Little known MBE facts: MBE crystal growth optimisation

Faebian Bastiman

In the first instance MBE appears to be a rather complicated affair involving pumps, LN2, valves, fluxes and vacuums. A high purity vacuum and effusion sources are essential and require great attention,  however when an MBE system is well maintained and operated the problem of MBE growth optimisation becomes trivially two dimensional: A III:V MBE grower ultimately only has control over two parameters (1) the growth temperature and (2) the III:V flux ratio.

Most miscible materials can therefore be optimised within less than 10 sample runs. Take GaAs/GaAs(100) as a thought exercise. A simple assessment of a direct band gap semiconductor like GaAs is a room temperature PL investigation. To differentiate between the epilayer and the substrate we will want to confine carriers and possibly shift the wavelength away from the ~872nm of bulk GaAs at RT. A suitable test structure is shown in figure 1 (below) with a RT PL of ~810nm.

The first thing to do is discern a favourable starting point. The oxide remove temperature is a good starting temperature for optimal GaAs growth (~580°C) and a flux ratio that gives a slightly weak 2×4 pattern and >20 RHEED oscillations before damping is a good flux. The flux ratio can also be estimated by (carefully and quickly) finding the dynamic (2×4)/(4×2) transition by dropping the As flux for a given growth rate. This corresponds to an As:Ga  of ~1:1. A good starting point is ~1.6:1, hence the As beam equivalent pressure of the 1:1 can be simply multiplied by 1.6.

Once these basic conditions have been found, a bench mark test structure can be obtained. Then the optimisation can begin. Increase and decrease the As flux by ~10-15% and increase and decrease the growth temperature by 10-20°C. Which of these 5 is the best? Which of these 5 gives the brightest PL with the narrowest FWHM? The cycle can then be repeated until no further improvement in RT PL is observed.

A myriad of III-Vs can be optimised either directly (by RT PL) or indirectly (in the case of AlGaAs) by the effect they have on RT PL of the confined layer. The only encountered notable exception is GaAsBi. Where the miscibility, ordering and phase separation are just some of the added complexity. GaAsBi is actually an excellent system to study as it possesses every complication to MBE growth. The idea being once you can grow bismide you can grow anything.

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