ECSE-6290 Semiconductor Devices and Models II Lecture 20: Laser Diodes

1. ECSE-6290Semiconductor Devices and Models IILecture 20: Laser Diodes Shayla M. Sawyer Bldg. CII, Room 8225 Rensselaer Polytechnic Institute Troy, NY 12180-3590 Tel. (518)276-2164 FAX (518)276-2990 e-mail: ssawyer@ecse.rpi.edu

2. Lecture Outline • Introduction • Main Concepts • Stimulated Emission • Population Inversion • Optical Gain • Optical Resonator • Threshold Current • Laser Diode Types

3. Concepts: Population Inversion • Switch from two energy levels to two separate continuous bands • Electron concentration as a function of energy, determined by Fermi-Dirac distribution and Density of States Population Inversion T>0K Population Inversion T=0K Equilibrium

4. Concepts: Population Inversion and Optical Gain Photons above EFn-EFp are absorbed

5. Concepts: Population Inversion and Optical Gain

6. Laser Physics: Population Inversion • Necessary requirements and conditions for lasing • In order for equation below to be positive FC>FV and EFn>EFp (population inversion) • Photon energy must be larger than the bandgap • For current pumped laser diode the quantity (EFn-EFp) is equal to the bias voltage, bias is limited to the built up potential of the junction (ψBn+ ψBp) Fermi-Dirac For homojunction: one side must be doped todegeneracy

7. Concepts: Optical Resonator • Need to build up stimulated emissions by a optical resonator • Provided by cleaved and polished ends of the crystal

8. Concepts: Optical Resonator • Requirement is a structure to trap the light and build up intensity inside • Like a Fabry-Perot etalon, two parallel walls perpendicular to the junction • Longitudinal modes: multiple resonant frequencies • Separation of modes in wavelength and frequency Only multiples of the half wavelength can exist in the cavity Output spectrum is determined by the optical cavity, and optical gain vs. wavelength characteristics

9. Concepts: Optical Gain • Optical gain (g) due to stimulated emission is compensated by optical loss due to absorption (α) • Net gain/loss as a function of distance • For a given system R1, R2, and α are fixed, the only parameter to vary is overall gain • To keep gain positive Threshold gain

10. Concepts: Threshold Current • The relationship between optical gain and bias current can be described by the equation • Linear increase of optical gain with bias current Nominal current density Threshold value

11. Concepts: Threshold Current • Lasing oscillations occur only when the optical gain in the medium can overcome the photon losses from the cavity • Cavity modes vary with increasing current

12. Laser Types • single • heterostructure double heterostructure homostructure

13. Both Carrier and Optical Confinement!

14. Example: Modes in an optical cavity length Consider an AlGaAs based heterostructure laser diode which has an optical cavity of length 200 microns. The peak radiation is at 870 nm and the refractive index of GaAs is about 3.7. a) What is the mode integer m of the peak radiation and the separation between modes of the cavity? b) If the optical gain vs. wavelength characteristics has a FWHM wavelength width of about 6 nm, how many modes are there within this bandwidth? c) How many modes are there if the cavity length is 20 μm?

15. Stripe contact increases current density in the active region. • The widths of the active region or the optical gain region is defined by current density from the stripe Gain guided: optical gain is highest where current density is greatest

16. Active layer is surrounded by lower index AlGaAs and behaves like a dielectric waveguide • Ensures that photons are confined to the active or optical gain region • Increases rate of stimulated emission Index guided: optical power confined to waveguide

17. Index guided LD: changes from multiple mode to single mode with increasing optical power • Gain guided: remain multimode even at high diode currents

18. Single Frequency Solid State Lasers: DBR laser • Frequency selective dielectric mirrors a cleaved surfaces. • Only allow a single mode to exist • Periodic corrugated structure that interfere constructively when the wavelength corresponds to twice the corrugation periodicity (Bragg wavelengths)

19. Single Frequency Solid State Lasers: DFB laser • The corrugated layer, called the guiding layer, is now next to the active layer • In the DFB structure traveling wave are reflected partially and periodically as they propogate.

20. Constant 2D density of states means a large concentration of electron can easily occur at E1 (and holes at the minimum valence band energy) • Population inversion occurs quickly without the need for a large current to bring a large number of electrons • Benefits: Threshold current reduced, linewidth is narrower

22. Optical cavity axis along the direction of current flow rather than perpendicular to current flow • Radiation emerges from the surface of the cavity rather than from its edge • Reflectors at the edges of the cavity are dielectric mirrors • 20-30 layers for mirror, MQW active region

23. Summary • Laser concepts: Stimulated Emission, Population Inversion, Optical Resonator, Optical Gain, and Laser Threshold Current • Types of laser diodes include • Homostructure • Single heterostructure • Double heterostructure • Gain guided (stripe geometry) • Index guided (buried heterostructure) • Distributed Bragg reflection • Distributed feedback • Quantum well, multiple quantum well • Vertical cavity surface emitting

24. ECSE-6290Semiconductor Devices and Models IILecture 21: Photodetectors Prof. Shayla M. Sawyer Bldg. CII, Room 8225 Rensselaer Polytechnic Institute Troy, NY 12180-3590 Tel. (518)276-2164 FAX (518)276-2990 e-mail: ssawyer@ecse.rpi.edu

25. Lecture Outline • Introduction to Photodetectors • Photodiodes • General • p-i-n and p-n • Metal-Semiconductor • Avalanche • Metal-Semiconductor-Metal Photodetector • Quantum Well Infrared Photodetector • Summary

26. Introduction • Photodetectors are semiconductor devices that can detect optical signals through electronic processes • Three main processes: • Carrier generation by incident light • Carrier transport and/or multiplication by current-gain mechanism • Extraction of carriers as terminal current to provide the output signal • Desired: High sensitivity, high response speed, minimum noise, compact size, low biasing voltage and current

27. Introduction • Wavelength relation to transition energy • ΔE is the transition of energy levels • Depending on photodetector type can be: • Energy gap of the semiconductor • Barrier height as in a metal semiconductor photodiode • Transition energy between impurity level and band edge as an extrinsic photoconductor Often minimum wavelength for detection

28. Introduction • Important Factors/Parameters • Absorption Coefficient • Response Speed • Quantum Efficiency • Responsivity • Gain • Noise • Detectivity

29. Introduction • Absorption coefficient • Determines whether light can be absorbed for photoexcitation • Determines where light is absorbed • High value means near surface • Low value means deeper penetration

30. Introduction • Response Speed • Shorter carrier lifetime yields fast response at the expense of higher dark current (noise) • Depletion width should be shortened to reduce transit time at the expense of capacitance • Quantum Efficiency • Number of carriers produced per photon • Iph is the photocurrent, Φ is the photon flux (=Popt/hv) and Popt is the optical power

31. Introduction • Responsivity: Photocurrent generated per incident optical power • Gain and response time for common photodetectors

32. Introduction • Noise ultimately determines minimum detectable signal strength • Sources of noise • Dark current • Thermal noise • Shot noise • Flicker noise • Generation recombination noise • Figure of Merit Noise Equivalent Power • Detectivity A is the Area • B is the Bandwidth NEP-incident rms optical power required to produce a signal-to-noise ratio of one in a 1 Hz bandwidth (minimum detectable light power)

33. Introduction • Detectivity • The signal-to-noise ratio when one watt of light power is incident on a detector of area 1 cm2 measured over 1 Hz bandwidth • Normalized to area, noise is generally proportional to the square root of area • Detectivity depends on • Detector sensitivity • Spectral Response • Noise • Is a function of wavelength, modulation frequency and bandwidth

34. Photodiodes: General • Photodiodes have depleted region with a high electric field that separates photogenerated electron-hole pairs • Tradeoff between speed of response and quantum efficiency (depletion layer: transit time, absorbance area) • Reverse biasing often employed to reduce carrier transit time and lower diode capacitance • All photodiodes except Avalanche has a maximum gain of one a) p-i-n photodiodes b) pn photodiode c) Metal-i-n photodiode d) Metal-semiconductor photodiode e) Point contact photodiode

35. Photodiodes: General • Important characteristics • Quantum efficiency • Absorption coefficient strong dependence on wavelength • Long wavelength cutoff given by energy gap of semiconductor • Short wavelength cutoff given by large value of α (surface where recombination is likely) • Response Speed • Limited by • Drift time in the depletion region • Diffusion of carriers • Capacitance of detection region • Optimized when the depletion layer is chosen so the transit time is on the order of one half the modulation period WD ~ 1/α

36. Photodiodes: General • Device Noise • Shot noise • IP average photocurrent, IB background radiation, ID dark current due to thermal generation of electron hole pairs in the depletion region • Thermal noise where • Rj Junction resistance • Ri Input resistance of amplifier • RL External load resistor

37. Photodiodes: General • Signal to Noise for 100% modulated signal with average power Popt • Minimum optical power required to obtain a given signal-to-noise ratio is (setting Ip=0) • Noise equivalent power (S/N=1; B=1 Hz)

38. Photodiodes: p-i-n and p-n • Depletion layer thickness (intrinsic layer) can be tailored to optimize the quantum efficiency and frequency response • Total photocurrent density through reverse biased depletion layer • Total current density is the sum of Idr inside the depletion region and Idiff outside the depletion region

39. Photodiodes: p-i-n and p-n • Quantum efficiency • Reduced from unity from • Reflection R • Light absorbed outside the depletion region • High quantum efficiency, low R and αWD>>1 is desirable • For WD>>1/ α transit time delay may be considerable

40. Photodiodes: p-i-n and p-n • Frequency Response • Phase difference between photon flux and photocurrent will appear when incident light intensity is modulated rapidly • Assume light is absorbed at surface, applied voltage is high enough to ensure saturation velocity • Response time is limited by the carrier transit time through the depletion layer • Compromise for high frequency response and quantum efficiency • Absorption region of thickness 1/α to 2/α • Large portion of light is absorbed within the depletion region

41. Photodiodes: p-i-n and p-n • Frequency Response • 3-dB frequency • Illustrates trade off between response speed and quantum efficiency at various wavelengths by adjusting the depletion width • Smaller WD, shorter transit time, higher speed, but reduced η Shows internal quantum efficiency of the Si p-i-n photodiode as a function of the 3-dB frequency and depletion width

42. Photodiodes: p-i-n and p-n • p-n photodiode • Thin depletion layer means some light can be absorbed outside • Light more than a diffusion length outside do not contribute at all to photocurrent • Reduces quantum efficiency • Diffusion process is slow • Time require to diffuse a distance x • Lower response speed than p-i-n • Neutral region contributes to noise

43. Photodiodes: Heterojunction • Advantages • Large bandgap material can be transparent and used as a window for transmission of incoming optical power • Quantum efficiency is not dependent on distance of junction from surface • Unique material combinations so quantum efficiency and response speed can be optimized for a given optical wavelength • Reduced dark current J.H. Jang et al., Journal of Lightwave Technology, Vol. 20, No. 3, March 2002.

44. Photodiodes: Metal-Semiconductor • Has a threshold of qΦB, • when it gets to the energy gap value the quantum efficiency jumps to a much high value • Operates in two modes • hυ>Eg : radiation produces electron hole pairs (similar to pin photodiode) • hυ<Eg : photoexcited electrons surmount barrier

45. Photodiodes: Metal-Semiconductor • Quantum Efficiency in two modes • hυ>Eg : radiation produces electron hole pairs (similar to p-i-n) • hυ<Eg : photoexcited electrons surmount barrier, internal photoemission • Internal photoemission has typical quantum efficiencies of less than 1% CF is the Fowler emission coefficient

46. Photodiodes: Metal-Semiconductor • Configurations • Advantageous for band-to-band • Diode illuminated through thin metal contact with antireflection coating • Use low doping i layer similar to p-i-n • Point contact diode reduces active volume, drift time and capacitance are small • Very high modulation frequencies

47. Photodiodes: Metal-Semiconductor • Main advantages • High speed and long wavelength detection capability without having to use a semiconductor with a small energy gap • Not limited by charge storage of minority diffusion current • Ultrafast Schottky barrier photodiodes beyond 100 GHz have been reported • Useful in the visible and UV

48. Photodiodes: Avalanche • Operate at high reverse bias voltages where avalanche multiplication takes place • Creates internal current gain • Can respond to light modulated at microwave frequencies • Current gain-bandwidth product of an APD can be higher than 300 GHz • High gain comes at the price of noise • Low frequency avalanche gain αn and αp are electron and hole ionization rates

49. Photodiodes: Avalanche • For equal ionization coefficients (α =αn=αp) multiplication takes the simple form • In a practical device, the dc multiplication at high light intensities is limited by series resistance and space-charge effect Breakdown when αWD=1 I total multiplied current IP unmultiplied current ID unmultiplied dark current IMD multiplied dark current VR Reverse bias voltage VB Breakdown voltage

50. Photodiodes: Avalanche • Current gain mechanism multiples the signal current, background current and dark current indiscriminately • Signal to noise power ratio