GB2044519A - A laser emission element - Google Patents

Abstract

A laser emission film element 32 consists of a monocrystalline zinc oxide to which is added lithium to form an impurity level having an extremely narrow energy band. The laser emission element emits red laser radiation having a wavelength of 0.695 mu m by transition of electrons between a metastable energy level formed by Vo<-> centres and a ground state formed by Li. Pumping is effected by an electron beam source 33. A resonator is formed by polished end surfaces of the film, by a half mirror at each end, or by the combination of a half mirror 35 and a grating 36 on a major surface of the film. <IMAGE>

Description

SPECIFICATION A laser emission element This invention relates to a luminescent element and more particularly to a laser emission element which is made of monocrystalline zinc oxide (ZnO) doped with lithium (Li) and emits laser light or radiation at room temperature.

Various kinds of lasers have been developed and tested as an optical source or optical amplifier for use in optical communication systems, optical integrated circuits, or optical data processing.

As the optical source or optical amplifier in such systems, a semiconductor laser has considerable attractions, and research activities have been made to put it to practical use, because it is small in size and is capable of being made in a solid state.

In a semiconductor laser, for example, a junction laser using a hetero or p-n junction, laser emission is obtained from the recombination of electrons and positive holes at the junction. Accordingly, in order to increase the emission efficiency, it is necessary to prepare a semiconductor crystal which is monocrystalline and free from defects in its crystal structure. The production process of such semiconductor crystals is extremely complicated, requiring delicate production controls. Thus, production costs are rather high.

According to one aspect of the present invention, a laser emission element comprises monocrystalline zinc oxide to which is added lithium to form an impurity level.

The invention may be carried into practice in various ways, but one apparatus and method for manufacturing laser emission elements embodying the invention, and a number of laser emission elements embodying the invention, will now be described, by way of example, with reference to the accompanying drawings, in which:: Figure 1 is a schematic illustration of an apparatus for carrying out a reactive cluster ion beam deposition process adapted to produce a laser emission element embodying the present invention; Figure 2 is a block diagram of an apparatus for measuring the spectral emission characteristics of a laser emission element embodying the present invention; Figure 3 is a diagram illustrating the relationship between luminous wavelength and relative luminous intensity of a laser emission element embodying the present invention; Figure 4 is a three dimensional diagram showing the spectral emission characteristics of a laser emission element in a region where a sharp emission is observed;; Figure 5 is a diagram illustrating the relationship between the acceleration voltage of an electron beam used for stimulating the laser emission element, and the luminous intensity of the light emitted by the laser emission element; and Figures 6(a) to 6(d) show actual arrangements of a laser emission element embodying the present invention.

A laser emission element embodying the present invention can be produced by various methods. In one particular method which will now be described, the laser emission element is produced by a reactive cluster ion beam deposition process which is well suited to forming a monocrystalline thin film, and makes it easy to add impurities.

Fig. 1 is a schematic illustration of an apparatus for use in the reactive cluster ion beam deposition process. The whole apparatus of Fig. 1 is housed in an enclosure within which a high vacuum is maintained. In operation, an ionised beam of zinc (Zn), lithium (Li) and oxygen (02) is directed towards a substrate 4 of monocrystalline sapphire, to form on the substrate a layer 1 3 of monocrystalline zinc oxide (ZnO) doped with lithium.

The zinc and lithium for forming the ionised beam are contained, in a highly pure molten state, as shown at 2, in-a closed crucible 1 which has one or more nozzles 1 a. The proportion of lithium may be from 0.001% to 10% by weight, and preferably 0.1% to 0.5%. It would alternatively be possible to provide two crucibles containing, respectively, zinc and lithium. The crucible 1 is kept heated by a heater 9, at a temperature sufficiently high that a jet of zinc and lithium vapour issues from the nozzle 1. The oxygen for the ionised beam is supplied through a pipe 11 to a nozzle 11 a adjacent the crucible nozzle 1.

A heated annular cathode 5 is arranged close to the path of the vapour jet from the nozzle 1. Therm ion emitted by the cathode 5 are accelerated by an electrode 6, which is maintained at a positive potential relative to the cathode 5, to impinge on the vapour jet, thereby ionising the jet. The electrode 6 has the form of an apertured sleeve lying within the cathode 5 and around the vapour jet. The ionised vapour is then accelerated by an acceieration electrode 7, which is kept at a negative potential relative to the crucible 1, to travel towards the substrate 4, which is supported by a substrate holder 3, and heated by a heater 1 2. A movable shutter 8 allows the ionised beam to be blocked from reaching the substrate 4.

In operation, the crucible 1 is heated so that the vapour pressure within the crucible 1 is at least 102 times as high as the pressure prevailing around the crucible 1. The pressure in the enclosure housing the crucible 1 will normally be between 10-5 Torr and 10-2 Torr, so that the corresponding range of minimum values of vapour pressure within the crucible 1 is 10- Torr to 1 Torr. In the cluster ion beam deposition process, it is desirable that the pressure difference between the vapour pressure within the crucible 1 and the pressure prevailing around the crucible 1 is as high as possible.

The materials 2 heated and vaporized in the crucible 1 are jetted through the nozzle 1 a into the high vacuum outside the crucible 1.

When jetted, the vaporized materials are subjected to a supercooling effect caused by the adiabatic expansion thereof, and the atoms thereof are loosely bonded to one another by van der Waal's forces, being formed into a number of atom groups or clusters each normally consisting of about 500 to 2,000 atoms. Considerable kinetic energy is imparted to each cluster as a result of the materials 2 being discharged from the nozzle 1 a as a jet.

As explained hereinabove, the nozzle 11 a of the oxygen supply pipe 11 is close to the nozzle 1 a of the crucible 1. The pipe 11 supplies a small amount of oxygen (02) gas into the vapour jetted from the nozzle 1 a, and the resultant mixture moves towards the substrate 4. The pressure within the surrounding enclosure after the introduction of the oxygen may be 10-5 Torr to 10-2 Torr as explained above, and it is desirable that the clusters into which the oxygen gas is introduced are not broken while travelling towards the substrate 4 within the enclosure.

The clusters consisting of Zn and Li vapour and 02 gas are subjected, as they pass through the region of the ionisation electrode 6, to a bombardment of electrons which are emitted from the cathode 5 and accelerated by the electrode 6, thereby ionising one of the atoms of each cluster and obtaining ionised cluster. The ionised clusters then travel towards the substrate 4 together with 02 ions, non-ionised neutral clusters and 02 while being accelerated by the action af the electric field produced by the acceleration electrode 7, which is connected to a high acceleration voltage which is negative with respect to the crucible 1.The ionised clusters and O2 therefore collide with the substrate 4 together with the neutral clusters and 02. When the ionised clusters come into collision with the substrate 4, they disintegrate into individual atomic particles which move on the substrate 4 to facilitate the epitaxial growth of the highly crystalline film 1 3 of ZnO with Li impurity (in epitaxy with the monocrystalline substrate 4).

This is due to the so-called surface migration effect which is inherent in the cluster ion beam deposition process and the chemical action of the 02 gas introduced into the clusters.

With the reactive cluster ion beam deposition process, epitaxial growth can be satisfac torily produced even at substrate temperatures as low as 200"C, because of the activated reaction resulting from the ionisation of Zn, Li and O2 and the high impact speed of the cluster particles which are formed of Zn and Li vapour and jetted from the crucible 1. In addition, an epitaxial ZnO film 1 3 which is extremely smooth on its surface can be obtained, owing to the migration energy (surface diffusion energy at the time of vapour deposition) of the clusters.

In the preferred method of producing a laser emission element, a high quality epitaxial ZnO film 13 is produced by vapour deposition under the following conditions: Number of nozzles in the crucible: One Electron current emitted from the cathode 5 for 250 mA to 300 mA ionisation: Acceleration voltage applied to the 0 to 1 KV acceleration electrode 7: Temperature of the substrate: 200"C to 300"C Degree of vacuum around the crucible 1 after the 4 x 10-4 to introduction of 2 gas by the gas supply pipe 8 X 1 0- Torr 11: In order to examine the characteristics of the film 1 3 produced by this deposition process, the laser emission element may be subjected to spectrochemical analysis of its emission spectrum.Fig. 2 shows schematically a form of apparatus for carrying out this analysis. In Fig. 2, the laser emission element, shown at 21, is bombarded by electrons from an electron gun 22. The accelerating voltage for the electron gun 22 is supplied by a high voltage source 23, and can be adjusted from 0 to 20 KV. The radiation emitted by the element 21 as a consequence of the bomardment passes to a grating monochromator or analyser 24. Whatever wavelength of radiation is separated out by the monochromator is then detected by a photo-multiplier tube 25, which is supplied with power by a high voltage source 29. The output from the photomultiplier 25 is amplified by an amplifier 26, whose output is supplied to a lock-in or sampling amplifier 27. Control signals are supplied to the sampling amplifier 27 through an optically-coupled arrangement comprising an electro-optical pulse generator 30 and an optical pulse receiver 31; these signals cause the amplifier 27 to sample the output of the amplifier 26. The output of the amplifier 27 is recorded by a recorder 28, which is also supplied with a signal from the monochromator 24 indicating the wavelength of the radiation being analysed.

Fig. 3 shows the emission characteristics of the laser emission element 21 embodying the invention. In Fig. 3, the radiation wavelength is plotted as the abscissa, while the relative radiation intensity is plotted as the ordinate.

The full-line curve (a) represents the emission characteristics of the laser emission element 21 embodying the present invention, and the broken-line curve (b) represents the emission characteristics of the monocrystalline sapphire substrate, which is measured for comparison.

The spectrochemical analysis was carried out on a laser emission element which was prepared by depositing ZnO, with the Li impurity, on the (1 102) plane of the sapphire substrate 4 using the apparatus shown in Fig.

1 under such conditions that the ionisation current for ionising the clusters of zinc, lithium and oxygen is 300 mA, the acceleration voltage applied to the acceleration electrode 7 is 7 KV and the temperature of the substrate 4 heated by the heating source 1 2 is 230"C.

The film thickness thus obtained is approximately 0.35 ym and the electrical sheet resistivity is approximately 10Q (for a square sheet), and it is recognized from X-ray locking curve before the measuring the characteristic of the spectral intensity of the emission spectrum and "RHEED" pattern that the C axis of ZnO is orientated substantially in parallel with the (1102) plane of the sapphire substrate 4.

As shown in the curve (b) of Fig. 3, an emission having a peak in the ultraviolet region at approximately 0.380 lim due to free excitons and an emission having a peak in the green part of the visible spectrum at approximately 0.527 jum, which seems to be due to oxygen defects, are produced by the sapphire substrate 4. On the other hand, as shown in the curve (a) of Fig. 3, an emission having a relatively wide peak at approximately 0.330 jum at room temperature due to bound excitons, and a red emission having a sharp peak at 0.695 jum are produced by the grown ZnO film. The acceleration voltage Vb of the electron beam for exciting the laser emission element 21 in this case is 10 KV, and the beam current is approximately 20 yA.

In order to examine the nature of the emission having the sharp emission peak at 0.695 jum which is observed in the grown ZnO film, the spectral emission of the laser emission element 21 is measured with different acceleration voltages; the results are shown in Figs.

4 and 5.

Fig. 4 is a three-dimensional diagram showing the characteristics of the emission in the region where the sharp emission is observed.

In Fig. 4, the luminous wavelength is shown along the X axis, the acceleration voltage of the exciting electron beam is shown along the Y axis, and the radiation intensity is shown along the Z axis.

Fig. 5 shows the relationship between the radiation intensity at the wavelength of 0.695 m and the acceleration voltage of the exciting electron beam. The measurement is made at room temperature and the beam current Ib is 20 yA.

As is apparent from Figs. 4 and 5, the sharp emission peak which can be observed at 0.695 ym appears when the acceleration voltage Vb of the electron beam is above 6 KV.

This means that the critical voltage of the laser emitting element 21 is 6 KV. As shown in Fig. 4, the half width of the peak (width at half the amplitude of the peak) decreases with an increase in the acceleration voltage Vb, and when the acceleration voltage Vb of the electron beam is 1 2 KV, the half width is less than approximately 30 .

The emission peak at 0.695 ym is only observed when the film 1 3 is epitaxially grown on the (1 102) plane of the sapphire substrate 4. If the crystalline state of the grown ZnO film 1 3 is poor, or if amorphous glass is used as the substrate 4, no emission peak at 0.695 ym is observed.

As a result of the mode measurement and study of light emitting mechanism of the Li added ZnO film, it can be recognised that there is coherence in the emission peak, and that the emission is related to crystalline state of the ZnO growth film 13, a kind of the substrate to be used and addition of Li, and that there is a critical voltage to excite the ZnO film. In view of these facts, it can be concluded that the emission is laser.

The emission of laser radiation can be theoretically explained as follows: First, consideration will be given to the function of Li within the grown ZnO film 1 3.

The Li substitutes, as Li+, for Zn2+ in a hexagonal wurtzite structure (hexagonal closepacked, with each Zn2+ (or Li+) ion associated with four 02- ions, but with of these 02ions associated with only two Zn2+ ions, while the other three 02- ions are each associated with six Zn2+ ions). One of four 02- ions traps a positive hole resulting from the presence of an Li+ ion, to thereby form an acceptor level.

If the crystalline state of the grown ZnO film 1 3 is inferior, such as, for example, a polycrystalline state, the position hole is trapped by all 02- ions in an equal probability distribution, which makes the acceptor level not very sharply defined. However, if the ZnO growth film 1 3 is monocrystalline, the positive hole is preferentially trapped by those 02- ions whose bonds are orientated in the direction of the C axis (the axis perpendicular to the planes of closest packing), and have a lower energy level (approximately 1 52 meV lower energy) than the positions of the remaining three 02- ions. Thus, the acceptor level will be extremely sharply defined.

The energy level formed by Li+ acts as a ground state for the injected electrons. Thus, the localised narrow acceptor level is a fundamental requirement for obtaining the sharp emission peak at 0.695 ym explained hereina bove.

It is noted from the temperature characteristic of the electrical conductivity of the highly crystalline ZnO layer that the energy for ionis ing Li is 0.24 eV. That is to say, the acceptor level formed by Li+ is formed at the level of 0.24 eV above the valence band.

The highly crystalline state of the grown film has an important effect upon the energy state of bound excitons and free excitons in the vicinity of the conduction band and the emission of vertical mode of optical phonon (LO phonon) associated therewith. That is, the emission in the ultraviolet region which is associated with these energy levels can be observed in an epitaxial grown film of high quality, but if the crystalline state is poor, no emission of this type is observed.

The emission having the peak of 0.330 ym in the curve shown in Fig. 3 is considered to be the emission associated with the energy levels explained hereinabove.

Next, consideration will be given to the metastable level required for the emission of laser radiation. The existence of such a metastable state can be conjectured from the luminous emission peak of 0.695 m observed.

According to a rough calculation, the metastable level is at an energy level of approximately 1.8 eV above the ground state formed by Li+, or approximately 1.4 eV below the conduction band of ZnO (the energy band gap width is 3.2 eV). This energy level is thought to correspond to oxygen defects (hereinafter referred to as Vo-) in the ZnO crystal.

When the electrons trapped by the Vo- are preferentially orientated to the C axis in the same manner as Li+ ions, the life time of the electrons in this energy level is 10-' to 102 second longer than in the other energy levels, and the Vo- centre is thought to be the metastable level for the injected electrons.

This can be proved from experiments on the monocrystalline ZnO by resonance of electron spin.

As is apparent from the considerations explained hereinabove, the laser emission element 21 embodying the present invention emits radiation at 0.695 ym by shifting the exciting electrons injected by the electron beam first from the conduction band to the metastable level formed by the Vo- centre, in a nonradiative transition and then be recombining the electrons with the ground state positive hole formed by the Li + ions.

In order to produce laser radiation by stimulated emission in the course of the recombination between the electrons and positive holes, the life time of the injected electrons in the metastable level must be one order of magnitude longer than the life time in the ground state. However, the life time of the electrons in the ground state is less than 10-3 use., thus, the above-mentioned condition can easily be satisfied.

Consideration will be further given to the depth to which the electrons are injected into the grown ZnO film at the critical voltage of 6 KV, because this may be helpful in understanding the emission phenomenon used in the present invention.

It is well known that the mean depth ito which accelerated electrons can be injected into a grown crystalline film is proportional to the second power of the acceleration voltage Vb and is inversely proportional to the density a of the material. The equation for 1 is as follows: 1=2.5 X 10-2cr' Vb2(cm) The value of Iat an acceleration voltage Vb of 6 KV (critical voltage) and with a = 4.1 is approximately 0.31 mm. This is substantially the same as the 0.35 jilm thickness of the grown ZnO film 13 of the laser emission element 21. This means that the injected electrons reach the sapphire substrate 4 when the acceleration voltage of the exciting electron beam is about 6 KV.When the injected electrons are distributed through the entire grown film, the sharp emission can be observed. In other words, the critical voltage of the exciting electron beam depends upon the film thickness of the grown ZnO film 1 3.

The results of the various measurements explained above are obtained from a laser emission element 21 which has been prepared by simply epitaxially growing the ZnO film 1 3 on the sapphire substrate 4, with no treatment to increase the photon density. Accordingly, the half width of the laser emission peak may be as wide as 30 A.

In semiconductor lasers, such as, for example, GaAs lasers, simply forming a p-n junction, the laser emission peak is approximately 100 A which is three times wider than the half peak width of the present laser emission element 21. In a conventional semiconductor laser, the half width of the emission peak can be reduced to as little as 1 A with the use of a cavity resonator (Fabry-P6rot resonator) using a reflecting mirror which increases the photon density. Accordingly, in a laser emission element of the present type, it is also effective to polish two surfaces which are parallel to one another and perpendicular to the substrate or to conduct a suitable grating treatment so as to increase the photon density and to make the half width of the emission peak less than 1 A.

Fig. 6 schematically shows a laser emission apparatus using a laser emission element embodying the present invention which makes the half width of the emission peak 1 A by the application of the above treatment.

The apparatus shown in Fig. 6 (a) comprises a substrate 31 made of sapphire on which a Li doped ZnO film 32 is epitaxially grown by the reactive cluster ion beam deposition process and an electron gun 33 for generating electron beams to excite the ZnO film 32. The substrate 31 and the electron gun 33 are contained in a vacuum housing 34 which has at least one transparent output window 34a, and which is maintained under a high vacuum condition. As shown in Fig. 6 (b), means for increasing the photon density is provided at two opposite ends of the grown ZnO film 32. The surfaces which extend perpendicular to the substrate 31 at these ends may be polished, or a half mirror 35 may be provided at one end, and gratings 36 may be formed in the vicinity of the end opposite to the half mirror 35.

The gratings 36 are, in cross-section, substantially equilateral triangles having their peaks spaced by a distance 1, which is equal to one-half of the laser emission wavelength, which is 0.695/2 = 0.3475 lim in the present invention. The length 1 of the grown ZnO growth film between its reflecting ends is an integral multiple of the laser emission wavelength and is 1.5 to 2 times the width 1 of the film 32 (see Fig. 6 (b)).

By the provision of the gratings described above, red laser radiation having a wavelength of 0.695 ym and a half peak width of less than 1 A is obtained.

It is to be understood that a half mirror may be provided at the end where the gratings 36 are shown in Fig. 6 (b) instead of the gratings. Furthermore, in the above embodiment, if light is introduced into one end of the resonance space using a prism coupler, and an output is taken from the other end, an optical amplifier for amplifying the introduced light, for instance, for amplifying He-Ne laser light having a wavelength of 0.6328 pm, can be formed.

In the described embodiment, the Li doped grown ZnO film is prepared by a reactive cluster ion beam deposition process, which can form a monocrystalline epitaxial film on the substrate at substrate temperatures as low as 200"C. However, it should be understood that this process for preparing the ZnO film is not essential to the present invention; rather, the ZnO film may be prepared by various other processes, such as, for example, high frequency or direct current sputtering methods, or CVD method, so long as a highly crystalline epitaxial growth Li doped ZnO film is formed on the substrate. Also, it should be understood that the substrate need not be sapphire, but can instead be any of various materials on which a monocrystalline thin film can be formed. Furthermore, in the embodi ment described, an electron beam is used as an excitation source for the laser emission element, but laser radiation can be obtained by using other excitation sources, for instance light or an electric field.

Claims (6)

1. A laser emission element comprising a monocrystalline zinc oxide to which is added lithium to form an impurity level.

2. A laser emission element as claimed in Claim 1 in which the lithium forms an impurity level which is preferentially orientated to the C axis of the zinc oxide.

3. A laser emission element as claimed in Claim 1 or Claim 2 in which the lithium is present in the zinc oxide in an amount of 0.001% to 10% by weight.

4. A laser emission element as claimed in Claim 1 or Claim 2 or Claim 3, in which the zinc oxide is epitaxially grown on a monocrystalline sapphire substrate.

5. A laser emission element substantially as specifically described herein with reference to Figs. 1 to

6.