JP3198474B2 - Method for evaluating laminated structure of semiconductor substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for measuring light emission generated in an extremely small area in a laminated structure buried deep under the surface of a semiconductor substrate by electrons injected from a probe into the semiconductor substrate. By detecting the electron injection energy and the light emission amount at each electron injection position while scanning the electron injection position, the thickness distribution of the quantum structure can be measured in nanometers.
The present invention relates to a method for evaluating a laminated structure of a semiconductor substrate, which enables measurement with a high spatial resolution on the order.

[0002]

2. Description of the Related Art In recent years, with the progress of semiconductor crystal growth and processing techniques, it has become possible to produce a laminated structure (= quantum structure) having an electron wavelength size. In a quantum structure, electrons exhibit a remarkable quantum effect, and a device having a new function (= quantum device) may be realized by controlling and using this effect.

In quantum devices, structural fluctuations on the order of nanometers, which are about the same as the electron wavelength size, greatly affect device characteristics. To improve device characteristics, a quantum structure must be evaluated with a spatial resolution on the order of nanometers. Is required.

As a conventional means for evaluating a quantum structure, there is a method in which light or electrons are injected into the quantum structure from the outside, and light emission (= exciton light emission) generated when excitons generated recombine is used.

Since excitons have a size of about 10 nm, which is almost the same as an electron, and the emission energy changes sensitively to a slight change in the quantum structure, the state of the quantum structure can be measured with extremely high precision by measuring the exciton emission. Can be found at

As actual measuring methods, photoluminescence (hereinafter abbreviated as “PL”) using light as means for generating electron-hole pairs and cathodoluminescence (hereinafter abbreviated as “CL”) using high-energy electrons are used. (Abbreviated).

[0007]

However, these PLs and CLs have problems of exciton diffusion length and probe diameter (= excitation beam diameter) as described below, and deteriorate spatial resolution.

First, the effect of exciton diffusion will be described.
In both cases of the PL and the CL, the exciton diffuses far from the place where the exciton is generated within a period from generation to emission of light. The range of diffusion has an extent of submicron or more, making it difficult to improve the spatial resolution of PL and CL.

In addition, since excitons have the property of diffusing into a region having a lower energy (= thick quantum structure) than the place where they are generated, light emission from a thick quantum structure inevitably becomes stronger.
There is a problem that the light emission amount distribution does not always accurately reflect the thickness distribution.

Next, the effect of the excitation beam diameter will be described.
In the PL, visible light or infrared light having a wavelength longer than that is usually used as the excitation light, and the wavelength of these lights is about 1 micron.

In consideration of the fact that the spot size of the excitation light cannot be smaller than the wavelength of the light, and in consideration of the spread caused by other factors in the configuration of the apparatus, it is actually difficult to reduce the beam diameter to the order of microns or less.

In the CL, since the electron energy is high, the electrons are scattered over a wide range in the sample, and the electrons after the scattering are electrons / electrons.
Since a hole pair is generated, it is difficult to reduce a region (= generation volume) for generating an electron-hole pair even if the electron beam diameter is reduced.

As described above, in these methods, since the diffusion length of the exciton and the diameter of the excitation beam are large, it is difficult to improve the spatial resolution from the submicron order, and the measurement of an extremely small area on the order of nanometers is performed. Could not be applied to

Optical STM and photon scanning microscopes are used as optical evaluation means having a high resolution spatial capability on the order of nanometers. In each case, the measurement area is limited to the sample surface and is buried deep under the surface. The evaluation of the structure
In principle impossible.

As described above, at present, there is no device capable of measuring the thickness (= confinement energy) of a quantum structure buried deep under the surface with a spatial resolution of the order of nanometers.

It is an object of the present invention to provide a method for evaluating a laminated structure of a semiconductor substrate, which enables a quantum structure to be measured with ultra-high resolution on the order of nanometers.

[0017]

The above object can be attained by adopting the following novel characteristic construction method of the present invention. That is, a first feature of the method of the present invention is that, first, electrons having different energies are injected into a semiconductor substrate having a stacked structure by using a tunnel current,
The current value injected into the semiconductor substrate is measured for each injection energy, and subsequently, the amount of light emitted from the semiconductor substrate obtained at the time of injection of electrons having different energies is measured for each injection energy, and further obtained for each injection energy. This is a method for evaluating a laminated structure of a semiconductor substrate, in which a normalized luminescence intensity from a quantum structure placed at a different depth is measured by dividing an amount of emitted light by a current value injected into the semiconductor substrate. .

According to a second feature of the method of the present invention, the injection of electrons having different energies into the semiconductor substrate in the first feature of the method of the present invention is first changed while controlling the injection point. The normalized luminescence intensity from the quantum structures placed at different depths is measured by measuring the luminescence amount at a point for each injection energy and dividing by the current value injected into the semiconductor substrate, and subsequently, This is a method for evaluating a laminated structure of a semiconductor substrate, in which a standardized emission intensity is displayed corresponding to each injection point position to measure a spatial distribution of a thickness of a quantum structure of the semiconductor substrate.

[0019]

According to the present invention, the novel method described above is employed.
Electrons are injected into the sample from the probe with a predetermined energy, and the injection current and the quantity or energy of light emitted by the electrons reaching the quantum structure buried deep below the surface of the sample are measured. Thickness of nanometer
It is possible to measure with ultra high spatial resolution of the order.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Apparatus used in the method of the present invention) Before describing the method of the present invention, an example of a laminated structure evaluation apparatus used in the method of the present invention will be described with reference to the drawings for the case of evaluating a well structure in a quantum structure. I will explain. FIG. 1 is a diagram showing a schematic configuration of a laminated structure evaluation apparatus. In the figure, A is a laminated structure evaluation device, P is a measurement point, 1 is a probe, 2 is a tunnel current, 3 is a tunnel current detector, 4 is a variable bias power supply, 5 is a space, 6 is a sample, and 6a is a sample surface. , 7 is sample 6
, A sandwich-like quantum structure comprising upper and lower barrier layers 7a and an intermediate well layer 7b sandwiched between the upper and lower barrier layers 7a;
Reference numeral 8 denotes light emission generated by recombination of electron-hole pairs, 9 denotes a photodetector including a spectroscope and a photoelectric converter for receiving light emission 8 emitted from the inside of the sample 6 to the outside, and 10 denotes a tunnel current detector. 3 and output measurement signals S1 and S2 from the photodetector 9.
, A probe driving mechanism using, for example, a piezo element, and 12, a probe driving mechanism 11 receiving an operation command signal S3 from the signal processing apparatus 10.
Is a control device that issues a control operation signal S4 for controlling the control operation.

Next, the operation principle of the laminated structure evaluation apparatus A will be described. First, the probe 1 is placed very close to the surface 6a of the sample 6, and a variable bias power supply is provided between the probe 1 and the surface 6a of the sample 6 so that the probe 1 has a positive potential with respect to the surface 6a of the sample 6. 4 applies a bias voltage.

The implantation energy is set within a range lower than the potential energy of the barrier layer 7a by adjusting the bias voltage. Thereby, the surface of the sample 6 from the tip of the probe 1
A tunnel current 2 is emitted through the space 5 toward a.

Most of the tunnel current 2 injected into the sample 6 by obtaining the injection energy determined by the bias voltage travels a certain distance in the sample 6 without losing energy.

The electrons that move in the solid without losing energy are called "ballistic electrons". The average value of the distance that the electron travels without losing energy is referred to as “mean free path”.

If the quantum structure 7 exists within the mean free path from the surface 6a of the sample 6, the ballistic electrons reach the quantum structure 7 while maintaining the injected energy. When the quantum structure 7 has an energy level that resonates with a ballistic electron, the emission intensity increases.

Reference [LD Bell and WJ Kaiser, Phy
According to s. Rev. Let. 61, 2368 (1988)], in a region where the potential changes (hereinafter, referred to as a “potential boundary”), an electron whose traveling direction deviates from the vertical by a certain angle with respect to the potential boundary is Since the electrons cannot pass through the potential boundary, electrons that can pass through the potential region can be considered to be ballistic electrons that have not been scattered before. The lateral spread of ballistic electrons that have passed through the potential boundary is extremely narrow, and can be suppressed to the order of nanometers.

In the laminated structure evaluation apparatus A, the barrier layer 7
a (which can be regarded as a potential boundary) and the quantum structure 7
When the energy of the ballistic electrons reaching the well layer 7b of the first layer coincides with the energy of the electron-hole pairs, a large number of electron-hole pairs are generated. The size of the region where the electron-hole pairs are generated is extremely narrow, on the order of nanometers, which is about the diameter of a ballistic electron beam.

The generated electron-hole pairs recombine after forming excitons and emit light. The amount of light emission 8 is proportional to the number of electron-hole pairs created by the ballistic electrons reaching the well layer 7b of the quantum structure 7. The thickness of the well layer 7b of the quantum structure 7 can be measured by measuring the relationship between the amount of emitted light 8 and the ballistic electron energy.

Incidentally, the tunnel current 2 is applied to the surface 6 of the sample 6.
It is extremely sensitive to the state of a, and a slight change in the surface state greatly changes the tunnel current 2. This change is irrelevant to the quantum structure 7 of the sample 6 to be measured, and results in a measurement error. Therefore, in order to remove the influence of the tunnel current 2 fluctuation, the signal processor 10 reduces the amount of light emission 8 by the tunnel current 2. Is standardized (converted to the amount of light emission per unit tunnel current). This value is hereinafter referred to as “normalized light emission intensity” in the present application.

Also in the laminated structure evaluation apparatus A, the electron-hole pairs are diffused in the quantum structure 7 until the light emission disappears in the same manner as in the conventional PL or CL. However, the laminated structure evaluation apparatus A measures the energy of the place where the electron-hole pair is generated, not the energy of the place where light is emitted after the diffusion of the electron-hole pair like the conventional CL. It is possible to measure the information of the quantum structure 7 at the position where the electron-hole pair is generated irrespective of the diffusion length.

The spread of the positions where the electron-hole pairs are generated is as follows.
It is about the diameter of a ballistic electron beam, which is on the order of nanometers, and is significantly narrower than conventional PLs and CLs. That is, in the laminated structure evaluation apparatus A,
Since the influence of diffusion such as hole pairs can be eliminated and the beam diameter can be made nanometer size, it is possible to increase the final spatial resolution of the measurement to the order of nanometers.

(Example of Method) Next, an execution procedure of an example of the method of the present invention will be described. First, a procedure for obtaining the thickness of the well layer 7b in the quantum structure 7 will be described. The probe 1 is positioned at the measurement point P on the sample 6 having a laminated structure, and the photodetector 9 such as a spectroscope is set so that the light emission 8 emitted outside the sample 6 can be detected. Next, the injection energy of electrons is changed by controlling the bias voltage by the variable bias power supply 4, the amount of light emission 8 and the injection tunnel current 2 are measured, and the “normalized light emission intensity” is obtained by the signal processing device 10.

Thereafter, similarly, while controlling the bias voltage to change the injection energy, the luminescence 8 and the tunnel current 2 are measured for each injection energy, and the relationship between the “normalized luminescence intensity” and the injection energy is measured. Ask for.

Since the injection energy at the time when the amount of light emission 8 reaches a peak represents the thickness of the quantum structure 7 at the measurement point P,
Thereby, the well layer 7 of the quantum structure 7 at the measurement point P
The thickness of b is required.

Here, a case where the spatial distribution of the thickness of the well layer 7b of the quantum structure 7 is measured will be described. Further, the bias voltage of the probe 1 is controlled by the variable bias power supply 4 to set the electron injection energy to a value corresponding to the confinement energy of the quantum structure 7 to be measured. Subsequently, the next measurement point P of the control device 12 which has received the operation command signal S3 of the signal processing device 10 while holding the injection energy is maintained.
The probe 1 is scanned by the probe drive mechanism 11 on the sample 6 according to the instruction of the plot control operation signal S4.

In addition, the amount of light emission 8 and the tunnel current 2 are measured at each measurement point P, and the “normalized light emission intensity” of the light emission amount 8 is obtained using the tunnel current 2. In addition, when this "normalized emission intensity" is displayed in correspondence with the position of each measurement point P plotted, the spatial distribution of the thickness of the well layer 7b of the quantum structure 7 of the sample 6 to be measured is finally obtained. Can be

In the above embodiments, description has been made regarding the evaluation of a semiconductor substrate having a quantum well structure. However, evaluation of other quantum structures, for example, a quantum wire structure, a quantum box structure, etc., is similarly performed in the present invention. If you evaluate using the example method,
It goes without saying that the evaluation can be performed with several levels of accuracy higher than in the past.

[0038]

As described above, according to the method for evaluating a laminated structure of a semiconductor substrate of the present invention, the "normalized light emission amount" is detected by the injection energy of the tunnel current and the tunnel current, so that a deep portion below the surface can be obtained. It has excellent usefulness, such as being able to measure the thickness of a certain quantum structure with ultra-high spatial resolution on the order of nanometers.

[Brief description of the drawings]

FIG. 1 is a schematic structural view illustrating a method example of the present invention.

[Explanation of symbols]

 A: Lamination structure evaluation device P: Measurement point S1, S2: Measurement signal S3: Operation command signal S4: Control operation signal 1: Probe tip 2: Tunnel current 3: Tunnel current detector 4: Variable bias power supply 5: Space 6: Sample 6a: Sample surface 7: Quantum structure 7a: Barrier layer 7b: Well layer 8: Light emission 9: Photodetector 10: Signal processing device 11: Probe driving mechanism 12: Control device

Continuation of front page (56) References JP-A-7-120483 (JP, A) JP-A-5-343023 (JP, A) JP-A-6-201584 (JP, A) JP-A-6-74899 (JP, A) JP-A-5-136238 (JP, A) JP-A-5-133897 (JP, A) JP-B-53-9114 (JP, B2) JP-B-3-12772 (JP, B2) U.S. Pat. (US, A) Berndt, R .; R. Schl ittler, and J. et al. K. Gim zewski, "Photon emission scanning tunneling microscopy", Journal of Vacuum Science & Technology, Technology, Vol. 573-577 Tsukada, K., "Recent Development of Scanning Tunneling Microscope", Journal of the Physical Society of Japan, August 1993, Vol. 48, No. 8, p. 615-623 Yoichi Uehara, Shikatsu Shioda, "Emission Phenomena of Scanning Electron Microscope", Applied Physics, 1991, Vol. 60, No. 12, p. 1235-1238 (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01N 13/10-13/24 G01N 21/62-21/74 JICST file (JOIS)

Claims (2)

(57) [Claims] At first, electrons having different energies are injected into a semiconductor substrate having a laminated structure using a tunnel current, and then a current value injected into the semiconductor substrate is measured for each injection energy. By measuring the amount of light emission from the semiconductor substrate obtained at the time of injection of different electrons for each injection energy, and further dividing the amount of light emission obtained for each injection energy by the current value injected into the semiconductor substrate, A method for evaluating a laminated structure of a semiconductor substrate, comprising measuring a standardized light emission intensity from a quantum structure placed at a depth. 2. Injecting electrons having different energies into the semiconductor substrate, first changing the injection points while controlling the injection points, and then measuring the amount of light emitted at each injection point for each injection energy to be injected into the semiconductor substrate. The normalized luminescence intensity of the quantum structures at different depths is measured by dividing by the current value obtained, and then the normalized luminescence intensity is displayed in correspondence with each injection point position 2. The method according to claim 1, further comprising measuring a spatial distribution of a thickness of the quantum structure of the semiconductor substrate.
13. The method for evaluating a laminated structure of a semiconductor substrate according to the above.