Nicholas E. Grant

Passivation of a (100) Silicon Surface by Silicon Dioxide Grown in Nitric Acid

This letter investigates silicon dioxide layers grown at low temperature in concentrated nitric acid using a two-step process developed by Imai <etal/> for thin-film transistors. With photoconductance measurements, we find that, prior to an anneal, nitric acid oxidation does not passivate the silicon surface, but, after a 30-min nitrogen anneal at 1100degC, a surface recombination velocity (SRV) of 107 cm/s (at Deltan = 1015 cm-3) is attained on 1-Omega ldr cm n-type silicon. The SRV is further decreased to 42 cm/s after a 30-min forming gas anneal (FGA) at 400degC, which is equivalent to a thermal oxide under similar annealing conditions, although it is not stable and returns to its pre-FGA state over time. Capacitance-voltage and photoconductance measurements suggest that the oxides contain a high positive fixed charge-particularly after a 1100degC N2 anneal-which aids the passivation of n-type and intrinsic silicon but harms the passivation of low-resistivity p-type silicon.

Grown-in defects limiting the bulk lifetime of p-type float-zone silicon wafers

This work has been supported by the Australian nRenewable Energy Agency (ARENA) fellowships program and nthe Australian Research Council (ARC) Future Fellowships nprogram.

Low Surface Recombination Velocity on (100) Silicon by Electrochemically Grown Silicon Dioxide Annealed at Low Temperature

This letter investigates silicon dioxide (SiO2) layers that are electrochemically grown in nitric acid (HNO3) at room temperature. It examines the dependence of surface recombination velocity (SRV), oxide charge, interface states, and oxide thickness on the concentration of HNO3. The results show that an SRV of less than 40 cm/s can be attained after SiO2 is annealed at 400°C in oxygen first and then forming gas. This SRV is similar to that attained by the best thermal oxides. Photoconductance and capacitance-voltage measurements indicate that the low SRV is caused by a large positive charge rather than a low interface state density. The SRV is found to degrade due to a decrease in charge and an increase in interface states, where the rate depends on the HNO3 concentration in which the SiO2 layer was grown.

Effect of a post-deposition anneal on Al 2 O 3 /Si interface properties

While Al<inf>2</inf>O<inf>3</inf> has been proven to provide an excellent level of surface passivation on all sorts of p-type doped silicon surfaces, the passivation mechanism of this layer and especially the influence of the post-deposition anneal on the Al<inf>2</inf>O<inf>3</inf>/Si interface properties is not yet completely understood. A great increase in the surface passivation is observed after a post-deposition anneal, i.e. a post-deposition anneal is mandatory to activate the surface passivation. Thus, the influence of this anneal on the interface properties, density of negative fixed charges Q<inf>f</inf> and density of interface traps D<inf>it</inf>, will be investigated and correlated to the measured minority carrier lifetime. In the case of plasma enhanced ALD, Q<inf>f</inf> is already high in the as-deposited state and the annealing process only has a minor effect on Q<inf>f</inf> (Q<inf>f</inf> increases by 20-50 %, depending on the annealing temperature). The D<inf>it</inf> however is strongly reduced by the post-deposition anneal, decreasing by two orders of magnitude. This large reduction in D<inf>it</inf> is a prerequisite for benefiting from the strong field effect induced by the high density of negative charges of the Al<inf>2</inf>O<inf>3</inf>.

Light-induced activation and deactivation of bulk defects in boron-doped float-zone silicon

In this paper, we present new insight in the degradation and subsequent recovery of charge carrier lifetime upon light soaking at 75u2009°C observed in float-zone silicon wafers. Variations of doping type, dielectric passivation schemes and thermal treatments after layer deposition were performed. The degradation was only observed for p-type float-zone silicon wafers passivated with passivation schemes involving silicon nitride layers. An influence of thermal treatments after deposition was found. N-type wafers did not degrade independent of their passivation scheme. Room temperature re-passivation experiments showed the degradation to affect the wafer bulk, and photoluminescence studies demonstrated fine lateral striations of effective lifetime. We conclude that the degradation is caused by bulk defects that might be related to hydrogen complexes.

Charge Density in Atmospheric Pressure Chemical Vapor Deposition TiO 2 on SiO 2 -Passivated Silicon

The charge density of a TiO 2 film deposited on a SiO 2 -passivated silicon wafer is determined. The TiO 2 is deposited by atmospheric pressure chemical vapor deposition at 400°C, and the Si0 2 is grown thermally at 950°C. This TiO 2 ―SiO 2 stack is a useful coating for the front surface of a silicon solar cell, as it has a high optical transmission and a low density of interface states D it (E) at the SiO 2 ―Si interface. While these properties are beneficial to high efficiency solar cells, so too is a large charge density, as what occurs in Si 3 N 4 ―SiO 2 (+10 12 cm ―2 ) and Al 2 O 3 ―SiO 2 (―10 13 cm ―2 ) stacks. The D it (E) and charge density of TiO 2 -coated and SiO 2 -passivated silicon are evaluated by capacitance―voltage and Kelvin probe measurements. The charge density of the TiO 2 is within the conservative limits of ―8.5 and ―1 × 10 11 cm ―2 after deposition and of ―10 and +1 × 10 11 cm ―2 after a subsequent 800°C oxygen anneal. Photoconductance measurements suggest that the dangling-bond defects at the SiO 2 ―Si interface are predominantly donorlike and, hence, that the change density in the TiO 2 is closer to the upper limits (less negative); this charge is too small to benefit solar cells.

Influence of Annealing and Bulk Hydrogenation on Lifetime-Limiting Defects in Nitrogen-Doped Floating Zone Silicon

A recombination active defect is found in as-grown high-purity floating zone n-type silicon wafers containing grown-in nitrogen. In order to identify the properties of the defect, injection-dependent minority carrier lifetime measurements, secondary ion mass spectroscopy measurements, and photoluminescence lifetime imaging are performed. The lateral recombination center distribution varies greatly in a radially symmetric way, while the nitrogen concentration remains constant. The defect is shown to be deactivated through high temperature annealing and hydrogenation. We suggest that a nitrogen-intrinsic point defect complex may be responsible for the observed recombination.

Quantifying the optical losses in back-contact solar cells

A procedure to quantify the optical loss mechanisms in back-contact solar cells is presented. It incorporates recent developments in optical simulation that yield rapid and precise results. The procedure includes spectrophotometry, ellipsometry, quantum efficiency measurements, ray tracing, and the thin-film matrix method. The paper shows how experiments and simulation can be combined to quantify reflection from the front surface, absorption in the antireflection coatings, non-ideal light trapping, and free-carrier absorption-all in terms of a `lost generation current. The procedure is demonstrated on a back-contact solar cell with single-layer and double-layer antireflection coatings. It is extendable to other cell structures.

Superacid Passivation of Crystalline Silicon Surfaces

The reduction of parasitic recombination processes commonly occurring within the silicon crystal and at its surfaces is of primary importance in crystalline silicon devices, particularly in photovoltaics. Here we explore a simple, room temperature treatment, involving a nonaqueous solution of the superacid bis(trifluoromethane)sulfonimide, to temporarily deactivate recombination centers at the surface. We show that this treatment leads to a significant enhancement in optoelectronic properties of the silicon wafer, attaining a level of surface passivation in line with state-of-the-art dielectric passivation films. Finally, we demonstrate its advantage as a bulk lifetime and process cleanliness monitor, establishing its compatibility with large area photoluminescence imaging in the process.

A Contactless Method for Determining the Carrier Mobility Sum in Silicon Wafers

In this paper, we present a new method to determine the simultaneous injection and temperature dependence of the sum of the majority and minority carrier mobilities in silicon wafers. The technique is based on combining transient and quasi-steady-state photoconductance measurements. It does not require a full device structure or contacting but only adequate surface passivation. The mobility dependence on both carrier injection level and temperature, as measured on several test samples, is discussed and compared with well-known mobility models. The potential of this method to measure the impact of dopant concentration, compensation ratio, injection level, and temperature on the mobility is demonstrated.