2.1 3D growth mode

GaN growth by the two-dimensional growth mode has been made possible by the nitridation of the sapphire surface and by the two-step process. Reference Amano, Sawaki, Akasaki and Toyoda[11] Basically, the growth conditions have been optimized in order to obtain a continuous coverage of the substrate by the GaN nucleation layer (NL) and thereby producing flat GaN films at high temperature. This growth process however, leads to high defect densities (~10¹⁰ dislocations/cm²) and therefore better quality material is needed to achieve high performance and reliable devices.

By essentially using the epitaxial overgrowth principle, i.e. involving localized but maskless lateral overgrowth, a new growth process, referred to as the “3D growth process”, has been implemented to obtain better quality GaN materials. Silicon has been reported to act as a “antisurfactant”, i.e. it plays an important role in changing the growth mode from 2D to 3D. Reference Tanaka, Iwai and Aoyagi[12] Reference Shen, Tanaka, Iwai and Aoyagi[13] Reference Nakamura, Senoh, Nagahama, Iwasa, Yamada, Matsushita, Kiyoku, Sugimoto, Kozaki, Umemoto, Sano and Chocho[3] Using this antisurfactant effect of silicon, GaN quantum dots on Al_xGa_1−xN surfaces have been realized by MOVPE. Reference Tanaka, Iwai and Aoyagi[12]

The exposure of the sapphire substrate prior to the deposition of a GaN NL under simultaneous silane and ammonia flows fundamentally influences the quality of GaN epilayers grown by MOVPE. This step of the growth process will be referred to as “Si/N treatment”. Reference Vennegues, Beaumont, Haffouz, Vaille and Gibart[6] Reference Haffouz, Beaumont, Vénnègues and Gibart[9] The NL is deposited between 500 and 600°C. During the annealing up to 1080°C of this NL, the Si/N process induces a dramatic morphological change of the GaN NL from a flat layer fully capping the substrate to a high density of 3D GaN islands (200-400 nm large and 100−200nm high) surrounded by the bare sapphire substrate (see Figure 1(a)).

Figure 1a. AFM (5×5µm²) scan showing the layer morphology for 180 sec SiN treatment time. The GaN islands density is about 1×10⁹ cm⁻².

Figure 1b. AFM (5×5µm²) scan showing the layer morphology for 300 sec SiN treatment time. The GaN islands density is about 5×10⁸ cm⁻².

Figure 1c. AFM (5×5µm²) scan showing the layer morphology for 480 sec SiN treatment time. The GaN islands density is about 3×10⁸ cm⁻².

Figure 1d. AFM (5×5µm²) scan showing the layer morphology for 720 sec SiN treatment time. The GaN islands density is about 1×10⁸ cm⁻².

It has been observed that two parameters are of critical importance to induce the 3D growth process:

1. 1. the composition of the carrier gas (N₂ or N₂+H₂) and

2. 2. the duration of the Si/N treatment. Reference Vennegues, Beaumont, Haffouz, Vaille and Gibart[6] Reference Haffouz, Beaumont, Vénnègues and Gibart[9]

The GaN island formation is achievable only when H₂ is present in the carrier gas. H₂ seems to act as a “morphactant” as Eaglesham et al. Reference Eaglesham, Unterwald and Jacobson[14] called impurities which favoured particular equilibrium shapes of islands. Recent studies Reference Han, Ng, Biefeld, Crawford and Follstaedt[15] have also reported that the appearance of islands is strongly related to the H₂ concentration in the growth chamber.

GaN epilayers grown following this process show X-ray rocking curves with FWHM in the 180-360 arcsec range (for asymmetric reflections). Hall electron mobility in the 500-700 cm²/V.s range at 300K, with background carrier concentration in the low 10¹⁶cm⁻³, were obtained for GaN films grown with an adequate Si/N treatment time. When measuring the PL intensity of 3D GaN layers, an increase in the intensity by a factor of about 20 is observed as compared to the 2D-growth mode. This gain in radiative efficiency is well correlated to the decrease by a factor of about 50 in the dislocation density, thus confirming the non-radiative recombination nature of some of the dislocations in GaN. Reference Rosner, Carr, Ludowise, Girolami and Erikson[16] Reference Sugahara, Sato, Hao, Naoi, Kurai, Tottori, Yamashita, Nishino, Romano and Sakai[17] This Si/N treatment at the early stage of the growth was subsequently implemented by other groups and led to TD density in the low 10⁸cm⁻² range. Reference Haffouz, Kirilyuk, Hageman, Macht, Weyher and Larsen[18] Reference Sakai, Wang, Morishima, Naoi and Crystal Growth[19]

2.2 Improvement of the Si/N treatment

The density of defects in material grown with a 3D mode was recently reduced down to the mid 10⁷cm⁻² range as measured by atomic force microscopy by an improved Si/N treatment. Growth experiments were carried out in a 3×2″ low pressure MOVPE reactor using Trimethyl gallium (TMG) (308µmole/min) as gallium source, and ammonia for providing N, a V/III ratio of 1305 and H₂ as carrier gas. After the Si/N treatment, a GaN nucleation layer was deposited at 525°C. The NL was then annealed at the growth temperature at 1000°C, and afterwards growth proceeded. As described and analyzed in previous papers, Reference Haffouz, Beaumont, Vénnègues and Gibart[9] the whole growth process is monitored in real time by in situ laser reflectometry, Figure 2. In short, the nucleation layer deposited at low temperature on the Si/N treated sapphire surface experienced a 2D-3D transition; this therefore increases the diffuse scattering of the laser beam and results in a continuous decrease of the reflectivity. Afterwards, the growth of GaN proceeds from the islands by lateral and vertical expansion to coalescence. These two stages correspond to the profound dip in the reflectivity and to the subsequent recovering of the reflectivity level, respectively. In the improved Si/N treatment, the duration of the supply of SiH₄ and NH₃ lasts 360 sec. During the growth of this kind of samples, the full recovery of the reflectivity takes about 4 hours. The AFM scan of Figure 1 shows the size of islands for different Si/N treatment durations. These islands are formed after deposition of amorphous SiN from SiH₄/NH₃ on the sapphire substrate, low temperature deposition of the GaN nucleation layer and then annealing at the growth temperature. As expected, the size of the 3D nuclei increases with the deposition time of SiN. However, it cannot significantly increase beyond the actual value; otherwise the recovery time would be too long to obtain layers without pits. Nethertheless, such pitted layers exhibit a dislocation density in the low 10⁷cm⁻²(<4×10⁷cm⁻²)

Figure 2. Comparison between reflectivity spectra recorded during the growth of GaN/sapphire standard epilayer and Ultra Low dislocation (ULD) GaN/sapphire. Arrows indicate where the growth starts.

Delaying the coalescence has been recently achieved using a different approach, Reference Sakai, Wang, Morishima, Naoi and Crystal Growth[19] Reference Figge, Böttcher, Einfeldt, Hommel and Growth[20]. Tuning the V/III ratio was used to control the coalescence of the island nucleation; TD densities of 1.5×10⁸cm⁻² have been reported. Reference Barna[21]

The TD densities of the thick ULD GaN layers grown onto the nucleation layers shown in Figure 1(c) were determined by AFM scan and cathodoluminescence. Both methods give 7×10⁷cm⁻².

3.1 High resolution transmission electron microscopy

Transparent specimens for Transmission Electron Microscopy (TEM) were prepared by low angle ion milling Reference Barna[21] using Ar⁺ bombardment at 10 kV. The energy of the impinging Ar⁺ ions was decreased to 3 keV after perforation and even further down to 500 eV in order to minimize ion beam damages. The specimens were investigated in a JEOL 3010 UMR microscope operating at 300 kV. High resolution images reveal that there is an amorphous region between GaN and sapphire, most likely SiN, (EDS shows some silicon). However, the amorphous material at the interface is a discontinuous layer. Indeed, the interface SiN layer is build of grains (20 nm height and 20-40 nm long). In figure 3 two small amorphous inclusions inside the GaN layer at the interface are seen. Actually the amorphous SiN layer acts as a micro mask, thereby leading to an ELO process at the micrometer scale.

Figure 3. High Resolution image of the interface in a ULD GaN/sapphire sample.

The amorphous nature of the SiN discontinuous layer can be seen clearly. Indeed, the SiN layer acts as randomly distributed (in size and in location) openings, thereby inducing a lateral overgrowth process. The GaN layer over these “masks” is hexagonal and follows the usual epitaxial relationship to sapphire (30° rotation around the 0001 axis). Sometimes the cubic sequence is also observed in grains of the NL between two SiN “inclusions” (Figure 3). However, these regions are also overgrown by hexagonal GaN. As further proof of the occurrence of an ELO mechanism, bending of threading dislocation occurs in this micro-ELO process as shown in Figure 4. More precisely, Figure 4 shows two TDs which propagate horizontally after bending.

Figure 4. Cross-sectional bright field image of the ULD GaN/sapphire sample showing the interface region. The arrow indicates the SiN coverage.

Plan view samples of the top region of GaN were also prepared for TEM. Dislocation density was determined and value of 6×10⁷cm⁻² was obtained. As AFM probes a larger area, the value of 7×10⁷cm⁻² determined by AFM measurement is in good agreement with TEM data.

3.2 Photoluminescence

Figure 5 displays the near band gap low temperature PL spectra of GaN/sapphire grown following the present technology. The PL spectrum is dominated by the so-called I₂ line, assigned to the donor-bound exciton, lying ~6 meV below the PL from the 1s-state of the free exciton with hole in the A valence band (X_A). This assignment is confirmed by the reversal of intensity between these two lines above T = 40 K, due to thermal escape of excitons from the donor traps. The positions of the X_A and X_B lines prove that the layers are under biaxial compression Reference Gil, Hamdani and Morkoc[22] thus pushing the X_Cexciton towards much higher energies. The line at 3.512 eV is then assigned to the 2s state of the A exciton. The fluctuations of the strain are certainly at the origin of the present broadening of the lines. Nevertheless, apart from the broadening, this spectrum contains many of the features that are usually observed for high-quality homoepitaxial GaN layers Reference Kornitzer, Ebner, Grehl, Thonke, Sauer, Kirchner, Schwegler, Kamp, Leszczynski, Grzegory and Porowski[23], Reference Korona[24], including the two-electron satellite of the donor-bound exciton red-shifted from the I₂ line by exactly the same amount (21.6 meV) as for the homoepitaxial layers of Ref.  Reference Korona[24]. These observations are further proof of the high quality of the layer.

Figure 5. Near band gap low temperature photoluninescence of ULD GaN/sapphire

3.3 Time-resolved PL

The recombination dynamics of excess carriers in group III-nitrides is a key issue for the optimization of blue-light emitting diodes and laser diodes devices based on nitrides. Therefore, the relationships between growth parameters of GaN and optoelectronic properties are of critical importance. The recombination dynamics of GaN have been widely studied by several groups and reviewed in ref Reference Im, Moritz, Steuber, Haerle, Scholz and Hangleiter[25]. The main features are the following:

• • The near-band edge emission spectrum appears dominated by excitonic recombination’s. In most cases, the salient contribution arises from donor-bound excitons (D⁰X or I2) while free excitons associated with the two upper valence bands (X_A and X_B) have been also identified in high quality material.

• • In most cases, fast decay times have been reported. At 10K, they range from 14 ps to 35 ps for MOVPE GaN/sapphire Reference Im, Moritz, Steuber, Haerle, Scholz and Hangleiter[25] Reference Harris, Monemar, Amano and Akasaki[26] Reference Chen, Smith, Lin, Jiang, Asif Khan and Sun[27] Reference Shan, Schmidt, Yang, Song and Goldenberg[28] Reference Hangleiter, Im, Forner, Härle and Scholz[29] Reference Okada, Yamada, Tagushi, Sasaki, Kobayashi, Tani, Nakamura and Shinomiya[30] Reference Kawakami, Peng, Narukawa, Fujita, Fujita and Nakamura[31] Reference Meyer, Volm, Wetzel, Eckey, Holst, Maxim, Heitz, Hoffmann, Broser, Mokhov, Baranov and Qiu[32] Significantly different values, ranging from 6 ps Reference Eckey, Heitz, Hoffmann, Broser, Meyer, Hiramitsu, Detchprohm, Amano and Akasaki[33] to ~170-200 ps Reference Bergman, Monemar, Amano, Akasaki, Hiramatsu, Sawaki and Detchprohm[34] and even to ~300-500ps have been measured for thick HVPE layers. Reference Buneau, Herzog, Ünlü, Goldberg and Molnar[35]

• • For ELO, due to high quality of such materials, values ranging from 400 to 700 ps were reported Reference Allègre[36] Reference Holst, Kaschner, Christen, Hiramatsu, Shibata and Sawaki[37] Reference Hess, Walraet, Taylor, Ryan, Beaumont and Gibart[38] Reference Wraback, Shen, Eiting, Carrano and Dupuis[39]

All fast decay times have been unanimously assigned to parasitic non-radiative processes. Similar results have been reported for the I2 line. Decay times of ~50-260 ps Reference Harris, Monemar, Amano and Akasaki[26] Reference Chen, Smith, Lin, Jiang, Asif Khan and Sun[27] Reference Shan, Schmidt, Yang, Song and Goldenberg[28] Reference Hangleiter, Im, Forner, Härle and Scholz[29] Reference Okada, Yamada, Tagushi, Sasaki, Kobayashi, Tani, Nakamura and Shinomiya[30] Reference Kawakami, Peng, Narukawa, Fujita, Fujita and Nakamura[31] Reference Meyer, Volm, Wetzel, Eckey, Holst, Maxim, Heitz, Hoffmann, Broser, Mokhov, Baranov and Qiu[32] Reference Eckey, Heitz, Hoffmann, Broser, Meyer, Hiramitsu, Detchprohm, Amano and Akasaki[33] Reference Bergman, Monemar, Amano, Akasaki, Hiramatsu, Sawaki and Detchprohm[34] have been reported for MOCVD and decay times of up to ~600 ps have been reported for the best MBE-grown layers. While convincing attempts have been made to correlate fast decays to various parameters such as the intensity of the low-energy “yellow luminescence” Reference Okada, Yamada, Tagushi, Sasaki, Kobayashi, Tani, Nakamura and Shinomiya[30] Reference Song, Shan, Schmidt, Yang, Fisher, Hwang, Taheri, Goldenberg, Horning, Salvador, Kim, Atkas, Botchkarev and Morkoç[40] or the presence of defects near the substrate-layer interface Reference Buneau, Herzog, Ünlü, Goldberg and Molnar[35], up to now, no significant attempt to reach a limit close to the high value reported in MBE has been done using standard MOVPE.

Figure 6 shows the PL decays recorded on an ULD layer, at T = 13 K, on a time-scale of ~5 ns. The I2 line exhibits a nonexponential decay with initial decay time of ~0.17 ns, followed by a much slower and exponential decay, with time constant of 1.06 ns. This result is totally comparable to that obtained on homoepitaxial (dislocation-free) GaN layers. Reference Im, Moritz, Steuber, Haerle, Scholz and Hangleiter[25] From the interpretation in Ref.  Reference Im, Moritz, Steuber, Haerle, Scholz and Hangleiter[25], the slow decay is the intrinsic lifetime of the donor-bound exciton, but at short delays the recombination rate is enhanced by interaction of bound excitons with free ones. Even more remarkably, the decay dynamics of the X_A line starts with a rather fast exponential decay, with ô ~ 0.08 ns, again comparable to the homoepitaxy of Ref.  Reference Korona[24] but clearly with a long-lived component. The latter could not have been be observed if the decay was merely controlled by exciton trapping at nonradiative defects, as it is usually the case for lower-quality epitaxial layers. Instead, we believe that this slow decay may correspond to re-absorption phenomena, or rather, to the exciton-polariton nature of the recombining entity. This type of property can only be observed for high-quality samples, for which the nonradiative lifetime is much larger than the radiative lifetime. The figure also shows the fast decays recorded for the X_B line and for the 2s state of X_A.

Figure 6. Decays of the free-excitons and I2 PL intensities recorded at 13K

Experiments performed using a smaller time window (1 ns) have allowed us to confirm the assignment of the lines: after the excitation laser pulse, the first line reaching its maximum intensity is the X_B line. Then, 28 ps later, the X_A line and the higher energy line simultaneously reach their maximum, thus confirming the assignment to the 2s state of the X_A exciton. Finally, due to an evident transfer mechanism commented on in previous works, Reference Im, Moritz, Steuber, Haerle, Scholz and Hangleiter[25] the I2 line reaches its maximum, with a delay of 42 ps from the X_A line. This delay rapidly drops near zero when the temperature is raised to ~ 40 K.