Tuesday, April 28, 2009

MBE Native Oxide Desorption

Native oxide desorption process depends on various conditions such as the following:

1 Temperature monitor calibration: including pyrometer and desorption mass spectroscopy (DMS); one has to take the viewport deposition, reflectance from heated effusion cells, etc. into consideration when measuring temperature using pyrometer, while the calibration of DMS is not easy, either.

2 The gas species used for overpressure: Basically, using the same group V species as the group V component of the substrate will results in lower desorption temperature [1-3].

2.1 For InP substrate, according to [1] under P2 overpressure the oxide (thermal oxide) desorbs within 5 min at 490oC (actually at 473±6oC the (2×4) surface is observed and the oxygen is below detection limit for the XPS measurement) and a (2×4) reconstruction is observed. This report suggests that under P2 overpressure, the appearance of the (2×4) reconstruction can be regarded as a sign for the complete desorption of the oxide. On the other hand, heating in a flux of As4 molecules results in the formation of an InAs layer at the top of the oxide, which strongly reduces the oxide desorption rate. The complete removal of the oxygen requires at least 520oC and typically leaves a 1-nm-thick overlayer of InAs. In this case RHEED gives no indication whether the surface is oxide free.

2.2 Authors in [2] suggest from In-P-O phase diagram that the UV-ozone oxide prepared on InP substrate should have a composition of InPO4. The (2×4) reconstruction emerges at the temperature of 528oC under P2 overpressure. They claim the removal of thin oxides from InP surface is probably not a matter of simple decomposition of the oxide but rather involves a reaction with the overpressure species or the substrate. The results suggest that the most probable reaction is with atomic phosphorus to form volatile phosphorus oxide, of which the most prevalent one would be P2O3, and atomic In as following formula:

3InPO4 + 5P 4P2O3 + 3In

The phosphorus from the substrate is the dominant driving force for the reaction and the reaction is both temperature and time dependent. During the desorption process there is a coexistence of areas covered by oxide and of some oxide-free regions which gradually increase in size until the oxide is desorbed. The phosphorus overpressure prevents decomposition of the oxide-free, exposed InP substrate surface. The free In atom from the reaction will evaporate rather than staying on the InP substrate. However, to the extent that a P2 overpressure does not entirely replace all the atomic P lost from the substrate, which is due to thermo-etching at overheated temperature, even desorption under a P2 overpressure would not result in a perfect interface between substrate and subsequent epilayer, namely In-ball will form.

2.3 In [3] As4 and/or Sb4 are used to protect the surface during InP desorption. The authors show that even if the deoxidation is feasible using one of the different beam pressures, the less critical temperature control leading to a good surface quality is obtained under (As4+Sb4) combined pressures. However, in my opinion, using different group V species when desorbing native oxides is not recommended and should be avoided.

3 Substrate temperature ramp rate: Some studies use very slow ramp rate (1oC/min) [2] while some studies use fast ramp rate (~50oC/min) [3]. The ramp rate should be established by oneself according to each system.

4 The composition and the thickness of the native oxide: Thermal oxide will have a lower desorption temperature than ozone plasma oxide [4], the thermal oxide grown on GaAs substrate has a desorption temperature of 582oC (in this report the temperatures were measured using a Pt/Pt13%Rh thermocouple and the thermocouple was calibrated at the melting point of aluminum: 660oC). Whereas ozone oxide has a desorption temperature of 638oC. The authors attribute the 56oC difference to the composition difference of the two oxides. And desorption temperature can be used as an accurate thermal reference point in the MBE growth of GaAs. As for thickness, the thinner, the better, in order to reduce the desorption time [2].


[1] R. Averbeck et al., “Oxide desorption from InP under stabilizing pressures of P2 and As4,” Appl. Phys. Lett., vol. 59, pp. 1732-1734, 1991.

[2] P. G. Hofstra et al., “Desorption of ultraviolet-ozone oxides from InP under phosphorus and arsenic overpressures,” J. Appl. Phys., vol 77, pp. 5167-5172, 1995.

[3] A. Godefroy et al., “X-ray and UV photoelectron spectroscopy of oxide desorption from InP under As4 and/or Sb4 overpressures: exchange reaction AsรณSb on InP surfaces, “ J. Crystal Growth, vol. 179, pp. 349-355, 1997.

[4] A. J. SpringThorpe et al., “Measurement of GaAs surface oxide desorption temperatures,” Appl. Phys. Lett., vol. 50, pp. 77-79, 1987.

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