fabrication and characterization of schottky diodeadhabal/v1/reports/mod phy project.pdfiii. cv is...

Fabrication and Characterization of

Schottky Diode

Arnab Dhabal

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Acknowledgements

I would like to express my greatest gratitude to the people who have helped and supported

me in this project. I wish to thank Prof. Damodaran and Prof. Anjan K. Gupta for helping

throughout right from giving invaluable suggestions and comments to making arrangements

for obtaining the samples.

I also thank Prof. Y.N.Mohapatra, for giving advice on our method of fabrication and providing

me with the silicon sample.

Special thanks go to Mr. Ramesh, whose vast experience with the Vacuum Deposition Unit was

of great help, and Mr. Upendra, for helping me in getting the project done smoothly and in

time.

The data acquisition and characterization part would have been unsatisfactory without the

assistance of Mr. Indranuj Dey, who not only helped set up the electric instruments, but also

took interest in discussing the results, thereby providing deeper insight as I worked on the

project. I am extremely grateful to him.

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Contents

Introduction ………………………………………………………………………………………………..…… 4

Fabrication of a Schottky Diode ………………………………………………………………………. 4

Choice of Material 4

High Vacuum System Overview 5

Steps of Operation 5

Electrical Characterization …………………………………………………………………………..…… 8

Current-Voltage Characteristics 8

Current Variation with Time due to Heating 12

Voltage Characteristics on Application of Square Wave Pulses 13

Discussion and Scopes of Improvement …………………………………………..…………….. 21

References ..…………………………………………………………………………………….………………. 22

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Introduction

A Schottky diode is a special type of diode with a very low forward-voltage drop and a very

fast switching action. When current flows through a diode, there is a small voltage drop across

the diode terminals. A normal diode has between 0.7-1.7 volt drops, while a Schottky diode

voltage drop is between approximately 0.15-0.45 – this lower voltage drop translates into

higher system efficiency.

Chemically, a Schottky diode has a Schottky contact between a semiconductor and some

appropriate metal. The other end of the semiconductor has an Ohmic contact, with a metal. To

ensure that the two ends of the semiconductor form different junctions, a gradient in the

doping concentration is required within the semiconductor, such that the end with the Ohmic

contact has more carriers than the Schottky contact.

Fabrication of a Schottky Diode

Choice of material

SiC performs best as a semiconductor for Schottky diodes. However, only Si wafers were

available within IIT Kanpur. On reading a few papers, it was understood that n-typed Si works

better as a diode than p-type Si. Also a Schottky contact is formed on Si by depositing Au, while

the Ohmic contact could be of any metal, like Aluminium. The initial plan was to take a glass plate

as the base, and to sputter deposit the following components as 2mm x 5mm strips in the order

given:

• Aluminium

• Heavily doped n- Si (1018 /cc)

• Low doped n- Si (1016 /cc)

• Gold

This had to be done in such a way that later the wires can be soldered to the Gold and

Aluminium ends.

However this plan was rejected due to two major reasons:

i. Unavailability of n-Si of two different types of doping concentration.

ii. The possibility that on sputter depositing the Si, the doping and the Si might not

evaporate and get deposited in the same ratio as in the original wafer sample.

So it was decided that I should start with a Si sample that already has a gradient in the

doping concentration. Au was to be sputter deposited onto the lowly doped (shiny) Si surface.

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The Ohmic contact was to be made of Indium solder, while the wire to be drawn from the Gold

side was to be attached to it by Silver paste.

High Vacuum System Overview

The HINDIVAC system at IIT Kanpur Modern Physics Lab utilizes 3 pumping devices in

stages:

i. The rotating Mechanical Pump

- Primary source for creating vacuum. Can reach only up to 10-3 mbar

ii. The Diffusion Pump

- It uses hot oil and has the advantage of reaching up to 10-7 mbar but

must be backed by a rotary pump.

iii. The cold trap

- It reduces pressure by condensing, or freezing out condensable

vapours that may exist in the system. It also prevents oil vapour in

diffusion pump from back streaming into the system. Liquid N2 is

used for the purpose.

Other system components include valves and baffles to aid the control of action of these

pumps. The valves allow Roughing and Backing modes of operation. In the roughing mode,

only a rough vacuum is obtained by the rotary pump. The Foreline valve and the Hi-Vac valve

isolates the diffusion pump, and the cold trap, from the chamber. After completion of roughing

(Pressure < 0.005 mbar), the Foreline Valve is opened to start the Backing mode.

Within the Vacuum chamber there are electrodes on which the material to be deposited

are kept in boats, and high Power is given to them continuously until the material starts

evaporating. The substrate which is placed above the electrodes gets coated. The system also

has a Digital Thickness Monitor, which should be placed in the vacuum chamber at the same

distance from the material being sputtered as the substrate. The Acoustic Impedance and

Density of material are given as input. It displays the thickness of material deposited in

Angstroms, and the rate of Deposition.

Steps of Operation

Initially the vacuum chamber is tested for leaks. This is done by just running the Rotary

pump, with the Combination Valve in Roughing. Possible sources of leaks are identified and

attended to.

The substrate is prepared by cleaning it using acetone, and placing it appropriately on a

plate, using cleaned blades for support. The Aluminium boat is placed on the electrode with

the gold after rinsing it in acetone as well.

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After closing the chamber properly, the vacuuming process is started:

i. The power supply and the main MCB is switched on. All valves (High-Vacuum

valve - HV and Combination Valve- CV) are closed.

ii. The Rotary Pump is switched on.

iii. CV is put to roughing

iv. When vacuum reaches ~0.005 mbar in the roughing line, CV is changed back to

Backing.

v. Water supply is turned on, and then the Diffusion Pump is switched on.

vi. In around 20 minutes, the pressure in the backing region also falls to 0.005

mbar.

vii. CV is changed to Roughing for 1-2 minutes and again brought back to Backing.

viii. When the pressure reaches 10-4 mbar, HV is opened.

ix. Liquid N2 is poured into the cold trap.

x. The pressure is allowed to stabilize at <10-5 mbar for 30 minutes.

Figure 1 - High Vacuum Generator and Thin Film Depositor

Now current is passed through the electrode for heating up the Au to evaporating

temperatures. The current is increased at the rate of 2 Amp/minute, till deposition temperature is

reached. The DTM was not functioning properly, so it was used only for getting a rough estimate,

by seeing the rate of deposition from time to time. This is carried out over a period of 30 minutes,

and the current rate was reduced again at a steady rate of 2 Amp/minute. It was estimated that

800nm – 1200 nm of Gold had been deposited on the Silicon surface.

Figure 2 – Gold sputtering in process in 10-5

mbar pressure

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After 10 minutes, HV is closed, and the DP is switched off. After around 20 more minutes,

CV is closed and Rotary pump and water supply are also switched off.

The air admittance valve is opened for fast release of the vacuum. The sample is

collected out. The gold coating on it is visible.

It is found that gold had been deposited at the edges of the Silicon piece, which can cause

unexpected characteristics. So the edges with depositions are chipped off. Indium is used to

solder a thin copper wire onto the rough surface of the Silicon (for the Ohmic contact). On the

other surface, Silver paste is used to attach a thin copper wire on to the Gold.

For ease of use, the sample thus produced is mounted on a PCB. The Au coated Si piece is

vertically attached to the PCB using insulating glue. Thicker wires are soldered on-to the PCB

and connected to the two thin wires of the diode. These two terminals can now be directly

plugged into bread-boards for electrical characterization.

Figure 4 – Design schematic of Fabricated diode

Figure 3 – The fabricated diode mounted on a PCB with connections

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Electrical Characterization

1) Current Voltage Characteristics

The following circuit was implemented for the normal Voltage Current characteristics:

Figure 5 - Circuit Diagram A

The circuit with the diode A was made on a breadboard. A Voltage source, 2 Multimeters, and

Wires were the other requirements

Figure 6 - Circuit Set-up A

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Readings for Forward Bias:

Voltage in mV Current in µA



10.6 0.04

319.0 17.9

903 105

31.7 0.15

339.0 20.3

959 116

41.3 0.22

361.6 23.1

1039 133

62.0 0.41

386.4 26.3

1078 141

82.8 0.70

406.9 28.9

1152 156

99.4 1.03

432.1 32.1

1215 177

131.2 1.97

451.2 34.5

1294 196

178.2 4.33

487.5 39.2

1405 220

198.5 5.76

546.8 46.8

1720 290

221.8 7.64

570.1 49.9

2090 401

259.2 11.2

656.0 64.4

2410 469

291.0 14.6

728.0 75.4

3450 743

301.0 15.8

764.0 81.1

3920 941

310.2 16.9

824.0 91.2

5700 2064

Readings for Reverse Bias:




-33.8 -0.116

-137.2 -0.303

-384.0 -0.627

-53.6 -0.159

-173.9 -0.367

-445.5 -0.717

-71.4 -0.185

-197.5 -0.401

-501.2 -0.824

-83.9 -0.206

-208.1 -0.418

-594.2 -1.058

-95.4 -0.227

-245.1 -0.465

-722.5 -1.68

-109.5 -0.253

-294.2 -0.516

-856.9 -2.62

-114.8 -0.264

-358.5 -0.586

-1015.6 -3.80

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Figure 7. Current vs Voltage characteristics in both forward reverse regions

• The I vs V characteristic shows that current flow in one direction is definitely

favoured over the other.

• In the reverse bias, there is flow of current, but substantially low (<5 µA at 1 V)

• However, it can be seen that the positive region does not have a very steep slope

as expected from normal diodes. This is indicative of an inbuilt Resistance in series,

possibly at one of the junction of the diodes, or also because of the thick layer of

Silicon.

• We can roughly say that sans the series resistance the Voltage drop across the diode

would have been 0.19 V.

• The resistance changes with temperature. So at higher voltages when there is

substantial heating, the current reading gradually goes up, for (>5 minutes at 2 V

forward bias) indicating that the resistance is lowered. If the current is switched

off for some time, the resistance returns to its room temperature value. So when

switched on again, the current returns to its low starting value.

It was found that above 500 mV, the heating effects caused too unstable

current values, and hence the readings taken in that region were not very reliable,

and hence has been excluded from the plot (Figure 7).

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The V-I equation for an ideal diode is :

−= 1exp

nkT

qVII oD

Correction for resistance:

−

−= 1

V

)IRV(expII

0

o where V0 = ��

i.e. V = ��

+ 1� + ��

Figure 8 - Plot of Forward biased region after curve fitting

Using OriginLab 7.5, Curve fitting was carried out for the above equation and found the

coefficients as:

R = 6.2 + 0.1 kΩ

I0 = 0.134 + 0.005 µA

V0 = 42.7 + 0.5 mV

It is noted that the resistance of 6200 Ω is quite high in comparison to normal diodes.

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2) Current variation with time due to heating

The rate of change of resistance value is quite high as the following dataset comprising of

current values noted down at intervals of 10 seconds at a voltage of 2.0V indicates. These

characteristics were repeated if the circuit was given sufficient time to cool down.

Figure 9 – Plot of Diode Current against time at Voltage = 2.0 V

The possibility of this effect being caused by a capacitor discharging was also considered, but

given the high time period it is highly unlikely of a capacitor of capacitance ~ 0.025F to be

present alongside the diode. Besides, the results of the following section show that there is

capacitance but of the order of nanoFarads.

Time in secs Current in mA



0 0.3342

145 0.5679

285 0.695

15 0.3629

155 0.5796

295 0.7029

25 0.3836

165 0.5914

305 0.7099

35 0.4022

175 0.6021

315 0.7163

45 0.4244

185 0.612

325 0.7227

55 0.4427

195 0.6222

335 0.7279

65 0.4599

205 0.6298

345 0.7338

75 0.4757

215 0.6381

355 0.7395

85 0.4903

225 0.6463

365 0.746

95 0.504

235 0.6559

375 0.751

105 0.5192

245 0.6656

395 0.7645

115 0.534

255 0.6739

405 0.768

125 0.5444

265 0.6808

425 0.7794

135 0.5573

275 0.6884

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3) Voltage characteristics on application of square wave pulse:

The following circuit was implemented, with the use of a Function Generator at 1kHz and the

characteristics were studied by using a Oscilloscope:

Figure 10 - Circuit Diagram B

Figure 11 – Circuit Set-up B

The experiment was once carried out for -3V to 3V and for 0V to 6V using both the sample

made and a normal LED. The data were collected on a PC and analyzed separately.

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Vin = -2.96 V to +2.96 V

LED:

Figure 12 – Plot of V(in) and V(out) for LED and V(in) changing from -3 V to +3V

Voltage across LED in forward bias= 2.96 – 1.16 = 1.80 V

Diode A:

Figure 13 - Plot of V(in) and V(out) for Diode A and V(in) changing from -3 V to +3V

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Voltage drop across the diode = 2.96 V – 0.56 V = 2.4 V.

Resistance of Diode in forward bias = (2.4/0.56)*1 = 4.3 kΩ

• The sample prepared follows the first requirement of a diode. It allows current to flow

in the forward direction and prevents it from flowing in the reverse direction.

• There is substantial capacitance in the diode, because of which we get a curve like that

of a differentiator circuit. The charge stored in the capacitor continues to discharge

even after the voltage is reversed.

• In the reverse direction, the slope of the curve changes twice in the course of the

value returning to zero. Initially it was thought that it is indicative of the fact that there

are 2 capacitors at work, the effect of one of which changes with time. But on closer

inspection it was noticed that the input voltage had developed a kink in that region.

Somehow, due to momentary heavy current drawing, the Voltage source fails to keep

the same negative Voltage for a period of about 66 µs. It was further found that this

effect reduced and became indiscernible, as the Voltage amplitude was brought down

to zero. Because of this analysis of the negative regime would largely prove to be futile.

Figure 14 – Magnified view of region of voltage direction changing

It is to be noted that these magnified values were recorded after some heating had

already taken place in the circuit and hence the resistance being lower, voltage drop

across the diode is lesser and that across the 1kΩ resistor is more.

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The capacitance in the forward direction was obtained by curve fitting of an exponentially

decaying curve using OriginLab.

In the curve, y = V0 + A*exp(-t/τ) , subjected to the constraint V0 + A = 5.9 (theoretical

Voltage value from which capacitor starts discharging), the coefficients obtained by

Levenberg-Marquardt iterations are as follows:

V0 = 0.96+ 0.07 V

A = 4.9 + 0.1 V

τ = (4.8 + 0.1 )x 10-7

s

From equation, at t = ∞, Voltage drop across diode = 2.96 – 0.96 = 2.00V

Thus, diode resistance in forward bias R = (2/0.96)*1 kΩ = 2.1 kΩ

Hence from the following model I of

circuit (in forward biased region), we

get

By circuit analysis, Equivalent

resistance = 3.1 kΩ

Capacitance C = τ/R = 1.6 nF

Another model II of circuit (in

forward biased region), is given

alongside.

From this,

By circuit analysis, Equivalent

resistance = (1*2.1)/(1+2.1) = 0.68

kΩ

Capacitance C = τ/R = 7.1 nF

Figure 15 – Possible simple equivalent circuit A

Figure 16 – Possible simple equivalent circuit B

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(i) Vin = 0 V to +5.92 V

LED:

Figure 17 – Plot of V(in) and V(out) for LED and V(in) changing from -3 V to +3V

Voltage across LED in forward bias= 5.92 – 3.92 = 2.00 V

Diode A:

Figure 18 – Plot of V(in) and V(out) for Diode A and V(in) changing from 0 V to +6V

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Voltage drop across diode = (5.92 – 1.52) V = 4.40 V

Resistance of Diode in forward bias = (4.4/1.52)*1 = 2.9 kΩ

It is evident from the curves that the slope in the forward and reverse biasing are

different. Thus it indicates that the Time constant during the 2 cases are different. If we

model the diode as having only one capacitance, it must be having a resistance in parallel

to it, that becomes infinite on reverse biasing. This might be in addition to another series

resistance within the diode. So we may model the diode as follows:

Figure 19 - Model for diode circuit that can explain the results of both the forward and reverse biases

Figure 20 – Magnified view of region of voltage direction changing for forward bias, V(in) changing from 0 V to +6V

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Note: The input voltage is a square pulse from 0.08 V to 5.80 V.

The forward bias region of the curve, was fitted on to an exponentially decaying polynomial

curve, y = V0 + A*exp(-t/τ) , subjected to the constraint V0 + A = 5.8 (theoretical

Voltage value from which capacitor starts discharging), the coefficients obtained by

Levenberg-Marquardt iterations are as follows:

V0 = 1.4 + 0.1 V

A = 4.4 + 0.1 V

τ1 = (5.6 + 0.1 )x 10-7

s

From equation, at t = ∞, Voltage drop across diode = 5.8 – 1.4 = 4.4 V

Thus, diode resistance in forward bias = (4.4/1.4)*1 kΩ = 3.1 kΩ

From Figure 19, R1 + R2 = 3.1 kΩ …(1)

Equivalent resistance as seen from capacitor = (R1*(R2 +1))/(R1 + R2 +1)

Thus Time constant τ1 = 5.6 x 10-7

= C(R1*(R2 +1))/4.1 … (2)

Figure 21 – Magnified view of region of voltage direction changing for reverse bias, V(in) changing from 0 V to +6V

The reverse bias region of the curve, was also fitted on to an exponentially decaying

polynomial curve, y = V0 + A*exp(-t/τ) , subjected to the constraint V0 + A = 5.8

(theoretical Voltage value from which capacitor starts discharging), the coefficients

obtained by Levenberg-Marquardt iterations are as follows:

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V0 = 0.1 + 0.1 V

A = 5.7 + 0.1 V

τ2 = (2.7 + 0.3 )x 10-7

s

Now from Figure 19, the circuit is only a discharging circuit having a capacitor C and

resistors R2 and 1 kΩ. So τ2 = 2.7 x 10-6

= C*(R2 +1) …(3)

From (1), (2) and (3),

R1 = 0.85 kΩ, R2 = 2.25 kΩ, C = 0.8 nF

Figure 22 - Model for diode circuit that can explain the results of both the forward and reverse biases

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Discussion and Scopes of Improvement

The diode that was fabricated acted like a diode but it had many defects most visibly the high

resistance and capacitance. None of the results regarding the calculation of the resistances

and capacitances are very conclusive as no two results exactly support each other. However

we can say that that the diode has a resistance of the order of kΩ and a capacitance of the

order of nF.

A better analysis could have been possible had the diode been studied for a few more ranges

of square waves.

The capacitance and high resistance can be arising from :

a. Formation of oxide layer on the silicon wafer. This possibility could have been

removed by treating the Silicon wafer with HF before using it as a substrate.

b. An intrinsic property of the silicon wafer, caused during the formation of the

gradient in the doping concentration

Also there is substantial effect of temperature on the resistance of the diode, which again

makes the results dependent on how long the experimentation is on. This effect can be

reduced by using some mechanism of dissipating away the heat generated, in the thick Si

layer.

Since the voltage sources are not very sharp in the transition from +ve to –ve Voltage, it was

not possible to exactly measure the reverse recovery time, which is supposed to zero for

Schottky diodes.

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References

Papers:

• Metal-semiconductor Contacts for Schottky Diode Fabrication – Mark D. Barlow

• Comparison of Current-Voltage Characteristics of n- and p-Type 6H-SiC Schottky Diodes

– Q. Zhang, V. Madangarli, M. Tarplee, and T.S. Sudarshan

• Schottky Contact Barrier Height Enhancement on p-Type Silicon by Wet Chemical Etching

– G. A. Adegboyega, A. Poggi, E. Susi, A. Castaldini, and A. Cavallini

Web-sites:

• http://en.wikipedia.org/wiki/Schottky_diode

• http://www.radio-electronics.com/info/data/semicond/schottky_diode/schottky_barrier_diode.php

• http://journals.iop.org/

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