UDC 541.1+620.18

PHYSICS

N. V. ALEKSEEV, L. S. MILEVSKII, L. P. NOVIKOVA

X-RAY DIFFRACTION STUDY OF THE STRUCTURAL PERFECTION OF INDIUM ARSENIDE SINGLE CRYSTALS

(Presented by Academician N. V. Ageev on 5 February 1970)

Methods of X-ray topography are widely used for studying structural defects in single crystals of germanium ($^1$), silicon ($^2$), and also semiconductor compounds of the type $A_{\mathrm{III}}B_{\mathrm{V}}$ ($^{3,4}$). The causes of dislocation formation during impurity diffusion in germanium ($^5$) and silicon ($^6$) single crystals have been studied by the method of anomalous transmission of X-rays. During impurity diffusion in germanium and silicon, as a rule, rectilinear dislocations are formed parallel to the external surface of the plate. They lie in the region with the maximum concentration gradient of the diffusing impurity; moreover, with increasing duration of the diffusion anneal, the depth of occurrence increases ($^5$), and the dislocations seem to follow the diffusion front. The formation of dislocations during arsenic diffusion in germanium was discussed in ($^7$).

In single crystals of various types, curved dislocations oriented at small angles to the growth axis are observed more often ($^8$).

In the present work, the perfection of InAs single crystals was investigated by the method of anomalous transmission of X-rays.

Method for obtaining topograms. The single crystals studied were grown by the Czochralski method with growth axis [111], specific resistivity 0.00024 ohm·cm, doped with Te, with a dislocation density of $3.6 \cdot 10^3\ \text{cm}^{-2}$ and an impurity concentration of $4 \cdot 10^{18}\ \text{cm}^{-3}$.

For the investigations, plane-parallel plates 800 μ thick were cut, the surfaces of which coincided with the crystallographic planes (111), (110), (100). The plates cut from the ingot were ground on a fine powder on both sides and brought to the required thickness with the aid of a polishing etchant of the following composition: 1 part HF, 3 parts HNO$_3$, 1 part H$_2$O. For the investigations, the Borrmann anomalous-transmission X-ray technique was used, with a Lang-type camera ($^9$), using MoK$_\alpha$ radiation and reflections of type {220}.

Experimental results. Figure 1 presents a topogram of a {100} plate cut at an angle to the growth axis $\langle 111\rangle$, with reflection from the {220} plane. The characteristic bands indicate layered growth with a sharply varying concentration of the dopant impurity in neighboring layers. The region shown in Fig. 1 is a transition region from the flat face located in the central part of the ingot to the peripheral part adjoining one of the lateral sides. The layering is formed by the superposition of oscillations that differ noticeably in period and, as follows from the figure presented, arose as a result of the addition of at least two frequencies. Similar effects had previously been observed in silicon ($^{10}$), but were revealed from the distribution of electrically active impurities, i.e., of the specific resistivity.

The dislocation structure of the crystals studied consists of chaotically distributed curvilinear dislocations of mixed type (Fig. 2) and rectilinear dislocations lying in the plane of the crystallization front, the plane ($\bar{1}\bar{1}\bar{1}$), along lines of intersection with the planes {111} inclined to the growth axis, i.e., along the directions [110].

Straight-line dislocations have a two-color contrast: white and black, similar to dislocations in germanium that arise during the diffusion of arsenic (5).

Fig. 1. Topogram of a plate with surface orientation (100), reflection \(\langle 220\rangle\), Borrmann method, magnification \(20\times\). On the left is the region of the central part of the ingot; on the right, the region of the peripheral part of the ingot.

Upon changing the sign of \((\bar{g}\bar{b})\), the contrast of these dislocations changes to the opposite. They are 60-degree dislocations with slip plane \(\{111\}\) and Burgers vector in the \([110]\) direction.

Fig. 2. Dislocations of various types and particles of a second phase in an InAs crystal. Surface orientation (111), reflection \(\langle 2\bar{2}0\rangle\), Borrmann method. \(20\times\)

It is seen in Fig. 2 that the intensity along the lines changes noticeably from one segment to another, and also at the points where dislocations from different systems intersect.

In addition to straight-line dislocations, particles of a second phase are often observed in InAs crystals; their strain field is visible in Fig. 3.

The rosettes formed by these particles have four clearly pronounced petals—two black and two other white ones, arranged crosswise.

Discussion. The mechanism of dislocation formation during the diffusion of arsenic into germanium, proposed in (7), makes it possible to understand the reasons for the formation of regions of rectilinear dislocations in InAs crystals grown with heavy tellurium doping. According to this mechanism, an impurity diffusing into the bulk of a plate causes dislocation climb in the regions penetrated by the diffusion layer. In the region with an increased impurity concentration, an extra atomic half-plane is built up by interstitial atoms from the supersaturated solid solution. This leads to the appearance of dislocation segments parallel to the front of the diffusing impurity.

Fig. 3. Image of dislocations, rosettes, and second-phase particles. Surface (111), reflection (220), Borrmann method. 20×.

An analogous process is possible when a dislocation lying in a \(\{111\}\) plane inclined to the growth axis and emerging with one end at the crystallization front intersects a crystal layer with an increased impurity concentration. This leads to climb of the dislocation segment in the region adjoining the layer, with the formation of a segment of rectilinear dislocation parallel to the crystallization front.

The presence of second phases (Figs. 2 and 3), apparently formed by the accumulation of Te atoms, indicates a considerable supersaturation of the solid solution in regions with an increased impurity concentration.

The process described above, first, leads to the appearance in the crystal of dislocation regions parallel to the crystallization front and, second, at a sufficiently high concentration of the doping impurity, facilitates the removal of most dislocations from the crystal. It is seen in Fig. 2 that some dislocations, after passing through a large part of the section under study, emerged at the external surface of the ingot.

Apparently, not all impurities act in InAs analogously to tellurium. However, it may be hoped that boron and phosphorus in Si, arsenic in Ge, and zinc in GaAs—i.e., impurities that cause the appearance of rectilinear dislocations during diffusion into the corresponding material—should facilitate the removal of growth dislocations from the crystal during pulling.

Institute of Metallurgy named after A. A. Baikov
Academy of Sciences of the USSR
Moscow

Received
29 I 1970

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