InP-BASED MQW-LASER

Abstract: An InP-based MQW-laser for the 1.3 mu m band has at least one, preferably several, well layers sandwiched between barrier layers having larger band gaps. The well layers and the barrier layers both have essentially the same content composition of group-III elements, but different content compositions of group-V elements. The well layers have lattice constants which deviate from the substrate within the limits of possible voltages, so-called mismatching, whereas the well layers have a smaller mismatch.

Description: InP-based MQ - laser. The present invention relates to a semiconductor laser of the kind defined in the preamble of Claim 1. Semiconductor lasers of this general kind have been known for a relatively long time and the basic principle of such lasers will be apparent from the monograph "Long Wavelength Semiconductor Lasers" by G.P. Agrawal and N.K. Dutta (New York 1986), this monograph herewith being declared as constituting part of the present description. It has been possible to produce MQW-type (Multiple Quantum Well) semiconductor lasers for the wavelength band 1.55 μm, which is of interest in the communications field (optical fibres). However, in the case of the wavelength band 1.3 μm, which is also of interest, no practical types of laser have been proposed that will function without being cooled. An object of the invention is to rectify this deficiency. Such lasers are required to function uncooled at relatively high powers and modulation rates. This implies low threshold current values that preferably do not rise with increasing ambient temperatures. This requires radiation-free transitions, Auger transitions being normal in this respect. One factor in this regard is that the different well layers shall be able to take-up the same number of holes and electrons without an exaggerated collection of holes in the wells closest to the p-contact in turn attracting electrons to the same wells. The non-radiating recombination (e.g. the Auger recombination) namely rises much more rapidly than linear as a function of the charge carrier density, whereas the rise in light amplification is lower than linear at this density. (not understood) Consequently, one should strive for uniform well distribution. The crystalline substances concerned in this regard are substances that include a mixture of group 3 atoms and group 5 atoms: Fig. 1 is a diagrammatic illustration of the former combination, in which dotted-line curves show the energy gap expressed in a wavelength measurement in a bulk alloy as a function of x and y (in thin layers in the con iguration relevant in respect of MQW-lasers, the wavelength will be shorter due to the quantised energy states in the wells present in the conduction and valence bands). The full lines in Fig. 1 denote equilines for lattice constants of equal size, namely such that the line which passes from origo to lm (lattice matched) signifies compositions that have the same lattice constant as a substrate of indium phosphide (InP). Those lines to the left of this line, reference 1% cs and 2% cs, show reduced lattice constants, also called compressive (constrictive) in relation to the substrate, whereas the line to the right of said line signifies corresponding extended or increased lattice constants, called tensile constants ?. It will be realised, and is also well known, that epitaxial crystal growth is not possible between layers where the difference in lattice constant is excessively large (also called mismatching), while retaining single crystalinity - a rule of thumb is that a difference of 20 nm% should not be exceeded, in other words the layer thickness measured in nm and multiplied by the mismatch in percent (strain) should be kept beneath this value. The reason why one no longer sticks to the lattice- match line hangs together with improved theoretical laser properties in MQW-lasers having lattice mismatch wells, due to the separation of heavy and light holes in the valence band and the change in the hole masses that cause the state density in the valence band to approach the state density in the conduction band, which leads to the requirement of reduced inversion. A well known solution with respect to MQW-lasers for the 1.55 μm band is to place both the well layers and barrier layers on a horizontal line in accordance with the Fig. 1 diagram, with opposing 1% mismatches. Tests have shown that a corresponding placement, on a lower arsenic level, is unsuccessful by virtue of being unable to detect any layers in X-ray diffraction, therewith excluding the use of such material for an MQW-laser. The invention solves the aforesaid problem by arranging the well layers and barrier layers of an InP-based MQW-laser of the aforesaid kind in accordance with the characterising clause of Claim 1. Further embodiments of the invention will be apparent from the dependent Claims . It will be understood that the manufacturing methods used and available at present are generally encumbered with serious difficulties with respect to precision. The quantitative magnitudes recited herein shall not therefore be considered to be more precise than what is meaningful in relation thereto, therewith bestowing an explanation and definition of the word "essentially" or "generally" as used in this document . BRIEF DESCRIPTION OF THE DRAWINGS As before mentioned, Fig. 1 is a graphic illustration of the InGaAsP system and shows how the lattice constant and energy gap respectively vary with different compositions in bulk. Fig. 2 illustrates schematically and in section an MQW-laser with the thickness shown along the X-axis and the energy gap shown along the Y-axis. Fig. 3 illustrates the influence of the effective heavy well depth on the lasering threshold current and the optical power output. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The invention will now be described in more detail with reference to illustrative examples thereof. Manufacture of epitaxial layers of the kind meant here is well known and will not therefore be described in great detail. Gaseous compounds of the various substances are passed at a low pressure (normally 20-100 bar, in the present case 50 mbar) over a monocrystalline InP substrate towards the plane <100> heated to about 700°C (standard method temperature of 600-700°C), wherewith the gases are decomposed and the relevant atoms released so as to build up a layer which follows the existing crytalline lattice. According to MOVPE (Metal Organic Vapour Phase Epitaxy) there is used phosphine (PH 3 ), arsine (AsH 3 ) and TMIn, TMGa and possibly TMA1 , entrained in a hydrogen gas flow. Fig. 2 illustrates schematically one such epitaxial structure, where the substrate 1 has been doped to either p- conduction or n-conduction, and successive barrier layers 2 and well layers 3 are provided for connection with an electrode layer 4 of opposite conduction to the substrate. Although the X-axis has been given a length measurement, the axis is not according to scale. For instance, the wells may be very shallow, e.g. 6.5 nm, and the barrier layers will be normally thicker, e.g. 8 nm. The outermost barriers 2' may have a much greater thickness, e.g. a thickness of 100 nm, and function as optical waveguides through the medium of a high optical refractive index, which encloses the optical field across the wells, i.e. the active area. EXAMPLE As before mentioned, there is used in accordance with the invention combinations of respective wells and barriers that have mutually the same content of groups-Ill atoms, while varying the content of group-V atoms. The system shown in Fig. 1 was used in a test series, with x = 0.10 and y = 0.52 in respect of the wells, y was varied between 0.08 and 0.22 in several stages in respect of the barriers. It was possible to show in earlier tests with the aid of high resolution, X-ray diffraction and photoluminescence that it is not possible to obtain a good structure with a constant content of group-V atoms and a varying content of group-Ill atoms (horizontally situated points in Fig. 1), which according to known technology functions well within the 1.55 μm wavelength range. On another occasion, a test was run with a constant content of group-Ill atoms, x = 0.1, y = 0.52 in respect of the wells, x = 0.1, y = 0 in respect of the barriers, and a particularly good structure was established in X-ray fraction and also in photoluminescence from the well package, although the lasers still exhibited very low internal efficiencies. Lasers having a width of 50 μm and lengths of 300-1200 μm with mirrors in the form of fission/?/ surfaces were produced from the coated substrates. The hole constriction energy/?/ in respect of heavy holes in the wells could be varied between 130 and 215 meV, by changing the barrier composition while maintaining the same well composition. Lasers having 5, 8 and 12 well layers were produced at a hole constriction energy/?/ of 130 meV, otherwise lasers having five well layers were produced. The results are evident from Table 1. In the table, the first row and the third from the end refer to the same laser. The constant T 0 is a measurement of the thermal drift of the threshold current according to the idealised relationship I exp T/T 0 calculated for different temperature ranges. Table 1 Threshold Calc. Number of Internal Internal density at T 0 T 0 hole depth well layers ef f iciency losses/? inf inite? 2 0-60°C 2 0-80°C [meV] [%] [%] [A/cm 2 /well] [K] [K] 130 9 . 7 65 83 67 154 97 11 . 8 71 74 67 186 77 66 63 215 90 60 58 130 85 9 . 7 65 83 67 1 3 0 89 13 63 67 70 1 3 0 1 2 98 23 60 77 79 It is evident that the barrier height, which determines the calculated well depth of the holes, is highly significant to the result and it is preferred to keep the barrier height as low as possible in practice and at least beneath 170 meV, although it should not be too low either, since the advantage of having wells would then disappear and the laser would begin to liken a bulk laser. As will be evident from Fig. 3, which shows with solid squares the threshold density for lasering in A cm " (the scale to the left) and with circles the optical power output in mW (the scale to the right) at otherwise equal test conditions (mW) as functions of effective hole well depth in meV. Hole transport times across the well package can be measured in a manner taught by Tessler et al, "Vertical carrier transport in InGaAsP Multiple-quantum - Well Laser Structures: Effect of p-doping", IEEE Journ. Of Selected Topics in Quant. Electron., Vol. 3, No. 2, pp. 315-319. It is well known that the total transfer velocity of the charge carriers is determined by the hole dynamics. It has been found possible to obtain extremely good geometric properties with respect to the well and barrier layers by means of the inventive MQW-laser, and also well-functioning barrier conditions that are directed towards suppressing tendencies towards radiation-free transitions, so-called Auger effects, and low lasering threshold current densities, these factors both contributing towards high efficiency and low losses, such as to enable controllable laser light sources to be driven at usable intensities without special cooling, even in the wavelength band around 1.3 μm. It is assumed that the good result is due to the combination of lattice-mismatched wells having relatively high electron constrain ent energy/?/ (which is highly significant with respect to high temperature properties where it is the electrons that can leave the well most easily due to thermal escape) and optimisation of a uniform charge distribution across the well packet, achieved by practical tests that have been able to ensure a high material quality./?/ This results in lasers that have low internal losses and thus low threshold currents, and a threshold current that is temperature-stable . It will be understood that the invention is not restricted to the exemplified embodiment thereof and that the scope of the invention is restricted solely by the contents of the following Claims.

Claims: CLAIMS 1. An InP-based MQW-laser including the group-III elements In, Ga and possibly Al, and the group-V elements P and As and operating in the 1.3 μm band and built-up on an InP-substrate and having at least one epitaxial well layer with a lattice constant mismatch in relation with the substrate in sandwich formation with barrier layers that have a larger energy gap than said well layers, wherein said well layers have an energy transition from a first quantised state in its conduction band to a first quantised state in its valence band that corresponds to the laser wavelength, and wherein said well layer and barrier layer have essentially the same group Ill-element content, characterised in that the mismatch of the barrier layers is tensile or lattice-adapted by variation of the group-V element content of the barrier layer such that the effective hole barrier will be less than 170 meV and preferably less than 150 meV. 2. An MQW-laser according to Claim 1, characterised in that the content of the group-III element Ga is 5-20%. 3. An MQW-laser according to Claim 1 or Claim 2, characterised in that the content of group-III element Ga and/or Al is 5-20%. 4. An MQW-laser according to Claim 1 or Claim 2, characterised in that the group-III element Ga is 10%. 5. An MQW-laser according to Claim 1 or Claim 2, characterised in that the content of the group-III element Ga and/or Al is 10%. 6. An MQW-laser according to any one of the preceding Claims, characterised in that the content of the group-V element As in the barriers lies between 8-22%. 7. An MQW-laser according to any one of the preceding Claims, characterised in that it includes a plurality of well layers. 8. An MQW-laser according to any one of the preceding Claims, characterised in that the mismatch of said well layer in relation to the substrate is greater than 0.5%.

Inventors: LANDGREN GUNNAR, SILFVENIUS CHRISTOFER, LANDGREN, GUNNAR, SILFVENIUS, CHRISTOFER
Applicants: ERICSSON TELEFON AB L M, LANDGREN GUNNAR, SILFVENIUS CHRISTOFER, LANDGREN, GUNNAR, SILFVENIUS, CHRISTOFER, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: H01S3
IPC codes: H01S5