A. Fischer-Colbrie

Pulse-doped AlGaAs/InGaAs pseudomorphic MODFETs

MODFETs have been fabricated using heterojunctions consisting of AlGaAs and pseudomorphic InGaAs, grown on GaAs substrates. The large conduction band discontinuity (about 0.46 eV for 25% In and Al concentration) leads to a 2-D electron density as high as 2.3*10/sup 12/ cm/sup -2/, with electron mobilities of 7000 and 16000 cm/sup 2//V-s at 300 and 77 K, respectively. Such a high electron density in combination with reasonable transport properties leads to MODFETs with exceptional characteristics. Devices with 0.15-0.25- mu m gate length have room-temperature drain currents as high as 600 mA/mm and room-temperature transconductance as high as 500 mS/mm. The f/sub T/ is as high as 98 GHz, as determined by 20-dB/decade extrapolation of microwave data taken to 25 GHz. A comparison of the effect of bias on the total delay through standard and pseudomorphic MODFETs suggests that the excellent microwave performance exhibited by the pseudomorphic device arises from a reduction in parasitic and drain delays and not from a higher electron velocity under the gate. >

Nucleation of GaAs on Si: Experimental evidence for a three‐dimensional critical transition

The initial nucleation and growth stages of ultrathin GaAs films grown by molecular beam epitaxy upon Si(100) substrates are studied by high‐resolution transmission electron microscopy. It is found that for a growth temperature of 405 °C, films initially nucleate as islands associated with steps on the substrate surface. In the earliest stages of growth, these islands are strained coherently to the substrate lattice, i.e., are free of misfit dislocations. As the islands grow, they become faulted. The transition from strained to dislocated states is shown to be a function of both the lateral and vertical island dimensions.

Anisotropic and inhomogeneous strain relaxation in pseudomorphic In0.23Ga0.77As/GaAs quantum wells

The structural properties of pseudomorphic In0.23Ga0.77As/GaAs single quantum wells are investigated with x‐ray double‐crystal diffractometry. Anisotropic tilting of lattice planes along dislocations and anisotropic reflectivity of fully relaxed domains coexisting with strained domains are reported for the first time. Due to the anisotropic strain relaxation the crystal symmetry changes from tetragonal in the fully strained case to monoclinic in a partially relaxed quantum well.The structural properties of pseudomorphic In0.23Ga0.77As/GaAs single quantum wells are investigated with x‐ray double‐crystal diffractometry. Anisotropic tilting of lattice planes along dislocations and anisotropic reflectivity of fully relaxed domains coexisting with strained domains are reported for the first time. Due to the anisotropic strain relaxation the crystal symmetry changes from tetragonal in the fully strained case to monoclinic in a partially relaxed quantum well.

Long wavelength (λ∼1.5 μm) native‐oxide‐defined InAlAs‐InP‐InGaAsP quantum well heterostructure laser diodes

Native oxidation (‘‘wet’’ oxidation via H2O vapor+N2) of InAlAs is employed to fabricate long wavelength (λ∼1.5 μm) InAlAs‐InP‐InGaAsP quantum well heterostructure laser diodes. Data are presented on gain‐guided native‐oxide‐defined stripe‐geometry lasers (40 μm×500 μm) with threshold currents of 200 mA (1 kA/cm2) emitting with multiple longitudinal modes centered at λ∼1.5 μm. The threshold currents, approximated as Ith=I0 exp(T/T0), exhibit a characteristic temperature of T0∼49 K and an operating temperature as high as T=70 °C. Maximum continuous output powers of 140 mW/facet (uncoated facets) and a differential quantum efficiency of 38% are achieved.

Effect of substrate surface structure on nucleation of GaAs on Si(100)

We study the effect of surface structure upon the nucleation of GaAs in molecular beam epitaxy growth on vicinal Si(100) surfaces. In general, cross‐sectional transmission electron microscopy reveals that nucleation of GaAs islands is associated with surface steps produced by the deliberate substrate misorientation. Furthermore, it is found that standard in situ cleaning of the Si surface prior to deposition can result in steps grouping together, producing local surface facets. GaAs nucleation then occurs on these facets, the nucleation sites being correspondingly further apart than for an equilibrium distribution of monolayer steps.

Total reflection x-ray fluorescence spectroscopy using synchrotron radiation for wafer surface trace impurity analysis (invited)

Trace impurity analysis is essential for the development of competitive silicon circuit technologies. Current best methods for chemically identifying and quantifying surface and near‐surface impurities include grazing incidence x‐ray fluorescence techniques using rotating anode x‐ray sources. To date, this method falls short of what is needed for future process generations. However, the work described here demonstrates that with the use of synchrotron radiation, total reflection x‐ray fluorescence methods can be extended to meet projected needs of the silicon circuit industry until, at least, the year 2000. The present results represent over an order of magnitude improvement in detection limit over what has been reported previously. A double multilayer monochromator on a high flux wiggler beam line resulted in a detection limit for Ni of 3×108 atoms/cm2. This is to be compared with a detection limit of 5×109 atoms/cm2 obtained with a rotating anode system. This is due to the greatly improved signal to bac...

Dependence of structural and optical properties of In0.23Ga0.77As/GaAs quantum wells on misfit dislocations: Different critical thickness for dislocation generation and degradation of optical properties

The critical layer thickness Lc for dislocation generation and the characteristic thickness for consequent modification of the optical properties of strained quantum wells are shown to be remarkably different. We visualize the misfit dislocations directly using scanning cathodoluminescence and transmission electron microscopy. The dislocations are found to be asymmetrically distributed within the (001) interface plane. The critical thickness for In0.23Ga0.77 As on GaAs is determined to be Lc ≊15 nm in agreement with the theory of Matthews and Blakeslee [J. Cryst. Growth 27, 118 (1974)]. The deterioration of quantum efficiency, a change of recombination dynamics, the increase of spectral broadening and a shift of the band gap occur at much larger thickness than Lc since these quantities are barely affected by low dislocation densities. The relaxation is heterogeneous; strained and fully relaxed domains coexist. Using double crystal x‐ray diffraction we find that partially relaxed layers with thickness larg...

Growth and characterization of AlGaAs/InGaAs/GaAs pseudomorphic structures

We have grown pseudomorphic modulation‐doped Aly Ga1−y As (0.25≤y≤0.30)/Inx Ga1−xAs (x≤0.50) on GaAs structures by molecular‐beam epitaxy and obtained two‐dimensional electron gas densities as high as 2.6×1012/cm2 with mobilities of 6600 and 14 900 cm2/V s at 300 and 77 K, respectively. In conjunction with this work, Rutherford backscattering measurements were made to study the substrate temperature dependence of In incorporation. Cross‐section transmission electron microscopy (TEM) measurements were made to determine layer morphology (planar versus island growth) as a function of substrate temperature and composition. Dislocation density as a function of InGaAs layer thickness was studied using plan‐view TEM. Hall measurements are consistent with the composition, morphology, and structural measurements of the material and indicate that there is a limited composition‐dependent thickness and temperature range over which both InGaAs and AlGaAs of high quality can be grown.

Improved microwave performance in transistors based on real space electron transfer

Experimental results on an improved type of transistor based on real space electron transfer are presented. Microwave measurements through 25 GHz show an extrapolated fT of 60 GHz and a measured fMAX of 18 GHz. These gain‐bandwidth products are approximately twice as high as any previously reported for this relatively new class of device. This improvement in performance results from a novel device design which incorporates a doped, pseudomorphic InGaAs channel, a GaAs collector drift region, and a collector‐up structure.

High output conductance of InAlAs/InGaAs/InP MODFET due to weak impact ionization in the InGaAs channel

InAlAs/InGaAs/InP MODFETs with short gatelength suffer from a high output conductance, G/sub 0/, and low breakdown voltage. In addition, the devices often show a kink effect in the I-V curve. The authors show by numerical calculations that weak impact ionization occurs in the In/sub .53/Ga/sub .47/As channel under relatively moderate fields. The high G/sub 0/ and the kink are mainly caused by mechanisms associated with weak impact ionization. The calculations also show that the low breakdown voltage of the device is caused by the drain-source breakdown in the InGaAs channel. The occurrence of the impact ionization process in the channel was experimentally verified by detection of photons from the device under moderate drain bias.<<ETX>>