RU2277736C1 - Semiconductor element emitting light in visible-spectrum blue region - Google Patents

Abstract

FIELD: semiconductor light-emitting devices such as diodes.

SUBSTANCE: proposed invention relates, in particular, to light-emitting diodes built around broadband compounds of A^IIIB^V type that function to emit light in blue region of visible spectrum whose wavelength is 450 - 500 nm. Element structure incorporates substrate, buffer layer made of nitride material, n-contact layer made of Si-doped GaN, active layer with two or more quantum wells made of In_xGa_{1 - x}N, and barriers separating these wells and made of Si-doped nitride material, emitter layer made of Mg-doped Al_YGa_{1 - y}N, and p-contact layer made of Mg-doped nitride material. Molar fraction of indium X linearly reduces in In_xGa_{1 - x}N through thickness of quantum wells from 0.35 to 0.1 toward n-contact layer. Molar fraction of aluminum Y in emitter layer composition linearly reduces from its maximum value of 0.3 + Z on emitter layer surface bordering the active layer to Z on emitter layer surface bordering p-contact layer, where Z is molar fraction of aluminum in p-contact layer composition using Al_ZGa_{1 - Z}N as nitride material, where 0 ≤ Z ≤ 0.1. Active layer is doped with Si, where Si concentration is minimum 5 x 10¹⁸cm^-1; thickness "h₁" of active-layer quantum wells is 1.5 ≤ h₁ ≤ 3 nm; thickness "h₂" of barriers separating quantum wells is 5 ≤ h₂ ≤ 15 nm, and thickness "h_c" of emitter layer is 10 ≤ h_c ≤ 30 nm.

EFFECT: enhanced efficiency of light-emitting diodes.

1 cl, 1 dwg, 1 tbl

Description

The invention relates to the field of semiconductor emitting devices, and more particularly to LEDs based on wide-gap nitride compounds of type A ^III B ^V.

Known semiconductor light emitting element containing a substrate, a buffer layer, an n-contact layer with high conductivity, doped with silicon, an active layer comprising a structure of several quantum wells, barrier layers and a p-contact layer, US 6,515,313.

This technical solution provides a local decrease in the electric fields generated by polarization charges at the boundaries of layers with different compositions, and potentially contributes to an increase in the internal quantum efficiency of the structure. However, a simple decrease in polarization charges at heterointerfaces does not solve the main problem of creating an effective light-emitting element, since the efficiency of a light device in a complex way depends on the nature of the functioning of all layers of the structure.

A semiconductor element emitting light in the blue region of visible light is also known, the structure of which consistently includes a sapphire substrate, a buffer layer made of nitride material (AlN), an n-contact layer made of GaN doped with Si, an n-emitter layer of AlGaInN , an active layer containing one or more quantum wells made of nitride material and separated by barriers made of nitride material (AlGaInN), an emitter layer made of Al and Ga nitrides, doped with Mg, and a p-contact layer made of yielded from a nitride material doped with Mg, US 6,265,726.

In this design, to increase the quantum efficiency of the LED, the molar fractions of aluminum, gallium, and indium in the composition of the layers are chosen in such a way as to reduce the mismatch of the constant crystal lattices of the overlying layers with respect to the n-contact layer and thereby reduce the polarization charges at the heterointerfaces. In addition, the composition and concentration of dopants in the emitting active layer and adjacent layers are chosen so as to increase the likelihood of radiative recombination, mainly by increasing the density of the number of carriers in the active layer.

This technical solution is taken as a prototype of the present invention.

However, an increase in the concentration of dopants has a natural limit, which does not allow a sufficiently high level of concentration of charge carriers in the case of layers with hole type conductivity due to the high activation energy of the acceptor impurity. In this case, the relatively low hole mobility also prevents the increase in the injection of holes into the active layer.

The present invention is based on the solution of the problem of increasing the level of concentration of charge carriers in layers with hole type conductivity and increasing the efficiency of injection of holes into the active layer.

According to the invention, this problem is solved due to the fact that in a semiconductor element emitting light in the blue region of the visible spectrum, the structure of which consistently includes a substrate, a buffer layer made of nitride material, an n-contact layer made of GaN doped with Si, an active layer with two or more quantum wells made of In _X Ga _1-X N and separating these wells with barriers made of nitride material doped with Si, an emitter layer made of Al _Y Ga _1-Y N doped with Mg, and p contact layer, vol ying of nitride material doped with Mg, in the In _X Ga _1-X N X mole fraction of indium is linearly reduced thickness of the quantum wells from 0.35 to 0.1 in the direction from the n-contact layer, emitter layer composed of the mole fraction of aluminum Y decreases linearly from the maximum value of 0.3 + Z on the surface of the emitter layer adjacent to the active layer to the value Z, on the surface of the emitter layer adjacent to the p-contact layer, where Z is the molar fraction of aluminum in the composition of the p-contact layer, in the nitride material of which is used Al _Z Ga1- _Z N, where 0≤Z≤0,1 while the active layer is doped with Si, with a Si concentration of at least 5 · 10 ¹⁸ cm ^-3 , the thickness "h ₁ " of the quantum wells of the active layer is 1.5≤h ₁ ≤3 nm, the thickness of the "h ₂ " barriers separating quantum well, is 5≤h ₃ ≤15 nm, and the thickness of the "h _e " emitter layer is in the range of 10≤h _e ≤30 nm.

The applicant has not identified sources containing information about technical solutions identical to the present invention, which allows us to conclude that it meets the criterion of "novelty."

In the proposed design, the gradient composition of the emitter layer (the molar fraction of Al is maximum on the surface of the emitter layer adjacent to the active layer, and then decreases in thickness of the emitter layer to its boundary with the p-contact layer) provides an increase in the hole concentration at the boundary of the active layer, since the composition gradient the emitter layer leads to an additional distributed polarization p-doping near the active layer.

The proposed thickness of the emitter layer from 10 to 30 nm, firstly, allows you to effectively limit the penetration of electrons into the p-contact layer, while not substantially reducing the injection of holes into the active layer, and, secondly, eliminates stress relaxation in the semiconductor element by cracking emitter layer.

To improve the spectral characteristics of radiation in an optimized structure, the width of the quantum well is limited to 1.5-3 nm. The use of quantum wells with a width of up to 3 nm in the structure allows one to get rid of the second electronic level in the quantum well and to obtain a narrow emission spectrum, but with a thickness of less than 1 nm the efficiency of carrier capture into the quantum well decreases. The choice of the width of the quantum well within the specified limits gives good spectral characteristics of the device, and a certain decrease in the efficiency of carrier capture into a narrow quantum well is compensated by the fact that the number of quantum wells should not be less than two. The thickness of the barriers separating the quantum wells is chosen in such a way as, firstly, to significantly limit the tunneling current between the wells, which negatively affects the efficiency of the device, and secondly, to prevent deterioration of the current-voltage characteristics of the device.

The composition gradient creates a pulling field, which contributes to a better injection of charge carriers into the active layer, and also, which is essential for materials based on nitrides of the third group, it creates a distributed polarization charge in the bulk of the material.

Thus, a significant additional polarization p-doping occurs near and inside the active region, which helps to increase the quantum efficiency of the proposed LED structure.

An increase in the energy barrier for electrons in the emitter layer with a high AlN content prevents the penetration of electrons into the p layers, which significantly reduces the probability of nonradiative recombination of charge carriers. Moreover, the proposed composition of the emitter layer and its geometric characteristics make it possible to exclude a noticeable deterioration in the current-voltage characteristics of the semiconductor element.

The applicant has not found any sources of information containing information about the impact of the claimed distinctive features on the technical result achieved as a result of their implementation. This, according to the applicant, indicates that this technical solution meets the criterion of "inventive step".

The invention is illustrated in the drawing, which shows a diagram of the layered structure of the semiconductor element in cross section.

The semiconductor element in a specific embodiment, in all examples, has a structure that includes in series:

- a substrate 1 made of sapphire with a thickness of 500 μm;

- buffer layer 2 of AlN, a thickness of 20 nm;

- n-contact layer 3, made of GaN, doped with silicon with a concentration of 2-5 · 10 ¹⁸ cm ^-3 , a thickness of 3 μm;

- active layer 4, containing two quantum wells 2 nm thick, made of In _X Ga _{1- ×} N, where the molar fraction of indium X linearly decreases along the thickness of the quantum wells in the direction from the n-contact layer from 0.35 to 0.1 , and a barrier between them, 10 nm thick, made of GaN and doped with silicon with an atom concentration of 5 · 10 ¹⁸ cm ^-3 ;

- an emitter layer 5 doped with magnesium with a concentration of 10 ¹⁹ cm ^-3 , made of Al _y Ga _1-y N, where the molar fraction of aluminum Y linearly decreases from the value of 0.3 + Z on the surface of layer 5 bordering layer 4 to Z values on the surface of layer 5 adjacent to the p-contact layer 6;

- p-contact layer 6, made of Al _z Ga _1-z N, where Z is the molar fraction of aluminum doped with magnesium with a concentration of 5 · 10 ¹⁹ cm ^-3 , a thickness of 100 nm.

The semiconductor element is a two-sided LED heterostructure with a variable composition of quantum wells and emitter layer, which allows to obtain an internal efficiency of 90% in a wide range of current densities from 10 ^-4 · 10 ³ A / cm ² , with a dislocation density of ~ 10 ⁸ -10 ⁹ cm ^-2 . It should be noted that an increase in the dislocation density in the structure leads to a significant decrease in its internal efficiency.

For testing, heterostructures were grown on a sapphire substrate by the method of MOS hydride epitaxy at subatmospheric pressure and temperatures from 1000 to 1100 ° С, InGaN quantum wells were grown at temperatures of 700-800 ° С, n-contact layers were doped with Si to a concentration of 5 × 10 ¹⁸ cm ^{- 3} , which was established using SIMS (secondary ion mass spectrometry). The active layer was doped with Si to a concentration of 2 · 10 ¹⁹ cm ^-3 , the emitter and p-contact layers were doped with Mg to a concentration of 5 · 10 ¹⁹ cm ^-3 .

After the growth process, the structure was subjected to ion etching in order to form a mesa to a depth corresponding to the level of the n-contact layer. Then, the n and p contacts, which are multilayer metal compositions, respectively Ti / Al / Pt / Au and Ni / Au, were applied to the etched and remaining parts of the structure, respectively. The contacts were burned in an atmosphere of nitrogen at a temperature of 850 ° C for 30 seconds.

Further, individual LEDs were cut from the structure, which were mounted on the heat sink with the p-contact down, and gold electrodes were soldered to them to supply electric current.

To study the luminescent characteristics of LEDs, a KSVU-12 spectrometer was used. An FEU-100 photomultiplier was used as a detector. The signal from the photomultiplier, through the Sch1413 digital voltmeter, was transmitted to a computer for final processing.

The accuracy of radiation intensity measurements was no worse than 0.02%.

To measure the external efficiency of the semiconductor element, a calibrated photodetector based on amorphous Si: H (doped with hydrogen) was used. The measurements were carried out with a fixed experimental geometry, which made it possible to quantitatively compare the radiation of various samples.

The electroluminescence of LEDs was measured when radiation was output through a sapphire substrate.

In examples 1-7, the width of the quantum well is from 1 to 3 nm, the width of the emitter layer varies from 10 to 30 nm, the molar fraction of aluminum in the composition of the emitter layer on the surface adjacent to the active layer is from 0.4 to 0.2, and the molar fraction of aluminum in the p-contact layer in all examples except example 4 was chosen equal to zero, in example 4 the molar fraction Z = 0.1.

The characteristics of semiconductor light-emitting elements obtained as a result of the tests are shown in Table 1.

Table 1 Example Number Semiconductor element parameters Internal quantum efficiency at a current density of 1 to 10 ² A / cm ² The half-width of the emission spectrum (nm) Peak of emission spectrum (nm) one Width of quantum wells 1 nm Emitter Layer Width 0.72 7 420 20 nm Al share in
emitter layer 0.3-0 2 Width of quantum wells 2 nm Emitter Layer Width 0.78 8 456 20 nm Al share in
emitter layer 0.2-0 3 Width of quantum wells 2 nm Emitter Layer Width 0.95 12 456 20 nm Al share in the composition
emitter layer 0.3-0 four Width of quantum wells 2 nm Emitter Layer Width 0.85 fourteen 440 20 nm Al share in
emitter layer 0.4-0.1 5 Width of quantum wells 2 nm Emitter Layer Width 0.83 8 450 10 nm Al share in
emitter layer 0.3-0 6 Width of quantum wells 2 nm Emitter Layer Width 0.84 10 456 30 nm Al share in
emitter layer 0.3-0 7 Width of quantum wells 3 nm Emitter Layer Width 0.81 eighteen 465 30 nm Al share in
emitter layer 0.3-0

The above examples confirm the high radiation efficiency in the blue part of the visible spectrum.

To implement the method used standard industrial equipment, which determines the compliance of the invention with the criterion of "industrial applicability".

Claims (1)

A semiconductor element emitting light in the blue region of the visible spectrum, the structure of which consistently includes a substrate, a buffer layer made of nitride material, an n-contact layer made of GaN doped with Si, an active layer with two or more quantum wells made of In _X Ga _1-X N, and with barriers separating these wells made of nitride material doped with Si, an emitter layer made of Al _Y Ga _1-y N doped with Mg, and a p-contact layer made of nitride material doped with Mg , featuring ysya in that the In _X Ga _1-X N X mole fraction of indium is linearly reduced thickness of the quantum wells from 0.35 to 0.1 in the direction from the n-contact layer linearly decreases in the composition of the emitter layer aluminum mole fraction Y of the maximum values of 0.3 + Z on the surface of the emitter layer adjacent to the active layer to the value of Z on the surface of the emitter layer adjacent to the p-contact layer, where Z is the molar fraction of aluminum in the composition of the p-contact layer, the nitride material of which was used Al _Z Ga _1-Z N, 0≤Z≤0,1, wherein the active layer is doped with Si conc Si ntratsiey not less than 5 × 10 ¹⁸ cm ^3, the thickness «h _1" of the quantum wells of the active layer is 1,5≤h ₁ ≤3 nm thickness «h _2" barriers between quantum wells is 5≤h ₂ ≤15 nm and the thickness “h _e ” of the emitter layer is in the range of 10≤h _e ≤30 nm.