Wurtzite phonons and the mobility of a GaN/AlN 2D hole gas

Abstract

To make complementary GaN electronics more than a pipe dream, it is essential to understand the low mobility of 2D hole gases in III-Nitride heterostructures. This work derives both the acoustic and optical phonon spectra present in one of the most prominent p-channel heterostructures (the all-binary GaN/AlN stack) and computes the interactions of these spectra with the 2D hole gas, capturing the temperature dependence of its intrinsic mobility. Finally, the effects of strain on the electronic structure of the confined 2D hole gas are examined and a means is proposed to engineer the strain to improve the 2D hole mobility for enhanced p-channel device performance, with the goal of enabling wide-bandgap CMOS.Decades after the celebrated invention of Mg p-doping 1 in Gallium Nitride (GaN) and the subsequent development of GaN-based LEDs, the manipulation of holes in GaN remains a fundamental challenge. Consequently, despite the expected dominance of GaN High Electron Mobility Transistors (HEMTs) in the coming generation of power electronics 2 and communications systems 3 , there is no complementary p-channel device which can be readily integrated. This incompleteness restricts the possible circuit topologies and system designs acheivable in GaN electronics, but arises quite straightforwardly from the physics of the GaN valence band. These bands, both heavy and deep in energy, have proven difficult to contact with typical metal workfunctions 4 , difficult to dope with high efficiency 5 , and difficult to flow current through with high conductivity. Nonetheless, the commercial interest in generating complementary GaNbased circuits 6 and scientific interest in studying highlydegenerate hole physics 7 have prompted great recent progress in p-channel devices 8 .Of the varied structures 6,9-16 which have been proposed as a platform for p-channel III-Nitride electronics, the single GaN/AlN heterojunction field-effect transistor has received recent attention for its high sheet conductance 7 and excellent device performance 8 . In this structure, depicted in Fig 1(a), the all-binary materials provide a straightforwardly repeatable growth with no possible parasitic electron channels, a tremendous holeinducing polarization-charge for low sheet-resistance, and maximal bandgaps for extreme voltage-handling capability. Given the recent reports of temperature-dependent transport studies in this heterostructure 7 , and recent first-principles suggestions of possible enhancements to hole mobility in bulk p-GaN 17 , this work presents a model to explain the measured mobility of the 2D hole a) sjb353@cornell.edu; http://sambader.net 2.5 0.0 2.5 ← k y [1/nm] k x → (b) HH LH 0 2 4 DOS [eV −1 nm −2 ] 150 100 50 0 50 Energy [meV] HH LH (c) 0 5 10 15 20 Depth [nm] E C E V E F p GaN 13nm AlN ↓ 2DHG 0.5 1.0 1.5 2.0 2.5 Effective mass [m e ] HH LH (d) (a) 0 1 f(k x ) f(k y ) FIG. 1. (a) A 13nm GaN layer on top of a thick AlN buffer induces a two-dimensional hole gas of density ∼ 4.4×10 13 /cm 2 , which is confined at the interface by strong p...

High conductivity Polarization-induced 2D hole gases in Undoped GaN/AlN Heterojunctions enabled by Impurity Blocking Layers

“…The samples show a similar trend of mobility as a function of temperature. This is explained by the freeze out of acoustic and optical phonons 17 , and agrees with previous report 4 of GaN/AlN 2DHG. However, the 2DHG mobility in sample B, C is consistently 2-3× higher that of sample A, with the highest mobilities of 85 cm 2 /Vs and 64 cm 2 /Vs respectively at 20 K. Other samples under similar growth conditions with IBLs have exhibited even higher mobilities, the highest 4 among them is also plotted in figure 3 for reference.…”

“…It reduces from µ imp ≥ 100 cm 2 /V•s at N imp ∼ 5 × 10 18 cm −3 to µ imp ∼ 5 cm 2 /V•s at N imp ∼ 1.5 × 10 20 cm −3 . The dominant intrinsic scattering mechanism at room temperature for GaN/AlN 2D holes is due to acoustic phonons (AP) 17 through the deformation potential coupling. For a simplified single valence band model, the AP-limited hole mobility of for a 2DHG at temperature T is 18 :…”

Molecular Beam Epitaxy Growth of Large‐Area GaN/AlN 2D Hole Gas Heterostructures

“…The lowest 2DHG sheet resistances measured were ≈6 kΩ sq −1 at 300 K and ≈2 kΩ sq −1 at 77 K. These are some of the highest conductivity hole channels reported so far in III‐nitrides. We previously reported the temperature‐dependent transport of similar 2DHGs, showing that their conductivity increases as the temperature is lowered, down to ≈1 kΩ sq −1 at 10 K. In recent reports, Poncé et al and Bader et al derived that the 300 K transport of the GaN/AlN 2DHG is dominated by acoustic phonons (AP) scattering. At the low temperatures of 77 K, phonons freeze out and extrinsic scattering mechanisms such as due to interface roughness (IR) dominate.…”

Very High Density (>1014 cm−2) Polarization‐Induced 2D Hole Gases Observed in Undoped Pseudomorphic InGaN/AlN Heterostructures

“…[11,40] Additionally, the in-plane strain in these regrown InGaN regions can be engineered to test the effect on 2DHG mobility. [26] The survival of the InGaN/AlN 2DHGs at cryogenic temperatures offers a unique platform of studying alloy disorder effects on the hole transport when phonons are frozen out. This study presents the first such investigation of the alloy disorder scattering of holes in III-nitride semiconductors.…”

“…Hence, a path to higher 2DHG mobility through alloying might not be feasible. However carefully engineered in-plane strain [26,38] could potentially be used to boost the mobility of these InGaN/ AlN and decrease their sheet resistances further. Application of uniaxial, in-plane 1) compressive strain in the direction of hole transport, or 2) tensile strain applied perpendicular to the hole transport have been predicted [26] to reduce the valence band hole effective mass m ⋆ , which should translate to a higher alloy scattering limited hole mobility through Equation (3).…”