ELECTRICAL-PROPERTIES OF BORON-DOPED DIAMOND FILMS AFTER ANNEALING TREATMENT

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DIAMOND

MATERIALS

Diamond and Related Materials 4 (1995) 451-455

Electrical properties of boron-doped diamond films after annealing

treatment

Chia-Fu Chen, Sheng Hsiung Chen

Institute of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan

Abstract

Trimethylborate (B(OCH,),) was used to obtain boron-doped (p-type) polycrystalline diamond thin films on a silicon substrate using microwave plasma-assisted chemical vapor deposition. The effect of the doping agent (trimethylborate) was characterized in terms of electrical properties, and the current-voltage (I-V) characteristics of the boron-doped diamond films using aluminum as contacts at room temperature were examined. A surprising result was that annealing improved the electrical properties of boron- doped diamond films. As the boron concentration increased, the forward current decreased because of the effects of impurity scattering, and the I-V characteristics were still rectified. After the samples were annealed in a helium ambient at 900 “C for 30 min, the I-Vcharacteristics changed: the higher the concentration of boron in the doped diamond films, the more ohmic was the observed behavior. The differences in the I-Vcharacteristics of annealed and non-annealed films were subsequently confirmed by Fourier transform infrared spectroscopy (FTIR), capacitance-voltage (C-V) measurements, and cathodoluminescence (CL) experiments. The results of these experiments demonstrated that the annealing treatment caused boron atoms initially in inactive sites (e.g. interstitial or clustering sites) to be incorporated into substitutional sites. The increase in substitutional boron atoms altered the I-V characteristics of the boron-doped diamond films.

Kepordst Doping p-type; Infrared absorption; Polycrystalline diamond films

1. Introduction

It is well known that diamond films possess semicon-

ductor properties. Because of their large energy gap

(5.45 eV), chemical inertness, and temperature stability diamond films are a prospective material for microwave,

power and high-speed applications under severe condi-

tions such as high temperature and irradiation environ- ment. In particular, diamond films are also attractive

materials for high-frequency and high-power devices

because of their large saturation velocity (2.8 x 10’ cm s-l) [ 11, low dielectric constant (5.5), high breakdown

field ( 106-10’ V cm-‘) [2,3] and high electron and

hole mobility (2000 cm2 VP’ s-l and 1800 cm2 V-’ s-l respectively) [4]. To take advantage of these excellent properties, adequate control of doping, in particular by the in-situ method, is required. At present, only polycrys-

talline diamond films can be deposited on large areas

and only p-type doping (by boron) is possible. P-type

diamond films presenting good semiconductor proper-

ties have been reported [S]. The most common doping

procedure for CVD diamond films is to add a boron-

containing compound to the reaction atmosphere. Solid,

liquid, and gaseous boron-containing compounds such

as diborane (B2H6) [6] and boron trioxide (B,O,) have

been used [7]. In addition, boron doping of diamond

films can also be performed by means of ion implantation with boron ion beams followed by annealing for activa- tion of the dopant atoms and recovery of the implant-

ation damage [8]. The effectiveness of doping in

polycrystalline CVD diamond by 10’3-10’6 crnp2 boron

implantation followed by annealing at 800 “C has been

studied by Z(T/,T) measurements [9]. Our previous

report [lo] demonstrated the use of trimethylborate

(B(OCH,),) mixed with CH,-CO, gas mixtures to

synthesize good quality, uniformly doped diamond films.

The purpose of the research reported here was to

investigate the electrical properties of boron-doped dia-

mond films synthesized by a microwave plasma CVD

method using B(OCH,), as a doping source. In order

to investigate the effects of annealing on the Z-v

characteristics of the boron-doped diamond films, the

as-deposited diamond films were annealed in helium

ambient at 900 “C for 30 min. Fourier transform infrared

spectroscopy (FTIR), capacitance-voltage (C-V) meas-

urements, and cathodoluminescence (CL) experiments

were used to examine the effects of annealing on boron- doped diamond.

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452 (I-F Chm. S. H. ChenjDiumond und Related Mutrriuls 4 ( 1995) 4.51-455

2. Experimental details

The samples used in this work were fabricated by

standard microwave-plasma enhanced chemical vapor

deposition processes [ 111, The experimental conditions are listed in Table 1. N-type Si (100) substrates were

scratched using diamond powder (approx. l-2 mm) to

enhance the nucleation density of deposits. The scratched

Si substrates were then dipped into a solution of

HF:H,O= 1:lOO for a few seconds to remove the native oxide of the silicon. A vapor source of trimethylborate

was introduced to the CH,-CO2 gas mixtures. The flow

rate of trimethylborate was varied from 0 to 5 seem

using a mass flow meter.

The as-deposited diamond films were annealed in a

helium ambient at 900 “C for 30 min. In an effort to

remove any non-diamond surface layer, the diamond

films were cleaned in a saturated solution of CrO, in

H,SO, at 170 “C for 10 min followed by a rinse in a 1:l

boiling solution of H,O, and NH,OH. The diamond

films were then thoroughly cleaned with DI water and

dried with a jet of nitrogen gas. Conventional photoli-

thography was used to pattern thin photoresist in a

positive mask of circles of various sizes (approx.

7.5 x 10e4 cm2) on the surface of the diamond films. The patterned samples were hardbaked at 140 “C for 30 min. Aluminum contacts (Z 5000 A) were formed by the lift- off method.

An HP-4145B semiconductor parameter analyzer was

used to measure the current-voltage (1-v) characteristics

of the aluminum-diamond contacts. For every experi-

ment, the applied voltage was varied from ~ 10 to 10 V.

The maximum operating temperature is 300 “C, achieved

by heating the measurement stage.

The infrared absorption characteristics of diamond

films were studied using a Nicolet 510 FTIR spectrome-

ter with a Michelson-type interferometer. Infrared data

were taken with a resolution of 2 cm-i over the range

4000-400 cm

’ .

Free-standing diamond films were pro-

duced from as-grown Si (100) substrate by the dissolu-

tion of the silicon using a mixture of HF and HNO,

(3:l) diluted by about 50% using DI water. Infrared

spectra recorded by using pellets of free-standing

Table I

Experimental conditions

Microwave power Substrate temperature Reaction pressure Gas flow rate CO, CH, B(OCH,), Type of substrate Reaction time 400 w 820 ‘C (approx.) 25 Torr 30 seem 19.2-22 seem o-5 seem N-Si ( 100) 1.5 h

diamond films compressed with KBr provide better

resolution of the boron-doped diamond films.

An IBM PC controlled HP4275 LCR meter was used

to measure the capacitance-voltage (C-V) characteristics

of the aluminum-diamond contacts. The calibration of

measurement units was executed carefully to reduce the

measurement error. A frequency of 1 MHz was used in

this work. The doping concentration profile was deter-

mined through the following equations [ 121:

w=:

-lxC3 N,=

q&A2(dC/dV)

where E, is the permittivity of GaAs, NA is the doping concentration, Vbi is the built-in potential and W is the depletion width.

The calculation of the doping profile through the

above equations is the so-called depletion approxima-

tion. In fact, the doping concentration profile measured from the depletion approximation is the majority carrier

concentration profile rather than the doping concen-

tration N,.

3. Results and discussion

3.1. I--l/characteristics and eflects ~f’annealing lliamonrl ,jilms

The 1-l’ characteristics of aluminum contacts on

boron-doped polycrystalline diamond films deposited

from gas mixtures with different boron concentrations are shown in Fig. 1. To explore the differences in I&I/

characteristics, the trimethylborate flow rates used in

the experiments were varied from 0.5 to 2 seem. The

I-V characteristics were obtained by applying a voltage

to the Al contact on the diamond film surface while

earthing the rear of the Si substrate. Poor rectifying characteristics were obtained as a result of excess reverse

leakage current. As the boron concentration increased,

the forward current decreased. This deterioration of the forward current may be explained by impurity scattering

in the diamond film. For a given temperature, the

mobility decreased with increasing impurity concentra-

tion because of enhanced impurity scattering [ 131. The smaller forward current was due mainly to the smaller hole and electron mobility.

The I-V characteristics of the as-deposited diamond

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C.-F. Chen, S. H. Chen/Diamond and Related Materials 4 (1995) 451-455 453 6 / ’ ” I ’ ” I ” I I / ” f ---B ~undoped 4 _- _{-C : Ei(OCH3)J=0.5 scan} - - -D : B(OCH3)3=1.0 scan g 2 - E B(OCH3)J=2.0 scan -4 -6 -10 -8 -6 -4 -2 0 2 4 Voltage, v

Fig. 1. 1-V characteristics of Al contacts to “as-deposited” diamond films with various dopant concentrations.

a constant annealing temperature of 900 “C in helium

ambient for 30 min are shown in Fig. 2. After the

annealing treatment, there was no deterioration in the

Z-T/ characteristics of the undoped diamond films. For

the boron-doped diamond film (B(OCH,), =2 seem),

the leakage current for the 10 V forward voltage

increased by a factor of 2 and deterioration in the

rectifying characteristics was observed. Increasing

the B(OCH,)3 flow to 5 seem caused the diamond films

to exhibit more ohmic behavior. We are interested in

the differences in the Z-v characteristics of the boron-

doped diamond films caused by annealing prior to

metallization. We speculated that the annealing would

cause boron atoms doped into the diamond films and

located in inactive sites (e.g. interstitial and clustering

30 20 ^4 lo V 2 g 0 5 -10 -20 -30 ““,“‘,‘,‘,,‘,,“‘,,,’ ~ B : B(OCH3)34 scan I _ -C : E3@CH3)+0 scan r’ _ _ -D B(OCH&=5.0 scan : I 1’ / .lO -8 -6 -1 -2 0 2 4 Voltage, V

Fig. 2. I-V characteristics of Al contacts to “as-deposited” diamond films with various dopant concentrations after 900 “C annealing treatment.

sites) to be incorporated into substitutional sites in the films. The increase in the number of substitutional boron atoms would then change the Z-I/ characteristics of the

boron-doped diamond films because of the tunnelling

effect. This hypothesis was examined in our FTIR and

CL experiments from the viewpoint of bonding behavior, carrier concentration, and CL emission intensity. 3.2. FTIR analysis of diamond films

FTIR experiments were carried out to examine the

difference in the bonding behavior of boron atoms in

the diamond films that resulted from annealing in helium ambient at 900 “C for 30 min.

To more clearly resolve the absorption due to the

incorporation of boron in the lattice of the diamond

films, spectra were taken from samples compressed with

free-standing diamond films and dry KBr powder.

As shown in Fig. 3(a), a broad band was detected in

the region of 3000-3500 cm-’ due to O-H stretching

vibrations of molecular water physisorbed on the surface

of the diamond or diamond-KBr pellets. A peak due to

O-H bending vibrations was also found at 1650 cm- I.

This result suggests that the diamond surface readily

adsorbs atmospheric water on the polar carbonyl groups

[ 141. The weaker band appearing at 2450 cm-’ was

identified as the O=C=O stretching vibrations band.

This means that a small amount of CO, was physically

adsorbed onto the sample surface from the atmosphere.

A sharp peak at 1380 cm-’ and a small peak at around 2980 cm-’ were due to bending and stretching vibration

bonding of C-H, respectively. The peaks at 620, 1180

and 1360 cm- 1 were due to boron present in the boron-

doped diamond films. These well-known bands have

been defined as the O-B-0 vibration [15] and B-H

deformation bands [ 161.

When the diamond films were annealed and then

compressed with KBr powder into transparent pellets,

r

Wavenumber (cm-’ )

Fig. 3. Infrared absorption spectra of pellets formed from diamond films compressed with KBr powder. The doped diamond films were deposited using CO,= 30 seem mixed with CH,= 19.2 seem, B(OCH3)3 = 2 seem (a) as-deposited (b) after annealing.

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454 C.-F Chen, S.H. ClwnlDiamond and Related Matrrials 4 i 1995) 451 -455

infrared absorption spectra of the annealed films were

as shown in Fig. 3(b). After annealing, the spectra of the diamond films contained significant bands in the region

of 2800-2980 cm I, which can be ascribed to C-H

stretching vibrations of sp3 hybridized bonding. This

indicates that the annealing caused hydrogen atoms

present in the diamond films to migrate from inactive to active sites and bond with carbon atoms.

3.3. Carrier concentration measured by CL measurement

Cathodoluminescence spectroscopy were used to

examine the difference between the boron atoms in

annealed and non-annealed diamond films. The CL

spectrum of undoped diamond film shows a main emis-

sion peak at 2.8 eV. For the boron-doped diamond films, the emission peaks were located at 2.352.40 eV and the

intensity was higher than that of undoped diamond

films. The variation of the emission band between

2.0-3.5 eV can be explained directly by the distance

between D-A pairs [ 171. The increase in the CL inten-

sity at 2.4 eV can be explained by the increase of

substitutional boron, which becomes an acceptor in the

D-A pair recombinations. As shown in Fig. 4, the intensity of CL at 2.4 eV increased dramatically after annealing was performed. From the change in the intensity of CL at 2.4 eV, it appears that the amount of substitutional

boron in the boron-doped diamond films increased

because of the annealing treatment. The number of

substitutional boron atoms increased, resulting in an

increase in the CL intensity at 2.4 eV after annealing.

This result confirmed again that inactive boron atoms

were activated and the I-V characteristics of the boron-

A 0 Photon energy eV 3.0 2.5 22 20 18 I I I I I 400 500 600 700 Wavelength (nm)

Fig. 4. CL spectra of diamond films deposited using CH,!CO,= Fig. 6. Carrier concentration of boron-doped diamond film using 19.2130 seem and B(OCH,), = 2 seem: (a) as-deposited and (b) after CH,;CO,= 19.2!30 seem and B(OCH,),= 2 seem diamond film at

annealing treatment. 900 ‘C with annealing treatment.

doped diamond films were altered by the annealing

treatment.

3.4. Carrier concentration measured by C-I/method

The C-V curves of the Al-diamond contacts were

performed (not shown here) with boron concentration

equal to 3.8% in the gas phase. Assuming the dielectric constant c (=5.7 e,,) is uniform over the entire film, the

depth W from the surface is given by c/C. Inserting

experimental values in the expressions in W and N,, we

can derive the depth profile of the activated boron

concentration beneath the Al electrode. as shown Fig. 5 and Fig. 6. Fig. 5 shows the depth profile of carrier

concentrations in the boron-doped diamond film (B =

3.8%) before the annealing treatment. It is interesting to

compare the mean N, values obtained from the

1o19 I 1 I t

10’6

1

/ 42 44 47 49 52 Depth (elm)

Fig. 5. Carrier concentration of boron-doped diamond tilm using CH,!CO, = 19.2;30 seem and B(OCH,), = 2 seem

1

30 31.1 32 3 _33.4 _34.5

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C.-F. Chen, S. H. ChenlDiamond and Related Maierials 4 (1995) 451-455 455

depletion approximation measurement, representing a

selected region about 0.4 pm in depth, with those

obtained previously from SIMS measurement. The

boron concentration derived from the B = 3.8% diamond

films using depletion approximation is 6 x 1016 cmP3

and that obtained using SIMS measurement is

2.2 x 102’ cme3. It is apparent that the SIMS measurement overestimates the value of the boron concentration by a factor of almost 4. We believe the SIMS measure-

ment detected the quantity of the boron atoms in the

diamond films. These boron atoms perhaps located at

the substitutional, interstitial, or clustering sites in the diamond films. Unlike the SIMS technique, the depletion

approximation technique directly measures the number

of charges in the depletion layer. As a result, it is

reasonable for there to be a difference between these

two values.

Fig. 6 shows the depth profile of the annealed boron-

doped diamond film using the depletion approximation

technique. It is evident that the depth distribution of carriers is uniform in the region of 0.31 pm to 0.345 pm.

The carrier concentration in the annealed boron-doped

diamond film is about 1 x lo’* cmm3, which is almost

two orders of magnitude larger than the value for the

non-annealed diamond film. This large increase in the

amount of carriers probably results from the large

number of acceptors. In the boron-doped diamond films,

boron atoms are the dominant acceptors, and the

number of acceptors increases after annealing.

4. Conclusions

At room temperature, the 1&-I/ characteristics of

boron-doped diamond films using aluminum as contacts

exhibited rectifying behavior. As the boron concen-

tration increased, the forward current decreased, owing

to impurity scattering and rectifying behavior. After the

samples were annealed in a helium ambient of 900 “C

for 30 min, the 1-V characteristics changed: the higher

the boron concentration in the doped diamond films,

the more ohmic behavior they presented. CL measure-

ments confirmed that the intensity at 2.4 eV increased

after the annealing treatment was performed. These data

indicate that more substitutional boron atoms were

obtained as a result of annealing and that annealing

altered the Z-T/ characteristics of the boron-doped

diamond films.

Acknowledgments

The authors are pleased to acknowledge financial

support from the National Science Council of China

under contract number NSC 83-0416-009-014.

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