JP2004015072A - Nitride semiconductor light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the output of a nitride semiconductor light emitting element containing In (indium) in a well layer. <P>SOLUTION: In the nitride semiconductor light emitting element having an active layer on a substrate according to the present invention, the active layer includes the well layer and a barrier layer and at the same time, includes an intermediate layer between the well layer and the barrier layer arranged in order from the substrate side, the well layer contains In, and the band gap energy of the intermediate layer is equal to or higher than that of the barrier layer. By thus providing the intermediate layer between the well layer and the barrier layer, the crystallinity of the barrier layer becomes excellent and the output of light emission is improved. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a light emitting diode (LED), a laser diode (LD), a superluminescent diode (SLD), etc. composed of a nitride semiconductor (InXAlYGa1-X-YN, 0≤X, 0≤Y, X + Y≤1). The present invention relates to an element and a light emitting device such as a signal lamp and a full color display using the light emitting element.

Nitride semiconductors have been put into practical use as blue LEDs since the end of 1993, and subsequently put into practical use as green LEDs in the middle of 1994. Together with GaAs and AlInGaP-based red LEDs, they have already been installed in various places as full-color displays. The current blue LED and green LED have an active layer composed of a single quantum well structure having one InGaN well layer or a multiple quantum well structure in which an InGaN well layer and a GaN or InGaN barrier layer are stacked. It has a double hetero structure sandwiched between the mold nitride semiconductors. The emission wavelength of the LED is determined by increasing or decreasing the In composition of the well layer.

It is known that, in a wide-gap semiconductor such as a nitride semiconductor, the emission region changes from ultraviolet to red by changing the In composition, which is one of the compositions of the active layer. A technique of forming a plurality of active layers having different In compositions to emit light of multiple colors with a single light emitting element is disclosed in, for example, Japanese Patent Application Laid-Open No. 7-183576. In addition, display devices using three types of LED elements, blue LED, green LED, and red LED, have already been installed in various places. The blue component and the green component of this display device are made of a nitride semiconductor.

In addition to LEDs, we announced that we fabricated a nitride semiconductor laser device containing an active layer on a nitride semiconductor substrate and achieved more than 10,000 hours of continuous oscillation at room temperature for the first time in the world ( ICNS'97 Proceedings, October 27-31, 1997, P444-446, and Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L1568-1571, Part 2, No. 12A, 1 December 1997).

As described above, blue and green colors have already been put to practical use in LEDs using nitride semiconductors. For example, the output of a blue LED at 20 mA is about 5 mW, and that of a green LED is about 3 mW. Further, in the case of a nitride semiconductor, the output tends to decrease as the wavelength becomes longer in the order of yellow-green, yellow, and orange. This is because the crystallinity of the well layer deteriorates as the In composition of the well layer increases.

For example, a signal lamp requires yellow or yellow-orange, but when trying to realize yellow with a nitride semiconductor, the output is somewhat insufficient compared to the outputs of other green LEDs and red LEDs. In a full-color display, lower power consumption can be realized if the output of the green LED is improved. The present invention has been made in view of such circumstances, and an object of the present invention is to improve the output of a nitride semiconductor light emitting device containing In in a well layer.

The present invention provides a nitride semiconductor light emitting device having an active layer on a substrate, wherein the active layer has a well layer and a barrier layer, and further has an intermediate layer between the well layer and the barrier layer arranged in order from the substrate side. Wherein the well layer contains In, and the intermediate layer has the same or larger band gap energy as the barrier layer.

The well layer is preferably composed of InXGa1-XN (0 <X <1), and its thickness is preferably 100 Å or less, more preferably 70 Å or less, and most preferably 50 Å or less. An element is obtained. In addition, it may be made of ternary mixed crystal InAlN and quaternary mixed crystal InAlGaN.

(4) The barrier layer is formed of a nitride semiconductor having a thickness larger than that of the well layer or having a large band gap energy, and is preferably InYGa1-YN (0 ≦ Y <1, Y <X). The preferred thickness of the barrier layer is less than 200 angstroms, more preferably less than 100 angstroms, and most preferably less than 70 angstroms. Further, the barrier layer may be made of ternary mixed crystal InAlN and quaternary mixed crystal InAlGaN having a larger band gap energy than the well layer.

In an active layer having a well layer made of a nitride semiconductor containing In, the number of well layers is increased as the wavelength is increased, that is, as the In composition of the well layer is increased. This relates to the crystallinity of the well layer. For example, in InGaN, as the In composition increases, the crystallinity deteriorates, and the output of the light emitting element tends to decrease. However, when a barrier layer having a larger band gap energy than the well layer is stacked on the well layer, the crystallinity of the well layer is improved by the barrier layer. That is, since the barrier layer has a smaller amount of In than the well layer, the crystallinity is good, and the crystallinity of the entire active layer is improved by stacking the barrier layer. Therefore, in a well layer having a large In composition, by increasing the number of the well layers and simultaneously increasing the number of the barrier layers, the light emission output is improved as compared with a well layer having a small number of the well layers. In addition, when the In composition of the well layer is increased, the strain increases and the light is not emitted even by the piezo effect. For this reason, the number of well layers is increased, the strain related to each well layer is reduced, and the piezo effect is alleviated, so that there is an effect of further shining.

According to the present invention, further, by inserting an intermediate layer having the same or larger band gap energy as that of the barrier layer between the well layer and the barrier layer, the crystallinity of the barrier layer is improved and the light emission output is improved. Is improved.

FIG. 1 is a schematic sectional view showing the structure of the LED element according to the present embodiment. Hereinafter, the first embodiment will be described with reference to FIG.

A nitride semiconductor substrate 1 made of 1-inch square Si-doped GaN is prepared. This nitride semiconductor substrate 1 is grown as follows.
(Nitride semiconductor substrate 1)
A 2 inch φ heterogeneous substrate 1 made of sapphire having a C-plane as a main surface is set in a MOVPE reaction vessel, the temperature is set to 500 ° C., and a buffer made of GaN using trimethylgallium (TMG) and ammonia (NH3) is used. The layer is grown to a thickness of 200 Å. After the growth of the buffer layer, the temperature is raised to 1050 ° C., and an underlayer of GaN is grown to a thickness of 4 μm.

After the growth of the underlayer, the wafer is taken out of the reaction vessel, and a protective film made of SiO2 having a stripe width of 10 μm and a stripe interval (window portion) of 2 μm is formed on the surface of the underlayer. After the formation of the protective film, the wafer is set again in the MOVPE reaction vessel, the temperature is set to 1050 ° C., and an undoped GaN layer is grown to a thickness of 5 μm using TMG and ammonia to cover the surface of SiO 2. After the growth, the wafer is transferred from the MOVPE apparatus to the HVPE apparatus, and an n-type GaN layer doped with 1 × 10 18 / cm 3 of Si is grown to a thickness of 200 μm using Ga metal, ammonia, HCl and silane gas. After the growth, the sapphire substrate is polished from the sapphire substrate side to remove the sapphire substrate, the buffer layer, the underlayer, and the protective film, thereby producing a nitride semiconductor substrate 1 made of Si-doped GaN having a total thickness of 170 μm. It is desirable that the Si concentration of Si-doped GaN be adjusted in the range of 5 × 10 17 to 1 × 10 19 / cm 3.
(Buffer layer 2)
The nitride semiconductor substrate 1 manufactured as described above is transferred to a MOVPE apparatus, and ammonia and TMG are used, and silane gas is used as an impurity gas. On the surface of the nitride semiconductor substrate on the AS-GROWN side, 1 × 10 18 Si is applied at 1050 ° C. A buffer layer 2 made of GaN doped with / cm 3 is grown to a thickness of 2 μm. When a nitride semiconductor such as a substrate having a thickness of 100 μm or more is grown on a heterogeneous substrate made of a material different from the nitride semiconductor in this way, and then the heterogeneous substrate is removed to produce a nitride semiconductor substrate, If GaN is first grown to a thickness of 10 μm or less on the AS-GROWN surface of the nitride semiconductor substrate (the side opposite to the side from which the heterogeneous substrate is removed) to form a buffer layer, then the crystallinity of the nitride semiconductor to be grown next is good. Tend to be.
(Active layer 3)
Next, at 800 ° C., a barrier layer made of n-type In0.2Ga0.8N doped with 1 × 10 18 / cm 3 of Si is grown to a thickness of 100 Å. Subsequently, at 750 ° C., a well layer made of undoped In0.4Ga0.6N is grown to a thickness of 30 Å. Next, while maintaining the temperature at 750 ° C., TMA is added to the source gas, and an intermediate layer made of undoped Al 0.1 Ga 0.9 N is grown to 10 Å. Next, the temperature is raised to 800 ° C., and a barrier layer made of n-type In0.2Ga0.8N doped with 1 × 10 18 / cm 3 of Si is grown to 100 Å.

After growth of the barrier layer, the temperature is lowered to 750 ° C., and then a well layer of undoped In0.4Ga0.6N is grown to a thickness of 30 Å, followed by an intermediate layer of undoped Al0.2Ga0.8N at 750 ° C. The layer is grown for 10 Å, and then the temperature is raised to 800 ° C. to grow the barrier layer made of Si-doped In0.01 Ga0.99 N to 100 Å.

活性 Thus, an active layer 3 having a multiple quantum well structure (three well layers) having a total film thickness of 800 angstroms in which barrier + (well + intermediate + barrier) × 3 is stacked is grown.

In the active layer 3, between the well layer made of InGaN and the barrier layer having a larger band gap energy or a larger thickness than the well layer, the barrier layer has a band gap energy equal to or less than 30 angstroms, It is desirable to grow a larger intermediate layer made of AlGaN or GaN (Al x Ga 1 -xN, 0 ≦ x <1). Generally, the decomposition temperature of the intermediate layer is higher than that of the well layer. Therefore, when a barrier layer made of GaN, InGaN (in this case, the In composition ratio is smaller than that of the well layer) or the like is grown on the intermediate layer having a high decomposition temperature, when the barrier layer is grown as a thick film, Crystallinity is improved. In addition, when a nitride semiconductor layer or a GaN layer containing Al exists between the well layer and the barrier layer, the light emission starting voltage tends to decrease. Therefore, by growing an active layer having a multiple quantum well structure in which a well layer + intermediate layer + barrier layer is repeatedly stacked, a device having a long wavelength can be obtained, and the output of the device is improved.
(P-side cladding layer 4)
Subsequently, a p-side cladding layer 4 of p-type Al0.05Ga0.95N doped with Mg at 1.times.10.sup.19 / cm.sup.3 is grown at 850.degree. The p-side cladding layer 4 can be omitted.
(P-side contact layer 5)
Finally, a p-side contact layer 5 made of p-type GaN doped with Mg at 1 × 10 20 / cm 3 at 850 ° C. is grown to a thickness of 500 Å. The p-side contact layer can be composed of p-type InXAlYGa1-X-YN (0≤X, 0≤Y, X + Y≤1). Preferably, if GaN or InGaN doped with Mg is used, the p-electrode 6 The most favorable ohmic contact is obtained. It is desirable that the Mg concentration be higher than that of the p-side cladding layer 4 in order to lower Vf.

After the growth, the wafer is taken out of the reaction vessel, and a transmissive p-electrode 6 made of Ni / Au for ohmic purposes is formed on the surface of the uppermost p-side contact layer 5 to a thickness of 200 Å, and Au is formed thereon. A p pad electrode 7 for bonding is formed. On the other hand, an n-electrode 8 made of Ti / Al is formed on almost the entire back surface of the nitride semiconductor substrate.

(5) After the electrodes were formed, the LED chip was separated into rectangular chips, which showed green light emission of 520 nm at 20 mA, a forward voltage of 3.2 V, and a light emission output of 4.2 mW.

On the other hand, for comparison, the LED element in which the composition of the well layer was the same and the active layer of a single quantum well structure composed of one well layer was grown, the forward voltage at 20 mA was 3.4 V, and the light emission output was It was 3.5 mW.

In addition, apart from the above-mentioned green LED element, when the LED active layer 3 is grown in the step of manufacturing the above-mentioned LED element, the composition of the well layer is In0.15Ga0.85N, and a single well layer consisting of only one well layer is formed. A blue LED having a quantum well structure is manufactured. This blue LED had a forward voltage of 3.4 V at 20 mA and an emission output of 7 mW. As described above, when the structure is the same, the output is improved by increasing the number of well layers as the wavelength becomes longer.

{Furthermore, separately from the above-mentioned green LED element, the step of growing the LED active layer 3 in the step of manufacturing the above-mentioned green LED element is performed as follows. That is, an n-type In0.2Ga0.8N barrier layer doped with 1.times.10.sup.18 / cm3 of Si is grown to a thickness of 100 .ANG., And a well layer of undoped In0.6Ga0.4N is grown at 750.degree. Let it. Next, while maintaining the temperature at 750 ° C., TMA is added to the source gas, and a second nitride semiconductor layer made of undoped Al0.2Ga0.8N is grown to 10 Å. Next, the temperature was raised to 800 ° C., and a barrier layer (third nitride semiconductor layer) made of n-type In0.2Ga0.8N doped with 1 × 10 18 / cm 3 of Si was grown to 100 Å, and barrier + (well + The active layer 3 having a multiple quantum well structure (the number of well layers: 5) having a total film thickness of 800 Å in which (second + barrier) × 5 is stacked is grown. This LED element emitted red light of 650 nm, and had a forward voltage of 3.2 V and an emission output of 1.5 mW at 20 mA.

On the other hand, a red LED having a single quantum well structure having the same composition of InGaN well layers had a forward voltage of 3.5 V and an emission output of 0.5 mW.

A full-color LED display was manufactured using the blue LED of the well layer 1, the green LED of the well layer 3, and the red LED of the well layer 5 obtained as described above. Compared to a display using a blue LED, a green LED, and a red LED, white luminance was improved by 1.2 times or more, and power consumption was reduced by 10% or more.

In the first embodiment, the following steps are performed when growing the active layer of the green LED. That is, at 800 ° C., a barrier layer made of n-type GaN doped with 1 × 10 18 / cm 3 of Si is grown to a thickness of 100 Å, and then at 750 ° C., a well layer made of undoped In0.4Ga0.6N (the 1 nitride semiconductor layer) is grown to a thickness of 30 Å. Next, while maintaining the temperature at 750 ° C., a second nitride semiconductor layer made of undoped GaN is grown by 10 Å. Next, the temperature is raised to 800 ° C., and a barrier layer (third nitride semiconductor layer) made of n-type GaN doped with 1 × 10 18 / cm 3 of Si is grown to 100 Å.

Others A green LED was obtained in the same manner as in Example 1, and a green LED having substantially the same characteristics as the green LED of Example 1 was obtained.

Further, a full-color LED display was manufactured using this green LED, the blue LED obtained in Example 1, an AlInGaP-based red LED with an output of 5 mW, and the like. With a small number of red LEDs, the white luminance was doubled and the power consumption was further reduced.

FIG. 2 is a schematic sectional view showing another embodiment of the present invention. This figure shows a nitride semiconductor device capable of emitting multicolor light in the same device, in which an n-type contact layer 11 is laminated on a substrate 10, and independent blue light-emitting portions 12B, 13B are provided on the n-type contact layer 11. , 14B, green light emitting portions 12G, 13G, 14G and red light emitting portions 12R, 13R, 14R. Reference numeral 12 denotes an n-type cladding layer, 13 denotes an active layer containing InGaN, and 14 denotes a p-type cladding layer, each of which has a double hetero structure. Note that 15 is a p-electrode and 16 is an n-electrode. The blue light emitting portion active layer 13B has a multiple quantum well structure having two InGaN well layers, the green light emitting portion active layer 13G has a multiple quantum well structure having three InGaN well layers, and a red light emitting portion active layer. 13R has a multiple quantum well structure having five InGaN well layers. The In composition of each well layer is adjusted to be longer as the wavelength becomes longer. The present invention can be applied to a multicolor light emitting element having a plurality of active layers in the same element.

In addition, although the LED element has been mainly described in this specification, it goes without saying that the present invention can be applied not only to an LED but also to a laser element, an SLD element, and the like.

FIG. 1 is a schematic cross-sectional view showing the structure of an LED element according to one embodiment of the present invention. FIG. 9 is a schematic cross-sectional view illustrating a structure of an LED element according to another embodiment of the present invention.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 ... Nitride semiconductor substrate 2 ... Buffer layer 3 ... Active layer 4 ... p-side cladding layer 5 ... p-side contact layer 6 ... p-electrode 7 ... p-pad electrode 8 ..... n electrode

Claims (1)

In a nitride semiconductor light emitting device having an active layer on a substrate,
The active layer has a well layer and a barrier layer, and further has an intermediate layer between the well layer and the barrier layer arranged in order from the substrate side,
The nitride semiconductor light emitting device, wherein the well layer contains In, and the intermediate layer has the same or larger band gap energy as that of the barrier layer.

Images