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EP-2927966-B1 - Optical device including three-coupled quantum well structure

EP2927966B1EP 2927966 B1EP2927966 B1EP 2927966B1EP-2927966-B1

Inventors

  • Cho, Yongchul
  • LEE, YONGTAK
  • Na, Byunghoon
  • Park, Changyoung
  • Ju, Gunwu
  • PARK, YONGHWA

Dates

Publication Date
20260513
Application Date
20141118

Claims (11)

  1. An optical modulator comprising: a lower reflection layer (102,202,320,420,520,620) doped with a first conductive-type dopant; an active layer (110,210,330,430,530,630,650) on the lower reflection layer and including at least two outer barriers (112,212) and at least one coupled quantum well between the at least two outer barriers; and an upper reflection layer (103,203,340,440,540,640) on the active layer and doped with a second conductive-type dopant that is electrically opposite to the first conductive-type dopant, wherein each coupled quantum well includes a first quantum well layer (113a), a first coupling barrier (114a), a second quantum well layer (113b), a second coupling barrier (114b), and a third quantum well layer (113c), wherein the second quantum well layer (113b) is disposed between the first quantum well layer (113a) and the third quantum well layer (113c), the first coupling barrier (114a) is disposed between the first quantum well layer (113a) and the second quantum well layer (113b), and the second coupling barrier (114b) is disposed between the second quantum well layer (113b) and the third quantum well layer (113c), the thicknesses of the first and third quantum well layers are less than a thickness of the second quantum well layer; characterised in that band gaps of the first and third quantum well layers (113a, 113c, 213a, 213c) are greater than the band gap of the second quantum well layer (113b, 213b), and wherein, when the reverse bias voltage is not applied to the active layer, a coupling-barrier energy of the first and second coupling barriers (114a, 114b, 214a, 214b) is higher than an energy level of a first electron (e1) and lower than well-barrier energy of the outer barriers (112, 212) in a conduction band, wherein, when the reverse bias voltage is not applied to the active layer, a coupling-barrier energy level of the first and second coupling barriers (114a, 114b, 214a, 214b) is higher than an energy level of a first heavy hole (hh1) and lower than a well-barrier energy of the outer barriers (112, 212) in a valence band, wherein one of the outer barriers (112, 212) is directly adjacent to the first quantum well layer (113a, 213a) and another of the outer barriers (112, 212) is directly adjacent to the third quantum well layer (113c, 213c).
  2. The optical modulator of claim 1, wherein thicknesses of the first and second coupling barriers (114a, 114b,214a,214b) are equal to or less than thicknesses of the first and third quantum well layers (113a, 113c,213a,213c).
  3. The optical modulator of claim 1 or 2 wherein the first and third quantum well layers (213a,213c) comprise Al z Ga 1-z As where 0<z<1, the second quantum well layer (213b) comprises GaAs, the first and second coupling barriers (214a,214b) comprise Al y Ga 1-y As where z<y<1, and the outer barriers (212) comprise Al x Ga 1-x As where z<y<x≤1.
  4. The optical modulator of claim 1, 2 or 3 further comprising a substrate (101,201,310,410,510, 610), wherein the first to third quantum well layers (113a, 113c,213a,213c) include a material having compressive strain with respect to the substrate, and the outer barriers (112,212) are formed of a material having tensile strain with respect to the substrate,
  5. The optical modulator of claim 4 wherein the first to third quantum well layers (113a, 113b, 113c) comprise In x Ga 1-x As where 0.1≤x≤0.2, the first and second coupling barriers (114a, 114b) comprise GaAs, and the outer barriers (112) comprise GaAs 1-y P y or In y Ga 1-y P where 0.4≤y≤0.5.
  6. The optical modulator of any of claims 1 to 2, 4 or 5, wherein the first to third quantum well layers (113a, 113b, 113c) comprise at least one of In1-xGaxAs and In 1-x-y Ga x Al y As, the first and second coupling barriers (114a, 114b) comprise at least one of In 1-x' - y' Ga x' Al y' As where x'<x and y<y' and In 1-x' Ga x' As z P 1-z where x'<x, and the outer barriers (112) comprise at least one of In 1-x"-y" Ga x" Al y" As where x"<x'<x and y<y'<y" and In 1-x" Ga x" As z' P 1-z where x"<x'<x, and z<z', and 0<x, z<1.
  7. The optical modulator of any preceding claim, further comprising at least one microcavity layer (525) in at least one of the lower and upper reflection layers, wherein, when a resonance wavelength of the optical modulator is λ, the active layer and the at least one microcavity have an optical thickness that is an integer multiple of λ/2.
  8. The optical modulator of any preceding claim, wherein the at least one coupled quantum well comprises a first coupled quantum well and a second coupled quantum well, each of the first coupled quantum well and the second coupled quantum well including the first quantum well layer, the first coupling barrier, the second quantum well layer, the second coupling barrier and the third quantum well layer, wherein a thickness of the second quantum well layer of the first coupled quantum well is different from a thickness of the second quantum well layer of the second coupled quantum well.
  9. The optical modulator of any preceding claim, wherein a reflectance of the lower reflection layer (102,202,320,420,520,620) is higher than a reflectance of the upper reflection layer (103,203,340,440,540,640).
  10. The optical modulator of any preceding claim, further comprising an anti-reflection coating disposed under the lower reflection layer.
  11. An optical modulator according to claim 1, wherein the active layer is a first active layer (630), further comprising: a second active layer (650) on the upper reflection layer (640) comprising at least two outer barriers and at least one coupled quantum well inserted between the at least two outer barriers; and an uppermost reflection layer (660) on the second active layer and doped with the first conductive-type dopant, wherein each coupled quantum well includes a first quantum well layer, a first coupling barrier, a second quantum well layer, a second coupling barrier, and a third quantum well layer, wherein the second quantum well layer is disposed between the first quantum well layer and the third quantum well layer, the first coupling barrier is disposed between the first quantum well layer and the second quantum well layer, and the second coupling barrier is disposed between the second quantum well layer and the third quantum well layer, the thickness of the first and third quantum well layers are less than a thickness of the second quantum well layer; and the band gaps of the first and third quantum well layers are greater than the band gap of the second quantum well layer.

Description

BACKGROUND Field Example embodiments relate to an optical device including a three-coupled quantum well structure, and/or to an optical device including a three-coupled quantum well structure, which may improve the light absorption intensity in a multiple quantum well structure without increasing a driving voltage. Description of the Related Art 3D cameras typically have not only a general image capturing function, but also a function of measuring a distance from a plurality of points on a surface of an object. A variety of algorithms for measuring the distance between an object and a 3D camera have recently been suggested. A typical algorithm is a time-of-flight (TOF) algorithm. According to the TOF algorithm, illumination light is emitted onto an object, and a flight time until the illumination light reflected from the object is received by a light-receiving unit is measured. The flight time of illumination light may be obtained by measuring a phase delay of the illumination light. A high-speed optical modulator is used to accurately measure the phase delay. An optical modulator having superior electro-optical response characteristics is typically used to obtain a 3D image with high distance accuracy. Recently, a GaAs-based semiconductor optical modulator is used. The GaAs-based semiconductor optical modulator has a P-I-N diode structure in which a multiple quantum well (MQW) structure is disposed between a P-electrode and an N-electrode. In the structure, when a reverse bias voltage is applied between the P-N electrodes, the MQW structure forms excitons in a particular wavelength band and absorbs light. An absorption spectrum of the MQW structure characteristically moves toward a long wavelength as a reverse bias voltage increases. Accordingly, a degree of absorption at a particular wavelength may vary according to a change in the reverse bias voltage. Thus, according to the above principle, the intensity of incident light having a particular wavelength may be modulated by adjusting the reverse bias voltage that is applied to an optical modulator. In the optical modulator, a distance accuracy increases as a contrast ratio, for example, a demodulation contrast, indicating a difference in the degree of absorption between when a voltage is applied and when the voltage is not applied, increases. Driving at a low voltage is advantageous to reduce or prevent performance deterioration due to heat. In general, an increase in the contrast ratio may be achieved by increasing the light absorption intensity and transition energy in the MQW structure. The light absorption intensity is inversely proportional to the thickness of a quantum well layer and is proportional to a value obtained by normalizing a square of a degree of superimposition between a hole's wave function and an electron's wave function in the quantum well layer by a sum of the area of each wave function. Also, transition energy that indicates a degree of an absorption spectrum moving toward a long wavelength is proportional to the fourth power of the thickness of a quantum well layer and to the square of an applied voltage. However, when the thickness of a quantum well layer is reduced to increase the light absorption intensity, the transition energy decreases and an applied voltage increases in order to compensate for a decrease in the transition energy. Reversely, when the thickness of a quantum well layer is increased to increase the transition energy, the degree of superimposition between a hole's wave function and an electron's wave function decreases, and the generation of excitons by electron-hole pairs is reduced so that absorption intensity decreases. Thus, since a high absorption intensity and a low driving voltage are in a trade-off relationship, it is difficult to simultaneously achieve improvement of the absorption intensity and reduction of the driving voltage. In a TOF-type 3D camera, light having a wavelength of about 850 nm in an infrared range is generally used as illumination light. Since a GaAs substrate is not transparent with respect to the 850 nm wavelength light, a complicated process of removing the GaAs substrate is added to the process of manufacturing an optical modulator. Recently, to omit the complicated additional process, there have been efforts to use light having a wavelength of about 870 nm or more, for example, about 940 nm, which transmits through the GaAs substrate, as illumination light. However, since a lattice constant of the material of a quantum well layer and a barrier suitable for an optical modulator having a 940 nm resonance wavelength does not match the resonance wavelength of the GaAs substrate, stress and strain may be generated. Unless the stress and strain are compensated, quantum wells of a large number of layers are not stacked and thus it is difficult to increase the absorption intensity of an optical modulator. US 5,416,338 teaches an optical device with variable light absorbtion with three quan