Molecular Monolayer Doping Research Articles

De-carbonization of self-assembled molecular monolayers doping in silicon

Unintentional C-related contamination can be readily introduced into the substrate in self-assembled molecular monolayer doping. These C contaminants can bind with dopants, forming interstitial defects, which will in return electrically deactivate the dopants. This issue will exacerbate when the dopant concentration is low. In this paper, a low temperature oxidation method (550 °C for 30 min) is introduced to remove carbon before the phosphorus dopants are driven into silicon in a rapid thermal annealing process. The samples with and without the pre-oxidation process are characterized by the Van der Pauw, low-temperature Hall effect measurements, and secondary ion mass spectrometry analysis. The results indicate that the surface carbon concentration in silicon is nearly completely removed with the pre-oxidation process, as a result of which the electrical activity of phosphorus is indeed increased.

Full Activation of Dopants by Carbon-free Self-Assembled Molecular Monolayer Doping

Full Activation of Dopants by Carbon-free Self-Assembled Molecular Monolayer Doping

Sub-2 nm boron doping in silicon using novel ultra-thin SiO2 film produced by sol–gel dip coating as a capping layer

Advancements in semiconductor electronics and sensor technologies demand a more robust doping approach and process to fabricate ultra-shallow doping with abrupt depth, particularly on complex structured surfaces. Molecular monolayer doping (MLD) is a potential approach, but is limited by the difficulty in depositing a high-quality SiO2 capping film without damaging the dopant molecular layer. In this work, a relative humidity condition during deposition of the film in the sol–gel dip-coating process is found to produce continuous, dense and stoichiometric ultra-thin SiO2 films of quality equivalent to that produced by complicated thermal oxidation/etching and sophisticated chemical vapor deposition and atomic layer deposition methods. Applying these ultra-thin SiO2 films as capping layers in the molecular MLD method, B doping in Si with sub-2 nm effective depth and sub-5 nm abrupt depth is achieved. Remarkably, around 82%–86% of doped B atoms in Si are found to be electrically active as estimated from sheet resistance measurements. The established sol–gel dip coating conditions to deposit ultra-thin SiO2 films are generic and can be extended to produce high-quality ultra-thin films of other metal oxide materials required for advanced technological applications.

Silicon nanowire core–shell PN junction phototransistors by self-assembled monolayer doping

Nanoscale photoconductors often have extremely high gain in quantum efficiency but suffer from the difficulty to design the density of surface states that cause the high photogain. In this Letter, we created high-gain photoconductors by forming a core–shell PN junction in silicon nanowires via self-assembled molecular monolayer doping. The highly doped n-type shell deactivates all the surface states by filling with electrons so that the n-type shell as a well, instead of the surface states, captures and emits photogenerated minority electrons under ON/OFF light illumination. The corresponding excess majority holes are accumulated in the nanowire channel and thus modulate the channel width, resulting in the experimentally observed high photogain (∼108). The photoresponses of these phototransistors were systematically investigated as a function of the nanowire width and light illumination intensity. The results show that the nanowire channel is pinched off for the nanowires narrower than 73 nm due to the core–shell PN junction. We further derived analytical equations based on the PN junction device principle, finding the explicit gain equation that governs the photogain as a function of light intensity and other physical parameters of the nanowires. The explicit gain equations can fit well with the experimental data and allow us to design the core–shell nanowire phototransitors with desired performance.

Full Activation of Boron in Silicon Doped by Self-Assembled Molecular Monolayers

Self-assembled molecular monolayer (SAMM) doping has great potential in state-of-the-art nanoelectronics with unique features of atomically precision and nondestructive doping on complex 3D surface...

Dopant activation and photoresponses of boron-doped silicon by self-assembled molecular monolayers

Self-assembled molecular monolayer doping is an emerging doping technique. In this work, we investigated the activation rate and photoresponses of boron doped silicon by self-assembled molecular monolayer doping. By using low temperature Hall effect measurements and by secondary ion mass spectroscopy, we find that the activation rate of boron in these samples is in the range of 91%–54%, depending on the doping concentration. Interestingly, the photoresponsivity of the boron doped samples is also significantly higher than that of the phosphorus doped samples even though the same doping technique is used. The intriguing photoresponses are closely related to the trapping of photogenerated minority carriers by the defects in the p-type silicon.

Shallow Si N + P junction diodes realized via molecular monolayer doping

A conformal and controlled semiconductor doping is needed for applications in next generation nanoscale devices, low contact resistivity metal semiconductor junctions such as selective emitters in solar cells. Molecular monolayer doping (MLD) in silicon is a novel technique based on the formation of self-assembled monolayer of dopant – containing molecules on surface of crystalline silicon, followed by rapid thermal anneal. The technique is capable of forming ultra-shallow junctions with high atomic accuracy and minimum defects in silicon. A container and process was developed which successfully doped 6in. diameter silicon wafers using MLD for the in-house CMOS fabrication facility. The phosphorus monolayer is grafted on hydrogen terminated p-type silicon followed by rapid thermal anneal. Average sheet resistance ~670Ω/sq. and junction depth ~25nm are achieved. N+P junctions are fabricated using MLD and current-voltage characteristics are measured and analyzed using unified diode model.

Deep level transient spectroscopic investigation of phosphorus-doped silicon by self-assembled molecular monolayers

It is known that self-assembled molecular monolayer doping technique has the advantages of forming ultra-shallow junctions and introducing minimal defects in semiconductors. In this paper, we report however the formation of carbon-related defects in the molecular monolayer-doped silicon as detected by deep-level transient spectroscopy and low-temperature Hall measurements. The molecular monolayer doping process is performed by modifying silicon substrate with phosphorus-containing molecules and annealing at high temperature. The subsequent rapid thermal annealing drives phosphorus dopants along with carbon contaminants into the silicon substrate, resulting in a dramatic decrease of sheet resistance for the intrinsic silicon substrate. Low-temperature Hall measurements and secondary ion mass spectrometry indicate that phosphorus is the only electrically active dopant after the molecular monolayer doping. However, during this process, at least 20% of the phosphorus dopants are electrically deactivated. The deep-level transient spectroscopy shows that carbon-related defects are responsible for such deactivation.

Controlling the dopant dose in silicon by mixed-monolayer doping.

Molecular monolayer doping (MLD) presents an alternative to achieve doping of silicon in a nondestructive way and holds potential for realizing ultrashallow junctions and doping of nonplanar surfaces. Here, we report the mixing of dopant-containing alkenes with alkenes that lack this functionality at various ratios to control the dopant concentration in the resulting monolayer and concomitantly the dopant dose in the silicon substrate. The mixed monolayers were grafted onto hydrogen-terminated silicon using well-established hydrosilylation chemistry. Contact angle measurements, X-ray photon spectroscopy (XPS) on the boron-containing monolayers, and Auger electron spectroscopy on the phosphorus-containing monolayers show clear trends as a function of the dopant-containing alkene concentration. Dynamic secondary-ion mass spectroscopy (D-SIMS) and Van der Pauw resistance measurements on the in-diffused samples show an effective tuning of the doping concentration in silicon.

Behavior of phosphorous and contaminants from molecular doping combined with a conventional spike annealing method

The fabrication of future nanoscale semiconductor devices calls for precise placement of dopant atoms into their crystal lattice. Monolayer doping combined with a conventional spike annealing method provides a bottom-up approach potentially viable for large scale production. While the diffusion of the dopant was demonstrated at the start of the method, more sophisticated techniques are required in order to understand the diffusion, at the near surface, of P and contaminants such as C and O carried by the precursor, not readily accessible to direct time-of-flight secondary ion mass spectrometry measurements. By employing atom probe tomography, we report on the behavior of dopant and contaminants introduced by the molecular monolayer doping method into the first nanometers. The unwanted diffusion of C and O-related molecules is revealed and it is shown that for C and O it is limited to the first monolayers, where Si-C bonding formation is also observed, irrespective of the spike annealing temperature. From the perspective of large scale employment, our results suggest the benefits of adding a further process to the monolayer doping combined with spike annealing method, which consists of removing a sacrificial Si layer to eliminate contaminants.

Wafer-Scale, Sub-5 nm Junction Formation by Monolayer Doping and Conventional Spike Annealing

We report the formation of sub-5 nm ultrashallow junctions in 4 in. Si wafers enabled by the molecular monolayer doping of phosphorus and boron atoms and the use of conventional spike annealing. The junctions are characterized by secondary ion mass spectrometry and noncontact sheet resistance measurements. It is found that the majority ( approximately 70%) of the incorporated dopants are electrically active, therefore enabling a low sheet resistance for a given dopant areal dose. The wafer-scale uniformity is investigated and found to be limited by the temperature homogeneity of the spike anneal tool used in the experiments. Notably, minimal junction leakage currents (<1 microA/cm(2)) are observed that highlights the quality of the junctions formed by this process. The results clearly demonstrate the versatility and potency of the monolayer doping approach for enabling controlled, molecular-scale ultrashallow junction formation without introducing defects in the semiconductor.