Ge doping of liquid phase epitaxial In0.53Ga0.47As

Journal of Crystal Growth 94(1989>270 2~2 North-Holland. Amsterdam

270

LE’VT’ER TO THE EDITORS Ge DOPING OF LIQUID PHASE EPITAXJAL In

0 53Ga047As

S.S. CHANDVANKAR and B.M. ARORA Tata Institute of Fundamental Research, Horn: Bhabha Road, Bombay 400 005, India Received 25 August 1988

Germanium doping of In0 53Ga047As grown by liquid phase epitaxy compensates the nominally n-type layers. At higher doping levels, thc Lyeis become p-type with signilkant wmpensdtlon.

1n051Ga047As is a material of considerable potential for microwave and optical devices. The as-grown material by liquid phase epitaxy (LPE) is generally n-type. Long bakeouts of the solutions are used by many workers to reduce the autodoping level to values about 1016 cm or less. Silicon is considered to be the impurity most responsible for the high level of autodoping [1]. Silicon is an amphotenc impurity, but seems to prefer the cation site. Among the other group IV impurities, Sn is well studied and found to be predominantly n-type. The behaviour of Ge in In0 53Ga047As, however, is still unexplored. Ge is known to be a p-type dopant for LPE grown GaAs. Its behaviour in InP is n-type and we have earlier reported its n-type behaviour in LPE In072Ga028As060P040 [2]. We find, however, that the Ge doping behaviour of LPE In0 51Ga0 47As departs considerably from the other members of the In1 ~Ga~As~, P1 (y 2.2x) alloy series. Details of this experiment are reported in this letter. The epitaxial layers are grown on (100) onented semi-insulating InP substrates (4 x 4.5 x 0.25 mm). The growth experiments are carried out over a temperature interval 599 592°C for 10 mm duration. The growth procedure is similar to the one described earlier [2] with some specific details outlined below. The solution is prepared according to the liquidus composition at 600°C: In 94.12 mol%, Ga 2.01 mol% and As 3.87 mol%, as given by Kondo et al. [3]. We generally use about 250 mg In, 2.75 mg GaAs/ 100 mg In and

3.33 mg InAs/100 mg In. We have used two different initial solution baking schedules before carrying out growths in the two sets of data reported here: (a) 650°C, 5 h and (h) 675 700°C, 24 h. Ge is added after the hake. In all cases, the growth substrate is loaded Just prior to the growth run. An additional bake of 650°C. 30 mm is done just prior to each growth. We can carry out a limited number of (two to three) successful growth experiments from a given solution. Therefore, the data reported are compiled from several solutions containing increasing amounts of Ge in each set. The grown layer is delineated by cleaving and staining in 1HF: 1H202: 10H20 solution for 2 mm. The typical epi-layer thickness is about 4 p~m. Close lattice matching is observed between the epi-layer and the substrate from X-ray diffraction measurements. All layers are characterized by optical transmission measurement for the hand edge at 0.74 eV. Composition of some selected layers is also checked by energy dispersive X-ray measurement and found to he close to the nominal value stated. The electrical behaviour of the doping is evaluated by resistivity and Hall effect measurements over the temperature range 77 300 K. Four indium contacts are alloyed at he corners of the sample at a temperature of 410°C in flowing atmosphere. Fig. 1 summarizes the results of Hall effect and resistivity measurements at room ternperature for two sets of samples grown from solutions containing an increasing amount of

0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

S.S. Chanduankar, B.M. Arora

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germanium. We see that in both sets of data, addition of Ge reduces the free electron concentration.which This are effect is seen in the solutions baked lessmore and clearly give a higher electron concentration ~ 1O~~ cm3) without Ge doping. The effect of the Ge doping is more dramatic on the mobility of the samples. For example, in the set II in which the reduction of free electron concentration is relatively small, mobility reduction is as prominent as in the set I. We are therefore led to the conclusion that the Ge doping produces strong compensation, i.e. the amphoteric nature is quite predominant with Ge atoms occupying both the cation and the anion sites. An estimate shows that at X~e 0.005 Ge atom fraction in the solution, the ND + NA is nearly 1019 cm , so that the compensation (ND + NA)/(ND NA) is nearly 100 or more as compared with the starting compensation of about 10 —

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or less. Increasing the Ge atom fraction X~e> 0.01 causes mobility to fall tothere valueisofa change nearly 25 2/V.the s and simultaneously in cm sign of the Hall coefficient indicating that the the samples become p-type. There is a slight increase in the net hole concentration with further increase in the Ge concentration in the solution with simultaneous decrease in the mobility value. Although the samples become p-type at high Ge mole fraction X~e~they are still highly cornpensated as shown by the low mobility values. Further insight into the nature of mechanism limiting the mobility is obtained from the temperature dependence measurements shown in fig. 2. We see that in the undoped n-type sample I, the mobility has the form of a lattice limited type, i.e. the mobility decreases with an increase in temperature. In sample II with X~e 0.0051, which is still n-type, the temperature dependence is re-

272

S.S. Chanduankar, B.M Arora

versed, the mobility increasing with temperature. The p-type sample also shows the same impurity scattering limited behaviour with a high degree of compensation. Pearsall et al. [4] have earlier studied the p-type dopants by using various epitaxial techniques. Their experiments with Zn, Cd and Mg also show that compensation is a common feature of p-type doping in all cases. In order to further characterize the Ge acceptor behaviour, we have plotted in fig. 3 the carrier concentration of two samples: (i) undoped and (ii) Ge doped, as a function of the inverse of temperature. The electron concentration of the undoped sample changes very little over the range of temperatures used here, indicating the shallow nature of the unknown donor energy levels. Similar curves are obtained for the samples which are grown from solutions up to X~0 — 0.05 and remain n-type. However, the carrier concentration versus 1000/T plot of the sample grown from a solution with X~e 0.0201, in which the conversion to p-type has occurred, shows a large temperature dependence. The activation energy obtained from the slope of this graph is — 26 meV. Since the samples are heavily compensated, we expect the hole concentration to follow a temperature dependence given by NA

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where /3 is the degeneracy factor of the acceptor [5]. Therefore, the activation energy obtained from fig. 3 is equal to LA, the Ge acceptor energy above the valence hand maximum. This EA value is very similar to the value of the Zn acceptor level in this material, as reported by Rao [6] from photoluminescence measurements.

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In conclusion, we have found that Ge behaves as a net acceptor type dopant in LPE 1n1153Ga047 As. It compensates the n-type background in small Ge concentrations and produces p-type behaviour when the Ge concentration in the solution exceeds 0.01 atom fraction. The materials are, however, highly compensated.

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References

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[1] PA. Houston, J Mater. Sc: 16 (1981) 2935. [2] Chandvankar, R.and Rajalakshmi. AK. Srivastava [3] S.S. S. Kondo. T. Amano H. Nagai. J. Crystal Growthand 64 B.M. Arora, J. Crystal Growth 88 (1988) 303. (1983) 433. [4] T.P. Pearsall, G. Beuchet, J.P. Hertz, N. Visentin and M Bonnet, in: Proc. Compounds, Vienna, 8th 1980, Intern.Inst. Symp. Phys. on Conf. GaAs Ser. and Related 56, Ed. H.W. Thim (Inst. Phys, London Bristol, 1981) p. 639. [5] J.S. Blakemore, Semiconductor Statistics (Pergamon. Ox ford, 1962) p. 139. [6] M.V Rao, J. AppI. Phys. 58 (1985) 4313