G. D. Pettit

Nucleation mechanisms and the elimination of misfit dislocations at mismatched interfaces by reduction in growth area

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‐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 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...

Ohmic contacts to n‐GaAs using graded band gap layers of Ga1−xInxAs grown by molecular beam epitaxy

Ohmic contacts were studied on structures which utilize the fact that for InAs surfaces Fermi level pinning occurs at or in the conduction band. It was found that an epitaxial layer of n‐Ga1−xInxAs grown by molecular beam epitaxy on n‐GaAs which is graded in composition from x = 0 at the GaAs interface to 0.8?x?1.0 at the surface will produce a structure with a nearly zero Schottky barrier height for the metal–Ga1−xInxAs interface and hence a low resistance ohmic contact. A transmission line measurement of non‐alloyed contact resistance of 5×10−7<Rc<5×10−6 ohm cm2 was obtained for a Ag/n‐Ga1−xInxAs/n‐GaAs MESFET structure.

An In 0.15 Ga 0.85 As/GaAs pseudomorphic single quantum well HEMT

This letter describes high electron mobility transistors (HEMTs) utilizing a conducting channel which is a single In<inf>0.15</inf>Ga<inf>0.85</inf>AS quantum well grown pseudomorphically on a GaAs substrate. A Hall mobility of 40 000 cm²/V.s has been observed at 77 K. Shubnikov-de Haas oscillations have been observed at 4.2 K which verify the existence of a two-dimensional electron gas at the In<inf>0.15</inf>Ga<inf>0.85</inf>As/GaAs interface. HEMTs fabricated with 2-µm gate lengths show an extrinsic transconductance of 90 and 140 mS/mm at 300 and 77 K, respectively-significantly larger than that previously reported for strained-layer superlattice In<inf>x</inf>Ga<inf>1-x</inf>As structures which are nonpseudomorphic to GaAs substrates. HEMTs with 1-µm gate lengths have been fabricated, which show an extrinsic transconductance of 175 mS/mm at 300 K which is higher than previously reported values for both strained and unstrained In<inf>x</inf>Ga<inf>1-x</inf>As FETs. The absence of Al<inf>x</inf>Ga<inf>1-x</inf>As in these structures has eliminated both the persistent photoconductivity effect and drain current collapse at 77 K.

Unpinned (100) GaAs surfaces in air using photochemistry

We have unpinned the Fermi level at the surface of both n‐ and p‐type (100) GaAs in air. Light‐induced photochemistry between GaAs and water unpins the surface Fermi level by reducing the surface state density. Excitation photoluminescence spectroscopy shows a substantial decrease in both surface band bending and surface recombination velocity in treated samples, consistent with a greatly reduced surface state density (≂1011 cm−2). Capacitance‐voltage measurements on metal‐insulator‐semiconductor structures corroborate this reduction in surface state density and show that the band bending may be controlled externally, indicating an unpinned Fermi level at the insulator/GaAs interface. We discuss a possible unpinning mechanism.

Structure and recombination in InGaAs/GaAs heterostructures

The defect structure of lattice‐mismatched 1‐μm InxGa1−xAs (x≊0.12, misfit Δa/a≊8.5×10−3) epilayers on GaAs was studied with scanning cathodoluminescence (CL), transmission electron microscopy (TEM), high‐voltage electron microscopy, and scanning electron microscopy. CL shows that nonradiative recombination lines exist in the GaAs buffer layer as far as 4000 A from the interface. The density of these defects is independent of substrate dislocation density. Plan‐view TEM analysis indicates that the majority of these dislocations in the buffer layer are sessile edge half‐loops. Cross‐sectional TEM shows that loops also extend into the InGaAs epilayer, but the majority of the loops are located on the buffer layer (substrate) side of the interface. A model is proposed to explain sessile edge dislocation formation in the buffer layer. A comparison of CL and high‐voltage electron microscopy images from the same interface area reveals that the dark nonradiative recombination lines seen in scanning luminescence i...

Some optical properties of the AlxGa1−xAs alloys system

Optical transmission data covering the Γ15V–Γ1C absorption edge are presented, together with photoluminescence (PL) results, for AlxGa1−xAs crystals of high purity [n (293 K) <1017 cm−3] for 0<x<0.9. The results indicate that the fundamental absorption edge is domnnated by electron‐hole interaction even in an an alloy. In the direct‐gap region the bound‐exciton ground‐state peak is clearly resolved. In the indirect‐gap region the relaxation of crystal momentum conservation in optical transitions for an alloy apparently has significant effects on the Γ15V–Γ1C absorption edge, and the discrete peak in absorption spectra disappears. From the variation of the absorption coefficient αth at the Γ15V–Γ1C edge with x, we can deduce the corresponding variation of the excitonic binding energy Eex, excitonic effective mass m*, and electron mass mc for the Γ1C minimum. Extrapolated values for these quantities in AlAs are Eex=5 meV, m*=0.060 m0, and mc=0.11 m0 for a value αth = 2.3×104 cm−1. Further, there is a shift ...

ESR AND OPTICAL ABSORPTION STUDIES OF ION‐IMPLANTED SILICON

The g value, line shape, and linewidth of an ESR signal in Si layers which have been damaged by ion implantation of Si, P, or As at room temperature are found to be identical to those of the electron states observed in amorphous Si films prepared by rf sputtering. Interference phenomena observed in the optical absorption spectra allow a determination of the depth to which the Si has been damaged by the energetic heavy ions. These two techniques together provide a new tool for investigating lattice disorder in ion‐implanted Si layers.

EFFICIENT VISIBLE ELECTROLUMINESCENCE AT 300°K FROM Ga1‐xAlxAs p‐n JUNCTIONS GROWN BY LIQUID‐PHASE EPITAXY

Efficient visible light emitting diodes have been fabricated from Ga1‐xAlxAs. Epitaxial layers were obtained by a modified solution growth technique. External quantum efficiencies of up to 3.3% have been measured at room temperature on diodes, which had their emission at 1.70 eV. The switching time for the light emission at 300°K was measured to be 60 nsec.

Photoreflectance study of the surface Fermi level at (001)n-and p-type GaAs surfaces

We report a photoreflectance study of Fermi level pinning (VF) on (001) n‐ and p‐type GaAs with large, uniform electric fields. Surface photovoltage (Vs) effects were evaluated as a function of temperature (77 K<T<450 K), pump beam wavelength (633 and 407 nm) and W–metal coverage (in situ). The dependence of the measured barrier height, Vb (=VF−Vs), on T can be explained by a modification of the theory of M. Hecht [Phys. Rev. B 41, 7918 (1990)] yielding values of VF =0.77±0.02 V and VF =0.75±0.02 V for n‐ and p‐type GaAs, respectively, at 300 K. In addition, by introducing the ratio (r) of the area of the surface states to the illuminated area into the theory of Hecht we have been able to estimate the density of surface states on the GaAs surface. The effect of metal coverage is to increase r and reduce the influence of Vs.

Photoreflectance study of surface photovoltage effects at (100)GaAs surfaces/interfaces

We report a photoreflectance study of surface photovoltage (VS) effects on the determination of Fermi level pinning (VF) on (100) n‐GaAs in air and with W‐metal coverage (in situ) as a function of temperature (77 K<T<450 K) and light intensity (I). The dependence of VS on T and I can be explained by a modification the theory of M. Hecht [Phys. Rev. B 41, 7918 (1990)] yielding a value of VF=0.73±0.02 V. The effect of metal coverage is to reduce the influence of VS.