Strain-induced changes in AlGaN/GaN two-dimensional electron gas structures with low surface state densities

Abstract

We report on the effects of applied external strain on AlGaN/GaN two dimensional electron gas structures exhibiting a negative strain dependence of the sheet carrier density (ns). Flexible AlGaN/GaN heterojunctions, grown on two-dimensional boron nitride (BN)-on-sapphire templates, were released from the substrate via strain-induced separation at the weak BN van der Waals interface and then transferred to flexible substrates. By releasing the AlGaN/GaN layer from the substrate, residual strain was removed which allowed for isolation and study of the effects of the externally applied strain. By bending samples, uniaxial strain up to 0.15% was applied as measured by the shifts in the GaN E2H Raman mode. Hall effect measurements revealed a 2.5% decrease in ns with 0.11% applied tensile strain, which is contrary to the increase expected from the piezoelectric effect. The observed decrease in ns is attributed to a relatively large increase in the AlGaN surface barrier height. This effect, which is rarely reported, is observable due to a low surface state density (2.2 × 1012 cm−2 eV–1) in the samples. Illumination was found to dramatically alter the ns-strain dependence, an effect potentially related to detrapping of electrons in the GaN buffer.

Graphite Nanocomposite Substrates for Improved Performance of Flexible, High-Power AlGaN/GaN Electronic Devices

“…67 Because of the much lower modulus of the PDMS composite substrate (∼MPa) than the GaN film (∼200 GPa 68 ) and the strong adhesion of the AlGaN/GaN HEMT to the PDMS, the interfacial stress is primarily accommodated by the PDMS with likely little transfer of bending stresses to the device (infinitesimally small values of strain, if any at all). 42 This curvature (R) is comparable to other reported flexible AlGaN/ GaN HEMT demonstrations, experimentally (R = 11, 12, and 16 mm 4,17,18 ) and theoretically (R = 2.5 mm 69 ). Most works regarding AlGaN/GaN HEMTs on flexible substrates claim flexibility without demonstrating the performance of the device while flexed.…”

“…The device in this work are high-performance, high-power electronic devices that, because of their ∼2 μm thickness, are inherently more flexible than the bulk and have been demonstrated to withstand small strains (<1%). 4,42 3.1. Nanocomposite Fabrication and Characterization.…”

“…For both the PDMS and PDMS composite substrates, the GaN HEMTs were transferred to the substrates without the need for an adhesive layer, as the ultraconformal nature of PDMS effectively binds the thin AlGaN/GaN HEMT device to the substrate. 42,54 The complete transfer procedure is detailed in ref 41.…”

“…31−33 There has been some work regarding the AlGaN/GaN HEMTs transferred to flexible substrates that have inherently poor thermal conductivities and low maximum use temperatures and/or require an adhesive layer that further increases the thermal resistance. 4,17,18,27,34,35,42 Other recent reports have demonstrated devices on thin Cu substrates, which act as a heat spreading layer but may pose an issue when incorporated into wearable applications with soft substrates. 36 This work demonstrates the improved performance of the transferrable AlGaN/GaN HEMTs on a thermally conductive polymeric composite substrate without the need for an adhesive layer.…”

Analysis of Light‐Emission Polarization Ratio in Deep‐Ultraviolet Light‐Emitting Diodes by Considering Random Alloy Fluctuations with the 3D k·p Method

“…[13][14][15] As the Al composition increases in the template, the in-plane strain of a QW changes from tensile to compressive and the TE mode becomes dominant. [16] Several studies have tried to improve the electrical and optical properties through simulations. For example, by studying the behavior of the trap density in the structure with a 1D simulation, researchers have tried to modify the polarization charges in the QW/quantum barrier (QB) interface.…”