WO2005050802A1

WO2005050802A1 - Algaomas straomed mqw laser diode

Info

Publication number: WO2005050802A1
Application number: PCT/FI2004/000701
Authority: WO
Inventors: Ville Vilokkinen; Mika Saarinen; Pekka Savolainen
Original assignee: Modulight, Inc.
Priority date: 2003-11-20
Filing date: 2004-11-19
Publication date: 2005-06-02
Also published as: FI20031691A7; FI20031691L; FI20031691A0

Abstract

The invention is a laser diode with improved slope efficiency and decreased threshold current. The diode laser produces light at IR-region of the radiation spectrum. It comprises two cladding layers of opposite conductivity types on opposite sides of an active layer. The active layer has at least eight compressively strained AlGaInAs quantum wells and at least seven AlGaInAs barrier layers placed between the quantum wells. The benefits of the invention are related to the improved temperature behavior of laser diodes. A laser diode such as disclosed herein has reduced threshold current and better slope efficiency at higher operating temperatures up to 85 C. Due to better temperature behavior the laser diodes can be utilized in equipments without active cooling.

Description

AlGaOmAs straomed MQW laser diode

FIELD OF THE INVENTION The invention relates to semiconductor lasers and optical transmission technology. Particularly, the invention relates to active AlGalnAs layer structures operational at bit rates equal or higher than 10 Gbit/s in a broad temperature range up to 85 C without active cooling.

BACKGROUND OF THE INVENTION In the last few years the requirements for consumer bandwidth have grown rapidly. To meet the demand for increased bandwidth new optical transmission technologies have been developed. Optical transmission utilizes semiconductor laser diodes at the transmitting end and uses an optical fiber as the transmission medium. The transmitter semiconductor lasers operate mainly at infrared (IR) wavelength region between the wavelengths 1.3 μ and 1.55 μm. Reference is now made to Figure 1, which illustrates the functionality of a simplified prior art optical transmission system. The optical transmission system comprises a semiconductor laser diode 100, which produces a laser beam 150 to be conveyed in an optical fiber 152. At the receiving end there is a photodiode 154, which is connected to a voltage source 156 and a high-frequency electrical signal output 158. Semiconductor laser diode 100 is connected to a current source 160 and a high-frequency electrical signal input 162. Semiconductor laser diode 100 comprises an active region 105, which is sandwiched between P-type layers 170 and N-type layers 171. P-type layers and N- type layers act as cladding and wave-guides that confine and guide the light wave within the active region 105. Active region 105 usually comprises a number of quantum wells. On the one end of the semiconductor la- ser diode 100 there is a high reflectance coating and on the other end there is a lower reflectance coating. Between the mirrors an optical cavity is formed. The motion of photons between the reflecting ends contrib- utes to the lasing effect in the active region. Current source 160 produces a current that exceeds the threshold of semiconductor laser diode 100. This causes lasing effect in active region 105, which produces light beam 150. In active region 105 absorption occurs when an electron from valence band absorbs an incident photon and moves to the conduction band. The photon has sufficient energy to raise the electron to the conduction band when absorbed by the molecule. Stimulated emission in active region 105 occurs when an incoming photon stimulates an electron-hole pair to emit a second in-phase photon. Stimulated emission is possible when an electron and a hole in the electron-hole pair have an energy gap that corresponds to the energy of the incoming photon. The stimulated emission rate depends on the incident photon flux, the density of correct energy level electrons in the conduction band and the density of correct energy level holes in the valence band. Optical gain is possible in electrical pumping stace. In electrical pumping state there is a sufficient density of electrons and holes in the conduction and valence band states. In this state the stimulated emission rate exceeds the absorption rate. Reference is now made to Figure 2, which il- lustrates the density of electron and hole states in bulk semiconductor and a quantum well. The y-axis above origin (E_e) ar-d below origin (E_h) represents the electron and hole energies respectively. The x-axis

the crystal momentum k. In the case of Fig- ure 2 the semiconductor material has direct band gap of Eg. The electron energy E_e is obtained from h²k²

E_e = — , wherein m^* _e denotes the electron effective 2m_e mass and k is the scalar crystal momentum. The hole h²k² energy Eh is obtained from E_h = — , wherein m^* de- 2m_h notes the hole effective mass. In bulk semiconductor the density of allowed electron and hole states is larger than in a quantum well. The electron and hole motions are restricted inside the quantum well. Due to the Pauli exclusion principle, the allowed electron and hole energy states are quantized and occur on dis- crete energies. The precise discrete energies can be computed from the Schrδdinger wave equation from quantum mechanics. In Figure 2 quantum well electron energies are E¹ _e, E² _e and E³ _e. The hole energies are E^, E² _h and E³ _h. In bulk semiconductor the electron states are located in the area above the parabola E_e and hole states are located in the area below parabola E_h- In quantum well the electrons fill up the available states and reach higher energy with same volume density of carriers. This is illustrated in Figure 2. Let D be a given volume density of carriers. The volume density D is sufficient to reach the energy level D^3D _e in bulk semiconductor, whereas in quantum well the energy level reaches level D^2D _e, which is a higher level. The same is valid for holes in inverse -direction. Due to the discrete electron and hole energies the energy i.e. the wavelength of photons emitted can be better controlled. Reference is now made to Figure 3, which illustrates the quantum well and barrier energies. The y-axis above origin (E_e) and below origin (Eh) represents the electron and hole energies respectively. The x-axis represents the z-dimension L_z of the quantum well. The quantum well illustration comprises electron confinement 300 and hole confinement 302. Ideally, the difference in potential between quantum well 300 and barrier region 350 (or between 302 and 352 respectively) is infinite, which causes electron (or hole) confinement. In practice, the materials for the barrier region and the quantum well have to be picked so that the conduction band offset ΔE_C between the quantum well and the barrier region is high enough. Too low conduction band offset ΔE_C causes carrier leakage, which makes the performance of the lasers worse . In Figure 3 the gap energy between conduction band and valence band in the quantum well region is E_g ^w. In barrier region the gap energy is E_g ^b. Stimulated photon emission occurs in the quantum well r-egion with energies that are determined by the energy transitions between electron energies E¹^, E² _e, E³ _e and hole energies E^X _h, E² _h, E³ _h. Due to the transitions occurring between the discrete electron and hole energies, energies of emitted photons are discrete, which contributes to efficient lasing in the active region. Reference is now made to Figure 4a, which il- lustrates an example of the quantum well and barrier energies in a prior-art InGaAsP (Indium Gallium Arsenide Phosphide) laser diode. In Figure 4a there are five quantum wells, which exhibit relatively low barrier energy E_b compared to the quantum well energies Eg. It should be noted that the number of quantum wells in prior art has depended on the material used in the active region. Reference is now made to Figure 4b, which illustrates cross-sectional view of a prior-art laser diode and its layer structure. The illustrated laser diode operates at infrared (IR) wavelength region between 1.3 μm and 1.5 μm, desirable for optical communication. The laser diode in Figure 4b is manufactured using a combination of materials InP (Indium Phos- phide) and InGaAsP (Indium Gallium Arsenide Phosphide) or AlGalnAs (Aluminium Gallium Indium Arsenide) . Laser diode 100 comprises an active region 105, which is made of InGaAsP or AlGalnAs. Active region 105 comprises a number of quantum well layers (for example, quantum well layer 107) sandwiched between barrier layers (for example, barrier layer 106) . If AlGalnAs has been used as the material in active region 105 and the barrier layers have not been lattice matched, in prior art there have been six or less quantum wells in the active region. In prior art, if AlGalnAs has been used in active region 105 and the barrier layers have been lattice matched, there have been even up to ten quantum wells in the active region. The barrier layers have been lattice matched, which allows only relatively thin quantum well layers, namely thinner than 6 nm. If InGaAsP has been used, in prior art there have been from four up to fifteen quantum wells in active region 105. Active region 105 has been sandwiched between InGaAsP or AlGalnAs wave-guide layers 108 and 104. Wave-guide layer 108 is coated with p-InP cladding 110. P-InP cladding 110 has an etched ridge structure on top of it. The ridge confines the light laterally and forms a single-mode optical wave-guide. The ridge has been insulated on both sides using dielectric material 112 such as silicon dioxide (Si0₂) . The ridge top is covered with a p-InGaAs contact layer 111. Dielectric material 112 and contact layer 111 are covered with a metal contact layer 114. On top of the metal layer there is a thick plated gold pad 118, which mainly spreads heat. Negative contact is located below the thinned n-InP substrate. There are drawbacks in prior art laser diodes such as illustrated in Figure 4b. The use an InGaAsP active region is sensitive to temperature variations. The performance of InGaAsP active region laser diodes decreases severely as the operation temperature in- creases. The performance factors include threshold current and efficiency. A natural solution to avoid the problems associated with increased temperature is to introduce active cooling to the system involving the laser diodes. However, this naturally increases the overall system costs and power consumption. Reference is now made to Figure 5, which il- lustrates on Y-axis the output power of a given laser diode and on X-axis the input current to the same given laser diode. Below a certain threshold current the laser diode emits only spontaneous emission (working as light emitting diode, LED) and the output power is effectively at zero. In this condition no lasing occurs at the quantum wells of the active region 105. When the threshold current is exceeded, the laser starts emitting stimulated light, in other words it starts lasing, at a power that depends on how much the input current exceeds the threshold. The increase in output power is directly proportional on the increase of input current to the laser diode - naturally provided that the input current exceeds the threshold. The rate of increase in output power versus a given increase in input current i.e. curve derivative is called slope efficiency. However, when temperature is increased, the threshold current increases. Also similarly the slope efficiency is decreased. This results in demand for increased current to maintain a given output power. In Figure 5 is illustrated the threshold currents and slope efficiencies at temperatures 25 deg C and 85 deg C for a given laser diode. The temperature properties of laser diodes have been improved in prior art by compressive or ten- sile strain in quantum wells. Compressive strain lowers the threshold current and improves the slope efficiency. Compressive strain is achieved to a semiconductor layer by growing epitaxially a second grown layer onto a first adjacent layer, which has lower lattice coefficient. The second layer compressively adapts to the adjacent first layer provided that the second layer is not too thick. If critical thickness is exceeded, the strain relaxes at the interface between the first and the second layer and the lattices are no longer in synchrony i.e. have only one lattice structure. Instead, crystal defects like several in- compatible lattice structures form at the layer interface. Also additional quantum wells have also been added to improve the temperature stability. However, the number of compressively strained quantum wells is limited by the critical thickness of the active re- gion. When the critical thickness is exceeded, the strain is relaxed causing crystal defects. Although the number of quantum wells can be enlarged in InGaAsP lasers, the temperature behavior will remain insufficient for operation without active cooling. The insufficient temperature behavior is illustrated in Figure 4a. The five quantum wells have insufficient conduction band offset, which causes carrier leakage in higher temperatures. As the temperature increases, the quantum wells get increasingly shallow and the thermal speed of carriers increases. The capability to hold already caught carriers is reduced due lowered potential difference. Similarly, the capability to catch new carriers is reduced. Further, the lower number of quantum wells contributes to the reduced carrier confinement. Adding an extra quantum well does not largely result in better carrier confinement. In prior art also AlGalnAs-based lasers have been found to be much more stable with respect to op- eration temperature. With this material it is possible to increase barrier height and also to introduce strain to the quantum wells. In prior art typical Al- GalnAs-diode laser operates around 1310 nm wavelength and has six or less quantum wells. For instance, the reference publication US

5,541,949 discloses the use of three 0.98 % tensile strained quantum wells or five 1.43 % compressively strained quantum wells. Reference publication US 5,381,434 discloses the use of five compressively strained quantum wells. With this number of quantum wells it is pos- sible to obtain a relatively good temperature behavior for the lasers. Although each quantum well increases the threshold current, the slope efficiency remains about at the same level. The maximum modulation speed of these structures is not necessarily high enough for 10 Gbit/s operation. Increasing the number of quantum wells improves the high-frequency properties of the active region. Each strained layer added contributes to the overall strain in the active region. If the total thickness of the strained layers exceeds a criti- cal thickness, the structure will get relaxed. Adding highly strained layers requires the addition of opposite compensating strain to the layer structure. By varying compensating strains in the layer structure, the total strain of the entire layer structure becomes zero. The problem with such compensating strain structures is that they are difficult to manufacture. It is possible to carefully manufacture individual laboratory samples, but they will not be mass-production commodities . Reference publication "Well-Thickness Dependence of High-Temperature Characteristics in 1.3 μm AlGalnAs-InP Strained-Multiple-Quantum-Well Lasers" in IEEE Photonics Technology Letters, Vol. 10, No. 12, December 1998 discloses the use of ten AlGalnAs quan- turn wells with lattice matched barrier layers. In this reference publication the quantum well thickness has been 2, 4 or 6 nm and the respective wavelengths produced using these quantum well thickness values have been 1140 nm, 1310 nm and 1410 nm. The use of lattice matched barrier layers imposes an upper limit for the thickness of the quantum well layers and thus requires the use of thin quantum well layers. Such thin quantum well layers are difficult to manufacture. The relative thickness variations in such thin quantum well layers are always larger, which leads to variations in the wavelength of the light emitted between each quantum well in the structure or from run-to-run variations between different structures.

PURPOSE OF THE INVENTION The purpose of the invention is to solve the problems discussed before. Particularly, the purpose of the invention is to improve the temperature behavior of laser diodes including reduced threshold current and better slope efficiency. The purpose of the invention is to improve the stability of the slope ef- ficiency so that there is observed less than 20 % decrease in the slope efficiency when operating temperature is increased from 25 deg C to 85 deg C. In addition the purpose is to improve the spectral properties of laser diodes so that spectral linewidth at operat- ing output power is 1 nm or below at the same temperature range .

SUMMARY OF THE INVENTION The invention is a laser diode with improved slope efficiency and decreased threshold current at high temperature capable of producing optical signals with bit rates equal or higher than 10 Gbit/s. The laser diode produces light at IR- egion of the radiation spectrum. It comprises two cladding layers of opposite conductivity types on opposite sides of an active layer i.e. region. The active region further comprises at least eight compressively strained AlGalnAs quantum well layers the thickness of which is greater than 6 nm and less than or equal to 12 nm; and at least seven AlGalnAs barrier layers placed between the quantum well layers. In one embodiment of the invention the compressive straining of AlGalnAs quantum well layers is higher than 1 %. In yet another embodiment of the invention the laser diode is a ridge wave-guide Fabry- Perot laser. In one embodiment of the invention the Al¬

GalnAs barrier layers have strain between 0.5 % and -0.5 %. In one embodiment of the invention the laser diode is a ridge wave-guide Distributed Feedback la- ser. In another embodiment the laser diode is a buried heterostucture Distributed Feedback laser. In yet another embodiment the laser diode is a buried heterostucture Fabry-Perot laser. The benefits of the invention are related to the improved temperature behavior of laser diodes. A laser diode such as disclosed herein has reduced threshold current and better slope efficiency at higher operating temperatures up to 85 deg C, and even above. Usually 85 deg C is required by standards, but this invention is not limited by this temperature requirement. Due to better temperature behavior the laser diodes can be utilized in equipment without active cooling. Therefore, equipment cost and size is reduced significantly. Similarly, electrical current consumption and total power consumption is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings: Fig. 1 (PRIOR ART) is a block diagram of prior art showing the structure of a laser diode and the manner, in which it is used in optical communica- tion. Fig. 2 (PRIOR ART) is a graph illustrating energy versus crystal momentum in prior art bulk semiconductor and quantum well. It also illustrates carrier state densities for bulk semiconductor and quan- turn well. Fig. 3 (PRIOR ART) is a graph illustrating discrete energies for electrons and holes in a prior art quantum well. Fig. 4a (PRIOR ART) is a graph illustrating energies for quantum wells and barriers in a prior art laser diode active region. Fig. 4b (PRIOR ART) is a block diagram illustrating the cross-section and layers of a prior art laser-diode. Fig. 5 (PRIOR ART) is a graph illustrating input current versus output power in a typical prior art laser diode. Fig. 6 is a block diagram illustrating the cross-section of the core layers in a laser diode in accordance with the invention. Fig. 7a is a graph illustrating energies for quantum wells and barriers in the active region of a laser diode in accordance with the invention. Fig. 7b is a block diagram illustrating the cross-section and layers of a laser diode in one embodiment of the invention. Fig. 8 is a block diagram illustrating the longitudinal view of a laser diode in accordance with the invention. Fig. 9 is a graph illustrating input current versus output power in a typical prior art laser diode and a laser diode in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Figure 6 illustrates the cross-section of the core layers 600 in a laser diode in accordance with the invention. Core layers 600 comprise multi-quantum well active region 605, which comprises eight or more AlGalnAs quantum well layers (such as 607) and their accompanying AlGalnAs barriers (such as 606) . AlGalnAs wave-guide layer 604 below active region 605 is n- doped. The AlGalnAs wave-guide layer 608 above active region 605 is p-doped. The doping -concentrations in layers 604 (AlGalnAs n-doped) and 608 (Al-GalnAs p- doped) have opposite doping concentrations. The quantum wells are epitaxially grown and highly compres- sively strained (> 1 %) AlGalnAs thin films. The thickness of each quantum well layer is greater than 6 nm and equal or less than 12 nm. The quantum well layers are surrounded by AlGalnAs barrier layers (such as 606) and wave-guiding layers 604 and 608. The bar- rier layers and wave-guiding layers 604 and 608 have slightly different composition than the quantum wells. The quantum wells and barriers in the active region produce light at IR-region of electromagnetic radiation spectrum, namely at wavelengths 1.0 μm - 1.6 μm. The core layers 600 of the invention can be used in all kinds of lasers such as for example Ridge Wave- guide (RWG) , Buried Heterostructure (BH) Fabry-Perot (FP) or Distributed Feedback (DFB) lasers. Figure 7a illustrates the carrier confinement properties of active region 605, which are improved when compared to Figure 4a. There are eight quantum wells, which exhibit relatively high barrier energy E compared to the quantum well energies E_q. It should be noted that in one embodiment of the invention the number of quantum wells may be higher than eight. Figure 7b illustrates the cross-section of a Ridge Wave-guide Fabry-Perot (RWG-FP) laser 700 utilizing the core layers 600 from Figure 6 in one embodiment of the invention. The laser 700 comprises AlGalnAs multi-quantum well active region 605 sandwiched between partly p-doped AlGalnAs wave-guide 608 and partly n-doped AlGalnAs wave-guide 604. Figure 7b depicts eight quantum wells in active region 605. Below the wave-guide 604 there is n-InP or n-AlInAs cladding 702. Below the cladding 702 is an n-InP substrate. In the p-InP or p-AlInAs cladding 710 a ridge wave-guide, which confines the light emitted from active region 605 laterally, is formed. It acts as a single-mode optical wave-guide. The sides of the ridge are covered with dielectric material 712 such as silicon dioxide (Si0₂) . The ridge top is covered with p-InGaAs contact layer 716. Dielectric material 712 and contact layer 716 are covered with a metal contact layer 714. On top of the metal layer there is a thick plated gold pad 718, which spreads heat. Negative contact is located below the thinned n-InP substrate 701. Figure 8 illustrates the longitudinal view of a Fabry-Perot laser diode 700 in accordance with the invention. At he ends of laser diode 700 there are mirrors i.e. facets. End 801 of the laser diode is a high reflectance surface. End 800 is a low reflectance surface, which reflects only part of the photons. The laser beam is mainly emitted from the low reflectance end 800. When utilizing laser diode 700, the contact 716 on the topside is connected to the positive terminal of a current driver, whereas the ground or negative terminal is connected to the backside contact. When current is driven through the laser diode, electrons will travel through the quantum well region. Part of the electrons get trapped to the bonded state of the quantum wells where they can then drop to a lower energy level releasing the energy in the form of a photon. The photons are guided by the wave-guide layers 604 and 608 in the other direction and by the ridge in the other direction and they will travel along the cavity. Part of the photons emitted is reflected from the end mirrors and the photon density starts to dominate and lasing takes place. The beam coming out of the edge of the laser has a narrow 1-2 nm spectral width and the beam shape is defined by the optical cavity. Typically, the beam shape is elliptical . Figure 9 is a graph illustrating input current versus output power in a typical prior art laser diode and a laser diode in accordance with the invention. Y-axis represents the output power and X-axis represents the input current. Graphs 902 and 908 rep- resent the slope efficiencies of a prior art laser diode at 25 deg C and 85 deg C, respectively. -Graphs 904 and 906 represent the slope efficiencies of a laser diode in accordance with the invention at 25 deg C and 85 deg C, respectively. Figure 9 points out the major difference of this invention compared to prior art laser diodes such as the one shown in Figure 4b. Threshold current at room temperature remains about the same or slightly increases in a laser diode in accordance with this invention due to additional quantum wells. However, the threshold current at a high temperature (like 85 deg C) is much smaller and the slope effi- ciency is much better than in devices based on prior art. This leads to the overall benefits of this invention. Two most-commonly known growth methods suitable for the growth of the quantum wells are Molecular Beam Epitaxy (MBE) and Organometallic Vapour Deposition (OMCVD, MOCVD) . MBE is based on either solid elemental material sources (SSMBE) or gaseous sources (GSMBE) , which are evaporated as molecular beams on the surface of the substrate, where they form compound semiconductors such as AlGalnAs. In MOCVD the material elements are parts of gases and after chemical reactions on the surface of the substrate they form desired compound semiconductors. It is obvious to a person skilled in the art that with- the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims .

Claims

CLAIMS 1. A laser diode producing light at IR-region of the radiation spectrum, comprising two cladding layers of opposite conductivity types on opposite sides of an active region, the active region further comprising: at least eight compressively strained AlGalnAs quantum well layers the thickness of which is greater than 6 nm and less than or equal to 12 nm; and at least seven AlGalnAs barrier layers placed between the quantum well layers. 2. A laser diode according to claim 1, wherein said compressive straining of AlGalnAs quantum wells is higher than 1 %. 3. A laser diode according to claim 1, wherein said AlGalnAs barrier layers have strain between 0.5 % and -0.5 %. 4. A laser diode according to claim 1, wherein said laser diode is a ridge wave-guide Fabry- Perot laser. 5. A laser diode according to claim 1, wherein said laser diode is a ridge wave-guide Distributed Feedback laser. 6. A laser diode according to claim 1, wherein said laser diode is a buried heterostucture

Distributed Feedback laser. 7. A laser diode according to claim 1, wherein said laser diode is a buried heterostucture Fabry-Perot laser.