ALBERT

Notes: A procedure for the fabrication of two-dimensional carrier (electron and hole) gases in modulation doped GeSi/Si heterostructures is presented. The best 4.2 K mobilities measured for the two-dimensional electron and hole gases are 180 000 cm2/V s and 18 000 cm2/V s, respectively. Recently, two-dimensional hole gases with mobilities as high as 55 000 cm2/V s have been obtained. The carrier gases are fabricated on top of relaxed, compositionally graded GexSi1−x buffer layers with low threading dislocation densities (≈106 cm−2). Experimental evidence indicates that the function of the graded buffer is to promote dislocation propagation while suppressing nucleation. A comparative analysis is carried out for two dimensional electron gases in GeSi/Si/GeSi and in AlGaAs/GaAs structures. Although molecular beam epitaxy is used to grow the samples, the principle discussed here is independent of growth technique.

Notes: A method for measuring the mean free path of laterally gliding threading dislocations, based on the growth of lattice mismatched epitaxial layers on patterned substrates has been presented. The method requires no post growth annealing and permits an accurate measurement of the glide process as it occurs during epitaxial growth. This method has been applied to an InGaAs/GaAs heterostructure with a lattice mismatch of 0.5%. Mean free paths of 960 μm for α type dislocations and 1000 μm for β type dislocations were measured. It was concluded that the interaction of threading dislocations with clusters of misfit dislocations and/or edge type dislocations was the primary mechanism of glide length attenuation. © 1996 American Institute of Physics.

Notes: Strain-relaxed Ge0.3Si0.7/Si, grown by rapid thermal chemical-vapor deposition, has been investigated with deep-level transient spectroscopy (DLTS) and bias-dependent electron-beam-induced current (EBIC). A single electron trap and several hole traps have been detected in these samples. The apparent electron capture cross section is found to be ∼2×10−13 cm2, which is several orders of magnitude larger than the apparent hole capture cross sections (∼10−17 cm2), and is responsible for the detection of the minority-carrier electron trap even under reverse-bias majority-carrier capture conditions. All observed traps which were investigated as a function of filling pulse time exhibit logarithmic capture kinetics, as expected for extended defects, and the bias-dependent DLTS peak height and EBIC relative defect contrast are consistent with the spatially varying dislocation density. Moreover, the trap concentration, as determined by DLTS, is correlated to the dislocation density, as determined by EBIC measurements. Based on a comparison of Arrhenius plots, the observed logarithmic capture kinetics, the correlation of trap density to dislocation density, and the observed bias dependence, the electron trap appears to be related to dislocation core states, while two of the hole traps appear to be related to either dislocation core states or Cottrell atmospheres. © 1995 American Institute of Physics.

Notes: The capture kinetics and trapping properties of a dislocation related electron trap detected in strain-relaxed, compositionally graded Ge0.3Si0.7/Si grown by rapid thermal chemical-vapor deposition are investigated by deep-level transient spectroscopy (DLTS). The volume DLTS trap concentration scales linearly with the areal threading dislocation density, as determined by electron-beam-induced current measurements on samples with different compositional grading rates, indicating that the detected trap is most likely associated with dislocation core states in these graded structures. The dislocation related trap exhibits both the logarithmic dependence of DLTS peak height on fill pulse time tp, and broadened DLTS peaks which typically characterize carrier trapping at dislocations. These effects are quantified and analyzed to gain insight into the trapping properties of dislocations in GeSi/Si heterostructures and to investigate the effects of dislocation related carrier trapping on DLTS measurements. It is demonstrated that the peak broadening, as characterized by the dimensionless broadening parameter FWHM/Tp, where FWHM and Tp are the full width at half-maximum of the DLTS peak and the DLTS peak temperature, respectively, monotonically decreases with decreasing fill pulse duration, and approaches point-defectlike behavior for tp〈100 μs.The observed broadening is asymmetric about Tp, and occurs predominantly on the low-temperature side of the DLTS peak. This asymmetric broadening is shown to shift the "apparent'' trap activation energy, as determined by Arrhenius analysis, from EC−0.6 eV to EC−0.9 eV (relative to the bulk conduction-band edge) as tp decreases from 5 ms to 50 μs. These observations are explained by the presence of a dislocation related distribution of energy levels within the GeSi band gap and the consequent fill-pulse-dependent local band bending. The lowest-energy states within this distribution are preferentially filled with electrons for short fill pulse times. The Arrhenius-determined "apparent'' activation energy is hence interpreted as being a measure of the average energy of the filled defect states, weighted by the density of states distribution in this energy band and by the related fill-pulse-dependent local band bending. It is further demonstrated that the minority-carrier capture cross section may be enhanced by the presence of an attractive coulombic barrier for minority carriers at the dislocations, and we use the logarithmic capture equations to derive a value of 4×10−12 cm2 for this "effective'' capture cross section. © 1995 American Institute of Physics.

Notes: The Si substrates were patterned and etched to create arrays of squares separated by 10 μm trenches. Some edges of these squares were ion implanted with a large dose of Ge to create damaged regions that act as misfit dislocation nucleation centers during molecular beam epitaxy of a 700 nm Si0.90Ge0.10 lattice mismatched layer. Low temperature photoluminescence spectra of the material were obtained with an illuminated area selected so that only one type of edge implant was studied at a time. Sub-band gap luminescence peaks associated with dislocations, D lines, were found only in the squares with implants, consistent with electron microscopy results that showed that the control squares had few misfit dislocations. A large D1 luminescence peak was observed in samples that consisted of intersecting arrays of misfit dislocations, while only a small peak was present in specimens implanted such that dislocations lay in only one in-plane direction. These results are interpreted to mean that the D1 peak is intrinsic to the dislocations themselves and not to impurity gettering. These observations further infer that D1 luminescence is related to dislocation kinks. © 1998 American Institute of Physics.

Notes: To investigate the effect of growth area on interface dislocation density in strained-layer epitaxy, we have fabricated 2-μm-high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5×105, 104, and 102 cm−2. 3500-, 7000-, and 8250-A(ring)-thick In0.05Ga0.95As layers, corresponding to 5, 10, and 11 times the experimental critical layer thickness as measured for large-area samples, were then deposited by molecular-beam epitaxy. For the 3500-A(ring) layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 cm−1 for mesas as large as 100 μm. A pronounced difference in the linear interface dislocation densities along the two interface 〈110〉 directions indicates that α dislocations nucleate about twice as much as β dislocations. For samples grown on the highest dislocation density substrates, the linear interface-dislocation density was found to vary linearly with mesa width and to extrapolate to a zero linear interface-dislocation density for a mesa width of zero. This behavior excludes dislocation multiplication or the nucleation of surface half-loops as operative nucleation sources for misfit dislocations in these layers. Only nucleation sources that scale with area (termed fixed sources) are active. In specimens with lower substrate dislocation densities, the density of interface dislocations still varies linearly with mesa size, but the slope becomes independent of substrate dislocation density, indicating that surface inhomogeneities now act as the dominant source for misfit dislocations.Thus, in 3500-A(ring)-thick overlayers, substrate dislocations and substrate inhomogeneities are the active fixed nucleation sources. Since only fixed nucleation sources are active, a single strained layer will dramatically reduce the threading dislocation density in the epilayer. For the 7000-A(ring) layers, we observe a superlinear increase in linear interface-dislocation density with mesa size for mesas greater than 200 μm, indicating that dislocation multiplication occurs in large mesas. For mesas less than 200 μm in width, linear interface-dislocation density decreases linearly with mesa size, but extrapolates to a nonzero linear interface-dislocation density for a mesa size of zero. This nonzero extrapolation suggests an additional active source which generates a dislocation density that cannot be decreased to zero by decreasing the mesa size. Cathodoluminescence (CL) images using radiative recombination indicate that the additional source is nucleation from the mesa edges. Despite a doubling in epilayer thickness from 3500 to 7000 A(ring), the linear interface-dislocation density for mesas 100 μm in width is still very low, approximately 1500 cm−1. The 8250-A(ring) layers possess interface-dislocation densities too high to be accurately determined with CL. However, increases in CL intensity as mesa width is reduced indicate that the interface-dislocation density is decreasing and that growth on small areas produces higher-quality layers than growth on large areas. Our investigations show that different sources for misfit dislocations become active at different epilayer strain levels. The critical thickness depends on which type of nucleation source becomes activated first; therefore, different critical thicknesses can be observed depending on which kind of source is present in a specimen.

Notes: Modulation-doped Si/GexSi1−x/Ge/GexSi1−x structures were fabricated in which a thin Ge layer was employed as the conduction channel for the two-dimensional hole gas. The strained heterostructure was fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The best mobility of the two-dimensional hole gas is 55 000 cm2/V s at 4.2 K with a concentration-dependent hole effective mass of ≤0.10m0. The effect of the Ge/GeSi interface roughness on the 2DHG mobility was studied.

Notes: Deep level transient spectroscopy (DLTS) and cathodoluminescence (CL) were used to study the hydrogen passivation of misfit dislocations in In0.06Ga0.94As/GaAs heterostructures. The CL observations indicate that hydrogen plasma exposure passivates most, but not all, of the dark line defects existing in the specimen prior to hydrogenation. The concentration of deep level defect states that cannot be passivated is below the detection limit of the DLTS instrument (approximately 4×1012 cm−3). We find the passivation is stable after anneals at temperatures as high as 600 °C, indicating that hydrogen passivation of misfit dislocations is at least as stable as that of the isolated point defect studied previously with DLTS [W. C. Dautremont-Smith, J. C. Nabity, V. Swaminathan, M. Stavola, J. Chevalier, C. W. Tu, and S. J. Pearton, Appl. Phys. Lett. 49 1098 (1986)].