SYNTHESIS AND PROPERTIES OF NEW SEMICONDUCTOR ALLOYS FOR LONG WAVELENGTH COHERENT EMITTERS ON GALLIUM ARSENIDE

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1 UNIVERSITÀ DEGLI STUDI DI TRIESTE XVI CICLO DEL DOTTORATO DI RICERCA IN FISICA SYNTHESIS AND PROPERTIES OF NEW SEMICONDUCTOR ALLOYS FOR LONG WAVELENGTH COHERENT EMITTERS ON GALLIUM ARSENIDE DOTTORANDO Alessandro Cristofoli COORDINATORE DEL COLLEGIO DEI DOCENTI CHIAR.MO PROF. Gaetano Senatore, Università degli Studi di Trieste TUTORE/RELATORE CHIAR.MO PROF. Alfonso Franciosi, Università degli Studi di Trieste

2 Abstract Ga (1 x) In x As (1 y) N y semiconductor alloys have recently shown promise as a new materials system for implementation of long wavelength coherent emitters operating in the optical-fiber communication windows 1.3 and 1.55 µm on GaAs. In this work GaAs (1 y) N y and Ga (1 x) In x As (1 y) N y epilayers and quantum wells (QWs) with GaAs barriers were grown by plasma-enhanced molecular beam epitaxy (MBE) and investigated by high resolution x-ray diffraction (HRXRD) and low temperature photoluminescence (PL) spectroscopy. Pseudomorphic ternary alloy layers were used to determine the best growth conditions for nitrogen concentrations y 0.05 and study the effect of post-growth annealing. The latter was found to be an efficient method to improve the PL emission, increase the radiative efficiency and decrease the PL linewidth. Quaternary alloy QWs with different degrees of lattice mismatch relative to GaAs were produced to study the dependence of the PL emission intensity on the nitrogen and indium concentrations. Finally, nitrogen incorporation in InAs quantum dots (QDs) was successfully used as an alternative method to produce long wavelenght emitters.

3 Contents Introduction 3 1 Effects of nitrogen incorporation 5 2 Molecular beam epitaxy Molecular beam epitaxy Growth system Reflection High Energy Electron Diffraction The MBE growth process Growth of III-V alloys Our growth system Characterization techniques Photoluminescence High resolution x-ray diffraction X-ray photoelectron spectroscopy Other techniques Ternary alloy samples Literature survey Sample structure GaAs (1 y) N y characterization Effects of growth conditions Post-growth annealing Quaternary alloy samples Literature survey Samples description Determination of alloy composition Ga (1 x) In x As (1 y) N y characterization Post-growth annealing

4 5.6 Emitters at 1.3 µm Quantum dot samples Literature survey Samples description Quantum dots characterization Comparison of optical properties Conclusions 101 2

5 Introduction In the middle of 1960s the communications industry realized that the existing copper wire infrastructure used to transfer data and voice would not have enough bandwidth for the impending increase in traffic. This demand stimulated an intense international effort to find a solution to overcome copper s limitations. The use of laser beam seemed very promising, especially for the enormous bandwidth that would be available. Among the different solutions experimented, in 1966 a technique was developed that used a continuous glass fiber as a guidance system to travel light signals: optical fiber communication was born. But it was only with the advent of information technology, in the last two decades, that optical fiber communications become firmly estabilished in most countries of the world. The structure of an optical communication channel is similar to other types of communication system: a transmitter, that contains an optical source and a means of modulating the output with the signal to be transmitted; a transmission medium, in which the informations travel; a receiver, where the optical signal is converted first in an electrical waveform and then in a form suitable for use. One of the main problem in optical fiber communications technology, is the attenuation of the signal along the travel. By developing ways to eliminate absorbing impurities from the fiber, it was possible to reach very low absorption in the infrared region of the electromagnetic spectrum, inside two windows centered at 1.3 and 1.55 µm. For this reason, great interest had the development of long-wavelength semiconductor lasers working at these minima. Semiconductor alloys composed by the elements of group III and V of periodic table such as gallium arsenide (GaAs) are suitable to produce this type of devices. Most of them, in fact, has a direct band gap, that makes them useful to produce optoelectronic devices, and a band gap energy low enough to produce infrared emitters. 3

6 In particular, the incorporation of nitrogen in III-V semiconductors have in recent years emerged as a subject of considerable theoretical and experimental research interest, due to their very unique physical properties and a wide range of possible device applications. Unlike all conventional ternary III-V semiconductor alloys, N-containing III-V alloys exhibit a huge bowing in the band gap energy. This causes a large band gap reduction together with a lattice parameter reduction with increasing N concentration. These remarkable fundamental properties found applications in the quaternary GaInAsN semiconductors. By optimizing the In and N contents, in fact, it is possible to tailor the material properties, such as the lattice parameter and the bandgap energy. The quaternary GaInAsN semiconductors is promising for the production of emitters operating at wavelengths of 1.3 and 1.55 µm, i.e., within the optical-fiber communication windows. Moreover, the alloy lattice parameter can be made to approach that of GaAs, allowing laser fabrication on GaAs wafers, bringing mature GaAs technologies to bear on advanced optical fiber communications. The synthesis and the characterization of GaInAsN semiconductors is the main goal of this work. 4

7 Chapter 1 Effects of nitrogen incorporation Since the elements of the group III and V of the periodic table are almost totally miscible in the solid state, ternary and quaternary alloys can be formed. Ternary alloys can be cation-mixed (III-III -V) or anion-mixed (III-V-V ). In this work we adopt the convention of indicating with x the cationic content ratio (A (1 x) B x C) and with y the anionic content ratio (AC (1 y) D y ). For the following discussion III-III -V alloys are considered, but the same holds also for the III-V-V type. In absence of composition-induced structural phase transition in the alloys or electronic direct-to-indirect band gap crossover, physical properties such as the band gap energy E g, the lattice parameter a 0, elastic constants, etc. are traditionally assumed to be simple continuous functions of the composition x [1]. Fig. 1.1 shows the relationship between a 0 and E g for conventional III-V alloy semiconductors, such as Al x Ga (1 x) As, Ga (1 x) In x As, Ga (1 x) In x P, etc. The bandgap energy E g (x) for most of these alloys A (1 x) B x shows only a small deviation from the composition weighted linear average of the band gaps E g (A) and E g (B) of the parental binary compounds A and B: E g (x) = (1 x) E g (A) + x E g (B), known as Vegard s law [2]. This deviation is usually described by a correction term E g (x) = E g (x) E g (x) that depends quadratically on chemical composition of the material: E g (x) = b x(x 1) (1.1) b is called bowing coefficient, it is composition independent and it has a typical value of a fraction of an electronvolt [1, 3, 4]. 5

8 Figure 1.1: The relationship between bandgap energy and lattice constant in conventional III-V alloy semiconductors. 6

9 From Fig. 1.1 one can note that conventional III-V alloys have a tendency toward increasing bandgap energy with decreasing lattice constant. Since GaN has a high bandgap energy (3.2 ev for zincblende structure [5]) compared to the other III-V binary compounds, the growth of III-Vnitrides alloys was initially motivated to fabricate light emitters covering the entire visible spectral range based on the direct band-gap III-V materials [6]. Surprisingly, GaAs (1 y) N y material showed a quite large redshift of the emission energy, instead of the expected blueshift, that increases with N content, for low y values [7, 8]. This behavior can be justified assuming a huge value of the bowing coefficient b, that has to be of the order of ev for y < Moreover, b strongly depends on the N content and on the internal strain in the GaNAs epilayer [4]. The first theoretical works [9] that explained the observed strong reduction in the bandgap energy applied the virtual crystal approximation of Van Vechten [10]. A giant bowing in the conduction band energy of the alloy, with b = 20 ev, is caused by the large electronegativity of N atoms, compared to the other group V anions. The model, based on a constant value of the bowing coefficient, provides very good agreement with the experimental results for the lowest nitrogen compositions (y < 0.02), but predicts a negative bandgap for < y < 0.867, in contrast with the clear reduction of the bowing coefficient for N content y > 0.02 showed by later experiments [11, 12]. Now it is commonly accepted that the giant bandgap bowing is predominantly due to the downshift of the minimum of the conduction band (CB), while the effects on the valence band (VB) states is negligible [3, 4]. But the exact physical mechanism that explains this downshift is still controversial, and it has been a subject of intense debate in the last few years. Two main models are currently discussed: Local density approximation (LDA) calculations of the band structure [1, 3, 13] show that the dilute nitrides alloy the band edge wave functions are localized impurity-like states. The localization reflects the large differences between atomic orbital energies and sizes of the As and N atoms. The model predicts two regions in the bandgap variation as a function of N content: (i) a band-like region, where the bowing coefficient is relatively small and nearly constant, and (ii) an impurity-like region, where the bowing coefficient is considerably larger and composition dependent. The band anticrossing (BAC) model [14], according to which the incorporation of N atoms introduces perturbation of highly localized na- 7

10 ture. The strong interaction between the CB and a narrow resonant band formed by the N states results in a reduction of the fundamental band gap and in a splitting of the CB into two subbands, one having mainly delocalized CB-like character, and the other having more localized character. The giant bowing in the band gap energy, makes GaAs (1 y) N y unique between III-V alloys, since it has the property to decrease the bandgap energy decreasing lattice constant, at the opposite of other alloys. This has a great potential for applications in optoelectronics and photonics, because enormously extends the region of conventional III-V alloys, and significantly increases freedom in designing semiconductor devices. In particular, combining the effects of the introduction of N in GaAs, i.e., lattice parameter decrease and band gap reduction, with those coming from In incorporation in GaAs, i.e., lattice parameter increase and band gap reduction, it is possible to achieve lattice-matching to GaAs and a strong decrease of the band gap. The possibility to grow coherently GaInNAs on GaAs enables fabrication of vertical cavity surface-emitting lasers (VCSELs) based on the high refractive index contrast AlGaAs/GaAs Bragg reflectors, and the low band gap allows to obtain emission at the optical-fiber communication wavelength window. 8

11 Chapter 2 Molecular beam epitaxy 2.1 Molecular beam epitaxy Molecular beam epitaxy (MBE) is a technique developed for the production of high quality epitaxial structures. Introduced in the 1970s as a tool for growing high-purity semiconductor films, now it is used also for the production of epitaxial layers of metals, insulators and superconductors as well, both in the research and in the industrial production. MBE takes place in a reactor in which source materials are introduced in the form of molecular beams. Molecular beams are created by heating solid source materials. A gas source may be used instead of a solid source, in which case the source material is introduced into the reactor through a gas injector nozzle. Due to the ultra high vacuum (UHV) environment of the reactor, when the source materials escape from the crucibles their molecules form a series of directed beams that are able to travel without collision until they make impact with the substrate surface. Here, the molecules decompose into the constituent atoms of the source materials. Because the substrate is heated during the process, there is sufficient kinetic energy for the atoms to rearrange themselves into a single crystal structure replicating the crystal structure of the underlying substrate. The key advantages of the MBE process include: Atomic precision control, with the thickness of each surface layer being as thin as one or two atoms. Uniformity across the sample surface. Ability to produce abrupt transitions between epilayers of different compound semiconductor crystals. 9

12 Very high purity of the grown material. Furthermore, UHV makes MBE compatible with the use of electron diffraction probes, which provide fundamental information on the growth mechanisms. 2.2 Growth system The basic components of a generic MBE system are presented in Fig Figure 2.1: Schematic drawing of a generic MBE system. The growth chamber is made of stainless-steel walls. It is connected, through a gate valve, to a preparation chamber, in which substrates are degassed before the growth, and a load lock module, for transferring samples inside and outside the UHV environment. Since to minimize outgassing 10

13 from the internal walls, the growth chamber is subjected to a bake-out, it is necessary that all its components resist to temperatures up to 200 C for extended periods of time. UHV conditions are necessary to minimize residual impurities during material growth. Since typical MBE growth rate for III-V type semiconductors is of the order of 1 µm/h, equivalent to 1 monolayer (ML) per second, the impurity partial pressures must be reduced below Torr, assuming a unity sticking coefficient [15]. In practice, base pressure is reduced to the Torr range, with the residual gas being essentially H 2. UHV is supported by a pumping system, composed of ion pumps, cryogenic pumps, and auxiliary Ti-sublimation pump. Pressure inside the growth chamber is monitored by an ionisation gauge (IG), placed far from the beam fluxes. The chamber internal walls are covered by a cryopanel, cooled by liquid nitrogen (LN 2 ), to prevent the re-evaporation of particles from parts other than the material sources. Another cryopanel surround the material sources, with the further task of their thermal isolation. Moreover, cryopanels provide additional pumping of the residual gas. The material fluxes are provided by the effusion cells. They must provide excellent flux stability and uniformity, and material purity. To limit daily flux variations to less than 1%, and day-to-day variations less than 5%, the temperature control must be of the order of ±1 C at a cell temperature near 1000 C [16]. Furthermore, the cell geometry must be chosen in a way that the material flux does not drift appreciably as the source is depleted. A typical MBE system contains 6 10 cells, placed on a source flange, that are co-focused on the substrate heater, to optimise flux uniformity. A typical effusion cell is composed by the following parts: (i) A crucible, usually made of pyrolitic boron nitride, which can stand temperatures of up to 1300 C without appreciable degassing. It can be cylindrical or conical in shape and has to be big enough to provide several months of operation before the depletion of the material. Both shape and size depend on the material to be evaporated. (ii) One or two Ta filament, that provides the heating of the material in the crucible. (iii) Ta foils, surrounding the crucible, provide heat shielding. (iv) A thermocouple measures the material temperature. The temperature is controlled by high-precision proportional-integral-derivative (PID) regulators. In front to each cell a shutter is placed to trigger the flux. A larger one, called main shutter, can stop the fluxes from all of the cells simultaneously. Shutters are usually made of Ta or Mo, and are activated mechanically or pneumatically. Their movements has to be much faster than the growth of 1 ML (typically 1 s), and computer control is used to guarantee growth reproducibility from run to run. Shutters must be designed not to outgas 11

14 when heated from the cells, and not to constitute an appreciable heat shield, giving rise to flux transients after opening. The substrate holder is mounted on a manipulator, placed in front of the cells. Manipulator rotation around its axis provides uniformity across the wafer during the growth. Behind the substrate holder there is a filament that provides heating of the sample. Substrate temperature is monitored by a thermocouple and controlled by a PID regulator. Opposite to the substrate holder there is an IG. Rotating the manipulator by 180, the IG can be used to monitor the beam fluxes for day-to-day calibration of the molecular beam intensities. The substrate holder is made of Mo or Ta, and the substrate is glued on it with an In film, or it is clamped with a ring. Several analysis tools are available for MBE systems, that can provide analysis during the growth process. Between these, a quadrupole residual gas analyser (RGA) is an essential complement to ionisation gauges, since RGA spectra provide signatures for possible air leaks, give a measure of the system cleanliness and detect impurities originating, e.g., from an insufficient system bake, or inefficient pumping, or from heating up the material sources. 2.3 Reflection High Energy Electron Diffraction The most valuable analysis tool inside the growth chamber is reflection high energy electron diffraction (RHEED). A high energy (up to 20 kev) electron beam is directed on the sample surface at grazing incidence. The electrons are scattered only by the uppermost few atomic layers, and then they go to impinge on a fluorescent screen on the opposite side. The image formed, hence, is a diffraction pattern that gives informations about the surface structure. Moreover, thanks to the grazing geometry, the RHEED apparatus does not interfere with the molecular beams, making the technique suitable for analysis during growth. In the ideal case, with electrons interacting only with the first atomic layer of a perfectly flat and ordered surface, the resulting Ewald construction on the screeen should consist in a series of points placed on a half circle. Since the crystal is subject to thermal vibrations and lattice imperfections, and the electron beam suffers of divergence and dispersion, in the real case the diffraction pattern consisting in a series of streaks with modulated intensity (see Fig. 2.2). If the surface is not flat, many electrons will be transmitted through surface asperities and scattered in different directions, resulting in a RHEED 12

15 Figure 2.2: RHEED geometry and formation of a diffraction pattern. pattern constituted by many spotty features. Therefore, a first important information provided by RHEED regards the flatness of a surface. Furthermore, it is evident that diffraction from an amorphous surface (such as an oxide on top of a semiconductor) gives no diffraction pattern at all, and only a diffuse background will result. This is important, for example, for evaluating oxide desorption when a new substrate is initially heated up prior to growth in the MBE chamber, exposing the underlying, crystalline semiconductor surface. The RHEED diffraction pattern consists in streaks of different intensity. The stronger are generated by the bulk lattice periodicity, due to penetration of the beam electrons in the atomic layers under the surface. Weaker lines are caused by surface reconstruction: due to the interruption of the crystal symmetry, atoms in the topmost layers rearrange the plane periodicity to minimize their free energy. The kind of reconstruction depends critically on the material, the surface orientation and the surface termination, that in turn is a function of sample temperature and composition of the gas phase. Therefore, RHEED provides fundamental information about surface geometry and chemistry, both in static conditions and during growth. Typical reconstruction of a (001) GaAs surface is a (2 4) pattern. RHEED can also be used to measure the growth rate. Plotting, as a function of time, the intensity of the specular spot in the 2 direction of a (2 4) GaAs surface. Opening the Ga shutter, under an As flux, the intensity oscillates periodically, with damped intensity, until shutters are closed again. The origin of the oscillations is explained in Fig The initial surface is flat, and reflectivity of the specular spot is relatively high. As layer-by-layer growth starts, the incident electron beam 13

16 Figure 2.3: Mechanism of intensity RHEED oscillations. gets partially scattered by the island steps of the forming ML, thus reducing the reflected intensity. At half ML coverage the scattering is maximum, then it decreases while completing of the surface flattens. As a consequence the reflected intensity returns to a maximum value. However, the oscillations damp as growth proceeds, and eventually disappear. This is because at higher coverages the growth front becomes statistically distributed over more and more layers (a new ML starts before the preceding one completes), yielding eventually a constant surface roughness. As the period of RHEED intensity oscillations is the time in which 1 ML is deposited on the surface, this method gives the value the growth rate, and allows its calibration on a daily basis. 2.4 The MBE growth process In general, three different phases can be identified in the MBE process [17]: the crystalline phase of the growing substrate; the gas phase of the molecular beams; the near-surface phase, where the phenomena most relevant to the MBE process take place. The first phase is simply the crystal structure of the substrate and of the deposited material, where short- and long-range order exists. On the other side, the molecular beams represent the most disordered phase. In this phase the mean free path of the atoms and molecules evaporated from the cell is much larger than the distance from the source to the substrate (some tens of cm), caused by the very low pressures of the residual gas and of the molecular beams typical of MBE conditions. As a consequence, no homogeneous reactions in the gas phase can occur, also in the zone where there is a mixing of the beams. 14

17 The third phase occours on the growth surface, where the impinging molecular beams interact with the hot substrate and can be adsorbed. There are two types of adsorption. The physical adsorption (physisorption), when there is no electron transfer between the impinging particle and the substrate, and the bound is due to van der Waals forces. The chemical adsorption (chemisorption), when there is electron transfer, i.e. a chemical reaction occours. Physisorbed or chemisorbed species can then diffuse on the surface, or re-evaporate, or form two-dimensional clusters with other atoms, or be incorporated in the material. The energetics of each event, and thus time and length scale, depend on a number of factors. Three possible modes of crystal growth can be distinguished [17]: The Volmer-Weber mode, in which small clusters are nucleated directly on the substrate surface and grown into islands. This happens when the deposited particles are more strongly bound to each other than to the substrate. The Frank-van der Merwe mode, in which the growth occours layer by layer, completing the forming layer before the starting of a new layer. In this case the deposited particles are more strongly bound to the substrate than to each other. The Stranski-Krastanow mode, that is a intermediate case. After forming few monolayers, subsequent growth occours by islands. Typically, the growth of semiconductors in MBE occours in the Frank-van der Merwe mode. In heteroepitaxy, with a material deposited over a different substrate, effects due to the lattice mismatch between the two materials can occour. The difference in lattice spacings produces strain energy due to the elastic distorption of the crystal cell, that becomes stronger with increasing the layer thickness. Typically, this energy is low enough to allow Frank-van der Merwe growth mode in the first stages, with the formation of a pseudomorphic layer, and the lattice constant adapting to the substrate one. However, as growth proceeds, a critical thickness is reached above which strain energy is relaxed through formation of misfit dislocations or the instauration of the Stransky- Krastanow growth mode, with development of 3-dimensional islands. The latter phenomenon can be observed for example in GaInAs/GaAs growth with high In content and, due to the 3-dimensional quantum confinement properties of the resulting islands, has been used to produce selforganized quantum dots. 15

18 2.5 Growth of III-V alloys Much of the knowledge we have today about the mechanisms of MBE is due to studies performed on GaAs growth on (001) surfaces. Some of the first fundamental studies have been performed by Foxon and Joice by means of modulated flux mass spectrometry some decades ago [18, 19], but their conclusions remain still valid. The incorporation of As 4 is quite complicated, involving a second-order process: two surface tetramers must meet to generate four chemisorbed As atoms on Ga sites, and a residual As 4 molecule that is desorbed (Fig. 2.4). Therefore, the maximum sticking coefficient for As 4 is 0.5. Figure 2.4: Model of GaAs growth. Due to the higher As species volatility, with respect to Ga, growth is usually performed with an As/Ga beam flux ratio much higher than 2. This flux imbalance does not affect the one-to-one crystal stoichiometry, since As atoms do not stick if Ga atoms are not available on the surface for bonding. This means that the growth rate is ultimately determined by the Ga atoms flux, since the Ga sticking coefficient is normally one. In fact, only for extremely low As fluxes or high temperatures, outside of the so-called MBE window ( C), significant Ga re-evaporation takes place. Moreover, the As sticking coefficient is an increasing function of the Ga flux and, with no Ga flux at all, As does not incorporate on the surface. Growth of III-III -V alloys, like GaInAs, follows the same mechanisms, and an optimal growth window can be found where sticking coefficients of both group-iii atoms are unity, with no mutual interference of the two species. The resulting growth rate and composition are simply derived from 16

19 the two binary growth rates. Things are much more complicated in the case of III-V-V alloys. No unequivocal film composition can be derived from the two individual group- V fluxes, since one species is absorbed more efficiently than the other, and there is mutual interference of the sticking coefficients [20, 21]. Doping of semiconductors is a fundamental aspect in MBE, since it allows carrier transport in electronic or optoelectronic devices. In III-V semiconductors, doping can be achieved by using group II (p-type), IV (p- or n- type) and VI atoms (n-type). For p-type doping, elements of the IIB column (Zn, Cd) have a too high vapour pressure at usual growth temperatures, that leaves elements of the IIA column, and Be in particular, as the universal choice. For n-type doping, group-vi atoms are not the most common choice, because of surface segregation and re-evaporation problems [15]. Group-IV atoms are amphoteric, i.e., they can act as donors (if they accommodate on group-iii sites) or acceptors (on group-v sites). Among these, C is an acceptor, but has a very low vapour pressure, hence it must be evaporated at very high temperatures (above 2000 C), while the amphoteric behaviour of Ge is difficult to control, and Sn presents a too high surface segregation. Therefore, the universal n-type dopant is Si, as in standard growth conditions on (Ga,In)As (001) it occupies group-iii sites, and doping levels up to about cm 3 can be obtained, before compensation (i.e., substitution of Si on As sites as well) is observed. One problem with Si (and Be as well) is diffusivity towards the surface at doping levels higher than about cm 3, which could be a problem in obtaining sharp doping profiles [22, 23]. 2.6 Our growth system Our growth system is part of a multi-chamber facility dedicated to the fabrication of compound semiconductors by MBE. The facility includes interconnected Riber 32P MBE chambers for the growth of II-VI and III-V semiconductors, a chamber for the deposition of metallic layers (such as Au, Fe, Mn and Al), an analysis chamber with a monochromatic x-ray photoemission spectroscopy (XPS) facility, and a class 100 clean room. The III-V growth chamber is equipped with standard effusion cells for As, Ga, In, Al, Si and Be. It is provided of a 10 kv RHEED equipment to monitor the structural quality of the material during the growth process and a quadrupole residual gas analyzer. UHV conditions are mantained by an ionic pump and a cryogenic pump, with an additional Ti sublimator that is 17

20 used for removing specific gas species. A heating stage, outside the growth chamber, but in UHV conditions, is used to desorbe residual gas from the sample before starting the growth. An Applied EPI UniBulb radio frequency (rf) plasma source, operating at MHz, is used to generate active nitrogen species. It is a one-piece bulb with a fixed aperture pattern optimized for pure nitrides growth. As it is a very efficient atomic nitrogen source, it was initially chosen for the growth of gallium nitride. In contrast, the growth of diluite nitride-arsenide compounds requires much less atomic nitrogen. However, it is possible to use a technique that reduces the amount of active nitrogen through the use of inert gas dilution [24]. The N 2 source gas was diluted with argon, that is a gas readily available, relatively inexpensive and can be pumped with conventional cryopumps, like that used in our system. By plasma spectroscopy on gas mixture, it has been shown that introduction of Ar in gas source does not change active N species, thus argon acts as a pure dilutant. The effect is that Ar reduces the amount of active nitrogen plasma [25], making it is possible to control active N produced by rf cell. Ultra pure N 2 and Ar (both six nines purity) were passed through a gettering furnace to remove residual contaminating substances, such as water and oxygen, and were collected in a tank. Gas mixture composition were controlled by using a ratio of pressures. A single gas mixture could be used to grow a large number of samples. 18

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