Patent Application
20150318666
The present invention generally relates to a vertical-cavity surface-emitting transistor laser (T-VCSEL) and methods for producing the same. It also covers the use of the same in applications such as high-speed data communication.
The transistor laser (TL) is a fundamentally new device type that since its first demonstration in 2005 has received significant interest [1]. Unlike the monolithic integration of a transistor and a laser diode on the same chip where the transistor performs as the driver of the laser [2], the TL relies on the fusion of the two components into a single device where the base recombination is used to provide stimulated emission. Due to an altered carrier dynamics in the base/cavity region and the three-terminal configuration, the TL has a number of attractive properties and potential advantages compared to conventional diode lasers, many of which have already been demonstrated and that can be briefly summarized as follows:
Transistor lasers have received significant attention during recent years. Based on the monolithic integration of a heterojunction bipolar transistor (HBT) in a semiconductor laser they provide a number of unique properties as compared to conventional diode lasers, see ref [1, 3]. A particularly attractive feature is the potential for increased laser modulation bandwidth due to the altered charge dynamics in the base region, see ref [6]. Given the growing demand for broadband capacity in optical communication networks this may find important applications. Single-channel data rates of 40 Gbit/s and beyond are, e.g., presently considered for both local area networks and interconnects. Due to the tough requirements on cost- and power-efficiency, vertical-cavity surface-emitting lasers (VCSELs) are the preferred light-sources for these applications. Whereas VCSELs have been demonstrated with 3 dB-bandwidth of 28 GHz, see ref [17] and VCSEL-based optical links with modulation speeds up to 55 Gbit/s [18], this is approaching fundamental limits. To reach such high and even higher modulation rates over an extended temperature range and with sufficient output power, radically new design concepts are required. Transistor-VCSELs (T-VCSELs) and their potential for high-speed modulation were evaluated numerically by Shi et al. [14], and very recently the first experimental demonstration of a T-VCSEL at low temperature was reported [20], including the voltage controlled operation of such lasers [21].
The T-VCSEL combines the functionality and performance advantages of transistor lasers with the inherent advantages of conventional diode-type VCSELs widely used in optical communication networks, such as cost- and power-efficiency, option for two-dimensional arrays, circular beam profile, etc. Hence, it provides the potential for low-cost, low-power consumption digital and analogue applications with improved performance, simplified monitor and driver designs, as well as options for transistor-based circuit design techniques.
Whereas VCSELs with 3 dB-bandwidth of 28 GHz [17] and VCSEL-based optical links with modulation speeds up to 55 Gbit/s have been demonstrated [18], this is approaching fundamental limits. To reach such high and even higher modulation rates over an extended temperature range with sufficient output power suitable for real system environments, radically new design concepts are required, thus opening up an enabling role for T-VCSELs in high-speed data communication.
Similar to conventional VCSELs, T-VCSELs rely on the confinement of the injected carriers to a very small region, typically 10·10 μm2 or less. In conventional VCSEL technology this has been achieved in different ways. VCSEL devices typically consist of a cavity region and an amplifying medium sandwiched between two conducting distributed Bragg reflectors (DBRs) through which the drive current is injected. By making most of the topmost DBR non-conducting except for a very small region, the current can be confined to this area. In the early days of VCSEL technology this was usually accomplished using masked proton implantation whereas modern VCSELs use a selective oxidation process. In this scheme, mesas are etched into the VCSEL structure throughout most of the topmost DBR. These mesas are subject to a wet oxidation process where one of layers in the topmost DBR has a composition which corresponds to a significantly higher oxidation rate as compared to the rest of the DBR stack. In this way, a small aperture is formed that confines the current to the central region of the mesa.
The oxidation-confined design has some specific drawbacks. First, the requirement of conducting DBRs also leads to higher optical loss imposed by the doping. Secondly, the oxidation process leads to expansion and stress around of oxidized layer and thereby reliability concerns. Lasers are minority current-based devices and are thereby extremely sensitive to non-radiative recombination through lattice defects. As a consequence, the oxidation layer is usually put some distance apart from the amplifying medium which results in current spreading and loss of efficiency. Thirdly, the selective oxidation is a difficult-to-control process requiring precise control of timing and temperature to monitor the progress of the oxidation front with micrometer precision. This is in contrast to the general low-cost microelectronic-style processing methods that rely on photolithographic techniques to define lateral dimensions. Nonetheless, oxidation-confinement is presently the technology of choice for mass-fabrication of VCSELs with state-of-the-art performances.
In case of T-VCSELs the situation is more complicated. These are three-terminal devices that require the separate contacting of the emitter, base and collector regions, thereby making the contacting through conducting DBRs more difficult. In particular, the thin base region is very difficult to assess through a thick semiconductor DBR structure. Moreover, a proper operation of the device requires well-controlled biasing of these three regions which is difficult to achieve due to lateral voltage drops inside the device. Finally, due to the necessity of a high doping concentration close to the active gain medium, the structure is more sensitive to optical loss which puts additional requirements on an optimized design.
A three-terminal T-VCSEL is disclosed in U.S. Pat. No. 7,693,195 which provides the layer configuration for a functional device and also discusses its operation but omits any details related to the current confinement. Another configuration of a three-terminal T-VCSEL is disclosed in U.S. Patent Application Publication No. 2013/0177036 which makes use of transparent conducting oxide contacts and selective oxidation for the current injection. Three-terminal VCSELs have also been discussed in the open scientific literature. Shi et al. analyzed the performance of oxidation-confined T-VCSELs and demonstrated the enhanced bandwidth as compared to a conventional VCSEL [19] but since this is a purely numerical study any practical difficulties in realizing such a device is not considered, and no experimental result on such a design has so far been demonstrated. Wu, Feng and Holonyak reported the first experimental realization of T-VCSELs in two papers published in the summer of 2012 [20,21]. However, these T-VCSELs had an insufficient current confinement and could only be operated at high threshold and low power. In a subsequent paper [22], the same group introduced a selective oxidation process to lower the threshold current and increase the optical output power but at expense of a complicated and difficult-to-implement processing scheme.
Consequently, there is a need for a revised T-VCSEL design with a current confinement scheme which allows high-performance operation and also corresponds to a robust fabrication procedure. In addition, there is also a need for a viable confinement technology that is not restricted to selective oxidation, which is difficult to implement for other materials systems than AlGaAs/GaAs.
There is an object to provide a Transistor Vertical Cavity Surface Emitting Laser T-VCSEL that fulfils at least some of these sought for features. There is also an object to provide a method for producing such a T-VCSEL.
According to a first aspect of the invention there is provided a transistor vertical-cavity surface-emitting laser, T-VCSEL, comprising, a collector comprising a bottom substrate and a base contacting layer,
an emitter and a base arranged between the emitter and the collector, and comprising a light emitting active layer, arranged between layers of a base material, and a collector contacting layer. The emitter comprises a current confining blocking layer that is comprising a first material layer, having a first type of doping and comprising a current confining means and a second material layer having a second type of doping and being provided on the base side of the material layer whereby electrical carriers injected from the emitter flow through the current confining blocking layer confined by the current confining means to arrive at the base to activate the light emitting active layer.
According to a second aspect of the invention there is provided a method for producing T-VCSEL. The method comprises the following steps:
depositing a bottom Distributed Bragg Reflector, bottom DBR, on a substrate
depositing, on the bottom DBR, a p-doped collector layer
depositing a base layer, part of which is n-doped and comprises a light-emitting active layer, on top of the p-doped collector layer
depositing first and second p-doped layers on top of the base layer
etching away a volume through the second p-doped layer, thereby creating a central mesa region
covering the central mesa region with a masking material preventing epitaxial regrowth
growing, by epitaxial regrowth, an n-doped layer on top of the central mesa region covered by the masking material;
removing the masking material from the top of the central mesa region
growing, by epitaxial regrowth, a third p-doped layer over both the central mesa region and the n-doped layer
etching away a volume through the third p-doped layer, the n-doped layer, the first p-doped layer, the base layer and partly through the p-doped collector layer, thus exposing part of the p-doped collector layer
etching away a volume through the third p-doped layer, the n-doped layer, the first p-doped layer and partly through the base layer above the light-emitting active layer, thus exposing part of the base layer;
attaching, an electrical contact on the exposed part of the p-doped collector layer, and an electrical contact on the upper faces of the third p-doped layer;
attaching an electrical contact on the exposed part of the base layer; and
depositing a top distributed Bragg Reflector, top DBR, on the third p-doped layer in such a way that said DBR at least overlies said central mesa region.
The proposed technology thus provides a type of Vertical Cavity Surface Emitting Transistor Laser (T-VCSEL). One particular feature of the T-VCSEL relates to the confinement scheme of the current injection which relies on photolithography and epitaxial regrowth rather than selective oxidation. This yields a T-VCSEL with improved properties compared to prior art. For example, the T-VCSEL according to the invention is easy to manufacture and are more reliable since it avoids the mechanical strain imposed by a selective oxidation step. Furthermore, it has specific performance advantages since the confinement means can be positioned closer to the active region. Also, the T-VCSEL according to the invention is not restricted to AlGaAs/GaAs-based materials but can equally well be applied to other materials systems, which extends the range of possible emission wavelengths, and it does not require electrically conducting top and bottom DBRs. This is an advantage since electrically conducting DBRs must be made out of semiconductor materials which restricts the number of available materials and thereby might lead to non-optimal DBR designs. Also, the requirement on electrically conducting DBRs means that they need to be doped which increases their optical absorption and thereby decreases the efficiency of the device. Finally, the T-VCSEL according to the present invention is easier to manufacture than oxidation-confined T-VCSELs since the base is not covered by a thick semiconductor DBR and can therefore be reached with a shallow and well-controlled etching-step.
a shows a diagram giving the room-temperature light-current characteristics and optical spectra for a 10·10-μm2 device according to the present invention. The two spectra have been vertically displaced for clarity.
c gives a diagram of a Temperature-dependent operation of a T-VCSEL according to the invention using IB or VCE as control signals.
The above stated objects together with other objects, advantages and features of the present invention will be more readily understood from the following detailed description of embodiments thereof, when considered in conjunction with the drawings, in which like reference numerals indicate identical structures throughout the several views unless there is explicitly stated otherwise.
The invention will first be described in conjunction with
In
In
In an exemplary embodiment of a T-VCSEL according to above the first material layer 112 and the second material layer 19 are layers of a semi-conducting material.
In a particular embodiment of a T-VCSEL according to the above given embodiments will the first material layer 112 comprise an n-doped material layer and the second material layer 19 will comprise a p-doped material layer.
With reference to
In an exemplary version of this embodiment of a T-VCSEL, the current confining means 111 will comprise a square-shaped section 111 extending through the first material layer 112, and containing at least part of the third material layer 113 to thereby confine carriers injected from the emitter to the base section to flow in section 111. This embodiment of the current confining means is schematically disclosed in
In a possible version of a T-VCSEL comprising a third material layer, is the material of the first material layer 112 GaAs, the material of the second material layer 19 AlGaAs and the material of the third material layer 113 GaAs. This is schematically shown in
Still another possible embodiment of a T-VCSEL is shown in
In all the earlier disclosed embodiments of a T-VCSEL the second material layer 19 can be provided with a reduced or non-uniform doping concentration that is higher closer to the base 16. In this way the lateral resistance of the material layer 19 will be high, thus leading to a low lateral current spreading while the vertical current blocking properties of the current confining layer are preserved.
The invention as described above details a vertical-cavity surface-emitting transistor laser (T-VCSEL) based on a pnp-type blocking layer for current confinement. Beyond this, the invention also embodies variations of the invention where the heterojunction bipolar transistor has an n-p-n doping sequence for the emitter-base-collector regions, respectively, and thereby the blocking layer consequently has an npn configuration. In this case, the corresponding layer and device structure remains the same as the device shown in
In other words, another possible version of a T-VCSEL will be described in what follows, Thus, in
In
In a T-VCSEL schematically shown in
In an exemplary embodiment of a T-VCSEL the base contacting layer 172 of the collector section 13 is a p-doped GaAs layer and the collector contacting layer 171 of the base section 16 is a GaAs layer.
A version of the above described T-VCSEL is schematically shown in
In still another version of a T-VCSEL is the collector contacting layer 171 a GaAs layer and said base contacting region 172 is an n-doped GaAs layer.
In all of the earlier disclosed embodiments of a T-VCSEL the second material layer 19 can be provided with a reduced or non-uniform doping concentration that is higher closer to the base section 16. In this way the lateral resistance of the material layer 19 will be high, thus leading to a low lateral current spreading while the vertical current blocking properties of the current confining layer are preserved.
Any of the earlier described embodiments of a T-VCSEL can be provided with a collector section 13 that comprises a multi-layered distributed Bragg reflector, DBR, 12 provided between the bottom substrate 11 and the base contacting layer 172. This distributed Bragg reflector 12 might comprise a multi-layered structure with alternating AlGaAs layers and GaAs layers. Alternatives of a T-VCSEL with such distributed Bragg reflectors are shown in
A T-VCSEL according to earlier described embodiments might also comprise an emitter section that comprises a multi-layered distributed Bragg Reflector, DBR, 115 arranged on the current confining blocking layer 110 on the side farthest from the base section 16. This is shown in, for example,
The above described multi-layered DBR might comprise a-Si layers alternated with SiO2 layers.
In all of the earlier described embodiments the bottom substrate 11 of the collector section 13 of the T-VCSEL might comprises a GaAs substrate.
It is also possible to provide a variation of earlier described embodiments of a T-VCSEL where the type of doping of the various layers has been exchanged, or substituted for each other. In other words a T-VCSEL wherein p-doped materials are exchanged with corresponding n-doped materials and wherein said n-doped materials are exchanged with corresponding p-doped materials.
Moreover, as will be described more detailed below, any of the earlier described embodiments of a T-VCSEL might further comprise a collector contact 114, corresponding to a first terminal arranged on the emitter 14, a base contact 18, corresponding to a second terminal, arranged on the base 16 and a collector contact 140, corresponding to a third terminal, arranged on the collector 13.
The collector 13 of the earlier described embodiments of a T-VCSEL might modulation doped for efficient collector current injection while minimizing the optical absorption.
The earlier described embodiments of a T-VCSEL might also be provided with a light-emitting active layer 17 that consists of quantum-dots, quantum dashes or any other active layer material suitable for semiconductor lasers.
The earlier described embodiments of a T-VCSEL might also be provided with a third material layer 113 that is modulation doped to obtain efficient emitter-current injection and minimize optical losses.
A version of the earlier described embodiments of a T-VCSEL could also have the indicated doping types reversed, for example, an n-type collector section 13 instead of a p-type; a p-type base section 16 instead of an n-type, an n-type emitter section 19 instead of a p-type; instead of a p-type third material layer an n-type third material layer 113, and finally a p-type first material layer 112.
The earlier described embodiments of a T-VCSEL could preferably have a second material layer 19 that consists of a semiconductor compound with similar or larger band-gap as compared to the base region of the base section 16 and collector region of the collector section 13.
A version of the earlier described embodiments of a T-VCSEL could also be provided where the combined thicknesses of the layers 19, 111 and 113, which might be semi-conducting, are larger than the combined thickness of the possibly semiconducting layers 19, 110 and 113 at the peripheral region of the T-VCSEL thereby providing a lateral variation in the cavity thickness which will confine the laser light to the central region.
In a particular embodiment of a T-VCSEL according to the earlier described embodiments, the surface of the third material layer 113 could have a convex lens-like shape. This particular shape might spontaneously form during epitaxial regrowth of the third material layer 113.
To facilitate a deeper understanding of a T-VCSEL according to the invention in what follows there will be provided descriptions regarding the functionality of the various embodiments as well as descriptions for manufacturing procedures for obtaining particular embodiments of a T-VCSEL according to the invention. These descriptions are merely intended to provide a, hopefully, clear description of the invention and anything explicitly stated in the examples should not be construed as limiting features.
Seen in
To laterally confine the laser light mode to the central region of the device so that it overlaps with the gain in the light-emitting active layer, the thickness of the first regrown layer 112 is in a particular embodiment smaller than the mesa etch depth that defines the square-shaped section, or the central region, 111. In this way a structure is created with laterally varying optical properties that laterally confine the laser light to the region 111.
If the thickness of the first regrown layer 112 is made smaller than the mesa etch depth, this has additional consequences. During growth of the regrown layer 113, a crystallographic plane of high lateral growth rate will spontaneously be formed at the edge of the central region 111. As the growth of the layer 113 proceeds, this moves the step in the total cavity thickness (i.e. combined thicknesses between the bottom DBR 11 and top DBR 115) outwards, i.e. away from the central region 111. Such a change in lateral position of the step has useful properties. The lateral size of the laser light modes that are confined by the step will increase as the step moves outwards. The gain in the light-emitting active layer will, however, remain in the central region below the square-shaped section, or the central region, 111. This means that the optical gain will overlap more with, and thereby provide more modal gain, to laser light modes that have higher optical intensity in the central region, e.g. the fundamental laser light mode. This can be used to force the structure to emit light in the fundamental laser light mode only, which is desirable in many applications.
Another feature of the invention is that as the step in total cavity thickness moves away from the central mesa region 111 during growth of the regrown third material layer 113, the surface of the regrown third material layer 113 in the central region confined by the step in total cavity thickness spontaneously becomes curved with a convex lens-like shape. Such a shape is desirable as it provides optical confinement of the laser light without the diffraction losses induced by the step in the total cavity thickness mentioned above. The spontaneous formation of such a curved shape is a big advantage as deliberate formation of such a shape, e.g. by etching, is difficult, especially if a high degree of uniformity within a wafer or between wafers is needed.
The radius of the regrown third material layer 113 will determine the area to which the laser light will be confined. A smaller radius of curvature will confine the laser light to a smaller area while a larger radius of curvature will confine the laser light to a larger area. The radius of curvature can be tailored by varying the difference between the mesa etch depth and the thickness of the regrown third material layer 113. A smaller difference will give a larger radius of curvature while a larger difference will give a smaller radius of curvature. In addition, the radius of curvature can be controlled by the growth parameters, e.g. the growth temperature or growth rate, which controls lateral diffusion of adatoms and thereby the lateral-to-vertical growth rate ratio.
In another embodiment of the invention, lateral current spreading in the peripheral part of the second material layer 19, or emitter region 19, is reduced by a reduced or non-uniform doping of this layer. If the peripheral region of the layer 19 in the blocking layer 110 is highly conductive, the injected current will spread laterally before reaching the light-emitting active layer 17 which leads to a reduced efficiency of the laser. This can be avoided from a reduced doping concentration in the layer 19 so that this layer is almost fully depleted (and thus non-conducting) by the reverse-bias over the np-junction between layer 110 and the peripheral part of layer 19. However, this may lead to problems if the T-VCSEL is used in application where the bias between the emitter, base and collector contacts varies in time, which is e.g. the case for directly modulated light sources in fiber-optical communication systems. In such a case, the depletion layer, and thereby the conductivity of the peripheral layer 19 will vary with the applied voltage. As a consequence, the current spreading and thereby the efficiency of the laser will depend on the drive current of the laser, which is undesirable since it e.g. will make the design of the drive circuitry more difficult.
The above described effect of varying efficiency with applied voltage can be reduced if the doping concentration in the layer 19 has a non-uniform doping concentration that is lower towards the top of this layer and higher towards the bottom of the layer, i.e., towards the light-emitting active layer 17. In this configuration, the upper, peripheral part of layer 19 of the blocking layer 110 will always be fully depleted regardless of the applied bias (as long as the polarity of the bias is such that the emitter-base junction, i.e. the junction between the layer 19 and the base section 16, is forward-biased), while the lower part only will be depleted to a small extent that does not vary much with the applied bias (at least under normal drive conditions). The lower part of layer 19 with high doping concentration can therefore made thin and will consequently have a relatively high lateral resistance, thus limiting the current spreading to such an extent that the efficiency of the laser is reduced.
An additional benefit of keeping the highly doped part of layer 19 thin is that this can be used to reduce the optical absorption in that layer. The optical absorption in semiconductor layers usually increases with doping, especially p-type doping, and decreases the efficiency of the laser. However, the optical absorption in a laser structure can be reduced if the doped layer is positioned in a node of the standing wave optical field, but the thicker this layer is, the more it will penetrate regions where the standing wave optical field intensity in the laser is larger and thereby affect the efficiency of the laser. Consequently, the efficiency of the laser can be increased from a non-uniform doping concentration of layer 19 if the highly doped part of this layer is positioned close to a node of the standing wave optical field.
With reference to the T-VCSEL of
a discloses a GaAs-based Pnp-type 980-nm T-VCSEL.
For this particular T-VCSEL continuous-wave operation is demonstrated up to 50° C. with a room-temperature (RT) output power of 1.8 mW for a 10·10-μm2 device, controlled by the base current in combination with the collector-emitter voltage. To the best of the inventors knowledge, this is the first demonstration of the room-temperature operation of a T-VCSEL.
Device design of the T-VCSEL and fabrication of the same: A schematic drawing of such a T-VCSEL is shown in
a-5c illustrates some measured characteristics for a particular type of T-VCSEL. The particular type of T-VCSEL is schematically given in
That is,
b shows IC and VBE as function of IB for different values of VCE. For VCE=0, IC is negative throughout the range of IB since both junctions (EB and CB) are forward biased with corresponding hole injection into the base. For VCE>0, IC initially increases with increasing IB but then saturates and starts to gradually decrease. Considering the detailed geometry and the biasing configuration of the device, this behaviour is attributed to a gradual turn-on of the CB junction, starting from the edge close to the collector contact and progressing towards the centre region with increasing IB. The current injected close to the collector contact will not contribute to the lasing modes since it is only pumping the peripheral part of the active region outside the optical cavity. Beyond the kinks in the IC-versus-IB characteristics, IC is composed of two parts; a reverse current at the central part of the device and a forward current in the peripheral region that adds to the rapidly increasing IB, eventually summing up to IC<0. In the central region, holes injected from the emitter and electrons diffusing from the base contact provide the optical gain.
c shows temperature-dependent current as well as voltage-controlled operation of the T-VCSEL. The high-temperature performance can even further be improved by increasing the spectral offset between the active layer gain maximum (λg) and the cavity resonance (λcav). The minimum in threshold current occurs at around 10° C., corresponding to a slight negative tuning at RT (λg−λcav≈−15 nm). For the voltage-controlled operation, IB is set to 8 mA while Pout versus VCE are recorded at different temperatures. Similar to the current-controlled operation, a pronounced lasing threshold is observed and Pout is limited by self-heating and thermal roll-over at high voltages. This voltage-controlled operation is unique to transistor lasers and may find important applications [19].
In another embodiment of the invention, the current confinement (carrier injection from the emitter to the base) is arranged by the insertion of a reverse-biased p+n+ tunnel diode, as e.g. described in the case of conventional diode vertical-cavity surface-emitting lasers (diode VCSELs); see, e.g., Y. Onishi et al., IEEE J. Sel. Top. Quantum Electron, 15 (3), 838 (2009).
In yet another variant of the invention, tunnel junctions are used both to confine and inject carriers into the base from the emitter and also to extract the injected carriers from the base to the collector. The advantage of this approach is that the structure is almost exclusively n-doped, leading to high carrier mobility and suppressed free-carrier absorption.
Modulation doping in grown semiconductor layers are used to minimize optical absorption and/or improve the carrier distribution. Highly doped regions are positioned at a node of the standing wave optical field of the lasing light, and vertically varying doping levels are used to control the lateral resistance and thereby the current spreading.
That is, in another embodiment of the invention, the current constricting structure makes use of a highly conductive reverse-biased buried tunnel junction 211 positioned in the central region of the device, while the current is blocked in the peripheral region due to a reverse-biased pn-junction. A tunnel junction might consist of a highly doped p+n+-diode and has the very interesting property that the conduction through it relies on direct electron tunnelling form the valance band in the p-doped side to the conduction band in the n-doped side. This is made possible by the degenerate doping concentrations of the semiconductor on both sides of the junction so that filled electron states in the valance band on the p-side overlap with empty electron states on the n-side. An increased reverse bias will increase this overlap and the current will thus increase rapidly with increasing reverse bias.
In yet another embodiment of the invention, also the base-collector junction consists of n+p+-bilayer, thus forming a forward-biased tunnel junction. This configuration has the advantage that almost the entire device can be made n-type (for a pnp-type T-VCSEL) except for the p+-layer in the tunnel junction itself and the thin second material layer 19. Also, the doping and band alignment of the tunnel junction will set a limit for the maximum applied voltage across this junction since an increasing forward bias eventually will decrease the overlap between occupied electron states in the conduction band on the n-side and un-occupied electron states in the valence band on the p-side. Also this device configuration is less suitable for a npn-type T-VCSEL since that would result in an almost completely p-type device and thereby an overall high optical absorption.
A specific advantage of the embodiments of the present invention is that, in contrast to oxidation-confined T-VCSELs in the prior art which preferably are made out of the AlGaAs/GaAs materials system since selective oxidation is difficult in other materials systems, lend itself equally well to alternative materials systems such as GaAs, InP, GaN, GaSb, etc, and thereby can be used to fabricate emitters for a wide wavelength range, including but not limited to 850, 980, 1310, 1490 and 1550 nm.
The fabrication procedure of the T-VCSEL according to a first embodiment of the invention will now be described with reference to
With reference to
The current constricting layer 312 is then deposited by area-selective epitaxial regrowth while the top of the current confining blocking layer 311 is covered by a masking material 317 that prevents epitaxial growth from occurring there, this is schematically illustrated in
Thereafter, the top DBR 315 is deposited on the regrowth third material layer 313, in such a way that it is overlying the central mesa region 311. The DBR 315 could also be deposited so that it also at least partly overlies the peripheral regions of layer 313 and layer 312. The end product is schematically illustrated in
In other words, there is provided a method for producing a T-VCSEL, the method comprises the following steps:
depositing Si a bottom Distributed Bragg Reflector, bottom DBR, 32, on a substrate 31;
depositing S2, on said bottom DBR 32, a p-doped collector layer 372;
depositing S3 a base layer 36, part of which is n-doped and comprises a light-emitting active layer 37, on top of said p-doped collector layer 372;
depositing S4 first 39 and second 311′ p-doped layers on top of said base layer 36;
etching S5 away a volume 38, 38′ through said second p-doped layer 311′, thereby creating a central mesa region 311;
covering S6 said central mesa region 311 with a masking material preventing epitaxial regrowth;
growing S7, by epitaxial regrowth, an n-doped layer 312 on top of the central mesa region 311 covered by the masking material;
removing S8 the masking material from the top of the central mesa region 311;
growing S9, by epitaxial regrowth, a third p-doped layer 313 over both the central mesa region 311 and the n-doped layer 312;
etching S10 away a volume 318, 318′ through the third p-doped layer 313, the n-doped layer 312, the first p-doped layer 39, the base layer 36 and partly through the p-doped collector layer 372, thus exposing part of the p-doped collector layer 372;
etching S11 away a volume 325, 325′ through the third p-doped layer 313, the n-doped layer 312, the first p-doped layer 39 and partly through the base layer 36 above the light-emitting active layer 37, thus exposing part of the base layer 36;
attaching S12, an electrical contact 340 on the exposed part of the p-doped collector layer 372, and an electrical contact 314 on the upper faces of the third p-doped layer 313;
attaching S13 an electrical contact 38 on the exposed part of the base layer 36; and
depositing S14 a top distributed Bragg Reflector, top DBR, 315 on the third p-doped layer 313 in such a way that said DBR 315 at least overlies said central mesa region 311.
The fabrication of the T-VCSEL according to another embodiment of the invention is described with respect to
In other words there is provided a method for producing T-VCSEL comprising the following steps:
depositing S1 a bottom Distributed Bragg Reflector, bottom DBR, 42, on a substrate 41;
depositing S2, on the bottom DBR 42, a p-doped collector layer 472;
depositing S3 a base layer 46, part of which is n-doped and comprises a light-emitting active layer 47, on top of the p-doped collector layer 472;
depositing S120 a p-doped layer 49 on top of the base layer 46;
depositing S130 an n+p+-tunnel-junction bilayer 411′ on top of the p-doped layer 49;
etching S140 away a volume 417, 417′ through said n+p+-tunnel-junction bilayer 411′, thereby creating a central mesa region 411;
growing S150, by epitaxial regrowth, an n-doped layer 412 on the peripheral region of the layer 49 and on top of the central mesa region 411;
etching S160 away a volume 419, 419′ through the n-doped layer 412, the p-doped layer 49, the base layer 46 and partly through the p-doped collector layer 472, thereby exposing part of the p-doped collector layer 472; and
etching S170 away a volume 421, 421′ through the n-doped layer 412, the p-doped layer 49 and partly through the base layer 46 above the light-emitting active region 47, thereby exposing part of the base layer 46;
attaching S180, an electrical contact 440 on the exposed part of the p-doped collector layer 472, and an electrical contact 414 on the upper faces of the n-doped layer 412;
attaching S190 an electrical contact 48 on the exposed part of the base layer 46; and
depositing S200 a top DBR 415 on the n-doped layer 412 in such a way that it at least overlies the central mesa region 411.
The method could also comprise a further step S2″ following the step S2 in order to add a forward-biased tunnel junction in a layer arranged between layers 472 and 46. It would be obvious for a person skilled in the art to alter the various doping type of the layers to accommodate such a forward-biased tunnel junction.
The fabrication methods described above, with reference to drawings 9-13 and 14-17, respectively, would work equally well if all doping types were reversed. That is all layers with a p-type of doping were replaced with layers of an n-type at the same time as layers with an n-type doping were replaced with layers with a p-type doping.
The attached electrical contacts above could be either p-type for 314, corresponding to the emitter contact, and for 340, corresponding to the collector contact, and n-type for 38, corresponding to the base contact, or vice versa.
As is clear from the above description the method steps S1, S2 and S3 are common for the methods described.
The fabrication sequence of the device is such that the cavity region is exposed before the final regrowth and/or deposition of the top DBR, thus making it possible to engineer the cavity shape for improved optical mode control, e.g. as discussed by X. Yu et al, Proc. SPIE 7720, 772021 (2010). The etching depth for mesa formation and/or regrowth thicknesses can also be chosen for a lateral variation of the cavity thickness, e.g., to focus the laser light to the central region, possibly through the spontaneous formation into a convex-shaped lens of the topmost regrown layer.
The shape of the mesa around which the regrowth is done is very important. This is due to the anisotropic properties of the mesa. Growth around a circular mesa proceeds with very different speeds in different crystallographic directions, causing very non-uniform regrowth shapes. Instead, the mesa is made square-shaped with sidewalls parallel with the [011] crystallographic directions of the substrate. Regrowth around such a mesa produces desirable regrowth shapes. One drawback with the square mesa is that it is invariant to a 90 degree rotation. This means that the laser light will not have any preferred polarization direction as both polarizations (which are rotated by 90 degree with respect to each other) will be influenced equally by the square mesa. In order to introduce anisotropy into the mesa shape, it can be made slightly rhombic with the sidewalls facing a few degrees off from the [011]-like directions. The regrowth shape around such a mesa does not differ from the regrowth shape around a perfectly square-shaped mesa.
The invention has been described with reference to exemplary embodiments but is not limited to those embodiments. Various modifications or alternations of the embodiments can easily be made by those skilled in the art without departing from the scope of the invention. It should for example be noted that the present invention lends itself equally well to alternative materials systems, such as InP, which is difficult to realize as based on selective oxidation that otherwise is a popular technology for GaAs-based conventional VCSELs. Thereby it is also suitable for a broad range of emission wavelengths, from the visible throughout the near-infrared communication wavelengths, e.g., including but not limited to 850, 1310, 1490 and 1550 nm.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SE2013/051451 | 12/4/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61733441 | Dec 2012 | US |
