JP2751468B2 - Semiconductor heterojunction structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a semiconductor heterojunction structure used for a semiconductor device such as a diode or a high-luminance blue-ultraviolet light-emitting device.

[Prior art]

Semiconductor light emitting devices such as light emitting diodes (LEDs) and semiconductor lasers (LDs) have excellent features such as high reliability, high speed, and small size. LEDs have already been developed in the infrared to visible range.
Infrared LEDs are in the field of optical communication and optical information processing equipment, and visible LEDs are indispensable devices as various display elements. Semiconductor lasers from infrared to red have been developed. This also plays an important role that can be said to be the center of the optical communication and optical information processing equipment fields. These semiconductor light emitting devices are mainly manufactured using a III-V group compound or a mixed crystal semiconductor thereof. As for visible LEDs, the red LED is made of GaP (Zn, O), AlGaAs, GaAsP, the yellow LED is made of GaAsP (N), and the green LED is made of GaP (N).

The biggest problem of the semiconductor light-emitting device that has made a dramatic progress is the development of a blue LED. As mentioned above, visible LEDs are currently being developed to green, but there is no blue LED yet. Therefore, a color display using each LED as a pixel has not been realized. If a high-brightness blue LED can be obtained, a revolutionary display with high speed, controllability, and reliability, or an ultra-thin display can be developed. To obtain a blue LED, a material with a wide bandgap (Eg> 2.6 eV) is required. In addition, it is desired that a direct transition type and low resistance p and n type can be formed. At present, crystals such as GaN, 6H-SiC, ZnS, and ZnSe have been studied as materials for blue light-emitting devices, and LED prototypes have been made. However, GaN, ZnS, and ZnSe cannot obtain a low-resistance p-type. Therefore, a good pn junction cannot be formed. 6H-SiC is the most promising material, but its conversion efficiency is poor due to the indirect transition type. Thus, each of the already known materials has advantages and disadvantages, and a high-brightness blue LED has not yet been obtained. At present, there is a report that a light emitting diode made of 6H-SiC has obtained a light output of 12mcd, which is the highest value at the laboratory level. Diamond has a wide band gap (5.5 eV) and is expected as a material for blue light emitting devices. In recent years, it has become possible to form a high-quality diamond thin film by the microwave plasma CVD method. Therefore, expectations for a material for a blue light emitting device are increasing. P-type diamond is made by doping boron (B). It has a relatively low resistance. However, there are two main problems that must be solved to make a light-emitting device from diamond. One is that a low-resistance n-type diamond has not yet been obtained. Another drawback is that diamond is an indirect semiconductor. For this reason, at present, only Schottky diodes using a junction of p-type diamond and a metal have been prototyped. This has been observed to emit green to blue light when a forward current is applied. However, the luminance of blue light emission is extremely low. The drive voltage is high (50-100V). These problems make it useless in practice.

[Means for Solving the Problems]

In the present invention, a p-type cubic boron nitride (c-BN) crystal is bonded to one surface of a p-type diamond crystal, and an n-type cubic boron nitride crystal is bonded to the opposite surface of the p-type diamond crystal. It is a joined semiconductor heterojunction. That is, it is a double hetero junction of n-type c-BN / p-type diamond / p-type c-BN. Thus, a high-luminance blue-ultraviolet light-emitting element is obtained. Since c-BN has a larger forbidden band width than diamond, carriers can be confined by this double heterostructure.

[Operation]

As described above, diamond does not have a low-resistance n-type. BN is a substance with properties similar to diamond. For example, it is formed on a metal or Si substrate by a CVD method or the like. BN with various structures can be made depending on the manufacturing conditions. Cubic BN (abbreviated as c-BN) A cubic structure of zinc blende (ZnS) type. Cubic BN (h-BN) A hexagonal structure is repeated in a certain plane, and this plane is laminated to form a layered structure. In most cases, this h-BN is easily formed. t-BN (turbostratic structure) A structure in which BN is connected but the structure is disordered, and the hexagonal structure is disordered. a-BN (amorphous) It may become amorphous depending on the temperature of the substrate. However, h-BN corresponds to graphite with C skeleton, and c-BN corresponds to diamond with C skeleton. It can be said that it corresponds. h-BN is mechanically weak, while c-BN has a high hardness for diamond.
However, c-BN does not exist in nature and is all made artificially. Many methods have been proposed, such as hollow cathode discharge, reactive ion beam plating, and reactive pulse plasma. As described above, diamond cannot have a low resistance n-type. On the other hand, c-BN has a wider bandgap (about 7.5 eV),
Both p-type and n-type are dopable materials. Therefore, a heterojunction having a wide band gap can be obtained by forming a junction between p-type diamond and n-type c-BN. This is a single hetero structure, but a double hetero (DH) structure can be formed by bonding the p-type c-BN to the surface of the p-type diamond opposite to the n-type c-BN. It has a structure of n-type c-BN / p-type diamond / p-type c-BN. In this case, the quality of the interface characteristics between diamond and c-BN becomes a problem. Diamond is a diamond-type cubic crystal with a lattice constant of 3.567 °. c-BN has a lattice constant of 3.61
It is a 5% zinc-blende cubic system. Lattice mismatch when both are joined is as small as 1.3%. Moreover, if two different atoms of the zinc blende type are replaced with one type of atom, they become diamond type, so that they have crystal structures very similar to each other. For this reason, good interface characteristics can be obtained in the bonding of diamond / c-BN. The present invention is a double hetero structure in which such a hetero junction is provided on both sides of the diamond layer. FIG. 1 shows a band diagram of the DH structure of n-type c-BN / p-type diamond / p-type c-BN. The right end is p-type c-BN, the center is p-type diamond, and the left end is n-type c-BN. FIG. 1A shows a case of zero bias, and FIG. 1B shows a case of forward bias. There is a Fermi level 7 below between the conduction band 1 and the valence band 2 of p-type c-BN. The conduction band 3 and the valence band 4 of the p-type diamond are continuous with the conduction band 1 and the valence band 2 at the junction interface 8. Since the bandgap of c-BN is wider than that of diamond, the conduction band 3 of p-type diamond is slightly lower than the conduction band 1 of p-type c-BN. The valence band 4 of p-type diamond is p-type c-
It is slightly above the valence band 2 of BN. The conductor 5 and valence band 6 of n-type c-BN are also continuous at the interface 9 with the conduction band 3 and valence band 4 of p-type diamond. Since the interface 9 is a pn junction surface, the discontinuity of the conduction band and the valence band is remarkable. At zero bias, the bands of p-type c-BN, p-type diamond, and n-type c-BN are stepped. At the time of forward bias, an n-type c-BN band is present at the pn junction surface. As shown in FIG. 1B, electrons are injected from n-type c-BN into p-type diamond. These electrons are effectively confined in the conduction band 3 because the forbidden band width of diamond is small. On the other hand, the holes in the p-type diamond are confined in the valence band 4 of the diamond by the barriers at the heterojunction surfaces on both sides. With this DH structure, a high-luminance blue-ultraviolet light-emitting element can be realized due to the following factors. Since a pn junction is formed, the injection of minority carriers is significantly promoted during forward bias. This minority carrier injection is the most important item in semiconductor light emission. By injection of minority carriers, luminescent electron-hole recombination occurs near the pn junction to emit blue light. The above-mentioned diamond Schottky diode emits blue light, but the brightness is low because in the Schottky junction, the current is mainly carried by majority carriers and the injection of minority carriers is extremely small, so that luminescent electron-hole recombination is unlikely to occur. is there. The carrier is confined in the p-type diamond region which is the active layer by the DH structure. As shown in FIG. 1 (b), at the time of forward bias, p and p have low energy for both electrons and holes.
Trapped in the diamond-shaped region, the radiative recombination probability is significantly increased. Since the refractive index (2.42) of diamond is larger than the refractive index (2.12) of c-BN, light is also confined in the diamond region. This is an effective characteristic particularly in the case of an edge-emitting device. For the reasons described above, a blue to ultraviolet light emitting device with high luminance can be obtained by the DH structure of n-type c-BN / p-type diamond / p-type c-BN. This DH heterojunction is not limited to light-emitting elements,
It can be applied to various semiconductor devices such as transistors. Diamond and c-BN have high mechanical strength, excellent heat resistance, and high carrier mobility of diamond, so that it is possible to realize a semiconductor device having environmental resistance, heat resistance, high output, and high speed.

【Example】

High-pressure synthetic p-type c-BN substrate (Be-doped, resistivity 5 × 10 ²
A p-type diamond layer (B-doped, resistivity 20 Ωcm, thickness 0.5 μm) was grown on Ωcm, 1 × 2 mm, thickness 500 μm). An n-type c-BN layer (Si-doped, resistivity 10
Ωcm, thickness 0.7 μm) to form a DH structure. As the electrodes, an AuBe electrode was provided on the back surface of the p-type c-BN substrate, and an AuSi electrode was provided on the upper surface of the n-type c-BN layer. FIG. 2 is a sectional view of the structure of the DH junction diode. p-type diamond growth layer 1 on p-type c-BN substrate 11
2, there is an n-type c-BN growth layer 13, AuSi electrode 15 on the upper surface,
An AuBe electrode 14 is provided on the lower surface. Here, the diamond thin film was formed by a microwave plasma CVD method. The growth conditions were source gas CH ₄ , H ₂ , B ₂ H ₆ gas pressure 40 Torr substrate temperature 900 ° C. microwave power 350 W. The c-BN thin film was formed by an activated reactive vapor deposition method using an HCD (hollow cathode discharge) gun. The growth conditions are: raw material solid boron, N ₂ , H ₂ , SiH ₄ substrate temperature 600 ° C HCD gun 20V 85A substrate bias voltage −120V. AuBe electrodes and AuSi electrodes were formed by vacuum evaporation. When a forward current is applied to the diode thus fabricated, 44
Blue light emission having a peak near 0 nm was observed. FIG. 3 shows the relationship between the driving voltage and the luminous intensity at this time.
The horizontal axis is the driving voltage (V), and the vertical axis is the luminous intensity (mcd). FIG. 3 shows p as a comparative example in addition to the embodiment of the present invention.
A graph showing the light emission characteristics of a Schottky junction diode of type diamond and W (a cross-sectional view is shown in FIG. 4) is also shown.
In FIG. 4, a p-type diamond growth layer 22 is formed on an insulating diamond substrate 21 and a W electrode is formed thereon.
23, a Ti electrode 24 is provided. For Schottky junction diode, drive voltage is 50-100V
Finally, the emission is about 0.1mcd to 0.3mcd. In the case of the DH junction diode of the present invention, light emission started at a driving voltage of 4 V, and luminous intensity of 20 to 30 mcd was obtained at a driving voltage of 7 V. This value is as high as 2 to 3 times the maximum luminous intensity (12 mcd) of blue light emission achieved by the conventional 6H-SiC LED. It is considered that the reason why the high luminous intensity is obtained at the low driving voltage is that the pn junction is formed and the carrier and light are effectively confined by the DH structure, as described above. It is expected that higher luminous intensity or lower driving voltage can be achieved by improving the crystallinity of each layer and optimizing the element structure. The light emitting diode according to this embodiment has a wavelength of ~ 280 nm.
Ultraviolet light emission to the extent was also observed. Therefore, it is possible to develop an ultraviolet LED from the light emitting device structure of the present invention by taking an appropriate manufacturing process and device structure. For example, a light-emitting center can be created by doping impurities into diamond, or a vacancy can be created by irradiating diamond with an electron beam, and a short-wavelength light can be emitted by creating a light-emitting center that combines this with impurities. Make it possible. Here, the case where the p-type diamond layer and the n-type c-BN layer are grown on the high-pressure synthesized p-type c-BN substrate has been described, but p-type c-BN formed by vapor phase synthesis may be used for the substrate. Further, a p-type c-BN layer, a p-type diamond layer, an n-type c-BN
The thing which grew a layer may be sufficient.

【The invention's effect】

According to the present invention, n-type c-BN / p-type diamond / p-type c
-Because the BN structure of BN is formed, the blue color having about 2-3 orders of magnitude higher luminous intensity at a driving voltage one order lower than the conventional blue light emitting device
LED is obtained. This is because carriers and light can be effectively confined by having a pn junction and a p-type diamond layer. Since short-wavelength light is also emitted, an unprecedented ultraviolet LED can be realized by adopting an appropriate manufacturing process. Further, the DH structure of the present invention can be applied to semiconductor devices such as rectifiers, lasers, and transistors. In this case, a semiconductor device having environmental resistance, heat resistance, high speed, and high output can be manufactured.

[Brief description of the drawings]

FIG. 1 shows the p-type c-BN / p-type diamond / n-type c- of the present invention.
The band diagram of the DH structure composed of BN. FIG. 1A is a band diagram at the time of zero bias, and FIG. 1B is a band diagram at the time of forward bias. FIG. 2 is a sectional view of a p-type c-BN / p-type diamond / n-type c-BN DH junction diode according to an embodiment of the present invention. FIG. 3 is a graph showing the relationship between the drive voltage and the luminous intensity of the DH junction diode according to the embodiment of the present invention and the conventional Schottky junction diode. FIG. 4 is a sectional view of a Schottky junction diode according to a conventional example. 1 ... conduction band of p-type c-BN 2 ... valence band of p-type c-BN 3 ... conduction band of p-type diamond 4 ... valence band of p-type diamond 5 ... n-type c-BN 6 conduction band of n-type c-BN 7 Fermi level 8 bonding interface of p-type diamond and p-type c-BN 9 bonding of n-type c-BN and p-type diamond Interface 11 p-type c-BN substrate 12 p-type diamond growth layer 13 n-type c-BN growth layer 14 AuBe electrode 15 AuSi electrode 21 insulating diamond substrate 22 p-type Diamond growth layer 23 W electrode 24 Ti electrode

Claims (1)

(57) [Claims] A p-type cubic boron nitride crystal is bonded to one surface of a p-type diamond crystal, and an n-type cubic boron nitride crystal is bonded to a surface of a p-type diamond crystal on the opposite side. A semiconductor heterojunction structure, characterized by being provided.