W. G. Lyons

Use of a superlattice to enhance the interface properties between two bulk heterolayers

Single interface modulation‐doped Alx Ga1−xAs /GaAs heterostructures with the binary on top of ternary were grown by molecular beam epitaxy. By incorporating a 150‐A‐thick Alx Ga1−xAs /GaAs three‐period superlattice in place of an undoped Alx Ga1−xAs spacer layer, 10‐K mobilities of up to 256 000 cm2/Vs were obtained. This value is about 6.5 times that of the previous best value. This dramatic improvement is tentatively attributed to the relief of strain caused by the small but significant lattice mismatch although impurity trapping by the superlattice may also play a role. Normal modulation‐doped structures where the ternary is grown on top of binary also showed mobility improvement (about 30%) when the undoped AlGaAs spacer layer is replaced with a three‐period superlattice of the same thickness. This concept should have a significant role in heterojunction bipolar transistors, field‐effect transistors, lasers, and other heterojunction devices.

Double heterojunction GaAs/AlxGa1−xAs bipolar transistors prepared by molecular beam epitaxy

Double heterojunction AlGaAs/GaAs bipolar junction transistors (DHBJT’s) grown by molecular beam epitaxy (MBE) were fabricated and tested. The incorporation of a graded base‐collector junction improved the dc performance of a DHBJT over that obtained with an abrupt base‐collector junction. Maximum current gains of 500, 900, and 1650 were obtained using graded collector and emitter junctions and base widths of 0.05, 0.2, and 0.1 μm, respectively. The reduction in current gain in going from a 0.1‐μm base width to a 0.05‐μm base width is attributed to a lowering of the electron lifetime resulting from the high base doping level used. The value of 1650 is comparable to the current gains obtained from liquid phase epitaxy (LPE)‐grown HBJT’s and is the best obtained from MBE‐grown HBJT’s. Common‐emitter transistor turn‐on voltages were found to correspond to a difference in the collector and emitter junction turn‐on voltages. High collector growth temperature improved dc performance with 700 °C as the optimum g...

Characteristics of modulation‐doped AlxGa1−xAl/GaAs field‐effect transistors: Effect of donor‐electron separation

The dc characteristics of modulation‐doped AlxGa1−xAs/GaAs field‐effect transistors have been studied experimentally and theoretically to determine the effect of the thickness of the undoped AlxGa1−xAs spacer layer commonly left at the heterointerface. Increasing the thickness of the spacer layer decreases charge transfer and increases mobility. Current transport in short channel transistors, however, is limited by the electron saturation velocity which is independent of the spacer thickness. Due to increased charge transfer, decreasing the spacer thickness from 100 to 20 A doubled the maximum saturation current and transconductance. This should allow faster switching speeds to be obtained. A maximum current of 24 mA was obtained for a gate width of 145 μm with a 40‐A spacer and a transconductance of 250–275 mS/mm was obtained for a device with a 20‐A spacer. Theoretical results indicate that intrinsic transconductances greater than 900 mS/mm are possible. Preliminary small‐signal rf measurements indicate...

Use of a GaAs smoothing layer to improve the heterointerface of GaAs/AlxGa1−xAs field‐effect transistors

The effect of a GaAs smoothing layer on the performance of GaAs/AlxGa1−xAs field‐effect transistors prepared by molecular beam epitaxy was investigated. The GaAs smoothing layer was inserted between the channel layer and the AlxGa1−xAs buffer layer in an attempt to reduce the dependence of interface quality on growth conditions. Current‐voltage characteristics of field‐effect transistors with 1‐μm gate lengths were used to characterize the properties of the heterointerface. Without the GaAs smoothing layer, extremely sharp interfaces, as indicated by the electron velocity, were obtained when the structures were grown at 700 °C. However, the interface sharpness was very sensitive to growth conditions and in particular to the substrate temperature, decreasing from 50 A at 700 °C to 70 A at 640 °C and to 260 A at 580 °C. Incorporation of a 200‐A‐thick undoped GaAs smoothing layer at the heterointerface decreased this sensitivity to growth conditions. With the smoothing layer, interface sharpnesses of 150, 60...

An explanation for anomalous donor activation energies in Al0.35Ga0.65As

Transport properties of Si‐doped Al0.35Ga0.65 As epitaxial layers grown by molecular beam epitaxy were studied and the activation energy of the Si donors determined. Previously reported values for the Si‐donor activation energy for x = 0.35 have varied considerably and values as high as 100 meV have been suggested. For x values above and below x = 0.35, the activation energies reported are much lower, and show better consistency. Anomalously high activation energies for Se (300 meV) and Te (150 meV) doped AlxGa1−x As near the direct‐indirect transition have also been reported. In this letter, a possible explanation for these anomalies in previous results based on the modulation doping effect is described. Results obtained for layers grown so as to avoid modulation doping indicate the activation energy of Si in Al0.35Ga0.65 As is less than 14 meV. When modulation doping effects were not suppressed, apparent activation energies as high as 125 meV were obtained. Finally, electron mobilities in samples not di...

Development of a tunnelling model of charge-density-wve depinning

Abstract The tunneling model of transport by charge-density waves in quasi-one-dimensional metals has been developed from a close interaction between theory and experiment. Depinning by coherent Zener tunneling of electrons in the CDW condensate in many parallel chains, first applied to NbSe 3 ,and TaS 3 , was suggested by the field dependence of the dc current. In 1981, a program initiated at Illinois to study detection, mixing and harmonic generation was successful in showing that photon-assisted tunneling theory accounts for a wide range phenomena. The only inputs are a scaling parameter and the dc I-V characteristic. Recent measurements by Gruner, Reagor baret al. of ϑ(ω) in the range 1–100 Ghz suggested a modification to account for CDW metals with high pinning frequencies, such as (TaSe 4 ) 2 I and the alloys Ta 1−x Nb x S 3 . The dc and low-frequency ac response can be derived from parameters measured at high frequencies. The theory accounts for the “narrow band” noise that accompanies dc current flow and the subharmonic steps of constant current in the dc I-V characteristic measured in the presence of an applied ac.

Mixing experiments in NbSe3

Abstract The response of NbSe 3 to combined a.c. and d.c. fields has been characterized at T = 125 K below the first charge-density wave (CDW) transition. The a.c. and d.c. conductances were measured, along with the rectification and harmonic mixing at megahertz frequencies due to the CDW non linearity. These experimental results are shown to be consistent with a revised tunneling theory of CDW depinning.

Properties of silicon-doped AlxGa1-xAs grown by molecular beam epitaxy

Abstract The incorporation of silicon in AlxGa1-xAs grown by molecular beam epitaxy was investigated. In Al0.25Ga0.75As and Al0.35Ga0.65As doping levels were proportional to the incident silicon flux for effusion cell temperatures below 1125°C. At 1125°C the electron concentration saturated at about 2×1018 cm-3 and then decreased slightly. For cell temperatures above 1150°C silicon precipitation observable with an optical microscope took place. Silicon incorporation decreased with increasing growth temperature above 650°C with an activation energy of 2.1 eV. Electron mobilities in excess of 1000 cm2V-1 s-1 were obtained for doping levels above 1×1018 cm-3. Donor activation energies were typically between 5 and 20 meV and increased slightly as the silicon flux was increased.

Frequency- and bias-dependent ac conductivity of charge-density waves in TaS3

Abstract The charge-density wave contribution to the dc and the small-signal ac conductivity of orthorhombic TaS3 has been completely characterized for a single crystal. The ac conductivity is found to be independent of dc bias below threshold. The tunneling theory of charge-density wave depinning successfully predicts the real and imaginary parts of the ac conductivity as functions of both frequency and applied dc bias, using only a fit to the dc I–V data and one adjustable parameter.

Scanning tunnelling microscopy of 1-D and 2-D charge-density wave systems

We have used a new variable temperature scanning tunnelling microscope (STM) to study quasi‐1D and 2‐D charge‐density wave (CDW) systems. The 1‐D systems, typified by NbSe3 and TaS3, are of special interest since they exhibit unusual transport phenomena associated with moving CDW above a threshold electric field. In the case of NbSe3, room temperature STM images show both major and subtle details of the lattice structure. At present, however, images taken below the Peierls transition temperature of TP=144 K resolve major lattice details but are not sufficiently clear to resolve the CDW. On the other hand, for the fully gapped CDW system orthorhombic‐TaS3, the CDW modulation superimposed on the lattice structure and having the correct period of four times the S‐S spacing of 3·3 Å, is observed below TP=215 K. Above TP, the main observable feature is the S‐S spacing along the chains. STM measurements have also been performed on the 2‐D CDW system 1T‐TaS2 in its incommensurate, nearly commensurate, fully commensurate and trigonal phases. For the nearly commensurate phase, STM images show uniform commensurability with a relatively low concentration of small, time‐varying discommensurations in contrast to models pradicting a regular domain structure. In the trigonal phase, however, evidence is seen for the striped phase composed of long, nearly parallel discommensurations.