Ultrafast electronic processes in CVD diamonds and GaAs:picosecond photoconductivity and high voltage switching.

s.v. Garnov a S.M. Klimentov a S.M. Pimenov a v.i. Konov a v.v. Kononenko a O.G. Tsarkova a

S. Gloor b w. Luthy b H.P. Weber b

a General Physics Institute ofRussian Academy of Sciences, Vavilov st. 38, 117942 Moscow, Russiab Institute ofApplied Physics, Sidlerstrasse 5, CH-301 2 Bern, Switzerland

ABSTRACT

An "electrode-free" transient photoconductivity technique was applied to investigate excitation, drift andrecombination of non-equilibrium free charge carriers in high quality synthetic polycrystalline diamond films, naturaldiamond crystals and low-conductive GaAs with a time resolution better than 200 ps. Picosecond laser pulses of UV, visibleand IR spectral range were applied for single-photon excitation of free charge carriers with initial concentrations of (10121019) cm3. Dependences of amplitude and duration of photocurrent on laser intensity/carrier density were measured.Lifetimes, drift mobilities and carrier photoexcitation cross sections as a function of electron concentration were estimated.Computer calculations of conduction and displacement currents, induced space charge and electric field spatial distributionhave been performed for the real experimental conditions.

Based on the obtained results, high voltage diamond-based switches triggered by ultra-short laser pulses have beendesigned. Special attention was paid to metal-dielectric interface investigation and ohmic contacts formation. The developeddiamond-base module permitted to switch electric fields as high as 100 kV/cm within a time interval less than 200 ps. Theamplitude ofphotocurrent reached 100 A and the electrical resistance reduced by a factor of 1O'°.

Key words: CVD diamond films, GaAs, photoconductivity, high voltage switching, picosecond pulses.

1. INTRODUCTION

The goal of the work was to develop and apply a new "electrode-free" experimental technique of transientphotoconductivity (PC) to obtain information on charge carries behavior in polycrystalline diamond films (DF), naturaldiamond single crystals and GaAs, which is necessary for the designing of high voltage fast optoelectronic devices —voltage/current switches and radiation detectors.

In recent years the interest to diamond films as to the elements of high voltage ultra-fast optoelectronic devices hasbeen steadily growing due to the apparent progress in CVD diamond technology providing controllable synthesis of highquality thick diamond layers, which optical and electrical parameters become very close to those of high quality naturaldiamond single crystals.

The potential advantages of diamond-based electronic and optoelectronic devices are certain to result from the uniquecombination of diamond properties: high thermal conductivity (that is five times that of copper), high electric breakdownfield strength (more than lO V/cm), wide band gap, superior resistance to radiation damage, high mobility of charge carriers(compared to those in metals), ultra-short free carrier life-times belonging to sub-nanosecond -picosecond range. The mainfeature that makes the use of diamond materials ideal in high power optoelectronics is a unique combination of highresistance at normal conditions and high electric conductivity arising after illumination of a specimen by laser radiation. Itmeans that diamond actually reveals both dielectric and metallic properties.

The comparison of diamonds with GaAs, traditionally used in fast electronic devices, is quite important todemonstrate both the advantages and limitations inherent to the materials.

SPIE Vol. 3287 • 0277-786X1981$1O.OO67

68

2. SAMPLES

The materials tested were the high quality CVD diamond films, natural diamond single crystals and GaAs wafers.Different samples of free-standing diamond films grown by arc-jet deposition technique were examined 1,2 Both

"as-grown", grainy films (with the average grain size of 20- 1 00 microns) and mechanically-polished specimens were tested.The thickness of the samples varied from 230 to 890 microns. Before measurements, the samples were treated with H2-plasma to remove residual carbon and after that were annealed in air at 500 C during 1 hour to reduce (by orders ofmagnitude) the induced surface conductivity associated with the H-enriched layer formation . The only feature observed inthe Raman spectra ofCVD films was a strong single peak near 1332 cm' which showed no noticeable amorphous carbon orgraphite inclusions '. The samples of natural diamond were: i) the type ha highest quality gemstone with the totalconcentration of impurities (nitrogen centers) as low as iO' cm3 and ii) the type Ia crystal with the noticeable density ofnitrogen centers about 1 021 cm3.

GaAs samples were low-conductive ((3-6)lO cm) high quality 450 micron thick wafers.

3. TRANSIENT PHOTOCONDUCTIVITY TECHNIQUE

It is well known, that photoconductivity is the most informative technique of charge carriers investigation, whichfeatures are the highest sensitivity and temporal resolution. The PC demonstrates the apparent superiority over altemativemethods of free carriers detection (e.g. ones based on luminescence and photoacoustic measurements), because theinformation on the mechanisms of radiation absorption, presence of electrically active impurities, processes of free carriersexcitation and ftirther dynamics ofexcited states, can be directly extracted from the experimental results in a straightforwardway without, as a rule, employing any additional theoretical hypothesis on mechanisms of radiation-matter interaction.These features of PC techniques are especially useful for investigations ofwide-band gap materials exhibiting relatively lowvalues oftransition probabilities and very short recombination times, belonging to the nano-picosecond range.

The extensive study of electrical properties of polycrystalline diamond films using a time-resolvedphotoconductivity method has been carried out in . A microstripe transmission line was chosen for current measurementsand picosecond laser pulses of 5 ps pulsewidth and 6. 1 1 eV photon energy were applied for a sub-surface carriers excitation.It has been demonstrated, that the drift mobilities and lifetimes of free carriers were strongly dependent on the quality ofdiamond films. The combined electron-hole mobility as high as 4000 cm2/Vsec and the carrier lifetimes ranged from 150 psto 1 ns have been measured in the best samples. The obtained values were comparable to those measured in the ha diamondsingle crystals. Both the drift velocity saturation and electron-hole scattering were observed. The estimated drift distance ofcarriers was as high as 20 microns at 1 0 kV/cm. It was concluded, that the significant improvement of the electronicproperties ofDF could be achieved with the increase in the film thickness.

However, certain limitations peculiar to the applied microstripe transmission line technique (e.g. strong absorption ofUv radiation in a thin surface layer) limited the accuracy of the measurements and obscured the features of charge carriersbehaviour inside the specimen volume far from the surface.

In the reported work we have developed and applied a transient photoconductivity method allowing measurements ofphotocurrent inside a bulk of a transparent material with the time resolution better than 200 picosecond and sensitivity(minimal detectable free carrier concentration) better than 1 012 cm3. As compared to other commonly usedphotoconductivity techniques, the applied method permits to avoid the main limitation connected with the problem ofelectrical contacts deposition and metal-dielectric interface formation. Because the conventional PC based techniques implythe recording of an electric current flowing through the ohmic (metallic) electrodes, the "quality" of the contact and theproperties of the interface can strongly affect the obtained results. For example, the illumination of a tested sample near thecontacts may give rise to an uncontrollable excitation of charge carriers at the interface area. Any imperfections of thecontacts, resulting in a non-ohmic interface resistance, cause the formation of a space charge which, in turn, blocks thecurrent flowing and masks the intrinsic electronic processes.

Meanwhile, there is an apparent way enables one to avoid, to some extent, the problem of contacts. It consists indirect measurement of a displacement current inside the specimen volume. Indeed, any alternating current, induced in aclosed electric circuit, keeps its value in every point of the circuit even though the real charge movement may not beprovided through a certain element of the circuit (for example, through a series capacitor). In other words, if free chargecarriers, excited inside a specimen bulk far from the interface, are moving under the action of applied electrical field, thecorresponding electric current is flowing both inside and outside the specimen in all external elements of a closed electriccircuit. This current is composed of a conduction and a displacement parts and its magnitude and temporal shape aredependent on the intrinsic specimen properties and the electric circuit parameters. The simplest configuration providing free

carriers recording in a such way, consists of three consequently connected elements: a voltage supply, a tested specimen,placed between two flat metallic electrodes, and a load (active) resistor. The process of measurement can be qualitativelydescribed as follows. Prior to specimen illumination (before free carriers excitation) there is no current in any place of thecircuit. (It is supposed, that the dark conductance of a specimen is low enough and the bias voltage does not cause anynoticeable tuimeling effects or initiate any sort of discharge. For diamonds and GaAs these assumptions are quitereasonable). Under the illumination of the tested area by a short laser pulse, the excited free carriers start to be moving inthe bias electrical field. If the laser pulse duration is less than the typical carrier relaxation/recombination times, theamplitude of the induced current is governed, at this stage, exclusively by the bias field, the carrier drift mobility, carrierconcentration and the electrical parameters ofthe circuit (the value of intrinsic capacity, load impedance, etc.). Moreover, ifthe typical time-scale of the process (carrier lifetime) is less than the Maxwell relaxation time (the time of electrostaticshielding), the temporal behavior ofthe recorded current adequately describes the intrinsic kinetic of carriers.

Taking into account all the mentioned above, we have developed an experimental setup for high speed, highsensitive investigation of electronic processes in wide gap dielectrics and semiconductors. It was successfully applied earlierfor the detailed experimental studies of multiphoton carriers excitation in the perfect solid state dielectrics -alkali halidecrystals 1,5 One-, two,- three- and four-photon processes of free electrons excitation have been recorded applying similartransient PC method and the corresponding parameters - excitation cross sections, drift mobilities, recombination times, havebeen measured.

The principle scheme ofthe experimental setup is shown at Fig. 1.

Fig. 1 . Experimental arrangement for transient photoconductivity measurements.

A picosecond YAG:Nd laser emitting high stable, spatially and temporarily homogenous Gausian pulses at thebasic (1064 nm), second, third and fourth harmonics was used as a source of excitation light. The pulsewidth at 1064 nmwas 50 picoseconds and consequently decreased after each stage of frequency conversion according to the "square-rootlaw". The temporal profile ofthe laser pulses was checked by a streak camera with the time resolution of2 picoseconds.

To record the photoconductivity signals, a specimen was positioned between flat electrodes to which an externalpulsed voltage with the amplitude up to 5 kV and duration of 100 microseconds was applied synchronously with the excitinglaser pulses. The pulsed external voltage permits to reduce essentially both a surface current leakage and an electrostatiêshielding resulting from the dark conductivity of the tested materials. The shielding reveals itself only at times higher thantdark CØC/Oj , where c0 is the permittivity of vacuum, c is the dielectric constant, ck is the dark conductivity. In the caseof GaAs we have: 0k 3 i09 Q1 cm' ,c13. 1 and td,k 400 microseconds, i.e. tdk is much higher the external voltage

69

SHG THG/FHG

X2=532nm X314355/266nm

External pulsed voltaget0= 100 ts, U0= 3.6 kV

Wedge Lens ( F=25 cm)

Co-axial shield

Energymeter

Load

Computer-controlled data Oscilloscopeacquisition/processing system

70

pulsewidth and the corresponding electrostatic shielding does not affect the results. This also is valid for diamonds, whichdark conductivity is much lower the 10b0 T'cm'

The distance between electrodes could be varied from 10 to 2 mm and the applied field strength reached 5kV/cm.The induced photocurrent caused the corresponding voltage signal on a load resistor (a cable with the wave impedance of50 � served as the load), which was recorded with a high-speed oscilloscope. The intrinsic temporal resolution of theelectrical circuit was about 1 00 ps, as it was measured using a sampling technique. The oscilloscope reduced the resolutionto 250 p5 or 1 ns depending on the amplifier applied. The laser pulse parameters were measured by a calibrated photodiode,a streak camera and a pyroelectric energy meter connected to a computer controlled data acquisition/processing system.

It was shown in 1,5 that the transient PC voltage signal U(t) may be presented as:

I 1 r e.iU0

UpC(t)=[UpC(O)j.i,i(t)=[a.Nk2.RQj.W(t) (1)

Here, e and are the charge and the combined drift mobility of electrons and holes, U0 is the external voltageapplied to a specimen, R is the load resister, L is the distance between electrodes (specimen height), a is a geometricalparameter describing a deviation of the real external field spatial distribution from the plane capacitor approximation, Nk isthe total number of free carriers, c is the dielectric constant. Nk is linear dependent on the product of the k-photonabsorption coefficient 13k and the k-power of excitation intensity . For impurity mechanisms of excitation (k=l) the totalnumber ofcarriers N1 is:

Nk1 [tpuise10]ø, (2)

where, ha) is the laser photon energy, "tpu1se 5 the laser pulsewidth (for the Gaussian pulse). The expression in thebrackets is the initial concentration of carries n0 V is the excited volume.

As it is seen, the particular laser pulse intensity profile being known, the relationship (1) allows the value of r 3k tobe determined by measuring the signal amplitude U(O).

The expression (1) has been obtained taking into account all the mentioned above and the following assumption:t<<'tM where tM is the electrostatic shielding time (TM eco/n0ep). In this case the time-dependent parameter 'J'(t) correctlydescribes the intrinsic dynamics of free charge carriers. At high initial density n0 and high mobility .t the simple analyticformula (1) is no more valid and it is necessary to perform the numerical calculations of U(t). These calculations havebeen performed "from the first principles" for the one-dimensional model.

4. COMPUTER SIMULATION OF PHOTOCONDUCTIVITY

The completed system of equations describing the process of free charge carries drift is:

. - p(1,t)div(E) =

ôp(1,t) . -:at +div(j)=O (3)

j =a•E

c = e.[p(i,t).th —fl(r,t).LeI

p(,t) = e .[Pt— n(,t)e Jp(,t)dt—1. Jn(t)dt]0 te 0

Here, E is the electric field strength, 1c* ? ,t) is the charge density, ? is the space coordinate, C is the permittivityof vacuum, c is the dielectric constant, j is the conduction current density, a is the conductivity, .th and are the hole and

electron mobility, p(? ,t) is the free holes density, n( i ,t) is the free (conduction band) electron density. th is the linearrecombination time of holes, te is the linear recombination time of electrons determined as:

an(?,t) 1 -=——.n(r,t)j

For the one-dimensional model the system (3) is simplified and one can obtain the following equation describing,for example, free electrons drift and its linear recombmation in the case of an impurity mechanism of excitation:

3n(x,t)x

ôn(x t) [ 1 1

5te•E0J(X,t,fl)dX . + ——(x,t,n) •n(x,t)=O, (4)

where, expression (x,t,n) is:

(x,t,n) = ee[P(x) n(x,t)—1. Jn(xt)dt]. (5)

The expressions (4)-(5) were calculated numerically for the Gaussian exciting laser pulse with the intensitydistribution I(x,t):

I(x,t) I .exp—

•

exp[ ' J,xo Tpulse

where, tpulse 5 the laser pulsewidth, and the corresponding initial electron density distribution is:

( x2 ( t2no (x) 7 .I .

exp—) . 2 Jdt0 - pulse

where, ' is the excitation rate.The spatial distributions of carrier density, charge density and induced electric field strength as a function of time

so as the corresponding photocurrent J(t) flowing inside and outside the specimen have been obtained. Thephotoconductivity voltage signal is: U(t)R&J(t). The calculations were performed for the real experimental conditions(laser beam radius x0 =1 mm, pulsewidth 'tpulse =30 ps, applied external voltage U0 =3.6 kV, R0 =50 Q, L =6 mm) using themodel of the electric circuit shown at Fig. 2. The results of calculations are shown at Figs. 3-5.

71

Specimen

J(t)

Eexternai++++

+

electrons

UoExcited volume

x=0 x

Fig. 2. Principle scheme of photoconductivity computer simulation.

0

T\LiI'

2:.

fH

Fig. 3 . Computer simulation of free carriers drift in external field in diamond specimen.a) free electron density distribution, n0=1014cm3; b) space charge density distribution; c) electric field strength

distribution; d) photoconductivity voltage signal. Laser beam radius x0 = 1 mm, pulsewidth tpulse 30 p5, applied externalvoltage Uo =3.6 kV, R =50 Q, L = 6 mm, electron drift mobility lte 1000 cm2/Vsec, liner recombination time 'trec 1 ns.

Distance, mm Distance, mm

a) b)

>Q00000.0

Distance, mm

c)

2x10'° 4x10'° 6x10'° 8x10'° lxlO9

time, sec

d)

0>008Cs

Cs

55

>

00000

Fig. 4. Calculated amplitude of photoconductivitysignals in diamond as a function of initial free carrierdensity for different carrier mobility.

Specimen thickness 0.5 mm; L=6 mm, U0 =3.6 kV;electrode size (in plane) 10x15 mm2.

10CC 1012 i' tO'4 10C5

Initial free carriers density, cm3

0.0 0.5 1.0 1.5 2.0 2.5

time, nanosecond

72

Fig. 5. Calculated photoconductivity signalsin diamond. Shortening of signals as a result ofelectrostatic shielding/induced space charge formation.

Specimen thickness 0.5 mm; L=6 mm, U0 =3.6 kV;electrode size (in plane) lOxl5 mm2.

0

10'0

- 10°0(0 10

102

- 100C

iü

10

5. PHOTOCONDUCTIVITY EXPERIMENTAL RESULTS

The results of photoconductivity measurements are presented at Figs. 6 a, b. The dependences of PC (voltage) signalamplitude versus excitation laser intensity/energy: U0(O)°°f(I0), are demonstrated.

Intensity, I, W/cm2 Intensity, I, W/cm2

1.3x10° 1.3x104 1.3x105 1.3x10° 1.3x10° l.3x108 1.3x10° 1.5x104 1.5x105 1.5x10° 1.5x10° 1.5x108

—— I I I 10

102

— -y Natural Ia diamond 0. 1 mm, 3= 200 cm'

—I U=8* I 0' I*(3)*J n0=6.5* 1 O°I, [cm°]

—L52*10cm/V*sec, .t=2.5*102 cm2/V*sec

\ / I!J,'

. Natural ha diamond, 0.55 mm, 0.1 cm'

4 U=4I0'°(t)I0,n33*I07*I [cm3]

/11 3t=I02 crn/V*sec, 1=1 crn2/V*sec

0

C. 0

'0C

0.E(0

210'

>

'00 102

10'

• Natural Ila diamond o=0.55 mmiCVD diamond # I, ,0.23 mm

0 CVD diamond # 2, &0.89 mm

10-8 io b" io° io10-° 10' I0 l0° 10° 10" 10°

Energy, J Energy,

a) b)

Fig. 6. Photoconductivity amplitude dependences measured in natural and CVD diamonds.A is the specimen thickness. The curves at Fig. 6b are normalized to "the equal" thickness of 890 microns.Natural diamonds and CVD sample # 1 were polished ones; CVD #1 was "as grown". Nitrogen centerconcentration is about lO'7cm3 in type ha diamond and about 1021 cm'° in type Ia diamond. Specimen size inplane: 5x4(5) CVD samples were cleaned in H2 plasma to remove residual carbon and annealed in air at500 C during 1 hour to reduce surface conductivity. Laser pulses parameters: 2 =266 nm; pulsewidth = 30 ps;spot size diameter = 1 .7 mm. Applied external pulsed voltage: U0 3.6 kV; external electric field strength =l.l1O V/cm.

Both the natural and CVD diamonds reveal a linear U=f(I0) dependence at low initial concentration of carriers. Itmeans that free electrons arise in the conduction band of diamond specimens due to impurity mechanisms ofphotoexcitation. The saturation of the dependences at high intensities can be explained by a space charge formation, as itfollows from the calculations (see Fig. 4) . Note, that a possible field-induced drift mobilities decrease did not manifestitself in our experiments because a moderately low external electric field (about 1 O V/cm) was applied.

Fig. 6a shows the results obtained in natural diamond single crystals. Taking into account the expressions (l)-(2)one can obtain from Fig. 6a the recorded amplitude ofPC signals at the linear region. For Ia type diamond sample we have:U(O)=8 I 0" "13.t'Io , and, correspondingly, for ha type diamond sample: U(O)'4 lO'°f3tIo (U [VJ). Here, [1/cm] isthe linear absorption coefficient, t [cm2Nsec] is the drift mobility, I [W/cm2] is the laser intensity. Accordingly, thecorresponding product 35.2 iO cmlVsec (for the Ia sample) and 3t 1 102 cmfVsec (for the ha sample). The valuesof 3 were obtained independently in direct optical measurements. For Ia type sample: 3 =200 cm"; for ha type sample: 3=0.1 cm". Thus, it is possible to estimate the drift mobility values: t=25O cm2/Vsec for Ia type and tlOOOcm2/Vsec forlIa type specimen. Here we assume that all absorbed photons create free charge carriers. Because the optically measuredvalue 13 is actually consist ofboth an intraband absorption, causing no "free" carriers formation, and an absorption resultingin carriers transitions from impurity levels to the conduction band, the real value of mobility could be noticeably different(higher) from the estimated one. The dependences of initial carrier concentration on laser intensity (expression (2)) are : n0= 6.5lOI for Ia diamond and n0 3.31OI for ha diamond (no [cm]). Substituting the lowest and the highest intensity

73

values, one can obtain the ranges of the initial carrier concentration: (5 '1012 8 1018 ) cm3 and (2 1012 2 1016) cm3 for Iaand ha diamonds, correspondingly.

Fig. 6b, demonstrates the PC dependences obtained in different CVD diamond films at different specimen thicknessA. Besides, the latter figure presents the U=f(Io) dependence for high quality ha natural diamond crystal. To compare thesamples of different thickness the curves were normalized to "the equal" thickness of 890 microns.

It is interesting to note, that both polished and unpolished ("as grown") films demonstrate approximately the samebehaviour of PC signals. As one can see the electronic properties of the tested high quality CVD films are very close tothose ofthe most perfect ha natural diamond crystal.

The typical recorded signals are shown at Figs. 7-8.

74

1 2 mU/div 1.76 mU

a)

1 1 mU/div 3.50 mU

b)

Fig.7. Photoconductivity signals recorded in GaAs and CVD diamond film.a) GaAs, 1064 nm; time resolution 500 ps , recombination time rrec 8 iO sec;b) CVD diamond, X 355 nm; time resolution 500 ps, recombination time trec <i0 sec.

Fig. 8. Photoconductivity signals recorded in natural high quality ha type diamond.X=355 nm; time resolution 500 ps , recombination time Trec iO sec.

a) low initial concentration of free carriers : cm3b) high initial concentration of free carriers : n03 1 0 12cm3.

TTTTTTT— — — — — i — — — — — — — —

::———— — —.

——

— — — — — — — — — — — — — — — — —1.68 mU

a)

50 mU/div 50.9 mU

b)

6. HIGH VOLTAGE SWITCHING WITH DIAMONDS

The experimental studies of optoelectronic switching with diamonds have been conducting since the end of theseventieth. Among the other publications it worth to mention the works performed with natural diamond crystals 6,7 andrecently investigations with CVD diamond films8'9.

In 6 the laser induced high voltage switching in natural ha type diamond has been investigated using 7 ns KrC1excimer laser pulses at 222 nm wavelength. Switching efficiencies up to 50%were obtained for dc bias voltage up to 8 kV atroom temperature. Later , the investigation of optoelectronic switching in insulating diamonds (types Ia, Ib, and ha) andsemiconducting diamonds (type JIb) has been carried out with picosecond pulses of YAG:Nd laser and its second, third andfourth harmonics. It has been established that the insulating diamonds exhibited nearly the same response time of about 150PS, while for the semiconducting crystals the time varied from 50 to 300 ps. The output voltage was linear proportional to thebias (switched) voltage for 2 kV excepting ha type diamond, the latter being supposed due to space charge effect. For allwavelengths, the value of photocurrent was found to be linear with laser intensity up to 100 MW/cm2, indicating thatmultiphoton absorption was negligible and the main carrier excitation mechanism was the impurities photoexcitation. Thebehaviour of photocurrent observed in6 with nanosecond pulse excitation has been ascribed to the space charge accumulationnear the cathode. It has been concluded that for the purposes of high voltage optoelectronic switching in diamonds muchresearch is to be carried out with respect to the space-charge effects in these materials.

The photoconductive properties of CVD DF based optoelectronic switches, including grain boundary effects, wereinvestigated in . The proposed and applied over-coatmg of metal electrodes and photoconductive gap (which size was 10microns) by a thin diamond layer allowed to reduce the leakage current to less than i0 A and to switch high electric fieldsup to 2 106 V/cm within tens of picoseconds. The switch impedance as a function of laser intensity (KrF laser, X=248 nm)and average grain size (varied from 1 01 1 0 microns), has been measured. It was found that the carrier mobility-lifetimeproduct was linearly proportional to the dc electric field and decreased with an average gram size. The latter was attributedto the decrease in mobility and lifetime values inside the grain.

It seems, the reported in 8,9 results demonstrate the highest dc electric field switching made with diamonds. At that,the maximal switched voltage value was just a moderate one -about 2 i0 V, and the corresponding switched current wasabout 20 A. However, for many applications it is important to provide a switching of much higher voltages when switchedcurrent reaches hundreds of amperes. Up to now the problem of such high power diamond optoelectronic devicesdevelopment is far from the solution.

The investigations in this direction have been undertaken in our studies. Contrary to the previously applied andinvestigated configurations of photoconductive zone consisted in surface and sub-surface area photoactivation, the excitationof the whole bulk of a thick diamond film, mounted between electrodes, was suggested. This geometry allows to performswitching and transmitting currents as high as 100 A. The experimental arrangement of switching studies and the obtainedresults are shown at Figs. 9-10.

The mentioned above important problem of metal contacts deposition on a low-conductive dielectric surfacedistinctively manifests itself in optoelectronic switches designing. Contrary to photoconductivity experiments this problembecomes especially important because a switch has to provide the commutation ofthe real current flowing through the metal-dielectric interface. It is clear that the quality of the contacts must be the highest to provide the commutation without anycurrent losses, including the ohmic one, causing the heating and failure of electrodes.

Two techniques ofmetal deposition onto a diamond surface were applied in our studies: i) a direct metallization ofsurface area by a metal-organic liquid solution with the following specimen annealing at high temperature in air, and ii) alaser-assisted method consisting in a preliminary laser-induced etching of diamond surface (causing its partial graphitization)with the following metal-organic liquid deposition and annealing. The method ofactivation of diamond surface with laseretching has been found to combine the possibility ofthe deposition ofhighly adherent metal coatings with the producing ofstable ohmic contacts onto diamond surface.

The laser-assisted method reveals apparent advantages over the direct metallization technique. It permited tofabricate the contacts with the smallest residual contact resistance about several ohms. The contact resistance so as theswitched pulse amplitude were calculated from the main relationship for for a charged co-axial cable line:Ur = U0[ ZoI(2Z0 + R + Ri)], where, Ur and U0 are the recorded peak voltage and the initial charged line voltage,

respectively; Z0 is the cable-line impedance (75 Q); R is the residual specimen impedance; R is the contact resistance.

75

71)

b)

Charoedugh-voltage/current Load

100 MQ cahIeline DF switch module resistor-75Q

— Laser pulse radiationCapacity (2=26o/355 nm t3Ops)

Fig. 9. Ultra fast high-voltage switching with diamond films.a) experimental setup b) diamond film with metal contacts;c)the output ("switched") electrical pulse.

Fig. 10. An interdigitated electrode configuration fabricated at the mechanically polished surface of a 350 u.lm thickdiamond film by selective etching with a Cu-vapor laser and metallization with electroless copper.

Wide-band oscilloscopeH

Attenuator

A0-5GHz)

0.5-I mma)

Metalic contacts covered

__________ V with a polimer dieectric film1 JIr[Irl II

Diamond film 0.2-1 mm

Laser radiation

c)

-—- )200_jim

7. CONCLUSIONS

The "electrode-free" transient photoconductivity technique was successfully applied to investigation of free chargecarriers dynamics in polycrystalline diamond films, natural diamond crystals and GaAs with a time resolution better than200 ps. Dependences of amplitude and duration of photocurrent on laser intensity/carrier density were measured. Computersimulation of conduction and displacement currents, induced space charge and electric field spatial distribution have beenperformed for the real experimental conditions. The results of calculations of photocurrent coincide with the experimentaldata.

High voltage diamond-based switches triggered by ultra-short laser pulses have been designed. Special attentionwas paid to metal-dielectric interface investigation and ohmic contacts formation. It permitted to switch electric fields ashigh as 100 kV/cm within a time interval less than 200 ps. The amplitude of photocurrent reached 100 A and the electricalresistance reduced by a factor of 1010.

8. ACKNOWLEDGMENTS

This work was supported by Russian RFBR grant # 97-02-17710 and by the Swiss National Science Foundationunder the Project 7SUPJ048239.

9. REFERENCES

1 . S.V. Garnov, S.M. Pimenov, V.G. Ralchenko, S.M. Klimentov, V.1 Konov, KG. Korotoushenko, E.D. Obraztsova, S.P.Plotnikova, D.M. Sagatelyan, 'Picosecond photoconductivity of natural and CVD diamonds," Laser induced damage inoptical materials: 1994 ", SPIE Proc., 2428, pp. 134-143, 1995.

2. V.G. Raichenko, A.A. Smolin, V.1. Konov, K.F. Sergeichev, l.A. Sychov, 1.1. Vlasov, V.V. Migulin, S.V. Voronina, A.V.Khomich, " Large-area diamond deposition by microwave plasma", Diamond and Related Materials, 6, pp.4 17-42 1,1997.

3. MI. Landstrass, K.V. Ravi, "Hydrogen passivation of electrically active defects in diamond", Appi. Phys. Lett., 55(14),pp. 139l-1393, 1989.

4. a) L.S. Pan, DR. Kania, P. Pianetta, and O.L. Landen, "Carrier density dependent photoconductivity in diamond", App!.Phys. Lett., 57, 6, pp. 623-626, 1990.

b) L.S. Pan, DR. Kania, P. Pianetta, M. Landstrass, O.L. Landen and L.S. Plano, "Photoconductive measurements onmicrowave assisted plasma-enhanced chemically vapor deposited diamond films", Surface and Coating Technology,47, pp. 356-364, 1991.

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