Modulation Doping of Silicon using Aluminium-induced Acceptor States in Silicon Dioxide

Abstract

All electronic, optoelectronic or photovoltaic applications of silicon depend on controlling majority charge carriers via doping with impurity atoms. Nanoscale silicon is omnipresent in fundamental research (quantum dots, nanowires) but also approached in future technology nodes of the microelectronics industry. In general, silicon nanovolumes, irrespective of their intended purpose, suffer from effects that impede conventional doping due to fundamental physical principles such as out-diffusion, statistics of small numbers, quantum- or dielectric confinement. In analogy to the concept of modulation doping, originally invented for III-V semiconductors, we demonstrate a heterostructure modulation doping method for silicon. Our approach utilizes a specific acceptor state of aluminium atoms in silicon dioxide to generate holes as majority carriers in adjacent silicon. By relocating the dopants from silicon to silicon dioxide, Si nanoscale doping problems are circumvented. In addition, the concept of aluminium-induced acceptor states for passivating hole selective tunnelling contacts as required for high-efficiency photovoltaics is presented and corroborated by first carrier lifetime and tunnelling current measurements.

Figure 1. DFT results on SiO₂ modulation doping with Al acceptors.

(a,b) Si₁₀ nanocrystal (cyan) in three ML SiO₂ (O is red, Si grey, H white) with Al atom (yellow) replacing Si in outermost SiO₂ shell, showing β-HOMO (a) and β-LUMO (b) as iso-density plots of 4 × 10⁻⁴ e/cubic Bohr radius. (c) Electronic DOS (blue, red) of that approximant (energy scale refers to vacuum level E_vac). DOS for pure SiO₂ embedding (cyan, orange) is shown for comparison along with the HOMO and LUMO of a 1.9 nm Si nanocrystal fully terminated with OH groups. The two possible MO spin orientations are noted by α and β due to unpaired electron configuration caused by the acceptor (doublet). (d) Band structures showing the principle of direct modulation doping for a Si nanocrystal (NC) in SiO₂ and (e) for Si bulk terminated with SiO₂:Al layer. Occupation of electronic states are described by chemical potential μ and Fermi level E_F, respectively.

Figure 2. Electronic characterization of SiO₂ modulation doping with Al acceptors using Al/SiO₂/Al-O/SiO₂/Si MOS structures.

(a) Sample structure showing Al modulation acceptors charged from Si substrate by electron tunnelling. (b) CV curves of reference sample Al0 (no Al-O), Al1 (1 ML Al-O) and Al2 (2 ML Al-O) measured at T = 300 K; ΔV_fb and ΔQ_fix due to negatively ionized Al shown by coloured arrows. (c) Band structure scheme of charge transient measurements for electron release [scheme I] and for electron capture [scheme II] by Al in DLTS. (d) Electron release of Al in SiO₂ with pulse time t_p = 205 μs, transient time T_W = 31 ms and pulse voltage V_p = +0.5 V as function of reference voltage V_R measured at T = 102 K to freeze out inelastic scattering and trap-assisted processes for maximum energy resolution by direct electron tunnelling into Si. (e) Electron capture with transient time T_W = 3.63 s, pulse voltage V_p = −4 V and reference voltage V_R = 0 V as function of pulse time t_p measured at T = 502 K to activate all transport paths (hopping, direct and trap assisted tunnelling) for maximum occupation probability of Al in SiO₂. Arrows show sub-peaks in accord with Al-O MLs. Capacitance per area scale changes from pF/cm² in (d) to nF/cm² in (e).

Figure 3. Effect of SiO₂:Al modulation doping on effective minority carrier (hole) lifetime and hole tunnelling current density.

(a) Double-side polished 1.6 Ωcm phosphorus-doped 525 μm Cz-Si wafers with RTO/Al-O/SiO₂ stacks on both sides (sample Al1) show more than 2 orders of magnitude higher lifetimes compared to the RTO/SiO₂ reference sample (Al0). The effective minority carrier lifetime of Al1 at an excess minority carrier concentration corresponding to 1 sun illumination (Δp = 10¹⁵ cm⁻³) is τ_hole = 1 ms. (b) Hole tunnelling current density under accumulation bias on boron-doped Cz-Si wafers with RTO/Al-O/SiO₂ stacks (3 nm total thickness). The Al-O monolayer in sample Al1 enables 1 order of magnitude higher hole current densities at small bias as compared to sample Al0 (RTO/SiO₂ reference sample).

Figure 4. Application examples for Si modulation doping.

(a,c) Undoped Si fin-FETs can be provided with holes as majority carriers from Al acceptors in the buried oxide layer forming the base of the fin (a), eliminating any size limit due to conventional doping. The band diagram of such fin-FET (c) shows its working principle as hole depletion (p self-conducting) transistor. (b,d) HIT Si solar cells can be equipped with massively enhanced hole contacts where SiO₂ is the optimum choice in terms of chemical bond interface passivation (b). Moreover, SiO₂ provides a much increased minority (electron) barrier while accelerating holes through a low tunnelling barrier thanks to negatively charged acceptors located about 0.5 eV below the valence band of c-Si. The band diagram of such a hole contact shows its working principle to provide much increased conversion efficiencies of HIT solar cells (d).