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Behaviour of Mn in GaSb grown by the Bridgman method
IIMNHli & ELSEVIER
Materials Science and Engineering B27 (1994) 47-51
B
Behaviour of Mn in GaSb grown by the Bridgman method T. Adhikari, S. B a s u Semiconductor Preparation and Processing Laboratory, Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India Received 30 March 1994
Abstract Mn-doped gallium antimonide crystals were grown by the vertical Bridgman method. The surface morphology was studied with the high resolution microscope and the etch pit density (EPD) was determined by the image analysis method. The absolute distribution coefficient kabsonuteand the active distribution coefficient kactlve of Mn were determined from impurity analysis by the inductively coupled plasma method and from the hole concentration obtained by Hall coefficient measurements respectively. The impurity activation energy of Mn in GaSb was calculated from the slope of the variation in both conductivity and hole concentration with temperature. The variation in hole mobility with temperature was also studied to predict the nature of the scattering. It was concluded that Mn produces a shallow acceptor level ( 17-18 meV ) in GaSb and its inclusion did not reduce the EPD.
Keywords:Gallium antimonide; Doping effects; Manganese; Electrical measurements.
1. Introduction Investigations on dopants for deep levels in III-V semiconductors to obtain a high resistivity has created interest in studies of transition metal doping. Consequently, there has been a large number of papers related to the effect of transition metal impurities in III-V materials, a significant fraction of these being devoted to 3 d impurities [1-3]. However, most of them dealt with GaAs, InP and GaP and relatively less information is available on the behaviour of transition metal impurities in GaSb. It should be pointed out that there are differences in the influence of transition metal impurities on different hosts and hence it is necessary to investigate the role of each of them separately in every specific material. The solubility limit of transition metal impurities in III-V materials is in general rather low, around 1017 c m - 3 . It was found [4] that the introduction of Fe into GaAs at more than the intrinsic acceptor concentration, e.g. n = 1017 cm -3, results in an inhomogeneous distribution and precipitation in the form of the eutectic inclusions. On the contrary, Mn solubility in GaP in excess of 1019 cm -3 without the precipitation of a second phase has been reported by Abagyan et al. [5]. This paper reports for the first time the surface morphology and electrical charac0921-5107/94/$7.00 © 1994 - Elsevier Science S.A. All rights reserved
SSD10921-5107(94)01107-S
terizations of Mn-doped GaSb grown by the Bridgman method.
2. Experimental details The Mn-doped GaSb crystals were grown by the vertical Bridgman method. The required amounts of Ga and Sb (purity, 99.999%) after proper etching and drying were placed in a conical tipped (0 = 30 °) high purity quartz ampoule. Mn of 99.98% purity (Koch Limited, England) was incorporated into the charge as a dopant (doping concentration, 10 TM cm-3). Initially the synthesis was carded out at 850 °C for 24 h using a wire resistance furnace with a suitable temperature profile. Then the temperature was reduced to 800 °C and it was held for 6 h for homogenization. The ampoule was then lowered down the temperature profile with three consecutive rates, e.g. 0.94 mm hup to 660 °C, 2.7 mm h-1 up to 520 °C and 9.4 mm h-1 up to 200 °C at the tip end. The temperature was precisely controlled to within + 1 *C with a programmable temperature controller. Finally the furnace was switched off to attain room temperature. The nucleation and growth started preferentially at the conical tip of the ampoule. After the growth had been completed,
48
T.Adhikari, S. Basu / MaterialsScience and Engineering B27 (1994) 47-51
an ingot of 3.2 cm length and 1.2 cm diameter was removed from the ampoule without any problem of sticking to the wall. The formation of the crystal was confirmed by X-ray diffraction using CuKa radiation (Philips PW 1840 model). No separate peaks of Ga, Sb and Mn were found, thereby confirming complete synthesis and formation of the crystal. A surface morphological study was carried out to determine the dislocations which are normally produced during the growth and subsequent processing. The etch pits were revealed by chemical etching with I H N O 3 : I H F : I H 2 0 for 15 s at room temperature and they were observed with a high resolution microscope (Reichert MeF 2 universal camera microscope, Austria). Wafers cut from the tip end and from 3.0 cm away from the tip end were used for this purpose. Fig. 1 shows typical etch pits and their variation from the core to the periphery. The average etch pit density (EPD) and its variation along the length and the diameter of the ingot were determined by the computerized image analysis method using a texture analysis system (Leitz, Germany). The Mn content in the grown crystals was determined by inductively coupled plasma (ICP) analysis and was compared with
the manganese content actually added to the melt to determine the absolute distribution coefficient as kabs°lute --
[Mn] in the crystal [Mn] in the melt
( 1)
The electrically active manganese can be computed from the following relation utilizing the value of hole concentration from the Hall effect measurements: kactive= (NA-- No)(lO0 AWMn)(WMn)-l(NdGaSb)- 1
(2)
where N A - N D is the net acceptor concentration, AWMnthe atomic weight of Mn, N Avogadro's number, WMnthe weight percent of Mn present in the melt and doasb the density of GaSb. The resistivity and the Hall effect experiments were done using the Van der Pauw configuration in the temperature range between 100 and 296 K. Ohmic contacts were made by In evaporation and subsequent annealing in a hydrogen atmosphere. The sample was mounted inside a liquid-nitrogen cryostat the temperature of which was measured with a calibrated copperconstantan thermocouple. The resistivity and the flail effect were measured at zero field and in a field of 2-3 kG provided by an electromagnet (accuracy, + 1% in the most sensitive range). The constant current was provided by a Keithley 220 programmable current source and the voltage was measured by a 196 system DHM voltmeter. The Hall effect measurements were carried out in both current directions in order to eliminate errors due to thermoelectric and thermomagnetic effects. The results given in this report incorporate experimental data for different wafers with maximum 2% variations as tested by repeated experiments. The hole concentration was calculated from the standard relation p=(qRH) -1 (RH is the Hall coefficient) assuming that the ratio of the Hall mobility to the drift mobility is unity.
3. Sample preparation
Fig. 1. Optical micrograph of etch pits revealed by chemical etching.
The wafers were sliced perpendicular to the axis of the ingot with a diamond tip saw cutter (South Bay Technology, USA). The surface damage produced during cutting was removed by lapping on a glass plate and then on a Rodel polishing pad with 800 mesh silicon carbide abrasive powder. After thorough washing with deionized water the wafers were mechanically polished to mirror fmish with 0.3 p m suspension of AI203 using a Rodel polishing pad. Finally polished wafers were etched in 1HF:lHNO3:10H20 for 10 s followed by thorough washing with deionized water and drying. However, to reveal the etch pits the etchant used was 1HF: 1HNO3:1H20 for 15 s.
T. Adhikari, S. Basu
/
Materials Science and Engineering B27 (1994) 47-51
4. Results and discussion
The typical EPD of the Mn-doped GaSb crystals was found to be 3.3 x 105 cm -2 and it is of the same order of magnitude as the EPD in an undoped GaSb crystal. The variation in EPD along the length of the ingot is shown in Fig. 2(a). Each point on the curve is the average EPD across the diameter of a particular position from the tip end of the ingot. While growing crystals from the melt a major part of the crystallization heat is conducted away axially from the ingot. This creates a temperature gradient between the growth interface and the cooler part of the ingot, thereby producing an axial thermoelastic stress. This stress is responsible for the formation of etch pits and their variation along the ingot. Fig. 2(b) shows the EPD distribution along the diameter at two positions of the ingot. The open squares and open circles indicate the variation in EPD at the tip end and at 3 cm from the tip end respectively. It is seen from the variation in both cases that the EPD increases from the periphery to the core of the crystal. This is further evident from the micrograph presented in Fig. 1. Thus the radial
x 105 3.6 3.5 3./, ~E ca
3.3
Q. IJJ
3.2 3.
( O ,
dlir~ti°n 1.6
0
(a)
3.2
Distance from tip end (cm)
106
i
0 ]~ i
LJ
E u
rl~ 0.2{m Scan direction
n LU
105 0
(b)
3.0 cm
,
I
I
I
0. z, 0.8 Distance (cm)
J
I
1.2
Fig. 2. Variation in EPD (a) along the length of the ingot and (b) along the diameter at two positions of the ingot.
49
thermoelastic stresses are also responsible for the formation of dislocations and its distribution. Harsy et al. [6] in their growth experiments on GaSb reported similar results and proved this by growing low EPD crystals under microgravity conditions where the radial heat loss is minimum. Recently there has been a trend to investigate the role of dopants in reducing the dislocation density [7]. In the present case, however, the Mn dopant did not reduce the dislocation density. The insignificant effect of Mn as a dopant in GaSb, as far as dislocations are concerned, is explained by the bond model suggested by Seki et al. [8]. According to this model, the dislocations introduced into the grown crystal from the periphery by thermal stress will be pinned by the impurity, if the latter forms relatively stronger bonds with the host atoms compared with the bonds between the host atoms themselves. The bond energies of Mn-Sb and Ga-Sb are calculated as 36.37 kcal mol- ] and 42.82 kcal mo1-1 respectively. This difference between bond energies indicates that the Mn-Sb bond is weaker than the Ga-Sb bond and thus, according to Seki et al., Mn should not reduce the dislocation density of GaSb. The sample showed p-type conductivity as confirmed by both the hot-probe method and the sign of the Hall coefficient. A resistivity of 0.07 f2 cm and a hole concentration of 2.35 x 1017 cm -3 were obtained. The corresponding room-temperature hole mobility was 370 cm 2 V- 1 s- 1. The atomic percentage of Mn in the grown crystal and in the melt are 0.38 x 10 -6 and 3.49x 10 -5 respectively. Thus the value of kabsolute obtained from Eq. (1) is 1.10x 10 -2. This can be compared with the distribution coefficient values of Mn in other Ga-based III-V compounds, e.g. 0.05 in GaAs [9]. From the value of hole concentration found from Hall effect measurements, kactive was calculated using Eq. (2) and is 3.49 x 10 -3. So kactive is an order of magnitude smaller than kabso~ute and thus a fraction (32%) of the total Mn content in the crystal is electrically active. The temperature variations in electrical parameters such as conductivity o, hole concentration p, Hall coefficient R H and Hall mobility/~ are shown in Figs. 3(a), 3(b), 3(c) and 3(d), respectively. The activation energy as determined from the slope of the o = f(T)-1 curve in Fig. 3(a) is found to be 17 meV. Fig. 3(b) shows the variation in hole concentration with temperature and from the slope of the curve an activation energy of 18 meV was obtained. This is in close agreement with the activation energy found from the log o vs. inverse temperature plot (Fig. 3(a)). It has been reported [10] that Si and Cu lie 10 meV above the valence band in GaSb. So an attempt was made to find out the concentrations of Cu and Si in the present
50
T. Adhikari, S. Basu
/
Materials Science and Engineering B27 (1994) 47-51 17.42
1.16
'7
f
t
E
to
E 17.18 O
1.06
O.,
b _J
0.g6
I
I
I
I
I
I
Z.
5
6
7
8
g
(a)
1000/T
16.94 10
3
(b)
(K -l)
I 4
I 5
I 6
1000/T
I 7
I 8
I 9
10
{K -11
2.78
60
z
E o
0
,Y,
50
\ 2.67 E
40
t)
cl
~
30
20
(c)
..J
I 4
I 5
1000/T
I 6
I 7
I 8
2.56 2.0,
g
(K -1) ~
(d)
I
2.26
2.48
Log T (K ]
Fig. 3. (a) Variationof conductivitywith inverse temperature. (b) Variationin hole concentration with inverse temperature. (c) Variation in Hall coefficientwith inverse temperature. (d) Variationin hole mobilitywith temperature.
samples. In fact, ICP analysis confirmed that the Si and Cu concentrations were less than 0.1 ppm and hence had a much lower concentration than did Mn. Thus the energy of 17-18 meV is presumably the activation energy of the acceptor level due to Mn in GaSb sample. From Fig. 3(c) it is observed that RH decreases slowly with increasing temperature, thereby signifying the ionization of holes from the Mn-associated impurity level to the valence band. It has been observed that most 3d impurities in Gabased III-V compounds act as deep acceptors and the activation energies for these 3d acceptors in GaAs and GaP are listed in Table 1. The Mn doping is, however, a special case because the Mn acceptor level in different III-V hosts is the shallowest compared with the level due to other transition metals. So it is expected that Mn should produce a shallow level in GaSb also. Our experiments further supported this view as we obtained activation energies of 17 meV and 18 meV from the temperature variations in conductivity and hole concentration respectively. Thus we may conclude
Table 1 Activation energiesof 3d acceptors in GaP and GaAs Activation energy(eV)
Cr Mn Fe Co Ni
GaP
GaAs
1.13 0.40 0.86 0.41 0.50
0.80 0.11 0.49 0.16 0.20
that electrical conduction in Mn-doped GaSb is dominated by thermally activated holes from a shallow acceptor level. The variation in mobility with temperature is shown in Fig. 3(d). The behaviour is similar to that of undoped GaSb [11] where the shape of the curve is due to a combination of optical phonon scattering and ionized impurity scattering. For temperatures between 180 and
T. Adhikari, S. Basu /
MaterialsSeience and Engineering B27 (1994) 47-51
296 K it is possible to fit the curve with a simple power law p = p T - " and the value of n obtained from the linear region of the curve is 0.75. So, we can say that the mixed scattering mechanism accounts for the temperature variation in hole mobility in Mn-doped GaSb. The reported electron paramagnetic resonance [12] and optical spectroscopy [13] data together with the electrical experiments [13] have confirmed that the transition elements in III-V semiconductors act as substitutional impurities and are located in the sites of the crystal lattice of the group HI element. Thus three electrons are needed in the bonding orbitals and two situations are possible. Firstly, the two 4s electrons and one hole may participate in the bonding orbital which generally corresponds to a shallow state of hydrogenic character. Secondly, one d electron together with two 4s electrons may take part in the bonding orbitals. Thus a hole is created in the 3d shell which corresponds to a well-localized acceptor state having no hydrogenic character. The main problem is to determine whether the neutral state of Mn is dominated by a configuration of 3d 5 + hole in a delocalized orbit or by a 3d 4 configuration in a localized orbit. In view of the low activation energy (shallow character) and the stability of a 3d 5 configuration the hole may favour a more delocalized orbit. In that case the neutral ground state of Mn seems to be Mn 2÷ (3d 5 + hole).
5. Conclusion Manganese as dopant in bulk GaSb behaves as an acceptor impurity. It did not effectively reduce the dislocation density in GaSb and it was estimated from
51
the bond energies. A resistivity of 0.07 Q cm, a hole concentration of 2.35 x 1017 cm -3 and a hole mobility of 370 cm 2 V-1 s-1 were obtained at room temperature. The absolute distribution coefficient kabso~uteand the active distribution coefficient kacave of Mn were found to be 1.10 x 10 -2 and 3.49 x 10 -3 respectively. It was observed that Mn forms a shallow acceptor level in GaSb with an activation energy of 17-18 meV. From this shallow character of the Mn acceptor, a neutral state of Mn as 3d 5 + hole was suggested. The temperature variation in the mobility was accounted for by the mixed scattering mechanism.
References [1] W. J. Brown, Jr., and J. S. Blakemore, J. AppL Phys., 43 (1972) 2242. [2] K. Suto and J. Nishizowa, J. Appl. Phys., 43(1972) 2247. [3] M. Hamera, W. Walukiewicz and D. D. Note, Phys. Rev. B,39(1989) 10114. [4] E. M. Omel'yanovskii, V. I. Fistul, L. A. Balagurov, V. S. Ivleva, V. V. Karataev, M. G. Mil'vidskii and A. N. Popkov, Sov. Phys.--Semicond., 9 (1975) 381. [5] S. A. Abagyan, G. A. Ivanov, G. A. Korokeva, Yu. N. Keiznersov and Yu. A. Okunev, Sov. Phys.--Semicond., 9 (1975) 243. [6] M. Harsy, E Koltai and T. Gorong, Acta Phys. Hung., 57 (1985)245. [7] D. Laister and G. M. Jenkins, J. Mater. Sci., 8(1973) 1218. [8] Y. Seki, J. Matsui and H. Watanabe, J. Appl. Phys., 47 (1976) 3374. [9] L.J. Vieland, J. Appl. Phys., 33(1962) 2007. [10] I.I. Burdiyan, Sov. Phys.--Semicond., 7 (1973) 449. [11] D. Baryen, A. Raymond, B. Pistoulet and J. C. Robert, J. Phys. Chem. Solids, 34(1973) 2231. [12] J. Schneider, U. Kaufmarm, W. Wilkenig and M. Baeumber, Phys. Rev. Lett., 59(1987) 240. [13] M. Ilegems, R. Dingle and I. W. Rupp, Jr., J. Appl. Phys., 46 (1975)3059.
Materials Science and Engineering B27 (1994) 47-51
B
Behaviour of Mn in GaSb grown by the Bridgman method T. Adhikari, S. B a s u Semiconductor Preparation and Processing Laboratory, Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India Received 30 March 1994
Abstract Mn-doped gallium antimonide crystals were grown by the vertical Bridgman method. The surface morphology was studied with the high resolution microscope and the etch pit density (EPD) was determined by the image analysis method. The absolute distribution coefficient kabsonuteand the active distribution coefficient kactlve of Mn were determined from impurity analysis by the inductively coupled plasma method and from the hole concentration obtained by Hall coefficient measurements respectively. The impurity activation energy of Mn in GaSb was calculated from the slope of the variation in both conductivity and hole concentration with temperature. The variation in hole mobility with temperature was also studied to predict the nature of the scattering. It was concluded that Mn produces a shallow acceptor level ( 17-18 meV ) in GaSb and its inclusion did not reduce the EPD.
Keywords:Gallium antimonide; Doping effects; Manganese; Electrical measurements.
1. Introduction Investigations on dopants for deep levels in III-V semiconductors to obtain a high resistivity has created interest in studies of transition metal doping. Consequently, there has been a large number of papers related to the effect of transition metal impurities in III-V materials, a significant fraction of these being devoted to 3 d impurities [1-3]. However, most of them dealt with GaAs, InP and GaP and relatively less information is available on the behaviour of transition metal impurities in GaSb. It should be pointed out that there are differences in the influence of transition metal impurities on different hosts and hence it is necessary to investigate the role of each of them separately in every specific material. The solubility limit of transition metal impurities in III-V materials is in general rather low, around 1017 c m - 3 . It was found [4] that the introduction of Fe into GaAs at more than the intrinsic acceptor concentration, e.g. n = 1017 cm -3, results in an inhomogeneous distribution and precipitation in the form of the eutectic inclusions. On the contrary, Mn solubility in GaP in excess of 1019 cm -3 without the precipitation of a second phase has been reported by Abagyan et al. [5]. This paper reports for the first time the surface morphology and electrical charac0921-5107/94/$7.00 © 1994 - Elsevier Science S.A. All rights reserved
SSD10921-5107(94)01107-S
terizations of Mn-doped GaSb grown by the Bridgman method.
2. Experimental details The Mn-doped GaSb crystals were grown by the vertical Bridgman method. The required amounts of Ga and Sb (purity, 99.999%) after proper etching and drying were placed in a conical tipped (0 = 30 °) high purity quartz ampoule. Mn of 99.98% purity (Koch Limited, England) was incorporated into the charge as a dopant (doping concentration, 10 TM cm-3). Initially the synthesis was carded out at 850 °C for 24 h using a wire resistance furnace with a suitable temperature profile. Then the temperature was reduced to 800 °C and it was held for 6 h for homogenization. The ampoule was then lowered down the temperature profile with three consecutive rates, e.g. 0.94 mm hup to 660 °C, 2.7 mm h-1 up to 520 °C and 9.4 mm h-1 up to 200 °C at the tip end. The temperature was precisely controlled to within + 1 *C with a programmable temperature controller. Finally the furnace was switched off to attain room temperature. The nucleation and growth started preferentially at the conical tip of the ampoule. After the growth had been completed,
48
T.Adhikari, S. Basu / MaterialsScience and Engineering B27 (1994) 47-51
an ingot of 3.2 cm length and 1.2 cm diameter was removed from the ampoule without any problem of sticking to the wall. The formation of the crystal was confirmed by X-ray diffraction using CuKa radiation (Philips PW 1840 model). No separate peaks of Ga, Sb and Mn were found, thereby confirming complete synthesis and formation of the crystal. A surface morphological study was carried out to determine the dislocations which are normally produced during the growth and subsequent processing. The etch pits were revealed by chemical etching with I H N O 3 : I H F : I H 2 0 for 15 s at room temperature and they were observed with a high resolution microscope (Reichert MeF 2 universal camera microscope, Austria). Wafers cut from the tip end and from 3.0 cm away from the tip end were used for this purpose. Fig. 1 shows typical etch pits and their variation from the core to the periphery. The average etch pit density (EPD) and its variation along the length and the diameter of the ingot were determined by the computerized image analysis method using a texture analysis system (Leitz, Germany). The Mn content in the grown crystals was determined by inductively coupled plasma (ICP) analysis and was compared with
the manganese content actually added to the melt to determine the absolute distribution coefficient as kabs°lute --
[Mn] in the crystal [Mn] in the melt
( 1)
The electrically active manganese can be computed from the following relation utilizing the value of hole concentration from the Hall effect measurements: kactive= (NA-- No)(lO0 AWMn)(WMn)-l(NdGaSb)- 1
(2)
where N A - N D is the net acceptor concentration, AWMnthe atomic weight of Mn, N Avogadro's number, WMnthe weight percent of Mn present in the melt and doasb the density of GaSb. The resistivity and the Hall effect experiments were done using the Van der Pauw configuration in the temperature range between 100 and 296 K. Ohmic contacts were made by In evaporation and subsequent annealing in a hydrogen atmosphere. The sample was mounted inside a liquid-nitrogen cryostat the temperature of which was measured with a calibrated copperconstantan thermocouple. The resistivity and the flail effect were measured at zero field and in a field of 2-3 kG provided by an electromagnet (accuracy, + 1% in the most sensitive range). The constant current was provided by a Keithley 220 programmable current source and the voltage was measured by a 196 system DHM voltmeter. The Hall effect measurements were carried out in both current directions in order to eliminate errors due to thermoelectric and thermomagnetic effects. The results given in this report incorporate experimental data for different wafers with maximum 2% variations as tested by repeated experiments. The hole concentration was calculated from the standard relation p=(qRH) -1 (RH is the Hall coefficient) assuming that the ratio of the Hall mobility to the drift mobility is unity.
3. Sample preparation
Fig. 1. Optical micrograph of etch pits revealed by chemical etching.
The wafers were sliced perpendicular to the axis of the ingot with a diamond tip saw cutter (South Bay Technology, USA). The surface damage produced during cutting was removed by lapping on a glass plate and then on a Rodel polishing pad with 800 mesh silicon carbide abrasive powder. After thorough washing with deionized water the wafers were mechanically polished to mirror fmish with 0.3 p m suspension of AI203 using a Rodel polishing pad. Finally polished wafers were etched in 1HF:lHNO3:10H20 for 10 s followed by thorough washing with deionized water and drying. However, to reveal the etch pits the etchant used was 1HF: 1HNO3:1H20 for 15 s.
T. Adhikari, S. Basu
/
Materials Science and Engineering B27 (1994) 47-51
4. Results and discussion
The typical EPD of the Mn-doped GaSb crystals was found to be 3.3 x 105 cm -2 and it is of the same order of magnitude as the EPD in an undoped GaSb crystal. The variation in EPD along the length of the ingot is shown in Fig. 2(a). Each point on the curve is the average EPD across the diameter of a particular position from the tip end of the ingot. While growing crystals from the melt a major part of the crystallization heat is conducted away axially from the ingot. This creates a temperature gradient between the growth interface and the cooler part of the ingot, thereby producing an axial thermoelastic stress. This stress is responsible for the formation of etch pits and their variation along the ingot. Fig. 2(b) shows the EPD distribution along the diameter at two positions of the ingot. The open squares and open circles indicate the variation in EPD at the tip end and at 3 cm from the tip end respectively. It is seen from the variation in both cases that the EPD increases from the periphery to the core of the crystal. This is further evident from the micrograph presented in Fig. 1. Thus the radial
x 105 3.6 3.5 3./, ~E ca
3.3
Q. IJJ
3.2 3.
( O ,
dlir~ti°n 1.6
0
(a)
3.2
Distance from tip end (cm)
106
i
0 ]~ i
LJ
E u
rl~ 0.2{m Scan direction
n LU
105 0
(b)
3.0 cm
,
I
I
I
0. z, 0.8 Distance (cm)
J
I
1.2
Fig. 2. Variation in EPD (a) along the length of the ingot and (b) along the diameter at two positions of the ingot.
49
thermoelastic stresses are also responsible for the formation of dislocations and its distribution. Harsy et al. [6] in their growth experiments on GaSb reported similar results and proved this by growing low EPD crystals under microgravity conditions where the radial heat loss is minimum. Recently there has been a trend to investigate the role of dopants in reducing the dislocation density [7]. In the present case, however, the Mn dopant did not reduce the dislocation density. The insignificant effect of Mn as a dopant in GaSb, as far as dislocations are concerned, is explained by the bond model suggested by Seki et al. [8]. According to this model, the dislocations introduced into the grown crystal from the periphery by thermal stress will be pinned by the impurity, if the latter forms relatively stronger bonds with the host atoms compared with the bonds between the host atoms themselves. The bond energies of Mn-Sb and Ga-Sb are calculated as 36.37 kcal mol- ] and 42.82 kcal mo1-1 respectively. This difference between bond energies indicates that the Mn-Sb bond is weaker than the Ga-Sb bond and thus, according to Seki et al., Mn should not reduce the dislocation density of GaSb. The sample showed p-type conductivity as confirmed by both the hot-probe method and the sign of the Hall coefficient. A resistivity of 0.07 f2 cm and a hole concentration of 2.35 x 1017 cm -3 were obtained. The corresponding room-temperature hole mobility was 370 cm 2 V- 1 s- 1. The atomic percentage of Mn in the grown crystal and in the melt are 0.38 x 10 -6 and 3.49x 10 -5 respectively. Thus the value of kabsolute obtained from Eq. (1) is 1.10x 10 -2. This can be compared with the distribution coefficient values of Mn in other Ga-based III-V compounds, e.g. 0.05 in GaAs [9]. From the value of hole concentration found from Hall effect measurements, kactive was calculated using Eq. (2) and is 3.49 x 10 -3. So kactive is an order of magnitude smaller than kabso~ute and thus a fraction (32%) of the total Mn content in the crystal is electrically active. The temperature variations in electrical parameters such as conductivity o, hole concentration p, Hall coefficient R H and Hall mobility/~ are shown in Figs. 3(a), 3(b), 3(c) and 3(d), respectively. The activation energy as determined from the slope of the o = f(T)-1 curve in Fig. 3(a) is found to be 17 meV. Fig. 3(b) shows the variation in hole concentration with temperature and from the slope of the curve an activation energy of 18 meV was obtained. This is in close agreement with the activation energy found from the log o vs. inverse temperature plot (Fig. 3(a)). It has been reported [10] that Si and Cu lie 10 meV above the valence band in GaSb. So an attempt was made to find out the concentrations of Cu and Si in the present
50
T. Adhikari, S. Basu
/
Materials Science and Engineering B27 (1994) 47-51 17.42
1.16
'7
f
t
E
to
E 17.18 O
1.06
O.,
b _J
0.g6
I
I
I
I
I
I
Z.
5
6
7
8
g
(a)
1000/T
16.94 10
3
(b)
(K -l)
I 4
I 5
I 6
1000/T
I 7
I 8
I 9
10
{K -11
2.78
60
z
E o
0
,Y,
50
\ 2.67 E
40
t)
cl
~
30
20
(c)
..J
I 4
I 5
1000/T
I 6
I 7
I 8
2.56 2.0,
g
(K -1) ~
(d)
I
2.26
2.48
Log T (K ]
Fig. 3. (a) Variationof conductivitywith inverse temperature. (b) Variationin hole concentration with inverse temperature. (c) Variation in Hall coefficientwith inverse temperature. (d) Variationin hole mobilitywith temperature.
samples. In fact, ICP analysis confirmed that the Si and Cu concentrations were less than 0.1 ppm and hence had a much lower concentration than did Mn. Thus the energy of 17-18 meV is presumably the activation energy of the acceptor level due to Mn in GaSb sample. From Fig. 3(c) it is observed that RH decreases slowly with increasing temperature, thereby signifying the ionization of holes from the Mn-associated impurity level to the valence band. It has been observed that most 3d impurities in Gabased III-V compounds act as deep acceptors and the activation energies for these 3d acceptors in GaAs and GaP are listed in Table 1. The Mn doping is, however, a special case because the Mn acceptor level in different III-V hosts is the shallowest compared with the level due to other transition metals. So it is expected that Mn should produce a shallow level in GaSb also. Our experiments further supported this view as we obtained activation energies of 17 meV and 18 meV from the temperature variations in conductivity and hole concentration respectively. Thus we may conclude
Table 1 Activation energiesof 3d acceptors in GaP and GaAs Activation energy(eV)
Cr Mn Fe Co Ni
GaP
GaAs
1.13 0.40 0.86 0.41 0.50
0.80 0.11 0.49 0.16 0.20
that electrical conduction in Mn-doped GaSb is dominated by thermally activated holes from a shallow acceptor level. The variation in mobility with temperature is shown in Fig. 3(d). The behaviour is similar to that of undoped GaSb [11] where the shape of the curve is due to a combination of optical phonon scattering and ionized impurity scattering. For temperatures between 180 and
T. Adhikari, S. Basu /
MaterialsSeience and Engineering B27 (1994) 47-51
296 K it is possible to fit the curve with a simple power law p = p T - " and the value of n obtained from the linear region of the curve is 0.75. So, we can say that the mixed scattering mechanism accounts for the temperature variation in hole mobility in Mn-doped GaSb. The reported electron paramagnetic resonance [12] and optical spectroscopy [13] data together with the electrical experiments [13] have confirmed that the transition elements in III-V semiconductors act as substitutional impurities and are located in the sites of the crystal lattice of the group HI element. Thus three electrons are needed in the bonding orbitals and two situations are possible. Firstly, the two 4s electrons and one hole may participate in the bonding orbital which generally corresponds to a shallow state of hydrogenic character. Secondly, one d electron together with two 4s electrons may take part in the bonding orbitals. Thus a hole is created in the 3d shell which corresponds to a well-localized acceptor state having no hydrogenic character. The main problem is to determine whether the neutral state of Mn is dominated by a configuration of 3d 5 + hole in a delocalized orbit or by a 3d 4 configuration in a localized orbit. In view of the low activation energy (shallow character) and the stability of a 3d 5 configuration the hole may favour a more delocalized orbit. In that case the neutral ground state of Mn seems to be Mn 2÷ (3d 5 + hole).
5. Conclusion Manganese as dopant in bulk GaSb behaves as an acceptor impurity. It did not effectively reduce the dislocation density in GaSb and it was estimated from
51
the bond energies. A resistivity of 0.07 Q cm, a hole concentration of 2.35 x 1017 cm -3 and a hole mobility of 370 cm 2 V-1 s-1 were obtained at room temperature. The absolute distribution coefficient kabso~uteand the active distribution coefficient kacave of Mn were found to be 1.10 x 10 -2 and 3.49 x 10 -3 respectively. It was observed that Mn forms a shallow acceptor level in GaSb with an activation energy of 17-18 meV. From this shallow character of the Mn acceptor, a neutral state of Mn as 3d 5 + hole was suggested. The temperature variation in the mobility was accounted for by the mixed scattering mechanism.
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