Jia-Lin Shieh

Material properties of compositional graded InxGa1−xAs and InxAl1−xAs epilayers grown on GaAs substrates

The residual strain, crystallographic tilt, and surface topography of InxGa1−xAs and InxAl1−xAs (0<x<0.3) epilayers grown on GaAs substrates are investigated. The residual strain of the InxAl1−xAs grown on graded InyAl1−yAs is shown to be strongly dependent on the thickness of the underlying‐graded buffer layers and is larger than that of the InGaAs of the same structure. The crystallographic tilt of the InGaAs epilayers with respect to GaAs substrate is found to be strongly dependent on the growth temperature as well as the layer structure of the underlying buffer layer, while that of InAlAs is insensitive to these two factors. This behavior is attributed to the different roughness of the growth front between these two material systems and is consistent with the observation by atomic force microscopy.

Formation of self-organized In0.5Ga0.5As quantum dots on GaAs by molecular beam epitaxy

Abstract Self-organized In 0.5 Ga 0.5 As quantum dots have been successfully grown on vicinal GaAs substrates by molecular beam epitaxy. The density of the quantum dots can be changed by nucleating the dots under different As overpressure. Substrate tilt angle of 15° results in much larger dot size and density than that of 4° due to the closely spaced step edges on the surface. Through investigations of the dots grown on In 0.1 Ga 0.9 As buffer, the strain energy of the buffer layer is also found to be an important factor that affects the size and density of the quantum dots.

Overall performance improvement in GaAs MSM photodetectors by using recessed-cathode structure

We report the experimental work on the GaAs metal-semiconductor-metal photodetectors with recessed cathode and/or anode. The recessed-cathode detectors exhibit much superior dc and speed performance to the conventional one because of the enhancement of hole transport in this structure. Full-width at half-maximum and fall time of the temporal response were measured to be 21 and 13 ps at 5 V bias on a 50/spl times/50 /spl mu/m/sup 2/ detector with a finger width and spacing of 3 /spl mu/m.

HIGH-SPEED GAAS METAL-SEMICONDUCTOR-METAL PHOTODETECTORS WITH RECESSED METAL ELECTRODES

The GaAs metal‐semiconductor‐metal photodetector (MSM‐PD) with recessed metal electrodes have been fabricated and characterized. The recessed structure allows very short photocarrier sweep‐out time because of both a strengthened electric field in the active region and a shortened distance for the photocarriers to reach the electrodes. Improved high‐speed performance and enhanced peak amplitude in the temporal response can thus be obtained simultaneously. There is about a 60% and 50% improvement in the fall time and peak amplitude of the temporal response at 5 V bias over the conventional device, respectively. The measured results also show that high‐speed operation can be achieved at a lower bias voltage for the GaAs MSM‐PD with recessed metal electrodes as compared to the conventional one. Two‐dimensional simulation was carried out to give an insight into the operation principle of this device.

D.c. and microwave characteristics of In0.32Al0.68As/In0.33Ga0.67As heterojunction bipolar transistors grown on GaAs

Abstract The d.c. and microwave characteristics of graded and abrupt junction In 0.32 Al 0.68 As/In 0.33 Ga 0.67 As heterojunction bipolar transistors (HBTs) grown on GaAs were investigated. A step-graded In x Ga 1− x As buffer was employed to effectively suppress the threading dislocations resulting from the lattice mismatch between In 0.33 Ga 0.67 As and GaAs. These devices exhibited a small turn-on voltage of collector current and a high collector–emitter breakdown voltage (BV CEO >9.5 V) for a 0.35 μm-thick collector, demonstrating excellent quality of the base–emitter and base–collector junctions. Less size-dependence on current gain was observed for these metamorphic HBTs even without the emitter ledge. The peak common-emitter current gain at a collector current density of 40 kA/cm 2 is 53 for the graded junction device with an emitter size of 2×4 μm 2 and a base doping of 2×10 19 cm −3 . An F max of 56 GHz was measured for this device.

Characteristics of pseudomorphic AlGaAs/InxGa1−xAs (0≤x≤0.25) doped‐channel field‐effect transistors

The pseudomorphic properties of doped‐channel field‐effect transistors have been thoroughly investigated based on AlGaAs/InxGa1−xAs (0≤x≤0.25) heterostructures with different In contents. Through various analytical schemes and device characterization, we observed that by introducing a 150 A strained In0.15Ga0.85As channel we can enhance device performance; however, this strained channel is not stable after high‐temperature heat treatment. By further increasing the In content up to x=0.25, the device performance started to degrade, which is associated with strain relaxation in this highly strained channel.

Defects in metamorphic InxAl1−xAs (x<0.4) epilayers grown on GaAs substrates

Defects in Si-doped InxAl1−xAs (0<x<0.4) epilayers on GaAs substrates were systematically investigated by deep-level transient spectroscopy. Three electron traps, ranging from 0.22 to 0.89 eV, were observed in InxAl1−xAs grown by molecular beam epitaxy. Their energy levels can be extrapolated from those in the InxAl1−xAs/InP system. A low-temperature-grown buffer layer as well as thermal annealing were found to be capable of reducing the defects in the active layer effectively.

Reduction of hole transit time in GaAs MSM photodetectors by p-type /spl delta/-doping

The temporal responses of the undoped, n-type, and p-type /spl delta/-doped GaAs metal-semiconductor-metal photodetectors were systematically studied. The full-width at half-maximum of the temporal response is significantly improved at low bias for the /spl delta/-doped detectors compared to the conventional undoped one. The p-type /spl delta/-doped detector exhibits the smallest fall-time, and hence the highest bandwidth, because the induced electric field in the absorption region facilitates the transport of the photo-generated holes. In addition, the p-type /spl delta/-doped detector also gives the highest peak amplitude of the temporal response. Two dimensional simulation on the internal electrical field distribution in the detectors is consistent with the experimental results.

A comparative study of the passivation films on AlGaAs/GaAs heterojunction diodes and bipolar transistors

A series of Si-based thin films, including amorphous Si, SiC, as well as the conventional SiO/sub x/ and SiN/sub x/, was investigated in terms of the electrical characteristics of GaAs/Al/sub 0.3/Ga/sub 0.7/As heterostructure diodes and heterojunction bipolar transistors (HBTs). All the films were found effective in reducing the leakage current and long term degradation. Less size-dependence of the current gain was found for the HBTs passivated by amorphous Si and SiC. In addition, the devices passivated by amorphous Si and SiC films exhibited better performance during high power operation. This is attributed to the high thermal conductivity of these two materials.

Characteristics of a In/sub 0.52/(Al/sub x/Ga/sub 1-x/)/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As(0/spl les/x/spl les/1) heterojunction and its application on HEMT's

The quaternary In/sub 0.52/(Al/sub x/Ga/sub 1-x/)/sub 0.48/As compound on InP substrates is an important material for use in optoelectronic and microwave devices. We systematically investigated the electrical properties of quaternary In/sub 0.52/(Al/sub x/Ga/sub 1-x/)/sub 0.48/As layers, and found a 10% addition of Ga atoms into the InAlAs layer improves the Schottky diode performance. The energy bandgap (E/sub g/) for the In/sub 0.52/(Al/sub x/Ga/sub 1-x/)/sub 0.48/As layer was (0.806+0.711x) eV, and the associated conduction-band discontinuity (/spl Delta/E/sub c/), in the InAlGaAs/In/sub 0.53/Ga/sub 0.47/As heterojunction, was around (0.68/spl plusmn/0.01)/spl Delta/E/sub g/. Using this high quality In/sub 0.52/(Al/sub 0.9/Ga/sub 0.1/)/sub 0.48/As layer in the Schottky and buffer layers, we obtained quaternary In/sub 0.52/(Al/sub 0.9/Ga/sub 0.1/)/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As HEMTs. This quaternary HEMT revealed excellent dc and microwave characteristics. In comparison with the conventional InAlAs/InGaAs HEMTs, quaternary HEMTs demonstrated improved sidegating and device reliability.