Study of InGaN multiple quantum dots by metal organic chemical vapor deposition

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Study of InGaN Multiple Quantum Dots by Metal Organic Chemical Vapor Deposition

Te-Chung WANG1;2, Hao-Chung KUO1, Tien-Chang LU1, Ching-En TSAI2, Min-Ying TSAI1, Jung-Tsung HSU2 and Jer-Ren YANG3

1_{Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China}

2_{Opto-Electronics and System Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, Republic of China} 3_{Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, Republic of China}

(Received September 16, 2005; accepted December 22, 2005; published online April 25, 2006)

We reports a study of InGaN multiple quantum dot layers. Using the in-situ SiNxtreatment process, InGaN multiple quantum

dot layers were successfully developed. The InGaN multiple quantum dot layers were constructed with SiNxdot mask layers,

InGaN dot layers, and GaN cap layers on a 2-mm-thick GaN underlying layer on a sapphire substrate. Optical properties including room temperature photoluminescence (PL), temperature dependent PL, and low power power-dependent PL were examined and discussed. The structure was also analyzed by transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) line scan. [DOI: 10.1143/JJAP.45.3560]

KEYWORDS: InGaN QDs, MOCVD

1. Introduction

GaN and related materials are currently the subject of intense research due to their applications in laser diodes (LDs) and light-emitting diodes (LEDs) that operate between the ultraviolet and the visible regions.1) InGaN/GaN quantum well (QW) structures have successfully been used as active layers in LEDs and LDs.1)_{However, the threshold} current density is high for LDs with InGaN QW structure. Having quantum dots (QDs) instead of QWs as the active layer is expected to improve the performance of LDs. LDs with QD structures in the active layer have been theoret-ically predicted to have superior characteristics, including lower threshold currents and narrow spectra.2) _Moreover, because of the localization of carriers trapped at disloca-tions, QD structures have been expected to increase the efficiency of the luminescence of LDs and LEDs.3) To ensure suitability for QD applications, QD layers with high spatial density and of uniform size must be grown.4)Several approaches have been investigated for fabricating InGaN QDs, including the Stranski–Krastanow growth mode5,6)and growth using an anti-surfactant.7,8) _{The deposition of a} silicon anti-surfactant or a SiNx nano-mask alters the morphology of the AlGaN films from that of step flow to that of a three dimensional island, facilitating the formation of GaN7)_{QDs and InGaN QDs}8)_{on the AlGaN.}

On the other hand, the properties of the InGaN-based red emission device are dominated by both the band-filling effect and the screening effect of the quantum confined stark effect (QCSE) due to the high indium composition and the large thickness of the InGaN well layer. Therefore, InGaN multiple quantum well light emitting devices are very difficult to achieve. However, the InGaN quantum dot devices possess the potential to overcome limitations to make the efficiency higher and the wavelength longer. Using the in-situ SiNx treatment process that we have reported,9) red emission InGaN multiple quantum dots with ultra-high density have been successfully developed. In this report, we discuss the optical characteristics of InGaN red emission multiple quantum dots and their structure determined by TEM and EDX line scan.

2. Experimental

Despite many phenomenon of InGaN localization and segregation that have been reported,10,11)the efficiency and intensity of high indium-content devices at room temper-ature are still quite low. It is very difficult to make a high efficiency red emission device. According to previous results,9) _{InGaN QD density of about 2 10}11_cm2 _has been achieved. We attempted to grow a five period, multi-stack structure to enhance the intensity of PL and decrease the deposition temperature of InGaN QDs to make the wavelength longer. The InGaN multiple quantum dot layers were constructed with SiNx dot mask layers, InGaN dot layers, and GaN cap layers as shown in Fig. 1. At first, a 30-nm-thick low-temperature GaN nucleation layer was grown on the c-plane of a sapphire substrate at 550_{C by metal} organic chemical vapor deposition (MOCVD). Then, the reactor temperature was increased to 1000_{C to grow a} 2-mm-thick Si-doped GaN underlying layer, providing a step flow grown surface. An island-like random-formation SiNx in-situ nano-mask was deposited on the n-type GaN under-lying layer. During the 420 s treatment of the SiNx nano-mask, the flow rates of NH3 and diluted Si2H6 were 5 slm and 50 sccm, respectively. The temperature was then ramped down to 750_{C to grow 2 nm In}

xGa1xN layers, and ﬁnally 3-nm-thick GaN was deposited as the cap layer. The temperature was increased 1000_{C to repeat the process}

GaN

Sapphire

Insitu-SiNx mask

InGaN QDs

GaN cap layer

Fig. 1. Scheme for InGaN multiple quantum dot layers. _{E-mail address: [email protected]}

Japanese Journal of Applied Physics Vol. 45, No. 4B, 2006, pp. 3560–3563 #2006 The Japan Society of Applied Physics

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and produce a ﬁve period multi-stack structure. During the growth of the InGaN layers, the vapor phase ratio TMIn=ðTMIn þ TEGaÞ was ﬁxed higher than previous work at 0.40. We investigated the optical properties of the InGaN multiple quantum dot layers by room-temperature PL using Accent rpm 2000, which contains a He–Cd 5 mW laser. Then we analyzed the InGaN MQD behavior by temper-ature-dependent PL and low-power power-dependent PL. Finally, we analyzed the structure by high resolution TEM and EDX line scan.

3. Results and Discussion

The room temperature PL of InGaN multiple quantum dot layers, shown in Fig. 2, was measured by Accent rpm 2000; the PL spectrum reveals a broadened emission from 480 to 800 nm. The additional peaks between the blue emission and red emission are the results of Fabry–Perot interference fringes. By Gaussian peak ﬁtting, the spectrum was divided into seven individual wavelengths, and the 2to 7indicated the same interference relationship. Consequently, the spec-trum could be divided into two parts: emissions 1 at 531 nm, and main at 618 nm. The two emissions may be excited by diﬀerent energy states. To further verify the origin of the emission, the bandgap of InGaN MQDs was calculated based on the equation provide by Wu et al.:12)

EgðInxGa1xNÞ ¼ Eg(InN)xþEgðGaNÞ

ð1 xÞ 1:42xð1 xÞ ð1Þ where x is the indium concentration, and Eg(InN) and Eg(GaN) represent the bandgap energies of InN (0.77 eV) and GaN (3.42 eV), respectively. Following the calculation, the indium compositions of the two energy states are 0.30 and 0.40. However, the high resolution X-ray diﬀraction measurement of the MQD structure is too broad to analyze, and this yields the same result as reported,11) _{for phase} separation in InGaN/GaN multiple quantum wells. The high temperature of the SiNx treatment step may lead the formation of diﬀerent InGaN phases and the lower temper-ature in-situ SiNx nano-mask technique should be studied further in the future. The red emission at room temperature

is quite strong; it is evidence that ultra-high density InGaN multiple quantum dots have the potential to be used in red emitting devices.

Figure 3 shows the temperature-dependent PL of InGaN MQDs. Three different PL peaks are present, and all intensities increase as temperature decreases. By connecting the main peak positions, the trends of the three peaks shift with the temperature as shown in Fig. 4. The left peak is obviously blue shifted from 540 to 525 nm as the temper-ature decreases like a general uniform quantum well. In comparison with the structure, this could be the wetting layer of quantum dots generated by InGaN phase separation. The middle peak is stable around 567 nm; this may come from the nature of defects in GaN referred to as yellow luminescence. Because the power density of the exciting laser is higher than rpm 2000, deeper natural defects were excited. The right peak above 600 nm is blue shifted first and red shifted as the temperature decrease. This luminescence exhibits an ‘‘S-shaped’’ emission shift in position with temperature. The redshift–blueshift phenomenon was first explained by Cho13)_{in terms of inhomogeneity and carrier} localization in InGaN. Consequently, we may conclude the in-situ SiNxtreatment of InGaN multiple quantum dot layers 350 400 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 λ5 λ7 λ4 λ3 λ2 λ6 λ1

λ

main

=618nm

λ1

=531nm

Intensity (arb. units.) Wavelength (nm) Fig. 2. Room temperature PL of InGaN MQDs.

450 500 550 600 650 1000 10000 100000 20K 30K 50K 70K 90K 110K 130K 150K 170K 190K 210K 230K 250K 270K 300K Intensity (arb. units) Wavelength (nm)

Fig. 3. Temperature-dependent PL of InGaN MQDs.

0 50 100 150 200 250 300 520 530 540 550 560 570 580 590 600 610 W a velength (nm) Temperature (K) left peak middle peak right peak

Fig. 4. Trends of main peak shifts in the temperature-dependent PL of InGaN MQDs.

Jpn. J. Appl. Phys., Vol. 45, No. 4B (2006) T.-C. WANGet al.

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causes the same behavior of carrier localization, and the wavelength could be pushed toward a red emission. At the same time, ultra-high density of InGaN QDs of approx-imately 2 1011_cm2 _{also could be achieved by this} procedure. As Fig. 5 shows, low-power power-dependent PL spectra of InGaN MQDs at 20 K have three emission peaks as well, and all intensities increase as power increases. The left peak is blue shifted from 530 to 525 nm as power increases, and the behavior is still like that of a wetting layer of quantum dot; the middle peak demonstrates yellow luminescence. The position of the right peak is almost the same as the rise in exciting power. The phenomenon shows the emission generated by InGaN MQDs was subjected to a very low piezoelectric field, and it may be possible to make a red emitting device by this method. Figure 6 shows the high resolution TEM image of InGaN MQDs. An obvious interface exists between the high temperature GaN under-lying layer and the five pairs of MQDs, which consist of 2-nm-thick InGaN layers and 3-2-nm-thick cap layers. Unfortu-nately, the boundary between InGaN and SiNx can not be identified in this image. Because of randomness in size and in position, isolated InGaN QDs and SiNxdomains were not

easily found in the cross-sectional view. As the EDX line scan in the horizontal direction of MQDs shows in Fig. 7, the indium composition fluctuates in regular size about 20 nm, and the size is almost the same as indicated by the AFM data in previous work.9)_{It is strong evidence toward} that establishing SiNx is suitable to use as a nano-mask to control the InGaN QD size. On the other hand, as the EDX line scan in the vertical direction of the MQDs shows in Fig. 8, the indium composition does not obviously fluctuate. This may result from indium diffusing into the GaN barrier layer during high temperature SiNx treatment. The low temperature in-situ SiNx nano-mask technique should be studied further, and low the temperature p-GaN contact layer should be made available for LED device structure or else the high temperature will damage the dot layers.

4. Conclusions

An in-situ SiNx nano-mask technique has been success-fully used for InGaN MQD growth by MOCVD. A red emission from InGaN MQDs was demonstrated at lower growth temperatures and higher indium/gallium ratios. InGaN MQDs based on a SiNx in-situ nano-mask may be under the inﬂuence of a low piezoelectric ﬁeld, and it may be

450 500 550 600 650 100000 1000000 1E7 24mw 16mw 8mw 4mw 2mw 1mw 512µw 256µw 128_µw 64µw 32µw 16µw 8µw 4_µw Intensity(arb. units) Wavelength (nm)

Fig. 5. Low-power power-dependent PL of InGaN QDs at 20 K.

MQDs

GaN

Epoxy

~20nm

Fig. 6. High resolution TEM image of InGaN MQDs. Fig. 7. EDX line scan in the horizontal direction of InGaN MQDs.

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possible to use this phenomenon to develop red LEDs. During MQD growth, SiNx is stable enough to conﬁne the InGaN QDs. However, the indium in InGaN QDs diﬀuses easily into the GaN barrier layer during the high temperature growth of SiNx. Further study of the low temperature in-situ SiNx nano-mask technique should be carried out.

Acknowledgement

The authors thank the Ministry of Economic Aﬀairs, Taiwan, Republic of China, for ﬁnancial support.

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