Optoelectronic integrated circuits incorporating negative differential resistance devices

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1 Optoelectronic integrated circuits incorporating negative differential resistance devices José Figueiredo Centro de Electrónica, Optoelectrónica e Telecomunicações Departamento de Física da Faculdade de Ciências e Tecnologia Universidade do Algarve In collaboration with Bruno Romeira (CEOT), Thomas J. Slight and Charles N. Ironside (University of Glasgow, Scotland). 1

2 Outline Positive versus negative resistance Voltage and current controlled negative resistance Old ways of getting negative resistance Tunnel diodes and resonant tunneling diodes Integration of a RTD with an optical waveguide (RTD-OW) Optical modulation with a RTD-OW Optical detection with a RTD-OW Laser emission controlled by a RTD 2

3 Positive resistance I I linear V + - Z I=G(V)V slope >0 G: Conductance V Ohm s law : V>0 I 0 Non-linear G (V) 0 Dynamic resistance R>0 Power P diss =v I >0 = losses 3

4 Negative resistance (voltage controlled) I N shape IV V + - Z I=GV I slope<0 slope >0 G: Conductance V Ohm s law : Non-linear V>0 I < 0 G < 0 Dynamic resistance R=1/G <0 Dynamic resistance R<0 Power P diss =v i<0 = GAIN 4

5 Negative resistance (current controlled) I S shape IV I + - Z V=G(I)I I slope>0 G(I) : Conductance slope<0 Ohm s law: I>0 V can be < 0 G(I): <0 Dynamic resistance R=1/G<0 Example: gas tubes V 5

6 Old ways of getting negative resistance With transistors With amp-ops R in =V s /I s = - R 3 (R 1 /R 2 ) A real generator behave like an ideal one, and the generator's entire current will go to the loadz L, whatever the value of Z L andr S. 6

7 Novel ways to get negative resistance Tunnel diodes or Esaki diode These diodes have a heavily doped p-n junction only some 10 nm wide. The heavy doping results in a broken band-gap, where conduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side. A small applied voltage induces the electron tunneling from the n-side conduction band to the p-side valence band. Resonant tunneling diodes (RTDs) A resonant tunnel diode (RTD) is a device which uses quantum effects and negative differential resistance (NDR). As an RTD is capable of generating a terahertz wave at room temperature, it can be used in ultra high-speed circuitry. RTDs are formed as a single quantum well structure surrounded by very thin layer barriers. This structure is called a double barrier structure. This structure can be grown to by molecular beam heteroepitaxy. GaAs and AlAs in particular are used to form this structure. AlAs/InGaAs or InAlAs/InGaAs can be used. 7

8 What is a Resonant Tunnelling Diode (RTD)? (from an electron point of view) Single Quantum barrier Tunnelling AlGaAs GaAs AlGaAs Conduction band minimum profile Double Barrier Quantum Well GaAs AlAs GaAs AlAs Resonant tunnelling GaAs 8

9 Tunnelling versus Resonant Tunnelling Electron Transmission Probability 1.0 E f E c 1 E c 2 E E c 0.5 E f E c 1 E c 2 E n GaAs AlAs GaAs -d 0 L L+d E c AlAs n GaAs Z (d=1.4 nm, L=7.0 nm, E c =1.0 ev) E-E c1 (ev) 9

10 How does a resonant tunnelling diode work? DBQW-RTD Structure Double-Barrier { Collector Emitter n+ GaAs n GaAs AlAs GaAs AlAs n GaAs n+ GaAs ~10 nm Collector I I p I v NDC GaAs V p V v V Γ-Conduction Band profile E F E 0 E C E F E F E C E C V=V p V=V v Zero Bias Resonance Off Resonance E F E C E F E C E F E C 10

11 RTD as a voltage controlled negative resistance I N shape I-V I slope<0 V + - Z I=G(V) slope>0 G(V) : Conductance Vp V V V V>0 I can be < 0 G(V): <0 Vp<V<Vv: the dynamic resistance R=1/G(V)<0 Gain P diss =v i<0 self-oscillation 11

12 Advantages of new negative resistance devices Reduce circuit complexity Reach terahertz operating frequencies (high speed switching) High frequency bistable or multistable much simpler circuits as relaxation oscillators, single-pulse generators, and sinewave generators. Create multiple-peak current-voltage characteristics allowing functional applications such as multi-state memories... 12

13 Why optoelectronic RTD based devices? Traditionally, the change in the current or electrical field through a pn junction laser diodes, modulators, switches and detectors are imposed by a quite complex drive circuit. Integrating a DBQW-RTD with a laser diode, for example, the light emission can be controlled by the RTD. The DBQW-RTD: is the fastest purely electronic solid state device exhibits Negative Differential resistance (NDR) up to terahertz frequencies can be easily integrated with conventional electronic and optoelectronic devices 13

14 Integration of a RTD with an optical waveguide InGaAlAs/InP waveguide RTD n+ InGaAs n+ InAlAs n InGaAs } AlAs n InGaAlAs InGaAs 1 um AlAs n InGaAlAs 300mm n InGaAs (Si: 2x10 16 cm -3 ) SI InP n+ InP W Integration advantages: The electric field distribution across the waveguide section is strongly dependent on the bias voltage. Electrical control of waveguide optical properties, such as losses/transmission. Possibility of electric field self-oscillation at high frequency due to the gain in the NDR region. High-speed operation achievable due to the small RC time constant. collector contact SI InP emitter contact light (AuGe)NiAu silica 500mm 14

15 RTD-OW wafer structure In 0.53 Ga 0.47 As/AlAs (Si: cm -3 ) (Si: cm -3 ) (Si: cm -3 ) (Si: cm -3 ) Expected performance: h Operation at 1300 nm or at 1550 nm h Bandwidth > 50 GHz h Modulation depth >20 db h Bandwidth-to-drive-voltage ratio > 50 GHz/V 15

16 Implemented RTD-OW Ridge Waveguide gold RTD-OW Side View Ridge Waveguide W 50 W CPW Line silica 0,5 mm 0,4 mm 50 W CPW Line SMA Gold Wire Light 16

17 RTD-OW typical I-V characteristic electrical gain The curve shows signs of self-oscillation. PVCR=7 and J= 13.5 ka/cm 2 PVCR: peak-to-valley current ratio 17

Resonant Tunnelling Relaxation Oscillator Diagram of the RT Relaxation Oscillator Working Principle RTD Transmission Line (electrical delay t

18 Resonant Tunnelling Relaxation Oscillator Diagram of the RT Relaxation Oscillator Working Principle RTD Transmission Line (electrical delay t d ) R L E Oscilloscope and/or Streak Camera RTD I-V Characteristic I a b V p V v V Voltage across the RTD I Current across the RTD 4t d time 18

19 Optical modulation with the RTD-OW 19

20 RTD-OW electroabsorption modulator (RTD-EAM) 20

Self and direct modulation with a RTD-OW Coaxial cable 15 cm long Coaxial cable 10 cm long Light Transmission (a.u.) 200 150 100 50 0 0 3750 7500 11250 time (ps) Light Transmission (a.u.) 150 100 50 0 0 3750 7500 11250 time (ps) RF injected: 950 MHz - amplitude 0.

21 Self and direct modulation with a RTD-OW Coaxial cable 15 cm long Coaxial cable 10 cm long Light Transmission (a.u.) time (ps) Light Transmission (a.u.) time (ps) RF injected: 950 MHz - amplitude 0.6 V RF injected: 16 GHz - amplitude 0.4 V Light Transmission (a.u.) time (ns) RF power (db) Frequency 16 GHz Figure of Merit: 40 GHz/V 21

22 Electrical field induced absorption change Γ-Conduction band profile: E F E F E C depletion region E E C 0 E F E F V=V p Zero Bias Resonance E C E C E F E C V=V v ε Off Resonance E F E C E E c Waveguide Transmission (a.u.) E v E g λ g ' (ε) >λ g ε λ g' E g' V v D (depleted region) z V=0 V V~V p V~V v Wavelength 22

23 Optical detection with the RTD-OW 23

24 RTD-OW photo-detector 24

25 RTD-OW-PD results at 1 GHz 25

26 RTD-OW-PD results at 5 GHz 26

27 Integration of a RTD with a pin photo-detector* Energy Conduction Band Diagram RTD AlAs Barriers InAlAs Window Contacts *T. S. Moise et al, IEEE Photonics Technology Letters, 1997, 9 (6), 803. J.F. Martins-Filho et al, 40 db Photodetection Gain in a Resonant Tunneling Diode Optical Waveguide, submited to IEEE Photonics Technology Letters. Light + hv InAlAs Window Und InGaAs n InGaAs InP Substrate n+ InGaAs Distance I-V Characteristic I (ma) Responsivity (A/W) mm GHz Dark V (volt) V (volt) 27

28 Laser emission controlled by a RTD 28

29 Hybrid integration of a RTD with a laser diode * RTD, LD, RTD+LD I-V characteristics Light output due to relaxation oscillation * Integration of a Resonant Tunnelling Diode and an Optical Communications Laser, PhD Thesis, T. J. Slight,

* Integration of a Resonant Tunnelling Diode and an

30 Monolithic integration of a RTD with a LD * Possible implementations RTD-OW wafer structure RTD-LD schematics * Integration of a Resonant Tunnelling Diode and an Optical Communications Laser, PhD Thesis, T. J. Slight,

31 Conclusion Successful design and implementation of optoelectronic devices incorporating RTDs. Demonstration of RTD waveguide modulator: direct high frequency (up to 16 GHz) modulation (up to 18 db). Operation as a relaxation oscillator optical modulator. Demonstration of RTD waveguide detector up to 5 GHz Successful integration of a RTD with a laser diode. Possible applications of a RTD-LD: Chaotic light source to generate pseudo-random bit sequence Clock recovery hen operated as a forced oscillator driven by a digital data source Direct data encoding using small perturbation signals 31

32 Thank you 32

Resonant tunneling diode optoelectronic integrated circuits

Invited Paper Resonant tunneling diode optoelectronic integrated circuits C. N. Ironside a, J. M. L. Figueiredo b, B. Romeira b,t. J. Slight a, L. Wang a and E. Wasige a, a Department of Electronics and