JP5120861B2 - Optical semiconductor device and manufacturing method thereof - Google Patents

Description

  The present invention relates to an optical semiconductor device and a method for manufacturing the same, and more particularly, to a deep ultraviolet light emitting device technology using a III-V group compound semiconductor.

  In recent years, research and development have been conducted in the field of semiconductor light emitting devices such as light emitting diodes and laser diodes in which nitride semiconductors containing nitrogen as a group V element use pn junctions. Nitride semiconductors such as AlN, GaN, and InN are direct transition type semiconductors. Furthermore, in the case of ternary mixed crystals and quaternary mixed crystals, the composition is appropriately set and the band gap is changed to change the depth from the infrared. It has a feature that it can emit light up to ultraviolet.

  Further, a semiconductor light-emitting element that emits light in the ultraviolet region using AlGaInN quaternary mixed crystal as a material for the light-emitting layer has attracted attention (see, for example, Patent Document 1). Although the AlGaInN layer contains In, it is reported that the emission peak wavelength can be set in a wavelength region of 360 nm or less, and the internal quantum efficiency can be improved to the same extent as the InGaN layer.

JP-A-9-64477

  However, when manufacturing a semiconductor light emitting device using a nitride semiconductor, it is difficult and expensive to produce a high quality and large area epitaxial growth substrate made of a nitride semiconductor. It is necessary to use as a substrate. FIG. 14 is a graph showing the relationship between the lattice constant of InAlGaAs-based quaternary mixed crystal and the band gap energy (corresponding to the wavelength). In addition, the emission wavelength of the ultraviolet gas laser is also shown. As shown in FIG. 14, in order to produce a solid-state light emitting device on the short wavelength side, it is necessary to increase the band gap energy by increasing the Al composition of AlGaN. Therefore, the difference in the lattice constant from the sapphire substrate is in the increasing direction. For this reason, the mismatch of lattice constants increases, which causes a problem that the density of threading dislocations in the nitride semiconductor thin film increases. The presence of threading dislocations causes a decrease in the internal quantum efficiency of the semiconductor light emitting device, and thus it is necessary to improve the internal quantum efficiency of the semiconductor light emitting device.

  An object of the present invention is to provide an elemental technique for increasing the emission intensity of deep ultraviolet light while using an AlGaInN-based material, particularly an AlGaN-based material, as a material for a light-emitting layer.

  According to one aspect of the present invention, a first III-V group layer formed on a substrate, which is formed on a polarity control layer formed under a group III-rich growth condition, and on the polarity control layer. And a first III-V layer. A semiconductor structure is provided.

  In addition, a first group III nitride layer formed on the sapphire substrate, the polarity control layer formed under a growth condition that is rich in group III, and the first group III formed on the polarity control layer And a nitride layer. The group III rich layer to which the group III source is intermittently supplied may be provided for five or more periods. The group III source is preferably TMAl.

  The polarity control layer has a structure in which threading dislocations extending from the base are U-turned in the vicinity of the interface with the first III-V group layer. A high temperature growth AlN layer formed between the substrate and the first III-V group; The III-V group crystal layer is made of an InAlGaN-based semiconductor crystal.

  The present invention may be an optical semiconductor element formed in the semiconductor structure described above. For example, an optical semiconductor element that emits light in a wavelength region between 250 nm and 350 nm.

  According to another aspect of the present invention, a step of preparing a substrate, a first period of supplying a group III source and a group V source in order from the substrate side, a group III sheath and a group V source, The second period in which the supply is stopped, the third period in which only the group III source is supplied without supplying the group V source, and the surface is polarized to the group III polarity by repeating the second and third periods. There is provided a method for forming a semiconductor structure, characterized by having a period of control.

  A step of preparing a sapphire substrate; a first period of supplying a first Al source, a Ga source, and an N source to the sapphire substrate; a second period of stopping the supply of all the sources; A third period of supplying only the second Al source; and a step of forming a polarity control layer having a surface of Al polarity by repeating the second and third periods. A method of forming a semiconductor structure is provided.

  A step of preparing a sapphire substrate; a first period of supplying a first Al source, a Ga source, and an N source to the sapphire substrate; a second period of stopping the supply of all sources; A method for forming a semiconductor structure comprising: a third period for supplying only two Al sources; and a step of forming a polarity control layer by repeating the second and third periods. Is done. It is preferable to include a step of forming an AlN buffer layer at a high temperature between the sapphire substrate and the polarity control layer.

  Furthermore, it is preferable to have a step of forming an optical semiconductor element made of InAlGaAs on the polarity control layer.

  According to the present invention, a high-quality AlGaN buffer for an ultraviolet LED can be produced. Group III polarity can be obtained by pulsed supply of TMAl. Further, TMAl pulse supply has an effect of reducing threading dislocations.

  By using a template formed by using a high-quality AlGaN buffer layer formed by the TMAl pulse supply method, the maximum output of the ultraviolet LED is greatly improved compared to the conventional one.

It is sectional drawing of the atomic structure of the III group atom and V group atom which were formed on the sapphire substrate. FIGS. 2A and 2B are diagrams showing a configuration example with TMA pulse supply (A) and without pulse supply (B) according to the present embodiment. It is a growth sequence diagram of the TMAl layer. 4A and 4B are surface photographs obtained by observing the surface corresponding to FIGS. 2A and 2B with an optical microscope. 2 is a cross-sectional structure showing a structural example of a semiconductor structure according to the present embodiment. FIG. 4 is a diagram illustrating a result of growing a TMAl layer in FIG. 3 by changing parameters such as 0 period, 5 periods, and 10 periods in order to examine the period dependence of TMAl pulse supply. FIG. 7 is a diagram showing the growth period dependence of the TMD TMAl layer corresponding to threading dislocations. It is a cross-sectional TEM photograph of the structure shown in FIG. It is the elements on larger scale of FIG. 8A. It is a figure which shows one structural example of the semiconductor structure by this Embodiment. FIG. 10 is a sequence diagram of growth of the semiconductor structure shown in FIG. 9. It is a figure which shows the AlGaN growth time dependence of threading dislocation TDD. It is typical sectional drawing which shows a TMAl pulse supply structure and the LED element produced on it. It is a figure which shows the EL spectrum characteristic of an LED element. It is a figure which shows the relationship between the lattice constant of InAlGaAs type | system | group quaternary mixed crystal, and band gap energy (corresponding to a wavelength).

  Before describing the embodiments of the present invention, the inventors' consideration regarding deep ultraviolet light emitting devices will be described. The inventor has conceived that polarity control in group III nitrides such as AlGaN is very important. For example, when growing AlN on a sapphire substrate, the polarity is difficult to stabilize, and it is important to control the polarity to a stable group III polarity. It was also found that stable group III polarity can be realized by supplying TMAl in pulses.

  Hereinafter, an optical semiconductor device and a manufacturing method thereof according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view of an atomic structure of group III atoms and group V atoms formed on a sapphire substrate. As shown in FIG. 1, by inserting a TMAl pulse supply layer into a group V polarity sample, an Al-rich layer can be formed and the polarity can be reversed to the group III polarity. In this way, the surface can be stabilized with group III polarity.

  Next, a semiconductor structure related to polarity control and a growth method thereof will be described. FIGS. 2A and 2B are diagrams showing a configuration example with TMA pulse supply (A) and without pulse supply (B) according to the present embodiment. The structure shown in FIG. 2A is a periodic structure 5 of a sapphire (0001) substrate 1, an HT-AlN layer 3 formed thereon, and an AlGaN / TMAl pulse supply layer formed thereon, And an AlGaN layer 7. On the other hand, the structure shown in FIG. 2B has a sapphire (0001) substrate 1, an AlN layer 3 formed thereon, and an AlGaN layer 11 formed thereon. That is, the structure does not have the periodic structure 5 with the AlGaN / TMAl pulse supply layer. The AlGaN / TMAl pulse supply layer is a polarity control layer that controls the polarity of the surface to be a group III polarity. Further, the periodic structure 5HT-AlN is High Temperature AlN, and refers to AlN grown at a high temperature. The high temperature in this case is usually 1100 ° C. or higher, and in this example, it grows at 1270 ° C.

Table 1 is a table showing growth conditions for AlGaN and TMAl pulse supply layers. As shown in Table 1, the growth temperature is set to 1270 ° C., the growth pressure is set to 76 Torr (133.32 Pa = 1 Torr), and the supply amounts are 1 sccm for TMG and 50 sccm for TMA (see FIG. 1). 1 sccm = 1.667 × 10 ⁻⁵ l / s), NH ₃ is 40 sccm, and the TMA pulse is 1 sccm.

  The details of the MOCVD apparatus used can be implemented by referring to FIG. 2 and FIGS. 7 to 9 of Japanese Patent Application Laid-Open No. 2004-228489 and the description thereof, for example. Then, explanation is omitted.

FIG. 3 is a growth sequence diagram of the TMAl layer. According to the sequence diagram shown in FIG. 3, first, the AlN layer 3 is grown on the sapphire surface 1. The AlN layer 3 grows under NH ₃ rich conditions. For example, in the TMAl pulse supply sequence, after the growth of the AlGaN layer was performed for 10 seconds, the growth was interrupted for 5 seconds in order to remove NH ₃ , and then TMAl was introduced at 1 sccm for 5 seconds. After that, the growth was interrupted again for 5 seconds. The above sequence was regarded as one period, and growth was performed for a total of five periods. By performing growth in this way, Al can have a rich polarity. The above sequence is an example, and various modifications are possible, but Al polarity can be basically realized by a process of repeating growth interruption and supply of Al source. It is assumed that the growth interruption includes a method for greatly reducing the flow rate of NH ₃ .

  4A and 4B are surface photographs obtained by observing the surface corresponding to FIGS. 2A and 2B with an optical microscope. Since the surface morphology in FIG. 4A is much better, it can be confirmed that group III polarity was obtained by supplying TMAl pulses. Furthermore, it was confirmed that when these two samples were surface-treated with KOH for 10 minutes, for example, a group III polar surface having a smooth surface was obtained instead of a rough surface group V polar surface. .

  Next, a technique for reducing threading dislocations in the AlGaN layer will be described. FIG. 5 is a cross-sectional structure showing a structural example of the semiconductor structure according to the present embodiment. The semiconductor structure shown in FIG. 5 includes a sapphire substrate 21, an AlN layer 23, an AlGaN / TMAl pulse supply layer 25, an n-type AlGaN layer 27, and an n-type AlGaN layer 31.

  Table 2 shows the growth conditions of the AlGaN and TMAl pulse layers, and the same conditions as in Table 1.

  FIG. 6 is a diagram showing an experimental result based on a sequence diagram in which the TMAl layer is grown by changing parameters such as 0 period, 5 periods, and 10 periods in FIG. 3 in order to investigate the period dependence of TMAl pulse supply. is there. FIG. 7 is a diagram showing the growth period dependence of the TDD TMAl layer corresponding to threading dislocations by growth based on the sequence diagram. As shown in FIG. 7, it can be seen that threading dislocations are reduced to about half when pulse supply with a period of TMAl greater than about 5 is performed.

  8A and 8B are cross-sectional TEM photographs of the structure shown in FIG. As shown in FIG. 8A, an HT-AlN layer 23, a TMAl pulse supply layer and AlGaN layers 25 and 27, and an nAlGaN layer 31 are stacked on a sapphire substrate 21. As shown in the enlarged photograph of FIG. 8B, the threading dislocations formed in the HT-AlN layer 23 on the sapphire substrate 21 are TMAl pulse supply layers and AlGaN layers 25 and 27 (superlattice layers are observed). It can be seen that the loop 33 is formed (the U-turn does not extend to the nAlGaN layer 31) and returns to the HT-AlN layer 23 again. This loop structure shows that the threading dislocation density in the upper n-AlGaN layer 31 is greatly reduced. Further, as a result of surface observation, it has been found that the surface morphology is improved by introducing the TMAl layer, and this shows that the surface is controlled to have a stable Ga (III) group polarity.

  Hereinafter, the interval dependency of TMAl pulse supply will be described. FIG. 9 is a diagram illustrating a configuration example of a semiconductor structure. As shown in FIG. 9, an AlN layer 43, an AlGaN / TMAl layer 45, and an AlGaN layer 47 are grown on a sapphire substrate 41. As shown in Table 3, the growth conditions are the same as in Tables 1 and 2.

  FIG. 10 is a sequence diagram of the growth of the semiconductor structure shown in FIG. Here, an example is shown in which the supply time of the AlGaN layer is changed to 5 seconds, 10 seconds, and 20 seconds without changing the TMAl layer introduction sequence. In such a case, as shown in FIG. 11, the dependency of threading dislocation TDD on the AlGaN growth time is minimized in about 10 seconds, and the threading dislocation is minimized in about 10 seconds of AlGaN growth time. I understand.

As described above, threading dislocations can be summarized as follows.
1) The threading dislocations are reduced by the TMAl pulse supply (for example, 5 cycles, 10 cycles).
2) The threading dislocation is minimized in about 10 seconds as the TMAl pulse supply interval. If the reason why threading dislocations are reduced is estimated, it can be presumed that threading dislocations bend due to a difference in lattice constant between the TMAl pulse supply layer and the AlGaN layer, and threading dislocations in the vicinity collide and disappear.

  In addition, as to why there is a period number dependency, it is considered that a certain degree of periodic structure is necessary in order to reduce dislocation, and it is presumed that it will be saturated at some point. This is presumably because threading dislocations in the vicinity have disappeared.

  Further, the reason why there is an optimum value for TMAl pulse supply is expected to be that the change in the lattice constant becomes too steep when the AlGaN growth time is 5 seconds and the crystal structure is disturbed. When the AlGaN growth time is 20 seconds, it is expected that the dislocation curve becomes insufficient due to the long interval of the lattice constant change.

  Below, the technique which forms LED on the template created with the said TMAl supply technique is demonstrated.

  FIG. 12 is a schematic cross-sectional view showing the TMAl pulse supply structure and the LED element formed thereon. As shown in FIG. 12, a TMAl pulse supply structure including an HT-AlN layer 53, a TMAl pulse supply AlGaN layer 55, and an n-AlGaN layer 57 is formed on a sapphire substrate 51 in order, A quantum well structure in which a set of an n-AlGaN layer 59 and an InAlGaN barrier layer 61a / InAlkGaN quantum well layer 63a is formed over a plurality of periods, and an InAlGaN cap layer formed on the final InAlGaN barrier 65 of this quantum well structure 67, a p-InAlGaN electron blocking layer 71, and a p-AlGaN composition gradient layer 73. Further, an In electrode 77 is formed on the n-AlGaN layer, and a Ni / Au electrode 75 is formed on the p-AlGaN graded layer 73. Such a quantum well structure and a pn junction constitute an LED element. As described above, good LED characteristics can be expected by controlling the polarity and reducing the threading dislocation density.

  FIG. 13 is a diagram showing the EL spectral characteristics of the LED element. As shown in FIG. 13, a single peak having a peak at a wavelength of about 351 nm and a maximum output of 1.95 mW was obtained at room temperature. In the case where there is no TMAl layer, the maximum output is about 1.14 mW, which shows that the output is greatly increased.

  As described above, according to the present embodiment, a high-quality AlGaN buffer for an ultraviolet LED could be manufactured by introducing a TMAl pulse supply layer. Group III polarity could be obtained by TMAl pulse supply.

  Also, the effect of reducing threading dislocations (TDD is about ½) by supplying TMAl pulses was confirmed. Furthermore, by using the TMAl pulse supply template, the maximum output is about 1.71 times (1.14 mW to 1.95 mW), and high performance can be achieved.

  In the above embodiment, the introduction of the TMAl pulse supply layer has explained the improvement of the characteristics of the LED as an example of the device using the InAlGaN compound layer on the sapphire substrate. Needless to say, the characteristics are improved when a device is created. Moreover, it is applicable also to other III-V group compounds, such as a group III nitride.

  The present invention can be used for an InAlGaN-based optical device.

DESCRIPTION OF SYMBOLS 1 ... Sapphire (0001) board | substrate, 3 ... HT-AlN layer, 5 ... Periodic structure of AlGaN / TMAl pulse supply layer, 7 ... AlGaN layer.

Claims (6)

A first step of forming an AlN layer on a sapphire substrate;
A second step of supplying a first Group III source, a second Group III source different from the first Group III source, and a Group V source on the AlN layer in a first period; A third step of stopping the supply of the first and second group III sources and the group V source in a second period following the first period; and a third period following the second period A fourth step of supplying only Al as the first group III source without supplying a group V source to the first group , and the first and second groups III in a fourth period following the third period. A fifth step of stopping the supply of the source and the group V source, and repeating the second to fifth steps a predetermined number of times after the first step to form III on the AlN layer Forming a polarity control layer that reverses the polarity of the family,
A method of forming a semiconductor structure, comprising forming a buffer layer using the first and second group III sources and group V sources as raw materials. A first step of forming an AlN layer on a sapphire substrate;
A second step of supplying an Al source, a Ga source and an N source on the AlN layer in a first period; and the Al source, the Ga source and the N source in a second period following the first period. A third step of stopping the supply of N, a fourth step of supplying only the Al source in a third period following the second period, and a fourth period following the third period. And a fifth step of stopping the supply of the source, and the second to fifth steps are repeated a predetermined number of times after the first step, so that the polarity is controlled to be inverted to the Al polarity on the AlN layer. Forming a layer,
A method of forming a semiconductor structure, comprising forming an AlGaN buffer layer.   3. The method of forming a semiconductor structure according to claim 1, wherein the first period in the second step is 5 to 10 seconds. 4. The method of forming a semiconductor structure according to claim 1, wherein the first step includes a step of forming an AlN layer at 1100 ° C. or higher. 5.   5. The method of forming a semiconductor structure according to claim 1, wherein the predetermined number of times is 5 or more.   6. The method of forming a semiconductor structure according to claim 1, further comprising a step of forming a light emitting layer made of InAlGaN on the buffer layer.