Nanoassembler Path - Diamond Direct-Doped Growth
This section contains information on various potential mechanisms and technology for growing diamond-like material with dopants. Part of the Nanoassembler top-level project.
- 1 Growth Mechanisms
- 1.1 Existing CVD Methods
- 1.2 Our Growth Methods
- 1.2.1 Proposed Surface Termination/Passivation Stripping Methods
- 1.2.2 Proposed Substrate Conductivity Mechanisms
- 1.2.3 Proposed Surface Growth Mechanisms
Existing common methods to grow simple (nanostructure-free) diamond films for scientific purposes primarily involve the use of Chemical Vapour Deposition of various forms, often involving microwave assistance or other forms of energisation of the relevant gases. There are other, less relevant methods (like high-pressure diamond construction) that are used for commercial synthesis, but the primary mechanism we care about are ones that can extend a surface in a controlled manner.
Typically, for diamond growth, the CVD involves a hydrocarbon gas of some kind plus hydrogen (a large amount) which supposedly avoids the formation of graphene (though there is some questions on this, in some science papers I'm not allowed to link yet - might not be necessary for what we're doing).
Existing CVD Methods
The wikipedia page on CVD is a good resource on various existing CVD methods. There are older and newer methods - older ones tend to require more heat overall, while newer ones often use plasmas, microwaves, or arcing to energise the CVD gases with less heat. All these methods - as exist right now - are designed for uniform, bulk construction tasks. That is, they construct layers of uniform composition.
These methods have to solve several problems they have to solve:
- Strip the terminator structures off the surface of a material - crystalline materials like diamond and silicon don't terminate in a free radical (carbon/silicon/etc. atom with one bond not made), so a CVD process either has to strip the other atoms (usually hydrogen, oxygen, or some combo) off the surface before the reaction or otherwise account for them before new deposition can occur.
- Prevent the formation of graphene - at the low-to-atmospheric pressures that CVD often occurs at, graphene is technically a more stable chemical state than diamonds. This means that a diamond growth process usually has to have some way to reduce the probability of the production of graphene/graphite layers. In typical CVD, this is accomplished by some amount of hydrogen (though there are some questions on the level of necessity here)
- Prevent contamination from the containment chamber in the produced material - for any process needed for our purposes, this is a much less significant issue because we need to create conditions on a micro/nano scale and this already implies containment, especially if the tools used are made of the same material as that which is being grown. Though a related issue is avoiding corrosion or other breakdown of the structures used to grow the resultant material.
- Substrate heating - a lot of methods require heating of the growth substrate. This is undesirable for any methods we pull from.
Our Growth Methods
The growth mechanisms we use must solve most of the above problems, but they also require a couple other specific properties:
- Variable growth rate - While for very small scale, high-detail structures it's acceptable for a growth mechanism to only do a few nanometres per second, being able to adjust the rate to be capable of producing more coarsely-structured regions faster is a necessary quality. This is required so any mechanism we provide is capable of producing at least some amount of bulk material (not necessarily massive blocks, but at least a few centimetres thickness in a non-absurd timescale). It may be acceptable to produce somewhat degraded material (e.g. mixed graphite/diamond rather than pure diamond) in such cases, but it's much better if such a thing can be avoided.
- Directability - Being able to direct the growth is extremely important, as otherwise it's of no use for producing nanoscale structures. This directability must also be of sub-constructor resolution (i.e. can be used to construct devices of more fine resolution than was used to build the constructor)
- Non-Extreme Environmental Conditions - Bulk environmental conditions should not need to be extreme (in a "leaky" way e.g. via heat conduction or vacuum) for it to function. It shouldn't require any super exotic materials.
- Reasonably power efficient - Shouldn't require megawatts of power to build 1cm2 of material continuously.
Proposed Surface Termination/Passivation Stripping Methods
Any growth mechanism needs a way to strip the diamond surface of it's passivation/termination-atom layer, to enable reaction with the Carbon structure below. In the ideal case, the mechanism itself does this "automatically". However, should this not be quite so doable, there are some potential methods:
Argon/Noble Gas Stripping
This method is used in Low Energy Plasma-Enhanced Chemical Vapour Deposition (see the wikipedia page for details) - this knocks off the surface passivation/termination layer with minimal damage.
If we decide to add Argon into the element mix (and it'll probably be Argon, as it's cheap and abundant in atmosphere, and in theory we could print a fractional distillation system once we have semiconductor printing using the Peltier effect), this has the major benefit that a pure Argon cold plasma can also be used for surface cleaning (and probably also surface nanoleveling, as argon will not bond with the surface and hence the only reaction it can do is stripping off surface deviations, if we can do some cleverness with current-equalization for flow rate or something).
Electric Field Catalysis of Terminator Stripping
This method is based on some obscure papers from 1999 (doi:10.1016/S0039-6028(99)00377-5), which compute the electric fields at the surface to initiate the dissolution of C-H bonds.
Photochemical Bond Dissolution
Identify the bond energy for the passivation/termination layer, and shoot ultraviolet radiation with appropriate photon energy/wavelength to induce the bond to split into free radicals. This is a very flexible method, fairly comparable to the Argon-based stripping, though it lacks the benefit of surface leveling.
This is the same way CFCs would destroy ozone in the atmosphere (they'd split a chlorine-carbon bond and the chlorine free radicals would induce ozone dissolution by bonding to the component oxygens more permanently).
Proposed Substrate Conductivity Mechanisms
The primary proposed mechanism for nanostructured deposition relies on the existence of an electric field between the nanodeposition nozzle array and the diamond growth substrate. Furthermore, this substrate must be conductive during construction to prevent rapid charge buildup and the end of the electric field between the construction plate and the actual growth surface. To do this, there are some possible methods.
Direct Avalanche Breakdown
In this proposed mechanism, a controlled avalanche breakdown is initiated in the substrate directly (with a resistor to prevent everything melting).
This renders the substrate conductive. To avoid large voltages, an array of alternating electrodes could be used instead of one on either end.
Once a material is in avalanche breakdown state, that can be maintained with much lower voltages, and it has an absurdly low resistance, so the material ends up having minimal internal electric field, which means it is usable as a minimal-resistance ground.
However, should a mechanism require dynamic reversal of the electric field between the growth substrate and the deposition head, you'd need to ensure the avalanche breakdown state did not dissipate.
Indirect Photoconductivity via External Avalanche Breakdown
Instead of directly supplying current through the substrate, this method proposes using a square array of the same diamond material just on the outsides of the substrate - then causing avalanche breakdown within that.
By pushing sufficient voltage across it (probably resonant A/C to avoid extreme energy dissipation), this should make it fluoresce at the right wavelength with photons that can push electrons in the substrate up into the conduction band via direct photon absorption (with no need for a phonon/thermal transition even though diamond is an indirect semiconductor).
This has the advantage that the photoconductive state can be induced without needing to run any current through the substrate that is not for the purposes of controlling the growth process. However, it loses some power efficiency due to the radiation leaking, and it does involve EM radiation of ionising wavelengths (in diamond, the energies involved correspond to ultraviolet wavelengths in the 150 -> 280nm range). However, other processes are also likely to produce this sort of radiation and hence shielding is already required anyway.
Proposed Surface Growth Mechanisms
This is the section containing information on actual mechanisms that extend the surface of the substrate, as well as the means of precisely directing them.
Electroguided Ion-Plasma Chemical Vapour Deposition (EIP-CVD)
This is the primary mechanism I've been researching and analyzing the most. It seems to have a very high potential capability, though simulation and modelling needs to be performed to determine the practical parameters.
The basic principle is as follows:
- Hold some pre-doped growth gases at a strong negative voltage, charging them with excess electrons (to become negative ions). The technical details for this are complex but existing and fairly accessible methods of doing this exist (e.g. via thermionic emission, though we want to avoid any grounding and build up a high density, highly-ionised, low-energy cold gas in-atmosphere - avoiding the stripping of electrons from molecules, which would cause positive ions to be deposited on chamber walls - essentially, we want a non-neutral plasma of negative ions).
- Direct the gas using electric fields from surfaces at various degrees of negative voltage, down to a large array of nanoscale electrically-charged and controlled nozzles. These nozzles are gated by nanoscale electrodes, which enables the control of when and where and how a given amount of gas is held above the substrate. More details will be clear in a basic diagram to be uploaded soon.
- Hold the growth substrate (diamond) at ground voltage. Methods for making diamond conductive are discussed above (as it's an insulator).
- The strong electric field at the diamond's surface should aid in catalysing the spontaneous dissociation of any terminating atoms and enable reaction - this is the most experimental part of this concept, as the papers on this are fairly obscure and computational and from 1999. However, there are other methods documented above that should be just as usable (especially the Argon method, which has major benefits, though a combo Ar/Photocatalyzed dissolution may be an even better process)
- By holding the ionised gas in controlled positions and gating it on and off, you can control exactly where growth happens.
- As the surface grows, the array of nanodeposition nozzles moves upward (or the surface plate moves downward) at a rate appropriate to avoid one bump on the surface from causing exponential growth increase and an eventual short-out.
In terms of mechanics, probably the closest concept that exists today (on a macro scale) is Low-Energy Plasma-Enhanced Chemical Vapor Deposition. This typically relies on a plasma of a noble gas (like Argon) to create free radicals and ease the deposition reactions instead. The noble gas also deals with the surface terminator structure problem automatically.
The proposed method also carries aspects of the use of CVD where the gas energisation comes from electrical arcing. As, essentially, the entire process goes from a negative electrode in the source gases, down through a complex system of electrically-charged nanofluidic control mechanisms, through a very small gap to the surface, where the excess electrons (plus ions) get pulled down through the substrate being grown as the ion bonds to the surface.
It may also be the case that it's necessary to flip the polarity - either because it may be easier to create a positively charged plasma, or because of the requirements of the electric field direction to sufficiently increase the probability of the disassociation of hydrogen from the diamond surface. If these are conflicting, there are several other options to deal with the surface termination layer, as detailed above.
Either way, the control of this method then goes to the manipulation of electric fields within the gap between the hypothetical construction array and the grounding substrate growth surface. This can occur in one of several modes:
- Sub-cell resolution - In this case, the desire is to grow a structure with resolution smaller than a single constructor head nozzle "cell". To do this, my proposal is that structures of this sophistication are built in 9 stages for each layer, where the relevant matrix of construction cells has only one of the 9 positions active in each 3x3 grid at a time. The other surrounding 8 are gated closed and their voltages are held at high magnitudes and used to compact the area that the ions from the central constructor nozzle cell are guided to on the growth surface. By varying the precise voltage, it becomes possible to grow structures at very tiny spots. This, naturally, requires modelling and simulation to account from field distortions on any existing grown potrusions.
- Cell resolution - In this case, the desire is to grow within a single cell, fully. This provides some problems with beam shaping, that may require it to occur using the same mechanism as the sub-cell resolution mechanism (in particular, as the beam shape is likely to be circular, a square cell would not be covered fully without some redirectio).
- Supercell resolution - For bulk growth of material, the idea here is to open the floodgates and release more of the ionised gas through a whole collection of cells simultaneously. There are several potential ways to reduce the circle shaping issue - moving the substrate back and forth, or altering the hypothetical nozzle to add smaller beam diverters.
There is also the control aspect of gating the rate of current and ionised gas flow to increase or decrease the growth rate. Furthemore, it may be possible to create nozzles which allow both a cylindrical beam shape and a square beam shape (for instance adding a second electrode in a square shape on the bottom of the nozzle that can be enabled or disabled to squeeze the beam).
It should also be noted that while this system does entail the use of ions, the term "beam" is a slight misnomer. The ions here are not intended for acceleration to high velocities as in ion-bombardment doping mechanisms. The means of bonding with the existing surface is not implantation but chemical reaction - the ionic status is primarily to facilitate chemical reactions as well as provide a guidance mechanism via electric fields, to precisely control the location of reactions (and hence growth) while mitigating beam dispersion due to electrostatic repulsion. This does entail some acceleration - to get ionised gas out of the nozzle and into the area between the growth surface and the nozzle array - but not the kind of hyper-acceleration involved with ion beam deposition.