Critical Disconnect Between Structural and Electronic Recovery in Amorphous GaAs during Recrystallization

TL;DR

The paper investigates how Ne++ irradiation induces a well-defined amorphous GaAs layer atop a crystalline substrate and how recrystallization proceeds upon heating. Using 4D-STEM with angular cross-correlation, unsupervised clustering, EELS, and DBS, it reveals two distinct regrowth regimes: a slow, epitaxial front at low temperatures and a rapid, nanotwin-dominated regime above ~250°C. Importantly, while the recrystallized region reattains the original [101] orientation and partial long-range order, its electronic structure remains degraded relative to pristine GaAs, with plasmon shifts and broader band-edge features indicating persistent defect states. The results show a critical disconnect between structural recovery and electronic functionality, driven by residual strain and point defects, and demonstrate that nanoscale structural memory (paracrystallinity) can template regrowth but does not guarantee electronic restoration. The work highlights the need for multi-modal characterization to assess damage and recovery in compound semiconductors and has implications for designing defect-tolerant devices under extreme conditions.

Abstract

Understanding the evolution of structure and functionality through amorphous to crystalline phase transitions is critical for predicting and designing devices for application in extreme conditions. Here, we consider both aspects of recrystallization of irradiated GaAs. We find that structural evolution occurs in two stages, a low temperature regime characterized by slow, epitaxial front propagation and a high-temperature regime above dominated by rapid growth and formation of dense nanotwin networks. We link aspects of this structural evolution to local ordering, or paracrystallinity, within the amorphous phase. Critically, the electronic recovery of the materials is not commensurate with this structural evolution. The electronic properties of the recrystallized material deviate further from the pristine material than do those of the amorphous phase, highlighting the incongruence between structural and electronic recovery and the contrasting impact of loss of long range order versus localized defects on the functionality of semiconducting materials.

Figures (8)

• Figure 1: (a) Cross-sectional STEM image of [001]-oriented GaAs irradiated with 400 keV Ne$^{++}$ with a thin recrystallized layer at the top surface. A approximately 510 nm thick amorphous layer extends below the recrystallized layer, underlain by a dense band of dislocation loops within the partially damaged crystal, and finally by undamaged pristine GaAs. EDS maps from the same region show uniform Ga and As distributions across both amorphous and crystalline zones. Ne is sparsely distributed, with a peak concentration of 1 at.% located near the dislocation loop zone. (b) SRIM simulations of displacement damage (dpa) and implanted Ne concentration as a function of depth. The dpa profile (replotted in (a)) shows a maximum of roughly 8 dpa at 480 nm, consistent with the depth of observed amorphization. (c) Over time, partial recrystallization occurs, primarily from the surface downward and to a lesser extent from the crystalline base upward. The recrystallized regions adopt the original [101] out-of-plane orientation of the cross-section of the undamaged GaAs. Inset: HRTEM image of undamamaged GaAs with resolved Ga and As atomic columns. After annealing at 400°C, the formerly amorphous layer becomes largely recrystallized, forming extensive twin domains aligned along the [111] directions and small polycrystalline pockets. Representative 4D-STEM diffraction patterns are shown (top to bottom) from the recrystallized upper layer, amorphous region, undamaged prsitine crystal, and twinned zones.
• Figure 2: (a) GaAs irradiated with 400 keV Ne$^{++}$ ions exhibits an amorphous layer, with partial recrystallization occurring within the top approximately 200 nm. This recrystallization progresses over time at room temperature and closely matches the crystalline orientation of the pristine single-crystal GaAs. Beneath the recrystallized region lies an amorphous layer approximately 200 nm thick, followed by a zone containing dislocation loops embedded in the underlying pristine GaAs. 4D-STEM data was collected from the two indicated regions. (b) As the temperature increases, the amorphous layer begins to recrystallize. At 200°C, the crystalline--amorphous interface advances into the amorphous layer. By 250°C, nanocrystalline twinning becomes the dominant recrystallization mechanism, occurring along the {111} family of planes. Observations over a 12-minute interval at 250°C reveal that recrystallization dynamics are time-dependent in addition to being temperature-dependent. Panels in (b) are from the same field of view as the region in (a).
• Figure 3: Order--disorder maps from regions 1 and 2, as shown in Fig. \ref{['fig:haadf']}a, indicate the relative degree of crystallinity across the sample. Each pixel corresponds to a single probe position and its associated diffraction pattern. The 25°C case shows the distribution of ordering prior to heating. Between 150°C and 200°C, the crystalline fronts of both the bulk and recrystallized regions advance epitaxially into the amorphous layer. At approximately 250°C, twins begin to rapidly extend into the amorphous region, resulting in near-complete recrystallization. These temperature steps were selected because the majority of recrystallization occurs within this range and is complete by 300°C. In Region 1, the more ordered spot appearing at 150°C on the right side of the amorphous layer (indicated with an arrow) was created by the electron beam and served as a fiducial marker. Time steps are shown in the upper left corner of each map; it takes approximately 12 minutes to collect each dataset.
• Figure 4: Polar-transformed diffraction patterns from representative regions of (a) the recrystallized layer, (b) the amorphous layer, and (c) the undamaged GaAs at 25°C. Dotted white lines indicate the range of scattering vectors (k) used to compute the cross-correlation function (CCF), encompassing the first diffuse halo in the amorphous region and the 111 diffraction peaks in the crystalline regions. (d) CCF amplitude, C(k,$\Delta\theta$), was calculated as a function of angular shift to quantify the angular periodicity of diffracted intensity at a given spatial frequency. The amorphous region exhibits the lowest CCF amplitude, consistent with a lack of long-range order. Both crystalline regions show dominant two-fold symmetry; however, the recrystallized layer displays broader and less coherent peaks, indicating increased lattice disorder and orientational variation relative to the undamaged crystal. CCF measurements were taken from regions deep within both the recrystallized and undamaged layers, as well as near the amorphous-recrystallized and dislocation loop-amorphous interfaces. In both cases, regions further from the interface exhibit stronger angular periodicity and more defined long-range order.
• Figure 5: N-fold angular correlation maps are generated from 4D-STEM datasets of region 1 by measuring periodicity in each diffraction pattern. The scattering vector range extends beyond the {111} reflections, capturing higher-order angular correlations. Shown are 2- and 4-fold maps across the thermal regime in which {111} nanotwins nucleate and grow from crystalline regions into the amorphous layer, leading to near-complete recrystallization. Room temperature maps are shown for comparison. At 200°C, rotational symmetry signals increase in crystalline regions. At 250°C, 2-fold symmetry becomes more prevalent across the field of view, including the amorphous layer, though it remains suppressed at the amorphous--crystalline interfaces. By 300°C, twinning extends through most of the amorphous region. The abrupt increase in 2-fold symmetry suggests a structural transition driven by twin network formation. These twins produce streaked diffraction patterns with dominant 2-fold symmetry, explaining the lack of a corresponding rise in 4-fold symmetry. A single polycrystalline pocket (bright yellow in 4-fold maps) persists in the now-recrystallized region; amorphous content is negligible. Differences in angular correlation strength between the recrystallized and undamaged regions arise from slight misorientation, increased lattice distortion, and higher defect densities. Angular correlation maps for region 2 are shown in Figure S4 of the Supplemental Information.