JPS5953703B2 - Method for measuring minority carrier lifetime in semiconductors - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the lifetime of minority carriers in a semiconductor.

Conventionally, as a method for measuring the minority carrier lifetime in semiconductors, P
A contact measurement method using the reverse recovery characteristics of N-junctions and MOSc- characteristics, etc., and the temporal decay process of minority carriers excited by irradiation with light from xenon, LED, etc. using microwaves as probes. A non-contact method for measuring is known.

By the way, in the non-contact minority carrier lifetime measurement system, when measuring the minority carrier lifetime from the change in the reflected power of the microwave irradiated to the sample, the present inventors can measure the minority carrier lifetime by measuring the irradiation time of the minority carrier injection light. If the initial elapsed time of minority carrier decay is at least several times the minority carrier lifetime, the microwave reflected power change Δ
It has been found that P(), that is, the obtained signal, is given by the following equation.

ΔP()4-Π3-■(1+) exp(--)・・・(1)△
P(O)doτ where do is the diameter of the excitation light source, D is the diffusion constant, and τ is the minority carrier lifetime.

Therefore, in this case, if the diameter do of the excitation light source is made smaller in order to be able to measure the minority carrier lifetime in as small an area as possible, the second term in equation (1) cannot be ignored and becomes The resulting signal becomes a non-exponential function.

Therefore, a problem arises in that the decay time constant of the signal no longer represents the minority carrier lifetime τ. 1. The present invention proposes a method for measuring the lifetime of minority carriers in a semiconductor, which can solve these problems and satisfactorily measure the lifetime of minority carriers in a semiconductor substrate.

That is, according to the present invention, irradiation with excitation light is continued until a steady state is achieved in which the spatial distribution of excited minority carriers hardly changes over time, and then the irradiation is stopped. By observing the attenuation process, the term corresponding to the first term in equation (1) is always kept constant, so even if the i diameter of the excitation light source is made smaller, the exponential nature of the signal can be confirmed. will not be damaged.

Therefore, the time required for the obtained signal to decay from the value of = 0 to 1/e matches the lifetime of the minority carrier, and the minority carrier lifetime can be directly read from the signal decay time constant. Become. The present invention will be described below with reference to Examples.

As shown in Figure 1, a sample 1a made of a semiconductor substrate is placed on a sample stage 1 that is transparent to microwaves, and an LED 1 whose irradiation time and interval can be made variable is irradiated from above by an LED drive circuit 6. , to excite minority carriers in the sample. On the other hand, a microwave circuit 4 irradiates the sample 1a with microwaves from the lower side of the sample stand, detects a change in reflected power, and observes it with an oscilloscope 5. Here, the microwave circuit 4 is composed of a Gunn diode 4a that oscillates microwaves, a waveguide 4b that transmits the microwaves, a circulator 4C that branches the microwaves, and a mixer diode 4d that detects the microwaves. Ru. When the diameter DO of the aperture 3 is large, the signal observed by the oscilloscope 5 becomes an almost exponential function, as shown in the experimental data shown in FIG. 3A, for example, regardless of the LED irradiation time.

This experimental data shows that the LED irradiation time was 100 ns. When d0 is 5 mm, the vertical axis shows intensity and the horizontal axis shows time. On the other hand, when the diameter DO of the aperture 3 is made smaller, the conventional method deviates from the exponential bifunction as shown in FIG. 3B, for example, if the LED irradiation time is short. This experimental data shows that the LED irradiation time is 100 ns, and the diameter DO is DO = 0 from the top.
．． In the case of 7, 1, 1.5, and 2 mm, the coordinates are the same as in FIG. 3A. Next, in accordance with the present invention, L shown in FIG.
ED. l In the drive circuit, if the pulse width and period are changed by adjusting the variable resistors RVl and RV2 of the square wave oscillation circuit 7, and the LED irradiation time is made as long as necessary through the current booster 8, the deviation of the signal from the exponential function can be reduced. This makes it possible to directly read the minority carrier lifetime from the decay time constant of the obtained signal.

Note that R1 to R4 are resistors, 0P is an operational amplifier, Dl, D2
is a diode. In the embodiments described above, a variable resistor was used to change the pulse width of the square wave oscillation circuit 7, but a variable capacitor may be used instead of the capacitor C.

Furthermore, it can be implemented in all square wave oscillator circuits in which the pulse width can be varied.

In addition, the excitation light is not limited to LEDs,
As long as minority carriers can be generated in the semiconductor, the effects of the present invention will not be impaired in any way. a region extended by at least the diffusion length of the minority carriers, and the duration of light irradiation is at least one times the lifetime of the minority carriers so that the spatial distribution of the minority carriers is almost in a steady state. By doing so, the reflected microwave signal intensity becomes a function only of the carrier concentration, so that the lifetime of minority carriers can be directly read from the attenuation curve of the microwave signal.

[Brief explanation of the drawing]

Figure 1 is a schematic configuration diagram of an apparatus for carrying out the method of the present invention, Figure 2 is a detailed circuit diagram of the LED drive circuit in Figure 3, and Figure 3A
, B are diagrams showing experimental data. 1...Sample stage, 2...LED (light emitting diode), 3...Aperture, 4...Mital wave circuit, 5... oscilloscope.

Claims (1)

[Claims] 1. In a system in which minority carriers are excited in a semiconductor substrate by light irradiation, microwaves are emitted to the semiconductor substrate, and the recombination lifetime of the minority carriers is measured from the time change in the power of the reflected wave or transmitted wave. The irradiation region of the wave includes the irradiation region of the light that excites the minority carriers and the region extending outward in the normal direction along the contour by at least the diffusion length of the minority carriers, and the irradiation region of the light A method for measuring the lifetime of minority carriers in a semiconductor, characterized in that the duration of irradiation is at least one time longer than the lifetime of the minority carriers.