Effect of ion-induced damage on GaNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy

The effect of ion-induced damage on GaNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy employing a DC plasma as the N source was investigated. Ion-induced damage results in: (i) an observed disappearance of pendellosung fringes in the X-ray diffraction pattern of the sample; (ii) a drastic decrease in intensity and a broadening in the full-width at half-maximum of photoluminescence spectra. It was shown that ion-induced damage strongly affected the bandedge potential fluctuations of the QWs. The bandedge potential fluctuations for the samples grown with and without ion removal magnets (IRMs) are 44 and 63 meV, respectively. It was found that the N-As atomic interdiffusion at the interfaces of the QWs was enhanced by the ion damage-induced defects. The estimated activation energies of the N-As atomic interdiffusion for the samples grown with and without IRMs are 3.34 and 1.78 eV, respectively. (C) 2001 Elsevier Science B.V. All rights reserved.

Optical properties and band lineup in GaNxAs1-x/GaAs single quantum wells

The optical properties and the band lineup in GaNAs/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL) technique were investigated. It was found that the low-temperature PL is dominated by the intrinsic localized exciton emission. By fitting the experimental datawith a simple calculation, band offset of the GaN0.015As0.985/GaAs heterostructure was estimated. Moreover, DeltaE(c), the discontinuity of the conduction band was found to be a nonlinear function of the nitrogen composition (chi) and the average variation of DeltaE(c) is about 0. 110eV per % N, such smaller than that reported on the literature to (0.156 similar to 0.175 eV/N %). In addition, Qc has little change whtn N composition increares, with an experimential relation of QC approximate tox(0.25). The band bowing coefficient (b) was also studied in this paper. The measured band bowing coefficient shows a strong function of chi, giving an experimental support to the theoretic calculation of Wei Su-Huai and Zunger Alex (1996).

Variational calculations of ionized-donor-bound excitons in GaAs-AlxGa1-xAs quantum wells

Using a two-parameter wave function, we calculate variationally the binding energy of an exciton bound to an ionized donor impurity (D+,X) in GaAs-AlxGa1-xAs quantum wells for the values of the well width from 10 to 300 Angstrom, when the dopant is located in the center of the well and at the edge of the well. The theoretical results confirm that the previous experimental speculation proposed by Reynolds tit al. [Phys. Rev. B 40, 6210 (1989)] is the binding energy of D+,X for the dopant at the edge of the well. in addition, we also calculate the center-of-mass wave function of the exciton and the average interparticle distances. The results are discussed in detail.

Circular polarization of excitonic luminescence in CdTe quantum wells with excess electrons of different densities

The circular polarization of excitonic luminescence is studied in CdTe/Cd1-xMgxTe quantum wells with excess electrons of low density in an external magnetic field. It is observed that the circular polarization of X and X- emissions has opposite signs and is influenced by the excess electron density. If the electron density is relatively high so that the emission intensity of the negatively charged excitons X- is much stronger than that of the neutral excitons X, a stronger circular polarization degree of both X and X- emissions is observed. We find that the circular polarization of both X- and X emissions is caused by the spin polarization of the excess electrons due to the electron-spin-dependent nature of the formation of X-. If the electron density is relatively low and the emission intensity of X- is comparable to that of X, the circular polarization degree of X and X- emissions is considerably smaller. This fact is interpreted as due to a depolarization of the excess electron spins, which is induced by the spin relaxation of X-.

Influence of dual incorporation of In and N on the luminescence of GaInNAs/GaAs single quantum wells

The optical properties of above- and below-band-edge transitions have been investigated by incorporating In atoms into GaNAs/GaAs single quantum wells. The experimental results show that with increasing In concentration the interband luminescence is improved and the luminescence intensity below the band edge in GaInNAs/GaAs decreases significantly. An interpretation is given that N atoms are preferable to form a covalent bond with In than with Ga atoms in a GaInNAs alloy, due to the compensation of the atomic-size difference between In and N atoms on the GaAs substrate. The photoreflectance spectra of the GaInNAs/GaAs single quantum well support the assignment of an intrinsic mechanism to the high-energy luminescence peak. (C) 2000 American Institute of Physics. [S0003- 6951(00)01752-6].

Photoluminescence studies on pseudomorphic delta-doped AlGaAs/InGaAs/GaAs quantum wells

Photoluminescence (PL) measurements were performed on several series of single-side Si-doped pseudomorphic high electron mobility transistors (p-HEMTs) quantum well (QW) samples, with different spacer layer widths, well widths and Si delta -doped concentrations , under different temperatures and excitation power densities. The dynamic competitive luminescence mechanism between the radiations of e2-hh1 and e1-hh1 was discussed in detail. The confining potential, subband energies, corresponding envelope functions, subband occupations and transferring efficiency etc., were calculated by self-consistent finite differential method at different temperatures in comparison with the present experiment results. The relative variation of the integrated luminescence intensity of the two transitions (e1-hh1 and e2-hh1) was found to be dependent on the temperature and the structure's properties, e. g. spacer layer width, dopant concentration and well width.

Effect of rapid thermal annealing on GaInNAs/GaAs quantum wells grown by plasma-assisted molecular-beam epitaxy

We have studied the effect of rapid thermal annealing (RTA) on GaInNAs/GaAs quantum wells (QWs) grown by molecular-beam epitaxy using a dc plasma as the N source. It was found that RTA at low temperature (LT, 650 degrees C) and high temperature (HT, 900 degrees C) could both improve the QW quality significantly. To clarify the mechanism of quality improvement by RTA, a magnetic field perpendicular to the path of the N plasma flux was applied during the growth of the GaInNAs layers for the sake of comparison. It was found that LT-RTA mainly removed dislocations at interfaces related to the ion bombardment, whereas, HT-RTA further removed dislocations originating from the growth. LT-RTA caused only a slight blueshift of photoluminescence peak wavelength, probably due to defect-assisted interdiffusion of In-Ga at the QW interfaces. The blueshift caused by HT-RTA, on the other hand, was much larger. It is suggested that this is due to the fast defect-assisted diffusion of N-As at the QW interfaces. As defects are removed by annealing, the diffusion of In-Ga at interfaces would be predominant. (C) 2000 American Institute of Physics. [S0003- 6951(00)01535-7].

Evolution from point defects to arsenic clusters in low-temperature grown GaAs/AlGaAs multiple quantum wells

Optical transient current spectroscopy (OTCS), photoluminescence (PL) spectroscopy and excitonic electroabsorption spectroscopy have been used to investigate the evolution of defects in the low-temperature grown GaAs/AlGaAs multiple quantum well structures during the postgrowth rapid thermal annealing. The sample was grown at 350 degrees C by molecular beam epitaxy on miscut (3.4 degrees off (001) towards (111)A) (001) GaAs substrate. After growth, the sample was subjected to 30s rapid thermal annealing in the range of 500-800 degrees C. It is found that the integrated PL intensity first decreases with the annealing temperature, then gets a minimum at 600 degrees C and finally recovers at higher temperatures. OTCS measurement shows that besides As,, antisites and arsenic clusters, there are several relatively shallower deep levels with excitation energies less than 0.3 eV in the as-grown and 500 degrees C-annealed samples. Above 600 degrees C, OTCS signals from As,, antisites and shallower deep levels become weaker, indicating the decrease of these defects. It is argued that the excess arsenic atoms group together to form arsenic clusters during annealing. (C) 2000 Elsevier Science B.V. All rights reserved.

Characteristics of circular waveguide photodetectors using SiGe/Si multiple quantum wells

We report on the fabrication of circular waveguide photodetectors with a response near 1.3 mu m wavelength using SiGe/Si multiple quantum wells. The quantum efficiency of the circular waveguide photodetector is improved when compared with that of the rib waveguide photodetector in the same wavelength at 1.3 mu m The frequency response of the photodetectors is simulated. The emciency-bandwidth product of the circular waveguide photodetectors is improved correspondingly. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Binding energy of ionized-donor-bound excitons in the GaAs-AlxGa1-xAs quantum wells

The binding energy of an exciton bound to an ionized donor impurity (D+,X) located st the center or the edge in GaAs-AlxGa1-xAs quantum wells is calculated variationally for the well width from 10 to 300 Angstrom by using a two-parameter wave function, The theoretical results are discussed and compared with the previous experimental results.

Observation of quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells

Quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells are suggested by the bias dependence of the photocurrent spectra of p-i-n photodiodes. Both Stark redshift and blueshift have been observed for the same sample in the different ranges of electric fields applied to the quantum wells. The turnaround point corresponds to a certain electric field (named "critical" field). This phenomenon was generally predicted by Austin in 1985 [Phys. Rev. B 31, 5569 (1985)] and calculated in detail for SiGe quantum structure by Kim recently [Thin Solid Films 321, 215 (1998)]. The critical electric field obtained from the photocurrent spectra is in reasonable agreement with the theoretical prediction. (C) 2000 American Institute of Physics. [S0021-8979(00)03711-7].

Effect of rapid thermal annealing on InGaAs/GaAs quantum wells

We have studied the effect of rapid thermal annealing (RTA) on highly strained InGaAs/GaAs quantum wells by using photoluminescence (PL) and double-crystal X-ray diffraction (DCXRD) measurements. It is found that a distinct additional PL emission peak can be observed for the annealed samples. This PL emission possesses features similar to the PL emission from InGaAs/GaAs quantum dots (QDs) with the same indium content. It is proposed that this emission stems from QDs, which were formed during the annealing process. This formation is attributed to the favorable diffusion due to the inhomogeneous strain distribution in the InGaAs layer intersurface. The DCXRD measurements also confirm that the dominant relaxation is strain enhanced diffusion under the low annealing temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-delta-doped GaAs/Al0.3Ga0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E-LO at B > 27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E-LO + (E-2 - E-1), where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm(-1) around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LI, of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence: of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Ultrafast low-temperature grown AlGaAs/GaAs photorefractive quantum wells using point defects as capture centers

At a medium substrate temperature of 400 degrees C and a lower As flux, we have grown an ultrafast AlGaAs/GaAs photorefractive multiple quantum well (MQW) structure by molecular beam epitaxy. The as-grown sample exhibits strong photorefractive effect under the transverse Frantz-Keldysh geometry. A peak electroabsorption of 2100 cm(-1) is measured in the as-grown sample in an 11 kV/cm dc electric field, and the peak photorefractive diffraction efficiency can be 1.2%. After postgrowth annealing, the photorefractive effect becomes weak and disappears in samples annealed above 700 degrees C. Using optical transient current spectroscopy, deep levels are measured in these samples. It is found that deep levels are stable against annealing until 700 degrees C. Using a pump-probe technique, carrier lifetimes are measured at room temperature. We find that the as-grown sample has a lifetime of 20 ps, while the 700 degrees C annealed sample has a lifetime of more than 200 ps. The ultrafast lifetime in the as-grown sample is caused by point defects, not by As clusters. Our result show that AlGaAs/GaAs MQW structure grown around 400 degrees C has better performance of the photorefractive effect. (C) 1999 American Institute of Physics. [S0003-6951(99)04036-X].

Observation of field-driven blue shift in delta-doped Si n-i-p-i multiple quantum wells

We report the observation of the field-driven blue shift at near absorption edge in the photo-current response spectra of delta-doped Si n-i-p-i multiple quantum wells due to the widening of the effective energy gap. This phenomenon differs from the observed results in GaAs/AlGaAs and GeSi/Si superlattices, because the physical mechanisms of forming energy band in these superlattice samples are different. Our experimental results are interpreted satisfactorily by the theoretical calculation. (C) 1999 Elsevier Science Ltd. All rights reserved.