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Si1−Xgex Single Crystals Grown by the Czochralski Method: Defects and Electrical Properties T.S

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Si1−Xgex Single Crystals Grown by the Czochralski Method: Defects and Electrical Properties T.S Vol. 124 (2013) ACTA PHYSICA POLONICA A No. 2 Special Anniversary Issue: Professor Jan Czochralski Year 2013 Invited Paper Si1−xGex Single Crystals Grown by the Czochralski Method: Defects and Electrical Properties T.S. Argunovaa;b, J.H. Jeb;∗, L.S. Kostinaa, A.V. Rozhkova and I.V. Grekhova aIoe Physical-Technical Institute, RAS, Polytekhnicheskaya st. 26, 194021 St. Petersburg, Russia bX-ray Imaging Center, Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31 Hyoja-dong, Namku, 790-784 Pohang, Republic of Korea Defects in Si1−xGex single crystals (28.5 at.% Ge) grown by the Czochralski method are investigated by synchrotron white beam topography and phase contrast imaging techniques. As the Ge concentration increases, dislocation structure evolves from individual dislocations to slip bands and sub-grain boundaries. We discuss the eect of dislocations on the electrical characteristics such as resistivity ρv, the Hall hole mobility µp and carrier lifetime τe. Diodes are fabricated by bonding p-Si1−xGex to n-Si wafers to investigate IV characteristics and reverse recovery process. IV characteristics are not deteriorated in spite of a ve times decrease in τe with Ge concentration. A small reverse recovery time (determined by the accumulated charge) can be achieved for an optimised preset Ge concentration. DOI: 10.12693/APhysPolA.124.239 PACS: 61.72.Ff, 61.72.Lk, 72.15.Eb 1. Introduction results in the formation of a relaxed SiGe layer with a relatively low (105 to 107 cm−2) threading dislocation density on which a strained Si channel is grown [1, 3]. Si1−xGex (SiGe) solid solutions are presently used to fabricate high-power high-speed bipolar transistors The foremost critical challenge in the FET technology is the control of dislocation defects in the epitaxial lay- (HBTs) [1], pin diodes [2], eld-eect transistors (FETs) [3], and integrated circuits for operation at ultra- ers. The method includes preliminary fabrication of SiGe high frequencies. SiGe layers are grown on silicon sub- layers on Si substrates using epitaxial technology. strates by epitaxial growth technologies such as ultra- In this review, we suggest an alternative approach, di- -high vacuum chemical vapor deposition or molecular rect binding of SiGe and Si single-crystal wafers, to form beam epitaxy [1, 3]. SiGe single crystals are becoming the relaxed Si1−xGex/Si compositions with satisfactory available as well [46], though their commercial produc- structural and electrical characteristics [79]. Mist dis- tion is not set yet. locations in such structures are localized in a thin layer In bipolar transistors, a SiGe base with a graded SiGe near the interface where strain relaxation occurs without prole is used [1]. The increase in Ge content brings generating threading dislocations. about high lattice mismatch between SiGe layer and Si Structural defects in SiGe bulk aect the electrical substrate, giving rise to high elastic stress. By the relax- properties of the bonded structures. We studied the evo- ation of the stress, mist and threading dislocations are lution of dislocations in SiGe single crystals by the in- generated, which aect device performance and produc- crease in Ge concentration. Good quality single crystals tion yield. up to 50 mm in diameter with Ge concentrations of up to In FET structures, a tensile strained silicon channel 15 at.% were produced with the Czochralski (Cz) tech- is formed on a relaxed SiGe buer layer by epitaxial nique [4]. The main diculty in Cz growth is caused by growth [3]. The carrier mobility in the channel is then uctuations of the crystallization rate at the solidliquid signicantly enhanced by a factor of 3 to 5, comparing interface. Together with the eect of a high segregation to bulk silicon, due to reduced eective masses of car- coecient of germanium in silicon and the risk of consti- riers and weak intervalley scattering, which cause high tutional supercooling, uctuations lead to the formation frequency characteristics of FET transistors. of Ge striations [10]. Striations then induce strain [11] Specically, to fabricate tensile strained silicon chan- and, depending on Ge content, may cause plastic defor- nel, a SiGe buer layer is rst grown on Si wafer by grad- mation in the lattice [12]. ually increasing Ge content (1530% Ge) until the strain The electrical parameters of bulk Si1−xGex were pre- relaxes by the generation of high density of mist and viously investigated [4, 13, 14], but multiplication of dis- threading dislocations. Then the continued SiGe growth locations with Ge content or any correlation between the structure and the properties are largely unexplored. We discuss the perfection of Cz-grown Si1−xGex single crys- tals (0:02 < x ≤ 0:085) and compare the structural qual- ∗corresponding author; e-mail: [email protected] ity with the electrical resistance and the carrier lifetime measured in the pre-studied samples [15, 8]. We also an- (239) 240 T.S. Argunova et al. alyze currentvoltage characteristics and reverse recov- Images were obtained in several ways (Fig. 1). Phase- ery processes from the bonded diodes p-Si1−xGex=n-Si -contrast images were always recorded on the detector [7, 16]. by transforming X-rays into visible light using a 200 µm thick CdWO4 scintillator. The CCD array had 1600 pix- 2. Samples and techniques els × 1200 pixels and a sensitivity of 12 bit. Images were magnied before recording with an objective lens. Si1−xGex single crystals were grown by the Czochral- Depending on the objective magnication, the eld of ski method [4]. The germanium concentration changed view could be changed from several millimeters to sev- from 2 to 8.5 at.%. Crystals (with n- or p-type conduc- eral dozens of micrometers. The eective pixel size was tivity) were lightly doped with phosphorus or boron to 0.19 µm for high-resolution images. The scintillator a concentration of approximately 1015 cm−3. The oxy- sample distance was chosen in the range of 610 cm. gen content was at a level of 1017 cm−3. The growth Topographs in the Laue or Bragg geometries were was performed in the h111i (0:02 < x < 0:08) and h100i recorded using either a photographic lm or the detec- (x = 0:085) directions. The samples were cut perpendic- tor. A high-resolution KODAK SR-45 lm was placed at ularly to the growth direction and polished at both sides a distance of 812 cm behind the sample. The beam size to a thickness of 0.4 mm. on the sample was 10 mm × 10 mm. Topographs were To fabricate p-SiGe/n-Si heterostructures, mirror- magnied in an optical microscope. When recording on -polished p-SiGe wafers were bonded to dislocation-free the detector, the eld of view did not exceed 1.5 mm × n-type Si wafers of the same orientation. Prior to bond- 1.5 mm [19]. ing, an articial relief in the form of an orthogonal net When polychromatic radiation is allowed to fall on of grooves was photolithographically prepared on the Si a single crystal, several diracted beams are recorded wafer surfaces [9]. After RCA cleaning, the wafers were simultaneously. Topographs in various reections were joined under the deionized water surface, spin-dried and used to determine the dislocation Burgers vectors b ac- heated at 95 ◦C for 4 h. Bonding annealing was performed cording to the visibility criteria [20]. in air in two steps: at 1000 ◦C (1 h) and 1150 ◦C (2 h). The electrical resistivity, the carrier lifetime, the Structural defects in the SiGe bulk and at the SiGe/Si currentvoltage (IV ) characteristics, and the reverse re- interfaces were investigated by X-ray topography com- covery process wave forms were measured. The germa- bined with phase contrast imaging [8, 15, 17]. The ex- nium distribution was investigated by means of energy periments were performed on the 7B2 X-ray microscopy dispersive X-ray (EDX) analysis on a JEOL JSM-6330F beamline of the Pohang Light Source, which is third- FESEM system. -generation synchrotron radiation (SR) source, in Po- hang, Republic of Korea. A bending magnet was used to 3. Investigation of lattice defects in SiGe crystals oer an eective source size of 160 µm × 60 µm (H × V) at a distance of 34 m from the sample. The initial spec- trum showed a monotonic decrease in the range from 6 We performed a comparative study of defects in Si Ge crystals with . As a rule, to 40 keV. There were no optical elements between the 1−x x 0:02 < x ≤ 0:085 source and sample. Partially coherent imaging with a dislocations were not observed in the range x ≤ 0:05, white beam was feasible due to the fact that the eective where Ge striations were the main structural defects. spectrum was shaped with a maximum by the absorption Dislocations appeared for high Ge concentrations (x > in a Be window and in the sample [18]. 0:05). Figure 2 represents white beam topographs of Si0:946Ge0:054 (111) taken from a region between the cen- ter and the periphery of the wafer. The 200 (a) and 020 (b) reections were feasible because of twinning and the change in the growth axis from h111i to h115i. One can see variations of the dislocation density in the 200 and the 020 topographs. In particular, the dislocations that strongly stand out in (b) but disappear in (a) are pre- sumably produced by a FrankRead source [21], as can be seen in the inset to (b) for a FrankRead source nearby but outside the scope of the image. The dislocations were invisible in the 311 topograph (data not shown), indicat- ing that they belong to the (111), [011] slip system.
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