MORPHOLOGY OF SELF-ORGANIZED InAs ISLANDS ON InP

As already pointed out for the case of InAs on AlAs, one of the attractive features of MBE growth is that it is possible to engineer the bandgap by choice of material.  For example, for device applications, it is of interest to investigate InAs 3-D island formation on different starting substrates.  InP is particularly interesting since it would permit the shifting of the emission wavelength to the technologically important 1.3 to 1.5µm range when using the binary InAs as the low bandgap material. In addition to the different emission range, coherent 3-D InAs islands on InP will result in a significantly smaller carrier effective mass and a correspondingly larger intersubband energy level spacing.        

In addition to bandgap engineering, by choosing an InP substrate, the self-assembled InAs/InP system provides us control over the quantum nanostructures, i.e. either quantum wire or quantum dot. Direct deposition of several MLs of InAs on the substrate induces self-assembled nanowires. The nanowires can also be produced by exchange of substrate phosphorous with incoming arsenic. The nanowires, either by direct deposition or by exchange, transform into dots when they are annealed under no flux.  The most significant difference between the InAs/InP and the InAs/AlGaAs system is the observations of stable InAs quantum wires and their evolution into quantum dots. This morphological change enables us to select either quantum wire or quantum dot in the InAs/InP system.

Growth

InAs coherently strained islands on InP were grown in the same MBE chamber described in 1.2.1. We used n-doped InP wafers previously degassed at 250°C in a separate chamber. After loading a commercial n-type, planar InP (001) wafer into the MBE chamber, the oxide layer on the wafer is removed by annealing the wafer above 480°C under a cracked (950°C) phosphorous (P2) beam equivalent pressure (BEP) of 10–5 Torr. The resultant surface yields 2´4 RHEED patterns, similar to those of 2´4 patterns of a GaAs (001) surface as in previous sections.

Once the oxide is removed, a 0.5mm thick buffer layer of InP is grown on the substrate at 470°C.  To have a smooth InP (001)-2´4 surface, the substrate is annealed at 500 °C under a P2 pressure of 1´10–5 Torr for 15 minutes and then cooled to 480°C.  At 480°C, the valve for the P2 flux is closed and the residual phosphorous is pumped out for about 3 minutes. During this period the RHEED continuously indicates a stable 2x4 surface reconstruction that is independent of the P2 overpressure. After preparing an InP surface, the sample is rapidly cooled down below 250°C and transferred to the STM.  STM images are taken for filled states (-3V on the sample) with tunneling current around 100 pA and confirm a flat and smooth surface.

On this smooth surface of InP, we deposit a desired amount of InAs with growth rate ~0.3ML/s and As4 flux around 1´10–5 Torr. The sample is, either immediately or after some delay for annealing, cooled down below 250°C and transferred to the STM chamber through the ultra high vacuum modutrack.  STM images are taken for filled states (-3V on the sample) with tunneling current around 100 pA. Observation of RHEED patterns during the growth indicates that the surface reconstruction remains the same 2x4. Upon closing the arsenic valve, however, the RHEED pattern immediately changed to 4x2, indicating an In rich surface. Nanowires were also produced by simple application of high As4 flux around 1´10–5Torr on InP surface without indium deposition, utilizing the intermixing or exchange of V-elements.

InP Starting Surface and Nanowires

The STM images of the starting surface of InP (001), just before deposition of InAs, are shown in Figure 29-(a) and (b). The images show terraces, vacancy islands, monolayer (ML) high steps and 2x4 surface reconstruction with dimer rows, the inset in Figure 29-(b), which are along the [10] direction. The STM images indicate a beautiful InP (001) surface even after annealing without phosphorous for 15 minutes although there are vacancy islands. The shape of the vacancy island is anisotropic, elongated more along the dimer row direction of the 2x4 surface reconstruction. No 4x2 surface reconstruction on the InP (001) surface is observed, even after 15 minutes of annealing without a phosphorous flux at 480°C, either by RHEED or by STM.

Application of a high arsenic flux at 480°C on the InP (001)-2x4 surface produces a high density of nanowires as shown in Figure 30-(a). The substrate temperature and the As BEP were 480°C and 10-5 Torr respectively. The dimer rows on the top surface of nanowires are clearly visible as seen in Figure 30-(b), indicating the arsenic-terminated 2x4 surface reconstruction. This STM picture was taken after the substrate was annealed under the growth conditions for 8 minutes and then quenched under enough arsenic flux to support the 2x4 surface reconstruction as monitored by RHEED. The necessary time period of arsenic application is about 3 minutes to form nanowires under the high arsenic BEP, indicating that the nanowires are stable when the arsenic flux is high enough to maintain a 2x4 surface reconstruction. The typical wire length is over 1 mm and the density of the nanowires is about 50/mm. The average height of the nanowires, along with the standard deviation, is 1.9±0.5nm. The width and average separation between nanowires are 14.3±1.9nm and 3.7±1.6nm respectively while the shape is a trapezoid as shown in the line profile along the [110] direction.

InAs growth on the InP (001)-2x4 surface at 480°C produces a high density of nanowires, similar to nanowires produced by the exchange process described above.  When the nanowires are cooled down under no arsenic flux, the surface reconstruction changes to 4x2 from 2x4 still preserving the nanowire morphology as shown in Figure 31-(a). The dimer rows on the top surface of nanowires are clearly visible as seen in Figure 31-(b), indicating the indium-terminated 4x2 surface reconstruction.

Imaging of InAs 3-D Islands

While the nanowires were stable under high As flux, this was not the case for a low flux. For example, when the needle valve of the arsenic cell was closed, the As-flux dropped below 10-9 Torr in several seconds. This resulted in an In-rich 4x2 surface reconstruction for the nanowires as in Figure 31-(a) and (b). As expected, the RHEED pattern changed immediately from 2x4 to 4x2 upon closing the arsenic valve. Continuing this condition of no flux at the same temperature transforms the nanowires into dots as shown in Figure 31-(c) and (d) as well as in Figure 30-(c) and (d). A residual of the nanowires, now 2D islands (only 1ML high), are also visible in the Figure 30-(c) and 31-(c). Interestingly, the dots are expanded more along the [110] direction, the dimer row direction of the 4x2 surface reconstruction. The 4x2 surface reconstruction on the rectangular flat-top of the dot is clearly visible in Figure 30-(d) and 31-(d). The same surface reconstruction is also observed on the wetting layer between dots as well as on the residual of nanowires.

The density of the transformed dots in Figure 30 along with the standard deviation is 3.6±1.3x109/cm2. The average height of the dots is 4.7±0.7nm. The dots have a larger width along the dimer row direction of the 4x2 surface reconstruction as shown in Figure 30-(d) and the line profile below. The base and top widths are 58.1±8.3nm and 32.1±5.1nm respectively along the [10] direction while 75.5±8.0nm and 38.4±12.4nm along the [110] direction. The ratio of length/width has changed from ~70 (nanowire) to 0.77 (dot), where the width is measured along the [110] direction, resulting in a rectangular based, truncated pyramid elongated along the dimer row direction. Also the height has increased by 2.5 times over the nanowires.       

The morphology change from wire to dot is correlated to the change in surface reconstruction from 2x4 to 4x2. The change from wires to dots, only and always occurred, with a change in surface reconstruction.  One plausible explanation for this behavior is as follows: Initially the morphological change from film to wires is driven by the gain in elastic relaxation energy of the coherent islands which overcompensates the energy cost due to the increase of surface area. This would be reasonable if the energy cost due to the increase of surface area for wires is less than that for dots and if a 4x2 surface is a higher energy cost than a 2x4 surface [117]. In this case, four possible stable morphologies would be 2x4 wires oriented [10], 2x4 dots, 4x2 wires along [110], and 4x2 dots.  However, in our experiment, the change in surface reconstruction from 2x4 to 4x2 produces 4x2 wires along [10]. This could be a very high unstable energy configuration that falls to the next lowest stable morphology, that of 4x2 dots.  

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