Type II Band Alignment inSi1−xGex/Si(001)

Abstract

We present experimental verification of type II band alignment in a coherently strained Si 0.7 Ge 0.3 ͞Si͑001͒ quantum well by studying photoluminescence energy shifts under external strains. A recent determination of type I band alignment from a similar experiment is shown to result from band-bending effects due to high excitation. In high quality samples, the type II luminescence can be observed in the absence of external stress by using extremely low excitation. The type II luminescence differs from the well known type I spectrum in a dramatic but as yet unexplained change in the relative intensities of the phonon replicas. [S0031-9007(97)03558-8] PACS numbers: 73.20.Dx, 78.66.Db The growth and properties of strained Si 12x Ge x , and more recently Si 12x2y Ge x C y heterostructures on (001) Si has attracted intense interest, not only because of the technological promise of combining band gap engineering with a materials system compatible with standard Si processing, but also due to the fact that this is the prototypical indirect band gap heterostructure system. Until recently, little was known about the optical properties of such structures, compared to the exhaustively studied direct gap heterostructures, but the discovery of well-resolved neargap photoluminescence (PL) [1,2] led to a speedy adaptation of PL as a standard assessment tool, and to a rapid increase in our understanding of the physical processes involved [2][3][4][5][6][7]. As in bulk, relaxed, Si 12x Ge x alloys [3], the PL of typical samples is dominated by impuritybound excitons (BE) at liquid He temperatures, and "free" excitons (FE) at higher temperatures [2], remembering that these FE move in a random potential due to alloy disorder whose width exceeds kT at low temperatures. Related to this, localized excitons (LE) associated with local band gap minima have been discovered and shown to have much higher PL quantum efficiency and longer lifetimes than BE (dominated by Auger recombination [4,5]). While most descriptions of the now ubiquitous PL spectra of these systems limit discussion to the BE͞FE species, the long carrier lifetimes make it easy to reach high excitation conditions in the quantum well (QW), and the importance of biexcitons and electron-hole plasmas in the PL processes under normal excitation conditions has been demonstrated [6,7].One of the remaining contentious issues regarding the physics and optical properties of these systems involves the band-edge alignments. While it was known early on that the majority of the band gap difference was taken up in the valence-band (VB) discontinuity, the only certainty regarding the conduction band (CB) edge was that the discontinuity was small. Theoretical studies have predicted both type I band alignments [8,9], with electrons ͑e͒, and holes ͑h͒ localized in the Si 12x Ge x , and type II alignments [10,11], with h localized in the Si 12x Ge x and e in the Si. Some PL studies claimed evidence for type I behavior, using line shifts [12] and hydrostatic pressure effects [13], respectiv...

Direct gap Ge1-ySny alloys: Fabrication and design of mid-IR photodiodes

“…37 This was done due to the dearth of theoretical and experimental data for a-Sn. More recently, 40 imply E v,av (Si) ¼ À0.800 eV, and subsequent capacitancevoltage measurements at Si/Ge 1-x Si x interfaces are also in very good agreement with this value. 34 These results provide strong support for Li's theoretical results.…”

Effects of interdiffusion on the band alignment of GeSi dots

“…For example, even when the excitation power density increases from 1 to 4 W/cm 2 , the blueshift was as large as ϳ3.6 meV, in a 3-nmtype-II Ge 0.3 Si 0.7 /Si quantum well. 12 This NP dot PL peak blueshift ͑1.5 meV͒ of sample B is similar to the blueshift of the GeSi wetting layer, whose band alignment is type I, 4 so, the band alignment of sample B could be reasonably explained by a type-I band alignment and the blueshift is due to the band filling effect, 11 which is due to the finite density of states of the GeSi dots layer. The power dependence of the integrated PL intensities of the dots as shown in Fig.…”

“…The power-dependent PL measurements were widely used to study the band alignment of the GeSi quantum dots, [2][3][4] SiGe/Si quantum well, 11,12 and III-V heterostructures. 13 In the type-II band alignment quantum structure, a band bending occurs at the interfaces due to the Hartree potential, as seen in the insets of Fig.…”

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Impact on SiGe Light Emission

“…It has also been found that, compared to two-dimensional (2D) Si/SiGe NSs, the PL and EL quantum efficiency in 3D Si/SiGe NSs is higher (up to 1%), especially for T > 50 K (Cerdeira et al, 1972;Apetz et al, 1995;Schittenhelm et al, 1995;Schmidt et al, 1999b). Despite many successful demonstrations of PL and EL in the spectral range of 1.3-1.6 μm, which is important for optical fiber communications, the proposed further development of 3D Si/SiGe-based light emitters was discouraged by several studies, indicating a type II energy band alignment at Si/SiGe heterointerfaces (Van de Walle and Martin, 1986;Thewalt et al, 1997;Schittenhelm et al, 1998;El Kurdi et al, 2006), where the spatial separation of electrons (located in Si) and holes (localized in SiGe) (see 1) was thought to make carrier radiative recombination very inefficient. Later, it was also shown that 3D Si/SiGe NSs exhibit an extremely long (of the order of 10 −2 s) luminescence lifetime (Kamenev et al, 2005), which is of the order of a million times longer than in III-V semiconductors and their NSs.…”

“…So far, intense luminescence has been observed only at low temperatures in Si1−xGex in single and multiple QWs for x ≤ 0.2 [see Robbins et al (1992)]. There have been several studies of the energy band alignment (i.e., to determine whether it is type I or type II; see Figure 1) in these NSs, but no final conclusions have yet been reached (Houghton et al, 1995;Thewalt et al, 1997;Shiraki and Sakai, 2005). Si and Ge interdiffusion during the growth of Si/Si1−xGex QWs broadens the heterointerfaces, but this interdiffusion can be reduced by using moderately low growth temperatures (less than 550°C).…”