Results of Angular Scan Curves

The angular scan of channeling experiment was performed along the [100] direction, which is normal to the sample surface, and the [110] direction, 45^o to the surface normal direction. An angle scan for the [100] beam was performed along the(011)surface. Another angular scan was performed for the [110] direction beam along the (001) surface. In this way, we were able to probe the strain in the QDs in both the vertical and lateral directions. Fig. 3.3 shows schematically how the channeling beam probes a buried strain layer.

FIG.. 3.3. Schematic diagram of how a buried strained layer is probed by ion beam.

Backscattering spectrum shows the In and GaAs signal are separated completely. Figure 3.4 shows the angular scan curve, the curve provide strain information. In signal is the integral over the whole In peak In backscattering spectrum from 1200 to 1300 channel. GaAs signal is integral from 800 nm to 40nm channel.

FIG. 3.4. Backscattering spectrum shows the In and GaAs signal are separated completely.

Tight figure shows the angular scan curve, the curve provide strain information.

In signal is the integral over the whole In peak In backscattering spectrum from 1200 to 1300 channel. GaAs signal is integral from 800 nm to 40nm channel.

FIG. 3.5. Angular scan spectra along (a) [100] and (b) [110] axes of the as-grown QD sample. The angular shift was determined by the relative change in angle position of the half maximum of the In and Ga/As curves on either side of the channeling curves. From the sign of (∆θ)right -(∆θ)left, we determine which direction the curves shift relative to each other. No obvious difference was observed between the curves of In and Ga/As in the [100] direction. In the [110] direction, we observe an angular shift of the In signal relative to the Ga/As signal toward the [100] direction.

We first looked at a sample with a single QD layer buried under a 50 nm GaAs cap. Fig.

3.5(a) and (b) show the angular scan of the In and the Ga/As signals for the as-grown sample along [100] and [110] directions. While the In signal is from the QDs and the wetting layer, the Ga/As signal was taken from the top 40 nm of the sample. The wetting layer is one monolayer and fully strained. The geometry of the wetting layer influences only the half width of the In angular scan (narrowing) and minimum yield for the [110] direction [38]. No significant difference was observed between the curves of In and Ga/As in the [100] direction.

However, an angular shift was observed in the [110] scan, which indicates a lattice distortion in the buried QD layers. This is the first time that an angular shift was observed for samples with a buried strain QD layer.

The size of the dots, measured from TEM and AFM images, was about 4 nm in height and 20 nm in width. The calculated shadow cone length is 4 nm and 2.5 nm in the [100] and [110] directions, respectively. In [100] direction, because it is perpendicular to the strained layer, no angular shift in the angular scan can be obtained. Furthermore because the shadow cone length is about the same as the height of the QDs, according to the ion channeling theory [39], the In atoms in the QDs behave like In impurities in a GaAs matrix. The channeling behavior of the In signal from the QD layer is determined by the position of In atoms in the flux pattern emerged from the overlying GaAs capping layer. If the In atoms deviate from their equilibrium positions, they could cause the dip of the angular scan to be narrower. The fact that the angular scans of In and Ga/As are nearly identical indicates that there is no strain relaxation in the QDs in the lateral direction. For channeling along the [110] direction, because the shadow cone length is smaller than the QD size, it becomes possible to observe the lattice distortion directly from the shift of the angular scan spectrum. The angular shift was determined by the relative change in angle position of the half maximum of the In and Ga/As curves on either side of the channeling curves. From the sign of (∆θ)right -(∆θ)left, we

determine which direction the curves shift relative to each other. From Fig. 3.5(b), we indeed see a slight shift of the In signal relative to the Ga/As signal toward the [100] direction. This shift provides a directly evidence that the lattice of the InAs QDs is larger than that of the GaAs matrix in [100] or the crystal growth direction.

The effect of thermal annealing on strain relaxation in QDs was then studied. The as-grown sample was annealed at 650°C and 750°C for 30sec with a rapid thermal annealing furnace. The angular scan spectra after the sample was annealed at 650°C are shown in Fig.

3.6(a) and 3.6(b). In the [100] direction, we again did not see any difference between the In spectrum and the Ga/As spectrum. In the [110] direction, the angular shift of the In signal, however, becomes larger than that of the as-grown sample. So the InAs lattice remains the same in the lateral direction but becomes larger, less strained, in the growth direction after annealing. In other words, we start to see some strain relaxation in vertical direction.

The angular scan spectra of the sample annealed at 750°C are shown in Fig 3.7(a) and 3.7(b). The shape of the dip in the angular scans for the In signal in the [100] direction is clearly different from that of Ga/As. It has become narrower, which is a clear evidence of displaced In lattice. In the [110] direction, the angular scan spectra are similar to those of the sample annealed at 650°C. So annealing at 750°C caused the strain to relax not only in the vertical direction but also in the lateral direction. From these observations, we reach the following conclusion. The as-grown QDs have the same lattice constant as that of GaAs in the plane perpendicular to the growth direction. In the growth direction, the InAs lattice is larger than that of GaAs. After 650°C annealing, the InAs lattice of the QDs becomes larger or relaxed in the growth direction, but in the in-plane direction, the QDs remain strained with the lattice constant the same as that of GaAs. After 750°C annealing, however, the InAs lattice of the QDs not only relaxes in the growth direction but also becomes relaxed in the lateral direction.

FIG. 3.6. Angular scan spectra of the QDs annealed at 650^oC. The In and Ga/As signal again match in the [100] direction. In the [110] direction, the angular shift of the In signal becomes larger than that of the as-grown sample.

FIG. 3.7. Angular scan spectra of the QDs annealed at 750^oC. In the [100] direction, the In signal is narrower than that of the Ga/As signal. In the [110] direction, the angular scan spectra are similar to those of the sample annealed at 650°C.