WO2007071809A1

WO2007071809A1 - Semiconductor device and method to fabricate thereof

Info

Publication number: WO2007071809A1
Application number: PCT/FI2005/050482
Authority: WO
Inventors: Oleg Okhotnikov; Mircea Guina; Anatoly Grudinin
Original assignee: Reflekron Oy
Priority date: 2005-12-22
Filing date: 2005-12-22
Publication date: 2007-06-28

Abstract

The invention concerns a design of a semiconductor saturable absorber that offers a convenient and reliable way to decrease absorption recovery time. The design employs a multiple-layer heterostructure where at least one layer, called nonlinear layer, is used for absorbing a signal light. Additional layer(s) adjoin the nonlinear layer(s) and have a smaller band-gap then the nonlinear layer(s). The role of these narrow band-gap layers is to collect the carriers generated within the nonlinear layer(s) thus reducing the recovery time of absorption in nonlinear layers. We call these layers collectors. The control of the recovery time is achieved by means of the following measures a) band-gap engineering of the nonlinear layers such that the signal light will be absorbed in this layers that exhibit higher band-gap then surrounding layers and allow for an efficient sweep-out of the carriers towards surrounding layers; b) band-gap engineering of layers surrounding the nonlinear layers so that they will act as effective collectors for the carriers generated by absorption of the signal light in the nonlinear layers. The width of the absorber and collector layers is optimized to further enhance the nonlinearity and speed of the device The absorber can be used either in transmission or reflection mode. In the later configuration, the absorber is attached to a mirror that for example can be grown by molecular-beam-epitaxy monolithically to the absorbing layers, or it is obtained by deposition of dielectric or metallic layers.

Description

Semiconductor Device and Method to Fabricate Thereof

FIELD OF THE INVENTION

The present invention relates generally to semiconductor devices comprising saturable absorber mirrors, and in particular to a concept to reduce the absorption recovery time of such a mirror, a method to fabricate thereof, and the use of such a device in mode-lock lasers.

BACKGROUND OF THE INVENTION

Semiconductor saturable absorbers are nonlinear optical elements that have found applications in a large variety of fields ranging from passively mode-locked lasers, as demonstrated for example by Collings et al. in J. SeI. Top. in Quantum Electron., vol. 3, pp.1065- 1075, 1997, to all-optical regeneration, and noise suppression in optical telecommunication links. These devices impose an intensity-dependent attenuation on a light beam incident upon it; an incident radiation of low intensity is preferably absorbed, while a high intensity radiation passes the saturable absorber with much less attenuation. For practicality a semiconductor saturable absorber is usually integrated with a semiconductor, dielectric or metallic mirror forming a semiconductor saturable absorber mirror (SESAM).

It can be gathered from prior art that an optically saturable absorber is provided by a semiconductor material whose energy band-gap is set to match the wavelength of the optical signal to be controlled. See for example U.S. Pat. No. 5,237,577 to Keller et al. and U.S. Pat. No. 5,627,854 to Knox et al. The absorbing material is usually embedded within semiconductor material(s) with a higher band-gap(s) that do not absorb the optical signal. The thickness of a single absorbing layer is typically in the range of few nanometers so that quantum-mechanical effects are enabled (in this case the absorbing layers are called quantum-wells, QWs). The whole absorber region may comprise a number of quantum-well layers representing the so-called multiple- quantum-wells structure. Additional design features can include positioning of the nonlinear absorbing layer within a Fabry-Perot cavity as well as means to apply an electrical field to the structure to control its absorption properties, as shown by Heffernan et al. in Appl. Phys. Lett., vol. 58, pp. 2877-2879, Jun. 1991. An external optical source that provides a control beam can be used to vary the optical properties of the saturable absorber whereas the control beam can be absorbed also in the material surrounding the saturable absorber as initially thought by Chemla el al in U.S. Pat. No. 4,860,296.

Passive mode-locking using semiconductor saturable absorber mirror has been proven to be a powerful technique as it supplies short pulses in a simple laser cavity, as described by Collings et al. in J. SeI. Top. in Quantum Electron., vol. 3, pp.1065-1075, 1997. When used in a laser cavity, a SESAM introduces intensity-dependent losses. Because the laser tends to operate with minimum cavity loss per round-trip, the longitudinal modes of the lasers become phase-locked corresponding in time domain to high intensity short optical pulses. A mode-locking technique which relies upon the use of the nonlinear reflectivity of a SESAM eliminates the need for critical cavity alignment, it can be designed to operate in a wide spectral range, and can have relatively large nonlinear reflectivity changes. Ultrashort optical pulses have been produced with this technique using different semiconductor structures and mirror designs. See for example U.S. Pat. 5,627,854 to Knox and U.S. Pat. 5,237,577 to Keller.

If the absorber design employs means to apply an electrical field across the structure or if the absorber's parameters can be controlled with an external light, which is absorbed at least in the saturable absorption region, then the absorber can be used for active mode- locking. Active mode-locking can be achieved in this way at the fundamental frequency of the laser cavity or its higher harmonics. All the benefits of the active mode-locking can be exploited, e.g. locking an optical pulse train to an external clock generator.

In order to provide efficient pulse shaping, the saturable absorption should recover to its initial value in a short time, see for example Hayduk et al., LEOS '97 10th Annual Meeting, Conference Proceedings., IEEE, vol. 2 , pp. 458-459, 1996. For efficient and self- starting mode-locking (apart of other applications), a recovery time in a range from few picoseconds to few tens of ps, depending on gain medium and laser cavity is desirable. The recovery time for typical compound semiconductors, in particular multiple-quantum-well structures, is about nanosecond, see for example Gray et al., Opt. Lett., vol. 21 , pp. 207-209, 1996. Therefore, SESAMs fabrication usually includes special measures to reduce (control) the recovery time. The methods for reducing the absorption recovery time reported so far include low-temperature growth as shown by Gupta et al., IEEE J. Select. Topics Quantum Electron., vol. 10, pp. 2464-2472, 1992, Be- doping, for example shown by Qian et al., Appl. Phys. Lett., vol. 17, pp. 1513-1515, 1997, proton bombardment, see for example Gopinath, et al., Proceedings CLEO, 2001 , pp. 698-700, and ion bombardment, as shown by Delponet al., Appl. Phys. Lett., vol. 72, pp. 759-761 , 1998. Each of these techniques brings in certain constrains related to fabrication cost, device reliability, degradation of nonlinearity and increased non-saturable loss. Low temperature growth is very efficient in reducing the recovery time of GaAs-based devices, as evidenced by Siegner et al. in Appl. Phys. Lett., vol. 69, pp. 2566-2568, 1996, however, low temperature growth usually produces a non-uniform distribution of the recovery time over the sample, see for example Hayduk et al., Proceedings LEOS, 1997, vol.2 paper ThU3, pp. 458 - 459, 1996. Proton bombardment was demonstrated to reduce the recovery time down to ~ 1 ps, however, the recombination centers created by proton bombardment have low activation energies that may lead to long-term instabilities induced under exposure to intense optical radiation. Ion implantation is a simple method but it also poses certain problems due to its low reproducibility and uniformity. The long penetration depth of the ions can lead to structural degradation of the semiconductor mirror and results in an increase of the non-saturable loss. Both proton bombardment and ion implantation requires post- growth fabrication steps using expensive equipment.

SUMMARY OF THE INVENTION

It is the object of the present invention to propose a design, a fabrication method and implementation of a semiconductor saturable absorber that offers a convenient and reliable way to decrease the absorption recovery time. The invention is a multiple-layer heterostructures, where at least one layer, called nonlinear layer, is used for absorbing a signal light. Additional layers are positioned on one side or on both sides of the above mentioned nonlinear layer(s) and have a smaller band-gap than the nonlinear layer(s). The role of these narrow band-gap layers is to pump-out (remove) the photo- carriers from the nonlinear layer(s) thus reducing the recovery time of absorption of nonlinear layer(s). We call these layers as collectors of the photo-carriers. The control of the recovery time is achieved by means of the following measures a) band-gap engineering of the nonlinear layers such that the signal light will be absorbed preferably in these layers that have higher band-gap than surrounding layers and allow for an efficient sweep-out of the carriers towards surrounding layers; b) band-gap engineering of layers surrounding the nonlinear layers so that they will act as effective collectors for the carriers generated by absorption of the signal light in the nonlinear layers. The aim is to have as large as possible band-gap difference between the "nonlinear" and "collector" layers to enable an effective (rapid) sweep- out of photo-carriers towards high-capacity collectors. This is because with a large band-gap-energy difference between nonlinear and collecting layers, higher number of states are expected in a collecting energy well. Therefore, such "high-capacity drain" can accommodate larger number of photo-carriers without saturation. It is interesting to observe that the device can be used as a fast saturable absorber at a wavelength that matches the absorption edge of the high band-gap material but act as a slow saturable absorber at a wavelength that corresponds to the low band-gap energy of the collector layers.

This invention suggests a number of semiconductor materials that enables the implementation of the proposed design by epitaxial growth of semiconductors layers with a large band-gap difference. The invention also provides an application example where a semiconductor saturable absorber developed according to the proposed design is used for mode-locking of a fiber laser. To put it more precisely, the semiconductor device according to the present invention is primarily characterised in that the band-gaps of said second layers are smaller than the band-gap of said first layer, and that the absorber region comprises a stack of alternating first and second layers so that the first and the last layer of the stack is the second layer, that the energy band-gap of the first layer is set to match the wavelength of the optical signal to provide saturable absorption, and that said second layers are adapted to collect carriers generated within the first layer.

To put it more precisely, the method according to the present invention is primarily characterised in that the method also comprises forming the band-gaps of said second layers smaller than the band-gap of said first layer, forming the absorber region as a stack of alternating first (6) and second layers so that the first and the last layer of the stack is the second layer, the energy band-gap of the first layer is set to match the wavelength of the optical signal to provide saturable absorption, and said second layers are meant to collect carriers generated within the first layer.

To put it more precisely, the laser according to the present invention is primarily characterised in that the band-gaps of said second layers are smaller than the band-gap of said first layer, that the absorber region comprises a stack of alternating first and second layers so that the first and the last layer of the stack is the second layer, that the energy band-gap of the first layer is set to match the wavelength of the optical signal to provide saturable absorption, and that said second layers are meant to collect carriers generated within the first layer.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the invention is provided by the description of the specific illustrative embodiments and the corresponding drawings in which: Fig.1 shows a cross-section of a semiconductor saturable absorber mirror where the absorbing region is designed according to this invention.

Fig.2 is a schematic description of the band-gap profile of a semiconductor saturable absorber according to a first embodiment of the invention.

Fig.3 depicts another embodiment of the invention where a modified band-gap profile is used to enhance the carrier sweep-out from the "nonlinear" absorbing region.

Fig.4 depicts an exemplary embodiment of the invention where a low band-gap GaInNAs compound semiconductor is used as a collector and the nonlinearity is provided by InGaAs.

Fig.5 illustrates an embodiment of a passively mode-locked fiber laser employing a semiconductor saturable absorber fabricated according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference now to the drawings, Fig. 1 shows a cross section of a semiconductor saturable absorber device described in the art. It includes a compound semiconductor substrate 1 , for example GaAs or InP, suitable for growing epitaxial layers. The first layers to be grown are compound semiconductors with alternate high and low refractive indices composing a distributed Bragg reflector (DBR) 2 designed to ensure a high reflectivity over a large spectral range around the wavelength of an optical signal to be processed. In the another "non- monolithic" arrangement, the semiconductor DBR 2 can be replaced by high-reflective dielectric or metallic mirror. The device includes the semiconductor multi-layers region 3 comprising layer(s) with energy band-gap that matches the wavelength of the optical signal and provides saturable absorption. Additional compound semiconductors 4 are placed above and/or below the region 3 to control the optical field distribution in respect to the position of the absorbing layers within region 3 and to control the thickness of the Fabry-Perot cavity defined by the DBR mirror and the top surface of the device. The structure can be terminated by depositing dielectric mirrors 5 with a required reflectivity.

Fig. 2 represents a first embodiment of the invention and shows the band-gap profile of a multilayer heterostructure S that provides saturable absorption with a fast recovery time. The structure is comprised of at least one layer, called nonlinear layer 6, which is used for providing saturable absorption at the signal wavelength. Layer(s) 7 (at least one) with a smaller band-gap than nonlinear layer adjoin the nonlinear layer(s). The role of these narrow band-gap layers is to collect the carriers generated within the nonlinear layer(s) thus reducing the recovery time of absorption within the nonlinear layers by fast removal of the photocarriers from absorbing layer(s) 6. We call these layers collectors. The reduction of the recovery time is due to a sweep-out of the carriers towards collectors where they recombine. The mechanisms involved in the sweep-out process include carrier drift, carrier diffusion, carrier thermalisation and decay towards lower energy levels available at collectors. The role of the high band-gap absorbing semiconductor layer(s) 6 two fold: it provides saturable absorption at signal wavelength and also quantum-confinement of the carriers within collectors 7 by defining the barrier(s) for energy well.

The time constant that is associated with the process of collecting the carriers generated within nonlinear region 6 is called capture time. From the previous analysis of the carrier capture time in quantum-wells semiconductor laser it is known that the capture time for structures similar with the one described here is in the range of few picoseconds and below. See for example Bennett et al., IEEE J.Q.E., Vol. 33, No. 1 1 , pp. 2077-2083, 1997. Such an analysis offers grounds to expect that suitable engineering of the band-gaps and thicknesses of nonlinear and collector layers leads to a reduction of the absorption recovery time of nonlinear layers to levels suitable for at least mode- locking applications. The central objective in designing the band-gap profile of the region 3 is to ensure as large as possible a band-gap difference between the nonlinear and collector layers to enable an efficient (rapid) sweep-out of photo-carriers towards collectors. In addition, the escape of carriers from collectors 7 towards nonlinear layers 6 decreases exponentially with the said band-gap difference. The width of the absorbing layer(s) 6 is assumed to be optimized for providing a required saturation fluence of the absorption and to ensure a low value of the capture time. The thickness of the collector should be optimized in respect with the following condition: a low signal absorption and efficient collection of the carriers generated in the nonlinear layer(s). The cap and spacer layers 4 shown in Figs. 1 and 2 have larger band-gap energies and are highly transparent at the signal wavelength.

Fig. 3 represents the band-gap profile of a semiconductor saturable absorber according to a second embodiment of the invention. The structure employs graded band-gaps 8 between the nonlinear 6 and collector 7 layers that improve the carriers sweep-out from the nonlinear layers. The graded band-gaps give rise to a "built-in" electric field in the transition region as initially thought by H. Kroemer, in RCA Review, vol. 18, pp. 332-342, 1957. The electric fields in the conduction and valence bands have different polarization therefore electrons and holes flow in the same direction, towards collectors. In simple terms, the photo-carriers generated in the nonlinear regions get captured by the collectors faster due to the field assisted drift. Additional enhancement of the electrical field can be obtained by suitable doping of the nonlinear and collector layers.

Fig. 4 shows a schematic representation of the band-gap profile corresponding to an exemplary embodiment of the invention. The epitaxial layers are grown on a monocrystaline GaAs wafer. The DBR mirror can be comprised of a number of GaAs/AIAs quarter-wavelength pairs. The cap and spacer layers consist of GaAs. For operation at the signal wavelength around 0.9-1.24 μm, the nonlinear layers are comprised of ln_{1 -x}Ga_xAs with different indium compositions.

The selection of materials for the collector layer(s) is generally governed by the need of layer(s) with a smaller band-gap than the band-gap of the nonlinear layer(s), while the material should be grown with good quality, i.e. it should be latticed- or quasi-latticed-matched, with the GaAs wafer. A promising solution for fabricating saturable absorbing devices with an enhanced speed due to photo-carriers sweep-out effect is based on the use of the so-called dilute nitride compound semiconductors. Substitutional^ random dilute nitride alloys, Ga_xln_{1 -x}N_yAs_1-y, pseudomorphically grownable on GaAs substrates has recently challenged the InP technology. See for example Nakahara et al. Electron. Lett. vol. 32, p. 1585-1586, 1996 and Pessa et al., IEE Proc.-Optoelectron. 12, p. 150, 2003. GaAs devices are cost-effective and robust with good thermal conductivity, and allows for growth of dilute nitride vertical-cavity surface-emitting lasers (VCSELs) and semiconductor saturable absorber mirrors (SESAMs) in a single growth run. Ga_xln_1-xN_yAs_{1 -y} at a dilute limit (y ≤ 2 %) is a proper material for the quantum-wells of SESAMs at λ > 1.3 μm. The GalnNAs/lnGaAs interface may have a large conduction band discontinuity, which offers excellent confinement of electrons in the quantum-wells, while the DBR of GaAs/AIAs exhibits a high reflectivity and broad bandwidth.

Absorber-collector structures based on InGaAs/GalnNAs material system is expected to show improved performance due to extremely large band-gap difference achievable with these material compositions. This would allow to increase significantly the internal field by grading the band-gap at the absorber-collector interface and to improve the capacity of "drain" provided by the low band-gap Ga_xIn₁ ^N_yAs_{1 -y} collector.

According to an application example revealed in Fig. 5 a SESAM designed according to the present invention is used to passively mode- lock a laser. Here the gain medium 9 is pumped electrically or optically to generate a signal beam. The laser cavity is defined by a SESAM at one side of the gain region and another mirror 10 at the other side of the gain region. The SESAM can be butt-coupled to the cavity, as shown in the Fig. 5, or lens coupled. One of the mirror can be used to provide dispersion compensation.

Claims

1. A semiconductor device (S) comprising a semiconductor mirror (2) and a multilayer heterostructure (3, 4) providing saturable absorption for a predetermined optical signal, the multilayer heterostructure comprising an absorber region (3), the absorber region (3) of the semiconductor device comprises:

- at least one first layer (6), and

- at least one second layer (7) of different material composition and energy band-gap than said first layer (6) on both sides of said first layer (6), characterised in that the band-gaps of said second layers (7) are smaller than the band-gap of said first layer (6), and that the absorber region (3) comprises a stack of alternating first (6) and second layers (7) so that the first and the last layer of the stack is the second layer (7), that the energy band-gap of the first layer (6) is set to match the wavelength of the optical signal to provide saturable absorption, and that said second layers (7) are adapted to collect carriers generated within the first layer (6).

2. The semiconductor device according to claim 1 , characterised in that at least part of the band-gaps between said first (6) and second layers (7) are at least partly linearly graded.

3. The semiconductor device according to claim 1 or 2, characterised in that the thickness of the second layer (7) is optimized to minimize the signal absorption and to ensure efficient collection of the carriers generated in the first layer (6).

4. The semiconductor device according to any of the claims 1 to 3, characterised in that the first layers (6) are made of ln_1-xGa_xAs.

5. The semiconductor device according to any of the claims 1 to 4 characterised in that the second layers (7) are made of Ga_xIn_{1 -x}N_yAs_{1 -y}.

6. A method for producing a semiconductor device (S) comprising: forming a semiconductor mirror (2), a multilayer heterostructure (3, 4) providing saturable absorption for a predetermined optical signal, and forming an absorber region (3) to the multilayer heterostructure, forming to the absorber region (3) of the semiconductor device:

- at least one first layer (6), and

- at least two second layers (7) of different material composition and energy band-gap than said first layer (6) on two sides of said first layer (6), characterised in that the method also comprises: forming the band-gaps of said second layers (7) smaller than the band- gap of said first layer (6), forming the absorber region (3) as a stack of alternating first (6) and second layers (7) so that the first and the last layer of the stack is the second layer (7), the energy band-gap of the first layer (6) is set to match the wavelength of the optical signal to provide saturable absorption, and said second layers (7) are meant to collect carriers generated within the first layer (6).

7. A laser comprising at least gain medium (9), means to pump the gain medium, a mirror that can be also the out-put port (10) the semiconductor device (S) for passively mode-locking the laser, the semiconductor device (S) comprising a semiconductor mirror (2) and a multilayer heterostructure (3, 4) providing saturable absorption for a predetermined optical signal, the multilayer heterostructure comprising an absorber region (3), the laser further including a semiconductor device with an absorber region (3) that comprises:

- at least one first layer (6), and

- at least one second layer (7) of different material composition and energy band-gap than said first layer (6) on two sides of said first layer (6) to initiate pulsed operation of the laser, characterised in that the band-gaps of said second layers (7) are smaller than the band-gap of said first layer (6), that the absorber region (3) comprises a stack of alternating first (6) and second layers (7) so that the first and the last layer of the stack is the second layer (7), that the energy band-gap of the first layer (6) is set to match the wavelength of the optical signal to provide saturable absorption, and that said second layers (7) are meant to collect carriers generated within the first layer (6).