Search

Si has dominated semiconductor industry for decades. However, new materials are appearing in this field such as Ge, GaAs and InxGa1-xAs. In semiconductor processing several wet chemical steps are used including cleaning, etching and stripping. Knowledge of etch rates in these solutions is of great importance. In this paper, an etch rate study was performed for Ge, GaAs and InxGa1-xAs in several wet chemical cleaning solutions. The chemistries studied include acids (HCl, HF, HNO3, H2SO4, H3PO4, H2O2) and bases (NH4OH) and peroxide based mixtures. It has been found that etch rates are much higher than for Si due to the formation of soluble oxides. Therefore, "fine-tuning" of the chemistries is a prerequiste when introducing these materials in microelectronic industry.

We are reporting on a growth procedure for selective growth of GaAs on Ge substrates by organometallic vapor phase deposition. The precursors used for the growth are Tertiarybutylarsine (TBAs) and Trimethylgallium (TMGa). As a mask material 200 nm thick polycrystalline SiO2 was deposited on a 6° miscut Ge(001) substrate. The patterns available on the mask are various structures with feature sizes ranging from a fraction of a micron to a mm square. The filling factor of the mask, defined by the ratio of open area on the substrate to the area covered by SiO2, is approximately 2⁄3. This means that most of the wafer is covered with SiO2, making loading effects due to enhanced concentration of growth species at the mask openings very apparent. The most critical processing step during the selective growth of GaAs on Ge is the growth of the nucleation layer. This nucleation layer needs to be grown at high pressure in order to maximize the As partial pressure, which avoids formation of anti phase domains during the initial nucleation stages. Unfortunately, at high pressure, the nucleation of GaAs on the SiO2 is largest as well. As a result, during the initial growth steps, a small amount of GaAs is nucleated on the SiO2 area, which should be avoided. By keeping the high pressure nucleation layer as thin as possible and by reducing the pressure in the reactor as fast as possible, this nucleation can be avoided. A detailed study of the growth sequence for this nucleation layer has been undertaken, showing the tradeoff between thin high pressure nucleation layer and presence of anti-phase domains in the final selectively grown GaAs layer. Loading effects due to enhanced growth at the boundaries of the SiO2 layer are studied as well. Characterization of the material was done with X-ray diffraction, defect etching, cross-section scanning electron microscopy, photoluminescence spectroscopy and Nomarski microscopy. The final results show that the growth of high quality anti phase domain-free GaAs on Ge is possible, with no GaAs nucleation on the SiO2 mask material. A loading effect at the boundaries of the GaAs is still present and can not be eliminated with the present growth precursors. Figure 1: Nomarski microscope picture of a selectively grown GaAs structure on a 6° miscut Ge substrate. The mask material is a 200 nm thick SiO2 film.

The electrical properties of GeO1-xNx/HfO2 stack deposited by atomic beam on Ge are reported. The incorporation of N in the GeOx layer is found to be quite beneficial in reducing the gate leakage current, improving the uniformity of the device characteristics and reducing the equivalent oxide thickness of the gate stack down to about 0.8 nm. On the other hand, N incorporation also leads to a large density of fixed positive charges and slow states, which can be reduced by post- deposition anneals in O2 or N2. These gate stacks were successfully integrated into a simple transistor flow with TiN gates. Both n and p-channel transistors show promising electrical characteristics, with Ion/Ioff ratios of about 4 to 5 orders of magnitude. However, the mobility of the n-channel devices is found to be extremely low, which is most likely due to a large density of acceptor-like interface states in the upper-part of the Ge band-gap.