Laser comprising stacked laser diodes produced by epitaxial growth inserted between two bragg mirrors

[0002] 1. Field of the Invention

[0003] The field of the invention is that of lasers comprising a stack of semiconductor lasers making it possible to achieve powers of the order of several kilowatts.

[0004] According to the prior art, lasers of this type comprise laser diodes produced by epitaxial growth, on a semiconductor substrate C_ndoped with impurities of a certain type (generally of the n type), of a quantum-well emissive structure PQ then a layer C_pdoped with impurities of the opposite type (generally of the p type).

[0005] FIG. 1 illustrates a stack according to the prior art of two laser diodes: metal contacts ME₁, and ME₂are deposited on either side of the laser diodes, and the laser diodes are stacked, layer C_non layer C^p, via the metal contacts ME₁and ME₂.

[0006] Generally, metal mechanical jaws enclose the set of diodes in order to dissipate the heat due to the strong thermal effects which take place in each laser diode. The diodes are electrically connected by soldering between a metallization ME₁and a metallization ME₂. The electrons pass by the tunnel effect through the Schottky junction corresponding to the interface of a layer C_nand a metallization ME₁while the holes can pass by the tunnel effect through the Schottky junction corresponding to the interface between the layer C_pand the metallization ME₂. This is illustrated in FIG. 2, which represents a diagram of the energy bands of the layers C_n, ME_iand C_pwhich are used in a stack of laser diodes according to the prior art.

[0007] The quantum-well emissive structures are spaced apart by at least the thickness of a substrate layer (typically a layer C_nrepresented in FIG. 1), that is to say by several hundreds of microns. In this configuration, the diodes have to be driven by pulses which are short enough for the diffusion lengths of the temperature gradients Ld not to overlap, in order to avoid excessive local temperature rises, the length Ld being given by the following equation (1): $\mathrm{Ld}={\left(\frac{k\tau }{\rho K_p}\right)}^{1/2}$

[0008] with k: thermal conductivity

[0009] τ: pulse length

[0010] ρ: semiconductor density

[0011] K_p: semiconductor heat capacity.

[0012] For substrates about 400 μm thick, lengths Ld of the order of 200 μm are typically obtained for pulse lengths of the order of 70 to 100 microseconds.

[0013] These configurations for stacked laser diodes have the drawback that they require the use of external means for carrying out assembly (in particular soldering), and are of a size which it is difficult to minimize further, because of the thickness of the substrate of each elementary diode.

[0014] In order to solve these problems, the invention proposes a laser structure in which the laser diodes are stacked periodically by epitaxial growth of a set of semiconductor layers, requiring no manual intervention.

[0015] More precisely, the invention relates to a laser comprising a stack of N laser diodes DL_i, each having a quantum-well emissive structure. S_i, characterized in that:

[0016] the stack of diodes DL_iis a stack of epitaxial semiconductor layers which is inserted between two mirrors so as to produce a laser cavity;

[0017] the ohmic contact between a laser diode DL_iand a laser diode DL_i+1is provided by an Esaki diode junction formed of an n-doped layer with a very high doping level and of a p-doped layer with a very high doping;

[0018] the optical electric field of the mode created in the laser cavity is periodically cancelled at the Esaki diode junctions.

[0019] More precisely, the junctions being of the Esaki diode type, this laser according to the invention may comprise a stack of N laser diodes, characterized in that the stack of epitaxial layers comprises layers S_i, n-doped layers C′_ni, p-doped layers C′_pi, a layer C′_nibeing adjacent to a layer C′_pi+1, the doping level t′_niof the layers C′_niand the doping levels t′_piof the layers C′_pibeing such that an electron flow is capable of passing by the tunnel effect from a layer C′_pito a layer C′_ni+1so as to provide the ohmic contact between a diode DL_iand a diode DL_i+1.

[0020] Since the free carriers, present in very large numbers at the Esaki diode junction, absorb the optical wave which is generated, the invention proposes a laser structure designed so that the optical electric field of the mode is cancelled at the position of the Esaki diode junctions.

[0021] This solution is advantageous in so far as it makes it possible to retain laser structures with small dimensions in comparison with a laser structure in which the choice is made to distance the junctions between diodes of the laser emission zone.

[0022] According to an alternative embodiment, the stack of diodes (DL_i) is inserted between two plane mirrors which are parallel to the plane of the epitaxial semiconductor layers, so as to create laser emission perpendicular to the plane of the layers.

[0023] Advantageously, the mirrors may be Bragg mirrors, one of which has a reflectivity close to 100% for the optical wave which is created, the other Bragg mirror having a lower reflectivity so as to allow the laser emission to emerge.

[0024] According to another variant of the invention, the stack of diodes (DL_i) is inserted between two plane mirrors which are perpendicular to the plane of the epitaxial semiconductor layers, so as to create laser emission parallel to the plane of the layers.

[0025] The optical wave is confined in the stack of diodes by inserting the set of epitaxial layers between two confinement structures or layers.

[0026] These confinement structures may in particular consist of Bragg mirrors having close to 100% reflectivity for the optical wave created in the laser cavity.

[0027] The invention will be understood more clearly, and other advantages will emerge, on reading the following description which is given by way of nonlimiting example and by virtue of the appended figures, in which:

[0028] FIG. 1 illustrates a stack of laser diodes according to the prior art;

[0029] FIG. 2 represents the energy band diagram of the layers C_n, ME_iand C_pwhich are used in a stack of laser diodes according to the prior art;

[0030] FIG. 3 illustrates an example of a laser diode stack according to the invention, using Esaki junction;

[0031] FIG. 4aillustrates the energy band diagram of the stack of laser diodes which are used in a laser according to the invention, comprising Esaki junctions in the absence of voltage;

[0032] FIG. 4billustrates the energy band diagram of the stack of laser diodes which are used in a laser according to the invention, comprising Esaki junctions in the presence of an applied voltage;

[0033] FIG. 5 illustrates an example of a laser according to the invention, comprising two maximum-reflection Bragg mirrors and Esaki diodes, and emitting in a plane parallel to the plane of the diodes;

[0034] FIG. 6 illustrates the optical field created by the laser according to the invention, close to the laser (near field) and far from the laser (far field);

[0035] FIG. 7 illustrates an example of a laser according to the invention with surface emission, using a partially transparent Bragg mirror;

[0036] FIG. 8 illustrates the variation in the temperature of a laser according to the invention, at the centre of the said laser, as a function of time during a pulse;

[0037] FIG. 9 illustrates the maximum pulse length permitted as a function of the thickness of a laser structure according to the invention, for an allowed temperature difference To=10 K.

[0038] According to the invention, the laser structure comprises a set of semiconductor layers grown epitaxially so as to produce a stack of laser diodes, two adjoining laser diodes being electrically connected by means of Esaki diode junctions, such a junction being formed by a very heavily p-doped region and a very heavily n-doped region. It is actually known, as explained for example in the book by S. M. Sze, Physics of Semiconductor Devices, John Wiley (New York, 1983) that such a junction when reverse-biased (as is the case in the stacked structure of the invention) allows electrons to pass from the valence band on the heavily p-doped side to the conduction band on the heavily n-doped side.

[0039] The emissive structure S_iof each laser diode may be contained between two optical confinement layers with a lower index, one of the layers C_nibeing n-doped, the other layer C_pibeing p-doped. FIG. 3 shows the way in which a laser diode D_i+1is stacked on a laser diode D_iby means of the heavily doped layers C′_niand C′_pi+1. The electric field vector {overscore (E)} indicates the electrical bias direction to be applied during operation in order to allow electron-hole recombinations at the emissive structures S_i.

[0040] FIG. 4aillustrates the energy band diagram of the set of semiconductor layers allowing creation of the stack of laser diodes according to the invention, without an applied voltage.

[0041] FIG. 4billustrates the energy band diagram of the set of semiconductor layers allowing creation of the stack of laser diodes according to the invention, with an applied voltage.

[0042] Several quantum-well structures have been represented in order to demonstrate the movement of electrons at each junction. Four laser diodes are thus illustrated here by a succession of layers C_ni, quantum-well structures PQ_iand layers C_pi.

[0043] FIG. 4ashows the energy band diagrams of such a structure. At zero voltage, the junctions which are in series have periodic potential barriers. Under a forward-bias voltage applied to the overall structure, the Esaki junctions are all reverse-biased. As indicated by FIG. 4b, for a high enough voltage, the system assumes a flat band configuration. Once in flat bands (with small potential drops at the junctions), the structure allows the current to flow and the layers start to lase. The operating principle can be interpreted in the following way: once it has made an optical transition at the junction S_i, the electron is injected from the region C_pito the region C_ni+1by the Esaki junction C_pi/C_ni+1. It will be noted that the Esaki diode can be produced in a semiconductor having a smaller gap than the substrate, in order to reduce the contact resistance of the diode, by using an InGaAs alloy on GaAs, for example.

[0044] According to a first variant, the stack of diodes is inserted between two plane mirrors which are perpendicular to the plane of layers. A first mirror has close to 100% reflectivity, and the other mirror has lower reflectivity.

[0045] In order to cancel the optical electric field of the mode which is created at the Esaki junctions, the stack of diodes may advantageously be integrated between two series of Bragg mirrors which are parallel to the plane of the layers, having close to 100% reflectivity. The structure constitutes a guide such that the guided mode has a zero electric field at the Esaki junctions JT_i, the laser beam being emitted parallel to the layers (see FIG. 5), with E(x) representing the electric field of the optical wave in the laser cavity. By way of example, for 10 stages, the total thickness would then be 10 times the halfwavelength divided by the optical index, that is to say of the order of 2 μm.

[0046] In this configuration, the laser layers are very strongly coupled to one another, and their emissions can naturally be set in phase. This provides a vertical array of lasers in phase, which is extremely difficult to obtain in a horizontal geometry. It is known that these arrays can emit beams whose divergence is much less than in a single laser (A. Yariv, Quantum Electronics, Wiley Interscience). In this regard, FIG. 6 illustrates the extent of the “near” optical field of the laser and the extent of the “far” optical field.

[0047] We will now provide an illustrative embodiment. The substrate is heavily n+-doped GaAs (typically 10¹⁸cm⁻³). An n+ GaAs buffer layer is produced according to the rules of the art then the component is formed by a stack of the following elementary structures: a strongly p-doped AlGaAs barrier followed by a GaAs layer doped n++ (a few 10¹⁹cm⁻³) then doped p++. Each elementary layer has a thickness of 1 μm. The n++/p++ contact resistance is typically 10⁻⁴ohm.cm⁻². Since the laser currents are of the order of 15 kA/cm², there is a 100 millivolt potential drop at each Esaki junction. This potential drop can be reduced by using an In_0.2Ga_0.8As Esaki diode which, by virtue of the gap reduction in comparison with that of GaAs, leads to a 30 mV voltage drop.

[0048] According to another variant of the invention, the laser emission is obtained perpendicular to the plane of the diodes, and for this the stack of diodes may be integrated between two Bragg mirrors, one of which is slightly transparent, the plane of the said mirrors being parallel to the plane of the diodes.

[0049] The structure is produced in the form of a mesa with small dimensions. The laser emission then takes place through the surface, and the system is therefore of the VCSEL type (Vertical Cavity Surface Emission Laser) (see FIG. 7). Let us note that, in this case, the two Bragg mirrors may be n-doped, which considerably reduces the access resistances and greatly facilitates the VCSEL production process.

[0050] The stacks of epitaxial laser diodes according to the invention have different thermal behavior from the stacked laser diodes according to the prior art:

[0051] In spite of the increased proximity of the emissive structures to one another, and of the maximum heating zones, the structures according to the invention are capable of dissipating the heat satisfactorily.

[0052] In conventional structures using a plurality of diodes which are prefabricated then stacked, the temperature decreases linearly in each layer over a distance given by equation (1) which was defined above (typically over a distance of 200 μm for a length of time of 100 μs).

[0053] In the epitaxial structure according to the invention, the temperature may be assumed to be uniform over the entire thickness of each laser diode after a very short time, because of the short distances separating two emissive regions S_i. The metal bases constitute heat reservoirs.

[0054] It may thus be considered that the active structure is placed between two heat reservoirs at a relative temperature T=0 and that during a current pulse the heat power dissipated is distributed uniformly throughout the active structure.

[0055] If x corresponds to the distance from the center of the structure in the direction of a heat reservoir, the temperature field T (x, t) as a function of this distance and of time is then determined by the diffusion equation (2):

[0056] ∂T/∂t=D∂²T/∂x²=P/ρc_pL

[0057] with D: thermal diffusion constant

[0058] P: heat power divided by the laser cross section

[0059] L: overall laser thickness corresponding to the epitaxial structure.

[0060] The boundary conditions for this equation (2) are:

[0061] T(−L/2,t)=T(L/2,t)=0

[0062] and T(x,0)=0

[0063] By redimensioning with L as the length unit, L²/D as the time unit and PL/ρC_pD (or alternatively PL/K, K being the thermal conductivity) as the temperature unit, the following equation (3) is also obtained:

[0064] ∂T′/∂t′=∂²T′/∂x′²+1

[0065] The steady-state solution to equation 3 is the following equation (4):

[0066] T′∞(x′)=⅛−x′²/2

[0067] and the maximum steady temperature is therefore T_max=PL/8K.

[0068] In this regard, FIG. 8 illustrates the results of the variation of the curve T′ (0, t) at the center of the structure. Two ranges are clearly demonstrated: for small t′, the temperature rises linearly, indicating that the heat is by no means dissipated to the reservoirs, and for larger t′, the temperature stabilizes at the steady-state value, indicating that the heat is continuously dissipated.

[0069] These principles can be applied in the specific case of a laser structure according to the invention based on GaAs. Taking into account the permissible temperature rise T₀and a given laser thickness L, it is possible on the basis of a curve such as the one represented in FIG. 6 to determine the maximum pulse lengths which the laser structure can tolerate on the basis of imposed parameters such as the permissible temperature rise.

[0070] An example is illustrated in FIG. 9, the constants of the GaAs material being as follows:

[0071] D=2.5×10⁻⁵m²/s

[0072] K=46 W m⁻¹xK⁻¹

[0073] P=2 kW/0.06×10⁻⁶m²

[0074] T₀=10K

[0075] The curve of FIG. 9 gives the maximum pulse length τ in seconds as a function of the thickness L of the laser, expressed in metres. The curve illustrated in this figure relates to a temperature T₀=10 K.