[Paper Review] Understanding H-defect complexes in ZnO

R. Vidya, P. Ravindran+1 more|arXiv (Cornell University)|Sep 20, 2013

ZnO doping and properties3 citations

TL;DR

This study uses hybrid-functional density functional theory to demonstrate that hydrogen (H) in ZnO exhibits amphoteric behavior—acting as cationic, anionic, or neutral—through defect complexes with intrinsic (e.g., V_O) and extrinsic (e.g., B, Al, Li) defects. These complexes explain persistent n-type conductivity and experimentally observed Raman/IR peaks, including 3326 cm⁻¹ and 3611 cm⁻¹, by stabilizing shallow donor levels via H-related complexes.

ABSTRACT

From state-of-the-art density-functional calculations using hybrid functionals we show that, persistent $n$-type conductivity in ZnO is due to defect complexes formed between H with intrinsic and extrinsic defects. H exhibits cationic, anionic, and electrically-inactive character on interacting with defects in ZnO. The electrically-inactive molecular hydrogen can contribute to $n$-type conductivity in ZnO by activating deep donor levels into shallow levels. By calculating local vibrational mode frequencies, we have identified origins of many H-related Raman and infra-red frequencies and thus confirmed the amphoteric behavior of H.

Motivation & Objective

• To resolve the long-standing puzzle of persistent n-type conductivity in ZnO despite the instability of isolated interstitial hydrogen (H_i).
• To identify the microscopic origin of experimentally observed H-related vibrational modes (e.g., 3326 cm⁻¹, 3611 cm⁻¹, 4145 cm⁻¹) in ZnO using first-principles calculations.
• To clarify the amphoteric nature of hydrogen in ZnO by examining its charge character (cationic, anionic, neutral) in different defect environments.
• To link H-defect complexes to the activation of deep donor levels into shallow levels, thereby sustaining n-type conductivity at high temperatures.
• To provide a definitive assignment of H-related IR and Raman frequencies through comparison of calculated local vibrational mode (LVM) frequencies with experiments.

Proposed method

• Employed projector-augmented wave (PAW) method within VASP for full structural and electronic structure optimization using GGA-PBE functional.
• Calculated local vibrational mode (LVM) frequencies via the frozen-phonon method with atomic displacements of 0.001–0.005 Å to account for anharmonicity.
• Used the HSE06 hybrid functional (screening parameter a = 0.375) to accurately reproduce the experimental band gap and structural parameters of ZnO.
• Computed defect formation energies and thermodynamic transition levels using HSE06 total energies under Zn-rich conditions.
• Systematically modeled H in various configurations: interstitial (H_i), substitutional (H_O), molecular (H₂,int), and in complexes with intrinsic (V_O) and extrinsic defects (Li, B, C, N, Al, Ga).
• Validated calculated LVM frequencies against experimental IR and Raman data to assign the origin of observed peaks.

Experimental results

Research questions

• RQ1 What is the origin of the persistent n-type conductivity in ZnO, particularly at high temperatures, despite the thermal instability of isolated H_i?
• RQ2 Why do experimental IR and Raman spectra show prominent H-related peaks at 3326 cm⁻¹ and 3611 cm⁻¹, and what defect structures give rise to these frequencies?
• RQ3 How does hydrogen exhibit amphoteric behavior in ZnO, and what determines whether it acts as a cationic, anionic, or neutral species in different defect environments?
• RQ4 Can H-defect complexes explain the activation of deep donor levels into shallow levels, thereby sustaining n-type conductivity?
• RQ5 What is the role of extrinsic impurities (e.g., B, Al, Li) in stabilizing H-related complexes and influencing the observed vibrational modes in ZnO?

Key findings

• H-defect complexes—particularly H_i with B_Zn or Al_Zn—produce LVM frequencies of 3621 cm⁻¹ and 3337 cm⁻¹, respectively, in excellent agreement with experimental peaks at 3611 cm⁻¹ and 3326 cm⁻¹.
• The H₂,int molecule aligned along the c-axis (z-direction) has a calculated Raman frequency of 4047 cm⁻¹, close to the experimentally observed 4145 cm⁻¹ for 'hidden' hydrogen.
• Formation energy calculations show that H_i and H_O have lower formation energies than H₂,int, but H₂,int at V_O forms a stable, neutral complex that induces a shallow donor level.
• H exhibits cationic character when bound to B_Zn or Al_Zn (e.g., 3621 cm⁻¹ mode), anionic character when bound to Li_Zn (H_O near Li_Zn), and neutral character in H₂,int at V_O.
• The complex H_i at AB⟂ near Al_Zn yields a calculated LVM of 3337 cm⁻¹, matching the experimental 3326 cm⁻¹ peak, and SIMS data confirm Al presence in melt-grown samples with this peak.
• Despite its amphoteric character, H always induces n-type conductivity via defect complexes, explaining the persistent n-type behavior in ZnO across a range of conditions.