JPS6328512B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a novel semiconductor thin film light emitting device, and in particular, it is an object of the present invention to provide a novel semiconductor thin film light emitting device that can obtain a desired luminescent color and is suitable as a light emitting device for various display devices. . Summary of the Invention The semiconductor thin film light emitting device of the present invention is a light emitting device using thin film layers of two types of semiconductors having different forbidden band widths, that is, the semiconductor thin film layer with the smaller forbidden band width is used as an active layer, and the semiconductor thin film layer with the larger forbidden band width is used as an active layer. The semiconductor thin film layer is stacked in a sandwiched manner between two semiconductor thin film layers, and the semiconductor thin film layer consisting of the semiconductor with a larger forbidden band width is doped with an impurity that forms an energy level in the middle of the forbidden band of the semiconductor. It is characterized by becoming. As a result, by appropriately changing the thickness of the active layer, it is possible to obtain a light-emitting element that emits light in a desired color, and it is also possible to obtain a light-emitting element that emits light in a desired color by appropriately changing the layer thickness of the active layer. By adding impurities that form energy levels, electrons are more easily excited, and the luminous efficiency is greatly improved. EXAMPLES The details of the semiconductor thin film light emitting device of the present invention will be described below with reference to illustrated examples. Reference numeral 1 denotes a transparent substrate, which is made of a glass plate or a single crystal substrate of silicon (Si), gallium-arsenide (GaAs), or the like. A transparent electrode 2 is made of, for example, a thin film of indium-tin oxide, and is formed on one surface of the transparent substrate 1. 3 is a semiconductor thin film, such as zinc sulfide (ZnS).
It is a thin film of about 100 to 1000 angstroms (Å). This semiconductor thin film layer is doped with impurities that form an energy level in the middle of the forbidden band of the semiconductor material. Reference numeral 4 denotes an active layer, which is a thin film made of a semiconductor material whose forbidden band width is smaller than that of the semiconductor material of the semiconductor thin film 3, and is in a heterojunction with the semiconductor thin film 3. The semiconductor material of this active layer 4 has a smaller forbidden band width than the semiconductor material of the semiconductor thin film 3. For example, when the semiconductor material of the semiconductor thin film 3 is zinc sulfide (ZnS) as described above, selenium A mixed crystal of zinc (ZnSe) or ZnSxSe ( ₁ -x) is considered. The thickness of this active layer 4 is 10 to 200
Angstrom (Å). Reference numeral 5 denotes a semiconductor thin film made of the same semiconductor material as that of the semiconductor thin film 3 (ZnS in the above example), which is heterojunctioned with the active layer 4 and has a layer thickness of
It is between 50 and 500 angstroms (Å). This semiconductor thin film layer 5 is also doped with impurities that form an energy level in the middle of the forbidden band of the semiconductor material. Reference numeral 6 denotes a back electrode provided in contact with the back surface of the semiconductor thin film 5. This back electrode 6 may be transparent, translucent, achromatic, chromatic, or any other suitable material. FIG. 2A is a cross-sectional view showing the laminated body 7 consisting of the semiconductor thin films 3 and 5 and the active layer 4 sliced thinly in the thickness direction, and FIG. It shows the distribution. As can be seen from Figure 2, when a layered structure is created in which a thin film made of a semiconductor material with a small bandgap is sandwiched between thin films made of a semiconductor material with a larger bandgap, a quantum well QW is formed and the bandgap is narrower. Electrons and holes are confined in a layer of a semiconductor material with a narrower band width, where they undergo radiative recombination, resulting in light emission of a predetermined wavelength. Further, according to the quantum well structure as described above, electrons and holes are confined in the active layer 4 and are easily recombined by light emission, so that the luminous efficiency is increased and, therefore, the driving voltage can be lowered. In addition, as shown in Figure 3, impurities added to the semiconductor thin film layer with a larger forbidden band form an energy level in the middle of the forbidden band, which increases the number of excited electrons and causes light emission. Efficiency is further improved.
In other words, when no impurities are added, the only excitation from the valence band to the conduction band is direct excitation as shown by arrow a, and energy at least equivalent to the forbidden band width must be applied by an electric field. . However, if impurities are added to form an energy level within the forbidden band, this level will be used as a stepping stone to excite electrons from the valence band to the conduction band.
Both of the excitation processes shown by arrows b ₁ and b ₂ are one process, and each process b ₁ and b ₂ gives the electron energy △E ₁ and △E ₂ smaller than the forbidden band width Eg. It will happen if you do it. Therefore, if the same amount of energy is applied, the number of excited electrons will increase compared to when no impurity is added. Then, the electrons lifted from the valence band to the conduction band are confined in the quantum well QW, where they undergo radiative recombination. Furthermore, according to the semiconductor thin film light emitting device of the present invention,
By changing the thickness L of the active layer 4, it is possible to change the emitted light color, and the two constituent materials, i.e.
It is possible to emit all intermediate colors of the characteristic light emission of the semiconductor material with the larger forbidden band width used for the semiconductor thin film layers 3 and 5 and the semiconductor material with the smaller forbidden band width used for the active layer 4. That is, as described above, as shown in Figure 2A, semiconductors with different forbidden band widths, such as ZnS and
When ZnSe is stacked, a quantum well QW is formed as shown in Figure B. As shown in Figure 2B, the energy Ee held by electrons in a quantum well with a well in one direction and free directions in the remaining two dimensions is Ee = 〓 ² / 2me (Kx ² + Ky ² ) + 〓 ² / 2me(nπ/L) ² ...(1) n=1, 2, 3,... Here, L is the thickness of the semiconductor layer with a small forbidden band width (i.e., the width of the quantum well QW), n is the quantum number,
Kx and Ky are the x and y components of the wave number vector. Similarly, the energy Eh of a hole in the valence band quantum well QW is Eh=〓 ² /2mh (Kx′ ² +Ky′ ² ) +〓 ² /2mh (n′π/L) ……(2 ) n'=1, 2, 3,... In the above equations (1) and (2), ≡h/2π, where h is Planck's constant and me and mh are the effective masses of electrons and holes. Since Kx and Ky take the values 0<Kx, Ky, (Kx′, Ky′)<∞, (Kx, Ky, Kx′, Ky′〜0, n,
n' = 1), the minimum energy that an electron can take in the conduction band is 〓 ² (π/L) ² /2me, and the minimum energy that a hole can take in the valence band is 〓 ² (π/L) ² /
2mh, and the actual forbidden band width Eg is Eg=Eg(ZnSe)+〓 ² /2me(π/L) ² +〓 ² /2mh(
π/L) ² . Therefore, by changing L, the forbidden band width Eg can be changed within the range of Eg(ZnSe)<Eg<Eg(ZnS). Therefore, in the above example (laminate of ZnSe and ZnS),
By changing the width of the ZnSe layer, we can change the conduction band from the conduction band-valence band emission wavelength (4640 Å) of a semiconductor with a small band gap (ZnSe in this case) to that of a semiconductor with a large band gap (ZnS in this case). Emission colors ranging up to the band-valence band emission wavelength (3390 Å) can be obtained. Basically, any combination of semiconductor materials used for the laminate in the semiconductor thin film light emitting device of the present invention may be used as long as they have different forbidden band widths. The lattice mismatch between different materials, which is the key to determining the performance of other heterojunction devices, does not have as large an effect as in other heterojunction devices because each layer is extremely thin in the stack of semiconductor thin film light emitting devices of the present invention. does not occur. Therefore, it is possible to apply not only combinations conventionally used for heterojunctions but also a wider range of combinations. For example, the above-mentioned ZnS-ZnSe combination produces blue light, the GaAlP-GaP combination produces green light, the GaP-GaAsP combination produces red light, and the combination produces infrared light. GaAlAs―
Examples include a combination of GaAs. In addition to these, combinations of 2-binary, 1-binary, 3-binary, and 4-binary compounds are also conceivable, and their range of application is extremely wide. The attached table shows some semiconductor combinations along with their forbidden band widths, lattice constants, and interband emission wavelengths.

[Table] The semiconductor layers 3, 4, and 5 may be manufactured by any appropriate method, such as vacuum evaporation using an electron beam method, resistance heating method, etc., hot wall epitaxial method (HWE), or molecular beam method. Possible methods include epitaxial method (MBE), CVD method, MOCVD method, and atomic layer epitaxial method. FIG. 4 shows a second embodiment of the semiconductor thin film light emitting device of the present invention. In this embodiment, the laminate 7a has semiconductor layers 3, 3 (slightly thicker than 100 mm thick) having a larger forbidden band width on the outside.
1000 Å) between which the semiconductor layer with the smaller forbidden band width (thickness 10 to 200 Å), that is, the active layer 4
and semiconductor layer 5 (thickness 50~
500 Å) are laminated alternately. With this multilayer structure, more powerful light emission can be obtained. FIG. 5 shows a modification of the second embodiment of the semiconductor thin film light emitting device of the present invention. In this modification, the active layer 4 of the laminate 7b is
The layer thickness of the layer 4 ₁ located on the transparent electrode 2 side is thin,
The layer thickness of the layer 4n located on the back electrode 6 side is made thicker, and the layer thickness of the active layer 4 between these layers is made thicker as it is closer to the back electrode 6. With this configuration, the effective forbidden band width Eg is widened where the active layer 4 is thin (Eg1>Eg2...>Egn), and light emission with a shorter wavelength can be obtained. Therefore, each active layer 4 ₁ , 4 ₂ , 4 ₃
...The emitted light color in 4n is different, and in this device, each of these active layers 4 ₁ , 4 ₂ , 4 ₃ ,
... Luminescence with a composite color of 4n luminescent colors can be obtained. In the case of this modification, it is necessary to emit light with a longer wavelength as the distance from the transparent electrode 2 increases. Therefore, the layer thickness of the active layer 4 needs to be increased as it is closer to the back electrode 6. There is. In addition, in what is shown in FIGS. 4 and 5 above,
Of course, the semiconductor thin film layers 3 and 5 are doped with impurities that form an energy level in the middle of the forbidden band of the semiconductor material.

[Brief explanation of the drawing]

FIG. 1 is a schematic side view showing a first embodiment of the semiconductor thin film light emitting device of the present invention, and FIG. 2 shows the relationship between each layer of the laminate and the distribution state of energy levels in the semiconductor thin film light emitting device of the present invention. , A is a cross-sectional view showing a layered product sliced thinly in the thickness direction, B is a view corresponding to A showing the distribution of energy levels in each layer, and FIG. 3 explains the operation of the semiconductor thin film light emitting device of the present invention. Energy level distribution diagram for 4th
The figure is a side view schematically showing a second embodiment of the semiconductor thin film light emitting device of the present invention, and FIG. 5 is a side view schematically showing a modification of the semiconductor thin film light emitting device of the present invention. Explanation of symbols, 2...Transparent electrode, 3...Semiconductor thin film with larger forbidden band width, 4...Active layer, 5...
Semiconductor thin film with larger forbidden band width, 6... Back electrode, 7, 7a, 7b... Laminated body.

Claims (1)

[Claims] 1. A light emitting device using thin film layers of two types of semiconductors with different forbidden band widths, that is, a semiconductor thin film layer with a smaller forbidden band width is used as an active layer, and is stacked in such a manner that it is sandwiched between two semiconductor thin film layers with a larger forbidden band width. A semiconductor thin film light emitting device characterized in that the semiconductor thin film layer having a larger forbidden band width is doped with an impurity that forms an energy level in the middle of the forbidden band of the semiconductor.