Evidence of temperature dependence of initial adsorption sites of Ge atoms on Si(111)-7×7

Abstract

Small amounts of Ge atoms are deposited on Si͑111͒-7 ϫ 7 surfaces at room temperature ͑RT͒ and at 100°C to clarify the initial adsorption sites using scanning tunneling microscopy. At RT Ge atoms are adsorbed at high coordination B 2 sites around the rest atoms, as predicted by Cho and Kaxiras ͓Surf. Sci. 396, L261 ͑1998͔͒. On one hand, at 100°C Ge atoms are adsorbed on corner adatom sites. With increasing Ge coverage the corner sites are gradually occupied, followed by Ge adsorption at center adatom sites, resulting in Ge cluster growth with a size of the half unit cell. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2198109͔The stable periodic structure of a Si͑111͒-7 ϫ 7 surface with a comparatively large size unit cell ͑UC͒ offers a good template for fabricating self-assembled nanostructures. [1][2][3][4]6 For example, Si magic clusters and self-assembled metal nanostructures confined in a UC over the Si͑111͒-7 ϫ 7 surface have been extensively studied. [3][4][5][6][7][8][9][10][11][12][13][14] In addition, there are many reports on the initial Ge growth in the UC of Si͑111͒-7 ϫ 7 accompanied by the formation of clusters with various shapes and sizes at submonolayer coverages, 3,4,[13][14][15][16][17][18][19][20] followed by Stranski-Krastanov ͑SK͒ growth mode with increasing Ge coverage owing to Si-Ge lattice mismatch.At present, there is a considerable controversy regarding the initial Ge adsorption site and the bonding coordination over the Si͑111͒-7 ϫ 7 surface. Dev et al. have performed x-ray standing wave measurements at submonolayer Ge coverage and proposed that Ge adatoms directly bond to Si adatoms and rest atoms. 12 However, using first principles density-functional theory ͑DFT͒ calculations, Cho and Kaxiras suggested that a bridge ͑B 2 ͒ site among the high coordination sites has the minimum energy for Ge adsorption, even compared to those of Ge adatoms bounded with dangling bond of adatoms and rest atoms. 21 On the other hand, Wang et al. 20 explored the Ge adsorption over Si adatom sites in detail as well as sites other than the B 2 site. They concluded that Ge atoms are replaced with Si adatoms, and the kickedout Si atoms migrate to near B 2 sites within a basin on a half unit cell ͑HUC͒: This is in agreement with earlier reports. 3,4,15,20 In this letter we report on the initial stages of Ge adsorption on Si͑111͒-7 ϫ 7 at room temperature ͑RT͒ and at 100°C, which are observed by scanning tunneling microscopy ͑STM͒. The results show that Ge atoms are adsorbed over B 2 sites at RT, while they are adsorbed at corner adatom sites at 100°C by being presumably replaced with the Si adatoms, preferably over a faulted half unit cell ͑FHUC͒.Experiments were performed using a homemade ultrahigh vacuum ͑UHV͒ STM having a base pressure Ͻ2 ϫ 10 −11 Torr, equipped with a field emission microscope 22 ͑FEM͒ to evaluate STM tips, and with a resistive-type Ge evaporation source. N-type Sb-doped Si͑111͒ substrates were used ͑thickness= 375 m, = 0.02 ⍀ cm͒. Clean Si͑111͒-7 ϫ 7 surfaces were prepared by the ...

Low-flux elucidation of initial growth of Ge clusters deposited onSi(111)−7×7observed by scanning tunneling microscopy

“…[1][2][3][4][5][6][7][8][9][10] While Ge nanostructures have been widely studied on Si͑001͒ surfaces, 1-3 there has been growing interest for Ge nanostructures on a Si͑111͒-7 ϫ 7 reconstructed surface with dimer rows, adatoms, and stacking fault ͑DAS͒ explained by the DAS model. 11 Many reports of Ge growth on Si͑111͒-7 ϫ 7 surfaces presented thermodynamic fluctuations on morphology of nanostructures grown at different temperatures and fluxes; [6][7][8][9][10][12][13][14][15][16][17][18][19] the morphology delicately changes with growth conditions.…”

“…On the other hand, at 100°C a Ge atom was initially substituted for a Si adatom, and with increasing Ge deposition various types of clusters were formed on half unit cells ͑HUCs͒ of the 7 ϫ 7 reconstruction as a result of complex and diverse bonding configurations. [13][14][15][16][17]19 The diverse Ge nanostructures indicate that formation of self-organized nanostructures crucially depends on coverage, temperature, flux, quality of substrate, and so on, which are key factors to produce well-ordered and uniform nanostructures. [6][7][8][9][10][12][13][14][15][16][17][18][19] To elucidate the growth conditions and mechanism, systematic study is strongly recommended.…”

“…12,15 The STM chamber was equipped with a field emission microscope for tip evaluation and a resistive heating type of Ge source. Cuts ͑12ϫ 2 ϫ 0.5 mm 3 ͒ from an n-type Sb-doped Si͑111͒ wafer with a resistivity of ϳ0.02 ⍀ cm were used as substrate.…”

Self-assembly of periodic nanoclusters of Si and Ge along atomically straight steps of a vicinal Si(111)

“…It was reported that the preferential nucleation site of various atoms on wide 7 ϫ 7 terraces of the Si͑111͒ surface is the faulted half. [27][28][29][30][31] Therefore, the confinement of the nanoclusters on the unfaulted half in the present study results from the presence of the decorated step edge: The nucleation at the upper step edge is initiated by the atomic wire, which, together with the FIG. 2.…”

One-dimensional ordering of Ge nanoclusters along atomically straight steps of Si(111)

“…24 Ge nanocluster formation is also observed on the terraces as reported before for the Ge growth on the nominal Si͑111͒ surfaces. [25][26][27][28][29][30][31][32][33] It is interesting to note that nucleation of the Ge nanoclusters at the edges occurs predominantly on the unfaulted halves of the 7 ϫ 7 structure, while nucleation on the terraces occurs preferentially on the faulted halves. 26,27,31 We find that, for the particular miscut of our samples ͑the average terrace width of 18 nm͒, the substrate temperature T s = 250°C during Ge growth leads to the optimum ordering of the Ge clusters along the step edges.…”