JPH0552668B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to the depth of an electron or hole trapping level (hereinafter referred to as an impurity level) due to impurities or defects present in a luminescent material such as a phosphor (i.e.,
The present invention relates to a method for measuring activation energy (activation energy), and in particular aims to rapidly detect impurity levels that control the characteristics of light-emitting materials.

Conventionally, a thermal glow method is known as a method for measuring impurity levels in luminescent materials. However, in this thermal glow method, it is necessary to strictly control the temperature increase rate, and this is difficult, making it difficult to obtain quantitative data from the measurement results.

Furthermore, for semiconductor materials, methods such as the DLTS method and the optical capacitance method are conventionally known. However, these methods require a bonded interface and electrodes for measurement, so samples that cannot form a bonded interface, such as powdered samples such as phosphors, and electrode formation techniques have not yet been developed. The drawback is that it cannot be applied to new semiconductor materials.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for quickly and reliably measuring the depth of impurity levels in a luminescent material by eliminating such drawbacks.

In order to achieve such an object, in the present invention, the sample to be measured is stimulated by a stimulation source, and after the stimulation is stopped, at least two sampling times t _s
Find the _temperature dependence of _{the afterglow rate at} (Here, k is the Boltzmann constant, C is a parameter that depends on the substance, and Ea is the activation energy of the impurity level, that is, the depth of the impurity level, e = 2.718)
Determine the depth of the impurity level by

Embodiments of the present invention will be described in detail below with reference to the drawings.

In general, when a luminescent center is stimulated by ultraviolet light, free electrons and holes are generated, and when these recombine, they emit fluorescence, but some free electrons (or holes) enter the conduction band (or valence band). electron band) and is captured by the impurity level. After stopping the stimulation, these electrons (or holes) are thermally released and re-emitted into the conduction band (or valence band), where they fluoresce when recombining with holes (or free electrons). . This fluorescence is called afterglow.

Figure 1 shows the mechanism of light emission after stimulation stops. In this figure, 1 is the conduction band, 2 is the valence band,
3 represents an impurity level, and 4 represents light emission. Also, a represents the probability that an electron (or hole) captured in the impurity level is thermally released from there to the conduction band (or valence band), and a=Sexp(-Ea/kT)... It is given by (1). In this equation, S is the frequency factor, Ea is the activation energy of the impurity level, k is Boltzmann's constant, and T is the absolute temperature. In this figure, r is the probability that free electrons and holes recombine (the sum of the probabilities of radiative transition and non-radiative transition)
represents.

Now, if the impurity level and the number of electrons (or holes) in the conduction band (or valence band) after stopping the stimulation are n _r and n, respectively, the rate equation for this system is dn _T /dt =−an _T +bN _T n ……(2) dn/dt=an _T −bN _T n−rn ……(3) Given. In this equation, b represents the probability that free electrons are captured in the impurity level, and N _T represents the concentration of the impurity level. Note that since the measuring method according to the present invention targets a phosphor, the probability of transition from an impurity level to a valence band, which is considered not to directly contribute to light emission, is ignored.

We solve this rate equation below. First, solve equations (2) and (3) as n _T (t)=n _TO e ^a ′ ^t , a′=a/(1+bN _T /r)
...(4) n(t)=Be ^-a ′ ^t ...(5) Then, substituting equations (4) and (5) into equations (2) and (3), B=a′/rn _TO ... …(6) is obtained. From equations (4) and (5), the emission intensity I can be determined as I=r _R n=(r _R a′/r)n _TO e ^-a ′ ^t (7). However, r _R indicates the probability of recombination due to light emission among the recombination probabilities r. Also, if the luminescence intensity and stimulation intensity during stimulation are I ₀ and G, respectively, then I ₀ = (r _R /r)G, so from equation (7), I/I ₀ = (n _TO /G) a ′e ^-a ′ ^t ……(8).

Next, when [∂(I/I ₀ )/∂T] _t=ts = 0 is found, a′=1/t _s holds, and from this and equation (1), log ₁₀ t _s = (1/k log ₁₀ e) Ea/T+C...(9) is obtained. In this equation (9), T _s is the time measured after the stimulation stops, and C represents a parameter dependent on the substance. As is clear from the fact that this equation satisfies [∂(I/I ₀ )/∂T] _t=ts = 0, _T in equation (9) is determined by , is the temperature T _n at which the emission intensity peaks when the temperature dependence of the emission intensity is determined.

In addition, from a′=1/t _s and equation (8), I/I ₀ =n _TO /eGt _s ...(10) holds, and from this, the approximate concentration N _T of the impurity level, that is, N _T n _TO = eGt _s (I/I ₀ ) ...(11) can be known.

The impurity level measuring method according to the present invention will be explained below.

FIG. 2 is a waveform diagram for explaining the principle of the present invention. In the measurement method according to the present invention, a sample to be measured is intermittently stimulated with a rectangular stimulation source such as ultraviolet rays, an electron beam, or an electric field as shown in FIG. In this case, the light emission from the sample has the waveform shown in 2. If the luminescence intensity during stimulation is I ₀ and the afterglow intensity during the period when stimulation is stopped is I, I/I ₀ (hereinafter referred to as afterglow rate) is the temperature T (K) of the sample and the period when stimulation is stopped. It is a function of the subsequent elapsed time t.

Certain elapsed time t=t _s (sampling time)
When measuring the temperature dependence of the afterglow rate at , there is a temperature (T _n ) at which the afterglow rate shows a maximum. The relationship that holds between this peak temperature T _n (K), sampling time t _s (s), and activation energy Ea (eV) of the impurity level is expressed by equation (9), and two or more sampling times, For example, by measuring the temperature dependence of the afterglow rates _{I 1 /} I ₀ and I ₂ /I ₀ at t 1 and t ₂ shown in FIG. 2, and finding the peak temperatures T ₁ and T ₂ , (9) Activation energy Ea and concentration of impurity level can be calculated from equation (11) and equation (11).
N _T can be determined.

FIG. 3 is an example of an apparatus for measuring the temperature dependence of the afterglow rate. In this figure, L is a stimulating light source, and the ultraviolet light emitted from this becomes an intermittent light beam by a rotating chopper C and a filter _F1 , and is focused on a measurement sample on a sample holder H to stimulate it.
At this time, the sample holder H is in the vacuum chamber B, and the sample temperature can be changed from low temperature to about 200°C. The light emitted from the sample is collected by a lens LN, passed through a selection filter _F2 according to the emission spectrum of the sample, and detected by a photomultiplier tube D.

The output waveform of the photomultiplier tube D is measured by a sampling device S such as a boxcar integrator (2 channels or more) as I ₀ , I ₁ , I ₂ ... shown in FIG. Afterglow rate I ₁ /I ₀ , I ₂ /I ₀
are recorded respectively. As the X input of the X-Y recorder R, the thermoelectromotive force of the thermocouple TC attached to the sample holder H is used.

The activation energy Ea can be determined from the above measurement results and equation (9).

FIG. 4 shows an example of measurement results for Y ₂ O ₂ S/Tb phosphor using the measurement method of the present invention. As the sampling device S, a transient memory controlled by a microcomputer was used. Also,
As a stimulus source, ultraviolet rays mainly having a wavelength of 254 nm from a low-pressure mercury lamp were used.

FIG. 5 is an analysis of the measurement results shown in FIG. 4. That is, in order to find the activation energy using equation (9) from the results in Figure 4, t _s and
Figure 5 shows the relationship between T _n . As a result, there are three types of trap levels a, b, and c in Y ₂ O ₂ S:Tb, and their depths are 0.17 and 0.17, respectively.
It was possible to measure 0.52 and 1.33 eV. Further, from FIG. 4 and equation (11), it was found that the concentrations of a, b, and c were approximately equal.

Regarding this phosphor, it has been qualitatively known that there are 2 to 4 impurity levels based on measurements using the usual glow method.
There is no example of quantitatively measuring its depth.

Figure 6 shows GaP/Zn-O obtained by the measurement method of the present invention.
Examples of measurement results for crystals are shown below. A two-channel boxcar integrator was used as the sampling device S, and the temperature was changed three times. As the stimulation source, light with a wavelength of 488 nm from an Ar laser was used.
From this example, two depths of 0.12 eV and 0.23 eV were observed as impurity levels of this material.

Note that the 0.23 eV level has also been found by the DLTS method, but the 0.12 eV level has not been known so far. In this way, the measurement method of the present invention can more easily obtain the same results as the DLTS method.

As is clear from the above, the impurity level measurement method according to the present invention does not require precise temperature control, and does not require electrode attachment, etc., which is required in the DLTS method used for semiconductor materials. Since no preparation is required, the impurity level in the luminescent material can be measured relatively easily.

Furthermore, according to the present invention, it is possible to quantitatively measure the depth of impurity levels in phosphors, etc., which has been difficult with conventional glow methods.

Furthermore, according to the measuring method of the present invention, it is also possible to roughly estimate the concentration of impurity levels.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the light emission mechanism after stimulation stops.
2. FIGS. 1 and 2 are waveform diagrams for explaining the principle of the present invention, FIG. 3 is a configuration diagram showing an example of an apparatus for implementing the present invention, and FIGS. 4 to 6 are one embodiment of the present invention. FIG. 3 is a diagram showing measurement results according to an example. 1...Conduction band, 2...Valence band, 3...Impurity level, 4...Light emission, L...Ultraviolet light source, C...Optical chopper, _F1 ...Ultraviolet transmission filter, H...Sample holder , TC...Thermocouple, B...Vacuum tower,
LN...Condenser lens, _F2 ...Ultraviolet cut filter, D...Photodetector (photomultiplier tube), S...Sampling device, R...X-Y recorder.

Claims (1)

[Scope of Claims] 1. After stimulating the sample to be measured with a stimulation source and stopping the stimulation, the temperature dependence of the afterglow rate is determined at at least two or more sampling times _ts , and the temperature dependence of the afterglow rate is determined from the temperature dependence. In addition to obtaining the temperature T _n at which the afterglow rate is maximum, log ₁₀ t _s = [1/k log ₁₀ e] Ea/T _n +C (where k is Boltzmann's constant, e is the base of the natural logarithm, and C is An impurity level measurement method characterized by determining the depth of an impurity level from Ea, which is a parameter depending on the substance (Ea is the activation energy of the impurity level, that is, the depth of the impurity level). 2. Claim 1, characterized in that the stimulation source is an ultraviolet ray, an electron beam, an electric field, or an electric current.
Impurity level measurement method described in section.