JP6337686B2 - GaN substrate and method of manufacturing GaN substrate - Google Patents

Description

  The present invention relates to a GaN substrate and a method for manufacturing the GaN substrate.

In the manufacture of nitride semiconductor devices, nitride semiconductor thin films having various compositions are epitaxially grown on the GaN substrate by MOCVD or MBE.
A nitride semiconductor is a compound semiconductor represented by the general formula Al _x Ga _y In _z N (x + y + z = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1). It is also called a physical compound semiconductor, a nitride-based III-V group compound semiconductor, a GaN-based semiconductor, or the like.

In the case of a C-plane GaN substrate, the Ga polar plane, that is, the main surface on the [0001] side of the two main surfaces of the substrate is used for epitaxial growth of the nitride semiconductor.
When the C-plane GaN substrate was developed, it was thought that it was difficult to planarize by a chemical mechanical polishing method (CMP) because the Ga polar surface was chemically very stable. Therefore, a method has been developed to flatten the Ga polar surface by mechanically polishing the substrate until the RMS roughness (average square root roughness) is about 1.0 nm and then heat-treating the substrate in an ammonia-containing atmosphere. (Patent Document 1). According to this method, atomic rearrangement occurs due to thermal energy, and the Ga polar face is flattened.
On the other hand, a technique for flattening the Ga polar face of the C-plane GaN substrate by CMP has also been developed (Patent Document 2). The Ga polar surface of GaN can be made flat at the atomic level without a damaged layer by CMP using colloidal silica as abrasive particles.

JP 2003-327497 A Special table 2004-530306 gazette

The present inventors made a trial production of a C-plane GaN substrate having a Ga polar face flattened by CMP and epitaxially growing GaN thereon by MOCVD. While a flat GaN film is obtained, it has been found that when the substrate temperature is lowered to 1000 ° C., the frequency of occurrence of defects in which the surface of the GaN film becomes rough increases.
The present invention has been made in the process of solving such a problem, and even when a nitride semiconductor layer is grown at a relatively low temperature, the nitride semiconductor layer is used for forming a semiconductor device. The main object of the present invention is to provide a GaN substrate that does not cause surface roughness that hinders the performance.

(1) It has a first main surface made of GaN crystal and a second main surface opposite to the first main surface, and the RMS roughness of the first main surface measured by AFM is 0.8 nm in a measurement range of 10 μm × 10 μm. And the secondary ion mass spectrum of the first main surface is obtained under the conditions of (A) below by time-of-flight secondary ion mass spectrometry (TOF-SIMS). A GaN substrate having an intensity ratio SiO ₂ ⁻ / ⁶⁹ Ga ⁷¹ GaN ⁻ of 0.12 or less when the peak area intensity of SiO ₂ ⁻ and ⁶⁹ Ga ⁷¹ GaN ⁻ is determined.
(A) For the primary ion, the ion species is Bi ₃ ⁺⁺ (Bi trimer divalent ion), the acceleration voltage is 25 kV, the irradiation current is 0.1 pA, the irradiation area is 200 um × 200 um, Has a negative polarity and a collection time of 98 seconds.
(2) In the emission spectrum of the first main surface obtained by photoluminescence measurement, the peak intensity at the wavelength corresponding to the band gap of GaN is at least twice the intensity of the broad peak appearing in the visible wavelength range. The GaN substrate according to (1).
(3) The GaN substrate according to (1) or (2), wherein a low index plane that is parallel or closest to the first main surface is (0001).
(4) The GaN substrate according to any one of (1) to (3), which is a GaN single crystal substrate.
(5) The GaN substrate according to any one of (1) to (4), which is a GaN layer bonded substrate.
(6) The method for producing the GaN substrate according to any one of (1) to (5), wherein the main surface to be the first main surface is planarized by CMP using colloidal silica as abrasive particles. The manufacturing method which has a CMP process to perform and the washing | cleaning process performed after this CMP process.

  According to a preferred embodiment, even when a nitride semiconductor layer is grown on the nitride semiconductor layer at a relatively low temperature, the surface roughness is such that the nitride semiconductor layer is hindered from being used for forming a semiconductor device. A GaN substrate that does not generate is provided.

The GaN substrate referred to in the present invention is a general term for a GaN single crystal substrate and a GaN layer bonded substrate.
The GaN single crystal substrate is a single crystal substrate composed only of GaN (gallium nitride) and is also called a self-standing GaN substrate.
The GaN layer bonded substrate is a composite substrate in which a GaN single crystal layer is bonded to a different composition substrate having a chemical composition different from that of GaN. For details of the structure and manufacturing method of the GaN layer bonded substrate, reference can be made to JP-A-2006-210660, JP-A-2011-44665, and the like.
Both the GaN single crystal substrate and the GaN layer bonded substrate have a main surface made of a GaN crystal.

The GaN substrate of the present invention has a first main surface made of a GaN crystal and a second main surface opposite to the first main surface. In the case of a GaN single crystal substrate, the second main surface is also made of a GaN crystal. The second main surface of the GaN layer bonded substrate is the surface of the different composition substrate.
In the GaN substrate of the present invention, the main surface scheduled to be used for the growth of the nitride semiconductor layer is the first main surface. In other words, the first main surface is the “front surface” and the second main surface is the “back surface”.

In the GaN substrate of the present invention, the RMS roughness of the first main surface measured by AFM is usually less than 0.8 nm, preferably less than 0.5 nm, more preferably less than 0.3 nm in a measurement range of 10 μm × 10 μm.
Such a flat surface can be obtained by performing mechanical polishing such as grinding or lapping and then performing CMP using colloidal silica as abrasive particles as a finishing process.
Preferably, the CMP processing amount is set so that the damaged layer generated by the mechanical polishing is sufficiently removed.

Numerous dark spots corresponding to dislocations are observed in the cathodoluminescence image of the GaN surface that has been mechanically polished. When CMP processing is performed on such a GaN surface, the dark spot density decreases as the processing amount increases. After the damaged layer is removed, the dark spot density does not change even if the CMP processing amount is further increased.
The degree of residual damage layer can also be known from the emission spectrum of the first main surface obtained by photoluminescence measurement. When the damaged layer is present, the emission spectrum has a broad peak in the visible wavelength region. This broad emission peak is also called a yellow band, and appears in a wavelength range including a wavelength (550 to 580 nm) corresponding to yellow light.
When the damaged layer is removed by CMP, in the emission spectrum, the ratio of the intensity of the yellow band to the intensity of the peak at the wavelength corresponding to the band gap of GaN decreases. Preferably, CMP processing is performed until this ratio becomes 1/2 or less, further 1/5 or less, and further 1/10 or less.

The most characteristic point in the GaN substrate of the present invention is that a secondary ion mass spectrum of the first main surface is obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS), and SiO _{2 is} obtained from the mass spectrum. ^- a ⁶⁹ Ga ⁷¹ GaN ^- when seeking the peak area intensity, the intensity ratio ^{_{SiO 2 - / 69 Ga 71 GaN}} - is that it is 0.12 or less.
Here, the conditions for acquiring the secondary ion / mass spectrum are as follows. That is, for the primary ions, the ion species is Bi ₃ ⁺⁺ (Bi trimer divalent ions), the acceleration voltage is 25 kV, the irradiation current is 0.1 pA, the irradiation area is 200 μm × 200 μm, and the secondary ions are The polarity is negative and the collection time is 98 seconds.

TOF-SIMS is known as an analytical method for obtaining information on the extreme surface of a sample, and is generally said to have low quantitativeness. However, CMP-finished GaN substrate surface foreign matter is extremely small, because they are composed only of substantially GaN, SiO ₂ derived from the signal ^- the intensity of the (SiO ₂ peaks), GaN derived signal ⁽⁶⁹ Ga ⁷¹ GaN ^- normalized by the intensity of the peak) is believed to be an indicator of the presence of SiO _2.

The fact that the surface roughness of the epitaxial layer is suppressed when the intensity ratio SiO ₂ ⁻ / ⁶⁹ Ga ⁷¹ GaN ⁻ is smaller than a predetermined value suggests that the main cause of such surface roughness is SiO ₂ contamination.
What is likely to cause SiO ₂ contamination is, for example, colloidal silica used as abrasive particles in CMP processing. Colloidal silica remains on the substrate surface if the cleaning after CMP treatment is insufficient.
In addition, colloidal silica floating in the environment can also be a source of SiO ₂ contamination. For example, colloidal silica may be suspended in a room where a chemical solution (for example, CMP slurry) containing colloidal silica is used or stored.

Therefore, in order to prevent SiO ₂ contamination, it is necessary to pay sufficient attention not only to cleaning the GaN substrate sufficiently, but also to the environment for drying, packing and storing the substrate after cleaning. There is.
When CMP of a GaN substrate is performed using colloidal silica as abrasive particles, it is preferable to use a cleaning solution containing hydrofluoric acid and a cleaning solution containing a surfactant for cleaning the substrate after processing. A higher concentration of hydrofluoric acid in the cleaning liquid containing hydrofluoric acid is preferable.
The conditions such as the composition of the cleaning liquid, the cleaning temperature, and the cleaning time are as follows. The surface of the cleaned GaN substrate is subjected to TOF-SIMS analysis under the above-described conditions, and the intensity ratio SiO ₂ ⁻ / ⁶⁹ Ga ⁷¹ GaN ⁻ is 0.12 or less. What is necessary is just to determine.

  In the GaN substrate of the present invention, the plane orientation of the main surface made of the GaN crystal is not particularly limited, but preferably, the low index plane parallel or closest to the main surface is (0001), (10-10), (20-21), (20-2-1), (30-31), (30-3-1), and the like. (0001) is a so-called C-plane, and (10-10) is a so-called M-plane. The low index plane means a crystal plane in which the absolute values of integers h, k, m, and l in the Miller index (hkml) are all 3 or less.

  The semiconductor device that can be formed by epitaxially growing a nitride semiconductor thin film on the GaN substrate of the present invention is not limited. For example, a light emitting device such as a light emitting diode or a laser diode, a rectifier, a bipolar transistor, a field effect transistor, HEMT ( Electronic devices such as High Electron Mobility Transistor), temperature sensors, pressure sensors, radiation sensors, semiconductor sensors such as visible-ultraviolet light detectors, SAW (Surface Acoustic Wave) devices, vibrators, resonators, oscillators, MEMS (Micro Electro) Mechanical System) parts, voltage actuators, etc.

[Example 1]
A single crystal GaN ingot with a diameter of 2 inches (50 mm) was sliced using a wire saw. The N polar face ([000-1] side main surface) of the obtained as-sliced C-plane substrate was etched with an aqueous KOH solution to remove the damaged layer. Next, lapping and CMP treatment were sequentially performed on the Ga polar face. In CMP, a slurry containing colloidal silica as abrasive particles was used.
After CMP, the GaN substrate was cleaned and dried in a clean room where neither chemical solution containing colloidal silica was used nor stored.
Cleaning is performed by immersing the GaN substrate in isopropyl alcohol, scrubbing with a surfactant (pH = 10), and then immersing in an aqueous solution containing 5% hydrofluoric acid and rinsing with pure water. went.

When the Ga polar face of the C-plane GaN substrate produced by the above procedure was observed using AFM, no scratch was found. From the cathodoluminescence image, it was confirmed that there was no damage layer on the Ga polar face. The RMS roughness of the Ga polar surface measured by AFM was less than 0.2 nm in a measurement range of 10 μm × 10 μm.
Furthermore, the secondary ion mass spectrum of the Ga polar face of the C-plane GaN substrate was obtained by TOF-SIMS. The conditions are as follows: for the primary ions, the ion species is Bi ₃ ⁺⁺ , the acceleration voltage is 25 kV, the irradiation current is 0.1 pA, the irradiation area is 200 μm × 200 μm, and for the secondary ions, the polarity is negative and the collection time is It was set to 98 seconds.
When the peak area intensity of SiO ₂ ⁻ and ⁶⁹ Ga ⁷¹ GaN ⁻ was determined from the obtained mass spectrum, the intensity ratio SiO ₂ ⁻ / ⁶⁹ Ga ⁷¹ GaN ⁻ was 0.01.

  Next, an undoped GaN layer having a thickness of 2 μm was epitaxially grown on the Ga polar face of the C-plane GaN substrate by MOCVD. Although the substrate temperature was 1000 ° C., the surface of the undoped GaN layer was smooth, and the surface roughness Ra measured with an optical interferometer was less than 0.01 μm.

<Example 2>
After washing and drying, a C-plane GaN substrate was produced in the same manner as in Example 1 except that it was exposed to a room atmosphere using a chemical solution containing colloidal silica for a short time. Gets the secondary ion mass spectra of the Ga-polar surface under the same conditions as in Example 1, SiO ₂ ^- and ⁶⁹ Ga ⁷¹ GaN ^- result of obtaining a peak area intensity of the intensity ratio SiO ₂ ^- / ⁶⁹ Ga ⁷¹ GaN ^- it was 0.12.
Next, when an undoped GaN layer was epitaxially grown on the C-plane GaN substrate under the same conditions as in Example 1, the surface was smooth as in Example 1.

<Comparative example>
After cleaning and drying, a C-plane GaN substrate was produced in the same manner as in Example 2 except that the time for exposure to an indoor atmosphere using a chemical solution containing colloidal silica was extended. Gets the secondary ion mass spectra of the Ga-polar surface under the same conditions as in Example 1, SiO ₂ ^- and ⁶⁹ Ga ⁷¹ GaN ^- result of obtaining a peak area intensity of the intensity ratio SiO ₂ ^- / ⁶⁹ Ga ⁷¹ GaN ^- it was 0.55.
Next, when an undoped GaN layer was epitaxially grown on this C-plane GaN substrate under the same conditions as in Example 1, the surface was rough, and the surface roughness Ra measured by an optical interferometer exceeded 0.1 μm. The parts were scattered.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited to the embodiments explicitly disclosed in the present specification, and various modifications can be made without departing from the spirit of the present invention. can do.

Claims (5)

A first major surface that is used for the growth of Do Ri nitride semiconductor layer of GaN crystal, and a second major surface on the opposite side, the RMS roughness of the first main surface measured by AFM, the measurement The secondary ion mass spectrum of the first main surface is less than 0.8 nm in the range of 10 μm × 10 μm, and by the time-of-flight secondary ion mass spectrometry (TOF-SIMS) under the following condition (A) acquires, SiO ₂ from the mass spectrum ^- when seeking the peak area intensity, the intensity ratio SiO ₂ ^{- -} and ^{69 Ga 71 GaN / 69 Ga 71} GaN - a nitride semiconductor layer grown is 0.12 or less A method of manufacturing a GaN substrate for use ,
A preparation step of preparing a GaN substrate before finishing having a main surface to be the first main surface, a CMP step of flattening the main surface by CMP using colloidal silica as abrasive particles as finishing processing, Having a cleaning step performed after the CMP step; and
The finished GaN substrate is not exposed to the atmosphere in the room where the colloidal silica is used or stored in the cleaning step and the subsequent steps.
The manufacturing method characterized by this .
(A) For the primary ion, the ion species is Bi ₃ ⁺⁺ (Bi trimer divalent ion), the acceleration voltage is 25 kV, the irradiation current is 0.1 pA, the irradiation area is 200 um × 200 um, Has a negative polarity and a collection time of 98 seconds. In the cleaning step, the strength ratio SiO ₂ ^- / ⁶⁹ Ga ⁷¹ GaN ^- The manufacturing method according to claim 1, wherein a cleaning condition determined so as to be 0.12 or less is used. 3. The manufacturing method according to claim 1, wherein the nitride semiconductor layer growth GaN substrate has a (0001) low index plane parallel or closest to the first main surface. The manufacturing method according to claim 1, wherein the GaN substrate for growing a nitride semiconductor layer is a GaN single crystal substrate. The manufacturing method according to claim 1, wherein the nitride semiconductor layer growth GaN substrate is a GaN layer bonded substrate.