Carbondoped impurity induced layer disorder 0.98 μm lasersR. B. Bylsma, W. S. Hobson, J. Lopata, G. J. Zydzik, M. Geva, M. T. Asom, S. J. Pearton, P. M. Thomas,P. M. Bridenbaugh, M. A. Washington, D. D. Roccasecca, and D. P. Wilt Citation: Journal of Applied Physics 76, 590 (1994); doi: 10.1063/1.357048 View online: http://dx.doi.org/10.1063/1.357048 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/76/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical losses of Alfree lasers for λ=0.808 and 0.98 μm Appl. Phys. Lett. 69, 2983 (1996); 10.1063/1.117751 Buried heterostructure 0.98 μm InGaAs/InGaAsP/InGaP lasers Appl. Phys. Lett. 63, 2183 (1993); 10.1063/1.110577 Estimation of the reliability of 0.98 μm InGaAs/GaAs strained quantum well lasers J. Appl. Phys. 72, 2119 (1992); 10.1063/1.351599 Lowthreshold InGaAs strainedlayer quantum well lasers (λ=0.98 μm) with GaInP cladding layers preparedby chemical beam epitaxy Appl. Phys. Lett. 61, 755 (1992); 10.1063/1.107788 A novel technique for lowthreshold and highpower InGaAs/GaAs strainedlayer 0.98μm buriedheterostructure laser fabrication J. Appl. Phys. 71, 1525 (1992); 10.1063/1.351398

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.216.129.208 On: Mon, 24 Nov 2014 22:19:20

Carbon-doped impurity induced layer disorder 0.98 pm lasers R. 9. Bylsma, W. S. Hobson, J. Lopata, G. J. Zydzik, M. Geva, M. T. Asom, S. J. Pearton, P. M. Thomas, P. M. Bridenbaugh, M. A. Washington, D. D. Roccasecca, and D. P. Wilt AT&T Bell Laboratories, Murray Hill, New Jersey 07974

(Received 15 November 1993; accepted for publication 10 March 1994)

We have fabricated high power carbon-doped InGaAslAlGaAs lasers using an impurity-induced layer disordering process to define the active region. The advantage of carbon doping is that it exhibits significantly lower diffusivity compared to other p-type dopants, thereby avoiding displacement of the p-n junction, even at the high temperatures and long diffusion times required by the disordering process. Secondary ion mass spectrometry (SIMS) measurements before and after Si diffusion show the p-n junction position to be unchanged during processing. The carbon was introduced using Ccl, as an extrinsic precursor, giving improved control over doping levels and ternary growth conditions that is not available with intrinsic carbon doping. Thresholds of 20 mA and slope efficiencies of 0.44 mW/mA at 25 “C were obtained for lasers with cavity lengths of 500 ,um and coated facets.

There has been much interest in the last few years in Er-doped fiber amplifiers (EDFA), and consequently, an in- terest in high power lasers for pumping the doped fiber. The two wavelength bands used for pumping the amplifiers are centered at 1.48. and 0.98 ,um. Lasers emitting at 0.98 pm are attractive because they are more efficient and might give lower noise figures in EDFA systems.’ While there has been much work on 0.98 ,um lasers and many device structures have been investigated, the most promising are those which are planar structures and do not require additional process- ing, such as etching and regrowth. A process consistent with this and attractive in principle is impurity induced layer dis- order (IILD), first discovered” in 1981. In this process the interdiffusion of layers is enhanced by the presence of an impurity. In the most common case, thin GaAs/AlGaAs quantum wells are intermixed by diffusing Si into the epitax- ial layers, resulting in a material whose band gap has been increased by the addition of Al atoms at the expense of Ga atoms. This process has also been used in material with strained InGaAs quantum wells to selectively disorder the field regions between the laser stripes, leaving the active re- gions of the lasers intact.3 IILD is attractive as a process because only one epitaxial growth is needed, no etching of the Al containing clad layers is required, and formation of nonabsorbing mirrors by disordering near the facets to in- crease the catastrophic optical damage threshold is an option.

An issue with the IILD process is that it involves holding the wafer at 850 “C for a few hours to disorder the quantum wells. During this time, commonly used p-type dopants such as Be, Zn and Mg will diffuse, causing the p-n junction position to move. We have used carbon as a p-type dopant because of its low diffusivity at high temperatures, leading to better control of the carrier profile in the final device. There are two previous reports of carbon-doped lasers fabricated by IILD.4*5 In one case (Ref. 4), the layers were doped using the carbon byproducts of the Al and Ga precursor pyrolysis com- bined with adjustments to the Group V/Group III precursor tlow ratio and growth temperature to get the correct carrier density and conductivity type. In the second case,5 Ccl, was

used to extrinsically dope the layers, but the authors were not able to fabricate lasers using the IILD process. The authors speculated that oxygen contamination may have been the cause. In this communication we demonstrate that using Ccl, as a carbon source need not increase the oxygen levels at the C-doping concentrations typical of p-clad layers and that high quality lasers can be fabricated. Excellent control of the p-n junction placement is confirmed by secondary ion mass spectroscopy (SIMS) profiles. These spectra showed that the carbon concentration profile after processing was identical to the as-grown profile. Lasers fabricated using this process had slope efficiencies of 0.44 mW/mA and a thresh- old of 20 mA for devices 500 pm long with coated facets.

The InGaAs/AlGaAs laser structures were grown by or- ganometallic vapor phase epitaxy in a low pressure (30 mTorr) vertical geometry reactor described elsewhere.6 Tri- methylgallium, trimethylaluminum and trymethylindium were used as the gallium, aluminum and indium sources, respectively. Arsine was used as the arsenic source at a par- tial pressure of 0.4 mTorr. Carbon tetrachloride (500 ppm dopant mixture in hydrogen) and disilane were used as the C and Si dopant precursors, respectively. The GaAs growth was kept constant at 27 nrn/min-‘. The structure was grown at 760 “C except for the InGaAs quantum well and 5 nm of the GaAs confinement layer, which were grown at 625 “C. A growth interruption was used prior to and after the growth of the active layer in order to allow time for the temperature change.

Figure 1 depicts the growth sequence and layer compo- sitions. The a-clad doping was -2X1O’8 cmw3 and the p-clad doping was -7X10i7 cmm3. The Graded Index (GRIN) layers were doped at --2X1O’7 cmd3. To process the material into lasers, SiN, was first deposited by plasma en- hanced chemical vapor deposition (PECVD) and patterned Into stripes. A very thin A~(-40 A) layer was then deposited by molecular beam epitaxy and followed by e-beam evapo- ration of 500 A of Si and 1000 A of SiN, was deposited using PECVD. The wafer was then sealed in an evacuated ampoule which had enough solid As to give a -1 atm over-

590 J. Appt. Phys. 76 (l), 1 July 1994 0021-8979/94/76(1)/590/3/$6.00 Q 1994 American Institute of Physics

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.216.129.208 On: Mon, 24 Nov 2014 22:19:20

GaAs 0 -- - 800 A

1500 ii IOOA ioow 100 A

1500 i

n-GaAs SUBSTRATE

FIG. 1. Schematic diagram of low-pressure metalorganic chemical-vapor deposition grown heterostrucure for 0.98 pm lasers

pressure at the diffusion temperature. This was then heated in a furnace for -5 h at 850 “C. The Si and SiN, were removed with a combination of wet and dry etching. A full surface Zn diffusion using a Zn-doped spin-on glass and a proton im- plant of the field region were used to provide a p-contact layer and current blocking, respectively. The fabrication was completed by thinning the wafer, metallizing, cleaving into 500 pm long chips, bonding p-down on studs and coating the facets. The output facet had a 1% low reflectivity coating and the rear facet had a 90% high reflectivity coating.

The diffusion of Si at a high temperature and for rela- tively long times is a necessary step in the laser fabrication process. This thermal cycle causes most p-type dopants to diffuse, as well as causing some interdiffusion of the quan- tum well in the masked off region. Carbon was used in these structures because of its much smaller diffusion coefficient relative to other p-type dopants such as Be and Zn. Figure 2 shows SIMS atomic concentration profiles of the material used to make the lasers and compares the profiles before and after annealing. Profiles (a), (b), and (d) are Al, C, and Si profiles, respectively, before annealing. The downward spike in the Al profile identifies the position of the single InGaAs quantum well between the GRIN regions. The metallurgical p-n junction, identified by where the doping changes from p type to n type, is coincident with the quantum well. Trace (c)

SIMS PROFILE 0.98 IILD LASER I

Al (arb. units) % -$ IO’ E I? s

4

DEPTH (Fm)

FIG. 2. SIMS atomic profiles showing carbon depth profile before and after 5 h Si diffusion. The Al profile la) in arbirtrary units identifies the position of the InGaAs quantum well.

40 0.5 -

-2 30 0.4 5

g

.E.

0.3 5 E .20 is

h 0.2 g Ii x 0.98 pm IILD LASER

10

- 0.44 mW/mn 1 0

0 40 80 120 1 0-W

FIG. 3. Light power vs current and slope efficiency vs current curves for a 0.98 pm laser.

shows the carbon profile after diffusion at 850 “C for 5 h. It is virtually identical to the as-grown profile (b). This allows very good control of the p-n junction position, an important parameter in making low threshold, highly efficient lasers. The oxygen concentration (not shown) was 1.7X1017 cme3 in the p-clad layer and -l.5X10r7 cmm3 in the n-clad layer. This indicates that the oxygen incorporation at the carbon doping level used here is not principally determined by the Ccl4 source, but most likely by aluminum alkoxides in the Al source or possibly from trace quantities of oxygen from the reactor. Nevertheless, the oxygen concentration obtained here is only slightly larger than our typical best values (--1x1or7 cme3). Since we have produced state-of-the-art InGaAs/AlGaAs and GaAs/AlGaAs lasers with oxygen con- centrations in the low 1017 cmB3 range we do not see this as a limitation in the performance of the lasers reported here.7 Figure 3 shows the light output versus current for one of the lasers. A threshold of -20 mA and a slope efficiency of 0.44 mW/mA were measured with 1% and 90% facet coatings. Lasers with narrow actives (-3 pm) were single mode with far-field angles of 30 o parallel to the layers by 45 ’ perpen- dicular to the layers. Wider actives resulted in higher order modes being observed.

A scanning electron microscopy (SEM) cross section of the laser is shown in Fig. 4. This was taken in the voltage contrast mode where the number of secondary electrons col- lected depends on the relative potential of the material and, consequently, n- and p-type material appear on the SEM micrograph with different intensities. In this figure the dark regions are n type and the lighter regions are either p type or semi-insulating. The top layer in Fig. 4 is the p-type contact metallization. Below that, the field regions have had a Si diffusion, a shallow Zn diffusion, and a proton implant. The field region appears, starting at the top, as a light p-type region which is indistinguishable from the semi-insulating region just below it and then the n-type Si diffused region. The disordered quantum well is visible below this layer be- cause of the compositional variation from the In and reduced AI content relative to the AlGaAs cladding layers. The last two layers are the n-type AlGaAs cladding layer and the n-type GaAs substrate. The central portion of the picture shows the Zn and C-doped cladding layer above the virgin

J. Appl. Phys., Vol. 76, No. 1, 1 July 1994 Bylsma et al. 591 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.216.129.208 On: Mon, 24 Nov 2014 22:19:20

H+ IMP1 AuBe p -CONTACT

.ANT/Zn DIFFUSED Si DIFFUSED

--i I--- /

AlGaAs : C --GRIN/

%cTUM WELL/

FIG. 4. Voltage contrast SEM microgragh of a facet of an IILD laser.

single quantum well active, which, for this particular laser, the active is -5 pm wide. Just next to the p cltid above the active it can be seen that there was some lateral Si diffusion and that the proton implant mask protected this .acea so that the n-type region rises to just below the p contact. Electron beam induced current traces showed the p-n junction to be at the active within the resolution of the instrument, cocrobo- rating the SIMS results.

Two important criteria for EDFA pump lasers ace control of the emission wavelength and resistance to catastrophic optical damage (COD). These lasers emitted at 950 nm, slightly shorter than the desired 980 hm. This is in part due to interdiffusion of the well during the disordering process and can be compensated for during growth of the epitaxial base material. It is likely that the time required to disorder the quantum well can be reduced by optimizing the As ovec- pressure, encapsulant material, and surface preparation. Re- duction of diffusion time will~ceduce the interdiffusion of the quantum well in the protected region, giving better wave- length control. The lasers experienced COD at powers of -80 mW. This is not surprising since no precautions, other than facet coating, were taken to protect against this failure mechanism, such as nonabsorbing mirrors or flared facets. Incorporation of either of these improvements should raise the COD threshold and along with the low diffusivity of the carbon dopant and the simplicity of the IILD process provide

for a high yield well controlled fabrication process. Single quantum well strained layer carbon-doped lasers

have been fabricated with low thresholds and high quantum efficiency using the ITLD process. We have shown that Ccl, can be successfully used as a carbon source for growing laser hetecostcuctuces. Carbon has the significant advantage of low diffusivity in devices that require processing at high tempeca- tuces for relatively long periods of time. We have shown that the p-n junction position can be well controlled during fab- rication of laser using the IILD process and so high power lasers can be reproducibly made using this technique, which is planar and needs only one epitaxial growth.

‘M. Yamada, M. Shimuzu, MI Okayasu, T. Takeshita, M. Horiguchi, Y. Tachikawa, and E. Sugita, IEEE Photon. Technol. Lett. 2, 205 (1990).

‘W. D. Laidig, N. Holonyak, Jr., M. D. Camras, K. Hess, J. J. Coleman, P. D. Dapkus, and J. Bardeen, Appl. Phys. Lett. 38, 776 (1981).

‘W. X. Zou, J. L. Merz, R. J. Fu, and C. S. Hong, IEEE Photon. Technol. Lett. 3, 400 (1991).

‘L. J. Guido, G. S. Jackson, D. C. Hall, W. E. Piano, and N. Holonyak, Jr., Appl. Phys. Lett. 52, 522 (1988).

‘I. Szafranek, J. Major, Jr., B. T. Cunningham, L. J. Guido, J. Holonyak, Jr., and G. E. Stillman, Appl. Phys. Lett. 57, 2910 (1990).

bW. S. Hobson, T. D. Harris, C. R. Abernathy, and S. J. Pearton, Appl. Phys. I.&t. 58, 77 (1991).

‘W. S. Hobson, M-C. Wu, Y. K. Chen, M. A. Chin, M. Geva, and K. S. Jones, Appl. Phys. Lett. 60,598 (1992); W. S. Hobson, M. C. Wu, and Y. K. Chen, Semicond. Sci. Technol. 7, 1425 (1992).

592 J. Appt. Phys., Vol. 76, No. 1, 1 July 1994 Bylsma et al. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.216.129.208 On: Mon, 24 Nov 2014 22:19:20