Homogeneity of Thermally-Annealed Lightly Fe-Doped SI InP
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concentration of electrically active Fe close to 106 cm- is needed in order to get semi-
insulating material. However the amount of iron to be added to the starting polycrystals is much higher (>1019 cm-3) since the distribution coefficient of iron is very small (KFe = 0.001). Because of this low coefficient, Fe results inhomogeneously distributed in the crystal: growth striations and concentration gradients are typical of Fe-doped InP [1]. Furthermore, extended defects, like dislocations, are involved in the fluctuation of dopant distribution and electrical compensation [2]. Finally, it appears that a nonnegligible fraction of the incorporated iron is not electrically active [3]. All these factors imply that the concentration of Fe in the substrates is higher than the one strictly necessary for the compensation of residual donors and this may adversely affect the electrical properties of the epilayers grown on those substrate, as the Fe in excess can diffuse from the substrate to the epilayers. Therefore any attempt to reduce the iron content, while maintaining good electrical properties, can be seen as a substantial contribution towards the improvement of semi-insulating InP substrates. Among the different attempts, the thermal treatment of semiconducting InP with a small Fe content, i.e. below the threshold necessary to achieve as-grown Sl material, seems very promising: the resistivity increases above 10 7 Qcm and the mobility approaches 4000 cm 2/Vs [4]. In the present paper, the characterization of InP made semi-insulating by thermal annealing is further extended in order to assess its homogeneity and its suitability as substrate for electronic applications. Some semiconducting InP samples with low iron content (3-4x10 15cm 3 ) have been annealed at 900 0C for 60 hours in a 111
Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
phosphorus ambient. The resistivity increased several orders of magnitude up to the semiinsulating range (_107 Qcm) after annealing. An increase of the mobility was also observed. The homogeneity of the iron distribution was studied by Scanning Photocurrent (SPC) and Scanning Photoluminescence (SPL) techniques. EXPERIMENTAL PROCEDURE AND RESULTS Two lightly iron doped InP samples were taken from the upper part of the same InP crystal, about 3 cm from the seed. The samples were initially semiconducting (see Table I) and were submitted to the same thermal treatment, except for the cooling rate. The annealing was carried out under 2 bar phosphorous pressure, at a temperature of 900°C for 60 hours. The cooling rate was 30°C/h for the sample R34 and 50°C/h for the R37. The results of Hall effect measurements after annealing are shown in Table I. It can be clearly observed that the resistivity of both samples increased several orders of magnitude after the annealing, reaching the 10 7 Qcm range. The Hall mobility resulted improved: in sample R37 it increased by a factor of two whereas in sample R34 the increase was less than 30%. It is important to note that the mobilit
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