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

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-1408416

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

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 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) 084004.

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, 2015. 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, 2015. Solved by growth optimization 7

Challenges for III-Nitride UV LEDs Thermal Injection Na=1 x 10 19 cm -3 GaN: 140 mev, Na - =7 x 10 17 cm -3 AlN: 630 mev, Na - =6 x 10 13 cm -3 Challenges: Lack of high quality substrates Poor hole injection Poor light extraction Poor p-type contact Rass, Jens, et al. SPIE OPTO, 2015. Solved by growth optimization Caused by high acceptor activation energy in (Al)GaN 9

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

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 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

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 0 20 40 60 80 100 Depth (nm) 13

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 0 20 40 60 80 100 Depth (nm) Electrons Holes Electron blocking layer is used to increase Ƞ inj 14

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 0 20 40 60 80 100 Depth (nm) N- Al 0.3 Ga 0.7 N V a Electron blocking layer is used to increase Ƞ inj 15

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

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

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

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

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, 026801 (2009) (Notre Dame) -σ +σ

Overview of the tunnel junction technology 10 2 TJ resistance (Ω cm 2 ) 10 0 10-2 10-4 10-6 GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN/AlN GaN/AlN GaN GdN/GaN InGaN/GaN Nano Lett., 13, 2570 (2013) APL 102, 113503 (2013) 10-8 1 2 3 4 5 Bandgap (ev) Resistance down to 10-4 Ohm cm 2 achieved for GaN tunnel junctions. 23

Overview of the tunnel junction technology 10 2? TJ resistance (Ω cm 2 ) 10 0 10-2 10-4 10-6 GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN/AlN GaN/AlN GaN GdN/GaN InGaN/GaN Nano Lett., 13, 2570 (2013) APL 102, 113503 (2013) 10-8 1 2 3 4 5 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

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) 2 0-2 -4 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

Modeling: tunneling current Energy (ev) 4 2 0-2 -4 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 0.01 1E-3 0 0 1 2 3 Voltage (V) 1E-4 1 10 100 1000 Current Density (A/cm 2 ) 26

Beyond the GaN bandgap: Design of AlGaN TJs TJ resistance (Ω cm 2 ) 10 2 10 0 10-2 10-4 10-6 10-8 GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN/AlN GaN/AlN GaN 1 2 3 4 5 Bandgap (ev) GdN/GaN? InGaN/GaN TJ Resistance (Ω cm 2 ) 10-3 10-4 10-5 10-6 70% 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% 0.2 0.4 0.6 0.8 InGaN composition MODEL 27

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 10 19 cm -3 15 nm n+ AlGaN [Si] = 1 X 10 20 cm -3 4 nm In 0.25 Ga 0.75 N 15 nm p+ -Al 0.3 Ga 0.7 N [Mg] = 5 X 10 19 cm -3 50 nm p-al 0.3 Ga 0.7 N [Mg] = 2X 10 19 cm -3 12 nm p type Al 0.46 Ga 0.54 N QWs 50 nm n-al 0.3 Ga 0.7 N [Si] =3 X 10 18 cm -3 +c TJ QWs 0 50 100 150 Depth (nm) 50nm 300 nm n-al 0.3 Ga 0.7 N [Si] =1.2 X 10 19 cm -3 N-Al 0.3 Ga 0.7 N on Sapphire -4-2 0 2 4 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 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 0.4 Wavelength (nm) 0.2 0.2 0 5 10 15 20 Current (ma) Power (mw) 0.6 0.4 WPE (%) 0.6 0.5 0.4 0.3 0.2 0.1 On-wafer measurement 0.0 0 5 10 15 20 Current (ma) 1.4 Single peak emission at 327 nm 1.2 Peak EQE and WPE are 1.5% and 1.0 1.08%, 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, 141103 (2015)

TJ-UV LED electrical characteristics Current Density (A/cm 2 ) 2k Full metal coverage L shape metal 1k Current Density (A/cm 2 ) 1k 10 100m 1m 10µ 100n 1n -6-4 -2 0 2 4 6 Voltage (V) 0-6 -4-2 0 2 4 6 Voltage (V) Resistance (Ω cm 2 ) 10 0 10-1 10-2 10-3 10-4 Experiment Simulation 1 10 100 1000 Current Density (A/cm 2 ) Voltage @ Voltage @ R @ 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

TJ-UV LED electrical characteristics Current Density (A/cm 2 ) 2k Full metal coverage L shape metal 1k Current Density (A/cm 2 ) 1k 10 100m 1m 10µ 100n 1n -6-4 -2 0 2 4 6 Voltage (V) 0-6 -4-2 0 2 4 6 Voltage (V) Resistance (Ω cm 2 ) 10 0 10-1 10-2 10-3 10-4 Experiment Simulation 1 10 100 1000 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

TJ-UV LED Sub-300 nm emission 300 nm n-al 0.55 Ga 0.45 N [Si] = 5 X 10 19 cm -3 15 nm n+ AlGaN [Si] = 1 X 10 20 cm -3 4 nm In 0.2 Ga 0.8 N 15 nm p+ -AlGaN [Mg] = 5 X 10 19 cm -3 20 nm p-al 0.55 Ga 0.45 N [Mg] = 2X 10 19 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 10 18 cm -3 600 nm graded to n-al 0.55 Ga 0.45 N [Si] =1.2 X 10 19 cm -3 Al 0.78 Ga 0.22 N on Sapphire +c -4-2 0 Energy (ev) TJ QWs 2 Voltage @ 20A/cm 2 is 7.1 V. Resistance @ 1kA/cm 2 is 1.6E-3 Ohm cm 2 4 0 30 60 90 120 Depth (nm) Current Density (A/cm 2 ) Resistance (Ohm cm 2 ) 1200 1000 800 600 400 200 0 1 0.1 0.01 1E-3 Current Density (A/cm 2 ) 1000 100 10 1 0.1 0.01 1E-3-6 -4-2 0 2 4 6 8 10 12 Voltage (V) -6-4 -2 0 2 4 6 8 10 12 Voltage (V) 1E-4 1 10 100 1000 Current Density (A/cm 2 ) 34

TJ-UV LED Sub-300 nm emission 300 nm n-al 0.55 Ga 0.45 N [Si] = 5 X 10 19 cm -3 15 nm n+ AlGaN [Si] = 1 X 10 20 cm -3 4 nm In 0.2 Ga 0.8 N 15 nm p+ -AlGaN [Mg] = 5 X 10 19 cm -3 20 nm p-al 0.55 Ga 0.45 N [Mg] = 2X 10 19 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 10 18 cm -3 600 nm graded to n-al 0.55 Ga 0.45 N [Si] =1.2 X 10 19 cm -3 Al 0.78 Ga 0.22 N on Sapphire +c -4-2 0 Energy (ev) TJ QWs 2 4 0 30 60 90 120 Depth (nm) Current Density (A/cm 2 ) 1200 1000 800 600 400 200 0 Current Density (A/cm 2 ) 1000 100 10 1 0.1 0.01 1E-3-6 -4-2 0 2 4 6 8 10 12 Voltage (V) -6-4 -2 0 2 4 6 8 10 12 Voltage (V) Voltage @ 20A/cm 2 is 7.1 V. 10-6 Resistance @ 1kA/cm 2 is 1.6E-3 Ohm cm 2 10-8 1 2 3 4 5 TJ resistance (Ω cm 2 ) 10 2 10 0 10-2 10-4 GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN Bandgap (ev) Al0.3 Ga 0.7 N Al0.55 Ga 0.45 N This work 35

TJ-UV LED Sub-300 nm emission Intensity 4x10 4 3x10 4 2x10 4 1x10 4 0.3 ma to 6 ma 30um device 0 250 300 350 400 Wavelength (nm) Power (mw) 0.12 0.10 0.08 0.06 0.04 0.02 On-wafer measurement 0.00 0 200 400 600 Current (A/cm 2 ) 36 EQE (%) 0.4 0.3 0.2 0.1 0.0 0 200 400 600 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.

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 ) 38 10 2 10 0 10-2 10-4 10-6 10-8 GaSb/InAs InP GaAs AlGaAs/InAlGaP GaN Al0.3 Ga 0.7 N Al0.55 Ga 0.45 N 1 2 3 4 5 Bandgap (ev) This work Intensity 4.5x10 4 4.0x10 4 3.5x10 4 0.3 ma to 6 ma 3.0x10 4 2.5x10 4 2.0x10 4 1.5x10 4 1.0x10 4 5.0x10 3 0.0 250 300 350 400 Wavelength (nm) 30um device Current Density (A/cm 2 ) 1200 1000 800 600 400 200 0 Current Density (A/cm 2 ) 1000 100 10 1 0.1 0.01 1E-3-6 -4-2 0 2 4 6 8 10 12 Voltage (V) -6-4 -2 0 2 4 6 8 10 12 Voltage (V)

UV Tunnel Junction LEDs Backup slides

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 1 10 5 cm -1 3.9% 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

Tunable wavelength Intensity 20% AlGaN QW 10% AlGaN QW FWHM 16.0 15.5 15.0 14.5 14.0 13.5 13.0 12.5 12.0 10% AlGaN QW 20% AlGaN QW 0 2 4 6 8 10 Current (ma) 260 280 300 320 340 360 380 400 420 Wavelength (nm) 1 41

Output power with time Power (uwatts) Power (uwatts) 52 51 50 49 48 47 46 45-1 0 1 2 3 4 5 6 7 8 9 151 150 149 148 147 146 145 144 143 142 141 1 ma 40A/cm 2 Time (hr) 4 ma 160A/cm 2 0 1 2 3 4 5 6 7 8 9 Time (hr) Power (uwatts) 103 102 101 100 99 98 97 2 ma 80A/cm 2 96 0 1 2 3 4 5 6 7 8 Time (hr) Power increases by about 6% and 4% with time for 1mA and 2mA, respectively. Power decreases by 4% for 4mA. 2 42

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 5 082101 (201

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 2 327 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 2 @ 120 A/cm 2 @ 5.9 V 4 44

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