JP3304903B2 - Semiconductor quantum dot device and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a semiconductor quantum dot device used for a semiconductor device such as a semiconductor laser or a field effect transistor, and a method of manufacturing the same.

[0002]

2. Description of the Related Art A quantum dot structure in which charge carriers are confined three-dimensionally has a small energy spread of a state density. Due to this energy concentration, a device using quantum dots has better characteristics than a device having a general quantum well structure, and for example, a quantum dot laser or the like that oscillates at an extremely low current is realized.

As a method of manufacturing this quantum dot structure, a method is known in which quantum dots made of a semiconductor having a different lattice constant from the semiconductor are formed on a barrier layer made of the semiconductor in a self-forming manner (Applied Physics Letters, Vol. 63, p. 3203 (1993)). This method utilizes the fact that the growth of a semiconductor having a different lattice constant from the semiconductor of the substrate naturally forms a dot structure by the action of stress.
This is called anski-Krastanow (SK) mode growth.
Quantum dots formed by this method are formed by only one growth, and do not require steps such as processing a substrate before growth or processing a dot shape after growth. Therefore, not only the process is simple, but also dots having good quality crystals free from defects such as processing damage can be manufactured. In addition, since dots can be formed on the substrate surface at a high density of 10 10 or more per square centimeter and can be stacked, sufficient optical gain is obtained, for example, when used as an active layer of a laser. be able to.

[0004]

However, when quantum dots are conventionally formed in the SK mode in a self-forming manner, a large number of dots formed on the substrate surface have a variation in size of 10 dots.
% Or more. Although the density of states of one quantum dot is concentrated on a certain energy by three-dimensional confinement, the energy distribution of a large number of quantum dots has a spread of about 60 meV due to the existence of size variation. The dot size is such that the bottom diameter is about 30 nm and the height is 10 nm, and the height is smaller than the diameter. For this reason, the energy level of the quantum dot is substantially controlled by the height of the quantum dot, and the energy distribution of the quantum dot spreads by about 60 meV due to the variation in the height. As a result, it has been difficult to realize the device characteristics of quantum dots that have been conventionally expected, for example, a low threshold current operation of a laser.

Therefore, an object of the present invention is to form a large number of holes having a uniform depth in a semiconductor film and form quantum dots having a uniform height in the holes, thereby forming a sharp peak-like shape having a narrow energy width. An object of the present invention is to provide a semiconductor quantum dot device provided with quantum dots having a state density at a high density.

[0006]

According to the present invention, a semiconductor quantum dot device is formed by etching a dot structure made of a second semiconductor having a lattice constant different from that of a first semiconductor formed in a semiconductor film made of a first semiconductor. And a quantum dot structure comprising a third semiconductor formed so as to fill the hole.

According to the manufacturing method of the present invention, the lattice constant of the semiconductor crystal made of the first semiconductor is different from that of the first semiconductor.
A step of forming a dot structure made of a second semiconductor which can be removed without impairing the structure and function of the crystal of the first semiconductor by etching, and covering the dot structure on the semiconductor crystal in a range where the top is exposed. Forming a semiconductor film made of the first semiconductor while removing the dot structure by etching to form a hole in the semiconductor film; and forming a quantum dot structure made of the third semiconductor so as to fill the hole. It is characterized by including.

Alternatively, in the manufacturing method, the etching of the dot structure made of the second semiconductor is performed by thermal etching performed by maintaining the second semiconductor at a temperature at which the second semiconductor thermally desorbs.

[Operation] When a III-V compound semiconductor having a different lattice constant from the III-V compound semiconductor is grown on the surface of the III-V compound semiconductor by the crystal growth apparatus, the strain is relaxed when the film thickness exceeds the strain limit. As a result, the film aggregates to form island-like (dot-like) crystals. For example, when an InAs semiconductor crystal having a lattice constant larger than this by about 7% on a GaAs substrate is grown, I
nAs has a dot shape to reduce distortion. The size of the formed dots is a hemispherical shape with a bottom diameter of about 30 nm and a height of about 10 nm, and is formed at a density of 10 10 or more per square centimeter. When a GaAs semiconductor is grown on the dot, the GaAs does not grow immediately above the dot, but grows around the dot to form a GaAs film because the GaAs and InAs do not have lattice mismatch. This GaAs film has a flatness on the order of an atomic layer as in the case of ordinary layer growth. If this GaAs film is grown with a thickness higher than that of the InAs dots, the GaAs film will
s begins to grow, but has a lower thickness than InAs dots,
For example, when the GaAs film is grown to a thickness of 7 nm,
A structure in which the dots protrude by 3 nm is formed.

[0010] Comparing the temperature at which desorption occurs from the surface of the GaAs crystal and the surface of the InAs crystal, the GaAs temperature of about 670 ° C and the InA
s is about 570 ° C., about 100 ° C. lower. Therefore, if the structure in which the InAs dots protrude from the GaAs film at an intermediate temperature, for example, 600 ° C.,
Only nAs is desorbed (ie, thermally etched) and removed. As a result, the GaAs film has a diameter of about 30 nm and a depth of 7 n.
m holes are formed. Its density is the same as that of the originally existing InAs dots.

Next, when InGaAs having a relatively small In composition is grown on the GaAs film having the holes, three-dimensional island growth does not occur because the lattice constant difference from GaAs is not so large. grow up. At this time, the distance between the holes is In,
Since they are sufficiently smaller than the diffusion length of Ga atoms (several μm), those atoms fall into the pores, and the InGaAs crystal grows preferentially in the pores. This InGaAs crystal is made to have a hole depth of 7n.
When grown to a thickness of 8 nm, which is larger than m, InGaAs
In the crystal, an InGaAs film is formed evenly over the holes and the GaAs film by 1 nm by filling all the holes. When a GaAs crystal is grown thereon, a structure is obtained in which the entire surface is capped with GaAs.

As a result, InGaAs grown preferentially in the pores
All the crystals from the bottom of the hole to the GaAs cap layer interface
It has a cylindrical shape having a height of 9 nm including the InGaAs film of 1 nm. The diameter is equal to the diameter of the thermally etched InAs dots and is about 30 nm. Therefore, the cylindrical InGaAs crystal has a quantum dot structure in which carriers are confined in three-dimensional directions, surrounded by GaAs having a large energy gap. The uniformity of the height of the InGaAs quantum dots reflects the flatness of the layer growth of the InGaAs crystal, and the variation of the atomic layer thickness is less than 0.3 nm, that is, the variation of the film thickness is 3% or less. I have. On the other hand, the diameter is equal to the diameter of the thermally etched InAs dots, and the uniformity is 10%, the same as that of the InAs dots.
There is a variation of However, since the size in the diameter direction is larger than that in the height direction and the quantum confinement effect is weak, the size of the quantized energy level is hardly affected by the variation in the diameter direction. Therefore, the energy level distribution of the InGaAs quantum dots formed in a large number is 20 meV or less, reflecting the variation of 3% in the height direction. This is a narrow value compared to about 25 meV or more, which is the energy spread width of the quantum well at room temperature or higher.

If the size of the quantum dots is made uniform in this way, the quantum dot device characteristics reflecting the idealized state density of the quantum dots can be obtained. Dot laser can be manufactured.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a structural sectional view of a semiconductor laser according to a typical embodiment of the present invention. From below, an n-type GaAs substrate 1, an n-type AlGaAs cladding layer 2, a GaAs barrier layer 3,
aAs film 4, InGaAs quantum dots 5, InGaAs film 6, Ga
As barrier layer 7, p-type AlGaAs cladding layer 8, p-type Ga
As cap layer 9, further AlGaAs buried layer 10, p
It comprises an electrode 11 and an n-electrode 12.

In the n-type AlGaAs cladding layer 2,
The composition ratio of Al is preferably 0.2 to 0.4, for example, 0.3.
And the thickness is preferably 1 to 4 μm, for example 2 μm,
The carrier concentration is preferably 5 × 10 ¹⁷ to 2 × 10 ¹⁸ c
m ⁻³ , for example, 1 × 10 ¹⁸ cm ⁻³ .

The thickness of the GaAs barrier layer 3 is preferably 15 to 50 nm, for example, 20 nm.

The thickness of the GaAs film 4 is preferably 5 to 9 nm, for example, 7 nm.

In the InGaAs quantum dots 5, the composition ratio of In is preferably 0.1 to 0.3, for example, 0.2, the thickness is preferably 6 to 10 nm, for example, 8 nm, and the in-plane diameter is preferably. Is 20 to 50 nm, for example, 30 nm.

In the InGaAs film 6, the composition ratio of In is preferably 0.1 to 0.3, for example, 0.2, and the thickness is preferably 0.5 to 2 nm, for example, 1 nm.

The thickness of the GaAs barrier layer 7 is preferably 15 to 50 nm, for example, 20 nm.

In the p-type AlGaAs cladding layer 8,
The composition ratio of Al is preferably 0.2 to 0.4, for example, 0.3.
And the thickness is preferably 1 to 3 μm, for example 1.5 μm.
m and the carrier concentration are preferably 1 × 10 ¹⁷ to 1 × 10 ¹⁸
cm ⁻³ , for example, 5 × 10 ¹⁷ cm ⁻³ .

The thickness of the p-type GaAs cap layer 9 is preferably 0.1 to 0.5 μm, for example, 0.2 μm, and the carrier concentration is preferably 1 × 10 ^{18 to} 5 × 10 ¹⁸ c.
m ⁻³ , for example, 2 × 10 ¹⁸ cm ⁻³ .

Further, in the AlGaAs buried layer 10, the Al composition ratio is larger than the Al composition ratio of the AlGaAs cladding layers 2 and 8, and is preferably 0.3 to 0.5, for example 0.35.

As the p-electrode 11, any material can be used as long as it is Au or an Au alloy.
Au.

As the n-electrode 12, any material can be used as long as it is Au or an Au alloy.
Au.

The thickness of the InGaAs quantum dots 5, including the thickness of the InGaAs film 6, is preferably 6 to 10 nm, for example, 8 n.
m.

The InGaAs quantum dot 5 has a square centimeter
Preferably 5 × 10 in number per Torr ⁹~ 1 × 10¹¹Pieces, example
For example, 10^TenExists in the plane at the density of the individual, and varies in the thickness direction
Is preferably 1 to 4%, for example, about 3%.

When a current is applied to this laser structure, light is emitted by a combination of electrons and holes having uniform energy in an active region composed of quantum dots having a uniform size, and oscillates at a low current to achieve highly efficient light emission characteristics. Is obtained.

Further, the GaAs film 4 and the In
The structure of the quantum dots may be multilayered by repeating the GaAs quantum dots 5, the InGaAs film 6, and the GaAs barrier layer 7 many times. In this case, a laser having high optical output characteristics in addition to low current oscillation operation can be obtained by the active region of the multilayered quantum dot.

Further, the semiconductor crystals of the GaAs barrier layer 3 and the GaAs film 14 may be replaced with AlGaAs. In this case, since the energy gap of AlGaAs is larger than that of GaAs, leakage of carriers from the quantum dots at high temperatures is reduced.

FIG. 2 is a manufacturing process diagram of a laser having a quantum dot structure. First, an n-type substrate 1 doped with a group IV, for example, Si is introduced into a molecular beam epitaxial growth apparatus. On this substrate, a group IV-doped n-type AlGaAs cladding layer 2 and a GaAs barrier layer are grown under heating (FIG. 2).
(a)). The heating temperature is preferably 550-630C, for example, 600C.

When InAs 13 is grown under heating as shown in FIG. 2 (b), lattice mismatch with the underlying GaAs layer is 7%, so that the critical thickness (0.6 nm) is exceeded. At this point, the distortion is relaxed, and the dot 14 is formed as shown in FIG. The heating temperature is preferably 500 to 530 ° C,
For example, it is 5100C.

Thereafter, as shown in FIG.
When 4 is grown at the same temperature, it does not grow directly on the InAs dot but grows so as to fill around it, because of lattice mismatch with InAs. For example, if the grown film thickness of the GaAs film 4 is 7 nm, the InAs dots 14 are 3 nm thick from the GaAs film.
The structure protrudes by m. Here, since the GaAs film grows in a layered manner, its thickness is flat at the atomic layer level.

Next, the substrate is preferably 570-630.
C., for example, to 600.degree. C. and maintained for 3 minutes. Ga
The thermal desorption temperature of As is 670 ° C, and the thermal desorption temperature of InAs is 57
At 0 ° C., only InAs thermally desorbs at this heating temperature. Therefore, a hole is formed in the GaAs film 4 where the InAs dot exists. The diameter of the holes is preferably between 20 and 50
nm, for example 30 nm, and the depth is preferably 5 to 9 nm, for example 7 nm.

InGaAs is grown on the GaAs film 4 having the holes 16 preferably at a temperature of 500 to 530 ° C., for example, 510 ° C. (FIG. 2F).

InGaAs has lattice mismatch with GaAs,
For example, when the In composition is 0.2, it is about 1%.
Up to about m, it grows in a layer without strain relaxation.
When the distance between the holes is 100 nm or less, the diffusion distance of In or Ga
(Several μm), In and Ga fall into the hole, and InGaAs grows preferentially from the bottom of the hole.

As the growth of InGaAs continues, the growth rate of the dots present at high density varies, but when one hole is filled with InGaAs, the atoms that diffuse on the GaAs film are not completely filled. It flows into the hole.
Then, after all the holes are filled with InGaAs, the InGaAs film 6 is grown so as to be flat on the holes and on the GaAs film.

Therefore, after the holes are filled, InGaAs is formed.
Stop growing InGaAs by growing 1nm. As a result, the depth of the hole (for example, 7 nm) and the InGaA
A cylindrical InGaAs quantum dot structure having a thickness (for example, 8 nm) obtained by adding 1 nm of the thickness of the s film is formed. The thickness of the dot (for example, 8 nm) varies by 3 because the bottom of the hole and the surface of the InGaAs film are flat at the atomic level.
%.

On the other hand, the diameter of the quantum dot is the diameter of the hole (for example, 30 nm), and the variation thereof is about 10%. However, the energy level of the quantum dot is almost determined by the confinement of the small size in the thickness direction. Therefore, the variation of the quantized energy level is 20 meV or less,
25 meV which is the energy spread of the quantum well at room temperature
Smaller than.

Subsequently, the GaAs barrier layer 7 is formed on the InGaAs film 6.
Grow. Further, a Be-doped p-type AlGaAs cladding layer 8 and a p-type GaAs cap layer 9 are grown. The growth substrate is taken out of the growth apparatus, and stripe mesa processing, formation of the buried layer 10, and formation of the electrodes 11, 12 are performed (FIG. 2 (h)).

Through the above steps, a quantum dot laser is manufactured. When a current is applied to the laser, the light gain from the dots of uniform size has a narrow energy line width and a very high intensity. Therefore, a laser which oscillates at a low current and has high efficiency can be obtained.

In the above method, the InAs dots 14 are used to form the holes 16, but InP dots may be used instead. InP has a lattice constant difference of about 4% from GaAs.
For this reason, dots are formed on GaAs in the same manner as InAs. Since the thermal desorption temperature of InP is lower than that of GaAs, only InP can be removed by thermal etching. Therefore, it is possible to form holes in the GaAs film as in the case of using InAs dots.

[0044]

According to the present invention, a quantum dot having a highly uniform thickness distribution is formed using a crystal film having holes of a uniform depth. As a result, quantum dots having the same energy of the quantization level can be obtained at a high density, and, for example, a high-performance semiconductor laser using these as an active layer is realized.

[Brief description of the drawings]

FIG. 1 is a structural sectional view of a semiconductor laser of the present invention.

FIG. 2 is a diagram illustrating a manufacturing process of the present invention.

[Explanation of symbols]

Reference Signs List 1 n-type GaAs substrate 2 n-type Al _0.3 Ga _0.7 As cladding layer 3 GaAs barrier layer 4 GaAs film 5 InGaAs quantum dot 6 InGaAs film 7 GaAs barrier layer 8 p-type Al _0.3 GaAs _0.7 cladding layer 9 p-type GaAs cap layer 10 Al _0.35 Ga _0.65 As buried layer 11 p-type electrode 12 n-type electrode 13 InAs 14 InAs dots 15 SiO ₂ insulating film 16 holes

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-42481 (JP, A) JP-A-4-180283 (JP, A) JP-A-7-202164 (JP, A) JP-A-10-108 12975 (JP, A) JP-A-8-236501 (JP, A) JP-A-10-289996 (JP, A) JP-A-11-87687 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 H01L 29/06 H01L 29/201 H01L 29/80

Claims (6)

(57) [Claims] 1. A hole formed by etching a dot structure made of a second semiconductor having a lattice constant different from that of a first semiconductor formed in a semiconductor film made of a first semiconductor, and a hole formed by filling the hole. A method of manufacturing a semiconductor quantum dot device having a quantum dot structure made of a third semiconductor formed in the above, wherein a lattice constant of the semiconductor crystal made of the first semiconductor is different from that of the first semiconductor, and etching is performed. Forming a dot structure made of a second semiconductor that can be removed without impairing the structure and function of the crystal of the first semiconductor, and covering the dot structure on the semiconductor crystal in a range where the top is exposed. A step of forming a semiconductor film made of the first semiconductor, a step of removing a dot structure by etching to form a hole in the semiconductor film, and filling the hole. Characterized in that it comprises a step of forming a quantum dot structure consisting of 3 of the semiconductor manufacturing method of a semiconductor quantum dot device. 2. The semiconductor quantum dot according to claim 1, wherein the etching of the dot structure made of the second semiconductor is performed by thermal etching performed by holding the second semiconductor at a temperature at which the second semiconductor thermally desorbs. Device manufacturing method. 3. The first semiconductor, the second semiconductor,
A method for manufacturing a semiconductor quantum dot device, wherein the third semiconductor is a III-V compound semiconductor. 4. The method according to claim 1, wherein the first semiconductor is Al _x Ga _{1 -x} As (0 ≦
x ≦ 1), wherein the second semiconductor is InAs, and the third semiconductor is In _y Ga _1−y As (0 <y <1). 5. A method for manufacturing a semiconductor quantum dot device, wherein said second semiconductor is InP. 6. The method for manufacturing a semiconductor quantum dot device according to claim 1, wherein the quantum dot structure is multi-layered by repeating a combination of the respective steps a plurality of times.