A novel technique to grow Ge quantum dots on porous Si by ultrahigh vacuum chemical vapor deposition

Abstract

Semiconductor quantum dots have attracted worldwide attention now due to their suitability for quantum physics study and their potential device applications in both future microelectronics and optoelectronics industry. Ge and SiGe quantum dots with their technological compatibility to the Si industry are especially important [1, 2]. Recent research in this field led to very promising results, such as the efficient Si-SiGe dry etched quantum dot based light emitting diodes, which could potentially be used in all Si-based optical interconnects on Si microchips. The investigation on semiconductor quantum dots is one of the most attractive areas in semiconductor science and technology. A variety of techniques have been developed to fabricate semiconductor quantum dots [3–13], among them the self-assembled growth of nanometer size islands in highly strained system is one of the methods on which people’s interests are currently focused. Ge/Si is also a strain system with the misfit in the lattice constant of 4.2%. Several groups have achieved the self-organizing growth of Ge-rich islands on Si by using molecular beam epitaxy or chemical vapor deposition. However, it is very difficult to control the size and position, To overcome this difficulty and to accurately control dots in both size and position, certain methods have been proposed. One of them is to define SiGe/Si quantum dots by electron beam lithography and reactive ion etching after growing quantum well structure. Another method is to grow dots in the Stranski-Krastanov mode on cleaved edges utilizing the periodic strain modulation resulting from the underlying SiGe/Si multiple quantum wells. A method considered to be more effective is to fabricate dots in selective areas on patterned substrates. By using selective epitaxial growth on patterned substrate, it is generally possible to control the size and the position. Here, we used a very low cost way to grow Ge quantum dots on porous silicon, substituting for a patterned substrate, as its substrate successfully. In this work, the Ge dots were grown via an ultrahigh vacuum chemical vapor deposition (UHV/CVD) system, although can be done by molecular beam epitaxy and other techniques. The porous silicon is formed by anodic dissolution of crystalline silicon in 50% hydrofluoric acid solution diluted by alcohol with the equipment shown in Fig. 1. The wafers were diameter 75 mm Czochralski silicon with 〈100〉 orientation, p-type (boron doped, 10 cm). Before anodization, aluminum alloy layers were formed on the back of silicon wafers as the low resistivity contact layer in order to make the anodic current distribution uniform. We know that the density and size of porous silicon are affected mainly by silicon resistivity, current density, hydrofluoric acid concentration, and etching time [14, 15]. The uniform and quantum size porous silicon was obtained with 5 mA/cm2 current density and 5 min etching time. Sample was grown by a UHV/CVD system. Reactants were high pure germane (GeH4). The pressure during film growth was in the range of 5–10 mTorr. Prior to being introduced into the sample chamber, the porous silicon wafer was submitted to a chemical cleaning process, followed by a dip in dilute hydrofluoric acid (10% HF) without being rinsed. After being transferred to the growth chamber, the substrate was baked at 720 ◦C in the 5 sccm flow GeH4 environment for 5 min. This process had two results: one was to remove residue oxygen [16], the other was to deposit Ge islands uniformly. The size of Ge islands grown on the porous silicon was small than that of Si substrate. The structure of Ge dots on porous silicon was characterized by a Philip X’pert X-ray diffraction (XRD). The surface morphology and the dots’ dimension were investigated by atomic force microscopy (AFM). Photoluminescence (PL) spectra were measured in standard lock-in configuration with a He-Ne laser for exciton and with a liquid-nitrogen cooled Ge detector. Fig. 2 shows the measured X-ray diffraction rocking curve of the sample of Ge quantum dots on porous silicon. We can see that the dominant peak at 2θ = 69.12 ◦ (peak 1, plane space d1 = 1.363 A) is the (400) diffraction from Si substrate and peak 2 is due to Cu Kα2 line from the Si substrate. The peak 3 at 2θ = 68.87 ◦ (plane space d3 = 1.368 A) is identified as the porous silicon diffraction. The lattice parameter of the porous silicon layer is a little bigger than that of silicon. Therefore, porosity affects the crystal lattice by producing a