Aidong Shen

(Ga,Mn)As: A new diluted magnetic semiconductor based on GaAs

A new GaAs‐based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy. The lattice constant of (Ga,Mn)As films was determined by x‐ray diffraction and shown to increase with the increase of Mn composition, x. Well‐aligned in‐plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements revealed the occurrence of anomalous Hall effect in the (Ga,Mn)As layer.

Epitaxy of (Ga, Mn)As, a new diluted magnetic semiconductor based on GaAs

Abstract GaAs-based diluted magnetic semiconductor, (Ga, Mn)As, with Mn composition x up to 0.07 was prepared by molecular-beam epitaxy on GaAs substrate at temperatures ranging from 160 to 320°C. Clear reflection high-energy electron diffraction oscillations were observed at the initial growth stage, indicating that the growth mode is two-dimensional. The lattice constant of (Ga, Mn)As films determined by X-ray diffraction showed a linear increase with the increase of Mn composition. Well-aligned in-plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements also revealed the presence of ferromagnetic order in the (Ga, Mn)As layer. The easy axis of magnetization can be reversed by changing the strain direction in (Ga, Mn)As. GaAs (Ga, Mn)As superlattice structures with high crystal perfection and good interface quality were also prepared.

Spontaneous splitting of ferromagnetic (Ga, Mn)As valence band observed by resonant tunneling spectroscopy

Current–voltage characteristics of AlAs/GaAs/AlAs double barrier resonant tunneling diodes with ferromagnetic p-type (Ga, Mn)As on one side and p-type GaAs on the other have been studied. A series of resonant peaks have been observed in both polarities, i.e., injecting holes from p-type GaAs and from (Ga, Mn)As. When holes are injected from the (Ga, Mn)As side, spontaneous resonant peak splitting has been observed below the ferromagnetic transition temperature of (Ga, Mn)As without magnetic field. The temperature dependence of the splitting is explained by the the spontaneous spin splitting in the valence band of ferromagnetic (Ga, Mn)As.

High absorption (>90%) quantum-well infrared photodetectors

The intrinsic short carrier lifetime (∼5 ps) makes the quantum-well infrared photodetector (QWIP) well suited for high speed and high frequency applications. In such cases, since lasers are commonly used, a high dark current can be tolerated. The most important parameter is then the absorption efficiency. For system simplicity and potential wide use, room temperature operation is desirable. Using GaAs/AlGaAs QWIPs, high absorption (∼100%) and up to room temperature operation are achieved in devices having high doping densities and 100 quantum wells.

INAS SELF-ORGANIZED QUANTUM DASHES GROWN ON GAAS (211)B

We have grown InAs self-organized quantum dots and quantum dashes on GaAs (211)B substrates by molecular beam epitaxy. The growth temperature dependence of InAs nanostructures were studied by in situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy. In the studied temperature range from 400 to 510 °C, the RHEED pattern changed from streaky to spotty after deposition of 6 ML of InAs, showing the formation of nanostructures. The quantum dots grown at lower growth temperatures (from 400 to 490 °C) showed bimodal dot size distribution. At higher growth temperatures, a drastic change from quantum dots to quantum dashes was observed. The quantum dashes have an asymmetric hutlike shape and align themselves along the [011] direction. The quantum dash width increases dramatically, whereas the average length and density increases slightly on further deposition of InAs.

GaAs/AlGaAs quantum-well photodetector for visible and middle infrared dual-band detection

We present experimental results on quantum-well photodetectors for visible and infrared dual-band detection. Large band gap top contacts were used on a standard GaAs/AlGaAs quantum-well infrared photodetector so that visible light could reach the quantum-well region and be absorbed via interband transitions. Two designs were investigated, using a high Al fraction AlGaAs and a short period GaAs/AlAs superlattice contact layer. The dual-band response spectral regions are 0.55–0.7 and 7–10 μm. Measured responsivities are about 0.7 A/W at 8.3 μm and 0.1 A/W at 0.63 μm under −6 V bias voltage.

(Ga, Mn)As/GaAs Diluted Magnetic Semiconductor Superlattice Structures Prepared by Molecular Beam Epitaxy

A (Ga, Mn)As/GaAs superlattice, a semiconductor-based ferromagnetic/non-magnetic multilayer system, was prepared by low-temperature molecular beam epitaxy. X-ray diffraction measurements showed that the superlattice structure has high crystal perfection and good interface quality. Magnetotransport measurements revealed the presence of ferromagnetic order in the multilayer system at low temperatures.

Interlayer exchange in (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As semiconducting ferromagnet/nonmagnet/ferromagnet trilayer structures

Magnetic properties of all-semiconductor (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As trilayer structures are studied. The interactions between the two ferromagnetic (Ga,Mn)As layers are investigated by magnetotransport measurements in a number of samples with different GaAs thickness or with different Al content in the intermediary nonmagnetic (Al,Ga)As layer. The results indicate that carriers present in the nonmagnetic layer mediate the coupling between the two ferromagnetic layers.

Reflection high-energy electron diffraction oscillations during growth of GaAs at low temperatures under high As overpressure

Reflection high-energy electron diffraction oscillations were observed during molecular beam epitaxy of GaAs at temperatures as low as 150 °C under various V/III beam equivalent pressure ratios using As4 as an arsenic source. At the As/Ga beam equivalent pressure ratio of 40, the amplitude of the oscillations was shown to first decrease with the decrease of substrate temperature and then increase when the temperature was further lowered to below 300 °C. The oscillation characteristics at 200 °C were comparable to those recorded at around 600 °C. At fixed temperatures in the low temperature region (<350 °C), the oscillations were shown to be greatly affected by the V/III ratio, with the maximum amplitude at a certain V/III ratio which depends on the substrate temperature used. The strong oscillations at low temperatures and at high V/III ratios were tentatively explained by assuming that the migration of Ga atoms is enhanced on the surface passivated by excess As.

Heterojunction wavelength-tailorable far-infrared photodetectors with response out to 70 μm

Results are presented on the performance of a heterojunction interfacial workfunction internal photoemission (HEIWIP) wavelength-tailorable detector. The detection mechanism is based on free-carrier absorption in the heavily doped emitter regions and internal emission across a workfunction barrier caused by the band gap offset at the heterojunction. The HEIWIP detectors have the high responsivity of free-carrier absorption detectors and the low dark current of quantum well infrared photodector type detectors. For a 70±2 cutoff wavelength detector, a responsivity of 11 A/W and a D*=1×1013 cmHz/W with a photocurrent efficiency of 24% was observed at 20 μm. From the 300 K background photocurrent, the background limited performance (BLIP) temperature for this HEIWIP detector was estimated to be 15 K. This HEIWIP detector provides an exciting approach to far-infrared detection.