Kazuo Nanbu

A New Field-Effect Transistor with Selectively Doped GaAs/n-AlxGa1-xAs Heterojunctions

Studies of field-effect control of the high mobility electrons in MBE-grown selectively doped GaAs/n-AlxGa1-x As heterojunctions are described. Successful fabrication of a new field-effect transistor, called a high electron mobility transistor (HEMT), with extremely high-speed microwave capabilities is reported.

Improved Electron Mobility Higher than 106 cm2/Vs in Selectively Doped GaAs/N-AlGaAs Heterostructures Grown by MBE

Electron mobility of quasi-two dimensional electron gas (2DEG) in selectively doped GaAs/N-AlxGa1-xAs (x=0.3) heterostructures grown by MBE was investigated as a function of thickness of an undoped AlxGa1-xAs spacer-layer (0–200 A) introduced between a Si-doped AlGaAs layer and an undoped GaAs layer, at 77 K and 5 K. Mobility of 2DEG as high as 2,120,000 cm2/Vs at 5 K was achieved with a spacer-layer thickness of 200 A. This electron mobility is higher than any observed so far in semiconductor materials.

Effect of Thermal Etching on GaAs Substrate in Molecular Beam Epitaxy

Thermal etching of GaAs substrates prior to epitaxial growth by molecular beam epitaxy has been used to reduce carrier depletion at the substrate and epitaxial layer interface. The amount of carrier depletion between a Si-doped n-GaAs substrate and a Si-doped n-GaAs epitaxial film, measured by a C-V carrier profiling technique, was proved to decrease significantly with increased etched depth at a substrate temperature of 750°C. The origin of the carrier depletion is believed to be the carbon acceptor, from the results of C-V measurement and secondary ion mass spectrometory. Based on these results, thermal etching was successfully applied to semi-insulating GaAs substrates to improve mobility and sheet concentrations of two-dimensional electron gas in the selectively doped GaAs/N-Al0.3Ga0.7As heterostructures with very thin GaAs buffer layers (0.2 µm).

Very low threshold current GaAs–AlGaAs GRIN‐SCH lasers grown by MBE for OEIC applications

Very highly efficient GaAs–AlGaAs GRIN–SCH lasers were grown on an n‐GaAs substrate as well as on a semi‐insulating GaAs substrate by MBE. The threshold current density Jth of the lasers was found to be minimum when the thickness of the GaAs quantum well active layer is 6 nm. The lowest Jth of 260 A/cm2 was achieved for the broad‐area Fabry–Perot laser (the Al composition of the cladding layer x=0.7, the cavity length L=400 μm). A ridge‐waveguide (5 μm wide stripe) GaAs–AlGaAs GRIN–SCH laser, which is monolithically integrated with GaAs MESFET’s on a semi‐insulating GaAs substrate, exhibited cw operation with a threshold current as low as 19 mA at room temperature.

Improved 2DEG Mobility in Inverted GaAs/n-AlGaAs Heterostructures Grown by MBE

Electrical properties of an inverted heterojunction interface in selectively Si-doped GaAs/n-AlGaAs heterostructures were investigated. It was found that the origin of reduced 2DEG mobility in an inverted heterostructure is not interface roughness and/or background impurity pileup at the interface, but is diffused profile of doped Si impurities. Larger enhancement of 2DEG mobility at low temperature was obtained even in an inverted heterostructure by introducing a thick undoped AlGaAs spacer layer (Δt=100 A) and lowering doping concentration of Si (Nd=5×1017cm-3); electron mobility as high as 7×104 cm2/Vs was observed at 5K with sheet electron concentration of 1×1012cm-2.

Extremely High Mobility of Two-Dimensional Electron Gas in Selectively Doped GaAs/N-AlGaAs Heterojunction Structures Grown by MBE

Selectively Si-doped GaAs/N-AlGaAs heterojunction structures have been grown by MBE. Two-dimensional electron gas accumulating at the interface of the heterojunction showed mobilities as high as 69,000 cm2/Vs at 77 K and 100,000 cm2/Vs at 4.2 K, with a sheet electron concentration of 5.5×1011 cm-2, which are higher than any reported so far. An enhancementmode high electron mobility transistor (E-HEMT), which was first fabricated from the heterojunction material, showed a field effect mobility of 49,300 cm2/Vs at 77 K, suggesting that this heterojunction material has potential for application to low-power, high-speed integrated circuits.

MBE as a production technology for HEMT LSIs

Abstract Molecular beam epitaxy (MBE) is widely used to produce high quality epitaxial layers for advanced heterostructure devices. High electron mobility transistors (HEMTs) based on selectively-doped AlGaAs/GaAs heterojunction structures are one of the most successful applications of MBE. This paper reviews MBE techniques developed in our laboratories for HEMT LSI fabrication. To meet material requirements for submicron-gate AlGaAs/GaAs HEMT LSIs, we designed a multiwafer MBE system that enables good uniformity and high throughput. Epitaxial growth is done simultaneously on three 3-inch wafers mounted in a 7.5-inch holder. The variations of layer thickness and carrier concentration over the holder area were less than ±1%. One-touch substrate mounting without indium solder was developed for the t.5-inch holder. In-situ monitoring of growth rates by RHEED instensity oscillation with an automated growth system was introduced to control epitaxial parameters precisely. The density of oval defects due to Ga sources was stably controlled to less than 10 cm -2 and carrier depletion at the substrate-epitaxial layer interface was significantly reduced. These excellent material characteristics make it possible to develop high-performance HEMT logic and memory LSI circuits.

Electronic States in Selectively Si-Doped N-AlGaAs/GaAs/N-AlGaAs Single Quantum Well Structures Grown by MBE

Electronic states and transport properties of 2DEG were investigated in selectively Si-doped N-AlGaAs/GaAs/N-AlGaAs single quantum well (SQW) structures grown by MBE. The number of the quantum levels below Fermi energi in the SQW structures varied from one to three with increasing thickness of the GaAs quantum well layer from 60 to 1000 A. The effect of unintentionally asymmetric Si-doping in the SQW structures due to the surface segregation of Si was found to be essential in explaining the observed results.

Dependence of the Mobility and the Concentration of Two-Dimensional Electron Gas in Selectively Doped GaAs/N-AlxGa1-xAs Heterostructure on the AlAs Mole Fraction

The mobility and the concentration of two-dimensional electron gas (2DEG) were measured by Hall measurements (4.2 K, 77 K) and Shubnikov-de Haas measurements (4.2 K) in selectively doped GaAs/N–AlxGa1-xAs (0.1x0.4) heterostructures grown by MBE. The electron conduction in the Si-doped N–AlxGa1-xAs layer was eliminated by careful step etching of this layer. When x was increased from 0.1, the 2DEG mobility considerably increased, and it reached a maximum mobility (243,000 cm2/Vs at 4.2 K; 107,000 cm2/Vs at 77 K) at x=0.25-0.3. However, the sheet electron concentration of 2DEG was not sensitive to x, and showed broad peak (6×1011 cm-2) at around x=0.25.

Room-Temperature Mobility of Two-Dimensional Electron Gas in Selectively Doped GaAs/ N -AlGaAs Heterojunction Structures

Room-temperature mobility of two-dimensional electron gas accumulating at a heterojunction interface in selectively doped GaAs/N-AlGaAs grown by MBE was shown to be potentially as high as 8,600 cm2/Vs with a sheet electron concentration of 5.5×1011 cm-2. This is almost twice as high as the mobility in conventional GaAs FETs with typical carrier concentrations.