K. H. Chang

Two‐color infrared photodetector using GaAs/AlGaAs and strained InGaAs/AlGaAs multiquantum wells

A two‐color infrared detector using GaAs/AlGaAs and strained InGaAs/AlGaAs multiquantum wells is demonstrated. The response peak of the GaAs/AlGaAs quantum well is at 8 μm, and that of the InGaAs/AlGaAs quantum well is at 5.3 μm. The responsivity of the detector is 1 A/W at 8 μm and 0.27 A/W at 5.3 μm; these are the best values reported for a two‐color quantum well infrared detectors (QWIPs) with peak sensitivities in the spectral regions of 3–5.3 μm and 7.5–14 μm. Single‐colored 5.3 and 8 μm QWIPs were also fabricated to study the bias dependent behavior. This behavior can be explained using the concept of current continuity. Because of a higher electrical resistance, a high electric field domain is always formed first in the shorter wavelength quantum well stack.

Quantum well infrared photodetectors with bi‐periodic grating couplers

Multiple quantum well infrared photodetectors (QWIPs) with bi‐periodic grating couplers have been demonstrated. The bi‐periodic grating consists of two gratings with different periods. The response linewidths of such QWIPs are found to be much wider than those of conventional grating coupled QWIPs. By using bi‐periodic gratings for coupling the radiation into QWIPs, the influence of variation of material and grating quality on the performance of QWIP can be largely eliminated.

Precise determination of aluminum content in AlGaAs

The Al composition of AlGaAs has been determined by four methods: high‐resolution transmission electron microscopy (HRTEM), reflection high‐energy electron diffraction (RHEED), photoluminescence (PL), and double‐crystal x‐ray diffraction (DCXRD). HRTEM is direct and the most accurate method because it does not involve any formula or extrapolation. Using the result obtained from HRTEM as a standard, we have calibrated the results from other methods. RHEED intensity oscillation is found to be accurate and reliable, if the growth conditions are correctly chosen. Comparing the PL results with those determined from HRTEM and RHEED, we suggest three formulas to determine the Al contents at different temperatures. We also proposed a polynomial to determine the Al concentration using the DCXRD measurement.

Photoreflection study on the surface electric field of delta‐doped GaAs grown by molecular beam epitaxy

Photoreflectance spectroscopy has been used to study the surface electric‐field strength and the surface potential of delta‐doped GaAs. Franz–Keldysh oscillations in the reflectance spectra were clearly observed and the oscillation periods were used to calculate the internal electric fields of the delta‐doped samples. Based on the measured results and the self‐consistent calculation, a surface potential of 0.6 eV is obtained.

Direct observation of Si delta‐doped GaAs by transmission electron microscopy

Direct observation of the Si delta‐doped layer in GaAs has been achieved by high resolution transmission electron microscopy. Samples with different Si doses, from half a monolayer to two monolayers, were studied. The observed spreading of the delta‐doped layer showed that Si atoms are largely confined in five monolayers at most (in the highest dose case), indicating excellent confinements of dopants in GaAs. From the images, the Si atoms were uniformly distributed in the doped layer, no cluster formation was observed. For delta‐doped GaAs grown at low temperature (480 °C), stacking faults originated from the doped layers were observed. These faults were thought to be caused by the large unrelaxed strain in the low‐temperature grown GaAs.

A novel PHL-emitter bipolar transistor—Fabrication and characterization

Abstract The polysilicon high-low-emitter (PHL-emitter) bipolar transistors have been fabricated and characterized. It is shown that the fabricated PHL-emitter bipolar transistors exhibit higher maximum current gain with lower activation energy, higher emitter-base breakdown, and better hot-carrier reliability than those of the fabricated conventional bipolar transistor. However, a little degradation in cutoff frequency for the fabricated PHL-emitter bipolar transistors due to the presence of the high-low emitter junction has also been observed. The effects of the n − implant dose on the electrical characteristics of the fabricated PHL-emitter bipolar transistors have been examined and discussed.

TRANSMISSION ELECTRON-MICROSCOPY STUDY OF HEAVILY DELTA-DOPED GAAS GROWN BY MOLECULAR-BEAM EPITAXY

N‐type and p‐type delta‐doped GaAs grown by molecular beam epitaxy with rather significantly high doses of Si and Be have been investigated by transmission electron microscopy (TEM). The amount of doses ranged from half a monolayer to two monolayers. The microscopic structures of the delta‐doped regions and the adjacent epilayers were directly observed by TEM. The effect of impurity spreading on the heterointerfaces and superlattices was also studied. Si atoms present in Si delta‐doped samples were confined to within a few atomic layers. The Be atoms present in Be delta‐doped samples, however, spread over a quite wide region and caused rough heterointerfaces and wavy superlattices to form. Spreading of Be was attributed to segregation and diffusion which occurred during growth. Stacking faults were found in the delta‐doped samples when they were grown at low temperatures. They could be attributed to local strain caused by heavy doping.

Improved AlGaAs/GaAs double-barrier resonant tunneling structures using two-dimensional source electrons

We report the enhancement of peak‐to‐valley current ratios (PVCRs) of double‐barrier resonant tunneling structures (DBRTSs) based on the AlGaAs/GaAs material system. The PVCRs as high as 25.4 and 18 have been obtained at 77 K for superlattice and alloy barrier structures with 0.2‐μm undoped electrodes, respectively. These are the largest PVCRs to date for AlGaAs/GaAs DBRTSs. The large band bending across the undoped electrodes causes size quantization of the accumulation layer, resulting in better resonant tunneling characteristics.

A novel technique for low‐threshold and high‐power InGaAs/GaAs strained‐layer 0.98‐μm buried heterostructure laser fabrication

A novel fabrication technique has been developed for InGaAs/GaAs strained‐layer buried heterostructure lasers. Dielectric masks and Zn diffusion are not required in this technique. This novel fabrication process is much easier than the conventional approach and yields excellent laser results. A low threshold of 3 mA and high‐power operation for lasing wavelength of 9800±20 A have been achieved with graded index separate confinement heterostructure devices using this novel technique.

Electrical characteristics of double-barrier resonant tunneling structures with different electrode doping concentrations

Abstract The electrical characteristics of AlGaAs/GaAs double-barrier resonant tunneling structures with various electrode doping concentrations have been studied. It was found that the peak current and the valley current are not sensitive to the change in the doping level of the electrodes. The peak-current voltage and the valley-current voltage, however, increase when the electrode doping level is lowered. These behaviors are explained using the band-bending effect in the electrodes. For devices with lightly doped or undoped electrodes, the quantum size effect in the accumulation layer due to the strong band bending causes additional kinks in the current-voltage characteristics. They are attributed to the resonant tunneling of electrons from the quantized levels in the accumulation layer. The quantum effect also accounts for the improved negative differential resistance behavior.