AlInGaN alloy based laser diode

Abstract

The invention relates to an AlInGaN alloy based laser diode, which uses a gallium nitride substrate. It also includes a lower cladding layer, a lower light-guiding layer-cladding, a light emitting layer, an upper light-guiding-cladding layer, an upper cladding layer, and a subcontact layer. The lower light-guiding-cladding layer and the upper light-guiding-cladding layer have a continuous, non-step-like and smooth change of indium and/or aluminum content.

Claims

1. An AlInGaN alloy based laser diode, comprising: a gallium nitride substrate; a lower cladding layer with n-type electrical conductivity; a lower light-guiding-cladding layer with n-type electrical conductivity; a light emitting layer; an upper light-guiding-cladding layer with p-type electrical conductivity; an upper cladding layer with p-type electrical conductivity; and a subcontact layer with p-type electrical conductivity; wherein the lower light-guiding-cladding layer and upper light-guiding-cladding layer have a continuous, non-step like and smooth change of indium and/or aluminium content.

2. The laser diode according to claim 1, wherein the lower light-guiding-cladding layer has a continuous, non-step-like and smooth change of the refractive index nn, described by the equation (W1): $\begin{array}{cc}{n}_n=\frac{a_1-a_4}{1+e^{\frac{-\left(x-a_3\right)}{a_2}}}+a_4,& \left(\mathrm{W1}\right)\end{array}$ where x is distance from the light emitting layer; a.sub.1 is in range of 2.38 to 2.66; a.sub.2 is in range of 1 to 100; a.sub.3 is in range of 800 to 0; a.sub.4 is in range of 2.32 to 2.57.

3. The laser diode according to claim 1, wherein the lower light-guiding-cladding layer has a continuous, non-step-like and smooth change of the silicon doping profile dop.sub.n, described by the equation (W3): $\begin{array}{cc}{\mathrm{dop}}_n=\frac{-c_1}{1+e^{\frac{-\left(x-c_3\right)}{c_2}}}+c_1,& \left(W3\right)\end{array}$ where x is distance from the light emitting layer; c.sub.1 is in range of 110.sup.18 to 110.sup.20; c.sub.2 is in range of 1 to 100; c.sub.3 is in range of 800 to 0.

4. The laser diode according to claim 1, wherein the upper light-guiding-cladding layer has a continuous, non-step-like and smooth change of the refractive index np, described by the equation (W2): $\begin{array}{cc}{n}_p=\frac{b_4-b_1}{1+e^{\frac{-\left(-x-b_3\right)}{b_2}}}+b_1,& \left(\mathrm{W2}\right)\end{array}$ where x is distance from the light emitting layer; b.sub.1 is in range of 2.32 to 2.57; b.sub.2 is in range of 1 to 100; b.sub.3 is in range of 0 to 800; b.sub.4 is in range of 2.38 to 2.66.

5. The laser diode according to claim 1, wherein the upper light-guiding-cladding layer has a continuous, non-step-like and smooth change of the silicon doping profile dop.sub.p, described by the equation (W4): $\begin{array}{cc}{\mathrm{dop}}_p=\frac{d_1}{1+e^{\frac{-\left(x-d_3\right)}{d_2}}},& \left(\mathrm{W4}\right)\end{array}$ where x is distance from the light emitting layer; d.sub.1 is in range of 110.sup.18 to 110.sup.20; d.sub.2 is in range of 1 to 100; d.sub.3 is in range of 0 to 800.

6. The laser diode according to claim 4, wherein the diode has a ridge structure, wherein the ridge is created to the depth in range from the light emitting layer to at least the first derivative of function (W2), preferably to the depth in range of 99% to 80% of the maximum value of the refractive index n.sub.p, described by equation (W2).

7. The laser diode according to claim 1, wherein between the lower light-guiding-cladding layer and the light emitting layer there is a lower light guiding layer, which can have an n-type electrical conductivity or be an undoped layer.

8. The laser diode according to claim 1, wherein between the light emitting layer and the light-guiding-cladding layer there is an upper light guiding layer, which can have a p-type electrical conductivity or be an undoped layer.

9. The laser diode according to claim 1, wherein the upper light-guiding-cladding layer contains a region blocking the escape of electrons.

10. The laser diode according to claim 1, wherein the subcontact layer is doped with acceptors above the concentration level of 10.sup.20 cm.sup.3.

11. The laser diode according to claim 1, wherein it emits light with wavelength in range of 380 nm to 555 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention together with the above and other objects and advantages will be best understood from the following detailed description of the preferred embodiment of the invention shown in the accompanying drawings, wherein:

(2) FIG. 1 shows a cross section of the laser diode according to one embodiment of the present invention;

(3) FIG. 2 shows refractive index profile of the laser diode layers, according to one embodiment of the present invention;

(4) FIGS. 3a and 3b show the doping profile of the laser diode, according to one embodiment of the present invention, for silicon and magnesium, respectively;

(5) FIGS. 4a-4e show the consecutive technological steps of manufacturing of the laser diode ridge, according to one embodiment of the present invention;

(6) FIGS. 5a and 5b how the optical power dependence on current, for the laser diode according to one embodiment of the present invention and a reference laser diode, respectively.

DETAILED DESCRIPTION

(7) The technical problem faced by the present invention is proposing such a construction of an AlInGaN alloy based laser diode, which would be characterised by improved opto-electric parameters, particularly a low threshold current and the highest possible increase of power as a function of current, above the threshold current of the laser diode. Simultaneously, there is a desire for a problem solution that would not involve any significant change in the present manufacturing process, thereby not causing a significant increase of the cost of a single device. Unexpectedly, the mentioned technical difficulties were solved by the presented invention.

(8) The present invention relates to AlInGaN alloy based laser diode, comprising a gallium nitride substrate, a lower cladding layer with n-type electrical conductivity, a lower light-guiding-cladding layer with n-type electrical conductivity, a light emitting layer, an upper light-guiding-cladding layer with p-type electrical conductivity, an upper cladding layer with p-type electrical conductivity, and a subcontact layer with p-type electrical conductivity, characterised in that the lower light-guiding-cladding layer and upper light-guiding-cladding layer have a continuous, non-step-like and smooth change of indium and/or aluminium content. In a preferred embodiment of the invention the lower light-guiding-cladding layer has a continuous, non-step-like and smooth change of the refractive index n.sub.n, described by the equation (W1):

(9) $\begin{array}{cc}{n}_n=\frac{a_1-a_4}{1+e^{\frac{-\left(x-a_3\right)}{a_2}}}+a_4,& \left(\mathrm{W1}\right)\end{array}$ where: x is the distance from the light emitting layer; a.sub.1 is in range of 2.38 to 2.66; a.sub.2 is in range of 1 to 100; a.sub.3 is in range of 800 to 0; a.sub.4 is in range of 2.32 to 2.57.

(10) In another preferred embodiment of the invention, the lower light-guiding-cladding layer has a continuous, non-step-like and smooth change of the silicon doping profile dop.sub.n, described by the equation (W3):

(11) $\begin{array}{cc}{\mathrm{dop}}_n=\frac{-c_1}{1+e^{\frac{-\left(x-c_3\right)}{c_2}}}+c_1,& \left(W3\right)\end{array}$ where: x is the distance from the light emitting layer; c.sub.1 is in range of 110.sup.18 to 110.sup.20, c.sub.2 is in range of 1 to 100; c.sub.3 is in range of 800 to 0.

(12) In the next preferred embodiment of the invention the upper light-guiding-cladding layer has a continuous, non-step-like and smooth change of the refractive index n.sub.p, described by the equation (W2):

(13) $\begin{array}{cc}{n}_p=\frac{b_4-b_1}{1+e^{\frac{-\left(-x-b_3\right)}{b_2}}}+b_1,& \left(\mathrm{W2}\right)\end{array}$ where: x is the distance from the light emitting layer; b.sub.1 is in range of 2.32 to 2.57; b.sub.2 is in range of 1 to 100; b.sub.3 is in range of 0 to 800; b.sub.4 is in range of 2.38 to 2.66.

(14) Preferably, the upper light-guiding-cladding layer has a continuous, non-step-like and smooth change of the silicon doping profile dop.sub.p, described by the equation (W4):

(15) $\begin{array}{cc}{\mathrm{dop}}_p=\frac{d_1}{1+e^{\frac{-\left(x-d_3\right)}{d_2}}},& \left(\mathrm{W4}\right)\end{array}$ where: x is the distance from the light emitting layer; d.sub.1 is in range of 110.sup.18 to 110.sup.20, d.sub.2 is in range of 1 to 100; d.sub.3 is in range of 0 to 800.

(16) In the presented equations (W1), (W2), (W3) and (W4), coefficients: a.sub.1 and b.sub.1 specify the parameter defining the maximal refractive index, a.sub.2 and b.sub.2 specify the parameter defining the spatial change of the refractive index between the region with the lowest and the highest refractive index, a.sub.3 and b.sub.3 specify the parameter defining the location of the point of the inflection of the light-guiding-cladding layer refractive index profile, a.sub.4 and b.sub.4 specify the parameter defining the minimal refractive index, c.sub.1 and d.sub.1 specify the parameter defining the maximal doping level, c.sub.2 and d.sub.2 specify the parameter defining the spatial change of doping between the region with maximal doping level and undoped layers, c.sub.3 and d.sub.3 specify the parameter defining the location of the point of the inflection of the doping profile.

(17) In a preferred embodiment of the invention the laser diode has a ridge-type structure, wherein the ridge is created up to the depth in a range from the light emitting layer to at least the first derivative of function (W2), preferably to the depth in range of 99% to 80% of the maximum value of the refractive index n.sub.p, defined by equation (W2).

(18) In the next preferred embodiment of the invention, between the lower light-guiding-cladding layer and the light emitting layer, there is a lower light-guiding layer, which can have an n-type electrical conductivity or be an undoped layer.

(19) Preferably, between the light emitting layer and the upper light-guiding-cladding layer, there is an upper light guiding layer, which can have a p-type electrical conductivity or be an undoped layer.

(20) In a preferred embodiment of the invention the upper light-guiding-cladding layer contains a region blocking the escape of electrons. Alternatively, the region blocking the escape of electrons can be present outside of the upper light-guiding-cladding layer, e.g. above it. The region blocking the escape of electrons can be acceptor doped in range of 10.sup.19 cm.sup.3 to 10.sup.20 cm.sup.3.

(21) In another preferred embodiment of the invention the subcontact layer is doped with acceptors concentration above 10.sup.20 cm.sup.3.

(22) In the next preferred embodiment of the invention the laser diode emits light having the wavelength in range of 380 nm to 555 nm.

(23) In one of the embodiments of the invention the lower and/or upper light-guiding-cladding layer, as well as lower and/or upper light guiding layer, can be made of material GaN, InGaN or AlGaN, wherein the In content in InGaN alloy does not exceed 15%, the Al content in AlGaN alloy does not exceed 20%.

(24) In the present invention the laser diode is grown using epitaxy process based on an AlInGaN alloy. Continuous, non-step-like and smooth change of the refractive coefficient of individual layers can be obtained by changing the flow rate of the proper carrier gas during layer growth, as well as by changing the temperature during the growth of a particular layer, or by simultaneous changing of both parameters during the growth of a particular layer. The electron blocking layers and the doping of these layers, according to the present invention, are treated as local variations of the refractive index and doping concentration, and are not taken into account in equation W2 and W4, respectively. By applying a continuous, non-step-like and smooth change of indium and/or aluminium content in lower and upper light-guiding-cladding layer, an analogical, continuous, non-step-like and smooth change of refractive index in these layers was obtained. That, in turn, allowed obtaining a higher coverage of the optical mode with the active region (the light emitting layer) and, due to its non-step-like character, avoiding an interface arising between the waveguide and cladding, resulting in an energy barrier in the bands, inhibiting the carrier movement to the active region. The parameters used in equations W1-W4 provide an optimal change of the refractive index in layers with epitaxial growth and an optimal mode coverage with the laser diode active region. Using, in construction of the laser diode according to the present invention, a lower light guiding layer, distributed between the lower light-guiding-cladding layer and the active layer, allowed providing proper reactor conditions for optimal well growth (temperature change), due to which high quality quantum wells were obtained. Whereas the upper light guiding layer, distributed between the active layer and the upper light-guiding-cladding layer, was protecting the quantum wells from thermal decomposition during growth (the upper light guiding layer is grown in a temperature similar to the active layer growth). Moreover, both the lower light guiding layer and the upper light guiding layer allow optimizing the location of the optical mode maximum in relation to the active layers, thereby increasing the light filling coefficient of the active region. Additionally, these layers prevent the diffusion of dopants (particularly magnesium) to the active region, preventing the forming of non-radiative recombination centres in the light emitting region. The invention allows obtaining a laser diode with a lower current threshold, arising from a higher coverage of the optical mode with the active region, improving carrier injection to the active region and avoiding the arising of interfaces between the light guide cladding and the light guide.

(25) Exemplary embodiments of the invention are presented in a drawing, where FIG. 1 shows a cross section of the laser diode according to one embodiment of the present invention, FIG. 2 shows refractive index profile of the laser diode layers, according to one embodiment of the present invention, FIG. 3a and FIG. 3b show the doping profile of the laser diode, according to one embodiment of the present invention, for silicon and magnesium, respectively, FIG. 4 shows the consecutive technological steps of manufacturing of the laser diode ridge, according to one embodiment of the present invention, FIG. 5a and FIG. 5b show the optical power dependence on current, for the laser diode according to one embodiment of the present invention and a reference laser diode, respectively.

Example 1

(26) One of the possible embodiments of the present invention is a laser diode, emitting electromagnetic waves with 425 nm wavelength, manufactured on a uniform GaO.sub.xN.sub.1-x substrate, obtained in high pressure growth, with a structure shown in FIG. 1. In the first step a GaO.sub.0.0005N.sub.0.9995 substrate was made by means of growing from a gallium solution of nitrogen under the pressure of 1000 MPa and at temperature of 1500 C. The obtained crystal was cut and polished so as to obtain a parallel-flat plate with typical thickness of 200 m. After appropriate mechano-chemical polishing, the crystal surface with gallium polarity had an atomic smoothness, demonstrated by the atomic steps in an Atomic Force Microscope image. The crystal surface was misoriented by 0.5 in relation to the direction of the crystallographic axis c of the GaN hexagonal (wurtzite) structure.

(27) Next, the substrate 1 was placed in a MOVPE reactor, where a 300 nm thick Ga.sub.0.92Al.sub.0.08N cladding layer 2 doped with silicon to the 510.sup.18 cm.sup.3 level at the temperature of about 1050 C. was created. Afterwards, the growth was continued, changing the carrier gases flow and the growth temperature in a continuous way, creating a 350 nm thick light-guiding-cladding layer 3. This way a continuous content change (lowering the aluminium content) was obtained, corresponding to the refractive index n.sub.n profile, shown in FIG. 2 (region 3). The refractive index profile of the created light-guiding-cladding layer 3 is described by equation W1 with following set parameters: a.sub.1=2.5195, a.sub.2=30, a.sub.3=205, a.sub.4=2.489. Simultaneously, a silicon dopant was introduced to the light-guiding-cladding layer 3 in order to ensure an n-type conductivity according to equation W3, where the parameters had following values: c.sub.1=510.sup.18, c.sub.2=30, c.sub.3=205. Doping profile dop.sub.n of the lower light-guiding-cladding layer 3 is presented in FIG. 3a. Next, an about 20 nm thick undoped GaN lower light guiding layer 4 was created, functioning also as a lower waveguide. After lowering the temperature to 820 C., a In.sub.0.1Ga.sub.0.9N/In.sub.0.02Ga.sub.0.98N multi quantum well region was crated, having a thickness of 2.5 nm and 7.4 nm, accordingly, which constituted the light emitting layer 5, where the number of quantum wells was three. Next, the temperature was raised to 900 C. and a 45 nm thick undoped GaN upper light guiding layer 6 was created, being a part of the light guide region. The growth was continued, changing the flow of carrier gases and temperature in a continuous manner, in order to obtain the content changes corresponding to the refractive index n.sub.p profile shown in FIG. 2. (region 7). The changes included introducing aluminium to the layer and increasing its amount. The refractive index n.sub.p profile of the created upper light-guiding-cladding layer 7 is described by equation W2, for which the specific parameters have following values: b.sub.1=2.5192, b.sub.2=25, b.sub.3=175, b.sub.4=2.502, and its thickness is equal to 250 nm. Simultaneously, a magnesium dopant was introduced to the upper light-guiding-cladding layer 7, in order to unsure a p-type conductivity, according to equation W4, where the coefficients had following values: d.sub.1=110.sup.19, c.sub.2=25, c.sub.3=175. Doping profile dop.sub.p of the upper light-guiding-cladding layer 7 is shown in FIG. 3b. During the growth of the upper light-guiding-cladding layer 7, in the distance of 48 nm from the last quantum well, the reactor temperature was raised abruptly to the level of 1050 C., in order to create layer blocking the escape of electrons made of Al.sub.0.2Ga.sub.0.8N:Mg alloy, where the doping was performed on the level of 510.sup.19 cm.sup.3. The next layer was a 125 nm thick upper cladding layer 8 having a constant Al.sub.0.05Ga.sub.0.95N composition, doped to the level of 110.sup.19 cm.sup.3. The structure growth was finished by an 80 nm thick GaN:Mg thin subcontact layer 9 with magnesium concentration higher than 10.sup.20 cm.sup.3. After completion of the structure growth the reactor was cooled in nitrogen atmosphere.

(28) The next technological step was depositing of the upper 10 and lower contact (on substrate 1) made of nickel-titan-gold or nickel-gold or nickel-molybdenum-gold or nickel-palladium-gold alloy, having 100-500 nm thickness. Afterwards, photolithography defining the ridge shape was performed, in a form of straight strip perpendicular to the crystal cleavage planes (the future light guide exit windows).

(29) Process of creating the ridge and upper contact is shown schematically in FIG. 4. The first technological step was depositing a photoresist layer 11a, having a thickness of 0.5 m to 5 m, on the designed light guide area. Next, a two-step dry etching of the crystal, using active ions, was performed. In the first step, the gold not covered by photoresist is etched entirely. In the second step, a ridge is etched to the depth, for which the first derivative of equation W2 has a minimum. For the presented example, that value was equal to 173 nm from the active region, that is 360 nm from the structure surface.

(30) For another example, etching of the ridge to the point, where refractive index value is in range of 99% to 80% of the maximum value of the refractive index n.sub.p, defined by equation W2, is preferred. That means, for the presented example, this range is from 90 nm to 178 nm from the active region that is from 443 nm to 355 nm from the structure surface.

(31) In this manner, a ridge was created in the upper contact 10, subcontact layer 9, upper cladding layer 8, and partly in the upper light-guiding-cladding layer 7, which is shown on a diagram in FIG. 4b. Afterwards, a 250 nm thick insulator layer 11 made of SiO.sub.2, shown in FIG. 4c, was deposited on the whole crystal. Due to the high thickness of the photoresist, its side edges are not fully covered by the insulator 11. Wet etching allows uncovering of the ridge (FIG. 4d), while simultaneously leaving the insulator 11 on the ridge side walls and the area outside the ridge. The next technological step was electrolytic deposition of the upper contact 12, having a thickness of 1 m to 8 m.

(32) Then, the crystal was separated along the crystal cleavage planes, forming strips containing many devices, wherein the separation took place along the designed locations of the light guide windows of individual devices. The first step enabling the separation was scratching the crystal along the intended division lines. Next, due to the mechanical stresses, the crystal was broken along the cleavage planes.

(33) The next step was separating the strips on the individual devices performed in an analogical way to the division on the strips, except not along the cleavage planes, but perpendicularly to them.

(34) The last step was the assembly of the devices in a standard TO-56 housing. A thin layer of SnPb solder, or a pad made of AlN, covered with an AuSn thin film or other solder was placed on the housing socket. The device was placed on said layer with the substrate side (FIG. 1, region 1), covered with the contact, towards the solder. Process of annealing in a temperature above the solder melting point allowed a permanent connection of the device and support. Afterwards, an electrical contact with the upper contact 12 (FIG. 1, region 12) material was made, using the ball-bonding technique. Then, the housing of the laser was hermetically closed, using protective atmosphere preventing water condensation inside the housing.

(35) Due to applying the lower and upper light-guiding-cladding layer 3 and 7, with non-linear change of the refractive index and non-linear doping profile, the laser threshold current was decreased and the power as a function of current, above the threshold current of the laser diode, was increased which is shown in FIG. 5a, in relation to the reference laser diode with identical layer structure of the device, having a conventional step-like waveguide structure, for which the opto-electric dependence is shown in FIG. 5b. Thereby the presented technical problem was solved.

(36) It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. While the dimensions and types of materials described herein are intended to define the parameters of the invention, they are by no means limiting, but are instead exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms including and in which are used as the plain-English equivalents of the terms comprising and wherein. Moreover, in the following claims, the terms first, second, and third, are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. 112, sixth paragraph, unless and until such claim limitations expressly use the phrase means for followed by a statement of function void of further structure.