Implant isolation of AlGaAs multilayer DBR

Transcription

1 Nuclear Instruments and Methods in Physics Research B 218 (2004) Implant isolation of AlGaAs multilayer DBR A.V.P. Coelho a, *, H. Boudinov a, T. v. Lippen b, H.H. Tan c, C. Jagadish c a Instituto de Fısica, UFRGS, Av. Bento Gonclaves 9500, Porto Alegre, RS, Brazil b Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands c Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia Abstract AlGaAs distributed Bragg reflector (DBR) structures isolation by proton irradiation was studied. The evolution of n- and p-type DBR structures lateral sheet resistance with the irradiated proton dose was measured. The vertical isolation behavior was also obtained and compared to the lateral one. No significant differences were observed. The implantation maximum energy for these structures was estimated. Thermal stability of DBR vertical isolation was studied. A 500 C stability was obtained for samples implanted to a dose 2.5 times greater than the threshold dose of isolation. Ó 2003 Elsevier B.V. All rights reserved. PACS: Jv; Tm; Eq Keywords: AlGaAs; VCSEL; DBR; Implantation; Isolation 1. Introduction GaAs AlGaAs is one of the most widely used compound semiconductor structures for a broad range of device applications. High mobility of the electrons at the interface of modulation doped GaAs AlGaAs heterostructures has led to the fabrication of high electron mobility transistors (HEMT [1]). GaAs AlGaAs structures are also widely used to fabricate heterostructure bipolar transistors (HBT [2]) and various kinds of lasers, from edge emitting lasers to vertical cavity surface emitting lasers (VCSEL[3]). There is much interest on multiple section lasers, which allow generation * Corresponding author. address: arturvpc@terra.com.br (A.V.P. Coelho). of picosecond laser pulses [4]. For this purpose, the carrier lifetime in the absorber section needs to be minimized. This could be achieved by introducing damage in the material, in order to create deep level centers and to decrease nonradiative emission lifetime. It is also necessary to electrically isolate the gain and absorber sections. Both these properties can be achieved by ion implantation. Proton and nitrogen bombardment of laser diode facets has been reported [5]. Ion implantation is an essential process for the production of modern compound semiconductor devices and circuits and has been proven as a successful method to convert a conductive layer into a highly resistive one. Due to its simplicity, precise depth control and compatibility with planar technologies, ion implantation is a potential alternative for mesa etching. Selective masking of X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi: /j.nimb

2 382 A.V.P. Coelho et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) the semiconductor surface with photoresist followed by ion irradiation is an efficient and practical way to isolate closely spaced devices. The isolation of GaAs by ion irradiation has been extensively studied [6 10]. It was shown that the threshold dose to convert a conductive layer to a highly resistive one and the thermal stability of irradiated AlGaAs do not depend on the Al content [11]. Some of the very early VCSELdesigns were implant defined. These have the advantages of simpler processing steps, which are suitable for mass production. In fact, because the device aperture is completely controlled by photolithography, device to device uniformity is easy to achieve. These factors, coupled with a lower manufacturing cost made the implant defined VCSELthe first VCSELdesign to be mass produced commercially. There is still a great need to optimize the implantation steps, in order to help improve this VCSELdesign. Traditionally, proton implantation has been used to form electrical isolation in VCSELs. This is usually done because protons are known to create point defects in GaAs, and, due to the lower stopping power for protons, they require lower energies to penetrate sufficiently deeply into the top distributed Bragg reflector (DBR). This is not expected to impact significantly on the optical properties of the material, since the defects concentration is too low to significantly scatter the light. In this work, high resistive n- and p-type DBR layer formation by proton bombardment was studied in an attempt to better understand the isolation process and optimize this technological step. 2. Experimental All DBR structures were grown in the Australian National University Metalorganic Epitaxial Chemical Vapor Deposition (MOCVD) reactor on semi-insulating (SI) or p þ vertical gradient freeze (VGF) GaAs for p-type DBR and SI or n þ VGF GaAs for n-type DBR. The heterostructure of the DBR was made by repeating 15 times the following structure: a layer of Al 0:1 Ga 0:9 As (494 A), a layer with an Al gradient from 0.1 to 0.9 (200 A), Al 0:9 Ga 0:1 As (590 A) and a layer with Al gradient from 0.9 to 0.1 (200 A) (see Fig. 1). The final structure was capped with a 50 A GaAs layer to prevent oxidation and the total DBR thickness was 2.23 lm. The doping concentrations were in the range of cm 3 for p-type (Zn doped) and cm 3 for n-type (Si doped) DBRs (measured by electrochemical capacitance voltage (ECV) method). These samples were cleaved in pieces of 6 3 mm 2 for the preparation of resistors to measure the lateral resistance. For this purpose samples with SI substrates were used. The same structure was grown on a highly doped substrate to measure the perpendicular resistance. Circular Au contacts were applied on the top of the DBR structure by electron beam deposition using a mechanical mask. The diameter of the contacts was 0.8 mm. GaAs capping (50 Å) 494 Å 0.0 x 1.0 Al 0.1 Ga 0.9 As Al 0.9 Ga 0.1 As 590 Å 200 Å Al0.1 Ga 0.9 As 14 x GaAs substrate 15 times 1484 Å Al 0.9 Ga 0.1 As Fig. 1. Schematic overview of DBR structure.

3 A.V.P. Coelho et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) InGa alloy is applied on the backside for making ohmic contact with the substrate. The prepared samples were irradiated at RT with protons in an energy range of kev with various doses. The energy was varied in order to optimize the implantation parameters for current confinement. The current density was varied between 80 and 300 na/cm 2. During implantation the samples were tilted 15 C away from the beam axis to minimize channeling effects. The lateral resistance values were measured in-situ during irradiation. The I V curves of the vertical isolated samples were measured after the implantation, using HP4140B pa meter/dc voltage source. Threshold doses and thermal stability of the isolation of perpendicular and lateral DBR structures were investigated. 3. Results and discussion The maximum implantation energy is a critical parameter for the performance of the VCSELdevice because one needs to electrically isolate layers of the top DBR structure without creating nonradiative recombination centers in the active region. To find out this maximum value for our DBR resistors, cm 2 dose steps with different energies were accumulated in p-type DBR resistors on SI substrate, leading to an evolution of the lateral resistance as shown in Fig. 2. After each implantation step of this dose, the energy was increased. When the critical energy was achieved and if the dose step used was high enough (this step must isolate the entire region from the surface to the defect peak depth), the lateral resistance will be in the order of 10 9 X, and the complete DBR structure will be isolated. The evolution of lateral resistance shows very little increment until 300 kev. After this energy the lateral resistance suddenly increased to X, which means the DBR layer is completely isolated. According to these measurements, the critical energy for proton isolation of the structure shown in Fig. 1 is 300 kev. Fig. 3 presents the estimated [12] defect distribution for different proton implantation energies. Another important result is shown in Fig. 4. In these data, we used 600 kev proton implanted p- and n-type DBR structures to compare the isolation behaviors obtained in vertical and lateral measurements. This energy was chosen to ensure a practically uniform defect distribution in the DBR layer (see Fig. 3). Although some differences were expected, due to the heterojunction barriers for the vertical transport measurements, the results presented similar increases in the resistance with the dose accumulation for both lateral and vertical experiments. Let take a closer look at each case: for lateral resistance, the samples can be compared to a parallel resistors structure, one of these Lateral resistance [Ω] Lateral resistance of DBR Dose = 1 x 10 14, j = 130 na/cm Energy [kev] Fig. 2. Lateral resistance versus implantation energy in p-type DBR on SI substrate. Fig. 3. Defect profile TRIM [12] simulation for different proton energies.

4 384 A.V.P. Coelho et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) R S [Ω/sq] or diff R[Ω] 1x x x10 8 1x10 6 1x10 4 1x10 2 1x10 0 n-type lateral n-type vertical p-type lateral p-type vertical Dose [cm -2 ] Fig. 4. Lateral and vertical resistance versus dose accumulation in p- and n-type DBR samples. 600 kev H þ. resistors corresponding to the DBR layer and another being the SI bulk. Initially, the DBR layer is much more conductive than the bulk, and the total sheet resistance measured is practically the value related to this first region. When the proton dose steps are accumulated, trapping centers are introduced and reduce the conductivity in the DBR layer, increasing the obtained sheet resistance. This goes on until the DBR resistance becomes comparable to the bulk one. Further increases in the proton dose, and, consequently, in the resistivity of the DBR structure, wonõt produce any appreciable change in the total sheet resistance of the sample because this value will, then, correspond to the bulk, and a plateau is observed in the dose versus sheet resistance curve. The dose for which the DBR resistivity becomes similar to the bulk one is called threshold dose ðd th Þ. The plateau ends when the defect concentration turns out to be high enough for hopping conduction [13] to take place; in the case of vertical measurements, the samples correspond to a series resistors structure. For this samples, the bulk is always more conductive than the DBR layer and the total resistance obtained corresponds to the layer. With the proton fluence increase, the DBR resistivity and the total resistance of the sample will also increase. This picture goes on until hopping conduction takes place. The dissimilar initial values presented in Fig. 4 for n- and p-type DBR resistances are due to differences in both mobility and initial carrier concentrations. Note that parallel and perpendicular resistance curves have practically the same behavior up to D th in both n- and p-type cases. Hopping conduction was observed for higher doses in all samples. The thermal stability of the isolation for p-type DBR vertical resistance was studied using three different doses: , , cm 2 (see Fig. 5). The proton implantation energy used was 340 kev. This energy was chosen to ensure complete DBR structure isolation. The doses correspond to 0:5D th, D th and 2:5D th, respectively, for the parallel resistance case (the dose versus parallel sheet resistance figure for the 340 kev implantation case wasnõt shown). After each annealing step the differential resistance was measured by I V curves in the voltage interval )0.1; 0.1 V. For the sample irradiated to the lowest dose (0:5D th ), the perpendicular resistance is already decreased significantly after 300 C. A further decrease also takes place after annealing at 400 C. The conductivity is enhanced by about 2 orders of magnitude during this step. For the D th case, the 300 C recovery step is practically absent, but the 400 C one becomes more pronounced. A new recovery step is observed at 600 C. The sample irradiated with 2:5D th shows a resistance increase after annealing at 300 C, due to a partial recovery of the hopping effect responsible defects. Further Diff Resistance [ Ω ] DBR on p + -substrate E = 340 kev Dose 1 x cm Dose 2 x cm -2 Dose 5 x cm Temperature [ C] Fig. 5. Thermal stability of 340 kev proton isolated p-type DBR, measured vertically.

5 A.V.P. Coelho et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) annealing behavior shows no great difference from the D th case. The differential resistance for these two last samples ends up at the same resistance after annealing at 600 C. 4. Conclusions The evolution of the sheet resistance of conductive Al 0:1 Ga 0:9 As/Al 0:9 Ga 0:1 As multilayer DBR structures during proton irradiation and the stability of the electrical isolation during post-irradiation annealing was studied. The maximum proton implantation energy for these structures was estimated to be 300 kev. No significant differences were observed in the lateral and vertical isolation of p- and n-type DBR. A 500 C thermal stability of vertical isolation can be achieved by a proton implantation to a 2:5D th dose. Acknowledgements This work was partly supported by Brazilian Agencies FAPERGS, CAPES and CNPq. References [1] H. Morkocß, P.M. Solomon, IEEE Spectrum 21 (1984) 28. [2] M.F. Chang, P.M. Asbeck, D.L. Miller, K.C. Wang, IEEE Electron Dev. Lett. EDL-7 (1986) 8. [3] C.W. Wilmsen, H. Tenkin, L.A. Coldren, Vertical Cavity Semiconductor Emitting Laser: Design, Fabrication, and Applications, Cambridge University Press (Trd), [4] D.J. Derickson, R.J. Helkey, A. Mar, J.R. Karin, J.G. Wasserbauer, J.E. Bowers, IEEE J. Quantum Electron. 28 (1992) [5] E.L. Portnoi, A.V. Chelnokov, Sov. Tech. Phys. Lett. 15 (1989) 432. [6] S.J. Pearton, Int. J. Mod. Phys. B 7 (1993) [7] J.P. de Souza, I. Danilov, H. Boudinov, Nucl. Instr. and Meth. B 122 (1997) 51. [8] J.P. de Souza, I. Danilov, H. Boudinov, J. Appl. Phys. 81 (1997) 650. [9] H. Boudinov, A.V.P. Coelho, J.P. de Souza, J. Appl. Phys. 91 (2002) [10] H. Boudinov, A.V.P. Coelho, H.H. Tan, C. Jagadish, J. Appl. Phys. 93 (2003) [11] J.T.v. Lippen, H. Boudinov, H.H. Tan, C. Jagadish, Appl. Phys. Lett. 80 (2002) 264. [12] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Range of Ions in Solids, Vol. 1, Pergamon, Oxford, [13] Y. Kato, T. Shimada, Y. Shiraki, K.F. Komatsubara, J. Appl. Phys. 45 (1974) 1044.