W. T. Masselink

Growth and properties of GaAs/AlGaAs on nonpolar substrates using molecular beam epitaxy

Using molecular beam epitaxy, we have successfully grown device quality GaAs/AlGaAs on (100)‐oriented Ge and Si substrates. Modulation doped field effect transistors have been fabricated from these layers which exhibit room‐temperature transconductances as high as 160 and 170 mS/mm for layers on Ge and Si, respectively, and showed no looping in either case. At 77 K, these values rose to 345 and 275 mS/mm for Ge and Si, respectively. Analysis by transmission electron microscopy of layers grown on Ge showed that the antiphase disorder was contained within the 250‐A‐thick initial layer which was grown at a 0.1‐μ/h growth rate at a substrate temperature of 500 °C. For both the layers grown on Si and Ge specular surface morphologies were obtained. The photoluminescence of GaAs/AlGaAs quantum wells grown on Si and Ge was similar in intensity to the same quantum well structures grown on GaAs. In quantum wells grown on Ge, the luminescence was dominated by a donor‐acceptor recombination at 1.479 eV, which appears...

Optical properties of GaAs on (100) Si using molecular beam epitaxy

Undoped GaAs layers have been grown on (100) oriented Si by molecular beam epitaxy and have been studied using photoluminescence, photographic, and x‐ray measurements. To minimize antiphase disorder, an As primer layer was successfully used to initiate the polar on nonpolar growth. Photoluminescence spectra show the presence of five bands into which the luminescence from each sample falls. The energy variation within any given band was less than 4 meV. X‐ray and photoluminescence analysis appear to indicate some strain in the crystal. Although very preliminary, the results obtained indicate that GaAs on Si can potentially be used for a number of hybrid ciruits.

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/Al0.15Ga0.85As modulation-doped field effect transistors (MODFETs) exhibiting extremely good dc characteristics have been successfully fabricated, dc transconductance in these strained-layer structures of 270 mS/mm were measured for 1-µm gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.

Ultrafast all‐optical gate with subpicosecond ON and OFF response time

An all‐optical logic gate consisting of a GaAs‐GaAlAs multiple quantum well structure inserted in a 1.3‐μm‐thick Fabry–Perot cavity is demonstrated to perform with subpicosecond on and off switching time. The use of a purely optical field effect allows for a recovery time as rapid as the switch‐on time.

Determination of interfacial quality of GaAs‐GaAlAs multi‐quantum well structures using photoluminescence spectroscopy

Well size fluctuations have been observed in high quality GaAs‐Ga0.75Al0.25As multi‐quantum well structures having very sharp photoluminescence transitions. The effect of well size fluctuations appears as multiple peaks in both the heavy hole free exciton and the heavy hole donor bound exciton transitions. The observed energy separation of the peaks corresponds to what would be expected for a change in well thickness of 1/2 monolayer. The observed linewidths and the well size fluctuations suggest that these are interlayer rather than intralayer fluctuations. These results are different from earlier reported work. The very narrow emission lines reflect the excellent structural quality of the layers. A model to explain the effective half‐monolayer well size fluctuations is proposed.

Quasi-Fermi level bending in MODFET's and its effect on FET transfer characteristics

Using Shockleys diffusion/drift model we calculate the quasi-Fermi level (imref) bending in the depleted AIGaAs barrier layer of GaAs/AIGaAs MODFETs. We show that the assumption of a constant imref from the heterointerface through the barrier layer is not justified when the gate is moderately forward biased. Once the barrier-layer conduction band edge at the gate interface fails below that at the heterointerface, the imref changes both in the vicinity of the heterointerface and at the gate metal. This has important consequences for the MODFET transfer characteristics and necessitates new considerations for the gate control mechanism As a result, sheet electron concentrations in the MODFET channel exceeding the equilibrium concentrations are obtained. Despite the gate being forward biased with voltages close to or larger than the Schottky-barrier height, the gate current is suppressed not only by the barrier at the heterointerface but also by a barrier of about the same height present at the gate metal-semiconductor interface. Experimental results on AIGaAs/GaAs MODFETs are in good quantitative agreement with the theoretical calculations.

The dependence of 77 K electron velocity-field characteristics on low-field mobility in AlGaAs-GaAs modulation-doped structures

We have used the geometrical magnetoresistance method to measure the electron velocities and mobilities as functions of electric field in AlGaAs-GaAs modulation-doped structures at 77 K. These structures had a variety of AlGaAs mole fractions and undoped setback layers resulting in different low-field mobilities in the different structures. We find that higher low-field mobility does not lead to a higher high-field velocity, and in fact, that at electric fields found in operating MODFETs, the mobilities and velocities were about the same in various AIGaAs-GaAs MODFETs. The results indicate that high low-field mobility is not an important parameter for MODFET design and does not even significantly reduce the parasitic source resistance in an operating FET. Furthermore, because of the strong electric field dependence of the mobility, low-field source resistance measurements are not adequate for determining the source resistance in an operating FET.

Temperature dependence of sharp line photoluminescence in GaAsAl0.25Ga0.75As multiple quantum well structures

Abstract Photoluminescence properties of sharp emission lines (the heavy- and light-hole excitons, the excitons bound to neutral donor and acceptor, and the heavy hole-to-neutral donor transition) present in high quality GaAsAl0.25Ga0.75As multiple-quantum well structures have been determined as a function of temperature from 2 to 296 K. The four multiple- quantum well structures studied each consist of 100, 200, 300 and 400 A GaAs wells separated by 100 A Al0.25Ga0.75As barriers. The exciton bound to a neutral donor located at the center of the well is suggested to be the predominant photoluminescence emission in the temperature range of 2–296 K. The photoluminescence in the peak region consists mainly of the heavy-hole exciton and donor-bound excitons.

Measurement of the electron velocity-field characteristic in modulation-doped structures using the geometrical magnetoresistance method

We describe a method for measuring the electron mobility, velocity, and sheet carrier concentration in modulation-doped structures as functions of electric field through the use of the geometrical magnetoresistance effect. Because the geometry of the structures is identical to that of ungated FETs, these measurements are well suited for studying the electron velocity in MODFETs. We see that the mobility quickly decreases from its low field value with increasing electric field and observe significant electron injection from the contacts. The electron velocity increases to about 1.4 × 107cm/s at 3000 V/cm at 77 K before domain formation prevents accurate measurements at higher fields.

Determination of carrier saturation velocity in high-performance In y Ga 1-y As/Al x Ga 1-x As modulation-doped field-effect transistors (0 ≤ y ≤ 0.2)

We describe a new method for determining the carrier saturation velocity υ<inf>sat</inf>in modulation-doped field-effect transistors (MODFETs). High-performance pseudomorphic In<inf>y</inf>GA<inf>1-y</inf>As/AlGaAs MODFETs (0 ≤ y ≤ 0.20) grown by MBE were tested, and the dependence of υ<inf>sat</inf>on InAs mole fraction y was obtained. It was found that each device with y > 0 had a higher υ<inf>sat</inf>than a conventional GaAs/ AIGaAs MODFET (y = 0). Further, we believe that there is an optimum InAs mole fraction such that υ<inf>sat</inf>is maximized. A pseudomorphic MODFET structure optimizing υ<inf>sat</inf>may also optimize overall device performance.