device at 350-400°C that could lead to diffusion of In atoms into the gate dielectric. In this work low-temperature RF plasma treatment (RFPT) [2] is proposed to improve S/D contacts and enhance the properties of JL MOSFETs.
A structure consisting of a 32-nm-thick n- In0.53Ga0.47As (Nd = 2×1018 cm-3) on a 500-nm-thick p-In0.52Al0.48As (Na = 8×1015 cm-3) barrier was grown by metal organic vapour-phase epitaxy (MOVPE) on a p+-InP wafer (Fig. 1 (a)). In order to form a channel for the JL devices, the In0.53Ga0.47As layer was thinned using a 10% H2O2/10% HCl digital wet etching process to achieve channel thicknesses of 24, 20 and 16 nm. A gate enclosed device layout was employed to simplify the fabrication process flow (Fig. 1(b-e)). A surface passivation in 10% (NH4)2S for 30 min was performed before atomic layer deposition (ALD) of an 8.5-nm-thick Al2O3 gate oxide. A Pd gate was formed by e-beam evaporation and lift-off. The Al2O3 on the S/D contact areas was etched in dilute HF. The S/D contact formation was performed by e-beam evaporation of a Au/Ge/Au/Ni/Au stack and lift-off. The RFPT (13.6 MHz) was performed in forming gas (10%H2+90%N2) with additional heating of the sample holder (up to 200°C). Temperature of the samples at the RFPT did not exceed 250°C [2].
Well behaved Id-Vg characteristics measured on the 24-nm-thick In0.53Ga0.47As channel device are shown in Fig. 2. A threshold voltage (VT) of -0.55 V was extracted using the second-derivative method. Scaling the In0.53Ga0.47As channel thickness down to 16 nm improved the subthreshold swing (SS) but degraded the maximum Id and ION/IOFF due to the combined effect of S/D series resistance (RSD) and mobility degradation. As a result, the lowest SS value (115 mV/dec) was extracted on the 16-nm-thick In0.53Ga0.47As channel device and 190 mV/dec - on the 24-nm-thick In0.53Ga0.47As channel one, but the highest ION/IOFF value (1.5×105) was obtained on the 20-nm-thick In0.53Ga0.47As channel device and 2.0×104 - on the 24-nm-thick In0.53Ga0.47As channel one. The gate-to-channel Cgc-Vg characteristic featured a low frequency dispersion near the accumulation region, suggesting a low density of interface traps (Dit) in the upper part of the In0.53Ga0.47As bandgap. Moreover, the conductance analysis yielded low Dit values ranging from 1.5×1012 to 3.5×1012 cm-2×eV-1 from the conductance band edge to midgap, correspondingly. Measurements of RSD on circular transfer length method (CTLM) structures yielded the value of 12 kOhm on the 20-nm-thick In0.53Ga0.47As channel devices and contacts were strongly rectifying. RF plasma treatment with power 0.5 W/cm2 at 150°C resulted in Ohmic SD contact, a strong decrease of RSD down to 45 Ohm (Fig. 3) and increases the ION by two orders of magnitude (Fig. 2). Additionally, SS for the plasma treated devices decreases from 190 mV/dec to 150 mV/dec, and the threshold voltage shifts to positive values by 0.4 V (Fig.2). This is associated, correspondingly, with a decrease of Dit and negative charge in the gate dielectric. The conductance analysis yielded reduction of Dit at midgap down to 1.0×1012 cm-2×eV-1.
[1] Colinge J.-P., Lee C.-W., Afzalian A., Akhavan N.D., Yan R., Ferain I., Razavi P., O'Neill B., Blake A., White M., Kelleher A.-M., McCarthy B., Murphy R. Nanowire Transistors Without Junctions. Nature Nanotechnology. Vol. 5, P. 225-229 (2010).
[2] Nazarov A.N., Lysenko V.S., Nazarova T.M. Hydrogen Plasma Treatment of Silicon Thin-film Structures and Nanostructured Layers. Semiconductor Physics, Quantum Electronics & Optoelectronics. Vol. 11, P. 101-123 (2008).

Figure 1