- Email: [email protected]
GaAs layers
Nuclear Instruments and Methods in Physics Research A 434 (1999) 164}168
Preparation and properties of thick not intentionally doped GaInP(As)/GaAs layers D. Nohavica*, P. Gladkov, K. Z[ d'aH nskyH Institute of Radio Engineering and Electronics, Academy of Sciences of the Czech Republic, Chaberska& 57, 182 51 Praha 8, Czech Republic
Abstract We report on liquid-phase epitaxial growth of thick layers of GaInP(As), lattice matched to GaAs. Layers with thicknesses up to 10 lm were prepared in a multi-melt bin, step-cooling, one-phase con"guration. Unintentionally doped layers, grown from moderate purity starting materials, show a signi"cant decrease in the residual impurity level when erbium is added to the melt. Fundamental electrical and optical properties of the layers were investigated. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The compound semiconductors generally of interest for room-temperature radiation detectors have band gap energies between 1.35 and 2.55 eV [1]. The GaInAsP lattice matched to GaAs is a promising alternative to AlGaAs/GaAs for electronic and optoelectronic device applications. It could also be considered as an appropriate material for radiation detectors since the photoelectric absorption coe$cient of the GaInAsP/GaAs, which is commonly proportional to Z\ [2], is positively in#uenced by the presence of In (Z"49) which occupies one half of the `cationa sublattice. Film thicknesses have to be as high as possible since the e$ciency of high-quality epitaxial detectors is limited by the material volume available. The leakage current of a semiconductor radiation detector is primarily determined by the band gap energy of the material, the concentration of the majority carriers and the device structure. Liquid-phase epitaxy * Corresponding author. Fax: (0042-2)-664-102-22. E-mail address: [email protected] (D. Nohavica)
(LPE) o!ers the possibility to grow thick, practically completely disordered layers and hence to obtain the widest band gap energy of the GaInP(As)/GaAs, approaching &2 eV, which is essential for the operation at room temperature or elevated temperature. The energy gap (at 300 K) of the quaternary system GaInP(As)/GaAs increases in the range (1.57}1.89) eV as the arsenic concentration decreases [3]. However, a material that has a wide band gap may still have a relatively high free carrier concentration. Low carrier concentration in the III}V semiconductor materials can be achieved by appropriate introduction of deep energy levels into the material to compensate the shallow impurity levels, thereby producing high resistivity material. Unfortunately, the introduction of deep and shallow energy levels into a semiconductor can have adverse e!ects on the charge carrier mean free drift times and internal electric "eld [1]. Another known way to reduce the background impurity concentration in the III}V materials and their solid solutions is by adding to the melt rare earth elements, which have been found to act as an e!ective getter for the background impurities [4].
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 4 5 6 - 8
D. Nohavica et al. / Nuclear Instruments and Methods in Physics Research A 434 (1999) 164}168
The aim of the present work was to verify the possibility for growth of thick GaInP(As) layers on GaAs substrates, both unintentionally doped and doped with Er, to study their electrical and optical properties and hence the applicability of these layers for particle detectors.
2. Experimental 2.1. Crystal growth The epitaxial layers were grown in a graphite boat [5] placed into a horizontal furnace. As source materials we used `"ve ninesa purity In, InP, GaP and InAs and as substrates (1 0 0) GaAs wafers doped with Si, Zn or Cr. The optimized composition melts were prepared in advance in a special dosing boat. Before the growth, the boat was loaded with the source melts and the substrate, the reactor tube was evacuated and #ushed with H . It was then heated and kept at 8003C for 1 h for the source melts to become homogeneous. Evaporation of P and As during this homogenization process was suppressed to a negligible level by means of graphite caps covering the melt bins. After the melt was moved to the growth position, the temperature was decreased to 790}7953C and the supercooled melt was brought into contact with the substrate and kept on it during the growth. In terms of the LPE growth of multicomponent solid solutions, the melt during homogenization was `single phasea and the growth process was realized in the `step coolinga variant. The experimentally optimized melt composition is between calculated equilibrium and `coherenta phase diagram data [6]. We used melts containing no more than 0.05 mol% of InAs. The thickness of the layers grown from one melt bin was 2}3 lm. To increase the layer thickness, multiple growth was performed from more bins containing melts of the same composition. In the `step coolinga growth variant the number of bins is practically limited by furnace properties and cost-e!ectiveness considerations. In our experiments we apply growth from four melt bins. To decrease the residual donor impurity concentration, we attempted to use the `getteringa e!ect of
165
a rare earth (Er) added to the melt in the range 0.003 to 0.05 at%. 2.2. Electrical measurements The conduction type and shallow donor concentrations were determined from capacitance}voltage characteristics measured at room temperature with a mercury probe. Layers grown on semi-insulating substrates were characterized by Hall e!ect measurements using the Van der Pauw technique on a computer-controlled system with high impedance inputs. The temperature dependence of the Hall e!ect was measured in the range from room temperature down to 10 K in a helium closed cycle cryostat designed for insertion in the gap between the magnet poles. In this arrangement a magnetic "eld of 0.8 T was used for measurements of the carrier Hall concentrations and mobilities. 2.3. Optical measurements The photoluminescence (PL) measurements were made using a set-up consisting of a variable temperature optical cryostat, (based on the cryorefrigerator KelCool-type UCH 130), which allows measurements in the range (3.4}300) K, a 1 m single grating monochromator (Jobin Yvon THR 1000), and a photomultiplier, with an S1 liquid nitrogencooled photo-cathode, connected to a lock-in registration system. The excitation of the layers was made using the 514.5 and 496.5 nm lines of an Ar> ion laser, while the InGaP(As)/GaAs heterointerface was excited by the 632.8 nm line of a He}Ne laser.
3. Results The surface morphology of the layers grown from the optimized composition melts was mirror like. Lattice mismatching, as determined by double crystal X-ray di!ractometry, exhibits a value of *a/a in the range (6}11);10\. The growth rate corresponds to a growth rate constant K&0.17 lm K\ min\ . When four melt bins were used, the total layer thickness approached 7}10 lm.
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Table 1 Concentrations of shallow donors N determined from C}< " characteristics, concentrations of free electrons n determined from the Hall e!ect measurements at the room temperature, and the compensation ratio N /N (with N * concentration of " acceptors) determined from the N and n " InGaP No.
N "
n
N /N "
1038 1040 1042 1044 1046 1048 1050 1051
3.7E#16 5.5E#16 1.8E#16 6.9E#16 4.2E#16 4.9E#16 2.1E#16 1.5E#15
6.2E#15 9.8E#15 1.1E#16 2.0E#16 1.2E#16 7.8E#15 3.4E#15 3.1E#14
0.83 0.82 0.39 0.71 0.71 0.84 0.84 0.79
Not intentionally doped layers obtained under the above growth conditions show n-type conductivity with a carrier concentration of the order of 4;10 cm\ as determined by the Hall and (C}<) measurements. The concentrations of shallow donors and free electrons measured at room temperature in the layers grown on semi-insulating substrates are given in Table 1. The last column of Table 1 gives the compensation ratio, i.e. the ratio of the concentrations of acceptors to shallow donors, determined from the measured values. Over the temperature range studied (300}25 K), the electron concentration decreases with decreasing temperature by about one order of magnitude for the not intentionally doped samples, as illustrated in Fig. 1 by the data from sample 1038. Adding 0.005 at% of Er results in a reduction (by a factor of +1.5) in the carrier concentration, indicating that higher concentrations of Er would be needed for e!ective reduction of the background carrier concentration. The layer No. 1051, (grown with 0.03 at% of Er), demonstrates a much more e!ective reduction of the donor concentration, to 1.5;10 cm\ (see Table 1). The electron concentration in this layer decreases by several orders of magnitude as the temperature decreases to 10 K, as can be seen in Fig. 1. Fig. 2 shows a PL spectrum of the layer 1050 recorded at 4 K and an excitation density of 0.04 W/cm. This is a typical spectrum for all not intentionally doped layers. The PL spectrum is
Fig. 1. Temperature dependence of the electron concentration n of the two InGaP(As) samples: 1038 and 1051. The solid line is a "t to the data points of sample 1051, yielding 44 meV activation energy for the background donor involved.
Fig. 2. PL spectrum of sample 1042, recorded at 0.04 W/cm excitation with the 514.5 nm line and 3.8 K temperature. The doted line is a "t of the BE line shape following the theory given in Refs. [8,9].
dominated by a PL line with a maximum at 1.984 eV and FWHM of 7 meV, labelled in the "gure as L1. A second band with a maximum at 1.941 eV (FWHM"22 meV), labelled as L2, is characteristic of all PL spectra recorded and is accompanied by a low intensity feature peaked at 46$1 meV below it, labelled as L3. For the purpose of identi"cation of the PL lines we studied the excitation density and temperature dependence of the PL spectra. Fig. 3 shows the PL spectra of the InGaP(As)/ GaAs heterointerface excited from the epitaxial layer top surface with the 632.8 nm line of
D. Nohavica et al. / Nuclear Instruments and Methods in Physics Research A 434 (1999) 164}168
Fig. 3. PL spectrum of the sample 1042 heterointerface excited with the 632.8 nm line at 4 K.
a He}Ne laser. Clearly, at this wavelength the absorption takes place in the GaAs substrate and the PL is generated in the substrate, but it could also originate partially from the InGaP layer in close proximity to the substrate due to non-equilibrium carriers supplied by di!usion from the substrate. Besides the known PL lines de"nitely originating from the substrate, a weak PL intensity was observed, with a maximum at about 1.41 eV, which proved to be not from the substrate but originating from the near interface region of the InGaP layer.
4. Discussion A "t of the experimental temperature dependence of the electron concentration for sample 1051 (the solid line in Fig. 1) yields an activation energy of +44 meV. A "t of the temperature dependence of the free carrier concentration for the not intentionally doped samples, (for instance sample 1050), taking into account certain compensation and non-degenerate statistics yields the smaller activation energy +4 meV which we attribute to the gap between the conduction band and the unavoidable impurity band expected at concentrations of the order of 4;10 cm\. The PL excitation density dependence measured at a constant temperature of 3.6 K over four orders of magnitude variation of the excitation density reveals a constant positions for the maxima of the L1 and L2 lines. The intensity of the L1 peak rises
167
super-linearly with the excitation while the intensity of the L2 peak shows a nearly linear dependence. All this suggests an interpretation for the L1 line as due to recombination of bound excitons (BE) and for the L2 line as due to bound-to-band recombination i.e. (D, h). The constant position of the L3 peak relative to the L2 peak and the apparent similarity between the line shapes implies an identi"cation for the L3 line as a phonon replica of the L2 line. The separation between L2 and L3 is 46$1 meV in good agreement with known values of 45.2 meV for the InP like LO phonon and 47.6 meV for the GaP-like LO phonon in InGaP [7]. The L3 line is probably a mixture of the two phonon replicas. The L1 line shape and FWHM can be reasonably well "tted (see the dashed line in Fig. 2) using the theoretical expression for the BE line width developed for the case of Al Ga As in Ref. [8] V \V and later generalized for other III}V ternary compounds, e.g. for the case of InGaAs [9]. Applying this approach we obtained a theoretical FWHM of 8 meV for the BE line in InGaP using the available values for the reduced excitonic e!ective mass in InGaP, m "0.083m [10] and dE /dx"1.35 eV (derived for x"0.5 from Ref. [11]). This agrees well with the experimental value of 7 meV. Based on the measured temperature shift of the L1 peak and a "t of the experimental points for the L1 line maximum for temperatures above 60 K with the Varshni dependence for E [12], we evaluated the dissociation energy of an exciton bound to a donor in InGaP as +5 meV. Compared to the corresponding value in GaAs this is a surprisingly small value but it agrees well with the results for Al Ga As obtained in Ref. [13]. It can be underV \V stood as a result of micro-scale compositional #uctuations, respectively, lowering the binding energies via internal electric "elds and the related Poole} Frenkel e!ect associated with these random alloy #uctuations. Both L1 and L2 peaks exhibit thermal quenching. The thermal quenching of L1 is slowed down at about 20 K accompanied by a shift of the L1 peak to higher energies. Above this temperature the L1 peak obviously represents band-to-band transitions. A "t of the thermal quenching of the line L2 gives an apparent activation energy for the donor involved of +35 meV.
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D. Nohavica et al. / Nuclear Instruments and Methods in Physics Research A 434 (1999) 164}168
Usually the activation energy derived in this way can vary from a half to almost the full ionization energy, determined independently, where the latter holds only for extremely low impurity concentrations [14]. Consequently, the optically derived activation energy for the donor correlates with the activation energy determined from Hall measurements for sample 1051. We suppose some group VI elements in the `anionica and Si in the `cationica sublattices as dominant background donors in our layers. Taking account of the results and the analysis of the Hall e!ect measurements for the not intentionally doped samples, we conclude that we should consider the presence of a donor impurity band at these concentrations rather than isolated donors and the transition responsible for the L2 line is most likely (Donor impurity band}Valence band). A strong PL line with a maximum at 1.41 eV, (comparable with the intensity of the near-band luminescence from the substrate), was reported in gas-source MBE grown heterostructures of InGaP/GaAs or vice versa [15]. The nature of this line is identi"ed as due to trapping of carriers on traps in the InGaP/GaAs interface region arising from intermixing of As and P atoms in the initial stage of the growth process. The existence of electron traps in the interface region of InGaP/GaAs heterostructures was also shown by means of DLTS measurements on thin layers ((5000 As ) [16]. The PL spectra of our layers show the intensity at 1.41 eV suppressed by 100 times relative to the near-band edge lines of the substrate, which we consider as a proof of the interface quality of our layers.
5. Conclusions Thick InGaP(As) layer growth was demonstrated by the multiple bins, step-cooling variant of LPE. As expected, doping with Er leads to a signi"cant reduction of the carrier concentration. The samples are nearly completely disordered as the PL measurements demonstrate and show a low concentration of the interface traps. A further reduction of the background carrier concentration is expected when electronic grade starting materials are used in combination with optimized Er-doping.
Additional investigations of the e!ect of the duration of individual layer segment growth on layer composition homogeneity will be performed to extend the growth duration of the individual growth segments. The application for particle detectors of GaInAsP/GaAs with higher As content than used in this work we expect to be more promising because of higher expected growth rates and the higher average atomic number of the system. Acknowledgements The authors wish to thank Dr. J. Novak (Electrotechnical Inst. Slovak Acad. of Sci.) for helpful comments and for measuring the lattice mismatch of the samples reported in this work. This work was partially supported by Grant Agency of the Czech Acad. of Sci. under contract GAAV no. A 1010807/1998.
References [1] D.S. McGregor, H. Hermon, Nucl. Instr. and Meth. A 395 (1997) 101. [2] G.F. Knoll, Radiation Detection and Measurement, 2nd Edition, Wiley, New York, 1989. [3] S. Mukai, M. Matsuzaki, J. Shimada, Jpn. J. Appl. Phys. 19 (1980) L505. [4] W. Gao, P.R. Berger, J. Appl. Phys. 80 (1996) 7094. [5] D. Nohavica, J. TemmH novaH , Crystal Prop. Prep. 32}34 (1991) 630. [6] V.V. Kuznetsov, P.P. Moskvin, V.S. Sorokin, J. Crystal Growth 88 (1988) 241. [7] M.C. DeLong et al., J. Appl. Phys. 73 (1993) 5163. [8] E.F. Schubert, E.O. GoK bel, Y. Horikoshi, K. Ploog, H.J. Queisser, Phys. Rev. B 30 (1984) 813. [9] E.F. Schubert, W.T. Tsang, Phys. Rev. B 34 (1986) 2991. [10] A. Gomio, H. Hitoshi, I. Hino, S. Kawata, K. Kobayashi, T. Suzuki, Jpn. J. Appl. Phys. 28 (1989) L1330. [11] D. Auvergne, P. Merle, H. Mathieu, Solid State Commun. 21 (1977) 473. [12] Y.P. Varshni, Physica 34 (1967) 149. [13] E.F. Schubert, K. Ploog, J. Phys. C 18 (1985) 4549. [14] P.J. Dean, in: J.O. McCaldin, G. Somorjai (Eds.), Progress in Solid State Chemistry, Vol. 8, Pergamon press, New York, 1973, p. 64. [15] T. Kitatani, Y.Yazava, S. Watahiki, K. Tamura, J. Minenura, T. Warabisako, Sol. Energy Mater. Sol. Cells 50 (1998) 221. [16] E.C. Paloura, A. Ginoudi, G. Konstandinidis, G. Kiriakidis, Sensors Actuators 33 (1992) 63.
Preparation and properties of thick not intentionally doped GaInP(As)/GaAs layers D. Nohavica*, P. Gladkov, K. Z[ d'aH nskyH Institute of Radio Engineering and Electronics, Academy of Sciences of the Czech Republic, Chaberska& 57, 182 51 Praha 8, Czech Republic
Abstract We report on liquid-phase epitaxial growth of thick layers of GaInP(As), lattice matched to GaAs. Layers with thicknesses up to 10 lm were prepared in a multi-melt bin, step-cooling, one-phase con"guration. Unintentionally doped layers, grown from moderate purity starting materials, show a signi"cant decrease in the residual impurity level when erbium is added to the melt. Fundamental electrical and optical properties of the layers were investigated. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The compound semiconductors generally of interest for room-temperature radiation detectors have band gap energies between 1.35 and 2.55 eV [1]. The GaInAsP lattice matched to GaAs is a promising alternative to AlGaAs/GaAs for electronic and optoelectronic device applications. It could also be considered as an appropriate material for radiation detectors since the photoelectric absorption coe$cient of the GaInAsP/GaAs, which is commonly proportional to Z\ [2], is positively in#uenced by the presence of In (Z"49) which occupies one half of the `cationa sublattice. Film thicknesses have to be as high as possible since the e$ciency of high-quality epitaxial detectors is limited by the material volume available. The leakage current of a semiconductor radiation detector is primarily determined by the band gap energy of the material, the concentration of the majority carriers and the device structure. Liquid-phase epitaxy * Corresponding author. Fax: (0042-2)-664-102-22. E-mail address: [email protected] (D. Nohavica)
(LPE) o!ers the possibility to grow thick, practically completely disordered layers and hence to obtain the widest band gap energy of the GaInP(As)/GaAs, approaching &2 eV, which is essential for the operation at room temperature or elevated temperature. The energy gap (at 300 K) of the quaternary system GaInP(As)/GaAs increases in the range (1.57}1.89) eV as the arsenic concentration decreases [3]. However, a material that has a wide band gap may still have a relatively high free carrier concentration. Low carrier concentration in the III}V semiconductor materials can be achieved by appropriate introduction of deep energy levels into the material to compensate the shallow impurity levels, thereby producing high resistivity material. Unfortunately, the introduction of deep and shallow energy levels into a semiconductor can have adverse e!ects on the charge carrier mean free drift times and internal electric "eld [1]. Another known way to reduce the background impurity concentration in the III}V materials and their solid solutions is by adding to the melt rare earth elements, which have been found to act as an e!ective getter for the background impurities [4].
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 4 5 6 - 8
D. Nohavica et al. / Nuclear Instruments and Methods in Physics Research A 434 (1999) 164}168
The aim of the present work was to verify the possibility for growth of thick GaInP(As) layers on GaAs substrates, both unintentionally doped and doped with Er, to study their electrical and optical properties and hence the applicability of these layers for particle detectors.
2. Experimental 2.1. Crystal growth The epitaxial layers were grown in a graphite boat [5] placed into a horizontal furnace. As source materials we used `"ve ninesa purity In, InP, GaP and InAs and as substrates (1 0 0) GaAs wafers doped with Si, Zn or Cr. The optimized composition melts were prepared in advance in a special dosing boat. Before the growth, the boat was loaded with the source melts and the substrate, the reactor tube was evacuated and #ushed with H . It was then heated and kept at 8003C for 1 h for the source melts to become homogeneous. Evaporation of P and As during this homogenization process was suppressed to a negligible level by means of graphite caps covering the melt bins. After the melt was moved to the growth position, the temperature was decreased to 790}7953C and the supercooled melt was brought into contact with the substrate and kept on it during the growth. In terms of the LPE growth of multicomponent solid solutions, the melt during homogenization was `single phasea and the growth process was realized in the `step coolinga variant. The experimentally optimized melt composition is between calculated equilibrium and `coherenta phase diagram data [6]. We used melts containing no more than 0.05 mol% of InAs. The thickness of the layers grown from one melt bin was 2}3 lm. To increase the layer thickness, multiple growth was performed from more bins containing melts of the same composition. In the `step coolinga growth variant the number of bins is practically limited by furnace properties and cost-e!ectiveness considerations. In our experiments we apply growth from four melt bins. To decrease the residual donor impurity concentration, we attempted to use the `getteringa e!ect of
165
a rare earth (Er) added to the melt in the range 0.003 to 0.05 at%. 2.2. Electrical measurements The conduction type and shallow donor concentrations were determined from capacitance}voltage characteristics measured at room temperature with a mercury probe. Layers grown on semi-insulating substrates were characterized by Hall e!ect measurements using the Van der Pauw technique on a computer-controlled system with high impedance inputs. The temperature dependence of the Hall e!ect was measured in the range from room temperature down to 10 K in a helium closed cycle cryostat designed for insertion in the gap between the magnet poles. In this arrangement a magnetic "eld of 0.8 T was used for measurements of the carrier Hall concentrations and mobilities. 2.3. Optical measurements The photoluminescence (PL) measurements were made using a set-up consisting of a variable temperature optical cryostat, (based on the cryorefrigerator KelCool-type UCH 130), which allows measurements in the range (3.4}300) K, a 1 m single grating monochromator (Jobin Yvon THR 1000), and a photomultiplier, with an S1 liquid nitrogencooled photo-cathode, connected to a lock-in registration system. The excitation of the layers was made using the 514.5 and 496.5 nm lines of an Ar> ion laser, while the InGaP(As)/GaAs heterointerface was excited by the 632.8 nm line of a He}Ne laser.
3. Results The surface morphology of the layers grown from the optimized composition melts was mirror like. Lattice mismatching, as determined by double crystal X-ray di!ractometry, exhibits a value of *a/a in the range (6}11);10\. The growth rate corresponds to a growth rate constant K&0.17 lm K\ min\ . When four melt bins were used, the total layer thickness approached 7}10 lm.
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Table 1 Concentrations of shallow donors N determined from C}< " characteristics, concentrations of free electrons n determined from the Hall e!ect measurements at the room temperature, and the compensation ratio N /N (with N * concentration of " acceptors) determined from the N and n " InGaP No.
N "
n
N /N "
1038 1040 1042 1044 1046 1048 1050 1051
3.7E#16 5.5E#16 1.8E#16 6.9E#16 4.2E#16 4.9E#16 2.1E#16 1.5E#15
6.2E#15 9.8E#15 1.1E#16 2.0E#16 1.2E#16 7.8E#15 3.4E#15 3.1E#14
0.83 0.82 0.39 0.71 0.71 0.84 0.84 0.79
Not intentionally doped layers obtained under the above growth conditions show n-type conductivity with a carrier concentration of the order of 4;10 cm\ as determined by the Hall and (C}<) measurements. The concentrations of shallow donors and free electrons measured at room temperature in the layers grown on semi-insulating substrates are given in Table 1. The last column of Table 1 gives the compensation ratio, i.e. the ratio of the concentrations of acceptors to shallow donors, determined from the measured values. Over the temperature range studied (300}25 K), the electron concentration decreases with decreasing temperature by about one order of magnitude for the not intentionally doped samples, as illustrated in Fig. 1 by the data from sample 1038. Adding 0.005 at% of Er results in a reduction (by a factor of +1.5) in the carrier concentration, indicating that higher concentrations of Er would be needed for e!ective reduction of the background carrier concentration. The layer No. 1051, (grown with 0.03 at% of Er), demonstrates a much more e!ective reduction of the donor concentration, to 1.5;10 cm\ (see Table 1). The electron concentration in this layer decreases by several orders of magnitude as the temperature decreases to 10 K, as can be seen in Fig. 1. Fig. 2 shows a PL spectrum of the layer 1050 recorded at 4 K and an excitation density of 0.04 W/cm. This is a typical spectrum for all not intentionally doped layers. The PL spectrum is
Fig. 1. Temperature dependence of the electron concentration n of the two InGaP(As) samples: 1038 and 1051. The solid line is a "t to the data points of sample 1051, yielding 44 meV activation energy for the background donor involved.
Fig. 2. PL spectrum of sample 1042, recorded at 0.04 W/cm excitation with the 514.5 nm line and 3.8 K temperature. The doted line is a "t of the BE line shape following the theory given in Refs. [8,9].
dominated by a PL line with a maximum at 1.984 eV and FWHM of 7 meV, labelled in the "gure as L1. A second band with a maximum at 1.941 eV (FWHM"22 meV), labelled as L2, is characteristic of all PL spectra recorded and is accompanied by a low intensity feature peaked at 46$1 meV below it, labelled as L3. For the purpose of identi"cation of the PL lines we studied the excitation density and temperature dependence of the PL spectra. Fig. 3 shows the PL spectra of the InGaP(As)/ GaAs heterointerface excited from the epitaxial layer top surface with the 632.8 nm line of
D. Nohavica et al. / Nuclear Instruments and Methods in Physics Research A 434 (1999) 164}168
Fig. 3. PL spectrum of the sample 1042 heterointerface excited with the 632.8 nm line at 4 K.
a He}Ne laser. Clearly, at this wavelength the absorption takes place in the GaAs substrate and the PL is generated in the substrate, but it could also originate partially from the InGaP layer in close proximity to the substrate due to non-equilibrium carriers supplied by di!usion from the substrate. Besides the known PL lines de"nitely originating from the substrate, a weak PL intensity was observed, with a maximum at about 1.41 eV, which proved to be not from the substrate but originating from the near interface region of the InGaP layer.
4. Discussion A "t of the experimental temperature dependence of the electron concentration for sample 1051 (the solid line in Fig. 1) yields an activation energy of +44 meV. A "t of the temperature dependence of the free carrier concentration for the not intentionally doped samples, (for instance sample 1050), taking into account certain compensation and non-degenerate statistics yields the smaller activation energy +4 meV which we attribute to the gap between the conduction band and the unavoidable impurity band expected at concentrations of the order of 4;10 cm\. The PL excitation density dependence measured at a constant temperature of 3.6 K over four orders of magnitude variation of the excitation density reveals a constant positions for the maxima of the L1 and L2 lines. The intensity of the L1 peak rises
167
super-linearly with the excitation while the intensity of the L2 peak shows a nearly linear dependence. All this suggests an interpretation for the L1 line as due to recombination of bound excitons (BE) and for the L2 line as due to bound-to-band recombination i.e. (D, h). The constant position of the L3 peak relative to the L2 peak and the apparent similarity between the line shapes implies an identi"cation for the L3 line as a phonon replica of the L2 line. The separation between L2 and L3 is 46$1 meV in good agreement with known values of 45.2 meV for the InP like LO phonon and 47.6 meV for the GaP-like LO phonon in InGaP [7]. The L3 line is probably a mixture of the two phonon replicas. The L1 line shape and FWHM can be reasonably well "tted (see the dashed line in Fig. 2) using the theoretical expression for the BE line width developed for the case of Al Ga As in Ref. [8] V \V and later generalized for other III}V ternary compounds, e.g. for the case of InGaAs [9]. Applying this approach we obtained a theoretical FWHM of 8 meV for the BE line in InGaP using the available values for the reduced excitonic e!ective mass in InGaP, m "0.083m [10] and dE /dx"1.35 eV (derived for x"0.5 from Ref. [11]). This agrees well with the experimental value of 7 meV. Based on the measured temperature shift of the L1 peak and a "t of the experimental points for the L1 line maximum for temperatures above 60 K with the Varshni dependence for E [12], we evaluated the dissociation energy of an exciton bound to a donor in InGaP as +5 meV. Compared to the corresponding value in GaAs this is a surprisingly small value but it agrees well with the results for Al Ga As obtained in Ref. [13]. It can be underV \V stood as a result of micro-scale compositional #uctuations, respectively, lowering the binding energies via internal electric "elds and the related Poole} Frenkel e!ect associated with these random alloy #uctuations. Both L1 and L2 peaks exhibit thermal quenching. The thermal quenching of L1 is slowed down at about 20 K accompanied by a shift of the L1 peak to higher energies. Above this temperature the L1 peak obviously represents band-to-band transitions. A "t of the thermal quenching of the line L2 gives an apparent activation energy for the donor involved of +35 meV.
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Usually the activation energy derived in this way can vary from a half to almost the full ionization energy, determined independently, where the latter holds only for extremely low impurity concentrations [14]. Consequently, the optically derived activation energy for the donor correlates with the activation energy determined from Hall measurements for sample 1051. We suppose some group VI elements in the `anionica and Si in the `cationica sublattices as dominant background donors in our layers. Taking account of the results and the analysis of the Hall e!ect measurements for the not intentionally doped samples, we conclude that we should consider the presence of a donor impurity band at these concentrations rather than isolated donors and the transition responsible for the L2 line is most likely (Donor impurity band}Valence band). A strong PL line with a maximum at 1.41 eV, (comparable with the intensity of the near-band luminescence from the substrate), was reported in gas-source MBE grown heterostructures of InGaP/GaAs or vice versa [15]. The nature of this line is identi"ed as due to trapping of carriers on traps in the InGaP/GaAs interface region arising from intermixing of As and P atoms in the initial stage of the growth process. The existence of electron traps in the interface region of InGaP/GaAs heterostructures was also shown by means of DLTS measurements on thin layers ((5000 As ) [16]. The PL spectra of our layers show the intensity at 1.41 eV suppressed by 100 times relative to the near-band edge lines of the substrate, which we consider as a proof of the interface quality of our layers.
5. Conclusions Thick InGaP(As) layer growth was demonstrated by the multiple bins, step-cooling variant of LPE. As expected, doping with Er leads to a signi"cant reduction of the carrier concentration. The samples are nearly completely disordered as the PL measurements demonstrate and show a low concentration of the interface traps. A further reduction of the background carrier concentration is expected when electronic grade starting materials are used in combination with optimized Er-doping.
Additional investigations of the e!ect of the duration of individual layer segment growth on layer composition homogeneity will be performed to extend the growth duration of the individual growth segments. The application for particle detectors of GaInAsP/GaAs with higher As content than used in this work we expect to be more promising because of higher expected growth rates and the higher average atomic number of the system. Acknowledgements The authors wish to thank Dr. J. Novak (Electrotechnical Inst. Slovak Acad. of Sci.) for helpful comments and for measuring the lattice mismatch of the samples reported in this work. This work was partially supported by Grant Agency of the Czech Acad. of Sci. under contract GAAV no. A 1010807/1998.
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