Phosphorus ionization in silicon doped by self-assembled macromolecular monolayers

Individual dopant atoms can be potentially controlled at large scale by the self-assembly of macromolecular dopant carriers. However, low concentration phosphorus dopants often suffer from a low ionization rate due to defects and impurities introduced by the carrier molecules. In this work, we demonstrated a nitrogen-free macromolecule doping technique and investigated the phosphorus ionization process by low temperature Hall effect measurements. It was found that the phosphorus dopants diffused into the silicon bulk are in nearly full ionization. However, the electrons ionized from the phosphorus dopants are mostly trapped by deep level defects that are likely carbon interstitials.

The precise control of individual dopants at arbitrary location is the key to developing atomic scale devices.Hydrogen lithography by scanning tunneling microscopy (STM) 1 and single ion implantation technique 2 have been demonstrated to manipulate single dopant atoms.However, these techniques are time-consuming serial processes and difficult to control atoms at large scale.Previously, we proposed to utilize the self-assembly of macromolecular dopant carriers to control single dopant atoms at large scale. 34][5] Without a high activation rate, individual dopants, although controlled at large scale, will not function properly in electronic devices.Nitrogen is one of the main sources that lower the activation rate of phosphorus dopants. 6We have shown that nitrogen introduced by the carrier molecules or coupling reagents will significantly deactivate the P dopants in silicon. 3,5n this work, we demonstrated a nitrogen-free macromolecule doping technique and investigated the phosphorus ionization process.Hyperbranched polyglycerols (hbPGs) that do not contain nitrogen were first synthesized.Each hbPG macromolecule carries one phosphorus atom.The hbPGs molecules were then grafted onto H-terminated silicon surfaces without using N-containing coupling reagents.Secondary ion mass spectroscopy and low temperature Hall effect measurements were employed to analyze the sample.It was found that the phosphorus dopants diffused into the silicon bulk are in nearly full ionization.However, the electrons ionized from the P dopants are mostly trapped by deep level defects that are likely carbon interstitials, resulting in a low nominal ionization rate of phosphorus dopants.
Hyperbranched polyglycerols (hbPGs) were synthesized by anionic ring-opening multibranching polymerization 7 of glycerols (J&K Scientific) with diphenyl-phosphinyl hydroquinone (TCI (Shanghai) Development Co., Ltd) as the core to initiate the reaction.The monomer/core molar ratio is set at ∼210 to produce high molecular weight hbPGs.After purified by dialysis bag (molecular weight cut-off > 50 kDa) in DI water, hbPGs were characterized by nuclear magnetic resonance (NMR) techniques ( 1 H NMR and 13 C NMR) and dynamic light scattering (see the results in our previous work 3 ).The results indicate that the number average molecular weight of hbPGs is approximately 84000 g/mol and the diameter of globular hbPGs is ∼11 nm. a To whom correspondence should be addressed: yaping.dan@sjtu.edu.cn.The doping protocol for the self-assembled macromolecule monolayers is depicted in Fig. 1.It starts with a silicon wafer cleaned by Piranha solution and is then followed by HF etching to obtain H-terminated silicon surfaces.To graft functional molecular groups on the H-terminated silicon surfaces, coupling reagents that contain nitrogen (e.g.[10][11] From this potential source, nitrogen contamination might be introduced into the silicon samples, resulting in significant electrical deactivation of P dopants as we observed previously. 3,6Here, we adopted a nitrogen-free coupling strategy in which hbPGs molecules (dissolved in methanol) were directly grafted onto an intrinsic silicon wafer (10000 Ω•cm, Institute of Electronic Materials Technology) in Ar ambient.After grafting, X-ray photoelectron spectroscopy (XPS) was employed to analyze the sample surface.The results show that a major carbon peak in the C1s scan (Fig. 2a) at 286.6 eV is observed, which is attributed to the etheric carbon. 12This observation confirms that the hbPGs were successfully immobilized to the silicon surface.To analyze the coverage of hbPGs on the silicon surface, atomic force microscopy (AFM) was applied to investigate the surface morphology (Fig. 2b).The AFM image indicates that a dense hbPGs film is formed on the silicon surface.
After the analysis, the hbPGs-modified silicon surface was then coated with spin-on-glass (SOG IC1-200, Futurrex Inc.) to form a 200 nm thick SiO 2 capping layer.The purpose of the capping layer is to prevent possible external contamination in the rapid thermal annealing (RTA) process (1050 • C, 30 s), during which phosphorus dopants were diffused into the silicon wafer.In the end, the SiO 2 capping layer was removed in buffered oxide etch (BOE) to obtain oxide-free Si surfaces for secondary ion mass spectroscopic (SIMS) analysis and low temperature Hall effect measurements.
To quantify the P dopants in silicon, SIMS was performed to profile the distribution of P dopants, as shown in Fig. 2c.The first few points near the surface are generally not accurate due to the inherent technical issue of SIMS.The P concentration starts from ∼8×10 16 cm -3 a few nanometers below the surface and exponentially drops to ∼ 10 15 cm -3 within ∼ 80 nm below the surface.By integrating all the P dopants in this thin layer, we found that the area concentration of phosphorus is 1.97×10 11 cm -2 .We will discuss the carbon profiles shown in Fig. 2d later.
To analyze the electrical activities of the phosphorus dopants in the sample, low temperature Hall measurements were conducted in a physical property measurement system (Quantum Design EverCool-II).As shown in Fig. 3a, the Hall resistance is linearly dependent on the magnetic field at room temperature but the dependence becomes increasingly nonlinear at lower temperature.The nonlinearity is caused by the magnetoresistance 13,14 of the sample.The actual Hall resistance was extracted following the method that we previously reported. 6The electron concentration per unit area obtained from Hall measurements is plotted as a function of temperature in Fig. 3b.In Fig. 3b, the electron concentration of the doped silicon sample decreases from ∼5.25×10 10 cm -2 at 300 K to ∼2.48×10 9 cm -2 at 200K, approximately a decrease of 21 folds in electron concentration.The P dopants in silicon have an energy level of 45 meV below the conduction band.The ionization rate of P dopants will decline at most 2.4 times when the temperature is lowered from 300K to 200K.Clearly the ionization process in the doped silicon is dominated by some deep energy level dopants or defects.The chemicals used in the synthesis are of CMOS grade.A blank intrinsic silicon wafer does not show any significant change in electrical conductivity after going through the whole process except the surface modification of hbPGs monolayers.Therefore, any detectable concentration of extrinsic dopants is not possible.Using deep level transient spectroscopy, we recently detected a fairly high concentration of carbon-related defects introduced into silicon by the self-assembled molecular monolayer. 15These carbon-related defects not only capture electrons ionized from the P dopants  but also electrically deactivate part of P dopants by forming interstitial carbon and substitutional P (C i -P s ) pairs.Indeed, we observed a relatively high concentration of carbon impurities introduced by the molecular monolayer as shown in Fig. 2d.The average areal concentration of carbon impurities is ∼7×10 12 cm -2 in the first 80nm thick layer where most phosphorus dopants were located.As a result, the ionization process in Fig. 3b is dominated by deep level carbon-related defects in addition to the electrically active P dopants.
If we assume that only deep level donor-type dopants (C i -P s pairs for example) dominantly contribute to the electrical activity observed in Fig. 3b, the extracted concentration for this type of dopants is significantly larger than the total concentration of phosphorus profiled by SIMS in Fig. 2c.As a result, it is not possible that C i -P s pairs as donor-type defects play a dominant role here.It is more likely that deep level acceptor-type defects (carbon interstitials) and shallow donor-type dopants (P) are playing the main role.In this case, the electron concentration is governed by the following equation (1).
where n e is the electron concentration in the conduction band, N d the electrically active dopant concentration, N t the defect concentration, E F the Fermi energy level, E d the donor energy level, E t the defect energy level, k the Boltzmann constant and T the absolute temperature.
The above analysis on the electron concentration as a function of temperature in Fig. 3b indicates that the deep level defects dominate the ionization process between 200K and 300K.Therefore, the temperature dependence of the P ionization plays a negligible role here.For simplicity, we can assume that the P dopants always remain complete ionization.The temperature dependent term exp E t −E F kT can be rewritten as exp n e and (E C E t ) is defined as the activation energy ∆E t of the defects.In the end, eq.( 1) can be rewritten as eq.( 2) in the following form.
where N C is the effective density of states associated with the conduction band (N c ≈ w * kT 3/2 , and w is the corresponding constant multiplied with the integration length which is 80 nm in our case).Eq. ( 2) is a quadratic equation, the real positive solution of which is as following: By fitting Eq. ( 3) into the data in Fig. 3b, we find the defect concentration, defect activation energy and the concentration of electrically active P dopants as listed in Table I.
The SIMS data indicate that the P concentration in the bulk is 1.97×10 11 cm -3 , which is surprisingly close to the extracted concentration for the electrically active P dopants 1.98×10 11 cm -3 (Table I).This is unlikely a coincidence.It implies that the P dopants diffused into the silicon bulk are almost all electrically active.This is possible, because the P dopants in this work is ∼10 16 cm -3 or lower, meaning that the average distance between P atoms is 46 nm or more.Fig. 2d clearly shows that the carbon concentration within the diffusion distance of P dopants is ∼ 10 18 cm -3 .The average distance between carbon atoms is ∼10 nm.The chances that carbon binds with phosphorus or carbon are low, meaning that carbon atoms are most likely in substitutional and interstitial forms.A certain portion of phosphorus may bind with carbon, forming deep level donor-type C i -P s defects.But this portion must be small because the fitting results do not support a dominant role of C i -P s defects.What's more, we previously observed 15 that only 20% of all P dopants can form C i -P s pairs with the interstitial carbon in a circumstance that the concentration of P dopants and C impurities are similarly high at ∼10 18 cm -3 .A lower concentration of P dopants in this work shall result in at most the same or likely a smaller probability for P to bind with C. At room temperature, the electron concentration is ∼ 5.25×10 10 cm -2 .If compared to the phosphorus SIMS data, it appears that the nominal activation rate is only 26.7% (0.525/1.97).But the above Hall effect analysis indicates that the P dopants are in fact nearly completely ionized.This seemingly contradiction is due to the fact that the electrons ionized from the P dopants are mostly trapped by the acceptor-type of defects that has a concentration of 1.05×10 12 cm -2 (Table I).The defect energy level is located at 133.56 meV below the conduction band.Although it is a single energy level, it is more likely a combination effect of multiple defect energy levels.The defects are probably standing alone carbon interstitials 16 and unlikely the C i -P s pairs that all have defect energy levels larger than 250 meV. 17This is in line with the above analysis on the average atom distance and the finding that P dopants in the silicon bulk are in nearly full ionization.Fig. 2d shows that carbon impurities have diffused much farther into the bulk (500 nm) than phosphorus dopants (80 nm).The carbon interstitials at deep locations will not get involved in the electron trapping because the built-in electric field due to the separation of electrons and phosphorus ions will confine most electrons near the P dopants.As a result, it is the carbon impurities within the first ∼80 nm layer (∼7×10 12 cm -2 ) that contribute to the electrically active carbon interstitials (∼1.05×10 12 cm -2 from Table I).It means that ∼15% of carbon impurities are interstitial and the rest ∼85% are substitutional.
The carbon interstitials will distort the electronic activity of phosphorus dopants by trapping electrons.As a result, electronic devices based single dopants will not function properly unless the concentration of carbon impurities introduced by the molecular monolayers is significantly reduced.One possible approach is to anneal the sample in high-purity oxygen at appropriate temperature (500 • C for instance) that is high enough to oxidize carbon into gaseous CO 2 and CO, but not too high to diffuse carbon impurities into the silicon substrate.After carbon is removed, rapid thermal annealing is then applied to diffuse the desired dopants into the substrate.Without carbon impurities, devices based on single dopants are expected to function properly.
In conclusion, we developed a nitrogen-free monolayer doping technique by self-assembled hbPGs molecules.From the SIMS analysis and low temperature Hall effect measurements, we found that the P dopants are in nearly full ionization.Most of the electrons excited from the P dopants are trapped by acceptor-type of defects that are likely carbon interstitials.Removing the carbon-related defects is the key to the development of a defect-free doping technique by self-assembled molecular monolayers.

FIG. 2
FIG. 2. a) XPS narrow scan of C1s spectra for hbPGs-modified silicon surfaces.The main peak at 286.6 eV is attributed to etheric carbons (C-(O)), and the side shoulder at 285.0 eV is assigned to exogenous carbon contamination (C-C) from atmosphere.b) AFM image for the surface morphology of hbPGs-modified silicon surfaces.c) Phosphorus profile in silicon by performing SIMS twice (black and red).Inset: Phosphorus profile in log scale.d) Carbon profile in silicon by SIMS for doped (black) and blank (red) sample that are cleaned by oxygen plasma right before SIMS profiling.

FIG. 3
FIG. 3. a) Hall resistance dependence on magnetic field at different temperature (from 200K to 300K).b) Charge carrier concentration per unit area as a function of temperature in kT.Red curve is the fitting curve of eq.(3).Inset: Diagram showing ionization and trapping process.

TABLE I .
Extracted parameters by fitting eq.(3) to Fig.3b.Note: R-square is equal to 0.999.