KHz MHz C C

Fig. 4-3 displays the definition of dispersion and dispersion trend of Pt/Gd2O3/GaAs MOS capacitor with sulfided conditions. It is observed that NH4OH and sulfidation(80^oC) can reduce frequency dispersion. Plotted in Fig.

4-4(a) is the 10k Hz frequency curve with different treatment, and the accumulation capacitance increases with NH₄OH etching solution because of Ga-terminated surface remain stable after post dielectric anneal (PDA).On the other hand, sulfide concentration increase 10% will decrease accumulation capacitance because of residual sulfur element on the Gd2O3/ GaAs interface.

59

Fig.4-4(b) shows the C-V curve hysteresis which place value on sulfided temperature. The resultant difference in hysteresis value was evaluated at Cfb , and 80^oC condition resulted in less hysteresis below to 50mV. In Fig. 4-5(a), we study the leakage distribution with different treatment, and the condition:

HCl+NH4OH+Sulside (1%, 80^oC) can obtain the lowest leakage current. In contrast, higher sulfide concentration (10%) and low temperature (RT) led to the larger leakage current for the Pt/Gd2O3/GaAs MOS capacitor. Fig. 4-5(b) is leakage current trend at Vg =+1V result from Fig. 4-5 (a).These differences will be discussed in depth later in combination with the material analysis. In Fig. 4-6, we studied the variation of density of state (Dit) using single-frequency method [55], and equation is shown below.

(4.3)

Where Gmax is the maximum conductance in the G-V plot with its corresponding capacitance (Cm).The Dit shows the decreasing trend with sulfided temperature and the input of NH4OH solution.

Fig. 4-7 (a)-(d) show multi-frequency C-V and hysteresis width

max

60

characteristics of Pt/Gd2O3/GaAs MOS capacitor with two best clean methods and the NH4OH concentration was chosen as 10% because of less As element compared to 1% NH4OH/D.I.W.. Their corresponding frequency dispersion is estimated by the equation (4.1) and (4.2), as shown in Fig. 4-8(a).Plotted in Fig.

4-8 (b) is the hysteresis as functions at 10k Hz frequency of the two clean way with the same sulfide condition. The two clean ways mean that they are HCl_NH4OH and NH4OH procedure. In Fig. 4-8 (c), we study the variation of density of state (Dit). This shows that NH4OH/D.I.W. (10% ) concentration only less As element, but worst electrical characteristics. According to equation below, the As-O will react with GaAs substrate, so we need to think about this important thing.

(4.4)

In Fig. 4-9(a), we study the leakage distribution with different NH4OH solution and sulfidation. In contrast, higher sulfide concentration (2%) and NH4OH/H2O2/D.I.W. led to larger leakage current, whereas result in the smaller leakage current for Gd2O3/GaAs gate stacks, especially for the case of 1%

combined with NH4OH/D.I.W. Fig. 4-9(b) is Weibull current distribution at Vg

2 3

2

4

As O + GaAs → Ga O + As

61

=+1V result from Fig. 4-9(a).The slope of fitting line is not sharp, especially in the NH4OH with Sulfide method. We find that leakage distribution result from non-uniform wet chemical clean and sulfidation.

In previous work, an improved C-V curves and I-V leakage current are observed with NH4OH solution. We find that the As element will effect electrical characteristics, and surface oxides also significantly determine properties because of reaction between oxides and GaAs substrate. Following are the electrical properties of Pt/Gd2O3/GaAs MOS capacitor with the NH4OH/D.I.W.(1%) solution, which solution can suppress smaller oxide but a little As element compared to the NH4OH/D.I.W.(10%) solution.

Three possible clean methods were prepared to explain the observed difference between the Gd2O3 /GaAs interface. Fig.4-10 plots multi-frequency C-V frequency characteristics of Pt/Gd2O3/GaAs MOS capacitor with three cleaning methods: (i) HCl (ii) HCl+NH4OH (iii) NH4OH .According to the Chaptr 2 and Chapter 3, the Ga-termined surface can obtain by NH4OH etching solution, and Ga-termined compound is thermal stable compared to HCl cleaned As-termined one. Fig. 4-11 (a) and (b) show the C-V dispersion and hysteresis of Pt/Gd2O3/GaAs MOS capacitor. Clearly, the NH4OH solution and sulfidation can reduce more dispersion and hysteresis because cleaned Ga-termined surface

62

likes to react with high temperature sulfur element. The Ga-S bonds can against the 500-600 ^oC PDA temperature. Next, from the single frequency method, we observe that the density of state on the Gd2O3/GaAs MOS capacitor in Fig. 4-11 (c). The lowest density of state by wet chemical and sulfur passivation on GaAs substrate in this work has reduce to 6x10¹² cm^-2e V^-1. According to (4-4) and (4-5) equilibrium reaction equation, leakage current result from the metal-like As element.

(4-5)

Equation (4.5) shows that the more As-O and As-S components increasing, and the right hand of (4.5) equation will produce more As element. In Fig. 4-12(a) is the leakage current with different clean methods. The NH4OH solution and sulfidation method is the best condition because of less arsenic compound component. Remarkably, the Gd2O3/GaAs with surface passivation revealed extremely low Jg range from 1e^-3 to 1e^-5 of magnitude with about CET equal to 2nm shown in Fig. 4-12(b).We find that Gd2O3 on Si and GaAs substrate locate on the same distribution line of this figure. Figure 4-13 shows the Jg distribution for annealed Gd2O3 film under moisture test. Because that rare-earth oxides will

2 3

3 3 5

As S + GaAs → GaS + As

63

absorb moisture contamination, and gate dielectric will degrade with time in the electrical characteristic behaviors. According to the leakage current test, in our work, the Gd2O3 film also keeps the same leakage current after 1week or 1 month. So we think that the Gd2O3 film don’t absorb moisture contamination during a short period time. Finally, we show the TEM picture and EDS signal with NH4OH solution under optimum sulfidation fabricated MOS capacitor which is shown in Fig.4-14 and Fig.4-15. The interfacial layer between Gd2O3/GaAs interface is about 0.5nm, and oxide thickness is 0.96nm.

Following is the PDA conditions at different gas ambient. Fig. 4-16 (a)-(e) plot multi-frequency C-V frequency characteristics of Pt/Gd2O3/GaAs MOS capacitor with different gas ambient: (i) O2 500^oC (ii) Ar500^oC (iii) N2 500^oC (iv) Ar /O2 500^oC (v) Ar /O2 600^oC. Clearly, the O2 gas ambient led to Gd2O3/GaAs interface oxidized quickly during 10 second. Fig. 4-17 (a)-(e) show the hysteresis width of Pt/Gd2O3/GaAs MOS capacitor. Fig. 4-18 is the 10k Hz frequency C-V curve and hysteresis trend. The 600^oC anneal led to Gd2O3 film quality more dense because of higher activation to fill Gd2O3 vacancy site. In Fig.

4-19(a) is the C-V dispersion and leakage current with different gas ambient methods.

64

4-3-2 xps characteristic of Gd2O3 film on GaAs substrate

Fig. 4-20 shows As and Ga 3d core level spectra from GaAs substrate with different surface treatment after PDA 10s with Ar/O2 ambient at 500^oC. In the As 3d, three oxides components appeared, one could be identified as As2O3, two as As2O5, and three as As(Gd-O) form their typical chemical shifts;

approximately 2、3.5 and 6 eV, respectively [55]. In the Ga 3d, three oxides components appeared, one could be identified as GA2O, two as Ga2O3, and three as Ga(Gd-O) form their typical chemical shifts. After PDA at 500^oC, the interface still contains GaAs oxides and GaAs(GdO)-like compound. When Gd2O3 film has deposited on the GaAs substrate with 500^oCanneal temperature, we can’t exactly describe the magnitude of every component. This is because interface GaAs oxides have desorbed after 500^oC anneal, and the Gd2O3 film combined with this residual element. From the Fig.4-21(a), the Gd-O oxide in the Gd 3d orbit is assigned in 1188 eV which is one of the spin orbit of Gd 3d, and Auger electron spectra of As LMN is 20 eV distance from Gd 3d orbit. We find that a large number of As-O was found in the spectra regardless of surface treatment. Fig.4-21(b) shows the Gd 4d spectra, the two peaks are known as Gd 4d spin orbit peak [52]. In the Fig. 4-22 spectra, we discuss the NH4OH solution effect to remove cleaned residual native oxides. We know that the As-O of (ii)

65

and (iii) methods are less than (i) one after cleaned GaAs wafer. When the film and PDA process has finished, according to equation (4-4), the As-O will be desorbed, and it transfers to Ga-O component and remains As element. In the Fig. 4-22 (b), the O1s spectrum has two peaks, they are deconvoluted into one with a binding energy of around 529.8eV (Gd-O[I]) and 532.5eV(Gd-O[II]). It was reported the Gd-O[II] correspond to both the oxygen contaminants and interfacial GaAs oxides whereas Gd-O[I] correspond to the O-Gd bond in the amorphous Gd2O3.Fig. 4-23illustrates the Gd2O3 film contaminants, the (ii) and (iii) methods contain less Ga2O component because of NH4OH solution.Fig.4-24 is the XRD spectra, which show that no serious crystallization behavior on the Ar/O2 500^oC annealed Gd2O3 film on GaAs substrate.

4-3-3 surface morphology of Gd2O3 film on GaAs substrate

Fig. 4-25 represents the 1x1 um AFM images of the different surface treatments GaAs substrate surface after the deposition of the Gd2O3 films, and images show a relatively smooth surface morphology with a measured Rms roughness ranging from 0.24 to 0.37nm.Because the surface roughness affects the properties of a thin oxide or device, it is desirable to preserve a smooth surface during the process. Fig. 4-26 shows the clean method effect on Gd2O3

66

film under same silfidation, and the Rms roughness ranging from 0.25 to 0.29nm

4-4 Summary

We systematically investigated the surface treatment effect on the electrical and material characteristics of GaAs MOS capacitors with Gd2O3 gate dielectric.

The higher sulfur temperature and NH4OH solution was found to obtain the lower EOT of Gd2O3/GaAs gate stack, however, with a smaller hysteresis width.

A lower EOT of 20Å with a low leakage current of 1 x 10^-3 to 1 x 10^-5 A/cm² @ Vg = +1V, which has been achieved after 500°C annealing for 10 seconds. The density of state can reduce to 6 x 10¹²cm^-2e V^-1 and lowest leakage current about 1 x 10^-5 A/cm² @ Vg = +1V for the best surface treatment. We believed that the continuous optimization of the interface structure through process modification is expected to further improve the electrical performance of the Gd2O3/GaAs gate stack, which thus be considered as a promising gate dielectric of GaAs device.

67

(a)

(c) (d)

(b)

Fig. 4-1 (a)-(d) Multi-frequency C-V characteristics of Pt/Gd₂O3/GaAs MOS capacitor with different surface treatment.

-2 -1 0 1 2 3 4

HCl+NH₄OH+Sulf.(10%,80^OC)

Capatance (pF)

68

(a) (b)

(c) (d)

Fig. 4-2 (a)-(d) The hysteresis of 10kHz frequency of Pt/Gd₂O3/GaAs MOS capacitor with different surface treatment.

-2 -1 0 1 2 3 HCl+NH₄OH+Sulf.(1%,80^oC)

Capatance (pF) HCl+NH₄OH+Sulf.(10%,80^oC)

Capatance (pF)

Applied voltage (V) inv.-to-acc.

acc.-to-inv.

Pt / Gd2O3 / n-GaAs

69

(a) (b)

Fig. 4-3 (a) Multi-frequency C-V curve with HCl+NH₄OH +Sulfide treatment (b) The C-V dispersion with different surface treatments for Pt/Gd2O3/GaAs MOS capacitors.

70

(a) (b)

Fig. 4-4 (a) The 10kHz frequency C-V and (b)hysteresis characteristics of Pt/Gd₂O3/GaAs MOS capacitor with different clean and sulfide method.

-2 -1 0 1 2 3 4

71

(a) (b)

Fig. 4-5 (a) The I-V curve characteristics and (b) Jg(@V=+1v) of Pt/Gd₂O3/GaAs MOS capacitor with different clean and sulfide method.

0 1 2 3

Sulf. ^{HCl+NH4OH+ Sulf.}

72

Fig. 4-6 Density of state on Gd₂O3/GaAs interface with different clean and sulfide method.

10¹² 10¹³ 10¹⁴

(c)

D it (cm-2 eV-1 )

(1%, RT) HCl + Sulf.

(10%, 80^oC) (1%, 80^oC)

(1%, RT)

HCl+NH₄OH+ Sulf.

Sulfide conditions

73

(a) (b)

(c) (d)

Fig. 4-7 (a)-(d) Multi-frequency C-V and hysteresis using 10k Hz frequency characteristics of Pt/Gd₂O3/GaAs MOS capacitor with two cleaning method: (i) HCl+NH4OH (ii) NH4OH HCl+NH₄OH+Sulf.(1%,80^OC)

-2 -1 0 1 2 3

200 400 600

10 KHz HCl+NH₄OH+Sulf.(1%,80^oC)

Capatance (pF)

74

(a) (b)

(c)

Fig. 4-8 (a) Multi-frequency C-V (b) the hysteresis using 10k Hz frequency and (c) density of state characteristics of Pt/Gd₂O3/GaAs MOS capacitor with two cleaning method: (i) HCl+NH4OH (ii) NH4OH

75

(a) (b)

Fig. 4-9 (a) The I-V curve characteristics and (b) Jg(@V=+1v) of Pt/Gd₂O3/GaAs MOS capacitor with different clean methods and sulfide methods.

76

(a) (b)

(c)

Fig. 4-10 (a)-(c) Multi-frequency C-V frequency characteristics of Pt/Gd₂O3/GaAs MOS capacitor with three cleaning methods: (i) HCl (ii) HCl+NH4OH (iii) NH4OH HCl+NH₄OH+Sulf.(1%,80^OC)

-2 -1 0 1 2 3 4

NH₄OH + Sulf. treatment

77

(a) (b)

(c)

Fig. 4-11 (a)-(c) The C-V characteristics and Dit of Pt/Gd2O3/GaAs MOS capacitor with three cleaning methods: (i) HCl (ii) HCl+NH4OH (iii) NH4OH under optimum sulfidation.

78

(a)

(b)

Fig. 4-12 (a)-(b) The leakage current characteristics and J_g v.s CET of Pt/Gd2O3/GaAs MOS capacitor with three cleaning methods: (i) HCl (ii) HCl+NH4OH (iii) NH4OH under optimum sulfidation.

10^-8 10^-6 10^-4 10^-2 10⁰

Leakage current density J g(A/cm2 )

CET (Angstrom) Si control ,J_g@ +1V

HCl + NH4OH + Sulf. ,J_g@ +1V NH4OH + Sulf. ,J_g@ +1V

79

Fig. 4-13 The Weibull distribution of Pt/Gd₂O3/GaAs MOS capacitor with three cleaning methods: (i) HCl (ii) HCl+NH4OH (iii) NH4OH after 1 week/1 month.