JP2014197582A - Pretreatment method of group iii nitride substrate and leakage current control method in group iii nitride device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pretreatment method of a base substrate composed of group III nitride crystals which can reduce leakage current in an electronic device structure formed on the base substrate composed of the group III nitride crystals.SOLUTION: As a pretreatment performed on a group III nitride substrate for formation of a group III nitride device, the group III nitride substrate is immersed in hydrofluoric acid with a concentration of 15 wt% and over, preferably not less than 20 wt% and not more than 50 wt%.

Description

  The present invention relates to a pretreatment method for a group III nitride substrate, and more particularly to a pretreatment method for a group III nitride substrate used for forming an electronic device.

  Group III nitride crystals (single crystals) represented by GaN (gallium nitride) are used in electronic devices such as HEMT (high electron mobility transistors), light emitting elements such as LEDs (light emitting diodes), and light receiving elements. It is widely used as a base layer of an optical device and a functional layer that develops desired device characteristics in these devices.

  Further, by performing pretreatment on the base substrate, a method of changing the surface state of the substrate before the formation of the group III nitride crystal layer into a so-called epi-ready state (see, for example, Patent Document 1 and Non-Patent Document 1) or Group III nitriding An etching method (for example, see Patent Document 2 and Non-Patent Document 1) for a substrate made of a physical crystal is already known.

Special table 2012-52503 gazette JP 2007-324549 A

“Dry etching of GaN substrates for high-quality homoepitaxy” M. Schauler, F. Eberhard, C. Kirchner, V. Schwegler, A. Pelzmann et al., Appl. Phys. Lett. 74, 1123 (1999)

  In an electronic device (group III nitride device) produced by forming a group III nitride crystal layer on a base substrate made of group III nitride crystal, a current that should not flow originally flows between the electrodes. So-called leakage current (leakage current) may occur. Since the inventors of the present invention have found that the occurrence of leakage current varies from substrate to substrate, in the present invention, an attempt was made to solve the problem by pretreatment of the substrate. In the conventional substrate pretreatment method, there are many methods of exposing to the etching gas atmosphere in the growth apparatus, but it is not efficient from the viewpoint of productivity. In manufacturing electronic devices, the inventors of the present invention have studied a substrate pretreatment method that can suppress the generation of leakage current and can be introduced at low cost.

  The present invention has been made in view of the above problems, and a base substrate made of a group III nitride crystal capable of reducing leakage current in an electronic device structure formed on the base substrate made of a group III nitride crystal. An object of the present invention is to provide a pre-processing method.

  In order to solve the above-mentioned problems, the invention of claim 1 is a pretreatment method for a group III nitride substrate for forming a group III nitride device, wherein the group III nitride substrate has a concentration of 15 wt% or more. It is immersed in hydrofluoric acid.

  The invention of claim 2 is the pretreatment method for a group III nitride substrate according to claim 1, wherein the group III nitride substrate is immersed in hydrofluoric acid having a concentration of 20 wt% or more and 50 wt% or less. It is characterized by that.

  The invention of claim 3 is a method for suppressing the occurrence of leakage current in a group III nitride device formed by forming a group III nitride crystal layer on a group III nitride substrate, the group III nitride substrate Is immersed in hydrofluoric acid having a concentration of 15 wt% or more, and then the group III nitride crystal layer is formed on the group III nitride substrate.

  The invention of claim 4 is a method for suppressing leakage current in a group III nitride device according to claim 3, wherein the group III nitride substrate is immersed in hydrofluoric acid having a concentration of 20 wt% or more and 50 wt% or less. It is characterized by that.

  According to the first to fourth aspects of the invention, it is possible to suitably suppress the occurrence of leakage current in the group III nitride device.

It is a figure which shows the method (procedure) of the pre-processing performed with respect to a base substrate. 1 is a diagram schematically showing a configuration of an electronic device structure 10. FIG.

  FIG. 1 is a diagram showing a pretreatment method (procedure) for a base substrate in the present embodiment. Such pretreatment is performed for the purpose of suppressing the occurrence of leakage current (leakage current) in an electronic device (group III nitride device) structure made of group III nitride manufactured using a base substrate as a base substrate. .

  The base substrate that is the target of the pretreatment method according to the present embodiment is a so-called self-supporting substrate made of a group III nitride crystal (single crystal) represented by, for example, GaN. Preferably, the base substrate 1 has a (0001) plane that is flat at the atomic level, that is, a C plane. The base substrate may be made of AlN, InN or the like in addition to GaN, or may be made of a mixed crystal thereof. Alternatively, the base substrate may be a so-called template substrate in which the above-described group III nitride crystal layer is epitaxially formed on a substrate made of a different material such as sapphire or silicon. The thickness of the base substrate is preferably about 400 μm to 1000 μm.

  In addition, the electronic device structure is manufactured by a so-called multi-cavity method in which a plurality of device structures are obtained from one mother substrate having a relatively large size. That is, one or more crystal layers made of a group III nitride are appropriately formed on a pretreated base substrate according to functions and crystal quality required for developing device characteristics, The electrode pattern is formed so that the electrodes are arranged at predetermined positions in the regions to be the individual electronic device structures, and then the device is separated.

  In the present embodiment, the base substrate made of the group III nitride crystal as described above is first immersed in hydrofluoric acid (hydrogen fluoride aqueous solution) (step S1). The concentration of hydrofluoric acid is preferably 15 wt% or more. The immersion time may be about 2 to 20 minutes.

  After soaking in hydrofluoric acid, rinsing by soaking in ultrapure water (step S2) is repeated 2 to 4 times, followed by rinsing with running water for 2 to 20 minutes (step S3). Next, drying by nitrogen blowing is performed (step S4).

  By performing the pretreatment for the base substrate by the above procedure, the occurrence frequency of the leakage current in the electronic device structure manufactured using the base substrate can be 5% or less.

  However, in the present embodiment, the frequency of occurrence of leakage current is expressed by the ratio of the number of electronic device structures produced using a base substrate that has been subjected to pretreatment as a mother substrate.

In addition, the occurrence of leakage current means that the current density between the electrodes exceeds 1 × 10 ⁻⁵ A / mm ² when a voltage of 300 V or less is applied between the electrodes of the electronic device structure. .

  Preferably, the concentration of hydrofluoric acid is 20 wt% or more and 50 wt% or less. In such a case, the occurrence frequency of leakage current is reduced to 1.5% or less. The reason why the concentration of hydrofluoric acid is 50 wt% or less is that it is not preferable from the viewpoint of safety to handle hydrofluoric acid having a higher concentration. In addition, since the effect of reducing the frequency of occurrence of leakage current is almost constant when it is 20 wt% or more (the frequency of occurrence does not change even if the concentration of hydrofluoric acid is increased), hydrogen fluoride is practically used. It is sufficient that the acid concentration is 20 wt% or more and 30 wt% or less.

  On the other hand, when the concentration of hydrofluoric acid is less than 15 wt%, the occurrence frequency of leakage current is lower than that in the case of no treatment, but the effect is not always sufficient. This means that it is effective to use hydrofluoric acid having a relatively high concentration of 15 wt% or more, or 20 wt% or more for reducing the leakage current.

The reason why the leakage current is reduced when an electronic device structure is manufactured using a base substrate that has been pre-processed by the above procedure is that when the base substrate is polished prior to being subjected to such manufacturing. The work-affected layer generated on the outermost surface of the substrate and the Si oxide (for example, SiO ₂ ) contained in the abrasive used for polishing the surface are suitably removed by such pretreatment, so that the crystal This is considered to be because the generation of defects (dislocations) and the propagation in the thickness direction are suppressed. Regarding the latter, the inventors of the present invention confirmed that there is a certain correlation between the magnitude of the pit density, which reflects the degree of crystal defects in the group III nitride crystal layer, and the frequency of occurrence of leakage current. Has been.

  In addition to hydrofluoric acid, as a chemical solution that can be used for pretreatment of the base substrate, in addition to an aqueous solution of TMAH (tetramethylammonium hydroxide) disclosed in Patent Document 2, an aqueous solution of ammonium hydroxide, hydrochloric acid, However, it is confirmed by the inventor of the present invention that even if immersion treatment is performed in these chemical solutions, a sufficient effect cannot be obtained for suppressing leakage current.

  As described above, according to the present embodiment, a hydrofluoric acid having a relatively high concentration of 15 wt% or more prior to manufacturing an electronic device structure using a base substrate made of group III nitride. By immersing the base substrate in, the generation of leakage current in the electronic device structure can be suitably suppressed. In particular, when immersed in hydrofluoric acid having a concentration of 20 wt% or more and 30 wt% or less, the occurrence frequency of leakage current in the electronic device structure can be suppressed to 1.5% or less.

  In this example, a plurality of base substrates subjected to immersion treatment with various chemical solution conditions were prepared, and an electronic device structure was prepared and evaluated using each base substrate.

  Specifically, first, a plurality of C-plane GaN single crystal substrates (hereinafter, referred to as GaN substrates) having a diameter of 2 inches and a thickness of 400 μm are prepared as base substrates, and eight kinds of chemical solutions for immersion treatment having different concentrations are prepared. Hydrofluoric acid (concentrations of 5 wt%, 10 wt%, 15 wt%, 17.5 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%), an aqueous ammonium hydroxide solution with pH = 10, A total of 12 types of 005 mol / L TMAH aqueous solution, 20 wt% hydrochloric acid, sulfuric acid / hydrogen peroxide mixture (96 wt%) and hydrogen peroxide (30 wt%) mixed at 20: 1 were prepared.

  And after immersing a GaN board | substrate in each chemical | medical solution for 10 minutes, immersion in ultrapure water was repeated twice and the running water rinse for 5 minutes was performed. Thereafter, the GaN substrate was dried by nitrogen gas blowing.

  Next, in addition to these processed GaN substrates (12 types in total), 13 types of device structures were formed targeting untreated GaN substrates (1 type).

  FIG. 2 is a diagram schematically illustrating the configuration of the electronic device structure 10 that is an evaluation target of leakage current in the present embodiment.

  The electronic device structure 10 is formed by forming a GaN layer 2 with a thickness of 3 μm on each base substrate 1 and then forming circular electrodes (Pt) made of Pt on the surface on the base substrate 1 side and the surface on the GaN layer side, respectively. An electrode pattern in which diameters 3 and 4 are repeatedly arranged in a two-dimensional manner is formed by electron beam evaporation, and device isolation is performed. From one base substrate, 250 electronic device structures 10 were obtained.

  When the GaN layer 2 was formed, the substrate surface was observed with a polarizing microscope, and the pit density on the surface was evaluated based on the result.

Further, current-voltage measurement was performed on the obtained individual electronic device structures 10. When a maximum voltage of 300 V was applied, it was determined that leakage current was generated when the current density between the electrodes exceeded 1 × 10 ⁻⁵ A / mm ² .

  Table 1 shows a list of conditions for each chemical solution and the frequency of occurrence of leakage currents obtained from the corresponding pit density measurement results and current-voltage measurement results.

  As shown in Table 1, in the case of the treatment conditions a-1 to a-8 where the immersion treatment was performed using hydrofluoric acid having different concentrations, all of them were from the case of the treatment condition b (untreated). However, the pit density decreased and the value of the pit density decreased as the concentration of hydrofluoric acid increased. In addition, the frequency of occurrence of leakage current also tends to decrease as the concentration of hydrofluoric acid increases, and in the case of treatment conditions a-3 to a-8 where the concentration of hydrofluoric acid is 15 wt% or more. 5% or less. This is a value of approximately 1/9 to 1/10 or less of the occurrence frequency of leakage current in the case of no processing. However, in the case of treatment conditions a-5 to a-8 in which the concentration of hydrofluoric acid was 20 wt% or more, the occurrence frequency of leakage current was almost constant at 1.3% or less. This is approximately 1/30 or less of the occurrence frequency of leakage current in the unprocessed case.

  On the other hand, in the case of the treatment conditions cf in which the immersion treatment is performed using a chemical solution other than hydrofluoric acid, the pit density is smaller than in the case of the treatment condition b only in the case of the treatment condition d using the TMAH aqueous solution. In addition, the frequency of occurrence of leakage current was reduced, but the degree of reduction was small compared to the processing conditions a-5 to a-8. In the case of the processing conditions c, e, and f, the pit density was not much different from that in the processing condition b, and the frequency of occurrence of leakage current was rather increased.

  The above results show that immersion treatment in hydrofluoric acid having a concentration of 15 wt% or more with respect to the base substrate suppresses the occurrence of leakage current in an electronic device structure manufactured using the base substrate. It is effective in that point.

DESCRIPTION OF SYMBOLS 1 Ground substrate 2 GaN layer 3, 4 Electrode 10 Electronic device structure

Claims (4)

A pretreatment method for a group III nitride substrate for forming a group III nitride device,
Immersing the group III nitride substrate in hydrofluoric acid having a concentration of 15 wt% or more,
A pretreatment method for a group III nitride substrate. A pretreatment method for a group III nitride substrate according to claim 1,
Immersing the group III nitride substrate in hydrofluoric acid having a concentration of 20 wt% or more and 50 wt% or less,
A pretreatment method for a group III nitride substrate. A method for suppressing the occurrence of leakage current in a group III nitride device formed by forming a group III nitride crystal layer on a group III nitride substrate,
Forming the group III nitride crystal layer on the group III nitride substrate after immersing the group III nitride substrate in hydrofluoric acid having a concentration of 15 wt% or more;
A method for suppressing leakage current in a group III nitride device. A method for suppressing leakage current in a group III nitride device according to claim 3,
Immersing the group III nitride substrate in hydrofluoric acid having a concentration of 20 wt% or more and 50 wt% or less,
A method for suppressing leakage current in a group III nitride device.

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