Room‑Temperature SILAR Synthesis of Feather‑Like ZnO Hierarchies for High‑Performance UV Photodetectors

Background

Zinc oxide is prized for its wide bandgap (~3.37 eV) and large exciton binding energy (~60 meV), enabling UV and blue light‑emitting diodes. Recent research has focused on three‑dimensional (3D) ZnO architectures—flowers, tubes, dendrites, and feather‑like structures—because their high surface area boosts light absorption and enhances optical, electronic and catalytic properties. Conventional synthesis routes such as hydrothermal or solvothermal methods typically require a seed layer or metal catalyst and high‑temperature annealing, complicating the process and risking impurity incorporation. A truly scalable, room‑temperature method that eliminates these steps remains a key challenge.

In this work, we present the first room‑temperature SILAR synthesis of feather‑like ZnO on Si(100) without seed layers or catalysts, elucidate the growth mechanism, and evaluate the resulting heterojunction’s photo‑electronic performance.

Methods

Si(100) wafers were cleaned ultrasonically in ethanol for 10 min. A 0.01 M zinc acetate solution was prepared by dissolving Zn(CH₃COO)₂ in 100 mL deionized water; ammonia hydroxide was added to reach pH ≈ 11, forming a transparent precursor solution. The wafer was dipped for 30 s, rinsed in deionized water for 20 s, and washed 20 times with ultrapure water to remove unbound Zn(OH)₂. A 90 °C water bath for 1 min converted the adsorbed ion complex into ZnO. This SILAR cycle was repeated 20 times. Crystal structure was analysed by XRD; morphology by SEM and TEM; elemental composition by EDS. Photo‑response measurements used a 12 nm semitransparent Cu electrode (5 mm × 5 mm) deposited by thermal evaporation. The heterojunction schematic is shown in Fig. 4c.

Results and Discussion

SEM images (Fig. 1a) reveal a feather‑like morphology with trunk lengths of 300–800 nm and branch widths of 200–400 nm. TEM (Fig. 1c) shows single‑crystalline nanorod segments forming the branches, confirming the hierarchical nature. EDS (Fig. 1e) indicates the structure consists of Zn, O, C and Si, confirming pure ZnO deposition. XRD (Fig. 1f) matches wurtzite ZnO (JCPDS 36‑1451) with a dominant (002) peak, evidencing preferred c‑axis orientation.

When Si(100) is replaced by Si nanowires, feather‑like ZnO does not form (Fig. 3), underscoring the critical role of substrate crystallography in nucleation.

The growth mechanism (Fig. 4) involves initial adsorption of Zn(OH)₄²⁻ on the substrate, followed by dehydration to form ZnO nuclei. Excess OH⁻ stabilises the {110} surface, promoting rapid growth along [100] and producing nano‑sheet trunks. Defect‑rich edges of these trunks then serve as heterogeneous nucleation sites for perpendicular branches, culminating in feather‑like structures.

Photoluminescence (Fig. 5a) shows a UV near‑band‑edge peak at 384 nm and a weaker green emission at 443 nm, the latter attributed to oxygen vacancies. Reflectance spectra (Fig. 5b) demonstrate a significant drop from 40% to 10% over 300–400 nm, confirming superior anti‑reflection.

Photo‑diode I‑V characteristics (Fig. 5d) exhibit rectification ratios of 535 (−1 V) and 1695 (−2 V) in the dark, with a photo‑current to dark‑current ratio of ~90 at −2 V. Compared to nano‑dot ZnO/Si (Fig. 6a), the feather‑like heterojunction shows markedly improved rectification, indicating reduced carrier recombination. The energy band diagram (Fig. 6b) illustrates a 0.3 eV conduction‑band offset and 2.54 eV valence‑band offset, facilitating efficient charge separation.

Under 365 nm illumination at 1 V bias, the feather‑like ZnO/p‑Si device yields a 0.10 mA response current, a 90% increase over bare Si. The rise and decay times (Fig. 5c) are also extended, reflecting effective carrier trapping at the hierarchical interfaces. Overall, the feather‑like ZnO demonstrates a ten‑fold sensitivity improvement relative to planar Si.

Conclusions

We have demonstrated a seed‑layer‑free, catalyst‑free SILAR method that produces feather‑like ZnO hierarchies at room temperature. A two‑stage nucleation‑growth mechanism explains the morphology. The resulting heterojunction delivers outstanding anti‑reflection, high UV sensitivity and robust photo‑current, positioning these structures as strong contenders for next‑generation photodetectors.