n‐type silicon‐doped epitaxial layers of gallium arsenide grown by molecular‐beam epitaxy (MBE) or metal‐organo chemical vapor deposition (MOCVD) have been investigated by measurements of the Hall effect and the strengths of the localized vibrational modes (LVM) of silicon impurities using both Fourier transform absorption spectroscopy and Raman scattering at an excitation energy of 3 eV close to the E1 band gap. Lines from Si(Ga) donors, Si(As) acceptors, Si(Ga)‐Si(As) pairs, and Si‐X, a complex of silicon with a native defect, were detected and correlated for the two techniques. The maximum carrier concentration [n] found for samples grown under standard conditions was 5.5×1018 cm−3. At higher doping levels Si‐X becomes dominant and acts as an acceptor, so reducing [n]. An integrated absorption of 1 cm−2 in the Si(Ga) LVM line corresponds to 5.0±4×1016 atoms cm−3: a similar calibration applies to the Si(As) line, but for Si‐X, an absorption of 1 cm−2 corresponds to only 2.7±1.0×1016 defects cm−3. Possib...

The calibration of the strength of the localized vibrational modes of silicon impurities in epitaxial GaAs revealed by infrared absorption and Raman scattering

Secondary Ion Mass Spectrometry (SIMS) is a mature surface analysis technique with a vast range of applications in Materials Science. In this review article the SIMS process is described, the fundamental SIMS equations are derived and the main terminology is explained. The issue of quantification is addressed. The various modes of SIMS analysis including static SIMS, imaging SIMS, depth profiling SIMS and three-dimensional (3D) SIMS are discussed as are specialized analysis strategies such as the imaging of shallow bevels and cross-sections and reverse side analysis. SIMS is shown to be a useful sample preparation tool based on ion beam milling (with SIMS and Scanning Electron Microscopy (SEM) analysis providing end-point detection). The case studies shown illustrate the application of SIMS to several important materials including semiconductors, superconductors, glass, stainless steel, micrometeoroids, solid oxide fuel cell components, museum artifacts, aerospace alloys and biomaterials. Strategies for introducing SIMS into undergraduate education and thus increasing awareness are described. Finally some informed guesses are made as to the future directions of SIMS.

Applications of Secondary Ion Mass Spectrometry (SIMS) in Materials Science

Dopant segregation in the well region of a multiple quantum well intersubband photodetector can cause an asymmetry in the observed forward and reverse current‐voltage characteristics. We compensate for the segregation by shifting the position of the Si δ doping in the well and model the effect with good agreement for a range of shift values. For samples grown at a substrate temperature of 605 °C, we find that the observed behavior is best described by assuming that the Si δ‐doping profile smears in the growth direction resulting in an asymmetric broadening of about 27 A.

Segregation of Si δ doping in GaAs-AlGaAs quantum wells and the cause of the asymmetry in the current-voltage characteristics of intersubband infrared detectors

High-quality GaMnAs films grown with arsenic dimers

Shubnikov-de Haas and persistent photoconductivity measurements have been performed as a function of hydrostatic pressure to study the saturation of the free electron concentration and the mobilities of the individual subbands at high doping densities in very thin sheets (2, 5, 10 nm) of silicon donors in MBE GaAs. The samples were grown at very low temperature (400 degrees C) in order to limit dopant diffusion, and silicon concentrations were close to the solubility limit at this temperature. As has been shown previously with spike-doped GaAs(Si), the relative occupancies and the mobilities of the lower subbands are very sensitive to the spreading of the dopant distribution. A routine was developed for the analysis of the Fourier transforms of the complex pattern of Shubnikov-de Haas peaks in order to provide quantitative values for the mobilities of the individual subbands. The results of this analysis are compared with values deduced from the magnetic field dependence of the resistivity and Hall effect. On applying hydrostatic pressures of the order of 15 kbar in the dark, a decrease of the free electron concentration of a factor of two was observed. This was accompanied by an increase in the mobility of all the subbands due to the change in the charge state of the silicon donors in the doping slab. With the two thinnest slabs the mobility at ambient pressure is so low in the i=0 subband that Shubnikov-de Haas peaks from this subband could not be detected at fields up to 15 T, although strong peaks could be observed from the higher order subbands. After illumination of the thinnest sample at high pressure the measured free electron concentration is not restored to the zero pressure value. One possibility is that the missing electrons occupy a localized non-metastable Si state resonant with the conduction band rather than a DX centre. The pressure coefficient of the carrier density yields an extrapolated position for the energy level for the Si localized state of 270+or-10 meV above the Gamma -conduction band edge at ambient pressure.

https://iopscience.iop.org/article/10.1088/0268-1242/6/6/023/pdf

Subband dependent mobilities and carrier saturation mechanisms in thin Si doping layers in GaAs in the high density limit

Si atomic plane or delta ( delta )-doping of GaAs during MBE has been investigated using SIMS profiling optimised for high depth resolution. For layers in which almost all the Si atoms act as donors, post-growth diffusion occurs at the growth temperature with a diffusion coefficient estimated to be 9*10-17 cm2 s-1 at a substrate temperature of 550 degrees C. At temperatures greater than about 550 degrees C, a marked preferential migration towards the surface is observed, which may be due to surface segregation or possibly enhanced diffusion.

http://iopscience.iop.org/article/10.1088/0268-1242/3/6/019/pdf

Migration of Si in δ-doped GaAs

Delta-doping of GaAs and Al0.33Ga0.67As with Sn, Si and Be: a comparative study

A GaAs layer grown by MBE at a substrate temperature of 520 degrees C and containing three delta -doped planes with Si concentrations of 0.4, 1 and 4*1013 atoms/cm2 has been post-growth annealed in a furnace, up to temperatures of 648 degrees C. Secondary ion mass spectroscopy (SIMS) and capacitance voltage (cv) measurements have been carried out to measure profile-broadening. The most lightly doped plane gave a near-Gaussian diffusion profile with a diffusion coefficient comparable with literature values for simple diffusion of isolated SiGa atoms. The more heavily doped planes exhibit a complex profile shape with two components, a proportion of the atoms being confined to the original plane, together with an almost square-shaped profile of fast-diffusing atoms. Comparison of the CV and SIMS data suggests that formation of Si islands is taking place during deposition of the delta -doped plane, giving electrically inactive atoms which can subsequently diffuse into the surrounding GaAs during heat-treatment. This model is supported by preliminary local vibrational mode measurements which have been made on a set of multiplane samples.

https://iopscience.iop.org/article/10.1088/0268-1242/4/12/021/pdf

Post-growth diffusion of Si in delta -doped GaAs grown by MBE

Si migration effects in GaAs/(Al,Ga)As heterojunction and δ-doped structures

The authors have used high depth-resolution SIMS profiling to investigate the broadening of delta-doped planes of Be and (Be+Si) in GaAs grown by molecular beam epitaxy. They have confirmed that concentration-dependent diffusion is the dominant broadening process for Be at growth temperatures <or=600 degrees C, and by incorporating Si atoms on the same plane, they have shown that this broadening can be completely inhibited. This suggests that the rapid diffusion process may be the result of mutual repulsion between the BeGa- ions, and is prevented by the reverse field due to the SiGa+ ions, or by the formation of low-mobility SiGa+-BeGa- complexes. The rapid diffusion of Si as SiGa-SiAs pairs is also reduced, and they believe this is a Fermi-level effect, the compensation by Be reducing the probability of SiAs formation. The surface segregation of Si is not affected, whereas that of Be is reduced, indicating that surface fields during growth contribute to this effect for Be, but not for Si.

R B Beall

Papers

Migration of Si in δ-doped GaAs

Delta-doping of GaAs and Al0.33Ga0.67As with Sn, Si and Be: a comparative study

Post-growth diffusion of Si in delta -doped GaAs grown by MBE

Si migration effects in GaAs/(Al,Ga)As heterojunction and δ-doped structures

Migration processes of co-doped Si and Be delta-planes in MBE-grown GaAs