Electrical activity at the AlN/Si Interface: identifying the main origin of propagation losses in GaN-on-Si devices at microwave frequencies

Abstract

AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 10 18 cm -3 is estimated from Capacitance–Voltage (C–V) measurements while the volume hole concentration of several 10 17 cm -3 is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the p -type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a p -doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this p -type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.

Origin(s) of Anomalous Substrate Conduction in MOVPE-Grown GaN HEMTs on Highly Resistive Silicon

“…In contrast, a fast decrease from 1824 to 869 Ω/□ can be seen for increasing TMAl predose from 6 to 12 s. As R SH = ρ channel /t, where ρ and t denote the channel resistivity and thickness, considering t of the SCM-resolved p-type layer to be ∼3 μm and approximating a uniform distribution, these values reveal an associated resistivity of ∼0.2−0.6 Ω cm, suggesting 51 10 16 −10 17 cm −3 acceptors on average for the whole layers. Notably, such magnitudes of unintentional doping agree well with both the estimated volume concentration of holes 32 in the p-type layer and the ranges of the reported 26,27,29 SIMS depth profiles for group-III species. Also, in conjunction with the Hall effectmeasured n-type carrier concentrations (i.e., mid-10 13 cm −3 ), these values of p-type carriers should result in a p−n junction depletion region of ∼4 μm, quite consistent with those observed in SCM.…”

“…Very recently, the presence of a p-type layer followed by a depletion region that is formed with the FZ-grown n-type substrate has been evidenced by Bah 32 et al through coupled SCM and SSRM experiments. For AlN grown with a fixed NH 3 predose, those authors had observed a proportional relationship between the AlN growth temperature and the width of the p-type layer.…”

“…Separately, group-III elemental diffusion into the silicon substrate (and acting as acceptors) has been attributed as the causal mechanism by several authors 27−33 alongside suggestions of diffusion partly arising from residual species in the reactor. 27,29,32 Arguments negating the role of the substrate also exist, with the AlN nucleation layer itself held accountable for being conductive. 34 Irrespective of the origin, the presence of a capacitively coupled buried channel greatly undermines the potential of GaN HEMTs by inducing power loss not only for the transistors but also for the passive components and the connecting transmission lines of the integrated circuits (ICs); for example, investigations on structures grown by molecular beam epitaxy (MBE) process have identified that controlling the epi/ substrate interface is essential to minimize parasitic conduction and radio frequency (RF) loss.…”

“…For instance, an inversion layer of electrons , arising from an abrupt AlN/Si interface and oxy-nitride acceptor traps due to a nonideal AlN/Si interface have both been independently proposed to explain the presence of mobile carriers. Separately, group-III elemental diffusion into the silicon substrate (and acting as acceptors) has been attributed as the causal mechanism by several authors − alongside suggestions of diffusion partly arising from residual species in the reactor. ,, Arguments negating the role of the substrate also exist, with the AlN nucleation layer itself held accountable for being conductive . Irrespective of the origin, the presence of a capacitively coupled buried channel greatly undermines the potential of GaN HEMTs by inducing power loss not only for the transistors but also for the passive components and the connecting transmission lines of the integrated circuits (ICs); for example, investigations on structures grown by molecular beam epitaxy (MBE) process have identified that controlling the epi/substrate interface is essential to minimize parasitic conduction and radio frequency (RF) loss. , Given the superior growth rates and wafer throughput but also the different thermodynamics and reaction kinetics of the MOVPE technique, understanding and controlling the physical origin of the parasitic conduction path in MOVPE-grown structures have become important to the demonstration and large-scale production of high-performance GaN-on-Si RF electronics.…”

Assessing the electrical activity of individual ZnO nanowires thermally annealed in air

“…SCM measurements are performed using a Bruker Dimension Icon AFM apparatus equipped with a Nanoscope V controller. The SCM technique is a useful tool dedicated to the 2D electrical profiling of carrier concentration in semiconductor devices, especially at material interfaces [57]. SCM experimental setup includes AFM conductive probe, ultra-high frequency (~ 1 GHz) capacitance sensor, lock-in amplifiers, multi-channel signal processing systems and 2D map plotters.…”