Reducing Auger recombination in semiconductor optical devices

Abstract

A semiconductor optical device (40, 50, 60) includes a first region, the first region including an active region, the active region being configured such that when a voltage is applied to the device, electrons and holes recombine in the active region to generate photons. The device includes at least one second region (43, 44, 53, 54, 62, 63), the second region including a quantum well structure configured to capture only electrons, only holes, or different numbers of electrons and holes. The second region is arranged at a distance sufficiently close to the first region such that when a voltage is applied to the device, a charge imbalance is generated in the first region, thereby reducing Auger recombination in the first region.

Description

Technical Field

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

[0002] In many applications, the temperature of semiconductor lasers must be carefully controlled using cooling devices to stabilize the output power. Such cooling devices can use a large amount of electrical energy, for example, significantly exceeding the electrical energy of the laser itself. Furthermore, cooling devices such as piezoelectric coolers can be the first component of the semiconductor to fail over time, effectively limiting the lifetime of the laser system.

[0003] The applicant's U.S. patent (Patent No.: US 8,937,978 B2) 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 the ratio of electrons and holes that recombine across the band gap Eg to emit photons at the lasing wavelength decreases as the temperature of the optical cavity increases. The band gap Ec of the third material is arranged so that the ratio of electrons and holes that recombine across the band gap Eg of the second material to electrons and holes that recombine across the band gap Ec of the third material increases as the temperature of the optical cavity increases. Within the temperature range of the semiconductor laser's intended operation, the increased ratio compensates for the decreased ratio, thereby reducing variations in the ratio of electrons and holes that recombine across the band gap Eg of the second material to emit photons at the lasing wavelength as a function of the cavity temperature.

[0004] In this way, the output power of the laser in US 8,937,978 is less sensitive to temperature variations than conventional semiconductor lasers, but this comes at the expense of an increased lasing threshold.The development of low-threshold, temperature-insensitive semiconductor lasers has challenged many 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.

[0006] Various embodiments described herein reduce Auger recombination in semiconductor optical devices.

[0007] In one exemplary embodiment, a semiconductor optical device includes a first region comprising an active region configured such that when a voltage is applied to the device, electrons and holes recombine in the active region to generate photons. The device also includes at least one second region comprising a quantum well structure configured to capture only electrons, only holes, or different numbers of electrons and holes.

[0008] Thus, the quantum well structure of the second region can be designed to capture charge carriers in the form of electrons and / or holes, wherein more than 50% of the charge carriers captured by the quantum well structure can be of the same type, i.e., electrons or holes. Alternatively, in various embodiments, 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 captured by the quantum well structure can be of the same type, i.e., electrons or holes.

[0009] In some examples, the quantum well structure in the second region includes a type II quantum well structure that provides quantum wells for holes rather than electrons. In other examples, the quantum well structure in the second region includes a type II quantum well structure that provides quantum wells for electrons rather than holes.

[0010] The second region is close enough to the active region to alter the charge balance within the active region, but far enough away from the active region that it does not recombine with the charge.

[0011] Assuming the second region is positioned sufficiently close to the first region, when a voltage is applied to the device, a charge imbalance is created in the first region. The charge imbalance reduces Auger recombination in the first region.

[0012] Those skilled in the art 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 processes and are therefore inactive in emitting photons. Therefore, 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.

[0013] In various embodiments, the second region is generated under tensile strain or under substantially no strain. The first region may be generated under compressive strain.

[0014] In some embodiments, two or more second regions may be provided, for example, on opposite sides of the first region. Each second region may cause an imbalance in the charge generated in the first region.

[0015] The device may comprise a semiconductor laser or an amplifier. For example, the device may comprise a quantum well laser.

[0016] The term "optical" in the term "optical device" should not be understood to mean that the device must generate light in the visible portion of the electromagnetic spectrum; those skilled in the art will understand that the optical devices described herein can generate radiation within the visible range or outside the visible range, such as infrared radiation, including radiation with wavelengths greater than 2 μm, or ultraviolet radiation, or radiation at other wavelengths within the electromagnetic spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order that the present disclosure may be more fully understood, various embodiments thereof will now be described with reference to the accompanying drawings, in which:

[0018] Figure 1 shows the energy-momentum diagram of the first Auger recombination process (CHCC process);

[0019] Figure 2 shows the energy-momentum diagram of the second Auger recombination process (CHSH process);

[0020] Figure 3 is a general schematic diagram of a known quantum well laser;

[0021] Figure 3a Shown Figure 3 Bandgap diagram of the conduction and valence bands of a quantum well laser; the diagram shows the lowest allowed energy for electrons in the conduction band and the highest allowed energy for holes in the valence band as a function of position in the structure;

[0022] Figure 4 Schematically shows a quantum well laser according to a first embodiment;

[0023] Figure 4a Shown Figure 4 Band gap diagram of the conduction band and valence band of the quantum well laser;

[0024] Figure 5 schematically illustrates a quantum well laser according to a second embodiment;

[0025] Figure 5a Shown Figure 5 Band gap diagram of the conduction band and valence band of the quantum well laser;

[0026] Figure 6 schematically illustrates a quantum well laser according to a third embodiment;

[0027] Figure 7 shows the bandgap diagram and alloy structure of a quantum well laser configured to operate at near-infrared wavelengths, and

[0028] Figure 8 Shown are the bandgap diagram and alloy structure of a quantum well laser configured to operate at mid-infrared wavelengths. DETAILED DESCRIPTION

[0029] Figure 1 The Auger recombination process (CHCC process) is shown, in which an electron 10 and a hole 12 recombine non-radiatively by transferring their energy to another electron 14. Since two electrons are involved, the probability of this process occurring is related to n 2 p is proportional to n, where n is the electron density and p is the hole density.

[0030] Figure 2 Another Auger recombination process (CHCC process) is shown, in which an electron 20 and a hole 22 recombine non-radiatively by transferring their energy to another electron 24. Since two electrons are involved, the probability of this process occurring is related to p 2 n, where p is the hole density and n is the electron density.

[0031] The probability of both the CHCC and CHSH processes increases with increasing temperature. However, depending on the lasing wavelength for a particular quantum well laser design, the material properties of the laser may make one of these processes more likely to occur than the other and may dominate. For example, for short-wavelength materials (i.e., materials designed to emit laser light at shorter wavelengths), the band gap may be larger than the spin-orbit splitting energy, and therefore, the CHSH process dominates. However, for long-wavelength materials (i.e., materials designed to emit laser light at longer wavelengths), the band gap may be smaller, and therefore, the CHSH process becomes less likely or even impossible. In this case, the CHCC process may dominate.

[0032] Figure 3 FIG3 is a general schematic diagram of a known quantum well laser 30 having a quantum well structure 32 configured so that when a voltage is applied to electrodes (not shown) of the laser, electrons and holes recombine in the quantum well structure to produce photons. 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) so that the CHSH process dominates over the CHCC process. Figure 3a The band gap diagram shows 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, making the quantum well structure 32 charge neutral.

[0033] Figure 4A quantum well laser 40 according to a first embodiment is schematically shown. The quantum well laser 40 comprises a layer structure 41 including an active layer in the form of a first quantum well structure 42, which is configured such 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 in that it is designed to operate at a relatively short wavelength (e.g., a near-infrared wavelength such as 1.55 μm or 1.3 μm), so that the CHSH process dominates over the CHCC process.

[0034] The quantum well structure 42 is configured to trap electrons and holes. Thus, the electrons can recombine with the holes and emit photons. Thus, the quantum well structure 42 is the active well of the laser 40.

[0035] 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.

[0036] The second and third quantum well structures 43 and 44 are designed to accommodate holes rather than electrons. That is, the quantum well structures 43 and 44 act as "traps" for holes rather than electrons. Since the second and third regions capture holes but not electrons, the carriers stored therein do not undergo any recombination and therefore do not emit photons, and thus can be referred to as "inactive."

[0037] Figure 7 One possible alloy structure for realizing a hole-assisted well is shown, along with the corresponding calculated bandgap diagram. However, it should be understood that this example is not intended to be limiting, and that other alloy structures may be used to realize a suitable assisting well for holes.

[0038] return Figure 4 , note that the second quantum well 43 and the third quantum well structure 44 are close enough 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. For illustration purposes, Figure 4a A simplified diagram of the band gap diagram of the layer structure 41 is shown in . As can be seen, a large number of holes are trapped in the second and third quantum wells 43 , 44 .

[0039] In general, the device 40 is charge neutral, so 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 quantum well structure 42, the second quantum well structure 43, and the third quantum well structure 44, respectively. That is, n = p + p2 + p3. It can be seen that 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. In other words, due to the presence of the second and third quantum well structures 43 and 44 that trap holes, the charge of the first quantum well structure is unbalanced (there are fewer holes than electrons). Since the rate of the CHSH Auger process is related to p 2 is proportional to n, so Figure 3 The rate of the CHSH Auger process in quantum well laser 40 has been reduced compared to the quantum well laser 30 shown. It will be appreciated that as the hole density p decreases, the electron density n should be increased to achieve lasing so that the product np remains approximately the same. However, since p 2 As n decreases, the rate of the CHSH Auger process decreases.

[0040] The rate of the CHCC process is related to n 2 is proportional to p and therefore increases due to charge imbalance (because there are more electrons than holes). However, as described above, in the quantum well laser 40, the CHSH process dominates over the CHCC process, thereby reducing the overall Auger recombination.

[0041] For longer wavelength lasers, the CHCC process may dominate over the CHSH process. In this case, the second and third quantum well structures can be designed to trap electrons rather than holes.

[0042] Figure 5 A longer wavelength quantum well laser 50 according to a second embodiment is schematically shown. The quantum well laser 50 comprises a layer structure 51 including an active layer in the form of a first quantum well structure 52, which is configured such that electrons and holes recombine in the quantum well structure 52 to generate photons. The quantum well structure 52 is designed for longer wavelengths (e.g., mid-infrared wavelengths, e.g., above 2-3 μm), such that the CHCC process dominates over the CHSH process.

[0043] The quantum well structure 52 is configured to trap electrons and holes. Thus, the electrons can recombine with the holes and emit photons. Thus, the quantum well structure 52 is the active well of the laser 50.

[0044] The layer structure 51 further comprises a second quantum well structure 53 and a third quantum well structure 54 , which are located on opposite sides of the first quantum well structure 52 .

[0045] The second quantum well structure 53 and the third quantum well structure 54 are designed to accommodate electrons rather than holes. That is, the quantum well structures 53 and 54 act as "traps" for electrons rather than holes. Since the second and third regions capture electrons but not holes, the carriers stored therein do not undergo any recombination and therefore do not emit photons, and thus can be referred to as "inactive."

[0046] Figure 8 One possible alloy structure for realizing an auxiliary well for holes is shown, along with the corresponding calculated band gap diagram. However, it should be understood that this example is not intended to be limiting, and other alloy structures may be used to realize suitable auxiliary wells for electrons.

[0047] return Figure 5a , note that the second quantum well 53 and the third quantum well structure 54 are close enough 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. For illustration purposes, Figure 5a A simplified diagram of the band gap diagram of the layer structure 51 is shown in . As shown, a large number of holes are trapped in the second and third quantum wells 53 , 54 .

[0048] Overall, the device 50 is charge neutral, so 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. That is, p = n + n2 + n3. It can be seen that 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. In other words, due to the presence of the second and third quantum well structures 53 and 54, the charge of the first quantum well structure 52 is unbalanced (there are fewer electrons than holes). Since the rate of the CHCC Auger process is related to n 2 is proportional to p, so Figure 3 The rate of the CHCC Auger process in the quantum well laser 40 has been reduced compared to the quantum well laser 30 shown. The rate of the CHSH process is related to p 2 n is proportional to n and therefore increases due to charge imbalance. However, as described above, in the quantum well laser 50, the CHCC process dominates over the CHSH process, thereby reducing the overall Auger recombination.

[0049] As described above, various embodiments of the present disclosure provide second and third quantum well structures that are located close enough to the first quantum well structure to allow charge carriers to move between the first, second, and third structures. However, if the second and third quantum well structures are too close to the first quantum well structure, Auger recombination may occur between the different quantum well structures, which may reduce efficiency. For example, in Figure 4 In the case of , the electrons in the first quantum well structure 42 and the corresponding holes in the second or third quantum well structure 42, 43 may recombine non-radiatively. Figure 5 In the example shown in FIG. 5 , holes in the first quantum well structure 52 and corresponding electrons in the second or third quantum well structures 53 , 54 can recombine non-radiatively.

[0050] To avoid this, the second quantum well structure and the third quantum well structure can be arranged close enough to the first quantum well structure to allow charge carriers to move between them, but far enough away 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 structures.

[0051] exist Figure 4 In the example of , the second and third quantum well structures 43, 44 are designed to accommodate holes rather than electrons, that is, the second and third quantum well structures 43, 44 act as "traps" for holes rather than "traps" for electrons. However, even if the second and third quantum well structures 43, 44 capture some electrons, if the quantum well structures 43, 44 capture more holes than electrons, a reduction in Auger recombination can still be achieved. Similarly, even if Figure 5 The quantum well structures 53 and 54 capture some holes. If the quantum well structures 53 and 54 capture more electrons than holes, then Figure 5 The reduction of Auger recombination can still be achieved in the example.

[0052] During fabrication, layers comprising quantum well structures configured to capture only one type of charge carrier (i.e., electrons or holes) or to capture more of one type of charge carrier than the other can be generated under tensile strain or in unstrained conditions. Because tensile strain increases the density of states, more of one type of charge carrier can be trapped in the tensile strained layer. Thus, the second and third quantum well structures can include tensile strained layers. The active layer can be generated under compressive strain, i.e., the first quantum well structure can include a compressive strained layer.

[0053] although Figure 4 and Figure 5 The second and third quantum well structures are shown separately, but Auger recombination may still be reduced if one of the second or third quantum well structures is omitted.

[0054] Figure 6A semiconductor optical device 60 according to a third embodiment is schematically shown. The device may include a semiconductor laser or an amplifier. The device includes a first region 62, which includes an active region. The active region is configured such that when a voltage is applied to an electrode (not shown) of the device, electrons and holes recombine in the active region to generate photons. The device also includes at least one second region, which includes a quantum well structure 63 configured to preferentially capture either electrons or holes. For example, the quantum well structure 63 can be configured to capture only electrons, only holes, or different numbers of electrons and holes (i.e., the quantum well structure does not capture the same number of electrons or holes). The quantum well structure 63 is arranged at a distance sufficiently close to the first region 62 so that when a voltage is applied to the device 60, a charge imbalance is generated in the first region 62, thereby reducing Auger recombination in the first region. The second region can 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, the at least one second region may include two quantum well structures located on opposite sides of the first region 62 .

[0055] In some embodiments, the semiconductor optical device 60 may include additional structures, such as those described in U.S. Pat. No. 8,937,978. The first region 62 may have a first band gap, the first band gap being arranged such that the ratio of electrons and holes that recombine across the first band gap to emit photons decreases as the temperature increases. The additional structure includes one or more additional band gaps, the first and one or more additional band gaps being arranged such that the ratio of electrons and holes that recombine across the first band gap to electrons and holes that recombine across the one or more additional band gaps increases as the temperature increases. Within the temperature range over which the semiconductor optical device is expected to operate, the increased ratio compensates for the decreased ratio, thereby reducing the temperature-dependent variation in the rate at which electrons and holes recombine across the first band gap to emit photons.

[0056] In this way, a low-threshold semiconductor laser can be provided whose output power has reduced sensitivity to temperature.

[0057] Many modifications and variations will be apparent to those skilled in the art and are intended to fall within the scope of the following claims.

Claims

1. A semiconductor optical device, comprising: a first region comprising an active region configured such that when a voltage is applied to the device, electrons and holes recombine in the active region to generate photons; and at least one second region comprising a quantum well structure configured to capture only electrons, only holes, or different numbers of electrons and holes, The second region is arranged at a distance sufficiently close to the first region so that a charge imbalance is generated in the first region when a voltage is applied to the device, thereby reducing Auger recombination in the first region.

2. The semiconductor optical device according to claim 1, wherein The second region includes a type II quantum well structure configured to capture only electrons or only holes.

3. The semiconductor optical device according to claim 1 or 2, wherein: The first region includes a quantum well structure configured to capture both electrons and holes.

4. The semiconductor optical device according to claim 3, wherein The first region includes an I-type quantum well structure.

5. The semiconductor optical device according to any one of the preceding claims, wherein: The second region is arranged at a distance from the first region that is sufficiently large to prevent or limit Auger recombination between first charge carriers in the first region and second charge carriers in the second region.

6. The semiconductor optical device according to any one of the preceding claims, 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 in the first region, wherein the charge imbalance causes a rate of the first Auger recombination process to increase and a rate of the second Auger recombination process to decrease, such that overall Auger recombination in the first region is reduced.

7. A semiconductor optical device as claimed in any one of the preceding claims, comprising at least two second regions located on opposite sides of the first region, wherein each second region is arranged at a distance sufficiently close to the first region so that when a voltage is applied to the device, a charge imbalance is generated in the first region, thereby reducing Auger recombination in the first region.

8. The semiconductor optical device according to any one of the preceding claims, wherein: The second region includes a tensile strained layer.

9. The semiconductor optical device according to any one of claims 1 to 7, wherein: The second region includes an unstrained layer.

10. The semiconductor optical device according to any one of the preceding claims, wherein: The first region includes a compressively strained layer.

11. The semiconductor optical device according to any one of the preceding claims, wherein: The second region includes a quantum well structure, which is 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 captured by the quantum well structure are of the same type, wherein the charge carriers of the same type are one of electrons or holes.

12. The semiconductor optical device according to any one of the preceding claims, wherein: The first region has a first band gap, the first band gap being arranged such that a proportion of electrons and holes that recombine across the first band gap decreases with increasing temperature, the device comprising: an additional structure having one or more additional band gaps, the first and one or more additional band gaps being arranged such that a ratio of electrons and holes recombining across the first band gap to electrons and holes recombining across the one or more additional band gaps increases as the temperature of the device increases, Wherein, within the temperature range in which the semiconductor optical device is expected to operate, the increased ratio compensates for the decreased ratio, thereby reducing the temperature variation of the rate at which electrons and holes recombine across the first band gap to emit photons.

13. A semiconductor optical device as claimed in any one of the preceding claims, wherein: The device includes a semiconductor laser or a semiconductor optical amplifier.

14. A semiconductor optical device as claimed in any one of the preceding claims, wherein: The first region comprises a quantum well, and wherein the device comprises a quantum well laser.

15. The semiconductor optical device according to any one of the preceding claims, wherein: The second area is an inactive area.

16. A semiconductor optical device as claimed in any one of the preceding claims, wherein: The first region is the only active region of the device.

17. A method for manufacturing a semiconductor optical device, comprising: A first semiconductor region and at least one second semiconductor region are formed, wherein: The first region includes an active region configured such that when a voltage is applied to the device, electrons and holes recombine in the active region to generate photons, and At least one second region includes a quantum well structure configured to capture only electrons, only holes, or different numbers of electrons and holes, each or the second region being arranged at a distance sufficiently close to the first region so that when a voltage is applied to the device, a charge imbalance is generated in the first region, thereby reducing Auger recombination in the first region.

18. The method of claim 17, comprising forming the quantum well structure under tensile strain or in a substantially strain-free condition.

19. A method as claimed in claim 17 or 18, comprising forming the first region under compressive strain.

20. The method according to any one of claims 17 to 19, wherein: The second area is an inactive area.

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