A systematic experimental investigation of the influence of charge modulation on transport phenomena in pseudomorphic AlxGa1-xAs/InyGa1-yAs/GaAs field effect transistors as a function of In mole fraction (y) in the channel is presented. There is evidence that improved charge modulation can totally account for increases in both effective electron velocity and low-field drift mobility with increasing y. Transistor performance was observed to improve by 30% for y = 0.25, in comparison to the conventional A10.3Ga0.7As/GaAs HEMT. A state- of-the-art measured f(T) of 120 GHz was achieved with an f(max) of 200 GHz for y = 0.25 with a 0.18-mu-m gate length.

The electrical properties of Al0.3Ga0.7As/InxGa1−xAs modulation doped heterostructures grown on GaAs substrates were studied. We found for the normal and inverted heterostructures principal differences of the transport properties. For an InAs mole fraction of 0.2 the inverted modulation doped heterostructures show a stronger decrease in the electron mobility of the two-dimensional electron gas if the critical layer thickness of the In0.2Ga0.8As layer is exceeded, in comparison to the normal heterostructures. This behavior can be explained by the relaxation process of the In0.2Ga0.8As layer. For InxGa1−xAs heterostructures with x≳0.3 the growth mode changes from two-dimensional to three-dimensional growth, which leads to interface roughness, degrading the transport properties of the normal heterostructure. Thus for high InAs mole fractions the inverted heterostructures show better transport properties in comparison to the normal heterostructures.

ABSTRACTLattice mismatched InxGa1−xAs layers with InAs mole fractions below 0.25 grow in a two dimensional growth mode on GaAs. If the thickness of these layers is beyond the critical layer thickness the strain relaxes through misfit dislocations. The misfit dislocation density in the <011> and <01-1> direction differs for n-type layers. This results in a highly anisotropic electron mobility for GaAs/InxGa1−xAs/Al0.3Ga0.7As inverted HEMT structures. A higher electron mobility in the < 011 > direction is measured in comparison to the <01-1> direction. The resistance ratio in the two perpendicular directions exceeds 105. For a three dimensional growth mode, the InxGa1−xAs layer shows interface roughness which degrades the transport properties of the normal Al0.3Ga0.7As/ InxGa1−xAs/ GaAs HEMT structures more than the inverted GaAs/InxGa1−xAs/ Al0.3Ga0.7As HEMT structures. For a three dimensional growth mode, an anisotropic electron mobility for Al0.3Ga0.7As/InxGal, As/GaAs HEMT structures is also observed. For these structures the highest electron mobility is measured in the < 01-1 > direction. This anisotropy could be explained by anisotropic growth rates in the <011> and < 01-1 > directions which results in growth islands with asymmetric extensions.

The anisotropic conduction of GaAs/In0.2Ga0.8As/Al0.3Ga0.7As inverted high-electron-mobility transistor (HEMT) structures has been investigated. The heterostructures were grown by molecular-beam epitaxy on (100) GaAs substrates. The thickness of the pseudomorphic layer was increased stepwise (150–300 Å) beyond the critical layer thickness as determined by the appearance of misfit dislocations. These mixed 60° dislocations surrounded by depletion regions were observed as straight dark lines in cathodoluminescence. The measured resistance Rs was higher in the [01̄1] direction than in the perpendicular [011] direction. At T=30 K the conduction ratio of these two directions exceeded 105 in the 300-Å-thick layer. The magnitude and anisotropy of Rs was correlated with the anisotropic dislocation patterns resulting from the preferential generation of the α dislocations ∥ [011] as compared to the orthogonal β dislocations ∥ [01̄1]. In both directions Rs depended exponentially on the number of dark lines perpendicular to the probing current. Simultaneously, the functional form of the temperature-dependent Rs(T) strongly varied with layer thickness. The thin, still elastically strained layers showed the usual behavior of HEMT structures. For the thicker layers a completely different temperature dependence was gradually developing, eventually leading to an exponential increase of Rs with inverse temperature between 300 and 100 K. Below this range Rs(1/T) changed more slowly and leveled off at 30 K. All these features are convincingly explained by a model assuming that the electrons can surmount the insulating depletion barriers in the conducting channel by a thermally induced tunneling mechanism.

Anisotropic electron mobilities in Al0.3Ga0.7As/In(x)Ga(1-x)As/GaAs high electron mobility transistor structures grown using molecular beam epitaxy were studied using Hall effect measurements. The electron mobility depends on the orientation of the Hall bar in relation to the crystal axes. The lowest mobility was observed for the <011> direction. Up to an angle of 90-degrees corresponding to the <01-1> direction, the mobility increases with increasing angle. Beyond 90-degrees it decreases until it reaches its initial value again at 180-degrees. The mobility ratio mu-<01-1>/mu-<011> increases with increasing width of the quantum wells of In(x)Ga(1-x)As layers for a constant x value. The mobility ratio also depends on the in content. With increasing in content of the In(x)Ga(1-x)As layer and for a quantum well width near the critical layer thickness, an increase in the mobility ratio was observed.

Sub-0.1 micrometers mushroom-shaped gates (T-gates) have been realized with a three-layer resist technique using e-beam exposure. The exposure was carried out on a Philips EPBG-3 system operating at 50 kV. The resist system and writing strategy were investigated. Test exposures on SiN-capped GaAs wafers with ohmic contacts having the same topography as active devices were carried out. Using this T-gate lithography, pseudomorphic AlGaAs/InGaAs/GaAs HEMTs were fabricated. These devices have transit frequencies of 120 GHz.

Anisotropic electron mobilities for GaAs/In0.2Ga0.8As/Al0.3Ga0.7As inverted high electron mobility transistor structures were observed using Hall effect measurements. If the In0.2Ga0.8As quantum-well thickness is below the critical layer thickness, a higher electron mobility in the <01-1> direction is observed in comparison to the <011> direction. Exceeding the critical layer thickness of the In0.2Ga0.8As quantum well results in a change in the behavior of the anisotropy, and a highly anisotropic electron mobility with a higher electron mobility in the <011> direction, in comparison to the <01-1> direction, is observed. With increasing In0.2Ga0.8As quantum-well width, the anisotropy increases. An increase of the anisotropy was also observed if the Hall-effect measurements were carried out at lower temperatures. The anisotropy in the electron mobility can be correlated to the occurrence of a highly asymmetric-dislocation density. The asymmetry in the dislocation density was observed using wavelength-selective catholuminescence measurements.

Modulation doped Al0.3Ga0.7As/In(x)Ga1-xAs/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for Al0.3Ga0.7As/In(x)Ga1-xAs/GaAs high electron mobility transistor structures with a constant In(x)Ga1-xAs quantum well width was investigated as a function of InAs mole fraction. If the In(x)Ga1-xAs quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided delta-doped structures was obtained for the structures with delta-doping on both sides of the In(x)Ga1-xAs quantum well. Al0.3Ga0.7As/In(x)Ga1-xAs/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0-0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25-0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15-mu-m a f(T) of 115 GHz was measured.

Modulation doped AlGaAs/InGaAs/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for AlGaAs/InGaAs/GaAs high electron mobility transistor structures with a constant InGaAs quantum well width was investigated as a function of InAs mole fraction. If the InGaAs quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided δ-doped structures was obtained for the structures with δ-doping on both sides of the InGaAs quantum well. AlGaAs/InGaAs/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0–0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25–0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15 μm a of 115 GHz was measured.

We report on optical and electrical properties of GaAs/AlGaAs heterostructures prepared by molecular beam epitaxy (MBE). For fixed Ga and As fluxes the damping of the oscillations of the reflection high energy electron diffraction (RHEED) pattern is strongly dependent on substrate temperature. The minimum of the damping of the intensity oscillations has been observed at 750°C. The best electrical properties of high electron mobility transistors (HEMT) have been found at this substrate temperature. The maximum electron mobility was 120000 cm2/Vs at 77 K for a spacer of 50 Å and an electron concentration of 1∗1012 cm−2. For the presented quantum well (QW) structures we obtained the best results at a substrate temperature of 740°C. In photoluminescence we obtained a full width at half maximum (FWHM) for example for a 20 nm, 4.5 nm, 2 nm and 1 nm wide QW of 0.27 meV, 1.5 meV, 5 meV and 7 meV respectively.

We have performed a comparative study of low temperature photoluminescence (PL), photoluminescence excitation spectroscopy (PLE), transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS) on GaAs/AlGaAs quantum wells (QW) grown by molecular beam epitaxy (MBE). The QW widths derived from reflection high energy electron diffraction (RHEED) intensity oscillations are compared with TEM results and SIMS measurements. In order to study the effect of growth interruption on optical properties we have grown QWs without and with growth interruption. For QWs grown without growth interruption we obtained an interface roughness of one monolayer. QWs grown with growth interruption exhibit a series of emission lines in PL which are attributed, based on the comparison of PL and PLE data, to free and bound exciton recombination. The above assignment is confirmed by excitation power density dependent PL. For QWs grown with growth interruption we obtained atomically flat interfaces.

Nonresonant carrier tunneling is investigated by time-resolved and time-averaged optical methods for a series of samples with various barrier thicknesses. The electron tunneling times decrease exponentially with the decrease of barrier thickness from 8 to 3 nm, and the trend is well described by a semiclassical model. Additional efficient hole tunneling is observed in the 3 nm barrier sample, and the time constant is of the order of 50 ps.

Modulation doped Alo.aGao.vAs/In~Gax _~As/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for Alo.3Gao.vAs/InxGa~ _~As/GaAs high electron mobility transistor structures with a constant InxGa~ _~As quantum well width was investigated as a function of InAs mole fraction. If the In~Gal_~As quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided 6-doped structures was obtained for the structures with f-doping on both sides of the In~Gaa_~As quantum well. Alo.3Gao.7As/In~Gal_xAs/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0-0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25-0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15 gm afT of 115 GHz was measured. PACS: 6855, 7280E, 7340L Pseudomorphic AlrGal_rAs/InxGal_~As/GaAs high electron mobility transistor (HEMT) structures grown on GaAs are of great interest for high speed device applications. A superior electron confinement and a higher conduction band offset resulting in higher sheet densities can be achieved in comparison to the conventional AlyGal _rAs/GaAs HEMT structure [1]. The conduction band offset between the AlrGal -r As layer and the In~Gal_~As layer increases with increasing InAs mole fraction. Thus it is desirable to increase the InAs mole fraction. However, the lattice mismatch between the GaAs substrate and the ln~Gax_~As layer can only be accommodated within the layers by elastic strain if the In~Gal_=As layer thickness is below the critical layer thickness (CLT) [2]. If the layer thickness is above the CLT the strain energy can force the formation of misfit dislocations. A high dislocation density leads to a strong degradation of the electrical [3] and optical [4] properties. * Dedicated to H.-J. Queisser on the occasion of his 60th birthday For best performance of AlyGal_rAs/InxGaa_~As/ GaAs HEMT structures for device application, it is necessary to know the, dependence of the CLT of the InxGax_xAs layers on the InAs mole fraction in the range 0.2-0.5. For InAs mole fractions below 0.2 the CLT is above 15 nm [5]. As for device applications a quantum well (QW) width of only 15 nm is necessary the CLT below 0.2 was not investigated. Previous work has been concerned with the determination of the CLT with different measurement methods, such as X-fay [6], photoluminegcence [4, 5], photoluminescence microscopy [7], and Hall effect measurements [8]. These methods yield different values for the CLT, due to the varying sensitivity of the different methods [9]. This paper reports on the growth of Alo.aGao.TAS/ In~Gal_~As/GaAs HEMT structures for electrical devices. The CLT is determined by Hall effect measurements for different InAs mole fractions in the range 0.2-0.5 with respect to the electrical properties. Using the results for the determination of the CLT, Alo.aGao.yAs/InxGal_~,As/GaAs HEMT structures for