Mahitosh Biswas

A low temperature investigation of the optical properties of coupled InAs quantum dots with GaAsN/GaAs spacers

Epitaxially-grown 10-layer coupled InAs quantum dots with GaAsN/GaAs barrier layers have been investigated. The PL spectra was seen to be a complex convolution of bimodal distribution of QDs along with an asymmetric signature introduced by incorporation of nitrogen into the structures. Reducing the GaAsN/GaAs barrier thickness (from 2/16nm to 2/8nm) resulted in an improvement of PL linewidth as low as 20meV of the dominant PL peak for the sample with thinnest barrier layer. A blueshift in emission was observed due to higher indium intermixing as a result of an increase in overall strain in the multilayer structure. The highly asymmetric exponential tail signature evident from the PL spectra of as-grown samples indicated a higher presence of localized N-induced excitonic states near the conduction band edge. Samples with thicker barriers showed relatively lower asymmetry compared to samples with thinner barriers. Also, samples with thinner barriers showed an arrest in blueshift in the PL spectra with annealing temperature indicating thermal stability.

Low-temperature photoluminescence studies in epitaxially-grown GaAsN/InAs/GaAsN quantum-dot-in-well structures emitting at 1.31 μm

We report a single layer GaAsN/InAs/GaAsN quantum-dot-in-well (DWELL) structure with PL emission at 1.31μm important for applications in communication lasers. This extension has been achieved with a nitrogen composition of only 1.8% and QDs embedded within 1/6nm GaAsN which is higher compared to single layer QDs with GaAs and GaAsN capping layers as a result of confinement reduction on both sides of the QD energy levels. The structures remain as QDs till 800°C of annealing temperature alongwith a drastic enhancement in PL intensity as a result of annihilation of N-induced crystal defects which provide non-radiative recombination centers for carriers in the as-grown sample which is responsible for degraded luminescence. A typical highly asymmetric PL signature observed in dilute nitride structures is seen with a sharp cut-off at lower wavelengths and a large exponential tail at higher wavelengths in the as-grown and 650°C annealed samples. This is due to the presence of localized excitonic states extending into the bandgap close to the band edges. For higher annealing temperatures, this asymmetry disappears indicating an improvement in uniformity of nitrogen distribution and absence of localized states; which is also confirmed from a smaller blueshift in excitation intensity-dependent PL spectra of these samples. Well-resolved ground and first excited states in the PL spectrum of 700°C annealed sample indicates an improvement in QD confinement.

A low-temperature photoluminescence study of GaAs1-xNx/GaAs multiple quantum-wells

Five-period GaAs1−xNx/GaAs multiple quantum wells (MQWs) were grown on GaAs(001) substrates under different nitrogen background pressures through solid-source molecular beam epitaxy and the structural and optical properties at low temperature were investigated. High resolution x-ray diffraction revealed sharper satellite peaks observed for GaAs0.978N0.022/GaAs MQWs as compared to GaAs0.982N0.018/GaAs MQWs, indicating better interfaces. The MQWs with higher nitrogen content exhibited high photoluminescence (PL) intensity, whereas a degraded PL intensity was observed for the latter, attributed to reduction in surface recombination with high nitrogen incorporation. Moreover, the spectrum for the MQWs with higher nitrogen content was observed to be consisted of several Gaussian spectra, indicating thickness variation of QWs caused by randomness in distribution of N atoms. In the low energy regime of PL, a long asymmetric tail was observed because of nitrogen introduced potential fluctuations. Rapid thermal annealing enhanced PL intensity by multi-fold and substantially reduced the full width at maximum because of homogenization of MQWs. This investigation could enhance understandings of the MQWs-based optoelectronic devices.

Impact of rapid thermal annealing on dilute nitride (GaAsN)-capped InAs/GaAs quantum dots exhibiting optical emission beyond ~1.5 μm

We report here self-assembled 2.6 ML InAs QDs capped with GaAsN0.021 on GaAs (001) substrate grown under high arsenic overpressure and high power by solid source molecular beam epitaxy. With variation in GaAsN0.021 layer thickness, InAs/GaAs QDs were studied by photoluminescence (PL) spectroscopy. It was found that with InAs dot density of 3 ×1010 cm-2 and 4 nm GaAsN capping layer, emission wavelength was possible to extend beyond 1.5 μm at 300K. Rapid thermal annealing was carried out in nitrogen ambient for 30 sec at temperatures ranging from 700°C to 800°C and a continuous blue-shift for the nitride-capped QDs was observed at 19 K PL spectra, and the sample annealed at 800°C exhibited highest intensity with narrowest full width at half maximum (FWHM). Both the as-grown and annealed samples exhibited asymmetric PL behavior in low energy region at low temperature, associated to the N-related states or cluster of N atoms. The peak emission wavelength at the annealing temperature domain of 750-800°C was remained constant, attributed to no In/Ga diffusion at the interface between the dot and the barrier. Hence, the InAs/GaAs dots capped with 4-nm GaAsN0.021 layer could be implemented in lasers in the temporal range of 750-800°C.