Gravity-Driven Convection Studies in Compound Semiconductor Crystal Growth by Physical Vapor Transport
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GRAVITY-DRIVEN CONVECTION STUDIES IN COMPOUND SEMICONDUCTOR CRYSTAL GROWTH BY PHYSICAL VAPOR TRANSPORT
JOHN A. ZOUTENDYK AND WESLEY M. AKUTAGAWA Jet Propulsion Laboratory, California Institute of Technology, Drive, Pasadena, California, 91109, USA
4800 Oak Grove
ABSTRACT Gravity-driven convection can alter the diffusive-advective mass transport behavior in the growth of crystals by physical vapor transport. Specially designed and constructed transparent furnaces are being used in our laboratory to study the effects of gravity in crystal growth of the compound semiconductors PbTe and CdTe.
INTRODUCTION Compound semiconductor crystal growth by physical vapor transport (PVT) has been done for many years [1]. Materials which exhibit reasonably-behaved evaporation characteristics (e.g., nearly-congruent evaporation) at temperatures below their melting point may (at least in theory) be grown by PVT. Most semiconductor materials which meet this requirement fall into either the IV-VI comSimply pound (e.g., PbTe, GeTe) or II-VI compound (e.g., CdTe, CdS) groups. stated, PVT is merely the sublimation of a material from a source held at a given temperature TS to a surface (e.g., seed, crucible) held at a lower temCrystal growth by PVT is generally done in a closed-tube cylinperature Ts-AT. drical ampoule usually made of quartz. Most PVT crystal growth has been done in conventional insulated furnaces. However, these furnaces do not permit the direct observation of crystal growth. More recently, transparent furnaces have been used [2,3] in order to visually monitor crystal growth and to perform insitu growth rate measurements as well as to observe nucleation, crystal-surface breakdown, etc. We are performing PVT experiments which utilize specially designed transparent furnaces to measure and observe the effects of gravitydriven (natural) convection. In this paper, we describe the furnaces used in our laboratory and summarize some experimental results obtained for the growth of PbTe crystals.
VAPOR TRANSPORT BEHAVIOR-THEORETICAL BACKGROUND Vapor-solid behavior of compound materials The vapor-solid behavior of specific compounds is all important in mass transport by PVT. We will discuss here two binary compounds, PbTe and CdTe, which represent two different modes of vaporization; i.e. 9 solid PbTe(s) vaporizes mainly into molecular PbTe(v) [4], whereas solid CdTe(s) dissociates primarily into the atomic vapor species Cd(v) and diatomic Te 2 (v) [5].
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TEMPERATURE (GC)
Fig.
1.
P-T Diagram for PbTe [4].
0
Lu ir (0
10 3 /T(K)
TEMPERATURE (°C) 900 700
1100 Fig. 2.
P-T diagram for CdTe [I1].
LM Cu
cc u. c.
10 3 /T(K)
451 Pressure-temperature (P-T) diagrams for PbTe and CdTe are shown in Figures 1 and 2, respectively. The solid-vapor (s-v) regions within the open-looped curves indicate the partial-pressure ranges of the major vapor components for compositions on both sides of the congruent (maximum) melting temperature. Also shown are the stoichiometric vapor pressures. The major differences between PbTe an
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