Electrical behavior of MIS devices based on Si nanoclusters embedded in SiO x N y and SiO2 films
© Jacques et al; licensee Springer. 2011
Received: 20 September 2010
Accepted: 24 February 2011
Published: 24 February 2011
We examined and compared the electrical properties of silica (SiO2) and silicon oxynitride (SiO x N y ) layers embedding silicon nanoclusters (Sinc) integrated in metal-insulator-semiconductor (MIS) devices. The technique used for the deposition of such layers is the reactive magnetron sputtering of a pure SiO2 target under a mixture of hydrogen/argon plasma in which nitrogen is incorporated in the case of SiO x N y layer. Al/SiO x N y -Sinc/p-Si and Al/SiO2-Sinc/p-Si devices were fabricated and electrically characterized. Results showed a high rectification ratio (>104) for the SiO x N y -based device and a resistive behavior when nitrogen was not incorporating (SiO2-based device). For rectifier devices, the ideality factor depends on the SiO x N y layer thickness. The conduction mechanisms of both MIS diode structures were studied by analyzing thermal and bias dependences of the carriers transport in relation with the nitrogen content.
Silicon heterojunctions have been extensively studied for the understanding of the physics of the device as well as their applications to majority of the carrier rectifier , photodetectors , solar cells , and indirect gap injection lasers . Because of its indirect band gap, silicon is a highly inefficient material for a light emitter. However, to overcome this problem, different approaches were developed in this last decade for the fabrication of Si-based light emitting sources made of silicon nanoclusters (Sinc) embedded in silica or silicon oxynitride (SiO2-Sinc or SiO x N y -Sinc) matrix.
Due to quantum confinement effects, Sinc are characterized by an energy band gap which is enlarged with respect to bulk silicon and by an intense room temperature photoluminescence that can be obtained in the visible-near infrared (IR) range [5, 6]. Previous study  recently reported that the presence of incorporated nitrogen species influences the silicon nanoclusters' growth and affects the photoluminescence of the SiO x N y -Sinc layer. In addition, IR light emitting properties were also reported in matrix embedding Sinc and rare earth [8–11]. In such system, the emitting rare earth ions benefit from the quantum confinement properties of the carriers generated within Sinc to be efficiently excited by an energy transfer mechanism. Electroluminescence of SiO x N y -Sinc-based IR light emitting devices is limited by the difficulty in carrier injection. Therefore, prior to developing IR light emitting devices, a good understanding of optimum carrier injection in SiO x N y -Sinc type layers is needed. In this way, previous works have been recently reported on electrical properties in metal-insulator-semiconductor (MIS) devices fabricated with such silicon-rich oxide layers either deposited by (1) plasma-enhanced chemical vapor deposition technique  or by (2) magnetron co-sputtering of Si, SiO2 . In the first case, rectifying behavior was observed, but not in the second. In addition, thermal dependence of the carrier transport was not studied.
In this present work, we report a detailed study of the carrier transport governing electrical properties of SiO2-Sinc and SiO x N y -Sinc layers integrated in MIS devices. Layers are deposited by reactive magnetron sputtering of a pure SiO2 cathode. The thermal and the bias dependences of the carrier transport are analyzed. The aim of such study consists in fabricating a thin layer for future electroluminescent devices doped with rare earth ions. Thus, one of the key parameters is to overcome the insulating characteristic of the SiO2 matrix by incorporating nitrogen.
Device technology and experimental details
Static electrical characteristics J(V) are collected by using an HP 4155B semiconductor parameter analyzer. For temperature measurements from -70°C to 230°C samples are placed in a cryostat under vacuum (10-5-10-4 Pa). All measurements were made in darkness on more than 20 devices homogeneously located over the 2-in. surface substrate. The area of each tested device is 0.32 mm2.
where q is the electron charge, k B the Boltzmann's constant, n the ideality factor dealing with current dominated by carrier diffusions (n = 1) and/or by carriers recombination processes at defects (n = 2), R the global serial resistance and J 0 the saturation current density. Ideality factor n and serial resistance R are deduced by fitting our experimental results with the theoretical model (1). The resistance R is likely to arise from minority carrier space charge, the bulk resistances, and finally contact resistance. In our case, n was estimated to n ≈ 1.2 indicating that carrier injection is dominated by the carrier diffusion process. For such N-rich devices J(V) plots have an excellent rectifying ratio (> 104 at V = ± 1 V) (Figure 1b) leading to a higher injected current level than reported in the literature [16–18]. In addition, at high voltages (2 V > V > 0.8V), current deviates from the exponential behavior due to the low global resistance series (20 Ω < R < 40 Ω).
where J is the current density, N the density of trapping sites, μ the effective carrier mobility, E(= V/d) the local electric field, Φ 0 the zero field trapped energy barrier depth and β PF the PF coefficient.
The permittivity obtained is also compared to the value deduced from quasi static C-V measurements. The coefficients deduced from the PF relation provides ε r = 5.6 and ε r = 4.4 for the 30-nm and the 65-nm-thick SiO x N y -Sinc layers, respectively while from C-V measurements, the corresponding permittivity obtained are ε r = 5.1 and ε r = 4.3. Such similar results are consistent with values obtained with the PF model to explain the reverse current behavior (V < -0.2 V). The difference of permittivity noticed could be explained either by a change of the density of Si nanoclusters (for the same Si content) or by a modification of the Si excess with the thickness. Considering that we observe an increase of the refractive index from 1.61 to 1.75 for 65 nm and the 30-nm layer thicknesses respectively, it suggests that during the first step of the deposition process, the starting growth process conditions could promote the incorporation of Si within a few nanometres thick due to the vicinity of the Si substrate.
where C is a constant both depending on the elementary charge q, the barrier height Φ B, and the Planck's constant h, and where m* stands for the carriers' effective mass. This conduction mechanism could be effective between the silicon nanoclusters through the silicon oxynitride.
where ε is the permittivity of the material and V TFL is the voltage at which the current significantly increases. The permittivity extracted from C-V measurements is ε r = 3.95. V TFL is defined as the intersection between the linear parts of the second and the third region. The value of the trap density n t, acting as quality factor of the SiO2-Sinc layer, has been estimated to be 5.39 × 1016 cm-3. The presence of trap centers could be associated to the density of Si nanoclusters in the silicon oxide matrix as it has been reported in a previous work . In the case of SiO x N y layer, as previously discussed, conduction mechanism appears to be different. In such a layer, the nitrogen is suspected to passivate the trap centers close to the silicon nanoclusters and thus promoting N-type doping effect responsible of pn junction creation between the active layer and the P-doped silicon substrate.
Conduction mechanisms of SiO2-Sinc and SiO x N y -Sinc layers have been studied and compared. The use of silicon oxynitride with embedded silicon nanoclusters has been validated in order to achieve diode with high rectifying behavior. Nitrogen significantly modifies the electrical behavior of the layer. It is suspected to be both responsible of a of (1) a defect passivation at the interface of silicon oxide matrix and silicon nanoclusters and (2) to act as N-doping specie and to promote a pn junction creation between active layer and P-doped silicon substrate.
This study has shown the interest to use nitrogen in silicon matrix with silicon nanoclusters to improve the current injection in the MIS structure. This effect could be interesting for an energy transfer to the rare earth ions for an infrared emission in such structures based on silicon-rich oxynitride layer doped with rare earth.
This work was financially supported by the Agence National de la Recherche (France) through the program PNANO-ANR-08-NANO-005 entitled DAPHNES (Dispositifs Appliqués à la Photonique à base de Néodyme et de Silicium).
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