- Email: [email protected]
AlGaAsSb heterostructures
Vacuum 57 (2000) 171}178
Sul"de treatment of GaSb surface: in#uence on the LPE growth of InGaAsSb/AlGaAsSb heterostructures E. Papis *, A. Piotrowska , E. KaminH ska , K. Go"aszewska , W. Jung , J. Ka9 tcki , A. Kud"a , M. Piskorski , T.T. Piotrowski , J. Adamczewska Institute of Electron Technology, Al. Lotniko& w 46, 02-668 Warsaw, Poland Institute of Physics PAS, Al. Lotniko& w 46, 02-668 Warsaw, Poland
Abstract The in#uence of Na S and (NH ) S treatments on the surface properties of GaSb have been investigated through the etch rate, ellipsometry and Schottky barrier measurements. XRD and TEM analysis were performed to examine the e!ect of sul"de pretreatment of GaSb substrate on the LPE growth of InGaAsSb/AlGaAsSb heterostructures and their structural quality. Additionally, C}V characteristic of Hg-Schottky barriers were measured to evaluate carrier concentrations in non-intentionally doped InGaAsSb epilayers. Na S preepitaxial treatment has been applied to LPE growth of n In Ga As Sb /p-Al Ga As Sb LED heterostructures for j "2.06 lm and n N In Ga As Sb /p-Al Ga As Sb PD heterostructures for wavelength j"2.0}2.4 lm. As a result, LEDs which with increased by a factor of 4 quantum e$ciency and total power of 6 mW were obtained. Mesa-structure photodiodes were characterised by detectivity DH "4;10 cmHz W\ H l at room temperature and dark current density j "16 mA/cm at a reverse bias of !0.5 V. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Heterostructures based on Sb-containing III}V semiconducting compounds are particularly attractive for the fabrication of a wide variety of electronic and optoelectronic devices such as resonant tunneling diodes, light emitting diodes, laser diodes and photodetectors operating in the mid-infrared range of wavelength. During the last few years, much e!ort has been focused on
* Corresponding author. E-mail address: [email protected] (E. Papis). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 1 6 - 0
172
E. Papis et al. / Vacuum 57 (2000) 171}178
increasing the device performance including optimisation of epitaxial structure design and growth. In spite of that, the procedure of chemical preparation of GaSb surface prior to epitaxy is still far from satisfactory. In standard preparations, GaSb etching is characterised by high oxidation rate associated with a low solubility of its oxides. In this context, the sul"de treatment with its proved ability to deoxidize III}V semiconductor surface and remove carbon contaminants appears as a promising candidate as a technique of preparation of epiready grade surface. (NH ) S chemical treatment was successfully applied to molecular beam epitaxy (MBE) regrowth of GaAs [1,2] and to the selective regrowth of the GaAs base layer of heterojunction bipolar transistor [3]. Na S treatment was demonstrated to allow liquid-phase epitaxy (LPE) regrowth of AlGaAs [4] and to improve LPE growth of InGaAsSb/AlGaAsSb heterostructures [5,6]. Following this approach, in this work we have investigated the preparation of GaSb surface in Na S and (NH ) S solutions and its applicability as preepitaxial treatment prior to LPE growth of InGaAsSb/AlGaAsSb photonic device structures. Light emitting diodes (LEDs) and photodetectors (PDs) operating at wavelength 2.0}2.4 lm were of particular interest. 2. Experimental GaSb substrates, (100) oriented, tellurium doped to the concentration n"5;10 cm\ were used in the experiments. Prior to sul"de treatment, the samples received standard surface cleaning consisting of degreasing in hot organic solvents, and etching in a solution of HCl#HNO " 30#1, ¹"53C, t"20 s, followed by etching in 5% HCl for 60 s. Three procedures of sul"dation have been compared: (1) dipping in the 1M Na S aqueous solution, pH"13.1 at RT; (2) dipping in the 21% (NH ) S aqueous solution, pH"9 at RT; (3) anodic sul"dation in the 21% (NH ) S aqueous solution, pH"9 at current density 16 lA/cm. The e!ects of sul"dation of GaSb surface were characterised through the etch rate, ellipsometry and Schottky diode properties. Etch rate was determined on patterned GaSb samples using TENCOR ALPHA-STEP pro"ler. Ellipsometric analysis were performed in the spectral range 240}1100 nm at two angles, 653 and 753, using J.A. Woollam variable angle spectroscopic ellipsometer (VASE). For Schottky barrier measurements samples were patterned with Au dots having a diameter of 200 lm and one large area backside AgTe ohmic contact. Their characteristics were measured using a QuadTech 7600 Precision LCR Meter and Keithley 237 High Voltage Source Measure Unit. The LPE growth experiments were performed in horizontal sliding boat system described in detail in Ref. [7]. 6N Ga, In, Sb, Al, as well as undoped GaSb, InSb, GaAs and InAs were used as source materials. Ge was employed as p-type dopant. LPE growth of InGaAsSb and InGaAsSb/AlGaAsSb heterostructures was carried either at ¹"5283C in case of In Ga As Sb single epilayers or at ¹"5933C in case of In Ga As Sb single epilayers, In Ga As Sb /Al Ga As Sb photodiode, and the In Ga As Sb /Al Ga As Sb LED heterostructures. Before beginning of the growth process GaSb substrates were annealed in #owing H during 1 h at temperature 5903C and 6403C, respectively. X-ray di!raction (high-resolution Philips Material Research) and Transmission Electron Microscopy (JEM 200 CX Jeol) analysis were performed to examine the e!ect of sul"de
E. Papis et al. / Vacuum 57 (2000) 171}178
173
pretreatment of GaSb substrate on the LPE growth of InGaAsSb/AlGaAsSb heterostructures and their structural quality. Additionally, C}V measurements of Hg-Schottky barrier were carried out to evaluate carrier concentrations in non-intentionally doped InGaAsSb epilayers. The atomic composition of epilayers was determined by electron microprobe analysis (EPXMA). The performances of device structures were characterised through the measurements of their electrooptical properties: I}V, spectral, and power characteristics for LEDs and I}V and detectivity for PDs. LED emission was detected with SPM-2 monochromator and PbS PC detector. The optical spectra of PDs were measured using SPM-2 monochromator with NaCl prism and a globar as a light source.
3. Results and discussion Etch depth of GaSb subjected to sul"de treatment, for time up to 30 min, is a linear function of the process duration, with etch rate dependent on su"de solution and method of sulfuration. Etching in 1M Na S solution is characterised by the lowest v"2.2 nm/min etch rate. Sul"de treatment in 21% (NH ) S etches GaSb surface with a rate of 12.2 nm/min (for chemical treatment) and 14 nm/min (for electrochemical treatment). The results of ellipsometric investigations were interpreted in terms of two models. First, the Bruggeman E!ective Medium Approximation for two constituents, GaSb-oxide and Cauchy material (in which refraction index and extinction coe$cient are represented by slowly varying function of wavelength and exponential absorption tail). Second, the bilayer model in which GaSb-oxide and Cauchy material formed separate layers, with Cauchy layer on top. Such models allowed the best "t of calculated and measured data of ( and * (estimated using mean-squared error factor de"ned as in the WVASE 32 program,) and contained a limited number of unknown parameters (seven in case of absorbing layers). It should be mentioned here that recent experimental XPS data of Lin et al. [8] suggesting the formation of two-layer structure, Ga O and Sb S on GaSb surface after sul"de treatment, would support the applica bility of bilayer model. Optical index of GaSb and its oxide were taken from WVASE program library. In Fig. 1 spectroscopic characteristics * indicating the thickness of super"cial layer formed on GaSb after various sul"de treatments are shown. For comparison, the data for standard surface preparation (etching in HCl#HNO "30#1, ¹"53C, t"20 s, followed by etching in 5% HCl for 60 s) are included. The results of ellipsometric investigations are summarized in Table 1. They indicate that the thickness of super"cial layer formed on GaSb surface under sul"de treatment increases with the duration of sul"dation process. They also show that its growth rate is much higher in 21% (NH ) S solution compared to 1 M Na S treatment. The most important "nding is that the thickness of super"cial oxide on sul"de-treated and H -annealed GaSb surface (in LPE reactor) is about 0.8 nm. Since the ellipsometric measurements have been performed ex-situ it might suggest the ex-post formation of native oxide, rather than the presence of residual one. Another important feature of sul"de-treated surface is its long-term stability. No change of ellipsometric characteristics of electrochemically sulfurized GaSb surface has been observed up to 12 days of air exposure, while the oxide thickness on HCl-treated GaSb surface rapidly increased within "rst several hours.
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E. Papis et al. / Vacuum 57 (2000) 171}178
Fig. 1. Spectroscopic ellipsometric characteristics * of (100) GaSb surface after various surface reparation: (1) theoretical curve from J.A. Woollam library, (2) 1 M Na S chemical sulfuration, t"45 s followed by annealing at 6403C for 1 h under H #ow, (3) HCl#HNO (30#1) treatment, (4) 1 M Na S chemical sulfuration, t"45 s, (5) 21%(NH ) S chemical sulfuration, t"45 s, (6) 21%(NH ) S electrochemical sulfuration, t"30 min, j"16 lA/cm.
Table 1 Formation of a super"cial layer on (1 0 0) n-GaSb surface under sul"de treatment Sul"de treatment
Thickness of super"cial layer (nm) Bilayer model
Solution/method
Time (min)
Oxide
Cauchy layer
Bruggeman E!ective Medium Approximation
1 M Na S
0.25 0.75 1.0
1.17$0.095 1.46$0.86 1.58$0.10
0.66$0.093 1.67$0.10 1.97$0.11
2.21$0.027 2.98$0.043 3.72$0.049
21% (NH ) S
0.25 0.75 1.0
1.37$0.19 2.18$0.095 4.85$0.13
2.74$0.21 2.88$0.10 3.37$0.14
5.23$0.16 5.83$0.12 8.26$0.30
21% (NH ) S, j"16 lA/cm
1.0 15 30
4.17$0.082 8.37$0.32 6.81$0.31
2.89$0.08 12.05$0.36 16.70$0.35
7.30$0.054 21.58$0.25 25.57$0.43
0.26$0.21
1.34$0.21
*
0.84$0.021
*
*
HCl # HNO ; t"0.33 min 5% HCl t"1 min 1 M Na S# 6403C, 1 h, H
0.75
E. Papis et al. / Vacuum 57 (2000) 171}178
Fig. 2. Forward current}voltage characteristics of the Schottky barriers for various GaSb surface treatments.
175
Fig. 3. X-ray di!raction spectra of In Ga As Sb grown on: (1) HCl#HNO (30#1) treated GaSb surface, (2) 1 M Na S (t"45 s) treated GaSb sur face.
The e!ect of the sul"de treatment on the properties of Au/n-GaSb Schottky diodes is illustrated in Fig. 2. The ideality factor was found to be n"1.2}1.3 for Schottky diodes formed on non-sul"dized surface, slightly lower, n"1.18 after 1 M Na S treatment (10 min), and consider ably reduced, n"1.06 for (NH ) S electrochemical treatment (30 min) and n"1.03 for (NH ) S chemical treatment (30 min). The height of the Schottky barrier increased from 0.45 eV for non-sul"dized surfaces to 0.49 eV for (NH ) S chemical treatment, and to 0.52 eV for 1 M Na S chemical and (NH ) S electrochemical treatments. At the same time, the reverse current of Schottky diodes formed on sulfurized surfaces decreased by a factor of 10}20 compared to non-treated ones. This is in good agreement with the data published by Dutta et al. [9] and Perotin et al. [10], who observed improved behaviour of Au/n-GaSb Schottky barriers formed on sul"de treatment GaSb surface. The in#uence of sul"de treatment on the structural quality of LPE-grown GaSb/InGaAsSb heterostructures is illustrated in Figs. 3 and 4. X-ray di!raction rocking curve (Fig. 3) of InGaAsSb epilayer grown on sul"de-treated and H -annealed GaSb substrate exhibits oscillations testifying improved abruptness of heterojunction. XTEM micrographs (Fig. 4) prove better quality of GaSb/InGaAsSb interface when surface pretreatment in (NH ) S followed by in-situ annealing in #owing H at 6403C were applied before LPE growth of InGaAsSb. Table 2 shows the results of measurements of lattice mismatch at GaSb/InGaAsSb interface and of the carrier concentration in non-intentionally doped n-type InGaAsSb epilayers. It should be noticed that low doping level in n-InGaAsSb is of crucial importance from the point of view of their applications in PDs structures. The obtained results show that good matching between GaSb substrate and InGaAsSb epilayers has been obtained. Moreover, they indicate that sul"de preepitaxial treatment, especially in 1 M Na S solution enables not only to decrease the lattice
176
E. Papis et al. / Vacuum 57 (2000) 171}178
Fig. 4. Cross-sectional TEM micrographs of In Ga As Sb grown on: (a) HCl#HNO (30#1) treated GaSb surface, (b) 1 M Na S (t"45 s) treated GaSb surface. Table 2 Process parameters and properties of InGaAsSb epilayers LPE-grown on (100)GaSb LPE parameters
Sulfuration Solution/process
Solid composition: In Ga As Sb Growth temperature ¹ "5933C Liquid atomic fractions x "0.0973 % x "0.400 ' x "0.0027 x "0.500 1
1 M Na S 21% (NH ) S
1 M Na S 21% (NH ) S 21% (NH ) S j "16 lA/cm
Carrier concentration (cm\)
!3.7 !5.5 !6.5
2.6;10 1.6;10 1.3;10
!4.0 !5.0 !4.2 !4.4
4.7;10 2.4;10 2.8;10 5.1;10
#3.0 #3.5 #4.5 #1.3 #1.2
2.0;10 2.1;10 2.9;10 4.7;10 4.6;10
#1.3 #2.2
1.1;10 1.4;10
Time (min)
Solid composition: In Ga As Sb Growth temperature T "5283C Liquid atomic fractions: x "0.149 % x "0.595 C x "0.0017 x "0.2546 1
Lattice mismatch *a/a;10\
0.25 0.5 1 1
0.75 10.0 15 15
mismatch and to improve the reproducibility of the process but also to considerably reduce the carrier concentration in InGaAsSb layer. Better matching between the substrate and epilayer probably results from better quality of GaSb surface after sulfurization/annealing preepitaxial treatment. The presence of sulphur in the epitaxial solution, which has been shown to greatly
E. Papis et al. / Vacuum 57 (2000) 171}178
177
Fig. 5. Spectral characteristic of In Ga As Sb /Al Ga As Sb LED measured at room temperature.
in#uence the LPE growth of GaSb [11,12] might be bene"cial for the reduction of native acceptor centers in InGaAsSb. 1 M Na S preepitaxial treatment was applied in LPE growth of n In Ga As Sb /p-Al Ga As Sb LED heterostructures for j " N 2.06 lm and n-In Ga As Sb /p-Al Ga As Sb PD heterostructures for wavelength j"2.0}2.4 lm. As a result, LEDs, with increased by a factor of 4 quantum e$ciency and total power of 6 mW were obtained. Fig. 5 shows an example of spectral characteristic of the obtained GaSb/InGaAsSb/AlGaAsSb LED. Mesa-structure photodiodes of working area 3.2;10\ cm were characterised by detectivity DH "4;10 cmHz W\ at room temH l perature and dark current density j "16 mA/cm at a reverse bias of !0.5 V. 4. Conclusions The e!ect of sul"de treatment in Na S and (NH ) S on the surface properties of GaSb have been analysed and the applicability of sul"de pretreatment prior to LPE growth of InGaAsSb/AlGaAsSb photonic device structures have been investigated. It has been shown sul"de treatment GaSb results in etching of GaSb surface and formation of super"cial layer improving both, the quality of Schottky barriers and structural quality of semiconductor heterostructure. As a result, n-InGaAsSb/p-AlGaAsSb LEDs for j "2.06 lm with increased by a factor of N 4 quantum e$ciency and total power of 6 mW were obtained. Mesa-structure n-InGaAsSb/pAlGaAsSb photodiodes for wavelength j"2.0}2.4 lm were characterised by detectivity DH "4;10 cmHz W\ at RT and dark current density j "16 mA/cm at reverse bias H l of !0.5 V.
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Acknowledgements This research was partially supported by the State Committee for Scienti"c Research Grant No. 9 T10C 020 14.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Carpenter MS, Melloch MR, Cowans BA, Dardas Z, Delgass WN. J Vac Sci Technol B 1989;7:845. Melloch MR, Carpenter MS, Dungan TE, Li D, Otsuka N. Appl Phys Lett 1990;56:1064. Sik H, Driad JR, Legay P, Juhel M, Harmand JC, Launay P, Alexandre F. J Vac Sci Technol B 1996;14:147. Berkovits VL, Lantratov VM, Lvova TV, Shakiashvili GA, Ulin VP, Paget D. Appl Phys Lett 1993;63:970. Lvova TV, Andreev IA, Kunitsyna EV, Mikhailova MP, Yakovlev YuP, Ulin VP. J Electrochem Soc 1997;97:351. Lvova TV, Andreev IA, Kunitsyna EV, Mikhailova MP, Ulin VP, Yakovlev YuP. IEEE Proc Optoelectron 1998;145:303. Piskorski M, Adamczewska J, Golaszewska K, Piotrowski TT, Swiatek K. Electron Technology 1996;29:351. Lin CL, Su YK, Se TS, Li WL. Jpn J Appl Phys 1998;37:L1543. Dutta PS, Sangunni KS, Bhat HL, Kumar V. Appl Phys Lett 1994;65:1695. Perotin M, Coudray P, Gouskov L, Luquet H, Llinares C, Bonnet JJ, Soonckindt L, Labert B. J Electron Mat 1994;23:7. Novak J, Kucera M, Lauer S, Benz KW. J Crystal Growth 1996;158:1. Zytkiewicz ZR, Dobaczewski L, Gomez D, Briones F. Acta Phys Polon A 1997;92:1083.
Sul"de treatment of GaSb surface: in#uence on the LPE growth of InGaAsSb/AlGaAsSb heterostructures E. Papis *, A. Piotrowska , E. KaminH ska , K. Go"aszewska , W. Jung , J. Ka9 tcki , A. Kud"a , M. Piskorski , T.T. Piotrowski , J. Adamczewska Institute of Electron Technology, Al. Lotniko& w 46, 02-668 Warsaw, Poland Institute of Physics PAS, Al. Lotniko& w 46, 02-668 Warsaw, Poland
Abstract The in#uence of Na S and (NH ) S treatments on the surface properties of GaSb have been investigated through the etch rate, ellipsometry and Schottky barrier measurements. XRD and TEM analysis were performed to examine the e!ect of sul"de pretreatment of GaSb substrate on the LPE growth of InGaAsSb/AlGaAsSb heterostructures and their structural quality. Additionally, C}V characteristic of Hg-Schottky barriers were measured to evaluate carrier concentrations in non-intentionally doped InGaAsSb epilayers. Na S preepitaxial treatment has been applied to LPE growth of n In Ga As Sb /p-Al Ga As Sb LED heterostructures for j "2.06 lm and n N In Ga As Sb /p-Al Ga As Sb PD heterostructures for wavelength j"2.0}2.4 lm. As a result, LEDs which with increased by a factor of 4 quantum e$ciency and total power of 6 mW were obtained. Mesa-structure photodiodes were characterised by detectivity DH "4;10 cmHz W\ H l at room temperature and dark current density j "16 mA/cm at a reverse bias of !0.5 V. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Heterostructures based on Sb-containing III}V semiconducting compounds are particularly attractive for the fabrication of a wide variety of electronic and optoelectronic devices such as resonant tunneling diodes, light emitting diodes, laser diodes and photodetectors operating in the mid-infrared range of wavelength. During the last few years, much e!ort has been focused on
* Corresponding author. E-mail address: [email protected] (E. Papis). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 1 6 - 0
172
E. Papis et al. / Vacuum 57 (2000) 171}178
increasing the device performance including optimisation of epitaxial structure design and growth. In spite of that, the procedure of chemical preparation of GaSb surface prior to epitaxy is still far from satisfactory. In standard preparations, GaSb etching is characterised by high oxidation rate associated with a low solubility of its oxides. In this context, the sul"de treatment with its proved ability to deoxidize III}V semiconductor surface and remove carbon contaminants appears as a promising candidate as a technique of preparation of epiready grade surface. (NH ) S chemical treatment was successfully applied to molecular beam epitaxy (MBE) regrowth of GaAs [1,2] and to the selective regrowth of the GaAs base layer of heterojunction bipolar transistor [3]. Na S treatment was demonstrated to allow liquid-phase epitaxy (LPE) regrowth of AlGaAs [4] and to improve LPE growth of InGaAsSb/AlGaAsSb heterostructures [5,6]. Following this approach, in this work we have investigated the preparation of GaSb surface in Na S and (NH ) S solutions and its applicability as preepitaxial treatment prior to LPE growth of InGaAsSb/AlGaAsSb photonic device structures. Light emitting diodes (LEDs) and photodetectors (PDs) operating at wavelength 2.0}2.4 lm were of particular interest. 2. Experimental GaSb substrates, (100) oriented, tellurium doped to the concentration n"5;10 cm\ were used in the experiments. Prior to sul"de treatment, the samples received standard surface cleaning consisting of degreasing in hot organic solvents, and etching in a solution of HCl#HNO " 30#1, ¹"53C, t"20 s, followed by etching in 5% HCl for 60 s. Three procedures of sul"dation have been compared: (1) dipping in the 1M Na S aqueous solution, pH"13.1 at RT; (2) dipping in the 21% (NH ) S aqueous solution, pH"9 at RT; (3) anodic sul"dation in the 21% (NH ) S aqueous solution, pH"9 at current density 16 lA/cm. The e!ects of sul"dation of GaSb surface were characterised through the etch rate, ellipsometry and Schottky diode properties. Etch rate was determined on patterned GaSb samples using TENCOR ALPHA-STEP pro"ler. Ellipsometric analysis were performed in the spectral range 240}1100 nm at two angles, 653 and 753, using J.A. Woollam variable angle spectroscopic ellipsometer (VASE). For Schottky barrier measurements samples were patterned with Au dots having a diameter of 200 lm and one large area backside AgTe ohmic contact. Their characteristics were measured using a QuadTech 7600 Precision LCR Meter and Keithley 237 High Voltage Source Measure Unit. The LPE growth experiments were performed in horizontal sliding boat system described in detail in Ref. [7]. 6N Ga, In, Sb, Al, as well as undoped GaSb, InSb, GaAs and InAs were used as source materials. Ge was employed as p-type dopant. LPE growth of InGaAsSb and InGaAsSb/AlGaAsSb heterostructures was carried either at ¹"5283C in case of In Ga As Sb single epilayers or at ¹"5933C in case of In Ga As Sb single epilayers, In Ga As Sb /Al Ga As Sb photodiode, and the In Ga As Sb /Al Ga As Sb LED heterostructures. Before beginning of the growth process GaSb substrates were annealed in #owing H during 1 h at temperature 5903C and 6403C, respectively. X-ray di!raction (high-resolution Philips Material Research) and Transmission Electron Microscopy (JEM 200 CX Jeol) analysis were performed to examine the e!ect of sul"de
E. Papis et al. / Vacuum 57 (2000) 171}178
173
pretreatment of GaSb substrate on the LPE growth of InGaAsSb/AlGaAsSb heterostructures and their structural quality. Additionally, C}V measurements of Hg-Schottky barrier were carried out to evaluate carrier concentrations in non-intentionally doped InGaAsSb epilayers. The atomic composition of epilayers was determined by electron microprobe analysis (EPXMA). The performances of device structures were characterised through the measurements of their electrooptical properties: I}V, spectral, and power characteristics for LEDs and I}V and detectivity for PDs. LED emission was detected with SPM-2 monochromator and PbS PC detector. The optical spectra of PDs were measured using SPM-2 monochromator with NaCl prism and a globar as a light source.
3. Results and discussion Etch depth of GaSb subjected to sul"de treatment, for time up to 30 min, is a linear function of the process duration, with etch rate dependent on su"de solution and method of sulfuration. Etching in 1M Na S solution is characterised by the lowest v"2.2 nm/min etch rate. Sul"de treatment in 21% (NH ) S etches GaSb surface with a rate of 12.2 nm/min (for chemical treatment) and 14 nm/min (for electrochemical treatment). The results of ellipsometric investigations were interpreted in terms of two models. First, the Bruggeman E!ective Medium Approximation for two constituents, GaSb-oxide and Cauchy material (in which refraction index and extinction coe$cient are represented by slowly varying function of wavelength and exponential absorption tail). Second, the bilayer model in which GaSb-oxide and Cauchy material formed separate layers, with Cauchy layer on top. Such models allowed the best "t of calculated and measured data of ( and * (estimated using mean-squared error factor de"ned as in the WVASE 32 program,) and contained a limited number of unknown parameters (seven in case of absorbing layers). It should be mentioned here that recent experimental XPS data of Lin et al. [8] suggesting the formation of two-layer structure, Ga O and Sb S on GaSb surface after sul"de treatment, would support the applica bility of bilayer model. Optical index of GaSb and its oxide were taken from WVASE program library. In Fig. 1 spectroscopic characteristics * indicating the thickness of super"cial layer formed on GaSb after various sul"de treatments are shown. For comparison, the data for standard surface preparation (etching in HCl#HNO "30#1, ¹"53C, t"20 s, followed by etching in 5% HCl for 60 s) are included. The results of ellipsometric investigations are summarized in Table 1. They indicate that the thickness of super"cial layer formed on GaSb surface under sul"de treatment increases with the duration of sul"dation process. They also show that its growth rate is much higher in 21% (NH ) S solution compared to 1 M Na S treatment. The most important "nding is that the thickness of super"cial oxide on sul"de-treated and H -annealed GaSb surface (in LPE reactor) is about 0.8 nm. Since the ellipsometric measurements have been performed ex-situ it might suggest the ex-post formation of native oxide, rather than the presence of residual one. Another important feature of sul"de-treated surface is its long-term stability. No change of ellipsometric characteristics of electrochemically sulfurized GaSb surface has been observed up to 12 days of air exposure, while the oxide thickness on HCl-treated GaSb surface rapidly increased within "rst several hours.
174
E. Papis et al. / Vacuum 57 (2000) 171}178
Fig. 1. Spectroscopic ellipsometric characteristics * of (100) GaSb surface after various surface reparation: (1) theoretical curve from J.A. Woollam library, (2) 1 M Na S chemical sulfuration, t"45 s followed by annealing at 6403C for 1 h under H #ow, (3) HCl#HNO (30#1) treatment, (4) 1 M Na S chemical sulfuration, t"45 s, (5) 21%(NH ) S chemical sulfuration, t"45 s, (6) 21%(NH ) S electrochemical sulfuration, t"30 min, j"16 lA/cm.
Table 1 Formation of a super"cial layer on (1 0 0) n-GaSb surface under sul"de treatment Sul"de treatment
Thickness of super"cial layer (nm) Bilayer model
Solution/method
Time (min)
Oxide
Cauchy layer
Bruggeman E!ective Medium Approximation
1 M Na S
0.25 0.75 1.0
1.17$0.095 1.46$0.86 1.58$0.10
0.66$0.093 1.67$0.10 1.97$0.11
2.21$0.027 2.98$0.043 3.72$0.049
21% (NH ) S
0.25 0.75 1.0
1.37$0.19 2.18$0.095 4.85$0.13
2.74$0.21 2.88$0.10 3.37$0.14
5.23$0.16 5.83$0.12 8.26$0.30
21% (NH ) S, j"16 lA/cm
1.0 15 30
4.17$0.082 8.37$0.32 6.81$0.31
2.89$0.08 12.05$0.36 16.70$0.35
7.30$0.054 21.58$0.25 25.57$0.43
0.26$0.21
1.34$0.21
*
0.84$0.021
*
*
HCl # HNO ; t"0.33 min 5% HCl t"1 min 1 M Na S# 6403C, 1 h, H
0.75
E. Papis et al. / Vacuum 57 (2000) 171}178
Fig. 2. Forward current}voltage characteristics of the Schottky barriers for various GaSb surface treatments.
175
Fig. 3. X-ray di!raction spectra of In Ga As Sb grown on: (1) HCl#HNO (30#1) treated GaSb surface, (2) 1 M Na S (t"45 s) treated GaSb sur face.
The e!ect of the sul"de treatment on the properties of Au/n-GaSb Schottky diodes is illustrated in Fig. 2. The ideality factor was found to be n"1.2}1.3 for Schottky diodes formed on non-sul"dized surface, slightly lower, n"1.18 after 1 M Na S treatment (10 min), and consider ably reduced, n"1.06 for (NH ) S electrochemical treatment (30 min) and n"1.03 for (NH ) S chemical treatment (30 min). The height of the Schottky barrier increased from 0.45 eV for non-sul"dized surfaces to 0.49 eV for (NH ) S chemical treatment, and to 0.52 eV for 1 M Na S chemical and (NH ) S electrochemical treatments. At the same time, the reverse current of Schottky diodes formed on sulfurized surfaces decreased by a factor of 10}20 compared to non-treated ones. This is in good agreement with the data published by Dutta et al. [9] and Perotin et al. [10], who observed improved behaviour of Au/n-GaSb Schottky barriers formed on sul"de treatment GaSb surface. The in#uence of sul"de treatment on the structural quality of LPE-grown GaSb/InGaAsSb heterostructures is illustrated in Figs. 3 and 4. X-ray di!raction rocking curve (Fig. 3) of InGaAsSb epilayer grown on sul"de-treated and H -annealed GaSb substrate exhibits oscillations testifying improved abruptness of heterojunction. XTEM micrographs (Fig. 4) prove better quality of GaSb/InGaAsSb interface when surface pretreatment in (NH ) S followed by in-situ annealing in #owing H at 6403C were applied before LPE growth of InGaAsSb. Table 2 shows the results of measurements of lattice mismatch at GaSb/InGaAsSb interface and of the carrier concentration in non-intentionally doped n-type InGaAsSb epilayers. It should be noticed that low doping level in n-InGaAsSb is of crucial importance from the point of view of their applications in PDs structures. The obtained results show that good matching between GaSb substrate and InGaAsSb epilayers has been obtained. Moreover, they indicate that sul"de preepitaxial treatment, especially in 1 M Na S solution enables not only to decrease the lattice
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Fig. 4. Cross-sectional TEM micrographs of In Ga As Sb grown on: (a) HCl#HNO (30#1) treated GaSb surface, (b) 1 M Na S (t"45 s) treated GaSb surface. Table 2 Process parameters and properties of InGaAsSb epilayers LPE-grown on (100)GaSb LPE parameters
Sulfuration Solution/process
Solid composition: In Ga As Sb Growth temperature ¹ "5933C Liquid atomic fractions x "0.0973 % x "0.400 ' x "0.0027 x "0.500 1
1 M Na S 21% (NH ) S
1 M Na S 21% (NH ) S 21% (NH ) S j "16 lA/cm
Carrier concentration (cm\)
!3.7 !5.5 !6.5
2.6;10 1.6;10 1.3;10
!4.0 !5.0 !4.2 !4.4
4.7;10 2.4;10 2.8;10 5.1;10
#3.0 #3.5 #4.5 #1.3 #1.2
2.0;10 2.1;10 2.9;10 4.7;10 4.6;10
#1.3 #2.2
1.1;10 1.4;10
Time (min)
Solid composition: In Ga As Sb Growth temperature T "5283C Liquid atomic fractions: x "0.149 % x "0.595 C x "0.0017 x "0.2546 1
Lattice mismatch *a/a;10\
0.25 0.5 1 1
0.75 10.0 15 15
mismatch and to improve the reproducibility of the process but also to considerably reduce the carrier concentration in InGaAsSb layer. Better matching between the substrate and epilayer probably results from better quality of GaSb surface after sulfurization/annealing preepitaxial treatment. The presence of sulphur in the epitaxial solution, which has been shown to greatly
E. Papis et al. / Vacuum 57 (2000) 171}178
177
Fig. 5. Spectral characteristic of In Ga As Sb /Al Ga As Sb LED measured at room temperature.
in#uence the LPE growth of GaSb [11,12] might be bene"cial for the reduction of native acceptor centers in InGaAsSb. 1 M Na S preepitaxial treatment was applied in LPE growth of n In Ga As Sb /p-Al Ga As Sb LED heterostructures for j " N 2.06 lm and n-In Ga As Sb /p-Al Ga As Sb PD heterostructures for wavelength j"2.0}2.4 lm. As a result, LEDs, with increased by a factor of 4 quantum e$ciency and total power of 6 mW were obtained. Fig. 5 shows an example of spectral characteristic of the obtained GaSb/InGaAsSb/AlGaAsSb LED. Mesa-structure photodiodes of working area 3.2;10\ cm were characterised by detectivity DH "4;10 cmHz W\ at room temH l perature and dark current density j "16 mA/cm at a reverse bias of !0.5 V. 4. Conclusions The e!ect of sul"de treatment in Na S and (NH ) S on the surface properties of GaSb have been analysed and the applicability of sul"de pretreatment prior to LPE growth of InGaAsSb/AlGaAsSb photonic device structures have been investigated. It has been shown sul"de treatment GaSb results in etching of GaSb surface and formation of super"cial layer improving both, the quality of Schottky barriers and structural quality of semiconductor heterostructure. As a result, n-InGaAsSb/p-AlGaAsSb LEDs for j "2.06 lm with increased by a factor of N 4 quantum e$ciency and total power of 6 mW were obtained. Mesa-structure n-InGaAsSb/pAlGaAsSb photodiodes for wavelength j"2.0}2.4 lm were characterised by detectivity DH "4;10 cmHz W\ at RT and dark current density j "16 mA/cm at reverse bias H l of !0.5 V.
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Acknowledgements This research was partially supported by the State Committee for Scienti"c Research Grant No. 9 T10C 020 14.
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