GB2442767A

GB2442767A - A vertical cavity surface emitting optical device

Info

Publication number: GB2442767A
Application number: GB0619976A
Authority: GB
Inventors: Torsten Wipiejewski
Original assignee: Firecomms Ltd
Current assignee: Firecomms Ltd
Priority date: 2006-10-10
Filing date: 2006-10-10
Publication date: 2008-04-16
Also published as: GB0619976D0; WO2008043526A1

Abstract

A vertical cavity surface emitting optical device has an optical confinement structure, for example, a top electrode 50, 55, or a mesa structure formed in the upper layers or, an ion implanted region. The optical confinement structures are specially shaped to provide an optical output that has a shape orthogonal to the beam axis of the optical device which shape is substantially non-circular, e.g. polygonal, so as to enable direct projection of predetermined, complex shapes and patterns of light in the form of letters, numbers, symbols, logos, trademarks and other information bearing patterns. The device may also comprise regions of different reflectivity within a Bragg reflector formed on the top side and or bottom side of the cavity (Figure8). The device may also include an external or integrated optical lens.

Description

SURFACE EMITTING OPTICAL DEVICES

The present invention relates to vertical cavity surface emitting optical devices such as vertical cavity surface emitting lasers (VCSELs).

VCSELs differ from conventional edge emitting lasers in that the resonant cavity is not formed. by the natural cleavage planes of the semiconductor material but is formed (usually) by epitaxially produced distributed Bragg reflector (DBR) mirrors.

For reference, a schematic diagram of a VCSEL is shown in Figure 1. An active region 1 is sandwiched between a p-type DBR 2 and an n-type DBR 3. Both DBRs are highly reflecting. The device is grown epitaxially onto, for example, a GaAs substrate 4. Contact electrodes such as n-and p-contacts 6 and 7 respectively conduct current through the device, the current being confined to a small volume by an aperture 5 in an oxide layer 8. The cavity of the VCSEL is much smaller than that of an edge emitter -e.g. of the order of 1 wavelength (i.e. < 1 micron) -compared to several hundred microns for a conventional edge emitter.

The n-type distributed Bragg reflector 3 (hereinafter also referred to as the n-DBR) typically comprises multiple pairs of alternating A./4n layers 9, 10 of suitable material such as AlAs / A1(0.5)Ga(0.5)As, where ? is the optical output wavelength of interest and n is the refractive index of the constituent layer at the wavelength of interest. The n-DBR is typically lattice matched to the GaAs substrate 4.

On top of the n-DBR 3 is grown the active region I. Typically, the active region I comprises a number of layers forming a succession of quantum wells, not shown, bounded by cladding layers 14, 15.

The p-type distributed Bragg reflector mirror 2 (hereinafter also referred to as the p-DBR) typically comprises multiple pairs of alternating A/4n layers 11, 12 of suitable material such as Al(0.95)GaAs / Al(0.5)GaAs, where X and n are as defined above.

Disposed within the lower layers of the p-DBR is the current confinement aperture 5 defined in the oxide layer 8.

On top of the p-DBR 2, a p-type contact 7 is formed, typically of suitable metal layers such as Ti, Pt and Au.

Using photolithography and etching process steps well known in the art, the p-DBR 2 together with the oxide layer 8 and p-type contact 7 are formed as a mesa structure 16. The current confining aperture 5 is typically formed by lateral oxidation of the oxide layer 8 leaving a central region or aperture' 5 unoxidised in the centre of the mesa. Drive current between the top electrode (p-contact 7) and the bottom electrode (n-contact 6) for the active region 1 is laterally confined by this oxide current confinement aperture'.

In plan view, the mesa 16 and current confinement aperture are circular. More specifically, the p-type contact defines a circular ring around the peripheral edge of the mesa thereby defining a circular optical emission window' 17 that provides the optical output 18 of the device when in operation. Typical diameters of the emission window are of the order of 5-20 microns.

Other forms of electrical isolation or current confinement structures are known in the art rather than the mesa structure 16 with laterally grown oxide aperture 5.

For example, in figure 2, an implanted current confinement aperture is shown. The n-type contact 26, substrate 24, n-DBR 23, active region 21 with cladding layers 34 and 35, p-DBR 22, p-type contact 27 and optical emission window 37 for optical output 38 may be the same or similar in function to those described in connection with figure 1. However, instead of providing electrical isolation and current confinement by way of a mesa structure and current confining oxide aperture, an ion-implanted region 39 of the p-DBR renders it effectively electrically non-conducting in at least the forward conduction direction. Thus, drive current between the top electrode (p-type contact 27) and the bottom electrode (n-type contact 26) for the active region I is laterally confIned by this implanted portion of the upper DBR layers.

Like the arrangement of figure 1, in plan view, the current confined portion of the device (upper DBR 22) is conventionally circular. As shown in the perspective view of figure 3, the p-type contact 27 defines a circular ring around a peripheral edge of the current confined portion thereby defining the circular optical emission window' 37 that provides the optical output 38 of the device when in operation.

As shown in figure 3, the p-type contact typically includes an electrically conductive track 30 leading to a bond pad 31 for wire bonding to a suitable carrier or substrate for external electrical connection.

The prior art VCSELs have typically been directed towards providing an optical output having a high degree of symmetry, i.e. a substantially circular beam profile.

For example, conventional VCSEL output may be a single transverse mode output in which the output beam 18, 38 has a circular profile with the intensity distribution orthogonal to the beam axis (the propagation axis in the direction of the arrows of figures 1 to 3) which follows a Gaussian distribution. Alternatively, conventional VCSEL output may be a circular profile multi-mode output in which the output beam 18, 38 has a multi-lobed intensity distribution orthogonal to the beam axis that follows a higher order distribution with multiple radial axes of symmetry.

The formation of VCSEL devices with circular optical apertures has hitherto been required for conventional use of the devices, for example in coupling the outputs to external optical elements such as lenses, waveguides and the like.

it is an object of the present invention to provide VCSEL devices suitable for new and varied applications.

According to one aspect, the present invention provides a vertical cavity surface emitting optical device comprising a cavity adapted for generating optical output, the device having an optical confinement structure that has a shape orthogonal to the beam axis of the optical device which shape is substantially non-circular.

According to another aspect, the present invention provides a vertical cavity surface emitting optical device comprising a cavity adapted for generating optical output, the device having an optical confinement structure that defines a beam shape orthogonal to the beam axis.of the optical device, the device further including, within the boundary of the optical confinement structure as viewed on the optical axis, at least one region of different reflectivity so as to locally modify the optical output intensity within that region.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic cross-sectional diagram of a conventional VCSEL structure with a mesa-based electrical confinement structure; Figure 2 is a schematic cross-sectional diagram of a conventional VCSEL structure with ion implanted electrical confinement structure; Figure 3 is a perspective schematic view of the conventional VCSEL structure of fIgure 2; Figures 4a and 4b are schematic plan views of alternative shapes of optical confinement structures suitable for modifying the shape of optical output of a VCSEL; Figures 5a and 5b are schematic plan views of alternative shapes of optical confinement structures suitable for modifying the shape of optical output of a VCSEL; Figure 6 is a schematic plan view of an array of VCSEL devices suitable for generating a display matrix for displaying alphanumeric characters; Figure 7 is a schematic plan view of a VCSEL having an optical confinement structure and plural regions of different reflectivity for modifying the optical output of the device of figure 4a; and Figure 8 is a schematic cross-sectional view of a VCSEL with a surface structure suitable for providing the regions of different reflectivity of figure 7.

One application for VCSEL devices proposed herein is to provide shaped light beams which can be projected in space, for example as display pointers, information displays and the like. It has been recognised that projection of such shaped light beams can be effected directly by the device without the use of complex optical sub-systems such as lenses and gobos or the like.

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The devices as described in connection with figures 1 to 3 are modified to shape the light emission aperture 37 into a non-circular or polygonal form so that direct projection and display of suitable shapes, logos and information patterns is possible from the device itself. More particularly, polygonal shapes having four or more sides are particularly preferred.

With reference to figure 4a, in a preferred arrangement, the p-type contact 40 is configured to define a polygonal optical emission window' 41 that provides the optical output of the device when in operation. In this arrangement, the optical emission window 41 is arrow shaped, ideally suited for a laser pointing device.

With reference to figure 4b and figure 6, the optical emission windows 42 are rectangular. Plural optical devices 60 are formed onto a single substrate 61 in an array, each having the rectangular shape. Figure 6 illustrates the electrical contacts 43, 44 and conductive traces 45 required to drive each device independently. The driver circuitry may be located off-chip' by suitable electrical connection to bond pads 46. Alternatively, the driver circuitry could be located on-chip' by suitable circuitry formed elsewhere on the substrate 61 using known integration techniques.

The array of devices may be a regular array or irregular array for producing more complex projection display patterns. The example of figure 6 illustrates an irregular seven segment array useful for presenting a display matrix suitable for displaying alphanumeric characters.

Figure 5 illustrates further p-contact layouts 50, 55, in which the p-type contact layer 50, 55 defines a star shaped optical emission window 56 or plural optical emission windows 51 of various shapes in order to generate particular logos, characters or pictures. These arrangements are particularly useful for generating projected advertising displays, for example.

Various ways of producing a required optical confinement structure in the shape of the optical output required can be used. In the examples discussed above, the upper p-type contact 7, 27 of figures 1 and 2 has been adapted in shape to provide the requisite shaped window. As an alternative, or in addition, the optical confinement

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structure may be effected by shaping at least the upper layers of the device, e.g. by forming a mesa in the shape desired, such as the shapes illustrated in figures 4 to 6.

As another alternative, the optical confinement structure may be effected by shaping the ion implanted region 39 (figure 2) to define the optical output shape desired.

Thus, in a general aspect, it will be understood that these various techniques serve to define one or more optical confinement structures that each have a shape, orthogonal to the beam axis of the optical device, which shape is non-circular and preferably polygonal. The expression optical confinement structure' is intended to encompass any feature that can be used to define the lateral extent of the optical output (transverse to the beam axis), such as a non-transparent upper contact, a mesa shape, an ion implant layer etc. Such features can also be used in combination with one another.

It will also be understood that electrical confinement may be used to assist in the optical confinement process. For example, the oxide aperture 5 described in connection with figure 1 can be modified into the desired shape, e.g. by lateral oxidation from a mesa shape that also defines the desired shape. Similarly, an ion imp]anted current confinement layer 39 as described in connection with figure 2 can be modified into the desired shape.

Such optical confinement structures and electrical confinement structures can generally be defined using photolithographic processes and therefore can conveniently and economically be modified into complex shapes. Particularly, the upper contact 40, 50, 55, defining the optical emission window can readily be formed by thin film metal deposition, photolithography and etch steps to form an opaque border to the upper surface of the device, defining the window that defines the optical output shape.

Further definition or modulation of the optical emission is possible. In figure 7, a generally polygonal shaped optical emission window 70 is provided with plural regions 71 of different reflectivity to that of the main window area 72. Thus, the optical output of the device varies across the optical confinement structure as a

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function of the different reflectivities. Each region 71 may have the same reflectivity as each other region 71, albeit different to the reflectivity of the main window area 72. Alternatively, some or each of regions 71 may have different reflectivities, e.g. to provide a gradual change in optical output intensity of the plural regions 71 from one end of the shape to another. For example, the device could be configured to project an optical output beam in the form of an arrow with dots that have increasing intensity towards the pointed end of the arrow.

The regions of different reflectivity within the confines or boundary of the optical confinement structure can be conveniently formed, again using photolithographic processes, by modifying the composition and/or thickness of one or more layers in the device structure.

With reference to figure 8, preferably an anti-reflection surface coating layer 80 is formed on the top surface of p-DBR 82 and this antireflection coating layer is at least partially, or possibly wholly etched away in designated regions 88 to iocally increase optical output in those regions 88. Alternatively, the top layer or layers of the p-DBR 82 could be etched away in the designated regions 88 to create reduced reflectivity and therefore increased optical output in the desired pattern. The other features in figure 8, such as the oxide layer 83 defining a current confinement aperture, active region 84, n-DBR 85, substrate 86 and n-contact 87 may be fabricated in similar manner to the corresponding features of figures 1 and 2.

it will be understood that the regions 88 of different reflectivity lie within the boundary of the optical confinement structure as viewed on the optical axis of the device, although not necessarily in exactly the same plane as the optical confinement structure. For example, the optical confinement structure can comprise the contact electrode 81 and the window formed therein, whereas the regions of different reflectivity could be defined by the p-DBR 82 or n-DBR 85 at lower layers of the device.

Although embodiments of the invention have been described based on suitable modification of the general device structures of figures 1 to 3, it will understood that other constructions of VCSEL device could be modified in similar manner. Further, the device structure as described in connection with figures 1 and 2 could be inverted.

in other words, the substrate 4 could be a p-type substrate, bottom DBR 23 could be a p-type mirror, and top DBR 22 could be an n-type mirror.

Although preferred embodiments comprise vertical cavity surface emitting optical devices having coherent laser output, it will be understood that the techniques described can also be used for incoherent surface emitting optical devices such as resonant cavity LEDs (RECLEDs).

The VCSELs described herein may be used to form projection devices for projecting predetermined, complex shapes and patterns of light in the form of letters, numbers, symbols, logos, trademarks and other information bearing patterns. Such projection devices may be able to forego conventional focussing optics, shadow masks, gobos, dc, or may be further enhanced thereby. Therefore, in preferred arrangements, the optical output of the vertical cavity surface emitting optical devices as described here would be in the visible range of the optical spectrum, e.g. 400 nm to 700 nm wavelength, or even 380 mu to 780 mu. However, other wavelengths of optical output could be used for other purposes.

Although the vertical cavity surface emitting optical devices as described here can be used to avoid the need for additional optical elements such as lenses and the like, it will be understood that the devices could also be formed with or used with additional optical elements such as lenses. Such additional optical elements could be integrated into the device or be separate external elements attached thereto.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

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I. A vertical cavity surface emitting optical device comprising a cavity adapted for generating optical output, the device having an optical confinement structure that has a shape orthogonal to the beam axis of the optical device which shape is substantially non-circular.
2. The vertical cavity surface emitting optical device of claim I further including an electrical confinement structure that has a cross-sectional shape orthogonal to the beam axis which shape is substantially non-circular.
3. The vertical cavity surface emitting optical device of claim I or claim 2 in which the optical and/or electrical confinement structure has a shape orthogonal to the beam axis of the optical device that is a polygon.
4. The vertical cavity surface emitting optical device of claim 3 in which the polygon is a rectangular shape, an arrow shape or a star shape.
5. The vertical cavity surface emitting optical device of claim I or claim 2 in which the optical confinement structure includes a metal contact layer forming a window that defines said shape.
6. The vertical cavity surface emitting optical device of claim 3 further including an array of such devices formed on a single semiconductor substrate, each device having said polygonal optical and/or electrical confinement structure.
7. The vertical cavity surface emitting optical device of claim 6 including a plurality of devices in the array each having a substantially rectangular optical and/or electrical confinement structure, the devices being disposed on the substrate so as to present a display matrix suitable for displaying alphanumeric characters.
8. The vertical cavity sirface emitting optical device of claim 7 further including circuitry for enabling independent drive of each device in said display matrix.
9. The vertical cavity surface emitting optical device of claim I in which the device cavity includes one or more regions, within the boundary of the optical confinement structure as viewed on the optical axis, of different reflectivity so that, in use, the optical output varies across the optical confinement structure as a function of the different ref lectivities.
10. The vertical cavity surface emitting optical device of claim 9 in which the one or more regions of different reflectivity are defined within a distributed Bragg reflector formed on the top side and/or bottom side of the cavity.
11. The vertical cavity surface emitting optical device of claim 9 in which the one or more regions of different reflectivity are defined by a surface layer formed on the top of the cavity.
12. The vertical cavity surface emitting optical device of any preceding claim in which the device is a laser.
13. The vertical cavity surface emitting optical device of any one of claims 1 to 11 in which the device is a resonant cavity LED.
14. The vertical cavity surface emitting device of any preceding claim in which the optical output is in the visible range of the optical spectrum.
15. A projection device for projecting a light beam having a predetermined polygonal shape comprising the vertical cavity surface emitting optical device of claim 3.
16. A projection device for projecting a plurality of light beams each having predetermined polygonal shapes comprising the anay of vertical cavity surface emitting devices of claim 6 or claim 7.
17. The projection device of claim 16 in which each of said plurality of devices forms a seven segment indicator display.
18. A vertical cavity surface emitting optical device comprising a cavity adapted for generating optical output, the device having an optical confinement structure that defines a beam shape orthogonal to the beam axis of the optical device, the device further including, within the boundary of the optical confinement structure as viewed on the optical axis, at least one region of different reflectivity so as to locally modif' the optical output intensity within that region.
19. The vertical cavity surface emitting device of claim 18 including plural said regions of different reflectivity.
20. The vertical cavity surface emitting device of claim I further including an external optical lens coupled thereto for receiving said optical output.
21. The vertical cavity surface emitting device of claim I further including an integrated optical lens to receiving said optical output from the cavity.
22. An optical device substantially as described herein with reference to the accompanying drawings. Ii