Epitaxial Growth of Ge Strain Relaxed Buffer on Si with Low Threading Dislocation Density
Authors: Abedin Ahmad , Asadollahi Ali , Garidis Konstantinos , Hellström Per-Erik , Ostling Mikael 
Published: February 2020

Germanium, with its superior electron and hole mobility compared to Si, is considered as a channel material in future CMOS technology [1]. The band gap of Ge makes it attractive in optoelectronic devices. However, integration of crystalline Ge on Si wafers is challenging due to the high lattice mismatch, leading to misfit and threading dislocations in Ge strain relaxed buffers (SRBs) grown on Si. Threading dislocations reaching the device layer can degrade the carrier mobility and increase the junction leakage current of MOSFETs and photodetectors. Several groups have reported that threading dislocation density (TDD) can be reduced by implementing a two-step growth (Step 1: T~400°C, Step 2: T~600-800°C) and using post growth anneal and/or cyclic thermal annealing TDDs~107cm-
2 has been demonstrated for SRB thicknesses ~1µm [1]. Even lower TDDs~106cm-2 has been achieved by increasing the SRB thickness >3µm [2]. In this work, we show that further reduction of the Ge growth temperature during the initial step to less than 300°C together with Ge2H6 as precursor enables a 0.5µm thin Ge SRB with TDDs as low as 4·104cm-
2.The Ge SRB was grown on (100) oriented Si wafers by reduced pressure (20 torr) chemical vapor deposition (CVD) using Ge2H6 as precursor and H2 as carrier gas. Dopant sources were PH3 and B2H6 for n- and p-type Ge SRB, respectively. HF last Si wafers were loaded into the CVD reactor followed by a 2 min H2 bake at T=1050°C. The temperature was ramped down to 280°C and Ge2H6 was introduced in the chamber followed by a temperature ramp up to 300°C for 20 min growing a 40nm strained relaxed Ge layer. The temperature was ramped up to 600°C in H2 followed by the growth of a 460nm thick Ge layer. For comparison, a 3µm thick Ge SRB was deposited with the conventional method; i.e. first layer grown at T=400°Cfollowed by the second layer at T=600°C and post growth annealing at T=800°C.The grown Ge SRB was subjected to wet chemical etching using SECCO solution (K2Cr2O7:H2O:HF) in order to delineate the threading dislocations. TDD was determined by counting delineated defects on images obtained from dark field optical microscope, AFM and SEM. High Resolution Reciprocal Mapping (HRRLM) was used to compare crystalline quality and XTEM was implemented to image the defects at the interfaces. Surface morphology and roughness were investigated using AFM.Fig. 1 shows AFM images of the surface morphology for temperature ramped (T=280-300°C) Ge SRB (left image) and conventional (T=400°C) Ge SRB (right image). The images indicate that the strain relaxation occurred through pit formation in the temperature ramped Ge SRB while in the conventional Ge SRB, the strain relaxed through island and valley formation. Similar relaxation through pit formation has been found in Ge grown on Si at T=290°C [3]. The pits are formed in the first grown Ge layer at T=280-300ᵒC where the low temperature allows Ge growth in 2D mode instead of 3D. The 2D growth mode enables low TDD in the Ge layer grown at T=600°C. Fig. 2 shows dark field optical microscope images of the Ge SRB after delineation of the threading dislocations by SECCO etching. For the conventional 3µm thick Ge SRB (right image) a TDD~107cm-2 was measured. The Ge SRB with the temperature ramped (T=280-300°C) exhibits a comparably low TDD~4·104cm-2. Fig. 3 displays reported TDDs in grown Ge SRBs together with the TDDs achieved in this work. The conventionally 3µm thick Ge SRB (T=400°C) exhibits a TDD well in accordance with what has been reported in the literature. The Ge SRB grown with an initial temperature ramp (T=280-300°C) exhibits about 3 decades lower TDD at the same thickness compared to literature data.In conclusion, we have shown that by using Ge2H6 and optimizing the temperature profile of the initial Ge growth it is possible to grow a 0.5µm thin Ge SRB on Si with a threading dislocation density ~4·104cm-2without any post growth annealing.
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Figure 1