Effects of thermal and hydrogen treatment on indium segregation in InGaNÕGaN multiple quantum wells

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 11 1 JUNE 2001 Effects of thermal and hydrogen treatment on indium segregation in InGaNÕGaN multiple quantum wells Yong-Tae Moon, Dong-Joon Kim, Keun-Man Song, Chel-Jong Choi, Sang-Heon Han, Tae-Yeon Seong, and Seong-Ju Park a) Department of Materials Science and Engineering and Center for Optoelectronic Materials Research, Kwangju Institute of Science and Technology, Kwangju , Korea Received 15 November 2000; accepted for publication 13 March 2001 The effects of indium segregation and hydrogen on the optical and structural properties of InGaN/GaN multiple quantum wells, grown by metalorganic chemical vapor deposition were investigated. Photoluminescence and high-resolution transmission electron microscopy analysis showed that two types of indium-rich regions can be formed in the InGaN well layers. Self-assembled quantum dot-like indium-rich regions were found in the well layer grown at a normal growth temperature. These regions behaved as luminescent centers, showing a maximum indium content at the center of indium-rich region. However, randomly-distributed indium-segregated regions, which formed near the upper interface of the InGaN well layers during the subsequent high-temperature annealing process led to the degradation of the optical properties by generating defects such as misfit dislocations. The use of hydrogen during the growth interruption was found to be very effective in suppressing the formation of indium-segregated regions in the InGaN well layers American Institute of Physics. DOI: / I. INTRODUCTION InGaN alloys have been used as active layer materials in III-nitride-based light-emitting diodes LEDs and laser diodes LDs. 1 The growth and characterization of InGaN films 2 5 and the phenomenon of InGaN phase separation 6 10 are subjects of considerable interest, since it has been reported that InN-rich regions play a significant role in the mechanism of emission of nitride-based blue or green LEDs and LDs However, the growth and properties of InGaN ternary alloys represent an area that is not well understood. In particular, the growth of epitaxial thick InGaN films with high indium contents is very difficult, since the structural and optical properties of InGaN thick films undergo degradation with increasing film thickness The degradation of the structural and optical properties of an InGaN thick film can be related to the accumulation of indium atoms on the growing surface 14 and the generation of misfit dislocation in the film with a thickness over a critical film thickness. 15 Recently, evidence for strong indium surface segregation has been reported for the cases of InGaN/GaN heterostructures 17 and InGaN/GaN multiple quantum wells MQWs 18 grown by molecular beam epitaxy. Compositional fluctuationinduced indium-rich regions has also been reported in the InGaN wells of MQWs with high indium contents. 13,19 Ponce et al. reported that the microstructure of In x Ga 1 x NQWs with a well thickness of 1.5 nm x 0.28 and 0.52 are highly inhomogeneous with respect to indium composition and strain distribution. 19 The inhomogeneities in composition and strain distribution can degrade the thermal stability of InGaN QWs. Recently, Romano et al. reported on the InGaN/GaN MQWs were grown in a rotating disk metalorganic chemical vapor deposition MOCVD system Ema Electronic-mail: sjpark@kjist.ac.kr phase stability and thermal degradation of 4.5 nm In 0.33 Ga 0.67 N/10 nm GaN MQWs, prepared by annealing at 975 C for 4 h. 20 Although highly homogeneous InGaN/GaN MQWs with low indium contents can be grown by optimizing the growth parameters e.g., by the increase of growth temperature, V/III ratio, 3 the decrease of well thickness, 21 and by the use of H 2 carrier gas, 22 etc, itremains difficult to obtain completely homogeneous InGaN wells with high indium contents, since films with indium contents of over 22% are thermodynamically unstable at normal growth temperatures. 7 Thus, in order to obtain homogeneous InGaN wells with high indium contents, a study of the dependence of the formation of compositional fluctuationinduced indium-rich regions on growth parameters and the effect of indium-segregated regions on thermal stability, as well as optical properties is necessary. In this work, we report on an investigation of the effect of indium segregation in InGaN QWs on the optical and structural properties of InGaN/GaN MQWs by analyzing the profiles of indium compositions in the quantum wells using high-resolution transmission electron microscopy HRTEM and energy dispersive x-ray spectroscopy EDS. In addition, the effects of the growth interruption before the growth of GaN barrier on an InGaN well and the use of H 2 during the growth interruption on the optical properties were examined. We found that the use of H 2 is a very effective method for suppressing the segregation of indium as well as for preventing the optical degradation of InGaN/GaN MQWs. II. EXPERIMENT /2001/89(11)/6514/5/$ American Institute of Physics

2 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Moon et al core D-125 system at a low pressure of 200 Torr, using a c-plane sapphire ( -Al 2 O 3 as a substrate. Trimethylgallium TMG, trimethylindium TMI, and ammonia (NH 3 ) were used as sources of Ga, In, and N, respectively. The substrate was preheated in a stream of H 2 at 1030 C for 3 min, on which a 30 nm thick GaN buffer layer was grown at 560 C. This was followed by the growth of a 1.5 m thick Si-doped GaN epilayer at 1020 C and then five periods of InGaN/GaN MQWs at 730 C. When the growth temperature was decreased from 1020 C to 730 C for the growth of InGaN layer H 2 carrier gas was replaced with N 2 in the reactor. The flow rate of the TMG, TMI, NH 3, and N 2 used in the growth of the InGaN well were 10.8 mol/min, 11.1 mol/min, 406 mmol/min; and 196 mmol/min, respectively. To investigate the segregation of the surface indium, the growth of InGaN/GaN MQWs was interrupted for 0.5 min, at the every stage of interface formation between InGaN and GaN. NH 3 and N 2 were continuously supplied during the growth interruption. When we investigated the effect of H 2 on the indium segregation in the MQWs, a H 2 flux in addition to NH 3 and N 2 was introduced during the growth interruption. To investigate the thermal effect on In segregation in the MQW, a 0.25 m thick Mg-doped p-type GaN layer was grown at 1010 C for 6.5 min on the 6 nm In 0.25 Ga 0.75 N/8 nm GaN MQWs, which is basically identical to a blue LED structure. The indium content in the QW was determined to be 25% by x-ray diffraction measurements usingacuk x-ray source. Photoluminescence PL measurements were carried out at room temperature RT using a He Cd laser operating at 325 nm. A cross sectional TEM examination was performed on the InGaN/GaN MQWs using a field emission JEM 2010F instrument, operating at 200 kv. The distribution of indium in the QW was measured by EDS OXFORD, ISIS with an electron beam probe which was 0.5 nm in diameter. The EDS point analysis was performed on the cross section of QWs in the MQWs using a 0.5 nm electron beam probe with a current density of approximately 7 pa/cm 2. The effective diameter of sampling area inside the specimen was estimated to be less than 0.7 nm. III. RESULTS AND DISCUSSION Figure 1 a and b show, respectively, the RT PL spectra of 5 (6 nm InGaN/8 nm GaN MQWs grown with and without interruption sequences at the InGaN/GaN interfaces in the MQWs. For the MQWs grown with an interruption time at the interface formation, a strong InGaN PL peak was observed at 484 nm as shown in Fig. 1 a. For the MQWs grown without an interruption process, however, a broad PL spectrum with a dominant peak at the low energy side is evident Fig. 1 b. Considering the fact that indium atoms can be segregated on the surface during growth, 23,24 the lower-energy peak can be assigned to the indium-rich regions formed near the upper interface of the InGaN. These results indicate that the formation of surface segregationinduced indium-rich regions can be suppressed by the FIG. 1. The RT PL of InGaN/GaN MQWs grown with a t i 0.5 min, d w 6nm, b t i 0 min, d w 6 nm, and c t i 0 min, d w 12 nm. t i and d w indicate interruption time and well thickness, respectively. growth interruption, since the surface segregated-indium atoms are easily evaporated during the growth interruption at the surfaces. The formation of surface segregation-induced indiumrich regions can be influenced by the changes in the well layer thickness. Thus, to investigate the effect of indium surface segregation due to the increase in the well layer thickness on the optical properties of the MQWs, the thickness of a 6 nm InGaN well was increased to 12 nm and a RT PL spectrum was obtained. As shown in Fig. 1 c, the increase in well thickness results in a significant line broadening and a large redshift of the PL spectrum, compared to the PL spectrum shown in Fig. 1 b. These changes in the PL spectrum can be related to an increase in inhomogeneities in the indium composition and misfit strain distribution. 21 When the thickness of the InGaN well increases beyond a critical thickness, the generation of misfit dislocations at the interface of InGaN/GaN is favored. However, the plastic relaxation can not be complete and homogeneous in all regions of the InGaN well, resulting in an inhomogeneous strain distribution within the well layer. 19 The inhomogeneous strain distribution could, in turn, enhance the local diffusion of indium atoms and thereby spinodally increase the compositional fluctuation. Hence, considering the effects of quantum confinement and piezoelectric field 25 in InGaN/GaN MQWs which are inhomogeneous in composition and strain distribution, the inhomogeneous strain-induced compositional fluctuation, in addition to the indium surface segregation, could be partially responsible for the significant changes of PL spectrum shown in Fig. 1 c. To further investigate the possible origin for this broad PL spectrum shown in Fig. 1 c, HRTEM and EDS analyses were performed on the MQW with a well thickness of 12 nm. Figures 2 a and 2 c show a cross sectional TEM image and typical EDS results for the InGaN/GaN MQWs. Figure 2 c clearly shows that the indium content in the InGaN well layer increases along the growth direction, resulting in a nonuniform indium distribution in the well layer. Kaspi and Evans also reported that similar indium-segregation behavior leads to a graded indium distribution in the InGaAs on GaAs. 26 It is noteworthy that the upper interface is planar and abrupt in composition, while the lower interface is slightly rough and diffuse. Figure 2 b reveals a HRTEM

3 6516 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Moon et al. FIG. 3. a The RT PL spectra of a blue LED structure with a 0.25 m thick p-gan layer and InGaN 6 nm /GaN 8 nm MQWs without the p-gan layer. The MQWs were grown with an interruption time of 0.5 min. b The effect of H 2 introduction on the RT PL of blue LED structure. The H 2 was introduced during the interruption time of 0.5 min. The PL intensity increases with the amount of H 2 flux. FIG. 2. Cross sectional images and EDS quantization results of InGaN 12 nm /GaN 8 nm MQWs. a The upper interfaces of the InGaN are planar, while the lower interfaces are slightly rough. b The well layer contains quasiperiodically-aligned quantum dot-like regions. Indium contents of the marked areas of a and b are plotted in c and d, respectively. image obtained from a different region of the same MQW sample. This image shows that the well layer contains quasiperiodically-aligned quantum dot-like regions. 13 The indium composition of the selected area in Fig. 2 b was characterized by EDS. Figure 2 d shows that the indium content is at a maximum at the center of the dot-like region and is decreased toward the edges. These quantum dot-like In-rich regions are considered to be responsible for the lower-energy peak observed in the PL spectrum in Fig. 1 c. Therefore, it is believed that the nonuniform indium distribution in InGaN and the formation of the self-assembled quantum dot-like InN-rich regions are responsible for the significant broadening of the PL spectrum shown in Fig. 1 c. When an InGaN/GaN MQW blue LED is fabricated, a p-type GaN contact layer should be grown at a high temperature of about 1010 C on top of the InGaN/GaN MQWs which were grown at a low temperature of about 730 C. Thus, to investigate the effect of the subsequent hightemperature growth process on the optical and microstructural properties of MQWs, a blue LED structure, which consists of a 0.25 m thick p GaN, five periods of 6 nm InGaN/ 8 nm GaN MQWs, and a 1.5 m thick n-gan layer, was grown using an interruption time of 0.5 min for each interface in the MQWs. Figure 3 a shows the RT PL spectra of a LED structure and MQWs. It is noteworthy that the PL intensity of the LED structure is much weaker than that of the MQWs. The decrease in the PL intensity of the LED can be related to 1 the intrinsic absorption of incident laser light in the p-gan, and 2 the thermal degradation of MQWs due to the subsequent high-temperature growth of a p-gan electrode layer on the MQWs. The p-gan electrode layer can absorb the 325 nm incident light 27 and the blue light emitted from the MQWs. In the following experiment, it was found that the decrease in PL intensity of the LED is mostly attributed to the thermal degradation of MQWs. To understand

4 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Moon et al how the thermal degradation of MQWs affects the PL emission, we investigated the effect of H 2 on the removal of excess indium atoms segregated to the upper interfaces of InGaN wells during the growth interruption time of 0.5 min. Figure 3 b shows the RT PL spectra of a blue LED with a p-gan layer grown on the MQW as a function of H 2 flow rate. As shown in Fig. 3 b, the PL intensity drastically increased with increasing the H 2 flow rate during the growth interruption. It should be noted that the PL spectrum corresponding to the 0 sccm H 2 flux in Fig. 3 b is the same as that corresponding to the LED structure in Fig. 3 a. Considering that all the LED structures corresponding to the spectra in Fig. 3 b have the p-gan layers with a same thickness of 0.25 m on the MQWs, the remarkable increase of PL intensity by the use of H 2 in Fig. 3 b indicates that the decrease in PL intensity of the LED structure in Fig. 3 a is mainly attributed to the thermal degradation of interfaces in the MQWs. Considering that the upper interfaces of InGaN wells can be more fluctuated in indium composition due to indium surface segregation than the lower interfaces, it is believed that the formation of indium-rich precipitates can be mainly initiated near the upper interfaces. Thus, in order to enhance the thermal stability of InGaN wells, indium-rich regions near the upper interfaces should be eliminated. This work shows that the indium-rich regions near the upper interfaces can be mostly eliminated by a proper use of growth interruption and H 2 in the growth of MQWs. However, if the interruption time is further increased, it may cause surface roughening 28 and the incorporation of additional impurities from the growth atmosphere. 29 To further understand the effect of H 2 on the thermal degradation and the indium segregation at the interface, a TEM examination was performed on the blue LEDs which had been grown using an interruption process with and without H 2 gas. Figures 4 a and 4 b show the cross sectional TEM images of the MQW in the LED structures grown with and without H 2 during growth interruption, respectively. For the LED structure grown in the presence of 400 sccm of H 2 Fig. 4 a, the TEM image reveals well-defined QW structures. However, for the case of the LED structure grown without H 2, dark and nonuniform regions are present in the TEM image of the InGaN layers as shown in Fig. 4 b. One of the dark regions was quantitatively analyzed by EDS as shown in Fig. 4 c. Figure 4 c shows that the dark region contains an average indium content of 34% but the area outside the dark region is depleted of indium. The indium distribution shown in Fig. 4 c indicates that the indium atoms are segregated in the film, forming the dark regions during the high temperature growth of the p-type GaN capping layer. The segregated indium on the upper interface of In- GaN as shown in Figs. 2 a and 2 c may initiate the formation of the dark regions in the well layer during the subsequent high temperature growth of p-type GaN. The indiumsegregated dark regions were observed to be randomly distributed along the InGaN well layers. The morphology and distribution of indium composition in the dark regions were very different from those of the self-assembled quantum dot-like regions, which are shown in Fig. 2 b. Furthermore, a detailed TEM examination of MQWs revealed that FIG. 4. Cross sectional images of InGaN 6 nm /GaN 8 nm MQWs grown a with 400 sccm H 2,and b without H 2 during the interruption time of 0.5 min. The MQWs were capped by a 0.25 m-thick p-type GaN layer. The areas marked in b were analyzed by EDS and the quantization results are plotted in c. the misfit dislocations were generated at the interfaces between the well and barrier layers near the dark regions. Most of the misfit dislocations lying along the direction were not uniformly distributed at the interfaces. In a region of the MQWs, the average spacing between dislocations was about 6.7 nm. Misfit dislocations are known to be responsible for the optical degradation of InGaAs quantum dots embedded in GaAs matrix by forming deep level defects. 30 Therefore, it is believed that the misfit dislocations could be partially responsible for the nonradiativity of the indiumsegregated regions, resulting in the decreased intensity of PL from the LED structure with p-gan on MQWs as shown in Fig. 3 a. The small excess-indium clusters 31 and/or point defects which can be segregated near the boundary area of the dark regions or misfit dislocations 30 might be also responsible for the nonradiativity of the precipitates. As a result, the use of H 2 during the growth interruption time was found to efficiently remove the surface-segregated indiums on the upper interface. This process appears to prevent the formation of indium-segregated dark regions and dislocations in the well layer, and thereby the PL intensity can be drastically increased with an increase in the H 2 flow rate during the growth interruption as shown in Fig. 3 b. The thermally degraded MQWs showed a dark surface over the whole 1 in. 1 in. square sapphire substrate by visual inspection. A number of MQWs samples were grown under the same growth conditions and the dark surface was

5 6518 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Moon et al. consistently observed. The possible origins of the formation of dark regions can be related to the growth conditions of MQWs. McCluskey et al. reported that when 2 nm In 0.27 Ga 0.73 N/4 nm GaN MQWs were annealed for 4 min at 1100 C and for 40 h at 950 C, indium-rich phases could be observed. 10,32 In our study, a p-gan layer was grown on 6 nm In 0.25 Ga 0.75 N/8 nm GaNMQWs for 6.5 min at 1010 C. Thus, the formation of dark regions in our MQWs samples, which have a high indium content, can be partly attributed to the relatively thick wells. As mentioned earlier, the increase in well thickness can increase the inhomogeneities in indium composition 21 and strain distribution 33 in InGaN QWs, resulting in the degradation of thermal stability. To confirm the thickness dependence of thermal stability, the same LED structures were grown with a reduced well thickness of 3 nm. This structure was highly transparent and no dark surface was observed over the 1 in. 1 in. square sapphire substrate. This result indicates that the formation of dark regions are related to the extent of compositional fluctuation in the In- GaN well layers. This compositional fluctuation of indium in the InGaN well layer can be increased by an increase in the indium content as well as the well thickness in the MQWs. When the InGaN was grown at a higher temperature, or grown with a higher V/III ratio, the dark surface was also not observed. Under these growth conditions, the indium contents in InGaN were decreased significantly, implying that the thermal degradation of MQWs with high indium contents is more profound than that of MQWs with low indium contents due to an increase in compositional fluctuation. IV. CONCLUSION The effect of indium segregation on the optical and structural properties of InGaN/GaN MQWs grown by MOCVD was investigated by means of PL, HRTEM, and EDS. Two types of indium-rich regions were observed in the MQW. The self-assembled quantum dot-like InN-rich regions in InGaN well layers could act as luminescent centers. However, the indium-segregated regions, which were formed in the InGaN well layer during the subsequent hightemperature growth process used for LED fabrication were found to act as nonradiative recombination centers and these could be effectively removed by introducing H 2 gas during the growth interruption period in the growth of MQW. ACKNOWLEDGMENTS The authors thank S. H. Oh at Pohang University of Science and Technology for the HRTEM and EDS measurements. This work was supported by the Korea Research Foundation through Grant No E00073 and by the Brain Korea 21 project. 1 S. Nakamura and G. Fasol, The Blue Laser Diode Springer, Berlin T. Matsuoka, N. Yoshimoto, T. Sasaki, and A. Katsui, J. Electron. Mater. 21, S. Keller, B. P. Keller, D. Kapolnek, U. K. Mishra, S. P. DenBaars, I. K. Shmagin, R. M. Kolbas, and S. 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