Two promising high-mobility substrate materials, Ge and GaAs, are investigated in this dissertation. We devoted our efforts to the effects of wet-chemical clean, surface passivation, and thermal processing on the physical and electrical properties of various alternative high-k gate dielectrics on these two substrates. This dissertation is divided into seven chapters and organized as follows:
In Chapter 1, a brief overview of the background and motivation is described. We reviewed the MOSFET scaling roadmap and the possible challenges emerged in the nanoscale devices. Next, we discussed the significance of high mobility substrates in further application of the advanced CMOS technologies; meanwhile, the process difficulties and material issues in the accomplishment of the Ge and GaAs III-V devices were brought up.
In Chapter 2, we performed the physical and electrical analyses to systematically examine the HfOxNy thin films sputtered on Ge substrates and the admittance properties of their MIS capacitors. Both the NH3 plasma treatment and Si2H6 thermal annealing are proposed in an attempt to improve the properties of HfOxNy high-k films on Ge substrates.
We observed that not only the out-diffusion of Ge substrate and charge trapping phenomenon of entire HfOxNy/Ge gate stack are relieved, but also the thermal stability is enhanced after these two passivation methods.
In Chapter 3, we investigated the structural and electrical properties of Al2O3 thin films
grown through ALD system on Ge substrates. It was observed that variation of the substrate temperature strongly influenced the Al2O3 film qualities on Ge, including the density, stoichiometry, and the degree of the Al2O3-GeO2 intermixing. Such a temperature effect on the interface, capacitor, and gate leakage characteristics of the Al2O3/Ge structures is examined as well. Furthermore, we explored the minority-carrier response behavior of Ge MOS capacitors through MEDICI simulations and device experiments, and further compare their electrical differences with those in traditional Si MOS capacitors.
In Chapter 4, we modified the wet-chemical clean and (NH4)2S sulfidizing treatment on GaAs substrate and characterized the effects of the surface modification on the interfacial and electrical improvements of the e-gun evaporated Gd2O3/GaAs MOS capacitors. In addition, we further compared the gate leakage characteristics of the fabricated Gd2O3/GaAs structures to the reported performance of various high-k dielectric materials on GaAs substrates.
In Chapter 5, the deposition of ALD-Al2O3 gate dielectrics on GaAs substrates was studied. Next, the impact of interfacial (NH4)2S treatment integrating with different sulfidizing solvents was investigated through analyses of the surface chemistry and capacitor characteristics of the Al2O3/GaAs structure. In the following, we performed the post-deposition annealing in O2 and N2 ambient to understand the changes of the structural and electrical properties; the differences were clarified in terms of identifying the underlying thermochemical mechanisms.
In Chapter 6, both the inversion-mode Ge p- and n-MOSFETs and the depletion-mode GaAs n-MOSFET with the ALD-Al2O3 gate dielectrics were fabricated. Herein, we studied effects of the forming gas annealing on the Ge junction and device properties; the correlation between the n-type dopant activation and the origins of n+p junction leakage and source/drain resistance is also analyzed in more details. On the other hand, the transfer and output characteristics of ALD-Al2O3/GaAs n-MOSFET were demonstrated and we employed the (NH4)2S-C4H9OH chemical passivation to improve the GaAs device performance further.
In Chapter 7, we summarized the experimental results in the thesis and gave the conclusions and the suggestions for future studies.
References (Chapter 1)
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Table. 1.1 Material properties of Si, Ge, GaAs, and InAs at 300 K.
Fig. 1.1 Transistor scaling and research roadmap demonstrated by R. Chau, Intel Corp.
Fig. 1.2 Scheme of device issues in the nanoscale MOSFETs.
Chapter 2
Sputtered HfO
xN
yDielectric Films on Bulk Ge Substrates
2.1 Introduction
Recently, Ge-channel devices—including bulk Ge [1], [2], strained Ge [3], and Ge-on-insulator (GOI) [4] systems—integrated with high-k gate dielectrics have attracted considerable research interest. Although transistors were originally fabricated on Ge substrates, the lack of a stable Ge native oxide has been an obstacle in complementary metal-oxide-semiconductor (CMOS) device realization with Ge. Therefore, silicon has been used in CMOS technology for many decades because of the better qualities of its native oxide, such as a low leakage current, low interface state density, and good thermal stability. With the further scaling of device and gate oxide dimensions down to the nanometer range, however, the leakage current density in SiO2 has become much higher than 2 mA/cm2, which is the maximum concession for low power applications [5]. Consequently, higher dielectric-constant materials with a thicker physical thickness are introduced to suppress the concern of excessive gate leakage while maintaining the equivalent-oxide-thickness (EOT) of the scaled devices.
Presently, hafnium-based oxides or oxynitrides, e.g., HfO2, HfON, and HfSiON, are the uppermost candidates for application among all of the potential high-k dielectrics. Both Si and SiGe MOSFETs integrated with Hf-based gate dielectrics exhibit admirable properties [6]-[8], but they also reveal undesirable surface carrier mobility degradation behavior [9], [10].
Changing the substrate from Si to Ge might be a possible solution to this problem because Ge has a higher carrier mobility relative to that of Si. From recent advances in the deposition of high-k materials, Ge MOSFETs incorporating high-k gate dielectrics have exhibited some promising performance [11], [12]. In this session, we investigated the physical and electrical
characteristics of HfOxNy sputtered films on bulk Ge substrates and then determined the impact of thermal annealing processing on the entire capacitor structure. Meanwhile, recent reports have described that annealing a cleaned Ge substrate in a NH3 [13] or SiH4 [14] gas ambient, prior to deposition of a high-k dielectric, further improves the MOS properties on Ge.
Here, we proposed the NH3 plasma pretreatment to passivate a Ge substrate and investigated the passivation efficiency. Actually, the overall MOS structures had higher thermal stability and showed the improved electrical characteristics. On the other hand, we also speculate that the incorporation of nitrogen atoms may lead to incomplete passivation of the dangling bonds on the Ge surface; such a surface would not fully inhibit the growth of GeOx because of the lower thermal stability of Ge–N bonds [15]. Therefore, we also attempted to adopt the Si2H6 passivation onto Ge surfaces, in which several monolayers of Si exist between the gate dielectric and Ge substrate, for inhibiting the formation of GeOx and suppressing hysteresis phenomena in high-k/Ge MOS capacitors; we also presented an energy band diagram to explain the charge trapping model.