WO2004095662A2

WO2004095662A2 - Bandgap engineering

Info

Publication number: WO2004095662A2
Application number: PCT/GB2004/001727
Authority: WO
Inventors: Yee Loy Lam; Yuen Chuen Chan
Original assignee: Denselight Semiconductors Pte Ltd; Finnie, Peter, John
Priority date: 2003-04-23
Filing date: 2004-04-23
Publication date: 2004-11-04
Also published as: WO2004095662A3; GB0309228D0

Abstract

A method for achieving large localized bandgap energy differences at the wafer-level scale, with fine bandgap control, through a combination of regrowth and quantum well intermixing processes. The technique allows fabrication of a photonic integrated circuit on a wafer, wherein epitaxial layers of different composition are formed on separate regions to optimise the associated energy bandgap at a different centre wavelength. Quantum well intermixing of those parts of the structure containing quantum wells allows localised fine tuning of the bandgap, either to correct for inaccuracies during deposition or growth, or intentionally to detune the bandgap to achieve a certain functionality such as greater transparency or responsivity.

Description

BANDGAP ENGINEERING

Field of the Invention

The present invention relates to a technique for bandgap engineering in the fabrication of photonic integrated circuits and in particular, a technique for achieving both large regional bandgap differences and fine control of more local differences.

Introduction to the Invention

The development of photonic integrated circuits (PICs) is being advanced intensely because of the potential of additional functionality, compact size and other attendant advantages associated with monolithic integration.

Monolithic PICs involve the integration of both active devices, such as laser diodes and photodetectors, and passive devices, such as waveguides, couplers and spot-size converters. To achieve optimal performance from each device, the energy bandgap of the material must be varied locally to meet the requirement of the device. For example, the energy bandgap of a passive device should be made larger than the laser source so as to realize low optical absorption loss. On the other hand, the energy bandgap of a monitoring photodetector will have to be matching or smaller than that of the laser source to obtain high responsivity. There are a number of techniques available to change the energy bandgap locally on a chip or wafer. In one such technique, the wafer undergoes repeated cycles of growth, lithographic patterning and etching before the next growth step over the patterned wafer. Although this growth and regrowth technique is laborious, it nevertheless remains the most commonly used technique. A second technique uses selective area epitaxy to achieve modification of the bandgap. Here, a dielectric mask is patterned onto a wafer, leading to growth only on the exposed regions. By varying the width and separation of such dielectric films, it is possible to control the thickness of the epitaxial layers grown in between the dielectric mask, since the source atoms migrate from the dielectric mask towards the edge to the semiconductor surface. Hence, a smaller separation between two dielectric patterns leads to a thicker epitaxial layer and, when coupled with the quantum well structure, the effective energy bandgap is decreased. This selective area epitaxy technique requires very precise control of the epigrowth conditions and is not easily managed.

A further technique for bandgap tailoring is through quantum well intermixing (QWI). Here, vacancy defects are introduced into the semiconductor crystal lattice to promote interdiffusion of atoms between the quantum well and barrier layers. A consequence of the intermixing process is that the rectangular shaped quantum well profile becomes graded and the effective bandgap of the intermixed quantum well is increased. The intermixing process is usually followed by an annealing step to restore good crystallinity to the semiconductor.

There are many known approaches to activating the intermixing process. Such techniques include employing impurities (as in impurity-induced disordering), dielectric films (as in impurity-free vacancy disordering), laser light (as in laser induced disordering) and plasma. Quantum well intermixing as a post-growth bandgap tuning technique is both simple and relatively easy to control.

The QWI process is typically performed on the full device structure, which may comprise multiple epitaxial layers such as cladding and waveguide, contact layer, p-doped cladding, undoped active waveguide and n-doped cladding. Figure 1 shows such a structure after complete epitaxial growth. As a (multiple) quantum well based optical waveguide is normally situated more than a micron below the surface, vacancies have to be generated and diffused all the way down to the quantum well layers.

The wavelength shift realizable in QWI is determined primarily by the material bandgap energy difference between the quantum well and barrier layers. For the InGaAsP based material system, a bandgap shift of 200nm is achievable for quantum well structures emitting at 1.55 μm. However, the achievable bandgap shift is much reduced for similar InGaAsP based quantum well structures emitting at 1.31 μm, a result of material composition constraints for the quantum well and barrier layers. Consequently, it is very difficult to integrate devices operating over a wide wavelength range by simply using quantum well intermixing. This is particularly true where the devices are intended for use within different ITU telecommunications bands such as the short (S), centre (C) and long (L) wavelength bands, each of which span some 5 THz.

There is therefore a need for a technique by which devices operating at widely different frequencies and performing quite different functions can be integrated on a single wafer and optically coupled where necessary to form a single photonic integrated circuits

Summary of the Invention

According to a first aspect of the present invention a method of fabricating a photonic integrated circuit on a wafer comprises the steps of: forming a base structure having a layer with a first bandgap energy, at least a portion of the base structure including a quantum well; removing regions of the base structure by photolithography, masking and etching; performing regrowth in said regions to form material with a second bandgap energy, at least a portion of said regions including a quantum well; and, performing quantum well intermixing on portions of the wafer to tune the local bandgap energy.

Using this technique a photonic integrated circuit can be fabricated on a wafer, wherein epitaxial layers of different composition are formed on separate regions of the wafer with the intention that the energy bandgaps of the different regions are optimised for a different centre wavelength. Quantum well intermixing of those parts of the structure that contain quantum wells allows localised fine tuning of the bandgap, either to correct for inaccuracies during deposition or growth, or intentionally to detune the bandgap to achieve a certain functionality such as greater transparency or responsivity.

If regions with more than two different bandgap energies are required, the material removal and regrowth steps can be repeated.

Therefore, preferably, the step of performing quantum well intermixing is preceded by the further steps of: removing regions of the wafer structure by photolithography, masking and etching; and, performing regrowth in said regions to form material with a third bandgap energy, at least a portion of said regions including a quantum well. The final layers of the structure may be formed either before or after the quantum well intermixing step. However, the QWI process is more difficult to perform afterwards because of the penetration depth required.

Preferably, the method further comprising the step of forming one or more upper layers of the photonic integrated circuit after performing the step of quantum well intermixing. Preferably, the upper layers include at least one of a cladding layer and a contact layer.

There are many techniques suitable for forming the base structure. Preferably, the base structure is formed by metal-organic chemical vapour deposition (MOCVD). Alternatively, the base structure may be formed by molecular beam epitaxy (MBE). The step of QWI may be performed either sequentially on individual regions of the wafer or in a single step on the whole wafer, thereby simultaneously intermixing all regions of the wafer that contain quantum wells.

Preferably, QWI is simultaneously performed on all regions of the wafer that contain quantum wells.

There are many techniques for performing QWI, which include employing impurities (as in impurity-induced disordering), dielectric films (as in impurity-free vacancy disordering), laser light (as in laser induced disordering) and plasma. Equally, there are numerous methods for controlling the degree of QWI. Preferably, the degree of quantum well intermixing is determined by the thickness of a mask applied to the wafer. A mask of variable thickness may be used to achieve the required local degree of intermixing.

The QWI process can be used to fine-tune the local bandgap energy, shifting the corresponding wavelength. Preferably, the bandgap energy of a region of the photonic integrated circuit is blue shifted by quantum well intermixing.

The QWI process may also be employed to smooth refractive index discontinuities between a quantum well region and an adjacent region, thereby reducing unwanted reflections at the interface. The adjacent region may be another quantum well region or simply comprise bulk material, as might be used for a passive waveguide.

According to a second aspect of the present invention a photonic integrated circuit fabricated according to the first aspect of the present invention.

The photonic circuit may be designed so those components operating at approximately one centre wavelength are grouped in a different region to those operating at a different centre wavelength. For example, components in two separate regions may be designed to operate at a wavelength in the telecommunications C band and S band, respectively.

Preferably, regions of the photonic integrated circuit with the first bandgap energy operate at a first predetermined optical wavelength and regions with the second bandgap energy operate at a second predetermined optical wavelength.

Within regions of material that have been deposited, or grown, for operation at one particular centre wavelength, QWI may be used for localised fine-tuning of the bandgap. This may be either to correct for inaccuracies during deposition or growth, or intentionally to detune the bandgap so as to achieve certain functionality such as greater transparency or responsivity. Preferably, sub-regions of the first and second regions of the photonic integrated circuit are quantum well intermixed to form photonic devices selected from a group that includes: laser diode, optical amplifier, optical modulator, photodetector, optical switch and passive waveguide. The photonic integrated circuit can be implemented using a variety of well- known material systems.

Preferably, the photonic integrated circuit is formed from the quaternary Indium Gallium Arsenide Phosphide (InGaAsP) material system.

Brief Description of the Drawings

Examples of the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates the structure of a typical InGaAsP photonic device containing a multiple quantum well; and, Figure 2 shows the device of Figure 2 before fabrication of the upper cladding and contact layers.

Detailed Description

The present invention provides a method of achieving large localized bandgap energy differences at the wafer-level scale through a combination of quantum well intermixing and regrowth processes. Through this technique both large energy bandgap differences, as well fine bandgap control, can be realized on the same wafer.

Prior to fabrication the layout of the photonic integrated circuit is designed so as to organise regions with the desired wide range of bandgaps into groups. The different groups will be fabricated by epitaxial growth, whereas constituent bandgaps within each group will be determined by quantum well intermixing.

In the first process step the base device structure is grown by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) and will include a first quantum well structure designed for a first energy bandgap group. Following this, photolithography is used to expose the regions where another energy bandgap group is desired. Semiconductor etching through a dielectric mask is performed to remove the existing quantum well structure. Selective regrowth is regions not masked allows the formation of a new set of quantum well structures with a second energy bandgap. If required, the above process of etching and regrowth can be repeated to fabricate further groups of regions with other energy bandgaps. During fabrication of the base structure, the device structure could be completed up to the upper layers, including the cladding and contact layers, as shown in Figure 1. Alternatively, fabrication of the base structure could stop just above the light confining waveguide layers as shown in Figure 2. While both methods are workable, the disadvantage of base growth up to the cladding and contact layers, as in Figure 1 , is that subsequent regrowth would need to be carried out to replace the cladding and contact layers that were etched away prior to the regrowth. Good continuity of these layers then requires careful control of the regrowth process. However, if the base growth were only carried out up to the waveguide layers of Figure 2, the subsequent regrowth would also only need to be implemented up to the waveguide layers with the new quantum well structure. For this latter case, the substrate would only require one final regrowth over the whole wafer, with any dielectric mask removed, to fabricate the cladding and contact layers. Once the base structure is complete, the next process is to carry out quantum well intermixing onto the wafer so as to realize fine control of a localized bandgap shift. The resulting shift in energy bandgap equates to a blue shift in the corresponding wavelength. By performing QWI on the whole wafer the differing local bandgaps can be engineered simultaneously. Control of the local amount of bandgap shift is achieved by selecting the degree of quantum well intermixing to be implemented. For certain techniques of quantum well intermixing, such as that by ion implantation, the degree of intermixing can be determined by the thickness of an implantation mask deposited on top of wafer.

In addition, quantum well intermixing carried out in the interface region between two sets of quantum well structures can reduce the refractive index discontinuity at the interface. This will minimize the back-reflection from such an interface when light propagates through it.

It should be noted that through this process, the requirement for large localized bandgap differences, such as for a photonic integrated circuit operating in the S, C and L telecommunication bands, can easily be met. In addition, fine control of the localised bandgap, by quantum well intermixing, allows the realization of various functional devices, including amplifier, modulator and detector, operating within each of these bands. Different groups of devices can operate around different centre wavelengths, with individual devices, bandgap engineered for optimal performance. Quantum well intermixing can also help to reduce reflections at interfaces as light propagates from one device to another and from one section to another.

Claims

1. A method of fabricating a photonic integrated circuit on a wafer comprising the steps of: forming a base structure having a layer with a first bandgap energy, at least a portion of the base structure including a quantum well; removing regions of the base structure by photolithography, masking and etching; performing regrowth in said regions to form material with a second bandgap energy, at least a portion of said regions including a quantum well; and, performing quantum well intermixing on portions of the wafer to tune the local bandgap energy.

2. A method according to claim 1, wherein the step of performing quantum well intermixing is preceded by the further steps of: removing regions of the wafer structure by photolithography, masking and etching; and, performing regrowth in said regions to form material with a third bandgap energy, at least a portion of said regions including a quantum well.

3. A method according to claim 1 or claim 2, further comprising the step of forming one or more upper layers of the photonic integrated circuit after performing the step of quantum well intermixing.

4. A method according to claim 3, wherein the upper layers include at least one of a cladding layer and a contact layer.

5. A method according to any of claims 1 to 4, wherein the base structure is formed by metal-organic chemical vapour deposition (MOCVD).

6. A method according to any of claims 1 to 4, wherein the base structure is formed by molecular beam epitaxy (MBE).

7. A method according to any preceding claim, wherein the step of quantum well intermixing is simultaneously performed on all regions of the wafer that contain quantum wells.

8. A method according to any preceding claim, wherein the degree of quantum well intermixing is determined by the thickness of a mask applied to the wafer.

5 9. A method according to any preceding claim, wherein the bandgap energy of a region of the photonic integrated circuit is blue shifted.

10. A method according to any preceding claim, wherein the step of quantum well intermixing smoothes a refractive index discontinuity at an interface between a o quantum well and another region of the photonic integrated circuit.

11. A photonic integrated circuit fabricated according to the method of any preceding claim.

5 12. A photonic integrated circuit fabricated according to claim 11, wherein regions with the first bandgap energy operate at a first predetermined optical wavelength and regions with the second bandgap energy operate at a second predetermined optical wavelength.

0 13. A photonic integrated circuit fabricated according to claim 12, wherein sub- regions of the first and second regions are quantum well intermixed to form photonic devices selected from a group that includes: laser diode, optical amplifier, optical modulator, photodetector, optical switch and passive waveguide.

5 14. A photonic integrated circuit according to any of claims 11 to 13, wherein the photonic integrated circuit is formed from the quaternary Indium Gallium Arsenide Phosphide (InGaAsP) material system.