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InP quantum well laser structures by photoreflectance and photoluminescence
Materials and Engineering B101 (2003) 137 /141 www.elsevier.com/locate/mseb
Investigation of dielectric cap induced intermixing of Inx Ga1x Asy P1y /InP quantum well laser structures by photoreflectance and photoluminescence R. Kudrawiec a,*, G. Se˛k a, W. Rudno-Rudzin´ski a, J. Misiewicz a, J. Wojcik b, B.J. Robinson b, D.A. Thompson b, P. Mascher b b
a Institute of Physics, Wroclaw University of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wroclaw, Poland Department of Engineering Physics, Centre for Electrophotonic Materials and Devices, McMaster University, Hamilton, Ont., Canada L8S 4L7
Abstract The quantum well intermixing (QWI) of 1.55 mm laser structure through thermal treatment, utilising various cap layers, has been investigated by both photoluminescence (PL, emission-like) and photoreflectance (PR, absorption-like) experiments. A blue shift of the QW ground state transition has been observed for post-grown-modified laser structures. The influence of the cap stoichiometry on QWI has been analysed. It has been found that the magnitude of the blue shift strongly depends on the stoichiometry of the dielectric film. In PR, besides the blue shift of the fundamental transition, a blue shift of the excited state transitions has been observed. The blue shift is evidently stronger for the ground state transition than for the higher energy ones. The character of the recombination process at room temperature has been found as free carrier recombination for both as-grown- and post-grownmodified laser structures. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Photoreflectance spectroscopy; InGaAsP compound
1. Introduction InP-based structures have recently attracted increasing interest both in optoelectronic and photonic device fabrication for optical communication of 1.3 and 1.55 mm wavelengths. Besides the control of device operating wavelength by well width and/or well and barrier composition, a post-growth modification of the QW profile is a very promising technique to improve the optical properties of devices. Quantum well intermixing (QWI) is a well-known method for the post-growth modification of bandgaps in QW heterostructures [1]. To date, number of QWI techniques have been reported including impurity-induced disordering [2], impurityfree vacancy disordering [3], ion implantation-induced inter-diffusion [4], and several laser-induced disorderings [5]. Most of them have been used for modification
* Corresponding author. Tel.: /48-71-3202358; fax: /48-713283696. E-mail address: [email protected] (R. Kudrawiec).
of GaAs-based QW, whereas relatively little effort has been devoted to modification of InP-based heterostructures. In this paper, the intermixing of 1.55 mm laser structures through a thermal treatment, utilising various cap layers, have been investigated by both photoreflectance (PR, absorption-like) and photoluminescence (PL, emission-like) experiments. In investigated samples, the dielectric cap layer is deposited on top of the structure and followed by a rapid thermal annealing (RTA) process, during which some group-V element species, diffuse across QW interfaces [6]. In consequence, the QW profile changes and the electronic levels in QW are shifted. PL is a commonly used method for the investigation of these effects [6], and hence only a blue shift of the ground state transition could be observed. Absorption-like experiments are rarely used in such investigations. Therefore, the energy shifts of excited states have not been investigated yet. In this paper, the blue shift of the ground state transition has been determined from both PL and PR. According to earlier results [7,8], it has been shown that the stoichiometry of the dielectric cap influences QWI. The influence of the
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00683-9
138
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
dielectric cap stoichiometry on the magnitude of the blue shift has been analysed, also for excited state transitions.
2. Experiment Samples used in this study were grown by gas source molecular beam epitaxy (GSMBE) on n-doped (1 0 0) InP substrates. The laser structure is shown schematically in Fig. 1. The active region is composed of three compressively strained 5 nm thick QWs of In0.76Ga0.24As0.85P0.15 separated by 10 nm In0.76Ga0.24As0.52P0.48 barriers lattice-matched to InP. Such structure emits at 1568 nm as determined from room temperature PL. The compositions have been chosen so that the In/Ga ratio remains constant throughout the wells and barriers. This ‘partial’ laser structure is completed with 80 nm upper and lower cladding regions of a 1.15 quaternary layer (In0.76Ga0.24As0.39P0.61) doped with 5 /1017 cm 3 Be and Si, respectively, and capped with undoped InP, In0.53Ga0.47As and InP layers as shown in Fig. 1. The InP shield was removed before the deposition of the dielectric film. Silicon oxynitride ˚ thick films were deposited by electron cyclotron 1000 A resonance plasma-enhanced chemical vapour deposition (ECR-PECVD). The technical details of the deposition system have been described elsewhere [9]. The plasma gas precursors, introduced at the top of the ECR chamber, were mixtures of 10% O2 in Ar and of 10% N2 in Ar. The silicon precursor, 30% SiH4 in Ar, was
introduced into the deposition chamber through a dispersion ring, below the plasma generation region. The pressure in the chamber during deposition was maintained at a value of 2.4 mTorr. The silane flow was kept constant, at about 11 sccm, while the oxygen and nitrogen flows were varied in a range from 10 to 40 sccm, yielding films with refractive indices ranging from 1.5 to 1.85, as determined by ellipsometry. The total gas flow rate during deposition was between 50 and 75 sccm and the microwave power was set to 500 W. The samples were not intentionally heated, resulting in a surface temperature of about 120 8C. The deposition rate ˚ min 1. An in situ rotating varied between 42 and 62 A compensator ellipsometer that operates at wavelength of 633 nm was used for thickness control and monitoring of the refractive index of the film. All structures were annealed at a temperature of 780 8C for 60 s. The short description and the numbering of samples are presented in Table 1. PR measurements were performed in the so-called bright configuration, where the sample was illuminated by white light from a halogen lamp (100 W) serving as a probe beam source, at near normal incidence. The reflected light was dispersed through a 0.55 m focal length single grading monochromator and detected by a germanium photodiode. For photomodulation, 488 nm line of an Ar laser was used as a pump beam. Before optical characterisation for all samples, the dielectric film and next two layers (In0.53Ga0.47As, InP) had been removed to improve the optical signal.
3. Results and discussion Fig. 2 shows the room-temperature PL spectra for asgrown (1), as-grown annealed (2), and dielectric film cap-annealed (3 /6) samples. For all samples, one asymmetrical peak is observed. The line shape of the peak is typical for the free carrier recombination process [10] between the first electron sub-band and the first heavy hole sub-band. The energy of the PL peak maximum, for all investigated samples, is shown in Table 2. In comparison to as-grown sample, the peak shifts toward shorter wavelengths after the post-growth modification. The annealing of the as-grown sample (2) generates 16 meV (31 nm) blue shift, whereas in the case Table 1 Samples description
Fig. 1. Layer structure of 1.55 mm 3QW laser. 1.15Q and 1.24Q mean quaternary alloys emitting at 1.15 and 1.24 mm, respectively.
Number of samples
Refractive index
˚) Thickness (A
O2/N2 ratio
3 4 5 6
1.838 1.884 1.547 1.705
1003 995 118 1004
0.147 0.317 2.67 1.01
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
139
Fig. 2. Room-temperature PL spectra.
of dielectric film-capped samples the blue shift is evidently stronger. The shift depends also on the stoichiometry of the dielectric cap and varies from 25 to 55 meV. The PL line of the laser structure capped with film having 1.71 refractive index (6) is mostly shifted. For the more detailed analysis of the optical properties of post-grown modified laser structures, PR spectra have been measured. Fig. 3 shows the room-temperature
Fig. 3. Room-temperature PR spectra: (open circles) experimental points; (solid lines) PR fitting curve.
PR spectra, for the same samples as in Fig. 2. Generally, it is clearly seen that PR can provide more information than PL. Two additional transitions related to QW region are observed, besides the ground state one. The transition energies have been obtained from the fitting procedure according to the first-derivative Gaussian line shape, the most appropriate shape in the case of
Table 2 Experimental results obtained from PR spectra As-grown
As-grown annealed
SiOx Ny (n/1.84)
SiOx Ny (n/1.88)
SiOx Ny (n/1.55)
SiOx Ny (n/1.71)
1C /1HH transitiona Energy (eV) 0.790 Blue shift (meV) / Blue shift (nm) /
0.806 16 31
0.815 25 48
0.822 32 61
0.839 49 92
0.845 55 102
1HH /1C transitionb Energy (eV) 0.790 Blue shift (meV) / Blue shift (nm) /
0.806 16 31
0.815 25 48
0.820 30 57
0.838 48 90
0.843 53 99
2HH /1C transitionb Energy (eV) 0.888 Blue shift (meV) / Blue shift (nm) /
0.890 2 3
0.892 4 5
0.892 4 5
0.905 17 21
0.916 19 24
1LH /1C transitionb Energy (eV) 0.910 Blue shift (meV) / Blue shift (nm) /
0.921 11 16
0.931 21 25
0.932 21 25
0.933 23 27
0.919 43 50
n , refractive index. a From PL. b From PR.
140
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
confined state transitions at room temperature [11]. In order to identify the features in the PR spectra in Fig. 3, the calculations have been performed. Schro¨dinger equation has been solved for the as-grown structure in a effective mass approximation using simple envelope function model. The energy levels were obtained assuming a single square QW (neglecting the coupling effects between the wells due to thick separating barriers), tilted by a built-in electric field of about 57 kV cm 1, estimated from the Fermi level location at acceptor and donor energy levels in the p- and n-type materials at each side of the active region. This way, the origin of these three PR resonances has been explained. The first feature at 0.790 eV (1568 nm), labelled as 1HH /1C in Fig. 2, is associated with the transition between first heavy hole and first electron sub-bands. The two following resonances at higher energies (0.888 and 0.910 eV) are related to transitions between the second heavy hole sub-band and the first electron sub-band (2HH /1C) and the first light hole and the first electron sub-bands (1LH /1C). Generally, the 2HH /1C transition is the forbidden one, but in this case, the electric field of the p /i/n structure breaks the selection rules making possible its observation. The PR fitting parameters obtained for modified structures are shown in Table 2. The blue shift observed between the as-grown and the post-growth-modified samples has been derived and also is shown in Table 2. Analysis and discussion of the results, which are included in Table 2, are found below. The migration of semiconductor atoms from the QW region to the dielectric cap region could generate some defect states, which could be observed in emission. The absorption experiment, such as PR, is not sensitive on defect states, and a comparison between PL and PR gives information about a character of the recombination process. Within the experimental error in PL and PR experiments, the ground state transition is observed at the same energy for all samples. It means that for all investigated structures the emission line is attributed to the free carrier recombination process at room temperature without a significant contribution of defect states. In PR spectra, besides the blue shift of the ground state transition (1HH /1C), a blue shift of the 2HH /1C and 1LH /1C transitions is observed. In Fig. 3, these QW transitions are marked by arrows. Generally, for all post-growth-modified samples, the blue shift of the ground state transition is stronger than that for the 1LH /1C and 2HH /1C ones (see Table 2). Such behaviour is expected and is connected with a change of QW profile caused by atom migration. It is obvious that the change of QW profile from a square-like to a parabolic-like has to shift different energy levels in different way. In investigated structures, the blue shift is associated with the above-discussed shift of energy levels inside the potential well and a change of the
bandgap energy of the well material. The value of the bandgap depends on the stoichiometry of the well material, which changes during the post-growth modification. It has been shown [6] that in InGaAsP QW, the intermixing of group-V atoms take place. In consequence, the bandgap increases for the well material and decreases for the barrier material near the QW interface. It causes that the bandgap changes gradually in the growth direction in contrast to sharp step-like potential of the perfect square QW. We can write a simple equation for the energy of the QW transition in the post-growth modified structure. For the ground sate transition, it is E1HH1C E g? DE g? E1HH E1C ;
(1)
where E ?g is the energy taken between the top of valence band and the bottom of the conduction band in the QW profile before the post-growth modification, DE ?g is an increase of the E ?g value caused by modification, E1HH the energy of the first heavy hole sub-band, and E1C the energy of the first electron sub-band. For the 1LH /1C and 2HH /1C transitions, the energy of the first light hole sub-band (E1LH) and the second heavy hole subband (E2HH) substitute the E1HH value of the first heavy hole sub-band in Eq. (1). The value of DEg cannot be determined on the basis of obtained results. It is certain that the increase of the bandgap of QW material should be greater than zero. If DE ?g was sufficiently large, the value of the energy of the levels would decrease in relation to the sub-band energy of the as-grown sample. Generally, the observed blue shift of the QW transitions is a sum of an increase of E g? value (/DE g? ) and an increase or decrease of the level energy. In our case, this sum is positive and in consequence the blue shift of all the three QW transitions has been observed. From the energy difference between the second and the first heavy hole transitions, the energy difference
Fig. 4. Energy difference between the second and the first heavy hole levels versus the blue shift of the ground state transition.
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
between the second and the first heavy hole levels (E2HH/E1HH) can be obtained. This E2HH/E1HH energy difference versus the blue shift of the ground state transition is presented in Fig. 4. For structures with stronger blue shift of the ground state transition, the difference between heavy hole levels is lower. Such result is expected and means that strongly modified QW are wider and more shallow.
4. Conclusions In conclusion, the influence of SiOx Ny cap films on the intermixing effect for 1.55 mm Inx Ga1x Asy P1y / InP QW laser structures has been investigated by PR and PL spectroscopies. The free carrier recombination process has been observed at the room temperature for the as-grown and post-growth-modified laser structures. The blue shift is strongly correlated with the stoichiometry of the cap film and is observed for both ground and excited state transitions, but the ground state transition is evidently more shifted than the other QW transitions. A maximum in the blue shift of the ground state transition has been observed for one particular content of the cap layer. It can be useful for the postgrowth dielectric cap-induced correction of the laser emission wavelength.
141
Acknowledgements One of the authors (G.S.) acknowledges the financial support from the Foundation for Polish Science.
References [1] E.H. Li, Semiconductor Quantum Well Intermixing, Gordon and Breach, Amsterdam, 1997. [2] W.D. Laidig, N. Holonyak, Jr., M.D. Camras, K. Hess, J.J. Coleman, P.D. Dapkus, J. Bardeen, Appl. Phys. Lett. 38 (1981) 776. [3] G. Deppe, L.J. Guido, N. Holonyak, K.C. Hseih, R.D. Burnham, L. Thornton, T.L. Paoli, Appl. Phys. Lett. 49 (1986) 510. [4] J.Z. Wan, J.G. Simmson, D.A. Thompson, J. Appl. Phys. 81 (1997) 765. [5] J.D. Ralston, A.L. Moretti, R.K. Jain, F.A. Chambers, Appl. Phys. Lett. 52 (1817) 1987. [6] A.S.W. Lee, M. MacKenzie, D.A. Thompson, J. Bursik, B.J. Robinson, G.C. Weatherly, Appl. Phys. Lett. 78 (2001) 3199. [7] J.F. Hazell, D.A. Thompson, N. Bertsch, J.G. Simmons, B.J. Robinson, G.I. Sproule, Semicond. Sci. Technol. 16 (2001) 986. [8] J. Wojcik, B.J. Robinson, D.A. Thompson, P. Mascher, J. Vac. Sci. Technol. A 20 (2002) 1076. [9] M. Boudreau, M. Boumerzoug, P. Mascher, P.E. Jessop, Appl. Phys. Lett. 63 (1993) 3014. [10] M. Colocci, M. Gurioli, A. Vinatteri, J. Appl. Phys. 68 (1990) 2809. [11] F.H. Pollack, Modulation spectroscopy of semiconductor and semiconductor microstructures, in: M. Balkanski (Ed.), Handbook on Semiconductors, vol. 2, Elsevier, Amsterdam, 1994, pp. 527 /635.
Investigation of dielectric cap induced intermixing of Inx Ga1x Asy P1y /InP quantum well laser structures by photoreflectance and photoluminescence R. Kudrawiec a,*, G. Se˛k a, W. Rudno-Rudzin´ski a, J. Misiewicz a, J. Wojcik b, B.J. Robinson b, D.A. Thompson b, P. Mascher b b
a Institute of Physics, Wroclaw University of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wroclaw, Poland Department of Engineering Physics, Centre for Electrophotonic Materials and Devices, McMaster University, Hamilton, Ont., Canada L8S 4L7
Abstract The quantum well intermixing (QWI) of 1.55 mm laser structure through thermal treatment, utilising various cap layers, has been investigated by both photoluminescence (PL, emission-like) and photoreflectance (PR, absorption-like) experiments. A blue shift of the QW ground state transition has been observed for post-grown-modified laser structures. The influence of the cap stoichiometry on QWI has been analysed. It has been found that the magnitude of the blue shift strongly depends on the stoichiometry of the dielectric film. In PR, besides the blue shift of the fundamental transition, a blue shift of the excited state transitions has been observed. The blue shift is evidently stronger for the ground state transition than for the higher energy ones. The character of the recombination process at room temperature has been found as free carrier recombination for both as-grown- and post-grownmodified laser structures. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Photoreflectance spectroscopy; InGaAsP compound
1. Introduction InP-based structures have recently attracted increasing interest both in optoelectronic and photonic device fabrication for optical communication of 1.3 and 1.55 mm wavelengths. Besides the control of device operating wavelength by well width and/or well and barrier composition, a post-growth modification of the QW profile is a very promising technique to improve the optical properties of devices. Quantum well intermixing (QWI) is a well-known method for the post-growth modification of bandgaps in QW heterostructures [1]. To date, number of QWI techniques have been reported including impurity-induced disordering [2], impurityfree vacancy disordering [3], ion implantation-induced inter-diffusion [4], and several laser-induced disorderings [5]. Most of them have been used for modification
* Corresponding author. Tel.: /48-71-3202358; fax: /48-713283696. E-mail address: [email protected] (R. Kudrawiec).
of GaAs-based QW, whereas relatively little effort has been devoted to modification of InP-based heterostructures. In this paper, the intermixing of 1.55 mm laser structures through a thermal treatment, utilising various cap layers, have been investigated by both photoreflectance (PR, absorption-like) and photoluminescence (PL, emission-like) experiments. In investigated samples, the dielectric cap layer is deposited on top of the structure and followed by a rapid thermal annealing (RTA) process, during which some group-V element species, diffuse across QW interfaces [6]. In consequence, the QW profile changes and the electronic levels in QW are shifted. PL is a commonly used method for the investigation of these effects [6], and hence only a blue shift of the ground state transition could be observed. Absorption-like experiments are rarely used in such investigations. Therefore, the energy shifts of excited states have not been investigated yet. In this paper, the blue shift of the ground state transition has been determined from both PL and PR. According to earlier results [7,8], it has been shown that the stoichiometry of the dielectric cap influences QWI. The influence of the
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00683-9
138
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
dielectric cap stoichiometry on the magnitude of the blue shift has been analysed, also for excited state transitions.
2. Experiment Samples used in this study were grown by gas source molecular beam epitaxy (GSMBE) on n-doped (1 0 0) InP substrates. The laser structure is shown schematically in Fig. 1. The active region is composed of three compressively strained 5 nm thick QWs of In0.76Ga0.24As0.85P0.15 separated by 10 nm In0.76Ga0.24As0.52P0.48 barriers lattice-matched to InP. Such structure emits at 1568 nm as determined from room temperature PL. The compositions have been chosen so that the In/Ga ratio remains constant throughout the wells and barriers. This ‘partial’ laser structure is completed with 80 nm upper and lower cladding regions of a 1.15 quaternary layer (In0.76Ga0.24As0.39P0.61) doped with 5 /1017 cm 3 Be and Si, respectively, and capped with undoped InP, In0.53Ga0.47As and InP layers as shown in Fig. 1. The InP shield was removed before the deposition of the dielectric film. Silicon oxynitride ˚ thick films were deposited by electron cyclotron 1000 A resonance plasma-enhanced chemical vapour deposition (ECR-PECVD). The technical details of the deposition system have been described elsewhere [9]. The plasma gas precursors, introduced at the top of the ECR chamber, were mixtures of 10% O2 in Ar and of 10% N2 in Ar. The silicon precursor, 30% SiH4 in Ar, was
introduced into the deposition chamber through a dispersion ring, below the plasma generation region. The pressure in the chamber during deposition was maintained at a value of 2.4 mTorr. The silane flow was kept constant, at about 11 sccm, while the oxygen and nitrogen flows were varied in a range from 10 to 40 sccm, yielding films with refractive indices ranging from 1.5 to 1.85, as determined by ellipsometry. The total gas flow rate during deposition was between 50 and 75 sccm and the microwave power was set to 500 W. The samples were not intentionally heated, resulting in a surface temperature of about 120 8C. The deposition rate ˚ min 1. An in situ rotating varied between 42 and 62 A compensator ellipsometer that operates at wavelength of 633 nm was used for thickness control and monitoring of the refractive index of the film. All structures were annealed at a temperature of 780 8C for 60 s. The short description and the numbering of samples are presented in Table 1. PR measurements were performed in the so-called bright configuration, where the sample was illuminated by white light from a halogen lamp (100 W) serving as a probe beam source, at near normal incidence. The reflected light was dispersed through a 0.55 m focal length single grading monochromator and detected by a germanium photodiode. For photomodulation, 488 nm line of an Ar laser was used as a pump beam. Before optical characterisation for all samples, the dielectric film and next two layers (In0.53Ga0.47As, InP) had been removed to improve the optical signal.
3. Results and discussion Fig. 2 shows the room-temperature PL spectra for asgrown (1), as-grown annealed (2), and dielectric film cap-annealed (3 /6) samples. For all samples, one asymmetrical peak is observed. The line shape of the peak is typical for the free carrier recombination process [10] between the first electron sub-band and the first heavy hole sub-band. The energy of the PL peak maximum, for all investigated samples, is shown in Table 2. In comparison to as-grown sample, the peak shifts toward shorter wavelengths after the post-growth modification. The annealing of the as-grown sample (2) generates 16 meV (31 nm) blue shift, whereas in the case Table 1 Samples description
Fig. 1. Layer structure of 1.55 mm 3QW laser. 1.15Q and 1.24Q mean quaternary alloys emitting at 1.15 and 1.24 mm, respectively.
Number of samples
Refractive index
˚) Thickness (A
O2/N2 ratio
3 4 5 6
1.838 1.884 1.547 1.705
1003 995 118 1004
0.147 0.317 2.67 1.01
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
139
Fig. 2. Room-temperature PL spectra.
of dielectric film-capped samples the blue shift is evidently stronger. The shift depends also on the stoichiometry of the dielectric cap and varies from 25 to 55 meV. The PL line of the laser structure capped with film having 1.71 refractive index (6) is mostly shifted. For the more detailed analysis of the optical properties of post-grown modified laser structures, PR spectra have been measured. Fig. 3 shows the room-temperature
Fig. 3. Room-temperature PR spectra: (open circles) experimental points; (solid lines) PR fitting curve.
PR spectra, for the same samples as in Fig. 2. Generally, it is clearly seen that PR can provide more information than PL. Two additional transitions related to QW region are observed, besides the ground state one. The transition energies have been obtained from the fitting procedure according to the first-derivative Gaussian line shape, the most appropriate shape in the case of
Table 2 Experimental results obtained from PR spectra As-grown
As-grown annealed
SiOx Ny (n/1.84)
SiOx Ny (n/1.88)
SiOx Ny (n/1.55)
SiOx Ny (n/1.71)
1C /1HH transitiona Energy (eV) 0.790 Blue shift (meV) / Blue shift (nm) /
0.806 16 31
0.815 25 48
0.822 32 61
0.839 49 92
0.845 55 102
1HH /1C transitionb Energy (eV) 0.790 Blue shift (meV) / Blue shift (nm) /
0.806 16 31
0.815 25 48
0.820 30 57
0.838 48 90
0.843 53 99
2HH /1C transitionb Energy (eV) 0.888 Blue shift (meV) / Blue shift (nm) /
0.890 2 3
0.892 4 5
0.892 4 5
0.905 17 21
0.916 19 24
1LH /1C transitionb Energy (eV) 0.910 Blue shift (meV) / Blue shift (nm) /
0.921 11 16
0.931 21 25
0.932 21 25
0.933 23 27
0.919 43 50
n , refractive index. a From PL. b From PR.
140
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
confined state transitions at room temperature [11]. In order to identify the features in the PR spectra in Fig. 3, the calculations have been performed. Schro¨dinger equation has been solved for the as-grown structure in a effective mass approximation using simple envelope function model. The energy levels were obtained assuming a single square QW (neglecting the coupling effects between the wells due to thick separating barriers), tilted by a built-in electric field of about 57 kV cm 1, estimated from the Fermi level location at acceptor and donor energy levels in the p- and n-type materials at each side of the active region. This way, the origin of these three PR resonances has been explained. The first feature at 0.790 eV (1568 nm), labelled as 1HH /1C in Fig. 2, is associated with the transition between first heavy hole and first electron sub-bands. The two following resonances at higher energies (0.888 and 0.910 eV) are related to transitions between the second heavy hole sub-band and the first electron sub-band (2HH /1C) and the first light hole and the first electron sub-bands (1LH /1C). Generally, the 2HH /1C transition is the forbidden one, but in this case, the electric field of the p /i/n structure breaks the selection rules making possible its observation. The PR fitting parameters obtained for modified structures are shown in Table 2. The blue shift observed between the as-grown and the post-growth-modified samples has been derived and also is shown in Table 2. Analysis and discussion of the results, which are included in Table 2, are found below. The migration of semiconductor atoms from the QW region to the dielectric cap region could generate some defect states, which could be observed in emission. The absorption experiment, such as PR, is not sensitive on defect states, and a comparison between PL and PR gives information about a character of the recombination process. Within the experimental error in PL and PR experiments, the ground state transition is observed at the same energy for all samples. It means that for all investigated structures the emission line is attributed to the free carrier recombination process at room temperature without a significant contribution of defect states. In PR spectra, besides the blue shift of the ground state transition (1HH /1C), a blue shift of the 2HH /1C and 1LH /1C transitions is observed. In Fig. 3, these QW transitions are marked by arrows. Generally, for all post-growth-modified samples, the blue shift of the ground state transition is stronger than that for the 1LH /1C and 2HH /1C ones (see Table 2). Such behaviour is expected and is connected with a change of QW profile caused by atom migration. It is obvious that the change of QW profile from a square-like to a parabolic-like has to shift different energy levels in different way. In investigated structures, the blue shift is associated with the above-discussed shift of energy levels inside the potential well and a change of the
bandgap energy of the well material. The value of the bandgap depends on the stoichiometry of the well material, which changes during the post-growth modification. It has been shown [6] that in InGaAsP QW, the intermixing of group-V atoms take place. In consequence, the bandgap increases for the well material and decreases for the barrier material near the QW interface. It causes that the bandgap changes gradually in the growth direction in contrast to sharp step-like potential of the perfect square QW. We can write a simple equation for the energy of the QW transition in the post-growth modified structure. For the ground sate transition, it is E1HH1C E g? DE g? E1HH E1C ;
(1)
where E ?g is the energy taken between the top of valence band and the bottom of the conduction band in the QW profile before the post-growth modification, DE ?g is an increase of the E ?g value caused by modification, E1HH the energy of the first heavy hole sub-band, and E1C the energy of the first electron sub-band. For the 1LH /1C and 2HH /1C transitions, the energy of the first light hole sub-band (E1LH) and the second heavy hole subband (E2HH) substitute the E1HH value of the first heavy hole sub-band in Eq. (1). The value of DEg cannot be determined on the basis of obtained results. It is certain that the increase of the bandgap of QW material should be greater than zero. If DE ?g was sufficiently large, the value of the energy of the levels would decrease in relation to the sub-band energy of the as-grown sample. Generally, the observed blue shift of the QW transitions is a sum of an increase of E g? value (/DE g? ) and an increase or decrease of the level energy. In our case, this sum is positive and in consequence the blue shift of all the three QW transitions has been observed. From the energy difference between the second and the first heavy hole transitions, the energy difference
Fig. 4. Energy difference between the second and the first heavy hole levels versus the blue shift of the ground state transition.
R. Kudrawiec et al. / Materials and Engineering B101 (2003) 137 /141
between the second and the first heavy hole levels (E2HH/E1HH) can be obtained. This E2HH/E1HH energy difference versus the blue shift of the ground state transition is presented in Fig. 4. For structures with stronger blue shift of the ground state transition, the difference between heavy hole levels is lower. Such result is expected and means that strongly modified QW are wider and more shallow.
4. Conclusions In conclusion, the influence of SiOx Ny cap films on the intermixing effect for 1.55 mm Inx Ga1x Asy P1y / InP QW laser structures has been investigated by PR and PL spectroscopies. The free carrier recombination process has been observed at the room temperature for the as-grown and post-growth-modified laser structures. The blue shift is strongly correlated with the stoichiometry of the cap film and is observed for both ground and excited state transitions, but the ground state transition is evidently more shifted than the other QW transitions. A maximum in the blue shift of the ground state transition has been observed for one particular content of the cap layer. It can be useful for the postgrowth dielectric cap-induced correction of the laser emission wavelength.
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Acknowledgements One of the authors (G.S.) acknowledges the financial support from the Foundation for Polish Science.
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