Press 1971. Vol. 14, Pp. 1259-1263.
Printed in Great Britain
ELECTRICAL PROPERTIES OF MELT-GROWN SILICON MONOARSENIDE* T. L. CHU, A. KUNIOKAt and R. W. KELM. JR. Electronic Sciences Center, Southern Methodist University, Dallas, Texas 75222, U.S.A. (Received7
May 1971; in revisedform
14 June 1971)
Abstract-Silicon monoarsenide crystallizes in the monoclinic structure and has an optical energy gap of 2.2 eV. Single crystal ingots of silicon monoarsenide have been grown by the Bridgman technique. These crystal cleaved readily along a (701) plane. The electrical resistivity and Hall coefficient of cleaved specimens have been measured over a wide temperature range. Silicon monoarsenide was found to exhibit very pronounced resistivity anisotropy, the resistivity along the direction perpendicular to the (201) being more than 500 times that along the b-axis. From the resistivity data, an acceptor level with an ionization energy of 0,135eV was deduced, and the energy gap of silicon monoarsenide was determined to be 2.03 eV at 850°K. R6sum6-Le monoarsenure de silicium se cristallise dans la structure monoclinique et possede un espace d’bnergie optique de 2,2 eV. Des lingots de cristaux simples de monoarsenure de silicium ont 6te cultivks p& la-technique de Bridgman. Ces cristaux ont Cti aisCment coup& au long d’un plan (301). La r&istivit& Clectrisue et le coefficient de Hall d’6chantillons COUP& ont 6t6 mesutis sur une &a&e gamme de temp&at&es. Le monoarsenure de silicium a montrk uie anisotropie de r&stivit6 marqute, la r6sistivit6 dans le sens perpendiculaire au plan (201) &ant plus de 500 fois celle au long de l’axe b. Des r&ultats de r&.istivit& un niveau d’accepteur ayant une tnergie d’ionisation de 0,135 eV a Ctb dkduit, et l’espace d’tnexgie du monoarsenure de silicium a Ct6 trouvC &tre de 2,03 eV g 850°K. Zusammenfassung-Siliziummonoarsenid kristallisiert mit monokliner Struktur und hat einen optisthen Bandabstand von 2,2 eV. Es wurdcn einkristalline Proben nach der Bridgeman-Technik gezogen. Diese Kristalle spalten leicht entlang (201)-Ebenen. Der spezifische Widerstand und der Hal&&or wurde an gespaltenen Proben in einem weiten Tempemturinterwall gemessen. Da_s Material zeigt eine ausgepriigte Anisotropie des spezifischen Widerstandes, der senkrecht zur (ZOl>Ebene iiber 500-mal gr6Ber ist als in Richtung der b-Achse. Aus den MeBergebnissen wurde ein Akzeptor ermittelt, dessen Aktivierungsenergie 0,135 eV [email protected]
Der Bandabstand des Siliziummonoarsenids wurde bei 850°K zu 2,03 eV bestimmt. 1. INTRODUCTION
of this relatively large energy gap, silicon mono-
SEVERALmembers of the binary compounds formed by the group IV and group V elements are semiconductors. For example, the combination of silicon and arsenic yields silicon monoarsenide and silicon diarsenide [ 11. The cubic modification of the diarsenide behaves like a semimetal, and the monoarsenide is a semiconductor with an optical energy gap of approximately 2-2 eV [33. Because
arsenide has potential applications for high temperature devices and for optoelectronic devices. Silicon monoarsenide crystallizes in the monoclinic structure with a = 15.98 A, b = 3668A, c = 9.529 A, p = 106-O”,and the unit cell contains twelve formula units . Its structure may be described as consisting of layers of distorted arsenic octahedra parallel to the (201) plane. In each layer, the arsenic tetrahedra are joined by sharing edges, and each octahedron contains two silicon atoms. Thus, each silicon atom is bonded tetrahedrally to three arsenic atoms and one silicon atom, and each arsenic atom has three silicon
*This work was supported
by the National
Foundationunder Grant GK3944. ton leave from the AoyamaGakuin University, Tokyo, Japan.
T. L. CHU,
atoms as the nearest neighbors. The shortest distance between arsenic atoms of adjacent layers is 360 A, indicating that the binding force between layers is presumably of the van der Waals type. In earlier investigations, silicon monoarsenide crystals were grown by the chemical transport technique in the form of thin ribbons. These ribbons were not suitable for electrical characterization because of their small thicknesses. Furthermore, the vapor grown ribbon was found to be twinned with respect to the (201) plane, parallel to its main faces. Recently, single crystalline silicon monoarsenide ingots, up to 9 mm diameter, have been grown from the melt by the Bridgman technique[S]. In the present work, the electrical resistivity and Hall coefficient of melt-grown silicon monoarsenide have been measured over a wide-temperature range to determine the energy gap of silicon monoarsenide and the ionization energy of impurities in the melt-grown crystal. The results are discussed in this paper. 2. CRYSTAL
Silicon monoarsenide crystals were grown from the melt by the Brigdman technique. Since silicon monoarsenide decomposes at about 700°C and above, the melt-growth must be carried out under an arsenic pressure equal to or higher than its dissociation pressure. Briefly, polycrystalline silicon monoarsenide was prepared by the reaction of silicon, arsenic, and iodine in a closed tube. The polycrystalline material and a suitable amount of arsenic were then placed in a fused silica Bridgman tube of 9 mm i.d., evacuated to less than 10e5 Tort-, and sealed. The amount of arsenic was sufficient to provide an arsenic pressure of about 3 atm at the melting point of silicon monoarsenide, 1083” C. The tube was heated in a vertical furnace with a controlled temperature gradient, and the molten silicon monoarsenide was allowed to solidify from the tapered end, thus yielding a single crystal. The grown crystal exhibits a metallic luster and cleaves readily along its length. The cleaved specimens were confirmed to be single crystalline silicon monoarsenide by X-ray oscillation and Weissenberg techniques. The growth direction was deduced to be parallel to the b-axis, and the cleavage occurred along the (201) plane. Cleaved specimens were used in most of the subsequent experiments. When a flat surface was desirable, the cleaved face was mechanically polished with
and R. W. KELM, JR. alumina abrasives and chemically etched with a chromic acid-hydrofluoric acid mixture. 3. OHMIC CONTACTS
The melt-grown silicon monoarsenide crystals were found to be p-type by thermoelectric probe measurements. The use of aluminum as an ohmic contact was investigated. Aluminum was evaporated onto the cleaved surface of silicon monoarsenide from a tungsten filament under a pressure of less than 10-5T~rr, and the specimen was heated at 600°C for the alloying of the contact. The resulting contacts exhibited linear currentvoltage characteristics over several orders of magnitude of current range. To measure the contact resistance, five aluminum contacts were made on the cleaved surface of a silicon monoarsenide specimen of 12 X 8 X O-2 mm in size, with its length parallel to the b-axis, and lead wires were bonded to the contacts by using a silver epoxy adhesive. The geometry of the specimen is shown in Fig. l(a). Using a current of 1 mA through the specimen, the voltage drop across the various contacts were measured at room temperature. This voltage drop is a linear function of the distance between contacts, as shown in Fig. l(b). An extrapolation of this relation to zero bias yields a contact resistance of 5 0 or a specific contact resistance of 0.2 &cm*. Thus, aluminum provides an ohmic contact of relatively low resistance compared with the resistance of the melt-grown silicon monoarsenide. 4. ELECTRICAL
The current-voltage characteristics of silicon monoarsenide specimens with cleaved main faces were measured at 77” and 300°K with the current flow along different crystallographic directions in the specimen. Figure 2(a) shows the geometry of two of the samples used for the measurements. Sample A with two electrodes on the same surface was used for the Z-V measurements along the direction of the b-axis, and sample B with two electrodes on the opposite faces was used for the measurements along the direction perpendicular to the (201) plane. These samples were prepared from the neighboring regions of a crystal, and their log I vs. log V plots are shown in Fig. 2(b). At room temperature, the slope is unity over four orders of magnitude of current range in both cases, designating the ohmic region. At 77”K, however, the slope of the log Z vs. log V plot for sample B
Fig. l(b). Fig. 1. (a) Geometry of the siliconmonoarsenidespecimen
used for contact resistance measurements. (b) Voltage drop across aluminum contacts on silicon monoarsenide as a function of distance between contacts by using a currentof 1mA. increased to two at bias higher than about 35 V/cm, indicating a space-charge-limited current situation. The space-charge-limited current was also observed at room temperature at biases higher than those shown in Fig. 2(b). The room temperature resistivity of silicon monoarsenide calculated from the I-V data of sample A is l-5 n-cm, and that of other samples measured in the direction of the b-axis is also similar. The resistivity calculated from the data of sample B is approximately lOOOC!-cm; however, that of other samples measured in the direction perpendicular to the (201) plane varies in the range of lo- 1000 &cm. The low resistivity is presumably related to the presence of defects which serve as low resistance paths. Thus, the ratio of the
SSEVoL 14,Na 12-F
Fig. 2(b). Fig. 2. (a) Geometry of silicon monoarsenidesamples for I-V measurements along the direction of the&-axis (A) and along the direction perpendicularto the (201) plane (B). (b) Current-voltage characteristics of silicon monoarsenide crystals at 77” and 300°K.
T. L. CHU, A. KUNIOKA
resistivity along the direction perpendicular to the (201) plane to that along the direction of the b-axis is very high, about 650. The room temperature resistivity of silicon monoarsenide in the [IO21 direction, perpendicular to the two directions under discussion, was also measured. The ratio of the resistivity along the  direction to that along the direction of the b-axis is approximately 2. The electrical resistivity of silicon monoarsenide along the direction of the b-axis was also measured over the temperature range 774350°K. A cleaved silicon monoarsenide sample was used, and its geometry was similar to that of sample A shown in Fig. 2(a). The sample was mounted on a ceramic disc by using Saureisen cement, and the aluminum contacts on silicon monoarsenide were connected to the metal contacts on the disc by evaporated aluminum. Figure 3 shows a plot of the logarithm of electrical resistivity vs. the reciprocal of the absolute temperature. The room temperature resistivity of this sample is approximately 1.5 &cm. The temperature dependence of the resistivity at temperatures below 25O”K indicates that two or more acceptor levels are present. The slope of the linear portion of the plot corresponds to an ionization energy of 0.13 eV. At temperatures
and R. W. KELM, JR.
above 8OO”K, intrinsic ionization dominates, and the logarithm of the intrinsic resistivity is plotted vs. the reciprocal of the absolute temperature in Fig. 4. The energy gap of silicon monoarsenide was determined from this plot to be 2.03 eV at 850”K, in agreement with the room temperature optical gap of 2.2 eV [31.
Fig. 4. Electrical resistivity of silicon monoarsenide in the direction of the b-axis in the intrinsic region.
Fig. 3. Electrical resistivity of a melt-grown silicon monoarsenide sample in the direction of the b-axis as a function of temperature.
5. HALL MEAguREMlWTs The Hall coefficient of silicon monoarsenide was measured in the temperature range 77-300°K. A conventional bridge-shaped specimen was fabricated from a cleaved silicon monoarsenide crystal of 8 X 2.5 X 0.25 mm in size. Aluminum contacts were used, and the current flow was parallel to the b-axis. To correct for the spurious ohmic drop associated with the misalignment of the potential contacts, the measurements were first carried out with both the magnetic field and steady current in the positive direction, then with the magnetic field positive and steady current reversed, then with both the magnetic field and current reversed, and tinally with the field reversed and current positive. In the case of anisotropic materials, such as silicon monoarsenide, the interpretation of Hall data in terms of material properties is not simple, and the carrier concentration is not defined by a single Hall coefficient. Thus, the
logarithm of l/&e is plotted vs. the reciprocal of the absolute temperature in Fig. 5. The linear portions of this plot yields activation energies of 0.135 and 0.75 eV. Figure 6 shows a plot of Rllp as a function of temperature; however, the interpretation of this data as a meaningful mobility is not certain.
‘5 d d \
Fig. 6. RI/p of a silicon monoarsenide crystal along the direction of the b-axis as a function of temperature.
Fig. 5. l/R,e of a silicon monoarsenide crystal along the
direction of the b-axis as a function of temperature. 6. SUMMARY The electrical properties of silicon monoarsenide crystals grown by the Bridgman technique have been studied. The melt-grown crystals have been found to be p-type, and alloying with aluminum has produced ohmic contacts with a specific contact resistance of 0*2CI-cm*. The electrical resistivity of silicon monoarsenide is highly anisotropic with high resistivity along the direction
perpendicular to the (20 1) plane. The energy gap of silicon monoarsenide at 850°K was deduced to be 2.03 eV from high temperature resistivity measurements. Hall measurements were carried out in the direction of the b-axis in the temperature range 77-300°K; however, no definitive conclusions can be made at this time.
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