| Summary: | 博士 === 國立成功大學 === 電機工程學系 === 89 === A new liquid phase chemical-enhanced oxidation (LPCEO)
technique for gallium arsenide (GaAs) device applications
has been proposed.
Unlike the other oxidation technique, the oxide film can be grown at relatively high oxidation rate (1000A within 1 h, at 70℃) and near room temperatures (30-70℃) under room-light by the LPCEO technique without any assisted energy source like electrical potential or plasma.
Reliable growth of featureless and uniform oxides on heavily doped, undoped, (100), (110), and (111) oriented GaAs wafers or epitaxial layers have been achieved by employing this simple and effective technique.
As for the factors having influence on the oxidation,
we have found the lowest oxidation rate at (110) oriented surface because of its lowest available bond density as compared with the other orientations, and a higher oxidation rate for n-type GaAs rather than p-type or undoped.
Owing to motion of holes near the surface,
light illumination enhances the oxidation for n-type GaAs by introducing electron-hole pairs, whereas that for p-type or undoped is relatively insignificant.
In addition, sulfide treatments effectively passivate the GaAs surface from oxide growth by LPCEO.
We have obtained a roughness of 19.2A in rms values for a 684A thick oxide and uniformities of 1.6A in thickness and 0.6\% in refractive index for a ~1230A thick oxide.
All of them are associated with pH value and stirring during oxide growth.
By X-ray spectroscopic measurements, it has been determined that the oxide is composed of Ga2O3 and As2O3 at the surface and with elemental As accumulated within the oxide.
The refractive index (n) of as-grown oxides is around 1.6 and significantly increasing (up to 2.3) with higher concentration of As2O3 within the oxide.
Based on current-voltage characteristics of metal-oxide-semiconductor structures,
a leakage current density about 2-3x1E-6 A/cm2 at
an electric field of 1 MV/cm and a pre-breakdown leakage current density of 1-2E-3 A/cm2 have been obtained.
With increasing the refractive index of oxides,
the breakdown fields increase up to 6-7 MV/cm.
The results show comparable insulating properties with those prepared by ultrahigh vacuum deposited Ga2O3 and thermal oxidation in As2O3 vapor.
In addition, relative permitivities of 6.5-11 and also improved interface properties after thermal annealing were determined based on capacitance-voltage measurements.
Both volatization and densification processes play dominant roles in post-oxidation annealing effects on the LPCEO-grown oxide.
At annealing temperatures below 600℃, loss of As2O3 leads to
shrinking of thickness for all oxides.
The oxides tend to be composed of Ga2O3 only and with refractive indices approaching to 1.8, the refractive index of amorphous Ga2O3.
The dominant process is considered to be volatization for original n>1.8, since there is a higher As2O3 concentration within the oxide.
On the other hand, densification dominates for n<1.8 because the accumulated As compounds is composed of mostly elemental As which is capped within the oxide and does not evaporate at the temperature.
Measurements of oxide depth profiles support the above points of view.
The situation is drastically different at 600℃ annealing,
since both the elemental As and bulk GaAs start to evaporate.
Destruction of the oxide structure leads to the decrease of the refractive index and increase of the thickness.
We have also found formation of a few amount of arsenic pentoxide (As2O5) after annealing at 600℃.
The discussion of mechanism is divided into two parts,
including models for surface chemistry and growth kinetics.
The surface chemistry model is based on pH-dependent hydrolysis behaviors of gallium, arsenic and their oxides in aqueous solutions.
According to the experimental observations, we have establish a model that the oxidation is initiated by a ionic reaction of gallium with the solution to form hydrated gallium cations.
The cations transform to oxides or hydroxides or remain cations
at pH values higher than 4.5 or lower than 4.
Within the range, a pH window for the optimum oxide growth conditions has been found.
As the mainframe of the oxide constructed by the gallium oxides,
both elemental arsenic and As2O3 start to accumulate
within the oxide, which the later is more significant with decreasing of the pH value.
By a pH-controlled procedure,
the oxidation rates remain constant for a
longer oxidation time, and also weaker As2O3 accumulation is obtained.
In addition, models for the growth kinetics, at all oxidation stages, are established to illustrating the oxide formation processes and compositions.
Finally, based on the LPCEO method and the characteristics of the oxide, device applications based on the oxides have been proposed.
For example, a convenient method of selective oxidation using photoresist or metal as masks has been realized.
In addition, we have established a technique of planarized GaAs device isolation using the selectively
grown oxide as insulators.
As for the fabrication of electronic devices,
GaAs metal-oxide-semiconductor field effect transistors (MOSFETs) of n-channel and depletion-mode with gate oxides grown by the LPCEO method have been demonstrated.
The fabrication processes of the GaAs MOSFETs are reliable and with a automatical side-wall passivation characteristic.
The MOSFETs also exhibit almost fully pinch-off current-voltage characteristics.
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