R. W. Fathauer

Visible luminescence from silicon wafers subjected to stain etches

Etching of Si in a variety of solutions is known to cause staining. These stain layers consist of porous material similar to that produced by anodic etching of Si in HF solutions. We have observed photoluminescence peaked in the red from stain‐etched Si wafers of different dopant types, concentrations, and orientations produced in solutions of HF:HNO3:H2O. Luminescence is also observed in stain films produced in solutions of NaNO2 in HF, but not in stain films produced in solutions of CrO3 in HF. The luminescence spectra are similar to those reported recently for porous Si films produced by anodic etching in HF solutions. However, stain films are much easier to produce, requiring no special equipment.

Electronic structure of light‐emitting porous Si

Characterization of light‐emitting porous Si films with x‐ray photoelectron spectroscopy is reported. Only traces of O are detected on HF‐etched samples, in contradiction to an earlier report that oxides are a significant component of porous Si. Si 2p and valence‐band measurements demonstrate that the near‐surface region of high porosity films which exhibit visible luminescence consists of amorphous Si.

Hydrogen-terminated silicon substrates for low-temperature molecular beam epitaxy

Abstract The preparation of hydrogen-terminated silicon surfaces for use as starting substrates for low-temperature epitaxial growth by molecular beam epitaxy is examined in detail. The procedure involves the ex-situ removal under nitrogen of residual oxide from a silicon substrate using a spin-clean with HF in ethanol, followed by the in-situ low-temperature desorption (150°C) of physisorbed etch residues. The critical steps and the chemical basis for these steps are examined using X-ray photoelectron spectroscopy. Impurity residues at the epilayer-substrate interface following subsequent homoepitaxial growth are studied using Auger spectroscopy, secondary ion mass spectrometry, and transmission electron microscopy. Finally, scanning tunneling microscopy is used to examine the effect of cleaning methods on substrate morphology.

Growth and characterization of single crystal insulators on silicon

Abstract Epitaxial insulators have a number of potential applications in the semiconductor industry. These include semiconductor-on-insulator (SOI) structures, three-dimensional (3-D) and/or high-density integrated circuits, improved gate insulators, and optoelectronic applications such as integrated wave guides. Because of the potential for such applications, there are a number of approaches to epitaxial insulators that are being pursued at different laboratories. Much of this activity has centered on the growth of epitaxial Group-I1 fluorides by molecular beam epitaxy (MBE). However, alternatives such as vapor phase epitaxy (VPE) of spinel are also being pursued. In any case, the mechanisms for good heteroepitaxy must be understood in order to grow good material. Applications in the semiconductor industry will also require that the electrical properties, such as dielectric breakdown, current transport across the interface, and current transport along the interface, be understood. These topics are review...

Microstructural investigations of light-emitting porous Si layers

The structural and morphological characteristics of visible‐light‐emitting porous Si layers produced by anodic and stain etching of single‐crystal Si substrates are compared using transmission electron microscopy and atomic force microscopy (AFM). AFM of conventionally anodized, laterally anodized and stain‐etched Si layers show that the layers have a fractal‐type surface morphology. The anodized layers are rougher than the stain‐etched films. At higher magnification 10 nm sized hillocks are visible on the surface. Transmission electron diffraction patterns indicate an amorphous structure with no evidence for the presence of crystalline Si in the near‐surface regions of the porous Si layers.

Room‐temperature codeposition growth technique for pinhole reduction in epitaxial CoSi2 on Si (111)

A solid phase epitaxy technique has been developed for the growth of CoSi2 films on Si (111) with no observable pinholes (103 cm−2 detection limit). The technique utilizes room‐temperature codeposition of Co and Si in stoichiometric ratio, followed by the deposition of an amorphous Si capping layer and subsequent in situ annealing at 550–600 °C. CoSi2 films grown without the Si cap are found to have pinhole densities of 107–108 cm−2 when annealed at similar temperatures. A CF4 plasma etching technique was used to increase the visibility of the pinholes in the silicide layer. This plasma technique extends the pinhole detection resolution to 103 cm−2 and is independent of the pinhole size.

Visible photoluminescence of porous Si1−xGex obtained by stain etching

We have investigated visible photoluminescence (PL) from thin porous Si(1-x)Ge(x) alloy layers prepared by stain etching of molecular-beam-epitaxy-grown material. Seven samples with nominal Ge fraction x varying from 0.04 to 0.41 were studied at room temperature and 80 K. Samples of bulk stain etched Si and Ge were also investigated. The composition of the porous material was determined using X-ray photoemission spectroscopy and Rutherford backscattering techniques to be considerably more Ge-rich than the starting epitaxial layers. While the luminescence intensity drops significantly with the increasing Ge fraction, we observe no significant variation in the PL wavelength at room temperature. This is clearly in contradiction to the popular model based on quantum confinement in crystalline silicon which predicts that the PL energy should follow the bandgap variation of the starting material. However, our data are consistent with small active units containing only a few Si atoms that are responsible for the light emission. Such units are present in many compounds proposed in the literature as the cause of the visible PL in porous Si.

Endotaxial growth of CoSi2 within (111) oriented Si in a molecular beam epitaxy system

A new mode of growth is reported in which buried metallic layers can be fabricated within a single‐crystal semiconductor through preferential subsurface growth on previously‐grown ‘‘seed’’ regions. The deposition of Co at 800 °C at a rate of 0.01 nm/s on (111) Si substrates containing buried CoSi2 columns 40–100 nm below the Si surface results in the growth and coalescence of these subsurface columns. The formation of a CoSi2 layer on the Si surface is suppressed by this growth mode. It is proposed that the high diffusion rate of Co at 800 °C, coupled with the high growth rate of CoSi2 at the subsurface columns, is responsible for this preferred ‘‘endotaxial’’ growth mode. This growth technique was used to produce a continuous buried single‐crystal layer of CoSi2 under a single‐crystal Si capping layer.

Long-wavelength PtSi infrared detectors fabricated by incorporating a p(+) doping spike grown by molecular beam epitaxy

By incorporating a 1‐nm‐thick p+ doping spike at the PtSi/Si interface, we have successfully demonstrated extended cutoff wavelengths of PtSi Schottky infrared detectors in the long wavelength infrared (LWIR) regime for the first time. The extended cutoff wavelengths resulted from the combined effects of an increased electric field near the silicide/Si interface due to the p+ doping spike and the Schottky image force. The p+ doping spikes were grown by molecular beam epitaxy at 450 °C using elemental boron as the dopant source, with doping concentrations ranging from 5×1019 to 2×1020 cm−3. Transmission electron microscopy indicated good crystalline quality of the doping spikes. The cutoff wavelengths were shown to increase with increasing doping concentrations of the p+ spikes. Thermionic emission dark current characteristics were observed and photoresponses in the LWIR regime were demonstrated.

Molecular‐beam epitaxy of CrSi2 on Si(111)

Chromium disilicide layers have been grown on Si(111) in a commercial molecular‐beam epitaxy machine. Thin layers (10 nm) exhibit two epitaxial relationships, which have been identified as CrSi2(0001)//Si(111) with CrSi2[1010]//Si[101], and CrSi2(0001)//Si(111) with CrSi2[1120]//Si[101]. The latter case represents a 30° rotation of the CrSi2 layer about the Si surface normal relative to the former case. Thick (210 nm) layers were grown by four different techniques, and the best‐quality layer was obtained by codeposition of Cr and Si at an elevated temperature. These layers are not single crystal; the largest grains are observed in a layer grown at 825 °C and are 1–2 μm across.