Hydrothermal Phosphorus‑Doped ZnO Nanorods: Structural, Electrical, and Optical Characterization for Optoelectronic Devices

Abstract

We report the successful hydrothermal synthesis of phosphorus‑doped ZnO nanorods and systematically investigate how the dopant concentration alters their crystal structure, morphology, electrical behavior, and photoluminescence (PL) across the ultraviolet (UV), visible, and near‑infrared (NIR) ranges. X‑ray diffraction (XRD) reveals that increasing ammonium dihydrogen phosphate (NH4H2PO4) concentration enlarges both the nanorod diameter and length, confirming effective phosphorus incorporation. Hall‑effect measurements show a transition from intrinsic n‑type to p‑type conductivity around 0.3 % NH4H2PO4, illustrating the amphoteric nature of phosphorus in ZnO. PL spectra demonstrate a red‑shifted near‑band‑edge (NBE) emission attributable to donor‑acceptor pair (DAP) transitions, while deep‑level emissions (visible and NIR) remain largely unchanged. These findings establish phosphorous‑doped ZnO nanorods as versatile light‑emitting platforms for UV to NIR optoelectronic devices.

Introduction

Zinc oxide (ZnO) is a wide‑bandgap semiconductor prized for its tunable electrical and optical properties, making it a cornerstone for LEDs, lasers, and photodetectors.1–11 Its intrinsic n‑type behavior, however, limits the realization of ZnO homojunctions that require p‑type material.12–14 Recent efforts to introduce p‑type conductivity have explored dopants such as antimony, arsenic, nitrogen, and phosphorus, but many suffer from deep‑level acceptors, low solubility, and instability.15–18 Phosphorus has emerged as a promising candidate: thermal activation via rapid annealing can stabilize p‑type ZnO, and long‑term studies report stability over 16 months.19–21 Moreover, phosphorus incorporation often activates visible‑range PL associated with oxygen‑vacancy defects.22–24 Yet a comprehensive study linking structural, electrical, and multi‑regime optical behavior remains scarce. This work addresses that gap by hydrothermally doping ZnO nanorods with controlled phosphorus levels, assessing crystal quality, carrier type, and emission across UV, visible, and NIR wavelengths.

Methods

Seed Layer Preparation

Al‑doped ZnO (AZO) films (~100 nm) were deposited on cleaned quartz via RF sputtering from a 2 % Al2O3/ZnO target (Fig. 1a). Substrates were ultrasonically cleaned in acetone and isopropyl alcohol, then dried under nitrogen. Sputtering ran for 40 min at 90 W RF power and 60 SCCM Ar flow. AZO offers higher conductivity and transparency than pure ZnO, providing an ideal template for vertical nanorod growth.

Growth of ZnO Nanorods

Undoped nanorods were synthesized from 0.06 M zinc nitrate hexahydrate and 0.06 M hexamethylenetetramine (HMTA) stirred for 2 h. Phosphorus doping was achieved by adding NH4H2PO4 at 0–1 % molar ratios relative to zinc nitrate (0 %, 0.05 %, 0.1 %, 0.2 %, 0.5 %, 1 %). AZO‑coated quartz was immersed in these solutions and maintained at 90 °C for 10 h (Fig. 1b). After rinsing with deionized water and nitrogen drying, vertically aligned ZnO nanorods were obtained (Fig. 1c).

Characterization

Scanning electron microscopy (SEM) assessed surface morphology. Powder XRD examined crystalline changes. Hall‑effect measurements (0.5 T magnetic field) evaluated conductivity type, carrier concentration, and mobility. Room‑temperature PL employed a 266 nm Nd:YAG pulsed laser (150 mW). Reflectance spectra were collected in diffuse mode to determine bandgaps via Kubelka–Munk (KM) and Tauc analysis.

Results and Discussion

Structural Evolution

XRD patterns (Fig. 1d) show characteristic ZnO peaks at 34.36°, 44.27°, 62.80°, and 72.45° for (002), (111), (103), and (004) planes. The (002) peak remains at the same angle across all doping levels, but its integrated intensity decreases with increasing NH4H2PO4 (Fig. 1e), indicating reduced crystallinity or misalignment. At 1 % doping, additional (100), (101), and (102) peaks emerge, confirming significant structural modification. The full width at half maximum (≈0.25°) remains unchanged, suggesting that lattice strain is minimal.

Morphology

SEM images (Fig. 2a–f) reveal uniform hexagonal nanorods whose diameter grows from ~60 nm (undoped) to ~145 nm (1 % doping), while length increases from 1.35 µm to 2.5 µm (Fig. 3a,b). This trend plateaus beyond 0.1 % NH4H2PO4, consistent with phosphorus solubility limits in ZnO. The growth mechanism involves Zn2+ and OH– reacting to form ZnO, while phosphate ions compete with Zn2+ for lattice sites, forming Zn3(PO4)2 precipitates that suppress excessive phosphorus incorporation.

Electrical Properties

Hall‑effect data (Fig. 3c–e) show negative carrier concentrations up to 0.2 % doping (n‑type) and positive values above 0.3 % (p‑type), reflecting phosphorus’s amphoteric behavior. At 0.5 % doping, the carrier concentration peaks (~7.8 × 10^15 cm⁻²) before dropping sharply to 1.67 × 10^9 cm⁻² at 1 % due to self‑compensation when phosphorus exceeds the solubility limit. Hall coefficients (Fig. 3d) and mobilities (Fig. 3e) follow the same trend, confirming the conductivity transition.

Optical Bandgap

Diffuse reflectance spectra (Fig. 4a) show a steep drop near 380 nm, corresponding to the ZnO bandgap. After converting to the KM function and applying the Tauc relation (Fig. 4b), bandgaps range from 3.28 eV (undoped) to 3.18 eV (0.1 % doping), then rising to 3.26 eV at 1 % doping. The slight bandgap narrowing at low doping is attributed to carrier‑induced band‑gap renormalization; the subsequent increase reflects phosphorus‑induced lattice expansion.

Photoluminescence

Normalized PL spectra (Fig. 4c) exhibit two dominant features: a near‑band‑edge (NBE) peak (≈379 nm for undoped, shifting to 384 nm at 1 % doping) and a deep‑level emission (DLE) spanning violet (≈389 nm), yellow (≈580 nm), red (≈700 nm), and NIR (≈1.2 µm). Deconvolution attributes these to Zn interstitials, oxygen interstitials, oxygen vacancies, and the Al‑doped seed layer, respectively. The NBE red‑shift aligns with donor‑acceptor pair transitions involving phosphorus. Importantly, DLE wavelengths remain unchanged across doping levels, offering a stable visible‑range emission for device integration.

Implications for Device Design

The persistence of DLE wavelengths irrespective of phosphorus concentration allows independent tuning of carrier injection (via doping) and emission color (via post‑growth annealing). Such decoupling simplifies the fabrication of p–n junction LEDs with targeted visible colors while maintaining efficient carrier transport.

Conclusions

Phosphorus doping via a hydrothermal route successfully converts ZnO nanorods from n‑type to p‑type, with optimal conductivity near 0.5 % NH4H2PO4. Doping enlarges rod dimensions, modifies XRD intensity, and shifts the NBE PL peak due to donor‑acceptor pair formation, while preserving stable visible and NIR DLE emissions. These multifunctional optical signatures, combined with controllable electrical properties, position phosphorous‑doped ZnO nanorods as promising candidates for UV–NIR optoelectronic devices.

Abbreviations

DLE

Deep level emission

HMTA

Hexamethylenetetramine

KM method

Kubelka–Munk method

NBE

Near‑band‑edge emission

Nd-YAG

Neodymium‑doped yttrium aluminum garnet

NH4H2PO4

Ammonium dihydrogen phosphate

NIR

Near‑infrared

PL

Photoluminescence

P_O

Oxygen sites in ZnO

P_Zn

Zinc sites in ZnO

RF

Radio frequency

SEM

Scanning electron microscope

UV

Ultraviolet

XRD

X‑ray diffraction

Zn(NO3)2

Zinc nitrate hexahydrate

ZnO

Zinc oxide