Background doping and memory effect

To evaluate the contribution to epilayer carrier density coming from a constant background doping, intrinsic silicon layers have been grown on silicon substrates of different resistivity. To exclude any contamination from parasitic deposits on the quartz carrier or quartz tube these parts were wet chemically cleaned and baked in hydrogen before the depositions were carried out. Quartz carrier and dummy wafers which were used for the experiment were capped with an intrinsic silicon CVD layer.

5.3 Experimental carrier concentration profiles 69

The test wafers were used in the order of increasing doping level to exclude any memory effects from proceeding samples. Figure 5.6 shows the measured carrier concentration profiles for intrinsic epilayers grown on different silicon substrates.

All curves in Figure 5.6 (left) show the same characteristic: the carrier concentration profiles consist of a steep drop until a concentration of approximately 3x10¹⁴ cm^-3 is reached, followed by a more gradual decrease and a dip before the final constant doping level is approached. For each substrate type the minimum carrier density in the dip is located at 6-7x10¹² cm^-3 and the final epilayer doping concentration reaches a constant value of 1x10¹⁴ cm^-3. The whole transition region from substrate until the constant part of the profile begins, expands over 7-8 µm for all depositions (Figure 5.6, right). The on-substrate steep decrease is not present for the substrate of 100 Ωcm resistivity.

25 30 35 40 45 50

Figure 5.6: Carrier concentration profiles of intrinsic epilayers deposited on silicon substrates of different resistivity.

The independence of the measured profiles on the resistivity of the substrates indicates a negligible dopant contribution from the backside or edges of the samples. All epitaxial layers are p-type with a carrier concentration of 1x10¹⁴ cm^-3, representing the background doping level. Interpolating the empirical function given in eqn. (5.5), a diborane concentration of 16 ppb is necessary to produce this doping concentration. A contamination through impurities present in the trichlorosilane can be excluded since the trichlorosilane which has been used, is of electronic grade purity with a specified boron content below 0.1 ppba. Other sources for boron contamination may be the quartz carrier, the surrounding quartz tube or the gas system.

Compared to the schematic doping profile illustrated in Figure 5.3, the experimental curves show a distinct dip in carrier concentration within the transition region. The interpretation of this characteristic is not straightforward and will be discussed in the next section.

To extract the influence of system memory effects on the carrier concentration of epilayers, two depositions with a different conditioning process of the surrounding carrier and dummy wafers were carried out. In the first case, an intrinsic pre-deposition was applied while in the second experiment the reaction chamber was conditioned by depositing a doped silicon layer. The resulting carrier concentration profiles are shown in Figure 5.7.

zoom

The sample with intrinsic pre-deposition shows the same characteristics as in Figure 5.6 (right). The conditioning of the reaction chamber by a doped silicon layer (doped predep.) resulted in a carrier density profile with on-substrate peak. The peak doping density was determined to 6x10¹⁵ cm^-3. The formation of the peak is assumed to result from a diffusion of adsorbed boron atoms from the surface into the bulk of the sample during the heating-ramp and the prebake (see section 5.3.1) i.e. before the deposition process begins. The adsorbed boron atoms are provided from boron evaporation of the doped surrounding. The final shape of the peak is determined by solid state diffusion of boron from the peak into the lower doped neighboring regions during the deposition process and the following high-temperature purge and annealing step.

10 12 14 16 18 20 22 24

10¹² 10¹³ 10¹⁴ 10¹⁵ 10¹⁶ 10¹⁷

Carrier Density [cm-3 ]

Depth [µm]

Reactor conditioning intrinsic predep.

doped predep.

Figure 5.7: Influence of memory effect on the carrier concentration profiles of intrinsic epilayers.

To evaluate the impact of gas phase autodoping caused by the residence time of the dopant gas in the reactor system, a high-low doping deposition process was carried out where the growth of a low resistivity epilayer was directly followed by the deposition of an intrinsic layer. The resulting carrier concentration profile of the entire epilayer and the corresponding setting of the diborane gas flow rate are illustrated in Figure 5.8.

The abrupt change in diborane gas flow results in an immediate drop in carrier density, followed by a steady decrease. Within the deposition time of 2 min, the final steady state featuring a constant doping concentration of 1x10¹⁴ cm^-3 was not reached.

The residence time of dopant gas in the reaction volume and in the gas lines determine the transition of the high-low junction. These parameters are intrinsic characteristics of each deposition system, depending on reactor geometry, gas inlet and gas system. The residence time of the process gas in the RTCVD100 reaction chamber is assumed to be comparatively short, due to the high total gas flow rates and the associated high gas velocities in the range of several 10 cm/s. The gradual decrease in carrier density is supposed to be mainly related to the design of the gas system (see section 5.3.4).

After closing the diborane valve, the diborane gas line is still filled with dopant gas which slowly diffuses into the main gas line feeding the reactor.

5.3 Experimental carrier concentration profiles 71

5 10 15 20 25

10¹⁵ 10¹⁶ 10¹⁷ 10¹⁸ 0 2 4 6 8 10

Carrier Density [cm-3 ]

Depth [µm]

B 2H 6 [sccm]

Figure 5.8: Diborane gas flow rate set for a high-low doping deposition process (top) and corresponding carrier density profile (bottom).

For the preparation of epitaxial silicon thin-film solar cells, epilayers with carrier concentrations in the range of 4x10¹⁶-1x10¹⁷ cm^-3 are deposited onto highly doped substrates. Compared to these concentrations, the observed background and gas phase autodoping and memory effect are negligible.