Sub 300 nm Wavelength III-Nitride Tunnel-Injected Ultraviolet LEDs

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1 Sub 300 nm Wavelength III-Nitride Tunnel-Injected Ultraviolet LEDs Yuewei Zhang, Sriram Krishnamoorthy, Fatih Akyol, Sadia Monika Siddharth Rajan ECE, The Ohio State University Andrew Allerman, Michael Moseley, Andrew Armstrong Sandia National Labs Jared Johnson, Jinwoo Hwang MSE, The Ohio State University Funding: NSF EECS

2 Outline: Tunneling injected UV LED Motivation Polarization engineered III-Nitride tunnel junctions Tunneling junction for hole injection into UV LEDs. Electrical characteristics Optical characteristics Sub-300 nm emission Summary 2

3 Outline: Tunneling injected UV LED Motivation Polarization engineered III-Nitride tunnel junctions Tunneling junction for hole injection into UV LEDs. Electrical characteristics Optical characteristics Sub-300 nm emission Summary 3

4 Motivation 100 nm 280 nm 315 nm UV C UV B UV A Disinfection Medical imaging UV curing Sterilization Sensing Protein analysis Drug discovery DNA sequencing Printing Sensing Phototherapy 400 nm 4

Motivation 100 nm 280 nm 315 nm UV C UV B UV A Disinfection Medical imaging UV curing Sterilization Sensing Protein analysis Drug discovery DNA sequencing Printing

5 Motivation 100 nm 280 nm 315 nm UV C UV B UV A Disinfection Medical imaging UV curing Sterilization Sensing Protein analysis Drug discovery DNA sequencing Printing Sensing Phototherapy 400 nm UV lighting market is increasing. UV LEDs are replacing the traditional UV lamps. 5 Y. Muramoto, Semicond. Sci. Technol. 29 (2014)

6 Challenges for III-Nitride UV LEDs Thermal Injection Challenges: Lack of high quality substrates Poor hole injection Poor light extraction Poor p-type contact Rass, Jens, et al. SPIE OPTO,

7 Challenges for III-Nitride UV LEDs Thermal Injection Challenges: Lack of high quality substrates Poor hole injection Poor light extraction Poor p-type contact Rass, Jens, et al. SPIE OPTO, Solved by growth optimization 7

8 Challenges for III-Nitride UV LEDs Thermal Injection Challenges: Lack of high quality substrates Poor hole injection Poor light extraction Poor p-type contact Rass, Jens, et al. SPIE OPTO, Solved by growth optimization Caused by high acceptor activation energy in (Al)GaN 8

9 Challenges for III-Nitride UV LEDs Thermal Injection Na=1 x cm -3 GaN: 140 mev, Na - =7 x cm -3 AlN: 630 mev, Na - =6 x cm -3 Challenges: Lack of high quality substrates Poor hole injection Poor light extraction Poor p-type contact Rass, Jens, et al. SPIE OPTO, Solved by growth optimization Caused by high acceptor activation energy in (Al)GaN 9

10 P-contact and light extraction Current designs p GaN p AlGaN MQW n AlGaN p AlGaN/AlGaN SL p AlGaN MQW n AlGaN Absorption loss LED Electrical loss LED Increased absorption losses Increased voltage drop Trade-off between ƞ injection & ƞ LEE 10

11 P-contact and light extraction Current designs p GaN p AlGaN MQW n AlGaN p AlGaN/AlGaN SL p AlGaN Absorption loss LED Electrical loss Tunneling injection TJ-UV LED n AlGaN Tunnel Junction Thin p AlGaN MQW LED n AlGaN MQW n AlGaN LED 11

Non-equilibrium injection Current designs p GaN p AlGaN MQW n AlGaN p AlGaN/AlGaN SL p AlGaN Absorption loss LED Electrical loss Tunneling injection TJ-UV LED n AlGaN Tunnel Junction Thin

12 Non-equilibrium injection Current designs p GaN p AlGaN MQW n AlGaN p AlGaN/AlGaN SL p AlGaN Absorption loss LED Electrical loss Tunneling injection TJ-UV LED n AlGaN Tunnel Junction Thin p AlGaN MQW LED n AlGaN MQW n AlGaN LED Replace p-type contact using tunneling contact. Non-equilibrium injection. e- E c e- V LED Reduced light absorption loss Better contacts. h+ E v 12

13 Electron and hole injection imbalance Ƞ inj = = ~ JJ p JJ n Electrons injected into active region Total electrons Holes injected into active region Total electrons P-Al 0.3 Ga 0.7 N J p Ea=0 mev J n N- Al 0.3 Ga 0.7 N V a For ideal junction, equal amount of e/ h are supplied to active region. Ƞ inj = 1 Carrier Concentration (cm -3 ) 1E18 1E17 1E16 1E15 Electrons Holes Depth (nm) 13

14 Electron and hole injection imbalance Ƞ inj = = Electrons injected into active region Total electrons Holes injected into active region Total electrons ~ JJ p JJ n Real case: JJ n < JJ p Ƞ inj << 1 For PN junction, hole current is much lower than electron current. Low injection efficiency. Carrier Concentration (cm -3 ) 1E18 1E17 1E16 1E15 P-Al 0.3 Ga 0.7 N J p Ea=0 mev Ea=0.22 mev J n N- Al 0.3 Ga 0.7 N V a Depth (nm) Electrons Holes Electron blocking layer is used to increase Ƞ inj 14

15 UV LED real junction Ƞ inj = = Electrons injected into active region Total electrons Holes injected into active region Total electrons ~ JJ p JJ n Real case: JJ n < JJ p Ƞ inj << 1 For PN junction, hole current is much lower than electron current. Low injection efficiency. Injection efficiency decreases with increasing bandgap. Carrier Concentration (cm -3 ) 1E18 1E17 1E16 1E15 P-Al 0.3 Ga 0.7 N J p Ea=0 mev Ea=0.22 mev J n Electrons Holes Depth (nm) N- Al 0.3 Ga 0.7 N V a Electron blocking layer is used to increase Ƞ inj 15

16 Tunneling injection into UV LEDs TJ-UV LED J n e- n AlGaN Tunnel Junction e- V a Thin p AlGaN MQW n AlGaN LED h+ J p = J tunnel Ƞ inj ~ JJ p JJ n ~ JJ tunnel JJ n Tunneling injection enables high hole current. Increased injection efficiency. Injection efficiency not sensitive to the increasing bandgap. 16

17 Tunneling injection into UV LEDs TJ-UV LED J n e- n AlGaN Tunnel Junction e- V a Thin p AlGaN MQW n AlGaN LED h+ J p = J tunnel Ƞ inj ~ JJ p JJ n ~ JJ tunnel JJ n Tunneling injection enables high hole current. Increased injection efficiency. Injection efficiency not sensitive to the increasing bandgap. Required TJ characteristics Voltage drop across TJ should be low On-resistance should be minimal Optical absorption should be minimal 17

18 What WPE can we achieve for UV/DUV LEDs? Conventional UV LED 18

19 What WPE can we achieve for UV/DUV LEDs? Conventional UV LED Tunneling injected UV LED 19

20 Outline: Tunneling injected UV LED Motivation Polarization engineered III-Nitride tunnel junctions Tunneling junction for hole injection into UV LEDs. Electrical characteristics Optical characteristics Sub-300 nm emission Summary 20

21 Polarization engineering for tunnel junctions Standard p+/n+ TJ Large Eg wide depletion region Doping limitations Large energy barrier for tunneling Low tunneling current density 21

22 Polarization engineering for tunnel junctions Standard p+/n+ TJ Large Eg wide depletion region Doping limitations Large energy barrier for tunneling Low tunneling current density Polarization TJ p + (Al)GaN InGaN n + (Al)GaN High density polarization sheet charge depletion width greatly reduced. Tunnel barrier reduced due to InGaN. 22 AlN barrier TJ: Previous Work M. J. Grundmann, PhD Dissertation (UCSB) J. Simon et.al., PRL 103, (2009) (Notre Dame) -σ +σ

23 Overview of the tunnel junction technology 10 2 TJ resistance (Ω cm 2 ) GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN/AlN GaN/AlN GaN GdN/GaN InGaN/GaN Nano Lett., 13, 2570 (2013) APL 102, (2013) Bandgap (ev) Resistance down to 10-4 Ohm cm 2 achieved for GaN tunnel junctions. 23

24 Overview of the tunnel junction technology 10 2? TJ resistance (Ω cm 2 ) GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN/AlN GaN/AlN GaN GdN/GaN InGaN/GaN Nano Lett., 13, 2570 (2013) APL 102, (2013) Bandgap (ev) Resistance down to 10-4 Ohm cm 2 achieved for GaN tunnel junctions. What would happen when we go to wider bandgap (AlGaN)? 24

25 Modeling: tunneling current 4 N- Al 0.55 Ga 0.45 N P-Al 0.55 Ga 0.45 N φ n In 0.2 Ga 0.8 N Energy (ev) φ p Depth (nm) Self-consistent Schrodinger Poisson solution WKB approximation for tunneling probability calculation. n p n n ( ρ ρ ) J = q f f v T de T p n z wkb 25

26 Modeling: tunneling current Energy (ev) φ n φ p In 0.2 Ga 0.8 N N- Al 0.55 Ga 0.45 N P-Al 0.55 Ga 0.45 N Depth (nm) Self-consistent Schrodinger Poisson solution WKB approximation for tunneling probability calculation. n p n n ( ρ ρ ) J = q f f v T de T p n z wkb Resistance reaches 7E-4 Ohm cm 2. High current density could be achieved with low voltage drop. Current Density (A/cm 2 ) Resistance (Ohm cm 2 ) 2k 1k E Voltage (V) 1E Current Density (A/cm 2 ) 26

27 Beyond the GaN bandgap: Design of AlGaN TJs TJ resistance (Ω cm 2 ) GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN/AlN GaN/AlN GaN Bandgap (ev) GdN/GaN? InGaN/GaN TJ Resistance (Ω cm 2 ) % Low resistance TJ could be created for high composition AlGaN. Hole injection could be achieved through high bandgap AlGaN TJs. 50% Al x Ga 1-x N 30% 20% 10% InGaN composition MODEL 27

28 Outline: Tunneling injected UV LED Motivation Polarization engineered III-Nitride tunnel junctions Tunneling junction for hole injection into UV LEDs. Electrical characteristics Optical characteristics Sub-300 nm emission Summary 28

29 MBE-grown TJ-UV LED n-algan top contact TJ Active region 100 nm n-al 0.3 Ga 0.7 N [Si] = 5 X cm nm n+ AlGaN [Si] = 1 X cm -3 4 nm In 0.25 Ga 0.75 N 15 nm p+ -Al 0.3 Ga 0.7 N [Mg] = 5 X cm nm p-al 0.3 Ga 0.7 N [Mg] = 2X cm nm p type Al 0.46 Ga 0.54 N QWs 50 nm n-al 0.3 Ga 0.7 N [Si] =3 X cm -3 +c TJ QWs Depth (nm) 50nm 300 nm n-al 0.3 Ga 0.7 N [Si] =1.2 X cm -3 N-Al 0.3 Ga 0.7 N on Sapphire Energy (ev) TJ as a tunneling contact to p-algan Enables extraction from top surface no need for flip chip bonding Low spreading resistance in n-algan reduced metal coverage 29

MBE-grown TJ-UV LED n-algan top contact TJ 5nm Active region 50nm Flat and sharp interfaces Embedded p-algan layer MBE is a better technique for TJ-UV LED growth

30 MBE-grown TJ-UV LED n-algan top contact TJ 5nm Active region 50nm Flat and sharp interfaces Embedded p-algan layer MBE is a better technique for TJ-UV LED growth TJ as a tunneling contact to p-algan Enables extraction from top surface no need for flip chip bonding Low spreading resistance in n-algan reduced metal coverage 30

TJ-UV LED optical characteristics 31 Intensity (a.u.) 5x10 4 4x10 4 3x10 4 2x10 4 RT, CW 0.1mA to 20mA 1x10 4 50µm device 0 280 300 320 340 360 380 400 420 EQE (%) 1.6 1.4 1.2 1.0 0.8 0.6 0.

31 TJ-UV LED optical characteristics 31 Intensity (a.u.) 5x10 4 4x10 4 3x10 4 2x10 4 RT, CW 0.1mA to 20mA 1x µm device EQE (%) Wavelength (nm) Current (ma) Power (mw) WPE (%) On-wafer measurement Current (ma) 1.4 Single peak emission at 327 nm 1.2 Peak EQE and WPE are 1.5% and %, respectively. 0.8 At 120 A/cm 2, voltage is 5.9 V, power is 6 W/cm 2. Proof of efficient hole injection through tunneling. Y. Zhang, Appl. Phys. Lett. 106, (2015)

32 TJ-UV LED electrical characteristics Current Density (A/cm 2 ) 2k Full metal coverage L shape metal 1k Current Density (A/cm 2 ) 1k m 1m 10µ 100n 1n Voltage (V) Voltage (V) Resistance (Ω cm 2 ) Experiment Simulation Current Density (A/cm 2 ) 2 ka/cm 2 J=20 A/cm 2 J=2 ka/cm 2 (50um*50um) 4.8 V 7.47 V 7.5E-04 Ohm cm 2 Lowest TJ resistance of 5.6 x 10-4 Ohm cm 2 is obtained for Al 0.3 Ga 0.7 N TJ Forward Resistance = R series + R TJ + R c 1.9E-04 ~ 1E-06 Ohm cm 2 32

10-1 10-2 10-3 10-4 Experiment Simulation 1 10 100 1000 Current Density (A/cm 2 ) Lowest TJ resistance of 5.

33 TJ-UV LED electrical characteristics Current Density (A/cm 2 ) 2k Full metal coverage L shape metal 1k Current Density (A/cm 2 ) 1k m 1m 10µ 100n 1n Voltage (V) Voltage (V) Resistance (Ω cm 2 ) Experiment Simulation Current Density (A/cm 2 ) Lowest TJ resistance of 5.6 x 10-4 Ohm cm 2 is obtained for Al 0.3 Ga 0.7 N TJ Polarization engineered TJ enables orders of magnitude lower resistance. 33

34 TJ-UV LED Sub-300 nm emission 300 nm n-al 0.55 Ga 0.45 N [Si] = 5 X cm nm n+ AlGaN [Si] = 1 X cm -3 4 nm In 0.2 Ga 0.8 N 15 nm p+ -AlGaN [Mg] = 5 X cm nm p-al 0.55 Ga 0.45 N [Mg] = 2X cm -3 8 nm p type Al 0.72 Ga 0.28 N QWs 50 nm n-al 0.55 Ga 0.45 N [Si] =3 X cm nm graded to n-al 0.55 Ga 0.45 N [Si] =1.2 X cm -3 Al 0.78 Ga 0.22 N on Sapphire +c Energy (ev) TJ QWs 2 20A/cm 2 is 7.1 V. 1kA/cm 2 is 1.6E-3 Ohm cm Depth (nm) Current Density (A/cm 2 ) Resistance (Ohm cm 2 ) E-3 Current Density (A/cm 2 ) E Voltage (V) Voltage (V) 1E Current Density (A/cm 2 ) 34

35 TJ-UV LED Sub-300 nm emission 300 nm n-al 0.55 Ga 0.45 N [Si] = 5 X cm nm n+ AlGaN [Si] = 1 X cm -3 4 nm In 0.2 Ga 0.8 N 15 nm p+ -AlGaN [Mg] = 5 X cm nm p-al 0.55 Ga 0.45 N [Mg] = 2X cm -3 8 nm p type Al 0.72 Ga 0.28 N QWs 50 nm n-al 0.55 Ga 0.45 N [Si] =3 X cm nm graded to n-al 0.55 Ga 0.45 N [Si] =1.2 X cm -3 Al 0.78 Ga 0.22 N on Sapphire +c Energy (ev) TJ QWs Depth (nm) Current Density (A/cm 2 ) Current Density (A/cm 2 ) E Voltage (V) Voltage (V) 20A/cm 2 is 7.1 V kA/cm 2 is 1.6E-3 Ohm cm TJ resistance (Ω cm 2 ) GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN Bandgap (ev) Al0.3 Ga 0.7 N Al0.55 Ga 0.45 N This work 35

36 TJ-UV LED Sub-300 nm emission Intensity 4x10 4 3x10 4 2x10 4 1x ma to 6 ma 30um device Wavelength (nm) Power (mw) On-wafer measurement Current (A/cm 2 ) 36 EQE (%) Current (A/cm 2 ) Single peak emission at 295 nm. Peak EQE is 0.4%. EQE curve indicates high non-radiative recombination in active region. Al 0.55 Ga 0.45 N/ In 0.2 Ga 0.8 N TJ is demonstrated for the first time.

37 Outline: Tunneling injected UV LED Motivation Polarization engineered III-Nitride tunnel junctions Tunneling junction for hole injection into UV LEDs. Electrical characteristics Optical characteristics Sub-300 nm emission Summary 37

Summary First report of tunneling hole injection through wide band gap Al 0.55 Ga 0.45 N tunnel Junctions with Eg ~ 4.7 ev. Single peak emission at 295 nm Tunneling injection gives EQE of 0.

38 Summary First report of tunneling hole injection through wide band gap Al 0.55 Ga 0.45 N tunnel Junctions with Eg ~ 4.7 ev. Single peak emission at 295 nm Tunneling injection gives EQE of 0.4% (on-wafer) Lowest TJ resistance of 1.6E-3 Ohm cm 2 Tunnel Junctions are promising for high efficiency UV/ DUV LEDs TJ resistance (Ω cm 2 ) GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN Al0.3 Ga 0.7 N Al0.55 Ga 0.45 N Bandgap (ev) This work Intensity 4.5x x x ma to 6 ma 3.0x x x x x x Wavelength (nm) 30um device Current Density (A/cm 2 ) Current Density (A/cm 2 ) E Voltage (V) Voltage (V)

39 UV Tunnel Junction LEDs Backup slides

40 Absorption losses due to TJ p AlGaN Hole injected back into the active region n AlGaN recycled Absorbed/injected back/ emitted recursively II II =II 0 exp ( ααt) absorption coefficient (α) of cm % photons absorbed in one pass 0.039*IQE is emitted again, and absorbed 1 Loss= T + T(1-T)R + T(1-T)2R2+ T(1-T)3R3 + + T(1-T)NRN Absorption loss = 2%,assuming IQE 50% 0 40

41 Tunable wavelength Intensity 20% AlGaN QW 10% AlGaN QW FWHM % AlGaN QW 20% AlGaN QW Current (ma) Wavelength (nm) 1 41

42 Output power with time Power (uwatts) Power (uwatts) ma 40A/cm 2 Time (hr) 4 ma 160A/cm Time (hr) Power (uwatts) ma 80A/cm Time (hr) Power increases by about 6% and 4% with time for 1mA and 2mA, respectively. Power decreases by 4% for 4mA. 2 42

43 What WPE can we achieve for UV/DUV LEDs? TJ-UV LED n AlGaN Tunnel Junction Thin p AlGaN MQW n AlGaN LED Input Power 100% ƞ inj ƞ IQE < 80% < 70% Non-equilibrium injection Crystal quality (TDD) Active region ƞ LEE < 80% Minimal absorption (similar to visible LEDs) Output Power ~ 45% 43 p GaN p AlGaN MQW n AlGaN p AlGaN/AlGaN SL p AlGaN MQW n AlGaN Absorption loss LED Electrical loss LED Input Power 100% ƞ inj ƞ IQE ƞ LEE < 50% < 70% Low hole density P-contact Crystal quality (TDD) Active region < 25% Absorption loss Reflection Output Power < 8 % Highest value < 5.5% M. Shatalov, et al. APEX (201

44 Key Results First report of Al 0.3 Ga 0.7 N interband Tunnel Junctions (TJ) for hole injection in UV LEDs Low TJ resistance of 5.6 x 10-4 ohm cm nm LEDs with 0.58 mw at 20 ma (on-wafer) Peak EQE 1.5%, Peak WPE 1.08% Stable output power of 6 W/cm 120 A/cm 5.9 V 4 44

45 Backup slides Absorption losses.. Calculation details Exact quantum well design.. Tunable wavelength.. Stability/ reliability of output power! All previous tunnel junction work! Latest MOCVD Work! 5 45

Graded P-AlGaN Superlattice for Reduced Electron Leakage in Tunnel- Injected UVC LEDs

Graded P-AlGaN Superlattice for Reduced Electron Leakage in Tunnel- Injected UVC LEDs Yuewei Zhang, Sriram Krishnamoorthy, Fatih Akyol, Zane Jamal-Eddine Siddharth Rajan ECE, The Ohio State University