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The effect of short term anneals on the cathodoluminescence of GaAs Third, Christine Elizabeth


This study examines the effect of furnace annealing and rapid thermal annealing (RTA) on the room-temperature cathodoluminescence (CL) image of liquid encapsulated Czochralski (LEC) GaAs substrates. Furnace annealed samples were heated in a tube furnace for 20 minutes at temperatures from 600 to 950 °C. RTA annealed samples were heated in a commercial RTA furnace for 5 s at temperatures from 650 to 950 °C. The times used for both methods are typical of those used for post-ion implantation annealing although selected samples were RTA annealed at times from 10 to 160 s. The temperature range examined has been extended beyond typical post-ion implantation anneal temperatures to investigate the effect of temperature on the substrate. Examination of RTA annealed GaAs using CL has not been reported previous to this investigation. The CL images of the annealed samples are compared with those of the as-received (un-annealed) material. The CL images of the LEC GaAs wafers prior to annealing have dark spots which correspond to the location of dislocations with regions of higher CL intensity surrounding them. These regions of higher CL intensity are referred to as 'halos'. The remaining material has a CL intensity lower than these halos. The dislocations in LEC GaAs form into cellular networks to reduce the strain energy in the crystal. When viewed at low magnification the overlap of the halos makes the cell walls appear bright and the cell interiors appear dark in a CL image. The furnace annealed substrates show an increase in CL intensity in the interior of the cells. The halos are still present at the cell walls but a region of low CL intensity persists outside the halos making the cell walls appear dark with a brighter interior. This behaviour was seen in all the furnace annealed material although the contrast decreases with increasing anneal temperature. The RTA anneal samples show similar behaviour to the furnace annealed samples at temperatures below 800 °C. Above this temperature the halos are no longer noticeable in the surface CL images, although the region of lower CL intensity can be seen along some cell walls. When a cleaved cross-section is examined using CL, there are regions of higher CL intensity adjacent to both surfaces. These regions typically extend from 100 to 200 μm in from the surface and are nearly uniform in depth. The centre region of the sample appeared the same as the as-received material with dislocation spots, surrounding halos and low CL intensity in the interiors of the cells. The bright regions seen in the cross-section CL images of the RTA samples were examined using photoluminescence at liquid helium temperatures. This investigation found a correlation between the bright regions and the presence of Cu. In addition, the amount of residual Cu on the surface has a significant affect on the depth of the bright bands. Low residual Cu levels results in shallower band depths than high residual Cu levels. It is proposed that the presence of the Cu acts to increase the recombination rate thus increasing the brightness of the CL image.

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