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GaAs double heterostructures under controlled arsenic vapour pressure
596
Journal of Crystal Growth 65 (1983) 596-601 North-Holland, Amsterdam
LIQUID PHASE EPITAXIAL GROWTH OF (Ga,AI)As/GaAs DOUBLE HETEROSTRUCTURES UNDER CONTROLLED ARSENIC VAPOUR PRESSURE J. NOVOTNY, F. SROBAR, F. MORAVEC and J. ZELINKA Institute of Radio Engineering and Electronics, Czechoslovak Academy of Sciences, Lumumbova 1, 18251 Prague 8, Czechoslovakia
It is shown that the structural and lasing characteristics of (Ga,Al)As/GaAs double heterostructures are favourably influenced by applying controlled arsenic vapour pressure in the liquid phase epitaxial apparatus during growth. Previously found experimental evidence for an influence of vapour pressure on various physical properties of grown materials is confirmed. An improvement in lasing characteristics may be due to a reduced dislocation density, which is caused, in turn, by a reduction in point-defect density as a consequence of the application of optimum arsenic vapour pressure.
1. Introduction
Even though bulk nonradiative recombination is believed to be negligible in the best state-of-theart LPE (Ga,A1)As/GaAs double heterostructures (DH), high concentration of point defects, may prevent good performance and reliability of DH injection lasers. Point defects, which can favour the formation and propagation of dislocations, are not necessarily bound to impurity atoms. Studies of the ~olidus curves [1,2] showed that the III-V compound stability ranges, albeit narrow, are of finite width; hence the formation of intrinsic stoichiometric defects at the crystal growth temperatures is a consequence of thermodynamic laws. In GaAs, due to the high fugacity of arsenic, As vacancies (acting as donors) are the dominant defects species. It is natural, therefore, to expect that the inherent tendency towards nonstoichiometry can be combatted by the introduction of excess arsenic pressure. Therefore, the added degree of freedom contributed by nonstoichiometry, provides an opportunity to control crystal properties by the ambient arsenic atmosphere. A Ga melt layer of several millimeters thick which covers the crystal surface in LPE growth procedure is sufficiently transparent for diffusing arsenic molecules [3]. In the mid-seventies, Nishizawa and coworkers [3,4] took
up an extensive study of this issue. They found that various crystal properties (surface morphology, dislocation density, lattice constant, carrier concentration and mobility) can be optimized when the arsenic pressure takes a suitable value Popt in pascal. The experimental data for GaAs layers grown at temperature Tg were fitted by the relation Popt = 3.5 × l0 s e x p ( - 12200/Tg).
(1)
This so-called controlled vapour pressure (CVP) method was also used to grow (Ga,A1)As/GaAs single heterostructures [5]. It is interesting to note that the presence of arsenic in the annealing atmosphere also reduces the surface decomposition of heat-treated GaAs [6]. This paper presents the results of our experim e n t s with the C V P p r e p a r a t i o n of (Ga,A1)As/GaAs DH lasers on medium-quality GaAs substrates. Firstly, the modified LPE growth apparatus is described. It is shown that presence of arsenic vapour slows down the growth of the (Ga,A1)As layers, but does not have a notable influence on the growth rate of GaAs layers. Subsequently we report on the surface and cross section morphology, dopant concentration profiles and lasing characteristics of the prepared double heterostructures. The CVP method favourably affects both the magnitude and the temperature dependence of the laser threshold current.
0022-0248/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
597
J. Novotnj~et aL / LPE growth of (Ga,Al)As/ Ga.4s double heterostructures
2. LPE system and growth procedure
The arrangement used by Nishizawa for growing epilayers of I I I - V compounds under CVP conditions employs a vertical temperature gradient in the melt. This approach is not directly transferable to the growth of multilayer structures, which require a multiple-bin boat. That would require maintaining a defined vertical temperature gradient along the whole length ( - 20 cm) of the boat. Also, the temperature stability of about 0.05°C would be difficult to achieve, but is required for growing submicron active layers of D H lasers. We therefore based our apparatus on the near equilibrium technique [7], which we modified to allow CVP for layer growth. A graphite cover (fig. 1) connected by a fused silica tube with the fused silica arsenic container is located on top of the carbon boat. The latter is placed strategically in the oven, so as to produce the required arsenic pressure over the Ga melt. The arsenic source temperature is monitored by a thermocouple. GaAs substrates of medium quality with a dislocation density n e a r 1 0 4 cm -2 were used. Otherwise the manufacturing process follows established procedures. The substrate is cleaned and chemically polished. Then it is baked and loaded into the
furnace at 820°C for 5 h and subsequently reloaded into the boat with added dopants, dummy and growth substrates. The system is equilibrated at 800°C for 4 h (temperature stability of + 0.05°C). This is followed by a 20 min contact of the melt with the dummy substrate. At the end of the latter period the automatic cooling program starts, providing a uniform cooling rate of 0.15°C/min. When the CVP regime is required, the arsenic source is maintained at a temperature near 400°C to provide an As vapour pressure over the Ga melt of about 103 Pa.
3. Growth kinetics
Experimental thickness versus growth time data points, together with the theoretical straight-line relationships, are shown in fig. 2 (for GaAs) and in fig. 3 (for (Ga,AI)As). The thickness values were measured on SEM micrographs of the sample cross sections. In accordance with expectation based on relevant experimental conditions, there is fairly good agreement with the equilibrium cooling growth law [8]
t 3/2.
d =
(2)
Here R is the cooling rate, c s the arsenic concentration in the solid, D the arsenic diffusion coefficient in the solution, and 1 / m = d x L J d T graphite cover
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Fig. 2. Dependence of GaAs layer thickness on the growth time. (O) Experimentaldata for growth without CVP. (O) CVP conditions, straight line by eq. (2) with the parametervalues as indicated and discussed in the text.
J. Novotn~ et al. / LPE growth of (Ga,Al)As/GaAs double heterostructures
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growth time: (O) growth without CVP; (zx)CVP conditions. Straight lines were computed for parameter values indicated in the figure, and discussed in the text.
the slope of the liquidus surface. Numerical values of these parameters are indicated in the figures. The value of D has been chosen in accordance with Dawson [7], c s = 0.5 (neglecting small deviations from stoichiometry). This leaves m as the only variable parameter. For similar growth conditions, there is a good agreement of the best-fit values of m with those given by Hsieh [9] (for GaAs layers) and by Isozumi et al. [10] (for the (Ga,AI)As layers). Suppression of the (Ga,A1)As growth rate by the presence of As vapour, apparent in fig. 3, is an interesting observation, especially in conjunction with the fact that no such effect has been detectable in the GaAs data (fig. 2). Because the nucleation phenomena do not play a controlling role in layer growth under the given conditions, this occurrence must be ascribed to the influence of arsenic pressure on the slope of the liquidus surface. The latter effect is apparently enhanced by the presence of A1 in the melt.
4. Surface and cross-section morphology (Ga,AI)As/GaAs double heterostructures
of
The surface of the D H chips intended for stripe-contact injection laser fabrication must be planar, smooth and mirror-like. These require-
Fig. 4. Surface optical micrograph of (Ga,A1)As DHs grown: (a) with and (b) without optimum arsenic vapour pressure. Magnification ca. 100 x .
ments are dictated mainly by the photolithographic process used to form striped ohmic contacts of 5-10/~m width. Previously the best results have been obtained with the near-equilibrium LPE method [11]; however, there still remains considerable scope for improvement. Fig. 4 demonstrates that the CVP method may be helpful in this respect. The perfect surface in fig. 4a has been obtained under optimum arsenic vapour-pressure conditions. By contrast the surface in fig. 4b, pertaining to a sample grown by the conventional method, exhibits pits, hillocks and linear grooves. Formation of these discrete features is apparently associated with higher point-defect concentration. These results are similar to those reported by Nishizawa et al. [3] for simple epitaxial layers. The beneficial effect of the CVP method on planarity
J. Novotn~ et aL / LPE growth of (Ga,A1)As / GaAs double heterostructures
P - GaAs _ Gao.TAto3As ~-GaAs 0.2/Jm active foyer
q - GOo.TA!.o~jAs N-GaAs Substrate (a)
P - GaAs P - Gao.TAtozAs P-GaAs ~ active layer
N- GaAs Substrate
(b) Fig. 5. SEM view of the chemicallyetched DH cross sections: (a) with and (b) without CVP.
and perfection of the (Ga,A1)As/GaAs DHs i s also seen on the SEM micrographs of the chemically etched cross sections in fig. 5. The extent of lda
......---.---
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i
i
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the irregularities in the direction perpendicular to the interfaces is estimated to be less than 15 nm for the sample shown in fig. 5a. Corresponding conclusions can be inferred from the capacitance-voltage measurements performed on the DH samples. The CVP samples exhibit linear C - " versus V plots (n = 2.36-2.66, which is quite close to the value, n = 2, valid for ideal rectangular step profiles). Samples manufactured by the conventional method show dependences that cannot be fitted into a broad voltage range by a law of this type. This means that employment of the CVP method favours the formation of abrupt and well defined dopant concentration profiles (fig. 6).
5. Lasing properties
N - Gao.7 Ato.3 As
5 % u
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;
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Fig. 6. Dopant concentration profiles in (Ga,A1)As DHs: ( . . . . . . ) withoutCVP; ( ) underoptimumarsenicvapour pressure; (-- -- --) under excess arsenic vapour pressure.
The prepared DH wafers were provided with stripe and broad-area contacts, cut into individual laser devices and mounted on standard gold-plated copper holders. Fig. 7 shows the light intensity versus current characteristics of two sets of laser samples (with a rather high value of the active layer thickness, d = 1 ~tm) prepared by the conventional (fig. 7a) and by the CVP (fig. 7b) method. The manufacturing procedures are otherwise identical. Threshold currents of the CVP samples are seen to be about half of those of the samples prepared with no arsenic pressure. We found the following laser construction to represent an optimum when using medium quality (dislocation density 104 cm -2) (100)+20' GaAs substrates (Si doped 2 x 1018 cm -3, supplied by the Sumitomo Corp.): first confining layer Gao.65Alo.35As:Sn (6 x 1017 cm -3) of thickness 2.5/~m, active layer GaAs : Ge (1 x 1017 cm -3) of thickness 0.25 /xm, second confining layer Ga0. 7 A10.3As : Ge (5 x 1017 cm -3) of thickness 1.8/~m and contacting layer GaAs : Ge (1.5 X 1018 cm -3) of thickness 1.0 /~m. The initial growth temperature was 800°C, and the arsenic vapour pressure within the epitaxial boat (1.3-5.2) x 103 Pa. The cw room temperature threshold current values of the lasers range typically from 100 to 200 mA with a differential quantum efficiency of 80-95%. The temperature dependence of the
600
J. Novotny)et al. / LPE growth of (Ga,AI)As/ GaAs double heterostructures
AGA - 2
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i
0.2
0.4
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Fig. 7. Dependence of emitted-light intensity on the laser current: (a) without and (b) with CVP. Characteristics measured on several devices fabricated from the identical wafer are displayed in each case.
threshold current in the range 10-50°C can be approximated by the customary empirical law I,h e x p ( T / T o ) , with the parameter TO values situated near to 350 K. This is a very good value for this
type of laser (cf. ref. [12]). The temperature dependence of Ith for some selected samples can be even smaller (fig. 8).
6. Conclusions 8
7
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200
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300 I (mA)
Fig. 8. Light-current characteristics of a selected laser device for a set of temperature values. P = optical power, 1 = current.
The experimental results presented in this publication show that controlled arsenic vapour pressure in the LPE apparatus favourably influences not only the structural characteristics of (Ga,A1)As based DHs, but also their lasing characteristics, such as the differential quantum efficiency, the threshold current and its temperature dependence. Studies aimed at establishing the influence of CVP on aging properties of the lasers are under way. A better understanding of the observed changes in growth kinetics will have to await clarification of the way the controlled arsenic vapour pressure influences the liquidus surface. The observed improvement in lasing characteristics is perhaps due to a reduced dislocation density, which in turn is caused by a reduction in the point-defect density. It should be noted that good laser properties were obtained, notwithstanding the fact that the sub-
J. Novotn~ et al. / LPE growth of (Ga,AI)As/ GaAs double heterostructures
strates were only of a moderate quality. The theoretical discussion will be published separately.
References [1] R.M. Logan and D.T.J. Hurle, J. Phys. Chem. Solids 32 (1971) 1739. [2] A.S. Jordan, R. Caruso, A.R. Von Neida and M.E. Weiner, J. Appl. Phys. 45 (1974) 3472. [3] J. Nishizawa, Y. Okuno and H. Tadano, J. Crystal Growth 31 (1975) 215. [4] J. Nishizawa and Y. Okuno, Technical Report TR-41,
601
Research Institute of Electrical Communications, Tohoku Univ., Sendal, 1978. [51 H. Tadano, Y. Okuno and J. Nishizawa, in: Abstracts of the Intern. Conf. on Vapour Growth and Epitaxy, Nagoya, 1978. [6] J. Ohsawa, K. Ikeda, K. Takahashi and W. Susaki, Japan. J. Appl. Phys. 21 (1982) 149. [7] L.R. Dawson, J. Crystal Growth 27 (1974) 86. [8] M.B. Small and J.F. Barnes, J. Crystal Growth 5 (1969) 9. [9] J.J. Hsieh, J. Crystal Growth 27 (1974) 49. [10] S. Isozumi, Y. Komatsu and T. Kotani, Fujitsu Sci. Tech. J. (June 1979) 85. [11] R.J. Nelson, Appl. Phys. Letters 35 (1979) 654. [12] H.D. Wolf, K. Mettler and K.-H. Zschauer, Japan. J. Appl. Phys. 20 (1981) 1693.
Journal of Crystal Growth 65 (1983) 596-601 North-Holland, Amsterdam
LIQUID PHASE EPITAXIAL GROWTH OF (Ga,AI)As/GaAs DOUBLE HETEROSTRUCTURES UNDER CONTROLLED ARSENIC VAPOUR PRESSURE J. NOVOTNY, F. SROBAR, F. MORAVEC and J. ZELINKA Institute of Radio Engineering and Electronics, Czechoslovak Academy of Sciences, Lumumbova 1, 18251 Prague 8, Czechoslovakia
It is shown that the structural and lasing characteristics of (Ga,Al)As/GaAs double heterostructures are favourably influenced by applying controlled arsenic vapour pressure in the liquid phase epitaxial apparatus during growth. Previously found experimental evidence for an influence of vapour pressure on various physical properties of grown materials is confirmed. An improvement in lasing characteristics may be due to a reduced dislocation density, which is caused, in turn, by a reduction in point-defect density as a consequence of the application of optimum arsenic vapour pressure.
1. Introduction
Even though bulk nonradiative recombination is believed to be negligible in the best state-of-theart LPE (Ga,A1)As/GaAs double heterostructures (DH), high concentration of point defects, may prevent good performance and reliability of DH injection lasers. Point defects, which can favour the formation and propagation of dislocations, are not necessarily bound to impurity atoms. Studies of the ~olidus curves [1,2] showed that the III-V compound stability ranges, albeit narrow, are of finite width; hence the formation of intrinsic stoichiometric defects at the crystal growth temperatures is a consequence of thermodynamic laws. In GaAs, due to the high fugacity of arsenic, As vacancies (acting as donors) are the dominant defects species. It is natural, therefore, to expect that the inherent tendency towards nonstoichiometry can be combatted by the introduction of excess arsenic pressure. Therefore, the added degree of freedom contributed by nonstoichiometry, provides an opportunity to control crystal properties by the ambient arsenic atmosphere. A Ga melt layer of several millimeters thick which covers the crystal surface in LPE growth procedure is sufficiently transparent for diffusing arsenic molecules [3]. In the mid-seventies, Nishizawa and coworkers [3,4] took
up an extensive study of this issue. They found that various crystal properties (surface morphology, dislocation density, lattice constant, carrier concentration and mobility) can be optimized when the arsenic pressure takes a suitable value Popt in pascal. The experimental data for GaAs layers grown at temperature Tg were fitted by the relation Popt = 3.5 × l0 s e x p ( - 12200/Tg).
(1)
This so-called controlled vapour pressure (CVP) method was also used to grow (Ga,A1)As/GaAs single heterostructures [5]. It is interesting to note that the presence of arsenic in the annealing atmosphere also reduces the surface decomposition of heat-treated GaAs [6]. This paper presents the results of our experim e n t s with the C V P p r e p a r a t i o n of (Ga,A1)As/GaAs DH lasers on medium-quality GaAs substrates. Firstly, the modified LPE growth apparatus is described. It is shown that presence of arsenic vapour slows down the growth of the (Ga,A1)As layers, but does not have a notable influence on the growth rate of GaAs layers. Subsequently we report on the surface and cross section morphology, dopant concentration profiles and lasing characteristics of the prepared double heterostructures. The CVP method favourably affects both the magnitude and the temperature dependence of the laser threshold current.
0022-0248/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
597
J. Novotnj~et aL / LPE growth of (Ga,Al)As/ Ga.4s double heterostructures
2. LPE system and growth procedure
The arrangement used by Nishizawa for growing epilayers of I I I - V compounds under CVP conditions employs a vertical temperature gradient in the melt. This approach is not directly transferable to the growth of multilayer structures, which require a multiple-bin boat. That would require maintaining a defined vertical temperature gradient along the whole length ( - 20 cm) of the boat. Also, the temperature stability of about 0.05°C would be difficult to achieve, but is required for growing submicron active layers of D H lasers. We therefore based our apparatus on the near equilibrium technique [7], which we modified to allow CVP for layer growth. A graphite cover (fig. 1) connected by a fused silica tube with the fused silica arsenic container is located on top of the carbon boat. The latter is placed strategically in the oven, so as to produce the required arsenic pressure over the Ga melt. The arsenic source temperature is monitored by a thermocouple. GaAs substrates of medium quality with a dislocation density n e a r 1 0 4 cm -2 were used. Otherwise the manufacturing process follows established procedures. The substrate is cleaned and chemically polished. Then it is baked and loaded into the
furnace at 820°C for 5 h and subsequently reloaded into the boat with added dopants, dummy and growth substrates. The system is equilibrated at 800°C for 4 h (temperature stability of + 0.05°C). This is followed by a 20 min contact of the melt with the dummy substrate. At the end of the latter period the automatic cooling program starts, providing a uniform cooling rate of 0.15°C/min. When the CVP regime is required, the arsenic source is maintained at a temperature near 400°C to provide an As vapour pressure over the Ga melt of about 103 Pa.
3. Growth kinetics
Experimental thickness versus growth time data points, together with the theoretical straight-line relationships, are shown in fig. 2 (for GaAs) and in fig. 3 (for (Ga,AI)As). The thickness values were measured on SEM micrographs of the sample cross sections. In accordance with expectation based on relevant experimental conditions, there is fairly good agreement with the equilibrium cooling growth law [8]
t 3/2.
d =
(2)
Here R is the cooling rate, c s the arsenic concentration in the solid, D the arsenic diffusion coefficient in the solution, and 1 / m = d x L J d T graphite cover
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Fig. 1. Modified carbon boat for the LPE process using controlled arsenic vapour pressure.
0,1
I 6
] 7
i B
I 9
L 10
L 15
I 20 t (rain)
25
Fig. 2. Dependence of GaAs layer thickness on the growth time. (O) Experimentaldata for growth without CVP. (O) CVP conditions, straight line by eq. (2) with the parametervalues as indicated and discussed in the text.
J. Novotn~ et al. / LPE growth of (Ga,Al)As/GaAs double heterostructures
598
5 4 3
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.
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.
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.
8
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.
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.
.
.
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20
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,/
O 5
Fig. 3. D e p e n d e n c e of Ga0.7A10.3As layer thickness on the
growth time: (O) growth without CVP; (zx)CVP conditions. Straight lines were computed for parameter values indicated in the figure, and discussed in the text.
the slope of the liquidus surface. Numerical values of these parameters are indicated in the figures. The value of D has been chosen in accordance with Dawson [7], c s = 0.5 (neglecting small deviations from stoichiometry). This leaves m as the only variable parameter. For similar growth conditions, there is a good agreement of the best-fit values of m with those given by Hsieh [9] (for GaAs layers) and by Isozumi et al. [10] (for the (Ga,AI)As layers). Suppression of the (Ga,A1)As growth rate by the presence of As vapour, apparent in fig. 3, is an interesting observation, especially in conjunction with the fact that no such effect has been detectable in the GaAs data (fig. 2). Because the nucleation phenomena do not play a controlling role in layer growth under the given conditions, this occurrence must be ascribed to the influence of arsenic pressure on the slope of the liquidus surface. The latter effect is apparently enhanced by the presence of A1 in the melt.
4. Surface and cross-section morphology (Ga,AI)As/GaAs double heterostructures
of
The surface of the D H chips intended for stripe-contact injection laser fabrication must be planar, smooth and mirror-like. These require-
Fig. 4. Surface optical micrograph of (Ga,A1)As DHs grown: (a) with and (b) without optimum arsenic vapour pressure. Magnification ca. 100 x .
ments are dictated mainly by the photolithographic process used to form striped ohmic contacts of 5-10/~m width. Previously the best results have been obtained with the near-equilibrium LPE method [11]; however, there still remains considerable scope for improvement. Fig. 4 demonstrates that the CVP method may be helpful in this respect. The perfect surface in fig. 4a has been obtained under optimum arsenic vapour-pressure conditions. By contrast the surface in fig. 4b, pertaining to a sample grown by the conventional method, exhibits pits, hillocks and linear grooves. Formation of these discrete features is apparently associated with higher point-defect concentration. These results are similar to those reported by Nishizawa et al. [3] for simple epitaxial layers. The beneficial effect of the CVP method on planarity
J. Novotn~ et aL / LPE growth of (Ga,A1)As / GaAs double heterostructures
P - GaAs _ Gao.TAto3As ~-GaAs 0.2/Jm active foyer
q - GOo.TA!.o~jAs N-GaAs Substrate (a)
P - GaAs P - Gao.TAtozAs P-GaAs ~ active layer
N- GaAs Substrate
(b) Fig. 5. SEM view of the chemicallyetched DH cross sections: (a) with and (b) without CVP.
and perfection of the (Ga,A1)As/GaAs DHs i s also seen on the SEM micrographs of the chemically etched cross sections in fig. 5. The extent of lda
......---.---
1017 8
5 i
i
i
i
/ ..., ~¢. .. t...r "'i."" i L
i
i
i
k
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the irregularities in the direction perpendicular to the interfaces is estimated to be less than 15 nm for the sample shown in fig. 5a. Corresponding conclusions can be inferred from the capacitance-voltage measurements performed on the DH samples. The CVP samples exhibit linear C - " versus V plots (n = 2.36-2.66, which is quite close to the value, n = 2, valid for ideal rectangular step profiles). Samples manufactured by the conventional method show dependences that cannot be fitted into a broad voltage range by a law of this type. This means that employment of the CVP method favours the formation of abrupt and well defined dopant concentration profiles (fig. 6).
5. Lasing properties
N - Gao.7 Ato.3 As
5 % u
599
;
02 x(~ml
Fig. 6. Dopant concentration profiles in (Ga,A1)As DHs: ( . . . . . . ) withoutCVP; ( ) underoptimumarsenicvapour pressure; (-- -- --) under excess arsenic vapour pressure.
The prepared DH wafers were provided with stripe and broad-area contacts, cut into individual laser devices and mounted on standard gold-plated copper holders. Fig. 7 shows the light intensity versus current characteristics of two sets of laser samples (with a rather high value of the active layer thickness, d = 1 ~tm) prepared by the conventional (fig. 7a) and by the CVP (fig. 7b) method. The manufacturing procedures are otherwise identical. Threshold currents of the CVP samples are seen to be about half of those of the samples prepared with no arsenic pressure. We found the following laser construction to represent an optimum when using medium quality (dislocation density 104 cm -2) (100)+20' GaAs substrates (Si doped 2 x 1018 cm -3, supplied by the Sumitomo Corp.): first confining layer Gao.65Alo.35As:Sn (6 x 1017 cm -3) of thickness 2.5/~m, active layer GaAs : Ge (1 x 1017 cm -3) of thickness 0.25 /xm, second confining layer Ga0. 7 A10.3As : Ge (5 x 1017 cm -3) of thickness 1.8/~m and contacting layer GaAs : Ge (1.5 X 1018 cm -3) of thickness 1.0 /~m. The initial growth temperature was 800°C, and the arsenic vapour pressure within the epitaxial boat (1.3-5.2) x 103 Pa. The cw room temperature threshold current values of the lasers range typically from 100 to 200 mA with a differential quantum efficiency of 80-95%. The temperature dependence of the
600
J. Novotny)et al. / LPE growth of (Ga,AI)As/ GaAs double heterostructures
AGA - 2
GA-16
~4 &.
&. 3
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i
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0.4
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0.8
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0.B
10 [ (A)
Fig. 7. Dependence of emitted-light intensity on the laser current: (a) without and (b) with CVP. Characteristics measured on several devices fabricated from the identical wafer are displayed in each case.
threshold current in the range 10-50°C can be approximated by the customary empirical law I,h e x p ( T / T o ) , with the parameter TO values situated near to 350 K. This is a very good value for this
type of laser (cf. ref. [12]). The temperature dependence of Ith for some selected samples can be even smaller (fig. 8).
6. Conclusions 8
7
cwo,~o~ioo of
lo
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(C-QAtlAs taser
20°C"~11
60oc
~6 E CL 5
I
I
100
200
J
300 I (mA)
Fig. 8. Light-current characteristics of a selected laser device for a set of temperature values. P = optical power, 1 = current.
The experimental results presented in this publication show that controlled arsenic vapour pressure in the LPE apparatus favourably influences not only the structural characteristics of (Ga,A1)As based DHs, but also their lasing characteristics, such as the differential quantum efficiency, the threshold current and its temperature dependence. Studies aimed at establishing the influence of CVP on aging properties of the lasers are under way. A better understanding of the observed changes in growth kinetics will have to await clarification of the way the controlled arsenic vapour pressure influences the liquidus surface. The observed improvement in lasing characteristics is perhaps due to a reduced dislocation density, which in turn is caused by a reduction in the point-defect density. It should be noted that good laser properties were obtained, notwithstanding the fact that the sub-
J. Novotn~ et al. / LPE growth of (Ga,AI)As/ GaAs double heterostructures
strates were only of a moderate quality. The theoretical discussion will be published separately.
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