Transparent Graphene Nanoplatelets for Charge Storage in Memory Devices
The memory cells are fabricated on an n+-type (111) (Antimony doped, 15-20 mµÙ-cm) Si wafer. 10-nm-thick Al2O3 is deposited at 250°C in Cambridge Nanotech Savannah-100 atomic layer deposition (ALD) system. Pristine graphene nanoplatelets (NanoIntegris PureSheets Quattro grade) are deposited by drop-casting technique. Samples are placed on hot-plate at 110°C and 2-2.5 ml of 0.05 mg/ml graphene solution is drop-casted slowly by using plastic pipette and samples are left to dry for 5 minutes on hot-plate. Then a 5-nm-thick Al2O3 tunnel oxide is ALD deposited at 250°C. A 400-nm-thick Al layer for the gate contact is sputtered using a shadow mask. A cross-sectional illustration of the fabricated memory device is depicted in Fig. 1a).
Then, the memory devices are electrically characterized by measuring the C-Vgate characteristics of the programmed and erased states at high frequency (1 MHz). Using the Agilent B1505A Semiconductor Device Parameter Analyzer, the memory cells gate voltage was first swept from -10 V forward to 10 V. The obtained C-V characteristic corresponds to the erased state as shown in Fig. 1b). Then, upon sweeping the gate voltage from +10 V to -10 V, there was a near parallel shift in the measured C-V curve in the positive direction as seen in Fig. 1b). The positive shift is due to electrons storage in the graphene nanoplatelets and the value of the Vt shift is 4.4 V. The Vt is extracted at a capacitance of 500 pF at the onset of the depletion region. The measured memory window at different operating voltages indicates a large Vt shift (3.6 V) at low operating voltages (8/-8 V). At 10/-10 V, the charge trap states density of the graphene is calculated and found to be 5.83×1012 cm-2 .
Furthermore, to identify the electron emission mechanism, the electric field across the tunnel oxide is calculated  and the natural logarithm of the Vt shift divided by the square of the electric field is plotted vs. the reciprocal of the electric field as shown in Fig. 2. In Fowler-Nordheim tunneling (F-N), the charges are injected by tunneling into the conduction band of the oxide through a triangular energy barrier and then are swept by the electric field into the charge trapping layer. The emission rate of charges in F-N tunneling is exponentially proportional to the reciprocal of the electric field times the electric field squared . Thus, the linear trend presented in Fig. 2 indicates that the dominating electron emission mechanism at electric fields larger than 5.6MV/cm (corresponding to a gate voltage of 6V) is F-N tunneling.
Finally, the fabrication of such graphene based memory structure is compatible with existing semiconductor processing thus has potential on low-cost integrated nanoscale memory applications.
Acknowledgment: This work was supported by ATIC, and TUBITAK Grants 109E044, 112M004, 112E052 and 113M815.
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