Bulk growth of gallium antimonide crystals by Bridgman method

Transcription

1 Bull. Mater. Sci., Vl. 13, Ns 1 & 2, March 1990, pp Printed in India. Bulk grwth f gallium antimnide crystals by Bridgman methd U N ROY and S BASU Materials Science Centre, Indian Institute f Technlgy, Kharagpur , India Abstract. Gallium antimnide crystals were grwn by the vertical Bridgman technique. Effects f ampule diameter and dpant impurities (Te, P and In) n grwth were studied. Crystal stichimetry and hmgeneity were verified with electrn-prbe micranalysis. Impurity distributin was investigated by secndary in mass spectrmetry (SIMS) and electrn prbe micr analysis. Variatins f etch pit density (EPD) alng the.length and the diameter were studied by image analysis methd. Resistivity, mbility and carrier cncentratins were measured alng the length f the crystal. Keywnrds. III-V semicnductr; gallium antimnide; crystal grwth; Bridgrnan technique; iselectrnic dping. 1. Intrductin High quality gallium antimnide (GaSb) crystals have grwn in technlgical imprtance in recent years fr the fabricatin f pht-detectrs, in the wavelength range /~m (Naga et al 1981), and heterstructure detectrs and lasers fr fibre ptic cmmunicatins (Capass et al 1980; Chiu et al 1986). GaSb has als gt prime attentin fr quantum-well and superlattice structures (Munekata et al 1986; Sants et al 1988). Fr such devices gd quality GaSb substrates are required. The mst widely used methd t grw substrate quality material is the Czchralski (CZ) technique, but the crystals grwn by this technique frequently shw micrfaceted grwth and twins (Kumagawa 1978; Miyajawa et al 1980). Striatins and hetergeneity f impurities, alng the grwth directin and transverse t it, are als bserved in CZ-grwn crystals (Chin and Bnner 1982; Sunder et al 1986). The travelling heater methd is less studied fr grwing GaSb crystal because f extremely slw grwth rate (Benz and Muller 1979). Recently single crystal GaSb has been grwn by emplying the hrizntal Bridgman grwth technique (Lewadwsky et al 1985). Crystals grwn by vertical Bridgman technique generally shwed plycrystalline nature (Harsy et al 1981; Lendvay et al 1985). Hwever, Lendvay et al (1985) culd grw bicrystals in space using the same grwth technique. They bserved the average etch pit density t be 6'5 105 cm -2 and 5.x 105 cm -2 fr terrestrial- and space-grwn samples. Relatively less effrt has been made t study crystal stichimetry, impurity distributin, etch pit density distributins etc. n Bridgman-grwn GaSb. The present investigatin thus reprts bulk grwth f GaSb by the vertical Bridgman technique and effects f ampule diameters and different dpant impurities n the quality f the crystal. The crystal stichimetry was verified by micrprbe analysis at different psitins f the ingt t study the crystal hmgeneity. The impurity and etch pit density distributins were studied by secndary in mass spectrmetry (SIMS), electrn prbe micr analysis (EPMA) and image analysis, respectively. Variatins f electrical prperties alng the length and the diameter f the crystal were investigated fr the first time fr Bridgman-grwn GaSb. 27

2 28 U N Ry and S Basu 2. Grwth All the crystals were grwn frm stichimetric melt with slight excess f antimny t cmpensate fr its lss during synthesis and grwth. The synthesis was dne frm 5N r 6N purity Ga and Sb either in an argn atmsphere r in a vacuum f 10-6 trt. The grwth prcedure has already been reprted (Basu and Ry 1986; Ry and Basu 1987), but is described briefly belw. The grwth was carried ut in a cnical-tipped quartz ampule, in which the required amunts f Ga and Sb were placed and sealed at a vacuum f 5 x 10-6 trr. Synthesis was carried ut either in an argn atmsphere at 900 C fr ne and a half hurs r in vacuum in a sealed quartz ampule at 800 C fr tw hurs. The temperature f the furnace was raised at a rate f 250 C/h. After the synthesis, the ampule was kept in the Bridgman furnace as shwn in figure 1 and heated t 740 C fr 2 h. The ampule was then lwered with the help f a slw speed mtr at the rate f 0.94 mm/h upt 510 C and 9.4 mm/h upt 300 C. Finally the furnace was cled t rm temperature at a faster rate and the ingt was remved frm the ampule. 3. Characterizatin Frmatin f GaSb crystals was cnfirmed by the X-ray pwder diffractin methd. The surface mrphlgy f the crystals were studied with a high reslutin ~Pully GaSb melt I I ~y ff / Qurtz mpule Furnace wll H at~g cil...,. -.) :i "~ lo- d :i 2O I f I 1 1 J Temperture ( C ) Figure 1. Schematic f vertical Bridgman grwth unit alng with the temperature prfile f the resistance-heated furnace.

3 Grwth f gallium antimnide crystals 29 ptical micrscpe. The variatin f etch pit density (EPD) was investigated by image analysis methd using a Texture Analysis System (Leitz, Germany). The Ga t Sb rati was determined by electrn prbe micranalysis. Impurity distributins, as already mentined, were studied by SIMS and EPMA. The type f cnductivity was determined by the ht prbe technique and the electrical parameters were measured emplying the Van der Pauw methd, using indium fr hmic cntact. 4. Results and discussin Different ampule diameters used were 1"5, 1.3, 1.1 and 0"8 cm. Effect f dpant impurities were studied using different dping elements like P, In (iselectrnic) and Te. The typical ingt is shwn in figure 2, grwn frm an ampule diameter f 0.8 cm. The results are given in table 1. While the undped GaSb grwn frm a 1'1 cm diameter ampule was a single crystal, increasing r decreasing the diameter resulted in plycrystallinity. A similar effect was als bserved by Harsy et al (1981) and it was suggested that fr larger ampule diameters, nn-unifrm heat cnductin results in plycrystallinity, and fr small diameters the wall effect is respnsible fr plycrystallinity. Incrpratin f impurities during grwth reduces the grain size f the ingts, as bserved frm table 1, which is mainly due t the strain develped because f the difference in the atmic sizes f the dpant species and the hst atms. The surface mrphlgy shwed the presence f twins in almst all the crystals. The single crystal, hwever, shwed a twin nly near the (cnical) tip. Figures 3 and 4 shw typical twin structure and etch pit pattern, respectively. The EPD values were fund t be higher near the cre f the crystal as reprted earlier (Ry and Basu 1988), and were attributed t the thermal stresses due t the radial temperature gradient. The EPD values were als bserved t increase frm the tip t the upper end. The micrprbe analysis shwed fairly gd stichimetry and hmgeneity f the crystal. Ga t Sb ratis varied between 0.99 and 1.02 frm the tip t the upper end. At the upper end slight antimny lss due t vlatilisatin during grwth was bserved. The impurity prfiles fr Te (fr Te-dped crystals), Si and O were Figure 2. Typical GaSb ingt grwn by the Bridgman methd.

4 Average grain size (cm) 0"51 0"66 0" "6 1"5 1"8 4"5 2"7 0 ~ Table 1. Electrical prperties f GaSb crystals grwn under different cnditins. Ampule diameter Nature f Type f Resistivity Hall mbility Sample* (cm) Dping element ingt cnductivity (hm. cm) (cm2/v s) 1 1 "5 Undped Plyerystal P Undped Plycrystal P Undped Single crystal P ) Te-dped Plycrystal P ) (4-4 x 1016 cm -3) Te-dped Plycrystal n x (3-5 x 1018 cm -3) 6 1" 1 P-dped Plycrystal P ) (8 x 1018 cm -3) In-dped Plycrystal P ) (2-1 x 1018 cm -3) Carrier cncentratin (cm ) 2"64-2"07 5"83-3'15 2"78-2' ' ~' "99 2'26 Average EPD (cm ) ~0 *Samples 1 and 7 were grwn frm 6N purity starting materials; Other samples were grwn frm 5N purity materials.

5 Grwth f ~Tallium antimnide crystals 31 Figure 3. Optical micrgraph f grain bundary and twin. Etchant -IHNO3+ 1HF + IH2 O fr 15 s. Figure 4. Optical micrgraph f etch pits. Etchant - IHNO3 + 1HF+ 1H20 fr 15 s. recrded by SIMS depth prfiling and EPMA, alng the length f the crystal, and transverse t it, N fluctuatin f Si and O was bserved frm the SIMS prfile upt 0' 15 pm, but a little fluctuatin f Te (,~ 0"02%) was bserved alng the length f the crystal as shwn in figure 5. The values f the electrical parameters e.g. resistivity, mbility and carrier cncentratins are given in table 1. The carrier cncentratins were fund t increase frm the tip t the upper end which might be due t the segregatin f impurities near the upper end f the ingt during the lwering f the ampule frm the Bridgman furnace. The highest electrn and hle mbilities were fund t be 2217 cm2/v.s and 520 cm2/v-s crrespnding t the electrn and hle cncentratins f 6.55x 1017 cm -3 and 2.07x 1017 cm -3, respectively. The lwest average EPD value (1"6 105 cm -2) s far reprted was bserved fr an undped single crystal.

6 m 32 U N Ry and S Basu Sputtering rate : 300A/min Ga -~4 i J B 0 ""x,\/,.-"----'-- Sb Si re J J. 1.0 I... l... J 2.O Sputter time (min) 5.0 Figure 5. SIMS depth prfile f Ga, Sb and impurities (Te, O and Si). 5. Cnclusin Gd quality GaSb single crystals culd be grwn by the vertical Bridgman methd. The EPD value, 1.6 x 105 cm- 2, was fund t be the lwest s far reprted fr Bridgman-grwn GaSb crystals. References Basu S and Ry U N 1986 Prc. E-MRS Cnf. Strasbur.q (Paris: Les Editin de Physique) p. 71 Benz K W and Muller G 1979 J. Cryst. Grwth Chin A K and Bnner W A 1982 Appl. Phys. Lett Capass F, Panish M B, Sumski S and Fy P W 1980 Appl. Phys. Lett Chiu T H, Tsang W T, Ditzenberger and Van der Ziel J P 1986 Appl. Phys. Lett Harsy M, Grg T, Lendvay E and Ktlai F 1981 J. Cryst. Grwth Kumagawa M 1978 J. Cryst. Grwth Lewavdwski W, Gajewska N, Pastuszka B and Bugajski M 1985 Acta Phys Lendvay E, Harsy M, Grg T, Gyur I, Pzsgai I, Ktlai F, Gyulai J, Lhner T, Mezey G, Ktai E, Paszti F, Hrjapv V T, Kultchisky N A and Regel L L 1985 J. Cryst. Grwth Miyazawa S, Knd S and Naganuma M 1980 J. Cryst. Grwth Munekata H, Mendez E E, Iye Y and Esaki L 1986 Surf. Sci Naga Y, Hariu T and Shibata Y 1981 IEEE Trans. Electrn Devices ED Ry U N and Basu S 1987 Prc. E-MRS Cnf. Strasburg (Paris: Les Editin de Physique) p. 165 Ry U N and Basu S 1988 Mater. Lett Sants P V, Sd A K, Cardna M, Plg K, Ohmri Y and Okamt 1988 Phys. Rev. B Sunder W A, Barns R L, Kmetani T Y, Parsey J M and Laudise R A 1986 J. Cryst. Grwth 78 9