W. Shi

Effects of arsenic beam equivalent pressure on InGaAsP grown by solid source molecular beam epitaxy with continuous white phosphorous production

Abstract We report the effects of arsenic beam equivalent pressure on lattice mismatch, electrical properties, surface roughness and morphology of InGaAsP grown by solid source molecular beam epitaxy using valve arsenic and phosphorous cracker cells with continuous white phosphorous production. Arsenic is found to have a higher sticking coefficient than phosphorous in almost all arsenic pressure employed in the growth. The incorporation of arsenic is found to fit a polynomial expression, Y =1.56 R −0.59 R 2 , with the beam equivalent pressure ratio R = f As /( f As + f P ). The incorporated arsenic elements significantly affect lattice mismatch and electrical properties. They also dominate surface construction of the quaternary material.

Physical properties of InGaAsP/InP grown by molecular beam epitaxy with valve phosphorous cracker cell

InGaAsP films grown on InP substrate by solid source molecular beam epitaxy (SSMBE) using a valve phosphorous cracker cell are investigated. It is found that the films grown at flux ratios f As /(f As + f p ) from 0.45 to 0.50 show a superior quality. It is also found that As pressure plays a crucial role in the scattering process; for the films grown at higher arsenic beam pressure (BEP), the Hall mobility μ is dominated by impurity scattering, polar phonon scattering and alloy scattering. For the films with high quality, optical scattering and alloy scattering dominate the mobility. The exponent of T for the films grown at low BEP is found to be as high as 2.54, which cannot be explained by impurity scattering alone. It is believed that, in addition to the impurity-related scattering, some defects associated with As vacancies also significantly contribute to the scattering, especially at low temperatures.

Effect of Be doping on the absorption of InGaAs/AlGaAs strained quantum-well infrared photodetectors grown by molecular-beam epitaxy

We report on the effect of Be doping in the well layers on the absorption of the p-type strained InGaAs/AlGaAs quantum-well infrared photodetectors. It is found that the absorption spectrum originated from the bound-to-bound intersubband transition shifts towards the low-wavelength side as the doping density is increased, due to the band gap shrinkage and widened well width. The full width at half maximum of the absorption spectrum increases with doping density due mainly to the increased roughness at the well–barrier interfaces. The observed results are in good agreement with the estimated values after taking the compressive strain, band gap shrinkage of the well layers, and the increased well width into account.

DOPING EFFECTS ON P-TYPE INGAAS/ALGAAS QUANTUM WELL STRUCTURES FOR INFRARED PHOTODETECTORS GROWN BY MOLECULAR BEAM EPITAXY

We report the effects of Be concentration incorporated into the well material on the p-type InGaAs/AlGaAs multiple quantum well structures. The increased Be doping is found to cause red shift of the dopant-related and excitonic luminescence in the wells of the structures, increase well width, deteriorate the interface quality and increase lattice mismatch. Those factors affect the positioning of the heavy and light holes in the valence band well, and thus the absorption wavelength resulted from the intersubband transition. These observations are useful for the design of p-type quantum well infrared photodetectors.

Effects of phosphorous beam equivalent pressure on GaInAsP/GaAs grown by solid source molecular beam epitaxy with a valve phosphorous cracker cell

Abstract We report the growth and characterization of GaInAsP films on GaAs substrates by solid source molecular beam epitaxy (SSMBE) using a valve phosphorous cracker cell at varied white phosphorous beam equivalent pressure (BEP). It is found that the GaInAsP/GaAs can be easily grown with the solid sources, and the incorporated phosphorous composition as a function of the beam equivalent pressure ratio, R=fP/(fP+fAs), can be well described by a parabolic relationship. With the increase of the incorporated phosphorous composition, the GaP-, InP-, InAs- and GaAs-like phonon modes shift towards opposite directions and their emission intensities also change. The first three modes shift to larger wave numbers while the last one shifts to smaller wave number. The lattice mismatch, Δa/a, of the materials grown with varied phosphorous BEP follows a linear relationship. Photoluminescence (PL) measurements reveal that as the phosphorous BEP ratio increases, the peak position or energy band gap of the material shifts towards higher energy; the full-width at half-maximum (FWHM) becomes narrower, and the luminescence intensity becomes higher. In addition, the materials also show smooth surfaces that do not change significantly with phosphorous beam equivalent pressure.

Strained p-type InGaAs/AlGaAs multiple quantum well infrared photodetectors

Strained p-type In0.15Ga0.85As/Al0.33Ga0.67As quantum well infrared photodetectors (QWIPs) with different Be concentrations in their wells, which detect normal infrared incidence, were investigated. The QWIPs with a Be doping density of 1018 cm-3 in the wells show a cut-off wavelength of 7.9 μm and basically symmetric detectives of about 8 x 108 cm.Hz½/W at 600 Hz. By increasing the Be doping density in the wells to 2 x 1019 cm-3, the cut-off wavelength is blue-shifted to about 7.25 meV and the photoresponsivity and detectivity become asymmetric. The detectivity is increased to about 1.4 x 109 cm.Hz½/W at positive biases but significantly reduced at negative biases. The blue shift in the cut-off wavelength for the QWIP devices with heavy doping concentration in the wells is mainly due to the bandgap shrinkage and the increased well width while the asymmetric behavior in the photoresponsivity and detectivity is likely due to the inhomogenity resulting from dopant diffusion at high doping.

Optical properties of p-type InGaAs/AlGaAs multiple quantum well structures

Abstract Be-doped InGaAs/AlGaAs multiple quantum well (MQW) structures with different concentrations in the well material are characterized by photoluminescence (PL) technique. Two significant luminescence peaks were observed from the device structures and they both showed a red shift and broading in line width as doping concentration increases. Strong intersubband absorption originated from intersubband transitions of heavy holes between the ground state E hh1 and the excited state E hh2 was also observed and the maximum absorption wavelength was found to shift from 8.35 μm to 8.00 μm as the Be concentration increased from 10 17 cm −3 to 2×10 19 cm −3 . These observations are in very good agreement with the theoretical estimation after taking the doping-induced changes in barrier height into account. Temperature dependence of the luminescence from the doped MQW structures indicates that the PL intensity can be described by an exponential relationship and the energy variation of the two PL peaks with temperature follow well with the Varshni’s equation.

Energy Dispersion Contour Approach to Calculate Optical Properties of Quantum Well Structures

In order to calculate optical properties i.e. dielectric function, refractive index and absorption coefficient, the evaluation of integration including ground and excited wavefunctions is required over entire k-space. The contour of energy dispersion was proposed to form the criteria to select and limit the value of k in the integration. With the contour approach the integration can be determined to truncate at certain k value where the weight factor of Fermi-Dirac distribution function become very small and the fraction of integration can be ignored. The approach was applied to AIGaAs/AlAs/GaAs double barrier quantum well structures with 14-band k.P Hamiltonian. By systematically modifing this quantum well structure the dependence of absorption peak width was investigated in bound-to-bound and bound-to-quasibound intersubband transitions. The energy dispersion contours of each involved state were illustrated in two dimensional k-space including the compositions in perpendicular and parallel directions to the interface. The calculated refractive index and absorption as a function of wavelength can be simply extracted from the contour characteristic especially at the constant Fermi energy surface.

Characterisation of p-doped InGaAs/AlGaAs quantum wells for infrared photodetector application

We report the I-V characteristics as a function of temperature and infrared absorption of the p-doped In 0.15 Ga 0.85 As/Al 0.45 Ga 0.55 As multiple quantum well structures grown by molecular beam epitaxy. The dark current I d of the structure is found to be basically symmetrical over a voltage range from - 10 to + 10 V. It is about 10 -9 A at a bias of 1 V at 80 K, about two to three orders of magnitude lower than that reported for p-doped GaAs/AlGaAs QW structures with the same size. It is also found that I d is proportional to T exp[-(E C - E F )/kT] at 70 K and above while at temperatures below 30 K, however, it does not change significantly, indicating two different transport mechanisms. The E C - E F decreases with the increase in bias in the form of 156 exp (-V/9), due likely to energy bandgap bending. A strong Infrared absorption with a peak wavelength of 10.7 μm has been observed at room temperature, in excellent agreement with the estimated value of 10.4 μm. Our observations indicate that the p-doped In 0.15 Ga 0.85 As/ Al 0.45 Ga 0.55 As material system is a promising candidate for long wavelength detection.

Be-doped InGaAs/AlGaAs strained quantum well structures grown by molecular beam epitaxy

The effects of doping concentration incorporated into the well material of the p-type InGaAs/AlGaAs multiple quantum well structures are investigated by photoluminescence and double crystal X-ray diffraction techniques. The increased Be doping is found to cause red shift of the dopant-related and excitonic luminescence in the well material of the structures, increase well width, deteriorate the interface quality and increase lattice mismatch. Those factors affect the positioning of the heavy and light holes in the valence band well, and thus the infrared wavelength resulted from the intersubband transition. These observations are useful for the design of p-type quantum well infrared photodetectors.