Chapter 3

 

Electrical contacts to semiconducting diamond

 

3.1. Introduction

 

For electrochemical studies an ohmic contact is required. An ohmic contact is considered as one that does not add a significant parasitic impedance to the structure and does not sufficiently change the equilibrium carrier concentration within the semiconductor or affect the device characteristics. In other words, an ohmic contact should have linear and symmetrical current-voltage relationship; it is characterized by having no potential barrier (hence no asymmetry) and an infinite surface recombination velocity (hence linearity). At an ohmic contact the electron and holes are at their thermal equilibrium values ¹.

 

In practice, the above definition is valid only for an ideal situation. In reality all the requirements should be taken in a much more approximate manner. Moazed et al. ^{2, 3} suggest that a contact is ohmic if the potential drop across it is small compared to that across the active portion of the device.

 

To obtain an ohmic contact one can match the metal work function (f) to the bands of the semiconductor. Alternatively one can decrease the electron affinity (c) of the semiconductor. In the case of diamond (see below) this is achieved by altering the surface termination.

 

Despite of the high number of studies published on the electrochemistry of highly boron doped diamond samples ^4-15 only a few consider the Ohmic contact ^16-20. This study shows different types of electric contacts generally used for boron doped polycrystalline diamond devices. Properties and applications are discussed. A new ohmic metal-semiconductor contact not described in the polycrystalline diamond electrochemistry literature is presented.

 

3.2. Indium/Gallium Eutectic Electrical Contacts

           

Indium/Gallium (In/Ga) eutectic is extensively used to make electrical contacts in silicon technology ¹. Also this method can be found as a procedure in the early stages of the diamond technology development ²¹.

 

This section describes how to build the electrical contact and discusses the doping range over which its application is useful.

                       

3.2.1. Construction of the electrical contact

 

The boron doped diamond substrate (conductive silicon) backside was scratched with a diamond scribe. The In/Ga eutectic was deposited on the silicon and a steel strip pressed onto the liquid eutectic. To protect the contact, the sample and metal strip were covered using a PTFE (polytetrafluoroethene, (C₂F₂)_n) tape (very resistant against acids, bases and strong oxidising agents), leaving exposed only the area on which to perform the electrochemical experiments.

                              

Silicon substrate samples had to be doped (n-type) and dipped in a hydrofluoric acid (HF) solutionª (just a few seconds) to remove any possible insulation layer of SiO₂ and rinsed in 18.2 MW cm ultrapure deionised water (Millipore) immediately prior to the contact formation.

 

 

3.2.2. Results

 

This type of contact was used successfully for highly doped diamond samples ([C]/[B] ratio of 2.8´10⁴ to 3.0´10⁴ p.p.m in the gas phase).

 

A poor performance for low boron doped diamond samples was found.

 

This technique was limited by the need to consider the effects of the liquid contact and interface between the silicon and the diamond.

 

3.2.3. Conclusions

 

The Ga/In eutectic contact was successful for highly conductive films but was limited by the need to use HF solutions and the poorly characterised interface between the silicon and the diamond.

 

Due to these limitations the technique was abandoned in favour of a simple technique: silver loaded epoxy resin electrical contacts.

 

3.3. Silver Loaded Epoxy Resin Electrical Contacts

 

Silver loaded epoxy resin electrical contacts have been used extensively as a general procedure for contacting boron doped polycrystalline diamond in electrochemistry ^{6-8, 22-36}.

 

A description of the construction method is detailed in this section. Silver loaded epoxy resin electrical contact is analysed using a simple technique such as current-voltage (i-V) curves. The range of application of this contact is considered in terms of the doping level of the diamond sample.

 

3.3.1. Construction of the electrical contact

 

Contact was made by simply applying silver loaded epoxy resin (“silver paint”) on a wire attached on top of the diamond surface. The contact was protected using a layer of a strong resin adhesive (Araldite Rapid) on top of the contact.

 

3.3.2 Results and discussion

 

Current-voltage (i-V) curves were employed to determine the quality of the contacts. These were recorded using a m-Autolab potentiostat (Eco Chemie B.V.) in two-electrode configuration. The scan rate was 50 mV/s.

 

Figure 3.1 shows the i-V curve of an as grown sample with two silver paint contacts. The concentration of boron in the gas phase during the growth of this diamond sample was 3´10³ ppm. The voltage range of the experiment was from   –5 V to 5 V. The plot is symmetrical with linear region between –0.5 V and 0.5 V. The resistance of the film calculated from the reciprocal of the slope in the linear region was 475 W.

 

Figure 3.1. Current-voltage characteristics for an as-grown moderately doped diamond film with two silver paint contacts (sample B111).

 

Figure 3.2 shows the i-V curve of an as grown sample with two silver paint contacts. The doping level of this diamond was unknown because no doping source was used during the growth just the boron contamination in the chamber. The experiment was done in the same conditions as above. The plot is asymmetrical without any linear region.

Figure 3.2. Current-voltage characteristics for an as-grown low doped diamond film with two silver paint contacts (sample B112).

 

Silver paint could be considered to yield ohmic contacts when used in samples with doping levels ([B]/[C] ration in gas phase) higher than 3´10³ ppm and over a narrow potential range. When silver paint was utilised for low doped diamond samples no Ohmic contact was observed due to a Schottky barrier formed at metal/semiconductor interface. (See figure 3.3).

 

This type of contact because it was on top of the diamond surface reduced the space available for electrochemical studies, causing difficulties when the dimensions of the sample were small.

 

Silver paint contacts eliminated the hazardous treatment of using HF solutions and simplified the process. There were no restrictions in possible pre or post-treatments because this contact was compatible with a range of surface treatment.

 

Figure 3.3. The metal/semiconductor interface

 

 

3.3.3. Conclusions

 

 

Successful contacts were made using silver paint in high doped diamond samples ([C]/[B] ratio of 3´10³ to 3.3´10⁴ p.p.m.). Silver paint electrical contact cannot be applied to low doped diamond samples because of the Schottky barrier formed at the metal/semiconductor interface.

 

The simple procedure of construction and ease of pre or post-treatment indicate that this contact is favourable for highly doped diamond samples.

 

3.4. Gold Electrical Contacts

 

In the literature it has been reported that gold forms a Schottky barrier when deposited on semiconducting diamond ³⁷. However, this behaviour is rarely observed and Ohmic contacts are more usually reported ³⁸.

 

A description of the gold electrical contact method is detailed in this section. Gold electrical contact is analysed using the simple technique of current-voltage    (i-V) curves. The range of application of this contact with respect to doping level of the diamond sample is discussed.

 

3.4.1. Construction of the electrical contact

 

Gold layers were deposited using an Edwards evaporator which was modified to incorporate a heating stage (further details are given in the next section: Formation of Three Metal Layers Contacts).

 

To increase the adhesion of the gold layer, samples were heated under vacuum to remove organic contaminants from the surface. After this stage, gold was deposited on the clean surface. The samples were cooled under vacuum.

 

The evaporator worked by passing a current through a tungsten three wire basket containing the metal (a ball of gold wire). A current of 30 A was enough to evaporate the gold stored in the basket. The diamond samples were covered with a mask to allow deposition in the shape of a circle (2 mm diameter) at the edge of the substrate. The distance between the holes and the mask was 1cm.

 

Gold was evaporated. When the metal started to cool down, gold vapour solidified and coated the inside of the evaporation chamber. Two gold spots were deposited on the diamond surface. The deposition of the metal was controlled to yield gold layers of approximately 100 nm in thickness.

 

Applying silver paint on a wire attached on top of the gold spot made the electrical contact. The contact was protected using a layer of a strong resin adhesive (Araldite Rapid) deposited on top of the contact.

 

Two pairs of diamond samples were grown. Each pair was grown in a single deposition run to minimise variation between the films. One sample from each pair was refluxed in chromic acid solution to oxidise the diamond surface, while the other sample remained “as grown”.

 

Samples B123a and B123b were grown with no boron in the gas phase, i.e. doping was achieved using that boron in the CVD chamber as contamination. Samples B128a and B128b were grown with a gas phase boron to carbon ratio of 50 ppm.

 

To remove contacts immersion in an aqua regia solution (1 HNO₃: 3 HCl (aq)) was sufficient.

 

3.4.2. Results and discussion

 

Current-voltage (i-V) curves were employed to describe the quality of the contacts using a m-Autolab potentiostat (Eco Chemie B.V.) in two-electrode mode configuration. The scan rate was 50 mV/s.

 

Results showed an asymmetrical non-linear behaviour. The moderately doped samples (B128a and B128b) gave an approximately ohmic response over a reduced potential range (-0.5 V to +0.5 V) while the low doped samples (B123a and B123b) gave a non-linear answer for all the potential range of the i-V plots. (Figures 3.4 and 3.5)

 

It was believed that the reflux in chromic acid had successfully modified the hydrogen termination of the diamond surface. A simple test with ultrapure water showed the “as grown” sample to be hydrophobic and the oxidised sample to be hydrophilic.

 

Figure 3.4. Current-voltage characteristics for diamonds films with two gold contacts (sample B123a and B123b).

 

Figure 3.5. Current-voltage characteristics for diamonds films with two gold contacts (sample B128a and B128b).

 

Gold electrical contact was limited by boron doping level of the diamond sample. When diamond samples were very low doped (hydrogen or oxygen terminated) a Schottky contact was described. But using moderately boron doped diamond samples the response of the gold contact was acceptable for a small range of potential. It should be noted that it was difficult to compare results because of the poor reproducibility of gold contact formation due to poor adhesion.

 

Poor adhesion of the gold layer was one of the biggest difficulties of this type of contact. Even if the diamond surface was cleaned either by chemical treatment (chromic acid) or by heating process. The gold did not adhere well on the diamond surface. Carefully handling of the diamond samples was a requirement after the gold deposition to avoid a possible delamination of the gold spots.

 

An advantage of this type of contact was that it did allow any kind of treatment of the diamond surface before the gold deposition.

 

As this type of contact was made on top of the diamond surface it reduced the available space causing difficulties when the dimensions of the samples were small.

 

3.4.3. Conclusions

 

An Ohmic response was found over a reduced applied potential range (-0.5 V to +0.5 V) for moderately boron doped samples. Poor performance was observed for low doped diamond.

 

Problems in the reproducibility when the contact was made complicate the analysis of the gold contact properties.

 

Gold contacts did not provide a significant improvement in performance over the simpler silver paint contact.

 

3.5. Three Layer Metal Electrical Contacts

 

Reaction at the interface to produce a third phase is a useful method of changing the nature of metal contacts on semiconductors (reduction of the Schottky barrier)^{39, 40}. For diamond, the third phase formed is a carbide, which could be a reliable contact as well as a good diffusion barrier for high-temperature application. Annealing process (heating up the carbide forming metal) promotes the formation of the carbide layer. 

 

Carbide forming metals including titanium, molybdenum, and tantalum have been used to obtain ohmic contact on diamond. Collins, Lightwowlers, and Williams used a gold tantalum alloy ²¹. Titanium (Ti) has been used by several groups of researchers and was often capped with platinum/gold (Pt/Au) overlayers ^{2, 3, 17, 18, 41}. Various combinations of carbide forming refractory metals have also been studied to obtain ohmic contacts of low resistivity  ^{2, 3}.

 

Titanium was selected as a metal representing the carbide forming metals for the following reasons: titanium is a transition metal which has a great affinity for carbon (carbide formation is thermodynamically favourable, DH= - 44 kcal/mol at 25ºC); it is estimated that the diffusion constant of carbon in titanium is orders of magnitude higher than the other carbide forming metals ⁴².

 

 In this section, Ti/Pt/Au (three layer metal, 3LM) contacts on boron doped polycrystalline diamond are detailed. As the literature described before, titanium was chosen as a metal representing the carbide forming metals that could be annealed to produce titanium carbide (TiC). A gold top layer was deposited to avoid the oxidation of titanium with traces of oxygen at high temperature (during the annealing process) and, more slowly, over the lifetime of the device. The presence of a third metal, platinum was required to prevent the interdiffusion between titanium and gold layers during the annealing step.

 

Wires could be attached to the surface of gold using silver paint, as was described before in the section on gold contacts.

A description of the 3ML electrical contact procedure is detailed in this section. 3ML electrical contacts were analysed using simple techniques such as current-voltage (i-V) curves and four point probes resistance measurements. The range of application of this contact is given in terms of the doping level of the boron doped polycrystalline diamond sample.

 

3.5.1. Construction of the electrical contact

 

Before any metal was deposited the diamond surface was oxidised and cleaned from any non-diamond carbon. For this task, the sample was refluxed in a chromic acid solution at 100°C for a period of about 4 hours. After the solution had cooled down, samples were rinsed with ultrapure water and dried in an oven for approximately 1 hour.

 

When the diamond samples were loaded into the evaporator (see figure 3.6), a heating treatment was necessary to remove any physisorbed species from the diamond surface to enhance adhesion prevent the oxidation of the metal. The samples were located in a heating stage inside the vacuum chamber of the evaporator. A mask was placed on top of the sample and a three-wire tungsten basket (Alfa Aesar) was loaded with titanium crystals (Alfa Aesar). The masks were made from 0.5 mm thick stainless steel contained circular holes  (diameter 2 mm; spacing 8mm).The vacuum chamber was insulated with the bell jar and pumped down to less than 2´10^-5 Torr. Samples were heated between 200 °C and 250 °C. An alternating current between 40 A to 48 A was passed through the tungsten basket to evaporate the titanium crystals. A layer of 100 nm to 150 nm of titanium was deposited at a rate of approximately 0.5 nm/min, monitored using a quartz crystal microbalance.

 

When the titanium deposition was over, the sample was allowed to cool down to room temperature slowly under vacuum. Normally this process was done  overnight. Although not employed in this study the cooling could be accelerated using a flow of a dry oxygen-free gas.

 

To reach the desired titanium thickness it was frequently necessary to use more than one run. The loading, heating, deposition and cooling processes were repeated many times.

 

Once the titanium deposition was complete a platinum layer was deposited. A    60 nm thick platinum layer was sputtered onto the titanium. A mask was used with a 4 mm diameter spot a diameter greater than the one that was used for the titanium deposition in order to ensure complete coverage of the lower metal.

 

Gold deposition was performed in the evaporator with an approximate thickness of 100 nm covering the whole platinum spot. To ensure complete coverage a mask which the diameter of the holes was 6 mm was employed.

 

After each step of the process, the three metal layer structure was examined by the naked eye and with an optical microscope to verify the alignment, coverage and quality of the metal layers.

 

The final stage of the process was annealing. Reviewing the literature data suggested that an annealing process of 500 °C in vacuum would be sufficient to form the titanium carbide layer ³.

 

Samples were loaded in a quartz cell designed with two taps to allow evacuated and fluxed gases through it. The annealing process was divided in two steps. The first part consisted of pumping down the cell using a rotary pump. The cell was flushed with oxygen-free nitrogen or helium (BOC Speciality Gases). The vessel was frequently flushed with nitrogen to eliminate the oxygen before the oven was switched on to heat up the cell to 250 °C. During this process the cell was evacuated by a rotary pump to remove species as soon they desorbed from the surface of the sample and the walls of the vessel. Whilst hot (temperature about 250 °C), the cell was transferred to a high temperature furnace for annealing at 500 °C once completed the cell was left to cool down to room temperature. A schematic diagram about this contact can be found in figure 3.7.

 

Figure 3.6. Schematic diagram of the metal evaporator

 

Next table (table 3.1) offers a schematic resume of the 3LM contact production process.

 

Figure 3.7. Schematic diagram of a three layer metal top contact (not to scale). Note that the plan view shows the metal layers in reverse order for illustrative purposes. The layers from top to bottom are Ag, Au, Pt and Ti.

 

Step	Description		Step
1	Nucleation pre-treatment	(a)    Abrade with diamond powder to provide nucleation sites	1.a
2	Removing diamond dust	(a)    Wipe with IPA soaked cotton sticks (b)   Place in a beaker of IPA in an ultrasonic bath	2.a
			2.b
3	Diamond growth	(a)    Load substrate into chamber (b)   Evacuate chamber (c)    Pre-heat substrate (d)   Deposit diamond for several hours (e)    Initially cool in hydrogen atmosphere (f)     Reach room temperature under vacuum	3.a
			3.b
			3.c
			3.d
			3.e
			3.f
4	Surface Pre-treatment	(a)    Sample reflux in chromic acid solution (b)   Rinse sample with ultrapure water	4.a
			4.b
5	Titanium deposition	(a)    Load samples into evaporator (b)   Load metal into the basket (c)    Place mask over the sample (d)   Evacuate chamber (e)    Heat sample to remove physisorbed species (f)     Evaporate titanium onto diamond surface (g)    Reach room temperature under vacuum (h)    Repeat from 2nd step until obtain desired thickness	5.a
			5.b
			5.c
			5.d
			5.e
			5.f
			5.g
			5.h
6	Platinum deposition	(a)    Load sample into the chamber (b)   Place mask over the sample (c)    Evacuate chamber (d)   Sputter platinum over the titanium spot	6.a
			6.b
			6.c
			6.d
7	Gold deposition	(a)    Load sample into evaporator (b)   Place mask over sample (c)    Heat sample to eliminate physisorbed species (d)   Evaporate gold over platinum spot (e)    Reach room temperature under vacuum	7.a
			7.b
			7.c
			7.d
			7.e
8	Annealing process	(a)    Put samples into quartz cell (b)   Evacuate cell (c)    Flush cell with nitrogen (d)   Repeat steps 2 and 3 several times (e)    Evacuate cell (f)     Heat sample until 250 °C (g)    Seal cell (h)    Transfer cell to high temperature furnace (i)      Anneal at 500 °C (j)     Reach room temperature under vacuum	8.a
			8.b
			8.c
			8.d
			8.e
			8.f
			8.g
			8.h
			8.i
			8.j
9	Attachment of wires	(a)    Attach copper wire with silver paint (allow to dry) (b)   Cover silver paint with epoxy resin for protection	9.a
			9.b

Table 3.1. Summary of the production process for 3LM

 

3.5.2. Results and discussion

 

Current-voltage (i-V) curves were employed to describe the quality of the contacts using a Solartron 1287 Electrochemical Interface potentiostat in two-electrode mode. The scan rate was 50 mV/s.

 

A series of four point probe measurements were taken to obtain values for the sheet resistance of the samples without problems of contact resistance. A Solartron 1287 Electrochemical Interface potentiostat was used in four electrode mode. A test rig was designed with four electrodes built into a PTFE block. The four spring-loaded brass electrodes had flat circular contact pads (1 mm diameter) and were aligned in a straight line and separated by 2 mm gaps. The samples were raised up to the electrodes on an adjustable ramp.

 

Sze ¹ provides an equation the calculation of the sheet resistance, which is reproduced below.

 

                       

 

                        where   r         =          sheet resistance (W cm)

                                               dV/di  =          reciprocal of the gradient of the current-

                                                        voltage plot  

                                               W         =          thickness of the film

                                               CF       =         correction factor dependant on the probe separation and the size of the sample (approximately equal to 4.54 for the system under investigation).

 

Figure 3.8 shows a current-voltage plot for low doped (50 ppm) diamond sample with three layer metal contact (3LM). Measurement was taken after the annealing process at 500 ºC. The response of the device was asymmetrical, but with a linear region between –0.5 V to +0.5 V. The contact appears approximately ohmic in that range of applied potential.

 

Figure 3.8. Current-voltage characteristics for post-annealed diamond film with two 3LM contacts (sample B129a).

 

Figure 3.9. Current-voltage characteristics for post-annealed oxidised diamond film with one 3LM contact and one gold contact (sample B129a).

 

Figure 3.9 shows a current-voltage was measured using one 3LM contact and the other contact in the gold spot on oxidised terminated sample. A Shottky barrier was observed.

 

Resistance measurements are shown in the next table:

 

 

Table 3.2. Data from four point probes resistance measurements of the different stages in the production of the three layer metal contact.

 

Analysing data from above table, resistance values for hydrogen and oxygen terminated samples for two 3LM contacts are different. Hydrogen terminated material presents a higher conductivity than the oxygen one ^43-46. When the hydrogen conductive layer is removed from the surface the resistance of the film increases ^47-50. If the data for the annealing process is analysed the two point probe has a bigger value than the four point probe and it is in a ratio 3:1. Two point probe measures the resistance of the sheet material and the contact resistance. Four point probe only measures the resistance of the sheet between the contacts. Proportionality between four and two point probes correspond approximately with the increase of the length between the contact points when the experimental set-up is modified from 4 point probe to 2 point probe. To explain the fact that the annealed sample is less resistance than the oxidised one reconstruction of the structure in the surface, possible graphitation or displacement of hydrogen from inner layers to the surface has been suggested ^{2, 3, 17, 51-58}.

 

Lack of accuracy was a problem during the deposition of the different metals. The main source was the vibration produced by the rotary pump in the evaporator chamber during the metal depositions this caused a slight displacement in the mask creating a faulty deposition and then a deficient contact.

Figure 3.10. Four point probe current-voltage characteristics for post-annealed diamond film with two 3LM contacts (sample B129a).

 

Figure 3.11. Current-voltage characteristics for post-annealed diamond film with two 3LM contacts (sample B129a).

 

Many steps of the production of the contacts not only were long in terms of time but the probability of a possible mistake was increased dramatically:

 

·        When the annealing was not performed correctly, no TiC was formed giving a contact very badly adhered on the surface sample. Frequently delamination was observed.

 

·        The heating stage was a delicate step because it could produce a change of surface diamond structure (appearance of graphite on the surface)

 

This 3LM electrical contact was limited to oxidised diamond surface. Some modifications were done trying to adapt the process to hydrogenated diamond surface. A PTFE cell (as described in the electrochemical chapters) was used in order to restrict the area to which hot chromic acid was applied to that of the contact.

 

The time consumed to produce this type of contact was high. At least seven working days were required to complete every single step in table 3.2. This was extended as access to sputter apparatus and the high temperature furnace was restricted.

 

3.5.3. Conclusions

 

An Ohmic response has been found over a reduced applied potential range (-0.5 V to +0.5 V) for low doped boron polycrystalline diamond samples. However, further investigation will be required in order to improve the quality and reliability of this type of contact. Some investment will be necessary to achieve a high enough level of accuracy and purity to generate the 3LM contact under laboratory conditions. Due to all restrictions of this 3LM electrical contact, this technique was abandoned in favour of a new technique: titanium under layer contacts.

 

3.6. Titanium under layer contacts

 

Same ideas about the TiC formation described in the 3LM electrical contacts were the starting point to develop a new method (not described in the literature before) to form ohmic contacts for low boron doped polycrystalline diamond.

 

A single strip of titanium was deposited onto the surface of the silicon substrate before the diamond deposition. Selective diamond growth on top of this metal layer was achieved by leaving a smooth section of silicon which provided few nucleation sites. As the diamond formed at raised temperatures, a metal carbide would form without the need for a separate annealing step after the growth process and, since the film was being grown in the absence of an oxygen containing species, no protective metals were needed to prevent oxidation of the titanium.

 

A description of the titanium under layer (TiUL) electrical contact fabrication is detailed in this section. TiUL electrical contact is analysed using simple techniques such as current-voltage (i-V) curves and four point probe resistance measurements. The range of application of this contact is given in terms of the doping level of the boron doped polycrystalline diamond sample.

 

3.6.1. Construction

 

A silicon substrate was abraded with diamond powder to create nucleation sites. The abrasion was not completed in the whole silicon surface. One strip of the sample was left smooth in order to suppress diamond growth on this region.

 

After removing the excess of diamond powder using IPA soaked cotton sticks the sample was further cleaned in an ultrasonic bath, only the abraded area was dipped in IPA solution. The substrate was then dried at 100 ºC in a vacuum oven and stored in an evaporated chamber. The silicon sample was covered with a mask such that a small fraction of the abrasion zone and the whole of the non-abraded silicon was exposed. The sample was heated under vacuum to remove any possible physisorbed species. In the next step titanium was deposited (approximately between 100nm and 150 nm) and then allowed to cool under vacuum to reach room temperature. The silicon sample was then transferred to the CVD diamond chamber. During the growth of the diamond film, an in situ generation of titanium carbide was achieved with no risk of possible oxidation due to the hydrogen atmosphere present in these conditions.

 

When the growth was over and the diamond sample had reached room temperature under vacuum conditions, a wire could be attached using silver paint and protected with an epoxy resin adhesive. Table 3.3 is a summary of the production process for TiUL contacts.

 

Table 3.3. Summary of the production process for TiUL contacts

 

A schematic diagram of the titanium under layer contact is shown in figure 3.12 and figure 3.13

Figure 3.12. Plane schematic view of titanium under layer contact (not to scale). Area a is a region of diamond growth on abraded silicon. Area b is a region of diamond growth on titanium coated silicon. Area g is a region of a titanium coated silicon with no diamond present (silicon was not abraded)

 

Figure 3.13. Perspective schematic view of titanium under layer contact (not to scale). Area a is a region of diamond growth on abraded silicon. Area b is a region of diamond growth on titanium coated silicon. Area g is a region of a titanium coated silicon with no diamond present (silicon was not abraded).

 

As explained in figures 3.12 and 3.13 the standard procedure to fabricate a titanium under layer contact was the deposition of a single strip of metal on the silicon substrate. This procedure was employed for electrochemical measurements in order to maximise the sample area. However, it was not adequate for the electrical characterisation. A double strip diamond sample (see figures 3.14 and 3.15) (B147a) was fabricated to properly characterise the electrical contact.

 

Figure 3.14. Plane schematic view of double strip under layer titanium contact (not scale). Area a is a region of titanium coated silicon with no diamond present (silicon was not abraded). Area b is a region of diamond growth on titanium coated silicon. Area c is a region of diamond growth on abraded silicon

 

Figure 3.15. Perspective schematic view of double strip under layer titanium contact (not scale). Area a is a region of titanium coated silicon with no diamond present (silicon was not abraded). Area b is a region of diamond growth on titanium coated silicon. Area c is a region of diamond growth on abraded silicon.

 

A series of structural characterisation experiments were performed on the titanium under layer double contacts using SEM and optical microscopy techniques.

 

The SEM equipment and the conditions to work with the diamond samples were described in section 2.2.10. A Zeiss Axiolab Optical Microscope fitted with Zeiss Epiplan lenses was frequently used to view the samples. Objective lenses of 5 ´, 10 ´, 50 ´ and 100 ´ coupled with an 10 ´ eyepiece to give a range of magnifications from fifty to one thousand.

 

Images were captured with a JVC TK-1280E Colour Camera attached to an Olympus BH2 Optical Microscope fitted with a 10 ´ eyepiece and 4 ´, 10 ´, 20 ´ and 50 ´ objective lenses.

 

The images shown below refer to the double contact sample, the regions are indicated on figures 3.14 and 3.15.

 

Figures 3.16 to 3.18 show a randomly oriented diamond film on area c. If the images are analysed in more detail no pinholes or cracks are presented. However diamond grit from the preparation of the substrate to grow diamond is found in the images (figures 3.16 and 3.18).   

 

Figure 3.16. Optical microscopy image of the central section (area c) taken with a 50 ´ objective lens (sample B147a).

 

Figure 3.17. Randomly oriented diamond film in central section (area c) (sample B147a).

 

Figure 3.18. Randomly oriented diamonds with an inclusion of diamond grit. SEM image was taken in central section (area c) (sample B147a).

 

 

The boundaries between the different growth areas are shown in figures 3.19 to 3.25.

 

Figures 3.19 to 3.22 show the boundary between areas a and b. Areas are quite well defined. Low diamond coverage can be seen in the titanium non-abraded zone and good coverage for the abraded part. SEM images compliment the optical images with more details.

 

Figures 3.23 to 3.25 show the boundary between areas b and c. The change in brightness allows identification of each zone in the optical images.

 

Figure 3.19. Optical microscopy image of the boundary between areas a and b taken with a 20 ´ objective lens (sample B147a).

 

Figure 3.20. Optical microscopy image of the boundary between areas a and b taken with a 10 ´ objective lens (sample B147a).

 

Figure 3.21. SEM image of the boundary between areas a and b (sample B147a).

 

Figure 3.22. SEM image of the boundary between areas a and b (sample B147a).

 

Figure 3.23. Optical microscopy image of the boundary between areas b and c taken with a 10 ´ objective lens (sample B147a).

 

Figure 3.24. Optical microscopy image of the boundary between areas b and c taken with a 20 ´ objective lens (sample B147a).

 

Figure 3.25. Optical microscopy image of the boundary between areas b and c taken with a 20 ´ objective lens (sample B147a).

 

3.6.2. Results and discussion

 

Current-voltage (i-V) curves were employed to describe the quality of the contacts using a Solartron 1287 Electrochemical Interface potentiostat in two-electrode mode. The scan rate was 50 mV/s.

 

A series of four point probe measurements were taken to obtain values for the sheet resistance of the samples without measuring the contact resistance (see section 3.5.2 for more details).

 

Figure 3.26 shows the current voltage plot for a low doped (50 pppm) diamond sample with two TiUL contacts (sample b147). The plot is linear over a potential range of approximately ten volts (see insets).

 

Figure 3.26. Current-voltage characteristics for an as-grown diamond film with two TiUL contacts (sample B147a)

 

Figure 3.27. Current-voltage characteristics for an oxidised diamond film with one TiUL contact and one gold contact (sample B147a).

 

Figure 3.27 shows a current-voltage was measured using one TiUL contact and the other contact is a gold spot that was deposited after the oxidation of the diamond surface. A Shottky barrier was observed as suggested in the literature for low doped diamond/gold barriers ^16-20.

Comparing the TiUL contact fabrication process against the 3ML one, following advantages are noted:

 

·        There was a drastic reduction in the average time necessarily to generate the contact on the sample. The whole process could be completed in three working days.

 

·        The number of steps was reduced thus decreasing the probability of contact failure.

 

·        Titanium carbide layer was synthesised at the same time that the diamond film was growing. In such conditions no oxidation was possible for the titanium strip giving durability, strength and a long life to the contact.

 

·        As this contact is underneath the diamond film, more diamond surface was available to perform electrochemical experiments.

 

The only disadvantage was that post-treatments were limited for the titanium under layer contact. It was not possible to apply a reflux in a chromic acid bath. Only by protecting very carefully the contact with PTFE adhesive film could the samples to be dipped in hot chromic acid. An alternative was the selective oxidation of a determinate zone using a PTFE cells (as presented and explained in chapter 5).

 

3.6.3. Conclusions

 

Ohmic response has been found over a wide applied potential range (-5 V to      +5 V) for low doped boron polycrystalline diamond samples. This range is over the limits of the electrochemical potentials applied in this study

 

Easy fabrication process, good reproducibility and long life contact suggests TiUL electrical contacts as the ones to be used in boron low doped polycrystalline diamond electrochemical studies.

 

3.7. Summary

 

Highly doped diamond films do not need any special electrical connection to obtain Ohmic contacts. However low doped diamond requires the carbide formation in the electrical contact to obtain the Ohmic properties. Different methods have been presented to reach it. Titanium underlayer contacts appears to be the quickest in the fabrication stage and most reliable of the electrical contacts studied.

 

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