Cathodoluminescence and cathodoelectroluminescence of amorphous SiO2 films

N

ELSEVIER

]OURNA L OF

Journal of Non-CrystallineSolids 220 (1997) 69-77

Cathodoluminescence and cathodoelectroluminescence of a m o r p h o u s S i O 2 films M. Goldberg a, H.-J. Fitting a,*, A. Trukhin b a Physics Dept., Rostock Unicersity, Unit~ersitatsplatz 3, D-18051 Rostock, Germany b Institute of Solid State Physics, Latcian State University, Kengaraga iela 8, LV-I063 Riga, Latvia

Received 10 December 1996; revised 30 April 1997

Abstract Cathodoluminescence of amorphous SiO 2 films thermally grown on a Silicon substrate has been observed in a scanning electron microscope using wavelength dispersed registration by a charge coupled device (CCD) camera. Spectra have three bands: at 650 nm (red), 460 nm (blue), and 285 nm (UV) whose intensities change during the initial period of electron beam excitation. Luminescence peak dose dependence has been investigated in a wide range of current density (10 -5 to 10 3 A cm 2) and temperature (90 to 500 K). An interpretation of the dose-temperature dependence is made by a model of precursor transformation via a metastable interlevel. Application of an electric field during continuous electron excitation (cathodoelectroluminescence) causes an enhancement up to five times of the blue band intensity. On the other hand, the red band decreases in the electric field. Based on these phenomena, the UV and the blue luminescence band are attributed to an internal electron impact excitation within a localized center, probably twofold-coordinated silicon in the SiO 2 network, whereas the red band is ascribed to band-to-band-recombination via localized levels attributed to non-bridging oxygen. © 1997 Elsevier Science B.V.

1. Introduction Several defects in amorphous SiO 2 (a-SiO 2) are optically active and can be studied by luminescence spectroscopy [1]. Cathodoluminescence (CL), the emission of light as a result of electron irradiation, is particularly suitable for investigation of thin films such as a-SiO 2 on a substrate, whereas photoluminescence (PL) is rather difficult to measure because of the small efficiency of PL excitation in thin films. By means of CL it is possible to obtain large excitation doses within a small layer. Several CL studies of a-SiO 2 films have been

* Correspondingauthor. Tel.: +49-381 498 1646; fax: +49-381 498 1667; e-mail: [email protected].

reported [2-9], but no unambiguous correlations of observed luminescence bands with specific luminescence centers and defects have been established yet [10]. CL spectra show three dominant bands, a UV peak at about 285 nm (4.3 eV), a blue peak at 455 nm (2.7 eV) and a red peak at 650 nm (1.9 eV). CL peak position and half-width are identical with those of photoluminescence (PL) [1 1-14]. Moreover, CL decay kinetics [15,16] is similar to PL [1 1-14,17]. Because of the similarity of CL spectra of thin a-SiO 2 films and PL spectra of bulk silica it is usually assumed that the same luminescence centers are acting in both cases [9,10]. Whereas the red band in PL is commonly ascribed to non-bridging oxygen [ 1,13,14], there are still two main models for the blue (and UV) luminescence band: the oxygen vacancy or

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M. Goldberg et al. / Journal of Non- Co'stalline Solids 220 (1997) 69- 77

70

E' center [3,4,18,19] and the twofold-coordinated silicon center, a silicon . atom with only two neighbor°

ing oxygen atoms O - Si - O , written Si ° [13,14], see also Ref. [1] for a review. According to the latter model the fast UV band (lifetime ~-< 5 ns) is attributed to the permitted singlet-singlet transition and the slow B band (~-= 10 ms) is due to the 'forbidden' triplet-singlet transition in a single center, the Si ° [13,14]. Mitchell and Denure [2] first reported an increase of the blue CL band with irradiation time for thin a-SiO 2 films. Later McKnight and Palik [7,8] have compared the CL intensity changes under electron bombardment at liquid-nitrogen (LNT) and room temperature (RT). In a previous paper we have presented the CL dose dependence of thermal a-SiO 2 films in a wide range of temperatures from 80 K to 500 K [20]. Here, we add a more detailed model for the temperature effects observed. Recently, studies of CL intensity changes during electron irradiation have been published for quartz (RT and LNT) [2123] and bulk silica (RT) [24,25], but both papers did not analyze the UV band. To specify the nature of the luminescence centers in a more extended way we present the influence of an external electric field on CL. Of course, an affect of electrical and optical properties by electric fields is expected in dielectric materials and especially utilized in electroluminescence (EL) [26]. On the other hand, the simultaneous application of an electric field and electron beam excitation is called cathodoelectroluminescence (CEL) [27,28]. To the knowledge of the authors, the present paper is the first report on this field phenomenon in a-SiO 2 films. Moreover, the variation of electron-beam energy and hence the CL excitation range provides information on the depth distribution of luminescence centers across the a-SiO 2 films. In this way we determine a CL 'dead layer' beneath the surface and define a measurement method to determine the thickness in which luminescence occurs.

2. Experimental The experimental set-up consists of an paraboloidal mirror collector (Oxford Instr.) attached

to a digital scanning electron microscope (Zeiss DSM 960), a fiber optics, a spectrograph (Spex-270M) and a LN2-cooled charge coupled device (CCD) camera (Princeton Instr., EEV 1024 × 256). CL spectra ranging from 200 to 850 nm are acquired by a single exposure of the CCD camera within an accumulation time of 2 s and a spectral resolution of 4 nm. A cooling and heating temperature stage (Oxford Instr.) provides sample temperatures between 77 and 650 K. Details of the experimental setup are given in Ref. [20]. CL measurements were performed with electron energies from 0.5 to 30 keV and current densities form 10 -s to 10 -3 A cm 2. The samples were thermal dry oxidized SiO 2 films on n-type Si with SiO 2 thicknesses, dox, between 15 and 500 nm. Spectra shown here are not corrected for spectral response of the apparatus. Due to the relatively fiat spectral efficiency of the CCD camera and the whole system the changes in relative intensities of the CL bands would be less than 50%. Each measurement consists of some hundreds of digital stored spectra. From these spectra the CL dependence on dose and on electric field are extracted for the three CL bands. In order to perform CEL, a thin, semitransparent top electrode of gold (dAu ~ 15 nm) was deposited (sputtered) on the SiO2-Si-samples. A special sample holder shielded the specimen and kept the front side with the gold layer at ground potential. The control voltage was applied to the back Si substrate.

3. Results

3.1. Dose-temperature effects It is well known that CL spectra of different S i O 2 modifications change during the initial period of excitation [2,7-9,20-25]. The time evolution of the CL spectrum of a thermal, dry oxidized a-SiO 2 film during electron irradiation at room temperature is presented in Fig. 1. The initial spectrum of a virgin, previously unirradiated sample is dominated by a peak around 650 nm (R band) that increases with irradiation time. However, the main change of the spectrum is given by the growing blue peak around 460 nm. This B band increases from almost zero to a constant intensity. A third band at 285 nm (UV

M. Goldberg et al. / Journal of Non-Cr),stalline Solids 220 (1997) 6 9 - 7 7

band) first increases and then decreases. The dose dependence of both the UV and the B band scales with excitation current density. Moreover, it is possible to get a universal dose behavior of CL depending only on the exposure, the product of current density and irradiation time (jot), shown in Ref. [20]. Assuming first-order-kinetics the transformation of precursors, N l to luminescence centers, N L (Eq. (1)) and activation of quenching centers, NQ, from their precursors, N 2, (Eq. (2)) under electron irradiation leads to the following set of equations:

dN~

J0

dt

e0

dUo

J0

dt

e0

6 5 ,

4

r

,

3

,

energy (eV) 2.5 2

1.8

1.6

,

,o

,,-.:.

10 min / "

.~ =60

1 min , J "- \ '.

f ,":".1

10s . o)

2s

\

"

I

200

300

400 500 600 wavelength (nm)

(a)

G,(N, --NL),

71

(1)

.

,

,

.

,

700

,

,

800

.

5 80

G2 ( N2 - NQ ) ,

(2)

N ( t) = U o + UL(t ) -- UQ(t),

60

g

"~ 40

(3)

B peak 460 nm

UV peak 285 nm

.~_ ..J

ICL(t)

O( N ( t )

N O -4"-N 1 1 -- e x p

o

e0

20

UV I

00

(b)

(4) with an initial center concentration, No, luminescence precursor centers, N~, with an electron beam activation cross-section, o-~, and quenching-centerprecursors, N2, with their transformation cross-section, o-2. By means of Eq. (4) it is possible to describe the exponential saturation of the B band (Fig. l b) as well as the turnaround behavior of the UV band. Once transformed, luminescence centers are relatively stable. At room temperature there is no measurable change in CL intensity after an irradiation break of several hours. Even a post-annealing in vacuum at 600 K for about 1 h does not change the CL intensity. The dose dependence has been investigated in a wide range of temperatures, Fig. 2, see also Ref. [20]. Decrease of temperature to liquid nitrogen leads to faster transformation of precursors, N~, as well as faster and more intensive activation of the quenching component, N 2. By fitting the measured dose curves with Eq. (4) the temperature dependence can be attributed to the cross-sections, o-~, (transformation, activation) and o-2 (destruction, quenching), respec-

I

200

I

I

400 600 irradiation time (s)

800

Fig. I. (a) Evolution o f the SiO 2 C L spectrum with electron irradiation time. (b) C L p e a k intensities vs. irradiation time. Note: the blue (B) peak is rising almost f r o m zero intensity (dox = 250 nm, E 0 = 5 keV, J0 = 8 X 10 4 A c m - 2 , RT).

tively. Obviously, the transformation of precursors occurs much faster at low temperatures; the crosssection o-~, increases by almost 2 orders of magnitude between 500 and 90 K, see Fig. 3. 9(3 K ' 400 = _>,

300

400 K j

o

~

'

~

300 K

200 0"

100'

0.0

.,~

011

500K

B peak

012 013 014 dose density (As/cm 2)

0.5

Fig. 2. Evolution o f the B p e a k intensity as a function of dose density for different temperatures 90 to 500 K. The lines are d r a w n as guides for the eye. (do~ = 2 5 0 nm, E o = 8 keV, J0 = 2.8 X10 4 A cm-2.)

M. Goldberg et al. / Journal of Non-Crystalline Solids 220 (1997) 69-77

72 '

i

.

.

.

.

oE

~

.

.

.

.

i

.

.

.

.

!i..],.. {.. ~ -

i

.

.

.

.

i

,

,

n 3 = N L, the luminescence center has been activated. The occupation of level n 3 in case of not too high excitation densities a << b, c is given by

Bpeak

co "7

UL(t)

o

quenching ~Q

"" "~...TT

=

[(

T

n3(t)

N, 1 - exp 0.1

100

200 300 400 temperature (K)

n, + n 2 + n 3 = N t ,

nl(t=0)=N,, n3(t) =

NL(t),

(5)

dn I

(6)

an 1 + bn2,

dt

dn 2 an,

-

bn 2 -

(7)

cn 2 ,

dn 3

a=

(8)

= cn2,

J0°'0

,

--

"

Comparing Eqs. (10) and (4) the temperature dependence of the precursor transformation cross-section, o-, is obtained: Or0

° ' l ( T ) = (1 + / e x p ( - A E / k T ) )

b=f~exp

{

-~

El )

,

e0

"

(11)

This temperature dependence is in good agreement with the experimental observation, see Fig. 3 (dotted line), assuming a thermal activation energy of A E = E , - E 2 ~ 0.1 eV. Moreover, this three-level-model excludes a simple thermal annealing of luminescence centers that have been transformed in accordance with experimental data.

(a) \ metastable/ t\ state / a

b

c

~x,a ~ . u m i n . / pre~urs0r n~~ n2 > n3 = NL metastable (b) n2 ,[ a

dt

dt

(fl/f2)exp(--(E1

)]

500

In order to explain the faster transformation of precursors N~ to N L at low temperatures we propose a model of center transformation via a metastable level (three-level-model), see Fig. 4a. The electron beam excites centers from the initial precursor state, n 1, to a metastable level, n 2. From this state the system can reach the final level, n 3, equal to the activated luminescence centers, N L or could return thermally activated to the initial state, n~. This three-level-model, Fig. 4b, can be described by the following set of differential equations:

-

1+

_oo-o.ot

(lO)

' ' '

Fig. 3. T e m p e r a t u r e dependence o f the cross sections o f C L p r e c u r s o r activation 0.1 (solid circles) and q u e n c h i n g 0-2 (open circles) c o m p o n e n t obtained b y fitting experimental dose curves as in Fig. 2 with Eq. (4). The dotted line is given by Eq. (11) with A E = 0.1 eV. (dox = 250 nm, E 0 = 8 keV.)

-

=

jo~o/eo

b

f, d~'/kT V

c f2

eE2/kT

stable nl precursor center n3=N, Fig. 4. (a) Potential scheme in configuration coordinates p r o p o s e d for transformation of the B peak luminescence center f r o m a p r e c u r s o r state. (b) Three level model o f the transformation of precursors n~ via a metastable state n 2 to stable luminescence

If the system has turned from state n~ to the state

centers n 3 = N L.

M. Goldberg et al. / Journal of Non-C~stalline Solids 220 (1997) 69-77 ,

2.5

3.2. Cathodoelectroluminescence ( CEL) Additional information on the nature of transformed luminescence centers can be obtained by investigation of the effect of an electric field on CL, called cathodoelectroluminescence (CEL). Electroluminescence (EL) is known to occur in thermal a-SiO 2 films at fields above 7 M V / c m , but published spectra differ [29,30]. For relatively thick a-SiO 2 films examined here (380 and 500 rim) we found no pure EL up to 6 M V / c m . However, we found a strong influence of an external electric field on CL. The B band CL intensity increased during field application up to five times at a field strength of 4 M V / c m and was independent of the field polarity, Fig. 5. Furthermore, there was no threshold visible in the field dependence, Fig. 5. The UV band had a similar enhancement with field. On the other hand, the R band intensity decreases in electric fields < 1.5 M V / c m , Fig. 6. However, at field strengths > 2 M V / c m the R band also increases.

3.3. CL depth profiling The lateral resolution of cathodoluminescence (CL) mapping in a scanning electron microscope (SEM) can be extended to spatial depth analysis by means of varying the electron beam energy E 0 and thus the excitation range of CL. For practical use the most convenient method for CL depth analysis is the 'constant power method' [31], where the power of the impinging electron beam ( E 010) is kept constant. In this case the CL intensity measured versus E 0 will remain constant if assuming a homogenous depth

= 2.0

.

i

.

i

.

i

.

i

.

N 2

E ~t

f

-4

,

i

-3

,

~

,

~

,

,

i

,

i

-2 0 1 2 field strength (MV/cm)

.

.

,

.

.

i

3

.

.

,

.

,

I

1.5

~ 1.0 0.5

. . . .

,

. . . .

h

. . . .

1 2 field strength (MV/cm)

,

3

Fig. 6. Field quenched and field enhanced CL of the R peak ( A = 6 5 0 nm). (dox = 5 0 0 nm, E 0 = 1 0 keV, j 0 = 1 × 1 0 -4 A cm -, RT.)

distribution of luminescence centers. Deviation of such a constant power behavior is easy to detect and should be interpreted in terms of CL center depth profiles. Fig. 7 shows the CL B peak intensity versus incident electron energy, E 0, of a 250 n m S i O 2 layer. Obviously, the B peak intensity has a maximum at about 4 keV. At larger E 0 an increasing part of energy is deposited within the Si substrate and is lost for excitation within SiO 2. However, there is also a decrease of the CL intensity at lower energies. This decrease is attributed to a 'dead layer' beneath the surface.

1.0

~

. . . . . . . . . . . . . . . . . . . . . . . . .

o m o g e n i ~ ' "

.

"-.

0.6

oo °

©

.

,

o x

i° fJ

B peak %

oxide thickness: • 500 nm o 380 nm

,

B peak

,, ,,,I

.

t

5 c 4 ._~ ~__) 3

.

R peak d = 500 nm

"~ 0.4 i

73

i

4

Fig. 5. Electric field enhanced CL B peak (A = 460 nm). ( E 0 = l0 keV, J0 < 5 × 10 - 6 A c m -2, RT.)

..../

4 beam energy E (keY)

;o

-t

12

Fig. 7. CL intensity as a function of electron beam energy E o applying the 'constant power method' Eolo =const. (symbols: measured), CL decrease towards small E 0 indicates a surface dead layer, ddeaa, decay towards larger E o expresses the finite luminescent SiO 2 layer thickness dox = 250 nm. Best fit, Eq. (16), is possible with a dead layer of 25 nm (solid line). The CL signal calculated for only SiO~-Si interface centers (IF, dashed line) and a homogeneous depth distribution across film (dotted line) are also shown.

M. Goldberg et al. / Journal of Non-C~,stalline Solids 220 (1997) 69-77

74

(a)

4. D i s c u s s i o n

e

(b)

(c)

(d)

,1,

1

)

s

~

4.1. Dose-temperature effects Assuming the twofold-coordinated silicon Si ° is the B band luminescence center then in Fig. 8 one possible way of its generation is presented. We propose the creation of a Si ° center from an oxygen vacancy by electron beam induced bond cleavage and subsequent reordering. In this way the correlation observed for oxygen vacancies as well as E' centers on the one hand and the blue luminescence band in a-SiO 2 [ 1,3,4,18,19] can be explained. Despite the similarities of the dose-temperature dependence of the UV and B hand [9,20] there are some differences in the saturation behavior, Fig. 1. The change of the peak ratio of these two bands under different kinds of ionizing radiation [32,33] was taken as an argument against the assumption that both bands are transitions in one and the same center [34]. Other authors [33] argue that the ratio change could be due to the influence of hydrogen or another mobile part in a more complex defect containing twofold-coordinated silicon. Note, hydrogen is present even in nominally dry a-SiO 2 films [35]. We think the slight differences in the dose behavior should be explained by internal weighing of the radiative and non-radiative relaxation channels within the Si °, see also Ref. [36] for a more detailed discussion.

4.2. Cathodoelectroluminescence Possible processes of ionization, excitation and capture of charge carriers for luminescence are shown in Fig. 9. Charge carriers created by the electron irradiation are heated in the field. Scattering processes cause energy losses and momentum transfer. At sufficiently large fields the gain in energy from

/

/

/Sip Si--O \

/-

...~°Si-.~

I

~

~Sig

"n o/S~

/

I

~ . Si.....

I

Fig. 8. Structural model proposed for creation of a twofold-coordinated silicon center Si ° via bond cleavage and subsequent reordering starting from an oxygen vacancy

u

.... VB Si

Fig. 9. Electronic processes in cathodoelectroluminescence CEL: (a) capture o f charge carders in a localized state, (b) trap ionization, (c) internal impact excitation and luminescence within a localized center, (d) impact ionization across the band gap (phonon losses are not shown).

the field exceeds the losses due to phonon scattering, mainly LO-phonon emission [37-40]. This phenomenon starting in S i O 2 at the critical fl eld strength F 0 ~ 1.5 M V / c m is called 'optical runaway'. Thus, more and more electrons will approach energies sufficient to create electron-hole pairs. During this process called interband impact ionization the energy of an exciting electron decreases to the bottom of the conduction band, Fig. 9d. Both the exciting and the excited electrons will gain energy again causing an avalanche multiplication process. In comparison to impact ionization across the S i O 2 band gap (Egap ~ 9 eV), a smaller energy is sufficient to ionize an impurity level, Fig. 9b, or to produce an internal impact excitation process within a localized center, Fig. 9c [26]. In the latter case primary or secondary electrons (SE) excite the luminescence center from the ground state to an excited state. Relaxation to the ground state is by emission of a photon. Such internal excitation processes also work without external electric fields when electrons with sufficient energies are available. However, a field will increase the mean energy of the exciting electrons. As a consequence, more centers will be excited and CL increases. This model of impact excitation seems to be in good agreement with the observed B peak increase, shown in Fig. 5. On the other hand, if one assumes a band-to-band recombination process via localized states (see Fig.

M. Goldberg et al. / Journal qf Non-Co,stalline Solids 220 (1997) 69-77

9a) then the capture cross-section of Coulomb-attractive luminescence centers is reduced by the field [41,42] and a decreasing recombination luminescence is expected. Therefore the R band quenching by the electric field could be due to such a band-to-band recombination process. The R band increase observed at higher field strengths is finally caused by avalanche multiplication of the exciting electrons compensating for reduced capture cross-sections. This assumption is supported by the turnaround point in Fig. 4 occurring at about 1.2 M V / c m that is close to the optical runaway field strength F 0 ~ 1.5 M V / c m [37-40]. As already mentioned the increase of the B peak with electric field can be explained by an impact excitation process. Further evidence is obtained by Monte Carlo (MC) calculations showing that the enhanced CL intensity is caused by the energy gain of the exciting inner SEs from the field [43]. Assuming a Bethe-like excitation cross-section of the luminescence center with excitation energy threshold E i, E o'i(E) =~Aln I[~],

(12)

the integral of the MC-modeled inner SE-energy distribution, fsE(E, F), and the excitation cross-section, o-i(E), should yield in the qualitative behavior of cathodoelectroluminescence CEL with field F, sc

ICEL(F) = c f

o'i( E)fsE ( E, F ) dE.

(13)

Ei

Results of the calculated CL enhancement are presented in Fig. 10 together with experimental data.

>" 4

exp

o

MC

* 5 eV

03 C

c

•

C

0m

7eV

0

o

oO

o

3 © o

9

O

c

o,tO o 1

°m

°

Q

0

. . . .

0

.

L

. . . .

.

.

]

.

. . . .

7 2 field strength (MV/cm)

i

3

Fig. 10. Monte Carlo calculation of field enhanced CL according to Eqs. (12) and (13) and experimental data for the blue band B of Fig. 3. (Parameter: impact excitation threshold energy E i = 5 and 7 eV.)

75

There, the excitation threshold energy has been varied: E i = 5 and 7 eV. Differences between MC calculation and observed CEL are due to the uncertainty in the excitation cross-section, o-i, of the luminescence centers.

4.3. CL depth profiling and thickness measurement For modeling the CL signal we use experimental spatial energy-transfer curves of an electron beam in target depth z, [44]: - - = d z l'544R---~0)exp - 7 . 5

-~-0.3

(14)

and the numerous equations for the maximum electron range R R = 90p-°SEoL3,

E 0 < 10 keV,

R=45p

E 0 < 10keY,

°gE0J7 ,

(15)

with R in nm as a function of the initial electron energy, E 0 in keV, and target mass density, p in g cm -3

The energy-dependent CL intensity is obtained by integration over the energy transfer, d E/d z, and the depth distribution of CL centers, n(z), r~dE

ScL( Eo) = cJo -g-/(z, Eo)n( z) dz.

(16)

We assume no reabsorption of luminescence because of optical transparency below the a-SiO 2 band gap of 9 eV. Moreover, diffusion of thermalized charge carriers can be neglected since the blue luminescence band is excited only by impact excitation as shown in Section 4.2. By means of an iterative approach we found the layer of reduced center concentration (dead layer) to be about 25 nm within a 250 nm a-SiO 2 film, shown in Fig. 7. The thickness of the surface dead layer increases with a-SiO 2 film thickness, starting from about 10 nm in a 15 nm film to about 40 nm in thick SiO 2 samples, do× = 500 nm. Since the B peak is not related to a charge carrier recombination process this dead layer is of another kind than the well-known surface dead layer in semiconductor luminescence caused by non-radiative surface recombination [31]. Most probably, the dead layer of the B and UV peak arises from an excess of oxygen near the surface preventing the formation of oxygen-deficient lumi-

76

M. Goldberg et al. / Journal of Non-Crystalline Solids 220 (1997) 69-77

nescence centers like oxygen vacancies [18,19] and twofold-coordinated silicon [13,14]. Whereas the CL decay towards small energies, E 0, is used to determine the surface dead layer, from the large energy slope we estimate the finite luminescent SiO 2 layer thickness, dox For prompt evaluation it is possible to use the fixed energy, El~ 2, where the CL intensity has dropped to one half intensity, always applying the constant power method (Fig. 1 l a). We found an empirical fit between layer thickness, dox, in nm and 'half-decay energy', El~ 2 in keV: dox : 7E~)7

(17)

demonstrated in Fig. 1 lb. Determination of the SiO 2 layer thickness is possible with an error better than 10%. Because of the affect of the surface dead layer the relative error of thickness measurement increases for thin films, e.g., 500 + 35 nm but 24_+ 4 nm. Indeed, this method is of less accuracy than optical techniques, e.g., ellipsometry, but contrary to such methods it can be used in combination with scanning

l ffl

electron microscopy in order to measure thicknesses in lateral areas with less than 1 i~m separation.

5. Conclusion The typical luminescence bands in S i O 2, the blue B peak at 460 nm and the UV peak at 285 nm have similarities in their dose-temperature and electric field dependence whereas the red R peak at 650 nm differs. Some interpretation of the dose-temperature behavior of the UV and B band seems to be possible by means of the luminescence center model of Skuja [13,14] based on a twofold-coordinated silicon atom Si °. The faster initial CL increase with irradiation time at low temperatures is explained by a two-step precursor transformation process via a metastable interlevel. From there the system can reach the stable final state (luminescence center) or it returns, thermally activated, to the initial state. The increase in luminescent emission produced by cathodoelectroluminescence is attributed to internal impact excitation processes in localized luminescence centers. The dependence of the R band intensity on electric fields is ascribed to a band-to-band recombination process via localized states.

i.m

0.5

_J

(..?

References

0 e-

0.0

E 1/2

(a)

beam energy E°

1000

_~ 100 ~2 o

10

(b)

.

.

.

.

,,

,

,i

10 "half-decay energy" El~2 (keV)

Fig. 11. Determinationof S i O 2 layer thickness, dox, by means of (a) the 'half-decay energy' El/2 method and (b) calibrationof dox vs. El~ 2, see Eq. (17).

[1] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 899. [2] J.P. Mitchell, D.G. Denure, Solid-State Electron. 16 (1973) 825. [3] C.E. Jones, D. Embree, J. Appl. Phys. 47 (1976) 5365. [4] C.E. Jones, D. Embree, in: The Physics of SiO, and Its Interfaces, ed. S.T. Pantelides(Pergamon, New York, 1978) p. 289. [5] H. Koyama, K. Matsuhara, M. Mouri, J. Appl. Phys. 48 (1977) 5380. [6] H. Koyama,J. Appl. Phys. 51 (1980) 2228. [7] S.W. McKnight, E.D. Palik, J. Non-Cryst. Solids 40 (1980) 595. [8] S.W. McKnight, in: The Physics of MOS Insulators,ed. G. Lucovsky et al. (Pergamon, New York, 1980) p. 137. [9] L. Skuja, W. Entzian, Phys. Status Solidi (a)96 (1986) 191. [10] B.G. Yacobi, D.B. Holt, CathodoluminescenceMicroscopy of Inorganic Solids (Plenum, New York, 1990). [11] L.N. Skuja, A.R. Silin, J. Mares, Phys. Status Solidi (a)50 (1978) K149. [12] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [13] L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Solid State Commun. 50 (1984) 1069.

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