Mass production of laser diodes by MBE

Microelectronics Journal, 25 (1994) 619-630

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Mass production of laser diodes by MBE Hiroshi Mataki and Haruo Tanaka R OHM Corporation, 21 Saiin Mizosaki-cho, Ukyo-ku, Kyoto 615,Japan

Molecular beam epitaxy (MBE) plays an important role in the mass production of A1GaAs laser diodes which have been widely used in a variety of opto-electronic applications. This paper provides an overview of self-aligned structure AIGaAs laser diodes fabricated by MBE ('SAM laser diodes') focusing on the GaAs passivation technique that we put into practical use for the first time, and on the modification of conventional MBE systems. Several features of SAM laser diodes for practical applications are also reviewed. 1. Introduction

aser diodes have been extensively used in a variety o f opto-electronic applications such L as optical communications, compact disks (CDs), video disks (VDs) and laser beam printers. Today it is estimated that more than 7 million laser diodes are being produced and sold each month. There are several types o f laser diode, but over 95% are A1GaAs laser diodes with wavelengths ranging from 770 to 860 nm. A1GaAs laser diodes are mainly used for the C D family, including audio CDs, C D - I and C D R O M , optical memories such as magnetooptical disks, write-once optical disks, C D - R and optical cards, VDs, laser beam printers, and optical measurement equipment. Three types o f epitaxial technique for the mass production of A1GaAs laser diodes are wellknown: molecular beam epitaxy (MBE), liquidphase epitaxy (LPE) and metal-organic vapor phase epitaxy (MOVPE). In the early days,

0026-2692/94/$7.00 © 1994 Elsevier Science Ltd

A1GaAs laser diodes were produced by LPE because the other two epitaxial techniques were still technologically immature. A m o n g these three epitaxial techniques, M B E was expected to be o f most practical use because it provides excellent uniformity and purity as well as precise controllability [1]. In 1974, C h o and Casey fabricated A1GaAs double-heterostructure (DH) laser diodes for the first time by the M B E technique [2]. Then, in 1979, Tsang succeeded in fabricating A1GaAs D H laser diodes by MBE whose threshold current was as low as that fabricated by LPE [3]. However, these achievements did not lead directly to the mass production o f A1GaAs laser diodes by the M B E technique because a structure for controlling transverse m o d e compatibility with the M B E technique had not been found, and because productivity was very low. In 1984, we succeeded in solving these problems for the first time, and in putting the MBE technique to practical use [4, 5]. T h e purpose of this paper is to give an overview o f MBE as a mass production technology for A1GaAs laser diodes. First, MBE techniques for fabricating A1GaAs laser diodes are reviewed, with emphasis on the laser diode structure for controlling the transverse mode. Then, several modifications of the M B E system to improve the productivity o f the M B E technique are shown. Finally, several features o f A1GaAs laser diodes

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H. Mataki and H. Tanaka~Producing laser diodes by MBE

produced by MBE are illustrated in relation to their practical applications. 2. Structure and fabrication method

There are three types of laser diodes for controlling the transverse mode [6]: a strongly index-guiding structure, a weakly index-guiding structure and a gain-guiding structure. Among these three types of structure, a weakly indexguiding structure has been thought to be the best for A1GaAs laser diodes in practical applications because its great flexibility of design means that it can meet a variety of requirements, which differ from application to application. That is, in a weakly index-guiding structure, the major characteristics such as far-field patterns and astigmatism, as well as longitudinal mode, can easily be controlled by changing the structural parameters of the laser diodes such as the thickness of each layer, the density of dopants (impurities) and the stripe width. In the early stages of our investigation [7], we tried to fabricate two types of laser diode with a weakly index-guiding structure: one is a channelled-substrate planar waveguide laser diode, and the other is a self-aligned structure laser diode. In MBE, unlike LPE, the crystal growth rate does not depend on the orientation of the GaAs substrate. Therefore, for the channelledsubstrate planar waveguide laser diode, the shape of the epitaxially grown layers was decided by

the channel in the substrate (see Fig. l(a)), resulting in distorted far-field pattern. For the self-aligned structure laser diode, on the other hand, the epitaxially grown layers can be kept flat (see Fig. l(b)). In fact, the best transverse mode was obtained for the self-aligned structure laser diode grown by MBE. However, it is necessary to take out the wafer from the chamber of a MBE system during crystal growth to form a striped region by chemical etching when a self-aligned structure laser diode is fabricated. That is, two-step epitaxy is needed to fabricate self-aligned structure laser diodes. Once the pA1GaAs cladding layer is exposed to air after the stripe formation process is complete, the exposed p-A1GaAs cladding layer will easily be degraded by oxidation. For this reason, in the early stages of our investigation, we could not obtain laser diodes with high ef~ciency and high reliability. To avoid this problem in the fabrication of selfaligned structure laser diodes, we investigated the properties of GaAs and A1GaAs layers, and finally found that the sublimation ofAlxGal _ xAs (x>~ 0.15) is negligible at a temperature of 740°C and in a vacuum of about 1 x 10 -7 torr under As4, while the sublimation rate of GaAs is about 2 #m h -1 under the same conditions [4]. The difference in sublimation behavior at elevated temperatures indicates that the GaAs layer can only be thermally etched in the very beginning of the second epitaxy ifa thin GaAs layer is

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Microelectronics Journal, t/ol. 25

left on the A1GaAs cladding layer during chemical etching to form a striped region. We refer to this procedure as the 'GaAs passivation technique'. Figure 2 shows the entire flow of the two-step epitaxy for fabricating self-aligned structure laser diodes using the GaAs passivation technique. Crystal growth is carried out on a (100) oriented Si-doped GaAs substrate in a modified Riber 32P MBE system at a vacuum of around 1 × 10 -7 torr under As4. The active layer is grown at a temperature of 680°C. Under these

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conditions, the crystal growth rates of GaAs and A1GaAs are 1.2 #m h -1 and 3.0/~m h -1, respectively. In the first epitaxy, the following six AlxGal _xAs layers were grown successively: (1) n-AlxGal _xAs (x = 0.6, Si: 3 × 1017 cm-3) cladding layer, 1.3/tm; (2) un-doped AlxGal_~As (x = 0.15) active layer, 0.07/lm; (3) p-AlxGal _~As (x = 0.6, Be: 5 × 1017 c m - 3 ) , 0.35 #m; (4) n-GaAs (Si: 5 × 10 TM cm -3) current blocking layer, 0.4 #m; (5) nAlxGal_xAs ( x = 0 . 1 5 , Si: 1 × 1018cm -3) evaporation preventing layer, 0.07 #m; (6) undoped GaAs (Si: 1 × 10 TM cm-3), 0.04/tin. After these six layers have been grown, the wafer is once taken out of the MBE system to form a striped region, with a width of 4 #m along the <110> direction, by a chemical etching technique of conventional photolithography. Here, a H2SO4:H202:H20 mixture is used as the etchant. In chemical etching, a thin GaAs layer with a thickness of 1000-2000 A is left for the purpose of preventing the oxidation of the pA1GaAs layer. Then the wafer is put into the MBE system again for the second epitaxy. Prior to crystal regrowth, the GaAs thin layers left on both the p-A1GaAs and the n-A1GaAs evaporation preventing layer are thermally etched at a temperature of 740°C under As4. Next, two additional layers are grown: (1) p-Al=Gal_xAs (x = 0.6, Be: 5 x 10 TM cm -3) cladding layer, 1.3#m; (2) p+-GaAs (Be: 1 × 10rgcm-3) contact layer, 1.6#m. After this two-step epitaxy is over, the wafer is lapped down to a thickness of about 60/am. Then Ti-Au and alloyed Au-Ge layers are formed on the wafer as electrodes. Finally, laser diode chips with a cavity length of 250/~m are obtained by cleaving and sawing. We refer to a self-aligned structure laser diode fabricated in this way as a 'SAM (Self-_aligned structure by MBE) laser diode'. The most remarkable feature of the GaAs passivation technique is that, unlike chemical etching in which etching is carried out with an accuracy of + 10% thickness, the GaAs thin layers left on the p-

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H. Mataki and H. Tanaka~Producing laser diodes by MBE

A1GaAs and n-A1GaAs layers can be etched to an accuracy of atomic scale. This means that the boundary between the first epitaxial layer and the second epitaxial layer in the p-A1GaAs cladding layer has excellent smoothness, which is essential to realize high lasing efficiency. In reality, the structural parameters mentioned above vary slightly from type to type (namely, from application to application) of our products.

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3. Improvement of productivity 0

Production by MBE systems on the market had the following problems: (1) up-time was short (about 15 days), mainly because of the small volume of metal source cells and of mechanical failures; (2) the measurement accuracy of the substrate temperature was poor (roughly +50°C); (3) to set and remove a GaAs substrate on and from a holder was time-consuming and complex. To solve these problems, we modified the MBE system in the following way.

3.1 Operation efficiency The enlargement of metal source cells was the quickest way to elongate the up-time. However, the metal source deposited on the shutters can obstruct the movement of the shutters when the up-time is elongated, thereby leading to mechanical failures. To avoid this problem, we changed the shape of the shutters from flat to conical so that the deposited metal would not brush against the rim of a source cell. As a result, the up-time could be elongated from 15 to 100 days. Figure 3 shows the photoluminescent intensity change of mass-produced SAM laser diode chips at room temperature during 100 days of operation after the degree of vacuum reached the specified condition. Before the MBE system is evacuated, the crystal growth chamber of the MBE system is baked at a temperature of 200°C for 2 days in order to eliminate the metal deposited inside the chamber. Above the broken

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line in Fig. 3, high-quality SAM laser diode chips can be obtained. This means that 2 days of initial operation is enough to optimize crystal growth conditions .and to obtain high-quality SAM laser diodes. 3.2 Substrate mounting

Two other problems arose from the same cause: namely that a substrate had to be set on a mount using In as solder. To avoid the conventional procedure for substrate mounting, we experimented with several types of substrate mounting [8]. Figure 4 shows schematic diagrams of three types of substrate mounting, including the conventional type (Fig. 4(a)). Although the GaAs substrate was easily mounted on a holder using a Mo pin and screw in the mounting shown in Fig. 4(b), it was found that the

Microelectronics Journal, Vol. 25

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(c) Fig. 4. Schematic diagram of substrate mountings: (a) conventional type in which the GaAs substrate is fixed on the Mo block by In solder; (b) the GaAs substrate is fixed on the conventional type of Mo block by Mo pins and screws; (c) the GaAs substrate is fixed on the modified Mo block by Mo pins and screws. substrate temperature could not be elevated higher than 600°C because o f the n o n - u n i f o r m contact between the substrate and the M o block. This was a fatal problem because the substrate temperature had to be elevated higher than 600°C not only to obtain high-quality laser diodes, but also to carry out the GaAs passivation technique. T h e procedure depicted in Fig. 4(c), on the other hand, allowed the GaAs substrate to be elevated to a temperature higher than 740°C. In addition, from measurement o f the distribution o f the threshold current density in a GaAs substrate, it was found that the uniformity o f the substrate temperature was within +10°C. This result suggested that the procedure depicted in

4. Features of SAM laser diodes for practical uses 4.1 For compact disk players For C D players, the main requirements for practical laser diodes are: (1) low threshold and operating currents; (2) longitudinally multimode oscillation; (3) narrow distribution in characteristics. T h e low threshold and operating currents are realized by optimizing the dopant concentrations in the p-A1GaAs cladding layer so as to minimize the diffusion o f the carriers in the lateral direction o f the striped region [9]. Optimization o f the dopant concentration in the p-A1GaAs cladding layer as well as optimization o f the stripe width is also needed to control the longitudinal mode. Figures 5 and 6 show the distribution o f threshold and operating currents, respectively, o f SAM laser diodes for C D players. N o t only are the threshold and operating currents low, but also the distribution o f characteristics is narrow. Thanks to the improved uniformity and precise controllability o f the M B E technique, the distribution o f characteristics are generally seen to be about one-third o f those for the laser diodes produced by the LPE technique. In a C D player, the laser light reflected from a C D can enter the laser diode, inducing fluctuations o f phase and amplitude [10]. These fluctuations, mainly arising from the hopping o f longitudinal modes, are detected as noise called 'feedback noise'. T o reduce m o d e - h o p p i n g induced feedback noise, laser diodes are usually designed to oscillate with longitudinal multimodes. Figure 7 shows the longitudinal-mode

623

H. Matald and H. Tanaka/Producing laser diodes by MBE

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spectrum of a SAM laser diode for CD players at an optical output of 3 mW, and Fig. 8 shows that the feedback noise is kept low over feedback rates up to 10%. 4.2 For optical memories Optical memories, such as magneto-optical (MO) disks, write-once-read-many (WORM) disks and optical cards, require a high optical power above 20 mW. To obtain such high optical power while still keeping the fundamental transverse mode, the degree of optical confinement in a laser diode must be kept strong. However, this strong optical confinement was believed to lead to longitudinal single-mode oscillation. As described above, single-mode oscillation is not favorable for the reduction of feedback noise. Most conventional high-power laser diodes have been used in

624

optical memories with high-frequency modulation to multiply the longitudinal mode [11, 12]. We investigated the relation between the structural variation of a SAM laser diode and the longitudinal mode in order for a high-power laser diode to exhibit longitudinal multi-mode oscillation. We found that it is possible to balance the longitudinal multi-mode oscillation and the optimum optical confinement in order to keep the fundamental transverse mode by precisely controlling the thicknesses of the active layer, the cladding layers and the current blocking layer, the dopant concentration and profile in the p-A1GaAs cladding layer, and the stripe width [13]. Figure 9 shows the longitudinal-mode spectra of a high-power SAM laser diode with a maximum optical power of 35 m W at optical powers of 3 m W and 5 m W compared with a conven-

Microelectronics Journal, Vol. 25

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4.3 For optical I.AN InGaAsP laser diodes play a major role in optical fiber communications. However, their high cost has been a barrier to the spread o f laser diodes in a variety o f optical communications including short-haul, multi-nodes optical local area networks (LAN). Recently, a number o f attempts have been made to use AIGaAs laser diodes as low-cost light sources for optical LANs. ANSI X3T9.3 'Fibre Channel' Standard is one such optical L A N which has been put to practical use [14, 15]. Fibre Channel is expected

Microelectronics Journal, Vol. 25

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the dopant profile in the p-A1GaAs cladding layer being controlled very precisely in order to keep the operating current low, and also the great purity o f the materials, which are the main features o f the M B E technique. Figure 12 shows the response o f a SAM laser diode for optical L A N w h e n operated at a peak power o f 3 m W . T h e relaxation oscillation (KO) frequency is estimated to be about 3 GHz, thereby enabling the laser diode to modulate up to more than 1 Gbps. T h e K O frequency can be controlled by optimizing the dopant concentration in the p-A1GaAs cladding layer. Figure 13 shows the distribution o f the R O frequencies o f SAM laser diodes for optical LAN.

5. Concluding remarks T h e key technologies in fabricating A1GaAs laser diodes based on the MBE techniques were

627

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reviewed. The main features of SAM laser diodes were also shown, with emphasis on the precise controllability and excellent uniformity of the MBE technique for obtaining the best performance in each application.

Because of the high reliability of the MBE .technique, the next step will be to consider future technologies to boost its potential. In fact, higher performance will be required from AIGaAs laser diodes as they spread in a variety of opto-electronic fields. We think that other processing technologies, such as the dry-etching technique, should be developed and improved in order to be used in conjunction with the MBE technique.

Acknowledgements The authors acknowledge helpful contributions made by Mr. Mushiage, Mr. Shakuda, Mr. Ikawa, Mr. Uchida and Mr. Yamaji. References [1] H.P. Meier, Role of molecular epitaxy in the optoelectronic field, Mater. Sci. Eng., (1991) 77. [2] A.Y. Cho and H.C. Casey, Jr., (;aAs-Al.,Gal .,As double-heterostructure laser prepared by molecularbeam epitaxy, Appl. Phys. Lett., 25 (1974) 288. [3] W.T. Tsang, Low-current-threshold and high-lasing uniformity GaAs-AI.,Gal _.,.As double-heterostructure lasers grown by molecular beam epitaxy, Appl. Phys. Lett., 34 (1981) 473. [4] H. Tanaka, M. Mushiage, Y. Ishida and H. Fukada, Single-longitudinal-mode selfaligned (AlGa)As

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H. Mataki and H. Tanaka~Producing laser diodes by MBE

[5] [6] [7] [8]

[9] [10]

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double-heterostructure laser-fabricated by molecular beam epitaxy,Jpn. J. Appl. Phys., 24 (1985) L89. H. Tanaka, MBE as a mass production technology for AIGaAs lasers, Extended Abstracts 1991 Int. Conf. Solid State Devices and Materials, Yokohama, 1991, 396. G.P. Agrawal and N.K. Dutta, Semiconductor Lasers, 2nd edn., Van Nostrand Reinhold, New York, 1993. H. Tanaka and M. Mushiage, Development of semiconductor lasers by a production-type MBE, Oyobutsuri, 54 (1985) 1208 (in Japanese). Y. Ishida, M. Mushiage, H. Fukada, M. Muranishi and H. Tanaka, Development of laser diodes by MBE with In-free substrate mounting method, Shinku, 10 (1985) 759 (in Japanese). H. Tanaka and M. Mushiage, MBE as a production technology,J. Cryst. Growth, 111 (1991) 1043. R. Lang and K. Kobayashi, External optical feedback

[11] [12]

[13] [14] [15] [16]

effects on semiconductor injection laser properties, IEEEJ. Quantum Electron., QE-16 (1980) 347. T. Kanada, Theoretical study of noise reduction effects by superimposed pulse modulation, Trans. IECEJpn., 68 (1985) 180. G.R. Gray, A.T. Ryan, G.P. Agrawal and E.C. Cage, Control of optical-feedback-induced laser intensity noise in optical data recording, Opt. Eng., 32 (1993) 739. S. Uchida and H. Mataki, to be published. P.E. Green, Jr., Fiber Optic Networks, Prentice-Hall, Englewood Cliffs, NJ, 1993. D.C. Hanson, Progress of fiber optic LAN and MAN standard, IEEE LMS Mag., May (1990) 17. H. Mataki, T. Yamaji and H. Tanaka, Mass production of AIGaAs laser diodes using MBE, SPIE Proc., 22 (90) (1994).