S. L. Wright

A GaAs gate heterojunction FET

A new semiconductor-insulator-semiconductor field-effect transistor has been fabricated. The device consists of a heavily doped n-type GaAs gate with undoped (Al,Ga)As as the gate insulator, on an undoped GaAs layer. This structure gives the device a natural threshold voltage near zero, well suited for low-voltage logic. The threshold voltage is, to first order, independent of Al mole fraction and thickness of the (Al,Ga)As layer. The layers were grown by MBE and devices fabricated using a self-aligned technique involving ion-implantation and rapid thermal annealing. A transconductance of 240 mS/mm and a field-effect mobility of about 100 000 cm2/V-s were achieved at 77 K.

The capture barrier of the DX center in Si-doped AlxGa1−xAs

We report measurements of the capture barrier for the DX center in Si‐doped AlxGa1−xAs as a function of the alloy composition. A model of the capture process which requires a distribution of capture barrier heights has been fit to the data for samples with x=0.35. A simple technique is used to extract the average capture barrier height from data for samples with AlAs mole fraction ranging from x=0.27 to x=0.55. The barrier height varies strongly with the composition and has a minimum at x=0.35. The implications of these results are discussed.

Effect of local alloy disorder on emission kinetics of deep donors (DX centers) in AlxGa1−xAs of low Al content

We report measurements by deep level transient spectroscopy of electron emission from the deep donor level (DX center) in Si‐doped GaAs and AlxGa1−xAs of very low Al content. For the first time, discrete emission rates corresponding to different local configurations of Ga and Al atoms around the Si donor are resolved. The large change in emission kinetics previously observed between GaAs and AlxGa1−xAs (x≥0.14) is thus shown to arise from the local alloy disorder which is absent in GaAs.

Noise spectroscopy of deep level (DX) centers in GaAs-AlxGa1−xAs heterostructures

We have measured the generation‐recombination noise from the donor‐related DX centers in current biased GaAs/AlxGa1−xAs heterostructures from 1 Hz to 25 kHz and from 77 to 330 K. A significant noise contribution from these traps is observed even at Al mole fractions below 0.2, where the trap level is resonant with the conduction band. The activated behavior of the noise spectrum from this resonant level is very similar to that observed at higher Al mole fractions, when the level lies deep in the fundamental gap. This result can be predicted, based on the recently elucidated relationship of the trap level to the band structure of AlxGa1−xAs. In accordance with other experimental results, the noise spectra demonstrate that the emission and capture kinetics of the level are unperturbed by its resonance with the conduction band. We briefly discuss some implications of these results for heterostructure transistor design.

In situ contacts to GaAs based on InAs

We report preliminary electrical results on n+‐InAs/n‐GaAs contact structures grown by molecular beam epitaxy. The data indicate that the conduction‐band discontinuity is sufficiently small to allow the formation of an ohmic contact to n‐type GaAs for very heavily doped InAs layers. The structures require a short‐term anneal to obtain a low resistance contact. An InAs layer which is only 200 A thick is sufficient to provide a specific contact resistance of 10−6 Ω cm2. The contacts appear to be thermally stable for short‐term anneals up to 900 °C.

Electron mobility in p‐type GaAs

The mobility of electrons in p‐type GaAs, μPn has been determined by measuring the common emitter cutoff frequency fT of heterojunction bipolar transistors with a wide, uniformly doped base. At 295 K, μPn =1150 cm2/(V s) is found for a hole concentration of 3.6×1018 cm−3. At 77 K, μPn =6000 cm2/(V s). The room‐temperature value is considerably smaller and the 77 K value considerably larger than the electron mobility in comparably doped n‐type material.

Effects of the local environment on the properties of DX centers in Si-doped GaAs and dilute AlxGa1-xAs alloys

Si‐doped GaAs and dilute AlxGa1−xAs alloys under hydrostatic pressure have been studied using deep level transient spectroscopy (DLTS). In GaAs the DLTS spectrum of the DX center is a single peak. In AlGaAs however, multiple peaks, resulting from different thermal emission rates from donors having different numbers of Al atoms as near neighbors, are observed. The pressure dependence of the electron occupation of individual DX levels shows that the larger the number of Al atoms near the Si donor, the lower the energy position of the DX level.

Effect of the silicon doping concentration on the recombination kinetics of DX centers in Al0.35Ga0.65As

The recombination (electron capture) kinetics of the ionized DX center in AlxGa1−xAs have been measured as a function of temperature and silicon doping concentration. It is shown that for x≂0.35, the silicon concentration dependence of the recombination kinetics is dominated by effects of the electron distribution in the conduction band, and is insensitive to changes in the trap characteristics. In a model kinetic calculation consistent with the data the trap is found to capture through a level 0.202 eV from the bottom of the conduction band with a width of 0.045 eV, independent of DX center concentration.

Electron velocity and negative differential mobility in AlGaAs/GaAs modulation‐doped heterostructures

We have measured the electron velocity in low‐doped GaAs and in AlGaAs/GaAs modulation‐doped heterostructures at electric fields up to 8000 V/cm at both 300 and 77 K. In order to avoid the charge domain formation which occurs in dc measurements at these fields, this measurement uses 35 GHz radiation to supply the electric field. These measurements indicate that the peak velocity for electrons in the heterostructures is lower than for electrons in bulk low‐doped GaAs. This result is explained in terms of modified intervalley transfer, real space transfer, and an enhanced scattering with polar optical phonons.

Origin of the nonexponential thermal emission kinetics of DX centers in GaAlAs

Direct evidence has been found, via hydrostatic pressure experiments, that the random distribution of Al and Ga atoms (alloy broadening) is the main cause of the nonexponential behavior of thermal emission processes from DX centers in Ga1−xAlxAs alloys (0.19≤x≤0.74). Isothermal single‐shot emission transients at constant capacitance were used to measure the nonexponential behavior. Experimental values of the degree of nonexponentiality at ambient pressure, as a function of the Al content, are in good agreement with an alloy broadening model. When hydrostatic pressure up to 11 kbar is applied, the nonexponential behavior does not change, confirming its independence from variations in the conduction‐band structure.