Ž .Journal of Non-Crystalline Solids 227–230 1998 1164–1167

Room-temperature electroluminescence of Er-dopedhydrogenated amorphous silicon

Oleg Gusev a,), Mikhail Bresler a, Alexey Kuznetsov a, Vera Kudoyarova a,Petr Pak a, Evgenii Terukov a, Konstantin Tsendin a, Irina Yassievich a,

Walther Fuhs b, Gerhard Weiser c

a A F Ioffe Physico-Technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russian Federationb Hahn-Meitner Institut, Abteilung PhotoÕoltaik, Rudower Chaussee 5, D-12489 Berlin, Germany

c Phillips-UniÕersitat Marburg, Fachbereich Physik, D-35032 Marburg, Germany

Abstract

We have observed room-temperature erbium-ion electroluminescence in erbium-doped amorphous silicon. Electricalconduction through the structure is controlled by thermally activated ionization of deep Dy defects in an electric field and

0 Ž .the reverse process of capture of mobile electrons by D states. Defect-related Auger excitation DRAE is responsible forexcitation of erbium ions located close to dangling-bond defects. Our experimental data are consistent with the mechanismsproposed. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: Erbium-ion electroluminescence; Amorphous silicon; Dangling-bond defects; Defect-related Auger excitation; Ionization

1. Introduction

Ž .The idea to fabricate light emitting diodes LEDsintegrable into silicon electronics and emitting at thewavelength of 1.5 mm corresponding to the absorp-tion minimum of optical fibers attracted attention tothe luminescent properties of erbium-doped siliconw x1a,1b . Recently, we have demonstrated that thefilms of erbium-doped amorphous hydrogenated sili-

Ž Ž ..con a-Si:H Er prepared by cosputtering haveroom-temperature photoluminescence and the life-time of erbium ions in this material is considerablyshortened by the effect of amorphous-matrix disorder

) Corresponding author.

w x2 . We have reported also on the first observation ofEr-luminescence in a diode structure based on a-

Ž . w xSi:H Er 3 . Here we present more detailed resultsof these electroluminescence studies.

2. Experimental procedures

Electroluminescent structures on the basis of er-bium-doped amorphous hydrogenated silicon wereprepared by cosputtering, applying the magnetron-as-

Ž .sisted silane-decomposition MASD techniquewhere mixtures of Ar and SiH are used as the4

w xsputtering gas 4 . The films of 1 mm thickness weredeposited on n-type silicon substrate with a donorconcentration of 5=1017 cmy3. The erbium concen-

0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0022-3093 98 00283-X

( )O. GuseÕ et al.rJournal of Non-Crystalline Solids 227–230 1998 1164–1167 1165

tration measured by Rutherford back scattering wasabout 1020 cmy3. The diameter of aluminium con-tacts to the amorphous film was approximately 1mm. A typical I–V characteristic at 300 K hadconventional diode shape with rectifying properties.Electroluminescence was measured in the tempera-ture interval 77 to 300 K with square-shaped currentpulses at 25 to 100 Hz and with a duty cycle of 1:2,both in forward and reverse bias. The emission wasobserved through the crystalline silicon substrate.The measurements were done in the constant-currentregime. The forward direction corresponds to nega-tive voltage at the n-type substrate.

3. Experimental results

Erbium luminescence corresponding to a line atŽ .1.54 mm the energy of 805 meV is observed only

for reverse bias. A typical spectrum measured atroom temperature for reverse bias is shown in Fig. 1.Besides the line corresponding to the 4 I ™

4 I13r2 15r2

transition in the 4f-shell of erbium ions a band withgreater width at 0.8–0.9 eV can be seen. This bandcorresponds to defect-related luminescence of the

w xa-Si:H matrix 5 . Erbium luminescence is nearlylinear in excitation current.

The dependence of erbium luminescence intensityon electric field is presented in Fig. 2. At larger

2 Želectric fields it approaches an E -dependence. Dueto contact potential and resistance of the amorphous

Fig. 1. Electroluminescence spectrum for reverse bias at 300 KŽ .experiment .

Fig. 2. Electroluminescence intensity as a function of electric fieldŽ .experiment . Dashed line indicates the asymptotic behavior ofthis dependence.

material, the voltage applied to the structure de-.creases practically only across the amorphous layer.

This behaviour is unusual for amorphous silicon andindicates the occurrence of thermally activated ion-

Ž .ization of deep defect or impurity states well knownin crystalline semiconductors.

The temperature dependence of the erbium lumi-nescence intensity for different currents through thestructure is shown in Fig. 3. This result is alsocompletely unexpected: whereas the intensity ofphoto- and electroluminescence usually decreases at

Žhigher temperatures i.e., suffers temperature.quenching , in our case, it is much smaller at liquid

nitrogen temperatures but increases while approach-ing room temperature.

Fig. 3. Temperature dependence of erbium electroluminescence atŽ .constant current experiment .

( )O. GuseÕ et al.rJournal of Non-Crystalline Solids 227–230 1998 1164–11671166

4. Discussion

The introduction of erbium ions into the amor-phous matrix is accompanied by the formation of alarge number of dangling-bond defects with concen-tration ;1018 cmy3 as determined in our measure-

w xments of optical absorption 2 . Therefore, we as-sume that dangling-bond defects are associated withthe erbium ions. According to our dark-conductivity

w xmeasurements 3 , the Fermi level in erbium-dopedamorphous silicon at room temperature is displacedupward from the middle of the gap, i.e., the dopedsamples are slightly n-type. In this case, defectsconnected with the dangling bonds occur in D0 orDy states. These levels are situated near the midgapand are separated by ;0.2 eV.

In the case of reverse bias, electrons are excitedinto the conduction band due to thermally activatedionization of Dy-defects in an electric field. Whilemoving in the amorphous layer, these electrons ex-cite erbium and defect-related luminescence by cap-ture of electrons from the conduction band by D0-de-

w xfects 3 .The transition eqD0 ™Dy can occur by radia-

Ž .tive or nonradiative multi-phonon process. In thecase when dangling-bond states are correlated witherbium ions, a third recombination channel is opened,i.e., nonradiative transition accompanied by erbium-ion excitation due to Coulomb interaction betweenthe electron being captured to the D0 center and thef-electron of the erbium-ion. This defect-related

Ž .Auger excitation DRAE process is specially effec-tive because of its nearly resonant character: theenergy difference of radiative transition between eqD0 and Dy states is close to the energy difference of4 I and 4 I states of the 4f-shell. The excess13r2 15r2

energy of the transition is transferred to localphonons.

In the steady state, we can write a balance equa-tion for thermally activated ionization of Dy statesŽ .the dependence on electric field is given explicitly ,and capture of electrons from the conduction bandby D0 defects

b exp E2rE 2 Ny scnN 0 , 1Ž .Ž .i c D D

where b and c are the ionization and capture coeffi-i

cients, Ny, N 0, and n are concentrations of Dy-de-D D

fects, D0-defects, and electrons in the conductionband, E is the electric field applied to the structure,E is a characteristic electric field. Using the detailedc

balance condition to establish the correspondencebetween b and c we arrive at the resulti

nsn exp E2rE 2 , 2Ž .Ž .0 c

which indicates that the concentration of electrons inthe conduction band rises exponentially in high elec-

Ž . Žtric field n is the equilibrium concentration . The0

increase of the concentration of the free electronswas detected directly from the measurements of the

. w xvoltage drop across the structure 3 .Now the intensity of erbium luminescence is

t t0 2 2 0I sc nN sc n exp E rE N , 3Ž .Ž .L A D A 0 c D

t tR R

in agreement with the electric field dependence ofŽerbium electroluminescence shown in Fig. 2. c isA

the contribution to the capture coefficient from theDRAE process, t and t are total and radiativeR

.lifetimes of erbium ions in the excited state. Thecharacteristic field determined from the plot of Fig. 2is E s1.8=105 Vrcm.c

The temperature dependence of the intensity oferbium luminescence in the constant-current regimeis

´ yz j ty 0I sc N exp , 4Ž .L A D ž /kT qmE tj R

where the electric field, E , applied to the structurej

at current, j , is a weaker function of temperature0

than the exponent, q is the electron charge, m is themobility of electrons. From Fig. 3, we estimate theenergy of the Fermi level counted from the level ofthe Dy-defect: ´ yzf110 meV in reasonableyagreement with the n-type property of the amorphoussilicon layer. In this simple model, we have ne-

Ž .glected the power temperature dependence of theelectric field, E , and of the mobility of electronsj

above the mobility edge. Then, the temperature de-pendence of the erbium luminescence intensity isdetermined by population of D0 states.

( )O. GuseÕ et al.rJournal of Non-Crystalline Solids 227–230 1998 1164–1167 1167

5. Conclusions

In conclusion, we have first observed room-tem-perature erbium-ion electroluminescence in amor-phous-silicon-based semiconductor structure. Theproperties of the structure seem promising for furtherdevelopment of LEDs emitting at 1.5 mm and inte-grable into silicon electronics. The mechanism ofelectrical current conduction through the structure isdescribed; it is controlled by thermally activatedionization of deep Dy-defects in high electric fieldsand the reverse process of capture of mobile elec-trons by D0 states. Defect-related Auger excitationŽ .DRAE is responsible for excitation of erbium ionslocated close to dangling-bond defects. The wholeset of our experimental data is consistent with themechanisms proposed.

Acknowledgements

This work was partially supported by VolkswagenŽStiftung under the project Ir71 646, by INTAS the

.project number 93-1816 , and Russian FoundationŽfor Basic Research Grants No. 95-02-04163a and

.No. 96-02-16931a .

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