Joseph G. Pellegrino

Spectroscopic ellipsometry determination of the properties of the thin underlying strained Si layer and the roughness at SiO2/Si interface

The existence of both the strain and microroughness at the interface of thermally grown SiO2 films on Si was ascertained unambiguously for the first time by high accuracy spectroscopic ellipsometry. The dielectric function of the interface was determined by a comprehensive data analysis procedure. By carefully examining the dielectric function obtained by our model, the strain was seen to cause a red shift of 0.042 eV of the interband critical point E1 compared with the bulk silicon value. The thickness of the interface region was found to be 2.2 nm of which a significant part is due to the strain.

Unique properties of molecular beam epitaxy silicon on sapphire using in situ high-temperature substrate annealing compared with chemically vapor deposited silicon on sapphire

Abstract The dramatically different and superior properties of molecular beam epitaxy (MBE) of silicon on sapphire (SOS) have been investigated and compared with standard commercially available chemically vapor deposition (CVD) SOS. X-ray diffraction reveals a 24% reduction in the strain of the MBE SOS relative to the CVD SOS. The MBE SOS has a 40% higher electron Hall mobility at room temperature. At liquid N 2 temperatures, the electron Hall mobility of the MBE SOS increases instead of decreasing as in CVD SOS. The microtwin differential volume fraction profile for MBE SOS is lower by more than an order of magnitude compared with that of CVD SOS; it decreases faster with distance from the Si-sapphire interface; and it effectively goes to zero at 0.3 μm from the interface. The average Si-sapphire interface charge for MBE SOS is −8.0×10 10 cm −2 , which is negative and more than an order of magnitude lower than the interface charge of 2×10 12 cm −2 for CVD SOS. Some of the unique features of the Naval Research Laboratory VG80 Si MBE/Surface-Analytical System are discussed.

Strain and relaxation in InAs and InGaAs films grown on GaAs(001)

The strain and relaxation of InAs and InGaAs films grown on GaAs(001) have been examined by the x‐ray standing wave and extended x‐ray absorption fine‐structure techniques. While 1 monolayer (ML) films of both InAs and InGaAs are found to be tetragonally distorted in accordance with the prediction of macroscopic‐elastic theory, thicker InAs films are found to collapse to their natural‐lattice constant past a critical thickness Tc, of ∼2 ML’s. By 8 ML’s, bond‐length strain is no longer evident, and a large degree of structural disorder is observed.

Characterization of vertical‐cavity semiconductor structures

Several analytical tools are applied to characterize vertical‐cavity surface‐emitting laser structures grown on GaAs wafers. These epitaxial structures are amenable to x‐ray, electron‐beam, and optical metrologies. Cross‐sectional scanning electron microscopy and transmission electron microscopy were used to measure layer thicknesses and uniformity. Photoluminescence wafer mapping was used to determine alloy composition uniformity across the wafer. Photoreflectance was also used to determine alloy composition. Cross‐sectional microphotoluminescence was used to measure average alloy compositions in the top and bottom mirrors. Reflectance spectroscopy was used to characterize the cavity resonances and mirror layers. Double‐crystal x‐ray diffractometry (DCXRD) was used to characterize mirror layer dimensions, uniformity, and average alloy composition. Excellent agreement was found among these measurement techniques and between simulations and measurements. The results demonstrate the accuracy of the device s...

X‐ray reflectivity determination of interface roughness correlated with transport properties of (AlGa)As/GaAs high electron mobility transistor devices

To explore the role of interface scattering in high electron mobility transistor (HEMT) device performance, a series of samples consisting of both a superlattice and a HEMT structure were grown by molecular beam epitaxy (MBE) at temperatures ranging from 500 to 630u2009°C. Hall measurements indicate a trend toward higher mobilities in samples grown at higher temperatures. Subsequent x‐ray reflectivity measurements were made, and the data were fitted by least‐squares refinement of a calculated reflectivity curve determined from a model of the sample structure to obtain the composition profile along the growth direction. These results indicate smoother interfaces for the samples with higher mobilities.

High-accuracy determination of the dependence of the photoluminescence emission energy on alloy composition in AlxGa1−xAs films

In an effort to improve the accuracy of photoluminescence (PL) measurements of the Al mole fraction (x) of AlxGa1−xAs alloys, the PL peak emission energy, EPL,peak, was measured at room temperature for molecular-beam epitaxy-grown AlxGa1−xAs films with 0⩽x<0.37, and correlated with independent measurements of x by in situ reflective high-energy electron diffraction (RHEED) and also by ex situ wavelength-dispersive x-ray spectroscopy in an electron microprobe analyzer (WDS/EMPA). The measurement uncertainty of EPL,peak was minimized through the following procedures: Accurate calibration of the photon energy (or wavelength) scale, correction of the measured spectra for the spectrometer response function, fitting the data with a well-chosen line shape function, and compensation for the effect of ambient temperature drift. With these procedures, the 2σ measurement uncertainty of EPL,peak was of the order 5×10−4u2009eV for most samples. From correlation of the PL and WDS/EMPA composition data, the slope ∂EPL,peak/...

Relief of compressive biaxial strains in thin films via microtwins

For heteroepitaxial systems, such as silicon on sapphire, microtwins can usually be observed in the epitaxial layer. It has also been suggested that microtwins play a significant role in strain relief in these systems. From a knowledge of the differential volume fraction of microtwins occurring in a heteroepitaxial systems, it is possible to estimate the greatest possible strain relief due to microtwins. Measurements of the differential volume fraction of microtwins in silicon‐on‐sapphire, however, indicate that the strain relief due to microtwins cannot be greater than 0.7%, even though the lattice mismatch between silicon and sapphire is an order of magnitude larger. Therefore, if the silicon/sapphire interface is coherent, the misfit strain must be relieved by another mechanism.

Optimization of thickness and doping of heterojunction unipolar barrier layer for dark current suppression in long wavelength strain layer superlattice infrared detectors

Suppression of generation-recombination dark current and bias stability in long wavelength infrared (LWIR) strained layer superlattice (SLS) detectors, consisting of a lightly doped p-type absorber layer and a wide bandgap hole barrier, are investigated with respect to the wide bandgap barrier layer thickness and doping profile. Dark current IV, photoresponse, and theoretical modeling are used to correlate device performance with the widegap barrier design parameters. Decreased dark current density and increased operating bias were observed as the barrier thickness was increased. This study also identifies key device parameters responsible for optimal performance of heterojunction based SLS LWIR detector.

An X-ray standing wave study of ultrathin InAs films in GaAs(0 0 1) grown by atomic layer epitaxy

Abstract X-ray standing wave and X-ray diffraction measurements were used to determine the structure of nominal 1 monolayer and 1/2 monolayer InAs films buried in GaAs(0xa00xa01). The films were grown by atomic layer epitaxy using trimethylgallium, tertiarybutylarsine and trimethylindium. For the full monolayer sample the standing wave measurement shows that the indium atoms reside 1.577±0.014xa0A above the GaAs(0xa00xa04) substrate planes. A calculation based on the macroscopic elastic theory suggests that this corresponds to a single In x Ga 1− x As layer with x =0.794±0.068. The coherent fraction of 0.766±0.051 indicates a reasonably abrupt interface, as confirmed by the In-excitonic photoluminescence full width at half maximum of 5.74±0.01xa0meV. The half monolayer sample is less strained, as expected, with the indium atoms at 1.502±0.030xa0A above the substrate planes, corresponding to an In x Ga 1− x As layer with x =0.446±0.145, and a coherent fraction of 0.88±0.12. This study exemplifies the complimentary nature of XSW and XRD.

Molecular beam epitaxy versus chemical vapor deposition of silicon on sapphire

Molecular beam epitaxy (MBE) of Si on sapphire (SOS) has dramatically different and superior properties compared to chemical vapor deposited (CVD) SOS. The strain in the Si epilayer decreases by 21%. A 40% higher electron Hall mobility occurs at room temperature. At LN2 temperatures the electron mobility increases to a level which is more indicative of bulk Si than of CVD SOS. The microtwin differential volume fraction profile is lower by more than an order of magnitude, and decreases below the detectable limit at 300 nm from the interface. The average Si/sapphire interface charge for MBE SOS is −8.0×1010 cm−2, while the interface charge of CVD SOS is 2×1012 cm−2.