Aluminium nitride nanowires for electronic and photonic applications

Abstract

Aluminium nitride (AlN) has attracted particular research enthusiasm for their promising application as short wavelength light emission devices and high power transistors. AlN has the largest band gap of ~6.0 eV among all the group III nitrides, and has excellent thermal, mechanical and chemical stability, and low electron affinity. It is a promising candidate for deep-ultraviolet (DUV) light emitting devices. DUV light sources have attracted considerable attention because of their potential uses in environmental protection equipment, nano-fabrication technology, high-density optical data storage, water and air purification, and sterilization. Semiconductor nanowires have been demonstrated significant potential as fundamental building blocks for nanoelectronics and nanophotonic devices. As compared to AlN epilayers, one-dimensional (1D) AlN nanostructures have the following appealing features: They can be single-crystalline, relatively defect-free, having atomically smooth surfaces and able to accommodate large lattice mismatch. In this work, we investigate the synthesis and characterization of AlN nanowires and pay particular attention in preparing Mg-doped AlN nanowires (AlNNWs). Mg is a potential p-type dopant for AlN. It is predicted that Mg could also be doped into AlN to make it ferromagnetic. AlNNWs and zigzag AlNNWs were synthesized by chemical vapor deposition. Randomly aligned nanowires and zigzag nanowires with single-crystalline structure were synthesized on sapphire substrates at 1450 °C and 1350 °C respectively under the flow of nitrogen (N₂) gas. It is found that when the substrate was located at a low temperature zone, it was relatively easier to dope Mg into the nanowires and the growth of zigzag nanowires was imitated. The structural, magnetic and optical properties of the nanowires and zigzag nanowires were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) cathodoluminescence (CL) and photoluminescence (PL). The high resolution transmission electron microscopy (HR-TEM) and the selected area electron diffraction (SAED) revealed that the nanowires were single crystalline, with the lattice spacing matched wurtzite hexagonal AlN (h-AlN). The growth axis of the nanowires was identified to be the hexagonal a-axis along the [100] direction. As detected by the energy-dispersive X-ray spectroscopy (EDX), the nanowires contained no Mg dopant. The diameter and length of the nanowires were ranging from 20 nm to 100 nm and 10 μm to 50 μm, respectively. Two vibration modes A₁ (TO) and E₂ (high) located at 612 and 656 cm⁻¹ respectively were observed from the Raman spectrum, which fitted well with the stress-free bulk AlN crystal. Room temperature CL showed a strong and broad emission peaked at around 3.50 eV. Room temperature PL spectrum also showed a similar emission peak at around 3.51 eV when the samples were excited by a quadrupled Ti-sapphire femtosecond laser (195 nm). The emission was attributed to oxygen substituting the nitrogen vacancy (ON). The HR-TEM images and the SAED patterns revealed that the zigzag nanowires were also single crystalline, with the lattice spacing matched wurtzite h-AlN. The growth axis of the zigzag nanowires was identified to be the hexagonal c-axis along the [001] direction. About 5 at.% of Mg was estimated in the zigzag nanowire. The diameter and length of the nanowires were ranging from 50 nm to 100 nm and 10 μm to 50 μm, respectively. Slight redshift of the two vibration modes A₁ (TO) and E₂(high) located at 616 and 660 cm⁻¹ respectively were observed from the Raman spectrum, which could be attributed by the incorporation of Mg. It is noted that ferromagnetic properties can be obtained only from the zigzag nanowires. The spontaneous saturated magnetization and coercivity measured in the magnetization versus magnetic field (M-H) loop of the zigzag nanowires were estimated to be ~50 μemu/cm² and 3 kOe respectively when the field was applied perpendicular to the sample. Both room temperature CL and PL spectrum showed a strong and broad defect-related emission at around 3.47 eV. A heterojunction light emitting device (LED) based on AlNNWs and p-type silicon carbide (p-SiC) had been demonstrated. When an applied voltage greater than 8 V was applied to the LED, a broad band emission peaked at 417 nm could be observed. The peak deconvolution by Gaussian curve fitting revealed four emission peaks centered at ~400, ~420, ~468, and ~525 nm. These emission peaks were attributed to the nitrogen vacancies (VN) and ON trap-levels of the nanowires.