Interdiffusion

Interdiffusion between III-V and other group materials, i.e. germanium, has been a key challenge for the reliability and performance of devices. TEM-EDS line concentration profile of indium(purple), arsenic (blue), germanium (green), gallium (light green), and phosphorous (red) along the line drawn across the Ge/In0.5Ga0.5P/GaAs structure was characterized in Figure 4.9. The profile has clear separation at the edge of each interface. Sharp interface with only a few nanometers of interdiffusion is demonstrated.

For the fabrication of Ge p-channel MOSFETs, the two-dimensional hole gas (2DHG) will form in the Ge layer near edge of bottom interface; therefore, good interface

characteristics including minimal interdiffusion and low defect density are of immense importance.

41 4.6 Photoluminescence

N-type doping of Ge will compensate the 0.136 eV difference in energy between Г and L valleys. Figure 4.10 is the room temperature PL infrared emission of 1.8 μm thick Ge

epitaxial layer on In0.5Ga0.5P/GaAs substrate with 330-mW PL incident laser power. The PL peak at 0.8 eV indicates the electrons in the Г valley recombine with holes in the valence band that makes the direct band-gap emission occur. The emission at the range of 650 to 750 meV is also detected in the spectrum that revealed indirect emission and had much lower intensity.

The detection of direct emission in the PL measurement indicates that the defect density of the Ge film is very low. Otherwise, non-radiative recombination due to the large number of defects would decrease the intensity of the L emission.

42

Figure 4.1 (top) Plot of Ge film thickness versus growth time on different surface coverage InGaP layers. The incubation time are 29.4 and 38.9 minutes for indium coverage of 27.82%

and 45.84%, respectively. (bottom) The analysis of composition ratio on InGaP surfaces revealed by XPS

43

Figure 4.2 Plot of Ge film thickness versus growth time with GeH4 gas flow rate of 10 and 20 sccm, respectively, during growth

Figure 4.3 An illustration of the basic processes of vapor deposition on a surface of a substrate

44

Figure 4.4 Top view surface morphology of Ge grown on In0.5Ga0.5P/GaAs characterized by SEM at growth time of (a) 20, (b) 38, and (c) 45 minutes

45

Figure 4.5 Cross-sectional view surface morphology of Ge grown on In0.5Ga0.5P/GaAs characterized by TEM at thickness of (a) 40, and (b) 190 nm

46

Figure 4.6 Plot of surface roughness and film thickness of Ge versus growth time

Figure 4.7 Plot of surface roughness of Ge versus growth time in different growth conditions

47

Figure 4.8 Cross-sectional high-resolution TEM image and diffraction pattern of 190 nm Ge epitaxial layer

Figure 4.9 High-resolution TEM microstructure and the EDS line scan profile across two interfaces of Ge/In0.5Ga0.5P/GaAs structure

400

300

Ge InGaP

GaAs

200

100

0

0 0.5 1.0 Position (µm)

Counts

48

Figure 4.10 Room temperature photoluminescence infrared emission from the structure of 1.8 μm Ge film on In0.5Ga0.5P/GaAs substrate. The direct band gap emission occurs at 0.8 eV.

0.60 0.65 0.70 0.75 0.80 0.85 0.90

0.00E+000

600 650 700 750 800 850 900

Energy (meV)

49

Chapter 5

Conclusions

High quality epitaxial Ge films were successfully grown on In0.5Ga0.5P/GaAs (100) by UHVCVD, as confirmed by TEM. This is the first study of Ge grown on InGaP layer to provide with some characteristics of this structure. A longer incubation time is needed for high indium surface coverage of InGaP. The growth mode of Ge on In0.5Ga0.5P/GaAs (100) is the Volmer-Weber growth, calculated by the thermodynamic theory of capillarity, as well as examined by top-viewed SEM and cross-sectional TEM images. With continuous growth of Ge on InGaP, rough surface would formed as soon as the InGaP surface is completely covered by Ge because the Ge-Ge attachment has lower adatom bonding energy than the formation energy of bonds between Ge and InGaP, resulting in the intensively enhancement of growth rate as well as surface roughness. The main reason is that the surface diffusion is reduced because of high growth rate. As a consequence, for good surface morphology of the Ge film, growth conditions of high temperature and low growth rate are suggested.

The Ge epitaxial film on In0.5Ga0.5P/GaAs (100) has shown sharp interface with

interdiffusion depth as low as the requirement of device applications [25]. And a direct band gap emission at 0.8 eV was detected by PL. This structure studied is useful for the future integration of Ge p-channel and III-V n-channel MOSFETs on the same GaAs template for beyond Si-CMOS logic applications.

50

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