Recently, there has been a growing scientific and technological interest in the II-VI compound semiconductor ZnO. This is due to the wide-ranging applicable properties of ZnO such as high electron mobility, high transparency, piezoelectricity, large direct band gap around 3.4 eV, in addition to luminescence due to its large exciton binding energy (around 60 meV) [1, 2]. Furthermore, fabricating ZnO nanostructures with different dimensions in the nanometer range allow tuning of the ZnO properties due to quantum confinement effects [3,4]. In this work, 1 to 5 nm ZnO nanoparticles are characterized and studied for MOS memory application.

 

The ZnO nanoparticles (NPs) are first characterized by dip coating a quartz wafer in the solution which contains ZnO nanoparticles stabilized by polyacrylate sodium. Amplitude Modulation-Atomic force microscopy (AM-AFM) measurements are conducted on the sample and the height map (depicted in Fig. 1) shows that the majority of the ZnO nanoparticles have an average height of 1.8 nm and a width > 15-nm. This means that there is a single layer of NPs which are agglomerated in the horizontal direction only. It has been reported that some nanoparticles agglomerations cause a change in the properties of the nanomaterials [5]. In this case, since the thickness of the agglomerations is in the range of the bohr radius of the ZnO exciton (~2.3 nm) [6], 2-D quantum confinement effects are expected to occur in the agglomerations. The reflectance and transmittance spectra are measured using a UV-Vis-NIR spectroscopy. The Kubelka-Munk function is used to extract the bandgap of the agglomerations, and the extracted value is 3.7 eV which is larger than the ZnO bulk bandgap (3.4 eV). The result is consistent with the 2-D quantum confinement effects.

Then, MOS-memory devices are fabricated on an n⁺-type (111) (Antimony doped, 15-20 mΩ-cm) Si wafer. 4-nm tunnel oxide Al₂O₃ is deposited at 300°C in an Oxford FlexAL system. Then, the ZnO NPs are dip-coated on the sample. Next, 9-nm blocking oxide Al₂O₃ is deposited by plasma assisted ALD at 200°C. Finally, a 350-nm Al layer for the gate contact is e-beam evaporated using a shadow mask with feature size down to 10 μm which eliminated the need for any lithography steps. A cross-sectional illustration of the fabricated MOS-memory device is shown in Fig. 2.

The electrical characterization of the fabricated memory devices is conducted by measuring the C-V_gate characteristics of the programmed and erased states at 1 MHz using the Agilent-B1505A Semiconductor Device Parameter Analyzer. The memory cells gate voltage (V_g) was first swept from -10 V forward to 10 V then backwards. An 8.2 V threshold voltage (V_t) shift is measured as shown in Fig. 3. The C-V measurements are repeated at different gate sweeping voltages as shown in Fig. 4. The curve shows that the ZnO-NPs are providing high density charge trapping states in the 2-D quantum well, and a large memory window of 4 V is obtained at a low program/erase voltage of 7/-7 V. The charge emission-mechanism during the program-operation is also studied. The natural-logarithm of the V_t shift divided by the square of the electric field across the tunnel oxide (E_ox) is plotted vs. the reciprocal of E_ox as depicted in Fig. 5. The linear trend indicates that the prevailing charge emission-mechanism at a V_g > 5 V (corresponding to E_ox=5.01 MV/cm) is Fowler-Nordheim tunneling (F-N). In F-N tunneling, the charges are emitted by first tunneling into the conduction/valence band of the oxide through a triangular energy barrier and then are swept into the charge trapping layer by the electric field. Also, the linear trend observed in Fig. 6 which depicts the V_t shift vs. the square of E_ox indicates that Phonon-Assisted Tunneling is the dominant charge emission-mechanism at V_g < 5 V. Finally, the results also show that the electric field across the tunnel oxide dictates the charge emission mechanism and therefore the operating voltage of the memory. The results also confirm that ZnO nanoparticles are promising in future nano-memory applications.

This work was supported by Masdar Institute of Science and Technology. N. El-Atab acknowledges financial support provided by L’Oréal-UNESCO For Women in Science Middle East Fellowship.

[1]     U. Özgür, et al., J. Appl. Phys. 98, 041301, 2005,

[2]     N. El-Atab, et al., Appl. Phys. Lett. 104, 013112, 2014.

[3]     N. El-Atab et al., Appl. Phys. Lett. 104, 253106, 2014.

[4]     N. El-Atab et al., Appl. Phys. Lett. 105, 033102, 2014.

[5]     P. Francis et al., The European Physical Journal D, 67(7), 1-7, 2013.

[6]     Y. Gu et al., Appl. Phys. Lett., 85, 3833, 2004.