Current-Voltage Characteristics

4.3.1 Leakage Current

Fig. 4-7 shows the J-V characteristics of p-type HfO2 capacitors with different die areas (6.25×10^-6, 2.5×10^-5 and 1×10^-4 cm²) under 400℃-15 minutes oxidation condition from 0 V to -1 V. We observed that the gate leakage current density becomes only a little larger while die area decreases. From table 4-1, 4-2 and 4-3, EOT of these three devices are 17.3 Å, 21.0 Å and 22.3 Å respectively. Fig. 4-8 shows J-V characteristics of p-type HfO2 capacitors with 6.25×10^-6 cm² die area under different oxidation conditions (400℃-15 minutes, 400℃-30 minutes, 500℃-15

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minutes and 500℃-30 minutes) from 0 V to -1 V. The EOT of these four devices are 17.3 Å, 17.4 Å, 23.8 Å and 23.9 Å respectively. Apparently, the device under higher oxidation temperature and longer oxidation time has lower gate leakage current. In fig.

4-7, 5 Å increase of EOT (from 17.3 Å to 22.3 Å) only reduces a little gate leakage at VG = -1 V. But in fig. 4-8, 6.5 Å increase of EOT (from 17.3 to 23.8 Å) reduces gate leakage even more than 2 orders at VG = -1 V. Thus, from fig. 4-7, three similar magnitudes of gate leakage reveal that the thickness of devices with three different die area on the same wafer might be the same. As shown in fig. 4-8, the main reason of such a large repression of gate leakage between 400℃-15 minutes and 500℃-15 minutes oxidation conditions is the increase of thickness. Higher oxidation temperature could also make HfO2 film have stronger chemical bonding to effectively resist gate leakage. Besides, the devices under 500℃-15 minutes (EOT = 23.8) and 500℃-30 minutes (EOT = 23.9) oxidation condition have almost the same thickness but have about an order difference of gate leakage at VG = -1 V. Thus, the longer oxidation time mainly increases the intensity of chemical bonding.

Fig. 4-9 shows measured and simulated J-V characteristics of NMOSFET with SiO2 gate insulator. Gate leakage current density of 20 Å SiO2 gate insulator at VG = 1 V is about 3×10^-2 A/cm². From fig.4-9, however, gate leakage current density of the HfO2 capacitor with EOT = 17.3 Å at VG = -1 V is only about 3×10^-4 A/cm². Even thinner EOT of HfO2 capacitor but has less gate leakage than SiO2 device about 2 orders. Consequently, replacing SiO2 with HfO2 for gate insulator could effectively reduce gate leakage.

Then, we make plots of gate leakage versus EOT for further discussing. As shown in fig. 4-10, we could obviously find that, whether for n-type or for p-type HfO2 capacitors, gate leakage at Vg = 1 V of different die areas are almost the ∣ ∣ same, even if their EOT are not the same. With the increase of EOT, however, the gate leakage doesn’t decrease. Consequently, we think the EOT of devices with different die areas on the same wafer should be the same in fact. From fig. 4-11, we observe that, whether for n-type or for p-type HfO2 capacitors, while the oxidation temperature rises, the EOT becomes larger and the leakage becomes less. Higher oxidation temperature makes the HfSiO interfacial layer become thicker, as shown in fig. 4-6. The thicker interfacial layer causes the larger EOT of HfO2 gate insulator.

Higher oxidation temperature also makes the chemical bonding stronger. Thicker physical thickness and stronger chemical bonding both contribute to the decrease of gate leakage current. Longer oxidation time slightly increases EOT and decreases gate leakage current. In addition, another interesting phenomenon is that under the same

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oxidation condition, n-type HfO2 capacitor has larger EOT but larger gate leakage current than p-type HfO2 capacitor. The larger EOT might result from the larger diffusion rate of hafnium in company with oxygen atoms in n-type substrate than in p-type substrate. Generally, the device with larger EOT has less leakage current. Thus, we think the HfO2 layer grown on p-type substrate has better quality of resisting gate leakage than on n-type substrate.

Fig.4-12 shows J-V characteristics of n-type and p-type HfO2 capacitors with 6.25×10^-6 cm² die area from -1 V to +1 V. For p-type, the leakage current under positive gate bias is much lower than negative case by 3 orders. For n-type, the leakage current under negative gate bias is lower than positive case by 1 order. The reverse leakage is thought to be dominated by surface leakage [3]. The surface generation current has been reported to be linearly correlated to the interface state density [4]. These interface states are caused by the dangling bonds at the Si/SiO2

interface [5][6]. This component is often masked by surface leakage, especially at low temperatures [7][8]. This is illustrated in fig. 4-13.

Fig. 4-14 shows J-V characteristics of n-type HfO2 capacitors with different die areas under 400℃-30 minutes oxidation condition from 0 V to +10 V. In this figure, we could see the devices with 6.25×10^-6, 2.5×10^-5 and 1×10^-4 cm² die area, generate breakdown at VG = 3.85 V, 3.50 V and 3.10 V respectively. The device with smallest die area has the biggest breakdown voltage (VBD), which means the best quality of resisting leakage current. We think more ratio of leakage current could pass through the gate insulator from edge side between HfO2 layer and isolation oxide in the device with smaller die area. The leakage current which doesn’t pass through HfO2 layer doesn’t destroy the structure of HfO2 layer. Thus, the device with less ratio of leakage current passing through the HfO2 layer could have larger breakdown voltage. Fig.4-15 shows J-V characteristics of p-type HfO2 capacitors with 6.25×10^-6 cm² die area under different oxidation conditions (400℃-15 minutes, 400℃-30 minutes, 500℃-15 minutes and 500℃-30 minutes), correspond separating EOTs with 17.3 Å, 17.4 Å, 23.8 Å and 23.9 Å, from 0 V to -10 V. Breakdown voltages of these four devices are -6.05 V, -6.30 V, -7.15 V and -7.50 V. We could find that the device with thinner thickness and weaker chemical bonding generates breakdown more easily. In addition, like SiO2, thicker HfO2 shows more abrupt breakdown characteristics compared to thinner HfO2.

Fig. 4-16 displays gate current as a function of the gate voltage for four oxide degradation stages in a 2000 µm² SiO2 NMOSFET. After SBD, a large increase of the

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substrate current is observed in the whole voltage range [9]. Fig. 4-17 shows four oxide degradation stages of p-type HfO2 capacitors with 6.25×10^-6 cm² die area under 400 15 minutes oxidation condition.℃ Like SiO2 NMOSFET, HfO2 capacitor has four degradation stages. This hints HfO2 might have similar breakdown mechanism with SiO2.

4.3.2 Band-gap Diagram and Conduction Mechanism

Selecting a gate dielectric with a higher permittivity than that of SiO2 is an essential choice. The required permittivity must also be balanced against barrier height to limit tunneling. For electrons traveling from the silicon substrate to the gate, this barrier is the conduction band offset, ∆EC. A gate dielectric must have a sufficient

∆EC value to poly-Si, and to other gate materials, in order to obtain low off-state currents (leakage). If the experimental ∆EC is < 1.0 eV, it will likely preclude the oxide's use in gate dielectric applications because thermal emission or tunneling would lead to unacceptably high leakage currents [10]. Among the several materials that have been investigated as gate dielectrics, as shown in Table 1-3, the dielectric constant generally exhibits an inverse relationship to the energy band gap.

Fig. 4-18 shows the energy band diagram of the SiO2 MIS capacitor with Al gate.

The energy band gap of SiO2 is 8.9 eV. Fig. 4-19 shows the energy band diagram of the HfO2 MIS capacitor with Al gate. The energy band gap of HfO2 is 5.7 eV [10].

The electron affinity 2.82 eV for HfO2 is obtained from the measurement of Fowler-Nordheim tunneling current of metal/ HfO2/Si MIS capacitors [11]. Taking the work function of Al as ФAl = 4.1 eV, the barrier height of Al/ HfO2 is ФAl/ HfO2 = 1.28 eV and the barrier height of HfO2/Si is ФSi/ HfO2 = 1.13 eV.

There are many possible conduction mechanisms in insulators [12]. For SiO2, the dominate conduction mechanism was believed to be Fowler-Nordheim tunneling in the medium field (6~10MV/cm), low temperature region (T＜200 ) [℃ 13]. As the thickness scales down, it would show the direct tunneling characteristics [14]. The weak temperature dependence of this tunneling process is well-known. The schematic illumination of conduction mechanism is shown in fig. 4-20, including Schottky emission, F-N tunneling, and Frenkel-Poole emission.

The leakage current governed by the Schottky emission is as following:

JSK = A* T² exp{ -q[φB - ( qE / 4πεoεd )^1/2 ] / kT }

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where A* is a constant, φB is the potential height on the surface, E is the electric field, ε0 is the permittivity in vacuum, εd is the dynamic dielectric constant, T is the temperature, and k is the Boltzmann constant. Fig. 4-21 shows the Schottky plot of n-type HfO2 capacitors with 6.25×10^-6 cm² die area under different oxidation conditions. The gray filled circles on each curve indicate where the individual slope of the curves becoming constant, which means the generation of Schottky emission happened. For the substrate electron injection case, the experimental results fit the Schottky emission theory well. While electric field is between 1 MV/cm to 16 MV/cm, the fitting slopes are almost constant. N-type HfO2 capacitors under 300℃-15 minutes oxidation condition begins to generate Schottky emission at VG = 0.09 V. Even for n-type capacitors under 500℃-30 minutes oxidation condition, which have largest EOT, Schottky emission occurs at very low bias VG = 0.82 V.

Fowler-Nordheim (FN) tunneling is the flow of electrons through a triangular potential barrier illustrated in fig. 4-20. Tunneling is a quantum mechanical process similar to throwing a ball against a wall often results that the ball goes through the wall without damaging the wall or the ball. It also loses no energy during the tunnel event. The probability of this event happening, however, is extremely low, but an electron incident on a barrier typically several nm thick has a high probability of transmission. The FN current (IFN) is given by the expression [15]:

⎟⎟ ⎠

⎥⎦⎤

where mox is the effective electron mass in the oxide, m is the free electron mass, and Φ is the barrier height at the silicon-oxide interface given in units of eV in the B

expression for B. Φ is actually an effective barrier height that take into account _B barrier height lowering and quantization of electrons at the semiconductor surface.

Rearranging

I

21 the oxide is pure Fowler-Nordheim conduction [15]. The slop of linear F-N plot gives A and the intercept yields B. Fig. 4-22 shows F-N plot of n-type HfO2 capacitors with 6.25×10^-6 cm² die area under different oxidation conditions. The gray filled circles on each curve indicate the slopes of the each curves becoming constant, which means the generation of F-N tunneling happened. N-type HfO2 capacitors under 300℃-15 minutes oxidation condition begins to generate F-N tunneling at VG = 1.42 V. We could find that device with thinner EOT generates F-N tunneling at smaller gate bias.

It is consistent with our prediction.

The leakage current governed by Frenkel-Poole emission is as following:

JFP = B E exp{ ‧ ‧ -q[φt - ( qE / πεoεd )^1/2 ] / kT }

where B is a constant, φt is the barrier height of trap level. The Frenkel-Poole emission is due to the field-enhanced thermal excitation of trapped electrons into the conduction band. Fig. 4-23 shows Frenkel-Poole plot of n-type HfO2 capacitors with 6.25×10^-6 cm² die area under different oxidation condition. The gray filled circles on each curve indicate the slopes of the curves becoming constant, which means the generation of Frenkel-Poole emission happened. N-type HfO2 capacitors under 300℃-15 minutes oxidation condition begins to generate Schottky emission at VG = 0.21 V. While electric field is between 4 MV/cm to 16 MV/cm, the fitting slopes are almost constant, indicating Frenkel-Poole emission is one conduction mechanism of leakage current. Even for n-type capacitors under 500℃-30 minutes oxidation condition, which have largest EOT, Schottky emission occurs at very low bias VG = 0.92 V. From above, we could find that Schottky emission occurs at very low gate bias and F-N tunneling occurs at higher gate bias than both Schottky emission and Frenkel-Poole emission. In addition, n-type HfO2 capacitors under 300℃-15 minutes oxidation condition always generate Schottky emission, F-N tunneling or Frenkel-Poole emission happened earliest because of smallest EOT.