EFFECTS OF ULTRAVIOLET AND VISIBLE LIGHT ON SYNTHETIC SAPPHIRE

(1)

HAL Id: jpa-00216908

https://hal.archives-ouvertes.fr/jpa-00216908

Submitted on 1 Jan 1976

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

EFFECTS OF ULTRAVIOLET AND VISIBLE LIGHT

ON SYNTHETIC SAPPHIRE

N. Kristianpoller, A. Rehavi

To cite this version:

N. Kristianpoller, A. Rehavi. EFFECTS OF ULTRAVIOLET AND VISIBLE LIGHT ON

(2)

EFFECTS OF ULTRAVIOLET AND VISIBLE LIGHT

ON SYNTHETIC SAPPHIRE

N. KRISTIANPOLLER and A. REHAVI

Department of Physics and Astronomy, Tel-Aviv University, Ramat-Aviv, Israel

Résumé. — Des cristaux de a-AhCh ont été soumis à la température de l'azote liquide et sous

vide à un rayonnement U. V. dans le domaine 1100-1 800 Â.

Alors qu'aucun changement notable n'a pu être détecté dans le spectre d'absorption, une thermo-luminescence (TL) a pu être excitée pour des longueurs d'onde inférieures à 1 600 A. Les résultats sont comparés à l'irradiation aux rayons X. On a également étudié la photostimulation de la thermo-luminescence pour l'ultraviolet proche et le visible. Les échantillons ont été irradiés à la température de l'azote liquide, aux rayons X, chauffés jusqu'à 500 K environ et refroidis, puis exposés, à la température de l'azote liquide, à un rayonnement monochromatique dans le domaine 2 000-6 000 Â. Malgré quelques différences dans l'intensité des pics, les mêmes pics principaux apparaissent entre 80 et 500 K après photostimulation et après irradiation X ou V. U. V.

Abstract. — Single crystals of a-Al203 were irradiated at LNT by monochromatic vacuum U. V.

radiation in the spectral range 1100-1 800 A. While no notable changes could be detected in the absorption spectrum, thermoluminescence (TL) could be excited by wavelengths shorter than 1 600 A. Results were compared with those obtained for X-rayed samples.

Photostimulation of TL by near U. V. and visible light was also investigated. For this, the samples were X-rayed at LNT, heated to about 500 K then recooled and exposed at LNT to monochromatic light in the range 2 000-6 000 A. Despite differences in the peak intensities, essentially the same main glow peaks appeared between 80 and 500 K, after this photostimulation as after sole X or V. U. V. irradiation. The efficiency of photostimulation was also measured as a function of the exciting wavelength and showed maxima at about 230 and 400 nm.

Introduction. — The effects of monochromatic ultra violet (U. V.) irradiation on various ionic crystals have recently been studied in this laboratory [1, 2]. The monochromatic light appeared to be of advantage for the study of the energies connected with the creation of colour centres. The use of vacuum U. V. enabled us to include in the investigations the range of non-ionizing and ionizing radiation.

Methods of optical absorption as well as thermally stimulated luminescence (TL) and conductivity (TSC) were applied. The relatively high sensitivity of TL and TSC was found to be very useful in the study of defect induced by the weak U. V. radiation. Excita-tion spectra of various glow peaks revealed signifi-cant information concerning the processes of defect creation.

In course of these studies the effects of U. V. irra-diation on nominally pure sapphire were investigated in the present work.

Much attention has recently been given to study of radiation induced defects in A L203 ; this from

the point of view of basic research as well as from the aspect of possible technological applications [3-6].

Most of the previous investigations of A L203

concentrated on effects of high energy irradiation (such as neutrons, electrons, y or X-rays) at room and higher temperatures.

Levy [3] studied colour centres induced in A1203

by reactor irradiation. In addition to absorption bands, which appeared in the single crystals even before irradiation, he found that some bands were induced by y-rays, but most bands only by fast neutrons and ionizing radiation in the reactor.

It has been theoretically estimated that the mini-mum energy required to eject an Al or 0 atom from its normal lattice position is of about 50 to 100 eV [4]. It has also been found that irradiation at LNT pro-duces much more centres than at RT and that about 90 % of the defects produced at RT anneal out imme-diately. Govinda [5] found that X-irradiation at RT induced additional optical absorption bands at about 225 nm and 400 nm. Various glow peaks appeared during heating between 160 °C-428 °C.

Turner and Crawford [6, 7] concluded from optical and thermal bleaching experiments that the broad absorption band which appears in y irradiated crystals, around 410 nm., is a composed V band. The radiation induced absorption band at 227 nm they attributed to a Cr2 + impurity, and the glow peak at 170 °C,

which accompanies the decay of this band, to a Cr2 + -> Cr3 + transition.

In the present work the <x-Al203 crystals were

irradiated at LNT by X-rays as well as by mono-chromatic light. TL was measured after sole X or

(3)

EFFECTS OF ULTRAVIOLET AND VISIBLE LIGHT ON SYNTHETIC SAPPHIRE C7-2 1 3

V. U. V. irradiation as well as after combined X-irradiation and photostimulation with near U. V. or visible light. From the dependence of the efficiency of the photostimulation on the wavelength and from results of thermal annealing, conclusions are drawn concerning the defects and processes involved.

Experimental procedure. - For our experiments nominally pure synthetic sapphire crystals from Insaco Inc. were used. The specimens were of a cross- section of 9 x 11 mm and about 0.3 mm thickness. Absorption measurements were carried out with a Cary-17 spectrophotometer.

For the V. U. V. irradiations the samples were kept in a vacuum cryostat attached to a 1 m normal incident vacuum U. V. monochromator of a linear dispersion of 8.3 A/mm. (McPherson model 225). The slits width was 1 mm. The crystals were irradiated a t 80 K by monochromatic U. V. light obtained from a 1000 W hydrogen arc lamp. A schematic diagram of this arrangement is shown in figure 1.

FIG. 1.

-

Schematic diagram of the experimental set-up for V. U. V. excitation.

For X-irradiations a tungsten tube operated at 45 kV and 15 mA was used. The V. U. V. or X-irradiated crystals were heated at a constant rate of 10 K/min from 80 K to 500 K and TL was recorded by a standard technique [I]. For some measurements the X-irradiated samples were heated to about 500 K, then recooled to 80 K and irradiated by monochromatic light obtained from a 0.25 m Ebert monochromator (Jarrel-Ash model 82-410). The grating of this monochromator was blazed for 3 000

A

and its linear dispersion was 33 A/mm ; the slit width was 2 mm. The light sources for these experiments were a 50-A Sylvania deuterium lamp (in the spectral range from 1950-3 200

A)

and a Peck 75 Xenon arc lamp (in the spectral range from 2 700-6 000 A). The photon flux of the deuterium lamp was monitored by a Sodium-Salicylate screen and the energy of the Xenon lamp was calibrated by a radiation thermopile. Filters were used to eliminate higher order spectra.

The TL obtained after this procedure was measured between 80-500 K and compared to that obtained after sole X or V. U. V. excitation.

Thermal annealing of the irradiated crystals was performed by heating the crystal in vacuum oven

gradually t o about 400 OC. At various intermediate temperatures the crystals were returned to the vacuum cryostat and again illuminated at LNT with near U. V. or visible light ; effect of the annealing o n TL was measured.

Results. - X-irradiation at LNT caused the appearance of a strong TL. The main glow peaks appeared at 250 K, 310 K and 450 K. Irradiation with monochromatic V. U. V. light caused the appearance of weaker glow peaks at the same temperatures, while no noteable changes could be detected in the absorption spectra of the V. U. V. irradiated crystals.

Glow curves obtained after X and after V. U. V.

(A = 1 320

A,

9.3 eV) are given for comparison in figure 2. TL could be excited also with somewhat

TEMPERATURE ( K )

FIG. 2. - TL glow curves of A1203 excited at 80 K : a) by X-rays ; b) by 1 = 1 320 A.

longer wavelengths ; in this case the weaker 310 K

peak became dominant. This glow peak could be excited with wavelengths up t o 1 600

A.

No TL could be excited with wavelength longer than 1 600

A.

However, samples which were X-irradiated at LNT, heated to about 500 K, then recooled and illuminated with near U. V. or visible light up to about 5 600

A

showed a noteable TL during the subsequent heating. This is shown in figure 3 ; curve a was obtained after

TEMPERATURE ( K )

FIG. 3.

-

TL glow curves of A1203 obtained after photostimulation at 80 K (see text) : a) with 1 = 230nm ; b) with

1

-

400 nm.

(4)

RG. 4. - Excitation spectra of photostimulated glow peaks :

a) for the 250 K glow peak ; b) for the 310 K glow peak ; c) for the 450 K glow peak.

ANNEALING TEMPERATURE(%)

Fro. 5.

-

Intensity of the 310 K glow peak obtained after annealing to different temperatures (T).

illumination with

A

= 230 nm and curve b with

1 = 400 nm. It can be seen that main glow peaks appeared in this case also at 250 K, 310 K and 450 K.

The intensities of the various glow peaks were measured as function of wavelength of the illuminating light. Curves a, b, c, of figure 4 show this dependance for the 250, 310 and 450 K glow peaks respectively. A maximum appears for all glow peaks at about 230 nm. An additional maximum appeared at about 400 nm for the 250 K and 450 K gIow peaks only.

A step-by-step annealing of the X-irradiated crystal to temperatures above 300 O C caused a gradual decrease of the TL. This is shown in figure 5 for a crystal which was first X-irradiated at LNT, then heated to certain temperatures, recooled to LNT and illuminated at a given wavelength (I = 230 nm) with a constant photon flux. No TL could be excited by this procedure after heating to about 700 K.

Discussion. - It is well established that in A120,

high energy radiation, such as fast neutrons or electrons, is required for the displacement of an atom from its normal position and the generation of new defects.

Ionizing radiation such as X or y-rays, which are known to be effective in the creation of vacancies or interstitials in alkali-halides, will normally cause in AI2O3 free electrons and holes ; these may then be trapped at the sites of existing lattice defects, or at impurities.

Ionizing V. U. V. radiation is obviously not expected to create new defects in this crystal, but may cause, changes in the charge state of impurities or the filling of existing vacancies by free electrons or holes. Very little is known about the influence of V. U. V. radiation in A1203.

In the present investigation it has been shown that the V. U. V. radiation at LNT induces, except for differences in intensity, essentially the same TL that X-irradiation. This indicates that the same defects are responsible for the V. U. V. as for the X induced TL. This conclusion is supported by the preliminary measurements of TL emission spectra which indicate that the same emission band appear after X and V. U. V. irradiation.

The energy gap of A1203 is known to be about 9 eV [3]. Some TL peaks could be excited also with somewhat smaller photon energies by irradiation into the long wavelength tail of the fundamental absorption band. Similar behaviour has been found in aklali-halides and explained by an excitonic mecha- nism [I, 21,

(5)

EFFECTS OF ULTRAVIOLET AND VISIBLE LIGHT ON SYNTHETIC SAPPHIRE C7-215 light is therefore attributed to a process of photosti-

mulation. It is assumed that part of the charge carriers created by previous X-irradiation remain trapped at deep traps, which are stable to above 500 K. By subsequent near U. V. or visible radiation at low temperatures part of this carriers are transferred to shallow traps. This assumption is supported by the experiments of gradual annealing which have shown that after heating to about 700 K no TL could be re-excited by this procedure. This fits also the findings of other research group that some colour centres in

Al,O, are stable to high temperatures and decay gradually during thermal annealing [5, 71. It should be noted that the photostimulated glow peaks in our work appeared at the same temperatures as the main V. U. V. or X-excited peaks. The dependence of intensities of various glow peaks on the wavelengths of photostimulation gives indication regarding the energies most effective for this process and the energy levels of the trapping centres. Excitation maxima for the 250 K and 450 K glow peaks appear at 230 nm and at about 400 nm. These wavelengths coincide approximately with two known absorption bands [5]. It has previously been suggested that the band near 230 nm is due to a trapped electron. Govinda attributes this absorption band to an F centre. Turner and Crawford [6] to an electron trapped at a site of a Cr-ion impurity. It is generally agreed, that the broad absorption band near 400 nm is due to a V centre.

The appearance of our 250 K and 450 K glow peaks after photostimulation into this V band can be explained as follows : holes which remain at 500 K still trapped at deep traps are transferred by the LNT illumination to shallower traps and are then released thermally at 250 K and 450 K. According to the model of Turner and Crawford, the 450 K glow peak is due to a recombination of a thermal released hole with an electron trapped at the Cr2', which results in the emission of the excited Cr3+. It appears that the 250 K peak is due to an analog process. This glow peak could not be recorded in the work of Turner and Crawford since the y irradiations were performed at RT and above.

We found a correlation between the 250 K and the 450 K glow peaks ; both appeared under the same conditions and their intensities were propor- tional. The meaning of the second excitation maximum of these two glow peaks is not yet clear. The 310 K glow peak showed, however, a different behaviour. This glow peak became dominant by V. U. V. irradiation into the long wavelength tail of the fundamental absorption band. Its excitation spectrum for photostimulation had a sharp maximum near the 230 nm absorption band. It appears that this TL peak is due to the release of an electron. We expect to obtain more detailed information from measurements of the TL emission spectra which are presently being investigated for various conditions of excitation.

References

[I] KRISTIANPOLLER, N. and ISRAELI, M . Phys. Rev. B 2 (1970) [51 GOVINDA, S., Phys. Status Solidi(a) 32 (1975) K95.

1 1 7 C

L I 1 2 .

[2] ISRAELI, M. and KRISTIANPOLLER, N . , Sofid State Commun. [6] TUR"Ry T. J. and CRAWFORD~ J- H. Jr.y Phys. Rev. l3

9 (1971) 1749. (1976) 1735.

[3] LEVY,P. N., Phys. Rev. 123 (1961) 1226.

[4] ARNOLD, G. W. and COMPTON, W, D , phys. Rev. Lett. 4 171 TURNER, T. J. and CRAWFORD, J. H. Jr., Solid state ~ o m m u n .