Effects of Pressure and NH 3 Flow on the Two-Dimensional Electron Mobility in AlGaN/GaN Heterostructures

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1 Journal of the Korean Physical Society, Vol. 42, No. 5, May 2003, pp Effects of Pressure and NH 3 Flow on the Two-Dimensional Electron Mobility in AlGaN/GaN Heterostructures Dong-Joon Kim Optical Device Group, Samsung Electro-Mechanics, Yongin Yong-Tae Moon, Min-Su Yi, Do-Young Noh and Seong-Ju Park Department of Materials Science and Engineering and Center for Photonic Materials Research, Kwangju Institute of Science and Technology, Kwangju (Received 28 November 2002) The dependence of the two-dimensional electron gas mobility in AlGaN/GaN heterostructures on the growth conditions of the AlGaN films grown by using metalorganic chemical-vapor deposition was investigated. X-ray reflectivity and X-ray diffraction measurements revealed that the growth pressure and the NH 3 flow rate during the deposition of the AlGaN layer critically influenced the interface roughness between the AlGaN and the GaN layers and the stacking order of the AlGaN layer in the growth direction. The Al 0.15Ga 0.85N/GaN heterostructure grown at a low pressure and a high NH 3 flow rate showed an abrupt interface and an enhanced stacking order in the film, resulting in an increase in the two-dimensional electron gas mobility up to 1340 cm 2 /V s at room temperature and 5200 cm 2 /V s at 130 K. PACS numbers: Bk, Gh, Kp Keywords: AlGaN/GaN heterostructures, Metalorganic chemical vapor deposition I. INTRODUCTION III-nitride semiconductor materials have been largely developed for use in optoelectronic devices such as light emitting diodes (LED) and laser diodes (LD) [1]. The AlGaN/GaN heterostructure has recently been the subject of considerable interest in the fabrication of highpower electronic devices because of its large band-gap energy, high breakdown field, and high thermal stability [2]. The first demonstration of an AlGaN/GaN heterostructure which showed a high two-dimensional electron gas (2DEG) mobility [3] stimulated enormous interest in the area of electronic devices, such as modulationdoped field-effect transistors (MODFETs) [4, 5]. This is mainly because of the large band offset in the Al- GaN/GaN system and the strong piezoelectric constants of III-nitrides, which enable the formation of a stable 2DEG at the interface between the AlGaN and the GaN layers [6]. The strong piezoelectric field, particularly in the case of AlGaN, permits the achievement of a very high 2DEG density, even in an undoped AlGaN/GaN heterostructure. This suggests that a 2DEG with high mobility can be obtained by circumventing the electron scattering, which is caused by ionized ions in the case of a modulation-doped AlGaN/GaN heterostructure. sjpark@kjist.ac.kr Most studies to date have focused on the analysis of the electrical conduction of a 2DEG as a functions of the temperature and of process parameters, such as the Al composition in the AlGaN layer [7, 8]. Meanwhile, the effects of interface roughness in the AlGaN/GaN heterostructure and defects, such as dislocations, on the mobility of the 2DEG have been examined via theoretical calculations based on a 2-dimensional, as well as a 3-dimensional, approximation [9 13]. In addition, several reports have appeared on the properties of AlGaN bulk layers grown under various conditions and on interface control in AlGaN/GaN layered structures for optical devices [14 16]. However, experimental research on the effect of the growth conditions of AlGaN on the electrical properties, such as the mobility of the 2DEG, in the AlGaN/GaN heterostructure has been minimal. In this work, the effect of the growth pressure and the NH 3 flow rate during the deposition of the AlGaN layer on the interface roughness of and the 2DEG s mobility in an AlGaN/GaN heterostructure grown by metalorganic chemical vapor deposition (MOCVD) was investigated by means of X-ray reflectivity and X-ray θ-2θ diffraction (XRD) measurements and by atomic force microscopy(afm) II. EXPERIMENT

2 -692- Journal of the Korean Physical Society, Vol. 42, No. 5, May 2003 Table 1. Results of Hall measurements, full width at half maximum of the X-ray rocking curves for the AlGaN (0002) plane, and AFM surface analysis of AlGaN/GaN heterostructures. Electron r.m.s. Sample 2DEG mobility concentration FWHM roughness (cm 2 /V s) (cm 2 (arcsec) ) (Å) A B C D DEG mobility was measured at room temperature. AFM r.m.s. roughness was measured for an area of µm 2. The Al 0.12 Ga 0.88 N/GaN heterostructure was grown by using MOCVD in a high-speed rotating disk reactor (Emcore D-125 T M ). Hydrogen was used as the carrier gas, as well as the ambient atmosphere, for the growth of all AlGaN/GaN samples. Trimethylgallium (TMGa), trimethylaluminium (TMAl), and ammonia were used as source gases. Prior to the deposition of a lowtemperature nucleation layer, a c-plane sapphire substrate was pre-heated in an atmosphere of H 2 gas. The grown structure was composed of a 30-nm-thick layer of undoped AlGaN and a 1.2-µm-thick nominally undoped GaN buffer layer with a 30-nm-thick low-temperature GaN nucleation layer on the c-plane of the sapphire substrate. The detailed growth procedures for the undoped GaN have been described elsewhere [17]. In order to investigate the effects of the growth conditions for AlGaN on the mobility of the 2DEG in Al- GaN/GaN heterostructures, we grew the AlGaN layers at 1020 C at growth pressures of 200 Torr and 30 Torr, and we varied the ammonia flow rate at each growth pressure. Since the Al composition of AlGaN can be greatly altered due to the formation of adducts (TMAl + ammonia) in the gas phase during the growth of films, the flows of the TMGa and the TMAl source gases were carefully controlled so that all AlGaN layers had the same Al composition of 12 %, irrespective of the growth conditions. The electron concentrations of the AlGaN layers were about cm 3. The surface and the interface roughness of the Al 0.12 Ga 0.88 N/GaN heterostructure were characterized by using AFM and X-ray reflectivity measurements. An x-ray θ-2θ scan was also performed in order to examine the interfacial structural properties of the AlGaN/GaN heterostructure. III. RESULTS AND DISCUSSION Table 1 shows the Hall measurements at room temperature, the full widths at half maximum (FWHM) of the X-ray rocking curves, and the AFM root-mean-square Table 2. Growth conditions for the AlGaN layers. Sample Growth pressure (Torr) NH 3 flow rate (sccm) A B C D (r.m.s.) roughnesses of the AlGaN/GaN heterostructures grown under different conditions. Typical growth parameters used to grow the AlGaN layers are summarized in Table 2. The two-dimensional electron concentration was high in the case of the AlGaN/GaN heterostructure grown at a high pressure with a high NH 3 flow rate (sample D). Increases in the growth pressure and the NH 3 flow rate are known to increase adduct formation between TMAl and NH 3 [18], which reduces the effective N source concentration in the gas phase and may cause an increase in the electron concentration by forming nitrogen vacancies in the film. Since the Al compositions of these samples had constant values of 12 %, changes in the two-dimensional electron concentration due to differences in the piezoelectric field are not expected at the AlGaN/GaN interface. Therefore, this increase in the carrier concentration in AlGaN layers can be attributed to an enhanced gaseous adduct formation between TMAl and NH 3. However, the electron concentration of the AlGaN/GaN heterostructure grown at low pressure was reduced by increasing the NH 3 flow rate (samples A and B). This result can be attributed to a suppression of the formation of nitrogen vacancies, which was also observed in the case of GaN growth using a high NH 3 flow rate [19]. In the case of low-pressure growth (samples A and B), we conclude that increasing the NH 3 flow is very effective in reducing the formation of nitrogen vacancies because adduct formation is suppressed at a low pressure, which can also improve the crystalline quality of the AlGaN layer. The 2DEG s mobility in the AlGaN/GaN heterostructure grown at low pressure was much higher than that in the structure grown at high pressure, as shown in Table 1. To further investigate this result, we measured X- ray reflectivity curves for the AlGaN/GaN heterostructures because X-ray reflectivity measurements provide information on the surface and the interface roughness [20]. As shown in Fig. 1, the X-ray reflectivity intensity slowly decreased for AlGaN/GaN heterostructures grown at low pressures (curves (a) and (b)) compared to those grown at high pressures (curves (c) and (d)). This suggests that the surface and the interface roughnesses of the AlGaN/GaN heterostructures grown at low pressures are more abrupt than those of the heterostructures grown at high pressures, which is consistent with the r.m.s. surface roughness measured by AFM, as shown in Table 1. Zhang et al. suggested that the interface roughness, in the case of an AlGaN/GaN heterostruc-

3 Effects of Pressure and NH 3 Flow on the Two-Dimensional Dong-Joon Kim et al Fig. 2. AFM surface images (5 5 µm) of AlGaN/GaN heterostructures grown under NH 3 conditions of (a) 6500 sccm, 30 Torr and (b) 6500 sccm, 200 Torr. Fig. 1. X-ray reflectivity curves for AlGaN/GaN heterostructures grown under various NH 3 conditions: (a) 3000 sccm, 30 Torr, (b) 6500 sccm, 30 Torr, (c) 3000 sccm, 200 Torr, and (d) 6500 sccm, 200 Torr. ture, has a strong effect on the 2DEG s mobility based on their charge control and mobility model [12]. Therefore, the increase in the mobility of the 2DEG in the Al- GaN/GaN heterostructures grown at low pressures can be attributed to an enhanced interface smoothness. For an AlGaN/GaN heterostructure grown at a high pressure, however, the mobility of the 2DEG is severely decreased, presumably due to a large interface roughness. Figure 2 shows the AFM surface images of AlGaN/GaN heterostructures grown at both low and high pressures. As shown in Fig. 2(a), the AlGaN/GaN heterostructure grown at low pressure had a step-like flat surface, which is indicative of enhanced lateral growth. However, a very rough surface structure was evident for the sample grown at high pressure, as shown in Fig. 2(b). In addition, a significant number of open cores, the origin of which was screw dislocations [21], were also formed in the AlGaN/GaN heterostructures grown at high pressures while relatively few cores were found in the Al- GaN/GaN heterostructures grown at low pressures. In the case of the AlGaN/GaN heterostructures grown at high pressures (samples C and D), the 2DEG s mobility was further decreased by the presence of numerous open cores since charged dislocation lines, which represent the origin of the open cores, can act as mobility-limiting scattering centers [13]. In the X-ray reflectivity analysis, no discernible difference was detected between AlGaN/GaN heterostructures grown using low and high NH 3 flow rates (Figs. 1(a) and (b)) at low pressures, even though the mobilities of the 2DEGs in the two samples were drastically different, as shown in Table 1. The FWHM values of X-ray rocking curves for the (0002) plane of the AlGaN layers were in consistent with the values of the mobilities Fig. 3. X-ray θ-2θ measurements for AlGaN/GaN heterostructures grown under NH 3 conditions of (a) 3000 sccm, 30 Torr and (b) 6500 sccm, 30 Torr. of the 2DEGs for samples A and B, as shown in Table 1. However, the difference between the two FWHM values was too small to account for the large difference in mobilities of the 2DEGs. In order to examine the difference in the mobilities of the 2DEGs of AlGaN/GaN heterostructures grown at different NH 3 flow rates at low pressures, we conducted X-ray θ-2θ diffraction measurements. Figure 3 illustrates the X-ray θ-2θ diffraction curves for Al- GaN/GaN heterostructures grown at two different NH 3 flow rates at low pressure. This figure shows distinct Bragg peaks for the GaN (0002) layer and shoulder peaks for the AlGaN layers on the right side of the GaN (0002) peaks. The peak positions of the AlGaN layers were determined by curve fitting, and the Al compositions, as calculated from the peak positions, were found to be in the range of %. It is generally known that the

4 -694- Journal of the Korean Physical Society, Vol. 42, No. 5, May 2003 oscillation peaks on both sides of the GaN (0002) peaks provide information on the lattice coherence of the Al- GaN layers and the interface smoothness between the AlGaN and the GaN layers. The number and the intensites of these oscillation peaks represent a measure of the interface roughness and the coherence lattice length of the AlGaN layer on the c-axis [22,23]. A large coherence lattice length indicates that the stacking order of an epitaxial film is improved, i.e., that the domains are well aligned to the surface normal direction without defects. For the AlGaN/GaN heterostructures grown at low pressures, the sample grown at a high NH 3 flow rate exhibited more discernible, strong oscillations [Fig. 3(b)] than the other sample, which was grown at a low NH 3 flow rate [Fig. 3(a)]. However, Figs. 1(a) and (b) revealed that in the case of low-pressure growth the interface roughness of the AlGaN/GaN heterostructure grown at a low NH 3 flow rate was comparable or slightly smoother than that of the sample grown at a high NH 3 flow rate. From these results, we conclude that the lattice coherence of AlGaN in the AlGaN/GaN heterostructure also plays an important role in the 2DEG s mobility. Thus, the increased mobility of the 2DEG in an AlGaN/GaN heterostructure grown at a low pressure with a high NH 3 flow rate can be attributed to an enhanced structural continuity of the AlGaN layer, probably due to the suppression of defects, such as nitrogen vacancies and threading dislocations, with increasing NH 3 flow rate, which is similar to the case for GaN growth [19]. Using the same growth conditions for sample B, we were able to grow an Al 0.15 Ga 0.85 N/GaN heterostructure which showed very high mobilities of 1340 and 5200 cm 2 /V s at room temperature and 130 K, respectively, by increasing the molar flow rate of TMAl. In this sample, the piezoelectric field appeared to increase with increasing Al content of the AlGaN/GaN heterostructure from 12 to 15 %, resulting in an increase in the 2DEG s density from up to cm 2. This increased electron density led to an increase in the electron mobility from 1020 to 1340 cm 2 /V s, presumably by enhancing the screening effect of ionized scattering centers. IV. CONCLUSIONS The effect of the growth pressure and the NH 3 flow rate during the growth of an AlGaN layer on the mobility of the 2DEG in an AlGaN/GaN heterostructure was investigated by means of X-ray reflectivity and θ-2θ diffraction measurements and by AFM surface analysis. 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