Emission characteristics variation of GaAs0.92Sb0.08/Al0.3Ga0.7As strained multiple quantum wells caused by rapid thermal annealing

Rapid thermal annealing is an effective way to improve the optical properties of semiconductor materials and devices. In this paper, the emission characteristics of GaAs0.92Sb0.08/Al0.3Ga0.7As multiple quantum wells, which investigated by temperature-dependent photoluminescence, are adjusted through strain and interfacial diffusion via rapid thermal annealing. The light-hole (LH) exciton emission and the heavy-hole (HH) exciton emission are observed at room temperature. After annealing, the LH and HH emission peaks have blue shift. It can be ascribed to the variation of interfacial strain at low annealing temperature and the interfacial diffusion between barrier layer and well layer at high annealing temperature. This work is of great significance for emission adjustment of strained multiple quantum wells.


Results and discussion
The temperature-dependent PL spectra of all samples are shown in Fig. 1a. Emission peaks of four samples at 10 K locate at 1.407 eV, 1.410 eV, 1.413 eV and 1.427 eV, respectively. As temperature increasing from 10 to 300 K, the peak position has first blue shift and then red shift, which is attributed to the exciton fission and temperature dependent energy shrinkage 8 . The variation of emission peaks for four samples follows the same tendency. In order to reveal the emission mechanism, the relationship of peak position and temperature is analyzed. The evolution characteristics of PL peak position could be well described by Eq. (1), which is derived by O'Donnell and Chen 18 where E g (0) is the band gap at 0 K, S is a dimensionless coupling constant and < ћω > represents an average phonon energy involved the radiative recombination process 18,19 . The experiment results and fitting curves of temperature dependent emission peak position are shown in Fig. 1b. The solid lines are fitting curves with Eq. (1) and the discrete points are experimental results. The fitting parameters are worked out and shown in Table 2. The experimental results are well consistent with the fitting curve, which indicate that the main peak of PL spectra originate from near band emission. Figure 2a shows the room temperature PL spectra of all samples. The main peaks of spectra locate at 1.321 eV, 1.322 eV, 1.326 eV and 1.345 eV, respectively. Notably, the shape of emission peak is asymmetry and the peak positions after annealing have a tiny blue shift compared with as grown sample. There must be more than one type of radiative recombination. Then, we employ Gaussian fitting to analyze the emission spectra and reveal the mechanism. Figure 2b shows the Gaussian fitting results of PL spectra at room temperature. The emission peak contains three parts for all samples. According to our previous work 20 , the left two parts are related with the splitting of valence band in strained MQWs. The peak (P1) located at low energy side comes from the radiative recombination of electrons and heavy holes. The peak (P2) located at high energy side is attributed to the radiative recombination of electrons and light holes. The peak (P3) located at ~ 1.38 eV is attributed to the interfacial radiative recombination. Notably, the P3s in the spectra of as grown, RTA 600 and RTA 700 are so weak that can be neglected.
Then, the origins of blue shift are investigated in detail to reveal the emission mechanism of strained MQWs. The increase of compressive strain in MQWs may be the significant factor. A triple crystal XRD measurement is performed to study strain characteristics at room temperature, as shown in Fig. 3. Five satellite peaks are observed in samples as grown, RTA 600 and RTA 700, which suggested that the qualities of MQWs are perfect. But, only -1st and -2nd satellite peaks are observed in sample RTA 800. It indicates that the periodic structure of MQWs has been slightly damaged. And, more remarkable, the satellite peaks in all spectra are asymmetry. It means that the compressive strains exist in samples. As the lattice parameter of Al 0.3 Ga 0.7 As (5.655 Å) is similar to GaAs (5.653 Å), the strain between AlGaAs and GaAs buffer could be ignored. So, the real strain in MQWs is originated from the mismatch of AlGaAs barrier layers and GaAsSb well layers. According to X-ray diffraction theories, the real strain can be estimated by following equations 21 :   is the band gap of GaAs 1-x Sb x well layer without strain, a c and a v are the conduction band and valence band deformation potential, b is the shear deformation potential, C 11 and C 12 are elasticity modulus. The parameters values of GaAs, GaSb and AlAs are shown in Table 3 22 .
The band gap of GaAs 1-x Sb x bulk without strain can be estimated by Vegard's law 24 : When the thickness of well layer reduced to the Bohr radius of excitons, the quantum confinement effect will lead to broadening of band gap. The band gap of well layer material can be estimated by following equation 25 :    www.nature.com/scientificreports/ However, the calculation results for RTA 700 and RTA 800 are smaller than experimental results. And the difference between theory and experimental results gradually increases with annealing temperature increasing. Especially for RTA 800, although its strain is smaller than that of RTA 700, the emission peaks have blue shift in experiment results. Therefore, there must be other factors affecting the emission properties.
The interfacial diffusion, which existed in semiconductor heterojunction interface, may be the other significant factor for the blue shift of emission peak in sample of RTA 800. It has been confirmed by the broken of periodicity structure of MQWs from XRD spectra. Owing to the concentration gradient between well layers and barrier layers, Sb atom migrates from GaAsSb to AlGaAs, which leads to the decrease of Sb component in well layer 26,27 . So, the band gap of GaAsSb is broadened and the height of valence-band offsets is reduced. In this case, the interface related emission is enhanced and the peak position has blue shift (Fig. 4). Furthermore, when the annealing temperature increase up to 800 °C, the degeneration of interface caused by interfacial diffusion will greatly increase interfacial radiative recombination, which is consistent with the PL spectra in Fig. 2b. In addition, as the interfacial emission of quantum well increasing, the full width of half maximum (FWHM) of PL spectra will greatly broaden (Insert of Fig. 2a).
As is known to all, the diffusion rate depends on annealing temperature. Meanwhile, it can be also enhanced by the compressive strain in MQWs and vacancy concentration. The diffusion coefficient related to temperature is according to Arrhenius equation 28 : where D 0 is the temperature-independent preexponential, Q d is the activation energy. As can be seen from the equation, the diffusion coefficients increase with increasing of annealing temperature. Under conditions of nonsteady state, Fick's second law is used to calculate the distribution of concentration. The Sb component is estimated to be about 0.068 in GaAs 1-x Sb x of sample RTA 800. And then, the Sb component is substituted into the Eqs. (4)- (8) to calculate the HH and LH emission peaks. The calculation values are also shown in Fig. 4 (star symbol), which is consistent with experiential data. Considering the above analyses, we conclude that the interfacial diffusion is the main significant factor for the blue shift of emission peak of RTA 800.

Conclusions
In summary, the effect of rapid thermal annealing on strained GaAs 0.92 Sb 0.08 /Al 0.3 Ga 0.7 As MQWs is analyzed in detail. The crystal quality and strain properties of samples are analyzed by using XRD. The elastic continuum theory is employed to calculate the strain and the band gap of strained MQWs. After annealing, the light holes exciton emission and heavy holes exciton emission have blue shift. At low annealing temperature, the blue shift is attributed to the increase of interfacial strain. But, when annealing temperature is higher than 800 °C, the blue shift is mainly due to the interfacial diffusion between barrier layers and well layers. Our work presents a detailed study on the emission characteristics variation of strained MQWs caused by rapid thermal annealing, which is significance for enhancing the performance of GaAsSb based MQWs laser.