DE3005536A1 - LASER EMISSION ELEMENT - Google Patents

Description

description

The invention relates to a luminescent or luminous element and in particular to a laser emission element.

Various types of lasers have already been developed and used as optical source or amplifier for optical communication or message systems, optical integrated circuits (integrated optics) or for optical data processing.

As an optical source or optical amplifier in those discussed above High expectations are placed on a semiconductor laser for its usefulness, and research and systems Development activities have been undertaken to make this practical because it is smaller in size and can be made in a solid state.

In the semiconductor laser, e.g. in the junction or boundary layer laser with heterojunction, laser emission is obtained by adding light to the outside, that occurs during the recombination of electrons injected via the boundary layer and positive electrons Holes is stimulated. Accordingly, in order to increase the emission efficiency, it is absolutely necessary to use a semiconductor crystal which is single crystal and excellent in crystal state. The manufacturing process of such a semiconductor crystal is extremely complex, requiring sensitive manufacturing controls. Thus, the manufacturing effort is considerable high.

030034/0776

It is therefore the object of the invention to provide a preferably red laser radiation specify emitting laser emission element that can be produced with reduced effort and for the conventional Semiconductor laser can be used; this laser emission element should in particular radiation with a peak value of a narrow Half width or a narrow profile at normal temperature in comparison with the conventional semiconductor laser just hand in.

This object is achieved according to the invention in a laser emission element according to the preamble of claim 1 by the in its Characteristic part specified features solved.

Advantageous developments of the invention are in the claims 2 to 4 indicated.

The invention enables a laser emitter element to function effectively as an optical source or amplifier for optical communication systems or optical integrated circuits is applicable. Furthermore, the invention enables a solid-state laser emission element, which is smaller in size and can be fabricated in a thin film to be easily handled and divided into various optical systems to be incorporated. In addition, the laser emission element according to the invention can be easily manufactured and Manufacturing control can be produced with reduced manufacturing costs.

The invention thus provides a laser emission element which has monocrystalline zinc oxide (ZnO), which is a smaller one Amount of lithium (Li) is added to thereby form a narrow level of foreign matter in the zinc oxide (ZnO) in the energy band. The ZnO laser emission element according to the invention is particularly radiant red laser radiation with a peak value wavelength of approx. 695 nm at room temperature, in that the foreign-

030034/0776

Substance level is used.

The invention thus provides a monocrystalline laser emission element Zinc oxide, to which lithium is added to form a level of impurities that has an extremely narrow energy band Has. The laser emission element emits in particular red laser radiation with a wavelength of 0.695 pm through transition between a metastable level formed by a Vo ~ center and a ground state formed by Li.

The invention is illustrated below with reference to the drawing, for example explained in more detail. Show it:

1 shows a schematic representation of an example of an apparatus for a reactive agglomerate jet deposition process, which is suitable for manufacturing the laser emission element of the present invention;

2 shows a block diagram of an example of a device for measuring the characteristic curve of the emission spectrum sensitivity a laser emission element according to an embodiment of the invention;

Fig. 3 is a diagram for explaining the relationship between the light wavelength and the relative luminous intensity of the invention Laser emission element;

4 shows a three-dimensional diagram with the characteristic curve of the spectral sensitivity of the emission in a region, in which a sharp emission is observed;

Fig. 5 is a diagram for explaining the relationship between the accelerating voltage and the luminous intensity of the present invention Laser emission element; and

030034/0776

Figures 6 (a) to 6 (d) actual arrangements of the invention Laser emission element.

The present ZnO laser emission element can be implemented by various Methods are made. In one embodiment, which is described below, the ZnO laser emission element produced by an agglomerate jet deposition process with reactive ionic agglomerates, the is preferably applicable to form a single crystal thin film and to which foreign matter can be easily added.

Fig. 1 is a schematic representation of an apparatus for the aforementioned agglomerate jet deposition process according to Invention. 1 shows a closed crucible 1 which is equipped with one or more nozzles 1a. Of the Crucible 1 contains materials 2 to be evaporated, such as zinc (Zn) and lithium (Li) in the present exemplary embodiment. In In this case, it is possible to prefer two crucibles each of which contains Zn and Li.

A substrate holder 3 holds a substrate 4 which is positioned or adjusted with respect to the nozzle 1 a of the crucible 1. A cathode 5 is provided on the path of the steam emerging from the nozzle 1a and can emit thermions when heated. One The ionization electrode 6 is kept at a positive potential with respect to the cathode 5 and can be that of the cathode 5 accelerate emitted electrons, whereby they hit the vapor, so that the vapor can be ionized. Farther an accelerating electrode 7 is provided for accelerating the ionized vapor. A lock or a lock

030034/0776

BATH ORIGINAL

3005538

shields the substrate 4 from the bombardment of the steam, if necessary. A heating device 9 is provided in the vicinity of the crucible 1 and can heat the crucible 1, to evaporate the material 2. Near the nozzle 1a there there is a gas supply pipe 11 with a gas spray nozzle 11a on its Final end facing nozzle 1a. The gas supply pipe 11 supplies a gas corresponding to that exiting from the nozzle 1a Steam is reactive. Eine.Heizeinrichtung 12 is used for heating of the substrate 4. A bell jar or vacuum bell (not shown) takes all of the above-mentioned components under a high vacuum atmosphere on.

The following is a practical example of the ZnO laser emission element of the present invention manufactured by the above-mentioned apparatus described in more detail.

First, the materials 2, which are composed of a high-purity Zn and a metallic lithium, which are in a range from 0.001% by weight to 10% by weight, preferably 0.1% by weight to 0.5% by weight with respect to the Zn, is brought into the crucible 1, and the crucible 1 is heated by the heating device 9 in order to evaporate the material 2. The one made from a monocrystalline sapphire The substrate 4 to be ordered is attached to the substrate holder 3.

In this case, the crucible 1 is heated so that the vapor pressure

2 inside the crucible 1 is at least 10 times as high as the atmospheric pressure around the crucible 1. That is, the temperature for Heating of the crucible 1 is controlled so that the vapor pressure within the crucible 1 is at least in a range of 0.1 Pa to 100 Pa (10 Torr to 1 Torr) when the atmospheric pressure around the crucible 1 after the supply of the reactive gas from

-3 -5 -2

Gas supply pipe 11 approx. 10 Pa to 1 Pa (10 Torr to 10 Torr)

amounts to. In the agglomerate jet deposition process, it is useful to that the pressure difference between the vapor pressure in the crucible 1

030034/0776

BATH ORIGINAL

and the atmospheric pressure around the crucible 1 is as high as possible.

The materials 2 heated and vaporized in the crucible 1 are ejected through the nozzle 1a into the outer space of the crucible 1, which is kept on a high vacuum. During ejection, the evaporated materials are exposed to supercooling, the caused by their adiabatic expansion, and their atoms are loosely related to each other by van der Waals' Forces connected, resulting in a number of agglomerates, each usually made up of approx. 500 to 2,000 atoms exist, which are given a large kinetic energy when ejected from the nozzle 1a, so that they to the substrate 4 to fly.

As explained above, the gas supply nozzle 11 a, the gas supply pipe 11, is directed towards the nozzle 1 a of the crucible 1. The gas supply pipe 11 feeds a small amount of oxygen (0 _? ) Gas into the steam ejected from the nozzle 1a, and the resultant steam flies to the substrate 4. The pressure in the bell jar after the introduction of the oxygen gas is preferably

- 3 - 5 - 2

about 10 Pa to 1 Pa (10 Torr to 10 Torr), as explained above, and it is appropriate that the agglomerates, into which the oxygen gas is introduced, inside the bell jar not be broken.

The agglomerates consisting of Zn and Li vapor and O ₂ ^- GaS are bombarded with a beam of electrons which are emitted by the heating wire 5 and accelerated by the ionization electrode 6 during the passage through the space of the ionization electrode 6, thereby forming individual atoms ionize the agglomerates to form ionized agglomerates. The ionized agglomerates fly to the substrate 4 together with one

030034/0776

BATH ORIGINAL

0 ₂ -lon, non-ionized agglomerates and O ₃ / while they are accelerated by the action of an electric field.

The accelerating electrode 7 is due to a high accelerating voltage which is negative with respect to the crucible 1, and the ionized agglomerates receive kinetic energy from the Bosch voltage to strike the substrate 4 together with the neutral agglomerates and O ₂ . When the ionized agglomerates collide with the substrate 4, the ionized agglomerates disintegrate into individual atomic particles, which are due to the so-called surface migration effects /

which occur due to the agglomerate jet deposition process and the chemical effect of the O ₂ gas introduced into the agglomerates, move on the substrate 4 in order to facilitate the epitaxial growth of the highly crystalline ZnO film 13 provided with Li.

In the agglomerate jet deposition process with reactive ionized agglomerates can undergo epitaxial growth even at a substrate temperature as low as about 200 ° C due to the activated or excited reaction that takes place on the ionization of Zn, Li and O- and the rapid injection speed of the agglomerates or their parts is based, which are formed by the Zn and Li vapor and from Crucible 1 are ejected. In addition, the epitaxially grown ZnO film 13, which is extremely smooth on its surface is due to the migration energy (surface diffusion energy obtained during the vapor deposition) of the agglomerates will.

According to the invention, the epitaxially grown ZnO film 13 high quality by the vapor deposition among the following Conditions generated:

030034/0776

BAD ORIQfNAL

Number of nozzles in crucible:

Electron current emitted by the filament 5 for the ionization

Accelerating voltage applied to the accelerating electrode 7

Heating temperature of the substrate:

Vacuum around crucible 1 after introducing the O ^ gas through the gas supply pipe 11:

one

250 mA to 300 mA

0 to 7 kV 200 ° C to 300 ° C

0.05 to 0.1 Pa

(4 to 8 χ 10 Torr)

The ZnO laser emission element according to the invention is under the generated above condition. To a spectrochemical To carry out analysis of the emission spectrum to test the properties or characteristics of the ZnO film produced in this way this excited by electron beams.

In Fig. 2, which schematically shows an apparatus for performing the spectrochemical analysis, is a ZnO laser emission element 21 is provided to which Li is added and which is produced by the above-described process. The laser emission element 21 is excited by electron beams from an electron gun 22 are issued. The electron gun 22 is connected to a high voltage source 23 which can control the output voltage to drive the electron gun within a range of 0 V to 20 kV. The emission or radiation from the laser emission element 21 radiates on a grating monochromator to the spectrochemical Perform analysis of the emission spectrum. A photoelectron amplifier tube 25, an amplifier, is also provided 26 for amplifying the output signal from the photoelectron amplifier tube 25, a take-out amplifier 27 for taking out of signals from the output of amplifier 26 and

030034/0776

for amplifying the signals, a writer 28 and a direct current high voltage source 29 for controlling the photoelectron amplifier tube 29. Furthermore, there are electro-optical pulse generators 30 and 31 for generating control signals for the driver amplifier 27 or an optical pulse receiver is provided.

The characteristics of the spectral sensitivity of the emission spectrum of the laser emission element 21 obtained by the measuring instrument shown in FIG. 2 are illustrated with reference to FIG explained in more detail below.

in Fig. 3, the light wavelength is on the abscissa and the relative Luminous intensity plotted on the ordinate. A full line curve (a) represents the characteristic of the laser emission element 21 according to the invention, and a broken line curve (b) indicates the characteristic curve of the substrate made of single-crystal sapphire, which is measured for comparison purposes.

The laser emission element 21 shown in Fig. 2 is made by evaporation prepared by ZnO provided with Li, which is formed on the (1102) plane of the sapphire substrate 4 by means of that shown in FIG Device is to be deposited under such conditions that the ionization current to ionize the agglomerates of the materials 2 ejected from the nozzle 1a and the reactive gas 300 mA fed from the gas supply nozzle

is / that the acceleration voltage at the acceleration electrode 7 has the value 7 kV, and that the temperature of the through the heat source 12 of heated substrate 4 is 230 ° C. The so the film thickness obtained is about 0.35 μm, and the electrical resistance value is about 10-Ώ-; from the X-ray blocking or locking curve before the measurement of the characteristic curve of the spectral sensitivity of the emission spectrum and the "RHEED" pattern it is recognized that the C-axis of ZnO is essentially

030034/0776

300S536

is oriented parallel to the (ΐΤθ2) plane of the sapphire substrate 4.

As shown in curve (b) of FIG. 3, with respect to c? Es Sapphire substrate 4 the emission with the peak value in the ultraviolet range 0.380 pm due to a free exciton and the emission with a peak value in the green band of approx. 0.527 µm can be observed, which appears to indicate oxygen defects is based. On the other hand, as shown in curve (a) of Fig. 3, the emission may have a relatively wide peak of about 0.330 pm at room temperature due to a bound exciton and the sharp red emission of 0.695 pm with respect to the ZnO growth films can be observed. The accelerating voltage Vb of the electron beams for exciting the laser emission element 21 is 10 kV in this case, and the electric one Current measures approx. 20 pA.

To the type of emission with the sharp emission peak 0.695 pm observed in the ZnO growth film, becomes the spectral sensitivity of the laser emission element 21 when different acceleration voltages are present there, the result being shown in FIGS is.

Fig. 4 is a three-dimensional diagram showing the characteristics shows the spectral sensitivity of the emission strength in the region where the sharp emission is observed. In 4 shows the light wavelength on the X-axis and the acceleration voltage of the electron beams before the excitation on the Y-axis and the light intensity is plotted on the Z axis.

Fig. 5 shows the relationship between the luminous intensity of the wavelength of 0.695 pm and the acceleration voltage of the electron beams for the excitation. The measurement takes place at

030034/0776

BATH ORIGINAL

Room temperature, and the electric current Ib of electron beams is 20 pA.

As can be seen from Figures 4 and 5, there occurs the sharp emission peak observed at the wavelength of 0.695 µm when the accelerating voltage Vb of the electron beams for excitation is over 6 kV. This means, that a critical voltage of the laser emission element according to the invention is 6 kV. As shown in Fig. 4, falls the half width of the peak value decreases in accordance with the increase in the accelerating voltage Vb, and when the accelerating voltage Vb of the electron beams for exciting the laser element measures 12 kV, the half-width is less than approx.

3 nm (30 Å) ·.

The emission peak at the wavelength of 0.695 µm (see above) can only be observed when the Li-provided ZnO film 13 is epitaxial on the (1TO2) plane of the sapphire substrate

4 grew up. When the crystalline state of the ZnO growth film 13 is weak or amorphous glass is used for the substrate 4, there will be no emission peak at the wavelength of 0.659 µm is observed.

As a result of the mode measurement and the investigation of the light emission sequence of the Li-provided ZnO film can be recognized that there is coherence in the emission peak that the Emission from the crystalline state of the ZnO growth film 13, the type of substrate to be used and the addition of Li depends, and that this is a critical voltage for exciting the ZnO films exist. In view of these facts, it can be concluded that the emission is a laser emission.

In this case, the laser emission can theoretically be explained as follows:

030034/0776

First, the function of Li in the ZnO growth film 13 is determined

2+
seeks. Li is substituted by Zn, which has a hexagonal

2-

nalen wurtzite structure, and one of the four O ions that

Surrounding Zn ⁺ captures a positive hole to thereby form an acceptor level. In this case, when the crystalline state of the ZnO growth film 13 is low such as a polycrystalline state, the positive hole becomes through all of them

2-O ions captured with the same probability distribution, which broadens the acceptance level. However, when the ZnO growth film 13 is single crystal, all Li ions are uniform

2_
arranged in the position of the 0 ions, which in energy thermals are stable and oriented in the direction of the C-axis (about 152 meV less energy than the positions of the other three

2
O ions). Thus, the acceptor level width to be formed becomes extremely narrow.

The energy level of Li acts as a ground state with respect to the injected electrons. So that is localized, narrow Acceptor level a basic requirement to get the sharp emission peak of the wavelength of 0.695 µm like this above was explained.

From the temperature characteristic of the electrical conductivity of the high quality ZnO growth layer provided with Li it follows that that the energy for ionizing Li is 0.24 eV. In particular, the acceptance level formed by Li becomes Level of 0.24 eV formed above the valence band.

The highly crystalline state of the growth film has a significant one Influence on the energy state of a bound and a free exciton in the vicinity of the conduction band .and the herewith associated emission of a vertical mode of an optical Phonons (LO-Phonon). That is, the emission in the ultraviolet range, which is assigned to these energy levels, can be in the epitaxial

030034/0776

High quality growth film can be observed; but when the crystalline state is weak or low, no emission in question is recognized.

The emission with the peak of 0.330 µm in that shown in FIG Curve is considered to be the emission associated with the energy levels discussed above.

In the following, a metastable level required for the emission of the laser is considered in more detail, which is derived from the observed Light emission peak of 0.695 µm can be assumed.

According to a rough calculation, the metastable level at the energy level of approx. 1.8 eV is above that formed by Li Basic state. This means that at the energy level of approx. 1.4 eV it is below the conduction band of ZnO (the energy bandwidth is 3.2 eV). An oxygen defect (hereinafter referred to as Vo ") in the ZnO crystal is considered to correspond to this energy level viewed.

When the electrons trapped by the Vo center are preferred are oriented to the C axis in the same way as Li ions, the lifetime of the electrons in this energy

-1 2
The energy level is 10 to 10 seconds longer than that in the other energy levels, and the Vo center is considered to be the metastable level with respect to the injected electrons. This can be demonstrated by an experiment on single-crystal ZnO by means of electron spinning resonance.

As follows from the above explanations, the invention emits Laser emission element 21 do radiation of 0.695 by moving the electron beams for excitation injected electrons from the conduction band to the metastable level formed by the Vo center due to a non-

030034/0776

"f3AP ORIGINAL

3QQ5536

radiating transition and by recombining the electrons with the positive hole formed by the Li ions in the ground state.

To in this case the laser emission by the induced emission during the recombination between the electrons and To excite the positive holes, the lifetime of the metastable level storing the injected electrons must be increased be one digit or order of magnitude longer than that of the basic state. However, the lifetime of the electrons is im Ground state less than 10 ~ jus; thus the above condition can be fully met. The emission from the invention Laser emission element can be considered to be on the transition from the metastable level formed by the Vo ~ center to the can be considered based on the ground state formed by Li.

In the following, the depth to which the electrons are driven by the collision of the electrons on the Li-provided ZnO growth film will be considered be injected at the critical voltage of 6 kV, since this is suitable for checking the emission phenomenon in the invention.

It has already been pointed out that the mean value -C of the depth in a crystalline growth film into which accelerated electrons can be injected is proportional to the square of the accelerating voltage Vb and inversely proportional to the density QS of the material. When the proportionality becomes constant:

—2

constant 2.5 χ 10, € can be expressed as follows

& = 2.5 χ 10 " ² G" ¹ Vb ² (cm)
The value ^ is calculated by adding the value 6 kV (critical

030034/0776

300553S

Voltage) and for 6 "the value 4.1 is used. The result is then £ f & 0.31 µm. This is essentially the same value as the film thickness of 0.35 nm of the ZnO wax turns filnios 13 dos Lascremissionsclementes 21 designed to be used in the measurement by the device shown in Fig. 2. This means that the injected electrons reach the sapphire substrate 4 when the accelerating voltage of the electron beams for excitation is about 6 kV injected electrons are distributed over the entire growth film, the sharp emission can be observed, that is, the critical voltage of the electron beams for excitation varies depending on the film thickness of the ZnO growth film 13, and the critical voltage occurs according to the film thickness.

The results of various measurements explained above are obtained from the laser emission element 21, which is produced by epitaxial Growing the Li-provided ZnO film 13 on the sapphire substrate 4 is prepared by the reactive cluster ion beam deposition process, and no treatment for enhancement is given the photon density. Accordingly, the half width of the laser emission peak is about 3 nm (30 Å).

In semiconductor lasers such as GaAs, which only have a P-N interface form, the laser emission peak is approx. 10 nm (100 Å), which is three times wider than the half-width of the laser emission element 21 according to the invention. In the conventional semiconductor laser, the half width of the emission peak to about 0.1 nm (1 A) using a cavity resonator (Fabry Perot - resonator), which uses a reflection mirror that increases the photon density. Is accordingly In the laser emission element of this type, it is advantageous to polish two surfaces that are parallel and perpendicular to the substrate

030034/0776

BATH ORIGINAL

300553B

or to carry out a suitable grating treatment in order to increase the photon density and the half width of the

ο Make the emission peak smaller than 0.1 nm (1 A).

6 schematically shows a laser emission element according to the invention using laser emission device which the

ο Half width of the emission peak to 0.1 nm (1 A) through the Applying the above treatment makes.

The device shown in Fig. 6 (a) comprises one made of sapphire Substrate 31, on which a ZnO film 32 provided with Li by the deposition of reactive ionized agglomerates

is epitaxially grown, and an electron gun 33 for generating electron beams around the ZnO film 32 to stimulate, the electron beam generator 33 being provided opposite the ZnO film 32. The substrate 31 and the electron gun 33 are contained in a vacuum housing 34 with at least transparent laser removal part 34a, wherein the vacuum housing 34 is maintained under a high vacuum condition. As shown in Fig. 6 (b), the treatment is performed for Increasing the photon density on surfaces of the ZnO growth film 32, which are vertically opposite to the substrate 31, such that one of the surfaces on the side of the removal of the The laser beam may be polished, or there may be a half mirror 35 and grating 36 that are close to the surface opposite to the Half mirror 35 executed ..are provided.

The gratings 36 are substantially equilateral triangles with distances 1- between the corners corresponding to a half-width of the laser emission wavelength which is 0.695 / 2 = 0.3475 µm in the present embodiment of the invention. The length 1 ″ of the ZnO growth film is an integral multiple of the laser emission wavelength and is 1.5-2 times with respect to the width I ₃ .

030034/0776

The arrangement of the grids explained above results in a red laser emission with a wavelength of 0.695 µm and a half width of less than 0.1 nm (1 Å).

A half mirror can be provided on the surface in place of the grating where the grids 36 shown in Fig. 6 (b) are arranged. If furthermore in the above embodiment light from one end of the vibration or oscillation space introduced by means of a prism coupler and removed from the other end an optical amplifier for amplifying the introduced light, e.g. for amplifying a Hl-Ne laser with the Wavelength of 0.6328 µm can be obtained.

In the above embodiment, provided with Li ZnO-growth film is prepared by depositing reactive ionized agglomerates that can form the single-crystalline epitaxial growth film on the substrate at the low substrate temperature of 200 ⁰ C. However, the process or the method for making or preparing the ZnO film is not essential to the invention; Rather, the ZnO film can be produced by other processes, such as, for example, by high-frequency or direct current sputtering (or sputtering) processes or by a CVD process (CVD = chemical vapor deposition), provided that it is highly crystalline on the substrate , epitaxially grown, Li-provided ZnO-FiIm is formed. It should also be noted that the substrate is not limited to sapphire; rather, it is possible to use other single crystal semiconductor substrates or substrates of various materials on which a single crystal thin film can be formed. Furthermore, in the above embodiment, the electron beams are used as an excitation source for emitting laser radiation; however, the laser emission can be obtained by another excitation source such as light or an electric field.

030034/0778

BATH ORIGINAL

Claims (4)

PATENT LAWYERS jUUOOOO KLAUS D. KIRSCHNER WOLFGANG LARGE DIPL.-PHYSICIST D 1 P L.-l N G E N I E U R APPROVED REPRESENTATIVES BEFORE THE EUROPEAN PATENT OFFICE HERZOG-WILHELM-STR. 17th Futaba Denshi Kogyo Kafaushiki Kaisha d-8 Munich 2 Mobara-shi, Chiba-ken / Japan your mark YOUR REFERENCE: OUR MARK: K 3728 K / dp OUR REFERENCE: DATE: February 14, 1980 Laser emission element Expectations Laser emission element, characterized by single crystalline zinc oxide to which lithium is added to form a level of impurities. 2. Laser emission element according to claim 1, characterized in that the lithium forms the foreign matter level, which is preferably is aligned with the C-axis of the zinc oxide. 3. Laser emission element according to claim 1 or 2, characterized in that the lithium of the zinc oxide is added in a range from 0.001% by weight to 10% by weight, based on the zinc oxide. 4. Laser emission element according to claim 1 or 2, characterized in that that the zinc oxide is grown epitaxially on a single crystal sapphire substrate. 030034/0776