Charge and Mobility Enhancements in In-Polar InAl(Ga)N/Al(Ga)N/GaN Heterojunctions Grown by Metal–Organic Chemical Vapor Deposition Using a Graded Growth Strategy

Abstract

In this paper, we investigated the Ga incorporation effect in InAl(Ga)N/Al(Ga)N/GaN heterojunctions grown by a close coupled showerhead metal–organic chemical vapor reactor and proposed a grading growth strategy, where the indium composition was graded from Al(Ga)N to InAl(Ga)N, to mitigate the deleterious effect of Ga carry-over on the transport properties of two dimensional electron gas (2DEG). In contrast to non-graded samples grown by conventional growth strategy without grading, hall measurements revealed significant charge and mobility enhancements for the graded samples, with an electron mobility of 1300 cm2 V-1 s-1, a sheet charge density of 2.35 ×1013 cm-2 and a resultant low sheet resistance of 205 Ω/□ compared to the non-graded sample with an low sheet charge density of 1.4 ×1013 cm-2 and mobility of 1100 ±50 cm2 V-1 s-1. The reason of the enhancements were then analyzed by transmission electron microscopy (TEM) and atom probe techniques, which revealed that grading strategy led to a higher average Al composition in the barrier layer.

Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD

“…It is clear that increasing the total flow rate from 8000 sccm to 24000 sccm acts to suppress Ga incorporation in the InAl(Ga)N layers by ~ 50 %. This suggests a higher gas flow prevents lingering contaminants from reacting on the surface of the wafer, supporting an argument [7,8] that unwanted Ga in the group III sublattice originates from TMGa sticking to the reactor and gas delivery system walls and partially redepositing on a wafer surface during subsequent growth runs. The small measured difference in Ga fraction between layers B and C (with proportions spanning 11 -14%) is much less than the higher values of ~ 25% Ga content obtained for Sample A.…”

“…It is clear that increasing the total ow rate from 8000 sccm to 24 000 sccm acts to suppress Ga incorporation in the InAl(Ga) N layers by $50%. This suggests a higher gas ow prevents lingering contaminants from reacting on the surface of the wafer, supporting an argument [5][6][7][8][9] that unwanted Ga in the group III sublattice originates from Ga-containing material sticking to the reactor walls, susceptor and/or gas delivery pipes and partially redepositing on a wafer surface during subsequent growth runs.…”

“…InAlN layers and other III-nitride materials are commonly prepared by metalorganic chemical vapour deposition (MOCVD) where group III precursors, typically trimethylalkyls, react with NH3 in carefully controlled conditions on a substrate surface, such as sapphire, silicon carbide or silicon. Recently it has been reported that unintentional Ga incorporation [5,6] in InAlN layers can occur during MOCVD growth, potentially attributed to ¬¬¬left-over Ga precursor residue from previous growth on the reactor walls/susceptor [5,7,8] and the decomposition of preceding Ga-containing layers [5,9]. Each proposed reason has convincing arguments, particularly when the geometry of the reactor and the use of Ga in the preparation of buffer or device layers prior to the InAlN growth are considered.…”

“…Each proposed reason has convincing arguments, particularly when the geometry of the reactor and the use of Ga in the preparation of buffer or device layers prior to the InAlN growth are considered. Unwanted Ga has implications for both the structural and electrical properties of InAl(Ga)N epilayers [7][8][9]. The band gap and polarisation of the layer both depend on the composition fraction, and are critical parameters in determining the wavelength and efficiency of light emitted by an optoelectronic device and also the current handling capabilities of a power transistor.…”

Dual barrier InAlN/AlGaN/GaN-on-silicon high-electron-mobility transistors with Pt- and Ni-based gate stacks

“…Lower gate leakage current and sub‐threshold swing can be achieved for these InAlN/AlGaN/AlN/GaN structures compared to devices that use an InAlN/AlN/GaN based structure with similar barrier heights . Barrier layers incorporating InAlN and AlGaN have been utilized in both Ga‐ and N‐polar GaN‐based heterojunctions .…”