Germanium (Ge) has attracted much interest in terms of application to high mobility channel devices due to its more symmetric and higher carrier mobilities than Si, which could enable us to boost the driving current of transistors. Recently, high electron mobility in Ge n-MOSFETs exceeding the Si universal mobility have been reported ^[1-6], and the key technology for high performance Ge CMOS is how to effectively improve the Ge gate stacks. It is found that interface passivation of near the conduction band edge is more problematic, which limits the electron mobility in Ge n-MOSFETs. Both high electron and hole mobilities in Ge MOSFETs have only been achieved in Ge/GeO₂-based gate stacks ^[5-8], and GeO₂ has received a great deal of attention as an interfacial layer of Ge gate stack. It has been reported that the degradation of electrical properties of Ge/GeO₂-based gate stacks is attributed to the GeO desorption at the Ge/GeO₂ interface ^[9]. After that, extensive research efforts for Ge interface passivation have been focused on the mild oxidation. Several oxygen passivation methods of Ge/GeO₂ interface for reducing the interface trap density (D_it) have been successfully demonstrated ^[10-15], where the relatively low temperature (below 450^oC) is employed to avoid GeO desorption. However, since the stoichiometric oxide layer is generally formed by high temperature oxidation, these approaches raise the question of whether GeO₂ bulk properties and reliability are good enough to meet the requirements of electronic device.

On the basis of thermodynamic and kinetic consideration of Ge-O system, the high-pressure oxidation (HPO) on Ge was proposed to suppress the GeO desorption during the thermal oxidation and significant improvements of Ge/GeO₂-based gate stacks have been achieved ^[16]. In this paper, the advantage of HPO on the formation of Ge/GeO₂ stack is reviewed in terms of Ge/GeO₂ interface and GeO₂ bulk properties.

To study the electrical properties of Ge/GeO₂ stacks, MOSCAPs with Ge/GeO₂ stacks were fabricated. Ge (100) wafers were chemically cleaned using methanol, HCl, H₂O₂+NH₄, and dilute HF solution in sequence before the thermal oxidation, in which HPO was carried out in a specialized furnace in a wide range of temperature and O₂ partial pressure (P_O2). Note that P_O2 in this work is defined at room temperature before increasing the furnace temperature, and that the actual O₂ pressure in the furnace during the oxidation should be higher than the P_O2. After thermal oxidation, Au and Al were deposited by vacuum evaporation for the gate electrode and substrate contact of MOSCAPs, respectively.

Figure 1(a) shows bidirectional capacitance-voltage (C-V) characteristics of p-type MOSCAPs with Ge/GeO₂ stack grown by atmospheric-pressure oxidation (APO, P_O2 = 1 atm) at 550^oC, where a large hysteresis and frequency dispersion are observed. It is inferred that a huge amount of interface and bulk traps are generated during a conventional thermal oxidation. Figure 1(b) shows a schematic of Ge/GeO₂ stack grown by APO. Under the APO condition, the Ge oxidation and GeO desorption occur simultaneously, resulting in the interface and bulk defects in Ge/GeO₂ stack. The thermodynamic consideration of Ge thermal oxidation is discussed in more detail ^[16]. Figure 2 shows bidirectional C-V characteristics of p-type MOSCAPs with Ge/GeO₂ stack grown by HPO (P_O2 = 70 atm) at 550^oC, in which nearly no hysteresis and frequency dispersion are observed. The D_it at the Ge/GeO₂ interface formed by HPO was estimated by low-temperature conductance method at 200 K and 100 K to avoid the minority carrier response. The minimum value of D_it near the mid gap is about 2e11 eV^-1cm^-2 without any post oxidation treatment, indicating that the high-quality Ge/GeO₂ stack is formed by suppressing the GeO desorption at the Ge/GeO₂ interface.

Alternatively, GeO desorption can be suppressed by lowering the oxidation temperature since the equilibrium vapor pressure of GeO at the Ge/GeO₂ interface is thermodynamically fixed independent of P_O2 at a given temperature due to Gibb’s phase rule ^[17]. The vapor pressure of GeO can be reduced by three orders with a decrease in the annealing temperature from 550^oC to 400^oC. To investigate the efficacy of suppressing GeO desorption at the interface, the post oxidation annealing in O₂ ambient at 400^oC for Ge/GeO₂ stack grown by APO (550^oC, P_O2 = 1 atm) is carried out.

Figure 3(a) shows the hysteresis of C-V characteristics as a function of post oxidation annealing time, where the hysteresis is extracted at flat-band voltage. It shows a significant reduction of hysteresis in Ge/GeO₂ stack grown by APO, suggesting that Ge/GeO₂ interface is improved by post oxidation anneal. The bidirectional C-V characteristics of n-type MOSCAPs with Ge/GeO₂ stack grown by APO, followed by post oxidation anneal for 60 min are shown in Figure 3(b). A very small hysteresis and frequency dispersion indicate that the superior Ge/GeO₂ interface can be achieved by optimizing the post oxidation annealing even if interface defects are generated by APO. A low temperature conductance method was used to quantitatively estimate the D_it at the Ge/GeO₂ interface, as shown in Figure 4, where measurements were carried out in the temperature range of 250 K to 100 K to evaluate a wide range of D_it spectrum in the energy band gap of Ge. It is shown that the minimum value of D_it near the mid gap is below 1e11 eV^-1cm^-2, which is close to the Ge/GeO₂ interface formed by HPO. Thus, it can be inferred that post oxidation annealing can effectively passivate the dangling bonds at the Ge/GeO₂ interface generated by GeO desorption during APO. It is worthy to note that post oxidation annealing temperature should be around 400^oC to avoid GeO desorption at the interface.

In order to investigate the GeO₂ bulk properties in the Ge/GeO₂ stack, the same thickness of GeO₂ film was thermally grown on Ge (100) wafers by APO and HPO, respectively. For the interface passivation of Ge/GeO₂ stacks, the post oxidation annealing at 400^oC for 60 min was carried out for both HPO-grown and APO-grown Ge/GeO₂ stacks. Since Ge/GeO₂ interface properties are superior enough and similar to both HPO-grown and APO-grown Ge/GeO₂ stacks with the post oxidation annealing, the influence of Ge/GeO₂ interface properties can be excluded from the discussion of GeO₂ bulk properties. The conduction band offset (Φ_B) is one of the most important factors as the gate insulator to suppress the gate leakage current. In order to extract and compare the Φ_B between GeO₂ and Ge substrate, both n-type MOSCAPs with APO-grown GeO₂ and HPO-grown GeO₂ are investigated. Figure 5(a) shows a plot of ln(J_FN/E²) versus (1/E), where J_FN is the current density and E is the electric field of GeO₂, known as the Fowler-Nordheim (F-N) plot. The F-N tunneling current density J_FN can be expressed as follows ^[18]:

,

where and . Here, h is the Planck constant, q is the electron charge, E_ox is the electric field in the GeO₂, Φ_B is the barrier height, m is the free electron mass, and m_ox is the effective electron mass in the GeO₂. Since m_ox is an unknown factor, m_ox is assumed to be 0.42m, which is generally accepted as the effective electron mass in the SiO₂. If the F-N plot is linear, the intercept of this F-N plot gives A and the slope yields B. The calculated Φ_B from above equations is 1.5 eV for HPO-grown GeO₂ on Ge and 1.2 eV for APO-grown GeO₂ on Ge, respectively. The value of Φ_B for Ge/GeO₂ stack with APO is well consistent with other report ^[19], where GeO₂ was thermally oxidized at 550^oC in 1-atm O₂ ambient. Figure 5(b) shows the calculated Φ_B from F-N plot as a function of m_ox . It is expected that the Φ_B from Ge/GeO₂ stack grown by HPO is about 0.3 eV higher than that of Ge/GeO₂ stack grown by APO, regardless of m_ox . This difference of Φ_B in the Ge/GeO₂ stacks might be originated from bulk defects in the GeO₂ film, which is generated by GeO desorption during the conventional thermal oxidation.

To further investigate the GeO₂ bulk properties, the GeO₂ film density was estimated by grazing incidence X-ray reflectometry. Figure 6(a) shows the GeO₂ film density as a function of P_O2 in the thermal oxidation. It clearly shows that GeO₂ film density increases with the increase of P_O2. For comparison, the density of amorphous GeO₂ film is included ^[20], which is in good agreement with APO-grown GeO₂ film. It is well known that there is a strong correlation between dielectric film density and their wet etch rate. Since GeO₂ is water soluble and readily dissolved in deionized water, we prepared the dilute deionized water solution (C₂H₅OH + H₂O) to study the wet etch rate of GeO₂ films grown by APO and HPO, respectively. Figure 6(b) shows the wet etch rate of GeO₂ film in dilute deionized water solution. As expected from the film density, HPO-grown GeO₂ film shows the slower wet etch rate (~0.6 nm/min) than APO-grown GeO₂ film (~0.75 nm/min).

Since GeO₂ film is easily etched in water, a strong influence of moisture absorption in the air on Ge/GeO₂ stack is expected. Figure 7(a) shows the hysteresis of C-V characteristics extracted at flat-band voltage as a function of the air exposure time, where Ge/GeO₂ stacks were exposed to the air before the gate electrode formation. A clear difference of electrical degradation rate between HPO-grown Ge/GeO₂ stack and APO-grown Ge/GeO₂ stack against the air exposure is observed. The effects of air exposure on electrical properties in Ge/GeO₂ stack in terms of water absorption and hydrocarbons are comprehensively discussed in the literature ^[21], which is well consistent with our results. After 5-day air exposure, the desorbed H₂O was measured by thermal desorption spectroscopy, as shown in Figure 7(b). It shows that APO-grown GeO₂ film has about 40% more H₂O molecules than HPO-grown one, indicating that HPO-grown GeO₂ film has a higher hygroscopic tolerance. From the results of GeO₂ bulk properties, it is concluded that one of the primary advantages of HPO on Ge is the improvement of GeO₂ bulk properties by suppressing GeO desorption and accelerating the oxidation process at high temperatures. Dangling bonds at the Ge/GeO₂ interface can be effectively passivated by post oxidation annealing at low temperature, while robust GeO₂ bulk properties can only be achieved by HPO. The optimization of HPO and post oxidation annealing, which can further improve both GeO₂ bulk and Ge/GeO₂ interface properties, will be the most suitable for the formation of high-quality Ge/GeO₂ stack.

The Ge thermal oxidation for the formation of high-quality Ge/GeO₂ stack has been studied. It is found that the Ge/GeO₂ interface can be improved by the post oxidation anneal at low temperatures that GeO desorption doesn’t occur. It is helpful to passivate the interface defects at the Ge/GeO₂ interface generated by the conventional thermal oxidation. However, high-quality GeO₂ bulk properties can only be achieved by high-pressure oxidation (HPO) that grows GeO₂ at high temperatures with suppressing GeO desorption. This was achieved by taking care of thermodynamic and kinetic control of the Ge/GeO₂ interface. The optimization of HPO and post oxidation annealing, which can further improve both GeO₂ bulk and Ge/GeO₂ interface properties, will be the most suitable for the formation of high-quality Ge/GeO₂ stack.