Bull Mater Sci (2020) 43:234 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12034-020-02181-9Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

Gallium nanocrystal formation in Si3N4 matrix by synthesis

MANOJ KUMAR RAJBHAR1, SARAVANAN RAJAMANI1, S K SINGH1, SERGEY SURODIN2, DMITRY NIKOLICHEV2, RUSLAN KRYUKOV2, DMITRY KOROLEV2, ALYONA NIKOLSKAYA2, ALEXEY BELOV2, ALEXEY NEZHDANOV2, ALEXEY MIKHAYLOV2, DAVID TETELBAUM2 and MAHESH KUMAR1,* 1 Department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur 342011, India 2 Lobachevsky University, 603950 Nizhny Novgorod, Russia *Author for correspondence ([email protected])

MS received 23 October 2019; accepted 16 March 2020

Abstract. Synthesis of in insulators attracts tremendous attention due to their unique electrical and optical properties. Here, the gallium (Ga) and (GaN) nanoclusters have been synthesized in the nitride matrix by sequential ion implantation (gallium and ) followed by either furnace annealing (FA) or rapid thermal annealing (RTA). The presence of Ga and GaN nanoclusters has been confirmed by Fourier-transform infrared, Raman and X-ray photoelectron spectroscopy. Thereafter, the effect of RTA and FA on the conduction of charge carriers has been studied for the fabricated devices. It is found from the current–voltage measurements that the carrier transport is controlled by the space charge limited current conduction mechanism, and the observed values of parameter m (trap and the distribution of localized state) for the FA and RTA devices are *2 and *4.1, respectively. This reveals that more defects are formed in the RTA device and that FA provides better performance than RTA from the viewpoint of opto- and nano-electronic applications.

Keywords. Ion implantation; nanocrystals; rapid thermal annealing; furnace annealing; space charge limited conduction.

1. Introduction the area of semiconductor technology [4]. Sequential implantation of two or more elements can produce Gallium nitride (GaN) is one of the most promising semi- metastable compounds and complex microstructures con- conductor materials for the next-generation microelectron- sisting of finely dispersed nanoscale precipitates in the near- ics and optoelectronics, in particular, for creating high- surface region of the matrix material [5]. Hence, this tech- power and high-frequency devices, light-emitting diodes, nique is quite favourable to form nanocrystals inside the lasers, and UV photodetectors [1]. This semiconductor has a matrix, which facilitates the encapsulation and passivation. wide (3.4 eV), high chemical and radiation The ion implantation technique also has some other resistance and high thermal conductivity. The possibility of advantages like the ability to control the concentration and growing GaN layers and nanocrystals on silicon substrates distribution of implanted species, which allows altering the would make it possible to preserve silicon as the base optical, electrical, magnetic and other properties of mate- material of advanced electronics/optoelectronics in combi- rials. Current activities in the area of optoelectronics (in- nation with the mentioned advantages of GaN. However, tegrated optical circuits) and tailoring of properties due to a very low reactivity between Ga and N, mismatch of for the specific application have led to the renewed interest crystal lattices and difference in thermal expansion coeffi- in the use of ion implantation [6–17]. There are certain cients of GaN and Si, many difficulties arise in the growth issues related to the damage associated with implantation, of GaN nanostructures on Si substrate [2,3]. but to reduce the damage, the implanted samples can be The ion implantation is an extensively used technique to subjected to annealing. change the properties of semiconducting and insulating Recently, several attempts have been made to synthesize materials. By ion implantation, any element can be intro- GaN nanostructures in Si and Si-based substrates by co- duced into a in a controlled and reproducible manner. implanting Ga? and N? ions followed by annealing in The ion implantation technique has shown great success in nitrogen or ammonia atmosphere [17–21]. One of the 234 Page 2 of 9 Bull Mater Sci (2020) 43:234 challenges of ion synthesis of GaN nanostructures is the distribution of gallium and nitrogen ions according to the intensive loss of gallium introduced into silicon during post- SRIM calculation. According to the sample details given in implantation annealing due to out-diffusion [20]. Prelimi- table 1, the ion-irradiated sample N32 was subjected to FA nary ion synthesis of a silicon nitride (Si3N4) film reduces in nitrogen atmosphere for 30 min, and the ion-irradiated the gallium loss. However, ion synthesis of Si3N4 is asso- sample N34 was subjected to RTA in the presence of argon ciated with the formation of radiation defects, which are not gas for 30 s. Both samples were annealed at 800°C. The ion completely removed during subsequent annealing and can implantation and subsequent annealing strongly modified have a negative effect on the formation of GaN. In this the properties of Si3N4 matrix compared to the pristine regard, it is of interest to study the possibility of ion syn- sample. As shown schematically in figure 1a–d for the ion- ? ? thesis of GaN by Ga and N ion implantation into a Si3N4 implanted and annealed samples as well as fabricated film pre-deposited on a Si substrate. At the same time, Si3N4 electrode pattern, the top electrodes of the devices were with a relatively large band gap (5.19 eV) is transparent, formed by thermal evaporation of circular Au contacts of and ion synthesis of GaN nanostructures in Si3N4 matrix diameter 0.3 mm on the surface of implanted layer, and Al offers encapsulation and confinement, which can be useful was used to form the bottom electrodes (not shown). The in optoelectronic applications. modified electrical properties of the fabricated devices were In the present work, a thin layer of silicon nitride Si3N4 studied by using the semiconductor characterization system (200 nm) was grown over the c-Si substrate by using the Keithley 4200-SCS. low pressure chemical vapour deposition (LP-CVD) tech- Fourier-transform infrared (FTIR) transmission mea- ? ? nique. Then, Ga and N2 ions were sequentially implanted surements were performed at room temperature on a Varian in Si3N4 film, and the influence of different kinds of Excalibur 4100 Fourier spectrometer with a spectral reso- annealing treatments—furnace annealing (FA) and rapid lution of 4 cm-1. The signal-to-noise ratio was increased thermal annealing (RTA) on carrier transport characteristics using repeated signal accumulation. was studied and discussed. The current–voltage (I–V) char- Raman spectroscopy of the films studied was imple- acteristics were analysed to see the effect of sequential ion mented on an NT-MDT NTEGRA Spectra system at room implantation of Ga and N ions in the Si3N4 matrix in terms temperature. The excitation emission of laser with a of implantation doses and annealing conditions. wavelength of 473 nm was focused by a 1009 objective lens with a 0.90 numerical aperture. The spectra were recorded in a reflection scheme in the range of 2. Experimental 50–900 cm-1 with a resolution of 0.8 cm-1. X-ray photoelectron spectroscopy (XPS) investigation The thin (about 200 nm in thickness) Si3N4 layer was of phase and chemical composition of the ion-implanted deposited on the c-Si substrate by using the LP-CVD. The Si3N4 films was carried out by using the ultrahigh-vacuum silicon substrates were cleaned with the standard cleaning Omicron Multiprobe RM system with the pressure lower than process before the film deposition. To form the nanostruc- 10-10 mbar. The details of XPS measurements and analysis ? ? ture, the Ga and N2 ions were sequentially implanted into were reported in our previous paper [23]. To carry out the depth 16 -2 the Si3N4 film at the doses of 5 9 10 ions cm and 2.5 9 profiling of the implanted layers, the material of the sample was 1016 ions cm-2 with the energies of 80 and 40 keV, subjected to ion sputtering with an Ar? ion source at an respectively. In accordance with the SRIM calculation [22], accelerating voltage of 1 kV. The beam 20 mm in diameter ? ? the mean projected range for Ga and N ions in Si3N4 was exhibited uniform radial distribution of the ion current. *60 nm. The ion implantation was done by using an ILU- 200 ion implanter (Russia). The trimethylgallium Ga(CH3)3 and nitrogen gas (N2) were used as source materials for ion 3. Results and discussion ? implantation. The molecular N2 ions were used instead of ? ? N1 ions to reduce the duration of implantation (every N2 3.1 FTIR and Raman characterization ion splits into two ions with equal energy at the first colli- sion with target atoms). Energy and doses of implanted ions Figure 2 shows the FTIR transmission spectra of the were chosen to obtain the good overlap and depth sequentially ion-implanted and annealed samples N32 (FA

Table 1. Parameters of ion implantation and annealing.

Samples Ion Dose, cm-2 Energy, keV Implantation and annealing sequence

? 16 ? ? FA Ga 5 9 10 80 N2 ? Ga ? FA (800°C) ? 16 N2 2.5 9 10 40 ? 16 ? ? RTA Ga 5 9 10 80 N2 ? Ga ? RTA (800°C) ? 16 N2 2.5 9 10 40 Bull Mater Sci (2020) 43:234 Page 3 of 9 234

Figure 1. Schematic structure of the sample after (a) ion implantation, (b) RTA, (c) FA and (d) fabrication of electrodes. at 800°C) and N34 (RTA at 800°C). The broad peak Figure 3 shows the Raman spectra of the same samples observed at 844 cm-1 is evidently related to Si–N bond in N32 and N34. The peaks at 300 and 520 cm-1 correspond -1 Si3N4 film. The small scattering band observed at 640 cm to the crystalline silicon substrate used as a substrate for the is close to the position of Ga–O peak, but it is difficult to film deposition. According to theoretical calculations, the expect the oxidation of implanted Ga in a high-quality zone-centre phonon frequencies of hexagonal GaN are E2 -1 -1 -1 Si3N4 film. No FTIR absorption related to Ga–N bonds is (low) 143 cm ,B1 (low) 337 cm ,A1 (TO) 541 cm ,E1 -1 -1 -1 observed in figure 2. (TO) 568 cm ,E2 (high) 579 cm ,B1 (high) 720 cm , 234 Page 4 of 9 Bull Mater Sci (2020) 43:234

3.2 XPS characterization

The XPS measurements have been performed to detect the elemental composition of the implanted species and their chemical interaction in the Si3N4 film. The depth profiles of species in the Si3N4 film after ion implantation and annealing at 800°C are shown in figure 4a. The profiles indicate the presence of implanted Ga atoms with the con- centration up to 10 at% in the near-surface region. There is a uniform distribution of nitrogen atoms with the concen- tration of up to 50 at% in the Si3N4 film. To determine the chemical state of various atoms, the Gaussian decomposition of experimental data has been performed by the spectral fit refinement technique [23]. Figure 4b shows the example of fitting the Ga 2p3/2 pho- toelectron line by Gaussian functions. The main photo- ? ? Figure 2. FTIR transmittance spectra of Ga and N2 co- electron line of Ga is identified as related to the elemental implanted and subsequently thermally annealed samples N32 (FA Ga0 and the content of GaN is less. Figure 4c and c0 shows at 800°C) and N34 (RTA at 800°C). the depth profiles of silicon and gallium atoms in different chemical states—silicon bonded to nitrogen, elemental gallium and gallium bonded to nitrogen. These data evi- dence that the main part of the implanted film is stoichio- metric Si3N4, but less than 5 at% of silicon represents sub- stoichiometric phase SiNx with x \ 1.3. The depth of maximum concentration of Ga implanted in the Si3N4 film corresponds well to the SRIM calculation. The significant amount of implanted gallium atoms (up to 9 at% at the depth of 60 nm) is found to be in an elemental state and about 3 at% of gallium atoms is bonded to nitrogen. Prob- ably, very low mobility of Ga atoms in stoichiometric sil- icon nitride is responsible for the small observed concentration of Ga–N bonds, which at the same time can also constitute GaN precipitates and small nanoclusters. In addition, gallium atoms interact with each other to form chemically stable Ga–Ga complexes or Ga nanoclusters. Implanted N atoms diffuse into the silicon nitride film much more easily. During annealing, a part of them is embedded into the surrounding matrix damaged by ion irradiation, Figure 3. Raman spectra of the samples after FA (N32) and restoring it to Si3N4, other nitrogen atoms form bonds with RTA (N34). gallium.

-1 -1 A1 (LO) 748 cm and E1 (LO) 757 cm [24]. Among 3.3 Current–voltage characterization these phonon modes, one A1,oneE1 and two E2 modes are Raman active. However, according to the experimental From I–V measurements, it can be concluded that there is data, the most intense are the coupled phonon modes E1 no conduction in pristine and ion-implanted devices after (TO) and A1 (LO) at frequencies at around 540 and FA (N32) and RTA (N34) when biasing between the top 740 cm-1, respectively. In the case of our spectra, the and bottom electrodes. In contrast, when both contacts are presence of an intense photo luminescent background taken from the top electrodes, it is observed that the ion- associated with defects in the implanted layer does not implanted and annealed devices show conduction, while the allow them to be identified. pristine device does not show any measurable conduction. Therefore, we cannot conclude on the formation of GaN So, one can conclude that the origin of conduction in these ? ? during ion implantation based only on FTIR and Raman devices is closely related to the sequential (Ga ? N2 ) ion spectra. This may be due to the insufficient sensitivity of the implantation. used FTIR and Raman techniques for detecting small con- Due to the miscibility of the implanted gallium, nitrogen centrations of Ga–N bonds. ions and their chemical interactivity with the host matrix, Bull Mater Sci (2020) 43:234 Page 5 of 9 234

Figure 4. XPS data for the ion-implanted samples. (a) The depth profiles of species in the Si3N4 film co-implanted with Ga and N ions and annealed at 800°C. (b) The decomposed Ga 2p3/2 XPS line recorded at the depth of 45 nm: 1 – experimental data; 0 2 – fitting curve; 3 – Ga in Ga ;4–GainGaN.(c) The depth profiles of silicon atoms in different chemical states: 1 – in SiNx (x\ 0 0 1.3);2–inSi3N4.(c ) The depth profiles of gallium atoms in different chemical states: 3 – Ga in Ga ;4–GainGaN.

[25–28]. From the measured XPS data (figure 4), it is observed that the implanted Si3N4 thin layer contains Ga and GaN nanoclusters after FA and RTA, respectively. Moreover, the sub-stoichiometric phase of SiNx is also present in the samples. It is expected that, at high temper- ature, thermal annealing recrystallizes the amorphous layer, as well as reduces the radiation-induced defects and enhances the electrical activity of implanted atoms. Figure 5 shows the semi-log I–V characteristics of the sequentially ion-implanted samples N32 and N34 after FA and RTA, respectively, measured in a two-terminal con- figuration between circular Au electrodes with a diameter 0.3 mm on implanted Si3N4 film. There are many transport mechanisms which explain the conduction in an insulator. The most basic conduction mechanisms are Fowler–Nord- heim (FN) tunnelling conduction, Schottky emission, Poole–Frenkel conduction and space charge limited con- Figure 5. Semi-log I–V characteristics of ion-implanted devices duction (SCLC). FN tunnelling model explains that tun- after FA and RTA. nelling is caused by field ionization of trapped electron into nucleation and growth of clusters occur. In general, the conduction band and the I–V characteristics are independent amount of defects (lattice disorder) during implantation of temperature [29,30]. While the measured I–V character- depends on the energy, ion mass and dose. At high ion istics of devices (N32 and N34) are highly sensitive to doses ([1014 cm-2), the amorphous layer is formed temperature, which ruled out the possibility of existence of 234 Page 6 of 9 Bull Mater Sci (2020) 43:234

Figure 6. Temperature-dependent semi-log I–V characteristics of (a) furnace-annealed device, (b) rapid thermal- annealed device, (c) room temperature I–V curve of the FA device (log I–V1/2) and (d) room temperature I–V curve of the RTA device (log I–V1/2).

FN tunnelling as the possible conduction mechanism. Figure 6a and b shows the temperature-dependent semi-log I–V characteristics of devices N32 and N34, respectively. Current due to tunnel emission can be expressed as [30,31]: I  V2 exp ðÀb=VÞ; ð1Þ where value b holds within an effective mass term and barrier height. In general, the barrier height at the electrode interface is the most important parameter in the Schottky emission conduction mechanism [30–32]. In fig- ure 5 the non-linear and symmetric I–V curve suggests the rectifying contacts between the implanted Si3N4 matrix and Au (metal) interface. When the polarity of applied bias is changed, symmetry in the current variation indicates the absence of any significant barrier interfacial layer between the film and electrodes [28]. Basically, the mechanism involved in the Poole–Frenkel emission is similar to the Schottky emission, except that it is applied to thermal excitation of electrons from trap into the conduction band of insulator [32,33]. The log I vs. V1/2 characteristics is Figure 7. Schematic diagram of carrier injection and distribution expected to be linear in nature in the Poole–Frenkel and in the SCLC: (a) weak injection (V [ 0) and (b) strong injection Schottky emission [32–34]. Figure 6c and d shows the log I 1/2 (V [ Vtr). vs. V characteristics of measured devices (N32 and N34), Bull Mater Sci (2020) 43:234 Page 7 of 9 234

Figure 8. Forward-biased experimental and fitted I–V characteristics of the FA device N32 (a, c) and RTA device N34 (b, d). which represent those characteristic curves are non-linear in applied voltage, while at higher voltage the second term nature. It rules out the possibility of existence of Schottky controls the current–voltage characteristics. The current emission or Poole–Frenkel emission as possible conduction increase with voltage obeys the power law relation I * V m, mechanisms. Non-linear (exponential), symmetric beha- where the parameter m is related to the trap density and the viour of current–voltage curve in figure 5 is probably distribution of the localized states (traps) in the insulator related to SCLC. Generally, in an insulating substance, [30,37]. The value of parameters a and b can be expressed when carriers are injected and no compensating charge is as [35,36,38–40]: present, the SCLC can occur [28,30,35]. This conduction qnl a ¼ ; ð3Þ mechanism dominates, once the applied voltage (V) reaches d to the transition voltage (V ) and, at this voltage, the con- tr 9lhe e centration of injected charge carriers in the vicinity of r 0 b ¼ 3 ; ð4Þ source becomes equivalent to the number of thermally 8d generated ones, while, at low applied voltage (V \ Vtr), the where q is the electronic charge, n is the density of ther- current–voltage characteristics follow the ohmic conduc- mally generated charge carriers, l is the mobility of carri- tion, which implies that thermally generated charge carriers ers, d is the spacing between two electrodes, h is the ratio of dominate the injected carriers [32,36]. Schematic diagram the free carrier density to total carrier (free and trapped) of the SCLC for applied voltage V[0 and V  Vtr is shown density, er is the dielectric constant and e0 is the permittivity in figure 7a and b, respectively. In the presence of these two of free space. mechanisms, the total current is represented as [30,35,36]: However, the FA device (N32) shows high current compared to the RTA device (N34) in figure 5. Four distinct I ¼ aV þ bVm; ð2Þ regions on the I–V curve can be seen in figure 8a and b. A where the parameters a and b are related to the ohmic linear ohmic dependence I / V is observed in the region of transport and SCLC, respectively. The first term in equation V \ 0.51 V and V \ 1.52 V (region I) for the FA and RTA (2) shows the ohmic behaviour (I * V) for lower values of device, respectively. The smaller value of transition voltage 234 Page 8 of 9 Bull Mater Sci (2020) 43:234 of the device N32 (FA) compared to the device N34 (RTA) The I–V characteristics support the SCLC mechanism of suggests the low trap density in the FA device. Current carrier transport through the implanted layer. The transport increases exponentially with voltage in region II. In region of charge carrier depends on the kind of annealing (FA and III the current increases with voltage satisfying the power RTA), and in the first case, a more complete damage law relation I / Vm. Figure 8a and b shows that the I–V annealing occurs. characteristics are governed by the relation I / Vm. It also shows the transition voltage of devices, which represent the transition from ohmic conduction to SCLC with traps, and Acknowledgements the temperature-dependent behaviour suggests a possibility of exponential distribution of traps [29,41]. Generally, the J–V relationship in the presence of traps state that are The study was supported by the Ministry of Education and exponentially distributed in energy is given by the follow- Science of the Russian Federation (RFMEFI58414X0008) ing equation [29,41]: and the Department of Science and Technology, India (INT/  RUS/RMES/P-04/2014). The Raman spectroscopy study 1=l l lþ1 lNv 2l þ 1 ere0 l V was performed at the Laboratory of Functional Nanomate- J ; ¼ lÀ1 2lþ1 q l þ 1 Nt l þ 1 d rials (Lobachevsky University). ð5Þ where l is the carrier mobility, q is the electronic charge, Nt References is the trap density, Nv is the effective density of state and l is an exponent, should be greater than one. For traps free SCLC m should be equal to 2, while for exponentially distributed [1] Morkoc¸ H (ed) 2009 Handbook of nitride semiconductors traps SCLC m=l? 1[2[29,41]. In order to examine the and devices, GaN-based optical and electronic devices (Weinheim: Wiley-VCH Verlag GmbH) SCLC of measured devices (N32 and N34) at room tem- [2] Kumar M, Roul B, Bhat T N, Rajpalke M K, Misra P, perature, the forward-biased I–V curves of the devices N32 Kukreja L M et al 2010 Mater. Res. Bull. 45 1581 (FA) and N34 (RTA) are shown in figure 8c and d, respec- [3] Kumar M, Rajpalke M K, Roul B, Bhat T N, Sinha N, tively. The dotted symbols are the experimental data point Kalghatgi A T et al 2011 Appl. Surf. Sci. 257 2107 and solid lines are fitting curves according to the relation I * [4] Appleton B R, Naramoto H, White C W, Holland O W, V m. It is observed that the values of parameter m for FA and McHargue C J, Farlow G et al 1984 Nucl. Instrum. Methods RTA devices are m * 2 and m * 4.1, respectively. The Phys. Res., Sect. B 1 167 value of m[1 exhibits the SCLC behaviour in both devices [5] Van Hassel B A and Burggraaf A J 1989 Appl. Phys. A 49 (N32 and N34). The value of parameter m suggests the trap 33 free SCLC in the FA device, while exponentially distributed [6] White C W, Boatner L A, Sklad P S, McHargue C J, Rankin traps SCLC in the RTA device. Moreover, it is expected that J, Farlow G C et al 1988 Nucl. Instrum. Methods Phys. Res., Sect. 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