Record negative photoconductivity in N-polar AlGaN/GaN quantum-well heterostructures

TL;DR

The paper addresses the scarce occurrence of negative photoconductivity in AlGaN/GaN heterostructures by introducing a GaN/AlN superlattice back barrier. Using MOCVD-grown, N-polar structures with varying SL period $n$ and infrared excitation, the authors demonstrate a PPC-to-NPC transition at room temperature, with a peak photoconductivity yield $Y_{PC}$ exceeding 85% and robustness up to 400 K. They propose a hot-electron trapping mechanism in the SL as the dominant cause, where infrared-excited electrons from the 2DEG are captured in the SL, heating the near-interface region and reducing mobility. The findings suggest a viable path to NPC-based devices such as infrared photodetectors and photoelectronic memory, leveraging the large, persistent NPC response and high-temperature stability provided by the SL back barrier engineering.

Abstract

The AlGaN/GaN quantum-well heterostructures typically exhibit a positive photoconductivity (PPC) during the light illumination. Surprisingly, we found that introducing the GaN/AlN superlattice (SL) back barrier into N-polar AlGaN/GaN quantum-well heterostructures induces a transition in these heterostructures from PPC to negativie photoconductivity (NPC) as the SL period number increased at room temperature. This transition occurred under an infrared light illumination and can be well explained in terms of the excitation of hot electrons from the two-dimensional electron gas and subsequent trapping them in a SL structure. The NPC effect observed in N-polar AlGaN/GaN heterostructures with SL back barrier exhibits photoconductivity yield exceeding 85 % and thus is the largest ones reported so far for semiconductors. In addition, NPC signal remains relatively stable at high temperatures up to 400 K. The obtained results can be interesting for the development of NPC related devices such as photoelectric logic gates, photoelectronic memory and infrared photodetectors.

Figures (8)

• Figure 1: (a) Schematic illustration of the fabricated heterostructures used in this study. (b) Current-voltage characteristics of investigated structures. Inset of (b) shows the optical image of an actual device with marked illuminated area by the dashed red lines.
• Figure 2: (a) Dependencies of warp parameter, w, as a function of n and (b) dependencies of 2D electron density vs. $n$. Inset in (a) shows wafer flattering phenomenon: deposition of SL caused compressive stress which reduced tensile strain. Arrow A ($n$=2) indicates the starting point of warp decreasing (a) and 2D electron density increasing (b).
• Figure 3: SIMS depth profile of investigated N-polar AlGaN/GaN heterostructure with 4 period SL and 1 $\mu$m thick GaN buffer layer. For comparison, a depth distribution of [O] for N-polar AlGaN/GaN heterostructure with 4 period SL and 400 nm thick GaN buffer layer is shown. The atomic concentration was determined for O and C.
• Figure 4: Room temperature PL spectra of investigated N-polar AlGaN/GaN heterostructures with SL structures ($n$ varied from 0 to 8). Inset compares the intensity of near-band emission (NBE) and yellow emission (YL) peaks vs. $n$.
• Figure 5: The transient photocurrent characteristics of N-polar AlGaN/GaN heterostructures with SL period $n$=0, 2, 4, 6 and 8 obtained under the illumination with wavelengths: (a) 430 nm, (b) 610 and (c) 990 nm. Inset of (a) shows schematically definition of the photocurrent $\Delta I$.