JP2551369B2 - Film thickness measuring method and composition analyzing method of HgCdTe crystal thin film - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film thickness measuring method and a composition analyzing method for a HgCdTe crystal thin film used for an infrared detector and the like.

[0002]

2. Description of the Related Art As a film thickness measuring method and a composition analyzing method of a non-destructive HgCdTe crystal thin film (hereinafter simply referred to as HgCdTe film), a method obtained from an infrared transmission spectrum is very often used [Applied Physics Letters ( Applied / Physics / Letter
s52 (1988) p. 2151)]. In this infrared transmission method, first, the film thickness d _MCT is obtained from d _MCT = 1 / (2n _MCT Δκ) using the fringe period _Δκ of the infrared transmission spectrum. Next, when the absorption coefficient α is set to 500 cm ^-1 , the wavelength of the transmittance equal to the value obtained by multiplying the maximum transmittance by exp (-αd _MCT ) is obtained. The energy of this wavelength is HgCdTe
It is experimentally known that it is equal to the band gap of the crystal. Therefore, from the value of the band gap obtained here, the Journal of Applied Physics (Journal of Applied Physics) is used.
The composition x of the Hg _1-x Cd _x Te film is obtained by using the relational expression with the composition reported in cs53 (1982) page 7099.

[0003]

In the infrared transmission method, the measurement can be performed only by measuring the HgCdTe film from which the substrate is removed or by using a substrate transparent to infrared light.
Even if the substrate is transparent to infrared light, if its surface is not mirror-polished, the spectrum changes due to the scattering of infrared light. However, at present, in addition to CdTe substrate, S
HgC on various substrates such as i, GaAs, sapphire
A dTe film is grown and a substrate that does not transmit infrared light or a substrate whose back surface is rough is often used. In that case, composition analysis by the conventional infrared transmission method cannot be performed. In contrast, the infrared reflection spectrum is less affected by the substrate,
In principle, compositional analysis should be possible in the same manner as the infrared transmission method. However, HgCdTe film growth via a buffer layer such as CdTe is very well performed, but the infrared reflection spectrum in this case is very complicated as shown in FIG. There are no reports on the method of determining the composition from the external reflection spectrum.

The present invention has been made in view of the above conventional circumstances, and is capable of nondestructively measuring the thickness and composition of a HgCdTe film via a buffer layer regardless of the type of substrate. An object of the present invention is to provide a method for measuring the film thickness and a composition analysis method.

[0005]

[Means for Solving the Problems] HgC according to the first invention
The film thickness measuring method of the dTe crystal thin film is the method of measuring the film thickness of the HgCdTe (MCT) crystal thin film grown on the substrate through the buffer layer by the infrared reflection spectrum.
Wavenumber kappa ₁ the maximum amplitude in the wave number range of the absorption coefficient of CdTe crystal thin film can be regarded as 0 is obtained, the fringe period △ kappa _MCT near the maximum amplitude, and a distance △ kappa _B minimum amplitude calculated, the buffer layer And the wavenumber κ ₁ of HgCdTe crystal thin film
Using the refractive indices n _B and n _MCT in the above, the film thickness d _{B of the} buffer layer and the film thickness d _MCT of the HgCdTe crystal thin film can be calculated by the following two equations: d _B = 1 / (2n _B Δκ _B ) d _MCT = ( it is characterized in that obtained from _{1-n B d B △ κ} MCT) / (2n MCT △ κ MCT).

Further, the composition analysis method of the HgCdTe crystal thin film of the second invention is such that the HgCdTe crystal thin film is grown on the substrate via the buffer layer.
The film thickness d _MCT of the HgCdTe crystal thin film was measured by infrared reflection spectroscopy.
The maximum amplitude A at wave number κ ₁ after the Toll method
And a wave with a certain value of absorption coefficient of HgCdTe crystal thin film
Maximum obtain an amplitude B and the wave number kappa ₂ in which in a few range, vibration
The following equation for obtaining the ratio Ra (= B / A) of the width B and the amplitude A , Ra = R ₁₂ R ₂₂ R ₃ exp (−αd _MCT ) / [(1-
r ₂₂ ) (1-r ₃₂ ) R ₄₂ R ₅₂ ] However, R ₁ = 1-r ₁ r ₂ + r ₂ r ₃ −r ₃ r ₁ R ₂ = 1 + r ₁ r ₂ + r ₂ r ₃ + r ₃ r ₁ R _{3 = (1-r 22)} (1-r 3 2) + [1-exp (-
2αd _MCT )] (r ₂ + r ₃ ) ² R ₄ = 1-r ₁ r ₂ exp (-αd _MCT ) + r ₂ r ₃ -r ₃
r ₁ exp (−αd _MCT ) R ₅ = 1 + r ₁ r ₂ exp (−αd _MCT ) + r ₂ r ₃ + r ₃
r ₁ exp (−αd _MCT ) r ₁ : complex reflection coefficient of Fresnel in reflection at air and HgCdTe crystal interface r ₂ : complex reflection coefficient of Fresnel in reflection at HgCdTe crystal and buffer layer interface r ₃ : buffer layer and The absorption coefficient α of the HgCdTe crystal thin film is obtained by using the Fresnel complex reflection coefficient at the reflection at the substrate interface, and then the composition x is obtained from the relation line between this α and Hg _1-X Cd _X Te x. It is what

[0007]

In the method of the present invention equipped with such means, the parameters shown in FIG. 1 are first measured from the infrared reflection spectrum. The values to be measured are the amplitude A at the maximum amplitude and the wave number κ ₁ there, the amplitude B at the second maximum amplitude and the wave number κ ₂ there, the fringe period Δκ _MCT near the maximum amplitude, and the interval between the minimum amplitudes. Δκ _b . A and κ ₁ , Δκ _MCT , Δκ _b
Is measured on the long wavelength side where the absorption coefficient α of the HgCdTe film can be considered to be almost zero. In FIG. 1, the horizontal axis is the wave number κ and the vertical axis is the reflectance R.

Next, the reflectance of the thin film structure shown in FIG. 2 will be calculated. This thin film is composed of a HgCdTe film 1, a buffer layer 2, and a substrate 3. Furthermore, the surface of the HgCdTe film is in contact with air 4. Let the complex refractive index of each layer be N _sub , N _b , N _MCT , and N _Air . These indices are complex and a function of wavenumber κ. The reflection coefficient at each interface (Fresnel's complex reflection coefficient) is r ₁ for reflection at the interface between air and the HgCdTe film, r ₂ for reflection at the interface between the HgCdTe film and the buffer layer, and the interface between the buffer layer and the substrate. It is defined as r ₃ in the reflection at. The reflection coefficient can be calculated from the refractive indexes of two kinds of substances forming the interface, and for example, the reflection coefficient r ₂ at the interface between the HgCdTe film and the buffer layer is represented by the following equation. r ₂ = (N _b −N _MCT ) / (N _b + N _MCT ), where the phase delay Δ ₁ in the HgCdTe film and the phase delay Δ ₂ in the buffer layer are Δ ₁ = 4π, respectively.
κN _MCT d _MCT, expressed as △ ₂ = 4πκN _b d _b. H
The total reflection coefficient r for the light vertically incident on the surface of the gCdTe film is as shown in Expression (1). _{r = [r 1 + r 2} exp (-i △ 1) + r 3 exp {-i (△ 1 + △ 2)} + r 1 r 2 r 3 exp (-i △ 2)] / [1 + r 1 r 2 exp ( _{-i △ 1) + r 2 r} 3 exp (-i △ 2) + r 3 r 1 exp {-i (△ 1 + △ 2)}] ......... (1) where, i is the imaginary number.

The reflectance R appearing in the infrared reflection spectrum is the reflection coefficient r multiplied by its complex co-reduction. Although this calculation is very complicated, it can be approximated as follows by ignoring only the imaginary term of the reflection coefficient, that is, the phase change term. First, assuming that the absorption coefficient of the HgCdTe film is α and the absorption coefficient of the buffer layer is 0, using the real number terms n _MCT and n _b of the complex refractive index of the HgCdTe film and the buffer layer, the reflectance R can be calculated by the formula (2). Can be expressed as _{R = 1- (1-r 1} 2) {(1-r 2 2) (1-
r ₃ ² ) + R ₆ } / R ₇ ...... (2) However, R ₆ = (1-exp (-2αd _MCT )) {(r ₂ ² + r
₃ ² ) + 2r ₂ r ₃ cos δ ₂ } R ₇ = 1 + r ₁ ² r ₂ ² exp (-2αd _MCT ) + r _{2
^{2 r 3 2 + r 3 2}} r 1 2 exp (-2αd MCT) + 2r
₁ r ₂ (1 + r ₃ ² ) exp (−αd _MCT ) cos δ <sub>1
+ 2r 2 r 3 (1 +</sub> r 1 2) exp (-2αd MCT) c
osδ ₂ + 2r ₃ r ₁ exp (−αd _MCT ) cos (δ
₁ + δ ₂ ) + 2r ₁ r ₂ ² r ₃ exp (-αd _MCT ) c
_{os (δ 1 + δ 2)} δ 1 = 4πκn MCT d MCT δ 2 = 4πκn b d b absorption coefficient α 0, when m is an arbitrary integer, Equation (2)
Therefore, the reflectance R has a maximum amplitude when δ ₂ = ₂ mπ, and δ ₂ =
The amplitude becomes minimum when (2m + 1) π. Therefore, the film thickness d _{b of the} buffer layer can be obtained from the equation (3). d _b = 1 / (2n _b Δκ _b ) ……………………………………………… (3) Next, the film thickness d _{MCT of the} HgCdTe film is obtained. In most cases, CdTe or CdZnTe doped with Zn of about 4% is used for the buffer layer. At this time, n _MCT > n _b . In addition, the film thickness is also HgCdTe
Since the film is usually thicker, assuming that δ ₁ is much larger than δ ₂ , d _MCT is given by equation (4). _{d MCT = (1-n b} d b △ κ MCT) / (2n MCT △ κ MCT) ............... (4) where amplitude B (absorption coefficient alpha) and amplitude A (absorption coefficient 0) The ratio Ra (= B / A) of is expressed by equation (5). Ra = R ₁ ² R ₂ ² R ₃ exp (-αd _MCT ) / [(1-r ₂ ² ) (1-r ₃ ² ) R ₄ ² R ₅ ² ] …………………………………… …………………………………… (5) However, R ₁ = 1-r ₁ r ₂ + r ₂ r ₃ −r ₃ r ₁ R ₂ = 1 + r ₁ r ₂ + r ₂ r ₃ + r ₃ r ₁ R ₃ = (1-r ₂ ² ) (1-r ₃ ² ) + [1-exp
(−2αd _MCT )] (r ₂ + r ₃ ) ² R ₄ = 1−r ₁ r ₂ exp (−αd _MCT ) + r ₂ r ₃ −
r ₃ r ₁ exp (−αd _MCT ) R ₅ = 1 + r ₁ r ₂ exp (−αd _MCT ) + r ₂ r ₃ +
r ₃ r ₁ exp (−αd _MCT ) Here, when the ratio Ra of the amplitude B and the amplitude A obtained from the infrared reflection spectrum is substituted into the equation (5), the absorption coefficient α at the wave number κ ₂ is obtained. FIG. 3 is a graph showing the relationship between Ra and αd _MCT obtained from equation (5). The relationship between the absorption coefficient α, the wave number κ, and the composition x of the Hg _1-x Cd _x Te film can be obtained by measuring the infrared transmission spectra of several types of HgCdTe films whose compositions are known.
In addition, the journal of applied physics (J
individual of Applied Physics
71 (1992) p. 3955 can be utilized to determine the composition. FIG. 4 is a graph showing the relationship between the absorption coefficient α and the composition x at four types of wave numbers κ.

If there is no buffer layer, the second maximum amplitude does not exist. Therefore, an arbitrary amplitude at α ≠ 0 may be obtained as B.

[0011]

The present invention will be described below with reference to the drawings. First, a film thickness measuring method will be described as a first embodiment.

As shown in FIG. 2, CdT is formed on the Si substrate 1.
The HgCdTe film is grown by the molecular beam epitaxy method via the buffer layer 2 of e. Further, by partially masking the substrate during the growth, an ungrown portion, that is, a region of only the buffer layer 2 was formed. This is for evaluating the reliability of the film thickness with a step gauge. Since the Si substrate 1 is a substrate having a low resistivity, it is opaque to infrared light and its composition cannot be measured by the infrared transmission method. air,
The refractive indices of HgCdTe, CdTe, and Si are n.
_AIR = 1.00, n _MCT = 3.55, n _b = 2.68,
n _si = 3.44. The wavenumber dependence of the refractive index is not considered here.

The infrared reflection spectrum was measured by a Fourier transform infrared spectroscope. The result is shown in FIG. The measurement wave number is 400 to 4000 cm ⁻¹ . In this example, the measured values obtained from the infrared reflection spectrum of FIG. 1 were as follows. A = 0.4120, κ ₁ = 1040c
m ^-1 , B = 0.0466, κ ₂ = 1730 cm ^-1 , Δκ
_MCT = 150 cm ⁻¹ , Δκ _b = 684 cm ⁻¹ . Note that κ
The value of ₁ is not used in this example because it is assumed that the refractive index near the measured wave number is constant. Substituting these values into the equations (3) and (4), the film thickness d _{b of the} buffer layer 2
= 2.73 μm, the film thickness of the HgCdTe film 1 d _MCT = 8.
36 μm was obtained. The film thickness value by the step gauge is 2.6 each
The values were μm and 8.5 μm, which were almost reasonable values in consideration of the measurement error and the scattering of the molecular beam during growth due to the mask.

Next, a composition analysis method will be described as a second embodiment. First, using the measured value in the first embodiment, the ratio Ra = 0.1131 of the amplitude B and the amplitude A was substituted into the equation (5) to obtain 2607 cm ₋₁ as the value of the absorption coefficient α (αd
_MCT = 2.18). This value is the wave number κ ₂ = 1 in FIG.
At 730 cm ^-1 , the Cd composition x corresponds to 0.229. The Cd of the Hg _1-x Cd _x Te film 1 is
The value of the composition value x could be obtained. The composition values of other elements can be uniquely determined assuming that stoichiometry holds. The composition values obtained here were evaluated by composition analysis by electron probe microanalysis (EPMA). As a result, the Cd composition value x of the HgCdTe film 1 was 0.230, which was in agreement with the value obtained by the method of this example within the error range.

Table 1 shows the film thickness, the composition value, and the values obtained by other evaluation methods, which were obtained by the above-mentioned first and second embodiments. Table 1 proves that the reliability of this embodiment is high.

[0016]

[Table 1]

[0017]

As described above, according to the method of the present invention, HgCdT with a buffer layer is used regardless of the type of substrate.
The film thickness and composition of the e-crystal thin film can be easily measured nondestructively. Therefore, the light receiving wavelength of the infrared detector can be accurately determined.

[Brief description of drawings]

FIG. 1 is a diagram showing an infrared reflection spectrum of a HgCdTe film via a buffer layer.

FIG. 2 is a cross-sectional view showing the structure of a sample used in an example.

FIG. 3 is a diagram showing a relationship between Ra and αd _MCT .

FIG. 4 is a diagram showing the relationship between absorption coefficient α and composition x at four types of wave numbers κ.

[Explanation of symbols]

 1 HgCdTe film 2 Buffer layer 3 Si substrate 4 Air

Claims (2)

(57) [Claims] 1. H grown on a substrate through a buffer layer
In the film thickness measurement method by infrared reflection spectrum of the gCdTe (MCT) crystal thin film, the maximum amplitude in the wave number range where the absorption coefficient of the HgCdTe crystal thin film can be regarded as 0 is obtained.
That the wave number κ _1, the period of the fringe in the vicinity of the maximum amplitude △ κ
_{The MCT} and the interval Δκ _B of the minimum amplitude are obtained, and the refractive index n _{B of} the buffer layer and the HgCdTe crystal thin film at the wave number κ ₁ ,
with n _MCT, the thickness of the buffer layer d _B and HgCdT
The film thickness d _MCT of the e crystal thin film is _calculated by the following two equations: d _B = 1 / (2n _B Δκ _B ) d _MCT = (1−n _B dB _B Δκ _MCT ) / (2n _MCT Δκ _MCT ) A method for measuring a film thickness of a HgCdTe crystal thin film, comprising: 2. Hg grown on a substrate via a buffer layer
After obtaining the film thickness d _MCT of the CdTe crystal thin film by the infrared reflection spectrum method, the maximum amplitude A at the wave number κ ₁ and H
Wavenumber range where the absorption coefficient of gCdTe crystal thin film has a certain value
The maximum amplitude B at and the wave number κ ₂ at
The following expression for obtaining the ratio Ra (= B / A) of the amplitude A , Ra = R ₁₂ R ₂₂ R ₃ exp (−αd _MCT ) / [(1-
r ₂₂ ) (1-r ₃₂ ) R ₄₂ R ₅₂ ] However, R ₁ = 1-r ₁ r ₂ + r ₂ r ₃ −r ₃ r ₁ R ₂ = 1 + r ₁ r ₂ + r ₂ r ₃ + r ₃ r ₁ R _{3 = (1-r 22)} (1-r 3 2) + [1-exp (-
2αd _MCT )] (r ₂ + r ₃ ) ² R ₄ = 1-r ₁ r ₂ exp (-αd _MCT ) + r ₂ r ₃ -r ₃
r ₁ exp (−αd _MCT ) R ₅ = 1 + r ₁ r ₂ exp (−αd _MCT ) + r ₂ r ₃ + r ₃
r ₁ exp (−αd _MCT ) r ₁ : complex reflection coefficient of Fresnel in reflection at the interface between air and HgCdTe crystal r ₂ : complex reflection coefficient of Fresnel in reflection at interface between HgCdTe crystal and buffer layer r ₃ : buffer layer and The absorption coefficient α of the HgCdTe crystal thin film is obtained using the Fresnel complex reflection coefficient at the reflection at the substrate interface, and then the composition x is obtained from the relation line between this α and Hg _1-X Cd _X Te x A method for analyzing the composition of a HgCdTe crystal thin film.