JP2996928B2 - Optical semiconductor device and method for manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor device, and more particularly, to a light emitting device, a laser device, a color display device and a method of manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, materials which are practically used in light emitting devices in the blue / ultraviolet region or are expected to be practically used are mainly group III-nitride, group II-selenide / sulfide compound semiconductors. Only.

Hereinafter, the various light emitting elements will be described.

(1) (Al) as [III-nitride]
In the case of the -Ga-In) N mixed crystal system, (a) light is emitted despite a very high defect density.
Its emission wavelength is 390-430 nm.

(B) A blue light emitting diode having an emission wavelength of 450 nm has already been put to practical use.

(C) It has been reported that a blue laser diode having an emission wavelength of 408 nm continuously oscillates at room temperature. It has such features.

[0007] (2) - in the case of (Zn-Cd-Mg) ( Se-S) mixed crystals as [Group II selenide-vulcanization of Product], good lattice matching with the (a) GaAs substrate, Growth can also be performed at low temperatures.

(B) The emission wavelength is 430 to 500 nm. It has such features.

[0009]

However, in the compound semiconductors of group III-nitride and group II-selenide / sulfide, mixing of oxygen is a serious problem.

Also, individually, (1) in the case of (Al—Ga—In) N mixed crystal system, (a) high temperature (up to 1200 ° C.) is required for fabrication.

[0011] (b) sapphire substrate and cleavage are different,
It is difficult to form a cavity end face.

(C) There is no suitable substrate for lattice matching.

(D) The active layer is made of In-doped GaN, and the emission wavelength becomes longer.

(E) The laser oscillation threshold is large.

(2) (Zn-Cd-Mg) (Se-S)
In the case of a mixed crystal system, (a) the light emission lifetime is short.

(B) Considering the progress made during a very long research period (from 1980), practical application is difficult. Had such a problem.

An object of the present invention is to eliminate the above problems and to provide an optical semiconductor device based on the laser oscillation phenomenon of ultraviolet light in a greenhouse of zinc oxide (ZnO) and a method of manufacturing the same.

[0018]

Means for Solving the Problems The present invention, in order to achieve the above object, (1) in an optical semiconductor device, a thin film made of zinc oxide with a light emitting layer, resonant grain boundaries existing in the thin film vessel
Enable laser oscillation by excitons at room temperature
It is a thing.

[ 2 ] In the optical semiconductor device described in the above [1] , the thin film is a zinc oxide hexagonal column nanocrystal thin film obtained by a laser molecular beam epitaxy method.

[ 3 ] In the optical semiconductor device described in the above [1], color display is possible in which ultraviolet light or blue light generated from the light emitting layer is used as excitation light to cause the fluorescent layer to emit light.

[ 4 ] In the method for manufacturing an optical semiconductor device,
When forming a thin film made of zinc oxide sapphire c-plane substrate, in which so as to form a thin film separating the step of growing the nucleation step and the nucleic determining the size and density of nanocrystals .

[0022]

Embodiments of the present invention will be described below in detail.

As the photofunctional oxide, a so-called "optical crystal" such as a solid laser crystal or a photorefractive crystal is typical. Most of them are insulators having a band gap of 4 eV or more, and have a function of controlling light by electricity, light, magnetism, and sound using ferroelectricity and piezoelectric magnetism.

The optical functional oxide to be treated here has an optical function as a semiconductor, that is, a function of generating photons by combining carriers.

According to the present invention, there is provided an optical semiconductor device exhibiting a laser oscillation phenomenon of ultraviolet light in a greenhouse of ZnO.

Hereinafter, embodiments of the present invention will be described in detail.

ZnO has an absorption edge (E _g =
(3.2 eV). Currently being studied as a material for compound semiconductor lasers in the blue and ultraviolet regions with the aim of improving optical recording density II
Table 1 shows the physical property parameters in comparison with the Group I nitride and the Group II selenide sulfide.

[0028]

[Table 1]

Compared to these materials, for example, GaN belonging to the same wurtzite is similar in terms of band gap, lattice constant, and the like, but a remarkable point is the binding energy (E _b ^ex ) Is as large as 60 meV.

An exciton is an elementary excitation unit excited in a crystal, and is formed from a pair of electrons and holes like a hydrogen atom. Since excitons are Bose particles, it is expected that a laser diode with very high efficiency will be realized. At low temperatures, the light emission process due to exciton recombination is dominant, but at room temperature, thermal energy (24 meV) makes excitons with weak bonds unable to exist.

As a result, for example, GaAs / AlGaAs
The laser emission process is governed by the high-density state of dissociated electrons and holes (electron-hole plasma), like the red semiconductor laser. From this point of view, the very large exciton binding force of ZnO is attractive not only from the viewpoint of device application but also from the viewpoint of studying physical properties of exciton systems. In fact, the exciton emission and laser oscillation of ZnO at low temperatures have been studied considerably, but there have been no reports of exciton laser oscillation at room temperature.

The oxide is controlled at the atomic level epitaxial thin film active laser molecular beam epitaxy to grow with (MBE), ZnO thin films fabricated sapphire c-plane substrate, X-ray rocking curve FWHM of Is 0.
It has a very high crystallinity of 1 °. The thin film is n-type,
The carrier density was 4 × 10 ¹⁷ / cm ³ .

As shown in the atomic force microscope (AFM) image shown in FIG. 1, the thin film formed under the condition that the oxygen pressure at the time of forming the thin film was fixed at 10 ^-6 Torr exhibited a wurtzite-type crystal habit. Reflecting this, it has a structure in which hexagonal prism-shaped nanocrystals of uniform size are densely packed. A spiral structure with a step of one unit cell height (0.5 nm) is observed on each nanocrystal, suggesting that the nanocrystal is grown under conditions close to thermodynamic equilibrium. By controlling the growth conditions, the lateral size of the nanocrystals is 70 nm to 250 n.
m. In FIG. 1, an AFM image (1 μm) of a thin ZnO nanocrystal having a thickness of 200 nm is shown.
m × 1 μm). The average size of the nanocrystals is around 250 nm. The area in the figure is 100 nm x 1
Arrows outside the figure at 00 nm indicate the crystal orientation. The crystal orientation exactly matches the hexagonal orientation of the thin film surface.

FIG. 2 shows a histogram of the size distribution of the nanocrystals. When the thickness of the thin film is 200 nm,
The average size is 257 nm, and the spread is 37
% And very uniform nanocrystals. That is, the size distribution of hexagonal nanocrystals was determined from the AFM image. The value Δφ obtained by dividing the size variation by the average size of the nanocrystal (φ _average ) is 3
7.3% and other materials (GaAs, GaN, InAs, etc.) made by other methods (MBE, MOCVD)
The degree of fixation is smaller than the generally reported value of the size variation in c).

FIG. 3 shows the thickness dependence of the size of the nanocrystal. As the film thickness increases, the size of the nanocrystals increases
It gradually increases from about 0 nm. Such a thin film can be formed when a repeatedly used ZnO target is used. The surface of this ZnO target is Zn
O is partially reduced, and zinc metal is deposited on the surface.
If the ZnO target is not used, smaller (50 nm) nanocrystals can be formed as shown in FIG. Regarding (I), the sizes of a plurality of thin films manufactured by a devised manufacturing method are shown.
Only the film that enters the area (I) shows exciton-exciton emission (P∞ emission) described later. The dashed line indicates the change in nanocrystal size versus film thickness for films made by conventional methods.

FIG. 4 shows an AFT image (1 μm × 1 μm) of the thin film, and FIG. 5 shows the size and histogram of the particles. FIG. 5 is similar to FIGS. 1 and 2. The difference is that the film thickness is 50 nm, the average size is 45 nm,
The distribution is 33%, which means that the size of the nano-sized crystal can be reduced.

The effect is that when sufficiently oxidized particles are supplied onto the substrate, particles having a small diffusion distance on the surface and high-grain size particles are formed, whereas insufficiently oxidized metal zinc atoms are formed. When mainly supplied, it can be explained by the fact that the diffusion distance at the surface becomes longer, the coarse particle form is formed, and the particles coalesce as the thickness of the thin film increases.

As a method for obtaining a fine particle thin film, there is the following method.

(1) ZnO whose surface is sufficiently oxidized
In addition to the method using a target, the following method can be used.

(2) Regardless of the surface condition of the target, the oxygen pressure at the time of forming the thin film is selected in the range of 10 ^-5 to 1 Torr, a thin film of about 1 nm is formed, nucleation is performed at an arbitrary density, and The remaining thin film is formed at 10 ^-6 to 10 ^-3 Torr to form high quality nanocrystals. The rate of nanocrystal fusion is controlled by the second stage oxygen pressure.

[0041] 50nm approximately nanoparticle thin film of a cross section having the shape shown in FIG. 4 (transmission electron microscope image) shown in FIG. This figure shows a cross section in a direction parallel to the sides of the hexagon, and the arrow in the figure indicates a grain boundary (grain boundary).
s) is shown. The position of the grain boundary is indicated by A in FIG.
FIG. 4 and FIG. 6 correspond to black portions in the FM image.
From this, it can be seen that the shape of the nanocrystal is a hexagonal prism box having a thickness of 50 nm and a lateral size of 50 nm.

As described above, a clear grain boundary is formed perpendicular to the substrate surface at the boundary between individual nanocrystals. The reason why the grain boundaries are formed will be described below.

The arrangement of the ZnO and sapphire c-plane atoms is
As shown in FIG. 7, it has an 18% mismatch. But,
11 lattices of ZnO and 13 lattices of sapphire, or Z
Comparing the five lattices of nO with the six lattices of sapphire, the mismatch is as small as 0.085% or 1.4%.

Such matching having a repetition period of a common multiple of the lattice distance between the substrate and the thin film is called higher-order epitaxy, and is performed on a Si substrate.
Although reported on the formation of N thin films, there is no report on the interface between oxides.

FIG. 8 shows the generation mechanism of grain boundaries in higher order epitaxy. For the sake of simplicity, the drawing shows a diagram in which four lattices of the film crystal are matched with three lattices of the substrate crystal. Nuclei satisfying the higher order epitaxy condition are randomly generated on the substrate, and when the nuclei grow in the lateral direction, the grains collide with each other. At this time, since there is no correlation between the phase of the nucleus and the common multiple of the nucleus, a gap of a fraction of the common multiple is generated. In this gap, the crystal lattice is disturbed to form a grain boundary. Each grain is completely in the same directional relationship, but is displaced in the lateral direction. This grain boundary functions as a confinement barrier for excitons, as described later, and constitutes a mirror of the laser resonator.

This thin film showed clear exciton emission in the ultraviolet region at room temperature. FIG. 9 shows Nd: YAG.
3 shows a photoexcitation-emission spectrum by a 波長 wavelength laser (355 nm, 30 psec). The emission spectrum is
With increasing pump power, free exciton emission (E _ex ) followed by exciton-exciton (P ₂ ) spontaneous emission, followed by a threshold intensity (I _th ) of 24 kW / cm ² to the eighth power of the pumping intensity , The intensity of the spectrum increases due to the strong light emission (P∞) in which the intensity increases and the light emission (N) whose intensity increases in the fifth power from I _th = 55 kW / cm ² .

The P ₂ and P∞ emission lines can be explained by light emission due to recombination of excitons accompanied by exciton collision and scattering processes. Two excitons in the state of n = 1 collide, and one exciton is n = 2 (P ₂ emission) or n = 2
Raised to a state of ∞ (P∞ emission), the remaining exciton recombine to emit light. The N emission line is considered to be the above-mentioned electron-hole plasma emission because the N emission line shows a red shift and a broadening with an increase in the excitation intensity.

[0048] The light emission spectrum when the size of nanocrystals is large as about 250nm shown in FIG. 10.

I _{th at which} laser oscillation occurs is 62 kW / c
m very large and ^2. In addition, P ₂ or P∞ emission was not observed, and N emission occurred immediately. Efficient P∞
Emission was only observed when the size of the nanocrystals was controlled at 40-60 nm.

In addition, when the threshold value of laser oscillation is very small and when the nanocrystal size is large, P∞ emission is not observed, so that the oscillator strength increases due to the confinement of carriers (excitons) by the nanocrystal. I can guess.

The P∞ emission is clearly observed even when the spectrum is taken from the lateral direction of the thin film. At this time, the pumping laser excites only the thin film inside the physical end face, and there is no artificial resonant mirror.

FIG. 11 shows TE (⊥c) and TM (// c)
2 shows a P∞ emission spectrum measured with polarized light. That is,
It is a spectrum when P∞ emission is viewed from the lateral direction, and I _th
= 24 kW / cm ² . The emission line shows a strong concentration on the TE polarization and a finely split spectral structure. As shown in FIG. 12, the split interval (ΔE) is the length of the excited region (excitation length: L).
It depends on. This indicates that P∞ emission is laser oscillation from a longitudinal mode cavity having mirrors at both ends of the excitation region.

FIG. 13 shows the angle dependence of the laser oscillation intensity. Nanocrystals were formed on a disk-shaped substrate, and the sample was rotated to measure the laser light intensity. When the excitation region can use a hexagonal column crystal grain boundary as a resonator mirror, strong light emission can be observed, whereas when it is shifted by 30 °, light emission becomes weak. This indicates that the grain boundaries are acting as resonator mirrors.

The origin at which both ends of the excitation region act as mirrors constituting a resonator will be explained as follows.

The excited region has a large carrier density and therefore has a larger refractive index than the unexcited portion. At the edge of the excitation region, the excitation intensity changes continuously, and it is considered that the refractive index of the thin film has a similar distribution. Assuming a certain refractive index threshold, light is reflected only at the grain boundary between the nanocrystal rows in the refractive index continuous change region, and a Fabry-Perot resonator is formed between both end faces.

From the cross-sectional TEM observation, the grain boundary is formed equilibrium on the c-axis from the interface between the substrate and the thin film to the surface.
It is considered that the grain interface of the nanocrystal functions as a mirror surface having high parallelism and reflectance. This self-forming cavity mirror not only does not require a mirror forming process by cleavage or etching, but also allows an active region to be selected arbitrarily and a resonator to be installed by attaching an electrode of an appropriate shape to the diode. Benefits can be given.

FIG. 14 shows the temperature dependence of the exciton laser oscillation threshold described above. For the thin film having a thickness of 50 nm, the value of the threshold excitation intensity of the lateral P∞ emission with respect to the crystal temperature is plotted, and the vertical axis indicates the Log scale. As is clear from FIG.
K), the threshold gradually increases,
Up to about 0 K (100 ° C.), laser oscillation of exciton generation was observed. From the slope of this graph, the characteristic temperature is
157K, which is larger than ZnSe's 124K, which indicates that it is excellent as a laser material.

FIG. 15 shows the transition of the laser oscillation threshold in GaN and ZnO.

In the GaN-based laser diode, the threshold value is reduced year by year by improving the quality of a thin film and adopting a structure such as a quantum well and a clad layer.

The laser oscillation threshold of the naturally formed ZnO nanocrystal thin film according to the present invention is smaller than the best value. Therefore, it can be said that the ZnO thin film is very promising as a short wavelength laser material.

The gallium arsenide double hetero laser diode is currently applied to a compact disk pickup laser or the like and is the most popular laser diode. It is listed as a guideline for the laser threshold that can be actually put to practical use. The oscillation threshold of a laser diode that actively uses exciton emission is expected to exceed this. It is thought that the efficiency of exciton emission can be further improved by using a double heterostructure or an optical confinement structure in combination with a ZnO diode in the future.

The present invention is not limited to the above embodiment, and various modifications and applications can be considered.

For example, magnesium or cadmium may be added instead of adding ZnO.

Further, when ultraviolet light emission is used from the ZnO thin film, the following color display can be formed.

Conventionally, a color display has been formed by combining diodes for generating the three basic colors of blue, green, and red. Therefore, a process of assembling the display using a very large number of diodes was necessary.

Since ultraviolet light can be emitted from ZnO, a large number of ZnO ultraviolet light emitting diodes are integrated on a large-area display substrate, and blue, green,
By adding a red fluorescent layer and causing the fluorescent layer to emit light by ultraviolet light excitation from a ZnO diode, a color display can be formed. Since ZnO can grow a crystal even at a low temperature of 600 ° C. or less, there is also an advantage that an inexpensive and large-area substrate such as a glass substrate can be used.

The present invention relates to an optical semiconductor device (LED, laser diode) that emits light in the blue to ultraviolet region,
Optical storage disc (CD, MO, DVD) pickups for laser, a flat panel display using a light-emitting diode, a display unit of the traffic signal or the like using a light emitting diode, as the optical semiconductor element used in the illumination or the like of the germicidal lamp, etc., extensive Application fields.

The present invention is not limited to the above-described embodiment, but various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0069]

As described above in detail, according to the present invention, (1) it is not necessary to manufacture a resonator for laser oscillation, and an optical semiconductor device having a simple structure can be obtained.

(2) The active region can be arbitrarily formed only by taking an electrode for the diode.

(3) Laser oscillation occurs at room temperature by an exciton-exciton scattering process.

(4) The laser oscillation threshold value of the semiconductor laser device can be significantly reduced.

[Brief description of the drawings]

FIG. 1 shows a ZnO film having a thickness of 200 nm according to an embodiment of the present invention.
It is a figure which shows the atomic force microscope image of a nanocrystal thin film.

FIG. 2 shows a ZnO film having a thickness of 200 nm according to an embodiment of the present invention.
It is a figure which shows the distribution state of the nanocrystal size in a thin film.

FIG. 3 is a diagram illustrating a relationship between a film thickness and a nanocrystal size according to an example of the present invention.

FIG. 4 is a view showing an atomic force microscope image of a 50 nm-thick ZnO nanocrystal thin film showing an example of the present invention.

FIG. 5 is a diagram showing a distribution of nanocrystal sizes in a ZnO thin film having a thickness of 50 nm according to an example of the present invention.

FIG. 6 is a view showing a cross-sectional transmission electron microscope image of a 50 nm-thick ZnO thin film showing an example of the present invention.

FIG. 7 is a schematic diagram (higher order epitaxy) of an interface between a ZnO thin film and a sapphire substrate showing an example of the present invention.

FIG. 8 is a diagram illustrating generation of incoherent grain boundaries according to the embodiment of the present invention.

FIG. 9 is a view showing an emission spectrum of a 50 nm-thick ZnO nanocrystal thin film according to an example of the present invention.

Thickness 2 showing an embodiment of the invention; FIG 5 0 nm of Zn
It is a figure showing an emission spectrum in an O nanocrystal thin film.

FIG. 11 is a diagram showing a longitudinal mode polarization spectrum of P∞ emission showing an example of the present invention.

FIG. 12 is a diagram illustrating a relationship between a mode interval and an excitation length according to an embodiment of the present invention.

FIG. 13 is a diagram showing the angle dependence of P∞ oscillation intensity showing an example of the present invention.

FIG. 14 is a diagram illustrating a relationship between a crystal temperature and a laser oscillation threshold according to an example of the present invention.

FIG. 15 is a diagram showing a transition of a laser oscillation threshold value in GaN and ZnO.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H01L 33/00 H01L 33/00 D (56) References Proceedings of the Japan Society of Applied Physics Vol. 57, No. 1, p. 190-191 (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 3/18 C09K 11/00 H01L 33/00 JICST file (JOIS)

Claims (4)

(57) [Claims] In an optical semiconductor device, zinc oxide is used.
A thin film as a light emitting layer, and a grain boundary existing in the thin film.
Laser oscillation by excitons at room temperature
An optical semiconductor device characterized by being made possible . 2. A optical semiconductor device according to claim 1 Symbol placement,
An optical semiconductor device, wherein the thin film is a zinc oxide hexagonal column nanocrystal thin film formed by a laser molecular beam epitaxy method. 3. The optical semiconductor device according to claim 1, wherein
An optical semiconductor device capable of emitting a fluorescent layer by using ultraviolet light or blue light generated from the light emitting layer as excitation light so as to emit light. The manufacturing method of claim 4. The optical semiconductor device, when forming a thin film made of zinc oxide Sahu <br/> § b A c-plane substrate, the nucleation step and the determining the size and density of nanocrystals A method for manufacturing an optical semiconductor device, wherein a thin film is formed by separating a step of growing a nucleus.