Improvement of Characteristic Temperature for AlGaInP Laser Diodes Presenter: Hsiu-Fen Chen ( 陳秀芬 ) Authors: Man-Fang Huang ( 黃滿芳 ), Meng-Lun Tsai ( 蔡孟倫 ), Yen-Kuang Kuo ( 郭艷光 ) Institute of Photonics National Chunghua University of Education 國立彰化師範大學 光電科技研究所

2Paper Content  Introduction  Requirement of DVD Laser Diodes  Disadvantage of DVD Laser Diodes  Approach for High Temperature Operation  Theoretical Analysis  Experimental Results  Conclusion

3Paper Introduction  AlGaInP laser diodes (LDs) are widely used in DVD-ROM, DVD- R/RW and DVD player.  However, the requirement for the operation temperature of the AlGaInP LD has been increased from 70 ºC in the past to the more recent 100 ºC, especially for the outdoor applications such as portable players, computers or vehicle-used player.  The main reason to prevent AlGaInP LD from high operation temperature is the electron overflow from active region to p- cladding.  This study will focus on how to minimize the leakage current and improve the operation temperature for AlGaInP LD.

4Paper Specification of DVD LD Refer to the web site:  AlGaInP LD with high operation temperature for outdoor application is still under development. ApplicationDVD-ROM/playerDVD-R/RWDVD Vedio Wavelength (nm) /654 dual Power (mW)Up to 1060 (x4)7 Operation Temperature( ℃ ) -10 ~ 70 (indoor) -10 ~ 100 (outdoor)* -10 ~ 70 FFP(  ⊥ ) Aspect Ratio~31.7~ 3 Mode PatternSingle transverse mode

5Paper Disadvantage of AlGaInP LD  Small conduction band offset (  Ec=0.27 eV)  Result in bad electron confinement  Increase leakage current over p-cladding layer  Low Zn-doping concentration when Al is increased  Increase leakage current over p-cladding  Large thermal resistivity (14~19 K  cm/W)  Cause heat dissipation problem  Increase threshold and operation currents  Ec Leakage current

6Paper Approach for High Operation Temperature  Utilize strained multiple quantum well (strained-MQW) to reduce threshold current    Optimize quantum well numbers to minimize electron overflow    Increase P-doping concentration to reduce leakage current    Utilize multiple quantum barrier(MQB) to block overflow electrons    Our Work  Optimize barrier/confining layer composition along with quantum well numbers    Change confining layer structure (or SCH) to graded-index separate confinement hetero-structure (GRIN-SCH) to enhance carrier confinement.

7Paper Strain-Induced Effect  Strain-induced effect  Split of HH band from LH band  density of states in valence band   I th  HH LH E HH LH E Band mixing Unstrained Strained Density of states E fv Ev Density of states E fv Ev

8Paper Wavelength Design  For DVD application,  A compressive strain of 0.5% ( In 0.55 Ga 0.45 P) and a well width of 5 nm is used for the well region Optimized (angstrom)

9Paper Well Number vs. Operation Temperature  Well number   carrier overflow   characteristic temperature (T 0 )  However, Threshold Current   There is a trade-off 3 wells4 wells 2 wellsElectron Overflow

10Paper  Barrier height =  Eg -  p - (  n +  p ) and  p decreases by increasing p-doping [~  kT ln(p/N v )]  Therefore, doping concentration of the p-cladding layer   quasi-Fermi level (  p )   conduction barrier height   leakage current  P-doped Concentration vs Leakage Current  n  p E c E v  p g E activep-claddingn-cladding barrier height Jr J nr ~ J o exp(-barrier height/kT)

11Paper P-doping Concentration Effect  Use Mg as p-type dopant  Carrier Concentration up to 1~2 x cm -3  (1) External differential efficiency ~ 90 % (2) Characteristic temperature > 100 K (Man-Fang Huang et al, IEDMS, Vol. B, 1996)

12Paper Multi-quantum Barrier (MQB)  Multi-quantum barrier (MQB) in p-cladding  Bragg reflector for electrons  reduce electron overflow  High operation temperature (IEEE QE-29, p.1844, 1993)

13Paper Some Issues  In reality, diffusion of p-dopants causes reliability issue  Un-dope the “p-type” cladding layer for more than one thousand angstroms  P-doping concentration at the interface between the active layer and p-cladding layer cannot be too high  Leakage current cannot be ignored  Control for MQB thickness accuracy and uniformity is NOT easy  Inaccuracy in MQB thickness may cause the increase in leakage current.  Careful control of thickness of the MQB is critical to obtain high- performance LDs

14Paper Study Goal  Theoretical analysis is done using LASTIP software  Key parameters including  Quantum barrier composition  Quantum well number  Confining layer structure  The optimization of the structure is based on constant emission wavelength and far-field pattern.  =654 nm (lasing) or 645 nm (spontaneous)   =29° single transverse mode

15Paper Laser Diode Structure  Cavity length = 450  m; Ridge width =5  m  No facet coating n-GaAs (0.3  m, 1x10 18 cm -3 ) n-(Al 0.7 Ga 0.3 )InP (1.3  m, 1x10 18 cm -3 ) (Al x Ga 1-x )InP confining layer (undoped) 5 nm Ga 0.45 InP well n-GaAs substrate (200  m, 1x10 18 cm -3 ) (Al x Ga 1-x )InP barrier (Al x Ga 1-x )InP confining layer (undoped) p-(Al 0.7 Ga 0.3 )InP (0.17  m, 1x10 18 cm -3 ) p-(Al 0.7 Ga 0.3 )InP (1.13  m, 1x10 18 cm -3 ) P-InGaP (0.05  m, 5x10 18 cm -3 ) P-GaAs(0.1  m, 1x10 19 cm -3 )

16Paper Far Field Pattern and Optical Confinement Perpendicular FWHM=29  Angle (degree)  To achieve a constant vertical emission angle of 29 o  The optical confinement factor is about 0.3. FFP Parallel FWHM = 9.2 ° Perpendicular FWHM = 29°

17Paper Effect of Barrier Composition and Quantum Well Number  At RT, 4QW & x=0.4 demonstrates the lowest threshold current  However, at 80 ℃, 5QW & x=0.5 shows the lowest threshold current  Al increases  threshold current increases as well [x in (Al x Ga 1-x )InP]

18Paper Effect of Barrier Composition  x=0.4 in the (Al x Ga 1-x )InP barrier layer  a lower quantum barrier  uniform stimulated emission rates  x=0.4 in the (Al x Ga 1-x )InP confining layer  a higher cladding barrier  lower electron overflow Therefore, a high average stimulated emission rate is achieved

19Paper Leakage Current  5QW  the leakage currents are smaller than those of 4QW  small difference among various aluminum compositions.  Al=0.6 has a higher leakage current due to smaller confining barrier

20Paper Simulated Characteristic Temperature  The threshold current of 4-QW LD is increased faster than 5-QW LD  6-QW has similar temperature characteristics; however, the threshold current is too large  5-QW with x=0.5 is a better choice for high operation temperature application

21Paper Experimental Characteristic Temperature  There is a crossover point between 4QW and 5QW  A characteristic temperature of as high as 110 K is obtained for 5-QW

22Paper Different Confining Structures  GRIN-SCH is widely used and generally combined with SQW ∵ A reduction in the density of states in the optical confinement region  Threshold current can be reduced  Mostly, linear-GRIN-SCH is employed  However, we will demonstrate that parabolic-GRIN-SCH shows a better choice for AlGaInP LD in terms of high operation temperature STEP-SCH GRIN-SCH LinearParabolic

23Paper Active Region Structure  The optimization is based on a fixed far-field pattern (FFP).  The confining layer thicknesses are different for different confining structures.  Confining layer thicknesses for different QW number and GRIN- SCH combinations are given as follows: Step-SCH Linear-GRIN- SCH Parabolic- GRIN-SCH Spacer (nm) QW#

24Paper Band Diagrams  Step-SCH  A dip at the interface between the n-cladding and the confining layer  Some of carriers are confined in this dip  GRIN-SCH  No dip  better carrier injection  Graded confining structure  carrier distribution is non-uniform n-sidep-siden-sidep-side

25Paper Carrier Distribution  Electrons accumulate in the n-cladding/confining interface for SCH  Injection efficiency is poor  GRIN-SCH has non-uniform electron distribution in the confining region  Less electron overflow in the p-cladding  Better carrier confinement

26Paper Stimulated Emission Rate  Stimulated emission rates (SER) at 20 ºC are almost the same for different GRIN-SCHs.  At 80 ºC,  SERs become more uniform among different quantum wells due to increase in thermionic transport  Parabolic-GRIN-SCH has higher SERs than linear-GRIN-SCH

27Paper Leakage Current for GRIN-SCH  Linear-GRIN-SCH shows higher leakage current than parabolic- GRIN-SCH.  Parabolic-GRIN-SCH has better carrier confinement.

28Paper Experimental Results  Threshold Current  GRIN-SCH is lower than Step-SCH  Characteristic Temperature  GRIN-SCH-4QW is similar to SCH-5QW  GRIN-SCH-4QW is the best choice for lower threshold current and higher operation temperature

29Paper Conclusions  We have done the optimization for the AlGaInP LD under the same waveguide confinement.  The simulation results suggest that five quantum wells are good enough to inhibit the electron overflow.  We theoretically show that the parabolic GRIN-SCH has a better carrier injection and smaller overflow than other SCH.  Experimental results show that LD with GRIN-SCH-4QW demonstrates the best performance. The characteristic temperature can be as high as 110K.  This work is supported by the National Science Council of the Republic of China, Taiwan, under grant NSC E

For questions, please contact Prof. Man-Fang Huang at