Photoluminescence of Si:HSINx:H superlattices

Photoluminescence of Si:HSINx:H superlattices

Superlattices and Microstructures, PHOTOLUMINESCENCE Gen Moscow Chun, OF a-Si:H/a-SiNx:H V.E. State University, luminescence a decrease Depart...

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Superlattices

and Microstructures,

PHOTOLUMINESCENCE Gen Moscow

Chun,

OF a-Si:H/a-SiNx:H V.E.

State

University,

luminescence a decrease

Department

than

a 4 nm,

between also

not

the

only

line The

states. effects.

is

Ro2,

results

The

study the

of amorphous

last

several

attracted the attention tories [l - 91. X-ray investigations optical

and photoelectrical

that role

quantum-size effects In thin a-Si:H layers

fects

can be revealed

experiments

[l,

3,

energy study.

1983)

combined

measurements

with showed

play an Important in SLs. These ef-

in photoluminescence

(PL)

5 - 71.

However, properties diative recombination of defect tion with

(SL)

(after

of a number of laboraand electron microscope

of SL structures

spectra of amorphous SLs need more Some results of different authors are

This paper gives the results of PL studies of a-Si:H/a-SiNx:H SLs and confirms the role effects. The influence of these

on deep levels of defects

is

particu-

larly important. 2. Preparatlon

caused

by

towards

at

the

higher

transitions by

d

less

transitions energies,

from

position in the

varied (x =

the

but

deep

level

quantum-size

of the a-Si:H

&l-line layers.

is

from 1.5 to 8 nm, thickness 0.3)

periods

layers

was

prepared

and

Thick

were

Si.

made

was 4 nm, the

50 to 40.

taneously

for

a-SiNx:H

The substrates

pairs

layers

of number

of

SL

of simul-

of samples were quartz of a-Sl:H

comparison

and

(see Table

a-SiNx:H 1).

The

thicknesses of the layers were calculated based on technological conditions. Structural control measurements were made on one sample and proved the

and interface states, their interacNitrogen and Hydrogen, and models of

of quantum-size

layers,

correctness

of thickness

calculations.

of radiative and nonrain a-Si:H SLs, the role

contradictory.

effects

super-lattices

a-Si:H

chamber was about 10-S Torr, flow of SiH4 was about 5 ml/min; thickness of a-Si:H layers

superlattices years

1990)

be explained on the impurity

1. Introduction

over

flul,

by the

can

Physics,

a-Si:H/a-SiNx:H d of

shifted

caused

Some influence by the nitrogen

produced

line

of

USSR

August

of

width a

tail-states, a

73

spectra of

A.E.Yunovich

Moscow,

(Received

In the

SUPERLATTICES

Kaznacheev,

119999,

with

191

Vol. 9, No. 2, 7991

of samples and their structural analysis

Parameters

Table 1 of samples

__-----------_-__N d(a?isH) _-----_-_--_---___ 31 15 32 25 33 40 2 :: 37 2300 38 ---------__-_-~-__

of

d(a-SiNx:H) R 40 40 40 40 40 2400

sperlattices

of

Number periods 60 60 60 60 40 1 1

SL-samples were prepared by glow discharge in the laboratory of the University of Science and

Fig. 1 shows the spectra of a X-ray diffraction and an electron microphoto of this sample.

Technology

Three diffraction

0749-6036/91

of China (USTC). The pressure

I0201 91+04$02.00/O

in the

maxima give evidence of

01991

good

Academic Press Limited

192

Superlattices

and Microstructures,

Vol. 9, No. 2, 199 7

fO0 .

2

I 3

4

Fig. 1

s

X-ray

6

diffractlon

spectrum

sample of a-Si:H/a-SINx:H

of

the

superlattlce

(N 31, see Table 1). The insert shows a photo of the sample made in transparent electron

microscope,

an arrow shows

a

defect.

perlodlcity

of

the

period was calculated d=

SL. The

thickness

of

a

from the formula

(h/2)/(sln gktl - sin gk).

h = 0.154 nm. The photo also gives evidence of

a

perio-

dlcity of SL, the roughness of interfaces can be estimated to be about 0.2 nm. The arrow indicates sane defect which has grown from

the

substrate through several layers of the SL. 3. Spectra of photolumlnescence PL

and discussion

was excited by an argon-ion

laser

(A

??

Spectra were 480 nm, Wo = 0.5 - 1 W/cm2). slgnal measured with a grating monochrcmator, was detected by a photomultiplier and photoncounting cjrcuit. Usual signal-to-noise ratio was 15. Most measurements were performed at a temperature

T =

110 - 115 K. The Intensity

of

PL decreased with T, only for the best samples we could detect PL up to room temperature. A typical spectrum of the sample with a thin (2.5 nm) a-Si:H layer is shown in Fig. 2. There are two peaks in the spectrum with maxima Rwl = 1.35 eV and flwp = 1.05 eV.

Fig. 2

A typical spectrum of photoluminescence of an a-Si:H/a-SiNx:H superlattlce (N 32. see Table 1); T = 115 K, Wo = 1.0 W/cm2.

193

Super/a ttices and Microstructures, Vol. 9, No. 2, 199 1

Fig.

3

Dependence of maxima In the

the positions of luminescence spectra

a-Si:H/a-SiNx:H

superlattlces

the of

versus

superlattice

sitions

which are

to to

is

can describe

illustrated

corresponds band tail

the

known for

the of

of

the

the

tail:

experlmental

4.

The peak Rwl

peaks

effective

with

101.

band edge Using

effective

conduction

seems

that

caused

the

with

decreasing

??

0.6

specpoten-

m. in the

the

valence

+ (mo/mv*)l

Is

such an estimation

justified

in

the

case of a real amorphous SL? The effective-mass rectangular potencalculation and the model

- with

be used

a

in the

deep levels. More rigoamorphous SLs have not

Is an important Rup-peak takes

the

origin

flw2 band,

semiconductor

observation, place when

of is

interfaces.

band intensity

a

with inter-

with

the

defects,

connected

ulth

The increase

decreasing

d

and

of the

increase due to the change of the substrate from Si to quartz both indicate this cause. besldes should also be noted that It quantum-size effects a possible influence of N impurities in the a-Si:H layers can take place. In Fig. 3 the results of papers [2, 61 are shown In comparison the

peak f’lwl in

barriers (x Is to considerable

For thickness d = 1.5 - 6 nm such an estimagives A (flu) = 0.45 - 0.016 eV. These are of the same order as the experlmant-

ones.

the

with-

distances.

higher energies Is clear, that

tion values al

the

of

-

of the defect is comparable d, that Is about several

which

1.0 mo in

band [6] we can obtain A(flw) = AEc t AEv mo).[(mo/mc*) = (2 fl2/d2.2

possible size the thickness

It of

recombination

in continuous

masses mc*

been performed. that the shift

It

to the

line

can not

with for

interpre-

a Kronlg-Penney-model

band and mv* ;

the defect states rous calculations

atomic

dashed

speaking,

density line

quasi-continuous tail states localized real space. The same can be said about

conductive-

radiative

the

strictly

case of in the

of

a solid

superlattice,

this

[ill.

of

tial,

change

in a-Si:H:

from

d are actually caused by quantum-size then we can estimate analogous shlfts

19,

tial

by Fjg.

spectra layers. tran-

The peak flwp corresponds from deep defect levels to

valence-band tail If the shifts

tra

the

transitions

valence-band

a-Si:H [ll]. transitions

thickness effects,

out

peak fk~l.

We could not detect any ljnes In the which could be connected with a-SINx:H Mechanisms of radiative recombination

in the

Hypothetlcal

the

these curves is an Increase of both maxima with decreasing thickness d, at d lower than m 4 nm.

tation

4

states

In Fig. 3 the dependence of the energy of maxima on d 1s shown. An important feature

results

Flg.

the width of a-Si:H layers. Two upper curves are taken from [2] and [6] for

with

the

ours.

both

cases

with the

decreasing increase

changed from 0.3 shifts in energy 4.

It is shown that of a-Si:H/a-SiNx:H crease of the width

We see,

Is shifted d. of

that

towards Though, N in

to 1.5) flui.

It the leads

Conclusions In the luminescence spectra a desuper-lattices, with d of the a-SI:H layers at d

194 less the

Superlattices

than

4 nm not only

transitions

a line

Rwl,

between tail-states,

caused

by

is shifted

towards caused

higher energies, but also a line Awe by the transitions from deep level

states.

These observations

the quantum-size position impurity

effects.

can be explained Some influence

of the line Flu1 is produced of Nitrogen in a-Si:H layers.

Acknowledgments

- The authors

by

on the by

the

are thankful

to

Wu Zi-Qln, Wu Zi-Qian, Lu Ron-Ty (USTC) for sample preparation and to I. P. Zvlagin, A. G. Katanski,

I.

A. Kurova for

helpful

dlscusslon.

References 1.

F3.Abeles, Letters,

T *Tied je. 1.983, v.51,

Physic,al Review N 2, p.2003.

2.

and Microstructures,

Vol. 9, No. 2, 1991

T.Tiedje, B.Abeles, P.D.Persans, 0 .G .Srooks, G .D .Gody . Journal of Noncrystallied Solids, 1984, v.66, p .345. 3. T.Tiedje, B.Abeles, S.G.Brooks. Physical Review Letters, 1985. v.54. N 23. D .2545. 4. K.Ibaraki, H.Fritzsche. Physical Review 8, 1984, v.30, N 15, p.5791. 5. W.C.Wang, H.Fritzsche. Journal of Noncrystallied Solids, 1987, v.9798, p.919. 6. S .Kalem. Ph,ysical review 0, 1985, v.37, N 15, p.8837. Superlattices and Microst7. S .Kalem. ructures, 1988, v.3, p.325. 8. W.Cheng, S.Wen, J.Feng, H.Fritzsche Journal of Noncrvstallied Solids, 1985>, v.77-78, p:lG61. 9. K.Morigaki, S.Mitta. ISSN 0082-4798, Ser.A, N 1056, 1984. IEEE Journal of Quantum 10. L.Esaki. l986., v.QE-22, N 9, Electronics, p.1611. 11. K.Morig,aki, S.Mitta. Technical Peport of ISSP, 1984.