JP4691722B2 - Diamond semiconductor light emitting device - Google Patents

Description

  The present invention relates to a diamond semiconductor light emitting device expected to be put to practical use as an ultraviolet light emitting device.

  A light-emitting element based on a semiconductor based on a pn junction uses a light-emitting process caused by recombination of electrons and holes in the semiconductor. Whether or not light is emitted when electrons and holes recombine is determined. Depends strongly on the nature of the semiconductor material and its quality. For this reason, in order to obtain a highly efficient light-emitting element, a direct transition type semiconductor that usually emits light when electrons and holes are recombined is used. In this case, the wavelength of light proportional to the energy of the emitted light or the inverse thereof depends on the size of the band gap of the semiconductor material. Therefore, in order to obtain a light emitting element having a short wavelength, a direct transition type semiconductor having a large band gap is a candidate. From this point of view, light-emitting elements using wide-gap semiconductors such as GaN and GaAlN, which are well known for blue light-emitting diodes, have been developed and put into practical use.

  However, the wavelength of these light-emitting elements in practical use is 400 nm or more, and a light-emitting element having a wavelength shorter than this is difficult to manufacture the material, and the element formation process becomes more difficult as the gap becomes wider. Not yet commercialized.

  On the other hand, diamond has great semiconductor and optical properties in addition to its mechanical, chemical and thermal properties, so it has attracted a great deal of attention as an electronic device material and a light emitting device material. Yes. In particular, when viewed as a light emitting device material, diamond has a large band gap of 5.5 eV, and is expected as a light emitting element that outputs light with the above short wavelength. Since diamond is composed of a single element, a high-quality material can be obtained.

By the way, diamond is an indirect transition type semiconductor that cannot make a direct transition when electrons and holes are recombined. Therefore, a light emission process based on the same principle as a conventional light emitting element cannot be expected. As an alternative to this, for the diamond, a proposal of an ultraviolet light emitting element using a light emission process by excitons has been made by the present inventors, for example, in Patent Document 1 below. The light emitted by the excitons described in Patent Document 1 is an ultraviolet ray of 235 nm and is observed even at room temperature.
JP 2001-35804 A

  However, the light emission of the above diamond has not reached a practical level, and moreover, light having an energy higher than the band gap of 5.5 eV that the diamond material has, that is, an emission wavelength predicted from the band gap. A light-emitting element that can emit light having a wavelength shorter than 226 nm is in a situation where it cannot be expected.

  The present invention has been proposed in view of the above, and can emit light having a wavelength shorter than the emission wavelength predicted from the band gap even though it is an indirect transition type, and raises the emission of diamond to a practical level. An object of the present invention is to provide a diamond semiconductor light emitting device capable of achieving the above.

In order to achieve the above object, an invention according to claim 1 is formed by a planarized diamond layer formed on a substrate by a microwave plasma CVD method and on the planarized diamond layer by a microwave plasma CVD method. A p-type diamond semiconductor layer, an n-type amorphous silicon layer formed on the p-type diamond layer, an active region formed at an interface between the p-type diamond semiconductor layer and the n-type amorphous silicon layer, And a wavelength shorter than 226 nm, which is an emission wavelength predicted from a band gap of the diamond material, by injecting current into the electrodes formed in the p-type diamond semiconductor layer and the n-type amorphous silicon layer, respectively. Is output in the extending direction of the active region .

  In the present invention, even if it is an indirect transition type diamond semiconductor, recombination of electrons and holes by current injection can be directly caused to obtain highly efficient ultraviolet light emission.

  Further, it is possible to emit light having a wavelength shorter than the emission wavelength predicted from the band gap. In other words, it is an epoch-making thing that overturns the conventional theory that a light emitting element that emits light having energy higher than the band gap is impossible.

  Therefore, the light emission of diamond can be raised to a practical level, and it becomes possible to realize a fluorescent lamp which does not require mercury, and to be applied to a high density optical memory, an ultraviolet microscope, lithography and the like.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram schematically showing the configuration of a microwave plasma CVD apparatus used for manufacturing the diamond semiconductor light emitting device of the present invention. In the figure, a microwave plasma CVD apparatus 100 is of an end launch type in which microwaves are incident from the normal direction of the substrate 11, and the microwave source 1 oscillates microwaves of 2.45 GHz with a maximum output of 1 At .5 kW, the output can be adjusted as needed. A circulator 2 and a dummy load 3 are provided at the subsequent stage of the microwave source 1, and the reflected wave returned to the waveguide 12 among the microwaves emitted from the microwave source 1 is thermally absorbed as a water load. The reflected wave is prevented from adversely affecting the oscillator of the microwave source 1. In addition, a tuner 4 is provided in the subsequent stage of the circulator 2 and the impedance of the waveguide 12 is adjusted by three bars, so that reflection of microwaves can be suppressed and total incident power can be consumed by plasma. Further, an applicator 5 having an antenna protruding into the waveguide 12 is provided at the subsequent stage of the tuner 4 to convert the TE10 mode microwave traveling through the waveguide 12 into a concentric TM01 mode. By setting the microwave to the TM01 mode, the microwave is aligned with the cylindrical reaction vessel 13 and a stable plasma can be obtained.

  The source gas is a mixed gas of methane gas, which is a carbon source, hydrogen gas, and an impurity doping gas supplied as necessary. From each gas cylinder 15,..., A pressure reducing valve (not shown) and a mass flow controller 16,. Then, the gas is introduced from the gas introduction pipe 6 to the reaction vessel 13 and introduced into the reaction vessel 13 as a gas shower from the shower head 19 above the reaction vessel 13. As the mass flow controller 16 on the methane gas side, a high-precision one is used to obtain a mixing ratio (ratio of methane gas to hydrogen gas) of 0.5% or less.

  During the CVD diamond synthesis process, the process pump 18 is evacuated, and the gas pressure in the reaction vessel 13 is controlled to advance the diamond synthesis by plasma CVD. The turbo pump 7 is used for obtaining a high vacuum in preliminary exhaust, the rotary pump 17 is used for exhaust during synthesis, and the high-frequency induction heater 10 is used for temperature control of the substrate 11. The substrate 11 is set at a predetermined position by opening the sample exchange door 14.

  In the microwave plasma CVD apparatus described above, a thin-film diamond semiconductor was manufactured with a low methane gas concentration, and a light-emitting element was configured using the diamond semiconductor.

FIG. 2 is a diagram showing a configuration example of the diamond semiconductor light emitting device of the present invention. In the figure, a diamond semiconductor light emitting device 20 of the present invention is formed on a high-quality planarized diamond layer 22 formed on a substrate 21, a high p-type diamond semiconductor layer 23 doped with boron B, and a top thereof. And an amorphous silicon layer 24 formed in contact therewith.

  An active region 29 is formed at the interface between both the p-type diamond semiconductor layer 23 and the amorphous silicon layer 24.

  On the p-type diamond semiconductor layer 23 and the amorphous silicon layer 24, electrodes 25 and 26 each formed by laminating three layers of Ti, Pt, and Au are formed to form ohmic contacts. The current flows through the electrode 25, the p-type diamond semiconductor layer 23, the amorphous silicon layer 24, and the electrode 26.

  In the diamond semiconductor light emitting device 20 having this configuration, when current is injected into the electrode 25, holes are injected from the p side to the n side and electrons are injected from the n side to the p side. Then, the active region 29 in the middle of p and n becomes a region having a high electron and hole density, light emission appears, and ultraviolet light is output from the active region 29.

  The junction interface between the p-type diamond semiconductor layer 23 and the amorphous silicon layer 24 of the diamond semiconductor light emitting device 20 is a physically extremely steep pn due to the formation of the amorphous silicon layer 24 as an upper layer. A junction is formed and has a steep band change. FIG. 3 shows the IV characteristics at the pn junction, where the horizontal axis represents the bias voltage and the vertical axis represents the current flowing between the junction interfaces.

  The measurement results of the emission spectrum of the diamond semiconductor light emitting device 20 are shown in FIG. The horizontal axis represents wavelength, and the vertical axis represents EL emission intensity. As shown in FIG. 4, the diamond semiconductor light emitting element 20 emits ultraviolet light having a wavelength of 215 nm, which is shorter than the light emission wavelength 226 nm predicted from the band gap of the diamond material. That is, an epoch-making result was obtained that overturned the conventional theory that a light-emitting element that emits light having energy higher than the band gap is impossible.

  Such a result can be attributed to the following phenomenon.

  1. When a voltage is applied to the pn diode and electrons are injected into the p-type from the n-type side, the injected electrons may be trapped in a certain quantum state by receiving an electrostatic potential generated at the junction.

  2. When electrons are confined in this quantum state and the state is satisfied, the electrons finally recombine with holes in the system and disappear. At this time, light cannot be recombined in general, and thus light is emitted.

  3. The quantum state of the electrons depends on the band structure of the semiconductor, but can be generated at the gamma point, which is the minimum of the momentum of the electrons.

  4). To realize this possibility, the pn junction is steep, the semiconductor band structure (especially near the conduction band edge occupied by free electrons) is composed of simple wave functions, and the quantum state is easily realized. It is necessary to be done.

  5. The band structure of diamond is composed of 2s and 2p electron orbits, which is the simplest electronic structure for a semiconductor. As in this example, a steep pn junction made of this diamond and amorphous silicon was formed. This is considered to be achieved.

  As described above, according to the present invention, light having a wavelength shorter than the emission wavelength predicted from the band gap can be emitted. Therefore, even if it is an indirect transition type diamond semiconductor, electrons and holes due to current injection are used. High-efficiency ultraviolet light emission can be obtained by directly causing recombination. Therefore, the light emission of diamond can be raised to a practical level, and it becomes possible to realize a fluorescent lamp which does not require mercury, and to be applied to a high density optical memory, an ultraviolet microscope, lithography and the like.

  In the above description, the n-type amorphous silicon layer 24 is formed on the p-type diamond semiconductor layer 23. However, instead of the n-type amorphous silicon layer, other materials other than diamond, such as crystalline silicon or An n-type semiconductor layer made of germanium, silicon carbide, or the like may be provided.

  Further, an n-type diamond semiconductor layer may be formed instead of the n-type amorphous silicon layer.

  In this case, it is preferable that a steep band change is obtained at the bonding interface with the p-type diamond semiconductor layer as in the case of the bonding interface with the amorphous silicon layer.

It is a figure which shows roughly the structure of the microwave plasma CVD apparatus used for manufacture of the diamond semiconductor light-emitting device of this invention. It is a figure which shows the structural example of the diamond semiconductor light-emitting device of this invention. It is a figure which shows the relationship between the bias voltage and electric current in the junction interface of a p-type diamond semiconductor layer and an n-type amorphous silicon layer. It is a figure which shows the result of having measured the emission spectrum of the diamond semiconductor light-emitting device of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Microwave source 2 Circulator 3 Dummy load 4 Tuner 5 Applicator 6 Gas introduction pipe 7 Turbo pump 10 High frequency induction heater 11 Substrate 12 Waveguide 13 Reaction container 14 Sample exchange door 15 Gas cylinder 16 Mass flow controller 17 Rotary pump 18 Process pump 19 Shower head 20 Diamond semiconductor light emitting element 21 Substrate 22 Planarized diamond layer 23 P-type diamond semiconductor layer 24 N-type amorphous silicon layer 25 Electrode 26 Electrode 29 Active region

Claims (1)

A planarized diamond layer formed on a substrate by a microwave plasma CVD method ;
A p-type diamond semiconductor layer formed on the planarized diamond layer by a microwave plasma CVD method ;
An n-type amorphous silicon layer formed on the p-type diamond layer;
An active region formed at the interface between the p-type diamond semiconductor layer and the n-type amorphous silicon layer,
By injecting current into the electrodes formed on each of the p-type diamond semiconductor layer and the n-type amorphous silicon layer, light having a wavelength shorter than 226 nm, which is an emission wavelength predicted from the band gap of the diamond material, is emitted. Outputting in the stretching direction of the active region ,
A diamond semiconductor light emitting device characterized by the above.

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