JP2533581B2 - Superlattice mixed crystal method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a superlattice mixed crystallizing method, which is a manufacturing technique of a semiconductor device using a superlattice.

[Prior Art] Mixed crystal is to break the superlattice structure and change the composition into a uniform liquid crystal, and it is possible to give physical characteristics different from those of the superlattice structure. This is an important process in manufacturing a waveguide.

As one of the conventional superlattice mixed crystallizing methods, there is a method in which a specific ion species is ion-implanted into the superlattice at room temperature and then heat-treated at a high temperature. As for the ionic species that can be mixed crystallized by this method, Hirayama et al.
Si ⁺ , Ga ⁺ , As ⁺ , F ⁺ and B ⁺ , as reported in Applied Physics (Vol.24 (1985), p1498).

Of these, B ⁺ has a low capacity for mixed crystallization, and other ionic species other than B ⁺ can be mixed 6 times or more as compared with mixed crystallization by thermal diffusion, while less than 2 times It is not practically effective because it has only the ability to promote liquid crystallinity.

The disadvantage of this method is that the region which can actually be mixed crystallized is a depth of at most 1 μm from the surface. The acceleration energy obtained by a commonly used ion implanter is 50.
Up to 0kV. Among the ions that are practically effective, even the lightest F ⁺ has a projective range of 6000 Å at an energy of 500 kV
Therefore, even if the spread of ions and the diffusion due to heat treatment after ion implantation are taken into consideration, it is difficult to form a mixed crystal at a position deeper than 1 μm from the surface.

In order to avoid this defect and manufacture an optical element, first,
A method of growing a crystal layer to a superlattice portion, implanting ions, and then stacking a necessary layer thereon is adopted. Although this technique is called a re-growth method, it has a drawback that it requires two epitaxial growths to form one device. Furthermore, the surface of the first grown layer is AlGaAs
As described above, when Al is contained, a stable Al oxide is formed on the surface, so that there is a drawback that it is difficult to regrow the crystal layer.

As another crystallization method of the superlattice, a method of heat-treating the superlattice using SiO ₂ and a Si ₃ N ₄ protective film and selectively crystallizing only the portion covered with SiO ₂ by DGDeppe et al. , Appli
ed Physics Letter (Vol.49 (1986) 510). As shown in FIG. 7, in this method, SiO ₂ is used for the portion to be mixed crystal and Si ₃ N _{4 is used for the} portion where the superlattice structure is to be left.
This is a method of covering with a film and performing heat treatment in a heating furnace 7 in an As atmosphere 6. Here, 4 is Al _0.5 Ga _0.5 As / GaAs superlattice, 5
Is a GaAs substrate.

According to this method, mixed crystals are formed up to a depth as deep as 1 μm, but there is a drawback that the mixed crystals proceed even in the portion 3 protected by the Si ₃ N ₄ film. For example, room temperature photoluminescence emission wavelength is 7910Å Al _0.35 Ga _0.65 As / Al _0.05 Ga
_{When 0.95} As (80Å) superlattice is heat-treated at 825 ℃ for 10 hours, S
It can be seen that the emission wavelength of the portion 4 covered with iO ₂ 2 shifts to the short wavelength side of 7300 Å, and the mixed crystallization progresses. At the same time, the emission wavelength of the part 3 protected by the Si ₃ N ₄ film also becomes 7600Å, indicating that mixed crystals have occurred.

[Problems to be Solved by the Invention] In the above-mentioned conventional method, when it is difficult to re-grow a crystal layer or to perform mixed crystallization by protecting with a protective film to prevent mixed crystallization, There is a problem that the mixed crystal proceeds even in the protected portion.

An object of the present invention is to solve the problems and to provide a method of mixed crystallization utilizing the advantages of the mixed crystallization technique by ion implantation.

[Means for Solving Problems] In order to achieve such an object, according to the present invention, when a semiconductor crystal having a superlattice structure is mixed, the semiconductor crystal is heated and light ions are added to the semiconductor. Ion implantation.

[Operation] In the present invention, since light ions are used as the ion species for ion implantation, the projective range of the ions is significantly increased, and the ions can reach deep positions. For example, when the acceleration voltage is 500 kV, the projected range of Ga ⁺ is 0.
The projection range of H ^{+ at} the same acceleration voltage is 4.4 μm, whereas the projection range can be greatly increased. Since the H and He atoms themselves do not have the ability to mix crystals, these light ion species have not been conventionally used in the process of mixing crystals.

By the way, a bulk semiconductor has the following phenomenon called “enhanced diffusion”. This is a phenomenon that when a bulk semiconductor is irradiated with light ions such as H ⁺ or He ^+, excess vacancies are formed from the thermal equilibrium state. Substrate temperature
At 500 ° C or lower, these vacancies condense and do not contribute to promotion of diffusion, but at temperatures higher than this, the diffusion coefficient of the constituent elements and impurities in the lattice position is increased by 10 ⁴ to 10 ⁸ times. Can also be increased.

By applying this phenomenon to the superlattice, it became possible to form a mixed crystal at a position deeper than the surface. For example, an Al _0.35 Ga layer having a thickness of 1 μm is formed on a 4-period superlattice layer composed of GaAs having a thickness of 80 Å and Al _0.3 Ga _0.7 As having a thickness of 120 Å.
Consider the case where _0.65 As and 1 μm GaAs are mounted. When H ⁺ is used as an ion species and an acceleration voltage of 260 kV is applied,
The projective range is 2.1 μm and its standard deviation is 0.25 μm.
All superlattices located 2 μm from the surface and having a total thickness of 0.1 μm will be exposed to H ⁺ irradiation.

The substrate heating temperature and the dose amount can be determined from the data of accelerated diffusion of impurities in Si. Enhanced diffusion is Si
It has been applied to the formation of a transistor in the microwave region using a substrate. In this case, when the H ⁺ beam was irradiated to Si at a heating voltage of 300 kV, an ion current density of 30 μA / cm ² , and a substrate temperature of 900 ° C., Sb in Si was The diffusion coefficient of is 10 ⁴ times larger. Applying this data to group III atoms (Ga and Al) in GaAs and AlGaAs, the diffusion coefficient at 750 ° C is 1.4 × 10 ^-21 (cm ²
/ s) for a group III atom, irradiation with a H ⁺ beam of 30 μA / cm ² increases the ^{concentration} to 1.4 × 10 ^-17 (cm ² / s). If ion implantation is continued for 1 hour in this state, the diffusion length [= 2
× {(diffusion coefficient) × (time)} ^1/2 ] is 45Å,
It is possible to mix GaAs / Al _0.3 Ga _0.7 As superlattice with s-layer thickness of 80Å. This is Al _0.3 Ga on both sides in GaAs.
_This is because Al diffuses from _0.7 As. Since the substrate temperature is as high as 750 ° C., the ion-implanted H ⁺ is immediately released into the vacuum and is not accumulated in the sample.

[Examples] Hereinafter, the present invention will be described in detail with reference to the drawings. First
FIG. 6 to FIG. 6 show an embodiment for manufacturing an embedded laser using the present invention.

FIG. 1 shows a cross section of a semiconductor structure obtained in a single growth process. Here, 8 is a GaAs cap layer (Be-doped:
˜2 × 10 ¹⁸ cm ⁻³ , layer thickness: 1 μm). 9 is Al _0.35 Ga
_0.65 As clad layer (Be doping: ~ 1 × 10 ¹⁸ cm ^-3 , Layer thickness: 1
μm). 10 is an undoped GaAs / Al _0.3 Ga _0.7 As superlattice, which forms a single quantum well layer. The layer thickness of GaAs is 80 Å, and the layer thickness of Al _0.3 Ga _0.7 As is 120 Å, which is 4 cycles. 11 is Al _0.3 Ga _0.7 As optical guide layer (Si-doped: ~ 1 ×
10 ¹⁸ cm ^-3 , layer thickness: 1 μm). Reference _numeral 12 is an Al _0.35 Ga _0.65 As clad layer (Si-doped: up to 1 × 10 ¹⁸ cm ⁻³ , layer thickness: 3 μm). 13 is a GaAs substrate (Si-doped: ˜1 × 10 ¹⁸ cm ⁻³ ).

Next, as shown in FIG. ₂ , SiO ₂ having a thickness of 1 μm is formed by a plasma CVD method, and then SiO _{2 in} a stripe shape having a width of 5 μm is formed.
Processed into mask 14.

Next, as shown in FIG. 3, a Si ₃ N ₄ film 15 having a thickness of 0.1 μm is formed.
Then, the whole sample was covered with it to form a protective film.

Next, as shown in FIG. 4, the H ⁺ beam 16 was ion-implanted while heating the sample to 750 ° C. using the heater 19.
The ion implantation conditions are the acceleration voltage 260k as described above.
V, ion current density 30 μA / cm ² , injection time 1 hour.
This acceleration voltage was set to a voltage at which the H ⁺ beam 16 reaches the superlattice through the GaAs cap layer 8, the AlGaAs cladding layer 9 and the Si ₃ N ₄ protective film 15. By this step, the superlattice in the portion not covered with the SiO ₂ mask 14 could be mixed crystallized. The advantage of using the ion implantation for the mixed crystal of the superlattice is that the acceleration voltage, the ion current, and the implantation time can be accurately controlled, so that the variation in the finished structure can be suppressed as compared with other methods. Further, since the ion depth distribution can be controlled by the accelerating voltage, it becomes possible to form a mixed crystal only in the superlattice at a specific depth.

Next, the Si ₃ N ₄ protective film 15 and the SiO ₂ mask 14 are removed, and the GaAs cap layer 8 is formed into a non-mixed superlattice 18 by using a photoresist technique and a selective etching solution as shown in FIG. Etch leaving only the top. Next, the entire surface is covered with a Si ₃ N ₄ film 20 which is an insulating film having a thickness of 0.1 μm, and then a portion of the GaAs cap layer 21 is removed by using a photoresist technique to form stripes, as shown in FIG. It has a different structure.

Next, as shown in FIG. 6, a P contact layer 22 made of Cr—Au and an n contact layer 23 made of AuGeNi are formed. Finally, this sample is cleaved to form a laser end face.

In the structure manufactured by the above-described steps, the diffusion length of the group III atoms (Ga and Al) is large in the portion where the H ⁺ beam 16 is not injected because it is masked by the SiO ₂ mask 14.
Since it is 0.64Å, the structure of the superlattice is maintained and it can be used as an active layer. On the other hand, the portion of the superlattice 17 which is not covered with the SiO ₂ mask 14 becomes a mixed crystal due to the irradiation of the H ⁺ beam 16, so that the refractive index is lowered and the energy gap is increased. Therefore, the confinement of light and the confinement of carriers occur at the same time, and an embedded laser having excellent characteristics can be manufactured.

Although hydrogen ions are used for ion implantation in this embodiment, it goes without saying that helium ions may be used. The heating temperature of the sample was 750 ℃, but
It may be in the range of ° C. Further, the ion accelerating voltage at the time of ion implantation is 260 kV, but needless to say, it may be 1 kV or more.

[Effects of the Invention] As described above, in the present invention, since ion implantation using light ions is used, while utilizing the characteristic of ion implantation having excellent controllability, it is possible to achieve deep position (1 μm or more) Mixed crystals are possible. Therefore, there is an effect that the optical element can be manufactured by the epitaxial growth only once without the need for the regrowth technique.

Further, since it is possible to form a mixed crystal only in the vicinity of a certain specific depth, there is an effect that it can be a basic technique for forming a three-dimensional stereoscopic optical element.

Furthermore, since electrically inactive H ⁺ or He ⁺ is used for ion implantation, there is also an effect that the electrical properties of the ion implantation region do not change.

[Brief description of drawings]

1 to 6 are views showing an embodiment for manufacturing an embedded laser by using the present invention, and FIG. 7 is an explanatory view of a conventional mixed crystal method. 8,21 ... GaAs cap layer, 9 ... Al _0.35 Ga _0.65 As clad layer, 10 ... undoped GaAs / Al _0.3 Ga _0.7 As superlattice, 11 ... Al _0.3 Ga _0.7 As optical guide layer, 12 ... Al _0.35 Ga _0.65 As clad layer, 13 …… GaAs substrate, 14 …… SiO ₂ mask, 15 …… Si ₃ N ₄ protective film, 16 …… H ⁺ beam, 17 …… Mixed crystal superlattice, 18… … Unmixed superlattice, 19 …… heater, 20 …… Si ₃ N ₄ film, 22 …… Cr-Au P contact layer, 23 …… AuGeNi n contact layer.

Claims (4)

(57) [Claims] 1. A mixed crystal method for a superlattice, which comprises ion-implanting light ions into the semiconductor crystal while the semiconductor crystal having a superlattice structure is mixed, with the semiconductor crystal being heated. 2. The superlattice mixed crystallization method according to claim 1, wherein the heating temperature is 500 to 800 ° C. 3. The light ion according to claim 1, wherein the light ion is a hydrogen ion or a helium ion.
A method for forming a mixed crystal of a superlattice according to the item. 4. The superlattice mixed crystallization method according to claim 1, wherein the ion implantation is performed at an acceleration voltage of 1 kV or more.