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Homoepitaxial growth of HVPE-GaN doped with Si
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Homoepitaxial growth of HVPE-GaN doped with Si M. Iwinska n, T. Sochacki, M. Amilusik, P. Kempisty, B. Lucznik, M. Fijalkowski, E. Litwin-Staszewska, J. Smalc-Koziorowska, A. Khapuridze, G. Staszczak, I. Grzegory, M. Bockowski Institute of High Pressure Physics PAS, Sokolowska 29/37, 01-142 Warsaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2016 Received in revised form 16 August 2016 Accepted 19 August 2016 Communicated by Jaime Andrade Freitas
Results of growth of high structural quality gallium nitride single crystals doped with silicon are described in this paper. Dichlorosilane was used as precursor of silicon in the hydride vapor phase epitaxy method. Crystallization runs with different flows of dichlorosilane were performed and compared. Oneinch free-standing HVPE-GaN crystals of high structural quality and high purity, previously grown on ammonothermal GaN substrates, were used as seeds. Structural, electrical, and optical properties of HVPE-GaN doped with silicon are presented and discussed in detail. A laser diode built on the homoepitaxially grown GaN is demonstrated. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A3. Hydride vapor phase epitaxy B1. GaN B1. Nitrides B2. Semiconducting III–V materials
1. Introduction Hydride Vapor Phase Epitaxy (HVPE) is the most popular method for production of GaN substrates with quality sufficient for optoelectronic and electronic devices [1,2]. Recently, it has been shown that ammonothermally grown GaN (Am-GaN) crystals can be successfully used as seeds for HVPE growth [3,4]. Structural properties of free-standing (F-S) HVPE-GaN crystals, sliced from Am-GaN seeds, do not differ from excellent structural properties of the seeds [5,6]. Additionally, HVPE-GaN is high-purity material, i.e. concentration of all impurities, including Si, is at the level of 1016– 1017 cm 3 [7,8]. As a result, the free carrier concentration in HVPEgrown crystal can be as low as 3–5 1016 cm 3 [8]. F-S HVPE-GaN can be successfully used as seed material for consecutive HVPE growth runs. The low level of impurities together with high crystallographic quality of F-S HVPE-GaN crystals enables the next step, namely introducing intentional doping to the growth process and obtaining highly conductive crystals suitable for laser diodes and vertical electronic devices. This work describes a method to produce silicon-doped HVPEGaN (HVPE-GaN: Si) crystals of high structural quality and low level of other impurities. One-inch F-S HVPE-GaN crystals (grown before on Am-GaN substrates) were used as seeds. Growth processes were carried out in a home-built quartz horizontal HVPE n
Corresponding author. E-mail address: [email protected] (M. Iwinska).
reactor. Dichlorosilane (H2SiCl2), used as the precursor of silicon, was transported together with gallium chloride (GaCl) into the growth zone. HVPE crystallization runs with different flows of H2SiCl2 were performed. A morphologically stable crystallization, with a hillock growth mode, was obtained. HVPE-GaN:Si crystals were characterized with X-ray diffraction (XRD), Raman spectroscopy, low-temperature photoluminescence (L-T PL), optical as well as transmission electron microscopies, Hall measurements, and Secondary Ion Mass Spectrometry (SIMS). It is shown that there is no significant change in silicon incorporation in HVPE-GaN when different flows of H2SiCl2 are employed. However, the highest free carrier concentration was measured for a sample with the lowest H2SiCl2 flow applied. This phenomenon will be explained in this work. Additionally, utility of F-S HVPE-GaN:Si crystals as substrates for laser diodes is demonstrated in this paper.
2. Experimental setup A home-built horizontal quartz HVPE reactor with a rotating quartz susceptor designed for a 2 in. substrate was used as growth apparatus. The reactor with two temperature zones is schematically presented in Fig. 1. In the low-temperature zone of the reactor ( 850 °C) hydrochloride (HCl) reacted with gallium to form gallium chloride (GaCl). Then, GaCl was transported by the carrier gas (H2) to the high-temperature zone. Here, at 1045 °C, GaCl reacted with ammonia to form GaN. The seed was placed in the
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Fig. 1. Scheme of the HVPE reactor; GaCl is supplied vertically with the use of shower head type quartz nozzles; NH3 is supplied by a quartz nozzle located on the same level as the susceptor; H2SiCl2 is added to the gallium source, thus it is transported with GaCl to the reaction zone.
center of the susceptor (see Fig. 1). GaCl was supplied vertically above the surface of the susceptor with the use of shower head type quartz nozzles. NH3 was supplied by a quartz nozzle located on the same level as the susceptor and a few millimeters distant from it. Dichlorosilane was added to the gallium source in the reactor (see Fig. 1). A special quartz tube was introduced to the tube where HCl reacts with gallium. Thus, dichlorosilane was flown with GaCl to the reaction zone. Three H2SiCl2 flows were used, 0.002, 0.04, and 0.8 ml min 1, with all other experimental conditions fixed and the same as presented in [5,7]. HCl flow was 48 ml min 1 and V/III ratio was equal to 20. The crystallization time varied from 2 to 5 h. One-inch F-S HVPE-GaN crystals with misorientation 0.3 degree to the m-direction were used as seeds. Their (0001) surfaces were prepared by mechano-chemical polishing and then cleaning to the epi-ready state. All crystals were of high structural quality with full width at half maximum (FWHM) values of X-ray rocking curves for (002) reflection of about 30–40 arc s in and bowing radii of (0001) crystallographic planes of about 20–30 m. The free carrier concentration of all seeds was of the order of 5 1016 cm 3. The HVPE samples were studied by means of optical microscopy with differential interference contrast (DIC) and XRD. The (002) symmetrical reflection X-ray rocking curves (omega scan) of the seeds and the deposited material were measured using a highresolution Philips X′Pert Pro X-ray diffractometer, equipped with a four-reflection Bartels monochromator. The X-ray beam had dimensions of 1 10 mm (1 mm is the width in the diffraction plane). Defect selective etching (DSE) in molten KOH-NaOH eutectic [9] was performed in order to reveal the etch pits and determine the etch pit density (EPD) of the HVPE-GaN crystals. SIMS was used to investigate the concentrations of impurities in the HVPE material. Raman spectroscopy was performed at room temperature, in a backscattering configuration, using a Jobin–Yvon
T64000 spectrometer equipped with a CCD camera. A 514.53 nm line of argon laser was used. Low-temperature ( 15 K) photoluminescence (LT-PL) spectra were obtained with a 325 nm line of a He–Cd laser and with power density of about 5 W/cm2 as an excitation source. The diameter of the spot was approximately 200 mm. Emission from the sample, collected in backscattering geometry and dispersed by a SPEX500M spectrometer, was detected by a photomultiplier (Hamamatsu photomultiplier tube r943–02). FEI TECNAI G2 F20 S-TWIN electron microscope was used for TEM investigation of a laser diode structure built on a F-S HVPE-GaN:Si substrate.
3. Results Fig. 2 presents an HVPE-GaN:Si crystals grown on F-S HVPEGaN seeds and their morphology, which was the same as in case of undoped HVPE-GaN. Hillock growth mode was observed on the crystal's growing surface, as was reported earlier for growth with no intentional doping [7,10]. However, in case of the highest H2SiCl2 flow (0.8 ml min 1) some instability (like step bunching growth mode) was observed on the sides of the hillock (see Fig. 2b). For the lowest H2SiCl2 flow (0.002 ml min 1) typical hillocks were observed on the surface (Fig. 2c). The structural quality of all HVPE-GaN:Si crystals reported in this paper was extremely high and comparable with the structural quality of the seeds. Two X-ray rocking curves, for the seed and HVPE-GaN:Si layer (as-grown, presented in Fig. 2a), are shown in Fig. 3a and b, respectively. FWHM value of rocking curve was of the order of 30 arc s for the seed as well as for the HVPE-GaN:Si layer. Etching the (0001) surface of the HVPE-GaN: Si, at 500 °C in molten KOH-NaOH solution after mechanical and chemo-mechanical polishing, revealed EPD on the order of 5 104 cm-2. Similar to undoped GaN samples [11], large, average, and small size
Fig. 2. a) 800 μm-thick layer HVPE-GaN:Si grown with the highest H2SiCl2 flow (0.8 ml min 1) on F-S HVPE-GaN seed; 1-mm grid; b) instabilities (step bunching mode) observed on the as-grown surface for H2SiCl2 flow of 0.8 ml min 1; c) hillocks visible on crystal grown with H2SiCl2 flow of 0.002 ml min 1.
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Fig. 3. XRD rocking curve, (002) reflection of a) F-S HVPE-GaN (seed); FWHM value: 28 arc s; b) HVPE-GaN:Si layer (as grown); FWHM value: 32 arc s.
Fig. 4. a) Raman spectra for undoped F-S HVPE-GaN (red line) and doped F-S HVPE-GaN:Si (black line); peaks LPP þ and LPP , E2 and A1(LO) are marked; b) E2 peak frequency on c-plane of F-S HVPE-GaN:Si in the center of the sample; The Raman spectra were acquired along the diameter on a distance of 9 mm with a 250 mm step. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
diameter pits were observed. Fig. 4a shows two Raman spectra, shifted on the intensity axis for clarification. The upper line (red) represents a Raman spectrum of a F-S HVPE-GaN seed. Two peaks are well visible: E2 and A1(LO). Raman spectrum for HVPE-GaN layer doped with Si (0.8 ml min 1 flow of H2SiCl2) is the lower line (black). It should be remarked that in this case the A1(LO) peak disappeared and two peaks: LPP þ and LPP , appeared. Fig. 4b shows the position of E2 peak measured on the F-S HVPE-GaN: Si. The crystal was investigated on its (0001) surface along the diameter, on a distance of 9 mm with a 250 mm step. Only very small changes are observed in the E2 peak frequency. In order to investigate electrical and optical properties of HVPEGaN:Si, the HVPE layer was sliced from the seed. Values of free carrier concentration were calculated from Raman spectra obtained on the c-plane of the investigated crystal. The free carrier concentration n, according to [12], was calculated from the following equation:
n=
ωp2ε∞ε0m* e2
(1)
where ε0 represents the vacuum permittivity, m* is the effective mass of electron and equals 0.2 of the electron mass, e is the charge of the electron, ε1 is the high frequency dielectric constant of GaN, and ωp represents the plasmon frequency. The last one was calculated from:
ωp =
ω−ω+ ωT
(2)
where ω and ω þ are determined from Raman spectra frequencies of LPP and LPP þ modes, respectively, and ωT is transversal phonon frequency. Calculated values of free carrier
Fig. 5. Distribution of free carrier concentration (calculated from Raman spectra) along the diameter of F-S HVPE-GaN:Si grown with 0.8 ml min 1 of H2SiCl2 flow.
concentration varied from 1 1019 cm 3 for GaN grown with the lowest H2SiCl2 flow (0.002 ml min 1) to 3 1018 cm 3 in case of higher H2SiCl2 flow (0.8 ml min 1). Fig. 5 presents free carrier concentration determined from Raman spectra measured for 50 points along the diameter of F-S HVPE-GaN:Si grown with the 0.8 ml min 1 of H2SiCl2 flow. Distribution of carrier concentration on the (0001) surface was very uniform. Small deviation from the mean value of 3.2 1018 cm 3 was observed only close to the edges. Hall measurements performed for a sample grown with 0.8 ml min 1 of H2SiCl2 gave the value of free carrier concentration of 1 1018 cm 3 with carrier mobility of 270 cm2 V 1s 1. SIMS measurements showed that Si concentration in F-S HVPEGaN:Si was always higher than 1 1019 cm 3 and slightly depended on the H2SiCl2 flow. Four SIMS depth profiles for F-S HVPE-
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Fig. 6. SIMS depth profiles for F-S HVPE-GaN:Si samples grown with different H2SiCl2 flows a) 0.8 ml min 1; b) 0.04 ml min 1; c) 0.002 ml min 1; d) 0 ml min 1 (undoped); hydrogen, oxygen, and carbon are on the same low level; only difference in silicon concentration is observed.
Table 1 Concentration of silicon and free carrier concentration in HVPE-GaN doped with silicon with different flows of H2SiCl2. H2SiCl2 flow [ml min 1]
Si concentration [cm 3]
Free carrier concentration [cm 3]
0.8 0.04 0.002
6 1019 3 1019 2 1019
3 1018 4 1018 1 1019
GaN:Si grown with the highest (0.8 ml min 1), medium (0.04 ml min 1), the lowest (0.002 ml min 1) H2SiCl2 flows, and undoped material are presented in Fig. 6a, b, c, and d, respectively. For the highest flow of H2SiCl2 the silicon concentration in HVPEGaN:Si was the highest, at the level of 6 1019 cm 3. With decreasing of dichlorosilane flow the silicon concentration in HVPEGaN slightly decreased to 2–3 1019 cm 3. In case of undoped material, concentration of silicon was at the level of 3 1017 cm 3. Values of silicon concentrations and the free carrier concentration for samples grown with different H2SiCl2 flows are summarized in Table 1. Concentrations of others impurities were at the same low level in all examined cases. Fig. 7 shows the L-T PL spectra for undoped (with Si concentration at the level of 3 1017 cm 3 [8]) and silicon-doped HVPE-GaN samples. The spectra were
Fig. 7. L-T PL spectra for F-S HVPE-GaN:Si grown with various H2SiCl2 flows.
normalized do the near band edge emission intensity in order to compare the intensities of yellow luminescence (YL) peaks visible at 2.2–2.3 eV. It is worth to mention that the intensity of YL increases with increasing of Si concentration in the material. For the undoped material the spectrum between 3 and 3.5 eV is dominated by recombination processes, associated with annihilation of
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Fig. 8. TEM of the MOCVD LD quantum wells structure; no defects are observed. Layers comprising quantum barriers (QBs), quantum wells (QWs), and cap are indicated.
free and bound excitons, and their phonon replicas. In case of Sidoped GaN a band between 3 and 3.3 eV appears. The visible peaks are connected to DAP recombination. Free-standing HVPE-GaN:Si crystal, with free carrier concentration of 3 1018 cm 3, was used as a substrate for a laser diode (LD) structure. The prepared substrate was 300 mm-thick and 1 cm2 of lateral size. The substrate was misoriented and the obtained misorientation angle was measured by XRD in 9 points on the surface. The offcut variation did not exceed 0.1 degree. The substrate was used for MOCVD growth of a GaN-based LD. Details of the device structure were described elsewhere [13]. The grown epitaxial layers (quantum wells) were examined by TEM (Fig. 8b). The analysis showed that the active region had a good structural quality and no defects propagated from the substrate. Etching of the p-type cap revealed the EPD of the order of 5 105 cm 2. This value is one order of magnitude higher than the EPD of the used substrate. The results of optical and electrical measurements (wavelength, voltage-current, and optical power-current characteristics) of the processed LD were as follows: wavelength λ ¼410 nm, threshold current Ith ¼50 mA, threshold voltage Uth ¼ 7 V, and slope efficiency ɳ ¼0.35 W/A.
4. Discussion The presented results show that intentional incorporation of silicon to HVPE-GaN did not change the high structural quality of obtained gallium nitride crystals. Thus, the high quality of the initial seed (ammonothermally grown GaN used for obtaining the undoped HVPE-GaN seed) was not lost. This was confirmed by XRD measurements (see Fig. 3). The etch pit density remained at the same low level as in the seed material, of the order of 5 104 cm 2. The revealed etch pits correspond to edge, screw, and mixed dislocations in the crystal [11]. Raman spectroscopy allowed examining biaxial strain in the deposited doped film by measuring the position of E2 peak on the c-plane (see Fig. 4). The results showed that there is no biaxial strain in the free-standing crystal. The grown HVPE layers were transparent and crack-free, as shown on Fig. 2a. The morphology also remained the same as in case of unintentionally doped HVPE-GaN. Hillock growth mode was observed on the surface of the growing crystal. For the highest
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H2SiCl2 flow employed some morphological instabilities were, however, observed. These may result from a high concentration of Si atoms on the surface disturbing the macro step flow on the sides of the hillock. For lower flows of H2SiCl2 the growth mode is the same as in case of undoped HVPE-GaN growth. The presented doping with silicon allowed obtaining highly conductive n-type HVPE-GaN crystals of very homogeneous free carrier concentration on their c-planes. Electron concentration was determined by Raman spectroscopy and Hall measurements. Data from both methods were in good agreement. Silicon concentration increased slightly with increasing the H2SiCl2 flow. On the other hand, the free carrier concentration decreased. Additionally, SIMS measurements showed that the Si concentration in the material is always higher than the free carrier concentration. Data presented in Table 1 clearly show that not all Si atoms are electrically active. This suggests that a large fraction of silicon donors was compensated by an acceptor level. Gallium vacancies may be partially responsible for this effect. Ab initio calculations carried out by Van de Walle and Neugebauer [14] showed that the energy barrier of gallium vacancy formation reaches minimum when the Fermi level is at the bottom of conduction band. This means that in highly doped n-type material gallium vacancies can be generated relatively easily. Van de Walle and Neugebauer also showed that the deep acceptor level introduced by the Ga vacancy (or related complexes) is responsible for the yellow luminescence (YL) in GaN. Additionally, the formation of Ga vacancies may be enhanced by formation of complexes with donor impurities. Similar effect was observed earlier for GaN grown by High Nitrogen Pressure Solution (HNPS) growth method [15]. For highly n-type crystals, which contained a lot of gallium vacancies (of the order of 5 1018 cm 3) as well as oxygen atoms (5 1019 cm 3), a strong increase in the YL signal was observed. In the case of the F-S HVPE-GaN, the oxygen level was extremely low, but concentration of Ga vacancies increased with formation of the complex: gallium vacancy – silicon in gallium position. Therefore, silicon incorporation enhanced the YL intensity, what was well observed (see Fig. 7). Obviously, the difference between the concentration of incorporated Si and the free carrier concentration can be decreased if appropriate H2SiCl2 flow is employed. Lower H2SiCl2 flow allows maintaining a high silicon concentration and increasing the free carrier concertation. However, Si concentration is still higher than the concentration of electrons. This phenomenon is also present in unintentionally undoped crystals. As was shown by Freitas et al. [8], silicon concentration on the c-plane of undoped HVPE-GaN is at the level of 2 1017 cm 3. Hall measurements on the c-plane of undoped HVPE-GaN give the free carrier concentration value at the level of 3–5 1016 cm 3 [5]. This suggests that part of Si atoms is compensated by acceptors states, most probably associated with gallium vacancies and these may be the reason for the very weak but visible YL peak in the undoped material presented in this work (Fig. 7) and described in other work [5]. The bands which appear in the doped material between 3 eV and 3.3 eV are assigned to DAP recombination. It indicates the presence of a shallow acceptor state in the samples. This may also be, apart from gallium vacancies, the reason for compensation of Si in both doped and undoped HVPE-GaN. The acceptor may be Mg, since the 3.27 eV band is associated to Mg in Si-doped GaN. As was shown in [7], for a very low growth rate Mg was detected by SIMS at the level of 2– 3 1017 cm 3. The origin of Mg in the presented GaN crystals can be the gallium source used in the HVPE reactor. Free-standing HVPE-GaN:Si was used as a substrate for a laser diode structure. Due to high crystalline perfection of the Si-doped material it was possible to miscut the substrate very uniformly. This allowed growing epitaxial layers with alloy compositions homogenous on the entire surface. It is well known that indium
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incorporation as well as hole concertation in the p-type layer of a LD depend on misorientation angle of the substrate [16,17]. Thus, a very high crystalline quality of the wafer enables a large reproducibility of devices. Laser diode chips based on HVPE-GaN:Si all had the same parameters. Additionally, the low defect density in the substrate allowed deposition of a LD structure with a low defect concentration. This, in turn, results in a long life time of the devices, as demonstrated by S. Uchida et. al [18]. It should be remarked that the EPD in the LD structure (measured on top of the laser diode in the p-type cup) increased only one order of magnitude compared to the EPD in the F-S HVPE-GaN:Si substrate. TEM image of the LD epitaxial layers (see Fig. 8) shows that the quantum wells were defect free. One can suppose, thus, that the increase of the EPD from 5 104 cm 2 to 5 105 cm 2 occurred in the GaN p-type layer doped with magnesium.
5. Summary Preliminary results of intentional incorporation of silicon into HVPE-grown GaN with maintained high structural quality of the crystals were demonstrated in this paper. Dichlorosilane, used as the precursor of Si, was transported with gallium chloride into the crystallization zone. One-inch F-S HVPE-GaN crystals of extremely high structural quality, previously grown on the Am-GaN seeds and sliced from them, were used as substrates. It was shown that for higher flow of H2SiCl2 silicon incorporation into HVPE-GaN crystal slightly increased but it was always higher than the free carrier concentration. This can be explained by the effect of compensation. A large fraction of elect silicon donors was compensated by acceptor levels, in this case, most probably, gallium vacancies. For lower H2SiCl2 flow the value of free carrier concentration increased and the difference between silicon concentration and electron concentration was reduced. Free carrier concertation was very uniform across the (0001) surface of the doped samples. This result, combined with the high crystalline quality of the crystallized material, allowed to use F-S HVPE-GaN: Si as a substrate for laser diode. Parameters of the obtained laser diode showed that this n-type material is promising for preparing substrates required for optoelectronic (laser diodes) and electronic (vertical transistors) applications.
Acknowledgment This research was partially supported by The National Center
for Research and Development: PBS1/B5/7/2012 and PBS3/B5/32/ 2015 and by Polish National Science Center through project No. 2012/05/B/ST3/02516. The authors would like to thank Dr. R. Jakiela from IP PAS for SIMS measurements and TopGaN processing team for their help in preparing the laser diode.
References [1] T. Paskova, M. Bickerman, Handbook of Crystal Growth, vol. 602, Second ed., Elsevier, 2015. [2] A. Koukitu, Y. Kumagai, Technology of Gallium Nitride Crystal Growth, vol. 31, Springer-Verlag, Heidelberg, 2010. [3] T. Sochacki, Z. Bryan, M. Amilusik, R. Collazo, B. Lucznik, J.L. Weyher, G. Nowak, B. Sadovyi, G. Kamler, R. Kucharski, M. Zajac, R. Doradzinski, R. Dwilinski, I. Grzegory, M. Bockowski, Z. Sitar, Appl. Phys. Express 6 (2013) 075504. [4] T. Sochacki, M. Amilusik, B. Lucznik, M. Fijalkowski, J.L. Weyher, B. Sadovyi, G. Kamler, G. Nowak, E. Litwin-Staszewska, A. Khachapuridze, I. Grzegory, R. Kucharski, M. Zajac, R. Doradzinski, M. Bockowski, Jpn. J. Appl. Phys. 53 (2014) 05FA04. [5] T. Sochacki, Z. Bryan, M. Amilusik, Milena Bobea, M. Fijalkowski, I. Bryan, B. Lucznik, R. Collazo, J.L. Weyher, R. Kucharski, I. Grzegory, M. Bockowski, Z. Sitar, J. Cryst. Growth 394 (2014) 55. [6] L. Kirste, A.N. Danilewsky, T. Sochacki, K. Kohler, M. Zajac, R. Kucharski, M. Bockowski, P.J. McNally, ECS J. Solid State Sci. Technol. 4 (8) (2015) P324–P330. [7] T. Sochacki, M. Amilusik, M. Fijalkowski, B. Lucznik, J.L. Weyher, I. Grzegory, R. Kucharski, M. Iwinska, M. Bockowski, J. Cryst. Growth 407 (2014) 52. [8] J.A. Freitas Jr., J.C. Culbertson, N.A. Mahadik, T. Sochacki, M. Bockowski, M. Iwinska, Cryst. Growth Des. 15 (2015) 4837. [9] G. Kamler, J.L. Weyher, I. Grzegory, E. Jezierska, T. Wosinski, J. Cryst. Growth 246 (2002) 21. [10] T. Sochacki, M. Amilusik, B. Lucznik, M. Fijalkowski, J.L. Weyher, G. Nowak, B. Sadovyi, G. Kamler, R. Kucharski, M. Iwinska, I. Grzegory, M. Bockowski, J. Cryst. Growth 403 (2014) 32. [11] T. Sochacki, M. Amilusik, M. Fijalkowski, M. Iwinska, B. Lucznik, J.L. Weyher, G. Kamler, R. Kucharski, I. Grzegory, M. Bockowski, Phys. Status Solidi B 252 (5) (2015) 1172. [12] R. Lewandowska, J.L. Weyher, J.J. Kelly, L. Konczewicz, B. Lucznik, J. Cryst. Growth 307 (2007) 298–301. [13] I. Grzegory, M. Bockowski, T. Suski, P. Perlin, C. Skierbiszewski, M. Sarzyński, S. Krukowski, S. Porowski, III-Nitride Semiconductors and Their Modern Devices, vol. 18, Oxford Univ. Press, 2013. [14] J. Neugebauer, Ch.G. Van de Walle, Appl. Phys. Lett. 69 (4) (1996) 22, 503. [15] T. Suski, E. Litwin-Staszewska, R. Piotrzkowski, R. Czernecki, M. Krysko, S. Grzanka, G. Nowak, G. Franssen, L.H. Dmowski, M. Leszczynski, P. Perlin, B. Lucznik, I. Grzegory, R. Jakiela, Appl. Phys. Lett. 93 (17) (2008) 172117. [16] S. Keller, C.S. Suh, N.A. Fichtenbaum, M. Furukawa, R. Chu, Z. Chen, K. Vijayraghavan, S. Rajan, S.P. DenBaars, J.S. Speck, U.K. Mishra, J. Appl Phys. 104 (9) (2008) 093510. [17] T. Suski, P. Perlin, H. Teisseyre, M. Leszczynski, I. Grzegory, J. Jun, M. Bockowski, S. Porowski, T.D. Moustakas, Appl. Phys. Lett. 67 (1995) 2188. [18] S. Uchida, M. Takeya, S. Ikeda, T. Mizuno, T. Fujimoto, O. Matsumoto, S. Goto, T. Tojyo, M. Ikeda, IEEE J. Sel. Top. Quantum Electron. 9 (5) (2003) 1252.
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Homoepitaxial growth of HVPE-GaN doped with Si M. Iwinska n, T. Sochacki, M. Amilusik, P. Kempisty, B. Lucznik, M. Fijalkowski, E. Litwin-Staszewska, J. Smalc-Koziorowska, A. Khapuridze, G. Staszczak, I. Grzegory, M. Bockowski Institute of High Pressure Physics PAS, Sokolowska 29/37, 01-142 Warsaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2016 Received in revised form 16 August 2016 Accepted 19 August 2016 Communicated by Jaime Andrade Freitas
Results of growth of high structural quality gallium nitride single crystals doped with silicon are described in this paper. Dichlorosilane was used as precursor of silicon in the hydride vapor phase epitaxy method. Crystallization runs with different flows of dichlorosilane were performed and compared. Oneinch free-standing HVPE-GaN crystals of high structural quality and high purity, previously grown on ammonothermal GaN substrates, were used as seeds. Structural, electrical, and optical properties of HVPE-GaN doped with silicon are presented and discussed in detail. A laser diode built on the homoepitaxially grown GaN is demonstrated. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A3. Hydride vapor phase epitaxy B1. GaN B1. Nitrides B2. Semiconducting III–V materials
1. Introduction Hydride Vapor Phase Epitaxy (HVPE) is the most popular method for production of GaN substrates with quality sufficient for optoelectronic and electronic devices [1,2]. Recently, it has been shown that ammonothermally grown GaN (Am-GaN) crystals can be successfully used as seeds for HVPE growth [3,4]. Structural properties of free-standing (F-S) HVPE-GaN crystals, sliced from Am-GaN seeds, do not differ from excellent structural properties of the seeds [5,6]. Additionally, HVPE-GaN is high-purity material, i.e. concentration of all impurities, including Si, is at the level of 1016– 1017 cm 3 [7,8]. As a result, the free carrier concentration in HVPEgrown crystal can be as low as 3–5 1016 cm 3 [8]. F-S HVPE-GaN can be successfully used as seed material for consecutive HVPE growth runs. The low level of impurities together with high crystallographic quality of F-S HVPE-GaN crystals enables the next step, namely introducing intentional doping to the growth process and obtaining highly conductive crystals suitable for laser diodes and vertical electronic devices. This work describes a method to produce silicon-doped HVPEGaN (HVPE-GaN: Si) crystals of high structural quality and low level of other impurities. One-inch F-S HVPE-GaN crystals (grown before on Am-GaN substrates) were used as seeds. Growth processes were carried out in a home-built quartz horizontal HVPE n
Corresponding author. E-mail address: [email protected] (M. Iwinska).
reactor. Dichlorosilane (H2SiCl2), used as the precursor of silicon, was transported together with gallium chloride (GaCl) into the growth zone. HVPE crystallization runs with different flows of H2SiCl2 were performed. A morphologically stable crystallization, with a hillock growth mode, was obtained. HVPE-GaN:Si crystals were characterized with X-ray diffraction (XRD), Raman spectroscopy, low-temperature photoluminescence (L-T PL), optical as well as transmission electron microscopies, Hall measurements, and Secondary Ion Mass Spectrometry (SIMS). It is shown that there is no significant change in silicon incorporation in HVPE-GaN when different flows of H2SiCl2 are employed. However, the highest free carrier concentration was measured for a sample with the lowest H2SiCl2 flow applied. This phenomenon will be explained in this work. Additionally, utility of F-S HVPE-GaN:Si crystals as substrates for laser diodes is demonstrated in this paper.
2. Experimental setup A home-built horizontal quartz HVPE reactor with a rotating quartz susceptor designed for a 2 in. substrate was used as growth apparatus. The reactor with two temperature zones is schematically presented in Fig. 1. In the low-temperature zone of the reactor ( 850 °C) hydrochloride (HCl) reacted with gallium to form gallium chloride (GaCl). Then, GaCl was transported by the carrier gas (H2) to the high-temperature zone. Here, at 1045 °C, GaCl reacted with ammonia to form GaN. The seed was placed in the
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Fig. 1. Scheme of the HVPE reactor; GaCl is supplied vertically with the use of shower head type quartz nozzles; NH3 is supplied by a quartz nozzle located on the same level as the susceptor; H2SiCl2 is added to the gallium source, thus it is transported with GaCl to the reaction zone.
center of the susceptor (see Fig. 1). GaCl was supplied vertically above the surface of the susceptor with the use of shower head type quartz nozzles. NH3 was supplied by a quartz nozzle located on the same level as the susceptor and a few millimeters distant from it. Dichlorosilane was added to the gallium source in the reactor (see Fig. 1). A special quartz tube was introduced to the tube where HCl reacts with gallium. Thus, dichlorosilane was flown with GaCl to the reaction zone. Three H2SiCl2 flows were used, 0.002, 0.04, and 0.8 ml min 1, with all other experimental conditions fixed and the same as presented in [5,7]. HCl flow was 48 ml min 1 and V/III ratio was equal to 20. The crystallization time varied from 2 to 5 h. One-inch F-S HVPE-GaN crystals with misorientation 0.3 degree to the m-direction were used as seeds. Their (0001) surfaces were prepared by mechano-chemical polishing and then cleaning to the epi-ready state. All crystals were of high structural quality with full width at half maximum (FWHM) values of X-ray rocking curves for (002) reflection of about 30–40 arc s in and bowing radii of (0001) crystallographic planes of about 20–30 m. The free carrier concentration of all seeds was of the order of 5 1016 cm 3. The HVPE samples were studied by means of optical microscopy with differential interference contrast (DIC) and XRD. The (002) symmetrical reflection X-ray rocking curves (omega scan) of the seeds and the deposited material were measured using a highresolution Philips X′Pert Pro X-ray diffractometer, equipped with a four-reflection Bartels monochromator. The X-ray beam had dimensions of 1 10 mm (1 mm is the width in the diffraction plane). Defect selective etching (DSE) in molten KOH-NaOH eutectic [9] was performed in order to reveal the etch pits and determine the etch pit density (EPD) of the HVPE-GaN crystals. SIMS was used to investigate the concentrations of impurities in the HVPE material. Raman spectroscopy was performed at room temperature, in a backscattering configuration, using a Jobin–Yvon
T64000 spectrometer equipped with a CCD camera. A 514.53 nm line of argon laser was used. Low-temperature ( 15 K) photoluminescence (LT-PL) spectra were obtained with a 325 nm line of a He–Cd laser and with power density of about 5 W/cm2 as an excitation source. The diameter of the spot was approximately 200 mm. Emission from the sample, collected in backscattering geometry and dispersed by a SPEX500M spectrometer, was detected by a photomultiplier (Hamamatsu photomultiplier tube r943–02). FEI TECNAI G2 F20 S-TWIN electron microscope was used for TEM investigation of a laser diode structure built on a F-S HVPE-GaN:Si substrate.
3. Results Fig. 2 presents an HVPE-GaN:Si crystals grown on F-S HVPEGaN seeds and their morphology, which was the same as in case of undoped HVPE-GaN. Hillock growth mode was observed on the crystal's growing surface, as was reported earlier for growth with no intentional doping [7,10]. However, in case of the highest H2SiCl2 flow (0.8 ml min 1) some instability (like step bunching growth mode) was observed on the sides of the hillock (see Fig. 2b). For the lowest H2SiCl2 flow (0.002 ml min 1) typical hillocks were observed on the surface (Fig. 2c). The structural quality of all HVPE-GaN:Si crystals reported in this paper was extremely high and comparable with the structural quality of the seeds. Two X-ray rocking curves, for the seed and HVPE-GaN:Si layer (as-grown, presented in Fig. 2a), are shown in Fig. 3a and b, respectively. FWHM value of rocking curve was of the order of 30 arc s for the seed as well as for the HVPE-GaN:Si layer. Etching the (0001) surface of the HVPE-GaN: Si, at 500 °C in molten KOH-NaOH solution after mechanical and chemo-mechanical polishing, revealed EPD on the order of 5 104 cm-2. Similar to undoped GaN samples [11], large, average, and small size
Fig. 2. a) 800 μm-thick layer HVPE-GaN:Si grown with the highest H2SiCl2 flow (0.8 ml min 1) on F-S HVPE-GaN seed; 1-mm grid; b) instabilities (step bunching mode) observed on the as-grown surface for H2SiCl2 flow of 0.8 ml min 1; c) hillocks visible on crystal grown with H2SiCl2 flow of 0.002 ml min 1.
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Fig. 3. XRD rocking curve, (002) reflection of a) F-S HVPE-GaN (seed); FWHM value: 28 arc s; b) HVPE-GaN:Si layer (as grown); FWHM value: 32 arc s.
Fig. 4. a) Raman spectra for undoped F-S HVPE-GaN (red line) and doped F-S HVPE-GaN:Si (black line); peaks LPP þ and LPP , E2 and A1(LO) are marked; b) E2 peak frequency on c-plane of F-S HVPE-GaN:Si in the center of the sample; The Raman spectra were acquired along the diameter on a distance of 9 mm with a 250 mm step. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
diameter pits were observed. Fig. 4a shows two Raman spectra, shifted on the intensity axis for clarification. The upper line (red) represents a Raman spectrum of a F-S HVPE-GaN seed. Two peaks are well visible: E2 and A1(LO). Raman spectrum for HVPE-GaN layer doped with Si (0.8 ml min 1 flow of H2SiCl2) is the lower line (black). It should be remarked that in this case the A1(LO) peak disappeared and two peaks: LPP þ and LPP , appeared. Fig. 4b shows the position of E2 peak measured on the F-S HVPE-GaN: Si. The crystal was investigated on its (0001) surface along the diameter, on a distance of 9 mm with a 250 mm step. Only very small changes are observed in the E2 peak frequency. In order to investigate electrical and optical properties of HVPEGaN:Si, the HVPE layer was sliced from the seed. Values of free carrier concentration were calculated from Raman spectra obtained on the c-plane of the investigated crystal. The free carrier concentration n, according to [12], was calculated from the following equation:
n=
ωp2ε∞ε0m* e2
(1)
where ε0 represents the vacuum permittivity, m* is the effective mass of electron and equals 0.2 of the electron mass, e is the charge of the electron, ε1 is the high frequency dielectric constant of GaN, and ωp represents the plasmon frequency. The last one was calculated from:
ωp =
ω−ω+ ωT
(2)
where ω and ω þ are determined from Raman spectra frequencies of LPP and LPP þ modes, respectively, and ωT is transversal phonon frequency. Calculated values of free carrier
Fig. 5. Distribution of free carrier concentration (calculated from Raman spectra) along the diameter of F-S HVPE-GaN:Si grown with 0.8 ml min 1 of H2SiCl2 flow.
concentration varied from 1 1019 cm 3 for GaN grown with the lowest H2SiCl2 flow (0.002 ml min 1) to 3 1018 cm 3 in case of higher H2SiCl2 flow (0.8 ml min 1). Fig. 5 presents free carrier concentration determined from Raman spectra measured for 50 points along the diameter of F-S HVPE-GaN:Si grown with the 0.8 ml min 1 of H2SiCl2 flow. Distribution of carrier concentration on the (0001) surface was very uniform. Small deviation from the mean value of 3.2 1018 cm 3 was observed only close to the edges. Hall measurements performed for a sample grown with 0.8 ml min 1 of H2SiCl2 gave the value of free carrier concentration of 1 1018 cm 3 with carrier mobility of 270 cm2 V 1s 1. SIMS measurements showed that Si concentration in F-S HVPEGaN:Si was always higher than 1 1019 cm 3 and slightly depended on the H2SiCl2 flow. Four SIMS depth profiles for F-S HVPE-
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Fig. 6. SIMS depth profiles for F-S HVPE-GaN:Si samples grown with different H2SiCl2 flows a) 0.8 ml min 1; b) 0.04 ml min 1; c) 0.002 ml min 1; d) 0 ml min 1 (undoped); hydrogen, oxygen, and carbon are on the same low level; only difference in silicon concentration is observed.
Table 1 Concentration of silicon and free carrier concentration in HVPE-GaN doped with silicon with different flows of H2SiCl2. H2SiCl2 flow [ml min 1]
Si concentration [cm 3]
Free carrier concentration [cm 3]
0.8 0.04 0.002
6 1019 3 1019 2 1019
3 1018 4 1018 1 1019
GaN:Si grown with the highest (0.8 ml min 1), medium (0.04 ml min 1), the lowest (0.002 ml min 1) H2SiCl2 flows, and undoped material are presented in Fig. 6a, b, c, and d, respectively. For the highest flow of H2SiCl2 the silicon concentration in HVPEGaN:Si was the highest, at the level of 6 1019 cm 3. With decreasing of dichlorosilane flow the silicon concentration in HVPEGaN slightly decreased to 2–3 1019 cm 3. In case of undoped material, concentration of silicon was at the level of 3 1017 cm 3. Values of silicon concentrations and the free carrier concentration for samples grown with different H2SiCl2 flows are summarized in Table 1. Concentrations of others impurities were at the same low level in all examined cases. Fig. 7 shows the L-T PL spectra for undoped (with Si concentration at the level of 3 1017 cm 3 [8]) and silicon-doped HVPE-GaN samples. The spectra were
Fig. 7. L-T PL spectra for F-S HVPE-GaN:Si grown with various H2SiCl2 flows.
normalized do the near band edge emission intensity in order to compare the intensities of yellow luminescence (YL) peaks visible at 2.2–2.3 eV. It is worth to mention that the intensity of YL increases with increasing of Si concentration in the material. For the undoped material the spectrum between 3 and 3.5 eV is dominated by recombination processes, associated with annihilation of
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Fig. 8. TEM of the MOCVD LD quantum wells structure; no defects are observed. Layers comprising quantum barriers (QBs), quantum wells (QWs), and cap are indicated.
free and bound excitons, and their phonon replicas. In case of Sidoped GaN a band between 3 and 3.3 eV appears. The visible peaks are connected to DAP recombination. Free-standing HVPE-GaN:Si crystal, with free carrier concentration of 3 1018 cm 3, was used as a substrate for a laser diode (LD) structure. The prepared substrate was 300 mm-thick and 1 cm2 of lateral size. The substrate was misoriented and the obtained misorientation angle was measured by XRD in 9 points on the surface. The offcut variation did not exceed 0.1 degree. The substrate was used for MOCVD growth of a GaN-based LD. Details of the device structure were described elsewhere [13]. The grown epitaxial layers (quantum wells) were examined by TEM (Fig. 8b). The analysis showed that the active region had a good structural quality and no defects propagated from the substrate. Etching of the p-type cap revealed the EPD of the order of 5 105 cm 2. This value is one order of magnitude higher than the EPD of the used substrate. The results of optical and electrical measurements (wavelength, voltage-current, and optical power-current characteristics) of the processed LD were as follows: wavelength λ ¼410 nm, threshold current Ith ¼50 mA, threshold voltage Uth ¼ 7 V, and slope efficiency ɳ ¼0.35 W/A.
4. Discussion The presented results show that intentional incorporation of silicon to HVPE-GaN did not change the high structural quality of obtained gallium nitride crystals. Thus, the high quality of the initial seed (ammonothermally grown GaN used for obtaining the undoped HVPE-GaN seed) was not lost. This was confirmed by XRD measurements (see Fig. 3). The etch pit density remained at the same low level as in the seed material, of the order of 5 104 cm 2. The revealed etch pits correspond to edge, screw, and mixed dislocations in the crystal [11]. Raman spectroscopy allowed examining biaxial strain in the deposited doped film by measuring the position of E2 peak on the c-plane (see Fig. 4). The results showed that there is no biaxial strain in the free-standing crystal. The grown HVPE layers were transparent and crack-free, as shown on Fig. 2a. The morphology also remained the same as in case of unintentionally doped HVPE-GaN. Hillock growth mode was observed on the surface of the growing crystal. For the highest
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H2SiCl2 flow employed some morphological instabilities were, however, observed. These may result from a high concentration of Si atoms on the surface disturbing the macro step flow on the sides of the hillock. For lower flows of H2SiCl2 the growth mode is the same as in case of undoped HVPE-GaN growth. The presented doping with silicon allowed obtaining highly conductive n-type HVPE-GaN crystals of very homogeneous free carrier concentration on their c-planes. Electron concentration was determined by Raman spectroscopy and Hall measurements. Data from both methods were in good agreement. Silicon concentration increased slightly with increasing the H2SiCl2 flow. On the other hand, the free carrier concentration decreased. Additionally, SIMS measurements showed that the Si concentration in the material is always higher than the free carrier concentration. Data presented in Table 1 clearly show that not all Si atoms are electrically active. This suggests that a large fraction of silicon donors was compensated by an acceptor level. Gallium vacancies may be partially responsible for this effect. Ab initio calculations carried out by Van de Walle and Neugebauer [14] showed that the energy barrier of gallium vacancy formation reaches minimum when the Fermi level is at the bottom of conduction band. This means that in highly doped n-type material gallium vacancies can be generated relatively easily. Van de Walle and Neugebauer also showed that the deep acceptor level introduced by the Ga vacancy (or related complexes) is responsible for the yellow luminescence (YL) in GaN. Additionally, the formation of Ga vacancies may be enhanced by formation of complexes with donor impurities. Similar effect was observed earlier for GaN grown by High Nitrogen Pressure Solution (HNPS) growth method [15]. For highly n-type crystals, which contained a lot of gallium vacancies (of the order of 5 1018 cm 3) as well as oxygen atoms (5 1019 cm 3), a strong increase in the YL signal was observed. In the case of the F-S HVPE-GaN, the oxygen level was extremely low, but concentration of Ga vacancies increased with formation of the complex: gallium vacancy – silicon in gallium position. Therefore, silicon incorporation enhanced the YL intensity, what was well observed (see Fig. 7). Obviously, the difference between the concentration of incorporated Si and the free carrier concentration can be decreased if appropriate H2SiCl2 flow is employed. Lower H2SiCl2 flow allows maintaining a high silicon concentration and increasing the free carrier concertation. However, Si concentration is still higher than the concentration of electrons. This phenomenon is also present in unintentionally undoped crystals. As was shown by Freitas et al. [8], silicon concentration on the c-plane of undoped HVPE-GaN is at the level of 2 1017 cm 3. Hall measurements on the c-plane of undoped HVPE-GaN give the free carrier concentration value at the level of 3–5 1016 cm 3 [5]. This suggests that part of Si atoms is compensated by acceptors states, most probably associated with gallium vacancies and these may be the reason for the very weak but visible YL peak in the undoped material presented in this work (Fig. 7) and described in other work [5]. The bands which appear in the doped material between 3 eV and 3.3 eV are assigned to DAP recombination. It indicates the presence of a shallow acceptor state in the samples. This may also be, apart from gallium vacancies, the reason for compensation of Si in both doped and undoped HVPE-GaN. The acceptor may be Mg, since the 3.27 eV band is associated to Mg in Si-doped GaN. As was shown in [7], for a very low growth rate Mg was detected by SIMS at the level of 2– 3 1017 cm 3. The origin of Mg in the presented GaN crystals can be the gallium source used in the HVPE reactor. Free-standing HVPE-GaN:Si was used as a substrate for a laser diode structure. Due to high crystalline perfection of the Si-doped material it was possible to miscut the substrate very uniformly. This allowed growing epitaxial layers with alloy compositions homogenous on the entire surface. It is well known that indium
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incorporation as well as hole concertation in the p-type layer of a LD depend on misorientation angle of the substrate [16,17]. Thus, a very high crystalline quality of the wafer enables a large reproducibility of devices. Laser diode chips based on HVPE-GaN:Si all had the same parameters. Additionally, the low defect density in the substrate allowed deposition of a LD structure with a low defect concentration. This, in turn, results in a long life time of the devices, as demonstrated by S. Uchida et. al [18]. It should be remarked that the EPD in the LD structure (measured on top of the laser diode in the p-type cup) increased only one order of magnitude compared to the EPD in the F-S HVPE-GaN:Si substrate. TEM image of the LD epitaxial layers (see Fig. 8) shows that the quantum wells were defect free. One can suppose, thus, that the increase of the EPD from 5 104 cm 2 to 5 105 cm 2 occurred in the GaN p-type layer doped with magnesium.
5. Summary Preliminary results of intentional incorporation of silicon into HVPE-grown GaN with maintained high structural quality of the crystals were demonstrated in this paper. Dichlorosilane, used as the precursor of Si, was transported with gallium chloride into the crystallization zone. One-inch F-S HVPE-GaN crystals of extremely high structural quality, previously grown on the Am-GaN seeds and sliced from them, were used as substrates. It was shown that for higher flow of H2SiCl2 silicon incorporation into HVPE-GaN crystal slightly increased but it was always higher than the free carrier concentration. This can be explained by the effect of compensation. A large fraction of elect silicon donors was compensated by acceptor levels, in this case, most probably, gallium vacancies. For lower H2SiCl2 flow the value of free carrier concentration increased and the difference between silicon concentration and electron concentration was reduced. Free carrier concertation was very uniform across the (0001) surface of the doped samples. This result, combined with the high crystalline quality of the crystallized material, allowed to use F-S HVPE-GaN: Si as a substrate for laser diode. Parameters of the obtained laser diode showed that this n-type material is promising for preparing substrates required for optoelectronic (laser diodes) and electronic (vertical transistors) applications.
Acknowledgment This research was partially supported by The National Center
for Research and Development: PBS1/B5/7/2012 and PBS3/B5/32/ 2015 and by Polish National Science Center through project No. 2012/05/B/ST3/02516. The authors would like to thank Dr. R. Jakiela from IP PAS for SIMS measurements and TopGaN processing team for their help in preparing the laser diode.
References [1] T. Paskova, M. Bickerman, Handbook of Crystal Growth, vol. 602, Second ed., Elsevier, 2015. [2] A. Koukitu, Y. Kumagai, Technology of Gallium Nitride Crystal Growth, vol. 31, Springer-Verlag, Heidelberg, 2010. [3] T. Sochacki, Z. Bryan, M. Amilusik, R. Collazo, B. Lucznik, J.L. Weyher, G. Nowak, B. Sadovyi, G. Kamler, R. Kucharski, M. Zajac, R. Doradzinski, R. Dwilinski, I. Grzegory, M. Bockowski, Z. Sitar, Appl. Phys. Express 6 (2013) 075504. [4] T. Sochacki, M. Amilusik, B. Lucznik, M. Fijalkowski, J.L. Weyher, B. Sadovyi, G. Kamler, G. Nowak, E. Litwin-Staszewska, A. Khachapuridze, I. Grzegory, R. Kucharski, M. Zajac, R. Doradzinski, M. Bockowski, Jpn. J. Appl. Phys. 53 (2014) 05FA04. [5] T. Sochacki, Z. Bryan, M. Amilusik, Milena Bobea, M. Fijalkowski, I. Bryan, B. Lucznik, R. Collazo, J.L. Weyher, R. Kucharski, I. Grzegory, M. Bockowski, Z. Sitar, J. Cryst. Growth 394 (2014) 55. [6] L. Kirste, A.N. Danilewsky, T. Sochacki, K. Kohler, M. Zajac, R. Kucharski, M. Bockowski, P.J. McNally, ECS J. Solid State Sci. Technol. 4 (8) (2015) P324–P330. [7] T. Sochacki, M. Amilusik, M. Fijalkowski, B. Lucznik, J.L. Weyher, I. Grzegory, R. Kucharski, M. Iwinska, M. Bockowski, J. Cryst. Growth 407 (2014) 52. [8] J.A. Freitas Jr., J.C. Culbertson, N.A. Mahadik, T. Sochacki, M. Bockowski, M. Iwinska, Cryst. Growth Des. 15 (2015) 4837. [9] G. Kamler, J.L. Weyher, I. Grzegory, E. Jezierska, T. Wosinski, J. Cryst. Growth 246 (2002) 21. [10] T. Sochacki, M. Amilusik, B. Lucznik, M. Fijalkowski, J.L. Weyher, G. Nowak, B. Sadovyi, G. Kamler, R. Kucharski, M. Iwinska, I. Grzegory, M. Bockowski, J. Cryst. Growth 403 (2014) 32. [11] T. Sochacki, M. Amilusik, M. Fijalkowski, M. Iwinska, B. Lucznik, J.L. Weyher, G. Kamler, R. Kucharski, I. Grzegory, M. Bockowski, Phys. Status Solidi B 252 (5) (2015) 1172. [12] R. Lewandowska, J.L. Weyher, J.J. Kelly, L. Konczewicz, B. Lucznik, J. Cryst. Growth 307 (2007) 298–301. [13] I. Grzegory, M. Bockowski, T. Suski, P. Perlin, C. Skierbiszewski, M. Sarzyński, S. Krukowski, S. Porowski, III-Nitride Semiconductors and Their Modern Devices, vol. 18, Oxford Univ. Press, 2013. [14] J. Neugebauer, Ch.G. Van de Walle, Appl. Phys. Lett. 69 (4) (1996) 22, 503. [15] T. Suski, E. Litwin-Staszewska, R. Piotrzkowski, R. Czernecki, M. Krysko, S. Grzanka, G. Nowak, G. Franssen, L.H. Dmowski, M. Leszczynski, P. Perlin, B. Lucznik, I. Grzegory, R. Jakiela, Appl. Phys. Lett. 93 (17) (2008) 172117. [16] S. Keller, C.S. Suh, N.A. Fichtenbaum, M. Furukawa, R. Chu, Z. Chen, K. Vijayraghavan, S. Rajan, S.P. DenBaars, J.S. Speck, U.K. Mishra, J. Appl Phys. 104 (9) (2008) 093510. [17] T. Suski, P. Perlin, H. Teisseyre, M. Leszczynski, I. Grzegory, J. Jun, M. Bockowski, S. Porowski, T.D. Moustakas, Appl. Phys. Lett. 67 (1995) 2188. [18] S. Uchida, M. Takeya, S. Ikeda, T. Mizuno, T. Fujimoto, O. Matsumoto, S. Goto, T. Tojyo, M. Ikeda, IEEE J. Sel. Top. Quantum Electron. 9 (5) (2003) 1252.
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