El derecho a la vida

For ease of reference, mean values for tfie impurity gradients deriving using the N(W) and A^/C^ vs V techniques are repeated in Table 5.3.

(100) Lower (cm "*) (100) Upper (cm'"*)

N(W); Double-sided 1 .6 2 x ic f3 2.03 X i c f 3

A^/C^ vs V 1.63 X ic f3 5.48 X 1Cf2 Table 5.3. Mean impurity gradient values for the (100) lateral junctions.

The main assumption used in the N(W) analysis technique was that the net doping concentrations on either side of junction were of equal magnitude. This could equally be the case for a linearly-graded junction, which possessed a smooth transition in doping type between symmetrically doped n- and p-type materials. Due to close agreement between the calculated mean impurity gradients for the lower junctions, it seems probable that this is the incorporation behaviour in the (100) lower junction region, i.e. a linear transition in doping type between symmetrically doped, uncompensated p-type (111)A and n-type (100) material. This conclusion suggests that the (100) material on the n-side of the lower junction is not compensated by Ga migration, as the doping levels on both sides of the junction are comparable and the (111)A material is uncompensated by the migration (Section 5.2).

In the case of the (100) upper junctions, the large deviation between the mean gradients values (3x) required further analysis on the C-V data. The main assumption used in deriving the N(W) gradients was that the doping distribution across the junction plane was symmetrical in nature. Ga migration in the upper junction region could have heavily compensated the n-type (411)A surface during growth. Silicon incorporation of this type was outlined in Section 5.2. The n-side of the upper junction would therefore exhibit a much lower net impurity concentration than the p-type (311)A material and the majority of depletion would occur within the (411)A material. This would render the (100) upper junction side-sided in nature i.e. p^-n and thus require the C-V data to be re­ analysed using equations (5.4)-(5.5).

Table 5.4. contains impurity gradient values calculated from single-sided N(W) and A^/C^ vs V analysis for the (100) upper junctions.

m63 Upper 1 (cm '') m63 Upper 2 (cm"'')

N(W); Single-sided 5 .74 X 10^^ 4 .8 7 X 10^^

A W vs V 5 .72 X 10“ 5.2 2 x 10^^

Table 5.4. Comparison of impurity gradients for the (100) upper lateral junctions Good agreement exists between the two sets of impurity gradients, strongly pointing towards the upper (100) junction possessing a single-sided nature. The impurity grade only existed on the n-side of the junction and the average dopant

density given by equation (5.4) was the donor density ( N d - N a ) present on the (411 )A surface. Impurity profiles deduced from the single-sided N(W) analysis are therefore donor impurity profiles on the (411)A surface and these are shown in the below figure for the selected (100) upper junctions:

- » - m 6 3 U p p e r 1 —* - m 6 3 U p p e r 2

1. 5 10® 1. 6 10®

D e p le tio n D e pth (cm )

Figure 5.8. Donor profiles on the (411) A surface for the (100) upper Junctions.

The donor concentration rises in an apparently linear fashion away from the junction plane. This supports the Ga migration compensation model proposed in Section 5.2 because the number of excess Ga atoms present on the (411)A surface due to migration from the (311)A surface would decrease in distance from the (411)A/(311)A surface boundary, as the migration process is diffusive in nature. If one assumes that the degree of n-type compensation is determined by the density of excess Ga present on the (411)A surface during growth, then the degree of n-type compensation would also therefore fall in distance away from the surface intersection, as is seen in practice. Compensation of the doped material on the p-side of the upper junction is not seen, due to the one-sided nature of the upper junction. Ga adatom density on the (311)A surface outside of the zero bias depletion region width must be sufficient to produce uncompensated p-type material.

Hall carrier densities measured for Sa12m63 were n(ioo) = Nd-Na = 2.4x1017 cm'^ and P(3h)a = Na-Nq = 1.9x10^^ cm'^, respectively (Chapter 4, Appendix A).

Doping concentrations derived from the N(W) analysis for both upper and lower junctions are considerably in-excess of these carrier concentrations (Figures 5.8 & 5.6(a)). A possible explanation for this doping behaviour is that, in the same manner as Ga adatoms, silicon adatoms preferentially migrate towards the lower and upper (100) surfaces. A build-up of incorporated silicon in the junction regions would occur, thus explaining the high doping levels seen in both junctions. Impurity profiles for both (100) junction types suggest a linear increase in doping concentration exists away from the junction plane. It is proposed that, distant from the junction regions, the silicon concentration levels decay back to those seen on the planar surfaces.

The C-V technique can only accurately characterise doped material outside of the zero bias depletion region width, since high forward bias currents prevent accurate measurement of the impurity profile within the volume depleted by the junction’s own built-in voltage. Using the conclusions outlined previously, dopant profiles for the (100) lower and upper junctions can be generated i.e. solid lines depicting the measured profiles and dotted lines the speculated concentration variations; (100) Lower Junction (100) Upper Junction (411)A n-type (311)A

p-type (111)A (_n-type100) p-type Hall density Hall density C -V profile C -V profile » X Hall density X Hall I density

Figure 5.9. Dopant distribution for the (100) lateral junctions.

Similar findings to ours were reported by Inai et al, in their paper on the electrical properties of p(111)A-n(311)A silicon-doped G a As lateral junctions [19]. They profiled the upper junction as it possessed a sharp demarcation, thus allowing accurate metallisation and found it to possess a linearly-graded structure. The authors proposed that the upper junction doped to similar levels on both sides of the junction, in the same manner as the (100) lower junctions. The difference in doping distribution with regards to junction location is due to the different combination of surfaces used in creating the lateral p-n junction devices i.e. ( I ll) A - ( 3 1 1 )A and (100)-(111 )A.

Takamori and Kamijoh, in their study of Si-doped (100) GaAs lateral p-n junctions, proposed that the upper junction was situated between the p-type (311)A facet and a heavily compensated n-type (411)A surface [16-17]. The upper (100) lateral junction therefore exhibited a p-n nature. They speculated that the lower (100) junction was formed between the p-type (311)A facet and n- type (100) surface, which doped to similar levels thus realising a p-n dopant distribution [16-17]. These dopant distributions are in accordance with our finding. However, they used electrical and EBIC measurements to reach their conclusions and did not measure the actual distributions by the C-V technique.

W e have reported the first Capacitance-Voltage profiling on (100) Si-doped GaAs lateral p-n junctions. It was demonstrated that the junction profile was dependant on the physical location on the facet, which was explained in terms of Ga adatom migration and dopant compensation in the junction regions during growth. It is proposed that an abrupt p'^-n' junction was formed at the upper junction, with heavy compensation of the n-type (411)A surface accounting for the difference in doping level across the junction interface. In the lower junction, it is thought that a linear transition existed between symmetrically doped p-type (111)A and n-type (100) surfaces, thus giving a linearly graded junction structure.

5.4

Summary

• Ga migration between different combinations of surfaces was discussed, with the development of minor facets at the surface boundaries highlighted.

• On the (100) substrate, a (411 )A surface was seen to grow at the upper (100)-(311)A intersection, while a (111)A surface developed at the lower flat- facet interface.

• A new (110) surface was observed at the upper (1 lO)-(IOO) boundary on the (110) patterned substrates. At the lower facet-flat boundary, the surface intersection point was seen to move along the (110) plane.

• Silicon incorporation in the vicinity of the junction regions was discussed, with dopant compensation from the Ga migration forming the main subject area. • Capacitance-Voltage profiles of the (100) upper and lower junctions were

presented. The (110) junctions possessed too high a reverse leakage current for accurate C-V measurements.

• Lower (100) junctions exhibited a linearly-graded transition between symmetrically doped n- (100) and p-type (111)A surfaces

• Upper (100) junctions possessed a single-sided (p'^-n') dopant distribution, with the (411)A surface being heavily compensated by Ga migration directed from the (311)A to (100) surface.

• The donor distribution on the (411)A surface was derived from the C-V data, with a linear increase in donor density evident away from the junction plane. This supported the Ga migration compensation picture, as the compensation reduced away from the actual junction.

• Silicon migration and subsequent build-up in the junction region was proposed to explain the high carrier densities measured in the junction region.

References for Chapter 5

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Chapter 6