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

Saptarsi Ghosh,Menno J Kappers,Alexander Hinz,Simon M Fairclough,Khaled Elgaid,Saiful Alam,Bogdan F Spiridon,Abdalla Eblabla,David J Wallis,Rachel A Oliver,Michael A Casbon

doi:10.1021/acsaelm.0c00966

Abstract

The performance of transistors designed specifically for high-frequency applications is critically reliant upon the semi-insulating electrical properties of the substrate. The suspected formation of a conductive path for radio frequency (RF) signals in the highly resistive (HR) silicon substrate itself has been long held responsible for the suboptimal efficiency of as-grown GaN high electron mobility transistors (HEMTs) at higher operating frequencies. Here, we reveal that not one but two discrete channels distinguishable by their carrier type, spatial extent, and origin within the metal-organic vapor phase epitaxy (MOVPE) growth process participate in such parasitic substrate conduction. An n-type layer that forms first is uniformly distributed in the substrate, and it has a purely thermal origin. Alongside this, a p-type layer is localized on the substrate side of the AlN/Si interface and is induced by diffusion of group-III element of the metal-organic precursor. Fortunately, maintaining the sheet resistance of this p-type layer to high values (∼2000 Ω/□) seems feasible with particular durations of either organometallic precursor or ammonia gas predose of the Si surface, i.e., the intentional introduction of one chemical precursor just before nucleation. It is proposed that the mechanism behind the control actually relies on the formation of disordered AlSiN between the crystalline AlN nucleation layer and the crystalline silicon substrate.

Highlights

Access to a plethora of physical properties within the same semiconductor family and simultaneous opportunity to design application-specific heterostructures has generated tremendous interest in III-nitride (Al, Ga, and In being the group-III elements)-based electronics in the past two decades
Similar to radio frequency (RF) Si-complementary metal−oxide−semiconductor (CMOS) devices, for high-frequency GaN-on-Si applications, it is customary to grow upon highly resistive (HR) silicon wafers so that the power loss due to substrate conductivity is at par with semi-insulating GaAs.[37]
A nominally 250 nm thick AlN nucleation layer was grown by metal-organic vapor phase epitaxy (MOVPE) under conditions typical of our standard high electron mobility transistors (HEMTs) structures.[15,41,42]

Summary

Introduction

Access to a plethora of physical properties within the same semiconductor family and simultaneous opportunity to design application-specific heterostructures has generated tremendous interest in III-nitride (Al, Ga, and In being the group-III elements)-based electronics in the past two decades. The adverse electrical implications of these additional layers have proven to be a reason for concern, necessitating further scrutiny. In this context, though the illeffects of the AlGaN buffers in the form of leakage[14,15] and trapping[16,17] are relatively well understood and controlled, the suspected association of the AlN nucleation layer with the formation of a conductive channel at the AlN/Si interface is still under debate. Apart from thermal conductivity, the power loss due to this channel has been alleged to be the remaining bottleneck[18,19] for the high-frequency performance (i.e., maximum power gain frequency and power output) of record-breaking GaN-on-Si HEMTs20,21 still lagging behind the devices grown on SiC.[22,23]

Methods

Results

Conclusion