JP3367546B2 - Manufacturing method of resonant tunnel structure - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for fabricating a resonant tunnel structure by metal organic chemical vapor deposition or molecular beam growth. 2. Description of the Related Art A resonance tunnel structure can be applied to high-speed electronic switches, high-frequency oscillators, functional elements, and the like. This structure consists of a few molecular barrier layers and a slightly thicker well layer, and the resonant tunneling characteristics are very sensitive to the thickness of these layers, especially the thickness of the barrier layers. For example, a change in the thickness of one molecular layer in the barrier layer changes the electron tunneling probability by about 50%. Therefore, not only precise film thickness control, but also a technique for forming a flat interface between the barrier layer and the well layer with atomic precision is important. As a film forming method for producing the resonance tunnel structure, first, there is a metal organic chemical vapor deposition method. Regarding the film thickness control technique in this method, a method of monitoring the reflection intensity of oblique X-rays applied to the surface on which a film is formed has been proposed. However, this is not easy due to the device configuration. Therefore, the film thickness control in the metal organic chemical vapor deposition method is usually controlled by the film formation time. As a film forming method for producing a resonance tunnel structure, there is a molecular beam growth method. In this film formation method, there is a method of controlling the film thickness by monitoring the reflection electron diffraction intensity of the electron irradiated on the surface on which the film is formed, and is actually used in many cases. However, as described above, the method of controlling the film thickness using an X-ray or an electron beam only obtains information in a very narrow area. The film thickness is not controlled for the entire surface on which the film is formed. For this reason,
Even if two-dimensional islands or three-dimensional islands are frequently formed on the grown crystal surface, this state could not be grasped, and it was difficult to obtain a flat interface with atomic precision. . By the way, an example in which the relationship between the flatness of the interface between the barrier layer and the well layer forming the resonance tunnel structure, that is, the uniformity of the film thickness of those layers, and the resonance tunnel characteristics has been experimentally clarified. There is no. The reason for this is that it was difficult to control the flatness of the interface, and the actual observation of the interface was not performed, and the degree of flatness was estimated in reverse from various characteristics. Here, the following has been recently elucidated by scanning atomic force microscopy (Reference: Shinohara et al. Appl. Phys. Lett. 65 (11) 141).
8 (1994)). For example, in the molecular beam crystal growth method, two-dimensional islands are easily formed at the above-described interface. On the other hand, in the metal organic chemical vapor deposition method, a flat terrace is more suitable than the molecular beam crystal method. Etc. have been elucidated. FIG. 5 is a sectional view showing a state of a heterointerface of a resonance tunnel structure formed by a conventional method. In the figure, reference numeral 51 denotes a barrier layer, and 52 denotes a well layer. Also, 2
0 is a flat terrace with the same height formed at the interface, 21
Is a step corresponding to one molecular layer, 22 is a multi-step in which the step 21 is overlapped several steps, and 23 is a terrace 20
A two-dimensional island 24, which is a local region generated above, is a three-dimensional island in which two-dimensional islands overlap two or more layers in the height direction. FIG. 6 is a perspective view showing a surface state of the barrier layer 51 of FIG. 5, and a cross section taken along a line II in FIG.
It is indicated by 1. As shown in FIG. 5, when a two-dimensional island 23 is generated, the change in the layer thickness in that region is equivalent to one molecular layer. Then, when the multi-step 22 or the three-dimensional island 24 occurs,
The change in the layer thickness in that region is equivalent to several molecular layers, and the flatness (uniformity of the film thickness) is significantly deteriorated. Step 21
6 are formed linearly when viewed from above, as between C and C in FIG. 6, and are not formed linearly due to the occurrence of protruding portions 21 a and recessed portions 21 b is there. When the step position at the upper interface and the step position at the lower interface are shifted from each other, a change in the film thickness of the well layer 52 occurs as shown by a position B in FIG. In normal growth, even if the steps are formed linearly, the step positions at the upper interface and the lower interface are generally displaced. Is inevitable. As described above, the resonance tunnel characteristic is very sensitive to the thickness of the layer to be formed, but conventionally, since the layer thickness varies as described above, the characteristic of the element using the formed resonance tunnel structure However, there was a problem that it was not as good as expected. The present invention has been made to solve the above problems, and has as its object to improve the characteristics of an element using a resonance tunnel structure. According to the method for manufacturing a resonant tunnel structure of the present invention, first, the surface is set to <1 from the (100) plane.
A first barrier layer made of a first material having a larger band gap energy than the compound semiconductor substrate is crystal-grown at a predetermined temperature on the compound semiconductor substrate inclined in a range of 0.5 degrees to 2 degrees in the 10> direction. . Then, on the first barrier layer, a second bandgap energy having a smaller bandgap energy is formed.
The well layer made of the above material is grown at a predetermined temperature. Then, on the well layer, a second barrier layer made of the first material is crystal-grown at a predetermined temperature to form a resonance tunnel structure. However, the predetermined temperature mentioned above is the first and
Formed on the growth surface when the second material grows crystal
Step flow growth where the step height is one molecule
This is the range of the state. Here, the substrate is made of GaAs,
In some cases, AlGaAs is used for the second material and GaAs is used for the second material, and in other cases, InP is used for the substrate, InAlAs or AlAs is used for the first material, and InGaAs is used for the second material. As described above, the interface between the well layer and the layers above and below the well layer is formed by steps regularly arranged with the terrace. Then, no two-dimensional islands or three-dimensional islands are generated on this terrace, and the step of each step is 1 step.
It is a molecular layer, and the width of this terrace is twice or less the Fermi wavelength of electrons. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view showing a part of a resonance tunnel structure manufactured according to the present invention. In the figure, 15 is a well layer made of GaAs, 16 is Al _0.5 G
a barrier layer made of a _0.5 As, showing a state in which the barrier layer 16 is grown on the well layer 15. Although not shown, a barrier layer made of Al _0.5 Ga _0.5 As and a substrate made of GaAs are provided below the well layer 15.
The plane is slightly inclined from the (100) plane in the [1-10] direction. Thus, [1-1] from the (100) plane
[0] direction, the crystal grows on a plane slightly inclined in the direction.
A monolayer step 21c having a straight end is formed on the aAs surface, and a monolayer step 21d having a wavy end is formed on the surface of AlGaAs. Usually, Ga
The surface of a substrate made of a compound semiconductor such as As is slightly inclined slightly from the (100) plane due to the limit of the manufacturing accuracy. For this reason, steps (terraces) are formed on the surface of the layer on which the crystal is grown, to some extent. In such a state, under a condition of crystal growth, a two-dimensional island is more likely to be formed on the terrace as the growth temperature is lower. On the other hand, the higher the growth temperature, the more easily multi-step steps are formed. The multi-steps are more likely to be formed as the crystal plane of the substrate surface is greatly inclined from the (100) plane. That is, if the temperature is higher than the temperature at which two-dimensional islands are easily formed and lower than the temperature at which multi-steps are easily formed, that is, if crystal growth is performed in a step flow growth state, a uniform terrace (monomolecular layer step) is formed. The crystal can be grown in the state in which it is formed. [0015] As described above, the structure shown in FIG.
This figure shows a configuration in which there is no dimensional island or multi-step, that is, when the growth temperature and the tilt angle of the substrate can be formed within a certain limited range of conditions. This is because the inclination angle of the substrate is 0.
The film could be formed in the range of 05 to 2 degrees and the growth temperature in the range of 600 to 730 ° C. In addition, the growth temperature is 630-700.
When the temperature is set to ° C., the formation of two-dimensional islands and multi-steps can be suppressed more reliably. At this time, the inclination direction of the substrate is [1-1].
0] direction, and the respective layers were grown by metal organic chemical vapor deposition using trimethylgallium as a Ga source, trimethylaluminum as an Al source, and arsine as an As source. Here, it is almost impossible to align the positions of the monolayer step 21d at the upper interface and the monolayer step 21c at the lower interface shown in FIG. Therefore, there is always a region d1 having a different layer thickness. However, if the tilt angle in the <110> direction from the (100) plane of the substrate surface is set to 0.5 degree or more and the region d1 is set to be equal to or less than the Fermi wavelength of the electrons, the change in film thickness is not captured by the electrons. This is a region where the electrons cannot be obtained, and a layer having a uniform film thickness is formed for electrons. The width d2 of the terrace to be formed becomes smaller as the inclination angle from the (100) plane of the substrate surface on which the crystal grows becomes larger, and is set to 1.0 degree or more (<2 degrees). It can be less than the electron Fermi wavelength. If the width d2 of the terrace is equal to or smaller than the Fermi wavelength of the electrons, the width of the region d1 having a different layer thickness is also equal to or smaller than the Fermi wavelength of the electrons. The relationship between the resonance tunnel structure showing the effect of the present invention and the characteristics will be described below. FIG. 2 is a cross-sectional view of a resonance tunnel diode manufactured by applying the above-described results, and FIG. 3A is a sectional view of the diode (element area 20 × 20 μm).
² ) is a characteristic diagram showing current-voltage characteristics at 83 K in FIG. FIG. 2 shows a cross section parallel to the direction of the terrace formed on the surface of the growing crystal. [0019] In FIG. 2, 1 is Si as an impurity is made of n-type GaAs of 5 × 10 ¹⁷ cm ^-3 substrate, 2,8
Is an n-type GaAs layer having a concentration of Si as an impurity of 5 × 10 ¹⁷ cm ^-3 and a thickness of 300 nm;
GaAs layers of 4 nm and 5 nm, 4 and 6 are barrier layers of 12 molecular layers of Al _0.5 Ga _0.5 As, 5 are well layers of GaAs of 14 molecular layers, and 9 is Si as an impurity. With a concentration of 3 × 10 ¹⁸ cm ⁻³ and a thickness of 50 nm
The n ^{+ -type} GaAs layer 10 is an electrode. The tilt angle of the substrate used at this time is 1.0 degree, and the growth temperature is 650 ° C. As is well known, the peak voltage Ip appears in the current-voltage characteristics of the resonant tunneling diode as the applied voltage increases, and when the voltage is further increased, the current decreases and the ballet current Iv appears. As shown in FIG. 3A, the peak current / valley current ratio of the negative resistance portion representing the figure of merit of the resonant tunneling diode is 7.2, and the peak current density is 5
A result of 5000 A / cm ² was obtained. This characteristic is the most excellent value obtained by the metal organic chemical vapor deposition method. This is shown in FIG.
In the structure having the characteristic (a), the width of the terrace is 16 nm.
Since this value is smaller than 24 nm, which is the Fermi wavelength of the electrons of the resonant tunneling diode, the electrons are considered as a flat surface of the interface, and the scattering of the electrons at the interface is suppressed, and the characteristics are improved. is there. In contrast to the above, FIG. 3B shows a resonance tunnel diode (element area 20 ×) manufactured by a conventional method affected by a change in the layer thickness of one molecular layer.
It is a characteristic diagram which shows an example of the current-voltage characteristic at 83K of 20 μm ² ). As shown in FIG. 3B, conventionally, the values of the peak current / valley current ratio and the peak current density are as small as 3.6,30000 A / cm ² . FIG. 4 shows the effect of improving the resonance tunnel characteristic by making the layer thickness change region equal to or less than the Fermi wavelength of electrons as described above. FIG.
'S I _rt, those determined from _{I rt = I p -I v /} 2 in the tunnel current. As described above, if the terrace width is reduced in a state where the multi-steps and the two-dimensional islands are not formed, the tunnel current and the I _p / I _v ratio increase accordingly. This terrace width is less than the (100) plane of the substrate surface.
The greater the inclination angle to the 110> direction, the greater the value. When the inclination angle is increased to some extent and is about 0.5 degree, the terrace width is about 32 nm, and the peak current / valley current ratio and the value of the peak current density are significantly increased. At this time, the width d1 of the change in the layer thickness shown in FIG.
Is estimated to be at most 16 nm. Since this value is equal to or smaller than the Fermi wavelength of the electron as described above, it is a region where the change in the film thickness cannot be recognized for the electron.
Therefore, the characteristics are improved. Furthermore, the terrace width is 16 nm
In this case, as shown in FIG. 3, the terrace width itself is equal to or less than the Fermi wavelength of the electrons, so that even if there is a step, the electrons cannot be captured, and the characteristics are further improved. By the way, squares, circles, and triangles painted in black in FIG. 4 indicate a case where a substrate having an inclination angle of 2 degrees in the <110> direction from the (100) plane is used. It can be seen that the characteristic deterioration has begun. This is because a multi-step step in which the step has a step of two or more molecules has begun to be formed. (1)
Even if the angle of inclination in the <110> direction from the (00) plane is 0.5 ° or more, if it exceeds 2 °, multi-steps will be formed for the first time, and even if this width is as small as 16 nm or less, The characteristics will be degraded. In the above description, GaAs / AlGaAs
The case where a system material is used has been described. However, the present invention is not limited to this. An InP substrate is used, and an InAlA
The same applies to the case where a barrier layer made of s or AlAs and a well layer made of InGaAs are formed.
In this case, the growth temperature is set to 530 to 650 ° C. in order to obtain a step flow growth state. As described above, according to the present invention, a compound semiconductor substrate whose surface is inclined from 0.5 degree to 2 degrees in the <110> direction from the (100) plane can be formed on a compound semiconductor substrate. By setting the crystal growth temperature to a predetermined value, a resonance tunnel structure including a first barrier layer, a well layer, and a second well layer is formed. The predetermined temperature described above is such that the first and second materials
During crystal growth, the height of the steps formed on the growth surface
Range of the step flow growth state where
It is an enclosure. For this reason, the interface between the well layer and the layers above and below the well layer is composed of steps regularly arranged on the terrace, and no two-dimensional islands or three-dimensional islands are generated on this terrace. The step is a single molecular layer, and the width of this terrace is twice or less the electron Fermi wavelength. For this reason, for example, the region where the layer thickness changes due to the mismatch of the step positions at both interfaces of the quantum well becomes equal to or less than the Fermi wavelength of the electron and cannot be captured by the electron. In other words, a state in which the layer thickness does not change is formed for electrons, which has the effect of improving the characteristics of the device using the resonance tunnel structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing a part of a resonance tunnel structure manufactured according to the present invention. FIG. 2 is a cross-sectional view showing a configuration of a resonant tunnel diode manufactured according to the present invention. FIG. 3 shows a resonant tunneling diode (element area 20 × 2)
FIG. 7 is a characteristic diagram showing current-voltage characteristics at 83 K of 0 μm ² ). FIG. 4 is a correlation diagram showing a relationship between a step width and a resonance tunnel characteristic. FIG. 5 is a cross-sectional view showing a state of a heterointerface of a resonance tunnel structure formed by a conventional method. FIG. 6 is a perspective view showing a state of a hetero interface of a resonance tunnel structure formed by a conventional method. [Description of Signs] 1 ... substrate, 2,8 ... n-type GaAs layer, 3,7 ... GaAs
Layers 4,6 ... barrier layer, 5 ... well layers, 9 ... n ⁺ form GaAs
Layers, 10 ... electrodes.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-213170 (JP, A) JP-A-7-6957 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/20-21/205 H01L 29/06 H01L 29/66 H01L 29/68 H01L 29/88

Claims (1)

(57) [Claim 1] A compound semiconductor substrate having a surface inclined from 0.5 degree to 2 degrees in the <110> direction from a (100) plane by a band from the compound semiconductor substrate. A first step of crystal-growing a first barrier layer made of a first material having a large gap energy at a predetermined temperature; and forming a first material on the first barrier layer from a second material having a smaller band gap energy. a step of crystal growth of the well layer at a predetermined temperature at which, on the well layer, a second barrier layer made of the first material have a a step of crystal growth at a predetermined temperature, the predetermined temperature Means that the first and second materials are crystalline
As the length increases, the height of the steps formed on the growth surface
Step flow growth range for one child
A method for fabricating a resonant tunnel structure.