Through the use of low temperature Si molecular beam epitaxy (MBE), we have fabricated high performance Si1−x Gex/Si heterojunction bipolar transistors (HBTs) and bipolar inversion-channel field effect transistors (BICFETs). Our growth method employs a high temperature Si-assisted desorption followed by MBE growth at a temperature only slightly in excess of the critical temperature for two-dimensional layer-by-layer growth. [1] At this temperature, segregation of Sb has previously been shown to be kinetically limited. [2] In addition, significantly more strain can be frozen into such an epitaxial layer as compared with those grown at higher temperatures.[3] Secondary ion mass spectroscopy (SIMS) data verify the abruptness of the Sb doping profiles in our device structures. High resolution x-ray diffraction (HRXRD) data are consistent with planar, coherently strained Si1−x Gex layers in our HBTs. A gain of 2690 (3210) is observed in our BICFETs at 300 K (7 K).

Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) has been successfully used to grow GexSi1−x/Si (x = 0.1 – 1.0) heteroepitaxial structures at low temperatures (∼450°C). This technique utilizes a noble gas (Ar or He) r-f plasma to decompose reactant gases (SiH4 and GeH4) and drive the chemical deposition reactions in the gas phase as well as on the substrate surface. Growth of pure Ge on Si is of great interest because it provides a promising technique for making suitable low-cost substrates for thin-film Ge photodetectors as well as GaAs devices on Si substrates. The realization of these applications depends on the ability to grow high-quality epitaxial Ge layers on Si substrates. Since GaAs is lattice matched to Ge, growth of Ge layers on Si substrates with good crystalline perfection would permit the integration of GaAs and Si devices. Islanding was observed after the growth of pure Ge films directly on Si(100) for a wide range of plasma powers (7W ∼ 16W) in RPCVD. Cross-sectional TEM analysis showed that the islands have complicated facet structures, including {311} planes. Graded Gex Si1−x buffer layers with different Ge profiles have been used prior to the growth of Ge. It was found that uniform Ge films can be obtained using a buffer with an abrupt Ge profile, and the dislocation density in the Ge film decreases with increasing distance from the substrate.

Boron diffusion in Sia7 Gea3 strained layers has been studied as a function of annealing temperature and is compared to boron diffusion in unstrained Si epitaxial layers. A lower diffusivity was observed in Sia7 Gea3 strained layers as compared to the Si control samples. Using the well-accepted model for boron diffusion via charged point defects in Si, the intrinsic diffusivities of the boron atoms in the Sia7 Gea3 layers were derived and an activation energy of 4.4 eV was estimated for the diffusion process.

A new approach to describe the diffusion process is also presented here. In this model, we relate the lower diffusivity to a corresponding change in the charged point defect equilibrium concentration caused by a change in band-gap due to the strain and the existence of foreign atoms in a Si matrix.

1007×100 μm2 n+− Si/p-Si1−x Gex diodes fabricated on conventional p-type Si substrates are irradiated at room temperature with 1-MeV electrons with fluences from 1.0×1014 to 1.0×1015 e/cm2 in a high voltage transmission electron microscope. The boron concentration and germanium fraction of the Si1−x Gex epitaxial layer used for the diodes in this study are 5×1017 cm−3 and x = 0.12, respectively. The degradation of diodes is investigated by means of current/voltage and capacitance/voltage measurements. The characteristics of the electrically active defects induced in the Si1−x Gex epitaxial layer by irradiation are also examined by using deep level transient spectroscopy and capacitance/temperature measurements. The degradation of the diode performance and the presence of deep levels are investigated as a function of electron fluence. In order to examine the recovery process, an isochronal thermal anneal is performed in the temperature range between 100 and 400°C.

In this study, photoreflectance (PR) spectroscopy and x-ray rocking curves measurements were used to study the variation in strain configuration, defect propagation, structural properties and direct electronic transition energies in Sim Gen superlattices (SL) and nearly relaxed Si1−x Gex buffer layers grown on < 100 > Si as a function of annealing temperature. The in-plane (a│) and perpendicular (a┴) lattice constants of the alloy buffer layers are found to vary only slightly with anneal temperature, TA, up to a temperature To. For TA To, the in-plane strain changed from roughly zero a│ ≈ a┴ (relaxed) or a┴ > a│ (compressive) to a┴ > a│ (tensile). This change in strain configuration is believed to be caused by the difference in thermal expansion coefficients between the epilayer and the Si substrate. The anneal temperature T0 is also correlated with the disappearance of higher order x-ray harmonics from the SL. This point was also correlated with a large energy shift and broadening of the PR spectra from the SL. The shift in energy of the PR spectra is explained in terms of the interdiffusion of Si and Ge at SL heterointerfaces, and to a lesser degree, the strain induced by the above mentioned difference in thermal expansion coefficients. The PR spectra of the alloy E0 transitions are also observed to shift to higher energy with increasing TA.

We report Hall mobilities (at T = 4.2K) as high as 180,000cm2V−1 s−1 in modulation-doped Si layers in Si1−x Gex heterostructures grown by Molecular-Beam Epitaxy. These mobilities reflect dramatic improvements in the quality of relaxed Si1−xGex buffer layers (with x'30%) grown by gradual grading of composition at high temperature. The resulting moderate threading dislocation densities (< 106 cm−2 ) appear to cause no mobility degradation. The strong damping of Shubnikov de Haas oscillations, as well as the increase of mobility with carrier density, indicate predominantly small-angle scattering. This suggests that residual Coulomb scattering from background impurities limit the mobility, rather than interface-roughness scattering as for the Si/SiO2 interface. The reduced interfacial scattering, as well as the strain-induced splitting of the valley degeneracy to select the two low-effective-mass valleys, significantly enhance room-temperature transport as well, with μHall ' 2,100cm2 V−1 s−1. We also observe a small splitting of the remaining twofold valley degeneracy using the integral quantized Hall effect. As a further indication of the high sample quality, measurements to 17T at 0.3K show indications of the v = 2/3 fractional quantum Hall effect.

Si1−xGex alloys and multilayers synthesized by solid source MBE on Si(100) substrates have been characterized by low temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. PL spectroscopy over the temperature range 2K to 100K has been used to characterize Si1−xGex/Si heterostructures exhibiting both types of PL spectra. Thin alloy layers exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. In single quantum wells confinement shifts up to ∼200 meV were observed (1.2 nm wells with x = 0.38) and NP linewidths down to 1.37 meV were obtained. However, the broad PL band (peak energy ∼120meV below the strained bandgap) was predominant when the alloy layer thickness was greater than 2 – 10nm, depending on x, growth temperature, and substrate surface preparation. The strength of the broad PL band was correlated with the areal density of strain perturbations (∼109cm−2 per quantum well; local lattice dilation ∼1.5 nm in diameter) observed in plan-view TEM. The role of MBE growth parameters in determining optical properties was investigated by changing growth temperature, substrate preparation procedures and exploring the effect of surface passivation in a hydrogen ambient. In addition, post growth anneals at temperatures in the range 700°C to 1 100°C were carried out, where interdiffusion removes interfacial asperities and the broad luminescence band decays to zero intensity.

The effect of the Si:Ge ratio in SiGe/Si heterostructures on the structural and optical properties of SiGe/Si planar waveguide are reported here for Ge concentrations from 1 to 33.6%. The high propagation loss at 1.15 pm is due to band edge absorption, which increases as the Ge concentration increases, while the loss at longer wavelengths (1.523 pm) increases with decreasing Si concentration, due to the reduced optical confinement of the waveguide structure.

Recent studies on strained layer heteroepitaxy in high misfit systems (eg. Ge on Si) have clearly indicated the importance of elastic strain on a number of surface-mediated growth effects. In this work we consider the transition from ideal, 2-D layer-by-layer growth to the initial stages of a 3-D growth morphology as a precursor to misfit dislocation injection in GexSi1−x/Si heterostructures with x as low as 0.15. Experimentally, we have studied a wide range of MBE-and CVD-grown single and multilayer GexSi1−x/Si (x<0.5) structures using transmission electron microscopy and photoluminescence spectroscopy. Firstly, we describe a new mechanism for the heterogeneous nucleation of misfit dislocations in strained epitaxial layers, the ‘double half-loop’ source, which originates from atomic-scale (<1.5 nm) interfacial perturbations. Secondly, the atomic-scale dilatational perturbations, which can exist in areal densities up to ∼109 cm−2, have been identified as the origin of intense, broad-band PL from MBE-grown GexSi1−x/Si strained layers. The change in PL behaviour with increasing strained layer thickness has been used to study the effects of elastic strain-induced surface roughening at low misfits.

Room temperature Raman spectra are reported of Si1−xGex layers on Si substrates (for 0.08≤x≤0.2). The samples were grown using atmospheric pressure chemical vapor deposition techniques. Layer thicknesses varied from 0.1 — 10μm. The relative frequency shift of the Si-Si phonon mode for the SiGe strained epilayers from an incommensurate pseudo-alloy of the same composition is used as a quantitative measure of the lattice strain. For thicknesses below a critical value the Raman data indicate that the films are highly strained and the growth is commensurate with the substrate whereas thicker films are partially or fully relaxed. Phonon lines are sensitive to the interfaces and alloy layers. The results are consistent with other characterization studies, such as TEM and X-ray rocking curve measurements, of the same samples.

The rate of solid-phase epitaxial regrowth has been studied using time-resolved reflectivity in three different types of SiGe/Si epilayers amorphized by ion implantation. In two of these cases, the alloy epilayer contained either 12% or 20% Ge, and the amorphization depth was greater than the thickness (2000 Å) of the SiGe alloy layer. Time-resolved reflectivity measurements showed that the rate of regrowth was not constant in these two cases, but first decreased after passing the SiGe/Si interface, and then increased. The minimum regrowth rate occurred closer to the SiGe/Si interface in the epilayers with the larger Ge atomic fraction. In the third type of sample, the alloy epilayer thickness was ∼7μm, so that the initial epilayer (15% Ge) had the lattice constant of the bulk alloy. Furthermore, amorphization and regrowth occurred entirely within the relaxed alloy layer. In this case, the regrowth rate was constant. The composition dependence of the regrowth-rate transient in the strained layers is discussed in the context of a ‘critical-thickness’ model of strain relaxation.

Microwave remote plasma oxidation system was used to study the oxidation of SiGe samples at low temperatures. The extent of Ge segregation at oxide/SiGe interface was investigated by using SIMS depth profiles. By comparing the segregation factors, the Ge segregation in the samples oxidized by atomic oxygen at 500 °C was much less than that in the samples oxidized without plasma at 950 °C. The Ge segregation has been largely suppressed by atomic oxygen oxidation at lower temperature.

The paper presents the results of preliminary experiments on the production of GexSi1−x/Si structures using deposition of a thin Ge film on a Si substrate, implantation of Si ions and rapid electron-beam annealing. The conditions under which monocrystalline layers form have been found. It is supposed that the large depth of Ge penetration into Si is due to enhanced diffusion of Ge conditioned by the high density of point defects in the doped Si. It has been established that high-dose implantation of C ions into Si and subsequent electron beam annealing result in the formation of a monocrystalline layer of the SiC phase in the case of pulsed (∼0.7 μs) heating and liquid-phase recrystallisation and a polycrystalline SiC layer in the case of prolonged annealing.

A patterning process compatible with conventional Si electronics technology, which has resolution better than 5 μm, has been developed in order to perform selected-area anodic etching for producing luminescent porous Si layers (PSL). Correlations between the anodic etching and photolithographic parameters have been identified, and their effects on the resolution and luminescence of porous Si layers have been studied. Finally, the first monolithic processing, i.e.,. true wafer-scale integration, of a Si-based visible light-emitting diode (LED) and a photodetector using conventional Si technology has been demonstrated.

Novel porous amorphous/crystalline superlattices were produced by the etching of mesas containing superlattices of alternating layers of Si and Si1−xGex. These layers were grown by molecular beam epitaxy on (100) Si substrates and etched in an aqueous HF:HNO3 solution. Preferential attack and amorphization of the Si1−x Gex layers was observed, leading to the formation of alternating layers of single crystal Si and porous amorphous Si1−xGex. The etchant is highly selective and it was possible to etch extremely thin (5nm) Si0.7Ge0.3 layers between 30nm Si layers. Complete conversion of the Si0.7Ge0.3 layers to the porous amorphous state was seen in lμm wide mesas. The role of composition and thickness of the Si1−xGex layers was studied. The variation in the lateral etch depths of the Si1−xGex layers in the superlattices demonstrates that lattice strain in these layers is an important factor in the selectivity of the etch process. As the thickness of the Si1−xGex layers is decreased, transport of the etchant to and the etch products from the reaction front is reduced, limiting the penetration of the etching process. The porosities of these etched Si1−xGex layers were determined to be comparable to measured values for thick etched alloy layers.

We have developed a new, minimal damage approach for examination of luminescent porous Si layers (PSLs) by transmission electron microscopy (TEM). In this approach, chemically etched PSLs are fabricated after conventional plan-view TEM sample preparation. A diffraction pattern consisting of a diffuse center spot, characteristic of amorphous material, is primarily observed. However, crystalline, microcrystalline, and amorphous regions could all be observed in selected areas. A crystalline mesh structure could be observed in some of the thin areas near the pinhole. The microcrystallite sizes were 15–150 Å and decreased in size when located further from the pinhole.

The effect of Si doping on the formation of stain-etched porous Si and its photoluminescent properties was studied. Porous Si is obtained by purely chemical etching of crystalline Si in a solution of HF:HNO3:H2O in the ratio of 1:3:5. We have observed that an incubation time (ti) exists between the insertion of Si into the solution and the onset of porous Si production. This incubation time was found to be a strong function of hole concentration in both n- and p-Si. In p-Si, the ti decreased rapidly with increasing conductivity, whereas for n-Si the opposite (but not as pronounced) trend was found to be the case. For example in (B-doped) p-Si, ti, is only ∼0.5 min for 250 (Ω-cm)−1 but increases to ∼ 5 min for 0.2 (Ω-cm)−1. In (P-doped) n-Si substrates ti was ∼ 8 min for 0.2 (Ω-cm)−1 increasing to ∼ 10 min for 7 (Ω-cm)−1. Photoluminescence (PL) measurements of the porous Si obtained on substrates of various conductivity (p and n) show similar spectra, namely a peak at around 1.94 eV with a full width at half-maximum (FWHM) of about 0.5 eV. Based on the ti difference, we have fabricated localized photoemitting porous Si patterns by Ga+ focused ion beam (FIB) implantation doping and B+ broad beam (BB) implantation doping of n-type Si. Using 30 kV FIB Ga+ implantation, sub-micron photoemitting patterns have been obtained for the first time.

Optical luminescence in porous silicon induced by soft X-ray and vacuum UV excitation with energies in the vicinity of the Si K-edge (1838 eV) and the Si L-edge (99 eV) has been observed. The luminescence has been used, together with total electron yield, to record X-ray absorption fine structure (XAFS) in the near-edge region of both Si edges. The near- edge spectra recorded simultaneously with either luminescence or total electron yield were compared, and the implications of these measurements for the structure of porous silicon are discussed.

Electroluminescent nanoporous silicon is investigated by accelerator techniques. Rutherford Backscattering Spectroscopy (RBS) and Nuclear Reaction Analysis (NRA) are used to measure the oxygen content. The stoichiometry is Si1O1.8 in the top layer and Si1O0.8 in 6 μm depth. Channeling experiments show that only very weak channels are present.

The use of a nitrogen rf plasma source for p-type ZnSe grown by MBE, has allowed a variety of pn junction based devices to be realized. The pn junctions have been combined with (Zn,Cd)Se quantum wells to implement semiconductor injection lasers, operating in the blue/green portion of the spectrum, which were reported by 3M and the Brown/Purdue group in the summer of 1991. In the past year the field has moved rapidly. In particular, we can now report CW operation at low temperatures as well as pulsed operation at room temperature (490nm) using a Zn(S,Se)-based device configuration. Laser power output per facet for some designs is above 300 mW, and threshold current densities are as low as 1000A/cm 2 at room temperature. Lasing was demonstrated from devices grown on both p and n-type GaAs substrates. X-ray rocking curves of theII-VI regions exhibit FWHM values below 20 arcsec for specific samples. Dislocation densities are less than 105 cm−2, below the threshold of TEM plan view imaging. The blue LEDs provide power outputs in excess of 100μW while exhibiting external quantum efficiencies of 0.1% at room temperature.