CN110071197A - A kind of high polarization degree spin LED and preparation method thereof based on non-polar plane gallium nitride - Google Patents

Abstract

The spin LED and preparation method thereof for the high circular polarization polarizability based on non-polar plane gallium nitride that the invention discloses a kind of, non-polar plane (face m or the face a) gallium nitride is used to prepare the spin LED of inverted structure for substrate, the circular polarization polarizability upper limit and Study of Electron Spin Relaxation Time service life are effectively increased, so that the practical circular polarization polarizability that front goes out light increases substantially.

Description

A kind of high polarizability spin LED based on non-polar surface gallium nitride and preparation method thereof

technical field

The invention belongs to the technical field of semiconductors, and in particular relates to a structure of a gallium nitride spin LED with high circular polarization at room temperature and a preparation method thereof.

Background technique

Nitride semiconductors have long intrinsic spin lifetimes due to their wide band gap, which makes the spin-orbit coupling weak. At the same time, the nitride material with wurtzite structure has a strong polarization field, so that it can generate a strong Rashba spin-orbit coupling effective magnetic field in a specific crystal orientation. The long spin lifetime is beneficial to the preservation of the spin signal, while the strong effective magnetic field is beneficial to the regulation of the spin signal. These advantages make nitride semiconductors an ideal material system for the fabrication of spintronic devices.

At present, the further reduction of integrated circuit size has encountered many problems, including: quantum tunneling effect in small scale affects transistor switching performance, heat dissipation caused by high integration, and RC delay caused by parasitic capacitance in small scale. Room-temperature electronic devices based on electron spin degrees of freedom are a good choice to solve the above problems and continue Moore's law. The research on spintronic devices such as spin LEDs, lateral spin valves, and spin field effect transistors (spin FETs) based on cubic gallium arsenide started earlier. Mature. However, the working temperature of the above-mentioned devices mostly stays in the low temperature environment of the laboratory. Compared with the spintronic devices of the gallium arsenide system, the advantage of nitride is that it can work at room temperature. Spontaneous circularly polarized light sources have important applications in the study of biochemically active molecules, visible light communication, and detection of magnetic media. A typical device to realize spontaneous circularly polarized light source is spin LED. The limitations of the common gallium arsenide-based spin LEDs are mainly reflected in two aspects: 1. The range of emission wavelengths is limited and cannot include the short-wave band from blue to ultraviolet; 2. It can only maintain a relatively high circular polarization at low temperatures degree of polarization, while there is almost no circular polarization at room temperature.

The spin LED based on gallium nitride will effectively overcome the limitations in the spin LED of the gallium arsenide system, making the spin LED one step closer to industrial application. However, there are also two problems to be solved in the realization of spin LED based on gallium nitride ([1]Chen W M et al., Applied Physics Letters. 87(19):2599(2005).): 1. In the case of front light emission (The light exit direction is parallel to the C-axis direction) The upper limit of the circular polarization degree is only about 3%; 2. The stress-induced polarization field in the GaN light-emitting quantum well is large, so that the spin relaxation lifetime is only a few tens of picometres magnitude of seconds. In order to solve the above problems, a scheme of using nano-pillar arrays to combine semi-metal ferric oxide nanoparticles has been adopted internationally ([2] Chen JY et al., Nano Letters. 14(6): 3130-3137 (2014). ). However, this solution requires etching to prepare nano-pillar arrays, which causes great damage to materials and reduces luminous efficiency. Moreover, the process is incompatible with the existing conventional gallium nitride LED process, which is not conducive to practical industrial applications.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the problems that the upper limit of circular polarization degree of GaN-based light-emitting quantum wells is low and the spin relaxation life in the quantum wells is short in the case of front-side light emission. a) Scheme of gallium nitride to fabricate spin LEDs. This design can increase the upper limit of the circular polarization degree of the front light from 3% to 33%, and at the same time can effectively increase the spin relaxation lifetime, making the actual circular polarization degree of the light output closer to the upper limit.

In order to achieve the above object, the present invention adopts the following technical solutions:

A high-polarization spin LED, which is characterized by an inverted LED structure based on non-polar plane GaN, including in order from bottom to top: a non-polar plane GaN substrate, a P-type GaN layer, a P-type AlGaN electron blocking layer, InGaN/GaN light-emitting quantum well layer, N-type GaN layer, MgO tunneling layer, ferromagnetic layer and heavy metal protective layer.

The above-mentioned non-polar plane GaN substrate may be m-plane GaN or a-plane GaN. The P-type GaN layer, P-type AlGaN electron blocking layer, InGaN/GaN light-emitting quantum well layer, and N-type GaN layer are sequentially obtained by epitaxial growth on a non-polar GaN substrate by MOCVD (metal organic chemical vapor deposition) method.

Wherein, the P-type GaN layer is a Mg-doped P-type GaN layer, the thickness is preferably 500-800 nm, and the Mg doping concentration is preferably controlled at the order of 10E8/cm ³ .

The P-type AlGaN electron blocking layer is a Mg-doped P-type AlGaN layer, and the thickness is preferably 5-15 nm, and the doping concentration of Mg is preferably controlled at the order of 10E7/cm ³ . The P-type AlGaN layer acts as an electron blocking layer to prevent electrons injected from the N-type GaN layer from entering the P-type GaN layer and reducing the luminous efficiency.

The InGaN/GaN light-emitting quantum well layer is generally an InGaN/GaN quantum well with 3-5 cycles, and the thicknesses of the potential barrier and potential well in one cycle are 3.5-5 nm and 10-15 nm, respectively.

The N-type GaN layer is a Si-doped N-type GaN layer, and the thickness is preferably 50-150 nm, and the doping concentration of Si is preferably controlled at the order of 10E8/cm ³ .

The above-mentioned MgO tunneling layer, ferromagnetic layer and heavy metal protective layer are spin injection layers prepared on the inverted LED structure by the method of magnetron sputtering. Among them, the thickness of the MgO tunneling layer is preferably 1-4 nm, which is used to alleviate the conductance mismatch problem between the ferromagnetic metal and the N-type GaN layer on the upper layer and improve the spin injection efficiency. The ferromagnetic layer can be a Co film or a ferromagnetic thin film such as CoFe, CoFeB, NiFe, etc. The thickness of the ferromagnetic layer is preferably 20-60 nm, and the ferromagnetic layer is used to generate spin-polarized electrons. The heavy metal protective layer can be a Pt film, or a heavy metal film such as Ta and Au can be used as the protective layer, and its thickness is preferably 10-40 nm, which is used to protect the ferromagnetic layer and prevent it from being oxidized.

After the growth of each epitaxial layer and the sputtering of the spin injection layer on the non-polar surface GaN substrate in turn, the traditional LED lithography process can be used to prepare the high circularly polarized spins based on the non-polar surface GaN. LED devices.

In the above structure, the magnetization direction of the Co film or other ferromagnetic thin film as the ferromagnetic layer is perpendicular to the surface, so the electron spin polarization direction injected into the N-type GaN layer and the quantum well through the ferromagnetic layer and the MgO tunneling layer Also perpendicular to the sample surface. From the analysis of the energy band structure of the wurtzite structure nitride, it can be found that the difference in the upper limit of the circular polarization degree of front emission of the c-plane and m-plane (or a-plane) GaN-based quantum wells. According to the Kane model, the coefficients related to the band-edge transition amplitudes of wurtzite-structured GaN can be calculated as ([3]Chuang, S.L et al., Physical Review B 54.4 (1996).):

where E ₊ has the form:

In the above formula, Δ ₁ is the crystal field splitting energy, and Δ ₂ and Δ ₃ are the splitting energies related to the spin-orbit coupling. Assuming that the electron spins injected into the quantum well do not relax, in the case of the c-plane, the transition of the electron through the conduction to the crystal field splitting band is almost negligible, while the transition of the electron to the light and heavy hole bands will produce different For circularly polarized light in the polarization direction, the transition amplitude ratio of the two is 1:a for GaN. According to the structural parameters of GaN, 1:a≈1:0.96 can be obtained, so the upper limit of the final circular polarization degree is On the other hand, in the case of non-polar plane (m-plane or a-plane) GaN, the transition of electrons to the crystal field splitting band will not be negligible. The transition of electrons from the conduction band to the crystal field splitting band and the heavy hole band will generate circularly polarized light of the same polarity, but the transition from the conduction band to the light hole band will generate circularly polarized light of the opposite polarity. The ratio of the three transition amplitudes is 1:(a+b):(a+b)=1:1:1, and the upper limit of the circular polarization degree is

On the one hand, the actual circular polarization degree of spin LED is related to the upper limit of the circular polarization degree of polarization of the system, and on the other hand, it also depends on the spin relaxation lifetime in the luminescent quantum well. The actual circular polarization degree of light emitted by the spin LED can be expressed as: Pc=Pmax/(1+τe/τs), where Pc is the actual circular polarization degree, Pmax is the upper limit of the system circular polarization degree, τe , τs are the radiative recombination lifetime and spin relaxation lifetime of electrons, respectively. Considering that the polarization electric field that induces Rashba spin-orbit coupling in GaN materials is along the C-axis direction, and the dominant spin relaxation mechanism in GaN materials is the DP mechanism, the electron spin relaxation lifetimes in different polarization directions can be inferred. will be anisotropic. In the embodiment of the present invention, it is proved by means of time-resolved Kerr spectroscopy that when the spin polarization direction is along the m direction, the lifetime is much longer than that when the polarization direction is along the C direction. To sum up, the non-polar plane GaN can not only increase the upper limit of the circular polarization degree, but also increase the spin relaxation lifetime of electrons, both of which are beneficial to the improvement of the actual circular polarization degree.

Description of drawings

FIG. 1 is a schematic structural diagram of an inverted m-plane gallium nitride spin LED;

Figure 2 shows the time-resolved Kerr spectroscopy measurement results of GaN with the spin-polarized direction precessing around the C-axis (A) and the spin-polarized direction parallel to the C-axis (B).

Detailed ways

The technical solution of the present invention is described in detail below with a spin LED structure based on non-polar m-plane GaN, but does not limit the scope of the present invention in any way.

As shown in Figure 1, the structure of the high-polarization spin LED includes, from bottom to top, a non-polar m-plane GaN substrate, a P-type GaN layer, a P-type AlGaN electron blocking layer, and 3-5 cycles of InGaN /GaN quantum well, N-type GaN layer, MgO tunneling layer, ferromagnetic metal Co layer and anti-oxidation Pt layer.

The specific preparation process is as follows:

(1) Select an undoped non-polar m-plane GaN epitaxial substrate with a thickness of 350-500 microns;

(2) A Mg-doped P-type GaN epitaxial layer with a thickness of 500 nm to 800 nm is grown on the m-plane GaN substrate by MOCVD, and the doping concentration is roughly controlled at the order of 10E8/cm ³ ;

(3) Next, a P-type AlGaN electron blocking layer of about 5 nm to 15 nm is grown, and the doping concentration of Mg is controlled at the order of 10E7/cm ³ ;

(4) Next, grow 3-5 cycles of InGaN/GaN quantum wells, the thickness of the potential barrier and potential well in one cycle are 3.5nm-5nm and 10nm-15nm, respectively, and the composition of In in InGaN is 13%-20% ;

(5) Next, a Si-doped N-type GaN epitaxial layer of 50 nm to 150 nm is grown, and the doping concentration is controlled at the order of 10E8/cm ³ ;

(6) Transfer the sample from the MOCVD chamber to the magnetron sputtering chamber, and sputter a layer of MgO tunneling layer of 1 nm to 4 nm first, which is used to relieve the friction between the ferromagnetic metal and the contacting N-type GaN. Conductance mismatch;

(7) Sputtering a layer of 20nm-60nm ferromagnetic thin film Co, and then covering a layer of 10nm-40nm Pt as a protective layer to prevent oxidation of Co in contact with air.

After going through the above process, it is necessary to etch a part of the sample to the depth of the P-type GaN layer by a conventional ultraviolet lithography process. After the etching process is completed, common metal electrodes are drawn from the two polar ends of the spin LED through photolithography and lift-off processes, and the metal electrodes can be Ti/Au films evaporated by electron beams. Finally, the whole sample was annealed under nitrogen atmosphere at 200-400 degrees Celsius to improve the contact between the Co film, the MgO thin film and the N-type GaN layer.

To compare the spin relaxation in the polarization directions along the C and m directions, we measured the spin lifetimes for different spin polarization states in GaN by time-resolved Kerr spectroscopy. The light from the femtosecond laser in the time-resolved Kerr spectroscopy experiment was split into two beams. One beam of light (pump light) is photoelastically converted into circularly polarized light, which is used to excite electrons in GaN to generate spin polarization; the other beam of linearly polarized light (probe light) reaches the surface of the sample after passing through the time delay line, and spins The polarized electrons rotate the plane of linear polarization of this light (the Kerr effect). Finally, the rotation angle of the linear polarization plane under different time delays is detected by a photodetector, thereby realizing the measurement of the spin lifetime. The measurement results are shown in Figure 2. The spin polarization direction excited by the pump light in A is parallel to the C-axis, but by applying a magnetic field perpendicular to the C-axis direction, the polarization directions perpendicular to the C-axis and parallel to the C-axis appear at the same time, so the measured The signal is the superposition of the perpendicular C-axis and the parallel C-axis, resulting in a spin relaxation lifetime of 363 ps. The spin polarization direction excited by the pump light in B is parallel to the C axis and there is no external magnetic field, so the measured signals are all from the polarization direction parallel to the C axis, and the obtained spin relaxation lifetime is 196ps. The spin relaxation is isotropic in the plane perpendicular to the C-axis, so the above measurement results prove that the spin lifetime of the polarization along the m-direction is much larger than that of the polarization along the C-axis.

The above-mentioned embodiments are only to illustrate the technical ideas and characteristics of the present invention, and the descriptions are more specific and detailed, and the purpose is to enable those of ordinary skill in the art to understand the content of the present invention and implement them accordingly. to limit the protection scope of the present invention. It should be pointed out that, for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can also be made, that is, any changes made according to the spirit disclosed in the present invention should still cover within the protection scope of the present invention.

Claims (10)

1. The LED 1. a kind of high polarization degree spins, it is characterized in that the inversion LED structure based on nonpolar face GaN, is successively wrapped from the bottom to top It includes: nonpolar face GaN substrate, p-type GaN layer, p-type AlGaN electronic barrier layer, InGaN/GaN luminescent quantum well layer, N-type GaN Layer, MgO tunnel layer, ferromagnetic layer and heavy metal protective layer.
2. The LED 2. high polarization degree as described in claim 1 spins, which is characterized in that the nonpolar face GaN substrate is the face m GaN Substrate or the face a GaN substrate.
3. The LED 3. high polarization degree as described in claim 1 spins, which is characterized in that the p-type GaN layer is the p-type of Mg doping GaN layer, doping concentration are controlled in 10E8/cm³Magnitude, with a thickness of 500~800nm.
4. The LED 4. high polarization degree as described in claim 1 spins, which is characterized in that the p-type AlGaN electronic barrier layer is Mg The p-type AlGaN layer of doping, doping concentration are controlled in 10E7/cm³Magnitude, with a thickness of 5~15nm.
5. The LED 5. high polarization degree as described in claim 1 spins, which is characterized in that the InGaN/GaN luminescent quantum well layer is The InGaN/GaN Quantum Well in 3~5 periods, in a cycle the thickness of potential barrier and potential well be respectively 3.5~5nm and 10~ 15nm。
6. The LED 6. high polarization degree as described in claim 1 spins, which is characterized in that the N-type GaN layer is Si doped N-type GaN Layer, with a thickness of 50~150nm, the doping concentration of Si is controlled in 10E8/cm³Magnitude.
7. The LED 7. high polarization degree as described in claim 1 spins, which is characterized in that the material of the ferromagnetic layer be Co, CoFe, CoFeB or NiFe；The heavy metal protective layer uses Pt, Ta or Au metal film.
8. The LED 8. high polarization degree as described in claim 1 spins, which is characterized in that the MgO tunnel layer with a thickness of 1~ 4nm, ferromagnetic layer with a thickness of 20~60nm, heavy metal protective layer with a thickness of 10~40nm.
9. 9. the preparation method of any high polarization degree spin LED of claim 1~8, first passes through metallo-organic compound chemistry Vapor deposition method successively epitaxial growth p-type GaN layer, p-type AlGaN electronic barrier layer, InGaN/ on nonpolar face GaN substrate GaN luminescent quantum well layer, N-type GaN layer, are then sequentially prepared MgO tunnel layer, ferromagnetic layer and a huge sum of money by the method for magnetron sputtering Belong to protective layer, the preparation of electrode is finally completed using photoetching process.
10. 10. a kind of method for improving spin LED and going out optical circular polarizing polarizability, using nonpolar face GaN as substrate, and using inverted LED structure.