Characterization of Epitaxial GaN Thin Films by Positron [612585]

Characterization of Epitaxial GaN Thin Films by Positron
Spectroscopy
Doru Dinescu1,2,a)and Nikolay Djourelov1,b)
11Extreme Light Infrastructure Nuclear Physics, Horia Hulubei National Institute for Physics and Nuclear
Engineering, 30 Reactorului Street, 077125 Magurele, Ilfov county, Romania
2University Politehnica of Bucharest, Faculty of Applied Sciences, Splaiul Independentei, nr. 313, 060042,
Bucharest, Romania
a)Corresponding author: [anonimizat]
b)[anonimizat]
Abstract. GaN presents all the characteristics required for it to be considered a suitable material for field assisted e+moderation.
Even though typical e+diusion lengths in semiconductors such as Si or Ga are known to be between 200 and 300 nm, reported
values for L+in GaN are below 60 nm.
The interest in GaN for various applications
Due to the interest shown for GaN in other research fields, such as the development of high-electron-mobility transistors,
there have been improvements in reducing the defects in the crystal structure of the GaN thin films.
In this study positron annihilation experimental results using a slow positron beam are used to determine the L+and quality
of four epitaxial GaN wafers that are mass produced for device manufacturers.
INTRODUCTION
Since the 1990’s GaN has been one of the most promising materials in the development of semiconductor based
devices [1]. The wide direct band gap, high displacement energy and high thermal stability made GaN a key material
in the development of optoelectronics, high temperature and high power devices [2]. Perfect lattice semiconductors
such as Si or Ga are known to have L+=200 – 300 nm [3]. The wide band gap of GaN (3.4 eV) should translate to a
L+longer than the average semiconductor L+due to the epithermal positron di usion. The long e+diusion length,
the fact that GaN presents a negative e+workfunction and the high breakdown voltage ( 1 kV) means that GaN might
be a suitable material for field assisted e+moderation [4]. On the other hand, L+reported in the literature for GaN
are below 60 nm [5]. It is well known that during the di usion process e+get trapped in defects [6]. This a ects the
L+and implicitly the moderation e ciency, so it is very important for the moderation material to have a low defect
concentration. Positron annihilation spectroscopy is a powerful non-destructive method of studying the defects in a
material. By using the slow positron beam (SPB) technique depth profiling of the studied sample can be achieved.
GaN epitaxial wafers are produced for the manufacturing of high-electron-mobility transistors (HEMT). Due to the
constant development in HEMT research, over the years, there have been improvements in reducing the defects in the
crystal structure of the GaN thin films. The aim of the present study is to investigate by the Doppler broadening using
SPB the progresses made in the fabrication of GaN thin films which might be useful for the positron moderation.
SAMPLES AND EXPERIMENT
The GaN thin films studied in the present work are produced by NTT Advanced Technology Corporation (NTT AT).
For each sample the GaN thin film was grown using the molecular beam epitaxy technique on three kinds of substrates.
By precisely controlling the epitaxial growth conditions NTT AT provides great quality GaN epitaxial wafers for
device manufacturers, with high uniformity and high breakdown voltage. The studied samples will further be referred

to as it follows: GaN300 /Si, GaN700 /Si, GaN500 /SiC, GaN300 /Sp, where the number in the name represents the GaN
thickness in nm, as given by the producer, and Sp stands for Sapphire.
The Doppler Broadening Spectroscopy (DBS) and the Coincidence Doppler Broadening Spectroscopy (CDBS)
experiments were performed at the slow positron beam line at the Institute of High Energy Physics in Beijing, China.
For the DBS measurements, the gamma energy spectra were recorded by a HPGe detector, with a resolution of
FWHM =0.97 keV estimated for 511 keV . The detector was placed perpendicularly in respect to the positron beam
axis at a distance of 20 cm from the sample. The incident positron energy was controlled from E+=0.5 to 25 keV .
Each of the 35 experimental spectra was collected over a period of 8 minutes for a fixed E+, resulting in statistics
of 5105counts in the 511 keV region. The shape of the 511 keV peak was analyzed by the sharpness parameter
(S) and by the wings parameter ( W). Due to the correlation between the mean e+implantation depth zmandE+
(zm=(36=)E1:62
+nm, whereis the sample density in g cm3), the experimental data represents the depth profile of
S. To fit the S(E+) and W(E+) data the VEPFIT software was used [7]. We used the model 5 available in VEPFIT with
2 or 3 layers in addition to the surface e+states. The CDBS experiments were performed with two HPGe detectors
facing each other in order to detect the energies E
1andE
2of the annihilation
-rays in coincidence. The detectors
were set up at distance of 20 cm from the sample perpendicular to the e+beam direction. The resolution of the second
detector was FWHM =1.02 keV . The CDBS spectra were acquired for only two fixed E+per sample over a period of
14 hours for each measurement. The total statistics in each spectrum is 1107counts. The longitudinal momentum
distribution, M(pL) , of the annihilation pair, pL=2
E=c, was extracted from the 3D CDBS data by histograming the
dierence
E=E
1E
2with the condition j2m0c2(E
1+E
2)j<2:4 keV keV . In order to magnify the di erences
in the high momentum region, the momentum distributions are often presented as a ratio to a reference sample by
R(pL)=Msample(pL)=Mreference(pL).
RESULTS AND DISCUSSION
The presented best fits from Table 1 are obtained with two layer model, i:e:a single GaN layer and the Si substrate.
TABLE 1. Best fit parameters obtained by VEPFIT fitting of S(E+) and W(E+) depth profiles.
Sample GaN Substrate
L+(nm) S W d (nm) L+(nm) S W
GaN300 /Si 66(10) 0.4550(3) 0.0931(2) 347(7) 245 0.5257(3) 0.0447(1)
GaN700 /Si 96(7) 0.4540(3) 0.0944(1) 754(6) 245 0.5273(6) 0.0440(3)
GaN500 /SiC 60(6) 0.4562(3) 0.0929(2) 500(6) 150 0.4658(7) 0.0709(4)
1 0.4775(25) 0.0783(14) 1
GaN300 /Sp 37(3) 0.4504(20) 0.0950(25) 109(20)80 0.4232(3) 0.1077(2)
26(19) 0.4656(25) 0.0857(14) 173(16)y
near-surface.ynear-substrate.
The e ective L+, in Si(100) was fixed to 245 nm in order to reduce the uncertainties [8]. It can be seen that
the film thickness, d, as obtained by the fit overestimates the one given by the producer of the thin films. One of the
reason for the overestimation is the existence of an interfacing layer between the film and the substrate. Introducing
this bu er layer in the VEPFIT analysis leads to higher uncertainties so we preferred to stick to the simplest model.
In the case of GaN film on SiC the scatter of the data of the measured depth profiles is large (specially for S(E+)),
which is a result of the relatively close values of the characteristics SandWof the film and the substrate (see Fig. 1
a)). This caused problems in the VEPFIT analysis and we were forced not only to fix the L+in SiC to 150 nm and also
to introduce a highly defective layer, i.e. to apply a fit by a 3-layer model. It should be mentioned that according to
our experience, the introduction in the VEFIT analysis of an interface layer with small thickness and short di usion
length both of few nm (similarly to [9]) leads to unreliable best fit results since the last are strongly dependent on the
initial parameters. Whether the data in the presented in 1 for GaN500 /SiC obtained with the mentioned constraints
is trustworthy or not will be discussed below. A good fit of the experimental data of GaN film on sapphire was also
obtained by a 3-layer model. The di usion length in sapphire was fixed to 80 nm [10].
In Fig. 1 b) are plotted the characteristic SWpoints. The points for GaN are closely grouped except for the
point of the near-substrate layer in GaN300 /Sp indicating a higher defect concentration. Although the di erences in

FIGURE 1. a) The depth profiles S(E+) and W(E+) for the GaN thin films on all four substrates. The lines represent the best fit
obtained by VEPFIT; b) SWpoints of the characteristic best fit parameters for the GaN layer and the substrates.
the characteristic S(and W) of the GaN films are rather small, the parameters show an ordinary correlation with
L+. The only exception is the relatively short L+=37 nm in the near-surface layer for GaN300 /Sp which does not
correspond to the high S(respectively low W) parameter. Lack of correlation between SandL+was reported in
[5]. They suggested that the observed low L+values are due to e+interaction in dislocations. Even though such an
explanation sounds reasonable, we cannot draw firm conclusion if it is relevant for our case or the short L+in the
near-surface layer for GaN300 /Sp is an artificial e ect due to the complexity of the applied VEPFIT model. CDBS
spectra were measured at selected E+=7.5 keV (and 12.5 keV for GaN700 /Si) with a goal to get information for the
GaN films avoiding the surface influence as much as possible, and at E+=25 keV (the maximum possible) in order
to get information for the substrates. Due to the fact that the e+implantation profile width increases significantly with
theE+, due to the long L+, the information for the film is mixed with the one of the substrate i:e:
S(E+)=fepi(E+)Sepi
GaN+fsurf(E+)Ssurf+fGaNSGaN+fsub(E+)Ssub: (1)
The same equation is valid for the Wparameters and for the momentum distributions, as well. If we consider that
the momentum distributions for GaN in GaN300 /Si and GaN700 /Si are undistinguishable, which is supported by the
very close SWpoints for GaN (Fig. 1 b)), we easily find the characteristic momentum distributions MGaN(pL) and
MSi(pL) by solving the following system of linear equations:
MGaN300=Si(pL;)=fGaN300=Si
GaN()MGaN(pL)+fGaN300=Si
Si()MSi(pL); (2)
MGaN700=Si(pL;)=fGaN700=Si
GaN()MGaN(pL)+fGaN700=Si
Si()MSi(pL); (3)
wherestands for E+=25 keV and the coe cients fare extracted from the experimental data.
Further, using the CDBS data for E+=7.5 keV (12.5 keV), in an analogical way the momentum distributions
which comes from thermal e+annihilation on the GaN surface is extracted. The extracted characteristic momentum
distributions are shown in Fig. 2 a). The momentum distributions shown in Fig. 2 a) are in good agreement with
the data available in the literature for perfect GaN and it definitely di ers from the momentum distribution in case
of saturation trapping of e+in Ga vacancies [11]. Consequently, we may consider that the studied GaN films on Si
present a low defect concentration.
The momentum distributions for E+=7.5 keV in GaN300 /Si, GaN300 /Sp and GaN500 /SiC as a ratio to the data
taken at E+=12.5 keV in GaN700 /Si are shown in Fig. 2 b). The fraction from the thermal e+annihilating at the
GaN surface ranges from 0.07 to 0.17. The lines represent the experimental points corrected for the influence from the
substrates. Di erences in the quality of the GaN films on Si and SiC is not observed. However, the shape of the ratio
for GaN on sapphire definitely resembles the ratio shape for e+trapped in GaN defects like V Ga, VGa-VNand V Ga-ON
[11].

0 5 10 15 20 25 3010−510−410−310−210−1
pL (10−3 m0c)Probability

Si perfect
GaN perfect
VGa in GaN
Si extracted
GaN extracteda)
0 5 10 15 20 25 300.70.80.911.11.2
pL (10−3 m0c)Ratio to corrected GaN700/Si ( E+= 12.5 keV)

GaN700/Si ( E+= 12.5 keV)
GaN300/Si ( E+= 7.5 keV)
GaN300/Sp ( E+= 7.5 keV)
GaN500/SiC ( E+= 7.5 keV)b)FIGURE 2. a) Momentum distributions for GaN and Si extracted from the experimental CDBS data together with reproduced from
literature data represented by the lines; b) Momentum distributions obtained for samples GaN300 /Si, GaN500 /SiC, and GaN300 /Sp
measured at E+=7.5 keV and for GaN700 /Si at E+=12.5 keV as a ratio to corrected GaN700 /Si data. The thick lines represent
the pure GaN data, i:e:after the correction of the measured momentum distribution for the small contributions from the substrates.
CONCLUSIONS
In the present study PAS was used to characterize four epitaxial GaN wafers. The short L+of GaN thin films compared
to the typical L+in semiconductors was confirmed. On the other hand it was proven that improvements have been made
in the crystal structure of GaN thin films. ( L+(GaN700 /Si)=967 nm). The defect concentration in GaN300 /Sp is
higher than in the films on Si and SiC. The minimum level of the ratio is 0.8 well above 0.6, the level expected to be
seen in case of saturation e+trapping by defects in GaN compared to perfect lattice GaN [11]. The enhanced level can
be explained by a superposition of the two layers found by VEPFIT, the near-substrate which is highly defective with
the near-surface good quality layer.
ACKNOWLEDGMENTS
The authors wish to acknowledge, the funding of this work by the Research and Innovation Ministry, under the contract
27-ELI /2016, in the frame of the ELI-RO Program as well as the support of the team from the Positron Spectroscopy
Laboratory at the Institute of High Energy Physics in Beijing, China, especially Prof. Baoyi Wang, Prof Xingzhao
Cao, and all the PhD students which helped during the experiment.
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