Fig. 3-10 The relation between Al composition and hillock size.
Chapter 4 GaN Nanodots Growth
Because of the lack of lattice-matched substrates, the growths of GaN, InGaN and AlGaN materials on sapphire substrates are known to contain numerous defects, such as threading dislocations, stacking faults and inversion domain boundary in the epilayer, accompanied generally by a high concentration of nonradiative recombination centers. The existence of such defects seems not to affect significantly the efficiency of band-edge luminescence in InGaN/GaN blue/green light-emitting diodes. It is ascribed to the formation of self-assembled In-rich islands during InGaN film growth, which form dotlike states and lead to a marked gain enhancement in their optical process [1]. However, this is not the case for ultraviolet AlGaN materials. To date, no evidence has shown that Al segregation exists. Even though Al-rich nanoislands indeed occur, because of their higher band-gap energy feature, it would not improve the carrier confinement and hence luminescence efficiency in AlGaN films. From published ports on AlGaN ternary [2,3], we consider that the presence of a high concentration of threading dislocations and the absence of self-assembled lower energy dotlike structures are the two most detrimental factors that cause the poor quantum efficiency of AlGaN-based UV-light-emitting devices. Thus, the successful fabrication of GaN or AlGaN dotlike structures operating in the ultraviolet range is an essential step for the implementation of high-brightness UV-LEDs.
Despite the numerous studies on InGaN dots, the published reports on GaN dots, particularly on specific sample preparation procedure, are still quite limited. The growth of GaN dots on AlN using the commonly used Stranski-Krastanow (S-K) growth mode was not reported until 1997 by Daudin et al.[4] using molecular beam epitaxy (MBE), and more recently by Miyamura et al. [5] using metalorganic vapor phase epitaxy (MOVPE) by maximizing the advantages of the driving force induced
by the lattice mismatch between GaN and AlN. Probably because of the insufficient lattice mismatch provided by the underlying layer, very few updated reports have been published recently on the GaN island growth on AlGaN ternary. That imposes a strict restriction for its practical use in UV-light-emitting devices. To overcome this problem, an interesting approach, called antisurfacant method, has been used to grow GaN dots on AlGaN ternary [6]. By supplying a small amount of Si antisurfacants on an AlxGa1-xN surface, the GaN growth is found to change from the step-flow growth feature to the three-dimensional island growth, resulting in the formation of nanoscale GaN dot structures on the AlGaN surface. In this letter, we present another feasible method for preparing GaN dots on the AlGaN surface. Preliminary results indicate that by alternating the source precursors during the MOVPE epitaxial growth, a dotlike GaN structure can be obtained on an Al0.11Ga0.89N epilayer. This method is proved to be a simple and yet effective way for preparing dotlike structures in the GaN material system and may find potential use in fabricating the ultraviolet GaN-based light-emitting devices.
4-1 experimental details
In this study, the uncapped GaN dots were grown on Al0.11Ga0.89N/sapphire (0001) substrates by AIX 200/4 RF-S horizontal-reactor MOVPE system. The substrate temperature is measured by inserting the S-type thermal couple into a susceptor, which has a deviation of approximately ±2oC. Trimethylgallium (TMGa), trimethylaluminum (TMAl), and ammonia (NH3) were used as the source precursors of Ga, Al, and N, respectively. Hydrogen was used as a carrier gas. Prior to material growth, the sapphire substrate was annealed to remove any residual impurities on the surface in a H2 ambient at 1120 ℃ for 10 min. A nominal 25-nm-thick AlN nucleation layer was deposited at 650 ℃ . The substrate temperature was then increased to 1120℃ to grow a 0.5 μm Al0.11Ga0.89N layer. The GaN dots were deposited at temperatures ranging from 840 to 960oC by flow-rate modulation epitaxy (FME) technique. The gas flow sequence for FME, basically consists of four steps: 20 s Ga source step, 10 s NH3 source step and intervening 5 s purge steps in between. To suppress such reevaporation, we intended to introduce a 1/10 nominal flow rate of NH3 during the entire FME growth cycle except the N step, where full NH3 flux (1.79
× 105 μmol/min) was used. Such arrangement can conceivably suppress, if any, the re-evaporation of N atoms to a minimum extent. After the dot growth, the substrate temperature was then decreased to room temperature under a continuous flush of NH3
gas. The other detail growth conditions were described in Table 4-1.
The theory of the periodic flow-rate modulation epitaxy growth according to this study is shown in Fig. 4-1. First, substrate 1 is provided (Fig. 4-1(a)), and buffer layer 2 is grown on the substrate (Fig. 4-1(b)). Purge gas 3 is turned on and the first reactant 4 is modulated to a range below the first molar flow rate (Fig. 4-1(c)), so that
the second reactant 5 turned on thereafter forms metal or metal-rich compound islands 6 on the buffer layer (Fig. 4-1(d)). After the formation of said island, purge gas is turned on to clean remaining second reactant which does not form islands (Fig.
4-1(e)). Subsequently, the first reactant 4 is modulated to a range above the second molar flow rate (Fig. 4-1(f)), so that said islands form high quality nanoparticles 7 with excellent structure under sufficient first reactant molar flow rate, and a growth cycle is completed. The geometric shape and size can be controlled through modulating the first and second reactant molar flow rates and the turn-on time.
The time chart of reactant precursor flow sequence is shown in Fig. 4-2. In the first purge step (as shown in Fig. 4-2(a)), purge gas (hydrogen 3) was turned on to clean remaining excess NH3 reactant, and NH3 reactant 4 was modulated to the lower first molar flow rate range (1.79x10-2 mole/min), so that TMGa reactant turned on in next step was able to grow metal gallium or Ga-rich islands on Al0.11Ga0.89N buffer layer below the first molar flow rate without the formation of 2D grown GaN film.
Further, NH3 reactant was modulated to the lower first molar flow rate to avoid re-evaporation of nitrogen atoms on the surface of Al0.11Ga0.89N buffer layer in low grown temperature environment of 900℃, and the nitrogen vacancy defect on the surface of Al0.11Ga0.89N buffer layer was reduced.
In TMGa reactant turn-on stage (as shown in Fig. 4-2(b)), TMGa reactant 5 was turned on with molar flow rate setting of 8.84x10-5 mole/min for 20 seconds, to form metal gallium or Ga-rich islands on Al0.11Ga0.89N buffer layer. In the second purge stage (as shown in Fig. 4-2(c)), TMGa reactant was turned off, and purge gas 3 was turned on for 5 seconds to clean remaining TMGa reactant which did not form metal gallium or Ga-rich islands. In NH3 reactant turn-on stage (as shown in Fig. 4-2(d)), the molar flow rate of NH3 reactant 4, which was modulated from below the first
molar flow rate to above the second molar flow rate (1.79x10-1 mole/min), was turned on for 10 seconds, so that islands formed GaN nanoparticles 7 with excellent quality at sufficient NH3 reactant molar flow rate.
Table 4-1: The growth conditions of GaN dots on AlGaN films.
Time
4-2 Growth Temperature Effect
With regard to GaN growth, Stephenson et al.[7] reported that the typical GaN growth on a nearly lattice-matched substrate by conventional MOVPE undergoes mainly three different growth modes with increasing substrate temperature, namely, three-dimensional (3D) island growth, two-dimensional (2D) layer-by-layer growth, and one-dimensional (1D) step-flow growth. The corresponding transition temperatures are approximately 800 and 1000oC, respectively. Since growth temperature is one of the critical parameters affecting the growth mode, to verify whether the FME technique is suitable for GaN dot growth, we conducted GaN growth on AlGaN at 840, 870, 900, 930, 940 and 960℃ with two FME growth cycles.
The flow rates of TMGa and NH3 during the source exposure steps were kept at 88.4 and 1.79× 104 μmol/min, respectively. The resulting surface morphologies are shown in Fig. 1. Unlike the conventional MOVPE layer-by-layer 2D growth feature at the high-growth-temperature region (>800oC), by employing FME, we do observe that GaN dots can be formed on the slightly lattice-mismatched Al0.11Ga0.89N epilayer in the temperature range from 840 to 940oC.
Dot density as a function of reciprocal temperature is shown in the inset of Fig. 2.
As anticipated, the dot density is very sensitive to the substrate temperature. There are two distinct regions in our dot density curve, divided by a temperature of ~915oC. As can be seen in the figure, the density decreases gradually from 1.7× 109 to 3× 108 cm-2 as the temperature is increased from 840 to 900oC, which then tends to drop sharply with further increasing temperature, and eventually becomes zero, i.e., no dot growth, if the substrate temperature is beyond 960oC. From the island nucleation mechanism proposed by Robinson and Robins et al.[8], we learn that the dot density
at low growth temperatures is governed by the diffusion capability of the adatom, while that at high temperatures is determined predominately by the re-evaporation rate of adatoms, hence the binding energy of the adatom to the adsorbed site. The respective characteristic equations are
where Ns is dot density, N0L and N0H pre-exponential parameters, Ed activation energy of diffusing Ga adatom, and Ea binding energy of Ga adatom to the adsorbed sites.
The calculated Ed and Ea for the Ga adatom are ~0.64eV and ~1.20eV, respectively, for GaN growth on AlGaN film by FME, which are considerably smaller than the simulated value of Ed = 1.8 eV[9] and the experimental value of Ea= 2.2 eV obtained by the conventional MBE growth system,[10] and the value of Ea= 3.0 eV by the conventional MOCVD system [7]. Our result is also similar to the case of GaAs growth in which the activation energy during the FME growth is smaller than that during the conventional MOCVD growth [11]. It was also noted that the smaller Ea of 0.4 eV was reported in the GSMBE system [12].
Aside from the dot density, the other dot parameters that concern us are height and diameter. One can find in Fig. 2 that by using FME, the GaN dot diameter tends to increase slowly with increasing temperature and increases marked as the growth temperature exceeds the dividing temperature of 915oC. In contrast, the dot height almost remains constant of 30nm at low temperatures, becomes reduced with further increasing the growth temperature and vanishes completely at temperatures above 960oC. The observation extremely large dot diameter, concurrently with essentially zero height for T>960oC, indicates that the growth manner for FME-GaN on AlGaN
starts to transform from 3D to 2D growth mode at ~960oC. That is, by using FME, we can extend the upper limit of the growth temperature for GaN island growth from
~800oC in the conventional MOVPE to 960oC. Since a higher growth temperature usually yields a better film quality, this will make the FME a very promising method for preparing nanodot structures.
4-3 Size Control
For light-emitting devices, the size of low-band-gap dot structure used in the active layer is a matter related to the device luminescence efficiency. The use of a smaller dot structure will certainly result in a better carrier confinement, and even provoke quantum excitonic effects to improve the quantum efficiency. For this reason, we thus performed another series of GaN dot growth on Al0.11Ga0.89N at 900oC by decreasing the TMGa exposure time in FME, with all the other growth parameters kept the same. Figure 3 shows the atomic force microscopy (AFM) images for films grown at different TMGa exposure times of 20, 15, 10, 7, 5 and 0 s per cycle. The resulting variation of GaN dot height and diameter are shown in Fig. 4. The average dot diameter and height are clearly decreased as the Ga exposure time is decreased.
By decreasing the Ga exposure time to 5 s, we are able to attain a GaN dot structure with a height of 5 nm; a strong quantum effect is expected to dominate the light transition.
4-4 Growth Mode
Based on the growth theory, there are two growth modes responsible for the self-organized island growth, namely, Stranski-Krastanow and Volmer-Weber (V-W) growth modes [13,14]. Regardless of which growth mode, the sum of the surface free energy of the deposited film (σfilm) and the interface strain energy (σinterface), including interface energy (σif) due to lattice mismatch between substrate and deposited film, and strain energy (σst(t)) due to induced strain caused by the wetting layer, has to be greater than the surface free energy of the substrate (σsubstrate); otherwise, the layer-by-layer growth manner will occur during deposition. The primary difference between these two island growth modes is the thickness of the wetting layer. If the required wetting layer thickness for producing island growth is greater than one lattice layer, the associated island growth mode is categorized into the S-K growth mode; if not, then the V-W mode.
If the SK mode is the case for our FME GaN dot structure grown on Al0.11Ga0.89N, the critical thickness of the GaN wetting layer should be at least of 330Å[15] to accumulate sufficient strain energy to induce island growth. This seems unlikely to happen in our study, because such a thickness can never be achieved with the same growth time and source supplies under our nominal MOVPE growth method, not to mention the FME method, where the growth rate is considered to be lower due to the high desorption nature of the reactants.
Thus, we consider that our FME GaN dot growth on Al0.11Ga0.89N is via the Volmer-Weber growth mode. The explanation for this is as follows. It is known that the V-W island growth will proceed if the surface free energy of the substrate (σsubstrate) is less than the sum of the surface free energy of the depositing film (σfilm) and the
interface strain energy (σinterface).
σsubstrate< σfilm + σinterface (3)
Because of the special alternating gas supply feature in FME, during the Ga source step, it is the Ga metal, not the GaN, that is deposited on the underlying AlGaN layer, which will be converted lately into GaN film during the following NH3 source step.
The surface free energies of Al0.11Ga0.89N and Ga metal [16] are ~107 and 45 meV/Å2, respectively. To meet the V-W island growth constraint, it would need at least an additional ~62 meV/Å2. Since the interface strain energy includes an interface energy and a strain energy of one lattice layer, [17]
σinterface = Y
( )
twhere the first term is interface energy (σα-Ga/AlGaN) and the second term is strain energy σst(t), Y is modulus of Ga metal (5.69×1010 N/m2) [18], εa in-plain strain (aα-Ga-aAlGaN/aAlGaN), t the thickness of one lattice layer of Ga metal (cα-Ga=7.64 Å, aα-Ga=4.51 Å [19] and aAlGaN=3.18 Å). The calculated interface energy and strain energy of one lattice layer are 31.5 and 237 meV/Å2, respectively. The resulting total interface strain energy is 268.5 meV/Å2, which is far beyond the need for V-W island growth mode. Thus, we consider that this is the reason that accounts for the successful growth of the GaN dot on AlGaN in our study by FME method.
4-5 Conclusions of GaN Nanodots Growth
In summary, we have demonstrated that the GaN dots can be grown on a slightly lattice-mismatched Al0.11Ga0.89N epilayer using flow-rate modulated epitaxy. The dot growth in this method is found to be controlled primarily by surface diffusion of adatoms at substrate temperatures below ~915oC and by desorption at higher temperatures. Because of the alternating gas supply nature in FME, we consider that the dot growth studied here is mainly via the Volmer-Weber growth mode, not through the Stranski-Kranstanow growth mode. Our results indicate that the FME growth technique is a very promising tool for preparing self-organized quantum dot structures for most practical devices due to the release of requirement of large lattice mismatch between the grown dot structure and substrate.
References
[1] H. K. Cho, J. Y. Lee, N. Sharma, C. J. Humphreys, G. M. Yang, C. S. Kim, J. H.
Song and P. W. Yu: Appl. Phys. Lett. 79 (2001) 2594.
[2] T. Akasaka, T. Nishida, Y. Taniyasu, M. Kasu, and T. Makimoto: Appl. Phys. Lett.
83 (2003) 4140.
[3] S. Tanaka, Jeong-Sik Lee, Peter Ramvall and Hiroaki Okagawa: Jpn. J. Appl. Phys.
42 (2003) L885.
[4] B. Daudin, F. Widmann, G. Feuillet, Y. Samson, M. Arlery, and J. L. Rouvière: Phys.
Rev. B56 (1997) 7069.
[5] M. Miyamura, K. Tachibana, and Y. Arakawa: Appl. Phys. Lett. 80 (2002) 3937.
[6] Satoru Tanaka, Sohachi Iwai, and Yoshinobu Aoyagi: Appl. Phys. Lett. 69 (1996) 4096.
[7] G. B. Stephenson, J. A. Eastman, C. Thompson, O. Auciello, L. J. Thompson, A.
Munkholm, P. Fini, S. P. Denbaars, and J. S. Speck: Appl. Phys. Lett. 74 (1999) 3326.
[8] V. N. E. Robinson and J. L. Robins: Thin Solid Films 20 (1974) 155.
[9] Tosja Zywietz, Jörg Neugebauer, and Matthias Scheffler: Appl. Phys. Lett. 73 (1998) 487.
[10] S. Guha, N. A. Bojarczuk, and D. W. Kisker: Appl. Phys. Lett. 69 (1996) 2879.
[11] Naoki Kobayashi, Toshiki Makimoto and Yoshiji Horikoshi: Jpn. J. Appl. Phys. 24 (1985) L962.
[12] Keith R. Evans, Ting Lei and Charles R. Jones: Solid-State Electron. 41 (1997) 339.
[13] D. J. Eaglesham and M. Cerullo: Phys. Rev. Lett. 64 (1990) 1943.
[14] D. Winau, R. Koch, A. Führmann, and K. H. Rieder: J. Appl. Phys. 70 (1991)
3081.
[15] William G. Perry, M. B. Bremser, T. Zheleva, K. J. Linthicum and R. F. Davis:
Thin Solid Films 324 (1998) 107.
[16] L. Z. Mezey and J. Giber: Jpn. J. Appl. Phys. 21 (1982) 1569.
[17] Kazuo Nakajima, Toru Ujihara, Satoru Miyashita, and Gen Sazaki: J. Appl. Phys.
89 (2001) 146.
[18] K. Gschneidner, Jr.: Solid State Phys. 16 (1964) 275.
[19] X. G. Gong, Guido L. Chiarotti, M. Parrinello, and E. Tosatti: Phys. Rev. B43 (1991) 14277.
Fig. 4-1 A scheme showing the principle of the periodic flow-rate modulation epitaxy growth GaN dots on AlGaN films.
Fig. 4-2 A time chart showing the modulation of reactant molar flow rate in the periodic flow-rate modulation epitaxy.
Time
Time
Time Reactant flow rate
(a) (b) (c) (d)
Fig. 4-3 AFM images of GaN dots grown at (a) 840, (b) 870, (c) 900, (d) 930, (e) 940 and (f) 960 ℃.
840 860 880 900 920 940 960
930 900 870 840Temperature (oC)
Dot density ( x108 cm-2 )
Fig. 4-4 Dependence of average diameter and height of GaN dots on growth temperature. The inset shows island density as a function of reciprocal temperature.