Figure 5.14 shows the Ga 2p3/2 and As 2p3/2 photoemission spectra of GaAs substrates subjected to the (NH4)2S-C4H9OH sulfidizing treatments with different concentrations and temperatures, respectively. As seen, the deconvoluted peaks of Ga-S, Ga2O, and Ga2O3 were observed in Ga 2p3/2 spectra at the respective binding energy of 0.65, 1.0, and 1.6 eV above the GaAs substrate. While the As 2p3/2 spectra also presented the signals of As-As, As-S, As2O3, and As2O5 at the respective binding energy of 1.0, 1.7, 2.95, and 3.95 eV above the GaAs substrate. Table 5.1 summarizes the signal ratios of these surface chemical species on GaAs. It was found that the NH4OH rinse decreased the content of As-As species and native oxides obviously, as compared to the results presented in the DIW-rinsed sample. Subsequent immersion of (NH4)2S-based solution can make the amount of As-related species reduce continuously. Here, we are of more interest in the surface modification of GaAs by replacing H2O with C4H9OH as sulfidizing solvent; evidently, not only the removal efficiency of AsOx
species accelerated but also more sulfur ions bonded to GaAs with either raising the concentration to 10% and/or the temperature to 60 °C. These observations can be attributed to the stronger electrostatic interaction between the sulfur ion and the GaAs surface by decreasing the solvent dielectric constant ε from 80.1 (H2O) to 12.5 (C4H9OH). Even if V. N.
Bessolov et al. [17] brought up that the change of the alcoholic solvents in (NH4)2S solution, relative to that in Na2S solution, had a minor enhancement on the sulfur passivation on GaAs,
a higher degree of the sulfur coverage was actually observed from XPS analysis as a result of the increased chemical activity in sulfidized solution. As compared to the results in the RT case, a remarkable feature was the amount of As-As species that increased slightly from 8.4%
to 10.9% at higher temperature of 60 °C. Such an increase in the elemental As on GaAs surface did lead to severe leakage degradation of the ALD-Al2O3 deposited film (not shown here). In general, the resultant suppression of GaAs defects is able to boost the interface and film qualities of the deposited insulator. As the AFM images displayed in Fig. 5.15, the flatter morphology of GaAs surface was observed with the help of the sulfidized immersion, giving the evidence of the interface improvement. The root-mean-square (RMS) roughness of NH4OH-cleaned GaAs surface was ca. 0.28 nm and then reduced to ca. 0.22 nm by rinsing in (NH4)2S-C4H9OH solution (10%) at 60 °C.
Figure 5.16 presents the TEM image of as-deposited Pt/ALD-Al2O3/GaAs capacitor, in which the thickness of Al2O3 film is ca. 62 Å for 60 deposition cycles. We did not observe the existence of an obvious IL between the Al2O3 and the GaAs, which should be correlated to the self-cleaning of GaAs native oxides at the initial stage of ALD deposition [44], [45]. Such behavior of the bonding replacement between ALD metal precursor and As2Ox(Ga2Ox) has been also manifested in the recent studies of ALD-HfO2/GaAs growth [46], [47].
5.6.2 Capacitor and Gate Leakage Characteristics
Furthermore, the impact of the sulfidizing solvent and PDA on the capacitor characteristics was examined. As the frequency–dependent C-V curves shown in Figs. 5.17(a) and 5.17(b), the alcoholic-(NH4)2S solution increased the value of accumulation capacitance, and subsequent O2 PDA improved the C-V dispersion further. For the NH4OH-cleaned GaAs capacitor, the capacitance-equivalent-thickness (CET) extracted at 1 kHz was ca. 3.78 nm and its values were reduced to ca. 3.54 and 3.75 nm for the sulfidized GaAs samples before and
after 600 °C thermal annealing, respectively. We qualitatively inspected the improvement of capacitor characteristics through variations of the △C and △V that were defined as follows:
△C(@Vg = 4 V) = 1 - Cacc(@100 kHz) / Cacc(@1 kHz) (5.1)
△V(@CFB) = Vg(@1 kHz) - Vg(@100 kHz), (5.2) where △C is the deviation of the accumulation capacitance measured at 1-kHz and 100-kHz for the Vg of 4 V, while △V is the voltage difference between 1-kHz and 100-kHz C-V curves achieving the value of the flatband capacitance (CFB). As the results plotted in Fig. 5.17(c), both values of △C and △V showed the decrement with the C4H9OH solvent used, an increased sulfidizing concentration, and in particular the implement of post thermal annealing.
The lowest values of △C and △V obtained were 13.6% and 0.69 V, respectively, by performing 10% (NH4)2S-C4H9OH interfacial treatment with 600 °C O2 PDA; a corresponding Dit value of 5 × 1012 cm-2eV-1 was evaluated.
Figure 5.18(a) displays the Jg characteristics and the results indicate that the sulfur passivation is effective to suppress the dielectric leakage current and the alcoholic-(NH4)2S solution can boost the thermal stability further. The treatment of 10% (NH4)2S-C4H9OH apparently led to the better leakage performance that the respective values of Jg at Vg = (VFB + 1) V were ca. 7 × 10-7 and 1 × 10-5 A/cm2 before and after 600 °C PDA in O2 ambient. By plotting the Jg versus CET, as shown in Fig. 5.18(b), the fabricated ALD-Al2O3/GaAs capacitors exhibited the comparable Jg characteristics to the previous studies of the sputtered HfxSi1-xO/n-GaAs [2] and HfOxNy/p-GaAs [48] structures, respectively. However, the capacitor performance was inferior than the results of ALD-Al2O3 [49], -HfAlO [50], -HfO2 [6]
films and metal-organic chemical vapor deposition HfO2 film [51] on GaAs substrates from other research groups, indicating that the film quality of the deposited Al2O3 itself still needs the growth optimization.
5.7 Conclusions
In Chapter 5, the ALD-Al2O3 films deposited on GaAs substrate at 300°C relative to that at 100°C showed the nearly four orders of magnitude reduction in gate leakage current at the capacitance-equivalent-thickness of 40 Å. Next, we examined the interfacial chemistry of the Al2O3/GaAs and the impact of sulfidization and thermal annealing on the properties of the resultant capacitor. It was observed that sulfidized passivation actually improved the effect of the EF pinning on the electrical characteristics, thereby providing a higher oxide capacitance, smaller frequency dispersion, and reduced surface states, as well as decreased interfacial charge trapping and gate leakage current. Photoemission analysis indicated that the (NH4)2S-treated GaAs improved the quality of the as-deposited Al2O3 thin film and preserved the stoichiometry of the dielectric during subsequent high-temperature annealing. This behavior was closely correlated to the diminution of GaAs native oxides and elemental-As defects and their unwanted diffusion. In addition, thermal processing under an O2 atmosphere, relative to that under N2, decreased the thickness of the Al2O3 gate dielectric and relieved the Jg degradation induced by metallic arsenic; as a result, superior dielectric reliability was attained. We discussed the underlying thermochemical reactions that account for these experimental observations. Furthermore, it was also found that degreasing GaAs into (NH4)2S solution with C4H9OH used as solvent clearly reduced the amount of As-As and AsOx surface species and formed more sulfur bondings to GaAs; the flatter surface morphology was also characterized. A higher electrical improvement was indeed presented by replacing H2O with C4H9OH as sulfidizing solvent, again thanks to the elimination of more As-related defects as well as the higher sulfur coverage on GaAs. It is believed that the (NH4)2S-C4H9OH treatment can be adopted on the InGaAs/InSb substrates or integrates with other passivation methods to provide the better surface quality for realizing high-performance high-k/III-V devices.
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Table. 5.1 Chemical ratios of As 2p3/2 and Ga 2p3/2 spectra for the GaAs substrate after undergoing different wet-chemical procedures. Note that the surface of all samples has been exposed to air for ca. 10 min prior to chemical analysis.
Table. 5.2 Calculated relative concentrations of GaAs oxides and stoichiometric O-to-Al ratios in as-deposited ALD-Al2O3 thin films before and after chemical treatment with (NH4)2S. Note that the As 2p3/2, Ga 2p3/2, Al 2p, and O 1s core levels were used by considering the atomic sensitivity factor.
Table. 5.3 Chemical bonding ratios of As 3d and Ga 3d core levels determined by fitting the XPS data from Fig. 5.5. Note that the contribution of the O 2s emission has been excluded.
Table. 5.4 Solid state chemical reactions of Ga–As–O associated systems. All these equations are separated into four parts.
Table. 5.4 (Conti.) Solid state chemical reactions of Ga–As–O associated systems. All these equations are separated into four parts.
Table. 5.5 Calculated relative concentration of GaAs oxides and stoichiometric O-to-Al ratios in the N2- and O2-annealed ALD-Al2O3 thin films with and without (NH4)2S surface passivation.
Table. 5.6 Chemical bonding ratios of As 3d and Ga 3d core levels determined by fitting the XPS data from Fig. 5.12. Note that the contribution of the O 2s emission has been excluded.
-3 -2 -1 0 1 2 3 4 5
Fig. 5.1 (a) Multi-frequency C-V and (b) bidirectional C-V (10 kHz) curves of Pt/ALD-Al2O3/n+-GaAs capacitors with different cleaning preparation and annealing processes. The Al2O3 films were deposited at 100 °C for 60 cycles.
0 1 2 3
Fig. 5.2 Gate leakage current Jg characteristics of Pt/Al2O3/n-GaAs capacitors. The Al2O3 thin films were deposited at 100 °C for 60 cycles.
-3 -2 -1 0 1 2 3 4
Fig. 5.3 (a) C-V curves of GaAs MOS capacitors with ALD-Al2O3 deposited at 300°C for 60 cycles and (b) Comparison of Jg versus CET characteristics of ALD-Al2O3/GaAs capacitors deposited at 100 and 300 °C with different surface preparation and annealing processes.
Fig. 5.4 As 2p3/2 and Ga 2p3/2 XPS spectra of as-deposited ALD-Al2O3 thin films on GaAs substrates without and with (NH4)2S sulfide passivation.
Fig. 5.5 As 3d and Ga 3d XPS spectra of as-deposited ALD-Al2O3 thin films on GaAs substrates with and without (NH4)2S sulfide passivation.
Fig. 5.6 (a) Multi-frequency and (b) bi-directional C–V characteristics and (c) gate leakage current (I–V) curves of GaAs MOS capacitors with as-deposited Al2O3 thin films after (NH4)2S passivation and PMA at 400 °C.
Fig. 5.7 HRTEM images of Pt/Al2O3/sulfidized-GaAs structures with different deposition cycles and PDA conditions: (a) 100 cycles, N2 PDA; (b) 100 cycles, O2 PDA; (c) 60 cycles, O2 PDA. PDA temperature: 600 °C.
Fig. 5.8 Multi-frequency C–V curves of sulfidized-GaAs MOS capacitors with Al2O3 thin films annealed for (a) 100 and (b) 60 deposition cycles. Each inset displays the respective C–V hysteresis measured at 100 kHz.
Fig. 5.9 I–V Characteristics of the MOS capacitors analyzed in Fig. 5.8. Inset: Plot of Jg (at Vg = 2 V) versus CET.
Fig. 5.10 Stress time dependence of the gate leakage Jg for Pt/ALD-Al2O3/GaAs MOS capacitors under a constant Vg stress of 5 V (at the left y-axis) and a constant Eox stress of 8.5 MV cm–1 (at the right y-axis), respectively.
Fig. 5.11 As 2p3/2 and Ga 2p3/2 XPS spectra of 600 °C-annealed Al2O3 thin films on the sulfidized GaAs substrate. The contributing chemical components were extracted and are displayed in the respective spectra.
Fig. 5.12 As 3d and Ga 3d XPS spectra of 600 °C-annealed Al2O3 thin films on the sulfidized GaAs substrate. The contributed chemical components were extracted and are displayed in the respective spectra.
Fig. 5.13 SIMS depth profiles of the Al2O3/GaAs samples analyzed in Fig. 5.12. The diffusion of As- and Ga-related chemical species into overlying high-k film was observed.
1122 1120 1118 1116 1114
Fig. 5.14 Ga 2p3/2 and As 2p3/2 XPS spectra of GaAs substrate subjected to different wet-chemical procedures. Four components were extracted in Ga 2p3/2 spectra:
GaAs, GaSx, Ga2O, and Ga2O3, while five components were extracted in As 2p3/2 spectra: GaAs, elemental As, AsSx, As2O3, and As2O5.
Fig. 5.15 AFM images of the NH4OH-cleaned GaAs substrates (a) before and (b) after receiving (NH4)2S-C4H9OH(10%, 60 °C) chemical treatment, respectively.
Fig. 5.16 TEM image of the as-deposited Pt/ALD-Al2O3(60 deposition cycles)/GaAs MOS structure with (NH4)2S-C4H9OH(10%, RT) chemical treatment.
Fig. 5.17 Multi-frequency C–V curves of Pt/Al2O3/n-GaAs capacitors (a) with different wet-chemical procedures; (b) before and after 600 °C PDA. (c) Variations of the frequency response of △C and △V values.
0 1 2 3
Fig. 5.18 (a) The I–V characteristics of Pt/Al2O3/n-GaAs capacitors with various sulfidizing conditions (solid symbols) before and (open symbols) after 600 °C PDA in O2 ambient, respectively. (b) Comparison of Jg versus CET or EOT characteristics for our work with other’s published data [2], [6], [48]-[51].
Chapter 6
Inversion-Mode Ge p- and n-MOSFETs and Depletion-Mode GaAs n-MOSFET with
Atomic-Layer-Deposited Al
2O
3Gate Dielectrics
6.1 Introduction
When Si complementary metal-oxide-semiconductor (CMOS) technologies gradually approximate to 22 nm node, the field of high mobility semiconductor materials is renewed in MOS field effect transistor (MOSFET) applications to pursue much higher device performance. In particular, Ge- and III-V-based channels featuring various prevailing gate dielectrics are promising structures to use in place of conventional Si MOSFETs.
Nevertheless, several formidable challenges remain if we are to realize state-of-the-art Ge devices. The first bottleneck is the water-solubility and poor thermal stability of its native oxide. Even though GeO2–Ge interfaces can be prepared with excellent quality [1], subsequent thermal processing is likely to degrade the gate stack integrity. Fortunately, recent developments in high-k material deposition and surface pretreatment methods—e.g., thermal annealing in SiH4 ambient [2] and atomic N radical plasma [3]—have expanded the possibilities in this field. Other key obstacles are the smaller band gap of Ge and the higher intrinsic carrier concentration (ca. 1013 cm–3; cf. ca. 1010 cm–3 in Si), which lead to larger junction leakage currents [4]. In addition, because a higher thermal budget is required during n-type dopant activation, more rapid dopant diffusion (either out of the surface or into the substrate) and lower electrical activation are generally observed. A shallow source/drain (S/D)
junction possessing an acceptable off-state current, within the range from 10–4 to 10–7 A cm–2 [5], will be required if we are to achieve higher device performance.
Although superior high-k/Ge p-FET characteristics have been reported recently [6], [7], the fabrication of promising Ge n-FETs remains challenging because of the resulting low electron mobility. Such n-FET degradation behavior can be explained in terms of the
Although superior high-k/Ge p-FET characteristics have been reported recently [6], [7], the fabrication of promising Ge n-FETs remains challenging because of the resulting low electron mobility. Such n-FET degradation behavior can be explained in terms of the