Little known MBE facts: Making a static reconstruction map

Faebian Bastiman

A static reconstruction map is one of the most useful items in your MBE arsenal. The map charts RHEED reconstructions against group V flux and applied power (or inferred temperature). Most reconstructions are stable over a wide set of temperatures and fluxes, however luckily for us they undergo very abrupt transitions. The map can enable you to return to any flux-temperature reference point with high accuracy even in the absence of any knowledge of the actual absolute temperature. However, with suitable inferences the absolute temperature can be stated to within ±10 °C in most cases.

The actual map is different for every substrate and depends on the number of reconstructions. Undoped GaAs is a good substrate to start with since it has 4 static reconstructions in the flux-temperature range of interest to an MBE grower: i.e. 300 – 650°C. Note that doped GaAs absorbs thermal energy more readily than undoped GaAs, which means for a given applied power doped GaAs is always hotter than undoped. Thus the map must be repeated for the doped and undoped version. That is true for all substrates, though is less obvious for those with narrow band gaps.

The map itself is generated in a several stages. It begins with defining your parameters. For GaAs this range of As fluxes (0 – 300 mil of movement in the As cracker’s needle valve in my case) and the range of your heater power (0 – 100W in my case). The second stage involves gathering flux data. A monitoring ion gauge (MIG) can be used to record the beam equivalent pressure (BEP) for each As needle valve position (see table for example data and ranges). The BEP needs converting into system independent ML/sGaAs or preferably atoms/nm2/s utilising the method in Little known MBE facts: flux determination.

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The Omicron MBE-STM system has no reliable thermocouple so the heater power is used in place of temperature in the first instance. Most MBE systems possess a thermocouple located behind the heater element that accurately tracks the temperature at a fixed position from the heater element. However both form a suitable reference and either can be used.

The third stage is sample preparation. Remove the oxide as described in Little known MBE facts: oxide removal and deposit enough material (50 nm is sufficient) to create a clear (2×4) reconstruction. This can then be annealed under a lower As flux (0.5 ML/s at normal growth temperature is sufficient) to achieve a flat surface. Cool the sample until a strong c(4×4) is observed and then (with the As flux at 1ML/s) switch off the heater power and periodically check the RHEED until an amorphous pattern is observed and all 1x spots have vanished. You have just deposited an As cap. Once the RHEED is amorphous the As valve can be fully closed. A nominal heating power can be supplied to prevent the manipulator freezing in the presence of the LN2 cooling 0.2W in my case. This is not essential but certainly preferable.  Just make sure the nominal heating does not desorb the cap! Once all As has been purged from the system (typically several hours) the next stage can begin.

The fourth stage is data gathering. Create a 2D table with As flux on the y axis and heater power on the x axis. With no As flux, start heating the sample in small steps (0.05A in my case). Watch for the As cap desorbing with the RHEED and a c(4×4) RHEED pattern emerging. Mark it down in the table. This represents 300±10°C.  Continuing with no As flux, continue incrementing the heater power/current. C(4×4) is stable with no As flux until 400±10°C, at which point on the [-110] azimuth the 2x near the top will transform into a 4x pattern. This is caused because the patches of (2×4) coexist with the c(4×4) on the surface. The is the c(4×4)/(2×4) transition, mark it as “mix” for short. Continue with no As flux until (2×4) replaces the c(4×4). The (2×4) will remain until around 475±25°C where a (nx6) pattern emerges. The pattern is weak and it is unclear whether n is 3 or 4. At this point, to avoid damaging the sample, open the As to the first position (10 mil in my case). The surface should immediately become (2×4) once more.

At this point keep the heater power fixed and increase the As flux in small steps. The (2×4) will mix once more at an As flux of ~0.8ML/s and will thereafter give way to c(4×4). Continue to chart out the range of fluxes and temperature and eventually the data should resemble the table below.

The fifth and final stage involves interpretation of the raw data. We have already marked 300, 400 and 500°C with some error bar. The earlier oxide remove power can be marked as 600±20°C. Plotting the power vs temperature allows the other temperatures to be extracted from a line of best fit. The temperatures from ~400 to ~540°C can now be readily located with a known As flux by utilising the c(4×4)/(2×4) “mix” transition. If your aim is publication or dissemination you can now create a graphical plot of your data, though for personal use the table retains greater fidelity.

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4 thoughts on “Little known MBE facts: Making a static reconstruction map

  1. Pingback: Little known MBE facts: Flux determination | Dr. Faebian Bastiman

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

  3. Pingback: How to growth your first sample: Oxide remove | Dr. Faebian Bastiman

  4. Pingback: How to grow your first sample: As capping | Dr. Faebian Bastiman

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