1Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University,
Hsinchu 30010, Taiwan, R.O.C.
2Institute of Photonic System, College of Photonics, National Chiao-Tung University, No.301, Gaofa 3rd. Road, Guiren
Township, Tainan County 71150, Taiwan.
3The Photonics Technology Center, Dept. of Electrical and Electronic Engineering, Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong. Abstract:
We investigate the optical and electrical characteristics of the GaN-based light emitting diodes (LEDs) grown on Micro and Nano-scale Patterned silicon substrate (MPLEDs and NPLEDs). The transmission electron microscopy (TEM) images reveal the suppression of threading dislocation density in InGaN/GaN structure on nano-pattern substrate due to nanoscale epitaxial lateral overgrowth (NELOG). The plan-view and cross-section cathodoluminescence (CL) mappings show less defective and more homogeneous active quantum well region growth on nano-porous substrates. From temperature dependent photoluminescence (PL) and low temperature time-resolved photoluminescence (TRPL) measurement, NPLEDs has better carrier confinement and higher radiative recombination rate than MPLEDs. In terms of device performance, NPLEDs exhibits smaller electroluminescence (EL) peak wavelength blue shift, lower reverse leakage current and decreases efficiency droop compared with the MPLEDs. These results suggest the feasibility of using NPSi for the growth of high quality and power LEDs on Si substrates.
Introduction:
The wide band gap GaN-based semiconductors have received enormous attention for various applications, such as short-haul optical communication, traffic and signal lights, back lights for liquid-crystal displays, and indoor/outdoor lightings. Typically, GaN-based light emitting diodes (LEDs) were grown on sapphire or SiC substrate by heteroepitaxial techniques in a metal-organic chemical vapor deposition (MOCVD) system [1]-[3]. However, the low thermal and electrical conductivities make sapphire less perfect as a substrate for the GaN epilayers, meanwhile the high price and mechanical defects hinder SiC substrate’s acceptability in the LED market. Silicon has been considered as an alternative substrate materialdue to its low manufacturing cost, availability of large size wafers, and good thermal and electrical conductivities. Thus, many efforts have been dedicated to the realization of GaN based LEDs on Si substrates.Even though good progress has been made, there are still several problems when using Si substrate for GaN epitaxial layers. The large lattice mismatch between GaN and Si (almost 17%) leads to high threading dislocation densities (TDDs) (around 108-1010 cm-2) in the subsequent GaN epilayers. The other major problem is the thermal expansion
coefficient difference (56%) between two materials, which induces a high tensile stress during the thermal cycling in MOCVD and often results in cracks and damages of epilayers. To reduce the density of cracks and threading dislocations of GaN grown on Si, a number of approaches have been reported, such as using AlN multilayer combined with graded AlGaN layer as buffer,epitaxial lateral overgrowth of GaN on micro-patterned Si, and nanoheteroepitaxial (NHE) lateral overgrowth of GaN on nanopore array Si,etc.. These methods effectively reduce the tensile stress and thus the crystal quality of GaN was greatly improved. Recently, our co-workers reported fabrication of GaN-based device structure on a nano-scale patterned silicon substrate [4]that shows significant improvement on reduction ofTDDs, surface morphology and light emission. In the mean time, the optical and electrical properties of InGaN/GaN MQWs grown on these patterned silicon substrates have not been fully studied yet. In this paper, we examine various optical and electrical characteristics of GaN based LEDs grown on micro
Optical and electrical properties of GaN-based light emitting diodes grown on micro and
nano-scale patterned Si substrate
Ching-Hsueh Chiu1, Chien-Chung Lin2, Dongmei Deng3, Hao-Chung Kuo1, Kei-May Lau3
Eleventh International Conference on Solid State Lighting, edited by Matthew H. Kane, Christian Wetzel, Jian-Jang Huang, Proc. of SPIE Vol. 8123, 81231F · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.893047
Proc. of SPIE Vol. 8123 81231F-1
and us to Resu First analy type NPS 2.5× n-Ga fewe botto drop NPS epila layer well Fig. 1 NPSi regar be ob samp nonr biexp nano-scale pa o believe that N
ult and discus t step to comp yze the detail s of devices in Si sample is r 1010 cm−2 at aN layer and 6 er dislocations om of the n-G p down to 5.7 Si over MPSi ayer/NPSi. As r or near the e region was m 1. TEM images o using g=(0002). Potential var rded as how f btained from ples was show radiaive recom ponential deca (t I atterned Si sub NPLED is in ssion: pare these two led epitaxial l n Fig. 1. A co reduced much the bottom o 6.2×108 cm−2 s are observab GaN layer is a ×108 cm−2, an is about 10 t s can be seen epilayer/NPSi much lower. of LEDs grown o riation affects fast the carrier decaying beh wn in Fig. 2. mbination pr aying function exp ) 0 ( ) I1 t = bstrates (MPL general superi o material gro layer quality, omparison of F h more than t of the n-GaN in the p-GaN ble within the about 1.1×101 nd it is only 8 times. Fig. 1(c in Fig. 1(c), t i interface. As n (a)MPSi and (b how easy th rs can recomb havior of photo Because the m rocess could n: [5] ) 0 ( ) ( 2 1 τt +I − LEDs and NPL ior to its micr owth methods
we used TEM Fig. 1(a) and that of MPSi layer, and it region. On th e range of vie 10 cm−2; howe 8.8×107 cm−2 i c) and 1(d) ar there are man s a result, the
b)NPSi；(c) and he carrier can bined. The info
oluminescenc measurement be excluded ). exp( ) 2 τ t − LEDs), and th o-scale counte is to check th M to compare 1(b) shows th i’s. The TDD decreases to he other hand, ew. As shown ever, the TDD in the p-GaN re TEM imag ny dislocations density of TD (d) region of be be confined, ormation abou e. The low tem
was carried o d. The TRPL he experiment erpart. heir material q e the cross se hat the disloca Ds for MPSi
4.6×109 cm−
for the epilay n in Fig. 1(b) Ds at the top o region. The r ges are taken s bent and ter DDs in the su
tween AlGaN lay
and the com ut carrier reco mperature TR out at 10K, th L results can (1)
tal results can
quality. In ord ction between ation density i is estimated t −2 at the top o yer grown on N ), the TDDs a of the n-GaN reduction of T at the interfa rminated in A ubsequent qua
yer and Si substr
mbining rate ca ombination rat RPL decay for he influence o n be fitted n lead der to n two in the to be of the NPSi, at the layer TDDs ace of lGaN antum rate for an be te can r both of the by a
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, wh The relax =3.2 comm for M gene in th can s NPL evid effic LED show forw 20m 62% 20% to re strain Fig. 3 respec
here I(t) is the fast decay tim xation of QW 2 and 1 ns f munication be MPLEDs and erally shorter t he potential m still be enhanc LEDs, much h ence of strong cient light-emi Fig The final tria D devices with ws the light o ward current d mA/cm2. Howe % of its maxim % efficiency dr educed polariz n in overgrow 3. Integrated EL ctively. PL intensity me constant ( W excitons from for MPLEDs etween localiz d NPLEDs, r than MPLEDs minima of QW ced even thou higher radiativ ger localized itter. g. 2. The compar al of this nano h a chip size o output intensit density for bo ever, it rolls o mum value wh roop with incr zation field w wn layers on N
intensity and nor
at time t ; τ1
(τ1) usually re
m free or ext and NPLED zed states and respectively. s’ at low temp Ws can be refe ugh the wave
ve recombina confinement i rison of low-temp o-size template of 350×350 μm ty and normal th samples. T over beyond 2 hen the curren
reasing the inj which also ech NPSi template rmalized EQE as and τ2 represe epresents the tended states Ds, respectivel d localized ex In both fast perature. S. Ch erred to as loc function over ation rate obs in NPLEDs th
perature TRPL be e is to test the m2 were fabri
lized external The light outp 20mA/cm2 wi nt density at 10 jection curren hoes to weake [6]. s a function of fo
ent the charac radiative reco toward locali ly. The slow xcitons. The f and slow c hichibu, et. al. calized excito rlap is weaken served in TRP han MPLEDs etween MPLEDs light emitting icated on both l quantum eff put-current cur ith a reduced 00mA/cm2. In nt density to 1 er QCSE und orward current de cteristic lifetim ombination o zed states. Ou w decay time fitting shows onstants, NP . reported the ons, and the e ned. In the cas PL can be in , and also an and NPLEDs. g efficiency fr h MPLEDs an ficiency (EQE rve of MPLE EQE. The EQ n contrast, the 00 mA/cm2. I
der the circum
ensity for (a)MPL
mes of the car of excitons an ur fitting show (τ2) account τ2 =9.4 and 3 LEDs’ lifetim electron-hole emission effic se of MPLED nterpreted as d indication of
rom the real d nd NPLEDs. F E) as a functio EDs is linear u QE is decreas e NPLEDs exh It can be attrib mstance of red LEDs and (b)NP rriers. nd the ws τ1 ts for 3.2 ns me is pairs iency s and direct more device. Fig. 3 on of under sed to hibits buted duced PLEDs,
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In conclusion, the optical and electrical properties of LEDs grown on micro and nano-scale patterned Si substrate were investigated. We demonstrated a more homogeneous growth of InGaN/GaN active layers under this nano-scale template by plan-view and cross-section CL mapping. From temperature dependent PL and low temperature TRPL measurement, NPLEDs has better carrier confinement and higher radiative recombination rate than MPLEDs. On theactual device performance, NPLEDs exhibits smaller peak wavelength blue shift, lower reverse leakage current and decreases efficiency droop compared with the MPLEDs. The results suggest a weaker QCSE due to relaxation of strain in the epitaxial layers on nano-scale patterned substrate, which can be really useful for the next generation of large area, Si-based heteroepitaxy of GaN related optoelectronic devices.
Reference:
[1] E. F. Schubert, “Light Emitting Diodes”, (Cambridge University Press, Cambridge, 2003), 2nd ed, pp 21-22.
[2] J. Han, M. H. Crawford, R. J. Shul, J. J. Figiel, L. Zhang, Y. K. Song, H. Zhou and A. V. Nurmikko,“AlGaN/GaN quantum well ultraviolet light emitting diodes,”Appl. Phys. Lett., vol. 73, pp. 1688, 1998.
[3] S. Nakamura, S. Pearton, and G. Fasol,“The Blue Laser Diode,” A The Complete Story, 2nd ed. Berlin, Germany: Springer, 2000. pp. 48.
[4] Dongmei Deng, Naisen Yu, Yong Wang, Xinbo Zou, Hao-Chung Kuo, and Kei May Lau,“InGaN-based light-emitting diodes grown and fabricated on nanopatterned Si substrates,” Appl. Phys. Lett., vol. 96, pp. 201106, 2010.
[5] W. Z. Lee, G. W. Shu, J. S. Wang, J. L. Shen, C. A. Lin,W. H. Chang, R. C. Ruaan, W. C. Chou, C. H. Lu and Y. C. Lee, “Recombination dynamics of luminescence in colloidal CdSe/ZnS quantum dots,” Nanotechnology, vol. 16, pp. 1517–1521, 2005.
[6] Y. B. Tao, Z. Z. Chen, F. F. Zhang, C. Y. Jia, S. L. Qi, T. J. Yu, X. N. Kang, Z. J. Yang, L. P. You, D. P. Yu, and G. Y. Zhang,“Polarization modification in InGaN/GaN multiple quantum wells by symmetrical thin low temperature-GaN layers”,
J. Appl. Phys., vol. 107, pp. 103529, 2010.
Conclusion:
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