In this work the results of high pressure solution growth of GaN on various patterned substrates are presented. The growth on GaN/sapphire substrates patterned ...
Invited Paper
Growth of GaN on patterned GaN/sapphire substrates with various metallic masks by high pressure solution method M. Boćkowski I. Grzegory, G. Nowak, G. Kamler, B. Łucznik, M. Wróblewski, P. Kwiatkowski, K. Jasik, S. Krukowski and S. Porowski Institute of High Pressure Physics PAS, 01-142 Warsaw, Poland, Sokołowska 29/37 ABSTRACT In this work the results of high pressure solution growth of GaN on various patterned substrates are presented. The growth on GaN/sapphire substrates patterned in GaN parallel stripes and with SixNy and Mo masks between stripes is studied and analyzed. The results are compared with the growth on patterned substrates without any mask, thus with a bare sapphire between stripes. The usefulness of tungsten and iridium for masking is also determined. The HVPE free standing GaN substrates with high stripes, up to 10 µm, are examined in details. The stripes growth modes are shown and described. Keywords: High-pressure growth from solution, Seeded growth, Lateral overgrowth, Gallium nitride
1. INTRODUCTION The progress in blue laser diodes and UV light sources is still limited by the lack of high quality and large (2 or 3 inches) gallium nitride wafers. GaN melts at high nitrogen pressure (6 GPa) and high temperature (2500 K). These conditions were determined experimentally by W. Utsumi et. al. [1]. Due to such extremely high melting temperature and associated N2 pressure, the crystallization of GaN from the stoichiometric melts (industrial growth methods like Czochralski or Bridgman) seems technically impossible at present. Therefore, the GaN crystals should be grown by ways allowing lower pressures and temperatures. There are many promising methods to obtain GaN substrates, for example HVPE [2,3,4] succeeded in 2 or even 3 inches substrates, however with inhomogeneous dislocation density varied from 5x105 cm-2 to 5x108 cm-2. The crystals with homogeneous and the lowest dislocation density in the world-102 cm-2 have been obtained by high pressure solution (HPS) method [5]. The spontaneous growth of GaN by this method is strongly anisotropic and results in crystals having form of hexagonal platelets. The growth rate in directions perpendicular to the c-axis is about one hundred times faster than in the c-direction. However, these crystals are still too small for commercialization. Their maximal size achieves 3 cm2 and the thickness rarely exceeds 150 µm. Recently, a seeded growth (a directional crystallization, the technique similar to the liquid phase epitaxy) of GaN on sapphire/GaN templates by HPS method was examined [6,7,8]. The main motivation for GaN crystallization under pressure on the templates was the possibility of use of 2 inches substrates. A new design, applied for directional crystallization of GaN on sapphire/GaN templates was based on the use of a crucible with a baffle plate [8]. It allowed the formation of an uniform, flat crystallization front on the substrate and consequently to maintain a flat GaN surface during a long crystallization run. However, the main problems of this crystallization were the slow growth rate in the cdirection (1-2 µm/h) and that it yielded GaN with the average defect density of the order of 5x107 cm-2. Therefore, the high pressure GaN growth on stripe-patterned GaN/sapphire substrates has been explored. It has been expected that the lateral growth should be relatively fast and that in the laterally overgrown material the dislocation density should decrease significantly. The first experiments showed that the initial stripes increased their width from 20 µm to 60 µm during 5 hours and the dislocation density in the laterally overgrown material decreased more than two orders of magnitude, from 108 cm-2 to 5x105 cm-2. However, if the GaN stripes were fully merged or coupled to the GaN template, the dislocation density increased to 5x107 cm-2 [9]. The coalescence between stripes can be avoided by optimization of the growth conditions, mainly the crystallization time or the distance between stripes, as will be described in this paper. In order to avoid the direct coupling of laterally overgrown material to the underlying template, a mask between GaN stripes may be used. The mask should be easy to process, exhibit smooth surface, very good adhesion to the substrate and must be thermally and chemically stable at high nitrogen pressure and high temperature. The liquid gallium and also GaN should not react with the mask and GaN should not nucleate on it. The first candidate was silicon nitride SixNy used Gallium Nitride Materials and Devices, edited by Cole W. Litton James G. Grote, Hadis Morkoc, Anupam Madhukar, Proc. of SPIE Vol. 6121, 612103, (2006), 0277-786X/06/$15 · doi: 10.1117/12.645066 Proc. of SPIE Vol. 6121 612103-1
successfully for substrate masking in MOCVD epitaxy lateral overgrowth (ELOG) technology [10]. It seemed also that high refractory metal films like molybdenum and tungsten or iridium might be very useful. On the other hand, the direct coupling may be also avoided using the patterned thick HVPE free standing GaN substrates. In this case, the high stripes (up to 20 µm height) ought to be created on the crystal surface (c-plane) and the growth should take place in lateral directions from the edges of the stripes and without coupling to the substrates. In this paper the growth in lateral direction on patterned GaN/sapphire substrates with SixNy and Mo masks is analyzed. The usefulness of tungsten and iridium for substrate masking is determined. Thick HVPE free standing GaN substrates with high stripes are also examined. A defect selective etching (DSE) method is used to determine the average dislocation density on the surface of laterally overgrown material and in the areas corresponding to the initial GaN stripes. The results are compared with the existing data on the growth on patterned GaN/sapphire templates without any mask, with GaN or bare sapphire between stripes [ 9].
2. METHODOLOGY The high-pressure crystallization processes were carried out in a vertically positioned high pressure chamber of internal diameter of 40 mm. This chamber was connected to the three stage hydraulic gas compressor generating nitrogen hydrostatic pressure. In this system, a maximum pressure of 1.5 GPa can be obtained. A two-zone cylindrical graphite furnace, capable of reaching temperature up to 2000 K, was used in the crystallization experiments. The temperature during the crystal growth process was measured by PtRh6%-PtRh30% thermocouples. The manganine gauge, positioned in the low-temperature zone of the high pressure system, was used to control the nitrogen pressure. The pressure and temperature were stabilized with an accuracy of ±1 MPa and ±0.2 K, respectively. As substrates we used: 1. 3 µm thick GaN MOCVD layers (grown on sapphire) patterned by conventional photolithography and reactive ion etched to form stripes. The 20 µm wide and 3 µm high stripes were arranged along the direction. The distance between the stripes was 60 µm. The surface between the stripes was bare sapphire. Fig.1a presents a plan view of such a substrate. 2. 3 µm thick GaN MOCVD layers (grown on sapphire) covered by SixNy of 0.5 µm thick with a grid of parallel line openings cut in SixNy film by photolithography and reactive ion etching. The 20 µm wide line seeds were arranged along the direction. The distance between seeds was 60 or 300 µm. 3. 3 µm thick GaN MOCVD layers (grown on sapphire) patterned by conventional photolithography and reactive ion etched to form stripes of 20 µm wide and arranged along the direction with the distance between them of 300 µm. The surface between the stripes was covered by molybdenum. The Mo film of 0.7 µm was sputtered on the substrates. A grid of parallel line opening was cut in the Mo film precisely at input GaN stripes by photolithography and chemical etching. 4. 0.5 mm 3N iridium platelet 5. 0.2 mm 3N tungsten platelet In these cases the nucleation of GaN on the metals surfaces and reaction between liquid gallium and the metals were investigated. 6. 500 µm thick HVPE free standing crystals patterned by conventional photolithography and reactive ion etched to form stripes. The 20 µm wide and from 5 to 10 µm high stripes were arranged along and directions. The distance between them was 300 µm. The positive (direct) temperature gradient configuration with the substrates placed at the bottom of the crucible was applied. The crucible with a baffle plate positioned close to the seed was used. The experimental arrangement is presented schematically in Fig 1b. The temperature profile was evaluated by three thermocouples arranged in the crucible wall as shown in the Fig 1b. Before the crystal growth experiment, the high pressure system was evacuated to 10-3 Pa at 500 K and filled many times with nitrogen to atmospheric pressure. After that, the N2 pressure of 650 MPa was generated in the system and the heating procedure started. During the crystallization, the temperature gradients from 20 to 50 K/cm along the crucible axis at N2 pressure of 1 GPa were applied. The maximum temperature in the solution was always 1723 K. The time of crystal growth experiments varied from 1 to 5 h. After the growth, the substrate with the new deposited material, was removed from the crucible and etched in boiling HCl or HNO3/HCl acid solution. The newly deposited material was characterized by means of optical microscopy (Zeiss), scanning electron microscopy (SEM-LEO 1530) with an energy-dispersive X-ray analyzer (EDS), X-ray diffraction (XRD) and by defect selective etching (DSE). In DSE method, the etching processes were performed in the liquid eutectic of KOH and NaOH
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solution at temperatures ranging from 673 to 723 K. Then, the number of etched pits on a given area was calculated and the dislocation density determined.
crucible
tube baffle plate
subsfrate"'
IiI1
TI T2T3
T
Thermocouples positions Fig. 1a Nomarski microphotograph (DIC) of sapphire substrate Fig. 1b Schematic illustration of the direct temperature gradient with GaN stripes (bright areas). The 20 µm wide and 3 µm high configuration with baffle plate. T-z diagram represents the stripes were arranged along direction. temperature profile measured by three thermocouples located in the wall of the crucible.
3. DATA 3.1 Growth on patterned substrates with sapphire between stripes The first observations of crystallization on patterned substrates with a distance between stripes of 300 µm, reported in details in ref. [9], showed that during 5 hours the stripes increased their size 3-5 times in the c-direction and 3 times laterally for crystallographic directions used i.e. and . Additionally the DSE showed that the dislocation density in the areas corresponding to the initial stripes was 108 cm-2, however the dislocation density in the laterally overgrown material was of the order of 106 cm-2. Figure 2a presents a plan view of the sample with GaN stripes aligned in the direction. Between the stripes one can see small, up to 50 µm, tilted GaN grains. This polycrystalline nitride material, grown on sapphire, disturbed and stopped the lateral growth of the GaN stripes. Fig 2b shows X-ray scan with a step of 20 µm through this area presented in Fig. 2a. The intensity of 006 reflected X-ray beam was measured in the function of the beam (diameter of 20 µm) location.
2200
Intensity [counts/s]
2000 1800 1600 1400 1200 1000 800 600 400 200 -1,0
-0,5
0,0
0,5
1,0
X [mm]
Fig. 2a SEM photographs of the sapphire sample with Fig. 2b 006 X-ray scan with a beam of 20 µm and with a step of oriented GaN stripes after 5 hours crystallization at 1 GPa of N2 20 µm through the sample area presented in Fig. 2a. The peaks of high intensity represent the GaN overgrown stripes. and 1700 K.
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It is clearly visible that between high quality crystalline material (peaks of high intensity) there are polycrystalline GaN (many peaks of low intensity). Each high intensity peak is correlated with the overgrown GaN stripe. In this case the FWHM of a peak represents the stripe’s width. One can see that it varies from 0.05 to 0.07 mm. Expecting that the lateral growth of stripes is faster than the spontaneous nucleation on a sapphire surface the distance between stripes was changed and decreased to 60 µm. Fig 3a represents oriented in direction and still separated stripes after 5 hours of crystallization. The stripes are not coalesced yet although one can see some connection points between them. However, the most important observation here is that the edges of stripes are not straight, but some flounces, typical for cellular growth, are observed. The DSE showed that in the laterally overgrown material the defect density was of the order of 106 cm-2. Two types of dislocation in laterally overgrown material were determined. They were represented by small and big etch pits. It is clearly visible in Fig. 3b. Most likely, the small EPs are correlated to the subgrain boundaries, thus they are the edge dislocations. The big EPs represent the screw or mixed dislocations. It should be noted that etched pits density decreased with stripes wide what is evidently shown in Fig 3b.
Fig. 3a SEM photographs of the stripes oriented in Fig. 3b The edge of the stripe. The small and big etch pits are direction after crystallization at 1 GPa of N2 and 1700 K after 5 visible. The small EPs are the edge dislocations. The big EPs represent the screw or mixed dislocations. hours. The distance between initial stripes was 60 µm.
3.2 Growth on patterned substrates with silicon nitride between stripes Fig. 4a presents the general view of GaN deposited on the substrates with SixNy mask and GaN seeds (windows) arranged along direction, with a distance of 60 µm between each other. The GaN grew up to 40 µm in the cdirection during 5 hours. It can be seen that it was not typical bulk growth.
Fig. 4a SEM photographs of GaN deposited on the patterned Fig. 4b SEM photographs of GaN deposited on the patterned MOCVD GaN sapphire template with SixNy mask after MOCVD GaN sapphire template with SixNy mask after crystallization at 1 GPa and 1673 K during 5 hours. crystallization at 1 GPa and 1673 K during 2 hours.
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The quite wide grooves (up to 10 µm width) periodically distributed on the sample are clearly visible. The depth of such grooves was approximately 20 µm. The lateral distance between them was 60 µm. Examination of the interface between sapphire/GaN template with SixNy mask and new deposited GaN by SEM and EDS allowed to detect only some traces of Si in some points of the interface. Therefore, it seemed that the SixNy mask was almost totally etched by liquid gallium at the beginning of the crystallization process. The DSE analysis showed that the dislocation density in new deposited GaN was approximately 108 cm-2. The experiment of crystallization carried out in the same p-T conditions as the previous one but in shorter growth time (only 2 hours) confirmed this hypothesis. In this case GaN was deposited on the substrate with SixNy mask and GaN windows arranged along direction with a distance between each other of 300 µm. The nitride nucleated everywhere, not only on the GaN seeds, but also at places where SixNy mask should have existed, what is clearly seen in Fig. 4b. 3.3 Growth on patterned substrates with molybdenum between stripes Fig. 5a represents one of the overgrown stripe with its surroundings. Although the GaN stripes increased their lateral size by a factor two, the crystallization of GaN was also observed between stripes. The GaN crystallized at places of the Mo mask. There were many coalesced GaN islands. The measurement of dislocation density on these islands showed that the density was of order of 108 cm-2. The dislocation density on the overgrown parts of stripes was 5x106 cm-2. The EDS analysis of the region between stripes (in many points) allowed to detect only the traces of gallium and nitrogen. No traces of the Mo were detected. The SEM analysis of the Mo mask sputtered on sapphire/GaN template showed that the Mo was cracked just after sputtering process and before the crystallization. This is presented in Fig. 5b.
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,
N.
Mag= 1.OOKX LE01530-CBWPAN
Mag= 5.OOKX LEO 1530-CBW PAN
Fig. 5a SEM photograph of the GaN stripe grown at 1 GPa and 1700K during 5 h.
Fig. 5b SEM photograph of cracked Mo mask. Mo was sputtered on sapphire/GaN template.
3.4 Growth on iridium and tungsten platelets After crystallization process at 1 GPa and 1700 K during 5 h the iridium platelet, used as a seed, was not found in the crucible.
Fig. 6a SEM photograph of IrGa grain Fig. 6b SEM photograph of W platelet Fig. 6c SEM photograps of W platelets obtained after reaction between Ir and Ga before high pressure-high temperature after high pressure high temperature under 1 GPa of N2 and 1700K. process and contact with hot liquid gallium contact with gallium and nitrogen.
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At its place many grains silver in color of few millimeters in diameters were presented. The weight of these grains, even after the etching in HCl acid, was bigger than the weight of input Ir platelet of about 40%. Fig. 6a presents the typical habit and morphology of the obtained grains. The EDS analysis showed that these grains consist of Ir and Ga. According to SEMQuant Results (the software attached to EDS analyzer), these grains were IrGa. More promising results were obtained using the tungsten platelet as a seed. After typical crystal growth experiment (1GPa, 1700K, 5h) the weight and habit of W platelet did not change. The recrystallization of the tungsten grains were observed what can be seen comparing Fig. 6b and Fig. 6c. Fig. 6b represents the morphology of the tungsten platelet before crystallization process and Fig. 6c after that. The morphology is totally different. The grains after annealing are bigger. It should be noted that a parasitic nucleation was observed on W platelet only in one small area, most probably where the supersaturation was the biggest. This parasitic nucleation was in form of small GaN platelets grown directly on the tungsten surface. 3.5 Growth on HVPE free standing crystals patterned to high stripes Fig. 7a shows the GaN grown on free standing HVPE crystals patterned in high stripes. In this case the stripes had 5 µm height. The stripes increased their lateral size by a factor two or even three. Unfortunately, they were entirely coupled to the substrates and the DSE measurements showed the same results of dislocation density in the regions between stripes and directly on them. It was 5x106 cm-2. It should be mentioned that the dislocation density in free standing HVPE crystal used as a seed was 2x107 cm-2. Thus, due to high pressure growth, the density of etch pits decreased more than 3 times. For higher stripes (10 µm height) on the free standing HVPE crystals a totally different mode of stripes overgrowth was observed, although the growth conditions were the same. It is shown in Fig. 7b. Thin GaN film (up to 30 µm) was nucleated in lateral direction from the edges of the stripe. In this case the stripes were arranged along direction. The new material was not coupled to the HVPE substrate. The maximum observed lateral size of this thin crystal was 50 µm. The DSE measurements showed that such GaN wings were dislocation free.
Fig. 7a Plane view of the GaN on the free standing HVPE crystal Fig. 7b 10 µm high stripe with thin GaN film nucleated from the patterned in stripes of 5 µm high (Nomarski microphotograph- stripe’s edge (Nomarski microphotograph-DIC). The sample DIC). The sample crystallized at 1 GPa, 1698 K and during 5 h. crystallized at 1 GPa and 1698 K during 5 h.
4. RESULTS The high-pressure directional crystallization of GaN on sapphire/GaN templates yielded GaN with the defect density of the order of 5x107 cm-2 [7,8]. Therefore, we have started to grow GaN on stripe-patterned GaN/sapphire substrates. We have assumed that the solution growth would proceed selectively from the edges of the stripes and mainly in directions parallel to the substrate. In this manner, the laterally overgrown regions should be nearly defect free, as takes place for example in GaAs ELOG by Liquid Phase Epitaxy [11]. Our assumption was based on the fact that spontaneous growth of GaN by high pressure method is strongly anisotropic and results in crystals having forms of thin hexagonal platelets. The growth rate in directions perpendicular to the c-axis is ~100 times faster than in the c-direction [5]. The conditions needed for high pressure growth on patterned substrates were indicated in the reference [9]. There were: the growth in unstable mode when the crystal is growing at the conditions of constitutional supercooling, the
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lateral growth without coalescence of the overgrown stripes and the use of SixNy mask between GaN stripes in order to avoid the direct coupling of overgrown GaN to the underlying template and to avoid the parasitic (spontaneous) GaN nucleation between stripes on bare sapphire. The results of GaN grown on SiC [6] suggested that GaN does not nucleate on SixNy and does not react with it. Therefore SixNy was a first candidate for masking. However before masking we wanted to investigate if there are some experimental conditions where lateral overgrowth of stripes is faster than spontaneous nucleation between stripes. It was already known that if the distance between stripes was 300 µm the lateral overgrowth of stripes was stopped and disturbed by the spontaneous nucleation of the columnar GaN grains (see also Fig2a). Therefore the distance between stripes was changed and decreased to 60 µm. For our experimental conditions i.e. 1 GPa and 1700 K and the crystallization time of 5h this distance has seemed to be correct. As was shown in Fig. 3a the stripes increased their size from 20 µm to 70 µm or more and coalesced only in some points. One can maintain that for the distance between GaN stripes of 60 µm and bare sapphire between them, the crystallization time of 5 h is an upper limit. The stripes are not coalesced yet and their overgrowth is not strongly disturbed. Thus the lateral overgrowth is faster than parasitic nucleation. Although some flounces on the stripes edges were observed (see Fig. 3a). These flounces might be caused by parasitic nucleation or by non uniform lateral front of crystallization and hence the cellular growth. They might be also caused by an interaction between laterally overgrown stripes due to a consumption of nitrogen by each other or by wrong processing procedure and bad stripes sides preparation (photolithography and reactive ion etching were used). The substrates with the stripes were never investigated by SEM before crystallization process. Nevertheless it can be concluded that the lateral growth of GaN stripes will be faster than the spontaneous nucleation if the distance between stripes is shorter or equal to 60 µm and the crystallization time is shorter than 5h. Unfortunately, in this case the stripes can overgrow only for 30 µm per side. The low dislocation areas are small what makes a construction of devices on them difficult. Therefore, the substrates with GaN windows and SixNy mask were prepared. Herein the nitride nucleated everywhere, not only on the GaN seeds, but also at places where SixNy mask should have existed. It is clearly seen in Fig. 4b. No doubts, the SixNy mask was etched by hot liquid gallium. At the beginning of the crystallization process the nucleation took place selectively on the GaN seeds and the growth proceeded in the cdirection. However at the same time SixNy mask was etched and dissolved in the liquid gallium. As soon as the crystallization front exceeded the top surface of the mask, the growth in lateral direction over masking film started. Unfortunately, the GaN growth at the dissolved areas of the masking region started too (see Fig. 4b). The coalescence between overgrown stripes and GaN islands nucleated at the mask areas was possible. Additionally the growth rate in the c-direction was bigger than the lateral one, thus probably the {1-102} facets were formed at the stripes sides and hindered the lateral growth. Then, the coalescence of the stripes led to the formation of grooves, those well visible in Fig. 4a. Due to this coalescence and also coupling of GaN to the template the DSE analysis showed that the dislocation density in new deposited GaN was 108 cm-2. The next candidate for masking GaN was molybdenum. The results were not promising. GaN crystallized at the mask areas. Those coalesced islands presented in Fig. 5a were probably grown from the cracking paths in the Mo mask (see Fig. 5b). Additionally the Mo mask was etched and dissolved by hot liquid gallium. It is suggested by EDS analysis (no Mo traces were detected) and by the fact that the dislocation density was of order of 108 cm-2 everywhere between stripes. If the Mo mask had existed at high temperature the dislocation density would have been lower on GaN islands due to the nitride growth over Mo film, thus without coupling to the substrate (a situation of a self-ELOG). Since it is complicated to prepare patterned GaN/sapphire substrate with a mask (photolithography, ion and chemical etching) at the beginning the tungsten and iridium platelets as seeds were tested as potential candidates for masking. The nucleation of GaN on the metal surface and the reaction between liquid gallium and solid metals were examined. The tungsten has the highest melting point and lowest vapor pressure of all metals, and at temperatures over 1923 K has the highest tensile strength. It has excellent corrosion resistance and is attacked only slightly by most mineral acids [12]. Iridium was chosen due to its good chemical resistant properties. Ir is not attacked by any of the acids including aqua regia [13]. However hot liquid gallium reacts with iridium quite fast. After crystallization run at 1 GPa, 1700 K and only during 5 h Ir platelet was not found in the crucible. It reacted with gallium to IrGa. Probably, the Ir platelet, consisting of sintered Ir grains, was penetrated by liquid gallium. Then, it came apart and the agglomerates of Ir grains reacted with Ga. This reaction took place with 100% degree of conversion since the traces of free Ir were not detected in synthesized material. More promising result was obtained using W as a seed. After crystallization run at 1 GPa, 1700 K and during 5 h the only recrystallization of the tungsten grains in the platelet was observed. The weight and habit of W platelet was not changed. It suggests that tungsten does not react with gallium and does not dissolve in it even at 1700 K. Some parasitic GaN nucleation on the W surface can be eliminate by the crystallization process at lower supersaturation. As was mentioned this parasitic GaN crystals were grown at the area of the sample where the supersaturation was the biggest.
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Among the already tested materials, the tungsten seems to be the best candidate for GaN masking in the layer. Obviously, there are some differences in structural properties between sputtered tungsten and the bulk platelet. Our experience with SixNy shows a possibility of a mistake. During GaN crystallization process on SiC substrates some silicon nitride crystals were formed at the crystal surface. GaN did not nucleate on them [6]. Therefore we concluded that GaN does not nucleate on SixNy and does not react with it. In the case of sputtered SixNy this turned out not to be. The sputtered SixNy was dissolved in liquid gallium. The sputtered tungsten may behave in the same way. Thus it seems that finding a good mask for LPE of GaN at 1700 K, due to strong reactivity of liquid gallium, may be impossible. Therefore the free standing HVPE crystals patterned in high stripes were prepared. The high stripes should make the direct coupling of the overgrown material to the substrate difficult or even impossible. For the stripes height of 5 µm this assumption presented above has appeared to be false. As shown in Fig. 7a the stripes were coupled to the substrates. The DSE measurements showed 5x106 cm-2 thus it was the same dislocation density in the region between stripes as directly on them. The totally different mode of the stripes overgrowth was observed for the free standing substrate patterned in stripes of 10 µm height. Then the growth of thin GaN film from the edges of the stripes was observed (see Fig 7b). This result shows how important is the GaN nucleation on the edges exposed to the incoming nitrogen flux in the liquid solution. The edges may be the seeds for new material growth and the coupling between new grown material and the initial seeds can be only at the edge line what guaranties zero dislocation density in new grown material. This crystal shown in Fig. 7b is not coupled to the substrate at all. This crystal is a high pressure crystal nucleated directly from the stripe’s edge. These two growth modes presented above are the results of different shapes of the crystallization front on the growing crystal surface caused in first approximation by a difference in stripes heights. Thus, caused by a different distribution of supersaturation near the crystal. As the GaN crystal grows, the nitrogen is consumed at the crystal surface and the nitrogen concentration at the interface between the crystal and liquid gallium can be lower than that in the liquid phase. The lack of nitrogen at the interface results in a reduction of the equilibrium liquidus temperature in this region. Therefore, for a relatively small temperature gradient at the crystallization front, the supersaturation in the liquid at some distance from the substrate can be higher than that just at the growing crystal surface. Thus, any perturbation of the interface will tend to cause faster growth because it will experience a higher supersaturation. Since the initial substrate surface is not flat in our case, the supersaturation at the stripe surfaces and their edges is higher than at the surfaces between stripes. Therefore, the stripes grow faster in the lateral and c-direction at the beginning of the crystallization run. As they are bigger and higher, the supersaturation in the solution can change and three growth modes may be observed. They are presented in Figs. 8a, 8b and 8c. The first mode (Fig. 8a) is characterized by overgrown stripes with a trapezoidal shape and well formed {1-102} facets (see Fig. 8a). In this case the stripes are coupled to the substrate like during the growth on the HVPE substrate with 5 µm stripes height (see Fig 7a). The supersaturation next to the stripes edges become equal or lower than that on the seed surface and therefore the patterned structure grows in the c-direction in a stable mode and the stripes integrate with the substrate in time. However if the supersaturation near to the stripes edges is higher than that on the seed surface the crystallization of stripes in lateral direction will be observed (see Fig. 8b). It should be noted that such growth mode has only occurred for the samples with the distance between stripes of 60 µm [9]. For very high stripe in starting material (10 µm) the third growth mode takes place (see Fig. 8c). The supersaturation near to the stripe edges is higher that on the seed surface and the GaN platelets nucleate just from the stripe’s edges (see also experimental result shown in Fig. 7b).
Fig. 8a Overgrown stripes with trapezoidal Fig. 8b Overgrown stripes. They are not Fig. 8c GaN platelets nucleate from the stripe’s edges. shape. Stripes coupled to the substrate. coupled to the substrate.
Only in third growth mode the laterally overgrown material is dislocation free. Unfortunately this material is too thin for its further application. In turn the others modes lead to thicker overgrown GaN but with defect density of about 106 cm-2. This value eliminates an usefulness of that material as substrates. Although the defect density decreases with increasing of the stripe’s width (see Fig. 3b) but up to now, due to the lack of a good mask, we cannot widen stripes more than 3 times (maximal width 60-70 µm). In our opinion for the substrate applications the low dislocation regions of minimum value of 100 µm are needed.
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5. CONCLUSIONS In this paper the high pressure growth from solution in the lateral direction on patterned GaN/sapphire substrates with various masks like SixNy, Mo, Ir and W was examined and analyzed. It seems that from tested materials only the tungsten can be used as a mask. Tungsten does not react with gallium and does not dissolve in it at 1700 K. However it should be noted that we have not tested the real tungsten mask yet, but only examined W behavior during a contact with the hot liquid gallium. The SixNy and Mo masks dissolve in the liquid gallium at 1700 K. Iridium reacts with Ga to form of IrGa at 1700 K. Thick HVPE free standing GaN substrates with high stripes and patterned substrates with bare sapphire between stripes, thus without mask, were also examined in this paper. It was shown that the lateral growth of 20 µm GaN stripes placed on bare sapphire will be faster than the spontaneous nucleation on the sapphire if the distance between stripes is shorter or equal to 60 µm and the crystallization time is shorter than 5h. However in this case the stripes can overgrow only for 30 µm per side. Additionally the dislocation density in the overgrown material is about 106 cm-2 what makes its unuseful from the technological point of view. More promising results were obtained using the HVPE free standing GaN substrate with high (10 µm) stripes. Then, the growth of thin (30 µm) GaN platelets directly from the stripes edges was observed. These platelets were dislocation free since they were typical high pressure crystals nucleated from the edges. The main disadvantage of this material is its thickness. However it seems that just the growth on HVPE free standing GaN substrates but with the stripes of about 100 µm height will be developed by the authors in the future.
AKNOWLEDGMENTS The authors would like to thank professor J. Szmidt and Dr. R. Mroczyński from Institute of Microelectronics and Optoelectronics of Warsaw University of Technology for sputtering of SixNy layers on MOCVD GaN/sapphire substrates.
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