JPH0577334B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an apparatus for measuring deep levels formed by defects in a crystal.

[Conventional technology]

Conventionally, a method of measuring current using thermal excitation method (TSC,
Thermally Stimulated Current) and DLTS (Deep Level
Transient Spectroscopy) and TSCMP
(Thremally Stimulated Capacitance).

[Problem that the invention attempts to solve]

Among the various defect evaluation devices mentioned in the previous section,
Thermal excitation methods such as the DLTS method use heat to excite deep levels in the sample and measure capacitance changes or currents caused by changes in charge state as a result of carrier emission or capture due to thermal excitation. The accuracy of measuring the absolute temperature of the sample directly affects the error in the deep energy level _E It is difficult to measure the spectrum, and the actual sample temperature change rate dT/dt is much slower than the excitation rate dL/dT due to light irradiation. As a result, the charge change rate dQ/dt due to carriers excited from deep levels is dt=
The large βdT is one of the reasons why the signal is small and it is difficult to obtain an accurate spectrum. On the other hand, the measurement method using optical excitation has a higher motive force than the thermal excitation method.
Because it uses quantum effects, it is possible to excite almost instantaneously, and monochromatic light with extremely good monochromaticity can be easily obtained using a spectrometer, so the dt is smaller than that of thermal excitation methods. Since dQ/dt is also large, the signal strength obtained is also large and more accurate E _T
It is clear that it also becomes possible to measure

For example, the PHCAP method using a constant bias method, which has such advantages, measures changes in C caused by changes in the charge state of deep levels due to photoexcitation at different wavelengths, and thereby determines the direction of changes in C. By measuring the magnitude and the threshold value hv of ΔC, it is found that excitation of deep levels occurs with optical energy hv. However, in general, transitions do not necessarily occur between deep levels and conduction bands or filling bands, but may occur at deep level transitions, so information about the transition process cannot be directly obtained from PHCAP measurements alone. It had some disadvantages that were not available.

[Purpose of the invention]

In order to solve the problems of the above-mentioned thermal excitation method and optical excitation method, an object of the present invention is to provide a device that measures a photoexcitation current at the same time as PHCAP measurement.

[Effect]

In the defect evaluation device configured as described above,
Accurate value of deep level E _T by PHCAP measurement,
If we measure the density N _T with depth resolution and measure the photoexcitation current at a sufficiently low temperature, for example, n
Taking a deep donor level in a type semiconductor crystal as an example, if the conduction band is excited from a filled deep donor level, a photoexcitation current is detected at hν = E _{L −ET} , and the excited If the excitation is from the filled zone of the later vacant deep donor level, the photoexcitation current flows as hν=E _{g −ET} . Furthermore, by integrating this current with a current integrator to find the amount of charge, N _T can also be found.

On the other hand, deep donor level and other deep e.g.
If it is a transition with the level, it will be detected by PHCAP as a change in ΔV _ph , but the photoexcitation current will not be detected.

〔Example〕

Hereinafter, the present invention will be explained in more detail based on the drawings. This example describes an example in which a GaAs crystal plate with n-type conductivity was measured in a wavelength range of 0.50 to 1.60 eV. FIG. 1 shows a block diagram of the present invention, in which the light source uses a halogen lamp in the wavelength range of the embodiment, the monochromatic light irradiation device is a grating monochromator, and the optical system focuses the light onto the sample. A mirror system is used to irradiate the area.
In this example, the sample used was an n-type GaAs substrate crystal, and 1 mm of AuGeNi/Au with a diameter of 500 μm was placed on the back surface of the sample.
After mask deposition at intervals and ohmic contact obtained by sintering at 450°C for 30 seconds in Hz, the surface is
A Schottky diode with a structure in which Al was deposited by electron beam evaporation was used. In the example, monochromatic light was irradiated from the back surface of the sample.

For measurements, the sample is cooled to 45 K in the dark while a forward bias is applied to keep the depletion layer short, and at a low temperature, a reverse bias voltage V _R is applied to sweep the wavelength from low energy. Therefore, deep in the n-type semiconductor
The donor is filled with electrons and is in a neutral state. Now, let us consider the case where deep donor levels with activation energies E _T1 and E _T2 exist in the sample crystal, as shown in FIG. 2. And E _T2
According to the inventor's considerations, is the E _L2 level, which is a defect level involving interstitial As atoms in the GaAs crystal,
The excited state E _T2 is formed as shown in Figure 2.
As in normal PHCAP measurement, when irradiated with light hν = E _C - E _T1 , the charge state of E _T1 changes from neutral to positively charged, and the excited electrons from E _T1 move into the conduction band. is excited to flow into the neutral region by the reverse bias voltage applied to the junction. At this time, if the current is measured at the same time using the configuration of the present invention, a current flows as the deep level is photoexcited, so it can be confirmed that there is indeed a transition between the deep donor level and the conduction band. On the other hand, hν ₂ = E _T2 ^* −
When irradiated with E _T2 light, the charge state of the deep level E _T2 changes from neutral to positive, and PHCAP measurements show that E _T2 is excited. Since this is a transition between the ground state and the excited state of _T2 , photoexcited electrons do not exist in the conduction band, and with the configuration of the present invention, no photoexcited current flows.
At the same time as the PHCAP measurement, it can be confirmed that this is a so-called intra-center transition.

In addition, in light irradiation with hν = E _C − _{E T} , if thermal excitation can be ignored, only an optical transition from E _T1 → E _C is generated, so the photo-excited current will increase until the deep donor level is excited and becomes empty. It flows, but it does not flow steadily and shows a time change as shown in Fig. 3. and deep level density
N _T is given by N _T =∫ ^tmax ₀ idt. On the other hand, in the light irradiation of hν ₁ ′=E _g −E _T1 , as shown in Figure 2, a process may occur in which electrons are excited from the filled zone to the empty deep donor, so the photoexcitation current is increased as shown in Figure 4. is distinguished from the excitation process due to the nearly steady flow hν ₁ . Figure 5 shows an example of steady-state PHCAP measurement results.
Deep levels of E _T3 , E _T4 , E _T5 , E _T6 , and E _T7 have been detected. According to the inventor's consideration, _ETS is
A single EL2 level in the so-called GaAs crystal
This is a photoquenching phenomenon that occurs along with the optical transition from a charge state to a double charge state, and E _T6 is a deep level different from the EL2 level, which has photoexcitation energy and is the photoexcitation energy region. In addition, E _T3 , E _T4 , and E _T7 are also levels different from the EL2 level, and E _T4 in particular is a thermal level.
The emission cross section is as small as 10 ^-14 cm ^-2 , and when measured using the DLTS method, which is a type of thermal excitation method, the signal intensity becomes extremely small as shown in Figure 6. On the other hand, α ^t/n of E _T3 and E _T5 (EL2) is 10 ^-13 cm
^-2 , so the DLTS signal is larger than E _T4 . FIG. 7 shows the photoexcitation current measured simultaneously with the configuration of the present invention, and it can be seen that the current flows steadily at the same time as E _T6 is excited. Therefore, from this, it is considered that the E _T6 response in FIG. 5 is caused by optical excitation of VB→D ⁺ e → CB.

As described above, according to the present invention, PHCAP measurement provides a spectrum with energy resolution that is difficult to obtain with the thermal excitation method, and at the same time, important optical transitions in related deep levels that cannot be obtained with PHCAP alone can be obtained. You can gain knowledge.

In this example, a GaAs with metal-semiconductor contact is used.
Although only the case where a crystal is used as a sample is shown, it goes without saying that it can be applied not only to metal-semiconductor contact but also to any structure having at least one p-n junction, n ⁺ n junction of the same conductivity type, or heterojunction. , various semiconductor devices can be evaluated non-destructively as they are, and the knowledge of deep levels has high industrial value.

〔Effect of the invention〕

As explained in detail above, the present invention measures the sample current due to monochromatic light irradiation and simultaneously measures the change in junction capacitance, thereby determining the photoexcitation energy and its density of deep levels formed by crystal defects, as well as determining the photoexcitation process. It has a great effect on obtaining detailed information about.

For example, it can be determined whether there is a transition between only a deep level and a conduction band, a transition from a filled band to a deep level, a transition from a deep level to a conduction band, or a transition between levels, using photoexcitation energy, etc. The effect can be known at the same time as the measurement.

[Brief explanation of the drawing]

Figure 1 is a block diagram of the configuration of the present invention, Figure 2 is an optical transition diagram in a semiconductor with a deep level, and Figure 3 is a photoexcitation current when only optical transition between the deep donor level and the conduction band occurs. Figure 4 showing the time course of
The figure shows the time course of the photoexcitation current when optical transitions between the full band and the deep donor level and between the deep donor level and the conduction band occur simultaneously. Figure 5 is the PHCAP spectrum of GaAs bulk crystal. Figure 6 teeth
DLTS spectrum of GaAs bulk crystal. Figure 7 shows the photoexcitation current spectrum of GaAs bulk crystal.

Claims (1)

[Scope of Claims] 1. A semiconductor junction comprising: a temperature control device that maintains the semiconductor junction at at least one constant temperature; and a device that irradiates the semiconductor junction with sustained monochromatic light; A light irradiation measuring device equipped with a device for recording the saturation value or time integral value of the generated current. 2. The light irradiation measuring device according to claim 1, wherein the device for recording the time integral value is a current integrator. 3. The light irradiation measuring device according to claim 1 or 2, further comprising a capacitance measuring device for measuring the capacitance of the semiconductor junction.