Impurities in epitaxial layers

Both p and n type doping have been studied in InAlAs. The p dopants are Zn and C, where the source for the latter is the carbon tetrabromide. The n dopant is Si. The investigation on p doping is underway and there are so far only preliminary results.

In the case of n-doping, a linear dependence of the both the net charge and the free carrier concentrations with the silane flux during growth is observed. In fact the data obtained with Hall effect and CV measurements coincide. This indicates the absence of deep levels. No signs of saturation were observed up to a doping concentration of 5 x 10¹⁸ cm^-3.On the other hand, the PL spectra of the as-grown samples and also after subjecting them to a one hour anneal at 800ºC show an emission at 41 meV attributed to a free to bound transition involving the SiAs acceptor. The intensity of such an emission increases with the silane flux and with the heat treatment. Annealing the samples introduces As vacancies which can be eventually filled with Si atoms. A decrease in the net charge concentration is observed after heat-treatment. One concludes that an autocompensation effect is always present and that it can be optically detected. In addition, a SiAs ionization energy in lattice matched InAlAs of 60 meV was obtained with a tight-binding calculation. If one takes into account the wide spread of the available values of the hole effective mass in InAs, then the theoretical ionization energy is in good agreement with the experimental value of 41 meV.

Fig. 1 Dependence of the free carrier density (n) and net charge concentration (N) on the normalized input silane partial pressures measured respectively by Hall (squares) and CV (empty circles). The mobility (full circles) is also included.

The p-doping in InAlAs layers, where the dopant is C, was studied as a function of growth temperature. We have observed that for temperatures below 620°C the layers are p-type, while for growth temperatures above this value an n-type layer is obtained. This effect is related to the C amphotericity, the presence of passivating H atoms and the incorporation of deep levels. The concentration of such deep levels is directly correlated to the concentration of C atoms.

Fig. 2 Atomic, free carrier and net charge concentrations as a function of growth temperature. A transition from a p-type to an n-type material can be observed at 620°C.

Doping of C was also studied in AlGaAs as a function of the III/V ratio. By varying this parameter doping goes from intrinsic to residual doping. We found optimal growth conditions to obtain control over the doping level. A location of the C atoms in a region lower than 8 Å was obtained in the layers of planar doping AlGaAs.

Fig. 3 Net charge concentration as a function of CBr₄ flux for different values of the ratio III/V.

Fig. 4 Full width at half maximum (FWHM) of the charge distribution as a function of the 2D doping level. The solid curve represents a calculation assuming a localization of the impurities within an 8 Å thick region, equivalent to 70 % of the Bohr radius, a_B.