Junji Saito

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.

Electrical Properties of Si-Doped AlxGa1-xAs Layers Grown by MBE

Electrical properties of Si-doped AlxGa1-xAs layers grown by MBE were investigated as a function of AlAs mole fraction (0x0.5). Although ionization energy of Si donor remained constant at a few meV in the range of 0x0.25, it rapidly became larger with increasing x for x0.25. It was found to be 60 meV for x=0.3, which is the composition used in HEMT. The overall trend was similar to the case of Sn- or Te-doped AlxGa1-xAs layers. However, the ionization energy was found to be independent of doping concentration, unlike the case of Sn-doped AlxGa1-xAs layers grown by MBE.

Effects of etching with a mixture of HCl gas and H2 on the GaAs surface cleaning in molecular‐beam epitaxy

In situ cleaning of GaAs substrates with a HCl gas and hydrogen mixture prior to molecular‐beam epitaxy has been investigated. The chemical reaction during etching was monitored using a quadrupole mass spectrometer. After etching, reflection high‐energy electron diffraction patterns revealed (2×4) arsenic‐stabilized surfaces and (4×2) gallium‐stabilized surfaces as reconstructed structures in the gas‐etched substrate surface. These structures suggest that the gas‐etched substrate surface is atomically flat, resembling an epitaxial layer surface. To study the effect of gas etching, the carrier depletion layer and the residual carbon impurity around the substrate epitaxial interface were measured by capacitance‐voltage carrier profiling and secondary‐ion mass spectroscopy. After gas etching, the carrier depletion was greatly reduced, from 1.2×1012 to 1×1010 cm−2. The carbon impurity around the interface also decreased by one order of magnitude. We discussed the surface‐cleaning mechanism using the atomic hy...

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).

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.

The Effect of Annealing on the Electrical Properties of Selectively Doped GaAs/N-AlGaAs Heterojunction Structures Grown by MBE

The effect of annealing on the electrical properties of selectively doped GaAs/N-AlGaAs heterojunction structures grown by MBE has been studied. Very high electron mobility in this structure at 77 K decreased considerably after annealing at about 700°C. However, it remained at a high value even after annealing when the Si doping concentration in N-AlGaAs was reduced to about 1×1018 cm-3. These results are discussed in terms of diffusion of Si impurities doped in N-AlGaAs.

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.

Reduction of backgating effect in MBE-Grown GaAs/AlGaAs HEMT's

We have investigated the backgating effect in high electron mobility transistors (HEMTs) fabricated on MBE-grown GaAs/AlGaAs layers, which is undesirable for LSI fabrication. Comparing five different types of devices, we related the backgating effect to the interface between the GaAs substrate and the undoped GaAs buffer layer. By using a thermally etched GaAs substrate, we successfully reduced the backgating to the same order as that of ion-implanted GaAs MESFETs.

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.