Nanowires: First NW sample

Faebian Bastiman

After the initial system calibration (see Nanowires: Pre campaign calibrations) we are ready for our first attempt at self-seeded GaAs NW on Si(111). In this particular case the As flux was tuned to be approximately equal to the Ga flux of 0.11ML/s (0.69 atoms/nm2/s). The epiready Si(111) wafer was loaded into the sample holder (platen) with no ex situ processing. The platen was then placed inside the fast entry lock (FEL) and the FEL was outgassed to 125°C for 3 hours to remove moisture. The platen was then transferred to the preparation chamber’s outgas stage and heated to 400°C for 2 hours to outgas the platen and sample prior to growth. Finally the platen was transferred to the growth chamber, ready to grow.

Well not quite ready to grow. The sample is a single side polished 275 µm thick P-doped n+ Si(111) from CrysTec. It comes in a box of 25, in a batch of 8 boxes all processed at the same time, in the same way from the same ingot. Before NW growth the oxide needs “conditioning” in a very specific manner. The actual process is not well understood, but probably the oxide reduces (in a REDOX reaction) and likely reduces in overall oxide thickness. It is possible small “pin holes” open up in the oxide. Oxide conditioning is critical and whilst it is reproducible in wafers from the same batch, it varies slightly from batch to batch and significantly from supplier to supplier. Hence it is a good idea to order 200 or so wafers from a single supplier and from a single batch. You can then use 10 or so wafers trying to condition the oxide, and know that once it is established the same method can be used on the remaining 190 wafers. The difference is due to changes in both the doping (which affects the absolute substrate temperature vs thermocouple temperature) and also the composition of the native Si oxide (which will alter the temperature required to condition the oxide).

For this particular Si(111) substrate the oxide was conditioned at 655°C on the pyrometer, which equates to 675°C on the thermocouple. This same thermocouple reading corresponded to 630°C on the pyrometer on a GaAs(100) wafer. After conditioning for 20 minutes, the Si(111) substrate was cooled to 580°C on the pyrometer (592°C at the thermocouple) and the As valve was opened to double the required valve position for 2 minutes. The As valve was then closed to the required valve position and left to stabilize for a further 10 minutes. At this point the RHEED pattern displayed Kikuchi lines and a single 1x spot on the major azimuths zero order diffraction bars.

The Ga cell was then opened and after 19 seconds… The RHEED pattern revealed the twinned zinc blende (ZB) pattern affiliated with ZB NWs. We had NWs on the first attempt: A combination of good fortune and good calibration. In order to prevent disruption from rotation stops, the images of the RHEED pattern were only recorded after 30 minutes, during sample cooling and upon the cessation of growth. The twined ZB RHEED pattern is shown in Figure 1. The upper spot corresponds to the NW’s ZB structure, the lower spot corresponds to its rotated twin. The rotated twin occurs around a stacking fault. A third spot located between the two ZB spots denotes the presence of “significant” wurtzite (WZ) regions. As you can see no significant WZ is present. So we are also fortunate to have stumbled across the parameters to grow primarily ZB phase GaAs NWs. What else does the RHEED tell us?

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Figure 1: Twinned ZB NW RHEED pattern after 30 minutes of NW growth

Well the nucleation time was around 19s, which is relatively short. This means the oxide is suitably conditioned and the As:Ga ratio is suitably close to 1:1. The nucleation time can be optimized later by increasing the oxide conditioning anneal temperature by 10 to 20°C and adjusting the As:Ga ratio. First one should focus on optimising the NW growth.

One thing that was noted between 5-10 minutes of NW growth was the presence of additional spots between the Si 1x and the NW twinned ZB spots. These are indicative of “parasitic” 2D growth on the oxide. Probably due to the growth temperature being too low or on an incorrect Ga:As ratio. One cannot be sure whether it is Ga or As deficient at this point.

Lastly the twined ZB spots show elongation perpendicular to the Si 1x pattern. This is scattering from the {110} NW side facets. Hence we can assume that the facets are well formed and the NWs will be straight, and hence probably hexagonal.

All that is left to do is to cool the sample down and see what SEM images can tell us. To preserve the Ga droplet on the NW the sample was cooled to 100 °C at 200 °C/min. First the Ga and As shutters were closed simultaneously, then the As valve was immediately closed, then the sample cooled immediately thereafter. If the sample is held under an As flux, the Ga droplet is “consumed” and replaced with heavily faulted (possibly WZ) single crystalline GaAs.

The sample was cleaved along two parallel (111) planes and mounted for SEM imaging using silver paste. An edge on image is shown in Figure 2a with a tilted image shown in figure 2b. There is clearly a lot of parasitic growth, largely in the form of Ga droplets on the surface. Also the wires are very short. 0.11ML/s planar growth calibrated on GaAs(100) equates to around 800nm long NW after 30 minutes of growth. These wires are only 400nm long. The wires are also reverse tapered (ice-cream cone shaped) with a large Ga droplet on top. All these 4 points indicate that the As flux is too low.

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Figure 2: SEM images of GaAs NWs grown self-seeded on Si(111) epiready substrate

In order to perform a systematic study the As flux will be increased by 33% and the substrate temperature will be held constant for the 2nd NW sample. The results should be NW of a similar density but with 133% the length and similar degree of parasitic growth. You can see how the second sample turned in the following article: Nanowire:  Second NW sample.

3 thoughts on “Nanowires: First NW sample

  1. Pingback: Nanowires: Pre campaign calibrations | Dr. Faebian Bastiman

  2. Pingback: Nanowires: Second NW sample | Dr. Faebian Bastiman

  3. Pingback: Nanowires: Third NW sample | Dr. Faebian Bastiman

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