A Study of Metal-Oxide-Semiconductor Structure with Embedded Ge Nanocrystals by Using a Trap Model
Date Issued
2005
Date
2005
Author(s)
Lu, Shau-Wei
DOI
en-US
Abstract
In this work, we utilize a single-level trap model to study impact of the shallow traps, resulting in dynamic behavior of charge exchange and charge loss mechanism in a MOS capacitor containing germanium nano-crystals (Ge NCs) performed by using frequency dependence capacitance-voltage (C-V) and conductance-voltage (G-V) measurement. Firstly, we present a study on the fabrication process of Ge NCs embedded in SiO2 matrix where the Ge NCs are grown by a two-step process in the UHV/CVD system. Si nuclei are first deposited on SiO2 surface. Then, Ge is selectively grown on the Si nuclei. Secondly, we also use a PVD method to synthesize the Ge NCs. They are formed by furnace annealing to an E-gun evaporated ultra-thin Ge layer at 900oC for 300 s in a dry N2 or O2 ambient. From SEM and HRTEM measurements, we ensure that Ge NCs are exactly pronounced.
From C-V measurement, the devices formed by the above two methods both show counterclockwise hysteresis phenomena, indicating charge storage effect clearly. For the characteristics of charge retention, the devices which are grown by the first method after PDA treatment show improvement. In addition, the ones formed by the second method with the condition in a dry O2 ambient to E-gun evaporated Ge layer shows even better retention characteristic compared to the ones formed by the same method but under the condition of a dry N2 ambient.
We also utilize a single-level trap model based on SRH theory and introduce an additional charge loss time constant in the model. Hence, there are two time constants in our model. The exchange time constant in original model indicates the time needed fro the trap to exchange the carriers with the external circuit. The other represents the additional charge loss path due to the hopping of carriers in the adjacent traps. We utilize our trap model to compare the characteristics of charge exchange and charge loss between the devices with/without PMA treatment. The behavior of charge exchange between shallow states of NCs/traps and Si substrate is by way of thermal assisted tunneling differently from pure MOS structure by way of directly generation and recombination process .Trap capacitance and trap conductance in the device without PMA treatment is larger than that with PMA treatment. The result suggests the defects or surface states around Ge NCs are annealed out by PMA treatment, resulting in smaller amount of communication of charges between NCs/traps caused by the reduction of conductive paths. The charge loss time constant for the devices with PMA treatment is larger than that for the device without PMA treatment due to the reduction of surface state density or oxide defects around Ge NCs, resulting in the suppression of trap-assisted tunneling through the tunnel oxide to decrease the charge loss behavior. During PMA treatment, the charge retention is not obviously improved due to the charges storing in the deeper traps or deeper states in NCs. From the admittance measurement between 10K Hz and 1M Hz , we could just estimate the time constant between 10-4 and 10-7 seconds and the order of magnitude belongs to the short time constant. Therefore, we can detect the magnitude of the time constant associated the depth of the shallow traps from 10-4 to 10-7 seconds. From our admittance measurement, we can only obtain the shorter time constant arising from shallow traps while the measurement of charge retention focuses on the results from deeper traps or deeper states in the NCs.
Subjects
陷阱
鍺奈米晶體
金氧半
非揮發性記憶體
trap
Ge nanocrystals
MOS
nonvolatile memory
Type
thesis
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