Effect of growth temperature of AlN buffer layer

The AlN buffer layer was grown at temperatures from 800 ℃ to 1250 ℃, while the thickness and V/III ratio of it were fixed to be 110 nm and 470 respectively. GaN was grown in 2D mode with constant conditions to test the effects of growth conditions of AlN buffer layer.

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From Fig. 5.1a, it is very clear that the quality of GaN was strongly dependent on the growth temperature of AlN buffer layer. In Fig. 5.2 we can see that GaN cannot grow properly on AlN buffer grown at 800 ℃ and 900 ℃. As shown in Fig. 5.2c and d, on 800-℃ grown AlN buffer layer, instead of an epitaxial film, GaN grew on it as crystal grains with various orientations and size ranged from tens of nanometers to several micrometers. On

900-℃ grown AlN buffer, in Fig. 5.2a and b, the quality of GaN improved with uniform orientation to (0002) plane, but still not coalesced completely within the thickness of 1.75 um. FWHM of XRD rocking curve for GaN grown on 800-℃ AlN buffer cannot be measured, it is only available above 900 ℃. As TAlN was elevated from 900 ℃ to 1100 ℃, the quality of GaN was improved significantly. From the knowledge of XRD rocking curve measurement, peak width of rocking curve relates to the out-of-plane and in-plane

Fig. 5.1 (a) Dependence of FWHM of XRD rocking curve of GaN and AlN buffer on the growth temperature of AlN buffer layer (TAlN); (b) curvature transition dependence on TAlN.

Fig. 5.2 SEM images of the surface and cross section of GaN on AlN buffer layer grown at 800 ℃ and 900 ℃.

(a)

(c)

10 um

10 um

(b)

(d)

2 um

5 um T_AlN= 900 ℃

T_AlN= 800 ℃

T_AlN= 900 ℃

T_AlN= 800 ℃ GaN

Si

AlN

GaN

Si AlN

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misorientations of crystallites in the epitaxial layers which originates from screw and edge threading dislocations respectively. For c-axis orientated GaN film, FWHM of the plane of (0002) and (10-10) corresponds to the tilt of (0002) plane caused by screw dislocation and twist of (10-10) plane caused by edge dislocations [40]. The densities of screw and edge dislocations in GaN can be calculated from the following equations [41]:

𝜌𝜌𝑠𝑠𝑐𝑐𝑟𝑟𝑒𝑒𝑑𝑑 =_{2𝜋𝜋𝑏𝑏} ^𝛼𝛼2

𝑠𝑠𝑠𝑠𝑠𝑠𝑎𝑎𝑠𝑠

2 ∙ln(2), 𝜌𝜌𝑒𝑒𝑑𝑑𝑔𝑔𝑒𝑒 =_{2𝜋𝜋𝑏𝑏} ^𝛽𝛽2

𝑎𝑎𝑒𝑒𝑒𝑒𝑎𝑎2 ∙ln(2) . (5-1) 𝜌𝜌 is the dislocation density, 𝛼𝛼 and 𝛽𝛽 correspond to tilt angle and twist distortion angle of the planes of (0002) and (10-10) respectively which are proportional to the values of FWHM, 𝑏𝑏 is the Burgers vector for screw and edge dislocations which are 5.185 Å and 3.189 Å respectively for GaN. Therefore, for GaN on AlN buffer layer grown at temperatures from 900 ℃ to 1100 ℃, both the density of screw and edge dislocations dropped rapidly. While as T_AlN increased from 1100 ℃ to 1200 ℃, the density of screw dislocations didn’t change much but the density of edge dislocations decreased significantly. As T_AlN was elevated further from 1200 ℃ to 1250 ℃, even with only increase of 50 ℃, the quality of GaN on it was enhanced substantially, with great reduction of the density of both screw and edge dislocation.

Fig. 5.3 SEM images of AlN buffer layer surface: grown at temperatures from 800 ℃ to 1250 ℃. 2.40E+09 cm^-2

2.90E+10 cm^-2 1.70E+10 cm^-2

1 um 1200 ℃ 1250 ℃

1100 ℃

500 nm 500 nm 500 nm

1000 ℃ 900 ℃

800 ℃

1 um 1 um

(a) (b) (c)

(d) (e) (f)

LT HT

Fig. 5.4 AFM of AlN buffer layer surface grown at 1100 ℃, 1200 ℃ and 1250 ℃.

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To investigate the improvement of GaN quality, it is necessary to discuss the crystal quality enhancement of AlN buffer layer. Since no structural techniques like 3D growth mode or multi-layer buffer was applied, the improvement of GaN quality is brought simply by the quality enhancement of AlN buffer layer by the growth temperature elevation here. In Fig.

5.1a, as indicated by the FWHM of XRD rocking curve of plane (0002) (𝜃𝜃0002) of AlN buffer layer, as TAlN was raised from 1100 ℃ to 1250 ℃, the quality of AlN buffer also improved considerably. Because 𝜃𝜃₀₀₀₂ of AlN buffer layer grown under 1100 ℃ cannot be measured, so the surface of them have also been characterized by SEM and AFM. Fig. 5.3 are the surface images of AlN buffer layer grown at temperature from 800 ℃ to 1250 ℃. In it we can clearly see the transition of growth mode and crystal quality improvement of AlN buffer. At 800 ℃, AlN grew on Si as polycrystalline grains with size of about 20×100 nm². As T_AlN increased to 900 ℃, it was still poly-crystallites but with larger size. It might be that because these two AlN buffer layers consisted of crystallites with various orientations, so the GaN grown on them also was of grains with various facets and sizes, and could not coalesce in large thickness of 1.75 um, as shown in Fig. 5.2. There was a crystalline phase transition for the growth of AlN on Si when the growth temperature was elevated from 900 ℃ to 1000 ℃. At T_AlN of 1000 ℃, AlN could be grown epitaxially on Si without any grains, while the pit density was very large around 10¹¹ cm^-2. As TAlN increased to 1100 ℃, the crystal quality was enhanced with significant reduction of pit density to about 1.7×10¹⁰ cm^-2 along with huge quality improvement of GaN on it. From the slope of curvature curves in Fig. 5.1b, it should be noticed that the tensile stress of AlN buffer layer grown at 1100 ℃ was the highest. This was brought by the crystal quality improvement of it. However, as TAlN was elevated higher to 1200 ℃, pit density was almost doubled. The roughness from AFM images in Fig. 5.4 was also doubled. But from the result of XRD rocking curve, the crystal quality of it was actually improved. This may explain why the screw dislocation density didn’t change much but the edge dislocation density was reduced very much. From 1200 ℃ to 1250 ℃, with only an increase of 50 ℃, but the pit density was reduced to 2.4×10⁹ cm^-2, which was one tenth of that at 1200 ℃. The surface morphology also became very smoother, especially the part without pits. Consequently, the quality of GaN on it was also improved substantially. It is not clear that why a small temperature increase of 50 ℃ caused so significant crystal quality improvement both for AlN buffer layer and GaN. It can be supposed that the temperature range of 1200 ℃ ~ 1250 ℃ is a critical temperature window to enhance the quality of AlN.

Based on the observation above, it can be concluded that temperature is a decisive factor of the growth mode and quality of AlN. At low temperature, the surface mobility and diffusion length 𝑛𝑛𝑠𝑠 of ad-atoms is very small, especially for Al adsorbed atoms; it is also easy to form clusters and move to the surface. These reasons cause easy atom accumulation on the surface and leads to 3D growth and poly-crystalline grains. At higher temperature above 1000 ℃, 𝑛𝑛𝑠𝑠

can be enhanced and epitaxy with single orientation is available. So, for AlN on Si, roughly, the growth temperature can be divided into several stages. As T_AlN increased from low and up to 900 ℃, it is amorphous firstly and then poly-crystalline with larger and larger grains. From 900 ℃ to 1000 ℃, there is a phase transition from poly-crystal to single crystal epitaxy. From 1000 ℃ to 1200 ℃, there is a steady crystal quality improving stage but it is not as sharp as at

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higher temperature. Above 1200 ℃, there is a sharp quality enhancement, both for the crystal quality and surface morphology.

The relationship between the dislocation density of buffer layer (AlN) and epilayer (GaN) is evident that high-quality buffer layer leads to high-quality epilayer. A part of defects on the surface of the buffer layer can be the source of dislocations in the epilayer or propagate into the epilayer, such as threading dislocations including screw and edge dislocations. Some of them will be multiplied by mechanisms like Frank-Read source, spiral source and Hagen-Strunk multiplication [42]. Threading dislocations can propagate to the epilayer and they can be annihilated by bending or increasing the epilayer thickness. The dislocation density in the epilayer is proportional to that in the buffer layer. Consequently, for simple epitaxy of GaN on AlN/Si substrate without special techniques like 3D growth or masking, the quality of AlN buffer layer should be as high as possible.

Another point should be discussed is the correspondence of crystal quality and the curvature (stress-strain) behavior. This is a complementary discussion to the part of 3.2.1 in chapter 3. Higher quality of GaN corresponds to more compressively stressed GaN layers.

Based on the strain relaxation mechanisms introduced in the part of 3.1.3 in chapter 3, GaN on AlN grown at higher T_AlN should undergo less compressive stress due to the thermal expansion of AlN buffer and the reduction of ∆𝑎𝑎𝐺𝐺𝑎𝑎𝐺𝐺−𝐴𝐴𝑡𝑡𝐺𝐺. But ∆𝑎𝑎𝐺𝐺𝑎𝑎𝐺𝐺−𝐴𝐴𝑡𝑡𝐺𝐺 is only one of the factors for strain relaxation. Another major mechanism is the relaxation by the formation of dislocations. Higher misfit dislocation density causes more rapid relaxation and smaller strain. As indicated by the theoretical analysis in chapter 4, the large dislocation density at the interface of between GaN and AlN grown at 1000 ℃ and 1100 ℃ caused rapid strain relaxation in GaN. The stress in GaN is affected by the AlN buffer and Si substrate. It was directly tensilely strained by AlN buffer firstly. The amount of tensile strain in GaN was determined by the perfection of its lattice. The lattice of GaN with higher quality is more perfect than low-quality one. High-quality GaN goes through compressive strain in larger thickness. After it has completely relaxed, it would be free from the constraint of underlying AlN buffer and strained by Si substrate, and then tensile strain occurs. This is the transition from compressive stress to tensile stress for GaN on AlN buffer grown at 1000 ℃, 1100 ℃ and 1200 ℃. From low to high T_AlN, the transition became slower and slower and the critical thickness of this transition of strain from compression to tension also increased. GaN on 1250-℃ grown AlN buffer, it was compressively strained through the thickness of 1.75 um.

So the enhancement of compressive strain brought by the improvement of GaN and AlN quality overtook the relaxation of it caused by the reduction of ∆𝑎𝑎𝐺𝐺𝑎𝑎𝐺𝐺−𝐴𝐴𝑡𝑡𝐺𝐺.