JP2014183112A - Method for manufacturing semiconductor light-emitting element - Google Patents

Abstract

【Task】
An object of the present invention is to reduce the luminous efficiency when a large current is passed through a semiconductor light emitting device in which an optical semiconductor stack made of a nitride semiconductor is grown on a growth substrate composed of GaN on the surface. It is to suppress.
[Solution]
The semiconductor light emitting device includes a) a step of preparing a growth substrate having at least a surface made of gallium nitride, and b) a step of heating the growth substrate to a crystal growth temperature while supplying ammonia gas to the growth substrate surface. The supply of ammonia gas to the surface of the growth substrate is started from the time when the growth substrate starts to be heated until the crystal growth temperature is reached, and c) is heated to the crystal growth temperature. And a step of growing an optical semiconductor stack including a nitride semiconductor layer having a light emitting property and made of Al _x In _y Ga _z N on the surface of the growth substrate.
[Selection] Figure 2

Description

  The present invention relates to a method for manufacturing a semiconductor light emitting device.

  A semiconductor light emitting device (nitride LED) using a nitride semiconductor such as gallium nitride (GaN) can emit ultraviolet light or blue light, and can emit white light by using a phosphor. it can. Such a nitride LED is used for illumination, for example.

A nitride LED is formed by growing a photosemiconductor stack formed by stacking a plurality of nitride semiconductor layers on a growth substrate. As the growth substrate, for example, a sapphire substrate or a GaN substrate is used. The growth substrate is generally heat-treated in an atmosphere containing ammonia (NH ₃ ) before growing the optical semiconductor stack (for example, Patent Documents 1 and 2).

JP 2001-015808 A JP 2011-198849 A

  In order to realize brighter illumination using the nitride LED, it is necessary to pass a large current through the nitride LED. However, the nitride LED has a problem that when a large current flows, the light emission efficiency decreases (so-called “Drop phenomenon”).

  An object of the present invention is to reduce the luminous efficiency when a large current is passed through a semiconductor light emitting device in which an optical semiconductor stack made of a nitride semiconductor is grown on a growth substrate composed of GaN on the surface. It is to suppress.

According to the main aspect of the present invention, a) preparing a growth substrate having at least a surface made of gallium nitride, and b) supplying an ammonia gas to the growth substrate surface, Heating to the surface of the growth substrate, wherein the supply of ammonia gas to the surface of the growth substrate is started after the growth substrate starts to be heated until the crystal growth temperature is reached; c) the crystal growth Growing a photosemiconductor stack having a light emitting property and including a nitride semiconductor layer made of Al _x In _y Ga _z N on the surface of the growth substrate heated to a temperature. Is provided.

  A decrease in light emission efficiency in the semiconductor light emitting element when a large current is passed is suppressed.

FIG. 1A is a conceptual diagram of a metal organic chemical vapor deposition (MOCVD) apparatus, and FIGS. 1B to 1E are cross-sectional views illustrating how a semiconductor light emitting device is manufactured using the MOCVD apparatus. FIG. 2A is a graph group showing the temperature profile of the growth substrate and the NH ₃ gas supply start timing in the pre-growth treatment step, and FIG. 2B is a graph showing the Drop characteristics of several manufactured semiconductor light emitting devices. .

  With reference to FIG. 1A-FIG. 1E, the manufacturing method of the semiconductor light-emitting device (LED) by an Example is demonstrated. FIG. 1A is a conceptual diagram of a metal organic chemical vapor deposition (MOCVD) apparatus used for manufacturing an LED, and FIGS. 1B to 1E are cross-sectional views illustrating how the LED is manufactured using the MOCVD apparatus.

  Hereinafter, the structure of the MOCVD apparatus used for manufacturing the LED will be described.

  As shown in FIG. 1A, the MOCVD apparatus 100 has a configuration in which, for example, a susceptor 102, a heater 103 for heating, a gas supply unit 104, and a gas exhaust unit 106 are disposed inside a reaction vessel 101.

  The susceptor 102 is supported by a rotating shaft connected to a rotating mechanism, and has a mechanism on which the growth substrate 10 can be placed. By driving the rotation mechanism, the susceptor 102 or the growth substrate 10 placed thereon rotates.

  The heater 103 heats the growth substrate 10 via the susceptor 102. The heater 103 is provided with a control mechanism that monitors the temperature of the susceptor 102 or the growth substrate 10 and maintains the temperature at a desired temperature.

  The gas supply unit 104 supplies various gases to the surface of the growth substrate 10 placed on the susceptor 102. The gas supply unit 104 includes, for example, gas supply sources 105a to 105c, a mass flow controller (MFC), a gas supply pipe (gas injection pipe), and the like. Various gases supplied from the gas supply sources 105a to 105c and adjusted to a desired flow rate by the MFC are supplied to the surface of the growth substrate 10 through the gas supply pipe.

The gas supply source 105a is a supply source of an organic source gas (group III element material gas) such as trimethylgallium (TMG), for example. The gas supply source 105b is a supply source of a reactive gas (Group V element material gas) such as ammonia (NH ₃ ), for example. The gas supply source 105c is a supply source of a carrier gas such as H ₂ or N ₂ , for example. Note that the gas supply source is not limited to these, and an impurity material gas supply source such as disilane (Si ₂ H ₆ ) may be added.

  The gas exhaust unit 106 exhausts various gases supplied from the gas supply unit 104 from the reaction vessel 101. The gas exhaust unit 106 includes, for example, a gas exhaust pipe and an exhaust pump including a pressure control mechanism that controls the pressure in the reaction vessel 101.

  Hereinafter, an LED manufacturing method using such an MOCVD apparatus will be described.

  First, as shown in FIG. 1B, a growth substrate 10 is prepared. The growth substrate 10 has, for example, a two-layer structure in which a GaN layer 12 is formed on the surface of the template substrate 11. As the template substrate 11, for example, a substrate composed of sapphire, Si, SiC, nitride semiconductor (GaN, AlGaN, InGaN, etc.) or the like can be used. The growth substrate 10 may be a GaN substrate having a single layer structure. That is, the growth substrate 10 only needs to have a surface made of GaN.

Next, an optical semiconductor stack made of a nitride semiconductor is grown on the growth substrate 10 by MOCVD. The optical semiconductor stack includes, from the growth substrate 10 side, an n-type semiconductor layer, a strain relaxation layer having a superlattice structure, an active layer (light emitting layer) having a multiple quantum well structure, a cladding layer, a p-type semiconductor layer, and a contact layer. It has a multilayer structure to be laminated. Each layer of the optical semiconductor stack is composed of a nitride semiconductor represented by Al _x In _y Ga _z N (x + y + z = 1).

First, the prepared growth substrate 10 is placed on a susceptor disposed in a reaction vessel of an MOCVD apparatus. Thereafter, the inside of the reaction vessel is replaced with an inert gas, for example, N ₂ gas, and the susceptor (or growth substrate 10) is rotated at, for example, 10 rpm.

Subsequently, as shown in FIG. 1C, while supplying ammonia (NH ₃ ) gas to the surface of the growth substrate 10 (GaN layer 12), the growth substrate 10 is grown at a temperature at which the nitride-based semiconductor grows, specifically 1200. Heat to ° C. The NH ₃ gas supply timing and the like will be described later (see FIG. 2A).

Subsequently, as shown in FIG. 1D, an n-type semiconductor layer 21 is grown on the growth substrate 10. Specifically, while maintaining the temperature of the growth substrate 10 at 1200 ° C., TMG (trimethyl gallium) gas (flow rate: 15 sccm), NH ₃ gas (flow rate: 4 SLM), and disilane (Si ₂ H ₄ ) gas ( (Flow rate: 0.2 sccm) is supplied for 60 minutes to grow an n-type semiconductor layer 21 made of a GaN crystal doped with Si.

Thereafter, a strain relaxation layer 22 having a superlattice structure is grown. Specifically, the temperature of the growth substrate 10 is set to 900 ° C., and TEG (triethylgallium) gas (flow rate: 35 sccm) and NH ₃ gas (flow rate: 4 SLM) are supplied for 50 seconds to grow a GaN layer. Subsequently, TEG gas (flow rate: 35 sccm), TMI (trimethylindium) gas (flow rate: 50 sccm), and NH ₃ gas (flow rate: 4 SLM) are supplied for 50 seconds to grow an InGaN layer. Similar steps are repeated alternately to grow, for example, a strain relaxation layer 22 having a five-layer structure.

Thereafter, an active layer (light emitting layer) 23 having a multiple quantum well structure is grown. Specifically, the temperature of the growth substrate 10 is set to 850 ° C., TEG gas (flow rate: 5 sccm) and NH ₃ gas (flow rate: 4 SLM) are supplied for 150 seconds to grow a GaN layer (barrier layer). Subsequently, TEG gas (flow rate: 5 sccm), TMI gas (flow rate: 90 sccm), and NH ₃ gas (flow rate: 4 SLM) are supplied for 150 seconds to grow an InGaN layer (well layer). Similar processes are repeated alternately to grow, for example, an active layer 23 having a 10-layer structure.

Thereafter, the cladding layer 24 is grown. Specifically, the temperature of the growth substrate 10 is set to 1000 ° C., TMG gas (flow rate: 1 sccm), TMA (trimethylaluminum) gas (flow rate: 1 sccm), NH ₃ gas (flow rate: 4 SLM), and Cp 2 Mg (biscyclohexane). A clad layer 24 made of AlGaN crystal doped with Mg is grown by supplying a pentadienyl magnesium) gas (flow rate: 15 sccm).

Thereafter, the p-type semiconductor layer 25 is grown. Specifically, the temperature of the growth substrate 10 is set to 1100 ° C., and TMG gas (flow rate: 3.5 sccm), NH ₃ gas (flow rate: 4 SLM), and Cp 2 Mg gas (flow rate: 15 sccm) are supplied for 300 seconds. A p-type semiconductor layer 25 made of GaN crystal doped with Mg is grown.

Finally, the contact layer 26 is grown. Specifically, TMG gas (flow rate: 1.75 sccm), NH ₃ gas (flow rate: 4 SLM), and Cp 2 Mg gas (flow rate: 200 sccm) are supplied for 20 seconds while maintaining the temperature of the growth substrate 10 at 1100 ° C. Then, a contact layer 26 made of a GaN crystal doped with Mg is grown.

  As described above, the optical semiconductor stack 20 made of a nitride-based semiconductor is grown on the growth substrate 10. Note that the optical semiconductor stack 20 is not limited to the layer configuration described above. After the optical semiconductor stack 20 is grown, the growth substrate 10 on which the optical semiconductor stack 20 is grown is taken out of the reaction vessel of the MOCVD apparatus.

  Next, as shown in FIG. 1E, a part of the optical semiconductor stack 20 is etched to expose the n-type semiconductor layer 21. For the etching of the optical semiconductor stack 20, for example, a dry etching method using chlorine gas can be employed.

  Thereafter, the n-side electrode 31 is formed on the exposed n-type semiconductor layer 21. A p-side electrode 32 is formed on the contact layer 26. For the formation of the n-side electrode 31 and the p-side electrode 32, for example, a photolithography method or a lift-off method can be employed. The n-side electrode 31 is made of, for example, TiAl. The p-side electrode 32 is configured by a laminated structure of indium tin oxide (ITO) and NiAu, for example.

  Thus, the LED according to the embodiment is completed. In addition, the planar shape of LED is a square shape whose one side is 1 mm, for example.

FIG. 2A is a graph group showing the temperature profile of the growth substrate and the supply start timing of the NH ₃ gas in the step (pre-growth treatment step) shown in FIG. 1C. In the graph shown in the first stage, the horizontal axis indicates time, and the vertical axis indicates the temperature of the growth substrate. In the graphs shown in the second to fourth stages, the horizontal axis represents time, and the vertical axis represents the flow rate of NH ₃ gas.

  The growth substrate 10 is heated from the time point Tr to the time point Tg as shown in the first stage of FIG. 2A in the pre-growth processing step. At this time, the growth substrate 10 is heated from the temperature Hr (room temperature) to the temperature Hg (1200 ° C.). The time (Tg-Tr) from the start of heating the growth substrate 10 until the temperature of the growth substrate 10 reaches the growth temperature (temperature Hg) is, for example, 14 minutes, and the temperature of the growth substrate 10 is about 1 per second. Increased by 4 ° C. Note that the growth of the optical semiconductor stack 20 (n-type semiconductor layer 21) is started at time Tg or later.

The inventor produced three types of samples S1 to S3 having different NH ₃ gas supply start timings in the pre-growth treatment step, and measured the Drop characteristics of the samples S1 to S3. Here, the Drop characteristic refers to a reduction characteristic of the external quantum efficiency (light emission efficiency) of the LED with respect to the current density of the current flowing through the LED (particularly, its active layer). The external quantum efficiency refers to the ratio of the number of photons emitted outside the LED to the number of electrons injected into the active layer of the LED.

The first sample S1 is an LED according to the reference example, and as shown in the second stage of FIG. 2A, the supply of NH ₃ gas is started at the time point Tr. That is, it is a sample in which the supply of NH ₃ gas is started almost simultaneously with the heating of the growth substrate 10. The second and third samples S2 and S3 are the LEDs according to the embodiment, and as shown in the third and fourth stages of FIG. 2A, respectively, the time point between the time point Tr and the time point Tg, specifically the growth substrate. In this sample, the supply of NH ₃ gas was started at time points T1 and T2 when the temperature of 10 became H1 (500 ° C.) and H2 (800 ° C.). The flow rate of NH ₃ gas is, for example, about ₃ SLM in any of the samples S1 to S3.

FIG. 2B is a graph showing the Drop characteristics of samples S1 to S3. The horizontal axis represents the current density (A / cm ² ) of the current supplied to the samples S1 to S3, and the vertical axis represents the external quantum efficiency (arbitrary unit) of the samples S1 to S3. In the figure, the Drop characteristic of the sample S1 is indicated by a solid line. In addition, the Drop characteristics of the samples S2 and S3 are indicated by a one-dot chain line and a broken line, respectively. Note that the Drop characteristics of the samples S1 to S3 are normalized at the maximum value of the external quantum efficiency.

From the graph shown in FIG. 2B, the luminous efficiency of sample S2 (NH ₃ gas supply from time T1) is higher than that of sample S1 (NH ₃ gas supply from time Tr) at a relatively large current density of 15 A / cm ² or more. It can be seen that the decrease is suppressed (the loop characteristics are good). For example, at a current density of 35 A / cm ² , the sample S2 is suppressed from being reduced in luminous efficiency by about 1% than the sample S1 (the luminous efficiency is improved by about 1% compared to the sample S1). In addition, at a current density of 70 A / cm ² , the sample S2 is suppressed from being reduced in luminous efficiency by about 1.5% than the sample S1 (the luminous efficiency is improved by about 1.5% compared to the sample S1). ).

Furthermore, it can be seen from this graph that the sample S3 (NH ₃ gas supply from the time point T2) is more suppressed in light emission efficiency at a relatively large current density of 15 A / cm ² or more than the samples S1 and S2. I understand. For example, at a current density of 35 A / cm ² , the decrease in luminous efficiency of sample S3 is suppressed by about 1.5% compared to sample S1 (the luminous efficiency is improved by about 0.5% compared to sample S2). ing). In addition, at a current density of 70 A / cm ² , the decrease in luminous efficiency of sample S3 is suppressed by about 2.0% compared to sample S1 (the luminous efficiency is improved by about 0.5% compared to sample S2). ing).

From the measurement results of the above Droop characteristics, the pre-growth treatment step (FIG. 1C), than the sample feed was started at the same time the NH ₃ gas and the heating of the growth substrate, the supply of the NH ₃ gas after starting to heat the growth substrate It was found that the sample that started the process has better Drop characteristics. From this measurement result, NH ₃ gas is used at any point in time from when the growth substrate starts to be heated until reaching the growth temperature of the n-type semiconductor layer (temperature at which the nitride semiconductor grows) from the viewpoint of improving the Drop characteristics. It is presumed that it is preferable to start supplying. Further, it is presumed that the NH ₃ gas preferably starts to be supplied when the temperature of the growth substrate is higher.

According to further examination by the present inventor, in consideration of optical characteristics or electrical characteristics other than the Drop characteristics, the NH ₃ gas is supplied at a temperature within the range of 500 ° C. to 850 ° C. of the growth substrate. It has been found preferable to start at a certain point. The supply of the NH ₃ gas, the temperature of the growth substrate is to be started at around the time of the 800 ° C. is a decomposition temperature of the NH ₃ gas was found to be most preferable.

Based on the above results, a mechanism for improving the Drop characteristics by adjusting the supply start timing of the NH ₃ gas will be considered.

When the growth substrate is heated without supplying NH ₃ gas, N (or N ₂ ) evaporates from the growth substrate surface (GaN layer). For this reason, the crystallinity or flatness of the growth substrate surface (GaN layer) deteriorates. The crystallinity or flatness of the growth substrate surface (GaN layer) is considered to have a great influence on various characteristics of LEDs manufactured using the growth substrate, in particular, Drop characteristics.

When the growth substrate is heated while supplying NH ₃ gas to the growth substrate surface (GaN layer), it is considered that N of the NH ₃ gas supplements N evaporated from the growth substrate surface (GaN layer). However, if the temperature of the growth substrate starts to supply NH ₃ gas at a relatively low point, without the NH ₃ gas is decomposed, since NH ₃ molecules attack the growth substrate surface (GaN layer), the growth substrate surface ( It is considered that deterioration of crystallinity or flatness in the GaN layer is not effectively suppressed.

When the supply of NH ₃ gas is started when the temperature of the growth substrate is relatively high (500 ° C. to 850 ° C., particularly 800 ° C.), the NH ₃ gas is decomposed into N (or N ₂ ) and H (or H ₂ ). Therefore, it is considered that the attack to the growth substrate surface (GaN layer) by NH ₃ molecules is reduced. Further, since the growth substrate surface (GaN layer) is effectively supplemented with N decomposed from the NH ₃ gas, deterioration of crystallinity or flatness on the growth substrate surface (GaN layer) is effectively suppressed. it is conceivable that. For this reason, it is considered that the LED manufactured by starting supplying NH ₃ gas at a time when the temperature of the growth substrate is relatively high (500 ° C. to 850 ° C., particularly 800 ° C.) has a favorable improvement in the Drop characteristics.

  As mentioned above, although this invention was demonstrated along the Example and the modification, this invention is not limited to these. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 10 ... Growth substrate, 11 ... Template substrate, 12 ... GaN layer, 20 ... Optical semiconductor lamination | stacking, 21 ... N-type semiconductor layer, 22 ... Strain relaxation layer, 23 ... Active layer (light emitting layer), 24 ... Cladding layer, 25 ... p-type semiconductor layer, 26 ... contact layer, 31 ... n-side electrode, 32 ... p-side electrode, 100 ... MOCVD apparatus, 101 ... reaction vessel, 102 ... susceptor, 103 ... heating heater, 104 ... gas supply unit, 105a- 105c ... Gas supply source, 106 ... Gas exhaust part.

Claims (5)

a) preparing a growth substrate having at least a surface composed of GaN;
b) a step of heating the growth substrate to the crystal growth temperature while supplying ammonia gas to the growth substrate surface, wherein the ammonia gas is supplied to the growth substrate surface after the growth substrate starts to be heated. A process that starts before the crystal growth temperature is reached;
c) growing an optical semiconductor stack including a nitride semiconductor layer made of Al _x In _y Ga _z N on the surface of the growth substrate heated to the crystal growth temperature;
A method for manufacturing a semiconductor light emitting device having   2. The semiconductor light emitting device according to claim 1, wherein in step b), the supply of ammonia gas to the growth substrate surface is started when the temperature of the growth substrate reaches a temperature in the range of 500 ° C. to 850 ° C. 3. Production method.   The method for manufacturing a semiconductor light emitting device according to claim 1, wherein the growth substrate is composed of a GaN substrate.   The growth substrate includes a template substrate made of at least one material selected from the group consisting of sapphire, Si, SiC, GaN, InGaN and AlGaN, and a gallium nitride layer formed on the template substrate surface. A method for producing a semiconductor light emitting device according to claim 1 or 2.   5. In the step c), the optical semiconductor stack is grown by metal organic chemical vapor deposition using an organic source gas as a group III element material gas and an ammonia gas as a group V element material gas. A method for producing a semiconductor light emitting device according to claim 1.