C. E. Stutz

Annealing dynamics of molecular‐beam epitaxial GaAs grown at 200 °C

By separating a 2‐μm‐thick molecular‐beam‐epitaxial GaAs layer grown at 200u2009°C from its 650‐μm‐thick substrate, we have been able to obtain accurate Hall‐effect and conductivity data as functions of annealing temperature from 300 to 600u2009°C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged AsGa concentrations. Comparisons of all of these quantities as a function of annealing temperature TA show a new feature of the annealing dynamics, namely, that the dominant acceptor (probably VGa related) strongly decreases and then increases as TA is increased from 350 to 450u2009°C. Above 450u2009°C, ND, NA, and [AsGa] all decrease, as is known from previous studies.

Deep traps in molecular‐beam‐epitaxial GaAs grown at low temperatures

Deep‐level transient spectroscopy has been performed on Si‐doped GaAs layers grown by molecular‐beam epitaxy at substrate temperatures of 400–450u2009°C. The λ effect is taken into account and overlapping peaks are analyzed numerically. An 0.65 eV electron trap of concentration 2×1016 cm−3 is believed to be related to the AsGa‐associated 0.65 eV Hall‐effect center, and also to the trap EB4 found in electron‐irradiated GaAs.

Analytical Two-Layer Hall Analysis - Application To Modulation-Doped Field-Effect Transistors

The classical magnetic‐field‐dependent Hall coefficient and conductivity equations are inverted to give the mobilities μ1 and μ2 and carrier concentrations n1 (or p1) and n2 (or p2) in two degenerate bands. The two‐band solution holds for arbitrary magnetic‐field strength as long as quantum effects can be ignored (i.e., kT≳ℏeB/m*), and it is argued that the analysis can also be applied to two separate layers up to reasonable field strengths. The results are used to determine the two‐dimensional electron gas mobility and carrier concentration in a modulation‐doped field‐effect transistor with a highly doped cap layer.

Magneto‐Hall characterization of delta‐doped pseudomorphic high electron mobility transistor structures

Conventional Hall‐effect determination of the two‐dimensional electron gas (2DEG) concentration n2D in pseudomorphic high electron mobility transistor structures is invalid because of interference from the highly doped GaAs cap. Furthermore, the usual methods of dealing with this cap‐interference problem, namely, (1) etching off the cap totally, (2) etching the cap until the mobility reaches a maximum, or (3) growing a separate structure with a thin, depleted cap, in general, give n2D values that are too low. However, we show here that magnetic‐field‐dependent Hall (M‐Hall) measurements can separately determine the carrier concentrations and mobilities in the cap and 2DEG regions, as verified by comparison with a self‐consistent, four‐band, k⋅p calculation and also by electrochemical capacitance‐voltage measurements in structures with different cap and spacer thicknesses.

On Hall Scattering Factors for Holes in GaAs

Hall scattering factors for electrons and holes in molecular beam epitaxial GaAs layers have been determined by comparing carrier concentrations measured by the Hall effect with those measured by the electrochemical capacitance–voltage technique. The conclusion is that both the electron and hole scattering factors are near unity for n ranging from 2×1016 to 7×1017 cm−3, and p ranging from 5×1016 to 4×1019 cm−3. This conclusion is consistent with the present theory for electrons, but not with that for holes.

Effects of In profile on material and device properties of AlGaAs/InGaAs/GaAs high electron mobility transistors

The molecular‐beam‐epitaxial growth of InxGa1−xAs on GaAs or AlyGa1−yAs leads to a variation of In content with depth, due to In segregation. However, by predepositing In at the beginning of InxGa1−xAs growth, and also thermally removing the excess In at the end, we can produce a layer with the ideal ‘‘square’’ In profile. We find that the performance of AlyGa1−yAs/InxGa1−xAs/GaAs high electron mobility transistors is most enhanced by the predeposition step alone.

Saturation spectroscopy of electronic states in a magnetic field in InAs/AlxGa1−xSb single quantum wells

Abstract We have carried out saturation spectroscopy of cyclotron resonance in a semiconducting InAs/Al 0.5 Ga 0.5 Sb single quantum well using the UCSB free electron laser and have extracted an effective Landau level lifetime using an n -level rate equation model. The effective lifetime shows strong oscillations (>an order of magnitude) with frequency. Minima are shifted to higher frequencies than those given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. We have also used this technique to investigate the origins of two lines: the X-line and cyclotron resonance in a “semimetallic” InAs/Al 0.1 Ga 0.9 Sb single quantum-well structure. Results show that the two lines are of different origin.

Characterization of AlAs/GaAs superlattice barriers using electrical barrier height analysis

Electrical barrier height measurements on n+‐GaAs–insulator–n‐GaAs structures with short‐period AlAs/GaAs superlattices forming the insulator show the effective conduction‐band discontinuity (ΔEC) of a superlattice barrier (SLB) to be defined by the lowest superlattice energy state. Five structures with different AlAs and GaAs SLB layer thicknesses are investigated. A SLB with GaAs layers greater than 10 monolayers is found to have a ΔEC defined by Γ‐valley states in the GaAs layers, while a SLB with GaAs and AlAs layers less than 10 monolayers and with thicker AlAs layers than GaAs layers is found to have a ΔEC defined by X‐valley states in the AlAs layers. The SLB with GaAs and AlAs layers less than 10 monolayers and thicker GaAs layers than AlAs layers behaves as a random alloy. Negative differential resistance is observed in the current‐voltage characteristic of the sample whose barrier height is defined by Γ‐valley states in the GaAs layers.

Effect of radial growth rate variation on resonant tunneling diode current-voltage characteristics

The uniformity of electronic device characteristics is dependent on the uniformity of the epitaxial material. Uniformity is particularly important for resonant tunneling diodes where small changes in well or barrier thickness can have profound effects on the diode current-voltage characteristics. The variability of these characteristics due to growth rate nonuniformity for epitaxial structures grown by molecular beam epitaxy has been documented and the magnitude of the thickness variations analyzed using photoluminescence and a theoretical model. An increase of 17 meV was observed in the quantum well ground state, corresponding to a 15% thinning of the well from the center to the edge of the substrate.