Spectral-kinetic properties of the near ultraviolet luminescence in nominally undoped Y3Al5O12 crystals

Solid State Communications, Vol. 54, No. 1, pp. 61--64, 1985. Printed in Great Britain.

0038-1098/85 $3.00 + .00 Pergamon Press Ltd.

SPECTRAL-KINETIC PROPERTIES OF THE NEAR ULTRAVIOLET LUMINESCENCE IN NOMINALLY UNDOPED Y3AlsO12 CRYSTALS A.V. Puj~ts, L.M. Kuzmina, Z.A. Rachko and J.L. Jansons Institute of Solid State Physics, Latvian State University, Riga*, USSR

(Received 29 July 1984 by 1~.A. Kaner) Spectral-kinetic properties of the near-u.v, cathodoluminescence band in nominally undoped Y3AlsO12 Crystals are investigated and analysed. This complex luminescence band in the near-u.v, and its separation into elementary bands as well as the temperature-dependence of these elementary bands are analysed on the computer using the experimental values. 1. INTRODUCTION

The present work proposes to investigate this complex near-u.v, luminescence band in nominally updoped Y3A1 s O 12 crystals.

BOTH NOMINALLY UNDOPED and activated by rare-earth yttrium aluminium garnet (Y3 AI s O12) crystals have been extensively studied in recent years because 2. EXPERIMENTAL they are widely used as active laser media. So far optical properties have been studied mainly The near-u.v, luminescence was excited by an in doped crystals Y3AlsOx2. The optical properties of electron beam with ampere density of about I A/m s and doped crystals have been shown [2, 3] to be affected the electron gun potential was 8 kV. Luminescence by defect centers present also in undoped Y3AlsO12. spectra were analysed by Seya-Namioka type vacuum Electronhole recombination at the defect centers gives monochromator with a spherical concave grating (R = 0.5 m, 1200mm -1 ; fit square 40 x 50ram2). The rise to a cathodoluminescence (CL) band in the near-u.v, whose peak energy shifts from 270 nm at resolution of this vacuum monochromator was no 12K to 333 nm at 45 K. It has been suggested [1, 4] worse than 1 nm. The single electron pulse strobe that at least three different defect centers are involved. counting method was used for the treatment of the Some properties of these centers are given in Table 1 [1]. output signal of a photomultiplier tube FEU-106 with an input window of MgF2. Spectral measurements are Table 1. Thermalization activation energies for defect not corrected for system response. Decay kinetics were centers in Y3AIs012 crystals investigated by the single photon statistical method. 100 ns excitation pulses and pulses recurring with a Center Significant thermalization Activation frequency of 10 kHz were used. The near front of the range (K) energy (meV) excitation pulses permitted measuring kinetics whose D1 50-220 ~ 16 decay time was not less than 7 ns. An interactive minicomputer system was used for D2 220-300 ~ 130 resolving the overlapping spectrum lines or bands in D3 > 300 ~ 450 the near-u.v, band of luminescence [6]. This system comprises a minicomputer, a graphical display and a Based on optical detection of EPR of recombination display for quick communication between the computer and the operator while processing the spectra. The user centers, it has been suggested [4] that this near-u.v. of the above system examines the experimental emission could be due to decay of excitons trapped by oxygen ions near cation vacancies, in terms of symmetry, spectrum on the display, defines the number of the occupy three different positions relative by to the spectrum bands, chooses the band shape function and sets the initial values for the parameters of each band. oxygen ions. After full'filing each instruction the user can see the The assumption has also been expressed in previous work [5] that centers of capture and recombination in results on the graphical display. The Gauss function was chosen for describing Y3AlsO12 are capable of stoichiometric deviations. the near-u.v, spectrum bands: *8 Kengaraga Str., 226063 Riga, USSR. 61

62

UNDOPED Y3AlsO12 CRYSTALS

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where A is the maximum height of the band, B is the position of the maximum, C is FWHM. The damped least-squares method described by Levenberg was used for obtaining the optimal parameters [7]. This method involves a simultaneous minimization of the sum of squares of residuals and corrections to the estimated parameters: ~1

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Fig. 1. CL spectra of the nominally undoped Y3AlsO 12 crystal at diverse temperatures.

F(xl, a) - the model function, a = ( a l , a 2 , . . • ,am) - vector of parameters, Aa]

- : a j + 1 - - Eli - -

corrections to the parameters.

If the initial choice of parameters is close to the optimum only two or three cycles of iteration are needed, if the initial choice is poor, the processes may not converge. Participation of an operator in setting the initial parameters helps to overcome this difficulty. Using the process iteration until the parameter adjustments are negligibly small leads to a distortion of the true position of the spectrum band. This difficulty can be overcome, if the user looks through the results on the graphical display and interrupts the process of optimizations at a suitable moment according to his opinion. The interactive system has a real advantage over the noninteractive method in accomplishing, because participation of the operator enables the a priori information about the spectrum to be used and this helps to achieve a solution having the maximum physical meaning. 3. RESULTS AND DISCUSSION Figure 1 shows the near-u.v, cathodoluminescence spectra of the nominally undoped Y3AlsO12 crystal at temperatures between 61 and 300 K. The steady shift of the peak in the band envelope to shorter wavelengths, as the temperature decreases, is in good agreement with the earlier published ones [ 1] and indicate the complicated nature of this band. To clarify to what degree the behaviour of the feasible elementary bands depends on temperature, the CL emission intensity has been investigated in the nominally undoped YaAlsOa 2 crystal at three different wavelengths; 242 nm, 261 nm and 302 nm (Fig. 2). The decay kinetics has been measured in both the short-wavelength (243 nm) and the long-wavelength (300 nm) sides of the near-u.v, luminescence band in

YaAlsO12 at temperatures between 60 and 380 K. The obtained data allowed three basic decay exponentials to be singled out as plotted in Fig. 3. Within these coordinates the obtained curves are straight lines are also obtained for curves of the thermal decay of the CL plotted as the logarithm from the luminescence intensity inversely to temperature (Fig. 4). Measurements were taken on both the short-wavelength (243 nm) and the long wavelength (300 nm) sides of the near-u.v. luminescence band because the elementary bands strongly overlapped and singling out credibly a third thermalization activation energy was a failure. Thus the probability of the non-radiative transition is in exponential dependence inversaly to temperature. The activation energies Q, for nonradiative transition (Table 2) are estimated from the slope of the corresponding straight lines. The long wavelength side of the near-u.v. luminescence band varied from sample to sample, considerably at higher temperatures. This region of the near-u.v, band probably is dependent of uncontrolled defects, therefore the long-wavelength side has not been thoroughly investigated.

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Fig. 2. CL emission intensity dependence of the temperature in a nominally undoped Y3AlsO12 crystal at three different wavelengths 242, 261 and 302 nm.

63

UNDOPED YaAlsO12 CRYSTALS

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Table 2. Thermalization activity energies for defects centers in Y3Al s012 crystals

Significant thermalization range(K) 80-140

Activation energy a (meV) ~

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160-260

~ 135

280-380

~ 330

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Fig. 3. Dependence of decay time (T) inversely to temperature in a nominally undoped Y3 AlsO 12 crystal, measured at 300 and 243 nm with the corresponding activation energies (Q). The obtained experimental values suggest the appearance in the near-u.v, band of decay kinetics of three basic kinds decay kinetics. Hence it follows that the given near-u.v, band consists of three elementary bands, which is in good agreement with earlier reports [1,4], and of one more band which is dependent of uncontrolled defects.

deviations, i.e., at yttrium ions in site of yttrium ions in the structure of garnet (intersite defects). Earlier published [4] reports on optical detection of EPR and opinions on the symmetry of defects in relation to the ions of oxygen do not contradict this assumption. Stoichiometric deviations from the structure of Y3AlsOI~ are reported in works [8, 9, 10]. I

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Fig. 5. Experimental CL spectrum at 80 K (curve 5) and that of distribution in elementary bands (curves 1, 2, 3, 4) by computer.

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Fig. 4. Dependence of the logarithm of intensity of luminescence (lg 1) inversely to temperature in a nominally undoped YaAlsOl2, measured at 300 and 243 nm with the corresponding activation energies (Q). The fact that this complex near-u.v, luminescence band is the nearest one to the fundamental absorption edge testifies that the former is related to transitions from the shallower levels. In our opinion vacancies strongly perturb the crystal lattice and create deeper levels. Hence luminescence due to such centers should be observed in the longer wavelength region of the spectrum. We think that the three elementary bands making up the near-u.v, luminescence band are related to excitons localized at defects of stoichiometric

As seen above, the near-u.v, luminescence band in question consists at least of four elementary bands, but separate elementary bands were not obtained directly by experiment. A computer was used for separating these heavily overlapping elementary bands. In Fig. 5 are shown the experimental spectrum and its separation into elementary bands at 80 K. Fig. 6a shows the intensity (I) corresponding to the elementary bands as a function of temperature, which is in good agreement with the experimental results. In Fig. 6b is seen the FWHM corresponding to elementary bands as a function of temperature. The location of the maximum of the band envelope as a function of temperature is shown in Fig. 6c. To judge by the obtained results the complex band is very well separated into 4 elementary bands. The mean-square error in the difference of the sum of the obtained elementary bands from the experimental complex band equals 0.005. The above considerations suggest that the considerable shifts of the peak in the band envelope to shorter-wavelength, as the temperature

Vol. 54, No. 1

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of aluminium ions and at aluminium ions in site of yttrium ions in the structure of garnet (intersite defects) rather than due to vacancies [4]. The fourth elementary band with maximum of the band envelope at 330 nm may be due to uncontrolled defects. The considerable shifts of the peak in the band envelope to shorter-wavelengths, as the temperature decreases, are to be related to the temperaturedependence of the intensities of different elementary bands.

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Acknowledgements - The authors are grateful to Dr. J. Valbis for valuable discussions and Mr. U. Abeltin~ for translating the article into English.

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Fig. 6. Dependence of intensity (a), FWHM (b), locations of, the maximums ~m~x (c) on temperature of the corresponding elementary bands obtained by computer. Curves 1, 2, 3, 4 for elementary bands with maximums 254, 274,297 and 326 nm respectively. decreases, are due to the temperature-dependence of the intensities of different elementary bands. 4. FINAL CONCLUSIONS The investigated near-u.v, luminescence band is a complicated one and consists at least of four elementary bands. Three elementary bands are expected to be connected with excitons localized in intersite defects of stoichiometric deviations, i.e., at yttrium ions in site

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