(a) Typical TEM image of a ∼650 nm thick Ge film deposited on a Si(001) substrate at a constant temperature of 730 °C; (b) A zoom taken near the interface region, illustrating the extension of the defected region to more than 200 nm thick.
RHEED patterns taken along the  azimuth illustrating the growth mode transition in the Ge/Si system when increasing the film thickness. sp indicates the specular or (00) streaks, (1 × 1) corresponds to the position of the Ge bulk-like streaks and ½ the half-order streaks of the Ge(001) 2 × 1 reconstructed surface; (a) streaky pattern observed during the formation of the wetting layer and (b) 3D pattern observed for film thicknesses larger than the critical thickness. The dotted lines represent a unit cell of the Ge(001) plane, which is parallel to the (001) plane of the Si substrate.
(a) and (b) RHEED patterns along two main  and [1–10] azimuths observed throughout the growth of a ∼200 nm thick Ge layer at 300 °C. The observation of long streaks in RHEED patterns clearly indicates that the formation of 3D islands has beencompletely suppressed; (c) Measurement of the RHEED intensity inside the rectangular region shown in (a); (d) Evolution of the (1 × 1) streak spacing of Ge when increasing the film thickness. The Ge epilayer is found to be fully strained up to a thickness of ∼0.65 nm, beyond which a progressive strain relaxation takes place and a fully relaxed Ge epilayer is obtained for a thickness of ∼1.1 nm.
(a) Cross-sectional TEM image of a 200 nm thick Ge layer grown on Si at 300 °C, both the film surface and interface are relatively smooth, threading dislocations are almost absent; (b) A zoom taken near the interface region. A high density of misfit dislocations is present, which have allowed the Ge layer to be fully relaxed.
Evolution of Ω-2θ XRD scans around the Ge(004) reflection with the growth temperature. The doted XRD scan corresponding to a sample grown at 300 °C is shown for comparison. As the growth temperature increases, the Ge(004) peak linearly shifts to higher angles, reaches a saturation value at 700 °C and finally remains almost constant for further increasing the temperature to 770 °C. The in-plane tensile strain observed in the temperature range of 700–770 °C is 0.24%.
Evolution of the Ge(400) peak position versus film thickness. Only a slight increase of the tensile strain (∼0.01%) is observed when the film thickness increases from 150 to 600 nm.
(a) Typical TEM image of a ∼300 nm thick Ge layer grown following a two-step growth at 300 and 730 °C. Threading dislocations, indicated by white arrows, appear in the layer during the increase to the high temperature growth step. However, the layer exhibits a low density of misfit and threading dislocations; (b) A zoom taken near the interface region, illustrating the flatness of the interface.
SEM images of the surface after defect etching in CrO3/HF/H2O for 5 min; (a) of a ∼650 nm thick Ge deposited on Si(001) at 730 °C. Noodle-like features are found to be formed, which can be attributed to a high density of threading dislocations. Shown in the inset is a zoom taken inside these features. Square pyramidal pits characteristic of threading dislocations are clearly observed; (b) of a ∼300 nm Ge layer grown using two-step growth at 300/730 °C. Defected regions having a square shape are present on the surface. In the inset, a detailed view of a square defected region is shown. If we take each square defected region as a threading dislocation unit, a defect density well below 104 cm−2 is measured.
Comparison of Ω-2θ XRD scans around the Ge(004) reflection measured with annealing at 900 °C for different durations and cyclic annealing between 780 and 900 °C with an annealing time of 10 min at each temperature. The dotted scan represents the as-grown sample grown at 650 °C. Cyclic annealing is shown to generate a larger tensile strain as compared to annealing at 900 °C.
Evolution of Ω-2θ XRD scans with various annealing times during cyclic annealing. The highest tensile strain, 0.30%, is obtained for an annealing time of 3 min. For an annealing time of 20 min, the Ge(001) reflection shifts to much higher angles and at the same time its width broadens.
SIMS profiles of the as-grown sample grown at 730 °C (blue curves) and after 10 cyclic anneals at 780 and 900 °C for a duration at each temperature of 3 min (red curves) and 20 min (green curves). No Ge/Si interdiffusion can be observed for the as-grown sample and after 3 min annealing, while after 20 min annealing, the deposited layer is no longer made of pure Ge but of a SiGe alloy. The Si concentration is found to continuously decrease from the interface region towards the film surface.
(a) TEM image taken near the interface region illustrating the growth of three multilayers of C (0.3 nm)/Ge (18 nm); (b) Atomically resolved TEM image taken in the vicinity of the carbon layer. Both the underneath and the upper Ge layers are perfectly epitaxial, no presence of defects can be detected. It can be seen that carbon atoms are distributed over a distance of ∼2 nm.
Comparison of Ω-2θ XRD scans of two samples annealed by ten 780/900 cycles during 20 min. The red curve corresponds to a sample without carbon deposition, and the blue curve corresponds to a sample containing three C/Ge multilayers deposited near the interface region. The (004) reflection of the sample with C/Ge multilayers becomes narrower and shifts to a lower angle. The corresponding tensile strain is 0.28%.
Variation of the in-plane tensile strain in Ge films versus the growth temperature during the second step growth. All samples have a total thickness of ∼300 nm, which consists of a ∼50 nm thick Ge layer deposited at 300 °C followed by another 250 nm thick Ge layer grown at various temperatures: 300, 400, 500, 600, 650, 700, 750, and 770 °C.
Variation of the in-plane tensile strain in Ge films as a function of the annealing time with two annealing methods: long anneal at 900 °C and cyclic anneals between 780 and 900 °C. All samples have a total thickness of ∼300 nm, which consists of a ∼50 nm thick Ge layer deposited at 300 °C followed by another 250 nm thick Ge layer deposited at 600 °C. Prior to annealing, the as-grown sample has a tensile strain of 0.16%. The highest value of the in-plane tensile strain obtained for each annealing method is shown in bold type. For cyclic anneals, the highest value of the tensile strain (0.30%) is obtained for 3 min, beyond which a decrease of the tensile strain is observed, which can be attributed to the beginning of Ge/Si interdiffusion.
Article metrics loading...
Full text loading...