Fabrication and characterization of dry and wet
etched InGaAs/InGaAsP/InP long wavelength
semiconductor lasers
Bulent Cakmak Ataturk University, Faculty of Engineering, Department of Electronics and Telecommunication
25240 Erzurum, TURKEY Tlf:+ 90 442 231 3951 [email protected]
Abstract: This paper presents fabrication and characterization of ridge
waveguide InGaAs/InGaAsP/InP lasers with an operating wavelength of 1.5µm using reactive ion etching (RIE), chemically assisted ion beam etching (CAIBE) and wet etching techniques. Characterization results of the lasers with 2µm-wide waveguides are given of the two etching methods comparatively using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), L-I-V (Light-current-voltage) and spectral measurement techniques. Additionally, a comparison of wet and RIE etched lasers with 20µm-wide waveguide is also discussed. Highly smooth (2.1+0.4nm rms surface roughness) and vertical (∼90o) structures are obtained using RIE, in which the 2µm-wide fabricated devices exhibit better performance over the CAIBE etched ones.
2002 Optical Society of America
OCIS codes: (140.5960) Semiconductor lasers; (220.4000) Microstructure fabrication; (120.6660) Surface measurements, roughness; (180.5810) Scanning microscopy; (300.6210) Spectroscopy, atomic
References
1. D. E. Ibbotson and D. L. Flamm, “Plasma etching for III-V compound devices.2,” Solid State Tech. 31, 105 (1988).
2. J. W. Lee, J. Hong, C. R. Abernathy, E. S. Lambers, S. J. Pearton, W. S. Hobson and F. Ren, “Cl2/Ar
plasma etching of binary, ternary, and quaternary In- based compound semiconductors,” J. Vac. Sci. Tech. B 14, 2567-2573C (1996).
3. S. Allen and P. Silverberg, “Methane-hydrogen III-V metal-organic reactive ion etching,” Semiconductor Sci. Tech. 6, 287-289 (1991).
4. C. S. Whelan, T. E. Kazior and K. Y. Hur, “High rate CH4:H2plasma etch processes for InP,” J. Vac. Sci.
Tech. B 15, 1728-1732 (1997).
5. Y. Yuba, K. Gamo, H. Toba, X. G. He and S. Namba, “Ion beam etching of InP. I. Ar ion beam etching and fabrication of grating for integrated circuits,” Jap. J. of Appl. Phys. 22, 1206-1210 (1983).
6. R. A. Mikhnev, S. K. Shtandel, M. I. Martynov and E. D. Olshanskii, “ How ion-beam etching affects the surface quality of optical articles,” J. of Optical Technology 66, 1032-1034 (1999).
7. W. Katzschner, U. Niggebrugge, R. Lofflerr and H. Schroter-Janssen, “Reactive ion beam etching of InP with N
2and N2/O2mixtures,” Appl. Phys. Lett. 48, 230-232 (1986).
8. F. Frost, A. Schindler and F. Bigl, “Reactive ion beam etching of InSb and InAs with ultrasmooth surfaces”, Semiconductor Science and Tech. 13, 523-527 (1998).
9. J. Daleiden, K. Czotscher, C. Hoffmann, R. Kiefer, S. Klussmann, S. Muller, A. Nutsch, W. Pletschen, S. Weisser, G. Trankle, J. Braunstein and G. Weimann, “Sidewall slope control of chemically assisted ion-beam etched structures in InP-based materials,” J. Vac. Sci. Tech. B 16, 1864-1866 (1998)
10. G. Vollrath, A. Schlachetzki and F. Fiedler, “Ion-beam milling of InGaAsP alloys with N2/O2-mixtures,”
Jap. J. of Appl. Phys. Part 1-Regular Papers Short Notes & Review Papers, 37, 1715-1720 (1998) 11. J. E. Schramm, D. I. Babic, E. L. Hu, J. E. Bowers and J. L. Merz, “Fabrication of high-aspect-ratio
InP-based vertical-cavity laser mirrors using CH4/H2/O2/Ar reactive ion etching,” J. Vac. Sci. Tech. B 15,
2031-2036 (1997).
12. N. Nunoya, M. Makamura, M. Morshed, S. Tamura and S. Arai, “High-performance 1.55µm wavelength GaInAsP-InP distributed-feedback lasers with wirelike active regions,” IEEE J. Selected Topics in Q. Elec 7, 249-258 (2001).
13. C. S. Sundararaman, H. Lafontaine, S. Poulin, A. Mouton and J. F. Currie, “Reactive sputtering of InP in N
2
and N
2/O2plasmas,” J. Vac. Sci. Tech. B 9, 1433-1439 (1991).
14. C. F. Calstrom, G. Landgren and S. Anand, “Low energy ion beam etching of InP using methane chemistry,” J. Vac. Sci. Tech. B 16, 1018-1023 (1998).
15. B. Cakmak and I. H. White, “Ion beam and chemically assisted ion beam etching of InP with anisotropic and smooth structures,” Semicond. Sci. Technol 16, 139-146 (2001).
16. S. Yu, P. Heard, B. Cakmak, R. V. Penty and I.H. White, “Surface diagnostics of dry etched III-V semiconductor samples using focused ion beam and secondary ion mass spectrometry (FIB-SIMS),” J. Vac. Sci. Tech. B 17, 3080-3084 (1999).
17. W. Katzschner, A. Steckenborn, R. Loffler and N. Grote, “Ion beam milling of InP with an Ar/O
2-gas
mixture,” Appl. Phys. Lett 44, 352-354 (1984).
18. J. Gowar, Optical communication systems (Prentice Hall International Ltd., Hertfordshire U.K, 1993), Chap. 19.
1. Introduction
InP-based devices have started to dominate opto-electronics because lasers and related devices with InGaAsP/InP heterostructures are suitable for integrated optics and low-loss fibre communications around the 1.5µm wavelength region where attenuation is about 0.2dB/km. The fabrication of integrated optoelectronic devices necessitates pattern transfer techniques with a high degree of precision and a variable anisotropy. Wet chemical etching is a simple technique that offers high selectivity and prevents deep damage to QW layers when compared with dry-etching techniques. However, features with dimensions less than 3µm are usually patterned using dry etching techniques because wet etching rates can be the same in all directions (isotropic) which causes undercut, and thus wet etching can hardly be used reliably below 3µm feature size.
It is necessary to use anisotropic (vertical) dry-etching in order to produce a uniform etch profile along the entire structure because sidewall verticality (anisotropy) is extremely important for applications such as etched laser facets. Smooth structures are also required because surface roughness can cause short-circuit between patterns that can fail the operation of lasers. Therefore, various dry etching techniques such as plasma etching [1,2], reactive ion etching (RIE) [3,4], ion beam etching (IBE) [5,6], reactive ion beam etching (RIBE) [7,8] and chemically assisted ion beam etching (CAIBE) [9,10] have been successfully used to fabricate InP-based devices to date.
Of these methods, reactive ion etching (RIE) and chemically assisted ion beam etching (CAIBE) are the most frequently used ones. RIE of InP-based structures has been reported widely in the literature using non-corrosive/non-toxic hydrocarbon chemistries (CH4/H2) that provide residue-free morphologies over Cl2-based halogen chemistries [11,12]. Smooth
etching of InP has also been demonstrated using N
2 and N2/O2 [13] plasmas. But these
plasmas not only reduce etch rates significantly (3 to 8nm/min) when compared with CH
4/H2
(typically 40-50nm/min) mixtures but also generate a certain amount of sidewall etching. The second method used for dry etching is CAIBE in which flexibility for profile definition is possible by varying the angle of incidence of beams. In this process, the inadequate performance, i.e. the surface and sidewall roughness, of pure Ar ion beam etching (IBE) can be eliminated by using a mixture of Ar/CH4/H2[14], N2/H2/CH4[14], Ar/H2[15] or
N2/O2[10] gas(es). CAIBE of InP-based structures has been performed using Ar/H2/CH4and
N2/H2/CH4 atmospheres by using an rf inductively coupled plasma (ICP) ion source [14].
Although the roughness in the Ar process was more severe than that for the nitrogen counterpart, the anisotropy was higher for the Ar process [14]. Vollrath and others produced good quality laser facets by CAIBE using mixtures of nitrogen and oxygen [10].
Here, fabrication of long wavelength (1.5µm) ridge-waveguide semiconductor lasers that overcomes short lifetime and low efficiency of oxide-stripe lasers are presented from InGaAs/InGaAsP/InP material system. Comparative experimental results of the lasers are discussed using the three etching (RIE, CAIBE and wet) methods. Anisotropy and surface
roughness of the etched structures are investigated using scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM) techniques. The fabricated lasers with 2µm waveguide-width are characterized using light-current-voltage (L-I-V) and spectra measurements. Finally, wet and RIE etched lasers with a waveguide-width of 20µm are studied comparatively.
2. Experiment 2.1 Material structure
The wafer was grown by metal organic vapour phase epitaxy (MOVPE) at the EPSRC III-V central facility at the University of Sheffield. The epitaxial layers were grown on an n+ Si doped InP substrate. From the substrate, the layer specifications are as follows: a 1µm n-type InP lower cladding layer, a 295nm waveguide core, a 1.2µm p-type InP upper cladding layer, a 50nm InGaAsP transition layer, and finally an In0.53Ga0.47As contact layer.
The active layer contains five 60ÅIn0.53Ga0.47As quantum wells with six 120Å InGaAsP
barriers. The wells are surrounded in both directions with a step graded index region, consisting of a 50nm InGaAsP and an 80nm InGaAsP quaternary layers. All the layers are lattice matched to InP.
2.2 Fabrication of InGaAs/InGaAsP/InP lasers using RIE and CAIBE
We prepared ridge-waveguide InGaAs/InGaAsP/InP lasers in three steps: Etch mask fabrication, etch process to define ridge and metal contact deposition. The etch mask was formed by classical photolithography-defined patterns (2µm straight waveguides) of resist on 160nm and 50nm thick layers of sputter-deposited SiO2 and Ni. These patterns were then
transferred to the substrate by RIE with C2F6, followed by an O2plasma to strip the resist. In
the second step, the ridge waveguide was formed by etching 1.2µm of the top layers with RIE or CAIBE as described below.
RIE of InGaAs/InGaAsP/InP material system was carried out in a Plasma Therm dual electrode system operating at a frequency of 13.56MHz. The rf power was applied to the bottom electrode while the other electrode and the chamber wall were kept at ground potential. Both electrodes and the chamber wall are made of aluminum. During etching, the samples were placed on the bottom electrode which was un-cooled. CH4/H2 gases were
introduced through small holes around the lower electrode and evacuated via a 3.2cm diameter hole in the center. In CH4/H2discharge, hydrogen atoms, organic radicals, and ions
are produced that interact on InP surfaces to etch the material [16].
CAIBE was done with an Ion Tech Ltd ion beam etching system where a saddle-field DC discharge ion gun was used to generate the ion beam. Ions were generated in the discharge chamber by electron bombardment of the neutral Ar atoms. To ignite the discharge, a potential difference was applied between cathode and anode. A combination of reactive gases (H2/CH4) were introduced directly to the sample to improve etch quality. Samples were rotated during etching to enhance etch uniformity. In both etching techniques, several parameters were optimized to produce the highest level of anisotropy and surface smoothness without compromising the etch rate. The optimized parameters are tabulated in Table 1.
Table 1. Optimized parameters during RIE and CAIBE etching of InGaAs/InGaAsP/InP
RIE CAIBE
RF power 400W Acceleration voltage 850V
DC self bias voltage 500V Discharge current 40mA CH
4/H2 10/20sccm CH4/H2 0.5/4.5sccm
Base pressure 0.5mTorr Base pressure 10-6Torr Ion incidence angle 20o
While a maximum etch rate of 50nm/min was observed in RIE, it was 40nm/min in the CAIBE. The etch depth was measured to be 1.2µm for the both processes. In RIE, a polymer layer accumulated in the chamber, requiring oxygen plasma (rf power 150W, 50sccm) cleaning after each run. In RIE and CAIBE, etching occurs by both chemical due to the formation of volatile products and physical because of sputtering of surface by the ions.
In the final stage of the fabrication, the two-layered mask (SiO2/Ni) was removed by
H2SO4and HF prior to the deposition of SiO2. After applying second photolithography and
contact window mask, the SiO2layer on the waveguide was removed by HF. Finally, a 20nm
titanium, 20nm platinum and 150nm gold metal alloy were used in the p-contact recipe, followed by the deposition of a 14nm gold, 14nm germanium, 14nm gold, 11nm nickel and 200nm gold on the n-side. Annealing at 400oC for 1 min. was the optimum condition to obtain the best lasing characteristics since otherwise the devices gave very poor characteristics.
2.3 Fabrication of InGaAs/InGaAsP/InP lasers using wet etching
Two etching steps were applied to fabricate wet etched ridge waveguide lasers. The constituent of the first etch (etch 1) is a mixture of acetic acid, hydrochloric acid (HCl), hydrogen peroxide (H2O2) and water with a ratio of 40:20:3:15. To form the ridge, a second
etch step (etch 2) containing orthophosphoric acid (H3PO4) and HCl (3:1) mixture was used.
To fabricate the devices, first 5µm wide waveguide mask was applied followed by S1805 photoresist spinning as an etch mask. Then, etch 1 was used to remove the contact layer. Prior to Al2O3(300nm) deposition, the ridge (etch depth=1.2µm) was defined by etch 2. Following
the removal of the resist in the acetone, p and n-contact metal deposition was performed by using the same compositions as that of the dry-etched ones.
3. Results and discussion
3.1 SEM and AFM measurements of the fabricated devices
SEM and AFM images of the RIE and CAIBE etched structures are given in Fig. 1 and Fig. 2.
(a) (b) (c)
Fig. 1. (a) Surface/sidewall structure, (b) AFM measurement, and (c) etch profile of the RIE etched lasers
(a) (b) (c)
Fig. 2. (a) Surface/sidewall structure, (b) AFM measurement, and (c) etch profile of the CAIBE etched lasers
2.0µm
2.0µm
2.0µm
Surface roughness in both RIE and CAIBE was quantified by AFM technique. The rms surface roughness was obtained to be 10.8+1.7nm with the CAIBE in Fig. 2(b). A dramatic improvement of the surface morphologies of the etched surfaces was recorded in the case of RIE where the rms roughness was only 2.1+0.4nm as illustrated in Fig. 1(b). This was very close to the initial roughness of an un-etched InP surface which was about 1.3+0.4nm.
Sidewall profiles of the RIE and CAIBE etched structures are shown in Fig. 1(c) and Fig. 2(c) respectively. In Fig. 1(c), a highly anisotropic (∼90o) sidewall profile to the surface parallel in the RIE was achieved while sidewall angle (anisotropy) was 83oin the CAIBE process (Fig. 2(c)). As a result, the best surface quality and anisotropy were obtained in the case of RIE. How these results affect the L-I-V characteristics is discussed below.
3.2 L-I-V characteristics of the fabricated lasers etched by RIE, CAIBE and wet chemical
The light-current-voltage (L-I-V) characteristic of the RIE and CAIBE etched 2µm wide and 400µm long ridge-waveguide lasers, shown in Fig. 3(a) and (b), was measured in order to characterize the operation of the fabricated lasers. A pulse generator, which generates electrical pulses with pulsewidth of 500ns and pulse period of 100µs, was used for driving the lasers. The output of the lasers was coupled either to an Anritsu optical powermeter or an Advantest optical spectrum analyzer via an optical fiber.
(a) (b)
Fig. 3. L-I-V characteristics of 2µm wide and 400µm long (a) RIE and (b) CAIBE etched lasers, and (c) spectral characteristic for both
A spectral characteristic of the device is also illustrated in Fig. 3(c) in which center wavelength was recorded to be 1.56µm. In Fig. 3(a), a threshold current as low as 30mA and 13mW maximum optical power at room temperature was observed for the RIE etched laser. However, the CAIBE laser yielded a threshold current of 65mA and output power of just about 7.5mW. This shows that CAIBE etched lasers have a higher threshold current than the
-80 -70 -60 -50 -40 -30 1,53 1,55 1,57 Wave le ngth (µµµµm ) Po wer (d B m) CAIBE etched (2µµµµm *400µµµµm ) 0 2 4 6 8 0 50 100 150 200 250 300 Current (m A) Po w e r (m W ) 0 0,5 1 1,5 2 Vo lt a g e (V) RIE etched (2µµµµm *400µµµµm ) 0 2 4 6 8 10 12 14 0 30 60 90 120 150 180 210 Current (m A) Po w e r (m W ) 0 1 2 3 Vo lt a g e (V) (c)
RIE counterparts which may be caused by roughness induced loss in the CAIBE process. In the case of CAIBE, there was also a decrease by 12.5% in the external quantum efficiency. It was assumed that the RIE process caused lower damage than that of CAIBE possibly due to less physical sputtering.
The forward bias characteristics (I-V) of the RIE and CAIBE etched lasers were also taken as depicted in Fig. 3(a) and (b). The typical turn-on voltage of 1.2V in InGaAsP/InP laser diodes [18] was recorded for the RIE etched lasers (Fig. 3(a)) in our measurements. However, this was reduced to 0.75V in the CAIBE etched lasers (Fig. 3(b)) which implies that either the optimized RIE etch did not cause the hydrogen passivation effect (the passivation of electrical activity) on the sidewalls or it was removed by the annealing process. The above results shows that RIE etched lasers are superior to the CAIBE etched ones.
Since the fabricated wet etched lasers whose feature sizes were less than 3µm produced very poor characteristics due to probably undercut caused by the wet etch, a comparison of wet and RIE etched semiconductor lasers with 20µm wide waveguide is given in Table 2.
Table 2. Material parameters of wet and RIE etched InGaAs/InGaAsP/InP lasers with 20µm width and 400µm length
ηin(%) Jth(A/cm2) αi(cm-1) Turn-on voltage (V)
Wet etched 18.4 1450 10.4 1.4
RIE etched 26 1625 14 1.3
Table 2 demonstrates that a slight increase in both the internal loss (αi) and the threshold
current density was observed in the case of RIE. This implies that RIE caused a slight damage, which was not removed by the post etch annealing process. However, higher internal quantum efficiency (ηin) and slightly better turn-on voltage were obtained from the RIE
etched lasers. Overall results obtained shows that RIE can be preferable to the other etching techniques to produce highly vertical and smooth structures that are required for laser mirrors.
4. Summary and conclusions
Fabrication and characterization of RIE and CAIBE etched 2µm wide ridge-waveguide long wavelength (1.5µm) semiconductor lasers from InGaAs/InGaAsP/InP material system have been presented comparatively using some characterization techniques such as SEM, AFM, spectra and L-I-V. In addition, 20µm wide devices were fabricated by using wet and RIE etching to obtain comparative results. Prior to the fabrication of the dry-etched lasers, much attention was paid to optimize the etching conditions of RIE and CAIBE processes to produce efficient and reliable lasers. Highly smooth and anisotropic surface morphologies were obtained by using RIE that produced just 2.1+0.4nm rms surface roughness and∼90osidewall angle while CAIBE resulted in an rms roughness of 10.8+1.7nm and an anisotropy of 83o. Thus, for 2µm wide devices performance of the RIE etched devices was observed to be better than the CAIBE etched ones. This is likely due to the higher damage and surface roughness caused by the CAIBE. Since 2µm-wide wet etched lasers failed, a comparison was made for 20µm RIE and wet etched devices where wet etching resulted in a slightly better performance over RIE because of probably a slight damage caused by the RIE. However, as a result, highly vertical and smooth structures are required for applications such as corner mirrors in optical crosspoint switches and etched laser mirrors, and therefore the advantage of using dry etching techniques preferably RIE largely outweighs the risk of damaging the material.
Acknowledgments
I wish to acknowledge the financial support of the Ataturk University in Turkey via project 2001/42. Prof. Dr. Richard Penty, Dr. Siyuan Yu and Prof. Dr. Ian White are acknowledged for their helpful discussions and useful comments. Thanks are also due to Matthew Hearn for assistance with the AFM measurements.
