J. E. Oh

Molecular‐beam epitaxial growth of high‐quality InSb on InP and GaAs substrates

Epitaxial layers of InSb were grown on InP and GaAs substrates by molecular beam epitaxy. The dependence of the epilayer quality on flux ratio, J sub Sb4/J sub In, was studied. Deviation from an optimum value of J sub Sb4/J sub In (approx. 2) during growth led to deterioration in the surface morphology and the electrical and crystalline qualities of the films. Room temperature electron mobilities as high as 70,000 and 53,000 sq cm /V-s were measured in InSb layers grown on InP and GaAs substrates, respectively. Unlike the previous results, the conductivity in these films is n-type even at T = 13 K, and no degradation of the electron mobility due to the high density of dislocations was observed. The measured electron mobilities (and carrier concentrations) at 77 K in InSb layers grown on InP and GaAs substrates are 110,000 sq cm/V-s (3 x 10(15) cm(-3)) and 55,000 sq cm/V-s (4.95 x 10(15) cm(-3)), respectively, suggesting their application to electronic devices at cryogenic temperatures.

The dependence of the electrical and optical properties of molecular beam epitaxial In 0.52 Al 0.48 As on growth parameters: interplay of surface kinetics and thermodynamics

The optical and transport properties of In0.52Alo.48As grown by molecular beam epitaxy have been studied as a function of growth temperature in the range of 300-520° C. It is evident that under our growth conditions, thermodynamic considerations become important, and combined with surface kinetics, clustering effects become most severe for growth temperatures around 400° C. The clustering effects are manifested by changes in low-temperature photoluminescence, Hall transport and in the properties of Schottky diodes made on the films and the relevant parameters show a peaking for growth at 400° C. In particular, the Hall mobility exhibits a turning point forT > 300 K, beyond which the mobility increases with increasing temperature. In addition, the Hall electron concentration exhibits an anomalous reduction in value in the same high-temperature range. Measurements were also made on In0.52Al0.48As grown at 620-650° C by metalorganic chemical vapor deposition. While these films exhibit the same turning point in Hall mobility, the reduction in carrier concentration is significantly absent. Analysis of these data therefore indicates that the turning point in the mobility, which is present for both growth techniques, is caused by small clusters (~35Å) of phases slightly different from the mean composition. The reduction in electron concentration, seen only in the molecular beam epitaxial samples, suggest a more severe phase separation. A simple analysis for the sample grown at 400° C indicates that the compositions In0.60Al0.40As and In0.44Al0.56As might be present, in addition to the mean lattice-matched composition.

In‐plane hole effective masses in InxGa1−xAs/Al0.15Ga0.85As modulation‐doped heterostructures

We have determined the strain dependence of the in‐plane hole effective mass in pseudomorphic Inx Ga1−x As/Al0.15 Ga0.85As modulation‐doped heterostructures by low‐temperature Shubnikov–de Haas measurements. An effective mass equal to 0.18m0 is measured for x=0.2. The measured values are in good agreement with theoretical calculations.

Piezoreflectance characterization of resonant tunneling and modulation-doped heterostructures

We used piezoreflectance, a stress modulation technique, to optically characterize both double-barrier resonant tunneling structures and modulation-doped heterostructures. This technique easily probed the single quantum wells buried below the ∼0.5-μm-thick n+ GaAs surface layers required for ohmic contacts. In fact, the positions of salient piezoreflectance spectral features denoted subtle variations of well width and barrier height and width; lineshapes indicated interface roughness. Since piezoreflectance spectra of our modulation-doped samples had sharp features associated with quantum confinement, and the corresponding photoreflectence spectra had Franz-Keldysh oscillations that depended on the built-in electric field, we could calculate energy levels that reflected not only growth parameters but experimentally determined potential profiles as well. Moreover, the sensitivity of piezoreflectance to carbon and silicon acceptor states permitted us to assess the material quality.

Nonalloyed and alloyed low‐resistance ohmic contacts with good morphology for GaAs using a graded InGaAs cap layer

Using a thin graded layer of InGaAs starting on GaAs and becoming InAs on the top, low‐resistance alloyed and nonalloyed ohmic contacts have been achieved on n+‐GaAs epilayers grown by molecular‐beam epitaxy on a semi‐insulating GaAs substrate. In addition, by a suitable choice of the multilayer ohmic metals and by an optimization of the alloying process, good surface morphology was obtained. The transmission‐line model is used to extrapolate contact resistances from measurements on test patterns with multiple gap spacings varying from 1 to 10 μm. The nonalloyed contact resistance is found to be 0.025 Ω mm for a GaAs layer doped to 1×1018 with a 500‐A graded InGaAs layer. Alloying the contact at 475 °C for 60 s produces a contact resistance of 0.019 Ω mm. This represents a substantial improvement over the contact resistances obtained by just using an ungraded cap layer of InAs on GaAs. Assuming that the sheet resistance under the contact is the same as the sheet resistance in the top semiconductor layer (...

Improved strained HEMT characteristics using double-heterojunction In/sub 0.65/Ga/sub 0.35/As/In/sub 0.52/Al/sub 0.48/As design

The DC and microwave properties of strained In/sub 0.65/Ga/sub 0.35/As/In/sub 052/Al/sub 0.48/As HEMTs (high electron-mobility transistors) with double-heterojunction design are presented. The high sheet carrier density and good carrier confinement give rise to excellent device performance with very low output conductance. For 1*150- mu m/sup 2/ long-gate HEMTs, the measured cutoff frequency f/sub T/ and maximum frequency of oscillation f/sub max/ are as high as 37 and 66 GHz, respectively.<<ETX>>

Piezoreflectance characterization of double‐barrier resonant tunneling structures

The piezoreflectance technique has been used to optically characterize resonant tunneling structures that utilize isolated single quantum wells. The heavy‐ and light‐hole transitions associated with the quantum wells were prominent in the spectra of samples with barrier widths ranging from 50 to 34 A. Their spectral positions depended not only on quantum well and barrier thicknesses, but also significantly on the amount of carrier confinement produced by barrier height. Furthermore, variations in the magnitude of impurity transitions could be observed in the spectra of different samples.

The effect of molecular‐beam‐epitaxial growth conditions on the electrical characteristics of In0.52Al0.48As/In0.53Ga0.47As resonant tunneling diodes

We have investigated the dependence of the performance characteristics of In0.52 Al0.48As/ In0.53 Ga0.47 As resonant tunneling diodes upon molecular‐beam‐epitaxial growth parameters. The roughness of the growth front, leading to intrawell‐size fluctuations and the V/III flux ratio at a fixed growth temperature are found to be important parameters affecting the performance of these devices. By means of a simple model, we have semiquantitatively related the peak current to the interface roughness. Defects and traps in the In0.52 Al0.48 As barriers, on the other hand, produced partially by nonoptimal V/III flux ratios, may produce shunt paths for tunneling, again reducing the resonant tunneling current peak‐to‐valley ratio. Under optimum growth conditions we have measured current peak‐to‐valley ratios of 6.1 and 21.6 at 300 and 77 K, respectively. These are the best values reported so far for this heterostructure system.

Calculations of the electric field dependent far‐infrared absorption spectra in InAs/AlGaSb quantum wells

Excitonic and band‐to‐band absorption spectra are calculated for vertical incident radiation for the InAs/AlGaSb multiple quantum well structures. Due to the special band lineup of this heterostructure, the absorption spectra can be tailored to respond in far infrared. The electric field dependence of the spectra shows blue shift and enhanced absorption in contrast to the situation in type I quantum wells. Applications to far infrared detectors are discussed.

A critical examination of the molecular‐beam‐epitaxial growth of InxGa1−xAs/GaAs strained quantum well structures

We have investigated the molecular‐beam‐epitaxial growth and optical properties of InxGa1−xAs/GaAs (0.07≤x≤0.20) single and multiple quantum well structures. Photoluminescence and absorption measurements were made to characterize the various structures. Low‐temperature excitonic linewidths as small as 1.2–2.4 meV have been obtained in 80–120‐A InxGa1−xAs/GaAs (0.07≤x≤0.20) single and multiple quantum wells up to total thicknesses of 2.0 μm. The Stokes shift in these samples is ∼1–2 meV. This result is independent of the absence or presence of an intermediate composition buffer layer and indicates that the latter does not influence the optical properties of strained multiquantum wells. The growth kinetics and growth modes are more important factors in this respect.