We have recently developed a method of local stress measurement that can be applied at the submicron level to measure differences in intrinsic stress at both free surfaces and buried interfaces. The method exploits the fact that transmission electron microscopy readily reveals local bulk strain fields produced by surface or interfacial stress differences. Using quantitative image analysis and simulation, we can determine the value of this stress difference. We have successfully applied this technique to the clean Si(111) surface during 7×7-1×1 phase coexistence and have determined the stress difference to be 30±5 meV/Å2.

The local environments of CAs acceptors in InxGa1−xAs and AlxGa1−xAs have been determined from the localized vibrational modes (LVMs) of both isolated CAs impurities and H-CAs pairs using infrared (IR) absorption and Raman scattering techniques. In as-grown layers of InxGa1−xAs (x<0.1), a single LVM due to isolated CAs acceptors was observed. The introduction of hydrogen led to the formation of H-CAs pairs and a single A1−-mode (stretch) and a single A1+-mode (XH) were observed for the InxGa1−xAs layers. All the LVMs were identified with carbon in CAsGa4 cluster configurations implying that less than 5 % of the detectable carbon atoms were present in clusters incorporating one or more CAs-In bonds. For AlxGa1−xAs, five stretch modes and five X-modes of the H-CAs pairs were observed for 0<x<1 and each mode was assigned to configurations for which the originally unpaired CAs had 0,1,2,3 or 4 Al nearest neighbors. These results show that carbon does not appear to form bonds with In atoms for the InxGa1−xAs samples investigated and this can explain the difficulty found in incorporating CAs acceptors in InxGa1−xAs with x>0.1 for some growth techniques. CAs acceptors can form strong bonds with both Al and Ga atoms, however, leading to a high solubility of carbon in AlxGa1-xAs over the full compositional range.

The effects of strain-induced defects on excess carrier lifetime and transport in a nipi-doped In0.2Ga0.8As/GaAs multiple quantum well (MQW) structure were examined with a new method called electron beam-induced absorption modulation (EBIA) in which the kinetics of carrier transport and recombination are examined with a high-spatial, -spectral and -temporal resolution. The excess carrier lifetime and ambipolar diffusion were found to be reduced by factors of ∼1013 and ∼103 compared to theoretical values, respectively, and this is attributed to the presence of strain-induced defects. The MQW excitonic absorption coefficient sensitively depends on the carrier density in the QWs, as a result of screening of the electron-hole (e-h) Coulombic interaction. Likewise, ambipolar diffusion is found to depend on the excess carrier density in a nonlinear fashion, as a result of the e-h plasma-induced changes in the local depletion widths in the vicinity of structural defects.

We have prepared InAsSb/InGaAs strained-layer superlattices (SLS's) and InPSb confinement layers using metal-organic chemical vapor deposition (MOCVD) for use as infrared emitters. X-ray diffraction was used to determine lattice matching as well as composition and structure of the SLS's. Photoluminescence linewidth and intensity were used as a measure of the quality of the structures. Typical FWHM were less than 10 meV. The presence of interface layers were indicated by broadened x-ray diffraction peaks for samples grown under non-optimized conditions. Two types of interfacial layers apparently due to a difference in composition at the interfaces were observed with transmission electron microscopy (TEM). The width of the x-ray peaks can be explained by a variation of the interfacial layer thicknesses. Optimized growth resulted in SLS's with narrow x-ray peaks and high radiative efficiency. Room temperature LEDs operating between 4-5 μm have been prepared.

We show that high-quality strain-compensated InAsxP1−x/GayIn1−yP multiple quantum wells (MQWs) can be grown by gas-source molecular beam epitaxy on InP substrates even though the InAsxP1−x quantum wells have a large lattice mismatch (∼ 1.3%) with respect to InP. Very sharp satellite peaks in double-crystal X-ray diffraction and narrow line width in low-temperature photoluminescence (FWHM of 4 meV at 9 K) are obtained from MQWs of up to 50 periods. Strain compensation allows a wide range of the net strain around the ideally compensated point, where the net strain equals zero, without degrading crystalline quality.

Preliminary results are described about the growth, structure and properties of multiple quantum well(MQW) Quantum Confined Stark Effect (QCSE) modulators grown on GaAs substrates for operation at 1.3μm in wavelength. Step graded InAlAs buffer layers grown at low temperature by molecular beam epitaxy (MBE) with a total thickness of 0.3 μm are used to relieve the strain caused by the lattice-mismatch between the GaAs substrate and the In0.35Ga0.65As/In0.35Al0.65As MQW heterostructure. X-ray diffraction spectra show that significant lattice relaxation takes place in the buffer. A quantum confined Stark shift of the exciton absorption peak of 48meV was obtained with an applied electric field of 130KV/cm, measured in PIN diode structures consisting of 30 period 95ÅIn0.35Ga0.65As/100ÅIn0.35Al0.65As MQWs on a 3 stage compositionally step graded InxAl1−xAs buffer doped with Si to 5*1017/cm3 grown on a nominally 1018/cm3 n-type doped GaAs substrate.

Photoelastic optical waveguides using strain-compensated InAsP/InGaP multiplequantum-well (MQW) have been fabricated. Lateral light confinement for waveguiding is achieved by introducing stress into semiconductor heterostructures with stable WNi surface stressor stripes. The waveguides have been characterized at both 1.52 μm and 1.32 μm wavelength in term of TE/TM intensity ratio. At 1.52 μm, the waveguides favor the propagation of TE mode, and the TE/TM intensity ratio can be as large as 15 dB. At 1.32 μm, the TE and TM intensity can be comparable. Anisotropy of waveguides fabricated along [110] or [110] directions has also been observed in term of TE/TM intensity ratio, which suggests the presence of anisotropic property of the strain-compensated MQW.

The experimental application of strained-Si / relaxed-Si1−xGex heterostructures to n-MOSFETs is discussed, focusing on the enhanced mobility provided by the strain. This paper provides an overview of the theoretically-predicted electronic properties of these heterostructures, as well as their growth. Several practical issues which arise in MOS applications are covered, including the effect of the relaxed-Si1−xGex, buffer layers on diode performance, and the observation of self-heating effects in the output characteristics of the MOS transistors.

During the last few years, Si/SiGe heterostructures have been used by several groups worldwide in order to achieve high-mobility two-dimensional electron gases.1-3 To confine high-mobility electrons, these heterosystems are grown in such a way that the Si channel, epitaxially grown on a relaxed SixGe1−x buffer, undergoes a tensile strain. As such structures have a significant conduction band offset, the spatial separation of ionized dopant atoms from mobile carriers can be exploited. The confined electrons in the Si channel are then provided by a doped SiGe supply layer. The significant improvement in SiGe buffer quality has led to low temperature mobilities exceeding 100 000 cm2/Vs with best values above 350 000 cm2/Vs.2-5 However, for MOS applications, the most interesting point is the room temperature (RT) mobility of confined electrons in the Si channel. With this aim in view, RT-mobilities between 2000 and 2600 cm2/Vs have been reported.6 These values are more than twice larger than these of commercial MOSFETs and are also higher than the bulk mobility of intrinsic Si. However, these relatively high values are still lower than the predicted ones (3000 cm2/Vs).7 This is partially due to the fact that the RT-mobility measured on modulation doped heterostructures (MDHs) is never exactly that of the confined electrons in the Si channel. The conduction electrons are in fact not completely transferred from the supply layer to the channel. The latter is the primary and desired path for conduction, but the supply layer can become a second, parallel path, with a lower mobility because of donor impurities. Different methods can be used to reduce the contribution of this parallel path. First, the supply layer can be designed with the lowest possible doping level so as to ensure an almost complete electron transfer at room temperature.8 Secondly, gated Hall devices can be fabricated using Schottky or Metal-Insulator-Semiconductor gates.9-11 These devices allow both the electron density and electron mobility in the strained Si channel to be measured for low gate voltage.

The first measurements of Si/SiGeC valence band offsets have been performed using heterostructure p-type MOS capacitors. Single crystalline epitaxial layers of SiGeC and Si were grown by the rapid thermal CVD technique, limited reaction processing, using ethylene as the carbon source. Films with carbon contents of up to ∼1.2% were studied. X-ray diffraction analysis shows the strain compensating effect of carbon in SiGeC. Substitutionality of carbon in SiGe was also confirmed by Fourier transform infrared spectroscopy measurements. MOS capacitors were fabricated by thermal oxidation (750°C) of the epitaxial Si cap on top of the SiGeC layer. Quasi-static and high frequency capacitance-voltage measurements show wellbehaved characteristics for the samples with carbon contents up to approximately 1%. Onedimensional Poisson simulations of the C-V curves were used to extract the valence band offset between Si and SiGeC. For a fixed germanium fraction, the offset decreases with increasing carbon content. However, for a given lattice mismatch, the offset for Si/Si1−x−yGexCy is larger than the measured value for Si/Si1−zGez.

One of the limiting factors in the performance of future CMOS integrated circuits is the low hole mobility which causes the transconductance of p-channel transistors to be inferior to those of n-channel devices. One promising way of enhancing hole mobility is to introduce a buried SiGe layer under the gate of p-MOS transistors. The mobility improvement thus results from, i) reduction in surface scattering (by moving carriers away from scattering sites at the Si/SiO2 interface), and ii) higher in-plane hole mobility in biaxally strained SiGe layers.

GexSi1−x, heterojunction internal photoemission devices for detection of infrared radiation have been fabricated from films grown by ultra high vacuum chemical vapor deposition (UHV/CVD). Films and devices were characterized by Fourier transform infrared spectroscopy (FTIR), photoresponse, and electrical measurements. Detector efficiencies were found to increase substantially (from ∼ 1.5 to ∼ 20% per eV) as the boron concentration in the absorbing layer was increased from 1×1020 to 2.5×1020 cm−3. Increases in boron concentration beyond 2.5×1020 cm−3 did not significantly improve device performance. This result is attributed to incomplete dopant activation at higher boron concentrations.

A systematic experimental investigation has been undertaken for the optimization of the wafer parameters and processing for silicon wafers intended for use as substrates for MBE growth, with emphasis on heteroepitaxial growth of GaAs-on- Si. Within this investigation, results are presented of an initial study focused on the optimization of the magnitude of the misorientation angle towards a <110> direction for the growth of GaAs on (001) Si wafers. This angle controls the structure of the stepped (001)Si surface and can influence the defect density and surface smoothness of the GaAs-on-Si layers. Silicon substrates misoriented from 0 deg. up to 9 deg. were cut to specification and subsequently used for the epitaxial growth of GaAs MESFET structures. MESFETs were fabricated and their dc and RF characteristics compared. The resistivity of the GaAs-on-Si buffer layers was evaluated and correlated to the results from device characterization. This work presents the effects of the magnitude of the angle of misorientation in the range from 0 to 9 deg.

We report Raman study of highly strained single InAs−In0.53Ga0.47As quantum wells grown by molecular beam epitaxy (MBE) on InP substrates with the well thickness between 4 and 15 monolayers. We have used Raman spectroscopy to characterize quality, disorder and strain of these structures which are of considerable interest for long wavelength optical communications.

In the Raman spectra we have observed an intense narrow line corresponding to the GaAslike LO mode of In0.53Ga0.47As cap layer and a narrow peak due to the LO phonon mode of the InAs layer. These dominant and sharp features characterize the high homogeneity of our samples. In addition to these features we have observed the appearance of distinct peaks with the increase of the InAs layer thickness. In our opinion the presence of these modes is indicative of a slight deterioration of the structural perfection of the sample with the increase of the well thickness. Our investigation shows the ability of Raman spectroscopy to describe these systems and the good quality of our structures.

Using finite elements we quantitatively calculate the inhomogeneous strain distribution associated with the so called crosshatch pattern. This pattern is a twodimensional, pseudoregular surface undulation that generally arises as an intermediate stage of heteroepitaxial growth of low misfitting semiconductor layers. This stage is characterized by an interplay of elastic and plastic relaxation processes. The calculations yield (i) the elastic strain energy density distribution and corresponding degree of elastic relaxation and (ii) the shear stress distribution in the glide system, in which misfit dislocations form. As a main result shear stresses due to the crosshatch pattern within the substrate are identified that cause misfit dislocations to multiply in the substrate.

Results are presented of a study on the degradation and recovery behavior of strained Si 1−xGex diodes and heterojunction bipolar transistors (HBTs) by electron and neutron irradiation. The degradation of device performance and the generation of lattice defects are reported as a function of germanium content and radiation source. Isochronal annealing is performed to study the recovery behavior of the irradiated devices. The radiation source dependence of the degradation is discussed taking into account the absorbed energy dunng irradiation.

Boron diffusion in in-situ doped Si and strained Si1−xGex (x < 0.20) epitaxial layers, subjected to inert-ambient furnace annealing, was investigated as a function of temperature (T = 750 °C - 850 °C). Boron diffusivity parameters were extracted from SUPREM IV, a process simulation program. We observed slower B diffusion in strained Si1−xGex relative to that in Si for B concentration levels ranging from 2×1017 to 3×1019 cm−3. Using relaxed graded Si1−xGex as “substrates”, we also characterized B diffusion in relaxed Si1−xGex (x < 0.60) at T = 800 °C. We propose a reaction of mobile B atoms pairing with Ge atoms to model the slower B diffusion in both fully strained and relaxed Si1−xGex.

In this work, the inert and implant enhanced diffusion of boron in silicon germanium (SiGe) was studied by means of a boron box-shaped profile grown inside Si0.905 Ge0.095 or Si0.785Ge0.22 layers. The SiGe layers were capped with silicon for stability and to ensure no lattice damage in the SiGe layers during the implant. Assuming boron moves by interstitial-mediated diffusion in SiGe, the results suggest that decreases in the mobile boron diffusivity, not decreases in interstitial concentration, cause much of the decrease in average boron diffusivity with increasing Ge fraction. Implant enhancements decreased as the germanium percentages increased, possibly indicating higher interstitial concentrations in SiGe than in Si.

This paper quickly reviews the structure of band-edge luminescence in Si/strained Si1−xGex heterostructures, and then focusses on two recent developments -- the origin of “deep” sub-bandgap luminescence which is sometimes observed in structures grown by Molecular Beam Epitaxy (MBE) and the understanding of the temperature dependence of the band-edge luminescence (up to room temperature). Strong evidence will be presented that the origin of the deep luminescence is radiation damage, and that generated defects are segregated or trapped in the SilxGex layers. The modelling of the temperature dependence by twocarrier numerical simulation is presented for the first time. The work and experimental data show convincingly that the strength of the luminescence at high temperature is controlled by recombination at the top silicon surface, which in turn can be controlled by surface passivation. At high pump powers and low temperatures, Auger recombination reduces the lifetime in the Si1−xGex layers, and leads to a luminescence vs. temperature which is flat up to 250 K and which is reduced only by a factor of three at room temperature.

Sample temperature dependence and excitation power dependence of the photoluminescence intensity were investigated with respect to growth temperature for SiGe single quantum wells grown pseudomorphically to (100)-oriented Si by molecular beam epitaxy. The determined excitation power exponents and thermal activation energies show unambiguously that defect incorporation is effectively reduced at higher growth temperatures. However, at higher growth temperatures the SiGe-related spectral distribution is found to be shifted to higher photon energy which is attributed to intermixing of Ge and Si at the heterointerfaces, governed by diffusion as well as Ge surface segregation during growth. The diffusion process is studied separately by photoluminescence measurements upon thermal annealing at different temperatures and a diffusion model is presented where the diffusion process is assumed to be composed of two different mechanisms, interdiffusion, i.e. lattice-site-exchange diffusion, and point-defectinduced diffusion. The determined activation energies for the two diffusion mechanisms are in good agreement with previous results which confirm that the model gives a realistic picture of the diffusion process.