Semiconductor laser device and its production method

Abstract: A semiconductor laser device (100) includes a Zn-doped p type semiconductor substrate (1a) in which more than 81% of Zn dopant atoms are activated to produce carriers, and semiconductor layers epitaxially grown on the Zn-doped p type semiconductor substrate (1a), including an active layer (2a) sandwiched between n type and p type cladding layers (15a,11a). Therefore, during the epitaxial growth of the semiconductor layers, diffusion of inactive Zn atoms from the substrate (1a) to the semiconductor layers is reduced, with the result that the internal loss due to free carrier absorption in the active layer (2a) and the underlying p type cladding layer (11a) is reduced and non-radiative recombinations of carriers in the active layer (2a) are reduced.

Description: The present invention relates to semiconductor laser devices fabricated by epitaxially growing semiconductor layers on a Zn-doped p type semiconductor substrate. Figure 4 is a perspective view, partially broken away, illustrating a distributed feedback laser (hereinafter referred to as DFB laser) having a buried heterojunction structure (hereinafter referred to as BH structure), which is disclosed in, for example, ELECTRONICS LETTERS, 21st May 1987, Vol. 23, pp. 546-547 and ELECTRONICS LETTERS, 2nd Feb. 1989, Vol. 25, No. 3, pp.220-221. In figure 4, reference numeral 1 designates a p type InP substrate to which Zn dopant is added. A double heterojunction structure including an InGaAsP active layer 2 sandwiched between a p type InP lower cladding layer 11 and an n type InP first upper cladding layer 12 is disposed on the InP substrate 1. An InGaAsP layer 13 having a diffraction grating structure 14 is disposed on the first upper cladding layer 12. An n type InP second upper cladding layer 15 is disposed on the diffraction grating structure 14. A first p type InP layer 3, an n type InP layer 4, and a second p type InP layer 5 are disposed on opposite sides of the active region, producing a p-n-p current blocking structure. An n type InP third upper cladding layer 6 is disposed on the n type InP second upper cladding layer 15 and the second p type InP layer 5. An n type InGaAsP contact layer 7 is disposed on the third upper cladding layer 6. An insulating film 8 is disposed on the n type InGaAsP contact layer 7 except for a region opposite the active region, extending on the side surfaces of the laser structure. An n side electrode 9 is disposed on the n type InGaAsP contact layer 7 and the insulating film 8. A p side electrode 10 is disposed on the rear surface of the substrate 1. A method for fabricating the DFB laser of figure 4 is illustrated in figures 5(a)-5(f). Initially, there are epitaxially grown on the Zn-doped p type InP substrate 1, the Zn-doped p type InP lower cladding layer 11 having a carrier concentration of 1 x 10 18 cm -3 or more, the InGaAsP active layer 2, the n type InP first upper cladding layer 12, and the InGaAsP layer 13 (figure 5(a)). After the epitaxial growth, the diffraction grating structure 14 is formed by conventional technique (figure 5(b)). For example, using photolithography, a photoresist is deposited on the InGaAsP layer, exposed to interference fringes of light, and developed as a mask with a periodic structure. Then, the layer 13 is etched through the mask, and the mask is removed. Thereafter, the n type InP second upper cladding layer 15 is epitaxially grown on the wafer, and a stripe-shaped dielectric film 20 is formed on the second upper cladding layer 15 (figure 5(c)). Then, the wafer is etched using the dielectric film 20 as a mask to form the double heterojunction structure in a mesa shape (figure 5(d)). Thereafter, the first p type InP current blocking layer 3, the n type InP current blocking layer 4, and the second p type InP current blocking layer 5 are successively grown on opposite sides of the stripe mesa by LPE (Liquid Phase Epitaxy) or the like, and the n type InP third upper cladding layer 6 and the n type InGaAsP contact layer 7 are successively grown on the surface of the wafer (figure 5(e)). Carrier concentrations of the p type InP current blocking layers 3 and 5 are in a range from 1 x 10 17 cm -3 to 1 x 10 18 cm -3 by doping Zn. Then, the laser structure is formed in a mesa shape by etching (figure 5(f)). Thereafter, an insulating film 8 is deposited on the entire surface and the side surfaces of the laser structure and a portion of the insulating film 8 opposite the active region is removed to expose the contact layer 7. Finally, the n side electrode 9 is formed in contact with the contact layer 7 and the p side electrode 10 is formed on the rear surface of the substrate 1, completing the DFB laser of figure 4. In the DFB laser thus fabricated, reactive current flowing outside the InGaAsP active layer 2 is reduced by the p-n-p current blocking layers disposed on opposite sides of the active layer 2. In addition, since the carrier concentration of the first p type InP current blocking layer 3 (1 x 10 17 - 1 x 10 18 cm -3 ) is lower than the carrier concentration of the p type InP lower cladding layer 11 (1 x 10 18 cm -3 or more), resistance of the current blocking layer 3 is higher than that of the lower cladding layer 11, whereby the reactive current during the laser operation is reduced. In fabricating the DFB laser of figure 4 according to the process steps of figures 5(a)-5(f), although the epitaxial growth of the semiconductor layers on the Zn-doped semiconductor substrate 1 is carried out under the same condition, if the Zn dopant concentration in the substrate 1 varies depending on lots, threshold current and efficiency of the laser completed unfavorably vary, resulting in a difficulty in manufacturing high output DFB lasers with high reproducibility. Threshold current density Jth and external differential quantum efficiency ηex of a semiconductor laser are represented by Jth = dAL βΓv (αi + In 1/R1·R2 2L )+ dAL · J0 ηex = ηi In 1/R1·R2 2Lαi + In 1/R1·R2 where dAL is the thickness of the active layer, Γv is the light confinement coefficient, L is the resonator length, R1 and R2 are reflectances at the front facet and the rear facet, respectively, JO is the current density when the gain is 0, β is the gain factor constant which is in proportion to the life of carrier, ηi is the internal differential quantum efficiency, and αi is the internal loss due to free carrier absorption in the crystal layers on the substrate. In order to reduce the threshold current and increase the efficiency of the semiconductor laser, the internal loss αi must be decreased, and the gain factor constant β must be increased by reducing non-radiative recombinations of carriers in the active layer. "IEEE Journal of Quantum Electronics", vol. QE-21, no. 5, May 1985, pages 452 to 457, discloses a semiconductor laser device in which an active layer is sandwiched between two cladding layers. These layers are epitaxially grown on an Zn-doped p-type semiconductor substrate. US-A-4,788,688 discloses a semiconductor laser of heterostructure type in which the active layers are grown by liquid phase epitaxy on a Zn-doped substrate. "Journal of Applied Physics", vol. 71, no. 2, January 15, 1992, pages 659 to 663, describes the electrical characteristics of Zn in heavily doped InP grown by the liquid-encapsulated Czochralski technique. It is an object of the present invention to provide a high output semiconductor laser with low threshold current and high efficiency, that reduces the internal loss αi due to the free carrier absorption in the epitaxial layers and increases the gain factor constant β by reducing non-radiative recombinations of carriers in the active layer, as well as to provide a method for producing such a laser. Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. Claim 1 defines a production method in accordance with the invention. Claim 4 defines a semiconductor laser in accordance with the invention. According to claim 4, the semiconductor laser device includes a plurality of semiconductor layers epitaxially grown on a Zn-doped p type semiconductor substrate in which more than 81% of the Zn dopant atoms are activated to produce a substrate carrier concentration of about 5 * 10 18 cm -3 . Therefore, diffusion of inactive Zn atoms from the substrate to the epitaxial layers during the growth is suppressed, with the result that the internal loss due to free carrier absorption in an active layer and an underlying p type cladding layer is reduced and non-radiative recombinations of carriers in the active layer are reduced. According to the present invention, the semiconductor laser device may include a double heterojunction structure including an active layer and p type and n type cladding layers, which are epitaxially grown on a p type semiconductor substrate. The p type cladding layer is grown while adding Zn dopant so that more than 81% of the Zn dopant atoms are activated to produce carriers. Therefore, diffusion of inactive Zn atoms from the p type cladding layer to the active layer during the epitaxial growth is suppressed, with the result that the internal loss due to free carrier absorption in the active layer and the p type cladding layer is reduced and non-radiative recombinations of carriers in the active layer are reduced. According to the present invention, the epitaxially grown semiconductor layers may be formed in a stripe shape mesa, and p-n-p current blocking layers are epitaxially grown on opposite sides of the mesa. During the epitaxial growth, Zn dopant is added to the p type current blocking layers so that more than 81% of the Zn dopant atoms are activated to produce carriers. Therefore, diffusion of inactive Zn atoms from the p type layers to the active layer and the cladding layer during the growth is suppressed, with the result that the internal loss due to free carrier absorption in the active layer and the p type cladding layer is reduced and non-radiative recombinations of carriers in the active layer are reduced. Figure 1 is a perspective view of a broad area mesa type semiconductor laser device in accordance with an embodiment of the present invention; Figure 2 is a graph illustrating relations between Zn dopant concentration and carrier concentration during producing a Zn-doped p type InP substrate by LEC (Liquid Encapsulated Czochralski) method; Figure 3(a) is a diagram illustrating Zn, As, and Ga SIMS (Secondary Ion Mass Spectroscopy) depth profile of the broad area mesa type semiconductor laser of figure 1, and figure 3(b) is a diagram illustrating Zn, As, and Ga SIMS depth profile of a semiconductor laser employing a Zn-doped p type InP substrate in which less than 81% of the Zn dopant atoms are activated; Figure 4 is a perspective view illustrating a DFB semiconductor laser with a BH structure in accordance with the prior art; and Figures 5(a)-5(f) are perspective views of steps in a method for producing the semiconductor laser of figure 4. Figure 1 is a perspective view illustrating a broad area mesa type semiconductor laser in accordance with an embodiment of the present invention. In the figure, the broad area mesa type laser 100 includes a p type InP substrate la to which Zn dopant is added to a concentration of 5.3 X 10 18 /cm 3 . More than 81 % of the Zn dopant atoms are activated. A p type InP lower cladding layer 11a about 2 µm thick, an InGaAsP active layer 2a about 0.1 µm thick, and an n type InP upper cladding layer 15a about 1 µm thick are successively disposed on the p type InP substrate 1a. An n side electrode 9a is disposed on the upper cladding layer 15a and a p side electrode 10a is disposed on the rear surface of the substrate la. Preferably, the resonator length is 600 µm, the width of the n side electrode 9 is 70 µm, and the composition of the InGaAsP active layer 2a is selected so that the energy band gap thereof corresponds to a wavelength of 1.48 µm. A description is given of a method for fabricating the semiconductor laser of figure 1. First of all, the p type InP substrate la is formed by LEC method. Figure 2 is a graph illustrating relations between the Zn dopant concentration and the carrier concentration of the p type InP substrate, i.e., figure 2 shows the rate of active Zn atoms to all the Zn atoms in the substrate la. As shown in figure 2, an increase in the Zn dopant concentration increases the rate of inactive Zn atoms to all the Zn atoms added and decreases the rate of active Zn atoms, i.e., carrier concentration/Zn dopant concentration. The doping of Zn atoms is usually performed to make a p type substrate and reduce the resistance of the substrate. Therefore, it is necessary to add the Zn dopant so that the carrier concentration of the substrate produced is about 5 x 10 18 cm 3 . In the embodiment of the present invention, in order to significantly reduce the resistance of the substrate, the Zn-doped p type InP substrate is formed so that the carrier concentration thereof is about 5 x 10 18 /cm 3 and the rate of active Zn atoms to all the Zn atoms exceeds 81 %, according to the characteristic curve shown in figure 2 by LEC method. More specifically, Zn dopant is added to the substrate to a concentration of 5.3 x 10 18 /cm 3 to form the Zn-doped p type InP substrate la having a carrier concentration of 5 x 10 18 /cm 3 and a Zn atom activated rate of 94 %. Then, the p type InP lower cladding layer 11a, the InGaAsP active layer 2a, and the n type InP upper cladding layer 15a are successively grown on the Zn-doped p type InP substrate la. Preferably, these layers are grown by MOVPE (Metal Organic Vapor Phase Epitaxy) at a temperature of 650 °C, pressure of 150 Torr, a V/III ratio of 140, and a growth rate of 1 µm/hr. Then, a stripe-shaped dielectric film (not shown) is formed on a prescribed region of the n type InP upper cladding layer 15a, and an etching is carried out using the dielectric film as a mask to form the epitaxially grown layers in a stripe shape mesa. After the dielectric film is removed, the p side electrode 10a is formed on the rear surface of the p type InP substrate 1a and the n side electrode 9a is formed on the n type InP upper cladding layer 15a, completing the structure of figure 1. A description is given of the operation. Figure 3(a) shows a Zn SIMS depth profile of the semiconductor laser of figure 1, and figure 3(b) shows a Zn SIMS depth profile of a semiconductor laser for comparison employing a Zn-doped p type InP substrate which is formed by adding Zn dopant to a concentration of 6.3 x 10 18 /cm 3 and activating more than 81 % of the Zn atoms to provide a carrier concentration of 5 x 10 18 /cm 3 . In both figures, As and Ga SIMS depth profiles are shown as references. In the Zn depth profile of figure 3(a) according to the present invention, an interface between the p type InP substrate la and the p type InP cladding layer lla is steep and clear, but it is not clear in the Zn depth profile of figure 3(b). The reason is as follows. In the semiconductor laser of figure 3(b) using the Zn-doped p type InP substrate with the Zn atom activated rate of below 81 %, inactive Zn atoms in the substrate are diffused into the p type InP layer lla, the InGaAsP layer 2a, and the n type InP layer 15a during the epitaxial growth, resulting in an increase in the internal loss αi due to the free carrier absorption in the epitaxially grown layers and an increase in non-radiative recombinations in the active layer. These two semiconductor lasers are operated by pulses with the pulse width of 0.1 µs and the duty ratio of 1/1000 to calibrate the threshold current, i.e., threshold current density Jth. While the threshold current density Jth of the semiconductor laser shown in figure 1 employing the Zn-doped p type InP substrate 1a with the Zn atom activated rate of 94% is 1.6 KA/cm 2 , the threshold current density Jth of the semiconductor laser for comparison employing the Zn-doped p type InP substrate with the Zn atom activated rate of 80 % is larger than 2.4 KA/cm 2 and, in this case, the laser would not oscillate with the driving current (1Amax) of the pulse generator employed. According to the present invention, since the rate of active Zn atoms to all the Zn atoms in the Zn-doped p type InP substrate 1a is as high as 94%, diffusion of inactive Zn atoms from the substrate to the epitaxially grown layers on the substrate is reduced, whereby the internal loss αi due to the free carrier absorption in the epitaxially grown layers are reduced and the non-radiative recombinations in the active layer are reduced to increase the gain factor constant β, resulting in a low-threshold and high-efficiency semiconductor laser. During epitaxially growing the p type InP layer 11a, the InGaAsP layer 2a, and the n type InP layer 15a, if the p type InP cladding layer 11a is grown with adding Zn dopant according to the characteristic curve shown in figure 2 so that more than 81% of the Zn dopant atoms are activated, the amount of inactive Zn atoms diffused into the active layer is further reduced, resulting in a further reduction in the non-radiative recombinations in the active layer. In the above-described embodiment, the ideal characteristic curve of the relation between carrier concentration and Zn dopant concentration in the formation of the Zn-doped p type InP substrate by LEC method is previously provided and then the substrate is formed by LEC method with adjusting the Zn dopant concentration according to the ideal characteristic curve to achieve a desired carrier concentration. In this method, however, the amount of Zn dopant atoms added to the substrate unfavorably varies according to lots for producing the substrate material, the process sequences and conditions, or the like. Therefore, before performing the epitaxial growth on the Zn-doped p type InP substrate, the carrier concentration of the substrate is measured by C-V profile measurement or Hall effect measurement and the Zn dopant concentration is measured by ICP method (inductive coupling high-frequency Ar gas plasma emission analysis) to find the rate of active Zn atoms to all the Zn atoms in the substrate. Then, the substrate with the Zn atom activated rate of 81% or more is selected and the laser structure is fabricated on the substrate, whereby the production yield is improved. While in the above-described embodiment the broad area mesa type laser is employed, the present invention may be applied to the semiconductor laser having the BH structure shown in figure 4. In this case, when the p-n-p current blocking layers are epitaxially grown on opposite sides of the stripe mesa structure including the active layer, if the rate of active Zn atoms to all the Zn atoms in the p type current blocking layers is 81 % or more, the diffusion of inactive Zn atoms from the current blocking layers to the stripe mesa is reduced, whereby the internal loss αi due to the free carrier absorption in the stripe mesa and the non-radiative recombinations in the active layer are further reduced.

Claims: A method for producing a semiconductor laser device including a step of epitaxially growing a p type InP cladding layer (11a), an InGaAsP active layer (2a), and an n type InP cladding layer (15a) on a Zn-doped p type InP substrate (la), characterized in that said Zn-doped p type InP substrate (1a) is formed by Liquid Encapsulated Czochralski (LEC) method with adding Zn dopant atoms to a concentration below 6.3 * 10 18 cm -3 and activating more than 81 % of the Zn dopant atoms added so that the carrier concentration in the InP substrate is about 5 * 10 18 cm -3 . The method of claim 1, wherein said p type cladding layer (11a) is grown with adding Zn dopant atoms and more than 81 % of the Zn dopant atoms added are activated to produce carriers. The method of claim 1 or 2, wherein said epitaxially grown layers are formed in a stripe shape mesa, and p-n-p current blocking layers are epitaxially grown at opposite sides of said mesa with adding Zn dopant atoms to said p type current blocking layers and activating more than 81 % of the Zn dopant atoms added to produce carriers. A semiconductor laser device (100) comprising a plurality of semiconductor layers including an active layer (2a) sandwiched between two cladding layers (11a, 15a), which semiconductor layers are epitaxially grown on a Zn-doped p type semiconductor substrate (1a), characterized in that more than 81 % of Zn dopant atoms in said Zn-doped p type semiconductor substrate (1a) are activated to produce carriers so that the carrier concentration in the substrate (1a) is about 5*10 18 cm -3 . The semiconductor laser device of claim 4, wherein said p type semiconductor substrate (1a) and said cladding layers (11a, 15a) comprise InP and said active layer (2a) comprises InGaAsP. The semiconductor laser device of claim 4 or 5, further including a stripe shape mesa of said semiconductor layers and a p-n-p current blocking structure disposed at opposite sides of said stripe mesa, wherein said current blocking structure includes Zn-doped p type layers in which more than 81 % of Zn dopant atoms are activated to produce carriers.

Inventors: MIZUOCHI HITOSHI, MIZUOCHI, HITOSHI, Mizuochi, Hitoshi, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H01S3
IPC codes: H01L33, H01S5