Reduction of Auger recombination in semiconductor optical devices

Abstract

To reduce Auger recombination in semiconductor optical devices. [Solution] The semiconductor optical device (40, 50, 60) comprises a first region 42 including an active region configured such that, when a voltage is applied to the device, electrons and holes recombine within the active region to generate photons. The device comprises at least one second region (43, 44, 53, 54, 62, 63) including a quantum well structure configured to trap only electrons, only holes, or different amounts of electrons and holes. The second region is positioned a distance from the first region that is sufficiently close to the first region such that, when a voltage is applied to the device, a charge imbalance develops within the first region, thereby reducing Auger recombination within the first region.

Description

[Technical Field]

[0001] SUMMARY The present disclosure relates to reducing Auger recombination in semiconductor optical devices such as quantum well lasers. [Background technology]

[0002] In many applications, the temperature of a semiconductor laser must be carefully controlled using a cooling unit to stabilize the output power. Such cooling units can use significantly more power than the laser itself, for example. In addition, cooling units, such as piezoelectric coolers, are the first semiconductor components to fail over time, effectively limiting the useful life of the laser system.

[0003] U.S. Patent No. 8,937,978, issued to the same applicant, describes a semiconductor laser having an active layer comprising alternating layers of a first and a second material. The n-side barrier layer and the p-side barrier layer each comprise alternating layers of a first material and a third material. The materials are selected so that the layers of the second and third materials form a quantum well between the layers of the first material. The band gap Eg of the second material is arranged so that a certain proportion of electrons and holes recombine across the band gap Eg to emit photons at the lasing wavelength, the proportion decreasing with increasing temperature of the optical cavity. The band gap Ec of the third material is arranged so that the ratio of electrons and holes recombine across the band gap Eg of the second material to those recombine across the band gap Ec of the third material increases with increasing temperature of the optical cavity. Over the range of temperatures over which the semiconductor laser is intended to operate, the increase in the ratio compensates for the decrease in the rate at which electrons and holes recombine across the bandgap Eg of the second material and emit photons at the lasing wavelength, reducing the change with temperature of the optical cavity.

[0004] Thus, the output power of the '978 laser is less sensitive to temperature fluctuations than conventional semiconductor lasers, but this comes at the cost of an increased lasing threshold. The development of low threshold, temperature insensitive semiconductor lasers has defied years of research and development. Summary of the Invention

[0005] Auger recombination is a fundamental physical process that is detrimental to the performance of semiconductor lasers and other semiconductor optical devices. More specifically, Auger recombination is a non-radiative process that reduces the efficiency of semiconductor lasers and increases their threshold current. Various embodiments described herein reduce Auger recombination in semiconductor optical devices.

[0006] In one exemplary implementation, a semiconductor optical device includes a first region including an active region configured such that electrons and holes recombine in the active region to generate photons when a voltage is applied across the device. The device further includes at least one second region including a quantum well structure configured to preferentially trap either electrons or holes. The quantum well structure may be configured to trap only electrons, only holes, or different numbers of electrons and holes.

[0007] Thus, the quantum well structure in the second region may be designed to trap charge carriers in the form of electrons and / or holes, and more than 50% of the charge carriers trapped by the quantum well structure may be of the same type, i.e., either electrons or holes. Alternatively, in various embodiments, more than 55%, 60%, 70%, 80%, 90%, 95%, or 99% of the charge carriers trapped by the quantum well structure may be of the same type, i.e., either electrons or holes.

[0008] In some examples, the quantum well structure in the second region comprises a Type II quantum well structure that provides a quantum well for holes rather than electrons, while in other examples, the quantum well structure in the second region comprises a Type II quantum well structure that provides a quantum well for electrons rather than holes.

[0009] The second region is located close enough to the active region to alter the charge balance within the active region, but far enough away from the active region so as not to recombine with the charge.

[0010] If the second region is located sufficiently close to the first region, a charge imbalance develops in the first region when a voltage is applied to the device, which reduces Auger recombination in the first region.

[0011] Those skilled in the art aware of this disclosure will understand that the active region of the device is the region where photons are generated. In the second region, carriers are stored but do not participate in any recombination process and are therefore not active in emitting photons. Thus, the or each second region may be referred to as an inactive region. In some embodiments, the first region may be the only active region of the device.

[0012] In various embodiments, the second region is grown under tensile strain or substantially no strain, and the first region may be grown under compressive strain.

[0013] In some embodiments, two or more second regions may be provided, for example, on opposite sides of the first region, and each second region may contribute to the charge imbalance that develops within the first region.

[0014] The device may comprise a semiconductor laser or amplifier, for example, the device may comprise a quantum well laser.

[0015] The term "optical," as used herein in the term "optical device," should not be understood to mean that the device must produce light in the visible portion of the electromagnetic spectrum. Those skilled in the art will understand that the optical devices described herein may produce radiation within or outside the visible range, e.g., infrared, including wavelengths greater than 2 μm, or ultraviolet, or other wavelengths within the electromagnetic spectrum. [Brief explanation of the drawings]

[0016] In order that the present disclosure may be more fully understood, various embodiments thereof will now be described with reference to the accompanying drawings. [Figure 1] The energy-momentum diagram of the first Auger recombination process (CHCC process) is shown. [Figure 2] The energy-momentum diagram of the second Auger recombination process (CHSH process) is shown. [Figure 3] 1 is a general schematic diagram of a known quantum well laser; [Figure 3a] 4 shows a bandgap diagram of the conduction and valence bands of the quantum well laser of Figure 3. The diagram shows the minimum allowed energy for an electron in the conduction band and the maximum allowed energy for a hole in the valence band as a function of position through the structure. [Figure 4] 1 shows a schematic representation of a quantum well laser according to a first embodiment; [Figure 4a] 5 shows a bandgap diagram for the conduction and valence bands of the quantum well laser of FIG. [Figure 5] 2 shows a schematic representation of a quantum well laser according to a second embodiment; [Figure 5a] 6 shows a bandgap diagram for the conduction and valence bands of the quantum well laser of FIG. 5. [Figure 6] 10 shows a schematic representation of a quantum well laser according to a third embodiment; [Figure 7] 1 shows a bandgap diagram and alloy structure for a quantum well laser configured to operate at near-infrared wavelengths. [Figure 8]1 shows a bandgap diagram and alloy structure for a quantum well laser configured to operate at mid-infrared wavelengths. DETAILED DESCRIPTION OF THE INVENTION

[0017] Figure 1 shows the Auger recombination process (CHCC process) in which an electron 10 and a hole 12 recombine non-radiatively by transferring their energy to another electron 14. Because two electrons are involved, the probability of this process occurring is n 2 It is proportional to p, where n is the electron density and p is the hole density.

[0018] Figure 2 shows another type of Auger recombination process (CHSH process) in which an electron 20 and a hole 22 recombine non-radiatively by transferring their energy to another hole 24. Because two holes are involved, the probability of this process occurring is p 2 It is proportional to n, where p is the hole density and n is the electron density.

[0019] The probability of both the CHCC and CHSH processes increases with temperature. However, depending on the lasing wavelength for which a particular quantum well laser is designed, the laser's material properties may make one of these processes more likely than the other and therefore dominate. For example, for short-wavelength materials (i.e., materials designed to lase at shorter wavelengths), the bandgap may be larger than the spin-orbit splitting energy so that the CHSH process dominates. However, for long-wavelength materials (i.e., materials designed to lase at longer wavelengths), the bandgap may be smaller so that the CHSH process becomes less likely or even impossible, in which case the CHCC process may dominate.

[0020] Figure 3 is a general schematic diagram of a known quantum well laser 30 having a quantum well structure 32 configured such that electrons and holes recombine within the quantum well structure to generate photons when a voltage is applied to the laser's electrodes (not shown). Quantum well lasers are designed to operate at relatively short wavelengths (e.g., near-infrared wavelengths such as 1.55 μm or 1.3 μm) whereby the CHSH process dominates over the CHCC process. Figure 3a shows a bandgap diagram for the conduction and valence bands of the quantum well structure 32. The number (and therefore density) of electrons, n, is equal to the number (and therefore density) of holes, p, whereby the quantum well structure 32 is electrically neutral.

[0021] 4 shows a schematic representation of a quantum well laser 40 according to a first embodiment. The quantum well laser 40 includes a layer structure 41 with an active layer in the form of a first quantum well structure 42 configured so that electrons and holes recombine in the quantum well structure 42 to generate photons. The quantum well structure 42 is similar to the quantum well structure 32, which is designed to operate at relatively short wavelengths (e.g., near-infrared wavelengths such as 1.55 μm or 1.3 μm) whereby the CHSH process dominates over the CHCC process.

[0022] The quantum well structure 42 is configured to trap both electrons and holes so that the electrons can recombine with the holes and emit photons. The quantum well structure 42 is therefore the active well of the laser 40.

[0023] The layer structure 41 further comprises a second quantum well structure 43 and a third quantum well structure 44. The second and third quantum well structures 43, 44 are located on opposite sides of the first quantum well structure 42.

[0024] The second and third quantum well structures 43, 44 are designed to accommodate holes but not electrons, i.e., the quantum well structures 43, 44 act as traps for holes but not electrons. Because the second and third regions trap holes but not electrons, the carriers stored therein do not recombine and therefore do not emit photons, and may therefore be referred to as "inactive."

[0025] One possible alloy structure for realizing the hole subwell, along with the corresponding calculated bandgap diagram, is shown in Figure 7. However, this example is not intended to be limiting, and it will be understood that other alloy structures can be used to realize suitable hole subwells.

[0026] Returning to Figure 4, note that the second and third quantum well structures 43, 44 are sufficiently close to the first quantum well structure 42 to allow thermal distribution of charge carriers from the first quantum well structure 42 to the second and third quantum well structures 43, 44. A simplified version of the bandgap diagram for layer structure 41 is shown in Figure 4a for illustrative purposes. As can be seen, a significant number of holes are trapped within the second and third quantum wells 43, 44.

[0027] Overall, the device 40 is electrically neutral, such that the electron density n in the first quantum well structure 42 is equal to the sum of the hole densities p, p2, and p3 in the first, second, and third quantum well structures 42, 43, and 44, respectively. That is, n=p+p2+p3. Thus, the hole density p in the first quantum well structure 42 is less than the electron density n in the first quantum well structure 42. That is, the first quantum well structure has a charge imbalance (fewer holes than electrons) due to the presence of the second and third quantum well structures 43 and 44, which trap holes. The rate of the CHSH Auger process is p 2 Since the rate of the CHSH Auger process is proportional to n, the rate is therefore reduced in quantum well laser 40 compared to quantum well laser 30 shown in Figure 3. It will be appreciated that because the hole density p is reduced, to achieve lasing, the electron density n must be increased so that the product np remains approximately the same. However, p 2 As n decreases, the rate of the CHSH Auger process is reduced.

[0028] The rate of the CHCC process is n 2Since ρ is proportional to p, it is therefore increased by the charge imbalance (because there are more electrons than holes). However, as explained above, in quantum well laser 40, the CHSH process dominates over the CHCC process, thereby reducing overall Auger recombination.

[0029] For long wavelength lasers, the CHCC process may dominate over the CHSH process, in which case the second and third quantum well structures may be designed as traps for electrons rather than holes.

[0030] 5 shows a schematic diagram of a long-wavelength quantum well laser 50 according to such a second embodiment. Quantum well laser 50 includes a layer structure 51 with an active layer in the form of a first quantum well structure 52 configured so that electrons and holes recombine within quantum well structure 52 to generate photons. Quantum well structure 52 is designed for longer wavelengths (e.g., mid-infrared wavelengths such as 2-3 μm or greater) so that the CHCC process dominates over the CHSH process.

[0031] Quantum well structure 52 is configured to trap both electrons and holes so that the electrons can recombine with the holes and emit photons. Quantum well structure 52 is therefore the active well of laser 50.

[0032] The layer structure 51 further comprises a second quantum well structure 53 and a third quantum well structure 54. The second and third quantum well structures 53, 54 are located on opposite sides of the first quantum well structure 52.

[0033] The second and third quantum well structures 53, 54 are designed to accommodate electrons but not holes, i.e., the quantum well structures 53, 54 act as traps for electrons but not holes. Because the second and third regions trap electrons but not holes, the carriers stored therein do not recombine and therefore do not emit photons, and may therefore be referred to as "inactive."

[0034] One possible alloy structure for realizing the electron sub-well, along with the corresponding calculated bandgap diagram, is shown in Figure 8. However, this example is not intended to be limiting, and it will be understood that other alloy structures can be used to realize suitable electron sub-wells.

[0035] Returning to Figure 5a, note that the second and third quantum well structures 53, 54 are sufficiently close to the first quantum well structure 52 to allow thermal distribution of charge carriers from the first quantum well structure 52 to the second and third quantum well structures 53, 54. A simplified version of the bandgap diagram for layer structure 51 is shown in Figure 5a for illustrative purposes. As can be seen, a significant number of electrons are trapped within the second and third quantum wells 53, 54.

[0036] Overall, the device 50 is electrically neutral, such that the sum of the electron densities n, n2, and n3 in the first, second, and third quantum well structures 52 is equal to the hole density p in the first quantum well structure 52, i.e., p=n+n2+n3. Thus, the electron density n in the first quantum well structure 52 is less than the hole density p in the first quantum well structure 52. That is, the first quantum well structure 52 has a charge imbalance (fewer electrons than holes) due to the presence of the second and third quantum well structures 53 and 54. The rate of the CHCC Auger process is n 2 The rate of the CHCC Auger process is therefore reduced in the quantum well laser 40 compared to the quantum well laser 30 shown in FIG. 3 because it is proportional to p. 2 n and is therefore increased by the charge imbalance. However, as explained above, in quantum well laser 50, the CHCC process dominates over the CHSH process, thereby reducing overall Auger recombination.

[0037] As noted above, various embodiments of the present disclosure provide second and third quantum well structures located sufficiently close to the first quantum well structure to allow charge carriers to travel between the first, second, and third structures. However, if the second and third quantum well structures are located too close to the first quantum well structure, Auger recombination may be possible between the different quantum well structures, potentially reducing efficiency. For example, in FIG. 4, electrons in the first quantum well structure 42 and corresponding holes in the second or third quantum well structures 43, 44 may recombine non-radiatively. For example, in FIG. 5, holes in the first quantum well structure 52 and corresponding electrons in the second or third quantum well structures 53, 54 may recombine non-radiatively.

[0038] To avoid this, the second and third quantum well structures may be located sufficiently close to the first quantum well structure to allow charge carrier movement between them, but far enough from the first quantum well structure to prevent or limit Auger recombination between the first quantum well structure and the second or third quantum well structure.

[0039] In the example of Figure 4, the second and third quantum well structures 43, 44 are designed to accommodate holes but not electrons, i.e., the quantum well structures 43, 44 function as traps for holes but not electrons. However, even if the quantum well structures 43, 44 trap some electrons, further reduction in Auger recombination can be achieved if the quantum well structures 43, 44 trap more holes than electrons. Similarly, even if the quantum well structures 53, 54 of Figure 5 trap some holes, further reduction in Auger recombination can be achieved in the example of Figure 5 if the quantum well structures 53, 54 trap more electrons than holes.

[0040] In fabrication, layers including quantum well structures configured to trap only one type of charge carrier (i.e., electrons or holes), or to trap one type of charge carrier more than the other, may be grown under tensile strain or under unstrained conditions. Because tensile strain increases the density of states, more of one type of charge carrier may be trapped in the tensile strained layer. Thus, the second and third quantum well structures may comprise tensile strained layers. The active layers may be grown under compressive strain, i.e., the first quantum well structure may comprise compressive strained layers.

[0041] Although Figures 4 and 5 show second and third quantum well structures, Auger recombination may be further reduced if one of the second or third quantum well structures is omitted.

[0042] FIG. 6 shows a schematic diagram of a semiconductor optical device 60 according to a third embodiment. The device may comprise a semiconductor laser or amplifier. The device comprises a first region 62 including an active region configured such that, when a voltage is applied to electrodes (not shown) of the device, electrons and holes recombine in the active region to generate photons. The device further comprises at least one second region including a quantum well structure 63 configured to preferentially trap either electrons or holes. For example, the quantum well structure 63 may be configured to trap only electrons, only holes, or different amounts of electrons and holes (i.e., the quantum well structure does not trap equal numbers of electrons and holes). The quantum well structure 63 is positioned a distance from the first region 62 that is sufficiently close to the first region such that, when a voltage is applied to the device 60, a charge imbalance develops in the first region 62, thereby reducing Auger recombination within the first region. The second region may be sufficiently far from the first region to prevent or limit Auger recombination between first charge carriers in the first region 62 and second charge carriers in the second region 63. In some embodiments, at least one second region may comprise two quantum well structures located on opposite sides of the first region 62.

[0043] In some embodiments, the semiconductor optical device 60 may include additional structures such as those described in U.S. Patent No. 8,937,978. The first region 62 may have a first bandgap arranged such that the rate at which electrons and holes recombine across the first bandgap to emit photons decreases with increasing temperature. The additional structures include one or more additional bandgaps, and the first bandgap and the one or more additional bandgaps are arranged such that the ratio of electrons and holes recombining across the first bandgap to the one or more additional bandgaps increases with increasing temperature. Over the temperature range at which the semiconductor optical device is intended to operate, the increase in the ratio compensates for the decrease in the rate at which electrons and holes recombine across the first bandgap to emit photons, reducing the temperature-dependent change in the rate at which electrons and holes recombine across the first bandgap to emit photons.

[0044] In this way, a low threshold semiconductor laser may be provided having an output power with reduced sensitivity to temperature.

[0045] Many modifications and variations within the scope of the following claims will be apparent to one of ordinary skill in the art.

Claims

1. A semiconductor optical device, comprising: a first region including an active region configured such that, when a voltage is applied to the semiconductor optical device, electrons and holes recombine within the active region to generate photons; at least one second region including a quantum well structure configured to trap only electrons, trap only holes, or trap different amounts of electrons and holes, the second region being an inactive region; the first region and the at least one second region as a whole are electrically neutral; each of the at least one second region is positioned a distance from the first region that is sufficiently close to the first region such that, upon application of a voltage to the semiconductor optical device, thermal distribution of charge carriers from the first region to the at least one second region develops a charge imbalance within the first region, thereby reducing Auger recombination within the first region; Semiconductor optical devices.

2. 10. The semiconductor optical device of claim 1, wherein the second region comprises a type II quantum well structure configured to trap only electrons or trap only holes.

3. 3. The semiconductor optical device of claim 1, wherein the first region includes a quantum well structure configured to trap both electrons and holes.

4. The semiconductor optical device of claim 3 , wherein the first region comprises a Type I quantum well structure.

5. 5. The semiconductor optical device of claim 1, wherein the second region is positioned at a distance sufficiently far from the first region to prevent or limit Auger recombination between first charge carriers in the first region and second charge carriers in the second region.

6. 6. The semiconductor optical device of claim 1, wherein the material properties of the first region are configured such that a first Auger recombination process and a second Auger recombination process contribute to Auger recombination within the first region, and the charge imbalance increases the rate of the first Auger recombination process and decreases the rate of the second Auger recombination process such that overall Auger recombination within the first region is reduced.

7. 7. The semiconductor optical device of claim 1, further comprising at least two second regions located opposite the first region, each of the at least two second regions being positioned at a distance from the first region that is sufficiently close to the first region such that a charge imbalance develops in the first region when a voltage is applied to the semiconductor optical device, thereby reducing Auger recombination in the first region.

8. The semiconductor optical device of claim 1 , wherein the second region comprises a tensile strained layer.

9. The semiconductor optical device of claim 1 , wherein the second region comprises an unstrained layer.

10. The semiconductor optical device of claim 1 , wherein the first region comprises a compressively strained layer.

11. 11. The semiconductor optical device of claim 1, wherein the second region comprises a quantum well structure configured such that more than 55%, more than 60%, more than 70%, more than 80%, more than 90%, more than 95%, or more than 99% of the charge carriers trapped by the quantum well structure are of the same type, the charge carriers being one of electrons or holes.

12. the first region has a first bandgap that is positioned such that a rate of electrons and holes that recombine across the first bandgap decreases with increasing temperature, and the semiconductor optical device comprises: an additional structure having one or more additional bandgaps, the first bandgap and the one or more additional bandgaps being arranged such that a ratio of electrons and holes recombining across the first bandgap to electrons and holes recombining across the one or more additional bandgaps increases with increasing temperature of the semiconductor optical device; over a range of temperatures at which the semiconductor optical device is intended to operate, the increase in the ratio compensates for the decrease in the proportion to reduce the variation with temperature of the rate at which electrons and holes recombine across the first band gap to emit photons.

12. A semiconductor optical device according to any one of claims 1 to 11.

13. 13. A semiconductor optical device according to any one of claims 1 to 12, comprising a semiconductor laser or a semiconductor optical amplifier.

14. 14. The semiconductor optical device of claim 1, wherein the first region includes a quantum well, and the semiconductor optical device comprises a quantum well laser.

15. 15. The semiconductor optical device of claim 1, wherein the first region is the only active region of the semiconductor optical device.

16. 1. A method for manufacturing a semiconductor optical device, comprising: growing a first region and at least one second region; the first region includes an active region configured such that, when a voltage is applied to the semiconductor optical device, electrons and holes recombine within the active region to generate photons; the at least one second region comprises a quantum well structure configured to trap only electrons, only holes, or different amounts of electrons and holes, each of the at least one second region is an inactive region, the first region and the at least one second region as a whole are electrically neutral, and each of the at least one second region is positioned a distance from the first region that is sufficiently close so as to develop a charge imbalance within the first region due to thermal distribution of charge carriers from the first region to the at least one second region when a voltage is applied to the semiconductor optical device, thereby reducing Auger recombination within the first region.

17. 17. The method of claim 16, comprising growing the quantum well structure under tensile strain or substantially no strain.

18. 18. A method according to claim 16 or 17, comprising growing the first region under compressive strain.

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