N. Zakharov et al/ Physica E 37(2007)148-152 2. Experiment The base of the whiskers was always located in triangular pits formed during the growth process. This We used <111> oriented 5" Si wafers cleaned by the clearly indicates that some surrounding silicon material conventional RCA (Radio Corporation of America) was definitely consumed by the growing whisker(see Fig procedure were used as substrates. Our MBE system la, b). During the growth Si atoms diffused upward to the includes three electron-beam guns for the evaporation of Au droplet and were then incorporated into the( 11) Au and Si as well as of Ge [14]. a thin Au film with a Si/Au droplet interface on the top This growth process ominal thickness of 2 nm was deposited on the substrate implies the presence of an ad-atom supersaturation at a substrate temperature Ts= 525C. During the nano Au kT>o due to a difference in the chemical potential wires (Nw) growth the constant Si flux II ranged in between the overgrown layer and the top of the whisker terval 0.013-0.108 nm/s. Two growth temperatures Normally grown whiskers are defect-free(see Fig. 5) Ts=525 and 545C were chosen. The vacuum in the however, they sometimes contain single inclined stacking chamber during the growth was 2 x 10 Pa. The samples fault coming from the interface between the substrate and were investigated by transmission electron microscopy the overgrown Si layer (TEM) and high-resolution scanning electron microscopy The intrusion of Au into the Si overgrown layer during (SEM). Reflection high energy electron diffraction whisker growth(see Fig. 3a)leads to an increase of the (RHEED)was used to monitor the evolution of the elastic energy and the formation of structural defects such surface structure during the growth process as twins, partial dislocations and small amorphous Si/Au eutectic particles inside the overgrown layer. The presence of an Au-enriched surface layer was also demonstrated by [15]. The very early stage of whisker growth is shown in 3. Results Fig. 3b, where the structural defects serving for elastic energy relaxation are indicated by arrows. It should be The gold deposition at T=525C resulted in the noted that the region underneath the growing whisker is formation of Au droplets on the Si surface. The diameters defect-free. This suggests that whisker growth results from of the droplets ranged from 20 to 400 nm. During the the relaxation of elastic stresses in overgrown layer subsequent Si deposition, NWs were formed on the Au In the cross-section TEM images at room temperature droplets(Fig. 1). Under these conditions their diameter d one can clearly see the presence of tiny amorphous droplets lated to the size of the droplet and ranged from 70 of Si/Au eutectic approximately 5nm in diameter(see to 200 nm(2re<d<2Rc)(see Fig. 2a). Smaller as well as Fig. 4). We assume that at a growth temperature of 525C larger droplets outside of this range did not initiate whisker they turn into a thin liquid layer on the Si surface. Thi growth at all. The length of the grown whiskers L seems to be the case in Fig. 5, where RHEED patterns was proportional to the growth time Lot and varied in taken at temperatures 150C (a), and 360C(b)are shown inverse proportion to the radius L1/R(Fig. 2b and c espectively) Even though the growth rate of the whiskers dL/dt 4. Discussion decreased with the Si flux I1, the ratio L/Ls increased as can be seen in Fig. 2d (Ls the thickness of overgrown Si layer The growth process of whiskers by MBE is shown ig. 6). This suggests that longer whiskers can be grown at schematically in Fig. 6. Two fluxes of the Si ad-atoms 11 low Si flux I1 and I2 can be distinguished. The uniform flux I, comes 100nm 400nn Fig 1. SEM (a)and TEM(b) images of Si whiskers. Each whisker is placed in the pit. They are defect-free. The surface of the whiskers is decorated by tiny Au droplets(see also Fig. 4)2. Experiment We used /111S oriented 500 Si wafers cleaned by the conventional RCA (Radio Corporation of America) procedure were used as substrates. Our MBE system includes three electron-beam guns for the evaporation of Au and Si as well as of Ge [14]. A thin Au film with a nominal thickness of 2 nm was deposited on the substrate at a substrate temperature TS ¼ 525 1C. During the nano wires (NW) growth the constant Si flux I1 ranged in interval 0.013–0.108 nm/s. Two growth temperatures TS ¼ 525 and 545 1C were chosen. The vacuum in the chamber during the growth was 2  107 Pa. The samples were investigated by transmission electron microscopy (TEM) and high-resolution scanning electron microscopy (SEM). Reflection high energy electron diffraction (RHEED) was used to monitor the evolution of the surface structure during the growth process. 3. Results The gold deposition at T ¼ 525 1C resulted in the formation of Au droplets on the Si surface. The diameters of the droplets ranged from 20 to 400 nm. During the subsequent Si deposition, NWs were formed on the Au droplets (Fig. 1). Under these conditions their diameter d related to the size of the droplet and ranged from 70 to 200 nm (2rcodo2Rc) (see Fig. 2a). Smaller as well as larger droplets outside of this range did not initiate whisker growth at all. The length of the grown whiskers L was proportional to the growth time Lt and varied in inverse proportion to the radius L1/R (Fig. 2b and c respectively). Even though the growth rate of the whiskers dL/dt decreased with the Si flux I1, the ratio L/Ls increased as can be seen in Fig. 2d (Ls the thickness of overgrown Si layer, Fig. 6). This suggests that longer whiskers can be grown at low Si flux I1. The base of the whiskers was always located in triangular pits formed during the growth process. This clearly indicates that some surrounding silicon material was definitely consumed by the growing whisker (see Fig. 1a,b). During the growth Si atoms diffused upward to the Au droplet and were then incorporated into the (1 1 1) Si/Au droplet interface on the top. This growth process implies the presence of an ad-atom supersaturation Dm/kT40 due to a difference in the chemical potential between the overgrown layer and the top of the whisker. Normally grown whiskers are defect-free (see Fig. 5); however, they sometimes contain single inclined stacking fault coming from the interface between the substrate and the overgrown Si layer. The intrusion of Au into the Si overgrown layer during whisker growth (see Fig. 3a) leads to an increase of the elastic energy and the formation of structural defects such as twins, partial dislocations and small amorphous Si/Au eutectic particles inside the overgrown layer. The presence of an Au-enriched surface layer was also demonstrated by [15]. The very early stage of whisker growth is shown in Fig. 3b, where the structural defects serving for elastic energy relaxation are indicated by arrows. It should be noted that the region underneath the growing whisker is defect-free. This suggests that whisker growth results from the relaxation of elastic stresses in overgrown layer. In the cross-section TEM images at room temperature one can clearly see the presence of tiny amorphous droplets of Si/Au eutectic approximately 5 nm in diameter (see Fig. 4). We assume that at a growth temperature of 525 1C they turn into a thin liquid layer on the Si surface. This seems to be the case in Fig. 5, where RHEED patterns taken at temperatures 150 1C (a), and 360 1C (b) are shown. 4. Discussion The growth process of whiskers by MBE is shown schematically in Fig. 6. Two fluxes of the Si ad-atoms I1 and I2 can be distinguished. The uniform flux I1 comes ARTICLE IN PRESS Fig. 1. SEM (a) and TEM (b) images of Si whiskers. Each whisker is placed in the pit. They are defect-free. The surface of the whiskers is decorated by tiny Au droplets (see also Fig. 4). N. Zakharov et al. / Physica E 37 (2007) 148–152 149