CN1438168A - Laser-inducing preparation of size-controllable high-density nano silicon quanta array of points - Google Patents

Abstract

The method of laser-induced preparation of size-controllable high-density nano-silicon quantum dot arrays: first prepare the multi-layer modulation structure: use plasma-enhanced chemical vapor deposition technology to prepare amorphous silicon or germanium/amorphous silicon nitride or silicon dioxide Single-layer or multi-layer modulation structure of silicon oxide, in which the thickness of a-Si:H sublayer is basically consistent with the size of quantum dots to be obtained after laser crystallization; then laser-induced crystallization: substrate temperature: 150-250°C. The present invention realizes restricted crystallization and can effectively control the formation and size distribution of silicon quantum dots. Since the thickness of the a-Si:H sublayer in the multilayer modulation structure can be manually designed, the precision can reach 0.5nm, and the controllability is strong, thus The size of the finally formed nc-Si quantum dots can also be artificially designed and controlled. Dielectric layer a-SiN _X : H or a-SiO ₂ sub-layer thickness can be as thin as 5nm,

Description

Laser-induced preparation of size-controllable high-density nano-silicon quantum dot arrays

1. Technical field:

The present invention proposes a new method for preparing a size-controllable high-density nano-silicon quantum dot array from two aspects of technical principle and implementation process, especially the preparation of a size-controllable high-density nano-silicon quantum dot array based on the principle of laser-induced restricted crystallization The array method, that is, the single-layer or multi-layer modulation structure of amorphous silicon or germanium/amorphous silicon nitride or silicon dioxide is prepared by plasma-enhanced chemical vapor deposition technology, which is a new generation of nano-electronics and nano-optoelectronics Key technologies in high-tech fields such as devices.

2. Background technology:

Semiconductor monocrystalline silicon material is the backbone material of the current microelectronics industry, and has an irreplaceable position for other materials. However, can silicon materials continue to play an important role in the future era of nanoelectronic devices, and can silicon monolithic optoelectronic integration be realized? This is a major topic of great concern to scientists and material scientists from all over the world. As one of the key technologies, how to obtain a controllable silicon quantum dot array is an urgent technical problem to be solved, especially the preparation principle and method of silicon quantum dots with three-dimensional scale limitation are particularly attractive. At present, there are mainly the following technical methods for preparing nanoscale silicon quantum dots: 1. "Top-down" technology, that is, using the current mature microelectronics technology, coupled with electron beam exposure and ultra-fine etching equipment , starting from a silicon single wafer to obtain a silicon quantum dot structure. Using this technology, the size of quantum dots can be precisely controlled, but its preparation cost is extremely expensive, and the smallest scale can only reach about 100 nanometers (nm) at present. 2. Using the stress existing during heterogeneous growth between different materials and controlling a certain growth temperature and air pressure, nano-silicon quantum dots can be obtained on substrates such as silicon dioxide, and their average size can reach several to tens of nanometers. However, the size of quantum dots cannot be controlled by this method, and the growth process is random, and generally requires a high temperature process. 3. Implanting silicon ions into the silicon dioxide film, and then performing subsequent treatment such as thermal annealing to obtain nano-silicon quantum dots embedded in silicon dioxide. The preparation process of this method is simple and convenient, but it is still unable to precisely control the size of quantum dots. At the same time, the long-term high-temperature treatment will also limit the application of this method in device applications. Therefore, it is the starting point of the present invention to find a new way to prepare silicon quantum dots that can effectively control the size distribution of quantum dots and can be applied to future nanoelectronics and optoelectronic devices. The laser-induced confinement crystallization principle we proposed is a method for preparing high-density nano-silicon quantum dots with controllable size. It is based on the multilayer modulation structure of amorphous semiconductor/insulating dielectric film. The quantum dots are formed by the structural phase transition from the amorphous state to the crystalline state, and the size of the obtained semiconductor quantum dots is controlled by the thickness of the original semiconductor layer by using the confinement effect of the dielectric layer.

3. Contents of the invention:

The research results of recent decades at home and abroad show that: to make nano-silicon quantum dots go out of the laboratory and be truly adopted by the industry, it is required to be able to control the size of quantum dots and passivate the surface of silicon quantum dots. , while also being compatible with current microelectronics process technologies.

The purpose of the present invention is exactly to the requirement of above three respects, proposes the new technology principle of preparation size controllable nano-silicon quantum dot, designs and provides a kind of new technical method in experiment simultaneously, utilizes this method not only with current microelectronic technology Compatible with the technology and avoiding the use of expensive ultra-fine processing technology, while obtaining high-density nano-silicon quantum dots with controllable size, it can also effectively passivate the surface of the nano-crystal grains to reduce the density of defect states, so it can show Quantum effects due to size changes. Quantum devices prepared based on this technology will have great application prospects and value in the field of nanoelectronics and nano-optoelectronic devices in the future.

principle of restricted crystallization

The present invention combines amorphous silicon (a-Si:H), amorphous silicon nitride (a-SiN _x :H) (silicon oxide (a-SiO ₂ )) multilayer modulation structures with laser crystallization technology, based on The principle of restricted crystallization realizes size-controllable high-density nano-silicon (nc-Si) quantum dot arrays. The principle of constrained crystallization is that in the process of laser-induced crystallization, it is required to allow amorphous silicon to absorb photon energy and transform into a crystalline state through phase transition without destroying the multi-layer modulation structure, so that nano-silicon quantum dots are affected by layer thickness during the growth process. Confinement to control the size of the resulting nano-Si quantum dots. The technical key is to select the crystallization energy density of the laser and the thickness of the a-Si:H sublayer in the multilayer modulation structure.

We theoretically estimated the critical thickness of the confined crystallized a-Si:H layer by the change of free energy during the crystallization process. Figure 1(a) is a schematic diagram of the nucleation and growth process of nc-Si quantum dots in a certain a-Si:H layer in a multilayer film during crystallization. After absorbing laser energy, amorphous silicon nucleates and then grows up. When the laser crystallization energy density is low, spherical quantum dot particles will be obtained. When the laser crystallization energy density is high, although the quantum dots are still restricted in the longitudinal direction , but will continue to grow in the lateral direction to form disc-shaped particles. In this process, due to the change of surface-to-volume ratio after crystallization to form silicon quantum dots, the influence of interfacial free energy increases. According to our theoretical calculations, when the thickness d of the a-Si:H sublayer is below 12nm, the control Appropriate process conditions (laser energy density) can make the size of the obtained nc-Si quantum dots limited by the thickness of the a-Si:H sublayer, and obtain spherical nc-Si quantum dots, thereby realizing laser-induced confined crystallization. process. When the thickness d of the a-Si:H sublayer is greater than 12 nm, since the free energy change (ΔG) is negative, it indicates that the confinement effect of the multilayer modulation structure will disappear, and the size of nc-Si quantum dots will no longer be controllable ( See Figure 1(b)).

Technical solutions:

Laser-induced preparation of size-controllable high-density nano-silicon quantum dot arrays:

1. Preparation of multilayer modulation structure: Utilize plasma-enhanced chemical vapor deposition technology to prepare amorphous silicon or germanium/amorphous silicon nitride or silicon dioxide (a-Ge: H/a-SiN _x : H, a -Si:H/a-SiN _x :H, a-Si:H/a-SiO ₂ ) single-layer or multi-layer modulation structure, wherein the thickness of a-Si:H sublayer can be precisely controlled to 1-10nm; its The thickness is basically consistent with the size of the quantum dots that are expected to be obtained after laser crystallization. Similarly, we can also prepare a three-layer sandwich structure with amorphous silicon in the middle, which has wider applications in nanoelectronic devices. It can also be a multi-layer sandwich structure with intervals. Generally speaking, the thickness of the amorphous silicon layer is 1-10 nm, and the thickness of the dielectric layer is 4-15 nm of amorphous silicon nitride or silicon dioxide.

2. Laser-induced crystallization: substrate temperature: 150-250°C; laser beam power is generally 1-5W, especially above 1.0-2.0W, Ar ⁺ laser with a wavelength of 514.5nm can be used as the laser light source, or can be used The laser wavelength is 248nm KrF excimer pulse laser as the laser light source. Laser-induced crystallization was performed on the prepared a-Si:H/a-SiN _x :H multilayer modulation structure by selecting appropriate laser irradiation conditions.

Realization of the principle of restricted crystallization: for Ar ⁺ laser, the a-SiN _x : H layer basically does not absorb laser energy, so the laser energy is basically absorbed by the a-Si: H layer; for KrF excimer pulsed laser, a-Si :H and a-SiN _x :H films have different absorption coefficients of 1-2 orders of magnitude, so the laser energy is mainly absorbed by the a-Si:H layer, while the a-SiN _x :H layer is basically not absorbed. In the a-Si:H/a-SiN _x :H multilayer modulation structure, since the a-SiN _x :H film is an insulating material with very good thermal stability, laser-induced crystallization of the a-Si:H layer During the crystallization process, the a-SiN _x : H film is basically unaffected, and can play a restrictive role, so that the nc-Si quantum dots are subjected to up and down in the vertical direction (the growth direction of the multilayer structure) during the nucleation and grain growth process. Due to the strong confinement of the two a-SiN _x : H insulating films, the crystal grain size is consistent with the thickness of the a-Si: H layer in the original deposited multilayer film structure, and laser-induced confinement crystallization is realized.

The advantage of the technology of the present invention:

The advantage of the technical solution of the present invention is that the formation and size distribution of silicon quantum dots can be effectively controlled by combining the multilayer modulation structure with the laser crystallization technology to realize the restricted crystallization. The thickness can be manually designed, the precision can reach 0.5nm, and the controllability is strong, so that the size of the finally formed nc-Si quantum dots can also be manually designed and controlled. The thickness of the dielectric layer a-SiN _x : H or a-SiO ₂ sublayer can be as thin as 5nm, so high-density nc-Si quantum dots (area density > 10 ¹² cm ^-2 , bulk density > 10 ¹⁸ cm ^-3 ). In addition, the surface of nc-Si quantum dots obtained by using this technical solution can be effectively protected by passivation due to the existence of hydrogen atoms and a-SiN _x : H dielectric film, reducing the interface state density, making it very suitable for building future nano device. Finally, this technical solution uses laser crystallization technology to avoid the high-temperature treatment process in conventional thermal annealing technology, which is especially beneficial for the future use of this technology in the integration of nano-optoelectronics and nano-electronics.

The technology of the invention is also applicable to the preparation of other semiconductor quantum dots.

4. Description of drawings:

Figure 1: Schematic of the principle of the confinement crystallization method.

Figure 1(a), a schematic diagram of the nucleation and growth process of nano-silicon quantum dots in a sublayer of a-Si:H/a-SiN _x :H multilayer structure; it can be seen that the silicon quantum dots are formed due to crystallization. The change of volume ratio increases the influence of interfacial free energy. By choosing different laser energy densities, the shape of the obtained quantum dots can change from spherical to disc-shaped, and its longitudinal size is always limited by the multilayer structure.

Figure 1(b), the theoretical curve of the change of free energy of silicon quantum dots with the change of grain radius r, different curves represent different a-Si:H sublayer thickness d. When the thickness of the a-Si:H sublayer is less than 12nm, the free energy change is positive, indicating that the size of the silicon quantum dots grown after nucleation can be limited by the layered structure, and the size can be controlled by controlling the appropriate laser energy density. spherical nano-silicon quantum dots. When the thickness of the a-Si:H sublayer is greater than 12nm, the free energy change is negative, indicating that the restrictive effect of the multilayer modulation structure will disappear, and the size of the nano-silicon quantum dots will no longer be controllable.

Figure 2: Schematic diagram of the laser crystallization setup.

1. Laser; 2. Laser beam; 3. Focusing lens; 4. Mirror;

5. Sample; 6. X-Y mobile stage;

Figure 3: Cross-sectional electron microscopy of a-Si:H/a-SiN _x :H multilayer modulation structure after KrF laser crystallization

photo.

(a), Sectional electron micrograph of the crystallized a-Si:H/a-SiN _x :H multilayer modulation structure. Wherein the a-Si:H thickness is 4 nm, and the a-SiN _x :H layer thickness is 6 nm. In the original amorphous silicon sub-layer, the nano-silicon quantum dots formed after crystallization can be clearly seen, and their size is limited by the upper and lower a-SiN _x : H insulating layers, which is equivalent to the thickness of the initial amorphous silicon sub-layer. And after laser irradiation, the multilayer modulation structure was not damaged either.

(b), A high-resolution electron microscope photograph of a section of a sublayer in a multilayer structure. In the photo, the atomic lattice image of nano-silicon quantum dots formed after crystallization can be clearly seen, and the quantum dots are spherical, and their size is limited by the layered structure.

5. Specific implementation methods:

1. Preparation of a-Si:H/a-SiN _x :H multilayer modulation structure

Using computer-controlled plasma-enhanced vapor deposition (PECVD) technology, using silane (SiH ₄ ), ammonia (NH ₃ ) and argon (Ar) as the reaction gas source; on single crystal silicon wafers and quartz or optical glass Deposit a-Si:H/a-SiN _x :H three-layer or multi-layer modulation structure on the substrate. Concrete process conditions during preparation are as follows:

Power source frequency: 13.56MHz

Power density during deposition: 0.2-0.3W/cm ²

             Pressure in the reaction chamber: 33-40Pa

Substrate temperature: 250°C

When depositing the amorphous silicon (a-Si:H) sublayer, it is formed by the glow decomposition reaction of SiH ₄ +Ar, wherein the flow rate of SiH ₄ is 8 sccm (standard cubic centimeter per minute), and the deposition rate is 0.10 nm/s; the amorphous silicon nitride (a-SiN:H) sublayer is formed by the reaction of SiH ₄ +NH ₃ +Ar mixed gas, and the gas flow ratio of NH ₃ /SiH ₄ is 5 during the reaction. The optical bandgap of the a-SiN:H film is about 3.0eV, and the deposition rate is 0.11nm/s. The thickness of the a-Si:H and a-SiN _x :H sublayers can be designed according to requirements. In our example, the thickness of the a-Si:H sublayer is 4nm, the thickness of the a- _SiNx :H sublayer is 6nm, and there are 72 periods in total. The most important thing in the deposition process is to use the computer to control the switch of the gas valve so that the gas in the reaction chamber can be exchanged rapidly when depositing different sub-layers.

2. Preparation of a-Ge:H/a-SiN _x :H multilayer modulation structure

The same technical method can be used to prepare a-Ge:H/a-SiN _x :H multilayer modulation structure. Wherein a- _SiNx : the preparation of H sub-layer is the same as before; when depositing amorphous germanium (a-Ge: H) sub-layer, reaction gas uses germane (GeH ₄ ), its flow rate is 2sccm, reaction chamber pressure It is 33Pa, and the deposition rate is 0.20nm/s.

3. Ar ⁺ laser crystallization process

For the a-Si:H/a-SiN _x :H and other multi-layer modulation structure samples obtained by the above technical method, use

The laser crystallization device shown in Figure 2 performs laser irradiation treatment, and its process conditions are as follows:

  Laser beam spot diameter:   100μm

  Laser beam scanning speed:   4.5cm/s

  Laser beam scanning overlap: ＞50%

    Laser beam power:                                                              

Substrate temperature: 160-180℃

After laser scanning irradiation, the a-Si:H sub-layer formed nano-silicon quantum dots through the nucleation and growth process, and the cross-sectional electron microscope inspection did form silicon quantum dots in the original amorphous silicon sub-layer, and the original multi-layer The layer modulation structure is not destroyed. By changing the thickness of the a-Si:H sublayer, silicon quantum dots with a size of 1-10 nm can be obtained by using the method.

4. KrF excimer pulse laser crystallization process

Using the laser irradiation device shown in Figure 2, the KrF excimer pulse laser can also be used for crystallization of a-Si:H/a-SiN _x :H and other multi-layer modulation structure samples. The wavelength of the KrF excimer pulse laser used is 248nm, and the pulse width is 30ns. In our practical example, the crystallization of a-Si:H/a-SiN _x :H by KrF excimer pulse laser can form silicon quantum dots with a size of about 4nm (see Figure 3(a) and (b) ). The specific process conditions are:

Laser irradiation area: 6×5mm ²

Laser irradiation power density: 500-1000mJ/cm ²

5. An embodiment where the dielectric layer is a-SiO ₂ ).

Preparation of a-Si∶H/a-SiO ₂ Multilayer Modulated Structure

The same technical method can be used to prepare a-Si:H/a-SiO ₂ multilayer modulation structure. The preparation of the a-Si:H sub-layer is the same as before; the silicon dioxide layer is obtained by plasma oxidation, using pure oxygen during preparation, the pressure in the reaction chamber is 33Pa, the power is 50W, the oxidation temperature is 250°C, and the oxidation rate is 5.4nm/s.

Claims (7)

1. Laser-induced preparation of size-controllable high-density nano-silicon quantum dot arrays: first prepare multi-layer modulation structures: use plasma-enhanced chemical vapor deposition technology to prepare amorphous silicon or germanium/amorphous silicon nitride Or a single-layer or multi-layer modulation structure of silicon dioxide, which is characterized in that the thickness of the a-Si: H sublayer is basically consistent with the size of the quantum dots that are expected to be obtained after laser crystallization; then laser-induced crystallization: substrate temperature: 150-250°C. 2. The method for preparing a size-controllable high-density nano-silicon quantum dot array according to claim 1, characterized in that a three-layer sandwich structure is prepared, and amorphous silicon is in the middle layer. 3. The method for preparing a size-controllable high-density nano-silicon quantum dot array according to claim 1, which is characterized in that a multi-layer sandwich structure with intervals is prepared. 4. The method for preparing a size-controllable high-density nano-silicon quantum dot array according to claim 1: It is characterized in that the amorphous silicon layer is 1-10nm, and the dielectric layer amorphous silicon nitride or silicon dioxide is 4-15nm . 5. The method for preparing a size-controllable high-density nano-silicon quantum dot array according to claim 1: it is characterized in that an Ar ⁺ laser with a wavelength of 514.5nm is used as a laser light source, and a KrF quasi with a laser wavelength of 248nm can also be used. The molecular pulse laser is used as the laser light source. 6. The method for preparing a size-controllable high-density nano-silicon quantum dot array according to claim 1: it is characterized in that by selecting appropriate laser irradiation conditions: laser beam spot diameter: 100 μm   Laser beam scanning speed:   4.5cm/s   Laser beam scanning overlap: ＞50%     Laser beam power:                                                                 Substrate temperature: 160-180° C. Laser-induced crystallization is performed on the prepared a-Si:H/a-SiN _x :H multilayer modulation structure. 7. The method for preparing a size-controllable high-density nano-silicon quantum dot array by light induction as claimed in claim 1: it is characterized in that the Ar ⁺ laser beam power is generally 1.0-5.0W, and the KrF excimer pulse laser irradiation power The density is 500-1000mJ/cm ² .

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