Improvement of Ge MOS Electrical and Interfacial Characteristics by using NdAlON as Interfacial Passivation Layer

LI Chunxia, ZHANG Weifeng

(Department of Microelectronic Technology, Shenzhen Institute of Information Technology, Shenzhen 518172, China)

Abstract: The Ge metal-oxide-semiconductor (MOS) capacitors were fabricated with HfO2 as gate dielectric. AlON, NdON, and NdAlON were deposited between the gate dielectric and the Ge substrate as the interfacial passivation layer (IPL). The electrical properties (such as capacitance-voltage (C-V) and gate leakage current density versus gate voltage (Jg-Vg)) were measured by HP4284A precision LCR meter and HP4156A semiconductor parameter analyzer. The chemical states and interfacial quality of the high-k/Ge interface were investigated by X-ray photoelectron spectroscopy (XPS). The experimental results show that the sample with the NdAlON as IPL exhibits the excellent interfacial and electrical properties. These should be attributed to an effective suppression of the Ge suboxide and HfGeOx interlayer, and an enhanced blocking role against interdiffusion of the elements during annealing by the NdAlON IPL.

Key words: Ge MOS capacitor; interfacial passivation layer (IPL); gate stacked dielectric; interface properties

1 Introduction

With the integration of Ge and different high-kdielectrics, the Ge MOSFETs offers a possible way to further scale down the physical limitation of the Si MOSFETs, ascribed to the higher mobility of electron and hole for Ge than Si[1]. Yet the above integration still suffers from the growth of the unstable GeOxbetween Ge and high-kdielectric, leading to a poor interface quality and low mobility of Ge MOSFETs[2,3]Therefore, much efforts were performed to find suitable passivation methods to improve the interfacial quality between high-kdielectric and Ge,eg, nitrogen incorporation, fluorine passivation, Si passivation,Al2O3passivation, Ta2O5passivation, La2O3passivation,Y2O3passivation[4-10]. Among these methods, the La/Y oxides are a promising candidate as interfacial passivation layer (IPL), due to their strong reaction with Ge to form a stable germinate, thus giving a good interface quality between Ge and high-kdielectric. Due to the similar chemical property between Nd and Y/La, the Nd oxide could be considered as an effective IPL with the weaker hygroscopicity and lower cost advantages. Moreover, it has been showed that the hygroscopicity of the NdON could be further improved by the Al incorporation, and the nitrogen incorporation in the IPLs could further improve the interfacial quality due to the suppression of the unstable GeOxgrowth,thus obtaining excellent interfacial quality[4,11]. So in this paper, the Ge MOS capacitors were fabricated with HfO2as the gate dielectric and AlON, NdON, and NdALON as the IPLs, respectively, and their electrical and interfacial properties were measured and compared.

2 Experimental

The NdON, AlON, and NdAlON samples with HfO2/NdN, HfO2/AlN, and HfO2/NdAlN gate stack were fabricated as the flowing method. First, the thin 2 nm IPLs of NdN, AlN, and NdAlN were deposited on the n-type (100) Ge wafers, by respectively sputtering of Nd target, Al target and co-sputtering Nd and Al targets in N2+Ar ambient. Then HfO2dielectric (8 nm)was sputtered on the IPLs by using the HfO2target in Ar ambient. A control sample of a 10 nm HfO2dielectric without any IPL was also fabricated for the comparison reason. Next, all the samples were annealed in N2+O2ambient for 5 min at 500 ℃, which reduced the oxygen vacancies in HfO2dielectric, and converted the IPLs into the oxynitrides of NdON, AlON and NdAlON. For electrical measurement, the circular Al gate electrodes with an area of 7.85×10-5cm2were fabricated on the gate dielectrics. At last, all the samples were annealed in the forming gas ambient at 300 ℃ for 20 min, as for lowing the contact resistance.

TheC-VandJg-Vgwere measured by HP4284A precision LCR meter and HP4156A semiconductor parameter analyzer. The reliability of Ge MOS was examined in terms of gate leakage increase after highfield stress applied to the gate stacked dielectrics by HP4156A. The gate stacked dielectric thickness was measured by the multi-wavelength elipsometer. The chemical states and interfacial quality of the high-k/Ge interface were investigated by XPS.

3 Results and discussion

In order to eliminate the charging effect, the control sample’s Ge 3dXPS spectrum was calibrated by fixing Ge 3dpeak of the Ge substrate at a binding energy (BE) of 28.8 eV, and shown in Fig.1[12]. There is an obvious peak at a BE of 31.86 eV in Fig.1, which is possibly attributed to HfGeOxformation during the PDA[13]. Additionally, the GeOxand GeO2peaks also occur at the BE of 30.6 eV and 32.4 eV, respectively (in Fig.1).

Fig.1 Ge 3d XPS spectra of the control sample

In order to investigate the roles of the above IPLs in suppressing the defective GeOxand HfGeOxgrowth, the Ge 3dXPS spectra of the four samples were measured and are depicted in Fig.2. A positive peak shift of 3.2 eV is observed for the control sample relative to the peak of the Ge substrate, which is due to the formation of the HfGeOxand Ge oxides between the HfON and the Ge substrate, as mentioned above. For the AlON sample, the same peak shift as the control sample is also observed, indicating the formation of the HfGeOxdue to an inter-diffusion of Hf and Ge through the AlON IPL. Comparing to the control sample, the peak-area ratio of the HfGeOxand GeO2to the Ge substrate is smaller for the AlON sample, as shown in Fig.2, suggesting a weak blocking role against interdiffusion of the elements for the AlON IPL. On the other hand, a germanate peak is observed for the NdON and NdAlON samples, as identified by a positive peak shift of ΔBE=2.1-2.2 eV relative to the Ge substrate, which should be the NdGeOxfrom[14,15].Compared with the control and AlON samples, the NdON and NdAlON samples have a smaller BE shift between the germanate and Ge-sub peaks, which is due to the lower electronegativity of Nd (1.14) than Hf (1.3). The NdGeOxformation can be explained from the Dimoulas’s theory: Ge is oxidized to form the GeOxdue to the oxygen in-diffusion, and then the GeOxmixes with Nd2O3to form the NdGeOx[15]. Since the Nd atoms are highly electropositive and the Nd2O3has lower concentration of the oxygen atoms (VO, 38.6 atom/nm3) than the GeO2(41.5 atom/nm3), the Nd atoms tend to attract the O atoms in the GeO2to form the NdGeOxinterfacial layer, suppressing the defective GeO2and HfGeOxgrowth and thus demonstrating a good passivation effect of NdON and NdAlON IPLs.

Fig.2 Ge 3d XPS spectra for all the samples

The Nd 3dXPS spectra of the NdON and NdAlON samples are shown in Fig.3. The NdAlON sample has a BE of 983.18 eV for the Nd 3d5/2peak,which shifts to higher binding energy (0.7 eV) relative to the Nd2O3peak, suggesting the formation of the NdOxNy[16,17]. As compared to the NdAlON sample,the NdON sample has a 0.58 eV positive shift for the Nd 3d5/2peak, suggesting the formation of the Nd hydroxides or Nd-OH, which has a higher BE than the Nd oxides[18]. With the Al incorporation in the NdON to form the NdAlON IPL, the moisture resistance is improved with the suppressed Nd hydroxides or Nd-OH, and thus the binding energy of the Nd 3d(Nd 3d5/2and Nd 3d3/2peaks) is decreased for the NdAlON sample, as shown in Fig.3. So, it can be concluded that the NdAlON IPL has a better passivation effect than the NdON IPL.

Fig.3 Nd 3d XPS spectra of (a)NdON and (b)NdAlON samples

The typical 1 MHz high-frequencyC-Vcurves and the gate dielectric-surface morphology of the four samples are depicted in Fig.4. It is observed that the control sample without any IPL has the largest stretch out in the depletion region, indicating poorC-Vbehavior and interface quality, ascribed to the growth of the unstable GeOxand defective HfGeOx,as discussed above. With the IPLs insertion, theC-Vcurves of the AlON, NdON, and NdAlON samples exhibit a sharper transition from the depletion to the accumulation, implying a better interface quality than the control sample. The AlON sample has the smallest accumulation capacitance per unit area (Cox), which could be attributed to lower permittivity of Al2O3(9).While the NdON and NdAlON samples have larger accumulation capacitances than that of the control sample, which is due to an effective suppression growth of the low-kGeOx, as mentioned above. The largest accumulation capacitance is achieved for the NdAlON sample, which is ascribed to its smooth dielectric morphology as shown in Fig.4(b) and improved hydroscopicity by the Al incorporation, thus suppressing the low-kinterfacial layer and forming a robust interface[11]. From Fig.4(b), it is observed that the gate dielectric surface roughness is improved for the samples with the IPL insertion. The NdON sample has a larger rms (0.33 nm) than that of the AlON and NdAlON samples (0.26 nm and 0.22 nm, respectively),which is possibly due to the growth of the Nd hydroxides or Nd-OH[18,19].

Fig.4 (a)The high-frequency C-V curves; (b)The gate dielec-tric surface morphology for all the samples

Table 1 lists the extracted electrical and physical parameters of the four samples from their 1 MHzC-Vcurves. The interface-state density (Dit) is extracted by Terman’s method for the comparison purpose[20]. The capacitance equivalent thickness (CET) is calculated byCET=k0kSiO2/Cox, where,k0andkSiO2are the vacuum permittivity and the dielectric constant of SiO2,respectively. The equivalent dielectric constant (keq) of the gate-stacked dielectric is calculated askeq=kSiO2×tdie/CET, where,tdieis the physical thickness of the gate stacked dielectric. The control sample has smallerCoxand lowerkeqvalue than those of the NdAlON and NdON samples, which should be attributed to its thicker low-kGeOx. Among the four samples, the NdAlON sample has the smallest CET and the highestkeqvalue,which is attributed for an effective suppression of the Nd-hydroxides and low-kGeOx. The equivalent oxide chargeQoxcan be calculated asQox=-Cox(Vfb-φms)/q, where, the flatband voltageVfbis determined from the flatband capacitance and φmsis the work function difference between Al electrode and Ge substrate.The negativeQoxof the control sample is possibly attributed to the oxygen vacancies in the gate dielectric of HfO2, which would trap the defects induced by the Ge atoms, leading to an intermixing between HfO2and Ge[21]. While the intermixing between HfO2and Ge is effectively suppressed for the NdON and NdAlON samples, due to the blocking role of their IPLs against the Ge, O, and Hf diffusions, thus giving smallQoxandVfb. The smallerDitfor the AlON, NdON, and NdAlON samples than the control sample could be explained by the suppression of the unstable GeOxand defective HfGeOxgrowth. However, theDitof the NdON sample is larger than that of the AlON and NdAlON samples,which is possibly due to the unstable Nd-hydroxide growth. Compared to the NdAlON sample, the AlON sample has largerDit, which is due to its insufficient passivation on the Ge surface, as mentioned above.Among all the samples, the NdAlON sample exhibits the smallestDit(9.6×1011cm-2eV-1), attributed to its more effective passivation than AlON and its improved hydroscopicity relative to NdON[11,16].

Table 1 Extracted parameters from the C-V curves for the four samples

The above results would be further supported by the frequency dispersion of theC-Vcurves for all the samples in Fig.5. The control sample has the largest frequency dispersion from the depletion to the accumulation, indicating the poorest interface quality, which would be associated with the growth of its defective HfGeOxand GeOx. With the insertion of the IPLs, theC-Vfrequency dispersion becomes smaller, indicating the improved interfacial properties.As compared to the AlON sample, the NdON sample has larger frequency dispersion, possibly due to its higher interface-state density originated from the Nd hydroxides. The smallest frequency dispersion of the NdAlON sample shows its best interface quality, which is ascribed to the Al incorporation to greatly improve the hydroscopicity of the NdAlON IPL.

Fig.5 Frequency dispersion of C-V curves for all the samples: (a)Control sample, (b)NdON sample, (c)AlON sample, and (d)NdAlON sample

Further, a high-field stressing of 7 MV/cm was applied to all samples’ gate dielectrics for 60 minutes to evaluate the reliability of the devices. The gate leakage current density (Jg) was measured before and after the stressing and depicted in Fig.6. The interface and near-interface traps is generated during the highfield stressing and theJgis increased after stressing[22].Among all the samples, the NdAlON sample has the highest reliability with the smallestJgincrease(1.6×10-5A/cm2atVfb+1V), further indicating its best quality of high-k/Ge interface.

Fig.6 Gate leakage current density vs. gate voltage both before and after high-field stressing (7 MV/cm) for the four samples

4 Conclusions

The Ge MOS capacitors were fabricated with HfO2as the gate dielectric without any IPL and with AlON, NdON, and NdAlON as the IPLs, respectively,and their electrical properties were measured and compared. The AlON and NdON IPLs show a poor passivation effect with the unstable HfGeOxNygrowth or the Nd hydroxide formation, and the NdAlON IPL exhibits an excellent passivation effect with the formation of the stable NdGeOxNyand greatly improved hydroscopicity by the Al incorporation. As compared with the other samples, the NdAlON sample exhibits the superior interfacial and electrical properties with low interface-state density (9.6×1011cm-2eV-1),small flatband voltage (0.35 V), highkvalue (19.1),low gate leakage current, smallC-Vdispersion and high device reliability. Therefore, it can be suggested that the NdAlON is a potential IPL for the preparation of the high-quality Ge MOS devices.