JP3862964B2 - Method and apparatus for measuring level in forbidden band by two-wavelength excitation photoluminescence - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a quantitative measurement method and apparatus for a non-radiative recombination level, which is indispensable for improving luminous efficiency of a light emitting material and a light emitting device.
[0002]
[Prior art]
Conventionally, there are technologies disclosed in the following fields.
[1] K. Hoshino et al. , J .; Lumin. 79 (1998) 39. , Jpn. J. et al. Appl. Phys. 37 (1998) 3210.
[2] Proc. Int. Works on Nitride Semiconductor. , IPAP Conf. Series 1 pp. 544-547, 2000.
When a light emitting material such as a semiconductor or a light emitting device sample is irradiated with excitation light (Above-Gap Excitation, AGE) having a band gap energy width or more, light emission (photoluminescence, PL) specific to the sample is generated. The PL intensity I _A is determined as a result of competition between the luminescence recombination rate and the non-radiation recombination rate via the forbidden band level.
[0003]
Next, in this state, when simultaneous excitation light (Below-Gap Excitation, BGE) light whose width is less than or equal to the forbidden band energy width, when the energy of the BGE light coincides with one of the levels in the forbidden band, only that level is by being selectively excited, non-radiative recombination rate changes in the sample, therefore the previous radiative recombination rate, PL intensity balance shifted in nonradiative recombination rate in I _{a + B} from I _a Change. Conventionally, the non-radiative recombination parameter of the level in the forbidden band is derived from the measurement result of the relative PL intensity I _{A + B} / I _A and its AGE intensity dependency and BGE intensity dependency.
[0004]
[Problems to be solved by the invention]
However, in the conventional method described above, since the number of data is insufficient, it is difficult to uniquely define four non-light-emitting recombination parameters per level.
[0005]
Further, in the conventional method for determining the non-radiative recombination parameter using only the steady-state value, by fitting the measurement result and the calculation result of the AGE and BGE intensity dependence of the relative PL intensity I _{A + B} / I _A , The parameters were defined. For this reason, there is a risk that the calculation accuracy is low and the result varies depending on the order of fitting.
[0006]
In order to solve the above-mentioned problems, the present invention provides a method for measuring a level in a forbidden band by two-wavelength excitation photoluminescence that enables determination of a non-radiative recombination parameter more easily and accurately. Objective.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides
[1] Levels in the forbidden band by two-wavelength excitation photoluminescence using excitation (Above-Gap Excitation, AGE) light that is greater than or equal to the forbidden band energy width, and excitation (Below-Gap Excitation, BGE) light that is less than or equal to the forbidden band energy width In the measurement method, the AGE or the BGE light is pulse-irradiated, and the non-radiative recombination level (forbidden band level) in the forbidden band due to the time dependence of the light emission (photoluminescence, PL) intensity specific to the sample. It is characterized by deriving recombination parameters.
[0008]
[2] In a forbidden band level measuring device using two-wavelength excitation photoluminescence, pulsed irradiation with excitation light greater than the forbidden band energy width or excitation light less than the forbidden band energy width, and time based on this pulse irradiation And a means for determining a time constant of the decomposition response and a means for measuring a parameter for determining a non-radiative recombination level based on the time constant.
[0009]
In the present invention, AGE or BGE light, which has conventionally been stationary light (light whose intensity does not change with time), is used as pulsed light. The time change of the PL intensity at this time between I _A and I _{A + B} or between I _B and I _{A + B} is measured and recorded. In addition to steady values (for example, I _A and I _{A + B} ), a time constant τ connecting them is newly added to the PL time response waveform. It was proved that the non-radiative recombination rate of the level can be easily calculated by using the value of this time constant τ.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0011]
FIG. 1 is a schematic diagram of two-wavelength excitation according to the present invention, and FIG. 1A shows the simplest case (one-level model) having one level in the forbidden band. In this case, the PL intensity increases due to the BGE light irradiation.
[0012]
When the PL intensity is reduced by BGE light irradiation, it can be handled by the two-level model shown in FIG. The rate equation based on the SRH statistics for the conduction band electron density n ′, the valence band hole density p ′, the electron occupancy functions f _t1 ′ and f _t2 ′ of levels 1 and 2 and the neutral condition of the charge are the most fundamental. Including the following terms:
[0013]
_{dn '/ dt = G 1 -Bn'p'} -C n1 n'N t1 (1-f t1') ... (1)
_{dp '/ dt = G 1 -Bn'p'} -C p1 N t1 f t1'p'-C p2 N t2 f t2 'p'
... (2)
df _t1 '/ dt = C _n1 n' (1-f _t1 ') -C _p1 f _t1 'p'-
G ₂ f _t1 'N _t2 (1-f _t2 ') (3)
df _t2 '/ dt = G ₂ N _t1 f _t1 ' (1-f _t2 ') -C _p2 f _t2 ' p '(4)
n '+ N _t1 f _t1 ' + N _t2 f _t2 '= p' + N _D (5)
Where G ₁ is the excitation density of AGE light, G ₂ is the excitation intensity of BGE light including the strength of coupling between deep levels, N _t1 and N _t2 are the concentrations of level 1 and level 2, respectively. C _p1 and C _p2 are hole capture rates, C _n1 is a level 1 electron capture rate, and _ND is a donor impurity density. B is called an emission recombination coefficient and is determined by the intraband distribution of electrons and holes and the interband dipole transition matrix element.
[0014]
The reason why 'is added to each variable in the above formula is to indicate that the value is the value at the time of BGE light irradiation. Since f _t2 = 0 when G ₂ = 0 (only AGE light irradiation),
_{dn / dt = G 1 -Bnp-} C n1 nN t1 (1-f t1) ... (6)
dp / dt = G ₁ −Bnp−C _p1 N _t1 f _t1 p (7)
df _t1 / dt = C _n1 n (1−f _t1 ) −C _p1 f _t1 p (8)
n + N _t1 f _t1 = p + N _D (9)
In general, the variables n ′, p ′, f _t1 ′, f _t2 ′ (at the time of BGE light irradiation) from the equations (1) to (5) and the variables n, n from the equations (6) to (9) The values are different from p, f _t1 and f _t2 (= 0) (when no BGE light is irradiated).
[0015]
FIG. 2 is a block diagram of an apparatus (system) for measuring levels in the forbidden band by time-resolved two-wavelength excitation photoluminescence according to an embodiment of the present invention.
[0016]
In this figure, 1 is an AGE light source, 2 is a BGE light source, 3 is a sample, 4 is a spectrometer, 5 is a photomultiplier tube, 6 is a digital oscilloscope, 7 is a boxcar integrator, and 8 is a computer.
[0017]
As an example, when GaN, which is a blue light emitting semiconductor, is evaluated as the sample 3, the AGE light source 1 is a combination of a deuterium lamp and an interference filter, and the BGE light source 2 is a wavelength-tunable Nd: YAG laser excitation light parametric oscillator ( Pulse oscillating lasers such as OPO), dye laser, and Ti: sapphire laser are used. The PL from the sample 3 is separated by the spectroscope 4 and then received by the photomultiplier tube 5, and the time response waveform is recorded in the computer 8 through the digital oscilloscope 6 or the boxcar integrator 7. The computer 8 is connected to the AGE light source 1 or the BGE light source 2 so as to obtain pulsed light.
[0018]
As described above, in the present invention, conventionally, AGE light or BGE light, which has been stationary light (light whose intensity does not change with time), is used as pulsed light. At this time, the time change of the PL intensity between I _A and I _{A + B} or between I _B and I _{A + B} is measured and recorded in the computer 8. In addition to steady values (for example, I _A and I _{A + B} ), a time constant τ connecting them is newly added to the PL time response waveform. It was proved that the non-radiative recombination rate of the level can be easily calculated by using the value of this time constant τ.
[0019]
Hereinafter, the description of the time-resolved two-wavelength excitation photoluminescence will be described sequentially using schematic diagrams.
[0020]
FIG. 3 is a diagram for explaining the action of the level in the forbidden band by the two-wavelength excitation photoluminescence according to the present invention by comparing it with the flow of water.
[0021]
First, in FIG. 3 (a) corresponding to the one-level model [FIG. 1 (a)], 101 is a conduction band n tank, 102 is a valence band p tank, and 103 is a non-radiative recombination level tank. , 104 is a tube (emission recombination) through which water (electrons) flows from the conduction band n tank 101 to the valence band p tank 102, and 105 is a non-emission recombination level tank 103 from the conduction band n tank 101. A tube through which water (electrons) flows (non-radiative recombination) (tube thickness Cn), 106 is a tube through which water (electrons) flows from the non-radiative recombination level tank 103 to the tank 102 in the valence band p. (Non-radiative recombination) (tube thickness Cp << Cn), 107 is a pump for pumping water (electrons) from tank 102 in valence band p to tank 101 in conduction band n, and 108 is tank 102 in valence band p Water (electrons) into the non-radiative recombination level tank 103 It is a pump to increase.
[0022]
So in the case of normal PL,
(1) Pumping water (electrons) into the tank 101 of the conduction band n with the pump 107.
[0023]
(2) Water (electrons) in the tank 101 in the conduction band n flows into the tank 102 in the valence band p through the light-emitting recombination tube 104 and the non-light-emitting recombination tubes 105 and 106.
[0024]
(3) At this time, the internal quantum efficiency η _int is
η _int = luminescent recombination (flow rate) / [luminescent recombination + non-radiative recombination (flow rate)]
Determined by
[0025]
(4) The non-light emitting recombination level tank 103 has a water level almost full (ft≈1). When water (electrons) flows through the tube 106 to the tank 102 in the valence band p, the water (electrons) in the tank 101 in the conduction band n immediately fills the empty space of the tank 103 in the non-radiative recombination level through the tube 105. That is, the flow rate of the non-radiative recombination level is limited by the smaller one of the thicknesses Cn and Cp of the tubes 105 and 106 (Cp in this case).
[0026]
Therefore, the pulse response will be described with reference to FIG. When only AGE light is irradiated,
(1) First, as shown in FIG. 4A, the pump 107 is driven only from t = 0 to Δt.
[0027]
(2) As described above, water (electrons) in the conduction band n flows to the valence band p through the light-emitting recombination tube 104 and the non-light-emitting recombination tubes 105 and 106.
[0028]
(3) As the water level of the conduction band n decreases, the flow decreases, and as shown in FIG. 4B, the light emission is attenuated by e xp ( ^{−t /} τ).
[0029]
(4) This time constant τ is determined by both the time responses of luminescent recombination and non-radiative recombination (especially the faster one). That is, 1 / τ = (1 / τ _r ) + (1 / τ _ur )
Next, in the case of two-wavelength excitation PL,
(1) In addition to normal PL, water (electrons) is pumped up to the tank (non-radiative recombination level) 103 by the pump 108 (BGE). At this time, water (electrons) in the tank 102 in the valence band p decreases (holes p increase), and thus the PL intensity Bnp increases (BGE effect).
[0030]
(2) The tank 103 is full (ft≈1), and even if the power of the pump 108 is further increased, water (electrons) does not enter the tank 103 (saturation phenomenon of the BGE effect). At this time, it can be seen that ft = 1.
[0031]
The pulse response will be described with reference to FIG.
[0032]
(1) First, as shown in FIG. 5B, the pump 108 is stopped at t = 0.
[0033]
(2) Next, in FIG. 3 (a), the water tank 103 accumulated in the above equilibrium water position AGE irradiation only by a pump 108 (electrons) gradually in the valence band p through the tube 106 Since it returns to the tank 102, as shown in FIG.5 (c), the water level of a valence band increases with the delay.
[0034]
(3) Then, as shown in FIG. 5 (c) and FIG. 5 (d), since the vacant water (electrons) in the tank 102 of the valence band p is the hole p, the hole p gradually decreases, Reflecting this, the PL intensity Bnp also gradually decreases. Since the response time constant τ ₂ at this time is determined by Cp, Cp can be determined from the time constant τ ₂ .
[0035]
In the explanatory diagram [FIG. 3B] of the case where the PL intensity is lowered by the BGE light irradiation [model of FIG. 1B], the explanations 103, 105, and 106 of FIG. In addition, water (electrons) from the level 2 tank 109, from there to the valence band tank 102 (non-radiative recombination) 110, and from the level 1 tank 103 to the level 2 tank 109 A BGE pump 111 is added.
[0036]
In normal PL, water pumped up to the conduction band tank 101 by the pump 107 passes through the luminescent recombination tube 104 and the non-radiative recombination tubes 105 and 106 as in FIG. The tank 102 is returned to. When there is no BGE light irradiation, the pump 111 does not operate and the level 2 tank 109 is empty.
[0037]
When the BGE light irradiation is performed, the pump 111 operates to draw water (electrons) from the level 1 tank 103 to the level 2 tank 109. Then, water flows from the level 2 tank 109 to the valence band tank 102 through the non-radiative recombination tube 110 anew. Further, since the water level of the level 1 tank 103 decreases, the flow of water from the conduction band tank 101 to the level 1 tank 103 increases. As a result, the water level of the tank 102 in the valence band increases (holes decrease), and the water level of the tank 101 in the conduction band decreases, so the PL intensity decreases. When the power of the pump 111 is increased, the water level of the level 2 tank 109 increases. However, when the tank 109 is full (f _{t2 is} approximately 1), the water does not enter the tank 109 even if the power of the pump 111 is further increased. (BGE effect saturation phenomenon).
[0038]
With respect to the pulse response when only AGE light is irradiated in this model, level 2 is irrelevant and is the same as described above with reference to FIG. That is, with respect to the excitation pulse of FIG. 4A, the PL intensity attenuates with a time constant τ as shown in FIG. 4B.
[0039]
Next, the pulse response in the two-wavelength excitation will be described with reference to FIG. 3B and FIG.
[0040]
(1) First, as shown in FIG. 5B, the pump 111 is stopped at t = 0.
[0041]
(2) Next, the amount of water that has been returned from the tank 109 to the valence band tank 102 in FIG. 3B gradually decreases because the supply to the tank 109 has stopped, and finally becomes zero. Therefore, as shown in FIG. 5 (e), the water level of the tank 102 in the valence band decreases with a time delay [time constant τ ₂ different from τ in FIG. The hole density increases), and returns to a steady value when there is no BGE light irradiation.
[0042]
(3) Then, as shown in FIG. 5 (e) and FIG. 5 (f), the hole density in the valence band increases, so the PL intensity increases. From the time constant, the property of level 2 (the tube 110 Thickness) can be observed.
[0043]
Finally, the method including the time response of the present invention will be described with reference to FIG.
[0044]
FIG. 6 is an explanatory diagram of a method for measuring a level in the forbidden band by time-resolved two-wavelength excitation photoluminescence (in the case of the two-level model in FIG. 1B) showing an embodiment of the present invention.
[0045]
201 is a normal PL section, 202 is a two-wavelength excitation PL section, 203 is a time-resolving response section of the two-wavelength excitation PL (in the simplest case, τ ₂ ∝1 / N _t C _p ), and 204 is a time constant τ ₂ . A calculation unit 205 is a fitting unit.
[0046]
Since it was configured in this manner, a new measurement amount representing the non-radiative recombination rate itself of time constant τ ₂ is used, so that fitting is performed only once in the final stage, and this uncertainty due to two fittings is eliminated. . That is, since one of the parameters to be determined for the non-radiative recombination level can be directly measured, all parameters can be derived more easily and accurately.
[0047]
[Experimental example]
Time-resolved recovery process of PL intensity after blocking BGE in PL and YL (yellow luminescence) emission near the band of bulk GaN by two-wavelength excitation PL method that measures changes in emission intensity due to AGE excitation light and BEG excitation light It was examined by measurement. The sample is GaN grown to 2.1 μm on the sapphire substrate by the atmospheric pressure TF-MOCVD method. When YL is observed, after blocking BGE, the YL intensity gradually recovers as shown in the inset of FIG. 7, and as shown by ● in FIG. 7, the recovery time constant τ ₂ increases as the AGE intensity increases. It was shortened from 28 seconds to 1 second. Further, when the AGE intensity was fixed and the BGE intensity was increased, as shown in FIG. 8, the time constant τ ₂ increased from 17 seconds to 31 seconds.
[0048]
In addition, this invention is not limited to the said Example, A various deformation | transformation is possible based on the meaning of this invention, and these are not excluded from the scope of the present invention.
[0049]
【The invention's effect】
As described above in detail, in the conventional method for determining the non-radiative recombination parameter using only the steady value, the measurement result and the calculation result of the AGE and BGE intensity dependency of the relative PL intensity I _{A + B} / I _A The parameters were determined by fitting the. For this reason, there is a risk that the calculation accuracy is low and the result varies depending on the order of fitting. On the other hand, in the present invention, since a new measurement amount representing the non-radiative recombination rate itself of time constant τ ₂ is used, the fitting is performed only once in the final stage, and this uncertainty is eliminated.
[0050]
According to the present invention, if the non-radiative recombination parameter in the forbidden band level can be quantitatively measured, not only whether there is a non-radiative recombination rate level in the forbidden band but also its energy, concentration, conduction band, valence. Since the strength and weakness of the connection with the electron band and the nature of the level are all clear, it is possible to know what it is based on (Ga vacancies, C residual impurities, etc.), and concrete measures to improve the process Can be specified. That is, it is a means that essentially brings about high efficiency, which is the greatest problem of light emitting materials and light emitting devices.
[0051]
Further, the method of the present invention is effective in common for various light emitting materials, phosphors, etc. in the ultraviolet, visible and infrared regions, and a wide ripple effect is expected.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of two-wavelength excitation according to the present invention.
FIG. 2 is a configuration diagram of an apparatus (system) for measuring a level in a forbidden band using time-resolved two-wavelength excitation photoluminescence according to an embodiment of the present invention.
FIG. 3 is a diagram for explaining the action of a forbidden band level by two-wavelength excitation photoluminescence according to the present invention, by comparing it with the flow of water.
FIG. 4 is an explanatory diagram of a pulse response (part 1).
FIG. 5 explains a pulse response (part 2).
FIG. 6 is an explanatory diagram of a method for measuring levels in the forbidden band by time-resolved two-wavelength excitation photoluminescence (in the case of the two-level model in FIG. 1B) according to an embodiment of the present invention. is there.
FIG. 7 is a diagram showing the AGE intensity density dependence of the time constant τ.
FIG. 8 is a graph showing the BGE intensity density dependence of the time constant τ.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 AGE light source 2 BGE light source 3 Sample 4 Spectrometer 5 Photomultiplier tube 6 Digital oscilloscope 7 Boxcar integrator 8 Computer 101 Tank of conduction band n 102 Tank of valence band p 103 Tank of non-radiative recombination level 1 104 Tube (luminescent recombination)
105,106 tubes (non-radiative recombination level 1)
107, 108, 111 Pump 109 Non-radiative recombination level 2 tank 110 tube (non-radiative recombination level 2)
201 Normal PL section 202 Two-wavelength excitation PL section 203 Time-resolved response section (τ ₂ ∝1 / N _t C _p ) of two-wavelength excitation PL
204 Calculation unit using time constant τ ₂ 205 Fitting unit

Claims (2)

  In two-wavelength excitation photoluminescence using excitation light with a forbidden band energy width or more and excitation light with a forbidden band energy width or less, the sample is pulsed with excitation light with the forbidden band energy width or with forbidden band energy width or less. A method for measuring a level in a forbidden band by two-wavelength excitation photoluminescence, wherein a recombination parameter of a non-radiative recombination level in a forbidden band is derived from the time dependence of emission intensity inherent to the sample. (A) means for irradiating pulsed excitation light with a forbidden band energy width or more or excitation light with a forbidden band energy width or less;
(B) means for obtaining a time constant of a time-resolved response based on the pulse irradiation;
(C) a device for measuring a level in a forbidden band by two-wavelength excitation photoluminescence, comprising means for measuring a parameter for determining a non-radiative recombination level based on the time constant.

Images