Memory Effect in Silicon Nitride in Silicon Devices

Amorphous silicon oxide (a-SiO2) and nitride (a-Si3N4) are two key dielectrics in microelectronic silicon devices [1]. The dominant dielectric used currently in silicon devices is a-SiO2 [1,2]. Application of silicon oxide for future devices will be impeded by several fundamental limitations which lead to low reliability of semiconductor devices and to the necessity of alternative dielectrics [1-3]. Amorphous silicon nitride and oxynitride are considered now as alternative to a-SiO2 in future devices [4]. One of the unique property of a-Si3N4 in comparison with a-SiO2 is the electron and hole capture by the deep traps with extremely long life time (10 years) in the captured state (the memory effect). This property is widely employed in memory devices and microprocessors in computers. Despite numerous efforts the nature of traps responsible for the memory effect in Si3N4 is so far unclear. In this paper we discuss the nature of such traps using the quantum-chemical simulation. The calculations show that the defects responsible for the electron and hole capture can be the Si-Si defects created by excess silicon atoms in Si3N4. Silicon devices in integrated circuits are the basic components of modern electronics and information technology. Silicon electronics has globally changed our life and will have great impacts in future. Currently the a-SiO2 films of about 5 nm thickness are used as a gate dielectric in silicon devices. The dielectric films with a thickness of 1.5-2.0 nm are required for future 1 Gbite devices [1]. The a-SiO2 with such thickness has low reliability and therefore cannot be applicable [1,2]. Unlike a-SiO2 that exists widely in the nature, amorphous silicon nitride has been specially synthesised for microelectronic applications and has several advantages over a-SiO2. Silicon nitride is used as insulator in silicon devices, as a mask against the impurity diffusion during fabrication, and as a passivation layer to protect the devices against the environment. Intrinsically a-Si3N4 contains a high density of the electron and hole traps and therefore is widely used in triple oxide-nitride-oxide (ONO) structures in silicon devices [5,6]. The stacked ONO structures are used as dielectric in memory capacitors of memory silicon devices and in microprocessors that are the main elements of personal computers. The advantage of the ONO dielectric over a-SiO2 is the high reliability due to the electron capture in Si3N4 [6]. This effect results in the increase of a breakdown field strength and the improved reliability of devices composed of ONO dielectrics. Besides, the ONO structures are the key components in silicon memory transistors based on the electron and hole capture in Si3N4 [7-9]. Information written in the transistor is stored and saved without the energy consumption more than ten years (similar to a magnetic memory). According to projections this type of the silicon memory devices will replace the floppy and hard magnetic discs in computers in future. The memory effect in a-Si3N4 was discovered in 1968, in the beginning of silicon microelectronics development [10]. As early as in 1994, the Hitachi Corporation has designed the 1 Mbit memory array based on the memory effect in a-Si3N4 [8], and Sony Corporation has designed memory devices for future 256 Mbit [9]. Furthermore, one of the unique advantages of a-Si3N4 over a-SiO2 is a high tolerance to radiation, in particular, to cosmic rays. Therefore, silicon devices, based on aSi3N4, are widely used in the space and nuclear technology with the harsh radiation environment [11]. The nature (atomic structure) of traps responsible for the memory effect in aSi3N4 is so far unclear in spite of the extremely technological importance, numerous applications, and studies. However, there is a consensus that the capture of electrons and holes is related with the defects created by excess silicon atoms in a-Si3N4 [12,13]. The first idea was to associate the memory effect with the existence of the neutral three-fold coordinated silicon atoms ≡Si* with unpaired electron (K center) which can capture either electron or hole. However, a very important physical information about the capturing properties of traps in a-Si3N4 was obtained by electron spin resonance (ESR) experiments, which allowed to observe the defects with unpaired electrons. It was found that the ESR signal was absent in as-prepared Si3N4 samples [12,13]. This means that there are no defects associated with unpaired electrons in as synthesed a-Si3N4. Consequently, the neutral isolated ≡Si* defects are absent in this materal. The next step was to assume that a pair of the three-fold coordinated ≡Si* neutral defects creates a pair of the negatively charged ≡Si: and positively charged ≡Si defects (K− and K centers) with the reaction [12,14,15] ≡ Si∗ + ≡ Si∗ → ≡ Si : + ≡ Si. (1) This model is called the ”negative correlation energy model” (NCE) that means that the reaction (1) is energetically favorable [12,14,15]. The NCE model explains the absence of the ESR signal in as synthesed a-Si3N4 because the charged defects ≡Si and ≡Si: do not contain unpaired electrons. The NCE model assumes that the reaction (1) may be favourable due to a lattice relaxation. According to this model the positively charged ≡Si defect is an electron trap and the negatively charged ≡Si: defect is a hole trap. Another model for interpretation of the memory effect in silicon nitride was considered in [7,13,16,17]. That model assumes the existence of the neutral Si-Si bonds in a-Si3N4 that are responsible for the electron or hole capture. The Si-Si defect model explains the absence of the ESR signal in as-prepared Si3N4 because two electrons of the neutral Si-Si bond are paired. The validity of the NCE and Si-Si models is so far under discussion [18]. We considered these models with the quantum-chemical MINDO/3 method which we applied early for the defect simulation [19]. Previously this method was used also for calculation of the electronic structure of the ≡Si* and ≡Si-Si≡ defects in a-SiO2 [20]. The 84 atomic (Si20N28H36) and 70 atomic (Si18N17H35) clusters were used for simulation of the a-Si3N4 bulk electronic structure to estimate the energy gain with capture of an electron or hole. The 56 atomic (Si14N12H30) and the 55 atomic (Si10N18H27) clusters were used for simulation of the ≡Si-Si≡ and ≡Si* defects. To understand the capturing properties of defects we added one electron or hole to the cluster. Atomic relaxation of the atoms in the first coordination sphere of defects was considered in the simulation. To verify if the reaction (1) is energetically favorable or not we calculated the difference between the total energies of the pairs of clusters (≡Si + :Si≡) and (≡Si* + ≡Si*). Calculations give the positive value for this difference ≈ 4.0 eV. Ab initio calculations at the 6-3G*/MP2 level for the cluster Si(NH2)3 give ≈ 5.5 eV [21]. Taking into account the energy of the long range polarization (estimated with ”classical model” [18]) we obtain the next values for the total energy gain ∆E ≈ 2.7 eV (MINDO/3) and ∆E ≈ 3.5 eV (ab initio). The recent first principle DFT calculations of the ≡Si* and ≡ Si2N* defects in silicon nitride [18] gave a positive correlation energy 0.9 eV for the reaction (1) with isolated ≡Si* defects. The small value of ∆E is explained by a strong relaxation effect observed in [18] for the positively charged defect ≡Si. The positive value of ∆E means that the charged ≡Si and ≡Si: states of a threefold coordinated silicon atom are energetically unfavorable in comparison with the neutral defects ≡Si*. We may conclude that the calculations do not support for the time being the widely accepted NCE model for a-Si3N4. To understand the capturing properties of the Si-Si bond we studied this defect in different charge states. The calculations show that the energy gain with capture of a hole by the Si-Si defect is about 0.34 eV. The capture of an electron is accompanied by the energy gain of 1.76 eV. Hence, the Si-Si bond in a-Si3N4 can capture either electron or hole. The absence of the ESR signal in a-Si3N4 with captured electrons or holes is explained by the interaction of electrons (holes) captured in the nearest occupied Si-Si defects [17]. According to our consideration the model of the Si-Si bond is the most favorable for explanation of the memory effect in a-Si3N4.