H. C. Lee

Minority-carrier lifetimes in undoped AlGaAs/GaAs multiple quantum wells

The lifetime of minority carriers in AlGaAs‐GaAs quantum well structures grown by metalorganic chemical vapor deposition has been studied at room temperature and carrier densities up to 3×1016 cm−3 using time‐resolved photoluminescence. Decay times are observed to be strongly dependent upon the quality of the AlGaAs barrier regions, excitation, and well width. Strong saturation of the decay times is observed with excitation indicating the presence of nonradiative traps. Saturated lifetimes as long as 650 ns have been measured in quantum wells with 200 A well widths.

Band‐edge absorption coefficients from photoluminescence in semiconductor multiple quantum wells

We describe a novel approach to determining absorption coefficients in thin films using luminescence. The technique avoids many of the difficulties typically encountered in measurements of thin samples, Fabry–Perot effects, for example, and can be applied to a variety of materials. We examine the absorption edge for GaAs/AlGaAs multiple quantum well structures with quantum well widths ranging from 54 to 193 A. Urbach parameters and excitonic linewidths are tabulated.

Single-pulse pump-probe measurement of optical nonlinear properties in GaAs/AlGaAs multiple quantum wells

Single-pulse ps-pump and ns-probe nonlinear transmission measurements provide carrier-density-dependent optical nonlinear spectra in GaAs/AlGaAs multiple quantum wells grown by metalorganic chemical vapor deposition. The use of the ps pump eliminates the need to know carrier lifetime to determine carrier density. The saturation behavior of changes in absorption coefficient and refractive index are modeled by a simple saturation equation to obtain saturation carrier density. The saturation spectra for different well thicknesses are obtained. The minimum saturation carrier density appears around 150 /spl Aring/. >

Nonlinear absorption in AlGaAs/GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition

We report the study of growth conditions for achieving the sharp exciton resonances and low intensity saturation of these resonances in AlGaAs‐GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition. Low growth temperature is necessary to observe this sharp resonance feature at room temperature. The optimal growth conditions are a tradeoff between the high temperatures required for high quality AlGaAs and low temperatures required for high‐purity GaAs. A strong optical saturation of the excitonic absorption has been observed. A saturation density as low as 250 W/cm2 is reported.

Carrier Lifetimes In A Hetero N-I-P-I Structure.

We discuss new studies of a multiple quantum well hetero n-i-p-i structure which combines the advantages of multiple quantum wells and doping superlattices. The structure exhibits large changes in its absorption coefficient for intensities of only a few mW/cm2. We have used the spectral dependence of the nonlinear absorption coefficient to deduce photogenerated carrier lifetimes as a function of intensity.

Impact of amplified spontaneous emission on carrier density for measurement of optical nonlinearities in GaAs/AlGaAs multiple quantum wells

It is shown that amplified spontaneous emission (ASE) can affect the measurement of nonlinear optical properties in semiconductors through its reduction of the excess carrier density. When the optical excitation area is large, lateral ASE can reduce the carriers within a much shorter period of time than the normal carrier lifetime. Just after ASE is over, the time-integrated surface photoluminescence signal may be used as a measure of the carrier density, which is the carrier density experienced by an ns probe in ps pump experiments. When the excited carrier density is at 10/sup 20/ cm/sup -3/, surface ASE is also possible and is observed, even with small spot sizes. >

Nonlinear Measurements In Multiple Quantum Wells Of Gaas/Algaas Fabricated By Mocvd

We discuss the saturation intensities and saturation densities as a function of well width for the room temperature excitonic absorption resonance of GaAs/A1GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition. The optimum growth temperature is considered. We discuss techniques for calculating absorption coefficients that remove Fabry-Perot effects as well as the effects of carrier diffusion. The difference between pulsed and CW excitation of the mutliple quantum wells is addressed. We discuss the calculation of the nonlinear index of refraction from a discrete set of absorption data. We introduce a new structure (a hetero n-i-p-i) which combines the advantages of multiple quantum wells and doping superlattices.

Electroabsorption in AlGaAs/GaAs multiple quantum well structures grown on a GaP transparent substrate

The first measurements of electroabsorption in AlGaAs/GaAs multiple quantum wells grown on transparent GaP substrates are reported. High quality quantum well materials with sharp, well defined excitonic resonances are grown by employing a GaAsP intermediate layer between the quantum wells and the substrate. Good surface morphology and abrupt interfaces have been achieved with etch pit densities of 4×105 cm−2. A 7.5‐meV shift of the absorption edge to lower energies is observed for field strengths of 8×104 V/cm. These structures are well suited for photonic switch array fabrication.

Room Temperature Excitonic Nonlinear Absorption in GaAs/AlGaAs Multiple Quantum Well Structures Grown by Metalorganic Chemical Vapor Deposition (MOCVD)

Room temperature excitonic nonlinear absorption in multiple quantum wells (MQW) has great potential for making low-power high-speed optical switching devices. The required ultrathin multilayers have been grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). Efficient, narrow-linewidth luminescence and low-threshold laser operation have been achieved in MQW’s with both techniques. However, virtually all of the reported work to date dealing with nonlinear optical properties has employed MBE-grown materials CHEMLA /1/, CHEMLA /2/, MILLER /3/. The implementation of arrays of photonic switches in optical computing and processing systems will require large area devices with uniformity that exceeds the capability of current technology. The potential advantages of MOCVD for large area, multiple wafer growth of these complex structures provides motivation to explore this technique for their fabrication. In this paper, the growth by MOCVD of GaAs/AlGaAs MQW structures that exhibit narrow-line, room temperature excitonic absorption are reported. We further describe the linear and nonlinear optical properties of these structures and discuss the implications of this work for the construction of large area arrays of switches.