K.L. Tsai

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.

Compositional disordering of InGaAs/GaAs heterostructures by low-temperature-grown GaAs layers

Compositional disordering of InGaAs/GaAs superlattices using a low‐temperature‐grown GaAs cap layer (LT‐GaAs) by molecular beam epitaxy has been studied. The disordering of the superlattice was verified by photoluminescence and double‐crystal x‐ray rocking curve measurements. The Ga‐vacancy‐enhanced interdiffusion due to the presence of LT‐GaAs was found to be the disordering mechanism. Diffusion equations and the Schrodinger’s equation were solved numerically to obtain the composition profile and the transition energies in the disordered quantum well, respectively. The simulated energy shifts for samples under different annealing conditions agreed very well with the experimental results. The calculated effective diffusivity for the In–Ga interdiffusion has an activation energy of 1.63 eV, which is smaller than the activation energy 1.93 eV, for intrinsic interdiffusion. The diffusivity for the enhanced In–Ga interdiffusion due to the presence of LT‐GaAs is about two orders of magnitude larger than the in...

Kinetics of compositional disordering of AlGaAs/GaAs quantum wells induced by low‐temperature grown GaAs

Compositional disordering of GaAs/AlGaAs quantum wells due to the presence of low‐temperature grown GaAs (by molecular beam epitaxy) was studied. Ga vacancy enhanced interdiffusion was found to be the mechanism underlying the observed intermixing. Diffusion equations were solved numerically to obtain the band profile after intermixing. The transition energies in the quantum wells under various annealing conditions were solved and agree very well with the observed photoluminescence emission peaks. The diffusivity of Ga vacancies and that of induced Al‐Ga interdiffusion were obtained. The vacancy induced interdiffusion diffusivity was found to have an activation energy of 4.08 eV, which is smaller than the activation energy of interdiffusion diffusivity of normal temperature grown GaAs/AlGaAs heterostructures. This is a clear indication of enhanced interdiffusion due to the presence of low‐temperature grown GaAs.

High current gain, low offset voltage heterostructure emitter bipolar transistors

Heterostructure Emitter Bipolar Transistors (HEBTs) were fabricated with improved characteristics by employing the emitter edge-thinning technique. Optimum design for the n-GaAs emitter and the ledge thickness were used to remove the potential spike and to prevent ledge conduction simultaneously. Current gains as high as 720, which is the highest current gain ever reported for HEBTs, and offset voltages down to 70 mV were obtained. Very large improvement in current gain, especially at low collector current, was obtained by using the emitter edge-thinning technique.<<ETX>>

The study of emitter thickness effect on the heterostructure emitter bipolar transistors

AlGaAs/GaAs heterostructure emitter bipolar transistors were grown with emitter thicknesses varied from 300 to 900 A and the emitter thickness effects on the current gain and offset voltage were studied. It was found that both the current gain and offset voltage are strongly dependent on the emitter thickness. The current gain decreases with increasing emitter thickness. For an emitter thickness smaller than 500 A, the offset voltage decreases with increasing emitter thickness, but for an emitter thickness larger than 500 A, the offset voltage stays at a nearly constant value. Offset voltage as low as 55 mV was obtained for an emitter thickness of 700 A. This low offset voltage, mostly contributed by the geometric effect, indicates that the base‐emitter junction is a homojunction. From the current gain and offset voltage considerations, the optimal emitter thickness was found to be about 300–500 A.

Heterojunction bipolar transistors with emitter barrier lowered by /spl delta/-doping

An npn heterojunction bipolar transistor with a Si-/spl delta/-doped layer at the emitter-base hetero-interface is demonstrated. The Si /spl delta/-doped layer reduces the potential spike at the emitter junction. This design reserves the abruptness of the heterojunction, reduces electron barrier and increases the hole barrier simultaneously. Experimental results show that the HBTs turn-on characteristics are greatly improved while the current gain remains high. The offset voltages as low as 44 mV have been obtained. Very high current gains at very low collector current densities are obtained.<<ETX>>

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.

The effect of barrier structure on the performance of double barrier quantum well infra-red photodetectors

Abstract The effects of the inner and outer barriers on the performance of mid-wavelength (3–5 μm) GaAs AlGaAs double barrier quantum well infra-red photodetectors (DBQWIPs) have been studied. It was found that the peak response wavelength had a significant red shift when the thickness of the AlAs inner barriers was below 10 A. This is attributed to the lowering of the upper state energy in the quantum wells. The responsivity of DBQWIPs decreases with the inner barrier thickness. This is because the optical gain decreases faster than the increasing rate of the quantum efficiency when the inner barrier thickness increases. The responsivities for devices with different Al compositions (0.35 and 0.24) in the outer barriers are in the same range, but the dark currents for devices with Al0.35Ga0.65As outer barriers are much lower than those with Al0.24Ga0.76As outer barriers. Nearly two orders of magnitude improvement in detectivity was observed when the Al composition in the outer barriers was changed from 0.24 to 0.35. A high detectivity of 1 × 1011 cm Hz1/2 W−1 at 77K was obtained for a device with 12 A AlAs inner barriers and 300 A Al0.35Ga0.65As outer barriers.

Influence of oxygen on the performance of GaAs/AlGaAs quantum well infrared photodetectors

The influence of oxygen on the performance of GaAs/AlGaAs quantum well infrared photodetectors (QWIP) has been studied. Photoluminescence and secondary ion mass spectroscopy were used to examine the relationship between the quality of AlGaAs and the oxygen content that were then correlated to the performance of QWIPs. It was found that oxygen is the dominant impurity in the GaAs/AlGaAs superlattice. Because oxygen atoms behave likes electron traps, which effectively reduce the carrier concentration in the material, both of the responsivity and the dark current of QWIP are reduced. The detectivities of QWIPs with lower oxygen concentrations are better than those with higher oxygen concentrations.

ASYMMETRIC DARK CURRENT IN QUANTUM WELL INFRARED PHOTODETECTORS

Asymmetric dark current versus voltage characteristics in quantum well infrared photodetectors have been studied. A model based on asymmetrical potential barriers was proposed. The asymmetrical potential barriers, which are most likely due to the accumulation of oxygen impurities at one of the interfaces, cause the asymmetrical I‐V characteristics. The height of the potential spike is found to increase with the Al content in the AlGaAs barriers. Calculations based on our model agree well with experimental results.