実験番号：2504053

低次元窒化物系半導体ヘテロ構造の原子レベル急峻性評価

天野 浩
名古屋大学大学院工学研究科電子情報システム専攻

1. 測定実施日25 年 11 月 21 日 9 時 – 18 時 00 分 (2 シフト) ，BL8S1
25 年 11 月 29 日 9 時 – 18 時 00 分 (2 シフト) ，BL8S1
25 年 12 月 11 日 9 時 – 18 時 00 分 (2 シフト) ，BL8S1
26 年 02 月 25 日 9 時 – 18 時 00 分 (2 シフト) ，BL8S1
26 年 02 月 26 日 9 時 – 18 時 00 分 (2 シフト) ，BL8S1
26 年 03 月 11 日 9 時 – 18 時 00 分 (2 シフト) ，BL8S1
2. 概要
Atomic-level control is becoming increasingly important for the realization ofultimate-performance and highest efficiency InGaN-based light emitting diodes (LEDs). An insitu atomic level surface monitoring tool such as reflection high-energy electron diffraction(RHEED) is effective for observing the atomic arrangement of a surface during growth.
Unfortunately, however, RHEED cannot be applied to the in situ monitoring of the metalorganic
vapor phase epitaxy (MOVPE) process because a high-vacuum condition is necessary. There areseveral in situ monitoring tools suitable for use during the MOVPE process using light such as
interference monitoring and spectroscopic ellipsometry. In the case of interference monitoring,
although we can estimate the surface thickness, roughness by monitoring the oscillation and its
amplitude, the resolution is limited to close to the wavelength of the light used. In the case ofspectroscopic ellipsometry, real-time monitoring is difficult because of the limited speed of themeasurement. Furthermore, the interface and any changes underneath the surface are nottraceable by these techniques. Therefore, in situ X-ray measurement techniques are expected tobe applied for further analyzing and understanding the growth processes of the heterostructures,such as surface and interface roughness, strain relaxation, defects, and indium distributions andfluctuations and so on.

3．背景と研究目的
We have conducted an atomic scale investigation on semiconductor interface structures using
X-ray CTR scattering measurement. Demonstration of X-ray CTR scattering measurement andanalysis at room temperature was from the sample measured using synchrotron radiation at thePhoton Factory in the High Energy Accelerator Research Organization at Tsukuba, Japan.
Information on the grown layers can be deduced from the curve fitting to the X-ray CTR scatteringspectra. The fitting results were almost the same as the designed values. In a word, the X-ray
CTR scattering measurement can reveal interface roughness, compositional distributions, and layer
thicknesses, quantitatively and non-destructively. In order to realize in-situ X-ray CTR
measurements, an MOVPE growth system for GaN and related semiconductors was installed in thelaboratory level X-ray measurement system. A more detailed description of the system can befound in references [1-3]. XRR is also a non-destructive technique used for evaluation of filmthickness, density, and surface and interface roughness of thin film layer structures. This technique
is also important for in-situ X-ray measurements at growth temperatures since it can be used foramorphous, crystalline, and liquid samples. We used XRR to investigate the wetting layer of liquid
indium on GaN substrate [4]. Until now, we have demonstrated that it was possible to conduct in-situ
XRD measurement, in-situ X-ray CTR scattering measurement, and in-situ XRR measurement under
the growth ambient at the growth temperatures [5].
We propose new tools for in situ monitoring of the MOVPE growth process at the atomiclevel. The tool is grazing angle X-ray reflectivity measurement, for which atomic-level resolution is
obtained because of the short wavelength of X-rays [6]. In addition, X-ray reflectivity is also
sensitive to the density of valence electrons in each atom; therefore we can in situ measure theatomic-level surface roughness and heterostructure interface in real time. We observed two criticalpoints at which the X-ray intensity corresponds to the roughness change. The first critical pointshows the roughening without lattice relaxation, and the second point indicates the lattice relaxation
caused by the generation of defects. Therefore, the purpose of this research is to demonstrate
which point is the critical thickness of the strain relaxation. Since the thickness of InGaN is verythin (about 10 nm) on the GaN template, the lab-based X-ray is difficult to get the good spectraof the in-plane measurement due to the strange background noise. The synchrotron radiation is a
powerful tool to know the strain relaxation of our samples.

4．実験内容InGaN epilayers were grown on c-plane GaN templates using the in situ MOVPE system at 150
Torr. The templates were MOVPE-grown (0001) hexagonal GaN (2 μm thick) epilayers on sapphire(0001). The full width at half maximum of the rocking curve of these templates was about 285
acrsec at the 0002 Bragg peak. The MOVPE precursors used were trimethylindium (TMIn),trimethylgallium (TMGa), and ammonia (NH3) with nitrogen carrier gases. For InGaN growth at 830oC, the fluxes of TMIn and TMGa were 0.043 and 0.19 μmol/min, respectively. The total V/III ratio
was about 800,000.
Synchrotron Radiation (SR) at the beam line BL8S1 was used to ex situ confirm the strain
relaxation of the thick In0.11Ga0.89N grown on GaN template in the Aichi Synchrotron Radiation
Center, Aichi Pref. Japan. The wavelength of the X-ray was set at 1.35326 Å. NaI scintillator
detector whose ability is over 700,000 cps was used to collect the X-ray data. Symmetric andasymmetric 2theta-omega scans were along [0001] and [10-15] crystal direction around the GaN(10-15) diffraction peak. The angular resolution is 0.0021. The incident slit was 0.4mm×0.8 mm.
Since there were two receiving slits 0.5 mm in height and two solar slits 0.541° between detector andsample, the intensity of the background diffuse scattering was effectively removed. The indium
composition of the obtained InGaN epilayers was determined by X-ray CTR scattering
measurements for the fully strained single quantum well (SQW) structure.

5．結果および考察
a. The indium composition of the obtained InGaN epilayers was determined by X-ray CTR
scattering measurements for the fully strained single quantum well (SQW) structure using ex situ
synchrotron radiation. The CTR simulation results are shown in Fig. 1. The detailed description isshown in Ref. 7

Fig. 1 The results of X-ray CTR scattering spectra measured using SR with well fitted curves and theindium composition profiles with analysis results.
b. Using in situ MOVPE system located in Akasaki Researcher Center, the reflected X-ray
intensity was measured as a function of the growth time shown in Fig. 2. The total growth timechanges in the film thickness d2 and surface roughness of the InGaN layer calculated by theParratt32 program, assuming the strained InGaN epilayer, is shown by the solid line. Because theexperimental curve is well fitted by the calculated one, the surface roughening as a function of thegrowth time can be obtained from the calculation, as shown in Fig. 2 using the narrower solid line.

1.0
0.8

X-ray reflectivityhc(r,2)hrhc(r,1)

0.6
0.4

Roughness [nm]Continuous in situ XRR on In0.11Ga0.89N single layer

0.2
Calculated continuous in situ XRR
Calculated surface roughening
At 830 ºC05101520250.030

Growth time [min]Fig. 2. Surface roughening of the thick In0.11Ga0.89N grown on GaN template as a function of thegrowth time, as derived from the fitted experimental in situ XRR curve, assuming a strained
In0.11Ga0.89N epilayer.
This curve clearly shows two inflection points, which are the In0.11Ga0.89N epilayer thickness(the growth time multiplied by the growth rate) hc(r,1) and hc(r,2). In order to figure out the growth
process of In0.11Ga0.89N epilayer shown in Fig. 2, the grown sample was ex situ measured by theBL8S1 of synchrotron radiation center around GaN (10-15) reflection along [0001] and [10-15]
directions for symmetric and asymmetric 2theta-omega scans, respectively, as shown in Fig. 3. The
conventional reciprocal space mapping by schematic diagrams is shown in the right side of Fig. 3.
Fig. 3(a) shows the symmetric 2θ/ω scan along [0001] direction. The observed peaks are GaN(10-15) and fully strained InGaN. As expected, the fully relaxed InGaN peak is also observed in theasymmetric 2θ/ω scan along [10-15] direction, as shown in Fig. 3(b). It demonstrates that the strain
relaxation occurred as the InGaN thickness increased. In another word, the thickness of InGaN
epilayer exceeded the critical thickness. Moreover, the result also shows that the region closer to theGaN/InGaN interface is nearly pseudomorphic to the GaN template, whereas the surface region isfully relaxed. Based on Vegard's law and Hooke's law in detail, and interplanar crystal spacing
equation, the indium composition of the fully strained InGaN is 0.114, and the fully relaxed InGaN
is 0.135. For the fully strained In0.114Ga0.886N epilayer, the indium content is in very good agreementwith X-ray CTR scattering measurement results for the fully strained SQW structure with the same
growth condition. However, for the surface region, the indium fraction x increases as the film
relaxed. This point has been reported by S. Pereira et al. [8]. Since the X-ray intensities of InGaN
diffraction peaks may vary strongly with their thicknesses. The fully strained InGaN peak intensity is2.3 times higher than the fully relaxed InGaN peak intensity based on Fig. 3(a) and (b). Assuming
the constant growth rate, the thickness hc(r,2) of the In0.11Ga0.89N epilayer in Fig. 2 was 14.8 ± 0.4
nm, calculated by multiplying the growth time of 21.2 ± 0.3 min b the growth rate of 0.70 ± 0.02
nm/min. The thickness hr is about 6.3 nm, which is from hc(r,2) to the top surface of InGaN epilayer.
So the thickness hc(r,2) is about 2.34 times thicker than the thickness hr. The thickness ratio is ingood agreement with the above-mentioned X-ray peak intensity ratio. Therefore, the thickness hc(r,2)
seems to be the critical thickness of In0.11Ga0.89N epilayer. In Fig. 2, when the thickness of theIn0.11Ga0.89N epilayer was below the thickness hc(r,1), the surface roughness changed very slowly. As
the thickness of the growing epilayer increased and neared the critical thickness hc(r,2), slightly
increased surface roughening took place at its surface. The roughening rate was about 0.07 nm/min.
After the inflection point of hc(r,2), the roughness was firstly decreased and then largely increased. Itmight be impossible. So the roughness curve might not reflect the true changing of roughness, since
the fully strained InGaN on GaN model was used to analyze the experimental in situ XRR curve.
This phenomenon also demonstrates that the inflection point at hc(r,2) was caused by strain
relaxation.

X-ray intensity [a.u.]

106

(a)

Symmetric scan
GaN (10-15)

GaN (10-15)

Symmetric scan

Qz

107

105

ω=0104103●● GaN template◒ ◒ Fully strainedInGaNFully strained
InGaN (10-15)Qx

102101107 (b)Asymmetric scan

GaN (10-15)

GaN (10-15)

X-ray intensity [a.u.]

10Asymmetric scan

Qz

6510●
◓

ω=1

● GaN templaterelaxed◓ Fully
InGaN

104
103

Qx

Fully relaxed
InGaN (10-15)

102
101
85

86

87

882θ/ω [degree]

89

90

91

Fig. 3 2θ/ω scans around the GaN (10-15) diffraction peaks for the thick In0.11Ga0.89N
grown on GaN template. (a) Symmetric scan along [0001] direction for the fully strained
InGaN. (b) Asymmetric scan along [10-15] direction for the fully relaxed InGaN.
Schematic diagrams in the right of figures illustrating the effect of strain in thesymmetric and asymmetric scan of InGaN.

6.今後の課題
The investigation of AlGaN grown on GaN using real-time X-ray reflectivity is

conducted during growth for strain relaxation, Al distribution at the interface, and soon.

7.参考文献[1] Guangxu Ju, Koji Ninoi, Hajime Kamiya, Shingo Fuchi, Masao Tabuchi, and Yoshikazu Takeda:
X-ray characterization at growth temperatures of InxGa1-xN growth by MOVPE, Journal of CrystalGrowth, Vol. 318, pp. 1143-1146 (2011).
[2] Koji Ninoi, Guangxu Ju, Hajime Kamiya, Shingo Fuchi, Masao Tabuchi, and Yoshikazu Takeda:
Novel system for X-ray CTR scattering measurement on in-situ observation of OMVPE growth ofnitride semiconductor heterostructures, Journal of Crystal Growth, Vol. 318, pp. 1139-1142 (2011).
[3] Yoshikazu Takeda, Koji Ninoi, Guangxu Ju, Hajime Kamiya, Tetsuya Mizuno, Shingo Fuchi,and Masao Tabuchi: X-ray characterization of GaN and related materials at growthtemperatures-system design and measurements, IOP Conf. Ser.: Material Science and Engineering,Vol. 24, 012002 (2011)[4] Guangxu Ju, Shingo Fuchi, Masao Tabuchi, and Yoshikazu Takeda: In situ X-ray reflectivity ofindium supplied on GaN templates by Metalorganic Vapor Phase Epitaxy, Journal of Applied
Physics, Vol.114, pp.124906-124913 (2013).
[5] Guangxu Ju, Shingo Fuchi, Masao Tabuchi, and Yoshikazu Takeda: In situ X-ray measurementsof MOVPE growth of InxGa1-xN single quantum well, Journal of Crystal Growth, Vol. 370, pp.
36-41 (2013).
[6] Guangxu Ju, Shingo Fuchi, Masao Tabuchi, Hiroshi Amano, and Yoshikazu Takeda: Continuousin situ X-ray reflectivity investigation on InGaN epitaxial growth by metalorganic vapor phase
epitaxy, Journal of Crystal Growth, Vol.407, pp. 68-73 (2014).
[7] Guangxu Ju, Yoshio Honda, Masao Tabuchi, Yoshikazu Takeda, and Hiroshi Amano: In situ
X-ray investigation of changing barrier growth temperatures on InGaN single quantum wells in
metal-organic vapor phase epitaxy, Journal of Applied Physics (AIP), Vol. 115, pp.094906 (2014).
[8] S. Pereira, M. R. Correia, E. Perreira, K. P. O’Donnell, C. Trager-Cowan, F. Sweeney, and E. Alves:
Compositional pulling effects in InxGa1-xN/GaN layers: A combined depth^-resolved
cathodoluminescence and Rutherford backscattering/channeling study, Physical Review B, 64,
pp.205311/1-5 (2011).